Oxides have attracted enormous attention for both application-driven and fundamental solid-state research owing to their broad range of unusual and unique properties. Oxides play an important role in countless applications, e.g., as catalysts or functional materials in electronic devices. The ability to produce high-quality epitaxial films is often crucial for these purposes. Various approaches to oxide epitaxy have been evolving for many years, each of which has its own features and strengths. However, oxide epitaxy also poses numerous challenges, the main ones being (1) the difficulty of finding a universal, versatile, and clean way to transfer an element from a source onto a substrate and (2) the ability to control the phase formation in a growing film. For oxides, this is an especially relevant topic due to the high oxidization potentials needed to achieve many desired compounds, the high temperatures required for numerous oxide phases to form, and the high temperatures necessary to grow films in adsorption-controlled growth modes. We provide a non-exhaustive overview of the state-of-the-art of oxide epitaxy within the context of these challenges. We also examine exciting advances and recent trends to overcome those challenges. Concluding, we discuss the implications of ongoing developments and the future of oxide epitaxy. An emphasis is put on thermal laser epitaxy and CO2 laser heaters, which we deem especially promising.

An important foundation of any technological development is the selection of materials whose properties are appropriate to the needs of specific applications. Owing to their wide variety of properties, oxides comprise some of the most promising materials for many key technologies, such as novel computing concepts (e.g., quantum1,2 and neuromorphic computing)3,4 and green energy applications (e.g., photovoltaics5,6 and catalysis).7–10 For example, these properties cover a broad range of electronic conductivity, from superconductivity11,12 to ultrawide-bandgap insulating behavior.13,14 Similarly, ionic conductivity can be very limited or superbly high.15,16 Oxides also exhibit an array of polar, electric, and magnetic properties.17,18 Therefore, it is not surprising that some of the most intriguing material properties have been discovered in oxides: high-Tc superconductivity,11,12 spin-ice frustrated order,19,20 and multiferroicity, to name but three examples.21,22

The understanding and application of these properties often necessitate the growth of high-quality epitaxial oxide films. There are numerous modes of epitaxial film growth, which are typically characterized by crystallization processes (e.g., step-flow, layer-by-layer, and Stranski–Krastanov growth) or methodologies used for stoichiometry control.23 Of the different stoichiometry control methods, adsorption-controlled growth plays an especially important role. Adsorption-controlled growth refers to the epitaxial growth of a film, during which the deposition parameters, i.e., source condition, substrate temperature Tsub, and chamber pressure, result in the volatility of all deposited species but one. This yields self-limiting growth, in which the growth rate is determined solely by the flux of the non-volatile species because all other species desorb if they are present in excess. This growth mode generally results in films with unsurpassed stoichiometric and phase purity.24,25

Thus, it is no coincidence that modulation-doped heterostructures of GaAs, the highest known electron mobility structures, are grown in an adsorption-controlled manner.26–30 In contrast to GaAs, epitaxial oxide films usually cannot compete with their corresponding single crystals in terms of perfection and purity. Various approaches have been proposed to overcome this drawback.31,32 An obvious solution is the adsorption-controlled growth of oxides. However, for most oxide constituents, volatility cannot be achieved in the deposition parameter space available to conventional deposition systems.33 For some oxides, epitaxial deposition already proves challenging, regardless of the growth mode. For example, pyrochlore iridates require crystallization temperatures far above 1000 °C combined with high oxidation potentials, which are impossible to realize with standard heaters,34,35 the usual limit of which is around 1000 °C Tsub. This restriction is also what has thus far limited the epitaxial growth of c-plane sapphire.36 Further complications are given by the fact that numerous elements show several oxidation states, which result in a plethora of possible crystal phases. Indeed, the most challenging oxides to grow are those that require high oxidation potentials and high crystallization temperatures. Outside the growth window of adsorption-controlled growth, big vapor pressure differences between the source materials may be problematic for maintaining the stoichiometry of the growing film.

This perspective provides an overview of the main challenges in connection with oxide epitaxy, which are (1) the quest for a universal, versatile, and clean way to transfer any element onto the substrate; and (2) a pure and controlled phase formation, which requires the ability to achieve high oxidation potentials and a high substrate temperature Tsub. We start by evaluating these challenges in more detail and then provide an overview of the state of the art of common oxide epitaxy methods with respect to these challenges. We discuss the most exciting trends to overcome these challenges and the resulting opportunities for oxide epitaxy. Due to space constraints, this analysis is not exhaustive but focuses on those topics that we consider the most important.

Epitaxial growth consists of two fundamental processes. First, in a film-growth system that ensures a high-purity environment, the source material is transferred to a substrate, where it arrives as adatoms. Then, the adatoms on the substrate crystallize in the desired phase and crystal orientation. Figure 1 illustrates these processes and provides desirable features and parameters of their essential components. Numerous mechanisms are used to transport the species from the source toward the substrate, as discussed below. Generally, the material can be introduced into the deposition chamber (Fig. 1, gray) either as a gas (light blue), a liquid, or a solid (blue, green, or yellow). In the latter case, these source materials must undergo a phase transition to be transported to the substrate (burgundy). In most cases, the formation of the epitaxial film with good crystallinity on the substrate requires a higher Tsub, which can be generated by various forms of heaters (red), the most common of which will be discussed.

FIG. 1.

Challenges and requirements posed by an epitaxial film growth system and associated with the heater are shown in pink, the recipient is shown in gray, the sources are shown in green, and the gas inlet is shown in blue.

FIG. 1.

Challenges and requirements posed by an epitaxial film growth system and associated with the heater are shown in pink, the recipient is shown in gray, the sources are shown in green, and the gas inlet is shown in blue.

Close modal

The transfer of an element to the substrate begins when it is introduced into the growth chamber. For oxide epitaxy, the most relevant gaseous source is usually the oxidizing agent (Fig. 1, light blue). High reactivity, i.e., a high oxidation potential, is desirable. This can be provided, e.g., by activated oxygen or ozone, which exposes the objects in the chamber to the risk of oxidation. Electronic filaments and heaters are particularly easily damaged by oxidation. Even worse effects may result from impurities generated by the oxidation processes of chamber components. Furthermore, oxidizing agents such as ozone pose a potential safety hazard due to their high reactivity.

It is desirable to be able to exchange the source of non-gaseous elements (Fig. 1, green) in the chamber without breaking the vacuum. This increases flexibility and throughput and decreases machine downtime. Once the element is loaded into the deposition chamber, the next challenge is to find a universal, versatile, and clean way for any element to be transferred to the substrate. This can prove difficult due to the elements’ drastically different vapor pressures and reactivities. For most deposition methods, element-specific solutions are, therefore, required for transferring elements to the substrate. In comparison to the film growth of the classic III–V and II–VI compound semiconductors, oxide epitaxy is often accompanied by the desire to access a wide range of elements. The versatility of the element transfer process is, therefore, an important specification.

Once the material reaches the substrate, epitaxial crystallization must be ensured by providing the required Tsub. Thus, in a versatile deposition system, a heater (Fig. 1, magenta) must accommodate a wide Tsub range that provides Tsub stability and homogeneity, ideally for any substrate material and size of interest. Providing very high Tsub proves especially challenging because the efficiency of most heaters declines with temperature, as does the coupling of the heat to the substrate. Simultaneously, more parasitic heat warms the components next to the heater, resulting in an increase in impurities released from the heater and its surroundings.

Therefore, for operation at higher substrate and source temperatures, the effects of parasitic heat must be considered in the design of the recipient system (Fig. 1, gray). The vacuum chamber must be able to withstand local hotspots and dissipate locally high heat loads. Generally, the recipient must provide a clean environment, i.e., background pressures that are as low as possible, together with inertness to the deposition conditions.

The increasing demand for epitaxial oxides is paralleled by the development of a variety of deposition techniques that address the associated requirements and challenges described above in their own unique ways. The film growth techniques most commonly used are pulsed laser deposition (PLD), sputtering, molecular beam epitaxy (MBE), metal–organic chemical vapor deposition (MOCVD), which, for epitaxial growth, is sometimes referred to as organometallic vapor-phase epitaxy (OMVPE), and the closely related atomic layer deposition (ALD). Other successful techniques include metal–organic aerosol deposition37,38 and chemical solution deposition.39,40 Several comprehensive reviews of PLD,41–44 sputtering,45–47 MBE,43,48–52 and MOCVD/ALD53–56 are available that include details of the operation and capabilities of the commonly used oxide epitaxy methods. Here, we focus on how they each address the requirements and challenges identified above.

In PLD, a target material (typically comprising the same stoichiometry as the desired film) is ablated by pulses of a laser beam with a typical wavelength of 248 nm. The target absorbs some of the laser pulses, resulting in a shock wave, ionization, and the formation of a plasma plume. This process is schematically depicted in Fig. 2(a). Details of the complex plasma and phase formation can be found elsewhere.42,57

FIG. 2.

Schematic depiction of the most commonly used oxide epitaxial growth systems using the same color scheme as shown in Fig. 1. (a) PLD with the laser shown in rose and the arrow indicating its direction. (b) Sputtering with argon; the sputter gas shown in purple. Black rings represent the random activation of some oxygen in these systems. (c) MBE with a shutter shown in gray. (d) MOCVD/ALD with arrows indicating precursor remains and the process gas exiting the reactor.

FIG. 2.

Schematic depiction of the most commonly used oxide epitaxial growth systems using the same color scheme as shown in Fig. 1. (a) PLD with the laser shown in rose and the arrow indicating its direction. (b) Sputtering with argon; the sputter gas shown in purple. Black rings represent the random activation of some oxygen in these systems. (c) MBE with a shutter shown in gray. (d) MOCVD/ALD with arrows indicating precursor remains and the process gas exiting the reactor.

Close modal

Despite the many parameters that influence the stoichiometry of the resulting epitaxial film, such as laser spot size,58–60 fluence,61–63 pulse repetition rate,64–66 and background atmosphere,67–69 the nominal, yet not always perfect, stoichiometric transfer of the target material to the substrate is a core strength of PLD.42 Typically, sintered polycrystalline targets that constitute a major source of impurities are used in PLD.70,71 These impurities can be strongly reduced by utilizing single-crystal targets, which are available in the required size only for a limited choice of materials. Although the stoichiometry of these targets, given the correct deposition parameters, is translated to a film with high reproducibility and reliability, this comes at the cost of flexibility. Experiments that study varying stoichiometries, dopants, or cation species usually require the synthesis of new targets for every new film composition.

It is generally more complex to control the oxygen stoichiometry in PLD than the cation stoichiometry. Owing to several loss mechanisms, typically only about 50% of the target’s oxygen is incorporated into the thin film.42,72 This loss of oxygen may be compensated by supplying oxygen as a background pressure because the oxide targets are robust against oxidation. If oxygen-resistant heaters are used as discussed below, background pressures in the range of 10−7 to 1 mbar may be applied (Fig. 3). The lower limit is set by the target outgassing and the recipient design, whereas the upper limit is set by the mean free path of the ablated species.73 However, the background pressure influences the cation stoichiometry in a complex way, as described in detail in Ref. 42.

FIG. 3.

Parameter space of the most common methods for epitaxial growth of oxide films in terms of applicable pressure p and substrate temperature Tsub. Boxes represent the currently available parameter space provided by most heaters, and the arrows indicate the expanded parameter space available through CO2 laser heating. State-of-the-art TLE systems (teal colored box) are equipped with CO2 laser heaters. The corresponding box, therefore, already expands over this parameter space. The arrow pointing upward indicates that the high-pressure limit has not been identified yet.

FIG. 3.

Parameter space of the most common methods for epitaxial growth of oxide films in terms of applicable pressure p and substrate temperature Tsub. Boxes represent the currently available parameter space provided by most heaters, and the arrows indicate the expanded parameter space available through CO2 laser heating. State-of-the-art TLE systems (teal colored box) are equipped with CO2 laser heaters. The corresponding box, therefore, already expands over this parameter space. The arrow pointing upward indicates that the high-pressure limit has not been identified yet.

Close modal

Owing to the species-dependent spatial composition of the plasma plume, the stoichiometry of the film may vary across the substrate area. For small samples (≤10 × 10 mm2), this is usually negligible. Upscaling PLD to large wafer sizes is not trivial, but significant progress in large-area PLD has been made over the past two decades.74–76 In industrial fabrication, PLD is a niche process despite the comparably high achievable growth rates (several 100 nm/h). In research and development, however, PLD is used as a workhorse, in particular, for the growth of complex oxides. Reviews of PLD used for oxide epitaxy can be found in Refs. 41–44.

The sputtering process utilizes plasma generated by electrical discharge. In this plasma, positively charged ions impact the negatively charged target, thereby ejecting the target species. The process is schematically depicted in Fig. 2(b). For enhanced sputter yields, magnetron configurations are used in which magnetic fields trap electrons in helical paths close to the target for efficient ionization of the sputter gas.45 

For oxide targets with at least a ternary composition, one of the constituent species is usually sputtered preferentially, resulting in a depletion of that species in the target. With time, this process equilibrates, and, in the ideal case, the resulting films have the desired stoichiometry. However, precise stoichiometry control in sputtering is challenging compared to PLD. Additional problems are oxygen back-sputtering and the high kinetic energy of the species hitting the growing film, which often causes defects. Again, the targets used for epitaxial oxide sputter processes are common sources of impurities.77 The possible high reproducibility and reliability of sputtering also come at the cost of flexibility. Sputtering requires a minimum background pressure to achieve stable plasma. Usually, additional oxygen is necessary to compensate for oxygen loss from preferential sputtering and related loss mechanisms.78,79 As a result, the typical total background pressure for oxide epitaxy by sputtering lies between 10−4 and 10 mbar (Fig. 3).80,81

Generally, sputtering can be upscaled in size, which, together with the stoichiometric transfer capability and comparably low cost, is a major advantage. While complex oxides can be sputter deposited, the most common application of sputtering is for binary oxides. This and the respectable achievable growth rates of several 100 nm/h make sputtering a commonly used method in industry.47,82 More details on the sputtering of oxides can be found in Refs. 45–47.

In MBE, the individual source materials are thermally sublimated or evaporated, typically from effusion cells, in which the source elements are located in crucibles ohmically heated by filaments. Effusion cells yield directed beams of the evaporated species, the so-called molecular beams. Molecular beams from sources that are hot but not used for growth at a given time are blocked by a shutter. The molecular beams are directed onto the substrates, where they adsorb and form the oxide films. The operating principle is depicted in Fig. 2(c).

Generally, MBE utilizes elemental sources, which have the advantage of having superior available purities compared to compound targets. Hence, of the deposition methods discussed in this section, MBE has the ability to produce the purest films.43,50 Furthermore, elemental sources provide flexibility. The modularity of the sources enables the combination of different elements available in the chamber. However, the size of the element portfolio is limited by the number of available source ports (typically ⪅9). Furthermore, sources cannot be exchanged easily in MBE. Without expensive special hardware, source changes even require opening the vacuum system to air. Ideally, the crucible material is inert under deposition conditions and does not release impurities. Therefore, the high temperatures needed to evaporate elements with low vapor pressures can drastically restrict the choice of crucible materials. In addition, the heater filaments must withstand deposition conditions while under high electric current densities. Therefore, for elements that yield a significant vapor pressure only above 1800 °C, standard effusion cells are inadequate. In this case, one commonly resorts to electron beam (e-beam) evaporation, which can evaporate most elements. e-beam evaporation, however, has the drawbacks of poor growth rate stability, high maintenance requirements, and proneness to impurity introduction (e.g., via impurities introduced by the filaments or electron-induced dissociation of residual hydrocarbon molecules).

The flexibility of source combinations in MBE comes at the cost of difficult stoichiometry control. The use of elemental sources requires excellent flux calibration to achieve well-controlled stoichiometries. For one single source, this calibration process may already take several hours. For most effusion cells, this requirement does not cause further difficulties because the flux drifts are smaller than 1%/h.83,84 However, some effusion cells, such as the ones used for Al and Ti evaporation or, even worse, e-beam sources, are not sufficiently stable to enable precise long-term calibration.83,85,86 To circumvent source instabilities and achieve stoichiometric films, the adsorption-controlled growth mode is preferable.87 As described in the introduction, the parameter space most oxides require for adsorption-controlled growth is unfortunately outside the substrate temperature range achievable with a standard MBE system.

Given the elemental sources used in MBE, an oxidizing agent is required to form the desired oxide phase on the substrate. The low kinetic energy of the species in the molecular beam and the failure of effusion cell filaments at high temperatures and oxygen pressures limit the background pressure in an MBE to <10−5 mbar, as depicted in Fig. 3. Applying this background pressure of molecular oxygen is insufficient to achieve many of the desired oxidation states, making it necessary to utilize more potent oxidizing agents such as ozone or oxygen plasma. Ozone is an unstable molecule that releases atomic oxygen, which makes it a much more potent oxidizing agent than molecular oxygen. This is apparent from Eq. (1), which yields the equivalent oxygen pressure (PO2) at a given ozone pressure (PO3) and Tsub,
PO2=PO32/3expΔG0RTsub,
(1)
where ΔG0 is the standard free energy of the formation of ozone from oxygen and R is the gas constant.87 Mixtures of 15% O3 and 85% O2 are readily achieved with commercial ozone generators, and higher ozone concentrations can be obtained through distillation. However, the reactivity of ozone is a potential source of impurities from reactions on the way to the vacuum chamber. It also poses safety hazards.88 A safer solution is to use oxygen plasma, as in plasma-assisted MBE (PAMBE). Oxygen plasmas contain up to 20% dissociated O.89 However, PAMBE introduces species with high kinetic energy into the system, which in turn causes defects in the growing films. In addition, the plasma generators themselves tend to generate impurities.

Upscaling MBE processes to large substrate areas is rather unproblematic. However, the achievable growth rates in MBE are relatively low, as required in order to maintain flux stability. On the other hand, the low growth rates and the resulting atomic precision make MBE a prime deposition technique for layered oxides. As a result, the strategy in the industry to combat low growth rates (<100 nm/h)90 and the resulting low throughput is to perform simultaneous growth on multiple wafers.91 More detailed information on oxide MBE can be found in Refs. 43 and 4852.

MOCVD and ALD are called chemical-deposition methods because the growth of the film is based on chemical reactions rather than physical deposition. All species are introduced into the recipient as gases. For those elements that are not available as gases, precursor molecules that contain the desired species are used. If the precursors are liquids, a carrier gas is required in addition. Within the recipient, in this case, also called a reactor, the precursors react under heat intake to form the desired phase on the substrate surface (pyrolysis). The precursor residues and carrier gas should ideally leave the reactor after the reaction of the desired species.56 This process is depicted in Fig. 2(d).

For any oxide to be grown epitaxially by MOCVD or ALD, suitable precursors of the involved cations are required that suit the parameter space demanded by the film growth. Indeed, multiple precursors exist for many cations. Using precursors poses the obvious challenge that precursor residues are introduced as impurities into the system and the growing films. This problem is especially relevant at lower Tsub, for which pyrolysis is less efficient.92,93 In addition, precursors are often a safety hazard.94 As precursors enter the reactor as gases, MOCVD and ALD are highly flexible. If a suitable precursor exists, elements can be combined at will for film growth without source swaps. Stoichiometry control of the injected gases is not trivial because it is difficult to predict exactly how the concentrations of the desired cations change as a function of time. One of the precursors can often be chosen to be more volatile, resulting in a growth mode somewhat equivalent to adsorption control,95 which is obviously more complicated for complex oxides.96 Multi-cation precursors can be a workaround, although this reduces flexibility and their availability is restricted.97 Thus, MOCVD is predominately used for binary oxides.

As the growth process in MOCVD and ALD is based on chemical reactions, the mean free path is of no concern. This allows a wide range of oxygen pressures to be applied. Considering that precursor fluxes add to the system pressure, the background pressure is usually much higher than in other deposition methods (10–100 mbar); see Fig. 3.98 The high applicable pressures and the usually self-regulating stoichiometry allow very high growth rates for MOCVD (≈2 µm/h).99 The unique ability of conformal growth and the effortless upscaling make MOCVD one of the most commonly used tools for the industrial growth of epitaxial oxides.56 More detailed information on oxide MOCVD and ALD is available in Refs. 53–56.

All film growth methods discussed here require substrate heating, which should preferably cover a wide range of Tsub and provide excellent control. In other words, there should be only a negligible difference between the real/measured Tsub and the setpoint value, both while maintaining a constant temperature and during temperature ramps. For this process, several types of heaters are used (Fig. 4).

FIG. 4.

Schematic depiction of important types of heaters: (a) resistive heater, (b) filament-based radiative heater, (c) quartz lamp, (d) infrared diode-laser heater, and (e) CO2 laser heater system. The back arrows indicate the direction of the laser beam.

FIG. 4.

Schematic depiction of important types of heaters: (a) resistive heater, (b) filament-based radiative heater, (c) quartz lamp, (d) infrared diode-laser heater, and (e) CO2 laser heater system. The back arrows indicate the direction of the laser beam.

Close modal

Resistive heaters are the simplest ones. An electric current is applied to a holder block to which the non-conductive substrate is attached [Fig. 4(a)]. In rare cases, the current is applied directly to the conducting substrate. Resistive heaters can be built to be rather oxygen-resilient and compatible with oxygen pressures of ≤1 mbar and to yield a maximum Tsub of up to 1000 °C. Convection increasing with pressure reduces the maximum achievable Tsub in the high-pressure regime. In the case of indirect heating, the maximum Tsub is decreased further by the temperature gradient at the substrate–block interface. This gradient is usually minimized by bonding the substrate to the heater block with metallic paint of Ag, Pt, or In. The limited temperature stability of these paints is an additional limiting factor for Tsub because they may diffuse or evaporate. Even worse, these paints are prone to contaminating the system. This process is exacerbated by the significant parasitic heat of such heaters. The thermal lag of the heating element is the limiting factor for the achievable ramp rates of Tsub. On the other hand, the large thermal mass supports maintaining a constant temperature, both in space and time. When using resistive heaters to which the substrates are glued, the entire heating element must be changed with each substrate change.

If thermal radiation is utilized for substrate heating, heat is usually produced by an electrically heated filament made of a Ni alloy, noble metal, or SiC. To couple the heat into the substrate effectively, the back of the substrate is usually coated with a metal film [Fig. 4(b)]. Depending on the filament in use and the applied pressure (up to 1 mbar), this setup allows for Tsub ≤ 1000 °C. Similar to resistive heaters, the thermal lag of the filament limits achievable Tsub ramp rates. For the filament itself and at high Tsub, the backside coating is also a possible source of impurities.36 As heat radiation is non-directional, parasitic heat management is difficult with such heaters at high Tsub. Filament-based radiative heaters work for all substrate sizes and are usually downward compatible in terms of size. A particular version of thermal radiation-based heaters features quartz lamps [Fig. 4(c)], which has the advantage that no electronic components are required in the chamber. Therefore, pressure is not limited by the heater. Damage to or impurities originating from electronic components are avoided. Quartz lamps enable Tsub ≤ 850 °C. Possible damage to the quartz lamps usually limits the Tsub ramps to a moderate ⪅5 °C/s.

Utilizing coherent light sources such as infrared diode lasers avoids most parasitic heat at the cost of more challenging scalability [Fig. 4(d)]. The issue of insufficient absorption for most oxides also applies to the wavelength of infrared diode lasers (λ = 940 nm), which is well absorbed by metals. Thus, the necessary backside coating limits the achievable Tsub to ≈1000 °C. Unlike non-coherent and resistive heaters, the achievable Tsub change rates are quasi unlimited; the only heated element is the substrate, which has a small thermal mass and can, therefore, quickly change temperature induced either by high-power heating or by radiative cooling. Therefore, Tsub ramps are exceptionally fast (≈100 °C/s). The insufficient absorption of the diode-laser light by oxide substrates is overcome by utilizing CO2 lasers instead, as these generate infrared light of longer wavelength (λ ≈ 10 µm); see Fig. 4(e). This wavelength is absorbed well by most commonly used oxide substrates (Table I), but not by metals, minimizing the necessity of absorbers for oxide epitaxy. The benefits of CO2 laser heaters will be discussed in detail in the next section. Substrate heater specs are adapted from Refs. 100–103.

TABLE I.

Substrates tested for heating by a ≈ 10 μm CO2 laser beam.

Well-absorbing substrates Al2O3, DyScO3, Ga2O3, GdScO3, KTaO3, LaAlO3, LaGaO3, LSAT, MgAl2O4, MgF2, MgO, NdGaO3, NSAT, SAGT, SLAO, SrTiO3, SrPrGaO4, SmScO3, TbScO3, YAlO3, Y2O3, YSZ, ZnO 
Substrates requiring high laser powers (poor absorption) AlN, Si, SiC 
Substrates that only work with some CO2 laser wavelengths (absorption edge close to 10 μm) TiO2 
Negligible absorption Diamond 
Well-absorbing substrates Al2O3, DyScO3, Ga2O3, GdScO3, KTaO3, LaAlO3, LaGaO3, LSAT, MgAl2O4, MgF2, MgO, NdGaO3, NSAT, SAGT, SLAO, SrTiO3, SrPrGaO4, SmScO3, TbScO3, YAlO3, Y2O3, YSZ, ZnO 
Substrates requiring high laser powers (poor absorption) AlN, Si, SiC 
Substrates that only work with some CO2 laser wavelengths (absorption edge close to 10 μm) TiO2 
Negligible absorption Diamond 

In the previous section, we described the most common oxide epitaxy methods with respect to their ability (1) to provide a universal, versatile, and clean transfer of any element onto the substrate and (2) to achieve sufficiently high oxidation potentials and temperatures to obtain the desired phases at high levels of quality and purity. Numerous efforts are ongoing to overcome the various limitations of existing techniques. In the following section, we will present recent trends that appear to be especially promising.

In hybrid MBE, MOCVD precursors are utilized as one or more molecular sources for MBE systems. Hybrid MBE was first developed in the early 1990s to obtain a stable flux of the low-vapor-pressure element Y, which was not achievable at that time with standard effusion cells.104,105 In subsequent years, hybrid MBE remained a fringe technique used for other low-vapor-pressure elements such as Zr and Nb.106,107 Nearly ten years later, the technique gained momentum when it was applied to circumvent the difficulties involved in evaporating Ti. It was then discovered that volatile metalorganic molecules can be utilized to achieve adsorption-controlled growth of SrTiO3.108,109 Importantly, Ti is already oxidized in the precursor. Therefore, less potent oxidizing agents suffice during deposition. Despite the impurities potentially introduced by the precursor, the adsorption control achieved by this approach has enabled the growth of oxide films with record electronic mobilities.110–112 Therefore, it is not surprising that hybrid MBE has since been applied to many material systems with impressive results.51,113 Most recently, for example, the growth of superconducting Sr2RuO4 has been demonstrated with hybrid MBE.114 Nevertheless, hybrid MBE is prone to several of the drawbacks encountered by MOCVD. For example, impurities are easily introduced by the precursor at low growth temperatures.93 As discussed in Sec. II D, precursors are also only applicable in a specific parameter range that ensures their effective pyrolysis.

In suboxide MBE, sources are applied to yield a phase-pure beam of suboxide species. These sources can be comprised of an oxide, a suboxide, or an oxide–metal mixture.33,115,116 Oxide sources were first used in MBE in the 1980s with the aim of providing a stable flux of easily oxidized elements.117 Shortly thereafter, the potential to use a highly volatile oxide of the low-vapor-pressure element boron was realized.118,119 Subsequently, however, oxide sources were used mainly for obtaining stable fluxes of easily oxidized elements.120–126 Suitable sources were identified by trial and error until thermodynamic modeling more recently allowed ideal oxide, suboxide, or oxide–metal mixed sources to be identified—first only for specific elements, then for all elements.33,127 This was the basis for a more systematic understanding and investigation of suboxide MBE. The development of suboxide MBE is motivated by the idea to utilize the volatility of suboxides to enable the evaporation of low-vapor-pressure elements as high-vapor-pressure oxides at much lower temperatures and to gain access to the adsorption-controlled growth regime while maintaining stable fluxes.116 Indeed, suboxide MBE has been utilized to grow adsorption-controlled Ga2O3 with unmatched crystal quality at MOCVD-like growth rates.115 In addition to the high growth rates obtainable with such sources, their flux is still reproducible and sufficiently controllable to grow complex ternary systems that have a high sensitivity to exact flux ratios.128 This newly gained knowledge about suboxide MBE has allowed ternary oxides with low-vapor-pressure constituents such as KTaO3 and SrMoO3 to be achieved.129,130 Currently, the remaining drawbacks of suboxide MBE are the limited choice of source elements and the lack of high-purity oxide and suboxide source materials.33,131

Whereas the two trends discussed in Secs. III A and III B represent developments of commonly used film-growth technologies, thermal laser epitaxy (TLE) is a separate technique. Indeed, the evaporation of source materials by lasers has a long history. In 1962, the growth of tellurides from compound sources evaporated with a ruby laser was first presented at the Optical Society of America meeting but not published until 1965.132 The same principle was quickly applied to growing metallic films from elemental sources.133 Upon the development of Nd:YAG lasers, they were commonly used for laser evaporation. Subsequently, this technology gained momentum and was used, for example, for growing tellurides.134–136 The technical boundary conditions at that time caused these lasers to be pulsed.137 Note that, because the wavelengths of the beams were not sufficiently short and the power densities of the pulses were small, these laser beams sublimated or evaporated the source materials rather than ablating them as done by PLD.138 Owing to the low growth rates and restrictions on film quality caused by the unfavorable deposition conditions used in the 1970s and 1980s for pulsed laser evaporation, this technique was mostly abandoned for PLD, by which epitaxial films of high-Tc superconductors could be grown with great success.139–141 Here, we note that the advantages of avoiding pulsed evaporation of source materials were already recognized in the 1960s. CO2 lasers were found to enable rather high growth rates and more uniform films.142 However, apparently due to high equipment costs and technical issues, the popularity of this technique eventually faded after 1990. A review of the work on laser evaporation in that era can be found in Ref. 138.

The concept of thermally evaporating sources by using lasers was revived in the late 2010s at the Max Planck Institute for Solid State Research in Stuttgart.143–145 The term “thermal laser epitaxy” (TLE) was then defined by the following characteristics: Evaporation (sublimation) of one or more sources by lasers, the entrance window being protected by an optical component, and the sources and substrate being exchangeable in situ.143 Because in PLD, the target is ablated rather than evaporated, PLD and TLE differ in their basic principles. In TLE, the substrate is also preferably optically heated to ensure the maximum available pressure range.143 This approach is schematically shown in Fig. 5(a) without the optical element protecting the entrance window of the source lasers. Figure 5(b) shows a 3D rendered image of the inside of a state-of-the-art TLE chamber, which, together with recent technological improvements, is described in the following. Beams of continuous-wave (CW) fiber lasers with a wavelength of ≈1 µm and a typical power of ⪅2 kW are aimed at elemental or compound sources, with one laser usually used for one source. Optical components mounted outside the deposition chamber allow the spot sizes on the sources to be adjusted. This beam control is important for evaporation from solid surfaces as well as from melt pools. Ideally, freestanding solid sources are used for evaporation, which is frequently possible for source materials that sublimate. For source materials that evaporate from the melt, tightly localized heating by the laser allows the formation of a melt pool contained in the source material itself, with some exceptions to be discussed below. The short thermal time scales in TLE make it possible to start, stop, and change the flux of the elemental sources quickly by changing the laser power, thus making shutters optional. As discussed below, the flux of the evaporated source material for a given geometry is a function of the incident power of the laser beam, which allows an estimation of the flux with first order accuracy.144 At present, TLE systems allow source materials to be changed in the same way as substrates. This avoids breaking the vacuum for source exchange and offers significant advantages in terms of throughput and flexibility. For (oxide) TLE systems, CO2 lasers are currently the ideal choice for most oxide substrate heating purposes (Table I).

FIG. 5.

(a) Sketch of a TLE system. Evaporation or sublimation of source materials can be “shuttered” by turning off the corresponding heating laser. The color scheme is the same as in Figs. 1 and 2. (b) 3D rendered image of the inside of a state-of-the-art TLE chamber.

FIG. 5.

(a) Sketch of a TLE system. Evaporation or sublimation of source materials can be “shuttered” by turning off the corresponding heating laser. The color scheme is the same as in Figs. 1 and 2. (b) 3D rendered image of the inside of a state-of-the-art TLE chamber.

Close modal

As discussed in Sec. II E, the ≈10 μm wavelength of the CO2 laser light is well absorbed by most commonly used substrates (Table I), but not by metals. Given the high laser powers and, particularly, the laser power densities available at present, the accessible Tsub parameter space of any deposition method is vastly extended if it is equipped with a CO2 laser heater, as illustrated in Fig. 3. The large Tsub space available by CO2 laser heating opens up new possibilities for oxide film growth via the adsorption control mechanism, for example, by using MBE, PLD, or TLE (see Sec. IV C). The long wavelength of CO2 lasers eliminates the need for beam absorbers thermally coupled to the substrates [Fig. 4(e)]. In this way, a significant source of impurities is avoided,36 which is a considerable advantage over diode laser heaters. The excellent Tsub control and high Tsub change rates of diode laser heaters are also achieved by CO2 laser heaters. Historically, CO2 laser heating was introduced for the PLD growth of YBa2Cu3O7.146–148 This technology was soon applied to substrate heating in CVD,149–151 where CO2 lasers had already been used for the enhancement of precursor dissociation.150–153 

A drawback of using CO2 lasers is the substantial investment costs, which are partially compensated by the higher available throughputs enabled by the high Tsub ramp rates. Improved equipment reliability and the commercial availability of deposition systems (first by SURFACE for PLD, to our knowledge)100 have recently revived interest in CO2 laser heaters. Therefore, it is not surprising that they are currently being used with all major epitaxy techniques.146–151,154,155

The new trends discussed in the previous section offer unique approaches to tackle the challenges of oxide epitaxy. Synthesis routes are being established that were previously thought impossible. In our view, CO2-laser substrate heating and TLE open a vast and fertile parameter space, and we will now focus on these two technologies. We do so by analyzing the characteristic features of TLE with respect to the transfer of source materials and the role of the oxidation potential of the background atmosphere during TLE growth. Then we present the opportunities provided by CO2 laser heaters for any film-deposition technology.

Generally, there are two fundamental strategies to ensure the desired stoichiometry of a mixture of species arriving on a substrate. First, one may try to transfer the stoichiometry of a target directly to the film (sputtering and PLD) or, second, one may adjust the flux of independent elemental sources to yield the desired stoichiometry (MBE, MOCVD, ALD, and TLE). As discussed above, stoichiometric targets usually introduce impurities into the system and inherently make it difficult to explore a multitude of different element combinations. Ensuring a stoichiometric transfer over time further proves problematic because the target species with a higher vapor pressure depletes preferentially. The greater the vapor pressure differences among the involved species, the greater the depletion effect. The challenges reported for the growth of KTaO3 by PLD illustrate this issue. In this specific example, the K deficiency caused by the high vapor-pressure discrepancy is compensated by using a K effusion cell, an approach referred to as hybrid-PLD.156 

Figure 6 compiles the vapor pressures of the chemical elements using a color scheme for the temperatures needed to achieve an elemental vapor pressure of 10−3 mbar. It is apparent in Fig. 6 that these vapor pressures cover a wide range. As discussed above, MBE elements that require T (p = 10−3 mbar) > 1800 °C necessitate the undesirable use of electron-beam evaporation. In comparison, the vapor pressure of the elements plays a minor role in MOCVD/ALD, where the challenge consists of finding a precursor that fulfills the deposition conditions. Table 1 of the Supplementary Material compares the (commercial) availability of source materials for MBE, MOCVD/ALD, and TLE sorted alphabetically, not considering elements that are gaseous under ambient conditions. For TLE, it has been found that all elements used so far are compatible with film growth by sublimation or evaporation.144 As the elements yet untested by TLE lie within the parameter range of the already tested ones, there is good reason to assume that nominally the entire Periodic Table is suitable for TLE (disregarding potential safety concerns).

FIG. 6.

Periodic table with elements colored according to temperature T necessary to achieve an elemental vapor pressure p of 10−3 mbar. Elements shaded gray are of limited interest for growing compound films because they are radioactive and/or noble gases.

FIG. 6.

Periodic table with elements colored according to temperature T necessary to achieve an elemental vapor pressure p of 10−3 mbar. Elements shaded gray are of limited interest for growing compound films because they are radioactive and/or noble gases.

Close modal

Technically, any CW laser is suitable for TLE if its light is absorbed by the source material. Indeed, we have successfully tested several different lasers for TLE. We have found CW fiber lasers operating at λ = 0.5–1 µm and power outputs of ⪅2000 W to be especially suitable. They enable the deposition of any element of interest. Considering Table 1 of the supplementary material, most elements in TLE chambers do not require a crucible and can, therefore, be used as freestanding sources. In this ideal case, contamination by crucible material is obviously excluded. Recent results for the epitaxial growth of sapphire by TLE reveal that, even for elements that require a crucible (Al in this case), the localized heating of the source by the laser leads to film purities even exceeding the purity of the underlying substrate single crystal.36 

In TLE, the flux of an elemental source is controlled by the incident laser power of the laser beam. This relationship is depicted in Fig. 7 for elements with widely different vapor pressures, using the same color scheme as shown in Fig. 6. It is evident that the laser power required to achieve a specific growth rate does not necessarily scale with the element’s vapor pressure. It is also clear that the source diameter plays a crucial role, as illustrated by comparing the iron fluxes. Additional factors that determine the required laser power are the element’s light absorption (or absorptance for larger irradiated areas) and its thermal conductivity. Despite the complexity invoked by this assortment of factors determining the relationship between laser power and flux, there seems to exist a rather linear relationship between the power of the incident laser beam on the source and the resulting local source temperature for the elements plotted in Fig. 7.144 To accurately assess the dependence of the flux on the laser power on a general basis, for example, in cases where the source material reacts in an oxidizing environment, an in-depth understanding of the evaporation or sublimation dynamics and oxide formation is required. Obviously, this is often complicated. Workarounds exist, such as the use of high laser powers, for which the source is less easily oxidized.85,157 This results in growth rates so high that for some applications, these may even be excessive. In our investigations, we have not yet observed any negative effects when using very high growth rates of up to 3 µm/h.

FIG. 7.

Arrhenius-type graph showing film growth rates as a function of the output power of the source laser for elements with drastically different vapor pressures. Data are shown for source diameters of 3 and 12 mm. Solid lines are fit to the Arrhenius-type behavior; the corresponding extrapolations are shown as dashed lines.

FIG. 7.

Arrhenius-type graph showing film growth rates as a function of the output power of the source laser for elements with drastically different vapor pressures. Data are shown for source diameters of 3 and 12 mm. Solid lines are fit to the Arrhenius-type behavior; the corresponding extrapolations are shown as dashed lines.

Close modal

Usually, the growth of complex oxides requires not only a controllable but also a stable flux. For some elements, this is currently a shortcoming of TLE. Flux variations are caused by changes in thermal mass over time, the evolving shape of the source during deposition, and changes in the source surface morphology. Controlling this evolution requires further research. Nevertheless, an elegant and powerful solution exists for the deposition of oxide films with complex stoichiometry because the extended parameter space provided by TLE with CO2-laser substrate heating enables the adsorption-controlled growth of most oxides. In that case, only the flux of the least volatile species must be controlled to stabilize the composition of the growing film. The applicability of this principle was recently demonstrated by the TLE growth of superconducting Sr2RuO4 films with excellent phase purity and very high residual resistivity ratios up to R300 K/R2 K = 88.158 

Finally, we point out an intriguing potential of TLE, which is provided by the capability to evaporate or sublimate nominally any element and the compatibility of TLE with robotic operation. These properties of TLE render it able to grow films and heterostructures of any element or compound with a very high level of automation. For example, one may envision artificial intelligence systems that design new films, heterostructures, compounds, or even devices and then fabricate them by robotic TLE film growth and a possible patterning process, with a subsequent AI-controlled analysis closing the loop. This principle is schematically shown in Fig. 8.

FIG. 8.

Schematic depiction of a growth system in which artificial intelligence (AI) is designing films, heterostructures, or even devices and controlling the growth and possibly even the patterning, e.g., by robotic mechanisms. A robotic TLE system can be provided with in situ access to all desired substrates and source materials, which may be elements of the periodic table, background gases, and all growth parameters, and produce the film. The film is analyzed, and the results are fed back to the AI, closing the feedback loop.

FIG. 8.

Schematic depiction of a growth system in which artificial intelligence (AI) is designing films, heterostructures, or even devices and controlling the growth and possibly even the patterning, e.g., by robotic mechanisms. A robotic TLE system can be provided with in situ access to all desired substrates and source materials, which may be elements of the periodic table, background gases, and all growth parameters, and produce the film. The film is analyzed, and the results are fed back to the AI, closing the feedback loop.

Close modal

In summary, TLE offers several unique opportunities to work with source materials. The availability of practically any element as a source paves the way to the epitaxial growth of oxides containing elements such as refractory metals, the elemental fluxes of which were previously difficult to produce. The practical in situ source exchange, paired with the availability of virtually all elements, further allows the growth of compounds that could not previously be grown by MBE. In addition, it provides unmatched throughput enabled by easy source exchange and high growth rates, which allow numerous compositions to be tested. Matching the ever-increasing speed of modeling materials’ properties by ab initio techniques,159 TLE offers rapid experimental feedback. Furthermore, the exceptional purity available through localized heating of sources is valuable for improving any (oxide) material for research and electronic applications.

Generally, two challenges are associated with oxidation in film growth: the stabilization of metastable oxidation states and oxidation states that require a high oxidation potential. The wide Tsub range of CO2 laser heaters allows access to a broad process window in which metastable oxides can be grown epitaxially. In addition, the extremely high cooling rate allows meta-stable phases to be preserved by suppressing further reactions or phase transitions during cooling.160,161

Figure 9 displays the oxygen pressure (PO2) required to achieve specific oxidation states of all elements considered relevant for film growth, based on a thermodynamic analysis of binary oxide formation reactions. The majority of these data were extracted from Shang et al.’s work,162 whereas data for Rh4+ were separately calculated based on input from Ref. 163. Underlying Ellingham diagrams for each element and oxidation state are provided in Fig. 1 of the supplementary material. Each panel in Fig. 9 is divided by groupings of integer oxidation states and sorted by the oxygen pressure necessary to achieve the respective oxidation states at 1000 °C. The required oxygen pressure is shown for three examples of Tsub at 500 °C (blue), 1000 °C (turquoise), and 2000 °C (magenta). The data show the general trend that high temperatures necessitate high oxygen pressure. The green line marks the maximum equivalent oxygen pressure compatible with MBE (≈10−5 mbar) for the case of using distilled ozone; see Eq. (1). Whereas many oxidation states are achievable, several of these states lie firmly outside the MBE parameter space. The limitation of the applicable oxygen pressure in MBE results from the mean free path and the necessity to protect electronic parts within the chamber.

FIG. 9.

O2 pressure PO2 required to achieve important oxidation states of elements relevant to epitaxial film growth. (a) 1+, (b) 2+, (c) 3+, (d) 4+, (e) 5+, and (f) 6+ and higher. O2 pressures are given for substrate temperatures of 500 °C (blue), 1000 °C (turquoise), and 2000 °C (magenta). Within a given oxidation state, the elements are sorted in descending order from left to right by the required O2 pressure for oxidation at 1000 °C Tsub. The green horizontal line indicates the equivalent O2 pressure for 10−5 mbar partial pressure of O3 at 1000 °C; see Eq. (1).

FIG. 9.

O2 pressure PO2 required to achieve important oxidation states of elements relevant to epitaxial film growth. (a) 1+, (b) 2+, (c) 3+, (d) 4+, (e) 5+, and (f) 6+ and higher. O2 pressures are given for substrate temperatures of 500 °C (blue), 1000 °C (turquoise), and 2000 °C (magenta). Within a given oxidation state, the elements are sorted in descending order from left to right by the required O2 pressure for oxidation at 1000 °C Tsub. The green horizontal line indicates the equivalent O2 pressure for 10−5 mbar partial pressure of O3 at 1000 °C; see Eq. (1).

Close modal

The compact design of TLE chambers elevates the pressure corresponding to the mean free path equaling the source substrate distance by an order of magnitude as compared to MBE for standard cases. Avoiding electronic components in the chamber also does not limit the available growth atmosphere. Growth at pressures as high as 10−1 mbar has been tested successfully. Thus, TLE allows previously unattainable oxidation states to be achieved over a wide Tsub range. Elements, such as Ag, that cannot be oxidized at all in an MBE system are potentially within reach for growth in oxide form by TLE.

The importance of the ability to cool quickly has been discussed above. Any laser heating system possesses this ability, and CO2 laser heaters are especially practical for this purpose. The key capability of CO2 laser heaters is the avoidance of absorbers, which eliminates thermal stress between absorber and substrate [Fig. 4(e)]. Table I lists substrates and their ability to absorb the 10 µm beam of a CO2 laser. If the absorption spectrum of a desired substrate material is known, its suitability can obviously also be predicted. If one wishes to use a substrate material that absorbs only weakly at this wavelength (e.g., a metal), one may resort to backside coating of the substrate, e.g., by amorphous Al2O3, which is clean and rather stable regarding heat and oxidation.

Generally, two sources of problems during epitaxial growth are eliminated if absorbers are avoided: the absorber itself and its parasitic heat. The excellent Tsub control and fast cooling are illustrated in Fig. 10 for the case of a sapphire substrate heated to 1700 °C in a vacuum in a TLE chamber equipped with cryo-shrouds. The top panel shows the temperature setpoint Tset, the center panel shows the real change of Tsub as a function of time measured by a backside pyrometer, and the bottom panel shows the resulting change of background pressure. Note that transferring the substrate has a greater impact on chamber pressure than heating the substrate. As a result of these excellent vacuum conditions, CO2 laser heating allows highly pure (oxide) films to be grown.

FIG. 10.

Performance of a CO2-laser based substrate heater system. (a) Controller set values for temperature Tset, (b) measured temperature Tsub, and (c) chamber background pressure (blue) for heating a sapphire substrate (5 × 5 cm2), all as a function of time. During the transfer and preparation of the substrate at 1700 °C, the pressure in the chamber remains in the lower 10−10 mbar regime.

FIG. 10.

Performance of a CO2-laser based substrate heater system. (a) Controller set values for temperature Tset, (b) measured temperature Tsub, and (c) chamber background pressure (blue) for heating a sapphire substrate (5 × 5 cm2), all as a function of time. During the transfer and preparation of the substrate at 1700 °C, the pressure in the chamber remains in the lower 10−10 mbar regime.

Close modal

The most significant benefit of CO2 laser heating is the range of available Tsub because this provides several opportunities for oxide epitaxy. First, it allows the in situ preparation and termination of the most commonly used and commercially available out-of-the-box substrates, which makes ex situ preparation (chemically or thermally) superfluous.164,176 The capability of in situ substrate preparation is accompanied by the ability to produce clean interfaces.36 In addition, many oxides require a high Tsub in order to form in the first place, e.g., pyrochlore iridates or sapphire.34–36,165 As described above, any oxide phase forming at >1000 °C is beyond the capability of most conventional heaters, whereas CO2 lasers allow these Tsub as well as the typical temperatures required for adsorption-controlled growth to be reached with ease. In many cases, the only factor limiting the achievable Tsub by CO2 laser heating is the melting point of the substrate.36 However, in some cases, undesired interdiffusion of substrate and film species can impose an additional obstacle (see the supplementary material, Fig. 2). Considering the volatility of all oxide binary species and the Tsub accessible by CO2 laser heating, the single remaining obstacle to growing oxides by adsorption-control is the oxide’s phase stability at the necessary Tsub.33 

An example that represents emerging opportunities is the homoepitaxial growth of c-plane sapphire. High temperatures beyond 1000 °C are necessary for the phase formation and adsorption-controlled growth.36,165 The high oxygen pressures available in TLE chambers allow for high growth rates even in the adsorption-controlled regime. Furthermore, TLE sources are able to overcome the issues of MBE Al sources in oxidizing environments.85 

Many of the trends and opportunities discussed here are neither specific nor restrictive to the epitaxial growth of oxides. For example, chalcogenides, pnictides, nitrides, carbides, and other material systems pose comparable challenges, which implies that we expect many of the potential solutions presented here to also apply to those systems. The currently available temperature regime of many film-growth technologies for non-oxide materials may be broader (<2000 °C) if a low reactivity of the involved species allows conventional heaters to reach higher temperatures. To provide a glimpse into materials beyond the oxides, we now briefly discuss implications for two other material systems: nitrides and carbides.

Compared to oxides, the growth of transition-metal nitrides and nitride ternaries has not been extensively explored. This is due in part to the fact that they often require deposition conditions outside of the standard parameter space of most deposition techniques. However, many of these nitrides have valuable properties, and epitaxial growth is crucial to unlocking their full potential.166 Recently, for example, the promising properties of refractory metal nitrides for superconducting qubits have resulted in increased efforts to grow high-quality samples of these nitrides.167,168 However, their epitaxial growth requires several parameters that remain challenging for standard deposition systems, namely, that refractory metals are difficult to evaporate and high-quality growth requires high Tsub.168 Both challenges are easily overcome by combining TLE and CO2 laser heating. A longer ongoing effort is the epitaxial growth of c-BN and h-BN, which faces the same complications, i.e., B is difficult to evaporate, and forming the desired phase requires high Tsub.169,170 It is expected that TLE can also significantly improve achievable film qualities in such cases.

Similarly, whereas the epitaxial growth of SiC is well established, the high-Tsub required in film growth implies that higher purities of the film are likely to be achieved with CO2 laser heaters.171 The growth of transition metal carbides and ternary carbides is arguably even less well understood than that of nitrides. The main challenge in this case is the lack of reactivity of the carbon species, which is commonly supplied as C60.172 In TLE chambers, carbon can be evaporated from a graphite source, which paves the way to resolving this issue.145 Another option is to utilize a gaseous carbon source of high reactivity, which the inertness of the TLE chamber would probably allow. Much like SiC, many other carbides require high temperatures in order to form, e.g., MoC >1800 °C.173 As described above, these Tsub are readily achieved by CO2 laser heating.

For one carbon-based material, there have been especially intense efforts to identify a method that can deliver high-quality epitaxial films. That material is diamond, whose unique properties not only make it the possibly ultimate ultrawide-bandgap material but also provide the basis to realize intriguing physical phenomena such as single-photon emission.174,175 The extreme environments needed for diamonds to form result in equally extreme parameters required for epitaxial deposition. The epitaxial films achieved so far lack the desired quality and growth speed.174 Beyond a doubt, the TLE growth of diamond films will be challenging, but this new deposition technique provides a promising new approach to meet that challenge.

This perspective provides an overview of two central challenges currently faced by oxide epitaxy. First is the difficulty of finding a universal, versatile, and clean way to transfer any element as individual atoms or molecules onto a substrate. Second is the phase formation in the growing film, especially regarding the high oxidization potentials needed to achieve many desired compounds and the high temperatures required for numerous oxide phases to form. Common oxide epitaxy methods have been evaluated, and some recent trends to overcome these challenges have been reviewed here. We have determined that CO2 laser heaters combined with TLE are one promising approach to overcome these challenges. To our understanding, this combination possesses the potential to achieve significant strides in oxide epitaxy, especially because of the high available purities.

Most notably, CO2 laser heaters are compatible with any epitaxy method. In particular, CO2 laser heating enables a huge parameter space to be explored (Fig. 3) and higher film purities to be achieved, making it the most promising substrate heating system for future research. However, if CO2 lasers are to become the predominant form of substrate heating even outside of research, it will be necessary to upscale their applicability to any wafer size.

TLE, in its current form, is still a relatively young technique. This implies that, along with the opportunities discussed here, the emergence of obstacles has to be expected. Currently, for some elements, the flux stability of TLE is not satisfactory. Corresponding improvements may utilize moving sources85 or a variable beam size to suit the respective source.158 Each of these approaches promises higher flux stability, so it remains to be seen which approach will be successful. Despite this obstacle, TLE presents a currently unmatched research opportunity for epitaxial film growth. As the first results are published, including results based on materials relevant to the industry, it will be exciting to see whether TLE will find a place in the industrial growth of epitaxial (oxide) films.

A general trend to overcome the weaknesses of individual growth methods is the effort to combine their respective strengths. In this paper, we have discussed the combination of MOCVD and MBE (hybrid MBE) and briefly mentioned the use of MBE effusion cells on PLD chambers (hybrid PLD).32 We expect hybrid systems of TLE and other epitaxial film growth systems to emerge. Possible scenarios also include replacing e-beam evaporators in MBE chambers with TLE sources and using low-temperature effusion cells in TLE systems.

See the Supplementary Material for the following: Table 1 lists sources available for MBE, TLE, and MOCVD/ALD; Fig. 1 shows the Ellingham diagram on which Fig. 9 is based; Fig. 2 shows the SIMS profile of a Sr2RuO4 film grown by TLE on LSAT at elevated temperatures, resulting in the diffusion of the substrate species into the first few nanometers of the films; and Table 2 compares the capabilities of the different methods with respect to handling sources.

The authors thank Varun Harbola, Hans Boschker, Y. Eren Suyolcu, and Darrell G. Schlom for many insightful discussions and Wolfgang Winter, Ingo Hagel, and Sabine Seiffert for the technical support. The authors are grateful to Tobias Schwaigert, Shun-Li Shang, and Zi-Kui Liu for assistance in establishing Fig. 9. The authors also thank Matthew R. Barone, Veronica Show, and Joe Falson and his team for additions to Table I, and Tolga Acartürk and Ulrich Starke for supplying data for Fig. 2 of the supplementary material.

W.B. and J.M. are partners in epiray, a company that sells CO2 laser heaters and TLE systems, W.B. also serves as CEO of that company, and B.D.F. as a consultant of epiray.

F. V. E. Hensling: Conceptualization (lead); Data curation (equal); Project administration (equal); Visualization (lead); Writing – original draft (lead). W. Braun: Conceptualization (equal); Data curation (equal); Project administration (equal); Writing – review & editing (equal). D. Y. Kim: Data curation (equal); Writing – review & editing (supporting). L. N. Majer: Data curation (equal); Visualization (supporting). S. Smink: Conceptualization (supporting); Data curation (supporting); Writing – review & editing (equal). B. D. Faeth: Conceptualization (supporting); Data curation (equal); Visualization (equal); Writing – review & editing (equal). J. Mannhart: Conceptualization (equal); Project administration (lead); Visualization (equal); Writing – review & editing (lead).

The data that support the findings of this study are available within the article and its supplementary material.

1.
J.
Levy
, “
Oxide-semiconductor materials for quantum computation
,”
Phys. Status Solidi B
233
,
467
471
(
2002
).
2.
A.
Barthelemy
et al, “
Quasi-two-dimensional electron gas at the oxide interfaces for topological quantum physics
,”
Europhys. Lett.
133
,
17001
(
2021
).
3.
X. L.
Hong
et al, “
Oxide-based RRAM materials for neuromorphic computing
,”
J. Mater. Sci.
53
,
8720
8746
(
2018
).
4.
S.
Yu
et al, “
Stochastic learning in oxide binary synaptic device for neuromorphic computing
,”
Front. Neurosci.
7
,
186
(
2013
).
5.
A.
Pérez-Tomás
,
A.
Mingorance
,
D.
Tanenbaum
, and
M.
Lira-Cantú
,
Metal oxides in photovoltaics: All-oxide, ferroic, and perovskite solar cells
,
The Future of Semiconductor Oxides in Next-Generation Solar Cells
(
Elsevier
,
2018
), pp.
267
356
.
6.
S.
Rühle
et al, “
All-oxide photovoltaics
,”
J. Phys. Chem. Lett.
3
,
3755
3764
(
2012
).
7.
J.
Scholz
et al, “
Tailoring the oxygen evolution activity and stability using defect chemistry
,”
Catalysts
7
,
139
(
2017
).
8.
M. L.
Weber
and
F.
Gunkel
, “
Epitaxial catalysts for oxygen evolution reaction: Model systems and beyond
,”
J. Phys.: Energy
1
,
031001
(
2019
).
9.
M.
Lorenz
et al, “
The 2016 oxide electronic materials and oxide interfaces roadmap
,”
J. Phys. D: Appl. Phys.
49
,
433001
(
2016
).
10.
M.
Coll
et al, “
Towards oxide electronics: A roadmap
,”
Appl. Surf. Sci.
482
,
1
93
(
2019
).
11.
J. G.
Bednorz
and
K. A.
Mueller
, “
Possible high Tc superconductivity in the Ba–La–Cu–O system
,”
Z. Phys. B: Condens. Matter
64
,
189
193
(
1986
).
12.
J. G.
Bednorz
,
M.
Takashige
, and
K. A.
Müller
, “
Susceptibility measurements support high-Tc superconductivity in the Ba-La-Cu-O system
,”
Europhys. Lett.
3
,
379
386
(
1987
).
13.
A.
Perez-Tomas
et al, “
Wide and ultra-wide bandgap oxides: Where paradigm-shift photovoltaics meets transparent power electronics
,”
Proc. SPIE
10533
,
105331Q
(
2018
).
14.
M. H.
Wong
,
O.
Bierwagen
,
R. J.
Kaplar
, and
H.
Umezawa
, “
Ultrawide-bandgap semiconductors: An overview
,”
J. Mater. Res.
36
,
4601
4615
(
2021
).
15.
H.
Hayashi
et al, “
Structural consideration on the ionic conductivity of perovskite-type oxides
,”
Solid State Ionics
122
,
1
15
(
1999
).
16.
S.
(Rob) Hui
et al, “
A brief review of the ionic conductivity enhancement for selected oxide electrolytes
,”
J. Power Sources
172
,
493
502
(
2007
).
17.
C. N. R.
Rao
, “
Transition metal oxides
,”
Annu. Rev. Phys. Chem.
40
,
291
326
(
1989
).
18.
H.
Boschker
and
J.
Mannhart
, “
Quantum-matter heterostructures
,”
Annu. Rev. Condens. Matter Phys.
8
,
145
164
(
2017
).
19.
S. T.
Bramwell
and
M. J. P.
Gingras
, “
Spin ice state in frustrated magnetic pyrochlore materials
,”
Science
294
,
1495
1501
(
2001
).
20.
M. J. P.
Gingras
and
P. A.
McClarty
, “
Quantum spin ice: A search for gapless quantum spin liquids in pyrochlore magnets
,”
Rep. Prog. Phys.
77
,
056501
(
2014
).
21.
A. C.
Lima
,
N.
Pereira
,
P.
Martins
, and
S.
Lanceros-Mendez
, “
Magnetic materials for magnetoelectric coupling: An unexpected journey
,” in
Handbook of Magnetic Materials
(
Elsevier
,
2020
), pp.
57
110
.
22.
D.
Khomskii
, “
Classifying multiferroics: Mechanisms and effects
,”
Physics
2
,
20
(
2009
).
23.
J. A.
Venables
,
G. D. T.
Spiller
, and
M.
Hanbucken
, “
Nucleation and growth of thin films
,”
Rep. Prog. Phys.
47
,
399
459
(
1984
).
24.
H.
Freller
and
K. G.
Günther
, “
Three-temperature method as an origin of molecular beam epitaxy
,”
Thin Solid Films
88
,
291
307
(
1982
).
25.
J. Y.
Tsao
,
Materials Fundamentals of Molecular Beam Epitaxy
(
Academic Press
,
1993
).
26.
K. G.
Guenther
, “
Aufdampfschichten aus halbleitenden III–V verbindungen
,”
Naturwissenschaften
45
,
415
416
(
1958
).
27.
J. R.
Arthur
, “
Interaction of Ga and As2 molecular beams with GaAs surfaces
,”
J. Appl. Phys.
39
,
4032
4034
(
1968
).
28.
R.
Heckingbottom
,
G. J.
Davies
, and
K. A.
Prior
, “
Growth and doping of gallium arsenide using molecular beam epitaxy (MBE): Thermodynamic and kinetic aspects
,”
Surf. Sci.
132
,
375
389
(
1983
).
29.
H.
Seki
and
A.
Koukitu
, “
Thermodynamic analysis of molecular beam epitaxy of III–V semiconductors
,”
J. Cryst. Growth
78
,
342
352
(
1986
).
30.
Y. J.
Chung
et al, “
Understanding limits to mobility in ultrahigh-mobility GaAs two-dimensional electron systems: 100 million cm2/V s and beyond
,”
Phys. Rev. B
106
,
075134
(
2022
).
31.
J. L.
MacManus-Driscoll
and
S. C.
Wimbush
, “
Processing and application of high-temperature superconducting coated conductors
,”
Nat. Rev. Mater.
6
,
587
604
(
2021
).
32.
J. L.
MacManus-Driscoll
et al, “
New approaches for achieving more perfect transition metal oxide thin films
,”
APL Mater.
8
,
040904
(
2020
).
33.
K. M.
Adkison
et al, “
Suitability of binary oxides for molecular-beam epitaxy source materials: A comprehensive thermodynamic analysis
,”
APL Mater.
8
,
081110
(
2020
).
34.
L.
Guo
et al, “
Searching for a route to synthesize in situ epitaxial Pr2Ir2O7 thin films with thermodynamic methods
,”
npj Comput. Mater.
7
,
144
(
2021
).
35.
W. J.
Kim
,
J.
Song
,
Y.
Li
, and
T. W.
Noh
, “
Perspective on solid-phase epitaxy as a method for searching novel topological phases in pyrochlore iridate thin films
,”
APL Mater.
10
,
080901
(
2022
).
36.
F.
Hensling
, “
Adsorption controlled growth of c-plane sapphire
,” (
2024
).
37.
I. V.
Khoroshun
et al, “
Characteristics of epitaxial Y-Ba-Cu-O thin films grown by aerosol MOCVD technique
,”
Supercond. Sci. Technol.
3
,
493
496
(
1990
).
38.
V.
Moshnyaga
et al, “
Preparation of rare-earth manganite-oxide thin films by metalorganic aerosol deposition technique
,”
Appl. Phys. Lett.
74
,
2842
2844
(
1999
).
39.
J. E.
ten Elshof
, “
Chemical solution deposition techniques for epitaxial growth of complex oxides
,” in
Epitaxial Growth of Complex Metal Oxides
, edited by
G.
Koster
,
M.
Huijben
, and
G.
Rijnders
(
Elsevier
,
2015
), pp.
69
93
.
40.
L.
Fei
,
M.
Naeemi
,
G.
Zou
, and
H.
Luo
, “
Chemical solution deposition of epitaxial metal-oxide nanocomposite thin films
,”
Chem. Rec.
13
,
85
101
(
2013
).
41.
D. H.
Lowndes
,
D. B.
Geohegan
,
A. A.
Puretzky
,
D. P.
Norton
, and
C. M.
Rouleau
, “
Synthesis of novel thin-film materials by pulsed laser deposition
,”
Science
273
,
898
903
(
1996
).
42.
R.
Dittmann
and
A.
Sambri
, “
Stoichiometry in epitaxial oxide thin films
,” in
Epitaxial Growth of Complex Metal Oxides
, edited by
G.
Koster
,
M.
Huijben
, and
G.
Rijnders
(
Elsevier
,
2022
), pp.
267
298
.
43.
D. G.
Schlom
,
L. Q.
Chen
,
X.
Pan
,
A.
Schmehl
, and
M. A.
Zurbuchen
, “
A thin film approach to engineering functionality into oxides
,”
J. Am. Ceram. Soc.
91
,
2429
2454
(
2008
).
44.
H. M.
Christen
and
G.
Eres
, “
Recent advances in pulsed-laser deposition of complex oxides
,”
J. Phys.: Condens. Matter
20
,
264005
(
2008
).
45.
M.
Dawber
, “
Sputtering techniques for epitaxial growth of complex oxides
,” in
Epitaxial Growth of Complex Metal Oxides
, edited by
G.
Koster
,
M.
Huijben
, and
G.
Rijnders
(
Elsevier
,
2015
), pp.
37
51
.
46.
P. R.
Willmott
, “
Deposition of complex multielemental thin films
,”
Prog. Surf. Sci.
76
,
163
217
(
2004
).
47.
A.
Brewer
et al, “
Uniform sputter deposition of high-quality epitaxial complex oxide thin films
,”
J. Vac. Sci. Technol. A
35
,
060607
(
2017
).
48.
Y. E.
Suyolcu
,
G.
Christiani
,
P. A.
van Aken
, and
G.
Logvenov
, “
Design of complex oxide interfaces by oxide molecular beam epitaxy
,”
J. Supercond. Novel Magn.
33
,
107
120
(
2020
).
49.
D. G.
Schlom
et al, “
Oxide nano-engineering using MBE
,”
Mater. Sci. Eng.: B
87
,
282
291
(
2001
).
50.
D. G.
Schlom
, “
Perspective: Oxide molecular-beam epitaxy rocks!
,”
APL Mater.
3
,
062403
(
2015
).
51.
M.
Brahlek
et al, “
Frontiers in the growth of complex oxide thin films: Past, present, and future of hybrid MBE
,”
Adv. Funct. Mater.
28
,
1702772
(
2018
).
52.
G.
Rimal
and
R. B.
Comes
, “
Advances in complex oxide quantum materials through new approaches to molecular beam epitaxy
,”
J. Phys. D: Appl. Phys.
57
,
193001
(
2024
).
53.
S.
Yamamoto
and
S.
Oda
, “
Atomic layer-by-layer MOCVD of complex metal oxides and in situ process monitoring
,”
Chem. Vap. Deposition
7
,
7
18
(
2001
).
54.
A. C.
Jones
and
P. R.
Chalker
, “
Some recent developments in the chemical vapour deposition of electroceramic oxides
,”
J. Phys. D: Appl. Phys.
36
,
R53
R79
(
2003
).
55.
A. C.
Jones
et al, “
Recent developments in the MOCVD and ALD of rare earth oxides and silicates
,”
Mater. Sci. Eng.: B
118
,
97
104
(
2005
).
56.
P. J.
Wright
et al, “
Metal organic chemical vapor deposition (MOCVD) of oxides and ferroelectric materials
,”
J. Mater. Sci.: Mater. Electron.
13
,
671
678
(
2002
).
57.
C. W.
Schneider
and
T.
Lippert
, “
PLD plasma plume analysis: A summary of the PSI contribution
,”
Appl. Phys. A
129
,
138
(
2023
).
58.
H. N.
Lee
,
S. S.
Ambrose Seo
,
W. S.
Choi
, and
C. M.
Rouleau
, “
Growth control of oxygen stoichiometry in homoepitaxial SrTiO3 films by pulsed laser epitaxy in high vacuum
,”
Sci. Rep.
6
,
19941
(
2016
).
59.
N.
Jaber
et al, “
Laser fluence and spot size effect on compositional and structural properties of BiFeO3 thin films grown by Pulsed Laser Deposition
,”
Thin Solid Films
634
,
107
111
(
2017
).
60.
T.
Ohnishi
,
M.
Lippmaa
,
T.
Yamamoto
,
S.
Meguro
, and
H.
Koinuma
, “
Improved stoichiometry and misfit control in perovskite thin film formation at a critical fluence by pulsed laser deposition
,”
Appl. Phys. Lett.
87
,
241919
(
2005
).
61.
S.
Wicklein
et al, “
Pulsed laser ablation of complex oxides: The role of congruent ablation and preferential scattering for the film stoichiometry
,”
Appl. Phys. Lett.
101
,
131601
(
2012
).
62.
C.
Xu
et al, “
Impact of the interplay between nonstoichiometry and kinetic energy of the plume species on the growth mode of SrTiO3 thin films
,”
J. Phys. D: Appl. Phys.
47
,
034009
(
2014
).
63.
D. J.
Keeble
et al, “
Nonstoichiometry accommodation in SrTiO3 thin films studied by positron annihilation and electron microscopy
,”
Phys. Rev. B
87
,
195409
(
2013
).
64.
H.
Schraknepper
,
C.
Bäumer
,
F.
Gunkel
,
R.
Dittmann
, and
R. A.
De Souza
, “
Pulsed laser deposition of SrRuO3 thin-films: The role of the pulse repetition rate
,”
APL Mater.
4
,
126109
(
2016
).
65.
M.
Pervolaraki
et al, “
Picosecond ultrafast pulsed laser deposition of SrTiO3
,”
Appl. Surf. Sci.
336
,
278
282
(
2015
).
66.
A.
Husmann
et al, “
Pulsed laser deposition of crystalline PZT thin films
,”
Surf. Coat. Technol.
97
,
420
425
(
1997
).
67.
R.
Groenen
et al, “
Research update: Stoichiometry controlled oxide thin film growth by pulsed laser deposition
,”
APL Mater.
3
,
070701
(
2015
).
68.
F. V. E.
Hensling
,
C.
Xu
,
F.
Gunkel
, and
R.
Dittmann
, “
Unraveling the enhanced oxygen vacancy formation in complex oxides during annealing and growth
,”
Sci. Rep.
7
,
39953
(
2017
).
69.
C. R.
Cho
and
A.
Grishin
, “
Background oxygen effects on pulsed laser deposited Na0.5K0.5NbO3 films: From superparaelectric state to ferroelectricity
,”
J. Appl. Phys.
87
,
4439
4448
(
2000
).
70.
M.
Sumiya
et al, “
Quantitative control and detection of heterovalent impurities in ZnO thin films grown by pulsed laser deposition
,”
J. Appl. Phys.
93
,
2562
2569
(
2003
).
71.
G. A.
Boni
et al, “
Accidental impurities in epitaxial Pb(Zr0.2Ti0.8)O3 thin films grown by pulsed laser deposition and their impact on the macroscopic electric properties
,”
Nanomaterials
11
,
1177
(
2021
).
72.
R.
Pérez Casero
,
R.
Gómez San Román
,
C.
Maréchal
,
J. P.
Enard
, and
J.
Perrière
, “
Laser ablation of oxides: Study of the oxygen incorporation by 18O isotopic tracing techniques
,”
Appl. Surf. Sci.
96–98
,
697
702
(
1996
).
73.
J.
Gonzalo
,
R.
Gómez San Román
,
J.
Perrière
,
C. N.
Afonso
, and
R.
Pérez Casero
, “
Pressure effects during pulsed-laser deposition of barium titanate thin films
,”
Appl. Phys. A: Mater. Sci. Process.
66
,
487
491
(
1998
).
74.
J. A.
Greer
and
M. D.
Tabat
, “
Large-area pulsed laser deposition: Techniques and applications
,”
J. Vac. Sci. Technol. A
13
,
1175
1181
(
1995
).
75.
D. H. A.
Blank
,
M.
Dekkers
, and
G.
Rijnders
, “
Pulsed laser deposition in Twente: From research tool towards industrial deposition
,”
J. Phys. D: Appl. Phys.
47
,
034006
(
2014
).
76.
Z.
Vakulov
et al, “
Towards scalable large-area pulsed laser deposition
,”
Materials
14
,
4854
(
2021
).
77.
D.
Depla
, “
Sputter deposition with powder targets: An overview
,”
Vacuum
184
,
109892
(
2021
).
78.
J. B.
Malherbe
,
S.
Hofmann
, and
J. M.
Sanz
, “
Preferential sputtering of oxides: A comparison of model predictions with experimental data
,”
Appl. Surf. Sci.
27
,
355
365
(
1986
).
79.
W. B.
Pennebaker
, “
RF sputtered strontium titanate films
,”
IBM J. Res. Dev.
13
,
686
695
(
1969
).
80.
S.
Fairose
,
S.
Ernest
, and
S.
Daniel
, “
Effect of oxygen sputter pressure on the structural, morphological and optical properties of ZnO thin films for gas sensing application
,”
Sens. Imaging
19
,
1
18
(
2018
).
81.
L.
Zhao
,
S.
Song
, and
L.
Li
, “
Effect of sputtering gas pressure on the performance of WO3 thin films electrochromic device
,”
J. Phys.: Conf. Ser.
1676
,
012037
(
2020
).
82.
P. J.
Kelly
and
R. D.
Arnell
, “
Magnetron sputtering: A review of recent developments and applications
,”
Vacuum
56
,
159
172
(
2000
).
83.
J.
Sun
et al, “
Canonical approach to cation flux calibration in oxide molecular-beam epitaxy
,”
Phys. Rev. Mater.
6
,
033802
(
2022
).
84.
Y. E.
Suyolcu
et al, “
a-axis YBa2Cu3O7−x/PrBa2Cu3O7−x/YBa2Cu3O7−x trilayers with subnanometer rms roughness
,”
APL Mater.
9
,
021117
(
2021
).
85.
T. J.
Smart
et al, “
Why thermal laser epitaxy aluminum sources yield reproducible fluxes in oxidizing environments
,”
J. Vac. Sci. Technol. A
41
,
042701
(
2023
).
86.
C. D.
Theis
and
D. G.
Schlom
, “
Cheap and stable titanium source for use in oxide molecular beam epitaxy systems
,”
J. Vac. Sci. Technol. A
14
,
2677
2679
(
1996
).
87.
H. P.
Nair
et al, “
Synthesis science of SrRuO3 and CaRuO3 epitaxial films with high residual resistivity ratios
,”
APL Mater.
6
,
046101
(
2018
).
88.
D. G.
Schlom
and
J. S.
Harris
,
MBE growth of high Tc superconductors
, in:
R. F. C.
Farrow
(Ed.),
Molecular Beam Epitaxy: Application to Key Materials
(
Noyes Publications
,
1995
), pp.
505
622
.
89.
J.-P.
Locquet
and
E.
Mächler
, “
Characterization of a radio frequency plasma source for molecular beam epitaxial growth of high-Tc superconductor films
,”
J. Vac. Sci. Technol. A
10
,
3100
3103
(
1992
).
90.
G.
Vinai
et al, “
An integrated ultra-high vacuum apparatus for growth and in situ characterization of complex materials
,”
Rev. Sci. Instrum.
91
,
085109
(
2020
).
91.
J. R.
Arthur
, “
Molecular beam epitaxy
,”
Surf. Sci.
500
,
189
217
(
2002
).
92.
P. D.
Dapkus
,
H. M.
Manasevit
,
K. L.
Hess
,
T. S.
Low
, and
G. E.
Stillman
, “
High purity GaAs prepared from trimethylgallium and arsine
,”
J. Cryst. Growth
55
,
10
23
(
1981
).
93.
A. P.
Kajdos
,
N. G.
Combs
, and
S.
Stemmer
, “
Hybrid oxide molecular beam epitaxy
,” in
Epitaxial Growth of Complex Metal Oxides
, edited by
G.
Koster
,
M.
Huijben
, and
G.
Rijnders
(
Elsevier
,
2022
), pp.
53
74
.
94.
P. V.
Chayka
,
Liquid MOCVD precursors and their application to fiber interface coatings
, in:
J. P.
Singh
(Ed.),
Proceedings of the 21st Annual Conference on Composites, Advanced Ceramics, Materials, and Structures—A: Ceramic Engineering and Science Proceedings
(
Wiley
,
1997
), pp.
287
294
.
95.
A. C.
Jones
et al, “
MOCVD and ALD of high-k dielectric oxides using alkoxide precursors
,”
Chem. Vap. Deposition
12
,
83
98
(
2006
).
96.
G.
Malandrino
et al, “
Phase-selective route to high Tc superconducting Tl2Ba2Can−1CunO2n+4 films: Combined metalorganic chemical vapor deposition using an improved barium precursor and stoichiometry-controlled thallium vapor diffusion
,”
Appl. Phys. Lett.
58
,
182
184
(
1991
).
97.
A. C.
Jones
,
H. C.
Aspinall
, and
P. R.
Chalker
, “
Molecular design of improved precursors for the MOCVD of oxides used in microelectronics
,”
Surf. Coat. Technol.
201
,
9046
9054
(
2007
).
98.
N. K.
Kalarickal
and
S.
Rajan
, “
β-(AlxGa(1−x))2O3 epitaxial growth, doping and transport
,”
Semicond. Semimetals
107
,
49
76
(
2021
).
99.
F. W.
Ainger
et al, “
Deposition of ferroelectric oxides by MOCVD
,”
Prog. Cryst. Growth Charact. Mater.
22
,
183
197
(
1991
).
100.
Surface Systems + Technology GmbH & Co. KG
, www.surface-tec.com,
2023
.
101.
Dr. Eberl MBE Komponenten
, www.mbe-komponenten.de,
2023
.
102.
Demcon TSST
, tsst.demcon.com,
2023
.
103.
Neocera
, www.neocera.com,
2023
.
104.
K.
Endo
,
S.
Saya
,
S.
Misawa
, and
S.
Yoshida
, “
Preparation of yttrium barium copper oxide superconducting films by metalorganic molecular beam epitaxy
,”
Thin Solid Films
206
,
143
145
(
1991
).
105.
L. L. H.
King
,
K. Y.
Hsieh
,
D. J.
Lichtenwalner
, and
A. I.
Kingon
, “
In situ deposition of superconducting YBa2Cu3O7−x and DyBa2Cu3O7−x thin films by organometallic molecular-beam epitaxy
,”
Appl. Phys. Lett.
59
,
3045
3047
(
1991
).
106.
N.
Izyumskaya
et al, “
Structural and electrical properties of Pb(Zr, Ti)O3 films grown by molecular beam epitaxy
,”
Appl. Phys. Lett.
91
,
182906
(
2007
).
107.
D.
Saulys
et al, “
An examination of the surface decomposition chemistry of lithium niobate precursors under high vacuum conditions
,”
J. Cryst. Growth
217
,
287
301
(
2000
).
108.
B.
Jalan
,
R.
Engel-Herbert
,
N. J.
Wright
, and
S.
Stemmer
, “
Growth of high-quality SrTiO3 films using a hybrid molecular beam epitaxy approach
,”
J. Vac. Sci. Technol. A
27
,
461
464
(
2009
).
109.
B.
Jalan
,
P.
Moetakef
, and
S.
Stemmer
, “
Molecular beam epitaxy of SrTiO3 with a growth window
,”
Appl. Phys. Lett.
95
,
032906
(
2009
).
110.
J.
Son
et al, “
Epitaxial SrTiO3 films with electron mobilities exceeding 30,000 cm2 V−1 s−1
,”
Nat. Mater.
9
,
482
484
(
2010
).
111.
B.
Jalan
,
S. J.
Allen
,
G. E.
Beltz
,
P.
Moetakef
, and
S.
Stemmer
, “
Enhancing the electron mobility of SrTiO3 with strain
,”
Appl. Phys. Lett.
98
,
132102
(
2011
).
112.
T. A.
Cain
,
A. P.
Kajdos
, and
S.
Stemmer
, “
La-doped SrTiO3 films with large cryogenic thermoelectric power factors
,”
Appl. Phys. Lett.
102
,
182101
(
2013
).
113.
W.
Nunn
,
T. K.
Truttmann
, and
B.
Jalan
, “
A review of molecular-beam epitaxy of wide bandgap complex oxide semiconductors
,”
J. Mater. Res.
36
,
4846
4864
(
2021
).
114.
R.
Choudhary
et al, “
Growing clean crystals from dirty precursors: Solid-source metal-organic molecular beam epitaxy growth of superconducting Sr2RuO4 films
,”
APL Mater.
11
,
061124
(
2023
).
115.
P.
Vogt
et al, “
Adsorption-controlled growth of Ga2O3 by suboxide molecular-beam epitaxy
,”
APL Mater.
9
,
031101
(
2021
).
116.
D. G.
Schlom
et al, “
Suboxide molecular beam epitaxy and related structures
,”
U.S. Patent Application No. 0122843
(April 21,
2020
).
117.
R. A.
Stall
, “
Growth of refractory oxide films using solid oxygen sources in a molecular beam epitaxy apparatus
,”
J. Vac. Sci. Technol. B
1
,
135
137
(
1982
).
118.
H.
Aizaki
and
T.
Tatsumi
, “
Boron doping in silicon molecular beam epitaxial film by coevaporation of boron oxide
,” in
Extended Abstracts of the 17th Conference on Solid State Devices and Materials
(
The Japan Society of Applied Physics
,
1985
), pp.
301
304
.
119.
R. M.
Ostrom
and
F. G.
Allen
, “
Boron doping in Si molecular beam epitaxy by co-evaporation of B2O3 or doped silicon
,”
Appl. Phys. Lett.
48
,
221
223
(
1986
).
120.
E. S.
Hellman
,
E. H.
Hartford
, and
T. T. M.
Palstra
, “
Superconducting (Rb,Ba)BiO3 thin films grown by molecular beam epitaxy
,”
Physica C
162–164
,
633
634
(
1989
).
121.
M.
Passlack
et al, “
Ga2O3 films for electronic and optoelectronic applications
,”
J. Appl. Phys.
77
,
686
693
(
1995
).
122.
M.
Hong
,
J.
Kwo
,
A. R.
Kortan
,
J. P.
Mannaerts
, and
A. M.
Sergent
, “
Epitaxial cubic gadolinium oxide as a dielectric for gallium arsenide passivation
,”
Science
283
,
1897
1900
(
1999
).
123.
Z.
Yu
,
C. D.
Overgaard
,
R.
Droopad
,
M.
Passlack
, and
J. K.
Abrokwah
, “
Growth and physical properties of Ga2O3 thin films on GaAs(001) substrate by molecular-beam epitaxy
,”
Appl. Phys. Lett.
82
,
2978
2980
(
2003
).
124.
G.
Rispens
and
B.
Noheda
, “
Ultra-thin lead titanate films grown by molecular beam epitaxy
,”
Integr. Ferroelectr.
92
,
30
39
(
2007
).
125.
R.
Kumaran
,
T.
Tiedje
,
S. E.
Webster
,
S.
Penson
, and
W.
Li
, “
Epitaxial Nd-doped α-(Al1−xGax)2O3 films on sapphire for solid-state waveguide lasers
,”
Opt. Lett.
35
,
3793
(
2010
).
126.
M.
Higashiwaki
,
K.
Sasaki
,
A.
Kuramata
,
T.
Masui
, and
S.
Yamakoshi
, “
Gallium oxide (Ga2O3) metal-semiconductor field-effect transistors on single-crystal β-Ga2O3 (010) substrates
,”
Appl. Phys. Lett.
100
,
013504
(
2012
).
127.
G.
Hoffmann
,
M.
Budde
,
P.
Mazzolini
, and
O.
Bierwagen
, “
Efficient suboxide sources in oxide molecular beam epitaxy using mixed metal + oxide charges: The examples of SnO and Ga2O
,”
APL Mater.
8
,
031110
(
2020
).
128.
F. V. E.
Hensling
et al, “
Epitaxial growth of the first two members of the Ban+1InnO2.5n+1 Ruddlesden–Popper homologous series
,”
J. Vac. Sci. Technol. A
40
,
062707
(
2022
).
129.
T.
Schwaigert
et al, “
Molecular beam epitaxy of KTaO3
,”
J. Vac. Sci. Technol. A
41
,
022703
(
2023
).
130.
T.
Kuznetsova
,
J.
Roth
,
J.
Lapano
,
A.
Pogrebnyakov
, and
R.
Engel-Herbert
, “
Growth of SrMoO3 thin films by suboxide molecular beam epitaxy
,”
J. Vac. Sci. Technol. A
41
,
053412
(
2023
).
131.
K.
Azizie
et al, “
Silicon-doped β-Ga2O3 films grown at 1 µm/h by suboxide molecular-beam epitaxy
,”
APL Mater.
11
,
041102
(
2023
).
132.
H. M.
Smith
and
A. F.
Turner
, “
Vacuum deposited thin films using a ruby laser
,”
Appl. Opt.
4
,
147
(
1965
).
133.
S.
Takemoto
,
T.
Kasai
,
Y.
Watanabe
, and
W.
Sakari
, “
Some experiments on optical processing of metals with ruby laser
,”
Bull. Univ. Osaka Prefect., Ser. A
12
,
61
67
(
1964
).
134.
J. T.
Cheung
and
T.
Magee
, “
Recent progress on LADA growth of HgCdTe and CdTe epitaxial layers
,”
J. Vac. Sci. Technol. A
1
,
1604
1607
(
1983
).
135.
J. T.
Cheung
,
M.
Khoshnevisan
, and
T.
Magee
, “
Heteroepitaxial growth of CdTe on GaAs by laser assisted deposition
,”
Appl. Phys. Lett.
43
,
462
464
(
1983
).
136.
S.
Gaponov
,
B. N.
Luskin
, and
N. N.
Salashchenko
, “
Homoepitaxial superlattices with nonoriented barrier layers
,”
Solid State Commun.
39
,
301
302
(
1981
).
137.
V. S.
Ban
and
D. A.
Kramer
, “
Thin films of semiconductors and dielectrics produced by laser evaporation
,”
J. Mater. Sci.
5
,
978
982
(
1970
).
138.
J. T.
Cheung
and
H.
Sankur
, “
Growth of thin films by laser-induced evaporation
,”
Crit. Rev. Solid State Mater. Sci.
15
,
63
109
(
1988
).
139.
D.
Dijkkamp
et al, “
Preparation of Y-Ba-Cu oxide superconductor thin films using pulsed laser evaporation from high Tc bulk material
,”
Appl. Phys. Lett.
51
,
619
621
(
1987
).
140.
X. D.
Wu
et al, “
Pulsed laser deposition of high Tc superconducting thin films: Present and future
,”
MRS Proc.
191
,
129
(
1990
).
141.
T.
Venkatesan
, “
Pulsed laser deposition—Invention or discovery?
,”
J. Phys. D: Appl. Phys.
47
,
034001
(
2014
).
142.
G.
Groh
, “
Vacuum deposition of thin films by means of a CO2 laser
,”
J. Appl. Phys.
39
,
5804
5805
(
1968
).
143.
W.
Braun
and
J.
Mannhart
, “
Film deposition by thermal laser evaporation
,”
AIP Adv.
9
,
085310
(
2019
).
144.
T. J.
Smart
,
J.
Mannhart
, and
W.
Braun
, “
Thermal laser evaporation of elements from across the periodic table
,”
J. Laser Appl.
33
,
022008
(
2021
).
145.
D. Y.
Kim
,
T. J.
Smart
,
J.
Mannhart
, and
W.
Braun
, “
Thermal laser epitaxy of carbon films
,”
Cryst. Growth Des.
23
,
8087
8093
(
2023
).
146.
C.
Romeo
et al, “
Superconducting properties of Y-Ba-Cu-O thin films grown in situ by laser ablation
,”
Physica C
180
,
77
80
(
1991
).
147.
K. H.
Wu
,
C. L.
Lee
,
J. Y.
Juang
,
T. M.
Uen
, and
Y. S.
Gou
, “
In situ growth of Y1Ba2Cu3O7−x superconducting thin films using a pulsed neodymium:yttrium aluminum garnet laser with CO2 laser heated substrates
,”
Appl. Phys. Lett.
58
,
1089
1091
(
1991
).
148.
K. H.
Wu
et al, “
Optimization of depositing Y1Ba2Cu3O7-δ superconducting thin films by excimer laser ablation with CO2 laser-heated substrates
,”
Physica C
195
,
241
257
(
1992
).
149.
M.
Hiramatsu
et al, “
Hydrogen-radical-assisted radio-frequency plasma-enhanced chemical vapor deposition system for diamond formation
,”
Rev. Sci. Instrum.
67
,
2360
2365
(
1996
).
150.
G.
Korevaar
,
A.
Goossens
, and
J.
Schoonman
, “
Synthesis of polycrystalline silicon films on metalized ceramic substrates with laser-assisted chemical vapor deposition
,”
J. Phys. IV
09
,
Pr8
-
757
(
1999
).
151.
H. S.
Tsai
et al, “
Laser-assisted plasma-enhanced chemical vapor deposition of silicon nitride thin film
,”
Surf. Coat. Technol.
132
,
158
162
(
2000
).
152.
H.
Lydtin
, in
Proceedings of the Third International Conference on Chemical Vapor Deposition
(
American Nuclear Society, Hinsdale, IL
,
1972
), p.
121
.
153.
S. D.
Allen
, “
Laser chemical vapor deposition: A technique for selective area deposition
,”
J. Appl. Phys.
52
,
6501
6505
(
1981
).
154.
A.
Llanos
et al, “
Supercell formation in epitaxial rare-earth ditelluride thin films
,”
Cryst. Growth Des.
24
,
115
121
(
2024
).
155.
M.
Lorenz
et al, “
Highly textured fresnoite thin films synthesized in situ by pulsed laser deposition with CO2 laser direct heating
,”
J. Phys. D: Appl. Phys.
47
,
034013
(
2014
).
156.
J.
Kim
et al, “
Electronic-grade epitaxial (111) KTaO3 heterostructures
,” arXiv.2308.13180 (
2023
).
157.
D. Y.
Kim
et al, “
Thermal laser evaporation of elemental metal sources in oxygen
,”
J. Appl. Phys.
132
,
245110
(
2022
).
158.
B.
Faeth
, “
Sr2RuO4 by TLE
,” (
2024
).
159.
G.
Pilania
,
C.
Wang
,
X.
Jiang
,
S.
Rajasekaran
, and
R.
Ramprasad
, “
Accelerating materials property predictions using machine learning
,”
Sci. Rep.
3
,
2810
(
2013
).
160.
B.
Petek
,
E. A.
Giess
, and
R. T.
Hodgson
, “
Measurement of Fe-Ga ion interchange by pulsed laser heating and fast cooling of magnetic bubble films
,”
J. Appl. Phys.
52
,
4170
4175
(
1981
).
161.
U.
Chitnis
et al, “
Microstructural and electrical investigation of polymorph stabilization and multistate transition in interface engineered epitaxial VO2 films
,”
Appl. Surf. Sci.
637
,
157916
(
2023
).
162.
S.-L.
Shang
,
S.
Lin
,
M. C.
Gao
,
D. G.
Schlom
, and
Z.-K.
Liu
, “
Predictions and correlation analyses of Ellingham diagrams in binary oxides
,” arXiv.2308.05837 (
2023
).
163.
K. T.
Jacob
and
D.
Prusty
, “
Thermodynamic properties of RhO2
,”
J. Alloys Compd.
507
,
17
20
(
2010
).
164.
W.
Braun
et al, “
In situ thermal preparation of oxide surfaces
,”
APL Mater.
8
,
071112
(
2020
).
165.
H.
Okumura
, “
Sn and Si doping of α-Al2O3 (10-10) layers grown by plasma-assisted molecular beam epitaxy
,”
Jpn. J. Appl. Phys.
61
,
125505
(
2022
).
166.
W.
Sun
et al, “
A map of the inorganic ternary metal nitrides
,”
Nat. Mater.
18
,
732
739
(
2019
).
167.
N. P.
de Leon
et al, “
Materials challenges and opportunities for quantum computing hardware
,”
Science
372
,
eabb2823
(
2021
).
168.
J.
Wright
et al, “
Unexplored MBE growth mode reveals new properties of superconducting NbN
,”
Phys. Rev. Mater.
5
,
024802
(
2021
).
169.
X.
Zhang
and
J.
Meng
, “
Recent progress of boron nitrides
,” in
Ultra-Wide Bandgap Semiconductor Materials
(
Elsevier
,
2019
), pp.
347
419
.
170.
C. B.
Samantaray
and
R. N.
Singh
, “
Review of synthesis and properties of cubic boron nitride (c-BN) thin films
,”
Int. Mater. Rev.
50
,
313
344
(
2005
).
171.
M.
Skowronski
and
T.
Kimoto
,
Silicon carbide epitaxy
,
Handbook of Crystal Growth
(
Elsevier
,
2015
), pp.
1135
1167
.
172.
U.
Jansson
et al, “
Low temperature epitaxial growth of metal carbides using fullerenes
,”
Surf. Coat. Technol.
142–144
,
817
822
(
2001
).
173.
J. P.
Palmquist
,
J.
Birch
, and
U.
Jansson
, “
Deposition of epitaxial ternary transition metal carbide films
,”
Thin Solid Films
405
,
122
128
(
2002
).
174.
M.
Kasu
, “
Diamond epitaxy: Basics and applications
,”
Prog. Cryst. Growth Charact. Mater.
62
,
317
328
(
2016
).
175.
D.
Das
et al, “
Diamond—The ultimate material for exploring physics of spin-defects for quantum technologies and diamondtronics
,”
J. Phys. D: Appl. Phys.
55
,
333002
(
2022
).
176.
S.
Smink
,
L. N.
Majer
,
H.
Boschker
,
J.
Mannhart
, and
W.
Braun
, “
Long-range atomic order on double-stepped Al2O3(0001) surfaces
,”
Advan. Mater.
(published online,
2022
).
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