Thermal laser evaporation (TLE) is a particularly promising technique for the growth of metal films. Here, we demonstrate that TLE is also suitable for the growth of amorphous and polycrystalline oxide films. We report on a spectrum of binary oxide films that have been deposited by laser-induced evaporation of elemental metal sources in oxygen–ozone atmospheres. The oxide deposition by TLE is accompanied by an oxidation of the elemental metal source, which systematically affects the source molecular flux. Fifteen elemental metals were successfully used as sources for oxide films grown on unheated substrates, employing one and the same laser optic. The source materials ranged from refractory metals with low vapor pressures, such as Hf, Mo, and Ru, to Zn, which readily sublimates at low temperatures. These results reveal that TLE is also well suited for the growth of ultraclean oxide films.

Oxide films are of great interest for realizing new functionalities due to their broad spectrum of intriguing and useful properties. Virtually all deposition techniques are used for the growth of oxide films, including electron-beam evaporation (EBE), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), sputtering, and atomic layer deposition. Thermal laser evaporation (TLE) has recently been demonstrated to be a promising technique for growing ultraclean elemental metal and semiconductor films1,2 because it combines the advantages of MBE, PLD, and EBE by thermally evaporating metallic sources with a laser beam.

By utilizing the adsorption-controlled growth mode, MBE is particularly suited for growing films of superior structural quality.3,4 In MBE, molecular fluxes of source materials are generated by evaporating the source materials. However, Ohmic heaters, which are preferred for this purpose, limit the use of reactive background gases. This restriction can be critical for the growth of complex metal oxides.5,6 Furthermore, elements with low vapor pressure, such as B, C, Ru, Ir, and W, cannot be evaporated by external Ohmic heating. To evaporate those elements requires EBE, but that technique is not optimal for achieving precise and stable evaporation rates. PLD transfers a source material onto a substrate via short-period, high-power laser pulses.7,8 Although PLD can operate with a high background pressure of reactive gases, the precise control of the material composition is challenging, in particular if the film composition is to be varied smoothly.

Laser-assisted evaporation had been proposed and attempted for film deposition right after the invention of the laser.9–15 However, the evaporation by continuous-wave (cw) lasers has been abandoned due to the formation of nonstoichiometric films.11,15 Film growth by high-power density pulsed laser was found to be much more successful and led to the invention of PLD. Along with the development of cw laser technology, TLE has been recently rediscovered as a candidate for the epitaxial growth of complex materials1,2 because TLE can combine the advantages of MBE, PLD, and EBE while eliminating their respective weaknesses. Lasers placed outside the vacuum chamber evaporate pure metal sources by local heating, which requires only a simple setup and allows the precise evaporation control of each source element, high purity of the source materials, and the almost unlimited choice of background gas composition and pressure. In many cases, the locally molten source forms its own crucible. By avoiding impurity incorporation from the crucible, the source is guaranteed to remain highly pure.1 The potential of TLE to deposit elemental metallic and semiconducting films has recently been demonstrated.1,2 Indeed, TLE has been used to grow a wide range of elements as films, ranging from high-vapor-pressure elements, such as Bi and Zn, to low-vapor-pressure elements, such as W and Ta.2 

Whereas using TLE to grow oxide films and heterostructures may also be highly advantageous, it is not obvious that it is possible in an oxidizing atmosphere. Oxidation of the heat source (filament), which plagues MBE and EBE, is trivial to avoid in TLE. However, the metal sources themselves are prone to oxidation when heated by a laser beam in an oxidizing atmosphere. If the source oxidizes, the laser radiation is no longer absorbed only by the original source material but also by its oxide. Indeed, the entire source or the surface of the source may oxidize, or the oxide may form a partial layer floating on a melt pool. In addition, the molecular fluxes of the source materials may be generated by both the metallic part of the source and the source material oxide. Whether films can be grown under stable conditions in configurations in which the laser irradiation interacts with such metal-oxide systems is an open question to be clarified.

To do so, we performed a series of evaporation experiments in which elemental metal sources having high or low vapor pressures were evaporated by laser irradiation in a variety of oxygen–ozone atmospheres. For simplicity in exploring the evaporation process, we used substrates of unheated Si (100) wafers coated by their native oxide. We readily succeeded in growing oxide films using the same laser optics and laser wavelengths of 1030–1070 nm for every element explored. Our experiments reveal that, despite the oxidation of the source, the evaporation of elemental sources in strongly oxidizing atmospheres is feasible and useful for oxide film growth. We also find that different oxide phases are obtainable in a given atmosphere by tuning the oxidizing atmosphere. The deposition process is furthermore found to display a characteristic variation as a function of oxygen–ozone pressure.

A schematic of the TLE chamber used in this study is shown in Fig. 1. The chamber shares its basic geometric features with the setup reported in Ref. 2. Separated by a 60-mm working distance (d in Fig. 1), a high-purity cylindrical metal source and the 2-in. Si (100) substrate are supported by Ta-based holders. We used a 1030-nm fiber-coupled disk laser and a 1070-nm fiber laser incident at 45° to the top surface to heat the sources. As determined by the availability of these lasers, we used the former laser to evaporate Ti, Co, Fe, Cu, and Ni and the latter for the other elements. No difference in the performance of the two lasers was noted. Both lasers illuminated approximately elliptical areas of ∼1 mm2 on the sources. For temperature sensing, we positioned type C W–Re thermocouples on the backside of the Si wafers and at the bottom of the sources.

FIG. 1.

Sketch of the TLE chamber for the oxide film growth.

FIG. 1.

Sketch of the TLE chamber for the oxide film growth.

Close modal

A flowing oxygen–ozone mixture and a cascaded pumping system comprising two turbomolecular pumps and a diaphragm pump connected in series16 were employed for the precise control of the chamber pressure Pox, which was varied between <10−8 and 10−2 hPa. Ozone accounted for ∼10 wt. % of the total flow provided by the glow-discharge continuous-flow ozone generator. The setting of the valve controlling this gas flow was held constant during each deposition to provide a constant flow. During the evaporation process, Pox and the temperatures of the source and substrate were monitored by pressure gauges and the thermocouples. Using the same deposition geometry, we used TLE to evaporate 15 different metal elements to deposit oxide films. Each element was evaporated in several runs using the same laser power and laser optics but different values of Pox ranging from 10−8 to 10−2 hPa. Scanning electron microscopy (SEM) was employed to measure the film thickness and to study its microstructure. The crystal structures of the deposited films were identified by x-ray diffraction. Photoemission spectroscopy was performed to reveal the oxidation states of the TLE-grown TiO2 films. If a film was found to be amorphous, it was later subjected to an additional 2-h Ar anneal at 500 °C for crystallization.

Owing to the consumption of the oxygen–ozone gas mixture caused by oxidation of the source and the evaporated material, Pox frequently decreased during deposition, as illustrated in Fig. 2. This figure shows Pox during the evaporation of Ti at several gas pressures. The laser irradiation time for TLE of Ti was 15 min, indicated by a gray rectangle in Fig. 2. Pox decreases as the laser is turned on at ∼300 s, and it quickly returns to the initial background value when the laser is turned off at ∼1200 s. Oxidation is more active at higher temperatures. The decrease in Pox has therefore to be attributed predominantly to the oxidation of the elemental source. This understanding is consistent with the observation that the maximum amount of oxygen required to oxidize the evaporated material is less than 1% of the inlet gas flow. Oxidation of the film can therefore not account for the observed pressure change. After deposition at 10−2 hPa with 160 W laser power (see Fig. S1 of the supplementary material), the Ti source is covered by a white substance, which most probably consists of TiO2. Other elemental sources are also oxidized after use, as shown in Fig. S1. This substantial oxidation of the sources, to which we referred in the Introduction, affects the absorption of the laser light, the evaporation process, and the vapor species deposited on the substrates.

FIG. 2.

Chamber pressure Pox measured during the laser evaporation of Ti, using a constant laser power and oxygen–ozone gas flow.

FIG. 2.

Chamber pressure Pox measured during the laser evaporation of Ti, using a constant laser power and oxygen–ozone gas flow.

Close modal

However, the decrease in the background pressure is not observed in all instances. The pressure change is small or even absent in two cases: first, if the source has already been fully oxidized at the beginning of the process and, second, if the oxidation of the source is intrinsically unfavorable. The thermal laser evaporation of Ni in the oxidizing atmosphere is an example of the first case. A decrease in Pox is observed only for Pox < 10−4 hPa. At higher pressure, the Ni source becomes covered by its oxide. Further oxidation is therefore suppressed, and the decrease in Pox disappears (supplementary material, S2). Figure S1(b) shows the correspondingly oxidized Ni source. The predominant vapor species obtained by heating Ni under strongly oxidizing conditions is therefore provided by NiO. The thermal laser evaporation of Cu is an example of the second case, as the oxidation of Cu is relatively unfavorable. Above 1000 °C and in an oxygen pressure range of 10−4–10−2 hPa, metallic Cu is more stable than its oxides.17 In the experiment, the source temperature in the irradiated area exceeds 1085 °C, as is evident from the fact that Cu is locally molten (see Fig. S3 of the supplementary material). At this temperature, liquid Cu is the thermodynamically stable phase, and elemental Cu is expected to provide the dominant vapor species. Indeed, no significant change in the chamber pressure occurs during the evaporation of Cu (Fig. S3). In agreement, the laser-irradiated area of a Cu source is metallic after the TLE process (Fig. S1).

We have tested 15 metallic elements as sources for the TLE growth of oxide films (Table I). Figure 3 shows the grazing-incidence XRD patterns of TLE-grown TiO2, Fe3O4, HfO2, V2O3, NiO, and Nb2O5 films. The gray lines in Fig. 3 indicate the expected diffraction peak positions of each oxide. These patterns are typical for all binary oxides investigated here. As shown, the films are polycrystalline and, in many cases, single-phase. Most elements provided polycrystalline films on the unheated Si substrates except for Cr, which formed an amorphous oxide. A subsequent 2-h 500 °C Ar anneal transformed this layer into a polycrystalline Cr2O3 film. Table I summarizes the observed oxide phases. The Ti, V, and Mo oxides formed several phases, with Pox determining which phase was obtained. In the case of V, for example, V2O3, VO2, or V2O5 films are obtained by increasing Pox from 10−4 to 10−2 hPa (supplementary material, S4). For the other elements, we observed only a single oxidation state within the Pox range used.

TABLE I.

List of the oxide thin films deposited by TLE in this work.

Elemental sourceFilm
Sc Sc2O3 
Ti TiO, TiO2a 
V2O3, VO2, V2O5 
Cr Cr2O3b 
Mn MnO 
Fe Fe3O4 
Co Co3O4 
Ni NiO 
Cu CuO 
Zn ZnO 
Zr ZrO2 
Nb Nb2O5 
Mo Mo4O11, MoO3 
Hf HfO2 
Ru RuO2 
Elemental sourceFilm
Sc Sc2O3 
Ti TiO, TiO2a 
V2O3, VO2, V2O5 
Cr Cr2O3b 
Mn MnO 
Fe Fe3O4 
Co Co3O4 
Ni NiO 
Cu CuO 
Zn ZnO 
Zr ZrO2 
Nb Nb2O5 
Mo Mo4O11, MoO3 
Hf HfO2 
Ru RuO2 
a

Both anatase and rutile phases were observed.

b

The film was annealed at 500 °C for 2 h in an Ar ambient.

FIG. 3.

Grazing-incidence x-ray diffraction patterns of (a) Ti-, (b) Fe-, (c) Hf-, (d) V-, (e) Ni-, and (f) Nb-oxide films grown by TLE on Si (100) substrates. The expected diffraction peak positions of each oxide are marked in each figure by gray lines.

FIG. 3.

Grazing-incidence x-ray diffraction patterns of (a) Ti-, (b) Fe-, (c) Hf-, (d) V-, (e) Ni-, and (f) Nb-oxide films grown by TLE on Si (100) substrates. The expected diffraction peak positions of each oxide are marked in each figure by gray lines.

Close modal

To investigate the structure of the films in more detail, we performed cross-sectional SEM. As shown in Fig. 4, which displays the SEM cross sections of the films of Fig. 3, most of the polycrystalline films have a columnar structure. The ratio between the measured substrate temperature and the melting point of the deposited oxide ranges from 0.05 to 0.2 (see Fig. S5 of the supplementary material). The observed columnar structure is therefore consistent with the zone model of film growth, which for the conditions used here predicts the formation of a columnar microstructure.18 The crystal structure of the deposited oxide nevertheless affects the film structure. Mo oxide films grown at 10−3 and 10−2 hPa comprise prismatic and hexagonal plates, respectively, as shown in Fig. S6 of the supplementary material. The films shown in Fig. 4 were grown at rates of several Å/s; these rates were chosen as typical for the growth of oxide films. The rates (see Fig. 4) were measured by dividing the film thickness at the wafer center by the laser irradiation time. The deposition rates are not limited to the values presented. Indeed, they increase super-linearly with laser power as specified in Fig. S7 of the supplementary material, consistent with Ref. 2.

FIG. 4.

Cross-sectional SEM images of several oxide films deposited by TLE. Each panel shows the value of Pox and deposition rate (ν). Most films have a columnar structure.

FIG. 4.

Cross-sectional SEM images of several oxide films deposited by TLE. Each panel shows the value of Pox and deposition rate (ν). Most films have a columnar structure.

Close modal

As the source is heated locally, it behaves like a flat, small-area evaporation source, providing as a function of emission angle a cosine-type flux distribution.19 Indeed, SEM measurements show that the films are thinner toward the wafer edge. With the evaporation parameters we used, the reduction in the film thickness toward the edge equals ∼20% in most cases, in reasonable agreement with the theoretically expected value of 28%.

Our studies show that, as expected, the phase of the deposited oxide is a function of oxidizing gas pressure. This behavior is illustrated for Ti and Ni films in Fig. 5. This figure provides the XRD patterns of such films grown in several different Pox. In the case of Ti, polycrystalline hexagonal Ti films are obtained if the deposition is made without oxygen–ozone. With increasing Pox, sub-stoichiometric TiO, rutile TiO2, and anatase TiO2 films are deposited. TiO is a well-known suboxide of Ti. It was formed in a weakly oxidizing environment with Pox ∼ 10−6 hPa. The peaks at 37.36°, 43.50°, and 63.18° [Fig. 5(a), red curve] indicate cubic TiO. Rutile TiO2 appears in the film for Pox ∼ 10−4 hPa. The gray lines mark the expected diffraction peak positions of rutile TiO2. At 10−3 hPa, anatase TiO2 is generated together with the rutile phase as marked by the purple stars in Fig. 5. Owing to its low surface free energy, the metastable TiO2 anatase phase is preferably obtained by most synthesis and deposition methods.20 High-energy conditions are usually required to transform the anatase phase to the rutile phase or to synthesize rutile-phase TiO2 directly.20,21 We observe the preferential formation of rutile-phase TiO2, although, given by the thermal energies of the evaporated atoms and molecules, TLE is a low-energy process. At 10−2 hPa, the deposited films lose their crystallinity, as discussed in greater detail in the supplementary material (Fig. S8).

FIG. 5.

Grazing-incidence x-ray diffraction patterns of TLE-deposited (a) Ti-oxide and (b) Ni-oxide films for several Pox values. As Pox increases, the Ti source produces TiO2 films in the rutile and anatase phases, whereas the Ni source forms partially oxidized Ni/NiO films. The gray lines and purple solid stars in (a) show the expected diffraction peak positions of TiO2 rutile and anatase phases, respectively. The gray lines in (b) show the expected peak positions of cubic NiO.

FIG. 5.

Grazing-incidence x-ray diffraction patterns of TLE-deposited (a) Ti-oxide and (b) Ni-oxide films for several Pox values. As Pox increases, the Ti source produces TiO2 films in the rutile and anatase phases, whereas the Ni source forms partially oxidized Ni/NiO films. The gray lines and purple solid stars in (a) show the expected diffraction peak positions of TiO2 rutile and anatase phases, respectively. The gray lines in (b) show the expected peak positions of cubic NiO.

Close modal

The oxidation states of the TLE-grown TiO2 films were analyzed by XPS and compared to TiO2 films grown by EBE. Whereas the as-deposited EBE sample comprises a significant amount of Ti3+, TLE samples contain mostly Ti4+ (supplementary material, S9). We attribute this phenomenon to the oxygen–ozone background, which suppresses the thermal dissociation of TiO2, TiO2(s) → TiO(g) + 1/2O2(g), and oxidizes the deposited material.

Interestingly, we have found that the oxidation behavior of TLE-grown Ni-oxide films differs markedly from that of Ti oxide films. Under UHV conditions, a cubic phase is found also for metallic Ni [Fig. 5(b)]. Although the Ni source surface is oxidized at Pox ∼ 10−6 hPa (see the decrease in the chamber pressure in Fig. S2), the obtained films show metallic behavior also at this Pox. We attribute this to the high oxidation potential of Ni5,22 and to Ni having a higher vapor pressure than NiO.23 The majority vapor species therefore originates from unoxidized Ni in the irradiated hot area. Furthermore, the Ni deposited on the substrate does not oxidize significantly at the low substrate temperature. The NiO phase evolves gradually with increasing Pox. Diffraction peak positions expected for the NiO phase are indicated by the gray lines in Fig. 5, showing the formation of cubic NiO. As evidenced by the presence of both metal and oxide peaks, the Ni film deposited at 10−5 hPa is partially oxidized to NiO. The NiO phase dominates at higher Pox.

Pox also affects the deposition rate of TLE-grown oxide films. Figure 6 shows the pressure dependence of the deposition rates of Ti- and Ni-based oxide films. Considering the oxygen incorporation into the film, we would expect the deposition rate to increase with increasing Pox. However, although the rate increases with Pox under specific conditions, the observed deposition rate behavior cannot be explained by oxygen incorporation alone. The growth rate of the Ti-based films increases with Pox from ∼0.6 Å/s at base pressure to 3.5 Å/s at 10−3 hPa. The sixfold increase in the deposition rate seen implies further factors to affect the growth rate. In contrast to the behavior of the Ti evaporation, the deposition rate of Ni-based oxide films increases only from 3.1 to 4.6 Å/s at 10−4 hPa and then drops drastically to 0.3 Å/s for Pox > 10−4 hPa. An increase in the oxide fraction in the film (Fig. 5) may be responsible for the initial increase in deposition rate but cannot explain the huge decrease in deposition rate at 10−3 hPa. The growth properties of Ti- and Ni-based films represent two characteristic modes observed for most of the films. Fe, Co, Nb, Zn, and Mo show the behavior of Ti, whereas Cr, Sc, Mn, and V show that of Ni.

FIG. 6.

Deposition rates of (a) Ti (oxide) and (b) Ni (oxide) measured at several Pox. The deposition rate of Ti increases with increasing Pox, whereas for Ni an increase in Pox > 10−3 hPa almost suppresses the evaporation process.

FIG. 6.

Deposition rates of (a) Ti (oxide) and (b) Ni (oxide) measured at several Pox. The deposition rate of Ti increases with increasing Pox, whereas for Ni an increase in Pox > 10−3 hPa almost suppresses the evaporation process.

Close modal

Why does Pox alter the deposition rate of oxide films grown by TLE in these two rather characteristic ways? We suggest that this behavior is controlled by the vapor pressure of the source’s oxidized surface layer. The deposition rate increases with Pox if the vapor pressure of the oxide formed at the source surface exceeds that of the metal. This corresponds to the Ti-like deposition rate behavior. The formation of TiO2 gas vapor, Ti(s) + O2(g) → TiO2(g), is an exothermic reaction,24 leading to effective generation of oxide vapor from the source. Because the metal oxidation rate increases with a power of Pox (oxidation rate Poxn),25 the deposition rate increases correspondingly with Pox, as observed for Fe and Nb (see Fig. S10 of the supplementary material). In contrast, the Ni-like scenario is found if the vapor pressure of the metal exceeds that of the oxide. As the vapor pressure of NiO is about one order of magnitude smaller than that of Ni,23 a NiO coverage of a source reduces the deposition rate by the same factor. This understanding is supported by the observation that the abrupt decrease in the deposition rate of Ni occurs at 10−3 hPa, the same pressure at which the pressure drop in the chamber disappears (see Fig. S2), revealing that the source is passivated by a NiO layer at this Pox.

We have demonstrated the growth of polycrystalline oxide films by TLE. The films having tunable oxidation states and a crystal structure can be grown by evaporating pure metal sources in oxygen–ozone pressures of up to 10−2 hPa, irrespective of possible oxidation of the sources. From a wide range of metal sources comprising low- and high-vapor-pressure elements, polycrystalline films in various oxidation states were deposited with growth rates of several Å/s on unheated Si (100) substrates. Determining the degree of source oxidation, the pressure of the oxidizing gas strongly affects the deposition rate as well as the composition and phase of the resulting oxide films. Our work paves the way to the TLE growth of epitaxial oxide heterostructures of ultrahigh purity.

See the supplementary material for the detailed experimental data and graphs.

The authors thank T. J. Smart and H. Boschker for many valuable discussions and support, H. Hoier for the x-ray diffraction of the films, and K. Küster for measuring and analyzing the x-ray photoemission spectra of the TiO2 films. The authors also acknowledge technical support by W. Winter, S. Seiffert, I. Hagel, and K. Lazarus and editing support by L.-M. Pavka.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Supplementary Material