The oxidation-related issues in controlling Si doping from the Si source material in oxide molecular beam epitaxy (MBE) are addressed by using its solid suboxide, SiO, as an alternative source material in a conventional effusion cell. Line-of-sight quadrupole mass spectrometry of the direct SiO-flux () from the source at different temperatures (TSiO) confirmed SiO molecules to sublime with an activation energy of 3.3 eV. The TSiO-dependent was measured in vacuum before and after subjecting the source material to an O2-background of (typical oxide MBE regime). The absence of a significant difference indicates negligible source oxidation in molecular O2. Mounted in an oxygen plasma-assisted MBE, Si-doped β-Ga2O3 layers were grown using this source. The at the substrate was evaluated [from 2.9 × 109 cm−2 s−1 (TSiO = 700 °C) to 5.5 × 1013 cm−2 s−1 (TSiO = 1000 °C)] and Si-concentration in the β-Ga2O3 layers measured by secondary ion mass spectrometry highlighting unprecedented control of continuous Si-doping for oxide MBE, i.e., from 4 × 1017 cm−3 (TSiO = 700 °C) up to 1.7 × 1020 cm−3 (TSiO = 900 °C). For a homoepitaxial β-Ga2O3 layer, a Hall charge carrier concentration of 3 × 1019 cm−3 in line with the provided (TSiO = 800 °C) is demonstrated. No SiO-incorporation difference was found between β-Ga2O3(010) layers homoepitaxially grown at 750 °C and β-Ga2O3(−201) heteroepitaxial layers grown at 550 °C on c-plane sapphire. However, the presence of activated oxygen (plasma) resulted in partial source oxidation and related decrease in doping concentration (particularly at TSiO < 800 °C), which has been tentatively explained with a simple model. Degassing the source at 1100 °C reverted this oxidation. Concepts to reduce source oxidation during MBE-growth are referenced.
Due to a high variety of functional properties, metal oxides are rising in popularity as material systems for innovative optoelectronic devices.1 Among metal oxides, monoclinic β-Ga2O3 is one of the most interesting ones, and it has been particularly intensely investigated in the past decade.2 In fact, due to a predicted breakdown field around 8 MV/cm as a consequence of its bandgap of about 4.8 eV, β-Ga2O3 is one of the most promising novel materials for power electronics.3 Moreover, it can be grown from the melt4–7 in turn allowing for β-Ga2O3 homoepitaxy of high quality layers.3
To obtain a broad range of devices for power electronic application, a thorough control on the electrical properties is needed.8 Considering the lack of p-type doping for Ga2O3,9 the main focus is on the group-IV elements in order to tune n-type doping. So far, Ge, Si, and Sn have been already successfully employed as dopants in homoepitaxial layers grown with different technique, such as molecular beam epitaxy (MBE),5,10–15 Metal Organic Chemical Vapor Deposition (MOCVD),16–19 and Metal Organic Vapor Phase Epitaxy (MOVPE).20,21
Lany has theoretically shown22 that Si can be considered as a better donor compared to Ge and Sn, since it is predicted to be the only truly shallow donor among them. From a survey of experimental data collected on semiconducting homoepitaxial β-Ga2O3 layers,8 the highest electron mobilities μ over a broad range of electron concentrations n are achieved with Si or Ge doping; the generally lower μ in Sn-doped films is likely related to a deep donor state identified for Sn.12 Controlled Si doping in β-Ga2O3 has been already demonstrated in MOCVD but not in MBE. This is most likely linked to the two limitations of Si doping in MBE systems.
The first one is due to the unintentional incorporation of Si, probably related to the quartz cavity of the plasma source in oxygen plasma-assisted MBE (PAMBE)23,24 and is limiting the low concentration side of the doping window (NSi < 1018 cm−3).
The second one is due to instable Si-doping concentration in PAMBE-grown Ga2O3 layers that depend on the oxygen background pressure rather than the Si source temperature as pointed out by Kalarickal et al.13 As an underlying process, they identified the oxidation of the elemental Si source resulting in the source flux to consist of the SiO suboxide, which has a higher vapor pressure than Si. Further oxidation of the Si surface into solid SiO2 was identified by Krishnamoorthy et al.15 to cause drastic flux reduction during layer growth due to the lower vapor pressure of SiO2. Based on the underlying mechanisms, we conjecture ozone-MBE not to suffer from the first limitation but from the second one.
Indeed, the oxidation of the heated elemental source in an oxygen background into its volatile suboxide happens for many elements, including In, Ga, Ge, or Sn.25 This suboxide formation at the source can determine the Ge- or Sn-doping of oxides (including in the Ga2O3 material system) or even the growth rates (GRs) for Ga2O3 during MBE deposition at relatively low Ga-source temperatures (e.g., <750 °C). Despite similar suboxide formation, the use of a Sn-metal source for Ga2O3 in PAMBE is more controllable with respect to a Si one12 but can involve segregation problems.26 For Ge-doping of Ga2O3 from a Ge-source in PAMBE, an additional strong dependence of doping concentration on the growth substrate temperature (Tg) has been shown.14 High concentrations (NGe ≈ 1020 cm−3) required relatively low Tg (600 °C), potentially placing limits on the crystal quality of the deposited layers.
An efficient solution to control Si doping in oxide MBE while avoiding massive oxidation of the source could be the use of an oxide or suboxide source material in the cell,27 similar to the use of SnO2, SnO, or mixtures of Sn and SnO2 to produce a SnO flux,26,28 or mixtures of Ga and Ga2O3 to produce a Ga2O flux.28,29 Major advantages in the use of a suboxide (or mixed elemental + oxide) sources over an oxide one are the significantly lower cell temperatures required and the absence of parasitic oxygen formation from the decomposition of the oxide source material.28 On the contrary, controllable Si doping would benefit from a SiO2 source that cannot oxidize further as SiO could. However, based on the vapor pressure of SiO2,27 the geometry of our growth chamber, and the temperature limit of conventionally employed effusion cells (1200 °C), the Si concentration in layers deposited at a typical growth rate of 4.5 nm/min would be limited to NSi ≤ 1019 cm−3 for a SiO2 source. For example, a high-temperature effusion cell running at = 1400 °C would be required to obtain NSi = 1 × 1020 cm−3 [considering that a growth rate (GR) of 1 Å/s in our MBE system correlates with a partial pressure of the source material in the effusion cell on the order of 10−3 mbar (Ref. 28)].
In comparison, a SiO source could reach NSi = 1.7 × 1020 cm−3 at TSiO = 900 °C, which prompted us to characterize a solid SiO source for PAMBE growth with a focus on doping β-Ga2O3 layers. In particular, we demonstrate the possibility to obtain Si-doping concentrations on a wide range—from 4 × 1017 cm−3 (limited by the background Si concentration in the PAMBE deposited layers) up to 1.7 × 1020 cm−3—in both homoepitaxial and heteroepitaxial β-Ga2O3 layers, with negligible dependence on the growth orientation and/or the growth temperature. For our study, an Al2O3 crucible loaded with 10 g of 3–6 mm SiO lumps (4 N purity, Alfa Aesar) was placed in a conventional single-filament effusion cell.
At first, we characterized the direct flux from this SiO source using a quadrupole mass spectrometer (QMS) mounted line in sight to the cell inside a custom-built system, which is schematically shown in the inset of Fig. 1 and described in Ref. 28. The composition of the flux from the SiO source at a base pressure in the low 10−8 mbar regime is reported in Fig. 1(a) for a cell temperature of TSiO = 1200 °C chosen to maximize the signal-to-noise ratio for the identification of the different desorbing species. Due to the natural abundance of stable isotopes of Si at 28 amu (92.23%), 29 amu (4.67%), and 30 amu (3.1%), the major signals shown in Fig. 1(a) can be clearly identified as SiO. A weaker Si signal is recorded and related to fragmentation inside the QMS.28 To remove interference with residual N2 (having the same mass per charge as 28Si), the measurements with closed cell shutter were subtracted from those with opened shutter. An Ar signal due to the background pressure is also visible. The same qualitative SiO spectra could be recorded down to TSiO = 880 °C where the 44SiO signal reaches the noise level.
To calculate the activation energy Ea for SiO sublimation, the 44SiO signal was recorded at TSiO ranging from 1200 to 800 °C in vacuum. The corresponding Arrhenius plot yields Ea = 3.29 eV [red line in Fig. 1(b)] in good agreement with the Ea = 3.41 eV [black line in Fig. 1(b)] extracted from the thermodynamic calculation of Adkison et al.27
To investigate potential source oxidation in molecular oxygen, the source was exposed to a controlled O2 background of p = 1 × 10−5 mbar (typical for PAMBE growths) for periods of 30–180 min at various TSiO. A potential surface oxidation to SiO2 during this exposure is expected to reduce the SiO flux () with respect to the untreated SiO source material. Figure 2 shows the comparison of the measured SiO partial pressure during the ramp up (performed in vacuum) from TSiO = 700 to 1200 °C before and after exposure of the source to an O2 background pressure of 1 × 10−5 mbar for 3 h at TSiO = 700 °C. The matching partial pressure recorded in both experiments indicates that the SiO source did not get oxidized. The initial (up to 15 min) higher values detected for the SiO partial pressure after the oxidation process (red dashed lines in Fig. 2) are due to the higher background pressure caused by residual oxygen in the chamber after the treatment and a limited mass filtering of the QMS.
Next, the crucible with the SiO source material was mounted in our PAMBE growth chamber for doping experiments. In order to predict the SiO-flux and consequential Si doping concentration in MBE deposited β-Ga2O3 layers as a function of the TSiO, we extrapolated the corresponding for TSiO = 1000 °C () toward lower TSiO by
with Boltzmann's constant k, activation energy Ea from the QMS experiment [red line in Fig. 1(b)], and extracted from the growth rate of an amorphous SiO2 layer as described in the supplementary material. The extrapolation described in Eq. (1) is shown as the solid line in Fig. 3. Knowing the β-Ga2O3 growth rate—in our case, a typical value is GRGa2O3 = 4.5 nm/min—it is possible to derive the expected Si doping concentration, e.g., ΝSi = 6.4 × 1021 cm−3 for TSiO = 1000 °C using
The predicted values obtained from Eqs. (1) and (2) were used as our guideline to investigate different doping concentrations in β-Ga2O3 layers on both Al2O3(0001) and β-Ga2O3(010) substrates grown at Tg of 550 and 750 °C, respectively (see the supplementary material). Depth profiles of the Si dopant were obtained by time-of-flight secondary ion mass spectrometry (ToF-SIMS IV, iontof GmbH, Germany). For quantification, the relative sensitivity factor for silicon in a gallium oxide matrix was obtained by measuring an implantation standard of Si into a nominally undoped β-Ga2O3 single crystal with a 1014 cm−2 fluence and an implantation energy of 80 keV.
In Fig. 3, symbols represent the different ϕSiO at the substrate as function of the corresponding TSiO, calculated as the product of NSi (measured by SIMS) and (obtained from SIMS profile) following Eq. (2). For reference, the corresponding volumetric Si concentrations are shown assuming a fixed growth rate of 4.5 nm/min (average GR of the deposited samples) on the right y-scale of Fig. 3. The obtained broad range of Si doping concentrations from 3–4 × 1017 cm−3 (TSiO = 700 °C) up to 1.7 × 1020 cm−3 (TSiO = 900 °C) demonstrates the possibility to use a SiO source charge for continuous doping in β-Ga2O3 thin films controlled by the source temperature in PAMBE. Nonetheless, it was not possible to demonstrate the possibility of achieving doping concentrations below 3 × 1017 cm−3 due to an unintentional Si incorporation in the nominally undoped β-Ga2O3 layers of around NSi = 2 × 1017 cm−3 detected by SIMS. We tentatively relate this unintentional Si incorporation to the plasma source as recently pointed out by Asel et al.24
A close inspection of the Si doping profiles of all the deposited β-Ga2O3 films reveals a systematically decreasing Si concentration toward the surface, as exemplarily shown in Fig. 4(a). The respective highest and lowest Si concentrations from the begin and end of the doped layers are reflected by the corresponding error bars in Fig. 3. Before the growth of the doped layers, the SiO cell was kept at the corresponding temperature for 15–60 min, in order to exclude the thermalization of the source as the possible origin of the slope. Consequently, we attribute the decreasing Si concentration in the doping profiles to progressive oxidation of the surface of the source material into SiO2 during PAMBE growth. This finding is just apparently in contrast to the absence of source oxidation in molecular oxygen (see Fig. 2), since it confirms the findings of Kalarickal et al.13 who suggested active oxygen to play a major role for the oxidation of the Si source into SiO2 in PAMBE. To further understand the impact of source oxidation, two different data sets are provided in Fig. 3. The empty symbols mark samples grown with a partially oxidized source, while the filled ones show data for samples where a high temperature cycle (TSiO = 1100 °C, t = 30–60 min) was carried out before the layer growth to degas and restore the SiO surface. At TSiO = 800 °C, the comparison between the two sets of data leads to a difference around 1.5 orders of magnitude for the (cf. Fig. 3). Notwithstanding, the Si concentrations from the begin of the layers grown with a freshly degassed cell (upper error bar of filled symbols in Fig. 3) follow the vapor pressure behavior of SiO, confirming the principal control of Si-doping by the source temperature rather than the background pressure. Doping values from the literature13 concerning Si doping using an elemental Si source inside a PAMBE system are also reported in Fig. 3 (filled orange hexagons); the comparison with the experimental data collected in this work permits to highlight the significantly increased control of a SiO-source compared to a Si one. In addition, the matching incorporated SiO-flux concentration at the same TSiO both on (0001) Al2O3 (Tg = 550 °C) and β-Ga2O3 (Tg = 750 °C) suggests (i) a negligible dependence of Si-incorporation on the Tg as was instead observed for Ge14 and (ii) a negligible dependence on the β-Ga2O3 growth orientation [i.e., (−201) and (010), respectively].
Further inspection of Fig. 4(a) suggests the slope of the decreasing Si-concentration to depend on the SiO cell temperature, being steeper for a TSiO = 700 °C compared to the one at 800 °C. In a simple oxidation model [pictorially explained in Fig. 4(c)], the unoxidized area of the source material (that can provide a SiO flux) decreases with a decay constant λ. Considering a fixed probability per unit time that a certain percentage of the SiO source surface will further oxidize into SiO2, as a function of TSiO and the oxygen background pressure in the MBE chamber, we expect an exponential decay of the accessible SiO surface, and hereby of the SiO flux. Consequently, an exponentially decreasing (which is proportional to this area) is expected during source oxidation,
where ΦSiO(0) corresponds to the flux at t = 0. Using Eq. (3), Eq. (2), and the relation between layer thickness d and corresponding growth time t (d =t), we extracted λ from the slopes of the SIMS profiles d(lnNSi)/d(d) as
Figure 4(b) shows the extracted λ for layers doped at different TSiO and suggests two different regimes: (i) for TSiO > 730 °C, almost flat doping profiles signify a low decay constant, (ii) for lower TSiO, a high λ indicates non-stable doping throughout the layer thickness (e.g., decrease in the doping concentration from 6.8 × 1017 cm−3 down to 2 × 1017 cm−3 in a deposition time t = 15 min, i.e., over a 90 nm layer thickness). These data suggest lower TSiO (within the investigated range) to promote oxidation of the source, similar to the observations of Kim et al. for the Sr source oxidation,30 and in very good agreement with Kalarickal et al.,13 that determined passive oxidation of an elemental Si source into SiO2 at TSi = 750 °C and active oxidation into SiO at TSi ≥ 800 °C during PAMBE growth of Si-doped Ga2O3. Hence, the low temperature and, consequentially, low doping regime requires further measures to reduce the oxidation of the source in order to achieve more uniform doping profiles.
In order to understand how the active oxygen in the chamber affects the oxidation of the source, a preliminary study of the decay constant, for a fixed TSiO = 800 °C, as a function of both the oxygen flux and the plasma power [inset Fig. 4(b)] is provided. More data will be needed in order to completely understand and model this behavior but, a reduction of the oxygen flux, from 1 sccm down to 0.33 sccm, clearly shows a significant reduction of the decay constant (around two times less at 400 W at the same TSiO). For the plasma power the data show some inconsistency. In fact, while at the lower flux Oflux = 0.33 sccm and low plasma power (150 W), a lower decay constant is found compared to the sample with the same oxygen flux, at higher plasma power (400 W, hence, a larger amount of active oxygen), for the higher supplied flux Oflux = 1 sccm, the trend is inverted. This unexpected behavior is still not clearly understood, and further experiment is needed in order to address this issue.
Finally, room temperature van der Pauw–Hall measurements (details in the supplementary material) were performed on a homoepitaxially grown sample with a 124 nm-thick Si-doped layer (NSi = 3 × 1019 cm−3). The extracted Hall electron concentration of n = 3 × 1019 cm−3 with an electron mobility μ of 25 cm2/V s and a sheet resistance Rs of 645 Ω/sq confirms effective Si-doping by our approach in line with the provided .
In conclusion, we have demonstrated the potential of a solid SiO source to provide a wide range of SiO fluxes controlled by the source temperature of a conventional effusion cell. Using this source, SiO fluxes at the substrate ranging from 5.5 × 1013 (TSiO = 1000 °C) to 2.9 × 109 (TSiO = 700 °C) cm−2 s−1 were used to grow a SiO2 layer as well as continuously Si doping β-Ga2O3 layers with concentrations ranging from 1.7 × 1020 (TSiO = 900 °C) cm−3 to 3 × 1017 (TSiO = 700 °C) cm−3 inside an oxide PAMBE. Hall measurements of a homoepitaxial Si-doped β-Ga2O3 layer deposited with a TSiO corresponding to an expected NSi = 3 × 1019 cm−3 (800 °C) revealed the same charge carrier density (n = 3 × 1019 cm−3) indicating effective doping. The Si-incorporation was essentially the same for Ga2O3(010) homoepitaxially grown at 750 °C and Ga2O3(−201) heteroepitaxially grown at 550 °C.
An oxidation of the SiO source, leading to a decreasing SiO flux, has been observed in active oxygen (plasma) but not in molecular oxygen at similar background pressures. The decreasing flux has been parametrized by a decay constant λ extracted from the measured Si-doping profiles. Our initial data indicate a strong oxidation regime (λ ≥ 0.14 min−1) for relatively low TSiO (≤730 °C) and a milder one (λ < 0.015 min−1) at higher TSiO (≥800 °C) at 250 W. The partial oxidation of the source can be reproducibly reverted by degassing at TSiO = 1100 °C. Reducing the supplied oxygen flux has been shown to reduce the oxidation of the source (lowest recorded λ = 0.003 min−1 at 0.33 sccm and 150 W), while the effect on the plasma power on the oxidation process is still unclear and further experiments will be needed in order to provide a definitive answer.
We believe that our approach can be also applied to O3-MBE26 as well as for doping of other oxides, e.g., In2O3. Suggested measures to reduce the source oxidation during growth by oxygen PAMBE include (i) an increased TSiO or (ii) a decreased activated oxygen pO2 at the SiO source. The (i) can be realized by a modified source geometry, e.g., using an aperture on the crucible31 or an increased source-to-substrate distance,30 to necessitate a higher SiO vapor pressure (and thus higher TSiO) inside the crucible to obtain the same SiO flux at the substrate. The (ii) can be realized by the same aperture as in (i), a differentially pumped SiO source,32 or by using growth conditions that require less oxygen,33 e.g., metal-exchange catalyzed MBE (MEXCAT-MBE).34–37
See the supplementary material for the growth of the amorphous SiO2 layer that was used in order to calibrate our flux predictions (Fig. 3). Furthermore, details on the growth of the β-Ga2O3 layers and additional information on the transport measurements are also provided.
We would like to thank Duc Van Dinh for critically reading the manuscript, as well as Hans-Peter Schönherr, Claudia Herrmann, Carsten Stemmler, and Steffen Behnke for their technical support on the MBE and test chamber. This work was performed in the framework of GraFOx, a Leibniz-ScienceCampus, and was funded by Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—Project No. 446185170.
Conflict of Interest
The authors have no conflicts to disclose.
Andrea Ardenghi: Data curation (equal); Formal analysis (equal); Investigation (equal); Writing – original draft (equal); Writing – review and editing (equal). Oliver Bierwagen: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Supervision (equal); Writing – review and editing (equal). Andreas Falkenstein: Investigation (equal); Methodology (equal); Writing – review and editing (supporting). Georg Hoffmann: Investigation (supporting); Writing – review and editing (supporting). Jonas Lähnemann: Investigation (supporting); Writing – review and editing (supporting). Manfred Martin: Investigation (supporting); Methodology (supporting); Writing – review and editing (supporting). Piero Mazzolini: Conceptualization (lead); Data curation (equal); Formal analysis (equal); Funding acquisition (lead); Investigation (equal); Project administration (lead); Supervision (lead); Writing – original draft (equal); Writing – review and editing (equal).
The data that support the findings of this study are available from the corresponding authors upon reasonable request.