NAMBE 2023 Session NM-MoM1: Oxide Semiconductors

We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow β-Ga₂O₃ at a growth rate of ~1 µm/h with control of the silicon doping concentration from 5x10¹⁶ to 10¹⁹ cm^–3. In S-MBE, pre-oxidized gallium in the form of a molecular beam that is 99.98% Ga₂O, i.e., gallium suboxide, is supplied. Directly supplying Ga₂O to the growth surface bypasses the rate-limiting first step of the two-step reaction mechanism involved in the growth of β-Ga₂O₃ by conventional MBE. As a result, a growth rate of ~1 µm/h is readily achieved at a relatively low growth temperature (T_sub ≈ 525 °C), resulting in films with high structural perfection and smooth surfaces (rms roughness of < 2 nm on ~1 µm thick films). Silicon-containing oxide sources (SiO and SiO₂) producing an SiO suboxide molecular beam are used to dope the β–Ga₂O₃ layers. Temperature-dependent Hall effect measurements on a 1 µm thick film with a mobile carrier concentration of 2.7x10¹⁷ cm^–3 reveal a room-temperature mobility of 124 cm² V^–1 s^–1 that increases to 627 cm² V^–1 s^–1 at 76 K; the silicon dopants are found to exhibit an activation energy of 27 meV. We also demonstrate working MESFETs made from these silicon-doped β-Ga₂O₃ films grown by S-MBE at growth rates of ~1 μm/h.

Ultra-wide band gap materials such as β-gallium oxide (β-Ga₂O₃) are promising for high power electronic devices since breakdown voltage scales with band gap. Within this material class, β-Ga₂O₃ is unique since β-Ga₂O₃ can be grown from the melt and demonstrates n-type conductivity with carrier concentration controllable between 10¹⁶ -10²⁰ cm^-3 with low donor activation energies.¹ However, oxidation of dopant material affects source vapor pressures which can impact dopant profile uniformity in β-Ga₂O₃ grown via PAMBE.^2,3 This work is focused on optimizing uniform dopant profiles in the low Si doping regime (<10¹⁸ cm^-3) of β-Ga₂O₃ films since Si doping concentrations show a gradient, increasing toward the surface of the thin films. SIMS and C-V measurements both show Si concentrations of ~3x10¹⁷ cm^-3. We also examine another aspect of Si incorporation in β-Ga₂O₃ that needs consideration when using traditional effusion cells as a Si dopant source. Previous work in PAMBE β-Ga₂O₃ growth demonstrated the importance of oxygen plasma power and Si cell temperature on the background amount of Si present in β-Ga₂O₃ films.⁴ In this work, we continued to explore sources of unintentional Si accumulation during the PAMBE growth process from sources like the quartz plasma bulb and Si effusion cell. The presence of Si at the interface between β-Ga₂O₃ substrate and film provides a parasitic conduction channel which is problematic for device performance. Attempts have been made to remove Si at β-Ga₂O₃ interfaces via etching. Our results indicate that removing interfacial Si may not be as simple since Si can re-accumulate during PAMBE, thus limiting the effectiveness of pre-growth surface treatments. Exposing a clean β-Ga₂O₃ surface to a quartz plasma bulb alone did not produce Si accumulation at the surface. On the other hand, a growth interrupt exposing a fresh β-Ga₂O₃ surface to a hot Si cell with shutter closed for several minutes produced Si accumulation equal to that of the interfacial Si between substrate and film. These results demonstrate that removing Si prior to loading into PAMBE may be inadequate to remove interfacial Si since Si can be re-introduced from the Si dopant cell during pre-deposition stages. These results point toward important challenges and potential solutions when growing Si-doped β-Ga₂O₃ thin films.

1 Neal, A. T. et al., Appl. Phys. Lett.113, 062101 (2018).

2 Kalarickal, N.K., et al., Appl. Phys. Lett.115, 152106 (2019).

3 McCandless, J.P., et al. Appl. Phys. Lett.121, 072108 (2022).

4 Asel, T. J., et al., J. Vac. Sci. Technol. A38, 043403 (2020).

β-Ga₂O₃ has been of interest due to its large bandgap and high critical electric field, making it an excellent candidate for power electronic and RF applications [1]. The community has made significant improvements in the growth of β-Ga₂O₃ via several techniques including molecular beam epitaxy (MBE). However, there has not been a study of the relationship between the β-Ga₂O₃ growth parameters and their effects on the defects present in the crystal and their impact on the electronic transport properties. Utilizing temperature dependent Hall Effect measurements and a self-consistent fitting of the temperature dependent carrier density and mobility data, we are able to quantify the concentration defect states, including compensating acceptors, deep level impurities, and unintentional donors, present in the epitaxial films. The Ga beam flux can control the oxygen to Ga ratio present in the chamber as our oxygen is held constant for each growth run. This allows for growths that occur in the “oxygen rich” regime where the amount of Ga supplied limits the growth rate and the “gallium rich” regime where the amount of O supplied limits the growth rate. In both regimes the formation of the volatile suboxide Ga₂O occurs, but in the gallium rich regime, the desorption of Ga₂O causes a lower growth rate than that seen in the oxygen rich regime, due to the excess of Ga presence preventing the second reaction step in Ga₂O₃ growth. Two preliminary samples were grown in the gallium rich (Ga Beam Flux = 1 × 10^-7 Torr) and oxygen rich (Ga Beam Flux = 6 × 10^-8 Torr) regimes. The samples had different doping densities of 4.25 × 10¹⁷ cm^-3 in the gallium rich sample and 2.60 × 10¹⁷ cm^-3 in the oxygen rich sample, this discrepancy is due to an inaccuracy in the Si doping source. The acceptor concentration in the gallium rich sample was calculated to be 7.26 × 10¹⁶ cm^-3 and 4.50 × 10¹⁵ cm^-3 in the oxygen rich sample, a factor of 16 difference based on the Ga beam. This is likely due to Ga vacancies that can form during desorption of Ga₂O during growth. These results indicate that there is significant impact on the electron transport properties of β-Ga­_2­O₃ with a change in the Ga to O ratio during growth, and that optimization of growth parameters is needed to optimize the electronic properties of MBE grown β-Ga­_2­O₃.

Ga₂O₃ has attracted significant interest due to its ultra-wide bandgap, high electron mobility, and large breakdown field. These properties exceed the current benchmarks set by materials such as SiC and GaN, making Ga₂O₃ optimal for next-generation power devices. Still, it has been proposed that the properties of Ga₂O₃can be extended further by alloying with Al to form (Al_xGa_1-x)₂O₃ which can raise the bandgap to 8.6 eV. This goal presents a challenge for the most researched phase, β, as β-Ga₂O₃thermodynamically prefers a monoclinic structure and α-Al₂O₃ is stable in the corundum structure. This structural mismatch limits the compositional range and the range of attainable bandgaps. In contrast, α-Ga₂O₃ occupies the corundum structure and has been shown to alloy over the full compositional range, enabling bandgaps from 5.3 - 8.6 eV. One method of growing α-(Al_xGa_1-x)₂O₃ is molecular-beam epitaxy (MBE). MBE is a powerful and highly controllable growth technique for α-(Al_xGa_1-x)₂O₃ thin films with drawbacks being slow growth rates of a few hundred nm/h and narrow adsorption-controlled growth windows. One method to increase the growth rate is the technique of suboxide MBE, which allows growth of β-Ga₂O₃thin films at rates exceeding 1 μm/h with large adsorption-controlled growth regimes.

Ultra-wide-bandgap (UWBG) semiconducting oxides are becoming more crucial in sustainable technologies due to their promising use in applications including transparent electronics and power switching. Among them, alkaline earth stannates such as SrSnO₃ with the perovskite crystal structure have gained much interest in recent years. However, the room-temperature mobility of SrSnO₃ thin films has been shown to be limited by defective surface scattering. By using a 4 nm undoped SrSnO₃ capping layer on 19 nm La-doped SrSnO₃ thin film, the measured room temperature mobility has been shown to improve. In this structure, charge spill over from the doped layer to the undoped layer is expected to happen as Fermi levels equilibrate. Here, we demonstrate a reversible and electrostatic doping of SrSnO₃ thin films grown by Hybrid molecular beam epitaxy with tunable carrier densities using electric-double-layer transistor (EDLT) configuration with ion gels. Using modeling and a discrete two-channel model, we show that the modulation due to gating is confined within 4 nm at the top capping layer and the modulation leads to an increase of mobility in SrSnO₃ up to 130 cm²V^-1s^-1 at 250 K. A detailed growth study combined with temperature-dependent Hall effect measurements and transport analysis will be presented.