GOX 2023 Session EG-MoM: Bulk/Epitaxial I

β-Ga₂O₃ has attracted extensive interest in power electronic applications owing to its large bandgap of ~ 4.9 eV, estimated high breakdown field of ~ 8 MV/cm, and availability of melt grown high quality β-Ga₂O₃ substrates. The growth of high-quality epitaxial films with low dislocation density and background impurity is critical to realize the projected device performances. Available epitaxial methods to grow β-Ga₂O₃ thin films include MBE, HVPE, and MOCVD. But, despite coming late to the field, the MOCVD method has proven to be suitable for producing high-quality epitaxial β-Ga₂O₃ films at a fast growth rate with uniform and controllable doping ¹. The highest purity β-Ga₂O₃ films have been reported from MOCVD with record low-temperature electron mobility exceeding 23,000 cm²/Vs and low~10¹³ cm^-3 compensating acceptors ². Also, a recent record-breaking result for lateral Ga₂O₃ MESFETs with a lateral figure of merit (LFOM) of 355 MW/cm² and a breakdown voltage of ∼2.5 kV ³, and a record low specific contact resistance ~10^-7 Ωcm²⁴ were reported based on MOCVD grown epitaxial Ga₂O₃ films.

This presentation will discuss recent progress in the growth of high-quality β-Ga₂O₃ thin films and related materials using MOCVD. The use of Ga precursors, including triethylgallium (TEGa) and trimethylgallium (TMGa), for the growth of Ga₂O₃ will be presented. Their advantages and disadvantages in realizing high-purity, carbon-free, epitaxial Ga₂O₃ films will be discussed. Critical process conditions and MOCVD reactor geometries on achieving high purity β-Ga₂O₃ films with high electron mobility and low background carrier concentration, including doping control in this range, will be discussed. This paper will also discuss the MOCVD growth of high Al composition (up to 30%) high quality strained β-(AlGa)₂O₃/Ga₂O₃ heterostructures and superlattices on various orientations of β-Ga₂O₃ substrates. The MOCVD growth of heavily doped (>10²⁰ 1/cm³), highly conductive β-Ga₂O_3, andstrained β-(Al_xGa_1-x)₂O₃/β-Ga₂O₃ heterostructures will be presented. We will also present the demonstration of record low resistance Ohmic contacts on heavily Si doped epitaxial β-Ga₂O₃ and strained β-(Al_xGa_1-x)₂O₃ epilayers with varying Al composition. A recent in-situ non-destructive etching of Ga₂O₃ in MOCVD followed by a regrowth process will also be discussed.

[1] F. Alema et al., Journal of Crystal Growth 475 (2017) 77-82.

[2] G. Seryogin et al., Applied Physics Letters 117 (2020) 262101.

[3] A. Bhattacharyya et al., IEEE Electron Device Letters 42 (2021) 1272-1275.

[4] F. Alema et al., EEE Electron Device Letters 43 (2022) 1649-1652.

Beta-gallium oxide (β-Ga₂O₃) is a promising ultra-wide bandgap (~ 4.8 eV) semiconductor material for the application field of power electronic converters. The theoretical breakdown field strength of up to 8 MV/cm^[1] can be best exploited using a vertical architecture for β-Ga₂O₃ based transistors.A high-quality homoepitaxial drift layer plays a crucial role in such a vertical device structure. Here, we report our developed process via metal-organic vapor phase epitaxy (MOVPE) to overcome the main issue associated with the homoepitaxial drift layer: (a) low doping (10¹⁶ cm^-3 range) concentrations, (b) layer thicknesses of several µm while maintaining a low density of structural and point defects and (c) high growth rates (µm/h).

A high growth rate process of up to 1.5 µm/h was achieved for Si-doped (100) β-Ga₂O₃ homoepitaxial films grown via MOVPE on Czochralski-grown semi-insulating^[2,3] and conductive^[4] (100) 4° off β-Ga₂O₃ substrates while maintaining the step-flow growth mode up to a film thickness of 4 µm^[5]. The enhanced diffusion channel due to forming a Ga adlayer was proposed as the possible growth mechanism^[6]. Furthermore, we also report the formation of parasitic particles as a killer issue during the growth, which can be suppressed by a close showerhead to substrate gap and a high total gas flow^[7]. With our optimized process, Si doping enabled precise control of the n-type conductivity of the layers with free electron concentrations ranging from 5 × 10¹⁶ cm^-3to 1.5 × 10¹⁹ cm^-3, and corresponding mobilities from 163 cm²·V^-1·s^-1 to 21 cm²·V^-1·s^-1 were measured by room temperature Hall measuerements. Temperature-dependent Hall measurements let expect a low background compensating acceptor concentration of 4 x 10¹⁵cm^-3, indicating a doping level down to mid 10¹⁵cm^-3 is still possible.

[1]M. Higashiwaki et al., Semicond. Sci. Technol. 2016, 31, 34001.

[2]Z. Galazka et al., Prog. Cryst. Growth Charact. Mater. 2021, 67, 100511.

[3]P. Seyidov et al., APL Materials 2022, 10, 111109 (2022)

[4]Z. Galazka et al., Appl. Phys. Lett. 2022, 120, 152101.

[5]T.-S. Chou et al., Jpn. J. Appl. Phys. 2023, 62, SF1004.

[6]T.-S. Chou et al., AIP Adv. 2021, 11, 115323.

β-Ga₂O₃ has emerged as a promising semiconductor candidate for future power electronic and radio frequency device applications, because of its ultra-wide bandgap (4.8 eV) and high critical field strength (8 MV/cm). Previously, metalorganic chemical vapor deposition (MOCVD) of β-Ga₂O₃ using triethylgallium (TEGa) as the Ga precursor has been demonstrated with record electron mobilities approaching the theoretical values. The use of trimethylgallium (TMGa) as the Ga precursor with higher vapor pressure enables the MOCVD growth of β-Ga₂O₃ with much faster growth rates (> 3μm/hr) for thick film growth.

In this work, a systematic study of the MOCVD growth of (010) β-Ga₂O₃ using TMGa was performed. Both TMGa molar flow rate and growth temperature play an important role on the β-Ga₂O₃ MOCVD growth rate. With a TMGa molar flow rate of 116 μmol/hr, β-Ga₂O₃ fast growth rate of 6.7 μm/hr was achieved at 950°C. The surface morphologies of the MOCVD β-Ga₂O₃ films were highly dependent on the growth conditions, particularly chamber pressure. For films grown at a growth rate of ~3 μm/hr, record room temperature electron Hall mobilities ranging between 190 cm²/Vs and 93 cm²/Vs were achieved with carrier concentrations between 1.6×10¹⁶ and 3.8×10¹⁹ cm^-3. Temperature-dependent charge transport characteristics revealed a low charge compensation level (~1.5×10¹⁶ cm^-3) and a decent low-temperature peak electron mobility (3425 cm²/Vs at 53K).

The impurities incorporation in the MOCVD grown (010) β-Ga₂O₃ thin films with different growth rates was probed by the quantitative secondary-ion mass spectroscopy (SIMS). SIMS results revealed that both [C] and [H] increase as the TMGa molar flow rate/growth rate increases. However, [C] increases at a much faster rate as compared to [H]. By comparing the electron Hall measurement results and the quantitative SIMS characterization, the net impurity concentration ([C]-[H]) matches well with the compensation level in the MOCVD β-Ga₂O₃ grown at the different growth rates. Therefore, the incorporated H forms neutral C-H complexes and thus passivates the compensation effect from pure C. This mechanism has been proposed from theoretical study based on the DFT calculations.

In summary, MOCVD growth of (010) β-Ga₂O₃ using TMGa as the Ga precursor was systematically studied. The role of C compensation and passivation from C-H complexes in the MOCVD grown films were proposed based on the experimental evidence.

Acknowledgment: The authors acknowledge the funding support from AFOSR (FA9550-18-1-0479) and the NSF (No. 2231026, No. 2019753).

One of the challenges in developing β-Ga₂O₃ lateral devices is the presence of parasitic charges at the epilayer-substrate interface due to the Si impurity that accumulates at the interface. One method that has been proposed recently to manage the interface Si is to etch the β-Ga₂O₃ substrate in hydrofluoric acid (HF) for an extended time before the growth of the films [1]. This method does not eliminate the Si at the interface but reduces its concentration just below the concentration of Fe in the substrate, leading to partial compensation. The other method to manage the interface Si is to compensate with deep acceptor dopants, such Mg, Fe, and N, among which N is less affected by thermal diffusion, making it a dopant of choice [2].

In this work, we report on controllable doping of N into Ga₂O₃ using nitrous oxide (N₂O) and ammonia diluted in nitrogen (NH₃/N₂) as sources for N. The incorporation efficiency, reproducibility, and controllability of N doping into Ga₂O₃ films will be discussed as a function of process conditions for both sources. Incorporating N into Ga₂O₃ is found to be sensitive to process conditions when N₂O is used as a source. A maximum N concentration of ~2×1019 cm-3 was achieved by growing the layer at a low substrate temperature. However, using NH₃/N₂, the doping of N into Ga₂O₃ is controllable through the molar flow rate. With the increase in the NH₃/N₂ molar flow rate from ~1.8×10^-8 mol/min to 1.45×10^-6 mol/min, the N impurities incorporated into the β-Ga₂O₃ layers increased from ~1×10¹⁸ cm^-3 to ~2×10²⁰ cm^-3. For both sources, hydrogen was found to incorporate into the films along with N, but annealing at reduced pressure helps to drive out the hydrogen from the film. For the NH₃/N₂ source, growing the layers at elevated temperatures (>900 C) also effectively reduced H incorporation into the film by as much as ~10× with no effect on the incorporation of N. This paper will also discuss the impact of exposing the surface of Ga₂O₃ substrate to NH₃/N₂ and halide based precursors to manage the interface Si.

[1] A. Bhattacharyya etal. C APL Materials 11, 021110 (2023).

[2] M.H. Wong et.al. Applied Physics Letters 113 , 102103 (2018).

In 2022, we demonstrated a high degree of doping control (1x10¹⁷ to 1x10²⁰/cm³) in β-Ga₂O₃ films grown by molecular beam epitaxy with mobilities of ~130 cm²/Vs in the lowest doped samples.¹ However, we discovered some samples where, despite being grown under the same conditions, the mobility would be ~1 cm²/Vs. To understand a potential impurity related origin of these low mobilities, secondary ion mass spectrometry (SIMS) was performed. Over 8 samples, we observed significant Fe variation within the substrate and significant Si variation at the surface. Depending on the substrate, the surface Si may or may not have been fully compensated by the Fe. In the worst case, a high-density of uncompensated free carriers exist at the interface with a low mobility which in turn affects the Hall effect measurement. Moreover, this same uncompensated charge can prevent confinement, or create a parallel conducting path for 2D electron gasses, thereby hindering the performance of high electron mobility transistors.²

The Si contamination is thought to arise from the polishing process and/or from siloxanes adsorbed from the air, and which are particularly difficult to remove.³

To quantify and study the Si accumulation and possible removal strategies, we investigated how Si accumulates on the film surface when exposed to air. We grew UID layers by molecular beam epitaxy, removed the sample and exposed it to air for different amounts of time. After less than 20 minutes of exposure to air, the accumulated Si on a clean surface had a sheet density (n_s) of ~2x10¹²/cm². The n_s continued to increase with longer exposure times in air, saturating at ~ 7x10¹²/cm² after an 8-hour exposure. Next, etching studies were performed to investigate possible removal of the SiO_x on the surface. β-Ga₂O₃ films were grown by metal-organic chemical vapor deposition and the surface was exposed to air for 2 hours to allow for the accumulation of Si. The surface was then etched in HF (49%) for varying times to remove the accumulated Si. After etching for 15 minutes, the Si sheet charge was reduced by ~1 order of magnitude to~ 3x10¹¹/cm².

1. APL, 121, 072108 (2022); 2. APEX, 10, 071101 (2017); 3. Chemosphere,92, 905-910 (2013)