GOX 2023 Session EG+BG+MD-WeM: Epitaxial III

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 Ga₂O₃ can 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-Ga₂O₃ thermodynamically 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 improve the growth rate is the technique of suboxide MBE, which allows growth of β-Ga₂O₃ thin films at rates exceeding 1 μm/h with large adsorption-controlled growth regimes.

Epitaxial growth of β-Ga₂O₃ using metalorganic chemical vapor deposition (MOCVD) has seen great advancements demonstrating high-quality films with low point defect concentrations and high mobility with low doping concentrations [1]. Here, we investigate the impact of buffer layer thickness for these MOCVD epitaxial films on electrical characteristics, thermal conductivity, and defect concentrations.

MOCVD films were grown on Novel Crystal Technology’s Fe-doped (010) β-Ga₂O₃ substrates using Agnitron Technology’s Agilis close-injection showerhead MOCVD (CIS-MOCVD). The unintentionally doped (UID) buffer layer thickness was varied on the 3 samples: A-300, B-500, and C-1000 nm. The UID layers were followed by a 10 nm thick n⁺ (~10¹⁹ cm^-3) Ga₂O₃ layer for improved channel conductivity. A 100 nm highly n⁺ layer was selectively regrown following ref. [2]. Ohmic contacts were formed in the regrown areas with an annealed 20/200 nm Ti/Au metal stack (470 °C, 1 min., N₂). Mesa isolation was formed with an etch of ~170 nm. Transmission line measurements (TLM) showed sample C had the lowest specific contact resistance of 2.25 × 10^-6Ω·cm² and sample A had the highest of 1.99 × 10^-4Ω·cm². Room temperature Hall effect measurement showed similar mobility for B and C of 115-116 cm²/V·s, while sample A showed a much lower mobility of 71 cm²/V·s. Samples B and C, both showed high open-gated source-drain current (I_D) (>0.05 A/mm at V_DS= 5 V) and low isolation (mesa-mesa) current (I_iso) of < 0.1 µA/mm at V_DS= 10 V. Sample A (300 nm thick buffer layer), showed 10X lower open-gated I_D and a high I_iso of ~3 mA/mm at V_DS= 10 V. Higher I_iso for samples with thin buffer layers, such as sample A, have been frequently attributed to a peak in Si concentration at the epilayer/substrate interface observed in secondary-ion mass spectroscopy [1]. Here, we offer further insight on this effect via frequency-domain thermoreflectance (FDTR) and positron annihilation spectroscopy (PAS). Preliminary FDTR data showed decreasing thermal conductivity for thicker epilayers. PAS data fitted with a 3-layer model consistently showed higher density of Ga-related vacancies in the epilayers compared to each substrate. More detailed measurements, including XRD and device-level FDTR, will be performed. This preliminary data suggested that MOCVD Ga₂O₃ was affected by both unintentional impurities and point defects in addition to the known issue of interfacial Si accumulation. [1] A. Waseem, et al., Physica Status Solidi (A), p. 2200616, 2022. [2] Z. Xia, et al.,IEEE EDL, 39(4), 568–571, 2018.

Gallium oxide has attracted attention as a candidate material for high-power diodes and transistors owing to its wide bandgap and high breakdown voltage. Homoepitaxial β-Ga₂O₃ has been successfully grown by plasma-assisted molecular beam epitaxy, however it is well-documented that unintentional Si donors at the epitaxial interface lead to the formation of an undesirable parasitic conducting channel. Mg and Zn are deep acceptor levels in β-Ga₂O₃ and Mg counterdoping by MBE has been shown to compensate unintentional donor impurities. However counterdoping with other elements such as Zn remains sparsely investigated.

We report on Mg and Zn counterdoping in homoepitaxial β-Ga₂O₃ grown by MBE on (010) Fe-doped (semi-insulating) and (001) Sn-doped (n-type) wafers. A valved cracker source is used for Mg while Zn is evaporated from a conventional effusion cell. Mg- and Zn-doped stacks are measured by secondary ion mass spectroscopy to calibrate the cell temperatures and valve positions to the dopant incorporation. A typical Ga₂O₃ growth temperature is 600 °C and growth rates are 0.47 – 0.70 Å/s. β-Ga₂O₃ samples composed of a ~2 nm Mg- or Zn-doped layer and a 300 nm unintentionally doped layer are grown with dopant fluxes ranging from 3.8×10^-9 to 2.0×10^-8 torr. Counterdoped samples grown on (001) Sn-doped and (010) Fe-doped wafers are processed into vertical and lateral Schottky devices, respectively. In both devices the Ohmic contact is formed by stable 5 nm Ti / 100 nm Au annealed under N₂ at 550 °C while the Schottky contact is formed by 30 nm Ni / 100 nm Au. The Schottky devices are characterized by capacitance-voltage (C-V) measurements at 20 kHz.

Optimizing the thermal anneal of Si implanted β-Ga₂O₃ is critical for low resistance contacts and selective area doping in advanced device structures. We report the impact of annealing time, temperature, and ambient on the activation of ion-implanted Si in β-Ga₂O₃ at concentrations from 5×10¹⁸ to 1×10²⁰ cm^-3, and in β-(Al_xGa_1-x)₂O₃ (x≤15%) at 5×10¹⁹ cm^-3. Nearly full activation (>90%) and high mobilities (>70 cm²/V-s) are achieved in β-Ga₂O₃ with contact resistances below 0.16 Ω-mm. In β-(Al_xGa_1-x)₂O_3, initial results are promising with moderate activation (50%) and high mobility (60 cm²/V-s).

UID β-Ga₂O₃ films were grown by plasma assisted MBE on Fe-doped (010) β-Ga₂O₃ substrates; comparable β-(Al_xGa_1-x)₂O₃ films were grown by MOCVD. Si was implanted at multiple energies to yield 65 or 100 nm box profiles with concentrations of 5×10¹⁸_, 5×10¹⁹, or 1×10²⁰ cm^-3_. To understand damage accumulation, low and high temperature implants were also studied. Anneals were performed in a UHV-compatible quartz furnace at 1 bar with well-controlled gas ambients.

To maintain β-Ga₂O₃ stability, P_O2 must be greater than 10^-9 bar (based on annealing in vacuum or forming gas). For 5×10¹⁹ cm^-3 Si, full activation is achieved for P_O2<10^-4 bar while 5×10¹⁸ cm^-3 tolerates ~10^-2 bar. Water vapor is critical even at 1 ppm; at 25 ppm active carriers are reduced by 10x. Optimal results were obtained with H₂O below 10 ppb. Based on recovery with subsequent “dry” anneals, we propose an OH-mediated defect compensating Si dopants.

Lattice recovery (mobility) occurs for T > 900 °C, with carriers and mobility increasing with temperature to 1050 °C. However, SIMS shows substantial Si diffusion above 1000 °C with 950 °C the optimal anneal temperature. Activation at 950 °C is maximized between 5 and 20 minutes with shorter times exhibiting slightly lower mobilities while longer times result in carrier deactivation; this “over-annealing” behavior occurs at all temperatures and becomes more significant at high concentrations. Room temperature implants to 1×10²⁰ cm^-3 are shown to fully activate under these optimal conditions.

To understand lattice damage recovery, implants at varying temperatures were characterized by XRD, Rutherford Backscattering Channeling (RBS/C), and STEM. XRD showed no second phases under any conditions. RBS/C and STEM showed only partial amorphization with remnant aligned β-Ga₂O₃. We propose a model to explain the efficient activation based on 3D lattice recovery in the absence of full amorphization.