The Effects of High Temperature Ion Implantation in β-Ga2O3

Abstract

Doping electronic oxides is often challenging due to issues like self-compensation, limited solubility, and defect formation. Additionally, there is typically an imbalance in favor of one type of conductivity, most commonly n-type. Another key factor is the impact of post-growth annealing, used for forming contacts or activating dopants, on material conductivity, which depends on the annealing environment. Ion implantation is an appealing technique for device processing because it enables precise control over dopant concentration and spatial distribution, allowing selective doping of specific regions within a material. While the effectiveness of high-temperature ion implantation in SiC is well-documented, this work focuses on investigating its influence on the conductivity and crystal structure of β-Ga2O3. This study highlights the benefits of performing silicon ion (Si+) implantation at elevated temperatures to achieve controlled, heavily doped regions in gallium oxide. Silicon implants were introduced into MBE-grown (010) β-Ga2O3 films at both room temperature (RT, 25 °C) and high temperature (HT, 600 °C) to form approximately 350 nm deep Si-doped layers with average concentrations of around 1.2 × 10^20 cm^-3. While the RT samples were too resistive to measure, the HT samples demonstrated remarkable results, achieving a Si dopant activation efficiency of 82.1%. They also exhibited a high sheet electron concentration of 3.3 × 10^15 cm^-2 and an excellent mobility of 92.8 cm^2/V ·s at room temperature. Additionally, X-ray diffraction analysis revealed that high-temperature implantation minimized the formation of secondary Ga2O3 phases and reduced structural defects and lattice damage. These findings underscore the potential of high-temperature ion implantation for fabricating ultra-low-resistance, heavily doped Ga2O3 layers. In this study, Fe-doped β-Ga2O3 substrates were implanted with Si+ ions at 275 and 425 keV to create a 300 nm thick doping profile, verified by SRIM simulations. Post-implantation annealing was performed at 970 °C and 1050 °C to activate the samples. Ohmic contacts were fabricated using a Ti/Au metal stack, with a 60 nm Ti layer followed by a 150 nm Au layer, and post-deposition annealing was conducted in a high-vacuum chamber at 450 °C for 1 minute. Samples annealed at 970 ◦C exhibited a linear I-V response between -0.2 V and +0.2 V, confirming the formation of ohmic contacts, while samples annealed at 1050 ◦C appeared more resistive. Schottky barrier diodes were also fabricated on in-situ Si-doped samples, showing a rectification ratio of 10^5 and a turn-on voltage of around 1 V. C-V measurements indicated a carrier concentration of 1.9 × 10^17 cm^-3, closely aligning with the target doping level of 2 × 10^17 cm^-3. This study investigates high-temperature (HT) Germanium (Ge) ion implantation to understand its impact on the structural morphology of β-Ga2O3. While prior work on Si ion implantation at 600 ◦C showed reduced lattice deformation compared to room temperature implantation, the influence of implantation-induced defects on material conductivity remains underexplored. Ge was implanted into Fe-doped β-Ga2O3 substrates with (010) orientation at room temperature and 600°C using a dose of 1.5 × 10^15 ions/cm^2. HRXRD analysis revealed that HT implantation resulted in less crystal deformation than room-temperature implantation. STEM analysis further identified dislocations at the interface of the MBE-grown β-Ga2O3 film and substrate, as well as within the film itself. Additional studies are necessary to fully characterize these dislocations and their effects on material properties.