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Abstract In this work, the out‐diffusion and uphill‐diffusion of Mg inside (100) β‐Ga2O3 epilayers and substrates are reported. The Mg accumulates toward the (100) surface upon annealing under an oxidizing environment, whereas the concentration profile changes with annealing temperatures and durations. Furthermore, the out‐diffusion of Mg from the substrate into the epilayer is observed at temperatures above 800 °C, which continues during the film growth. The substitutional‐interstitial‐diffusion (SID) mechanism is suggested to be the driving mechanism for the former, and the latter is related to the diffusion of mobile Mg interstitials. The accumulation profile of Mg can be used to identify the interface between the epilayer and the substrate. Furthermore, significant differences in device performance are observed for power transistors fabricated on annealed and non‐annealed epitaxial β‐Ga2O3 wafers. Increased breakdown voltages of annealed samples are attributed to the Mg diffusion into the first few nanometers of the epitaxial layer close to the interface to the semi‐insulating substrate, leading to compensation of residual dopants (donors) in that region.

Here, we investigate the effect of post-metallization anneal temperature on Ti/Au ohmic contact performance for (100)-oriented Ga2O3. A low contact resistance of ∼2.49 × 10−5 Ω·cm2 is achieved at an optimal anneal temperature of ∼420 °C for (100) Ga2O3. This is lower than the widely-used temperature of 470 °C for (010)-oriented Ga2O3. However, drastic degradation of the (100)-oriented contact resistance to ∼1.36 × 10−3 Ω·cm2 is observed when the anneal temperature was increased to 520 °C. Microscopy at the degraded ohmic contact revealed that the reacted Ti–TiOx interfacial layer has greatly expanded to 25–30 nm thickness and GaAu2 inclusions have formed between (310)-Ga2O3 planes and the Ti–TiOx layer. This degraded interface, which corresponds to the deterioration of ohmic contact properties, likely results from excess in-diffusion of Au and out-diffusion of Ga, concurrent with the expansion of the Ti–TiOx layer. These results demonstrate the critical influence of Ga2O3 anisotropy on the optimal post-metallization anneal temperature. Moreover, the observed Ti/Au contact degradation occurs for relatively moderate anneal conditions (520 °C for 1 min in N2), pointing to the urgent necessity of developing alternative metallization schemes for gallium oxide, including the use of Au-free electrodes.

The present work investigates the use of the refractory metal alloy TiW as a possible candidate for the realization of ohmic contacts to the ultrawide bandgap semiconductor β-Ga2O3. Ohmic contact properties were analyzed by transfer length measurements of TiW contacts annealed at temperatures between 400 and 900 °C. Optimum contact properties with a contact resistance down to 1.5 × 10−5 Ω cm2 were achieved after annealing at 700 °C in nitrogen on highly doped β-Ga2O3. However, a significant contact resistance increase was observed at annealing temperatures above 700 °C. Cross-sectional analyses of the contacts using scanning transmission electron microscopy revealed the formation of a TiOx interfacial layer of 3–5 nm between TiW and β-Ga2O3. This interlayer features an amorphous structure and most probably possesses a high amount of vacancies and/or Ga impurities supporting charge carrier injection. Upon annealing at temperatures of 900 °C, the interlayer increases in thickness up to 15 nm, featuring crystalline-like properties, suggesting the formation of rutile TiO2. Although severe morphological changes at higher annealing temperatures were also verified by atomic force microscopy, the root cause for the contact resistance increase is attributed to the structural changes in thickness and crystallinity of the interfacial layer.

A high growth rate process above 1 µm/h was achieved for Si-doped (100) β-Ga2O3 homoepitaxial films grown via metalorganic vapor phase epitaxy (MOVPE) while maintaining high crystalline perfection up to a film thickness of 3 µm. The main growth parameters were investigated to increase the growth rate and maintain the step-flow growth mode, wherein the enhanced diffusion channel due to the formation of a Ga adlayer was proposed to be the possible growth mechanism. Si doping allowed precise control of the n-type conductivity of the films with electron concentrations ranging from 1.5 × 1017 to 1.5 × 1019 cm−3 and corresponding mobilities from 144 to 21 cm2 V−1 s−1, as revealed by Hall effect measurements at room temperature. Secondary ion mass spectrometry confirmed homogeneous Si doping through the film and a one-to-one correlation between the Si concentration and the electron concentration. Low defect density in the films was determined by x-ray diffraction measurements. The demonstration of a high growth rate process of β-Ga2O3 films with μm level thickness and smooth surface morphology via MOVPE is critical for high power electronics with vertical device architecture.

We developed a kinetic Monte Carlo (KMC) model for the homoepitaxy of β-Ga_{2}O_{3}. It comprises adsorption, diffusion, and desorption and reflects the structure of β-Ga_{2}O_{3} with its two kinds of atoms: Ga and O. The knowledge gained from metal organic vapour phase experiments (MOVPE) experiments combined with AFM and TEM characterisation was used for the setup of rules and activation energies for the various surface processes. We performed a set of runs for the growth on flat and vicinal (100) surfaces. The nucleation on the flat surface requires a minimum ratio of the impingement rate of O_{2} and Ga. The behavior at different substrate temperatures was similar in experiment and in simulation. At high temperatures, we observe the formation of large islands whereas at low temperatures small islands are formed. The growth rate is increasing with decreasing temperature. On a vicinal surface (6^{∘}) different growth modes have been observed when using different desorption energies. Low desorption energy (high desorption rate) leads to step bunching, intermediate to step growth, and high energy (low desorption rate) to nucleation on terraces with a final configuration similar to step bunching.