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Analysis of Mechanical Strain in AlGaN/GaN HFETs
Herein, the influence of mechanical strain induced by passivation layers on the electrical performance of AlGaN/GaN heterostructure field-effect transistor is investigated. We studied the physical mechanism of a threshold voltage (Vth) shift for the monolithically fabricated on/off devices reported earlier by our group. For that, theoretical calculations, simulation-based analysis, and nano-beam electron diffraction (NBED) measurements based on STEM are used. Strain distribution in the gate vicinity of transistors is compared for a SiNx passivation layer with intrinsic stress from ≈0.5 to −1 GPa for normally on and normally off devices, respectively. The strain in epitaxial layers transferred by intrinsic stress of SiNx is quantitatively evaluated using NEBD method. Strain dissimilarity Δε = 0.23% is detected between normally on and normally off devices. Using this method, quantitative correlation between 1.13 V of Vth shift and microscopic strain difference in the epitaxial layers caused by 1.5 GPa intrinsic stress variation in passivation layer is provided. It is showed in this correlation that about half of the reported threshold voltage shift is induced by strain, i.e., by the piezoelectric effect. The rest of Vth shift is caused by the fabrication process. Therefore, various components/mechanisms contributing to the measured Vth shift are distinguished.
Refractory metal-based ohmic contacts on β-Ga2O3 using TiW
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.