Coherently grown AlN/GaN HEMT heterostructures on AlN buffer on SiC substrate—Impact of coherent growth on electrical and thermal characteristics

AlN/GaN high-electron mobility transistor (HEMT) heterostructures are advantageous for developing high-frequency and high-power devices, owing to their thinner barrier layers and high polarization-induced two-dimensional electron gas (2DEG) densities. Coherently grown AlN/GaN HEMT heterostructures on AlN substrates with intentional high Si δ-doping at the GaN/AlN interface have previously demonstrated enhanced electron mobility due to reduced dislocation densities and a lower electric field in the GaN channel. However, simultaneously implementing Si δ-doping and achieving a coherently grown GaN channel on AlN buffers on SiC substrates remains challenging because of the high dislocation density in the buffer layer. In this work, coherent growth of a GaN channel on an AlN buffer on SiC substrates is achieved using thin AlGaN and AlN/GaN superlattice-based stress transition layers (STLs). The resulting coherent AlN/GaN HEMT heterostructures with AlGaN-STLs exhibit a 2DEG mobility of 626 cm2 V−1 s−1, representing the highest value reported to date for a coherently grown GaN channel on an AlN buffer on SiC substrates. In contrast, AlN/GaN HEMTs with relaxed GaN channels exhibit significantly lower sheet resistance and reduced thermal resistance compared with devices with coherently strained channels. These results indicate that, provided device performance is not limited by misfit formation, relaxed GaN channels offer advantages for AlN/GaN HEMTs on SiC substrates. Otherwise, a trade-off emerges between thermal–electrical and misfit-induced degradation in device performance, governed by the choice between coherent and relaxed GaN channels.