A wet-etching technique based on a mixture of hydrochloric (HCl) and nitric (HNO₃) acids is introduced, demonstrating exceptional 42:1 selectivity for etching N-polar GaN over Al_0.24Ga_0.76N. In the absence of an AlGaN etch stop layer, the etchant primarily targets N-polar unintentionally doped (UID) GaN, indicating its potential as a suitable replacement for selective dry etches in the fabrication of GaN high-electron-mobility transistors (HEMTs). The efficacy and selectivity of this etchant were confirmed through its application to a gate recess module of a deep-recess HEMT, where, despite a 228% over-etch, the 2.6 nm AlGaN etch stop layer remained intact. We also evaluated the proposed method for the selective etching of the GaN cap in the n+ regrowth process, achieving a contact resistance matching that of a BCl₃/SF₆ ICP process. These findings underscore the applicability and versatility of the etchant in both the electronic and photonic domains and are particularly applicable to the development of N-polar deep-recess HEMTs.

Fully relaxed, crack free, smooth Al_xGa_1−xN layers with up to 50% Al composition were demonstrated on pseudo-substrates composed of dense arrays of 10 × 10 µm² compliant porous GaN-on-porous-GaN tiles. The AlGaN layers were grown in steps for a total of 1.3 µm. The growth conditions necessary to demonstrate high quality films at higher Al compositions also suppressed any sidewall growth.

This study investigates the effect of titanium nitride (TiN) interlayers on the performance of ruthenium (Ru)‐based Schottky diodes on N‐polar GaN. The Ru Schottky diodes exhibited the highest mean Schottky barrier height of 0.69 0.03 eV; however, they demonstrated a lack of uniformity in barrier height across varying device sizes. Introducing 2 and 10 nm TiN interlayers between Ru and GaN led to consistent Schottky barrier heights and leakage characteristics for all device sizes. Notably, the 2 nm TiN/40 nm Ru configuration achieved an optimal balance between the low leakage current and uniform barrier properties, highlighting the role of TiN in obtaining desirable Schottky barrier performance.

Al_xGa_1-xN layers with up to 53% Al composition and 1.3 µm total thicknesses were grown utilizing step-graded layers on dense arrays of 10 × 10 µm² GaN tiles. While the layers were fully relaxed and crack-free, the higher Al composition sample showed increased X-ray line width, which did not correlate with a lack of increased dislocation density (as measured by TEM).