Genhao Liang

In this letter we report the morphological, electrical and thermal transport properties of a high electron mobility transistor (HEMT) style epitaxial wafer, where an approximately 2000 nm thick GaN layer has been directly deposited on a bulk single crystal AlN (BCS AlN) substrate with no buffer layer in between, and also the experimental results of DC and RF properties of a HEMT device based on such a wafer. The sample achieved very smooth surface morphology and roughness down to Ra = 0.172 nm over an area of 1 μ m × 1 μ m in AFM measurements. Electrical transport measurements showed sheet carrier concentration of 7.3 × 10 ¹² cm ⁻² , Hall mobility of 2220 cm ² /(V·s) and sheet resistance of 386 Ω/sq. The measured maximum trans-conductance G _m of the fabricated HEMT device was 250 mS/mm at a gate bias voltage of −1.8 V. With a gate length of 500 nm and a gate-to-drain distance of 4.7 μ m, the f _T and f _max , derived from S -parameters measurements, are 25.9 and 54 GHz, respectively. Large-signal RF measurement exhibited a high linear power gain ( G _p ) of 25.2 dB and a peak output power ( P _out ) density of 7.2 W/mm@1.5 GHz, associated with a power-added efficiency (PAE) of 40.9%. Comparing with the structure with a 500 nm thick AlGaN buffer, the total thermal resistance of the structure in our device decreased by 44%. This work confirms the technical feasibility of fabricating GaN HEMT devices on BCS AlN substrates without any additional buffer layer, and the excellent electric and thermal transport properties of the simplified wafer structure indicate a bright future of BCS AlN-based GaN HEMT devices in ultra-high-frequency and high-power-density nitride electronics.

To achieve an atomically clean surface of single-crystal aluminum nitride (AlN) substrates, this study systematically evaluated the effects of each step in ex situ wet chemical cleaning (solvent, piranha solution, HF, HCl) and in situ hydrogen annealing. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analyses revealed that while the combination of solvent and piranha solution exposed step morphology, its effectiveness in removing organic contaminants was limited. HF cleaning efficiently removed the oxide layer but introduced fluorine residues, whereas HCl cleaning left no chlorine residues but exhibited lower efficiency in oxide removal. In situ hydrogen annealing significantly reduced carbon and oxygen contamination, albeit accompanied by a transformation of the surface morphology from step to island mode. By modulating the low V/III ratio during low-temperature metal–organic chemical vapor deposition (MOCVD) growth, a controlled transition from 3D island growth to 2D step-flow growth was achieved. This research provides a basis for optimizing AlN substrate surface treatment, offering important insights for advancing nitride-based optoelectronic and power devices.