Evaluation of DC Performance of Buffer-Free AlGaN/GaN HEMT on SiC Relative to the Thickness of AlN Nucleation Layer

Epitaxial layer engineering is fundamental to harnessing the benefits of gallium nitride (GaN) devices in power and high frequency applications. Various GaN epitaxial designs have been experimentally demonstrated. Recently, a buffer-free GaN-on-SiC epitaxial layer structure was introduced to improve the breakdown field and reduce parasitic parameters. Aluminium nitride (AlN) nucleation (NL) is commonly used in the epitaxial layer stack to initiate growth of GaN epitaxial structure on foreign substrates (Si, sapphire or SiC) and to mitigate growth stresses. Since this epilayer concept is free of a thick GaN buffer, AlN could influence the DC performance of the high electron mobility transistor (HEMT). In this report, we study the impact of the thickness of AlN NL on a buffer-free AlGaN/GaN/SiC HEMT using two identical wafers with different thicknesses (of AlN) of 60 nm (wafer 1) and 100 nm (wafer 2), respectively. A 2-finger HEMT was fabricated without passivation, having a gate length ( $\mathrm{L}_{\mathrm{g}}$ ) of 3 um, gate-drain $\mathrm{L}_{\text{gd}}$ ) and gate-source $(\mathrm{L}_{\text{gs}})$ I access regions of 5 um and 2 um, respectively. Higher breakdown voltage (>200 V), low gate and drain leakage currents were obtained from wafer 2. Wafer 1 shows much higher gate and drain current leakages of $4.0\times 10^{-3}\mu\mathrm{A}/\text{mm}$ and 150 uA/mm respectively. The gate leakage in wafer 2 $(\sim 1.4\times 10^{-3}\text{mA}/\text{mm})$ is $\sim 3\times$ lower than of wafer 1, and the off-state drain current density (F $\mathrm{I}_{\text{ds}}$ ) in wafer 2 is over $100\times$ lower than wafer 1 at $\mathrm{V}_{\text{ds}}=5\mathrm{V}$ and $\mathrm{V}_{\text{ds}}=$ 20 V respectively. The results indicate the suitability of wafer 2 with a thicker (100 nm) AlN NL for power applications.