Investigation of AlGaN/GaN Schottky barrier diodes on free-standing GaN substrate with low leakage current

Abstract

Benefiting from the excellent properties of GaN with a wide bandgap of 3.4 eV as well as high critical field of 3.3 MV/cm, GaN-based devices prove to be a promising candidate in extreme conditions. Especially, high-density high-mobility two-dimensional electron gas (2DEG) induced by spontaneous piezoelectric polarization in AlGaN/GaN heterostructure enables AlGaN/GaN device to lower on-resistance (<i>R</i>_ON). However, owing to the lack of free-standing GaN substrate with large size and high quality, the epitaxis of GaN is always based on hetero-substrate such as Al₂O₃, Si and SiC, which shows large lattice mismatch and thermal mismatch. The large mismatch between GaN and substrate leads to high dislocation as well as high leakage current (<i>I</i>_R) of GaN devices. In this work, high-performance AlGaN/GaN Schottky barrier diode with low <i>I</i>_R and low turn-on voltage (<i>V</i>_ON) is fabricated on a 3-inch free-standing GaN substrate with C-doping GaN buffer layer to suppress <i>I</i>_R. Owing to the suppressed dislocation density of the AlGaN/GaN epitaxial wafer on free-standing substrate, low Ohmic contact resistance (<i>R</i>_C) is difficult to achieve the suppressed penetration of Ohmic metal into 2DEG channel, which is adverse to the high current density. In this work, a low <i>R</i>_C of 0.37 Ω·mm is obtained by one-step self-aligned Ohmic process, including the etching of partial AlGaN barrier layer and lift-off of Ohmic metal. The 2DEG is formed under the effect of residual AlGaN barrier layer, and the short distance between 2DEG and Ohmic metal contributes to lowering the value of <i>R</i>_C. The groove anode region is defined by the low damaged inductively coupled plasma process with a low etching rate of 1 nm/min, and the total depth is 35 nm, confirmed by atomic force microscope. Fully removing the AlGaN barrier layer from the anode region makes the anode metal directly contact the 2DEG channel, thereby improving the performance of the fabricated AlGaN/GaN Schottky barrier diode (SBD) with a low <i>V</i>_ON of 0.67 V, low <i>I</i>_R of 3.6 × 10^–8 A/mm, and an <i>I</i>_ON/<i>I</i>_OFF ratio of up to 3 × 10⁷. The values of differential <i>R</i>_ON,sp are calculated to be 0.44, 0.86, 1.59, 2.55 mΩ·cm² for GaN SBDs with various values of <i>L</i>_AC of 6, 10, 15, 20 μm, and the values of <i>R</i>_ON,sp determined at an anode current density of 100 mA/mm are 1.27, 2.08, 3.29, 4.63 mΩ·cm², respectively. As the measured temperature increases from 300 to 425 K, the <i>I</i>_R is increased only by 3 times to 1.6 × 10^–7 A/mm, which shows the great potential for next-generation power electronics.