Effect of p-InGaN Cap Layer on Low-Resistance Contact to p-GaN: Carrier Transport Mechanism and Barrier Height Characteristics

This study investigated the low contact resistivity and Schottky barrier characteristics in p-GaN by modifying the thickness and doping levels of a p-InGaN cap layer. A comparative analysis with highly doped p-InGaN revealed the key mechanisms contributing to low-resistance contacts. Atomic force microscopy inspections showed that the surface roughness depends on the doping levels and cap layer thickness, with higher doping improving the surface quality. Notably, increasing the doping concentration in the p++-InGaN cap layer significantly reduced the specific contact resistivity to 6.4 ± 0.8 × 10−6 Ω·cm2, primarily through enhanced tunneling. Current–voltage (I–V) characteristics indicated that the cap layer’s surface properties and strain-induced polarization effects influenced the Schottky barrier height and reverse current. The reduction in barrier height by approximately 0.42 eV in the p++-InGaN layer enhanced hole tunneling, further lowering the contact resistivity. Additionally, polarization-induced free charges at the metal–semiconductor interface reduced band bending, thereby enhancing carrier transport. A transition in current conduction mechanisms was also observed, shifting from recombination tunneling to space-charge-limited conduction across different voltage ranges. This research underscores the importance of doping, cap layer thickness, and polarization effects in achieving ultra-low contact resistivity, offering valuable insights for improving the performance of p-GaN-based power devices.