The impact of dislocations on AlGaN/GaN Schottky diodes and on gate failure of high electron mobility transistors

Abstract

GaN epitaxially grown on Si is a material for power electronics that intrinsically shows a high density of dislocations. We show by Conductive Atomic Force Microscopy (C-AFM) and Defect Selective Etching that even for materials with similar total dislocation densities substantially different subsets of dislocations with screw component act as current leakage paths within the AlGaN barrier under forward bias. Potential reasons are discussed and it will be directly shown by an innovative experiment that current voltage forward characteristics of AlGaN/GaN Schottky diodes shift to lower absolute voltages when such dislocations are present within the device. A local lowering of the Schottky barrier height around conductive dislocations is identified and impurity segregation is assumed as responsible root cause. While dislocation related leakage current under low reverse bias could not be resolved, breakdown of AlGaN/GaN Schottky diodes under high reverse bias correlates well with observed conductive dislocations as measured by C-AFM. If such dislocations are located near the drain side of the gate edge, failure of the gate in terms of breakdown or formation of percolation paths is observed for AlGaN/GaN high electron mobility transistors.

Vertical field inhomogeneity associated with threading dislocations in GaN high electron mobility transistor epitaxial stacks

“…The density of the observed features in Fig. 3a showing a significantly lower surface potential is ~4 × 10 7 cm -2 which is consistent with reported densities of 3 -4 × 10 7 cm -2 for conductive dislocations 6,12,[17][18][19][20][21] .…”

“…However, those studies were conducted with a floating 2DEG, hence allowing the full applied voltage to be dropped across the top barrier layer. The conductive AFM measurement reported here suggests that the presence of the grounded 2DEG clamps the dislocation potential and connects any electrically conductive dislocations in the buffer to ground, similar to the suggestion in some previous reports 17,21 . Fig.…”

“…This was the case for measurements covering the entire substrate voltage range to the pinchoff voltage of -300 V with high contact force using high spring constant AFM probes to ensure good electrical contact between probe and sample. The conductive AFM result was somewhat unexpected since previous reports of conductive AFM using lower biases on GaN epitaxy revealed local leakage spots reaching the surface 6,12,[17][18][19][20][21] . However, those studies were conducted with a floating 2DEG, hence allowing the full applied voltage to be dropped across the top barrier layer.…”

“…Measuring the density of conductive dislocations, typically by conductive AFM, is often flawed as a leakage spot at the surface does not necessarily mean an underlying vertically conductive dislocation through the entire buffer due to shunting effects of the 2DEG 20 . It is also not understood how a single, potentially partially conductive dislocation with certain currentvoltage (IV) characteristics contributes to overall device breakdown 21 . Therefore, neither measuring buffer leakage nor investigating dislocation density or conductivity of different types is sufficient to characterize and model device breakdown.…”

Comprehensive Schottky Barrier Height Behavior and Reliability Instability with Ni/Au and Pt/Ti/Pt/Au on AlGaN/GaN High-Electron-Mobility Transistors

“…The electrical performances of GaNbased HEMT devices show sturdy functions of both Schottky and Ohmic metal contacts because the Schottky contact in AlGaN/GaN is controlled by the current conduction in the channel [7]. The gate reverse leakage current still remains a key concern for GaN-HEMT stability due to the interfacial trap states and strain-induced defects such as dislocations and cracks from large lattice mismatches [8][9][10][11][12]. Severe limitations such as current collapse, power slump, and poor long-term reliability are induced by the trapping effects at the interfacial trap states, trap sites in the AlGaN barrier layer, and deep-level traps in the GaN buffer layer [13][14][15].…”