Paper EM-WeA2
Schottky Metal-GaN and AlGaN-GaN Interface Issues Critical to HEMTs

Wednesday, November 11, 2009, 2:20 pm, Room B1

GaN devices promise advantages over other compound semiconductors including higher power amplification, increased linearity, and less temperature dependent degradation. But today current collapse, slow switching transients, and poor gain profiles are present and may be related to interface trap densities. To shed some light, we analyzed the interfaces found in GaN HEMTs and correlated results with amplifier performance characteristics and first principle atomic and electronic structure simulations.

MOCVD grown GaN Schottky diodes were mesa isolation etched, and KOH etched to remove Ga residue and surface defects. Then Ti/Al/Ti/Au Ohmic and Ni/Au Schottky contacts were deposited and annealed.

These blocked 340V in the off-state. KOH reduced surface roughness and improved the on-state performance (150mA at 3.5V in good devices). The undoped GaN layer had a free carrier concentration of 5x10¹⁶ cm^-3 from CV measurements. The reverse bias exhibits a soft breakdown due to an initial depletion followed by a slower field spreading. For three devices, the forward (2V) and reverse leakage (-1 V) currents were (A) 45mA, 3.4x10^-10A, (B) 17mA, 2.0x10^-9A, and (C) 4 mA, 3.6x10^-8A. Comparing the ideality factors for the three devices over a voltage range of 0 to 0.5 V, device (A) exhibited no bumps between 0.1 and 0.4 V (n = 1.04 at 0.2 V) and a smooth transition into the series resistance region, device (B) exhibited a bump at 0.4V with a peak n= 4.3. Device (C) exhibited a bump at 0.1V with a peak n=2.08.

These ideality factor bumps were seen with GaAs Schottky diodes and linked to interface trap densities. Our conductance measurements gave corresponding trap densities 0.3 eV from the band edge of (C) 1.02x10¹², and (B) 7.9x10¹¹ cm^-2 eV^-1, and for (A) much less. Also, the interface trap time constants are (C) 97 ns and (B) 81.9 ns and for (A) much longer.

Our first-principle simulation model of a planar bonded metal-semiconductor interface included an inhomogeneity at the interface and semiconductor interface defect interactions with H atoms and OH radicals during KOH etching. We assume that N antisite defects are common defects in GaN causing point defect pinning. After KOH etching these defects are passivated by two hydrogen atoms forming complexes H-N(Ga)-H and OH-N(Ga)-OH. The complexes do not have dangling bonds and do not participate in hybridization with extended states of a metal. Formation of the complexes could improve the interface state associated electrical properties of the Schottky diode, smooth the GaN (0001) surface and remove oxides residues.

Vendor provided GaN HEMTs were similarly analyzed and will be discussed.