Gate-recessed normally-off GaN-on-Si HEMT using a new O2-BCl3 digital etching technique

A new, semi-self-limiting, digital etch process using separate oxygen (O-2) and boron trichloride (BCl3) plasmas to sequentially remove layers of material from AlGaN/GaN high electron mobility transistors (HEMTs) on the order of a few angstroms per cycle is presented. This novel digital or atomic layer etching (ALE) technique was used for the conversion of AlGaN/GaN HEMT devices from depletion mode (normally-on operation) to enhancement mode (normally-off operation). For a fixed BCl3 time of 60 sec per cycle, the etch rate per cycle was increased from 1.4 nm/cycle to 2.5 nm/cycle by increasing the O-2 time per cycle from zero to 15 seconds, and remained fairly constant with higher O-2 time per cycle for a self-limiting etch process. Two GaN-on-Si wafers from the same CVD growth were processed side-by-side using the device layout and process steps, except for the depletion-to-enhancement conversion step, to compare ALE and fluorine treatment. The ALE-processed had a peak gm of 250 mS/mm, 67% higher than the fluorine-treatment process. The ALE process resulted in a threshold voltage variation of +/- 150 mV across the 3 inch wafer (sigma = 63 mV), which was less than half of that of the fluorine-treatment wafer. The three-terminal breakdown voltage of the ALE-processed wafer exceeded 1100 V, which is the first demonstration of such a high voltage device using GaN-on-Si and a gate recess technique, and the drain leakage current was in the mu A/mm range at 1100 V. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim