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 (O2) 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 O2 time per cycle from zero to 15 seconds, and remained fairly constant with higher O2 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 (σ = 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 μA/mm range at 1100 V. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)