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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)

The characteristics of III-nitrides grown on zinc- and oxygen-face ZnO by plasma-assisted molecular beam epitaxy were investigated. The reflection high-energy electron diffraction pattern indicates formation of a cubic phase at the interface between III-nitride and both Zn- and O-face ZnO. The polarity indicates that Zn-face ZnO leads to a single polarity, while O-face ZnO forms mixed polarity of III-nitrides. Furthermore, by using a vicinal ZnO substrate, the terrace-step growth of GaN was realized with a reduction by two orders of magnitude in the dislocation-related etch pit density to ∼108cm−2, while a dislocation density of ∼1010cm−2 was obtained on the on-axis ZnO substrates.

The metal modulated epitaxy ͑MME͒ growth technique is reported as a reliable approach to obtain reproducible large hole concentrations in Mg-doped GaN grown by plasma-assisted molecular-beam epitaxy on c-plane sapphire substrates. An extremely Ga-rich flux was used, and modulated with the Mg source according to the MME growth technique. The shutter modulation approach of the MME technique allows optimal Mg surface coverage to build between MME cycles and Mg to incorporate at efficient levels in GaN films. The maximum sustained concentration of Mg obtained in GaN films using the MME technique was above 7 ϫ 10 20 cm −3 , leading to a hole concentration as high as 4.5ϫ 10 18 cm −3 at room temperature, with a mobility of 1. GaN,7 unintentional hydrogen and oxygen doping, 8,9 a significant compensation of Mg acceptors at high dopant concentrations, 1 and a drastic dependence of incorporation upon the growth regime or III-V ratio.10,11 As a consequence, there is a narrow window of growth conditions, which yield electrically active p-type GaN. Furthermore, even low or moderate hole concentrations are often not consistently obtained and are difficult to reproduce due to the Mg incorporation sensitivity to growth conditions. These complications result in large and varied resistances and mobilities in GaN-based devices that rely on p-type layers. If hole carrier concentrations were to be increased, and p-type doping of GaN standardized, these devices could benefit from better performance and better reliability.Despite the complications associated with Mg-doped GaN, current state-of-the-art reports show promising results and give a reference for comparison. Mg incorporation as determined by secondary ion mass spectroscopy ͑SIMS͒ has been reported to be as high as 8 ϫ 10 20 cm −3 for metal organic chemical vapor deposition 12 and 3.5ϫ 10 20 cm −3 for molecular beam epitaxy ͑MBE͒.13 However, above approximately ͑2 -3.5͒ ϫ 10 20 cm −3 , the Mg incorporation begins to decrease with increased Mg flux 13 and inverts Ga-polar films to N-polar. 14 M-plane ͑1010͒ oriented substrates have been used to achieve a hole concentration as high as p = 7.2 ϫ 10 18 cm −3 .15 Electrical characterization for current stateof-the-art films grown on c-plane substrates is shown in Table I. [16][17][18] Reasonably high hole carrier concentrations are in the range of ͑1-3͒ ϫ 10 18 cm −3 . However, hole concentrations have been reported to be as high as 6 ϫ 10 18 cm −3 , 18 although the carrier concentration was metastable and significantly reduced upon heating during measurement. The metal modulated epitaxy ͑MME͒ growth technique was developed to specifically address reproducibility issues. [19][20][21][22] The MME growth technique is characterized by a group-III ͑metal͒ flux much higher than the group-V source, which would normally lead to droplets. However, the metal shutter is modulated to avoid droplet buildup and therefore build and deplete the metal bilayer on the growth surface, yielding smoother surfaces, larger grain sizes, and better repr...

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