Crystal growth and p‐type conductivity control of AlGaN for high‐efficiency nitride‐based UV emitters

Abstract

Microstructural analysis was carried out to clarify the compositional dependence of the generation of dislocations in AlxGa1‐xN on an underlying AlN layer grown by metalorganic vapor phase epitaxy. When the film thickness is less than 1.5 μm, the threading dislocation density (TDD) increases with decreasing AlN molar fraction. However, when the film thickness exceeds 1.5 μm, TDD becomes maximum at x around 0.5. The growth of AlGaN on a grooved AlN template is effective in reducing TDD for all AlN molar fractions. TDD in AlGaN, which is close to binaries such as GaN and AlN, is a few 107 cm–2, while for the intermediate composition with x around 0.5, TDD is still at mid 108 cm–2. The activation energy of Mg in AlGaN is found to show a strong Mg concentration dependence with a negative one‐third power law in Al0.25Ga0.75N and Al0.5Ga0.5N as well as in GaN. Overdoping of Mg causes an increase in the activation energy for every composition; from this, the optimum Mg concentration for realizing the highest hole concentration can be deduced. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Defect analysis in AlGaN layers on AlN templates obtained by epitaxial lateral overgrowth

“…Whereas the defect structure of pseudomorphic layers with a high Al content does not change significantly compared to the AlN underneath, strain relaxation through formation of additional threading dislocations is observed with decreasing Al content [22]. In the following, to visualize the defect distribution in Al x Ga 1 À x N layers grown on low dislocation density ELO-AlN templates, we have chosen Al x Ga 1 À x N with Al content x of 0.8 and 0.5 as two examples.…”

Deep traps in GaN-based structures as affecting the performance of GaN devices

“…Since the depth of Mg acceptors is quite high and causing severe problems in devices sensitive to the series resistance of the p-region, such as LEDs or LDs, changes in the structural quality and electronic properties of GaN:Mg occurring as the Mg concentration increases have been looked into in some detail. It turns out that, for high Mg concentrations approaching 10 2 cm À3 , a high density of inversion boundary domains (IBDs), stacking faults (SFs) and dislocations is generated due, as it is thought, to forming of Mg 2 N 3 precipitates limiting the Mg solubility [97][98][99][100][101]. The high density of IBDs causes the inversion of the dominant growth plane mode from Ga-polar at low Mg concentration to N-polar at high Mg concentration [99], with consequent increase in the uptake of Si and O shallow donors and increased compensation [97][98][99].…”

“…The Mg level was placed based on these measurements close to E v +(0.15-0.17) eV. There has been some published evidence on the level becoming more shallow with increasing Mg acceptors concentration [97]. The effect has been ascribed, as usual, to the overlap of the individual Mg acceptor wave functions resulting in the formation of an impurity band.…”

Microstructural analysis of thick AlGaN epilayers using Mg‐doped AlN underlying layer

“…The reduction of the threading dislocation (TD) density of AlGaN on AlN was observed when the thickness of AlGaN was increased. At the same time, it was also observed that Al 0.5 Ga 0.5 N on AlN still had the highest TD density, indicating that merely increasing the thickness is not very effective for reducing the TD density of Al 0.5 Ga 0.5 N. A further decrease in TD density can be achieved by using a grooved AlN template [2,3]. However, an additional etching process is necessary, and the growth of Al 0.5 Ga 0.5 N on the grooved AlN template might be complicated.…”

“…The compositional dependence of the generation of dislocations in Al x Ga 1-x N on underlying AlN has been reported [2]. The reduction of the threading dislocation (TD) density of AlGaN on AlN was observed when the thickness of AlGaN was increased.…”