Yellow Luminescence in GaN

The yellow luminescence (YL) in GaN is a broad luminescence band centred around 2.2 eV. The YL appears to be a universal feature it has been observed in bulk GaN crystallites as well as in epitaxial layers grown by different techniques. The intensity can vary over a wide range, with good samples exhibiting almost no YL. 

The origins of the YL have been widely debated. Ogino and Aoki [1] proposed a model in which the YL is a transition between a shallow donor and a deep acceptor level. We will see that a variety of experiments now confirm this model. Proposals for the microscopic nature of the deep level have included a complex between a Ga vacancy (VGa) and a carbon atom [1], an NGa antisite [2] and an isolated VGa [3,4] (or a complex between Vcja and oxygen [4]). 

At this time the gallium vacancy (in isolated form or complexed with an impurity) appears to be the most likely source of the yellow luminescence. In this Datareview we will summarise the available evidence. 

First-principles results for Vq, are presented in Datareview A8.2. The gallium vacancy is an acceptor-type defect, and hence its formation energy decreases with increasing Fermi level. Gallium vacancies are therefore more likely to occur in n-type than in p-type GaN. The Ga vacancy has a deep level (the 2-/3- transition level) about 1.1 eV above the valence band [4], as illustrated in FIGURE 1. Transitions between the conduction band (or shallow donors) and this deep level therefore exhibit the correct energy to explain the YL. 

FIGURE 1 Schematic illustration of levels involved in the yellow luminescence in GaN. Gallium vacancies introduce a deep acceptor level about 1.1 eV above the valence band Transitions between shallow donors and the deep acceptor level give rise to the YL. 

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A8.7 Yellow luminescence in GaN A8.8 Hydrogen and acceptor compensation in GaN A8.9 3d transition metals in GaN and related compounds A8.10 Er-doped GaN and AIN... 

Strong theoretical as well as experimental evidence is now available identifying gallium vacancies as the source of the yellow luminescence in GaN. 

In n-type GaN the lowest-energy native defect is the gallium vacancy (Vg ), a triple acceptor. This defect plays a role in donor compensation (see Datareview A8.1), as well as in the frequently observed yellow luminescence (see Datareview A8.7). 

The presence of a shallow acceptor level in GaN has been attributed to C substituting on an N site by Fischer et al [7], In luminescence experiments on GaN from high temperature vapour phase epitaxy in a C-rich environment donor-acceptor and conduction-band-to-acceptor transitions have been distinguished in temperature dependent experiments. From the separation of both contributions an optical binding energy of 230 meV close to the value of effective mass type acceptors was obtained. Hole concentrations up to 3 x 1017 cm 3 were achieved by C doping with CCU by Abernathy et al [10], In addition Ogino and Aoki [17] proposed that the frequently observed yellow luminescence band around 550 nm should be related to a deep level of a C-Ga vacancy complex. The identification of this band, however, is still very controversial. 

By this standard, PMBE-grown GaN with best room temperature mobilities of 300 to 410 cm2/V s [43,44] has yet to reach the quality of GaN grown by MOVPE or HVPE [45,46] where mobilities up to 900 cm2/V s have been reached. Despite these deficiencies, GaN with very low carrier concentration and exceedingly low levels of yellow luminescence have been routinely achieved by MBE. In RMBE, unintentionally doped GaN layers can be highly resistive, the highest mobilities (300 K) reported for undoped layers being 230 cm2/V s for free electron concentrations of 2 x 1017 cm 3 [10], n-Type doping with Si yields mobilities (300 K) or 255 cm2/V s and 150 cm2/V s for free electron concentrations of 5 x 1017 cm 3 [41] and 5 x 10l cm 3 [10], respectively. 

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