Low-temperature luminescence study of GaN films grown by MBE

Abstract

We report the results of low-temperature photoluminescence measurements on GaN films grown by molecular beam epitaxy on (0001) sapphire substrates. Samples were either nominally undoped or doped with Si. The spectra are generally dominated by a sharp peak at 3.47 eV which is attributed to excitons bound to neutral donors. A much weaker peak (or shoulder) near 3.45 eV probably arises from excitons bound to neutral acceptors. On raising the temperature to 50 K, in some samples free exciton peaks can be partially resolved on the high-energy side of the main line. In others we believe that these free excitons are recaptured onto neutral acceptors, thus enhancing the low-energy side of the line. A broader emission line appears in many samples at an energy near 3.42 eV which shows significant variation in position between samples. Our data show that it represents a free-to-bound, probably a free hole-to-donor, transition. This donor has previously been associated with oxygen. Of particular interest is the fact that some samples show a second sharp peak at 3.27 eV, together with a second broader peak at about 3.17 eV (also variable in energy). The sharp peak is energetically consistent with its being either a donor-acceptor or a free electron-to-bound hole transition, but subsidiary measurements rule out both these possibilities. We suggest that it may represent an exciton bound to a deep donor or a shallow donor-bound exciton in zinc blende GaN inclusions contained within the mainly wurtzite material. We tentatively interpret the 3.17 eV line as a phonon replica of this zinc blende line, the phonon energy being perturbed by the small size of the inclusions and by strain effects within these inclusions.

Effects of small amounts of Al in GaN grown on sapphire (0001) by molecular beam epitaxy

“…15 The third deconvoluted peak is at 3.45-3.46 eV, representing recombination from excitons bound to neutral acceptors (A 0 ,X). 13,16 The two additional peaks: A, at 3.41-3.43 eV and B ͑significant for GaN͒ around 3.36 eV, have clearly lower intensity and much larger linewidth compared to the previous ones. Peak A in the shoulder region is typical for free-to-bound emission involving shallow oxygen donors, 17 while the very broad peak B around 3.36 eV can be attributed to localized excitons, deeply bound to defects, such as dislocations close to the interface.…”

“…The measured peak has contributions from different important recombination channels. 13 In order to resolve these intensity contributions and to detect the detailed change due to the Al, the measured peaks were considered to be composed of several subpeaks, described by a set of Gaussian functions. Four Gaussian curves were extracted, except for the PL spectrum from the reference GaN sample, where five individual peaks were needed to give a fitting better than 99%.…”

“…11 Earlier photoluminescence ͑PL͒ investigations on Al x Ga 1Ϫx N (xϭ0% -22%) at low temperatures have shown that the neutral donor bound exciton emission (D 0 ,X) dominates in the PL spectrum. 12,13 In this work we have grown a series of GaN samples containing small amounts of Al. No systematic investigation of ͑Al͒GaN with small Al concentrations ͑ϳ0.1%-1%͒ has been undertaken before.…”

Low fraction of hexagonal inclusions in thick and bulk cubic GaN layers

“…Fig. 6 shows the PL spectra of the c-GaN samples measured at 4 K. Apparently, the ∼5 m thick c-GaN sample exhibits a broad signal at 3.28 eV due to the overlap of the broad peaks of the excitonic recombinations in c-GaN and some transitions in h-GaN [13]. The peak at 3.15 eV corresponds to the DA pair transitions in c-GaN [14].…”

Intrinsic defects in ZnO and GaN crystals

“…As the acceptors, both the isolated intrinsic defects V Ga (or complexes with V Ga ) and the impurities of second-group elements -Zn, Mg, and Cd -can serve. The band at 3.45 eV was detected in the spectra of p-GaN layers doped with Zn(Cd) [14,[22][23][24] and Mg [25,26] and in the spectrum of the impurity-free n-GaN [26,27]. The authors of [28] think that the band with a maximum at 3.45 eV is due to the gallium vacancies.…”