Quasi-ordering of composition fluctuations and their interaction with lattice imperfections in an optical spectra of dilute nitride alloys

Abstract

Using optical spectroscopy experiments, including conventional low-temperature photo-reflectance and photoluminescence (PL) together with time-resolved PL and near-field magneto PL, we distinguish between statistical composition fluctuations (CFs) and nitrogen clusters (Ncl) in i- and n-type GaAs1−xNx alloys (x = 0.01–0.03). We measured the size (l = 3–24 nm), activation energy (Ea = 15–30 meV) and density (nl ∼ 1011 cm−2) of CFs in i-type material. In n-type material we resolved the emission of nitrogen clusters (Ncl) having activation energy ∼70 meV and density ncl ∼ 1012 cm−2. Analysis of l and Ea values in i-GaAs1−xNx revealed a strong interaction between substitutional (Ns) and interstitial (Ni) nitrogen, which results in band gap splitting accompanied by activation of the heavy-hole exciton and Ncl emission. In the optical spectra of i-GaAs1−xNx, we suggest the identification of different Ni species and Ni-Ns arrangements, and in n-GaAs1−xNx we observed activation/suppression of Ncl/CF emission and interference between CFs and ionized donors. Using numerical analysis of amplitude, size and density of CFs we revealed unexpected quasi-ordering which results in modulation of CFs having period 2l. This is consistent with the high nl values observed. We show that this quasi-ordering, arising from statistical non-uniformity of the CFs, creates a set of latent states, which allows us to probe exciton properties, defect density and the structural homogeneity in GaAs1−xNx.