Luminescence studies on nitride quaternary alloys double quantum wells

Abstract

We present theoretical photoluminescence (PL) spectra of undoped and p-doped Al_xIn_1−x−yGa_yN/Al_XIn_1−X−YGa_YN double quantum wells (DQWs). The calculations were performed within the k.p method by means of solving a full eight-band Kane Hamiltonian together with the Poisson equation in a plane wave representation, including exchange–correlation effects within the local density approximation. Strain effects due to the lattice mismatch are also taken into account. We show the calculated PL spectra, analyzing the blue and red-shifts in energy as one varies the spike and the well widths, as well as the acceptor doping concentration. We found a transition between a regime of isolated quantum wells and that of interacting DQWs. Since there are few studies of optical properties of quantum wells based on nitride quaternary alloys, the results reported here will provide guidelines for the interpretation of forthcoming experiments.

Introduction

The III-nitride semiconductor materials have undergone an enormous development during the past few years. Of particular interest is the AlInGaN quaternary alloys as potential candidate for developing deep-ultraviolet (UV) light emission diodes and laser diodes [1], [2]. It has already been demonstrated the advantages of using AlInGaN quaternary layers, instead of GaN and AlGaN, for the fabrication of high quality quantum structures with strong UV emission at room temperature [3]. Additionally, applications of interest for the quaternary alloys involve long lifetime white lighting, biochemical agent detectors, environment purification, and others [4].

The AlInGaN quaternary alloys have also been studied because they may allow to minimize high densities of threading dislocations, which causes quantum efficiency deterioration. Also, these AlInGaN quaternary allows attaining higher quantum efficiencies as compared to the AlGaN ternary alloys with similar Al contents [4]. Different mechanisms have been proposed for the origin of the luminescence in the AlInGaN quantum well-based devices. It was claimed that the luminescence originates from localized states in In-rich nano-clusters embedded in the wells [5], [6]. Other investigations have argued that the optical transitions seen in photoluminescence (PL) spectra originate from quantum wells (QW) states under the influence of strain effects, specially for the wurtzite phase [7], [8].

Although there are many reports on the optical investigation of AlGaN/GaN and GaN/InGaN QWs [2], it has been proposed the study of AlGaN/GaN double quantum wells (DQWs) [9] and InGaN/GaN DQWs [10]. However, due to the piezoelectric and spontaneous polarization electric fields existent in the wurtzite phase of the systems, a large internal field is present in the heterostructures. Thus, a quaternary AlInGaN barrier has been used as one way to minimize the internal electric fields [1]. Most of devices are fabricated in the more stable phase, the hexagonal (wurtzite), but there are research efforts towards a more complete understanding of the cubic structures in the last years [11], [12]. The absence of polarization fields in the cubic-III-nitrides may be advantageous for some device applications. Moreover the cubic (c) ternary nitride alloys possess potential advantages, as compared to their wurtzite counterparts, such as for the fabrication of laser cavities with cleaved facets [12]. So we assumed the quaternary alloys in the zinc blend (cubic) structure for the calculations. An important feature in the Al_xGa_yIn_1−x−yN quaternaries is the possibility to control their energy band gap and lattice constant through the variation of the In and Al compositions in the alloy. We point out that, for the cubic phase, it is possible to reach the red region wavelength with less In content [13]. Another concern of particular interest is the doping in quaternary nitride alloys. Recently, it has been demonstrated that these quaternary alloys may be doped more easily as p-type, and due to the wavefunction localization the optical transition energies are higher in the alloys than in GaN [14].

In this work we investigate the theoretical photoluminescence spectra in undoped and p-doped Al_xIn_1−x−yGa_yN/Al_XIn_1−X−YGa_YN DQWs, in which the Al and In content, as well as the well and spike widths are varied. Fig. 1 shows a schematic diagram of the DQW systems investigated here. We use a self-consistent Kane method, developed to treat the case of quantum wells and superlattices, in conjunction with the Poisson equation for charge distribution in order to calculate the electronic and optical properties of the systems. Exchange–correlation effects are also included within the local density approximation [15], [16]. We assume that all systems are strained, so that the optical transitions are due to confinement effects. A detailed discussion of the assumptions made here can be found elsewhere [13]. Our results give important information about the PL spectra in undoped and p-doped AlGaInN-based DQWs, and may be relevant for the interpretation of forthcoming optical measurements.