Piezoelectric Field Influence on GaN/Al _x Ga _1—x N Quantum Well Optical Properties

Abstract

The absorption and luminescence properties of hetero-polarization GaN/Al x Ga 1--x N (x = 0.12 and 0.165) quantum well (QW) structures are studied by photoreflectivity, photoluminescence excitation spectroscopy (PLE), and photoluminescence at low temperature. The QW transition energy as a function of well thickness exhibits a quantum-confined Stark effect (QCSE) due to the presence of a strong built-in electric field (piezoelectricity and spontaneous polarization). An electric field strength of 120 kV/cm in the barrier and between 600 and 800 kV/cm in the well are obtained from the analysis of Franz-Keldysh oscillations in photoreflectivity spectra. These values are in good agreement with results from the fit of the QW transition energy versus the thickness, using the electric field as a parameter.Introduction With their large band gap the III nitride family is the most attractive to realize short wavelength devices. During the last decade, rapid breakthrough in different domains such as crystal growth, p-type doping has allowed the commercialisation of several devices as light emitting diodes and UV-blue laser diodes [1] based on those materials. One of the most specific properties in wurtzite GaN/Al x Ga 1--x N heterostructures is the presence of a large built-in electric field. This internal macroscopic field originates from the superposition of spontaneous polarization and piezoelectric effect, respectively due to the wurtzite structure and to the lattice mismatch between the GaN well and the AlGaN barrier [2,3]. In this work we study the implication of this field on the optical properties of quantum structures, using different optical characterisation techniques: photoluminescence (PL), photoreflectivity (PR) and photoluminescence excitation spectroscopy (PLE). The electric field value in the QW is calculated by two different techniques: from PR spectroscopy in the FKO regime and from the fitting of QW energy transition versus the well width using the electric field as a parameter.