Optical and structural study of deformation states in the GaN/AlN superlattices

Abstract

We report on the effect of strain on the optical and structural properties of 5-, 10-, and 20-period GaN/AlN superlattices (SLs) deposited by plasma-assisted molecular beam epitaxy. The deformation state in SLs has been studied by high resolution transmission electron microscopy (HRTEM), X-ray diffraction, and micro-Raman, Fourier transform infrared (FTIR), and photoluminescence spectroscopy. HRTEM images showed that the structural quality of the SL layers is significantly improved and the interfaces become very sharp on the atomic level with an increase of the SL periods. A combined analysis through XRD, Raman, and FTIR reflectance spectroscopy found that with increasing number of SL periods, the strain in the GaN quantum wells (QWs) increases and the AlN barrier is relaxed. Based on the dependence of the frequency shift of the E2High and E1TO Raman and IR modes on the deformation in the layers, the values of the biaxial stress coefficients as well as the phonon deformation potentials of these modes in both GaN and AlN were determined. With increasing number of SL periods, the QW emission considerably redshifted in the range lower than the GaN band gap due to the quantum confined Stark effect. The influence of strain obtained by the XRD, Raman, and FTIR spectra on the structural parameters and QW emission of GaN/AlN SLs with different numbers of periods is discussed.