Structural, optical, and transport properties of AlGaN/GaN and AlGaN/InGaN heterostructure on sapphire grown by plasma assisted molecular beam epitaxy

Abstract

The effects of indium on the strain states, surface morphologies, and polarization induced charges (and in turn on sheet carrier concentrations) at the interfacial channel layers of AlGaN/GaN and AlGaN/InGaN specimens have been investigated. Room temperature Raman spectroscopy was performed to explain the residual strains by the optical phonon frequency shift from their bulk values, and also to analyze the quality of the GaN and InGaN channel layer by the full width at half maxima of E2high and A1(LO) (longitudinal-optical) phonon lines. The strain state was also analyzed by room temperature photoluminescence (PL) spectra by observing the direct excitonic transition from Γ6v →Γ1c (A, B) for valance band to conduction band. The surface morphology was observed from atomic force microscopy imaging inferring pit densities of 4.4×107 cm−2 and 6×107 cm−2 for 5×5 μm area scans. In addition, the screw and edge type defect density were 3.28×107 and 5.85×109; 2.40×108 and 4.91×109 cm−2 as calculated from high resolution x-ray diffraction pattern (HRXRD) analysis for AlGaN/GaN and AlGaN/InGaN heterostructure, respectively. The homogeneity of the epilayers were confirmed by measuring (20 × 20 mm2) areal scans of omega-rel diffraction profiles. In addition, the theoretical estimation of carrier confinement at the interfacial two-dimensional electron gas concentration (2DEG) was carried out from analytical expressions accounting both polarization effects (spontaneous and piezoelectric) and interdependence of sheet density with Fermi level in the Ga(Al) face terminated AlGaN/GaN (or AlGaN/InGaN) airy quantum well. The calculated charge density for AlGaN/GaN and AlGaN/InGaN were found to be 1.60×1013 and 1.75×1013 cm−2 using in-plane strain values from HRXRD, and 1.20×1013 and 1.50×1013 cm−2 using in-plane strain values from room temperature PL band edge emission, respectively. Also, the calculated 2DEG concentrations were compared with the experimental outcomes using Hall-effect measurement based on van-der-Pauw geometry at 300 K. The experimental 2DEG concentrations were found to be 1.45×1013 and 2×1013 cm−2 for AlGaN/GaN and AlGaN/InGaN, respectively. The enhanced carrier concentrations from both analytical and experimental observations are attributed to the effect of “In” incorporation in the channel layer.