Optical characterization of Al_xGa_1-xN/GaN high electron mobility transistor structures

We present optical characterizations on three Al_xGa_1-xN/GaN high electron mobility transistor (HEMT) structures with different Al compositions using photoluminescence (PL), contactless electroreflectance (CER), photoreflectance (PR), photoconductivity (PC) and persistent photoconductivity (PPC) measurements. The samples used in this study were grown by metal organic chemical vapor deposition (MOCVD) on sapphire substrates, and were designated as samples A (x=0.07), B (x=0.11), and C (x=0.13). From PL and PR spectra the interband luminescences of GaN and Al_xGa_1-xN are identified, and hence the Al composition x can be determined. From CER spectra, we found a broad feature related to 2-dimensional electron gas (2DEG) and Al_xGa_1-xN bandgap transition with Franz-Keldysh oscillations (FKOs) in the high-energy end. The Al composition x is confirmed from the bandgap energy of Al_xGa_1-xN layer deduced from CER spectra, and the built-in electric field in the barrier layer can be determined by analyzing the period of FKOs. It has been shown that the internal electric field in Al_xGa_1-xN layer is enhanced as the Al composition is increased. For sample C, the PC and PPC spectra excited by different monochromatic lights have been done at room temperature and different decay times were evaluated.