Structural and optical properties of AlGaN/GaN quantum well structures grown by MOCVD on sapphire

AlGaN/GaN single quantum wells (QW) have been grown on 2 sapphire substrates (c-plane) by metal-organic chemical vapor deposition (MOCVD). The well width was varied between 20 and 40 A.U. for barriers containing 4 per cent and 16 per cent of aluminium. Cathodoluminescence (CL) and Photoluminescence (PL) spectra of the samples show, as expected, a shift of the quantum well emission to higher energies with decreasing well width, whereas the barrier luminescence stays at constant energy. Examination of the QWs by resonant Raman spectroscopy tuned to the gap of the well, clearly shows the GaN A1(LO) phonon besides the AlGaN A1(LO) phonon from the barrier. For a well width of 20 A.U. we observe a shift of the A1(LO) GaN phonon indicating a certain degree of intermixing at the GaN/AlGaN interface. Atomic Force Microscopy (AFM) reveals that the layers are growinng in a 2-dimensional step flow growth mode with step heights of 3 and 6 A.U. corresponding to mono- and biatomic steps. High Resolut ion Transmission Electron Microscopy (HRTEM) micrographs of the 40 A.U. well show a very low interface roughness of 1-2 atomic layers.