Shallow and Deep Centers in As-Grown and Annealed MgZnO/ZnO Structures with Quantum Wells

Shallow and deep centers in ZnO(P)/MgZnO/ZnO/MgZnO/ZnO(Ga) structures grown by pulsed laser deposition on sapphire were studied before and after annealing in oxygen atmosphere at high temperatures of 850A degrees C to 950A degrees C. In both as-grown and annealed structures, microcathodoluminescence spectra in the near-bandgap region demonstrate a blue-shift by 0.13 eV compared with bulk ZnO films, indicating carrier confinement in the MgZnO/ZnO/MgZnO quantum well (QW). Annealing strongly decreases the concentration of shallow uncompensated donors from similar to 10(17) cm(-3) to similar to 10(16) cm(-3) and makes it possible to probe the region of the QW by capacitance-voltage (C-V) profiling. This profiling confirms charge accumulation in the QW. The dominant electron traps in the as-grown films are the well-known traps with activation energies of 0.3 eV and 0.8 eV. After annealing, the electron traps observed in the structure have activation energies of 0.14 eV, 0.33 eV, and 0.57 eV, with the Fermi level in the n-ZnO(P) pinned by the 0.14-eV traps. The annealing also introduces deep compensating defects that decrease the intensity of band-edge luminescence and produce a deep luminescence defect band at 2.2 eV. In addition, a defect vibrational band becomes visible in Raman spectra near 650 cm(-1). No conversion to p-type conductivity was detected. The results are compared with the data for the structures successfully converted to p-type, and possible reasons for the observed differences are discussed.