Effect of group-III donors on high-resolution photoluminescence and morphology of ZnO nanowires grown by metalorganic vapour phase epitaxy

Extremely sharp low temperature photoluminescence (PL) linewidths as low as 0.17 meV are observed in nominally undoped ZnO nanowires grown by metalorganic vapour phase epitaxy (MOVPE). These are among the narrowest lines reported for ZnO PL, even for bulk growth, despite growth on highly mismatched sapphire substrates. MOVPE allows the control of group III dopants over a wide range of doping levels. At low concentrations the addition of indium and aluminum dopant resulted in the appearance of their corresponding neutral and ionized donor bound exciton transitions with linewidths comparable to those of nominally undoped material. At higher dopant levels, the axial growth rate decreases and the lateral growth rate increases for both dopants, resulting in eventual coalescence of the nanowires as well as a strong reduction in PL efficiency. The onset of lateral growth also coincides with the elimination of the so-called Y-line defect luminescence previously attributed to excitons localized at structural defects. Because of the lack of appreciable substrate induced strain, coupled with very high crystalline quality, nanowires provide an excellent tool for investigation of shallow dopant effects in ZnO by PL spectroscopy.