Contactless electroreflectance of ZnO layers grown by atomic layer deposition at low temperature

Room-temperature contactless electroreflectance (CER) has been applied to study optical transitions in ZnO layers grown by atomic layer deposition at low temperatures on glass, silicon and GaN substrates. A broad CER resonance was clearly observed at the energy of ~3.4 eV for layers deposited at low temperatures (100–240 °C) on glass or silicon. This resonance has been attributed to excitonic/band-to-band absorption in polycrystalline ZnO. A sharp excitonic resonance at ~3.32 eV was observed for monocrystalline ZnO layers deposited on GaN templates at higher temperatures (>250 °C). In addition, the broad CER resonance at ~3.4 eV was also observed for these layers but its intensity decreases when the growth temperature is increased, i.e. c-plane-oriented monocrystalline areas appear in the ZnO layer instead of polycrystalline areas with various surface orientations.