ZnO thin films prepared by atomic layer deposition at various temperatures from 100 to 180 °C with three-pulsed precursors in every growth cycle

• Three-pulsed precursors per ALD cycle provide more surface reactions of ZnO.
• Growth at 140 °C provides sufficient thermal activation energy of defects of ZnO.
• Significant changes of optoelectronic properties of ZnO occur at 140 °C.
• Growth at 100 °C can prevent activation and reduce stability of defects in ZnO.
• The lowest background electron concentration is 2.4 × 1016 cm−3 at 100 °C.

Zinc oxide (ZnO) thin films are grown at various temperatures from 100 to 180 °C by atomic layer deposition (ALD) with three-pulsed precursors per ALD cycle on sapphire (0001) substrates. In the texture analyses of x-ray diffraction (XRD), hexagonal (002) and (101) crystalline orientations are shown. The dominant diffraction is (002) above 140 °C and (101) at and below 140 °C. Hall effect measurements demonstrate remarkable decrease of free electron concentration below 140 °C and the lowest value is 2.4 × 1016 cm−3 at 100 °C. Photoluminescence (PL) spectra of samples display no defect-related luminescence band. The emission peak energy of PL spectra show blueshift with the increase of temperature from 100 to 140 °C, which is originated from band-filling (Burstein–Moss) effect. However, redshift occurred at temperature above 140 °C owing to bandgap renormalization (shrinkage) with the increase of free electron concentration. Enhancement of PL peak intensity and reduction of free electron concentration below 140 °C result from the large decline of intrinsic donor-like defects. 140 °C is a critical temperature for thermal activation of donors-like defects and alteration of dominant crystalline orientation as well as the electrical and optical behaviors in ZnO. The growth method also constitutes smooth surface of ZnO when it comes to the analysis of surface roughness of samples from topographic image forming by atomic force microscope (AFM).