Polar axis oriented crystal growth induced carrier transport characteristics in non-degenerate zinc oxide thin films grown on glass substrates

Abstract

Abstract The predominance of the methanol, ethanol, and 2-methoxyethanol solvents, with different boiling points and dielectric constants, are observed on the crystal growth orientation, optical behavior, carrier recombination, and carrier transport mechanism in sol-gel spin coated ZnO thin films. Multi-axis oriented crystal growth orientation was observed for methanol processed ZnO film as compared to that along c-axis only while using other solvents. The intensity of c-axis orientation, crystallite size as well as radiative recombination luminescence effects follows a direct (or inverse) relationship with the boiling point (or dielectric constant) of the involved solvent. Diverse nature of stress was seen for the films. Ideal band gap value (3.36 eV) was identified for 2-methoxyethanol processed oxygen defects induced stress free ZnO thin films. Time resolved photoluminescence (TRPL) spectra reveal a significant contribution of ultrafast and fast decay components in ethanol and methanol processed films as compared to the dominance of slow decay components in 2-methoxyethanol processed thin film. The temperature dependent electrical resistivity measurements unfold the coexistence of metallic and semiconducting behavior in all films. Dominant optical phonon scattering and piezoelectric scattering mechanisms provide metallic behavior to the films as confirmed by an increasing resistivity with increased temperature. The dominant grain boundary scattering mechanism controls the mobility of the carriers and results in the semiconducting behavior in all films at different values of higher temperatures. We have quantified the crystallite size induced band bending effect to understand the applicability of ZnO films in optoelectronic devices as well.