Fluorine dopant concentration effect on the structural and optical properties of spray deposited nanocrystalline ZnO thin films

Highlights

• Pure and Fluorine highly doped ZnO thin films deposited on glass by CSP at 450 °C.

• XRD studies ZnO and ZnO:F films showed polycrystalline nature and Hexagonal phase.

• Structural parameters were determined as a function of Fluorine dopant concentration.

• EDXS spectra well defined peaks refer to Zn, O, F and confirmed films stoichiometry.

• Optical results showed high transmittance films, F doped absorption edges blue shifted related to B–M effect.

Abstract

Pure and fluorine highly doped Zinc oxide with different concentrations (0.1,0.2,0.3 mol/ℓ) thin films were deposited on glass substrates heated up to 450 °C by chemical spray pyrolysis (CSP) technique. They were characterized using various techniques. X-ray diffraction pattern showed that all films had a polycrystalline structure Hexagonal wurtize phase; the doping processes did not show an obvious effect on the films structure. Structural parameters such as inter-planer distance, lattice constants, average grain size, dislocation density, the number of crystallites per unit surface area and strain could be determined as a function of Fluorine dopant concentration. The prepared films Grain size was found to be ranged from 39 to 52 nm, which indicates that all films have nanocrystalline structure. SEM images demonstrated a homogeneous and smooth uniform surface, and EDXS spectra confirm the stoichiometry of the deposited films. Optical properties data of the prepared films showed high transmittance values in the visible region (i.e. 40–99%), and this was increased after the doping process therefore the ZnO:F film was very suitable for solar cells applications . The energy gap (E_g) of direct electronic transitions was found equal to (3.082 eV) for pure ZnO and it increased to (3.163, 3.256, 3.271 eV) for ZnO:F, this behavior could be attributed to the Burstein–Moss effect.

Introduction

Zinc oxide (ZnO) is belonging to the II–VI group compound semiconductors. ZnO thin films are very promising materials and attracting considerable attention due to its suitable optical and electrical properties [1], [2]. It can be considered as an alternative for transparent conductive films if it doped with suitable impurities due to the fact that a high transmittance and low resistivity can be obtained simultaneously [3]. Transparent conducting oxide (TCO) films have an extensive applications in optoelectronic devices [4], TCO have a high electric conductivity and a high visible transmittance together with a wide direct optical band gap. Zinc oxide is practically useful as a transparent conducting oxide due to its large band gap (3.37 eV) [5], [6], high conductivity, easily doped and thermally stable [7]. Additionally Zinc oxide thin films have widely been investigated due to their important applications namely, as transparent electrode, antireflection coating and windows in solar cell, gas sensors and in optoelectronic and piezoelectric devices [3], [4], [5], [6], [7]. The effective way to enhance the optical and the electrical properties of ZnO is by doping it with Aluminum, fluorine etc. [3], [8], [9].

ZnO and ZnO:F can be prepared adopting many techniques including metal-organic chemical vapor deposition (MOCVD) [10], nonreactive radio frequency (RF) magnetron sputtering [4], DC magnetron sputtering [5],sol-gel spin coating [11], pulsed laser deposition (PLD) [12] and spray pyrolysis [13], [14], electron beam deposition [15]. In this study pure and Fluorine highly doped ZnO thin films were deposited by low coast spray pyrolysis technique at 450 °C substrate temperature; it is the most convenient method because of its simple,low cost, ease to add doping materials and the possibility of enhanced the film properties by the composition variation of starting solution.