Fluorine-doped zinc oxide transparent and conducting electrode by chemical spray synthesis

Abstract

Highly transparent and conducting fluorine-doped zinc oxide thin films, consisting of spherical nanometer-sized grains, were synthesized onto soda-lime glass substrate by using a chemical spray unit. The effect of fluorine doping concentration in starting solution was investigated. Both doped and undoped films were preferentially oriented along [002] direction. Electrical resistivity decreases from 5.7 x 10^-2 to 8.6 x 10^-3 ohm-cm after 1 at.% fluorine doping, and increases for higher doping concentration. However, surface morphology of films obtained at 3 at.% fluorine doping appeared smooth and uniform. A shift of the edge of the optical transmission in the ultraviolet region as a result of fluorine doping was obtained.

Introduction

Fundamental and practical invention of ZnO, one of the transparent conducting materials is at the glance due to its unique nature of low resistivity, high transmittance in the visible solar region, direct energy band gap (3.37 eV) with a large excitation binding energy of 60 meV, friendly non-toxicity, abundance in nature, low cost, easy fabrication, and hydrogen plasma stability for optoelectronic devices [1], [2]. Varistors, gas sensors, piezoelectric nanogenerators, heterojunction solar cells, and dye-sensitized solar cells, etc., are some of the additional reported applications of compact/porous ZnO [3], [4], [5], [6], [7]. It is widely accepted that in comparison with undoped, impurity doped (tin, fluorine, aluminum, indium, etc.) ZnO films are analogous to gallenium nitride [2]. These films can be a good replacement for indium–tin and fluorine–tin oxides which are unstable in hydrogen plasma treatment in addition to their economicity [8], [9].

Several deposition techniques such as pulsed laser deposition [10], [11], thermal evaporation [12], [13], chemical vapor deposition [14], [15], plasma enhanced chemical vapor deposition [16], sol–gel [17], [18], magnetron sputtering [19], [20], and chemical spray pyrolysis, etc., [21], [22], [23], [24], [25], [26] have been used for the synthesis of solid films of doped and undoped ZnO. According to review published by Patil [27] among these various techniques, chemical spray is advantageous on account of its low cost, easy-to-handle, easy to dope impurities and convenient for the large area deposition. A variety of binary and ternary metal oxides have been fabricated using this technique. On account of our ZnO synthesis experience, obtaining transparent and conducting ZnO thin films is really a great challenge. Keeping this aim, in this paper, effect of fluorine atomic percentage doping (F, at.%) on electrical, structural, geometrical, and optical properties of ZnO films are investigated for minimum resistivity and high transparency using chemical spray system and undoped ZnO properties are also inducted for the comparisons.