Effect of zinc addition and vacuum annealing time on the properties of spin-coated low-cost transparent conducting 1 at% Ga–ZnO thin films

Abstract

Pure and 1 at% gallium (Ga)-doped zinc oxide (ZnO) thin films have been prepared with a low-cost spin coating technique on quartz substrates and annealed at 500 °C in vacuum ∼10−3 mbar to create anion vacancies and generate charge carriers for photovoltaic application. Also, 0.5–1.5 at% extra zinc species were added in the precursor sol to investigate changes in film growth, morphology, optical absorption, electrical properties and photoluminescence. It is shown that 1 at% Ga–ZnO thin films with 0.5 at% extra zinc content after vacuum annealing for 60 min correspond to wurtzite-type hexagonal structure with (0001) preferred orientation, electrical resistivity of ∼9 × 10−3 Ω cm and optical transparency of ∼65–90% in the visible range. Evidence has been advanced for the presence of defect levels within bandgap such as zinc vacancy (VZn), zinc interstitial (Zni), oxygen vacancy (Vo) and oxygen interstitial (Oi). Further, variation in ZnO optical bandgap occurring with Ga doping and insertion of additional zinc species has been explained by invoking two competing phenomena, namely bandgap widening and renormalization, usually observed in semiconductors with increasing carrier concentration.

Investigation of characteristics of ZnO:Ga nanocrystalline thin films with varying dopant content

“…According to these values, it can be clearly proven that the covalent bond length between Zn and O ions has changed with Ga doping concentration inversely. These results are in accordance with other studies [29,31,33,34]. In Table 1, fluctuations can be observed in the c-axis orientations with Ga-doping concentration.…”

“…These values are compatible with the XRD diffractograms. To our knowledge, there is only one paper, published by Srivastava and Kumar, which addresses the degree of orientation values of Ga-doped ZnO thin films, derived from spin coating technique [29]. In this regard, these values seem to be a good reference for Ga-doped ZnO thin films by sol-gel spin coating.…”

“…When Zn 2 þ ions are substituted with Ga 3 þ ions, the Zn-O bond length is changed. The Zn-O bond length is calculated from the following expression [29]:…”

“…Additionally, the bond angles α 0 and β are represented as [29] α Table 1. According to these values, it can be clearly proven that the covalent bond length between Zn and O ions has changed with Ga doping concentration inversely.…”

Stable co-emission of UV, green and red light in ZnO thin films with rapid annealing treatment

“…Besides UV emitters, ZnO thin films are also the ideal materials for fabricating solid white light-emitting devices. This is because, besides the UV light, ZnO can also produce visible emissions with different wavelengths at the same time, such as blue [14], green [15] and red light [16]. In an appropriate proportion, these emissions can be mixed into a white emission.…”