Abstract
ZnO films with Ti atoms incorporated (TZO) in a wide range (0–18 at.%) have been grown by reactive co-sputtering on silicon and glass substrates. The influence of the titanium incorporation in the ZnO matrix on the structural and optical characteristics of the samples has been determined by Rutherford backscattering spectroscopy (RBS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The results indicate that the samples with low Ti content (<4 at.%) exhibit a wurtzite-like structure, with the Ti4+ ions substitutionally incorporated into the ZnO structure, forming Ti-doped ZnO films. In particular, a very low concentration of Ti (<0.9 at.%) leads to a significant increase of the crystallinity of the TZO samples. Higher Ti contents give rise to a progressive amorphization of the wurtzite-like structure, so samples with high Ti content (≥18 at.%) display an amorphous structure, indicating in the XPS analysis, a predominance of Ti–O–Zn mixed oxides. The energy gap obtained from absorption spectrophotometry increases from 3.2 eV for pure ZnO films to 3.6 eV for those with the highest Ti content. Ti incorporation in the ZnO samples <0.9 at.% raises both the blue (380 nm) and green (approx. 550 nm) bands of the photoluminescence (PL) emission, thereby indicating a significant improvement of the PL efficiency of the samples.
Highlights
Zinc oxide (ZnO) is a semiconductor material characterized by a broadband gap, a high transmission in the visible spectral range, a native n-type conductivity as well as by its excellent photoluminescence (PL) properties, which can be modified by the incorporation of impurities in its structure [1,2]
We propose the deposition of ZnO:Ti thin films in a wide range of Ti atomic concentrations by the means of the reactive DC magnetron co-sputtering technique and by using two separate targets of pure Ti and Zn and no intentional heating
The above results demonstrate a close relationship between the crystalline structure of sputtered
Summary
Zinc oxide (ZnO) is a semiconductor material characterized by a broadband gap (approx.3.2–3.3 eV), a high transmission in the visible spectral range, a native n-type conductivity as well as by its excellent photoluminescence (PL) properties, which can be modified by the incorporation of impurities in its structure [1,2]. The PL emission of ZnO films presents essentially two peaks, in the UV and in the visible spectrum, at approximately 380 nm and in the 450–730 nm range, respectively. It is well-known that the degree of crystallinity of the films has a strong influence on the UV emission efficiency [3,4]. N-type doping controlled by the incorporation of certain elements in the ZnO network is a recurrent method for tailoring both the bandwidth energy and electrical conductivity by increasing the concentration of carriers while keeping a high transparency in the visible range [9]. Most of previous work on doped ZnO films is related to doping with group
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