Electrical Characteristics of Fluorine-Doped Zinc Oxynitride Thin-Film Transistors
Authors: Kim Hyoung-Do , Kim Jong-Heon , Kim Hyun-Suk 
Published: February 2020

Increasing demands for cost effective switching transistors in the flat panel display industry have led to the development of metal oxide semiconductors such as In-Ga-Zn-O (IGZO). Thin-film transistors (TFT) that incorporate such materials exhibit field effect mobility exceeding 10 cm2/Vs, and are suitable for the fabrication of large size liquid crystal display (LCD) panels with ultra-high definition (4000 ´ 2000 pixels). However, further advances in display technology aim at producing organic light-emitting diodes (OLEDs) that require relatively high driving currents. In this regard, devices with even higher field effect mobility (> 30 cm2/Vs) must be integrated in the backplane array. Recent studies of a relatively new type of semiconductor, zinc oxynitride (ZnON), have shown that high mobility semiconductor films can be obtained by reactively sputtering a Zn metal target, which involves a simple and inexpensive process. The optimization of the associated TFT properties usually consists of controlling the nitrogen to oxygen anion ratio or thermally annealing the ZnON layer, so as to obtain sufficiently low leakage current levels while preserving sufficiently high field effect mobility. It is usually reported that the nitrogen vacant sites (VNs) act as the major source of free electrons and carrier traps that may degrade the device properties over prolonged bias stress. In order to passivate such defects, fluorine doping were attempted in nitride semiconductors.

The present work is a study on the effect of directly incorporating fluorine anions during the thin film growth, by co-sputtering a ZnF2 target alongside the Zn metal target. First-principles calculations indicate that fluorine may act as a carrier suppressor in the ZnON matrix by filling in the VN sites, which is an energetically favorable reaction. The properties of F-doped ZnON (ZnON:F) are investigated through various characterization methods, and the performance of the TFT devices incorporating ZnON:F active layers are studied next.

In order to examine how the chemical bond properties are influenced by the presence of fluorine, X-ray photoelectron spectroscopy (XPS) analyses were carried out. The results indicate that as the ZnF2 target power increases to 6 and 10 W, the ratio of fluorine content increases up to 3.47 and 9.22 at. %, respectively. The amount of nitrogen on the other hand decreases down to 20.5 and 19.2 at. %, respectively. Note that the oxygen content remains relatively constant (Table 1.). Figures 1 (a)-(c) consist of the XPS O 1s peak spectra of the pure ZnON and ZnON:F 6, 10 W films. The O 1s peak was deconvoluted into two into two sub-peaks labelled O1 and O2, which are generally known to occur from oxygen forming metal-oxygen bonds and oxygen near oxygen vacant sites (OV) in oxide semiconductors, respectively. No apparent variation in the relative area ratios of sub-peaks O1 and O2 is observed, which implies that the incorporated fluorine does not affect the oxygen-related bonding states.

Figures 1 (d)-(f) consist of XPS N 1s peak spectra of pure ZnON and the ZnON:F 6, 10 W layers. The N 1s peaks were deconvoluted into three different sub-peaks. The lowest energy sub-peak originates from nitrogen atoms in non-stoichiometric ZnxNy (including nitrogen vacancies), while the middle sub-peak arises from the nitrogen atoms in stoichiometric Zn3N2. The highest energy sub-peak represents mostly N-N bonds. The relative peak area ratios of the three sub-peaks within a single N 1s peak are represented in in table 2. Note that as the amount of incorporated fluorine increases, the relative fraction of nitrogen from stoichiometric Zn3N2 peak increases. Accordingly, the peak intensities from non-stoichiometric ZnxNy and N-N bonds diminish. The above results may be interpreted to occur from the fluorine anions passivating the vacant N sites that contribute to the sub-peak originating non-stoichiometric ZnxNy, so that the relative fraction of the latter appears to decrease with increasing F content, and consequently the sub-peak from stoichiometric Zn3N2 increases.

The transfer characteristics of the devices based on ZnON and ZnON:F are shown in figure 2. Because N-rich ZnON was synthesized as the host material, a relatively large number of nitrogen vacancies is formed so as to convey a high free carrier density. ZnON device exhibits high off-state current, which diminishes as the fluorine content in the semiconductor increases. The representative transfer parameters are listed in table 3. The threshold voltages shift towards positive values with increasing F content. A field effect mobility of 151 cm2/Vs is obtained using ZnON:F 6 W as the semiconductor, with sufficiently low off-state current and threshold voltage near zero, so that the device may be used as a switching element in electronic applications.

Figure 1