Synthesis And Characterization Of ZnOM (M=S, Se) Alloys And (Cu, S) Co-doped P-type W-Zn_1-yCu_yO_1-xS_x Alloy Films

ZnO, with hexagonal structure, whose band gap at room temperature is 3.37eV,and exciton binding energy is 60meV, has long been considered as an important wide band semiconductor material. Recently, there are many reports about the EL of the ZnO based homo- or hetero- p-n junction. However, there are still many fundamental physical problems about ZnO, such as ZnO p-type stability, repeatability, doping mechanism, intrinsic defects, and the reliability of p-type ZnO Hall characterization, many of which are still under debate. As we all know, if we want to achieve the application of ZnO based ultraviolet photoelectronic device, e.g. ZnO-based LED,LD, etc, the key problem is to obtain high–quality, stable p-type and n-type ZnO. Now, high quality n-type can easily be prepared to meet the requirements of device application, by IIIA group elements doping (such as Al, Ga, etc.). Due to self-compensation and asymmetric doping effects of ZnO, which are common in wide band semiconductor materials, p-type doping has encountered great difficulties, becoming the bottle neck, restricting ZnO application. Consequently, stable, repeatable and high quality p-type ZnO films always are the imperative key scientific problems.According to the theory calculation of Yan et al., the formation energies are very low for these group-IB elements on the substitutional sites, but rather high at the interstitial sites. Therefore, in quasi-equilibrium state, if we use IB group elements as p-type doping for ZnO films, it will favor us to control the stable doping state and growth condition during the p-type ZnO films growth. In the process of doping, most IB elements doped into ZnO films will take the substituent sites in ZnO lattice, avoiding the compensation effect of interstitial atoms. Research by Persson et al. revealed that the ZnO_1-xS_x alloys exhibits a very strong VB-offset bowing as a function of S content, and the VB offset Ev(x) increase strongly whereas the CB edge Ec(x) increases only weakly for small S incorporation. The strong VB-offset bowing can not only be utilized to reduce the acceptor level and restrict the compensation of intrinsic donors, but also can be used to reduce the forming energy of acceptors and increase the acceptor doping concentration. Consequently, through such bandgap engineering, we may be able to obtain high efficiency p-type ZnO-like alloys with lower resitivity and higher hole concentration.In this present research, we prepared amorphous Zn_xSe_1-xO₂ ternary, ZnO_1-xS_x alloys and p-type Cu and S co-doped w-Zn_1-yCu_yO_1-xS_x alloy films on quartz substrates, by sputtering high purity ZnSe, ZnO and ZnSe targets respectively while changing the ratio of working gas of Ar and O₂ and the amount of high purity copper wires fixed on the ZnS target. We did detailed research on the structure, doping state, electronic and optical properties of the films while changing the Se,S and Cu content in amorphous Zn_xSe_1-xO₂, ZnO_1-xS_x alloys and Cu and S co-doped w-Zn_1-yCu_yO_1-xS_x alloy films ,respectively. The detailed research content and conclusions are as following:1. We employed high pure ZnSe single chip as the sputtering target. No matter how we change the Ar and O₂ ratio, we always obtained amorphous Zn-O-Se ternary compounds. According to the thermodynamics and kinetic theory of thin film growth under non-equilibrium system, Gibbs formation energy of amorphous Zn-O-Se ternary compounds is smaller than crystalline ZnO_1-xSe_x alloy. So, in this work, we do not prepare crystalline ZnO_1-xSe_x alloy, which has band gap bowing effect, as Refs. have reported.2. According to EDS and XPS measurements of amorphous Zn-O-Se ternary compounds, we concluded that part of Zn lattice in ZnO₂ was replaced by Se atoms to form amorphous Zn_xSe_1-xO₂ ternary. The optical band gap of amorphous Zn_xSe_1-xO₂ ternary is 4 5eV, and with splendid high transparency, as high as 95% forλ>270nm.3. We employed high pure ZnS ceramic as sputtering target in this work. We synthesized a series of ZnO_1-xS_x alloy (0≤x≤1) on quartz substrates by changing the O₂ partial pressure. In O-rich side (0≤x≤0.23), ZnO_1-xS_x alloy films are hexagonal structure with (002) peak as the preferred peak, denoted by w-ZnO_1-xS_x. In the S-rich side (0.77≤x <1), ZnO_1-xS_x alloy films are cubic zinc blende structure with (111) peak as preferred peak, denoted asβ-ZnO_1-xS_x. For 0.231-xS_x alloys are constituted by w-ZnO_1-xS_x andβ-ZnO_1-xS_x two phases.4. ZnO_1-xS_x alloys show a band gap bowing effect while changing the sulfur content in the films, the bowing coefficient b=2.9eV. When S doped into ZnO lattice, the crystalline quality of ZnO_1-xS_x alloys get worse due to the increase of Vo concentration, which lead to the inferior photoluminescence intensity compared with un-doped ZnO films, as well as the resitivity ,carrier concentration and mobility of ZnO_1-xS_x alloys.5. We employed high pure ZnS ceramic as sputtering target. We prepared a series of stable p-type Cu and S co-doped single phase w-Zn_1-yCu_yO_1-xS_x alloy films with hexagonal structure, by changing the O₂ partial pressure and the amount of high pure copper wire fixed on the ZnS target. When y≥9% and x≥8.1%, the co-doped w-Zn_1-yCu_yO_1-xS_x alloy films show splendid p-type electronic properties. Compared with the p-type Cu-doped ZnO films, the electronic properties of p-type w-Zn_1-yCu_yO_1-xS_x can be greatly enhanced, by Cu and S co-doping, as well as the solid solution of Cu in ZnO. For p type w-Zn_1-yCu_yO_1-xS_x, the hole carrier density reaches 4.3～7.4×1019cm-3; resistivity is 0.29～0.57ohm cm; and the hall mobility is 0.14～0.5cm2/(V.s).According to Temperature dependent Hall, SEM and XPS analysis, as well as the post heat treat of stable p-type w-Zn_1-yCu_yO_1-xS_x , they revealed that the acceptors are form the material itself, not from the porous structure on the films surface. We supposed the shallow acceptor may come from the CuZn of Cu⁺¹ or the related intrinsic acceptor defects in the heavy doped degenerated w-Zn_1-yCu_yO_1-xS_x alloy film.