Nitriding Zinc Powder And Thin Films And Features

Zinc nitride powders were first synthesized by Juza and Hahn in 1940, but their properties had not been studied for the following 50 years. Zinc nitride is black in color and has the anti-scandium oxide (Sc₂O₃) structure, a derivation of the CaF₂ structure, where the nitrogen atoms occupy the calcium positions and the zinc atoms occupy three-fourths of the fluorine positions. In 1993, polycrystalline zinc nitride films were prepared by Kuriyama et al. These films have a large optical band gap of 3.2 eV, which is very close to that of zinc oxide films (3.3 eV). In 1997, the structures of Zn₃N₂ were refined by Partin et al. In 1998, the structural, electrical and optical properties of zinc nitride thin films were investigated by Futsuhara et al. who found that these films are a semiconductor of n-type with a small direct band gap of 1.23 eV. In the same year, the optical properties of zinc oxynitride (Zn_xO_yN_z) films were also investigated by Futsuhara et al. The optical band gap decreases from 3.26 to 2.30 eV with the increase of nitrogen concentration in the films.In 2003, p-type ZnO thin films prepared by oxidation of zinc nitride thin films were investigated by Wang et al. When the oxidation temperature is between 350 and 500 ℃, p-type ZnO thin films can be obtained. However, when the oxidation temperature increases to 550 ℃, n-type ZnO films can be obtained. In 2004, the nitrogen doped ZnO films fabricated directly by the thermal oxidation of zinc nitride films were investigated by Wang et al. To demonstrate that ZnO:N thin films are of a p-type, they have fabricated a Zn₃N₂/n-Si heterojunction structure for the first time, and observed the p-n junction rectification characteristics from â… -â…¤ measurements.Up to the present, very little research has been done on the zinc nitride material. Most characteristics of this material are not fully understood. For example, its optical band gap has remained a controversial issue for a long time. It is expected that zinc nitride will exhibit excellent electric and optical properties, so further intensive investigation is needed.In this dissertation, Zn₃N₂ powders of high quality have been synthesized through the nitridation reaction of Zn powder with NH₃ gas for the first time. A verycomprehensive investigation is conducted into the optimum nitridation condition, the structure, the component, the chemical bonding state, the surface morphology, and the microstructure of the Zn3N2 powders. A computer simulation of the Zn3N2 crystalloid structure is given for the first time. The thermal decomposition behaviors of zinc nitride powders in the air, in the pure nitrogen gas and in the deionized water are investigated for the first time. High quality polycrystalline zinc nitride films are prepared onto silicon and quartz substrates by reactive rf magnetron sputtering using a zinc nitride disk as the target and pure N2 as the working gas for the first time. The structural and optical characteristics of these films have been investigated.From the X-ray diffraction (XRD) patterns, it is found that the optimum nitridation condition is at a NH3 flow rate of 500 ml/min and at a nitridation temperature of 600 °C for 120 min. Zn3N2 powder belongs to the body-centeredcubic crystal system and has the x-axis glide symmetry. The space group is la 3, in which a = 0.9769 ran. The nitridation cannot occur below 500 °C, and will be incomplete at 550 °C. Zn3N2 can be decomposed and changed into ZnO partially at 650 °C, and can be been changed into ZnO completely above 750 °C. The chemical reaction formula is as follows,3Zn(s) + 2NH3(g)—^-^Zn3N2(s) + 2H2(g).According to Partin's model, zinc nitride has the antibixbyite structure. The metal atoms are in general positions, 48e (x, y, z;etc.). There are two kinds of N atom: N(l) in position 8b (1/4, 1/4, 1/4;etc.) and N(2) in positions 24d (x, 0, 1/4;etc.). The coordinate of Zn is (0.3957, 0.1489, 0.3759), including 48 atoms, and those of N(l) and N(2) are respectively (0.25, 0.25, 0.25) including 8 atoms and (0.9784,0,0.25) including 24 atoms. Here, the exact position of each atom inside the Zn3N2 crystal is calculated and the computer simulation of structure of Z^N2 is presented. Through comparing the computer simulations of HRTEM of Zn3N2 with the real picture of zinc nitride powders, the validity of Partin's model can be proved.From scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) images of Zn3N2 powders synthesized at the optimum nitridation condition, it can be found that the surface morphology of Zn3N2 powders are of various shapes, such as regular or irregular flakes, columns, stacks, solid spherical particles, hollow spherical shell particles, different segments of hollow spherical shellparticles, and nanowires in different diameters and different lengths. Under the standard atmosphere pressure, the melting point and the boiling point of Zn is 420 °C and 907 °C. At 600 °C, the vapour pressure of Zn is 0.014 atm and Zn has already reacted with NH3 and been changed to Zn3N2 on the surface of the Zn powder. There is solid Zn3N2 on the surface of the Zn powder and liquid Zn in its interior. So, the liquated state of Zn cannot be found. That is to say, now the surface of the Zn powder is in instability with plenty of Zn steaming. When Zn of nanowire shape sprays out and reacts with NH3, Zn3N2 of nanowire shape will be synthesized and the hollow spherical shell particles will be left;when Zn of round global shape reacts with NH3, Zn3N2 of solid spherical particle shape will be synthesized. In summary, Z^N2 powders of various shapes can be synthesized.Zinc nitride can be hydrolyzed easily by air moisture. It is unavoidable that a typical X-ray photoelectron spectroscopy (XPS) wide scan spectrum of zinc nitride includes Zn, N, O and C peaks, among which O and C peaks come from the surface adsorption of CO2, H2O and O2 from air.The thermal stability of Zn3N2 powders under three different conditions (air, nitrogen gas, and deionized water) are analyzed by using thermal gravimetric analysis (TGA), differential thermal analysis (DTA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). The thermal oxidation of zinc nitride powder in the open air follows the two-step reaction model. When the temperature is between 200 and 500 °C, compact zinc oxide (ZnO) or zinc oxinitride (ZnxOyNz) layers in the surface of zinc nitride powders are formed, which prevents the interior of zinc nitride powders from thermal oxidation. Therefore, thermal oxidation of zinc nitride powders is a very slow process. When the temperature is above 500 °C, fast thermal oxidation occurs in the interior of the zinc nitride powder. The interior thermal oxidization takes place at about 500 °C;the maximum oxidization rate is at about 681 °C;and the final thermal oxidization is delayed to 750 °C. In open atmosphere, oxygen is very active while others are inert. It is evident that the oxidization of zinc nitride will take place when the N3" is replaced by O2". The oxidization of zinc nitride is associated with the following chemical reaction formula:2Zn3N2 (s) + 3O2 (g) —==-> 6ZnO(s) + 2N2 (g) + QIn 2003, Wang et al. investigated the p-type ZnO thin films prepared by oxidation of zinc nitride thin films. When the thermal annealing temperature is between 350 °C and 500 "C, the surface of the zinc nitride films are oxidized to zinc oxynitride films (ZnO:N films), and so p-type ZnO:N thin films can be obtained. At 550 °C, however, the zinc nitride films are oxidized to zinc oxide films (ZnO films), and so an n-type ZnO film can be obtained.TGA and DTA in the environment of nitrogen gas indicate that zinc nitride powder is unstable at high temperature and will decompose into liquid state zinc and nitrogen gas. The thermal decomposition takes place at about 720 °C;the maximum decomposition rate is at about 767 °C;and the final decomposition is delayed to 800 °C. When the temperature is above 1080 "C, the liquid state metallic zinc will vaporize quickly, and there will be an endothermic reaction peak in DTA and a fast weight loss in TGA. Comparing the thermal oxidization of zinc nitride in the air with the thermal decomposition of it in nitrogen gas, it can be found that the thermal oxidization in air is an exothermic reaction, while the thermal decomposition in nitrogen gas is an endothermic reaction. The thermal decomposition of zinc nitride in nitrogen gas is associated with the following chemical reaction formula:^ 2(g) -QThe moist air also affects the stabilization of zinc nitride. When the zinc nitride powders are poured into boiling water, a large amount of ammonia will be released. Studies using XRD and FTIR show that ZnO can be composed from zinc nitride and water when zinc nitride powders are heated in boiling water. Zinc dihydroxides can be formed through the reaction between zinc nitride and water. As they are insoluble in water, the zinc dihydroxides can cover the surface of zinc nitride powders and prevent further reaction. This results in slow down the reaction between zinc nitride and water. The zinc dihydroxides can be air-dried, dehydrated and changed into zinc oxides. This reaction is associated with the following chemical equations:ZrhN2 (s) + 6H2O(l) heat >3Zn(OH)2 (s) + 2NH3 (g)Zn(OH)2 (s) """*- dehydration ) ZnO{s) + H2O(g) The zinc nitride films were deposited on silicon and quartz substrates in aJPGF-450 model radio frequency magnetron sputtering system with zinc nitride as the target and N2 (purity 99.999%) as the working gas. The base pressure is 2 X 10"3 Pa. A power supply operated at a crystal-controlled frequency of 13.56 MHz. The diameter of the target is 80 mm. The distance between the substrates and targets is 50 mm. During the film deposition, the nitrogen partial pressure is maintained at 1.2 Pa, the sputtering power is 110 W, the sputtering time is 90 min, and the substrates is kept at room temperature.The structural and optical properties are investigated at different sputtering powers and different sputtering times. The films are cubic in structure with the lattice constant a=0.979(l) nm and have preferred orientations with [321] and [442]. Absorption coefficients and film thickness are calculated from the transmission spectra. From analysis of the photon energy dependence of absorption coefficients, an indirect transition optical band gap of about 2.12 eV has been obtained. This work settles the controversy of the band gap value in zinc nitride films.