| ZnO is an importantⅡ-Ⅵdirect band gap semiconductor. It has excellent optical properties such as wide band gap (3.37 eV) and large exciton binding energy (60 meV). The binding energy is higher than those of GaN (25 meV), ZnSe (22 eV) and ZnS (40 meV). Therefore, as a UV emission material, ZnO is suitable for using at room temperature or higher temperature. It has a promising application in the field of LED and LD. As its band gap can be changed from 3.3 eV to 4.5 eV through doping, ZnO can be made into blue, green, purple and other light-emitting devices. In addition, for having the same wurtzite structure as GaN with a lattice mismatch less than 1.8% in [0001] direction, ZnO is a promising ideal substrate for high-quality GaN epitaxial material. As the substrate of ZnO films, homoepitaxial growth is excellent. So, it has great significance to grow high quality ZnO crystals both for the practical application and basic research.ZnO melts congruently at 1975℃and is very unstable at high temperature. So it is difficult to grow the ZnO crystal directly from its melts. Now the ZnO crystal is usually grown from a high-temperature solution or hydrothermal system at a temperature as low as possible. Comparing with the hydrothermal method, high-temperature solution method namely flux method has many advantages such as simple equipment and low cost. For high-temperature solution method, the key is to choose an appropriate flux. The advantages to grow the ZnO crystal in molten hydrous alkali solutions are obvious. Firstly, the reaction temperature is low, which is below 500℃. Secondly, the alkali can be easily removed by dissolving in water. However, the morphology of the ZnO crystal was usually needle-like and at the grade of millimeters. Therefore, it is valuable to develop further research including experiments and theory.Various parameters, which affect the growing of the ZnO crystal, have been investigated. On this basis, the growth unit polar bonding mechanism and the supersaturation competitive generating mechanism were put forward. Various ZnO crystals with different morphology have been grown in hydrous alkali solutions. Their photoluminescence, Raman spectra and conductivity were investigated also. The detail contents are shown as follows:1. The growth parameters, including the crucible material, crucible volume, heat treatment and system basicity, have been investigated. (1) As iron and nickel atoms are easy to enter the ZnO crystal lattices, the quality of the crystals grown in iron or nickel crucible is poor. As silver atoms are not easy to enter the ZnO crystal lattices, the quality of the crystals grown in silver crucible is good. Therefore, silver crucible is most suitable for growing the ZnO crystal. (2) The ZnO crystal can be grown above 400℃or below 350℃in molten hydrous alkali solutions. Above 400℃, the crystallizing of ZnO will not occur in the cooling process but in heating process. Below 350℃, ZnO can be grown by cooling. (3) The relative growth rate of various crystal faces depended strongly on the basicity of the growth system. So the growth habit of the ZnO crystal could be successfully controlled through adjusting the basicity of the solutions.2. Growth of various ZnO crystals. Using spontaneous nucleation method, various ZnO crystals were obtained. Brown, brown green and colorless needle-like crystals were obtained in relatively strong basic solutions. Colorless plate-like crystals were obtained in relatively weak basic solutions. Colorless granular crystals were obtained in relatively moderate basic solutions. In addition, ZnO crystals were also grown in waterless system.3. The growth mechanism of the ZnO crystal in molten hydrous alkali solutions. (1) The growth unit polar bonding mechanism. In molten hydrous alkali solutions the growth unit will change with the growth conditions. In relatively strong basic solution the dominate growth unit is [Zn(OH)4](aq)2-. Due to carrying negative charge, it is easiest to be attracted to the+c surface. So the growth rate of+c face is fastest and needle-like crystals have been obtained. In the relatively weak basic solution, there is a certain amount of [Zn(OH)3(H2O)](aq)-. Being a polar particle, [Zn(OH)3(H2O)](-aq)- is easiest to be adsorbed to m surfaces. As a result, the fastest growth rate is vertical the c-axis and plate-like crystals form. (2) Supersaturation competitive generating mechanism. There exists a competition in the molten hydrous alkali solutions. On the one hand, the ZnO solubility will increase in the heating process. The system will tend to become unsaturated. On the other hand, due to the volatilization of the fluxes the system will tend to become saturated. Above 400℃, volatilization of the fluxes is dominant. ZnO grows in heating process. Below 350℃, the decreasing of ZnO solubility with the temperature is dominant. ZnO grows in the cooling process.4. The formation mechanisms of various ZnO crystals. On the basis of the growth unit polar bonding and the supersaturation competitive generating mechanisms, the formation mechanisms of various ZnO crystals have been discussed. In addition, the morphology characteristics of the irregular ZnO crystals grown under unstable conditions were described.5. Characterization of various ZnO crystals. (1) Photoluminescence (PL spectra) of various ZnO crystals were tested at room temperature and low temperature. In the low temperature PL spectra, all the crystals display one strong and sharp UV peak centered at about 369 nm. In the room temperature PL spectra, all the crystals also appear with a weak green emission band. (2) Raman spectra of various ZnO crystals were tested at room temperature. The modes of the Raman peaks were identified. (3) Conductivity of various ZnO crystals were investigated. ICP-AES was adopted to test the impurities of the crystals. All the ZnO crystals contain p-type doping elements Li, Na, K and etc, but they still exhibit the n-type conductivity.
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