Physical properties of Zinc Oxide
Zinc oxide crystals have three structures: hexagonal wurtzite structure, cubic sphalerite structure, and relatively rare sodium chloride octahedral structure. Wurtzite structure is the most stable of the three, so it is the most common. Cubic sphalerite structure can be obtained by the gradual formation of zinc oxide on the surface. In both crystals, each zinc or oxygen atom forms a tetrahedral structure centered on its neighboring atoms. Octahedral structures have been observed only at high pressures of 10 billion pascals.
Wurtzite structure and sphalerite structure have central symmetry, but no axial symmetry. The symmetry of the crystal makes the wurtzite structure have piezoelectric effect and hot spot effect, and the sphalerite structure has piezoelectric effect.
The point group of wurtzite structure is 6mm (international symbol), and the space group is P63mc. In the lattice constant, a = 3.25 A, C = 5.2 A; the c/a ratio is about 1.60, approaching the ideal hexagon ratio of 1.633. In semiconductor materials, zinc and oxygen are mostly bonded by ionic bonds, which is one of the reasons for their high piezoelectric properties.
Zinc oxide is a relatively soft material with a hardness of about 4.5. The elastic constants of zinc oxide are smaller than those of gallium nitride and other III-V semiconductor materials. Zinc oxide has good thermal stability and conductivity, high boiling point and low coefficient of thermal expansion. It is useful in the field of ceramic materials.
Among all kinds of semiconductor materials with tetrahedral structure, zinc oxide has the highest piezoelectric tensor. This characteristic makes zinc oxide one of the important materials for mechanical and electrical coupling.
At room temperature, the energy band gap of zinc oxide is about 3.3 eV, so pure zinc oxide is colorless and transparent. High energy band gap brings high breakdown voltage, strong ability to maintain electric field, low electronic noise and high power. When zinc oxide is mixed with a certain proportion of magnesium oxide or cadmium oxide, the energy band gap varies between 3 and 4 eV.
Zinc oxide has the characteristics of N-type semiconductors even if no other substances are added. The characteristics of N-type semiconductors were once thought to be related to the incompatibility of compound atoms, while the study of pure zinc oxide was a counterexample. The properties of N-type semiconductors can be adjusted by using Al, Ga, Indium and other III major group elements or halogens such as chlorine and iodine. However, it is difficult to make zinc oxide into P-type semiconductor. The available additives include alkali metal elements such as lithium, sodium, potassium, nitrogen, phosphorus, arsenic and other V main elements, copper, silver and other metals, but all need to be effective under special conditions.