TABLE OF CONTENTS Zinc oxide (ZnO) single crystals show a wurtzite hexagonal structure (Fig. 5.2). These ZnO crystals are known as piezoelectric materials with a large electromechanical coupling factor and a low dielectric constant.[133] Typical physical properties of ZnO are listed in Table 5.5.
| Crystal System | Wurtzite (6 mm) |
|---|---|
| Space group | P6 3 mc |
| Lattice constant | a = 3.24265 , c = 5.1948 |
| Sublimation point | 1975 25 C |
| Hardness | 4 moh |
| Dielectric constants | ? S 11 = 8.55, ? S 33 = 10.20 10 ?11 F/m |
| Density | 5.665 10 3 kg/m 3 |
| Thermal expansion coefficient | ? 11 = 4.0, ? 33 = 2.1 ( 10 ?6/ C) |
| Optical transparency | 0.4 2.5 ?m |
| Refractive index | n 0 = 1.9985, n e = 2.0147 ( ? = 6328 ) |
| Electrooptic constant | r 33 = 2.6, r 13 = 1.4 ( 10 ?12 m/V, ? = 6328 ) |
Owing to these excellent piezoelectric properties, thin films of ZnO are used for making ultrasonic transducers in high-frequency regions. In the past fifteen years, many workers have investigated fabrication processes for ZnO, including sputter deposition, chemical vapor deposition,[134] and ion plating.[135] Among these processes, sputter deposition is the most popular for ZnO thin-film deposition.
Polycrystalline thin films. Polycrystalline ZnO thin films with a c-axis orientation are...
