Thin Film Materials Technology: Sputtering of Compound Materials

5.1: OXIDES

5.1 OXIDES

5.1.1 ZnO Thin Films

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.

Table 5.5: Physical Properties of ZnO Single Crystal

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 )


Figure 5.2: Crystal structure of ZnO single crystal.

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.

5.1.1.1 Deposition of ZnO

Polycrystalline thin films. Polycrystalline ZnO thin films with a c-axis orientation are...