Biomedical Engineering Reference
In-Depth Information
environment. For biosensing in liquids, it is necessary to generate an SH mode wave, where
the wave displacement is within the plane of the crystal surface (Kadoka and Miura, 2002;
Ya
n
agitani et
al
., 2005). For the generation of such a SH wave, other film textures such as
(
1120
) and (
1010
) are necessary (Yanagitani et al., 2004). Theoretical predictions indicate
that (1) (002), (004) oriented ZnO films generate a pure longitudinal mode; (2) (100), (110),
(200), (211), (300), and (220) oriented films a pure shear mode wave; (3) (101), (102), (103),
(112), (201) oriented films a combination of longitudinal and shear mode waves (Wu et al.,
2008).
Interlayer or Buffer Layer
An amorphous ZnO intermediate layer is normally formed before the growth of the crys-
talline ZnO layer on substrates, such as Ni, Cu, Si, Ti, Ni and glass, and this layer is about
10- to 50-nm thick depending on the type of substrates (Koch et al., 1997; Yoshino et al.,
2000). Self-textured (0001) ZnO films slowly grow on this amorphous layer, which is prob-
ably the reason why the ZnO films deposited on Ni, Cu, and Cr substrates show poor
texture. No such amorphous interlayer has been observed on Au, Ru, Pt, Al, and sapphire
substrates (Kim et al., 2006; Matsuda et al., 2008), and the ZnO films deposited on these
substrates show good (0001) orientation. Buffer layers are frequently used to enhance the
film crystalline quality and texture, as reported for AlN, MgO, Al
2
O
3
, GaN, DLC, SiO
2
, and
so on (see Table 8.5) (Lee et al., 2004; Yoshino, 2009; Jung et al., 2004), and these buffer layers
can also be used to promote the epitaxial growth of ZnO films.
Epitaxial Growth
Epitaxial growth on different substrates can lead to dissimilar ZnO orientations. The
growth of ZnO can be attributed to the competition between the lowest surface free energy
TABLE 8.5
Summary of Velocity of AlN Films on Different Substrates
Lattice
Difference
(%)
Substrate
Materials
Substrate
Materials
TemperatureExpansion
Coeficient(10
−6
K
−1
)
Structure
Velocity(m/s)
AlN/Si
Si (111)
Cubic
5000-5050
3
AlN/Al
Al (001)
Cubic
23.15
23.1
AlN/Pt
Pt (110)
Cubic
3
8.8
AlN/Au
Au (111)
Cubic
14
AlN/W
W (110)
AlN/Mo
0.87
4.8
AlN/SiO
2
Quartz
13.2/7.1
AlN/Sapphire
Al
2
O
3
(0001)
HCP
6000
8.4 (5.3)
AlN/LiNbO
3
LiNbO
3
(0001)
HCP
7.3 (18.1)
AlN/SiC
SiC (0001)
HCP
31.8
6500-7500
AlN/GaN
GaN
HCP
AlN/ZnO
ZnO (001)
HCP
4.1
2.9 (4.751)
AlN/DLC
DLC
Amorphous
AlN/Diamond
Diamond (111)
Cubic
10,000-12,000
1.18
