MOCVD also known as metal-organic vapor phase epitaxy (MOVPE), is one
of the major processes in the manufacturing of semiconductor devices. This technique
enables chemical vapor deposition of thin layers of atoms on flat substrates through
chemical reaction. It is excellent for large-area deposition, composition control and
film uniformity (Jones, 1998). The growth of single crystalline epitaxy InN was
achieved by Matsuoka et al. via this approach in 1989, with the microwave-excited
N2 gas (Matsuoka et al., 1990). In the MOCVD growth, the availability of suitable
precursors with sufficient volatility and stability is important. The precursor needs to
have the appropriate reactivity to decompose thermally (Neumayer and Ekerdt, 1996).
For the synthesis of InN, the common materials such as trimethylindium (TMI) and
NH3 were used as the indium and nitrogen sources, respectively. In addition, N2 can
be used as a carrier gas to promote the thermal decomposition of NH3 gas during InN
crystal growth (Ruffenach et al., 2010; Matsuoka et al., 2002).
Researchers pointed out that the InN crystal growth is dependent on the
MOCVD growth parameters, such as temperature, V/III ratio, and vapor pressure of
the sources (Yamamoto et al., 1994; Tuna et al., 2011). It was found that at the
relatively low growth temperatures (< 400 °C), the deposited thin film was dominated
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concluded that the growth at the low temperatures was impossible because of the
decreased in migration of the deposited materials (Ruffenach et al., 2010; Kadys et al.,
2015). Alternative approaches were proposed to promote the decomposition of NH3
gas, such as by laser-assisted activation and nitrogen plasma (Wintrebert-Fouquet et
al., 2004). However, the findings showed the obtained of poor crystalline quality of
InN and metallic-In was observed (Yamamoto et al., 2006; Wintrebert-Fouquet et al.,
2004). On the other hand, at the growth temperature of around 500 °C, both InN
crystal growth and In-droplets can be observed. The authors suggested that the
increase in temperature is able to promote thermal decomposition of NH3 gas and
prevent the formation of metallic-In. However, further increase in temperature at
around 600 to 650 °C has again caused to the formation of In-droplets. This
phenomena was due to the nitrogen desorption effect which leaved the metallic-In on
the film surface (Xie et al., 2007; Suihkonen et al., 2006). Hence, it can be deduced
that the suitable temperature range for the MOCVD growth of InN is very stringent,
at around 400 to 630 °C.
Although the growth rate could be enhanced with increasing temperature, at a
constant V/III ratio and even a greater flow of TMI will lead to a saturation of InN
formation, while the excess In-source will not able to form InN at the condition with
limited nitrogen source. Several studies demonstrated that variation in V/III ratio has
induced different growth modes and rates, and it is one of the important parameter in
controlling the film properties (Jamil et al., 2008). Matsuoka (1997) showed that at the low growth temperatures (≤ 600 °C), In-droplets were formed on a film surface at the V/III ratio lower than 1.6×104, while the In-droplet was not observed at increasing
V/III ratio to more than 1.6×105. It is known that the decomposition rate of NH3 gas
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is needed to produce an adequate amount of reactive nitrogen source for the chemical
reaction. On the other hand, the growth mechanism of InN is different at high growth
temperatures (approximately 650 °C). The low input V/III ratio is required at high
growth temperature, in which the dissociation of NH3 gas is greatly enhanced. Koukitu
et al. (1999) showed that the high decomposition rate of NH3 gas has led to the
increase of hydrogen partial pressure in the reactor, and reduce the driving force for
the deposition. As a result, the occurrence of negative driving force will induce the
etching mode, leading to InN crystal growth is prohibited. The deposition of InN is
more effective by using inert carrier gas to avoid the increase of H2 partial pressure.
Yang et al. (2002) proposed that the film quality can also be improved by
increasing a reactant gas velocity. Besides, Yamaguchi et al. (1999) reported that the
crystal quality of MOVPE grown InN thin films depends on lattice matching of
substrate and film thickness, where InN film was deposited on various substrates such
as GaN, AlN and sapphire, respectively. It was found that InN grown on GaN exhibits
the best crystalline structure. Furthermore, it was observed that the InN film with
thickness of around 400 to 1200 Å exhibits high screw dislocation, and it was
dominated by grain islands with different crystalline orientations. As the film
thickness exceeds 1200 Å, the residual strain was found to be gradually decreased.
The improvement in the structural properties was explained to be due to the reduction
in dislocation density and degree of disorientation (Lu et al., 2003a). According to
Khan et al. (2008), the MOCVD grown InN epilayers exhibited the excellent electrical
properties and could provide a platform for the future InN device applications. Yang
et al. (2002) reported that the InN thin film with Hall mobility of 250 cm2/Vs and a
carrier concentration of 1×1019 cm-3 at room temperature was obtained. Later,
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mobility of 1100 cm2/Vs and lowest carrier concentration of 4.5×1018 cm-3 was
obtained at a relatively low V/III molar ratio.