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carrier gas in the growth of InN. These theoretical and experimental results confirm that using a N2 carrier gas is preferred (and widely used) for successful InN growth.

2.4 Indium Nitride (InN) Growth Techniques

In this part, we review the several growth techniques commonly used for InN growth each of which deals briefly with the characteristics of the reaction system, the precursors, the chemistry, the applications, the disadvantages and advantage of each growth technique.

2.4.1 Chemical Vapor Deposition (CVD)

CVD involves the dissociation and/or chemical reactions of gaseous reactants in an activated (heated, plasma etc.) environment, followed by the formation of a solid film. The deposition involves homogeneous gas phase reactions, which occur in the gas phase, and heterogeneous chemical reactions which occur on a heated surface leading to the formation of epitaxial films.

In general, the CVD process involves the following key steps as shown in Fig. 2.11 [Cho00b, Cho03].

(1)Generation of active gaseous reactant species.

(2)Transport of the gaseous species into the reaction chamber.

(3)Gaseous reactants undergo gas phase reactions forming intermediate species:

(4)Absorption of gaseous reactants onto the heated substrate, and the heterogeneous reaction occurs at the gas—solid interface (i.e. heated substrate) which produces the deposit and by-product species.

(5)The deposits will diffuse along the heated substrate surface forming the crystallization centre and growth of the film.

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(6)Gaseous by-products are removed from the boundary layer through diffusion or convection,

(7)The unreacted gaseous precursors and by-products will be transported away from the deposition chamber.

Figure 2-11. Schematic illustration of the key CVD steps during deposition.

2.4.1.1 Metal-Organic Vapor Phase Epitaxy (MOVPE)

Metalorganic Vapor Phase Epitaxy (MOVPE) is one growth method among CVD, which has been classified according to the use of metalorganics as precursors. Compounds containing metal atoms bonded to organic radicals are known as “Metalorganics”. MOVPE can be used to deposit a wide range of materials in the form of amorphous, epitaxial, and polycrystalline films.

The schematic of MOVPE was shown in Fig. 2.12 where TMI is delivered by N2 carrier gas and NH3 is also delivered directly into MOVPE reactor and the thermal environment for the decomposition and/or deposition reaction of the precursors can be supplied using resistance heating, radio-frequency or infrared lamp heating. MOVPE tend to involve endothermic reactions, thus cold-wall reactors with a single temperature zone can be used.

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Figure 2-12. Schematic of horizontal cold-wall MOVPE system.

The metalorganic precursors generally undergo decomposition or pyrolysis reactions. In general, metalorganics precursors have lower decomposition or pyrolysis temperatures than halides, hydrides or halohydrides. Thus, metalorganic precursors enable MOVPE process to perform at a lower deposition temperature than conventional CVD, which generally uses halides or hydrides.

The source materials generally used for the MOVPE growth of InN, are trimethylindium (TMI) as In source, and ammonia (NH3) as N source.

The pyrolysis of TMI in MOVPE was first studied by Jacko and Price who founded that the decomposition occurred in three steps as each of the In-CH3 bonds were broken at the temperature above 400 oC [ Jac64]. The methyl radicals thus formed were then found to recombine to yield ethane (C2H6). This mechanism is given by Eq. (2-13) to (2- 16).

The other method was suggested by Koukitu and the reaction procedure is given as

follows [Kou97b]. First, thermodynamically, almost all the NH3 is decomposed into N2 and H2 at temperatures higher than 300 oC. However, it is well known that the decomposition rate of NH3 under typical growth conditions is slow without a catalyst and the extent of the decomposition strongly depends on the growth conditions. The mole

The chemical reaction which occurs at the substrate surface to form InN is the same as the former method given in Eq.(2-17).

MOVPE can be performed at atmospheric pressure and low pressure (about 2.7- 26.7 kPa). For a typical MOVPE process, the deposition is entirely kinetically controlled at very low deposition pressure (< 1 kPa), even though the deposition temperature is relatively high. At pressures above 1 kPa, the growth rate is predominantly controlled by diffusion-rate limited mechanism [Dup95].