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optimum conditions for achieving a particularly measure of film quality are different fro another film property. The quality factors included in this study are crystalline quality (XRD), surface roughness (AFM, SEM), background impurity level (PL, AES), and carrier type and mobility (Hall measurement). These factors are studied for each substrate and buffer layer.

5.1.1. Substrate Selection

The selection of substrate is one of the most important considerations for epitaxial growth. In this study, three substrates, Si (111), Al2O3 (0001), and GaN (5µm)/Al2O3

(0001), are explored for the epitaxial growth of InN. Since Al2O3 and Si have different crystal structures than InN, as well as different lattice constants values and thermal expansion coefficients, heteroepitaxy of InN on these substrates is expected to be a challenge. The mismatch in lattice constant important in heteroepitaxy is that which occurs at growth temperature, since growth mechanisms are influenced by the lattice spacing at growth temperature. Subsequent adhesion and cracking issues upon cooling are determined in part by the mismatch in thermal expansion coefficients.

Upon cooling, the mismatch in thermal expansion coefficients can lead to increasing strain, and possibly producing cracking. The lattice constants and thermal expansion coefficients of the substrates and InN are summarized in Table 5-1[aMor94, bDav02]. The lattice mismatch (f) between InN and substrate is defined by

where af is the lattice constant of film material (InN) in the bulk or unstrained state, as is the lattice constant of substrate.

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The linear thermal expansion coefficient (TEC) mismatch between InN and

substrate is calculated in the same way by

where af is the thermal expansion coefficient of film material in the bulk or unstrained state and as is the thermal expansion coefficient of substrate.

When the GaN film is rotated by 30o with respect to the sapphire substrate, the

minimum lattice mismatch in the GaN/Al2O3 system occurs so that the (0001)//(0001),

−−

[1010 ]//[1 2 10] GaN/Al2O3 interface is formed and the lattice constant of a = 5.524 Å at

room temperature as shown in Fig. 5.1[She02]. Bulk Si crystal has a diamond structure

with lattice constant of a = 5.43 Å at room temperature. The Si (111) surface, however,

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presents an equivalent hexagonal surface with lattice parameter a = 3.84 Å as shown in Fig. 5.2 [Shu00].

− −

Figure 5-1. Schematic for the deposition of the (0001)//(0001), [1010 ]//[1210 ] GaN/Al2O3 system.

Figure 5-2. Planes of Si (111) substrate.

5.1.1.1. Sapphire (c-Al2O3 (0001))

Sapphire is the most widely used substrate for the epitaxial growth of InN. Relatively large area, good quality crystals of sapphire are commercially available at a reasonable cost. Sapphire is transparent and stable at high temperature. High quality epitaxial InN films can be grown easily on sapphire substrates by popular growth methods such as MOVPE and MBE. Sapphire, however, has a large lattice mismatch of 25.7 % with InN. This large lattice mismatch and thermal expansion coefficient difference can result in an extremely high density of structural defects. Nevertheless, this

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disadvantage may be partially overcome with substrate pretreatment and buffer layers [Bhu03b]. For example, nitridation of the sapphire substrate surface significantly improves the crystalline quality of InN as a result of the formation of an AlN interfacial layer, which reduces the lattice mismatch from 25.7 % for InN/c-Al2O3 to 13 % for InN/AlN. Additionally, the InN/AlN has a comparatively small thermal expansion coefficient mismatch of 8.1 %.

This study aims to improve the crystallinity of InN on sapphire substrates through use of different buffer layers, different buffer layer temperature, different growth temperature, post-growth annealing, and modification of the inlet tube.

5.1.1.2. Gallium Nitride (GaN/c-Al2O3 (0001))

Gallium nitride has a small lattice mismatch of 10.9 % with InN compared with sapphire (25.7 %) and AlN (13 %) substrates, although it is greater than that of silicon (7.9 %). In addition, gallium nitride substrates generally lead to good coverage of InN, which is very difficult to achieve with silicon substrates. It was reported that the highest mobility of InN (~ 700 cm2/V s) was obtained from MOVPE on GaN substrates [Yam99a]. Therefore, this study explores the feasibility of the gallium nitride on sapphire substrate for the epitaxial growth. The thickness of the GaN layer on Al2O3 is 5 µm.

5.1.1.3. Silicon (Si (111))

Silicon is an excellent candidate as a substrate for the epitaxial growth of InN because it has a smaller lattice mismatch, 7.9 % for InN (0001)/Si (111), compared with the commonly used insulating sapphire substrates (25.7 % for InN (0001)/c-Al2O3

(0001), and 10.9 % for InN (0001)/GaN (0001)).

Si has been used as a substrate for the epitaxial growth of InN, but the film quality has been very poor to date and there has been no report of the FWHM of XRD principal