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Despite the high cost of precursors, MOVPE have been developed especially for the growth of epitaxy of III-V as well as II-VI and IV-VI semiconducting material for optoelectronic applications (e.g. light-emitting diode, heterojunction bipolar transistor, solar cells, photocathode advanced laser designs such as quantum well and double heterostructures, etc.).

2.4.1.2 Hydride Vapor Phase Epitaxy (HVPE)

Hydride vapor phase epitaxy (HVPE) has been important in the development of a variety of semiconductors including the III-arsenides, the III-phosphides and the III nitrides such as InN. HVPE occurs usually at atmospheric pressure (horizontal or

vertical).

Generally, HVPE using chloride sources provides a high growth rate (> 30µm) compared with that of MOVPE and MBE. Because of the group III element is transported to the substrate as a volatile compound (usually a chloride), this technique is often

referred to as chloride-transported vapor phase epitaxy.

The source material generally used for the HVPE growth of InN is liquid indium as indium source, which will react with HCl and form indium monochloride (InCl) or indium trichloride (InCl3) and ammonia (NH3) or monomethylhydrazine (MMHy) as nitrogen source. The source of InCl is formed by the reaction between metallic In and HCl at 780 oC and the InCl3 is presynthesized and is evaporated from the source boat in the temperature range from 325 to 375 oC (Fig. 2.13) [Tak97a]. The reaction chemistry

NH3

HCl

In (l)

Figure 2-13. Schematics of horizontal hot-wall hydride vapor phase epitaxy chamber. Theoretically, the chlorine in the HVPE chemistry should reduce the amount of

impurities from the system due to the formation of highly volatile species, producing films with low carrier concentrations. However, Si and O impurities from the quartz tube can cause highly n-type films.

2.4.1.3 Plasma Enhanced Chemical Vapor Deposition (PECVD)

Plasma Enhanced Chemical Vapor Deposition (PECVD) is also known as glow discharge chemical vapor deposition. It uses electron energy (plasma) as the activation method to enable deposition to occur at a low temperature and at a reasonable rate. Supplying electrical power at a sufficiently high voltage to a gas at reduced pressures (<1.3 kPa) results in the breaking down of the gas and generates a glow discharge plasma consisting of electrons, ions and electronically excited species.

Uncracked trimethylindium (TEI) carried by Ar or H2 as In source and uncracked N2 as N source are ionized and dissociated by electron impact, and hence generating chemically active ions (radicals). These radicals undergo the heterogeneous chemical reaction at or near the heated substrate surface and deposit the thin film (Fig.2.14) [Cho03].

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Figure 2-14. Schematics of PECVD.

The disadvantage of PECVD is that it requires the use of a vacuum system to generate the plasma, and a more sophisticated reactor to contain the plasma. PECVD is often more expensive and in general has difficulty in depositing high purity films. This is mostly due to the incomplete desorption of by-product and unreacted precursor at low temperatures, especially hydrogen which remains incorporated into the films.

However, PECVD can find applications where technology will balance the cost of fabrication and also where low deposition temperatures are required on temperature sensitive substrates, which can not be met by the conventional CVD.

The main advantage of PECVD over other CVD methods is that the deposition can occur at relatively low temperatures on large areas. It also offers flexibility for the microstructure of the film and deposition to be controlled separately. The ion bombardment can be substituted for deposition temperature to obtain the required film density. Such low temperature deposition is important for applications that involve the use of temperature sensitive substrates.

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2.4.2Molecular Beam Epitaxy (MBE) and Metalorganic Molecular Beam Epitaxy (MOMBE)

MBE is a common technique for thin epitaxial growth of semiconductors, metals and insulators. MBE has the capability of growing device quality layers of semiconductors with atomic resolution. The low growth temperature (300-600 °C) ensures negligible dopant diffusion. The apparatus consists of an UHV cold-wall chamber, independently controlled thermal or e-beam cells to supply the sources, in situ heating and cleaning, and in-situ monitors of growth and chemical analysis.

The simplest method for generating molecular beams is from heated Knudsen cells containing Ga, In, Al or dopant material. Shutters open to allow the molecular beams to leave the cells and the beams are directed at a heated substrate. Thermal beams of atoms or molecules react on a clean substrate surface to grow an epitaxial film under UHV conditions. The atomic species undergo adsorption and migration on the surface. MBE can be also equipped with a number of in-situ probes that monitor the growth real time including RHEED where high-energy electrons are diffracted off the growing surface and imaged to describe the nature of the epitaxy.

In the MBE growth of III-nitrides, the solid sources of the group III elements such as Ga, In, and Al are used in general, but the nitrogen is supplied by the gas source such as N2 and NH3. In general, this type of MBE system is called gas source MBE [Dav97].

When organometallic sources replace the group-III elemental source, it is called metalorganic molecular beam epitaxy (MOMBE) or chemical beam epitaxy [Abe97]. In both types of MBE, the key issue in the growth is the nitrogen source. The dissociation energy of N2 molecules is as high as 9.5 eV, therefore, the supply of N2 gas to the substrate surface with the group-III elemental beams can not induce any growth of the