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reflection to judge the crystal quality. Poor surface coverage of InN on Si is observed. It is believed that MOVPE growth of InN directly onto Si was unsuccessful because of formation of a SiNx interfacial layer. The Si substrate surface is nitrided during growth even at a low growth temperature (~ 400 oC) [Bhu03b]. Introduction of TMI into the reactor before flowing NH3 to prevent amorphous SiNx formation has been tried with the result . Although the previous work has not been encouraging, the high quality and low cost of silicon make it a very attractive substrate. The possibility of integrating optoelectronic InN devices with Si electronic devices is also attractive. This study aims to achieve high quality single crystalline InN growth on Si substrates by adjusting the growth conditions.

5.1.2. Substrate Preparation Procedure

Sapphire and GaN/sapphire substrates were degreased in boiling solvent in the following sequence, tri-chroloethylene, acetone, and then methanol for 5 min each. In the case of silicon, an etching step was added after the degreasing step in which the silicon substrate was etched in ammonium bifluoride (95 %) for 2 min to obtain an oxidefree and H-terminated silicon substrate. After degreasing and etching, all substrates were rinsed in de-ionized water and dried under nitrogen flow [Etc01].

5.1.3. Metal-Organic Vapor Phase Epitaxy (MOVPE) Reactor

A horizontal, cold-wall MOVPE reactor (Nippon Sanso) with a RF-induced heated susceptor was used in this study. Trimethylindium (TMI) (99.9995 %, Shipley) and ammonia (NH3) (99.9999 %, Solktronics) were used as precursors and nitrogen was used as a carrier gas. TMI is kept in the bubbler at 20 oC and is transferred into the reactor via carrier gas. Ammonia is carried directly into the reactor without carrier gas. The outer quartz wall is kept at 25 oC by circulating cooling water in the quartz jacket. The pressure

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of the reactor is controlled with a Baratron gauge. A schematic of the MOVPE reactor is shown in Fig. 5.3 and a more detailed description is given elsewhere.

Figure 5-3. Image and schematic of horizontal, cold-wall MOVPE reactor system.

5.1.4. Growth Chemistry and Conditions for InN Growth

The epitaxial growth of InN by MOVPE is a non-equilibrium growth process that relies on vapor transport of precursors to the surface of a heated substrate with subsequent reaction of typically group III alkyls and group V hydrides. The chemicals are transported as a dilute vapor to the surface of the heated substrate where pyrolysis reactions occur [Jac64, Tra78, and Lar85]. The reaction chemistry for deposition of InN

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was reviewed in detail in Chapter 3. The overall reaction involves trimethylindium reacting with NH3 to form InN and the reaction is given by

In(CH4)3 (g) + NH3 (g) = InN (s) + 3 CH4 (g) + 3/2H2 (3-5)

The N/In ratio (volumetric flow ratio of NH3 to TMI) is calculated by assuming ideal gas and solution behavior. Therefore, the volumetric flow ratio of NH3 /TMI at standard temperature and pressure (STP) is equal to the pressure ratio of NH3 /TMI at constant volume and temperature, according the equation:

(5-3)

Given that the total TMI bubbler pressure is kept at 500 Torr and the vapor pressure of TMI is given by

The pressure of MOVPE reactor was 100 Torr during the growth and two different

buffer layers of GaN and InN were also studied to check which buffer layer gave better structural quality InN. The range of growth conditions studied for depositing InN is summarized in Table 5-2. When the flow rate of TMI was kept at 0.26 sccm and the N/In ratio was varied from 3000 to 15,000 in the first growth condition set, indium droplets formed at the surface. To prevent indium droplet formation, a low flow rate of TMI (0.03~0.08 sccm) or a high N/In ratio (20,000~50,000) was used in the second growth condition set. In the second growth condition set, the narrower growth temperature range 530 to 570 °C was selected based on the results on the temperature influence on the InN

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structural quality obtained from the first growth condition set, which showed that the optimal growth temperature was around 550 °C .

Table 5-2. Range of growth conditions examined for growth of InN.

The growth sequence for each substrate is also shown in Fig. 5.4. For Al2O3 (0001), it is generally accepted that nitridation is required to obtain high quality InN by acting as a compliant layer. This effect will be discussed later. For Si (111), nitridation should be avoided because SiNx leads to polycrystalline InN. For GaN/Al2O3 (0001), the effect of nitridation will be discussed later.

Figure 5-4. Indium Nitride (InN) growth sequence for each of the three substrates.