Metal Organic Vapor Phase Epitaxy
MBE and MOVPE are the main epitaxial methods used today. For a long time MOVPE was thought to be more of a production instrument while MBE was the scientific tool. This happened because MBE could be dealt with straightforwardly with the available surface
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MBE and MOVPE are the main epitaxial methods used today. For a long time MOVPE was thought to be more of a production instrument while MBE was the scientific tool. This happened because MBE could be dealt with straightforwardly with the available surface science know-how while MOVPE was subject to the poor knowledge of the underlying chemistry, hydrodynamics and moreover surface analytical tools were not available. This situation has changed and both methods can be utilized for growing structures well defined on the atomic level. There are differences however which give preference to one of the methods for certain problems. Because of the large variety of chemical compounds for example MOVPE is essentially capable of depositing all solids while MBE with some elements (phosphorus, nitrogen) is problematic. On the other hand diffusion in MOVPE seems to be in general faster with the consequence of larger growth rates and thus the growth of very small structures (quantum dots) requires larger effort in MOVPE than in MBE. The larger possible growth rates together with the quasi-infinite supply of precursors makes MOVPE of course the ideal method for very thick layers in the micrometer regime and also for industrial production. Therefore the question of which ofthe methods should be used should be answered problem oriented. There are many excellent reviews on MOVPE and we specifically recommend also looking through the recent proceedings of the International Conference on MOVPE (ICMOVPE) [8.1-5] which give an excellent overview on the state of art of MOVPE. In this chapter we will follow the reviews of Stringfellow [8.6], Kisker and Kuech [8.7] as well as Richter and Zahn [8.8].
8.1 Basic Concepts The MOVPE process starts with a gas mixture which contains the molecular compounds, termed precursors, necessary for growth, and a carrier gas (Fig. 8.1). The latter is usually hydrogen (for special reasons also nitrogen), with an operating pressure between 103 Pa and atmospheric pressure (l05 Pa). For reasons of uniform deposition usually nowadays low pressures are preferred. This is a consequence of the more laminar flow pattern and the more homogeneous temperature field within the reactor (Sect. 6). The choice M. A. Herman et al., Epitaxy © Springer-Verlag Berlin Heidelberg 2004
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8. Metal Organic Vapor Phase Epitaxy
10-3 bar AsH3 10-4 bar Ga(CH 3h 10-3 -1 bar carrier gas
Fig. 8.1. Schematic picture of a MOVPE epitaxial growth process using the example of GaAs growth (taken from [8.8])
of precursors depends on the material to be deposited. For 111-V semiconductors the standard precursors are, for example, metalorganic compounds of the group III elements, like trimethylgallium (Ga(CH3h) (in abbreviated form TMGa), and hydrides of the group V elements, e.g., arsine (AsH 3). Thus, the general chemical reaction in the case of MOVPE of 111- V semiconductor compounds is of the type:
(8.1) where R is an alkyl or other organic group, X is usually a hydrogen atom, but can also be an organic radical or even a halogen atom, a
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