P4 4 InCl 2 H2 4 InP 4 HClEq

To achieve the growth of multilayered structures, the substrate must be moved between reactors that use different chemistries. It is therefore difficult, if not impossible, to achieve layer thicknesses on the order of 5 nm (50 A), as required for quantum-well or superlattice structures. Fine compositional manipulation favors the techniques of MBE or MOCVD, where the transport of source materials, rather than substrates, is controlled.

The MOCVD process uses at least one metal-organic chemical as a deposition precursor. The growth of Group III-V compounds from metal-organic and hydride sources was first reported by Manasevit in 1960 (Ref 2). In that experiment, InP was deposited from trimethylindium (TMIn) and PH3 in a closed-tube system. Later, it was established that many common compound semiconductors could be deposited from metal-organic materials (Ref 3, 4, 5, 6). Manasevit coined the MOCVD acronym, which is used in this article. Other authors refer to organometallic CVD (OMCVD). When applied to the growth of epitaxial films, this technique is sometimes called metal-organic vapor-phase epitaxy or organometallic VPE. As used for the epitaxial growth of compound semiconductors, MOCVD has advanced rapidly since the mid-1980s. It has become established as a unique and important epitaxial crystal growth technique, yielding high-quality, low-

dimension structures for fundamental semiconductor physics research and production of useful electronic and photonic semiconductor devices.

A typical MOCVD reactor is shown in Fig. 1. Gaseous precursors are introduced to the reaction chamber from the gas manifold. To achieve the growth of Group III-V compound semiconductors, Group III alkyls and Group V hydrides can be introduced to the reaction chamber. The substrate, located on a hot susceptor, has a catalytic effect on the decomposition of the gaseous products. Growth occurs primarily on this hot surface. The MOCVD technique is attractive because of its relative simplicity, when compared with other growth methods. Excellent control over film composition can be achieved by precisely metering the amounts of gaseous species introduced to the chamber. Thus, MOCVD can be used to produce heterostructures, multiquantum wells, and superlattices with very abrupt transitions in composition, as well as alloys with tailored doping profiles.

Fig. 1 Typical reactor design for metal-organic chemical vapor deposition. Source: Ref 7

Uniform layers with low background doping densities and sharp interfaces have been grown using MOCVD. Some reactor configurations in use are scalable to large areas, and are therefore attractive for commercial applications. This technique also has been used to produce multilayer structures with layers as thin as a few atomic layers (Ref 8, 9). These abrupt changes in composition produce quantum size effects (Ref 10, 11), and permit the study of two-dimensional electron gases (Ref 12, 13, 14, 15), two-dimensional hole gases (Ref 16, 17), and other charge-transport effects observed in a variety of Group III-V compound semiconductors, heterojunctions, and multilayers (Ref 18, 19). It is also possible to tailor the doping level or alloy composition of ternary and quaternary compounds. Varying the film composition results in a change in the bandgap (Ref 20). This ability to engineer the bandgap has created entirely new classes of electronic and photonic devices (Ref 21). Another recent advance is the ability to grow strained-layer superlattices, in which the crystal lattices of the two materials are purposely mismatched to produce a built-in strain in each layer (Ref 22, 23, 24, 25, 26).

A major disadvantage of MOCVD is the need for large quantities of toxic gases, such as AsH3 and PH3. However, less hazardous precursors, such as tertiarybutylphosphine (TBP) (Ref 27, 28) and tertiarybutylarsine (TBAs) (Ref 29, 30), are being developed to address this problem.

The MBE technique has been used to prepare epitaxial films of Group IV (Ref 31, 32), III-V (Ref 33, 34), and II-VI (Ref 35, 36) semiconductors, as well as metals (Ref 37, 38). In this technique, elemental sources are evaporated at controlled rates by heating and then condensed onto a crystalline substrate surface held at a suitable temperature. This is an ultrahigh vacuum (UHV) technique, in which the beams of evaporated molecules or atoms are focused on the substrate. UHV conditions are necessary to ensure sufficient film purity. The use of UHV has two advantages. First, atoms and molecules reach the growth surface in a very clean condition. Second, the growth process can be monitored by in-situ diagnostic techniques as the crystal grows one atomic layer at a time (Ref 39). The diagnostic techniques that are used include reflection high-energy electron diffraction (RHEED), Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), secondary-ion mass spectroscopy (SIMS), and ellipsometry.

The MBE technique is recognized as an excellent crystal growth technology for the production of complex and varied structures, especially for GaAs-based multilayer structures. It provides extremely precise control of layer thickness and doping profile. However, this technique is expensive because it requires UHV apparatus.

Another major problem is the incorporation of specific dopants. The typical approach is to add dopants by effusion of the elemental impurity, which leads to difficulty when the dopant vapor pressure is either too high (Ref 40) or too low (Ref 41) to be handled conveniently in an ultrahigh vacuum system. Because the MBE process is a near-equilibrium one, dopant incorporation can be hindered by low incorporation probabilities (Ref 42) or surface segregation effects (Ref 43). Phosphorus is one such dopant that suffers from a low incorporation probability. Instead of becoming incorporated into the growing film, it tends to "bounce around" the system, and eventually collects in the vacuum pumps. In addition, the growth of alloys containing both arsenic and phosphorus is particularly difficult because of surface segregation effects.

Hybrid MBE and CVD Techniques. Versatile growth techniques have been developed by combining the beneficial aspects of MBE and CVD. These hybrid techniques employ the gas-handling system of MOCVD and the growth chamber of MBE.

To overcome the limitations of growing phosphide compounds, elemental Group V sources are replaced by those that can decompose AsH3 and PH3. This technique is called gas-source molecular-beam epitaxy (Ref 44). In addition, Group III elemental sources can be replaced by simple metal-organic compounds to create the metal-organic molecular-beam epitaxy growth technique (Ref 45, 46). This technique is also known as chemical-beam epitaxy or metal-organic chemical-beam deposition (MOCBD).

The MOCBD technique has numerous advantages, when compared with MOCVD, including:

• Use of a fraction of the amount of PH3 and AsH3 for the growth of GaAs- and InP-based materials and the elimination of chemical waste-disposal systems, such as the scrubbers normally used in MOCVD

• Elimination of parasitic reactions in the gas phase because of UHV conditions

• Possible use of in-situ surface diagnostic techniques

• Improvement in homogeneity, composition uniformity, and reproducibility of InP-based materials over large-area substrates

• In-situ etching and removal of oxide during the growth of InP on silicon substrates

• Reduction of contamination from previous chemistries (the memory effect) during p-type doping

• Reduction of boundary-layer thickness on hot substrate surfaces

• Compatibility with other high-vacuum, thin-film processing techniques, such as plasma etching, metal evaporation, ion-beam etching, and ion implantation

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