Epitaxy of Compound Semiconductors

Compound semiconductor films have been grown epitaxially on single-crystal insulators (e.g., A1₂0₃, CaF₂) and semiconductor substrates. The latter case has attracted the overwhelming bulk of the attention and, unless indicated otherwise, will be the only systems discussed. The epitaxy of heterojunctions has been crucial in the exploitation of compound semiconductors for optoelec-tronic device applications. The term heterojunction refers to the interface between two single-crystal semiconductors of differing composition and doping level brought into contact. There have been two main thrusts with regard to the epitaxy of heterojunction structures. In the first, a single or generally limited number of different junctions is involved. The intent is to grow suitable binary, ternary, or even quaternary compound layers epitaxially on top of a similar compound substrate, or vice versa. The most common example is the AljGa^jAs-GaAs combination. As we shall see, good epitaxy is ensured because the two lattices are very well matched to each other (i.e., low misfit), even when the atom fraction x of Al substituted for Ga is high. From a device standpoint, it is significant that the energy band gaps of these materials are different; i.e., GaAs has narrower band gap than any of the Al^Ga^^As compounds. This means that charge carriers will be confined to the low-en-ergy-gap GaAs film when ciad by the wider-gap Al^Ga^^As heterojunction barriers. This makes carrier population inversión and láser action possible. Such layered structures are utilized in devices such as lasers (Fig. 7-11) and light-emitting diodes (Fig. 7-12), which have served as light sources in fiber optical communication systems.

Compound Semiconductor Materials

Materials employed for epitaxial optoelectronic devices have been drawn largely from a collection of direct-band-gap III-V semiconductors. Although the discussion will be primarily limited to them, the results are applicable to other materials as well (e.g., II-VI compounds). Table 7-1 contains a list of important semiconductors together with some physical properties pertinent to epitaxy. When light is emitted from or absorbed in a semiconductor, energy as well as momentum must be conserved. In a direct band-gap semiconductor, the carrier transitions between the valence and conduction bands occur without change in momentum of the two states involved. In the energy-momentum or equivalent energy-wave vector, parabola-like (E vs. k) representation of semiconductor bands (Ref. 10) (Fig. 7-13), emission of light occurs by a vertical electrón descent from the minimum conduction-band energy level to the máximum vacant level in the valence band. This is what occurs in the direct band-gap materials GaAs and InP. However, in indirect band-gap semiconductors like Ge and Si, the transition occurs with a change in momentum that is essentially accommodated by excitation of lattice vibrations and heating of the lattice. This makes direct electron-hole recombination with photon emission unlikely. But in direct-band-gap semiconductors, such pro-cesses are more probable, making them far more efficient (by orders of magnitude) light emitters.

In all semiconductors, a becomes negligible once the wavelength exceeds the cutoff wavelength. This critical wavelength \_c is related to the band-gap energy E_g by the well-known relation E - hc/\_c or X_c (pim) = [.24/E_g(eV). For direct-band-gap semiconductors the valué of a becomes large on the short-wavelength side of \., signifying that light is absorbed very cióse to the surface. For this reason even thin-film layers of GaAs are adequate, for example, in solar cell applications. In Si, on the other hand, a varies more gradually with wavelength less than X_c because of the necessity for phonon participation in light absorption-carrier generation processes. Therefore, effi-cient solar cell action necessitates thicker layers if indirect semiconductors are employed.

Optical Communications. Optical communication systems are used to transmit information optically. This is done by converting the initial electronic signáis into light pulses using láser or light-emitting diode light sources. The light is launched at one end of an optical fiber that may extend over long distances (e.g., 40 km). At the other end of the system, the light pulses are detected by photodiodes or phototransistors and converted back into electronic signáis that, in telephone applications, finally genérate sound. In such a system it is crucial to transmit the light with mínimum attenuation or low optical loss. Great efforts have been made to use the lowest-loss fiber possible and minimize loss at the source and detector ends. If optical losses are high, it means that the optical signáis must be reamplified and that additional, costly repeater stations will be necessary. The magnitude of the problem can be appreciated when transoceanic communications systems are involved. In silica-based fibers it has been found that mínimum transmission losses occur with light of approximately 1.3-1.5 ¿im wavelength. The necessity to opérate within this infrared wavelength window bears directly on the choice of suitable semiconductors and epitaxial deposition technology required to fabrícate the required sources and detectors.

Reference to Table 7-1 shows that InP is transparent to 1.3-ítm light, and this simplifies the coupling of fibers to devices. A very cióse lattice match to InP (a₀ = 5.869 A) can be effected by alloying GaAs and InAs. Through the use of Vegard's law, it is easily shown that the necessary composition is Ga₀₄₇In₀₅₃As. In the same vein, high-performance lasers based on the lattice-matched GalnAsP-InP system have recently emerged for optical communications use.

Silicon Heteroepitaxy (Ref. 8). Since the early 1960s, Si has been the semiconductor of choice. Its dominance cannot, however, be attributed solely to its electronic properties for it has mediocre carrier mobilities and only average breakdown voltage and carrier saturation velocities. The absence of a direct band gap rules out light emission and severely limits its efficiency as a photodetector. Silicon does, however, possess excellent mechanical and chemi-cal properties. The high modulus of elasticity and high hardness enable Si wafers to withstand the rigors of handling and device processing. Its great natural abundance, the ability to readily purify it and the fact that it possesses a highly inert and passivating oxide have all helped to secure the dominant role for Si in solid-state technology. Nevertheless, Si is being increasingly sup-planted in high-speed and optical applications by compound semiconductors. The idea of combining semiconductors that can be epitaxially grown on low-cost Si wafers is very attractive. Monolithic integration of III-V devices with Si-integrated circuits offers the advantages of higher-speed signal processing distributed over larger substrate áreas. Furthermore, Si wafers are more robust and dissipate heat more rapidly than GaAs wafers. Unfortunately, there are severe crystallographic, as well as chemical compatibility problems that limit Si-based heteroepitaxy. From data in Table 7-1, it is evident that Si is only closely lattice matched to GaP and ZnS. Furthermore, its small lattice constant limits the possible epitaxial matching to semiconductor alloys.

Epitaxy in ll-VI Compounds (Ref. 16). Semiconductor based on elements from the second (e.g., Cd, Zn, Hg) and sixth (e.g., S, Se, Te) columns of the periodic table display a rich array of potentially exploitable properties. They have direct energy band gaps ranging from a fraction of an electrón volt in Hg compounds to over 3.5 eV in ZnS, and low-temperature carrier mobilities approaching lO⁶ cm²/V-sec are available. Interest in the wide-gap II-VI compounds has been stimulated by the need for electronically addressable flat-panel display devices and for the development of LED and injection lasers operating in the blue portion of the visible spectrum. For these purposes, ZnSe and ZnS have long been the favored candidates. When the group II element is substituted by a magnetic transition ion such as Mn, new classes of materials known as diluted magnetic or semimagnetic semiconduc-tors result. Examples are Cd(Mn)Te or Zn(Mn)Se, and these largely retain the semiconducting properties of the puré compound. But the five electrons in the unfilled 3d shell of Mn give rise to localized magnetic moments. As a result, large magneto-optical effects (e.g., Zeeman splitting in magnetic fields, Fara-day rotation, etc.) occur and have been exploited in optical isolator devices. For this, as well as other potential applications in integrated optics, high-qual-ity epitaxial films are essential.

Ronellys Flores---CRF---libro the materials science of thin films