The second half of the twentieth century has experienced the semiconductor technology in all industrial fields. Although the majority of the revolution has been based on Silicon and its constituents, increasing demands in terms of speed, performance and functionality have generated great interests in alternative semiconductor materials. Among the most noteworthy materials, III-V, compound semiconductors such as GaAs and related alloys are the most developed in the view of fabricating devices. At present, III-V compound semiconductors provide the basic material for a number of well-established commercial technologies, as well as for fabricating novel electronic and optoelectronic devices. For examples, they include high electron-mobility transistors, heterostructure bipolar transistors, diode lasers, blue and white light-emitting diodes, photo-detectors, frequency-mixing components, electro-optic modulators and FIR detectors. The operating characteristics of these fabricated devices depend on the physical properties of the constituent materials which are confined into regions of the order of a few nanometers. The major advantages of ternary compounds compared with respect to binary compounds are the possibility to tune in the range of the band gap, effective mass, and the Landé g-factor by changing the alloy composition.
One of the most concentrated compound semiconductor heterostructures is Ga1?xInxAs, for optical and electronic applications. This material is lattice matched with InAs at an indium concentration 0.53 and is largely used today in light-emitters for optical fiber communications since its emission wavelength of 1.55 ?m is within the optimum transmission window of silica fibers. This special feature makes this system appealing for studying the electrical, optical, magnetic, and transport properties. Moreover, the Landé g-factor of the system increases towards the value of InAs at high indium concentrations. Eventually, it results from a large Zeeman spin-splitting under the application of weak magnetic fields, which attains this compound a promising candidate for the study of spintronics especially for spin-valve mesoscopic devices at relatively high temperatures. The high Landé g-factor of Ga1?xInxAs would make spin manipulation in Ga1?xInxAs based nanostructures more efficient, with respect to GaAs based semiconducting materials. The only drawback is the lack of substrates with suitable lattice parameter while developing these materials for making device fabrications. However, one must take into account the strain effects in order to bring these materials into real structures with good electrical properties.
Semiconductor crystals have characteristic electronic properties. The periodic array of the atom that forms the semiconductor crystal creates a periodic Coulomb potential. If an electron is free to move in the material, its motion is influenced by the presence of the periodic potential. Typically, electrons with energy near the conduction-band minimum or energy near the valence-band maximum have an electron dispersion relation. The kinetic energy of the electron in the crystal may, therefore, be written as E(k)= (?^2 k^2)/?2m?^* where m* is the electron effective mass. The value of m* can be greater or lesser than the mass of a bare electron moving in free space. Because different semiconductors have different periodic Coulomb potentials, different semiconductors have different values of effective electron mass. For example, the conduction-band effective electron mass in GaAs is approximately m* = 0.067m0 where m0 is the bare electron mass.
Semiconductor technology makes use of extrinsic methods to increase electrical conductivity to a carefully controlled and predetermined value. Electrons that are free to move in the material may be introduced into the conduction band by adding impurity atoms. This extrinsic process, called substitutional doping, replaces atoms of the pure semiconductor with impurity atoms, the effect of which is to add mobile charge carriers.
Epitaxial semiconductor crystal growth techniques may be used to grow thin layers of different semiconductor materials on top of one another to form a heterostructure. The interface between the two different materials is called a hetero-interface. By carefully designing a multi-layer heterostructure semiconductor, it is possible to create a specific potential as a function of distance in the energy bands. This periodically layered heterostructure forms a superlattice. New types of electronic and photonic devices can be designed. These devices make use of electron motion through potentials that change rapidly on a length scale comparable to the wavelength associated with the electron; these devices operate on the rules of quantum mechanics.
1.2 LOW-DIMENSIONAL SEMICONDUCTOR STRUCTURES
III-V compound semiconductors are given due interest in all the solid state device fabrications due to their optoelectronic properties. In order to have better optoelectronic properties of these materials, various low dimensional semiconductor nanostructures such as quantum wells, wires, and dots are developed and investigated to make use of quantum confinement effects. Compared to bulk materials, a quantum well structure is formed when a thin film with lower band gap energy is embedded on a barrier material having larger band gap energy. The thickness of the film is expected to be smaller than the de Broglie wavelength of electrons in the bulk material. Depending on the materials composition, the de Broglie wavelength is typical of the order of 100Å, and the electron is confined in one dimension giving rise to new exotic electronic properties. The energy dependence of the density of states changes from square-root to step-like dependence as mentioned below. Further reduction of the dimensionality makes these advantages more remarkable due to a more peak-shaped density of states as in quantum well wire systems. The structures in which carriers are confined in two or three dimensions are called quantum wires (1D free motion) or quantum dots (0D free motion).