Semiconductor Impurities, vibrational

Charge carriers in a semiconductor are always in random thermal motion with an average thermal speed, given by the equipartion relation of classical thermodynamics as m v /2 = 3KT/2. As a result of this random thermal motion, carriers diffuse from regions of higher concentration. Applying an electric field superposes a drift of carriers on this random thermal motion. Carriers are accelerated by the electric field but lose momentum to collisions with impurities or phonons, ie, quantized lattice vibrations. This results in a drift speed, which is proportional to the electric field = p E where E is the electric field in volts per cm and is the electron s mobility in units of cm /Vs. 

The heat transfer in a solid is due both to lattice vibrations (phonons) and to conduction electrons. Experiments show that in reasonably pure metals, nearly all the heat is carried by the electrons. In impure metals, alloys and semiconductors, however, an appreciable... 

In this section, I will devote attention to the question how does H move in semiconductors One type of motion consists of vibrations around a particular site in the lattice. Another type of motion is involved in the migration of the impurity through the lattice, in which barriers have to be surmounted (or tunneled through). Once again, most of the available information concentrates on Si. 

M. Stavola, Vibrational Spectroscopy of Light Element Impurities in Semiconductors... 

The specific heat of a semiconductor has contributions from lattice vibrations, free carriers and point and extended defects. For good quality semi-insulating crystals only the lattice contribution is of major significance. Defect-free crystals of group III nitrides are difficult to obtain, and thus the specific heat measurements are affected by the contributions from the free carriers and the defects. While the specific heat of AIN is affected by the contribution of oxygen impurities, the data for GaN and InN are affected by free electrons, especially at very low temperatures. 

At high concentrations of impurities, the band structure of the host material will inevitably be distorted. In general, the conductivity of a metal decreases as the temperature is raised because thermal vibration of the constituents decreases lattice order and hence the ease of movement of electrons. In contrast, thermal promotion of electrons in a semiconductor increases the number of charge carriers, thereby increasing the conductivity with temperature. 

Figure 15-14 (a) A simplified representation of a side view of a perfect crystal of a polar substance of 0 K. Note the perfect alignment of the dipoles in all molecules in a perfect crystal. This causes its entropy to be zero at 0 K. There are no perfect crystals, however, because even the purest substances that scientists have prepared are contaminated by traces of impurities that occupy a few of the positions in the crystal stracmre. Additionally, there are some vacancies in the crystal structures of even very highly purified substances such as those used in semiconductors (see Section 13-17). (b) A simplified representation of the same perfect crystal at a temperature above 0 K. Vibrations of the individual molecules within the crystal cause some dipoles to be oriented in directions other than those in a perfect arrangement. The entropy of such a crystalline solid is greater than zero, because there is disorder in the crystal. 

The absorption of impurity centres is observed in the transparency domains of semiconductors and insulators, which are limited by their intrinsic electronic and vibrational absorptions. Further, a brief account of the relevant physical processes and an overview of the intrinsic optical properties of these materials and of their dependence on temperature, pressure and magnetic field is given in this chapter. Some semiconductors have been or are now synthesized in quasi-monoisotopic (qmi) forms because of improvements in their physical properties like thermal conductivity. A comparison of their intrinsic optical properties with those of the crystals of natural isotopic composition is also given. The absorption related to free carriers, due mostly to doping is also discussed at the end of this chapter. A detailed account of the optical properties of semiconductors can be found in the books by Yu and Cardona [107] and by Balkanski and Wallis [4]. 

A detailed presentation of the piezospectroscopy of semiconductors can be found in [124]. Uniaxial stress is the most easily-produced perturbation (for experimental details, see Sect. 4.7.1), and the spectroscopy performed under stress is called piezospectroscopy. The relevant piezospectroscopic parameters for an impurity line are the number of components observed, their polarization characteristics and the amplitude of their shifts and splitting as a function of the value of the stress. Piezospectroscopy is useful when studying degenerate electronic transitions of the EM-like centres as it can lift intrinsic degeneracy. It can also lift the orientational degeneracy of electronic (and vibrational)... 

In the past five decades, a number of donors, acceptors, and their complexes with excitons in semiconductors have been discovered and delineated. Isoelectronic impurities and their localized vibrational modes have also been extensively studied in infrared absorption and Raman and luminescence spectroscopies. 

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