Indirect semiconductor

In many semiconductors employed for LEDs, and especially in mixed alloys, the direct and indirect minima ate separated by smaller energies than those of the purely direct and indirect semiconductors. As a result, finite electron concentrations exist within both minima. The total electron concentration, n, is given by equation 5 ... 

Fig. 2. Electron drift velocities as a function of electric field for A, GaAs and B, Si The gradual saturation of curve B is characteristic of all indirect semiconductors. Curve A is characteristic of direct gap semiconductors and at low electric fields this curve has a steeper slope which reflects the larger electron mobiUty. The peak in curve A is the point at which a substantial fraction of the electrons have gained sufficient energy to populate the indirect L minimum which has a much larger electron-effective mass than the F minimum. Above 30 kV/cm (not shown) the drift velocity in Si exceeds that in...

Serpone N, Lawless D and Khairutdinov R (1995) Size effects on the photophysical properties of colloidal anatase TiOz particles size quantization or direct transition in this indirect semiconductor. J Phys Chem 99 16646-16654... 

Energy band gaps for selected semiconductors are summarized in Table I. On the basis of the nature of the transition from the valence band to the conduction band, semiconductors are classified as direct or indirect. In a direct semiconductor, the transition does not require a change in electron momentum, whereas in an indirect semiconductor, a change in momentum is required for the transition to occur. This difference is important for optical devices such as lasers, which require direct-band-gap materials for efficient radiation emission (7, 8). As indicated in Figure 7, Si is an indirect semiconductor, whereas GaAs is a direct semiconductor. 

Denominators in both components are related to the number of phonons of Ep energy according to the Bose-Einstein statistics [21], Therefore the approximate method for bandgap determination in the case of indirect semiconductors has an intrinsic error of the order of Ep (see Figure 7.7b). 

This chapter reviews important aspects of inorganic LED structures. Section 1.2 introduces the basic concepts of optical emission. Band diagrams of direct and indirect semiconductors and the spectral shape of spontaneous emission will be discussed along with radiative and nonradiative recombination processes. Spontaneous emission can be controlled by placing the active region in an optical... 

The arrows on FIGURE 1 mark the principal phonons and a detailed discussion is provided in the later sections of this book. These indirect interband transitions reflect on the simultaneous absorption or emission of phonons. The mechanism of exciton transition for an indirect semiconductor may involve a bound exciton which may recombine without phonon emission during the transition. This is explained in detail in [4,5]. 

Si is an indirect semiconductor, thus not suitable for the fabrication of optoelectronic devices. Adding optical functionality to Si microelectronics is one of the most challenging problems but may revolutionize communication technology [9,10]. The key device would be an efficient emitter, i.e. a laser. 

Therefore, direct band gap materials, such as GaAs, AlAs, Gai.xAkAs, GaN and others are used for light-emitting diodes and solid-state lasers because in indirect semiconductors the electron-hole recombination in a p/n junction biased in the forward direction produces only negligible light. 

FIGURE 16 Formation of excitons (electron-hole pairs) by the addition of isoelectronic dopants N and ZnO to an indirect semiconductor. The excitons have a high probability to recombine radiatively. [From Flaitz, R. FI., Craford, M. G., and Weissman, R. FI. (1995). In Flandbook of Optics, 2nd ed., Vol. 1, McGraw-Flill, New York. With permission.]... 

In a multivalley indirect semiconductor the fundamental absorption process is phonon-assisted and proceeds by different scattering mechanisms involving EP as well as HP interactions. 

Direct semiconductors have strongly allowed optical transitions, with relatively short radiative rate constants and thus short-lived excited states, which can be highly emissive following excitation. For indirect semiconductors optical transitions are forbidden, absorption coefiftcients are low, they have relatively long radiative lifetimes and therefore potentially long-lived excited states, and deactivation after excitation is not usually emissive. 

Figure 9.6 Band structure of (a) silicon and (b) germanium. Both are indirect semiconductors, that is, valence band maxima and conduction band minima are not aligned in reciprocal space. In Si, the minima of the...

An indirect semiconductor has these extrema at different momenta (Figure 2.7b). 

The direct and indirect behaviors mentioned above are sufficiently important that they deserve special mention. The critical aspects of the energy band structures of these two types of semiconductor are shown schematically in Figure 2.8. The minimum energy of the conduction band in indirect materials is at a different momentum than that of the maximum energy of the valence band. Electrons in the conduction band rapidly relax to the minimum band energy. Holes equally rapidly move to the maximum energy of the valence band. Therefore, electrons and holes do not normally have the same momentum in an indirect semiconductor while in a direct-gap material these momenta are equal. This has consequences for the minority carrier lifetimes and optical properties of semiconductors. 

Theoretically, the opposite dependence of direct and indirect gaps on pressure could be used to convert indirect gap materials to direct gaps. However, a negative pressure (tensile stress) would have to be applied to achieve this conversion. Ceramics, including semiconductors, tend to be weaker in tension than in compression. Even by placing the indirect material in a strained-layer superlattice (see Chapter 7), which can achieve the highest tensile stress levels, it has been impossible to convert indirect semiconductors to direct gaps before the stress is relieved by formation of dislocations or by fracture. 

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