Direct bandgap semiconductors

Thin film photovoltaic devices (CdTe is a direct bandgap semiconductor with a bandgap energy of 1.5 eV at room temperature). 

The n parameter equals 1 for direct bandgap semiconductors or 4 for indirect bandgap semiconductors in the case of allowed fundamental transitions [22], Other values of n, 2 or 3, are valid only for forbidden transitions. The proper transformation allows estimation of the bandgap energy, Eg, for both types of crystalline semiconductors. Figure 7.7 presents the procedure of Eg evaluation. 

Figure 7.9 Recombination of electrons and holes in the case of (a) direct bandgap semiconductor and (b) indirect bandgap semiconductor. The energy E is a function of the wave vector k...

The first of these can be treated with a simple equivalent circuit of the form shown in Fig. 104. It is normally assumed that processes (b) and (c) are very fast compared with (d) and (e) and that, if a direct bandgap semiconductor is used, (a) can also be minimised. Under these circumstances, the initial condition is... 

Mercury cadmium teiluride (HgCdTe) is a direct bandgap semiconductor widely used as a material for infrared detectors due to his narrow variable band gap. The achievement of high-performance detectors depends critically on a low surface recombination velocity of the minority carriers. The chemical growth of a passivation oxidized superficial layer in an aqueous Fe(CN)g3- basic solution is studied in this work. The depth profiles of the different elements in the oxidized layer superficial layer and its thickness are studied by X-ray photoelectron spectroscopy. The electrical properties of the interface are evaluated from MIS devices. The conditions of oxidation have been optimized. 

Figure 2 shows the energy band diagram of a direct bandgap semiconductor where is lower in energy than Ex, and spontaneous emission of a photon is the most likely path for recombination. In Fig. 2, the F valley of the conduction band is directly above the holes where the momentum change in the transition is nearly zero. Electrons in the X valley of the conduction band have a different momentum and thus cannot directly recombine with... 

FIGURE 2 Energy vs. momentum band diagram for a direct bandgap semiconductor. (Ex - Er > ksT). The electrons (-) in the r valley of the conduction band recombine with the holes (+) in the valence band, and a photon is emitted to conserve energy. 

The most studied and technologically interesting band is the so-called S-band (S for slow decay times are rather long compared to those of direct bandgap semiconductors). It is electrically excitable and its properties (e.g., emission spectrum, efficiency, chemical activity) can be in principle engineered. Its main characteristics are summarized in Table 3. It originates mostly from exciton recombinations in Si nanocrystals as indicated by polarization memory of PL, PL saturation under... 

Direct-Bandgap Semiconductor see Indirect-Bandgap Semiconductor. 

The nature of the emissive transition. Typically, allowed transitions, such as fluorescence or direct bandgap semiconductor emission, have radiative lifetimes between a few and a hundred ns. Forbidden transitions, such as molecular phosphorescence, have radiative lifetimes longer than ps, usually much longer. 

For Si, in order for an electron at the bottom of the CB to recombine with a hole from the top of the VB, the momentum of the electron must shift from kcb to kyi, (Figure 4.9b). However, this is not allowed by the Law of Conservation of Momentum. Instead, an indirect recombination mechanism must take place, wherein the electron is captured by an interstitial defect with energy E, which facilitates its relaxation to the top of the VB. This process is accompanied by the emission of phonons, or lattice vibrations rather than light emission. In contrast, efficient electron-hole recombination may occur without any change in momentum for direct bandgap materials, resulting in the emission of photons. We will describe some important applications for direct bandgap semiconductors later in this chapter. 

M.G. Burt, S. Brand, C. Smith, R.A. Abram, Overlap integrals for Auger recombination in direct-bandgap semiconductors calculations for conduction and heavy-hole bands in GaAs and htP. J. Phy. C SoUd Stale Phys. 17(35), 6385-6401 (1984)... 

At equilibrium in the dark. Equation 5.6 holds. Application of either a bias or illumination increases norp (or both). For a direct bandgap semiconductor, transitions between the conduction and valence bands involve only the absorption/emission of photons. Transitions between the conduction and valence bands in indirect bandgap semiconductors require both absorption/emission of photons and phonons (lattice vibrations). Since both energy and momentum must be conserved, direct band-to-band transitions in indirect bandgap semiconductors are much less probable because of the additional requirement of momentum conservation, and so values of k tend to be much smaller than for direct bandgap semiconductors. 

Figure 3.2 Optical transitions in a direct bandgap semiconductor on the energy versus momentum (which also represents energy versus density of states though the functional forms deviate) diagram, which is pumped beyond transparency. The transitions 1= 1, 2, 3,...

Where a(v) is the absorption coefficient, B is constant and hv is incident photon energy n is the index that takes different values for different types of semiconductors and = 2 for direct-bandgap semiconductors. 

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