Direct semiconductor

Radiative recombination of minority carriers is tlie most likely process in direct gap semiconductors. Since tlie carriers at tlie CB minimum and tlie VB maximum have tlie same momentum, very fast recombination can occur. The radiative recombination lifetimes in direct semiconductors are tlius very short, of tlie order of tlie ns. The presence of deep-level defects opens up a non-radiative recombination patli and furtlier shortens tlie carrier lifetime. 

Light is generated in semiconductors in the process of radiative recombination. In a direct semiconductor, minority carrier population created by injection in a forward biased p-n junction can recombine radiatively, generating photons with energy about equal to E. The recombination process is spontaneous, individual electron-hole recombination events are random and not related to each other. This process is the basis of LEDs [36]. 

Direct and Indirect Energy Gap. The radiative recombination rate is dramatically affected by the nature of the energy gap, E, of the semiconductor. The energy gap is defined as the difference in energy between the minimum of the conduction band and the maximum of the valence band in momentum, k, space. Eor almost all semiconductors, the maximum of the valence band occurs where holes have zero momentum, k = 0. Direct semiconductors possess a conduction band minimum at the same location, k = O T point, where electrons also have zero momentum as shown in Eigure la. Thus radiative transitions that occur in direct semiconductors satisfy the law of conservation of momentum. 

Semiconductors can be divided into two groups direct and indirect band gap materials. In direct semiconductors the minimum energy in the conduction band and the maximum in the valence band occur for the same value of the electron momentum. This is not the case in indirect materials. The difference has profound consequences for the transitions of electrons across the band gap in which light is emitted, the radiative transitions, of interest here. 

Tellurium and cadmium Electrodeposition of Te has been reported [33] in basic chloroaluminates the element is formed from the [TeCl ] complex in one four-electron reduction step, furthermore, metallic Te can be reduced to Te species. Electrodeposition of the element on glassy carbon involves three-dimensional nucleation. A systematic study of the electrodeposition in different ionic liquids would be of interest because - as with InSb - a defined codeposition with cadmium could produce the direct semiconductor CdTe. Although this semiconductor can be deposited from aqueous solutions in a layer-by-layer process [34], variation of the temperature over a wide range would be interesting since the grain sizes and the kinetics of the reaction would be influenced. 

Microcrystallites of direct semiconductors usually show a simple exponential decay of the PL intensity P with time, with time constants r in the ps and ns range at RT. A similar simple exponential decay (r = 20ms at 2 K) is observed for PL from mesoporous silicon of high porosity, which shows a weak confinement effect... 

InSb is a direct semiconductor, and quantum dots of InSb, made under ultra-high vacuum conditions, have already been successfully studied for laser appHcations [32]. Quantum dots are widely under investigation nowadays and this is a rapidly growing research field. Definite electrodeposition from ionic Hquids would be an important contribution. 

As observed for direct semiconductors, the CdTe absorption spectrum is size dependent. A red shift in the absorption spectrum with increasing the particle size... 

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. 

If the lateral distances of the atoms are changed, this is called surface reconstruction. This is, for instance, observed with the (100) faces of Au, Ir, Pt, and W. Figure 8.4 shows two types of surface reconstruction that lead to a doubling of the lattice spacing in one direction. Semiconductor surfaces tend to exhibit surface reconstruction due to the directional character of the dangling bonds at the surface. 

The probability of direct electron-hole recombination even in direct semiconductors is low and various defects facilitate the light emission processes. In particular... 

For direct bandgap materials, the electron-hole recombination may occur without any change in momentum, resulting in the emission of photons. We will describe some important applications for direct semiconductors later in this chapter. 

The low-index layer is perforated by many small ohmic contacts that cover only a small fraction of the entire area. The array of microcontacts allows the electrical current to pass through the dielectric layer. Assuming that the ohmic contacts have an area of 1 % of the reflector, and that the alloyed ohmic contact metal is 50% reflective, the reflectivity of the ODR is reduced by only 0.5%. The ODR described here can be used with low-cost Si substrates or metal substrates using conductive epoxy or a metal-to-metal bonding process. These bonding processes have much less stringent requirements than direct semiconductor-to-semi-conductor wafer bonding processes. 

Figure 4-3. (a) Indirect and (b) direct semiconductor bandgap diagrams showing photon transition. 

Germanium is an important elemental semiconductor which exhibits an indirect band gap of 0.67 eV at room temperature in the microcrystalline phase. In contrast to the microcrystalline element, nanocrystalline germanium is a direct semiconductor and a promising material in the optoelectronic industry. The... 

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. 

In narrow-bandgap direct semiconductor of the Hgi cCd cTe and InSb type the dominant processes are Auger 1 (CCCH) and Auger 7 (CHHL) (Fig. 1.4). Here, the letters in the notation denote the bands containing the carrier taking part in the Auger process. The first two letters denote the initial state, and the second two the final one. C means the conduction band, H is the heavy holes band, and L is the light holes band. S means the spin split-off band. 

---