Indirect gap semiconductor

The occupied bands are called valence bands the empty bands are called conduction bands. The top of tire valence band is usually taken as energy zero. The lowest conduction band has a minimum along the A direction the highest occupied valence band has a maximum at F. Semiconductors which have the highest occupied k -state and lowest empty state at different points are called indirect gap semiconductors. If k = k, the semiconductor is call direct gap semiconductor. Gennanium is also an indirect gap semiconductor whereas GaAs has a direct gap. It is not easy to predict whether a given semiconductor will have a direct gap or not. 

Indigoid soluble dyes, 7 373t Indigo vat dye, 9 181 Indirect-arc furnaces, 12 297—298 Indirect coal liquefaction, 6 858-867 Indirect cooler evaporators, 21 537 Indirect extrusion, copper, 7 693 Indirect food additives, 12 29, 34 categories of, 12 31 Indirect-gap semiconductors, 14 837 ... 

Harrison D., Abram R. A. and Brand S. (1999), Characteristics of impact ionization rates in direct and indirect gap semiconductors , J. Appl. Phys. 85, 8186-8192. 

The expression for AEg (eq. 5.4) has often been nsed to calcnlate the size of small semicondnctor particnlates for comparison with sizes measnred by transmission electron microscopy. Expression 5.4 works well for particles of diameter above -50 A (see e.g. Serpone et al., 1995a). The reasonable congmence notwithstanding, there are other plausible explanations for the observed bine shifts in the bandgap. Studies by Serpone and co-workers (1995a, 1995b) contended that the earlier reports of bine-shifted absorption thresholds, taken as evidence for Q-size effects in small Ti02 particles, may in fact be direct (Franck-Condon) electron transitions in an otherwise indirect-gap semiconductor. 

In germanium and in other indirect-gap semiconductors, the direct T8+ —> r7 transitions from the VB to CB can be detected in the fundamental absorption region by an increase of the absorption cross-section, as shown in Fig. 3.5. 

Choosing the z axis along klnm and taking the electron energy origin at km n, the energy of a conduction electron near kmin for the indirect-gap semiconductors of Table 3.4 is ... 

In indirect-gap semiconductors, this phonon-assisted electronic absorption is revealed by kinks in the vicinity of the electronic absorption edge. They are due to the different energies of the momentum-conserving phonons involved as well as to the above-discussed different phonon processes. The evolution of this near band gap absorption with temperature can be seen in Fig. 3.7... 

In heavily doped indirect-gap semiconductors, it is possible to conserve momentum by a scattering process and phonon assistance becomes unimportant. The absorption coefficient becomes... 

Efficient recombination occurs in direct-gap semiconductors. The recombination probability is much lower in indirect-gap semiconductors because a phonon is required to satisfy momentum conservation. The radiative efficiency of indirect-gap semiconductors can be increased by isoelectronic impurities, e. g. N in GaP. Isoelectronic impurities can form an optically active deep level that is localized in real space (small Ax) but, as a result of the uncertainty relation, delocalized in k space (large Ak), so that recombination via the impurity satisfies momentum conservation. 

Nanoparticles of Si, BN, and SiC encapsulated in zeolites, such as ZSM-5, L, APO-5, and MCM-41, have been successfully prepared and strong visible photoluminescence (PL) have been observed at room temperature. Analysis of the PL spectra leads us to proposing that the transformation from indirect-gap semiconductor materials to quasi-direct-gap ones by encapsulation in the channels of zeolites is the possible origins of the strong PL. 

Silicon is the most widely used material in the electronics industry. To develop silicon-based devices for optoelectronic applications, one would like to make silicon a photon-emitting material. Unfortunately, silicon is an indirect gap semiconductor and, thus, the efficiency of photon emission is extremely low since the radiative recombination of the electron-hole pair is not allowed without the assistance of a momentum-conserving phonon. Moreover, the existence of defects leads to an almost total quenching of this rather unlikely process. 

Although transient absorption techniques are a sensitive probe of electron population dynamics in several systems, there are numerous situations where these techniques are not directly applicable. For example, type-II core/shell nanocrystals exhibit absorption spectra with indistinct features due to the occurrence of a number of accidentally degenerate transitions. Transient absorption methods also fail to shed much light on VB dynamics in most II-VI materials, presumably due to a higher density of state in the VB. Transient absorption is also ill-suited for probing dynamics in indirect gap semiconductor nanocrystals such as silicon where the band edge absorption is weak and unstructured. These problems make it necessary to supplement such studies with other alternate probes of carrier dynamics. 

However, R-doped semiconductors have evolved into a whole new field due mainly to the hope of obtaining silicon-based optoelectronic devices (Polman et al. 1993), that is, even using an indirect-gap semiconductor with no active optical property of its own. This field has been recently reviewed by Pomrenke et al. (1993). 

Diamond is an indirect-gap semiconductor, the lowest minima of the conduction band being located along the A axes. The valence band has the structure common to all group IV semiconductors, namely three bands degenerate at F. The spin-orbit splitting of these bands is negligible. 

Aluminium Antimonide (AlSb). Aluminium antimonide (like AlAs) is an indirect-gap semiconductor with camel s back conduction band minima near X (Fig. 4.1-69). 

Gallium Phosphide (GaP). Gallium phosphide is an indirect-gap semiconductor. The lowest set of conduction bands shows a camel s back structure the hand minima are located on the A axes near the zone boundary. The valence hands show the usual stracture characteristic of zinc blende semiconductors. 

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