Direct band gap semiconductors

Fig. 3. Spectra showing absorption coefficient as a function of the photon energy in a direct band gap semiconductor where (—) represents absorption,...

Eig. 1. Representation of the band stmcture of GaAs, a prototypical direct band gap semiconductor. Electron energy, E, is usually measured in electron volts relative to the valence, band maximum which is used as the 2ero reference. Crystal momentum, is in the first BriUouin 2one in units of 27r/a... 

A light-emitting diode (LED) is a forward-biasedp—n junction in which the appHed bias enables the recombination of electrons and holes at the junction, resulting in the emission of photons. This type of light emission resulting from the injection of charged carriers is referred to as electroluminescence. A direct band gap semiconductor is optimal for efficient light emission and thus the majority of the compound semiconductors are potential candidates for efficient LEDs. 

Optical studies on various direct band-gap semiconductor NWs have demonstrated that these NW materials can also exhibit excellent... 

Changes in intensity of semiconductor PL or EL can be used to detect molecular adsorption onto semiconductor surfaces [1,3]. PL occurs most efficiently when ultra-band-gap radiation excites electrons from the valence band to the conduction band of a direct-band-gap semiconductor and the electrons recombine radiatively with the holes left behind in the valence band. 

Deep dopant a dopant whose energy level is close to the middle of the semiconductor band gap Direct band gap semiconductors semiconductors that display a fully allowed electronic transition between the valence and conduction bands... 

For the purposes of this article we will limit our discussion to particles defined by a minimum of two dimensions less than 100 nm but usually with 2-dimenions less than 10 nm. Current interest in these materials can principally be traced to work by Luis Brus in the mid-1980s in which he pointed out that the band gap of a simple direct band gap semiconductor such as CdS should be dependent on its size once its dimensions were smaller than the Bohr radius [10]. Experimental work confirmed this suggestion. Initial samples were prepared by low temperature... 

Under strong band-gap excitation, the photo-neutralized ions can de-excite thermally, but in direct-band-gap semiconductors, they can also de-excite efficiently by radiative recombination of the bound electrons with the bound holes. Such photoluminescence (PL) lines are known as donor-acceptor pair (DAP) spectra. In a semiconductor with dielectric constant e, the energy of the photon emitted by a pair whose constituents, with ionization energies Ed and Ea, are both in the ground state and at a distance R is ... 

When a semiconductor is illuminated with the band-gap radiation, excess electrons and holes are photo-created. They can form free excitons or be trapped by ionized impurities, but their ultimate fate is their annihilation by thermal or radiative recombination. The formation of free excitons will be discussed in Sect. 3.3.2, but in direct band-gap semiconductors, electron-hole radiative recombination can also occur at an energy close to Eg if the pumping beam is kept at a low level. This can provide an accurate determination of Eg [87],... 

It has been mentioned at the end of Sect. 3.3.1 that for hydrostatic pressures > 4 GPa, GaAs turned from a direct-band-gap semiconductor with GB minimum at the T point into an indirect-band-gap semiconductor with an absolute CB minimum at the X point. This has consequences on the absorption spectrum of the shallow donors in this material discrete electronic absorption has been reported in GaAs at 487 (60.4 meV) and 405 cm-1 (50.2 meV) for the Sica and Snc a donors, respectively under hydrostatic pressures of about 6GPa, with a small pressure dependence of their positions (—0.5 and I 0.14 moVGPa 1 for the Si and Sn lines, respectively) [107]. This absorption is shown in Fig. 6.46 for GaAs Sn. 

With m in units of me, 7b = 4.2544 x 10 fi ( s/m )2 B(T). For shallow donors in multi-valley semiconductors, to is the electron transverse effective mass mnt of Table 3.4 and for QHDs in direct-band-gap semiconductors, it is the effective mass mn at the T minimum of the CB of Table 3.6. For the shallow acceptors where the effective Rydberg R oa is defined as Roo/li, Bo is equal to Rloa/jips- Values of Bo for shallow donors and acceptors in different semiconductors are given in Table 8.12. 

Light-emitting diodes and 111-V lasers are devices that use a direct-band-gap semiconductor to convert electrical energy into visible light. They have long been used for materials with Eg - 1 eV (the familiar red), but now GaN (with a wider band gap, large v, and smaller X) gives us a blue emitter. 

Porous III-V semiconductors are of great interest because of possible applications in optoelectronics and photonics. Study of the luminescence from porous direct band gap semiconductors such as GaAs and InP presents an additional interest because quantum confinement effects are well established in two-, one-, and zero dimensional electron systems prepared on crystalline III-V materials. Thus it would be possible to compare the light-emission properties between structures obtained by porous-etching and those produced in well-characterized lithographically defined structures directly. GaP was studied for comparison because it is an indirect band gap. 

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