Band gap direct

It is possible to identify particular spectral features in the modulated reflectivity spectra to band structure features. For example, in a direct band gap the joint density of states must resemble that of critical point. One of the first applications of the empirical pseudopotential method was to calculate reflectivity spectra for a given energy band. Differences between the calculated and measured reflectivity spectra could be assigned to errors in the energy band... 

Fig. 1. Schematic diagram of semiconductor materials showing band gaps where CB and VB represent the conduction band and valence band, respectively and 0 and 0, mobile charge. The height of the curve represents the probabiUty of finding an electron with a given momentum bound to an N-isoelectronic impurity, (a) Direct band gap the conduction band minimum, F, is located where the electrons have 2ero momentum, ie, k = 0. The couples B—B, D—A, B—D, and B—A represent the various routes for radiative recombination. See text, (b) Indirect band gap the conduction band minimum, X, is located...

Typical light output versus current (L—I) and efficiency curves for double heterostmcture TS AlGaAs LEDs lamps are shown in Eigure 8. The ir LED (Eig. 8a) is typically used for wireless communications appHcations. As a result, the light output is measured in radiometric units (mW) and the efficiencies of interest are the external quantum efficiency (rj y. = C y., photons out/electrons in) and power efficiency. As a result of the direct band gap... 

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

Another parameter of relevance to some device appHcations is the absorption characteristics of the films. Because the k quantum is no longer vaUd for amorphous semiconductors, i -Si H exhibits a direct band gap (- 1.70 eV) in contrast to the indirect band gap nature in crystalline Si. Therefore, i -Si H possesses a high absorption coefficient such that to fully absorb the visible portion of the sun s spectmm only 1 p.m is required in comparison with >100 fim for crystalline Si Further improvements in the material are expected to result from a better understanding of the relationship between the processing conditions and the specific chemical reactions taking place in the plasma and at the surfaces which promote film growth. 

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. 

Venus probe. References should be consulted for the details of the optical transparency of the different type diamonds (9,14,16—19). The direct band gap for diamond is 5.47 eV. Natural diamond exhibits many colors, and color modification by irradiation and annealing is common (36). Though cubic, most natural diamonds show strain birefringence under crossed polaroids. 

Shown in Figure 3 is the variation of the fundamental direct band gap (Fq) of Gai j j(As as a function of A1 composition (jt). These results were obtained at 300 K using electromodulation. Thus it would be possible to evaluate the A1 composition of this alloy from the position of Fq. 

Fig. 5. Calculated electronic structure by the LDA method of (a) an isolated Cuo molecule and (b) fee solid Cuo where the direct band gap at the X-point is 1.5 eV [60],...

Calculations for Ceo in the LDA approximation [62, 60] yield a narrow band (- 0.4 0.6 eV bandwidth) solid, with a HOMO-LUMO-derived direct band gap of - 1.5 eV at the X point of the fee Brillouin zone. The narrow energy bands and the molecular nature of the electronic structure of fullerenes are indicative of a highly correlated electron system. Since the HOMO and LUMO levels both have the same odd parity, electric dipole transitions between these levels are symmetry forbidden in the free Ceo moleeule. In the crystalline solid, transitions between the direct bandgap states at the T and X points in the cubic Brillouin zone arc also forbidden, but are allowed at the lower symmetry points in the Brillouin zone. The allowed electric dipole... 

Fig. 9. Direct band gap for [9,2] nanotube in vicinity of band gap. Wave number is dimensionless coordinate x, with onedimensional Brillouin zone for x defined —tt < x < tt.

Indium monoselenide, InSe, is a semiconductor with a weakly allowed direct band gap transition at 1.3 eV and an indirect at 1.2 eV, having a strongly anisotropic... 

Table 2. Surface density (0 ), direct band gap energy (Egd), and number of next nearest neighbors (N ) on WO -ZrOi calcined at 1073 K.

Optical band gap energies (Eg) for WOx-ZrOa samples calcined at 1073 K were obtained from UV-vis spectra using procedures based on direct and indirect transitions between valence and conduction bands [26]. Direct band gap energies (Egdecreased monotonically from 4.15 to 3.75 eV as the W loading increased from 3.05 to 15.0 W-atomsnm (Table 2). 

PbS has attracted much attention due to its special direct band gap energy (0.4 eV) and a relatively large exciton Bohr radius (18 nm) and their nanoclusters have potential applications in electroluminescent devices such as light-emitting diodes. PbS nanocrystals with rod like structures with diameters of 20-60 nm and lengths of 1-2 pm have been obtained using the sonochemical method and by using PEG-6000 [66]. Addition of PEG and the time of sonication have been found to play a key role in the formation of these rods. 

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

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