Bandgap composition dependence

In general, compositional dependence of the optical bandgap of ternary alloys is written as follows ... 

FIGURE 5 Composition dependence of energy bandgap Eg for AlxIni xN alloys (after [23]). 

Photocurrent spectroscopy is a useful tool for the determination of bandgapsof in semiconductor alloys (Hutton and Peter, 1993) or surface films on metals (Peter, 1980, 1987 and 1989 Campbell et al., 1992). Figure 12.7 illustrates the composition dependence of the photocurrent onset energy for a series of Gai cAl cAs alloy samples (Hutton and Peter, 1992). The relationship between bandgap and composition is known for this system, so photocurrent spectroscopy is a convenient tool for compos-... 

Both theoretical and experimental approaches have been developed to estimate and measure the composition dependence of the optical bandgap of glasses. Lucovsky found local extrema of the average bandgap for the chemically stoichiometric As2Se3 and GeSe2 compositions, but their relation to the topological order was not explored [102]. 

The compositional dependence of the bandgap parameter as well as the a and c lattice parameters for the Mgj Zni-KO alloy has been studied over the entire compositional range [2, 4, 7, 26-32]. As expected, the film quality is inferior in the wurtzite-cubic transition region where mixture of the two structures may be present. The bandgap data versus composition for both wurtzite and rocksalt portions are shown in Figure 6.5. Also shown is the fit to the T point E transitions for the wurtzitic ternary using... 

ZnS-CdS (bandgap = 2.3-2.4 eV) composite semiconductor photoelectrodes show a broad spectral response and n-type behavior, with saturation of the anodic photocurrent upon increasing anodic potential making the system suitable for use as a photoelectrochemical cell photoanode [72], Nanostructured ZnS-CdS thin film electrodes show that anodic photocurrent saturation can be attained with the application of a small, 0.1 V, bias [73], while hydrogen evolution is observed at the Pt cathode. The performance of the ZnS-CdS photoanodes appear strongly dependent upon the method of film preparation [72,73], with Zn rich films demonstrating superior photocurrent generation, and stability, in comparison to Cd rich films. 

Parker CA, Roberts JC, Bedair SM, Reed MJ, Liu SX, Masiy NA, Robbins FH (1999) Optical bandgap dependence on composition and thickness of InxGai-xN grown on GaN. Appl Phys Fett 75 2566-2568... 

FIGURE 6 Dependence of GaAsN alloy energy bandgap on the nitrogen composition (after [39]). 

FIGURE 7 Dependence of the GaPN energy bandgap as a function of the N composition (after [45]). 

Electrooptical properties of composites made of TiC>2 coated with PT based on in situ UV-vis spectroscopy were reported by Rammelt et al. [1097]. Optical properties of poly(3,4-ethylene-dioxythiophene)-sulfated poly(jS-hydroxyether) composite films prepared by electrochemical copolymerization were studied with in situ UV-vis spectroscopy by Yamato et al. [1098]. In the reduced state, a strong absorption around A = 600 nm, assigned to the 7T -) 77 transition, indicating a bandgap of A = 1.6 eV, was found. The actual position of the absorption maximum depended somewhat on the ratio of the components in the composite. The intensity of this band decreased upon oxidation. A fairly broad feature around A = 900-1000 nm appeared. 

Section II gives a short introduction of the different low-dimensional semiconductor systems discussed in this article and describes some of the most common fabrication pathways of these systems. Structures, which belong to the low-dimensional semiconductors, show spatial extensions which are restricted to a few nanometers in at least one dimension. Therefore, the mobility of the carriers and vibrations is reduced in this dimension. This results in a change of the energy levels of excited electron hole pairs and vibrations called confinement effect. Because of the latter, the bandgap energy and the vibrational properties of the low-dimensional semiconductors strongly depend on the sample constitution and size. Hence, it is possible to fabricate devices with well-defined optoelectronical properties only by variation of material parameters like composition, layer thickness, or crystallite size. 

A factor which has to be taken into consideration, especially in the MBE growth of layered systems, is the influence of effective pressure and strain on the vibronic properties of the semiconductors. Strain effects arise because of the differences in the lattice constants. This difference leads to a distortion of the crystal lattice, which causes a shift of the bandgap energy and, thus, directly affects the optical properties of the layered systems. Therefore, it is important to investigate strain effects quantitatively. The compression (expansion) of the lattice constants by strain effects is equivalent to a change in the interatomic distances and therefore leads to a shift of the LO-phonon frequency. Compressive (tensile) strain leads to a blueshift (redshift) of the phonon wave number. The amount of this shift is proportional to the strain. Strain shifts can mask confinement shifts or compositional shifts. Therefore, it is necessary to separate the strain-induced part of the shift from the other parts. In addition to the dependence on the difference in the lattice constants, the strain shows also a dependence on the thickness of the layer. The strain increases with increasing layer thickness up to a critical thickness. From this thickness onward, the strain decreases because of the formation of misfit dislocations. The appearance of misfit dislocations can be observed in the Raman spectra as an increase in the intensity of the symmetry-forbidden TO-phonon peak as well as a broadening of the LO-phonon peak. 

Figure 6.5 The bandgap dependence of Mg. Zni .,0 over the entire compositional range. Energies of the fundamental band-to-band transitions E of wurtzitic ZnO and those of rocksalt structure. It should be kept in mind that the layers in the region corresponding to wurtzite-cubic transitions might be of mixed structure, (a) Refs [37-39] (spectroscopic ellipsomet ), (b) Ref. [40] (transmission),...

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