Band-gap positions

Figure 6.2 Band-gap positions of some selected semiconductor materials at pH 1...

The redox potentials of both e- and h+ are determined by the relative positions of the conduction and valence bands, respectively. Band gap positions are material constants they are known for a variety of semiconductors. Electrons are better reductants in alkaline solutions, while holes have a higher oxidation potential in the acid pH range. With the right choice of semiconductor and pH, the redox potential of the e b can be varied from +0.5 to -1.5 V and that of the h+b from +1.0 to more than +3.5 V. Thus... 

For clarification of the type of junctions formed at the semiconductor-electrolyte, let us take an example of n-type semiconductor. In addition to possessing free electrons (referred to as the majority carrier), n-type semiconductor also possesses holes (referred to as the minority carrier). The concentration of holes is temperature-dependent and is equivalent to the intrinsic concentration of the carrier (which is related to the concentration of Frankel defects). It can be shown mathematically that the Fermi level of minority carrier hes at almost half the band gap position. On the other hand, the concentration of majority carriers as well as the Fermi level depends on doping concentration. Thus, the Fermi level of the majority carrier can he anywhere between the conduction hand edge and the intrinsic Fermi level that is situated at i g. 

Photodetectors exhibit well-defined, cutoff wavelength thresholds, the positions of which are determined by the magnitudes of the band gap activation energy, E, or impurity-activation energy, E. The cutoff wavelength, corresponds to a photochemical activation energy, E, where. 

Generally speaking tire mobilities of elecU ons and positive holes decrease and the band gaps increase as the bonding in the semiconductors becomes more... 

In Modulation Spectroscopy, which is mosdy used to characterize semiconductor materials, the peak positions, intensities and widths of features in the absorption spectrum are monitored. The positions, particularly the band edge (which defines the band gap)> are the most useful, allowing determination of alloy concentration. 

As an example, PL can be used to precisely measure the alloy composition xof a number of direct-gap III-V semiconductor compounds such as Alj Gai j, Inj Gai jfAs, and GaAsjfPj j(, since the band gap is directly related to x. This is possible in extremely thin layers that would be difficult to measure by other techniques. A calibration curve of composition versus band gap is used for quantification. Cooling the sample to cryogenic temperatures can narrow the peaks and enhance the precision. A precision of 1 meV in bandgap peak position corresponds to a value of 0.001 for xin AljfGai j, which may be usefiil for comparative purposes even if it exceeds the accuracy of the x-versus-bandgap calibration. 

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. 

There have been very few examples of PTV derivatives substituted at the vinylene position. One example poly(2,5-thienylene-1,2-dimethoxy-ethenylene) 102 has been documented by Geise and co-workers and its synthesis is outlined in Scheme 1-32 [133]. Thiophene-2,5-dicarboxaldehyde 99 is polymerized using a benzoin condensation the polyacyloin precursor 100 was treated with base to obtain polydianion 101. Subsequent treatment with dimethyl sulfate affords 102, which is soluble in solvents such as chloroform, methanol, and DMF. The molar mass of the polymer obtained is rather low (M = 1010) and its band gap ( ,.=2.13 eV) is substantially blue-shifted relative to PTV itself. Despite the low effective conjugation, the material is reasonably conductive when doped with l2 (cr=0.4 S cm 1). 

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

---