M-type semiconductors

FIGURE 21.11 MO energy levels for doped semiconductors, (a) An M-type semiconductor, such as silicon doped with phosphorus, has more electrons than needed for bonding and thus has negative electrons in the partially filled conduction band. 

The lack of photostability of M-type semiconductor electrodes is a severe problem when using aqueous solutions. Holes excited by light excitation and then driven toward the surface can be either used for the decomposition or are transferred to a redox system. In the case of large band-gap oxides, the stability is thermodynamically controlled i.e. the anodic decomposition occurs only at potentials that are considerably more positive than typical redox reactions. In all other cases, the competition between these two processes is kinetically controlled (Memming, 1990, 1994). In several cases, a decrease of the stability of n-type semiconductors with increasing... 

Antimonides of formulas CdSb and Cd2Sb2 have been reported. Both are usually prepared by direct union of the elements, the former is a hole-type semiconductor (9), with properties shown in Table 1, and finds use as a thermoelectric generator. Reagent-grade material costs 2.00/g in small lots. The band gap energy is 0.46 eV (2.70 J.m) (31) is 138 kj/mol (33.0 kcal/mol). Dicadmium triantimonideCd2Sb2, is a metastable, white... 

Interesting systems, mainly with respect to solid-state optoelectronics and chalco-genide glass sensors (due to ionic conductivity effects) are found among the Group IIIB (13) and IVB (14) chalcogenides, such as the p-type semiconductors MSe (M = Ga, In, Sn), SnS, and GeX (X = S, Se, Te). Some of the IIIB compounds. 

Bockris JO M, Uosald K (1977) The rate of the photoelectrochemical generation of hydrogen at p-type semiconductors. J Electrochem Soc 124 1348-1355... 

The Schottky-Mott theory predicts a current / = (4 7t e m kB2/h3) T2 exp (—e A/kB 7) exp (e n V/kB T)— 1], where e is the electronic charge, m is the effective mass of the carrier, kB is Boltzmann s constant, T is the absolute temperature, n is a filling factor, A is the Schottky barrier height (see Fig. 1), and V is the applied voltage [31]. In Schottky-Mott theory, A should be the difference between the Fermi level of the metal and the conduction band minimum (for an n-type semiconductor-to-metal interface) or the valence band maximum (for a p-type semiconductor-metal interface) [32, 33]. Certain experimentally observed variations of A were for decades ascribed to pinning of states, but can now be attributed to local inhomogeneities of the interface, so the Schottky-Mott theory is secure. The opposite of a Schottky barrier is an ohmic contact, where there is only an added electrical resistance at the junction, typically between two metals. 

Intrinsically conducting polymers, 13 540 Intrinsic bioremediation, 3 767 defined, 3 759t Intrinsic detectors, 22 180 Intrinsic fiber-optic sensors, 11 148 Intrinsic magnetic properties, of M-type ferrites, 11 67-68 Intrinsic photoconductors, 19 138 Intrinsic rate expressions, 21 341 Intrinsic semiconductors, 22 235-236 energy gap at room temperature, 5 596t Intrinsic strength, of vitreous silica, 22 428 Intrinsic-type detectors, cooling, 19 136 Intrinsic viscosity (TV), of thermoplastics, 10 178... 

Fig. 6-46. Differential capacity observed and computed for an n-type semiconductor electrode of zinc oxide (conductivity 0. 59 S cm in an aqueous solution of 1 M KCl at pH 8.5 as a function of electrode potential solid curve s calculated capacity on Fermi distribution fimction dashed curve = calculated capacity on Boltzmann distribution function. [From Dewald, I960.]...

Fig. 6-47. Mott-Schottky plot of electrode capacity observed for n-type and p-type semiconductor electrodes of gallium phosphide in a 0.05 M sulfuric add solution. [From Meouning, 1969.]...

Fig. 5-63. Flat band potential of two n-type semiconductor electrodes of zinc oxide in 1 M KCl (pH 8.5) as a function of donor concentration A= surface finished in 85 % H3PO4 B = surface finished in 2 M KOH = donor concentration. [From Dewald, I960.]...

Fig. 8-28. Cathodic polarization curves for several redox reactions of hydrated redox particles at an n-type semiconductor electrode of zinc oxide in aqueous solutions (1) = 1x10- MCe at pH 1.5 (2) = 1x10 M Ag(NH3) atpH12 (3) = 1x10- M Fe(CN)6 at pH 3.8 (4)= 1x10- M Mn04- at pH 4.5 IE = thermal emission of electrons as a function of the potential barrier E-Et, of the space charge layer. [From Memming, 1987.]...

Fig. 9-11. Polamation curves observed for anodic dissolution of n- pe and p-type semiconductor electrodes of germanium in 0.05 M NaOH solution = current of...

Fig. 10-11. Anodic photoexcited dissolution current of an n-type semiconductor electrode of gallium arsenide as a function of electrode potential in a 0.6 M sulfuric add solution lo - photon intensity = diotocurrent. [From Memming-Kelly, 1981.]...

In order for the photoelectrolytic decomposition of liquid water to proceed, the Fermi levels of the redox reactions in Eqns. 10-53a and 10- 3b need to be located within the band gap of the n-type semiconductor anode. In Fig. 10-26(a), we have assumed that the Fermi level ep(ac) of the n-type semiconductor anode at the flat band potential is higher than the Fermi level ep(h-/h2) of hydrogen redox reaction we have also assumed that the Fermi level e,(M) of the metallic cathode is lower than ekh /Hj)- Further, we have assiuned that the edge level of the conduction band is higher than the Fermi level of hydrogen redox... 

Fig. 10-26. Energy diagrams of a cell for photoelectrolytic decomposition of water consisting of a metal cathode (M) and an n-type semiconductor anode (n-SC) of which the Fermi level is higher than the Fermi level of hydrogen redox reaction ( R8o>ep(H /H2)) (a) cell circuit is open in the dark, (b) cell circuit is closed in the daric, (c) cell circuit is closed in a photoezdted state (cell reaction proceeds.). potential hairier of a space charge layer.

An STM probe has been used to isolate individual MS (M = Cd, Pb) particles and to measure electronic phenomena (55,56,81). The MS films were prepared either by exposure of metal ion/fatty acid films to H2S (55,56) or by transfer of a compressed DDAB-complexed CdS monolayer (81). All the films were transferred onto highly oriented pyrolytic graphite (HOPG) for the STM measurements. A junction was created at an individual CdS particle with the STM tip as one electrode and the graphite as the other, and the current/voltage characteristics of the panicles were measured. For the particle prepared in the fatty acid films the I/V curves exhibit step-like features characteristic of monoelectron phenomena. In the case of the DDAB-coated CdS particles the I/V measurements demonstrated n-type semiconductor behavior. The absence of steps in this system is probably a reflection of the larger size of the particles in the DDAB films (8 nm by AFM) compared to the 2-nm particle size typically found for MS particles formed in fatty acid films. 

Conversely, doping Ge with As introduces an extra electron that cannot be accommodated in the tetracovalent network (valence band), and this creates a narrow band of occupied donor levels, just below the conduction band in energy. The Fermi level is now located between the donor band and the conduction band, and electrons in the donor band can be readily excited thermally into the conduction band (Fig. 5.5). Thus, a negative or n-type semiconductor is created. Semiconductors can exhibit electrical conductivities in the range 10-3 to 104 S m 1, as compared to 103 to 107 S m 1 for metals. 

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