Semiconductors, disordered

N,N-bis(4-methylphenyl)-N,iV-bis(4-ethylphenyl)-[l,l -(3,3 -dimethyl)biphenyl]-4,4 -diamine [115310-63-9] (ETPD) (8) -doped PMPS (Table 1, entry 22), hole mobihty approaching 10 cm /Vs at 2.5 x 10 V/cm was observed (48). This is the highest recorded hole mobihty for disordered organic systems. From this perceptive, it is very interesting to study the carrier mobihty of polymers heavily doped with semiconductor nanoclusters. 

As is to be expected, inherent disorder has an effect on electronic and optical properties of amorphous semiconductors providing for distinct differences between them and the crystalline semiconductors. The inherent disorder provides for localized as well as nonlocalized states within the same band such that a critical energy, can be defined by distinguishing the two types of states (4). At E = E, the mean free path of the electron is on the order of the interatomic distance and the wave function fluctuates randomly such that the quantum number, k, is no longer vaHd. For E < E the wave functions are localized and for E > E they are nonlocalized. For E > E the motion of the electron is diffusive and the extended state mobiHty is approximately 10 cm /sV. For U <, conduction takes place by hopping from one localized site to the next. Hence, at U =, )J. goes through a... 

Fig. 3.4 Schematic representation of the cauliflower structure, showing the space charge layer in relation to the electrolyte and the semiconductor, and the pinching of cauliflowers , which is believed to be responsible for the disorder-dominated impedance. (Reprinted with permission from [71], Copyright 2009, The Electrochemical Society)...

G. E. N. Landweer and J. Bezemer, in Amorphous Silicon and Related Materials, Advances in Disordered Semiconductors 1 (H. Fritzsche, Ed.). World Scientific, Singapore, 1989. 

In amorphous semiconductors, information about the width of the band tail states (or disorder) may also be extracted from the optical absorption spectra. For photon energies near bandgap energy, the optical absorption coefficient of amorphous semiconductors exhibit an exponential dependence on the photon energy, following the so-called Urbach relationship ... 

We consider the existing models of adsorption response of electrophysical characteristics of ideal monocrystalline adsorbent, monocrystal with inhomogeneous surface as well as polycrystal adsorbent characterized by an a priori barrier disorder. The role of rechar g of biographic surface states in the process of adsorption charging of the surface of semiconductor is analyzed. 

Above theoretical analysis of adsorption effects on electric conductivity and VAC profiles in polycrystalline semiconductor adsorbent with accounting for its barrier disorder indicate that the value and kinetics of change in o(t) and yS(t) during adsorption of both acceptors and donors sharply differ from those predicted by theory both for the case of ideal monocrystal and for polycrystal considered from the standpoint of bicrystal model. 

Thus, all above experimental examples indicate that additionally to effect of disordering of the surface of adsorbent on its adsorption characteristics it is necessary to account for the effect of the adsorption itself on degree of adsorbent disordering controlling both its own electrophysical characteristics and the adsorption-caused change while deriving the theory of adsorption response of polycrystalline semiconductor adsorbent. 

The experimental studies of the surface properties of monocrystals of oxides of various metals recently conducted at well-controlled conditions [32, 210] enable one to proceed with detailed analysis of separate effects of various factors on characteristics of semiconductor gas sensors. In this direction numerous interesting results have been obtained regarding the fact of various electrophysical characteristics of monocrystalline adsorbents on the value of adsorption-related response. Among these characteristics there are crystallographic orientation of facets [211], availability of structural defects, the disorder in stoichiometry [32], application of metal additives, etc. These results are very useful while manufacturing sensors for specific gases with required characteristics. 

V. L. Bonch-Bruevitch, Electronic Theory of Disordered Semiconductors, Nauka, Moscow, 1981. 

In summary, NMR techniques based upon chemical shifts and dipolar or scalar couplings of spin-1/2 nuclei can provide structural information about bonding environments in semiconductor alloys, and more specifically the extent to which substitutions are completely random, partially or fully-ordered, or even bimodal. Semiconductor alloys containing magnetic ions, typically transition metal ions, have also been studied by spin-1/2 NMR here the often-large frequency shifts are due to the electron hyperfine interaction, and so examples of such studies will be discussed in Sect. 3.5. For alloys containing only quadrupolar nuclei as NMR probes, such as many of the III-V compounds, the nuclear quadrupole interaction will play an important and often dominant role, and can be used to investigate alloy disorder (Sect. 3.8). 

The work in this group has focussed mainly in antimony and bismuth because of the thermoelectric properties of the chalcogenides186 and as low temperature single-source precursors to related semiconductor materials.187 The use of bismuth compounds in the treatment of gastrointestinal disorders has lead to the study of several thiolate compounds as models to understand the bioactivity. 

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