Semiconductor, intrinsic defect states

Whereas in good-conducting doped or polymeric dyes ft-or -type conductivity can be explained without difficulty by analogy with inorganic semiconductors, the p- and -type photoconductivity in insulating (intrinsic) dye films cannot be explained in this manner. It is necessary to take into consideration the existence of defect states (lattice defects, dislocations, impurities etc.) distributed at different depths in the forbidden zone between valence and conduction band these defect states are able to trap electrons and holes, respectively, with different probability 10,11,88),... 

Under ordinary conditions, in particular when the electrode material is in contact with an electrolyte solution, adsorbed atoms or even layers are present on the surface moreover, real surfaces may contain structural defects. They all can exchange electrons with the semiconductor bulk to give rise to surface electron states of kinds and properties other than those inherent to intrinsic surface states. The former play an important role in adsorption and catalysis processes. 

The photophysical processes of semiconductor nanoclusters are discussed in this section. The absorption of a photon by a semiconductor cluster creates an electron-hole pair bounded by Coulomb interaction, generally referred to as an exciton. The peak of the exciton emission band should overlap with the peak of the absorption band, that is, the Franck-Condon shift should be small or absent. The exciton can decay either nonradiatively or radiative-ly. The excitation can also be trapped by various impurities states (Figure 10). If the impurity atom replaces one of the constituent atoms of the crystal and provides the crystal with additional electrons, then the impurity is a donor. If the impurity atom provides less electrons than the atom it replaces, it is an acceptor. When the impurity is lodged in an interstitial position, it acts as a donor. A missing atom in the crystal results in a vacancy which deprives the crystal of electrons and makes the vacancy an acceptor. In a nanocluster, there may be intrinsic surface states which can act as either donors or acceptors. Radiative transitions can occur from these impurity states, as shown in Figure 10. The spectral position of the defect-related emission band usually shows significant red-shift from the exciton absorption band. 

Although the appearance of red-shifted, broad-band luminescence is usually attributed to the presence of defects, this is not always so. As the size of the cluster becomes smaller, the concept of defect becomes meaningless. Red-shifted luminescence may be due to an intrinsic excited state of the cluster which is significantly distorted from the ground state. This is quite common for molecules. Since the surface structures of most of the semiconductor clusters synthesized to date are not precisely known, in most cases it is difficult to establish whether the observed red-shifted luminescence band is due to defects or the intrinsic excited state. 

The strategy to modify surface states, and thus the Vg and SRV, is based on interaction of chemically grafted molecules with these states. The key is to find molecules that will modify the semiconductor surface chemistry in a way that involves the surface states. In this respect, the origin of the surface states should be considered. Intrinsic surface states originate from the termination of the crystal bulk and the breaking of chemical bonds at the surface, whereas extrinsic surface states originate from crystal imperfections, such as missing surface atoms, line defects, or... 

A SOLID-STATE LASER is a device in which the active medium is based on a solid material. While this material can be either an insulator or a semiconductor, semiconductor lasers will be covered elsewhere in this volume. Solid-state lasers based on insulators include both materials doped with, or stoichiometric in, the laser ions and materials that contain intrinsic defect laser species. 

The situation becomes even more complicated when surface states are present the surface states may be intrinsic, defect induced, or of adsorbate-induced nature. The density can vary between less than 10 and 10 cm. Such a situation is illustrated in Figure 9.50. The left panel in the figure shows an n-doped semiconductor with a donator density Nq. All donators should be ionized, that is the saturation regime. 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

Recently, it was shown that transient absorption decay for hematite nanoparticles was very fast, 70% of the transient absorption disappeared within 8 ps and no measurable transient absorption remained beyond 100 ps [43]. This represented a much faster decay than many other semiconductors, which is consistent with the observed poor charge transfer properties in hematite. It should be mentioned that this decay was independent of the excitation power, which suggests alternative relaxation mechanisms compared to those observed for Ti02 and ZnO for instance [43]. Since the relaxation was independent of pump power, probe wavelength, pH and surface treatment the fast decay was interpreted to be due to intrinsic mid-bandgap states and trap states rather than surface defects. This is in agreement with earlier investigations [44]. 

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