Impurity photoconductivity

Impurity photoconductivity (extrinsic photoconductivity) is a type of absorption measurement where the detector is the sample itself. Classical photoconductivity occurs when the absorption of an electron or of a hole takes place between a discrete state and a continuum, where it can contribute to the electrical conductivity. When the final state of a discrete transition is separated from the continuum by an energy comparable to k T at the measurement temperature, the electron or the hole in this state can be thermally ionized in the continuum and give rise to photoconductivity at the energy of the discrete transition. This two-step process, which is temperature-dependent, is known as photo-thermal ionization spectroscopy (PTIS) and is discussed in more detail later in the section on extrinsic photoconductors. 

For impurity photoconductivity a relation similar to Eq. (1) holds (Raz and Jortner, 1969) ... 

In the case of undoped PVCa films, impurities and surface states dominate the photoconduction mechanism (6) leading one to question any study of intrinsic pKotoconduction in organic polymers. Poly(N-vinylcarbazole) films yellow under ambient laboratory conditions. Work in our laboratory (7) has shown that ageing of a purified sample of PVCa leads to an increase in photoresponse in the 350-450 nm region while there is an initial drop in photoresponse in the 250-... 

Peter R. Bratt, Impurity Germanium and Silicon Infrared Detectors E.H. Pulley, InSb Submillimeter Photoconductive Detectors... 

We now turn to photoconductive and photodiode detectors, both of which are semiconductor devices. The difference is that in the photoconductive detector there is simply a slab of semiconductor material, normally intrinsic to minimize the detector dark current, though impurity doped materials (such as B doped Ge) can be used for special applications, whereas by contrast, the photodiode detector uses a doped semiconductor fabricated into a p-n junction. 

The dark- and photoconductivity of organic compounds has long been regarded as a sort of side-effect arising from impurities, ionic carriers, or traces of adsorbed water. 

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),... 

In discussing the use of photoconduction measurements for testing the photo-bleaching process, it is important to recognize that the photoconduction of dyes is due to the formation and migration of electronic charge carriers and not to impurities of photochemical decomposition. However, the following facts should not be overlooked 91>. 

A principal obstacle to identification of defects is the difficulty of comparing the results from EPR, luminescence, absorption, and deep state experiments. Probably the least ambiguous is that between EPR and luminescence when, as for transition metal impurities, it is possible for optical Zeeman measurements of a sharp luminescence line to determine the ground state g factor. If the optical and EPR measurements give the same value, then the correlation is made (Watts, 1977). In some cases, when optical excitation enhances or quenches the EPR signal, there may be a similar response in the photoconductivity or luminescence excitation spectrum. 

A nonelectronic method of measuring impurity concentrations is that of absorption spectroscopy. From Eq. (36a) it is seen that ani = avnini0, where a i is the absorption constant due to electronic transitions from level i to the conduction band. The total impurity concentration Nt can be related to ni0 by a knowledge of EF. The photon-capture cross section doping experiments or by independently measuring Nt in some sample. This process has been carried out for Cr impurity (Martin, 1979) as well as (EL2) (Martin, 1981) in GaAs. The same considerations hold for photoconductivity measurements, except that t also needs to be known, as seen from Eq. (35). 

The impurity generation route is shown in Fig. 5 to the right of the line. The interaction of the relaxed states formed from m ami unexcited impurity t results in exiplex t formation. This exiplex can decay thermally or form coupled ion-radical pairs t, which may dissociate in the electric field. For explanation of the absorption and photoconductivity spectrum correlation it is necessary to assume a very high concentration of exiplex sites. 

Some types of the polymers were investigated in detail. The photoconductivity of polyethylene with quantum efficiency 10 5-10 10 is caused by impurities, Schottky type contact injection, and hole transport [82,83], The crystallinity increase is accompanied by a photocurrent increase. There is no clear correlation between the chemical structure and the photocurrent. 

At the early stages the photoconductivity of solid solutions of the leucobase of malachite green in various organic media was investigated [285]. In these systems, carrier transport occurs by direct interaction between the leucobase molecules. No direct participation of the organic matrix in the charge transfer was observed. A model was proposed which links charge transfer in these systems with impurity conduction in semiconductors. 

On the other hand, the photoconductivity of a poly(vinyl chloride) copolymer and of sucrose benzoate each containing the leucobase of malachite green was characteristic of the dyestuff and independent of the polymeric host (137). This is similar to the behavior of conventional impurity semiconductors. However, a distinction must be drawn between an impurity which acts solely as an impurity providing an easier path to the conduction band, one which acts only as a plasticizer and one which may act as both impurity and plasticizer. 

The most useful of the known photorefractives are LiNbC>3 and BaTiC>3. Both are ferroelectric materials. Light absorption, presumably by impurities, creates electron/hole pairs within the material which migrate anisotropically in the internal field of the polar crystal, to be trapped eventually with the creation of new, internal space charge fields which alter the local index of refraction of the material via the Pockels effect. If this mechanism is correct (and it appears established for the materials known to date), then only polar, photoconductive materials will be effective photorefractives. However, if more effective materials are to be discovered, a new mechanism will probably have to be discovered in order to increase the speed, now limited by the mobility of carriers in the materials, and sensitivity of the process. 

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