Photoconducting for

Probe measurements in silane discharges have been reported [296,297]. Apparently, no difficulties were experienced, as the deposited amorphous silicon layer on the tip was sufficiently photoconductive. For typical silane discharge conditions values for are found to be between 2 and 2.5 eV. Electron densities are around 1 x 10 cm - [296]. Probe measurement in the ASTER system failed due to strong distortions of the probe current, even after following cleaning procedures. 

Therefore an increase in conductivity upon illumination (photoconductivity) can be due to either an increase in carrier concentration and/or an increase in mobility. In general, it is believed that an increase in carrier (hole) concentration is the dominant cause for room-temperature photoconductivity for the lead chalco-genides and that an increase in mobility becomes increasingly important at low temperatures. The dark conductivity of films deposited with or without added oxidant were similar the difference in photoconductivity between them was ascribed to the formation of sensitizing centers (interband states) due to the oxidant. 

The time-resolved photoconductivity measurements shown in Fig. 15 give further support for a difference in the photoinduced charge transport in the polymerized samples versus the unpolymerized samples. For the incident laser of 100 mW/cm2 and a spot size of 2.5 mm, the decay time of the photoconductivity for the unpolymerized samples is 7.4 sec, whereas the photoconductivity of the polymerized samples does not significantly drop over a 30 sec period. Also, the photoconductivity of the polymerized sample is nearly twice that of the unpolymerized samples even at the peak of the unpolymerized photoconductive response. The unnormalized values for the dark conductivity in both samples is 1.7 x 10-10 S cm-1. The photoconductivity is 5.8 x 10-11 S cm-1 for the unpolymerized sample and 1.1 x 10-10 S cm-1 for the PSLC at an optical intensity of 2 W cm-2. 

There is much still to be understood about the photoconductivity of a-Si H. However, the measurements confirm that recombination through defects is the main mechanism, particularly when their concentration is high. Extrinsic effects further complicate the interpretation of photoconductivity. For example, surface recombination can dominate when the bulk recombination rate is low. These effects can arise from either the excess defects at the surface or from the band bending, which causes a field induced separation of the electron and hole distributions. Contacts, which are almost invariably non-ohmic, also modify the photoconductivity, in particular, the response time. 

Fig. 14. Optical absorption coefficient a determined from photoconductivity for a-Si H/ a-SiNjj H superlattice film 1.2 /im thick with alternating layers of 1200 A a-Si H and 35 A...

Figure 9.3 Temperature dependence of the in situ photoconductance for two Ceo films, using white light intensity of 2 mW/cm. Films were grown on sapphire substrates held at approx. 200°C. Starting Qo powder for film A was dried for a longer period. Inset shows temperature dependence of the dark conductances. (Reproduced by permission of the American Physical Society from ref...

From the viewpoint of photochemistry, fuiierenes are good electron acceptors and many photoinduced reactions have been reported by using these fuiierenes as acceptors [16-20]. The excellent acceptor ability of fullerene is a key feature of photoconductivity for fullerene-doped polymer films such as poly(Af-vinylcarbazole) and poly(p-phenylene vinylene) [4, 21]. Furthermore, many derivatives of the fuiierenes have been synthesized due to high reactivities of fuiierenes [22]. Fullerene oligomers and polymers are interesting materials as well as pristine fuiierenes [23, 24]. 

Free-electron lasers have long enabled the generation of extremely intense, sub-picosecond TFlz pulses that have been used to characterize a wide variety of materials and ultrafast processes [43]. Due to their massive size and great expense, however, only a few research groups have been able to operate them. Other approaches to the generation of sub-picosecond TFlz pulses have therefore been sought, and one of the earliest and most successfid involved semiconducting materials. In a photoconductive semiconductor, carriers (for n-type material, electrons)... 

Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses in electronic and solid-state applications. 

FURNACES,ELECTRIC - INDUCHON FURNACES] (Vol 12) -photoconductivity [PHOTOCONDUCTIVE POLYMERS] (Vol 18) for papermaking [PAPER] (Vol 18)... 

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

Lead sulfide is used in photoconductive cells, infrared detectors, transistors, humidity sensors in rockets, catalysts for removing mercaptans from petroleum distillates, mirror coatings to limit reflectivity, high temperature solid-film lubricants, and in blue lead pigments (82). 

Lead Telluride. Lead teUuride [1314-91 -6] PbTe, forms white cubic crystals, mol wt 334.79, sp gr 8.16, and has a hardness of 3 on the Mohs scale. It is very slightly soluble in water, melts at 917°C, and is prepared by melting lead and tellurium together. Lead teUuride has semiconductive and photoconductive properties. It is used in pyrometry, in heat-sensing instmments such as bolometers and infrared spectroscopes (see Infrared technology AND RAMAN SPECTROSCOPY), and in thermoelectric elements to convert heat directly to electricity (33,34,83). Lead teUuride is also used in catalysts for oxygen reduction in fuel ceUs (qv) (84), as cathodes in primary batteries with lithium anodes (85), in electrical contacts for vacuum switches (86), in lead-ion selective electrodes (87), in tunable lasers (qv) (88), and in thermistors (89). 

Nanoclusters/Polymer Composites. The principle for developing a new class of photoconductive materials, consisting of charge-transporting polymers such as PVK doped with semiconductor nanoclusters, sometimes called nanoparticles, Q-particles, or quantum dots, has been demonstrated (26,27). 

The avaHabihty of photoconductive polymers opens up many areas for research, in addition to electrophotography. These are relatively unexplored areas and represent promising future directions. 

This new optical data storage device is reported to be robust and nonvolatile. The response time for the write—read beam is in the subnanosecond range, and no refreshing is requked for long-term retention of trapped charges (95). The basic principle may be appHed to other, similar photoconductive materials. 

Fig. 9. Spectral sensitivity of detectors where the detector temperatures in K are in parentheses, and the dashed line represents the theoretical limit at 300 K for a 180° field of view, (a) Detectors from near uv to short wavelength infrared (b) lead salt family of detectors and platinum siUcide (c) detectors used for detection in the mid- and long wavelength infrared. The Hg CdTe, InSb, and PbSnTe operate intrinsically, the doped siUcon is photoconductive, and the GaAs/AlGaAs is a stmctured supedattice and (d) extrinsic germanium detectors showing the six most popular dopants.

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