Deep levels photoconductivity

Fig. 4. Energy below the conduction band of levels reported in the literature for GaP. States are arranged from top to bottom chronologically, then by author. At the left is an indication of the method of sample growth or preparation liquid phase epitaxy (LPE), liquid encapsulated Czochralski (LEC), irradiated with 1-MeV electrons (1-MeV e), and vapor phase epitaxy (VPE). Next to this the experimental method is listed photoluminescence (PL), photoluminescence decay time (PLD), junction photocurrent (PCUR), photocapacitance (PCAP), transient capacitance (TCAP), thermally stimulated current (TSC), transient junction dark current (TC), deep level transient spectroscopy (DLTS), photoconductivity (PC), and optical absorption (OA).

The threshold of the photocurrent is at the energy of 1 eV. This value is less than the absorption edge enagy, what proves the existence of deep levels inside the forbidden gap. The absence of the structure at the beginning of the interband transitions excludes the possibility that the photocurrent is due to the surface exciton dissociation. Attempts to observe good photoconduction in ris-(CH) were unsuccessful. The photocunent in cis-(CH)n was three orders of magnitude lower than in trans-(CH) . 

It should be noted that a-Si H exhibits a density of states that is neither constant nor discrete. Deep-level transient spectroscopy (DLTS) (Lang et al, 1982), optical absorption (Jackson and Amer, 1982), and photoconductivity (Jackson et al, 1983b) measurements have indicated that a rather broad defect band lies in the gap and is centered below the Fermi energy. This means that for cases in which the built-in potential is less than the width of... 

Cadmium Sulfide Photoconductor. CdS photoconductive films are prepared by both evaporation of bulk CdS and settHng of fine CdS powder from aqueous or organic suspension foUowed by sintering (60,61). The evaporated CdS is deposited to a thickness from 100 to 600 nm on ceramic substates. The evaporated films are polycrystaUine and are heated to 250°C in oxygen at low pressure to increase photosensitivity. Copper or silver may be diffused into the films to lower the resistivity and reduce contact rectification and noise. The copper acceptor energy level is within 0.1 eV of the valence band edge. Sulfide vacancies produce donor levels and cadmium vacancies produce deep acceptor levels. 

Films with low resistivity can be obtained by In doping /29/. Highly photoconductive films with high dark resistance can be made by Cu or Ag doping. These elements act as deep acceptors at an energy level between 0.8 and 1.2 eV above the valence band and hence cause a fractional compensation of the n-type conductivity /30/. 

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