Dark conductivity

Fig. 3. The room temperature dark conductivity, (Hem), and conductivity activation energy, AH in eV, plotted as A, a function of vppm of AsH ( ) B, PH (a) and C, B2H ( ) into the premix gas ratio of Sip4 H2 = 10 1. Thepton transition (left to right) refers to i -Si F H alloy, and D refers to doping...

The electrical properties (dark conductivity and photoconductivity) are reported to first decrease and then increase upon increasing power [361]. The optical bandgap increases with increasing power, due to the increase of the hydrogen content [63, 82, 362, 363]. However, at very high power levels, microcrystalline silicon is formed [364], which causes the hydrogen content (and, consequently, the bandgap) to decrease. 

Both the dark conductivity and the photoconductivity of the sample deposited at 40 W have been measured. The photoconductivity has been measured under AM 1.5 conditions. The dark conductivity and photoconductivity of the material deposited at 40 W are 1 x 10 and 1.2 x 10 " cm, respectively. These... 

Electrical data are shown in Figure 59 as a function of deposition rate for all frequencies, using the relation between deposition rate and power density as depicted in Figure 54. Both dark conductivity and photoconductivity decrease exponentially with increasing deposition rate. The data in this range of deposition rates can be fitted with (Td = 9 x 10 -exp( —1.5r[Pg.142]

FIG. 59. Dark conductivity ( d) and photoconductivity (Oph) as functions of deposition rate for all frequencies. 

Madan et al. [515] have presented the effect of modulation on the properties of the material (dark conductivity and photoconductivity) and of solar cells. They also observe an increase in deposition rate as a function of modulation frequency (up to 100 kHz) at an excitation frequency of 13.56 MHz, in their PECVD system [159]. The optimum modulation frequency was 68 kHz, which they attribute to constraints in the matching networks. Increasing the deposition rate in cw operation of the plasma by increasing the RF power leads to worse material. Modulation with a frequency larger than 60 kHz results in improved material quality, for material deposited with equal deposition rates. This is also seen in the solar cell properties. The intrinsic a-Si H produced by RF modulation was included in standard p-i-n solar cells, without buffer or graded interface layers. For comparison, solar cells employing layers that were deposited under cw conditions were also made. At a low deposition rate of about 0.2 nm/s, the cw solar cell parameters... 

The variation of deposition temperature has similar effects on the material properties to those on PECVD-deposited material. With increasing temperature (125-650°C), the material becomes more dense (the refractive index extrapolated to 0 eV increases from 3.05 to 3.65). and the hydrogen content is decreased (15 to 0.3 at.%), as well as the microstructure factor (0.4 to 0). The activation energy is 0.83 eV up to a deposition temperature of 500°C. The dark conductivity and AM 1.5 photoconductivity are about 5 x 10 " and 5 x 10 cm , respec-... 

Prototype electrostatic loudspeakers where the graphite is replaced by a-Si H have been made, where a Mylar foil (area 10 x 10 cm-, thickness 6 /im) is used [657]. Deposition of the a-Si H layer was carried out in the ASTER deposition system. Uniform deposition (standard deviation of thickness, 1.5%) was achieved by diluting the SiHa with Ht with SiHa Hi = 1 2 [370]. The deposition was at room temperature. The hydrogen content amounted to 18 at.%, and the bandgap was 1.81 eV. The dark conductivity and AM 1.5 photoconductivity were 7.5 X 10 and 1.8 x 10 cm" , respectively. In practice the film would not... 

A turning point in the study of amorphous semiconductors was reached with the discovery that the addition of hydrogen to amorphous silicon could dramatically improve the material s optical and electrical properties. Unlike pure amorphous silicon, which is not photoconductive and cannot be readily doped, hydrogenated amorphous silicon (a-Si H) displays a photoconductive gain of over six orders of magnitude and its dark conductivity can be changed by over ten orders of magnitude by n-type or p-type... 

In 1977 it was observed that extended illumination with visible light of a-Si H produced a decrease in photoconductivity and dark conductivity (the Staebler-Wronski effect), which is reversible upon annealing, as shown in Fig. 7 (Staebler and Wronski, 1977,1980). The effect can be quite dramatic, producing a decrease in dark conductivity of over four orders of magnitude, though the extent of the decrease depends on the initial defect density and doping level of the sample. The degraded conductivity state is... 

Fig. 7. Decrease of room temperature dark conductivity (solid circles) and photoconductivity (solid line) during illumination (the Staebler-Wronski effect) with 200mW/cm2 heat-filtered white light (Steabler and Wronski, 1977).

Recent studies of doped a-Si H have found that the background density of localized states, that is, the electrically active dopants and dangling bond defects, are metastable (Ast and Brodsky, 1979 Street et al., 1986, 1987a Muller et al., 1986). After annealing above 150°C in the dark, the dark conductivity at room temperature of n- and p-type doped a-Si H decreases by nearly a factor of two over a time scale of several weeks for n-type and several hours for p-type a-Si H. As shown in Fig. 9 (Street et al., 1987a), the relaxation rate of the occupied band tail density nBT is a sensitive function of temperature, so that the time to reach... 

Darenthin, molecular formula and structure, 5 96t Dark chocolate, 6 361 minerals content, 6 371t theobromine and caffeine content, 6 367t tocopherols, 6 370t typical formulation, 6 362t Dark conductivity ( D), of a-Si H, 22 ... 

Some Properties of CdS Films. It should be noted that aU properties discussed in this chapter refer to as-deposited films (not annealed) unless specifically stated otherwise. In general, annealing increases crystal size and reduces dark conductivity. The latter obviously depends to a large degree on the annealing atmosphere. 

ELECTRICAL PROPERTIES. Rcsistivity studies on CdSe are much less widespread than on CdS films. The dark conductivity of undoped films is high (10 O-cm is typical), and the photocurrent sensitivity is less than for CdS films (even under illumination, the films are normally very resistive). 

As is the case for the dark resistivity, the dependence of the sensitivity of the photoconductivity (defmed here as the ratio between light and dark conductivity) on the deposition parameters is far from clear-cut. Some observations can be made, however. The first (obvious) one is that for a high sensitivity, the dark resistivity must be high. Apart from this, there does seem to be a general trend (clear-cut in the triethanolamine and citrate baths and seen also by the lack of appreciable photoconductivity in the one low- (room-) temperature-deposited film reported [40]) of an increase in photosensitivity (due to decrease in light resistivity) with increasing deposition temperature. 

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. 

Finally, it is worth mentioning a comment made in a paper describing junctions between Ge and CD PbS [34]. It was noted that evaporated epitaxial PbS films were poorly, if at all, photoconducting, while CD films, with mobility lower by two orders of magnitude and much poorer structure, were much superior in this respect. In a way, this should not be surprising since, for good photoconductivity, low dark conductivity (and therefore either low mobility and/or low carrier concentration) is necessary. 

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