Photocurrent decays

The signal contained an initial decay in the form of a spike. We may consider three possibilities as originally cansing the photocurrent decay in the pretransit region. 

This is clearly illustrated in the case of p-GaP as shown in Figure 5. In a 0.1 M NaOH solution decomposition is by Ga loss from the surface as the hydroxide, and photocurrent decay is rapid as a phosphate layer builds on the surface. In 0.1 M HgPO solution decay is again rapid but in this case the Ga is not solubilized but deposits on the surface as the metal. In the more oxidizing acids such as H2SO4 both Ga and P are removed from the surface and the photocurrent remains high as the surface is essentially photoetched. 

As would be expected, the accumulation of Bi particles under an external bias leads to the gradual decrease of the photocurrent density owing to the rise of the recombination rate. It is obvious that such photocurrent decay cannot be detected during conducting the photoelectrochemical process in deaerated KHal solutions because the generated Bi atoms and nanoparticles are quickly oxidized by the dissolved oxygen with a regeneration of BiOHal. 

The photocurrent decay, under pulsed illumination, is strongly accelerated upon anodic or O-plasma oxidation of diamond surface. This effect is believed to be due to removal of the above-mentioned subsurface hydrogen [169]. 

Fig. 104. Equivalent circuits for the analysis of photocurrent-decay transients. The circuit elements are C, photocapacitor Rseries, total series resistance of the spectroelectrochemical cell Rl, load resistor Rin, internal leakage resistor R0, C , resistor and capacitor of counterelectrode solution interface Rd, resistance due to damaged surface layer.

In this section, we briefly consider the response of nanocrystalline semiconductor-electrolyte interfaces to either pulsed or periodic photoexcitation. Several points are worthy of note in this respect (a) the photocurrent rise-time in response to an illumination step is nonlinear. Further, the response is faster when the light intensity is higher, (b) The decay profiles exhibit features on rather slow time-scales extending up to several seconds, (c) The photocurrent decay transients exhibit a peaking behavior. The time at which this peak occurs varies with the square of the film thickness, d. (d) A similar pattern is also seen in IMPS data where the transit time, r, is seen to be proportional to d. ... 

A field effect is also seen in the photocurrent decay time at high light intensity, interpreted in terms of bimolecular carrier recombination. Finally we discuss the rise time of a photocurrent following a rectangular light pulse. The latter results pertain to the time domain where transport is barrier controlled and bear out the importance of photo-induced barrier crossing processes. 

Fio. 6. Electron photocurrent decay obtained by fitting together the current decays measured with three different amplifier bandwidths over three different overlapping time intervals. The sample was 3.8 /im thick, the applied bias was 16 V, and the temperature was 160 K. The numbers indicate the slope of the curve. [From Tiedje (1984).]... 

Fig. 8. Temperature dependence of the dispersion parameter a for electrons determined from the slope of the first branch of the photocurrent decay (solid circles) and the second branch (open circles).

Fig. 11, Temperature dependence of the dispersion parameter o for holes for the sample of Fig. 10 determined from the field dependence of the transit time (open circles) and the slope of the photocurrent decays (solid circles). [Reprinted with permission from Solid State Communications, Vol. 47, T. Tiedje, B. Abeles, and J. M. Cebulka, Urbach edge and the density of states of hydrogenated amorphous silicon. Copyright 1983, Pergamon Press, Ltd.)...

After the transit time, the nature of the photocurrent decay changes because on the average an electron that is thermally emitted from a trap near E after will be extracted at the back contact without being retrapped below . As a result, the photocurrent for t> is controlled by the rate of thermal emission of trapped electrons near E. For every factor of e increase in time, another slice of charge kT wide boils off the top of the trapped charge... 

FIG. 12 Photocurrent transient responses associated with the heterogeneous quenching of the dimer ZnTPPS/ZnTMPyP by tetracyanoquinodimethane (TCNQ) at a water DCE interface. The redox couple FelCNlg /FelCN)/ was used as supersensitizer in the aqueous phase. The back electron transfer reaction, responsible for the photocurrent decay in the on-transient, is significantly quenched in the presence of the supersensitizer. According to the redox diagram in (b), the overall process at A 4>= —0.11 V corresponds to the reduction of TCNQ by the redox couple in the aqueous phase photocatalyzed by the porphyrin complex. Reprinted from Ref. 109 with permission from Elsevier Science. 

Figure 2.56 Photocurrent decay (oscilloscope measurement) during the first initial oxidation phase of n-Si(lll) in dilute NH4F, pH 4 W-l lamp, ms shutter in this first oxidation phase, about 10 monolayers of oxide are formed (see text).

It is difficult to infer a reasonable trap parameter from isothermal photocurrent decay measurements, particularly in this case, where the traps are created at random in the material by ion irradiation. The parameters might be determined directly with sufficient knowledge of the ion beam and its effect on the irradiated material. We also assume that the solid is optically excited so that a portion of the traps above the equilibrium Fermi level contains electrons and a portion of those below the Fermi level contains holes. In the method of Simmons and Tam [182], a current is induced (/ ) in the external circuit given by... 

Figure 4 From photocurrent decays in T.S. such as that in the inset of fig. 2 are plotted decay curves at different applied voltages,...

Note that (25) looks exactly like the famous dispersive transport behaviour discovered by Scher and Montroll [16]. Here the deep traps act like boundaries and produce a knee in the photocurrent decay curve, the transit time tc scales however normally with with field, but the total photocurrent has a most interesting and unusual field dependence. Note that in 1-d [7] 6(t))ot). 

It is somewhat surprising in the light of this evidence that attempts to observe carrier transit have been unsuccessful [100 -102), After a short light pulse the photocurrent decays monoton-ically with no features attributable to carrier transit. Such a response, of the form t °, is in fact typical for amorphous materials an apparent paradox in view of the quality of the PDA crystals used. This problem has been resolved by recent theoretical and experimental work [103, 104). These show that the apparent anomaly is due to a breakdown in linear response theory. Carrier motion in ID in the presence of strong scattering centres is shown to be extremely sensitive to the density of scattering centres. 

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