Determination of surface state parameters from surface photovoltage transients: CdS

Surface photovoltage phenomena: theory, experiment, and applications

“…As noted in the previous sub-section, the time necessary for the equilibration of the surface and the bulk may vary over many orders of magnitude and results in the distinction between`slow' and`fast' states. A somewhat similar effect is related to the relaxation time of the sample back to its equilibrium surface voltage value following optical excitation [110,111]. If, for example, the SPV was a result of electron excitation from surface states into the bulk, its relaxation would necessarily involve the return of the same amount of electrons to the surface states.…”

“…(2.81) with Eq. (2.83) and the Poisson equation [110]. Based on these equations, it is readily observed that the change in n t (and hence the change in surface voltage) is, in general, non-exponential, because n The optical cross-section, ' opt n , is photon energy dependent (just like the band-to-band absorption coef®cient).…”

“…The relaxation process may also be hindered by very small thermal cross-sections (and hence capture coef®cients) of the surface states. As a result, very long relaxation times may be obtained, possibly extending over many hours [110].…”

“…When long response-time transitions are involved, their corresponding SPV features are likely to go undetected when using ac SPV because the illumination chopping rate may be too fast to allow any non-negligible population changes in the`slow' states involved [100,110,321,386]. In fact, Gatos et al [6] have been able to advance SPS much more than other early researchers [100,239] partly because they used dc, rather than ac, spectroscopy, thereby revealing a set of surface states not previously exposed.…”

“…Photosaturation has also been used for performing a V 0 s -determining step in SPV-based algorithms for extracting surface state energy distributions [565,568,569,572,602,603] and surface-state properties [110,319,604], which are described in Section 5.4. Similarly, it has been used as a step in an algorithm for constructing the band diagram of semiconductor heterojunctions, discussed in Section 5.5.3 [19,403,605].…”