Photovoltage measurement, surface

If c = 0, then VJ,h gives a measure of the flat-band potential provided r/re(i()x is known. In fact, this formula is very rarely obeyed in practice and deviations are both common and complex. Detailed theories of the potential distribution at the semiconductor-electrolyte interface have been presented, based on photovoltage measurements, but immense care needs to be taken in the interpretation of the photovoltage since kinetic effects apparently play a major role. This is especially true if surface recombination plays an important role [172]. 

Fig. 39. Surface photovoltage measurements, (a) Principle of the technique the two feedback systems FB1 and FB 2 are alternately active. FB1 serves for topography and FB 2 adds a potential u to Uy so as to nullify The SPV is f/ught- dark- (b) STM (left) and SPV (right) images taken on Si(111)-7x7. Low-SPV (dark region) corresponds to point defects in (a) (after [182]).

If strong illumination produces flat bands at the surface without changing X, that is, without aflfecting the surface dipole layer, then the saturation value of the surface photovoltage measured by the Kelvin method A should be... 

Fig. 10. Energy level diagram of the surface of (a) undoped and (b) phosphorus-doped a-Si H deduced from surface photovoltage measurements. [From Goldstein and Szostak (1980). Copyright North-Hoiland Publ. Co., Amsterdam, 1980.]...

Fig.9 Surface photovoltage measurement equipment. After ref. 85 with...

Surface photovoltage spectroscopy (SPS) in Fig. 6.5 was used to determine the surface acidity of JML-1 by measuring transition of electrons between the interface and the surface. The JML-I40 calcined at 550°C exhibited two peaks at 596 nm and 677 nm, whereas the sample without calcination had only one peak at 330 nm. The peak at 330 nm is assigned to the band-band electron transition and those at 596 nm and 677 nm are attributed to the surface-related transitions. The observation of these surface-related transitions indicates the presence of positive charges on the surface of the calcined sample, suggesting that the acidity of JML-1 catalyst is resulted from a large amount of SZ acidic sites on the silica surface. 

Surface treatments of CD CdSe films deposited from selenosulphate/NTA solutions have a pronounced effect on various optical, electrical, and optoelectronic properties of the films, due to interaction with or modification of such surface states. Mild etching (dilute HCl) of the films reverses the direction of current flow both in CdSe/polysulphide photoelectrochemical cells [108] and in Kelvin probe surface photovoltage (SPV) measurements in air [109], These studies are discussed in more detail in Chapter 9, in Section 9.2 on photoelectrochemical cells. At this point, it is sufficient to state that the effect is believed to be due to preferential trapping of either electrons or holes at surface states that are modified by the etching process. 

There are another type of photocells knowm as barrier layer photocells which work on an entirely different principle. They are semiconductor devices in which impinging photons promote the electrons from the valence band to the conduction band across the energy gap. A photovoltage is generated which can be measured by a voltmeter. Such photovoltaic devices can have a large surface area and are easy to operate. They are commonly used in many simple colorimeters and fluorimeters and as light Deters for cameras. 

The light-induced creation of recombination centers causes the diffusion length to decrease with exposure time as shown in Fig. 3 (Carlson et al., 1984b). The diffusion length was measured by the surface photovoltage method (Dresner et al., 1980), and similar results were obtained from an analysis of device characteristics (Faughnan et al., 1983). 

There are numerous techniques to measure the recombination lifetime. Some of the better known are photoconductive decay (13). diode reverse recovery (14). diode open circuit voltage decay (15). surface photovoltage (JL ) and forward-biased pn junction I-V characteristic (17. I will describe one particular photoconductive decay method, because it is a relatively new, non-contact method that requires no junctions. This makes it very suitable for a large number of measurements as for a process sequence characterization tool. 

The lower limit for short lifetimes in this technique is determined by the optical excitation source turn-off time to about 0.1 gs. For shorter lifetimes steady-state diffusion length measurements are more suitable. The diffusion leyth is related to the recombination lifetime by the equation L l/(Dt ). Suitable techniques are surface photovoltage and scanning electron microscope electron beam induced current. They lend themselves to lifetimes down to the nano-second range. 

A second technique that has been used is the measurement of photovoltage. The basis of this technique is that, on illumination under open-circuit conditions, the potential distribution will be modified so as to eliminate the potential drop within the depletion layer. In fact, as has been demonstrated by Kautek and Gerischer [7], the theory of the photovoltage effect is far from straightforward, especially in the presence of surface states. The effect is a steady-state rather than equilbrium phenomenon the potential distribution will change until the flux of holes to the surface is equal to the flux of electrons and the potential at which this occurs will depend on the recombination kinetics at the surface. Only when these kinetics are slow, i.e. when the surface states are slow and the main surface state equilbrium is with the redox couple in solution, is the technique likely to give results that can be interpreted within a consistent framework. 

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