Surface photovoltage

Surface Photovoltage. This technique was first proposed by Goodman (68), and has since been used both with solid and liquid junctions. It Is based on the observation that for most good semiconductors the photovoltage is related to the minority carriers diffusion length through the following expression  

A variance that usually appears in the literature as Surtece Photovoltage Spectroscopy (21) uses either a vibrating metal electrode in close proximity to the surface of the semiconductor (Kelvin probe), or a semitransparent metal electrode with a modulated light source to make contact with the semiconductor, and monitors the spectral response of the photovoltage with subband gap illumination. This technique can also be viewed as a non-contact technique that can be used for in situ characterization during device fabrication. 

Electromoduiated Infrared Spectroscopy. The form most widely in use is the electromoduiated attenuated-total-reflection-spectroscopy. This is the electromoduiated form of the multiple internal reflection spectroscopy that was previously discussed. The practice here is to modulate, by switching the potential between two values, and monitor the absorption changes by lock-in techniques. Description of this technique with an emphasis on metal-electrolyte interfaces can be found in (70). The modulation here is different from all the other techniques that were mentioned in the sense that the modulation is not a small perturbation of an equilibrium state but a shift between two equilibrium states. Whether 

---

PbS/In2S3 Pb(CH3COO)2 InCl3 Na2S Surface photovoltage spectra, I-V, quantum efficiency, SEM, ERDA 101,102... 

In conclusion, nanorods are a potentially interesting material, but present results still do not allow understanding of whether the nanostructure leads to an improvement of the intrinsic photocatalytic behaviour, or whether other factors (accessible surface area, enhanced adsorption, etc) are responsible for the observed differences. In ZnO nanorods have been shown quite recently by surface photovoltage spectroscopy that the built-in electrical field is the main driving force for the separation of the photogenerated electron-hole pairs.191 This indicates that the nano-order influences the photophysical surface processes after photogeneration of the electron-hole pairs. A similar effect could be expected for Titania nanorods. However, present data do not support this suggestion, mainly due to the absence of adequate photo-physical and -chemical characterization of the materials and surface processes. 

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. 

Balestra, CL, Lagowski, J., and Gatos, HC, Determination of Surface State Parameters from Surface Photovoltage Transients, Surface Science, 64, 457, 1977. 

M. Shimizu, Y. Kanai, H. Uchida and T. Katsube, Integrated biosensor employing a surface photovoltage technique, Sens. Actuators B Chem., 20(2-3) (1994) 187-192. 

Y. Ito, M. Takiue, H. Uchida, T. Katsube and E. Tamiya, An organic pollution sensor based on surface photovoltage, Sens. Actuators B Chem., 53 (1998) 163-172. 

Figure 5. Current, capacitance, and surface photovoltage vs. potential for galena. (Reproduced from Ref. 10.J...

The shift of EB can partly be removed by illumination of the sample with white light (W Hal lamp, 40 mW/cm2) (Fig. 4). As the observed surface photovoltage is considerably smaller than the shift of EB due to adsorption the formation of an inversion layer on this small bandgap semiconductor has to be assumed. The... 

Acetylcholineesterase The surface photovoltage technique was applied to the fabrication of a biosensor based on immobilized cholineesterase. The type of the cholineesterase used depends on the type of substrate. On the surface of the silicon wafer the cholineesterase layers were immobilized by using 3-aminopropyl triethoxysilane and glutaraldehyde. Characteristics of the sensor were studied in phosphate buffered saline containing 15 mM NaCl and 1 mM phosphate buffer (pH 7). 

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). 

Surface Photovoltage Experiments on SrTiCh Electrodes. The role of surface states in mediating charge transfer between electrode and electrolyte elucidated. 390... 

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]).

Eenzmann F., Krueger J., Bnrnside S., Gratzel M., Gal D., Riihle S. and Cahen D. (2001), Hot electron injection in dye-sensitised solar cells surface photovoltage spectroscopic evidence , J. Phys. Chem. B 105, 6347-6352. 

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. 

Chiesa M, Biirgi L, Kim J-S, Shikler R, Friend RH, Sirringhaus H (2005) Correlation between surface photovoltage and blend morphology in polyfluorene-based photodiodes. Nano Lett 5 559... 

---