Surface Photovoltage SPV Measurements

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. 

Surface photovoltage (SPV) measurements have been used to investigate electronic structures at the semiconductor surfaces for semiconductor-vacuum and semiconductor-gas interfaces. This method is applied to semiconductor-liquid interfaces in case of insulating liquids [39]. 

A number of characterization methods for wafers are used today. One group of experiments aims at the purely electronic parameters like DLTS and Photoluminiscence. While DLTS and related measurements give information on the trap energies, methods like surface photovoltage (SPV) or open circuit voltage decay (OCVD) give information on effective diffusion length. 

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]).

---