Light modulated microwave reflectivity

Light and potential modulated microwave reflectivity measurements offer a novel approach to the study of the semiconductor electrolyte interface. Perturbation of the density of electrons and holes in a semiconductor influences the conductivity and hence the imaginary component of the dielectric constant at microwave frequencies. For small perturbations, the change ARm in microwave reflectivity depends linearly on the change in conductivity [27, 28, 75). The application of frequency response analysis to light modulated microwave reflectance is relatively new [30]. Although the technique is analogous to IMPS, it provides additional information. 

For the very small perturbations involved in the measurements, the light modulated microwave reflectivity change, ARm is a linear function of the ac component of the photogenerated charge that accumulates at the sem-iconductor/electrolyte interface... 

IMPS and light modulated microwave reflectivity have been used to study the photo-evolution of hydrogen evolution on oxide free p-Si in acidic fluoride solution at low light intensities [30]. The set of IMPS plots in Fig. 8.23 can be compared with those for p-InP shown in Fig. 8.9. It can be seen that the response tends towards a point on the real axis as the potential becomes more negative and krec becomes negligible. Clearly all information about ktr is lost in this region. The behaviour of the frequency resolved microwave is quite different. As Fig. 8.24 shows, a semicircular microwave response is still observed at -1.0 V, which is well into the... 

Fig. 8.24. Set of light modulated microwave reflectivity plots for p-Si in 1.0 mol dm-3 HF. Contrast these plots with the IMPS plots in Fig. 8.23. It is clear that k can be derived from the microwave response even at -1.0 V, where the quantum efficiency is unity and the IMPS plot has condensed to a point.

LMMRS light-modulated microwave reflectance spectroscopy... 

S sensitivity factor in light modulated microwave reflectivity... 

Schlichthorl et al. [177] have used light modulated microwave reflectivity to derive the rates of interfacial electron transfer processes at the n-Si/electrolyte interface. In these measurements, the modulation frequency was constant, and the rate constants for charge transfer were derived from the potential dependent ARm response. Schlichthorl et al. [73] have extended the technique considerably by introducing frequency response analysis. The technique is therefore analogous to IMPS, although, as shown below, it provides additional information. 

Fig. 21. Experimental set-up for light modulated microwave reflectance based on X-band microwave system [177]. The apparatus can also be used for IMPS measurements.

If krec = 0, the photocurrent simply follows the illumination step and contains no information about the rate of charge transfer at the interface. The comparison shows that, unlike IMPS and PEIS, light modulated microwave reflectivity measurements still provide kinetic information at high band bending where recombination is negligible and the steady state photocurrent is described by the Gartner equation. 

Light modulated microwave reflectivity has been used to characterise hydrogen evolution on oxide free />-Si at low light intensities [73]. The cathodic photocurrent reaches a plateau at potentials more negative than -0.5 V, and it can be shown that this corresponds to complete collection of photogenerated electrons at the junction (i.e. no recombination). This is confirmed by the set of IMPS plots in Fig. 23 which collapse to a point on the real axis as the potential becomes more negative. 

Figure 12.33 illustrates the set-up for LMMRS. The frequency response analyser replaces the single frequency lock-in amplifier used in the potential and light modulated microwave measurements described in Section 12.3. LMMRS detects the frequency-dependent modulation of the microwave reflectivity AR associated with the photogenerated minority carriers. This concentration decays by interfacial charge fransfer k d and recombination kKc)- The LMMRS response is therefore a semicircle with a characteristic frequency otam = + rec)- The low-frequency intercept of the... 

The semiconductor wafer is mounted at the end of an X-band microwave waveguide so that microwave radiation probes the reflectivity of the sample. The ohmic contact is applied as a grid of thin lines in order to minimise microwave losses. The front of the wafer is in contact with an electrolyte solution, and a modulated light source (for example a light emitting diode) illuminates the sample. The changes in... 

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