The band gap was determined by absorption [3] and photoelectrochemical [7] methods and was found to be 1.7 and 1.75 eV respectively. The quantum yield vs. stimulation is shown in Fig. 1. A further reference [8] gives 1.5 eV band gap and indirect band transition. Ref. [9] gives 1.9 eV for band gap from photoluminescence measurement at 10 K. These strongly different photoelectrochemical and photoluminescence results are refined in this work.

The impedance measurements were performed in an electrochemical cell under potentiostatic control. The electrolytes were 0.05 M H2SO4 and 1 M KOH solutions. The impedance analysis was carried out with the perturbation of some mV. The modelling of an electrolyte — semiconductor junction is a difficult problem because the values of the circuit elements exhibited frequency dependence. In this work we determined the proper values of equalent circuit components with their physical meaning for the transfer function of the junction. The parameters of the equivalent circuit are very importat to know for device applications. A simple equivalent circuit with physical meaning was appropropriate for the calculation [5] which is contain three paralell branches. One of this branches is a resistance R|. The second branch is a swinging circuit R2C2 and the third branch is a capacity C3.

[oh»: The evaluation of the measured data

was carried out with the help of a computer program developed by us in Turbo Pascal language. The transfer function of the equivalent circuit has three solutions (one zero and two poluses). In the first step these three roots weree fitted in the same time with the help of least square method. The minimum of the error is determined with the help of gradient method, until the error become less than 1 Hz. The value of the constant in the transfer function was determined from the amplitude diagram with similar method [10]. The R1 resistance represents the charge transfer that is the electrochemical reaction at the interface. The value ranges between 6.2 and 7.1 kQ /mm2. R2 and C2 represent the surface levels and deep centres where the values

range between 0.6 and 1.6 kQ/mm2 and 5.3 2

Fig. 2. The amplitude (i) and phase (ii) diagrams of the Cd4GeSe6 and 0.05 M H2SO4 junction where the parameter of the model network are R1 = 51,100 Q (7229 Q/mm2), R2 = 4,227 Q (598 Q /mm2), C2 = 53.8 nF (7.61 nF/mm2), C3 = 4.4 nF (0.62 nF/mm2).

Acknowledgements

This work was supported by Hungarian National Scientific Foundation (OTKA) through Grant No. T 037509, which is very acknowledged.