Abstract: Electrodes were prepared with heating commercial pure iron chips buried in Mgo powder and showe dboth n-and p-type photoresponses.The photoresponses were so weak that in fact they were totally buried inthe noise.Hence asignal averager was used. After 1000 cycles'summation the signal to noise ratio was greatly improved and we obtained a wave form series showing the conversion of n-type, p-type photoresponses for the first time.
Fig.1(a): E=0.261V. The absolute peak-to-peak photovoltagevalue (ΔV_(ph)) equals 13.4 mV and shows a typical n-type photoresponse. (b); E=-0.04 V, ΔV_(ph)=1.24 mV, still an -n-type response, but when light is chopped off, the electrode potential reaches its stable value much faster than in (a). (c): When E=-0.076 V, which is only 0.036 V more negative than in (b), in the latter 85% part of illuminating period, electrode potential tends to increase, demonstrating some p-type character. (d): As E=-0.09 V, the magnitude of p-type response almost equals that of n-type. (e): As E=-0.12 V, the main response belongs to p-type, and ΔV_(ph) is only 0.634 mV. This electrode potential is much more positive than the flat band potential of the undoped iron oxide electrode (E_(fb) is about -0.5 V). (f): E=-0.17 V. There is still a small n-type response at the beginning of every illuminating period, and the whole curve appears to be p-type responnse. (g): E=-0.36 V. Typical p-type response appears. ΔV_(ph)=2.05 mV. (h) and (i): E=-0.40 V and -0.45 V. ΔV_(ph) drop to 1.55 mV and 1.05 mV respectively. the above data were reproducible, no retardation of photoresponse was observed when potential was changed.
An XPS surface analysis was performed thereafter on the same electrode. A peak of Mg1S at 1305 eV(BE) is apparent. Huge amount of Mg exists on the surface layer. the relative atomic concentration (s compared with Fe) reaches 15-20 AT%. After Ar~+ etching, the amount of Mg decreases. Fig.2 is the result after 9 minutes′ Ar~+ etching. A possible explanation of these experimental facts based on model of energy band of semiconductors was given.