Pulsed Electrochemical Capacitance Studies of the HgI2 ‐ NaCl Electrolytic Solution Junction

Abstract

We have studied the electrochemical capacitance characteristics of the HgI2-NaC1 electrolytic solution junction for short (0.1-100 ~s) voltage pulses. Under illumination and application of repetitive voltage pulses (-10V, -100 Hz), a permanent increase in sample conductivity was effected. The crystals then exhibited the behavior of a semiconductor, containing a continuous, nearly uniform distribution of traps across the bandgap, of density of about 5 • 1013 cm 3 (eV)-l. In 1983, Iwanczyk et al. (1, 2) reported on a successful use of HgI2 as a photoconductive detector in conjunction with scintillating crystals to form scintillation spectrometers. The energy resolution displayed by the scintillatorHgI2 detector combinations studied was about 20% for 511 keV photons (annihilation radiation from a positron source). The results were similar to the figures obtained using a photomultiplier as the light detector. Improvements in design and in energy resolution soon followed (3, 4). The use of HgI2 for this purpose requires that at least one of the electrodes be transparent, to permit the emitted light to reach the HgI2. One of the possibilities examined was that of a NaCI electrolytic solution (3, 4). It seemed to us desirable to study the electrical properties of the HgI2-NaCI electrolytic solution junction. We used the pulsed electrochemical capacitance measuring method, which has proved to be a valuable tool in studies of ZnO surfaces (5-8). We included the option of illuminating the crystal with weakly absorbed light, in order to carry out electrochemical photocapacitance spectroscopy studies (9, 10). However, a permanent increase in conductivity was incurred by the illumination. In the present work we concentrated on studying the characteristics of these samples. The conclusions provide a partial understanding, and a guidance for future studies, aimed at shedding light on the nature of the process which brings about the permanent conductivity increase. The work demonstrates the power of the pulsed electrochemical capacitance technique to reveal dynamics of surface charge processes. ExperimentalHgI2 crystals grown from vapor were supplied to us by EGG Energy Research Group, Santa Barbara Operations, California. The samples were in the form of plates of about 15 • 15 • 1 mm 3, cut normal to the crystallographic C direction (11). The samples were etched in a 20 weight percent aqueous solution of KI, cooled to about 10~ for about 2 rain (12). An "Aquadag" (Acheson Incorporated) electrical contact was pasted on one of the sample's large surfaces in the form of a ring, about 6 mm in internal diameter. A platinum lead, 0.2 mm in diam was attached to the contact, and both were pasted with a "Humiseal" (Columbia Chase) encapsulating resin. The last process ensures the mechanical stability needed for the contacts and the leads. The same resin was used to paste the entire crystal surface, except for a circular area on the surface just opposite the one with the contact. Again, the same resin was used to glue the sample to a Te...