Effects of Polarized Light on Photocurrents and Photovoltages in ZnS

Abstract

The anomalous short circuit photocurrents in ZnS have been measured using polarized light. In spite of the fact that for a given wavelength, light polarized perpendicular to the c axis is more strongly absorbed than light polarized parallel to the c axis, there are wavelength regions in which the absolute magnitude of the short circuit photocurrents are smaller for perpendicularly polarized light. The data are consistent with a double valence band model. P HOTOVOLTAGES larger than the energy gap have been reported for hexagonal ZnS. [1][2][3][4] In addition, the sign of the open circuit photovoltage (and the short circuit current) reverses with wavelength. 2,4 Similar reversals have been reported for CdS. 5 From measurements of the pyroelectric effect made in this laboratory and by Lempicki, 4 it has been definitely established that the sign of the photovoltage at a given wavelength is associated with the polarity of the c axis. These phenomena are related to general effects of the crystal symmetry on interactions with light.Another type of measurement falling into this category involves the polarization of light absorbed or emitted by the crystal. As an example we cite the work done on the dichroic optical absorption of ZnS and CdS 6-8 as well as the work on fluorescent measurements. 7,9 The results of the absorption measurements show that light polarized perpendicular to the c axis is more strongly absorbed than light polarized parallel. This is consistent with the optical selection rules derived theoretically by Birman 10 and Casella. 11 Since the electrical and optical properties are related to the crystal symmetry, it is reasonable to measure the electrical properties with polarized light. This has been done for the photoconductivity of CdS 12 with externally applied fields. We have measured the short circuit photocurrent as a function of the wavelength of incident light for the two directions of light polarization for ZnS. This communication presents the results. EXPERIMENTAL RESULTS AND DISCUSSIONThe measurements have been made by adding a polarizer to instrumentation which together with the sample preparation has been described in reference 2. It is important to discuss in some detail the nature of our crystal samples. Among workers in the field 2-4 there is some difference of opinion as to the role that crystalline disorder plays in the various properties of hexagonal ZnS. In an attempt to relate crystal disorder, such as inclusions of stacking faults and cubic phase, we have examined many samples for these inclusions. Sixty-five single crystals of hexagonal ZnS (40 unactivated and 25 activated with various combinations of Cu, Al, and Mn) were analyzed by x-ray diffraction techniques. (The x-ray radiation had no effect on the electrical properties.) Most of the crystals were about 4 mm long and exhibited uniform birefringent banding. With these, a one-mm spot in the middle of the crystal was chosen for x-ray examination. With crystals that exhibited heavy banding at one or more points, the one-mm spot in...

Semiconductors at IBM: Physics, Novel Devices, and Materials Science

“…S. P. Keller, G. Cheroflf, R. C. Enk, and G. D. Pettit measured optical properties of zinc sulfide [8][9][10], The most interesting result was the observation by Cheroflf and Keller [8] of photovoltages greater than the energy gap. Swanson [11] partially explained this result by a model involving a series of asymmetric p-n junctions in the material, but could not account for the reversal of sign of the photovoltage with photon energy observed by Cheroff and Keller.…”