Extrinsic Photoconductivity of Fast‐Electron‐Irradiated Undoped CdS Single Crystals

Abstract

The extrinsic photoconductivity of undoped platelet-type CdS crystals irradiated with 280 keV electrons was investigated a t liquid nitrogen and room temperatures in the photon energy range 0.1 to 2.5 eV, using polarized light in the range 0.6 to 2.5 eV. A sensitive ac method was used, superimposing a chopped measuring beam on a constant bias illumination level. At least six photoconductivity bands were found. The bands in the range 0.6 to 2.5 eV are weakly polarized, the higher intensity occurring for E 1 c. The fine structure of the low-energy quenching band previously observed by Hemila and Bube [28] was reproduced but was found to be caused by atmospheric absorption bands. Most of the bands in the photoconductivity spectrum correspond to absorption bands found by Boyn [8] in thick crystals. -The present results, assuming the threshold energies for atomic displacement in CdS determined by Kulp and Kelley [2,3] to be valid, together with the results of trap analysis made also in this work, absorption measurements [8], and our earlier investigations of the trap spectrum, electron lifetime, and dark conductivity of deuteronbombarded CdS crystals [27] suggest an assignment of the photoconductivity bands to intrinsic lattice defects and to donor-acceptor associates involving intrinsic defects and residual impurities.Die Storstellenphotoleitfahigkeit undotierter plattchenformiger CdS-Kristalle, die mit 280 keV-Elektronen bestrahlt worden waren, wurde bei der Temperatur des flussigen Stickstoffs und bei Raumtemperatur im Photonenenergiebereich von 0,l bis 2,5 eV untersucht, wobei im Bereich von 0,6 bis 2,5 eV mit polarisiertem Licht gearbeitet wurde. Zur Messung wurde eine empfindliche Wechsellichtmethode benutzt, bei der ein zerhackter Xe5strahl einem konstanten Gleichlicht uberlagert wurde. Mindestens sechs Photoleitfahigkeitsbanden wurden gefunden. Die Banden im Bereich von 0,6 bis 2,5 eV sind schwach polarisiert, die groBere Intensitat tritt fur E 1 e auf. Die Feinstruktur der niederener-

The Structure of the 0.9 eV Quenching band of Photoconductivity in CdS

“…2 shows the 0.9 eV band a t T = 295 O K plotted in a Gaussian scale. As seen from this figure, the 0.9 eV quenching band may be approximated by a Gaussian curve within the photon energy range 0.7 to 0.9 eV in agreement with results in [9]. For energies larger than 0.9 eV, however, a slight deviation from the Gaussian form is observed.…”

“…Hemila and Bube [l] attributed this structure tentatively to phonon satellites or crystal field splitting. On the other hand Wruck [9] suggests that this structure is caused by atmospheric absorption bands. I n our experiments (with spectral slit width of 5 meV at 0.9 eV) also a fine structure of the 0.9 eV band was observed.…”

Method for determining optical excitation densities due to photoionization of impurities and to interband transition in thin CdS-type crystals from photoconductivity measurements

“…The method described in the preceding sections has been used for investigating intrinsic defect centres produced by fast electron bombardment of ca. 50 pm 5, thick platelet-type CdS crystals [9]. For estimating the absorption coefficient the approximate method of Sect'ion 4.2 has been applied.…”

“…The degrees of modulation of the photocurrent varied between lo-' and Fig. 5 shows as an example the extrinsic absorption coefficient determined from measurements in [9] as function of photon energy for a CdS crystal at room temperature. The quantum yield is assumed to be unity, as intrinsic absorption coefficient a t h vo = 2.4 eV we use an approximate value of 100 cm-l according to the work of Dutton [lo].…”

“…Extrinsic absorption coefficicnh due to photoionization of impuritics of a CdS crjxfal at room temperature, determined from photoconductivit~y nicnsuremeiits of [ Y ]6 )In[9] a misprint led to an erroneous thickness value of 500 km.…”