Light-induced Modulation Research Articles

Photomechanical vibration of thin crystals of polar semiconductors

It was found that thin crystals of polar (non-centrosymmetric) semiconductors constitute a new type of photo-sensitive system in which incident illumination is converted into mechanical energy: thus, illumination-induced elastic deformation (bending) was observed on thin (00.1) CdS and (111) GaAs crystals; furthermore, by employing chopped light the crystals were excited to their resonant vibration (photomechanical vibration); the dependence of the amplitude of this vibration on the energy of the incident radiation was found to be similar to the dependence of the surface photovoltage on the energy of the incident radiation (surface photovoltage spectrum). The present findings are consistent with a model based on light-induced modulation of the piezoelectric surface stresses.

Photodetection by light-induced barrier modulation in Cu-diffused Au–CdS diodes

Modified Schottky barriers of the type Au–CdS : Cu were prepared by diffusing Cu to a depth of 0.1–0.2 μm into single crystals of CdS, prior to evaporating rectifying Au contacts. The electronic and photoelectronic properties of these junctions are adequately described by a simple model in which the Cu acts as an ionized acceptor, resulting in a ``humped'' potential barrier between the Au and the bulk CdS. Hole trapping by the acceptors under band-gap illumination reduces their ionization and, consequently, the hump height. The forward current density in the dark and under illumination can be accurately described by a thermionic emission model as J=A*T2 exp {—e[VB+VH(0)]/kT exp}(eV/βkT), where the hump potential VH (0) is light sensitive and where β (1<β<1.35) arises from a weak voltage dependence of the barrier height. Barrier height determinations, in the dark and under illumination with 5000-Å light, by means of internal photoemission, thermal activation energy, and J-V measurements unequivocally show that under saturating light conditions the hump in the potential barrier is reduced by 0.23±0.03 V. This barrier mechanism of photoconductive response results in steady-state electron/photon gains in excess of 106 at light intensities lower that 5×1011 photons cm−2 sec−1, where response times exceed 1 sec.

Photodetection on a Principle of Light-Induced Barrier Modulation

AbstractComprehensive electrical and optical measurements on certain lightly Cu diffused Au-CdSjunctions confirm photoconductive response on a mechanism of light-induced lowering of an electrostatic surface barrier. Electron per photon gain between 105 and 107 and threshold exposures in the range 1012 to 1013 cm-2 have been observed in junctions whose width is less than 5000 A. Arguments will be advanced which indicate a minimum exposure of about 5 x 109 cm-2 for this type of device. Implications for electrolytic imaging systems will be mentioned.

Light-Induced Modulation of Broad-Band Optical Absorption in CdS

The optical transmission of CdS single crystals has been changed using a laser. With ac techniques, these changes have been observed from 530 to 2000 nm in the transmission spectrum, and include both increased and decreased transmission. Comparison between the intensity dependence and the temperature dependence of photoconductivity and of change of transmission suggest a mechanism. The effect is a change of optical absorption produced primarily by the change in hole population of imperfection levels in the band gap. Changes in transmission due to population changes of approximately 1012 cm−3 have been observed.