Photocapacitance Effect Research Articles

Dielectric properties and index of refraction of lead iodide single crystals

The paper describes measurements of the dielectric constant ( E∥ c-axis) and loss as a function of temperature (110–450°K) and frequency ( 10 2 c s to 10 5 c s ), and the refractive index ( E⊥ c) from 0.5 μ to 15 μ on PbI 2 single crystals grown by a gel technique. Above 10 4 c s , K' = 6.21. The index of refraction was measured by an interferometric method for E⊥ c-axis from 0.5 μ to 15 μ where in the long wavelength region n E⊥ c = 2.50 ± 0.10. At 2.3μ Δn E⊥c ΔT = −2.0 × 10 −4 °K . The long wavelength value of the index of refraction for E∥ c-axis is shown by optical transmission measurements to be close to that for E⊥ c. The results show that the material is much less strongly ionic than hitherto supposed. Values of the dielectric constant previously reported vary within an order of magnitude. The origin of these discrepancies is traced and discussed. It is linked with the observation of photocapacitive effects.

Photokapazitive messungen an zinksulfid

To decide how far the photodielectric effect (of 1. order) and the photoconductive effect (photodielectric effect of 2. order) participate in photocapacitive phenomena in zinc sulphide, the real and imaginary parts of the complex dielectric constant of a zinc sulphide single crystal have been investigated between 78°K and 298°K. A photoconductive effect at 298°K may be inferred from these measurements; whereas the results at 78°K are described as a photodielectric effect. Especially remarkable are the thermal emptying of the photodielectric charge at low temperature as well as the increase in capacity by a factor of about 30 when increasing the temperature from 78°K to 298°K. The capacity at 298°K shows a photocapacitive effect of 100% with strong dispersion in the frequency range of 60–300 000 cs.

Der photokapazitive Effekt für verschiedene Orientierungen von ZnS-Kristallen mit Stapelfehlern

With the aid of the photocapacitive effect anisotropy of the conductivity of ZnS crystals with stacking faults is observed.

Photokapazitiver Effekt und Lumineszenz unter IR-Bestrahlung

The photocapacitive effect (change of dielectric constant and loss angle by uv-radiation) is modified by additional infrared radiation either during or after uv excitation. This is investigated in some detail for a phosphor with photodielectric effect where the capacitance change is essentially due to polarization of filled traps. There are indications for the influence of reversible occupation changes among traps. Results with a phosphor whose capacitance change is due to conduction electrons are not yet conclusive with respect to a detailed energy band model; they show, however, that both conduction band and valence band participate in the processes.

Photocapacitance effects in additively colored alkali halide crystals

Additively colored alkali halide crystals are placed between current blocking electrodes and an increase in the capacitance and conductance of the crystal is found on illumination. A measurement of these quantities at frequencies between 2 and 1000 c/s has shown that the observed behavior follows quite closely a linearized theory developed by J. R oss M acdonald. The region of application of the theory has been examined experimentally, and results show that the theory may be used for larger applied voltages than assumed in the theory. An analysis of the experimental data indicates that the field induced electrode breakdown for the larger voltages ( $ ̃ 5 V r.m.s.) is of such a nature that the equivalent circuit describing the crystal continues to be valid. This has also been observed by others. Using the results of the theory the mobility of KI and KBr crystals at room temperature has been measured. These determinations agree substantially with measurements by others using Hall effect methods. Measurements at low frequencies have revealed phenomena due to the finite recombination rate of the conduction electrons and ionized F-centers. From our analysis of this behavior, the capture cross section of an ionized F-center in KBr has been found to be 3.1 × 10 −15√γ cm 2, where γ is the ratio of the effective electron mass (polaron) to the free electron mass. The recombination rate constant in KBr has also been measured.