Microwave Photoconductivity Method Research Articles

The investigation of the kinetics of the losses of nonequilibrium current carriers in Cu-=SUB=-2-delta-=/SUB=-NiSnS-=SUB=-4-=/SUB=- (0≤delta≤0.2) quaternary copper compounds

By solid-phase synthesis from elemental Cu, Ni, Sn, and S samples Cu2-deltaNiSnS4 (0≤delta≤0.2) was prepared. The parameters of their crystal lattice have been refined. The lifetimes of photogenerated current carriers in Cu2-deltaNiSnS4 were estimated for the first time by the contactless time-resolved microwave photoconductivity method. This times turned out to be tau~7 ns, which is comparable with the literature data for the times in kesterites CZTS. Herewith, in kinetics of loses of photogenerated cureent carriers is observed predominance of bimolecular recombination processes over capture processes. Keywords: Cu2-deltaNiSnS4, onequilibrium current carriers, lifetimes, kinetics.

Исследование кинетики гибели неравновесных носителей заряда в четверных соединениях меди Cu-=SUB=-2-delta-=/SUB=-NiSnS-=SUB=-4-=/SUB=- (0≤delta≤0.2)

By solid-phase synthesis from elemental Cu, Ni, Sn, and S samples Cu2 - δNiSnS4 was prepared . The parameters of their crystal lattice have been refined. The lifetimes of photogenerated current carriers in Cu2 - δNiSnS4 were estimated for the first time by the contactless time-resolved microwave photoconductivity method. This times turned out to be τ ≈ 7 ns, which is comparable with the literature data for the times in kesterites CZTS. Herewith, in kinetics of loses of photogenerated cureent carriers is observed predominance of bimolecular recombination processes over capture processes.

Influence of Synthesis Conditions on the Lifetime of Excess Charge Carriers in Monograin Powders with Kesterite Structure

By solid-phase synthesis, macrocrystalline Cu2 − δZn2 − xSnxS4 − ySey monograin powders have been prepared, the chemical composition of which was shown by XRD and Raman spectroscopy to be different in powder fractions of different sizes formed during the synthesis. The influence of synthesis conditions on the decay kinetics of photogenerated charge carriers in different fractions has been studied using the frequency–time-resolved microwave photoconductivity method. The characteristic photoresponse half-life, τ1/2, increased with increasing grain size. The smallest values of the lifetime were observed for the fraction with sizes of 50–70 μm (τ1/2 <5 ns), and the largest values were for the fraction with a grain size of 70–90 μm (τ1/2 ~ 12 ns).

Two advanced research methods: Frequency-time-resolved microwave photoconductivity and broadband photodielectric spectroscopy

Two known methods of research of electric properties were applied on semiconducting materials and their methodology was significantly improved to obtain quantitative data on elementary reactions of charge carriers. The resonator method of microwave photoconductivity included the measurements of the frequency dependences of photoresponse. The modified frequency- and time-resolved microwave photoconductivity method gives the opportunity to separate contributions to a photoresponse of the free electrons which did not experience capture by acceptors and structural defects, and the electrons which are again released in a free state from traps. The use of a light source (λ = 350 ÷ 2500 nm) together with a broadband dielectric spectrometer provides an effective tool (the broadband photodielectric spectroscopy—BPDS) for research of relaxation processes in semiconductors caused by excess current carriers. Examples of investigation of the photoelectric properties of CdS, CdTe, and Cu(In,Ga)Se2 received with modified methods are presented.

The study of influence of doping by iodine or gallium on the lifetime of photogenerated current carriers in CdTe

This study investigated how doping by iodine or gallium affects processes associated with current carriers by undertaking a kinetics investigation of electron loss processes in the CdTe samples prepared by direct ampoule method. Owing to these processes, the current carriers generated by light can be trapped or recombined after a thermalization process and fail to reach electrodes during their lifetime and adversely impact the photovoltaic performance of solar cells. The microwave photoconductivity method provides an effective means of obtaining direct data on the lifetime and energy distribution of traps for current carriers. The obtained lifetimes in doped materials were compared to those in initial polycrystalline CdTe and monocrystalline CdTe. The experimental results indicate that the free electron lifetimes become higher after the doping. We found that the lifetime in the doped polycrystalline samples was equal (for I-doped samples) or even higher (for Ga-doped samples) than in CdTe monocrystals. The model based on the self compensation effect was suggested for the explanation of the experimental results.

The study of loss kinetics of current carriers in copper-indium-gallium selenide by microwave photoconductivity method

This study investigated how stoichiometry affects processes associated with current carriers by undertaking a kinetics investigation of electron loss processes in the copper-indium-gallium selenide (CIGS) samples prepared by PVD and sol-gel methods. Owing to these processes, the current carriers generated by light can be trapped or recombined after a thermalization process and fail to reach electrodes during their lifetime and adversely impact the photovoltaic performance of solar elements. The microwave photoconductivity method provides an effective means of obtaining data on the lifetime and energy distribution of traps for current carriers. Experimental results indicate that the free electron lifetimes in the best samples prepared in both methods are approximately the same, yet decrease with a decline in stoichiometry.

Effect of Annealing Temperature on the Properties of Silicon Crystal

In order to investigate the performance of silicon single crystal depended on the annealing temperature, the minority carrier lifetime, the resistivity and oxygen concentration after different temperature annealing in Ar ambient were examined. And the effect of oxygen and related defects formed during annealed on the minority carrier lifetime were analyzed by microwave photoconductivity method, Fourier transform infrared spectrometer and four-probe measurement. The results indicate that after 450°C annealing for 30h, the resistivity and minority carrier lifetime of silicon increase significantly, while the concentration of interstitial oxygen decreases. After the annealing at 650°C, oxygen donor can be removed and the resistivity and the minority carrier lifetime decrease. During the high-temperature (above 650°C) annealing, the oxygen precipitation can decrease the minority carrier lifetime silicon.

The role of underbarrier transitions in processes of annihilation of excess charge carriers in II–VI semiconductors

The loss kinetics of charge carriers generated by the emission pulses of nitrogen laser in II–VI semiconductor compounds, namely, n- and p-CdTe and CdS, is studied by the method of microwave photoconductivity. The decays of photoresponse after switching off the light consisted of fast (manifesting itself at times 50 ns) components. The shape of decay of a slow component was virtually independent of temperature, and at long times, the decay law corresponded to the linear dependence of the photoresponse on the logarithm of time. The assumption follows from the results that the slow component of the photoresponse decay reflects the loss process of charges captured in the traps due to the tunnel recombination.

On the possible role played by tunnel recombination in the loss processes of excess current carriers in cadmium telluride

The microwave photoconductivity method was used to study the kinetics of the decay of current carriers generated by nitrogen laser pulses in n- and p-type cadmium telluride. The dependences of the shape and amplitude of photoresponse decays on temperature and light intensity were studied. Photoresponse decays contained “fast” (at t 50 ns) components. At long times, the dependence of photoresponse on the logarithm of time was linear. The shape of slow component decays was almost independent of temperature. The slow component of photoresponse decay could correspond to the loss process of entrapped charges in tunnel recombination.

Ionic and Electronic Properties of Silver Iodide Grains

It has been observed in the preceding paper that the ionic conductivity of AgI grains with interstitial silver ions as carriers is comparable to that of octahedral AgBr ones. Since the activation energy of the migration of interstitial silver ions in AgI is much smaller than that in AgBr, it is estimated that the concentration of interstitial silver ions acting as shallow electron traps is much larger in AgI grains than in AgBr ones, providing an explanation for the observation by a microwave photoconductivity method that the photoconductivity with photoelectrons as carriers is smaller in AgI grains than in AgBr grains. The observation that latent image centers on AgI grains is oxidized at more negative redox potential than that on AgBr grains has been ascribed to the fact that the concentration of silver ions in the vicinity of the former is smaller than that in the vicinity of the latter owing to the difference in the solubility in water between them. A radiofrequency photoconductivity method has revealed that the activity of positive holes in AgI grains in an emulsion is higher than that in AgBr grains in an emulsion, and is depressed by increasing the concentration of silver ions in the emulsion. Although these results make it more difficult to form latent image centers on AgI grains than on AgBr ones in conventional photographic materials, the latent image formation on AgI grains could be enhanced by increasing the concentration of silver ions in conventional materials, and by using them in photothermographic materials, in which the concentration of silver ions was usually much higher than in conventional ones owing to the presence of silver carboxylate. Recent discovery that AgI grains are fixed during thermal development in photothermographic materials has merged with the above-stated results to reveal that AgI grains are suitable and useful for photothermographic materials.

Recombination rate constant of free electrons and holes in thin CdSe films

The decay kinetics of electrons generated in thin CdSe films by laser pulse (wavelength 337 nm, pulse duration 8 ns) at 295 K was studied by the microwave photoconductivity method (36 GHz). Based on analysis of the photoresponse decay kinetics and the reactions of free and trapped electrons, holes, and ions, a model for the processes was proposed and the recombination rate constant of free electrons and holes in cadmium selenide was determined, being (4–6)· 10–11 cm3 s−1.

The study of elementary steps of the photolysis of silver halides by the method of microwave photoconductivity

The major part of inconsistencies in the description of the mechanism of formation of nonhalide silver clusters under exposure to light is due to the lack of quantitative data on electronic-ionic processes in microdispersed silver halide systems. The use of the microwave photoconductivity technique partially fills the gap. The drift mobility of holes and electrons in powdered AgBr (0.8 and 60 cm2 V−1 s−1, respectively) and of electrons in powdered AgCl (40 cm2 V−1 s−1) and the rate constants of the recombination of localized electrons and free holes in powder AgBr (1.4 × 10−7 cm3 s−1) and of free electrons and holes in AgBr and AgCl (1.5 ± 0.5) × 10−11 and 2 × 10−12 cm3 s− 1, respectively) were determined. The range of the depth of traps formed by treatment with aqueous solutions of sodium thiosulfate in AgCl (0.45–0.63 eV) and by preliminary exposure of AgCl and AgBr grains (0.4 and 0.25–0.36 eV, respectively) was estimated. The rate constant of the hole capture by the centers formed by introducing AgI (7 × 10−9 cm3 s−1) was determined, and the lifetime of the Ag atom (10−4 s) was estimated. The paper summarizes the results obtained.

The role of inter- and intraparticle transport for the microwave photoresponse in nanocrystalline semiconductors

The photoinduced change of electrical conductivity and dielectric function were investigated in porous silicon and porous TiO2 by means of Complex Transient Microwave Photoconductivity Method. In porous TiO2, the photoexcitation leads to the increase of the dielectric function. The amplitude of this increase depends strongly on the size of the nanoparticles. The sign of the photoinduced change of the dielectric function is negative for mesoporous silicon and positive for nanoporous silicon. The photoinduced increase of the dielectric function in porous TiO2 is explained by the spatial separation of photoexcited charge carriers inside one nanoparticle while the interparticle transitions have very small probability during one microwave half period. In porous silicon, the confinement of charge carriers depends on the porosity. Therefore, the positive photoinduced change of the dielectric function is observed only in nanoporous silicon where most of photoexcited carriers are strongly localized.

Rate constant of free electron-hole recombination in thin cadmium sulfide films

The transient decay kinetics of electrons generated in thin cadmium sulfide films by short laser pulses was studied by the microwave photoconductivity method (9 and 36 GHz) at 295 K. The films were prepared by the pulverization method from thiocarbamide coordination compounds. At the high light intensity I 0 > 1014 photon cm–2 per pulse, the decay kinetics of photoelectrons corresponded to a reaction of the second order. Analysis of the kinetic data made it possible to determine the rate constant of recombination of free electrons and holes: k 3 ≈ 2(±1)·10–13 cm3 s–1.

Microwave photoconductivity in nanocrystalline porous titanium oxide subjected to pulsed laser excitation

A contactless method of microwave photoconductivity was used to study the relaxation of photoexcited charge carriers in nanocrystalline porous titanium oxide. A decrease in the amplitude and a change in the microwave photoconductivity relaxation time is observed with reduction in the nanocrystal size, which is explained by localization of carriers due to dielectric confinement. Capture of holes by the traps on the TiO2 surface increases the amplitude and prolongs the relaxation time of the microwave photoconductivity.

Diagnostics of high-resistivity GaAs wafers by microwave photoconductivity

It is demonstrated that the method of microwave photoconductivity developed previously for the study of ultrapure Si can be successfully employed in the qualitative diagnostics of pure high-resistivity GaAs. Data on the microwave photoconductivity of GaAs are in good agreement with the results of measurements of the Hall effect, C-V characteristics, and low-temperature photoluminescence.

Rate constant of the recombination of free electrons and holes in AgCl at 295 K

The transient kinetics of the loss of electrons generated by light pulses in powdered AgCl has been studied by the microwave photoconductivity method (36 GHz) at 295 K. At high light intensities,I0 > 1014 photon cm−2 per pulse, the kinetics obeys the second-order law. The rate constant of the recombination of free electrons and holes is equal to 2·10−12 cm3 s−1.

Photoeffects in cadmium sulfide and sulfoselenide powders: A reexamination by microwave photoconductivity

Cu doped CdS powders in electrophotographic devices have been characterized using the flash microwave photoconductivity method. The signals representative of these materials exhibited minima, as well as maxima, in microwave absorption, hence conductivity; the former were identified with the slow thermalization of deep trapped electrons, which allowed the trapping characteristics of the samples to be inferred. Correlation of the microwave photoconductivity results with electrophotographic measurement elucidated the role of the trapping states in interfering with electrophotographic response. From temperature and wavelength dependence of the photoconductivity it was inferred that (except at low temperatures) the conduction band is not populated directly on light absorption, even of band-gap wavelengths. A kinetic analysis revealed that the observed decay of the photoconductivity was controlled by hole trapping at the Cu(I) centers in the powder; thus the experimental data could be used to reveal a two-stage mechanism for hole trapping, and some features of the potential surface associated therewith are inferred.