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The study and characterization of the (opto)electronic properties of a-Si:H and µSi filmsby contactless transient photoconductivity measurements is presented. The importance ofminority carrier trapping is shown for the example of a-Si:H films prepared with different dopinglevels. It is shown that the microwave mobility determined by these measurements is a versatiletool for the characterization of the films. Examples are given by the study of µ Si filmsproduced by laser crystallization of a-Si:H films and the optimization of the substratetemperature for the Hot Wire deposition of µ Si films.

Contactless transient photoconductivity measurements of a-Si:H / c-Si heterojunctions are presented. It is shown that n a-Si:H / n c-Si junctions furnish an excellent passivation of the n c-Si surface. For i a-Si:H / n c-Si junctions the passivation is better for thicker films, whereas for very thin films (10 nm or less) a deviating behaviour probably due to inhomogeneities is observed. For the p a-Si:H / n c-Si junction separation of excess charge carriers in the space charge region is observed leading to a slowly decaying tail of the signal. This separation is also observed in thick i a-Si:H / n c-Si samples where electrons are injected from the a-Si:H film in the c-Si substrate.

By contactless transient photoconductivity measurements it is shown that i a-Si:H films, both of standard quality films and annealed low temperature films, passivate the c-Si surface for electron-hole pairs generated in the c-Si substrate. Films deposited at low temperature without annealing do not lead to passivation of the c-Si surface. The injection of excess electrons from a standard a-Si:H film into c-Si with a time constant of a few microseconds and a rather high efficiency was observed. For an annealed 120°C a-Si:H film a slower and less efficient injection was measured, whereas an annealed 50°C a-Si:H shows no appreciable injection.

Hydrogenated amorphous and microcrystalline silicon films were deposited on glass substrates at 250 °C by combining rf glow discharge decomposition of silane–helium gas mixtures and ultraviolet laser irradiation on the surface of the growing film. The effect of the laser fluence on the optical and electrical properties of the films was investigated with combined in situ ellipsometry and ex situ techniques. Particular attention was paid to the properties of the films deposited at low and high laser fluences. At a low laser fluence, the resulting hydrogenated amorphous silicon films display a reduced defect density as compared to unirradiated ones. At laser fluences above the melting threshold, we obtain microcrystalline silicon films with a high surface roughness, as observed by scanning electron microscopy, and high electron mobility, as deduced from time resolved microwave conductivity measurements. The enhanced optical absorption produced by the roughness along with the high electron mobility make these...

A simple model (the HTMT model) describing the interaction of excess mobile charge carriers with an exponential bandtail is presented. The new model differs from the commonly used intuitive thermalization (TROK) model only by the definition of the demarcation energy. By comparison of the results of the TROK and the HTMT model with numerical calculation of the interaction it is shown that the HTMT model yields an adequate description, whereas the TROK model does not reproduce the absolute values and also has a deviating time dependence at higher temperatures. The origin of these differences is discussed.

The plasma-induced deposition of hydrogenated amorphous silicon on crystalline silicon substrates has been investigated in situ by contactless transient photoconductivity measurements. In the initial stage of the deposition process the signals reflect charge-carrier kinetics in the substrate with the surface recombination rate modified by the deposition process. In the final stage the signal is dominated by mobile electrons in the film deposited. Between these extrema an additional contribution of excess charge carriers optically induced in the amorphous silicon film and transported to the substrate is observed. It is shown that this transport is inhibited by a high defect density in the film deposited and by a highly defective interface

In situ measurements of the transient photoconductivity during the deposition of amorphous silicon films on single-crystalline silicon substrates indicate a drastic increase of the surface recombination at the interface immediately after the start of the plasma discharge. This can be attributed to an initial damage of the substrate surface by impact of plasma-induced species on the silicon surface. The subsequent deposition of amorphous silicon films leads to a quenching of this plasma-induced surface recombination where this quenching process is faster at higher deposition temperatures. This passivation process is attributed to an interfacial relaxation occurring under influence of plasma-induced species even at an appreciable thickness of the a-Si:H film deposited. It is proposed that this process is an aspect of a general structure relaxation process in the material.

In-situ and ex-situ transient photoconductivity measurements of intrinsic a-Si:H films deposited from He-diluted SiH4 are presented. It is shown that material with good optoelectronic properties can be deposited at high deposition rates. The films can already be characterized during the deposition. It is shown that material with fairly different properties can be deposited with a relative low defect density.

Contactless transient photoconductivity measurements in a- Si:H/c-Si heterojunctions are presented. The experimental data are decomposed in the contribution of excess electrons in a-Si:H, the contribution of excess carriers in c-Si and the contribution of excess carriers injected from a-Si:H into c-Si. A value for the electron drift mobility of in a-Si:H of 1cm2 V-1 s-1 is determined. At low excitation densities a high injection efficiency is observed, which is quenched by bias illumination.

A ZnO-nanorod/In2S3/CuSCN radial heterostructure has recently shown promising photovoltaic conversion efficiencies. In this work, the charge separation and recombination in single ZnO/In2S3 and In2S3/CuSCN interfaces as well as the complete ZnO/In2S3/CuSCN structure were studied by time resolved microwave photoconductivity. Photoconductivity transients were measured for different thicknesses of the In2S3 light absorbing layer, under variation in the exciting light flux and before and after annealing of the ZnO nanorods at 450 °C. Upon excitation with 532 nm light, a long lived (millisecond) charge separation at the In2S3/ZnO interface was found, whereas no charge separation was present at the In2S3/CuSCN interface. The presence of the CuSCN hole conductor increased the initial amplitude of the time resolved microwave conductivity signal of the In2S3/ZnO interface by a factor of 8 for a 6 nm thick In2S3 layer, but the enhancement in amplitude dropped strongly for thicker films. The measurements show that the primary charge separation is located at the In2S3/ZnO interface but the charge injection yield into ZnO depends critically on the presence of CuSCN.

The signal of the Transient Microwave Detected Photoconductivity after laser pulse illumination is followed during the PECVD growth of intrinsic amorphous silicon on top of crystalline silicon (c-Si) substrates. Depending on the thickness of the amorphous layer the observed phototransients can be assigned to the excess charge carriers in different regions of the so grown heterojunction. For thin a-Si:H layers the signal amplitude decreases exponentially with deposition time. In the case of thick amorphous silicon the signal reflects only the properties of the amorphous silicon and the signal amplitude is constant. In an intermediate range a large contribution of charge carriers absorbed in the a-Si:H and subsequently transported to the crystalline part of the heterostructure to the measured signal can be observed. Due to their much higher mobility in the crystalline layer these excess charge carriers become visible only after reaching the crystalline silicon. This leads to the observed delay of the TRMC signal amplitude relative to the exciting laser pulse. The experiments show, however, that this charge carrier transfer takes place only in the case of a low defect a-Si:H layer to a measurable extent and is suppressed by the low carrier mobility in highly defective a-Si:H deposited at lower substrate temperatures.

A study of excess carrier kinetics in a-Si:H / crystalline n-Si heterojunctions by transient photoconductivity measurements as a function of the excitation density is presented. It is observed that excess carriers can be injected from the a-Si:H in the c-Si part of the junctions, where the time associated to this injection process can be clearly resolved. The number of injected carriers increases with the number of excess carriers induced in the a-Si:H at low excitation density reaching a limit in the moderate excitation range. The driving force of the injection process is tentatively ascribed to the space charge field at the interface. The injection efficiency is estimated to be about 10%.

The optoelectronic properties of a-SiC:H alloys with different carbon content are investigated with time resolved photoconductivity and photoinduced absorption measurements. It is shown that the electron drift mobility decreases with increasing carbon content. Also a weak increase of deep electron trapping with increasing carbon content is suggested by the experimental data. The recombination does not seem to be strongly changed by the presence of carbon at least at low carbon content.

The relaxation of pulse‐induced mobile charge carriers in a‐Si:H films is followed by transient photoconductivity measurements in the microwave frequency range (time resolved microwave conductivity measurements). It is shown that a‐Si:H films deposited at substrate temperatures lower than 150 °C are characterized by a lower electron drift mobility and a higher deep trap concentration for electrons than a‐Si:H films deposited at 250 °C, where films deposited at 150 °C have only a lower drift mobility. Annealing of a film deposited at 30 °C leads to a higher drift mobility and a lower deep trap concentration with increasing annealing temperature. However, the (opto)electronic properties of the annealed films are still inferior to those of films deposited at the annealing temperature.

The growth of n-i-p and p-i-n amorphous hydrogenated silicon structures, as used in solar cells, has been followed by measuring the transient photoconductivity with contactless microwave reflection measurements. The signals in p-layers are much lower than in intrinsic or n-type material due to the increased electron decay rate. Influences of a thin n- or p-layer on the transient photoconductivity signals during growth of a subsequently deposited intrinsic layer are discussed. The optical interference fringes can be used for in-situ determination of the deposition rate during growth.

The deposition process of multiple layer structures of intrinsic and p-type hydrogenated amorphous silicon was followed by measuring the microwave detected transient photoconductivity (TRMC) during the film growth. In an i-p-i structure we can show that after deposition of an upper layer of about 500 nm, former deposited layers do not influence the TRMC-signal any more. In an i-p+-i-p structure we can clearly distinguish between p-layers of different doping concentration.

Electrical modifications of InP samples induced by reactive ion etching (RIE) with CH4/H2 have been studied using (a) electrochemical capacitance–voltage measurements, (b) current–voltage characteristics of Schottky diode structures, and (c) transient reflected microwave conductivity measurements. From the capacitance–voltage measurements we confirm a depletion of holes near the surface of the dry-etched p-InP sample and an accumulation of electrons near the surface of the dry-etched n-InP sample. The hole concentration in p-InP sample can be recovered after a 400 °C, 1 min heat treatment in an Ar ambient. From the transient reflected microwave conductivity measurements we reveal new information on the modification of charge carrier mobilities and kinetics caused by dry etching and subsequent annealing procedures on p-InP, n-InP, and semi-insulating (Fe-doped) InP samples. The n-InP sample shows a loss in electron mobility after RIE and annealing; the p-InP sample shows a weakly n-type behavior after RIE ...

Transient photoconductivity measurements have been performed in situ during plasma-enhanced chemical vapor deposition of amorphous hydrogenated silicon by a contactless method that uses the change of the microwave reflection after laser pulse illumination. Through the use of the interference pattern of the amplitude of the transients of microwave reflection during the layer growth, the actual thickness of the amorphous film can be determined. In the case of crystalline silicon substrates, the change in the light absorption in the substrate modified by the growth of the amorphous layer is measured directly. An example of the optimization of antireflective layers on crystalline silicon substrates is shown. A good agreement is found between the experimental data and calculations of optical reflection and transmission on the multilayer structures.

Charge carrier kinetics in doped and undoped a-Si:H has been studied by contactless photoconductivity measurements. Comparison of signals obtained by above and sub-bandgap excitation is used to separate electron and hole kinetics. The strong dependence of charge carrier kinetics upon the doping level in a-Si:H is explained by the increase of structural disorder in a-Si:H with the doping level.

The influence of extended illumination on charge carrier decay processes in undoped and n-type a-Si:H was studied by a contactless time-resolved photoconductivity technique. In intrinsic a-Si:H light soaking decreases the effective electron lifetime by the generation of recombination centers. Additionally, an increase of the sub-bandgap absorption at 1064 nm was observed upon extended illumination. In n-type a-Si:H the influence of extended illumination is much less pronounced. However, also a decrease of the effective electron lifetime was observed.

Single crystals of the layer type compounds MoSe2 and WSe2 have been grown from various high temperature solutions. The fluxes used for crystal growth were either metals (Sb, Bi, Te), semiconductors (Se) or saltlike melts (PbCl2). The photoactivity of these materials was investigated by photoelectrochemical methods, time resolved microwave conductivity (TRMC) and temperature dependent DC conductivity. The comparison of crystals grown from different melts and from chemical vapor transport (CVT) indicates that foreign phase melts are deleterious for the optoelectronic properties. Melt grown crystals are less photoactive than selected CVT material. Possible origins of the observed differences are discussed.

Photoconductivity transients in a-Si: H measured in coplanar electrode geometry (PC) and measured with the time-resolved microwave conductivity (TRMC) method are identical. It can be concluded that with these techniques the dynamics of excess free electrons are investigated, where the influence of contacts in the PC-method can be neglected. The dependence of the signals on the exciting light density lead to a tentative recombination model.

Optical absorption studies of pulse-irradiated ice have been carried out with subnanosecond time resolution. A growth in the solvated electron absorption is observed with a risetime of 450 ps at -5/sup 0/C. The activation energy for relaxation of the matrix following localization is 0.3 eV. Below -35/sup 0/C localization at trapping sites becomes the rate-controlling step in solvation. Addition of 10/sup -2/M NH/sub 4/F considerably increases the overall solvation time by reducing the initial rate of dry electron localization at defects. A second absorbing species is produced immediately in the pulse with a yield Gepsilon/sub 660/ = 9 x 10 M/sup -3/ cm/sup -1/ (100 eV)/sup -1/, which is independent of temperature. This is tentatively ascribed to an exciton state. 4 figures, 1 table.

Ice is frequently taken as a model when factors controlling proton transport in hydrogen-bonded molecular networks are discussed. Such discussions have increased with the acknowledgement that proton transfer across cell membranes may play a significant part in energy conversion and storage in biological systems1–4 and that this transfer may involve hydrogen-bonded chains spanning the membrane5,6. However, there is still much uncertainty about the basic mode of proton displacement and the external factors which effect bulk proton transport in ice itself. We present here results of a microwave conductivity pulse radiolysis technique which has been used to determine the mobility of bare protons in ice, and thus reduce some of the uncertainty over proton mobility.

Several stationary and transient techniques are used to characterize a large number of intrinsic a-Si:H films prepared in a narrow range of production conditions. Correlation between stationary photoconductivity, the activation energy of the dark conductivity and the long-time range decay of the transient photoconductivity and photoinduced absorption is observed. The applicability of a-Si:H films for solar cells in view of these properties is discussed.