Photocarrier Grating Technique Research Articles

Further insight into the oscillating photocarrier grating technique: influence of the oscillation amplitude

Allowing for the simultaneous determination of the photocarriers drift mobilities and their small-signal recombination lifetime, the Moving photocarrier Grating Technique (MGT) is a useful characterization tool for photoconductive semiconductors. This technique is based on measuring the steady-state direct current induced by an interference pattern (IP) moving at a constant velocity between two coplanar ohmic contacts deposited on the semiconductor. The main drawback of the technique is the low level of the signal to be measured, which can be masked by noise. The Oscillating Photocarrier Grating technique (OPG), where the IP oscillates at a constant speed, has been proposed as an alternating current version of MGT, providing a higher signal-to-noise ratio. The IP oscillation is produced by the phase modulation of one of the interfering beams. Using the multiple trapping model we deduce the expression of the current density generated by OPG in a photoconductor. We observed theoretically and experimentally that OPG is not equivalent to MGT for the previously used amplitude of oscillation, especially when the IP moves at high speeds. However, we show that the desired equivalence between both techniques could be recovered by increasing the amplitude of oscillation. A phase modulator capable of achieving such amplitudes is required for the correct implementation of OPG.

The chopped moving photocarrier grating technique.

The Moving photocarrier Grating Technique (MGT) allows the simultaneous determination of the photocarrier drift mobilities and the small-signal recombination lifetime of photoconductive semiconductors. The technique measures the direct current (DC) induced by a monochromatic illumination consisting of a moving interference pattern superimposed on a uniform background of much higher intensity. A drawback of the technique is the low level of the signal to be measured, which can be masked by the noise at low temperatures or low light intensities. In this work, we propose implementing an alternating current (AC) version of the MGT by chopping the weak beam in the standard configuration. We call this new technique the Chopped Moving photocarrier Grating (CMG). In CMG, the AC signal can be measured with a lock-in amplifier for electrical noise removal. In this way, the signal-to-noise ratio can be increased compared to the standard DC technique. Assuming a multiple-trapping model for charge transport, we find the theoretical expression for the current density induced by CMG at fundamental frequency. By using a numerical simulation with parameters typical for hydrogenated amorphous silicon, we verify the expected equivalence between both techniques for low enough chopping frequencies. Then, we test experimentally this equivalence for an undoped hydrogenated amorphous silicon sample. For low signal levels, we demonstrate the superior performance of CMG.

Diffusion Length Measurement in CdS Thin Film by Steady State Photocarrier Grating Technique

Diffusion Length Measurement in CdS Thin Film by Steady State Photocarrier Grating Technique

Analysis of the moving photocarrier grating technique for semiconductors of high defect density

The moving photocarrier grating (MPG) technique allows us to determine the carrier drift mobilities and the recombination lifetime of semiconductors. This technique utilizes a spatially and temporally modulated light intensity for the generation of photocarriers, which leads to a modulation of the carrier densities. The different mobilities of electrons and holes introduce a phase shift between the charge distributions. The resulting internal electric field produces a short circuit current which depends on the modulation frequency. The theoretical calculation of that short circuit current includes the diffusion and drift currents, and the generation and recombination processes. In the original treatment of the MPG method recombination between electrons and holes was assumed to proceed only through band-to-band transitions. We analyze here the MPG method by explicitly considering recombination through dangling bond states. We simulate short circuit current curves and compare them with measurements. The inclusion of recombination through dangling bonds allows us to explain an apparent contradiction in the evolution of the recombination lifetime as a function of light-induced degradation.

Deposition of Device Quality Amorphous Silicon by Hot-Wire CVD

ABSTRACTIn this paper we present the results of the optimization of hydrogenated amorphous silicon films deposited by the hot-wire method in a larger area system. Using a two-wire design, we succeeded in depositing films that exhibit uniform electrical properties over the whole 4” x 4” Corning 7059 glass substrate. At a substrate temperature of 430 °C. and a pressure of 20 μbar we obtained a growth rate of ∼2 nm/s. The temperature of the tungsten filaments was kept at 1850 °C. The values for the photoconductivity and dark conductivity were 8.9×10−6 S/cm and 1.6×10−10 S/cm respectively, whereas the ambipolar diffusion length, as measured with the Steady-State Photocarrier Grating technique (SSPG), amounted to 145 nm. This value is higher than for our device quality glow-discharge (GD) films, which yield devices with efficiencies higher than 10%. The hydrogen content was 9.5%.We report on the density-of-states (DOS) distribution in the films, which was measured with the techniques of Thermally Stimulated Conductivity (TSC) and Constant Photocurrent Method (CPM). Furthermore, we describe the behavior of the electrical properties on light-induced degradation. Finally, we incorporated these films in solar cells, using conventional GD doped layers. Preliminary SS/n-i-p/ITO devices yielded efficiencies in excess of 3% under 100 mW/cm2 AM 1.5 illumination. Further work concerning the optimization of the interfaces is in progress.

The general Green’s-function solutions for the two carrier concentrations under any small signal nonuniform carrier generation

Thus far the general theory of the spatial distribution of photocarriers under the presence of a small signal inhomogeneous photocarrier generation has been limited to the one carrier case. In this work the more general and more complicated case of the spatial distribution of the two types of excess carriers is addressed. This distribution is pertinent in particular to the newly developed photocarrier grating (PCG) technique. The above two carrier problem and the PCG technique are discussed in a much broader context than previously given in the literature. A Green’s-function approach, applied here for the first time, provides a general recipe for the problem of inhomogeneous carrier generation and it gives a better physical insight into the corresponding system by showing how the local carrier concentration is determined by the carrier generation in the entire sample. Hence the mathematical and physical basis for the PCG technique is provided here. The generality and the usefulness of the present approach for the nontrivial carrier generations of the ‘‘single square pulse,’’ the ‘‘square wave,’’ and the ‘‘one period sinusoidal pulse,’’ which have not been considered previously, are demonstrated.

Application of Steady-State Photocarrier Grating Technique for Determination of Ambipolar Diffusion Lengths in Cu (In, Ga) (S, Se)<sub>2</sub> - Thin Films for Solar Cells

Application of Steady-State Photocarrier Grating Technique for Determination of Ambipolar Diffusion Lengths in Cu (In, Ga) (S, Se)<sub>2</sub> - Thin Films for Solar Cells

Electrical Transport in Mesoporous Silicon Layers

ABSTRACTIn order to clarify the role of the enlarged surface area of porous silicon on the electrical conductivity, we have studied the transport in mesoporous silicon layers, for which quantum confinement effects are negligible. We prepare free standing mesoporous films, from highly doped p-type Si wafers. The dark conductivity of the mesoporous layers is activated with an energy of 0.5 eV. Thermopower measurements show negative sign indicating electron conduction. The exposure of these layers to methanol vapor results in an increased conductivity and change of the thermopower magnitude. Photoconductivity measurements and the Steady-State Photocarrier Grating technique (SSPG) are used to evaluate the density of the surface states and the dynamics of the photo-excited carriers. All these results indicate that a large density of surface states exist, which results in a depletion of the free holes.

Moving Photocarrier Grating Technique for Mobility and Lifetime Measurements in Amorphous Semiconductors

ABSTRACTIn this paper we describe a new method for the determination of the carrier lifetime (r) and the carrier Mobilities (μn, μp) in semiconductors. This technique utilizes a moving intensity grating that is generated by superposition of frequency shifted laser beams for the illumination of the sample. The Material parameters are extracted from the short circuit current in the sample induced by the moving grating as a function of it's velocity (vgr). We solve the continuity equations and Poisson's equation in the small signal approach for the modulated electron and hole densities and show how these densities result in an electric field that in turn acts on the electrons and holes in order to yield a short circuit current density jsc. From a fit of this expression for jsc to experimental data we determine μn, μp and τ.

Ambipolar Diffusion Length in CuGaSe2 Thin Films for Solar Cell Applications Measured by Steady-State Photocarrier Grating Technique

The steady-state photocarrier grating technique is used to measure the transport properties of polycrystalline CuGaSe2 thin films. The influence of composition and growth temperature is analysed in bilayers grown exactly the same way as absorbers for solar cells. The photocurrent losses in solar cells in `substrate' and `superstrate' configuration are related to the measured diffusion lengths.

The Photocarrier Grating and Its Applications in the Study of A-Si:H Materials and Devices

ABSTRACTTo date the photocarrier grating (PCG) technique is widespread among a- Si:H researchers. This technique is presently the only available technique for the determination of the minority carrier diffusion length in materials where this length is in the submicron range. The basic idea or the PCG technique will be presented, and results which follow its fruitful use in the study of a-Si:H materials and devices will be reviewed. Finally some future studies of these materials by the PCG method will be suggested.

Accurate determination of the two carriers steady-state mobility-lifetime products in hydrogenated amorphous silicon

The photocarrier grating (PCG) technique is used for the first time in the high electric field regime. The results are shown to confirm previously unproven theoretical predictions for this regime. It is demonstrated that, by application of the PCG technique in the high-field regime, accurate values for the ratio of the two carriers mobility-lifetime products can be deduced. This is in contrast with the fact that their sum cannot be derived accurately from the measurement of photoconductivity because one cannot determine accurately the carrier generation rate. Combining the ratio, determined by the high field PCG, with the low-field data yields accurate values for the mobility-lifetime products of both carriers.

Theory of the small photocarrier grating under the application of an electric field.

Presently, a highly reliable method for the determination of the minority-carrier diffusion length in noncrystalline materials is the photocarrier grating (PCG) technique. The theory and the application of this technique have been essentially limited to the low-electric-field regime, i.e., where the carriers drift lengths are negligible in comparison with the carriers diffusion lengths. Recently, closed-form solutions that connect the microscopic mobility-lifetime (\ensuremath{\mu}\ensuremath{\tau}) products with the experimentally derived parameter \ensuremath{\beta} have been given for the above regime, thus allowing for the determination of the conditions under which the interpretation of the experimental results in terms of the minority carrier \ensuremath{\mu}\ensuremath{\tau} is justified. In contrast, for the high-electric-field regime only numerical solutions are available at present. These solutions are not very useful for the unique determination of \ensuremath{\mu}\ensuremath{\tau} products, and they do not provide physical insight into the transport and kinetic processes taking place in the PCG. In this paper we present analytic solutions for the PCG conductance in the high-field regime by using a diffusion-drift ``perturbation'' approach. The results reveal the physical processes in the grating and yield a general explicit relation between the measured quantities and the microscopic transport and kinetic parameters. The important prediction of the present theory is that the PCG technique in the high-field regime can yield direct and unique values for the majority-carrier \ensuremath{\mu}\ensuremath{\tau} product. The advantage of the PCG, in comparison with the conventional photoconductivity measurement, which is used for the same purpose, is that it is independent of experimental parameters, which are hard to determine. In view of the application of the PCG method to materials where trapping plays a dominant role, our analysis considers the PCG when trapping takes place, yielding closed-form solutions that include the trapping parameters.

A General Analysis of Steady State Photocarrier Grating Technique for the Determination of Ambipolar Diffusion Length

ABSTRACTA general photoconductivity formula is derived for the case of a semiconductor steady state photocarrier grating (SSPG)1. It is shown that, under the condition of weak applied electric field, the ambipolar diffusion length can be determined by the SSPG technique1without the lifetime-regime restriction2,3 if the lifetime of photocarriers is known. The general formula presented here is reduced to the simple lifetime-regime formula1–3 under the condition of fast dielectric relaxation.

Diffusion Lengths in a-SiGe:H and a-SiC:H Alloys from Optical Grating Technique

Ambipolar diffusion lengths in a-SiGe:H and a-SiC:H have been analyzed by Steady State Photocarrier Grating Technique. Diffusion lengths and photoconductivity of a-Si:H are considerably affected by alloying. Photoconductivity in a-SiGe:H can be improved by special deposition methods, diffusion lengths, however turn out to remain nearly unchanged. The comparison of diffusion lengths and -ho~oconductivity yields hole mobilities in a-Si 1-x Ge x :H of 10 -2 cm 2 /Vs for O≤×≤O.2. For a-Si:H changes in photoconductivity by generation of defects (light soaking) result in noticeable changes in diffusion lengths, whereas different photoconductivities in a-SiGe:H caused by different deposition methods end up in the same diffusion lengths. Consequently the improvement of a-SiGe:H photoconductivity by changes in deposition condition is by far a much more complex process than only decrease of density of midgap states.

Diffusion length measurements in a-Si:H using the steady state Photocarrier grating technique

The new Steady State Photocarrier Grating technique for minority carrier diffusion length measurements is briefly reviewed. The uncertainty about the magnitude of the surface recombination velocity is shown to introduce an error of less than 5% in the results. The ratio between the electron and hole drift mobilities in several samples of a-Si:H is determined by a simultaneous measurement of photoconductivity and the diffusion length on the same sample and it is found that there is no correlation high values of the photoconductivity and high values of minority carrier diffusion length.