Steady State Photocarrier Grating Technique Research Articles

Numerical modeling based on the analytical solution of the steady-state small signal photocarrier grating equations and analysis of field-dependent measurements for noncrystalline Si: H samples

The available experimental data of field-dependent steady-state photocarrier grating (SSPG) are numerically modeled employing the derived formula of [Formula: see text] (where [Formula: see text] represents the ratio of coherent to incoherent currents in SSPG technique) obtained from the analytical solution of the small signal steady-state grating problem by applying Galerkin’s method. This numerical analysis shows the extent of validity and success of using the derived formula of [Formula: see text] [R. I. Badran, Results Phys. 17, 103079 (2020)] in reproducing the field-dependent data for noncrystalline Si: H samples. The best quality of fittings is reproduced when [Formula: see text] value is minimum. This analysis provides us with approximate and corrected values for each of transport quantities, namely, the small-signal response lifetime, electron and hole mobilities that are found in agreement with the values obtained from the applications of other methods. For two polymorphous (pm) Si: H samples, the average estimated, corrected values for the carrier density and mobility-lifetime product, namely, [Formula: see text][Formula: see text]cm[Formula: see text] and [Formula: see text][Formula: see text]cm[Formula: see text] and ([Formula: see text][Formula: see text]cm2V[Formula: see text] and [Formula: see text][Formula: see text]cm2V[Formula: see text], respectively, are obtained. The analysis is also extended to analyze the field-dependent data for nanocrystalline (nc) Si: H samples at low temperatures. The extracted values of [Formula: see text] and ([Formula: see text] are [Formula: see text][Formula: see text]cm[Formula: see text] and [Formula: see text][Formula: see text]cm2V[Formula: see text].

Increasing the scope and precision of the steady-state photocarrier grating technique by measuring the photocurrents at several voltages

The steady-state photocarrier grating (SSPG) experiment is a popular technique for extracting the minority carrier diffusion length of photoconductive thin films in coplanar configuration. The diffusion length is basically obtained from the measurement of the steady-state photocurrent produced by a low applied voltage while the material is illuminated by two monochromatic laser beams of different intensities that interfere between the electrical contacts of the sample. Despite its simplicity and popularity, it is well known that the technique can overestimate the minority carrier diffusion length in some samples. In this paper, we show that the precision of the technique can be substantially increased by performing the same experiment at different voltages. Additionally, we show how to estimate fundamental material parameters from the experiment, such as the density of states at the majority carrier quasi-Fermi energy and the ratio between the recombination states’ capture coefficient and mobility of majority carriers. First, we show that the procedures found in the literature for correcting the overestimation produced by the standard technique do not work properly due to an oversimplification in the modeling. Then, we use a numerical simulation of an unintentionally-doped hydrogenated-amorphous-silicon-like material to evaluate the precision of the new formulas and procedures presented. We clarify the conditions under which the standard SSPG technique produces large overestimations. In these cases, we show that the precision of the new procedure can be more than ten times higher. Finally, we use the standard and the new method to characterize a hydrogenated amorphous (a-Si:H) and a hydrogenated polymorphous (pm-Si:H) silicon sample at different temperatures. We observe that the overestimations produced by the standard technique increase with the ratio between the majority and minority carrier diffusion lengths and the ratio between the recombination states’ capture coefficient and mobility of majority carriers.

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

An automated steady state photocarrier grating experiment

The design of an automated steady state photocarrier grating (SSPG) experiment is presented that matches most of the requirements of an industrial environment. We first briefly recall the bases of the SSPG technique and that the minority carrier diffusion length deduced from this experiment is a key parameter in the fabrication of thin film solar devices. We then underline the main drawbacks of the use of the classical SSPG technique as an industrial and systematic characterization technique mostly that all the adjustments have to be done manually. We show that this issue can be overcome to end with a compact (90 × 60 cm(2)), fast, reliable, and easy-to-use system that could become a routine characterization technique for research laboratories and thin film solar industry. We illustrate the possibilities of this new system by showing some of the results obtained with it on very different thin films.

Influence of spatial distribution of radiation on steady-state photocarrier grating measurement

The influence of spatial distribution of radiation intensity on results of steady-state photocarrier grating (SSPG) measurements is analyzed. This technique uses the interference pattern obtained over an illuminated sample surface when two coherent radiation beams hit the sample at different angles of incidence. The sample usually consists of a thin semiconductor film on a thick, parallel-sided substrate. In this paper, the interference of radiation internally reflected in the thin film as well as internally reflected in the substrate are taken into consideration. These phenomena evoke complicated spatial distributions of radiation in a sample, and consequently they affect the magnitude of photoconductivity. The value of SSPG response can be even a few times greater than the predicted one for a homogeneous distribution of light over the thickness of a semiconductor film. If one does not take this effect into account, the carrier diffusion lengths estimated with the SSPG technique may vary from those suggested by the measurement.

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.

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

Trapping effects in a-Si:H investigated by small-signal transient photoconductivity and the steady-state photocarrier-grating technique.

We report on trapping effects observed in small-signal photoconductivity and steady-state photocarrier-grating (SSPG) experiments. In the former case trapping effects appear since the small-signal photocurrent decays exponentially with time after the excitation is turned off, but with a decay time which depends on the ratio of free to trapped carriers and is determined by the intensity of the cw background illumination. In the case of (steady-state) SSPG measurements, carried out as a function of electric field, an electron drift mobility is obtained which is proportional to the ratio of free to trapped carriers and is therefore again determined by the background illumination. Both experiments show that around 0.4 eV below the electron mobility edge the density of states falls off much more slowly with energy towards midgap than near the mobility edge and may even exhibit a minimum.

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.

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.

Determination of surface recombination velocity of excess carriers in amorphous silicon using the SSPG technique

The steady state photocarrier grating (SSPG) technique is used to determine the surface recombination velocity in a-Si:H without changing the wavelength of the light producing the photocarriers. This method requires knowledge of the absorption coefficient and the ambipolar mobility of the carriers. In a-Si-:H, the surface recombination velocity is determined by this method to be about 100 cm/s.

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.