J. Toušek

Transport anisotropy in microcrystalline silicon studied by measurement of ambipolar diffusion length

We have studied charge transport anisotropy in microcrystalline silicon (μc-Si:H) by comparing diffusion length measured parallel to the substrate by steady stage photocarrier grating and perpendicular to the substrate by surface photovoltage method (SPV). We have developed a SPV evaluation procedure which allowed us to exclude the effect of light scattering at the naturally rough surface of the μc-Si:H. The procedure allows us to deduce not only the diffusion length, but also the depth of the depletion layer at the surface and recombination coefficients at both top and bottom interfaces of the film. With growing μc-Si:H film thickness the size of the crystallites increases, leading to higher roughness and thus also light scattering. At the same time density of grain boundaries decreases, resulting in an increase of the diffusion length and of the surface depletion layer depth. For all samples the diffusion length perpendicular to the substrate was several times higher than the diffusion length parallel to it, clearly confirming previous indication of the transport anisotropy resulting from the measurements of coplanar and sandwich conductivity.

Diffusion length in CdTe by measurement of photovoltage spectra in CdS/CdTe solar cells

Modified method of surface photovoltage (SPV), spectral response measurement, and constant photocurrent method (CPM) were applied to thin film CdS/CdTe solar cells with the aim of finding diffusion length of minority carriers (L) in the CdTe material. The SPV signal was theoretically calculated without constraints of absorption coefficients for the incident radiation and thickness of the sample assuming one space charge region (SCR) located on the CdS/CdTe interface. In addition to the diffusion length, the SPV is a function of the surface recombination velocity and the parameters of the SCR, which complicates the evaluation. Illuminating the back side of the solar cell (without ohmic contact) we obtain a photovoltage spectrum predominantly influenced by the diffusion length. On the other hand, the standard measurement using light penetrating from the CdS side strongly depends on the thickness of the SCR. The small signal approximation model presented here successfully explains both measured spectra and p...

Photovoltage spectroscopy of InAs/GaAs quantum dot structures

In addition to widely used photoluminescence spectroscopy photovoltaic measurement of quantum dot structures can give complementary information about electron and hole transitions. Structures with self-organized InAs quantum dots in GaAs matrix were grown by the Stranski–Krastanov mechanism using the low pressure metalorganic vapor phase epitaxy technique. Two types of samples were studied, with single and multiple quantum dot layers. We have shown that surface photovoltage spectroscopy can be used for the study of single, as well as multiple quantum dot layer structures.

Improved photovoltaic method for measurement of minority carrier diffusion length applied to silicon solar cells

Abstract The photovoltage spectrum measured on back illuminated silicon solar cells of the (passivated emitor solar cell) (PESC) type without original bottom ohmic electrode is evaluated with the aim to find the diffusion length of minority carriers in bulk of the absorber ( L ). Two junctions, namely pn + junction of the cell and that spontaneously created on the free surface generally exist in such samples. They give rise to two signals of opposite signs with one point of exact compensation. Six parameters (including L ) are needed to characterize the spectrum. Special simple arrangement removes influence of spontaneously created junction on the free surface, which, in this way, reduces the number of parameters needed for fitting to three and enhances reliability of the measurement.

Interpretation of minority carrier diffusion length measurements in thin silicon wafers

Diffusion length of minority carriers measured in thin wafers reflects not only properties of the material but also the state of its surface and, consequently, becomes an effective quantity Leff depending on the thickness of the sample. The function describing this dependence is presented here and compared with the data from surface photovoltage experiments. It allows one to find the correct diffusion length from measurement of two samples with different thicknesses.

Surface photovoltage measurements in μc-Si:H: Manifestation of the bottom space charge region

We discuss results of surface photovoltage (U) measurements for d=10 μm thick layers of undoped hydrogenated microcrystalline silicon (μc-Si:H). By applying excitation with low energetic photons (down to 1.1 eV), i.e., with small absorption coefficient α and a large penetration depth α−1, a photovoltage peak appears on a curve U=U(α). This peak is located at α≳d−1 and its occurrence depends critically on the substrate material. The peak is present in a μc-Si:H film grown on p+ crystalline silicon (c-Si), on the other hand it is missing in the μc-Si:H samples grown on n+ c-Si or ZnO film. We present a mathematical model that enables us to link this peak to photocharge separation in the bottom space charge region at the interface μc-Si:H/substrate. Besides the magnitude of the ambipolar carrier diffusion length L, a parameter particularly critical for the occurrence of the peak turns out to be the ratio n of reverse saturation current densities of the two diodes representing surface and bottom space charge ...

Current–voltage characteristics of GaSb homojunctions prepared by MOVPE

Mechanism of charge transport through GaSb p–n homojunctions prepared by low pressure metal organic vapour phase epitaxy technology was studied.Calculation of current–voltage characteristics based on Shockley–Read–Hall recombination model was performed accounting for specific features of the band structure of GaSb with the lowest minimum of the conduction band at C point and higher minima at L points.It is shown that this type of transport prevails if the samples were prepared at higher growth rate.Analysis of temperature dependencies of some parameters of the experimental characteristics gave, besides the expected value of band gap energy, also the activation energy of the recombination level as (Ec � 0:22) eV or (Ev þ 0:29) eV.The latter value agrees reasonably well with published data on deep levels in GaSb. On the other hand, the charge transport in more slowly grown samples was nearly temperature independent, especially in the reverse direction, probably due to prevailing tunnelling mechanism. 2003 Elsevier Science Ltd.All rights reserved.

Effect of silicon nitride layers on the minority carrier diffusion length in c-Si wafers

Silicon nitride layers prepared from silane and ammonia based gases by microwave assisted plasma enhanced chemical vapor deposition (PECVD) and by low pressure chemical vapor deposition (LPCVD) techniques on p-type c-silicon substrates were studied via the methods of surface photovoltage (SPV), Fourier transform infrared (FTIR), and secondary-ion-mass spectroscopy (SIMS). The effective diffusion length in silicon was evaluated by the SPV method, and it was strongly influenced by the deposited SiNx layer. The FTIR spectra show the form of chemical bond of hydrogen in the layer. Two absorption bands belonging to Si–H and N–H groups and their modification after temperature treatment were found in the spectra of PECVD samples, while in the spectra of LPCVD samples only N–H bonds were recognized. Transport of H from PECVD silicon nitride into Si subsurface layer during the annealing process is shown by SIMS profiles of hydrogen. Positive influence of the penetrated H manifests in passivation of defects in the ...

Influence of photon recycling on photovoltage spectra of GaSb diodes

A simple theory of photon recycling (PR) was applied to the calculation of the photovoltage spectra of GaSb diodes. The PR is analyzed here as a two stage process with primary electron-hole pair photogeneration and diffusion followed by an optical transport of secondary carriers induced by emission and reabsorption of photons. The latter process is responsible for different behavior of diodes with and without the PR mechanism. Low losses of the secondary processes in comparison with losses in the primary one are assumed. The secondary processes can change the character of the photovoltage spectra significantly, enhancing the signal in the short wavelength region. The calculated photovoltage spectral response agrees well with the experiment giving the possibility to evaluate parameters of the material and of the measured diodes. The diffusion length obtained in this way characterizes only diffusion of electron-hole pairs and the influence of PR can be separated.