Diffusion Length Measurements Research Articles

Effect of wafer stress on surface photovoltage diffusion length measurements

The surface photovoltage technique is an attractive method for measuring the minority carrier diffusion length. It has the advantage of not requiring permanent sample contacts. It does require, however, an accurate knowledge of the absorption coefficient-wavelength relationship. This relationship is well known for single-crystal silicon. However, for polycrystalline silicon we find the single-crystal data not to be applicable over the wavelength range for SPV measurements. We observe nonlinear SPV plots when using the well known α-λ data. Depending on which portion of the plot is used for diffusion length determination, one can obtain very different diffusion lengths. By making SPV, short circuit current, and spectral response measurements and comparing theory with experiment, we find a new α-λ relationship which is dependent on the stress in the sample. Since this stress varies spatially, it is difficult to use one particular set of α-λ data for SPV interpretation. We can find the low wavelength range of SPV plots to be most suitable for diffusion length measurements.

Study on surface photovoltage measurement of long diffusion length silicon: Simulation results

The limitations of surface photovoltage method (SPV), using the analytical solution for the minority-carrier distribution, are studied in detail. The principal source of error in thin wafers of long diffusion length material is the back surface recombination. The possibility of measuring large diffusion lengths in a reliable manner is discussed. With proper back surface passivation, diffusion lengths as long as several times the sample thickness can be measured. Evaluation of minor amounts of iron contamination in p-type material is possible even if the back surface recombination velocity is high, but a correction factor is required for the traditionally used relationship to convert the observed diffusion length change to iron concentration. The value of the correction factor varies between 1 and 1.5 for high back surface recombination velocity; the magnitude depends on the ratio between wafer thickness and the diffusion length of the minority carriers. The detection limit for iron is in the 109–1011 at/cm3 range, depending on the quality of the instrumentation and on the sample preparation; low back surface recombination gives better sensitivity. The use of SPV to measure the denuded zone width in precipitated material is briefly analyzed. It is shown that in heavily precipitated material an SPV measurement underestimates the denuded zone width, and the result will be very sensitive to the diffusion length in the bulk of the sample. Finally, the sensitivity of the measured diffusion length to different sources of errors and requirements for the sample preparation are discussed. The use of constant-flux method introduces some potential errors, which can be avoided through the use of constant bias illumination during the measurement.

Variable-area diode data analysis of surface and bulk effects in MWIR HgCdTe/CdTe/sapphire photodetectors

The authors investigate the separate dark current components which are dominant in the diffusion-limited regime in MWIR n/p HgCdTe/CdTe/sapphire photodetectors. Both mesa and planar configurations of variable-area diodes were fabricated and evaluated over the temperature range from 78 to 250 K. Simple analytical expressions are used to calculate the contributions of bulk, lateral and surface effects from the perimeter/area dependence of R0A and measurement of the minority carrier diffusion length. The analysis indicates that at 180 K the mesa diode results can be accounted for by bulk and lateral currents, but that the planar diodes are limited by surface currents. The 180 K median R0A for the mesa diodes ranges from 63 Omega cm2 for 500*500 mu m2 diode areas to 14 Omega cm2 for 30*30 mu m2 diodes at a cut-off wavelength of 4.64 mu m. Scanning laser microscope measurements determine the 180 K electron minority carrier diffusion length to be 17-18 mu m.

Photocurrent-capacitance method of diffusion length measurement

Abstract In the development of photovoltaic cells, it is helpful to monitor changes in the diffusion length of the minority carriers as fabrication changes are made. A convenient method of measurement of diffusion length in cells, where this quantity is relatively small, is the photocurrent-capacitance method. This method involves the measurement of the illuminated-to-dark photocurrent change of a cell illuminated with monochromatic bandgap light and junction capacitance as the applied reverse voltage bias is changed. Experiments with a CdOSe heterojunction photovoltaic cell were carried out to optimize the experimental conditions of wavelength, monochromator slit width and light chopping frequency. It was shown that the method is simple and enables the variation of the diffusion length in the absorber layer to be followed as fabrication changes are made to the cell.

Modulated photocarrier grating technique for diffusion length measurement in amorphous semiconductors

This article presents a method for investigating the diffusion of photocarriers in semiconductors by the analysis of phase shift between a temporally modulated illumination grating and its inducing photocurrent. Experiments on hydrogenated amorphous silicon prove that an accurate measurement of the diffusion length as well as an identification of ambipolar or nonambipolar diffusion can be acquired by using this technique.

Measurement of minority-carrier diffusion length and composition of Al xGa 1−xAs alloys by a microwave contactless method

Measurement of minority-carrier diffusion length and composition of Al xGa 1−xAs alloys by a microwave contactless method

Correlation Between Minority Carrier Diffusion Length and Microstructure in a-Si:H Thin Films

Aim of this work is to investigate the opto-electronic properties of amorphous hydrogenated silicon (a-Si:H). The deposition temperature nas been used as a driving force to modify the morphology and bonded hydrogen distribution. The influence of the hydrogen microstructure on the carriers μτ product has been examined. The majority and minority carrier μτ have been evaluated from the diffusion length measurement, by using the Steady State Photocarrier Grating (SSPG) technique, and from the photoconductivity in the steady state condition (SSPC). The μτ values have been correlated with the defect density and the Fermi level position. Some considerations are proposed to explain the carrier transport in terms of the compositional inhomogeneities in Si:H alloys due to the morphological variations.

Surface Photovoltage Measurement of Minority Carrier Diffusion Lengths Exceeding Wafer Thickness: Application to Iron Monitoring with Part Per Quadrillion Sensitivity

AbstractWe discuss an approach to iron concentration determination in silicon, based on wafer-scale surface photovoltage measurement of the minority carrier diffusion length in the millimeter range. The approach combines two novel aspects: it overcomes the diffusion length to wafer thickness ratio limitation of previous SPV methods, and it employs iron separation from other recombination centers using rapid photo-dissociation of iron-boron pairs. The wafer thickness limitation was eliminated by using the correct theoretical SPV wavelength dependence instead of simplified asymptotic diffusion length form adopted in all previous treatments and valid only for diffusion lengths much shorter than the wafer thickness. Photo-dissociation of Fe-B pairs and measurement of the corresponding decrease of the L value (caused by creation of iron intersticials) enables iron detection in typical silicon wafers in times of seconds with a sensitivity in the low 108 atoms/cm3 range.

Minority Carrier Diffusion Lengths for High Purity Liquid Phase Epitaxial GaAs

Measurements of minority carrier diffusion lengths for p-type and n-type GaAs were carried out using an electron beam induced current (EBIC) technique. The GaAs material was grown by liquid phase epitaxy (LPE) at the Australian Nuclear Science and Technology Organisation. The diffusion lengths measured for high purity p-type and n-type LPE-GaAs samples were observed to be longer than any previously reported.

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.

Confocal photoluminescence: A direct measurement of semiconductor carrier transport parameters

Using confocal microscopy a direct measurement of the minority carrier diffusion length of 5 μm is obtained for a 3-nm thick AlGaAs/GaAs graded-index separate confinement heterostructure. A photoluminescence spatial resolution of <1 μm is obtained by translating the collection aperture of the confocal microscopy arrangement. This technique provides a method for obtaining spatial resolution in photoluminescence from semiconductor structures that is limited only by the optics rather than by carrier transport effects. This resolution is illustrated by monitoring enhanced carrier transport for a transversely graded thickness quantum well formed by metalorganic chemical vapor deposition growth over a cleaved sample edge.

Minority carrier lifetime and diffusion length in HgTe/CdTe superlattices by molecular beam epitaxy

Transient minority carrier lifetime and steady state diffusion length measurements have been performed on n-type HgTe/CdTe superlattices grown on CdZnTe(100) substrates. The n-type (100) superlattice sample shows a lifetime of 90–100 ns at 77–160 K which then decreases with increasing temperature down to 50 ns near 300 K. p-type HgTe/CdTe superlattices show normal behavior for temperature-dependent lifetimes. The measured lifetime is approximately 20 ns at 77 K and increases continuously to 85 ns at 300 K. An independent measurement of minority carrier diffusion length in a p-type HgTe/CdTe superlattice has led to an excellent agreement with a transient lifetime value of 66 ns at 200 K. The minority carrier diffusion length in a direction parallel to the interfaces indicates that the lateral transport properties are not much different from the transport properties of bulk HgCdTe.

Effect of surface recombination and injection level on the diffusion length obtained by simulation of the SPV method

The procedure of measuring diffusion length by the surface photovoltage (SPV) method is modeled with a specially designed numerical simulation program. The effect of surface recombination and injection level on the obtained diffusion length is investigated using the simulation. The results show that at low injection levels the surface recombination does not influence the diffusion length, but at high injection levels the diffusion length is dependent on surface recombination velocity. It is shown that at high injection levels the effect of different surface recombination velocities on the diffusion length is different. For low surface recombination velocities raising the injection level tends to reduce the diffusion length, whereas for high surface recombination velocities it tends to raise the diffusion length. These simulation results are interpreted and discussed in detail.

Reply to ‘‘Comments on the steady state photocarrier grating technique to measure diffusion lengths’’

It is pointed out that the experimental pitfalls in carrying out steady state photocarrier grating measurements, mentioned by Prabhu et al. are apparently very rare since the diffusion lengths in a-Si:H determined by this method in many laboratories yield very similar results.

Comments on the steady state photocarrier grating technique to measure diffusion lengths

The steady state photocarrier grating technique has emerged as an important technique for measurement of the diffusion length in amorphous silicon. In this communication we show that morphological inhomogeneities lead to an overestimation of the magnitude of the diffusion length. The magnitude of this error cannot be easily estimated.

Effect of carrier transport on surface photovoltage in thin semiconductor films

Abstract An expression for surface photovoltage in thin semiconductor films has been obtained by considering the effect of coupling between the front and the back surface barriers because of carrier diffusion. Application of the result to the constant surface photovoltage method for the minority carrier diffusion length measurement has been discussed for thin films of hydrogenated amorphous silicon.

Theory of the photovoltage at semiconductor surfaces and its application to diffusion length measurements

This paper presents an analytical theory of the steady-state surface photovoltage, which takes into account recombination in the surface space charge region and at surface states, as well as bulk diffusion in the semiconductor. For a given wavelength of light used in the photo-excitation, the theory is able to predict the photon flux required to yield a specified surface photovoltage. The validity of the theory has been established by means of detailed comparison with exact numerical solutions. It is shown that for an Si specimen space charge recombination plays an important role in determining the surface photovoltage, particularly at photovoltages of the order 0.1 times the thermal voltage or less. The theory is applied to a rigorous examination of the validity of two standard test methods of the American Society for Testing and Materials for measuring the minority carrier diffusion length, which are based on the surface photovoltage. It is found that while the method due to Goodman works well in general and even in the presence of large surface recombination, the method due to Quilliet and Gosar does not always give the correct value of diffusion length, because of the importance of recombination in the space charge region and at the surface states—it does so only under certain restrictive conditions, i.e. the material must have a doping concentration greater than 10 15 cm −3 and a long minority carrier lifetime (> 10 μs), with a surface in depletion (but not in inversion) at equilibrium and a very low surface recombination velocity.

High-temperature annealing behavior of μτ products of electrons and holes in a-Si:H

The mobility-lifetime products of electrons and holes [(μτ)e and (μτ)h] in undoped hydrogenated amorphous silicon samples have been studied by photoconductivity and ambipolar diffusion length measurements. The density of dangling bonds Nd in the samples is changed over a range of 3×1015–2×1018 cm−3 by annealing at high temperatures. Nd and the Urbach tail slope Eov have been determined by the constant photocurrent method. In addition, the optical gap, the activation energy of dark conductivity, and the exponent governing the intensity dependence of σpc have been measured. The results show that there is a correlation between Nd and Eov which is consistent with equilibrium theory. (μτ)e and (μτ)h change in quite different ways as Nd increases, namely, (μτ)e decreases as a linear function of the inverse of Nd. However, (μτ)h remains almost constant when Nd≤5×1016 cm−3, then decreases fast for higher Nd. The asymmetric dependence of transport properties of electrons and holes on Nd suggests that for electrons recombination through dangling bond states is dominant; but, for holes, recombination mainly proceeds through deep band tail states, especially when Nd is relatively low.

Theory of the steady-state-photocarrier-grating technique for obtaining accurate diffusion-length measurements in amorphous silicon.

This paper presents a theory that is the basis of the steady-state-photocarrier-grating (SSPG) technique, as a means of determining the diffusion length of photocarriers in amorphous semiconductors. Solving the SSPG transport problem, including the formulation of small-signal photocurrent, by a second-order perturbation approach reveals deficiencies in the existing SSPG theory, which is based on first-order perturbation theory. It is also shown that the SSPG data analysis done routinely, assuming a priori that local space-charge neutrality prevails, yields a severe overestimate of the diffusion length when the condition is not met experimentally. An extension of the theory to measure accurately the diffusion length by removing these defects inherent in the original SSPG formulation is demonstrated by its successful application to hydrogenated amorphous silicon. The experiments carried out at various illumination levels show that the correct value of the diffusion length and its light-intensity dependence indeed differ to a significant degree from the results obtained with use of the previous method. A physical interpretation of the measured intensity dependence is also given, assuming a trap-controlled photocarrier transport model.

Transient Versus Steady State Measurements on the Transport Properties of a-SI:H Films

ABSTRACTTransient and steady-state measurements of the ambipolar diffusion length (L*) in undoped a-Si:H films have been carried out through Flying Spot Technique (FST) and Spectral Photovoltage Technique (SPT) using either Schottky or pin structures. The FST is based on the photovoltage response of a monochromatic light that moves with a constant velocity in the interface direction, while in SPT the optical excitation is achieved by changing the absorption coefficient of the incident light and so, the light depth penetration. In FST the additional photo-effect due to the light spot movement allows to infer separately L* and the effective lifetime, (τ*) while in SPT the curve shape of the spectral response allows to estimate the interface behavior and L*. The obtained results reveal that the transient measurement is useful for determining the transport properties of the bulk while the steady technique seems to be more useful to infer the role of the interface on device performances. Limitations of the above techniques will also be reported.