Minority Carrier Lifetime Measurements Research Articles

Correct Carrier Concentration for Lifetime Measurements with Local Illumination

Spatially resolved measurements of minority carrier lifetime are a valuable tool to monitor the quality of silicon wafers and of single processing steps. Common implementations of this technique generate excess minority carriers by scanning the variable focus of a pulsed light beam over the wafer surface. The resulting carrier distribution is spatially inhomogeneous; therefore minority carriers diffuse out of the illuminated area. However, most existing evaluation models neglect carrier diffusion, and hence overestimate the photogenerated minority carrier concentration by several orders of magnitude. This contribution presents a simulation model to calculate the correct local minority carrier concentration due to carrier diffusion for variable line and spot focus geometries. Carrier density distributions from spatially resolved photoluminescence measurements confirm the results of the model.

Defect Densities and Carrier Lifetimes in Oxygen doped Nanocrystalline Si

ABSTRACTWe report on the measurement of defect densities and minority carrier lifetimes in nanocrystalline Si samples contaminated with controlled amounts of oxygen. Two different measurement techniques, a capacitance-frequency (CF) and high temperature capacitance-voltage techniques were used. CF measurement is found to yield noisy defect profiles that could lead to inconclusive results. In this paper, we show an innovative technique to remove the noise and obtain clean data using wavelet transforms. This helps us discover that oxygen is creating both shallow and deep/midgap defect states in lieu with crystalline silicon. Minority carrier lifetime measured using reverse recovery techniques shows excellent inverse correlation between deep defects and minority carrier lifetimes through which hole capture cross section can be evaluated.

Characterization of laser-induced damage in silicon solar cells during selective ablation processes

Selective laser ablation of silicon nitride layers on crystalline silicon wafers was investigated for solar cell fabrication. Laser processing was performed with a nanosecond UV laser at various energy densities ranging from 0.2 to 1.5 J cm−2. Optical microscopy was used as a simple mean to assess the ablation threshold that was correlated to the temperature at the interface between the silicon nitride coating and the silicon substrate. Minority carrier lifetime measurements were performed using a microwave photo-conductance decay technique. Band to band photoluminescence spectroscopy proved to be a sensitive technique to qualify the laser-induced damage to the silicon substrate. The crystalline structure of silicon seemed to be maintained after silicon nitride ablation as shown by UV reflectivity measurements. Laser parameters corresponding to fluences of around 0.4 J cm−2 were found to achieve selective ablation of SiNx without causing detrimental damage to the surrounding material.

Fast, high resolution, inline contactless electrical semiconductor characterization for photovoltaic applications by microwave detected photoconductivity

Abstract The state-of-the art lifetime measurement technique MDP (microwave detected photoconductivity) is presented with its latest developments in sensitivity, measurement speed and data simulation. Several applications and examples in the field of inline material characterization, defect recognition and real time statistical process control in silicon bricks and wafers are presented, demonstrating the practical use of MDP measurements and of the data obtained by it. The measured lifetime itself combined with its spatial distribution and the measured steady state photoconductivity enable a good correlation to the cell efficiency. Furthermore, the paper presents a detailed summary of the properties of steady state and non-steady state microwave based minority carrier lifetime measurement techniques to complete this extensive study.

Phosphorus diffusion gettering process of multicrystalline silicon using a sacrificial porous silicon layer

The aims of this work are to getter undesirable impurities from low-cost multicrystalline silicon (mc-Si) wafers and then enhance their electronic properties. We used an efficient process which consists of applying phosphorus diffusion into a sacrificial porous silicon (PS) layer in which the gettered impurities have been trapped after the heat treatment. As we have expected, after removing the phosphorus-rich PS layer, the electrical properties of the mc-Si wafers were significantly improved. The PS layers, realized on both sides of the mc-Si substrates, were formed by the stain-etching technique. The phosphorus treatment was achieved using a liquid POCl3-based source on both sides of the mc-Si wafers. The realized phosphorus/PS/Si/PS/phosphorus structures were annealed at a temperature ranging between 700°C and 950°C under a controlled O2 atmosphere, which allows phosphorus to diffuse throughout the PS layers and to getter eventual metal impurities towards the phosphorus-doped PS layer. The effect of this gettering procedure was investigated by means of internal quantum efficiency and the dark current–voltage (I-V) characteristics. The minority carrier lifetime measurements were made using a WTC-120 photoconductance lifetime tester. The serial resistance and the shunt resistance carried out from the dark I-V curves confirm this gettering-related solar cell improvement. It has been shown that the photovoltaic parameters of the gettered silicon solar cells were improved with regard to the ungettered one, which proves the beneficial effect of this gettering process on the conversion efficiency of the multicrystalline silicon solar cells.

High-quality surface passivation of silicon using native oxide and silicon nitride layers

We report on the attainment of high quality surface passivation of crystalline silicon using facile native oxide and plasma enhanced chemical vapour deposition SiNx. Using systematic measurements of excess carrier density dependent minority carrier lifetime, it is observed that the inferred interface defect density decreases with increasing native oxide thickness while the interface charge density remains unchanged with thickness, which ranges from 0.2 Å to 10 Å. A surface recombination velocity of 8 cm/s is attained corresponding to a native oxide layer thickness of ∼10 Å. Similar chemically grown oxide layer followed by SiNx deposition is shown to yield comparable passivation, indicating practical viability of the passivation scheme.

Interplay of bulk and surface properties for steady-state measurements of minority carrier lifetimes

The measurement of the minority carrier lifetime is a powerful tool in the field of semiconductor material characterization as it is very sensitive to electrically active defects. Furthermore, it is applicable to a large variety of samples ranging from ingots to wafers. In this work, a systematic analysis of the effective measurable lifetime within the steady-state approach for samples of arbitrary thickness is presented and applied to experimental data. It is shown how the measured lifetime relates to the intrinsic bulk lifetime for a given material quality, sample thickness, and surface passivation. This approach makes the bulk properties experimentally accessible by separating them from the surface effects. A criterion for a critical sample thickness is given beyond which a lifetime measurement allows deducing the bulk properties for a given surface recombination. It is shown how the surface recombination effects lifetime measurements even for thick blocks. Furthermore, it is demonstrated under what conditions a lifetime measurement on unpassivated samples can give reliable bulk information. These results are of particular interest for semiconductor material diagnostics especially for photovoltaic applications but not limited to this field.

Parameterisation of injection-dependent lifetime measurements in semiconductors in terms of Shockley-Read-Hall statistics: An application to oxide precipitates in silicon

Injection-dependent minority carrier lifetime measurements are a valuable characterisation method for semiconductor materials, particularly those for photovoltaic applications. For a sample containing defects which obey Shockley-Read-Hall statistics, it is possible to use such measurements to determine (i) the location of energy levels within the band-gap and (ii) the ratios of the capture coefficients for electrons and holes. In this paper, we discuss a convenient methodology for determining these parameters from lifetime data. Minority carrier lifetime is expressed as a linear function of the ratio of the total electron concentration to the total hole concentration for p-type (or vice versa for n-type) material. When this is plotted on linear scales, a single-level Shockley-Read-Hall centre manifests itself as a straight line. The gradient and intercepts of such a plot can be used to determine recombination parameters. The formulation is particularly instructive when multiple states are recombination-active in a sample. To illustrate this, we consider oxide precipitates in silicon as a case study and analyse lifetime data for a wide variety of p-type and n-type samples as a function of temperature. We fit the data using both a single two-level defect and two independent single-level defects and find the latter can fit the lifetime curves in all cases studied. The first defect is at EV + 0.22 eV and has a capture coefficient for electrons ∼157 times greater than that for holes at room temperature. The second defect is at EC − 0.08 eV and has a capture coefficient for holes ∼1200 times greater than that for electrons at room temperature. We find that the presence of dislocations and stacking faults around the precipitates acts to increase the density of both states without introducing new levels. Using the analysis method described, we present a parameterisation of the minority carrier lifetime in silicon containing oxide precipitates.

Implications of alkaline solutions-induced etching on optical and minority carrier lifetime features of monocrystalline silicon

In this work, we search to optimize the surface textures of monocrystalline silicon (c-Si) intended to be used in silicon solar cells. For this purpose, we studied the morphology of formed etch hillocks during anisotropic etching of silicon using alkaline solutions based on sodium hydroxide (NaOH), potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH). Such treatments lead to the formation of various pyramids-like textures that can be well optimized to improve the photocurrent of c-Si-based solar cells. The produced textures were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–visible optical reflectivity and minority carrier lifetime measurements. These investigations allow evaluating the size and density of the formed pyramidal textures; the apex angles vary between 75° and 82°, while the heights and bases of the pyramids range from a few hundred nanometers to several micrometers. A minimum reflectivity value of about 6% was obtained at specific conditions using NaOH, whereas it was found that the apparent effective minority carrier lifetime (τeff) is sensitive to the injection level (Δn), which shows an apparent increase from 1.2μs to 2.4μs for a minority carrier density of about Δn=21014cm−3.

Measurement of Minority Carrier Lifetime in n-Type MBE HgCdTe on Variable Substrates

This paper correlates measurements of minority carrier lifetime using photoconductivity transient or photoconductive decay (PCD) with measurements and analysis of current–voltage (I–V) on P + n photodiodes of the same layer. This analysis is done at the primary temperature of operation near 77 K. Measurements of minority carrier lifetime in n-type molecular beam epitaxy Hg1−x Cd x Te (MCT) on variable substrates of GaAs and CdZnTe are presented. The minority carrier lifetime from I–V analysis of P + n long-wave infrared (LWIR) photodiodes on CdZnTe (CZT) and GaAs substrates is similar to PCD analysis. Deviations occur for MCT/GaAs in the low-temperature extrinsic region associated with Shockley–Read–Hall (SRH) at 78 K in contrast to MCT/CZT at similar temperatures. The x-values in the formula Hg1−x Cd x Te for samples grown on CdZnTe and GaAs are 0.22 and 0.235, respectively. I–V analysis of variable-area LWIR photodiodes shows a calculated minority carrier diffusion coefficient of 2.3 cm2/s on lattice-matched CZT and 3 cm2/s on non-lattice-matched GaAs with diffusion lengths of approximately 24 μm and 20 μm, respectively.

Influence of a-Si:H deposition power on surface passivation property and thermal stability of a-Si:H/SiNx:H stacks

The effectiveness of hydrogenated amorphous silicon (a-Si:H) layers for passivating crystalline silicon surfaces has been well documented in the literature for well over a decade. One limitation of such layers however has arisen from their inability to withstand temperatures much above their deposition temperature without significant degradation. This limitation is of importance particularly with multicrystalline silicon materials where temperatures of at least 400°C are needed for effective hydrogenation of the crystallographic defects such as grain boundaries. To address this limitation, in this work the surface passivation quality and thermal stability of a stack passivating system, combining a layer of intrinsic a-Si:H and a capping layer of silicon nitride (SiNx:H), on p-type crystalline silicon wafers is studied and optimized. In particular the sensitivity of different microwave (MW) power levels for underlying a-Si:H layer deposition are examined. Both effective minority carrier lifetime (ζeff) measurement and Fourier transform infrared (FTIR) spectrometry were employed to study the bonding configurations, passivating quality and thermal stability of the a-Si:H/SiNx:H stacks. It is established that the higher MW power could result in increased as-deposited ζeff and implied Voc (iVoc) values, indicating likely improved surface passivation quality, but that this combination degrades more quickly when exposed to prolonged thermal treatments. The more dihydride-rich film composition corresponding to the higher MW power appears to be beneficial for bond restructuring by hydrogen interchanges when exposed to short term annealing, however it also appears more susceptible to providing channels for hydrogen out-effusion which is the likely cause of the poorer thermal stability for prolonged high temperature exposure compared with stacks with underlying a-Si:H deposited with lower MW power.

The Development of Hydrazine‐Processed Cu(In,Ga)(Se,S)2 Solar Cells

AbstractThe hydrazine‐based deposition of Cu(In,Ga)(S,Se)2 (CIGS) thin films has attracted considerable attention in recent years due to its potential for the high‐throughput production of photovoltaic devices based on this absorber material. This article provides an introduction as well as presenting a complete picture of the current status of hydrazine‐based CIGS solar‐cell fabrication, including the three major steps of this deposition process: dissolution of the precursor materials in hydrazine, deposition of a film from the resulting precursor solution, and the completion and characterization of a photovoltaic device following absorber deposition. Recent discoveries are then discussed, regarding the dissolution chemistry of the relevant precursor complexes in hydrazine, which together represent the true foundation of this processing method. Recent studies on CIGS film formation are then summarized, including the control and analysis of the crystalline phase, electronic bandgap, and film morphology. Finally, the latest progress in high‐performance device fabrication is highlighted, with a focus on optoelectronic characterization including current–voltage, junction capacitance, and minority carrier lifetime measurements. Finally, a discussion and future outlook is provided.

Vanadium-based antireflection coated on multicrystalline silicon acting as a passivating layer

In this paper, we present important experimental results of a new efficient (ARC), leading to an efficient surface passivation that have not been reported before. Vanadium pentoxide V2O5 powder was thermally evaporated onto the front surface of mc-Si substrates, followed by a short annealing duration at 600°C, 700°C and 800°C under an O2 atmosphere. The chemical composition of the deposited vanadium oxide thin films was analyzed by means of Fourier Transform Infrared Spectroscopy (FTIR). Surface and cross-section morphology were determined by a scanning electron microscope (SEM). The effect of the deposited thin film on the electrical properties was evaluated by means of the internal quantum efficiency (IQE), minority carrier lifetime measurements which have been made using a WTC-120 photoconductance lifetime tester and we used dark current–voltage (I–V) characteristic to measure the defect density at a selected grain boundary (GB) in all samples and compared to an untreated wafer. The results show that the deposited thin film single layer gives the possibility of combining, in one processing step, an antireflection coating deposition along with efficient surface state passivation, as compared to a reference wafer.

Impact of a-Si:H hydrogen depth profiles on passivation properties in a-Si:H/c-Si heterojunctions

We explore the (near-)interface structure of amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions as employed in high-efficiency heterojunction solar cells. We make use of secondary-ion-mass-spectroscopy profiles and minority carrier lifetime measurements taken on undoped deuterated amorphous silicon [(i)a-Si:D] layers deposited on c-Si from deuterated silane at identical conditions as the hydrogenated layers we have analyzed previously [T. F. Schulze et al., Appl. Phys. Lett. 96 (2010) 252102]. We briefly discuss the implications of the local interface structure for the c-Si surface passivation as well as for the heterojunction band offsets, and identify a route towards optimization of (i)a-Si:H layers as passivating buffers in a-Si:H/c-Si high-efficiency heterojunction solar cells.

Influence of textured c-Si surface morphology on the interfacial properties of heterojunction silicon solar cells

For the HIT solar cells, the properties of interface between intrinsic thin film and c-Si are critical for the resulting device. The interfacial properties mainly depend on the surface passivation quality of c-Si, which is found to be affected by the morphology of textured surfaces. In this study, four kinds of textured c-Si substrates are fabricated: large pyramids without chemical polished (CP), large pyramids with CP, small pyramids without CP and small pyramids with CP. We investigated the effects of textured-surface morphology on the passivation of c-Si, the thin layer coverage and the interfacial properties of heterojunction prepared by HWCVD. Minority carrier lifetime measurements show that the wafer with small pyramids leads to better surface passivation than the one with large pyramids. The good coverage and contact between the thin film and the substrate can be achieved and no epitaxial growth occurs on the wafer with small pyramids through the study of TEM. Dark I-V measurements reveal that the heterojunction on wafer with small pyramids and CP has low recombination at the a-Si:H/c-Si interface. Our results indicate that the surface with small pyramids and low surface roughness is beneficial to the performance of HIT solar cells.

Influence Of Bulk And Surface Properties On Measurable Steady-State Carrier-Lifetime

Minority carrier lifetime measurements are a major characterization technique regarding the material quality of semiconductors. In particular, a large variety of electrically active defects can be detected at room temperature even if present in low concentrations down to 1010cm-3 only. Transient and (quasi) steady-state methods as well as combinations of both have been established. However, in either case, the measurable lifetime is influenced not only by the bulk lifetime itself but by a number of additional sample properties such as surface recombination, sample thickness, and wavelength of the carrier-generating light.An analytical model is investigated that relates the measurable lifetime to the bulk lifetime for samples of arbitrary thickness and surface quality when measured under steady-state conditions. It thus provides a generalization for the frequently used limiting expressions for thin Si-wafers and thick Si-blocks. In particular it allows the interpretation of data obtained for samples of intermediate thickness. Furthermore, it is shown under which conditions the surface properties can influence the lifetime measurements even for thick blocks. Finally, an approach is proposed that allows the simultaneous extraction of both bulk lifetime and surface recombination velocity. It is argued that reliable measurements of bulk properties are possible only for samples being thicker than some critical thickness while surface properties are best determined on samples thinner than this critical thickness. A quantitative estimate for this critical thickness is given. Experimental results are shown that demonstrate the applicability of the model.

Photoluminescence Imaging for Photovoltaic Applications

Photoluminescence (PL) imaging is a versatile technique for the characterisation of silicon samples across almost the entire photovoltaic (PV) value chain. Within only a few years after the first demonstration of PL imaging on large-area silicon wafers at the University of New South Wales in 2005, this measurement principle has quickly evolved into a standard method for process monitoring in R&D and is now being used at most PV research institutes and leading wafer and solar cell manufacturers. The first part of this paper provides a brief overview on PL imaging and discusses some specific applications and quantitative analysis methods, including spatially resolved calibrated minority carrier lifetime and series resistance measurements. The high resolution and short measurement time of PL imaging also allow a range of applications for inline process monitoring in production. Emphasis is given in the second part of this paper to PL imaging applications in solar cell manufacturing at an early stage of the PV value chain, specifically the characterisation of silicon bricks and ingots prior to wafer cutting and of as-cut wafers prior to solar cell processing.

Surface Passivation of Silicon by Electrochemically Formed Oxide Layers

Aluminum oxide and zinc oxide coatings of automatic high precision thickness control in the range of 10-200nm have been grown onto p-type Czochralski silicon crystalline wafers and solar cells by electrodeposition from organic electrolyte solutions at room temperature with current densities of 0.5-4mA/cm2 within 30-120seconds. Silicon oxide films have been formed on the surface of silicon wafers by anodic oxidation in organic electrolyte solution. Passivation effects have been investigated by means of minority carrier lifetime measurements. Silicon wafers and solar cells with formed oxides layers were exposed to a post-deposition anneal in forming gas at 300-475oC. Crystallographic structure and surface morphology of newly developed electrochemically formed coatings have been examined by X-ray diffraction and scanning electron microscopy.

Comparison of emitter saturation current densities determined by injection‐dependent lifetime spectroscopy in high and low injection regimes

ABSTRACTThe determination of the emitter saturation current density J0e of symmetrical test structures, analyzed by injection‐dependent lifetime spectroscopy (IDLS), in high as well as in low injection regimes is compared. A detailed investigation of the influence of different models for the Auger recombination on the evaluation of the measured minority carrier lifetime is performed. It can be concluded that a good agreement for the extraction of J0e under high and low injection conditions for lightly and highly doped emitters on textured and untextured surfaces was obtained if the Auger parameterization of Kerr et al. is applied. For deep‐diffused emitter profiles on textured surfaces a significant increase of J0e well above the geometrical surface area ratio of untextured and textured surfaces of 1.7 was observed. A verification of the experimentally determined values for J0e is performed by numerical device simulations. The simulated surface recombination velocity for the investigated emitter doping profiles is in good agreement to analytical calculations. Copyright © 2010 John Wiley & Sons, Ltd.

Limitations in the accuracy of photoconductance-based lifetime measurements

The accuracy of photoconductance-based minority carrier lifetime measurement techniques is studied in detail. Regarding their accuracy and comparability, the quasi steady state photoconductance (QSSPC) as well as the novel steady state microwave detected photoconductivity (MDP) method are compared with the traditional microwave detected photoconductance decay (μPCD). We show that differences in measurement conditions and analyzing procedures lead to deviating results. Calculations based on a generalized rate equation system are used to model the photoconductivity and the resulting effective lifetime for different measurement conditions and defect models. The simulation results are compared to measurements on several mono- and multicrystalline silicon samples with and without surface passivation. Our results clearly show, that for low as well as high injection conditions deviations occur for the measurement and analysis techniques investigated. To allow for a comparison of lifetime data, we recommend to report exact measurement conditions and analysis procedures as well as to perform measurements in a certain injection range. Only if no trapping effects are present and the penetration depth of the applied method is significantly larger than the sample width, accurate and comparable lifetime results can be achieved.