Quasi-steady State Photoconductance Research Articles

High Resolution inline Topography of Iron in p-Doped Multicrystalline Bricks by MDP

The iron density is one crucial parameter to verify the quality of the material. Brick measurements enable to check the material quality right at the beginning of the production line. This paper presents contact less high resolution inline topographic measurements of the iron density in multicrystalline silicon bricks by MDP (microwave detected photoconductivity). The measurement procedure is fully automatic and takes less than 5min a brick. The data obtained were compared to QSSPC (quasi steady state photoconductivity) measurements, however, with a loss in spatial resolution. Furthermore, passivated wafers were prepared from the bricks after the measurements and investigated individually. The consistency of all the data is remarkable.

Lifetime Measurements on Attached Epilayers and Detached Epifoils Grown on Reorganised Porous Silicon Showing a Bulk Lifetime Exceeding 100 μs

This paper discusses on-going efforts towards reliable lifetime measurements on epilayers and the subsequent decoupling of the bulk and surface recombination components, both while it is still attached to the p+ substrate on which it is grown (“attached epilayer”) and after its detachment from the substrate (“detached epifoil”). For the “attached epilayers”, microwave photoconductance decay (^-PCD) and simulation-assisted photoluminescence (sim-PL) were applied together with a variation in the epilayer thickness to evaluate the bulk lifetime (Tbulk) and total effective surface recombination velocity (Stot) of p-type epilayers. By applying linear fits to reciprocal effective lifetime versus reciprocal epilayer thickness data, Stot of all samples were reliably extracted, resulting in Stot of ∼265cm/s in the absence of porous silicon at the epilayer/substrate interface compared to ∼9220cm/s when present and ∼775cm/s when shielded by a thin back surface field. Based on these Stot values, sim-PL was used to estimate Tbulk to be ∼160 |us in the porous silicon area. For the n-type “detached epifoils”, quasi-steady state photoconductance (QSSPC) was used. However, reliable Tbulk could not be extracted, despite the lower Stot because Tbulk was too high. However, a lower limit to Tbulk was estimated to be >138 μs. Such high bulk lifetimes imply bulk diffusion lengths that are an order of magnitude longer than the epilayer thicknesses, potentially leading to high cell efficiencies.

Inline Bulk-lifetime Prediction on as-cut Multicrystalline Silicon Wafers

Solar cell production using multicrystalline silicon is rife with uncertainties about material quality and its impact on subsequent processing steps. However, inline electrical metrology can provide predictive information about bulk silicon quality before processing resources are applied, allowing later metrology to focus on process control. In this work, we show an industrial algorithm which enables bulk lifetime prediction in as-cut multicrystalline wafers, agnostic of wafer origin, using an inline Quasi-Steady-State Photoconductance (QSSPC) measurement setup from Sinton Instruments. In addition, we demonstrate a robust method to extract emitter saturation current density from post-diffusion wafer measurements. Finally, we extend the use of inline QSSPC with a corrected doping measurement that utilizes the theory of grain-boundary potential barriers to interpret excess conductance.

Surface Charge and Interface State Density on Silicon Substrates after Ozone Based Wet-Chemical Oxidation and Hydrogen-Termination

For more than 20 years, the application of ozone dissolved in pure deionised water (DIW-O3) has been investigated for wafer-cleaning and resist stripping applications in the semiconductor industry [, , ]. To reduce chemical consumption and disposal costs as well as to improve cleaning efficiency, DIW-O3 is used in semiconductor wet cleaning processes as an alternative to traditional sulphuric acid peroxide (H2SO4/H2O2) [] and RCA [] cleans. Silicon solar cell fabrication includes multiple wet cleaning steps involving large amounts of chemicals. In Si substrate manufacturing the wet-chemical oxidation of substrate surface is used mainly for three purposes: (i) the removal of surface contamination and surface micro-roughness by different cleaning and smoothing procedures in H2O2 containing solutions (RCA I and RCA II) [5], in H2SO4/H2O2 [4], (ii) the preparation of hydrophilic surfaces for subsequent layer deposition [,] and (iii) the fabrication of thin oxide layers []. Chemical consumption could be reduced by replacing some of these chemicals with a mixture of pure deionised water with dissolved ozone for cleaning and surface conditioning. The kinetics of wet-chemical oxidation in DIW-O3 were recently investigated by ellipsometrical measurements of oxide thicknesses (ox>) [3,6,7], by SPV-measurements [7,], by contact angle measurements as well as by quasi-steady-state photo conductance (QSSPC) measurements [7].

N+P photodetector characterization using the quasi-steady state photoconductance decay method

When a material is irradiated, it becomes more electrically conductive due to the absorption of the electromagnetic radiation. As a result, the number of free electrons and holes changes and raises its electrical conductivity. A simple but interesting phenomenon to characterise a fabricated n+p photodetector in order to determine its linearity (photoresponse) and photoconductance was employed. Using the transient decay when the irradiation source is switched off, the minority carrier concentration, effective lifetime and surface recombination velocity present at the surface of the detector were measured.

Comparing lifetime and photoluminescence imaging pattern recognition methodologies for predicting solar cell results based on as‐cut wafer properties

ABSTRACTWafer quality is extremely important in determining yield and efficiency of solar cells. Ideally, this wafer quality should be determined for incoming wafers before solar cell fabrication based on the electronic quality of the wafers. Recent papers have discussed methodologies for doing this by using lifetime measurement and pattern recognition of photoluminescence (PL) images. This paper compares results from quasi‐steady‐state photoconductance (QSSPC) lifetime measurements with PL imaging pattern recognition of dislocations. By using a more complete analysis of the lifetime and the PL data than performed in some recent publications, a more detailed physical picture is presented here, which reconciles contradictions between previous results. In particular, the differences between PL and QSSPC lifetime measurements on as‐cut wafers are discussed. The trends in voltage prediction based on measured lifetime, doping, and PL‐determined dislocation densities are shown. Copyright © 2012 John Wiley & Sons, Ltd.

Surface Passivation for Si Solar Cells: A Combination of Advanced Surface Cleaning and Thermal Atomic Layer Deposition of Al<sub>2</sub>O<sub>3</sub>

Thermal atomic layer deposition (ALD) of Al2O3 provides an adequate level of surface passivation for both p-type and n-type Si solar cells. To obtain the most qualitative and uniform surface passivation advanced cleaning development is required. The studied pre-deposition treatments include an HF (Si-H) or oxidizing (Si-OH) last step and finish with simple hot-air drying or more sophisticated Marangoni drying. To examine the quality and uniformity of surface passivation - after cleaning and Al2O3 deposition - carrier density imaging (CDI) and quasi-steady-state photo-conductance (QSSPC) are applied. A hydrophilic surface clean that leads to improved surface passivation level is found. Si-H starting surfaces lead to equivalent passivation quality but worse passivation uniformity. The hydrophilic surface clean is preferred because it is thermodynamically stable, enables higher and more uniform ALD growth and consequently exhibits better surface passivation uniformity.

Impact of Thermal Treatment on PECVD Al2O3 Passivation Layers

We present a detailed study about the influence of post-deposition temperature treatment on PECVD Al2O3 passivation layers. Fourier transform infrared spectroscopy (FTIR) and quasi-steady-state photoconductance (QSSPC) were applied for characterization. We observed several indications of structural changes of the Al2O3 layer with annealing duration, temperature and layer thickness. A shift and an increase of the Si-O vibrational mode (1100- 980cm-1) in the FTIR measured with changing temperature and layer thickness is presented. Structural changes of the Al2O3 layer with annealing duration, temperature, and layer thickness, caused by changes in the oxygen concentration within the SiOx layer are observed for the different FTIR spectra. We also detected a water band at 1650-1630cm-1 and other OH components around 3200cm-1-400cm-1 depending on annealing temperature and time. For a temperature of 820°C, a dramatic change of the Al2O3 peak (650-700cm-1) is observed. This is probably due to the crystallization of the film.

Al2O3 Surface Passivation Characterized on Hydrophobic and Hydrophilic c-Si by a Combination of QSSPC, CV, XPS and FTIR

In this work, the influence of the c-Si surface finishing (hydrophobic/hydrophilic) prior to the deposition of the Al2O3 passivation layer on the passivation quality is investigated. The samples are characterized by a combination of Quasi-Steady-State-PhotoConductance (QSSPC) Capacity-Conductance (CV), X-ray Photoelectron Spectroscopy (XPS) and Fourier Transformed InfraRed (FTIR) measurements. Furthermore, FTIR measurements are used to determine the thickness of interfacial SiOx layer.

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.

Effect of extended phosphorus diffusion gettering on chromium impurity in HEM multicrystalline silicon

We have investigated the extended phosphorus diffusion gettering (PDG) effect on chromium impurities (Cr) in p-type multicrystalline silicon (mc-Si) grown by Heat Exchanger Method (HEM). The study was made after phosphorous diffusion and according to different extended annealing temperatures. The secondary ion mass spectrometry (SIMS) analysis revealed a significant accumulation of 52Cr in heavily phosphorus doped (HPD) region. Using quasi-steady state photoconductance (QSSPC) technique, the apparent lifetime dependent minority carrier density curves have been obtained. The results showed an increment of the bulk minority carrier lifetime for specific annealing temperatures. Appropriate calculations based on QSSPC results allowed us to determine the lifetime curves associated to gettered impurities. Their fitting by Shockley-Read-Hall (SRH) model reveal that the origin of the lifetime increment is the reduction of interstitial chromium (Cri) density in the bulk. Furthermore, the estimation of electron to hole capture cross-section ratio (k=σn/σp) through the modelling of apparent lifetime curves using Hornbeck–Haynes model, confirmed the effectiveness of Cri gettering and identified the nature of dominant recombination centres after gettering process.

Growth and characterization of multicrystalline silicon ingots by directional solidification for solar cell applications

This study is focussed on the growth of multicrystalline silicon ingots with large grains by controlling the silicon melt cooling rate to initiate dendritic nucleation in the initial stage of the solidification. Two ingots were grown with different undercooling rates and compared with a reference ingot grown by standard cooling conditions. All ingots were grown in a lab scale directional solidification system. The wafers cut from all three ingots have been characterized for resistivity, minority carrier lifetime and dislocation density measurements by four point probe, quasi steady state photo conductance and PV Scan, respectively. The wafers were converted into solar cells and their electrical parameters have been measured. The cells fabricated from ingot 2 show slightly higher efficiencies in comparison with ingot 1 and the reference one. The present cooling rate was not enough to initiate the dendrite nucleation in the beginning of the solidification. Hence, there is no significant difference was observed in the crystal quality of the grown ingots 1 and 2.

Correlation Between Different Carrier-Lifetime Measurement Methods For Si-Blocks

Abstract A reliable material characterization in an early stage of fabrication is essential for further cost reduction in the photovoltaic industry. For the crystalline silicon technology, first investigations can be performed directly after cutting the Si-ingots into blocks (bricks). Thereby, valuable information about the electrical material properties can be gathered from carrier lifetime mappings in the beginning of the production chain. There are several methods available for carrier lifetime measurements for scientific research and industrial process screening. In this work we compare three different methods on Si-blocks of varying material quality quantitatively. The photoluminescence (PL) method has recently been introduced for the characterization of Si-blocks with commercially available measurement tools. It is compared to the widespread micro-wave detected photo conductance decay (μPCD) and the well-established quasi steady state photo conductance (QSSPC) methods. In order to get a better understanding for the interpretation of the measurements various types of silicon are compared. Different materials from standard p-type to compensated silicon are investigated. Within each of these blocks different regions from top to bottom are inspected and the measurement results are evaluated. A spatially resolved co-correlation analysis is used for pixel-wise comparison of the measured data. The results of this work show that in most cases the investigated methods correlate well to each other, i.e. justifying methods for quantitative PL measurements on Si-blocks. However, it is also demonstrated that for some regions within a single block the determined quantities show a rather low co-correlation. The possible reasons for the different response of the methods in certain materials respectively regions within a sample will be discussed.

Properties of amorphous silicon passivation layers for all back contact c-Si heterojunction solar cells

ABSTRACTLow temperature deposited Interdigitated All Back Contact a-Si:H/c-Si Heterojunction (IBC-SHJ) devices are a promising approach for high efficiency, low cost solar cells on thin wafers. Thin intrinsic a-Si:H films (i-a-Si:H) deposited below 300°C provide excellent surface passivation and high Voc. However, the optical properties of a-Si:H layers and electronic band alignment at the heterointerface are critical to reduce optical losses and transport barriers in IBC-SHJ solar cells. At the front illumination surface, a wide band gap (Eg) i-a-Si:H layer with good passivation is desirable for high Voc and Jsc while at the rear surface a narrower Eg i-a-Si:H layer with good passivation is required for higher FF and Voc as seen in 2D numerical simulation. Various substrate temperature, H2/SiH4 dilution ratio and plasma power conditions were explored to obtain i-a-Si:H with good passivation and desired Eg. All the deposited films are characterized by Variable Angle Spectroscopic Ellipsometry (VASE) to determine Eg and thickness and by Fourier Transform Infrared spectroscopy (FTIR) to estimate hydrogen content and microstructure factor. Passivation qualities are examined by quasi-steady state photoconductance (QSS-PC) measurement. The i-layer Eg, was varied in the range from ~1.65eV to 1.91eV with lifetime >1 ms. Lowest Eg is obtained just prior to the structure transition from amorphous to epitaxial-like growth. The FF of IBC-SHJ devices improved from 20% to 70% as Eg of the a-Si:H rear passivation layer decreased from 1.78 to 1.65 eV.

RIE‐induced carrier lifetime degradation

AbstractReactive Ion Etching (RIE) is used in the fabrication of some types of solar cells to achieve a highly directional etch. However, cells fabricated using RIE have lower than expected efficiency, possibly caused by increased carrier recombination. Characterisation of the carrier lifetime in solar cells was conducted using the quasi steady state photoconductance (QSSPC) measurement technique. Substantial effective lifetime degradation was observed for silicon samples processed by RIE. Lifetime degradation for samples where RIE etches into silicon is found to be permanent, while for samples where RIE etches only on dielectric layers of SiO2 grown on the wafer, the lifetime degradation is found to be reversible. The reversible degradation in RIE‐processed samples is associated with radiation damage. By reducing the proportion of a wafer exposed to RIE, the degradation of the effective lifetime of RIE‐etched silicon samples can be minimised, and the performance of silicon solar cells can be improved significantly. Copyright © 2010 John Wiley & Sons, Ltd.

Influence of different excitation spectra on the measured carrier lifetimes in quasi-steady-state photoconductance measurements

The interpretation of lifetime measurements on unpassivated silicon wafers by means of the quasi-steady-state photoconductance (QSSPC) technique is a practically highly relevant, but challenging task. We investigate QSSPC measurements on damage-etched samples with five different resistivities from 0.25 Ω cm p-type to 100 Ω cm n-type float-zone material with four different thicknesses each, ranging from 105 to 250 μm. The influence of different excitation spectra, realized by adding different long pass filters to the standard QSSPC Xenon flash spectrum, on the injection-dependent effective excess carrier lifetime is studied in detail. We observed that the measured effective lifetime is significantly dependent upon the excitation spectrum in use and is augmented by increasing the long pass filter cut-off wavelength, the latter being equivalent to more symmetrical excess carrier density profiles. Furthermore, the size and the injection dependency of the dominating surface recombination channels are investigated. Finally, two different approaches to accurately calculate the excess carrier generation rate within the sample are presented and compared.

Control of grain size and -orientation in multi-crystalline silicon ingots

Two different cooling rates have been imposed during the early solidification of two multi-crystalline silicon ingots with 250mm diameter and 100mm height in a pilot scale directional solidification furnace. This has been done by opening a variable heat leak system below the crucible in order to achieve a high initial cooling rate in one of the ingots. The grain-structure and -orientation of these two ingots have been investigated by light microscopy (LM) and electron backscattered diffraction (EBSD), and their electrical properties by quasi-steady state photo-conductance (QSSPC) and surface photovoltage (SPV) method. The ingot with the high initial cooling rate shows predominantly grains which are significantly larger than what is usually found in mc-Si. The minority carrier diffusion lengths measured on the large grains in the ingot with high cooling rate show higher values than those measured on the ingot with smaller grains. These results indicate that principles of grain size and -orientation control in mc-Si ingots can be applied to a pilot scale furnace, and the potential for up-scaling to industrial ingots with improved electrical properties and, thus, higher solar cell conversion efficiency.

Hydrogenated Amorphous Silicon Layer Formation by Inductively Coupled Plasma Chemical Vapor Deposition and Its Application for Surface Passivation of p-Type Crystalline Silicon

The satisfactory surface passivation properties of hydrogenated amorphous silicon (a-Si:H) prepared by inductively coupled plasma chemical vapor deposition (ICP-CVD) at a low temperature (400 °C) on p-type crystalline silicon wafers are reported. Certain parameters, such as SiH4/H2 ratio, annealing temperature, film thickness, and substrate temperature, were varied to determine their optimal levels. Completely amorphous layers with a broad transverse optic (TO) mode peak at approximately 480 cm-1 were identified by Raman spectroscopy, and an optical band gap of approximately 1.6 eV was determined from optical absorption data. A maximum carrier lifetime of 53 µs for an a-Si:H thickness of 15 nm and an annealing temperature of 450 °C was measured for 525-µm-thick p-type crystalline silicon (c-Si) substrates with a resistivity in the range of 1–20 Ω cm by the quasi-steady-state photoconductance (QSSPC) method. The lowest value of interface trapped charge density (Dit) of approximately 3.34 ×1011 cm-2 eV-1 was estimated by capacitance–voltage (C–V) measurement using a metal–insulator–semiconductor (MIS) structure. Furthermore, simple processing with satisfactory results can be achieved with substrate heating at 400 °C during deposition. The optimal conditions of a SiH4/H2 gas ratio of 1/1 and a substrate temperature of 400 °C were implemented for a passivation layer thickness of 5 nm.

Hydrogen transformations in Si-based solar structures studied by precise FTIR spectroscopy

In this work, an attempt to clarify the origin of a very pronounced effect of the minority carrier lifetime evolution in SiN x :H/a-Si:H/Si/a-Si:H/SiN x :H and a-Si:H/Si/a-Si:H solar cell structures prepared at temperatures around 200 °C upon heat treatments in the temperature interval of 445–550 °C is performed. For a comparison, heavily hydrogenated Si substrates were investigated as well. Analysis of all structures studied was performed by means of precise FTIR measurements and quasi-steady-state photoconductance (QssPC) techniques. Annealing-induced transformations of hydrogen-related states in Si-based structures upon heat treatments were monitored by a FTIR system with an enhanced sensitivity. Strong monotonic decrease of the intensity of various Si–H and N–H stretching bands was observed in all types of H-containing layers upon heat treatments applied. It is concluded that the transformations of H-related complexes in all structures studied are responsible for the observed changes in passivation efficiency of a-Si:H and SiN x :H/a-Si:H layers.

Investigation on the Surface Passivation of Intrinsic a-Si:H Thin Films Prepared by Inductively Coupled Plasma-Chemical Vapor Deposition for Heterojunction Solar Cell Applications

Intrinsic a-Si:H thin films, which can have passivation functions on the surface of crystalline Si, were deposited by inductively coupled plasma chemical vapor deposition (ICP-CVD). The properties of the films were investigated at deposition temperatures ranging from 50 to 400 degrees C. The Si--H stretching mode at 2000 cm(-1), which indicates good film quality, was found in the range of 150-400 degrees C, but the film quality was not good at deposition temperatures below 150 degrees C. The passviation quality was determined by measuring the effective carrier lifetime using the quasi-steady state photoconductance (QSS-PC) technique. Two, 5, 7.5 and 10 nm thick films were deposited at 150 degrees C and annealed at 200 degrees C for 1 hour. The carrier lifetime of these films was approximately 3 times higher than that observed before annealing. A p a-SiC:H/i a-Si:H/n c-Si hetero-structure solar cell with a 7.8% efficiency and approximately 85% quantum efficiency (QE) was obtained by inserting an intrinsic a-Si:H thin film (5 nm) between the interfaces. These results highlight the potential applications of a passivation layer to heterojunction solar cells.