Quasi-steady State Photoconductance Research Articles

NH3 Plasma Interface Modification for Silicon Surface Passivation at Very Low Temperature

The effect of passivation layers (SiOX, SiNX, etc.) using plasma-enhanced chemical vapor deposition (PECVD) for crystalline silicon solar cells showed high surface recombination with decreasing of deposition temperature (<300 °C). The surface of Czochralski (Cz) monocrystalline silicon (c-Si) wafer was exposed to an NH3 plasma before the low-temperature deposition of silicon nitride (SiNX) layer or amorphous silicon (a-Si:H) layer in a system respectively. The effect of NH3 plasma treatment for the interface was observed by microwave photoconductance decay (µ-PCD), quasi-steady-state photoconductance (QSSPC), Fourier transform infrared spectroscopy (FT-IR), secondary ion-microprobe mass spectrometry (SIMS), and X-ray photoemission spectroscopy (XPS). The effective lifetime (τeff) with NH3 plasma treatment exceeded τeff without the treatment. Even in the case of very low deposition temperature (100 °C), τeff is improved dramatically (about 38 times). This NH3 plasma treatment effect was caused by hydrogenation and carbon cleaning from the SIMS measurement. It was found that SiNX films with excellent surface passivation properties can be deposited at 100 °C.

How lifetime fluctuations, grain-boundary recombination, and junctions affect lifetime measurements and their correlation to silicon solar cell performance

Two-dimensional simulations of quasi-steady-state photoconductance (QSSPC), carrier density imaging (CDI), photoconductive decay (PCD), and solar cell performance are performed on silicon models that incorporate grain-boundary recombination or lifetime fluctuations on the distance scale of 5 μm to 5 mm. The relationships between the lifetime measurement, actual recombination rates, and solar cell performance vary widely based on beam size, measurement technique, and recombination profile. The strengths and weaknesses of different measurement methods and the ability of analytical models to predict aggregate solar cell performance are examined and compared with earlier studies. Lifetime measurements in the presence of a junction are shown to be distorted by charge separation.

Lifetime analysis of silicon solar cells by microwave reflection

Microwave photoconductive decay ( μPCD) has become a standard technique for measuring the carrier lifetime of silicon used in solar cells. Here, we have used μPCD to examine the carrier lifetimes at common doping levels used in the base region of silicon photovoltaic devices. For the conductivity range used in the p-type base of n +–p structures, the microwave penetration depth is less than the wafer thickness. In this case, the reflectance–conductivity relationship is very nonlinear. We will show that quasi-steady-state photoconductivity (QSSPC) and resonance-coupled photoconductive decay (RCPCD) lifetime measurements track over a wide range of injection level, and generally agree at higher injection levels. Our μPCD data will be compared with the transient RCPCD data over the same range. The data from the latter agree at low-injection levels, but show serious disagreement at higher injection levels. The conclusion is that μPCD must be limited to low-injection levels in the doping range used for solar cells.

PECVD‐ONO: A New Deposited Firing Stable Rear Surface Passivation Layer System for Crystalline Silicon Solar Cells

A novel plasma‐enhanced chemical vapour deposited (PECVD) stack layer system consisting of a‐SiOx:H, a‐SiNx:H, and a‐SiOx:H is presented for silicon solar cell rear side passivation. Surface recombination velocities below 60 cm/s (after firing) and below 30 cm/s (after forming gas anneal) were achieved. Solar cell precursors without front and rear metallisation showed implied open‐circuit voltages Voc values extracted from quasi‐steady‐state photoconductance (QSSPC) measurements above 680 mV. Fully finished solar cells with up to 20.0% energy conversion efficiency are presented. A fit of the cell′s internal quantum efficiency using software tool PC1D and a comparison to a full‐area aluminium‐back surface field (Al‐BSF) and thermal SiO2 is shown. PECVD‐ONO was found to be clearly superior to Al‐BSF. A separation of recombination at the metallised and the passivated area at the solar cell′s rear is presented using the equations of Fischer and Kray. Nuclear reaction analysis (NRA) has been used to evaluate the hydrogen depth profile of the passivation layer system at different stages.

Silicon nitride coating and crucible—effects of using upgraded materials in the casting of multicrystalline silicon ingots

AbstractA method for removal of metallic contaminants from commercial Si3N4‐powders has been developed. The method is based on acid leaching and shows promising results. Analyses show that the content of both intermetallic compunds as Fe, Ti and Al as well as O are significantly reduced. Clean, synthetic fused silica crucibles have been used along with normal sintered ones based on natural quartz. The crucibles were coated with normal, commercial silicon nitride powder and with purified Si3N4 in a cleanest possible environment. The crucibles were then used as vessels for directional solidification of multicrystalline silicon in a pilot scale furnace. The average lifetime of minority charge carriers in the cast silicon was determined by quasi steady state photoconductance (QSSPC) from the bottom to the top of the ingots. These varied in a systematic way, so that the materials cast in the pure environment had significantly higher values than the materials cast with conventional coating‐ and crucible materials. The maximum values for the lifetimes in the individual ingots varied from 7 to 135 µs. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Fixed charge density in dielectrics deposited on c-Si using space charge region dominated lifetime measurements

Depletion region modulation (DRM) effect is often observed in photoconductance lifetime measurements of crystalline silicon wafers passivated by dielectric films. This effect is closely related to the space-charge region electrostatically induced by fixed charges within the dielectric. This study proposes a model for dielectric-passivated c-Si wafers, which includes the DRM effect, to simulate and fit the effective lifetime vs excess minority carrier density curves obtained by quasisteady-state photoconductance techniques. The validity of the model is checked by applying it to different experimental samples, taking particular care of the mobility values of the surface carriers. It is, thus, demonstrated that the fixed charge within the dielectric film can be determined with improved accuracy and increased reliability if the DRM effect is included into the model.

Effect of oxygen precipitates in solar grade silicon on minority carrier lifetime and efficiency of solar cells

The effect of oxygen precipitates on minority carrier lifetime and performance of solar cell was studied by means of Fourier Transform Infrared Spectroscopy (FTIR), quasi-steady state photoconductance (QSSPCD), optical microscope, spectrum response and solar cell efficiency test. The minority carrier lifetime and performance of solar cell reduced depend on oxygen precipitates. A few of oxygen precipitates have formed after single-step annealing; and they do not impact the efficiency dramatically. Pre-annealing at 650 °C for 4 h enhances the oxygen precipitation when it is subjected to middle temperature annealing. The solar cells performance decayed sharply. Especially annealing at 950 °C for 3 h, the V os and I sc of cells decrease 12% and 25% respectively. Few oxygen precipitates have formed in silicon after high temperature annealing at about 1050 °C whether pre-annealing is used or not, and the performance of cells is not be affected.

Ge composition dependence of the minority carrier lifetime in monocrystalline alloys of Si 1−xGe x

In this work we have investigated the influence of the Ge composition on the minority carrier lifetime in Czochralski (Cz) grown monocrystalline Si 1− x Ge x alloys in the composition range of 0< x<0.07. The lifetime measurements were performed by both microwave photo conductance decay ( μ-PCD) measurements and the quasi-steady-state photoconductance (QssPC) technique. Low-temperature (450 °C) anneals as well as phosphorous diffusion gettering were applied in order to investigate the possibility of improving the minority carrier lifetime in this material. A surface passivation method based on deposition of a double layer stack consisting of a hydrogenated amorphous silicon thin film capped by a silicon nitride anti-reflection coating was used. It is established that the minority carrier lifetime in the investigated alloys decreases dramatically with increasing Ge concentration. It is further shown that phosphorous gettering is the most effective method for the enhancement of bulk lifetime values in the Si 1− x Ge x alloys studied.

Reactive ion etching of dielectrics and silicon for photovoltaic applications

This paper investigates the reactive ion etching of SiO2 ,S i3N4, and Si using CHF3/O2 plasma. In particular, we have characterized the time and rf power dependence of the carrier lifetimes in n- and p-type FZ Si. The time dependence of reactive ion etching (RIE) at different rf powers provide insight into the two competing processes of damage accumulation and damage removal in the near-surface region of the Si during plasma etching. The carrier lifetime, measured using the quasi-steady-state photoconductance (QSSPC) technique, has a quadratic dependence on the rf power, which can be related to changes in the dc self-bias generated by the plasma at different rf powers. The change in carrier lifetime is similar in both n- and p-type Si of the same doping concentration. Using this fact, together with the electronic properties of defects obtained by deep level transient spectroscopy (DLTS), we have modeled the injection-dependence of the measured carrier lifetimes using the Shockley– Read–Hall model. The isochronal annealing behavior of plasma etched Si has also been studied. Copyright # 2006 John Wiley & Sons, Ltd.

Characterization of magnetic Czochralski silicon radiation detectors

Silicon wafers grown by the Magnetic Czochralski (MCZ) method have been processed in form of pad diodes at Instituto de Microelectrònica de Barcelona (IMB-CNM) facilities. The n-type MCZ wafers were manufactured by Okmetic OYJ and they have a nominal resistivity of 1 kΩ cm. Concentrations of oxygen and carbon in MCZ wafers were measured by Secondary Ion Mass Spectroscopy (SIMS) technique to be compared to standard and oxygenated float zone (FZ) silicon material. The diodes were characterized by leakage current and capacitance measurements. The average full depletion voltage, V FD, of the pad detectors is 290±10 V, while the leakage current density at V FD+20 V is 7±3 nA/cm 2. Minority carrier lifetime of MCZ, standard FZ and oxygenated FZ substrates was measured by using a quasi steady-state photoconductance technique. The uniformity of the MCZ wafers was studied before the fabrication by mapping the resistivity of the wafer and after the fabrication process by measuring the full depletion voltage and leakage current as a function of diode position on the wafer. It was found that the fabrication process did not change the resistivity of the material although no donor-killing heat treatment was performed on the wafers.

Effect of Combined Oxygenation and Gettering on Minority Carrier Lifetime in High-Resistivity FZ Silicon

High oxygen concentration in silicon increases the resistance of radiation detectors to high radiation doses. Unfortunately, high-resistivity float zone (FZ) silicon, needed for radiation detectors, has too low an oxygen content. The solution to this is silicon oxygenation. There are different ways of incorporating oxygen into silicon. The most accepted one is by high-temperature diffusion from a thick layer. In this paper, we investigate the impact of this silicon oxygenation technique as well as the application of different gettering techniques to improve the minority carrier lifetime of high-resistivity FZ silicon substrates for radiation detectors. The minority carrier lifetimes before and after surface etching have been measured on samples subjected to different oxygenation and gettering treatments by using a quasi steady-state photoconductance technique. A lifetime improvement efficiency factor is defined for each treatment process. The lifetime efficiency factors behave independently, so that the lifetime efficiency factors associated with different sequential combinations of treatments can be estimated by a multiplicative combination. Different gettering techniques that improve or degrade the minority carrier lifetime are analyzed, and the best options for silicon radiation detector fabrication are determined. Oxygenated silicon with a minority carrier lifetime close to 1 ms can be obtained. © 2004 The Electrochemical Society. All rights reserved.

Electronic properties of the metastable defect in boron-doped Czochralski silicon: Unambiguous determination by advanced lifetime spectroscopy

By combining data from temperature- and injection-dependent lifetime spectroscopy (TDLS and IDLS) measured by means of the microwave-detected photoconductance decay technique and the quasi-steady state photoconductance technique, respectively, the exact electronic structure of the metastable defect in standard boron-doped Czochralski (Cz) silicon has been determined. A detailed Shockley–Read–Hall analysis of the entire TDLS curve reveals that the Cz-specific defect acts as an attractive Coulomb center [σn(T)=σn0T−2] which is localized in the upper band-gap half at EC−Et=0.41 eV and has an electron/hole capture cross section ratio k=σn/σp=9.3. The accuracy of this determination manifests itself by the fact that the corresponding IDLS curve can be simulated with the same parameter set.

Surface passivation of n-type crystalline Si by plasma-enhanced-chemical-vapor-deposited amorphous SiCx:H and amorphous SiCxNy:H films

Excellent passivation of n-type crystalline silicon surface is demonstrated by means of intrinsic amorphous silicon carbide (a-SiCx:H) thin films. An optimum CH4/SiH4 ratio is determined, leading to an effective surface recombination velocity, Seff, lower than 54 cm s−1. By adding a constant flow of N2 to the precursor gases, the surface passivation is improved to Seff⩽16 cm s−1. From infrared spectroscopy measurements of these films, it can be deduced that the N2 flow increases the carbon content of the layers for a constant CH4/SiH4 ratio. The dependence of the effective lifetime, τeff, on the excess charge carrier density, Δn, is measured using the quasisteady-state photoconductance technique, and these curves are simulated through an electrical model based on an insulator/semiconductor structure.

Measurement of differential and actual recombination parameters on crystalline silicon wafers [solar cells

In this paper, for the first time, measurements of differential and actual recombination parameters on crystalline silicon wafers are directly compared. In order to determine the differential bulk lifetime and the differential surface recombination velocity (SRV), small-signal light-biased microwave-detected photoconductance decay (MW-PCD) and modulated free-carrier absorption (MFCA) measurements are performed. The results obtained by these widespread techniques are compared with quasi-steady-state photoconductance (QSSPC) measurements, which directly determine the actual recombination parameters. On high-resistivity (1000 /spl Omega/cm) float-zone (FZ) n-type silicon at high injection levels, it is shown that the differentially measured Auger lifetime is a factor of three smaller than the actual Auger lifetime. This finding is in excellent agreement with the theory derived in this work. Thermally oxidized low-resistivity (/spl sim/1 /spl Omega/cm) p-Si wafers serve as an experimental vehicle to compare the differential and the actual injection-level dependent SRV of the Si-SiO/sub 2/ interface under low-injection conditions. Using two different integration procedures, the actual SRV is calculated from the differentially measured quantity. The actual SRV measured by the QSSPC technique is found to match perfectly the actual SRV obtained by integration.

Injection level dependence of the defect-related carrier lifetime in light-degraded boron-doped Czochralski silicon

The carrier recombination lifetime in light-degraded boron-doped 1 Ω cm Czochralski-grown silicon wafers is measured as a function of the bulk excess carrier concentration Δn. The measurements are performed with the quasi-steady state photoconductance method and cover a large injection level range between 1013 and 1.5×1017 cm−3. We observe a very strong increase of the carrier lifetime in the Δn range between 1014 and 2×1016 cm−3, which is attributed to boron–oxygen (BiOi) defect pairs. The observed strong increase of the defect-related carrier lifetime allows us to determine the previously unknown hole capture cross section σp of the BiOi pair. Our analysis gives a σp value of (0.45–1.2)×10−15 cm2, which is 2–3 orders of magnitude smaller than the corresponding electron capture cross section.

Expitaxial regrowth of (100) InP layers amorphized by ion implantation at room temperature: P Auvray et al,J Appl Phys, 53 (9), 1982, 6202–6207

This paper reports the use of injection-dependent local ideality factors, obtained from quasi-steady state photoconductance and photoluminescence measurements, to investigate the effects of various cell processing steps on recombination in solar cells, fabricated using boron-doped Czochralski silicon wafers, with an ∼ 100 Ω/□ phosphorus-doped emitter and silicon nitride passivation of both surfaces. It is shown that activation of boron-oxygen complexes in the cells by light soaking can impact the pseudo fill factor and is manifest in increased local ideality factors in the injection range between maximum power point and open circuit voltage. The introduced recombination was modeled as a single Shockley Reed Hall (SRH) recombination centre at Ec - Et = 0.41 eV and an electron:hole capture cross section ratio of ∼14. The effects of boron and phosphorus laser doping on the injection-dependent local ideality factor were also investigated. Boron laser doping was shown to introduce additional recombination in cells, as indicated by the increased local ideality factor in the injection range between maximum power point and open circuit voltage. However, in this case, the additional recombination was not well-modelled by a single SRH recombination centre, especially in the mid-injection range. Finally, it is shown that high temperature belt furnace anneals can place cells into a specific recombination state, and cells can be returned to this state by subsequent anneals even after additional recombination is introduced into cell by processes like laser doping.