Quasi-steady State Photoconductance Technique Research Articles

Application of Quasi-Steady-State Photoconductance Technique to Lifetime Measurements on Crystalline Germanium Substrates

Similar to other high quality crystalline absorbers, an accurate knowledge of surface passivation of crystalline Germanium (c-Ge) substrates is crucial for a straightforward improvement of photovoltaic device performance. For crystalline silicon devices, this information is typically obtained by quasisteady state photoconductance (QSS-PC) technique using Sinton WCT-120 tool. In this work, we explore the conditions to adapt this measurement technique to c-Ge substrates. Based on PC-1D simulations, we deduce that a minimum effective lifetime is needed corresponding to an effective diffusion length equal to the substrate thickness. Apart from this, an accurate estimation of the total photogeneration inside the c-Ge sample is also mandatory. This condition implies that the light intensity that impinges onto the sample must be measured with a c-Ge sensor, although the integrated c-Si sensor can be used for high flash intensities. Additionally, the optical factor used to evaluate sample reflectance must be also known, which is determined by measuring robust effective lifetime values under photoconductance decay conditions. Finally, knowledge about carrier mobility in c-Ge is also necessary to translate the measured photoconductance to the corresponding excess carrier density values. Lifetime measurements of passivated c-Ge substrates done by QSS-PC technique are validated by comparing them with the ones obtained by microwave photoconductance technique.

INFLUENCE OF THE ALKALI SURFACE TREATMENTS ON THE INTERFACE-STATES DENSITY AND MINORITY CARRIER LIFETIME IN Cz–SILICON WAFER

The chemical etching of the surface of silicon wafers is a critical step in the manufacturing process of all semiconductor devices. In this contribution, we investigate the effect of alkaline etching on minority carrier lifetime and interface-states density ([Formula: see text] of silicon wafers intended to be used as solar cell substrates. After alkali treatment, the surface morphology was analyzed using scanning electron microscopy (SEM) and UV-visible-NIR optical spectroscopy. Besides and as electrical characterizations, the minority charge carrier lifetime ([Formula: see text] was measured by the Quasi-Steady State Photoconductance technique (QSSPC), and the Electrochemical Impedance Spectroscopy was used to evaluate [Formula: see text]. These results were correlated with the surface recombination velocity (SRV) calculated by fitting the experimental data to the theory. The results of characterization showed a lower SRV and a higher apparent lifetime ([Formula: see text] obtained with 23[Formula: see text]wt.% KOH etching as compared to those obtained with 30[Formula: see text]wt.% NaOH; viz. 825[Formula: see text]cm[Formula: see text] against 1500[Formula: see text]cm.s[Formula: see text] and 32[Formula: see text][Formula: see text]s against 23[Formula: see text][Formula: see text]s, respectively. These findings were corroborated by [Formula: see text] measurements which gave [Formula: see text][Formula: see text]ev[Formula: see text]cm[Formula: see text] for KOH treatment and [Formula: see text][Formula: see text]ev[Formula: see text]cm[Formula: see text] for NaOH treatment.

PASSIVATION OF CRYSTALLINE SILICON ON POROUS SILICON SURFACE TREATED BY ERBIUM OXIDES (Er2O3) FOR ENHANCEMENT THE PHOTOVOLTAIC PROPERTIES

The aim of this paper is to study the effect of erbium oxide (Er2O3) on porous silicon (PS) wafers used for photovoltaic application. An immersion of PS wafers in Er2O3 solution can be used to enhance light trapping and form an efficient surface by passivation process. PS was prepared by the stain-etching method and doped by Er species. In fact, the topography was investigated by the scanning electron microscope (SEM). In addition, the spectral behaviors of the reflectivity and the photoluminescence were discussed. The dependence of minority carrier lifetime was evaluated by means of the Quasi-Steady-State Photoconductance technique (QSSPC). Besides, an enhancement in lifetime was observed. A framework is provided for estimating an efficiency improvement in studied films which will help to guide the development of improved energy-efficient.

Mechanical grooving effect on the gettering efficiency of crystalline silicon based solar cells

This paper examines a gettering process of Czochralski silicon (CZ) via mechanical texture, followed by two step heat treatment in the presence of porous silicon layer (PSL) under oxygen flow gas. It is shown that a process with PS has a positive trend of improvement in the electronic quality, and found to be more efficient when used in combination with mechanical grooving. We obtained a significant increase of the effective minority carrier lifetime and majority charge carriers mobility. Thus, there is an apparent decrease in the resistivity. These parameters were estimated through a The Quasi-Steady-State Photo-Conductance technique (QSSPC), the van Der Pauw method and Hall Effect. Particularly, we have made obvious that the large enhancement of the electronic quality of the wafers can be related to the presence of grooves, the influence during which the gettering process is of importance to overcome the unexpected saturation phenomena. The current voltage I-V characteristics of all samples had been measured under illumination. They were shown to enhance the photovoltaic properties of solar cells.

Enhancement of minority carrier lifetimes in n- and p-type silicon wafers using silver nanoparticle layers

The quasi-steady state photo conductance technique is employed to probe effective minority carrier lifetime (τeff) modifications after integrating silver nanoparticles (Ag NPs) on n-type and p-type silicon wafers with a native oxide surface. Our observations reveal that τeff modification is very sensitive to Ag NPs size, surface coverage and also wafer type. With an optimized Ag NPs, τeff is enhanced from 4.4 μs to 10 μs for a p-type silicon wafer, and from 8.1 μs to 14 μs for an n-type silicon wafer. We attributed the enhancement in τeff to the partial field effect passivation of the silicon surface by the surface plasmon resonance near-fields of Ag NPs after excitation. Our investigations demonstrate that an optimized Ag NPs on any silicon wafer with a native oxide layer can work as both a light trapping and a surface-passivating layer.

Low Temperature Annealing Effect on Electrical Properties of Multicrystalline Silicon Wafers for Photovoltaic Application

The aim of this work is to study the low temperature annealing effect on the electrical properties of p-type multicrystalline silicon grown by Heat Exchanger Method (HEM).The minority carrier lifetime variation, the transition metal elements behavior, the sheet resistivity and the interstitial oxygen concentration after different temperatures annealing under N2ambient were investigated using quasi-steady state photoconductance technique (QSSPC), secondary ion mass spectroscopy (SIMS), four-probe measurement and Fourier transform infrared spectrometer (FTIR), respectively. The obtained results indicate in the temperature range of 300°C to 700°C that the effective lifetime increases and reaches its maximum values of 28 μs at 500 °C and decreasing to 6 μs at 700 °C. This amelioration is due probably to metallic impurities internal gettering in the extended defects and in the oxygen precipitates as observed on SIMS profiles and the FTIR spectra. From 300 °C to 500 °C the sheet resistivity values rest unchanged at 30 Ω.cm-2and rises significantly to reach 45 Ω.cm-2for T> 500 °C.

Effect of nickel contamination on high carrier lifetime n-type crystalline silicon

The injection-dependent lifetimes of different levels of Ni-contaminated n-type Czochralski (CZ) silicon wafers were investigated using resonant-coupled photoconductance decay (RCPCD) and quasi-steady-state photoconductance technique (QSSPC). The lifetime degradation of the most heavily contaminated samples was caused by Ni silicide precipitates at the surface of the wafers. The impact on lifetime was determined by surface recombination velocities (SRV). SRV values from RCPCD were comparable to those extracted by the QSSPC technique. A direct correlation between minority carrier lifetime and the concentration of electrically active substitutional Ni and Ni silicide precipitate traps measured using deep level transient spectroscopy was established.

Surface recombination analysis in silicon-heterojunction solar cells

The origin of this work is the understanding of the correlation observed between efficiency and emitter-deposition temperature in single silicon-heterojunction solar cells prepared by depositing an n-doped hydrogenated-amorphous-silicon thin film onto a p-type crystalline-silicon wafer. In order to interpret these results, surface-recombination velocities have been determined by two methods, i.e. by fitting the current–voltage characteristics to a theoretical model and by means of the Quasi-Steady-State Photoconductance Technique (QSSPC). In addition, effective diffusion lengths have been estimated from internal quantum efficiencies. The analysis of these data has led to conclude that the performance of the cells studied is limited by back-surface recombination rather than by front-heterojunction quality. A 12%-efficient cell has been prepared by combining optimum emitter-deposition conditions with back-surface-field (BSF) formation by vacuum annealing of the back aluminium contact. This result has been achieved without using any transparent conductive oxide.

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.

Recombination of hot-electrons : Rob G. van Welzenis and Jan-Pieter Zeeuwen. Solid-St. Electron.21, 1591 (1978)

The origin of this work is the understanding of the correlation observed between efficiency and emitter-deposition temperature in single silicon-heterojunction solar cells prepared by depositing an n-doped hydrogenated-amorphous-silicon thin film onto a p-type crystalline-silicon wafer. In order to interpret these results, surface-recombination velocities have been determined by two methods, i.e. by fitting the current–voltage characteristics to a theoretical model and by means of the Quasi-Steady-State Photoconductance Technique (QSSPC). In addition, effective diffusion lengths have been estimated from internal quantum efficiencies. The analysis of these data has led to conclude that the performance of the cells studied is limited by back-surface recombination rather than by front-heterojunction quality. A 12%-efficient cell has been prepared by combining optimum emitter-deposition conditions with back-surface-field (BSF) formation by vacuum annealing of the back aluminium contact. This result has been achieved without using any transparent conductive oxide.