Microwave Photoconductivity Decay Research Articles

Study of process induced variation in the minority carrier lifetime of silicon during solar cells fabrication

A systematic study of the variation in the minority carrier effective lifetime in silicon associated with the different solar cell processing steps in a conventional industrial production line has been carried out using the microwave photoconductive decay (μ-PCD) technique. The solar grade silicon wafers used for this study presented bulk carrier lifetime of ∼10 μs and resistivity 0.5–3 Ω cm. Alkali texturing, phosphorus diffusion using POCl 3, thermal oxide growth for surface passivation, plasma etching for edge isolation, and APCVD of TiO 2 for surface passivation and antireflection coating were the major steps taken into consideration. The results clearly showed that the lifetime increased as the fabrication process proceeds from the bare wafer with the exception of the step associated to plasma edge isolation. The effective lifetime of the bare wafer was 4.04 μs, which increased to 16.67 μs after the antireflection coating and surface passivation with TiO 2. The results of a systematic study of the effective minority carrier lifetime of silicon due to different surface passivation processes are also reported. The results obtained are useful for the design and implementation of proper measures for minority carrier lifetime enhancement during silicon solar cell fabrication at the industrial scale.

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.

The uniformity of InGaAs in InP/InGaAs/InP by microwave photoconductivity decay (μ-PCD) carrier lifetime measurement

Carrier lifetime maps and distributions of doped n-InGaAs in a MBE-grown p-InP/n-InGaAs/n-InP double heterostructure (DH) were measured at 300 K and 85 K using the μ-PCD lifetime screening technique. The theoretical considerations required in this technique are discussed in detail. The average carrier lifetimes are 168.2 ns and 149.4 ns at 300 K and 85 K, respectively. The results are consistent with those of ZnS/n-InGaAs/n-InP DHs. InGaAs lifetime mapping of the InP/InGaAs/InP DH is easily obtained without destruction by μ-PCD carrier lifetime measurement. Therefore, it could be a potential method to characterize the uniformity of the wafers, which is necessary to fabricate InGaAs focal plane arrays (FPAs).

Recombination lifetime characterization and mapping of silicon wafers and detectors using the microwave photoconductivity decay (μPCD) technique

Recombination lifetime in oxidized high-resistivity silicon has been characterized by the microwave photoconductivity decay (μPCD) technique. In this technique, a silicon wafer is illuminated by a laser pulse that generates electron hole pairs. The transient of the decaying carrier concentration is monitored by using a microwave signal. The recombination lifetime is a measure of the material quality, i.e., defect/impurity concentration which affects the quality of the resulting detectors and their electrical properties. The μPCD technique is a non-contact and non-invasive technique that can map recombination lifetime of the entire wafer before it is selected for device/detector processing. The recombination lifetime mapping on a wafer is realized by a 2D color imaging code representing the range of the lifetime in μs. In general, the recombination lifetime is the smallest around the wafer edges (about 1000–2000 μs), and highest in the center of the wafer (5000–10,000 μs). The recombination lifetime has been measured under different injection levels and surface charge conditions. The results show that the lifetime in investigated n-type materials is almost independent of the injection level if the passivating SiO 2 film is corona charged.

Recombination activity of nickel in Czochralski silicon during rapid thermal process

The recombination activity of nickel in p- and n-type Czochralski silicon after rapid thermal process (RTP) in N 2, O 2 or Ar ambient has been investigated by the microwave photoconductivity decay technique. The effective lifetime of silicon decreases monotonically with increasing nickel in-diffusion temperature, and exhibits a step-like behavior at nickel in-diffusion temperature of 900 °C, which indicates that most of nickel atoms precipitate in the bulk no matter what kind of conducting type if the annealing temperature is above 900 °C under RTP. It is also found that the ambient during RTP almost has no effect on the recombination activity of nickel in silicon, which suggests point defects almost have no influence on the nickel precipitation.

High Carrier Lifetime Bulk-Grown 4H-SiC Substrates for Power Applications

AbstractA thermal anneal process has been developed that significantly enhances minority carrier lifetime (MCL) in bulk-grown substrates. Microwave photoconductivity decay (MPCD) measurements on bulk grown substrates subjected to this process have exhibited decay times in excess of 35 μs. Electron Beam Induced Current (EBIC) measurements indicated a minority carrier diffusion length (MCDL) of 65 μm resulting in a calculated MCL of 15 μs, well within the range of that measured by MPCD. Deep level transient spectroscopic (DLTS) analysis of samples subjected to this anneal process indicated that a significant reduction of deep level defects, particularly Z1/2, may account for the significantly enhanced lifetimes. The enhanced lifetime is coincident with a transformation of the original as-grown crystal into a strained or disordered lattice configuration as a result of the high temperature anneal process. PiN diodes were fabricated employing 350 μm thick bulk-grown substrates as the intrinsic drift region and thin p- and n-type epitaxial layers on either face of the substrate to act as the anode and cathode, respectively. Conductivity modulation was achieved in these diodes with a 10x effective carrier concentration increase over the background doping as extracted from the differential on-resistance. Significant stacking fault generation observed during forward operation served as additional evidence of conductivity modulation and underscores the importance of reducing dislocation densities in substrates in order to produce a viable bulk-grown drift layer.

Interface Study Of Nanocrystalline Silicon and Crystalline Silicon Using Microwave Photoconductivity Decay

AbstractThis paper presents a measurement technique for studying of the interface between a nanocrystalline silicon (nc-Si) film and a crystalline silicon (c-Si) substrate using microwave photoconductivity decay (MWPCD). The nc-Si films were deposited using plasma enhanced chemical vapor deposition of highly hydrogen-diluted silane. The films were deposited on both sides of the high purity float-zone (FZ) Si wafers. The high resolution transmission electron microscope (HRTEM) analysis of the interface and the characterization of the effective excess carrier lifetime of the samples using MWPCD revealed the following results: (i) The crystallinity of the deposited nc-Si films is very high. The nc-Si film follows the crystal orientation of the substrate such that not a well-defined boundary between nc-Si film and the c-Si substrate is observed. (ii) A surface recombination velocity of less than 10 cm/s was measured for the interface region of the nc-Si/c-Si junctions. (iii) A small discontinuity in the band-energy diagram of the interface region was observed.

EBIC Study of Electrical Activity of Stacking Faults in Multicrystalline Sheet Silicon

The electrical activity of stacking faults (SFs) in multicrystalline sheet silicon has been examined by correlating EBIC(electron beam induced current), preferential defect etching, and microwave photo-conductance decay (PCD) lifetime measurements. Following a three hour 1060 0C annealing the interstitial oxygen concentration decreased from 14 to 4.5 x 1017 cm-3, during which time a high density of SFs were generated in the center of individual large grains. Subsequent EBIC contrast variation within individual large grains was correlated with the local SF density revealed by preferential etching. In addition, a more quantitative intra-grain lifetime was obtained from high spatial resolution PCD measurements. It was found that an SF density of 1 to 2 x 106 cm-2 produces a lifetime limitation in sheet silicon which corresponds to a recombination lifetime of ~2 µs.

Excess Carrier Lifetime Measurement of Bulk SiC Wafers and Its Relationship with Structural Defect Distribution

Excess carrier lifetime in bulk 2-in. SiC wafers was measured by microwave photoconductivity decay (µ-PCD). The mapping technique was used to obtain the lifetime distribution in the entire wafer. We observed the birefringence image and X-ray topograph of the wafers in order to determine the structural defect distribution, and the net donor concentration distribution was also observed by capacitance–voltage measurements. By comparison of lifetime maps with the structural defect distribution, it was found that relatively long lifetime regions correspond to regions with high-density structural defects. The net donor concentration did not show a clear influence on the carrier lifetimes. We confirmed that surface recombination has a negligible effect on the carrier lifetimes, and therefore the lifetimes obtained from mapping measurements are mainly dominated by carrier recombination behavior in the bulk of the wafers.

Quantitative copper measurement in oxidized p-type silicon wafers using microwave photoconductivity decay

We propose a method to measure trace copper contamination in p-type silicon using the microwave photoconductivity decay (μ-PCD) technique. The method is based on the precipitation of interstitial copper, activated by high-intensity light, which results in enhanced minority carrier recombination activity. We show that there is a quantitative correlation between the enhanced recombination rate and the Cu concentration by comparing μ-PCD measurements with transient ion drift and total reflection x-ray fluorescence measurements. The results indicate that the method is capable of measuring Cu concentrations down to 1010cm−3. There are no limitations to wafer storage time if corona charge is used on the oxidized wafer surfaces as the charge prevents copper outdiffusion. We briefly discuss the role of oxide precipitates both in the copper precipitation and in the charge carrier recombination processes.

Characterization of 4H-SiC Epitaxial Layers by Microwave Photoconductivity Decay

A Microwave Photoconductivity Decay (M-PCD) technique which senses changes insample conductivity as carriers recombine following excitation by a laser pulse, has been used to determine the minority carrier recombination lifetime from the decay rate of carriers in 4H-SiC epitaxial layers. Decay times varying from 60 ns to 500 ns have been measured, with the decay increasing with thickness. Device simulations show that I-V characteristics of pin diodes fabricated with these epitaxial layers are compatible with the observed decay times.

Lifetime Mapping Technique for Ultrathin Silicon-on-Insulator Wafers

Highly sensitive lifetime mapping equipment using the UV laser excited differential microwave photoconductivity decay technique was developed to observe the defect and contamination distribution in ultrathin silicon-on-insulator (SOI) layers. The technique was successively applied to the evaluation of state-of-the-art SOI wafers. Through theoretical analysis, it was quantitatively confirmed that the lifetime is shortened as the SOI film becomes thin, due to an increase in interface recombination. The large improvement in sensitivity attained by the differential detection enabled us to evaluate nonuniformities in the active layer quality by overcoming the difficulty of detection normally experienced by the decrease in the thickness of the SOI layer. Using light illumination with an extremely short penetration depth and the barrier effect of the buried oxide layer, it was found that the generated carriers were confined to the active layer, and lifetime data reflecting the quality of the SOI layer could be obtained. In the lifetime map for an SOI layer with a thickness of 52 nm, numerous islandlike spots with long lifetime, which were believed to be caused by HF defects or thin silicon areas, were detected. © 2004 The Electrochemical Society. All rights reserved.

A theoretical analysis of sensitivity in semiconductor measurement by microwave photoconductance decay

The aim of this work is to determine the sensitivity in the semiconductor measurement by microwave photoconductance decay. The reflection coefficient and the sensitivity were determined by solving the Maxwell equations and the method of a transfer matrix, then the dependence of sensitivity on the dark conductivity of sample, the distance between sample and metal reflector, and the thickness of sample for the general and two special measurement configurations were discussed. It is found that for all conductivity samples, the case of putting a metal reflector behind the detected sample with an optimal distance helps the sensitivity higher.

Aluminum gettering in photovoltaic silicon

The effect of aluminum gettering on different silicon materials used for solar cells has been investigated by means of microwave photoconductivity decay (μ-PCD) and electron beam induced current (EBIC). μ-PCD measurement revealed that the lifetime of multicrystalline silicon (mc-Si) with a lower initial lifetime could be increased by high temperature gettering (1000 °C), while that of mc-Si with a higher initial lifetime could not be increased, but was even degraded. EBIC results revealed that no significant improvement of diffusion length was observed in both contaminated and uncontaminated wafers. while 850 °C Al gettering was employed. It is concluded that both the initial material quality and the thermal treatment have influences on the effect of Al gettering. In addition, dislocations with bright EBIC contrast were discovered in annealed mc-Si wafers, the origin of which is discussed.

Excess Carrier Lifetime Mapping for Bulk SiC Wafers by Microwave Photoconductivity Decay Method and Its Relationship with Structural Defect Distribution

Excess Carrier Lifetime Mapping for Bulk SiC Wafers by Microwave Photoconductivity Decay Method and Its Relationship with Structural Defect Distribution

Comparison of Silicon-on-Insulator Wafer Mappings between Photoluminescence Intensity and Microwave Photoconductivity Decay Lifetime

We characterized the nonuniformities of state-of-the-art ultrathin silicon-on-insulator (SOI) wafers by photoluminescence (PL) intensity mapping and microwave photoconductivity decay (µ-PCD) lifetime mapping. Both mapping techniques revealed characteristic patterns with extreme sensitivity. The PL and µ-PCD mapping patterns on the substrate were almost exactly alike for all the measured wafers. There was also a strong resemblance between the two mapping patterns on the top Si layer for most wafers. A higher PL intensity region corresponded to a longer lifetime area. The quantitative relationship between the PL intensity and µ-PCD lifetime was obtained not only for comparison within a wafer but also for wafer-to-wafer comparison. The mapping pattern on the substrates varied greatly depending on the wafer fabrication method. We believe that the pattern on the top Si layer originates in the distribution of microdefects such as HF defects in the layer, the variation in layer thickness, and/or the nonuniformity produced during the thermal process for surface passivation.

Excess Carrier Lifetime Measurements for GaN on Sapphire Substrates with Various Doping Concentrations and Surface Conditions by the Microwave Photoconductivity Decay Method

ABSTRACTWe have measured excess carrier lifetimes in GaN with various doping concentrations and surface conditions by the microwave photoconductivity decay method. GaN samples were grown by metalorganic chemical vapor deposition (MOCVD) without intentional doping and with Si doping or Mg doping on a-face sapphire substrates. By using the microwave photoconductivity decay method, we obtained 1/e excess carrier lifetimes of larger than 50 μs for the Si doped and undoped GaN and of less than 10 μs for the Mg doped GaN. We changed surface conditions for the samples by the inductively coupled plasma (ICP) etching and investigated effects of surface conditions on the carrier recombination behavior. The ICP etching has negligible effects on carrier lifetime in the Si doped GaN. On the other hand, in the undoped GaN, the ICP etching lengthened the carrier lifetime compared with the as-grown sample. On the contrary, the ICP etching shortened the carrier lifetime in the Mg doped GaN. The ICP etching seems to form hole traps and recombination centers at GaN surfaces and thus the carrier lifetime became longer in the undoped GaN and shorter in the Mg doped GaN.

Light Activated Copper Defects in P-Type Silicon Studied by PCD

We have studied copper defects in p-type silicon by measuring its precipitation kinetics by means of the microwave photoconductive decay (µPCD) technique. Copper atoms precipitated during high intensity light treatment at room temperature. We used the total reflection X-ray fluorescence (TXRF) and the transient ion drift (TID) techniques to determine the bulk concentration of copper. We estimated the density and the radius of the copper precipitates as well as the average capture cross-section for precipitated copper atoms from the measured copper precipitation time constant, bulk concentration of copper, and the change in the recombination rate. We also studied how the density of oxygen defect affects the copper precipitation. Our results show that copper precipitates at two different kinds of defects.

Sensitive Copper Detection in P-type CZ Silicon using μPCD

Conventional microwave photoconductive decay (μPCD) method in conjunction with high intensity bias light has been used to study out-diffusion and precipitation of copper in p-type Czochralski (CZ) silicon. The high intensity light is used to activate the precipitation of interstitial copper, which follows Ham’s kinetics and results in a distinctive decrease in the excess carrier lifetime. Results indicate that Cu concentration well below can be detected by this method. It is demonstrated that positive corona charge can be used to prevent out-diffusion of interstitial copper, while negative charge enables copper to freely diffuse to the wafer surfaces. It was observed that the precipitation rate of copper increased significantly when the bias-light intensity is raised above a certain critical level. In addition, the copper precipitation rate was discovered to be much higher in samples which have internal gettering sites. These findings suggest that high intensity light reduces the electrostatic repulsion between positively charged interstitial copper ions and copper precipitates enabling copper to precipitate in the wafer bulk even at a low concentration level, and during high intensity illumination, oxygen precipitates provide effective heterogeneous nucleation sites for copper. © 2003 The Electrochemical Society. All rights reserved.

Local series resistance mapping of silicon solar cells by microwave photoconductivity decay measurements

AbstractWe present a new non‐destructive mapping technique which yields information on the local series resistance in Si solar cells. The technique is based on microwave photoconductivity decay measurements performed on reverse‐biased Si solar cells. For reverse‐biased devices, the initial decay time measured after a high‐intensity laser pulse is shown to be directly proportional to the local series resistances between the excited area and the cell metallisation. We illustrate how high contact resistance between the fingers and the emitter, resistive back‐surface contacts or finger interruptions lead to an increase of the microwave detected decay time. Copyright © 2003 John Wiley &amp; Sons, Ltd.