Enhancement Of Minority Carrier Lifetime Research Articles

Enhancement of Minority Carrier Lifetime in Ultra-High Voltage 4H-SiC PiN Diodes by Carbon-Film Annealing

A novel process is developed for minority carrier lifetime enhancement in ultra-high 4H-SiC PiN diodes. It comprises two separate processes. Firstly, the ultra-thick epitaxial grown drift layer (200μm) covered with a protective thin carbon film is subject to a 1500°C high-temperature anneal process in Ar atmosphere for 2 hours. Secondly, a surface passivation process is adopted to reduce the surface recombination rate. μ-PCD tests show that after high-temperature anneal, the thick drift layer shows a minority carrier lifetime increase to about 1.6 μs. PiN diodes based on the novel process are fabricated and their electric characteristics are measured. Results show a low specific on-resistance of 16.3 mΩ·cm2 at 25°C and 14 mΩ·cm2 at 125 °C. Compared with simulation results, it is shown that its effective minority carrier lifetime increase to about 5μs .Our study demonstrates that the developed novel process is effective in minority carrier lifetime enhancement in ultra-voltage 4H-SiC PiN diodes.

Contactless Optical Characterization of Carrier Dynamics in Free-Standing InAs-InAlAs Core-Shell Nanowires on Silicon.

Contactless time-resolved optical pump-probe and external quantum efficiency measurements were performed in epitaxially grown free-standing wurtzite indium arsenide/indium aluminum arsenide (InAs-InAlAs) core-shell nanowires on Si (111) substrate from 77 to 293 K. The first independent investigation of Shockley-Read-Hall, radiative, and Auger recombination in InAs-based NWs is presented. Although the Shockley-Read-Hall recombination coefficient was found to be at least 2 orders of magnitude larger than the average experimental values of other reported InAs materials, the Auger recombination coefficient was reported to be 10-fold smaller. The very low Auger and high radiative rates result in an estimated peak internal quantum efficiency of the core-shell nanowires as high as 22% at 77 K, making these nanowires of potential interest for high-efficiency mid-infrared emitters. A greater than 2-fold enhancement in minority carrier lifetime was observed from capping nanowires with a thin InAlAs shell due to the passivation of surface defects.

Deleterious electrostatic interaction in silicon passivation stack between thin ALD Al2O3 and its a-SiNX:H capping layer: numerical and experimental evidences

This study focuses on the electrostatic interaction between the ALD Al2O3 passivation layer and the PECVD a-SiNX:H capping layer, while reducing ALD Al2O3 thickness. The first one embeds negative fixed charges while the second is well known to possess a fixed charge density with an opposite polarity. We reduced the Al2O3 thickness from 20 to 2 nm while keeping constant the a-SiNX:H thickness. To increase the magnitude of the field effect passivation provided by Al2O3, we used an original light-induced field effect enhancement strategy. QSS-PC was used to quantify the initial passivation properties. We monitored the evolution of the passivation quality under low-intensity light-soaking until its stabilization. We report here a minority carrier lifetime enhancement up to 900% and surface recombination velocity reduction below 10 cm.s-1. The evidence of field effect compensation at c-Si surface due to a-SiNX:H positive charges has been supported by numerical simulations (SILVACO ATLAS). While keeping the parameters of the a-SiNX:H layer constant, the configuration of the Al2O3 layer (thickness and fixed charge density) were varied. These results allow us to conclude that in order to use thinner Al2O3 passivation layer (2 nm), ideal capping layer has to be free of positive fixed charges and to release enough hydrogen at desired temperature to ensure the chemical passivation.

Effect of Thermal Annealing on Light-Induced Minority Carrier Lifetime Enhancement in Boron-Doped Czochralski Silicon**Supported by the National Natural Science Foundation of China under Grant No 61076056.

The effect of thermal annealing on the light-induced effective minority carrier lifetime enhancement (LIE) phenomenon is investigated on the p-type Czochralski silicon (Cz-Si) wafer passivated by a phosphorus-doped silicon nitride (P-doped SiNx) thin film. The experimental results show that low temperature annealing (below 300°C) can not only increase the effective minority carrier lifetime of P-doped SiNx passivated boron-doped Cz-Si, but also improve the LIE phenomenon. The optimum annealing temperature is 180°C, and its corresponding effective minority carrier lifetime can be increased from initial 7.5 μs to maximum 57.7 μs by light soaking within 15 min after annealing. The analysis results of high-frequency dark capacitance-voltage characteristics reveal that the mechanism of the increase of effective minority carrier lifetime after low temperature annealing is due to the sharp enhancement of field effect passivation induced by the negative fixed charge density, while the mechanism of the LIE phenomenon after low temperature annealing is attributed to the enhancement of both field effect passivation and chemical passivation.

The impact of thermal treatment on gettering efficiency in silicon solar cell

This paper describes a study to understand the impact of heat treatment on the crystal Silicon (c-Si) with developed surface by Aluminum /Porous silicon (Al/PS) and evaluating their influence on the solar cell performance. In fact, the heat treatment of silicon surfaces with PS/Al is an effective means of structural and electrical adjustment and performance improvement of c-Si solar cells. The new process could be accomplished by a two-step annealing in order to benefit from both the high and low temperature processes. The advantages of such a new getter in comparison with traditional getter were demonstrated in various devices. We can estimate that the resistivity using the standard Van Der Pauw, and LBIC measurements have been performed to determine the diffusion length. The resistivity of crystalline silicon decreased when the porous layer was removed at about a depth around 70μm. As a result, we found amelioration in the I–V characteristics and an enhancement of minority carrier lifetime. It has been shown that this simple method leads to improve the charge carrier collection and the electrical properties of c-Si based solar cells.

Enhancement of minority carrier lifetime of GaInP with lateral composition modulation structure grown by molecular beam epitaxy

We report the enhancement of the minority carrier lifetime of GaInP with a lateral composition modulated (LCM) structure grown using molecular beam epitaxy (MBE). The structural and optical properties of the grown samples are studied by transmission electron microscopy and photoluminescence, which reveal the formation of vertically aligned bright and dark slabs corresponding to Ga-rich and In-rich GaInP regions, respectively, with good crystal quality. With the decrease of V/III ratio during LCM GaInP growth, it is seen that the band gap of LCM GaInP is reduced, while the PL intensity remains high and is comparable to that of bulk GaInP. We also investigate the minority carrier lifetime of LCM structures made with different flux ratios. It is found that the minority carrier lifetime of LCM GaInP is ∼37 times larger than that of bulk GaInP material, due to the spatial separation of electrons and holes by In-rich and Ga-rich regions of the LCM GaInP, respectively. We further demonstrate that the minority carrier lifetime of the grown LCM GaInP structures can easily be tuned by simply adjusting the V/III flux ratio during MBE growth, providing a simple yet powerful technique to tailor the electrical and optical properties at will. The exceptionally high carrier lifetime and the reduced band gap of LCM GaInP make them a highly attractive candidate for forming the top cell of multi-junction solar cells and can enhance their efficiency, and also make them suitable for other optoelectronics devices, such as photodetectors, where longer carrier lifetime is beneficial.

Effect of substrate temperature on microstructure and optical properties of hydrogenated nanocrystalline Si thin films grown by plasma enhanced chemical vapor deposition

In this paper is presented the effect of substrate temperature (Ts) on the chemical composition, the morphology, the reflectivity and the carrier life time of hydrogenated nanocrystalline silicon (nc-Si:H) thin films, grown by radio frequency Plasma Enhanced Chemical Vapor Deposition (PECVD) on silicon single crystal (100) using a gas mixture of silane (SiH4) and hydrogen (H2). Substrate temperature was varied from room temperature to 450°C. Characterization of these films by X-ray diffraction (XRD) and Raman spectroscopy revealed that the crystallite size and at the same time the volume fraction of crystallites in the films tends to decrease with increasing Ts. The Fourier transform infrared spectroscopic analysis (FTIR) showed at low temperature the hydrogen is incorporated in the nc-Si:H films in the mono-hydrogen (Si–H) bonding configuration. With increasing Ts the hydrogen bonding in nc-Si:H films shifts from mono-hydrogen (Si–H) to di-hydrogen (Si–H2) and (Si–H2)n. The hydrogen content in the nc-Si:H films decreases with the increase of Ts. From the Atomic Force Microscopy (AFM), it was shown that the increase of Ts tends to increase the porosity and decrease the crystalline grain size. In order to more understand the effect of Ts on this structural change, minority carrier lifetime (MCL) measurement show that only the films with a nanocrystalline silicon structure present an enhancement in MCL which could be related to a quantum size effect and to the SiH-related bonds.

Study of different thermal processes on boron-doped PERL cells

Abstract In this paper, three kinds of thermal processes for boron-doped PERL cells were investigated. These are the forming gas annealing (FGA), the rapid thermal (RTP) and the low temperature annealing processes. FGA was introduced after laser ablation and doping in order to increase minority carrier lifetime by hydrogenating the trapping centers. Subsequent evaluation revealed considerable enhancement of minority carrier lifetime (from 150 μs to 240 μs) and the implied V oc (from 660 mV to 675 mV). After aluminum sputtering, three actual peak temperatures (370 °C, 600 °C and 810 °C) of RTP (as it occurs in the compressed air environment used in our experiment) were utilized to form a contact between the metal and the semi-conductor. It is concluded that only low temperature (lower than 600 °C) firing could create boron back surface field and high quality rear reflector. Lastly, a method of improving the performance of finished PERL cells which did not experience high temperature (over 800 °C) firing was investigated. Finished cells undergone low temperature annealing in N 2 atmosphere at 150 °C for 15 min produced 0.44% absolute increase in PERL cells. The enhancement of low temperature annealing originally comes from the activation of passivated boron which is deactivated during FGA.

SRH Recombination Strength of Interstitial Iron in Solar Grade Silicon

Shockley-Read-Hall (SRH) recombination is the main determinant for the effective minority carrier lifetime in silicon. This research has focused on the study to find the recombination strength of interstitial iron on low injection Shockley-Read-Hall recombination in compensated solar grade silicon by means of analytical modelling. This study found that strength of iron in SRH recombination is depends on the doping concentration and symmetry of minority carrier capture cross-section. This study shows that iron become more recombination active at higher doping and at lower symmetry of capture cross-section. Full Text: PDF References A.A. Istratov, T. Buonassisi, R.J. McDonald, A.R. Smith, R.Schindler, J.A. Rand, J.P. Kalejs, and E.R. Weber, "Metal content of multicrystalline silicon for solar cells and its impact on minority carrier diffusion length", J. Appl. Phys. 94, 6552 (2003). CrossRef T. Buonassisi, M.A. Marcus, A.A. Istratov, M. Heuer, T.F. Ciszek, B. Lai, C. Zhonghou, and E.R. Weber, "Analysis of copper-rich precipitates in silicon: Chemical state, gettering, and impact on multicrystalline silicon solar cell material", J. Appl. Phys. 97, 63503 (2005). CrossRef T. Buonassisi, A.A. Istratov, M. Heuer, M.A. Marcus, R. Jonczyk, J. Isenberg, B. Lai, C. Zhonghou, S. Heald, W. Warta, R. Schindler, G. Willeke, and E.R. Weber, "Synchrotron-based investigations of the nature and impact of iron contamination in multicrystalline silicon solar cells", J. Appl. Phys. 97, 74901 (2005). CrossRef T. Buonassisi et al., "Chemical natures and distributions of metal impurities in multicrystalline silicon materials", Prog. Photovolt: Res. Appl. 14, 513?531 (2006). CrossRef T. Buonassisi, A.A. Istratov, M.D. Pickett, J.P. Rakotoniaina, O. Breitenstein,M.A. Marcus, S.M. Heald, and E.R. Weber, "Transition metals in photovoltaic-grade ingot-cast multicrystalline silicon: Assessing the role of impurities in silicon nitride crucible lining material", Journal of Crystal Growth 287, 402?407 (2006). CrossRef A.A. Istratov, H. Hieslmair, and E. R. Weber, "Iron and its complexes in silicon", Appl. Phys A: Material Science and Processing 69, 13 ? 44 (1999). CrossRef D. Macdonald, PhD thesis, Australian National University (2001). K. Graff, "Metal impurities in silicon-device fabrication", Springer-Verlag, Berlin (1995). CrossRef R . Hall, "Electron-Hole Recombination in Germanium", Physics Review 87, 387 (1952). CrossRef W. Shockley and W. Read, "Statistics of the Recombinations of Holes and Electrons", Physical Rev.87, 835-842 (1952). CrossRef Y. Yoon, B. Paudyal, J. Kim, Y. Ok, and P. Kulshreshtha, "Effect of nickel contamination on high carrier lifetime n-type crystalline silicon", Journal of Applied Physics 111, 033702 (2012). CrossRef Libal J. et al, "Effect of compensation and of metallic impurities on the electrical properties of Cz-grown solar grade silicon", Journal of Applied Physics 104, 10450 (2008). CrossRef M. Ziaur Rahman and M. Jahangir Alam, "Enhancement of Minority Carrier Lifetime of Fe Contaminated Boron-Phosphorus Compensated p-Type SoG Silicon", Photonics Letters of Poland 5(2), 75 ? 77 (2013) DirectLink D. K. Schroder, American Society for Testing and Materials, ASTM STP 1340, 1998. J. Schmidt, C. Berge, A.G. ABerle, "Injection level dependence of the defect-related carrier lifetime in light-degraded boron-doped Czochralski silicon", Appl. Phys. Lett. 73 (15), 2167-9 (1998). CrossRef M. Ichimura et al., "Temperature Dependence of Carrier Recombination Lifetime in Si Wafers", J. Electrochem. Soc. 145 (9), 3265 ? 71 (1998). CrossRef

AlxGa1−xAs minority carrier lifetime enhancement at low temperatures

DX-centers in AlxGa1−xAs are known to reduce the minority carrier lifetime. Thus, DX-centers influence various semiconductor device characteristics. However, at temperatures below 100 K, we observed an unexpected improvement in the short circuit current density of an investigated Al0.37Ga0.63As solar cell. Using temperature-dependent capacitance measurements taken on a similar Al0.37Ga0.63As n−p+-diode, we correlated this behavior with the persistent photoconductivity effect. This effect derives from the suppressed silicon-DX-center generation inside the n-doped material at low temperatures.

Enhancement of Minority Carrier Lifetime of Fe Contaminated Boron-Phosphorus Compensated p-Type SoG Silicon

To understand the role of deliberate phosphorus doping on the minority carrier lifetime of iron contaminated boron-phosphorus-compensated p-type solar grade silicon, a numerical study has been performed. This study confirmed that compensation results in a significant increase in bulk lifetime of minority carrier. The gain in carrier lifetime is predicted due to the shift in Fermi energy level, carriers screening and reduction in net equilibrium hole concentration. The bulk lifetime of minority carrier reaches its maximum for phosphorus concentration around 10 15 cm -3 if the boron concentrations remain fixed at 10 17 cm -3 . Full Text: PDF References M.A. Green, Solar cells- operating principles, technology and system applications (Prentice-Hall 1986). F.E. Rougieux, D. Macdonald, A. Cuevas, "Transport properties of p-type compensated silicon at room temperature", Prog. Photovolt: Res. Appl. 19(7), 787 (2011). CrossRef F.E. Rougieux, M. Forster, D. Macdonald, A. Cuevas, B. Lim, J. Schmidt, "Recombination Activity and Impact of the Boron?Oxygen-Related Defect in Compensated N-Type Silicon", IEEE Journal of Photovoltaics 1, 54 (2011). CrossRef D. Leblanc, K. Putyera, "New resistivity/dopant density model for compensated-Si", Trans.Nonferrous Met. Soc. China 21, 1172 (2011). CrossRef S. Dubois, N. Enjalbert, J.P. Garandet, "Effects of the compensation level on the carrier lifetime of crystalline silicon", Appl. Phys. Lett. 93, 032114 (2008). CrossRef D. Macdonald, A. Cuevas, "Recombination in compensated crystalline silicon for solar cells", J. Appl. Phys. 109, 043704 (2011). CrossRef J. Veirman, S. Dubois, N. Enjalbert, J.P. Garandet, M. Lemiti, "Electronic properties of highly-doped and compensated solar-grade silicon wafers and solar cells", J. Appl. Phys. 109, 103711 (2011). CrossRef D. Macdonald, PhD Thesis (Australian National University 2001). S. Rein, Lifetime Spectroscopy (Springer 2005). A. A. Istratov, H. Hieslmair, E. R. Weber, "Iron and its complexes in silicon", Appl. Phys. A: Mater. Sci. Process. 69, 13 (1999). CrossRef W. Shockley, W. T. J Read, "Statistics of the Recombinations of Holes and Electrons", Phys. Rev. 87 (5), 835 (1952). CrossRef R.N. Hall, "Electron-Hole Recombination in Germanium", Phys. Rev. 87, 387 (1952). CrossRef A. Hangleiter, R. Hacker, "Enhancement of band-to-band Auger recombination by electron-hole correlations", Phys. Rev. Lett. 65 (2), 215 (1990). CrossRef

Light induced enhancement of minority carrier lifetime of chemically passivated crystalline silicon

In this work we present light effects on chemically passivated silicon surface. Di-iodine–ethanol (I–E) mixtures were used to passivate silicon dangling bonds. The passivation quality is sensitive to both silicon surface state and light irradiation. The minority carrier lifetime values vary from 2μs for unpassivated surfaces to about 40μs for chemically passivated ones. FTIR investigations show that light irradiation catalyses the passivation effect by forming the silicon-ethoxylate group (SiOC2H5). We suggest a mechanism to explain the passivation effect based on carrier-induced dissociation of I2.

Minority carrier lifetime enhancement in multicrystalline silicon by means of a dual treatment based on porous silicon and sputter-deposition of TiO2:Cr passivation layers

We report on a dual passivation approach of multicrystalline-silicon (mc-Si), which combines porous silicon (PS) treatment and sputter-deposition of TiO2:Cr passivation layer. At the optimal Cr content of 2at.%, the effective minority carrier lifetime was found to be enhanced significantly by more than 2 orders of magnitude (from 2 to ∼733μs). Our results demonstrate that this dual treatment not only provides strong passivation of the mc-Si substrate but decreases also the total surface reflectivity at 500nm (from 40% for untreated mc-Si samples to ∼19% for TiO2:Cr/PS treated ones).

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.

A novel approach for co-firing optimization in RTP for the fabrication of large area mc-Si solar cell

High-efficiency solar cells require printed metal contacts with adapted emitters to ensure high fill factors. Multi-crystalline silicon (mc-Si) substrates require adapting the temperature ramps of rapid thermal processing (RTP) to avoid minority carrier lifetime degradation from activated defect centers. Sufficient temperature difference is required between front and back surfaces during co-firing to prevent junction leakage on the front and proper aluminum back surface field (Al-BSF) on the back surface. Actual temperature profile for co-firing in RTP after screen printing is a key issue for high-quality metal–semiconductor contact for high efficiency solar cell. A novel approach for optimization of co-firing temperature profile in RTP is investigated by measuring the actual temperature of front and back surfaces of the solar cell. Rapid cooling in the spike firing profile is made possible by controlling the gas flow rate in the RTP chamber during co-firing. For the best front and back metal contact and enhancement of minority carrier lifetime, the actual temperature of the silver (Ag)-printed surface is 605 °C and that of Al-printed surface is 743 °C. Mc-Si solar cell with fill factor (FF) 0.79 and conversion efficiency 15.02% is fabricated using the optimized co-firing temperature of RTP in a conventional solar cell production line.

Enhancement of minority-carrier lifetime by an advanced high temperature annealing method

An advanced annealing method was proposed to enhance the lifetime of minority carriers by obtaining larger defect-free zone and keeping an amount of oxygen precipitates. We investigated the influence of annealing process on the defect-free zone and oxygen precipitates. In our experiments, the thickness of defect-free zone reached up to 100 μm. And an amount of oxygen precipitates, which play a great role on impurity gathering, were kept at the same time. It was found that the lifetime of minority-carrier was proportional to the thickness of defect-free zone. The lifetime of minority carriers was enhanced by reducing the grown-in defects in the defect-free zone.

Minority-carrier lifetime enhancement in edge-defined film-fed grown Si through rapid thermal processing-assisted reduction of hydrogen-defect dissociation

This paper demonstrates that a very short, 1-s, simultaneous firing of screen-printed Al at the back and SiNx antireflection (AR) coating at the front can significantly enhance the minority-carrier lifetime in edge-defined film-fed grown (EFG) ribbon Si via SiNx-induced hydrogen passivation of defects. It was found that 1-s firing in a rapid thermal processing system at an optimum temperature improved the average minority-carrier lifetime from 3to>80μs, resulting in ∼16% efficient 4-cm2 screen-printed EFG Si cells. It is proposed that rapid thermal firing enhances the retention of hydrogen at defect sites by minimizing the hydrogen-defect dissociation. A combination of simulations and experiments reveals that the dissociation of hydrogen is extremely rapid at conventional firing temperatures of ∼700°C. An activation energy of 2.4–2.6eV was determined for the hydrogen-defect dissociation in EFG Si. This activation energy, in conjunction with the room-temperature photoluminescence data, suggests that the impurity-decorated dislocations are the dominant hydrogenation and dehydrogenation sites in the EFG Si. Based on the above understanding, a manufacturable process, involving rapid co-firing of SiNx AR coating, screen-printed Al-doped back surface field (Al-BSF), and screen-printed Ag front grid, was developed in a conventional belt furnace to minimize the degree of dehydrogenation while producing good Al-BSF and ohmic contacts. This process produced 4-cm2 screen-printed EFG Si cells with an efficiency of 15.9%.

Properties of GaAs/InGaAs quantum well solar cells under low concentration operation

Multi-quantum well GaAs/In 0.19Ga 0.81As solar cells have been measured under low concentration levels (1–10 suns) of AM1.5 illumination. An efficiency of 22% has been obtained at a ratio of 4 suns as opposed to 18% under 1 sun AM1.5 conditions. We explain the improvements in conversion efficiency in terms of an enhancement in minority-carrier lifetime under concentration. Even when the concentration ratio is low, the high-injection regime can be achieved since the carrier concentration in the intrinsic layer is very low. The existence of a high concentration of defects at 0.36 eV below the conduction band in the base layer has been observed by the DLTS analysis. Enhancement of the minority-carrier lifetime under concentration is thought to be due to filling of recombination centers by the injection minority carriers.

Backside aluminisation effects on solar cell performance

In this work the effect of the back surface field (BSF) formation by high temperature treatment of backside aluminisation was analysed in silicon solar cells. Systematic I−V and spectral response (SR) characterisation shows an obviously superior performance of Al-diffused cells especially in the red near infrared region compared to those with conventional boron BSF. These findings along with the result of the performed XTEM, NRA (Resonance Nuclear Reaction Analysis), SIMS, and ECV (Electrochemical CV) analyses on the backside layer structure lead to the conclusion that the main contribution to the improvement, relative to the conventional reference sample, is the minority carrier lifetime enhancement by gettering effect in the Al-diffusion tail and the improved light trapping by backside roughening.

Enhancement of minority carrier lifetime in GaAs1−x P x (x=0.4; 0.65) by nitrogen implantation

Minority carrier lifetimes in nitrogen implanted GaAs1-x P x (x=0.4; 0.65) were measured at 77K by an optical phase shift method as a function of nitrogen dose and annealing temperature in order to investigate the dependence of the lifetime on the concentration of nitrogen isoelectronic traps. A large increase in the lifetime was observed after nitrogen implantation followed by annealing at and above 800°C. The maximum lifetimes were 22ns for GaAs0.35P0.65 and 6.7 ns for GaAs0.6P0.4. They were obtained by implantation to a dose of 5×1013 cm−2 in GaAs0.35P0.65 and 1013 cm−2 in GaAs0.6P0.4. The lifetime after nitrogen implantation followed by annealing was longer by a factor of 6–7 than that of the unimplanted sample.