Carrier Lifetime Degradation Research Articles

Lifetime-degrading boron-oxygen centres in p-type and n-type compensated silicon

Degradation of minority carrier lifetime under illumination occurs in boron-containing Czochralski silicon of both p- and n-type. In n-Si, the recombination centre responsible for degradation is found to be identical to the fast-stage centre (FRC) known for p-Si, where it is produced at a rate proportional to the squared hole concentration, p2. Holes in n-Si are the excess minority carriers—of a relatively low concentration; hence, the time scale of FRC generation is increased by several orders of magnitude when compared to p-Si. The degradation kinetics, which is non-linear, due to dependence of p on the current concentration of FRC, is well reproduced by simulations. The injection level dependence of the lifetime shows that FRC exists in 3 charge states (− 1, 0, + 1) possessing 2 energy levels. Comparison of n-Si samples of various electron concentrations shows that FRC emerges by the reconstruction of a latent BsO2 complex of a substitutional boron and an oxygen dimer (while the major recombination centre in p-Si denoted SRC was previously found to emerge by reconstruction of BiO2 defect involving an interstitial boron atom). A model of the BsO2 reconfiguration into FRC through an intermediate state accounts for the rate constant dependence on p, which is reduced to a p2 proportionality, under certain conditions.

The Nature of Lifetime-Degrading Boron-Oxygen Centres Revealed by Comparison of P-Type and N-Type Silicon

Illumination-induced degradation of minority carrier lifetime was studied in n-type Czochralski silicon co-doped with phosphorus and boron. The recombination centre that emerges is found to be identical to the fast-stage centre (FRC) known for p-Si where it is produced at a rate proportional to the squared hole concentration, p2. Since holes in n-Si are excess carriers of a relatively low concentration, the time scale of FRC generation in n-Si is increased by several orders of magnitude. The generation kinetics is non-linear, due to the dependence of p on the concentration of FRC and this non-linearity is well reproduced by simulations. The injection level dependence of the lifetime shows that FRC exists in 3 charge states (-1, 0, +1) possessing 2 energy levels. The recombination is controlled by both levels. The proper identification of FRC is a BsO2 complex of a substitutional boron and an oxygen dimer. The nature of the major lifetime-degrading centre in n-Si is thus different from that in p-Si - where the dominant one (a slow-stage centre, SRC) was found to be BiO2 – a complex involving an interstitial boron.

Germanium-doped Czochralski silicon for photovoltaic applications

Germanium (Ge)-doped Czochralski (GCZ) silicon has been grown for photovoltaic (PV) applications. It is found that Ge doping improves the mechanical strength of CZ silicon, resulting in the reduction of breakage during wafer cutting, cell fabrication and module assembly. Boron–oxygen (B–O) defects that lead to the light-induced degradation (LID) of carrier lifetime are effectively suppressed by Ge doping. The decrease in the maximum concentration of B–O defects increases with an increase of Ge concentration. The efficiency of GCZ silicon solar cells and the power output of corresponding PV modules both exhibit smaller loss under sunlight illumination. The current work suggests that GCZ silicon should be potentially a novel substrate for thin solar cells with low LID effect.

Ga co‐doping in Cz‐grown silicon ingots to overcome limitations of B and P compensated silicon feedstock for PV applications

AbstractIn this paper, we investigate gallium co‐doping during CZ crystallization of boron and phosphorus compensated Si. It is shown that the addition of gallium yields a fully p‐type ingot with high resistivity despite high B and P contents in the silicon. Segregation of doping impurities is consistent with theory. Minority carrier lifetime and majority carrier mobility measurements indicate that this material is suitable for the realization of solar cells with comparable efficiencies to standard material. Significant light‐induced degradation of minority carrier lifetime is however revealed to occur in this material. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Application of CL/EBIC-SEM Techniques for Characterization of Radiation Effects in Multijunction Solar Cells

We report the results of the characterization of irradiated InGaP2/GaAs/Ge multijunction (MJ) solar cells using the cathodoluminescence (CL) imaging/spectroscopy and electron beam induced current (EBIC) modes of scanning electron microscopy (SEM). These techniques were applied to verify the influence of irradiation damage on the optoelectronic properties of each subcell in the monolithic MJ structure and correlate them with the illuminated (AM0, 1 sun, 25°C) current-voltage (I-V) and quantum efficiency (QE) measurements. Minority carrier lifetime degradation data from CL measurements confirm that the GaAs subcell dominates the overall degradation of the 3J device. Also, a carrier removal mechanism in the GaAs subcell was revealed from the EBIC/CL measurements.

Impact of neutron irradiation on the RF properties of oxidized high-resistivity silicon substrates with and without a trap-rich passivation layer

This work investigates the influence of high-energy neutrons on oxidized high-resistivity silicon substrates (HR-Si). Two oxide thicknesses as well as the presence of a trap-rich passivation layer are considered. The impact of neutron irradiation is directly related to the competition between the generation of interface traps added to the mobility and carrier lifetime degradation, which are beneficial to reduce parasitic surface conduction (PCS) into the Si substrate similarly to the passivation layer, and accumulation of radiation-induced positive charges in oxide, which would unfortunately increase PSC. It is shown that under neutron irradiation, RF losses are strongly reduced in the case of thin oxide ( t ox = 50 nm), while substrates with a polysilicon passivation layer are almost insensitive to the neutron irradiation.

Charge carrier lifetime degradation in Cz silicon through the formation of a boron-rich layer during BBr3 diffusion processes

Boron diffusion is commonly associated with the formation of an undesirable boron-rich layer (BRL), which is often made responsible for degradation of the carrier lifetime in the bulk. We investigate the phenomenology of the BRL formation, which results from BBr3 boron diffusion processes, and its impact on sheet resistance and bulk lifetime. Our measurements show that boron silicate glass (BSG) and BRL thicknesses vary between 50 and 600 nm and 0 and 80 nm respectively within the two-dimensional wafer surface of one sample for one diffusion process. Both thicknesses strongly depend on the gas composition during composition and deposition time. Further results show that BRL formation is favored by high concentrations of BBr3 vapor and of oxygen during B2O3 deposition. Also, high drive-in temperatures promote the growth of the BRL. We find that a BRL of more than 10 nm thickness causes a degradation of the carrier lifetime in the bulk of the silicon wafer. In particular, we show that this bulk lifetime degradation occurs during the cool-down ramp after the diffusion process. We show that carrier lifetime degradation can be avoided either by limiting the process temperature to 850 °C and thus preventing BRL formation or through reconverting the BRL by a drive-in step in oxidizing atmosphere at 920 °C.

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.

Triple-axis X-ray reciprocal space mapping of In yGa 1−yAs thermophotovoltaic diodes grown on (1 0 0) InP substrates

Analysis of the composition, strain-relaxation, layer-tilt, and the crystalline quality of In y Ga 1− y As/InP 1− x As x thermophotovoltaic (TPV) diodes grown by metal-organic vapor phase epitaxy (MOVPE) is demonstrated using triple-axis X-ray reciprocal space mapping techniques. In 0.53Ga 0.47As ( E gap=0.74 eV) n/p junction diodes are grown lattice matched (LM) to InP substrates and lattice-mismatched (LMM) In 0.67Ga 0.33As ( E gap=0.6 eV) TPV diodes are grown on three-step InP 1− x As x (0< x<0.32) buffer layers on InP substrates. X-ray reciprocal space maps about the symmetric (4 0 0) and asymmetric (5 3 3) reciprocal lattice points (RELPs) determine the in-plane and out-of-plane lattice parameters and strain of the In y Ga 1− y As TPV active layer and underlying InP 1− x As x buffers. Triple-axis X-ray rocking curves about the LMM In 0.67Ga 0.33As RELP show an order of magnitude increase of its full-width at half-maximum (FWHM) compared to that from the LM In 0.53Ga 0.47As (250 vs. 30 arcsec). Despite the significant RELP broadening, the photovoltaic figure of merits show that the electronic quality of the LMM In 0.67Ga 0.33As approaches that of the LM diode material. This indicates that misfit-related crystalline imperfections are not dominating the photovoltaic response of the optimized LMM In 0.67Ga 0.33As material compared with the intrinsic recombination processes and/or recombination through native point defects, which would be present in both LMM and LM diode material. However, additional RELP broadening in non-optimized LMM In 0.67Ga 0.33As n/p junction diodes does correspond to significant degradation of TPV diode open-circuit voltage and minority carrier lifetime demonstrating that there is correlation between X-ray FWHM and the electronic performance of the LMM TPV diodes.

The Temperature Evolution of the Hydrogen Plasma Induced Structural Defects in Crystalline Silicon

Hydrogenated n and p doped Czochralski Si substrates have been studied by means of atomic force microscopy, scanning and transmission electron microscopy, Raman spectroscopy and microwave photoconductivity decay techniques. The measurements show that the surface is roughest in ndoped samples which are plasma treated at high frequency. The cone density was found to be highest on p-doped samples, which correlates well to the higher density of defects observed in pdoped samples. The surface cones were found to consist of nanograins, twins and stacking faults with random orientations, several hydrogen induced defects and bubbles. The size, density and formation depth of the subsurface defects were seen to depend on doping type, doping level, plasma frequency and hydrogenation time. Raman spectroscopy shows formation of nearly free hydrogen molecules, which are presumed to be located in nano-voids or platelets. These molecules dissolved at temperatures around 600°C. By means of the &amp;-PCD measurements, it is demonstrated that hydrogen-initiated structural defects act as active recombination centres, which are responsible for the degradation of the minority carrier lifetime.

Investigations on the long time behavior of the metastable boron–oxygen complex in crystalline silicon

AbstractBoron and oxygen contamination in Czochralski‐grown (Cz) silicon leads to a degradation of the minority charge carrier lifetime within short times due to the formation of recombination active complexes. The formation of these complexes is investigated for longer times showing a further development of the defect. This development called ‘regeneration’ is triggered by illumination or applied forward voltages and leads to a new state of the defect. This new state of the defect is proven to be less recombination active allowing higher stable minority carrier lifetimes and conversion efficiencies of solar cells. The influences of temperature and light intensity are discussed. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Axial Distribution of Gallium in Silicon Crystals for Photovoltaic Applications

The lifetime degradation induced by light illumination or carrier injection observed in Czochralski-grown silicon leads to a significant decrease in solar cell efficiency. The reduction in the extent of this effect has a high potential for the improvement of Czochralski-grown silicon solar cells. An approach to the substitution of boron with gallium in p-type Czochralski-grown silicon crystal has been pursued in a recent study. Since a reduced extent of light-induced degradation of minority carrier lifetime was observed, this approach seems to be very promising. The only disadvantage is the low segregation coefficient of gallium in silicon, resulting in a high resistivity variation over the crystal length compared with that in the case of boron doping. We propose a simple codoping (e.g., gallium and bismuth) method for controlling resistivity variation. Numerical simulations have been performed to study the transport phenomena of dopants in conventional and proposed silicon growth processes using the finite-element method and implicit Euler time integration. It has been demonstrated using mathematical models and numerical analysis that the axial distribution of effective gallium concentration can be modified in Czochralski-grown silicon for photovoltaic applications and made relatively uniform by the proposed doping method.

Investigation of reactive ion etching of dielectrics and Si in CHF3∕O2 or CHF3∕Ar for photovoltaic applications

Using a combination of etch rate, photoconductance, and deep level transient spectroscopy (DLTS) measurements, the authors have investigated the use of reactive ion etching (RIE) of dielectrics and Si in CHF3∕O2 and CHF3∕Ar plasmas for photovoltaic applications. The radio frequency power (rf-power) and gas flow rate dependencies have shown that the addition of either O2 or Ar to CHF3 can be used effectively to change the etch selectivity between SiO2 and Si3N4. The effective carrier lifetime of samples degraded upon exposure to a CHF3-based plasma, reflecting the introduction of recombination centers in the near-surface region. The extent of minority carrier lifetime degradation was similar in both types of plasmas, suggesting that the same defects were responsible for the increased recombination. However, the rf-power dependence of lifetime degradation in n- and p-type Si was different. Moreover, the lifetime degradation did not exhibit a linear rf-power dependence, suggesting that primary defects were not the dominant recombination centers responsible for the decrease in lifetime. Indeed, DLTS measurements have shown that secondary defects were formed in samples exposed to the plasma after annealing at 400°C, the temperature at which a SiN:H layer is deposited on samples to passivate their surfaces. The minority carrier lifetime degradation in RIE processed samples could be partially avoided using post-RIE chemical treatments.

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.

Electronically stimulated degradation of silicon solar cells

Carrier lifetime degradation in crystalline silicon solar cells under illumination with white light is a frequently observed phenomenon. Two main causes of such degradation effects have been identified in the past, both of them being electronically driven and both related to the most common acceptor element, boron, in silicon: (i) the dissociation of iron-boron pairs and (ii) the formation of recombination-active boron-oxygen complexes. While the first mechanism is particularly relevant in metal-contaminated solar-grade multicrystalline silicon materials, the latter process is important in monocrystalline Czochralski-grown silicon, rich in oxygen. This paper starts with a short review of the characteristic features of the two processes. We then briefly address the effect of iron-boron dissociation on solar cell parameters. Regarding the boron-oxygen-related degradation, the current status of the physical understanding of the defect formation process and the defect structure are presented. Finally, we discuss different strategies for effectively avoiding the degradation.

Fast-forming boron-oxygen-related recombination center in crystalline silicon

The mechanism of a fast carrier lifetime degradation effect proceeding within seconds in boron-doped Czochralski silicon is investigated. The decrease in the carrier lifetime is attributed to the formation of a deep boron-oxygen-related recombination center with a strongly asymmetric electron-to-hole capture cross section ratio of 100. The center is activated in a two-step process. In a first step, the electron quasi-Fermi level has to be shifted above an energy level at EV+0.635eV before, in the second step, the actual activation occurs via a thermally activated process with an energy barrier of 0.23eV. A defect model is proposed to explain the observed activation behavior.

Criteria for identifying radiation resistant semiconductor materials

An index that would reliably rank the relative radiation hardness of semiconductors would be very helpful for selecting materials for many applications. The concept of "concentration of primary defects" (CPD) has been proposed as such an index. Here we show that for the GaAs-InP family of semiconductors used in solar cells, CPD yields results that do not agree with experiment. The long-established concept of damage constants for various semiconductor properties is a more reliable index. For solar cells the relevant damage constant is that for the degradation of minority carrier lifetime. Although the damage constants reported for different semiconductors are specific to a particular type and energy of radiation, we show they can be extended beyond the type and particle energy for which they were originally determined by using nonionizing energy loss (NIEL), thereby greatly extending their possible application.

Characterization of the Initial Rapid Decay on Light-Induced Carrier Lifetime and Cell Performance Degradation of Czochralski-Grown Silicon

The rapid initial degradation of Czochralski-grown silicon (Cz-Si) solar cell performance has been investigated. The initial rapid degradation occurs at the beginning of the degradation process and is followed by a second slower degradation. The surface effect due to interface state degradation between the passivation layer and the Si substrate was investigated on several kinds of Cz- and float zone Si (FZ-Si) wafers. Carrier lifetime degradation induced by an illumination for 30 s was observed on every Cz-Si wafer, but no degradation was observed on FZ-Si wafers. Moreover, only the decrease of IR response of a cell was observed as a result of the initial degradation. The results suggest that the degradation occurs in the bulk. The magnitude and decay time of the degradation appear to depend on the wafer resistivity. Defects very similar to boron–oxygen complexes specific to Cz-Si were considered to be responsible. The distinctly small activation energy of 0.19 eV was obtained for the initial degradation, which suggests a fundamentally different defect reaction process. The activation energy of 1.6 eV for the defect annihilation process was obtained for both defects as well.

Development of RTP for industrial solar cell processing

Rapid Thermal Processing (RTP), originally developed for processing microelectronic devices has been investigated in the recent decade for its potential in the production of Si solar cells. This paper will discuss the use of RTP for industrial Si solar cells with screen-printed contacts. Printed metal contacts require adapted emitters when good fill factors should be achieved. Multi-crystalline Si substrates require to adapt the temperature ramps of RTP to avoid minority carrier lifetime degradation from activated defect centres. Finally, industrial processing requires high throughput that cannot be achieved with conventional RTP equipment. This paper will present an advanced selective emitter process and a recently developed continuous RTP system that meet for the first time the requirements to make RTP compatible with industrial solar cell processing. The limits of industrial RTP solar cell processing will be discussed.

Channel-width dependence of floating body effects in STI- and LOCOS-isolated MOSFETS

The narrow-channel effects (NCE) and their impact on the floating body effects (FBEs) are investigated by various static and dynamic measurements in both LOCOS and STI isolated devices. It is found that FBEs are reduced in narrow channel devices for both isolation technologies. Two possible mechanisms are found to be consistent with our results: a carrier lifetime degradation on the sidewalls or an increase of the source/body junction leakage on the edges. Both mechanisms are discussed on the basis of our experimental results and on previous studies in bulk or SOI devices.