Excess Carrier Lifetime Research Articles

Long-wavelength sensitivity limit in MBE-grown PbSnTe:In films: Correlation with the film structure and composition

PbSnTe:In films with a long-wavelength sensitivity limit of over 20 μm and low “dark” conductivity are studied. The spectral dependences of the photoconductivity obtained at different temperatures using a Fourier spectrometer are compared with data on the film composition determined by X-ray microanalysis. The established nonmonotonic temperature dependence of the long-wavelength sensitivity limit is attributed to the combination of the temperature dependence of the PbSnTe band gap and the Burstein–Moss effect making the largest contribution to low-temperature measurements due to the long lifetime of excess charge carriers. It is shown that the difference between the band-gap values determined from the measured composition and temperature dependences of the long-wavelength sensitivity limit can be related to the inhomogeneity of the composition of the films grown by molecular beam epitaxy.

Epitaxial stresses in an InGaAs photoconductive layer for terahertz antennas

The effect of epitaxial stresses on the excess-carrier dynamics and the terahertz radiation spectrum of the InyGa1–yAs films have been investigated by optical pump-probe and terahertz time-domain spectroscopy. It has been demonstrated that a InyGa1–yAs film with a higher mechanical stress has the shorter excesscarrier lifetime and broader terahertz radiation spectrum.

Advanced Local Characterization of Silicon Solar Cells

Solar cells made from multicrystalline silicon material, which still represent the majority of solar cells produced today, are by nature inhomogeneous devices. Their bulk excess carrier lifetime, which decisively influences the short circuit current density, the saturation current density, and the effective bulk diffusion length, may vary from position to position by an order of magnitude or more. Moreover, such cells may contain ohmic shunts and, in particular in the edge regions, positions of significant depletion region recombination current. The classical local characterization method for solar cells is light beam‐induced current (LBIC) mapping. This method does not give direct information on factors influencing the dark current density. A reliable quantitative local characterization of inhomogeneous cells can be performed by combining dark lock‐in thermography (DLIT) with electro‐ and photoluminescence (EL and PL) imaging. In this contribution, the well‐established and the most recent findings on the application of these methods are reviewed and typical application examples are presented. It is shown that luminescence imaging is most appropriate for local series resistance imaging and for qualitatively imaging of all kinds of recombination‐active defects. DLIT, on the other hand, shows a lower spatial resolution, but is most appropriate for imaging local two‐diode parameters like depletion region‐ and bulk saturation current densities, the latter allowing to draw conclusions also to the local short circuit current density. A comprehensive local characterization of silicon solar cells is possible by applying both DLIT and luminescence imaging, which complement each other.

Study of the bulk lifetime and material saturation current density of different p-type monocrystalline silicon materials

With efficiencies of industrial type PERC solar cells exceeding 22% and reduced electrical and optical losses in these high efficiency cells, it becomes more and more important for manufacturers to rely on high quality silicon wafers. In this contribution, we present data of the bulk excess carrier lifetime τbulk and a doping density independent material quality parameter at maximum power point, the material saturation current density j0,mat, of different p-type monocrystalline silicon materials. These values are obtained by measuring the effective carrier lifetime of neighbouring wafers of different thickness and thus eliminating the surface recombination contribution. We show that j0,mat is an adequate parameter to evaluate the bulk quality of a given material. To further validate these findings, we compare the results to PERC cell efficiencies obtained with the same material.

The germanium laser might be harder to implement than previously thought

Researchers address the impact of heavy doping on the excess carrier lifetime in Ge/Si layers.

The impact of donors on recombination mechanisms in heavily doped Ge/Si layers

Heavy n-type doping has been proposed as a route to achieve positive optical gain in germanium layers since it is supposed to enhance the Γc carrier density. Nevertheless, the impact of donor impurities on the excess carrier lifetime in heavy doped epitaxial Ge/Si layers has not yet been addressed in the literature. To elucidate this point, we investigate the optical properties of heavily doped Ge layers selectively grown on silicon by means of photoluminescence (PL) experiments and theoretical modelling. A self-consistent multi-valley effective mass numerical model for simulation of PL spectra has been implemented, taking into account the influence of dopants on the non-radiative recombination dynamics. Upon comparing measurements and modelling, we find a linear increase in the defect-related Shockley-Read-Hall (SRH) recombination rate as a function of the donor density. The non-radiative lifetime decreases from ∼30 ns in intrinsic Ge/Si samples to ∼0.1 ns for a doping density in the 1019 cm−3 range. As a consequence, we find that SRH is the dominant non-radiative recombination process up to a donor density of ∼5 × 1019 cm−3. Despite this reduced lifetime, we observe an overall positive impact of doping on the radiative recombination rate for donor densities up to an “optimal” value of ∼3 × 1019 cm−3, with a ×7 intensity enhancement compared to the intrinsic case. A further increase in the donor concentration brings about a worsening of the optical emission.

Measuring the carrier lifetime by using a quasi-optical millimeter- and THz-wave system

The existing method for contactless measurement of the photoconductivity decay time is limited in terms of sample selection according to the injection level or doping density. To solve this problem and improve the measurement sensitivity, we developed a quasi-optical photoconductivity decay (QO-PCD) technique based on millimeter- and terahertz-wave technology. A semi-insulating silicon (Si) wafer was used in a proof-of-concept experiment with the proposed QO-PCD system to find the initial excess carrier density and carrier lifetime based on the Drude–Zener model with a single decay function. The initial excess carrier density and carrier lifetime were measured to be 1.5 × 1015 cm−3 and 30.6 μs, respectively, in semi-insulating Si wafer (460 μm thickness). A 2D areal measurement of the decay time of the Si wafer was experimentally obtained. The proposed QO-PCD technique can provide more reliable and sensitive carrier lifetime measurement data for semiconductor wafers, which may impact the fields of photovoltaic solar cells and power electronics.

Reporting Effective Lifetimes at Solar Cell Relevant Injection Densities

Precise quantitative assessment of c-Si wafer quality is of crucial importance for the development and manufacturing of high efficiency solar cells. For this purpose, lifetime samples are typically fabricated with very well cleaned and passivated surfaces. Under those conditions the measured effective lifetime τeff is almost equal to the silicon bulk wafer lifetime τwafer, i.e. a material related quality parameter. Those lifetime measurements are typically carried out with a photo-conductance decay method (PCM) e.g. with a Sinton-WCT tool. The measurement result is an effective excess carrier lifetime τeff which typically exhibits a strong dependence on the excess carrier injection density Δn within the wafer. Stating τeff –values thus necessitates to specifiy Δn. The PV community typically reports at a fixed Δn in the range of 1×1014 cm-3 to 1×1016 cm-3 or for varying wafer doping density Ndop at Δn = Ndop/10. The latter allows for a comparison from the point of view of the Shockley-Read-Hall (SRH) formalism. Unfortunately, the impact of a certain lifetime for device performance changes with Ndop, due to the law of mass action. In this paper a wafer doping density dependent Δn which is relevant for the injection density at maximum power point (MPP) is derived. This Δn@MPP shows a contrary behaviour compared to the often used and accepted reporting method to set Δn = Ndop/10. Additionally, a wafer doping density independent material quality parameter, called material saturation current density j0,mat at MPP, is proposed to improve the comparability of measured effective lifetimes of differently doped wafers.

Excess Carrier Lifetime Improvement in c-Si Solar Cells by YAG:Ce3+-Yb3+**Supported by the Natural Science Foundation of Jiangsu Province under Grant No BK2011033.

Ce3+-Yb3+ doped Y3A5O12 (YAG) is a luminescent down-conversion material which could convert visible photons to near infrared photons. In this work, YAG:Ce3+ -Yb3+ is applied on the front surface of mass-produced mono crystalline Si solar cells. For the coated cells, the external quantum efficiency from the visible to the near infrared is improved, and the energy conversion efficiency enhances from 11.70% to 12.2% under AM1.5G. Furthermore, the phosphor down-conversion effect on the solar cell is characterized by the microwave detected photoconductivity technique on the n-type silicon wafer under the 977nm excitation. The down-conversion materials improve the average excess carrier lifetime from 22.5 μs to 24.2 μs and the average surface recombination velocity reduces from 424.5 cm/s to 371.6 cm/s, which reveal the significant reduction in excess carrier recombination by the phosphors.

An Excess Carrier Lifetime Extraction Method for Physics-based IGBT Models

An excess carrier lifetime extraction method is derived for physics-based insulated gate bipolar transistor (IGBT) models with consideration of the latest development in IGBT modeling. On the basis of the 2D mixed-mode Sentaurus simulation, the clamp turn-off test is simulated to obtain the tail current. The proposed excess carrier lifetime extraction method is then performed using the simulated data. The comparison between the extracted results and actual lifetime directly obtained from the numerical device model precisely demonstrates the accuracy of the proposed method.

Demonstration of c-Si Solar Cells With Gallium Oxide Surface Passivation and Laser-Doped Gallium p+Regions

Gallium oxide (Ga2O3) deposited by plasma-enhanced atomic layer deposition (PEALD) is shown to passivate crystalline silicon surfaces via a combination of a high negative charge and a reduction in the density of surface defects to below 1 × 1011 cm−2 eV−1 at midgap. The passivation, as determined by the injection-dependent excess carrier lifetime, is demonstrated to be commensurate to that of PEALD aluminum oxide. In addition, Ga2O3 is used as a gallium source in a laser-doping process, resulting in a device efficiency of 19.2% and an open-circuit voltage of 658 mV in a partial rear contact p-type cell design.

Device Architecture and Lifetime Requirements for High Efficiency Multicrystalline Silicon Solar Cells

We present a numerical simulation study of different multicrystalline silicon materials and solar cell architectures to understand today's efficiency limitations and future efficiency possibilities. We compare conventional full-area BSF and PERC solar cells to future cell designs with a gallium phosphide heteroemitter. For all designs, mc-Si materials with different excess carrier lifetime distributions are used as simulation input parameters to capture a broad range of materials. The results show that conventional solar cell designs are sufficient for generalized mean lifetimes between 40 – 90μs, but do not give a clear advantage in terms of efficiency for higher mean lifetime mc-Si material because they are often limited by recombination in the phosphorus diffused emitter region. Heteroemitter designs instead increase in cell efficiency considerable up to generalized mean lifetimes of 380μs because they are significantly less limited by recombination in the emitter and the bulk lifetime becomes more important. In conclusion, to benefit from increasing mc-Si lifetime, new cell designs, especially heteroemitter, are desirable.

Plasma enhanced atomic layer deposition of gallium oxide on crystalline silicon: demonstration of surface passivation and negative interfacial charge

Herein we report on the passivation of crystalline silicon by gallium oxide (Ga2O3) using oxygen plasma as the oxidizing reactant in an atomic layer deposition (ALD) process. Excess carrier lifetimes of 2.1 ms have been measured on 1.75 Ω cm p‐type silicon, from which a surface recombination current density J0 of 7 fA cm–2 is extracted. From high frequency capacitance‐voltage (HF CV) measurements it is shown that, as in the case of Al2O3, the presence of a high negative charge density Qtot/q of up to –6.2 × 1012 cm–2 is one factor contributing to the passivation of silicon by Ga2O3. Defect densities at midgap on the order of ∼5 × 1011 eV–1 cm–2 are extracted from the HF CV data on samples annealed at 300 °C for 30 minutes in a H2/Ar ambient, representing an order of magnitude reduction in the defect density compared to pre‐anneal data. Passivation of a boron‐diffused p+ surface (96 Ω/□) is also demonstrated, resulting in a J0 of 52 fA cm–2. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Carrier Lifetimes and Influence of In-Grown Defects in N-B Co-Doped 6H-SiC

The thick N-B co-doped epilayers were grown by the fast sublimation growth method and the depth-resolved carrier lifetimes have been investigated by means of the free-carrier absorption (FCA) decay under perpendicular probe-pump measurement geometry. In some samples, we optically reveal in-grown carbon inclusions and polycrystalline defects of substantial concentration and show that these defects slow down excess carrier lifetime and prevent donor-acceptor pair photo luminescence (DAP PL). A pronounced electron lifetime reduction when injection level approaches the doping level was observed. It is caused by diffusion driven non-radiative recombination. However, the influence of surface recombination is small and insignificant at 300 K.

A Novel Atomic Layer Deposited Al2O3Film with Diluted NH4OH for High-Efficient c-Si Solar Cell

In this paper, Al2O3 film deposited by thermal atomic layer deposition (ALD) with diluted NH4OH instead of H2O was suggested for passivation layer and anti-reflection (AR) coating of the p-type crystalline Si (c-Si) solar cell application. It was confirmed that the deposition rate and refractive index of Al2O3 film was proportional to the NH4OH concentration. Al2O3 film deposited with 5 % NH4OH has the greatest negative fixed oxide charge density (Qf), which can be explained by aluminum vacancies (VAl) or oxygen interstitials (Oi) under O-rich condition. Al2O3 film deposited with NH4OH 5 % condition also shows lower interface trap density (Dit) distribution than those of other conditions. At NH4OH 5 % condition, moreover, Al2O3 film shows the highest excess carrier lifetime (τPCD) and the lowest surface recombination velocity (Seff), which are linked with its passivation properties. The proposed Al2O3 film deposited with diluted NH4OH is very promising for passivation layer and AR coating of the p-type c-Si solar cell.BR

Excess carrier lifetimes in Ge layers on Si

The excess charge carrier lifetimes in Ge layers grown on Si or germanium-on-insulator are measured by synchrotron based pump-probe transmission spectroscopy. We observe that the lifetimes do not strongly depend on growth parameters and annealing procedure, but on the doping profile. The defect layer at the Ge/Si interface is found to be the main non-radiative recombination channel. Therefore, the longest lifetimes in Ge/Si (2.6 ns) are achieved in sufficiently thick Ge layers with a built-in field, which repels electrons from the Ge/Si interface. Longer lifetimes (5.3 ns) are obtained in overgrown germanium-on-insulator due to the absence of the defective interface.

Spectroscopy of radiation traps by temperature dependent photoconductivity and generation currents in Si

An analysis of excess carrier decay transients has been performed on the reactor neutron irradiated Si n-type material grown by the magnetic field applied Czochralski (MCZ) technology, using the microwave-probed photoconductivity (MW-PC) transient technique. The measurements of temperature dependent excess carrier lifetime variations were carried out on the just irradiated and annealed samples in order to identify the prevailing radiation defects and to predict trap behaviour under thermal treatments. The activation energy of the dominant carrier traps has been extracted. Barrier capacitance and generation current variations in neutron irradiated MCZ Si pin diodes have been controlled employing the barrier evaluation by the linearly increasing voltage (BELIV) technique, as well.

Spectroscopy of radiation traps by temperature dependent photoconductivity and generation currents in Si

An analysis of excess carrier decay transients has been performed on the reactor neutron irradiated Si n-type material grown by the magnetic field applied Czochralski (MCZ) technology, using the microwave-probed photoconductivity (MW-PC) transient technique. The measurements of temperature dependent excess carrier lifetime variations were carried out on the just irradiated and annealed samples in order to identify the prevailing radiation defects and to predict trap behaviour under thermal treatments. The activation energy of the dominant carrier traps has been extracted. Barrier capacitance and generation current variations in neutron irradiated MCZ Si pin diodes have been controlled employing the barrier evaluation by the linearly increasing voltage (BELIV) technique, as well.

Process Temperature Dependence of Al2O3Film Deposited by Thermal ALD as a Passivation Layer for c-Si Solar Cells

This paper presents a study of the process temperature dependence of Al₂O₃ film grown by thermal atomic layer deposition (ALD) as a passivation layer in the crystalline Si (c-Si) solar cells. The deposition rate of Al₂O₃ film maintained almost the same until 250 ℃, but decreased from 300 ℃. Al₂O₃ film deposited at 250 ℃ was found to have the highest negative fixed oxide charge density (Qf) due to its O-rich condition and low hydroxyl group (-OH) density. After post-metallization annealing (PMA), Al₂O₃ film deposited at 250 ℃ had the lowest slow and fast interface trap density. Actually, Al₂O₃ film deposited at 250 ℃ showed the best passivation effects, that is, the highest excess carrier lifetime (τPCD) and lowest surface recombination velocity (Seff) than other conditions. Therefore, Al₂O₃ film deposited at 250 ℃ exhibited excellent chemical and field-effect passivation properties for p-type c-Si solar cells.

Injection dependence of the effective lifetime of n-type Si passivated by Al2O3: An edge effect?

Aluminum oxide provides an excellent surface passivation on p- and n-type crystalline silicon. On n-type silicon, however, the effective excess carrier lifetime τeff is often found to be injection dependent. Our experimental results show that, in our case, this effect depends mainly on the size of the lifetime samples. The fixed negative charges present at the Al2O3/c-Si interface induce an inversion layer at the surface, which results in a p–n-junction close to the surface of the sample and the inversion layer couples the sensing area with the poorly passivated or damaged edge of the sample. For smaller samples stronger injection-dependent lifetimes are measured, whereas large samples show a smaller injection dependence. In addition, photoconductance-calibrated photoluminescence lifetime images show that for low injection levels the lifetime decreases towards the sample edge. Device simulations for different sample sizes including the edge recombination are in agreement with the measured injection-dependent lifetimes. Therefore, it is necessary to use sufficiently large samples or decouple the sensing area form the edge when evaluating the injection dependence of the lifetime. For the samples used in this contribution, the injection dependence of the lifetime did not even fully vanish for an edge length of 12.5 cm.