Excess Charge Carriers Research Articles

Quality control of as-cut multicrystalline silicon wafers using photoluminescence imaging for solar cell production

Measuring the lifetime of excess charge carriers gives the opportunity to access the electric quality of the material. However, on as-cut wafers before production this quantity is strongly limited by the surface of the material. On a batch of solar cells we show that the open circuit voltage of the finished cells only scales with the lifetime, measured on as-cut wafers, if the material quality is very low. The difference between moderate and high material quality cannot be resolved. Using photoluminescence imaging the lifetime can be acquired with high spatial resolution. We show that by analyzing crystallization-related features in the images, certain defects can be identified: Such features are defects of crystal growth (e.g. dislocations) and areas of reduced lifetime that form at the edges of the crystallization crucible or near the top or bottom of a brick. Those features can be detected before production and we show their influence on cell parameters. By recognizing and rating these features, a more accurate quality control for wafers can be introduced.

Polarons and Charge Carrier Solvation on Conjugated Carbon Chains: A Comparative Ab Initio Study

We study accommodation of an excess charge carrier on long even-N polyynic oligomers C(N)H(2) due to displacements of the underlying carbon lattice and polarization of the surrounding solvent in the context of carrier self-localization into a polaronic state. Spatial patterns of bond-length alternation, excess charge and spin densities are compared as derived with Hartree-Fock and two hybrid density functional theory methods (BHandHLYP and B3LYP) in conjunction with the polarizable continuum model. Quite distinct resulting pictures of carrier accommodation are found when contributions from different interactions are analyzed. Solvation robustly acts to promote excess charge localization.

The role of underbarrier transitions in processes of annihilation of excess charge carriers in II–VI semiconductors

The loss kinetics of charge carriers generated by the emission pulses of nitrogen laser in II–VI semiconductor compounds, namely, n- and p-CdTe and CdS, is studied by the method of microwave photoconductivity. The decays of photoresponse after switching off the light consisted of fast (manifesting itself at times 50 ns) components. The shape of decay of a slow component was virtually independent of temperature, and at long times, the decay law corresponded to the linear dependence of the photoresponse on the logarithm of time. The assumption follows from the results that the slow component of the photoresponse decay reflects the loss process of charges captured in the traps due to the tunnel recombination.

Solvation-induced one-dimensional polarons and electron transfer

When a one-dimensional (1D) semiconductor nanostructure is immersed in a sluggish polar solvent, fluctuations of the medium may result in the appearance of localized electronic levels inside the band gap. An excess charge carrier can occupy such a level and undergo self-localization into a large-radius adiabatic polaron surrounded by a self-consistent medium polarization pattern. Within an appropriately adapted framework of the Marcus theory, we explore the description and qualitative picture of thermally activated electron transfer involving solvation-induced polaroniclike states by considering transfer between small and 1D species as well as between two 1D species. Illustrative calculations are performed for tubular geometries with possible applications to carbon nanotube systems.

Optical and electrical quality improvements of undoped InAs∕GaSb superlattices

The performance and operating temperature of infrared (IR) detectors are largely limited by thermal generation and noise processes in the active region of the device. Particularly, excess background charge carriers enhance dark currents and depress the detector figures of merit. Therefore, reducing the overall defects and background carriers in the undoped region of p-i-n diodes is an important issue for developing high-operating temperature IR detectors. In this article, the authors discuss how several postgrowth annealing conditions and interface shutter sequences are optimized to reduce the density of nonradiative defect trap centers and background carriers and studied their relevance to the photoluminescence (PL) emission qualities of typical mid-IR InAs∕GaSb superlattices (SLs). Among the several in situ postgrowth annealing temperatures investigated, the SLs annealed at 450°C had the highest carrier density and the lowest PL intensity, while the SLs annealed at 475°C had the lowest carrier density and the highest PL intensity. In situ annealing did not change the carrier type, but slightly reduced the carrier densities as compared to the as-grown SL sample of 1.8×1011cm−2. While in situ annealing improves the surface morphological quality and the PL intensity, ex situ annealing creates many surface pits and significantly reduces the PL intensity. The carrier density was sensitive to the variation in interface composition control. With a minor variation in the interface shutter sequence, the carrier density dramatically increased from ∼2×1011to5×1012cm−2, and the corresponding mobility decreased from 6600to26cm−2∕Vs, indicating a degradation of the interfacial quality. All of the SL samples investigated for this study were p-type.

The Impact of Stationary Crossed Electrical and Magnetic Fields on Excess Charge Carriers Generated by Radiation in P-Type Mercury-Cadmium-Telluride Epitaxial Films (Review)

The main characteristics of infrared photodetectors on base of p-type mercury-cadmium-telluride epitaxial films are mainly governed by recombination and diffusion parameters of minor charge carriers. The investigation of excess charge carriers behavior in epitaxial films placed in stationary crossed electrical and magnetic fields is allows determining of this parameters. This paper reviews of such photoelecromagnetic methods for investigation of excess charge carriers’ recombination and diffusion properties as methods of photomagnetic effects and photoconductivity in magnetic fields for Voight and Faradey geometry.

Evaluation of soft error rates using nuclear probes in bulk and SOI SRAMs with a technology node of 90nm

Abstract The difference of soft error rates (SERs) in conventional bulk Si and silicon-on-insulator (SOI) static random access memories (SRAMs) with a technology node of 90 nm has been investigated by helium ion probes with energies ranging from 0.8 to 6.0 MeV and a dose of 75 ions/μm2. The SERs in the SOI SRAM were also investigated by oxygen ion probes with energies ranging from 9.0 to 18.0 MeV and doses of 0.14–0.76 ions/μm2. The soft error in the bulk and SOI SRAMs occurred by helium ion irradiation with energies at and above 1.95 and 2.10 MeV, respectively. The SER in the bulk SRAM saturated with ion energies at and above 2.5 MeV. The SER in the SOI SRAM became the highest by helium ion irradiation at 2.5 MeV and drastically decreased with increasing the ion energies above 2.5 MeV, in which helium ions at this energy range generated the maximum amount of excess charge carriers in a SOI body. The soft errors occurred by helium ions were induced by a floating body effect due to generated excess charge carriers in the channel regions. The soft error occurred by oxygen ion irradiation with energies at and above 10.5 MeV in the SOI SRAM. The SER in the SOI SRAM gradually increased with energies from 10.5 to 13.5 MeV and saturated at 18 MeV, in which the amount of charge carriers induced by oxygen ions in this energy range gradually increased. The computer calculation indicated that the oxygen ions with energies above 13.0 MeV generated more excess charge carriers than the critical charge of the 90 nm node SOI SRAM with the designed over-layer thickness. The soft errors, occurred by oxygen ions with energies at and below 12.5 MeV, were induced by a floating body effect due to the generated excess charge carriers in the channel regions and those with energies at and above 13.0 MeV were induced by both the floating body effect and generated excess carriers. The difference of the threshold energy of the oxygen ions between the experiment and the computer calculation might be due to the difference between the designed and real structures.

Study of Excess Charge Carrier Lifetime in Si/a-SiN[sub x]:H Heterojunctions under the Influence of an External Field

The surface recombination at the Si/amorphous hydrogenated silicon nitride (a-SiN x :H) heterojunction is investigated by transient photoconductance measurements as a function of the applied potential. A minimum of the photoconductance decay time is observed at the flatband potential. The fast decay at this potential, compared to the slow decays under accumulation and depletion regimes, indicates that surface passivation of silicon by an a-SiN x :H layer is mainly due to field passivation caused by the fixed charge in the a-SiN x :H layer. The present measurements suggest that the fixed charge is not stable under the application of a potential, at least in the electrochemical cell configuration studied here. .

Separation of local bulk and surface recombination in crystalline silicon from luminescence reabsorption

AbstractSpontaneous photoemission of crystalline silicon provides information on excess charge carrier density and thereby on electronic properties such as charge carrier recombination lifetime and series resistance. This paper is dedicated to separating bulk recombination from surface recombination in silicon solar cells and wafers by exploiting reabsorption of spontaneously emitted photons. The approach is based on a comparison between luminescence images acquired with different optical short pass filters and a comprehensive mathematical model. An algorithm to separate both front and back surface recombination velocities and minority carrier diffusion length from photoluminescence (PL) images on silicon wafers is introduced. This algorithm can likewise be used to simultaneously determine back surface recombination velocity and minority carrier diffusion length in the base of a standard crystalline silicon solar cell from electroluminescence (EL) images. The proposed method is successfully tested experimentally. Copyright © 2009 John Wiley & Sons, Ltd.

Photoconductivity in a magnetic field and photomagnetic effect in p-HgCdTe graded-gap photodetecting film structures

The photomagnetic effect and photoconductivity in a magnetic field are studied at a liquid nitrogen temperature for the Voigt geometry on p-HgCdTe films with graded-gap boundary regions where the content of cadmium (x) is increased above its mole fraction in the central area of the structure with a uniform distribution of x. A case with dominating Shockley-Read recombination is considered. It is demonstrated on the basis of experimental data that the film structure in studying these effects can be replaced by the central area with an identical value of x, by introducing effective velocities of surface recombination and surface generation of excess charge carriers on the interfaces between the central area and the graded-gap regions.

Determination of local minority carrier diffusion lengths in crystalline silicon from luminescence images

In 2007 Würfel et al. [J. Appl. Phys. 101, 123110 (2007)] introduced a method to determine spatially resolved minority carrier diffusion lengths in silicon solar cells from electroluminescence intensity ratios. The key feature of this method was the exploitation of reabsorption of luminescence within a solar cell through optical short pass filters. The first part of this work deals with some experimental challenges which we encountered with the practical application of this method. A procedure of removing an artifact due to typical lateral filter inhomogeneities is introduced. Moreover, temperature dependence of luminescence is discussed and incorporated into the underlying model. The second part of this work aims at a determination of spatially resolved carrier diffusion lengths from photoluminescence (PL) on silicon wafers. The shortcomings of a diffusion length determination from PL intensity ratios are discussed. A straightforward method to determine spatially resolved integral excess charge carrier density from single PL images is introduced, thereby rendering a calibration with another measuring technique unnecessary. We show that quantitative information about recombination properties, such as minority carrier diffusion length, can be directly extracted from single PL images of silicon wafers.

Model ab initio study of charge carrier solvation and large polaron formation on conjugated carbon chains

Using long CNH2 conjugated carbon chains with the polyynic structure as prototypical examples of one-dimensional (1D) semiconductors, we discuss self-localization of excess charge carriers into 1D large polarons in the presence of the interaction with a surrounding polar solvent. The solvation mechanism of self-trapping is different from the polaron formation due to coupling with bond-length modulations of the underlying atomic lattice well known in conjugated polymers. Model ab initio computations employing the hybrid B3LYP density functional, in conjunction with the polarizable continuum model, are carried out demonstrating the formation of both electron and hole polarons. Polarons can emerge entirely due to solvation but even larger degrees of charge localization occur when accompanied by atomic displacements.

Optical absorption from solvation-induced polarons on nanotubes

When an excess charge carrier is added to a one-dimensional (1D) wide-band semiconductor immersed in a polar solvent, the carrier can undergo self-localization into a large-radius adiabatic polaron. We explore the local optical absorption from the ground state of 1D polarons using a simplified theoretical model for small-diameter tubular structures. It is found that about 90% of the absorption strength is contained in the transition to the second lowest-energy localized electronic level formed in the polarization potential well, with the equilibrium transition energy larger than the binding energy of the polaron. Thermal fluctuations, however, can cause a very substantial--an order of magnitude larger than the thermal energy--broadening of the transition. The resulting broad absorption feature may serve as a signature for the optical detection of solvated charge carriers.

Near-surface muonium states in germanium

Abstract We used the low-energy μ SR technique (LE- μ SR ) to extend our previous studies on the energy dependence of muonium (Mu) states in Si and insulators [T. Prokscha, E. Morenzoni, D.G. Eshchenko, N. Garifianov, H. Gluckler, R. Khasanov, H. Luetkens, A. Suter, Phys. Rev. Lett. 98 (2007) 227401] to investigations on an undoped 0.15-mm-thick Ge (1 0 0) crystal. The Mu formation in the near-surface region from about 10 to 150 nm is probed with mean implantation energies between 2.5 and 17.4 keV. In this energy range the number of track electron–hole pairs varies as a function of energy between a few hundred and several thousand [R.C. Alig, S. Bloom, Phys. Rev. Lett. 35 (1975) 1522]. Similar behavior as in Si is observed between 30 and 150 K, i.e., a doubling of the diamagnetic fraction F D ( Mu + or Mu - ) on lowering the energy E from 17.4 to 2.5 keV, corresponding to mean implantation depths of 130 and 17 nm, respectively. The fraction of Mu at the tetrahedral interstitial site ( Mu T ) does not show a pronounced energy dependence. The change of F D therefore can be attributed to a corresponding change of the bond-center Mu ( Mu BC ) formation probability. This demonstrates that also in Ge the formation of Mu BC clearly depends on the availability of excess charge carriers which the muon creates during the stopping process. Surprisingly, below 50 K F D starts to increase again for E > 4 keV . Additionally, bulk μ SR studies on a piece cut from the same sample shows the opposite trend in F D below 50 K, and distinct final charge states. More investigations are necessary to clarify this difference.

Origin of trapping in multicrystalline silicon

Defect sites in silicon, which temporarily capture excess charge carriers (traps), are a promising source of information on defect structures relevant for photovoltaic application of the material. In this work the correlation between traps in p-type silicon, structural crystal defects, and impurities is explored in order to find the origin of these traps in multicrystalline silicon. The trap density is compared to the density of different impurities and structural crystal defects. These comparisons reveal that the trap density is positively correlated with the oxygen density and negatively correlated with the density of the metallic impurities analyzed. In addition we show that structural crystal defects are necessary but not sufficient for the existence of high trap densities. In summary, structural crystal defects that are decorated by oxygen precipitates arise as likely origin of trap centers.

Curie temperature and NMR of 53Cr nuclei in doped ferromagnetic cadmium chromium selenide semiconductors

The effect of alien-valence substitution on exchange interactions in ferromagnetic cadmium chromium selenide spinel is studied both experimentally and theoretically. The Curie temperature is calculated within the molecular field approximation. From comparing the calculated results with experimental dependences of the Curie temperature on impurity concentration and variation in NMR chromium spectra with temperature, a conclusion is drawn in favor of the model of magnetic clusters. The model suggests that, during the alien-valence substitution for nonmagnetic cations, the valence is compensated by a change in the valence state of chromium ions. The excess charge carriers hop between equivalent ions surrounding the impurity. By analogy with the double-exchange model, a detailed description of this hopping and its influence on the exchange interaction of magnetic ions is given.

Dynamic photoconductive gain effect in shallow-etched AlGaAs/GaAs quantum wires

We report on a dynamic photoconductive gain effect in quantum wires which are lithographically fabricated in an AlGaAs/GaAs quantum well via a shallow-etch technique. The effect allows resolving the one-dimensional subbands of the quantum wires as maxima in the photoresponse across the quantum wires. We interpret the results by optically induced holes in the valence band of the quantum well which shift the chemical potential of the quantum wire. The nonlinear current-voltage characteristics of the quantum wires also allow detecting the photoresponse effect of excess charge carriers in the conduction band of the quantum well. The dynamics of the photoconductive gain are limited by the recombination time of both electrons and holes.

Influence of electric field on the photoelectric effect in a Schottky barrier based on n-type cadmium diphosphide

The results of a study on photoelectric properties of a Schottky barrier based on n-type CdP2 are considered. The effect of the barrier electric field on photocurrents caused by photoelectron emission from metal and optical generation of excess charge carriers in the semiconductor was studied. It was found that the dependence of the photocurrent on the light-modulation frequency is controlled by level recharging times at the interfaces between the space-charge and quasi-neutrality regions and between semiconductor and metal. Good agreement between the calculated and experimental results was achieved.

Electroluminescence efficiency degradation of crystalline silicon solar cells after irradiation with protons in the energy range between 0.8 MeV and 65 MeV

AbstractStandard crystalline silicon homojunction solar cells have been irradiated with high energy protons at variable energies between 0.8 MeV and 65 MeV. As one possible criterion for the evaluation of the radiation induced degradation, we compared the suppression of the bandgap electroluminescence at a wavelength around 1150 nm for the solar cells, irradiated at different energies. The lowest electroluminescence efficiency has been observed for an intermediate proton energy of 1.7 MeV. At this energy value, however, crystalline silicon solar cells are not homogeneously damaged, as shown by the defect profiles, calculated by the SRIM code. We propose therefore a irradiation at a higher energy in order to obtain a relative homogeneous defect profile. At 65 MeV the projected range of the protons in silicon is about 2 cm and we obtain a nearly constant defect distribution in the solar cell and the irradiation can be performed in air. Subsequently we compared the fluence dependence of the degradation of the solar cell parameters after 65 MeV irradiation with other material parameters, such as effective diffusion lengths, as calculated by the quantum yield spectra and the effective excess charge carrier lifetime, as measured on silicon wafers by the contactless TRMC‐technique. Similar degradation has been found for the short circuit current, solar cell efficiency and electroluminescence efficiency and a good agreement between diffusion length and excess charge carrier lifetime changes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Charge carrier solvation and large polaron formation on a polymer chain revealed in modelab initiocomputations

When an excess charge carrier is added to a semiconducting polymer chain, it is well known that the carrier may self-trap into a polaronic state accompanied by a bond length adjustment pattern. A different mechanism of self-localization is the solvation of charge carriers expected to take place when the polymer chain is immersed in polar media such as common solvents. We use state-of-the-art ab initio computations in conjunction with the Polarizable Continuum Model to unequivocally demonstrate solvation-induced self-consistent charge localization into large-radius one-dimensional (1D) polarons on long $C_{N}H_{2}$ carbon chains with the polyynic structure. Within the framework used, the solvation results in a much more pronounced charge localization. We believe this mechanism of polaron formation to be of relevance for various 1D semiconductors in polar environments.