Hole Capture Research Articles

Carrier capture at the SiO2–Si interface: A physical model

The carrier (electron and hole) capture cross section of defects at the SiO2–Si interface is measured using trap-fill time in gigahertz charge-pumping experiment. The ability to use a highly asymmetric rectangular wave with very fast rise and fall times in our charge-pumping experiment greatly simplified data interpretation. Trap-fill times of less than or equal to 0.7ns are found for both electrons and holes. The corresponding capture cross section of 10−15cm2 or larger for both electron and hole cannot be explained by the usual multiphonon emission mechanism. A modified cascade capture model involving the strained Si–Si bonds around the Pb center is proposed to explain the large capture cross sections.

Carrier capture in InGaAsSb∕InAs∕InGaSb type-II laser heterostructures

Experimental studies of the electron and hole concentration dynamics in the barrier of GaSb-based type-II quantum-well (QW) heterostructures were performed. Capture of electrons and holes was studied separately in specially designed and grown laser heterostructures with QWs only for electrons or only for holes. The difference between electron and hole relaxation rates is explained by corresponding QW carrier confinement energies.

Doubly charged state of EL2 defect in MOCVD-grown GaAs

EL2 is the ubiquitous native defect in crystalline GaAs grown by a variety of different techniques. It has been proposed to be a doubly charged deep-level center with two states having distinct energy levels in the band gap. While the singly charged state has been the subject of many experimental studies and is, therefore, well established, the doubly charged state has only been occasionally alluded to in the literature. This paper provides evidence for a dominant inadvertent deep level in p-type GaAs most likely to be the doubly charged state of the EL2 center. Deep-level transient spectroscopy (DLTS) has been applied to characterize epitaxial layers of p-type GaAs grown on p + GaAs substrates by low-pressure metal organic chemical vapor deposition (LP-MOCVD). A pronounced peak is observed in the majority carrier (hole) emission deep-level spectra. Thermal emission rate of holes from the corresponding deep level is found to exhibit a strong electric field dependence, showing an increase of more than two orders of magnitude with an increase of the electric field by a factor of ∼2. The thermal activation energy for this level is found to vary from 0.29 to 0.61 eV as the electric field is varied from 2.8×10 5 to 1.4×10 5 V/cm. Direct pulse-filling measurements point to a temperature-dependent behavior of the hole capture cross section of this level. We identify this inadvertent deep-level defect, commonly observed in p-type GaAs grown by a variety of different methods, with the doubly charged state of the well-known As Ga antisite related defect, EL2.

On the origin of the 1-eV band in photoluminescence from Cd1−x Zn x Te

Photoluminescence (PL) at 77 K from Cd1−xZnxTe samples (x = 0, 0.005 and 0.01) annealed at 900°C and cadmium vapor pressure PCd = 3 × 104−2 × 105 Pa has been studied. It was found that the contribution of the 1-eV band to the spectrum-integrated PL from these samples is independent of PCd, in contrast to Cd0.95Zn0.05Te samples in which this contribution increases up to ∼90% as PCd grows. The band is not shifted to shorter wavelengths as x becomes larger. The conclusion that Zn vacancies are involved in the formation of Cd1−xZnxTe properties is confirmed. The 1-eV band is attributed to capture of free holes to acceptor levels related to vacancies of both cadmium and zinc. These levels are closely spaced and, therefore, are difficult to resolve.

Poly(3,4-ethylenedioxythiophene)−Semiconductor Nanoparticle Composite Thin Films Tethered to Indium Tin Oxide Substrates via Electropolymerization

Nanocomposite thin films are described which have been created from electropolymerized poly(3,4-ethylenedioxythiophene) (PEDOT) and new forms of ligand-capped CdSe nanoparticles (NP), using carboxylate ligands with 3,4-propylenedioxythiophene (ProDOT) capping groups (ProDOT-CA). A thin PEDOT film is first tethered to an indium tin oxide electrode, followed by electrochemical cross-linking of the ligand-capped nanoparticle with the PEDOT film. The resultant thin film material photoelectrochemically reduces solution C60 at potentials positive of its E°, where the PEDOT film is reduced (de-doped) and capable of hole capture from the photoexcited NP. This approach is distinctive from solution cast polymer/CdSe hybrid materials in that it enables direct wiring of the nanoparticle to the donor polymer and hole collecting electrode.

Kinetics of the electronically stimulated formation of a boron-oxygen complex in crystalline silicon

We present experimental data relating to the slow stage of the illumination-induced or electron-injection-induced generation, in crystalline $p$-type silicon, of the carrier-recombination center believed to be the defect complex ${({\mathrm{B}}_{s}{\mathrm{O}}_{2i})}^{+}$ formed by diffusion of oxygen interstitial dimers $\mathrm{O}_{2i}{}^{++}$ to substitutional boron atoms $\mathrm{B}_{s}{}^{\ensuremath{-}}$ and, taking account of those data, we consider a detailed theoretical model for the kinetics of the diffusion reaction. The model proposes that the generation rate of the ${({\mathrm{B}}_{s}{\mathrm{O}}_{2i})}^{+}$ defects is controlled by the capture of a majority-carrier hole by the dimer following the capture of a minority-carrier electron and by the Coulomb attraction of the $\mathrm{O}_{2i}{}^{++}$ to the $\mathrm{B}_{s}{}^{\ensuremath{-}}$ atom, and leads to predictions for the defect generation rate that are in excellent quantitative agreement with experiment.

On the dynamic formation of accreting intermediate-mass black holes

We compute the probability that intermediate-mass black holes (IMBHs) capture companions owing to dynamic interactions and appear as accreting sources and we explore the possibility that the accreting IMBHs would appear as ultraluminous X-ray sources.

Extremely small hole capture cross sections in HfO2∕HfxSiyOz∕p-Si structures

Defects in Al∕HfO2∕HfxSiyOz∕p-Si capacitors have been characterized using thermally stimulated current at temperatures between 30 and 300K. The hole activation energy and capture cross section were extracted from the results. The authors observed shallow traps that move with changing the discharging voltage, giving rise to activation energies in the range 0.03–0.14eV. Postmetallization anneal passivated these traps and a deeper trap appears with a significantly lower shift with the discharging voltage. Very small apparent capture cross sections (capture cross section times tunneling probability) have been extracted (10−26–10−18cm2). Simulations agree very well with experimental data.

On the dynamical formation of accreting intermediate mass black holes

We compute the probability that intermediate mass black holes (IMBHs) capture companions due to dynamical interactions and become accreting sources, and explore the possibility that the accreting IMBHs would appear as ultraluminous X-ray sources (ULXs). We focus on IMBHs originating from low-metallicity Population III stars. Two channels of IMBH formation are considered: from primordial haloes in the framework of hierarchical clustering, and from non-mixed, zero-metallicity primeval gas in galactic discs. IMBHs can form binary systems due to tidal captures of single stars and exchange interactions with existing binary systems in galactic discs. We find that neither formation mechanism of the accreting IMBH binary is able to provide enough sources to explain the observed population of ULXs. Even at sub-ULX luminosity, the total number of accreting IMBHs with L > 1036 erg s−1 with dynamically captured companions is found to be <0.01 per galaxy.

Electrical characterization of alpha radiation-induced defects in p-GaAs grown by metal-organic chemical-vapor deposition

Investigations of the alpha particle irradiation-induced defects in low-pressure metal-organic chemical-vapor deposition grown p-GaAs have been carried out. By employing deep-level transient spectroscopy, at least seven radiation-induced deep-level defects have been observed in the lower half of the band gap in the temperature range of 12−475 K. Double-correlation deep-level transient spectroscopy measurements show three prominent levels: two known radiation-induced levels namely, Hα1 and Hα5, and one inadvertent center HSA, present before irradiation, to exhibit a significant dependence of thermal emission rate on the junction electric field. For Hα1 and HSA the field-enhanced emission data are well fitted with a Poole-Frenkel model, using a three-dimensional square-well potential with radius r=3.2 and 1.43 nm, respectively. The field effect for Hα5 has been explained by a square-well potential in combination with a phonon-assisted tunneling process. Detailed data on the carrier capture cross section for all three levels have been obtained. The hole capture cross section for the levels Hα1 and Hα5 are found to be temperature independent, while for HSA, the hole capture data show a dependence on temperature. The dependence of hole capture cross section of HSA on temperature has been explained in terms of multiphonon capture mechanism, yielding a capture barrier of 0.13 eV and σ(∞)=1.5×10−14 cm2. These analyses lead us to conclude that the levels Hα1 and HSA are associated with a charged center, while the level Hα5 is most likely a substitutional defect in GaAs.

Radiative and nonradiative recombination processes in ZnCdSe∕ZnCdMgSe multi-quantum-wells

Carrier recombination through radiative and nonradiative processes in lattice-matched n-Zn0.5Cd0.5Se∕Zn0.21Cd0.19Mg0.6Se multi-quantum-wells (MQWs) was investigated by temperature-dependent time-resolved photoluminescence (PL) spectroscopy. The n-Zn0.5Cd0.5Se∕Zn0.21Cd0.19Mg0.6Se MQW samples with different well widths were grown on InP substrates by molecular beam epitaxy. The PL decay times and the PL intensities were measured as functions of temperature. For a doping level of 1×1018cm−3, the dominant mechanism of the radiative process was found to be free carrier recombination while excitonic recombination was absent due to the effect of strong carrier screening. The nonradiative mechanism was determined to be hole capture through multiphonon emission (MPE). The expressions of the nonradiative MPE recombination lifetime, the PL decay time, and the PL intensity were deduced as functions of temperature and were used to fit the measured temperature dependence of the PL decay times and the PL intensities. The MPE activation energies and relative defect densities for the samples with different well widths were obtained. A simple method is suggested to investigate the interfacial defects of quantum wells.

Kinetics of Charge Trapping and Emission in CIGS Solar Cells

AbstractIn this work we investigate bias - induced metastability in CIGS solar cells. Long - term capacitance transients have been measured for two baseline CIGS devices with different efficiencies and a CGS cell in order to analyze carrier trapping processes. Based on the results we discuss hole emission process which leads to metastable increase of net acceptor density and also hole capture related to its relaxation. Time constants and activation energies for hole emission and capture have been obtained. Apart of carrier trapping processes we have also distinguished an interface-related change of capacitance. Our results indicate processes involving thermal lattice relaxation. We explain them in light of properties of (VSe+VCu) divacancy, defect with negative correlation energy, which can exist in both donor and acceptor configuration.

Kinetics of an exciton-trion system in shallow GaAs/AlGaAs quantum wells

The formation of three-particle charged exciton complexes (trions) in shallow GaAs/AlGaAs quantum wells in the temperature range 1.7–15 K has been investigated by luminescence spectroscopy and resonant light scattering. The effect of the photon energy and the intensity of additional above-barrier illumination on the trion formation kinetics has been analyzed. It is established that, upon intrawell excitation, illumination leads to the formation of trions when the light photon energy corresponds to the regions of effective formation of trions in the photoluminescence excitation spectra. It is shown that, with an increase in the illumination level, the trion concentration first increases and then reaches a plateau since the quantum well acquires an electric charge whose field equalizes the electron and hole capture rates.

Bistability-Mediated Carrier Recombination at Light-Induced Boron-Oxygen Complexes in Silicon

A first-principles study of the BO2 complex in B-doped Czochralski Si reveals a defect-bistability-mediated carrier recombination mechanism, which contrasts with the standard fixed-level Shockley-Read-Hall model of recombination. An O2 dimer distant from B causes only weak carrier recombination, which nevertheless drives O2 diffusion under light to form the BO2 complex. Although BO2 and O2 produce nearly identical defect levels in the band gap, the recombination at BO2 is substantially faster than at O2 because the charge state of the latter inhibits the hole capture step of recombination.

Light Harvesting and Carrier Transport in Core/Barrier/Shell Semiconductor Nanocrystals

Excitation transfer pathways in colloidal core/barrier/shell nanomaterials were investigated in the CdSe/ZnS/CdSe system. Absorption of light in the outer CdSe shell results in emission from the band edge of the CdSe core. The CdSe quantum shell acts as a light harvester that indirectly increases the brightness of the CdSe quantum-dot core. Spectroscopic evidence suggesting that the CdSe core and shell are coupled by tunneling of excitons through the ZnS barrier is provided. Competition kinetic analysis shows that charge transport competes effectively with hole capture by pyridine at the outer CdSe shell and exciton relaxation within the outer CdSe shell.

Hole capture into self-organized InGaAs quantum dots

Hole capture into and emission from self-organized InGaAs∕GaAs quantum dots (QDs) are studied by means of charge-selective deep level transient spectroscopy. The authors observe hole capture and determine activation energies and apparent capture cross sections for emission and capture. The experimental findings indicate that the capture process into the QDs in the presence of an applied electric field is controlled by phonon-assisted tunneling. An apparent capture cross section (at infinite values of temperature T and electric field F) σF,T=∞≈7×10−12cm2 and an average time tc≈0.3ps (T=300K) for hole capture and relaxation are obtained.

Determination at 300K of the hole capture cross section of chromium-boron pairs in p-type silicon

Measurement of dissolved chromium concentration in p-type crystalline silicon by means of the change in carrier lifetime due to chromium-boron pair dissociation requires precise knowledge of the recombination parameters of dissolved chromium in silicon. This work, based on quasi-steady-state carrier lifetime measurements, aims to determine the value at 300K of the hole capture cross section of the chromium-boron pairs in p-type silicon, which is the only recombination parameter still unknown at this temperature. The complete knowledge of the recombination parameters allows the determination of dissolved chromium concentrations in p-type silicon as low as 1010cm−3, even when chromium atoms do not dominate carrier recombination.

Effects of energy distribution of interface traps on recombination dc current-voltage line shape

The effects of energy distributions of Si∕SiO2 interface traps in the energy gap of oxidized silicon on the current versus voltage line shape of the electron-hole recombination current are analyzed using the steady-state Shockley-Read-Hall kinetics. Slater’s [Insulators, Semiconductors and Metals; Quantum Theory of Molecules and Solids (McGraw-Hill, New York, 1967)] localized bulk perturbation theory applied by us to the interface anticipates U-shaped energy distributions of the density of neutral electron and hole interface traps from random variations of the Si:Si and Si:O bond angles and lengths. Conservation in dissipative transition energy anticipates the rate of electron capture into neutral electron trap to be faster for electron trap energy levels nearer the conduction band edge, and similarly, the rate of hole capture into neutral hole trap to be faster for hole trap energy levels nearer the valence band edge. Line shape broadening is analyzed for discrete and U-shaped energy distributions of interface trap energy levels. The broadened line shapes observed in past experiments, previously attributed to spatial variations of surface dopant impurity concentrations, could also arise from energy distributions of interface trap energy levels.

Defects Limiting Minorty Carrier Lifetime in 4H-SiC Epilayers

The defects controlling carrier lifetime in 4H-SiC n- epilayers have been investigated by DLTS and time- resolved photoluminescence. Measurements of the dependence upon layer thickness enabled the separation of bulk and surface contributions to the measured lifetimes. Only the Z1/Z2 defect exhibited consistent correlation with measured lifetimes, while the EH6/EH7 defect was correlated only in some cases. The dependence upon layer thickness revealed a bulk minority carrier lifetime that was linearly related to the Z1/Z2 concentration and a hole capture cross-section of 3x10-14 cm2. This corresponded well to DLTS measurements carried out in p-i-n diode structures, where a large hole capture cross-section, sp, was indicated for Z1/Z2 and a small sp for EH6/EH7. Consequently, these results suggest that Z1/Z2 acts alone as the lifetime killer in this material.

Structural and electronic properties ofFe3+andFe2+centers in GaN from optical and EPR experiments

This work provides a consistent picture of the structural, optical, and electronic properties of Fe-doped GaN. A set of high-quality GaN crystals doped with Fe at concentrations ranging from $5\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ is systematically investigated by means of electron paramagnetic resonance and various optical techniques. ${\mathrm{Fe}}^{3+}$ is shown to be a stable charge state at concentrations from $1\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. The fine structure of its midgap states is successfully established, including an effective-mass-like state consisting of a hole bound to ${\mathrm{Fe}}^{2+}$ with a binding energy of $50\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. A major excitation mechanism of the ${\mathrm{Fe}}^{3+}(^{4}T_{1}\ensuremath{\rightarrow}^{6}A_{1})$ luminescence is identified to be the capture of free holes by ${\mathrm{Fe}}^{2+}$ centers. The holes are generated in a two-step process via the intrinsic defects involved in the yellow luminescence. The ${\mathrm{Fe}}^{3+∕2+}$ charge-transfer level is found $2.863\ifmmode\pm\else\textpm\fi{}0.005\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the valence band, suggesting that the internal reference rule does not hold for the prediction of band offsets of heterojunctions between GaN and other III-V materials. The ${\mathrm{Fe}}^{2+}(^{5}E\ensuremath{\rightarrow}^{5}T_{2})$ transition is observed around $390\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at any studied Fe concentration by means of Fourier transform infrared spectroscopy. Charge-transfer processes and the effective-mass-like state involving both ${\mathrm{Fe}}^{2+}$ states are observed. At Fe concentrations from $1\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, additional lines occur in electron paramagnetic resonance and photoluminescence spectra which are attributed to defect complexes involving ${\mathrm{Fe}}^{3+}$. With increasing Fe concentration, the Fermi level is shown to move from near the conduction band to the ${\mathrm{Fe}}^{3+∕2+}$ charge-transfer level, where it stays pinned for concentrations from $1\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Contrary to cubic II-VI and III-V materials, both electronic states are effected by only a weak Jahn-Teller interaction.