Dominant Recombination Center Research Articles

Minority Carrier Lifetime Dependence on Resistivity in High‐Purity p‐Type Silicon

This work investigates the dependence of minority carrier lifetime on dopant concentration and injection level in p‐type silicon. For current high‐purity material grown by the conventional Czochralski technique or applying a magnetic field, iron is no longer the dominant recombination center. An approach based on Shockley‐Read‐Hall theory using published iron‐related recombination centers is shown to be unsuccessful in explaining the experimentally observed behavior. In this work, the variation of lifetime with doping concentration and injection level in high‐purity silicon is examined assuming the presence of a distribution of energy levels in the bandgap, possibly due to some kind of grown‐in recombination centers (high‐purity silicon recombination centers), and it is shown that this approach can indeed explain the experimental behavior. According to the results, even in the presence of a moderate iron contamination the contribution of high‐purity silicon recombination centers is necessary to fit the experimental data. The comparison between samples grown with and without magnetic field suggests that oxygen might be involved in the formation of these recombination centers. © 1999 The Electrochemical Society. All rights reserved.

Photoconductivity and electronic transport in tetrahedral amorphous carbon and hydrogenated tetrahedral amorphous carbon

The photoconductivity of tetrahedral amorphous carbon (ta-C) and hydrogenated tetrahedral amorphous carbon (ta-C:H) has been studied as a function of temperature, photon energy, and light intensity in order to understand the transport and recombination processes. ta-C and ta-C:H are found to be low mobility solids with μτ products of order 10−11–10−12 cm2/V at room temperature because of their relatively high defect densities. Deep defects tend to be the dominant recombination centers, but at high and moderate temperatures only a fraction of these centers or even tail states can act as recombination centers because the carrier demarcation levels do not always span the gap. For excitation by high energy UV photons, a peak in the photoconductivity is found at 200 K, similar to the thermal quenching effect found in a-Si:H, and attributed to competitive recombination between two classes of centers with very different capture cross sections.

Deep energy levels in power diodes introduced by iridium diffusion

The results of a study carried out on N-type silicon and P +PNN + silicon diodes diffused with iridium at temperatures ranging from 820 to 940°C are reported. The iridium-related deep energy levels were measured using DLTS method. The energy levels E c = 0.16, 0.28 and 0.54 eV were found, the dominant recombination centre is of energy E c − 0.28 eV. Besides the energy levels, the influence of iridium diffusion on carrier lifetime and basic parameters of the diode structures was studied. It was found that iridium diffusion may be used successfully for the fabrication of fast, soft reverse recovery power diodes.

Gap states in diamond-like amorphous carbon

Abstract The electronic structure of amorphous diamond-like carbon is reviewed. The sp2 sites form small, mainly chain-like clusters which control the bandgap. Detailed analysis shows that all π states within the σ-σ∗ gap are all localized; so the mobility gap greatly exceeds the optical gap. This accounts for the photoluminescence excitation spectrum. Dangling-bond states are calculated to possess some s orbital character, which lowers their energy level in the gap and accounts for the p-type conduction of tetrahedral amorphous carbon. The defect density is high in hydrogenated amorphous carbon and is found to be fairly well described by the weak-bond-dangling-bond conversion model. The photoluminescence mechanism is described and it is argued that the paramagnetic defects are still the most likely dominant recombination centre.

New interpretation of the dominant recombination center in platinum doped silicon

The midgap level in platinum doped n-type silicon, which was proposed to be the dominant recombination center, is identified as a platinum-hydrogen complex. Hydrogenation of the samples is achieved by wet-chemical etching at room temperature. Defect profiles, determined by deep level transient spectroscopy, clearly associate the level with the concentration profile of atomic hydrogen.

Electronic structure of diamond-like carbon

The electronic structure of amorphous diamond-like carbon is reviewed. Numerous calculations are giving a consistent description of the local atomic structure as containing small, mainly chain-like clusters of sp2 sites, which control the band gap. The experimental optical gap of all types of DLC is found to depend primarily on their sp2 content. Detailed analysis finds that all π states within the σ-σ∗ gap are all localized, so the mobility gap greatly exceeds the optical gap. This accounts for the photoluminescence excitation spectrum and the low carrier mobility seen in TFTs. Calculations also find that the dangling bond defects tend to possess some s orbital character, lowering their energy level in the gap. These states pin the Fermi level and cause undoped ta-C and ta-C:H to be p-type. The paramagnetic defect is the dominant recombination centre and the ESR linewidth is attributed to exchange narrowing.

Electron and proton irradiation-induced degradation of epitaxial InP solar cells

The degradation of epitaxial, shallow homojunction n +p InP solar cells under 1 MeV electron and 3 MeV proton irradiation is presented. The data measured under 3 MeV proton irradiation are analyzed in terms of displacement damage dose which is the product of the particle fluence and the calculated non-ionizing energy loss (NIEL)[1]. A characteristic proton degradation curve is derived from which the cell degradation under any energy proton irradiation can be calculated. The data measured under 1 MeV electron irradiation is also analyzed in terms of displacement damage dose. The electron irradiation-induced degradation is correlated with the proton degradation curve by determining electron to proton dose ratios for each of the photovoltaic (PV) parameters. A comparison of the characteristic degradation curves for InP and GaAs/Ge solar cells, which was determined previously, shows InP to be intrinsically more resistant to displacement energy deposition. The base carrier concentration was measured during the irradiations, and significant carrier removal was observed. When analyzed as a function of displacement damage dose, the reduction in carrier concentration under both the 1 MeV electron and the 3 MeV proton irradiation is shown to follow the same degradation curve. From this common degradation curve, a characteristic carrier removal rate is calculated for InP under any irradiation. The junction dark current was also measured during both irradiations, and the data were fit to a three-term diode dark current equation. From the fits, the diffusion current is determined as a function of particle fluence. Changes in the diffusion current under electron and proton irradiation are shown to correlate in terms of displacement damage dose in the same way as the cell maximum power. The junction recombination current is also determined from the dark current data, and the results show the energy level of the dominant radiation-induced recombination center to be approximately the same in both the electron and proton irradiated samples. In addition, the dark current analysis indicates that the relative changes in the hole and electron lifetimes are essentially the same under both the electron and the proton irradiations. Based on these results and the overall correlation between the electron and proton damage, a detailed description of the mechanism of the radiation response of InP is developed which describes the cell degradation under any particle irradiation.

Models of deep centres in gallium phosphide

The effects of varying the ammonia flux on the concentrations of background Si and free carriers and on deep traps for majority and minority carriers in n-GaP layers grown by vapour epitaxy have been studied by secondary ion mass spectrometry and deep level transient spectroscopy. The contribution from the background silicon to the free carrier concentration in samples variously doped with nitrogen is discussed. It is shown that the deep centre at eV may be attributed to silicon. The model of this centre in the form accounts for experimental results obtained in the present work and those already reported. The concentration of the dominant nonradiative recombination centre at eV is studied, depending on growth conditions, and its model is proposed in the form of a complex consisting of intrinsic defects.

Photoconductivity of a-Si:H as a Function of Doping, Temperature and Photocarrier Generation Rates Between 1013 and 1028cm-3s-1

AbstractThe steady state photoconductivity up of n-type, p-type and intrinsic a-Si:H has been studied up to photocarrier generation rates of G=5×1027cm-3s-1. In the 20ppm B2H6/SiH4 p-type sample photoconduction switches from holes at low G to electrons at high G. The electron photoconduction at high G is increased by n-type and decreased by p-type doping. This is explained by the charge state of the dominant electron recombination centers. The σp(G) curves of doped and intrinsic a-Si:H merge at the highest G used.

Gettering of FE By Aluminum In P-Type Cz Silicon

AbstractIn this study, we investigate the gettering process of Fe in p-type Cz silicon after iron has been introduced at the solubility limit at 1000°C. Deep Level Transient Spectroscopy (DLTS) was used to measure [FeB], a fingerprint of [Fei], at the center of samples. The minority carrier diffusion length and lifetime were calculated from Electron Beam Induced Current (EBIC) measurements. The fact that [FeB] is proportional to the negative second power of the minority carrier diffusion length at the high [FeB] regime confirms that FeB donors are the dominant recombination centers limiting solar cell performance with high Fe contamination. By quenching after heat treatment, we can maintain and measure the kinetics and thermodynamics of gettering exclusively. The getter/silicon interface was studied by comparison of the gettering rates of molten Al at 620°C, 700°C, and 800°C, and iron silicide at 700°C. We model Fe gettering with respect to temperature, time, solubility and precipitate nuclei density. In the early stage of Fe gettering, the process is dominated by precipitate formation around oxygen precipitate nuclei. The precipitate density is estimated to be on the order of 5×108cm−3. In later stages, Fe outdiffusion contributes to the [Fei] reduction. The early stage precipitation limits [Fei] reduction after short time to the solubility at the gettering temperature.

Evaluation of Oxygen‐Related Carrier Recombination Centers in High‐Purity Czochralski‐Grown Si Crystals by the Bulk Lifetime Measurements

Present day high‐purity Czochralski(CZ)‐grown Si crystals are characterized by the bulk carrier recombination life‐time measurements. The bulk lifetime of the p‐type as‐grown Si with resistivity ∼10 Ω‐cm is found to be as high as 500 μs for a CZ‐Si crystal with and as high as ∼1 ms or even higher for a low‐oxygen CZ‐Si crystal with. The as‐grown bulk lifetime is revealed to depend not only on resistivity under the framework of the Shockley‐Read‐Hall (SRH) relation, but also on the oxygen concentration. The dominant recombination center of the as‐grown crystal is found to be an SRH‐type deep‐level center which is considered to be an oxygen‐related defect complex. The complex is quenched easily by a heat‐treatment at a temperature as low as 650°C. Effective density of a recombination center, existing after the 650°C heat‐treatment in the p‐type crystal, correlates well with the amount of oxygen precipitation generated by a subsequent two‐step heat process . Thus the dominant recombination center after the 650°C heat‐treatment is considered to be closely related to the oxygen precipitation nuclei toward the subsequent precipitation formation heat process.

Exciton Luminescence of Single-Crystal GaN

ABSTRACTDetailed photoluminescence (PL) studies of high-quality MBE-grown single-crystal cubic and hexagonal GaN are presented. We identify free and bound exciton recombination. By means of a line-shape analysis, we quantitatively analyze our spectra, which were taken as a function of temperature (T = 4 - 300 K) and excitation density (Pex = 0.3 - 200W/cm2). We show the dominant recombination channel at 300 K to be free-excitonic in nature with an internal small-signal quantum efficiency of 6 · 10−3 for both cubic and hexagonal material. Based on a three-level model, activation energies for exciton dissociation are evaluated. Radiative (τrad ≈ 2 ns) and nonradiative lifetimes (τe ≈ 1μs, τh ≈ 20 ps) are determined, where in the latter case, electron and hole trapping are considered separately. Furthermore, we show that the dominant nonradiative recombination center, being a hole trap, saturates at Pex ≥ 20 W/cm2.

Observation of rapid direct charge transfer between deep defects in silicon.

Direct electron transfer is observed between two deep defects in silicon upon selective laser excitation with an energy lower than the band gap. This transfer is shown to be very efficient when one of the defects is a pseudodonor and the other is a dominant recombination center. It is argued that such processes must in general be considered when modeling capture and recombination processes of charge carriers in semiconductors.

Thermoluminescence spectra of doped Bi 4Ge 3O 12

Thermoluminescence of Bi 4Ge 3O 12 reveals a range of trapping levels which are common to pure and doped material. The relative intensities of the glow peaks observed above room temperature change with radiation dose and dose rate and the presence of impurities. Although the trapping sites are thought to be intrinsic oxygen defects, additions of impurities enhance the signals from different glow peaks. The emission spectra from pure and transition ion doped Bi 4Ge 3O 12 extend from 450 to 750 nm. If rare earth ions are present then they act as the dominant recombination centres and define the emission spectra. This is interpreted as resulting from direct charge transfer from intrinsic defect traps to rare earth recombination centres. The close interaction of trap and rare earth is evident by a temperature dependence of the 110 and 175°C glow peaks with the ionic radii of the impurities. Models for the charge traps are proposed and activation energies are given.

Correlation between optical spectroscopy and capacitance-voltage profile simulation applied to interface states in multilayer GaAs/AlGaAs heterostructures

We have determined the interface state density at each interface in a series of GaAs/AlGaAs multilayer heterostructures using capacitance-voltage profile simulation. We find that the values obtained correlate to the interface recombination velocities determined by time resolved photoluminescence and also to the strength of excitonic transitions observed in steady-state photoluminescence. The optical data therefore support the validity of our results. The results show that the interface state density follows a trend that decreases with the growth of each successive interface thereby supporting the view that the interfaces act as gettering planes for impurities during growth. In addition, the parallel application of electrical and optical techniques has allowed us to estimate the activation energy and minority carrier capture cross section of the dominant recombination centers.

Recombination Process in the As-Deposited State of Hydrogenated Amorphous Silicon

Intrinsic deep defect-related recombination process has been studied in a series of undoped hydrogenated amorphous silicon(a-Si:H) films grown under different deposition conditions. Steady-state photoconductivity (σph) was measured as a function of deep defect density Nd, Urbach energy Eu, and dark Fermi energy Ef. It was found that σph strongly depends on these parameters while Ef- stays at the energy levels lower than 0.82 eV below Ec, but it is nearly independent of those while Ef stays at above 0.82 eV. These behaviors were found to be independent of the sample deposition conditions. These results indicates that subgap defect states enclosed by E=0.82 eV and Ef are the dominant recombination centers.

Effect of non-stoichiometry on near-band-edge absorption and non- radiative recombination in bulk GaAs

We demonstrate that the total near-band-edge photoluminescence intensity of undoped GaAs grown by the liquid encapsulated Czochralski technique is reduced if the stoichiometry of the starting melt becomes more gallium rich. At the same time, concentrations of the native deep donor level EL2 are reduced, but there is an increase in the concentration of an unknown defect which causes absorption near the band edge in cooled samples. Such absorption is known as reverse contrast (RC). We produce evidence that the defects that give rise to this absorption, RC defects, are also the dominant non-radiative recombination centres in undoped GaAs. It is probable that these defects are gallium rich and may be related to arsenic vacancies.

Recognition of non-radiative recombination centres in semi-insulating GaAs

A novel technique, double-beam photoluminescence (DBPL), is introduced. This is used to investigate photoquenching effects of deep levels which affect near-band-edge photoluminescence (PL) in bulk-grown semi-insulating (SI) GaAs. The authors show that there is a remarkable increase in band-to-band radiative transition efficiency after photoquenching of near-band-edge absorption (also known as Reverse Contrast or RC) at low temperatures (less than 35 K) and suggest that RC absorption does indeed map concentrations of the dominant associate this effect with a reduction in non-radiative recombination centres. They suggest that RC absorption does indeed map concentrations of the dominant recombination centre in SI GaAs and that the observed increase in luminescence after low-temperature illumination with light of near 1 mu m wavelength is due to photoquenching of the RC defects.

Annealing behavior of deep-level defects in semi-insulating gallium arsenide studied by photoluminescence, infrared absorption, and resistivity mapping

Deep-level defects in as-grown, ingot-annealed, and wafer-annealed samples of semi-insulating gallium arsenide have been investigated by spatially resolved measurements of room-temperature photoluminescence, infrared absorption, free-carrier lifetime, and resistivity. High-temperature ingot annealing mainly causes a homogenization of the EL2 distribution. Rapid cooling from a wafer annealing process at T>900 °C suppresses the formation of the previously lifetime-limiting recombination center. After wafer annealing the EL2 defect may be the dominant recombination center, while in as-grown and ingot-annealed material lifetime is limited by a different trap. There is experimental evidence that this trap is related to the 0.8-eV luminescence band and that its density is spatially anticorrelated to the EL2 distribution. Based on lifetime measurements and a correlation of EL2 and photoluminescence topographs, we developed a recombination model, which explains the relationship between defect densities, and photoluminescence. The effect of surface recombination is described by a numerical calculation.

Suppression of the dominant recombination center in n-type GaAs by proximity annealing of wafers

n-type GaAs specimens have been annealed in sealed quartz ampoules and characterized with electron-beam-induced current and photoresponse measurements, deep-level transient spectroscopy, and photoluminescence spectroscopy. By correlating changes in the concentrations of defects with minority-carrier diffusion length (Lp) it is shown that the dominant recombination center in this material is a hole trap termed HCX (Ev+0.29 eV). Increases in Lp of up to a factor of 3, which can be achieved by proximity annealing at 950 °C for 16 h, are related to the loss of As from the specimen surfaces during the early stages of annealing. The beneficial effect of the annealing is associated with a limited source diffusion process since the total amount of As loss is determined by the ratio of the ampoule volume to the GaAs surface area. Proximity protection of the surfaces is necessary to prevent the generation of a Ga vacancy-related hole trap HCZ (Ev+0.57 eV).