Strong Recombination Centers Research Articles

Fundamental Interactions of Fe in Silicon: First-Principles Theory

Interstitial iron and iron-acceptor pairs are well studied but undesirable defects in Si as they are strong recombination centers which resist hydrogen passivation. Thermal anneals often result in the precipitation of Fe. Relatively little information is available about the interactions between Fe and native defects or common impurities in Si. We present the results of first-principles calculations of Fe interactions with native defects (vacancy, self-interstitial) and common impurities such as C, O, H, or Fe. The goal is to understand the fundamental chemistry of Fe in Si, identify and characterize the type of complexes that occur. We predict the configurations, charge and spin states, binding and activation energies, and estimate the position of gap levels. The possibility of passivation is discussed.

Current–Voltage Characteristics of p+–n SiC Junctions Fabricated by Low-Temperature Liquid Phase Epitaxy

p+-Layers containing Al at about 1 at. % were grown on an n-type substrate in Al–Si solution reacted with propane gas at 950 °C, and cleaved into prototype diodes. The current–voltage (I–V) characteristics showed a rectification behavior of a p–n junction formed between the grown layer and the substrate. It was implied that many defects acting as strong recombination centers exist over the entire grown p+ layer. Contamination by oxygen and nitrogen was also observed in the grown layer.

Carrier loss channels for non-collected carrier in InAsxP1−x∕InP multiquantum well solar cells

We present here the direct observation of majority carrier (electron) defects generation from chemical-beam epitaxy grown InAsxP1−x∕InP multiquantum-well solar cell structures by deep level transient spectroscopy. Four dominant majority carrier levels E1, E2, E3, and E4 at energy position EC−0.17eV, EC−0.25eV, EC−0.57eV, and EC−0.75eV below the conduction-band edge have been observed. Detailed analysis of double-correlation deep level transient spectroscopy (DDLTS) measurements shows that traps E1, E2, E2, and E4 are located at the well-barrier interfacial zone or/and extended over the whole region occupied by the quantum wells. The midgap center E3 has large capture cross sections and is found to exhibit significant dependence of the thermal emission rates on electric field, from DDLTS measurements. These characteristics indicate that the electron trap E3 at EC−0.57eV acts as a strong recombination center and could be responsible for the main carrier loss channel for non-collected carrier in the device.

Texture and electronic activity of grain boundaries in Cu(In,Ga)Se2 thin films

The influence of different film textures on the electronic properties of polycrystalline Cu(In,Ga)Se2 absorbers is studied by measuring the laterally resolved optoelectronic properties of differently textured Cu(In,Ga)Se2 films with Kelvin probe force microscopy and cathodoluminescence. The grain boundaries in (112)- and (220/204)-textured films behave differently. The work-function profile measured with the Kelvin probe across a grain boundary in (112)-textured films shows a dip indicating positive charges at the grain boundaries. In panchromatic cathodoluminescence mappings in a transmission electron microscope, such grain boundaries appear dark, i.e. the strongly reduced luminescence indicates that the grain boundaries represent strong non-radiative recombination centers. In contrast, grain boundaries in (220/204)-textured films give rise to a dip or a step in the work function indicating slightly negative charge or neutrality. Cathodoluminescence is reduced at such grain boundaries, but less dramatically than in the (112)-textured case. However, when Na is present in the (220/204)-textured films, the grain boundaries are almost invisible in cathodoluminescence mappings. This strong passivating action of Na occurs only in the (220/204)-textured films, due to a particular grain-boundary population. In (112)-textured films and films without pronounced texture, this passivation effect is much less noticeable.

Characteristics of the interaction of Cu-rich precipitates with irradiation-produced defects in α-Fe

The interaction between copper-rich precipitates in α-iron and either vacancies or self-interstitial atoms and their clusters is studied by atomic-scale modelling. Results are compared with predictions of elasticity theory and interpreted in terms of size misfit of precipitates and defects, and the modulus and cohesive energy differences between iron and copper. Interstitial defects are repelled by precipitates at large distance but, like vacancies, attracted at small distance. Hence, copper precipitates in iron can be sinks for both vacancy and interstitial defects, and can act as strong recombination centres under irradiation conditions. This leads to a tentative explanation for the mixed Cu–Fe structure of precipitates and the absence of precipitate growth under neutron irradiation conditions. More generally, both vacancy and interstitial defects may be strongly bound to precipitates with weaker cohesion than the matrix.

Prospects of Laser Operation in Erbium Doped Silicon

AbstractProspects of laser operation in erbium doped silicon has been analyzed by a Shockley-Read-Hall (SRH) model. Erbium atoms have been considered to be introducing strong recombination centers in the silicon lattice. Electron-hole recombination at these sites were considered to be the source of erbium excitation. A two level system was considered for calculation of optical gain and the laser threshold. For a laser cavity of 300 μm with mirror reflectivities of 90%, and an optimistic absorption coefficient of 5 cm-1, a population inversion of 1.4×1018/cm3 was estimated as the threshold value. Achievement of the lasing condition was found feasible, but only for certain conditions of erbium activation. Effects of nonradiative deexcitation routes have been analyzed. On the assumption of 1019/cm3 of active erbium sites, linear increase of optical power in the laser cavity has been estimated for injected carrier densities above 1018/cm3.

Carrier Trapping and Recombination at Point Defects and Dislocations in MOCVD n-GaN

We present the results of an approach that allows the analysis of grown-in defects in n-GaN grown by metal organic vapor phase epitaxy by relating the presence of various structural defects to the concentration and properties of distinct deep levels. Transmission electron microscopy and electron beam induced current (EBIC) microscopy together with post-growth hydrogenation are used to determine the threading dislocation density (TDD) and observe their local electrical activity. In conjunction, deep level optical and transient spectroscopies (DLOS and DLTS, respectively) are used to detect deep levels, determine their concentration and analyze their carrier trapping kinetics for these films. The comparison of EBIC analysis with trap spectra prior and after hydrogenation establishes a strong correlation between two specific levels, at Ec — Et = 0.58 and 1.35 eV, and recombination centers distributed in the field of the GaN films. Further, EBIC analysis shows that, independent of hydrogenation, TDs behave as strong recombination centers, and indicates that there must be a deep level associated with these regions. A level observed at Ec — 2.64/Ev + 0.87 eV is a good candidate to account for the electrical activity of the TDs because it captures both electrons and holes, which is characteristic of recombination centers. This is supported by DLTS analysis of the trapping kinetics for this level that reveals a behavior characteristic of a linear arrangement of point defects likely found along the TDs.

MBE/MEE growth and characterization of C 60-doped GaAs

For the first time, epitaxial growth by MBE/MEE (migration enhanced epitaxy) and the characterization of C 60 uniformly doped GaAs, C 60 δ-doped GaAs and C 60 δ-doped AlGaAs/GaAs superlattices are reported. Characterizations by TEM, SIMS, and other morphological or optoelectrical techniques show that the C 60-doping efficiency of MEE is much higher than that of MBE, and C 60 may introduce strong recombination centers. This possibly makes the materials good candidates for the application of fast response optical detector.

Rich chemistry of copper in crystalline silicon

The interstitial copper ion $({\mathrm{Cu}}_{i}^{+})$ is a very fast-diffusing impurity in Si. While the isolated interstitial is a shallow donor, it reacts with impurities and defects and these reactions affect the electrical properties of the material. Copper passivates shallow acceptors, forms pairs with various impurities, including itself, and precipitates at defects. Some Cu precipitates are strong electron-hole recombination centers. In this paper, interstitial and substitutional copper, copper-acceptor pairs, and the precipitation of copper at a model nanodefect, the ring hexavacancy, are studied in clusters at the ab initio Hartree-Fock level. The chemistry of copper in Si is not as simple as commonly believed, even in the case of ${\mathrm{Cu}}_{i}^{+}.$ The interstitial ion is not in the ${3d}^{10}{4sp}^{0}$ configuration as often assumed, but transfers some electrons into the $4sp$ shell, which ultimately is responsible for the various covalent interactions between copper and its host crystal. In addition to energy-optimized configurations and binding energies, a number of details of the chemical interactions shed light on the behavior of copper in silicon.

Hydrogen passivation of newly developed EMC-multi-crystalline silicon

Continuous casting with an electromagnetic cold crucible is a promising way of producing multi-crystalline (mc) silicon on an industrial basis for solar cell production. Casting equipment, capable of producing ingots of 130×130 mm 2 cross-section and 600 mm length has been developed and installed. Thorough characterisation of the produced material revealed a relatively high defect density (grain boundaries, dislocations) and small grain sizes of 1–4 mm in diameter. Although the effective minority carrier diffusion length is high on as-cut wafers (>150 μm) it decreases during solar cell processing to values of 50 μm. This can be attributed to standard high temperature processing steps that lead to redistribution and agglomeration of residual impurities (e.g. metals, carbon) at extended crystallographic defects which then act as strong recombination centres. In order to passivate these recombination centres, a PECVD SiN x-layer is deposited which acts as a source of hydrogen and also as an anti-reflective coating. During the firing of the screen-printed metal contacts through the SiN x-layer, atomic hydrogen is released from this layer and diffuses into the bulk of the wafers where it saturates dangling bonds and passivates impurities at crystal defects. After this treatment the minority carrier diffusion length can be restored to values of around 100 μm on finished solar cells.

Substrate free GaAs photovoltaic cells on Pd-coated silicon with a 20% AM1.5 efficiency

Thin-film epitaxial 2/spl times/2.5 mm/sup 2/ GaAs crack free solar cells grown with metal organic vapor phase epitaxy (MOVPE) have been successfully lifted-off from their original GaAs substrate using the epitaxial lift-off (ELO) technique and Van der Waals bonded on Pd-coated silicon substrates without degrading the electrical, optical and structural properties of the devices. A 20% AM1.5 energy conversion efficiency was measured for an anti-reflection coated device, with a fill factor of 0.79, a short-circuit current density of 21.84 mA/cm/sup 2/ with respect to the total area and a V/sub oc/ value of 957 mV at 25/spl deg/C. Electron beam induced current (EBIC) and cathodoluminescence (CL) measurements at room temperature were performed on the devices to study their structural properties. The EBIC signal image evidenced a very good homogeneous material with an EBIC contrast (C=(I/sub max/-I)/I/sub max/) below 1%. The variation of the penetration depth of the electron beam for a detection of the different layers of the cell gave nearly the same EBIC contrast. Only a low density of point like defects, that we assume to be structural defects, was evidenced. The CL image signal in comparison to the EBIC image didn't give any additional results for the existence of strong nonradiative recombination centers. Cross-sectional transmission electron measurements (TEM) of the GaAs-Pd interface showed a very intimate and homogeneous bonding between the grafted layer and its host substrate.

Effects of high energy radiation on group III–V compound semiconductors

The determination of the properties of compound semiconductors exposed to radiation from the environment is relevant because this exposure triggers optically induced degradation mechanisms which introduce strong non-radiative recombination centers. Two different III–V compound materials were studied: indium phosphide and indium gallium arsenide, both with interesting properties and well suited to optoelectronic applications. A highly doped InP:Sn (3 × 10 18 cm −3) substrate was exposed in different experiments to high energy electrons (3.8 MeV) and γ-rays (0–3.8 MeV) and annealed at room temperature and 473 K. Photoluminescence (PL) analyses showed a significant effect of radiation as well as of heat treatments on InP properties. Thick (3 μm) In 0.4Ga 0.6As epitaxial layers grown on a multistage strain-relief buffer system produced on GaAs substrates were exposed to the same irradiation doses employed for InP. It was expected these layers would not perform well after irradiation since they were grown on mismatched substrates. Nevertheless PL measurements indicated that In x Ga 1−x As was not affected by the γ-ray irradiation doses employed in this work, while it was quite sensitive to electron irradiation although almost complete recovery was possible after heat treatment.

Photostimulated gettering of deep band-gap impurities from semiconductors by resonance excitation: Fe from Cd0.98Fe0.02Se.

Impurities that lie deep in the band gap of a semiconductor (deep impurities) behave as a strong recombination center as shown by the Schockley-Read mechanism for surface recombination [A. Rothwarf and K. W. Boer, Prog. Solid State Chem. 10, 71 (1975)]. Thus, removing such impurities from the semiconductor is imperative for the optimal operation of photonic devices and solar cells, in particular. In the following work a method is proposed to remove such impurities. In this process resonance excitation of electrons (holes) from the impurity level into the conduction (valence) band leads to ionization of the impurity, which in turn leads to fast outdiffusion of the impurity and its dissolution by a chemical (electrochemical) reaction at the semiconductor surface. As a demonstration of this principle Fe, which is a deep donor in CdSe, is selectively removed from a ${\mathrm{Cd}}_{0.98}$${\mathrm{Fe}}_{0.02}$Se crystal. A giant diffusion constant (D=${10}^{\mathrm{\ensuremath{-}}9}$ ${\mathrm{cm}}^{2}$/sec) is measured for Fe in this experiment. The application of this process for the control of impurity profiles in various semiconductor devices is briefly discussed.

Defect Mechanisms in Degradation of Long Wavelength (1.30–1.55 Micron) Laser Diodes

ABSTRACTOptical degradation of long wavelength (1.30–1.55 micron) laser diodes during normal operation or accelerated aging test caused by lattice structural deterioration of the active region materials and mirror facet damage were investigated in detail. Extrinsic dislocation loops of 1/2<100>{010) types were observed in gradually degraded channeled-substrate-buried-heterostructure (CSBH) lasers. These dislocation loops, originated at the sidewall interfaces outside the active region, grew into the active region in the direction of minority carrier injection. A great enhancement of the loops' growth rate was observed after they entered the active region, indicating a nonradiative recombination enhanced defect reaction under the strong optical field. Furthermore, the <100> oriented extrinsic dislocation loops were confirmed to be dark-line-defects (DLDs). On the other hand, strong nonradiative recombination centers, created by mirror facet damage, or pre-existed internally inside the cavity, resulted localized melting on the {111} planes. The propagation of the localized melt-patch along the laser beam direction created a wormlike defect along the laser cavity, which degraded the laser device catastrophically. Various types of grown-in defects for CSBH and etched-mesa-buried-heterostructure (EMBH) laser devices are described and their effects on the device performance are demonstrated.

Characterization of RF Sputter‐Deposited Ti‐W Schottky Barrier Diodes in Boron‐Doped Silicon

The properties of defects introduced during RF sputter deposition of Ti‐W contacts on p‐Si were investigated using I‐V, deep level transient spectroscopy (DLTS), and electron‐beam‐induced current (EBIC) techniques combined with isochronal and isothermal annealing. It was found that the barrier height decreased from 0.56V before annealing to 0.5V after isochronal annealing. DLTS measurements indicate the presence of several sputter‐induced defect states. Annealing removed some of the defects, but second generation defects were still present after annealing at 500°C, now distributed deeper into the substrate. EBIC results showed that the diffusion length decreased with increasing annealing temperature. DLTS and EBIC results suggest that a hole trap at is a strong recombination center. This hole trap is very likely the same defect as the one identified as the K center .

First and second order twin boundaries in edge defined film growth silicon ribbon

The equilibrium defect structure of edge defined film-fed growth ribbons consists of linear twins and twin bundles lying parallel to the growth direction. These twins show similar etching behavior in spite of the fact that their electrical activity i.e. efficiency as recombination centers, can vary greatly. Correlated electron beam induced current microscopy and high voltage transmission electron microscopy show (i) that some of the linear boundaries consist of alternating sections of coherent first order twins and incoherent second order twins of the {111}/{115} type and (ii) that the first order twins are not electrically active, whereas the second order twins act as strong recombination centers. Structural models of the {111}/{115} interface indicate a large fraction of dangling bonds. Macroscopically, the alternating sections of these twin boundaries form single straight boundaries running parallel to the growth direction of the ribbon. In etching experiments the alternating sections of boundaries described above are therefore difficult to differentiate from coherent first order twins.

Reliability of high radiance InGaAsP/InP LED́s operating in the 1.2-1.3 µm wavelength

The reliability of high radiance InGaAsP/InP DH LED's operating in the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.2-1.3 \mu</tex> m wavelength and the defect structures observed in this quaternary alloy have been presented. Threading dislocations and misfit dislocations do not act as strong nonradiative recombination centers, in contrast with the case in GaAs or GaAlAs optical devices. Dark-spot defects (DSD's) were sometimes generated in the emitting area during aging at elevated temperatures. These defects were analyzed microscopically using a transmission electron microscope and were identified as precipitates. To investigate the homogeneous degradation, accelerated aging at the ambient temperatures of 20, 60, 120, 170, 200, and 230°C has been carried out for over 15 000 h at the current density of 8 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> using LED's without dark structures. The degradation rates were statistically calculated by assuming the normal distribution. The mean values of degradation rates and the values of standard deviation were determined at the temperatures above 170°C. The activation energy of homogeneous degradation was determined to be 1.0 eV and the extrapolated half-life in excess of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> h was estimated at the ambient temperature of 60°C.

Defects in natural diamonds: Recent observations by new methods

The observations presented concern both grown-in defects and those which develop after growth. They will deal principally with impurity platelets precipitated on {100} planes and with dislocations. The former defects are a notable feature in the most common variety of natural diamond (Type Ia.) The experimental techniques used are of the direct-imaging, topographic type and include ultraviolet absorption topography, X-ray topography, cathodoluminescence topography (both with visible and near infrared wavelengths) and transmission electron microscopy at 100 kV, 120 kV and 1 MeV. Spectrography and colour photography are informative in cathodoluminescence studies. The cathodolumisnecent images of individual dislocation lines can be photographed, whether the dislocations be grown-in or arise from plastic deformation post growth, provided that they lie in matrices sufficiently free from point defects acting as strong electron-hole recombination centres. In practice this requirement implies a low concentration of nitrogen impurity much of which is distributed in “small clusters” which give rise to the bright blue cathodoluminescent emission characteristic of Type Ia diamonds. The luminescence from individual dislocation lines may be either a violet-coloured emission, or an emission which is dominantly a system (known as “H3”) having a zero-phonon line at 2.46 eV and stretching from green to orange in the visible spectrum. The violet dislocation emission is strongly polarised with E vector parallel to the dislocation line. New observations on platelets have been mainly concerned with those attaining diameters of 1 μm or more which can be observed individually by X-ray topography and cathodoluminescence topography. However, evidence from electron and X-ray diffraction contrast indicates that these “giant platelets” produce the same matrix lattice displacements as the more familiar submicrometre-sized platelets in Type Ia diamond and may be presumed to have essentially the same structure and composition. Near infrared cathodoluminescence has been recorded from individual large platelets and it has been discovered that this emission is highly polarised with the E vector in the platelet plane. Available evidence on the structure and properties of the platelets weighs against a currently mooted hypothesis that they are composed of interstitial carbon, and does not conflict with an older idea, that they are composed of nitrogen impurity.

A deep center associated with the presence of nitrogen in GaP

The deep−level thermal activation energy spectrum of N−doped liquid−phase−epitaxial (LPE) and liquid−encapsulated−Czochralski (LEC) −grown GaP has been investigated using Schottky barrier thermally stimulated−current (TSC) measurements. We detect a center at Ec−Et=0.42 eV whose presence in the lattice depends on the deliberate addition of N to the material. The concentration of this center varies approximately as (Nd−Na)2 in n−type material, and the value of electron capture cross section for the center (St≈7×10−15 cm2) indicates that it could be a strong recombination center in N−doped p−type GaP. Separate experiments suggest that this center is strongly localized, and that it is not a simple complex involving shallow donors and native defects.

Evidence for a primarily nonradiative Si,O defect in GaP

Experimental evidence is presented to establish the existence of a Si,O defect impurity system in GaP. This defect is shown to be a strong nonradiative recombination center in n-type material. A photoluminescence band near 1.55 eV at room temperature has been associated with the Si,O defect.