Electron Beam Induced Current Images Research Articles

Role of defects in producing negative temperature dependence of breakdown voltage in SiC

Electron beam induced current (EBIC) techniques were employed in order to understand the role of defects on the breakdown characteristics of SiC. EBIC images revealed that certain defects caused enhanced multiplication leading to the catastrophic failures in SiC diodes. The impact ionization coefficients for holes measured at the defective site (αp,eff) were found to be higher than those measured at a nondefective site. Also, αp,eff measured at the defective site was found to increase with increasing temperature in contrast with a defect free diode where αp decreases with increasing temperature, clearly indicating that the defects produce the observed negative temperature coefficient of breakdown voltage in SiC.

EBIC observation of n-GaN grown on sapphire substrates by MOCVD

Si-doped GaN layers were grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD) and characterized by using electron beam-induced current (EBIC) and molten KOH etching. Dark spot densities in EBIC image are be 1.0 and 1.2×108cm−2 depending on the accelerating voltage. The dark spots in EBIC image correspond to recombination centers. When we compare dark spot density with etch pit density revealed molten KOH, this is the same order as the EPD revealed by molten KOH. We confirm there are many dislocations which influence recombination of minority carriers in the layers.

Tunable, Strongly Non parabolic Confinement in a Quasi-One-Dimensional Electron Gas Formed by Epitaxial Regrowth

A new type of quasi-one-dimensional electron gas (Q1DEG) characterised by a tunable, strongly non parabolic confining potential in the lateral direction has been produced, using molecular beam epitaxy to grow a high-mobility heterostructure on a (311)A GaAs substrate selectively etched to expose (100) facets. The electron gas on the (100) facets is confined in one dimension by the two-dimensional hole gases on the (311)A facets, forming a p-n-p structure. Infrared cyclotron resonance (CR), magneto resistance and electron-beam-induced current (EBIC) measurements have been made on these Q1DEGs for various biases (Vh) between the hole and electron gases to investigate its effects on the confinement potential. Two different regimes are present. For Vh>-1.9 V, a strong peak attributable to a confined magnetoplasmon (CPM), together with its higher-frequency harmonics, are observed in the CR spectra. For Vh<-1.9 V, a new mode appears and the CPM no longer fits the experimental data. The anomalous dependence of the resonance frequency on carrier density, together with the EBIC images, provides some understanding of the tunable, non parabolic nature of the confining potential.

Evaluating the Denuded Zone Depth by Measurements of the Recombination Activity of Bulk Defects

AbstractA method for measuring denuded zone depth from lifetime measurements is proposed and compared to observations by microscopy techniques and to Electron Beam Induced Current (EBIC) analyses. The surface photovoltage (SPV) method is chosen for measuring lifetime, as it is found to be extremely sensitive to oxygen precipitation. Samples prepared according to three typical thermal hystories are used as test vehicles.Correlating the results given by the different techniques, this study points out a few relevant features from the point of view of wafer processing. For instance, processes starting with a low temperature treatment are poorly reproducible for what concerns oxygen precipitation and denuding, as shown by SPV data, and do not guarantee a defect-free denuded zone, as seen in EBIC images. Additional high temperature treatments during the process increase the denuded zone depth and improve uniformity.Techniques which rely on a selective etching for defect detection, such as the Secondary Electron method of Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM), are limited by etching sensitivy. On the other hand, the Transmission Electron Microscopy (TEM) statistics is too poor for this sort of observations. Instead, as a technique based on the recombination activity of defects, EBIC has roughly the same sensitivity as SPV. Both these techniques detect the formation of a denuded zone even when other methods fail, and can be considered as complementary tools.

Understanding The Role Of Defects In Limiting The Minority Carrier Lifetime In Sic

AbstractDeep level Transient Spectroscopy (DLTS), Electron Beam Induced Current (EBIC), EBIC Diffusion Length Mapping (EBIC-DLM) and contactless Photoconductive Decay (PCD) were used to characterize both bulk substrates and epitaxially grown Silicon Carbide films. Traps as deep as 0.93 eV were observed via DLTS. These traps may play a role in the persistent photoconductivity effect. EBIC reveals the electrical activity of the well known triangular defects. However, only some of these defects display electrical activity consistent with that of 3C-SiC inclusions, others do not. Additionally, not all defects identified in the EBIC images are observable in the topographic SEM image, possibly indicating a new, yet unidentified defect. EBIC revealed the electrical activity of defects including micro-pipes, dislocations (or possibly growth step edge decoration), surface polish damage, and bulk defects. Diffusion length maps of SiC indicate wide variations in diffusion length on both microscopic and macroscopic scales. EBIC-DLMindicatedepitaxial 4H SiC resulted in diffusion lengths from 0.1 to 3 μm, while bulk values were less than 0.07 μm. PCD measurements indicate tens of nanosecond to microsecond variations in lifetime. Lifetime verses injection level variations are observed and explained on the basis of trap energy. The injection level dependence of lifetime was observed at various nitrogen doping concentrations. Finally, electron beam annealing is found to dramatically improve the minority carrier lifetime in epitaxial SiC.

Images of grain boundaries in polycrystalline silicon solar cells by electron and ion beam induced charge collection

Selected grain boundaries in a polycrystalline solar cell have been imaged by the electron beam induced current (EBIC) technique of the scanning electron micro-scope and the ion beam induced charge (IBIC) method, using a 2.0 MeV focused He + ion beam. The IBIC maps show lower resolution and weaker contrast in comparison to EBIC images, as a result of the larger spot size of the ion beam. However, IBIC allows an analysis of the height of the charge pulses produced by single ions; examples of the spectra thus obtained at different regions of the cell are given, and the relation between spectrum shape and local charge collection properties of the specimen is briefly discussed.

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.

Depletion Characteristics of Direct Schottky Contacts to Quantum Wells Formed by In Situ Selective Electrochemical Process

Depletion properties of direct Schottky/quantum well (QW) contacts formed by the in situ selective electro-chemical process were systematically characterized by the C-V and electron-beam-induced current (EBIC) techniques. The EBIC images clearly showed that the barrier exists at the edge of the QW layer. It was found that the capacitance of the Schottky/QW contact depends linearly on ln {1/(V bi-V)}, where the V bi is built-in voltage, and that the depletion width obtained from the EBIC signal is proportional to the applied voltage. These results demonstrate that well-behaved depletion characteristics of the Schottky/QW systems can be realized.

Microcharacterization of CdTe films deposited by close‐spaced sublimation

AbstractMicrocharacterization of the luminescent, structural, electrical and topographical properties of thin films of close‐spaced sublimation (CSS)‐fabricated CdTe was performed. the film morphology was found to be dependent on the film thickness, deposition conditions and post‐deposition treatment. the complementary use of electron beam‐induced current (EBIC) and cathodoluminescence (CL) analysis in the scanning electron microscope demonstrated large inter‐ and intragrain inhomogeneities in the luminescent and electrical properties of the films. Follow‐on plan‐view examinations with the transmission electron microscope revealed varying densities of structural dejects, such as stacking faults and threading dislocations, which could explain the variations observed in the CL and EBIC images. Consisting primarily of experimental observations and comments, this report is phenomenological in nature.

Low temperature identification of interfacial and bulk defects in Al/SiO/sub 2//Si capacitor structures by electron beam induced current

A low temperature electron beam induced current (EBIC) study using Al/SiO/sub 2//Si capacitors as probes of defects affecting the electrical properties of the bulk Si, SiO/sub 2/ interface and the SiO/sub 2/ layer is presented. The technique's relevance to current research on thin oxides and EBIC image enhancements obtained at reduced temperature are explained. The characteristic EBIC contrast representative of three capacitor bias conditions are reviewed as follows: 1) localized temperature dependent recombination at extended bulk defects for inversion bias, 2) spatial variation of the flat-band voltage due to nonuniform interfacial or oxide charge distributions for weak depletion bias, and 3) electron beam enhancement of SiO/sub 2/ leakage currents at defect sites for accumulation bias. Illustrations of these contrast modes are presented for samples containing buried epitaxial misfit dislocations and oxide interface defects. >

Apparatus for digital electron-beam-induced current imaging

Abstract A new electron-beam-induced current (EBIC) apparatus for studying charge carrier recombination at extended defects in semiconductors is described. It employs computer-automated measurement and control equipment to acquire high resolution beam-induced current images using a conventional scanning electron microscope. The EBIC images typically contain a matrix of 512 × 512 digitally measured points, each points, which is a result of multiple averaging. The images can be displayed on a workstation monitor and image processing using commercially available software packages can be performed. All the components of the system and software, except the data acquisition software routines, are readily available as off-the-shelf items. A method of calibrating the equipment in terms of quantum efficiency by scanning well-characterized solar cells of known material and physical properties was developed. As an example of an application, a modified quantitative analysis method is applied to the problem of precipitates that act as point recombination centers distributed through a Czochralski silicon solar cell. The analysis, based on non-linear curve fitting, results in a characterization of the strength of the charge carrier recombination at a given defect and the depth of the defect below the sample surface.

Use, applications and value of stereo-EBIC imaging

EBIC (electron beam induced current) is a widely used technique for studying the properties of semiconducting materials and devices. However, no attention has been paid so far to its use in connection with stereo-imaging. One reason might be that EBIC contrast depends mainly on physical parameters, not just on geometrical ones, as is rather the case of ordinary SEM and TEM images. Here we show that 3D EBIC images may provide a lot of information not available from EBIC flat images, even in cases where both physical and geometrical parameters contribute to the image contrast. A critical appraisal is then given of the advantages and limitations of the EBIC stereo-method, together with examples of application to crystal defects in p-n junctions and Schottky barriers. The examples include, in particular, measurements of defect depths and the elucidation of the cause of a novel type of mechanism which produces an image contrast inversion. Comparison is finally made between the information gathered from stereo-EBIC and that obtainable from images formed with secondary electrons, specimen current signals and 100 kV transmitted electrons.

Stacking faults under reoxidized nitrided oxides

Processing induced stacking faults below reoxidized nitrided oxides have been observed by electron beam induced current (EBIC) imaging and by secondary electron imaging after preferential etching. Comparative studies of stacking fault lengths with those of dry and nitrided oxides show that the growth rates of stacking faults are anomalously high during reoxidation. A large proportion of these stacking faults show electrical activity in EBIC images. This is reflected in the degradation of effective generation lifetimes as seen in transient capacitance measurements.

Formation of surface inversion layer in F +-implanted n-type silicon

Influences of fluorine ion implantation on the electrical properties of n-type silicon have been investigated by electron beam induced current (EBIC), Hall and metal-oxide-semiconductor (MOS) high frequency capacitance-voltage (HFCV) measurements. A grating mask was used to delineate the implantation region so that the F +-implanted and unirradiated areas were located in the same A1/n-type Si Schottky diode region. EBIC images, obtained with different electron beam energies, normal and parallel to the diode surface, and EBIC collection efficiencies in the implanted and unirradiated areas were recorded. Fitting data of energy dependent EBIC collection efficiency into the theoretical EBIC model of Schottky diode, the thickness of the metal layer, depletion layer width, minority carrier diffusion length in the substrate and EBIC collection efficiency in the deletion region were determined. The minority carrier recombination in the fluorine implanted area was found to be higher than that of the blank Si area under Schottky contact. Outside the Schottky contact, an inversion layer was observed to form at the surface of the implanted area. The structural perfection of the F + implanted area was investigated by cross-sectional transmission electron microscopy (XTEM). The p-type characterization in the surface layer of the F + implanted area was also confirmed by the Hall and HFCV measurements.

EBIC analysis of CuInSe 2 devices

This paper describes the electron-beam-induced-current (EBIC) technique and its application to the characterization of CuInSe 2 thin film solar cells. The collected current generated by a scannig electron microscope electron beam can be used to determine spatial non-uniformities, localized defects, p-n junctions, minority carrier lifetimes and diffusion lengths in solar cells. Spatial EBIC images permit detailed examination to 5000X magnification compared with 50X for optical-beam-induced current (OBIC) images.

Determination of the critical layer thickness of Si1−xGex/Si heterostructures by direct observation of misfit dislocations

The dependence of the critical layer thickness hc on mole fraction x of Si1−xGex/Si heterostructures is determined by direct observations of misfit dislocations by using the electron beam induced current (EBIC) technique and transmission electron microscopy (TEM). The EBIC images of the Si1−xGex/Si interfaces show square-grid patterns running in the 〈110〉 directions. These patterns are identified by TEM observation to be misfit dislocations generated at the Si1−xGex/Si interface. The dependence of hc on x is compared with the results reported by R. People and J. C. Bean [Appl. Phys. Lett. 47, 322 (1985) and 49, 229 (1986)]. The comparison reveals that inconsistency exists between them, especially in the range of x&amp;lt;0.3. This inconsistency is considered to arise from the difference in the techniques adopted to determine the hc’s.

Revealing depth information by processing EBIC images

The conventional use and interpretation of electron beam induced current (EBIC) images in the SEM suffer from several inherent problems. One such problem is the superposition of topographic details, e.g., scratches, dust, contact damage, on the subsurface EBIC signal. Moreover, the use of an accelerating voltage to select depth may not distinguish information from regions close in depth. This exists in depth dependent defect imaging in multi-layer heteroepitaxial structures where two or more defect layers may overlap in the EBIC image.To alleviate these problems, techniques of image processing may be employed. By digitizing and storing several images, for example in the first problem EBIC and secondary electron image and in the second problem EBIC images obtained at different accelerating voltages, it is possible to use image differencing to extract a more selective image that reveals the desired information. In principle, image differencing requires simple subtraction (or division) of the brightness of each pixel in one image from that in the second.

Enhanced electron-beam-induced current contrast of grain boundaries in silicon-on-insulator films

The direct electron-beam-induced current (EBIC) observation of electrical properties of grain boundaries in silicon-on-insulator (SOI) structures has always been hampered by the insulated structure to be measured. In this communication a new structure is proposed with which EBIC images of grain boundaries in as-grown SOI layers can be obtained. Implantation of fluorine in the layer causes an enhanced contrast of these boundaries.

EBIC contrast of defects in cadmium telluride. I. Experiments

Abstract Electron-beam-induced current (EBIC) contrast of freshly introduced dislocations and as-grown defects have been investigated in n-type CdTe (n ∼ 1015–1016cm–3). The carriers were collected by a Schottky diode perpendicular to the electron beam. The width of the space-charge region (SCR) of the diode was of the same order of magnitude as the penetration depth of the electrons of the scanning electron microscope. The contrast c was recorded as a function of the accelerating voltage E 0 (5–30 kV in 5kV steps). Variable recombination at dislocations in the SCR is evident from the observations. The EBIC contrast of dislocations perpendicular to the surface exhibits two kinds of behaviour with E 0, which are interpreted in terms of recombination in both the SCR and the bulk region of the diode. In the configuration studied, dislocations perpendicular to the surface and ‘point-like’ defects can produce not only similar EBIC images—circular dark spots—but also similar c = f(E 0) curves.

The interpretation of EBIC images using Monte Carlo simulations

summaryCharge collection microscopy, usually known by the acronym EBIC (Electron Beam Induced Current) imaging, is a powerful technique for the observation and characterization of semiconductor materials and devices in the scanning electron microscope. Quantitative interpretation of EBIC images is often difficult because of the problem of accurately representing the electron‐beam interaction with the semiconductor. This paper uses a Monte Carlo technique to simulate the electron‐beam interaction, and it is shown that this permits simple analytical point‐source solutions to be generalized to fully represent the experimental situation of an extended, non‐uniform, carrier source. The model is demonstrated by application to EBIC imaging in the Schottky barrier geometry.