Electron Beam Induced Current Experiments Research Articles

Experimental and theoretical EBIC analysis for grain boundary and CdS/Cu (In, Ga)Se2 heterointerface in Cu (In, Ga)Se2 solar cells

AbstractThe behavior of carriers for an electron‐beam‐induced current (EBIC) evaluation is experimentally and theoretically analyzed for the polycrystalline Cu (In, Ga)Se2 (CIGS) thin‐film solar cells. The experimental EBIC signal in‐depth profiles of the CIGS layers show four features: peaks at grain boundaries (GBs), narrowed peaks near the surface, shifted peaks near the surface, and double peaks at the GBs and surface. Operating principles for CIGS solar cell devices under electron–hole pair excitation are systematically revealed utilizing a two‐dimensional theoretical EBIC simulation: (i) The EBIC peaks at the GBs are caused by a high hole barrier, including a valence band offset (ΔEV) and a band bending at the GBs; (ii) narrowing of EBIC peaks near the surface is caused by high defect densities at the GBs, grading of a conduction band minimum, and grading of acceptor concentration (Na) along with the CIGS depth direction; (iii) a shift in EBIC peaks near the surface is caused by high donor‐like defect densities at the surface; (iv) double peaks at the GBs and surface are caused by a thick surface layer (SL) with the ΔEV and the hole barrier at GBs. These findings of the EBIC analysis will help to comprehensively understand the complex polycrystalline CIGS carrier transport mechanism in realizing the highest‐efficiency CIGS solar cells.

High-resolution planar electron beam induced current in bulk diodes using high-energy electrons

Understanding the impact of high-energy electron radiation on device characteristics remains critical for the expanding use of semiconductor electronics in space-borne applications and other radiation harsh environments. Here, we report on in situ measurements of high-energy electron radiation effects on the hole diffusion length in low threading dislocation density homoepitaxial bulk n-GaN Schottky diodes using electron beam induced current (EBIC) in high-voltage scanning electron microscopy mode. Despite the large interaction volume in this system, quantitative EBIC imaging is possible due to the sustained collimation of the incident electron beam. This approach enables direct measurement of electron radiation effects without having to thin the specimen. Using a combination of experimental EBIC measurements and Monte Carlo simulations of electron trajectories, we determine a hole diffusion length of 264 ± 11 nm for n-GaN. Irradiation with 200 kV electron beam with an accumulated dose of 24 × 1016 electrons cm−2 led to an approximate 35% decrease in the minority carrier diffusion length.

Depletion region surface effects in electron beam induced current measurements.

Electron beam induced current (EBIC) is a powerful characterization technique which offers the high spatial resolution needed to study polycrystalline solar cells. Current models of EBIC assume that excitations in the p-n junction depletion region result in perfect charge collection efficiency. However we find that in CdTe and Si samples prepared by focused ion beam (FIB) milling, there is a reduced and nonuniform EBIC lineshape for excitations in the depletion region. Motivated by this, we present a model of the EBIC response for excitations in the depletion region which includes the effects of surface recombination from both charge-neutral and charged surfaces. For neutral surfaces we present a simple analytical formula which describes the numerical data well, while the charged surface response depends qualitatively on the location of the surface Fermi level relative to the bulk Fermi level. We find the experimental data on FIB-prepared Si solar cells is most consistent with a charged surface, and discuss the implications for EBIC experiments on polycrystalline materials.

Dislocation gliding and cross-hatch morphology formation in AIII-BV epitaxial heterostructures

An approach for understanding the origin of cross-hatch pattern (CHP) on the surface of lattice mismatched GaMnAs/InGaAs samples grown on GaAs (001) substrates is developed. It is argued that the motion of threading dislocations in the {111} slip planes during the relaxation of InGaAs buffer layer is more complicated process and its features are similar to the ones of dislocation half-loops gliding in plastically deformed crystals. The heterostructures were characterized by atomic force microscopy and electron beam induced current (EBIC). Detailed EBIC experiments revealed contrast features, which cannot be accounted for by the electrical activity of misfit dislocations at the buffer/substrate interface. We attribute these features to specific extended defects (EDs) generated by moving threading dislocations in the partially relaxed InGaAs layers. We believe that the core topology, surface reconstruction, and elastic strains from these EDs accommodated in slip planes play an important role in the CHP formation. The study of such electrically active EDs will allow further understanding of degradation and changes in characteristics of quantum devices based on strained heterostructures.

Low temperature cathodoluminecence and electron beam induced current studies of single GaN nanowires

Single crystalline GaN nanowires, with 100 nm typical diameters, were grown by chemical vapor deposition method, using Pt catalyst, and characterized by cathodoluminescence and electron beam induced current (EBIC) measurements at 5 K. The near band edge emission was found to be asymmetric and broad, with full width half maximum of around 150 meV, peaking at 3.55 eV, well above the GaN bulk band gap. This blueshift was ascribed to band filling effect resulting from unintentional n-type doping in the range 1019–1020 cm−3. Despite of this heavy doping, EBIC experiments showed that minority carriers can diffuse over 0.2 μm.

On the use of Monte Carlo modeling in the mathematical analysis of scanning electron microscopy–electron beam induced current data

Electron beam induced current (EBIC) is often used to evaluate minority carrier properties in semiconductors. Various mathematical models have been proposed; the most advanced is that of Bonard and Ganière [J. Appl. Phys. 79, 6987 (1996)]. However, in order to apply this model to EBIC experiments, the lateral and depth distributions of the electron-hole-pair generation in the sample must be known. This letter presents a straightforward method based on Monte Carlo simulation of the electron beam interaction with the sample to evaluate these distribution parameters. A quantitative experimental example from a GaN-based light emitting diode is presented to test the proposed method.

Three-dimensional investigations of electrical barriers using electron beam induced current measurements

3D-investigations of electrically active interfaces in ZnO varistor materials have been performed using electron beam induced current (EBIC) contrast in the scanning electron microscope in combination with the focused ion beam workstation (FIB). The EBIC measurements were performed with spatial resolution better than 100 nm and the depth dependence of the signal was studied by variation of the acceleration voltage. The FIB was used for characterization of the subsurface geometry of electrically active grain boundaries. Experimental EBIC profiles, obtained by scanning the electron beam across grain boundaries, were compared with computer simulations using three different models for the electron–hole generation function. The EBIC contrast has been found to depend strongly on the geometric properties, i.e., the tilt of the grain boundary, and on asymmetries in the depletion region at the boundary. Calculations of the EBIC contrast taking these two effects into account show good agreement with experimental data. The “hot electron effect” close to the electrical breakdown voltage has been experimentally observed. The possibility to study asymmetry in the depletion region and the influence of the applied bias on the electrical properties of individual interfaces using the EBIC technique has been demonstrated.

Perturbation approach of the injection effect on the electron-beam-induced current efficiency in lightly doped semiconductors

Abstract The influence of the injection on the electron-beam-induced current (EBIC) collection efficiency η has been examined through a perturbation approach, assuming no recombination in the depletion zone but with a surface recombination speed which is independent from injection. This calculation investigates the hole (electron) concentration spatial distribution in the depletion zone as well as the consequences of the electric field variation in that zone, caused by the injection, on the EBIC efficiency of the lightly doped semiconductors. The results show that such an effect is far from weak. The EBIC experiments performed on Au-n-type Si Schottky diodes quantitatively agree with our model results. Finally, we come to the conclusion that the effect of the electron field variation on the collection efficiency is as important as the modification in the carrier recombination at higher levels.

Perturbation approach of the injection effect on the electron-beam-induced current efficiency in lightly doped semiconductors

Abstract The influence of the injection on the electron-beam-induced current (EBIC) collection efficiency η has been examined through a perturbation approach, assuming no recombination in the depletion zone but with a surface recombination speed which is independent from injection. This calculation investigates the hole (electron) concentration spatial distribution in the depletion zone as well as the consequences of the electric field variation in that zone, caused by the injection, on the EBIC efficiency of the lightly doped semiconductors. The results show that such an effect is far from weak. The EBIC experiments performed on Au-n-type Si Schottky diodes quantitatively agree with our model results. Finally, we come to the conclusion that the effect of the electron field variation on the collection efficiency is as important as the modification in the carrier recombination at higher levels.

Multiple quantum well GaAs/AlGaAs solar cells: transport and recombination properties by means of EBIC and cathodoluminescence

Multiple quantum well (MQW) p-i-n heterostructures are a new alternative to increase the quantum efficiency of solar cell devices. In such structures, the QW carrier capture, carrier escape and radiative recombinations are the phenomena governing the efficiency of the p-i(MQW)-n solar cell. In this contribution, in spite of the photon-induced current mode of work of such device, an electron beam-induced-current (EBIC) study allowing a very localized carrier excitation is reported. The EBIC measurements are shown to be able to estimate with high accuracy the QWs capture–escape and radiative lifetimes. The latter are determined as a function of the carrier injection level fitting experimental EBIC profiles to simulated ones. Values around 10 −10 and 10 −7 s respectively are obtained.

Quantitative interpretation of electron-beam-induced current grain boundary contrast profiles with application to silicon

An improved method is described for extracting material parameters from an experimental electron-beam-induced current (EBIC) contrast profile across a vertical grain boundary by directly fitting an analytical expression. This allows the least-squares values of the grain boundary recombination velocity and the diffusion length in each grain to be determined without the need for the reduction of the experimental profile to a few integral parameters, as is required in a previously reported method. Greater accuracy of the extracted values is expected since none of the information contained in the experimental contrast data is discarded and a less extensive spatial range of measured data is required than in the commonly used method. Different models of the carrier generation volume are used in the fitting and the effect of the choice of generation model on extracted values is investigated. In common with other EBIC approaches, this method is insensitive to changes in the diffusion length when the collection efficiency is high and diffusion lengths may not be reliably established in those cases.

Scanning electron microscope studies of AlGaN films grown by organometallic vapor phase epitaxy

Microcathodoluminescence (MCL) and electron beam induced current (EBIC) studies were carried out on AlGaN films grown by organometallic vapor phase epitaxy (OMVPE) on sapphire and SiC. It is shown that both non-uniformities of distribution of deep recombination centers and of electron concentration can exist in AlGaN layers and in some cases these non-uniformities can be associated with the presence of certain types of defects. For example, regions adjacent to pores in AlGaN films are often depleted of deep centers one of which is the center responsible for yellow luminescence in AlGaN. Electron concentration is enhanced in hexagonal hillocks commonly observed for unoptimized growth conditions of AlGaN. There were also found non-uniformities in concentration of deep centers or in electron concentration that could not be related to particular morphological features or structural defects. Diffusion lengths measurements for various AlGaN films are also reported. The diffusion lengths are shown to be in the 0.3–0.5 μm range for most of the samples. In some of the studied AlGaN films it was found necessary to introduce two diffusion length values to describe the experimental EBIC distribution profiles. The longer diffusion length is found to increase in value for lower donor densities. Minority carriers lifetimes are found to be longer in AlGaN compared to GaN films.

Structural and electrical investigations of silicon wafer bonding interfaces

The structure and the electrical properties of Si/Si interfaces obtained by wafer bonding are studied by transmission electron microscopy (TEM), electron-beam-induced current (EBIC) and spreading resistance (SR). Boron-doped Czochralsky-silicon (Cz-Si) (001) wafers and float zone-silicon (FZ-Si) (001) wafers are bonded after hydrophobic cleaning of the surfaces and annealed at 1200 °C for 2 h. The TEM experiments show that these interfaces are made of two dislocation networks: a grid of screw dislocations accommodating the misorientation between the two wafers and a set of 60 ° dislocations compensating a tilt component related to the vicinal surfaces. These dislocations are all located at the interface and no extended defects propagate in the Si wafers. The EBIC experiments show a large electrical activity of the Cz interfaces; SR profiles show a variation of carriers concentration near the interface. Large density of oxide precipitates located at the interface are seen for Cz-Si wafers. EBIC observations performed on FZ wafer show a weak electrical activity which seems to be related to a low content of oxygen in the interface.

Evaluation of diffusion length from a planar-collector-geometry electron-beam-induced current profile

The subject of this article is the determination of the minority carrier diffusion length L from a planar-collector-geometry electron-beam-induced current (EBIC) profile. Among extant techniques based on the analysis of a ln[IEBIC(x1)xα1] vs x1 plot, (x1 is beam-to-collector distance, and α is a constant), we find two major problems that demand immediate attention. First, the most widely used technique is found to be based on an invalid assumption, which results in the intrusion of large systematic errors into the values of L and surface recombination velocity sT. Therefore, this technique in its present form is no longer usable. Second, all these techniques are asymptotic (x1≳2L), a matter of great concern to experimentalists because large x1 means small signal-to-noise ratio and the proximity of adjacent active regions. We devise a nonasymptotic technique, based also on the analysis of a ln [IEBIC(x1)xα1] vs x1 plot, to evaluate L from a region close to the collector, as close as one-half, but no farther than two diffusion lengths away. This technique is applied to four separate sets of experimental EBIC profile data derived from three independent publications. In each case, L evaluated is either the same or nearly the same as the one obtained by curve fitting. We also show how this technique can be used to simultaneously evaluate L and η (=sTL/D,D=diffusion coefficient).

Electron Beam Induced Current Investigations of Transition Metal Impurities at Extended Defects in Silicon

The electron beam induced current (EBIC) mode of a scanning electron microscopy is a useful technique for studying gettering of impurities to extended defects, its high sensitivity allowing very low impurity concentrations to be studied. Extended defects, when studied by EBIC, normally exhibit one of two different kinds of carrier recombination behavior. In the most common case this is accurately described by the Wilshaw model in which the recombination mechanism is charge controlled. Analyzed in terms of this physical model, quantitative EBIC experiments indicate that the small amount of recombination associated with deformation induced dislocations produced at 650°C or above and at stacking faults is due only to residual impurities, whereas a state intrinsic to the dislocation is produced by deformation at 420°C. There also exists a less common second type of recombination behavior, often associated with nickel contaminates, which can dominate at low temperatures. This type of recombination is less well understood and cannot be modeled by simple Shockley Read Hall recombination statistics.

A simple specimen holder for EBIC imaging on the Hitachi S800.

Electron beam induced current (EBIC) technique has become widely used in the characterization of semiconductor materials and microelectronic devices. EBIC signals are measured in the form of a short circuit current, and this requires that both the front and the back surfaces of the specimen be electrically connected. The authors have developed a simple specimen holder to implement EBIC experiments on SEMs such as the Hitachi S800 which does not possess isolated electrical lead-outs. In this note the design of the specimen holder and illustrate the results by EBIC observations of polycrystalline silicon solar cells is discussed.

Minority carrier diffusion length and edge surface-recombination velocity in InP

A scanning electron microscope was used to obtain the electron-beam-induced current (EBIC) profiles in InP specimens containing a Schottky barrier perpendicular to the scanned (edge) surface. An independent technique was used to measure the edge surface-recombination velocity (Vs). These values were used in a fit of the experimental EBIC data with a theoretical expression for normalized EBIC [C. Donolato, Solid State Electron. 25, 1077 (1982)] to obtain the electron (minority carrier) diffusion length (Ln).

Characterization of the heavily doped emitter and junction regions of silicon solar cells using an electron beam

The electron-beam-induced-current (EBIC) technique is applied to the investigation of heavily doped emitter and junction regions of silicon solar cells via the beam specimen configuration in which the junction is parallel to the surface and the beam is perpendicular to it. It is demonstrated that this technique is very well suited to the investigation of the emitter and junction regions. Even though the experimental EBIC data are collected under three-dimensional conditions, it is shown analytically with the help of two numerical examples that the solutions obtained with one-dimensional numerical modeling are adequate. EBIC data for bare and oxide-covered emitter surfaces are measured and compared with theory. Good agreement is obtained when a cell quality factor of 0.89 is assumed, thus suggesting that ∼11% of the collected carriers recombined in the cell depletion region. This result is similar to Possin’s finding that there was evidence for ∼10% of recombination in the junctions of some of his silicon diodes with very shallow junctions [J. Appl. Phys. 50, 4033 (1979)]. It appears that further improvement of the solar cell conversion efficiency is possible if we can consistently make cells with an unity cell quality factor. The improvement in collection efficiency when an emitter surface is covered with an ∼100-A SiO2 film varies with beam energy. For a cell with a junction depth of 0.35 μm the improvement is ∼54% at 2 keV.

Evaluation of diffusion length and surface-recombination velocity from a planar-collector-geometry electron-beam-induced current scan

For performing electron-beam-induced current (EBIC) measurements on sufficiently large samples, the use of a ‘‘planar-collector geometry’’ (i.e., with the collector covering part of the irradiated surface itself) is very attractive. However, the pertinent theoretical EBIC curves for finite surface-recombination velocities s have so far been lacking. This paper presents the complete theoretical expressions for arbitrary values of s and diffusion length L. Simple asymptotic solutions are given for point- and finite-size generation sources. Easy methods are developed to facilitate the application of these solutions in the practical evaluation of L and s from experimental EBIC curves. These methods are applied to experimental data available through the literature.

Demonstration of excitation-dependent grain-boundary recombination velocity in polycrystalline silicon

The dependence of grain-boundary recombination velocity on the excitation (excess carrier density) at the grain boundary is experimentally demonstrated using electron-beam-induced-current (EBIC) measurements. To facilitate quantitative interpretation of the EBIC measurements, the underlying carrier transport problem in the adjacent grains, subject to the nonlinear boundary condition at the grain boundary, is solved using computer-aided numerical analysis. By comparing the theoretical and experimental EBIC responses, values for the surface-state density at unpassivated and passivated grain boundaries and for the diffusion length in the grains in Wacker (cast) polycrystalline silicon are derived.