Electron Beam Induced Current Profiles Research Articles

Modeling of the electron beam induced current signal in nanowires with an axial p-n junction

A tridimensional mathematical model to calculate the electron beam induced current (EBIC) of an axial p-n nanowire junction is proposed. The effect of the electron beam and junction parameters on the distribution of charge carriers and on the collected EBIC current is reported. We demonstrate that the diffusion of charge carriers within the wire is strongly influenced by the electrical state of its lateral surface which is characterized by a parameter called surface recombination velocity (v r). When the surface recombination is weak (i.e. low v r value), the diffusion of charge carriers occurs in one dimension (1D) along the wire axis, and, in this case, the use of bulk EBIC models to extract the diffusion length (L) of charge carriers is justified. However, when the surface effects are strong (i.e. high v r values), the diffusion happens in three dimensions (3D). In this case, the EBIC profiles depend on v r value and two distinct cases can be defined. If the L is larger than the nanowire radius (r a), the EBIC profiles show a strong dependency with this parameter. This gives evidence that the recombination of generated carriers on the surface through v r is the dominant process. In this situation, a decrease of two orders of magnitude in the EBIC profiles computed with a high and a low v r value is observed in neutral regions of the junction. For the case of L smaller than r a the dependency of the EBIC profiles on the v r is weak, and the prevalent recombination mechanism is the bulk recombination process.

Geometric effects on carrier collection in core–shell nanowire p–n junctions

We report electron-beam-induced current (EBIC) microscopy carried out on free-standing GaAs nanowire core–shell, p–n tunnel junctions. The carrier kinetics in both the n-type core and the p-type shell were determined by analyzing radial EBIC profiles as a function of beam energy. These profiles are highly sensitive to geometric effects such as facet width, shell and core thicknesses, and depletion widths. Combined with Monte Carlo simulations, they permitted measurement of the minority carrier diffusion lengths in the core and the shell, as well as the depletion widths as a function of radial direction. The relatively short minority carrier diffusion length in the core (50 nm), can be attributed to bulk point defects originating from low-temperature core growth (400 ∘C), or to interfacial recombination at traps at the p–n junction.

Analytical and numerical simulation of electron beam induced current profiles in p-n junctions

The electron beam induced current (EBIC) mode of a scanning electron microscope (SEM) is a widely used technique for the quantitative assessment of minority carrier diffusion length and surface recombination. Point source (one-dimensional) and extended source (two-dimensional) analytical models are two widely used approaches to assess this information in geometry where the electron beam (e-beam) is parallel to the p-n junction. In this article, a two-dimensional (2D) analytical model is evaluated and compared with 2D finite element numerical simulations, where the electron beam-solid interaction is modeled using a Monte Carlo simulation coupled with a drift-diffusion solver. The simulations are computed for both low and high level injection conditions. The effect of an e-beam injection level on the shape of EBIC profiles is analyzed to evaluate limitations of the analytical models.

Axial p–n junction and space charge limited current in single GaN nanowire

The electrical characterizations of individual basic GaN nanostructures, such as axial nanowire (NW) p–n junctions, are becoming indispensable and crucial for the fully controlled realization of GaN NW based devices. In this study, electron beam induced current (EBIC) measurements were performed on two single axial GaN p–n junction NWs grown by plasma-assisted molecular beam epitaxy. I–V characteristics revealed that both ohmic and space charge limited current (SCLC) regimes occur in GaN p–n junction NW. Thanks to an improved contact process, both the electric field induced by the p–n junction and the SCLC in the p-part of GaN NW were disclosed and delineated by EBIC signals under different biases. Analyzing the EBIC profiles in the vicinity of the p–n junction under 0 V and reverse bias, we deduced a depletion width in the range of 116–125 nm. Following our previous work, the acceptor Na doping level was estimated to be 2–3 × 1017 at cm−3 assuming a donor level Nd of 2–3 × 1018 at cm−3. The hole diffusion length in n-GaN was determined to be 75 nm for NW #1 and 43 nm for NW #2, demonstrating a low surface recombination velocity at the m-plane facet of n-GaN NW. Under forward bias, EBIC imaging visualized the electric field induced by the SCLC close to p-side contact, in agreement with unusual SCLC previously reported in GaN NWs.

Electron-beam-induced current measurements with applied bias provide insight to locally resolved acceptor concentrations at p-n junctions

Electron-beam-induced current (EBIC) measurements have been employed for the investigation of the local electrical properties existing at various types of electrical junctions during the past decades. In the standard configuration, the device under investigation is analyzed under short-circuit conditions. Further insight into the function of the electrical junction can be obtained when applying a bias voltage. The present work gives insight into how EBIC measurements at applied bias can be conducted at the submicrometer level, at the example of CuInSe2 solar cells. From the EBIC profiles acquired across ZnO/CdS/CuInSe2/Mo stacks exhibiting p-n junctions with different net doping densities in the CuInSe2 layers, values for the width of the space-charge region, w, were extracted. For all net doping densities, these values decreased with increasing applied voltage. Assuming a linear relationship between w2 and the applied voltage, the resulting net doping densities agreed well with the ones obtained by means of capacitance-voltage measurements.

EBIC investigations of dislocations in ELOG GaN

AbstractDislocations in the ELOG n‐GaN films have been studied by the Electron Beam Induced Current (EBIC) method. The effect of dopant concentration and diffusion length values on the dislocation EBIC profile is revealed. It is shown that the width of dislocation image in the EBIC mode decreases with a diffusion length decrease and the dopant concentration increase. In the crystals with the submicron diffusion length the individual dislocations can be separated in the EBIC images up to the dislocation densities higher that 109cm‐2. The radius of dislocation related defect cylinder was found to increase with the dopant concentration decreasing that could be explained under an assumption that dislocations in GaN are charged. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental and theoretical analysis of transport properties of core–shell wire light emitting diodes probed by electron beam induced current microscopy

We report a systematic experimental and theoretical investigation of core–shell InGaN/GaN single wire light-emitting diodes (LEDs) using electron beam induced current (EBIC) microscopy. The wires were grown by catalyst-free MOVPE and processed into single wire LEDs using electron beam lithography on dispersed wires. The influence of the acceleration voltage and of the applied bias on the EBIC maps was investigated. We show that the EBIC maps provide information both on the minority carrier effects (i.e. on the local p–n junction collection efficiency) and on the majority carrier effects (i.e. the transport efficiency from the excited region toward the contacts). Because of a finite core and shell resistance a non-negligible current redistribution into the p–n junction takes place during the majority carrier transport. A theoretical model for transport in a core–shell wire is developed, allowing to explain the dependence of the EBIC profiles on the experimental parameters (the electron beam acceleration voltage and the bias applied on the device) and on the structural parameters of the wire (core and shell resistance, shunt resistance, etc). Comparison between simulated and experimental profiles provides valuable information concerning the structure inhomogeneities and gives insight into the wire electrical parameters.

Why Lead Methylammonium Tri-Iodide Perovskite-Based Solar Cells Require a Mesoporous Electron Transporting Scaffold (but Not Necessarily a Hole Conductor)

CH3NH3PbI3-based solar cells were characterized with electron beam-induced current (EBIC) and compared to CH3NH3PbI(3-x)Clx ones. A spatial map of charge separation efficiency in working cells shows p-i-n structures for both thin film cells. Effective diffusion lengths, LD, (from EBIC profile) show that holes are extracted significantly more efficiently than electrons in CH3NH3PbI3, explaining why CH3NH3PbI3-based cells require mesoporous electron conductors, while CH3NH3PbI(3-Clx ones, where LD values are comparable for both charge types, do not.

Electron-beam-induced current at absorber back surfaces of Cu(In,Ga)Se2 thin-film solar cells

The present work reports on investigations of the influence of the microstructure on electronic properties of Cu(In,Ga)Se2 (CIGSe) thin-film solar cells. For this purpose, ZnO/CdS/CIGSe stacks of these solar cells were lifted off the Mo-coated glass substrates. The exposed CIGSe backsides of these stacks were investigated by means of electron-beam-induced current (EBIC) and cathodoluminescence (CL) measurements as well as by electron backscattered diffraction (EBSD). EBIC and CL profiles across grain boundaries (GBs), which were identified by EBSD, do not show any significant changes at Σ3 GBs. Across non-Σ3 GBs, on the other hand, the CL signals exhibit local minima with varying peak values, while by means of EBIC, decreased and also increased short-circuit current values are measured. Overall, EBIC and CL signals change across non-Σ3 GBs always differently. This complex situation was found in various CIGSe thin films with different [Ga]/([In]+[Ga]) and [Cu]/([In]+[Ga]) ratios. A part of the EBIC profiles exhibiting reduced signals across non-Σ3 GBs can be approximated by a simple model based on diffusion of generated charge carriers to the GBs.

Choice of Generation Volume Models for Electron Beam Induced Current Computation

The spatial distribution of the electron-hole pairs generated by the electron beam is commonly called the generation volume or interaction volume. The generation volume affects the electron beam-induced current (EBIC) profile. This effect of the generation volume on the EBIC profile is particularly true for regions near to the semiconductor junctions. Mathematical models have been proposed for use in the computation of EBIC profiles. Three pear-shaped generation volume distributions were analyzed by comparing the resulting EBIC profiles to the profile obtained from the data using the Monte Carlo simulations. The result shows that the Bonard model gives an EBIC profile that is closest to the one computed using the Monte Carlo simulation. This result is easily observed in the first and the second derivatives of the semilogarithmic EBIC profiles.

Analysis of V defects in GaN-based light emitting diodes by scanning transmission electron microscopy and electron beam induced current

In the fabrication of InGaN∕GaN multiple quantum well light emitting diodes so-called V defects are common, but little is known about their electrical activity. Scanning transmission electron microscopy is capable of directly observing these defects, while electron beam induced current (EBIC) techniques can be used to probe electronic behavior of semiconductor defects. These techniques were combined to obtain localized measurements and our results indicate that V defects suppress the EBIC signal near the core of the defect and produce a displacement in the p-n junction location. Furthermore, the EBIC profile suggests that minority carrier diffusion lengths are longer inside the defect.

A method of accurately determining the positions of the edges of depletion regions in semiconductor junctions

The technique of electron beam induced current (EBIC) in the scanning electron microscopy can be used to extract the depletion width in a variety of materials and devices. This article reviews current methods available for identifying the positions of the edges of the depletion layer and proposes an alternative methodology that uses the first derivative of the logarithmic EBIC profile. It was found that the precision of this method is only dependent upon the accuracy in which one can determine the lateral dimension of the beam-sample interaction. The proposed method is then applied to a gallium nitride light emitting diode device to verify the practicality of the technique.

Determination of Diffusion Lengths With the Use of EBIC From a Diffused Junction With Any Values of Junction Depths

Minority carrier diffusion lengths determine the performance of bipolar and photodiode devices. An electron-beam- induced-current (EBIC) method has been widely used to extract this parameter. The extraction of the diffusion lengths involves a p-n junction to collect the minority carriers. The most used configuration, which is called the normal collector, assumes that the junction has an infinitely large junction depth. However, in planar devices, the junction depth is comparable to the diffusion lengths of the material. This paper presents a simple and yet accurate method to determine the diffusion lengths of the material from a diffused junction with any values of junction depths. The diffusion length of the material is extracted from the negative reciprocal of the slope of the EBIC profile in semi-logarithmic plot. It was found that the proposed method is able to extract the diffusion lengths accurately for any values of the junction depths and surface recombination velocities. The maximum error in using this method is about 6%

Characterization of p–n junction formed at the boundary of facets in cubic-BN using scanning electron microscope

For the application of ultraviolet light emitting diodes, self-organized p–n junctions were formed at the boundaries of facets in cubic boron nitride. The optical and electrical characteristics of these junctions were studied using cathodoluminescence (CL) and electron-beam-induced current (EBIC) in a scanning electron microscope. The p and n regions were clearly distinguished by the variation of CL spectra and monochromatic CL images. A broad band with a peak at 4.0 eV (310 nm) was observed in p-region, but not in n-region. Monochromatic CL image at 4.0 eV showed that the Be impurity distribution in p-region was not uniform. Variation of EBIC profiles indicated the inhomogeneity of diffusion length along p-n junction. The inhomogeneous electroluminescence along the p–n junction under the reverse current condition was clarified by the inhomogeneities revealed by EBIC and CL characterizations.

Grain boundary structures in zinc oxide varistors

Lattice misorientations across electrically active interfaces in antimony and bismuth doped zinc oxide varistors have been determined by electron backscattered pattern analysis and coupled with the orientations of the associated grain boundary planes established by depth resolved electron beam induced current (EBIC) in order to index the crystal faces forming either side of the grain boundary plane. When correlated with the EBIC contrast observed at the grain boundary, it was found that a symmetrical EBIC profile was only found at interfaces where both crystal faces forming the grain boundary were oriented at a similar angle from the polar basal plane orientation. At the majority of grain boundaries the structure was such that one face lay close to the basal plane orientation with the other face much further away. In these cases the EBIC was suppressed on the side of the grain boundary whose face lay close to the basal orientation. The implications of these observations on the electrical structure of the interface and microstructural development are discussed.

Minority carrier diffusion lengths and optical self‐absorption coefficient in undoped GaN

AbstractIn this paper we report electron beam induced current (EBIC) measurements on Schottky diodes on undoped GaN samples grown by the metal organic vapor phase epitaxy method on porous silicon substrates. A 2D simulation model was applied to perform the theoretical EBIC profiles and to compare with the experimental one. A minority carrier diffusion length (L) for holes of 0.72 μm was measured in the space charge region, indicating constant diffusivity of 6.48 cm2/s and a lifetime coefficient of 0.8 ns. An increase of L is observed as the probe source is moved from the Schottky junction to the bulk material. This increase was related to the polarization effects that are preponderant in group III–nitride devices. Far from the depletion layer (greater than 0.91 μm) the measured current is produced by the reabsorbed recombination radiation process. This corresponds to an optical self‐absorption coefficient of ap = 0.112 μm−1. The results show that the transport parameters depend on the growth technology. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-resolution electron-beam-induced-current study of the defect structure inGaN epilayers

Electron-beam-induced-current (EBIC) investigations of GaN structures grown bymetal–organic chemical vapour deposition on (0001) sapphire substrates havebeen carried out. It is shown that the widths of the EBIC profiles forindividual extended defects can be as small as about 100 nm. This width isobserved to decrease with decreasing diffusion length and/or with increasingelectron beam energy. The high spatial resolution is explained by the smalldiffusion length in the samples under study. The diffusion length is smalleven in structures with dislocation densities of about 108 cm−3 and carriermobilities of about 600 cm2 V−1 s−1 at 300 K and 1800 cm2 V−1 s−1 at 125 K.

Characterization of a diamond p-n junction using electron-beam-induced current and cathodoluminescence

We have characterized a diamond p-n junction by means of electron-beam-induced current (EBIC) and cathodoluminescence (CL). The diamond p-n junction was fabricated by growing a B-doped p-type layer and a P-doped n-type layer on the {111} diamond substrate by microwave plasma enhanced chemical vapor deposition. The cross section of p-n junction was revealed by the mesa etching. The substrate, B- and P-doped layers were distinguished by CL spectra. The EBIC profiles across the p-n junction were recorded under various reverse bias conditions. These data confirmed that the p-n junction was actually formed at the interface between B- and P-doped layers. The energy-band profile suggests that the carrier concentration of P-doped layer is more than ten times higher than that of B-doped layer. The observed results strongly support the fact that the UV luminescence is emitted from the p-n junction region in diamond light-emitting diode.

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.

Electron-beam-induced-current study of artificial twist boundaries in bonded Si wafers

Several pairs of (1 0 0)Si wafers were directly bonded with known misorientation angles, between 0.1 and 11°, to form artificial twist boundaries. The structure and electrical property of these boundaries were studied by means of transmission electron microscopy (TEM) and electron-beam-induced-current (EBIC) method. TEM images showed that the dominant defects at boundaries were screw dislocations which compensate the twist component. All the twist boundaries were EBIC active at room temperature and their contrast values have increased with the increase in the twist angle. The EBIC profiles of twist boundaries were well fitted with the Donolato's model. Although EBIC contrasts of all the boundaries increased with decreasing temperature, the increase of 0.1° boundary was the most significant. Such temperature dependencies indicates that shallow levels are dominant in the smaller angle boundaries, while certain amounts of deep levels are introduced in larger angle boundaries due to dislocation interaction.