Electron Beam Induced Current Research Articles

Effects of Voltage Biasing on Current Extraction in Perovskite Solar Cells

AbstractThe recent rapid increase in efficiency of organic–inorganic perovskite solar cells (PSCs) has resulted in a need to develop a clear understanding of their stability and working mechanisms. In particular, it has been suggested that ion migration contributes to the commonly observed hysteresis in the current–voltage measurements of PSCs, but the rate of ion migration and its effects on the electronic properties of PSCs remain to be addressed. In this work, electron‐beam‐induced current (EBIC) is used to directly map changes in local current extraction in organic–inorganic PSCs under applied voltage. By combining EBIC mapping, standard current–voltage measurements, and external quantum efficiency measurements, it is shown that between the two potential roles that point defects play in device enhancement under voltage biasing, the effects caused by defect‐mediated ion migration outweigh the effects from the filling of trap states caused by these defects. Evidence is also provided for ion migration preferentially at local features such as extended defects. The measured timescale of tens of seconds for migration across a full device imply that ion migration contributes indirectly to the electronic capacitance of perovskite devices through interface charging.

Random walk on spheres method for solving anisotropic drift-diffusion problems

Abstract We suggest a random walk on spheres based stochastic simulation algorithm for solving drift-diffusion-reaction problems with anisotropic diffusion. The diffusion coefficients and the velocity vector vary in space, and the size of the walking spheres is adapted to the local variation of these functions. The method is mesh free and extremely efficient for calculation of fluxes to boundaries and the concentration of the absorbed particles inside the domain. Applications to cathodoluminescence (CL) and electron beam induced current (EBIC) methods for the analysis of dislocations and other defects in semiconductors are discussed.

Random walk on spheres method for solving anisotropic drift-diffusion problems

AbstractWe suggest a random walk on spheres based stochastic simulation algorithm for solving drift-diffusion-reaction problems with anisotropic diffusion. The diffusion coefficients and the velocity vector vary in space, and the size of the walking spheres is adapted to the local variation of these functions. The method is mesh free and extremely efficient for calculation of fluxes to boundaries and the concentration of the absorbed particles inside the domain. Applications to cathodoluminescence (CL) and electron beam induced current (EBIC) methods for the analysis of dislocations and other defects in semiconductors are discussed.

Electric field distribution using floating metal guard rings edge-termination for Schottky diodes

Diamond Schottky barrier diodes with floating metal guard ring as edge termination have been fabricated and characterized. The influences of spacing between two consecutive rings and number of rings on breakdown voltage and surface electric field distribution were investigated. Electron beam induced current (EBIC) analysis revealed the efficiency of floating metal guard rings edge termination for a fixed gap between two rings. The obtained high reverse leakage currents were mainly related to the presence of hotspots, influencing the breakdown voltage and avoiding avalanche phenomena from happening within the material. These high-field regions emerging under high reverse bias highlight the crowding of electric field causing a premature breakdown. The combination of the hotspots and the possible presence of electric arc between the rings at high bias may give rise to an increase of leakage current.

Electrical Activity of Extended Defects in Multicrystalline Silicon

The excess carrier lifetime (τ) distribution in multicrystalline silicon grown by the Bridgman technique from high-purity metallurgical silicon (HPMG-Si) is studied. The features of the variation in τ, caused by the grain-boundary structure of ingots, are revealed. The grain boundaries, dislocations, and impurity microinclusions are studied by electron probe microanalysis (EPMA) and scanning electron microscopy (SEM) using selective acid etching. The electrical activity of extended defects is measured by the electronbeam- induced-current (EBIC) method.

Nanoscale investigation of a radial p-n junction in self-catalyzed GaAs nanowires grown on Si (111).

One obstacle for the development of nanowire (NW) solar cells is the challenge to assess and control their nanoscale electrical properties. In this work a top-cell made of p-n GaAs core/shell NWs grown on a Si(111) substrate by Molecular Beam Epitaxy (MBE) is investigated by high resolution charge collection microscopy. Electron Beam Induced Current (EBIC) analyses of single NWs have validated the formation of a homogeneous radial p-n junction over the entire length of the NWs. The radial geometry leads to an increase of the junction area by 38 times with respect to the NW footprint. The interface between the NWs and the Si(111) substrate does not show any electrical loss, which would have led to a decrease of the EBIC signal. Single NW I-V characteristics present a diodic behavior. A model of the radial junction single NW is proposed and the electrical parameters are estimated by numerical fitting of the I-Vs and of the EBIC map. Solar cells based on NW arrays were fabricated and analyzed by EBIC microscopy, which evidenced the presence of a Schottky barrier at the NW/ITO top contact. Improvement of the top contact quality is achieved by thermal annealing at 400 °C, which strongly reduces the parasitic Schottky barrier.

Quantum Barrier Growth Temperature Affects Deep Traps Spectra of InGaN Blue Light Emitting Diodes

Electroluminescence (EL) efficiency, deep electron and hole traps spectra, microcathodoluminescence (MCL), electron beam induced current (EBIC) imaging, and MCL spectra were studied for blue GaN/InGaN multi-quantum-well (MQW) light emitting diodes differing by the temperature at which the GaN barriers of the MQW active region were grown. It was found that increasing the growth temperature from 850 to 920°C very strongly suppressed the formation of deep electron traps with level at Ec-1 eV in the GaN barriers and of the hole traps with levels at Ev+0.7 eV in InGaN QWs. The suppression of the formation of the Ec-1 eV electron trap, a known prominent nonradiative recombination center in n-GaN, improved the carrier injection efficiency into the InGaN QWs and increased the external quantum efficiency by about 9%. EBIC and MCL imaging showed that the density of threading dislocations and terminating them V-pits was relatively low and similar for both studied growth temperatures, close to 108 cm−2. The cross-sectional dimensions of the V-pits were measurably higher for increased growth temperature. However, the rather low dislocation density and rather high dimensions of the V-pits were believed to result in minor contribution of these defects to the observed EL efficiency changes.

Электрическая активность протяженных дефектов в мультикремнии

AbstractThe excess carrier lifetime (τ) distribution in multicrystalline silicon grown by the Bridgman technique from high-purity metallurgical silicon (HPMG-Si) is studied. The features of the variation in τ, caused by the grain-boundary structure of ingots, are revealed. The grain boundaries, dislocations, and impurity microinclusions are studied by electron probe microanalysis (EPMA) and scanning electron microscopy (SEM) using selective acid etching. The electrical activity of extended defects is measured by the electronbeam- induced-current (EBIC) method.

Proton irradiation effects on minority carrier diffusion length and defect introduction in homoepitaxial and heteroepitaxial n-GaN

Inherent advantages of wide bandgap materials make GaN-based devices attractive for power electronics and applications in radiation environments. Recent advances in the availability of wafer-scale, bulk GaN substrates have enabled the production of high quality, low defect density GaN devices, but fundamental studies of carrier transport and radiation hardness in such devices are lacking. Here, we report measurements of the hole diffusion length in low threading dislocation density (TDD), homoepitaxial n-GaN, and high TDD heteroepitaxial n-GaN Schottky diodes before and after irradiation with 2.5 MeV protons at fluences of 4–6 × 1013 protons/cm2. We also characterize the specimens before and after irradiation using electron beam-induced-current (EBIC) imaging, cathodoluminescence, deep level optical spectroscopy (DLOS), steady-state photocapacitance, and lighted capacitance-voltage (LCV) techniques. We observe a substantial reduction in the hole diffusion length following irradiation (50%–55%) and the introduction of electrically active defects which could be attributed to gallium vacancies and associated complexes (VGa-related), carbon impurities (C-related), and gallium interstitials (Gai). EBIC imaging suggests long-range migration and clustering of radiation-induced point defects over distances of ∼500 nm, which suggests mobile Gai. Following irradiation, DLOS and LCV reveal the introduction of a prominent optical energy level at 1.9 eV below the conduction band edge, consistent with the introduction of Gai.

Recombination properties of dislocations in GaN

The recombination activity of threading dislocations in n-GaN with different dislocation densities and different doping levels was studied using electron beam induced current (EBIC). The recombination velocity on a dislocation, also known as the dislocation recombination strength, was calculated. The results suggest that dislocations in n-GaN giving contrast in EBIC are charged and surrounded by a space charge region, as evidenced by the observed dependence of dislocation recombination strength on dopant concentration. For moderate (below ∼108 cm−2) dislocation densities, these defects do not primarily determine the average diffusion length of nonequilibrium charge carriers, although locally, dislocations are efficient recombination sites. In general, it is observed that the effect of the growth method [standard metalorganic chemical vapor deposition (MOCVD), epitaxial lateral overgrowth versions of MOCVD, and hydride vapor phase epitaxy] on the recombination activity of dislocations is not very pronounced, although the average diffusion lengths can widely differ for various samples. The glide of basal plane dislocations at room temperature promoted by low energy electron irradiation does not significantly change the recombination properties of dislocations.

Stateful characterization of resistive switching TiO2 with electron beam induced currents

Metal oxide resistive switches are increasingly important as possible artificial synapses in next-generation neuromorphic networks. Nevertheless, there is still no codified set of tools for studying properties of the devices. To this end, we demonstrate electron beam-induced current measurements as a powerful method to monitor the development of local resistive switching in TiO2-based devices. By comparing beam energy-dependent electron beam-induced currents with Monte Carlo simulations of the energy absorption in different device layers, it is possible to deconstruct the origins of filament image formation and relate this to both morphological changes and the state of the switch. By clarifying the contrast mechanisms in electron beam-induced current microscopy, it is possible to gain new insights into the scaling of the resistive switching phenomenon and observe the formation of a current leakage region around the switching filament. Additionally, analysis of symmetric device structures reveals propagating polarization domains.

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.

A Practical Example of GaN-LED Failure Cause Analysis by Application of Combined Electron Microscopy Techniques.

In this paper, we report a failure case of blue LEDs returned from a field application, and propose a practical way to identify the physical and structural reasons for the observed malfunction by a combination of different electron microscope techniques. Cathodoluminescence imaging and electron beam induced current (EBIC) imaging are employed in order to visualize conductive paths through the device in conjunction with subsequent energy dispersive x-ray analysis (EDS), revealing a metal deposition along cracks in the semiconductor layer which short-circuit the device. We demonstrate that the electron beam induced current imaging, in conjunction with other microscopic and analytical techniques at µm scale, is a powerful combination for clearly resolving and visualizing the cause of failure in the GaN LED chip. However, this represents a case study of a real application, which may not have been generally observed in laboratory testing environment.

Minority Carrier Transport in Lead Sulfide Quantum Dot Photovoltaics.

Lead sulfide quantum dots (PbS QDs) are an attractive material system for the development of low-cost photovoltaics (PV) due to their ease of processing and stability in air, with certified power conversion efficiencies exceeding 11%. However, even the best PbS QD PV devices are limited by diffusive transport, as the optical absorption length exceeds the minority carrier diffusion length. Understanding minority carrier transport in these devices will therefore be critical for future efficiency improvement. We utilize cross-sectional electron beam-induced current (EBIC) microscopy and develop methodology to quantify minority carrier diffusion length in PbS QD PV devices. We show that holes are the minority carriers in tetrabutylammonium iodide (TBAI)-treated PbS QD films due to the formation of a p-n junction with an ethanedithiol (EDT)-treated QD layer, whereas a heterojunction with n-type ZnO forms a weaker n+-n junction. This indicates that modifying the standard device architecture to include a p-type window layer would further boost the performance of PbS QD PV devices. Furthermore, quantitative EBIC measurements yield a lower bound of 110 nm for the hole diffusion length in TBAI-treated PbS QD films, which informs design rules for planar and ordered bulk heterojunction PV devices. Finally, the low-energy EBIC approach developed in our work is generally applicable to other emerging thin-film PV absorber materials with nanoscale diffusion lengths.

Atomistic Approach for Modeling the Wafer Curvature for Pseudo-Morphic and Non-Pseudo-Morphic Epitaxial Multilayer Structures

During last few decades, there have been tremendous progress in the development of III-V based and III-Nitride based devices, namely High Electron Mobility Transistors (HEMT) and Light Emitting Diodes (LEDs). Research interest has recently shifted towards III-V compound semiconductor epitaxy on large diameter (200 mm) Si substrates in order to realize III-V-on-Si and CMOS integration. Due to large lattice and thermal mismatch between III-V and Si materials, e g. GaN-on-Si wafers experience large internal stresses and significant wafer bow. The wafer bow is severe for large wafers (for a given radius of curvature, the wafer bow increases by 125% for 150 mm and 300% for 200 mm diameter wafers compared to 100 mm wafers). This results in wafer fragility issues, as well as difficulties in subsequent handling and processing of such wafers. These issues are typically addressed using thicker Si substrates and careful design of buffer layers. While there are many experimental efforts to overcome these challenges, there are only limited reports that try to properly explain and understand the mechanism of wafer curvature. An appropriate theoretical model which predicts the final wafer curvature for given epilayer designs would greatly improve our ability to design optimal buffer and device layers. There are so far two main models for epitaxial multilayers which relate the wafer curvature to the strain state and the material properties of each layer. Ohlsen et al. (J. App. Phys. 1977) derived an analytical expression for the stress in each layer and the final wafer curvature for a given layer stack. This model however does require one to define strain states in each layer which is not straightforward to predict prior to the actual hetero-epitaxy of complex layers with large misfit dislocation densities such as in the GaN-on-Si system. Usher et al. (Phys. Rev. B 2003) proposed an atomistic approach for analyzing pseudo-morphic multi-layer structures, and were able to provide a closed form equation for radius of curvature. However, none of the reports have explicitly considered the thermal strain in the calculation. In this presentation we would address these limitations and provide a complete description of the wafer curvature including the effects of both lattice mismatch and coefficient of thermal expansion mismatch. We extend the atomistic approach to calculate the radius of curvature for non-pseudomorphic systems like III/V-on-Si systems. The density of misfit dislocations at each interface is used as a parameter to define the effective lattice constant of partially relaxed epilayers at the interface. Subsequently the lattice constant at any point in each layer is defined with respect to the interface lattice constant and is used to calculate the strain. Then the total strain energy of the resulting system is calculated as a function of system curvature, with the minimum energy solution corresponding to the predicted final curvature of the wafer. For better accuracy each epitaxial layer may be divided to separate sublayers with individual misfit dislocation density values. For illustration GaN-3xAlGaN-AlN-Si system was considered and radius of curvature was predicted. The accuracy of the model depends on the accuracy with which the misfit dislocation density is known. Experimental techniques like TEM and electron beam induced current (EBIC) can be used to estimate or quantify the number of dislocated atoms. Finally, we will include the thermal effect within the atomistic approach to be able to predict in-situ curvature of III/V-on-Si systems which will be helpful in designing the growth experiments for various devices.

Modified electron beam induced current technique for in(Ga)As/InAsSb superlattice infrared detectors

Electron beam induced current (EBIC) measurements provide a powerful tool for characterizing semiconductor based materials and devices. By measuring the current generated by the electron beam of a scanning electron microscope (SEM), EBIC allows us to extract the minority carrier diffusion length (L) and the surface recombination velocity to diffusivity ratio (S/D) of a material. When combined with information on minority carrier lifetime (τ), for instance from time-resolved photoluminescence measurements, the minority carrier mobility of the material can be extracted. However, the EBIC technique relies heavily on the accurate modeling of the carrier generation and collection process. Traditionally, this was achieved using a combination of empirical analytical expressions (and later Monte Carlo simulations) for carrier generation and analytical diffusion/recombination expressions for carrier collection. This approach introduces significant uncertainties into the extracted material parameters. Here, we present a numerical approach to EBIC modeling which improves the spatial resolution of our model, while also retaining information regarding the relative EBIC signal as a function of incident beam energies and currents. We apply this technique to investigate the temperature dependent minority carrier mobility of InAs/InAsSb and InGaAs/InAsSb strained layer superlattice infrared detectors and compare our results to the values obtained using external quantum efficiency measurements of the same samples. Our approach not only allows for an improvement in the uncertainty of the extracted material parameters, but also offers insight into the material and device behavior as a function of nonequilibrium carrier concentration. The technique presented here offers potentially improved characterization of not only infrared detectors, but a range of semiconductor-based devices.

Random walk on spheres algorithm for solving transient drift-diffusion-reaction problems

Abstract We suggest in this paper a Random Walk on Spheres (RWS) method for solving transient drift-diffusion-reaction problems which is an extension of our algorithm we developed recently [26] for solving steady-state drift-diffusion problems. Both two-dimensional and three-dimensional problems are solved. Survival probability, first passage time and the exit position for a sphere (disc) of the drift-diffusion-reaction process are explicitly derived from a generalized spherical integral relation we prove both for two-dimensional and three-dimensional problems. The distribution of the exit position on the sphere has the form of the von Mises–Fisher distribution which can be simulated efficiently. Rigorous expressions are derived in the case of constant velocity drift, but the algorithm is then extended to solve drift-diffusion-reaction problems with arbitrary varying drift velocity vector. The method can efficiently be applied to calculate the fluxes of the solution to any part of the boundary. This can be done by applying a reciprocity theorem which we prove here for the drift-diffusion-reaction problems with general boundary conditions. Applications of this approach to methods of cathodoluminescence (CL) and electron beam induced current (EBIC) imaging of defects and dislocations in semiconductors are presented.

Imaging the Impact of Proton Irradiation on Edge Terminations in Vertical GaN PIN Diodes

Devices based on GaN have shown great promise for high power electronics, including their potential use as radiation tolerant components. An important step to realizing high power diodes is the design and implementation of an edge termination to mitigate field crowding, which can lead to premature breakdown. However, little is known about the effects of radiation on edge termination functionality. We experimentally examine the effects of proton irradiation on multiple field ring edge terminations in high power vertical GaN PIN diodes using in operando imaging with electron beam induced current (EBIC). We find that exposure to proton irradiation influences field spreading in the edge termination as well as carrier transport near the anode. By using depth-dependent EBIC measurements of hole diffusion length in homoepitaxial n-GaN we demonstrate that the carrier transport effect is due to a reduction in hole diffusion length following proton irradiation.

Temperature-dependent signals in STEM Electron Beam-Induced Current (EBIC) Imaging

Temperature-dependent signals in STEM Electron Beam-Induced Current (EBIC) Imaging

Microscopic electronic and structural analysis of femtosecond laser sulfur hyperdoped silicon

AbstractSulfur hyperdoping by fs laser irradiation is a promising process for improving absorption properties of crystalline silicon which indicates its suitability for manufacturing intermediate band silicon solar cells. Here, first results of a novel approach to study the electrical and structural properties of sulfur hyperdoped regions of p‐type silicon substrates are presented. In addition to electron beam induced current (EBIC) and transmission electron microscopy (TEM) in cross‐section geometry, shallow‐angle bevel polishing is used to employ surface sensitive techniques, such as atomic force microscopy (AFM) and Kelvin force microscopy (KFM). The latter reveals high lateral electric fields in regions beneath the crests of the surface topography, while EBIC shows an enhanced recombination activity at the location of the voltage drops. Both these effects are related to extended defects revealed by TEM and a possible lateral variation of sulfur concentrations.AFM image (top) and KFM map (bottom) showing a voltage drop on the surface of a terrace produced by bevel polishing of sulfur hyperdoped silicon.