Electron Beam-Induced Current (EBIC) in solution-processed solar cells

Electron Beam-Induced Current (EBIC) measurements were used to produce 2D maps for investigating the homogeneity of solar cells. These maps are acquired by scanning the electron beam of a scanning electron microscope over a small area and using a programmable sample stage to move the solar cell under the scan area. The electron beam generates electron-hole pairs in the solar cell much like light does in normal solar cell operation. Solution-processed solar cells where the active layer consisted of purely inorganic or purely organic materials were measured. Since the electron beam irreversibly damages organic material, it was important to ensure that the measurements were made before the materials were altered.     

Electron Microscope Studies Of Vpe Gainas Layer Structures Suitable For Use As Infrared Leds

Electron microscope investigations have been carried out on vapour grown (100) GaAs/GaInAs structures designed for use as infrared emitters of wavelength 1·06 μm. The structures consist of a GaAs substrate, a graded layer in which the indium concentration is increased from zero to 17 atomic %, and a constant composition Ga_0·83In_0·17As layer which contains a p-n junction. X-ray microprobe analysis of cross-sections of the slices established the uniformity of the grading. TEM analysis showed a dense and extensive asymmetric network of misfit dislocations (1 × 10¹² m^?2 (10⁸ cm^?2)) in the graded layer, threading dislocations and other anomalous contrast features extending from the graded layer through the p-n junction to the surface (local densities of 1 × 10¹¹–1 × 10¹² m^?2 (10⁷–10⁸ cm^?2)), and a planar network of dislocations just below the surface (spacing 0·2–2 μm). SEM EBIC and CL studies of the layer above the junction revealed dark spots, and a cross-grid of dark lines, which could be correlated with the threading defects, and the dislocation network just below the surface, respectively. The SEM results showed that these defects had a deleterious effect on the luminescent and electrical properties of the material in the vicinity of the p-n junction, and would therefore impair the performance of devices made from these layer structures.     

Advanced semiconductor diagnosis by multidimensional electron-beam-induced current technique

We present advanced semiconductor diagnosis by using electron-beam-induced current (EBIC) technique. By varying the parameters such as temperature, accelerating voltage (V(acc)), bias voltage, and stressing time, it is possible to extend EBIC application from conventional defect characterization to advanced device diagnosis. As an electron beam can excite a certain volume even beneath the surface passive layer, EBIC can be effectively employed to diagnose complicated devices with hybrid structure. Three topics were selected to demonstrate EBIC applications. First, the recombination activities of grain boundaries and their interaction with Fe impurity in photovoltaic multicrystalline Si (mc-Si) are clarified by temperature-dependent EBIC. Second, the detection of dislocations between strained-Si and SiGe virtual substrate are shown to overcome the limitation of depletion region. Third, the observation of leakage sites in high-k gate dielectric is demonstrated for the characterization of advanced hybrid device structures.     

High-resolution scanning near-field EBIC microscopy: application to the characterisation of a shallow ion implanted p+-n silicon junction

High-resolution electron beam induced current (EBIC) analyses were carried out on a shallow ion implanted p⁺–n silicon junction in a scanning electron microscope (SEM) and a scanning probe microscope (SPM) hybrid system. With this scanning near-field EBIC microscope, a sample can be conventionally imaged by SEM, its local topography investigated by SPM and high-resolution EBIC image simultaneously obtained. It is shown that the EBIC imaging capabilities of this combined instrument allows the study of p–n junctions with a resolution of about 20 nm.     

Reduction of electrical damage in specimens prepared using focused ion beam milling for dopant profiling using off-axis electron holography

GaAs specimens containing p-n junctions have been prepared using focused ion beam (FIB) milling for examination using off-axis electron holography. By lowering the FIB operating voltage from 30 to 8 kV, we have shown a systematic reduction of the electrically 'inactive' thickness from 220 to 100 nm, resulting in a significant increase in the step in phase measured across the junctions as well as an improvement in the signal-to-noise ratio. We also show that the step in phase measured across the junctions can be influenced by the intensity of the electron beam.     

Observation of dislocations and microplasma sites in semiconductors by direct correlations of STEBIC,STEM and ELS

A high voltage electron microscope, equipped with scanning transmission (STEM) attachment, electron beam induced conductivity (EBIC) facilities, and electron energy loss spectrometer (ELS), has been used to investigate semiconductor devices. The capability of STEM to produce, simultaneously or sequentially, conductive and transmission images of the same specimen region, which can also be ELS analysed, is exploited in order to establish direct and unambiguous correlations between EBIC and STEM images of defective regions (dislocations and microplasma sites) in silicon devices. The results obtained are discussed in terms of correlations, resolution, contrast, and radiation damage; in addition, a comparison is made between this method and the other correlation methods based on EBIC/SEM (scanning electron microscope) and TEM (transmission electron microscope).     

Electron Microscope Studies Of Ion Implanted Silicon And Gallium Arsenide After Laser And Furnace Annealing

In the present investigation, three separate projects are described. First, P+ implanted Si is laser and furnace annealed, and the resulting structures as observed by TEM are compared. Second, similar studies are made of Zn+ implanted GaAs, the laser annealing being performed either with or without a surface encapsulating layer. Third, As+ implanted Si is laser annealed, and the uniformity of the annealing process as determined by SEM EBIC studies is assessed. The results have shown that different damage structures can be produced by laser annealing compared to furnace annealing, and by different laser energy densities. For the laser annealed specimens, the TEM observations provided evidence for the formation of a molten surface layer. The results demonstrated for GaAs that encapsulation was still required for laser annealing, and for Si that laser annealing can give a varying p-n junction depth within individual slices. The latter is explained by a molten surface layer extending to different depths during the annealing process, this being attributed to spatial variations in laser intensity across the area illuminated.     

The remote electron beam-induced current analysis of grain boundaries in semiconducting and semi-insulating materials

When no charge collecting p-n junction or Schottky barrier is present in the specimen, but two contacts are applied, conductive mode scanning electron microscope (SEM) observations known as remote electron beam-induced current (REBIC) can be made. It was described as "remote" EBIC because the contacts to the specimen can lie at macroscopic distances from the beam impact point. In recent years, REBIC has been found to be useful not only for studies of grain boundaries in semiconducting silicon and germanium, but also in semi-insulating materials such as the wider bandgap II-VI compounds and electroceramic materials like varistor ZnO and positive temperature coefficient resistor (PTCR) BaTiO3. The principles of this method are outlined. Accounts are given of the five forms of charge collection and resistive contrast that appear at grain boundaries (GBs) in REBIC micrographs. These are (1) terraced contrast due to high resistivity boundary layers, (2) peak and trough (PAT) contrast due to charge on the boundary, (3) reversible contrast seen only under external voltage bias due to the beta-conductive effect in a low conductivity boundary layer, (4) dark contrast due to enhanced recombination, and (5) bright contrast apparently due to reduced recombination. For comparison, the results of the extensive EBIC studies of GBs in Si and Ge are first outlined and then the results of recent REBIC grain boundary studies in both semiconducting and semi-insulating materials are reviewed.     

Quantitative off-axis electron holography of GaAs p-n junctions prepared by focused ion beam milling

Focused Ion beam (FIB) prepared GaAs p-n junctions have been examined using off-axis electron holography. Initial analysis of the holograms reveals an experimentally determined built-in potential in the junctions that is significantly smaller than predicted from theory. In this paper we show that through combinations of in situ annealing and in situ biasing of the specimens, by varying the intensity of the incident electron beam, and by modifying the FIB operating parameters, we can develop an improved understanding of phenomena such as the electrically 'inactive' thickness and subsequently recover the predicted value of the built-in potential of the junctions.
PACS numbers: 85.30.De     

A computerized signal acquisition system for the quantitative evaluation of EBIC and CL data in a SEM

Electron beam-induced current (EBIC) and cathodoluminescence (CL) are widely used methods to obtain information about recombination properties of semiconducting materials and their defects on a micrometer length scale. In this article a computerized SEM (scanning electron microscope) setup is described, which enables us to perform simultaneous measurements of several signals and automatic temperature-dependent measurements. As examples for the performance of this system we present results obtained by simultaneous EBIC/CL experiments, allowing a reconstruction of the defect geometry. In a second example, the temperature dependence of the EBIC contrast is analyzed, introducing the method of EBIC spectroscopy.     

Determination of local deep-levels homogeneity using SEM

A method to study microhomogeneity of deep centres in high resistivity and low resistivity semiconductors is described herein. It uses measurements of current decay induced in Schottky diodes and p-n junctions by a probing electron beam of an SEM and is closely related to a method of photoelectron relaxation spectroscopy. The advantages of this method are demonstrated for an example of dislocation-related inhomogeneity in semi-insulating GaAs.     

EBIC microscopy of double-heterostructure laser materials and devices

The electron beam induced current (EBIC) mode of the scanning electron microscope (SEM) has been used to characterize double heterostructure laser materials and devices in GaAs/Ga_1–xAl_xAs. Scanning the electron probe across the cleaved face of the laser structure shows that displacement of the p-n junction with respect to the heterojunctions is not uncommon with displacements ～ 1 μm occurring. Concurrent measurement of the minority carrier diffusion length gives very short lengths of 0·3–0·4 μm, differing from those in much thicker layers. Scanning the electron probe in the contact plane indicates clearly that long-lived lasers exhibit marked heterogeneity during degradation. Considerable complexity and variation is recorded depending upon the fabrication details and degradation conditions adopted.     

Low-voltage cross-sectional EBIC for characterisation of GaN-based light emitting devices

Electron beam induced current (EBIC) characterisation can provide detailed information on the influence of crystalline defects on the diffusion and recombination of minority carriers in semiconductors. New developments are required for GaN light emitting devices, which need a cross-sectional approach to provide access to their complex multi-layered structures. A sample preparation approach based on low-voltage Ar ion milling is proposed here and shown to produce a flat cross-section with very limited surface recombination, which enables low-voltage high resolution EBIC characterisation. Dark defects are observed in EBIC images and correlation with cathodoluminescence images identify them as threading dislocations. Emphasis is placed on one-dimensional quantification which is used to show that junction delineation with very good spatial resolution can be achieved, revealing significant roughening of this GaN p-n junction. Furthermore, longer minority carrier diffusion lengths along the c-axis are found at dislocation sites, in both p-GaN and the multi-quantum well (MQW) region. This is attributed to gettering of point defects at threading dislocations in p-GaN and higher escape rate from quantum wells at dislocation sites in the MQW region, respectively. These developments show considerable promise for the use of low-voltage cross-sectional EBIC in the characterisation of point and extended defects in GaN-based devices and it is suggested that this technique will be particularly useful for degradation analysis.     

GaP LPE半导体材料的扫描电子显微镜研究

本文论述了用扫描电子显微镜研究GaP LPE半导体材料,二次电子像用于分析样品的表面形貌,电子束感生电流像(EBIC)用于显示p-n结的位置,定量EBIC用以确定少子扩散长度和表面复合速度等重要参量。     

Observation of electrostatic fields by electron holography: The case of reverse-biased p-n junctions

The technique of electron holography is applied to the investigation of microelectric fields such as those associated with reverse-biased p-n junctions. Suitable electron-optical conditions were adopted in order to minimize the effect of the electrostatic fringing field on the reference wave. The electron holograms were optically processed by the method of differential interferometry.     

Monte Carlo simulation of CL and EBIC contrasts for isolated dislocations

Electron beam-induced current (EBIC) and cathodoluminescence (CL) are widely used to investigate semiconductor materials and devices, particularly to obtain information on the recombination properties and the geometry of defects. This report describes a simple formulation of CL and EBIC contrasts based on the Born approximation of excess carrier density in the presence of a pointlike defect. Quantitative interpretation of the CL and EBIC images is often difficult because of a lack of accurate theory treating simultaneously both the details of the electron beam penetration in the semiconductor and the generation of EBIC and CL signals. To overcome this difficulty, the Monte Carlo approach to the phenomenon of the electron beam penetration in solids has been developed to calculate the CL and EBIC signals during a simulation of the electron trajectory. Results for an inclined dislocation in GaAs are presented.     

Complementary freeze-fracture, freeze-etch specimens

For a number of reasons, the specimens used to try to correlate the electrical and structural properties of commercial semiconductor devices often consist of a thick amorphous layer (e.g. silicon oxide) and a thinned layer of crystalline material (e.g. silicon) containing the p-n junctions. Diffraction patterns and diffraction contrast images from such specimens recorded in conventional and scanning transmission electron microscopes at 80 and 500 kV are shown. The visibility of these diffraction patterns and images depends on the mode of operation, the aperture sizes and the side on which the electrons enter the specimen. The interpretation of these results is discussed and the practical implications for imaging of such specimens are pointed out.     

The effect of electron range on electron beam induced current collection and a simple method to extract an electron range for any generation function

The effect of electron range on electron beam induced current (EBIC) is demonstrated and the problem of the choice of the optimal electron ranges to use with simple uniform and point generation function models is resolved by proposing a method to extract an electron range-energy relationship (ERER). The results show that the use of these extracted electron ranges remove the previous disagreement between the EBIC curves computed with simple forms of generation model and those based on a more realistic generation model. The impact of these extracted electron ranges on the extraction of diffusion length, surface recombination velocity and EBIC contrast of defects is discussed. It is also demonstrated that, for the case of uniform generation, the computed EBIC current is independent of the assumed shape of the generation volume.     

石墨烯纳米结构中负微分电阻效应研究

由于石墨烯具有高电子迁移率的特性,可以用来制备高频电子器件。利用传输矩阵方法,对石墨烯p-n结及方形势垒纳米结构中的负微分电阻效应进行了研究。证实了石墨烯p-n结中负微分电阻现象比传统半导体中的幅度要小,石墨烯中Klein隧穿过程的存在使负能量范围内的空穴对电流产生影响。石墨烯纳米方形势垒中发生负微分电阻效应的位置在费米面附近,势垒宽度越大,对载流子的阻挡越大,负微分电阻效应越明显。     

Observation of current-density filamentation in multilayer structures by EBIC measurements

The total current-voltage characteristics of the p⁺-n⁺-p-n^? and n⁺-p-n-p^? diodes under investigation show branches of negative differential resistance. Accompanied by the appearance of negative differential resistance is a filamentation of current-density and electric-field distribution. Electron beam-induced current (EBIC) measurements were used to examine the properties of filamentation from the point of view of self-organized pattern formation. Besides the detection of the spatial distribution of the electric field, EBIC measurements give information on current-density filamentation. Furthermore, the perturbation by the electron beam gives information on the dynamic behavior of the filamentary structure.