Electron Beam Induced Current Research Articles

Electron-Beam-Induced Current Study of Dislocations and Leakage Sites in GaN Schottky Barrier Diodes

This work aims to clarify the electrical activities of threading dislocations and their relation with leakage sites in homoepitaxial GaN Schottky barrier diodes based on the electron-beam-induced current (EBIC) technique and transmission electron microscopy (TEM). First, the recombination activities of threading dislocations in epilayers grown on different substrates are compared by EBIC. The dislocation type is characterized based on etch pit measurements and TEM. The dislocation density and character are strongly affected by defects in the substrate. The recombination strength of dislocations is revealed to be correlated with their type. It is found that single dislocations including both edge and mixed type exhibit weak (< 5%) EBIC contrast, while dislocation clusters show strong contrast (up to 30%). Second, leakage sites in Schottky diodes are visualized by EBIC under reverse bias. There is no direct correlation between the initial leakage sites and threading dislocations; whereas, instead of dislocations, a variety of initial leakage/breakdown sites are found, including grown-in pit defects as initial breakdown sites and hillocks at the Schottky interface acting as strong leakage sites.

Understanding and optimizing EBIC pn-junction characterization from modeling insights

In this paper, the physical mechanisms involved in electron-beam-induced current (EBIC) imaging of semiconductor pn-junctions are reviewed to propose a model and optimize the acquisition of experimental data. Insights are drawn on the dependence of the EBIC signal with electron accelerating voltage and surface conditions. It is concluded that improvements in the resolution of EBIC are possible when the surface conditions of the specimens are carefully considered and optimized. A lower accelerating voltage and an increase of the surface recombination velocities are quantitatively shown to maximize the EBIC lateral resolution in locating the pn-junction. The effect of surface band bending is included in the model, and it is seen to primarily affect the surface recombination. Introducing controlled surface damage is shown as a potential method for resolution enhancement via focused ion beam milling with Ga+ ions. These findings contribute to the understanding of this technique and can produce further improvements to its application in semiconductor device technology.

Transient characteristics of electron beam induced current in dielectric and semiconductor sample

The electron beam induced current (EBIC) characteristics of dielectric/semiconductor thin films under the electron beam (e-beam) irradiation is the important means of implementing the electron microscopic detection. The transient EBIC characteristics of the SiO&lt;sub&gt;2&lt;/sub&gt;/Si thin film irradiated by a high-energy e-beam are investigated by combining the numerical simulation and the experimental measurement. The scattering process of electrons is simulated by the Rutherford scattering model and the fast secondary electron model, and the charge transport, trapping and the recombination process are calculated by the current continuity equation and the Poisson equation. The transient charge distribution, EBIC and the transmission current are obtained, and influence of the beam current and the beam energy on them are analyzed. The results show that due to the electron scattering effect, the free electron density decreases gradually along the incident direction. The net charge density near the surface is positive and negative along the incident direction because of secondary electrons (SEs) emitted from the surface, and therefore the electric field intensity is positive near the surface and negative inside sample, which causes some electrons to be transported to the substrate and some SEs return to the surface. The negative charge density at the SiO&lt;sub&gt;2&lt;/sub&gt;/Si interface is higher than that in the nearby region because some electrons are trapped by the interface trap. With the decrease of the net charge density with e-beam irradiation, the charging intensity decreases gradually. Meanwhile, electrons are gradually transported to the substrate, and consequently EBIC and the sample current increase and the electric field intensity decreases with e-beam irradiation. However, due to the weak charging intensity, the surface emission current and the transmission current remain almost invariant with e-beam irradiation. The EBIC, the transmission current and the surface emission current are approximately proportional to the beam current. For the SiO&lt;sub&gt;2&lt;/sub&gt;/Si thin film in this work, the transmission current increases gradually to the beam current value with the increase of the beam energy, and the EBIC presents a maximum value at the beam energy of about 15 keV.

GaNP-based photovoltaic device integrated on Si substrate

Gallium phosphide is an important material in terms of III-V semiconductors integration on Si. In this work we study photovoltaic properties of GaP:Be/GaNP (Eg ~ 2.0eV)/GaP:Si p-i-n heterostructure grown on Si (100) with GaP buffer by plasma-assisted molecular beam epitaxy. Correlation between the structural and optoelectronic properties of the fabricated device was studied by scanning and transmission electron microscopy (TEM), electron beam induced current (EBIC) and deep-level transient spectroscopy (DLTS) techniques. The I–V characteristic of the fabricated mesa diode demonstrates short circuit current of 2.2 mA/cm2 and open circuit voltage of 0.8 V. TEM studies of the epilayer structural properties demonstrate high density of antiphase domains in the n-doped GaP buffer layer; however, they found to be annihilated in dilute nitride layer, where mainly threading dislocations were formed. EBIC investigation shows that crystalline imperfections of the heterostructure leads to poor carriers transport affecting the energy conversion efficiency. We performed numerical modeling concerning presence of the structural defects and discuss their influence on the diode performance. The carried out study is the initial step on the way to development of the growth technique of the GaP-based dilute nitride direct bandgap materials on Si for photonic and photovoltaic applications.

Development of atomic force microscopy combined with scanning electron microscopy for investigating electronic devices

Atomic force microscopy (AFM) was combined with scanning electron microscopy (SEM) to investigate electronic devices. In general, under observation using an optical microscope, it is difficult to position the cantilever at an arbitrary scan area of an electronic device with a microstructure. Thus, a method for positioning the cantilever is necessary to observe electronic devices. In this study, we developed an AFM/SEM system to evaluate an electronic device. The optical beam deflection (OBD) unit of the system was designed for a distance between the SEM objective lens and a sample surface to be 2 cm. A sample space large enough to place an actual device was created, using a scan unit fabricated with three tube scanners. The scanning ranges of the scan unit are 21.9 µm × 23.1 µm in the XY plane and of 2.5 µm for the Z axis. The noise density in the OBD unit was measured to be 0.29 pm/Hz0.5, which is comparable to noise density values reported for commercial AFM systems. Using the electron beam of SEM, the electron beam induced current (EBIC) is generated from a p–n junction of a semiconductor. Using the EBIC, the cantilever was positioned at the p–n-junction of a Si fast recovery diode (FRD). In addition, scanning capacitance force microscopy (SCFM) and Kelvin probe force microscopy (KFM) were combined with the AFM/SEM system. The SCFM and KFM signals were in qualitative agreement with the expected carrier density distribution of the p and n-regions of the Si-FRD.

Electron beam-induced current imaging with two-angstrom resolution

An electron microscope’s primary beam simultaneously ejects secondary electrons (SEs) from the sample and generates electron beam-induced currents (EBICs) in the sample. Both signals can be captured and digitized to produce images. The off-sample Everhart-Thornley detectors that are common in scanning electron microscopes (SEMs) can detect SEs with low noise and high bandwidth. However, the transimpedance amplifiers appropriate for detecting EBICs do not have such good performance, which makes accessing the benefits of EBIC imaging at high-resolution relatively more challenging. Here we report lattice-resolution imaging via detection of the EBIC produced by SE emission (SEEBIC). We use an aberration-corrected scanning transmission electron microscope (STEM), and image both microfabricated devices and standard calibration grids.

Scanning transmission electron microscope mapping of electronic transport in polycrystalline BaTiO3 ceramic capacitors

We report electronic transport mapping in a single dielectric layer of a polycrystalline BaTiO3 multilayer ceramic capacitor (MLCC) by electron beam induced current (EBIC) measurements using a scanning transmission electron microscope. Ga+ focused ion beam-lift out techniques with organometallic Pt-deposition are used to extract and electrically connect to these devices while maintaining high (&amp;gt;gigaohm) resistance between electrodes. Different modes of EBIC are observed depending on device resistivity. We demonstrate the use of EBIC resulting from secondary electron emission as a method for performing resistance contrast imaging (RCI), with resistive grain boundaries appearing as steps in EBIC contrast. These RCI maps are also used to calculate the potential and electric field of the device under an arbitrary bias. A mix of high- and low-resistance ohmic as well as rectifying grain boundaries is observed. These results help to better establish the distribution of resistivities critical to the prevention of performance-limiting current leakage in MLCCs.

Charge-induced pattern displacement in E-beam lithography

Electron beam lithography (EBL) requires conducting substrates to ensure pattern fidelity. However, there is an increasing interest in performing EBL on less well-conducting surfaces or even insulators, usually resulting in seriously distorted pattern formation. To understand the underlying charging phenomena, the authors use Monte Carlo simulations that include models for substrate charging, electron beam-induced current, and electric breakdown. Simulations of electron beam exposure of glass wafers are presented, exposing regular patterns which become distorted due to charge-induced beam deflection. The resulting displacements within the patterns are mapped and compared to experimental displacement maps obtained from patterns in PMMA resist on glass substrates. Displacements up to several hundreds of nanometers were observed at a primary beam energy of 50 keV. Also, various scan strategies were used to write the patterns, in the simulations as well as the experiments, revealing their strong effect on pattern distortion, in shape and in magnitude. A qualitative, in some cases even quantitative, good agreement was found between the simulations and the experiments, providing enough confidence in Monte Carlo simulations to predict charge-induced pattern displacement and shape distortion and to find smart scan strategies to minimize the effects of charging.

Electron-Beam-Induced Current Measurements of Thin-Film Solar Cells

The present tutorial review provides a practical guide to the analysis of semiconductor devices using electron-beam-induced currents (EBICs). The authors focus on cross-sectional EBIC measurements ...

Determination of Current Leakage Sites in Diamond p–n Junction

Current leakage sites in diamond p–n junction are determined by measuring an electron‐beam–induced current (EBIC) technique and conductive atomic force microscopy (C‐AFM). Current leakage sites in p–n diodes, particularly, can be determined by means of EBIC without destruction of the device structure. From EBIC observation, bright spots with higher signal intensity than these in surrounding region are observed in p–n diodes, and these spots strongly correlate with the leakage current. It is found that these bright spots are originating from pits of comet‐shaped defects observed by a scanning electron microscope. With C‐AFM, the leakage current is detected at pits of comet‐shaped defects. Reduction in comet‐shaped defects shall be reduced using the technique of reduction in dislocation and high‐quality crystal growth to suppress the leakage current.

Correlation of Structure and EBIC Contrast from Threading Dislocations in AlN/Si Films

Epitaxial growth of polar GaN is frequently made on AlN/Si substrates that contain a large density of threading dislocations induced by the mismatch of their lattice parameters and thermal expansion coefficients. In this paper, the structural and electrical properties of surface and extended defects found in 220 nm {0001} AlN films grown epitaxially on {111} oriented Si substrate by metalorganic chemical vapor deposition are characterized. Three types of surface depressions are recognized by atomic force microscopy (AFM) that correspond to V‐defects and their clusters and individual threading dislocations. Transmission electron microscopy studies prove that all V‐defects are attached to threading screw or mixed‐type threading dislocations. The electrical activity of the defects in the AlN film is assessed by measuring the electron beam induced current (EBIC) across Au/Ni/AlN Schottky barrier. The largest drop of EBIC compared to defect‐free regions is observed at clusters of V‐defects, which have a low density of (2.4 ± 0.4) × 107 cm−2. Only a very weak drop of EBIC is found at isolated threading dislocations, which have much higher density compared to clusters of V‐defects. Considering the defects studied, the electrical properties of AlN films are primarily affected by threading dislocations, despite the relatively weak recombination efficiency of individual dislocations.

Realization of p-type gallium nitride by magnesium ion implantation for vertical power devices

Implementing selective-area p-type doping through ion implantation is the most attractive choice for the fabrication of GaN-based bipolar power and related devices. However, the low activation efficiency of magnesium (Mg) ions and the inevitable surface decomposition during high-temperature activation annealing process still limit the use of this technology for GaN-based devices. In this work, we demonstrate successful p-type doping of GaN using protective coatings during a Mg ion implantation and thermal activation process. The p-type conduction of GaN is evidenced by the positive Seebeck coefficient obtained during thermopower characterization. On this basis, a GaN p-i-n diode is fabricated, exhibiting distinct rectifying characteristics with a turn-on voltage of 3 V with an acceptable reverse breakdown voltage of 300 V. Electron beam induced current (EBIC) and electroluminescent (EL) results further confirm the formation of p-type region due to Mg ion implantation and subsequent thermal activation. This repeatable and uniform manufacturing process can be implemented in mass production of GaN devices for versatile power and optoelectronic applications.

Investigation of defects influencing performance of type-II InAs/GaInSb superlattice based infrared PIN type photodetectors

We report on an investigation of dark current contributions from common microscale crystallographic defects in type-II InAs/GaInSb superlattice infrared PIN type photodiode structures grown on (100) GaSb substrates and have identified three general classifications. Defects on several wafers of varying design were examined from multiple perspectives to correlate electrical activity with structural properties, to develop the identification and classification scheme. Active defects were first identified by current density vs voltage (J-V) measurements and electron beam induced current (EBIC) scans of individual diodes with micrometer resolution. The EBIC scans were then correlated with plan-view optical and atomic force microscopy images, both before and after anisotropic etch-pit analysis using a newly developed etchant. The atomic scale structure of active and inactive defects was then compared using cross-sectional transmission electron microscopy (TEM) on vertical slices of defects extracted using focused ion beam milling. Analysis of the TEM images yielded important clues as to the structure and root causes of benign and active defects, in which only significant disruptions at the epi-substrate interface appear to play a key role in producing microscale defects that efficiently promote dark current.

Development of Electron Beam Induced Current Characterization of HgCdTe Based Photodiodes

In this paper, HgCdTe photodiodes are characterized by low temperature electron beam induced current (EBIC), with an emphasis on the impact of electron beam energy on the spatial resolution. Monte Carlo simulations are compared to experimental measurements using a comb shaped junction pattern. With a 15 keV electron beam, both converge to a resolution in the 1.3–1.4 μm range. On cross-section samples, lowering the beam energy to 2 keV leads to a 40 nm resolution. In regard to photodiode technologies, namely extrinsic p-on-n and intrinsic n-on-p Hg0.7Cd0.3Te, top view and cross-section typical EBIC characteristic decay lengths are measured. While EBIC exponential decays show no bias voltage dependence suggesting that the system is dominated by diffusion and not drift, the impact of the injection level on transport properties is discussed in regard to the literature.

The use of HfO2 in a point contact concept for front interface passivation of Cu(In,Ga)Se2 solar cells

We report on the use of a high bandgap metal-oxide at the front interface of Cu(In,Ga)Se2 (CIGS) solar cells in a point contact concept for reduced interface recombination. Highly resistive HfO2 is applied on the CIGS surface by atomic layer deposition (ALD). Aspects of the surface passivating effect of HfO2 on CIGS were investigated by time-resolved photoluminescence (TRPL), electron beam induced current (EBIC) and capacitance-voltage (C-V) measurements. Two structuring methods for point contact formation are compared, a lithographic top-down and a simple bottom-up approach using NaCl as template. The former method employed a plasma etch step which was found to degrade the performance of solar cells when applied on the CIGS surface. The template method omitted sputtering and allowed patterning of HfO2 up to 10 nm thickness without adversely impacting the open-circuit voltage (VOC). EBIC revealed an improved carrier collection due to the HfO2 coating and a long term stable PL decay was observed. Yet, the point contact concept with HfO2 was not significantly influencing the performance of a CIGS solar cell for the investigated parameter range.

Understanding the charge transport mechanisms through ultrathin SiOx layers in passivated contacts for high-efficiency silicon solar cells

We report on the microscopic structure of the SiOx layer and the transport mechanism in polycrystalline Si (poly-Si) passivated contacts, which enable high-efficiency crystalline Si (c-Si) solar cells. Using electron beam induced current (EBIC) measurements, we accurately map nanoscale conduction-enabling pinholes in 2.2 nm thick SiOx layers in a poly-Si/SiOx/c-Si stack. These conduction enabling pinholes appear as bright spots in EBIC maps due to carrier transport and collection limitations introduced by the insulating 2.2 nm SiOx layer. Performing high-resolution transmission electron microscopy at a bright spot identified with EBIC reveals that conduction pinholes in SiOx can be regions of thin tunneling SiOx rather than a geometric pinhole. Additionally, selectively etching the underlying poly-Si layer in contacts with 1.5 and 2.2 nm thick SiOx layers using tetramethylammonium hydroxide results in pinhole-like etch features in both contacts. However, EBIC measurements for a contact with a thinner, 1.5 nm SiOx layer do not reveal pinholes, which is consistent with uniform tunneling transport through the 1.5 nm SiOx layer. Finally, we theoretically show that reducing the metal to the c-Si contact size from microns, like in the p-type passivated emitter rear contact, to tens of nanometers, like in poly-Si contacts, allows lowering of the unpassivated contact area by several orders of magnitude, thus resulting in excellent passivation, as has been demonstrated for these contacts.

Charging effect of polymer thin film under irradiation of high-energy transmission electron beam

The serious charging effect of polymer film with a thickness of the order of microns under the radiation of high-energy transmission electron beam, on the reliability of the micro-nano electronic device in electron microscopy detection is investigated. The charging effect of the polymer film is numerically calculated in this paper. The scattering process is simulated by the Monte Carlo method. The elastic scattering is calculated with the Rutherford scattering model. The inelastic scattering is simulated with the fast secondary electron (SE) model and the Penn model. The transport, the capture, and the recombination process of the charges are treated with the finite difference method. The fourth-order Runge-Kutta method is used to solve the trajectory of the emitted SEs. The dynamic distributions of the net charge, the built-in electric field, the surface emission current, and the transmission current are investigated, and the influence of the film thickness and the beam energy on the charging characteristics are analyzed. The results show that due to the emission of electrons near the sample surface, the distribution of the net charge in the sample is first positive and then negative along the incident direction. In addition, under the irradiation,higher charge quantity is deposited in the sample, and the net charge density increases gradually. However, with long-time irradiation, the deposited electrons transport to the surface under the action of built-in electric field which reduces the surface net charge density. Therefore the net charge density tends to a stable value. The space potential is positive in the surface and negative inside the sample. Therefore some emitted SEs return to the surface, resulting in the electron beam-induced current. With the irradiation, the positive surface potential increases and tends to a stable value. Hence the actual surface emission current decreases to a stable value and the sample current increases to a stable value. The sample current remains unchanged due to the weak charging strength. Increasing the film thickness leads the transient time to increase, which contributes to the decline of the surface potential and the increase of the actual emission currentand sample current. The increase of the beam energy causes the transmission current to increase and the sample current to decrease. In addition, it reduces the positive surface potential and the actual surface emission current accordingly. The results conduce to the decrease of the charging effect of the polymer film under the radiation of high-energy electron beam in the electron microscopy.

Microstructural investigation of defects in photovoltaic cells by the electron beam-induced current method

This work aims to clarify the application of electron beam-induced current (EBIC) method for the morphological analysis and detection of local defects and impurities in semiconductor structures such as solar cells. One of the advantages of this method is to observe a leakage path and microplasma sites with nanometer resolution. This technique allows to precisely locate the affected area and determine the type of defect that cannot be commonly characterized with sufficient accuracy. Simultaneously, a focused ion beam could be used to determine junction by milling of the samples at the area of interest. The evaluation results of experimental measurement using these techniques on photovoltaic cells illustrates the applicability and importance of the EBIC method.

Growth optimization and characterization of regular arrays of GaAs/AlGaAs core/shell nanowires for tandem solar cells on silicon

With a band gap value of 1.7 eV, Al0.2Ga0.8As is one of the ideal III–V alloys for the development of nanowire-based Tandem Solar Cells on silicon. Nevertheless, growing self-catalysed AlGaAs nanowires on silicon by solid-source molecular beam epitaxy is a very difficult task due to the oxidation of Al adatoms by the SiO2 layer present on the surface. Here we propose a nanowire structure including a p.i.n radial junction inside an Al0.2Ga0.8As shell grown on a p-GaAs core. The crystalline structure of such self-catalysed nanowires grown on an epi-ready Si(111) substrate (with a thin native SiO2 layer) was investigated by transmission electronic microscopy and photoluminescence. I(V) measurements performed on single nanowires have shown a diode-like behaviour corresponding to the radial p.i.n junction inside the Al0.2Ga0.8As shell. Moreover, a current generation under the electron beam was evidenced over the entire radial junction along the nanowires by means of electron beam induced current (EBIC) microscopy. The same structure was reproduced on patterned substrates with a SiO2 mask, producing an ordered hexagonal array. High and uniform yields from 83% to 87% of vertical nanowires were obtained on 0.9 × 0.9 cm2 patterned areas. EBIC mapping performed on these nanowires confirmed the good electrical properties of the radial junction within the nanowires.

Ageing of kesterite solar cells 2: Impact on photocurrent generation

As yet results of ageing experiments of Kesterite solar cells have not been published. However, at least in crystalsol detailed investigations of monograin-based solar cells have been performed proving very different stabilities depending on the exact composition of the Kesterites. Completely stable solar cells even without any sealing have been demonstrated internally. However in this paper we report on ageing effects observed for internally called “highly unstable” Cu2ZnSnS4 (CZTS) solar cells aged for 8.5 and 95.5 h at 105 °C in air, already also reported in part 1 of these investigations. During these times the initial efficiency of 5.4% decreased by 76% and another 88%, respectively. The photocurrent generation was investigated in detail by light beam induced current (LBIC) measurements of the front side and electron beam induced current (EBIC) measurements of cross sections of the CZTS/CdS/ZnO active junctions. Besides a pronounced decrease of the LBIC and the EBIC signal also a shift of the EBIC signal much further into the Kesterite material was observed, which can be explained by the in part 1 already found diffusion of copper from the surface of CZTS into the CdS buffer layer acting finally as a strong charge carrier recombination center. Thereby the EBIC signal first observed mainly close to the interface has vanished, only a small EBIC signal in a depth of around 400 nm still remains visible, a depth, from which only minority carriers can still reach this main recombination center in the interface.