Remote Electron Beam Induced Current Research Articles

Growth and characterization of Mn-doped In2O3 nanowires and terraced microstructures

Mn-doped In2O3 micro- and nanostructures were grown by the thermal evaporation–deposition method using a precursor powder composed of In2O3 and a manganese compound. Terraced microstructures grow at 1200°C when using Mn2O3 in the precursor powders, while thinner micro- and nanowires have been obtained at temperatures between 560 and 780°C by using MnCO3 in the precursor mixture. The morphology, as well as the luminescencent, electrical and chemical properties, of the Mn-doped In2O3 micro- and nanostructures were characterized by means of X-ray diffraction, scanning electron microscopy, electron backscattered diffraction, cathodoluminescence (CL), energy dispersive X-ray spectroscopy, remote electron beam induced current (REBIC) and X-ray photoelectron spectroscopy (XPS). The amount of Mn incorporated in these low-dimensional structures is below 1at.%. The Mn-doped In2O3 structures show luminescence at 1.91 and 2.27eV, related to defects associated with an excess of oxygen and single ionized oxygen vacancies (VO+), respectively. An emission at 2.94eV, possibly due to the direct band gap of In2O3, has been also observed. The presence of VO+, which tend to accumulate at the edges of the terraces of the microstructures, has been traced by combined CL–REBIC–XPS techniques, and has been related to the incorporation of manganese as Mn2+.

Growth by thermal evaporation of Al doped ZnS elongated micro- and nanostructures and their cathodoluminescence properties

Wurtzite Al doped ZnS micro- and nanostructures have been grown by an evaporation method. Thermal treatments have been performed during 2h at a temperature of 1200°C in a N2 overpressure environment. Columns, rods and needles are the main morphologies observed. Their structural properties have been studied using Scanning Electron Microscopy (SEM), Electron Backscattering Diffraction (EBSD), X-ray Diffraction (XRD) and micro-Raman spectroscopy. Cathodoluminescence (CL) measurements of the structures show an increase of the emission related to the band edge, associated with Al doping. Correlated CL and Remote Electron Beam Induced Current (REBIC) images reveal the presence of charged planar defects and a non-uniform spatial distribution of the defect emission in the structures.

Characterization of Grain Boundaries of Lead-Free Semiconducting Ceramics Using BaTiO<sub>3</sub>-(Bi<sub>1/2</sub>K<sub>1/2</sub>)TiO<sub>3</sub> System

BaTiO3–(Bi1/2K1/2)TiO3 (abbreviated as BT-BKT) solid solution ceramics, as a lead-free PTC (positive temperature coefficient of resistivity) thermistor material usable over 130°C, has been synthesized by sintering in N2 atmosphere and after annealing in air over 1200°C. In the BT-BKT ceramics with PTC property, the impedance/modulus spectroscopic plots have revealed that a third resistance-capacitance (RC) response besides grains and grain boundaries. Using the remote electron beam induced current (REBIC) configuration, imaging has revealed EBIC contrast consistent with the presence of negatively charged electrostatic grain boundary barriers in the BT-BKT semiconducting ceramics.

Direct observation of potential barrier formation at grain boundaries of SnO2 ceramics

Remote electron beam induced current (REBIC) and cathodoluminescence (CL) modes in the scanning electron microscope (SEM) have been used to investigate SnO2 sintered samples. The study of the electrically active boundaries present in the oxide shows a characteristic peak and trough (PAT) contrast after thermal treatments in oxygen. Temperature-dependent measurements of the REBIC contrast show the presence of a shallow defect level 60 meV below the conduction band. This level is asigned to oxygen species adsorbed on the defect-rich boundaries. Evolution of REBIC contrast of the grain boundaries with excitation density enabled us to perform local measurements of minority carrier diffusion length.

Direct Characterization of Grain-Boundary Electrical Activity in Doped (Ba0.6Sr0.4)TiO3by Combined Imaging of Electron-Beam-Induced Current and Electron-Backscattered Diffraction

Simultaneous measurements of remote electron beam induced current (REBIC) and orientation imaging microscopy (OIM) in a scanning electron microscope (SEM) have been applied to a polycrystalline (Ba0.6Sr0.4)TiO3 with a positive temperature coefficient of resistivity (PTCR) to elucidate a grain-boundary character dependence of the potential barrier formation. The absence of electrical activity in a coherent Σ3 twin boundary is clearly imaged. The resistivity of individual grain boundaries estimated from a resistive contrast image is interpreted in terms of geometrical coherency, which is defined by the degree of coincidence in the reciprocal lattice points.

Conductive mode imaging of thermistor grain boundaries

PTC thermistors undergo a rapid increase in grain boundary resistivity, covering several orders of magnitude, at temperatures just above the Curie temperature, T C, which is associated with a ferroelectric to paraelectric phase transformation. In this study, hot-stage conductive mode scanning electron microscopy has been used to investigate the characteristics of individual thermistor grain boundaries over a range of temperatures around T C. Using the remote electron beam induced current (REBIC) configuration, imaging has revealed EBIC contrast consistent with the presence of negatively charged electrostatic grain boundary barriers for the first time in a commercial thermistor. Not all grain boundaries within the thermistor were found to be EBIC active and the EBIC contrast was only observed at temperatures above T C. EBSP analysis of grain boundary crystallography indicated that the EBIC active grain boundaries were predominantly high angle.

Study of carrier recombination at structural defects in InGaN films

A series of 100 nm thick InGaN films with Indium content up to 14% has been grown by MOVPE on SiC substrates. Cathodoluminescence (CL) and remote electron beam induced current (REBIC) in the scanning electron microscope have been applied to investigate with high spatial resolution the recombination of carriers at the structural defects present in the films. The observed defects are mainly pinholes formed at the surface. The density of pinholes increases with the In content in the layers, which can be explained by elastic relaxation at pinholes. CL images show the spatial distribution of the emission sites. For pinholes with diameter in the μm range we observe enhanced luminescence around the pinhole and a reduced luminescence at the apex. Pinholes are observed in REBIC images as dark spots occasionally surrounded by a bright halo. The halo spreads over an area larger than the pinhole, with a diameter of about 3–4 μm. Also a cell-like dislocation structure has been observed in some samples in the CL and REBIC images. CL spectra show, as common features of the samples, a complex emission in the blue range and a broad structured band centered around 670 nm. The influence of the inhomogeneous Indium incorporation on the luminescence of the films and of charged defects on the observed REBIC contrast is discussed.

Depth and lateral extension of ion milled pn junctions in CdxHg1−xTe from electron beam induced current measurements

Ion milling has been used to convert molecular beam epitaxy vacancy-doped CdxHg1−xTe from p- to n-type. Electron beam induced current and remote electron beam induced current (REBIC) measurements have been performed to study the pn junction depth and lateral extension dependence on the milling time, milling current, and vacancy concentration. The conversion depth is linear with the milling time and current and inversely proportional to the vacancy concentration in layers thinner than 10 μm. This shows that filling of Hg vacancies in this region during conversion is limited by the rate of supply of extra Hg from the milling. The lateral extension also increases linearly with the milling time, the ratio of the lateral extension to the depth being ∼0.5. One can therefore use REBIC on the top surface to determine the junction depth, which greatly simplifies the measurement and does not destroy the diodes.

STM-REBIC study of nanocrystalline and crystalline silicon.

ABSTRACTElectrically active regions of nanocrystalline silicon (nc-Si) films as well as of a p-type crystalline silicon (c-Si) wafer have been investigated by using a scanning electron microscope/scanning tunneling microscope (SEM/STM) combined instrument. The nc-Si films were obtained by boron implantation of amorphous silicon layers with an average nanocrystal size of about 10 nm. STM current constant images reveal a cell structure in the nc-Si films which was also revealed in the STM remote electron beam induced current (REBIC) images with a resolution of up to 20 nm. The contrast in the STM-REBIC images indicate the existence of space charge regions at the boundaries. The influence of the thermal treatment on the cell structure was studied. For comparison, SEM-REBIC and STM-REBIC images from c-Si wafer were obtained.

REBIC Studies of Heterojunction Gas Sensor

The elec. structure of a heterojunction gas sensor formed from p-type copper oxide and n-type zinc oxide has been investigated using hot-stage remote electron beam induced current (REBIC) microscopy. The heterojunction interface was found to be highly resistive compared with the bulk copper and zinc oxides. Elec. active barriers, displaying the characteristic EBIC contrast of a p-n junction, were obsd. at points of contacts between zinc oxide and copper oxide grains. The EBIC peak height was found to increase linearly with bias under conditions of reverse bias and decrease with forward bias. When the sensor was heated to 150 DegC contrast changes suggesting a broadening of the depletion region and an increase in the minority carrier diffusion lengths within the copper oxide and the zinc oxide were obsd. [on SciFinder (R)]

Crystallographic control of the barrier structure in zinc oxide varistors

Scanning electron microscope (SEM) based remote electron beam induced current (REBIC) microscopy has been used to investigate the electrical characteristics of individual grain boundaries in a zinc oxide based varistor. Although some grain boundaries showed the expected ‘bright and dark’ contrast consistent with symmetrical, opposed, electric fields on either side of a charged grain boundary, the majority of interfaces are electrically asymmetric showing only either bright or dark contrast. In these cases, the application of an external voltage bias to the grain boundary of several ten’s of millivolts is necessary to restore a symmetrical barrier structure. The orientations of grains on either side of each grain boundary were determined using electron backscattered pattern (EBSP) analysis and the grain boundary plane orientation was calculated by observing the lateral shift of the grain boundary EBIC contrast peak with SEM beam voltage and, hence, penetration depth. It was found that the electrical asymmetry is governed by the orientations of the grain boundary planes on either side of the interface, demonstrating some crystallographic control of the barrier structure.

Study of growth hillocks in GaN:Si films by electron beam induced current imaging

Remote electron beam induced current (REBIC) measurements have been carried out to investigate electrically active regions in Si doped GaN films. REBIC bright–dark contrast has been observed in the border of growth, round or pyramidal, hillocks, while pyramidal hillocks also show bright contrast at the center. The results are explained by the inhomogeneous distribution of charged point defects and impurities at the hillocks.

Scanning electron microscopy study of twins in ZnSe single crystals grown by solid-phase recrystallization

ZnSe single crystals were grown from n-type microcrystalline boules by a solid phase recrystallization (SPR) method. During SPR, twinned regions appear with different electronic recombination properties. The recrystallizations were performed under different atmospheres, Ar or Se, and pressures to investigate the influence of growth conditions on these structural features. Recombination properties were studied by means of cathodoluminescence (CL) and remote-electron beam induced current (REBIC). Wavelength dispersive X-ray (WDX) mappings were also performed to analyze possible differences in stoichiometry related to the presence of extended defects.

Crystal plane influence of the EBIC contrast in zinc oxide varistors

Scanning electron microscopy (SEM) based remote electron beam induced current (REBIC) microscopy has been used to investigate the electrical characteristics of individual grain boundaries in a zinc oxide based varistor. Although some grain boundaries showed ‘bright and dark’ contrast consistent with symmetrical, opposed, electric fields on either side of a charged grain boundary, the majority of interfaces were found to be electrically asymmetric showing only either bright or dark contrast. In these cases, the application of an external voltage bias across the grain boundary of several 10′s of mV was necessary to restore the symmetrical structure. The orientations of grains on either side of grain boundaries showing each of these contrast types were determined using electron backscattered diffraction (EBSD) analysis and the grain boundary plane orientation was established using depth resolved EBIC. It was found that the asymmetry in the electrical structure is governed by the orientations of the grain boundary planes on either side of the interface, demonstrating some crystallographic control of the electrical character of the barrier structure.

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.

Direct Observation of the Barrier Structure of a Praseodymium-Doped Grain Boundary in a Zinc Oxide Bicrystal by Charge Collection Microscopy

A zinc oxide bicrystal, in which the grain boundary was doped with praseodymium oxide, was prepared by hot pressing. The grain boundary showed characteristic varistor breakdown. Scanning electron microscope (SEM) observation revealed that the amount of visible praseodymium oxide along the interface was variable. Charge collection microscopy, using the remote electron beam induced current (REBIC) configuration, was carried out and the grain boundary was found to be electrically active in the praseodymium oxide rich regions, showing contrast consistent with the presence of a double Schottky barrier (DSB). In Pr poor regions the DSB contrast was lost. The observations are consistent with the idea of barrier enhancement with doping in varistors.

The effect of sintering temperature variations on the development of electrically active interfaces in zinc oxide based varistors

A series of zinc oxide based varistors containing 0.5 wt.% Bi 2O 3 and 0.5 wt.% Mn 2O 3 were prepared by a conventional mixed oxide route and sintered at temperatures between 950 and 1300°C. All samples showed the varistor effect, although as the sintering temperature was increased above 1000°C, the non-linear coefficient decreased from 22 to 3 at 1300°C. Local grain boundary property measurements were carried out using remote electron beam induced current (REBIC) configuration conductive mode scanning electron microscopy. The proportions of electrically active interfaces and those showing strong resistive contrast were found to increase with sintering temperature.

Electrical characterization of ZnO ceramics by scanning tunneling spectroscopy and beam-induced current in the scanning tunneling microscope

A correlative study of the electrically active grain boundary structure of ZnO polycrystals has been carried out using a scanning electron microscope/scanning tunneling microscope (SEM/STM) combined instrument. Current imaging tunneling spectroscopy (CITS) measurements reveal reduced surface band gaps, as compared with grain interiors, at the charged boundaries imaged by SEM-based remote electron beam induced current (REBIC). ZnO grain boundaries were also imaged in the STM-REBIC mode with a resolution of up to 20 nm. The contrast differences observed in the SEM-REBIC and STM-REBIC images are discussed in terms of the different experimental conditions used in both techniques.

Electron beam induced current and remote electron beam induced current assessment of chemical vapor deposited diamond films

In the present work, electron beam induced current (EBIC) has been applied to characterize several kinds of chemical vapor deposition diamond films. Regions of enhanced carrier recombination are detected in plan-view observations of thin films as well as in cross sections of thick films. Remote EBIC (REBIC) has been applied to obtain information about charged defects present in the samples. The dependence of EBIC and REBIC contrast on the contact configuration used, and on the observation conditions has been analyzed. Cathodoluminescence images of the same samples have been recorded for comparison.

Characterization of charged defects in CdxHg1−xTe and CdTe crystals by electron beam induced current and scanning tunneling spectroscopy

A correlative study of the electrically active defects of CdxHg1−xTe and CdTe crystals has been carried out using a scanning electron microscope/scanning tunneling microscope (SEM/STM) combined system. Charged structural and compositional defects were revealed by the remote electron beam induced current (REBIC) mode of the scanning electron microscope. The electronic inhomogeneities of the samples were analyzed with nm resolution by current imaging tunneling spectroscopy (CITS) measurements, which showed the existence of built-in electrostatic barriers as well as local variations of the surface band gap in the defect areas imaged by REBIC.