Comprehensive Characterization of Extended Defects in Semiconductor Materials by a Scanning Electron Microscope

Mapping of extended defects in B-doped (001) homoepitaxial diamond films by electron-beam-induced current (EBIC) and cathodoluminescence (CL) combination technique

Optically addressable point defects in semiconductor materials have been identified as promising single-photon sources and spin qubits in quantum information technologies. The traditional method of exploring the optical and spin properties of these defects is using a laser with a wavelength shorter than the point defects’ zero-phonon-line (ZPL) to Stokes exciting and detecting the Stokes photoluminescence (PL). On the other hand, anti-Stokes excitation with the pumping laser’s wavelength longer than the defects’ ZPL can also be used to investigate their optical and spin properties. The anti-Stokes excitation has shown many advantages and attracted great interest. Here, we provide a brief review of the anti-Stokes excitation of optically active point defects in semiconductor materials. The Stokes and anti-Stokes PL spectra of different point defect systems in semiconductor materials are compared. We then discuss the main mechanisms of the anti-Stokes excitation of different physical systems and conclude that the anti-Stokes excitation of the point defect system in the semiconductor is a single-photon absorption phonon-assisted process. Finally, we summarize some practical applications of anti-Stokes excitation, including laser cooling of semiconductor materials, high-sensitivity quantum thermometry, and enhancement of the readout signal contrast of the point defect spin states. The anti-Stokes excitation of point defects in semiconductors extends the boundary of quantum technologies.

Optical and electron beam studies of gamma-irradiated AlGaN/GaN high-electron-mobility transistors

Deep‐level defects in semiconductor materials usually induce carrier trapping and non‐radiative recombination. However, defects caused by unintentional contaminants in antimony selenide (Sb2Se3) solar cells have rarely been reported. Herein, the correlation between defect properties and unintentional impurities in the Sb2Se3 absorber is investigated, which is prepared by injection vapor deposition with Sb2Se3 source purities ranging from 99.9% to 99.9999%. The analysis of deep‐level transient spectra reveals that an increase in impurity concentration does not result in new defect types. Nevertheless, the higher impurity level causes an increase in both the defect density and the capture cross section of the original defect. To address this challenge, defect engineering is developed to regulate the growth of the Sb2Se3 absorber. This strategy completely suppresses deepest defect states E3, a mixture of intrinsic defect (SbSe) and impurity Si related defect. As a result, the carrier lifetime increases significantly from 0.37 to 3.4 ps. This enables to fabricate Sb2Se3 solar cells with a power conversion efficiency of 10.41%. This work uncovers the characteristics of unintentional contaminant‐related defects, and provides a strategy for fabricating highly efficient Sb2Se3 solar cells with low‐purity source materials.

While the use of electron probe microanalysis (EPMA) is widespread in the geological and metallurgical sciences, it remains less prevalent in the field of semiconductor research. For these materials, trace element (i.e. dopant) levels typically lie near or beneath the detection limit of wavelength-dispersive Xray (WDX) spectrometers, while alloy compositions of ternary mixtures and multilayer structures can more readily be determined using X-ray diffraction techniques. The electron beam measurements more commonly applied to semiconductors remain transmission electron microscopy (for structural characterization), and scanning electron microscopy (topographic, optical and electrical information). Despite this, there are many aspects of the EPMA that make it an attractive platform for all of thesetypes of semiconductor characterization, particularly when combining compositional information fromWDX with complementary and simultaneously-acquired signals. These advantages include: built-inlight optics; a stable, quantified and high-current beam; and a combined large-area and high-resolutionmapping capability. This allows the measurement of cathodoluminescence (CL), electron beam-inducedcurrent (EBIC) and electron channelling contrast imaging (ECCI) signals alongside WDX, which weapply to the investigation of visible and UV AlxInyGa1-x-yN materials, devices and nanostructures.

Cathodoluminescence and photoluminescence measurements are commonly accepted as revealing local properties of a specimen region excited by a beam of electrons or photons. However, in the presence of a strong electric eld (e.g. a junction) an electron (or light/laser) beam-induced current is generated, which spreads over the structure. A secondary non-local electroluminescence, generated by this current and detected together with the expected luminescence signal, may strongly distort measurement results. This was con rmed by cathodoluminescence measurements on test structures prepared by focused ion beam on AlGaAs/GaAs/InGaAs laser heterostructures. Methods for minimizing the distortion of measured luminescence signals are presented.

Analysis of Cu(In,Ga)(S,Se) 2 thin-film solar cells by means of electron microscopy

The state-of-the-art power switching devices made from SiC and GaN semiconductors contain a high density of crystal defects. Most of these defects are present in starting wafers and some are generated during device processing. There is little conclusive evidence so far on the exact role that the crystal defects play on device performance, manufacturing yield, and more importantly, long-term field-reliability especially when devices are operating under extreme stressful environments. This paper provides a review of the current state-of-the-art of SiC and GaN power semiconductor material technology, and the potential impact crystal defects may have on high-density power switching electronics. Group III-Nitride materials suffer from both extended and point defects, each of which will challenge the material’s application in both vertical and lateral power devices. The extended defects include vertical threading dislocations of both edge and screw type. The latter defects have been shown to be correlated to leakage in vertical two terminal device structures while the influence of the former is still undetermined and remains a critical research issue. Channel surfaces in vertical three terminal devices will also degrade due to vertical threading dislocations. These extended defects occur in all epitaxial layers grown on c-plane substrates (the predominant and largest area substrate type) and are the result of the lack of a high quality substrate bulk material as well as substrate surface. The present substrate technology after multiple efforts (either GaN, SiC or Si) still have a high defect density indicating that low-defect seeds may be non-existing. Point defects influence the background carrier concentration in low-doped layers and various recombination processes. The intrinsic nature of III-N materials makes nitrogen-vacancies a predominant point defect that automatically dopes the crystal n-type, but there are impurities (oxygen and carbon) from most growth environments that also contribute to conductivity. These defects are not well understood and not well controlled. Such defects must be minimized in order to realize thick epitaxial layers with drift regions that will support both high blocking voltages and low on-state resistances. A wide range of materials characterization methods have been applied to further understand these impurities. The semiconductor and dielectric materials used in the construction of power electronics switching devices employed for energy efficiency applications experience extreme electrical and thermal stresses during the on-state as well as when switching high voltages and high currents. For example, in a bipolar type power semiconductor switching device in order to achieve significant conductivity modulation and reduce the drift-region resistance, high-level injection of minority carriers is typical. If the semiconductor contains a high density of crystal defects, as is the case with WBG semiconductors, minority carrier recombination phenomenon can be adversely affected. For example, it has been shown more than a decade ago [3, 4] that basal plane dislocations (BPDs) present in SiC are excited by the energy released from minority carrier recombination process; the result is excess BPD generation and its glide within the semiconductor material that leads to an increase in the drift-region resistance and thermal run-away in a forward biased bipolar junction diode The reported experimental data for high-voltage SiC semiconductor strongly suggests that material defects have profound impact on its characteristics under extreme environments. Since GaN semiconductor has even higher density of material defects, this problem is further exacerbated especially in thick epitaxial GaN layers which are required for power switches beyond 5kV. The integration of the pillar junction scheme with the trench-gate GaN MOSFET processes developed at the University of Maryland will be reported. The thick pillar junction epitaxial layers have been grown by a commercial vendor, using a windowed GaN growth process and/or selective deep etching, followed by GaN regrowth to fabricate columns with opposite carrier polarities. The doping density of the n--pillars under the gate has been specified with background carrier concentration n ≤ 1x1016 cm-3, while the p-pillars will have a higher doping density (p ≥ 1x1018 cm-3) to maximize depletion in the n--pillar. The p-body and n+-source layers are grown by subsequent epitaxial growth across the whole structure. Our current research into optimized trench gate fabrication and gate dielectric processing has been used to fabricate gate trenches into the structure. These devices can be compared to traditional trench MOSFETs with extremely thick blocking layers of comparable thickness to the pillar junction layer in order to determine the role of defects in thick GaN epi layers in terms of limiting device performance and reliability.

The scanning electron microscope (SEM) techniques play a key role in the characterization of various inorganic and/or organic semiconducting materials, micro-/nanostructures and devices. The power of the SEM methods is mainly in imaging, characterization and diagnostics of local near surface properties. Among a variety of the SEM methods, Cathodoluminescence (CL) and Electron Beam Induced Current (EBIC) methods have been extensively used for characterization of generation/recombination phenomena and electrical properties of bulk semiconductors and semiconductor structures [1]. Exploitation of these methods allow to visualize the electrically active defects such as dislocations, grain boundaries and inhomogeneities and to estimate the diffusion length, lifetime and surface recombination velocity of minority carriers. In addition, EBIC can be used for investigation and diagnostics of SCR of Schottky and p-n junctions, which extends its applications, in combination with other techniques such as Voltage Contrast (VC), to functional diagnostics of electronic devices biased at operation point. Currently, various modifications and improvements of “standard” SEM methods are developed to improve their resolution and sensitivity for investigations of advanced structures and devices structural, optical and electrical properties. Various sample preparation and micromanipulation / microprobe techniques are examined to localize appropriate signals with the aim to improve the spatial resolution while for extreme depth resolution low acceleration voltages of SEM techniques are utilized [2]. In addition, complex signal acquisition and processing techniques are implemented to achieve time and frequency resolution and to gain required information about the sample properties.

In x Ga 1−x As/In y Ga 1−y As strain-balanced quantum well cells (QWCs) have been shown to be beneficial for photovoltaic applications in particular to extend the light absorption edge of a single-junction cell toward the near infrared with a lower reduction of the open-circuit voltage compared to a single band-gap cell. The strain-balancing condition ensures that the multi-quantum well as a whole does not relax. However, if the mismatch between wells and barriers exceeds a critical limit, the structure becomes vulnerable to morphological or compositional fluctuations, which can lead to a local structural breakdown with the generation of extended defects of a completely different nature from misfit dislocations. In this work, we investigated a series of strain-balanced InGaAs QWCs grown on InP for thermophotovoltaic applications by means of electron-beam-induced current (EBIC) and cathodoluminescence (CL) measurements. Despite being electrically active, these defects appear to have a minor impact on the dark current of the cells but cause a drop of the photocurrent at relatively low forward bias voltage. The higher carrier collection efficiency revealed both by EBIC and CL at the boundaries of the defects suggests that a notch in the valence band edge limits the collection of holes generated in the MQW and the energy states, induced by the defects inside the energy gap, assist the tunneling of holes through the notch. At zero bias, the overall reduction of the collection efficiency is of the order of a few percent but the rate of recombination of photogenerated carriers increases dramatically with increasing forward-bias voltage as the junction built-in field drops more rapidly where the density of in-gap states is higher.

We report on 0.93 eV luminescence observed in multicrystalline silicon. The spectral line is close to the well known D3 one, but its properties are different. The new feature shows a remarkable intensity at room temperature, exceeding the intensity of the band to band radiative transition. Moreover, it appears as a single line in the entire temperature range 10-300K, in contrast to the D3, which is usually accompanied by D4. Cathodoluminescence (CL) and electron beam induced current (EBIC) micrographs revealed that the centers causing 0.93 eV emission are irregularly distributed along certain grain boundaries. Electron backscattering diffraction examination showed that the 0.93 eV luminescence appears at grain boundaries characterized by a lattice rotation around a <344> axis. The EBIC contrast at those irregularities indicates strong total recombination. Based on an analysis of the temperature dependence of the CL intensity and the EBIC contrast we obtained an activation energy of about 120 meV.

Advancing photocatalytic performance for enhanced visible-light-driven H2 evolution and Cr (VI) reduction of g-C3N4 through defect engineering via electron beam irradiation

Applications of the scanning electron microscope EBIC mode to semiconductor device evaluation and failure analysis

High resolution electron beam induced current (EBIC) analyses were carried out on InAs quantum dots (QDs) grown on GaAs substrate, in a scanning electron microscope (SEM) and a conducting atomic force microscope (C-AFM) hybrid system. This scanning near-field EBIC microscope allows one to image a sample conventionally by SEM, to investigate by AFM the local topography and to simultaneously perform EBIC imaging. The EBIC capabilities of this combined instrument are evidenced by imaging plastic slip lines created by nanoindentation, and its performance in resolution is demonstrated to be of the order of 20nm by imaging the induced current flowing through the InAs∕GaAs QDs and the ringlike structures that surround them. The influence of the presence of QDs on the minority carrier diffusion length is also investigated. It is shown that the presence of QDs close to the surface sample increases the diffusion length.

Breakdown sites in multicrystalline Si solar cells have been studied by reverse-bias electroluminescence, electron beam induced current (EBIC) and laser beam induced current (LBIC), and Energy Dispersive X-Ray Spectroscopy methods. In the breakdown sites revealed by EL at small reverse bias (~5 V), the enhanced aluminum and oxygen concentration is revealed. Such breakdowns can be located inside the depletion region because they are not revealed by the EBIC or LBIC methods. Breakdowns revealed by EL at larger bias correlate well with extended defects in the EBIC and LBIC images.