Grain Boundaries In Semiconductors Research Articles

(Invited) Two-Dimensional Characterization of Wide-Bandgap Materials and Contact Interfaces by Using Scanning Internal Photoemission Microscopy

Scanning internal photoemission spectroscopy (SIPM) has been developed to map the electrical characteristics of metal/semiconductor interfaces nondestructively. Our experimental demonstrations of the mapping characterization are reviewed from the several aspects. (1) Thermal degradation: For the development of high-power devices, the thermal degradation is more important. In the case of Ni/n-SiC Schottky contacts, the local interfacial reaction between Ni and SiC was observed in the photoyield (Y)image after annealing at 500° C or higher. The lower Schottky barrier height was determined in such regions. In the case of Au/Ni/n-GaN Schottky contacts, we found that surface scratches on the metal dot enhanced interfacial degradation.(2) Device degradation by applying high-voltage: Applying reverse bias voltage (Vbias) down to −45 V during theYmeasurement is possible in our SIPM. For most of the Ni/n-GaN Schottky contacts formed on a thick n-GaN layer grown on a freestanding GaN substrate, uniform distribution of Y was observed over the electrode. On the other hand, for the contacts with a slightly larger reverse current, the Y distribution was also uniform at Vbias = 0 V, but over Vbias = −36 V, Y was intensively increased at small spots. After the SIPM measurements, the I–V characteristics became leaky, and the same spots were observed in the microscope image. These results indicate that SIPM is useful for in situ monitoring of the initial stage of the degradation under applying reverse bias voltage.(3) Process-induced surface damages: The electrical characteristics of the Schottky contacts have a sensitive nature to the damages located on the semiconductor surface. We applied SIPM to the damage characterization. Firstly, the surface damages were intentionally induced by focused ion beam, or selective ion-implantation: Ga ions to n-GaAs, N ions to n-GaN, and N ions to n-SiC. The implanted images were clearly observed as the same Y images, and no extension of the induced damages was found. For the GaAs and SiC contacts, Y increased in the implanted regions, while for the GaN, Y decreased. Different roles of the damages as generation or recombination centers were clarified.(4) Grain boundaries of semiconductors and printed metal particles: SIPM is also available for the contacts formed on a semiconductor surface with poor crystal quality. Among the SiC poly-types, 3C-SiC has the advantages of an isotropic crystal structure and high electron and hole mobilities for electron device applications. However, owing to a lack of 3C-SiC bulk crystals, heteroepitaxial growth on Si, 4H-, or 6H-SiC substrates is unavoidable. Thus, the crystal quality of 3C-SiC is not as good as those of 4H- and 6H-SiC. We measured Ni Schottky contacts on p-3C-SiC layers grown on 4H or 6H-SiC substrates. The sample surface consists of 3C-SiC domains with a flat top. The domain pattern was clearly visualized in a Y map. By combining Ymaps measured with red and green lasers, we found that Schottky barrier height is smaller and larger recombination occurs in the boundary regions than in the flat regions. (5) Expansion to semiconductor/ semiconductor and metal-insulator-semiconductor interfaces: Finally, the structural expansion of the samples is described. As long as a hetero junction and an electrical field at the interface exist, a photocurrent can be generated upon monochromatic light irradiation. We characterized p+-Si/n−-SiC heterojunctions formed by surface-activated bonding. In the internal photoemission spectra, a linear relationship was found between Y 1/2 and hν, and threshold energy was reasonably obtained to be 1.34 eV. In the SIPM results, Y maps were successfully obtained, and nanometer-deep scratches, which were formed on the SiC surface in wafer polishing, were sensitively visualized as a pattern.This technique was confirmed to be useful for the development of the wide-bandgap-semiconductor high-power devices.

(Invited) Two-Dimensional Characterization of Wide-Bandgap Materials and Contact Interfaces by Using Scanning Internal Photoemission Microscopy

Scanning internal photoemission spectroscopy has been developed to map the electrical characteristics of metal/semiconductor interfaces nondestructively. We conducted two-dimensional characterization of wide-bandgap Schottky contacts such as GaN, SiC, and oxide semiconductors. Our experimental demonstrations of the mapping characterization are reviewed from the aspects of (A) thermal degradation, (B) device degradation by applying high-voltage, (C) process-induced surface damages, (D) grain boundaries of semiconductors and printed metal particles, and (E) expansion to semiconductor/semiconductor and metal-insulator-semiconductor interfaces. This technique was confirmed to be useful for the development of the wide-bandgap-semiconductor high-power devices.

Defect states and passivation mechanism at grain boundaries of zinc-blende semiconductors

Grain boundaries (GBs) are significant in determining the electrical properties of polycrystalline semiconductors. However, the electronic structures and passivation mechanisms of polycrystalline semiconductors remain poorly understood. In this study, we systematically investigated the Σ3 (112) GB properties of several typical zinc-blende semiconductors via first-principles density functional calculations. We found significant differences of Σ3 (112) GB structures and properties between IV/III and V types, where dangling atoms formed new covalent bonds, and II–VI/I–VII types, where dangling atoms formed no new bonds. These different bonding configurations lead to different origins of defect states at GBs. We successfully designed a targeted doping approach to passivate such defect states for different types of semiconductors. We demonstrated the validity of the proposed approach in Σ3 (112) GB of the zinc-blende semiconductors. This work elucidates the defect states at GBs in common zinc-blende semiconductors, rationalizes diverse post-treatment approaches reported in previous experiments, and provides general guidance for defect passivation at the GBs of polycrystalline semiconductors.

Direct Microscopy Imaging of Nonuniform Carrier Transport in Polycrystalline Cadmium Telluride

Summary Inhomogeneous microscopic carrier transport is difficult to study, but important in many condensed-matter applications. For example, the role of grain boundaries (GBs) in polycrystalline semiconductors has been controversial for 20 years. In cadmium telluride (CdTe) solar cells, electron-beam-induced current (EBIC) measurements consistently demonstrate enhanced current collection along GBs, which is argued as evidence for interpenetrating CdTe p-n current-collection networks critical to high efficiency. Conversely, cathodoluminescence (CL) measurements consistently indicate that GBs are deleterious low-lifetime regions. Here, we apply transport imaging (TI) in conjunction with spatially correlated EBIC, CL, and scanning Kelvin probe force microscopy measurements to understand carrier drift, diffusion, and recombination in polycrystalline CdTe. We simultaneously observe GB potential wells, reduced carrier lifetime at GBs, and seemingly contradictory enhanced GB current collection, and then describe their coexistence with microscopic TI and physical arguments. The results provide visualization of inhomogeneous transport that is critical to understanding and engineering polycrystalline solar technology.

Anomalous 3D nanoscale photoconduction in hybrid perovskite semiconductors revealed by tomographic atomic force microscopy

While grain boundaries (GBs) in conventional inorganic semiconductors are frequently considered as detrimental for photogenerated carrier transport, their exact role remains obscure for the emerging hybrid perovskite semiconductors. A primary challenge for GB-property investigations is that experimentally they need to be performed at the top surface, which is not only insensitive to depth-dependent inhomogeneities but also could be susceptible to topographic artifacts. Accordingly, we have developed a unique approach based on tomographic atomic force microscopy, achieving a fully-3D, photogenerated carrier transport map at the nanoscale in hybrid perovskites. This reveals GBs serving as highly interconnected conducting channels for carrier transport. We have further discovered the coexistence of two GB types in hybrid perovskites, one exhibiting enhanced carrier mobilities, while the other is insipid. Our approach reveals otherwise inaccessible buried features and previously unresolved conduction pathways, crucial for optimizing hybrid perovskites for various optoelectronic applications including solar cells and photodetectors.

Self-passivation rule and structure of CdTe Σ3 (112) grain boundaries

The theoretical study of grain boundaries (GBs) in polycrystalline semiconductors is currently stalemated by their complicated nature, which is difficult to extract from any direct experimental characterization. Usually, coincidence-site-lattice models are constructed simply by aligning two symmetric planes ignoring various possible reconstructions. Here, we propose a general self-passivation rule to determine the low-energy GB reconstruction and find new configurations for the CdTe \ensuremath{\Sigma}3 (112) GBs. First-principles calculations show that it has lower formation energies than the prototype GBs adopted widely in previous studies. Surprisingly, the reconstructed GBs show self-passivated electronic properties without deep-level states in the band gap. Based on the reconstructed configurations, we revisited the influence of $\mathrm{CdC}{\mathrm{l}}_{2}$ post-treatment on the CdTe GBs and found that the addition of both Cd and Cl atoms in the GB improves the photovoltaic properties by promoting self-passivation and inducing $n$-type levels, respectively. The present study provides a new route for further studies of GBs in covalent polycrystalline semiconductors and highlights that previous studies on the GBs of multinary semiconductors, which are based on the unreconstructed prototype GB models, should be revisited.

Anchoring Molecules on the Surface of ZnO and Effects on Intergranular Transport Properties

Improving electrical conductivity in thin film semiconductor devices such as solar cells, thermoelectric modules, or transparent conducting devices, remains a key point for better performances. As polycrystalline films have the favor of the industry for their lower fabrication cost, the limitation of the current is most of the time attributed to charge scattering at the grain boundaries due to high energy barriers at the interface between grains.(1) This work focuses on the understanding of intergranular electrical conductivity in the view of improving thin film semiconductor performances deposited via a soft chemistry deposition routes. In this goal, ZnO:Al was chosen as a model doped semiconductor for our investigations as it is a reference material in many semiconductor applications (sensors, solar cells, transparent conducting oxides)(2). Many studies have already demonstrated the role of adsorbed species onto ZnO thin films and nanostructures surfaces to improve sensor selectivity or charge transfer in dye sensitized solar cells for instance (3). With this in mind, we have performed liquid phase adsorption studies of organic molecules onto ZnO particles through solution conductimetry and IR. A theoretical modeling of the energy level of anchoring molecules was performed in order to fit the LUMO level to the conduction band. Conjugated and thiophene based molecules with different substituents and lengths were selected. Conductivity on pressed powders was studied as a function of the applied pressure to show the effect of the grain boundaries and of the powder compaction. A conclusion was drown on the participation of these molecules and their LUMO levels in the electrical conductivity of ZnO:Al. This study allows quantitative determination adsorbates on ZnO surface and puts into perspective the investigation of better matches between LUMO and conduction band. Literature: 1. Greuter F, Blatter G. Electrical properties of grain boundaries in polycrystalline compound semiconductors. Semicond Sci Technol. 1990;5(2):111. 2. Klaus Ellmer, Andreas Klein, Bernd Rech. Transparent Conductive Zinc Oxide. Springer Series in Material Science. 2008. 3. Galoppini E. Linkers for anchoring sensitizers to semiconductor nanoparticles. Coord Chem Rev. 2004 juillet;248(13–14):1283–97.

Review: grain boundary faceting–roughening phenomena

Similar to free surfaces, the grain boundaries (GBs) in metals, semiconductors and insulators can contain flat (faceted) and curved (rough) portions. In the majority of cases, facets are parallel to the most densely packed planes of coincidence sites lattice formed by two lattices of abutting grains. Facets disappear with the increasing temperature (faceting–roughening transition) and the increasing angular distance from coincidence misorientation. The temperature of GB faceting–roughening transition T R decreases with the increasing inverse density of coincidence sites Σ. In case of fixed Σ, T R decreases with the decreasing density of coincidence sites in the GB plane. The intersection line (ridge) between facets or between facets and curved (rough) portions of surfaces can be of first order (two different tangents in the contact point) or of second order (common tangent, continuous transitions). The rough (curved) portions of GB can also form the first-order rough-to-rough ridges (with two tangents). GB facets control the transition from normal to abnormal grain growth and strongly influence the GB migration, diffusion, wetting, fracture and electrical conductivity.

Modeling of hole transport across grain boundaries in organic semiconductors for mesoscale simulations

Mesoscale carrier transport simulations for organic semiconductors with heterogeneous polycrystalline structures are very challenging because the charge transfer rates across the grain boundaries are unknown. To overcome this difficulty, we studied the process of hole transport across pentacene grain boundaries using large-scale quantum mechanics calculations. In this paper, we propose a simplified model for determining the hole transfer rates, which were found to be reduced considerably by hole barriers that were generated near the grain boundaries. By using this model, mesoscale simulations, such as the kinetic Monte Carlo method, can be applied to heterogeneous polycrystalline structures.

Physics of grain boundaries in polycrystalline photovoltaic semiconductors

Thin-film solar cells based on polycrystalline Cu(In,Ga)Se2 (CIGS) and CdTe photovoltaic semiconductors have reached remarkable laboratory efficiencies. It is surprising that these thin-film polycrystalline solar cells can reach such high efficiencies despite containing a high density of grain boundaries (GBs), which would seem likely to be nonradiative recombination centers for photo-generated carriers. In this paper, we review our atomistic theoretical understanding of the physics of grain boundaries in CIGS and CdTe absorbers. We show that intrinsic GBs with dislocation cores exhibit deep gap states in both CIGS and CdTe. However, in each solar cell device, the GBs can be chemically modified to improve their photovoltaic properties. In CIGS cells, GBs are found to be Cu-rich and contain O impurities. Density-functional theory calculations reveal that such chemical changes within GBs can remove most of the unwanted gap states. In CdTe cells, GBs are found to contain a high concentration of Cl atoms. Cl atoms donate electrons, creating n-type GBs between p-type CdTe grains, forming local p-n-p junctions along GBs. This leads to enhanced current collections. Therefore, chemical modification of GBs allows for high efficiency polycrystalline CIGS and CdTe thin-film solar cells.

Atomic and electronic structure of grain boundaries in crystalline organic semiconductors

Grain boundaries in organic crystalline semiconductors play an important role in charge carrier transport. We have developed a model for atomic and electronic structure calculations of grain boundaries in naphthalene polycrystals. Atomic structure was obtained by the Monte Carlo algorithm, while electronic structure was obtained by the charge patching method. The results for two naphthalene polycrystals are presented and discussed. They show the existence of trap states produced by grain boundaries.

Effect of applied bias voltage on grain boundary potential barrier height (Φ b ) in semiconductor nanocrystals

Carrier transport through electrically active grain boundaries was studied under biased condition using impedance spectroscopy in the frequency range 1 Hz to 1 MHz for a model system nanocrystalline CuSCN. Since, grain boundaries of semiconductors contain defects which often trap charge carriers and cause to form a potential barrier around them also plays a crucial role in the electrical properties of nanomaterials. The influence of bias voltage on relaxation times of grain (τg) and grain boundary (τgb) for our wet chemically prepared nanocrystalline CuSCN was estimated at a periodic increment of 0.35 V from 0 V to −4.2 V. In the static case with no applied bias voltage the grain boundary potential barrier height was found to be 0.144 eV. During each periodic increment of applied bias voltages, both the grain and grain boundary relaxation times are decreases it gives direct and unambiguous evidence on the suppression of grain boundary potential barrier height (Φb).

An EBIC model to describe two close grain boundaries in semiconductors

An EBIC model to describe two interacting grain boundaries (GBs) for semiconductor nanostructures is proposed. The effect of surface recombination velocities of the GBs, the distance between the GBs and the diffusion length of the semiconductor on the shape of the computed EBIC current are shown. The effect of the generation volume extension on the shape and also on the resolution of the computed EBIC current is studied.

Confined and Chemically Flexible Grain Boundaries in Polycrystalline Compound Semiconductors

AbstractGrain boundaries (GBs) in polycrystalline Cu(In,Ga)Se2 thin films exhibit only slightly enhanced recombination, as compared with the grain interiors, allowing for very high power‐conversion efficiencies of more than 20% in the corresponding solar‐cell devices. This work highlights the specific compositional and electrical properties of Cu(In,Ga)Se2 GBs by application of appropriate subnanometer characterisation techniques: inline electron holography, electron energy‐loss spectroscopy, and atom‐probe tomography. It is found that changes of composition at the GBs are confined to regions of only about 1 nm in width. Therefore, these compositional changes are not due to secondary phases but atomic or ionic redistribution within the atomic planes close to the GBs. For different GBs in the Cu(In,Ga)Se2 thin film investigated, different atomic or ionic redistributions are also found. This chemical flexibility makes polycrystalline Cu(In,Ga)Se2 thin films particularly suitable for photovoltaic applications.

Grain Boundary Effect on the Dielectric Properties of Nanocrystalline β-CuSCN

The physics of grain boundaries in semiconductors is of interest both from a fundamental as well as technical point of view because they play crucial role in controlling properties. The grain boundary effect in the nanocrystalline materials are more because the volume fraction of atoms lying at the grain boundaries of the nanocrystalline materials are more as compared with conventional coarse-grained polycrystalline materials. The most attractive hole transporting electrolyte material of dye sensitized solar cell is CuSCN because of its band gap, valence band edge position and p-type semiconducting nature. We have synthesised nanocrystalline beta phase of CuSCN using co-precipitation technique. The grain boundary effect on the dielectric behaviour has been investigated using impedance spectroscopy in a wide frequency range 1 Hz to 1 MHz under dc bias voltages 0 V to -4.2 V. The results are analyzed with double (or back-to-back) Schottky grain boundary barrier model and discussed in detail.

Transport at Grain Boundaries in Polymeric and Small Molecule Organic Semiconductors

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Surface potential images of polycrystalline organic semiconductors obtained by Kelvin probe force microscopy

P-type copper phthalocyanine (CuPc) and n-type hexadecafluorophthalocyanina-tocopper (F16CuPc) polycrystalline films were investigated by Kelvin probe force microscopy (KPFM). Topographic and corresponding surface potential images are obtained simultaneously. Surface potential images are related with the local work function of crystalline facets and potential barriers at the grain boundaries (GBs) in organic semiconductors. Based on the spatial distribution of surface potential at GBs, donor- and acceptor-like trapping states in the grain boundaries (GBs) of p-CuPc and n-F16CuPc films are confirmed respectively. In view of spatial energy spectrum in micro-scale provided by KPFM, it is going to be a powerful tool to characterize the local electronic properties of organic semiconductors.

Redox processes at grain boundaries in barium titanate-based polycrystalline ferroelectrics semiconductors

Barium titanate, which is characterized by a positive temperature coefficient of resistance (PTCR), is widely used in practice. At the same time, it is unknown why only a small percentage of the introduced donor dopant takes part in the formation of PTCR effect, which phases appear at grain boundaries, how the introduced acceptor dopants affect the properties of grains. Elucidation of the above questions is of considerable scientific and practical interest. It has been shown that the phases Bа6Ti17O40 and Y2Ti2O7 precipitate on grains of barium titanate doped with donor dopant (yttrium). We identified paramagnetic impurities (iron, manganese, chromium) in starting reagents. These impurities can occupy titanium sites. Therefore, the part of the donor dopant that is spent on the charge exchange of acceptor dopants does not participate in the charge exchange of titanium Ti4+ → Ti3+, which is responsible for the appearance of PTCR effect in barium titanate. It has been found that an extra acceptor dopant (manganese) is distributed mainly at grain boundaries and in the grain outer layer. It has been shown that manganese ions introduced additionally (as acceptor dopants) increase the potential barrier at grain boundaries and form a high-resistance outer layer in PTCR ceramics. The resistance of grains, outer layers, and grain boundaries as a function of the manganese content has been investigated.

Anomalous grain boundary physics in polycrystalline CuInSe2: the existence of a hole barrier.

First-principles modeling of grain boundaries (GB) in CuInSe2 semiconductors reveals that an energetic barrier exists for holes arriving from the grain interior (GI) to the GB. Consequently, the absence of holes inside the GB prevents GB electrons from recombining. At the same time, the GI is purer in polymaterials than in single crystals, since impurities segregated to the GBs. This explains the puzzle of the superiority of polycrystalline CuInSe2 solar cells over their crystalline counterpart. We identify a simple and universal mechanism for the barrier, arising from reduced p-d repulsion due to Cu-vacancy surface reconstruction. This discovery opens up possibilities for the future design of superior polycrystalline devices.

Role of Grain Boundaries in Exceptionally H 2 Sensitive Highly Oriented Laser Ablated Thin Films of SnO2

A comparative study on growth, electrical conductivity, and sensor characteristics of highly oriented (transparent) and randomly oriented thin films of grown by pulsed laser (KrF; ablation technique have been carried out. Sensors made of randomly oriented polycrystalline films (deposited at 725°C on alumina) exhibited a sensitivity of about 90% for 50 ppm of at sensor operating temperatures above 240°C with a good response (∼30 s) and retracing times (180 s). Sensors made of a axis orientated films [deposited on at 525°C] exhibited an exceptionally high sensitivity of 30 to 40% even for 1 ppm of at 310°C with a shorter response time of about 15 s. However, the retrace time was very long (about 20 min). Sensors made of predominantly (101) orientated films [grown at 525°C on sapphire (1102)] exhibited an exceptional sensitivity of 90% even for 5 ppm at 300°C also had remarkably short response times of a axis oriented films as well as the quick retracing times of polycrystalline films. Thin films, which exhibited exceptionally high sensitivity showed large changes in electrical conductivity and activation energy as function of oxygen partial pressure. Atomic force microscopy investigation reveals that the films are highly granular with average size of about 150-200 nm which is ten times larger than the critical size of 8 nm as being 8 nm for Analysis of results based on the model for carrier transport across the grain boundaries in polycrystalline semiconductors reveals that the surface barrier height of the grain boundaries is responsible for the large variation in activation energy and sensitivity. © 2001 The Electrochemical Society. All rights reserved.