High Density Of Extended Defects Research Articles

Limits to the strain engineering of layered square-planar nickelate thin films

The layered square-planar nickelates, Ndn+1NinO2n+2, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd6Ni5O12 thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n = 3 Ruddlesden-Popper compound, Nd4Ni3O10, and subsequent reduction to the square-planar phase, Nd4Ni3O8. We synthesize our highest quality Nd4Ni3O10 films under compressive strain on LaAlO3 (001), while Nd4Ni3O10 on NdGaO3 (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd4Ni3O10 on SrTiO3 (001). Films reduced on LaAlO3 become insulating and form compressive strain-induced c-axis canting defects, while Nd4Ni3O8 films on NdGaO3 are metallic. This work provides a pathway to the synthesis of Ndn+1NinO2n+2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy.

Anomalous electrical conduction and negative temperature coefficient of resistance in nanostructured gold resistive switching films

We report the observation of non-metallic electrical conduction, resistive switching, and a negative temperature coefficient of resistance in nanostructured gold films above the electrical percolation and in strong-coupling regime, from room down to cryogenic temperatures (24 K). Nanostructured continuous gold films are assembled by supersonic cluster beam deposition of Au aggregates formed in the gas phase. The structure of the cluster-assembled films is characterized by an extremely high density of randomly oriented crystalline nanodomains, separated by grain boundaries and with a large number of lattice defects. Our data indicates that space charge limited conduction and Coulomb blockade are at the origin of the anomalous electrical behavior. The high density of extended defects and grain boundaries causes the localization of conduction electrons over the entire investigated temperature range.

Effect of buried extended defects on the radiation tolerance of ZnO

Interaction between defects of different types often determines their evolution in semiconductors under ion irradiation. Here, we investigate the role of buried pre-existing extended defects in the damage accumulation in ZnO single crystals implanted at room temperature and at 15 K with 80 keV N ions. For the room temperature implants but not for the 15 K ones, the defect accumulation around the projected range is strongly suppressed while a pronounced build-up occurs in the region of the pre-existing defects. This is attributed to thermally activated migration of point defects from the implanted volume and subsequent trapping/annihilation by the pre-existing defects. The obtained results imply that a defect engineering scheme involving formation of a high density of extended defects outside the active volume of a ZnO-based device could be beneficial to enhance the radiation tolerance.

Sensitivity analysis explains quasi-one-dimensional current transport in two-dimensional materials

We demonstrate that the quasi-one-dimensional (1D) current transport, experimentally observed in graphene as measured by a collinear four-point probe in two electrode configurations A and B, can be interpreted using the sensitivity functions of the two electrode configurations (configurations A and B represents different pairs of electrodes chosen for current sources and potential measurements). The measured sheet resistance in a four-point probe measurement is averaged over an area determined by the sensitivity function. For a two-dimensional conductor, the sensitivity functions for electrode configurations A and B are different. But when the current is forced to flow through a percolation network, e.g., graphene with high density of extended defects, the two sensitivity functions become identical. This is equivalent to a four-point measurement on a line resistor, hence quasi-1D transport. The sensitivity analysis presents a formal definition of quasi-1D current transport, which was recently observed experimentally in chemical-vapor-deposition graphene. Our numerical model for calculating sensitivity is verified by comparing the model to analytical calculations based on conformal mapping of a single extended defect.

On the frequency dispersion of the capacitance-voltage behavior of epitaxial Ge on Si p+-n junctions

The electrically active defects in epitaxial germanium layers grown selectively on silicon-shallow trench isolation (STI) wafers have been investigated by means of p+-n junction characterization. The main focus is on the capacitance-voltage (C-V) analysis in reverse operation. A pronounced frequency dispersion in the C-V characteristics has been found for Ge Si-STI layers, while this is not the case for thick epitaxial Ge on Si layers, which contain a significantly lower density of threading dislocations. It is shown that the apparent free carrier density profile derived from the C-V plot at high frequency exhibits a pronounced overshoot in the vicinity of the Ge–Si heterointerface. However, this feature is not only determined by the band offset between the Ge and Si conduction band edges but also by the high density of extended defects and associated deep levels present near the interface. Finally, the impact of a postgrowth high-temperature annealing on the electrical properties of Ge Si-STI epitaxial layers will be discussed in view of the resulting change in extended defect density and profile.

Proton backscattering by point and extended defects in ion‐implanted Si

AbstractThe backscattering of 230‐keV protons, the depth dependences of the full width at the half minimum (FWHM) of proton angular scans and TEM were used to study the structural perfection of Si single crystals implanted with Er and Si ions at doses lower and higher than the respective amorphization thresholds. Implantation of 2‐MeV Er ions at a dose of 5x1013 cm–2 was found to produce high concentrations of point defects without amorphization of the Si layer. After the implantation of 100‐keV Si ions at a dose of 1x1017 cm–2 exceeding the amorphization threshold by two orders of magnitude, we observed a high density of extended defects but no amorphization. The dechanneling by point and extended defects was found to have different dependences (decreasing and increasing, respectively) of the scan FWHM on the scan depth. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Combined MEIS/TEM study of structural defects in Si implanted with a high dose of 100 keV Si ions

Medium-energy ion scattering and transmission electron microscopy have been used to study the structural perfection of a Si single crystal implanted with 100 keV Si ions at a dose of 1×10 17 cm −2, which exceeds the amorphization threshold by two orders of magnitude. The implantation of Si ions was found to produce a high density of extended defects without amorphization of the Si layer. The increasing depth dependence of the full width at half-minimum of the dip in angular scans of backscattered protons, was observed in a Si layer containing a high density of extended defects, in contrast to the decreasing dependence in the perfect Si crystal.

Structure and recombination properties of extended defects in the dislocation slip plane in silicon

AbstractIt is shown, that moving 60° dislocations in Si as well as in SiGe generate high density of extended defects in their slip plane. As a result, the plastic deformation introduces in the crystal volume in addition to the dislocations a very dense system of extended defects, spatially separated from dislocations. The EBIC imaging revealed strong recombination contrast associated with these extended defects. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of Defects in p-type Layers in III-nitride Laser Diode Structures Grown by Molecular Beam Epitaxy

ABSTRACTThis paper reports on the study of defects in p-type layers in III-nitride laser structures grown by molecular beam epitaxy. Characterization of the heterostructures was carried out using atomic force microscopy and transmission electron microscopy. The results show that a high density of extended defects – possibly inversion domains – exist in the p-type cladding layers of as-grown structures with either AlGaN/GaN superlattices or bulk AlGaN cladding layers. TEM analysis of operated and aged devices does not reveal any significant structural modification of the p-type material which might be the cause of deterioration in the lasing performance or failure.

Extended defects in Si films epitaxially grown by low-temperature ECRCVD

Si films grown epitaxially by low-temperature electron–cyclotron resonance chemical-vapor deposition (ECRCVD) were investigated by analyzing extended defects in these films. Extended defects were made visible by Secco etching and scanning (SEM) and transmission (TEM) electron microscopy were used to analyze them in detail. Films grown on Si(1 0 0) wafers show different kinds of etch pits with characteristic shapes, sizes and crystallographic alignments. Their correlation to crystallographic defects is discussed. The typical total etch pit density for boron-doped samples grown at 580 °C on Si wafers is in the range of 10 8 cm −2. Films grown on poly-Si seed layers on glass are strongly affected by Secco etch due to growth regions of different structural qualities and a high density of extended defects. Solar cell test structures were prepared from both types of samples and their results discussed in view to the density of extended defects.

Addition of nanoparticle dispersions to enhance flux pinning of the YBa2Cu3O7-x superconductor.

Following the discovery of type-II high-temperature superconductors in 1986 (refs 1, 2), work has proceeded to develop these materials for power applications. One of the problems, however, has been that magnetic flux is not completely expelled, but rather is contained within magnetic fluxons, whose motion prevents larger supercurrents. It is known that the critical current of these materials can be enhanced by incorporating a high density of extended defects to act as pinning centres for the fluxons. YBa2Cu3O7 (YBCO or 123) is the most promising material for such applications at higher temperatures (liquid nitrogen). Pinning is optimized when the size of the defects approaches the superconducting coherence length ( approximately 2-4 nm for YBCO at temperatures < or =77 K) and when the areal number density of defects is of the order of (H/2) x 10(11) cm(-2), where H is the applied magnetic field in tesla. Such a high density has been difficult to achieve by material-processing methods that maintain a nanosize defect, except through irradiation. Here we report a method for achieving a dispersion of approximately 8-nm-sized nanoparticles in YBCO with a high number density, which increases the critical current (at 77 K) by a factor of two to three for high magnetic fields.

Structural and cathodoluminescence study of mechanically milled silicon

The structural and luminescent properties of nanocrystalline silicon produced by high-energy ball milling of Si single crystals have been investigated using transmission electron microscopy (TEM), x-ray diffraction (XRD) and cathodoluminescence (CL) in a scanning electron microscope. XRD measurements show that the average size of the nanocrystals in the milled samples is about 30 nm but TEM reveals a wide range of size distribution including crystallites with the dimension of few nanometres. Ball milling causes the appearance of a visible luminescence band at 1.61 eV, attributed to the presence of nanocrystals, and a near-infrared band peaked at about 0.79 eV which is suggested to be related to the high density of extended defects formed during the mechanical treatment. These bands, attributed to processes in Si, are not observed in the cathodoluminescent spectra of untreated and ball-milled SiO2 powder.

Transmission Electron Microscopy Study of Planar Defects in Polycrystalline CdTe Thin Films

CdTe is a promising photovoltaic material due to its near optimum band gap and high absorption coefficient. Polycrystalline, thin-film CdTe/CdS solar cells have demonstrated an efficiency of 15.8%. High density of extended defects is often found in polycrystalline CdTe films grown by close-spaced sublimation (CSS). So far, most investigations of defects in CdTe have focused on epitaxially grown films, and the reported extended defects are mainly lamellar twins. However, epitaxially grown films generally have a different microstructure compared to CSS grown polycrystalline CdTe thin films. in this paper, we report our study of extended defects in CSSgrown polycrystalline CdTe thin films by high-resolution transmission electron microscopy (HRTEM). We found that the extended defects are mostly lamellar twins and stacking faults. The stacking faults always propagate across the grains, without ending at a partial dislocation inside the grains.

Cathodoluminescence from Nanocrystals in Mechanically Milled Silicon

Mechanical milling is a technique extensively used to prepare nanocrystalline metallic materials but it has been less frequently applied to semiconductors. The preparation of nanocrystalline silicon by different methods is a subject of increasing interest due to the luminescent properties of this material and its possible application in optoelectronic devices. In this work high-energy ball milling has been used to prepare nanocrystals from single crystalline silicon wafers. The structure of the milled samples has been assessed by X-ray diffraction, transmission electron microscopy (TEM) and Raman spectroscopy, while their luminescence has been investigated by cathodoluminescence (CL) in the scanning electron microscope (SEM). The samples consist of a powder of particles with sizes of hundreds of nanometers aggregated to form bigger particles of several microns. TEM reveals that the particles consist of nanocrystals with a wide range of sizes including crystallites with dimension of few nanometers. CL spectra of the milled samples show a band at 1.61 eV, attributed to the presence of nanocrystals through a quantum confinement effect. It is suggested that a milling induced infrared emission is related to a high density of extended defects present in the milled samples.

Influence of strain relaxation of the AlxGa1−xN barrier on transport properties of the two-dimensional electron gas in modulation-doped AlxGa1−xN/GaN heterostructures

Influences of the thickness of the Si-doped n-type Al0.22Ga0.78N barrier and the thickness of the Al0.22Ga0.78N spacer on mobility and density of the two dimensional electron gas (2DEG) in modulation-doped Al0.22Ga0.78N/GaN heterostructures were investigated. 2DEG mobilities of 1274 cm2/V s at 300 K and 4495 cm2/V s at 77 K were reached. Both 2DEG mobility and density decrease dramatically when the Al0.22Ga0.78N barrier becomes partially relaxed, indicating that transport properties of the 2DEG are influenced significantly by the piezoelectric polarization of the Al0.22Ga0.78N layer. From our results, the critical thickness of an Al0.22Ga0.78N layer on GaN is estimated to be between 65 and 75 nm, which is much higher than that predicted by theoretical calculation. This may be attributed to the interaction of misfit dislocations and the presence of a high density of extended defects in the Al0.22Ga0.78N layer.

Diamond growth by carbon ion implantation of diamond

Abstract Medium energy (5–25 keV) 13 C + ion implantation into diamond (100) to a fluence ranging from 10 16 cm −2 to 10 18 cm −2 was performed for the study of diamond growth via the approach of ion beam implantation. The samples were characterized with Rutherford backscattering/channelling spectroscopy, Raman spectroscopy, X-ray photoemission spectroscopy and Auger electron spectroscopy. Extended defects are formed in the cascade collision volume during bombardment at high temperatures. Carbon incorporation indeed induces a volume growth but the diamond (100) samples receiving a fluence of 4 × 10 17 to 2 × 10 18 at. cm −2 (with a dose rate of 5 × 10 15 at. cm −2 s −1 at 5 to 25 keV and 800 °C) showed no He-ion channelling. Common to these samples is that the top surface layer of a few nanometers has a substantial amount of graphite which can be removed by chemical etching. The rest of the grown layer is polycrystalline diamond with a very high density of extended defects.

AES and SIMS profiling of buried silicide layers formed by 6 MeV high dose nickel implantation into silicon

The composition of deep-buried conductive layers formed by 6 MeV high dose Ni implantation into silicon at 450 K has been studied using AES and SIMS. For a dose of 1.3 × 1018 Ni/cm2, AES analysis yields a Ni to Si ratio close to NiSi2 stoichiometry at profile maximum, as expected from high dose Monte Carlo simulations. In this region the shape of the Si LVV Auger line indicates the presence of NiSi2. TEM/XTEM investigations reveal a continuous NiSi2 layer, showing a high density of extended defects.

Microstructural studies of reactive ion etched silicon

The structures of Si near-surface damage induced after the removal of a thick SiO2 layer on Si using reactive ion etching with various etching gases were studied by transmission electron microscopy. Cross-sectional micrographs showed the presence of a fluorocarbon film on the Si surface after the SiO2 layer is etched away. No extended defects were observed in Si etched utilizing pure CF4 or CF4/20% H2 etching gas, even after a 25-min overetch into Si. For a CF4/40% H2 etching gas, no extended lattice defects were evident for overetch times of up to 5 min. However, extensive damage was found in the Si surface layer after a 10-min or longer overetch. This extensive damage consists of {111} planar defects distributing underneath the Si surface to a depth of 300 Å. The planar defects are highly decorated by impurities, likely H and possibly C, F. A pure H2 etching gas was found to introduce a heavily damaged layer and a high density of extended defects near the Si surface after only a 5-min overetch. These observations indicate that H ion bombardment plays an important role in the formation of Si near-surface damage during reactive ion etching.