Low Concentration Of Point Defects Research Articles

Enhancing Performance of Ultraviolet C Photodetectors Through Single-Domain Epitaxy of Monoclinic β-Ga2 O3 Films and Tailored Anti-Reflection Coating.

Implementing high-performance ultraviolet C photodetectors (UVC PDs) based on β-Ga2 O3 films is challenging owing to the anisotropic crystal symmetry between the epitaxial films and substrates. In this study, highly enhanced state-of-the-art photoelectrical performance is achieved using single-domain epitaxy of monoclinic β-Ga2 O3 films on a hexagonal sapphire substrate. Unlike 3D β-Ga2 O3 films with twin domains, 2D β-Ga2 O3 films exhibit a single domain with a smooth surface and low concentration of point defects, which enable efficient charge separation by suppressing boundary-induced recombination. Furthermore, a tailored anti-reflection coating (ARC) is adopted as a light-absorbing medium to improve charge generation. The tailored nanostructure, which features a gradient refractive index, not only substantially reduces the reflection, but also suppresses the surface leakage current as a passivation layer. This study provides fundamental insights into the single-domain epitaxy of β-Ga2 O3 films and the application of ARC for the development of high-performance UVC PDs.

Conducting interfaces between LaAlO3 and thick homoepitaxial SrTiO3 films for transferable templates

For the mechanical stability of heterostructure membranes such as LaAlO3 (LAO)/SrTiO3 (STO), which is well-known for two-dimensional electron transport at the interface, a thick template substrate layer is required. However, thick STO films typically suffer from the formation of undesired point defects, mainly due to its cation off-stoichiometry, which results in the insulating interface with LAO. Herein, we report the successful fabrication of a conducting interface between LAO and thick homoepitaxial STO films by pulsed laser deposition. The careful optimization of laser ablation parameters including laser energy density and laser spot size led to stoichiometric 500 nm–thick STO films that have a lattice structure identical to that of bulk single crystals of STO, as confirmed by X-ray diffraction and temperature-dependent Raman spectroscopy. The interface between 4 nm–thick LAO and 500 nm–thick stoichiometric STO films exhibited a metallic behavior at low temperature down to 4 K, implying a low concentration of point defects in the homoepitaxial STO film. Ultimately, a crack-free, millimeter-sized 500 nm–thick STO membrane was successfully fabricated. These results are expected to facilitate the integration of high-quality complex oxide heterostructure membranes with various substrates including silicon, enabling their practical applications.

Etchless Fabrication of High‐Quality Refractory Titanium Nitride Nanostructures

Nanosphere lithography has emerged as an efficient route to produce plasmonic nanostructures. Herein, the processes of nanosphere lithography and reactive magnetron sputtering that seem incompatible for a variety of reasons are reviewed and explained. However, understanding the physical obstacles of this combination enables us to identify a window of process parameters that make the deposition of well‐defined trigonal nanoislands of titanium nitride (TiN) feasible. TiN is used as a case study because it is a very good representative of refractory conductive nitrides, such as ZrN, HfN, NbN, and TaN. A UV ozone step is used to confine the triple‐junction vias of the polystyrene mask, and the reactive magnetron sputtering parameters are fine‐tuned to increase the directionality of deposited species, in particular the substrate‐to‐target distance is maximized to improve geometrical directionality, and a negative bias voltage is used to guide the ionic species deep into the vias. The proposed process produces well‐defined TiN nanoislands with low concentration of point defects that have similar structure with continuous films of high electrical conductivity and plasmonic performance.

Defect chemistry of Eu dopants in NaI scintillators studied by atomically resolved force microscopy

Activator impurities and their distribution in the host lattice play a key role in scintillation phenomena. Here a combination of cross-sectional noncontact atomic force microscopy (nc-AFM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) was used to study the distribution of Eu2+ dopants in a NaI scintillator activated by 3% of EuI2. Two types of precipitate structures were found. First, needle-shaped EuI2 precipitates with a layered structure are likely responsible for scattering the scintillation light. In transparent crystals with good scintillation properties, precipitates with a cubic crystal structure and a size below 4 nm were found. A surprisingly low concentration of point defects was detected in all of the investigated samples. Upon annealing, Eu segregates towards the surface, which results in the formation of an ordered hexagonal overlayer with the EuI2 composition and a pronounced, unidirectional moire pattern.

Impact of irradiation induced dislocation loops on thermal conductivity in ceramics

AbstractExperimental work aimed at understanding the role of dislocation loops in limiting phonon mediated thermal transport in ceramics is presented. Faulted dislocation loops, having diameters of a few nanometers, were introduced by irradiating a polycrystalline cerium dioxide sample with 1.6 MeV protons at 700°C. XRD analysis indicated that irradiated samples retained their crystalline structure and exhibit very little lattice expansion suggesting a low concentration of point defects. Further microstructure characterization using transmission electron microscopy revealed that interstitial type faulted dislocation loops were primarily created as expected for these irradiation conditions. Thermal conductivity of the damaged layer was measured using a modulated thermoreflectance approach. Analysis of the experimental data using the classical Klemens‐Callaway approach reveals that the conductivity reduction is primarily due to dislocation loops, while point defects and voids play only a minor role. These results provide experimental confirmation that faulted loops offer a unique arrangement for displaced atoms that leads to an unusually large reduction of thermal conductivity.

High power UVB light emitting diodes with optimized n-AlGaN contact layers

The influence of the n-AlGaN contact layer thickness and doping profile on the efficiency, operating voltage and lifetime of 310 nm LEDs has been investigated. Increasing the n-contact layer thickness reduces the operation voltage of the LEDs and increases the emission power slightly. Optimizing the n-doping profile yielded enhanced conductivity and reduced operation voltage with a simultaneous output power enhancement of the LEDs. Lifetime measurements have shown that even though the output power of the LEDs was enhanced the lifetimes were not negatively affected. Room temperature photoluminescence indicates a low concentration of point defects in the n-doping region yielding minimum AlGaN resistivity.

Mechanisms on the outstanding high temperature plasticity of AlNp/Al–0.4Cu composites induced by cryogenic treatment

The 8.2AlNp/Al–0.4Cu composites were solution treated by heating to 530 °C for 1 h and then quenched in water and liquid nitrogen, respectively. After quenching in liquid nitrogen, the composites possess nearly the same ultimate tensile strength (UTS) but a significant increase in elongation by 79% (at 350 °C) compared with that of the samples quenched in water. The impact of different quenching process on the microstructure evolution and high temperature tensile properties has been investigated systematically. After quenched in water, there are 69% low–angle grain boundaries (LAGBs), high point defect concentration and a small quantity of dislocations. This microstructure leads to the dynamic recrystallization as well as discordant deformation. The high percentage of high–angle grain boundaries (HAGBs), high dislocation density and low point defect concentration of the composites (quenched in liquid nitrogen) help improve the microstructural stability and contribute to the favorable elongation at 350 °C. The mechanisms revealed in this work may provide a paramount method to achieve a superior combination of strength and elongation for heat resistant Al alloys.

Self‐Powered n‐SnO2/p‐CuZnS Core–Shell Microwire UV Photodetector with Optimized Performance

AbstractHerein, a single‐crystal SnO2 microwire photodetector (PD) is demonstrated with a fast response speed owing to a low concentration of point defects. However, the presence of surface defects (e.g., oxygen vacancies) still limits its optoelectronic performance. To further improve the photoresponse of such device, a core–shell p–n junction is constructed by simply coating a new p‐type transparent conductive (CuS)0.35:(ZnS)0.65 nanocomposite film (CuZnS) on n‐type SnO2 microwire. As a result, not only the surface of SnO2 is modified, but also a space charge depletion region is formed at the interface, leading to an enhanced on‐off ratio of ≈1.3 × 103 and a faster speed of 45 µs/1.17 ms (rise time/decay time) in comparison with the plain SnO2 microwire PD (on‐off ratio of 30, response speed of ≈100 µs/1.5 ms). Besides, the n‐SnO2/p‐CuZnS core–shell microwire could steadily work as a self‐powered UV PD, with a maximum responsivity of 1.6 mA W−1 (at 0 V) and detectivity of 5.41 × 1011 Jones (at 0.05 V) toward 315 nm, which suggests its great potentials as a high‐performance self‐powered UV photodetector.

Fabrication and convergent X-ray nanobeam diffraction characterization of submicron-thickness SrTiO3 crystalline sheets

The creation of thin SrTiO3 crystals from (001)-oriented SrTiO3 bulk single crystals using focused ion beam milling techniques yields sheets with submicron thickness and arbitrary orientation within the (001) plane. Synchrotron x-ray nanodiffraction rocking curve widths of these SrTiO3 sheets are less than 0.02°, less than a factor of two larger than bulk SrTiO3, making these crystals suitable substrates for epitaxial thin film growth. The change in the rocking curve width is sufficiently small that we deduce that dislocations are not introduced into the SrTiO3 sheets. Observed lattice distortions are consistent with a low concentration of point defects.

Low-resistivity m-plane freestanding GaN substrate with very low point-defect concentrations grown by hydride vapor phase epitaxy on a GaN seed crystal synthesized by the ammonothermal method

An m-plane freestanding GaN substrate satisfying both low resistivity (ρ = 8.5 × 10−3 Ω·cm) and a low point-defect concentration, being applicable to vertically conducting power-switching devices, was grown by hydride vapor phase epitaxy on a nearly bowing-free bulk GaN seed wafer synthesized by the ammonothermal method in supercritical ammonia using an acidic mineralizer. Its threading dislocation and basal-plane staking-fault densities were approximately 104 cm−2 and lower than 100 cm−1, respectively. A record-long fast-component photoluminescence lifetime of 2.07 ns at room temperature was obtained for the near-band-edge emission, reflecting a significantly low concentration of nonradiative recombination centers composed of Ga vacancies.

HVPE GaN with Low Concentration of Point Defects for Power Electronics

ABSTRACTWe have studied photoluminescence (PL) from undoped GaN films grown by HVPE technique on sapphire. Several defect-related PL bands are observed in the low-temperature PL spectrum. The concentrations of the defects responsible for these PL bands are determined from the dependence of PL intensity on excitation intensity. The RL band with a maximum at 1.8 eV is often the dominant PL band in HVPE GaN. It is caused by an unknown defect with the concentration of up to ∼1017 cm-3. The concentrations of defects responsible for other defect-related PL bands rarely exceed 1015 cm-3.

Molecular beam epitaxy for high-performance Ga-face GaN electron devices

Molecular beam epitaxy (MBE) has emerged as a powerful technique for growing GaN-based high electron mobility transistor (HEMT) epistructures. Over the past decade, HEMT performance steadily improved, mainly through the optimization of device fabrication processes. Soon, HEMT performance will be limited by the crystalline quality of the epistructure. MBE offers heterostructure growth with highly abrupt interfaces, low point defect concentrations, and very low carbon and hydrogen impurity concentrations. Minimizing parasitic leakage pathways and resistances is essential in the growth of HEMTs for high-frequency and high-power applications. Through growth on native substrates with very low threading dislocation density, low-leakage HEMTs with very low on-resistance can be realized. Ga-rich plasma-assisted MBE (PAMBE) has been studied extensively, and it is clear that this technique has inherent limitations, including a high density of leakage pathways and a very small growth parameter space. Relatively new MBE growth techniques—high-temperature N-rich PAMBE and ammonia-based MBE—are being developed to circumvent the shortcomings of Ga-rich PAMBE.

Low threshold room-temperature lasing of CdS nanowires

The synthesis of CdS nanostructures (bands, wires, irregular structures) was investigated by systematic variation of temperature and gas pressure, to deduce a comprehensive growth phase diagram. The high quality nanowires were further investigated and show stoichiometric composition of CdS as well as a single-crystalline lattice without any evidence of extended defects. The luminescence of individual nanowires at low excitation shows a strong near band edge emission at 2.41 eV indicating a low point defect concentration. Sharp peaks evolve at higher laser power and finally dominate the luminescence spectrum. The power dependence of the spectrum clearly shows all the characteristics of amplified stimulated emission and lasing action in the nanowire cavity. A low threshold was determined as 10 kW cm−2 for lasing at room temperature with a slope efficiency of 5–10% and a Q factor of up to 1200. The length and diameter relations necessary for lasing of individual nanowires was investigated.

Elucidating the structure-dependent photocatalytic properties of Bi2WO6: a synthesis guided investigation

Polycrystalline microspheres and single-crystalline microplates of Bi(2)WO(6) have been synthesized by ultrasonic spray pyrolysis. Herein, these materials are evaluated as photocatalysts for the visible light mediated degradation of rhodamine B, a model pollutant, and the results compared to those obtained with Bi(2)WO(6) prepared by traditional methods. The microplates, which displayed the best crystallinity and highest surface area, were anticipated to facilitate the greatest rate of dye photodegradation. However, the polycrystalline microspheres outperformed both the Bi(2)WO(6) microplates and traditional samples. To understand the origin of this result, the local and macroscale structures of the Bi(2)WO(6) samples were comprehensively characterized by spectroscopy techniques (diffuse reflectance, fluorescence, Raman, and X-ray photoelectron spectroscopy) as well as electron microscopy and diffraction. This analysis found that the enhanced performance of the Bi(2)WO(6) microspheres results from the expression of a hydrophilic surface, a low concentration of point defects, and a moderate surface area. This finding highlights the significant role synthesis plays in imparting structure and functionality to solid materials.

Thermal conductivity of carbon nanotubes

This paper addresses the problem of modeling the thermal conductivity of carbon nanotubes (CNTs) of submicron length that have relatively low point-defect concentrations. Point defects are taken into account at the ends of these CNTs to model the commonly encountered situation where point defects are introduced unintentionally at the ends of the CNT under prevailing fabrication methods. Herein, previous models for thermal transport in graphite are adapted to investigate the thermal transport properties of CNTs.

X-ray diffraction analysis of the structure of antisite arsenic point defects in low-temperature-grown GaAs layer

A method for the structure analysis of point defects in a semiconductor layer is developed by combining x-ray diffraction and growth of a superlattice where the concentration of point defects is periodically varied in the growth direction. Intensities of satellite reflections from the superlattice depend predominantly on the atomic structure of point defects, and hence this method can be applicable to the case of a low concentration of point defects. By using this method, the atomic structure of antisite As point defects in GaAs layers grown by molecular-beam epitaxy at low temperatures has been analyzed. Measured intensity ratios of the first-order satellite reflection in the lower angle side to that in the higher angle side for a number of (hkl) reflections are compared with those calculated based on structure models. The analysis has shown that experimental intensity ratios cannot be reproduced by models which include only a uniform tetragonal lattice distortion and local atomic displacements around an antisite As atom. A fairly good agreement between measured and calculated intensity ratios is obtained with a model which account for both gradual change in the tetragonal lattice distortion in the (0 0 1) plane and displacements of neighboring atoms away from the antisite As atom.

Interface reactions and Kirkendall voids in metal organic vapor-phase epitaxy grown Cu(In,Ga)Se2 thin films on GaAs

Cu ( In 1 − x Ga x ) Se 2 (CIGS) films were grown on (001) GaAs at 570 or 500°C by means of metal organic vapor-phase epitaxy. All films were Cu-rich [Cu∕(In+Ga)>1] with pseudomorphic Cu2Se second phase particles found only on the growth surface. During growth, diffusion of Ga from the substrate and vacancies generated by the formation of CIGS from Cu2Se at the surface occurred. The diffusion processes lead to the formation of Kirkendall voids at the GaAs/CIGS interface. Transmission electron microscopy and nanoprobe energy dispersive spectroscopy were used to analyze the diffusion and void formation processes. The diffusivity of Ga in CIGS was found to be relatively low. This is postulated to be due to a comparatively low concentration of point defects in the epitaxial films. A reaction model explaining the observed profiles and voids is proposed.

Sliding friction: the contribution from defects

We study the influence of defects on the sliding friction for one-, two-, and three-dimensional elastic solids. We show that for 1D and 2D solids, perturbation theory breaks down at low sliding velocity. For a 1D solid with a low concentration of point defects we present an exact solution for the sliding friction, valid for arbitrary temperature and strength of the defect potential. We discuss the role of point defects in the linear (in the external driving force) sliding friction for Xe monolayers on metal surfaces.

Effects of Spallation neutron irradiation on single crystals of Iron, Nickel, Niobium and Stainless Steel

Abstract Single crystals of Iron, Nickel, Niobium and Stainless Steel (SS316) subjected to prolonged neutron irradiation in the Los Alamos Spallation Source for a period of one year have been studied using both small angle, and single crystal neutron diffraction. Small Angle Scattering show no evidence of any large radiation induced inhomogeneities in the pure metals, while regions of ∼ 65 A diameter are found in the irradiated SS316. Combining the s.a.s. measurements with the single crystal diffraction data these regions are attributed to the formation of coherent precipitates of Ni-Cr. However, a low concentration of point defects (<2.5 at %) is introduced in all the materials by the irradiation process. No significant radiation induced dilatation of the lattice is found in any of the materials.

A generalized overlap-damage model of amorphous phase and point defect generation during ion implantation into silicon

Abstract A generalized model is presented for describing point defect and amorphous phase generation kinetics during ion implantation into silicon. The model considers the heterogeneity of the damage clusters by dividing them into three different areas: into an amorphous core (a-Si), an inner periphery of high point defect concentration (undetectable by BPR), and an outer periphery of low point defect concentration (detectable by EPR). Weil-known experimental BPR results may be interpreted by this model.