High Hole Density Research Articles

Agrilus auroguttatus exit hole distributions on Quercus agrifolia boles and a sampling method to estimate their density on individual trees

AbstractIn recent decades, invasive phloem and wood borers have become important pests in North America. To aid tree sampling and survey efforts for the newly introduced goldspotted oak borer, Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), we examined spatial patterns of exit holes on the boles (trunks) of 58 coast live oak, Quercus agrifolia Née (Fagaceae), trees at five sites in San Diego County, southern California, United States of America. Agrilus auroguttatus exit hole densities were greater at the root collar than at mid-boles (6.1 m above ground). Dispersion patterns of exit holes on lower boles (≤1.52 m) were random for trees with low exit hole densities and aggregated for trees with high exit hole densities. The mean exit hole density measured from three randomly chosen quadrats (0.09 m2) provided a statistically reliable estimate of the true mean exit hole density on the lower bole, with <25% error from the true mean. For future sampling and survey efforts in southern California oak forests and woodlands, exit hole counts within a 0.09 m2 quadrat could be made at any three locations on lower Q. agrifolia boles to accurately estimate A. auroguttatus exit hole densities at the individual tree level.

Territoriality and Habitat Use of Common Buzzards (Buteo buteo) During Late Autumn in Northern Germany

We studied habitat use and spatial distribution of Common Buzzards (Buteo buteo) in Schleswig- Holstein, northern Germany during the autumn of 2000, to assess the raptors' territoriality and its consequenc- es for their use of space. For this purpose, we performed point counts covering an area of 20 km2 during each of 20 continuous days and recorded 1537 observations of Common Buzzards and 109 of Rough-legged Hawks (Buteo lagopus). Applying multivariate Poisson regression combined with randomization tests, we detected that Common Buzzards showed a preference for areas with a larger distance from nesting sites of the previous spring (P 5 0.019;n 5 290 grid units of 250 3 250 m in all three cases), a high density of vole holes (P , 0.001), and a low density of perch-sites (P 5 0.005). These preferences regarding vole holes and perches were also valid when only grid units far from the nesting sites were considered. Indirectly, the buzzards were also affected by the amount of grassland and by the amount of dry land, as these parameters were strongly related to vole-hole density. The distributions of the sympatric wintering species Rough-legged Hawk and Common Buzzard did not affect each other. Our study indicated that territorial sedentary breeding pairs displaced nonbreeders and wintering birds into suboptimal habitats, and it offers valuable clues on the constraints for Common Buzzards in areas where they occur in high abundance during late autumn and winter.

Hydrodynamic model for electron-hole plasma in graphene

We propose a hydrodynamic model describing steady-state and dynamic electron and hole transport properties of graphene structures which accounts for the features of the electron and hole spectra. It is intended for electron-hole plasma in graphene characterized by high rate of inter-carrier scattering compared to external scattering (on phonons and impurities), i.e., for intrinsic or optically pumped (bipolar plasma), and gated graphene (virtually monopolar plasma). We demonstrate that the effect of strong interaction of electrons and holes on their transport can be treated as a viscous friction between the electron and hole components. We apply the developed model for the calculations of the graphene dc conductivity; in particular, the effect of mutual drag of electrons and holes is described. The spectra and damping of collective excitations in graphene in the bipolar and monopolar limits are found. It is shown that at high gate voltages and, hence, at high electron and low hole densities (or vice-versa), the excitations are associated with the self-consistent electric field and the hydrodynamic pressure (plasma waves). In intrinsic and optically pumped graphene, the waves constitute quasineutral perturbations of the electron and hole densities (electron-hole sound waves) with the velocity being dependent only on the fundamental graphene constants.

Tool Breakage Feature Extraction in PCB Micro-Hole Drilling Using Vibration Signals

Tool breakage monitoring is crucial to automation fabrication, especially for high-density hole machining, such as PCB (Printed Circuit Board). A tool breakage feature extraction method in PCB micro-hole drilling is presented in this paper. The vibration signal is analyzed by wavelet transform. The decomposed signals energy ratio at each frequency band is computed as monitoring features. The monitoring performance of different features selection is given. The vibration signals are observed to provide the capability in distinguishing micro drill breakage with proper features extraction and classifier design.

Detection of Charge Storage on Molecular Thin Films of Tris(8-hydroxyquinoline) Aluminum (Alq3) by Kelvin Force Microscopy: A Candidate System for High Storage Capacity Memory Cells

Retention and diffusion of charge in tris(8-hydroxyquinoline) aluminum (Alq(3)) molecular thin films are investigated by injecting electrons and holes via a biased conductive atomic force microscopy tip into the Alq(3) films. After the charge injection, Kelvin force microscopy measurements reveal minimal changes with time in the spatial extent of the trapped charge domains within Alq(3) films, even for high hole and electron densities of >10(12) cm(-2). We show that this finding is consistent with the very low mobility of charge carriers in Alq(3) thin films (<10(-7) cm(2)/(Vs)) and that it can benefit from the use of Alq(3) films as nanosegmented floating gates in flash memory cells. Memory capacitors using Alq(3) molecules as the floating gate are fabricated and measured, showing durability over more than 10(4) program/erase cycles and the hysteresis window of up to 7.8 V, corresponding to stored charge densities as high as 5.4 × 10(13) cm(-2). These results demonstrate the potential for use of molecular films in high storage capacity nonvolatile memory cells.

Carbon‐doped p‐type (0001) plane AlGaN (Al = 6–55%) with high hole density

AbstractIn this paper, promising experimental results for the p‐type electrical properties of carbon‐doped (C‐doped) AlGaN are discussed. P‐type conductivity was experimentally achieved in C‐doped (0001) plane AlGaN layers with from a small amount to 55% solid Al composition, but not in (0001) plane GaN. The maximum free hole density (determined by van der Pauw geometry‐Hall effect measurement) achieved for an AlGaN layer with 10% solid Al composition was p = 3.2 × 1018 cm−3. The maximum net ionized acceptor densities (NIAD), $N_{{\rm A}}^{- } - N_{{\rm D}}^{ + } $, which were determined by capacitance–voltage measurement, for AlGaN with 6, 10, 27, and 55% solid Al compositions, were all in the range of (6–7) × 1018 cm−3. Moreover, the electrical activity of the carbon acceptors was estimated to be 55–71% from the NIAD and secondary‐ion microprobe mass spectrometry analysis data on the carbon concentration. A p–n junction was also fabricated using the C‐doped p‐type AlGaN.

Contact Behavior and Chemical Mechanical Polishing (CMP) Performance of Hole-Type Polishing Pad

A polishing pad plays an essential role in determining the chemical mechanical planarization (CMP) performance such as removal rate, planarization, and defectivity. Further, it is important for the polishing pad to maintain good CMP performance throughout its lifespan. In order to achieve high performance and durability, a hole-type pad was suggested as an alternative to the conventionally used pore-type pad. In this study, the effect of the hole density on the CMP performance was examined for a hole-type pad. Surface characterization of the hole-type pad showed that the hole density corresponded to the contact ratio between the pad and the wafer, and therefore, the contact characteristics could be controlled by a hole fabrication process. The experimental results showed that the number of holes play a key role in determining CMP performances. The removal rate decreased with an increase in the hole density. The level of dishing showed an increase with the hole density. The number of CMP scratches decreased at higher hole densities. Higher contact ratio and the presence of large particle trap sites explain the lower trend of the micro-scratch observed. An important advantage of the hole-type pad is that it retains the surface structure as CMP continues, which implies contact behavior is maintained.

Analysis of Dynamic Avalanche Phenomenon in SOI Lateral High-speed Diode during Reverse Recovery and Development of a Novel Device Structure for Suppressing Dynamic Avalanche

We have studied the dynamic avalanche phenomenon in an SOI lateral diode during reverse recovery by using a mixed-mode device simulation. In the study, it has been found that local impact ionization occurs near an anode-side field oxide edge, where a high-density hole current flows and a high electric field appears simultaneously. We propose that a p-type anode extension region (AER) along a trench side wall effectively sweeps out stored carriers beneath an anode p-diffusion layer during reverse recovery, resulting in reduction of the electric field and remarkable suppression of the dynamic avalanche. The AER reduces the total recovery charge and does not cause any increase in the total stored charge under a forward bias operation. This effect is verified experimentally by the fabricated device with AER. Thus, the developed SOI lateral diode is promising as a high-speed and highly rugged free-wheeling diode, which can be integrated into next-generation SOI microinverters.

Shubnikov-de Haas Oscillations in Proton-irradiated Graphite

Our study of the Shubnikov-de Haas (SdH) effect in highly-oriented pyrolytic graphite (HOPG) systems shows that low-density electrons may occupy a 3D Fermi surface and high-density holes a 2D surface in proton-irradiated HOPG while both carriers occupy a 2D Fermi surface in virgin HOPG. No massless Dirac fermions were evidenced by the SdH oscillations both in virgin HOPG and in proton-irradiated HOPG. Following the examination of the recent controversy on massless Dirac fermions in HOPG, we find that even a small amount of acceptor-like defects may play an important role in the interpretation of SdH oscillations.

Dependence of Mg acceptor levels in InN on doping density and temperature

Infrared (IR) reflectance and transmission spectra of Mg doped InN films are analyzed using a dielectric function consisting of the terms of phonon, plasmon, and electronic transition between the valence band and the acceptor levels. The reflectance spectra at lower temperatures than 200 K are highly affected by the electronic transition. Acceptor activation energy Ea decreases with the increase in ionized acceptors because of the Coulomb potential overlap of acceptors charged by the background electrons and/or hole generation by the temperature increase. It is found that Ea is 69(±5) meV at low Mg− density limit and decreases to 50 meV at 5 K because of the charging by the background electrons of the density of 1 × 1018 cm−3. Temperature increase causes the further decrease in Ea, which causes the high hole density of the order of 1018 cm−3 at room temperature in spite of the high degeneracy of the acceptor states. The heavy hole mass is obtained as 0.59(±0.06)m0.

Growth of &lt;i&gt;p&lt;/i&gt;-Type AlN Crystals by C and Si Codoping

Based on our earlier theoretical investigation,p-type C:Si codoped AlN crystals were grown on SiC substrates by a sublimation method in a improved growth reactor. Hall-effect measurement shows that the AlN crystals have a high hole density of 1.4×1014cm-3and mobility of 52 cm2V-1s-1in spite of the high resistivity (896 Ω•cm). In the AlN samples, Si dopants act as donors due to substituting Al atoms, and most of C dopants act as acceptors for replacing N atoms. It is also observed that the activation energy of C acceptors in C:Si codoped AlN is reduced by codoping Si donors, which agrees with the computational results.

Theory of Carrier Concentration-Dependent Electronic Behavior in Layered Cobaltates

A natural explanation for the carrier concentration-dependent electronic behavior in the layered cobaltates emerges within correlated-electron Hamiltonians with finite on-site and significant nearest neighbor hole-hole Coulomb repulsions. The nearest neighbor repulsion decreases hole double occupancy below hole density 1/3, but increases the same at higher hole densities. Our conclusion is valid for both single-band and three-band extended Hubbard Hamiltonians, and sheds light on concentration dependent e'(g) hole occupancy within the latter.

Time‐of‐Flight Studies of Electron‐Collection Kinetics in Polymer:Fullerene Bulk‐Heterojunction Solar Cells

AbstractThe charge‐collection dynamics in poly(3‐hexylthiophene:[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM) bulk heterojunctions are studied in thick (&gt;1 μm) devices using time‐of‐flight measurements and external quantum‐efficiency measurements. The devices show Schottky‐diode behavior with a large field‐free region in the device. Consequently, electron transport occurs by diffusion in the bulk of the active layer. At high applied biases where the depletion region spans the entire active layer, normal time‐of‐flight transients are observed from which the electron mobility can be determined. Here, the electron mobility follows Poole–Frenkel behavior as a function of field. At lower applied biases, where the depletion region only spans a small portion of the active layer, due to a high density of dark holes, the recombination kinetics follow a first‐order rate law with a rate constant about two orders of magnitude lower than that predicted by Langevin recombination.

P-type K-doped ZnO nanorods for optoelectronic applications

Single crystalline p-type K-doped ZnO nanorods (NR) have been synthesized by convenient and low-cost solution technique. X-ray diffraction analysis confirmed the hexagonal system of K-doped ZnO nanorods and a preferred a axis orientation. The diameter and length were found to be 30–50 nm and 150–200 nm, respectively, in electron microscopic studies. The p-type nature and high hole density of 2.36×1019 per cm3 was demonstrated by Hall studies. A sharp lower cutoff and reduction in the bandgap was observed in UV-Vis study which is linked to its smaller size and p-type conduction in NR. In Photoluminescence studies, a near band emission in the UV range and a yellow emission with a blueshift were detected in the visible region in K-doped ZnO NR. The variation in intensity of peaks is correlated with the dimensions of NR. The redshift observed in Raman peaks is explained in terms of the stress developed in ZnO nanostructure due to lattice mismatch by K doping. These results provide better insight of K-doped ZnO nanostructures for both fundamental research and technological applications in optoelectronic devices.

Mechanism of enhanced mobility and conductivity at donor–acceptor organic interfaces

The model of metallic type or enhanced hopping conductivity with reduced activation energy at the donor–acceptor interface formed by organic dielectrics is suggested. High density of electrons and holes at the interface arises due to the integer charge transfer from donor to acceptor molecules. Significant part of charge carriers can avoid thermal activation and tunneling in course of their transport if the interfacial energy barrier is low enough. The critical value of an interfacial energy barrier, density of free electrons, the values of two-dimensional mobility and conductivity of charge carriers are estimated by numerical modeling and compared with experimental data.

Quantum Supercurrent Transistors in Silicon Quantum Wells Confined by Superconductor Barriers

We present the findings of spin-dependent single-hole and pair-hole transport in plane and across the p-type high mobility silicon quantum wells (Si-QW), 2 nm, confined by the superconductor δ-barriers on the n-type Si (100) surface. The oscillations of the conductance in normal state and the zero-resistance supercurrent in superconductor state as a function of the top gate voltage are found to be correlated by on- and off-resonance tuning the two-dimensional levels of holes in Si-QW with the Fermi energy in the superconductor δ-barriers. The SIMS and STM studies have shown that the δ-barriers heavily doped with boron, 5 × 1021 cm–3, represent really alternating arrays of silicon empty and doped dots, with dimensions restricted to 2 nm. This concentration of boron seems to indicate that each doped dot located between empty dots contains two impurity atoms of boron. The EPR studies show that these boron pairs are the trigonal dipole centres, B+ - B–, that contain the pairs of holes, which result from the negative -U reconstruction of the shallow boron acceptors, 2B0 => B+ - B–. The electrical resistivity, magnetic susceptibility and specific heat measurements demonstrate that the high density of holes in the Si-QW (> 1011 cm–2) gives rise to the high temperature superconductor properties for the δ-barriers. The value of the superconductor energy gap obtained is in a good agreement with the data derived from the oscillations of the conductance in normal state and of the zero-resistance supercurrent in superconductor state as a function of the bias voltage. These oscillations appear to be correlated by on- and off-resonance tuning the two-dimensional subbands of holes with the Fermi energy in the superconductor δ-barriers. Finally, the proximity effect in the S-Si-QW-S structure is revealed by the findings of the quantization of the supercurrent and the multiple Andreev reflection (MAR) observed both across and along the Si-QW plane thereby identifying the spin transistor effect.

Origins of Hole Doping and Relevant Optoelectronic Properties of Wide Gap p-Type Semiconductor, LaCuOSe

LaCuOSe is a wide band gap (∼2.8 eV) semiconductor with unique optoelectronic properties, including room-temperature stable excitons, high hole mobility ∼8 cm(2)/(Vs), and the capability of high-density hole doping (up to 1.7 × 10(21) cm(-3) using Mg). Moreover, its carrier transport and doping behaviors exhibit nonconventional results, e.g., the hole concentration increases with decreasing temperature and the high hole doping does not correlate with other properties such as optical absorption. Herein, secondary ion mass spectroscopy and photoemission spectroscopy reveal that aliovalent ion substitution of Mg at the La site is not the main source of hole doping and the Fermi level does not shift even in heavily doped LaCuOSe:Mg. As the hole concentration increases, the subgap optical absorption becomes more intense, but the increase in intensity does not correlate quantitatively. Transmission electron microscopy indicates that planar defects composed of Cu and Se deficiencies are easily created in LaCuOSe. These observations can be explained via the existence of a degenerate low-mobility layer and formation of complex Cu and Se vacancy defects with the assistance of generalized gradient approximation band calculations.

(Invited) Recent Insights in the Diffusion of Boron in Silicon and Germanium

Experimental evidences and microscopic modelling of the mechanism of B diffusion in Si and Ge are given. In both the lattices B migrates by the mediation of self-interstitials (Is). In Si, B diffusion occurs mainly through the formation of a BI0 complex, after interactions of BS- with I0 or with I++ in intrinsic condition or high hole densities, respectively, followed by a proper charge exchange. A small contribution of the BI- complex is visible only under n-type doping, when BS- and I0 bind nonetheless the pairing of B with the n-dopants. Also in Ge, the B diffusion mechanism has been fixed, even if to a lower extent, revealing the need of self-interstitials to start the B motion. We evidenced the occurrence of the proton radiation enhanced diffusion (RED) and of the transient enhanced diffusion (TED) of B, modeling both the cases with the central role of Is.

Holey Silicon as an Efficient Thermoelectric Material

This work investigated the thermoelectric properties of thin silicon membranes that have been decorated with high density of nanoscopic holes. These "holey silicon" (HS) structures were fabricated by either nanosphere or block-copolymer lithography, both of which are scalable for practical device application. By reducing the pitch of the hexagonal holey pattern down to 55 nm with 35% porosity, the thermal conductivity of HS is consistently reduced by 2 orders of magnitude and approaches the amorphous limit. With a ZT value of ∼0.4 at room temperature, the thermoelectric performance of HS is comparable with the best value recorded in silicon nanowire system.

THE DISTRIBUTION OF STARS AND STELLAR REMNANTS AT THE GALACTIC CENTER

Motivated by recent observations that suggest a low density of old stars around the Milky Way supermassive black hole, models for the nuclear star cluster are considered that have not yet reached a steady state under the influence of gravitational encounters. A core of initial radius 1-1.5 pc evolves to a size of approximately 0.5 pc after 10 Gyr, roughly the size of the observed core. The absence of a Bahcall-Wolf cusp is naturally explained in these models, without the need for fine-tuning or implausible initial conditions. In the absence of a cusp, the time for a 10-solar-mass black hole to spiral in to the Galactic center from an initial distance of 5 pc can be much greater than 10 Gyr. Assuming that the stellar black holes had the same phase-space distribution initially as the stars, their density after 5-10 Gyr is predicted to rise very steeply going into the stellar core, but could remain substantially below the densities inferred from steady-state models that include a steep density cusp in the stars. Possible mechanisms for the creation of the parsec-scale initial core include destruction of stars on centrophilic orbits in a pre-existing triaxial nucleus, inhibited star formation near the supermassive black hole, or ejection of stars by a massive binary. The implications of these models are discussed for the rates of gravitational-wave inspiral events, as well as other physical processes that depend on a high density of stars or stellar mass black holes near Sagittarius A*.