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This is a copy of the slides presented at the meeting but not formally written up for the volume.
In order to pursue device applications of magnetoelectric Cr2O3, we have fabricated epitaxial Cr2O3 thin films on (001), (110) and (101) oriented Nb doped TiO2 by pulsed laser deposition. The Cr2O3 films with different thicknesses (0.3 1 ¦Ìm) showed extremely smooth surfaces with rms roughness ¡Ö 0.3 nm (for 10 X 10 ¦Ìm2) as measured by AFM for all 3 different orientations. The films display robust insulating properties at room temperature with leakage current density of 8.9 X 10-6 A/cm2 at 10 kV/cm for 300 nm thick films. In order to investigate exchange bias, we fabricated bilayer films of Cr2O3/Co with all 3 orientations. The magnetic properties of the films were measured using SQUID and the magnetic optical Kerr effect (MOKE). From the Cr2O3/Co film grown on a (110) oriented TiO2, we clearly observe exchange bias of ¡Ö 13 Oe with a coercive field of 115 Oe upon cooling from 320 K to 30 K in a 1 T magnetic field. The microstructural properties of the bilayers and the effect of electric field on the exchange bias behavior were investigated using TEM, VSM and MOKE. Comparison of exchange bias with BiFeO3 and TbMnO3 multiferroic thin films will also be discussed. This work is supported by W. M. Keck Foundation, ONR grant No. N00014-01-1-0761, N00014-04-1-0085, and the NSF under grants DMR-00-94265 (CAREER), NSF DMR-00-0231291, NSF 0095166, NSF-MRSEC Award No. DMR-00-0520471. We acknowledge use of the Nanoscale Imaging, Spectroscopy, and Properties (NISP) Laboratory for TEM characterization.
Inadequate protein quality may be a risk factor for poor growth. To examine the effect of a macronutrient–micronutrient supplement KOKO Plus (KP), provided to infants from 6 to 18 months of age, on linear growth, a single-blind cluster-randomised study was implemented in Ghana. A total of thirty-eight communities were randomly allocated to receive KP (fourteen communities, n 322), a micronutrient powder (MN, thirteen communities, n 329) and nutrition education (NE, eleven communities, n 319). A comparison group was followed cross-sectionally (n 303). Supplement delivery and morbidity were measured weekly and anthropometry monthly. NE education was provided monthly. Baseline, midline and endline measurements at 6, 12 and 18 months included venous blood draws, diet, anthropometry, morbidity, food security and socio-economics. Length-for-age Z-score (LAZ) was the primary outcome. Analyses were intent-to-treat using mixed-effects regressions adjusted for clustering, sex, age and baseline. No differences existed in mean LAZ scores at endline (−1·219 (sd 0·06) KP, −1·211 (sd 0·03) MN, −1·266 (sd 0·03) NE). Acute infection prevalence was lower in the KP than NE group (P = 0·043). Mean serum Hb was higher in KP infants free from acute infection (114·02 (sd 1·87) g/l) than MN (107·8 (sd 2·5) g/l; P = 0·047) and NE (108·8 (sd 0·99) g/l; P = 0·051). Compliance was 84·9 % (KP) and 87·2 % (MN) but delivery 60 %. Adjusting for delivery and compliance, LAZ score at endline was significantly higher in the KP v. MN group (+0·2 LAZ; P = 0·026). A macro- and micronutrient-fortified supplement KP reduced acute infection, improved Hb and demonstrated a dose–response effect on LAZ adjusting consumption for delivery.
The origins of the large Classic and Postclassic urban centres of Central Mexico remain poorly understood. Archaeological investigations at the Formative site of Tlalancaleca in Puebla (Mexico) provide the first detailed study of a large-scale urban centre of that period. Preliminary results suggest that the growth and development of this particular site may have influenced the subsequent growth of Teotihuacan itself. This study explores how urbanisation can be identified archaeologically by tracing the expansion of population and the emergence of monumental architecture.
Since the discovery of fading X-rays from Gamma-Ray Bursts (GRBs) with BeppoSAX (Piro et al. 1997, Costa et al. 1997), world-wide follow-up observations in optical band have achieved the fruitful results. The case of GRB 970228, there was an optical transient, coincides with the BeppoSAX position and faded (Paradijs et al. 1997, Sahu et al. 1997). These optical observations also confirmed the extended component, which was associated with the optical transient. The new transient are fading with a power-law function in time and the later observation of HST confirmed the extended emission is stable (Fruchter et al. 1997). This extended object seems to be a distant galaxy and strongly suggests to be the host.
It is now firmly established that a small anisotropy of the galactic cosmic rays exists, observable from Earth as a variation of intensity in sidereal time. The problem now is to determine more clearly the characteristics of the anisotropy and, in particular, its detailed spatial structure and how it depends upon the energy and composition of the cosmic rays. This is a very difficult task and, in the final analysis, may not be fully achievable from Earth-based observations. The purpose of the present paper is to describe briefly an installation now operating in Tasmania to provide further information on the spatial structure of the anisotropy.
We applied laser THz emission spectroscopy to study the effects of monolayer graphene on the THz emission from InAs. THz emission from graphene/InAs varies linearly with the laser excitation power in the low-intensity excitation regime. We found that unlike in graphene/SI-InP junctions, graphene and O2 adsorbates on graphene have no significant effect on the THz emission from graphene/InAs junctions because the THz radiation mechanism in InAs is by the photo-Dember effect, whereas for SI-InP is by the surge current effect. There is also a slight enhancement in the THz emission from both bare InAs and graphene/InAs by UV illumination, which is probably due to the additional photoexcited carriers by UV that somehow enhances the photo-Dember field.
We describe the current, 9-spacecraft Interplanetary Network (IPN). The IPN detects about
325 gamma-ray bursts per year, of which about 100 are not localized by any other missions.
We give some examples of how the data, which are public, can be utilized.
Magnetic properties of Co-doped rutile (Ti1-xCoxO2) film in combinatorial composition-spread form have been surveyed by means of a Scanning Superconducting-quantum-interference-device Microscope (SSM). As a consequence, we found magnetic domains in the spatial regions with x>0.05 without external field, giving strong evidence for ferromagnetism with finite spontaneous magnetization. The magnetic moment was monotonously increased with increasing doping level x from 0.05 to ∼ 0.13. On the other hand, it was almost unchanged for x > ∼ 0.13, suggesting that Co does not dissolve into rutile film beyond x ∼ 0.13. The SSM results on the rutile Ti0.95Co0.05O2 thin films with different thickness showed that the magnetic moment is proportional to film thickness, leading to a conclusion that the presently observed ferromagnetism does not result from Co or Co-based oxide particles on the film surface.
The damage build-up and amorphization behavior in wurtzite GaN films under a wide range of implant conditions are studied by Rutherford backscattering / channeling spectrometry, transmission electron microscopy, and cathodoluminescence spectroscopy. A strong surface peak of lattice disorder, in addition to the expected damage peak in the region of the maximum of nuclear energy loss, has been observed for all implant conditions of this study. Capping of GaN with SiOx and SixNy layers prior to implantation does not eliminate surface disordering. This may suggest that nitrogen loss is not the main reason for the observed enhanced surface disorder, but, rather, the GaN surface acts as a strong sink for migrating point defects. However, pronounced loss of N during ion bombardment is observed for high dose implantation when the near-surface region is amorphized. Moreover, after amorphization, annealing at temperatures above about 400°C leads to complete decomposition of the near-surface layer.
Recent advances in developing process modules for GaN power devices are reviewed. These processes include damage removal in dry etched n- and p-GaN, implant doping and isolation, novel gate dielectrics, improved Schottky and ohmic contacts and deep via etching of SiC for hybrid GaN/SiC structures.
Low resistance ohmic contacts are difficult to form to p-type GaN and AlGaN due to the unavailability of growth methods for highly p-doped GaN and AlGaN. A p-type carbon-doped GaAs regrowth on p-GaN prior to ohmic metallization has been shown in previous work to improve contact resistance to p-GaN . Applying the regrowth method to the p-base regions of npn structured bipolar transistors, AlGaN/GaN heterojunction bipolar transistors and GaN bipolar junction transistors have been demonstrated. GaN/AlGaN epilayers were grown with a molecular beam epitaxy system. Highly carbon-doped p-GaAs (1020 cm−3) was regrown on the devices (∼500 Å) in the base contact region by metal organic chemical vapor deposition after emitter mesa etching. Emitter and base mesa structures were formed by Inductively Coupled Plasma etching under low damage conditions with a Cl2/Ar chemistry. SiO2 was used for emitter sidewall formation to reduce leakage current to the emitter, as well as for a mask for GaAs base regrowth. Very high current densities were obtained for common base operation in both device types. The devices were operable at 250 °C.
A drift-diffusion model has been used to explore the performance capabilities of Npn AlGaN/GaN heterojunction bipolar transistors. Numerical results have been employed to study the effect of the p-type Mg doping and its incomplete ionization on device performance. The high base resistance induced by the deep acceptor level is found to be the cause of limited current gain values for Npn devices. Several computation approaches have been considered to improve their performance. Reasonable improvement of the DC current gain β is observed by realistically reducing the base thickness in accordance with processing limitations. Base transport enhancement is also predicted by the introduction of a quasi-electric field in the base.
We report on the dc performance of the first GaN pnp bipolar junction transistor. The structure was grown by MOCVD on c-plane sapphire substrates and mesas formed by low damage Inductively Coupled Plasma etching with a Cl2/Ar chemistry. The dc characteristics were measured up to VBC of 65 V in the common base mode and at temperatures up to 250°C. Under all conditions, IC ∼ IE indicated higher emitter injection efficiency. The offset voltage was ≤ 2 V and the devices were operated up to power densities of 13.9 kW·cm−2.
AlN layers were grown on 6H-SiC(0001) by molecular beam epitaxy using ammonia as the nitrogen source. Clean (√3×√3)R30° SiC surfaces was prepared by in-situ annealing alone and also by in situ annealing consisted of followed by Si deposition and subsequent annealing. The surface morphology of the AlN films observed by AFM was significantly changed by the nucleation procedure. When the AlN growth was initiated with Al flux exposure on a SiC surface prepared by thermal annealing, the surface roughness of the AlN was significantly reduced. Two-dimensional growth of AlN was observed with reflection high-energy electron diffraction from the very beginning. Atomically flat AlN surfaces with a RMS-roughness of ∼0.3 nm were obtained. On the other hand, when film growth was initiated with an ammonia flux exposure on a Si rich SiC surface, a high density of bumps was observed. The bumps seemed to originate from SiNx formation at the heteroepitaxial interface. It was found that control of the Si composition and the V/III ratio at the growth interface is crucial for the AlN film quality.
The fabrication of a wide variety of GaN-based photonic and electronic devices depends on dry etching, which typically requires ion-assisted removal of the substrate material. Under conditions of both high plasma flux and energetic ion bombardment, GaN etch rates greater than 0.5 νm/min and anisotropic etch profiles are readily achieved in Inductively Coupled Plasma (ICP) etch systems. Unfortunately, under these conditions plasma-induced damage often occurs. Attempts to minimize such damage by reducing the ion energy or increasing the chemical activity in the plasma often result in a loss of etch rate or profile control which can limit dimensional control and reduce the utility of the process for device applications requiring anisotropic etch profiles. It is therefore necessary to develop plasma etch processes which couple anisotropy for critical dimension and sidewall profile control and high etch rates with low-damage for optimum device performance. In this study we report changes in source resistance, reverse breakdown voltage, transconductance, and drain saturation current for GaN MESFET structures exposed to an Ar ICP plasma. In general, device performance was sensitive to ion bombardment energy and ion flux.
We have found that near-band-edge cathodoluminescence (CL) emission decreases with time for some nominally undoped GaN samples. The rate of intensity decrease depends on the incident beam current. It also depends on the size of the area being scanned, which is determined by the magnification used, although the electron beam voltage and current are held constant and similar regions of GaN are being examined. Faster decrease with time occurs with higher beam currents and higher magnifications. The reduced luminescence efficiency persists over at least 24 hour beam-off periods. The dependence of CL intensity on beam current and on scanned area may be a result of different levels of local charging, and structural modifications caused by this charging, for different beam current and magnification settings.
The arsenic dose dependence of electrical properties for implanted samples at 500°C and subsequently annealed at 1600°C for 30min has been investigated to derivate the activation energies of the arsenic donors in silicon carbide. Hall effect measurements were performed between 20K and 773K. Hall carrier concentration of implanted sample with high dose of 7×1015 cm−2 is independence of temperature, which indicates the formation of implanted layer with metallic conduction. For the sample with low dose of 1×1014 cm−2, the experimental Hall mobility varies directly as T3/2 below 80K and as T−3/2 above 150K. The activation energies of arsenic donors determined from the implanted sample with low dose using a least-squares fit of the charge neutrality equation are 66.8 meV for hexagonal site and 127.0 meV for cubic site, respectively.
This paper updates on-going experimental and theoretical investigations of non-micropipe defects imaged by synchrotron white beam X-ray topography (SWBXT) in SiC devices and epitaxial layers. Computer-based thermal modeling of screw-dislocation related breakdown in SiC diodes has been initiated to gain insights into internal temperature profiles as a function of microplasma power. A preliminary study of epitaxial 4H- and 6H-SiC p+n mesa diodes indicates that very low angle boundaries, whose electrical properties have not previously been reported, do not significantly impact DC I-V properties (forward and reverse) measured at biases less than 70% of the SiC breakdown field. The presence of very small growth pits on the surface of commercial 4H-SiC epitaxial layers, almost undetectable by high magnification optical microscopy, was revealed by atomic force microscopy and found to correspond to the locations of closed core screw dislocations imaged by SWBXT.
Electron cyclotron resonance (ECR) excited N2 plasma exposure were performed to explore a possibility of a robust surface passivation on Ar+ ion beam etched GaN. We adopted Taguchi orthogonal design to optimize the ECR operating parameters with respect to reverse breakdown voltage (VB) and then used the results to determine the VB dependence on the parameters close to the optimum operating point. Rather consistent results were obtained by Taguchi and normal experimental methods. It is clearly indicated that the radio frequency (rf) chuck power play the most important role on the plasma passivation. After N2 plasma treatment, the diode reverse breakdown voltage is drastically restored from 15V to 95V. With the aid of N2 plasma exposure, the Schottky characteristics of ion beam etched GaN can be thus effectively improved.