Elemental boron doping behavior in silicon molecular beam epitaxy

Boron-doped Si epilayers were grown by molecular beam epitaxy (MBE) using an elemental boron source, at levels up to 2×1020 cm−3, to elucidate profile control and electrical activation over the growth temperature range 450–900 °C. Precipitation and surface segregation effects were observed at doping levels of 2×1020 cm−3 for growth temperatures above 600 °C. At growth temperatures below 600 °C, excellent profile control was achieved with complete electrical activation at concentrations of 2×1020 cm−3, corresponding to the optimal MBE growth conditions for a range of Si/SixGe1−x heterostructures

Growth studies on Si0.8Ge0.2 channel two-dimensional hole gases

We report a study of the influences of MBE conditions on the low-temperature mobilities of Si/Si0.8Ge0.2 2DHG structures. A significant dependence of 2DHG mobility on growth temperature is observed with the maximum mobility of 3640 cm2 V−1 s−1 at 5.4 K being achieved at the relatively high-growth temperature of 640 °C. This dependence is associated with a reduction in interface charge density. Studies on lower mobility samples show that Cu contamination can be reduced both by growth interruptions and by modifications to the Ge source; this reduction produces improvements in the low-temperature mobilities. We suggest that interface charge deriving from residual metal contamination is currently limiting the 4-K mobility

Optical Gravitational Lensing Experiment. OGLE-1999-BUL-32: the Longest Ever Microlensing Event -- Evidence for a Stellar Mass Black Hole?

We describe the discovery of the longest microlensing event ever observed, OGLE-1999-BUL-32, also independently identified by the MACHO collaboration as MACHO-99-BLG-22. This unique event has an Einstein radius crossing time of 641 days. The high quality data obtained with difference image analysis shows a small but significant parallax signature. This parallax effect allows one to determine the Einstein radius projected onto the observer plane as rE^hat ~ 29.2AU. The transverse velocity projected onto the observer plane is about 79km/s. We argue that the lens is likely to be have a mass of at least a few solar masses, i.e., it could be a stellar black hole. The black hole hypothesis can be tested using the astrometric microlensing signature with the soon-to-be installed Advanced Camera for Surveys on board the Hubble Space Telescope. Deep X-ray and radio images may also be useful for revealing the nature of the object.Comment: submitted to Monthly Notice

The Optical Gravitational Lensing Experiment. Monitoring of QSO 2237+0305

We present results from 2 years of monitoring of Huchra's lens (QSO 2237+0305) with the 1.3 m Warsaw telescope on Las Campanas, Chile. Photometry in the V band was done using a newly developed method for image subtraction. Reliable subtraction without Fourier division removes all complexities associated with the presence of a bright lensing galaxy. With positions of lensed images adopted from HST measurements it is relatively easy to fit the variable part of the flux in this system, as opposed to modeling of the underlying galaxy. For the first time we observed smooth light variation over a period of a few months, which can be naturally attributed to microlensing. We also describe automated software capable of real time analysis of the images of QSO 2237+0305. It is expected that starting from the next observing season in 1999 an alert system will be implemented for high amplification events (HAE) in this object. Time sampling and photometric accuracy achieved should be sufficient for early detection of caustic crossings.Comment: 8 pages (including 4 figures and table), latex, emulateapj, submitted to ApJ, revised version - minor change

Antisite effect on ferromagnetism in (Ga,Mn)As

We study the Curie temperature and hole density of (Ga,Mn)As while systematically varying the As-antisite density. Hole compensation by As-antisites limits the Curie temperature and can completely quench long-range ferromagnetic order in the low doping regime of 1-2% Mn. Samples are grown by molecular beam epitaxy without substrate rotation in order to smoothly vary the As to Ga flux ratio across a single wafer. This technique allows for a systematic study of the effect of As stoichiometry on the structural, electronic, and magnetic properties of (Ga,Mn)As. For concentrations less than 1.5% Mn, a strong deviation from Tc ~ p^0.33 is observed. Our results emphasize that proper control of As-antisite compensation is critical for controlling the Curie temperatures in (Ga,Mn)As at the low doping limit.Comment: 10 pages, 7 figure

Evidence for quantum confinement in the photoluminescence of porous Si and SiGe

We have used anodization techniques to process porous surface regions in p-type Czochralski Si and in p-type Si0.85Ge0.15 epitaxial layers grown by molecular beam epitaxy. The SiGe layers were unrelaxed before processing. We have observed strong near-infrared and visible light emission from both systems. Analysis of the radiative and nonradiative recombination processes indicate that the emission is consistent with the decay of excitons localized in structures of one or zero dimensions

The Optical Gravitational Lensing Experiment. Short Distance Scale to the LMC

We present {\it UBVI} photometry of the eclipsing binary HV2274 - the system which has been recently used for distance determination to the LMC by Guinan et al. (1998). We determine the interstellar reddening to the star, E(B-V)=0.149+/-0.015 mag, based on observed colors of the star. This value is in excellent agreement with the mean reddening towards HV2274 obtained from photometry of the red clump stars in the surrounding field. The reddening is almost twice as large as determined by Guinan et al. (1998). We discuss the consequences of reddening underestimate. Most likely HV2274 is located much closer with the distance modulus to the star and the LMC: m-M = 18.22+/-0.13 mag supporting the short distance scale to the LMC. Such a distance modulus is in excellent agreement with the recent distance determinations with RR Lyr and red clump stars.Comment: 11 pages, Latex, 2 Figures. Accepted for publication in Astrophysical Journal Letters. New version - trimmed to fit ApJL. Additional determination of the reddening towards HV2274 with OB star

Reduction of CO_2 by Pyridine Monoimine Molybdenum Carbonyl Complexes: Cooperative Metal–Ligand Binding of CO_2

[(^(Ar)PMI)Mo(CO)_4] complexes (PMI=pyridine monoimine; Ar=Ph, 2,6-di-iso-propylphenyl) were synthesized and their electrochemical properties were probed with cyclic voltammetry and infrared spectroelectrochemistry (IR-SEC). The complexes undergo a reduction at more positive potentials than the related [(bipyridine)Mo(CO)_4] complex, which is ligand based according to IR-SEC and DFT data. To probe the reaction product in more detail, stoichiometric chemical reduction and subsequent treatment with CO_2 resulted in the formation of a new product that is assigned as a ligand-bound carboxylate, [(^(iPr2Ph)PMI)Mo(CO)_3(CO_2)]^(2-), by NMR spectroscopic methods. The CO_2 adduct [(^(iPr2Ph)PMI)Mo(CO)_3(CO_2)]^(2-) could not be isolated and fully characterized. However, the C_C coupling between the CO_2 molecule and the PDI ligand was confirmed by X-ray crystallographic characterization of one of the decomposition products of [(^(iPr2Ph)PMI)Mo(CO)_3(CO_2)])^(2-)