Real-time, in situ monitoring of surface reactions during plasma passivation of GaAs

Real-time, in situ observations of surface chemistry during the remote plasma passivation of GaAs is reported herein. Using attenuated total reflection Fourier transform infrared spectroscopy, the relative concentrations of -As-O, -As-H, -H2O, and -CH2 bonds are measured as a function of exposure to the effluent from a microwave discharge through NH3, ND3, H2, and D2. The photoluminescence intensity (PL) from the GaAs substrate is monitored simultaneously and used qualitatively to estimate the extent of surface state reduction. It was found that, while the -CHx(x = 2,3) and -As-O concentrations are reduced rapidly, the rates at which the -As-H concentration and the PL intensity increase are relatively slow. The concentration of -H2O on the GaAs surface increases throughout the process as surface arsenic oxides and the silica reactor walls are reduced by atomic hydrogen. These observations suggest that removal of elemental As by reaction with H at the GaAs–oxide interface limits the passivation rate

Cosmic Strings as Emitters of Extremely High Energy Neutrinos

We study massive particle radiation from cosmic string kinks, and its observability in extremely high energy neutrinos. In particular, we consider the emission of moduli --- weakly coupled scalar particles predicted in supersymmetric theories --- from the kinks of cosmic string loops. Since kinks move at the speed of light on strings, moduli are emitted with large Lorentz factors, and eventually decay into many pions and neutrinos via hadronic cascades. The produced neutrino flux has energy $E \gtrsim 10^{11} \rm{GeV}$, and is affected by oscillations and absorption (resonant and non-resonant). It is observable at upcoming neutrino telescopes such as JEM-EUSO, and the radio telescopes LOFAR and SKA, for a range of values of the string tension, and of the mass and coupling constant of the moduli.Comment: 13 pages, 2 figure

Effectiveness of Monetary Policy: Evidence from Turkey

Effectiveness of monetary policy depends on the degree to which policy interest rate affects all other financial prices, including the entire term structure of interest rates, credit rates, exchange rates and asset prices. An effective monetary policy framework can be seen as a pre-condition for well-functioning financial markets. However, effectiveness of the monetary policy is not straightforward to measure and requires empirical work to understand the effects of financial infrastructure, competitiveness of financial markets as well as current economic conditions. This paper examines the effectiveness of the monetary policy in Turkey by focusing on the interest rate pass-through behavior by means of an Interacted Panel Vector Autoregressive (IPVAR) approach. The results suggest that policy rate innovations transmit fully in less than eight months. Regulatory quality of the country, competition, liquidity, and profitability of banking sector, dollarization and exchange rate flexibility, inflation, and term structure have a positive effect on interest rate pass-through. Short-term credit ratio, GDP growth, monetary growth, and capital inflows have a negative effect

Effectiveness of Monetary Policy: Evidence from Turkey

Effectiveness of monetary policy depends on the degree to which policy interest rate affects all other financial prices, including the entire term structure of interest rates, credit rates, exchange rates and asset prices. An effective monetary policy framework can be seen as a pre-condition for well-functioning financial markets. However, effectiveness of the monetary policy is not straightforward to measure and requires empirical work to understand the effects of financial infrastructure, competitiveness of financial markets as well as current economic conditions. This paper examines the effectiveness of the monetary policy in Turkey by focusing on the interest rate pass-through behavior by means of an Interacted Panel Vector Autoregressive (IPVAR) approach. The results suggest that policy rate innovations transmit fully in less than eight months. Regulatory quality of the country, competition, liquidity, and profitability of banking sector, dollarization and exchange rate flexibility, inflation, and term structure have a positive effect on interest rate pass-through. Short-term credit ratio, GDP growth, monetary growth, and capital inflows have a negative effect

Ammonia plasma passivation of GaAs in downstream microwave and radio-frequency parallel plate plasma reactors

The poor electronic properties of the GaAs surface and GaAs–insulator interfaces, generally resulting from large density of surface/interface states, have limited GaAs device technology. Room-temperature ammonia plasma (dry) passivation of GaAs surfaces, which reduces the surface state density, is investigated as an alternative to wet passivation techniques. Plasma passivation is more compatible with clustered-dry processing which provides better control of the processing environment, and thus, improves interface integrity. Passivation was monitored in real-time and in situ using photoluminescence (PL). In addition, the passivated surfaces are inspected using x-ray photoelectron spectroscopy. Passivation with two different plasma excitation methods, downstream microwave (2.45 GHz) and rf (13.56 MHz) parallel plate, are compared, and effects of operating parameters such as pressure, flow rate, and power are examined. In both methods plasma-generated H atoms reduce the surface state density by removing excess As and As2O3 during the first few seconds of the plasma exposure. This step is followed by formation of Ga2O3 which takes place on a longer time scale (5–10 min). While the final passivation result appears to be similar for both methods, surface damage by ion bombardment competes with passivation in the parallel plate method, reduces the PL yield and adversely affects the long term stability of the passivated surface. Although it is common to heat the sample during passivation, we show that NH3 plasma passivation is possible at room temperature without heating. Low-temperature processing is important since passivation can be done at the end of device processing when it is undesirable to expose the device to elevated temperatures. The absence of ion bombardment damage combined with efficient generation of H atoms in the downstream microwave treatment, make this scheme a preferred dry passivation process, which could be easily and inexpensively clustered with existing GaAs processes

Manipulating multiple sequence alignments via MaM and WebMaM

MaM is a software tool that processes and manipulates multiple alignments of genomic sequence. MaM computes the exact location of common repeat elements, exons and unique regions within aligned genomics sequences using a variety of user identified programs, databases and/or tables. The program can extract subalignments, corresponding to these various regions of DNA to be analyzed independently or in conjunction with other elements of genomic DNA. Graphical displays further allow an assessment of sequence variation throughout these different regions of the aligned sequence, providing separate displays for their repeat, non-repeat and coding portions of genomic DNA. The program should facilitate the phylogenetic analysis and processing of different portions of genomic sequence as part of large-scale sequencing efforts. MaM source code is freely available for non-commercial use at ; and the web interface WebMaM is hosted at

In situ probing of surface hydrides on hydrogenated amorphous silicon using attenuated total reflection infrared spectroscopy

An in situ method based on attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) is presented for detecting surface silicon hydrides on plasma deposited hydrogenated amorphous silicon (a-Si:H) films and for determining their surface concentrations. Surface silicon hydrides are desorbed by exposing the a-Si:H films to low energy ions from a low density Ar plasma and by comparing the infrared spectrum before and after this low energy ion bombardment, the absorptions by surface hydrides can sensitively be separated from absorptions by bulk hydrides incorporated into the film. An experimental comparison with other methods that utilize isotope exchange of the surface hydrogen with deuterium showed good agreement and the advantages and disadvantages of the different methods are discussed. Furthermore, the determination of the composition of the surface hydrogen bondings on the basis of the literature data on hydrogenated crystalline silicon surfaces is presented, and quantification of the hydrogen surface coverage is discusse

Potential resolution to the doping puzzle in iron pyrite: Carrier type determination by Hall effect and thermopower

Pyrite FeS2 has outstanding potential as an earth-abundant, low-cost, nontoxic photovoltaic, but underperforms dramatically in solar cells. While the full reasons for this are not clear, one certain factor is the inability to understand and control doping in FeS2. This is exemplified by the widely accepted but unexplained observation that unintentionally doped FeS2 single crystals are predominantly n type, whereas thin films are p type. Here we provide a potential resolution to this “doping puzzle,” arrived at via Hall effect, thermopower, and resistivity measurements on a large set of FeS2 single crystals and films that span five orders of magnitude in mobility. The results reveal three main findings. First, in addition to crystals, the highest mobility thin films in this study are shown to be definitively n type, from both Hall effect and thermopower. Second, as mobility decreases an apparent crossover to p type occurs, first in thermopower, then in Hall measurements. This can be understood, however, in terms of the crossover from diffusive to hopping transport that is clearly reflected in resistivity. Third, universal behavior is found for both crystals and films, suggesting a common n dopant, possibly sulfur vacancies. We thus argue that n-type doping is facile in FeS2 films, that apparent p-type behavior in low mobility samples can be an artifact of hopping, and that the prevailing notion of predominantly p-type films must be revised. These conclusions have deep implications, both for interpretation of prior work on FeS2 solar cells and for the design of future studies