Electron transport and optical properties of shallow GaAs/InGaAs/GaAs quantum wells with a thin central AlAs barrier

Shallow GaAs/InGaAs/GaAs quantum well structures with and without a three monolayer thick AlAs central barrier have been investigated for different well widths and Si doping levels. The transport parameters are determined by resistivity measurements in the temperature range 4-300 K and magnetotransport in magnetic fields up to 12 T. The (subband) carrier concentrations and mobilities are extracted from the Hall data and Shubnikov-de Haas oscillations. We find that the transport parameters are strongly affected by the insertion of the AlAs central barrier. Photoluminescence spectra, measured at 77 K, show an increase of the transition energies upon insertion of the barrier. The transport and optical data are analyzed with help of self-consistent calculations of the subband structure and envelope wave functions. Insertion of the AlAs central barrier changes the spatial distribution of the electron wave functions and leads to the formation of hybrid states, i.e. states which extend over the InGaAs and the delta-doped layer quantum wells.Comment: 14 pages, pdf fil

Broad-band high-resolution rotational spectroscopy for laboratory astrophysics

We present a new experimental set-up devoted to the study of gas phase molecules and processes using broad-band high spectral resolution rotational spectroscopy. A reactor chamber is equipped with radio receivers similar to those used by radio astronomers to search for molecular emission in space. The whole range of the Q (31.5-50 GHz) and W bands (72-116.5 GHz) is available for rotational spectroscopy observations. The receivers are equipped with 16 × 2.5 GHz fast Fourier transform spectrometers with a spectral resolution of 38.14 kHz allowing the simultaneous observation of the complete Q band and one-third of the W band. The whole W band can be observed in three settings in which the Q band is always observed. Species such as CH3CN, OCS, and SO2 are detected, together with many of their isotopologues and vibrationally excited states, in very short observing times. The system permits automatic overnight observations, and integration times as long as 2.4 × 105 s have been reached. The chamber is equipped with a radiofrequency source to produce cold plasmas, and with four ultraviolet lamps to study photochemical processes. Plasmas of CH4, N2, CH3CN, NH3, O2, and H2, among other species, have been generated and the molecular products easily identified by the rotational spectrum, and via mass spectrometry and optical spectroscopy. Finally, the rotational spectrum of the lowest energy conformer of CH3CH2NHCHO (N-ethylformamide), a molecule previously characterized in microwave rotational spectroscopy, has been measured up to 116.5 GHz, allowing the accurate determination of its rotational and distortion constants and its search in space.We thank the European Research Council for funding support under Synergy Grant ERC-2013-SyG, G.A. 610256 (NANOCOSMOS). IT, VJH, and JLD acknowledge additional partial support from the Spanish State Research Agency (AEI) through grant FIS2016-77726-C3-1-P. JAMG, LM, and GS acknowledge additional partial support from the Spanish State Research Agency (AEI) through grant MAT2017-85089-C2-1R. We thank David López Romero for his help during the process of installation, commissioning, and cleaning of the chamber. We would like to thank Kremena Makasheva for the useful comments and suggestions during the experiments with Hexamethyldisiloxane. We would also like to thank Rosa Lebrón, Jesús Quintanilla, and Cristina Soria for providing us with the sample of N-ethylformamide. Sandra I. Ramírez acknowledges support from the FONCICYT under grant number 291842. Celina Bermúdez thanks the Spanish Ministerio de Ciencia Innovación y Universidades for the Juan de la Cierva grant FJCI-2016-27983

Characterization of an Nmr Homolog That Modulates GATA Factor-Mediated Nitrogen Metabolite Repression in Cryptococcus neoformans

Nitrogen source utilization plays a critical role in fungal development, secondary metabolite production and pathogenesis. In both the Ascomycota and Basidiomycota, GATA transcription factors globally activate the expression of catabolic enzyme-encoding genes required to degrade complex nitrogenous compounds. However, in the presence of preferred nitrogen sources such as ammonium, GATA factor activity is inhibited in some species through interaction with co-repressor Nmr proteins. This regulatory phenomenon, nitrogen metabolite repression, enables preferential utilization of readily assimilated nitrogen sources. In the basidiomycete pathogen Cryptococcus neoformans, the GATA factor Gat1/Are1 has been co-opted into regulating multiple key virulence traits in addition to nitrogen catabolism. Here, we further characterize Gat1/Are1 function and investigate the regulatory role of the predicted Nmr homolog Tar1. While GAT1/ARE1 expression is induced during nitrogen limitation, TAR1 transcription is unaffected by nitrogen availability. Deletion of TAR1 leads to inappropriate derepression of non-preferred nitrogen catabolic pathways in the simultaneous presence of favoured sources. In addition to exhibiting its evolutionary conserved role of inhibiting GATA factor activity under repressing conditions, Tar1 also positively regulates GAT1/ARE1 transcription under non-repressing conditions. The molecular mechanism by which Tar1 modulates nitrogen metabolite repression, however, remains open to speculation. Interaction between Tar1 and Gat1/Are1 was undetectable in a yeast two-hybrid assay, consistent with Tar1 and Gat1/Are1 each lacking the conserved C-terminus regions present in ascomycete Nmr proteins and GATA factors that are known to interact with each other. Importantly, both Tar1 and Gat1/Are1 are suppressors of C. neoformans virulence, reiterating and highlighting the paradigm of nitrogen regulation of pathogenesis

A 200 GHz medium power amplifier MMIC in cascode metamorphic HEMT technology

A 200 GHz power amplifier is presented. The millimeter-wave monolithic integrated circuit (MMIC) has been realized in a 35 nm InAlAs/InGaAs cascode metamorphic high electron mobility transistor (MHEMT) process in grounded coplanar waveguide technology (GCPW). The amplifier demonstrates an output power of 14 mW with 11.4 dB compressed power gain at 200 GHz. This represents an increase in output power in comparison to previous reported MHEMT-based MMIC amplifiers. The small-signal gain demonstrates a peak value of 20 dB and is above 15.9 dB from 185 to 215 GHz

LNA module reliability testing for the MetOp second generation satellites

The reliability of the equipment that will be used in the MetOp Second Generation satellites is crucial, since there are significant storage and in-operation times. To evaluate the reliability performance of these receivers ESA has set up projects to assess these issues in advance. The most critical parts within the receivers of the atmospheric sounder and imager instruments are identified to be the detectors and the low-noise amplifiers. These components have a dedicated reliability assessment program within the projects on module level and the results can help in evaluating the most critical reliability aspects that should be investigated more carefully. In this paper, the 54 GHz (Vband) and 118 GHz (F-band) low-noise amplifier module design and reliability test results are presented

GaAs microstrip-to-waveguide transition operating in the WR-1.5 waveguide band (500-750 GHz)

In this paper, we report on the development of a microstrip-to-waveguide transition for the WR-1.5 waveguide band (500-750 GHz). The microstrip lines and E-plane probes have been manufactured on 25 m thick GaAs substrates. The transmission loss per single microstrip-to-waveguide transition is only 1.0 dB @ 670 GHz. The measured return losses are better than 10 dB up to 720 GHz. The single transition includes a waveguide section with a length of 7.0 mm corresponding to the transitions which will be used in future submillimeter-wave MMIC modules

A 600 GHz low-noise amplifier module

A compact WR-1.5 (500-750 GHz) low-noise amplifier (LNA) circuit has been developed, based on a grounded coplanar waveguide (GCPW) technology utilizing 20 nm metamorphic high electron mobility transistors (mHEMTs). The realized six-stage LNA TMIC achieved a maximum gain of 15.4 dB at 576 GHz and more than 10 dB in the frequency range from 555 to 619 GHz. For low-loss packaging of the circuit, a waveguide-to-microstrip transition has been fabricated on a 20 µm thick GaAs substrate, demonstrating an insertion loss of only 1 dB between 500 and 720 GHz. The realized LNA module achieved a small-signal gain of 14.1 dB at 600 GHz and a room temperature (T = 293 K) noise figure of 15 dB at the frequency of operation