Searcharxiv⌕ Search

arXiv subjects

G. Brammertz

Publications and source records attributed to G. Brammertz.

11 recordsLinked to original sources
arXiv2007

Selective Epitaxial Growth of GaAs on Ge Substrates with a SiO2 Pattern

We have selectively grown thin epitaxial GaAs films on Ge substrates with the aid of a 200 nm thin SiO2 mask layer. The selectively grown structures have lateral sizes ranging from 1 um width up to large areas of 1 by 1 mm2. The growth is fully selective, thanks to an optimized growth procedure, consisting of a 13 nm thin nucleation layer grown at high pressure, followed by low pressure growth of GaAs. This growth procedure inhibits the nucleation of GaAs on the mask area and is a good compromise between reduction of loading effects and inhibition of anti phase domain growth in the GaAs. Nevertheless, both microscopic and macroscopic loading effects can still be observed on x-section SEM images and profilometer measurements. X-ray diffraction and low temperature photoluminescence measurements demonstrate the good microscopic characteristics of the selectively grown GaAs.

cond-mat.mtrl-sci↗
arXiv2004

Enhancement of photo-responsivity of small gap, multiple tunnelling superconducting tunnel junctions due to quasiparticle multiplication

We recently predicted the formation of a highly non-equilibrium quasiparticle (qp) distribution in low TC multiple tunnelling superconducting tunnel junctions (STJs) [1]. The situation arises through qp energy gain in cycles of successive forward and back tunnelling events in the absence of relaxation via sub-gap phonon emission. The qps can acquire sufficient energy to emit phonons, which break more Cooper pairs and release additional qps. In this paper we report theoretical and experimental studies of the effect of this process on photon detection by such an STJ. We derived a set of energy-dependent balance equations [2], which describe the kinetics of the qps and phonons, including the qp multiplication process described above. Solution of the balance equations gives the non-equilibrium distribution of the qps as a function of time and energy, and hence the responsivity of the STJ as a function of bias voltage. We compared the theoretical results with experiments on high quality, multiple-tunnelling Al STJs cooled to 35mK in an adiabatic demagnetisation refrigerator, and illuminated with monochromatic photons with wavelengths between 250 and 1000 nm. It was found that in the larger junctions with the longest qp loss time, both responsivity and signal decay time increased rapidly with bias voltage. Excellent agreement was obtained between the observed effects and theoretical modelling.

cond-mat.supr-con↗
arXiv2004

Development of low critical temperature superconducting tunnel jucntions for application as photon detectors in astronomy

This thesis describes the development of low-energy gap superconducting tunnel junctions (STJs) for use as photon detectors, with as a main goal the improvement of the energy resolution in both the optical and the x-ray energy domain. A new model for the photon detection process with STJs is presented, which includes the full energy dependence of all the quasiparticle processes occurring in the junctions. This model allows for the calculation of the time- and energy-dependent quasiparticle distribution from the moment of generation of the quasiparticles by the photon absorption process until the end of the current pulse, when all the quasiparticles have disappeared.

cond-mat.supr-con↗
arXiv2004

Optical photon detection in Al Superconducting Tunnel Junctions

We report on the successful fabrication of low leakage aluminium superconducting tunnel junctions with very homogeneous and transparent insulating barriers. The junctions were tested in an adiabatic demagnetisation refrigerator with a base temperature of 35 mK. The normal resistance of the junctions is equal to ~7 uohm cm2 with leakage currents in the bias voltage domain as low as 100 fA/um2. Optical single photon counting experiments show a very high responsivity with charge amplification factors in excess of 100. The total resolving power (including electronic noise) for 500 nm photons is equal to 13 compared to a theoretical tunnel limited value of 34. The current devices are found to be limited spectroscopically by spatial inhomogeneities in the detectors response

cond-mat.supr-con↗
arXiv2004

Critical temperature of superconducting bilayers: theory and experiment

A generalized model for the critical temperature Tc of superconducting bilayers is presented, which is valid with no restrictions to film thicknesses, Tc of the layers and interface resistivity. The model is verified experimentally on a series of Nb-Al and Ta-Al bilayers with Nb, Ta layer thicknesses of 100 nm and Al layer thicknesses ranging from 5 nm to 200 nm. Excellent agreement between theory and experiment was found for the energy gap and the Tc of bilayers. The results are important for designing practical superconducting devices.

cond-mat.supr-con↗
arXiv2004

Energy-dependent kinetic model of photon absorption by superconducting tunnel junctions

We describe a model for photon absorption by superconducting tunnel junctions in which the full energy dependence of all the quasiparticle dynamic processes is included. The model supersedes the well-known Rothwarf-Taylor approach, which becomes inadequate for a description of the small gap structures that are currently being developed for improved detector resolution and responsivity. In these junctions relaxation of excited quasiparticles is intrinsically slow so that the energy distribution remains very broad throughout the whole detection process. By solving the energy-dependent kinetic equations describing the distributions, we are able to model the temporal and spectral evolution of the distribution of quasiparticles initially generated in the photo-absorption process. Good agreement is obtained between the theory and experiment.

cond-mat.supr-con↗
arXiv2004

Future optical detectors based on Al superconducting tunnel junctions

Superconducting tunnel junctions are being developed for application as photon detectors in astronomy. We present the latest results on the development of very high quality, very low critical temperature junctions, fabricated out of pure Al electrodes. The detectors are operated at 50 mK in an adiabatic demagnetisation refrigerator. The contacts to the top and base electrodes of these junctions are fabricated either out of Nb or Ta, which has strong implications on the loss time of the quasiparticles. The Nb contacted junctions show quasiparticle loss times varying between 5 and 80 usec, depending on the device size. The bias range of the Nb-contacted junctions is limited to the range 0-100 uV, because of the set-in of strong non-equilibrium quasiparticle multiplication currents at higher bias voltages. The Ta-contacted junctions, on the other hand, show quasiparticle loss times in excess of 200 usec. These long loss times lead to very strong quasiparticle multiplication, which prevents the stable biasing of the junctions even at very low bias voltages. Junction fabrication and characterisation are described, as well as the response of the detectors to monochromatic light with wavelengths varying from 250 to 1000 nm. The energy resolution of the detectors is discussed.

cond-mat.supr-con↗
arXiv2001

Strong Quasiparticle Trapping In A 6x6 Array Of Vanadium-Aluminum Superconducting Tunnel Junctions

A 6x6 array of symmetrical V/Al/AlOx/Al/V Superconducting Tunnel Junctions (STJs) was fabricated. The base electrode is a high quality epitaxial film with a residual resistance ratio (RRR) of ~30. The top film is polycrystalline with an RRR of ~10. The leakage currents of the 25x25 mm^2 junctions are of the order of 0.5 pA/mm^2 at a bias voltage of 100 mV, which corresponds to a dynamical resistance of ~ 3 10^5 ohms. When the array was illuminated by 6 keV X-ray photons from a 55Fe radioactive source the single photon charge output was found to be low and strongly dependent on the temperature of the devices. This temperature dependence at X-ray energies can be explained by the existence of a very large number of quasiparticle (QP) traps in the Vanadium. QPs are confined in these traps, having a lower energy gap than the surrounding material, and are therefore not available for tunneling. The number of traps can be derived from the energy dependence of the responsivity of the devices (charge output per electron volt of photon input energy).

cond-mat.supr-con↗
arXiv2001

Development of practical soft X-ray imaging spectrometers

Cryogenic soft X-ray imaging spectrometers are currently being developed for applications in the fields of Astronomy and material sciences. In this paper we present experiments on optimized single devices, which show measured energy resolutions of 4.6 eV at 525 eV, 8.1 eV at 1.5 keV and 20.5 eV at 6 keV respectively. These energy resolutions combined with a quantum efficiency of more than 40 % in the energy range from 0.5 to 2 keV together with a count rate capability of 15 kHz demonstrate the overall good performance of single Superconducting Tunnel Junctions (STJs). Assembling these optimized single devices in a matrix read-out would provide the practical basis for a soft X-ray imaging spectrometer.

cond-mat.supr-con↗