We present a new type of a quantum detector, namely, the superconducting single-photon detector (SSPD), and demonstrate that it can be implemented in various single-photon counting applications, ranging from the visible light to near-infrared telecommunication wavelengths. We describe the physics of the photoresponse of a superconducting nanostripe to a flux of single optical photons and present the operation principle of SSPDs, stressing that, currently, they significantly outperform any competing, research or commercial devices in terms of their quantum efficiency, counting rate, jitter, and unwanted dark counts. SSPDs integrated with a cryogenic HEMT read-out circuit can provide some level of both the energy and number resolution of an unknown incident photon flux, making them uniquely suitable for various photon sensing applications.
AlGaN/GaN High Electron Mobility Transistors (HEMTs) are capable of achieving high breakdown voltage, low operating resistance and large switching speed due to the excellent performance shown by III-N structures. The paper presents selected details of technological experimental work on high voltage (HV) AlGaN/GaN-on-Si HEMTs fabricated with multifinger structures and gate widths of up to 40×1 mm. The electrical isolation of individual devices was elaborated using Al+ implantation. The ions were implanted up to a depth of 200 nm in order to produce an effective damage and isolation up to the non-conducting AlGaN buffer layer. The influence of the ion energy (in the range 208-385 kV) and the ion dose (in the range 8.5x1012-1.4x1013cm-2) on the effectiveness of the fabricated isolation was found. The properties of the fabricated ohmic contacts (using Ti/Al/Mo/Au and Ti/Al/TiN/Cu metallization schemes) with emphasis put on the technology of recess etching were studied. The impact of various pretreatment, applied before deposition of the gate metallization, on electrical parameters of multifinger devices was analysed. The tested pretreatment methods included oxide removal in HCl-based solution, and O2 or BCl3 plasma treatment, with the lowest gate leakage current obtained for the latter. The results of fabrication of the HV HEMTs with single field-plate structures with various dielectrics (Si3N4 or Al2O3) are discussed. The characterization results within the paper cover electrical (I-V characteristics), structural (TEM, XRD), topographical (AFM) and elemental (EDS mapping) analyses.
This work was supported by The National Centre for Research and Development under Agreement nr TECHMATSTRATEG1/346922/4/NCBR/2017 for project "Technologies of semiconductor materials for high power and high frequency electronics"
Novel optical sensors the most often require thin films or surface structures with strictly controlled properties, playing a critical role in them by initiating or modifying their sensorial responses. Selected results of research on atomic layer deposited (ALD) metallic oxides will be shown, regarding their applicability for thin functional coatings in lossy mode resonance (LMR) and long period grating (LPG) optical fiber sensors. Basically amorphous films of tantalum oxide (TaxOy), zirconium oxide (ZrxOy) and hafnium oxide (HfxOy) below 200 nm were deposited at relatively low temperature (LT) of 100°C. The optical, structural, topographical, tribological, hydrophilic and chemical stability properties of the films and their technological controllability were analysed. The TaxOy was selected and successfully applied as an oxide coating in LPG sensor. As chemically robust in alkali environment (pH over 9) it allowed to gain a potential for fabrication of regenerable/reusable biosensor. Additionally, ALD technique was tested as a tool for tailoring sensorial properties of LMR sensors. The double-layer coatings composed of two different materials were experimentally tested for the first time; the coatings were composed of plasma-enhanced chemical vapour deposited (PECVD) silicon nitride (SixNy) followed by much thinner ALD TaxOy. That approach yielded operating devices, ensuring fast overlay fabrication and easy tuning of the resonant wavelength at the same time. The LT ALD TaxOy films turned out to be slightly overstoichiometric (y/x approx. 2.75). Therefore, the issue of TaxOy chemical composition was studied by secondary ion mass spectroscopy, Rutherford backscattering spectrometry and x-ray photoelectron spectrometry.
Thermodynamic fluctuations of the superconducting order parameter in NbN/NiCu and NbTiN/NiCu superconductor/ferromagnet (S/F) thin bilayers patterned to microbridges are investigated. Plain NbN and NbTiN films served as reference materials for the analyses. The samples were grown using dc-magnetron sputtering on chemically cleaned sapphire single-crystal substrates. After rapid thermal annealing at high temperatures, the superconducting films were coated with NiCu overlays, using co-sputtering. The positive magnetoresistance of the superconducting single layers is very small in the normal state but with a sharp upturn close to the superconducting transition, a familiar signature of superconducting fluctuations. The fluctuation-enhanced conductivity (paraconductivity) of the NbN and NbTiN single layer films is slightly larger than the prediction of the parameter-free Aslamazov-Larkin theory for order-parameter fluctuations in two-dimensional superconductors. The addition of a ferromagnetic top layer, however, changes the magnetotransport properties significantly. The S/F bilayers show a negative magnetoresistance up to almost room temperature, while the signature of fluctuations is similar to that in the plain films, demonstrating the relevance of both ferromagnetic and superconducting effects in the S/F bilayers. The paraconductivity is reduced below theoretical predictions, in particular in the NbTiN/NiCu bilayers. Such suppression of the fluctuation amplitude in S/F bilayers could be favorable to reduce dark counts in superconducting photon detectors and lead the way to enhance their performance.
Critical current and current-voltage characteristics of epitaxial Nb(Ti)N submicron ultrathin structures were measured as function of temperature. For 700-nm-wide bridge we found current-driven vortex de-pinning at low temperatures and thermally activated flux flow closer to the transition temperature, as the limiting factors for the critical current density. For 100-nm-wide meander we observed combination of phase-slip activation and vortex-anti-vortex pair (VAP) thermal excitation. Our Nb(Ti)N meander structure demonstrates high de-pairing critical current densities ~107 A/cm2 at low temperatures, but the critical currents are much smaller due to presence of the local constrictions.
A. Klimov, R. Puźniak, B. Aichner, W. Lang, E. Joon, R. Stern, W. Słysz, M. Guziewicz, M. Juchniewicz, M. Borysiewicz, R. Kruszka, M. Węgrzecki, A. Łaszcz, A. Czerwinski, Roman Sobolewski
Performance of superconducting single-photon detectors based on resistive hotspot formation in nanostripes upon optical photon absorption depends strongly on the critical current density JC of the fabricated nanostructure. Utilization of an ultrathin, weak-ferromagnet cap layer on the top of a superconducting film enhances of the structure’s JC due to an extra flux pinning. We have fabricated a number of both NbN/NiCu and NbTiN/NiCu superconductor/ferromagnet (S/F) ultrathin bilayers and microbridges. NbN and NbTiN underlayers with thicknesses varying from 4 to 7 nm were grown using dc-magnetron sputtering on chemically cleaned sapphire single-crystal substrates. After rapid thermal annealing at high temperatures, the S films were coated with Ni0.54Cu0.46 overlayers with thicknesses of about 6 nm, using cosputtering. Compositions of the deposited films were confirmed by EDX spectroscopy analysis, while TEM studies demonstrated excellent epitaxial quality of our S layers with ~2-nm-thick F/S transition layer and atomically-sharp S/substrate interface. Magnetic properties of bilayers were studied using both the SQUID and Vibrating Sample Magnetometer techniques in low and high magnetic fields. Low-temperature tests confirmed that in all cases NiCu films were ferromagnetic with the Curie temperature of above 30 K. Below the bilayer critical temperature of approx. 12-13 K, the structures were fully proximitized with the strong superconducting signal. For superconducting transport properties characterization, we used bilayers patterned into 40-μm-long microbridges with the width varying from 0.4 μm to 2 μm. The same S/F nanostructures were also used to study their superconducting fluctuations. The temperature dependence of magnetoresistance demonstrated highly 2-dimensional character with an unusual negative region that extended almost to room temperature. In the S/F sample, the fluctuations were observed to be substantially below theoretical expectations.
The aim of our paper is to consider the possibility of applying pure Ag technology for assembly of SiC Schottky diode
into a ceramic package able to work at temperatures up to 350°C. Ag micropowder was used for assembly SiC structure
to DBC interposer of the ceramic package. Ag wire bonds as well as flip-chip technology using Ag balls were used as
material for interconnection systems. The parameters of I-V characteristics were used as a quality factor to determine the
Schottky diode after hermetization into ceramic package as well as after ageing in air at 350°C in comparison with
characteristics of bare SiC diode.
Progress in quality of ultrathin superconducting niobium nitride films for fabrication technology of single photon
detectors is here presented. The films deposited on Al2O3 single crystals reveal excellent both superconducting and
structure properties but the films deposited on Si single crystals have really worse parameters. High epitaxial quality of
NbN and NbTiN films grown on the Al2O3 substrates is proved by HRXRD and HRTEM studies. The results of the
studies on both NbN and NbTiN films reveal one cubic NbN phase with NaCl-type structure, and the planes of NbN are
correlated with Al2O3 crystal orientation. The critical temperatures of NbN and NbTiN films with thickness of few nm
grown on the Al2O3 and Si substrates are in range 4K ÷ 7K, but post-grown annealing of the films at 1000°C in Ar
increases temperature about 10K. Moreover, the NbTiN film deposited on sapphire at optimized conditions and annealed
discloses the best superconducting properties - critical temperature of 14 K as well as extremely high critical current
density of 12·106 A/cm2. This is the best results measured on so thin superconducting films and not reported up to now.
The improvement in superconductor parameters is explained here due to reduced strain and defects by high temperature
annealing of the film. Structural analysis on the annealed NbTiN films by XRD measurement confirms that FWHM of
the 111 Bragg reflection is extremely narrow, about value of 10 arcsec characterising the best single crystals.
The effects of various chemical treatments on (100) GaSb surface with the aim to develop procedures of polishing of GaSb substrates, surface preparation prior to LPE growth, metal and dielectric deposition, fabrication of patterns have been examined. We show that chemomechanical polishing in Br2 - ethylene glycol followed by anodic oxidation and oxide removal enables to fabricate damage free GaSb surface with the roughness of about 1.5 nm. Surface treatment in 30 HCL-1HNO3 followed by 5%HCL etch gives the best results for surface cleaning prior to metal deposition. The optimum pre-epitaxial treatment includes the use of 1M Na2S solution and H2 anneal. For features patterning 60HCL-1H2O2-1H2O enables etching at rate of approximately 4 micrometers /min, however, to achieve highly anisotropic etching of small size features the use of Ccl4/H2 plasma is the most suitable.
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