nBn Detectors Research Articles

Use of epitaxial unipolar barriers to block surface leakage currents in photodetectors

AbstractSurface leakage currents are one of the many causes of dark current exhibited by semiconductor devices and are a common limiting factor to device performance for many types of devices. Traditionally, these surface leakage currents are controlled through the post‐epitaxial insertion of a surface passivation layer, but surface passivation is often expensive, time consuming, and not entirely effective. Unipolar barriers may be incorporated into electronic devices as a less expensive and more effective alternative way to control surface leakage currents. Unipolar barrier devices are demonstrated here using MBE‐grown InAs nBn detectors and unipolar barrier photodiodes. A commercial InAs photodiode becomes surface leakage current limited at 220 K while the nBn detector shows no surface leakage current down to at least 135 K. The unipolar barrier photodiode shows no detectable surface leakage down to at least 130 K while a matching conventional photodiode is limited by surface conduction at a temperature of 150 K. This indicates the greatly improved performance of the unipolar barrier devices (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular beam epitaxy growth and characterization of type-II InAs/GaSb strained layer superlattices for long-wave infrared detection

The authors report on investigation of type-II InAs/GaSb and InAs/InxGa1−xSb strained layer superlattices (SLSs) for long-wave infrared detection. Growth conditions were optimized to obtain nearly lattice matched (Δa/a∼0.03%) 13 ML InAs/7 ML GaSb SLS nBn detector structure with cutoff wavelength of ∼8.5 μm (77 K). Dark current density was equal to 3.2×10−4 A/cm2 (Vb=+50 mV, 77 K) for this detector structure. Thin 10 ML InAs/6 ML In0.35Ga0.65Sb SLS was grown with zero lattice mismatch achieved by incorporation of 2.5 ML of GaAs in every SLS period.

Technology and Performance of THz Hot-Electron Bolometer Mixers

Hot-electron bolometer (HEB) mixers are a complex multi-layer thin film structure containing an ultra-thin superconducting film of NbN as a detecting element and a thick normal metal layer as an antenna structure. We have optimized the fabrication process starting with ultra-thin NbN films, Au films for antenna structures and their patterning using e-beam lithography and lift-off. The coupling between normal conducting antenna and NbN detector has been improved by introducing an intermediate NbN film to reduce proximity suppression of superconductivity in the detecting element. A critical temperature of about 9.5 K is reached for NbN films with a thickness between 5 nm and 6 nm. A twofold increase of the film thickness increases the critical temperature to 12 K. We have shown that a 20 nm thick buffer layer of NbN under a much thicker Au layer is sufficient to ensure a critical temperature of the bi-layer of 9 K. This value is close to the critical temperature of 5.5 nm thick HEB devices. The noise temperature of HEB mixer made using improved technology is about 800 K and was measured in a liquid cryogen free system with a quantum cascade laser as 2.5 THz local oscillator.

Niobium Superconducting Nanowire Single-Photon Detectors

We investigate the performance of superconducting nanowire photon detectors fabricated from ultra-thin Nb. A direct comparison is made between these detectors and similar nanowire detectors fabricated from NbN. We find that Nb detectors are significantly more susceptible than NbN to thermal instability (latching) at high bias. We show that the devices can be stabilized by reducing the input resistance of the readout. Nb detectors optimized in this way are shown to have approximately 2/3 the reset time of similar large-active-area NbN detectors of the same geometry, with approximately 6% detection efficiency for single photons at 470 nm.

Low noise superconducting single photon detectors on silicon

We have fabricated superconducting nanowire single photon detectors made of NbTiN on a silicon substrate. This type of material reduces the dark count rate by a factor of 10 compared to identical NbN detectors, enabling single photon detection with unprecedented signal to noise ratio: we report a noise equivalent power of 10−19 W Hz−1/2 at 4.2 K. The compatibility of our superconducting device with silicon enables its integration with complex structures.

Lateral diffusion of minority carriers in nBn based type-II InAs/GaSb strained layer superlattice detectors

We report on the investigation of lateral diffusion of minority carriers in nBn based InAs/GaSb strained layer superlattice photodetectors. Values of diffusion length of minority carriers were extracted from temperature dependent I-V measurements. The behavior of diffusion length as a function of applied bias, temperature, and composition of the barrier layer and the values ranging by two orders of magnitude from 77 to 250 K at the same value of applied bias were investigated. The obtained results suggest that at this point the lateral diffusion current could be the limiting factor of the MWIR nBn detector performance at low (<150 K) temperatures.

NBn detectors based on InAs∕GaSb type-II strain layer superlattice

We report on a type-II InAs∕GaSb strain layer superlattice photodetector using a nBn design with cutoff wavelength of ∼4.8μm at 250K. The surface component of dark current was eliminated. Using a shallow isolation etch, low temperature dark current was reduced by two orders of magnitude compared with conventional photodiode processing. Dark current densities were equal to 2.3×10−6 and 3.1×10−4A∕cm2 (Vb=0.1V, T=77K) for detectors with shallow isolation etch and conventional defined mesa, respectively. Quantum efficiency, responsivity, and spectral detectivity D* of the device are presented.

Superconducting tunneling-junction detectors of X-ray radiation. Issues concerning the energy resolution

The effect of the recombination-and edge-related losses of nonequilibrium quasiparticles on the energy resolution of superconducting tunneling detectors is studied. The dependence of the signal on the energy of X-ray photons is measured and the shape of instrument-related lines is studied for the Ti/Nb/Al/AlOx/Al/Nb/NbN detectors with the Ti/Nb passive electrode. Experimental data are analyzed using the diffusion-based model of tunneling detectors.

Fibre-coupled, single photon detector based on NbN superconducting nanostructures for quantum communications

We present a novel, two-channel, single photon receiver based on two fibre-coupled, NbN, superconducting, single photon detectors (SSPDs). The SSPDs are nanostructured superconducting meanders and are known for ultrafast and efficient detection of visible-to-infrared photons. Coupling between the NbN detector and optical fibre was achieved using a micromechanical photoresist ring placed directly over the SSPD, holding the fibre in place. With this arrangement, we obtained coupling efficiencies up to ∼30%. Our experimental results showed that the best receiver had a near-infrared system quantum efficiency of 0.33% at 4.2 K. The quantum efficiency increased exponentially with the photon energy increase, reaching a few percent level for visible-light photons. The photoresponse pulses of our devices were limited by the meander high kinetic inductance and had the rise and fall times of approximately 250 ps and 5 ns, respectively. The receiver's timing jitter was in the 37 to 58 ps range, approximately 2 to 3 times larger than in our older free-space-coupled SSPDs. We stipulate that this timing jitter is in part due to optical fibre properties. Besides quantum communications, the two-detector arrangement should also find applications in quantum correlation experiments.

N B n detector, an infrared detector with reduced dark current and higher operating temperature

This letter presents a type of infrared detector named the nBn detector. The nBn design essentially eliminates Shockley-Read-Hall generation currents. The result is greatly reduced dark current and noise, compared to other midwave infrared detectors, such as p-n photodiodes. This enables the nBn to operate at background-limited infrared photodetection conditions at significantly higher temperatures than conventional midwave infrared detectors and have greater detectivity near room temperature. The nBn is demonstrated in InAs and InAsSb materials, exhibiting cutoff wavelengths of 3.4 and 4.2μm, respectively.

The recombination of quasiparticles in x-ray superconducting tunnel detectors

The energy resolution of superconducting tunnel detectors is investigated from the point of view of the effect of recombination and boundary losses of nonequilibrium quasiparticles. Using the x-ray fluorescence method, the dependence of the signal on the energy of x-ray quanta is measured for Ti/Nb/Al/AlOx/Al/Nb/NbN detectors. The form of the apparatus line is also investigated. The experimental data are analyzed using the diffusion model of tunnel detectors.

Superconducting single-photon detector for the visible and infrared spectral range

We present results on dark count rates and spectral sensitivities of superconducting single-photon detectors in the visible and near-infrared spectral range. The active detector element is a nanometre-sized (a few nanometres thick and less than 100nm wide) meander line carrying a supercurrent. The superconducting materials are NbN and Nb, respectively. The NbN detector exhibited a flat spectral sensitivity up to about 2.4μm. Fluctuations of the superconducting order parameter are considered as a major source of dark count events. A simple model and its limitations to explain the observed dark count rates is discussed.

Superconducting single-photon detector for the visible and infrared spectral range

Abstract We present results on dark count rates and spectral sensitivities of superconducting single-photon detectors in the visible and near-infrared spectral range. The active detector element is a nanometre-sized (a few nanometres thick and less than 100nm wide) meander line carrying a supercurrent. The superconducting materials are NbN and Nb, respectively. The NbN detector exhibited a flat spectral sensitivity up to about 2.4μm. Fluctuations of the superconducting order parameter are considered as a major source of dark count events. A simple model and its limitations to explain the observed dark count rates is discussed.

Nano-structured superconducting single-photon detectors

NbN detectors, formed into meander-type, 10×10-μm2 area structures, based on ultrathin (down to 3.5-nm thickness) and nanometer-width (down to below 100nm) NbN films are capable of efficiently detecting and counting single photons from the ultraviolet to near-infrared optical wavelength range. Our best devices exhibit QE >15% in the visible range and ∼10% in the 1.3–1.5-μm infrared telecommunication window. The noise equivalent power (NEP) ranges from ∼10−17W/Hz1/2 at 1.5μm radiation to ∼10−19W/Hz1/2 at 0.56μm, and the dark counts are over two orders of magnitude lower than in any semiconducting competitors. The intrinsic response time is estimated to be <30ps. Such ultrafast detector response enables a very high, GHz-rate real-time counting of single photons. Already established applications of NbN photon counters are non-invasive testing and debugging of VLSI Si CMOS circuits and quantum communications.

Fast nonequilibrium induction detectors based on thin superconducting films

A new type of fast detector is proposed. The operation of the detector is based on the change induced in the kinetic inductance of NbN and YBa2Cu3O7−δ superconducting films by nonequilibrium quasiparticles produced by electromagnetic radiation. The speed of a NbN detector is essentially temperature-independent and is less than 1 ps. A model based on the Omen-Scalapino scheme describes well the experimental dependence of the voltage-power sensitivity of a NbN detector on the modulation frequency of the radiation. A low equilibrium quasiparticle density and a high quantum yield give detecting power D*=1012W−1 · cm · Hz1/2 at temperature T=4.2 K and D*=1016 W−1 · cm · Hz1/2 at temperature T=1.6 K. The time constant of the low-temperature YBaCuO induction detector is determined only by the electron-phonon interaction time τ e-ph d in the nodal regions.

Picosecond response of a superconducting hot-electron NbN photodetector

Abstract The ps optical response of ultrathin NbN photodetectors has been studied by electro-optic sampling. The detectors were fabricated by patterning ultrathin (3.5 nm thick) NbN films deposited on sapphire by reactive magnetron sputtering into either a 5×10 μm2 microbridge or 25 1 μm wide, 5 μm long strips connected in parallel. Both structures were placed at the center of a 4 mm long coplanar waveguide covered with Ti/Au. The photoresponse was studied at temperatures ranging from 2.15 K to 10 K, with the samples biased in the resistive (switched) state and illuminated with 100 fs wide laser pulses at 395 nm wavelength. At T=2.15 K, we obtained an approximately 100 ps wide transient, which corresponds to a NbN detector response time of 45 ps. The photoresponse can be attributed to the nonequilibrium electron heating effect, where the incident radiation increases the temperature of the electron subsystem, while the phonons act as the heat sink. The high-speed response of NbN devices makes them an excellent choice for an optoelectronic interface for superconducting digital circuits, as well as mixers for the terahertz regime. The multiple-strip detector showed a linear dependence on input optical power and a responsivity R =3.9 V/W.

Improved performance of a superconductive optical detector with planar antennas

A significant improvement in performance of an ultrathin-film NbN superconductive infrared detector was demonstrated by using a planar antenna concept. The output signal voltage is increased through improved optical coupling and the detector noise is decreased by reducing the total area of the NbN detector element. An enhancement in the signal-to-noise ratio by 32 dB was observed when comparing antenna-coupled detectors to area detectors (e.g., meandering lines). In addition, the authors observed a polarization selectivity of about 13 dB to linearly polarized light, which can provide additional discrimination. The primary noise mechanism in these films is confirmed to be voltage shot noise due to fluctuations in the motion of flux quanta in the film. This is a precise dual to current shot noise. >

Sensitive picosecond NbN detector for radiation from millimetre wavelengths to visible light

The authors report on the application of a broad-band NbN film detector which has high sensitivity and picosecond response time for detection of radiation from millimetre wavelengths to visible light. From a study of amplitude modulated radiation of backward-wave tubes and picosecond pulses from gas and solid state lasers at wavelengths between 2 mm and 0.53 mu m, they found a detectivity of 1010 W-1 cm Hz-1/2 and a response time of less than 50 ps at T=10 K. The characteristics were provided by using a 150 AA thick NbN film patterned into a structure of micron strips. According to the proposed detection mechanism, namely electron heating, they expect an intrinsic response time of approximately 20 ps at the same temperature.