We report electroluminescence at 14meV and 20meV from a n-type Ge/Si0.15Ge0.85 quantum cascade heterostructure on Si substrate grown by ultra-high vacuum chemical vapour deposition. The electroluminescence signal of the single quantum well active region design, extracted through diffraction gratings from mesa structures, is compared with its GaAs counterpart.The spectral features agree well with modeling based on Non-equilibrium Green's function calculations. The observed electroluminescence peaks show a full width at half maximum of 3meV and 4meV. These results are an important step towards the realization of an n-type THz quantum cascade laser on a non-polar material system.
A GaAs/AlGaAs distributed feedback semiconductor (DFB) laser with a laterally-coupled grating is demonstrated at a wavelength of 780.24 nm with an output power up to 60 mW. A mode expander and aluminum-free active layers have been used in the material epilayer to reduce the linewidth to 612 kHz while maintaining high output power. The fabricated laser demonstrates over 40 dB side-mode suppression ratio with tuning range > 0.3 nm, which is suitable for atom cooling experiments with the D2 87Rb atomic transition and provides substantial potential for the laser to be integrated into miniaturized cold atom systems.
In the last decade, silicon photonics has undergone an impressive development driven by an increasing number of technological applications. Plasmonics has not yet made its way to the microelectronic industry, mostly because of the lack of compatibility of typical plasmonic materials with foundry processes. In this framework, we have developed a plasmonic platform based on heavily n-doped Ge grown on silicon substrates. We developed growth protocols to reach n-doping levels exceeding 1020 cm-3, allowing us to tune the plasma wavelength of Ge in the 3-15 μm range. The plasmonic resonances of Ge-on-Si nanoantennas have been predicted by simulations, confirmed by experimental spectra and exploited for molecular sensing. Our work represents a benchmark for group-IV mid-IR plasmonics.
The addition of germanium to Si-based single-photon avalanche diode (SPAD) detectors can significantly increase the spectral range of these devices into the into the strategically important short-wave infrared (SWIR) region. We present the performance characteristics of small area (26 μm and 50 µm diameter) planar geometry Ge-on-Si SPAD detectors. There are many advantages for operating such SPAD detection in the SWIR region, these include: reduced eye-safety laser threshold, longer measurable ranges, improved depth resolution in range finding applications; and improved capability for imaging through obscurants such as precipitation and smoke. The time-correlated single-photon counting (TCSPC) technique has been utilized for the measurement of record low dark count rates (DCRs) and high single-photon detection efficiency. Specifically, the 26 µm diameter devices maintained DCR values < 100 kHz up to a temperature of 125 K for excess biases up to 6.6 %. The 50 µm diameter device consistently demonstrated DCRs a factor of approximately 4 times greater than 26 µm diameter devices, under identical operating conditions of excess bias and temperature, illustrating a dark count rate in proportion to the device volume. Single-photon detection efficiencies were found to reach a maximum of ~ 29 %, measured at a wavelength of 1310 nm and a temperature of 125 K. Due the record low dark currents observed, noise equivalent power values (NEP) down to 7.7 × 10-17 WHz-1/2 are obtained, significantly reduced when compared to both previous mesa geometry and larger area planar geometry Ge-on-Si SPADs, indicating much improved optical sensitivity levels attainable with these planar geometry devices. In addition to this, high speed operation was demonstrated, quantified by jitter values down to 134 ± 10 ps at a temperature of 100 K. These results demonstrate the potential of these devices for highly sensitive and high-speed LIDAR imaging in the SWIR.
This paper presents the performance of 26 μm and 50 μm diameter planar Ge-on-Si single-photon avalanche diode (SPAD) detectors. The addition of germanium in these detectors extends the spectral range into the short-wave infrared (SWIR) region, beyond the capability of already well-established Si SPAD devices. There are several advantages for extending the spectral range into the SWIR region including: reduced eye-safety laser threshold, greater attainable ranges, and increased depth resolution in range finding applications, in addition to the enhanced capability to image through obscurants such as fog and smoke. The time correlated single-photon counting (TCSPC) technique has been utilized to observe record low dark count rates, below 100 kHz at a temperature of 125 K for up to a 6.6 % excess bias, for the 26 μm diameter devices. Under identical experimental conditions, in terms of excess bias and temperature, the 50 μm diameter device consistently demonstrates dark count rates a factor of 4 times greater than 26 μm diameter devices, indicating that the dark count rate is proportional to the device volume. Single-photon detection efficiencies of up to ~ 29 % were measured at a wavelength of 1310 nm at 125 K. Noise equivalent powers (NEP) down to 9.8 × 10-17 WHz-1/2 and jitters < 160 ps are obtainable, both significantly lower than previous 100 μm diameter planar geometry devices, demonstrating the potential of these devices for highly sensitive and high-speed imaging in the SWIR.
The imaging and sensing technology operating in the THz region of the electromagnetic spectrum has a number of applications, with demonstrator products already available on the market for oncology imaging, production monitoring, and non-destructive test . However, the THz sources now at hand are still bulky and too expensive for expanding this technology to other proposed applications, which also include, among other, THz bandwidth photonics and security imaging. A higher level of integration with control electronics, a lower production cost, and a broader wavelength range of emission towards the far-infrared, are all desirable features to expand the fields of application of THz radiation. N-type Ge/SiGe quantum cascade structures grown on top of a Si(001) substrate are particularly promising for realizing a Si based THz source . The low effective mass and long non-radiative relaxation times due to the non-polar nature of the material, are expected to i) provide gain values close to those demonstrated in III-V quantum cascade structures at 4 K, and ii) to potentially enable 300 K operation. In this presentation we will discuss the optical and structural properties of n-type s-Ge/SiGe multi-quantum wells and asymmetric coupled quantum wells grown on Si(001) substrates by means of ultrahigh vacuum chemical vapor deposition . Extensive structural characterization obtained by scanning transmission electron microscopy (STEM), atomic probe tomography (APT) and X-ray diffraction shows the high material quality of strain-symmetrized structures (up to 5 micron active region thickness) and heterointerfaces (featuring interface roughness below 0.2 nm), down to the ultrathin barrier limit (about 1 nm). By performing THz absorption spectroscopy measurements combined to theoretical modeling on different asymmetric coupled quantum well systems (with varying large-well width or barrier thickness), we unambiguously demonstrated inter-well coupling and wavefunction tunneling . The agreement between experimental data and simulations allowed us to characterize the tunneling barrier parameters and, in turn, achieve a highly-controlled engineering of the electronic structure in forthcoming unipolar cascade systems based on n-type Ge/SiGe multi quantum-wells. Furthermore, by pump-and-probe, and time domain spectroscopic data with a thorough theoretical modeling, we will show that this material system is indeed promising as active material in quantum cascade lasers (QCL). We found i) narrow intersubband (ISB) absorption lines; ii) relatively long non-radiative ISB relaxation times at high temperature; iii) relaxation times for different ISB transitions favorable to population inversion. Leveraging on the promising results obtained by spectroscopy experiments, we theoretically investigate an electrically-pumped Ge/SiGe THz QCL through a non-Equilibrium Green Function formalism (nextnano.QCL), using as material parameters to model the scattering processes the values estimated from the analysis of the optical experimental data . As expected, due to the weaker interaction with the phonon field with respect to III-V based devices, we find a lower impact of the temperature on the gain spectrum. In addition, simulations show that the interface roughness values measured on our samples allows to achieve gain overcoming the losses of double-metal waveguides at room temperature. We believe that the present results will motivate new experimental efforts aimed at demonstrating room-temperature operation in group IV QCL THz devices.
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 C. Ciano, M. Virgilio, M. Montanari, L. Persichetti, L. Di Gaspare, M. Ortolani, L. Baldassarre, M. H. Zöllner, O. Skibitzki, G.Scalari, J. Faist, D. J. Paul, M. Scuderi, G. Nicotra, T. Grange, S. Birner, G. Capellini, and M. De Seta, Accepted in Phys. Rev. Appl., 11, 014003 (2019).
 T. Grange, D. Stark, G. Scalari, J. Faist, L. Persichetti, L. Di Gaspare, M. De Seta, M. Ortolani, D.J. Paul, G. Capellini, S. Birner and M. Virgilio, Submitted to Applied Physics Letters. Preprint available at https://arxiv.org/pdf/1811.12879.pdf.
We present innovative planar geometry Ge-on-Si single-photon avalanche diode (SPAD) detectors. These devices provide picosecond timing resolution for applications operating in the short-wave infrared wavelength region such as quantum communication technologies and three-dimensional imaging. This new planar design successfully reduces the undesirable contribution of surface defects to the dark current. This has allowed for the use of large excess biases, resulting in a single-photon detection efficiency of 38% when operated at 125 K using 1310 nm wavelength illumination. A record low noise equivalent power of 2 × 10-16 WHz-1/2 was achieved, more than a fifty-fold improvement compared to the previous best Ge-on-Si mesa geometry SPADs when operated under similar conditions. These Ge-on-Si SPAD detectors have operated in the range of 77 K to 175 K, and we will discuss ways in which the operating temperature can be raised to that consistent with Peltier cooling. We will present analysis of Ge-on-Si SPADs, which has revealed much reduced afterpulsing compared with SPAD detectors in other material systems. Laboratory trials have demonstrated these Ge-on-Si SPAD devices in short-range LIDAR and depth profiling measurements. Estimations of the performance of these detectors in longer range measurements will be presented. We will discuss the potential for the development of high efficiency arrays of Ge-on-Si SPADs for the use in eye-safe automotive LIDAR and quantum technology applications.
Low loss Ge-on-Si waveguides are demonstrated in the 8 – 14 μm atmospheric transmission window, a technology that will enable detection and sensing of unique molecular vibrations. Such a low cost platform would have applications in key markets such as pollution monitoring, explosives detection and point of care diagnostics. Rib-waveguides are fabricated using electron beam lithography and dry etching. The waveguides propagation losses are characterized using the Fabry-Perot technique, and are found to be below 5 dB/cm across the measurement range of 7.5 to 11 μm wavelength, reaching as low as ~ 1 dB/cm. The contribution to the losses are analyzed using the experimentally measured Si substrate losses, and the calculated scattering losses from an analytical model. The results verify the feasibility of the Ge-on-Si platform for integrated mid-infrared photonics and sensing.
High efficiency, Ge-on-Si single-photon avalanche diode (SPAD) detectors operating in the short-wave infrared region (1310 nm - 1550 nm) at near room temperature have the potential to be used for numerous emerging applications, including quantum communications, quantum imaging and eye-safe LIDAR applications. In this work, planar geometry Ge-on-Si SPAD designs demonstrate a significant decrease in the dark count rate compared to previous generations of Ge-on-Si detectors. 100 μm diameter microfabricated SPADs demonstrate record low NEPs of 2.2×10-16 WHz-1/2, and single-photon detection efficiencies of 18% for 1310 nm at 78 K. The devices demonstrate single-photon detection at temperatures up to 175 K.
Single photon avalanche detectors (SPADs) operating in gated-Geiger mode at near infrared wavelengths have applications in quantum key distribution (QKD), eye-safe light detection and ranging (LIDAR), 3D image sensing, quantum enhanced imaging and photonic based quantum information processing. Whilst InGaAs SPADs are commercially available, the high cost and lack of integrated SPADs limit the applications. We have previously demonstrated vertical Geiger mode Ge on Si SPADs at 1310 and 1550 nm operating at 100 K where the Ge is used as an absorber and the lower noise Si is used as the avalanche gain region. At 100 K and 1310 nm a single photon detection efficiency of 4% was demonstrated with a dark count rate (DCR) of 5 MHz.
Here we present first results on Ge on Si SPADs grown on top of silicon-on-insulator (SOI) substrates. Both vertical photodetectors and waveguide coupled detectors were investigated with designs aimed to reduce the DCR over previous results. Waveguides and avalanche regions were patterned in the top Si of a SOI wafer before being coated with silicon dioxide. Holes were then etched in the oxide to allow selective area growth of Ge inside these windows and on top of the Si waveguides for the waveguide coupled Ge SPADs. This approach reduces the threading dislocation density compared to bulk Ge growths which aids the reduction of the DCR. The fabricated devices have been tested at both 1310 nm and 1550 nm wavelengths and demonstrate improved performance over previous published results.
The UK National Quantum Technology Hub in Sensors and Metrology is one of four flagship initiatives in the UK National of Quantum Technology Program. As part of a 20-year vision it translates laboratory demonstrations to deployable practical devices, with game-changing miniaturized components and prototypes that transform the state-of-the-art for quantum sensors and metrology. It brings together experts from the Universities of Birmingham, Glasgow, Nottingham, Southampton, Strathclyde and Sussex, NPL and currently links to over 15 leading international academic institutions and over 70 companies to build the supply chains and routes to market needed to bring 10–1000x improvements in sensing applications. It seeks, and is open to, additional partners for new application development and creates a point of easy open access to the facilities and supply chains that it stimulates or nurtures.
We address the behavior of mid-infrared localized plasmon resonances in elongated germanium antennas integrated on silicon substrates. Calculations based on Mie theory and on the experimentally retrieved dielectric constant allow us to study the tunability and the figures of merit of plasmon resonances in heavily-doped germanium and to preliminarily compare them with those of the most established plasmonic material, gold.
The use of heavily doped semiconductors to achieve plasma frequencies in the mid-IR has been recently proposed as a promising way to obtain high-quality and tunable plasmonic materials. We introduce a plasmonic platform based on epitaxial n-type Ge grown on standard Si wafers by means of low-energy plasma-enhanced chemical vapor deposition. Due to the large carrier concentration achieved with P dopants and to the compatibility with the existing CMOS technology, SiGe plasmonics hold promises for mid-IR applications in optoelectronics, IR detection, sensing, and light harvesting. As a representative example, we show simulations of mid-IR plasmonic waveguides based on the experimentally retrieved dielectric constants of the grown materials.
Distributed simulation environments are increasingly using video to stimulate operational systems and their prototypical
equivalents. Traditionally, this video has been synthesized and delivered by an analog means to consuming software
applications. Scene generators typically render to commodity video cards, generate out of band metadata, and convert
their outputs to formats compatible with the stimulated systems. However, the approach becomes hardware intensive as
environment scale and distribution requirements grow. Streaming video technologies can be applied to uncouple video
sources from their consumers, thereby enabling video channel quantities beyond rendering hardware outputs. Moreover,
metadata describing the video content can be multiplexed, thereby ensuring temporal registration between video and its
attribution. As an application of this approach, the Night Vision Image Generator (NVIG) has been extended and
integrated with distribution architectures to deliver streaming video in virtual simulation environments. Video capture
hardware emulation and application frame buffer reads are considered for capturing rendered scenes. Video source to
encoder bindings and content multiplexing are realized by combining third party video codec, container, and transport
implementations with original metadata encoders. Readily available commercial and open source solutions are utilized
for content distribution and demultiplexing to a variety of formats and clients. Connected and connectionless distribution
approaches are discussed with respect to latency and reliability. Client side scalability, latency, and initialization issues
are addressed. Finally, the solution is applied to tactical systems stimulus and training, showing the evolvement from the
analog to the streamed video approach.
A review will be presented of recent work on Si/SiGe heavy-hole to heavy-hole quantum cascade emitters showing
progress towards a laser using the bound-to-continuum design for the active region. The sample was grown by
low energy plasma enhanced chemical vapour deposition in significantly less time than comparable structures
and designs in III-V or Si/SiGe technology using molecular beam epitaxy or more standard chemical vapour
deposition techniques. Clear intersubband electroluminescence is demonstrated at 4.2 K between 6.7 and 10.1
THz. This is inside the III-V restrahlung band where III-V materials cannot lase, unlike Group IV materials.
A review of waveguide losses will also be presented and some ideas of how to design an active region with gain
higher than the waveguide losses will be discussed.
There is strong interest in the development of sources that emit radiation in the far infrared (1-10 THz) frequency range for applications which include early detection of skin cancer, dental imaging, telecommunications, security scanning, gas sensing, astronomy, molecular spectroscopy, and the possible detection of biological weapons. While a number of THz sources are available, there are at present no compact, efficient, cheap and practical high-power solid-state sources such as light emitting diodes or lasers. Silicon is an excellent candidate for such a THz source since the lack of polar optical phonon scattering makes it an inherently low loss material at these frequencies. Furthermore, since over 97% of all microelectronics is presently silicon based, the realisation of a silicon based emitter/laser could potentially allow integration with conventional silicon-based microelectronics. In this paper THz electroluminescence from a Si/SiGe quantum cascade structure operating significantly above liquid helium temperatures is demonstrated. Fourier transform infrared spectroscopy was performed using step scan spectrometer with a liquid helium cooled Si-bolometer for detection. Spectra are presented demonstrating intersubband electroluminescence at a number of different frequencies. These spectral features agree very well with the theoretically calculated intersubband transitions predicted for the structure.
Terahertz (far-infrared) intersubband electroluminescence is reported in p-type Si/SiGe quantum wells and quantum cascade structures. Surface-normal emission (without the aid of a surface grating) from light hole - heavy hole intersubband transitions has been observed for the first time in a quantum cascade device. Edge-emission measurements have also been performed, which show emission from both heavy hole - heavy hole and light hole - heavy hole transitions, and have allowed demonstration of the polarisation dependence of the emitted power, according to the selection rules for the intersubband interactions. The electroluminescence is visible up to temperatures of ~150K, in the multiple quantum well structures, and >=77K in the quantum cascade structure.