Far-infrared Detectors Research Articles

Development of a highly sensitive detection module incorporating a charge-sensitive infrared phototransistor and a Ge hemispherical mirror

Abstract Charge-sensitive infrared phototransistors (CSIPs) based on GaAs/AlGaAs quantum well structures permit highly sensitive detection of radiation in the wavelength range of 10–50 μm. Despite this excellent sensitivity, their quantum efficiency remains limited to approximately 20%. In this study, we first developed a cryogenic measurement system for evaluating the quantum efficiency of CSIPs using a calibrated thermal radiation source (a heated chip resistor) in a liquid-helium-free refrigerator. Using this measurement system, which sufficiently suppresses background infrared radiation, we next attempted to enhance the quantum efficiency by combining the CSIP with a hemispherical mirror composed of a germanium hemispherical lens with a metal-coated convex surface. Mounting the hemispherical mirror on the CSIP surface improved the quantum efficiency to 35%. Moreover, by covering the frontside of the CSIP, the mirror further suppressed the infrared background radiation from the surrounding environment, reducing the detectable photon flux limit to 2×10^6 s^(-1), corresponding to a power of 35 fW. The proposed optical detection module with the dielectric hemispherical mirror is anticipated to prove valuable for developing highly sensitive mid- and far-infrared detectors.

Development and validation of a cryogenic far-infrared diffraction grating spectrometer used to post-disperse the output from a Fourier transform spectrometer.

Recent advances in far-infrared detector technology have led to increases in raw sensitivity of more than an order of magnitude over previous state-of-the-art detectors. With such sensitivity, photon noise becomes the dominant noise component, even when using cryogenically cooled optics, unless a method of restricting the spectral bandpass is employed. The leading instrument concept features reflecting diffraction gratings, which post-disperse the light that has been modulated by a polarizing Fourier transform spectrometer (FTS) onto a detector array, thereby reducing the photon noise on each detector. This paper discusses the development of a cryogenic (4K) diffraction grating spectrometer that operates over the wavelength range of 285 to 500μm and was used to post-disperse the output from a room-temperature polarizing FTS. Measurements of the grating spectral response and diffraction efficiency are presented as a function of both wavelength and polarization to characterize the instrumental performance.

Electronic and optical properties of InAs/InAs0.625Sb0.375 superlattices and their application for far-infrared detectors

We calculate the electronic and optical properties of InAs/InAs0.625Sb0.375 superlattices (SLs) within relativistic density functional theory. To have a good description of the electronic and optical properties, the modified Becke–Johnson exchange-correlation functional is employed to describe the band gaps correctly. First, we analyze the electronic and optical characteristics of bulk InAs and InSb, and then we investigate the InAs/InAs0.625Sb0.375 SL. The optical gaps deduced from the imaginary part of the dielectric function are associated with the characteristic interband transitions. We investigate the electronic and optical properties of the InAs/InAs0.625Sb0.375 SL with three lattice constants of the bulk InAs, GaSb and AlSb, respectively. It is observed that the electronic and optical properties strongly depend on the lattice constant. Our results support the presence of two heavy-hole bands with increasing in-plane effective mass as we go far from the Fermi level. We notice a considerable decrease in the energy gaps and the effective masses of the heavy-holes in the k x –k y plane compared to the bulk phases of the parent compounds. We demonstrate that the electrons are s-orbitals delocalized in the entire SL, while the holes have mainly p-Sb character localized in the In(As,Sb) side of the SL. In the SL, the low-frequency absorption spectra greatly increase when the electric field is polarized orthogonal to the growth axis allowing the applicability of III–V compounds for the long-wavelength infrared detectors.

High-operating temperature far-infrared Si:Ga blocked-impurity-band detectors

Silicon-based blocked impurity band (BIB) detectors have become the preferred candidate for the astronomical observation field because of their excellent ability for far-infrared detection, easy integration with the readout circuit, and potential for large-scale preparation. We fabricate Si:Ga BIB far-infrared detectors by a molecular beam epitaxy technique with an impressive blackbody specific detectivity of 4.21 × 1011 cm Hz1/2 W−1 at 10 K and nearly uniform broadband response between 2.5 and 20 μm. A response mechanism with variable temperature is described minutely by the varying temperature optoelectronic characterization and theoretical calculation as well as energy band diagram. The substantial results indicate that the responsivity of the detector can steadily maintain up to 26 K for far-infrared. This paper not only increases the accessibility of BIB detectors' fabrication tools but also provides an approach of high-operating temperature far-infrared detectors for astronomy explorations.

Middle- and far-infrared detector based on the plane collection of graphene strips

A concept of a middle- and far-infrared detector has been proposed. The detector is built as a planar collection of parallel graphene strips of different length and width. The feature of the detector scheme is the concurrent utilization of two different detection mechanisms: excitation in the given frequency range of low-frequency interband transitions inherent in armchair graphene strips and antenna resonances of strongly slowed-down surface waves (plasmon polaritons). It has been shown that matching these two resonances results in the essential detector signal amplification, thus providing an alternative way how to solve the problem of the low efficiency of resonant graphene antennas. An approach is proposed to analyze the design of such detectors, as well as to discuss the ways of tuning the both mechanisms.

Dark-Current-Blocking Mechanism in BIB Far-Infrared Detectors by Interfacial Barriers

We fabricate Ge:B-blocked impurity band (BIB) far-infrared detectors using near-surface processing techniques. The current–voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}$ </tex-math></inline-formula> ) characteristics and the energy band structure are investigated. It is found that there are three interfacial barriers in Ge:B BIB detectors due to the bandgap narrowing effect. The barrier height can be adjusted by changing the doping concentration in the contact region of BIB detectors. A new dark-current-blocking mechanism is proposed that the interfacial barriers can block the transport of carriers at low temperatures. Moreover, the greater the barrier height, the stronger the blocking effect. Utilizing this new blocking mechanism, the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}$ </tex-math></inline-formula> characteristics of Ge:B BIB detectors can be better explained. By increasing the barrier heights, the dark current is reduced significantly, almost six orders of magnitude within a certain voltage range, and the working temperature is increased from the usual 4.2–10 K, which is of great significance for extending the service life of BIB detectors on outer space observation platforms. Our findings will help to better design BIB detectors and improve their performance.

Carrier transport mechanism associated with the thickness of the absorbing layer in GaAs-based blocked‐impurity‐band (BIB) far‐infrared detectors

Carrier transport mechanism associated with the thickness of the absorbing layer in GaAs-based blocked-impurity-band (BIB) far-infrared detector has been investigated in detail. It is found that responsivity linearly increases with the increased thickness of absorbing layer first, and after achieving a peak value, and then starts dropping slowly. In order to explore the carrier transport mechanism behind the phenomena, the physical meaning of responsivity has been illuminated first, and then the vertical profiles of the optical generation rate, the electric field intensity, and the carrier mobility have been obtained, respectively. It is demonstrated that the carrier transport mechanism associated with the thickness of the absorbing layer can be attributed to the competing effects of the optical generation rate, the electric field intensity, and the carrier mobility.

МЕТОД ВИЗНАЧЕННЯ ДІЕЛЕКТРИЧНОЇ ПРОНИКНОСТІ ДІЕЛЕКТРИКІВ У ММ ТА СУБММ ДІАПАЗОНАХ ДОВЖИН ХВИЛЬ НА ПІДСТАВІ ВИМІРЮВАННЯ ПАРАМЕТРІВ ПЛАЗМОН-ПОЛЯРИТОННОГО РЕЗОНАНСУ

МЕТОД ВИЗНАЧЕННЯ ДІЕЛЕКТРИЧНОЇ ПРОНИКНОСТІ ДІЕЛЕКТРИКІВ У ММ ТА СУБММ ДІАПАЗОНАХ ДОВЖИН ХВИЛЬ НА ПІДСТАВІ ВИМІРЮВАННЯ ПАРАМЕТРІВ ПЛАЗМОН-ПОЛЯРИТОННОГО РЕЗОНАНСУ

An angle-scanned cryogenic Fabry-Pérot interferometer for far-infrared astronomy.

The sensitivity of state-of-the-art superconducting far-infrared detectors used in conjunction with cryogenically cooled space telescopes and instrumentation is such that spectroscopic observations are generally limited by photon noise from the astronomical source or by galactic foreground or zodiacal emission within the field-of-view. Therefore, an instrument design that restricts the spectral bandpass viewed by the detector must be employed. One method of achieving background limited, high resolution spectroscopy is to combine a high resolution component such as a Fabry-Pérot interferometer (FPI) with a lower resolution, post-dispersing system, such as a grating spectrometer, the latter serving to restrict the spectral bandpass. The resonant wavelength of an FPI is most often tuned by changing the spacing or medium between the parallel reflecting plates of the etalon. In this paper, we present a novel design for an FPI in which the wavelength is tuned by scanning the angle of incidence on a high refractive index etalon. This concept simplifies the cryomechanical design, actuation, and metrology. The first results from the realized instrument are presented and compared with theory. The effects on the spectral response as a function of the incident angle have been simulated and shown to agree well with the observation.

Far-infrared spectrally selective LiTaO3 and AlN pyroelectric detectors using resonant subwavelength metal surface structures

Plasmonic near-perfect absorbers, comprising metal films with a periodic array of subwavelength openings, were deposited on the surface of pyroelectric materials to create wavelength-selective far-infrared detectors. The detectors fabricated and investigated were based on one of two pyroelectric materials: (i) z-cut monocrystalline lithium tantalate (LiTaO3) wafers or, (ii) reactively sputtered aluminum nitride (AlN), with absorbers fabricated by contact photolithography. Spectrally selective absorption resonances were demonstrated by Fourier-transform spectroscopy. Spectrally-selective photoresponse was demonstrated with a tunable THz backward wave oscillator. Responsivity was estimated using a black body source to be ∼ 1 mV/W for AlN samples and ∼ 100 mV/W for LiTaO3 samples. Most similar work has focused on detectors for mid-wave and long-wave infrared spectral regions. Our focus on THz wavelengths beyond 20 μm is motivated by specific security and contraband sensing applications.

Temperature-dependent spectral response mechanism in GaAs-based blocked-impurity-band (BIB) far-infrared detectors

Temperature-dependent spectral response mechanism in GaAs-based blocked-impurity-band (BIB) far-infrared detectors has been investigated. Device structure, processing steps and physical models are described in detail. In this work, our discussion is mainly focused on the operation temperature (Tope) of BIB detector. It is demonstrated that the critical Tope and the optimal Tope both exist for GaAs-based BIB detector. It is only when the temperature-assisted photo electron excitation process is fundamentally equivalent to the temperature-assisted photo electron recombination process that the optimal Tope occurs, and it is only when the temperature-assisted photo electron excitation process is dominated by the temperature-assisted photo electron recombination process that the critical Tope occurs.

Photo-Nernst detection of cyclotron resonance in partially irradiated graphene

Cyclotron resonance of a Landau-quantized graphene can absorb a significant amount of infrared light. However, the application of this phenomenon to the photodetector had been limited due to the lack of efficient photon to the charge conversion scheme. Here, we demonstrate the detection of cyclotron resonance in a partially metal-masked monolayer graphene two-terminal device using the photo-Nernst effect. Due to the presence of the mask, incident infrared light is irradiated on only one-half of the graphene channel. This partial irradiation creates a temperature gradient perpendicular to the graphene channel. In the presence of an external magnetic field, thermopower is generated perpendicular to the temperature gradient due to the Nernst effect. Consequently, photo-Nernst voltage is generated along the graphene channel, which can be detected from the contacts on both ends of the channel. We demonstrate selective detection of the photo-Nernst effect while minimizing the other photovoltaic contributions, such as the photo-Seebeck effect. We investigate the dependence of the photo-Nernst effect on the magnetic field and excitation wavelength, which reveals a significant enhancement of the photo-Nernst signal at the cyclotron resonance conditions in graphene. Our finding could facilitate the realization of a far-infrared light detector using cyclotron resonance of graphene.

Charge Mobility and Recombination Mechanisms in Tellurium van der Waals Solid.

Trigonal tellurium is a small band gap elemental semiconductor consisting of van der Waals bound one-dimensional helical chains of tellurium atoms. We study the temperature dependence of the charge carrier mobility and recombination pathways in bulk tellurium. Electrons and holes are generated by irradiation of the sample with 3 MeV electrons and detected by time-resolved microwave conductivity measurements. A theoretical model is used to explain the experimental observations for different charge densities and temperatures. Our analysis reveals a high room temperature mobility of 190 ± 20 cm2 V–1 s–1. The mobility is thermally deactivated, suggesting a band-like transport mechanism. According to our analysis, the charges predominantly recombine via radiative recombination with a radiative yield close to 98%, even at room temperature. The remaining charges recombine by either trap-assisted (Shockley–Read–Hall) recombination or undergo trapping to deep traps. The high mobility, near-unity radiative yield, and possibility of large-scale production of atomic wires by liquid exfoliation make Te of high potential for next-generation nanoelectronic and optoelectronic applications, including far-infrared detectors and lasers.

Evaluation of the compensation ratio of heavily-Ga doped Ge for far-infrared detectors in astronomy

Ge-based blocked impurity band detectors fabricated with the surface-activated wafer bonding (SAB) method are expected to realize larger format arrays for far-infrared astronomical observations. To make sensitive p-type Ge detectors, the minor impurity (ND) concentration of a heavily-impurity doped Ge wafers is required to be <1014 cm−3 in addition to the major impurity (NA) concentration of ∼1016 cm−3. Thus, a compensation ratio K (= ND/NA) below 10−2 should be achieved in advance to SAB. We measured the resistivity and carrier density of heavily-Ga doped Ge wafers at temperatures of 3–300 K. However, ND and NA of the wafers cannot be obtained from the conventional model for p-type semiconductors due to the significant contribution of the impurity band conduction (IBC). Then, an extended model including the IBC process was constructed, which was found to well reproduce the measured data over the whole temperature range. From the values obtained from the model fitting, it is revealed that our heavily-Ga doped Ge wafers show NA ∼ 1016 cm−3 with ultra-low K below 10−3.

Experimental study of graphene-based plasmonic crystals in a strong-coupling regime

We have fabricated a large-area graphene-based transistor with top metal grating, which represents plasmonic crystal containing two-dimensional electron system gated by periodic metal grating. Using Fourier-transform infrared spectroscopy, we have studied the graphene plasmon modes in transmission spectra, which were efficiently excited by the incident light with the perpendicular polarization to metal grating. We have researched the dependence of resonance frequency of graphene plasmons on the bottom gate voltage. Here we demonstrate that excited graphene plasmons are strongly coupled to top metal grating and have the linear dispersion. These results not only prove the previous theoretical works but also demonstrate the possibility of significant reducing photon wavelength which can be controlled by modulating the carrier density of graphene via electrical gating. This result is important for design of highly effective tunable terahertz and far-infrared detectors.

A novel, highly efficient cavity backshort design for far-infrared TES detectors

In this paper we present a new cavity backshort design for TES (transition edge sensor) detectors which will provide increased coupling of the incoming astronomical signal to the detectors. The increased coupling results from the improved geometry of the cavities, where the geometry is a consequence of the proposed chemical etching manufacturing technique. Using a number of modelling techniques, predicted results of the performance of the cavities for frequencies of 4.3–10 THz are presented and compared to more standard cavity designs. Excellent optical efficiency is demonstrated, with improved response flatness across the band. In order to verify the simulated results, a scaled model cavity was built for testing at the lower W-band frequencies (75–100 GHz) with a VNA system. Further testing of the scale model at THz frequencies was carried out using a globar and bolometer via an FTS measurement set-up. The experimental results are presented, and compared to the simulations. Although there is relatively poor comparison between simulation and measurement at some frequencies, the discrepancies are explained by means of higher-mode excitation in the measured cavity which are not accounted for in the single-mode simulations. To verify this assumption, a better behaved cylindrical cavity is simulated and measured, where excellent agreement is demonstrated in those results. It can be concluded that both the simulations and the supporting measurements give confidence that this novel cavity design will indeed provide much-improved optical coupling for TES detectors in the far-infrared/THz band.

Single photon detection of 1.5 THz radiation with the quantum capacitance detector

Far-infrared spectroscopy can reveal secrets of galaxy evolution and heavy-element enrichment throughout cosmic time, prompting astronomers worldwide to design cryogenic space telescopes for far-infrared spectroscopy. The most challenging aspect is a far-infrared detector that is both exquisitely sensitive (limited by the zodiacal-light noise in a narrow wavelength band, λ/Δλ ~1,000) and array-able to tens of thousands of pixels. We present the quantum capacitance detector, a superconducting device adapted from quantum computing applications in which photon-produced free electrons in a superconductor tunnel into a small capacitive island embedded in a resonant circuit. The quantum capacitance detector has an optically measured noise equivalent power below 10−20 W Hz−1/2 at 1.5 THz, making it the most sensitive far-infrared detector ever demonstrated. We further demonstrate individual far-infrared photon counting, confirming the excellent sensitivity and suitability for cryogenic space astrophysics. The authors present a photon detector suitable for terahertz astronomy, with very high sensitivity, low power consumption and the ability to be configured into arrays. This device is demonstrably able to count individual far-infrared photons.

EVALUATION OF FAR-INFRARED BIB-TYPE GE DETECTORS FABRICATED WITH THE SURFACE-ACTIVATED WAFER BONDING TECHNOLOGY

To realize large-format compact array detectors covering a wide far-infrared wavelength range up to 200 µm, we have been developing Blocked-Impurity-Band (BIB) type Ge detectors with the room-temperature surface-activated wafer bonding technology provided by Mitsubishi Heavy Industries. We fabricated various types of <TEX>$p^+-i$</TEX> junction devices which possessed a BIB-type structure, and evaluated their spectral response curves using a Fourier transform spectrometer. From the Hall effect measurement, we also obtained the physical characteristics of the <TEX>$p^+$</TEX> layers which constituted the <TEX>$p^+-i$</TEX> junction devices. The overall result of our measurement shows that the <TEX>$p^+-i$</TEX> junction devices have a promising applicability as a new far-infrared detector to cover a wavelength range of <TEX>$100-200{\mu}m$</TEX>.

A strategy for the measurement of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; distribution in the stratosphere

Abstract. In this study we introduce a new strategy for the measurement of CO2 distribution in the stratosphere. The proposed experiment is based on an orbiting limb sounder that measures the atmospheric emission within both the thermal infrared (TIR) and far-infrared (FIR) regions. The idea is to exploit the contribution of the pure rotational transitions of molecular oxygen in the FIR to determine the atmospheric fields of temperature and pressure that are necessary to retrieve the distribution of CO2 from its rovibrational transitions in the TIR. The instrument envisaged to test the new strategy is a Fourier transform spectrometer with two output ports hosting a FIR detector devoted to measuring the O2 transitions and a TIR detector devoted to measure the CO2 transitions. Instrumental and observational parameters of the proposed experiment have been defined by exploiting the heritage of both previous studies and operational limb sounders. The performance of the experiment has been assessed with two-dimensional (2-D) retrievals on simulated observations along a full orbit. For this purpose, optimal spectral intervals have been defined using a validated selection algorithm. Both precision and spatial resolution of the obtained CO2 distributions have been taken into account in the results–evaluation process. We show that the O2 spectral features significantly contribute to the performance of CO2 retrievals and that the proposed experiment can determine 2-D distributions of the CO2 volume mixing ratio with precisions of the order of 1 ppmv in the 10–50 km altitude range. The error budget, estimated for the test case of an ideal instrument and neglecting the spectroscopic errors, indicates that, in the 10–50 km altitude range, the total error of the CO2 fields is set by the random component. This is also the case at higher altitudes, provided the retrieval system is able to model the non-local thermal equilibrium conditions of the atmosphere. The best performance is obtained at altitudes between 20 and 50 km, where the vertical resolution ranges from 3 to 5 km, and the horizontal resolution is of the order of 300–350 km depending on latitude.

Towards Background-Limited Kinetic Inductance Detectors for a Cryogenic Far-Infrared Space Telescope

Arrays of tens of thousands of sensitive far-infrared detectors coupled to a cryogenic 4–6 m class orbital telescope are needed to trace the assembly of galaxies over cosmic time. The sensitivity of a 4 Kelvin telescope observing in the far-infrared (30–300 $$\upmu $$ m) would be limited by zodiacal light and Galactic interstellar dust emission, and require broadband detector noise equivalent powers (NEPs) in the range of 3 $$\times 10^{-19}$$ W/ $$\sqrt{\mathrm{Hz}}$$ . We are fabricating and testing 96 element arrays of lumped-element kinetic inductance detectors (LEKIDs) designed to reach NEPs near this level in a low-background laboratory environment. The LEKIDs are fabricated with aluminum: the low normal-state resistivity of Al permits the use of very thin wire-grid absorber lines (150 nm) for efficient absorption of radiation, while the small volumes enable high sensitivities because quasiparticle densities are high. Such narrow absorption lines present a fabrication challenge, but we deposit TiN atop the Al to increase the robustness of the detectors and achieve a 95 $$\%$$ yield. We present the design of these Al/TiN bilayer LEKIDs and preliminary sensitivity measurements at 350 $$\upmu $$ m optically loaded by cold blackbody radiation.