Μm Detector Research Articles

Molecular beam epitaxial growth of GaAs 1− XN X with dispersive nitrogen source

The great bowing parameter of GaAsN makes this material hopeful for the fabrication of 1.3 and 1.55 μm laser diodes and detectors. However, the effect of energetic nitrogen ion bombardment during growth may have a deleterious effect on the material quality. To avoid the bombardment effect of energetic nitrogen ions, a modified mode for GaAsN growth using dispersive nitrogen is reported. High-quality GaAsN epilayers and abrupt GaAsN/GaAs interfaces were demonstrated using this growth mode. For the first time, a GaAsN/GaAs double quantum well sample was grown with dispersive nitrogen source. Under our annealing conditions of 750°C from 10 s to 10 min, no significant evidence of nitrogen out-diffusion from the material was found. The improvement in optical quality due to annealing is attributed to the annihilation of nitrogen-related defects through atomic rearrangement of the lattice, the effect of which on the observed blueshift in the photoluminescence peak needs further investigation.

PISCES: A Wide‐Field, 1–2.5 μm Camera for Large‐Aperture Telescopes11Observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution.

ABSTRACTWide‐field‐of‐view infrared cameras, operating on the new generation of large telescopes, offer unprecedented gains in the detection of faint sources and in observing efficiency for both direct imaging and spectroscopy. With a 1024 × 1024 pixel, 1–2.5 μm detector, the PISCES camera provides 8.′5 and 3.′16 fields at the 2.3 m Bok telescope and 6.5 m Multiple Mirror Telescope, respectively. Its refractive optics utilize all spherical surfaces and standard glasses. A cold pupil stop suppresses the thermal background produced by the emissive surfaces normally present in Cassegrain optical configurations. The optical design is directly extendable to wider fields with larger detector formats. Future upgrades include low‐resolution (R = 200–500) multiobject spectroscopy.

Extended wavelength InGaAs infrared (1.0–2.4 μm) detector arrays on SCIAMACHY for space-based spectrometry of the Earth atmosphere

Space qualified InGaAs solid-state array detectors, covering the near-infrared 1–2.4 μm wavelength range, have been developed for application in space-based spectroscopy of the Earth atmosphere. The SCIAMACHY atmospheric chemistry instrument on the ESA ENVISAT satellite (2002–2005) will be equipped with a series of these novel detectors. Detectors are arranged in linear arrays of 1024 pixels of 25×500 μm 2 dimension and meet requirements on modestly low operating temperature (150 K) and low levels of dark current and noise. In this paper the underlying physics of dark current and noise of the detector system is studied on the basis of a theoretical model in combination with measurements. At 2.4 μm wavelength the dark-current performance achieved is 20–100 fA at an operating temperature of 150 K and a bias voltage of −2 mV. This corresponds to a figure of merit for detector resistance R 0 times detector pixel area A of R 0 A=2.5–12.5 MΩ cm 2. This result has required the development of a customised multiplexer for parallel detector read-out at near-zero bias voltage. Further reduction of the operating temperature will not result in lowering the dark current and noise of the InGaAs detectors which are shown to be limited by tunnelling current. A route to future improvement is discussed.

Fast ion loss measurement by infrared television in reduced ripple experiment with ferritic inserts on JFT-2M (abstract)

In the medium size tokamak, JFT-2M, the ferritic steel boards (FBs) were inserted to reduce the toroidal field ripple which causes the fast ion losses. To evaluate the effect of FB insertion, two-dimensional (2D) infrared television (IRTV) system with high resolutions in time and space was developed, which is quite suitable to measure the first wall temperature increment caused by the ripple ion losses in a medium size tokamak. We adopt an IR thermal imager (Mitsubishi IR-M300) that provides a field time of 1/60 s. The detectable temperature range is 0–500 °C with resolution of 0.2 °C. The PtSi (26×20 μm) detector being sensitive to 3–5 μm IR radiation is composed of a 256×256 array. The optical system to view the first wall consists of an IR lens (f:25 mm, F:2.0), a reflecting mirror, and a sapphire vacuum window, in which distance from the camera position to the target wall is shortened to ∼3.5 m. Thus, we obtained a high spatial resolution of ∼3 mm. By using this system, the local hot spot due to the ripple trapped losses of fast ions was observed during neutral beam injection (36 kV, 500 kW) heating. The peak temperature increment ΔTs reached to ∼75 °C before FB insertion. After FB insertion, the ΔTs were reduced clearly. In the most optimized case, the temperature increment became negligibly small. These IRTV data make clear the effectiveness of FBs for reduction of fast ion losses.

Large VLWIR Hg1−xCdxTe photovoltaic detectors

Very long wavelength infrared (VLWIR; 15 to 17 µm) detectors are required for remote sensing sounding applications. Infrared sounders provide temperature, pressure and moisture profiles of the atmosphere used in weather prediction models that track storms, predict levels of precipitation etc. Traditionally, photoconductive VLWIR (λc >15 µm) detectors have been used for sounding applications. However, photoconductive detectors suffer from performance issues, such as non-linearity that is 10X – 100X that of photovoltaic detectors. Radiometric calibration for remote sensing interferometry requires detectors with low non-linearity. Photoconductive detectors also suffer from non-uniform spatial optical response. Advances in molecular beam epitaxy (MBE) growth of mercury cadmium telluride (HgCdTe) and detector architectures have resulted in high performance detectors fabricated in the 15 µm to 17 µmm spectral range. Recently, VLWIR (λc ∼ 17 µm at 78 K) photovoltaic large (1000 µm diameter) detectors have been fabricated and measured at flux values targeting remote sensing interferometry applications. The operating temperature is near 78 K, permitting the use of passive radiators in spacecraft to cool the detectors. Detector non-AR coated quantum efficiency >60% was measured in these large detectors. A linear response was measured, while varying the spot size incident on the 1000 µm detectors. This excellent response uniformity, measured as a function of spot size, implies that low frequency spatial response variations are absent. The 1000 µm diameter, λc ∼ 17 µm at 78 K detectors have dark currents ∼160 µA at a −100 mV bias and at 78 K. Interfacing with the low (comparable to the contact and series resistance) junction impedance detectors is not feasible. Therefore a custom pre-amplifier was designed to interface with the large VLWIR detectors operating in reverse bias. A breadboard was fabricated incorporating the custom designed preamplifier interfacing with the 1000 µm diameter VLWIR detectors. Response versus flux measurements were made on the large VLWIR detectors and non-linearity <0.15% was measured at high flux values in the 2.5×1017 to 3.5×1017 ph-cm−2sec−1 range. This non-linearity is an order of magnitude better than for photoconductive detectors.

GaAs homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared detectors

The recent development of p-GaAs homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared (>40 μm) detectors is briefly reviewed. The emphasis is on the detector performance, which includes responsivity, quantum efficiency, bias effects, cut-off wavelength, uniformity, noise, and negative capacitance characteristics. Promising results indicate that p-GaAs HIWIP detectors have great potential to become a strong competitor in far-infrared applications.

Charge-coupled devices as particle tracking detectors

Following a few years after the invention of the charge-coupled device (CCD) in 1970, the discovery of charmed particles established the need for very high precision (few μm) detectors for tracking high energy charged particles. This review describes the work which has evolved over the past 20 years from these disconnected events, both as regards the application of increasingly refined CCDs to particle tracking (in particular as vertex detectors for identifying heavy flavor quarks and tau leptons), and also the advances in CCD detector design stimulated by these requirements. The lessons learned in this work should provide guidance for the construction of large arrays of CCDs or active pixel devices in the future in a number of areas of science and technology.

High performance SWIR HgCdTe detector arrays

Short wave infrared (SWIR) devices have been fabricated using Rockwell’s double layer planar heterostructure (DLPH) architecture with arsenic-ion implanted junctions. Molecular beam epitaxially grown HgCdTe/CdZnTe multilayer structures allowed the thin, tailored device geometries (typical active layer thickness was ∼3.5 µm and cap layer thickness was ∼0.4 µm) to be grown. A planar-mesa geometry that preserved the passivation advantages of the DLPH structure with enhanced optical collection improved the performance. Test detectors showed Band 7 detectors performing near the radiative limit (∼3-5X below theory). Band 5 detector performance was ∼4-50X lower than radiative limited performance, apparently due to Shockley-Hall-Read recombination. We have fabricated SWIR HgCdTe 256 × 12 × 2 arrays of 45 um × 45 µm detector on 45 µm × 60 µm centers and with cutoff wavelength which allows coverage of the Landsat Band 5 (1.5−1.75 µm) and Landsat Band 7 (2.08−2.35 µm) spectral regions. The hybridizable arrays have four subarrays, each having a different detector architecture. One of the Band 7 hybrids has demonstrated performance approaching the radiative theoretical limit for temperatures from 250 to 295K, consistent with test results. D* performance at 250K of the best subarray was high, with an operability of ∼99% at 1012 cm Hz1/2/W at a few mV bias. We have observed 1/f noise below 8E-17 AHz 1/2 at 1 Hz. Also for Band 7 test structures, Ge thin film diffractive microlenses fabricated directly on the back side of the CdZnTe substrate showed the ability to increase the effective collection area of small (nominally <20 µm µm) planar-mesa diodes to the microlens size of 48 urn. Using microlenses allows array performance to exceed 1-D theory up to a factor of 5.

Growth of GaSb on GaAs/AlAs mirrors for 1.68 μm detectors

Distributed Bragg reflector mirrors (grown by MBE) are designed to give maximum reflectance at 1.68 μm. The DBR is incorporated into a resonant cavity enhanced (RCE) detector by growing GaSb (by MOVPE) on top of the mirror. Device results show such an RCE detector has enhanced quantum efficiency and photocurrent compared to a nonresonant detector grown at the same time. The initial nucleation and growth of GaSb on GaAs by MOVPE is studied by atomic force microscopy (AFM). Conditions are established for the nucleation of small islands of GaSb using TMGa and TMSb as precursors. However, the dark current suggests that the initial nucleation of GaSb is still not optimised. In order to nucleate smaller GaSb islands at lower temperatures, two new precursors are used. Triisopropylgallium (TIPGa) and trisdimethylaminoantimony (tDMASb) are used in combination for the first time to grow GaSb at 480°C. This results in nucleation of much smaller islands and subsequent improvement in the dark current and quantum efficiency of the nonresonant detector. Further work is needed on the equivalent (RCE) detector.

Study of characteristics of silicon detectors irradiated with 24 GeV/c protons between −20°C and +20°C

High resistivity ion-implanted silicon pad detectors have been irradiated at +20°C, +10°C, 0°C and −20°C with 24 GeV/c protons at a flux of ∼ 5 × 109cm−2s−1, up to fluences of ∼ 1.1 × 1014cm−2, and maintained at these temperatures during several months after the end of irradiation. The change of the diode reverse current, full depletion voltage and collection efficiency of the charge, deposited by relativistic electrons, are presented as a function of the proton fluence and of annealing time. It is found that operating the detectors below +10°C limits the diode reverse current and the bias voltage necessary to achieve full depletion. Moreover, at these temperatures, the charge collection efficiency for an integration time of 20 ns (typical of LHC operation) is better than 90% for 300 μm detectors irradiated to a fluence of 1014 cm−2 and biased at 160 V.

1 Gb/s Si high quantum efficiency monolithically integrable λ=0.88 μm detector

We propose and demonstrate a 1 Gb/s high quantum efficiency Si MSM metal-semiconductor-metal detector which is complementary metal-oxide semiconductor compatible. The detector absorbs over 50% of the light entering the active layer at a wavelength of λ=0.88 μm.

The retrieval of total optical depth and effective droplet radius of clouds from solar reflection measurements using the Along Track Scanning Radiometer‐2 (ATSR‐2)

A method is presented for the retrieval of liquid phase cloud microphysical properties; effective radius (reff) and total optical depth (τ) using measurements of reflected intensity in visible and near infrared bands produced by satellite radiometry. Previous techniques have been ambiguous for clouds made up of small water droplets i.e. reff ≤ 6µm. A method exploiting the dual look facility of the Along Track Scanning Radiometer ATSR‐2 has been used to remove this problem and determine a unique solution. Explicitly the 0.555µm and the 1.6 µm detectors of ATSR‐2 are used to retrieve reff (≥ 2µm) and τ (up to the semi‐infinite limit).

Charge sharing and resolution studies of double-sided silicon microstripe detectors with boron spray implants

Measurements of the signal to noise, charge sharing, cluster distribution properties, charge correlation and spatial resolution as a function of angle of incidence have been conducted in a high energy pion beam at Fermilab for a variety of DC-coupled double-sided silicon microstripe detectors (Micron, Inc.) utilizing a special boron-spray technique for ohmic side isolation. Readout was accomplished with the Berkeley SVX-D IC/SRS/SDA sequencer system. Stripe pitch configurations included 50 μm and 25 μm with every stripe read out (“50 ∗1”, “25 ∗1”), and also with every other stripe read out (“50 ∗2”, “25 ∗2”). One of the detectors under study was 158 μm thick, rather than the usual 300 μm. Of interest are the establishment, for 300 μm thick detectors, of sub-2.5 μm resolution perpendicular to the beam on an ohmic side with 38 mm long stripes for both 25 ∗1 and 25 ∗2 configurations, and diode side (58 mm stripes) resolutions ranging from below 3 μm at near normal incidence to about 11 μm at 60°. At large angle the 25 ∗2 configuration generally gave comparable or better resolution than 25 ∗1. The thin detector gave a resolution ranging from about 5.5 μm at 21° to about 7 μm at 45° for 25 ∗1 and 6.4 to 9.0 μm for 25 ∗2, which, for each of the three common angles of data, was better than the resolution yielded by the 300 μm detector to which it was compared. Signals and charge sharing results are compared to detailed Monte Carlo calculations based on an energy loss distribution due to Bichsel.

LPE GaAs as an X-ray detector for astronomy

Gallium arsenide detectors offer an attractive alternative to silicon for high energy X-ray (> 10 keV) astrophysics. Thick (130 μm) detectors fabricated from GaAs grown using the liquid phase epitaxy (LPE) process are now achieving an energy resolution which is approaching that needed in astrophysics. At the same time progress continues to be made in the field of true imaging optics at these high energies using multi-layer reflective surfaces and it is now not unreasonable to imagine a hard X-ray astrophysics mission with grazing incidence optics and a GaAs solid state position sensitive spectrometer in the focal plane. A brief introduction to hard X-ray astrophysics and optics is given. Results obtained with an LPE GaAs device are presented including both detector performance and the electrical characteristics. At room temperature a resolution of ∼ 2.8 keV HWHM was achieved.

Postgrowth tuning of quantum-well infrared detectors by rapid thermal annealing

The peak detection wavelength of an operational quantum-well infrared photodetector structure has been red shifted using rapid thermal annealing to partially intermix the well and barrier layers. Successive anneals at 850 °C were used to tune an 8.13 μm detector continuously out to 9.13 μm. All of the fabricated detectors were operational in spite of very long annealing times of up to 300 s. The peak spectral responsivity at a device current of 10 μA dropped from 0.62 to 0.12 A/W after the longest anneal time, but the broadband responsivity only dropped by a factor of 3 due to a simultaneous increase in the detection spectral bandwidth.

Nonresonant electron capture in GaAs/AlAs/AlGaAs double-barrier quantum well infrared detectors

We have investigated electron and hole capture times in n-type double-barrier quantum well (DBQW) infrared (3–5 μm) detector structures. Photoluminescence upconversion allows us to observe the relaxation of photoexcited carriers from AlAs/(AlGa)As/AlAs barriers into GaAs quantum wells. For 2 nm AlAs, 25 nm Al0.3Ga0.7As and 5 nm GaAs, the electron capture process is non-resonant, with a capture time of 47 ps at a temperature of 5 K and 28 ps at 77 K. Theoretical calculations show that electron tunneling across the 2 nm AlAs layers mainly occurs via the Γ-minimum, and that the Γ-X contribution is negligible.

The infrared space observatory camera

At the focal plane of the ISO 60 cm telescope, ISOCAM will take images of the sky in the wavelength range 2.5 to 17 μm. It features two independent channels, containing each a 32×32 array detector. The long wavelength (4–17 μm) detector, developed by LIR-LETI specifically for ISOCAM, is a Si:Ga array bonded with indium bumps to a DV readout circuit. The short wavelength (2.5–5.5 μm) detector, developed by SAT, is an InSb array with a CID readout.

GaAs/AlGaAs quantum well infrared photodetector arrays for thermal imaging applications

The performance of GaAs/AlGaAs multiple quantum well infrared detectors is studied theoretically and experimentally, with special emphasis on 8-12 μm thermal imaging applications. The dependence of detector performance on various factors like light coupling configurations (one and two dimensional reflection gratings or 45° polished edge), detector temperature, response wavelength and quantum well doping density is dealt with. An absorption quantum efficiency of 87% is demonstrated using a crossed grating and a waveguide (CGW). It is also found that an optimised 34 μm × 34 μm detector (a detector size suitable for large staring arrays, i.e. 256 × 256 or larger) with 9.0 μm cut-off wavelength, f# = 2 optics and 70% optical transmission reaches background limited operation at 74 K detector temperature.The potential of making highly uniform staring arrays utilising the mature GaAs material and processing technology is demonstrated by uniformity measurements of detector dark current. The experiments show that a metalorganic vapour phase epitaxy (MOVPE) grown structure can have a dark current standard deviation to mean value ratio over a 10mm long linear detector array of less than 2%.The staring array performance in terms of noise equivalent temperature difference (temporal NETD) is calculated to NETD < 20 mK at 77 K detector temperature and NETD < 10 mK at 70 K detector temperature.

Design considerations of metal-film bolometer with micromachined floating membrane

A metal-film microbolometer formed on a glass membrane suspended on a silicon substrate has been fabricated by the anisotropic etching technique. The rectangular glass membrane is supported by its four leads connected to the corners of an etched V-groove cavity. This structure allows flexible thermal management and sensitivity optimization of the bolometer for different kinds of application, with a trade-off between lead thermal impedance and device active area. Optimizations of the structure applied to both a focal-plane array and a single detector are interpreted. A thermal impedance as high as 4 × 10 5 °C/W with a response time of several milliseconds can be achieved. For a 320 μm x 320 μm detector, theoretical predictions give a responsivity and normalized detectivity of over 700 V/W and 10 9 cm √/Hz/W, respectively. For dimensions of 128 μm × 128 μm, suitable for a focal-plane-array detector, theory predicts a D* of 5.5 × 10 8. A discrepancy exists between the experiment and the theory, which is also discussed.

Charge non-neutrality in the quantum well region of a GaAs-AlGaAs intersubband 9 μm detector

Infrared transmission is employed to study intersubband transitions in a GaAs-AlGaAs quantum well detector with spectral response peaked at about 9 μm. Under dc bias, our experiments show directly that the density of electrons in the doped quantum wells changes significantly, i.e., a charge non-neutrality occurs in the multiquantum well region of the detector. This degrades the detector internal quantum efficiency. Theoretical calculations compared with our experiments show that under a dc bias, the intersubband transition energy shifts are dominated by many-particle effects when the electron density in the well is changed; while the Stark shift is expected to dominate when the electron density remains constant.