State-of-art detectors having response over 2.0+ μm wavelengths require cryogenic coolers to improve SNR and achieve high sensitivity. Next generation detectors are required to operate at room temperature (300 K) to fit small form factor sensor modules for SWaP-efficient space instruments and defense applications. III-V materials-based p-i-n detectors (e.g. InGaAs) over 2.0+ μm wavelengths suffer from lattice mismatch with substrate, resulting in high dark currents, low sensitivity (low SNR), and needing cooling. An avalanche photodiode (APD) with high gain will significantly increase sensitivity above p-i-n detector solutions; however, dark current also multiplies under high gain, leading to increased noise. An APD’s material system dictates noise associated with gain process and depends on ratio of ionization coefficient (hereinafter mentioned as k or k-factor) which dictate the APD excess noise. In APDs, dark current and excess noise limit useful gain and hence the SNR of detection systems. Therefore, it is essential to reduce k-factor and dark current of an APD. We present a first in the world low-noise, high-gain, and high bandwidth 2.0+ μm uncooled APD having k factor of <0.01 enabling low excess noise at a given gain. The APD uses a novel structure and a bandstructure-engineered multiplication region based on III-V material system to reduce k-factor to near zero. We present the APD SACM device design, their simulations, and experimental results of fabricated APD devices. The uncooled 2.0+μm APD matches advantages offered by HgCdTe APDs having k~ zero, but with low dark currents at near room-temperature operation, and also mitigates both HgCdTe APDs and standard III-V p-i-n deficiencies to become an ideal solution benefiting to extend short-wave IR band to 2.0 +μm for imaging applications.
High−speed photodetectors operating at short−wavelength infrared (SWIR) telecommunication waveband have been well studied with the development of optical fiber−based communication system. Recent innovations of photonic systems have raised new requirements on the bandwidth of photodetectors with cutoff wavelengths from extended short−wavelength infrared (eSWIR) to mid−wavelength infrared (MWIR). However, the frequency response and gain performance of photodetectors in these longer wavelength bands is less studied, and the performances of the current high-speed photodetectors in these bands are still not comparable with those in the telecommunication band. There are two major material systems are able to cover whole infrared spectrum from short− to long−wavelength infrared; HgCdTe well-developed ternary alloys and antimonide−based type−II superlattices (T2SLs). T2SLs are a developing new material system with intrinsic advantages such as great flexibility in bandgap engineering, low growth and manufacturing cost, high−uniformity, auger recombination suppression, and high carrier effective mass that are becoming an attractive candidate for infrared detection and imaging. Thanks to T2SLs’ extreme design flexibility one can demonstrate many different device architectures that could not been realized in other material systems to achieve higher gain and speed such as hetero−junction phototransistor (HPT) and avalanche photodiodes (APD) with bandstructure−engineered multiplication regions. We are going to present an overview of eSWIR and MWIR gain−based devices (such as HPTs and APDs) in T2SLs material system and possible routes to achieve higher gain and faster speed in these devices.
Reduction of dark current density in microjunction-based InAs/InAs1-xSbx type-II superlattice long-wavelength infrared photodetectors was demonstrated. A double electron barrier design was used to suppress both generation-recombination and surface dark currents. The photodetectors exhibited high surface resistivity after passivation with SiO2, which permits the use of small size features without having strong surface leakage current degrading the electrical performance. Fabricating a microjunction structure (25×25 μm2 mesas with 10×10 μm2 microjunctions) with this photodetector double barrier design results in a dark current density of 6.3×10-6 A/cm2 at 77 K. The device has an 8 μm cut-off wavelength at 77 K and exhibits a quantum efficiency of 31% for a 2 μm-thick absorption region, which results in a specific detectivity value of 1.2×1012 cm·Hz1/2/W at 77 K.
Most of reported HPTs in literatures are based on InGaAs compounds that cover NIR spectral region. However, InGaAs compounds provide limited cut-off wavelength tunability. In contrast, type-II superlattices (T2SLs) are a developing new material system with intrinsic advantages such as great flexibility in bandgap engineering, low growth and manufacturing cost, high-uniformity, auger recombination suppression, and high carrier effective mass that are becoming an attractive candidate for infrared detection and imaging from short-wavelength infrared to very long wavelength infrared regime. We present the recent advancements in T2SL-based heterojunction phototransistors in e– SWIR, MWIR and LWIR spectral ranges. A mid-wavelength infrared heterojunction phototransistor based on type-II InAs/AlSb/GaSb superlattices on GaSb substrate has been demonstrated. Then, we present the effect of vertical scaling on the optical and electrical performance of heterojunction phototransistors, where the performance of devices with different base width was compared as the base was scaled from 60 down to 40 nm.
InAs/InAs1-xSbx/AlAs1-xSbx type-II superlattices (T2SLs) is a system of multi-interacting quantum wells. Since its introduction, this material system has drawn a lot of attention especially for infrared detection. In recent years, InAs/InAs1- xSbx/AlAs1-xSbx T2SL material system has experienced incredible improvements in material quality, device structure designs and device fabrication process which elevated the performances of T2SL-based photodetectors to a comparable level to the state-of-the-art material systems for infrared detection such as Mercury Cadmium Telluride (MCT). In this paper, we will present the current status of InAs/InAs1-xSbx/AlAs1-xSbx T2SL-based photodetectors for detection in different infrared regions, from short-wavelength (SWIR) to long-wavelength (LWIR) infrared, and the future outlook of this material system.
We report InAs/InAs1-xSbx type-II superlattice base photodetector as high performance long-wavelength infrared nBn device grown on GaSb substrate. The device has 6 μm-thick absorption region, and shows optical performance with a peak responsivity of 4.47 A/W at 7.9 μm, which is corresponding to the quantum efficiency of 54% at a bias voltage of negative 90 mV, where no anti-reflection coating was used for front-side illumination. At 77K, the photodetector’s 50% cut-off wavelength was ~10 μm. The device shows the detectivity of 2.8x1011 cm.√Hz/W at 77 K, where RxA and dark current density were 119 Ω•cm2 and 4.4x10-4 A/cm2 , respectively, under -90 mV applied bias voltage.
We present a high-performance short-wavelength infrared n-i-p photodiode, whose structure is based on type-II
superlattices with InAs/InAs1-xSbx/AlAs1-xSbx on GaSb substrate. At room temperature (300K) with front-side
illumination, the device shows the peak responsivity of 0.47 A/W at 1.6mm, corresponding to 37% quantum efficiency at
zero bias. At 300K, the device has a 50% cut-off wavelength of ~1.8mm. For −50mV applied bias at 300 K the
photodetector has dark current density of 9.6x10-5 A/cm2 and RxA of 285 Ω•cm2, and it revealed a detectivity of
6.45x1010 cm•Hz1/2/W. Dark current density reached to 1.3x10-8 A/cm2 at 200 K, with 36% quantum efficiency which
leads to the detectivity value of 5.66x1012 cm•Hz1/2/W.
Low-frequency noise has been studied in a mid-wavelength infrared InAs/GaSb type-II superlattice-based focal plane array. Low-frequency noise is observed under reverse bias but not at zero bias, even in the presence of photo-current. The magnitude of low-frequency noise was separately measured as a function of operating temperature and operation bias. The low-frequency noise is linearly correlated with the generation-recombination component of the dark current. No correlation of low-frequency noise with photo-current or diffusion dark current was found.
We report a bias selectable dual-band Type-II superlattice-based short-wave infrared (SWIR) and mid-wave infrared (MWIR) co-located photodetector capable of active and passive imaging. A new double-layer etch-stop scheme is introduced for back-side-illuminated photodetectors, which enhanced the external quantum efficiency both in the SWIR and MWIR spectral regions. Temperature-dependent dark current measurements of pixel-sized 27 μm detectors found the dark current density to be ~1×10-5 A/cm2 for the ∼4.2 μm cut-off MWIR channel at 140 K. This corresponded to a reasonable imager noise equivalent difference in temperature of ∼49 mK using F⁄2.3 optics and a 10 ms integration time (tint), which lowered to ∼13 mK at 110 K using and integration time of 30 ms, illustrating the potential for high-temperature operation. The SWIR channel was found to be limited by readout noise below 150 K. An excellent imagery from the dual-band imager exemplifying pixel coincidence is shown.
We report a bias selectable dual-band mid-wave infrared (MWIR) and long-wave infrared (LWIR) co-located detector with 3 μm active region thickness per channel that is highly selective and can perform under high operating temperatures for the MWIR band. Under back-side illumination, a temperature evolution study of the MWIR detector’s electro-optical performance found the 300 K background-limit with 2π field-of-view to be achieved below operating temperatures of 160 K, at which the temperature’s 50% cutoff wavelength was 5.2 μm. The measured current reached the system limit of 0.1 pA at 110 K for 30 μm pixel-sized diodes. At 77 K, where the LWIR channel operated with a 50% cutoff wavelength at 11.2 μm, an LWIR selectivity of ∼17% was achieved in the MWIR wave band between 3 and 4.7 μm, making the detector highly selective.
Active and passive imaging in a single camera based on the combination of short-wavelength and mid-wavelength
infrared detection is highly needed in a number of tracking and reconnaissance missions. Due to its versatility in
band-gap engineering, Type-II InAs/GaSb/AlSb superlattice has emerged as a candidate highly suitable for this
multi-spectral detection.
In this paper, we report the demonstration of high performance bias-selectable dual-band short-/mid-wavelength
infrared photodetectors based on InAs/GaSb/AlSb type-II superlattice with designed cut-off wavelengths of 2 μm
and 4 μm. Taking advantages of the high performance short-wavelength and mid-wavelength single color
photodetectors, back-to-back p-i-n-n-i-p photodiode structures were grown on GaSb substrate by molecular beam
epitaxy. At 150 K, the short-wave channel exhibited a quantum efficiency of 55%, a dark current density of 1.0x10-9 A/cm2 at -50 mV bias voltage, providing an associated shot noise detectivity of 3.0x1013 Jones. The mid-wavelength
channel exhibited a quantum efficiency of 33% and a dark current density of 2.6x10-5 A/cm2 at 300 mV bias voltage,
resulting in a detectivity of 4.0x1011 Jones. The operations of the two absorber channels are selectable by changing
the polarity of applied bias voltage.
Free-space optical communication is a promising solution to the "last mile" bottleneck of data networks. Conventional
near infrared-based free-space optical communication systems suffer from atmospheric scattering losses and
scintillation effects which limit the performance of the data links. Using mid-infrared, we reduce the scattering and
thus can improve the quality of the data links and increase their range. Because of the low scattering, the data link
cannot be intercepted without a complete or partial loss in power detected by the receiver. This type of
communications provides ultra-high bandwidth and highly secure data transfer for both short and medium range data
links. Quantum cascade lasers are one of the most promising sources for mid-wavelength infrared sources and Type-II
superlattice photodetectors are strong candidates for detection in this regime.
The same way that that low scattering makes mid-wavelength infrared ideal for secure free space communications,
high scattering can be used for secure short-range free-space optical communications. In the solar-blind ultraviolet (<
280 nm) light is strongly scattered and absorbed. This scattering makes possible non-line-of-sight free-space optical
communications. The scattering and absorption also prevent remote eavesdropping. III-Nitride based LEDs and
photodetectors are ideal for non-line-of-sight free-space optical communication.
Recently, the type-II InAs/GaSb superlattice (T2SL) material platform is considered as a potential alternative for
HgCdTe technology in long wavelength infrared (LWIR) imaging. This is due to the incredible growth in the
understanding of its material properties and improvement of device processing which leads to design and fabrication of
better devices. In this paper, we report electrical low frequency noise measurement on a high performance type-II
InAs/GaSb superlattice 1024×1024 LWIR focal plane array.
We demonstrate widely tunable high power distributed feedback quantum cascade laser array chips that span 190 nm
and 200 nm from 4.4 um to 4.59 um and 4.5 um to 4.7 um respectively. The lasers emit single mode with a very narrow
linewidth and side mode suppression ratio of 25 dB. Under pulsed operation power outputs up to 1.85 W was obtained
from arrays with 3 mm cavity length and up to 0.95 W from arrays with 2 mm cavity length at room temperature.
Continuous wave operation was also observed from both chips with 2 mm and 3 mm long cavity arrays up to 150 mW.
The cleaved size of the array chip with 3 mm long cavities was around 4 mm x 5 mm and does not require sensitive
external optical components to achieve wide tunability. With their small size and high portability, monolithically
integrated DFB QCL Arrays are prominent candidates of widely tunable, compact, efficient and high power sources of
mid-infrared radiation for gas sensing.
Recent efforts have been paid to elevate the operating temperature of Type II superlattice Mid Infrared
photon detectors. Using M-structure superlattice, novel device architectures have been developed, resulting
in significant improvement of the device performances. In this paper, we will compare different
photodetector architectures and discuss the optimization scheme which leads to almost one order of
magnitude of improvement to the electrical performance. At 150K, single element detectors exhibit a
quantum efficiency above 50%, and a specific detectivity of 1.05x1012 cm.Hz1/2/W. BLIP operation with a
300K background and 2π FOV can be reached with an operating temperature up to 180K. High quality focal
plane arrays were demonstrated with a noise equivalent temperature difference (NEDT) of 11mK up to
120K. Human body imaging is achieved at 150K with NEDT of 150mK.
Infrared detection technologies entering the third generation demand performances for higher
detectivity, higher operating temperature, higher resolution and multi-color detection, all accomplished with
better yield and lower manufacturing/operating costs. Type-II antimonide based superlattices (T2SL) are
making firm steps toward the new era of focal plane array imaging as witnessed in the unique advantages and
significant progress achieved in recent years. In this talk, we will present the four research themes towards
third generation imagers based on T2SL at the Center for Quantum Devices. High performance LWIR
megapixel focal plane arrays (FPAs) are demonstrated at 80K with an NEDT of 23.6mK using f/2 optics, an
integration time of 0.13ms and a 300K background. MWIR and LWIR FPAs on non-native GaAs substrates
are demonstrated as a proof of concept for the cost reduction and mass production of this technology. In the
MWIR regime, progress has been made to elevate the operating temperature of the device, in order to avoid
the burden of liquid nitrogen cooling. We have demonstrated a quantum efficiency above 50%, and a
specific detectivity of 1.05x1012 cm.Hz1/2/W at 150K for 4.2μm cut-off single element devices. Progress on
LWIR/LWIR dual color FPAs as well as novel approaches for FPA fabrication will also be discussed.
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