In this communication, we report on electrical and electro-optical characterizations of InAs/GaSb Type-II superlattice (T2SL) LWIR photodetector, showing cut-off wavelengths at 11μm at 77K. The devices, made of barrier structures in XBp configuration, were grown by molecular beam epitaxy (MBE) on GaSb substrate. Experimental measurements on samples were made by photoresponse spectra, by capacitance-voltage (C-V) and dark current-voltage (I-V) characteristics performed as a function of temperature.
In this paper, results from the development of LWIR and VLWIR InAs/GaSb type-II infrared photodetector arrays are presented. Dark currents comparable to the HgCdTe benchmark (Rule07) have been observed and the quantum efficiencies of the detectors exceed 30 %. Bias and temperature dependencies of the QE have been studied showing very low turn on bias (~-25mV) and no variation of the peak QE value with temperature. These results show that there are no unintentional barriers in the detector structures and that the diffusion lengths are long enough to provide efficient collection of carriers. Initial results from the extension of the cut-off wavelength from 11 μm to 14 μm are also presented as well as initial results from photodiodes with thicker absorbers to enhance the QE.
Development towards higher operating temperature, smaller pitch and larger format arrays is ongoing for midwave (MW) InAs/GaSb superlattice detectors at IRnova. One part of this effort entails improvement in the MW detector design, which has resulted in increased quantum efficiency to 55-60 % in the entire 3-5 μm wavelength region, with dark current levels lower than 3×10-6 A/cm2 at 120 K. Furthermore, MW-MW dual band detectors have been realized by using pixel filters fabricated on top of regular MW FPAs. The pixel filters were designed to transmit infrared radiation in the 3.5 μm - 4.1 μm wavelength region and to completely block light shorter than 3.5 μm. By comparing the signals of filtered and unfiltered pixels, excellent contrast between the two bands were obtained. Long wave infrared detectors have also been realized with cut-off wavelength at 12.2 μm and dark current levels following the Rule07 trendline from 80 K to 160 K, with only two times higher dark current than Rule07 at 80 K.
Midwave-midwave dual color detection has been successfully demonstrated by using pixel filters fabricated on top of InAs/GaSb focal plane arrays (FPAs). The pixel filters used in these FPAs were designed to transmit infrared radiation in the 3.5 - 4.1 μm wavelength region and to completely block light shorter than 3.5 μm. By comparing the signals of filtered and unfiltered pixels, excellent contrast between the two bands were obtained. This design concept offers a great flexibility to tailor the transmission window to any wavelength range within the 3-5 μm wavelength region. In particular, this dual color detector concept has been used for gas detection of volatile organic compounds which have main absorption peaks at 3.3 μm.
IRnova has been manufacturing mid wave infrared (MWIR) detectors based on InAs/GaSb type-II superlattices (T2SL)
since 2014. Results from the first years of production of MWIR focal plane arrays (FPAs) with 320 x 256 pixels on 30
μm pitch using the ISC9705 readout integrated circuit (ROIC) is presented in terms of operability, temporal and spatial
noise equivalent temperature difference (NETD) and other key production parameters. Results on image stability of
T2SL detectors show that no deterioration of image quality over time can be observed. Furthermore it is shown that the
non-uniformity correction remains stable even after repeated detector temperature cycles. Spatial and temporal NETD
for fabricated mid wave arrays show a temporal NETD of 12 mK and a spatial NETD of 4 mK with f/2 optics and 8 ms
integration time. When studied over a large scene temperature, the spatial noise is still less than 60 % of the temporal
noise. Furthermore, 640 x 512 mid wave FPAs with 15 μm pitch using the ISC0403 ROIC are entering an
industrialization phase. Temporal and spatial NETD values of 25 mK and 10 mK, respectively, are obtained with f/4
optics and 22 ms integration time and the operability is 99.85 %. A status update on the development of T2SL detectors
for short wave, mid wave and long wave infrared wavelength regions for existing and new applications is given and
recent development towards higher operating temperature, smaller pitch and larger FPA formats is presented.
The extension of supercontinuum (SC) sources into the mid-infrared, via the use of uoride and chalcogenide optical fibers, potentially offers the high radiance of a laser combined with spectral coverage far exceeding that of typical tunable lasers and comparable to traditional black-body emitters. Together with advances in mid-IR imaging detectors and novel tunable filter designs, such supercontinua hold considerable potential as sources of illumination for spectrally-resolved microscopy targeting applications such as rapid histological screening. The ability to rapidly and arbitrarily select particular wavelengths of interest from a broad emission spectrum, covering a wide range of biologically relevant targets, lends itself to image acquisition only at key relevant wavelengths leading to more manageable datasets. However, in addition to offering new imaging modalities, SC sources also present a range of challenges to successful integration with typical spectral microscopy instrumentation, including appropriate utilisation of their high spatial coherence. In this paper the application of SC sources to spectrally-resolved microscopy in the mid-IR is discussed and systems-integration considerations specific to these sources highlighted. Preliminary results in the 3-5μm region, obtained within the European FP7 project MINERVA, are also presented here.
A VGA format type-II superlattice focal plane array (FPA) for the mid-wave infrared (MWIR) atmospheric window has been designed, manufactured and characterized. The detector material is based on a heterojunction structure with a barrier that effectively decreases the Shockley-Read-Hall based component of the dark current. A very effective passivation method has been used which successfully inhibits all surface leakage currents. The barrier structure has a 50 % cutoff at 5 µm and 65 % quantum efficiency without antireflective coating. The dark current density is 3×10-6 A/cm2 at -0.05 V bias and 120 K. The optical cavity of the detector has been optimized for maximum capture of available light in the MWIR window. A focal plane array with 640 by 512 pixels and 15 µm pitch was processed based on this barrier structure. High-quality imagery in a system with high F-number will be presented.
A mid wave infrared type-II superlattice focal plane array with 320x256 pixels, 30 μm pitch and 90 % fill factor was
fabricated in house, using a conventional homojunction p-i-n photodiode design and the ISC9705 readout circuit. High-quality
imaging up to 110 K is demonstrated with the substrate fully removed. The absorber is 2 μm thick, and no anti-reflection
coating was used, so there is still room for significant improvement of the quantum efficiency, which is in the
40 % range.
Studies of the dark current vs. temperature behavior indicate that the device is limited by Shockley-Read-Hall generation
from the depletion region. The activation energy of this dark current component is 0.13 eV, suggesting an unidentified
recombination center positioned halfway into the 0.24 eV bandgap.
Furthermore, we report on detectors with 100 % cut-off at 13 μm. The dark current density at 60 K and -50 mV bias is
2x10-4 A/cm2. Quantum efficiency, NETD and BLIP temperature are also calculated.
Position-sensitive photocurrent measurements on mesa-etched superlattice material were made at low temperatures using
a focused laser spot. The lateral diffusion length for holes was extracted and is reported.
A novel electrical and optical confinement scheme for surface emitting optoelectronic devices is presented. The scheme
is based on epitaxial regrowth of a pnp current blocking layer structure around a mesa etched in the vertical cavity region
of the device. The lateral size and orientation of the mesa is defined lithographically and dry etching is used to create
vertical mesa sidewalls. By orienting the mesa sidewalls in certain crystallographic directions, it is possible to selectively
grow a current blocking pnp layer structure on the exposed n-type lower cladding layer of the cavity whithout
obstructing the electrical injection into the active region. The concept is evaluated in 1.2-μm GaAs-based light emitting
diodes with InGaAs quantum wells. This type of structure can easily be used as the amplifying region of a vertical cavity
laser, providing a good alternative to selective oxidation confinement.
We describe the development of long-wavelength InGaAs/GaAs vertical-cavity surface emitting lasers (VCSELs). Using highly strained double-quantum wells (DQWs) in combination with negative gain-cavity detuning we have been able to realise such VCSELs with emission wavelength up to 1300 nm. High-performance device characteristics include mW-range output power, mA-range threshold currents, 10 Gbit/s data transmission and very good temperature stability with continuous-wave operation up to at least 140°C. Singlemode emission is realised using an integrated mode filter consisting of a patterned silicon layer on the out-coupling mirror surface, yielding output power and threshold currents for 1270-nm devices of 1.2 - 0.5 mW and 2.3 - 0.6 mA, respectively, over a temperature interval of 10 - 140°C. Multimode devices have been found to deliver more than 2 mW at 1290 nm. Preliminary lifetime measurements do not reveal any intrinsic reliability problems related to the highly strained quantum wells.
As the bit rates of high-speed data transmission increase, the dynamic properties of the VCSEL are becoming more important. High photon density is of great importance for high modulation efficiency and low noise. We will present a schematic of the Zarlink 850nm 10Gb/s selectively oxidized VCSEL design and results from small and large signal modulation. The modulation characteristics were improved by utilizing InGaAs quantum wells, which have better gain characteristics than GaAs wells. 10Gb/s transmission over 300m of OFS-Fitel LaserWavetm 300 Fiber for a 12micrometers aperture multimode VCSEL at 80 degree(s)C will be demonstrated. Bit error rate (BER) measurement with the corresponding power penalty will also be presented.
MITEL Semiconductor is developing the next generation low cost, high performance transceivers for data communication. The increasing quantity of data being transferred over the Internet demands very high capacity interconnects. A low cost, high-performance alternative is the use of parallel fiber interconnects where the light is, for example, coupled into a 12channel fiber-ribbon. Parallel interconnects require good uniformity in order to reduce escalating costs and complexity. In this paper we report on the static and the modulation properties of 850nm multimode oxide VCSELs for use in such Gb/s transceiver system. Static power-current-voltage characteristics with good uniformity were obtained for different structures, with threshold currents down to sub-mA. A maximum small signal 3-dB bandwidth of 10 GHz and a modulation current efficiency up to 8.4 GHztsJ[rnA] were measured. Single channel results are presented for VCSELs operated at data rates from 2.5-10Gb/s.
Vertical cavity surface emitting lasers (VCSEL) operating around 850 nm are finding escalating markets in fiber optical communication applications, currently mainly at data rates between 1 and 2,5 Gbit/s. More than 3 million device hours without failures at temperatures up to 100 degrees Celsius proves that the reliability of the VCSEL satisfies the requirements for these applications. Results for oxidized as well as implanted devices are discussed and designs for both common anode and common cathode driving conditions are described. We demonstrate the application of our VCSEL design to arrays with passive alignment for parallel data communication over fiber ribbon. As examples of visible components for communication over plastic optical fiber, results for resonant cavity light emitting diodes will be shown and compared to red VCSELs.
We report on two AlGaInP-based visible VCSEL designs based on different current confinement schemes, ion implantation and selective oxidation, and we compare the respective performances with a particular interest on the modulation properties. The implanted device operated continuous wave (CW) up to 40 degrees Celsius. Threshold current of 7 mA, threshold voltage of 2.5 V and maximum optical power of 0.3 mW were measured at room temperature. The small signal modulation responses were fitted using a 3-poles model, allowing the estimation of various parameters such as resonance frequency, damping factor and parasitic cut-off. The maximum 3dB- bandwidth was shown to be 2.1 GHz, limited both by thermal and parasitic effects. 'Error-free' transmission at 1 Gb/s was demonstrated through 50-meter of graded-index POF. The selectivity oxidized devices achieve much higher output power (1.8 mW for the 10 micrometer opening diameter) with threshold current as low a 1.5 mA and threshold voltage of 2.1 V at room temperature, and operate CW up to 49 degrees Celsius. The maximum 3 dB-bandwidth was 4.5 GHz. Modulation current efficiency factor up to 2.8 GHz/(root)[mA] was measured.
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