High-speed uncooled infrared (IR) detectors are highly demanded for many military and industrial applications such as: target detection systems, proximity fuzes, LIDARs, non-destructive testing and inspection techniques, monitoring of the chemical quality and process control, remote sensing, and free space communication. Commercially available uncooled IR imaging sensors use ferroelectric or microbolometer detectors which are inherently slow. Although photon detectors have gigahertz bandwidths, their high temperature detectivity is severely degraded due to several physical limitations. The existing infrared photon detectors can be categorized as interband, such as HgCdTe, and intersubband quantum well infrared detectors (QWIP). There are some fundamental limitations, namely fast Auger recombination rate in the interband detectors and high thermal generation rate in the intersubband detectors, which drastically decrease their performance and ability for high temperature operation.

At first, we developed the growth of high quality Type-II superlattices. In order to increase the operating temperature of detectors, we designed the superlattices for an effective suppression of Auger recombination, and confirmed it experimentally or the first time.

The progress of cooled and un-cooled type-II detectors at CQD

Using such designs, we demonstrated the first uncooled infrared detectors from Type-II superlattices. The measured detectivity is more than 1x10⁸ cm x Hz^½/W at 10.6 μm at room temperature which is higher than the commercially available uncooled photon detectors at similar wavelength. The measured carrier lifetime was about 27 nsec which is an order of magnitude longer than the bulk material due to the suppression of the Auger recombination. To use such devices in focal plane arrays (FPA) for imaging applications, we also developed Type-II photodiodes operating at zero bias.

Comparison with the state of the art and with the theoretical Auger limited detectivity.

Furthermore, we realized the first uncooled Type-II photodiode at λ=8 μm with a zero bias detectivity of 1 x 10⁸ cm·Hz^½/W. Recently we demonstrated uncooled photodiodes with 5 μm cutoff that have a zero bias detectivity above 1 x 10⁹ cm·Hz^½/W and a device quantum efficiency exceeding 30 %. These results indicate that Type II superlattice is an excellent candidate for uncooled applications.

Type-II Superlattice Infrared Detectors

Additionally, very high performance long-wavelength infrared (LWIR or 8~12 μm) and very-long-wavelength infrared (12 μm) detectors and imaging arrays are also in high demand for strategic missile defense, pollution monitoring, and space-based astronomy. Material uniformity complexities and strong leakage currents limit the viability of state-of-the-art HgCdTe detectors in this wavelength regime. For this reason, type-II superlattices are being pursued as an alternative third generation focal plane array technology.

Spectral response of photodiodes with different InAs/GaSb superlattices in their active layer. The thickness of GaSb layer is 40 Å for all of the superlattices, while the thickness of the InAs layer is shown for each device. ETBM calculations agree with experimental data.

We have successfully developed Type-II InAs/GaSb superlattice photodiodes for this challenging wavelength range.

For example, devices with a 50% cutoff wavelength of λ_c = 7 μm exhibited a peak zero bias detectivity exceeding 10¹² cm x Hz^½/W at 77 K.

Typical devices with a cutoff wavelength of ~11 μm exhibited detectivities of 2-3 x 10¹⁰ cm · Hz^½/W and quantum efficiencies in excess of 50 %.

A Type-II InAs/GaSb superlattice photodetector with a cutoff wavelength of 12.9 μm and a detectivity of ~4x10¹⁰cm·Hz^½/W.

Photodiodes with a 50 % cutoff wavelength of 12.9 μm exhibited an R₀A of ~7 ? .cm² and a detectivity of 4.03 x 10¹⁰ cm·Hz^½/W at 77 K.

For those with a 50 %cutoff wavelength of λ_c=22 μm, apeak current responsivity about 5.5 A/W at 80 K was observed. The 90% to 10% cut-off energy width of these devices is on the order of 2kT which is about four times smaller compared to the devices based on InAs/Ga_1-xIn_xSb superlattices.

Relative responsivity for Type-II superlattice detectors with cutoff wavelength of 32 μm at T=9.2 K.

For the first time also, we demonstrated Type-II photovoltaic detectors with cutoff wavelengths approaching 32 μm. These detectors have a peak responsivity of 3 A/W and a detectivity of 4.25 x 10¹⁰ cm·Hz^½/W at 15 μm under 40 mV reverse bias at 34 K.

The material is grown by molecular beam epitaxy on a GaSb substrate with excellent crystal quality as evidenced by x-ray diffraction and atomic force microscopy. The use of binary layers in the superlattice has significantly enhanced the uniformity and reproducibility of the energy gap. Our experimental results show excellent uniformity over a three inch wafer area.

World's First Type-II Superlattice Focal Plane Array

We have demonstrated the World's First focal plane array using Type-II superlattices with a NEΔTmin of 0.03 K. Building on our experience we have also recently demonstrated the world's first uncooled Type-II mid-wave infrared focal plane array operating with a 5 μm cutoff wavelength. This uncooled camera array operates with 25 % quantum efficiency, a detectivity of 8 x 10⁸ cm·Hz^½/W, and a NEΔT of 50 K.

MWIR Type-II InAs/GaSb superlattice FPA imaging obtained at 120 K, left. Uncooled infrared imaging of a hot soldering iron based on MWIR Type-II InAs/GaSb superlattices, right.

These superlattice FPAs provide high quality imaging of warm subjects, such as human beings, up to a temperature of about 130 K, significantly higher than the conventional operating temperature of 77 K. It can also provide fast uncooled imaging at room temperature for a hot soldering iron with demonstrated frame rate of 110 Hz and potentially up to 600 Hz.

Silicon dioxide surface passivation greatly reduces the surface leakage in VLWIR type-II photodiodes, making focal plane arrays in this wavelength regime a possibility.

One obstacle to the technological success of type-II infrared focal plane imaging arrays is solving the ever-present problem of surface passivation. The small dimensions of focal plane array pixels enhance the amount of leakage current caused by surface effects. With a proper electrically passivating layer surface states can reduced and leakage current decreased, greatly improving the overall device detectivity and performance. At CQD we have demonstrated successful silicon dioxide passivation in the MWIR, LWIR, and VLWIR on type-II photodiodes. Additionally, we have developed a sulfur-based technique that reduces the surface trap density by nearly 3 orders of magnitude.

MWIR Type-II InAs/GaSb superlattice FPA imaging obtained at 130 K. The left is the negative image of the right with more visible facial details.

Using our experience for FPA fabrication and our expertise on LWIR type-II InAs/GaSb superlattice single element detectors, we started developing FPAs in the LWIR (8-12μm). However, as the wavelength increases (the bandgap decreases), the devices become more sensitive to thermal and surface effects, making the FPA fabrication process more critical. Thanks to enhancement in our growing, etching, cleaning, hybridation techniques, we managed to fabricate the world's first 320x256 FPA based on type-II InAs/GaSb superlattice photodetectors with a cutoff wavelength higher than 8 μm. This FPA with the highest cutoff wavelength demonstrated yet for type-II superlattice is able to perform imaging of human beings from 81 K to 110 K. It can also perform imaging of soldering iron up to 185 K. The array is able to detect temperature differences as small as 270 mC.

LWIR Type-II InAs/GaSb superlattice FPA imaging obtained at 81K and 185K.

NEGATIVE LUMINESCENCE

Negative Luminescence (NL) is an expression describing the manipulation of the radiative balance of a low band gap minority carrier device with the ambient. This phenomenon can be observed by the extraction of carriers from the active region of a photodiode through the application of reverse bias. There are numerous potential applications of NL devices, which include dynamic cold shields for focal plane arrays and fast infrared scene simulators.

Spectra of electroluminescence and negative luminescence of LWIR Type-II photodetectors at 250 K.

NL has been observed in various infrared material systems mostly in the mid-infrared spectral range around 5 μm. At CQD, for the first time, NL was demonstrated in a III-V based system covering the entire infrared spectral range between 5 and 13 μm with longwavelength infrared (LWIR) InAs/GaSb superlattice photodiodes.

Nanoscale Structures for Enhanced Type-II Photodetectors

Highly uniform nanopillars of Type-II InAs/GaSb superlattices. The diameter of such nanopillars can be less than 50 nm.

To further enhance this technology, we are exploring nanoscale structures with the goal of achieving higher temperature operation and wavelength tunability. Theoretical calculations show that devices based on these quantum-size structures can be realized with much higher performance. However, a key technical issue is the fabrication of large arrays of "nanopillars" with good uniformity. We have successfully used electron beam lithography and plasma etching techniques to realize the first nanometer-sized structures in Type-II InAs/GaSb superlattice structures with excellent uniformity over large areas.

Published values of (left) D* and (right) R0A for type-II superlattice photodetectors compared against theoretical limits and state-of-the-art HgCdTe.

Last Updated 01/31/2007