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Infrared (IR) Photodetectors
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Achievement/Results
Infrared (IR) photodetectors are useful for a variety of military and civil applications, such as target acquisition, medical diagnostics, and pollution monitoring, to name just a few. So far most of the detectors in midwave (MWIR) and longwave (LWIR) IR regime need to be cooled to overcome the dark current effects. Detectors based on InAs/GaSb superlattices (SLS) have shown promising results, operating at higher temperatures as compared to the present day HgCdTe detectors. This is due to band-structure engineering and relatively large electron effective mass, which results in reduction of tunneling and Auger recombinations. So far, SLS photodetectors are based on photodiode (p-i-n) design. One of the main challenges with SLS structures is the lack of a mature passivation technology for the etched mesa surfaces especially for cutoff wavelengths longer than 8 µm. A class of IR detectors named nBn has recently shown promising results in eliminating the currents associated with Shockley-Read-Hall centers and mesa lateral surface imperfections, which have resulted in an increase of the operating temperature as compared to the p-i-n design. One of the advantages of the nBn structures is their unique processing technique which eliminates surface currents. Unlike p-i-n diodes where each pixel is defined with a deep etch (standard processing), in nBn processing (shallow etched devices), the diffusion length in the absorber is the parameter that isolates one device from the device next to it. This eliminates the presence of surface leakage currents. We have measured the current density for shallow etched (0.1 µm deep) and deep etched (2 µm deep) processed devices. A great reduction of current density in shallow etched device can be seen at lower temperatures in comparison with deep etched device. This is attributed to the reduction of leakage surface current. Also due to the band alignment of nBn, depletion region does not exist in the band structure which results in the elimination of SRH centers. We have investigated the multispectral detection capabilities of type II InAs/Ga(In)Sb SLS detector based on an nBn design. Multicolor detectors are desirable in a variety of IR applications related to remote sensing and object identification. Dual band, mid wave infrared (MWIR), and long wave infrared (LWIR), also have interesting applications such as computed-tomography imaging spectrometer. Multicolor capabilities have been demonstrated with mercury cadmium telluride (MCT) and quantum well infrared detector (QWIP) and more recently in the SLS system. Present day two color SLS detector requires two contacts per pixel leading to a complicated processing scheme and expensive specific readout circuits (ROICs). We have demonstrated two-color nBn structure having cut off wavelengths in midwave-longwave ( Lambda c1 ~3.5 µm and Lambda c2 ~ 8 µm ) using a LWIR absorbing layer. The two wavelengths regimes for this structure are obtained when the device is operated under different polarity of applied bias. Under forward bias, the LWIR absorber layer is the source of the signal, and under reverse bias the n-doped mid-wave contact layer is the source of the signal. To our knowledge, this is the first report of a bias dependent two-color response from a SLS detector. One of the advantages of two-color SLS nBn photodetectors is its standard single bump per contact processing that makes the fabrication of the device easier and more economical than previous attempts.
Address Goals
This research will advance the frontiers of knowledge. Specifically, there is a need to develop better infrared (IR) photodetectors which could be useful for target acquisition, medical diagnostics, and pollution monitoring, to name just a few. This research also provides a wonderful learning opportunity for IGERT fellow Arezou Khoshaklagh who utilized her interdisciplinary background to develop these novel photodetectors along with Professor Sanjay Krishna.
