Mid Infrared Focal Plane Arrays with Nanoscale Quantum Dots and ...

Mid Infrared Focal Plane Arrays With NanoscaleQuantum Dots and SuperlatticesS. KrishnaCenter for High Technology Materials, Department ofElectrical and Computer EngineeringUniversity of New Mexico,1313 Goddard SE, Albuquerque NM 87106Email: skrishna@chtm.unm.eduif desiredAbstract—Keywords- Molecular beam epitaxy, Nanoscale, Quantum DotsSuperlattices, Antimonides, Mid-infrared photodetector.I. INTRODUCTIONPresently, the state of the art photon detectors for the mid waveinfrared (MWIR, 3-5 µm) and long wave infrared (LWIR, 8-12µm) are based on interband transitions in bulk IndiumAntimonide (InSb) and Hg 1-x Cd x Te (MCT) alloys respectively.Two emerging technologies for next generation sensors thatoffer enhanced functionality include (i) intersubband detectorsbased on nanoscale quantum dots and (ii) interband transitionsin the Type II InAs/GaSb strain layer superlattice system.Infrared detectors based on InAs/GaSb strain layer superlattices(SLSs) were proposed by Smith and Mailhiot [1] in 1980s andappear as a promising alternative to the present-day infra-red(IR) detection technologies. SLSs offer numerous advantagesover existing detector technologies, including better uniformity,reduced tunneling currents, normal incidence absorption andsuppressed Auger recombination. SLSs are characterized by thebroken-gap type-II alignment. Presently all SLSs detectors arebased on a photodiode (p-i-n or n-i-p) design. During theconventional fabrication process of photodiodes, the deep etchthrough the absorbing region is utilized in order to define theoptical area of the detector. Electronic surface states within theenergy band gap of SLS are generated, resulting in large surfaceleakage currents. The suppression of these currents is the mostdemanding challenge for the SLS technology. A recentlyproposed nBn heterostructure design [2] implemented on SLSbaseddetectors showed a dark current reduction of two ordersof magnitude (at 77K) in comparison with conventionallyprosessed photodiodes [3]. We have recently fabricated a highperformance InAs/GaSb SLS detector with a P on N polarityand a 320x 256 MWIR FPA with a noise equivalent temperaturedifference of 24 mK at 77K based on this structure [4}In the quantum dots in a well (DWELL) heterostructure, InAsquantum dots are placed in a thin InGaAs quantum well that isin turn placed in a GaAs matrix. Three-color DWELL detectors,operating at 78K, with spectral response in the MWIR ( p1 ~ 4), LWIR ( p2 ~ 8 ) and VLWIR ( p3 ~ 23 ) regime havebeen fabricated in our group. Recently, we have fabricated thefirst long wave infrared and two-color quantum dot focal planearray (320x256 pixels).II.EXPERIMENTAL PROCEDUREThe SLS devices presented in this work were grown usingsolid source MBE on Te-doped epi-ready (100) GaSbsubstrates. The detector structure consists of an unintentionallydoped (n.i.d.) thick SLS absorber layer which isgrown on the top of n-type bottom contact layer and followedby the barrier layer. The structure is capped by a top contactlayer with the same superlattice composition, thickness, anddoping concentration as the bottom contact layer. The samplewas processed into FPAs with each die consisting of 320 x 256pixels with a 30 m pitch using standard photolithographytechniques. Unlike conventional photodiode processing, thesize of the device with nBn design is not defined by the etchdimensions but rather by the lateral diffusion length of minoritycarriers (holes).III. RESULTS AND DISCUSSIONThe measured FPA average dark current density was as 1 x10 -7 A/cm2 (V b =0.7V). This value is comparable to the stateof-the-artreports for MWIR SLS detectors based on aconventional photodiode design with a suitable passivationscheme. The noise equivalent temperature difference (NETD)was equal to 23.8 mK with a standard deviation of 10 mK at aFPA temperature of 77K. The external QE was obtained usingthe spectral response curve from the test diode on the FPA chipwith 380mm x 380mm optical area. Spectral measurementswere performed using a Fourier Transform InfraredSpectrometer (FTIR). Values of specific detectivity, QE andresponsivity as a function of wavelength are presented inFigure 1. Peak values of QE, responsivity and specificdetectivity were equal to 52%, 1.6 A/W, and 6.7 x 1011 Jones,respectively (77K, V b =0.7V, 3.8 mm). A thermal image takenwith 320 x 256 MWIR FPA at a detector temperature of 77 Kand integration time of 16.3 ms is shown in Figure 2.978-1-4244-2104-6/08/$25.00 ©2008 IEEE. 42

Figure 3. Images taken with a long wave infrared quantum dots in a wellfocal plane array (320x256).Figure 1. Spectral detectivity D*, responsivity and quantum efficiency for4.2 µm cut-off wavelength FPAACKNOWLEDGMENTWork supported by NSF, AFRL, AFOSR and DARPA.REFERENCESFigure 2. Thermal image taken with a MWIR 320x256 InAs/GaSb SLs nBncamera at a detector temperature of 77 K and integration time of 16.3 ms.Focal plane arrays were also made using nanoscalequantum dots using a similar procedure. Imaging obtainedfrom these arrays in the long wave infrared region are shown inFig. 3.[1] D.L. Smith and C. Mailhiot, J. Appl. Phys., vol. 62, 2545-2549 (1987).[2] S. Maimon and G. W. Wicks, Appl. Phys. Lett., vol. 89, 151109-15111(2006)[3] J.B. Rodriguez, E. Plis, G. Bishop, Y.D. Sharma, H. Kim, L.R. Dawsonand S. Krishna, Appl. Phys. Lett., vol. 91, 043514-043516 (2007).[4] H. S. Kim, E. Plis, J. B. Rodriguez, G. D. Bishop, Y. D. Sharma, L. R.Dawson, S. Krishna, J. Bundas, R. Cook, D. Burrows, R. Dennis, K.Patnaude, A. Reisinger, and M. Sundaram, Appl. Phys. Lett. 92, 183502(2008)43