Optimization of various growth parameters for Type-II GaSb (10MLs)/InAs(10MLs) nanoscale superlattices (SL) and
GaSb layers, grown by solid molecular beam epitaxy, has been undertaken. We present optical and structural
characterization for these heterostructures, using high resolution X-ray diffraction (HRXRD), photoluminescence (PL)
and atomic force microscopy (AFM). Optimized parameters were then used for growth of InAs/GaSb SLs photovoltaic
detectors (λcut-off ~5 μm) operating at room temperature. By controlling the nature of interfaces, the in-plane mismatch
between GaSb-buffer layer and SLs can be reduced enabling the growth of active regions up to 3μm. Normal incidence
single pixel photodiodes were fabricated using standard lithography with apertures ranging from 25-300 μm in diameter.
The spectral response from the SLs detector was observed at room temperature. This suggests the potential of the SLs
technology for realizing high operating temperature (HOT) sensors. Responsivity measurements were also undertaken
using a calibrated black body source, 400Hz optical chopper, SR 770 FFT Network signal analyzer and Keithley 428
preamplifier. We obtained current responsivity equal 2.16 A/W at V = -0.3V(300K). The Johnson noise limited D* at
300K was estimated to be 4.6x109 cm·Hz1/2/W at V = -0.3V
In this paper we report the use of a photonic crystal resonant cavity to increase the quantum efficiency, detectivity (D*) and the background limited infrared photodetector (BLIP) temperature of a quantum dot detector. The photonic crystal is incorporated in InAs/InGaAs/GaAs dots-in-well (DWELL) detector using Electron beam lithography. From calibrated blackbody measurements, the conversion efficiency of the detector with the photonic crystal (DWELL-PC) is found to be 58.5% at -2.5 V while the control DWELL detectors have quantum efficiency of 7.6% at the same bias. We observed no significant reduction in the dark current of the photonic crystal devices compared to the normal structure. The generation-recombination limited D* at 77K with a 300K F1.7 background, is estimated to be 6 x 1010 cmHz1/2/W at -3V bias for the DWELL-PC which is a factor of 20 higher than that of the control sample. We also observed a 20% increase in the BLIP temperature for the DWELL-PCs.
We report the first two-color 320 x 256 infrared Focal Plane Array (FPA), based on a voltage-tunable InAs/InGaAs/GaAs DWELL structure. The detectors, grown by solid source molecular beam epitaxy (MBE) comprise of a 15-stack asymmetric DWELL structure sandwiched between two highly doped n-GaAs contact layers, grown on a semi-insulating GaAs substrate. The DWELL region consists of a 2.2 monolayer deposition of n-doped InAs quantum dots (QDs) in an In0.15GaAs0.85As well, itself placed in GaAs. The well widths below and above the dots are 50Å and 60Å, respectively. The absorption region asymmetry results in a bias dependent spectral response, with the peak wavelength varying from 5.5 to 10 μm. Using calibrated black body measurements, mid-wavelength and long wavelength specific detectivities (D*) of top-illuminated test pixels at 78K were estimated to be 7.1 x 1010 cmHz1/2/W (Vb= 1.0V) and 2.8 x 1010 cmHz1/2/W (Vb= 2.5V), respectively. Subsequently, a 320 x 256 QDIP FPA array was fabricated on a 30 μm pitch and was hybridized with an Indigo 9705 ROIC. Thermal imaging was successfully carried out at an estimated FPA temperature of 80K, using different optical filters between 3-5 μm, and 8-12 μm, so as to demonstrate two-color operation. The operability of the FPA was greater than 99%, and the noise-equivalent temperature difference was estimated to be less than 100 mK for f#1 (3-5 μm) and f#2 (5-9 μm) optics.
Quantum-dot infrared photodetectors (QDIPs), based on intersubband transitions in nanoscale self-assembled dots, are perceived as a promising technology for mid-infrared-regime sensing since they are based on a mature GaAs technology, are sensitive to normal incidence radiation, exhibit large quantum confined stark effect that can be exploited for hyperspectral imaging, and have lower dark currents than their quantum well counterparts. High detectivity (D* = 1.0E11 cmHz1/2/W at 9 microns) QDIPs have been recently shown to exhibit broad spectral responses approximately 2-micron FWHM) with a bias-dependent shift in their peak wavelengths. This controllable, bias dependent spectral diversity, in conjunction with signal-processing strategies, allows us to extend the operation of the QDIP sensors to a new modality that enables us to achieve: (1) spectral tunability (single- or multi-color) in the range 2-12 microns in the presence of the QDIP's dark current; and (2) multispectral matched filtering in the same range. The spectral tuning is achieved by forming an optimal weighted sum of multiple photocurrent measurements, taken of the object to be probed, one for each bias in a set of prescribed operational biases. For each desired spectral response, the number and values of the prescribed biases and their associated weights are tailored so that the superposition response is as close as possible, in the mean-square-error sense, to the response of a sensor that is optically tuned to the desired spectrum. The spectral matching is achieved similarly but with a different criterion for selecting the weights and biases. They are selected, in conjunction with orthogonal-subspace-projection principles in hyperspectral classification, to nullify the interfering spectral signatures and maximize the signal-to noise ratio of the output. This, in turn, optimizes the classification of the objects according to their spectral signatures. Experimental results will be presented to demonstrate the QDIP sensor's capabilities in these new modalities. The effect of dark current noise on the spectral-tuning capability is particularly investigated. Examples of narrowband and wideband multispectral photocurrent synthesis as well as matched filtering are presented.
Spectrally tunable quantum-dot infrared photodetectors (QDIPs) can be used to approximate multiple spectral responses with the same focal-plane array. Hence, they exhibit the potential for real time adaptive detection/classification. In the present study, it is shown that we can perform the detection/classification operation at the adaptive focal-plane array (AFPA) based on QDIPs by fitting the QDIP's response to the correspondent operators. With a new understanding of spectral signature in the sensor space, the best fitting can be achieved. Our simulation results show how well QDIPs perform in different regions of the spectrum in the mid- and long wave infrared. The results indicate that the AFPA performance does not match that of the ideal filtering operators, but reliable measurement can be accomplished.
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