Current infrared imaging systems monitor emission from a given scene over a broad spectral range, which results with "black and white" images. As a result, there is ever increasing emphasis on the development of new, on the pixel level, infrared imaging technology that can provide spectral information. Attempts at creating a robust imaging system with spectral information have been made through a network of external optics, which results with a high cost and large system package. Here, we propose a metamaterial design that resonantly couples to an infrared photodetector for enhanced performance.
We have investigated optical properties and figures of merit of sub-monolayer quantum dots (SML-QD) infrared
photodetector and compared them with conventional Stranski-Krastanov quantum dots (SK-QD) with a similar design.
The purpose of this study is to examine the effects of varying the number of stacks(2,3,4,5 and 6) in SML-QD detector
on its device performance The peak of photoluminescence (PL) spectra of SK-QD and SML-QDs are observed at
1.07eV and 1.24~1.35eV at room temperature, respectively. The PL peak of 2 and 3 stacks SML QD are very close to
the GaAs band edge peak (1.42eV) and the full width at half maximum (FWHM) of all the SML-QD are much narrower
than SK-QD. Normal incidence photoresponse peak of 4 stacks SML QDIP are obtained at 7.5μm with responsivity of
0.5 A/W and detectivity of 1.2×10<sup>11</sup> cm.Hz<sup>1/2</sup>/W (77K, 0.4V, f/2 optics), which is much narrower than spectral response
of SK QDIP possibly due to bound-to-bound transition.
Systematic characterization of various types of intersubband transitions in the quantum dots in a well
(DWELL) infrared photodetectors has been presented. By changing the thickness of the quantum well,
the excited state energy can be tuned with respect to the barrier, without altering the quantum dot
ground state. Bound to continuum transitions offer very high extraction probability for photoexcited
electrons but poor absorption coefficient, while the bound to bound transitions have higher absorption
but poorer extraction probability. Bound to quasibound transition is optimum for intermediate values
of electric fields with superior signal to noise ratio. The bound to quasibound device has the detectivity
of 4×10<sup>11</sup> cm.Hz<sup>1/2</sup> W<sup>-1</sup> (+3V, f /2 optics) at 77 K and 7.4×10<sup>8</sup> cm.Hz<sup>1/2</sup> W<sup>-1</sup> at 200 K, which is highest
reported detectivity at 200 K for detector with long wave cutoff wavelength. High performance focal
plane arrays have been fabricated with noise equivalent temperature difference of 44 mK at 80 K for
6.1μm peak wavelength.
In this paper, we demonstrate a high operating temperature (HOT) quantum dot-in-a-well (DWELL) infrared
photodetector with enhanced normal incidence (s-polarization) radiation photocurrent. The s-to-p polarization ratio
was increased to 50%, compared to the 20% in conventional quantum dot detectors. This improvement was achieved
through engineering the dot geometry and the quantum confinement via post growth capping materials of the
quantum dots (QDs). The effect of the capping procedures was determined by examining the dot geometry using
transmission electron microscopy (TEM) and s-to-p polarization induced photocurrent in the DWELL structure
photodetector. The TEM image shows a quantum dot with a reduced base of 12 nm and an increased height of 8 nm.
The infrared photodetectors fabricated from this material shows a peak photodetectivity of 1×10<sup>9</sup> cmHz<sup>1/2/</sup>W at 77K
for a peak wavelength of 4.8 μm and 1×107 cmHz<sup>1/2</sup>/W at 300K for a peak wavelength of 3.2 μm. The dark current
density is as low as 2×10<sup>-4</sup>A/cm<sup>2</sup> and the photocurrent gain is 100 at the optimal operating bias.
Next generation infrared photodetector technology will require focal plane array (FPA) systems that have multi-spectral
imaging capabilities. One proposed approach to realizing these multicolor devices is to use plasmonic resonators.
However, device development and characterization are commonly addressed with large front side illuminated single
pixel detectors on a supporting epitaxial substrate. The focal plane arrays on the other hand are backside illuminated.
Moreover, in a front side illuminated device, there is significant substrate scattering of the incident light. Here, we
propose a method for the accurate measurement of device performance by using a hybridized chip design (hybrid chip)
that is similar to the fabrication of an FPA system, with the substrate completely removed through a combination of
mechanical polishing and subsequent wet etching techniques. The hybrid chip was also designed to precisely
characterize the effects of varying mesa size by incorporating square mesa structures that range from 25 to 200 μm in
width. This approach offers an advantage over conventional device characterization because it incorporates mesas that
are on the same scale as those normally used in FPA systems, which should therefore provide a fast transition of new
photodetector technology into camera based systems. The photodetector technology chosen for this work is a multi-stack
quantum dots-in-a-well (DWELL) structure designed to absorb electromagnetic radiation in the mid-infrared spectral
We report on the performance of multi-stack quantum dots in a well (DWELL) detectors. Present-day QD detectors are
limited by low responsivity and quantum efficiency (QE). This can be attributed to the low absorption efficiency of
these structures due to the small number of QD stacks in the detector. In this paper we examine the effect of the number
of stacks on the performance of the detector. In particular, we investigate the InAs/GaAs/AlGaAs D-DWELL (Dots-in-double-well) design, which has a lower strain per DWELL stack than the InAs/InGaAs/GaAs DWELLs thereby enabling
the growth of many more stacks in the detector. The purpose of the study detailed in this paper is to examine the effects
of varying the number of stacks in the InAs/InGaAs/GaAs/AlGaAs D-DWELL detector, on its device performance. The
numbers of stacks grown using solid source molecular beam epitaxy (MBE), were 15, 30, 40, 50, and 60. Once
fabricated as single pixel devices, we carried-out a series of device measurements such as spectral response, dark current,
total current, responsivity along with computing the photoconductive gain and the activation energies. The goal of these
experiments is to not only study the single pixel detector performance with varying number of stacks in a D-DWELL
structure, but to also understand the effect of the transport mechanism in these devices.
This paper discusses recent and future advancements in the field of quantum dots-in-a-well (DWELL) focal plane arrays (FPAs). Additionally, for clarity sake, the fundamentals of FPA figures of merit are reviewed. The DWELL detector represents a hybrid between a conventional quantum well photodetector (QWIP) and a quantum dot (QD) photodetector (QDIP). This hybridization, where the active region consists of QDs embedded in a quantum well (QW), grants DWELLs many of the advantages of its components. This includes normally incident photon sensitivity without gratings or optocoupers, like QDIPs, and reproducible control over operating wavelength through 'dial-in recipes' as seen in QWIPs. Conclusions, drawn by the long carrier lifetimes observed in DWELL heterostructures using femtosecond spectroscopy, have recently backed up by reports of high temperature operation results for DWELL FPAs. This paper will conclude with a preview of some upcoming advances in the field of DWELL focal plane arrays.