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.
In this paper, we report some of our recent results on improving the operating temperature of dots-in-a-well
(DWELL) infrared photodetectors. This was achieved by engineering the dot geometry and the interrelated quantum
confinement by varying the growth conditions and composition of the subsequent capping of the quantum dots
(QDs). The influence of these conditions was determined by examining the optical properties of the QDs directly
and indirectly with their function in a DWELL IR photodetector. Spectral response was observed until 250K with
spectral response peak at 3.2μm, and the peak detectivity is 1×10<sup>9</sup> cmHz<sup>1/2</sup>/W at 77K and ~ 1e8 cmHz<sup>1/2</sup>/W at 250K.
By varying the external bias, the DWELL heterostructure allows for the manipulation of the operating wavelength.
This tunability is a critical stepping stone towards creating multicolor imaging systems that can be used to take
images at multiple wavelengths from each pixel in a focal plane array.
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.
We report new results on the design, fabrication and characterization of a novel midinfrared sensor called quantum
dot avalanche photodiode (QDAP). The QDAP consists of a quantum dots-in-a-well (DWELL) detector coupled
with an avalanche photodiode (APD) through a tunnel barrier. In the QDAP, the photons are absorbed in the
DWELL active region while the APD section provides photocurrent gain. Spectral response and photocurrent
measurements at 77 K were taken to characterize the response of the device. The increase of the spectral response
and the nonlinear increase in the photocurrent as the APD voltage increases support theoretical predictions about
the QDAP capability to work in Geiger mode. The QDAP photocurrent is similar to the IV characteristic of the
APD section, indicating gain in the device.
We report the fabrication of low strain quantum-dots-in-a-double-well (DDWELL) infrared photodetector where the net
strain on the system has been reduced by limiting the total indium content in the system. The detector consists of InAs
dots embedded in In<sub>0.15</sub>Ga<sub>0.85</sub>As and GaAs wells with a Al<sub>0.1</sub>Ga<sub>0.9</sub>As barrier, as opposed to In<sub>0.15</sub>Ga<sub>0.85</sub>As wells and a
GaAs barrier in standard dots-in-a-well (DWELL) detector. The structure was a result of multilevel optimization involving
the dot, well layers above and below the dot for achieving the desired wavelength response and higher absorption, and
the thickness of the barriers for reduction in dark current. Detector structures grown using solid source molecular beam
epitaxy (MBE) were processed and characterized. The reduction in total strain has enabled the growth of higher number
of active region layers resulting in enhanced absorption of light. The detector shows dual color response with peaks in
the mid-wave infrared (MWIR) and the long-wave infrared (LWIR) region. A peak detectivity of 6.7×10<sup>10</sup> cm.√
was observed at 8.7μm. The detector shows promise in raising the operating temperature of DWELL detectors, thereby
enabling cheaper operation.
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.
A mid-infrared sensor is proposed in which an intersubband quantum-dot (QD) detector is integrated with an avalanche photodiode (APD) through a tunnel barrier. In the proposed three-terminal device, the applied biases of the QD and the APD are controlled separately; this feature permits the control of the QD's responsivity and dark current independently of the operational gain of the APD. It is shown theoretically that the proposed device can achieve a higher signal-to-noise ratio (SNR) over the QD detector without the APD component. Indeed, prior studies have revealed that although a heterostructure barrier lowers both the dark current and the photocurrent of the QD detector, the barrier has a greater impact on the dark current. Thus the dark-current-limited SNR is enhanced in the presence of the barrier. However, due to the reduced photocurrent, the SNR may not achieve its potential in the presence of Johnson noise, which may become dominant, for example, at low integration times or when detecting ultra-weak signals. In the proposed device, the APD component provides the necessary photocurrent gain required to elevate the SNR to the dark-current limit. This improvement, however, comes at a slight penalty in the SNR, due to the excess noise introduced by the APD. In this paper, guidelines for the SNR improvement are discussed in terms of the QD's operational bias voltage and the required APD gain. The higher SNR could be used to obtain a higher sensitivity at the same temperature, or to achieve a comparable performance at higher operating temperatures.