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 1.2×1011 cm.Hz1/2/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 4×1011 cm.Hz1/2 W-1 (+3V, f /2 optics) at 77 K and 7.4×108 cm.Hz1/2 W-1 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.
We report on surface passivation studies for type-II InAs/GaSb superlattice (SL) PIN detectors designed to
operate in the mid-wave infrared (MWIR) region and the long wavelength infrared (LWIR) spectrum. The two SL
structures were grown by molecular beam epitaxy and processed into mesa diodes using standard lithography. A simple
spin on photoresist, SU-8, was used to passivate the sample after a wet etch. Optical and electrical measurements were
then undertaken on the two devices. The dark current density of a single pixel device with SU-8 passivation is reduced
by four orders of magnitude and by a factor of eight compared to devices without any passivation for the MWIR and
LWIR pin detectors, respectively, at 77K.
We report on surface passivation using SU-8 for type-II InAs/GaSb strained layers superlattice (SLS) detectors
with a PIN design operating in mid-wave infrared (MWIR) spectral region (λ50% cut-off ~ 4.4 μm). Material growth and
characterization, single pixel device fabrication and testing, as well as focal plane array (FPA) processing are described.
High quality strain-balanced SLS material with FWHM of 1st SLS satellite peak of 36 arcsec is demonstrated. The
electrical and optical performance of devices passivated with SU-8 are reported and compared with those of unpassivated
devices. The dark current density of a single pixel device with SU-8 passivation showed four orders of magnitude
reduction compared to the device without any passivation. At 77K, the zero-bias responsivity and detectivity are equal to
1.1 A/W and 4 x 1012 Jones at 4μm, respectively, for the SU-8 passivated test pixel on the 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.
The InAs/GaSb Type II strained layer superlattice (SLS) is promising III-V material system for infrared (IR) devices due to the ability to engineer its bandgap between 3-30 μm and potentially have many advantages over current technologies such as high uniformity smaller leakage current due to reduced Auger recombination which are crucial for large IR focal plane arrays. However, an issue with this material system is that it relies on growth on GaSb substrates. These substrates are significantly more expensive than silicon, used for HgCdTe detectors, lower quality and are only available commercially as 3" diameters. Moreover it has to go through thinning down before it could be hybridized to readout integrated circuits. GaAs substrate is a possible alternative. We report on growth and characterisation of Type-II InAs/GaSb SLS photodiodes grown on GaAs substrates for mid-wave infrared with peak responses of 3.5 μm at 77K and 4.1 μm at 295K. Comparisons with similar structure grown on GaSb substrates show similar structural, optical and electrical characteristics. Broadening of X-ray rocking curves were observed on the structure grown on GaAs substrate. A full width half maximum (FWMH) of 25.2 arc sec. for the superlattice was observed near ~30.4 degree for the structure on GaSb substrate compared to near ~30.4 degree for structure grown on GaAs. However peak responsivity values of ~ 1.9 A/W and ~ 0.7 A/W were measured at 77K and 295K for devices grown on GaAs substrate. Room temperature responsivity suggests that these photodiodes are promising as high temperature IR detectors.
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 In0.15Ga0.85As and GaAs wells with a Al0.1Ga0.9As barrier, as opposed to In0.15Ga0.85As 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 6.7×1010 cm.√
Hz/W
was observed at 8.7μm. The detector shows promise in raising the operating temperature of DWELL detectors, thereby
enabling cheaper operation.
The development of type-II InAs/(In,Ga)Sb superlattice (SL) detectors with nBn design for single-color and
dual-color operation in MWIR and LWIR spectral regions are discussed. First, a 320 x 256 focal plane array (FPA) with
cutoff wavelength of 4.2 μm at 77K with average value of dark current density equal to 1 x 10-7 A/cm2 at Vb=0.7V (77
K) is reported. FPA reveals NEDT values of 23.8 mK for 16.3 ms integration time and f/4 optics. At 77K, the peak
responsivity and detectivity of FPA are estimated, respectively, to be 1.5 A/W and 6.4 x 1011 Jones, at 4 μm. Next,
implementation of the nBn concept on design of SL LWIR detectors is presented. The fabrication of single element nBn
based long wave infrared (LWIR ) with λc ~ 8.0 μm at Vb = +0.9 V and T = 100K detectors are reported. The bias
dependent polarity can be exploited to obtain two color response (λc1 ~ 3.5 μm and λc2 ~ 8.0 μm) under different polarity
of applied bias. The design and fabrication of this two color detector is presented. The dual band response (λc1 ~ 4.5 μm
and λc2 ~ 8 μm) is achieved by changing the polarity of applied bias. The spectral response cutoff wavelength shifts
from MWIR to LWIR when the applied bias voltage varies within a very small bias range (~100 mV).
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.
We report Quantum Dot Infrared Detectors (QDIP) where light coupling to the self assembled quantum dots
is achieved through plasmons occurring at the metal-semiconductor interface. The detector structure consists
of an asymmetric InAs/InGaAs/GaAs dots-in-a-well (DWELL) structure and a thick layer of GaAs sandwiched
between two highly doped n-GaAs contact layers, grown on a semi-insulating GaAs substrate. The aperture of
the detector is covered with a thin metallic layer which along with the dielectric layer confines light in the vertical
direction. Sub-wavelength two-dimensional periodic patterns etched in the metallic layer covering the aperture
of the detector and the active region creates a micro-cavity that concentrate light in the active region leading
to intersubband transitions between states in the dot and the ones in the well. The sidewalls of the detector
were also covered with metal to ensure that there is no leakage of light into the active region other than through
the metal covered aperture. An enhanced spectral response when compared to the normal DWELL detector
is obtained despite the absence of any aperture in the detector. The spectral response measurements show
that the Long Wave InfraRed (LWIR) region is enhanced when compared to the Mid Wave InfraRed (MWIR)
region. This may be due to coupling of light into the active region by plasmons that are excited at the metal-semiconductor
interface. The patterned metal-dielectric layers act as an optical resonator thereby enhancing the
coupling efficiency of light into the active region at the specified frequency. The concept of plasmon-assisted
coupling is in principle technology agnostic and can be easily integrated into present day infrared sensors.
Type-II InAs/GaSb superlattice photodiodes for mid-IR (3-5μm) region grown by solid-source
molecular beam epitaxy are reported. Different approaches for realization of high quality interfaces
between compositionally abrupt GaSb and InAs layers during the growth of the SLs are discussed.
Mid wave infrared (&lgr;c~ 4.5 µm at T=300K) P-on-N designs of SLs detectors were developed to
ensure compatibility with most present day readout integrated circuits (ROICs). Variable size diode
arrays were fabricated using standard photolithography technique and hybridized to silicon fanout
chip. The sizes of the detector mesas were varied from 29μm x 29μm to 804μm x 804μm. The
single pixel characterization was undertaken at Santa Barbara Focal Plane. Temperature-dependent
IV measurements revealed dark current density below 1 x 10-8 A/cm2 at 82K and below 2 x 10-5
A/cm2 at 240K. (Vbias = 0V). Dynamic resistance-area product at zero bias was found to be ~ 1 x 105
Ωcm2 at 82K and 0.24 Ωcm2 at 240K. Influence of protective silicon nitride coating on reduction
surface leakage currents of detectors was investigated. We found that rsurface was equal to ~ 3 x 106
Ωcm indicating the proper surface preparation followed by room temperature Si3N4 deposition is
effective in reduction of leakage currents in type-II MWIR InAs/GaSb superlattice photodiodes.
A systematic investigation of AgxSb2(1–x)Te3(1–x) (x=0.16, 0.18, and 0.20) is reported. The optical properties of the material were studied using a UV spectrophotometer. The optical behavior of a material is generally utilized to determine its optical constants such as reflectance, transmittance, and optical band gap E0. The value of E0 is estimated for these samples by measuring the absorption coefficient as a function of the wavelength of light in the range 300 to 1400 nm. Films of different Ag content were studied both before and after annealing. It was concluded that this material is a candidate for phase change optical memory.
A systematic investigation of glass forming tendency and crystallization kinetics of the Agx-Sb2(1–x)-Te3(1–x) (x=0.16, 0.18, and 0.20) is reported. The alloy of the Ag-Sb-Te glass system, obtained by the rapid quenching technique, is characterized by calorimetric measurements and different thermal analysis for different heating rates. Variation in two important parameters, namely, glass transition temperature Tg and glass forming tendency Kgl, are studied with the variation of Ag content. By analyzing the results that occur with the increase in Ag content, we see that the glass-forming tendency Kgl also increases. However, with the increase in heating rates, the glass transition helps us to find the suitability of an alloy to be used in phase change optical memories/switches. The results are analyzed using Kissinger's equation for the nonisothermal crystallization of materials.
A systematic investigation of the Ag x -Sb 2 (1-x) -Te 3 (1-x) (x=0.16, 0.18 and 0.20) is reported. The alloy of Ag-Sb-Te glass system, obtained by rapid quenching technique has been characterized by calorimetric measurements and differential thermal analysis (DTA) for different heating rates. The optical properties of the material were studied using UV spectrophotometer. The optical band gap (E o) was estimated for these samples by measuring absorption coefficient as a function of wavelength of light in the range 300 -900 nm. The films of the different compositions of Ag were studied for both the cases of before and after annealing. The present study is an attempt to understand the effect of alloying Ag into amorphous Te chalcogenide glasses.
Over recent years the demand for optical data storage devices with high speed has become increasingly more evident. Phase change optical storage is based on the rapid crystalline to amorphous (and vice versa) transition in a thin phase change layer enabled by laser induced heating. Among some of the potential candidates, AgSbTe alloy appears to be one of the latest promising materials that have drawn worldwide attention. Using this material as the active layer has other advantages such as the problem of material flow is reduced to a great extent. Moreover the marks written in AgSbTe based media have a well defined shape with sharp edges, leading to intrinsically lower jitter values than observed for GeSbTe based media. In the present work Ag x-Sb 2 (1-x)-Te 3 (1-x) alloys and films are developed for different composition. The present work describes the systematic study of thermal, structural and optical properties of amorphous Ag-Sb-Te system
Ag-Sb-Te alloy and films are developed as Optical recording material based on amorphous - crystalline phase transformation. The crystallization process of Ag-Sb-Te films is systematically studied through measurement of recording characteristics to solve the trade off problem between data stability and erasing sensitivity. Phase change optical recording disks have been found to demonstrate long thermal stability of the amorphous recording marks. In the present work, preparation and characterization of the chalcogenide alloy Ag x - Sb 2(1-x) - Te3(1-x) with different composition (x = 0.16, 0.18 and 0.20) has been presented. Samples were prepared using melt quenching technique and the films were grown by thermal evaporation system. The thermal Characterization of Ag-Sb-Te material was studied using differential thermal analysis (DTA) and Optical Characterization (Transmittance and reflectance) respectively. The films were studied for both cases: before and after annealing. The Differential thermal analysis curves were recorded for different compositions and glass transition temperature (Tg), crystallization temperature (Tc) and melting temperature (Tm) have been obtained. It may also be concluded that Tg/Tm ratio is closer to required condition for the phase change optical data storage material. Thermal and optical Characterization shows that the Ag-Sb-Te material is a potential candidate for phase change optical memory application. The optimized composition has also been obtained.
For phase change optical data storage, several chalcogenide-based materials have been reported and are expected to replace the conventional magnetic disk. In the present work, preparation and characterization of the chalcogenide allow Agx - Sb 2(1-x) - Te 3(1-x) with different composition (x = 0.16, 0.18 and 0.20) has been presented. Samples were prepared using melt quenching technique and the films were grown by thermal evaporation system. The crystallization process of Ag- Sb- Te material was studied using differential thermal analysis (DTA) and Optical analysis (Transmittance and reflectance) respectively. The films were studied for both cases: before and after annealing. The Differential thermal analysis curves were recorded for different compositions and Glass transition temperature (Tg), crystallization temperature (Tc) and melting temperature (Tm) have been obtained. It may also be concluded that Tg/Tm ratio is closer to required condition for phase change optical data storage material. Thermal and optical analysis shows that the Ag - Sb- Te material is a potential candidate for phase change optical data storage. The optimized composition has also been obtained.
Ag-Sb-Te alloy and films are developed as Optical recording material based on amorphous-crystalline phase transformation. The crystallization process of Ag-Sb-Te films is systematically studied through measurement of recording characteristics to solve the trade off problem between data stability and erasing sensitivity. Phase change optical recording disks have been found to demonstrate long thermal stability of the amorphous recording marks. In the present work, preparation and characterization of the chalcogenide allow Agx - Sb2(1-x) - Te3(1-x) with different composition (xequals0.16, 0.18 and 0.20) has been presented. Samples were prepared using melt quenching technique and the films were grown by thermal evaporation system. The crystallization process of Ag-Sb-Te material was studied using differential thermal analysis (DTA) and Optical analysis (Transmittance and reflectance) respectively. The films were studied for both cases: before and after annealing. The Differential thermal analysis curves were recorded for different compositions and Glass transition temperature (Tg), crystallization temperature (Tc) and melting temperature (Tm) have been obtained. It may also be concluded that Tg/Tm ratio is closer to required condition for the phase change optical data storage material. Thermal and optical analysis shows that the Ag-Sb-Te material is a potential candidate for phase change optical data storage. The optimized composition has also been obtained.
Phase change optical recording disks using chalcogenide alloy Ag-Sb-Te have been found to demonstrate long thermal stability of the amorphous recording marks. The crystallization process and nature of Ag-Sb-Te material were studied using Differential Thermal Analysis (DTA) and X Ray Diffraction (XRD) respectively. The films were studied for both the cases: before and after annealing and it was concluded that the alloy (Ag-Gb-Te) can be used as a phase change optical memory material.
Phase change optical recording disks using have been found to demonstrate long thermal stability of the amorphous recording marks. The thermal analysis of Ag-Sb-Te material was studied using DTA and structural analysis of the material were studied by x-ray diffraction, SEM and TEM respectively. The films were studied for both the cases: before and after annealing and it was concluded that the alloy could be used as a phase change optical memory material.
Phase change optical recording disks using have been found to demonstrate long stability of the amorphous recording marks. Structural analysis of the material were studied by X Ray Diffraction (XRD), Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) respectively. The films were studied for both the cases: before and after annealing and it was concluded that the alloy (Ag-Sb-Te) could be used as a phase change optical memory material.
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