Quantum structures base on type-II In(Ga)Sb quantum dots (QDs) embedded in an InAs matrix were used as active
material for achieving long-wavelength infrared (LWIR) photodetectors in this work. Both InAs and In(Ga)Sb are
narrow band semiconductor materials and known to possess a large number of surface states, which apparently play
significant impact for the detector’s electrical and optical performance. These surface states are caused not only by
material or device processing induced defects but also by surface dangling bonds, oxides, roughness and contaminants.
To experimentally analyze the surface states of the QD structures treated by different device fabrication steps, atomic
force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray
photoelectron spectroscopy (XPS) measurements were performed. The results were used to optimize the fabrication
process of the LWIR photodetectors in our ongoing project. The dark current and its temperature dependence of the
fabricated IR photodetectors were characterized in temperature range 10 K to 300 K, and the experiment results were
analyzed by a theoretic modeling obtained using simulation tool MEDICI.
We report on the device characterization of In(Ga)Sb/InAs quantum dots (QDs) based photodetectors for long wave IR
detectors. The detection principle of these quantum-dot infrared photodetectors (QDIPs) is based on the spatially indirect
transition between the In(Ga)Sb QDs and the InAs matrix, as a result of the type-II band alignment. Such photodetectors
are expected to have lower dark currents and higher operating temperatures compared to the current state of the art InSb
and mercury cadmium telluride (MCT) technology.
The In(Ga)Sb QD structures were grown using metal-organic vapour-phase epitaxy and explored using structural,
electrical and optical characterization techniques. Material development resulted in obtaining photoluminescence up to
10 μm, which is the longest wavelength reported in this material system. We have fabricated different photovoltaic IR
detectors from the developed material that show absorption up to 8 μm. Photoresponse spectra, showing In(Ga)Sb QD
related absorption edge, were obtained up to 200 K. Detectors with different In(Ga)Sb QDs showing different cut-off
wavelengths were investigated for photoresponse. Photoresponse in these detectors is thermally activated with different
activation energies for devices with different cut-off wavelengths. Devices with longer cut-off wavelength exhibit higher
activation energies. We can interpret this using the energy band diagram of the dots/matrix system for different QD sizes.
Narrow bandgap semiconductors GaSb, InAs, and InSb are important building blocks for infrared photodetectors based
on type-II InSb quantum dots or an InAs/GaSb strained layer superlattice. Understanding the surface chemical
composition of these materials can provide valuable information that enables optimization of device surface passivation
techniques leading towards surface leakage free IR photodetectors. We report on an investigation into Ga-, In-, Sb-, and
As-oxides and other chemical species on the surface of untreated, dry etched and thermally treated GaSb, InAs and InSb
samples by x-ray photoelectron spectroscopy. The experimental results reveal the presence of Sb- and Ga-oxides on the
surfaces of the untreated and treated GaSb samples. Both Sb- and In-oxides were observed on the surface of all InSb
samples, and especially the dry etched sample had thicker oxide layers. In the case of the InAs samples, not only In- and
As-oxides XPS signals were obtained, but also AsCl species were found on the ICP dry etched sample. These results
helped to analyze the dark current of our fabricated IR detectors.
A mid wave infrared type-II superlattice focal plane array with 320x256 pixels, 30 μm pitch and 90 % fill factor was
fabricated in house, using a conventional homojunction p-i-n photodiode design and the ISC9705 readout circuit. High-quality
imaging up to 110 K is demonstrated with the substrate fully removed. The absorber is 2 μm thick, and no anti-reflection
coating was used, so there is still room for significant improvement of the quantum efficiency, which is in the
40 % range.
Studies of the dark current vs. temperature behavior indicate that the device is limited by Shockley-Read-Hall generation
from the depletion region. The activation energy of this dark current component is 0.13 eV, suggesting an unidentified
recombination center positioned halfway into the 0.24 eV bandgap.
Furthermore, we report on detectors with 100 % cut-off at 13 μm. The dark current density at 60 K and -50 mV bias is
2x10-4 A/cm2. Quantum efficiency, NETD and BLIP temperature are also calculated.
Position-sensitive photocurrent measurements on mesa-etched superlattice material were made at low temperatures using
a focused laser spot. The lateral diffusion length for holes was extracted and is reported.
We report on the optical and structural characterization of InSb QDs in InAs matrix, grown on InAs (100) substrates, for
infrared photodetection. InSb has 7% lattice mismatch with InAs forming strained QDs, which are promising for longwave
IR applications, due to their type-II band alignment. This report contains material development results of InSb QDs
for increasing their emission wavelength towards long-wave IR region. Samples were grown by two techniques of MBE
and MOVPE, with different InSb coverage on InAs (100) substrates. Structures grown by MBE reveal QD related
photoluminescence at 4 μm. AFM investigations of the MBE grown structures showed uncapped dots of ~ 35 nm in size
and ~ 3 nm in height, with a density of about 2 x 1010 cm-2. Cross-section TEM investigations of buried InSb layers
grown by MBE showed coherently strained QDs for nominal InSb coverage in the range of 1.6 - 2 monolayers (MLs).
Layers with InSb coverage more than 2MLs contain relaxed QDs with structural defects due to large amount of strain
between InSb and InAs. Samples with such large dots did not show any InSb related luminescence. The MOVPE grown
InSb samples exhibit a strong QD related emission between 3.8 to 7.5 μm, depending on the amount of InSb coverage
and other growth parameters. We report the longest wavelength observed so far in this material system.
Historically IRnova has exclusively been a company, focused on manufacturing of QWIP detectors. Nowadays, besides
continuous improvements of the performance of QWIP FPAs and development of new formats IRnova is involved in
development of QWIP detectors for special applications and has started the development of the next generation infrared
detectors, as well.
In the light of the development of new formats we validate experimentally theoretical calculations of the response of
QWIPs for smaller pixel size. These results allow for the development of high performance megapixel QWIP FPA that
exhibit the high uniformity and operability QWIP detectors are known for. QWIP is also being considered for space
applications. The requirements on dark current and operating temperature are however much more stringent as compared
to the terrestrial applications. We show ways to improve the material quality with as a result a higher detector operating
temperature.
IRnova is also looking at antimony-based strained superlattice material for the LWIR region together with partners at the
IMAGIC centre of excellence. One of the ways to overcome the problem with surface currents is passivating
overgrowth. We will report the status and results of overgrowing the detector mesas with AlGa(As)Sb in a MOVPE
system. At the same centre of excellence a novel material concept is being developed for LWIR detection. This new
material contains a superlattice of vertically aligned and electronically coupled InAs and GaSb quantum dots.
Simulations show that it should be possible to have LWIR detection in this material. We will present the current status
and report results in this research.
In this paper we introduce a canonical minimised adder graph (CMAG) representation that can easily be generated
with a computer. We show that this representation can be used to efficiently develop code generation for
MAG graphs. Several code optimizations methods are developed in the computation of the non-output fundamental
sum (NOFS) computation, which allows the computation of all graphs up to cost-5 be accomplished in
a reasonable timeframe.
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