4.D IRG 2: Band-Engineered Antimonide and Bismide Materials
Faculty: Murphy (lead), Johnson, Santos, Sellers, Uchoa, Yang (OU), Salamo, Yu (UA), Borunda
(Oklahoma State University), 3 postdocs, 8 graduate students, 4 undergraduates.
Partners: University of Copenhagen (Denmark), Sandia National Laboratories, Tohoku University
(Japan), Amethyst Research Inc., Trinity College (Ireland).
Focus: To engineer the unique properties of bismide and antimonide materials for new types of
optoelectronic and photovoltaic devices, and to enable transport studies of topological solids.
Motivation: The insatiable worldwide demand for energy has created an acute need for devices
operating in the infrared. Advanced infrared devices are vitally essential both to more efficiently convert
the solar spectrum and waste heat to useable power, and to more efficiently operate as emitters and
detectors in applications for pollution sensing, process control and health and safety. Creative materials
engineering of antimonides and bismides provides a promising route to practicable devices by permitting
precise band offset and bandgap tuning over critical energy windows inaccessible in other systems.
This IRG will leverage pioneering design development of novel infrared devices (Yang) and full
knowledge of constraints for commercially viable devices (Sellers) with two world-class growth efforts
(Santos, Salamo, Yu) and a full complement of characterization and fabrication tools (Johnson).
Moreover, antimony- and bismuth-containing materials are central to the currently intense effort in
topological materials. These materials support spin-polarized, topologically-protected states which are
key elements for potentially revolutionary technologies. Hence the IRG’s growth expertise naturally
supports a complementary effort in topological transport studies (Murphy, Copenhagen, Japan)
supported with band-engineering (Ireland) and local theoretical support (Uchoa, Borunda).
Long Term Research Goals: The long term goals of IRG2 are:
 Harness the inherent potential of Sb- and Bi-containing semiconductors for infrared optoelectronics.
 Realize 3rd generation solar cells and thermophotovoltaics that exploit Type-II band offsets.
 Fully develop Sb- and Bi-containing semiconductors as model materials for studying electron transport
in topological-insulator materials and structures.
Planned Research Activities: Despite their significant potential for both fundamental physics and
applied research, there are surprisingly few groups worldwide that can grow antimonides. There are even
fewer who study dilute bismides, with no high-performance device demonstrations reported to date.
Below we relate specific planned research activities, grouped according to the three research goals above.
1. Antimonides and Bismides for Infrared Optoelectronics
There is intense demand for lasers and detectors operating in the infrared (IR) region, fueled by the
need for reliable and inexpensive hydrocarbon and general gas sensors. The market is global and rapidly
expanding, based on increasing concerns over issues such as pollution, health and safety, as well as in
industrial process control. The flexibility of the antimonides and bismides provides two complementary
avenues to address the need: interband cascade (IC) structures and the under-utlitized dilute bismides.
Interband Cascade Lasers: The IC structure, was originally proposed by YangD31 and is now the
technology of choice for semiconductor laser applications in the 3-4 m region.D32 IC lasers take
advantage of cascaded carrier transport for high quantum efficiency by using interband (valence to
conduction) tunneling facilitated by the unique variety of band alignments available in the
InAs/GaSb/AlSb material system. The alignments allow electron cascading by enabling formation of
multilayers for electron blocking, hole blocking, and interband tunneling. The optical transitions occur
across the broken band-gap type-II alignment in composite InAs/GaxIn1-xSb quantum wells (QWs) which
can have a bandgap smaller than for either component layer. Because InAs, GaSb, and AlSb are closely
lattice matched, an IC structure can be strain balanced to a GaSb or InAs substrate despite having
hundreds of layers. A schematic illustration of the layer structure is shown in Figure D5. Yang’s
established history with IC lasers places IRG2 at the forefront for this technology. He has developed an
extensive band structure calculation for InAs/GaSb/AlSb structures with excellent predictive powers due
to extensive feedback from the growth of numerous IC laser structures.
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Yang, Santos and Johnson are exploring new
waveguide architectures to extend the wavelength
band-gap
region for sensing important gases such as CO2
blocking
(4.3 m), CO (4.6 m), and SO2 (7.4 m). The
Interband
early stage devices already exhibit the lowest
tunneling
threshold-current densities compared with all mide IR semiconductor lasers in the region beyond 6
type-II band
m. The team’s goal is to extend the IC laser
alignment
operating range to the atmospheric transmission
window at 8-12 m, while maintaining low
threshold current densities.D33 In principle,
Fig. D5 Illustration of an IC laser.
incorporation of dilute amounts of Bi in the typeII wells (InAs1-yBiy/GaxIn1-xSb) is a promising route to longer wavelengths. Fortunately, Salamo and Yu
are experts in the growth of bismides. Additionally the performance at mid-IR wavelengths would be
improved by using type-I GaSb1-xBix wells. Development of GaSb1-xBix and InAs1-yBiy materials will be
an early focus of IRG2 with a goal of enhanced interband cascade devices, including detectors.
Temperature insensitive GaAsBi/GaAs QW lasers at 1.55 µm: InGaAsP/InP based laser diodes serve as
the workhorse for the telecommunication industry. The material system suffers from a high Auger
recombination loss enhanced by a resonance between the bandgap and the spin-obit splitting energy.
Therefore the laser has poor temperature stability and a thermoelectric cooler is required for a stable
operating temperature. Following the initial temperature-insensitive laser concept,D34 Yu and Salamo
propose to use GaAsBi/GaAs materials for making 1.55 µm lasers with reduced Auger losses due to a
spin-obit splitting energy that is much larger than the bandgap.D35 (See Fig. D6 for growth results) The
temperature insensitivity will eliminate the need for cooling and significantly simplify the system design.
Such devices are attractive for use with Si photonics for optical inter-connect applications. Low
temperature growth below 400°C is CMOS compatible and thus monolithic integration is feasible.
Obtaining light emission at 1.55 µm on a GaAs substrate will also enable leveraging of the mature
Vertical Cavity Surface Emitting Lasers (VCSEL) technology.
Bismide based infrared detectors: InSb is already an important IR detector material for thermal imaging
at ~5 µm. Adding Bi to InSb is predicted to further reduce the bandgap into the 8-12 µm range, but the
defect density is too high for detector applications when grown on conventional substrates. Santos and Yu
propose to grow free-standing InSbBi on a floating Si micro-template. Yu has recently developed a
monolithic integrated compliant substrate technique.D36 By using a low-temperature metal induced
crystallization technique, a single domain Si template with a size up to 200200 µm2 can be created by
converting amorphous Si into single crystalline Si on an arbitrary substrate. A subsequent epilayer can
accommodate large strain through lateral sliding along the substrate instead of through defect formation.
Additional advantages are that the material can be grown on insulating substrates and scaled up to a 2D
array. Because a bismide alloy is expected to have a low Auger recombination rate, we will also study
the reverse process, impact ionization, which is critical for detectors such as avalanche photodiodes.
Interband Cascade Infrared Detectors: We also propose to develop a new IR detector concept whose
proof-of-principle has only recently been demonstrated. Like IC lasers, IC infrared photodetectors
(ICIPs) can cover a wide wavelength spectrum by merely adjusting QW thickness (~3 to 15 m). ICIPs
are based on an entirely new operation principle that combines advantages of very fast intersubband
relaxation and interband tunneling for carrier transport and relatively slow interband transitions.D37
Unlike photodetectors based on intersubband transitions, ICIPs have a desirable polarization choice and
are sensitive to normal incidence radiation. Carrier transport/collection in the ICIP is insensitive to
background doping and material quality variations in contrast to conventional p-i-n junction photodiodes.
Because doping is unnecessary in ICIPs, the Shockley-Reed-Hall generation current (that usually
dominates in conventional photodiodes) is suppressed. Hence, the development of ICIPs by Yang,
Santos and Johnson will provide opportunities to investigate new phenomena associated with the
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underlying physics of IC structures and devices, and lead to low noise photodetectors that are able to
operate at elevated temperatures with high performance and fast response.
2. Third Generation Solar Cells and Thermophotovoltaic Devices
Efficient technologies for renewable energy address society’s pressing need for abundant, cheap
energy. The bandgaps and band alignments of the antimonides and bismides provide three complementary
avenues to improved energy conversion technologies:
Interband
Cascade
Thermophotovoltaic
Cells:
Thermophotovoltaic (TPV) systems are an attractive
technology that converts the otherwise-wasted radiant
energy from a heat source (at ~1000K) into useful
electrical energy. There is a surprisingly limited effort in
advancing TPV cell materials even though their efficiency
remains far below the predicted theoretical limit.D39 The
ICIP structure can be modified to operate as a TPV cell in a
similar way as a conventional p-n junction can be used as a
photodetector or PV cell. With multiple absorbers
connected in series and each individual absorber being
shorter than the diffusion length, radiant photons can be
effectively absorbed and photo-generated carriers can be
Fig. D6 Cross-sectional TEM image
collected more efficiently without the diffusion length
indicating smooth interfaces, and a growth
limitation. When the connected absorbers have different
temperature profile. A strong photobandgaps, a broader portion of the emitted spectrum can be
luminescence signal for the GaAsBi QW
captured. These quantum-engineered materials are a unique
indicated high material quality for both the
and promising approach to high-performance TPVs.
QW and low-temperature AlGaAs barrier.
Bi-N solar cells for next generation multi-junctions:
Currently the most viable technology for increasing the conversion efficiency of solar cells above 50%,
are multi-junction systems. Current triple junction technology based on Ge/GaAs/GaInP is, however,
limited to ~ 30%. GaInNAs has long been a candidate for a fourth junction. Despite recent success, these
materials still suffer from poor lifetimes, reproducibility, and yield. Quality should be improved by using
bismuth as a surfactant during GaInNAs growth. In addition, GaInNAsBi (absorbing at 1eV) can be
lattice matched to GaAs, increasing the efficiency of a quadruple-junction above 50%. Sellers, in
collaboration with Santos and Johnson, will leverage his experience in GaInNAs to investigate the
potential of bismuth containing alloys. The bismide growth experience of Salamo and Yu will be an asset
for the challenge of growing quaternary and quintinary compounds. The combination of N and Bi in a
single MBE system is available to few groups worldwide.
Sb structures for third-generation solar cells: Intermediate-bandD40 (IB) and multi-exciton generation
(MEG) are two mechanisms that have been proposed for third-generation solar cells, but have not yet
been realized. Specific bandgaps and offsets are required to realize each of these concepts. Quantum-dot
(QD) structures are candidates to form an IB within a single-gap material, but carrier extraction is a
limiting factor for InAs QDs in a GaAs matrix. Sellers and Santos propose to use an alternative matrix
material, GaAs1-xSbx (x~0.12) that is predicted to have no valence band offset with the InAs QDs, which
should greatly aid carrier extraction.D41 They also propose to use InAs/AlAs1-xSbx QDs on InP substrates
for MEG structures to overcome present issues related to the absorption of high-energy photons prior to
reaching the QD layers. While carrier extraction remains challenging, a pre-existing resonant tunneling
structureD42 and the small valence band degeneracy indicate these issues are not insurmountable. A
modified version of Yang’s band structure calculations for IC structures will guide the design of a full QD
multi-layer structure. The practical implementation of QD systems in IB or MEG solar cells would have a
substantial impact on the power efficiency in commercially viable devices.
3. Model Materials for Topological-Insulator Structures.
Interest in topological insulators (TIs) has exploded since they were first predicted in 2005,D43 with an
average of more than a preprint per day added to the condensed matter archive. These insulators fall into a
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different topological class than ordinary insulators; the two classes are distinguished by distinct
topological invariants (Z2). As a result, interfaces between topological and ordinary insulators (or
vacuum) must support ungapped metallic states within which the topological invariant can smoothly
evolve between the distinct insulating regimes. While ‘topological insulator’ is an accurate descriptive
name, it is the metallic interface/edge states that have spawned the tremendous interest. Spin-orbit
coupling which is time-reversal invariant, is the underlying mechanism for the unique topology of the TIs
and as a result the metallic interface states are helical (orthogonal spin and momentum) in addition to
being fundamentally protected from elastic backscattering. Moreover the surface states are predicted to
support Majorana fermions that have relevance for fault-tolerant quantum computing.
Carrier transport experiments should reveal a wealth of topological effects, but improvements in the
materials quality are needed to better suppress unwanted conduction through the bulk. Based on our
demonstrated proficiency in the growth of antimonides and bismides, Borunda, Murphy, Santos,
Salamo, Uchoa and Yu have targeted the following TI systems for growth and study:
Sb QWs for 3D TIs: Shortly after the prediction of TIs,
the topological surface states of bulk Sb were revealed
via ARPES,D44 however as bulk Sb is semimetallic,
transport is dominated by the topologically trivial bulk.
Band structure calculations predict that quantum
confinement in thin films (~10 nm) opens a gap,D45
changing the film from a semi-metal to an indirect gap
Fig. D7 Phase diagram showing the predicted
semiconductor. (See Fig. D7) Recently, Murphy and
topological states of thin Sb films, as a
Santos have exploited the near perfect lattice-match
function of film thickness.D45
between Sb and GaSb(111) to fabricate smooth and
continuous ultrathin Sb layers in which quantum confinement suppresses the bulk conductivity by
400.D46 They observed linear magneto-resistance to 18T and strong weak anti-localization consistent
with a 3D TI. To reveal the unambiguous hallmarks of the topological surface states, they propose
quantum interference studiesD47 of Sb nanowires fabricated lithographically from the planar samples. This
provides complete geometric control in contrast to the existing measurements on TI ribbons and flakes.
The appeal of Sb as model TI lies in its simple stoichometry, its inherent compatibility with pre-existing
high mobility electronic systems and the potential for superconducting topological states under modest
pressure.D48 They will be guided by band structure calculations as a function of thickness and interfacial
bonds by collaborators at Trinity College. Additionally Uchoa will analyze the nature of the many-body
ground state as the system is tuned though the TI phase and Borunda will model spin and charge
transport in these systems for making comparison between theory and experiment.
Bi2Te3 and Sb2Te3: Yu and Salamo have recently improved film quality when Bi2Te3 and Sb2Te3 films
are grown epitaxially on non-vicinal GaAs (111).D49 Unintentionally doping Sb2Te3 films are p-type
whereas Bi2Te3 films are n-type, creating the potential for a topological p-n junction, which may offer a
platform to realize an exciton condensate at the interface. We also propose to explore optoelectronic
device applications of TIs, including a surface state “laser” in which the electron-hole transition on the
surface directly couples to and amplifies ambipolar plasmon oscillations and a far-IR detector in which
the surface absorbs light and the bulk material serves as a high-efficiency thermo-electric cooler.
InAs/GaSb and InSb QWs for Majorana Fermion realization: We propose to build on our expertise on
InSb QWs and InAs/GaSb multilayers by collaborating with international experts. With Hirayama
(Japan), an expert on nanodevice processing, we will study spin interactions between electrons and atoms
in InSb QWs. Borunda, Uchoa and Marcus (Copenhagen), an expert on nanodevice processing and
quantum information processing systems, will explore the physics that emerges from combining strong
spin-orbit coupling, quantum confinement, and superconducting proximity effects.
Summary: IRG2 offers an integrated design-growth approach to bismide and antimonide materials with
the potential for both technologically relevant advances in infrared devices and for exciting breakthroughs
in fundamental science.
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