199 research outputs found
Theory of the electronic structure of dilute bismide and bismide-nitride alloys of GaAs: Tight-binding and k.p models
The addition of dilute concentrations of bismuth (Bi) into GaAs to form
GaBiAs alloys results in a large reduction of the band gap energy Eg
accompanied by a significant increase of the spin-orbit-splitting energy
(delta_SO), leading to an Eg < delta_SO regime for ~10% Bi composition which is
technologically relevant for the design of highly efficient photonic devices.
The quaternary alloy GaBiNAs offers further flexibility for band gap tuning,
because both nitrogen and bismuth can independently induce band gap reduction.
This work reports sp3s* tight binding and 14-band k.p models for the study of
the electronic structure of GaBiAs and GaBiNAs alloys. Our results are in good
agreement with the available experimental data.Comment: 2 pages, 1 figur
Hexagonal SixGe1-xas a direct-gap semiconductor
The band gap of germanium (Ge) is “weakly” indirect, with the L6c conduction band (CB) minimum lying only ≈150meV below the zone-center Γ7c CB edge in energy. This has stimulated significant interest in engineering the band structure of Ge, with the aim of realizing a direct-gap group-IV semiconductor compatible with established complementary metal-oxide-semiconductor fabrication and processing infrastructure. Recent advances in nanowire fabrication now allow growth of Ge in the metastable lonsdaleite (“hexagonal diamond”) phase, reproducibly and with high crystalline quality. In its lonsdaleite allotrope Ge is a direct- and narrow-gap semiconductor, in which the zone-center T8c CB minimum originates via back-folding of the L6c CB minimum of the conventional cubic (diamond) phase. Here, we analyze the electronic structure evolution in direct-gap lonsdaleite SixGe 1-x alloys from first principles, using a combination of alloy supercell calculations and zone unfolding. We confirm the Si composition range x≤ 25 % across which SixGe 1-x possesses a direct band gap, quantify the impact of alloy-induced band hybridization on the inter-band optical matrix elements, and describe qualitatively the consequences of the alloy band structure for carrier recombination
Theory of InGaBiAs dilute bismide alloys for highly efficient InP-based mid-infrared semiconductor lasers
Electronic structure of lonsdaleite SixGe1−x alloys
Conventional diamond-structured silicon (Si) and germanium (Ge) possess indirect fundamental band gaps, limiting their potential for applications in light-emitting devices. However, SixGe1-x alloys grown in the lonsdaleite ("hexagonal diamond") phase have recently emerged as a promising directgap, Si-compatible material system, with experimental measurements demonstrating strong room temperature photoluminescence. When grown in the lonsdaleite phase, Ge possesses a narrow (0:3 eV) "pseudo-direct"fundamental band gap. Lonsdaleite Si is indirect-gap (0:8 eV), creating the possibility to achieve direct-gap lonsdaleite SixGe1-x alloys across a Gerich composition range. We present a first principles analysis of the electronic and optical properties of lonsdaleite SixGe1-x alloys, elucidate the electronic structure evolution and direct-to indirect-gap transition, and describe the impact of alloy band mixing effects on inter-band optical transition strengths
Theory and optimisation of 1.3 and 1.55 μm (Al)InGaAs metamorphic quantum well lasers
The use of InGaAs metamorphic buffer layers (MBLs) to facilitate the growth of lattice-mismatched heterostructures constitutes an attractive approach to developing long-wavelength semiconductor lasers on GaAs substrates, since they offer the improved carrier and optical confinement associated with GaAs-based materials. We present a theoretical study of GaAs-based 1.3 and 1.55 μm (Al)InGaAs quantum well (QW) lasers grown on InGaAs MBLs. We demonstrate that optimised 1.3 μm metamorphic devices offer low threshold current densities and high differential gain, which compare favourably with InP-based devices. Overall, our analysis highlights and quantifies the potential of metamorphic QWs for the development of GaAs-based long-wavelength semiconductor lasers, and also provides guidelines for the design of optimised devices
Highly mismatched III–V semiconductor alloys applied in multiple quantum well photovoltaics
Adding dilute concentrations of nitrogen (N) or bismuth (Bi) into conventional III-V semiconductor alloys causes a large bowing of the bandgap energy due to the modification of the electronic band structure. This behaviour has attracted significant interest due to the resulting optical and electronic properties. Firstly, the authors present theoretical band structure models for GaAs-based dilute nitride, dilute bismide and dilute bismide-nitride alloys and then use them within current continuity equations to show the photovoltaic behaviour. To describe the band structures of these highly mismatched III-V semiconductor alloys, the authors introduce a 10-, 12and 14-band k · p Hamiltonian for dilute nitride, dilute bismide and dilute bismide-nitride semiconductors, respectively. The authors then use this approach to analyse GaBiAs multi-quantum well p-i-n structures for photovoltaic performance. Through theoretical analysis the authors can: (i) elucidate important trends in the properties and photovoltaic performance of GaBiAs QW structures and (ii) comment generally on the suitability of GaBiAs alloys and heterostructures for applications in multi-junction solar cells. In particular, the authors identify and quantify the limitations associated with current GaBiAs solar cells, and describe the improvements in performance that can be expected pending further development of this emerging class of devices
Theory and optimisation of radiative recombination in broken-gap InAs/GaSb superlattices
We present a theoretical analysis of mid-infrared radiative recombination in
InAs/GaSb superlattices (SLs). We employ a semi-analytical plane wave expansion
method in conjunction with an 8-band Hamiltonian
to compute the SL electronic structure, paying careful attention to the
identification and mitigation of spurious solutions. The calculated SL
eigenstates are used directly to compute spontaneous emission spectra and the
radiative recombination coefficient . We elucidate the origin of the
relatively large coefficients in InAs/GaSb SLs which, despite the presence
of spatially indirect (type-II-like) carrier confinement, are close to that of
bulk InAs and compare favourably to those calculated for mid-infrared type-I
pseudomorphic and metamorphic quantum well structures having comparable
emission wavelengths. Our analysis explicitly quantifies the roles played by
carrier localisation (specifically, partial delocalisation of bound electron
states) and miniband formation (specifically, miniband occupation and optical
selection rules) in determining the magnitude of and its temperature
dependence. We perform a high-throughput optimisation of the room temperature
coefficient in InAs/GaSb SLs across the 3.5 -- 7 m wavelength range,
quantifying the dependence of on the relative thickness of the
electron-confining InAs and hole-confining GaSb layers. This analysis provides
guidance for the growth of optimised SLs for mid-infrared light emitters. Our
results, combined with the expected low non-radiative Auger recombination rates
in structures having spatially indirect electron and hole confinement,
corroborate recently observed high output power in prototype InAs/GaSb SL
inter-band cascade light-emitting diodes.Comment: Published versio
Theory and design of InGaAsBi mid-infrared semiconductor lasers: type-I quantum wells for emission beyond 3 m on InP substrates
We present a theoretical analysis and optimisation of the properties and
performance of mid-infrared semiconductor lasers based on the dilute bismide
alloy InGaAsBi, grown on conventional (001) InP
substrates. The ability to independently vary the epitaxial strain and emission
wavelength in this quaternary alloy provides significant scope for band
structure engineering. Our calculations demonstrate that structures based on
compressively strained InGaAsBi quantum wells (QWs)
can readily achieve emission wavelengths in the 3 -- 5 m range, and that
these QWs have large type-I band offsets. As such, these structures have the
potential to overcome a number of limitations commonly associated with this
application-rich but technologically challenging wavelength range. By
considering structures having (i) fixed QW thickness and variable strain, and
(ii) fixed strain and variable QW thickness, we quantify key trends in the
properties and performance as functions of the alloy composition, structural
properties, and emission wavelength, and on this basis identify routes towards
the realisation of optimised devices for practical applications. Our analysis
suggests that simple laser structures -- incorporating
InGaAsBi QWs and unstrained ternary
InGaAs barriers -- which are compatible with established
epitaxial growth, provide a route to realising InP-based mid-infrared diode
lasers.Comment: Submitted versio
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