425 research outputs found
Impact of alloy disorder on the band structure of compressively strained GaBiAs
The incorporation of bismuth (Bi) in GaAs results in a large reduction of the
band gap energy (E) accompanied with a large increase in the spin-orbit
splitting energy (), leading to the condition that
which is anticipated to reduce so-called CHSH Auger
recombination losses whereby the energy and momentum of a recombining
electron-hole pair is given to a second hole which is excited into the
spin-orbit band. We theoretically investigate the electronic structure of
experimentally grown GaBiAs samples on (100) GaAs substrates by
directly comparing our data with room temperature photo-modulated reflectance
(PR) measurements. Our atomistic theoretical calculations, in agreement with
the PR measurements, confirm that E is equal to for
9. We then theoretically probe the inhomogeneous
broadening of the interband transition energies as a function of the alloy
disorder. The broadening associated with spin-split-off transitions arises from
conventional alloy effects, while the behaviour of the heavy-hole transitions
can be well described using a valence band-anticrossing model. We show that for
the samples containing 8.5% and 10.4% Bi the difficulty in identifying a clear
light-hole-related transition energy from the measured PR data is due to the
significant broadening of the host matrix light-hole states as a result of the
presence of a large number of Bi resonant states in the same energy range and
disorder in the alloy. We further provide quantitative estimates of the impact
of supercell size and the assumed random distribution of Bi atoms on the
interband transition energies in GaBiAs. Our calculations support
a type-I band alignment at the GaBiAs/GaAs interface, consistent
with recent experimental findings
GaAs-based dilute bismide semiconductor lasers:Theory vs. experiment
We present a theoretical analysis of the electronic and optical properties of near-infrared dilute bismide quantum well (QW) lasers grown on GaAs substrates. Our theoretical model is based upon a 12-band k·p Hamiltonian which explicitly incorporates the strong Bi-induced modifications of the band structure in pseudomorphically strained GaBi x As 1-x alloys. We outline the impact of Bi on the gain characteristics of ideal GaBi x As 1-x /(Al)GaAs devices, compare the results of our theoretical calculations to experimental measurements of the spontaneous emission (SE) and optical gain - a first for this emerging material system - and demonstrate quantitative agreement between theory and experiment. Through our theoretical analysis we further demonstrate that this novel class of III-V semiconductor alloys has strong potential for the development of highly efficient GaAs-based semiconductor lasers which promise to deliver uncooled operation at 1.55 μm
Dilute bismide alloys grown on GaAs and InP substrates for improved near- and mid-infrared semiconductor lasers
We present an analysis of dilute bismide quantum well (QW) lasers grown on GaAs and InP substrates. Our theoretical analysis is based upon a 12-band k·p Hamiltonian which directly incorporates the strong impact of Bi incorporation on the band structure using a band-anticrossing approach. For GaBiAs QWs grown on GaAs we analyse the device performance as a function of Bi composition, and quantify the potential to use GaBiAs alloys to realise highly efficient, temperature stable 1.55 μm lasers. We compare our calculations to measured spontaneous emission (SE) and gain spectra for first-generation GaBiAs lasers and demonstrate quantitative agreement between theory and experiment. We also present a theoretical analysis of InGaBiAs alloys grown on InP substrates. We show that this material system is well suited to the development of mid-infrared lasers, and offers the potential to realise highly efficient InP-based diode lasers incorporating type-I QWs and emitting at > 3 μm. We quantify the theoretical performance of this new class of mid-infrared lasers, and identify optimised structures for emission across the application-rich 3 - 5 μm wavelength range. Our results highlight and quantify the potential of dilute bismide alloys to overcome several limitations associated with existing GaAs- and InP-based near- and mid-infrared laser technologies
Device architecture engineering: progress toward next generation perovskite solar cells
Over the past decade, perovskite solar cells (PSCs) have quickly established themselves as a promising technology boasting both high efficiency and low processing costs. The rapid development and success of PSCs is a product of substantial research effort addressing compositional engineering, thin film fabrication, surface passivation, and interfacial treatments. Recently, engineering of device architecture has entered a renaissance with the emergence of several new bulk and graded heterojunction structures. These structures promote a lateral approach to the development of single-junction PSCs affording new opportunities in light management, defect passivation, carrier extraction, and long-term stability. Following a short overview of the historic evolution of PSC architectures, a detailed discussion of the promising progress of the recently reported perovskite bulk heterojunction and graded heterojunction approaches are offered. To enable better understanding of these novel architectures, a range of approaches to characterizing the architectures are presented. Finally, an outlook and perspective are provided offering insights into the future development of PSC architecture engineering
Development of time-resolved photoluminescence microscopy of semiconductor materials and devices using a compressed sensing approach
Charge carrier lifetime is a key property of semiconductor materials for photonic applications. One of the most established methods for measuring lifetimes is time-resolved photoluminescence (TRPL), which is typically performed as a single-point measurement. In this paper, we demonstrate a new time-correlated single photon counting method (TCSPC) for TRPL microscopy, for which spatial information can be achieved without requiring point-by-point scanning through the use of a compressed sensing (CS) approach. This enables image acquisition with a single pixel detector for mapping the lifetime of semiconductors with high repeatability. The methodology for signal acquisition and image reconstruction was developed and tested through simulations. Effects of noise levels on the reliability and quality of image reconstruction were investigated. Finally, the method was implemented experimentally to demonstrate a proof-of-concept CS TCSPC imaging system for acquiring TRPL maps of semiconductor materials and devices. TRPL imaging results of a semiconductor device acquired using a CS approach are presented and compared with results of TRPL mapping of the same excitation area measured through a point-by-point method. The feasibility of the methodology is demonstrated, the benefits and challenges of the experimental prototype system are presented and discussed
Optical gain in GaAsBi/GaAs quantum well diode lasers
Electrically pumped GaAsBi/GaAs quantum well lasers are a promising new class of near-infrared devices where, by use of the unusual band structure properties of GaAsBi alloys, it is possible to suppress the dominant energy-consuming Auger recombination and inter-valence band absorption loss mechanisms, which greatly impact upon the device performance. Suppression of these loss mechanisms promises to lead to highly efficient, uncooled operation of telecommunications lasers, making GaAsBi system a strong candidate for the development of next-generation semiconductor lasers. In this report we present the first experimentally measured optical gain, absorption and spontaneous emission spectra for GaAsBi-based quantum well laser structures. We determine internal optical losses of 10–15 cm−1 and a peak modal gain of 24 cm−1, corresponding to a material gain of approximately 1500 cm−1 at a current density of 2 kA cm−2. To complement the experimental studies, a theoretical analysis of the spontaneous emission and optical gain spectra is presented, using a model based upon a 12-band k.p Hamiltonian for GaAsBi alloys. The results of our theoretical calculations are in excellent quantitative agreement with the experimental data, and together provide a powerful predictive capability for use in the design and optimisation of high efficiency lasers in the infrared
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
Children's Hospital Association Consensus Statements for Comorbidities of Childhood Obesity
Background: Childhood obesity and overweight affect approximately 30% of US children. Many of these children have obesity-related comorbidities, such as hypertension, dyslipidemia, fatty liver disease, diabetes, polycystic ovary syndrome (PCOS), sleep apnea, psychosocial problems, and others. These children need routine screening and, in many cases, treatment for these conditions. However, because primary care pediatric providers (PCPs) often are underequipped to deal with these comorbidities, they frequently refer these patients to subspecialists. However, as a result of the US pediatric subspecialist shortage and considering that 12.5 million children are obese, access to care by subspecialists is limited. The aim of this article is to provide accessible, user-friendly clinical consensus statements to facilitate the screening, interpretation of results, and early treatment for some of the most common childhood obesity comorbidities. Methods: Members of the Children's Hospital Association (formerly NACHRI) FOCUS on a Fitter Future II (FFFII), a collaboration of 25 US pediatric obesity centers, used a combination of the best available evidence and collective clinical experience to develop consensus statements for pediatric obesity-related comorbidities. FFFII also surveyed the participating pediatric obesity centers regarding their current practices. Results: The work group developed consensus statements for use in the evaluation and treatment of lipids, liver enzymes, and blood pressure abnormalities and PCOS in the child with overweight and obesity. The results of the FFFII survey illustrated the variability in the approach for initial evaluation and treatment as well as pattern of referrals to subspecialists among programs. Conclusions: The consensus statements presented in this article can be a useful tool for PCPs in the management and overall care of children with overweight and obesity.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140335/1/chi.2013.0120.pd
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