Impact of defects on photoexcited carrier relaxation dynamics in GeSn thin films.

We report the results of a study that was conducted to investigate the recombination paths of photoexcited charge carriers in GeSn thin films. The charge carrier lifetime was predicted as a function of temperature from a description of photoconductivity transients, assuming co-influence of Shockley-Read-Hall and radiative carrier recombination paths. We identify that dislocations are the source of a band of electronic states with the highest occupied state at E V + (85÷90) meV that acts as Shockley-Read-Hall centers determining the charge carrier lifetime. The photoluminescence (PL) and photoconductivity spectroscopy have been applied to distinguish between the contribution of both band-to-band and dislocation-related electron transitions. The PL band was found to demonstrate a low-energy shift of about 80 ± 20 meV relative to the edge of the photoconductivity spectra in the indirect bandgap GeSn films with dislocations. The role of a different nature deeper acceptor level at E V + (140 ÷ 160) meV in the recombination processes of the GeSn layers with better structural quality and the Sn content higher than 4% was discussed. This detailed understanding of the recombination processes is of critical importance for developing GeSn/Ge-based optoelectronic devices.

Charge carrier relaxation in InGaAs-GaAs quantum wire modulation-doped heterostructures.

The time dependencies of the carrier relaxation in modulation-doped InGaAs-GaAs low-dimensional structures with quantum wires have been studied as functions of temperature and light excitation levels. The photoconductivity (PC) relaxation follows a stretched exponent with decay constant, which depends on the morphology of InGaAs epitaxial layers, presence of deep traps, and energy disorder due to inhomogeneous distribution of size and composition. A hopping model, where electron tunnels between bands of localized states, gives appropriate interpretation for temperature-independent PC decay across the temperature range 150-290 K. At low temperatures (T < 150 K), multiple trapping-retrapping via 1D states of InGaAs quantum wires (QWRs), sub-bands of two-dimensional electron gas of modulation-doped n-GaAs spacers, as well as defect states in the GaAs environment are the dominant relaxation mechanism. The PC and photoluminescence transients for samples with different morphologies of the InGaAs nanostructures are compared. The relaxation rates are found to be largely dependent on energy disorder due to inhomogeneous distribution of strain, nanostructure size and composition, and piezoelectric fields in and around nanostructures, which have a strong impact on efficiency of carrier exchange between bands of the InGaAs QWRs, GaAs spacers, or wetting layers; presence of local electric fields; and deep traps.

Surface effects of vapour-liquid-solid driven Bi surface droplets formed during molecular-beam-epitaxy of GaAsBi.

Herein we investigate a (001)-oriented GaAs1-xBix/GaAs structure possessing Bi surface droplets capable of catalysing the formation of nanostructures during Bi-rich growth, through the vapour-liquid-solid mechanism. Specifically, self-aligned "nanotracks" are found to exist trailing the Bi droplets on the sample surface. Through cross-sectional high-resolution transmission electron microscopy the nanotracks are revealed to in fact be elevated above surface by the formation of a subsurface planar nanowire, a structure initiated mid-way through the molecular-beam-epitaxy growth and embedded into the epilayer, via epitaxial overgrowth. Electron microscopy studies also yield the morphological, structural, and chemical properties of the nanostructures. Through a combination of Bi determination methods the compositional profile of the film is shown to be graded and inhomogeneous. Furthermore, the coherent and pure zincblende phase property of the film is detailed. Optical characterisation of features on the sample surface is carried out using polarised micro-Raman and micro-photoluminescence spectroscopies. The important light producing properties of the surface nanostructures are investigated through pump intensity-dependent micro-PL measurements, whereby relatively large local inhomogeneities are revealed to exist on the epitaxial surface for important optical parameters. We conclude that such surface effects must be considered when designing and fabricating optical devices based on GaAsBi alloys.

Optical Properties of a Quantum Dot-Ring System Grown Using Droplet Epitaxy.

Electronic and optical properties of InAs/GaAs nanostructures grown by the droplet epitaxy method are studied. Carrier states were determined by k · p theory including effects of strain and In gradient concentration for a model geometry. Wavefunctions are highly localized in the dots. Coulomb and exchange interactions are studied and we found the system is in the strong confinement regime. Microphotoluminescence spectra and lifetimes were calculated and compared with measurements performed on a set of quantum rings in a single sample. Some features of spectra are in good agreement.

Mechanism of periodic height variations along self-aligned VLS-grown planar nanostructures.

In this study we report in-plane nanotracks produced by molecular-beam-epitaxy (MBE) exhibiting lateral self-assembly and unusual periodic and out-of-phase height variations across their growth axes. The nanotracks are synthesized using bismuth segregation on the GaAsBi epitaxial surface, which results in metallic liquid droplets capable of catalyzing GaAsBi nanotrack growth via the vapor-liquid-solid (VLS) mechanism. A detailed examination of the nanotrack morphologies is carried out employing a combination of scanning electron and atomic force microscopy and, based on the findings, a geometric model of nanotrack growth during MBE is developed. Our results indicate diffusion and shadowing effects play significant roles in defining the interesting nanotrack shape. The unique periodicity of our lateral nanotracks originates from a rotating nucleation "hot spot" at the edge of the liquid-solid interface, a feature caused by the relative periodic circling of the non-normal ion beam flux incident on the sample surface, inside the MBE chamber. We point out that such a concept is divergent from current models of crawling mode growth kinetics and conclude that these effects may be utilized in the design and assembly of planar nanostructures with controlled non-monotonous structure.

Raman scattering reveals strong LO-phonon-hole-plasmon coupling in nominally undoped GaAsBi: optical determination of carrier concentration.

We report room-temperature Raman scattering studies of nominally undoped (100) GaAs1−xBix epitaxial layers exhibiting Bi-induced (p-type) longitudinal-optical-plasmon-coupled (LOPC) modes for 0.018 ≤ x ≤ 0.048. Redshifts in the GaAs-like optical modes due to alloying are evaluated and are paralleled by strong damping of the LOPC. The relative integrated Raman intensities of LO(Γ) and LOPC ALO/ALOPC are characteristic of heavily doped p-GaAs, with a remarkable near total screening of the LO(Γ) phonon (ALO/ALOPC → 0) for larger Bi concentrations. A method of spectral analysis is set out which yields estimates of hole concentrations in excess of 5×1017cm−3 and correlates with the Bi molar fraction. These findings are in general agreement with recent electrical transport measurements performed on the alloy, and while the absolute size of the hole concentrations differ, likely origins for the discrepancy are discussed. We conclude that the damped LO-phonon-hole-plasmon coupling phenomena plays a dominant role in Raman scattering from unpassivated nominally undoped GaAsBi.

Mechanism of strain-influenced quantum well thickness reduction in GaN/AlN short-period superlattices.

We report on the mechanism of strain-influenced quantum well (QW) thickness reduction in GaN/AlN short-period superlattices grown by plasma-assisted molecular beam epitaxy. Density functional theory was used to support the idea of a thermally activated exchange mechanism between Al adatoms and Ga surface atoms that is influenced by the strain state of the GaN QWs. These ab initio calculations support our experimentally observed reduction in QW thickness for different intrinsic strains.

Giant up-conversion efficiency of InGaAs quantum dots in a planar microcavity.

Self-assembled InGaAs quantum dots (QDs) were fabricated inside a planar microcavity with two vertical cavity modes. This allowed us to excite the QDs coupled to one of the vertical cavity modes through two propagating cavity modes to study their down- and up-converted photoluminescence (PL). The up-converted PL increased continuously with the increasing temperature, reaching an intensity level comparable to that of the down-converted PL at ~120 K. This giant efficiency in the up-converted PL of InGaAs QDs was enhanced by about 2 orders of magnitude with respect to a similar structure without cavity. We tentatively explain the enhanced up-converted signal as a direct consequence of the modified spontaneous emission properties of the QDs in the microcavity, combined with the phonon absorption and emission effects.

Effects of AlGaAs cladding layers on the luminescence of GaAs/GaAs1-xBix/GaAs heterostructures.

The structural and optical properties of GaAs1-xBix quantum wells (QWs) symmetrically clad by GaAs barriers with and without additional confining AlGaAs layers are studied. It is shown that a GaAs/GaAs1-xBix/GaAs QW with x ~ 4% and well width of ~ 4 nm grown by molecular beam epitaxy demonstrates efficient photoluminescence (PL) that becomes significantly more thermally stable when a cladding AlGaAs layer is added to the QW structure. The PL behavior for temperatures between 10 and 300 K and for excitation intensities varying by seven orders of magnitude can be well described in terms of the dynamics of excitons including carrier capture in the QW layer, thermal emission and diffusion into the cladding barriers. Understanding the role of these processes in the luminescence of dilute GaAs1-xBix QW structures facilitates the creation of highly efficient devices with reduced thermal sensitivity and low threshold current.

Strong passivation effects on the properties of an InAs surface quantum dot hybrid structure.

We report on an InAs quantum dot (QD) hybrid structure with a top surface QD layer coupled to two buried QD layers that is highly sensitive to surface passivation. After 180 min of passivation, the photoluminescence (PL) peak of the surface QDs shifts from 1545 to 1275 nm while its intensity decreases by one order of magnitude. Time-resolved PL reveals a significant decrease of carrier tunneling between the QD layers because of the surface state modification by chemical treatment. A simple model with rate equations is used to explain the observed optical performance. Our results show that the optical performance of this hybrid structure is very sensitive to the surface environment, making it a potential candidate for sensing applications.

Optical evidence of a quantum well channel in low temperature molecular beam epitaxy grown Ga(AsBi)/GaAs nanostructure.

A Ga(AsBi) quantum well (QW) with Bi content reaching 6% and well width of 11 nm embedded in GaAs is grown by molecular beam epitaxy at low temperature and studied by means of high-resolution x-ray diffraction, photoluminescence (PL), and time-resolved PL. It is shown that for this growth regime, the QW is coherently strained to the substrate with a low dislocation density. The low temperature PL demonstrates a comparatively narrow excitonic linewidth of ∼ 40 meV. For high excitation density distinct QW excited states evolve in the emission spectra. The origins of peculiar PL dependences on temperature and excitation density are interpreted in terms of intra-well optical transitions.

Anisotropic Confinement, Electronic Coupling and Strain Induced Effects Detected by Valence-Band Anisotropy in Self-Assembled Quantum Dots.

A method to determine the effects of the geometry and lateral ordering on the electronic properties of an array of one-dimensional self-assembled quantum dots is discussed. A model that takes into account the valence-band anisotropic effective masses and strain effects must be used to describe the behavior of the photoluminescence emission, proposed as a clean tool for the characterization of dot anisotropy and/or inter-dot coupling. Under special growth conditions, such as substrate temperature and Arsenic background, 1D chains of In0.4Ga0.6 As quantum dots were grown by molecular beam epitaxy. Grazing-incidence X-ray diffraction measurements directly evidence the strong strain anisotropy due to the formation of quantum dot chains, probed by polarization-resolved low-temperature photoluminescence. The results are in fair good agreement with the proposed model.

Band alignment tailoring of InAs1-xSbx/GaAs quantum dots: control of type I to type II transition.

We report the growth of InAs(1-x)Sb(x) self-assembled quantum dots (QDs) on GaAs (100) by molecular beam epitaxy. The optical properties of the QDs are investigated by photoluminescence (PL) and time-resolved photoluminescence (TRPL). A type I to type II band alignment transition is demonstrated by both power-dependent PL and TRPL in InAs(1-x)Sb(x) QD samples with increased Sb beam flux. Results are compared to an eight-band strain-dependent k x p model incorporating detailed QD structure and alloy composition. The calculations show that the conduction band offset of InAs(1-x)Sb(x)/GaAs can be continuously tuned from 0 to 500 meV and a flat conduction band alignment exists when 60% Sb is incorporated into the QDs. Our study offers the possibility of tailoring the band structure of GaAs based InAsSb QDs and opens up new means for device applications.

Aharonov-Bohm interference in neutral excitons: effects of built-in electric fields.

We report a comprehensive discussion of quantum interference effects due to the finite structure of neutral excitons in quantum rings and their first experimental corroboration observed in the optical recombinations. The signatures of built-in electric fields and temperature on quantum interference are demonstrated by theoretical models that describe the modulation of the interference pattern and confirmed by complementary experimental procedures.

Observation of PT-symmetry breaking in complex optical potentials.

In 1998, Bender and Boettcher found that a wide class of Hamiltonians, even though non-Hermitian, can still exhibit entirely real spectra provided that they obey parity-time requirements or PT symmetry. Here we demonstrate experimentally passive PT-symmetry breaking within the realm of optics. This phase transition leads to a loss induced optical transparency in specially designed pseudo-Hermitian guiding potentials.

Size and density control of In droplets at near room temperatures.

We report on the ability to control the size and density of In droplets on GaAs(100) substrates at near room temperatures using solid source molecular beam epitaxy. We specifically demonstrate the height, diameter and density control of In droplets as functions of substrate temperature (T(sub)) and monolayer (ML) coverage. For a range of density (approximately 10(9)-10(10) cm(-2)), the growth window is revealed to be between 20 and 70 degrees C. For a fixed ML coverage, the size and density of droplets can be controlled by controlling the T(sub). For a fixed T(sub), by controlling the ML coverage, droplet size and density can be controlled. Even at near room temperatures (20-70 degrees C), In atoms are extremely sensitive to surface diffusion and this enables the control of the size and density of droplets. This study provides an aid to understanding the formation of In droplets at near room temperatures and can find applications in the formation of quantum structures and/or nanostructures based on droplet epitaxy.

Spectroscopy of shallow InAs/InP quantum wire nanostructures.

A comprehensive investigation of the optical properties of InAs/InP(001) quantum wires (QWrs) and their parent quantum well system formed by the deposition of 4 ML (monolayers) of InAs on InP is carried out by means of temperature dependent photoluminescence (PL) and Fourier transform infrared spectroscopy. Unusual two-branch switching of the excitonic PL band maxima is revealed in the temperature dependence for both wires and wells. This is interpreted in terms of the thermal activation of excitonic ground states of the confined nanostructures. Strong modification of the absorbance line shape leading to the appearance of flat spectral regions in the room temperature spectrum of a QWr sample is interpreted in terms of thermally induced change of the dimensionality: from 1D to anisotropic 2D. This change of dimensionality is detected also in the polarized absorbance measurements through the disappearance or significant reduction of the polarization anisotropy in the regions of the hh1-e1 (hh: heavy hole; e: electron) and lh1-e1 (lh: light hole) transitions in QWrs.

InAs quantum dot clusters grown on GaAs droplet templates: surface morphologies and optical properties.

GaAs nano-mounds formed by droplet epitaxy are used as templates for growth of self-assembled InAs quantum dot clusters (QDCs). These QDCs are found to contain an average of thirteen dots per cluster, of which there are two families of different sized quantum dots. Excitation intensity-dependent photoluminescence (PL) demonstrates that there is no lateral coupling between the two different size quantum dots. Lateral transfer of carriers is observed between different size quantum dots due to thermal activation as seen in their different temperature-dependent optical behaviors.

Composite droplets: evolution of InGa and AlGa alloys on GaAs(100).

We present a comparative study for the evolution of utilizing indium gallium (InGa) and aluminum gallium (AlGa) alloys fabricated on GaAs(100) by means of simultaneous and sequential droplet formation. The composite alloys reported using the sequential approach lack the ability to precisely determine the final alloy composition as well as consistency in the density of the droplets. Further, the composition of the InGa alloy is not uniform, as seen by the size distribution using an atomic force microscope (AFM). Although this approach may be acceptable for materials with similar surface kinetics, as in the case of AlGa, it is not acceptable for InGa. This investigation reveals that the simultaneous approach for fabricating composite alloys is the optimum approach for producing InGa alloys with better control on composition for plasmonic applications such as plasmonic waveguides.

Improved photoluminescence efficiency of patterned quantum dots incorporating a dots-in-the-well structure.

InAs quantum dots embedded in InGaAs quantum well (DWELL: dots-in-the-well) structures grown on nanopatterned GaAs pyramids and planar GaAs(001) surface are comparatively investigated. Photoluminescence (PL) measurements demonstrate that the DWELL structure grown on the GaAs pyramids exhibits a broad quantum well PL band (full width at half-maximum ∼ 90 meV) and a higher quantum dot emission efficiency than the DWELL structure grown on the planar GaAs(001) substrate. These properties are attributed to the InGaAs quantum well with distributed thickness profile on the faceted GaAs pyramids, which introduces a tapered energy band structure and enhances carrier capture into the quantum dots.