Impact of modal gain and waveguide design on two-state lasing in quantum well-dot lasers.

We study the current-controlled lasing switching from the ground state (GS) to the excited state (ES) transition in broad-area (stripe width 100 µm) InGaAs/GaAs quantum well-dot (QWD) and quantum well (QW) lasers. In the lasers with one QWD layer and a 0.45 µm-thick GaAs waveguide, pure GS lasing takes place up to an injection current as high as 8 A (40 kA/cm).

Instantaneous growth of single monolayers as the origin of spontaneous core-shell InGaN nanowires with bright red photoluminescence.

Increasing the InN content in the InGaN compound is paramount for optoelectronic applications. It has been demonstrated in homogeneous nanowires or deliberately grown nanowire heterostructures. Here, we present spontaneous core-shell InGaN nanowires grown by molecular beam epitaxy on Si substrates at 625 °C.

Nucleation-Limited Kinetics of GaAs Nanostructures Grown by Selective Area Epitaxy: Implications for Shape Engineering in Optoelectronics Devices.

The growth kinetics of vertical III-V nanowires (NWs) were clarified long ago. The increasing aspect ratio of NWs results in an increase in the surface area, which, in turn, enhances the material collection. The group III adatom diffusion from the NW sidewalls to the top sustains a superlinear growth regime.

Interplay of Kinetic and Thermodynamic Factors in the Stationary Composition of Vapor-Liquid-Solid IIIVV Nanowires.

Compositional control over vapor-liquid-solid III-V ternary nanowires based on group V intermix (VLS IIIVV NWs) is complicated by the presence of a catalyst droplet with extremely low and hence undetectable concentrations of group V atoms. The liquid-solid and vapor-solid distributions of IIIVV NWs at a given temperature are influenced by the kinetic parameters (supersaturation and diffusion coefficients in liquid, V/III flux ratio in vapor), temperature and thermodynamic constants. We analyze the interplay of the kinetic and thermodynamic factors influencing the compositions of VLS IIIVV NWs and derive a new vapor-solid distribution that contains only one parameter of liquid, the ratio of the diffusion coefficients of dissimilar group V atoms.

Self-Consistent Model for the Compositional Profiles in Vapor-Liquid-Solid III-V Nanowire Heterostructures Based on Group V Interchange.

Due to the very efficient relaxation of elastic stress on strain-free sidewalls, III-V nanowires offer almost unlimited possibilities for bandgap engineering in nanowire heterostructures by using material combinations that are attainable in epilayers. However, axial nanowire heterostructures grown using the vapor-liquid-solid method often suffer from the reservoir effect in a catalyst droplet. Control over the interfacial abruptness in nanowire heterostructures based on the group V interchange is more difficult than for group-III-based materials, because the low concentrations of highly volatile group V atoms cannot be measured after or during growth.

Improved power and temperature performance of half-disk diode microlasers.

The power and temperature characteristics of Ø200 µm half-disk microlasers with a half-ring metal contact and high-density InGaAs/GaAs quantum dots are studied. In a continuous wave (CW) mode, the maximal optical power at 20°C was 134 mW, and the maximal CW lasing temperature reached 113°C. In a pulsed regime the maximal optical power of 1.

Control of Ge island coalescence for the formation of nanowires on silicon.

Germanium nanowires could be the building blocks of hole-spin qubit quantum computers. Selective area epitaxy enables the direct integration of Ge nanowires on a silicon chip while controlling the device design, density, and scalability. For this to become a reality, it is essential to understand and control the initial stages of the epitaxy process.

Circumventing the Uncertainties of the Liquid Phase in the Compositional Control of VLS III-V Ternary Nanowires Based on Group V Intermix.

Control over the composition of III-V ternary nanowires grown by the vapor-liquid-solid (VLS) method is essential for bandgap engineering in such nanomaterials and for the fabrication of functional nanowire heterostructures for a variety of applications. From the fundamental viewpoint, III-V ternary nanowires based on group V intermix (InSbAs, InPAs, GaPAs and many others) present the most difficult case, because the concentrations of highly volatile group V atoms in a catalyst droplet are beyond the detection limit of any characterization technique and therefore principally unknown. Here, we present a model for the vapor-solid distribution of such nanowires, which fully circumvents the uncertainties that remained in the theory so far, and we link the nanowire composition to the well-controlled parameters of vapor.

Can Nanowires Coalesce?

Coalescence of nanowires and other three-dimensional structures into continuous film is desirable for growing low-dislocation-density III-nitride and III-V materials on lattice-mismatched substrates; this is also interesting from a fundamental viewpoint. Here, we develop a growth model for vertical nanowires which, under rather general assumptions on the solid-like coalescence process within the Kolmogorov crystallization theory, results in a morphological diagram for the asymptotic coverage of a substrate surface. The coverage is presented as a function of two variables: the material collection efficiency on the top nanowire facet a and the normalized surface diffusion flux of adatoms from the NW sidewalls b.

Geometrical Selection of GaN Nanowires Grown by Plasma-Assisted MBE on Polycrystalline ZrN Layers.

GaN nanowires grown on metal substrates have attracted increasing interest for a wide range of applications. Herein, we report GaN nanowires grown by plasma-assisted molecular beam epitaxy on thin polycrystalline ZrN buffer layers, sputtered onto Si(111) substrates. The nanowire orientation was studied by X-ray diffraction and scanning electron microscopy, and then described within a model as a function of the Ga beam angle, nanowire tilt angle, and substrate rotation.

Composition of Vapor-Liquid-Solid III-V Ternary Nanowires Based on Group-III Intermix.

Compositional control in III-V ternary nanowires grown by the vapor-liquid-solid method is essential for bandgap engineering and the design of functional nanowire nano-heterostructures. Herein, we present rather general theoretical considerations and derive explicit forms of the stationary vapor-solid and liquid-solid distributions of vapor-liquid-solid III-V ternary nanowires based on group-III intermix. It is shown that the vapor-solid distribution of such nanowires is kinetically controlled, while the liquid-solid distribution is in equilibrium or nucleation-limited.

From Layer-by-Layer Growth to Nanoridge Formation: Selective Area Epitaxy of GaAs by MOVPE.

Selective area epitaxy at the nanoscale enables fabrication of high-quality nanostructures in regular arrays with predefined geometry. Here, we investigate the growth mechanisms of GaAs nanoridges on GaAs (100) substrates in selective area trenches by metal-organic vapor-phase epitaxy (MOVPE). It is found that pre-growth annealing results in the formation of valley-like structures of GaAs with atomic terraces inside the trenches.

An Overview of Modeling Approaches for Compositional Control in III-V Ternary Nanowires.

Modeling of the growth process is required for the synthesis of III-V ternary nanowires with controllable composition. Consequently, new theoretical approaches for the description of epitaxial growth and the related chemical composition of III-V ternary nanowires based on group III or group V intermix were recently developed. In this review, we present and discuss existing modeling strategies for the stationary compositions of III-V ternary nanowires and try to systematize and link them in a general perspective.

Modeling Catalyst-Free Growth of III-V Nanowires: Empirical and Rigorous Approaches.

Catalyst-free growth of III-V and III-nitride nanowires (NWs) by the self-induced nucleation mechanism or selective area growth (SAG) on different substrates, including Si, show great promise for monolithic integration of III-V optoelectronics with Si electronic platform. The morphological design of NW ensembles requires advanced growth modeling, which is much less developed for catalyst-free NWs compared to vapor-liquid-solid (VLS) NWs of the same materials. Herein, we present an empirical approach for modeling simultaneous axial and radial growths of untapered catalyst-free III-V NWs and compare it to the rigorous approach based on the stationary diffusion equations for different populations of group III adatoms.

Tuning the Liquid-Vapour Interface of VLS Epitaxy for Creating Novel Semiconductor Nanostructures.

Controlling the morphology and composition of semiconductor nano- and micro-structures is crucial for fundamental studies and applications. Here, Si-Ge semiconductor nanostructures were fabricated using photolithographically defined micro-crucibles on Si substrates. Interestingly, the nanostructure morphology and composition of these structures are strongly dependent on the size of the liquid-vapour interface (i.

Kinetic modeling of interfacial abruptness in axial nanowire heterostructures.

Kinetic modeling of the formation of axial III-V nanowire heterostructures grown by the Au-catalyzed vapor-liquid-solid method is presented. The method is based on a combination of kinetic growth theory for different binaries at the liquid-solid interface and thermodynamics of ternary liquid and solid alloys. Non-stationary treatment of the compositional change obtained by swapping material fluxes allows us to compute the interfacial abruptness across nanowire heterostructures and leads to the following results.

Criterion for Selective Area Growth of III-V Nanowires.

A model for the nucleation of vertical or planar III-V nanowires (NWs) in selective area growth (SAG) on masked substrates with regular arrays of openings is developed. The optimal SAG zone, with NW nucleation within the openings and the absence of parasitic III-V crystallites or group III droplets on the mask, is established, taking into account the minimum chemical potential of the III-V pairs required for nucleation on different surfaces, and the surface diffusion of the group III adatoms. The SAG maps are plotted in terms of the material fluxes versus the temperature.

Theory of MOCVD Growth of III-V Nanowires on Patterned Substrates.

An analytic model for III-V nanowire growth by metal organic chemical vapor deposition (MOCVD) in regular arrays on patterned substrates is presented. The model accounts for some new features that, to the author's knowledge, have not yet been considered. It is shown that MOCVD growth is influenced by an additional current into the nanowires originating from group III atoms reflected from an inert substrate and the upper limit for the group III current per nanowire given by the total group III flow and the array pitch.

Selective area epitaxy of GaAs: the unintuitive role of feature size and pitch.

Selective area epitaxy (SAE) provides the path for scalable fabrication of semiconductor nanostructures in a device-compatible configuration. In the current paradigm, SAE is understood as localized epitaxy, and is modelled by combining planar and self-assembled nanowire growth mechanisms. Here we use GaAs SAE as a model system to provide a different perspective.

Selective Area Epitaxy of GaN Nanowires on Si Substrates Using Microsphere Lithography: Experiment and Theory.

GaN nanowires were grown using selective area plasma-assisted molecular beam epitaxy on SiO/Si(111) substrates patterned with microsphere lithography. For the first time, the temperature-Ga/N flux ratio map was established for selective area epitaxy of GaN nanowires. It is shown that the growth selectivity for GaN nanowires without any parasitic growth on a silica mask can be obtained in a relatively narrow range of substrate temperatures and Ga/N flux ratios.

Modeling the Radial Growth of Self-Catalyzed III-V Nanowires.

A new model for the radial growth of self-catalyzed III-V nanowires on different substrates is presented, which describes the nanowire morphological evolution without any free parameters. The model takes into account the re-emission of group III atoms from a mask surface and the shadowing effect in directional deposition techniques such as molecular beam epitaxy. It is shown that radial growth is faster for larger pitches of regular nanowire arrays or lower surface density, and can be suppressed by increasing the V/III flux ratio or decreasing re-emission.

Theory of MBE Growth of Nanowires on Adsorbing Substrates: The Role of the Shadowing Effect on the Diffusion Transport.

A new model for nanowire growth by molecular beam epitaxy is proposed which extends the earlier approaches treating an isolated nanowire to the case of ensembles of nanowires. I consider an adsorbing substrate on which the arriving growth species (group III adatoms for III-V nanowires) may diffuse to the nanowire base and subsequently to the top without desorption. Analytical solution for the nanowire length evolution at a constant radius shows that the shadowing of the substrate surface is efficient and affects the growth kinetics from the very beginning of growth in dense enough ensembles of nanowires.