InGaAs Nanostructures Research Articles

Manipulating formation of different InGaAs/GaAs nanostructures via tailoring As4 flux

This research provides a flexible approach to manipulate formation of InGaAs nanostructures on the GaAs (100) surface by varying arsenic (As4) beam equivalent pressure (BEP). By selecting the As4/(In+Ga) BEP ratio to be 4, 8, 20, 50 and 100, we were able to obtain different quantum structures from quantum well (QW) to quantum dots (QDs), then to spatially ordered quantum dot chains (QD-chains), and finally to quantum wires (QWRs), respectively. This transformation of nanostructures was explained by anisotropic surface diffusion coupled with the strain relieving Stranski–Krastanov growth mode, while the anisotropy was modulated by increasing As4 flux and subsequently enhanced by multilayer-stacking growth with a suitable spacer thickness. Photoluminescence characteristics show correlation to the nanostructure morphology for each sample. In particular, the formation of QD-chains and QWRs results in anisotropic features that offer potential device applications.

Numerical study of valence band states evolution in AlxGa1-xAs [111] QDs systems

Quantum dots (QDs) are very attractive nanostructures from an application point of view due to their unique optical properties. Optical properties and valence band (VB) state character was numerically investigated with respect to the effects of nanostructure geometry and composition. Numerical simulation was carried out using the Luttinger–Kohn model adapted to the particular case of QDs in inverted pyramids. We present the source code of the 4-band Luttinger–Kohn model that can be used to model AlGaAs or InGaAs nanostructures. The work focuses on the optical properties of GaAs/AlGaAs [111] QDs and quantum dot molecules (QDMs). We examine the dependence of Ground State (GS) optical properties on the structural parameters and predict optimal parameters of the QD/QDM systems to achieve dynamic control of GS polarization by an applied electric field.

Highly uniform and symmetric epitaxial InAs quantum dots embedded inside Indium droplet etched nanoholes

III−V semiconductor quantum dots (QDs) obtained by local droplet etching technology provide a material platform for generation of non-classic light. However, using this technique to fabricate single emitters for a broad spectral range remains a significant challenge. Herein, we successfully extend the QD emission wavelength to 850–880 nm via highly uniform and symmetric InAs QDs located inside indium-droplet-etching nanoholes. The evolution of InGaAs nanostructures by high temperature indium droplet epitaxy on GaAs substrate is revealed. By carefully designing the appropriate growth conditions, symmetric QDs with the a small fine structure splitting of only ∼4.4 ± 0.8 μeV are demonstrated. Averaging over the emission energies of 32 QDs, an ensemble broadening of 12 meV is observed. Individual QDs are shown to emit nonclassically with clear evidence of photon antibunching. These highly uniform and symmetric nanostructures represent a very promising novel strategy for quantum information applications.

Buffer free InGaAs quantum well and in-plane nanostructures on InP grown by atomic hydrogen assisted MBE

We investigate the optical and electrical properties of InGaAs thin films and nanostructures grown directly on an InP semi-insulating substrate without any buffer layer using atomic hydrogen assisted molecular beam epitaxy. We confirm the positive influence of the atomic hydrogen flux during the deoxidization process as well as during the growth itself improving the photoluminescence properties of InGaAs quantum wells. We also study the effect of the atomic hydrogen flux on the electrical properties of buffer free undoped, Te or Si doped InGaAs epilayers. Eventually, we demonstrate that atomic hydrogen flux can be used to achieve InGaAs growth selectivity with respect to a SiO2 mask for a growth temperature as low as 470 °C and obtain in-plane InGaAs nanostructures on InP with good transport properties.

Rutherford backscattering spectrometry analysis of InGaAs nanostructures

In this work, the authors demonstrate that Rutherford backscattering spectrometry (RBS) can be extended from a metrology concept applied to blanket films toward a method to analyze confined nanostructures. By a combination of measurements on an ensemble of devices and extensive simulations, it is feasible to quantify the composition of InGaAs nanostructures (16–50 nm) embedded periodically in an SiO2 matrix. The methodology is based on measuring multiple fins simultaneously while using the geometrical shape of the structures, obtained from a transmission electron microscopy analysis, as input for a multitude of trajectory calculations. In this way, the authors are able to reproduce the RBS spectra and to demonstrate the sensitivity of the RBS spectra to the quantitative elemental composition of the nanostructures and to variations of their shape and mean areal coverage down to one nanometer. Thus, the authors establish RBS as a viable quantitative characterization technique to probe the composition and structure of periodic arrays of nanostructures.

(Invited) Band Gap Engineering and Optoelectronic Performance of Indium Gallium Arsenide Nanobelts

Low dimensional III-V compound semiconductors, such as GaAs, InAs, and their ternary alloys, exhibit unique optical and electronic properties. Their abilities of efficient carrier transport, light emission, and detection means they can be used in integrated transistors, lasers, light-emitting diodes, and detectors for infrared communications [1-3]. Very recently, intense research has been directed toward the fabrication and application of the III-V nanoplates/sheets owing to their realization of multiterminal electronic devices such as Hall bars, quantum point contacts, and so forth. However, the influence of growth condition on the formation of these nanostructures is still unclear. Furthermore, these nanoplates/sheets generally suffer from small and irregular morphology, demonstrating the difficulties in the controlled growth. Importantly, realizing the continuously tunable composition and heterojunction of these nanostructures remains great challenges, particularly for the technologically important InGaAs. It's well known that InGaAs is of great interest for high-performance device in electronics and optoelectronics [4,5]. Unfortunately, the transistors and photodetectors based on low dimensional InGaAs nanostructures still need improvement due to their low on/off ratio, poor photoresponsivity, and so on. To address these problems, we have realized the controlled growth of high-quality single-crystal InGaAs nanobelts with fully tunable composition/bandgap via an orientation selective vapor growth. Density functional theory calculations reveal that the surface energy variation induced by the chemical potential of arsenic (μ As) is critical for the formation of these nanobelts. When configured into transistors, the InGaAs nanobelt devices exhibit impressive electrical properties with the turn on/off ratio of 2×107, better than any value reported for a InGaAs nanowire device to date. The broadband phototransistor (covering 405-1,550 nm) were further fabricated with an ultrahigh photoresponsivity of >8000 A/W and an external quantum efficiency of >106 %, both of which are much higher than those of all the reported InGaAs nanowire photodetectors. These results suggest that the novel InGaAs nanobelts may open up new opportunities for various applications in integrated electronics, optoelectronics, and quantum electronics.

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.

Ultralow Surface Recombination Velocity in Passivated InGaAs/InP Nanopillars.

The III–V semiconductor InGaAs is a key material for photonics because it provides optical emission and absorption in the 1.55 μm telecommunication wavelength window. However, InGaAs suffers from pronounced nonradiative effects associated with its surface states, which affect the performance of nanophotonic devices for optical interconnects, namely nanolasers and nanodetectors. This work reports the strong suppression of surface recombination of undoped InGaAs/InP nanostructured semiconductor pillars using a combination of ammonium sulfide, (NH4)2S, chemical treatment and silicon oxide, SiOx, coating. An 80-fold enhancement in the photoluminescence (PL) intensity of submicrometer pillars at a wavelength of 1550 nm is observed as compared with the unpassivated nanopillars. The PL decay time of ∼0.3 μm wide square nanopillars is dramatically increased from ∼100 ps to ∼25 ns after sulfur treatment and SiOx coating. The extremely long lifetimes reported here, to our knowledge the highest reported to date for undoped InGaAs nanostructures, are associated with a record-low surface recombination velocity of ∼260 cm/s. We also conclusively show that the SiOx capping layer plays an active role in the passivation. These results are crucial for the future development of high-performance nanoscale optoelectronic devices for applications in energy-efficient data optical links, single-photon sensing, and photovoltaics.

Indium segregation during III–V quantum wire and quantum dot formation on patterned substrates

We report a model for metalorganic vapor-phase epitaxy on non-planar substrates, specifically V-grooves and pyramidal recesses, which we apply to the growth of InGaAs nanostructures. This model—based on a set of coupled reaction-diffusion equations, one for each facet in the system—accounts for the facet-dependence of all kinetic processes (e.g., precursor decomposition, adatom diffusion, and adatom lifetimes) and has been previously applied to account for the temperature-, concentration-, and temporal-dependence of AlGaAs nanostructures on GaAs (111)B surfaces with V-grooves and pyramidal recesses. In the present study, the growth of In0.12Ga0.88As quantum wires at the bottom of V-grooves is used to determine a set of optimized kinetic parameters. Based on these parameters, we have modeled the growth of In0.25Ga0.75As nanostructures formed in pyramidal site-controlled quantum-dot systems, successfully producing a qualitative explanation for the temperature-dependence of their optical properties, which have been reported in previous studies. Finally, we present scanning electron and cross-sectional atomic force microscopy images which show previously unreported facetting at the bottom of the pyramidal recesses that allow quantum dot formation.

Multilayers of InGaAs Nanostructures Grown on GaAs(210) Substrates

Multilayers of InGaAs nanostructures are grown on GaAs(210) by molecular beam epitaxy. With reducing the thickness of GaAs interlayer spacer, a transition from InGaAs quantum dashes to arrow-like nanostructures is observed by atomic force microscopy. Photoluminescence measurements reveal all the samples of different spacers with good optical properties. By adjusting the InGaAs coverage, both one-dimensional and two-dimensional lateral ordering of InGaAs/GaAs(210) nanostructures are achieved.

Self‐assembled InGaAs tandem nanostructures consisting of a hole and pyramid on GaAs (311)A by droplet epitaxy

AbstractWe report on the fabrication self‐assembled tandem (pyramidal‐ holed) InGaAs nanostructures on GaAs (311)A in comparison with the nanostructures on (100) surface. Under an identical growth condition, the fabricated nanostructures are characteristically dissimilar; ring‐shaped nanostructures on GaAs (100) and pyramidal‐holed nanostructures on (311)A. The underlying formation mechanism of these interesting nanostructures can be understood in terms of intermixing, dissolution of GaAs substrate and surface diffusion driven by a surface reconstruction. The size and density of nanostructures can be controlled by modifying the droplet size, density, and thermal energy applied during the fabrication of droplets and nanostructures. These unique InGaAs nanostructures can offer promising applications in optoelectronics.

Various Quantum- and Nano-Structures by III-V Droplet Epitaxy on GaAs Substrates.

We report on various self-assembled In(Ga)As nanostructures by droplet epitaxy on GaAs substrates using molecular beam epitaxy. Depending on the growth condition and index of surfaces, various nanostructures can be fabricated: quantum dots (QDs), ring-like and holed-triangular nanostructures. At near room temperatures, by limiting surface diffusion of adatoms, the size of In droplets suitable for quantum confinement can be fabricated and thus InAs QDs are demonstrated on GaAs (100) surface. On the other hand, at relatively higher substrate temperatures, by enhancing the surface migrations of In adatoms, super lower density of InGaAs ring-shaped nanostructures can be fabricated on GaAs (100). Under an identical growth condition, holed-triangular InGaAs nanostructures can be fabricated on GaAs type-A surfaces, while ring-shaped nanostructures are formed on GaAs (100). The formation mechanism of various nanostructures can be understood in terms of intermixing, surface diffusion, and surface reconstruction.

Shape control of InGaAs nanostructures on nominal GaAs(001): dashes and dots

A method for fabricating self-assembled InGaAs quantum dashes on a nominal GaAs(001)substrate is presented. InGaAs layers were grown on nominal GaAs(001) substratesat a low temperature to suppress the Stranski–Krastanov transition as well asindium segregation and indium desorption, then annealed at high temperatures toinduce self-assembly. While typical direct growth at the annealing temperaturehas yielded only quantum dot shapes, our approach has enabled us to controlthe shape of self-assembled nanostructures from quantum dashes to quantumdots and eventually quantum dot-chains. The major factor controlling the shapeof InGaAs nanostructures was found to be the thickness of the pseudomorphicIn0.4Ga0.6As layer.

Influence of valence band spin–orbit coupling on the entanglement of excitons in coupled quantum dots

We study the effect of valence band spin–orbit interaction on the exciton entanglement in vertically stacked quantum dots, using a multi-band k · p theory for holes. It is shown that the spin–orbit interaction reduces the exciton entanglement, establishing an intrinsic upper limit. For usual InGaAs nanostructures, however, this effect is generally small and does not pose a challenge for current attempts for the development of quantum information.

Quantum dot strain engineering of InAs∕InGaAs nanostructures

We present a complete study both by experiments and by model calculations of quantum dot strain engineering, by which a few optical properties of quantum dot nanostructures can be tailored using the strain of quantum dots as a parameter. This approach can be used to redshift beyond 1.31μm and, possibly, towards 1.55μm the room-temperature light emission of InAs quantum dots embedded in InGaAs confining layers grown on GaAs substrates. We show that by controlling simultaneously the lower confining layer thickness and the confining layers’ composition, the energy gap of the quantum dot material and the band discontinuities in the quantum dot nanostructure can be predetermined and then the light emission can be tuned in the spectral region of interest. The availability of two degrees of freedom allows for the control of two parameters, which are the emission energy and the emission efficiency at room temperature. The InAs∕InGaAs structures were grown by the combined use of molecular beam epitaxy and atomic layer molecular beam epitaxy; their properties were studied by photoluminescence and photoreflectance spectroscopies and by atomic force microscopy; in particular, by means of photoreflectance not only the spectral features related to quantum dots were studied but also those of confining and wetting layers. The proposed approach has been used to redshift the room-temperature light emission wavelength up to 1.44μm. The optical results were analyzed by a simple effective-mass model that also offers a rationale for engineering the properties of structures for efficient long-wavelength operation.

Dislocation networks adapted to order the growth of III‐V semiconductor nanostructures

We study by transmission electron microscopy shallowly buried dislocation networks (DNs). These DNs are grain boundaries (GBs) that form, between a thin GaAs layer and a GaAs substrate joined together by wafer bonding, in order to accommodate a tilt and a twist between these two crystals. These GBs are composed of a one-dimensional network of mixed dislocations and of a one dimensional network of screw dislocations (whereas a two-dimensional (2D) one is usually observed) forming hexagonal cells. These GBs generate a 2D surface pattern of dilatational and compressive strain adapted to the ordered growth of nanostructures which we demonstrate experimentally for GaAs and InGaAs nanostructures. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Stress‐engineered orderings of self‐assembled III–V semiconductor nanostructures

We report lateral orderings of self-assembled GaAs and InGaAs nanostructures by the stress field of a periodic dislocation network (DN) shallowly buried and parallel to the surface. This DN is a grain boundary (GB) that forms, between a thin GaAs layer and a GaAs substrate joined together by wafer bonding, in order to accommodate a tilt and a twist between these two crystals; both these misorientations are imposed in a controlled manner. This GB is composed of a one-dimensional network of mixed dislocations and of a one dimensional network of screw dislocations. The mixed DN accommodates the tilt and a part of the twist, while the screw DN accommodates the remaining twist. The ordered nanostructures we report result from a growth by metalorganic vapor phase epitaxy of a GaAs/InAs/GaAs sequence on the thin bonded layer. Using transmission electron microscope and atomic force microscope images, we prove that these nanostructures have same dimensions and orientations as the cells of the underlying DN and are thus correlated to the latter. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Buried dislocation networks designed to organize the growth of III-V semiconductor nanostructures

We first report a detailed transmission electron microscopy study of dislocation networks (DNs) formed at shallowly buried interfaces obtained by bonding two GaAs crystals between which we establish in a controlled manner a twist and a tilt around a k110l direction. For large enough twists, the DN consists of a twodimensional network of screw dislocations accommodating mainly the twist and of a one-dimensional network of mixed dislocations accommodating mainly the tilt. We show that in addition the mixed dislocations accommodate part of the twist and we observe and explain slight unexpected disorientations of the screw dislocations with respect to the k110l directions. By performing a quantitative analysis of the whole DN, we propose a coherent interpretation of these observations which also provides data inaccessible by direct experiments. When the twist is small enough, one screw subnetwork vanishes. The surface strain field induced by such DNs has been used to pilot the lateral ordering of GaAs and InGaAs nanostructures during metal-organic vapor phase epitaxy. We prove that the dimensions and orientations of the nanostructures are correlated with those of the cells of the underlying DN and explain how the interface dislocation structure governs the formation of the nanostructures.

Time resolved magneto-optical spectroscopy on InGaAs nanostructures grown on (311)A and (100)-oriented substrates

We present a time-resolved magneto-photoluminescence study of In0.5Ga0.5As self-organized nanostructures grown on (100) and (311)A-oriented substrates by molecular beam epitaxy. The (311)A-oriented samples have a corrugated surface realizing a sort of quantum wire array, whereas the (100) samples exhibit Stranski–Krastanow islands. The different morphology of the nanostructures is reflected in the different electron/hole wave-function confinement along the three directions (perpendicular and parallel to the growth direction). We discuss the effects of the magnetic field (up to 8 T) on the recombination mechanism in these InGaAs nanostructures and on the transient dynamics of photoluminescence. We observe a clear decrease of the photoluminescence decay time with magnetic field flux indicating the exciton nature of the radiative low-temperature recombination processes.

Interface profile optimization in novel surface passivation scheme for InGaAs nanostructures using Si interface control layer

This paper attempts to control and optimize the interface atomic profiles of a novel surface passivation scheme for InGaAs nanostructures, using a silicon interface control layer (ICL). An in-situ x-ray photoelectron spectroscopy characterization technique was used to establish a process sequence that satisfies the conditions of maintenance of pseudomorphic matching to InGaAs, prevention of direct oxidation of InGaAs, and formation of a good SiO2/Si interface with minimal suboxide components. It is shown that the above conditions can be satisfied by a new process that is a formation of the thermal SiO2 at the SiO2-Si interface by repetition of deposition/oxidation/annealing cycle. A large reduction of interface state density (Nss) was realized by the optimization of the new process, resulting in a minimum Nss of 4 × 1011 cm−2 eV−1. The silicon ICL technique was successfully applied to the passivation of InGaAs wire structures.