Droplet Epitaxy Method Research Articles

Controlled growth mechanism of ring-like In(Ga)As quantum Dot pairs on GaAs ring-ring-disk nanostructures templates

The ring-like In(Ga)As quantum dot pairs (QDPs) is prepared on GaAs ring-ring-disk (R-R-D) nanostructures template by droplet epitaxy (DE) method. The surface morphology of GaAs samples are characterized with Scanning Tunneling Microscope (STM), and studying local controlled growth mechanism of quantum dots (QDs) on GaAs R-R-D nanostructures. STM images show that In(Ga)As QDPs self-assembled a kind of complicated structure with one quantum dot (QD) pair in the center of nanostructures, other QDPs appearing both double rings and disk areas of original GaAs R-R-D nanostructures distributed like a exquisite ring. These In(Ga)As QDPs are uniformly arranged along the [10] direction of GaAs(001) surface. Combining Stranski-Krastanov (S-K) growth mode and the tensile effect of strain field in the [10] direction on sample surface, the nucleation position and distribution shape of ring-like In(Ga)As QDPs is locally controllable. It is found that high uniformly QDPs can be locally controlled by the original GaAs R-R-D nanostructures templates. These results can provide certain guiding significance for the controllable growth of QD by DE method.

Droplet epitaxy of semiconductor nanostructures for quantum photonic devices.

The long dreamed 'quantum internet' would consist of a network of quantum nodes (solid-state or atomic systems) linked by flying qubits, naturally based on photons, travelling over long distances at the speed of light, with negligible decoherence. A key component is a light source, able to provide single or entangled photon pairs. Among the different platforms, semiconductor quantum dots (QDs) are very attractive, as they can be integrated with other photonic and electronic components in miniaturized chips. In the early 1990s two approaches were developed to synthetize self-assembled epitaxial semiconductor QDs, or 'artificial atoms'-namely, the Stranski-Krastanov (SK) and the droplet epitaxy (DE) methods. Because of its robustness and simplicity, the SK method became the workhorse to achieve several breakthroughs in both fundamental and technological areas. The need for specific emission wavelengths or structural and optical properties has nevertheless motivated further research on the DE method and its more recent development, local droplet etching (LDE), as complementary routes to obtain high-quality semiconductor nanostructures. The recent reports on the generation of highly entangled photon pairs, combined with good photon indistinguishability, suggest that DE and LDE QDs may complement (and sometimes even outperform) conventional SK InGaAs QDs as quantum emitters. We present here a critical survey of the state of the art of DE and LDE, highlighting the advantages and weaknesses, the achievements and challenges that are still open, in view of applications in quantum communication and technology.

High‐temperature performance of non‐polar (11–20) InGaN quantum dots grown by a quasi‐two‐temperature method

Non‐polar (11–20) a‐plane InGaN quantum dots (QDs) are one of the strongest candidates to achieve on‐chip applications of polarised single photon sources, which require a minimum operation temperature of ∼200 K under thermoelectrically cooled conditions. In order to further improve the material quality and optical properties of a‐plane InGaN QDs, a quasi‐two‐temperature (Q2T) method has been developed, producing much smoother underlying InGaN quantum well than the previous modified droplet epitaxy (MDE) method. In this work, we compare the emission features of QDs grown by these two methods at temperatures up to 200 K. Both fabrications methods are shown to be able to produce QDs emitting around the thermoelectric cooling barrier. The sample fabricated by the new Q2T method demonstrates more stable operation, with an order of magnitude higher intensity at 200 K comparing to the comparable QDs grown by MDE. A detailed discussion of the possible mechanisms that result in this advantage of slower thermal quenching is presented. The use of this alternative fabrication method hence promises more reliable operation at temperatures even higher than the thermoelectric cooling threshold, and facilitates the on‐going development of high temperature polarised single photon sources based on a‐plane InGaN QDs.

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.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-016-1518-2) contains supplementary material, which is available to authorized users.

Formation and Temperature Effect of InN Nanodots by PA-MBE via Droplet Epitaxy Technique.

In this report, self-organized indium nitride nanodots have been grown on Si (111) by droplet epitaxy method and their density can reach as high as 2.83 × 1011 cm−2 for the growth at low temperature of 250 °C. Based on the in situ reflection high-energy electron diffraction, the surface condition, indium droplets, and the formation of InN nanodots are identified during the epitaxy. The X-ray photoelectron spectroscopy and photoluminescence measurements have shown the formation of InN nanodots as well. The growth mechanism of InN nanodots could be described via the characterizations of indium droplets and InN nanodots using scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. The density of the InN nanodots was less than that of the In droplets due to the surface diffusion and desorption of atoms during the nitridation and annealing process. The average size and density of InN nanodots can be controlled by the substrate temperatures during the growth. For the growth at lower temperature, we obtained the higher density and smaller average size of InN nanodots. To minimize the total surface energy, the coarsening and some preferred orientations of InN nanodots were observed for the growth at high temperature.

Infrared Absorption Enhancement by Charge Transfer in Ga-GaSb Metal-Semiconductor Nanohybrids.

We fabricated Ga-GaSb nanohybrids by the droplet epitaxy method and precisely tuned the interaction between the metal and semiconductor parts. Selective absorption enhancement from 1.2 to 1.3 μm was confirmed via ultraviolet-visible-infrared absorption spectra in all of the nanohybrids, which shows size and component dependence. Valence band spectra of the samples indicate that carrier separation occurs at the interface at the Schottky junction and the high density of states near the Fermi level in a semiconductor controls the process of charge transfer. Thus, the enhanced selective absorption in the infrared region will open up a broad prospect for applications in infrared detection and thermophotovoltaic cells.

Low Temperature Photoluminescence Kinetics of Double-Ring Structured GaAs Quantum Dots.

This work is focused on spectroscopically characterizing kinetic properties of concentric quantum-ring complexes of GaAs quantum dots. Quantum-ring (or double-ring) GaAs quantum dots, embedded in an Al0.3Ga0.7As barrier layer, were grown by a droplet epitaxy method during molecular beam epitaxy on a GaAs (001) substrate. Emission peaks of photoluminescence spectra with different excitation power, were measured as 702 nm at 0.17 mW and 690 nm at 400 mW, were blue-shifted as the excitation power increased. In addition, excitation laser power dependence of time-resolved photoluminescence of double-ring GaAs quantum dots at 10 K under 400 nm excitation wavelength was performed, revealing that photoluminescence lifetime slowly decreased in comparison to that of single disc-like quantum dots as excitation power increased, implying that carrier transfer between inner ring and outer ring could slow down the decay process. The luminescence lifetime at 10 K increased from 245 to 409 ps in the range from 0.17 to 400 mW of excitation power.

Growth of non-polar InGaN quantum dots with an underlying AlN/GaN distributed Bragg reflector by metal-organic vapour phase epitaxy

Non-polar (11–20) InGaN quantum dots (QDs) have been grown using a modified droplet epitaxy method by metal-organic vapour phase epitaxy on top of a 15-period AlN/GaN distributed Bragg reflector (DBR) on a-plane GaN pseudo-substrate prepared by epitaxial lateral overgrowth (ELOG), in which the QDs are located at the centre of a ca. 180 nm GaN layer. The AlN/GaN DBR has shown a peak reflectivity of ∼80% at a wavelength of ∼454 nm with a 49 nm wide, flat stop-band. Variations in layer thicknesses observed by cross-sectional scanning transmission electron microscopy have been identified as the main source of degradation of the DBR reflectivity. The presence of trenches due to incomplete coalescence of the ELOG template and the formation of cracks due to relaxation of tensile strain during the DBR growth may distort the DBR and further reduce the reflectivity. The DBR top surface is very smooth and does not have a detrimental effect on the subsequent growth of QDs. Enhanced single QD emission at 20 K was observed in cathodoluminescence.

Characterization and density control of GaN nanodots on Si (111) by droplet epitaxy using plasma-assisted molecular beam epitaxy.

In this report, self-organized GaN nanodots have been grown on Si (111) by droplet epitaxy method, and their density can be controlled from 1.1 × 1010 to 1.1 × 1011 cm-2 by various growth parameters, such as substrate temperatures for Ga droplet formation, the pre-nitridation treatment of Si substrate, the nitridation duration for GaN crystallization, and in situ annealing after GaN formation. Based on the characterization of in situ RHEED, we can observe the surface condition of Si and the formation of GaN nanodots on Si. The surface nitridaiton treatment at 600°C provides a-SiNx layer which makes higher density of GaN nanodots. Crystal GaN nanodots can be observed by the HRTEM. The surface composition of GaN nanodots can be analyzed by SPEM and μ-XPS with a synchrotron x-ray source. We can find GaN nanodots form by droplet epitaxy and then in situ annealing make higher-degree nitridation of GaN nanodots.

Investigation of Wetting Layers in InAs/GaAs Self-Assembled Nanostructures with Reflectance Difference Spectroscopy

As a system with both profound physics and promising application potentials, the strain-induced self-assembled semiconductor nanostructures have been investigated for decades of years. The optical and electrical properties of this system are mainly determined by the nanostructures, but can be greatly affected by the generally existent two dimensional structures from which the nanostructures evolve, the so called wetting layer (WL). The WL configurations can be varied with different growth conditions, and further influence the nanostructure morphology and properties. This is a reviewing article introducing some recent progresses in investigating the evolution of WLs in InAs/GaAs system with reflectance difference spectroscopy. Different kinds of WL evolution processes in Stranski-Krastanov growth mode are introduced. The segregation and desorption of indium atoms and the effect of growth interruption are mentioned. The existence and the evolution process of wetting layers in droplet epitaxy method are also discussed.

Photoluminescence studies of GaAs quantum dots and quantum rings

AbstractRecently, the growth of self‐assembled quantum structures has been intensively investigated for basics physics and device application. In our work, self‐assembled strain‐free GaAs quantum dots and quantum rings were investigated by the photoluminescence technique. The GaAs nanostructures are fabricated on AlGaAs (001) surface by droplet epitaxy method. Temperature dependent photoluminescence spectra were measured in the range of room temperature and 4 K. We give an explanation on the broadening of the photoluminescence peaks for different quantum structures. The calculated electronic structure is compared with the photoluminescence data (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of Rapid Thermal Annealing on the Optical Properties of GaAs Quantum Dots Grown by Using the Droplet Epitaxy Method

Effects of Rapid Thermal Annealing on the Optical Properties of GaAs Quantum Dots Grown by Using the Droplet Epitaxy Method

Surface mediated control of droplet density and morphology on GaAs and AlAs surfaces

AbstractWe present a comparative study of gallium (Ga) and aluminium (Al) droplets fabricated on GaAs (100) and AlAs (100) surfaces. Higher density of Ga droplets is achieved on AlAs surface compared with GaAs surface. Similarly, the density of Al nanostructures is higher on AlAs surface than on GaAs surface, even though different morphologies are obtained on each surface. Further, while uniform Ga droplets are formed on both GaAs and AlAs surfaces, Al rings and dots, with big inhomogeneity, are observed on GaAs and AlAs surface, respectively. This investigation suggests that size and shape of nanostructures grown by the droplet epitaxy method can be designed by employing different surfaces. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fine structure splitting reduction in droplet epitaxy GaAs quantum dots grown on (111)A surface

In this contribution we compare the geometrical symmetry of self-assembled GaAs quantum dots (QDs) grown by a refined droplet epitaxy (DE) method on (100) and (111)A AlGaAs substrate. By atomic force microscopy (AFM) we study the morphology of the QDs and by micro–photoluminescence spectroscopy (μPL) we compare the dependence of the fine structure splitting on QD size and shape. We show that: i) the refined DE enhances the elongation of QDs grown on (100) AlGaAs substrate increasing the fine structure splitting; ii) by growing the QDs on the (111)A AlGaAs surface it is possible to recover a good geometrical circular symmetry obtaining a small value of the fine structure splitting .

Different shape of GaAs quantum structures under various growth conditions

We report the influences of growth parameters on the characteristics of GaAs quantum rings (QRs) and quantum dots (QDs) formed on AlGaAs/GaAs by the droplet epitaxy (DE) method. After forming Ga droplets on the AlGaAs/GaAs surface, varying amounts of arsenic (As) flux were introduced to fabricate the GaAs quantum structures. By decreasing the As flux from 8 × 10 − 5 to 3 × 10 − 5 Torr, the shape of the GaAs quantum structures was changed from QDs to elongated QRs. With further decreasing As flux, the shape of the elongated QRs became symmetric. The formation characteristics of the GaAs QRs from the QDs with the amount of As flux were discussed in terms of migration behaviors of the gallium (Ga) atoms on the GaAs(001)- c(4 × 4) surface. The effects of the amount of Ga supply and the growth temperature for the deposition of Ga droplets on the formation of the GaAs quantum structures were also considered.

Ordering of high-quality InAs quantum dots on defect-free nanoholes

We demonstrate a promising way for the fabrication of high-quality InAs quantum dot (QD) arrays by combining atomic force microscope (AFM) tip-induced nano-oxidation, atomic-hydrogen etching/cleaning (AHE/C), and droplet epitaxy method. The highly aligned defect-free nanoholes as nucleation sites were fabricated by using AFM tip-induced nano-oxidation and subsequent AHE/C. Using the droplet epitaxy on the artificially patterned nanoholes, we fabricated laterally arrayed InAs QDs with an interdot distance of 100nm. The photoluminescence from the InAs QD arrays showed strong emission at 1.22eV, even though the processed interface directly faced the base of the InAs QDs, indicating that the process can reduce the physical and chemical defects/contaminants at the nanoholes and interface.

Near room temperature droplet epitaxy for fabrication of InAs quantum dots

By using the droplet epitaxy method, we succeed in fabricating the InAs quantum dots (QDs) with the spatial density of 4×1010∕cm2 and an average lateral size of 20nm on GaAs (001) at the droplets deposition temperature of 50°C and subsequent annealing process under As4 flux. These QDs shows the intense photoluminescence even at room temperature indicating that high-quality InAs QDs can be fabricated by near room temperature droplets deposition and crystallization process by 400°C in situ annealing.

Si-doping and annealing effects on In0.5Ga0.5As/GaAs quantum dots grown by heterogeneous droplet epitaxy

We investigated the Si-doped In0.5Ga0.5As quantum dots (QDs) grown by heterogeneous droplet epitaxy method and the annealing effects on the optical properties through photoluminescence (PL) spectroscopy. All together two or three peaks related to the QDs appear on the PL-spectra for undoped and lightly doped samples at low temperature. But, heavily doped sample, the free electron concentration is so high that it can screen the Coulomb interaction between electron and hole. Therefore, it is very difficult to form excitons. After, post-growth annealing, thermal intermixing was found to result in significant blue-shifts of the PL emission at annealing temperatures up to 800°C and a decrease of the intersublevel energy spacing (ΔE) as much as 22.7meV. In addition, the inhomogeneously broad line width of PL spectra become narrow with increasing annealing temperature. These results have been explained by the interdiffusion of In–Ga atoms at the interface between the QD and the GaAs barrier, which changes the composition of the QDs.

Indium segregation in the fabrication of InGaAs concave disks by heterogeneous droplet epitaxy

We have studied the effects of As 4 flux intensity during crystallization process and sample temperature during annealing process on the structural and optical properties of InGaAs (quantum dots) QDs fabricated by heterogeneous droplet epitaxy method. The QDs formation mechanism including In segregation and InAs–GaAs intermixing was investigated. We found the very wide optimum growth conditions, such as photoluminescence peak energy is constant, intensity is maximum value and full-width at half-maximum is minimum value.

InAs Quantum Dots Growth by Modified Droplet Epitaxy Using Sulfur Termination

We have applied the modified droplet epitaxy method using sulfur termination to fabricate InAs quantum dots on GaAs(001) substrates for the first time. It is observed that the S-terminated surface effectively prevents the two-dimensional growth of InAs, forming InAs nanocrystals. From the magneto-photoluminescence measurements of this structure, three-dimensional quantum confinement effect was confirmed. This modified droplet epitaxy method is promising for the fabrication of quantum dots, not only in the lattice-matched system but also in the lattice-mismatched system.