Molecular Beam Epitaxy Growth Technique Research Articles

Single SiGe quantum dot emission deterministically enhanced in a high-Q photonic crystal resonator.

We report the resonantly enhanced radiative emission from a single SiGe quantum dot (QD), which is deterministically embedded into a bichromatic photonic crystal resonator (PhCR) at the position of its largest modal electric field by a scalable method. By optimizing our molecular beam epitaxy (MBE) growth technique, we were able to reduce the amount of Ge within the whole resonator to obtain an absolute minimum of exactly one QD, accurately positioned by lithographic methods relative to the PhCR, and an otherwise flat, a few monolayer thin, Ge wetting layer (WL). With this method, record quality (Q) factors for QD-loaded PhCRs up to Q ∼ 105 are achieved. A comparison with control PhCRs on samples containing a WL but no QDs is presented, as well as a detailed analysis of the dependence of the resonator-coupled emission on temperature, excitation intensity, and emission decay after pulsed excitation. Our findings undoubtedly confirm a single QD in the center of the resonator as a potentially novel photon source in the telecom spectral range.

Growth and experimental evidences for doped nanofilms by X-ray photoemission spectroscopy data analyses

By using the molecular beam epitaxy growth technique, the impacts of growth temperatures for Bi2Te3 thin films were investigated, besides of the substrate temperatures on growth. Moreover, the full width half maximums, based on the X-ray photoemission spectroscopy measurements, have been shown clear results for Cr-Te bonds in chromium doped Bi2Te3 thin films. The current aim is a spotlight on enlightening how chromium is joined within Bi2Te3 epitaxial thin films in the process of doping concentration by providing detailed experimental evidences through the systematic structure and electronic investigations of the compound.

The influence of an AlN seeding layer on nucleation of self-assembled GaN nanowires on silicon substrates

Gallium nitride (GaN)-based nanowires (NWs) have attracted much attention for the fabrication of novel nanostructured devices. In this paper, the influence of an AlN seeding layer on the nucleation of self-assembled GaN NWs grown by plasma-assisted molecular beam epitaxy (MBE) on Si (111) substrates has been investigated. Not only is the formation of a two-dimensional compact GaN layer at the bottom of the NWs suppressed, but also a high density of vertically aligned well-separated GaN NWs originating from GaN islands are successfully obtained after introducing annealing and nitridation processes. Scanning electronic microscope and transmission electron microscope measurements show that the NWs have a high crystalline wurtzite structure nearly free of dislocations and stacking faults and the NW diameter remains constant over almost the entire length. Due to the temperature-dependent diffusion length of Ga adatoms during the nucleation process, the formation of well-separated NWs relies on the distribution and morphology of the underlying AlN seeding layer. Moreover, the SiNx layer served as mask to inhibit coalescence at the nucleation sites. The developed growth processes and the obtained results provide a viable path facilitating the use of MBE growth techniques to fabricate III-nitride NW-based materials and related devices on Si substrates.

Nucleation and growth of WSe2: enabling large grain transition metal dichalcogenides

The limited grain size (<200 nm) for transition metal dichalcogenides (TMDs) grown by molecular beam epitaxy (MBE) reported in the literature thus far is unsuitable for high-performance device applications. In this work, the fundamental nucleation and growth behavior of WSe2 is investigated through a detailed experimental design combined with on-lattice, diffusion-based first principles kinetic modeling to enable large area TMD growth. A three-stage adsorption-diffusion-attachment mechanism is identified and the adatom stage is revealed to play a significant role in the nucleation behavior. To limit the nucleation density and promote 2D layered growth, it is necessary to have a low metal flux in conjunction with an elevated substrate temperature. At the same time, providing a Se-rich environment further limits the formation of W-rich nuclei which suppresses vertical growth and promotes 2D growth. The fundamental understanding gained through this investigation has enabled an increase of over one order of magnitude in grain size for WSe2 thus far, and provides valuable insight into improving the growth of other TMD compounds by MBE and other growth techniques such as chemical vapor deposition (CVD).

MBE growth techniques for InAs-based nBn IR detectors

This manuscript describes an investigation of the effects of growth temperature on InAs epitaxial layers and InAs-based nBn detectors grown by molecular beam epitaxy (MBE). The motivation for this work is to improve the overall performance of InAs-based nBn detectors, which depends both on the bulk material quality of the individual device layers, particularly the infrared absorbing layer, as well as on the quality of the layer interfaces, particularly the interface between the absorber and barrier layers. Absorber layer bulk quality and absorber/barrier interface quality are presumably optimized by performing InAs growth at different temperatures, thus the preferred MBE growth strategy is not immediately apparent. InAs epitaxial layers of 2 μm thick are grown at several temperatures ranging from 420 to 490 °C, and are examined by differential interference contrast microscopy, atomic force microscopy, steady-state photoluminescence, and time-resolved photoluminescence measurements. Absorber layers of 2 μm thick in nBn detectors are also grown at the same temperatures as the InAs single layers, and the resulting devices are evaluated on the basis of dark current density. Competitively high InAs material quality and low nBn dark current densities have been achieved across the range of investigated growth temperatures. The material quality of the InAs single epitaxial layers is found to improve monotonically with growth temperature over the investigated range, and likewise, the reverse saturation dark current density of the nBn detectors is found to decrease monotonically with growth temperature. nBn detectors with dark current density within a factor of 5 of Rule 07 are reported. Finally, it is noted that this work uses an InAs growth rate of 0.9 μm/h, whereas many other studies have chosen to use InAs growth rates in the range of 0.2–0.5 μm/h. The results of this study demonstrate that high performance InAs-based detectors can be grown at this more convenient rate.

Periodic Two-Dimensional GaAs and InGaAs Quantum Rings Grown on GaAs (001) by Droplet Epitaxy.

Growth of ordered GaAs and InGaAs quantum rings (QRs) in a patterned SiO2 nanohole template by molecular beam epitaxy (MBE) using droplet epitaxy (DE) process is demonstrated. DE is an MBE growth technique used to fabricate quantum nanostructures of high crystal quality by supplying group III and group V elements in separate phases. In this work, ordered QRs grown on an ordered nanohole template are compared to self-assembled QRs grown with the same DE technique without the nanohole template. This study allows us to understand and compare the surface kinetics of Ga and InGa droplets when a template is present. It is found that template-grown GaAs QRs form clustered rings which can be attributed to low mobility of Ga droplets resulting in multiple nucleation sites for QR formation when As is supplied. However, the case of template-grown InGaAs QRs only one ring is formed per nanohole; no clustering is observed. The outer QR diameter is a close match to the nanohole template diameter. This can be attributed to more mobile InGa droplets, which coalesce from an Ostwald ripening to form a single large droplet before As is supplied. Thus, well-patterned InGaAs QRs are demonstrated and the kinetics of their growth are better understood which could potentially lead to improvements in the future devices that require the unique properties of patterned QRs.

Engineering Electronic Structure of a Two-Dimensional Topological Insulator Bi(111) Bilayer on Sb Nanofilms by Quantum Confinement Effect.

We report on the fabrication of a two-dimensional topological insulator Bi(111) bilayer on Sb nanofilms via a sequential molecular beam epitaxy growth technique. Our angle-resolved photoemission measurements demonstrate the evolution of the electronic band structure of the heterostructure as a function of the film thickness and reveal the existence of a two-dimensional spinful massless electron gas within the top Bi bilayer. Interestingly, our first-principles calculation extrapolating the observed band structure shows that, by tuning down the thickness of the supporting Sb films into the quantum dimension regime, a pair of isolated topological edge states emerges in a partial energy gap at 0.32 eV above the Fermi level as a consequence of quantum confinement effect. Our results and methodology of fabricating nanoscale heterostructures establish the Bi bilayer/Sb heterostructure as a platform of great potential for both ultra-low-energy-cost electronics and surface-based spintronics.

Effect of SiO&lt;sub&gt;2&lt;/sub&gt; Thickness on GaAs Nanowires on Si (111) Substrates Grown by Molecular Beam Epitaxy

The GaAs nanowires are grown on Si (111) substrates by Ga-assisted molecular beam epitaxy growth technique. The effect of SiO2 thickness on the structural properties of GaAs nanowires is investigated by Scanning Electron Microscope (SEM). The nucleation of GaAs nanowires related to the presence of a SiO2 layer previously coated on Si substrate. The results show that the density, length, and diameter of GaAs nanowires strongly depend on the oxidation time (or SiO2 thickness).

High net modal gain (&gt;100 cm^−1) in 19-stacked InGaAs quantum dot laser diodes at 1000 nm wavelength band

An InGaAs quantum dot (QD) laser diode with 19-stacked QDs separated by 20 nm-thick GaAs spacers was fabricated using an ultrahigh-rate molecular beam epitaxial growth technique, and the laser characteristics were evaluated. A 19-stacked simple broad area QD laser diode was lased at the 1000 nm waveband. A net modal gain of 103 cm(-1) was obtained at 2.25 kA/cm(2), and the saturated modal gain was 145.6 cm(-1); these are the highest values obtained to our knowledge. These results indicate that using this technique to highly stack QDs is effective for improving the net modal gain of QD lasers.

Molecular beam epitaxy for high-performance Ga-face GaN electron devices

Molecular beam epitaxy (MBE) has emerged as a powerful technique for growing GaN-based high electron mobility transistor (HEMT) epistructures. Over the past decade, HEMT performance steadily improved, mainly through the optimization of device fabrication processes. Soon, HEMT performance will be limited by the crystalline quality of the epistructure. MBE offers heterostructure growth with highly abrupt interfaces, low point defect concentrations, and very low carbon and hydrogen impurity concentrations. Minimizing parasitic leakage pathways and resistances is essential in the growth of HEMTs for high-frequency and high-power applications. Through growth on native substrates with very low threading dislocation density, low-leakage HEMTs with very low on-resistance can be realized. Ga-rich plasma-assisted MBE (PAMBE) has been studied extensively, and it is clear that this technique has inherent limitations, including a high density of leakage pathways and a very small growth parameter space. Relatively new MBE growth techniques—high-temperature N-rich PAMBE and ammonia-based MBE—are being developed to circumvent the shortcomings of Ga-rich PAMBE.

Gain Measurement of Highly Stacked InGaAs Quantum Dot Laser with Hakki–Paoli Method

A 147-µm-long cavity laser diode with 19 InGaAs quantum dot layers was fabricated by the ultrahigh-rate molecular beam epitaxial growth technique, and its gain properties were investigated using the Hakki–Paoli method below the threshold current (Ith) of 111.5 mA. At an injection current of 100.3 mA (0.9Ith), the positive net modal gain was in the range between 1005 and 1043 nm, corresponding to a photon energy of 45 meV. The maximum net modal gain and maximum modal gain were 46.5 and 60.5 cm-1, respectively. A differential net modal gain of as high as 3.8 cm-1/mA was observed at 0.77 times the threshold current. No gain saturation appeared below the threshold current, and injection currents higher than 78.4 mA (≈0.7Ith) were required to obtain a net modal gain.

Mid-infrared photoluminescence of InAsN dilute nitride alloys grown by LPE and MBE

Dilute nitride InAsN epitaxial layers were produced using both liquid phase and molecular beam epitaxial growth techniques. The spectral features in the photoluminescence of samples containing up to 1% N with emission energies in the mid-infrared spectral region are described and compared. The emission intensities of both LPE and MBE grown materials were found to be comparable. Increasing the N content in the InAsN alloys resulted in a significant increase in the activation energy for thermal quenching of the photoluminescence emission due to a combination of lowering of the conduction band edge and Auger de-tuning.

Tailoring of polar and nonpolar ZnO planes on MgO (001) substrates through molecular beam epitaxy

Polar and nonpolar ZnO thin films were deposited on MgO (001) substrates under different deposition parameters using oxygen plasma-assisted molecular beam epitaxy (MBE). The orientations of ZnO thin films were investigated by in situ reflection high-energy electron diffraction and ex situ X-ray diffraction (XRD). The film roughness measured by atomic force microscopy evolved as a function of substrate temperature and was correlated with the grain sizes determined by XRD. Synchrotron-based X-ray absorption spectroscopy (XAS) was performed to study the conduction band structures of the ZnO films. The fine structures of the XAS spectra, which were consistent with the results of density functional theory calculation, indicated that the polar and nonpolar ZnO films had different electronic structures. Our work suggests that it is possible to vary ZnO film structures from polar to nonpolar using the MBE growth technique and hence tailoring the electronic structures of the ZnO films.PACS: 81; 81.05.Dz; 81.15.Hi.

Highly stacked InGaAs quantum dot laser diodes fabricated by ultrahigh‐rate molecular beam epitaxial growth technique

AbstractUp to 19 stacks of high‐density (∼5 × 1010 cm‐2) self‐assembled InGaAs quantum dots (QDs) on a GaAs (001) substrate were fabricated by ultrahigh‐rate (1.0 ML/s) solid‐source molecular beam epitaxial (MBE) deposition. Using these QDs, stripe laser diodes were designed and fabricated for use in 1.0‐μm waveband applications, and their operation at room‐temperature was investigated. Laser diodes having a larger number of QDs in the active region exhibited lower threshold current densities. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Molecular beam epitaxy of metamorphic InyGa1−yP solar cells on mixed anion GaAsxP1−x/GaAs graded buffers

The authors have grown metamorphic InyGa1−yP on optimized GaAsxP1−x/GaAs graded buffers via solid source molecular beam epitaxy (MBE) for multijunction solar cell applications. In this work, the authors show that a previously developed kinetic growth model can be used to predict the composition of mixed anion GaAsxP1−x alloys on GaAs as a function of substrate temperature and group-V flux. The advantages of using a high growth temperature of 700 °C are then described, including the minimized dependence of composition on small temperature variations, a linear dependence of film composition on incident group-V flux ratio, and the ability to attain low threading dislocation densities of ≤106 cm−2. The authors then discuss the effect of faceted trenches, a morphological defect specific to tensile strain relaxation, on minority carrier properties, as well as strategies to eliminate them. Growth temperature effects, phase separation, and difficulties encountered in n-type doping of InAlP:Si are then described in the context of InyGa1−yP solar cell growth. The MBE growth techniques presented here have enabled the demonstration of 2.00 eV band gap metamorphic In0.39Ga0.61P solar cells, exhibiting open-circuit voltages as high as 1.42 V. These results indicate that metamorphic InyGa1−yP is a promising material for future multijunction solar cells.

Site-controlled lateral arrangements of InAs quantum dots grown on GaAs(001) patternedsubstrates by atomic force microscopy local oxidation nanolithography

In this work, we present a fabrication process that combines atomic force microscopy(AFM) local oxidation nanolithography and molecular beam epitaxy (MBE) growthtechniques in order to control both the nucleation site and number of InAs quantumdots (QDs) inside different motifs printed on GaAs(001) substrates. We find thatthe presence of B-type slopes (As terminated) inside the pattern motifs is themain parameter for controlling the selectivity of the pattern for InAs growth.We demonstrate that either single InAs QDs or multiple InAs QDs in a lateralarrangement (LQDAs) can be obtained, with a precise control in their position and QDnumber, simply by varying the fabricated oxide length along the [110] direction.

Quadra-quantum dots grown on quantum rings having square-shaped holes: Basic nanostructure for quantum dot cellular automata application

Preferable quantum dot (QD) nanostructure for quantum computation based on quantum cellular automata (QCA) is laterally close-packed quantum dot molecular (QDM) having 4 QDs at the corners of square configuration. We called this 4 QDs-set as quadra-quantum dots (QQDs). Aligned QQDs with 2 electron-confinements work like a wire for digital information transmission by Coulomb repulsion force which is high speed and consumes little power. Combination of QQDs in line and their cross-over work as logic gates and memory bits. We have developed a Molecular Beam Epitaxial (MBE) growth technique call “Droplet Epitaxy” for several quantum nanostructures such as quantum rings (QRs), quantum dot rings (QDRs). In this presentation, we prepare QRs with 20 ML In-Ga (15:85) droplet at 390 °C and droplet growth rate of 1ML/s. Arsenic flux (7–8 × 10 −6 Torr) is then exposed for InGaAs crystallization at 200 °C for 5 min. During droplet epitaxy at high value of droplet thickness and high temperature, out-diffusion from the centre of droplets occurs under anisotropic stain. This leads to quantum ring structures having non-uniform ring stripes and deep square-shaped nanoholes. Using these peculiar QRs as templates, 4 QDs situating at each corner of square shape are regrown. Two of these 4 QDs are aligned either [1 1 ¯ 0] or [1 1 0] which are preferable crystallographic directions of QD alignment in general.

GaAs Heteroepitaxy on SiGe-on-Insulator Using Ge Condensation and Migration Enhanced Epitaxy

A novel fabrication technique to grow device quality GaAs on Si substrate is proposed using thin graded SiGe-on-insulator (SGOI) substrate. Combination of low-temperature GaAs molecular beam epitaxy (MBE) growth technique, low-temperature GaAs migration enhanced epitaxy (MEE) technique, and modified two-step Ge condensation technique has demonstrated successful growth of 10-nm GaAs layer grown on a tensile strained 42-nm graded SGOI layer.

High-density, uniform InSb∕GaSb quantum dots emitting in the midinfrared region

The authors have developed a multistep molecular-beam epitaxy growth technique which allows them to grow InSb quantum dots with high structural perfection and high density. This technique consists in the deposition at a very low temperature followed by a properly designed annealing step. Fully strained InSb∕GaSb quantum dots with a density exceeding 7×1010cm−2 and lateral sizes in the 20–30nm range have been obtained. Narrow photoluminescence emission is obtained around 3.5μm up to room temperature.

ErAs island-stacking growth technique for engineering textured Schottky interfaces

We demonstrate a molecular beam epitaxy growth technique to create intentionally textured interfaces between semimetallic ErAs and GaAs that result in interface orientations that are different from the substrate. To grow the textured interfaces, ErAs is deposited on (100) GaAs and islands form in an island growth mode. Enough GaAs is then deposited to fill the space between islands with semiconductor. More ErAs is deposited on the ErAs∕GaAs surface and the new ErAs islands are thought to nucleate on the exposed top surface of the partially covered islands. The process is repeated to “stack” the islands. By quadratically increasing the amount of ErAs deposited in each repetition, the islands form into cones/pyramids that coalesce into a complete film with an intentionally roughened interface. Compared to a smooth interface between ErAs and GaAs, the Schottky barrier height between textured ErAs and n-GaAs can be reduced from ∼0.81to∼0.52eV, and the Schottky barrier height between ErAs and p-GaAs can be increased from ∼0.38to∼0.63eV.