As4 Flux 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.

Growth, crystal structures, and magnetic properties of Fe–As films grown on GaAs (111)B substrates by molecular beam epitaxy

We study epitaxial growth, crystal structures, and magnetic properties of Fe–As compound thin films grown on GaAs (111)B substrates at various values of the As4:Fe flux ratio γ, using molecular beam epitaxy. The samples grown at low As4 flux (γ = 0.3, sample A) show mainly a body-centered-cubic (bcc) crystal structure, exhibiting ferromagnetic properties similar to bcc Fe. Meanwhile, the Fe–As samples grown at medium γ (2.7–4.5, sample group B) comprise regions of Ni2In-type FeAs (a hexagonal crystal with lattice constants of a = 0.399 nm and c = 0.536 nm), which are grown at the bottom and interface with the GaAs buffer layer, and a layer of non-stoichiometric FeAs with a DO3 structure (a = 0.522 nm) formed on the top. The DO3-structure FeAs phase contains partially transformed regions, which are characterized by thin stripes in a scanning transmission electron microscopy image. Furthermore, in the sample grown with high γ = 8.5 (sample C), a hexagonal Fe–As crystal with a large in-plane lattice constant (a = 0.691 nm and c = 0.542 nm) and threefold screw axes are observed. None of these crystal structures of Fe–As compounds has ever been reported. While sample C shows no ferromagnetism, the samples in group B exhibit strong ferromagnetism with high Curie temperature TC above 400 K. These new ferromagnetic Fe–As compounds are promising for spintronic device applications.

Comparison of different passivation layers for GaInAs solar cells grown by solid-source molecular beam epitaxy

Ga0.47In0.53As (hereafter, GaInAs) solar cells with a 0.74 eV bandgap were grown by solid-source molecular beam epitaxy (MBE) using different passivation layer combinations. Al0.48In0.52As (hereafter, AlInAs) and InP with 1.50 and 1.35 eV bandgaps, respectively, are promising candidates for use as passivation layers. Since As2 and P2 are used in GaInAs/InP heteroepitaxy, the effect of exposing the InP and GaInAs surfaces to As2 and P2 fluxes was evaluated before the GaInAs solar cell growth to improve the heterointerface quality. It was found that an exposure time of 10 s was sufficient to obtain clean heterointerfaces by minimizing the As–P exchange reaction on each surface. Regarding the solar cell performance, by implementing an InP window and AlInAs back-surface field layers on a GaInAs solar cell, the recombination at both the front and rear surface can be effectively reduced. Consequently, 13.04% conversion efficiency with a 394 mV open-circuit voltage was obtained. This is the highest efficiency ever achieved in an MBE-grown GaInAs solar cell on InP.

Effect of mask-film properties on the initial GaAs nanowire growth stages

The initial self-catalyze GaAs nanowire (NW) growth stages were studied using Monte Carlo simulation. We analyzed the effect of the mask-film etching rate with liquid gallium for different thicknesses on the initial nanowire formation stages. At a high etching rate NWs do not form on thick mask-films. A high etching rate of a thin film-mask can lead to lateral Ga drop motion over the crystalline substrate surface, which delays the nanowire formation onset. It is shown that, for the NW formation, it is necessary to maintain the correct ratio between the film thickness, etching rate, Ga and As2 flux intensities.

Structural Properties of Superlattices {LTG-GaAs/GaAs : Si} on GaAs (100) and (111)A Substrates

The proposed new structure for photoconductive antennas is a multilayer epitaxial film grown on a GaAs (111) A substrate and consisting of alternating layers of undoped GaAs grown at low temperature (LTG-GaAs) and GaAs layers formed in the standard high-temperature regime and doped with silicon (GaAs : Si). The ratio of As4 and Ga fluxes (γ) was chosen such that the GaAs : Si layers hadp-type conductivity. LTG-GaAs layers were grown at an increased γ value. A similar structure was grown on a (100) substrate as a reference sample. Phase composition was investigated using high-resolution X-ray diffractometry, surface relief — by mean of atomic force microscopy and distribution of Si impurity over thickness — using secondary ion mass spectrometry.

Study of the initial stage of GaAs growth on FIB-modified silicon substrates

In this work, we studied the effect of the deposition thickness, growth rate, arsenic flux, and implantation dose on the morphology of the GaAs nanostructures grown on modified Si areas. It is shown that an increase in the growth rate at the initial stages of the growth process leads to the transition of the growth regime from layered-like to one-dimensional with the formation of nanowires. Studies of the effect of As4 pressure have shown that a change in the equivalent As4 flux in the range of 3.7 - 5.0 ML/s does not lead to any significant change in the structure of the GaAs layer in the modified areas. An increase in the implantation dose during processing with a focused ion beam led to disordering of the directions of the grown nanowires due to the degradation of the substrate crystal structure.

Influence of gallium and arsenic deposition rates on the GaAs planar nanowire morphology

Using the Monte Carlo simulation, the dependence of GaAs nanowire morphology on Ga and As2 flux intensities was analyzed. The self-catalyzed growth of planar GaAs nanowires on GaAs(111)A substrates was considered. Depending on the arsenic and gallium fluxes, three possible scenarios of the GaAs nanowire formation were shown. The regions of gallium and arsenic deposition rates necessary for obtaining a stable planar growth of GaAs nanowires were revealed. The reasons for the transition of planar GaAs nanowire growth to the nonplanar one were analyzed.

MBE growth strategy and optimization of GaAsBi quantum well light emitting structure beyond 1.2 μm

GaAs1−xBix/AlGaAs quantum wells (QWs) with varying As/Ga beam equivalent pressure (BEP) ratios were grown by solid source molecular beam epitaxy at a relatively high temperature of 350–400 °C intended for light emitting applications with wavelengths beyond 1.2 μm. Both the Bi content and the photoluminescence (PL) intensity were found to be highly dependent on As2 flux, especially for the case of growing GaAsBi at a relatively high temperature. A graded index separate confinement GaAsBi/AlGaAs single QW with 5.8% Bi exhibited a strong PL emission at 1.22 μm. The growth strategy to incorporate considerable Bi into GaAs at a relatively high temperature through meticulous control of the As/Ga BEP ratio and compensation of Bi flux is demonstrated to be effective in guaranteeing a high Bi content as well as an optimal optical performance of GaAsBi QWs.

The Growth of InAsxSb1 –x Solid Solutions on Misoriented GaAs(001) Substrates by Molecular-Beam Epitaxy

The effect of a substrate misorientation degree from a singular face on the composition and morphology of layers of InAsxSb1 –x solid solutions obtained by molecular-beam epitaxy on a GaAs surface has been investigated. The substrates of GaAs wafers with the orientation (100) misoriented in the direction [110] by 0°, 1°, 2°, and 5° are used. The heterostructures are grown at temperatures of 310°C and 380°C (respectively, the lower and upper boundaries of the temperature range in which structurally perfect InAsxSb1 –x films form). The effect of the molecular form of arsenic (As2 or As4) on the composition of the layers is studied. The composition and structural properties are investigated using high-resolution X-ray diffractometry (HRXRD) and atomic-force microscopy (AFM). It is established that, in the series of misorientation angles 0° → 5°, the arsenic fraction x increases consecutively when using fluxes of both As2 and As4 molecules. With the As2 molecular flux, the fraction x increases only a little (1.05 times) with increasing degree of misorientation, while, when using the As4 flux, the increase in x is 1.75 times. An increase in the growth temperature leads to growth in the arsenic fraction in the solid solution. The surface morphology improves with an increasing degree of misorientation at a low growth temperature and degrades at a high temperature.

Electrical and Photoluminescence Studies of {LT-GaAs/GaAs:Si} Superlattices Grown by MBE on (100)- and (111)A-Oriented GaAs Substrates

The results of studying semiconductor structures proposed for the first time and grown, which combine the properties of LT-GaAs with p-type conductivity upon doping with Si, are presented. The structures are {LT-GaAs/GaAs:Si} superlattices, in which the LT-GaAs layers are grown at a low temperature (in the range 280–350°C) and the GaAs:Si layers at a higher temperature (470°C). The p-type conductivity upon doping with Si is provided by the use of GaAs(111)A substrates and the choice of the growth temperature and the ratio between As4 and Ga fluxes. The hole concentration steadily decreases, as the growth temperature of LT-GaAs layers is lowered from 350 to 280°C, which is attributed to an increase in the roughness of interfaces between layers and to the formation of regions depleted of charge carriers at the interfaces between the GaAs:Si and LT-GaAS layers. The evolution of the photoluminescence spectra at 77 K under variations in the growth temperature of LT-GaAs is interpreted as a result of changes in the concentration of GaAs and VGa point defects and SiGa–VGa, VAs–SiAs, and SiAs–SiGa complexes.

Engineering the Size Distributions of Ordered GaAs Nanowires on Silicon.

Reproducible integration of III-V semiconductors on silicon can open new path toward CMOS compatible optoelectronics and novel design schemes in next generation solar cells. Ordered arrays of nanowires could accomplish this task, provided they are obtained in high yield and uniformity. In this work, we provide understanding on the physical factors affecting size uniformity in ordered GaAs arrays grown on silicon. We show that the length and diameter distributions in the initial stage of growth are not much influenced by the Poissonian fluctuation-induced broadening, but rather are determined by the long incubation stage. We also show that the size distributions are consistent with the double exponential shapes typical for macroscopic nucleation with a large critical length after which the nanowires grow irreversibly. The size uniformity is dramatically improved by increasing the As4 flux, suggesting a new path for obtaining highly uniform arrays of GaAs nanowires on silicon.

Monte Carlo simulation of the formation of AIIIBV nanostructures with the use of droplet epitaxy

A Monte Carlo lattice model is proposed to describe the formation of semiconductor nanostructures by the vapor–liquid–solid growth mechanism. This model is used to simulate the growth of GaAs nanostructures by the droplet epitaxy technique in the temperature range from 500 to 600 K in As2 fluxes with intensity of 0.005–0.04 ML/s. The morphology of the formed structures is demonstrated to depend on the growth parameters. Etching of the GaAs substrate by a gallium droplet is studied. The ranges of temperature and As flux rates necessary for the formation of GaAs nanorings are determined. The conditions of the formation of single and double concentric rings are analyzed.

Structural and photoluminescence properties of low-temperature GaAs grown on GaAs(100) and GaAs(111)A substrates

Undoped, uniformly Si-doped, and δ-Si-doped GaAs layers grown by molecular-beam epitaxy on (100)- and (111)A-oriented GaAs substrates at a temperature of 230°C are studied. The As4 pressure is varied. The surface roughness of the sample is established by atomic-force microscopy; the crystal quality, by X-ray diffraction measurements; and the energy levels of different defects, by photoluminescence spectroscopy at a temperature of 79 K. It is shown that the crystal structure is more imperfect in the case of GaAs(111)A substrates. The effect of the As4 flux during growth on the structure of low-temperature GaAs grown on different types of substrates is shown as well.

Molecular beam epitaxy of III-P x As1 − x solid solutions: Mechanism of composition formation in the sublattice of a group V element

The effect of substrate temperature, As2 and P2 molecular flux densities, and growth rate on the composition of III-P x As1 − x solid solution layers prepared by molecular beam epitaxy is experimentally investigated. Experimental data in a wide range of growth conditions are analyzed. The results obtained are presented in the form of a kinetic model for describing the process of formation of the composition in the Group V sublattice of the III-P x As1 − x solid solution upon molecular beam epitaxy. The model can be used for choosing the growth conditions of the III-P x As1 − x (001) solid-solution layers of a specified composition.

Ordering of InGaAs Quantum Dots Grown by Molecular Beam Epitaxy under As2 gas flux

ABSTRACTThe influence of the substrate temperature on the morphology and ordering of InGaAs quantum dots (QD), grown on GaAs (001) wafers by Molecular Beam Epitaxy (MBE) under As2 flux has been studied using Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM) and Photoluminescence (PL) measurements. The experimental results show that lateral and vertical orderings occur for temperatures greater than 520°C and that QDs self-organize in a 6-fold symmetry network on (001) surface for T=555°C. Vertical orderings of asymmetric QDs, along directions a few degrees off [001], are observed on a large scale and their formation is discussed.

Control of InGaAs and InAs facets using metal modulation epitaxy

Control of faceting during epitaxy is critical for nanoscale devices. This work identifies the origins of gaps and different facets during regrowth of InGaAs and InAs adjacent to patterned features. Molecular beam epitaxy near SiO2 or SiNx led to gaps, roughness, or polycrystalline growth, but low-arsenic metal modulated epitaxy produced smooth and gap-free (001) planar growth up to the gate. The resulting self-aligned field effect transistors (FETs) were dominated by FET channel resistance rather than source–drain access resistance. Higher As2 fluxes led first to conformal growth, then pronounced {111} facets sloping up away from the mask.

Fabrication of highly stacked quantum dots on vicinal (001) InP substrates using strain‐compensation technique

AbstractHighly stacked InAs quantum dots (QDs) were grown on vicinal (001) InP substrates by the strain‐compensation technique. The vicinal surface and As2 flux played important roles in changing quantum dashes into QDs on the (001) surface. Strain compensation was employed to stack 30 layers of QDs without degrading the quality of the crystal. In addition, the QDs exhibited strong photoluminescence emission at 1.55 µm and room temperature. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ring-to-dots transformation of InGaAs quantum rings grown by droplet epitaxy

Post-growth annealing in UHV can lead to the deformation of crystallized nanostructures. Annealing effects on structural and optical properties of InGaAs quantum rings (QRs) grown by droplet epitaxy was examined. The InGaAs QRs were fabricated by depositing 3 monolayer (ML) of In0.5Ga0.5 on GaAs (100) at the substrate temperature of 140^oC, and crystallized in As4 flux of ~7x10^-^6Torr at 180^oC for 5min. After the crystallization, the substrate temperature was ramped up to the desired annealing temperature (Ta) of 300-400^oC in As4 beam. In situ transformations of surface morphology were observed upon the evolution of RHEED patterns. Surface morphology was analyzed by AFM. As ramping up the annealing temperature, the QRs deformed and changed to numerous smaller QDs about the QR positions. Supposedly, there was segregation of group III atoms out of the QRs. At 380^oC, the QRs properly lost their actual shapes and burst into high-density small QDs. Most possible reasons of the segregation can be crystalline instability of the low-temperature-crystallized QRs, along with the different surface kinetics of In and Ga atoms.

As flux dependence on RHEED transients during InAs quantum dot growth

We study the role of As during InAs quantum dots (QDs) growth by molecular beam epitaxy using reflection high-energy electron diffraction (RHEED). We observe that the RHEED intensity transient onset and the transition to the saturation region occur at less InAs thickness under higher As4 flux. From the RHEED transients, we find that the diffusion time does not depend on the As4 flux while the diffusion barrier does. The diffusion length of In adatoms can be reduced by increasing the diffusion barrier using higher As4 flux, allowing us to grow InAs QDs at higher densities.

Molecular Beam Epitaxy Growth of GaAs/InAs Core–Shell Nanowires and Fabrication of InAs Nanotubes

We present results about the growth of GaAs/InAs core-shell nanowires (NWs) using molecular beam epitaxy. The core is grown via the Ga droplet-assisted growth mechanism. For a homogeneous growth of the InAs shell, the As(4) flux and substrate temperature are critical. The shell growth starts with InAs islands along the NW core, which increase in time and merge giving finally a continuous and smooth layer. At the top of the NWs, a small part of the core is free of InAs indicating a crystal phase selective growth. This allows a precise measurement of the shell thickness and the fabrication of InAs nanotubes by selective etching. The strain relaxation in the shell occurs mainly via the formation of misfit dislocations and saturates at ~80%. Additionally, other types of defects are observed, namely stacking faults transferred from the core or formed in the shell, and threading dislocations.