Molecular Beam Epitaxy Growth Parameters Research Articles

Asymptotic stage of self-catalyzed growth of III-V nanowires by molecular beam epitaxy

A new analytic theory is developed for asymptotic stage of self-catalyzed growth of III-V nanowires (NWs) by molecular beam epitaxy (MBE), where NWs collect all group III atoms deposited from vapor. The shadowing NW length is derived which corresponds for the full shadowing of the substrate surface in MBE. The NW length and radius are derived depending on the effective deposition thickness and MBE growth parameters. It is shown that the NW length increases, and their length decreases with decreasing the array pitch and increasing the V/III flux ratio. Keywords: II-V nanowires, shadowing effect, length, radius, surface density, modeling

Асимптотическая стадия роста автокаталитических III-V нитевидных нанокристаллов методом молекулярно-пучковой эпитаксии

A new analytic theory is developed for asymptotic stage of self-catalyzed growth of III-V nanowires (NWs) by molecular beam epitaxy (MBE), where NWs collect all group III atoms deposited from vapor. The shadowing NW length is derived which corresponds for the full shadowing of the substrate surface in MBE. The NW length and radius are derived depending on the effective deposition thickness and MBE growth parameters. It is shown that the NW length increases, and their length decreases with decreasing the array pitch and increasing the V/III flux ratio.

Monolayer 1T-NbSe2 as a 2D-correlated magnetic insulator.

Monolayer group V transition metal dichalcogenides in their 1T phase have recently emerged as a platform to investigate rich phases of matter, such as spin liquid and ferromagnetism, resulting from strong electron correlations. Newly emerging 1T-NbSe2 has inspired theoretical investigations predicting collective phenomena such as charge transfer gap and ferromagnetism in two dimensions; however, the experimental evidence is still lacking. Here, by controlling the molecular beam epitaxy growth parameters, we demonstrate the successful growth of high-quality single-phase 1T-NbSe2. By combining scanning tunneling microscopy/spectroscopy and ab initio calculations, we show that this system is a charge transfer insulator with the upper Hubbard band located above the valence band maximum. To demonstrate the electron correlation resulted magnetic property, we create a vertical 1T/2H NbSe2 heterostructure, and we find unambiguous evidence of exchange interactions between the localized magnetic moments in 1T phase and the metallic/superconducting phase exemplified by Kondo resonances and Yu-Shiba-Rusinov–like bound states.

Self-assembly of (111)-oriented tensile-strained quantum dots by molecular beam epitaxy

The authors report on a comprehensive study of the growth of coherently strained GaAs quantum dots (QDs) on (111) surfaces via the Stranski–Krastanov (SK) self-assembly mechanism. Recent reports indicate that the long-standing challenges, whereby the SK growth mechanism could not be used to synthesize QDs on (111) surfaces, or QDs under tensile strain, have been overcome. However, a systematic study of the SK growth of (111)-oriented, tensile-strained QDs (TSQDs) as a function of molecular beam epitaxy growth parameters is still needed. Here, the authors explore the effects of deposition amount, substrate temperature, growth rate, and V/III flux ratio on the SK-driven self-assembly of GaAs(111)A TSQDs. The authors highlight aspects of TSQD SK self-assembly on (111) surfaces that appear to differ from the SK growth of traditional compressively strained QDs on (100) surfaces. The unique properties of (111) QDs and tensile-strained QDs mean that they are of interest for various research areas. The results discussed here offer a practical guide for tailoring the size, shape, density, uniformity, and photon emission wavelength and intensity of (111) TSQDs for future applications.

Review Article: Molecular beam epitaxy of lattice-matched InAlAs and InGaAs layers on InP (111)A, (111)B, and (110)

For more than 50 years, research into III–V compound semiconductors has focused almost exclusively on materials grown on (001)-oriented substrates. In part, this is due to the relative ease with which III–Vs can be grown on (001) surfaces. However, in recent years, a number of key technologies have emerged that could be realized, or vastly improved, by the ability to also grow high-quality III–Vs on (111)- or (110)-oriented substrates These applications include: next-generation field-effect transistors, novel quantum dots, entangled photon emitters, spintronics, topological insulators, and transition metal dichalcogenides. The first purpose of this paper is to present a comprehensive review of the literature concerning growth by molecular beam epitaxy (MBE) of III–Vs on (111) and (110) substrates. The second is to describe our recent experimental findings on the growth, morphology, electrical, and optical properties of layers grown on non-(001) InP wafers. Taking InP(111)A, InP(111)B, and InP(110) substrates in turn, the authors systematically discuss growth of both In0.52Al0.48As and In0.53Ga0.47As on these surfaces. For each material system, the authors identify the main challenges for growth, and the key growth parameter–property relationships, trends, and interdependencies. The authors conclude with a section summarizing the MBE conditions needed to optimize the structural, optical and electrical properties of GaAs, InAlAs and InGaAs grown with (111) and (110) orientations. In most cases, the MBE growth parameters the authors recommend will enable the reader to grow high-quality material on these increasingly important non-(001) surfaces, paving the way for exciting technological advances.

Optimization of In2Se3/Si(111) Heteroepitaxy To Enable Bi2Se3/In2Se3 Bilayer Growth

Systematic optimization of molecular beam epitaxy growth parameters enabled high quality heteroepitaxy of In2Se3 on Si(111) surfaces. Surfaces of the best epilayers were characterized by atomically flat terraces that extended laterally for several hundred nanometers. These terraces were separated by single quintuple layer high steps. These In2Se3 films were suitable for subsequent high quality epitaxy of Bi2Se3. The quality of the In2Se3/Bi2Se3 interface was confirmed using atomic resolution transmission electron microscopy.

MBE-Grown ZnTe/Si, a Low-Cost Composite Substrate

Growth of ZnTe on Si using molecular beam epitaxy (MBE) has been pursued as a new approach for a lattice-matched, large-area, low-cost alternate substrate for both II–VI and III–V compound semiconductors with lattice constants very near 6.1 A, such as HgCdSe and GaSb-based type II strained-layer superlattices. In this paper, we report our findings on the systematic study of MBE growth parameters for both ZnTe(211) on Si(211) and ZnTe(100) on Si(100). Near-optimal growth procedures have been established for producing ZnTe/Si wafers with high crystallinity, low defect and etch pit densities, as well as excellent surface morphology. Using this baseline MBE growth process, we obtained ZnTe(211)/Si wafers with x-ray full-width at half-maximum as low as 70 arcsec.

MBE growth optimization of topological insulator Bi 2Te 3 films

We investigated the growth of the topological insulator Bi 2Te 3 on Si(1 1 1) substrates by means of molecular-beam epitaxy (MBE). The substrate temperature as well as the Bi and Te beam-equivalent pressure (BEP) was varied in a large range. The structure and morphology of the layers were studied using X-ray diffraction (XRD), X-ray reflectivity (XRR) and atomic force microscopy (AFM). The layer-by-layer growth mode with quintuple layer (QL) as an unit is accomplished on large plateaus if the MBE growth takes place in a Te overpressure. At carefully optimized MBE growth parameters, we obtained atomically smooth, single-crystal Bi 2Te 3 with large area single QL covering about 75% of the layer surface. Angular-resolved photoelectron spectroscopy reveals a linear energy dispersion of charge carriers at the surface, evidencing topologically insulating properties of the Bi 2Te 3 epilayers.

Growth mechanism of cuboid growth pits in lead selenide epilayers grown by molecular beam epitaxy

Microstructures and crystallographic orientations of cuboid growth pits in lead selenide (PbSe) epilayers grown on Si (1 1 1) by molecular beam epitaxy (MBE) have been studied by scanning electron microscopy and cross-sectional transmission electron microscopy. Cuboid density was found to be dependent on MBE growth parameters such as sample thickness and substrate temperature. Cuboid growth defects nucleate spontaneously on the PbSe growth surface probably at Pb droplets. This nucleation results in randomly oriented PbSe crystallites that preferentially grow along the [1 0 0] axis of the NaCl-type crystal structure. This preferential growth results in cuboid crystallites with cubic faces protruding from the epitaxial (1 1 1) face.

Low temperature transport in undoped mesoscopic structures

Undoped GaAs/AlGaAs heterostructures in which carriers are attracted from the Ohmic contacts by a voltage bias on an insulated top gate allows higher mobilities to be obtained at lower electron densities than is possible with modulation doped heterostructures. However a two level gating scheme and an Ohmic contacting process that maximizes lateral diffusion are necessary to fully exploit the advantages of the undoped system for fabricating lower dimensional mesoscopic structures. Ionized background impurities (at low densities) and interface roughness (at high densities) are found to be the dominant sources of scattering. An approximate length scale set by the number of impurities the interfacial wave function intersects is observed in the magnetoconductance of two-dimensional mesoscopic regions.

선택적 분자선 에픽택시 방법에 의한 1D-2DEG 혼성 나노선 FET의 구현

High quality three-dimensional (3D) heterostructures were constructed by selective area (SA) molecular beam epitaxy (MBE) using a specially patterned GaAs (001) substrate to improve the efficiency of tarrier transport. MBE growth parameters such as substrate temperature, V/III ratio, growth ratio, group V sources (As2, As4) were varied to calibrate the selective area growth conditions and the 3D GaAs-AlGaAs heterostructures were fabricated into the ridge type and the V-groove type. Scanning micro-photoluminescence measurements and the following analysis revealed that the gradually (adiabatically) coupled 1D-2DEG (electron gas) field effect transistor (FET) system was successfully realized. These 3D-heterostructures are expected to be useful for the realization of high-performance mesoscopic electronic devices and circuits since it makes it possible to form direct ohmic contact onto the (quasi) 1D electron channel.

Controllable growth of semiconductor nanometer structures

Self-assembled quantum dots and wires were obtained in the InxGa1-xAs/GaAs and InAs/In0.52Al0.48As/InP systems, respectively, using molecular beam epitaxy (MBE). Uniformity in the distribution, density, and spatial ordering of the nanostructures can be controlled to some extent by adjusting and optimizing the MBE growth parameters. In addition, some interesting observation on the InAs wire alignment on InP(001) is discussed.

Growth pyramids on Si(111) facets: A CVD and MBE study

The morphology of growth pyramids on Si(111) facets has been studied with an atomic force microscope. These facets with pyramids are formed upon chemical vapor deposition (CVD) growth on a hemispherical substrate. Spiral as well as concentric step edge patterns have been observed on the growth pyramids. Based on the step edge patterns the origin of the pyramids is attributed to different types of dislocations. Step edge patterns indicate that growth during CVD is governed by step edge processes and that surface diffusion is not rate limiting. The completely different growth mechanism of molecular-beam epitaxy (MBE) is illustrated by MBE overgrowth of a silicon layer on top of the CVD layer. MBE growth parameters have been studied by comparing the growth pyramid morphology with simulation results.

In situ composition control of III-As1 − xSbx alloys during molecular beam epitaxy using line-of-sight mass spectrometry

The interaction of a Sb2 molecular beam with the film surface during molecular beam epitaxy (MBE) of GaAs1 − xSbx layers was monitored by a quadrupole mass spectrometer (QMS) positioned in line-of-sight of the substrate. It was observed that the QMS signal from the monomeric Sb-species (massq = 121 or 123 amute) is proportional to the total Sb-flux leaving the film surface within a wide range of temperatures and surface composition. This enabled the accurate determination of the antimony incorporation coefficient, σ(Sb), and consequently the Sb-mole fraction during growth. Due to its applicability in growth conditions where σ(Sb) is less than unity, this compositional monitoring technique expands the practical range of MBE growth parameters for III-As1 − xSbx alloy growth. Real-time in situ control of alloy composition was demonstrated by implementing a feedback control scheme based on the QMS Sb signal. GaAs1 − xSbx alloys in which the Sb-fraction varied in a controlled manner in the direction of growth were deposited.

Modelling imperfections of epitaxial heterostructures by means of X-ray diffraction analysis

Interface intermixing of highly strained heterostructures was investigated using high-resolution double-crystal X-ray diffractometry combined with computer simulations. A method of modelling imperfections in semiconductor heterostructures was introduced to extract information from simulated and measured data in a fast and objective manner. The key steps of this method are i) the sensitivity analysis of experimental diffraction curves with respect to imperfections, and ii) the development of objective error criteria. The applicability of this method is demonstrated using highly strained [(InAs/GaAs)12/InGaAs]9 superlattices grown by molecular beam epitaxy (MBE) on InP substrates. The result of the modelling process enables a quantitative and objective determination of the intermixing at InAs/GaAs interfaces in relation to the MBE growth parameters which can be transferred to any other laboratory.

Exciton luminescence in Si1−xGex/Si heterostructures grown by molecular beam epitaxy

Coherent Si1−xGex alloys and multilayers synthesized by molecular beam epitaxy (MBE) on Si(100) substrates have been characterized by low-temperature photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM). Phonon-resolved transitions originating from excitons bound to shallow impurities were observed in addition to a broad band of intense luminescence. The broad PL band was predominant when the alloy layer thickness was greater than 40–100 Å, depending on x and the strain energy density. The strength of the broad PL band was correlated with the areal density (up to ∼109 cm−2) of strain perturbations (local lattice dilation ∼15 Å in diameter) observed in plan-view TEM. Thinner alloy layers exhibited phonon-resolved PL spectra, similar to bulk material, but shifted in energy due to strain and hole quantum confinement. Photoluminescence excitation spectroscopy, external quantum efficiency, time-resolved PL decay, together with the power and temperature dependence of luminescence intensity, have been used to characterize Si1−xGex/Si heterostructures exhibiting both types of PL spectra. The role of MBE growth parameters in determining optical properties was investigated by changing the quantum well thickness and growth temperature. The transition from phonon-resolved, near-band-gap luminescence in thin layers to the broad PL band typical of thick layers is discussed in terms of a strain energy balance model which predicts a ‘‘transition thickness’’ which decreases with increase in x.

EuTe/PbTe Superlattices: Mbe Growth and Optical Characterization

ABSTRACTThe heteroepitaxial growth of EuTe on PbTe (111) by molecular beam epitaxy (MBE) was investigated using in situ reflection high energy electron diffraction (RHEED). As a function of substrate temperature and Te2 flux rate, the resulting EuTe (111) surfaces exhibit several different surface reconstructions corresponding to Te-stabilized or Eu-stabilized surfaces. The Eustabilized surface shows a (2√3 × 2√3)R30° surface reconstruction. Because of the strain induced tendency for 3D islanding, only in a narrow window of MBE growth parameters perfect 2D layer-by-layer heteroepitaxial growth exists. Using such optimized MBE growth conditions, we have fabricated a series of PbTe/EuTe superlattices. In such superlattices the wide band gap EuTe layers act as barriers and the narrow band gap PbTe as quantum wells. The superlattices were investigated by high resolution x-ray diffraction, showing their high structural perfection. Modulated low temperature Fourier transform infrared reflection measurements were performed in order to determine the confined energy levels in the PbTe quantum wells. The measurements indicate that mini-subbands are formed in the PbTe quantum wells with a mini-band width of 22 meV in agreement with envelope function calculations.

Growth-induced shallow acceptor defect and related luminescence effects in molecular beam epitaxial GaAs

We report a study of a defect responsible for the ‘‘g’’ bound exciton line at 1.5112 eV that is frequently detected in photoluminescence spectra of GaAs grown by molecular beam epitaxy (MBE). A direct correlation has been observed between this line and a transition at 1.4946 eV, which is shown to result from a conduction band-to-acceptor recombination involving a shallow, unidentified acceptorlike defect that is labeled ‘‘A.’’ The activation energy of the defect is 24.8±0.2 meV, about 1.7 meV lower than that of CAs acceptor. Upon hydrogenation the defect is passivated more extensively than any known shallow acceptor species in GaAs. This result is analyzed in terms of a passivation model, from which it can be inferred that the A defect is not due to a simple substitutional Group II impurity on a Ga site. Incorporation of the A defect strongly affects the luminescence properties of the material. An almost complete quenching of the donor-bound exciton lines, profound changes in the line shape and relative intensity of the free exciton recombination, and appearance of a sharp transition of unknown origin at 1.5138 eV were observed with increasing defect concentration. Apparently ‘‘donorless’’ low temperature exciton recombination spectra are reported for defect-rich p-type MBE GaAs layers with donor concentrations as high as 7×1014 cm−3 and compensation ratios of ∼0.3. The dependence of the defect incorporation on MBE growth parameters is discussed. The feasibility of MBE growth of high purity, nearly shallow defect-free p-type GaAs layers at marginally As-stabilized surface conditions over an about 1–5 μm/h range of deposition rates is demonstrated.

Growth kinetics, impurity incorporation, defect generation, and interface quality of molecular-beam epitaxy grown AlGaAs/GaAs quantum wells: Role of group III and group V fluxes

A systematic knowledge of the influence of molecular-beam epitaxy growth parameters on the properties of AlGaAs/GaAs quantum wells grown under realistic growth conditions is important in order to obtain optimal performance of modern electronic and optoelectronic devices. Photoluminescence (PL) was used to investigate the influence of the Ga-controlled growth rate in the range below standard growth rates of 1 μm/h down to 0.1 μm/h, and of the As:Ga beam equivalent pressure ratio in the range of 10 to 60, on the growth kinetics, the interface quality, and the impurity incorporation, at a substrate temperature Ts =620 °C. As compared to in situ reflection high-energy electron diffraction (RHEED) measurements, where no sample rotation is possible, PL has the advantage that realistic growth conditions can be used. A careful line shape analysis, together with infrared and time-resolved PL measurements gives information on the interface roughness, the impurity incorporation, and the deep trap concentration. Non-negligible desorption of Ga during growth is found for the range of conditions under study. The desorption is found to increase upon a decrease of As:Ga ratio. The interface roughness as well as the impurity and trap incorporation are found to decrease with decreasing growth rate, an optimum interface quality being obtained below 0.5 μm/h. At this optimal growth rate, increasing the As4:Ga ratio leads to a decrease of shallow impurity concentration and thus to a narrower line width, but to a simultaneous increase of defect generation. Optimal growth conditions are found at a beam equivalent pressure ratio of 15. The observed desorption kinetics and interface properties can be explained in accordance with existing theoretical simulations. Finally, growth interruption was found to lead to optimal formation of flat growth islands when the overall growth rate is lowered.

Trends of deep level electron traps in AlxGa1−xAs grown by molecular beam epitaxy

Deep level transient spectroscopy is used to study one of the deep electron traps commonly seen in AlxGa1−xAs grown by molecular beam epitaxy (MBE). This trap, having the largest capture cross section of the commonly observed levels in MBE material, is measured in samples with Al composition varying from 8 to 40%. It is shown that the activation energy remains fixed with respect to the valence band, over the composition range studied. The trap is tentatively identified as a group III related vacancy. The effects of varying MBE growth parameters are discussed.