Use Of Molecular Beam Epitaxy Research Articles

HRXRD study of the effect of a nanoporous silicon layer on the epitaxial growth quality of GaN layer on the templates of SiC/por-Si/c-Si

Using High Resolution X-ray Diffraction (HRXRD) diagnostic techniques the influence of the transition layer of nanoporous silicon on the practical implementation and certain features of the epitaxial growth of GaN layers with the use of molecular beam epitaxy were investigated by means of plasma activation of nitrogen (MBE PA) on the templates of SiC/por-Si/c-Si. For the first time it was shown that introducing of the transition layer of nanoporous silicon in the template of SiC/por-Si/c-Si where the layer of 3C-SiC was obtained by substitution of the atoms had a number of indisputable advantages as compared with conventional silicon substrates. Particularly, such an approach, in fact, enabled a 90% reduction in the level of stresses in the crystalline lattice of the epitaxial GaN layer which was synthesized on SiC surface of SiC/por-Si/c-Si template by means of MBE PA technique as well as to decrease some of vertical dislocations within GaN layer.

Вертикально-излучающий лазер спектрального диапазона 1.55 μm с туннельным переходом на основе слоев n-=SUP=-++-=/SUP=--InGaAs/p-=SUP=-++-=/SUP=--InGaAs/p-=SUP=-++-=/SUP=--InAlGaAs

The design of the n++-InGaAs/р++-InGaAs/р++-InAlGaAs tunnel junction (TJ) for 1.55 μm range vertical-cavity surface-emitting lasers (VCSELs), developed by wafer fusion technique of InAlGaAsP/InP optical cavity with AlGaAs/GaAs distributed Bragg reflectors is proposed and realized. The presence of oxidation-resistant InGaAs layers allows the use of molecular-beam epitaxy at all stages of the heterostructure fabrication, including for regrowth of the TJ surface relief. In the case of using the n++-InGaAs/р++-InGaAs/р++-InAlGaAs TJ, a noticeable increase in the internal optical losses compared to the n++/р++-InAlGaAs TJ design was not obtained. The increase in internal optical loss in lasers can be avoided due to Burshtein-Moss effect in n++-InGaAs layers and thickness minimization of р++-InGaAs layer. As a result, the characteristics of fabricated lasers are comparable with characteristics of VCSELs with n++/p++-InAlGaAs TJ with a similar level of mirror losses.

Midwave infrared barrier detector based on Ga-free InAs/InAsSb type-II superlattice grown by molecular beam epitaxy on Si substrate

In this paper, we report on the demonstration of a midwave infrared (MWIR) Ga-free InAs/InAsSb superlattice (SL) barrier detector, grown monolithically on a Si substrate by molecular beam epitaxy (MBE). Prior to the use of MBE, the (1 0 0)-Si substrate was prepared in situ in a metal organic chemical vapor deposition (MOCVD) reactor, where an 800 nm thick GaSb buffer layer was grown. Next, a barrier structure with an active zone made of strain-balanced InAs/InAsSb SL, lattice-matched to GaSb, was epitaxied. Structural characterisations revealed that the material quality was degraded compared with the sample grown on GaSb. The processed Ga-free SL barrier detector has a 100% cut-off wavelength of 5 µm at 150 K, for a bias operation of −500 mV. A combined analysis involving electrical and electro-optical characterisation was performed to identify the operating regimes in this MWIR Ga-free SL barrier structure.

Normally-off operating GaN-based pseudovertical MOSFETs with MBE grown source region

In this report, the operation of a normally-off vertical gallium nitride (GaN) metal-oxide field effect transistor with a threshold voltage of 5 V is demonstrated. A crucial step during device fabrication is the formation of the highly n-doped source layer. The authors infer that the use of molecular beam epitaxy (MBE) is highly beneficial for suppressing diffusion of the magnesium (Mg) p-type dopants from the body layer grown by metal-organic vapor phase epitaxy into the source cap. Repassivation of the previously activated Mg acceptors by a hydrogen out-diffusion treatment is suppressed in the ultrahigh vacuum growth environment. Structural and electrical data indicate that the defect density of the GaN substrate is currently limiting device performance much more compared to other effects like varying surface morphology resulting from fluctuations in III/N stoichiometry during the MBE growth.

GaN/NbN epitaxial semiconductor/superconductor heterostructures.

Epitaxy is a process by which a thin layer of one crystal is deposited in an ordered fashion onto a substrate crystal. The direct epitaxial growth of semiconductor heterostructures on top of crystalline superconductors has proved challenging. Here, however, we report the successful use of molecular beam epitaxy to grow and integrate niobium nitride (NbN)-based superconductors with the wide-bandgap family of semiconductors-silicon carbide, gallium nitride (GaN) and aluminium gallium nitride (AlGaN). We apply molecular beam epitaxy to grow an AlGaN/GaN quantum-well heterostructure directly on top of an ultrathin crystalline NbN superconductor. The resulting high-mobility, two-dimensional electron gas in the semiconductor exhibits quantum oscillations, and thus enables a semiconductor transistor-an electronic gain element-to be grown and fabricated directly on a crystalline superconductor. Using the epitaxial superconductor as the source load of the transistor, we observe in the transistor output characteristics a negative differential resistance-a feature often used in amplifiers and oscillators. Our demonstration of the direct epitaxial growth of high-quality semiconductor heterostructures and devices on crystalline nitride superconductors opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic and piezoelectric properties of the group III/nitride semiconductor family.

Functional materials integrated on III–V semiconductors

The integration of ferroelectric oxides on semiconductors has been achieved through the use of molecular beam epitaxy and solution spin coating. The oxide layers were determined to be single phase and crystalline and were oriented with the (100) along the growth direction. Epitaxial SrTiO3 and BaTiO3 buffer layers were grown on GaAs and Si using a unique MBE process for nucleation that suppressed the formation of interfacial oxide. The deposition of ferroelectric Pb(Zr)TiO3, on STO buffered Si and GaAs was carried out using solution spin coating. The nucleation of the oxide growth by solution spin coating was promoted by the crystallinity of the surface structure of the SrTiO3 buffer layer. Ab initio Density Functional Theory calculations were used to determine the interfacial structure and band alignments for the STO/GaAs and BTO/GaAs interfaces. Initial electrical characterization of the PZT grown on GaAs was determined by measuring the polarization versus electric field which suggests evidence of ferroelectricity.

Incorporation and Activation of Arsenic Dopant in Single-Crystal CdTe Grown on Si by Molecular Beam Epitaxy

We report the use of molecular beam epitaxy to achieve p-type doping of CdTe grown on Si(211) substrates, by use of an arsenic cracker and post-growth annealing. A high hole density in CdTe is crucial for high efficiency II–VI-based solar cells. We measured the density of As in single-crystal CdTe by secondary ion mass spectroscopy; this showed that high As incorporation is achieved at low growth temperatures. Progressively higher incorporation was observed during low-temperature growth, presumably because of degradation of crystal quality with incorporation of As at such defect sites as dislocations and defect complexes. After As activation annealing under Hg overpressure, hole concentrations were obtained from Hall measurements. The highest doping level was ∼2.3 × 1016 cm−3, and near-1016 cm−3 doping was readily reproduced. The activation efficiency was ∼50%, but further optimization of the growth and annealing conditions is likely to improve this value.

Molecular Beam Epitaxial Growth and Properties of Bi2Se3 Topological Insulator Layers on Different Substrate Surfaces

Growth of high-quality Bi2Se3 films is crucial not only for study of topological insulators but also for manufacture of technologically important materials. We report a study of the heteroepitaxy of single-crystal Bi2Se3 thin films grown on GaAs and InP substrates by use of molecular beam epitaxy. Surface topography, crystal structure, and electrical transport properties of these Bi2Se3 epitaxial films are indicative of highly c-axis oriented films with atomically sharp interfaces.

Three-dimensional GaN templates for molecular beam epitaxy of nonpolar InGaN/GaN coaxial light-emitting diodes

This work highlights the development of three-dimensional (3D) GaN templates grown by metal organic vapor phase epitaxy (MOVPE). These templates are ideally suited for the subsequent growth of nonpolar (11¯00) m-plane InGaN-based coaxial wire/wall light-emitting diodes (LEDs) using molecular beam epitaxy (MBE). The use of MBE is expected to result in increased indium incorporation on the (11¯00) m-plane, compared with growth using MOVPE, which provides an attractive approach for the development of nonpolar green LEDs. While planar free-standing m-plane GaN substrates are prohibitively expensive (approximately ∼$500 per cm2), the coaxial LED approach offers an attractive lower-cost alternative. These nonpolar nanoscale LED templates are also free from threading dislocations and are expected to provide a number of benefits, including higher light extraction and a larger effective active region area. Previous work in our group has demonstrated the controlled growth of GaN nanowires using a catalyst-free selective-area MOVPE growth method. The present work extends this method to grow 3D GaN templates in various aperture geometries, resulting in smooth m-plane GaN using a scalable and industrially viable high-quality GaN growth technique. In addition to m-plane sidewalls, the authors also demonstrate semipolar (11¯01) inclined sidewalls. This orientation has been shown to incorporate indium at a higher rate during MOVPE growth, in comparison to both c-plane and m-plane.

Review of 3D topological insulator thin‐film growth by molecular beam epitaxy and potential applications

Abstractmagnified imageThin films of V–VI compound semiconductors (Bi2Se3, Bi2Te3 and Sb2Te3) have been synthesized recently as three‐dimensional topological insulators (TIs). Although these materials have been used as thermoelectric materials for many years, for future studies and applications of the topological surface states, a major bottleneck remains the lack of high‐quality bulk materials that have very few defects and the Fermi level can be moved to inside the bulk bandgap. In this paper, we review the use of molecular beam epitaxy (MBE) technique to achieve high‐quality TI materials. Furthermore, the use of layered growth in MBE affords us the fabrication of heterostructures, such as quantum wells and superlattices. Thus, it may further enable additional studies and applications, similar to those of conventional semiconductor heterostructures but with the novel properties of TI. We explore the growth mechanism, providing a detail discussion on the growth parameters of thin‐film synthesis by MBE. Then we discuss more complex cases, such as functional doping, heterostructures and superlattices. Potential new properties in such quantum structures are discussed. Finally, we give an outlook on this material system for both fundamental studies and applications. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Selective-Area Epitaxy of CdTe on CdTe/ZnTe/Si(211) Through a Nanopatterned Silicon Nitride Mask

We report here the use of molecular beam epitaxy (MBE) to achieve selective-area epitaxy (SAE) and coalescence of CdTe on nanopatterned substrates with 0.5-μm-pitch arrays of CdTe/ZnTe/Si(211) seeding areas, exposed through a silicon nitride mask. The nanopatterned substrate surface morphology, crystallinity, and chemical composition were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS) both before and after exposure to CdTe flux by MBE. We find a seven times wider (422) CdTe XRD rocking curve full-width at half-maximum (FWHM) for our patterned samples, when directly compared with unpatterned samples with identical growth and pregrowth conditions. We also observe that electron beam-induced carbon deposits can serve as a mask material for SAE of CdTe by MBE. Finally, we point to possible further improvements in patterned sample architectures of the future, for use in CdTe films on Si.

DOES HIGH TEMPERATURE IONIC SUPERCONDUCTIVITY EXISTS?

The basic idea is that we try to find some materials in which bosonic ions with sufficiently small effective mass are used as charge carriers instead of Cooper's pairs in order to provide high temperature ionic superconductivity. Ionic crystals LiCl , LiF , LiBr and LiI were considered with lithium isotope Li 6. Calculations show that Bose condensation temperature for lithium ions in these crystals is of the order of 10-34–10-43 K. If, however, the crystal is compressed so that the wave functions of neighboring lithium ions are sufficiently overlapped, then Bose-condensation temperature of Li 6-ions can be increased significantly. Our estimates show that by compressing the crystals by 20–22% in all three directions, one can raise the Bose-condensation temperature in all crystals considered to above room temperature. To realize materials with room temperature superconductivity in practice, the use of molecular beam epitaxy is proposed for the formation of heterostructures from thin and thick layers of thoughtfully chosen composition.

The use of molecular beam epitaxy for the synthesis of high purity III–V nanowires

The synthesis methods and properties of catalyst-free III–V nanowires with molecularbeam epitaxy (MBE) are reviewed. The two main techniques are selective-areaepitaxy (SAE) and gallium-assisted synthesis. The excellent structure and ultra-high puritycharacteristics of the grown nanowires are presented by Raman and photoluminescencespectroscopy.

Effects of temperature, nitrogen ions, and antimony on wide depletion width GaInNAs

GaInNAs is a promising candidate material to increase the conversion efficiency of triple junction solar cells, but the dilute nitrides suffer from low-to-nonexistent minority-carrier diffusion lengths. The use of molecular beam epitaxy grown p-i-n structures with wide depletion widths can achieve high photocurrents in dilute nitrides, but this requires background doping below 2×1014cm−3 in the i layer. Here, the authors report on a number of factors that increase the net background acceptor concentration, hindering the effects to realize wide depletion widths, including high substrate temperatures, ions from the rf plasma source used to provide active nitrogen, and the addition of Sb. In addition, low substrate temperatures lead to an increase in n-type conductivity. Solar cell results that show the deleterious effects of Sb on GaInNAs devices are presented, including decreased open-circuit voltage and fill factor.

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.

Development of GaAs-based MOSFET using molecular beam epitaxy

Enhancement mode, high electron mobility PHEMT-based metal oxide semi conductor field effect transistor (MOSFET) devices have been fabricated using an oxide high- κ gate dielectric stack developed using molecular beam epitaxy. A template layer of Ga 2 O 3 , initially deposited on the surface of the III–V device unpins the GaAs Fermi level while the deposition of a bulk ternary ( Gd x Ga 1 - x ) 2 O 3 layer forms the highly resistive layer to reduce leakage current through the dielectric stack. The use of molecular beam epitaxy allows for the control and uniformity of the oxide layers along the growth direction and deposition conditions were optimized for oxide surface morphology and interface quality. A midgap interface state density for the high- κ stack on GaAs of ≅ 2 × 10 11 cm - 2 eV - 1 and a dielectric constant of κ ≅ 20 are determined using electrical measurements. Enhancement-mode n-channel MOSFETs with a gate length of 1 μ m and a source-drain spacing of 3 μ m show a threshold voltage, saturation current, transconductance, and on-resistance of 0.11 V, 380 mA mm - 1 , 250 mS mm - 1 , and 4.5 Ω mm , respectively.

Gate dielectric on compound semiconductors by molecular beam epitaxy

An oxide-based high-κ gate dielectric stack for GaAs has been developed using molecular beam epitaxy. A template layer of Ga2O3, initially deposited on a GaAs(001) surface, serves to unpin the GaAs Fermi level while the deposition of a bulk ternary (GdxGa1−x)2O3 layer forms the highly resistive layer to reduce leakage current through the dielectric stack. The use of molecular beam epitaxy allows for the control and uniformity of the oxide layers along the growth direction and deposition conditions were optimized for oxide surface morphology and interface quality. The midgap interface state density for the high-κ stack on GaAs of ≅2×1011cm−2eV−1 and a dielectric constant of k≅20 are determined using electrical measurements. Metal oxide semiconductor field effect transistor layer structures were produced by replacing the GaAs cap layer in a pseudomorphic high electron mobility transistor-type layer structure with the high-κ gate dielectric stack.

Fabrication of combinatorial nm-planar electrode array for high throughput evaluation of organic semiconductors

We have fabricated a combinatorial nm-planar electrode array by using photolithography and chemical mechanical polishing processes for high throughput electrical evaluation of organic devices. Sub-nm precision was achieved with respect to the average level difference between each pair of electrodes and a dielectric layer. The insulating property between the electrodes is high enough to measure I–V characteristics of organic semiconductors. Bottom-contact field-effect-transistors (FETs) of pentacene were fabricated on this electrode array by use of molecular beam epitaxy. It was demonstrated that the array could be used as a pre-patterned device substrate for high throughput screening of the electrical properties of organic semiconductors.

Gate dielectrics on compound semiconductors

An oxide-based high-κ gate dielectric for GaAs has been developed using molecular beam epitaxy. A template layer of Ga2O3 is initially deposited on a GaAs(001) surface followed by the deposition of a bulk (GdxGa1−x)2O3 layer. The Ga2O3 layer serves to unpin the GaAs Fermi level while the GdGaO layer forms the high resistive layer to reduce leakage current through the dielectric stack. Deposition conditions during MBE were optimized for oxide surface morphology and interface quality. The use of molecular beam epitaxy also allowed for the control and uniformity of the ternary GdGaO layer along the growth direction. The midgap interface state density for the high-κ stack on GaAs of≅2×1011 cm−2 eV−1, and a dielectric constant of k≅20 were determined using electrical measurements. MOSFET devices were produced by replacing the GaAs cap layer in a PHEMT-type structure with a thick oxide layer. A contactless method was used to determine the mobilities from such devices which were comparable to those measured on a GaAs PHEMT using conventional Hall measurement.

Structure and tunnel magnetoresistance in Fe/MgF2/Co junctions with an oxide seed layer on an Fe bottom electrode

Fe/MgF 2 / Co magnetic tunnel junctions (MTJs) with an oxide seed layer (MgO, Fe–O) inserted between an Fe bottom electrode and a MgF2 barrier layer have been prepared through the use of molecular beam epitaxy, and the effects of the seed layers on structure of subsequently deposited MgF2 barrier layers and tunnel magnetoresistance (TMR) have been investigated. The crystallographic orientation of MgF2 layers depends significantly on the seed layers, and furthermore, it has been found that the surface roughness of MgF2 layer is reduced by inserting a MgO seed layer. In Fe/MgF2 (3–12 nm)/Co MTJs without any seed layer, top Fe and bottom Co electrodes contact each other through pin holes in MgF2 layers. On the other hand, however, Fe and Co electrodes are separated both magnetically and electrically in Fe/MgO (0.3–0.5 nm)/MgF2 (2–5 nm)/Co MTJs, i.e., MgF2-based MTJs have successfully been prepared by inserting a thin MgO seed layer, resulting in TMR of about 10% observed at 4.2 K.