Standard Molecular Beam Epitaxy Research Articles

Observation of an abrupt 3D-2D morphological transition in thin Al layers grown by MBE on InGaAs surface

Among superconductor/semiconductor hybrid structures, in situ aluminum (Al) grown on InGaAs/InAs is widely pursued for the experimental realization of Majorana Zero Mode quasiparticles. This is due to the high carrier mobility, low effective mass, and large Landé g-factor of InAs, coupled with the relatively high value of the in-plane critical magnetic field in thin Al films. However, growing a thin, continuous Al layer using the molecular beam epitaxy (MBE) is challenging due to aluminum's high surface mobility and tendency for 3D nucleation on semiconductor surfaces. A study of epitaxial Al thin film growth on In0.75Ga0.25As with MBE is presented, focusing on the effects of the Al growth rate and substrate temperature on the nucleation of Al layers. We find that for low deposition rates, 0.1 and 0.5 Å/s, the growth continues in 3D mode during the deposition of the nominal 100 Å of Al, resulting in isolated Al islands. However, for growth rates of 1.5 Å/s and above, the 3D growth mode quickly transitions into island coalescence, leading to a uniform 2D Al layer. Moreover, this transition is very abrupt, happening over an Al flux increase of less than 1%. We discuss the growth mechanisms explaining these observations. The results give new insights into the kinetics of Al deposition and show that with sufficiently high Al flux, a 2D growth on substrates at close to room temperature can be achieved already within the first few Al monolayers. This eliminates the need for complex cryogenic substrate cooling and paves the way for the development of high-quality superconductor-semiconductor interfaces in standard MBE systems.

Optical properties of lattice-matched GaAsPBi multiple quantum wells grown on GaAs (001)

Quaternary alloy GaAsPBi is a novel III–V compound with attractive optical properties and can in principle be grown lattice-matched to GaAs. However, the practical realization of the alloy by metal-organic vapor phase epitaxy and molecular beam epitaxy (MBE)—the two main growth technologies—is fraught with difficulties. Here, using standard solid-source MBE, GaAsPBi films, and GaAsPBi/GaAs multiple quantum wells (MQW) have been grown lattice-matched to (001) GaAs. The structural integrity of the films/MQW is investigated and confirmed by various in- and ex-situ diffraction and spectroscopic techniques. All GaAsPBi structures—films and MQWs—are luminescent at room temperature. Photoluminescence shows that all the samples exhibit an S-shape temperature dependency, indicating strong localizations. Of most significance to practical applications is the observation that the emission from GaAsPBi MQWs is more efficient than their non-quantum well (QW) counterparts (up to 30× at room temperature). These results confirm the long-known benefits of carrier confinements by QWs, demonstrated here for the first time in the GaAsPBi-based system despite the challenge of the crystal growths.

Analysis of Etched CdZnTe Substrates

State-of-the-art as-received (112)B CdZnTe substrates have been examined for surface impurity contamination and polishing residue. Two 4 cm × 4 cm and one 6 cm × 6 cm (112)B state-of-the-art as-received CdZnTe wafers were analyzed. A maximum surface impurity concentration of Al = 1.7 × 1015 atoms cm−2, Si = 3.7 × 1013 atoms cm−2, Cl = 3.12 × 1015 atoms cm−2, S = 1.7 × 1014 atoms cm−2, P = 1.1 × 1014 atoms cm−2, Fe = 1.0 × 1013 atoms cm−2, Br = 1.2 × 1014 atoms cm−2, and Cu = 4 × 1012 atoms cm−2 was observed on the as-received CdZnTe wafers. CdZnTe particulates and residual SiO2 polishing grit were observed on the surface of the as-received (112)B CdZnTe substrates. The polishing grit/CdZnTe particulate density on CdZnTe wafers was observed to vary across a 6 cm × 6 cm wafer from ∼4 × 107 cm−2 to 2.5 × 108 cm−2. The surface impurity and damage layer of the (112)B CdZnTe wafers dictate that a molecular beam epitaxy (MBE) preparation etch is required. The contamination for one 4 cm × 4 cm and one 6 cm × 6 cm CdZnTe wafer after a standard MBE Br:methanol preparation etch procedure was also analyzed. A maximum surface impurity concentration of Al = 2.4 × 1015 atoms cm−2, Si = 4.0 × 1013 atoms cm−2, Cl = 7.5 × 1013 atoms cm−2, S = 4.4 × 1013 atoms cm−2, P = 9.8 × 1013 atoms cm−2, Fe = 1.0 × 1013 atoms cm−2, Br = 2.9 × 1014 atoms cm−2, and Cu = 5.2 × 1012 atoms cm−2 was observed on the MBE preparation-etched CdZnTe wafers. The MBE preparation-etched surface contamination consists of Cd(Zn)Te particles/flakes. No residual SiO2 polishing grit was observed on the (112)B surface.

Congruent evaporation temperature of molecular beam epitaxy grown GaAs (001) determined by local droplet etching

The congruent evaporation temperature Tc of GaAs (001) is critical for many technological processes and is fundamental to the control and stability of Ga droplets for quantum structure fabrication. We apply the technique of local droplet etching (LDE) to measure Tc for technologically important molecular beam epitaxy (MBE) grown GaAs (001). Below Tc, Ga droplets deposited on the surface shrink and form nanoholes via LDE and thermal widening. Above Tc, droplets grow by capturing excess Ga. From the transition between both regimes, we determine Tc = 680 ± 10 °C. Additionally, we find that the nanohole/droplet densities follow an Arrhenius-type temperature dependence with an activation energy of 1.31 eV. The method probes the stability of pre-existing droplets formed by deposition and so avoids the complication of nucleation barriers and readily allows the measurement of Tc for technologically important planar GaAs surfaces in any standard MBE system.

A portable molecular beam epitaxy system for in situ x-ray investigations at synchrotron beamlines

A portable synchrotron molecular beam epitaxy (MBE) system is designed and applied for in situ investigations. The growth chamber is equipped with all the standard MBE components such as effusion cells with shutters, main shutter, cooling shroud, manipulator, reflection high energy electron diffraction setup, and pressure gauges. The characteristic feature of the system is the beryllium windows which are used for in situ x-ray measurements. An UHV sample transfer case allows in vacuo transfer of samples prepared elsewhere. We describe the system design and demonstrate its performance by investigating the annealing process of buried InGaAs self-organized quantum dots.

KFe2Se2is the Parent Compound of K-Doped Iron Selenide Superconductors

We elucidate the existing controversies in the newly discovered K-doped iron selenide (K(x)Fe(2-y)Se(2-z)) superconductors. The stoichiometric KFe(2)Se(2) with √2 × √2 charge ordering was identified as the parent compound of K(x)Fe(2-y)Se(2-z) superconductor using scanning tunneling microscopy and spectroscopy. The superconductivity is induced in KFe(2)Se(2) by either Se vacancies or interacting with the antiferromagnetic K(2)Fe(4)Se(5) compound. In total, four phases were found to exist in K(x)Fe(2-y)Se(2-z): parent compound KFe(2)Se(2), superconducting KFe(2)Se(2) with √2 × √5 charge ordering, superconducting KFe(2)Se(2-z) with Se vacancies, and insulating K(2)Fe(4)Se(5) with √5 × √5 Fe vacancy order. The phase separation takes place at the mesoscopic scale under standard molecular beam epitaxy conditions.

A valved cracking phosphorus beam source using InP thermal decomposition and its application to MBE growth

The principal design of a newly developed two-zone valved cracking phosphorus P2 molecular beam source with greatly improved performance based on InP thermal decomposition is outlined. Precise dimer phosphorus beam flux control is accomplished due to a thoughtfully designed and externally activated faucet placed between the InP decomposition zone and the cracking area of P4 vapors. Experimental tests show that the source can be easily incorporated into the standard ion-pumped molecular beam epitaxy (MBE) machine and can be used successfully for the MBE growth of device quality III–V single and multi-component phosphide epilayers incorporated into single- and multi-layer heterostructures with sharp interfaces.

A periodic thin film filters for acoustic phonons

We describe multilayer acoustic nanowave devices based on aperiodic stacks of GaAs and AlAs layers and achievable with standard Molecular Beam Epitaxy (MBE) technology. These nanostructures were designed to display optimized acoustic reflectivity curves in the terahertz range. We address the design and optimization of these devices using a downhill simplex algorithm. We analyze different strategies of optimization, and we study the different engineering parameters relevant for the conception of aperiodic filters.

Acoustic phonon nanowave devices based on aperiodic multilayers: Experiments and theory

We describe multilayer acoustic nanowave devices based on aperiodic stacks of GaAs and AlAs layers and achievable with standard molecular beam epitaxy (MBE) technology. These nanostructures were designed to display optimized acoustic reflectivity curves in the terahertz range. We discuss the use of different techniques for the design, optimization, and characterization of such acoustic phonon devices. Three optimized acoustic phonon devices were grown by MBE and characterized structurally by x-ray diffraction and photoluminescence: a broadband mirror, a color filter, and an edge filter. The acoustic phonon spectra were studied by Raman scattering in forward and backscattering geometries. We contrast the experimental results with simulations of the Raman spectra using a photoelastic model. We show that Raman spectroscopy provides a powerful tool to acoustically characterize complex aperiodic devices.

Regrowth of self-assembled InAs quantum dots on nanohole and stripe templates

Self-assembled InAs quantum dots (QDs) are grown on two types of templates by molecular beam epitaxy (MBE). A nanohole template is prepared by first growing InAs QDs on a GaAs substrate by a standard MBE process, then capping the QDs with a thin lattice-mismatched layer of GaAs. Subsequent annealing of the nanohole template results in a stripe template. We are able to transfer the patterns of self-assembled QDs onto these two types of templates: regrowth on the nanohole template results in uniform QDs situated in the nanoholes, while regrowth on the stripe template results in chains of QDs aligned along the stripes. Our results imply that self-assembled QDs can be grown onto in-situ prepared patterned substrates, with limited degree of randomness.

Si 1 − xGe x sputter epitaxy technique and its application to RTD

A Si 1 − x Ge x sputter epitaxy method using an ultra-high-vacuum-compatible magnetron sputtering technique with an Ar and H 2 mixture working gas has been introduced. The crystallinity and strain-relaxation controllability of a Si 1 − x Ge x layer grown on n-type Si(001) by our sputter epitaxy method is well comparable to those of a Si 1 − x Ge x layer grown by a standard molecular beam epitaxy (MBE) method. In the device level evaluation by forming an electron-tunneling Si/Si 1 − x Ge x asymmetric double quantum well resonant tunneling diode (ASDQW RTD), the fabricated RTD exhibits a high peak-to-valley current ratio (PVCR) of ∼ 26 with low background current, the performance of which is also close to that of our recent high PVCR ASDQW RTD fabricated by gas-source MBE. On the basis of these results, our proposed method is very promising for its application to various kinds of SiGe devices.

Magnetism of Fe clusters and islands on Pt surfaces

Clusters and islands of Fe atoms have been prepared by noble gas buffer layer assisted growth as well as by standard molecular beam epitaxy on Pt substrates. Xe buffer layers have been utilized to promote the formation of compact, relaxed Fe clusters with narrow size distribution. Without the Xe buffer, strained Fe islands with a characteristic misfit dislocation network are formed. Magnetization loops obtained by magneto-optical Kerr effect measurements reveal that in-plane easy magnetization axis is only found for the relaxed clusters, pointing out the important role of epitaxial lattice deformations for the magnetic anisotropy.

Reply to Comments by Buceta and Galeano Regarding the Article “The Universal Dynamics of Tumor Growth”

Reply to Comments by Buceta and Galeano Regarding the Article “The Universal Dynamics of Tumor Growth”

Ga-doping for β-FeSi2 films prepared by molecular beam epitaxy

β-FeSi 2 is expected as a new material for light emitting diode and optical sensor operating at 1.5 μm. However, doping techniques suited for various fabrication methods of β-FeSi 2 films have not been established. In this report, we present the results of Ga doping as a p-type impurity into β-FeSi 2 films using three molecular beam epitaxy (MBE)-associated growth methods. They are, (i) standard MBE method by depositing simultaneously Fe, Si and Ga molecular beams, (ii) alternating Fe/Si multilayers deposition method with an interval time between the Fe and Si depositions (migration enhanced epitaxy, MEE) in which Ga was deposited only in Si layer, (iii) standard alternating Fe/Si multilayers deposition (superlattice, SL) method in which Ga was deposited only in Si layer. All β-FeSi 2 films were prepared on Si substrates at room and elevated temperatures in MBE chamber and were subjected to post-annealing. Ga concentration was adjusted by changing its Knudsen effusion cell temperature or opening time of shutter for K-cell. Unintentionally doped β-FeSi 2 films prepared by SL method were n-type, having residual net electron concentration, |N D -N A | of ∼3×10 16 cm -3 and mobility of ∼400 cm 2 /Vs. When these β-FeSi 2 films were doped with Ga above a critical concentration, they exhibited p-type conductivity. By varying Ga concentration, net hole concentration, |N A -N D | ranging from 7×10 16 to 2x ×10 18 cm -3 with hole Hall mobility, μ h from 200 to 10 cm 2 /Vs were obtained. These results demonstrate that Ga is an effective p-type dopant for β-FeSi 2 films to fabricate various electronic and optoelectronic devices.

InAs-based bipolar transistors grown by molecular beam epitaxy

Large mobilities and electron saturation velocity make InAs a promising material for high speed devices. Investigations into materials characteristics of doped InAs show nonideal behavior with standard molecular beam epitaxy dopants, silicon, and beryllium. Critical thicknesses for cracking of AlxIn1−xAs on InAs were empirically determined as a function of x. Mesa pn junctions in InAs show no effects of surface Fermi level pinning and exhibit good rectification with low reverse leakage. Bipolar junction transistor and heterojunction bipolar transistor devices are presented, along with their dc electrical characteristics. Common emitter current gains of 100 have been achieved in these bipolar devices.

Transition of magnetic anisotropy in Ni/GaAs (0 0 1) observed by magnetization and ferromagnetic resonance

Polycrystalline thin Ni films deposited onto GaAs (0 0 1) show a transition of the magnetic anisotropy depending on its thickness. The anisotropy is perpendicular to the film plane for the thicknesses of the film ⩽12 nm. This becomes in-plane in the films having thicknesses ⩾15 nm. The films are deposited onto the n-type GaAs (0 0 1) substrate by the usual thermal evaporation method and also by the electron beam evaporation in ultra high vacuum onto a GaAs epilayer in the standard molecular beam epitaxy system. The magnetization and ferromagnetic resonance (FMR) are observed in the temperature range from 4.2 to 300 K. For the discussion of the microscopic origin of the anomalous properties in magnetization and FMR experiments, the experimental results are reviewed by introducing a uniaxial anisotropy, which is calculated from the easy-axis and hard-axis magnetization data. This calculated anisotropy is able to explain the temperature and angle dependency of the FMR spectra of the Ni films. Hence the magnetization and FMR spectra are in agreement with the type of the anisotropy and its temperature dependency. In addition to these, the temperature dependence of the in-plane magnetic anisotropy is able to explain the previously reported anomalous effect of reducing the squareness at low temperatures in Ni/GaAs.

Perpendicular Magnetic Anisotropy in Thin Ni Films on GaAs (001)

Polycrystalline thin Ni films deposited onto GaAs (001) show a transition of the magnetic anisotropy depending on its thickness. The anisotropy is perpendicular to the film plane for the thicknesses of the film equal to or less than 12 nm. This becomes in-plane in the films having thicknesses equal to or higher than 15 nm. The films are deposited onto the n-type GaAs (001) substrate by the electron beam evaporation in ultra high vacuum (UHV) in the standard molecular beam epitaxy (MBE) system. The experimental results are reviewed by introducing a uniaxial anisotropy, which is calculated from the easy-axis and hard-axis magnetization obtained by the superconducting interference device (SQUID) at various temperatures.

Ultrahigh vacuum preparation and characterization of TiO2/CdTe interfaces: Electrical properties and implications for solar cells

The semiconductor cadmium telluride (CdTe) has been grown in ultrahigh vacuum by standard molecular beam epitaxy on thin films of anatase titanium dioxide (TiO2) to investigate the interface properties of solid state injection type solar cells. These interfaces were studied by photoelectron spectroscopy and the band alignment was determined. The interface is characterized by a valence band offset ΔEV of 2.6 eV, a conduction band offset ΔEC of 0.7 eV, and a strong dipole δ of about 0.9 eV in contradiction to the electron affinity rule (Anderson model). Therefore this system is not very favorable for solar power generation.

Transmission Electron Microscopy Investigation of CdSe/ZnSe Quantum Dot Structures

Different techniques of transmission electron microscopy were applied to study the Cd distribution and the formation of defects in CdSe/ZnSe structures with a nominal CdSe layer thickness t of three monolayers (ML) and different ZnSe cap layer thicknesses which were varied from 20 to 60 nm. The samples were grown by standard molecular-beam epitaxy at 350 °C. We report on the configuration of defects which appear in the CdSe/ZnSe structure from a critical thickness t > 2.5 ML. The defects originate in Cd-rich regions close to the CdZnSe layer and consist of stacking fault (SF) pairs on {111} crystal planes. The lengths of the dislocation lines and the sizes of the SFs increase with the cap layer thickness. The quantitative measurement of the Cd distribution indicates segregation with an efficiency of R = 0.74 ± 0.04.

On the origin of the “coffee-bean” contrast in transmission electron microscopy images of CdSe/ZnSe quantum dot structures

The origin of the “coffee-bean” strain contrast is studied, that is observed in the plan-view transmission electron microscopy (TEM) images of CdSe/ZnSe quantum dot structures. The samples were grown by two different methods: standard molecular-beam epitaxy at 350 °C and atomic layer epitaxy at 230 °C with annealing at 340 °C after the CdSe deposition. The nominal CdSe thickness was above 3 ML. In situ reflection high energy electron diffraction during the growth or during the annealing shows the transition from the two- (2D) into the three-dimensional (3D) surface morphology for both samples. The coffee-bean contrast is usually assigned to three-dimensional islands which are generated after the morphological 2D/3D transition. It is found that the coffee-bean contrast in plan-view TEM images is alternatively associated with pairs of stacking faults on {111} lattice planes which are inclined against each other. The stacking faults, which are bound by Shockley partial dislocations, are preferably generated in the vicinity of the Cd-rich regions (large islands) of the CdZnSe layer where Cd concentrations of more than 40% are found.