Molecular Beam Epitaxy System Research Articles

Influence of the phenomena occurring at the soft/hard interface on the coercivity behavior in exchange-spring magnets

L 1 0 -ordered FePd thin films were grown onto MgO-(100) monocrystalline substrates at 450 and 650 °C by means of a molecular beam epitaxy system. Fe/FePd bi-layers were grown by covering the L 1 0 -FePd hard films with Fe layers of different thickness. For a soft/hard thickness ratio up to 1, these bi-layers show behavior typical of a single phase hard magnet while, for a higher thickness ratio, they behave as exchange-coupled magnets characterized by two critical fields. The bi-layers’ coercivity decreases as a function of the deposited Fe when it is controlled by the domain wall pinning mechanism, while it initially increases and then decreases, when the nucleation of reversed domains is the predominant coercivity mechanism. These different behaviors are due to the phenomena occurring at the soft/hard interface and in particular to the degree of Fe/FePd intermixing and to the composition of the thin interfacial layer.

The Fabrication of Ag Islands on AlN/GaN/AlN/Si(111) by Using Thermal Evaporator and Thermal Annealing Methods

In this paper, we studied growth of AlN/GaN/AlN on Si (111) by using plasma assisted molecular beam epitaxy (PA-MBE) system. The structural and optical characteristics of the sample have been investigated by using high resolution X-ray diffraction (HR-XRD), Raman spectroscopy and photoluminescence (PL). PL spectrum of the sample has shown sharp and intense band edge emission of GaN with the absence of yellow emission band, indicating good crystal quality of the sample. The silver (Ag) metal contact was then deposited on the sample followed by thermal treatment at 500°C and 700°C, respectively. Treated sample at 700°C showed good spherical Ag islands on sample compared to the treated sample at 500°C. The effect of Ag islands on the electrical characteristics of sample was also examined by usingI-Vmeasurement. The results showed that the treated sample at 700°C has decreased the photo-current of Schottky diode.

Molecular beam epitaxy grown GaNAsSb 1 eV photovoltaic cell

We report the performance of a 1 eV GaNAsSb-based photovoltaic cell grown using a molecular beam epitaxy system equipped with a radio frequency (RF) plasma-assisted nitrogen source. The 1 μm-thick photoabsorption layer contains 2% of N and 6% of Sb resulting in a GaNAsSb layer with bandgap energy of 1.0 eV. Under AM1.5G solar illumination condition with and without 850 nm long pass filter, the GaNAsSb-based photovoltaic cell demonstrates a J SC values of 15 and 32 mA/W, respectively. Deep level transient spectroscopy analysis reveals that the V OC of the photovoltaic cell could possibly be limited by the presence of arsenic antisite defects.

High-density NiSi nanocrystals embedded in Al 2O 3/SiO 2 double-barrier for robust retention of nonvolatile memory

NiSi nanocrystals of high density and good uniformity were synthesized by vapor–solid–solid growth in a gas source molecular beam epitaxy system using Si 2H 6 as Si precursor and Ni as catalyst. A metal–oxide–semiconductor memory device with NiSi nanocrystal–Al 2O 3/SiO 2 double-barrier structure was fabricated. Large memory window and excellent retention at both room temperature and high temperature of 85 °C were demonstrated.

Mechanisms of molecular beam epitaxy growth in InAs/InP nanowire heterostructures

InAs/InP axial nanowire heterostructures were grown by the Au-assistedvapour–liquid–solid method in a gas source molecular beam epitaxy system. Thenanowire crystal structure and morphology were investigated by transmissionelectron microscopy for various growth conditions (temperature, growth rate,V/III flux ratio). Growth mechanisms were inferred from the InAs and InP segment lengths as afunction of the nanowire diameter. Short InAs segment lengths were found to grow bydepletion of In from the Au particle as well as by direct impingement, while longersegments of InAs and InP grew by diffusive transport of adatoms from the nanowiresidewalls. The present study offers a way to control the lengths of InAs quantum dotsembedded in InP barriers.

Effects of substrate temperature and film thickness on properties of CuIn3Te5 thin films and solar cells

The effects of substrate temperature (Tsub) and film thickness (dCIT) on the properties of CuIn3Te5 thin films and solar cells were investigated. CuIn3Te5 thin films (dCIT = 1.8–4.0 μm) were grown on both bare and Mo-coated soda-lime glass substrates at Tsub of 250–400 °C by single-step co-evaporation using a molecular beam epitaxy system. The microstructural properties were examined using scanning electron microscopy and x-ray diffraction. Well-developed (112)-oriented CuIn3Te5 grains were obtained by increasing the dCIT for Tsub of 250 °C. Cathodoluminescence analysis and temperature-dependent Hall measurements indicate the formation of shallow defect levels in the CuIn3Te5 films grown at higher Tsub. An optimum solar cell fabricated using CuIn3Te5 thin films (Tsub = 250 °C, and dCIT = 4.0 μm) yielded a total area (0.504 cm2) efficiency of 6.28%. Recombination mechanisms in the CdS/CuIn3Te5 thin-film solar cells are discussed on the basis of the temperature-dependent electrical properties of the films and solar cells.

Epitaxial growth of InN film on intermediate oxide buffer layer by RF-MOMBE

In this paper, we report the studies on the hetero-epitaxial growth of wurtzite indium nitride (InN) thin films on oxide buffer layer by RF metal-organic molecular beam epitaxy (RF-MOMBE) system. Oxide buffer layer was pre-sputtered using RF sputtering technique. The structural properties and surface morphology were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). We also investigated the optical properties by temperature-dependence photoluminescence (PL). Near-infrared emission peak centered at 0.75 eV was observed from PL measurement. The irregular and asymmetric PL line shape was caused by absorbed moisture and surface electron accumulation in InN films. According to the fitting results of PL spectra measured at 20 K, we could estimate the bandgap and Fermi level is 0.65 eV and 113 meV, which confirm to previous reports. Our results reveal that the oxide thin film could be a suitable buffer layer for engineering the growth of InN on sapphire wafer, and it might be also applicable for other lattice-mismatched III-V hetero-epitaxial systems.

Quantum Hall Resistance Standards With Good Quantization at High Electron Mobilities

We study the magnetotransport properties and Hall resistance quantization in a set of AlGaAs/GaAs heterostructures grown in a high-mobility molecular beam epitaxy system. The data show that, with reduced spacer layer width, the electron mobility in such heterostructures can be reduced to moderate mobility values of ~6 × 105 cm2/(V · s). We And good resistance quantization, allowing for a metrological application not only for a quantum Hall device with moderate electron mobility but also for a high-mobility device with an electron mobility of 2.2 × 106 cm2/(V · s).

Epitaxial growth of high mobility Bi2Se3 thin films on CdS

We report the experiment of high quality epitaxial growth of Bi2Se3 thin films on hexagonal CdS (0001) substrates using a solid source molecular-beam epitaxy system. Layer-by-layer growth of single crystal Bi2Se3 has been observed from the first quintuple layer. The size of surface triangular terraces has exceeded 1 μm. Angle-resolved photoemission spectroscopy clearly reveals the presence of Dirac-cone-shape surface states. Magneto-transport measurements demonstrate a high Hall mobility of ∼6000 cm2/V s for the as-grown Bi2Se3 thin films at temperatures below 30 K. These characteristics of Bi2Se3 thin films promise a variety of potential applications in ultrafast, low-power dissipation devices.

On the Secondary Droplets of Self-Running Gallium Droplets on GaAs Surface

Self-running droplets by thermal evaporation GaAs (001) surface are studied and analyzed using a scanning electron microscope. The sample is prepared under high-temperature annealing in an ultrahigh vacuum molecular beam epitaxy system. Particularly, secondary droplets which formed along primary droplet running trails are investigated. The secondary droplets are found to initially move along the [1 ̅10] instead of [110] direction, but these droplets tend to turn into [110] direction as they grow bigger. The scanning electron microscope also captures nanoscale footprints of secondary droplets different from the main droplets.

High-performance metamorphic InGaAs resonant cavity enhanced photodetector grown on GaAs substrate

In this letter, we demonstrated a top illuminated 1.55 μm metamorphic InGaAs resonant-cavity-enhanced p-i-n photodetector grown on GaAs substrate. The photodetectors were grown by a solid-source molecular beam epitaxy system. The high quality linearly graded InxAl0.4Ga1−x−0.4As metamorphic buffer layer enabled photodiodes to achieve ultralow dark current densities of 2.3×10−6 A/cm2 at 0 V and 4.2×10−5 A/cm2 at a reverse bias of 5 V. A high quantum efficiency of 84.4% at resonant wavelength of 1542 nm, a full width at half maximum about 14 nm, and a −3 dB bandwidth up to 13 GHz were also obtained.

Design of monostable–bistable transition logic element using the BiCMOS-based negative differential resistance circuit

The monostable---bistable transition logic element (MOBILE) is a promising application for negative differential resistance (NDR) circuit. Previously reported MOBILE is constructed by resonant tunneling diode (RTD) that is implemented by the molecular beam epitaxy (MBE) process. However in this paper, we first propose a NDR circuit composed of standard Si-based metal---oxide---semiconductor field-effect transistor (MOS) and SiGe-based heterojunction bipolar transistor (HBT). Then we demonstrate the inverter, NAND, and NOR gate operations using this MOS---HBT---NDR-based MOBILE circuit. The great advantage of this NDR-based application is that we can implement it using the standard SiGe BiCMOS process without the need for the MBE system.

Efficient kinematical simulation of reflection high-energy electron diffraction streak patterns for crystal surfaces

An efficient program with a user-friendly graphical interface has been developed for simulating reflection high-energy electron diffraction patterns obtained on crystal surfaces. The calculations are based on a kinematical approach which considers single electron scattering events at the surface. This time-efficient approach is in most cases sensitive enough for distinguishing different structural models, even if they differ very subtly. The results are presented in realistic two-dimensional density plots which can be directly compared to experimental observations. This program provides a useful tool in studying different structural models for crystal surfaces. Program summaryProgram title: RHEEDsimCatalogue identifier: AEJC_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEJC_v1_0.htmlProgram obtainable from: CPC Program Library, Queenʼs University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 2752No. of bytes in distributed program, including test data, etc.: 367 205Distribution format: tar.gzProgramming language: Matlab (version > 7.6.0)Computer: Personal ComputersOperating system: Windows with Matlab environmentRAM: Greater than 1 MBClassification: 7.4Nature of problem: Reflection high-energy electron diffraction (RHEED) has been widely used in studying crystal surface structures all over the world, especially in combination with ultra-high vacuum molecular beam epitaxy systems. In addition to determining the surface smoothness, RHEED is also a very useful tool in studying surface reconstructions, which are often encountered at the growth surfaces of semiconductors and alloys. While the positions of the fractional streaks can be used to determine the basic information of the surface supercells, the intensity modulations on the fractional streaks provide further insights on the details within these unit cells. Kinematic approach is an efficient method for simulating the RHEED patterns based on various surface structural models, which can help unraveling the surface atomic structure.Solution method: The kinematic approach utilized here assumes single scattering events. Furthermore, the Ewald sphere is approximated into a planar surface for computing the streak intensities (which are most relevant to real experiments). Structure factors are calculated based on a given input of atomic species and their coordinates, with user modifiable form factors. In addition to the intensity modulation within the surface plane, additional modulations extending into the z direction is also taken into consideration, resulting in realistic density maps of the RHEED streaks, which can be directly compared to experimental observations.Unusual features: The main program RHEEDsim.m calls several local subprograms for certain computational tasks. As a result, all programs should be extracted into a single folder, and that folder should be set as the main directory in Matlab.Running time: The computing time is computer and user parameter dependent, but typically ranges from few seconds to few minutes.

Spontaneous exchange-bias in Fe/Mn thin multilayers

Fe/Mn multilayers were grown by means of a molecular beam epitaxy system onto quartz substrates changing the thickness of the elemental layers. A spontaneous unidirectional anisotropy develops for thickness of Fe or Mn layer of about 35 Å. Since the samples were no subjected to field cooling treatments during or after the growth, this kind of anisotropy can be explained considering besides the exchange coupling at the Fe/Mn interface, the structural disorder due to dislocations and defects. In effect, the appearance and strength of the exchange-bias field are depending on the surface roughness of the samples and are significantly enhanced by the formation of a structure constituted by islands showing a snake-like morphology. The fitting of the angular dependence of the exchange-bias field indicates that the associated anisotropy is due to the superposition of two contributions, the principal one with unidirectional symmetry and the other showing uniaxial characteristics.

Determination of the band gap and the split-off band in wurtzite GaAs using Raman and photoluminescence excitation spectroscopy

GaAs nanowires with 100% wurtzite structure are synthesized by the vapor-liquid-solid method in a molecular beam epitaxy system, using gold as a catalyst. We use resonant Raman spectroscopy and photoluminescence to determine the position of the crystal-field split-off band of hexagonal wurtzite GaAs. The temperature dependence of this transition enables us to extract the value at 0 K, which is 1.982 eV. Our photoluminescence excitation spectroscopy measurements are consistent with a band gap of wurtzite GaAs below 1.523 eV.

MONTE CARLO SIMULATION OF THE KINETICS IN THE GROWTH OF SEMICONDUCTOR QUANTUM DOTS

By means of kinetic Monte Carlo simulation, which is based on the random selection of the surface hops of single adatom, we investigate the atoms' kinetics during the growth of the semiconductor quantum dots in a molecular beam epitaxy system, the deposition, diffusion and nucleation are considered as the main relevant processes during the growth of the quantum dots, taking into account the contribution of the dangling bond of the adatoms in the simulation. The dependence of the quantum dot size on the temperature and flux as well as the atomic kinetic effects are discussed in detail. The simulation results are in good qualitative agreement with those of the experiment.

Molecular beam epitaxy based growth of cubic GaN quantum dots

AbstractZinc‐blende GaN/AlN quantum dots were grown in a molecular beam epitaxy system by two alternative methods. In method A the quantum dots were formed by the Stranski‐Krastanov process, whereas in method B the quantum dots were created by a vapor‐liquid‐solid process, respectively. The density of the Stranski‐Krastanov quantum dots was adjustable in a range of 5 x 109 cm‐2 to 5 x 1012 cm‐2. The density of the quantum dots grown by method B was controllable in the range of 5 x 108 cm‐2 to 5 x 1012 cm‐2. For both methods the samples with a high density of quantum dots have shown strong optical activity in photoluminescence spectroscopy. The emission energy of the quantum dots was tuneable in a range of 3.5 eV to 3.9 eV by alteration of the quantum dot height. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Layer-by-layer synthesis of large-area graphene films by thermal cracker enhanced gas source molecular beam epitaxy

A thermal cracker enhanced gas source molecular beam epitaxy system was used to synthesize large-area graphene. Hydrocarbon gas molecules were broken by thermal cracker at very high temperature of 1200 °C and then impinged on a nickel substrate. High-quality, large-area graphene films were achieved at 800 °C, and this was confirmed by both Raman spectroscopy and transmission electron microscopy. A rapid cooling rate was not required for few-layer graphene growth in this method, and a high-percentage of single layer and bilayer graphene films was grown by controlling the growth time. The results suggest that in this method, carbon atoms migrate on the nickel surface and bond with each other to form graphene. Few-layer graphene is formed by subsequent growth of carbon layers on top of existing graphene layers. This is completely different from graphene formation through carbon dissolving in nickel and then precipitating from the nickel during rapid substrate cooling in the chemical vapor deposition method.

Direct growth of hexagonal InN films on 6H-SiC by radio-frequency metal-organic molecular-beam epitaxy

Wurtzite InN films were prepared on a 6H-SiC substrate by a self-designed plasma-assisted metal-organic molecular-beam epitaxy system without a buffer layer. In this article, the authors investigate the structural and optical properties of InN films grown on a 6H-SiC substrate. The crystallinity and microstructure of the thin film were further characterized by x-ray diffraction (XRD), field-emission scanning-electron microscopy, and transmission-electron microscopy. Electrical and optical properties were evaluated by Hall and photoluminescence (PL) measurements. XRD results indicate that InN film grown at 500 °C is epitaxially grown along the c-axis orientation. The two-dimensional growth mode is clearly shown in scanning-electron microscope images. Room-temperature PL spectra show that the emission peak is located at ∼0.83 eV due to the Burstein–Moss effect. In addition, the crystalline InN samples crack and peel away from the substrate at elevated growth temperature. This phenomenon may be attributed to lattice mismatch and grain coalescence while increasing the growth temperature. The narrow window of the growth temperature plays an important role in engineering the InN epitaxial growth.

Texture analysis of GaAs nanowires

The technique of texture analysis by x-ray diffraction is demonstrated for GaAs nanowires grown by the vapor-liquid–solid process in a gas source molecular beam epitaxy system. This technique allows simultaneous and facile determination of nanowire growth direction and crystal structure over large sample areas and with improved statistics relative to scanning electron microscopy.