Molecular Beam Epitaxy System Research Articles

The optimization of a self-focusing e-beam evaporator for carbon evaporation and the application for graphene growth

A self-focusing e-beam evaporator for carbon flux source in a molecular beam epitaxy (MBE) system has been designed and optimized. It is found that the configuration of the e-beam evaporator is very important for the solid carbon evaporation. According to the electric field simulation and the followed suitable design, the self-focusing e-beam evaporator for carbon flux evaporation can be achieved with high efficiency and good stability. This easy designed e-beam evaporator is adopted to produce stable and controllable carbon flux in a MBE system for large-area graphene layers preparation, which shows promising application for graphene-based device fabrication in the future.

Single crystal Gd2O3epitaxially on GaAs(111)A

Gd2O3 films 2 and 6 nm thick, electron-beam evaporated from a compact powder target, were deposited on freshly molecular beam epitaxy (MBE) grown epi-GaAs(111)A in a multi-chamber MBE system. The oxide films are epitaxial with the underlying GaAs, as studied by in situ reflective high energy electron diffraction and high-resolution X-ray diffraction using synchrotron radiation. The films undergo a structure phase transformation from hexagonal to monoclinic when the film thickness increases from 2 to 6 nm; the 2 nm oxide has H-Gd2O3(0001)[100]‖GaAs(111)[4] orientation relationship and the 6 nm film follows M-Gd2O3(01)[102]‖GaAs(111) with 3 rotational variants anchored by the 3-fold symmetric substrate. Here, H and M denote the hexagonal and monoclinic phases, respectively.

Epitaxial Cu(001) films grown on a Cr/Ag/Fe/GaAs(001) buffer system

We present a procedure to prepare single-crystalline, high-purity Cu(001) films (templates) suitable as substrates for subsequent epitaxial thin-film growth. The template films were grown in a dedicated molecular-beam epitaxy system on a Cr/Ag/Fe/GaAs(001) buffer layer system. Low-energy electron diffraction and X-ray diffraction were applied to determine the surface orientation and the epitaxial relationship between all layers of the stack. Post-annealing at moderate temperatures enhances the quality of the film as shown by low-energy electron diffraction and atomic force microscopy. X-ray photoemission and Auger electron spectroscopy confirm that no atoms of the buffer layers diffuse into the Cu film during the initial preparation and the post-annealing treatment. The completed Cu(001) template system can be exposed to air and afterwards refurbished by Ar+-ion bombardment and annealing, enabling the transfer between vacuum systems. The procedure provides suitable conductive thin film templates for studies of epitaxial thin films, e.g. on the magnetic and magnetotransport properties of Co and Ni based films and multilayers.

<inline-formula> <tex-math notation="TeX">${\rm SiN}_{x}$ </tex-math></inline-formula>/InAlN/AlN/GaN MIS-HEMTs With 10.8 <inline-formula> <tex-math notation="TeX">${\rm THz}\cdot{\rm V}$ </tex-math></inline-formula> Johnson Figure of Merit

A high combination of three-terminal breakdown voltage (VBK) and current gain cutoff frequency (fT) was achieved with SiNx/InAlN/AlN/GaN metal-insulator- semiconductor high-electron mobility transistors (MIS-HEMTs). A 1-nm SiNx gate dielectric was deposited ex situ in a molecular beam epitaxy system and used to increase the carrier density of the 2-D electron gas under an ultrathin InAlN/AlN (2.3 nm/1 nm) barrier. Passivated MIS-HEMTs with a gate length of 80 nm exhibited a drain current density greater than 1.1 A/mm, a peak intrinsic transconductance gm,max of 800 mS/mm, and a maximum frequency of oscillation fmax of 230 GHz. The combination of f T of 114 GHz and VBK of 95 V provides a Johnson figure of merit of 10.8 THz · V, which is among the highest reported values for fully passivated GaN HEMTs. A peak power-added efficiency of 37.5% with an output power of 1.25 W/mm and an associated gain of 9.7 dB was obtained by load-pull measurements at 40 GHz.

Self-corrected sensors based on atomic absorption spectroscopy for atom flux measurements in molecular beam epitaxy

A high sensitivity atom flux sensor based on atomic absorption spectroscopy has been designed and implemented to control electron beam evaporators and effusion cells in a molecular beam epitaxy system. Using a high-resolution spectrometer and a two-dimensional charge coupled device detector in a double-beam configuration, we employ either a non-resonant line or a resonant line with low cross section from the same hollow cathode lamp as the reference for nearly perfect background correction and baseline drift removal. This setup also significantly shortens the warm-up time needed compared to other sensor technologies and drastically reduces the noise coming from the surrounding environment. In addition, the high-resolution spectrometer allows the most sensitive resonant line to be isolated and used to provide excellent signal-to-noise ratio.

(Invited) Novel Luminescent Materials Based on Semiconductor Nanowires

We report novel luminescent materials based on group III-V compound semiconductor nanowires. Semiconductor nanowires are essentially one-dimensional rods with length of several microns and diameter below 100 nm. Hence, nanowires exhibit interesting quantum confinement and carrier transport properties. Nanowires are grown using metal seed particles by the vapor-liquid-solid (VLS) process in a molecular beam epitaxy or metalorganic chemical vapor deposition system. By varying the material deposition during growth, axial or radial nanowire heterostructures and p-n junctions may be formed for various device applications including light emitting diodes, lasers, and photodetectors. Due to the large surface area to volume ratio of a nanowire, lattice mismatch strain may be accommodated by elastic distortion of the nanowire without detrimental misfit dislocations, which gives a much greater ability to perform bandgap engineering in nanowires as compared to thin films. Hence, unique heterostructures are possible in nanowires that would be impossible in thin films, opening up new device applications and possibilities in condensed matter physics. We will report our recent work on the photoluminescence properties of InAsP/InP nanowires. InP nanowires were grown on <111> Si substrates by the Au-assisted vapor-liquid-solid process in a gas source molecular beam epitaxy system. InAsyP1-y segments were grown in the middle of the InP nanowires, creating a multiple quantum dot structure or superlattice. The quantum dot dimensions and composition were determined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDX). Photoluminescence (PL) from the quantum dot structure could be tuned by the InAsyP1-y composition (y), or by the size of the quantum dot via the quantum confinement effect. Cathodoluminescence (CL) measurements confirmed localized emission from the quantum dots. To reduce detrimental surface states, the nanowires were passivated with an AlInP shell, which resulted in strong PL emission.The growth mechanism of the quantum dots were inferred from the InAsP and InP segment lengths as a function of nanowire diameter. Short InAsP segment lengths were found to grow by depletion of In from the Au particle as well as by direct impingement, while longer segments of InAsP and InP grew by diffusive transport of adatoms from the nanowire sidewalls. The present study offers a manner to engineer the lengths of InAsP quantum dots embedded in InP barriers to better control the PL or CL emission. A novel group III and V gas switching sequence is presented to improve compositional control of the QD.

Facility for low-temperature spin-polarized-scanning tunneling microscopy studies of magnetic/spintronic materials prepared in situ by nitride molecular beam epitaxy.

Based on the interest in, as well as exciting outlook for, nitride semiconductor based structures with regard to electronic, optoelectronic, and spintronic applications, it is compelling to investigate these systems using the powerful technique of spin-polarized scanning tunneling microscopy (STM), a technique capable of achieving magnetic resolution down to the atomic scale. However, the delicate surfaces of these materials are easily corrupted by in-air transfers, making it unfeasible to study them in stand-alone ultra-high vacuum STM facilities. Therefore, we have carried out the development of a hybrid system including a nitrogen plasma assisted molecular beam epitaxy/pulsed laser epitaxy facility for sample growth combined with a low-temperature, spin-polarized scanning tunneling microscope system. The custom-designed molecular beam epitaxy growth system supports up to eight sources, including up to seven effusion cells plus a radio frequency nitrogen plasma source, for epitaxially growing a variety of materials, such as nitride semiconductors, magnetic materials, and their hetero-structures, and also incorporating in situ reflection high energy electron diffraction. The growth system also enables integration of pulsed laser epitaxy. The STM unit has a modular design, consisting of an upper body and a lower body. The upper body contains the coarse approach mechanism and the scanner unit, while the lower body accepts molecular beam epitaxy grown samples using compression springs and sample skis. The design of the system employs two stages of vibration isolation as well as a layer of acoustic noise isolation in order to reduce noise during STM measurements. This isolation allows the system to effectively acquire STM data in a typical lab space, which during its construction had no special and highly costly elements included, (such as isolated slabs) which would lower the environmental noise. The design further enables tip exchange and tip coating without breaking vacuum, and convenient visual access to the sample and tip inside a superconducting magnet cryostat. A sample/tip handling system is optimized for both the molecular beam epitaxy growth system and the scanning tunneling microscope system. The sample/tip handing system enables in situ STM studies on epitaxially grown samples, and tip exchange in the superconducting magnet cryostat. The hybrid molecular beam epitaxy and low temperature scanning tunneling microscopy system is capable of growing semiconductor-based hetero-structures with controlled accuracy down to a single atomic-layer and imaging them down to atomic resolution.

Low resistance indium tin oxide contact to n-GaAs nanowires

Indium tin oxide (ITO) was deposited by RF sputtering on n-GaAs nanowires grown by the Au-assisted vapor–liquid–solid process in a molecular beam epitaxy (MBE) system. The ITO formed an Ohmic contact with n-doped (n = 8 × 1018 cm−3) GaAs nanowires with a specific contact resistance of <1.41 Ω cm2. Insertion of a 25 nm thick indium layer between 500 nm thick ITO and the GaAs nanowires resulted in a reduction of specific contact resistance to <0.13 Ω cm2 after annealing at 400 °C for 30 s. The In/ITO film had an average transmittance of 89% from 400 to 900 nm and a sheet resistance of 13 Ω/□, which is well suited for nanowire-based optoelectronic applications.

The effect of the In concentration on the surface morphology of InGaAs-GaAs heterostructures grown by MBE on GaAs substrate

A set of 3 heterostructures were formed by 10 periods of InGaAs-GaAs epitaxially grown on GaAs substrate by means of a molecular beam epitaxial system. Scanning electron microscopy (SEM) cross section images at high magnification show that the heterostructures present good periodicity. SEM micrographs of the surface morphology chemically etched show the coalescence effect of In due to an unequal etching rate of In and GaAs. Auger electron spectroscopy (AES) depth profiles show that the first GaAs layers in the 3 samples are off-stoichiometric and that the alloy layers present In square and triangular depth profiles.

(Invited) Novel Luminescent Materials Based on Semiconductor Nanowires

Recent work on the photoluminescence (PL) properties of InAsP/InP nanowires are reported. InP nanowires were grown on <111> Si substrates by the Au-assisted vapor-liquid-solid process in a gas source molecular beam epitaxy system. InAsyP1-y segments were grown within the InP nanowires, creating single or multiple quantum dots structures. The quantum dot dimensions and composition were determined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDX). PL from the quantum dot structures could be tuned by the InAsyP1-y composition (y), or by the size of the quantum dot via the quantum confinement effect. Cathodoluminescence (CL) measurements confirmed localized emission from the quantum dots. To reduce detrimental surface states, the nanowires were passivated with an AlInP shell, which resulted in strong PL emission.

Polarized infrared reflectance study of free standing cubic GaN grown by molecular beam epitaxy

Optical properties of free standing cubic gallium nitride grown by molecular beam epitaxy system are investigated by a polarized infrared (IR) reflectance technique. A strong reststrahlen band, which reveals the bulk-like optical phonon frequencies, is observed. Meanwhile, continuous oscillation fringes, which indicate the sample consists of two homogeneous layers with different dielectric constants, are observed in the non-reststrahlen region. By obtaining the first derivative of polarized IR reflectance spectra measured at higher angles of incidence, extra phonon resonances are identified at the edges of the reststrahlen band. The observations are verified with the theoretical results simulated based on a multi-oscillator model.

Spectroscopic characterization of high-purity polycrystalline Bi–Te films grown by thermal evaporation

Thin films of BixTey with various compositions have been grown on Si(100) substrates by thermal evaporation with the use of a Molecular Beam Epitaxy (MBE) system. The growth was performed in the co-deposition mode. The effect of stoichiometry and growth conditions on the structural and electronic properties of the films was studied. Films with compositions corresponding to the compound Bi2Te3 and with compositions rich in Te and Bi were studied. Two different phases which crystallized in the hexagonal family were recognized: trigonal Bi2Te3 with the lattice parameters of a=4.44Å and c=30.47Å and hexagonal BiTe with the lattice parameters of a=4.39Å and c=24.02Å. The analysis of photoemission from the Bi and Te core levels confirmed the structural studies. The layered structure of BixTey films caused that the found crystal phases are accompanied by layers of pure elements Te or Bi depending on the stoichiometry. Angle dependent photoelectron spectroscopy studies showed the tendency of segregation direction – towards the surface for Te layers and opposite one for metallic Bi.

Fabrication of polycrystalline Cu2ZnSnSe4 layers with strongly preferential grain orientation via selenization of Sn/Cu/ZnSe(001)/GaAs(001) structures

We report on a simple approach for the fabrication of polycrystalline Cu2ZnSnSe4 films on GaAs with strongly preferential grain orientation. Such layers are important for both fundamental spectroscopic investigations as well as studies of the impact of grain boundaries on the performance of solar cells. Cu2ZnSnSe4 (CZTSe) layers were fabricated by growing a Sn/Cu/ZnSe(001) stack on a GaAs(001) substrate in a molecular-beam epitaxy system followed by selenization in a tube furnace. Raman spectroscopy as well as X-ray diffraction measurements combined with a microstructural investigation indicate the presence of the kesterite phase. Further, X-ray analysis such as rocking curves and ϕ-scans proves a preferential grain orientation of the obtained CZTSe films in all three dimensions.

Closed-cycle cooling of cryopanels in molecular beam epitaxy

Closed-cycle cooling of the cryoshroud in a molecular beam epitaxy (MBE) system with a dimethyl polysiloxane heat transfer fluid has reduced liquid nitrogen consumption by an order of magnitude, significantly lowering operating costs. The temperature dependence of cryopanel pumping efficacy in the MBE system has been investigated. H2O, CO, CO2, and As4 are all pumped effectively by liquid nitrogen cooled cryopanels (−196 °C) in the MBE. At −78 °C, the operating temperature of the closed-cycle chiller, H2O and As4 are pumped effectively, while CO and CO2 are not. The pumping speed for H2O is found to increase exponentially with decreasing temperature. Below ∼−40 °C and ∼−95 °C, the pumping speeds for As4 and H2O saturate, respectively. AlGaAs layers grown with the closed-cycle-cooled shroud show strong photoluminescence, expected room temperature electron mobility, and background doping levels less than 4 × 1015 cm−3.

Growth and characteristic of high orientation indium nitride films grown on (100) silicon substrate

In this study, high orientation InN films were prepared on Si (100) substrates using plasma-assist metal-organic molecular beam epitaxy (RF-MOMBE) system. These buffers include the nitrided treatment of the Si substrate, a low-temperature AlN buffer layer and oxide layer. The crystalline structure, surface morphology and optical properties of the InN films were characterised by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscope (SEM), atomic force microscopy (AFM) and photoluminescence (PL) measurements. XRD results revealed that the wurtzite InN grows on oxide layer with preferential (0002) orientation. Bright-field XTEM images of InN films exhibit a hexagonal structure with highly preferred orientation along direction. InN film direct growth on Si substrate exhibited the formation of metallic indium droplets on the surface. AFM images revealed that InN films with oxide layer had a root-mean-square (RMS) roughness of 16.1 nm. The optical properties of InN/oxide layer were determined according to the photoluminescence, revealing a near band-edge of 0.719 eV.

980 nm半导体激光器n型波导结构优化

In order to raise the output power and lower the far field divergence angles of 980 nm diode laser, a high refractive index inner waveguide is introduced in the n-type waveguide structure, which is based on the asymmetric waveguide structure. The research is conducted by SimLastip. Meanwhile, the semiconductor laser is fabricated with the epitaxial layer grown by a solid source molecular beam epitaxy (MBE) system. The fabricated 980 nm laser diode with 100 m strip width and 1000 m cavity length has a threshold current of 97 mA and a slope efficiency of 1.01 W/A. When the injection current is 500 mA, the far field divergence angles are 29(vertical) by 8 (horizontal), which is consistent with the simulated result. The theoretical and experimental results indicate that the inner waveguide structure can achieve high power output, effectively reduce the far field divergence angle and improve the beam quality of the device.

Hydrogen evolution from water using AgxCu1−xGaSe2 photocathodes under visible light

Photoelectrochemical (PEC) water splitting using CuGaSe2 (CGSe) thin film photocathodes modified by partial substitution of Cu with Ag was investigated. The AgxCu1-xGaSe2 (ACGSe) thin films were deposited onto Mo-coated soda-lime glass substrates by means of co-evaporation using a molecular beam epitaxy (MBE) system. The valence band maximum (VBM) potential of ACGSe is deeper than that of CGSe, and its grain size is greatly increased compared to that of CGSe. A Pt and CdS modified ACGSe electrode (Pt/CdS/ACGSe) with a Ag/(Cu + Ag) ratio of about 5% showed a cathodic photocurrent of 8.1 mA cm(-2) at 0 VRHE and an onset potential of 0.70 VRHE (defined as a cathodic photocurrent of 0.05 mA cm(-2)) under simulated sunlight in a 0.1 M Na2SO4 solution (pH 9.5). Moreover, Pt/CdS/ACGSe exhibited a stable cathodic photocurrent for over 55 h, with no clear decrease.

Ordered phase formation in Sm-Ni thin film deposited on Cr(100) single-crystal underlayer

Sm17Ni83 (at. %) alloy thin films are prepared on Cr(100) underlayers hetero-epitaxially grown on MgO(100) single-crystal substrates by employing an ultra-high vacuum molecular beam epitaxy system. The effect of substrate temperature on the ordered phase formation is investigated. The films deposited below 300 °C consist of amorphous phase, whereas formation of SmNi5 ordered phase is recognized for the films deposited above 400 °C. The SmNi5 films with ordered phase consist of two types of (110) variant whose c-axes are lying in the film plane and rotated around the film normal by 90° each other. With increasing the temperature from 400 to 500 °C, the long-range order degree increases from 0.65 to 0.80. The ordered film formed at 500 °C shows an in-plane magnetic anisotropy reflecting the magnetocrystalline anisotropy of SmNi5 crystal.

Effect of Substrate Temperature on the Ordered Phase Formation in Sm–Ni Thin Film Deposited on Cu(111) Underlayer

Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sub> Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">83</sub> (at.%) alloy thin films are prepared on Cu underlayers hetero-epitaxially grown on MgO(111) single-crystal substrates by employing an ultra-high vacuum molecular beam epitaxy system. The substrate temperature is varied from 100 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C to 500 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. The film growth behavior and the detailed resulting film structure are respectively investigated by in situ reflection high-energy electron diffraction and by a combination of out-of-plane and in-plane X-ray diffractions. With increasing the temperature up to 300 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, formation of ordered SmNi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> phase is recognized. The films deposited above 400 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C consist of only epitaxial SmNi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> (0001) crystal, whereas the films deposited below the temperature include amorphous phase and/or a hexagonal ordered phase other than SmNi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> . When the temperature is increased from 300 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C to 500 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, the long-range order degree of SmNi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> film increases from 0.60 to 0.97. SmNi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> phase formation is enhanced with increasing the temperature.

Electronic processes in adatom dynamics at epitaxial semiconductor surfaces studied using MBE-STM combined system

Thermal desorption flux of cation adatoms is modeled in the context of the transition state theory. The model, on the basis of thermally excited electron tunneling to surface adatoms, suggests that the rate of phonon-stimulated electronic transition (In+ + e− → In0) by tunneling is essential to explain the desorption flux at the InAs surface. The adatom thermal desorption is electronic in nature. The importance of an electronic process as one of elemental dynamics of epitaxy, as well as collisional kinetic energy transfer among heavy nuclei of atoms and surface, is emphasized. By means of verifying the equivalence of electrons and phonons as excitation sources in the case of the electronic transition, usefulness of low-temperature scanning tunneling microscopy (LT-STM) as a tool for studying the crystal growth mechanism is explained. A molecular beam epitaxy (MBE) system equipped with the LT-STM provides a promising complement to in situ STM observation during MBE.