Two-step Molecular Beam Epitaxy Research Articles

Ordered L10-FeNi (111) epitaxial thin film on Al2O3 (0001) substrate: Molecular beam epitaxy growth and characterizations

Ordered hard magnetic L10-FeNi has attracted attention as a candidate for rare-earth-free permanent magnet with a large maximum magnetic energy product. Fabricating L10-FeNi is a big challenge using conventional methods due to its low order-disorder transition temperature, 320 °C. In this work, high-quality ordered L10-FeNi films were fabricated by using molecular beam epitaxy directly on Al2O3 (0001) substrate with a two-step co-deposition method below 250 °C. The films were epitaxially grown with [111] orientation, confirmed by reflection high energy electron diffraction and x-ray diffraction patterns. An order-disorder phase transition at 320 °C was observed in temperature dependent electrical resistivity curve. The two-step molecular beam epitaxy grown film exhibited high uniaxial magnetic anisotropy constant, Ku, up to 1.15 × 106 J/m3. The obtained Ku value is comparable to previous result for FeNi along [100] growth direction, indicating an isotropic Ku. This work provides an effective molecular beam epitaxy growth method of high-quality ordered L10-FeNi (111) film for the development of the next-generation rare-earth-free permanent magnet.

Suppression of three-dimensional pit formation of InAs on GaSb(0 0 1) by Sb-free two-step molecular beam epitaxy

Heteroepitaxial growth of InAs on GaSb(0 0 1) by molecular beam epitaxy was investigated. Pits composed of high-index crystal planes easily formed on InAs surfaces at low growth temperatures. They were suppressed by adding traces of Sb, which has a surfactant effect. However, this caused the content of Sb in the InAs layer to be inhomogeneous. On the other hand, without irradiation by Sb, the size of the pits became smaller as the growth temperature increased. The pits were completely suppressed by applying a two-step growth sequence: initial growth at 400 °C followed by subsequent bulk growth at 520 °C. In this sequence, Sb carryover from the underlying GaSb was negligible and did not contribute to the elimination of the pits as a surfactant. Two factors seemed to be essential for suppressing the formation of pits: (1) initial planar growth of InAs on GaSb, which is favored at low temperature, and (2) preference for the (0 0 1) plane over high-index crystal planes at high temperature.

Controlling the Polarity of the Molecular Beam Epitaxy Grown In-Bi Atomic Film on the Si(111) Surface

Synchrotron radiation core-level photoemission spectroscopy, scanning tunneling microscopy (STM), and first-principles calculations have been utilized to explore the growth processes and the atomic structure of the resulting films during the two-step molecular beam epitaxy (MBE) of In and Bi on the Si(111) surface. Deposition of 1.0-ML Bi on the In/Si(111)-(4 × 1) surface at room temperature results in Bi-terminated BiIn-(4 × 3) structures, which are stable up to ~300 °C annealing. By contrast, deposition of In on the β-Bi/Si(111)-(√3 × √3) surface at room temperature results in three dimensional (3D) In islands. In both cases, annealing at 460 °C results in the same In-terminated In0.75Bi/Si(111)-(2 × 2) surface. Our DFT calculations confirm that the surface energy of In-terminated In0.75Bi/Si(111)-(2 × 2) system is lower than that of Bi-terminated Bi0.75In/Si(111)-(2 × 2). These findings provide means for the control of the polarity of the MBE In-Bi atomically thick films.

Molecular beam epitaxy of quasi-freestanding transition metal disulphide monolayers on van der Waals substrates: a growth study

Based on an ultra-high vacuum compatible two-step molecular beam epitaxy synthesis with elemental sulphur, we grow clean, well-oriented, and almost defect-free monolayer islands and layers of the transition metal disulphides MoS2, TaS2 and WS2. Using scanning tunneling microscopy and low energy electron diffraction we investigate systematically how to optimise the growth process, and provide insight into the growth and annealing mechanisms. A large band gap of 2.55 eV and the ability to move flakes with the scanning tunneling microscope tip both document the weak interaction of MoS2 with its substrate consisting of graphene grown on Ir(1 1 1). As the method works for the synthesis of a variety of transition metal disulphides on different substrates, we speculate that it could be of great use for providing hitherto unattainable high quality monolayers of transition metal disulphides for fundamental spectroscopic investigations.

Raman scattering from GaAs/AlGaAs multiple quantum well structures grown by two-step molecular beam epitaxy

We report polarized and spatially resolved Raman scattering from GaAs/Al0.3Ga0.7As multiple quantum well (MQW) structures grown by a two-temperature step molecular beam epitaxy method, in which an initial GaAs buffer layer is deposited at low temperature and a subsequent GaAs layer is deposited at high temperature. Polarized Raman spectra revealed several transverse optical (TO) and longitudinal optical (LO) phonons, i.e., GaAs, GaAs-like, and AlAs-like TO and LO phonons. The Raman peaks related to the AlGaAs barriers were significantly enhanced under resonant excitation near the band gap energy of AlGaAs, In addition, resonant Raman scattering from the side edges of the samples exhibited symmetry-forbidden GaAs-like and AlAs-like LO phonons. To investigate stress and strain evolutions in the GaAs buffer layers, spatially resolved Raman measurements were performed along the growth direction, obtained via the side edges of the samples. Interestingly, Raman maps across the GaAs layers showed that the usually doubly degenerate GaAs TO phonons were split into two distinct phonons. Depending on the two-step growth temperatures, the peak energies of the TO phonons evolved differently along the growth direction. The built-in stress and strain depth-profiles in the GaAs buffer layers were estimated using a biaxial assumption.

Three-Dimensional GaN/AlN Nanowire Heterostructures by Separating Nucleation and Growth Processes

Bottom-up nanostructure assembly has been a central theme of materials synthesis over the past few decades. Semiconductor quantum dots and nanowires provide additional degrees of freedom for charge confinement, strain engineering, and surface sensitivity-properties that are useful to a wide range of solid state optical and electronic technologies. A central challenge is to understand and manipulate nanostructure assembly to reproducibly generate emergent structures with the desired properties. However, progress is hampered due to the interdependence of nucleation and growth phenomena. Here we show that by dynamically adjusting the growth kinetics, it is possible to separate the nucleation and growth processes in spontaneously formed GaN nanowires using a two-step molecular beam epitaxy technique. First, a growth phase diagram for these nanowires is systematically developed, which allows for control of nanowire density over three orders of magnitude. Next, we show that by first nucleating nanowires at a low temperature and then growing them at a higher temperature, height and density can be independently selected while maintaining the target density over long growth times. GaN nanowires prepared using this two-step procedure are overgrown with three-dimensionally layered and topologically complex heterostructures of (GaN/AlN). By adjusting the growth temperature in the second growth step either vertical or coaxial nanowire superlattices can be formed. These results indicate that a two-step method allows access to a variety of kinetics at which nanowire nucleation and adatom mobility are adjustable.

MBE growth and studies of hybrid heterostructures with II–VI/InAs heterovalent interfaces in the active region

InAs quantum wells (QWs) with AlAsSb and CdMgSe asymmetric barriers have been fabricated by a two-step molecular beam epitaxy on InAs(100) substrates. A 5-monolayer-thick ZnTe layer has been introduced between the InAs and CdMgSe layers as a buffer of the II–VI growth initiation. Ex-situ S-passivation technique applied to the uncapped InAs surface provides formation of defect free III–V/II–VI heterovalent interface. Photoluminescence study proves a strong type I band line-up at the ZnTe-mediated InAs/CdMgSe interface with the conduction and valence band offsets of about 0.6 eV and 1.0 eV, respectively. Transport properties of two-dimensional electron gas confined in a 16-nm-thick AlAsSb/InAs/ZnTe/CdMgSe QWs grown on GaAs (100) are compared with those of pure III–V 2DEG structures. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ge dots self-assembling: Surfactant mediated growth of Ge on SiGe (118) stress-induced kinetic instabilities

The ordering of islands on naturally or artificially nanostructured surfaces is one of the most recent objectives among actual nanotechnology challenges. We show in this letter that, by a combination of two approaches, i.e., a two-step molecular beam epitaxy (MBE) deposition process and surfactant-mediated growth, we are able to obtain chains of nicely ordered ultrasmall islands of lateral size below 50 nm. The two-step MBE process consists of vicinal Si(001) surface self-patterning by SiGe growth instability and Ge dot ordering by subsequent Ge deposition on a SiGe template layer. The surfactant-mediated growth consists of submonolayer Sb deposition prior to Ge growth, in order to reduce the island size up to 25 nm. The best ordering of Ge islands is obtained when the island size matches the wavelength of the template layer.

Visible-blind ultra-violet detectors with nano-second response time

A two-step molecular beam epitaxy growth approach was developed to fabricate a ZnSTe-based Schottky barrier photovoltaic ultra-violet (UV) detector array on GaP(1 0 0) substrate. The detectors show visible blind and UV sensitive response with peak UV responsivity of 0.13 A/W and 1.2×10 6 V/W at 320 nm. The built-in potential of the detector was determined to be 1.7 V. Temporal response of the device shows that the photocurrent decays with a time constant as fast as 1.2 ns, limited apparently by the RC constant of the detector structure.

ZnSTe-based Schottky barrier ultraviolet detectors with nanosecond response time

ZnSTe-based Schottky barrier photovoltaic detector arrays were fabricated on GaP(100) using a two-step molecular beam epitaxy growth approach. These detectors exhibit visible blind and ultraviolet (UV) sensitive response with a peak UV responsivity of 0.13 A/W and 1.2×106 V/W at 320 nm. The built-in potential of these detectors was determined to be 1.7 V. The temporal photocurrent response of a 400×400 μm2 detector was measured to be 1.2 ns, limited apparently by the resistance-capacitance (rc) constant of the detector structure.

InGaAs/AlGaAs strained quantum well laser with semi-insulating low temperature GaAs and lateral n-p-n current confinement structures grown by molecular beam epitaxy

A combination of a lateral n-p-n junction using an amphoteric doping of Si in GaAs and a semi-insulating GaAs grown at low temperature is applied for the first time to a novel current confinement structure for an index-guided InGaAs strained quantum well laser grown by two-step molecular beam epitaxy. A threshold current of 7.4 mA and total external differential quantum efficiency of 59% under room temperature continuous wave operation are achieved with devices fabricated by a self-aligned process.

An X-ray diffraction study of the effects of rapid thermal annealing on GaAs layers on Si substrates

The rapid thermal annealing method was applied to improve the crystalline quality of GaAs layers grown by molecular beam epitaxy (MBE) on Si (100) substrates. The annealed GaAs-on-Si samples exhibited remarkably better crystal structure than did the as-grown samples, as revealed by X-ray diffraction (XRD). The smallest XRD linewidth was 145 arc sec for a GaAs layer of 4 μm in thickness. The optimum temperature and time of annealing were found to be 930°C and 10 s for the layers having 1 μm in thickness but longer annealing times at this temperature were needed for thicker layers. Growth of a buffer layer by migration-enhanced epitaxy, prior to growing the final layer by MBE, also improved the crystalline quality when compared to that of the unannealed layers grown by the two-step MBE procedure.

Low temperature formation of Si/silicide/Si double heterostructures by self-aligned MBE growth

Abstract New formation technique of high quality Si/CoSi 2 /Si double heterostructures and fine patterning of CoSi 2 electrodes were developed using self-aligned and two-step molecular beam epitaxy (MBE). In addition, a permeable base transistor (PBT) was successfully fabricated, which showed a high mutual conductance ( g m ) of 42 mS mm −1 .

Formation of High-Quality Si/CoSi2/Si Double Hetero-Structures by Self-Aligned and Two-Step Molecular Beam Epitaxy

AbstractThe formation and application of Si/CoSi2/Si double heterostructures are comprehensively studied. A high-quality double heterostructure is formed by two-step molecular beam epitaxy of the Si over layer, i.e., low -temperature growth followed by high-temperature growth. The interfaces between CoSi2 and Si observed by cross-sectional transmission microscopy are atomically abrupt and smooth. In addition, a new fine patterning method of CoSi2 films using self-aligned and selective growth is developed. Finally, permeable base transistors (PBT) with high performance (gm=50 mS /mm) are fabricated using these new techniques.

Initial Nucleation Studies of Heteroepitaxial GaAs films on Si Substrates by Modulated Molecular Beam Epitaxy

AbstractNominal 100 Å and 150 Å thick GaAs layers were deposited on Si substrates by a modulated molecular beam technique and normal molecular beam epitaxy (MBE) at 300 °C and 375 °C respectively for plan view and cross-sectional transmission electron microscopy (TEM) examinations. From coverage of Moire fringes, it is found that the nucleated GaAs films grown by the modulated molecular beam technique were thinner, streaker and more two-dimensional than the MBE grown films. The same modulated molecular beam technique was also used for the deposition of the initial buffer layer of 3µm thick GaAs on Si films. Results from the 77k photoluminescence(PL) and double crystal X ray diffraction measurements showed that these films have superior optical and structural quality compared to similar films grown by normal two-step MBE. The improvement is attributed to a more two-dimensional nucleation of GaAs films associated with the modulated molecular beam growth technique.

New Approach to Growth of High-Quality GaAs Layers on Si Substrates

AbstractGaAs layers were grown on Si (001) substrates by molecular beam epitaxy (MBE) and migration enhanced epitaxy (MEE). They were examined by transmission electron microscopy, doublecrystal X-ray diffraction and Rutherford backscattering/channeling technique. Initial layer growth in both MEE and MBE was governed by three-dimensional nucleation but a stronger tendency for GaAs islands to align along the surface steps of Si was observed in the case of MEE. There was no measurable tilt between the (001) planes of GaAs and Si if growth was initiated by MEE at low temperature, prior to further growth by MBE at higher temperatures. On the contrary, the tilt angle was 0.34• when the entire structure was prepared by conventional two-step MBE. Rutherford backscattering measurements indicated a significant reduction in the density of defects throughout MEE/MBE GaAs in comparison with MBE GaAs.

Monolithic integration of a transverse-junction stripe laser and metal-semiconductor field-effect transistors on a semi-insulating GaAs substrate

A buried transverse-junction stripe laser diode and two metal-semiconductor field-effect transistors were monolithically integrated using two-step molecular beam epitaxy. This device had an almost flat surface well suited for the integration. High performance and high-speed operation (<1GHz) have been demonstrated in this integrated device for the first time.

Ridge waveguide AlGaAs/GaAs distributed feedback lasers

Ridge waveguide AlGaAs/GaAs distributed feedback lasers were fabricated by a two-step molecular beam epitaxy. A threshold current as low as 37 mA, which is the lowest ever reported, was obtained under a continuous wave condition at room temperature. The low threshold current is due to the larger coupling coefficient (91 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) and a good current and optical confinement. The effects of the facet coatings were investigated and a high-power and a high-temperature operation was obtained. The polarization and the dynamic behavior were also investigated.

Low-threshold current AlGaAs/GaAs-distributed feedback laser grown by two-step molecular beam epitaxy

AlGaAs/GaAs-distributed feedback (DFB) lasers with oxide-stripe structure were fabricated by a two-step molecular beam epitaxial (MBE) growth for the first time. The large coupling coefficient of 90 cm-1and the threshold current as low as 165 mA at room temperature were obtained with the second-order gratings. The characteristic temperature T 0 was as high as 210 K. Single longitudinal-mode oscillation was observed up to I/I_{th} = 1.5 and from 0 to 50°C without any mode hopping. The wavelength variation from device to device was ±5 A. The dependence of the coupling coefficient on the device structure was calculated, and it was shown that MBE is much more advantageous than LPE to enhance the coupling coefficient.

Low threshold current AlGaAs/GaAs distributed feedback laser grown by two-step molecular beam epitaxy

AlGaAs/GaAs distributed feedback lasers with oxide-stripe structure were fabricated by two-step molecular beam epitaxy (MBE) growth for the first time. The large coupling coefficient of 90 cm−1 was obtained by controlling precisely the thickness of each layer. As a result, the threshold current of 165 mA, which is the lowest ever reported, was obtained at room temperature. The characteristic temperature T0 was as high as 210 K. Single longitudinal mode oscillation over the temperature range of 50 K was observed without mode hopping. Due to the uniformity of the thickness and composition of the layers grown by MBE, uniform oscillation wavelengths were observed.