Molecular Beam Epitaxy System Research Articles

Bulk lattice parameter and band gap of cubic InXGa1−XN (001) alloys on MgO (100) substrates

InxGa1−xN (001) ternary alloys were grown on GaN/MgO (100) substrates in a plasma assisted molecular beam epitaxy system. We determined the in-plane [001] and in-growth [110] lattice parameters, as well as the bulk lattice parameter of the alloys for different In concentrations by high resolution X-ray diffraction. The In concentration was determined assuming Vegard׳s law for the alloy lattice parameter. The optical energy gap of InxGa1−xN has been determined by transmittance measurements from absorption edges for several In concentrations. Our results show that the alloys have a direct band gap for all In concentrations and a bowing parameter b=1.84.

Study of single crystal CuInSe2 thin films and CuGaSe2/CuInSe2 single quantum well grown by molecular beam epitaxy

High quality CuGaSe2 and CuInSe2 single crystalline layers are grown on GaAs (001) by employing the deposition sequence of migration enhanced epitaxy using a solid source molecular beam epitaxy system. When CuGaSe2 is grown on CuInSe2 at moderate temperatures, severe interdiffusion takes place at the heterojunction of CuGaSe2/CuInSe2. This problem has been solved by optimizing the growth temperature and deposition rates of the constituent elements. Thus, we have successfully grown CuGaSe2/CuInSe2 single quantum well with sharp interfaces on GaAs (001) for the first time. Intense photoluminescence from the single quantum well with 10nm well width is demonstrated.

Effect of SiNx gate insulator thickness on electrical properties of SiNx/In0.17Al0.83N/AlN/GaN MIS–HEMTs

The effect of SiNx thickness on device characteristics such as threshold voltage, carrier density, and carrier mobility have been determined for a metal–organic chemical-vapor-deposition grown In0.17Al0.83N/AlN/GaN structure with an ultra-thin In0.17Al0.83N/AlN (2.3/1nm) barrier layer. The SiNx gate dielectric was deposited ex situ in an RF plasma assisted molecular beam epitaxy system. The threshold voltage shifts negatively and the carrier density increases as the SiNx thickness is increased from 1 to 6nm due to the presence of a positive charge at the SiNx/In0.17Al0.83N interface. An interfacial charge of +3.84×1013cm−2 was extracted through the dependence of threshold voltage on insulator thickness. While remote charge scattering from the interfacial charge is shown to limit the carrier mobility, values as high as 1550cm2/Vs were achieved. Low gate and off-state drain leakage currents of less than 500nA/mm, a drain current ON/OFF ratio of approximately 107, and a normalized three terminal breakdown voltage of approximately 60–80V/μm gate–drain spacing were achieved on these ultra-thin In0.17Al0.83N/AlN barrier devices by implementing a thin SiNx gate insulator. The ability to maintain a short gate-to-channel distance while utilizing a gate insulator for reduced leakage current and improved breakdown can provide a pathway for higher power millimeter-wavelength amplifier performance.

Direct Structural Evidences of Epitaxial Growth Ge<SUB>1-X</SUB> Mn<SUB>X</SUB> Nanocolumn Bi-Layers on Ge(001)

Molecular Beam Epitaxy (MBE) system equipped with in-situ Reflection High-Energy Electron Diffraction (RHEED) has been used for (Ge, Mn) thin film growth and monitoring the surface morphology and crystal structure of thin films. Based on the observation of changes in RHEED patterns during nanocolumn growth, we used a real-time control approach to realize multilayer structures that consist of two nanocolumn layers separated by a Ge barrier layer. Transmission Electron Microscopy (TEM) has been used to investigate the structural properties of the GeMn nanocolumns and GeMn/Ge nanocolumns bi-layers samples.

Structural, optical and electronic properties of homoepitaxial GaN nanowalls grown on GaN template by laser molecular beam epitaxy

We have grown homoepitaxial GaN nanowall networks on GaN template using an ultra-high vacuum laser assisted molecular beam epitaxy system by ablating solid GaN target under a constant r.f. nitrogen plasma ambient.

Incorporation model of N into GaInNAs alloys grown by radio-frequency plasma-assisted molecular beam epitaxy

We present a Maxwell-Boltzmann electron energy distribution based model for the incorporation rate of nitrogen into GaInNAs grown by molecular beam epitaxy (MBE) using a radio frequency plasma source. Nitrogen concentration is predicted as a function of radio-frequency system primary resistance, N flow, and RF power, and group III growth rate. The semi-empirical model is shown to be repeatable with a maximum error of 6%. The model was validated for two different MBE systems by growing GaInNAs on GaAs(100) with variable nitrogen composition of 0%–6%.

Electronic structure of GaN nanowall network analysed by XPS

This work examines the changes in bonding of GaN nanowall network samples with varying morphology grown by rf-plasma assisted molecular beam epitaxy system. X-ray photoelectron spectroscopy is employed to study the valence band spectra of two samples and the difference in bonding with change in morphology is observed. Compared to a standard epilayer, the nanostructured samples display a higher binding energy for different hybridization levels appearing near the top of valence band. Higher than statistical branching ratio of Ga 2p spin split levels is also observed unlike in the case of epilayer. The higher bonding energy is understood as having more stable electronic structure which is possible because of reduction of defects in the nanostructured films.

Growth temperature induced physical property variation of InN films grown on nitrided sapphire substrate by PAMBE

InN epilayers were deposited on nitrided sapphire substrates using plasma-assisted molecular beam epitaxy (PAMBE) system. The physical properties of InN films under different growth temperatures were thoroughly studied. The XRD results indicated that as-prepared InN films without Indium droplets were preferentially oriented in the c-axis direction. The SEM results showed that the InN films grown in a two-dimensional mode with a thickness of ∼200 nm under the growth temperature of 460 °C. In addition, the optical absorption measurement exhibited that the energy bandgap of InN films was around 0.75–0.81eV and the PL spectra of the epilayers revealed an infrared emission. Moreover, the electrical properties of the films were discussed by Hall effect in detail.

Restoring pristine Bi2Se3 surfaces with an effective Se decapping process

High quality thin films of topological insulators (TI) such as Bi2Se3 have been successfully synthesized by molecular beam epitaxy (MBE). Although the surface of MBE films can be protected by capping with inert materials such as amorphous Se, restoring an atomically clean pristine surface after decapping has never been demonstrated, which prevents in-depth investigations of the intrinsic properties of TI thin films with ex situ tools. Using high resolution scanning tunneling microscopy/spectroscopy (STM/STS), we demonstrate a simple and highly reproducible Se decapping method that allows recovery of the pristine surface of extremely high quality Bi2Se3 thin films grown and capped with Se in a separate MBE system then exposed to the atmosphere during transfer into the STM system. The crucial step of our decapping process is the removal of the surface contaminants on top of amorphous Se before thermal desorption of Se at a mild temperature (∼210 °C). This effective Se decapping process opens up the possibility of ex situ characterizations of pristine surfaces of interesting selenide materials and beyond using cutting-edge techniques.

Nitridation effects of Si(1 1 1) substrate surface on InN nanorods grown by plasma-assisted molecular beam epitaxy

The InN nanorods were grown on Si(111) substrate by plasma-assisted molecular beam epitaxy (PAMBE) system, with a substrate nitridation process. The effect of nitriding time of Si(111) substrate on morphology, orientation and growth temperature of InN nanorods was characterized via scanning electron microscopy (SEM) and X-ray diffraction (XRD). The deviation angle of InN nanorods was measured to evaluate the alignment of arrays. The results showed that InN nanorods could not be formed with 1min nitridation of Si(111) substrate, but they could be obtained again when the nitriding time was increased to more than 10min. In order to get aligned InN nanorods, the growth temperature needed to increase with longer nitriding time. The vertical orientation of InN nanorods could be enhanced with increase in nitriding time. The influence of the substrate nitridation on the photoluminescence (PL) spectra of InN nanorods has been investigated.

Study of thermally activated reaction between Mn and GaAs(111) surface

We investigate the chemical reactivity of ultra-thin films of Mn on As-terminated (111) GaAs surface kept at ultra-high vacuum conditions in the temperature interval ranging from room temperature to 400°C. The experiments were performed using a customized molecular beam epitaxy system equipped with reflection high energy electron diffraction and X-ray photoelectron spectroscopy techniques. These analyses were complemented with X-ray diffraction measurements to put in evidence the formation of intermediate compounds. Only Ga1−xMnxAs and sub-arsenised MnAsx compounds are possibly formed below 200°C. The onset of the reactivity occurs around 200°C when ordered compounds such as MnAs and MnGa are observed. The formation of compounds more rich in Mn like Mn3Ga and Mn2As is found for deposition temperatures of 300 and 400°C.

Giant Magnetoresistance and Oscillations In Interlayer Exchange Coupling In Co/Cu/Co Multi-Layers

The oscillations in interlayer exchange coupling between ferromagnetic layers which are Separated by a nonmagnetic spacer layer, manifests n oscillatory magnetic properties which depend on the thickness of Copper interlayer. Using Molecular Beam Epitaxy system. We have made multilayer films with alternating cobalt and copper thin films. The preliminary results are reported on microstructure and magnetization properties. The conductance between first and third film depends on the relative orientation of magnetization between them. This optimizes to large magneto resistance changes called Giant Magneto-resistance effect.

Gallium loading of gold seed for high yield of patterned GaAs nanowires

A method is presented for maximizing the yield and crystal phase purity of vertically aligned Au-assisted GaAs nanowires grown with an SiOx selective area epitaxy mask on GaAs (111)B substrates. The nanowires were grown by the vapor-liquid-solid (VLS) method in a gas source molecular beam epitaxy system. During annealing, Au VLS seeds will alloy with the underlying GaAs substrate and collect beneath the SiOx mask layer. This behavior is detrimental to obtaining vertically aligned, epitaxial nanowire growth. To circumvent this issue, Au droplets were pre-filled with Ga assuring vertical yields in excess of 99%.

(Invited) Electrochemical Properties for UHV-Prepared Pt/M Well-Defined Single Crystal Surfaces

We investigated the electrochemical properties of various-monolayer (ML)-thick Pt-shell model surfaces. The m-ML-Pt shell/Au(111) (m = 1–3) was prepared through epitaxial growth of Pt on a clean Au(111) surface at 300 K, and the n-ML-Pt shell/Ni–Pt(111) (n = 1–4) was generated through thermal segregation of substrate Pt atoms during 1-ML-thick Ni deposition onto a clean Pt(111) surface at 823 K followed by additional epitaxial growth of Pt at 473 K in ultra-high-vacuum molecular beam epitaxy (UHV-MBE) systems. Surface structures of the Pt-shell surfaces were verified using a scanning tunneling microscope in UHV. Oxygen reduction reaction (ORR) activities and structural stabilities of the shell surfaces were evaluated in oxygen-saturated 0.1 M HClO4. The results obtained reveal that not only surface strains but also nanoscale surface morphologies of the most dense plane of Pt(111) are correlated with ORR enhancements and that the 3-ML-thick Pt shell stabilizes the topmost surfaces of Pt/M bimetallic systems.

Franz-Keldysh Effect in GeSn Detectors

The fundament for silicon based photonics is Ge grown on Si and SOI. A simple photonic integrated circuit (PIC) consist of an integrated light source, a waveguide, a modulator and a detector. Due to the fact that Ge is an indirect semiconductors, the light source is the main challenge for PICs. One concept is to add Sn into the Ge matrix. Theoretical calculations predict that Ge1-xSnx becomes a direct semiconductor for a Sn content of x = 10 % and therefore is an interesting material mixture for photonic devices integrated on Si. Recent publications have shown that detectors using GeSn as active material excess 40 GHz bandwidth. For PICs, also the modulation of the light is needed. Beside Mach-Zehnder modulators, the modulator can be realized using the Franz-Keldysh (FK) effect. Due to the FK effect, the absorption in the material can be changed by varying the applied electric field.In this paper we report the FK effect in GeSn PIN diodes with Sn concentrations up to 4.2 %. In order to analyze the absorption of GeSn, vertical PIN diodes with different Sn content were produced. First, the epitaxial layers were grown with a solid source molecular beam epitaxy system [1] on B-doped Si (100) substrates with a high specific resistance ρ > 1000 Ωcm. Fig. 1(a) shows the vertical layer sequence. The layer stack consists of a vertical PIN structure. Starting with a 400 nm B-doped Ge contact layer (1020 cm-3), the active layer with 300 nm intrinsic Ge1-xSnx and a 10 nm Ge spacer follow. The top contact consist of 100 nm Sb-doped Ge (1020 cm-3) and 100 nm Sb-doped Si (1020 cm-3). The three investigated samples have a Sn content of x = 0 %, 2 % and 4.2 %, respectively. The GeSn PIN heterojunction photodiodes are produced in a quasi-planar technology. The device processing by double mesa etching, passivation and metallization is similar to earlier reported GeSn pin photodiodes [2]. A schematic high frequency device structure of the GeSn pin photodetector with the typical ground signal ground (GSG) probe configuration with a pitch of 100 µm is shown in Fig 1(b).The electrical and optical characterization of the diodes is done with on-wafer measurement of the current-voltage characteristics. As a light source, a super-continuum laser with a tunable acousto-optical wavelength filter is used. The light couples into the diode through a lensed glass fiber. From measuring the dark current and photo current, the responsivity of the GeSn detectors were determined by dividing the photo current by the optical power.Fig. 2 shows the responsivity of the GeSn PIN diodes with Sn concentrations of x = 0 %, 2 % and 4.2 %. With increasing Sn content, the bandgap shifts to higher wavelengths and therefore, the responsivity increases for higher wavelengths. A voltage dependence of the optical responsivity is only shown near the direct bandgap Eg,Γ. This is due to the FK effect.From the measurement of the responsivity and reflectivity, the internal quantum efficiency can be calculated and further the absorption is determined. The electro-optical modulation from the FK effect was clearly observed in GeSn material. The maximum absorption ratio Δα/α0 for the 2 % Sn sample at room temperature was 1.5 at a wavelength of 1806 nm and a voltage swing of 3 V.

Dynamic layer rearrangement during growth of layered oxide films by molecular beam epitaxy.

The A(n+1)B(n)O(3n+1) Ruddlesden-Popper homologous series offers a wide variety of functionalities including dielectric, ferroelectric, magnetic and catalytic properties. Unfortunately, the synthesis of such layered oxides has been a major challenge owing to the occurrence of growth defects that result in poor materials behaviour in the higher-order members. To understand the fundamental physics of layered oxide growth, we have developed an oxide molecular beam epitaxy system with in situ synchrotron X-ray scattering capability. We present results demonstrating that layered oxide films can dynamically rearrange during growth, leading to structures that are highly unexpected on the basis of the intended layer sequencing. Theoretical calculations indicate that rearrangement can occur in many layered oxide systems and suggest a general approach that may be essential for the construction of metastable Ruddlesden-Popper phases. We demonstrate the utility of the new-found growth strategy by performing the first atomically controlled synthesis of single-crystalline La3Ni2O7.

MODULATION OF PTCDA NANOSTRUCTURE AND OPTICAL PROPERTY: DEPENDENCE ON GROWTH TEMPERATURE

3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) nanostructures with different morphologies are prepared on glass substrates at different substrate temperature (Ts) in a molecular beam epitaxy (MBE) system. Scanning/transmission/scanning transmission electron microscopy (SEM/TEM/STEM), X-ray diffraction (XRD), selected area electron diffraction (SAED) and nanobeam diffraction (NBD) techniques are employed in the systematical characterizations of the nanostructures. It is found that the PTCDA nanosheets (NSs), nanowires and nanorods are facile to produce at Ts = 350° C , 330°C and 240°C, respectively; the continuous films are obtained at 180°C and 50°C. XRD studies indicate that only the α-phase polymorph is formed regardless of the Ts. SAED and NBD results show that the nanowire and NS are single crystalline. The optical properties of the prepared PTCDA nanostructures are also found to be influenced by Ts and are correlated with the crystal quality and size. PTCDA nanowires and NSs exhibit an obvious redshift and broadening in the adsorption spectra, and enhanced emission intensity. The improved optical properties facilitate potential applications of these nanostructures in organic optoelectronic devices.

Mediation effect of sub‐monolayer carbon on interfacial mixing in Ge growth on Si(100)

AbstractThe mediation effect of carbon (C) on interfacial mixing in Ge growth was studied using C‐covered Si surface (Ge/C/Si) and C over thin Ge buffer layer (Ge/C/Ge/Si) systems. The samples were prepared by solid‐source molecular beam epitaxy system with electron beam gun for C sublimation and K‐cell for Ge evaporation. The interdiffusion at the interface and deterioration in crystallinity of Ge layer was evaluated by Raman spectroscopy and X‐ray diffraction, respectively. In the case of Ge/C/Si system, the interdiffusion of Si and Ge atoms was suppressed by more than 0.3 monolayer (ML)‐C depositions while crystallinity of Ge layer was deteriorates by the formation of Si‐C at the Si surface and the incorporation of C atoms in Ge layer. In the case of Ge/C/Ge/Si system, the crystallinity of Ge layer was maintained even at C coverage of a few MLs, however, the amount of C to suppress the Ge/Si interdiffusion was more than 3 ML. From the correlation between Ge crystallinity and surface roughness, Ge/C/Ge/Si structure is considered to be more effective to take advantage of C mediation. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and electronic properties of epitaxial GaN layer grown on sapphire (0001) using laser molecular beam epitaxy

Epitaxial GaN films were grown on sapphire (0001) substrates by an ultra-high vacuum laser assisted molecular beam epitaxy (MBE) system using GaN solid target with laser energy density of ∼3 J cm−2 at various growth conditions. The influence of growth temperature, layer thickness and growth rate on the structural properties of the GaN layers have been studied using high resolution x-ray diffraction, field emission scanning electron microscopy and scanning tunneling microscopy at room temperature. The epitaxial GaN layers grown at 700 °C exhibited good crystalline properties with a screw dislocation density of 3.1 × 108 cm−2 as calculated from the x-ray rocking curve measurements. The electronic properties such as core levels and valence band of GaN film were examined using x-ray photoelectron spectroscopy. Chemical composition of the GaN layer was determined using core level spectroscopy.

Characterizing the electron transport properties of a single 〈110〉 InAs nanowire

InAs nanowire with 〈110〉 orientation is proposed for use as an electron spin transport channel for application to spintronics devices, particularly the Datta–Das spin transistor. Stable zinc blende crystal NWs were grown using a molecular beam epitaxy system. Subsequently, global back-gate NW field effect transistors were fabricated, and the superiority of the electrical transport properties within our resultant 〈110〉 NWs was demonstrated by comparing the field-effect mobility with a 〈111〉 NW control sample. Additionally, single NW Hall-bar devices were fabricated, which allowed us to obtain the transport properties accurately, and Hall effect measurements were successfully taken at different temperatures.