MBE Chamber Research Articles

In situ photoemission spectroscopic study on La 1− xSr xMnO 3 thin films grown by combinatorial laser-MBE

We have performed an in situ synchrotron radiation photoemission spectroscopy (SRPES) on well-ordered surfaces of La 1− x Sr x MnO 3 ( x=0, 0.2, 0.4, 0.55) thin films grown using laser molecular beam epitaxy (laser-MBE). The laser MBE chamber is connected to a high-resolution SRPES system, where the grown films are transferred from the laser MBE to the SRPES chamber without breaking the ultrahigh-vacuum. In sharp contrast with the previous PES spectra recorded from scraped polycrystalline samples, the SRPES spectra exhibit an existence of prominent Mn 3d e g states near the Fermi level. On the other hand, O 1s X-ray absorption spectra for laser-MBE-grown samples are in good agreement with the scraped ones. These results clearly indicate the importance of in situ SRPES measurement on well-ordered surfaces of transition metal oxides for revealing their intrinsic electronic structures.

MBE lanthanum-based high-k gate dielectrics as candidates for SiO2 gate oxide replacement

In this work, we investigate La-based complex oxides such as perovskite-like LaAlO3 and pyrochlore La2Hf2O7, deposited on rapid thermal SiO2 (RTO)/Si substrates using rf plasma source of atomic oxygen in an MBE chamber. The C–V curves measured in MIS capacitors were analyzed solving self-consistently the Poisson and Schrodinger equations to take into account quantum mechanical effects. From the dependence of the equivalent oxide thickness (EOT) as a function of physical thickness tph, the dielectric permittivity was estimated to be 14 and 18 for perovskite-like LaAlO3 (LAO) and pyrochlore La2Hf2O7 (LHO), respectively, which are lower than expected probably due to slight deviations from the stoichiometric compositions. Values of EOT∼1.1nm and Jg∼2×10−3A/cm2 at 1V for the LAO, and EOT∼1.14nm and Jg∼4×10−5A/cm2 at 1V for the LHO were obtained.

High-Throughput Characterization of Combinatorial Thin Film Libraries using in situ Synchrotron-Radiation Photoemission Spectroscopy

We have constructed a high-resolution synchrotron-radiation photoemission spectroscopy system combined with a combinatorial laser molecular-beam epitaxy (laser MBE) thin film growth system, in order to realize high-throughput characterization of surface and interface electronic structures of transition-metal oxide thin films. The combinatorial laser MBE chamber is directly connected to the photoemission chamber so that a fabricated thin-film library can be transferred quickly into the photoemission chamber without breaking ultra high-vacuum. The combinatorial laser MBE system can be used for fabricating combinatorial thin film libraries, i. e. using physical masking during deposition to grow films under different deposition conditions with different compositions or with different thicknesses at different parts of a single substrate. Mapping electronic structures can be performed in a single growth-characterization cycle by scanning the synchrotron radiation beam over the thin film libraries. The capabilities of the system have been demonstrated by in-situ photoemission analysis of (1) the La1−xSrxMnO3 thin-film libraries with different compositions, (2) the La0.6Sr0.4MnO3 thin-film libraries with different growth conditions, and (3) the La0.6Sr0.4FeO3/La0.6Sr0.4MnO3 superlattice libraries with different thicknesses.

A high-resolution synchrotron-radiation angle-resolved photoemission spectrometer with in situ oxide thin film growth capability

We have constructed a high-resolution synchrotron-radiation angle-resolved photoemission (ARPES) spectrometer combined with a combinatorial laser molecular-beam epitaxy (laser MBE) thin film growth system in order to investigate the electronic structure of transition metal oxide thin films. An ARPES spectrometer GAMMADATA SCIENTA SES-100 was selected for the high-throughput and high-energy and angular-resolution ARPES measurements. A total energy resolution of 6.3 meV and a momentum (an angular) resolution of 0.02 Å−1 (0.2°) were obtained at a photon energy of 40 eV. The system is installed at the high-resolution vacuum-ultraviolet beamline BL-1C or the soft-x-ray undulator beamline BL-2C at the Photon Factory as an end-station. Another distinctive feature of this system is the direct connection from the spectrometer to a laser MBE chamber. Thin film samples can be transferred quickly into the photoemission chamber without breaking ultrahigh vacuum. Laser MBE is one of the best methods to grow thin films of many different transition metal oxides and to achieve well-ordered surfaces, which are indispensable for the ARPES measurements. The capabilities of the system and the importance of the in situ sample transfer between ARPES and laser MBE are demonstrated by studying the band structure of La0.6Sr0.4MnO3 thin films epitaxially grown on SrTiO3 substrates by laser MBE.

Stoichiometric and non-stoichiometric films in the Si–O–N system: mechanical, electrical, and dielectric properties

A novel low-temperature (600–850 °C), chemical vapor deposition method, involving a simple reaction between disiloxane (H 3Si–O–SiH 3) and ammonia (NH 3), is described to deposit stoichiometric, Si 2N 2O, and non-stoichiometric, SiO x N y , silicon oxynitride films (5–500 nm) on Si substrates. Note, the gaseous reactants are free from carbon and other undesirable contaminants. The deposition of Si 2N 2O on Si (with (1 0 0) orientation and a native oxide layer of 1 nm) was conducted at a pressure of 2 Torr and at extremely high rates of 20–30 nm min −1 with complete hydrogen elimination. The deposition rate of SiO x N y on highly-doped Si (with (1 1 1) orientation but without native oxide) at 10 −6 Torr was ∼1.5 nm min −1, and achieved via the reaction of disiloxane with N atoms, generated by an RF source in an MBE chamber. The phase, composition and structure of the oxynitride films were characterized by a variety of analytical techniques. The hardness of Si 2N 2O, and the capacitance–voltage ( C– V) as a function of frequency and leakage current density–voltage ( J L– V) characteristics were determined on MOS (Al/Si 2N 2O/SiO/ p-Si) structures. The hardness, frequency-dispersionless dielectric permittivity ( K), and J L at 6 V for a 20 nm Si 2N 2O film were determined to be 18 GPa, 6 and 0.05–0.1 nA cm −2, respectively.

Effect of As molecular species on inter-surface diffusion in GaAs MBE for ridge structure fabrication

The faceting and the shrinkage processes of ridge structure fabricated along [1 1 0] on GaAs (0 0 1) substrate with As 2 flux and As 4 flux were monitored by microprobe reflection high-energy electron diffraction/scanning electron microscopy (microprobe-RHEED/SEM) installed in the MBE chamber. It was found that the shrinkage of top size with As 4 flux was faster than that with As 2 flux. To analyze the result of the experiments, the calculation based on two-face inter-surface diffusion model was conducted. From this calculation, we show that the shrinkage of the top size is slower when the arsenic pressure is higher, simply because the lateral flow of Ga adatoms towards the top surface becomes smaller. Under the As 2 flux, the amount of Ga adatoms which migrate to (0 0 1) surfaces from (1 1 1)B surface becomes smaller compared with that under As 4 flux due to the higher reactivity of As 2 to Ga adatoms.

Nanometer patterning of thin CoSi 2-films by application of local stress

We have developed a self-assembly method for patterning thin CoSi 2 layers on Si(100) during their formation in a solid state reaction. This technique is based on anisotropic diffusion in a local stress field during rapid thermal processing. The stress is induced by a layer structure consisting of 30 nm SiO 2 and 300 nm Si 3N 4 which is patterned with conventional optical lithography. We have investigated two different silicide formation processes. Firstly, we deposited Co on Si in a UHV MBE chamber. Rapid thermal annealing leads to the formation of polycrystalline CoSi 2. Secondly, we used a titanium oxide mediated epitaxy process. For both processes we observed nanostructures with dimensions of about 100 nm showing wave-like separation edges in the first case and good uniformity in the second case.

COMBE: A Powerful New Tool for Materials Science

We describe a new technique, combinatorial molecular beam epitaxy (COMBE), for synthesis of thin films of complex oxides including high-temperature superconductors, ferroelectrics, ferromagnets, etc. The Oxxel COMBE system contains a modular MBE chamber, equipped with 16 metal sources, an ozone source, an electron microscope (LEEM / LEED / PEEM system), time-of-flight ion scattering and recoil spectroscopy (TOF-ISARS), a multiple-readout electron diffraction (scanning RHEED), and a scanning quartz crystal monitor (QCM). It allows for synthesis of large libraries of compounds with systematic variation in composition and thermodynamic parameters, and thus for fast combinatorial search for new materials for oxide electronics.

Synchrotron X-ray study of the epitaxial Co/Pt multilayers on Al 2O 3[formula omitted] substrates with Pt/Mo buffer layers

In this study, epitaxial [Co(t nm)/Pt(1 nm)] 30 multilayer samples ( t=1, 0.5, 0.4, 0.3, 0.25 and 0.2 nm) were studied using X-ray measurements. The samples were prepared on Pt(1 1 1)/Mo(1 1 0) buffered Al 2O 3(1 1 2̄ 0) substrates in the MBE chamber. It was found that the Pt layer in the multilayer has a compressible strain of about 2–3.5% along the in-plane direction and the saturation magnetization is roughly proportional to this strain. Polarized X-ray absorption spectroscopy study showed that the Co layer is more like an FCC pseudomorphic structure for Co thickness less than 0.5 nm.

Surface reconstruction of Fe 3O 4(001)

We have investigated the surface termination, structure, morphology and composition of Fe 3O 4(001) using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), low-energy He +-ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). The samples consisted of ∼5000 Å thick epitaxial films of Fe 3O 4(001) grown by oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE) on MgO(001) substrates. The ( 2 × 2 ) R45° surface reconstruction that is present on the as-grown surface is recovered by heating the sample in oxygen (10 −6–10 −7 mbar) at temperatures between 420 and 770 K after a through-air transfer from the MBE chamber. STM results are best interpreted by assuming an autocompensated B-layer termination, which consists of a layer of octahedrally coordinated iron and tetrahedrally coordinated oxygen, along with one oxygen vacancy per unit cell. Evidence for a vacancy-induced lateral relaxation of the adjacent octahedral iron ions is presented. Further annealing in ultrahigh vacuum causes a transformation to either a (1× n) or a (2 2 × 2 ) R45° structure. These surfaces can be reproducibly transformed back to the ( 2 × 2 ) R45° reconstruction by annealing in oxygen. Interestingly, at no time do we observe the other autocompensated termination, which consists of one-half of a monolayer of tetrahedrally coordinated Fe(III), despite its observation on the as-grown surface. Thus, it appears that the surface termination is critically dependent on the method of surface preparation.

Real-time observations of mesa shrinkage process in MBE of GaAs on (1 1 1)B patterned substrates and theoretical analysis

The faceting and shrinkage processes of mesa structures on GaAs (1 1 1)B substrates were monitored by microprobe reflection high-energy electron diffraction/scanning electron microscopy (microprobe-RHEED/SEM) installed in MBE chamber. To understand the experimental results, the calculation based on inter-surface diffusion model was conducted and the results are compared with the experiments. From the comparison we evaluated diffusion coefficient and incorporation lifetime of Ga adatoms. The lifetime and the diffusion coefficient of (1 1 1)B substrate were estimated, respectively, as 1/70 as small and 50 times as large as those of {1 1 0} side wall. The mesa shrinks faster when arsenic pressure is higher, because Ga adatoms on {1 1 0} side wall flow more towards (1 1 1)B top and less towards the bottom.

Growth of thick and pure cubic GaN on (0 0 1) GaAs by halide VPE

To obtain a thick and pure cubic GaN layer, a hybrid growth of the halide vapor phase epitaxy (HVPE) and the metal organic MBE (MOMBE) was performed. An about 20 nm thick GaN buffer layer was grown on (0 0 1) GaAs by MOMBE using tertiary butyl hydrazine in a MBE chamber. Thick GaN layers were grown on it by a conventional HVPE system using H 2 as a carrier gas. When the thickness was less than 2 μm, the hexagonal component in the grown layer was less than 1% for a V/III ratio of 200, but it increased exponentially with an increase of the growth time and it became about 10% at 5 μm (2 h growth) and about 40% at 10 μm (3 h growth). These thicknesses are about three times larger than those generally obtained by GSMBE or MOVPE with the same hexagonal content. The grown surface became rough with an increase of thickness and therefore of hexagonal content. Photoluminescence was very poor even for a pure cubic GaN probably due to autodoping by arsenic.

ＭＢＥ装置を用いたナノメータ金属膜の形成

The new functional materials have been studied to apply to micro machines or electronic devices. However in order to develop new materials, it is indispensable to control the film thickness of nano-meter or sub-nanometer and to deposit the fine film of the same roughness as substrate surface. This paper describes the experimental results, the possibility to fabricate metal films of nanometer order thickness and to synthesize new materials of multilayer structure using the Knudsen-cell in the high vacuum MBE chamber. The structure of the films was analyzed by X-ray diffraction.

Analysis of the variation in the composition as a function of growth parameters in the MBE growth of indium doped Hg1−xCdxTe

Compositional changes induced in growing Hg1−xCdxTe layers as a function of the changes in temperature of the indium source in the MBE chamber have been analyzed in terms of the Hg-In alloy thermodynamics and changes in the activity of indium over the growing films as the Hg flux changes. Compositional changes induced by changes in the Te source temperature have been analyzed in terms of the changes in the activity of Cd over the growing layers. These analysis are helpful in understanding the compositional changes with variation of the growth parameters in the MBE chamber.

Effects of growth interruption upon silicon delta layers in gallium arsenide

We describe a study of samples grown by molecular-beam epitaxy, designed to examine the limitations to silicon (Si) δ-doping of gallium arsenide (GaAs) at elevated growth temperatures. Samples (Si δ-doped to 1 × 1013cm−2) were grown at 630°C, during which the growth was paused (for times between 0 and 18 min) after depositing the Si. Total electron sheet densities were measured (using Hall and Shubnikov-de Haas (SdH) effect measurements) to determine the electrically active Si concentration. The free electron concentration showed a reduction from about 8 × 1012cm−2 for zero pause time, to 5 × 1012cm−2 for 18 min pause time. These results can be explained convincingly using simple models of impurity background in the MBE chamber or surface aggregation. Further experiments to distinguish these mechanisms are discussed.

Inhomogeneous growth of CaF2 adlayers on Si(111) at intermediate temperatures

The growth morphology of CaF2 adlayers deposited in the temperature range 500–650°C on Si(111) samples with 0.9° miscut in [112¯] direction has been studied by atomic force microscopy (AFM). After transferring the samples from the MBE chamber to ambient air we found an inhomogeneous film morphology: on one hand there are smooth islands wetting substrate terraces and on the other hand different substrate terraces show small clusters. The cluster formation is attributed to the reaction of the first unstable CaF2 layer after contact with ambient air while the smooth areas consist of thickness stabilized CaF2 islands. The smooth islands grow preferentially in the [11¯0] direction since the substrate steps block the growth in [112¯] direction.

UHV-AFM study of MBE-grown 10 nm scale ridge quantum wires

We evaluate the shape of ridge quantum wires (RQWIs) with nm-scale resolution by using SEM and ultra-high-vacuum (UHV) AFM system which is connected to MBE chamber. This MBE-AFM system provides us a detailed information about the evolution of size and uniformity of ridges grown under various conditions. In this report, we investigate systematically how the growth temperature T s and As flux affect the width and morphology of GaAs ridge structure. The results show that a very sharp and uniform GaAs ridge structures (W < 10 nm) can be obtained. On the basis of this understanding, we fabricated the ridge quantum wire of 10 nm width.

MBE growth of n-type ZnSe and ZnS using ethylchloride as a dopant

A new gas source n-type dopant, ethylchloride (EtCl), was used for MBE growth of ZnSe and ZnS. Carrier concentrations up to 3.2 × 1018 and 3.5 × 1017cm−3 were achieved for ZnSe and ZnS, respectively. The use of the gas source dopant is advantageous in maintaining the MBE chamber.

Design and Characterization of a UHV Arcjet Nitrogen Source

AbstractA UHV-compatible nitrogen arcjet suitable for the growth of III-nitrides by molecular beam epitaxy is described and characterized. The arcjet operates at powers between 10W and 300W (the highest power used for these studies); typical nitrogen flows range between 5sccm and 100sccm. Optical emission spectra show the presence of activated atomic (N*) and molecular (N2*) nitrogen. A collisional radiative equilibrium model has been employed to provide insight into the excitation state of the active nitrogen. These results indicate that the arcjet is capable of supplying atomic nitrogen fluxes consistent with growth rates on the order of several monolayers per second. Langmuir probe measurements conducted near the position of the sample holder in the MBE chamber show the charged particle flux density is very low. The arcjet operates over a large powerpressure parameter space, and properties of the arc can be systematically “tuned” to provide a source suitable for selected-energy-epitaxy.

Surface Reconstructions and III-V Stoichiometry: The Case of Cubic and Hexagonal GaN

ABSTRACTSurface reconstructions for MBE grown GaN are identified. Different cases are considered according to the type of substrate or crystal symmetry and surface phase diagrams are obtained. Through different examples, it is shown how growth monitoring can be efficiently achieved through the use of surface reconstructions. Finally, from the observation that a residual arsenic overpressure in the MBE chamber changes the surface reconstructions of cubic (001) GaN grown onto 3C-SiC (001) substrates to that commonly observed for GaN growth on (001) GaAs, it is proposed that arsenic might be a surfactant for nitride growth.