MBE Technique Research Articles

Surface Morphology of InxGa1-xAs/GaAs Relaxed Layers Characterized by Atomic Force Microscopy

ABSTRACTGa0.65In0.35As layers of a varying nominal epilayer thickness (10 – 1000 nm) have been grown by the MBE technique on GaAs (100) substrates and characterized by the combined use of atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXD). The surface roughness and morphology have been investigated. The GIXD and AFM results show that the thinnest films are characterized by an asymmetric strain relaxation along the two <110> directions with no surface crosshatched pattern but with a misfit dislocation network. AFM images on the thickest films show also well-oriented protrusions along the [110] direction, which increase in size and become more elongated as the nominal film thickness increases.

Strain and Interdiffusion Profiles in Epitaxied AU/NI (100) Multilayers Deduced From X-Ray Diffraction Experiments

AbstractAu(100)/Ni(100) periodic superlattices have been grown by the MBE technique specially for X-ray diffraction experiments. The nominal number of Ni planes deposited were 1.5, 2, 4 and 5 ML and the thickness of the gold layers was always 1.5 nm. The X-ray profiles have been simulated using a kinematical structure refinement program. The obtained strain profiles are in very good agreement with earlier HREM results. It is shown that a strong interdiffusion occurs during the growth.

Spin Dependent Photoconductivity in Silicon-on-Sapphire

Resonant and non resonant spin dependent photoconductivity is observed in(100) silicon films grown on sapphire by CVD and MBE techniques. The CVD films are either in their as-grown state or have undergone single or double solid phase epitaxial regrowth. For all samples, a resonant decrease in photoconductivity is observed at a field of about 0.34 T for a microwave frequency of about 9.7 GHz and at about 3.3 mT when the frequency is about 92 MHz. For all samples the maximum fractional change in photoconductivity is approximately 10-4 independent of magnetic field strength.

Direct submillimeter absorptivity measurements on epitaxial Ba 1−xK xBiO 3 films at 2K

We have used a bolometric technique to obtain accurate low temperature loss data for epitaxial thin films of Ba 0.6K 0.4BiO 3 from 30 cm −1 to 700 cm −1. These films were grown on MgO and SrTiO 3 substrates by MBE, off-axis sputtering and laser deposition techniques. All films show a strong absorption onset near the BCS tunneling gap of 3.5k BT c. We have analyzed these data using a Kramers-Kronig transformation and have corrected for finite film thickness effects. Our results indicate that the absorption onset is consistent with a superconducting energy gap. Comparison is made with predictions based on strong coupling Eliashberg theory using α 2F(ω) spectra obtained from the literature. While we are able to fit the overall measured absorptivity, we are unable to fit the structure observed in our data.

Symmetric self-electro-optic effect device array grown by metalorganic vapor phase epitaxy using GaAs/Al0.04Ga0.96As shallow quantum wells

A self-electro-optic effect device (SEED) epistructure grown by low-pressure metalorganic vapor phase epitaxy (MOVPE) is demonstrated. The symmetric SEED using GaAs/Al0.04Ga0.96As shallow quantum well (SQW) exhibits a contrast ratio of 2.9 at 5 V bias with 50 pairs of GaAs wells. This high contrast results from the low background carrier concentration of absorption region which is obtained by the temperature controlled compensation of the acceptors and donors. The absorption data suggest that the MOVPE technique is comparable to the MBE technique in growing the high performance SQW epistructure.

MOLECULAR BEAM EPITAXY OF Fe/Cu MULTILAYERED FILMS AND ITS MAGNETOOPTICAL PROPERTIES

Multilayered structures of 10 Fe layers (25 Å each) and 9 layers of Cu (50 Å) have been obtained by MBE technique using a 3-chambers MBE installation "Angara". For comparison Fe film with the thickness d = 250 Å have been prepared. The chemical composition and structure of the films were controlled by X-ray fluorescent analysis, Auger spectroscopy and electron microscopy measurements. Field dependencies of the magnetooptical Faraday effect were measured at different geometries. Spectral dependence of the Faraday effect revealed a maximum at 700 nm wavelength for this multilayered structure.

The electrical characteristics of Pb1-xEuxSe homojunctions

The current mechanism of Pb1-xEuxSe diodes prepared by the MBE technique was studied by measuring R0A of diodes with five different europium contents as a function of temperature. Good agreement was obtained between theoretical results and experimental data at high temperatures. At low temperatures, R0A is not dominated by band-to-band tunnelling, but by excess tunnelling. The authors favour the explanation of trap-assisted tunnelling since R0A is an exponential function of the energy gap. From the theoretical model, a minority carrier lifetime of the order of a nanosecond was derived.

Erbium Doped Gallium Arsenide a Self-Organising Low Dimensional System

ABSTRACTThe growth of erbium doped gallium arsenide by MBE at normal substrate temperatures (∼580°C) is constrained by a solubility limit of 8×1017 cm−3. This is much less than is desirable for optical emitters using the forbidden 4f transitions of Er3+ to produce radiation at 1.54μm. We have developed an MBE technique where it is possible to produce spherical mesoscopic precipitates containing erbium as a matrix element within the gallium arsenide. Structural and analytical studies indicate that the precipitate is cubic (rock salt) erbium arsenide. The physical size of the precipitates is self limiting as a result of surface migration occurring during MBE growth. By adjusting the growth conditions it is possible to produce an array of uniform erbium arsenide quantum dots of a size chosen from the range 10-20Å. The dot density can be varied by changing the erbium flux.

Growth of Cuinse2 on (lOO) Gaas by Molecular Beam Epitaxy

ABSTRACTDomain-free epitaxial layer of CuInSe2 has been successfully grown on (lOO) GaAs at the substrate temperature above 470°C. Both TEM and X-ray analyses were performed to verify this result. The CuInSe2/GaAs interface was examined by high resolution TEM. For films high in indium, InSe was identifed by X-ray diffraction. TEM observations showed that the InSe precipitates were rectangular in shape and the habit plane was determined to be (110). Photo-assisted MBE technique had also been used for the growth of CuInSe2 films with high crystalline quality. The epitaxial temperature was significantly reduced to 300°C.

Advanced III–V HEMT technology for microwave and millimetre-wave applications

This paper describes the current European status of HEMT technology for analogue MMIC applications at microwave and mm-wave frequencies. Results from three European Foundries (GEC-Marconi, Philips and Thomson) will be presented to illustrate the position. The key technology issues will be discussed, including the requirement for high quality epitaxial material. This latter point has been addressed commercially by both MBE (Picogiga) and MOVPE (Epitaxial Products International) growth techniques.

Novel Molecular System C60: Fullerites and Fullerides

Absorption spectra and TEM and STM images are observed on C 60 thin films prepared by MBE technique. Superconductivity behaviors of alkali metal-doped single crystals and thin films are also reported

Temperature dependence of R0A product for PbSe photodiodes fabricated by MBE and diffusion

The R 0 A product of PbSe photodiodes has been investigated experimentally and theoretically over a wide temperature region from 30 to 300 K. The photodiodes were made from PbSe bulk material by diffused and MBE techniques. Good agreement between experimental and theoretical results has been obtained. Evidence is presented that the R 0 A product is limited by depletion rather than diffusion (Auger recombination) at high temperatures, and at low temperatures by tunneling. Minority carrier lifetimes of the order of nanoseconds near room temperature are derived from a fit to experimental data, about 50% shorter for MBE diodes than for diffused diodes. Majority carrier concentration profiles near the p− n junction at low temperatures are obtained based on the band to band tunneling model roughly 70% higher for MBE diodes than that for diffused diodes, which is in reasonable accordance with Hall data.

Single-Crystal AU-NI Solid Solutions Grown by MBE

ABSTRACTEpitaxial solid solutions Au1-x Nix (100) have been obtained at room temperature on Au(100) substrates by the MBE technique. The layers are 10 nm-thick and the composition x has been varied up to 0.37. A large strain (2–3%) has been measured by X-ray diffraction. The lattice parameters have been measured in two perpendicular directions, perpendicular and parallel to the surface. In the latest case, the grazing-incidence technique has been used. Because this technique is very sensitive to the surface, the strain results may be re-interpreted if, the upperlOO nm are enriched in Au.

“Barrierless” Misfit Dislocation Nucleation in SiGe/Si Strained Layer Epitaxy

ABSTRACTThe study of the critical thickness/strain phenomenon inherent in metastable, layered heterostructures has led to the development of several models which describe elastic strain relaxation. Hitherto, the nucleation of misfit dislocations required for coherency breakdown is the least well understood aspect of strain relaxation, due to the paucity of experimental data. Moreover, existing theoretical calculations predict relatively large activation energy barriers (>10 eV) for misfit dislocation nucleation in relatively low misfit (<2%) systems. In this work it will be shown that the nucleation of misfit dislocations can occur spontaneously demonstrating a vanishingly small activation energy barrier. Specifically, experimental studies of a wide range of GexSi1−x/Si (x< 0.5) hetero-structures, grown by MBE and CVD techniques, have provided quantitative data from bulk specimens on the observed misfit dislocation nucleation rate and activation energy using large-area diagnostic techniques (eg. chemical etching/Nomarski microscopy). In parallel, the strained layer microstructure was studied in detail using crosssectional and plan-view electron microscopy in order to identify a new dislocation nucleation mechanism, the ‘double half-loop’ source. From the combined macroscopic and microscopic analyses, a theoretical treatment has been developed based on nucleation stress and energy criteria which predicts a “barrierless” nucleation process exists even at low misfits (< 1%). Accordingly, the observed misfit dislocation nucleation event has been found both experimentally and theoretically to be rate-controlled solely by Peierls barrier dependent, glide-activated processes with activation energies of ∼2 eV.

In-situ epitaxial growth of BiSr-Ca-CuO films and superlattices by MBE using an oxygen radical beam

Epitaxial BiSr-aCuO thin films and superlattices have been grown on MgO (100) and SrTiO 3 (100) substrates by the MBE technique using an oxygen radical beam produced by RF excitation. During the first deposition of Ca atomic layers on MgO, the epitaxial direction was found rotated by 45° between Ca-free and Ca-containing phases. Superlattices of Bi 2Sr 2caCu 2O 8 (2212)/BiSr 2CuO 6 (2201). in which 2212 layers with a thickness of m caxis half unit cells are separated by nonsuper-conducting 2201 layers of n half unit cells, were grown by this technique. We found that the superlattices exhibit similar superconductivity independent of both the m and n half-unit-cell numbers, indicating the two dimensionality of this superconductor..

In-situ growth of Bi 2Sr 2Ca n−1 Cu nOy superconducting ultrathin films prepared by MBE technique and their characterization

Ultrathin films of Bi 2Sr 2Ca n−1 Cu n O y (n=1>and2) were grown in situ by molecular beam epitaxy technique Even in the 1.5 nm-thick film of the n=2 phase, incommensurate modulated structures at the BiO layer were observed by reflection high-energy electron diffraction. Laue oscillations in X-ray diffraction were clearly observed and they corresponded to the number of primitive cells along the deposited directions.

Nz. sbnd;dependence of superconducting properties in Bi2Sr2Can−1 CunOy thin films prepared by in-situ MBE technique

Thin films of Bi 2 Sr 2 Ca n−1 Cu n O y ( n = 1 ∼ 4) were prepared in situ by molecular beam epitaxy. All films were grown with a sequential deposition technique in an identical condition. To investigate the n dependence of physical properties, Hall coefficient and electron energy loss spectroscopy were measured. Hall coefficient and energy loss spectra were systematically changed from n=1 to 3, but the results of the n=4 phase showed different behavior.

Origin of the near infrared luminescence in n-type AlxGa1-xAs alloys

A detailed analysis of the near-infrared luminescence has been performed in undoped and n-doped (Si, Te, Sn) AlGaAs samples grown by MBE and MOVPE techniques. A wide range of Al compositions (26-74%) and donor concentrations (1016-1018 cm-3) was examined. In the spectral region from 0.9 to 2.0 mu m in all samples there is a photoluminescence (PL) peak at 1.1 mu m (IR1). MOVPE samples show an additional PL peak at 1.5 mu m (IR2), which is independent of the Al mole fraction, the doping concentration and the donor species IR2 is not present in MBE-grown samples, where a third peak IR3 appears. None of the IR peaks is related to DX deep donor centres, since the amplitudes of all IR peaks are independent of the donor doping level. By optically detected ESR of Si-doped samples it was shown that the IR2 band is due to arsenic antisite defects and not to DX centres as previously assumed. Also, deep level transient spectroscopy measurements were performed on Si-doped samples.

Molecular beam epitaxy with synchronization of nucleation

A new MBE technique based on the possibility to maintain undamping RHEED oscillations by periodic increase of the surface supersaturation synchronized with the oscillations is presented. Surface supersaturation may be increased by any way that is suitable for a particular case. The technique has been referred to by the authors as MBE with nucleation synchronization and allows epistructures of any thickness to be grown with in situ control of their properties by RHEED or automatic ellipsometry. It can be applied both to elementary semiconductors and compounds like A IIIB V, A IIB VI, etc.

Molecular beam epitaxy with synchronization of nucleation

Abstract A new MBE technique based on the possibility to maintain undamping RHEED oscillations by periodic increase of the surface supersaturation synchronized with the oscillations is presented. Surface supersaturation may be increased by any way that is suitable for a particular case. The technique has been referred to by the authors as MBE with nucleation synchronization and allows epistructures of any thickness to be grown with in situ control of their properties by RHEED or automatic ellipsometry. It can be applied both to elementary semiconductors and compounds like A III B V , A II B VI , etc.