T. Tambo

Molecular Beam Epitaxy of SrTiO3 Films on Si(100)-2×1 with SrO Buffer Layer

Molecular beam epitaxy of SrTiO3 thin films on a Si(100)-2×1 surface has been studied using reflection high-energy electron diffraction (RHEED), an atomic force microscope (AFM) and X-ray diffraction (XRD) as a function of the thickness of the SrO buffer layer and growth temperature. Epitaxial SrTiO3 films do not grow directly on Si(100). Therefore, a SrO buffer layer was applied to grow SrTiO3 on Si(100). It was found that the SrO layer with a thickness of 100 A grown at 300 – 400°C in oxygen atmosphere of 5 ×10-8 Torr was sufficient to grow epitaxial SrTiO3 on Si(100). Then SrTiO3 thin films with a thickness of 2000 A were grown on the SrO(100A)/Si surface at 400 – 700°C using codeposition of strontium and titanium in oxygen atmosphere of 8×10-8 Torr. At 500°C, the best-quality SrTiO3(100) film grew parallel to Si(100), and numerous rectangular platelike crystals were observed on the surface in the AFM image. The crystallinity of the STO films was improved with increasing thickness of the SrO layer. The epitaxial relation between SrTiO3 and SrO/Si(100)-2×1 is also discussed using the RHEED patterns which show streaks and spots.

IMPROVEMENT OF ELECTRICAL PROPERTIES OF EPITAXIAL SRTIO3 FILMS ON SI(001)-2 1 BY IN SITU ANNEALING

We have studied the formation of a high-quality SrTiO3 (STO) film on a Si surface which is an appropriate buffer film for fabricating high-Tc superconductor devices on Si by molecular beam epitaxy. The STO films with thicknesses of 1300–6700 A are grown on a SrO buffer layer with a thickness of 100 A on Si(001)-2×1 in ∼10−8 Torr. The growth temperatures for the STO films and SrO layer are 500 and 400 °C, respectively. The as-grown STO films are insufficiently oxidized regarding the Ti–O bond which is confirmed by the observation of x-ray photoemission spectroscopy, although in situ reflection high-energy electron diffraction spectroscopy and ex situ x-ray diffraction (XRD) reveal a high-quality crystalline structure. The in situ postannealing for the as-grown STO films is performed at 500–900 °C for 60 min in an oxygen atmosphere of 1×10−6 Torr. The heat treatment promotes the oxidation of STO films and results in a high resistivity of 109–1011 Ω cm and a dielectric constant of 130 at 100 kHz at room temp...

Heteroepitaxial growth of InSb on Si(001) surface via Ge buffer layers

Abstract InSb was grown on the Si(001), Ge(001) and Ge Si(001) substrates by the coevaporation of elemental In and Sb sources. The grown films were characterized by AES (Auger electron spectroscopy), optical microscope, XRD (X-ray diffraction), SEM (scanning electron microscope) and ECP (electron channeling pattern), as a function of growth conditions, such as growth temperature and flux ratio of Sb In . The thickness of grown InSb films was about 0.8–1.2 μm. In contrast with the direct growth on Si(001) surface, InSb easily grows heteroepitaxially on Ge(001) and Ge Si(001) substrates, for wide growth conditions. The surface morphology and the crystal quality of the grown films strongly depend on the flux and/or composition ratio of Sb In . It is found that the optimized flux ratio is about 4.5 to obtain the stoichiometric InSb films with smooth surface at growth temperature of 300°C. However, the XRD spectrum and ECP pattern reveal that better crystal quality is obtained for the In-rich films rather than the stoichiometric films.

Surface structure evolution during Sb adsorption on Si(1 1 1)–In(4×1) reconstruction

The Sb adsorption process on the Si( I 1 1)-In(4 x 1) surface phase was studied in the temperature range 200-400 °C. The formation of a Si(1 1 1)-InSb (2 x 2) structure was observed between 0.5 and 0.7 ML of Sb. This reconstruction decomposes when the Sb coverage approaches 1 ML and Sb atoms rearrange to (3 x 3) and (2 × 1) reconstructions; released In atoms agglomerate into islands of irregular shapes. During the phase transition process from InSb(2 x 2) to Sb(3 × 3) (θ Sb > 0.7 ML), we observed the formation of a metastable (4 x 2) structure. Possible atomic arrangements of the InSb(2 x 2) and metastable (4 x 2) phases were discussed.

Growth temperature dependence of GaS thin films on GaAs(001) surface

The growth of GaS films on GaAs(001) surfaces by using thermal evaporation of layered-compound GaS has been investigated by Auger electron spectroscopy, low-energy electron-loss spectroscopy (LEELS), x-ray photoemission spectroscopy (XPS), and atomic force microscopy (AFM) as a function of deposition temperature. The LEELS spectrum of the films grown at lower temperatures (⩽400 °C) resembles that of a GaS single crystal, whereas that of the films grown at 450 °C reveals that GaAs surface was terminated by Ga2S3 layer. XPS spectra suggest that after annealing at 500 °C, S atoms moved from As atoms to Ga atoms to form stable Ga–S bonds at the interface and As–S bonds are observed to be less stable. The band discontinuity at the GaS/GaAs(001) interface estimated by XPS showed the straddling-type I band alignment. Surface morphology of the films studied by AFM reveals the layer-by-layer initial growth of GaS.

In(4?3) Reconstruction Mediated Heteroepitaxial Growth of InSb on Si(001) Substrate

In the present letter we report the epitaxial growth of 9000-?-thick InSb on a Si(001) surface via In(4?3) reconstruction by molecular beam epitaxy. The growth was monitored in situ using RHEED, Auger electron spectroscopy, scanning tunneling microscopy and the exsitu characterization was done using X-ray diffraction, atomic force microscopy, and optical microscopy. The heteroepitaxy of InSb on Si(001) surface is achieved despite the presence of a large lattice mismatch (over 19%) by incorporating a 0.5ML In(4?3) reconstruction at the InSb/Si interface. Based on these results a model for the interface formation is developed.

XPS Study on the Change of Surface Potential of GaSe and InSe Induced by Ar + Sputtering

The changes of surface potential, directly induced by Ar ion sputtering, have been studied using X-ray photoelectron spectroscopy for both p-type GaSe and n-type InSe with no dangling bonds on the cleaved surface. Since the sputtering makes the surface metal-rich in these compounds, it is then predicted that the surface potential dramatically changes. The change of work function due to sputtering is obtained by measuring the emission spectrum of slow secondary electrons. It decreases from 5.7 eV to 5.3 eV in GaSe and increases from 4.5 eV to 5.0 eV in InSe. The magnitude of band bending due to sputtering is estimated from the shift of core levels. It is found that the band bends downwards by about 0.2 eV in GaSe and upwards by about 0.6 eV in InSe. These experimental results are explained in terms of the thin metallic layer and the interface states on the sputtered surface.

Growth of Si0.75Ge0.25 alloy layers grown on Si(001) substrates using step-graded short-period (Sim/Gen)N superlattices

Short-period (Sim/Gen)N superlattices (SSLs) are grown step by step on a Si(001) substrates by solid source molecular beam epitaxy. Using the step-graded SSLs as buffer layers, 2000 A uniform Si0.75Ge0.25 alloy layers are grown on the same substrates. The growth temperature of the SSLs and uniform layers is 500 °C. In the SSLs layers, m and n are the number of monolayers of Si and Ge, respectively. N is the period of (Sim/Gen) bilayers. The samples grown are characterized by x-ray diffraction, atomic force microscopy (AFM), and transmission electron microscopy (TEM) as a function of the step number of SSL layers. The SSLs show very smooth surfaces [the root-mean-square (rms) surface roughness is between 7 and 12 A]. A dramatic decrease in roughness is observed in the uniform Si0.75Ge0.25 alloy layers, when even a one-step SSL is used as a buffer layer. A noticeable increase in rms roughness is seen in both SSL and alloy layers when the number of Ge monolayers is changed from one to two. AFM observation sh...

Growth of InSb films on a Si(001) substrate with Ge buffer layer

Abstract InSb films were grown on Si(001) substrates by molecular beam epitaxy using Ge buffer layers with thickness up to 1000 ML (monolayer). The surface morphology of Ge layers on which heteroepitaxy of InSb film is achieved, was observed using atomic force microscope (AFM). The density of Ge islands on Si(001) substrate rapidly increased with increase in the thickness of Ge layer ( d Ge ). X-ray diffraction (XRD) measurements were performed to characterize the InSb films. The dependence of the crystal quality of InSb films on d Ge was studied. The intensity of the InSb(004) peak was substantially enhanced with the increase in d Ge compared to the other diffraction peaks. These results indicate that the enhancement of intensity of the InSb(004) peak is correlated with the increase in the areal density of Ge islands which facilitate the heteroepitaxial growth of InSb films.

XPS study on the chemical shifts of crystalline III-VI layered compounds

Core electron binding energies of GaS(β), GaSe( e ) and InSe(γ) with respect to the vacuum level have been obtained by measuring the onset of secondary electron emission in the X-ray photoelectron spectroscopy (XPS). The chemical shift for the least bound core level of cation is estimated to be 3.4 eV for GaS, 2.0 eV for GaSe and 1.7 eV for InSe with respect to the level of metal. The Madelung constant of each compound is calculated to be 1.44, 1.60 and 1.48, respectively. The ionicity is estimated to be about 0.46, 0.45 and 0.49, respectively. The magnitude of chemical shift is discussed on the basis of the electrostatic model which reflects two-dimensional crystal structure.