C. Tatsuyama

Heteroepitaxial growth of Ge films on the Si(100)‐2×1 surface

The initial stage of Ge heteroepitaxy on a Si(100)‐2×1 surface has been investigated by low‐energy electron diffraction (LEED) and Auger‐electron spectroscopy (AES). The growth mode of the Ge films was studied by measuring the decrease in the Si(LVV) AES line at 92 eV with an increase in the Ge overlayer thickness. The Ge films deposited at room temperature exhibit layer‐by‐layer growth up to at least six monolayers. When the substrate is heated up to 350 °C, the growth mode is characterized by the Stranski–Krastanov type; i.e., the first three monolayers of growth is followed by island formation. Although these characteristics of the growth mechanism are similar to the case of Ge on Si(111)‐7×7 surfaces, annealing behavior of the Ge films suggests that the bond strength between Ge and Si is stronger on Si(100) than on Si(111) surfaces. In contrast to the case of Ge on Si(111) surfaces, where the original 7×7 superstructure of the Si surface is replaced by a new 5×5 pattern at about two‐monolayer coverage...

On the Electrochromism of Evaporated V2O5 Films

The electrochromism of amorphous V2O5 thin films prepared by the vacuum evaporation of V2O5 powder was studied through optical absorption, XPS and SIMS measurements. The films were colored and bleached with the aid of an electrolyte consisting of lithium perchlorate (LiCLO43H2O) in propylene carbonate (CH3CHOCOOCH2). The as-grown films were yellow, like V2O5 powder, while the colored films were greenish-gray, like VO2 powder. The XPS spectra of the O1s and V2p3/2 core levels of the as-grown films matched those of V2O5 powder, while those of the colored films were similar to those of VO2 powder. SIMS measurement revealed that Li+ ions are introduced into the films by coloration and expelled by bleaching. From these results, it is concluded that the coloration of a-V2O5 films is due to a change in the molecular state from V2O5 to VO2. This change is caused by the double injection of electrons and Li+ ions into the films.

Electrical and Optical Properties of GaSe-SnO2 Heterojunctions

The capacitance-voltage (C-V) and forward biased current-voltage (I-V) characteristics, electroluminescence (E.L) and photovoltaic effect of GaSe–SnO2 heterojunction diodes are measured. SnO2 layer is deposited on the c-plane of GaSe by using the spray method. The C-V and I-V characteristics of these diodes reveal the existence of a high resistivity layer, probably due to the diffusion of Sn into GaSe. The width of this layer is about 2.6 µm. And the current transport mechanism at low voltage is space-charge-limited. The trap density and the energy level of the trap from the valence band estimated by Lampert theory are about 5×1013~1×1014 cm-3 and 0.4~0.6 eV, respectively. The electroluminescence spectra at 275 K show one emission band due to free exciton recombination.

XPS and AES Studies on the Oxidation of Layered Semiconductor GaSe

We have studied the oxidation of layered compound GaSe with no dangling bonds on the cleaved surface using XPS and AES techniques. At room temperature, the cleaved surface is not oxidized in an oxygen atmosphere. When subjected to Ar ion sputtering, the surface starts to exhibit the behavior of metallic Ga owing to dissipation of the first sublayer of Se in the primitive layer, Se–Ga–Ga–Se. The thin layer of metallic Ga thus exposed is easily oxidized. In the thermal oxidation of cleaved GaSe in air, the oxygen diffuses into the primitive layer and combines with Ga, severing the intralayer bonding between the Se and Ga atoms. At temperature higher than 450°C, the oxygen is also intercalated between the primitive layers from the sides perpendicular to the layers. No Se oxides are observed under any of the oxidation conditions.

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.

Structural properties of heteroepitaxial Ge films on a Si(100)‐2×1 surface

Structural and electrical properties of epitaxial Ge films grown on a Si(100)‐2×1 surface have been studied by means of low‐energy electron diffraction (LEED), Auger electron spectroscopy (AES), x‐ray diffraction, and Raman scattering. The attempt has been made to find the growth conditions, such as the substrate temperature TSi and Ge thickness dGe, for the single‐crystalline Ge (100) on the Si surface. The sharp (2×1) LEED pattern of Ge (100) accompanied by the absence of any AES signals from the Si substrate is unambiguous evidence for the growth of single‐crystalline Ge films. The structural quality of the Ge films was examined by the peak position and width of the Ge(400) Bragg line in x‐ray diffraction. From these results, we found that more than 1000‐A Ge thickness is required to grow the high‐quality epitaxial Ge films on the Si substrate at elevated temperatures. This is due to island formation at the initial stage of Ge deposition. Therefore, we also studied the role of thin Ge buffer layers fab...

XPS Study on the Oxidation of InSe

The initial stage of the reaction of oxygen with the surface of layered semiconductor InSe has been studied by XPS. The cleaved surface of InSe does not react with oxygen at room temperature. However, when subjected to Ar ion sputtering, the surface becomes In-rich, and can then be oxidized at room temperature. By heating a sample with a cleaved surface in the atmosphere, the intralayer bonding between Se and In atoms is severed by the formation of In-oxides. In both cases, first, In2O3+x is formed at the surface due to the presence of In–O2 bonds. As oxidation proceeds, In2O3 grows under In2O3+x. In2O3-x is also observed in room-temperature oxidation. No evidence could be found for the presence of Se oxides on the surface under any oxidation conditions. It is found that a heterojunction cell of In2O3–InSe fabricated by the thermal oxidation of InSe exhibits the photovoltaic effect in the visible light region.

Heteroepitaxial Growth and Superstructure of Ge on Si(111)?7?7 and (100)?2?1 Surfaces

The initial stage of the heteroepitaxy, and the superstructure of Ge on Si surfaces, have been investigated by LEED and AES. Germanium was evaporated on to a clean Si(111–7×7 surface and also on to an Si(100)–2×1 surface. The layer-by-layer growth of Ge films on the Si surfaces was confirmed from the decrease in intensity of the Si(LVV)-AES signal with increase in Ge coverage. It was found that the (7×7) superstructure of the Si (111) surface is replaced by a (5×5) superstructure at about 2 monolayers coverage of Ge. The phase diagram of the superstructure of Ge on the Si(111) system was constructed for Ge coverage in monolayers evaporated at room temperature, versus annealing temperature. A similar study was also extended to Ge on an Si(100)–2×1 surface, where the original LEED pattern is not strongly distrubed at about 1–2 monolayers of Ge.

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...

STRUCTURAL CHARACTERIZATION OF SI0.7GE0.3 LAYERS GROWN ON SI(001) SUBSTRATES BY MOLECULAR BEAM EPITAXY

Structural properties of Si0.7Ge0.3 alloy films grown on Si(001) substrate by molecular beam epitaxy have been characterized by means of several available techniques. Different types of buffer layers were predeposited on a clean Si(001) substrate in order to relax the lattice mismatch between the topmost Si0.7Ge0.3 alloy layer and Si substrate. The effect of buffer layers on the structural quality of the overgrown Si0.7Ge0.3 was investigated by x-ray diffraction, x-ray photoemission spectroscopy, photoluminescence, and cross-sectional transmission electron microscope. It is confirmed that the threading dislocation density in the alloy layer drastically decreases by using buffer layers. The samples with step buffer layers relax the strain by introducing the dislocations at the interfaces, part of which goes through the alloy layer. On the other hand, the samples with superlattice buffer layers relax the strain by introducing the dislocations in the buffer layers which terminate at the interface of the supe...