Koichi Terashima

Near‐band‐gap photoluminescence of Si1−xGex alloys grown on Si(100) by molecular beam epitaxy

Photoluminescence spectra in the near‐band‐gap region of Si1−xGex alloys (x=0.04 and 0.15) grown on Si(100) substrates by molecular beam epitaxy have been measured at 4.2 and 12 K. Radiative recombinations of free and bound excitons in thin layers of Si1−xGex alloys have been clearly observed for the first time. No‐phonon transitions and transverse‐optical (TO) phonon‐assisted transitions have been identified.The luminescence lines become broader with an increase in excitation intensity; the broadening is interpreted to be due to the generation of the bound multiexciton complexes (BMECs). The position of the band‐edge luminescence lines is determined by the strain in the epitaxial layer as well as the alloy composition. The defect‐related L band appears in the case of x=0.15.

Factors determining the composition of strained GeSi layers grown with disilane and germane

Growth rates and compositions are reported for GeSi alloy films and superlattices epitaxially grown on both Ge(100) and Si(100) substrates using disilane and germane source gases in an ultrahigh vacuum chemical vapor deposition chamber. Although the growth rate changes rapidly with temperature the composition is nearly independent of it. Specifically, we find that the order of the adsorption reaction for disilane and germane is the same, resulting in the composition being determined by the partial pressures and by the ratio of the adsorption reaction rate constants. This ratio depends very weakly on temperature, if at all, and appears to vary slightly with the layer composition.

Hydrogen redistribution induced by negative-bias-temperature stress in metal–oxide–silicon diodes

Poly-Si/SiO2/Si diodes in which oxides were grown thermally under wet oxidation conditions and subsequently treated by a post-oxidation anneal (POA) have been characterized electrically and chemically before and after applying negative-bias-temperature stress (NBTS). It was confirmed that NBTS produces interface states and that POA suppresses the interface state production. Nuclear reaction analysis indicated that NBTS results in hydrogen redistribution within the oxide layer. POA was shown to partly suppress such hydrogen accumulation. Hydrogen is thus clearly shown to influence the stability against NBTS.

Structural analysis of imperfect GeSi superlattices grown on Ge(001) substrates

By combining the complementary techniques of x‐ray diffraction and transmission electron microscopy we have been able to accurately determine the structure of imperfect GeSi superlattices (SL’s). The samples were epitaxially grown on Ge(001) substrates using Si2H6 and GeH4 source gases. In this report, details of the x‐ray experiment and analysis are emphasized. In particular, a model is presented for calculating the diffracted intensity from a SL containing gradients in composition and thickness. Applying this model to the data it is found that between the first and last layers of each SL there exists a roughly 10% increase in the growth rate, without a corresponding change in the alloy composition. This is attributed to a slow increase in the substrate temperature, Tsub, of just a few degrees. A sample grown with a corresponding gradual decrease in the control temperature, TC, was found to be much more uniform.

Band‐edge emission from strained SiGe alloy layers on Ge(100) substrates

We have fabricated strained SixGe1−x/SiyGe1−y multiple quantum wells on Ge(100) substrates and measured the photoluminescence (PL) spectra, observing band‐edge emission from the SiGe alloy layers. The emission is due to the recombination of both bound excitons and free excitons in the quantum wells. From the positions of the observed PL lines, we have evaluated the band‐gap energies of the strained SiGe alloy layers, and found them to be smaller than those of bulk SiGe alloys. The band‐gap energy increases with the Si content of the alloy, reaching a maximum at about 15% Si, and subsequently decreases. These results agree well with the theoretical calculations for strained layers, and suggest a type II band alignment in some cases for SixGe1−x/SiyGe1−y heterostructures on Ge(100).

Dislocation-related photoluminescence in Si1-xGex/Si(100) grown by molecular beam epitaxy

Abstract Photoluminescence (PL) in Si 1−x Ge x alloys grown on Si(100) by molecular beam epitaxy (MBE) has been studied over the composition range 0≦χ≦0.5. For x ≧0.15, strong deep-level luminescence has been observed. Sharp lines which are similar to the dislocation-related D-lines in Si appear in the spectra for x =0.15 and 0.26. Transmission electron microscopy (TEM) for these samples reveals that there are a lot of dislocations at the Si 1−x Ge x /Si interface and in the Si substrate. The dislocations in the Si substrate are considered to be the origin of the sharp luminescence lines. These PL lines for x =0.15 cannot be observed in the case of thin layers. In that case, the density of the dislocations near the interface is much lower. For x =0.38 and 0.50, broad bands appear in the spectra. The TEM observations in these cases reveal that there are many dislocations in the Si 1−x Ge x layers and that the dislocation density in the Si substrate is very low. The broad bands are considered to originate from the dislocations in the Si 1−x Ge x layer.

YBa2Cu3Oy superconducting thin film obtained by laser annealing

A rapid and low-temperature process to obtain YBa2Cu3Oy superconducting thin film was studied using a laser annealing technique. As-deposited thin film prepared by ion beam sputtering was converted into the superconductive film using argon laser beam processing, where melting, quenching to 400°C and annealing at 400°C in air were carried out. The onset critical temperature was 80 K and the end-point critical temperature was 60 K. Structure, composition and surface morphology were studied by X-ray diffraction, electron probe micro-analysis and scanning electron microscope.

SIMS characterization of hydrogen transport through SiO2 by low-temperature hydrogen annealing

The variation of hydrogen distribution at the SiO 2 /Si interface by low-temperature hydrogen annealing was investigated using secondary ion mass spectrometry (SIMS). As the amount of hydrogen atoms incorporated at SiO 2 /Si is considered to be comparable to the silicon dangling bond density (1 x 10 10 to 1 x 10 12 atoms/cm 2 ), an analytical method with a high sensitivity is necessary for the detection of hydrogen at SiO 2 /Si. In this study, the experimental conditions of SIMS were optimized in order to obtain a sufficient reproducibility of interfacial hydrogen ion intensity. There are two main causes that influence the measurement reproducibility: (1) misalignment of the relative irradiation areas of the electron beam and ion beam and (2) the contribution of background hydrogen from surface contaminants and residual gas. We obtained a high measurement reproducibility within a 5.5% relative standard deviation (2σ). This enabled us to observe an increase of hydrogen at SiO 2 /Si by hydrogen annealing at 400 °C, which resulted from the incorporation of hydrogen from the ambient. From the results of nuclear reaction analysis (NRA) and thermal desorption spectroscopy (TDS), it was also found that the incorporated hydrogen had two chemical states.

Photoluminescence study of Si1−xGex/Si heterostructures grown by molecular beam epitaxy and ultrahigh vacuum chemical vapor deposition

Photoluminescence (PL) of Si1−xGex/Si heterostructures grown by conventional molecular beam epitaxy (MBE), gas‐source MBE (GSMBE), and ultrahigh vacuum chemical vapor deposition (UHV‐CVD) has been investigated in the wavelength region of 1000–1800 nm. Single and multiple quantum wells (SQW and MQW, respectively) and uncapped Si1−xGex alloy epitaxial (EPI) layers on Si substrates have been studied. In the case of GSMBE and UHV‐CVD, sharp band‐edge PL of Si1−xGex appears and deep‐level PL is very weak. PL lines due to bound excitons can be observed up to about 50 K for SQWs and MQWs but they disappear above 12 K for EPI layers. Broadening of the PL lines with an increase in excitation intensity due to bound multiple exciton complexes has been observed for the EPI layers while it can hardly be observed for SQWs and MQWs. In the case of conventional MBE, intense broad emission bands dominate the PL spectrum below the band gap of Si1−xGex for the SQWs and MQWs. Similar broad emission bands sometimes appear als...

Luminescence Centers in Indium-Implanted Silicon

We measured photoluminescence (PL) spectra of silicon implanted with 150-keV In+ to a dose of 1×1013 cm-2 and annealed at 400–1050°C. In addition to the In-bound exciton lines, defect-related P and R lines appear in the PL spectra of the samples annealed above 800°C. The P and R lines grow in intensity as a result of the annealing at the higher temperatures, while the intensity of the In-bound exciton line saturates above 800°C. We found that the intensities of the P and R lines decreased with time during storage at room temperature because of the effect of H2O in air. Considering the annealing behavior of the P and R lines and the change of their intensity during storage, we propose a model in which the luminescence centers of the P and R lines are the defect clusters with an X+–Y- structure, where X is mobile at room temperature and Y includes at least one In atom. In this model, the constituents of the defect clusters are weakly bound by Coulomb interaction, and the luminescence centers are dissociated by the influence of OH- near the Si/SiO2 interface during storage at room temperature.