Mitsuru Aoyama

Spectroscopic ellipsometry of epitaxial ZnO layer on sapphire substrate

Optical properties of epitaxial ZnO layers have been studied in the spectral region from 1.5 to 5.4 eV using four-zone null spectroscopic ellipsometry. An existing model dielectric function based on excitonic structure near direct band gap has been improved by including a high-energy absorption term. Surface layer, corresponding to the surface roughness, was found to be essential to fit the spectroellipsometric data obtained. Two kinds of samples have been studied: ZnO layers prepared on (0001) and (1120)-oriented sapphire substrates. The surfaces of the first ones were found to be more rough.

Doping effects on optical properties of epitaxial ZnO layers determined by spectroscopic ellipsometry

Optical properties of Al- and Ga-doped ZnO layers have been studied in the spectral range from 1.5 to 5.4 eV using a four-zone null spectroscopic ellipsometer and in the spectral range from 0.5 to 6.5 eV using near-normal incidence reflectivity measurements. The layers were prepared by RF magnetron sputtering onto (1 1 2 0) oriented single-crystal sapphire substrates. Al- and Ga-doping gives rise to a shift of the fundamental absorption edge from 3.4 to 3.7 eV. The model dielectric function (MDF) based on an excitonic structure derived by Tanguy [Phys. Rev. B 60 (1999) 10660] was completed by the Sellmeier and Drude terms. The Drude term describes a free-electron contribution originating from presence of the dopant. Spectroscopic ellipsometry and reflectometry are very sensitive to a surface roughness. The surface roughness was modeled by a surface layer of the Bruggeman effective medium and by diffraction theory.

Spectroellipsometric characterization of materials for multilayer coatings

The optical functions of titanium dioxide (TiO2), tantalum pentoxide (Ta2O5) and silicon dioxide (SiO2) have been determined in the spectral range from 1.5 to 5.4 eV (wavelength range from 230 to 840 nm). The ellipsometric spectra of 200 nm thick layers sputtered on a glass substrate were measured by a four-zone null spectroscopic ellipsometer. The data have been fitted by a Tauc–Lorentz model recently derived by Jellison and Modine for the optical functions of amorphous materials. The model dielectric function is based on a combination of the Tauc band edge and the Lorentz oscillator. The effects of the surface and interface layers and layer inhomogeneity on the measured data are discussed.

Optical characterization of TiN/SiO2(1000 nm)/Si system by spectroscopic ellipsometry and reflectometry

Titanium nitride (TiN) films are used as diffusion barrier layers in the semiconductor industry. There is a need for accurate data on optical properties, because film thickness are usually monitored by optical methods. There are further needs to characterize TiN/(interlayer dielectrics)/Si system simultaneously. Thus, TiN(35, 60, 90 and 105 nm)/SiO2(1000 nm)/Si samples were prepared using sputtering from a TiN target in argon and nitrogen atmosphere. Those samples were characterized using simultaneous fits of multiple angle of incidence spectroscopic ellipsometry and normal incidence spectroscopic reflectivity. TiN films partially transmit light that interferes in the thick thermal silicon oxide, which gives more detailed information on the optical properties of TiN. Consequently optical dielectric function of TiN as well as two thickness of TiN and SiO2 were successfully determined simultaneously.

Room Temperature InAsxP1-x-ySby/InAs Photodetectors with High Quantum Efficiency

Room temperature surface-illuminated InAsxP1-x-ySby/InAs photodiodes with an external quantum efficiency as high as 50–86% in a 1.83–3.53 µm wavelength range have been fabricated for the first time. Lattice matched heterostructures with a wide energy gap InAsPSb cap layer were grown on the InAs substrate using the liquid phase epitaxy technique. According to temperature dependence measurements for a 1 mm diameter photodiode, peak responsivities of 1.83–2.5 A/W have been realized in a temperature range of 296 to 200 K. The Johnson noise limited room temperature detectivities D* are deduced to be 1\endash6 ×109 cm·Hz1/2/W at zero bias. It is demonstrated that the only loss of external quantum efficiency is from the reflection of the entrance face.

Sulphur passivation of InAs(Sb)

Abstract The sulphur passivation for the InAs surface was studied by photoluminescence (PL) and spectroscopic ellipsometry (SE). Effects of the treatment are compared with I - V characteristics and photoresponse (PR) of MESA structure devices fabricated with LPE multilayer of InAs 1− y (Sb y ). It is expected that the dangling bonds at the surface are terminated in the forms of SIn, SSb bonds by the passivation, which results in a remarkable decrease of surface recombination center density. This phenomenon was confirmed by observing increases of PL intensity and of the PR of photodiodes. Four order reductions of dark current and remarkable increase of PR have been observed on sulphur passivated photodetectors. Stability of the effect of the sulphur passivation was also examined by SE as well as PR measurements. SE measurements showed that dielectric function of the surface layer for the InAs wafer has no absorption in the measured spectral range between 1.5 and 5.5 eV. After sulphur passivation, optical absorption appeared due to submonolayer sulphur atoms in good agreement with the results obtained from the InAs(Sb) photodetectors.

SiO2 Electret Generated by Potassium Ions on a Comb-Drive Actuator

Silicon dioxide electret generated by doping potassium ions will be demonstrated by forming it on a comb-drive actuator. The comb-drive actuator made of silicon on an insulator substrate is oxidized with bubbling a stream of KOH solution to form silicon oxide film including potassium ions uniformly on the etched side walls of comb electrodes. After a bias-temperature procedure at about 900–1000 K and 100 V was applied to the device, we confirmed a 40 V built-in potential difference between the opposing comb electrodes. The gradual decay of the potential was observed, but 35 V was maintained even after 1 month.

Possibility of Simultaneous Monitoring of Temperature and Surface Layer Thickness of Si Substrate by In Situ Spectroscopic Ellipsometry

The possibility of simultaneous monitoring of the temperature and the thickness of the surface layer of a crystalline silicon (c-Si) substrate by in situ spectroscopic ellipsometry (SE) is demonstrated using a surface adsorption layer (SAL) as an example. The model dielectric function (MDF) developed by Adachi and the tabular data of the dielectric function of fused silica were applied to a Si wafer and SAL, respectively. The best-fit curve has been obtained by fitting the measured \varPsi and Δ spectra simultaneously by adjusting the 12 MDF parameters and the thickness of SAL over the temperature range of 293–803 K. The best-fit MDF parameters indicate that the SAL thickness has almost no influence on the best-fit values of the MDF parameters. This fact can be used to monitor the temperature of a c-Si wafer independent of the thickness of surface layer by checking the shift of MDF parameters (e.g., E2) using the SE and the MDF. The measurement errors in thickness and temperature are roughly estimated to be 0.1 nm and less than 10 K, respectively.

Influence of Sulphidation Treatment on the Performance of Mid-Infrared InAsPSb/InAs Detectors

The influence of sulphidation treatment on the performance of room temperature InAsPSb/InAs photodetectors was investigated. Measurements of photoresponsivity, external quantum efficiency and detectivity of the detectors with and without the sulphidation treatment showed that after the sulphidation treatment the sensitivity of the detectors improved 1.5 times. A study of the chemical state of sulphur on an (NH4)2Sx-treated InAs(001) surface was performed by high-resolution X-ray photoelectron spectroscopy (XPS). The chemical state of sulphur bonded to indium (In) and arsenic (As) atoms for the as-treated surface was observed showing the termination of the dangling bonds at the surface by the sulphidation treatment. This gives rise to a remarkable decrease of surface recombination center density and results in the increase of photodetector sensitivity.

Optical Properties of High-Quality Ga1-xInxAs1-ySby/InAs Grown by Liquid-Phase Epitaxy

The photoluminescence (PL) of InAs and GaInAsSb/InAs epilayers grown by Gd-doped liquid-phase epitaxy (LPE) has been investigated using a double-modulation Fourier transform infrared spectrometer (FTIR). The spectra were composed of peaks due to band-to-band, band-to-impurities, and exciton recombinations. The excitation power, temperature and Gd concentration dependences of these features were studied. The temperature dependence of the band-to-band transition energies was examined by measuring FTIR transmission spectra and by comparing experimental results with those obtained with an empirical formula. A PL full width at half-maximum (FWHM) as narrow as 4.35 meV has been achieved for GaInAsSb epilayers grown from Gd-doped melt indicating high purity of epilayers. Raman scattering measurements of GaInAsSb epilayers showed a two-mode behavior for the optical phonons, indicating that homogeneous, high-quality epilayers were achieved.