Akihiro Moritani

Photoacoustic Spectroscopy Theory for Multi-Layered Samples and Interference Effect

The Rosencwaig-Gersho theory of photoacoustic spectroscopy has been expanded into the two-layer model and the photoacoustic signal in the multi-layer model has been deduced from its approximations in special cases. Furthermore, the photoacoustic signal is discussed considering multi-reflection of the light in a sample. This calculation accounts for the interference effect on photoacoustic signals. As an example, the interference theory was applied to a sample of silicon on sapphire (SOS), which showed considerable effect of interference.

Micro-Scale Characterization of Crystalline Phase and Stress in Laser-Crystallized Poly-Si Thin Films by Raman Spectroscopy

Laser crystallized poly-Si thin films were characterized using micro-Raman spectroscopy. Mapping measurements of the Si optical-phonon line were carried out for large size grains generated by the super-lateral growth, where the mapping interval was as small as 0.2 µm. It was found that the spectral intensity is extremely large at ridges located at grain boundaries. The map of peak-frequency shift indicated that the tensile stress tends to accumulate in grains and relax at grain boundaries. The map of spectral width revealed the abrupt variation of the crystalline phase from large-size grains to microcrystals.

Analysis of Stress in Laser-Crystallized Polysilicon Thin Films by Raman Scattering Spectroscopy

We investigated the tensile stress in poly-Si thin films crystallized by excimer-laser annealing (ELA). The stress was analyzed with the frequency shift of the optical-phonon line, Δω, detected by Raman scattering spectroscopy. The value of Δω is affected not only by stress but also by the presence of microcrystals. In the case of the full-width at half-maximum (FWHM) of the spectrum being smaller than 8 cm-1, the stress can be estimated from Δω without the influence of microcrystals. The primary origin of the stress was attributed to the thermal stress generated by selective heating of films by ELA. The presence of thermal stress in poly-Si was confirmed by comparing the stress in solid-phase crystallized films on fused quartz and on Si substrates. The stress in ELA poly-Si varied widely with conditions, such as irradiated-shot number, substrate temperature and postannealing temperature. It was deduced that the thermal stress being the dominant stress factor is relaxed by annealing performed after laser irradiation.

Epitaxial growth of 3C-SiC on Si(111) using hexamethyldisilane and tetraethylsilane

Hexamethyldisilane (HMDS, Si2(CH3)6) and tetraethylsilane (TES, Si(C2H5)4) were used as safe organosilane sources for the chemical vapor deposition of SiC films on Si(111) substrates. The surface morphology and crystalline quality of SiC films were investigated. On increasing temperature in H2 ambient after carbonization, voids appeared at the interface of SiC/Si causing the formation of hillocks on the grown SiC films. The formation of voids was prevented by supplying C3H8 and HMDS or TES during the heating process to the growth temperature, leading to the growth of SiC films with a good surface morphology. In the case of TES, the excess carbon incorporated in the film due to the high C/Si ratio was removed by reducing TES flow rate. It was found that the control of the flow rate of organosilane sources during heating plays a vital role in the formation of a high-quality SiC film with a smooth surface.

High‐speed ellipsometry of arsenic‐implanted Si during cw laser annealing

Ellipsometric study of solid phase epitaxial growth during cw Ar‐ion laser annealing in arsenic‐implanted Si is reported for the first time. Use of a high‐speed microscopic ellipsometer has made it possible to measure growth rates and growth temperature under laser irradiation.

Thermal Resistance and Electronic Characteristics for High Electron Mobility Transistors Grown on Si and GaAs Substrates by Metal-Organic Chemical Vapor Deposition

We report dc and microwave characteristics of 0.8 µm gate length GaAs/AlGaAs high electron mobility transistors (HEMTs) grown on Si substrates by metal-organic chemical vapor deposition (MOCVD) and demonstrate direct evaluation of thermal resistance for these HEMTs on Si for the first time. A maximum transconductance of 330 mS/mm is obtained, which is almost the same as that for HEMT/GaAs fabricated simultaneously. From microwave S-parameter measurements, a current gain cutoff frequency (fT) of 9.8 GHz is obtained for the HEMTs on Si, which is about 20% lower than that for HEMT/GaAs, because of the larger parasitic capacitances for HEMT/Si mainly caused by the device-process-related factors. Thermal resistance evaluation of these HEMTs utilizing the gate Schottky junction has revealed that thermal resistance for HEMT/Si is twice as small as that for HEMT/GaAs, namely 36±5 deg/W and 57±5 deg/W, respectively. From these results, it is shown that GaAs on Si is applicable to high-speed devices and very effective in improving the performances of CaAs ICs which are fairly influenced by the power dissipation.

Electroreflectance of Si-MOS

This paper describes the applicability of low-field electroreflectance (ER) to MOS capacitors to determine the surface properties of thermally oxidized silicon. The ER signal amplitude versus the dc gate voltage is calculated, including interface states. The flat-band voltage of the MOS structure is estimated from the ER method. This value is almost the same as that obtained from the conductance method within the experimental errors. It is shown that the relative change in reflectivity in the Si-MOS system includes not only the ER signal but also a signal possibly due to piezoreflectance ascribed to piezoelectrically active regions of the thermally grown Sio2. layer. This explanation is based on our observations of piezoelectric phenomena in Si MOS capacitors fabricated by thermal oxidation technique.

Electrostriction and Piezoelectricity of Thermally Grown SiO2 and Sputtered SiO2 Films

It is found that vibrating phenomena in a Si MOS sample and an Al– sputtered SiO2–Al–Si sample take place in a similar manner with a piezoelectric bimorph when an alternative electric field is applied to the samples. These vibrating phenomena are induced by stress fields which are generated on the SiO2 film. We report a measurement technique of the magnitude of this stress with the use of an optical method. It is shown that this stress field in thermally grown SiO2 and sputtered SiO2 is induced by both the electrostrictive and piezoelectric effect. We have also observed the induced voltage which agrees with the expected value from the measured electrostrictive and piezoelectric tensor element when an external mechanical stress is applied to the SiO2 film of the MOS sample.

Reflectance modulation in HgTe

Abstract Three kinds of modulation techniques, electroreflectance, thermoreflectance and piezo-reflectance are employed to determine energy band parameters in HgTe in the range of photon energy 0.5 to 4 eV. The value of the spin-orbit splitting Δ 0 is determined to be 1.08 ± 0.02 eV from the measurement of electroreflectance with the use of the electrolyte technique and is assigned with the help of electroreflectance measurement on Cd x Hg 1− x Te alloys. In thermoreflectance and piezoreflectance near E 1 , various information is obtained, such as broadening parameters, the energy gaps, the location of the transition edge and the ratios of deformation potential constants D 1 5 / D 1 1 = − 1.4 ± 0.5 and D 3 5 / D 3 3 = 2.3 ± 0.8 at room temperature.

Optical Constants of HgTe and HgSe

The optical constants of HgTe and HgSe are determined in the photon energy region 1.7~3.5 eV from ellipsometric measurements at room temperature. The structures associated with the E1 and E1+Δ1 band edges are found in the extinction coefficient spectra at energy values nearly coincident with the data of the normal incidence reflectivity for both materials. Tentative surface film corrections are made at several photon energies.