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Experimental results are reported on the frequency spectrum, the conversion efficiency, and the tunability of the center frequency of the Rayleigh-wave parametric generator on an n-Si/SiO2/ZnO/Al structure which we have recently proposed. It was found that this device can be used as a really tunable, wide-band center-frequency, and high Q generator of surface acoustic waves without micro-fabrication. Experimental results on the generator of the first higher-order Rayleigh mode (the Sezawa mode) are also reported. It was found that the threshold pumping power is smaller than that of the Rayleigh mode and is in fairly good agreement with the theoretical estimation.

Correlation between the distributions of nonradiative microdefects and dislocations in a Si-doped n-GaAs wafer has been investigated with a photothermal-radiation (PTR) microscope. We measured the PTR image with the wavelength of excitation light fixed at 895nm, and compared the PTR image with the dislocation-density distribution. The PTR image shows negative correlation with the dislocation-density profile. The PTR signal apparently originates from the nonradiative microdefects generated dominantly in the low-dislocation-density regions. The effect of annealing on the microdefects in the Si-doped n-GaAs wafers has been investigated with the PTR and photolurninescence(P1) microscope. We found that the PTR intensity decreases after annealing, while the PL intensity increases. Upon etching of the surface, the PL intensity decreases but the PTR intensity shows no appreciable change.

When the energies of photons approach the energygap in semiconductors, the Bri- llouin-scattering cross section shows a resonant enhancement. But, it is difficult to obtain the experimental data of Brill- ouin scattering for the photons with the energies very close to the energy gap. Even in such a case, it is possible to ob- tain the scattering cross section in pop- ulation-inverted semiconductors, since the photons can be amplified. Such a situati- on is realized actually in acoustic dist- ributed feedback lasers proposed and demo- nstrated by us. The main object of this work is to give a theoretical basis for such an investigation. We develop a the- ory of Brillouin scattering in population- inverted semiconductors with zinc-blende structure by solving the equation of moti- on of density matrix. Numerical estimates of photoelastic constants are given for GaAs. We point out the importance of the imaginary part of photoelastic constants, overlooked so far, and some new effects caused by the existence of such an imagi- nary part. Preliminary experiments in CdS are described.

We have determined the material constants of AlN,i.e., elastic stiffness, piezoelectricity and permittivity, by computer calculation from the experimental results on SAW phase velocities and electromechanical coupling coefficients. A1N films used were single-crystal films on the basal plane of sapphire grown by metalorganic chemical vapor deposition. Until now, very few of these material constants are known. We report, for the first time, all the matrix elements of the material constants. The calculated variations of SAW phase velocities and coupling constants with A1N film thickness on basal and R planes of sapphire were in good agreements with the reported experimental results. Next, using the evaluated material constants, it was shown that the highfrequency (UHF) and low-dispersion SAW characteristics (less than 0.1% velocity change for ~20% bandwidth) with a large coupling (4%) can be expected for AlN/Si and AlN/SOS, when the c-axis of A1N was aligned along the surface of (001) Si.

A novel principle is suggested for the optimum design of deep-submicron channel MOSFET operated at liquid-nitrogen temperature. Current-voltage characteristics of MOSFET are scaled down in proportion to operation temperature, while the distribution of the mobile carrier is kept constant. The results of a two-dimensional simulation confirm our scaling theory. A combination scaling law is also suggested which comprises the temperature-scaling law and a dimensional scaling law.

A photoacoustic (PA) and an absorption spectra of benzene vapor have been measured in the wavelength region from 280 to 220 nm. We determined nonradiative yields at the absorption peaks. We found that the relaxation process of benzene vapor at the wavelengths shorter than 242 nm, whose process has been called the third channel, is a nonradiative relaxation process with temperature rise of the benzene vapor. We also observed a drastic decrease of the nonradiative yields at wavelengths shorter than 233 nm, which seems to suggest an increase of quantum yield for photochemical isomerization.

Non-radiative states and defects in CdS and Si were studied at room temperature by means of a new photoacoustic (PA) technique. Signal detection was carried out by a ZnO transducer which operates in a wideband and high-frequency range. The fine structures due to A, B, C excitons and due to seven impurity states in CdS were observed as the maxima or minima in the PA spectra. It was also found that the spatial distribution of mechanical damage and defects in CdS and Si can be observed in the PA signals by scanning the laser beam spot across the sample.

Principles and recent experiments of a two-dimensional acoustic distributed feedback laser are described. In the experiments, a set of distributed Bragg reflectors were formed on a GaAs crystal and a surface acoustic wave at a frequency around 1.2 GHz was injected into the planar region between the Bragg reflectors. We also discuss a possibility of an injection laser with the acoustic distributed feedback.

A review is given of our investigation of nonradiative processes and defects in semiconductors by photoacoustic and photo-thermal-radiation techniques. The main results obtained so far are summarized as follows. (1) The spatial distribution of oxidation-induced stacking faults in Si wafers can be observed by a scanning photoacoustic topograph. (2) The spatial distribution of microdefects in GaAs wafers can be observed by a scanning photo-thermal-radiation microscope.

Epitaxial aluminum nitride (AlN) films have been grown on (111), (110) and (100)-oriented Si substrates using metalorganic chemical vapor deposition (MO-CVD). We found that (0001)-oriented AlN films were grown on the above three planes of Si substrates.

We have developed a new photothermal-radiation (PTR) microscope for noncontact and nondestructive evaluation of defects in semiconductors. We measured PTR topographs of a GaAs wafer and a thermally oxidized Si wafer with a HgCdTe infrared detector which is highly sensitive to the radiation in the wavelength range from 8 to 13 µm. We found that the PTR topograph has a correlation with the distribution of surface damage. We also unexpectedly found that the PTR signal decreases at a deep scratch in contrast to the increase of the photoacoustic signal reported previously.

An epitaxial film of AlN with a flat surface and good crystallinity was grown on an α-Al2O3 substrate by a new MOCVD. An initial-nitriding layer of the α-Al2O3 surface was employed as a buffer layer to reduce a lattice mismatch between the AlN film and the α-Al2O3 substrate. The initial-nitriding layer was formed by chemical conversion of the α-Al2O3 surface in NH3 gas at 1200°C.

A new type of scanning µ-RHEED (reflection high energy electron diffraction) microscope has been developed. Three diffraction spots of a RHEED pattern were simultaneously selected for imaging. The scanning µ-RHEED images were obtained from the intensity change of the diffraction spots. The distribution of normally rotated micrograins inside the same plane's grain parallel to the surface was successfully observed from the scanning µ-RHEED images. Micrograin structures of poly-Si and Cu thin films were observed with a resolution of less than one micron.

The optimum design for the monolithic Sezawa wave ZnO/Si convolvers is clarified on crystal cuts and SAW propagation directions in silicon. Theoretical analysis was carried out using an equivalent transmission line circuit model of SAW. The calculated bilinear convolution characteristics well agree with the experimental results. The terminal convolution efficiency FT=-47.5 dBm was obtained in the [001] direction on (110) Si. Dynamic range over 80 dB was observed in the [011] direction on (100) Si. The temperature and dc gate bias stability characteristics are described.

A vapor-phase decomposition mechanism for the trimethylaluminum(TMA) used in an Al CVD was investigated by infrared spectroscopy. In pyrolysis, the annihilation of TMA was a first-order reaction and a CH3 radical was generated. The activation energy of the annihilation was about 0.4 eV. The generation mechanism of CH4 from the intermediates was investigated quantitatively. It is suggested that carbon contamination can be reduced by using an H2 carrier gas since the CH3 radical which causes such contamination is changed into CH4 by hydrogenation. In photolysis, the annihilation of TMA is a first-order reaction and C2H6 is generated by the coupling of two CH3 groups. The reduction of carbon contamination is also suggested because the generation of a CH3 radical is suppressed by the generation of C2H6.

A theory of lossless parametric operation in surface-acoustic-wave convolver is developed by applying the treatment of parametric operation in varactor diodes. It is shown that much improvement in convolver efficiency can be achieved by the separation of the output circuit from the input circuit by filters, by the elimination of useless constant capacitance components by tuning with inductance in input and output circuits, and by the impedance matching in input and output circuits. Preliminary experiments on such a convolver in a ZnO/SiO2/Si system confirm our theory and show the highest internal efficiency among convolvers examined so far.

A nondestructive and noncontact method for observation of microdefects in GaAs wafers has been developed with a new photo-thermal-radiation (PTR) microscope. We measured the PTR signal as a function of excitation energy (PTR spectrum) and the spatial distribution of PTR intensity (PTR image) of n-GaAs wafers at room temperature. We found that the PTR spectra have a peak due to a nonradiative state of microdefects at wavelengths ranging from 895 to 903 nm. The present PTR image shows inhomogeneities of microdefect density in GaAs wafers which cannot be observed by X-ray topography.

Oxidation-induced stacking faults (OSF) have been observed quasi-nondestructively by photoacoustic topography. Photoacoustic (PA) signals were detected with a ZnO transducer made by sputtering on the back surface of a sample. We found a good correlation between the PA topograph and the distribution of OSF. The spatial resolution was about 60 µm in the present PA topography.

We have investigated the temperature dependence of surface acoustic wave (SAW) delay time on AlN films grown by metalorganic chemical vapor deposition (MO–CVD) on the basal plane of sapphire (Al2O3). The temperature coefficient of delay for AlN/Al2O3 structures for SAW propagation along the [11̄00] Al2O3 direction decreased with increasing k H, where k is the wave number and H is the thickness of AlN films. We have succeeded in realizing zero temperature coefficient SAW delay lines at a certain value of k H.

Efficient transduction from bulk to surface waves by a holographic grating on a GaAs crystal has been demonstrated. The beam profiles for 880 MHz surface waves propagating on the GaAs were measured by an optical probing method.

Epitaxial aluminum nitride (AlN) films were grown on the basal planes of sapphire substrates using metalorganic chemical vapor deposition (MO-CVD), and the following results were obtained. (1) The substrate temperature and the trimethylaluminum (TMA) concentration dependences of the growth rate suggested that the growth process is limited by mass transport. (2) The crystalline perfection of epitaxial films is strongly affected not only by the substrate temperature but also by the gas flow pattern. (3) Single-crystal AlN films which include no misoriented grains can be grown epitaxially at ∼1200°C under carefully-controlled gas flow patterns. (4) Crystalline imperfection is caused mainly by (1 1̄01)-oriented grains which correspond to the hillocks observed in the surface morphology.

Measurements of current-voltage and current-temperature characteristics have been performed on metal-aluminum nitride (AlN)-silicon (n+–Si) substrate structures with Au and Al as metal electrodes. Single-crystal AlN films were epitaxially grown on (111)-oriented Si substrates by metalorganic chemical vapor deposition (MO–CVD). At room temperature it was found that the current-voltage characteristics show Ohmic behavior at low fields and the resistivity is of the order of 1016 \varOmegacm. On the other hand, it was found at high fields that the current is controlled by the Poole-Frenkel conduction of holes or electrons. Values of the hole trap energy depth from valence band edge and the electron trap energy depth from conduction band edge were found to be about 0.5 and 0.9 eV, respectively.

A photo-assisted plasma CVD method was developed to deposit high quality silicon nitride films. Nitrogen gas excited in a high density plasma kept apart from a wafer was effectively transported to the deposition chamber under the Knudsen pressure region and reacted with silane gas to generate intermediates. The adsorbed intermediates on the wafer were photo-excited to produce silicon nitride film. Silicon nitride films with the hydrogen content of 3–7 atm%, the density of 2.9–3.3 g/cm3, the tensile stress of 0.5–3.0×109 dyn/cm2 and the Si/N composition of 0.75 were successfully deposited at 300°C.