Takashi Katoda

Raman spectra from Ga1−xInxAs epitaxial layers grown on GaAs and InP substrates

Raman spectra from Ga1−xInxAs epitaxial layers of various compositions were studied. Both disorder‐activated acoustic and optical phonons appeared in the midrange of composition independent of substrate materials. Broadening in the LO phonon due to stress was also observed near the interface region between the epitaxial layer and the substrate when there was lattice mismatch between them even if the amount was as small as 0.7%.

Effect of clusters on long‐wavelength optical phonons in Ga1−xInxAs

The phonon spectra of Ga1−xInxAs have been studied by measuring the infrared reflectivity at near‐normal incidence in the 180 ∼400‐cm−1 frequency range. Only one reststrahlen band corresponding to the GaAs‐like mode was observed for x≲0.2, and two bands were observed for the remaining composition. The data were analyzed by using the Kramers‐Kronig relationship and the dependence of the mode frequencies on alloy composition was interpreted on the basis of the modified cluster model including the effect of clusters. The fact that the calculated oscillator strength for the InAs‐like mode is substantially zero in the composition range x≲0.2 explains the reason why phonons of the InAs‐like mode are not detected experimentally for x≲0.2. The clustering parameter representing the effect of clusters is determined by fitting the measured mode frequencies to the calculated compositional dependence and takes the maximum value near the midrange of alloy composition.

Raman spectra from Si and Sn implanted GaAs

The variation of Raman spectra from Si and Sn implanted GaAs with various doses was studied. Si implanted GaAs showed one type of Raman spectra which indicates that the state is almost single crystalline with a high density of defects at a dose from 2×1012 to 1×1016 cm−2. Sn implanted GaAs showed, however, three types of Raman spectra corresponding to almost single‐crystalline, amorphous, and mixed states depending on the ion doses. A model of the implanted region which includes partly crystalline and amorphous layers was proposed. The annealing behavior of the implanted region could be explained with the model.

Effects of optically excited carriers on Raman spectra from InP

Raman spectra from InP were measured at various power densities of the exciting laser. Concentration of the optically excited excess carriers Δn calculated from the frequency of the coupled longitudinal optical phonon‐plasmon mode was about 0.5–1.7×1017 cm−3 at the laser power density of 5×102 W/cm2 for InP with equilibrium carrier concentration 0.9–5×1017 cm−3. Surface recombination velocity estimated from Δn assuming that the bulk lifetime was larger than 10−7 s was 1–8×103 cm/s. Such dependence of Raman spectra on laser power density could not be observed for GaAs.

Observation of the amorphous‐to‐crystalline transition in silicon by Raman scattering

Raman scattering, electron diffraction, and dark‐conductivity measurements have been made on so‐called a‐Si films deposited at various rf powers by a glow‐discharge technique. These measurements show that an abrupt transition between amorphous and polycrystalline states occurred between 350 and 370 rf voltages and films deposited at high rf voltages are polycrystalline. Dark conductivity of the silicon films changed largely with the amorphous‐to‐crystalline transition. It is also observed that some polycrystallized silicon films exhibited new peaks between 505 and 513 cm−1 in the Raman spectra.

Electrical activation and local vibrational mode from Si‐implanted GaAs

Local vibrational modes of lattice site Si implanted in GaAs were studied by laser Raman spectroscopy. From the dependence of the intensity of the local vibrational modes due to Si at As sites and Si pairs at Ga and As sites (SiGa‐SiAs) on annealing temperature, implanted Si are considered to occupy the lattice sites with the structural recovery by annealing at lower than 550 °C, and form SiGa‐SiAs pairs at higher than 550 °C besides the electrically active Si probably in Ga sites. The depth profiles of the intensity of the local vibrational mode and carrier concentration obtained after annealing at 850 °C indicate that Si at As sites prevent the generation of carriers, and Si at Ga sites have no contribution to the generation of carriers if they form the pairs with the nearest‐neighbor Si at As sites.

Effects of strain on crystallization of amorphous silicon characterized by laser Raman spectroscopy

Solid-phase crystallization at a surface of amorphous silicon and at an interface between amorphous silicon and a substrate was studied by laser Raman spectroscopy. Amorphous silicon was deposited on a fused silica substrate by a sputtering method. The thickness of the amorphous silicon film was about 3 μm. The samples were annealed at 700 or 1000°C. Amorphous silicon crystallized faster at the surface than at the interface. The results were confirmed by characterization of the cross section by micro Raman spectroscopy. It was found from Raman shift of the polycrystalline silicon that the effect of the strain is to suppress crystallization.

Characterization of interdiffusion coefficients in GaAs‐AlAs superlattices with laser Raman spectroscopy

A new method to estimate interdiffusion coefficients in superlattices (SLs) is proposed. The method is based on measurement of thicknesses of layers which remain without forming an alloy after an annealing that induced interdiffusion. The measurement was done from the frequency of phonons based on the Raman spectra. Values of interdiffusion coefficients obtained by the method were almost in the same order as those reported previously. It is also shown that the gallium atoms in GaAs‐AlAs SLs diffuse more rapidly into AlAs layers than aluminum atoms which diffuse into GaAs layers. Interdiffusion coefficients decreased at first with the annealing time and increased slightly when the annealing was done for more than 1.5 h at 860 °C.

Raman spectra from heat‐treated semi‐insulating GaAs

Raman‐spectra measurements were made on semi‐insulating (SI) GaAs heat‐treated in an atmosphere of H2 or Ar to study the effect of structural changes or strains on a thermal conversion of Si GaAs. Correlations between Raman spectra and electrical changes of SI GaAs indicate that strains in the interface region between SI GaAs and a dielectric film is, at least, one of the origins of the thermal conversion, while the structural changes accompanying the generation of the TO‐phonon line has no effect on the thermal conversion.

Characterization of ion‐implanted and rapidly thermal annealed GaAs by Raman scattering and van der Pauw measurement

The structures of microcrystalline or amorphous gallium arsenide (GaAs) introduced by 120‐keV Si+‐ or P+‐ion implantation with a dose of 1×1016 atoms/cm2 and the subsequent regrowth properties annealed by rapid thermal annealing in the range from 300 to 900 °C have been investigated by Raman scattering (RS) and by the van der Pauw measurement (sheet carrier concentration and sheet resistivity) [Phillips Res. Rep. 13, (1958)]. Raman spectra of the LO‐phonon mode observed for 514.5‐ and 457.9‐nm excitation of an Ar+ laser have been analyzed on the basis of a spatial correlation model [H. Richter et al., Solid State Commun. 39, 625 (1981); K. K. Tiong et al., Appl. Phys. Lett. 44, 122 (1984)]. The results show that the regrowth stages and the regrowth rates of the disordered GaAs in the annealing process depend on the excitation wavelength and annealing temperature, and the damaged layer regrows epitaxially toward the surface. Sheet carrier concentration was observed to increase steeply, and sheet resistivit...