Photoluminescence characterization with a surface treatment suggests that a reduction in the radiative recombination rate of GaAs nanowhiskers is caused by charge separation in depletion potential. Good agreement is obtained between photoluminescence characteristics and calculations based on self-consistent wavefunctions confined in the depletion potential. The radiative life time of 200-nm GaAs nanowhiskers at 77 K is estimated as short as 0.5 ns if the depletion potential is completely eliminated. Weak size dependence of photoluminescence spectra at 6 K is explained as a sign of band-gap reduction induced by the depletion potential.

The direct measurement of sidewall roughness on a ridge-type GaAs waveguide was performed using an atomic force microscope (AFM) combined with a scanning electron microscope (SEM). The ridge sidewall of a GaAs waveguide formed by wet-etching and the ridge sidewall formed after regrowth of a 2.45-µm GaAs/AlGaAs epitaxial layer on the same waveguide were observed using introducing the technique for sample slanting. The observed power spectral density was used to determine the scattering loss caused by the sidewall roughness. It was found that the ridge-type GaAs waveguide for light wave transmission had a scattering loss of 0.029 dB/cm in the as-etched ridge state and a scattering loss of 0.17 dB/cm after regrowing the cover GaAs/AlGaAs epitaxial layer. A leaky GaAs/AlGaAs waveguide for polariton-quantum-wave trans-mission had a scattering loss of 1.3l0-5 dB/cm, which means that the scattering loss is negligible. Furthermore, it was found that a periodical surface fluctuation (spatial frequency 2.2 µm-1) along the waveguide appeared after the regrowth of the epitaxial layer. Thus, this method is useful for direct observation of sidewall roughness and can be used to quantitatively determine the sidewall scattering loss.

We have developed a new HIGFET structure achieving an extremely high K-value of 13.3 S/Vcm with a gate length of 0.15 µm. Self-aligned ion implantation is excluded to suppress a short-channel effect. An i-GaAs cap layer and an n+-GaAs contact layer are employed to reduce source resistance. The threshold voltage shift is as small as 50 mV when the gate length is reduced from 1.5 µm to 0.15 µm. Source resistance is estimated to be 53 mΩcm. We have also developed a new fabrication process that can achieve a shorter gate length than the minimum size of lithography. This process utilizes an SiO2 sidewall formed on the n+-GaAs contact layer to reduce the gate length. A gate length of 0.15 µm can be achieved using 0.35 µm lithography.

Ultrathin GaAs, AlGaAs and GaAs/InAs wire crystals (whiskers) as thin as 20-50 nm are grown by organometallic vapor phase epitaxy (OMVPE) using Au as a growth catalyst. It is found that the whisker shape and width can be controlled by adjusting the thickness of the Au deposited on the substrate surface and the substrate temperature duing OMVPE. A new technique employing a scanning tunneling microscope (STM) for controlling the whisker growth position on the substrate surface is described. Photoluminescence spectra from the GaAs whiskers show a blue shift of the luminescene peak energy as the whisker width decreases. The amount of blue shift energy is rather small compared to that calculated by a simple square potential well model. The discrepancy is explained by the cylindrical potential well model including the surface depletion effect. Atomic composition within the portion of 1-20 nm along the AlGaAs and GaAs/InAs whiskers has been analyzed by energy dispersive X-ray analysis in combination with transmission electron microscopy. This shows the exsitence of Au at the tip of the whisker and the composition change occurs over a length of less than 5 nm at the GaAs/InAs heterojunction.