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Pre-tRNA splicing has been believed to occur in the nucleus. In yeast, the tRNA splicing endonuclease that cleaves the exon-intron junctions of pre-tRNAs consists of Sen54p, Sen2p, Sen34p, and Sen15p and was thought to be an integral membrane protein of the inner nuclear envelope. Here we show that the majority of Sen2p, Sen54p, and the endonuclease activity are not localized in the nucleus, but on the mitochondrial surface. The endonuclease is peripherally associated with the cytosolic surface of the outer mitochondrial membrane. A Sen54p derivative artificially fixed on the mitochondria as an integral membrane protein can functionally replace the authentic Sen54p, whereas mutant proteins defective in mitochondrial localization are not fully active. sen2 mutant cells accumulate unspliced pre-tRNAs in the cytosol under the restrictive conditions, and this export of the pre-tRNAs partly depends on Los1p, yeast exportin-t. It is difficult to explain these results from the view of tRNA splicing in the nucleus. We rather propose a new possibility that tRNA splicing occurs on the mitochondrial surface in yeast.

We have studied the effects of the roughness of a GaAs epitaxial surface on the characteristics of a surface oxide layer, which is used as a resist-mask in "in situelectron-beam (EB) lithography". The oxide layer formed on a microscopically rough surface resulted in degraded threshold characteristics of EB patterning and in an indented pattern edge. On the other hand, when we formed it on a flat epitaxial surface, obtained by step-flow growth using a misoriented substrate, the threshold characteristics were remarkably improved, and the resulting pattern exhibited a sufficiently sharp edge. These improvements are helpful for fabricating nanostructures by in situEB lithography.

Patterning of GaAs at the sub-100 nm size has been demonstrated using in situelectron beam (EB) lithography processes. Patterning is carried out by EB exposure of an ultrathin oxide layer on GaAs which is used as a mask material. The patterns are transferred into GaAs by Cl2gas etching. A high-brightness Schottky electron gun is used in the exposure process. The size of the etched feature is as small as 50 nm, which is equal to the diameter of the electron beam. The results show that in situEB lithography processes using an ultrathin oxide mask are very promising for fabricating nanometer-scale structures.

We describe cathodoluminescence (CL) spectroscopy and imaging studies of GaAs/AlAs quantum well (QW) structures and AlGaAs alloys grown by molecular beam epitaxy on GaAs (100) substrates patterned with mesa stripes along the [001] direction. GaAs wirelike structures were formed on these mesas due to the large diffusibility of Ga atoms from the {110} sidewalls to the (100) mesa-top plane. The spatial variation of the QW thickness and that of an AlGaAs alloy composition were investigated by carrying out CL measurements at low temperature. On the mesa-top region we found considerably large variations in the QW thickness and Al content, i.e., a 110% increase in the QW thickness and a 60% decrease in Al content, compared to the (100) bottom region. This is very useful for the fabrication of quantum-wire/box structures; thus, the use of the [001] mesa geometry is advantageous over other geometries.

The electron beam (EB)-induced modification of GaAs oxide for the in situlithography process has been investigated. The oxide formed on an as-grown GaAs surface is irradiated with an electron beam at doses in the range of 1.5×1017to 4×1019electrons/cm2at beam energies of 10-25 keV. Subsequent Cl2gas exposure results in selective etching only in the irradiated area. EB-induced modification of the GaAs oxide is found to involve a partial removal of oxygen by in situAuger measurements. The threshold dose for the etching depends on electron energy, whereas it is independent of the wafer temperature between 25 and 100°C during EB-irradiation. These results indicate that the modification is not a thermally activated process, but a direct interaction of the electron beam with the oxide.