Selective growth of GaAs on a nanoscale SiO2-patterned GaAs(001) substrate by molecular beam epitaxy is reported. Reduction of the lateral dimensions of a SiO2 pattern below the surface diffusion length of an incident Ga atom results in preferential migration from the SiO2 surface to stable bonding configurations at nearby open GaAs substrate surfaces. This intrinsic selectivity is achieved under high growth temperature with low growth rate where surface migration on the SiO2 surface is highly extended. A large-area nanoscale SiO2 pattern is realized by interferometric lithography. A 330-nm period two-dimensional array of GaAs disks having a height of 40 nm and a diameter of about 240 nm, selectively grown on a GaAs(001) substrate is presented. Variation of the nanoscale selective growth mode depending on the diameter of GaAs disks is also observed.

We have investigated solid phase crystallization behavior of the molecular beam epitaxy grown amorphous Si1−xGex (x=0 to 0.53) alloy layers using x-ray diffractometry and transmission electron microscopy (TEM). Our results show that the thermal budget for the full crystallization of the film is significantly reduced as the Ge concentration in the film is increased. In addition, we find that a pure amorphous Si film crystallizes with a strong (111) texture while that of the Si1−xGex alloy film crystallizes with a (311) texture suggesting that the solid phase crystallization mechanism is changed by the incorporation of Ge. TEM analysis of the crystallized film shows that the grain morphology of the pure Si is an elliptical and/or a dendrite shape with a high density of microtwins in the grains while that of the Si0.47Ge0.53 alloy is more or less equiaxed shape with a much lower density of defects. From these results, we conclude that the crystallization mechanism changes from a twin-assisted growth mode to a random growth mode as the Ge concentration in the film is increased.

Generally, voided reinforced concrete floor slabs in building structures have a lower shear strength compared with solid slabs owing to the reduction in their cross-sectional area. This study shows that the void shape and material of the void-shaper also influence the shear strength of voided slabs. To verify these assumptions, one-way shear tests were conducted for four test specimens. The first was a conventional solid slab and the others were voided slabs.The voids had toroidal or 'doughnut' shapes as well as non-doughnut shapes. The void-shaper was made of polypropylene plastic and glass-fibre-reinforced plastic. The test results showed that the shear strength of doughnuttype voided slabs was 73-78% of a solid slab and was superior to that of existing voided slabs. The shear crack angle changed with the void shape. A finite-element analysis was conducted to evaluate the influence of these parameters on the shear behaviour of the doughnut-type voided slab. Notation a shear span b v width of the void b w sum of smallest web thicknesses C Rd,c coefficient derived from tests D j diagonal length of the panel for measuring shear deformation d effective depth of the specimen d j depth of the panel for measuring shear deformation f ck specified compressive strength of concrete G shear modulus of elasticity H height of the slab h j width of the panel for measuring shear deformation k factor of size effect N number of doughnut-type void-shapers that are located in the shear crack region P cr measured load when first shear crack occurred R curvature radius of void edges V c nominal shear strength provided by concrete without shear reinforcement V HS shear strength of a doughnut-type void-shaper with varying materials V HS,Total sum of the shear strength of the doughnut-type void-shapers V n nominal shear strength of the voided slab without shear reinforcement V Rd,c nominal shear strength provided by concrete without shear reinforcement V u ultimate shear strength AE shear factor AE Doughnut shear factor of the doughnut-type voided slab ª 1 measurement of shear strain ª 2 measurement of shear strain ª s shear strain of the slab ª xy shear strain of the void-shaper ä 1 deformation of the panel in the diagonal direction ä 2 deformation of the panel in the diagonal direction å a strain of the void-shaper in the vertical direction å b strain of the void-shaper in the diagonal direction å c strain of the void-shaper in the horizontal direction Ł shear crack angle r tensile reinforcement ratio of the slab ô V shear stress of the void-shaper 336

정주홍 1) ‧ 강성훈 1) ‧ 이승창 2) ‧ 최창식 3) ‧ 최현기 4)*

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