Chanan Euaruksakul

Quantum Confinement, Surface Roughness, and the Conduction Band Structure of Ultrathin Silicon Membranes

We report direct measurements of changes in the conduction-band structure of ultrathin silicon nanomembranes with quantum confinement. Confinement lifts the 6-fold-degeneracy of the bulk-silicon conduction-band minimum (CBM), Delta, and two inequivalent sub-band ladders, Delta(2) and Delta(4), form. We show that even very small surface roughness smears the nominally steplike features in the density of states (DOS) due to these sub-bands. We obtain the energy splitting between Delta(2) and Delta(4) and their shift with respect to the bulk value directly from the 2p(3/2)-->Delta transition in X-ray absorption. The measured dependence of the sub-band splitting and the shift of their weighted average on degree of confinement is in excellent agreement with theory, for both Si(001) and Si(110).

Conduction band structure and electron mobility in uniaxially strained Si via externally applied strain in nanomembranes

Strain changes the band structure of semiconductors. We use x-ray absorption spectroscopy to study the change in the density of conduction band (CB) states when silicon is uniaxially strained along the [1 0 0] and [1 1 0] directions. High stress can be applied to silicon nanomembranes, because their thinness allows high levels of strain without fracture. Strain-induced changes in both the sixfold degenerate Δ valleys and the eightfold degenerate L valleys are determined quantitatively. The uniaxial deformation potentials of both Δ and L valleys are directly extracted using a strain tensor appropriate to the boundary conditions, i.e., confinement in the plane in the direction orthogonal to the straining direction, which correspond to those of strained CMOS in commercial applications. The experimentally determined deformation potentials match the theoretical predictions well. We predict electron mobility enhancement created by strain-induced CB modifications.

Heteroepitaxial growth on thin sheets and bulk material: exploring differences in strain relaxation via low-energy electron microscopy

Extremely thin single-crystal sheets have unique mechanical properties, which may influence the generation and behaviour of extended defects during heteroepitaxial growth. Using low-energy electron microscopy (LEEM) we investigate the earliest stages of inelastic strain relaxation in SiGe grown heteroepitaxially on Si-on-insulator (SOI) at a sensitivity not possible with other methods, employing a structure that forces dislocations, if they can form at all, to reside at the Si/oxide interface of SOI(0 0 1). LEEM confirms a lower dislocation line energy at the Si/amorphous oxide (SiO2) interface than at the crystalline SiGe/Si interface. The line energy is, however, nonzero, in contrast with earlier assumptions. The lower line energy makes the thermodynamic critical thickness for growth on SOI(0 0 1) lower than on bulk Si(0 0 1) for otherwise identical growth conditions. Nevertheless we can grow heteroepitaxial SiGe films on SOI(0 0 1) that are much thicker than even the thermodynamic critical thickness for growth on bulk Si(0 0 1), suggesting high kinetic barriers for dislocation formation or motion.