F. K. Men

Formation of regular step arrays on Si(111)7×7

Highly regular arrays of steps are produced on vicinal Si(111)7×7 surfaces. A tilt of the surface normal from (111) toward (112) produces single steps (0.3 nm high and typically 15 nm apart). The opposite tilt toward (112) produces bunched steps with adjustable height (1–5 nm) and a spacing of 70 nm. Preparation criteria for straight edges and regular spacings are determined, taking into account the miscut angle (azimuthal and polar), annealing sequence, current direction, and applied stress.

Regular step arrays on silicon

Highly regular arrays of steps are produced on vicinal Si(111)7×7. The step edges are atomically straight for up to 2×104 lattice sites. The terraces are single domain, which produces a minimum kink width of 2.3 nm (half a 7×7 unit cell) and thus a high barrier for creating kinks. Criteria for obtaining optimum step arrays are established, such as the miscut [≈1° towards (112)] and an annealing sequence which passes through step bunching regions quickly.

Model for an inversionless two-color laser

We propose a model, by taking advantage of quantum interference in a semiconductor quantum-well structure, for two-color lasing without population inversion. In the suggested three-well system, transitions of the lowest three excited states, being coupled to a common continuum by tunneling, to the ground state have been studied. Our results show that the emission spectrum can be arranged in accordance with the initial conditions of the excited states. With the emission peaks located in the vicinities of the absorption zeros resulting from destructive interferences, the nonreciprocal emission-absorption spectra provide a choice of inversionless lasing at two frequencies.

Surface step configurations under strain: kinetics and step-step interactions

Abstract Strain is an important ingredient in the physics of surfaces as has been shown in experiments with thin films and alloys. It is desirable to have strain as an externally applied and continuous variable. We have studied the effect of strain by loading a cantilevered bar and observing the effects on the surface with both LEED and the STM. On the Si(100) surface, strain produces a reversible asymmetry in the relative population of the 2 × 1 and 1 × 2 domains. This requires the motion of monatomic steps which are the domain boundaries. The effect is driven by the relaxation of the energy associated with a long-range strain field extending into the bulk due to the anisotropy of the intrinsic stress tensor of the two reconstructed domains. It is similar to magnetic domain structures reducing the magnetic field energy. These long-range strain fields have important consequences for a number of surface phenomena. Here we first report experiments on the terrace width distributions which gives information about the effective step-step interactions. Then we report experiments on the kinetics of the step migration which gives information about surface diffusion.

Linear arrays of CaF2 nanostructures on Si

Linear arrays of CaF2 stripes and dots, about 7 nm wide, are fabricated by self-assembly on stepped Si(111). Stripes are grown on a CaF1 passivation layer, dots directly on Si. The stripes have a precision of ±1 nm, are continuous, do not touch each other, and are attached to the top of the step edges. The stripe repulsion and their counter-intuitive attachment are explained via a reversal of the stacking at the CaF2/Si(111) interface. The dot density is 3×1011 cm−2=2 Teradots/in.2. These arrays may serve as masks in nanolithography.

UNOCCUPIED SURFACE STATES ON SI(111)3X3-AG

A nearly metallic surface state band is detected on Si~111!)3) Ag by inverse photoemission, Si 2p core level photoemission, and scanning tunneling spectroscopy. The band spans most of the bulk band gap of Si, from the Fermi level at 0.25 eV above the valence band maximum all the way to the conduction band minimum. The Fermi level is pinned over a wide doping range ~7310 cm p type to 1.2310 cm n type!. The data suggest that the surface band gap expected from the even electron count is filled in at room temperature, possibly due to thermal disorder or due to the finite domain size of 10–20 nm. A second, prominent surface feature at 2.2 eV above the valence band maximum is assigned to surface umklapp from K to Ḡ via a )3) reciprocal lattice vector. @S0163-1829~98!03204-4#

Variable low‐temperature scanning tunneling microscopy study of Si(001): Nature of the 2×1→c(2×4) phase transition

We have studied the Si(001) surface from 120 K to room temperature using a variable low‐temperature scanning tunneling microscope. Complementary investigations were carried out on two distinctly different types of surfaces: first, the normal 2×1 surface and second, the 2×n (4<n<12) surface. For the 2×1 surface, the defects are scattered randomly. By plotting out the fraction of buckled dimers as a function of temperature, we find a slow transition from predominantly c(2×4) at low temperature to mostly 2×1 at room temperature for a defect concentration of about 8.5%. For the 2×n surface, the much larger number of surface vacancies form long‐range ordered chains, dividing the surface into many short dimer segments. These dimer segments predominantly appear to be unbuckled at room temperature. Upon cooling to 190 K, we observe very little change in the amount of buckling. The implications of this result are discussed.

Scanning tunneling microscopy investigation of the dimer vacancy–dimer vacancy interaction on the Si(001) 2×n surface

Scanning tunneling microscopy has been used to investigate the formation of the 2×n vacancy line structure on Si(001). We find that quenching the surface from high temperatures results in the formation of vacancies. After further quenching, these vacancies nucleate into chains running perpendicular to the dimer rows. Finally, the vacancy chains connect and develop into vacancy lines which extend for many thousands of A’s. Each vacancy line consists of mainly two types of dimer vacancies: (1) a di‐vacancy and (2) a combination of a single vacancy and a di‐vacancy separated by an isolated dimer. All the vacancy lines together with the dimer rows form a 2×n structure with 6≤n≤12. Calculations using the Stillinger–Weber potential support the view that the formation of the vacancy line structure is due to the interaction between vacancies.

The effect of external stress on Si surfaces

To study the effect of strain on surface properties, we had externally applied a continuously variable stress by loading cantilevered bars. The first experiments1 on nominally flat Si(100) produced asymmetries in the population of 2×1 and 1×2 domains which depended on the strain and not the gradient. The kinetics for the production and annealing of the asymmetry were identical and thermally activated, but the final steady state was temperature independent. The sense of the asymmetry is that the domain compressed along the dimer bond is favored. Alerhand et al.2 have suggested that the mechanism is the reduction of the energy in long range strain field due to the intrinsic surface stress. In this paper we compare the experimental observations and the expectation of the theory. We have now done the same experiments on a vicinal surface miscut by 1°. We have also observed the changes in the step (domain boundary) configurations in the STM. With an inadvertant miscut of 0.1°, the steps remain single height an...

Self-assembled CaF2 nanostructures on silicon

A method for chemical imaging of CaF2, CaF1, and Si by scanning tunneling spectroscopy is presented. This method is utilized for identifying the growth regimes of CaF2 and CaF1 on stepped Si(111)7×7. For CaF2 on Si(111), we find random islands, stripes, and ordered islands, depending on the supersaturation. For CaF2 on a CaF1 monolayer on Si(111), we find regular stripes that are continuous and separated from each other. CaF2 structures are attached to the bottom edge of a step when growing directly on Si, but they prefer the top of a step edge when growing on a CaF1 buffer layer. These highly regular, linear arrays of CaF2 stripes and dots can serve as masks for assembling more sophisticated nanostructures.