# Presentation: 732975

## Structure and Reactivity of 1D Nanostructures on High-Index Silicon Surfaces Alison Baski, Virginia Commonwealth University, DMR 0207643

- Structure and Reactivity of 1D Nanostructures on High-Index Silicon Surfaces Alison Baski, Virginia Commonwealth University, DMR 0207643

**STABLE**Flat + Islands

**UNSTABLE**:
- Sawtooth Facets

- (001)

- (113)

- (5 5 12)

- (111)

- (112)

- 2×2 m2

- (112)

- (001)

- (111)

- (5 5 12)

**STABLE**Flat + Islands

- This project studies the nanometer-scale morphology changes causes by the controlled oxygen etching of a variety of silicon surface orientations. Silicon is the primary material used in the semiconductor industry for device fabrication. This fundamental study shows how oxygen processing can significantly tailor the surface morphology of oriented Si wafers.
- We have found that surface orientation strongly influences the morphology resulting from extended etching. The surface orientations that are _stable_ against etching and remain flat include Si(001), Si(111), and Si(113). It should be noted that the (113) orientation is the *only* high-index surface to remain stable after extended oxygen etching.
- The surface orientations that are _unstable_ against oxygen etching and facet to other orientations include Si(5 5 12) and Si(112). These surfaces form sawtooth facets that are primarily composed of the more stable (111) and (113) planes.
- By controlling the temperature and exposure during oxygen etching, it is therefore possible to form a variety of surface morphologies that can be used as nanotemplates for further growth.

- Atomic force microscopy images (2×2 m2) of various silicon surfaces after oxygen etching at 800°C with 200 L (Langmuir) exposure at a pressure of 3×10-7 Torr. The relative orientations are shown at the upper left.

**Notes:**
AIM OF THE PROJECT:
This project studies the nanometer-scale morphology changes caused by the controlled oxygen etching of a variety of silicon surface orientations. Since silicon is the “workhorse” of the semiconductor industry, it is a material of industrial interest. We use the techniques of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) to investigate how variables such as surface orientation, sample temperature, and dosage affect the resulting surface morphology.

RESEARCH RESULTS:
We have now studied both low and high dosage etching of five Si surface orientations: (001), (113), (5 5 12), (112), and (111). All of these orientations form stable, clean surface reconstructions after high temperature annealing, except for the (112) orientation which forms nanofacets. The oxygen etching was performed at sample temperatures between 650 °C and 950 °C, pressures of 3.3×10–7 and 1.5×10–7 Torr, and exposures of 50 up to 600 L (1 Langmuir is equivalent to 1 s exposure at 3.3×10–6).
At lower exposures (<50 L) and moderate temperatures (700 to 750 °C), oxygen etching on all of the surface orientations leads to step retraction and the appearance of islands on terraces and at step edges. These islands are presumably due to oxide-induced pinning sites that remain as the surrounding area is etched. The low-index surfaces show enhanced terrace etching and significantly modified step edges as compared to the high-index ones. The high-index surfaces have enhanced island nucleation along step edges, most likely due to their anisotropic surface reconstructions and subsequent diffusion behavior. 
At higher exposures (>200 L) and temperatures (see figure above), the various surface orientations have significantly different morphologies. Fewer kinetic limitations are present and the resulting morphologies reflect the stability of surface planes. The (001) and (111) surfaces are relatively flat with islands having two-fold or three-fold symmetry, respectively. The density of these islands increases at lower temperatures and higher pressures, and yield an effective activation energy of ~3 eV. The (113) surface is also flat with slightly anisotropic islands, consistent with its anisotropic reconstruction. In contrast, the less thermodynamically stable (5 5 12) and (112) surfaces are significantly etched to expose linear sawtooth structures primarily composed of short (111) and (113) planes. 

SIGNIFICANCE OF THIS WORK
We have found that surface orientation strongly influences the morphology resulting from extended etching. The surface orientations that are stable against etching and remain flat include Si(001), Si(111), and Si(113). It should be noted that the (113) orientation is the only high-index surface to remain stable after extended oxygen etching. The surface orientations that are unstable against oxygen etching and facet to other orientations include Si(5 5 12) and Si(112). These surfaces form sawtooth facets that are primarily composed of the more stable (111) and (113) planes.
By controlling the temperature and exposure during oxygen etching, it is therefore possible to form a variety of surface morphologies that can be used as nanotemplates for further growth.

## Structure and Reactivity of 1D Nanostructures on High-Index Silicon Surfaces Alison Baski, Virginia Commonwealth University, DMR 0207643

- Education
- This grant has helped to support one Ph.D. student (Jonathan Dickinson), five M.S. students (Ryan Hall, Elizabeth Morris, John Skrobiszewski, Mary Willis, Pat Woodworth), and three  undergraduates (Marcella Torres, Lindsay Hussey, Matt Sievert).  As part of their research, these students were trained to operate an ultra-high vacuum scanning tunneling microscope and/or ambient atomic force microscope.

- Outreach
- Graduate and undergraduate students in this grant have also been involved with the physical science outreach program directed by Dr. Baski. During the past three years, this program has delivered over 250 physical science lessons to the classrooms of participating teachers at Richmond City Elementary schools. Dr. Baski offers an intensive, 2-week summer course for the teachers, and then follows up with classroom visits during the school year. She has personally led over 140 of these lessons.

**Notes:**
For the past three years, Dr. Baski has developed an outreach program using hands-on activities to improve the physical science learning of Richmond City elementary school teachers and their students. She has taught over 80 teachers who have enrolled in her intensive, two-week summer course to learn about physical science topics such as motion & energy, electricity & magnetism, and sound & light. The summer course uses hands-on activities designed as part of this program.
During fall semesters, Dr. Baski teaches a service-learning course where a faculty leader and VCU students visit the elementary classrooms to teach 1-hour lessons based on the activities. This program has already delivered over 250 lessons to the classrooms of participating teachers, where Dr. Baski has personally led over 140 of these lessons. Graduate and undergraduate students who work in Dr. Baski’s laboratory also participate during select visits to gain teaching experience.