# Presentation: 340049

## Shape control of ordered semiconductor nanostructures

**Figure 2.****  Island shapes (AFM phase images) for different metal-species/Si-orientation combinations:  (a) Au/Si(001),  (b) Au/Si(111), (c) Sn/Si(110),  and (d) Au/Si(110) . **

**The organization of nanoscale elements into higher order systems remains a fundamental challenge in nanoscience and nanotechnology.**
**Dub****ó****n and co-workers have discovered a novel growth process that combines rudimentary metal patterning and natural self-assembly processes for the realization of germanium nanostructure arrays on silicon (Fig. 1).**

**Island-shape control within the patterned region is achieved by the choice of patterned metal species and crystallographic orientation of the Si surface (Fig. 2).  These investigations suggest an intimate relationship between Si-Ge intermixing and island morphology.**

**Figure 1.  (left) Optical diffraction from  Au-patterned Si covered with over 100,000 Ge islands.  (right) Representative atomic force microscopy (AFM) zoom-in image of Ge islands that have ordered into a 2D square lattice (height scale=100 nm).  The color (red) in the left image arises from the island spacing.**

**The surface-diffusion-based dynamics of the ordering process point to the possibility of forming a variety of nanostructure arrays in this and other semiconductor systems.**

**Shape control of ordered semiconductor nanostructures**
**DMR-****0349257****	****	****	****	****Oscar D. Dubón, Jr.**
**	****	****	****	****	****	****U.C. Berkeley**** **

**Notes:**
Aim of the project.  The goal of this project is to understand the effect of metal patterns and overlayers on the heteroepitaxial growth of semiconductor thin-films and nanostructures on other semiconductors and on crystalline insulators.  Metals can modify the wetting behavior of materials on each other and alter the crystal growth process.  Understanding and exploiting metal-induced effects may enable us to achieve unprecedented manipulation of thin-film and epitaxial nanostructure growth processes.

Significance/impact of this work to nanoscience.  The organization of nanoscale elements into higher order systems remains a fundamental challenge in nanoscience and nanotechnology.  The realization of macroscopic assemblies of nanostructures through direct manipulation, achieved by highly-reproducible patterning processes such as lithography, is not only impractical but also in general unfeasible.  Thus, considerable effort has been placed on the integration of spontaneous self-assembly processes that minimize the need to directly manipulate each nanoscale element but that are characterized by high defect densities.  We have discovered a semiconductor growth process based on rudimentary patterned metallization that represents a natural convergence between directed pattering techniques and self-assembly processes for the realization of ordered, epitaxial, semiconductor nanostructure arrays.  The simplicity and diffusion-based dynamics of the process means that growth temperature can be used to tune array assembly over a wide range of length scales while metal-pattern design can be exploited to form a variety of nanostructure arrays in perhaps a wide range of heteroepitaxial islanding systems .

Research results.  Focusing on the Ge on Si model semiconductor system, we have demonstrated the assembly of hundreds of thousands of Ge islands into a square lattice on a Si surface that has been patterned with sub-micron Au dots, which are produced simply by the evaporation of 1 nm of Au through a stencil mask containing arrays of square windows.  In the first year of this grant, our work focused on understanding the effects of i) substrate orientation and ii) metal species on array formation and island shape evolution processes.  By varying the metal species (e.g., using Sn rather than Au) and the crytallographic orientation of the Si surface, we have realized a wide range of unique island shapes including Ge nanorods, which may be suited for studies of low-dimensional electronic transport.  The structure of these islands has been investigated both at Berkeley and at Argonne National Lab's Advance Photon Source. 
Future plans.  Experiments point at intermixing between Si and Ge as a critical process facilitating the formation of the wide range of observed island shapes.  Detailed studies are underway to elucidate the connection between intermixing and island shape evolution.  In addition, we plan to investigate the effect of Si and Ge co-deposition on islanding dynamics and will eventually explore island formation on single-crystalline insulators, which would enable us to probe the electronic properties of individual (electrically isolated) nanostructures.  
We have recently proposed to study the ordering dynamics, structural properties and the composition of Ge islands on Au-patterned Si surfaces using in situ low-energy electron microscopy and x-ray photoemission electron microscopy at the Third Generation ELETTRA Synchrotron (Trieste, Italy).

## Shape control of ordered semiconductor nanostructures

**DMR-****0349257****	****	****	****	****Oscar D. Dubón, Jr.**
**	****	****	****	****	****	****U.C. Berkeley**** **

_**Education, training, outreach**_

***Integrating research concepts in the classroom***—Whether in graduate school or in industry, new college graduates will tackle problems requiring innovative solutions.  The successful engineer and future scientist must be able to gather information from a wide variety of sources,  synthesize new concepts from this information and convey his/her findings to a technical community and  even the public at large.  In the capstone course taught by Dubón, engineering seniors perform advanced library research, carry out peer collaborations and reviews, analyze original data produced in Dubón's research labs, and write a technical paper.  These activities cultivate important skills that will enable students to better meet challenges in science and technology.
***Training the future scientists through cutting-edge research experiences***—A major objective of this project is to train future scientists and engineers, who will become leaders on the global stage.  Both graduate and undergraduate students conduct original research and present their work at scientific conferences.  In addition, students are encouraged to expand their scientific horizons through programs such as NSF's East Asian and Pacific Institutes (EAPSI) Program.
***Reaching audiences beyond the classroom and laboratory***—Key to developing a strong, diverse workforce is to build an effective pipeline across education levels.  To this end, Dubón has participated in the Summer Undergraduate Program in Engineering Research at Berkeley (SUPERB), a program designed to provide research opportunities in engineering to students who have been historically underrepresented.  As a panelist, invited speaker, and SUPERB executive committee member, Dubón has sought to share his experiences and insights with program participants.  In addition,  Dubón served as a session co-organizer and co-chair (with Dr. J. Santiago-Aviles and Dr. L. Martinez-Miranda) for the 2004 National Meeting of the Society for the Advancement of Chicanos and Native Americans in Science.  The session "Engineering: Applied Physics, Materials Science, and Biophotonics" was sponsored in part by NSF.