Porous Silicon in Practice E-Book

M. J. Sailor

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch
• Veröffentlichungsjahr: 2011

Seitenzahl: 374

Beschreibung

By means of electrochemical treatment, crystalline silicon can be permeated with tiny, nanostructured pores that entirely change the characteristics and properties of the material. One prominent example of this can be seen in the interaction of porous silicon with living cells, which can be totally unwilling to settle on smooth silicon surfaces but readily adhere to porous silicon, giving rise to great hopes for such future applications as programmable drug delivery or advanced, braincontrolled prosthetics. Porous silicon research is active in the fields of sensors, tissue engineering, medical therapeutics and diagnostics, photovoltaics, rechargeable batteries, energetic materials, photonics, and MEMS (Micro Electro Mechanical Systems). Written by an outstanding, well-recognized expert in the field, this book provides detailed, step-by-step instructions to prepare and characterize the major types of porous silicon. It is intended for those new to the fi eld. Sampling of topics covered: * Principles of Etching Porous Silicon * Etch Cell Construction and Considerations * Photonic Crystals, Microcavities, and Bragg Stacks Etched in Silicon * Preparation of Free-standing Films and Particles of Porous Silicon * Preparation of Photoluminescent Nanoparticles from Porous Silicon * Preparation of Silicon Nanowires by Electrochemical Etch of Silicon * Surface Modifi cation Chemistry and Biochemistry * Measurement of Optical Properties * Measurement of Pore Size, Porosity, Thickness, Surface Area The whole is backed by a generous use of color photographs to illustrate the described procedures in detail, plus a bibliography of further literature pertinent to a wide range of application fi elds. For materials scientists, chemists, physicists, optical physicists, biomaterials scientists, neurobiologists, bioengineers, and graduate students in those fields, as well as those working in the semiconductor industry.

Leseprobe

Table of Contents

Cover

Related Titles

Title page

Copyright page

Preface

1 Fundamentals of Porous Silicon Preparation

1.1 Introduction

1.2 Chemical Reactions Governing the Dissolution of Silicon

1.3 Experimental Set-up and Terminology for Electrochemical Etching of Porous Silicon

1.4 Electrochemical Reactions in the Silicon System

1.5 Density, Porosity, and Pore Size Definitions

1.6 Mechanisms of Electrochemical Dissolution and Pore Formation

1.7 Resume of the Properties of Crystalline Silicon

1.8 Choosing, Characterizing, and Preparing a Silicon Wafer

2 Preparation of Micro-, Meso-, and Macro-Porous Silicon Layers

2.1 Etch Cell: Materials and Construction

2.2 Power Supply

2.3 Other Supplies

2.4 Safety Precautions and Handling of Waste

2.5 Preparing HF Electrolyte Solutions

2.6 Cleaning Wafers Prior to Etching

2.7 Preparation of Microporous Silicon from a p-Type Wafer

2.8 Preparation of Mesoporous Silicon from a p++-Type Wafer

2.9 Preparation of Macroporous, Luminescent Porous Silicon from an n-Type Wafer (Frontside Illumination)

2.10 Preparation of Macroporous, Luminescent Porous Silicon from an n-Type Wafer (Back Side Illumination)

2.11 Preparation of Porous Silicon by Stain Etching

2.12 Preparation of Silicon Nanowire Arrays by Metal-Assisted Etching

3 Preparation of Spatially Modulated Porous Silicon Layers

3.1 Time-Programmable Current Source

3.2 Pore Modulation in the z-Direction: Double Layer

3.3 Pore Modulation in the z-Direction: Rugate Filter

3.4 More Complicated Photonic Devices: Bragg Stacks, Microcavities, and Multi-Line Spectral Filters

3.5 Lateral Pore Gradients (in the x–y Plane)

3.6 Patterning in the x–y Plane Using Physical or Virtual Masks

3.7 Other Patterning Methods

4 Freestanding Porous Silicon Films and Particles

4.1 Freestanding Films of Porous Silicon-“Lift-offs”

4.2 Micron-scale Particles of Porous Silicon by Ultrasonication of Lift-off Films

4.3 Core–Shell (Si/SiO2) Nanoparticles of Luminescent Porous Silicon by Ultrasonication

5 Characterization of Porous Silicon

5.1 Gravimetric Determination of Porosity and Thickness

5.2 Electron Microscopy and Scanned Probe Imaging Methods

5.3 Optical Reflectance Measurements

5.4 Porosity, Pore size, and Pore Size Distribution by Nitrogen Adsorption Analysis (BET, BJH, and BdB Methods)

5.5 Measurement of Steady-State Photoluminescence Spectra

5.6 Time-Resolved Photoluminescence Spectra

5.7 Infrared Spectroscopy of Porous Silicon

6 Chemistry of Porous Silicon

6.1 Oxide-Forming Reactions of Porous Silicon

6.2 Biological Implications of the Aqueous Chemistry of Porous Silicon

6.3 Formation of Silicon–Carbon Bonds

6.4 Thermal Carbonization Reactions

6.5 Conjugation of Biomolecules to Modified Porous Silicon

6.6 Chemical Modification in Tandem with Etching

6.7 Metallization Reactions of Porous Silicon

Appendix A1. Etch Cell Engineering Diagrams and Schematics

Standard or Small Etch Cell-Complete

Standard Etch Cell Top Piece

Small Etch Cell Top Piece

Etch Cell Base (for Either Standard or Small Etch Cell)

Large Etch Cell-Complete

Large Etch Cell Top Piece

Large Etch Cell Base

Appendix A2. Safety Precautions When Working with Hydrofluoric Acid

Hydrofluoric Acid Hazards

First Aid Measures for HF Contact

Note to Physician

HF Antidote Gel

Appendix A3. Gas Dosing Cell Engineering Diagrams and Schematics

Gas Dosing Cell Top Piece

Gas Dosing Cell Middle Piece

Gas Dosing Cell Bottom Piece

Index

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The Author

Prof. Dr. Michael J. Sailor

University of California Mail C 0358

Chemistry and Biochemistry

9500 Gilman Drive

La Jolla, CA 92093-0358

USA

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Preface

This book is written for the beginner – someone who has no prior training in the field. It began as a series of summer tutorial lectures that I gave to my research group to familiarize them with the preparation and characterization of porous silicon. I found that the traditional undergraduate chemistry, biochemistry, bioengineering, physics, or materials science curriculum does not prepare one to work with porous silicon – most of my students would come into the group with no understanding of the electrochemical methods needed to carry out its synthesis, little appreciation for the fundamental semiconductor physics, electronics, chemistry, and optics principles needed to exploit its properties, and a sizable fear of the hydrofluoric acid used in its preparation. The tutorials resulted from my frustration that the basic conceptual and experimental “tricks of the trade” were not being passed from one student to the next. My goal was to provide my students with all that I thought they needed to know to get started in their research projects and survive the grilling of their second year oral committee. I provided laboratory “homework” experiments to get the students comfortable with the equipment and the techniques we use. The experiments in Chapters 1–5 are a direct result of these homework assignments. They are structured, step-by-step procedures with well-characterized results. I wrote them to allow me to correct obvious errors in laboratory technique or understanding before the student embarked on his or her independent research project, where errors are not as easily caught and carry significant consequences. The large increase in interest in porous silicon in the past few years, and the numerous email messages I have been receiving from students in groups around the world, asking me for details of our synthetic and optical analysis methods, gives me hope that more than my own research group members will make use of this material.

In the summer of 2004 I was fortunate to meet Esther Levy from Wiley-VCH, who, along with Martin Ottmar, encouraged me to convert my tutorial into a book. I thank them and the rest of the publishing team at Wiley-VCH for their patience during the several years spanning the writing and production of this work.

Many of my coworkers and collaborators contributed the ideas, concepts, and images that make up a large part of this book. In particular, I thank Gordon M. Miskelly, Giuseppe Barillaro, Andrea Potocny, Manuel Orosco, Sophia Oller, Ester Segal, M. Shaker Salem, Yukio H. Ogata, Stephanie Pace, Frederique Cunin, Jean-Marie Devoiselle, Luo Gu, Joseph Lai, Emily Anglin, Beniamino Sciacca, Michelle Y. Chen, Sara Alvarez, Anne M. Ruminski, Adrian Garcia Sega, and Vinh Diep.

Finally, I thank my family for putting up with the late nights, early mornings, and missed dinner appointments they suffered as I went through this process.

Michael J. Sailor

La Jolla

August 2011

1

Fundamentals of Porous Silicon Preparation

1.1 Introduction

Porous silicon was accidentally discovered by the Uhlirs, a husband and wife team working at Bell Laboratories in the mid 1950s. They were trying to develop an electrochemical method to machine silicon wafers for use in microelectronic circuits. Under the appropriate electrochemical conditions, the silicon wafer did not dissolve uniformly as expected, but instead fine holes appeared, propagating primarily in the <100> direction in the wafer. Since this did not provide the smooth polish desired, the curious result was reported in a Bell labs technical note [1], and then the material was more or less forgotten. In the 1970s and 1980s a significant level of interest arose because the high surface area of porous silicon was found to be useful as a model of the crystalline silicon surface in spectroscopic studies [2–5], as a precursor to generate thick oxide layers on silicon, and as a dielectric layer in capacitance-based chemical sensors [6].

Interest in porous silicon, and in particular in its nanostructure, exploded in the early 1990s when Ulrich Goesele at Duke University identified quantum confinement effects in the absorption spectrum of porous silicon, and almost simultaneously Leigh Canham at the Defense Research Agency in England reported efficient, bright red–orange photoluminescence from the material [7, 8]. The quantum confinement effects arise when the pores become extensive enough to overlap with each other, generating nanometer-scale silicon filaments. As expected from the quantum confinement relationship [9], the red to green color of photoluminescence occurs at energies that are significantly larger than the bandgap energy of bulk silicon (1.1 eV, in the near-infrared).

With the discovery of efficient visible light emission from porous silicon came a flood of work focused on creating silicon-based optoelectronic switches, displays, and lasers. Problems with the material’s chemical and mechanical stability, and its disappointingly low electroluminescence efficiency led to a waning of interest by the mid 1990s. In the same time period, the unique features of the material – large surface area, controllable pore sizes, convenient surface chemistry, and compatibility with conventional silicon microfabrication technologies – inspired research into applications far outside optoelectronics. Many of the fundamental chemical stability problems have been overcome as the chemistry of the material has matured, and various biomedical [10–18] sensor, optics, and electronics applications have emerged [10].

Porous silicon is generated by etching crystalline silicon in aqueous or non-aqueous electrolytes containing hydrofluoric acid (HF). This book describes basic electrochemical and chemical etching experiments that can be used to make the main types and structures of porous silicon. Beginning with measurement of wafer resistivity, the experiments are intended for the newcomer to the field, written in the form of detailed procedures, including sources for the materials and equipment. Experiments describing methods for characterization and key chemical modification reactions are also provided. The present chapter gives an overview of fundamentals that are a useful starting point to understand the theory underlying the experiments in the later chapters.