Preface ........................................................ XI
1 Fundamentals of Porous Silicon Preparation ................... 1
1.1 Introduction ............................................ 1
1.2 Chemical Reactions Governing the Dissolution of
Silicon ................................................. 2
1.2.1 Silicon Oxides and Their Dissolution in HF ....... 3
1.2.2 Silicon Oxides and Their Dissolution in Basic
Media ............................................ 3
1.2.3 Silicon Hydrides ................................. 4
1.3 Experimental Set-up and Terminology for
Electrochemical Etching of Porous Silicon ............... 5
1.3.1 Two-Electrode Cell ............................... 6
1.3.2 Three-Electrode Cell ............................. 6
1.4 Electrochemical Reactions in the Silicon System ......... 7
1.4.1 Four-Electron Electrochemical Oxidation of
Silicon .......................................... 8
1.4.2 Two-Electron Electrochemical Oxidation of
Silicon .......................................... 9
1.4.3 Electropolishing ................................ 10
1.5 Density, Porosity, and Pore Size Definitions ........... 11
1.6 Mechanisms of Electrochemical Dissolution and Pore
Formation .............................................. 13
1.6.1 Chemical Factors Controlling the
Electrochemical Etch ............................ 16
1.6.2 Crystal Face Selectivity ........................ 18
1.6.3 Physical Factors Controlling the
Electrochemical Etch ............................ 18
1.7 Resume of the Properties of Crystalline Silicon ........ 19
1.7.1 Orientation ..................................... 19
1.7.2 Band Structure .................................. 20
1.7.3 Electrons and Holes ............................. 21
1.7.4 Photoexcitation of Semiconductors ............... 22
1.7.5 Dopants ......................................... 23
1.7.6 Conductivity .................................... 24
1.7.7 Evolution of Energy Bands upon Immersion in an
Electrolyte ..................................... 24
1.7.8 Charge Transport at p-Type Si Liquid
Junctions ....................................... 26
1.7.9 Idealized Current-Voltage Curve at p-Type
Liquid functions ................................ 26
1.7.10 Energetics at n-Type Si Liquid Junctions ........ 28
1.7.11 Idealized Current-Voltage Curve at n-type
Liquid Junctions ................................ 28
1.8 Choosing, Characterizing, and Preparing a Silicon
Wafer .................................................. 28
1.8.1 Measurement of Wafer Resistivity ................ 29
1.8.2 Cleaving a Silicon Wafer ........................ 34
1.8.3 Determination of Carrier Type by the Hot-Probe
Method .......................................... 36
1.8.4 Ohmic Contacts .................................. 36
1.8.4.1 Making an Ohmic Contact by Metal
Evaporation ............................ 39
1.8.4.2 Making an Ohmic Contact by Mechanical
Abrasion ............................... 40
References ............................................... 40
2 Preparation of Micro-, Meso-, and Macro-Porous Silicon
Layers ...................................................... 43
2.1 Etch Cell: Materials and Construction .................. 43
2.2 Power Supply ........................................... 44
2.3 Other Supplies ......................................... 48
2.4 Safety Precautions and Handling of Waste ............... 48
2.5 Preparing HF Electrolyte Solutions ..................... 50
2.6 Cleaning Wafers Prior to Etching ....................... 51
2.6.1 No Precleaning .................................. 51
2.6.2 Ultrasonic Cleaning ............................. 51
2.6.3 RCA Cleaning .................................... 52
2.6.4 Removal of a Sacrificial Porous Layer with
Strong Base ..................................... 52
2.7 Preparation of Microporous Silicon from a p-Type
Wafer .................................................. 53
2.8 Preparation of Mesoporous Silicon from a p++-Type
Wafer .................................................. 57
2.9 Preparation of Macroporous, Luminescent Porous
Silicon from an n-Type Wafer (Frontside
Illumination) .......................................... 59
2.9.1 Power Supply Limitations ........................ 63
2.10 Preparation of Macroporous, Luminescent Porous
Silicon from an n-Type Wafer (Back Side
Illumination) .......................................... 64
2.11 Preparation of Porous Silicon by Stain Etching ......... 68
2.12 Preparation of Silicon Nanowire Arrays by Metal-
Assisted Etching ....................................... 73
References .................................................. 75
3 Preparation of Spatially Modulated Porous Silicon Layers .... 77
3.1 Time-Programmable Current Source ....................... 78
3.1.1 Time Resolution Issues .......................... 79
3.1.2 Etching with an Analog Source ................... 80
3.1.3 Etching with a Digital Source ................... 82
3.2 Pore Modulation in the z-Direction: Double Layer ....... 83
3.3 Pore Modulation in the z-Direction: Rugate Filter ...... 83
3.3.1 Tunability of the Rugate Spectral Peak
Wavelength ...................................... 88
3.3.2 Width of the Spectral Band ...................... 92
3.4 More Complicated Photonic Devices: Bragg Stacks,
Microcavities, and Multi-Line Spectral Filters ......... 94
3.4.1 Bragg Reflector ................................. 96
3.4.2 Multiple Spectral Peaks-"Spectral Barcodes" .... 100
3.5 Lateral Pore Gradients (in the x-γ Plane) ............. 104
3.6 Patterning in the x-γ Plane Using Physical or
Virtual Masks ......................................... 108
3.6.1 Physical Masking Using Photoresists ............ 109
3.6.2 Virtual Masking Using Photoelectrochemistry .... 112
3.7 Other Patterning Methods .............................. 114
References ............................................ 114
4 Freestanding Porous Silicon Films and Particles ............ 119
4.1 Freestanding Films of Porous Silicon-"Lift-offs" ...... 120
4.2 Micron-scale Particles of Porous Silicon by
Ultrasonication of Lift-off Films ..................... 120
4.3 Core-Shell (Si/SiO2) Nanoparticles of Luminescent
Porous Silicon by Ultrasonication ..................... 126
References ............................................ 130
5 Characterization of Porous Silicon ......................... 133
5.1 Gravimetric Determination of Porosity and Thickness ... 134
5.1.1 Errors and Limitations of the Gravimetric
Method ......................................... 137
5.2 Electron Microscopy and Scanned Probe Imaging
Methods ............................................... 138
5.2.1 Cross-Sectional Imaging ........................ 138
5.2.2 Plan-View (Top-Down) Imaging ................... 139
5.3 Optical Reflectance Measurements ...................... 139
5.3.1 Instrumentation to Collect Reflectance Data .... 139
5.3.1.1 Reflectance Optics .................... 140
5.3.1.2 Wavelength Calibration ................ 142
5.3.2 Principles of Fabry-Perot Interference ......... 143
5.3.3 Analyzing Fabry-Perot Interference Spectra
by Fourier Transform: the RIFTS Method ......... 150
5.3.3.1 Preparation of Spectrum for Fast
Fourier Transform ..................... 151
5.3.3.2 Interpretation of the Fast Fourier
Transform ............................. 153
5.3.4 Thickness and Porosity by the Spectroscopic
Liquid Infiltration Method (SLIM) .............. 154
5.3.4.1 Bruggeman Effective Medium
Approximation ......................... 155
5.3.4.2 Determination of Thickness and
Porosity by SLIM ...................... 156
5.3.4.3 Determination of Index of Refraction
of the Porous Skeleton ................ 156
5.3.4.4 Effect of Skeleton Index on Porosity
Determined by SLIM .................... 158
5.3.5 Comparison of Gravimetric Measurement with
SLIM for Porosity and Thickness
Determination .................................. 159
5.3.6 Analysis of Double-Layer Structures Using
RIFTS .......................................... 162
5.4 Porosity, Pore size, and Pore Size Distribution by
Nitrogen Adsorption Analysis (BET, BJH, and BdB
Methods) .............................................. 167
5.5 Measurement of Steady-State Photoluminescence
Spectra ............................................... 170
5.5.1 Origin of Photoluminescence from Porous
Silicon ........................................ 170
5.5.1.1 Tunability of the Photoluminescence
Spectrum .............................. 171
5.5.1.2 Mechanisms of Photoluminescence ....... 171
5.5.2 Instrumentation to Acquire Steady-State
Photoluminescence Spectra ...................... 173
5.6 Time-Resolved Photoluminescence Spectra ............... 173
5.6.1 Long, Nonexponential Excited State Lifetimes ... 173
5.6.2 Influence of Surface Traps ..................... 175
5.7 Infrared Spectroscopy of Porous Silicon ............... 176
5.7.1 Characteristic Group Frequencies for Porous
Silicon ........................................ 176
5.7.2 Measurement of FTIR Spectra of Porous
Silicon ........................................ 178
5.7.2.1 Transmission Mode Measurement Using
the Standard Etch Cell ................ 179
References ................................................. 181
6 Chemistry of Porous Silicon ................................ 189
6.1 Oxide-Forming Reactions of Porous Silicon ............. 190
6.1.1 Temperature Dependence of Oxidation Using
Gas-Phase Oxidants ............................. 190
6.1.2 Thermal (Air) Oxidation ........................ 191
6.1.3 Ozone Oxidation ................................ 192
6.1.4 High-Pressure Water Vapor Annealing ............ 193
6.1.5 Oxidation in Aqueous Solutions ................. 193
6.1.5.1 Aqueous Oxidation Induced by
Cationic Surfactants .................. 194
6.1.6 Electrochemical Oxidation in Aqueous Mineral
Acids .......................................... 194
6.1.7 Oxidation by Organic Species: Ketones,
Aldehydes, Quinones, and Dimethylsulfoxide ..... 195
6.1.8 Effect of Chemical Oxidation on Pore
Morphology ..................................... 196
6.2 Biological Implications of the Aqueous Chemistry of
Porous Silicon ........................................ 198
6.3 Formation of Silicon-Carbon Bonds ..................... 200
6.3.1 Thermal Hydrosilylation to Produce Si-C
Bonds .......................................... 200
6.3.2 Working with Air- and Water-Sensitive
Compounds-Schlenk Line Manipulations ........... 201
6.3.3 Classification of Surface Chemistry by
Contact Angle .................................. 203
6.3.4 Micro wave-Assisted Hydrosilylation to
Produce Si-C Bonds ............................. 204
6.3.5 Chemical or Electrochemical Grafting to
Produce Si-C Bonds ............................. 206
6.4 Thermal Carbonization Reactions ....................... 208
6.4.1 Thermal Degradation of Acetylene to form
"Hydrocarbonized" Porous Silicon ............... 208
6.4.2 Thermal Degradation of Polymers to Form
"Carbonized" Porous Silicon .................... 209
6.5 Conjugation of Biomolecules to Modified Porous
Silicon ............................................... 211
6.5.1 Carbodiimide Coupling Reagents ................. 211
6.5.2 Attachment of PEG to Improve
Biocompatibility ............................... 212
6.5.3 Biomodification of "Hydrocarbonized" Porous
Silicon ........................................ 213
6.5.4 Silanol-Based Coupling to Oxidized Porous
Silicon Surfaces ............................... 215
6.6 Chemical Modification in Tandem with Etching .......... 217
6.7 Metallization Reactions of Porous Silicon ............. 218
References ................................................. 219
Appendix Al. Etch Cell Engineering Diagrams and Schematics .... 229
Standard or Small Etch Cell-Complete ....................... 229
Standard Etch Cell Top Piece ............................... 230
Small Etch Cell Top Piece .................................. 231
Etch Cell Base (for Either Standard or Small Etch Cell) .... 232
Large Etch Cell-Complete ................................... 232
Large Etch Cell Base ....................................... 233
Appendix A2. Safety Precautions When Working with
Hydrofluoric Acid .......................................... 235
Hydrofluoric Acid Hazards .................................. 235
First Aid Measures for HF Contact .......................... 236
Note to Physician .......................................... 238
HF Antidote Gel ............................................ 239
Further Reading ............................................ 239
Appendix A3. Gas Dosing Cell Engineering Diagrams and
Schematics ................................................. 241
Gas Dosing Cell Top Piece .................................. 242
Gas Dosing Cell Middle Piece ............................... 243
Gas Dosing Cell Bottom Piece ............................... 244
Index ......................................................... 245
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