Characterization and integration of defect-mediated photodetectors for silicon photonic circuits
Subtitle: Ph.D. Thesis - Department of Engineering Physics, McMaster University, Hamilton, Canada
Publisher: Library and Archives of Canada
Published: 2011
Number of pages: 245
ISBN-13: 9780494739471
Description: Silicon photonic interconnects represent a possible solution to the problems associated with scaling electrical interconnects to higher data rates. Defect mediated optical detectors fabricated via inert ion implantation incorporated with optical waveguides in such a circuit allow control and monitoring of the optical signal and can be fabricated entirely using standard Complementery Metal-Oxide Semiconductor (CMOS) fabrication steps, thus simplifying the task of integrating optical functionality alongside electronic circuits on a chip.

In the work described in this thesis, the defects involved in the detection process at wavelengths at and around 1550 nm have been investigated. For low-dose ion implantation, the defect which dominates optical absorption and detection is shown to be an acceptor with an energy of 0.41 ± 0.02 eV below the conduction band, properties consistent with the silicon divacancy. A novel technique for optical characterization has been developed and used to determine the optical cross-section of this defect to be 6 x 10-17 cm2 for a wavelength of 1550 nm. Defect concentration versus thermal annealing temperature was evaluated, and waveguide integrated detectors were fabricated and used to characterize optical absorption and quantum efficiency as a function of the annealing conditions. A monolithically integrated device comprising a defect-enhanced photodiode and a variable optical attenuator was designed, modeled, fabricated, and demonstrated with the photodiode used to monitor and control the optical output power. To illustrate the potential of such a device, it was operated as a dynamic channel leveller with external software used to mimic an electronic feedback loop between the detector and attenuator. The channel leveller was able to maintain ± 1 dB output power variation across a 7-10 dB input dynamic range for wavelengths ranging from 1530 nm to 1570 nm.

Additionally, a tunable resonant defect-enhanced photodiode was designed, modeled, fabricated, and characterized. It is shown that resonant effects can be used to obtain similar responsivity from a small, lightly absorbent photodiode as would otherwise require a longer, highly absorbent photodiode, thus enabling very low minimum detectable power. The device responsivity was measured to be greater than 0.1 A/W for wavelengths ranging from 1510 nm to 1600 nm, with individual wavelengths selectable by tuning the resonance peak. The device was also demonstrated as a variable optical attenuator with 20 dB extinction, less than 44 mW switching power, and integrated monitoring.

The devices described in this thesis represent the first reported demonstrations of integrated optical power control, dynamic channel levelling, and tunable resonant detection using defect mediation in a silicon photonic circuit.

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Non nobis Domine, non nobis, sed Nomini Tuo da gloriam.
Ps. 113:9


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This thesis is dedicated to the memory of Marcus Berquist (1934-2010).
Requiescat in pace.

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Table of contents: title: List of Figures ...................................................................................................... xvi; title: List of Tables ....................................................................................................... xix; title: 1 Introduction..........................................................................................................1; title: 1.1 Optical versus electrical interconnects.................................................... 1; title: 1.1.1 Device density................................................................................. 2; title: 1.1.2 Bandwidth in competition with signal power ................................. 3; title: 1.1.3 Advantages of optical interconnects ............................................... 4; title: 1.1.4 Chip-chip considerations ................................................................ 5; title: 1.1.5 Challenges associated with optical interconnects ........................... 5; title: 1.2 Silicon photonics..................................................................................... 6; title: 1.2.1 Silicon-on-insulator......................................................................... 7; title: 1.2.2 Optical functions demonstrated in silicon....................................... 9; title: 1.2.3 Optical transmission........................................................................ 9; title: 1.2.4 Off-chip coupling.......................................................................... 10; title: 1.2.5 Light generation and amplification............................................... 14; title: 1.2.6 Modulation and variable attenuation ............................................ 15; title: 1.2.7 Photodetectors............................................................................... 18; title: 1.3 Defect-mediated photodetection ........................................................... 20; title: 1.3.1 Defect considerations.................................................................... 20; title: 1.4 Thesis purpose and approach ................................................................ 22; title: 1.4.1 Defect characterization ................................................................. 23; title: 1.4.2 Integration of multiple on-chip functions ..................................... 23; title: 1.5 Summary of contributions to the field .................................................. 24; title: 2 Theory and background .................................................................................25; title: 2.1 Defect states within the band gap ......................................................... 25; title: 2.1.1 Band gap ....................................................................................... 26; title: 2.1.2 Localized states............................................................................. 28; title: 2.1.3 Band-to-band optical absorption in silicon ................................... 29; title: 2.1.4 Carrier generation/recombination kinetics.................................... 30; title: 2.1.5 Generation, recombination and trapping....................................... 32; title: 2.1.6 Steady-state recombination rate.................................................... 32; title: 2.2 p-n junction theory................................................................................ 34; title: 2.2.1 Effect of trap density on junction current ..................................... 36; title: 2.3 Optical propagation in planar waveguides............................................ 37; title: 2.3.1 Ray optics approach...................................................................... 43; title: 2.4 Three-dimensional waveguides ............................................................ 44; title: 2.5 Effective index method ......................................................................... 44; title: 2.6 Beam propagation method .................................................................... 45; title: 2.6.1 BPM approach .............................................................................. 46; title: 2.6.2 Computing mode solutions using BPM ........................................ 47; title: 2.7 Coupled mode theory............................................................................ 48; title: 2.7.1 Derivation of coupled-mode equations......................................... 49; title: 2.7.2 Power transfer ............................................................................... 51; title: 2.7.3 Coupler transfer matrix ................................................................. 54; title: 2.8 CMOS processing ................................................................................. 55; title: 2.8.1 Materials ....................................................................................... 56; title: 2.8.2 Etching.......................................................................................... 57; title: 2.8.3 Lithography................................................................................... 57; title: 2.8.4 Ion implantation ............................................................................ 58; title: 3 Defect characterization ..................................................................................61; title: 3.1 Defects in silicon................................................................................... 61; title: 3.1.1 Known implantation-induced defects ........................................... 61; title: 3.1.2 Reported optical absorption near 1.5 μm...................................... 62; title: 3.1.3 Mechanism of absorption.............................................................. 63; title: 3.2 Standard defect characterization techniques......................................... 64; title: 3.2.1 Capacitance-voltage profiling....................................................... 64; title: 3.2.2 Current-voltage characteristics ..................................................... 66; title: 3.2.3 Thermally stimulated capacitance (TSCAP) ................................ 66; title: 3.2.4 Capacitance Transient Spectroscopy ............................................ 68; title: 3.2.5 Deep Level Transient Spectroscopy ............................................. 76; title: 3.3 Device fabrication................................................................................. 81; title: 3.3.1 Starting wafers and preparation .................................................... 81; title: 3.3.2 Junction definition and N+ doping ................................................ 81; title: 3.3.3 Ion implantation ............................................................................ 83; title: 3.3.4 Dicing and defect annealing.......................................................... 85; title: 3.3.5 Metal contacts ............................................................................... 85; title: 3.3.6 Bond pads...................................................................................... 86; title: 3.3.7 Fabrication issues.......................................................................... 87; title: 3.4 Results................................................................................................... 89; title: 3.4.1 Current-voltage measurements ..................................................... 89; title: 3.4.2 Capacitance-voltage measurements .............................................. 92; title: 3.4.3 DLTS measurements..................................................................... 95; title: 3.4.4 Equilibrium Capacitance Spectroscopy (ECS) Technique ......... 105; title: 3.4.5 Trap concentration profiles ......................................................... 113; title: 3.5 Waveguide integrated photodetectors................................................. 118; title: 3.5.1 Ion implantation .......................................................................... 119; title: 3.5.2 Characterization .......................................................................... 120; title: 3.5.3 Correlation of defect properties and photodetector performance 126; title: 3.6 Chapter summary ................................................................................ 128; title: 4 Photodiode integration with a variable optical attenuator ...........................129; title: 4.1 Introduction......................................................................................... 130; title: 4.1.1 Integration of VOA and detection functionality ......................... 130; title: 4.1.2 Dynamic optical channel equalization ........................................ 130; title: 4.2 Theory................................................................................................. 131; title: 4.2.1 Mach-Zehnder Interferometer transfer function ......................... 131; title: 4.2.2 MZI design considerations.......................................................... 133; title: 4.2.3 Tap Monitor Considerations ....................................................... 134; title: 4.2.4 Thermo-optic tuning ................................................................... 135; title: 4.2.5 Mathematical Equivalence of Thermal to Electrical Parameters 136; title: 4.2.6 Thermal Circuit Model ............................................................... 137; title: 4.2.7 Derivation of temperature as a function of lateral distance along; title: the SOI slab................................................................................................. 139; title: 4.2.8 Thermo-optical phase shift ......................................................... 142; title: 4.2.9 Thermo-optic tuning speed ......................................................... 144; title: 4.3 Device design...................................................................................... 148; title: 4.3.1 Waveguide geometry .................................................................. 148; title: 4.3.2 Directional couplers.................................................................... 148; title: 4.3.3 Photodiode doping regions ......................................................... 150; title: 4.3.4 Photodiode defect implantation .................................................. 151; title: 4.3.5 Thermo-optic tuners.................................................................... 153; title: 4.3.6 Integrated device......................................................................... 154; title: 4.4 Fabrication process ............................................................................. 154; title: 4.5 Integrated VOA / Photodiode Characterization.................................. 156; title: 4.5.1 Test setup .................................................................................... 156; title: 4.5.2 Variable attenuation .................................................................... 157; title: 4.5.3 Tuning Efficiency ....................................................................... 160; title: 4.5.4 Tuning Speed .............................................................................. 162; title: 4.5.5 Integrated photodiode results...................................................... 163; title: 4.5.6 Signal-to-noise ratio.................................................................... 165; title: 4.6 Channel Leveller................................................................................. 166; title: 4.6.1 Concept ....................................................................................... 166; title: 4.6.2 Output path considerations ......................................................... 167; title: 4.6.3 Levelling algorithm..................................................................... 168; title: 4.6.4 Test setup .................................................................................... 171; title: 4.6.5 Results......................................................................................... 171; title: 4.6.6 Limitations on dynamic range .................................................... 173; title: 4.7 Further work........................................................................................ 175; title: 4.7.1 Spatial filters ............................................................................... 175; title: 4.7.2 MZI power consumption............................................................. 175; title: 4.7.3 MZI couplers............................................................................... 176; title: 4.7.4 Alternate MZI tuning .................................................................. 176; title: 4.7.5 Carrier-injection attenuation ....................................................... 176; title: 4.7.6 Ring-resonator enhanced detection............................................. 177; title: 4.7.7 On-chip electronics ..................................................................... 177; title: 4.7.8 Cascaded devices ........................................................................ 177; title: 4.8 Chapter summary ................................................................................ 178; title: 5 Resonator integration ...................................................................................179; title: 5.1 Ring resonators in silicon.................................................................... 179; title: 5.2 Theoretical considerations .................................................................. 181; title: 5.2.1 Responsivity versus length ......................................................... 182; title: 5.2.2 Signal-to-noise ratio considerations............................................ 183; title: 5.2.3 Resonant power........................................................................... 184; title: 5.2.4 Cavity enhancement and MDP ................................................... 188; title: 5.2.5 Practical considerations regarding MDP .................................... 189; title: 5.2.6 Modeling resonant detection....................................................... 189; title: 5.3 Design and fabrication ........................................................................ 190; title: 5.3.1 Ring loss considerations ............................................................. 191; title: 5.3.2 Directional coupler design .......................................................... 192; title: 5.4 Test setup ............................................................................................ 193; title: 5.5 Results................................................................................................. 194; title: 5.5.1 Test structures ............................................................................. 194; title: 5.5.2 Leakage current........................................................................... 196; title: 5.5.3 Responsivity versus wavelength ................................................. 196; title: 5.5.4 Cavity enhancement and MDP ................................................... 200; title: 5.5.5 Comparison to other reported results.......................................... 201; title: 5.5.6 Performance over the C-band ..................................................... 201; title: 5.5.7 Tuning......................................................................................... 203; title: 5.6 Suggested improvements .................................................................... 206; title: 5.6.1 Tuning efficiency ........................................................................ 206; title: 5.6.2 Bandwidth................................................................................... 206; title: 5.7 Chapter summary ................................................................................ 207; title: 6 Conclusions..................................................................................................209; title: 6.1 Defect characterization ....................................................................... 209; title: 6.2 Device integration............................................................................... 210; title: 6.3 Further work........................................................................................ 211; title: 6.3.1 Deep donor states........................................................................ 211; title: 6.3.2 Bistability.................................................................................... 211; title: 6.3.3 SOI-specific characterization...................................................... 212; title: 6.3.4 Device integration....................................................................... 212; title: 6.4 Future of silicon photonics.................................................................. 213; title: A Photolithography.............................................................................................215; title: A.1 Photoresist recipes .............................................................................. 215; title: A.3.1 Shipley 1827 ............................................................................... 215; title: A.3.2 Futurrex NR9 .............................................................................. 216; title: A.2 Photomasks used for o-DLTS fabrication process.............................. 216; title: A.3 Photomasks used for LETI fabrication process .................................. 219; title: B Determination of photon flux density .............................................................223; title: B.1 Optical beam profile ........................................................................... 223; title: B.2 Calculation of average photon flux density at the diode..................... 225; title: B.2.1 Average irradiance corresponding to exposed region................. 225; title: B.2.2 Optical reflections....................................................................... 225; title: B.2.3 Photon flux density ..................................................................... 226; title: C LabVIEW code................................................................................................227; title: C.1 DLTS system ...................................................................................... 227; title: C.2 Channel leveller .................................................................................. 227; title: References............................................................................................................233