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Band-Gap Engineering of Germanium Monolithic Light Sources Using Tensile Strain and N-Type Doping

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Band-Gap Engineering of Germanium Monolithic Light Sources Using Tensile Strain and N-Type Doping UNIVERSITY OF SOUTHAMPTON FACULTY OF PHYSICAL SCIENCES AND ENGINEERING Electronics and Computer Science Nano-Electronics & Nano-Technology Research Group Band-gap Engineering of Germanium Monolithic Light Sources using Tensile Strain and n-type Doping by Abdelrahman Zaher Rateb Al-Attili Thesis for the degree of Doctor of Philosophy October 2016 UNIVERSITY OF SOUTHAMPTON FACULTY OF PHYSICAL SCIENCES AND ENGINEERING Electronics and Computer Science Nano-Electronics & Nano-Technology Research Group Band-gap Engineering of Germanium Monolithic Light Sources using Tensile Strain and n-type Doping by Abdelrahman Zaher Rateb Al-Attili Jury Supervisors Dr. Aleksey Andreev Prof. Shinichi Saito Prof. James S. Wilkinson Dr. Frederic Gardes Dr. Xu Fang Thesis for the degree of Doctor of Philosophy October 2016 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF PHYSICAL SCIENCES AND ENGINEERING Electronics and Computer Science Doctor of Philosophy BAND-GAP ENGINEERING OF GERMANIUM MONOLITHIC LIGHT SOURCES USING TENSILE STRAIN AND N-TYPE DOPING by Abdelrahman Zaher Rateb Al-Attili Band-gap engineering of bulk germanium (Ge) holds the potential for realizing a laser source, permitting full integration of monolithic circuitry on CMOS platforms. Tech- niques rely mainly on tensile strain and n-type doping. In this thesis, we focus on study- ing diffusion-based phosphorus (P) doping of Ge using spin-on dopants (SOD), and ten- sile strain engineering using freestanding micro-electro-mechanical systems (MEMS)-like structures. Process development of a reliable SOD recipe was conducted using furnace and rapid-thermal annealing, and successful doping up to 2:5 × 1019cm−3 was achieved, resulting in approximately 10× enhancement in direct-gap emission. A transition in Ge direct-gap photoluminescence (PL) behavior is observed upon doping, from being quadratically dependent on excitation power to linear. We have also demonstrated that the limited doping concentration of P in Ge using SOD is not source limited, but more probably related to the diffusion mechanism. The other part of the project concentrated on Ge strain engineering. Previous works reported high tensile strain values based on freestanding MEMS-like structures made of Ge, yet without embedding an optical cavity (until recently). In this project, we realize this combination by fabricating Ge micro-disks as an optical cavity on top of freestanding SiO2 structures, utilizing Ge-on- Insulator wafers (GOI). 3D computer simulations were used to understand and optimize the devices, in terms of strain and optical performance. Raman spectroscopy and PL measurements confirmed simulation results showing higher tensile strain for beams with shorter lengths, with a maximum uniaxial strain of 1.3%. Splitting of light and heavy- hole energy bands was observed by PL as the strain increases, agreeing with theoretical models. Direct-gap sharp-peak whispering-gallery modes (WGMs) were confined in 3 µm disks with a maximum quality-factor of ∼ 200. Two loss mechanisms could be distin- guished, red-shift of the absorption edge, and free-carrier absorption. In order to avoid these excitation-related losses, higher strain values combined with heavy n-type doping are required. A possible implementation using the same GOI platform is proposed for future work. Contents Declaration of Authorship xvii Acknowledgements xix Abbreviations xxi Nomenclature xxv 1 Introduction1 1.1 Motivation...................................1 1.2 Strategies for Realizing Ge Lasers.......................4 1.2.1 Aims of this Project..........................5 1.3 Thesis Outline.................................6 2 Background and Literature7 2.1 Introduction...................................7 2.2 Ge as a CMOS-Compatible Light Source...................8 2.3 Strain Application............................... 10 2.3.1 Band-Gap Deformation........................ 10 2.3.2 Application Methodologies...................... 13 2.3.2.1 Thermal Strain........................ 13 2.3.2.2 Buffer Layers or Virtual Substrates............ 14 2.3.2.3 Alloying............................ 15 2.3.2.4 Local Stress Liner or External Stressors.......... 16 2.3.2.5 MEMS-Like Structures................... 18 2.4 Heavy n-type Doping.............................. 21 2.4.1 Theoretical Effects of n-type Doping................. 22 2.4.2 Overview of Ge Doping Techniques.................. 24 2.4.2.1 Ion Implantation....................... 26 2.4.2.2 in-situ Doping........................ 26 2.4.2.3 Diffusion Based Techniques................. 27 2.5 Crystalline Quality: Wafer Options...................... 31 2.6 Photonic Cavity................................ 32 2.6.1 WGM Optical Cavities: Micro-Disks & Rings............ 33 2.6.1.1 Whispering Gallery Modes................. 33 2.6.1.2 Q-factor............................ 35 2.7 Optical Gain.................................. 36 vii viii CONTENTS 2.7.1 Strain & Doping: Feedback from Recent Highly-Strained Structures.......... 37 2.7.2 Further Approaches.......................... 38 2.8 Applications................................... 40 2.9 Summary.................................... 41 3 Spin-on Doping of Thin Germanium-on-Insulator Films using Furnace Annealing 43 3.1 Introduction................................... 43 3.2 Diffusion of Dopants.............................. 44 3.3 Spin-on Doping................................. 48 3.4 Experiment................................... 49 3.4.1 Spin-on Doping Process........................ 49 3.4.2 Water-Based Solution......................... 51 3.4.3 Alcohol-Based Solution........................ 52 3.5 Results and Discussion............................. 54 3.5.1 Raman Spectroscopy.......................... 55 3.5.2 Photoluminescence Measurements.................. 57 3.5.3 Excitation-Power Dependence..................... 58 3.6 Summary.................................... 62 4 Optimization of Spin-on Doping using Rapid-Thermal Annealing 63 4.1 Introduction................................... 63 4.2 High-Temperature Short-Duration Annealing................ 64 4.3 Reference Processing on Silicon........................ 66 4.4 Germanium Spin-on Doping using RTA................... 67 4.4.1 Filmtronics P507............................ 67 4.4.2 Higher Source Concentration: Filmtronics P509........... 70 4.4.2.1 Thin Si Interface Layer to Overcome Compatibility Issues 71 4.4.2.2 Ge-on-SOI Doping with 10-nm Si Interface Layer..... 72 4.5 Results and Discussion............................. 73 4.5.1 Raman Spectroscopy and Photoluminescence............ 75 4.6 Summary.................................... 77 5 Study of Strain Application using Freestanding SiO2 Beams 79 5.1 Introduction................................... 79 5.2 Freestanding MEMS-Like Structures for Strain Application............................. 80 5.3 Strain by Freestanding SiO2 Beams...................... 81 5.4 Finite-Element Simulations.......................... 84 5.4.1 Normal Strain Components and Volumetric Strain......... 85 5.4.2 Manipulation of Strain Orientation by Beam Design........ 86 5.4.3 Strain Uniformity........................... 88 5.4.4 Effect of Beam Dimensions on Strain Value............. 89 5.4.5 Strain Enhancement by BOX Layer Manipulation......... 91 5.5 Beam Designs.................................. 92 5.6 Strain Characterization by Raman Spectroscopy.............. 93 5.6.1 Power Dependence of Raman Shift.................. 93 CONTENTS ix 5.6.2 Effect of Beam Dimensions...................... 96 5.6.3 Uniaxial and Biaxial Beams...................... 97 5.7 Photoluminescence Measurements...................... 98 5.7.1 Power Dependence of Photoluminescence Spectrum......... 98 5.7.2 Effect of Beam Dimensions...................... 100 5.8 A Comparative Study with Ge-on-SOI Wafers................ 103 5.8.1 Finite-Element Simulations...................... 103 5.8.1.1 Strain Distribution...................... 104 5.8.1.2 Beam Length Dependence................. 108 5.8.1.3 PECVD Layer Engineering................. 109 5.8.2 Raman Spectroscopy.......................... 110 5.9 Summary.................................... 111 6 Germanium Micro-disks on Freestanding SiO2 Beams 113 6.1 Introduction................................... 113 6.2 Ge Micro-Disks & Rings............................ 114 6.3 Structure Layout and Fabrication....................... 116 6.3.1 Ge Patterning.............................. 117 6.3.2 Surface Passivation........................... 118 6.3.3 Beam Patterning and Suspension................... 121 6.4 Final Device Structure............................. 123 6.5 Strain Characterization by Raman Spectroscopy.............. 126 6.6 Whispering-Gallery-Mode Resonances.................... 128 6.6.1 Finite-Difference Time-Domain Simulations............. 128 6.6.2 Photoluminescence Measurements.................. 129 6.6.2.1 Excitation Power Dependence............... 131 6.6.2.2 Quality Factors....................... 132 6.6.2.3 Optical Losses & Required Improvements........ 135 6.7 Summary.................................... 136 7 Conclusions and Future Work 137 7.1 Conclusions & Contribution.......................... 137 7.2 Future Outlook................................. 140 7.2.1 Techniques for Higher Doping Levels................. 140 7.2.2 Optimization of Strain and Cavity.................. 141 7.2.3 Lower-Dimensions using the Same Platform............. 142 A Selected Fabrication Processes 143 A.1 Amorphous Si Deposition using PECVD................... 143 A.2 SiO2 Deposition using PECVD.......................
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