ENGINEERING OF SILICON AND GERMANIUM TUNNEL DIODES FOR INTEGRATED CIRCUIT APPLICATION A Dissertation Submitted to the Graduate School of the University of Notre Dame in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy by Jialin Zhao, B.E., M.S.E.E. ______________________________ Alan C. Seabaugh, Director Graduate Program in Electrical Engineering Notre Dame, Indiana April 2007 © Copyright 2007 Jialin Zhao ENGINEERING OF SILICON AND GERMANIUM TUNNEL DIODES FOR INTEGRATED CIRCUIT APPLICATIONS Abstract by Jialin Zhao In recent years, the tunnel diode has attracted interest from companies and researchers. Integrating tunnel diodes with transistors provide input-out isolation, gain and fan-out ability which the tunnel diode by itself lacks. The sparsity of tunnel diode fabrication processes compatible with transistor processing hinders the wider use of the device. Fabrication processes, which could be applied to integrated tunnel diode/transistor circuits on Si and Ge, are explored in this work. Silicon tunnel diodes were demonstrated in both vertical and lateral geometries using spin-on diffusants and rapid thermal processing. Silicon tunnel diodes were first formed in the substrate plane through an oxide window process, with peak current densities of approximately 1 µA/µm2 and peak-to-valley ratio of approximately 1.3. A self-aligned lateral fabrication process, which forms the junction perpendicular to the substrate plane, has also been successfully developed and yielded backward Si tunnel diodes with peak current densities of 30 nA/µm2. To the author’s knowledge, these accomplishments are the first demonstration of lateral Si tunnel diodes using spin-on diffusants and rapid thermal processing. Low current density tunnel diodes can find Jialin Zhao applications as zero biased detector. Germanium tunnel diodes were demonstrated both using a diffusion-based approach and an on-wafer liquid-phase regrowth approach. The diffusion-based approach utilized spin-on diffusants and rapid thermal processing. Germanium tunnel diodes with current densities up to 0.6 nA/µm2 and PVR of 1.1 were demonstrated for the first time using this approach. An on-wafer liquid-phase regrowth approach with a silicon nitride microcrucible was developed. Germanium TDs with current densities up to 1.2 mA/µm2 were demonstrated. A primary goal of this project, demonstration of a 1 mA/µm2 tunnel junction, was fulfilled. CONTENTS FIGURES…………………………………………...…….……………………………...iv TABLES……………………………………………………...……………………….......x ACKNOWLEDGEMENT………......................................................................................xi CHAPTER 1 INTRODUCTION ........................................................................................ 1 1.1 Motivation..................................................................................................................... 1 1.2 Prior art ......................................................................................................................... 2 1.3 New approaches for tunnel junction formation ............................................................ 4 1.4 Silicon vs. germanium .................................................................................................. 6 1.5 Applications .................................................................................................................. 7 CHAPTER 2 TUNNEL-JUNCTION TRANSPORT AND FORMATION PHYSICS .... 11 2.1 Interband tunneling current-voltage relations............................................................. 12 2.2 Tunnel-junction formation .......................................................................................... 21 2.2.1 Doping technologies ......................................................................................... 21 2.2.2 Heavy doping.................................................................................................... 23 2.2.3 Abrupt junction formation................................................................................. 27 2.3 Tunneling current density vs. junction doping density and gradient .......................... 29 2.4 Switching speed .......................................................................................................... 34 2.5 Curvature coefficient of backward tunnel diode......................................................... 36 CHAPTER 3 SILICON TUNNEL DIODE FABRICATION ........................................... 41 3.1 Spin-on diffusants and rapid thermal processing........................................................ 42 3.2 Diffused-junction tunnel diodes in a vertical geometry.............................................. 44 3.2.1 Vertical process flow......................................................................................... 44 3.2.2 Demonstration of diffused-junction silicon tunnel diodes................................ 47 ii 3.3 Lateral diffused-junction tunnel diodes ...................................................................... 53 3.3.1 Lateral process flow and mask set .................................................................... 53 3.3.2 First demonstration of lateral diffused-junction silicon tunnel diode............... 59 3.4 Limitations of the diffusion approach......................................................................... 66 CHAPTER 4 GERMANIUM TUNNEL DIODE FABRICATION.................................. 68 4.1 Prior art vs. new approaches ....................................................................................... 69 4.2 Demonstration of diffused-junction Ge TDs .............................................................. 71 4.2.1 Doping Ge from spin-on diffusants .................................................................. 71 4.2.2 Diffused Ge tunnel diodes process ................................................................... 77 4.2.3 Diffused-junction Ge tunnel diodes .................................................................. 81 4.3 Demonstration of on-wafer liquid-phase-epitaxy Ge tunnel diode process................ 90 4.3.1 On-wafer liquid-phase-epitaxy approach.......................................................... 90 4.3.2 Process flow...................................................................................................... 94 4.3.3 Demonstration of Ge TDs using on-wafer liquid-phase regrowth approach.... 95 4.4 Demonstration of Ge TDs exceeding 1 mA/µm2 current density............................. 101 4.5 Limitations of approach ............................................................................................ 109 CHAPTER 5 CONCLUSIONS ...................................................................................... 110 5.1 Summary of accomplishments.................................................................................. 110 5.2 Achievements relative to prior art..............................................................................111 5.3 Recommendation for further study ........................................................................... 112 APPENDIX 1 VERTICAL SILICON TD FABRICATION TRAVELER...................... 115 APPENDIX 2 LATERAL SILICON TD FABRICATION TRAVELER........................ 119 APPENDIX 3 GERMANIUM TUNNEL DIODE FABRICATION USING LIQUID-PHASE REGROWTH APPROACH TRAVELER.......................................... 126 APPENDIX 4 RAPID MELT GROWTH OF TUNNEL JUNCTIONS ......................... 129 REFERENCES ............................................................................................................... 153 iii FIGURES Figure 1.1 Schematic current-voltage characteristics of tunnel diode, backward tunnel diode and conventional p-n diode....................................................................................... 8 Figure 2.1 Schematic tunnel diode current-voltage curve with corresponding energy band diagrams. Shaded areas represent the filled electron states in the degenerately doped semiconductor. .................................................................................................................. 12 Figure 2.2 Triangular potential energy barrier. ................................................................. 15 Figure 2.3 Tunneling current density vs. electric field for a Si p-n junction comparing predicted by Eq. 2.5 with experimental data: ∆ Stork and Isaac [48] and▲ Fair and Wivell [26]. ....................................................................................................................... 17 Figure 2.4 Schematic current-voltage characteristic and energy band diagram of the tunnel diode biased at VP = (Vdn+Vdp)/3. ........................................................................... 18 Figure 2.5 Peak tunneling current density and internal electric field vs. effective doping density for an abrupt p+n+ junction.................................................................................. 20 Figure 2.6 SIMS profile of B diffusion into Si using SOD and RTP at 900 °C of 1 s. The gradient of diffusion is estimated at the concentration of 7 x 1020cm-3, which is the solid solubility of P at 900 °C.................................................................................................... 28 Figure 2.7 (a) Computed band diagram and (b) junction electric field for four doping profiles: an abrupt profile and 1, 2, and 4 nm/decade gradients from p to n with doping density of 1x 1020 cm-3. The calculation was made using W. R. Frensley’s BandProf..... 30 Figure 2.8 Junction