LOW RESISTANCE CONTACTS TO N-TYPE ... - Stanford University

[Pages:136]LOW RESISTANCE CONTACTS TO N-TYPE GERMANIUM

A DISSERTATION SUBMITTED TO THE DEPARTMENT OF ELECTRICAL ENGINEERING

AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

Jui-Yen Jason Lin June 2013

? 2013 by Jui-Yen Lin. All Rights Reserved. Re-distributed by Stanford University under license with the author.

This work is licensed under a Creative Commons AttributionNoncommercial 3.0 United States License. This dissertation is online at:

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I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.

Krishna Saraswat, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.

James Harris I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.

Yoshio Nishi

Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost Graduate Education

This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives.

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Abstract

The scaling of conventional silicon transistors faces several obstacles including the need to introduce materials such as germanium which have higher carrier mobilities. Several challenges need to be addressed in these new materials systems. In the case of germanium, contact resistance to n-type Ge is particularly problematic. This thesis presents two approaches to address this issue. The first is the use of metal-insulatorsemiconductor contacts, whereby inserting a thin insulator between the metal and semiconductor can reduce the barrier height and reduce contact resistance. Tunneling resistance and series resistance effects are studied experimentally and theoretically. The second approach is the use of germanide contacts in conjunction with high doping techniques. Low contact resistances are obtained due to the high dopant activation level and dopant segregation effects.

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Acknowledgements

I would like to thank my research advisor, Professor Krishna Saraswat, for all his support and guidance over the last several years. His door was always open, and I have received a tremendous amount of advice from him. I am especially grateful for his encouragement when I changed research directions half way through my time at Stanford. This thesis would not have been possible without his vast knowledge, patient mentoring, and gentle encouragement.

I have been very fortunate to interact with several faculty members. Professor Yoshio Nishi, Professor James Harris, and Professor Philip Wong not only served on my committee but gave me very helpful suggestions during my research. In addition to serving as my committee chair, Professor Paul McIntyre's expertise in TiO2 was a tremendous resource and I am very fortunate to have received his help.

This work was very much a collaborative effort. I wish to thank Dr. Arunanshu Roy, not only for his simulation expertise, but also for being a fantastic colleague. I will always cherish our debates regarding our simulated and experimental results. Also, Dr. Bin Yang's wealth of industrial experience was eye-opening, and I am still amazed at how he was able to point me to very useful papers after every discussion. I am also very fortunate to have collaborated with Suyog Gupta, whose energy and passion is as impressive as his technical expertise.

A large portion of this work was done at the Stanford Nanofabrication Facility (SNF) and the Stanford Nanocharacterization Laboratory (SNL), and it would not have been possible without the staff's support. I am especially grateful to J Provine for help with ALD and Dr. James McVittie for help with anything plasma related.

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I would like to thank Gail Chun-Creech for very efficient administrative help ranging from POs and reimbursements to room reservations and scheduling.

The Saraswat group members are definitely among the smartest and most dedicated individuals I know, and I am truly honored to call them my colleagues and friends. Our impromptu late night meetings in SNF are a prized memory, and I appreciate our mutual encouragement to persevere after failures.

Finally, I am very thankful to have a loving family, encouraging and supporting me every step of the way. With their unyielding integrity and hardworking spirit, my parents and brother have always been my role models. I dedicate this thesis to them.

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Table of Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Thesis Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Methods to Reduce Ge N-Type Contact Resistance . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 Properties of the Ge Material System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 Dopant Activation Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.2.1 Laser Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.2.2 Phosphorus & Antimony Coimplantation . . . . . . . . . . . . . . . . . . . . . . 12 2.2.3 Fluorine Vacancy Passivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3 MIS Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.3.1 Fermi Level Pinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.3.2 Fermi Level Depinning using Ultrathin Dielectrics . . . . . . . . . . . . . . 19 2.3.3 Resistance Due to Tunneling Through the Dielectric . . . . . . . . . . . . . 22 2.4 Chalcogen Passivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.5 Dopant Segregation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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3 TiO2 MIS Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.1 ALD Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.2 Band Offsets to Germanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 3.3 MIS Contacts on Epitaxial Germanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 3.3.1 Barrier Height Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.3.2 Effect of Metal Workfunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 3.3.3 Comparing MIS Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.4 TiO2 MIS Contacts on n+ Germanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.4.1 Measurement Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.4.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.4.3 Series Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 3.5 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 3.5.1 Ge N-Channel MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 3.5.2 Metal Source/Drain Transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 3.5.3 Asymmetric Metal-Semiconductor-Metal Photodetectors . . . . . . . . . . 55 3.5.4 Spin Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 3.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

4 Physics of MIS Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.2 Theory of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 4.2.1 Effect of Dipoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 4.3 Effect of Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 4.4 Effect of Series Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 4.5 Effect of High Semiconductor Doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 4.6 MIS Contact Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.6.1 Oxygen-Deficient TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.6.2 Indium Tin Oxide (ITO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 4.6.3 Oxygen-Deficient ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 4.7 Scalability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 4.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

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