Junction Mixing Scanning Tunneling Microscopy

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Transcript of Junction Mixing Scanning Tunneling Microscopy

University of Alberta

Library Release Form

Name of Author: Georey Mark Steeves Title of Thesis: Junction Mixing Scanning Tunneling Microscopy Degree: Doctor of Philosophy Year this Degree Granted: 2001

Permission is hereby granted to the University of Alberta Library to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientic research purposes only. The author reserves all other publication and other rights in association with the copyright in the thesis, and except as hereinbefore provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatever without the authors prior written permission.

. . . . . . . . . . . . . . . . . . . Georey Mark Steeves Department of Physics University of Alberta Edmonton, AB Canada, T6G 2J1

Date: . . . . . . . . .

University of Alberta

Junction Mixing Scanning Tunneling Microscopy

by

Georey Mark Steeves

A thesis submitted to the Faculty of Graduate Studies and Research in partial fulllment of the requirements for the degree of Doctor of Philosophy.

Department of Physics

Edmonton, Alberta Fall 2001

University of Alberta Faculty of Graduate Studies and Research

The undersigned certify that they have read, and recommend to the Faculty of Graduate Studies and Research for acceptance, a thesis entitled Junction Mixing Scanning Tunneling Microscopy submitted by Georey Mark Steeves in partial fulllment of the requirements for the degree of Doctor of Philosophy.

. . . . . . . . . . . . . . . . . . . Dr. Mark Freeman (supervisor) . . . . . . . . . . . . . . . . . . . Dr. Frank Marsiglio . . . . . . . . . . . . . . . . . . . Dr. Frank Hegmann . . . . . . . . . . . . . . . . . . . Dr. Richard Sydora . . . . . . . . . . . . . . . . . . . Dr. Arthur Mar . . . . . . . . . . . . . . . . . . . Dr. Geo Nunes Jr.

Date: . . . . . . . . .

AcknowledgementsI would like to thank my wife Wendy for her love, encouragement and support over the years both together and apart. I would like to thank my parents and my brother Matthew for their interest in, and encouragement of my research. Finally I would like to express deep gratitude to my advisor Mark Freeman. You were a quintessential mentor through my studies and you always made time for advise and discussions of physics and life.

AbstractResearch in the elds of nanotechnology and nanoelectronics is burgeoning. As nanodevices shrink in size and subsequently electronic operations on these length scales accelerate, new techniques will be required to study and characterize these devices. Techniques with atomic scale spatial resolution and femtosecond time resolution will soon be necessary. Looking to conventional scanning probe microscopy, the scanning tunneling microscope (STM) already possesses sucient spatial resolution to image any feature current nanotechnologies can produce (STMs are used to build the worlds smallest nano-devices). Similarly ultrafast pump/probe optical techniques exist, which can resolve femtosecond dynamics. The goal of this research is to wed ultrafast optical techniques with the scanning tunneling microscope to produce an aggregate probe with the ability to image nanoscale dynamics. This goal has been achieved using a technique known as junction mixing STM (JM-STM). Initial work using the junction mixing technique was performed to demonstrate the detection of a time-resolved signal using a home built scanning tunneling microscope. This work demonstrated an order of magnitude improvement in time resolution over previous experiments by utilizing ion implanted gallium arsenide substrates to generate fast electrical pulses. This work decisively demonstrated the STM tunnel junction as the origin of the measured time resolved signal, a crucial requirement in maintaining STM spatial resolution. Following these experiments, new test structures were designed to demonstrate combined STM spatial resolution with picosecond time resolution. Measurements were made on small titanium dots patterned onto a gold transmission line. The titanium provided electronic contrast to the gold, so that our time resolved signal was modulated as we scanned our STM across the titanium/gold interface. Combined 20 ns - 20 ps spatio-temporal resolution was achieved, the rst direct conrmation that

time-resolved STM was possible. These experiments were numerically reproduced using a lumped element circuit model of the non-linear tunnel junction in parallel with STM geometrical capacitance. Results from this model suggest the junction mixing technique should be able to yield time resolution in the hundreds of femtoseconds while maintaining atomic spatial resolution. To demonstrate the operation of this junction mixing technique eorts were directed to designing and building a low temperature high vacuum STM and a home built Ti/Sapph laser system. These systems are being incorporated into a new low temperature high vacuum time resolved scanning tunneling microscope.

Contents1 Introduction to Time-Resolved Scanning Tunneling Microscopy 1 1.1 Introduction to Scanning Tunneling Microscopy . . . . . . . . . . . . 1 1.1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.2 A typical STM and its applications . . . . . . . . . . . . . . . 4 1.1.3 STM Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . 9 1.2 History of Time Resolved STM . . . . . . . . . . . . . . . . . . . . . 18 1.2.1 Initial Developments in Time-Resolved Scanning Probe Microscopy 18 1.2.2 Time-Resolved Electrostatic Force Microscopy . . . . . . . . . 23 1.2.3 Time-Resolved Scanning Tunneling Microscopy . . . . . . . . 27 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 2 Advances in picosecond scanning mixing 2.1 Context . . . . . . . . . . . . . 2.2 Experimental Details . . . . . . 2.2.1 Anomalous Signal . . . . 2.3 Publication . . . . . . . . . . . References . . . . . . . . . . . . . . . tunneling microscopy via junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 49 50 57 59 69

3 Nanometer-scale imaging with an croscope 3.1 Context . . . . . . . . . . . . . . 3.2 Publication . . . . . . . . . . . . References . . . . . . . . . . . . . . . .

ultrafast scanning tunneling mi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 71 82 91

4 Circuit Analysis of an Ultrafast Junction Mixing Scanning Tunneling Microscope 93 4.1 Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 4.2 Publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 5 Design and Development of a Low Temperature High Vacuum STM with Optical Access 110 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 5.2 Mechanical Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 5.3 Electrical Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 5.4 Vacuum Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 5.5 STM investigations on NbSe2 . . . . . . . . . . . . . . . . . . . . . . 126 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 6 Conclusions 140 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 A Junction Mixing STM Control Software A.1 Introduction . . . . . . . . . . . . . . . . A.2 STM sample approach . . . . . . . . . . A.3 STM topographic scanning . . . . . . . . A.4 STM I/V and I/Z spectroscopic scanning A.5 Optical delay line scanning . . . . . . . . 144 144 146 150 156 159

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B Junction Mixing STM Optical Systems 163 B.1 The Ti/Sapphire Laser . . . . . . . . . . . . . . . . . . . . . . . . . . 163 B.2 The 8 Pulser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 C Modications to the C.1 Modied Model . C.2 Model Code . . . C.2.1 rbchvhyp.f Junction . . . . . . . . . . . . . . . . . . Mixing . . . . . . . . . . . . . . . Model 173 . . . . . . . . . . . . . . . . . . 173 . . . . . . . . . . . . . . . . . . 175 . . . . . . . . . . . . . . . . . . 177

List of FiguresPump pulse train repeatably stimulates the sample, which has a response to the stimulation shown on the top right quadrant of the gure. The probe pulse train is delayed by an amount T1, T2 or T3, and probes the response of the pumped system at these dierent times. By varying the delay time between pump and probe pulse trains the response of the system can be mapped out. . . . . . . . . . . . . . . 3 1.2 Basic elements of a Scanning Tunneling Microscope . . . . . . . . . . 6 1.3 77 reconstruction on Si (111); panel a) shows STM topographic scan; panel b) shows calculated structure. . . . . . . . . . . . . . . . . . . 8 dI 1.4 600 600 nm scan of NbSe2 at 1.8K in a eld of 1 Tesla, dV ranges from 1 108 0 (black) to 1.5 109 0 (white). . . . . . . . . . . . . 10 1.5 (a) Two metals well separated by a vacuum gap, the Fermi levels are separated by the dierence in work function between each metal. (b) Metals are brought close together and reach an electronic equilibrium where they share a common