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Electron Many-Body Effects in Quantum Point Contacts

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Electron Many-Body Effects in Quantum Point Contacts Electron Many-Body Effects in Quantum Point Contacts Zernike Institute PhD thesis series 2012-15 ISSN: 1570-1530 ISBN: 978-90-367-5600-6 (printed version) ISBN: 978-90-367-5601-3 (electronic version) The work described in this thesis was performed in the research group Physics of Nanodevices of the Zernike Institute for Advanced Materials at the University of Groningen, the Netherlands. It was supported with funding from a Pakistani HEC-Nuffic scholarship and the Zernike Institute, and did profit from collabora- tion with projects funded through a Vidi grant from the Netherlands Organization for Scientific Research (NWO) and the national NanoNed and NanoLab NL ini- tiatives. Thesis front cover: Linear conductance of a Quantum Point Contact as a func- tion of gate voltage, with traces that show a modulation of the 0.7 anomaly as a function of length (186 nm (left trace) to 608 nm (right trace)). Thesis back cover: Artistic image of a length-tunable Quantum Point Contact (modified from the original SEM image by Thomas Maassen). Printed by: Ipskamp Drukkers, Enschede Rijksuniversiteit Groningen Electron Many-Body Effects in Quantum Point Contacts Proefschrift ter verkrijging van het doctoraat in de Wiskunde en Natuurwetenschappen aan de Rijksuniversiteit Groningen op gezag van de Rector Magnificus, dr. E. Sterken, in het openbaar te verdedigen op vrijdag 13 juli 2012 om 11:00 uur door Muhammad Javaid Iqbal geboren op 1 januari 1982 te Narowal, Pakistan Promotor: Prof.dr.ir.C.H.vanderWal Beoordelingscommissie: Prof. dr. A.D. Wieck Prof. dr. D.M. Zumb¨uhl Prof. dr. ir. W.G. van der Wiel Dedicated to my (late) father whose memories are still a source of inspiration for me... vi Contents 1 Introduction 1 1.1 Charge and spin based electronics: from power plants to nanoscience 1 1.2 Thiswork: QuantumPointContacts . 3 1.3 Outlineofthisthesis ......................... 7 2 QPC transport properties, device fabrication and measurement techniques 11 2.1 Introduction.............................. 11 2.2 GaAs/AlGaAsheterostructure. 11 2.3 Quantum point contact and quantized conductance . 12 2.4 The0.7anomaly ........................... 15 2.5 TheZero-BiasAnomaly(ZBA) . 18 2.6 Reviewofliteratureonthe0.7anomalyandZBA . 20 2.7 Devicefabrication. 29 2.8 Lowtemperaturemeasurementtechniques . 34 3 Many-body effects with signs of two-impurity Kondo Physics in QPCs with a fixed length 41 3.1 Introduction.............................. 42 3.2 Experimentalrealization . 43 3.3 Part I: Correlations between QPC transport properties and geometry 44 3.4 PartII:Observationofadouble-peakZBA . 52 4 Emergent impurity states from many-body physics in clean quantum point contacts 59 4.1 Introduction.............................. 60 4.2 Experimental ............................. 61 4.3 Resultsfromsinglesplit-gateQPCs . 61 4.4 Resultsfromlength-tunableQPCs . 63 vii viii Contents 4.5 Conclusions .............................. 72 4.6 Appendix: Materialsandmethods. 73 4.7 Appendix: Earlier reports showing modulated 0.7 anomalies and double-peakZBAs .......................... 74 4.8 Appendix: Friedel oscillations and Fermi wavelength in theQPC . 76 4.9 Appendix: Analysis of ZBA peak positions at G = 0 .4, 0 .6, 0 .7 and 0 .85 (2 e2/h )........................... 78 · 5 Split-gate quantum point contacts with tunable channel length 81 5.1 Introduction.............................. 82 5.2 Designconsiderations. 84 5.3 Electrostaticsimulations . 86 5.4 Sample fabrication and measurement techniques . 91 5.5 Experimental realization of length-tunable QPCs . .... 92 5.6 Conclusions .............................. 95 6 The annealing mechanism of AuGe/Ni/Au ohmic contacts to a two-dimensional electron gas in GaAs/Al xGa 1−xAs heterostruc- tures 97 6.1 Introduction.............................. 98 6.2 Devicefabrication. 99 6.3 Electricalmeasurements . 100 6.4 Cross-sectionalTEMandEDXimaging. 103 6.5 Summaryofannealingmechanism. 104 6.6 Diffusionmodel ............................ 108 6.7 Contact-shapedependence . 110 6.8 Conclusions .............................. 113 7 Robust recipe for low-resistance ohmic contacts to a two-dimensional electron gas in a GaAs/AlGaAs heterostructure 117 7.1 Introduction.............................. 118 7.2 Experimentaldetails . 118 7.3 Device design, measurement schemes and methods . 120 7.4 Resultsanddiscussions. 125 7.5 Conclusions .............................. 128 A Wafer inventory 131 A.1 WSUMI301611 ............................ 131 Contents ix A.2 WSUMI301612 ............................ 131 A.3 WREUT1098 ............................. 132 A.4 WREUT1588 ............................. 132 A.5 WREUT12570............................. 133 B Device fabrication 135 B.1 Alignmentmarkers . 135 B.2 Mesaetching ............................. 136 B.3 Ohmiccontacts ............................ 137 B.4 Finegates ............................... 138 B.5 Largegates .............................. 139 C Biased cool down 141 Summary 145 Samenvatting 149 Acknowledgements 153 Curriculum vitae 157 List of publications 159 x Contents Chapter 1 Introduction 1.1 Charge and spin based electronics: from power plants to nanoscience The last few decades have been revolutionary for progress in condensed matter physics. The electronic properties of materials revealed very rich and interesting behavior that is fundamental to a wealth of applications in our society. At this stage, there is still a strong interplay between fundamental research in solid state physics and the development of new and improved devices. Already early in the nineteenth century, scientists discovered how to utilize the electron flow in materials for practical applications. For example, the first commercial electrical power station was opened in San Francisco in 1879, even before the discovery of the electron by J. J. Thomson in 1897. Electrons are fundamental particles that occur as building blocks in atoms. In isolated atoms, the electrons orbit around the nucleus. This can be pictured as having a similarity with the way our home planet Earth and other planets in the solar system revolve around the Sun. For detailed understanding of atoms, however, this analogy breaks down and the states of electrons in atoms can only be understood with quantum mechanical theory where the position of an electron is described by a wavefunction. The electron is a charged particle. Its charge was first accurately measured by Robert Millikan and Harvey Fletcher in 1909 in their famous oil drop experiments, and has the value 1.602 10 −19 C. In 1925, the two Dutch physicists George − × Uhlenbeck and Samuel Goudsmit discovered that the electron also has an intrinsic angular momentum, that has been given the name spin [1]. This can be pictured as having a similarity with the way the Earth revolves around its axis. The 1 2 Chapter 1. Introduction quantum mechanical nature of this spin gives that its value along a particular direction only shows one of two values when it is measured: + 1 ~ or 1 ~ (where 2 − 2 ~ the reduced Planck constant), which are known as spin-up and spin-down, respectively. Since the electron is a charged particle, its spin is directly linked to an intrinsic magnetic moment. This can be viewed as each electron being a tiny magnet with a North and South pole. In a magnetic field this magnetic moment splits the energies of the spin-up and spin-down states, the so-called Zeeman splitting. The charge and spin properties of the electron are at the heart of nearly all electrically powered devices that we use in our daily life. Most of the devices we are familiar with exploit the charge property of the electron, which causes the electron to move when it is subjected to an external electric field. Amplification and logic operations by cheap and robust electronic devices were made possible by the invention of the transistor by John Bardeen, William Shockley and Walter Brattain at Bell Labs in 1947. This made it possible to build electronic equipment like radio, TV, calculators, computers and is currently of importance for almost all other household and industrial equipment. The first electronic experiment that was in fact involving the spin of an elec- tron was also carried out before the discovery of the electron and its spin. Already in 1856, William Thomson (more commonly known as Lord Kelvin) discovered the Anisotropic MagnetoResistance (AMR) effect in bulk iron material [2]. The iron material showed a resistance change as a function of the orientation of the material with respect to an external magnetic field. Follow-up research in this direction led many years later, in 1988, to the discovery of the Giant MagnetoRe- sistance (GMR) effect [3, 4]. Also for the GMR effect the spin of the electron has an essential role. This effect appears in the electrical resistance of two adjacent ferromagnetic layers separated by a thin non-magnetic layer. This resistance can be switched to a low (or high) value when the magnetization of the layers is forced in the same (or opposite) direction by an applied external magnetic field. This effect was in recent years applied in the hard-disk read heads of almost all com- puters [5]. In addition, the discovery of the GMR effect led to the birth of a new resaerch field that is now called spintronics (or spin-electronics) [6, 7]. In 2007, Albert Fert and Peter Gr¨unberg were awarded the Nobel prize for the discovery of the GMR effect. Many of these developments in electronics became possible thanks to an ex- tremely impressive technological achievement: the ever increasing miniaturization of electronic devices
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