Schreiben und Niedertemperatur-Charakterisierung von Nanostrukturen
Summary
Oxide nanostructures provide new opportunities for science and technology. The interfacial conductivity between LaAlO3 and SrTiO3 can be controlled with near-atomic precision using a conductive atomic force microscopy technique. The protocol for creating and measuring conductive nanostructures at LaAlO3/SrTiO3 interfaces is demonstrated.
Abstract
Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions. Oxide interfaces exhibit a wide range of properties that can be controlled include conduction, piezoelectric behavior, ferromagnetism, superconductivity and nonlinear optical properties. Recently, methods for controlling these properties at extreme nanoscale dimensions have been discovered and developed. Here are described explicit step-by-step procedures for creating LaAlO3/SrTiO3 nanostructures using a reversible conductive atomic force microscopy technique. The processing steps for creating electrical contacts to the LaAlO3/SrTiO3 interface are first described. Conductive nanostructures are created by applying voltages to a conductive atomic force microscope tip and locally switching the LaAlO3/SrTiO3 interface to a conductive state. A versatile nanolithography toolkit has been developed expressly for the purpose of controlling the atomic force microscope (AFM) tip path and voltage. Then, these nanostructures are placed in a cryostat and transport measurements are performed. The procedures described here should be useful to others wishing to conduct research in oxide nanoelectronics.
Introduction
Oxide Hetero 1-5 zeigen eine bemerkenswert große Auswahl an emergent physikalische Phänomene, die sowohl wissenschaftlich interessanten und potentiell nützlich für Anwendungen, 4 sind. Insbesondere kann die Schnittstelle zwischen LaAlO 3 (LAO) und SrTiO3 (STO) 6 Isolations zeigen, Dirigieren, supraleitenden 7, ferroelektrischen artigen 8 und 9 ferromagnetischen Verhalten. Im Jahr 2006, Thiel et al zeigten, dass 10 gibt es eine scharfe Isolator-Metall-Übergang als die Dicke des LAO Schicht erhöht, mit einer kritischen Dicke von 4 Elementarzellen (4uc). Es wurde anschließend gezeigt, dass 3uc-LAO/STO Strukturen weisen eine Hysterese-Übergang, der lokal mit einem leitenden Rasterkraftmikroskop (AFM-c) Fühler 11 gesteuert werden kann.
Die Eigenschaften der Oxid-Schnittstellen wie LaAlO 3 / SrTiO 3 abhängig von der Abwesenheit oder Anwesenheit von leitendenElektronen an der Grenzfläche. Diese Elektronen können mit Top-Gate-Elektroden 12,13, Rücken-Tore 10 gesteuert werden, Oberflächenadsorbaten 14, ferroelektrischen Schichten 15,16 und c-AFM-Lithographie 11. Eine einzigartige Funktion von c-AFM-Lithographie ist, dass sehr kleine Nanostrukturen erstellt werden können.
Elektrische Top-Gating, kombiniert mit zweidimensionalen Haft wird oft verwendet, um Quantenpunkte in III-V-Halbleitern 17 erstellen. Alternativ können quasi-eindimensionalen Halbleiter-Nanodrähte elektrisch Nähe blendet werden. Die Verfahren zur Herstellung dieser Strukturen sind zeitaufwendig und in der Regel irreversibel. Demgegenüber ist die C-AFM Lithographietechnik reversibel in dem Sinn, dass eine Nanostruktur kann aus einem Experiment erzeugt werden, und dann "gelöscht" (ähnlich einer Tafel). Im Allgemeinen wird c-AFM Schreiben mit an die AFM Spitze angelegt positive Spannungen durchgeführt, während das Löschenwird unter Verwendung von negativen Spannungen. Die erforderlich ist, um eine bestimmte Struktur zu schaffen Zeit hängt von der Komplexität der Vorrichtung ist jedoch in der Regel weniger als 30 min; die meiste Zeit verbracht Löschen der Leinwand. Die typische räumliche Auflösung ist etwa 10 Nanometern, aber mit der richtigen Abstimmung bietet so klein wie 2 Nanometer erzeugt werden 18.
Eine detaillierte Beschreibung der Herstellung der nanoskaligen Verfahren folgt. Das Detail hier bereitgestellten sollte ausreichen, damit ähnliche Experimente von interessierten Forschern durchgeführt werden können. Die hier beschriebene Methode hat viele Vorteile gegenüber herkömmlichen lithographischen Methoden verwendet, um elektronische Nanostrukturen in Halbleitern zu schaffen.
Die hier beschriebene c-AFM Lithographieverfahren ist Teil eines viel umfassenderen Klasse von Rastersondenlithographie-basierte Bemühungen, einschließlich Scannen anodische Oxidation 19, Dip-Pen-Nanolithographie 20, piezoelektrische Muster21, und so weiter. Die c-AFM-Technik hier beschrieben, verbunden mit der Verwendung von neuen Oxid-Schnittstellen können einige der höchsten präzise elektronische Strukturen mit einer beispiellosen Vielfalt von physikalischen Eigenschaften herzustellen.
Protocol
Representative Results
Discussion
Successful creation of nanostructures depends on several critical steps. It is important that the LAO/STO samples are grown with a thickness that is known to be at the boundary between the insulating and conductive phase. (Details of sample growth fall outside the scope of this paper, but are crucial for overall success.) Second, it is important to have relative humidity within the range 25-45% for successful c-AFM writing. Values below 25% are unlikely to produce conductive nanostructures, while too high humidity will generally produce uncontrollably large features. Also, temperature control of the AFM is important if the c-AFM tip needs to achieve precise registry over long periods of time. Once the nanostructures are created, they must be placed in a vacuum environment if experiments lasting longer than a few hours are to be performed. For the experiments described here, the structure is created and within minutes transferred to a vacuum environment.
It is recommend before writing that a “writing test” be performed on all relevant electrodes. In such a test, two virtual electrodes are first created, and a single nanowire is written while simultaneously monitoring the conductance. A similar test of erasure can be performed by “cutting” the nanowire shortly afterwards. If the nanostructure is decaying rapidly, the issue is most likely due either to the interfacial contacts or the canvas itself. To distinguish between these two effects, a four-terminal measurement of the conductance should be performed, and the two-terminal conductance should be compared with the four-terminal conductance as a function of time. If the two-terminal conductance is decaying more rapidly than the four-terminal conductance, then the issue is related to the electrical contacts to the interface. If the four-terminal conductance is decaying at a comparable rate, then most likely the canvas is not suitable and should be replaced.
There are natural limitations of the current method for creating nanostructures. Specifically, the writing speed for the smallest devices is limited to a few hundred nanometers per second. Speeds far above that value lead to unpredictable results. Use of parallel writing techniques are possible27,28, but are not highly developed and have their own drawbacks. The size of nanostructures that can be created is naturally limited by the scan range of the AFM being used. A high-quality AFM with closed-loop feedback in the two scan directions is highly recommended. Tracking of point-like objects on the sample surface should be performed to monitor temporal drift of the sample.
Once creation of conductive nanostructures at oxide interfaces has been mastered, there are a wide range of experimental directions that can be explored. Using this technique, a wide variety of nanostructures and devices have already been demonstrated, including nanowires18, tunnel barriers29, rectifying junctions30, field-effect transistors18, single-electron transistors31, superconducting nanowires32, nanoscale optical detectors33, and nanoscale THz emitters and detectors34.
Disclosures
The authors have nothing to disclose.
Acknowledgements
The long-standing collaboration with Chang-Beom Eom at the University of Wisconsin-Madison, who provided the LAO/STO samples, is gratefully acknowledged. Video editing assistance from Christopher Solis is greatly appreciated. This work is supported by NSF (DMR-1104191, DMR-1124131), ARO (W911NF-08-1-0317), and AFOSR (FA9550-10-1-0524, FA9550-12-1-0268, FA9550-12-1-0057).
Materials
| Name | Company | Catalog Number | Comments |
| Equipment | |||
| Contact Aligner | Karl-Suss | MA6 | |
| Spinner | Solitec | 5110C | |
| Ion Mill | Commonwealth Scientific | 8C | |
| Sputtering System | Leybold-Heraeus | Z-650 | |
| Barrel Etcher | Branson/IPC | 3000C | |
| Wire Bonder | Westbond | 7700E | |
| AFM | Asylum Research | MFP-3D | |
| Dilution Refrigerator | Quantum Design | P850 | |
| Ultrasonic Wash Machine | Fisher Scientific | 15-335-6 | |
| Current Amplifier | Femto | DLPCA-200 | |
| Materials | |||
| LaAlO3/SrTiO3 | Prof. Chang-Beom Eom | N/A | 5mm x 1mm with ~3.4 unit cells of LAO (See Reference 18) |
| Photoresist | AZ Electronic Materials | P4210 | |
| Developer | AZ Electronic Materials | 400K | |
| Acetone | Fisher Scientific | A929SK-4 | |
| Isopropyl Alcohol | Fisher Scientific | A459-1 | |
| Deionized Water | Fisher Scientific | 23-290-065 | |
| Gold Wire | DuPont | 5771 | 1 mil diameter |
| Chip Carrier | NTK Technologies | IRK28F1-5451D |
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