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.
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.
Oxide hetero 1-5 utstillingen et bemerkelsesverdig bredt utvalg av emergent fysiske fenomener som er både vitenskapelig interessant og potensielt nyttig for applikasjoner fire. Spesielt kan grensesnittet mellom LaAlO 3 (LAO), og SrTiO 3 (STO) 6 oppviser isolerende, ledende, superledende 7, ferroelektrisk lignende 8, 9 og ferromagnetisk oppførsel. I 2006 Thiel et al viste 10 at det er en skarp isolator-til-metall-overgang som tykkelsen av LAO sjikt økes, med en kritisk tykkelse på 4 enhetsceller (4uc). Det ble senere vist at 3uc-LAO/STO strukturer oppviser en hysteretic overgang som kan styres lokalt, med en ledende atomic force-mikroskop (c-AFM) sonden 11..
Egenskapene til oksid grenseflate som LaAlO 3 / SrTiO 3 er avhengig av fravær eller tilstedeværelse av ledendeelektroner på grensesnittet. Disse elektronene kan kontrolleres ved hjelp av topp gate elektroder 12,13, rygg porter 10, adsorbates overflaten 14, ferroelectric lag 15,16 og c-AFM litografi 11. En unik funksjon i c-AFM litografi er at svært små nanoskala funksjoner kan opprettes.
Elektrisk toppen gating, kombinert med to-dimensjonale innesperring, blir ofte brukt til å lage kvanteprikker i III-V halvledere 17. Alternativt kan kvasi-en-dimensjonale halvledende nanotråder være elektrisk gated av nærhet. Metodene for fremstilling av slike strukturer er tidkrevende og vanligvis irreversibelt. Derimot, er det c-AFM litografi teknikk reversibel i den forstand at en nanostrukturen kan bli opprettet for ett eksperiment, og deretter "slettet" (ligner på en whiteboard). Vanligvis er c-AFM skriftlig utført med positive spenninger brukes på AFM spissen, mens, slettingutføres ved hjelp av negative spenninger. Den tid som kreves for å skape en spesiell struktur avhenger av kompleksiteten av anordningen, men er vanligvis mindre enn 30 min; meste av denne tiden tilbringes slette lerretet. Den typiske romlig oppløsning er ca 10 nanometer, men med riktig tuning funksjoner så liten som to nanometer kan opprettes 18.
En detaljert beskrivelse av nanoskala fabrikasjonsprosedyren følger. Detaljene gitt her bør være tilstrekkelig til å tillate tilsvarende forsøk som skal utføres av interesserte forskere. Metoden som beskrives her har mange fordeler i forhold til tradisjonelle litografiske metoder som brukes til å lage elektroniske nanostrukturer i halvledere.
Den c-AFM litografi metoden beskrevet her er en del av en mye større klasse scanning-probe-baserte litografi innsats, inkludert scanning anodisk oksidasjon 19, dip-penn nanolithography 20, piezoelektrisk mønster21, og så videre. Den c-AFM teknikk som er beskrevet her, samt ved bruk av nye oksid grenseflater, kan produsere noen av de høyeste presisjon elektroniske strukturer med en hittil ukjent rekke fysiske egenskaper.
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.
The authors have nothing to disclose.
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).
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 |