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 heterostrukturer 1-5 udviser en bemærkelsesværdig bred vifte af emergent fysiske fænomener, som er både videnskabeligt interessant og potentielt nyttige for applikationerne 4. Navnlig kan grænsefladen mellem LaAlO 3 (LAO) og SrTiO 3 (STO) 6 udviser isolerende, ledende, superledende 7 ferro-lignende 8 og 9 ferromagnetisk opførsel. I 2006 Thiel et al viste 10, at der er en skarp isolator-til-metal-overgang som tykkelsen af LAO lag forøges, med en kritisk tykkelse på 4 celler unit (4uc). Det blev senere vist, at 3uc-LAO/STO strukturer udviser en hysteresetab overgang, der kan styres lokalt med en ledende atomic-force mikroskop (c-AFM) sonde 11.
Egenskaberne af oxid grænseflader såsom LaAlO 3 / SrTiO 3 afhænger af fraværet eller tilstedeværelsen af ledendeelektroner i grænsefladen. Disse elektroner kan kontrolleres ved hjælp af top gateelektroder 12,13, ryg gates 10, overflade Absorbater 14, ferro lag 15,16 og c-AFM litografi 11. En unik funktion af c-AFM litografi er, at der kan skabes meget små nanoskala funktioner.
Elektrisk top gating, kombineret med to-dimensionelle indespærring, er ofte bruges til at skabe kvantepunkter i III-V halvledere 17. Alternativt kan kvasi-en-dimensionelle halvledende nanotråde være elektrisk gated ved nærhed. Fremgangsmåderne til fremstilling af disse strukturer er tidskrævende og generelt irreversibel. Derimod c-AFM litografi teknik er reversibel i den forstand, at der kan skabes for et eksperiment en nanostruktur, og derefter "slettet" (svarer til en tavle). Generelt er c-AFM skrivning udført med positive spændinger anvendes på AFM spidsen, mens sletningudføres ved hjælp af negative spændinger. Den tid, der kræves for at skabe en bestemt struktur afhænger af kompleksiteten af enheden, men er normalt mindre end 30 min; det meste af denne tid bliver brugt slette lærredet. Den typiske rumlige opløsning er omkring 10 nanometer, men med en ordentlig tuning funktioner så lille som kan oprettes 2 nanometer 18.
En detaljeret beskrivelse af nanoskala fremstillingsprocedure følger. Hvor detaljeret her bør være tilstrækkelig til, at lignende forsøg skal udføres af interesserede forskere. Den her beskrevne metode har mange fordele frem for traditionelle litografiske metoder anvendes til at skabe elektroniske nanostrukturer i halvledere.
C-AFM litografi her beskrevne metode er en del af et meget bredere klasse af scanning-probe-baserede litografi indsats, herunder scanning anodisk oxidation 19, dip-pen nanolithography 20, piezoelektriske mønster21, og så videre. C-AFM her beskrevne teknik, kombineret med anvendelsen af nye oxid grænseflader, kan producere nogle af de højeste præcision elektroniske strukturer med en hidtil uset række fysiske egenskaber.
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 |