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.
Oxidhetero 1-5 uppvisar ett anmärkningsvärt stort antal framväxande fysiska fenomen som både är vetenskapligt intressant och potentiellt användbara för tillämpningar 4. I synnerhet kan gränssnittet mellan LaAlOs 3 (LAO) och SrTiOs 3 (STO) 6 uppvisar isolerande, ledande, supraledande 7, ferroelektrisk liknande 8, och ferromagnetiskt 9 beteende. Under 2006, Thiel et al visade 10 att det finns en kraftig isolator-metall-övergång som tjockleken på LAO lagret ökas, med en kritisk tjocklek av 4 enhetsceller (4uc). Det har senare visat att 3uc-LAO/STO strukturer uppvisar en hysteres övergång som kan styras lokalt med en ledande atom-styrkamikroskop (c-AFM) sond 11.
Egenskaperna för oxid gränssnitt såsom LaAlOs 3 / SrTiOa 3 bero på frånvaro eller närvaro av ledandeelektroner i gränssnittet. Dessa elektroner kan styras med hjälp av toppgrindelektroderna 12,13, rygg-grindarna 10, adsorbater 14 yta, ferroelektriska skikten 15,16 och c-AFM litografi 11. Unikt för c-AFM litografi är som kan skapas mycket små nanoskala funktioner.
Elektrisk topp gating, kombinerat med två-dimensionell förlossningen används ofta för att skapa kvantprickar i III-V-halvledare 17. Alternativt kan kvasi-en-dimensionella halvledande nanotrådar elektriskt gated av närhet. Metoderna för framställning av dessa strukturer är tidskrävande och i allmänhet irreversibel. Däremot är det c-AFM litografi teknik reversibel i den meningen att en nanostruktur kan skapas för ett experiment, och därefter "raderat" (liknar en whiteboard). Generellt är c-AFM skrivande utförs med positiva spänningar som appliceras på AFM spets, medan, raderautförs med användning av negativa spänningar. Den tid som krävs för att skapa en viss struktur beror på komplexiteten av anordningen men är vanligtvis mindre än 30 min; mesta av den tiden går åt att radera duken. Den typiska rumsliga upplösningen är ungefär 10 nanometer, men med rätt tuning funktioner så små som 2 nanometer kan skapas 18.
En detaljerad beskrivning av nanotillverkningsprocedur följer. Detaljnivån här bör vara tillräcklig för att liknande experiment som ska utföras av intresserade forskare. Den metod som beskrivs här har många fördelar jämfört med traditionella litografiska metoder som använts för att skapa elektroniska nanostrukturer i halvledare.
Den c-AFM litografi metod som beskrivs här är en del av en mycket bredare klass av scanning-probe-baserade litografi insatser, inklusive scanning anodoxidation 19, dip-penna nanolithography 20, piezoelektriska mönstring21, och så vidare. Den c-AFM teknik som beskrivs här, i kombination med användning av nya gränssnitt oxid, kan producera några av de högsta precision elektroniska strukturer med en aldrig tidigare skådad mängd fysiska 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 |