लेखन और आक्साइड nanostructures की कम तापमान विशेषता
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
ऑक्साइड heterostructures 1-5 निशंक दोनों वैज्ञानिक दिलचस्प और अनुप्रयोगों 4 के लिए संभावित रूप से उपयोगी हैं जो आकस्मिक भौतिक घटना का एक उल्लेखनीय विस्तृत विविधता. विशेष रूप से, LaAlO 3 (लाओ) और SrTiO 3 (STO) 6 के बीच इंटरफेस, 8 ferroelectric की तरह, और ferromagnetic 9 व्यवहार 7 superconducting, संचालन, इन्सुलेट प्रदर्शन कर सकते हैं. 2006 में, थिएल एट अल लाओ परत की मोटाई के रूप में एक तेज इन्सुलेटर को धातु संक्रमण है कि वहाँ 10 से पता चला 4 इकाई कोशिकाओं (4uc) की एक महत्वपूर्ण मोटाई के साथ वृद्धि हुई है. यह बाद में 3uc-LAO/STO संरचनाओं एक प्रवाहकीय परमाणु बल सूक्ष्मदर्शी (C-AFM) जांच 11 के साथ स्थानीय रूप से नियंत्रित किया जा सकता है कि एक hysteretic संक्रमण है कि प्रदर्शन दिखाया गया था.
ऐसे LaAlO 3 / SrTiO 3 के रूप में ऑक्साइड इंटरफेस के गुणों का आयोजन की अनुपस्थिति या उपस्थिति पर निर्भरइंटरफेस में इलेक्ट्रॉनों. इन इलेक्ट्रॉनों वापस शीर्ष गेट इलेक्ट्रोड 12,13, फाटकों 10 का उपयोग कर नियंत्रित किया जा सकता है, सतह 14 adsorbates, ferroelectric परतों 15,16 और C-AFM लिथोग्राफी 11. C-AFM लिथोग्राफी की एक अनूठी विशेषता बहुत छोटे nanoscale सुविधाओं बनाया जा सकता है.
दो आयामी कारावास के साथ संयुक्त इलेक्ट्रिकल शीर्ष gating,, अक्सर III-V अर्धचालकों 17 में क्वांटम डॉट्स बनाने के लिए प्रयोग किया जाता है. वैकल्पिक रूप से, अर्ध एक आयामी semiconducting nanowires विद्युत निकटता से gated किया जा सकता है. इन संरचनाओं का निर्माण करने के लिए तरीकों समय लेने वाली और आम तौर पर अपरिवर्तनीय हैं. इसके विपरीत, C-AFM लिथोग्राफी तकनीक एक nanostructure एक प्रयोग के लिए बनाया है, और फिर (एक व्हाइटबोर्ड के समान) "मिट" किया जा सकता है कि समझ में प्रतिवर्ती है. मिटाकर, जबकि आम तौर पर, C-AFM लेखन, AFM टिप करने के लिए लागू सकारात्मक voltages के साथ किया जाता हैनकारात्मक voltages का उपयोग किया जाता है. एक विशेष संरचना बनाने के लिए आवश्यक समय डिवाइस की जटिलता पर निर्भर करता है, लेकिन आम तौर पर कम से कम 30 मिनट है; उस समय के सबसे कैनवास मिटाकर खर्च किया जाता है. ठेठ स्थानिक संकल्प के बारे में 10 नैनोमीटर है, लेकिन 2 नैनोमीटर 18 बनाया जा सकता है के रूप में साथ उचित ट्यूनिंग के रूप में छोटे सुविधाएँ.
Nanoscale निर्माण प्रक्रिया का एक विस्तृत विवरण इस प्रकार है. यहाँ प्रदान की विस्तार से इसी तरह के प्रयोगों रुचि शोधकर्ताओं द्वारा किया जा करने के लिए अनुमति देने के लिए पर्याप्त होना चाहिए. यहाँ वर्णित विधि अर्धचालकों में इलेक्ट्रॉनिक nanostructures के बनाने के लिए इस्तेमाल पारंपरिक lithographic दृष्टिकोण पर कई फायदे हैं.
यहाँ वर्णित C-AFM लिथोग्राफी विधि स्कैनिंग anodic ऑक्सीकरण 19, डुबकी पेन Nanolithography 20, piezoelectric patterning सहित स्कैनिंग जांच के आधार पर लिथोग्राफी प्रयासों का एक बहुत व्यापक वर्ग का हिस्सा है21, और इतने पर. उपन्यास ऑक्साइड इंटरफेस का उपयोग के साथ मिलकर यहां वर्णित C-AFM तकनीक, भौतिक गुणों का एक अभूतपूर्व किस्म के साथ उच्चतम परिशुद्धता इलेक्ट्रॉनिक संरचनाओं में से कुछ का उत्पादन कर सकते हैं.
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.
Declarações
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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