כתיבה ואפיון טמפרטורה הנמוכה של ננו Oxide
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
heterostructures תחמוצת 1-5 תערוכה מגוון רחב להפליא של תופעות פיזיות מתהוות אשר מעניינים גם מבחינה מדעית ועלולים להיות שימושיים עבור יישומים 4. בפרט, הממשק בין LaAlO 3 (לאו) וSrTiO 3 (STO) 6 יכול להפגין בידוד, ניצוח, מוליכי 7, 9 התנהגות כמו ferroelectric-8, ופרומגנטי. ב2006 Thiel et al הראה 10 שיש מעבר מבודדת למתכת חדה כעובי השכבה לאו הוא גדל, עם עובי קריטי של 4 תאי יחידה (4uc). זה לאחר מכן הראה כי מבני 3uc-LAO/STO תערוכת מעבר hysteretic שניתן לשלוט באופן מקומי באמצעות מיקרוסקופ כוח אטומי מוליך בדיקה (c-AFM) 11.
המאפיינים של ממשקי תחמוצת כגון LaAlO 3/3 SrTiO תלויים בהעדר או הנוכחות של ביצועאלקטרונים בממשק. ניתן לשלוט באלקטרונים אלו באמצעות אלקטרודות עליונה שער 12,13, גב שערים 10, משטח adsorbates 14, שכבות ferroelectric 15,16 ויתוגרפיה c-AFM 11. תכונה ייחודית של ליתוגרפיה c-AFM היא שניתן ליצור תכונות ננו קטנות מאוד.
gating העליון חשמל, בשילוב עם כליאה דו ממדים, משמש לעתים קרובות ליצירת נקודות קוונטיות במוליכים למחצה III-V 17. לחלופין, מוליכים למחצה nanowires מעין חד ממדי יכול להיות מגודרת חשמלי בקרבה. השיטות לייצור המבנים האלה הן זמן רב ובדרך כלל בלתי הפיך. לעומת זאת, טכניקת יתוגרפיה c-AFM היא הפיכה במובן זה שnanostructure ניתן ליצור עבור ניסוי אחד, ולאחר מכן "נמחק" (בדומה ללוח). באופן כללי, כתיבת c-AFM מבוצעת ברמות מתח חיובי מוחל על קצה AFM, בעוד, מחיקהמתבצע באמצעות מתח שלילי. הזמן הנדרש כדי ליצור מבנה מסוים תלוי במורכבותו של המכשיר, אך הוא בדרך כלל פחות מ 30 דקות; רוב הזמן הוא בילה למחוק את הבד. הרזולוציה מרחבית הטיפוסית היא כ 10 ננומטר, אך עם כוונון נכון תכונות קטנות כמו ניתן ליצור 2 ננומטרים 18.
תיאור מפורט של הליך ייצור ננו כדלקמן. הפירוט הניתן כאן צריך להיות מספיק כדי לאפשר ניסויים דומים שבוצעו על ידי חוקרים מעוניינים. יש השיטה המתוארת כאן יתרונות רבים על פני גישות מסורתיות ליתוגרפיות משמשות ליצירת ננו האלקטרוני במוליכים למחצה.
השיטה ליתוגרפיה c-AFM המתואר כאן היא חלק ממעמד הרבה יותר רחב של מאמצים ליתוגרפיה מבוססות סריקת בדיקה, כוללים חמצון anodic הסריקה 19, nanolithography לטבול עט 20, דפוסים פיזואלקטריים21, וכן הלאה. טכניקת c-AFM שתוארה כאן, יחד עם השימוש בממשקי תחמוצת רומן, יכולה לייצר כמה מהמבנים האלקטרוניים הגבוהה ביותר הדיוק עם מגוון חסר תקדים של תכונות פיזיות.
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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