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.
산화물 헤테로 1-5 전시 과학적으로 흥미로운 응용 프로그램 4 잠재적으로 유용한 응급 물리적 현상의 매우 다양한 종류. 특히, LaAlO3는 (LAO) 및 3 된 SrTiO3 (STO) (6) 사이의 인터페이스는 8 강유전체 조 및 강자성 구 동작 7 초전도, 전도성, 절연성을 나타낼 수있다. 2006 년 틸 등이 LAO 층의 두께와 같은 날카로운 절연체 – 금속 전이가 있음을 보여 주었다 (10)은 4 단위 셀 (4uc)의 임계 두께에 따라 증가한다. 그것은 이후 3uc-LAO/STO 구조가 전도성 원자 힘 현미경 (C-AFM) 탐침 (11)와 로컬로 제어 할 수있는 이력 전환을 나타내는 것으로 나타났다.
예컨대 LaAlO3는 / 3 된 SrTiO3 등의 산화물 인터페이스의 특성은 도전성의 부재 또는 존재에 의존계면에서 전자. 이 전자가 위로 게이트 전극 (12, 13), 게이트 (10)를 사용하여 제어 할 수 있습니다, 표면 (14) 흡착, 강유전체 층 (15, 16)과 C-AFM 리소그래피 11. C-AFM 리소그래피의 독특한 기능은 매우 작은 나노 기능을 생성 할 수 있다는 것입니다.
두 차원 감금과 함께 전기 톱 게이트는 종종 III-V 반도체 (17) 양자 도트를 만드는 데 사용됩니다. 대안 적으로, 준 일차원 반도체 나노 와이어는 전기적으로 근접하여 게이팅 될 수있다. 이러한 구조의 제조 방법은 시간 소모적이고, 일반적으로 비가 역적이다. 대조적으로, C-AFM 리소그래피 기술은 나노 구조물이 한 실험에 대해 생성 한 다음, (화이트 보드와 유사한) "소거"될 수 있다는 점에서 가역적이다. 삭제, 동안 일반적으로, C-AFM 쓰기는 AFM 팁에 적용 양의 전압으로 수행음의 전압을 사용하여 수행된다. 특정한 구조를 생성하는 데 필요한 시간은 디바이스의 복잡성에 의존하지만, 일반적으로 30 분 미만이고; 그 시간의 대부분은 캔버스를 삭제 소요됩니다. 일반적인 공간 해상도는 약 10 나노 미터입니다 만, 2 나노 미터 (18)를 생성 할 수있는 적절한 튜닝은 작은 있습니다.
나노 제조 절차의 상세한 설명은 다음과 같다. 여기에 제공된 내용은 비슷한 실험이 관심이 연구자에 의해 수행 될 수 있도록 충분해야한다. 여기에 설명 된 방법은 반도체에 전자 나노 구조를 만드는 데 사용되는 기존의 리소그래피 방법에 비해 많은 장점을 가지고 있습니다.
여기에 기재된 C-AFM 리소그래피 방법은 주사 양극 산화 (19), 딥 – 펜 나노 리소그래피 (20), 압전 패터닝 등 스캐닝 프로브 기반 리소그래피 노력의 훨씬 광범위한 클래스의 일부인21, 등등. 신규 산화물 인터페이스의 사용과 함께 여기에 설명 C-AFM 기술은, 물성의 전례 다양한 높은 정밀도의 전자 구조의 일부를 생성 할 수있다.
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 |