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.
氧化物异质1-5表现出了显着各种各样的突发物理现象这都是有趣的科学和应用4可能有用。特别是, 铝酸镧(LAO)和钛酸锶(STO)6之间的界面上可以显示出绝缘性,导电,超导7,铁电状8和9铁磁行为。在2006年,Thiel 等人发现10,有一个锐利的绝缘体-金属转变为LAO层的厚度增加时,为4的单元电池(4UC)的临界厚度。据事后证明3uc-LAO/STO结构表现出了可以在本地用导电原子力显微镜(C-AFM)探针11被控制的迟滞过渡。
氧化接口,如铝酸镧/ 钛酸锶的属性取决于传导的存在或不存在电子界面处。这些电子可以通过顶部栅电极12,13,背门10被控制,表面吸附物14,铁电层15,16和c-AFM光刻11。的C-AFM平版印刷的独特的特点是非常小的纳米级功能可以被创建。
电顶部门控,并结合2维限制,通常被用来创建在III-V族半导体17的量子点。备选地,准一维半导体纳米线可以电由接近门控。该方法用于制备这些结构是耗时的且通常不可逆的。与此相反,在C-原子力显微镜光刻技术是可逆的,即纳米结构可以为一个实验被创建,然后在“擦除”(类似白板)。通常,C-AFM写作与施加在针尖正电压进行的,而,擦除使用负电压被执行。创建特定结构所需的时间依赖于该设备的复杂性,但通常不超过30分钟;大部分时间都花在擦除画布。典型的空间分辨率约为10纳米,但2纳米可以创建18用适当的调整功能小。
纳米级的制造步骤的详细说明如下。这里所提供的细节,应足以允许类似的实验,以通过感兴趣的研究人员进行。这里所描述的方法具有超过用于在半导体制造的电子的纳米结构的传统光刻方法的许多优点。
这里所描述的C-AFM光刻方法是一个更广泛的类的扫描探针为基础的光刻技术的努力,包括扫描阳极氧化19,蘸笔纳米光刻20,压电图案的一部分21,依此类推。这里所描述的C-原子力显微镜技术,再加上使用的新型氧化物界面,可以产生一些最精密的电子结构与前所未有的各种物理性能。
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 |