Group of Prof. Wei Liu published two research articles in a leading international journal JACS


AuthorGuirong Su and Tao Shen

 

Recently, the research group of Wei Liu of Nano and Heterogeneous Materials Center, School of Materials Science and Engineering of our university has made a breakthrough in the field of interface contact of new-generation nano-electronic devices. On Jan. 4, 2019, the Journal of the American Chemical Society published the significant results of the group in the metal-organic interface. The title of the paper is: Switchable Schottky Contacts: Simultaneity Enhanced Output Current and Reduced Current. The first author of this paper is Ph.D. student Guirong Su, Lecturer Ji-Chang Ren and Professor Wei Liu are the corresponding authors, and Nanjing University of Science and Technology is the first affiliation.

 

Figure 1. The Schottky barrier heights of physisorbed and chemisorbed systems and the regulation mechanism between the two states.

Diodes are indispensable components in integrated circuits due to their unique amplitude-limiting and rectification properties. However, traditional p-n junction diodes made from semiconductor materials cannot satisfy the increased technical demands of integrated circuits with respect to low power consumption, high current and ultra-high speed. Instead, these heightened technical requirements can be fulfilled by utilizing metal-semiconductor contacts, also known as Schottky diodes, which exhibit extremely short reverse recovery time, low turn-on voltage, and high-frequency response characteristics.

Early Schottky contacts were formed from the junction of a metal anode (Mo, Cr, W, and Pt), with an n-type silicon cathode. However, continuous miniaturization of electronic devices has created a demand for alternatives to silicon-based devices which, due to physical limits, become sensitive and unstable. One possible route of eliminating silicon in these diodes is by replacement with organic components, especially p-conjugated molecules, which comparatively have more varied chemistry, higher adsorption coefficients, and fewer intrinsic defects. The organic component of a Schottky diode is then selected to maximize output current while minimizing reverse leakage. However, there is a long-established tradeoff between these two design criteria, as any decrease to the Schottky barrier of the system allows increased current in both directions. Consequently, there is high demand for new materials for the design of Schottky diodes which exhibit thermal stability, low forward barrier height and low leakage current.

The metal-molecule interface can have either a physisorbed or chemisorbed nature of bonding. For smaller (higher) adsorption distance, electron orbital interactions between the metal and the molecule are stronger (weaker), which may lead to a lower (larger) barrier height. A low Schottky barrier height would result in a large interfacial current while a high barrier height may suppress the current in the circuit. In this work, the author constructed a series of bistable surface adsorptions from metal surfaces and aromatic compounds, which are capable of overcoming a longstanding design trade-off between high forward current through the diode, and low leaking of reverse current. This study specifically considers the chemical phenomena which provide a new design paradigm for higher performing Schottky diodes, and our proposed general approach could also be applied in other aspects of surface science such as coatings and catalysis, as well as having implications for bulk solids with tunable electronic properties.

On Jan. 28, 2019, JACS published another latest development in the interface between two-dimensional semiconductor and metal heterojunctions from Wei Liu’s research group. The title of the paper is: van der Waals Stacking Induced Transition from Schottky to Ohmic Contacts: 2D Metals On Multilayer InSe. The first author is the Ph.D. student Tao Shen. Associate Professor Shuang Li and Professor Wei Liu are corresponding authors of the paper. All the authors are members of Wei Liu’s group of Nano and Heterogeneous Materials Center, School of Materials Science and Engineering.

With the rapid development of nanofabrication technology and nanomaterial preparation technology, the continuation of the performance of traditional silicon devices will face enormous challenges when the size of integrated circuit devices shrinks to the nanoscale limit. Device design featuring new materials, new structures, and new principles is at the forefront of nanomaterials and nanoelectronics science. Two-dimensional (2D) layered materials are currently the most promising solution to the scale bottleneck encountered by silicon-based materials in the development of chips. 2D materials can limit carriers to a space of 1 nm at the interface, and their single crystal properties ensure uniformity and reliability of device performance.

In particular, 2D InSe was recently produced by mechanical exfoliation and has been demonstrated to have high electron mobility (exceeding 103 cm2 V-1 s-1), comparable to that of black phosphorus, while remaining stable under ambient conditions. Practical utilization of 2D InSe in devices requires direct contact with metal electrodes to enable the injection of carriers. However, conventional metal-2D InSe contacts are often associated with the formation of a finite Schottky barrier which reduces carrier injection efficiency, increases contact resistance, and degrades device performance. Note that the contacts in InSe-based nanodevices are actually as important as InSe itself, whose excellent intrinsic properties would be masked by the high Schottky barrier at the hybrid interface. Therefore, the challenge of designing interfaces which form low-resistance Ohmic contact is of critical importance for the design, assembly, and fabrication of high-performance semiconductor devices.

In this work, the authors demonstrated that 2D van der Waals metal-semiconductor junction almost approaches the Schottky-Mott limit, realizing an effective tuning of the Schottky barrier. Importantly, the increase of InSe layer number induces a transition from Schottky to Ohmic contact, which is attributed to the decrease (rise) of conduction band minimum (valence band maximum) of InSe. Using density functional theory, authors provide a cautious explanation of the weak Fermi level pining and electronic properties between metal and semiconductor.

This design concept effectively solves the problem of contact resistance in electronic devices. At the same time, the depletion layer of the carriers generated at the contact interface is on the atomic scale, which will greatly increase the response frequency of the device. This work provides an effective solution to the contact problems of low-dimensional nanodevices. This work provides theoretical support for the design of two-dimensional metal materials.

 

Figure 2. The transition from Schottky to Ohmic contact induced by the stacking InSe layers.

The above work is supported by the NSF of China.

 

The paper links

https://pubs.acs.org/doi/abs/10.1021/jacs.8b11459

https://pubs.acs.org.ccindex.cn/doi/abs/10.1021/jacs.8b12212