For the last decade, RF electronics have become one of the fastest growing semiconductor segments due to the explosive growth of the wireless communication market. To further improve the performance of high speed transistors, novel materials and device structures could enable innovative technologies for faster, higher gain and more cost-effective transistors, changing the landscape of high speed analog circuit and telecommunication. 2D materials are the ultimate building blocks for bottom-up design of device functionalities1. Being atomically thin, these 2-D materials exhibit a wide range of properties spanning from semi-metals (graphene), semiconductors (MoS2, WSe2, etc.) to insulators (BN) and topological insulators (Bi2Te3)1. By exploring the potential of their unique electronic properties, atomic thickness and carefully engineered interfaces, ultra-thin, lightweight, high efficiency and cost-effective RF electronics could be achieved.
Our main objective is to create high performance 2D vertical tunneling transistors with high current density and high current gain which could potentially be used as high cut-off frequency (fT) and oscillation frequency (fmax) RF transistors. These transistors could also be made in flexible and transparent formats. High current density is required to obtain high performance RF tunneling transistor. In this sub project, either growing thin tunnel oxide (SiO2 thickness is about 1~2nm) or using effective tunnel oxide (such as MgO and Gd2O3), is planned to utilize for gaining high current density. Our preliminary results show that using ultra thin SiO2 as tunneling barrier, improvement from 10 mA/cm2 to 100A/cm2 can be achieved for tunneling current from the emitter electrodes. Band alignment at the base-collector interface is also important for vertical electron injection. By looking for the optimal combination of tunnel oxide, filter oxide and 2D materials, it is possible to further improve the tunneling probability for better performance. b. Improving the 2D material- metal contact for low base resistance Low base resistance is critical since it plays a direct role in determining the fT of RF transistor. In this sub project, we will focus on improving the contact between 2D materials and metal electrodes, which is regarded as one of the key areas in promoting the practical application of 2D materials in various electronics. To this end, we will focus on 1. Understand the fundamental physics governing the transport between 2D materials and metals; 2. Explore ways to promote reacted or hybridized 2D material-metal interface for a lower contact resistance. 3. Phase transformation of TMDs to semi-metallic phase to eliminate the Schottky barrier between the TMD material and the metals.
1. Energy barrier engineering for improving current density:
In order to achieve high current density, there are two alternative plans that may be adopted in tracking constraints. For the growth of ultrathin tunnel SiO2, rapid thermal anneal (RTA) is needed. The growth temperature and time is important for such thin tunnel layer. Various conditions will be tested and thus to determine the best quality of tunnel oxide for the purpose of high current density. For MgO and Gd2O3, the magnetron sputtering system is used for the growth of high quality tunnel oxide. In our lab, we have experienced on growth of MgO for spintronics. It is expected to efficiently grow high quality tunnel oxide for enhancement of tunneling current. The device performance will be compared between two methods.
2. Contact engineering for semiconductor 2D materials
To investigate vertical transport between the 2D materials and bulk materials, vertical stacks of highly doped silicon (acting as an atomically flat substrate as well as an electrode), graphene (to eliminate the Schottky barrier) and metals will be fabricated. The transport properties of the vertical stack will be investigated using the inelastic electron tunneling spectroscopy (IETS) at low temperature (1.9 K). The measurement will be carried out using our physical property measurement system (PPMS), as well as a custom-built IETS system. For improving the contact resistance between metal electrodes and 2D semiconductors, various surface treatments will be carried out including surface functionalization and annealing. To accurately extract the contact resistance, the transfer line analysis will be adopted. Devices based on exfoliated as well as CVD grown samples will be fabricated using e-beam and photo-lithography.
3. Phase transformation for low base contact resistance
Phase transformation from the 2H phase to the 1T phase will lead to changes in the vibrational modes and binding energy of TMD material, therefore Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) will be used to verify the transformation. Due to the change in the electronic structure from semiconducting to semi-metallic, photoluminescence (PL) is another effective tool to probe the transformation. Quantified data on the extent of phase transformation, defined as the ratio between the 1T phase and the transformed region, can be obtained from XPS analysis. Spatial distribution of the 1T and 2H phase can be visualized through Raman and PL mapping. The contact resistance between the TMDs and various metals with and without phase transformation will be probed using the standard transfer length method. Vertical tunneling transistors with 2H-TMD materials as the base region, and various degrees of phase transformation will be fabricated and characterized to investigate the effectiveness of phase transformation in improving both the DC and RF performance.