Advanced Logic and Memory Devices

For the past 50 years, transistors have been the main drivers for human life evolution. Moore's law depicts the brief history of science and technology in the modern era. From the first MOSFET to state of the art nanosheet FETs, everything changes except FETs as the basic unit for computing! There are many predictions on the end of Moore's law. However, before it does go to the tomb when the physical thickness of oxide is shrunk to the atomic scale, there are still a lot can be done. For example, 3D IC, high-mobility channel materials, heterogeneous integration, in-memory computing, etc. In QEL, we're aiming for the continuation of group IV-based materials such as GeSn for the extension of Moore's law. The advantages of GeSn are its compatibility to Si VLSI technology, high electron and hole mobilities, the potential of integration with Si photonics due to its direct-band nature. By the low-temperature chemical vapor deposition (CVD), high-quality GeSn epitaxial layers can be grown on Si substrates with a Ge relaxed buffer layer to accomodate dislocation defects at the Ge/Si heterointerfaces.

Hole mobility can be increased by compressive strains in the GeSn layer owing to the reduced hole effective mass. On the other hand, electron mobility can be much enhanced by tensile strains by the increased electron population in the direct band, where the effective mass is much reduced. We achieved record-high electron mobilities in GeSn n-MOSFETs in IEEE Electron Device Letters 2018, DOI and demonstrated the mobility enhancement by tensile strain for the first time (IEEE Electron Device Letters 2021, DOI). We also characterized the electron transport in n-GeSn films and demonstrated record-high mobility (> 6,000 cm2/V-s) at cryogenic temperatures (Advanced Electronic Materials 2025, DOI). While high electron mobility can be achieved in GeSn n-MOSFETs, the high contact resistivity in metal/n-GeSn contacts is still an issue. We used in-situ CVD to grow n-GeSn films with high doping concentrations and to form NiGeSn by post-annealing at low temperatures. We achieved a record-low contact resistivity of 1.5 x 10-7 Ohm-cm and determined the Schottky barrier height for the first time and found that it is pinnied at an energy of ~ 0.09 eV above the valence band for GeSn with different Sn fractions (ACS Applied Electronic Materials 2021, DOI).

As the transistor scaled down, the issue of power consumption due to subthreshold leakage becomes worse. Tunnel FETs have been proposed to reduce subthreshold conduction with benefits of the similarity of device structures and material compatibility with Si CMOS devices. We focus on the III-V broken-gap and GeSn tunnel FETs due to their capability of high tunnel rates. We proposed a vertical struture which separates two requirements of high ON current and low OFF current by heterobarrier or superlattice structure. The ON current is enhanced by the alignment of gate electric field and band-to-band tunneling (IEEE Electron Device Letters 2017, DOI). Furthermore, we investigated band-to-band tunneling (BTBT) in InAs/GaSb and GeSn Esaki diodes for high drive current applications. We demonstrated world record high peak tunnel current densitiesy of 10 MA/cm2 and 545 kA/cm2 in the InAs/GaSb (IEEE Transactions on Electron Devices 2021, DOI) and GeSn Esaki diodes (Advanced Materials 2022, DOI) among III-V and group-IV materials, respectively.

In addition to logic devices, there are high demands on the in-memory computing. We focus on the ferroelectric-based materials for non-volatile memory deivce applications. For example, we use hafnium-zirconium oxide (HZO) as an oxide layer sandwiched by metals to form FeRAM devices with high endurance. To further improve the device performance, the electrode/ferroelectrics interface is critical. We perform low-temperature measurement to investigate the ferroelectric properties of HZO films. We are also interested in HZO-based ferroelectric field-effect transistors (FeFETs) for future in-memory computing applications.