Prof. Ma's current research projects Advanced Gate Dielectrics The continued down scaling of MOS devices has created many challenges and opportunities, among them the formidable requirement for an ultra-thin gate oxide (<1.0 nm in thickness) within a few years. Since the tunneling leakage current for such an ultra-thin oxide will be unacceptable for mainstream ULSI applications, an alternative gate dielectric must be found to sustain the scaling trend. By the use of novel deposition techniques, Prof. Ma’s group has demonstrated gate-quality silicon nitride, TiO2, Zr02, and Hf02 films that exhibit very low leakage current (several orders lower than thermal SiO2), high resistance to impurity diffusion, high resistance to hot-carrier damage, and very low stress-induced leakage current. This work has attracted the attention of several major semiconductor companies who have conducted joint studies with Prof. Ma. Additional research remains to be performed to better understand the electronic properties of these advanced dielectric films. High on the list is the understanding of channel mobility degradation mechanisms and reliability of high-k gated MOSFETs. Flash Memory Devices Flash memory devices have found many new applications in recent years, and the list is growing rapidly. The majority of the flash memory devices are of the floating-gate variety. Over the past two decades, these devices have been scaling down aggressively in size to increase the packing density; but the industry is quickly approaching its scaling limit. The difficulty is in scaling down the tunnel oxide because of the stringent retention time requirement.  Prof. Ma’s group has demonstrated an alternative dielectric to replace the tunnel oxide, and the results so far suggest the possibility of continued scaling for at least 3 more generations beyond what's possible with the current technology. This alternative tunnel dielectric is based on silicon nitride and exhibits remarkable electronic properties, including very low stress-induced leakage current (SILC) and low densities of interface and bulk traps, very attractive for flash memory device applications.  MIS Devices Based on SiC, GaN, GaP, SiGe, and InAs To explore the potential of high-quality gate dielectrics for semiconductors other than Si, Prof. Ma’s group has done experiments with SiC, GaN, GaP, SiGe, and InAs, and the results all look encouraging. Among them the most extensive work has been done on SiC, as SiC is one of the most attractive semiconductors for high-temperature, high-power electronics. But, despite extensive research effort for over a decade, high-quality, high-reliability SiC CMOS transistors have remained elusive, due to the lack of a high-quality high-reliability gate dielectric for both n- and p-channel MOSFETs. For several years, Prof. Ma’s group has been investigating the use of a novel siliconoxide/nitride/oxide (ONO) stack as the gate dielectric for SiC and has achieved unmatched gate dielectric reliability at high temperatures (upto 450 ºC). More work is being carried out to understand the interface properties of SiC MIS systems and their effects on transistors' quality and reliability. In addition to SiC, Prof. Ma’s group has been working on another promising wide bandgap semiconductor, GaN, as well as on several other semiconductors, including SiGe, GaP, and InAs, with the goal of achieving a unified framework for high-quality gate dielectric deposition for all these technologically important semiconductors. Inelastic Electron Tunneling Spectroscopy (IETS) IETS is a promising technique for the study of microstructures of ultra-thin gate dielectrics where electron tunneling currents are non-negligible. It utilizes tunneling electrons to probe vibrational modes and electronic excitations involving phonons, defects, and impurities in the gate dielectric as well as near its interfaces on both sides. IETS spectra obtained on ultra-thin (1.5-1.8nm) SiO2/Si exhibit clearly resolvable Si phonons of various modes and Si-O bonding vibration modes plus structures attributable to impurities such as Si-F and Si-H bonds. IETS measurements on ultra-thin silicon nitride also revealed expected Si phonons and Si-N bonding vibrations. IETS results on high-k dielectrics, such as HfO2 and HfAlO, exhibit many features that are attributable to  bonding vibrations of the high-k oxide and their interfaces; in particular, recent IETS results have revealed soft optical phonons in HfO2 that couple strongly to electrons, which provides clear evidence to support the theory that soft optical phonons in a high-k gate dielectric may degrade carrier mobility in the channel below it. The ultimate aim of this project is to develop IETS into a broadly applicable characterization tool for a variety of ultra-thin dielectrics. Ferroelectric Thin Films for Memory Technology The key property of a ferroelectric material is spontaneous polarization that can be reversed by an applied field. This property is attractive for memory applications. Prof. Ma’s research in this area has involved the study of the properties of SBT (SrBi2Ta2O9) and PGO [Pb5Ge3O11] thin films, as well as the design, simulation, fabrication, and characterization of novel experimental devices based on such ferroelectric thin films. An invention from his group, based on the idea of a ferroelectric capacitor-less DRAM (FEDRAM) cell, won the 1998 National Collegiate Inventors Award sponsored by the B.F. Goodrich Corporation and the Inventors Hall of Fame. In collaboration with Prof. Charles Ahn of the Yale Department of Applied Physics, Prof. Ma is working on non-volatile memory devices based on single-crystal ferroelectric films. Ma homepage