P-type Oxides for CMOS Devices: Thermodynamics-based In-situ Synthesis and In-Situ Integration

Purdue University, West Lafayette IN

Investigators

Abstract

Nontechnical: Silicon metal oxide semiconductors have been the industry standard in electronic devices for decades. However, new non-silicon thin film semiconducting metal oxides have gained prominence in recent years. They are a promising new technology for electronic devices, particularly next generation displays. Oxide electronics have high carrier mobility, allowing them to conduct current efficiently, and can be fabricated at low-temperatures. This makes them compatible with flexible electronics. Doped semiconductors are n-type or p-type, depending upon if the majority charge carriers are electrons or holes. The vast majority of thin film oxide semiconductors are n-type, which limits their applications to unipolar devices. The development of more sophisticated circuits using complementary metal-oxide-semiconductor (CMOS) technology requires both p- and n-type devices. The goal of this project is to resolve the scientific questions that prevent the realization of high performance p- and n-type oxide semiconductors. This in turn will enable the development of low-temperature processed flexible CMOS inverters and other circuits. Fundamental device physics together with thermodynamic and kinetic considerations are the key components to enable in-situ synthesis of the oxides and fabrication of CMOS devices. The combination of fundamental materials processing and device fabrication has educational impacts in and out of the classroom at Baylor University. Low cost metal oxide technology is ideal for a prototype fabrication lab in an undergraduate course on electronic materials and devices. Strong connections between the PI and local industry will further enhance the value of the professional training experience for students. The project also includes outreach to local schools, such as The Annual Central Texas Science and Engineering Fair for middle and high school students. Technical: The recent development of several wide bandgap oxide semiconductors and the fabrication of basic thin film transistor (TFT) structures have garnered attention for applications in flexible electronics and high performance TFTs. However, research efforts of these oxide TFT devices are currently limited to n-type oxide TFTs. Recently, promising p-type oxides have emerged providing the opportunity to explore applications in oxide-based complementary metal-oxide-semiconductor (CMOS) devices. The development of reproducible p-type oxide semiconductors and their TFT devices will greatly accelerate flexible electronics and will pioneer the development of new oxide CMOS devices. A simple binary oxide (SnOx, 1<x<2) is an emerging candidate for a p-type semiconductor due to the possible formation of ns2 hybrid orbitals. However, the growth conditions for p-type SnOx are believed to be narrow, due to the formation of SnO2 (n-type) and the precipitation of metallic Sn. This research suggests reproducible approaches to synthesize p-type oxide semiconductors at low temperatures (T) below 200 ?C. Low-T (<200 ?C) in-situ synthesis of p-type SnOx (1<x<2) is a consequence of the thermodynamic instability of the metallization material in contact with SnO2. The same metallization material must be thermodynamically stable with n-type In(Ga)ZnO. This unique in-situ approach offers simple solutions to the complex challenges of synthesizing p-type oxides: the metallization of both p- and n-type oxide TFTs, and the low-T annealing processes (necessary for improving TFT performance). As a result, oxide CMOS devices will be fabricated in situ. This project also contains strategies for controlling hole carrier densities. The use of high pressure oxidation relates carrier density to the oxygen fugacity (i.e., effective reactivity), and the results will identify the defect-based doping mechanisms for p-oxides. The information obtained from these investigations will be carefully correlated with TFT and CMOS device performance in order to understand the relations between synthesis, composition, material properties and device characteristics. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.