As semiconductor chips are widely used in computers, phones, automobiles, appliances, etc., more capable yet more efficient semiconductor chips can have profound societal impact such as in reducing electric power consumption and in prolonging the battery life of portable electronics. Today, each semiconductor chip may contain billions of transistors that are made of silicon ‒ the most popular semiconductor. However, as transistors are made smaller and thinner so that more of them can be crammed on the same chip to perform more functions at a faster speed, silicon-based transistors will sooner or later run into physical limitations dictated by quantum mechanics. The physical limitations of silicon will cause transistors to fail to turn on and off efficiently and chips to consume more power and run hotter. To address the physical limitations of silicon, scientists have explored new materials such as graphene, made of a single layer of carbon atoms, as an alternative to silicon in making very small and very thin transistors. However, although graphene allows electrons to be highly mobile in a transistor, graphene lacks an energy gap that could be used to turn the transistor on and off efficiently. This shortcoming of graphene has motivated scientists to explore other atomic-layered materials such as transition-metal dichalcogenids which have an energy gap, but they turned out to have a different shortcoming in allowing electrons to have only low mobility. To solve this dilemma of atomic-layered materials, phosphorene, made of a single layer of phosphorus atoms, was recently discovered to have both a high electron mobility and a sizable energy gap. Thus, phosphorene is a promising replacement for silicon in future-generation semiconductor chips.
To this end, a team of scientists from Purdue University, Lehigh University and Michigan State University propose to explore phosphorene which can potentially overcome the challenges of silicon as well as other two-dimensional atomic-layered materials for ultra-scaled thin-body transistor applications thereby transforming the electronics industry. A collaborative and highly integrated interdisciplinary approach will be used to address three thrust areas: 1) exploration of phosphorene by exfoliation with focus on electrical and optical properties and device applications, 2) synthesis by chemical vapor deposition and nanomanufacturing in collaboration with government and industry labs, and 3) first-principles modeling to guide experiments and to interpret the results. Being the only other elemental material that can be exfoliated like graphene, phosphorene represents a unique opportunity as the basic material for future-generation devices. The initial exploration through exfoliation will guide the development for high-quality, defect-free materials and processes that enable safe and easy integration into device architectures. Additionally, the highly puckered structure of phosphorene dictates that each single layer comprises two tightly bonded atomic layers ? a property that can be exploited to develop a large-scale chemical vapor deposition manufacturing process. The puckered structure also makes phosphorene highly anisotropic ? a property that can be exploited for thermoelectric applications. To reduce the environmental sensitivity of phosphorene and to explore other novel architectures and properties, heterojunctions between phosphorene and graphene, hexagonal boron nitride, molybdenum disulfide, or other chalcogenides and oxides will be explored. For example, unlike most other atomic-layered materials, phosphorene is naturally p-type, and p-type phosphorene transistors can be combined with n-type molybdenum-disulfide transistors to form energy-efficient complementary circuits and tunneling transistors. Moreover, black phosphorus in bulk form has a direct band gap of 0.3 eV, which makes it a useful elemental infrared detector. When black phosphorus is thinned to a single phosphorene layer, the band gap increases monotonically to above 2 eV ? a property that can be exploited for efficient solar cells and tunable photodetectors. This award is co-funded by the Air Force Office of Scientific Research (AFOSR)
