Technical: This project aims for the discovery of new random materials that exhibit metal-insulator transitions when the sample size is comparable to electron's localization length. Such materials, recently observed in perovskite solid solutions and in SiO2 atomically dispersed with Pt, may find potential applications for resistance switching in non-volatile memory. These materials are solid solutions of an insulator and a metallic component. By varying the composition and the thickness of the thin films, we will engineer a tunable metal-insulator transition of the Anderson variety, being insulating at larger thickness, but metallic at smaller thickness. The transition can also be triggered by a voltage due to charge injection/removal, which alters the localization length. Local atomic structures and metallicity will be determined using synchrotron radiation, electron energy loss spectroscopy, and optical spectroscopy, whereas long range metallicity and transitions will be established using transport/dielectric measurements and atomistic simulations. Integrated research and educational activity built around PhD education, laboratory experience and remote laboratory access will benefit local and distant undergraduate and graduate students, as well as high school and university teachers.
The project explores fundamental material research at the nanoscale level where the prediction of quantum physics can be directly applied to material design with a potentially high payoff. The research is motivated by the insatiable need for new memory materials with a smaller footprint and a faster speed, to be used in computers and especially hand-held digital devices. Unlike conventional approaches that focus on materials of ever increasing perfection and purity, such as silicon single crystal, this project turns to random materials with a high concentration of impurities. If the project is successful, it will open a new avenue of technological opportunities. An education plan is integrated with the research: it emphasizes integration of instruction learning and laboratory learning, for both local and distant undergraduate and graduate students who, via on-line access, will gain laboratory experience.
PI: I-Wei Chen Awardee: University of Pennsylvania Award Number: 1104530 Award Expires: 07/31/2014 Program Officer Name: Charles Ying Program Officer Email Address: cying@nsf.gov Program Officer Phone Number: (703)292-8428 Research and Education activities Advanced integrated circuit devices have always demanded the highest purity, highest perfection materials. As the material technology has been driven in this way for the last five decades, the conventional wisdom does not regard random materials of random compositions to be device worthy. Indeed, random materials have indefinite energy levels and lower electron mobility, making them more difficult to control and slower to perform electronic functions. In the nano scale, however, random materials do undergo a size-dependent insulator to metal transition; moreover, the transition is reversible and can be electrically triggered at a modest voltage. This opens up the possibility of an entirely new class of electronic materials for device applications; many easy and inexpensive to fabricate by the prevailing CMOS process. This project is the first to explore such nano materials and their underlying physics offor potential applications of non-volatile resistance memory—memory that electrically switches between two resistance states, but remains unchanged when the voltage is withdrawn from the device. The project has successfully demonstrated the transition in a wide variety of atomic admixtures, made of an insulating oxide/nitride and a metal, the metal of a concentration as little as one tenth of the percolation threshold (typically 40%). Regardless of material, two-way switching is manifest at the same switching voltage over a broad range of temperature (2-350K), speed (1 ms to 1 ns) and device thickness (5-20 nm). Scaling laws for device resistance and capacitance, roles of electrodes and intermediate resistance states, and effects of UV irradiation and moist environment have all been determined. Electron-phonon interaction has been identified as a key enabling element that for switching and memory storage. This was directly verified to operate over a time scale of 1015 orders of magnitude, ranging from 100 s in a pressure test when the device is placed in a pressure vessel, to 10-13 s during mechanoplasmonic pulsing when the device is placed next to an ultrarelativistic electron bunch hundreds of mm away. Additional device engineering that lowered power consumption, improved endurance, and provided access to multibit memory has also been implemented. The project has trained two PhD students currently employed in the US electronic industry, and made available new non-volatile memory that has attracted industrial interest.
