Recent News

  • April 2020: Prof. Kapadia wins the 2020 AVS Peter Mark Memorial Award.

  • April 2020: Debarghya Sarkar wins the USC ECE Best Dissertation Award.

  • April 2020: Nick Mehlman wins the Philip Beigler Memorial Award.

Recent Papers

Metal Free Hot Electron Driven Electrode Assisted by Plasma Etched Graphene for Hydrogen Evolution Reaction

Hyun Uk Chae, Ragib Ahsan, Jun Tao, Stephen B. Cronin, and Rehan Kapadia

Advanced Materials Interfaces, 8 (6) 2001706, 2021

Recently, it has been shown that a semiconductor–insulator–graphene device can drive the hydrogen evolution reaction (HER) at the graphene surface with a reduced onset potential by injecting hot electrons into graphene. However, the catalytic properties of graphene are limited by the large hydrogen adsorption energy and lack of electrochemically active sites. To address these limitations, a n-silicon/insulator/plasma etched graphene device is investigated, where a dry etch process is used to increase the number of active sites on the graphene by creating a greater number of active edge sites, increasing hydrogen adsorption at a given potential. This has been shown to improve the properties of devices with cold electrons. However, here it is shown that this approach can improve the HER rate with hot electrons. The electrons injected into the graphene from the silicon shift the onset potential of HER by as high as ≈0.8 V reaching a current density of 90 mA cm−2 at an overpotential of -0.5 V versus RHE. Furthermore, the comparison between device with pristine graphene shows a ≈2X improvement in current density at high overpotentials. This result shows that hot-electron devices can be improved by modifying the catalytically active sites without metal catalysts.

Hot electron emission from waveguide integrated lanthanum hexaboride nanoparticles

Fatemeh Rezaeifar, Hyun Uk Chae, Ragib Ahsan, Rehan Kapadia

Applied Physics Letters 118 (7), 071108, 2021

Recently, it has been shown that hot-electron photoemission in waveguide-integrated graphene can occur at peak optical power densities many orders of magnitude lower than multiphoton and strong field emission. In this work, we study how the deposition of low-work function lanthanum hexaboride nanoparticles can alter the behavior of hot-electron emission from graphene and thin gold waveguide-integrated hot electron emitters. This approach is promising, as the graphene enables an electrically conductive platform on which to deposit the nanoparticles, while still enabling interaction between the nanoparticles and incident photons. Despite nonideal coatings of LaB6 nanoparticles on the waveguide integrated graphene and gold, there is a nearly order of magnitude improvement over previous graphene-based hot-electron emitters. This hybrid approach demonstrates how a combination of integrated photonics and low-work function coatings can improve the performance of the emerging class of hot-electron emitters.

Monolithic High-Mobility InAs on Oxide Grown at Low Temperature

Debarghya Sarkar, Jun Tao, Ragib Ahsan, Dingzhu Yang, Thomas Orvis, Sizhe Weng, Frank Greer, Jayakanth Ravichandran, Constantine Sideris, Rehan Kapadia

ACS Applied Electronic Materials 2 (7), 1997-2002, 2020

We demonstrate high electron mobility single-crystal InAs mesas monolithically integrated on amorphous dielectric substrates at a growth temperature of 300 °C. Critically, a room temperature mobility of ∼5800 cm2/(V s) was measured, the highest mobility reported for any thin-film semiconductor material system directly grown on a nonepitaxial substrate. Detailed modeling of the scattering mechanisms in the grown material indicates that the mobility is limited by surface roughness scattering, not the intrinsic material quality. We project that reducing the RMS surface roughness of the InAs from 1.8 to 1 nm would produce materials with room temperature mobilities of >10000 cm2/(V s), and RMS roughness of 0.5 nm would result in mobility of ∼20000 cm2/(V s), essentially identical with epitaxially grown materials. These results pave the way for growth of high-mobility materials directly onto the back end of silicon CMOS wafers and other nonepitaxial substrates such as glass, as well as polymers for flexible electronics.

Engineering Complex Synaptic Behaviors in a Single Device: Emulating Consolidation of Short Term Memory to Long Term Memory in Artificial Synapses via Dielectric Band Engineering

Jun Tao, Debarghya Sarkar, Salil Kale, Prakhar Kumar Singh, Rehan Kapadia
Nano Letters 20 (10) 7793-7801, 2020
As one of the key neuronal activities associated with memory in the human brain, memory consolidation is the process of the transition of short-term memory (STM) to long-term memory (LTM), which transforms an external stimulus to permanently stored information. Here, we report the emulation of this complex synaptic function, consolidation of STM to LTM, in a single-crystal indium phosphide (InP) field effect transistor (FET)-based artificial synapse. This behavior is achieved via the dielectric band and charge trap lifetime engineering in a dielectric gate heterostructure of aluminum oxide and titanium oxide. We analyze the behavior of these complex synaptic functions by engineering a variety of action potential parameters, and the devices exhibit good endurance, long retention time (>105 s), and high uniformity. Uniquely, this approach utilizes growth and device fabrication techniques which are scalable and back-end CMOS compatible, making this InP synaptic device a potential building block for neuromorphic computing.