Showing papers by "Myung-Ho Bae published in 2011"
TL;DR: The fabrication and design principles for using transparent graphene interconnects in stretchable arrays of microscale inorganic light emitting diodes (LEDs) on rubber substrates are described and several appealing properties of graphene are demonstrated, including its ability to spontaneously conform to significant surface topography.
Abstract: This paper describes the fabrication and design principles for using transparent graphene interconnects in stretchable arrays of microscale inorganic light emitting diodes (LEDs) on rubber substrates. We demonstrate several appealing properties of graphene for this purpose, including its ability to spontaneously conform to significant surface topography, in a manner that yields effective contacts even to deep, recessed device regions. Mechanics modeling reveals the fundamental aspects of this process, as well as the use of the same layers of graphene for interconnects designed to accommodate strains of 100% or more, in a completely reversible fashion. These attributes are compatible with conventional thin film processing and can yield high-performance devices in transparent layouts. Graphene interconnects possess attractive features for both existing and emerging applications of LEDs in information display, biomedical systems, and other environments.
325 citations
TL;DR: The authors' data indicate that thermoelectric effects account for up to one-third of the contact temperature changes, and that current crowding accounts for most of the remainder, andModelling predicts that the role ofCurrent crowding will diminish and the roles of thermoeLECTric effects will increase as contacts improve.
Abstract: The temperatures of the graphene–metal contacts in working transistors have been measured with a resolution of ∼10 nm, revealing the presence of both heating and cooling effects.
320 citations
TL;DR: In this paper, the authors demonstrate a reliable technique for counting atomic planes (n) of few-layer graphene (FLG) on SiO2/Si substrates by Raman spectroscopy.
Abstract: We demonstrate a reliable technique for counting atomic planes (n) of few-layer graphene (FLG) on SiO2/Si substrates by Raman spectroscopy. Our approach is based on measuring the ratio of the integrated intensity of the G graphene peak and the optical phonon peak of Si,I(G)/I(Si), and is particularly useful in the rangen>4wherefewmethodsexist.Wecompareourresultswithatomicforcemicroscopy(AFM)measurements and Fresnel equation calculations. Then, we apply our method to unambiguously identifynof FLG devices on SiO2andfind that the mobility (2000 cm 2 V 1 s 1 ) is independent of layer thickness forn>4. Ourfindings suggest that electrical transport in gated FLG devices is dominated by carriers near the FLG/SiO2interface and is thus limited by the environment, even forn>4.
140 citations
TL;DR: In this article, the authors used infrared thermal imaging and electrothermal simulations to find that localized Joule heating in graphene field effect transistors on SiO2 is primarily governed by device electrostatics.
Abstract: We use infrared thermal imaging and electrothermal simulations to find that localized Joule heating in graphene field-effect transistors on SiO2 is primarily governed by device electrostatics. Hot spots become more localized (i.e., sharper) as the underlying oxide thickness is reduced, such that the average and peak device temperatures scale differently, with significant long-term reliability implications. The average temperature is proportional to oxide thickness, but the peak temperature is minimized at an oxide thickness of ∼90 nm due to competing electrostatic and thermal effects. We also find that careful comparison of high-field transport models with thermal imaging can be used to shed light on velocity saturation effects. The results shed light on optimizing heat dissipation and reliability of graphene devices and interconnects.
95 citations
TL;DR: In this paper, the authors report measurements of magnetoresistance in bilayer graphene as a function of gate voltage (carrier density) and temperature, and demonstrate that the electron-electron Nyquist scattering is the major source of phase decoherence.
Abstract: We report measurements of magnetoresistance in bilayer graphene as a function of gate voltage (carrier density) and temperature. We examine multiple contributions to the magnetoresistance, including those of weak localization (WL), universal conductance fluctuations (UCF), and inhomogeneous charge transport. A clear WL signal is evident at all measured gate voltages (in the hole doped regime) and temperature ranges (from 0.25 to 4.3 K), and the phase coherence length extracted from the WL data does not saturate at low temperatures. The WL data is fit to demonstrate that the electron–electron Nyquist scattering is the major source of phase decoherence. A decrease in UCF amplitude with increase in gate voltage and temperature is shown to be consistent with a corresponding decrease in the phase coherence length. In addition, a weak positive magnetoresistance at higher magnetic fields is observed, and attributed to inhomogeneous charge transport.
17 citations
Posted Content•
TL;DR: In this paper, the authors studied the effect of microwave radiation on short superconducting nanowires and concluded that both of the two types of switching events are triggered by the same microscopic event, namely a single Little's phase slip.
Abstract: We study current-voltage (V-I) characteristics of short superconducting nanowires of length ~ 100 nm exposed to microwave radiation of frequencies between 100 MHz and 15 GHz. The radiation causes a decrease of the average switching current of the wire. This suppression of the switching current is modeled assuming that there is one-to-one correspondence between Little's phase slips and the experimentally observed switching events. At some critical power P* of the radiation a dissipative dynamic superconducting state occurs as an extra step on the V-I curve. It is identified as a phase slip center (PSC). With the dependence of the switching currents and the standard deviations observed at the transitions (i) from a constant supercurrent state to a normal state and (ii) from a constant superconducting state to a PSC state, we conclude that both of the two types of the switching events are triggered by the same microscopic event, namely a single Little's phase slip. We show that the Skocpol-Beasely-Tinkham model is not applicable to our microwave-driven phase slip centers, since it leads to an unphysical small estimated value of the size of the dissipative core of the PSC. Through the analysis of the witching current distributions at a sufficiently low temperature, we also present evidence that the quantum phase slip play a role in switching events under microwaves.
20 Jun 2011
TL;DR: In this paper, the authors examined the performance degradation caused by self-heating as a function of insulator (SiO 2 ) thickness and found that peak channel temperatures are proportional to oxide thickness for the unipolar case, but for ambipolar operation an optimum oxide substrate thickness exists (∼80 nm) which minimizes peak temperature, due to competing electrostatic and thermal effects.
Abstract: Recent studies using infrared (IR) imaging of graphene transistors [1,2] have revealed substantial Joule heating under realistic operating conditions for graphene-on-insulator (GOI) devices. Here we use simulations calibrated against experimental data to examine the trends of performance degradation caused by self-heating as a function of insulator (SiO 2 ) thickness. We also examine both unipolar and ambipolar operating conditions, and find that peak channel temperatures are proportional to oxide thickness for the unipolar case (as would be expected), but for ambipolar operation an optimum oxide substrate thickness exists (∼80 nm) which minimizes peak temperature, due to competing electrostatic and thermal effects.