Epitaxial Graphene and its Electronic Device Applications

Department of Electrical EngineeringGraphene is a two-dimensional material in which carbon atoms are bonded in honeycomb lattice. It has a unique electronic band structure that shows zero band gap energy and linear dispersion relation near the Dirac point. Because of to its outstanding electrical and mechanical properties, graphene has been actively studied in various fields. After successfully separating graphene from highly oriented pyrolytic graphite (HOPG), a variety of methods for obtaining high quality and large area graphene have been studied. Especially, a method of growing epitaxial graphene (EG) on a SiC substrate has attracted much attention as a material for next generation electronic devices. This allows the growth of large area graphene and it is not necessary for transfer process because semi-insulating SiC wafer can be used as a substrate. However, it has disadvantages of requiring high temperature (> 1300 ??C) for high quality EG growth and the single crystalline SiC substrate are too expensive. In order to overcome these problem, we propose two effective methods for growth of EG on SiC. Firstly, the high quality EG is grown on 4H-, 6H SiC substrate by molybdenum plate (Mo-plate) capping during annealing process. Mo-plate capping causes the heat accumulation on SiC surface by preventing loss of thermal radiations from SiC surface, and increase the Si vapor pressure on SiC surface by enclosing the sublimated Si atoms. Therefore, the temperature of the SiC surface becomes higher than surrounding temperature, and the Si sublimation rate is reduced. These factors enable high quality EG growth at relatively low power assumption (chamber temperature). The quality enhancement of the grown EG with Mo-plate capping is demonstrated by Raman spectra, compared to EG without Moplate capping. Secondly, the graphene is formed on SiC thin film surface at relatively low temperature by electron beam (e-beam) irradiation with low acceleration voltage. The e-beam irradiation with low acceleration voltage induces the heat accumulation within several layers of SiC thin film surface due to its shallow penetration depth. The thermalized electrons weaken the bond strength of the Si-C atoms while staying within a few layers of SiC thin film surface, which reduce the heat energy required for sublimating Si atoms. As the electron fluency increase, the crystallinity and uniformity of grown graphene are improved, which is confirmed by Raman spectra and scanning electron microscopy (SEM) images. We propose the cleanly patterning method for graphene using Al thin film as etching mask because general patterning methods such as electron beam lithography and photolithography induce the degradation of graphene quality due to polymer residue. The properties of fabricated graphene device using Al thin film are confirmed by Hall measurement and Raman spectra, compared with graphene sample patterned with conventional photolithography. In particular, the apparent Shubnikov-de Haas (SdH) oscillation measured in graphene device patterned with Al thin film demonstrates better homogeneity and 2DEG system. The carrier density and Hall mobility in Al patterned EG device are measured to be 9.16 ?? 10^12 cm-2 and ~ 2100 cm2/Vs, respectively. The complementary logic inverter having graphene channel is fabricated by using selective doping of graphene. Ti or Al adsorbed graphene is doped n-type, because Ti or Al with lower work function than graphene induces the charge transfer from the Ti or Al to graphene. On the other hand, the SiO2 adsorbed graphene is doped to p-type by the dangling bonds of SiO2 surface. The doping concentration and type of graphene are confirmed by Raman spectra and electrical measurements. We fabricated two kinds of inverter doped with Al-SiO2 and Ti-SiO2 materials. These inverters exhibit a clear voltage inversion as function of Vin at a wide range of VDD from 0.5 V to 20 V, and the highest voltage gains are ~0.93 and ~0.86, respectively. These properties can be improved by using insulating layer of higher dielectric constant and reducing thickness of gate oxide. We propose a new structure of multifunctional capacitive sensor to surmount the limitations the previous single-capacitor sensor. The proposed dual-capacitor sensor composes of two capacitors stacking vertically in a pixel which detects strength information and surface-normal directionality of external stimuli, and clearly classifies the types of stimuli. These properties have been demonstrated by detecting and distinguishing the curvature, pressure, touch and strain stimuli through the capacitances changes of the two capacitors. We successfully fabricated a stable n-type InAs NW FET with a very simple fabrication process using pre-deposition of Al2O3 layer. This oxide layer of 10 nm thickness is uniformly formed on entire surface of NW channel by ALD. It serves not only as a gate oxide but also as a protective layer of the NW channel. The structure of completed device is demonstrated by TEM images and EDX electron mapping. The n-InAs NW FET shows good current saturation and low voltage operation, the peak transconductance (gm) is extracted to be 13.4 mS/mm, the field effect mobility (??FE) is calculated to be ~1039 cm2/Vs at VDS = 0.8 V and current on/off ratio is about ~750.ope

Next-generation crossover-free quantum Hall arrays with superconducting interconnections

This work presents precision measurements of quantized Hall array resistance devices using superconducting, crossover-free and multiple interconnections as well as graphene split contacts. These new techniques successfully eliminate the accumulation of internal resistances and leakage currents that typically occur at interconnections and crossing leads between interconnected devices. As a result, a scalable quantized Hall resistance array is obtained with a nominal value that is as precise and stable as that from single-element quantized Hall resistance standards

Two-Terminal and Multi-Terminal Designs for Next-Generation Quantized Hall Resistance Standards: Contact Material and Geometry

In this paper, we show that quantum Hall resistance measurements using two terminals may be as precise as four-terminal measurements when applying superconducting split contacts. The described sample designs eliminate resistance contributions of terminals and contacts such that the size and complexity of next-generation quantized Hall resistance devices can be significantly improved

Accessing ratios of quantized resistances in graphene p–n junction devices using multiple terminals

The utilization of multiple current terminals on millimeter-scale graphene p–n junction devices has enabled the measurement of many atypical, fractional multiples of the quantized Hall resistance at the ν = 2 plateau (RH ≈ 12 906 Ω). These fractions take the form abRH and can be determined both analytically and by simulations. These experiments validate the use of either the LTspice circuit simulator or the analytical framework recently presented in similar work. Furthermore, the production of several devices with large-scale junctions substantiates the approach of using simple ultraviolet lithography to obtain junctions of sufficient sharpness.The utilization of multiple current terminals on millimeter-scale graphene p–n junction devices has enabled the measurement of many atypical, fractional multiples of the quantized Hall resistance at the ν = 2 plateau (RH ≈ 12 906 Ω). These fractions take the form abRH and can be determined both analytically and by simulations. These experiments validate the use of either the LTspice circuit simulator or the analytical framework recently presented in similar work. Furthermore, the production of several devices with large-scale junctions substantiates the approach of using simple ultraviolet lithography to obtain junctions of sufficient sharpness

Nonconventional Quantized Hall Resistances Obtained with ν = 2 Equilibration in Epitaxial Graphene p-n Junctions

We have demonstrated the millimeter-scale fabrication of monolayer epitaxial graphene p−n junction devices using simple ultraviolet photolithography, thereby significantly reducing device processing time compared to that of electron beam lithography typically used for obtaining sharp junctions. This work presents measurements yielding nonconventional, fractional multiples of the typical quantized Hall resistance at ν=2 (RH≈12906Ω) that take the form: (a/b)RH. Here, a and b have been observed to take on values such 1, 2, 3, and 5 to form various coefficients of RH. Additionally, we provide a framework for exploring future device configurations using the LTspice circuit simulator as a guide to understand the abundance of available fractions one may be able to measure. These results support the potential for drastically simplifying device processing time and may be used for many other two-dimensional materials

Differences in Trauma-Related Guilt in Females with History of Sexual Violence Based on Insomnia Severity

Background and Objective Females with history of sexual violence report a high percentage of insomnia. Guilt is a common symptom among this group. This study investigated differences in trauma-related guilt cognition between females of sexual violence with high or low symptoms of insomnia. Methods Participants were 43 females who reported having a history of sexual violence (mean age 26.56±7.81). All participants completed questionnaires about insomnia symptoms (Insomnia Severity Index, ISI), Posttraumatic stress disorder (PTSD) symptoms (PTSD Symptom Scale Self-Report), trauma-related guilt (Trauma-related Guilt Inventory, TRGI), depression (Beck Depression Inventory) and trauma-related information. The TRGI is consisted by global guilt, distress and guilt cognitions. Guilt cognitions can further be divided into Hindsight-Bias/Responsibility, Wrongdoing, and Lack of Justification subscales. Analyses were conducted using Pearson’s correlation coefficient and analysis of covariance. Results Results indicated ISI scores were significantly positively associated with PSS scores (r = 0.620, p < 0.01) and the distress subscale of the TGRI (r = 0.488, p < 0.01), and negatively associated with guilt cognitions (r = −0.423, p < 0.01). 53.5% (n = 23) of the sample met criteria for clinical insomnia using ISI cut-off scores of 15. Participants in the insomnia group scored significantly lower in overall guilt cognitions (p < 0.001) and significantly higher in distress (p = 0.001) than the non-insomnia group after controlling for depression. Among the subscales of guilt cognitions, hindsight-bias/responsibility was significantly lower in the insomnia group (p < 0.001). Conclusions Guilt can sometimes be adaptive in trauma patients as it may work as a catalyst in cognitively processing their trauma. Our results indicate that individuals with insomnia report lower guilt cognition. This may subsequently interfere with their ability to process the traumatic experience and effectively cope with their situation

The genus Arthrinium (Ascomycota, Sordariomycetes, Apiosporaceae) from marine habitats from Korea, with eight new species

Species of Arthrinium are well-known plant pathogens, endophytes, or saprobes found in various terrestrial habitats. Although several species have been isolated from marine environments and their remarkable biological activities have been reported, marine Arthrinium species remain poorly understood. In this study, the diversity of this group was evaluated based on material from Korea, using morphological characterization and molecular analyses with the internal transcribed spacer (ITS) region, β-tubulin (TUB), and translation elongation factor 1-alpha (TEF). A total of 41 Arthrinium strains were isolated from eight coastal sites which represented 14 species. Eight of these are described as new to science with detailed descriptions.This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (NRF-2017R1A2B4002071). Additional funding was provided by the project for the survey and excavation of Korean indigenous species of the National Institute of Biological Resources [NIBR201902113] under the Ministry of Environment, Republic of Korea, and the Marine Biotechnology Program of the Korea Institute of Marine Science and Technology Promotion (KIMST), funded by the Ministry of Oceans and Fisheries (MOF) (No. 20170431 & No. 20170325)

Impact of social isolation on behavioral health in elderly: Systematic review.

AimTo examine and compare the effects of subjective and objective social isolation on behavioral health in elderly adults.MethodsA systematic search of PubMed was performed for original research articles from peer-reviewed journals examining one of the following topics: "Social isolation and sleep disturbance", "social isolation and depression", or "social isolation and fatigue in older adults". Studies were selected following the criteria established based on the aim of this review. Data were extracted from the articles by two independent reviewers. Due to the heterogeneity in study designs and outcome measures of the included studies, qualitative and narrative analyses were conducted.ResultsThe set criteria were used to select a total of 16 studies for the review. Of the 16, 13 were cross-sectional studies. The characteristics of study populations were identified as follows. A total of 12 studies randomly selected subjects irrespective of pre-existing health conditions. Consequently, an unspecified number of the study subjects had chronic diseases in the studies compared. In addition, cultural and ethnic backgrounds of studies in this review were diverse, and included subjects living in North America, South America, Asia, Europe, and Oceania. Both subjective and objective types of social isolation increased behavioral symptoms, such as sleep disturbance, depressive symptoms, and fatigue in older adults. Furthermore, a few recent studies reported stronger effects of subjective social isolation than objective social isolation on sleep disturbance and depressive symptoms.ConclusionSocial isolation affects behavioral health in older adults. Compared to the objective social isolation, subjective social isolation contributes more significantly to sleep disturbance and depression