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Single CdTe microwire photodetectors grown by close-spaced sublimation method.
Yang, Gwangseok,Kim, Byung-Jae,Kim, Donghwan,Kim, Jihyun Optical Society of America 2014 Optics express Vol.22 No.16
<P>We demonstrate single CdTe microwire field-effect transistors (FETs) that are highly sensitive to ultraviolet (UV) light. Dense CdTe microwires were catalytically grown using a close-spaced sublimation system. Structural, morphological and transport properties in conjunction with the optoelectronic properties were systemically investigated. CdTe microwire FETs exhibited p-type behaviors with field-effect mobilities up to 1.1 10(-3) cm2 V(-1) s(-1). Optoelectronic properties of our CdTe microwire FETs were studied under dark and UV-illumination conditions, where photoresponse was highly dependent on the back-gate bias conditions. Our CdTe microwire FET-based photodetectors are promising for high-performance micro-optoelectronic applications.</P>
GaN-based light-emitting diodes on graphene-coated flexible substrates.
Yang, Gwangseok,Jung, Younghun,Cuervo, Camilo V?lez,Ren, Fan,Pearton, Stephen J,Kim, Jihyun Optical Society of America 2014 Optics express Vol.22 No.suppl3
<P>We demonstrate GaN-based thin light-emitting diodes (LEDs) on flexible polymer and paper substrates covered with chemical vapor deposited graphene as a transparent-conductive layer. Thin LEDs were fabricated by lifting the sapphire substrate off by Excimer laser heating, followed by transfer of the LEDs to the flexible substrates. These substrates were coated with tri-layer graphene by a wet transfer method. Optical and electrical properties of thin laser lift-offed LEDs on the flexible substrates were characterized under both relaxed and strained conditions. The graphene on paper substrates remained conducting when the graphene/paper structure was folded. The high transmittance, low sheet resistance and high failure strain of the graphene make it an ideal candidate as the transparent and conductive layer in flexible optoelectronics.</P>
Flexible graphene-based chemical sensors on paper substrates
Yang, Gwangseok,Lee, Chongmin,Kim, Jihyun,Ren, Fan,Pearton, Stephen J. The Royal Society of Chemistry 2013 Physical chemistry chemical physics Vol.15 No.6
<P>Graphene-based, flexible NO<SUB>2</SUB> sensors on paper substrates exhibited an immediate response (32–39%) once exposed to 200 ppm NO<SUB>2</SUB> gas under a strain of 0.5%. Chemical vapor deposition-grown graphene with a supporting poly(methyl methacrylate) layer was transferred onto paper substrates, followed by formation of two electrodes using silver paste. Current–voltage characteristics and dynamic sensing response were obtained under both relaxed and strained conditions. We demonstrate a facile method without complex photo-lithography and high vacuum processes for fabricating graphene-based flexible NO<SUB>2</SUB> sensors on paper substrates with high sensing response.</P> <P>Graphic Abstract</P><P>We demonstrate a simple method for fabricating graphene-based flexible chemical sensors on paper substrates with high sensing response. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cp43717a'> </P>
Transfer-Free Growth of Multilayer Graphene Using Self-Assembled Monolayers
Yang, Gwangseok,Kim, Hong-Yeol,Jang, Soohwan,Kim, Jihyun American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.40
<P>Large-area graphene needs to be directly synthesized on the desired substrates without using a transfer process so that it can easily be used in industrial applications. However, the development of a direct method for graphene growth on an arbitrary substrate remains challenging. Here, we demonstrate a bottom-up and transfer-free growth method for preparing multilayer graphene using a self-assembled monolayer (trimethoxy phenylsilane) as the carbon source. Graphene was directly grown on various substrates such as SiO2/Si, quartz, GaN, and textured Si by a simple thermal annealing process employing catalytic metal encapsulation. To determine the optimal growth conditions, experimental parameters such as the choice of catalytic metal, growth temperatures, and gas flow rate were investigated. The optical transmittance at 550 nm and the sheet resistance of the prepared transfer-free graphene are 84.3% and 3500 Omega/square, respectively. The synthesized graphene samples were fabricated into chemical sensors. High and fast responses to both NO2 and NH3 gas molecules were observed. The transfer-free graphene growth method proposed in this study is highly compatible with previously established fabrication systems;. thereby opening up new possibilities for using graphene in versatile applications.</P>
Self-aligned growth of CdTe photodetectors using a graphene seed layer.
Yang, Gwangseok,Kim, Donghwan,Kim, Jihyun Optical Society of America 2015 Optics express Vol.23 No.19
<P>We demonstrate the self-aligned growth of CdTe photodetectors using graphene as a pre-defined seed layer. Defects were generated in the graphene prior to growth to act as CdTe nucleation sites. Self-aligned CdTe structures were grown selectively on the pre-defined graphene region. The electrical and optoelectrical properties of the photodetectors were systematically analyzed. Our CdTe devices displayed Ohmic behavior with a low sheet resistance of 1.24 ?? 10<sup>8</sup> 곽/sq. Excellent photodetecting performances were achieved, including a high on-off ratio (~2.8), fast response time (10.4 s), and highly reproducible photoresponses. The fabrication method proposed here for these self-aligned device structures proves valuable for the development of next-generation graphene-semiconductor hybrid devices.</P>
Yang, Gwangseok,Jang, Soohwan,Ren, Fan,Pearton, Stephen J.,Kim, Jihyun American Chemical Society 2017 ACS APPLIED MATERIALS & INTERFACES Vol.9 No.46
<P>The robust radiation resistance of wide-band gap materials is advantageous for space applications, where the high-energy particle irradiation deteriorates the performance of electronic devices. We report on the effects of proton irradiation of beta-Ga2O3 nanobelts, whose energy band gap is similar to 4.85 eV at room temperature. Back-gated field-effect transistor (FET) based on exfoliated quasi-two-dimensional beta-Ga2O3 nanobelts were exposed to a 10 MeV proton beam. The proton-dose- and time-dependent characteristics of the radiation-damaged FETs were systematically analyzed. A 73% decrease in the field-effect mobility and a positive shift of the threshold voltage were observed after proton irradiation at a fluence of 2 x 10(15) cm(-2). Greater radiation-induced degradation occurs in the conductive channel of the beta-Ga2O3 nanobelt than at the contact between the metal and beta-Ga2O3. The on/off ratio of the exfoliated beta-Ga2O3 FETs was maintained even after proton doses up to 2 x 10(15) cm(-2). The radiation -induced damage in the beta-Ga2O3-based FETs was significantly recovered after rapid thermal annealing at 500 degrees C. The outstanding radiation durability of beta-Ga2O3 renders it a promising building block for space applications.</P>