• Journal of Sensor Science and Technology
• /
• v.28 no.2
• /
• pp.117-120
• /
• 2019

Graphene is recognized as a promising material for silicon photonics, since it has a wide optical-window that entirely covers the optical communication wavelength region ($1.3{\sim}1.6-{\mu}m$) and extremely high-carrier mobility that makes it possible to fabricate the high-speed photodetectors. However, the maximum absorbance of monolayer graphene is only 2.3%, which limits the photoresponse characteristics of graphene photodetectors. As a result, a low photoresponsivity of graphene photodetector is a critical issue limiting the use of graphene photodetectors in the optical communications field. In this paper, we investigated effects of optically-modulated metal-graphene contact on the photoresponsivity of graphene photodetectors. The optical modulation of the contact resistance mainly determined the photoresponse characteristics of graphene photodetectors. The Ni-contact graphene photodetector which has a characteristic of the significant optical modulation of metal-graphene contact showed a higher photoresponsivity than the Pd-contact device. This work will provide a way to improve the photoresponse characteristics of graphene-based photodetector and contribute to the development of high-speed/high-responsivity graphene photodetector.

• Journal of the Korea Institute of Information and Communication Engineering
• /
• v.24 no.10
• /
• pp.1306-1311
• /
• 2020

In this study, we proposed an anti-reflective nano-structure to improve the photoresponsivity of AlGaN UV photodiode that can be used as a receiver in a solar blind UV optical communication system. The anti-reflective nano-structure was fabricated by forming Ni nano-clusters on SiO2 film followed by etching the underneath SiO2 film. A sample with the anti-reflective nano-structure exhibited lower surface reflection along with less dependency on the wavelength in comparison with a sample without the nano-structure. Finally, a UV photodiode was fabricated by applying an anti-reflective structure produced by heat-treating a 2 nm-thick Ni layer. The photodiode fabricated with the proposed nano-structure exhibited noticeable improvement in the photoresponsivity at the wavelength range from 240 nm to 270 nm in comparison with the same photodiode with a SiO2 film without the nano-structure.

• Journal of the Korean Physical Society
• /
• v.73 no.9
• /
• pp.1351-1355
• /
• 2018

The photocurrent characteristics of zinc-oxide (ZnO) thin-film transistors (TFTs) prepared using radio-frequency sputtering and spin-coating methods were investigated. Various characterization methods were used to compare the physical and the chemical properties of the sputtered and the spin-coated ZnO films. X-ray photoelectron spectroscopy was used to investigate the chemical composition and state of the ZnO films. The transmittance and the optical band gap were measured by using UV-vis spectrometry. The crystal structures of the prepared ZnO films were examined by using an X-ray diffractometer, and the surfaces of the films were investigated by using scanning electron microscopy. ZnO TFTs were prepared using both sputter and solution processes, both of which showed photocurrent characteristics when illuminated by light. The sputtered ZnO TFTs had a photoresponsivity of 3.08 mA/W under illumination with 405-nm light while the solution-processed ZnO TFTs had a photoresponsivity of 5.56 mA/W. This study provides useful information for the development of optoelectronics based on ZnO.

• Current Applied Physics
• /
• v.18 no.12
• /
• pp.1496-1506
• /
• 2018

Organic/inorganic ultraviolet photodetector was fabricated using thermal evaporation technique. Organic/inorganic heterojunction based on thermally evaporated copper (II) acetylacetonate thin film of thickness 200 nm deposited on an n-type silicon substrate is introduced. I-V characteristics of the fabricated heterojunction were investigated under UV illumination of intensity $65mW/cm^2$. The diode parameters such as ideality factor, n, barrier height, ${\Phi}_B$, and reverse saturation current, $I_s$, were determined using thermionic emission theory. The series resistance of the fabricated diode was determined using modified Nord's method. The estimated values of series resistance and barrier height of the diode were about $0.33K{\Omega}$ and 0.72 eV, respectively. The fabricated photodetector exhibited a responsivity and specific detectivity about 9 mA/W and $4.6{\times}10^9$ Jones, respectively. The response behavior of the fabricated photodetector was analyzed through ON-OFF switching behavior. The estimated values of rise and fall time of the present architecture under UV illumination were about 199 ms and 154 ms, respectively. Finally, enhancing the photoresponsivity of the fabricated photodetector, post-deposition plasma treatment process was employed. A remarkable modification of the device performance was noticed as a result of plasma treatment. These modifications are representative in a decrease of series resistance and an increase of photoresponsivity and specific detectivity. The process of plasma treatment achieved an increment of external quantum efficiency from 5.53% to 8.34% at -3.5 V under UV illumination.

• Proceedings of the KAIS Fall Conference
• /
• 2006.05a
• /
• pp.207-208
• /
• 2006

분자선(分子線)에피택시법(法)으로 성장(成長)한 ZnO 산화물반도체(酸化物半導體)를 이용(利用)하여 제작(製作)한 쇼트키배리어 다이오드에 대하여 자외선(紫外線) 광(光)센서로서의 광특성(光特性)을 조사(調査)한다. 첫째, 백색광(白色光) 조사시(照射時) 포화전류치(飽和電流値)가 100배(培) 이상(以上) 증가(增加)하는 광(光)여기 특성(特性)을 나타낸다. 둘째, 조사(照射)하는 �셈� 파장(波長)에 대하 390nm의 차단장파장(遮斷長波長)을 갖으며 195nm 이상(以上)의 밴드폭을 갖는 파장감도특성(波長感度特性)을 나타낸다. 셋째, 자외선(紫外線)에 대해 0.36msec의 시정수(時定數) 갖는 것으로 평가(評價)된다. 따라서, ZnO 산화물반도체(酸化物半導體)는 향후(向後) 자외선(紫外線) 광(光)센서소자의 재과(材科)로서 기대(期待)되어진다.

• Bulletin of the Korean Chemical Society
• /
• v.33 no.5
• /
• pp.1653-1658
• /
• 2012

A new anthracene-containing conjugated molecule was synthesized through the Sonogashira coupling and reduction reactions. 1-Ethynyl-4-hexylbenzene was coupled to 2,6-bis((4-hexylphenyl) ethynyl)anthracene-9,10-dione through a reduction reaction to generate 2,6,9,10-tetrakis((4-hexylphenyl)ethynyl) anthracene. The semiconducting properties were evaluated in an organic thin film transistor (OTFT) and a single-crystal field-effect transistor (SC-FET). The OTFT showed a mobility of around 0.13 $cm^2\;V^{-1}\;s^{-1}$ ($I_{ON}/I_{OFF}$ > $10^6$), whereas the SC-FET showed a mobility of 1.00-1.35 $cm^2\;V^{-1}\;s^{-1}$, which is much higher than that of the OTFT. Owing to the high photoluminescence quantum yield of 2,6,9,10-tetrakis((4-hexylphenyl)ethynyl) anthracene, we could observe a significant increase in drain current under irradiation with visible light (${\lambda}$ = 538 nm, 12.5 ${\mu}W/cm^2$).

• Bulletin of the Korean Chemical Society
• /
• v.34 no.6
• /
• pp.1727-1734
• /
• 2013

Synthesis and characterization of some novel symmetric bis-azospiropyrans are reported in this study. These bis-azospiropyrans are bifunctional chromophores with two spiropyrans linked by a bis-azo extended aromatic system that produce more color strength (large molar absorption coefficient in mero forms) due to appending two azospiropyran chromophores on one molecule. Comparing to the molar absorption coefficients of the conventional spiropyran chromophores (${\varepsilon}=0.31{\times}10^4\;M^{-1}{\cdot}cm^{-1}$) and mono-azospiropyran chromophores ($1.35{\times}10^4\;M^{-1}{\cdot}cm^{-1}$), the novel synthesized photochromes showed astonishingly increased molar absorption coefficients ($2.3-3.8{\times}10^4\;M^{-1}{\cdot}cm^{-1}$) at the same conditions. Such high molar absorption coefficients confers high sensitivity to light and more color intensity of mero form, that leads to improvement of their light sensitivity and better discrimination of spiro (OFF) form from mero (ON) ones in molecular switches. The structures were deduced from their MS, FT-IR, and $^1H$-NMR spectroscopic data and CHN analysis. All the synthesized photochemically bifunctional compounds revealed fluorescent emission in their colorless form which was faded out after exposing to UV light. Fluorescence quantum yield values of the mero forms were 0.25-0.81 and two high fluorescence quantum yield values (0.60 and 0.81) were found in these series.

• Journal of Sensor Science and Technology
• /
• v.31 no.4
• /
• pp.228-237
• /
• 2022

Two-dimensional (2D) nanomaterials have demonstrated the potential to replace silicon and compound semiconductors that are conventionally used in photodetectors. These materials are ultrathin and have superior electrical and optoelectronic properties as well as mechanical flexibility. Consequently, they are particularly advantageous for fabricating high-performance photodetectors that can be used for wearable device applications and Internet of Things technology. Although prototype photodetectors based on single microflakes of 2D materials have demonstrated excellent photoresponsivity across the entire optical spectrum, their practical applications are limited due to the difficulties in scaling up the synthesis process while maintaining the optoelectronic performance. In this review, we discuss facile methods to mass-produce 2D material-based photodetectors based on the exfoliation of van der Waals crystals into nanosheet dispersions. We first introduce the liquid-phase exfoliation process, which has been widely investigated for the scalable fabrication of photodetectors. Solution processing techniques to assemble 2D nanosheets into thin films and the optoelectronic performance of the fabricated devices are also presented. We conclude by discussing the limitations associated with liquid-phase exfoliation and the recent advances made due to the development of the electrochemical exfoliation process with molecular intercalants.

• JSTS:Journal of Semiconductor Technology and Science
• /
• v.10 no.3
• /
• pp.213-224
• /
• 2010

An optically controlled silicon MESFET (OPFET) was fabricated by diffusion process to enhance the quantum efficiency, which is the most important optoelectronic device performance usually affected by ion implantation process due to large number of process induced defects. The desired impurity distribution profile and the junction depth were obtained solely with diffusion, and etching processes monitored by atomic force microscope, spreading resistance profiling and C-V measurements. With this approach fabrication induced defects are reduced, leading to significantly improved performance. The fabricated OPFET devices showed proper I-V characteristics with desired pinch-off voltage and threshold voltage for normally-on devices. The peak photoresponsivity was obtained at 620 nm wavelength and the extracted external quantum efficiency from the photoresponse plot was found to be approximately 87.9%. This result is evidence of enhancement of device quantum efficiency fabricated by the diffusion process. It also supports the fact that the diffusion process is an extremely suitable process for fabrication of high performance optoelectronic devices. The maximum gain of OPFET at optical modulated signal was obtained at the frequency of 1 MHz with rise time and fall time approximately of 480 nS. The extracted transconductance shows the possible potential of device speed performance improvements for shorter gate length. The results support the use of a diffusion process for fabrication of high performance optoelectronic devices.

• Korean Journal of Materials Research
• /
• v.29 no.7
• /
• pp.456-462
• /
• 2019

ZnO thin-films are grown on a p-Si(111) substrate by RF sputtering. The effects of growth temperature and $O_2$ mixture ratio on the ZnO films are investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and room-temperature photoluminescence (PL) measurements. All the grown ZnO thin films show a strong preferred orientation along the c-axis, with an intense ultraviolet emission centered at 377 nm. However, when $O_2$ is mixed with the sputtering gas, the half width at half maximum (FWHM) of the XRD peak increases and the deep-level defect-related emission PL band becomes pronounced. In addition, an n-ZnO/p-Si heterojunction diode is fabricated by photolithographic processes and characterized using its current-voltage (I-V) characteristic curve and photoresponsivity. The fabricated n-ZnO/p-Si heterojunction diode exhibits typical rectifying I-V characteristics, with turn-on voltage of about 1.1 V and ideality factor of 1.7. The ratio of current density at ${\pm}3V$ of the reverse and forward bias voltage is about $5.8{\times}10^3$, which demonstrates the switching performance of the fabricated diode. The photoresponse of the diode under illumination of chopped with 40 Hz white light source shows fast response time and recovery time of 0.5 msec and 0.4 msec, respectively.