Geun Young Yeom

Room-temperature GaN vertical-cavity surface-emitting laser operation in an extended cavity scheme

A GaN-based vertical-cavity surface-emitting laser (VCSEL) has been demonstrated in an extended cavity structure. A VCSEL device had a long extended cavity, which consisted of a sapphire substrate as well as a GaN epilayer and had an integrated microlens on one side. High-reflection dielectric mirrors were deposited on both sides of the laser cavity. The laser was optically pumped and operated at room temperature. The VCSEL device lased at a low threshold excitation intensity of 160 kW/cm2. In contrast to a conventional microcavity-VCSEL structure, the VCSEL operated in multiple longitudinal modes with mode spacing consistent with its physical thickness.

Highly efficient vertical laser-liftoff GaN-based light-emitting diodes formed by optimization of the cathode structure

Vertical GaN-based light-emitting diodes (LEDs) were fabricated using a laser-liftoff process and the effect of the cathode processing conditions on the properties of the LEDs was investigated. Surface roughening by 10%Cl2∕90%BCl3 plasma etching improved the light emission intensity at an operating current of 20mA; however, the forward operating voltage was increased due to the thin and rough n‐GaN layer. The use of an indium tin oxide (ITO) contact on the roughened n-type GaN surface decreased the forward voltage significantly, by decreasing the spreading resistance of the n-type GaN contact without decreasing the emission intensity. Through the combination of the ITO contact and the surface roughness of the n‐GaN layer, a 100% increase in the extraction efficiency was obtained compared to that of a lateral GaN device, with maintaining a similar forward operating voltage.

A study of transparent indium tin oxide (ITO) contact to p-GaN

Abstract In this study, indium tin oxide (ITO) thin film was evaporated on Mg-doped p-GaN layers with low 10 17 /cm 3 , grown by metalorganic chemical vapor deposition (MOCVD) on (0001) sapphire wafers, and its contact properties were investigated. The sheet resistance ( R S ) of the evaporated ITO films was several kΩ/□ before the annealing but the sheet resistance decreased to 40–50 Ω/□ after the annealing in N 2 using a rapid thermal annealing (RTA) system. I – V characteristics and contact resistivities of the evaporated ITO on p-type GaN were investigated as a function of substrate treatment, annealing time, and annealing temperature. The results showed that, in optimized conditions, the ITO contacts with the resistance in the range of low 10 −1 Ω cm 2 could be obtained, and which is probably applicable as the ohmic contact for GaN-based light emitting diodes. Also, at these conditions, the measured optical transmittances of ITO film were above 90% at a wavelength of 420 nm (blue).

Refractive sapphire microlenses fabricated by chlorine-based inductively coupled plasma etching.

We have fabricated refractive sapphire microlenses and characterized their properties for what we believe to be the first time. We use thermally reflown photoresist lenslet patterns as a mask for chlorine-based dry etch of sapphire. Pattern transfer to the mechanically hard and chemically inert sapphire substrate is made possible by an inductively coupled plasma etch system that supplies a high-density plasma gas. Processed sapphire microlenses exhibit properties close to the ideal and operate nearly in the diffraction limit.

Number of graphene layers as a modulator of the open-circuit voltage of graphene-based solar cell

Impressive optical properties of graphene have been attracting the interest of researchers, and, recently, the photovoltaic effects of a heterojunction structure embedded with few layer graphene (FLG) have been demonstrated. Here, we show the direct dependence of open-circuit voltage (Voc) on numbers of graphene layers. After unavoidably adsorbed contaminants were removed from the FLGs, by means of in situ annealing, prepared by layer-by-layer transfer of the chemically grown graphene layer, the work functions of FLGs showed a sequential increase as the graphene layers increase, despite of random interlayer-stacking, resulting in the modulation of photovoltaic behaviors of FLGs/Si interfaces.

Controllable Nondegenerate p-Type Doping of Tungsten Diselenide by Octadecyltrichlorosilane

Despite heightened interest in 2D transition-metal dichalcogenide (TMD) doping methods for future layered semiconductor devices, most doping research is currently limited to molybdenum disulfide (MoS2), which is generally used for n-channel 2D transistors. In addition, previously reported TMD doping techniques result in only high-level doping concentrations (degenerate) in which TMD materials behave as near-metallic layers. Here, we demonstrate a controllable nondegenerate p-type doping (p-doping) technique on tungsten diselenide (WSe2) for p-channel 2D transistors by adjusting the concentration of octadecyltrichlorosilane (OTS). This p-doping phenomenon originates from the methyl (-CH3) functional groups in OTS, which exhibit a positive pole and consequently reduce the electron carrier density in WSe2. The controlled p-doping levels are between 2.1 × 10(11) and 5.2 × 10(11) cm(-2) in the nondegenerate regime, where the performance parameters of WSe2-based electronic and optoelectronic devices can be properly designed or optimized (threshold voltage↑, on-/off-currents↑, field-effect mobility↑, photoresponsivity↓, and detectivity↓ as the doping level increases). The p-doping effect provided by OTS is sustained in ambient air for a long time showing small changes in the device performance (18-34% loss of ΔVTH initially achieved by OTS doping for 60 h). Furthermore, performance degradation is almost completely recovered by additional thermal annealing at 120 °C. Through Raman spectroscopy and electrical/optical measurements, we have also confirmed that the OTS doping phenomenon is independent of the thickness of the WSe2 films. We expect that our controllable p-doping method will make it possible to successfully integrate future layered semiconductor devices.

High-Performance 2D Rhenium Disulfide (ReS2) Transistors and Photodetectors by Oxygen Plasma Treatment

A high-performance ReS2 -based thin-film transistor and photodetector with high on/off-current ratio (10(4) ), high mobility (7.6 cm(2) V(-1) s(-1) ), high photoresponsivity (2.5 × 10(7) A W(-1) ), and fast temporal response (rising and decaying time of 670 ms and 5.6 s, respectively) through O2 plasma treatment is reported.

Low‐Temperature Synthesis of Large‐Scale Molybdenum Disulfide Thin Films Directly on a Plastic Substrate Using Plasma‐Enhanced Chemical Vapor Deposition

By plasma-enhanced chemical vapor deposition, a molybdenum disulfide (MoS2 ) thin film is synthesized directly on a wafer-scale plastic substrate at below 300 °C. The carrier mobility of the films is 3.74 cm(2) V(-1) s(-1) . Also, humidity is successfully detected with MoS2 -based sensors fabricated on the flexible substrate, which reveals its potential for flexible sensing devices.

Effect of N2O Plasma Treatment on the Performance of ZnO TFTs

Postfabrication rapid thermal annealing (RTA) and subsequent nitrous oxide (N 2 O) plasma treatment improved the performance of zinc oxide (ZnO) thin-film transistors (TFTs) in terms of off current and on/off current ratio by almost 2 orders of magnitude. The off current of 2 × 10 -8 A and on/off current ratio of 3 × 10 3 obtained after RTA were improved to 10 -10 A and 10 5 , respectively, by the subsequent N 2 O plasma treatment. X-ray photoelectron spectroscopy analysis of the TFT samples showed that the RTA-treated ZnO surface had more oxygen vacancies as compared to as-deposited samples, and the oxygen vacancies at the surface of RTA-treated ZnO were reduced by subsequent N 2 O plasma treatment. The reduction of oxygen vacancies at the top region of the ZnO channel is the cause of better off current and on/off current ratio of the TFTs.

Doped-Fluorine on Electrical and Optical Properties of Tin Oxide Films Grown by Ozone-Assisted Thermal CVD

Transparent conductive fluorine-doped tin oxide (FTO, SnO 2 :F) films were grown using a low-pressure metallorganic chemical vapor deposition (LP-MOCVD) with tetramethyltin (TMT), oxygen containing 2.96 mol % ozone (O 3 ), and hydrofluoric acid (HF) as a dopant. Using ozone contained oxygen instead of pure oxygen, the substrate temperature could be reduced by ∼ 100°C while maintaining a growth rate similar to that of tin oxide film. Growth rate of FTO film increased with the flow rate of TMT, however, adherence strength between the film and glass decreased. Resistivity of FTO thin films decreased with the TMT and HF flows over most of ranges investigated, while the resistivity increased rapidly with respect to excessive flow of the TMT over 300 seem. Optical transmittance of FTO film was about 80% at 550 nm, and the variation as a function of HF flow rate was not significant. FTO film prepared at an optimum condition showed a minimum resistivity of 1.09 X 10 -3 Ω cm, a mobility of 19 cm 2 /Vs, and a carrier concentration of 3.05 X 10 20 cm -3 .