Jung-Kyun Kim

Design and Characterization of a High Resolution Microfluidic Heat Flux Sensor with Thermal Modulation

A complementary metal-oxide semiconductor-compatible process was used in the design and fabrication of a suspended membrane microfluidic heat flux sensor with a thermopile for the purpose of measuring the heat flow rate. The combination of a thirty-junction gold and nickel thermoelectric sensor with an ultralow noise preamplifier, a low pass filter, and a lock-in amplifier can yield a resolution 20 nW with a sensitivity of 461 V/W. The thermal modulation method is used to eliminate low-frequency noise from the sensor output, and various amounts of fluidic heat were applied to the sensor to investigate its suitability for microfluidic applications. For sensor design and analysis of signal output, a method of modeling and simulating electro-thermal behavior in a microfluidic heat flux sensor with an integrated electronic circuit is presented and validated. The electro-thermal domain model was constructed by using system dynamics, particularly the bond graph. The electro-thermal domain system model in which the thermal and the electrical domains are coupled expresses the heat generation of samples and converts thermal input to electrical output. The proposed electro-thermal domain system model is in good agreement with the measured output voltage response in both the transient and the steady state.

Optical characterization of digital alloy In0.49Ga0.51P/In0.49(Ga0.6Al0.4)0.51P multi-quantum-wells grown by molecular beam epitaxy

An In0.49Ga0.51P∕In0.49(Ga0.6Al0.4)0.51P multi-quantum-well (MQW) structure grown by molecular beam epitaxy using a digital alloy method was parametrically investigated by photoluminescence (PL) measurement performed in a temperature range of 10–290K. The PL peak energies did not change with increasing temperature up to 60K, while the PL peak energy monotonously decreased with increasing temperature beyond 60K. From the curve fit of the linewidth full width at half maximum of the PL peak, it was observed that the homogeneous broadening of In0.49Ga0.51P∕In0.49(Ga0.6Al0.4)0.51P MQW with digital alloy barriers due to scattering by longitudinal optical phonons was smaller than that of InGaAs∕InGaAlAs MQW with digital alloy barriers. This is in accordance with the existence of a relatively weak phonon-related PL peak in the PL spectrum of InGaAlP digital alloy, as compared with InGaAlAs digital alloy. The fit of the integrated PL intensity shows the occurrence of a nonradiative recombination process with an ac...

Initiation and evolution of phase separation in GaP/InP short-period superlattices

We have investigated the initiation and evolution of lateral phase separation in GaP/InP short-period superlattices (SPSs). Cross-sectional scanning tunneling microscopy reveals lateral contrast modulations within the SPS region, presumably due to alloy phase separation. The wavelength of the modulations appears to be constant throughout the entire SPS structure. Interestingly, the wavelength is dependent on the thickness of the constituent layers of the superlattice, and is likely to be affected by an observed significant concentration of group V vacancies. Together, these results suggest that phase separation is initiated by compositional nonuniformities from excess surface adatoms due to incomplete coverage of the constituent layers of the superlattice, and that the phase separation process is assisted by In–Ga interdiffusion via P vacancies.

Compact Thermal Network Model of the Thermal Interface Material Measurement Apparatus With Multi-Dimensional Heat Flow

Accurate prediction of operating temperatures in electronic parts at the component, package, board and system level computational fluid dynamics simulations, has engaged the engineering community for several years. The primary challenge was the difficulty of implementing accurate operating temperature predictions directly into system designs using near-exact physical models (known as detailed thermal models) due to the wide disparity in length scales that result in large computational inefficiencies. A compact thermal model (CTM) attempts to solve this problem by reducing the detailed model to a far less grid intensive representation, while preserving accuracy in predicting the temperatures at key points with a short calculation time. A CTM is condensed using thermal network modeling with thermal resistance and thermal capacitance. This paper focuses on the derivation of compact thermal network models to predict the transient temperature response of complex electronic equipment. The system model is based on a thermal interface material (TIM) measurement apparatus taking into consideration the lateral heat flow in the TIM. The thermal impedance and the temperature difference between the junction and ambient nodes are used to estimate the heat flow division on each heat transfer path. The validity of the dynamic thermal network method and the simple thermal analysis model is confirmed based on the result of the transient thermal analysis. In this paper we describe an application of the dynamic compact thermal network method to analyze the transient thermal response of a TIM measurement apparatus in multi-dimensional heat flow, and we propose a simple modelling procedure for the parametric estimation as a preliminary thermal design tool.

Modeling and fabrication of thin film thermopile sensor

A thin film micro heat-flux sensor using thermopile, which can measure the heat flow, was fabricated by a complementary-metal-oxide-semiconductor-compatible process. The combination of a ten-junction polysilicon and aluminum thermoelectric sensor with an ultralow noise preamplifier and low pass Butterworth filter has enabled the resolution of 3.4μW power and shows the sensitivity of 2.43mV∕μW. In order to estimate the heat generation of samples from the output measurement of a micro heat-flux sensor, a methodology for modeling and simulating electro-thermal behavior in a micro heat-flux sensor with integrated electronic circuit is presented and validated. The electro-thermal model was constructed by using system dynamics, particularly the bond graph methodology. The electro-thermal system model, where the thermal domain and the electrical domain are coupled, expresses the behavior that the heat generation of samples converts thermal input to electrical output through the system model. The proposed electro...

Influence of dot size distribution and interlayer thickness on the optical property of closely stacked InAs/GaAs quantum dots with growth interruption

We investigated the effect of dot size distribution and interlayer thickness on the optical property of closely stacked self-assembled InAs/GaAs quantum dot (QD) structures with growth interruption for 30 s using an As2 source. The structural property was optimized by changing the growth parameters, such as growth temperature, growth time and group III/V ratio. As the stacking number was increased, the size of truncated pyramid-shaped QDs became larger in both height and width, maintaining an on-top vertical alignment with a dot density of ~5.2–5.9 × 1010 cm−2. Compared to the single QD layer, five closely stacked QDs with the GaAs interlayer are found to exhibit a significant improvement of their photoluminescent (PL) intensity, indicating a slight shift of the PL peak position toward the high-energy side. The use of a thin GaAs interlayer of 3 nm in the QDs enhanced the blue shift, which is attributed to the dominant strain-induced intermixing or loss of indium atoms in the InAs QD layers. For the interlayer thicker than about 7 nm, the blue shifts are correlated to the dominant high-energy excited state transitions due to the successive state filling of the ground and higher excited states in the QDs. The energy separation of double PL peaks, originating from two different excited states, was kept at around 50 meV at room temperature. A possible mechanism concerning this phenomenon was also discussed.

MBE growth and characteristics of self-assembled InAs/InGaAs/GaAs quantum dots

Self-assembled InAs QDs was grown on (001) GaAs substrate by MBE. The dot density was 5.2times1010/cm2, the height was 14 nm and the width was 30 nm. The peak-wave length and the FWHM were 1266 nm and 41.23 meV respectively

MBE growth and characterization of highly tensile-strained InGaAs/InGaAlAs multi-quantum well for 1.3 /spl mu/m laser diodes

Highly tensile-strained InGaAs/InGaAlAs MQW was grown by MBE using the digital-alloy technique. Two peaks corresponding to electron-light hole transition (E1-LH1) and electron-heavy hole transition (E1-HH1) were clearly observed in the PL spectrum of MQW structure