Sung Hyun Park

A robust desirability function method for multi-response surface optimization considering model uncertainty

A robust desirability function approach to simultaneously optimizing multiple responses is proposed. The approach considers the uncertainty associated with the fitted response surface model. The uniqueness of the proposed method is that it takes account of all values in the confidence interval rather than a single predicted value for each response and then defines the robustness measure for the traditional desirability function using the worst case strategy. A hybrid genetic algorithm is developed to find the robust optima. The presented method is compared with its conventional counterpart through an illustrated example from the literature.

High-rate chemical vapor deposition of nanocrystalline silicon carbide films by radio frequency thermal plasma

Abstract Silicon carbide films were deposited by radio frequency thermal plasma chemical vapor deposition (CVD) at rates up to several hundred micrometers per hour over a 40-mm diameter substrate. The films were primarily β-phase SiC. Film morphology was characterized by columnar growth terminating in hemispherical surfaces. The average crystallite size as determined by X-ray diffraction line broadening ranged from about 5 to 100 nm, and increased with increasing substrate temperature. The film growth rate varied linearly with the input flow rate of SiCl 4 precursor, and appeared to be independent of substrate temperature over the range 680–1215 °C.

3D Printed Stretchable Tactile Sensors

The development of methods for the 3D printing of multifunctional devices could impact areas ranging from wearable electronics and energy harvesting devices to smart prosthetics and human-machine interfaces. Recently, the development of stretchable electronic devices has accelerated, concomitant with advances in functional materials and fabrication processes. In particular, novel strategies have been developed to enable the intimate biointegration of wearable electronic devices with human skin in ways that bypass the mechanical and thermal restrictions of traditional microfabrication technologies. Here, a multimaterial, multiscale, and multifunctional 3D printing approach is employed to fabricate 3D tactile sensors under ambient conditions conformally onto freeform surfaces. The customized sensor is demonstrated with the capabilities of detecting and differentiating human movements, including pulse monitoring and finger motions. The custom 3D printing of functional materials and devices opens new routes for the biointegration of various sensors in wearable electronics systems, and toward advanced bionic skin applications.

Roll-to-Roll sputtered ITO/Cu/ITO multilayer electrode for flexible, transparent thin film heaters and electrochromic applications.

We fabricate high-performance, flexible, transparent electrochromic (EC) films and thin film heaters (TFHs) on an ITO/Cu/ITO (ICI) multilayer electrode prepared by continuous roll-to-roll (RTR) sputtering of ITO and Cu targets. The RTR-sputtered ICI multilayer on a 700 mm wide PET substrate at room temperature exhibits a sheet resistance of 11.8 Ω/square and optical transmittance of 73.9%, which are acceptable for the fabrication of flexible and transparent EC films and TFHs. The effect of the Cu interlayer thickness on the electrical and optical properties of the ICI multilayer was investigated in detail. The bending and cycling fatigue tests demonstrate that the RTR-sputtered ICI multilayer was more flexible than a single ITO film because of high strain failure of the Cu interlayer. The flexible and transparent EC films and TFHs fabricated on the ICI electrode show better performances than reference EC films and TFHs with a single ITO electrode. Therefore, the RTR-sputtered ICI multilayer is the best substitute for the conventional ITO film electrode in order to realize flexible, transparent, cost-effective and large-area EC devices and TFHs that can be used as flexible and smart windows.

Wide bandgap III-nitride nanomembranes for optoelectronic applications

Single crystalline nanomembranes (NMs) represent a new embodiment of semiconductors having a two-dimensional flexural character with comparable crystalline perfection and optoelectronic efficacy. In this Letter, we demonstrate the preparation of GaN NMs with a freestanding thickness between 90 to 300 nm. Large-area (>5 × 5 mm(2)) GaN NMs can be routinely obtained using a procedure of conductivity-selective electrochemical etching. GaN NM is atomically flat and possesses an optical quality similar to that from bulk GaN. A light-emitting optical heterostructure NM consisting of p-GaN/InGaN quantum wells/GaN is prepared by epitaxy, undercutting etching, and layer transfer. Bright blue light emission from this heterostructure validates the concept of NM-based optoelectronics and points to potentials in flexible applications and heterogeneous integration.

Less strained and more efficient GaN light-emitting diodes with embedded silica hollow nanospheres

Light-emitting diodes (LEDs) become an attractive alternative to conventional light sources due to high efficiency and long lifetime. However, different material properties between GaN and sapphire cause several problems such as high defect density in GaN, serious wafer bowing, particularly in large-area wafers, and poor light extraction of GaN-based LEDs. Here, we suggest a new growth strategy for high efficiency LEDs by incorporating silica hollow nanospheres (S-HNS). In this strategy, S-HNSs were introduced as a monolayer on a sapphire substrate and the subsequent growth of GaN by metalorganic chemical vapor deposition results in improved crystal quality due to nano-scale lateral epitaxial overgrowth. Moreover, well-defined voids embedded at the GaN/sapphire interface help scatter lights effectively for improved light extraction, and reduce wafer bowing due to partial alleviation of compressive stress in GaN. The incorporation of S-HNS into LEDs is thus quite advantageous in achieving high efficiency LEDs for solid-state lighting.

Estimation of the Change Point in the X¯ and S Control Charts

Abstract The X¯ and S (or R) control charts are most commonly used in practice for monitoring the process mean and variance, respectively. A method for detecting changes in the process mean or variance is to obtain a stopping time at which the control charts issue a signal. The potential delay in generating a signal from the control charts calls for the change point estimation combined with control charts. In this paper, when X¯ and S control charts issue a signal, a maximum likelihood joint estimator of the change point is suggested considering simultaneous change in the process mean and variance. The use of the proposed estimator is illustrated with an example.

Simultaneous Optimization of Multiple Responses Using a Weighted Desirability Function

The object of multiresponse optimization is to determine conditions on the independent variables that lead to optimal or nearly optimal values of the response variables. Derringer and Suich (1980) extended Harrington’s (1965) procedure by introducing more general transformations of the response into desirability functions. The core of the desirability approach condenses a multivariate optimization into a univariate one. But because of the subjective nature of this approach, inexperience on the part of the user in assessing a product’s desirability value may lead to inaccurate results. To compensate for this defect, a weighted desirability function is introduced which takes into consideration the variances of the responses.

Semipolar (20 2 ¯ 1) GaN and InGaN quantum wells on sapphire substrates

Here, we demonstrate a process to produce planar semipolar (20 2¯1) GaN templates on sapphire substrates. We obtain (20 2¯1) oriented GaN by inclined c-plane sidewall growth from etched sapphire, resulting in single crystal material with on-axis x-ray diffraction linewidth below 200 arc sec. The surface, composed of (10 1¯1) and (10 1¯0) facets, is planarized by the chemical-mechanical polishing of full 2 in. wafers, with a final surface root mean square roughness of <0.5 nm. We then analyze facet formation and roughening mechanisms on the (20 2¯1) surface and establish a growth condition in N2 carrier gas to maintain a planar surface for further device layer growth. Finally, the capability of these semipolar (20 2¯1) GaN templates to produce high quality device structures is verified by the growth and characterization of InGaN/GaN multiple quantum well structures. It is expected that the methods shown here can enable the benefits of using semipolar orientations in a scalable and practical process and can...

Catalyst-Free Growth of Vertically Aligned ZnO Nanostructures Arrays on Periodically Polarity-Inverted Substrate

We report vertically aligned ZnO nanostructures grown using periodically polarity-inverted (PPI) ZnO heterostructures on (0001) Al2O3 without the aid of a catalyst. In order to fabricate the PPI ZnO templates, the polarity control of ZnO film via in situ crystal polarity control methods using CrN and Cr2O3 buffer layers was used. Vertically aligned ZnO nanowires or nanowalls were formed only on the Zn-polar regions. The self-catalyzed process on the Zn-polar surface was most likely responsible for this selective growth. Finally, we determined the mechanism of nanowall formation on the basis of the coalescence of several nanowires in the defined area.