• Korean Journal of Optics and Photonics
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• v.1 no.1
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• pp.73-86
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• 1990

The technique of Spectroscopic Ellipsometry (SE) has been examined with emphasis on its inherent sensitivity to the existence of thin films or surface equivalents. A brief review of related theories like the Fresnel reflection coefficients, the effect of a multilayer upon reflectivities, together with the validity of the effective medium theory and the modelling procedure, is followed by a short description of the experimental setup of a rotating polarizer type SE as well as the necessful expressions which lead to tan and cos. Out of its numerous, successful applications, a few are exampled to convince a reader that SE can be applied to a variety of research fields related to surface, interface and thin films. Specifically, those are adsorption and/or desorption on metals or semiconductors, oxidation process, formation of passivation layers on an electrode, thickness determination, interface between semiconductor and its oxide, semiconductor heterojunctions, surface microroughness, void distribution of dielectric, optical thin films, depth profile of multilayered samples, in-situ or in-vitro characterization of a solid surface immersed in electrolyte during electrochemical, chemical, or biological treatments, and so on. It is expected that the potential capability of SE will be widely utilized in a very near future, taking advantage of its sensitivity to thin films or surface equivalents, and its nondestructive, nonperturbing characteristics.

• JSTS:Journal of Semiconductor Technology and Science
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• v.11 no.3
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• pp.182-189
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• 2011

In this work, a tunneling field-effect transistor (TFET) based on heterojunctions of compound and Group IV semiconductors is introduced and simulated. TFETs based on either silicon or compound semiconductors have been intensively researched due to their merits of robustness against short channel effects (SCEs) and excellent subthreshold swing (SS) characteristics. However, silicon TFETs have the drawback of low on-current and compound ones are difficult to integrate with silicon CMOS circuits. In order to combine the high tunneling efficiency of narrow bandgap material TFETs and the high mobility of III-V TFETs, a Type-I heterojunction tunneling field-effect transistor (I-HTFET) adopting $Ge-Al_xGa_{1-x}As-Ge$ system has been optimized by simulation in terms of aluminum (Al) composition. To maximize device performance, we considered a nanowire structure, and it was shown that high performance (HP) logic technology can be achieved by the proposed device. The optimum Al composition turned out to be around 20% (x=0.2).

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.306.1-306.1
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• 2016

ZnO semiconductor material has been widely utilized in various applications in semiconductor device technology owing to its unique electrical and optical features. It is a promising as solar cell material, because of its low cost, n-type conductivity and wide direct band gap. In this work ZnO/Si heterojunctions were fabricated by using pulsed laser deposition. Vacuum chamber was evacuated to a base pressure of approximately $2{\times}10^{-6}Torr$. ZnO thin films were grown on p-Si (100) substrate at oxygen partial pressure from 5mTorr to 40mTorr. Growth temperature of ZnO thin films was set to 773K. A pulsed (10 Hz) Nd:YAG laser operating at a wavelength of 266 nm was used to produce a plasma plume from an ablated a ZnO target, whose density of laser energy was $10J/cm^2$. Thickness of all the thin films of ZnO was about 300nm. The optical property was characterized by photoluminescence and crystallinity of ZnO was analyzed by X-ray diffraction. For fabrication ZnO/Si heterojunction diodes, indium metal and Al grid patterns were deposited on back and front side of the solar cells by using thermal evaporator, respectively. Finally, current-voltage characteristics of the ZnO/Si structure were studied by using Keithly 2600. Under Air Mass 1.5 Global solar simulator with an irradiation intensity of $100mW/cm^2$, the electrical properties of ZnO/Si heterojunction photovoltaic devices were analyzed.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.14 no.7
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• pp.1647-1652
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• 2010

A CdS film has been used as a window layer in CdTe and Cu(In,Ga)$Se_2$ thin films solar cell. Partial substitution of Zn for Cd increases the photocurrent and the open-circuit voltage by providing a match in the electron affinities of the two materials and the higher band gap. In this paper, CdZnS/CdTe and CdS/CdTe heterojunctions were fabricated and the electrical characteristics were investigated. Current-voltage-temperature measurements showed that the current transport for CdS/CdTe heterojunction was controlled by both tunneling and interface recombination. However, CdZnS/CdTe heterojunction displayed different current transport mechanism with the operating temperature. For above room temperature, the current transport of device was generation/recombination in the depletion region and was the leakage current and/or tunneling in the range below room temperature.

• Bulletin of the Korean Chemical Society
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• v.35 no.3
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• pp.913-918
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• 2014

Five mol % tungsten-doped tin oxide ($W_{0.05}Sn_{0.95}O_2$, TTO5) was prepared by co-precipitation of $SnCl_4{\cdot}5H_2O$ and $WCl_4$, followed by calcination at $1000^{\circ}C$. The as-prepared TTO5 was in the pure cassiterite phase with a particle size of ~50 nm and optical bandgap of 2.51 eV. Herein it was applied for the formation of TTO5/$TiO_2$ heterojunctions by covering the TTO5 surface with $TiO_2$ by sol-gel method. Under visible-light irradiation (${\lambda}{\geq}420$ nm), TTO5/$TiO_2$ showed a significantly high photocatalytic activity in removing gaseous 2-propanol (IP) and evolving $CO_2$. It is deduced that its high visible-light activity is caused by inter-semiconductor holetransfer between the valence band (VB) of TTO5 and $TiO_2$, since the TTO5 nanoparticle (NP) exhibits the absorption edge at ~450 nm and its VB level is located more positive side than that of $TiO_2$. The evidence for the hole-transport mechanism between TTO5 and $TiO_2$ was also investigated by monitoring the holescavenging reaction with 1,4-terephthalic acid (TA).

• Proceedings of the Korean Vacuum Society Conference
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• 2010.08a
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• pp.311-312
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• 2010

The fabricated photovoltaic cells based on PIN heterojunctions, in this study, have a structure of ITO/poly(3, 4-ethylenedioxythiophene)-poly(styrenesulfonate)(PEDOT:PSS)/donor/donor:C60(10nm)/C60(35nm)/2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline(8nm)/Al(100nm). The thicknesses of an active layer(donor:C60), an electron transport layer(C60), and hole/exciton blocking layer(BCP) were fixed in the organic photovoltaic cells. We investigated the performance characteristics of the PIN organic photovoltaic cells with copper phthalocyanine(CuPc), tetracene and pentacene as a hole transport layer. Discussion on the photovoltaic cells with CuPc, tetracene and pentacene as a hole transport layer is focussed on the dependency of the power conversion efficiency on the deposition rate and thickness of hole transport layer. The device performance characteristics are elucidated from open-circuit-voltage(Voc), short-circuit-current(Jsc), fill factor(FF), and power conversion efficiency($\eta$). As the deposition rate of donor is reduced, the power conversion efficiency is enhanced by increased short-circuit-current(Jsc). The CuPc-based PIN photovoltaic cell has the limited dependency of power conversion efficiency on the thickness of hole transport layer because of relatively short exciton diffusion length. The photovoltaic cell using tetracene as a hole transport layer, which has relatively long diffusion length, has low efficiency. The maximum power conversion efficiencies of CuPc, tetracene, and pentacene-based photovoltaic cells with optimized deposition rate and thickness of hole transport layer have been achieved to 1.63%, 1.33% and 2.15%, respectively. The photovoltaic cell using pentacene as a hole transport layer showed the highest efficiency because of dramatically enhanced Jsc due to long diffusion length and strong thickness dependence.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.53-53
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• 2011

This paper describes results for surface and bulk characterization of the most promising thin film solar cell material for high performance devices, (Ag,Cu) (In,Ga) Se2 (ACIGS). This material in particular exhibits a range of exotic behaviors. The surface and general materials science of the material also has direct implications for the operation of solar cells based upon it. Some of the techniques and results described will include scanning probe (AFM, STM, KPFM) measurements of epitaxial films of different surface orientations, photoelectron spectroscopy and inverse photoemission, Auger electron spectroscopy, and more. Bulk measurements are included as support for the surface measurements such as cathodoluminescence imaging around grain boundaries and showing surface recombination effects, and transmission electron microscopy to verify the surface growth behaviors to be equilibrium rather than kinetic phenomena. The results show that the polar close packed surface of CIGS is the lowest energy surface by far. This surface is expected to be reconstructed to eliminate the surface charge. However, the AgInSe2 compound has yielded excellent atomic-resolution images of the surface with no evidence of surface reconstruction. Similar imaging of CuInSe2 has proven more difficult and no atomic resolution images have been obtained, although current imaging tunneling spectroscopy images show electronic structure variations on the atomic scale. A discussion of the reasons why this may be the case is given. The surface composition and grain boundary compositions match the bulk chemistry exactly in as-grow films. However, the deposition of the heterojunction forming the device alters this chemistry, leading to a strongly n-type surface. This also directly explains unpinning of the Fermi level and the operation of the resulting devices when heterojunctions are formed with the CIGS. These results are linked to device performance through simulation of the characteristic operating behaviors of the cells using models developed in my laboratory.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.73-73
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• 2011

We will report a few methods to improve the efficiency and stability in small molecule based organic solar cells, including the formation of bulk heterojunctions (BHJs) through alternative thermal deposition (ATD), the use of a micro-cavity structure and interface modifications. By ATD which is a simple modification of conventional thermal evaporation, the thicknesses of alternative donor and acceptor layers were precisely controlled down to 0.1 nm, which is critical to form BHJs. The formation of a BHJ in copper(II) phthalocyanine (CuPc) and fullerene (C60) systems was confirmed by AFM, GISAXS and absorption measurements. From analysis of the data, we found that the CuPc|C60 films fabricated by ATD were composed of the nanometer sized disk shaped CuPc nano grains and aggregated C60, which explains the phase separation of CuPc and C60. On the other hand, the co-deposited CuPc:C60 films did not show the existence of separated CuPc nano grains in the CuPc:C60 matrix. The OPV cells fabricated using the ATD method showed significantly enhanced power conversion efficiency compared to the co-deposited OPV cells under a same composition [1]. We will also present by numerical simulation that adoption of microcavity structure in the planar heterojunction can improve the short circuit current in single and tandem OSCs [2]. Interface modifications also allowed us to achieve high efficiency and high stability OSCs.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.68-69
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• 2011

The process of charge transfer at the interface between two semiconductors or between a metal and a semiconductor plays an important role in many areas of technology. The optimization of such devices requires a good theoretical description of the interfaces involved. This, in turn, has motivated detailed mechanistic studies of interfacial charge-transfer reactions at metal/organic, organic/organic, and organic/inorganic semiconductor heterojunctions. Charge recombination of photo-induced electron with redox species such as oxidized dyes or triiodide or cationic HTM (hole transporting materials) at the heterogeneous interface of $TiO_2$ is one of main loss factors in liquid junction DSSCs or solid-state DSSCs, respectively. Among the attempts to prevent recombination reactions such as insulating thin layer and lithium ions-doped hole transport materials and introduction of co-adsorbents, although co-adsorbents retard the recombination reactions as hydrophobic energy barriers, little attention has been focused on the anchoring processes. Molecular engineering of heterogeneous interfaces by employing several co-adsorbents with different properties altered the surface properties of $TiO_2$ electrodes, resulting to the improved power conversion efficiency and long-term stability of the DSSCs. In this talk, advantages of the coadsorbent-assisted sensitization of N719 in preparation of DSSCs will be discussed.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.20 no.5
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• pp.135-141
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• 2021

As a trend of lightening automobiles and electronic products, several studies are currently underway to replace parts of metals with resins. In particular, heterojunctions between metals and resins are now under the spotlight. This study aims to evaluate the variation in bonding strength with process conditions when the polybutylene terephthalate (PBT) polymer is bonded to a specimen of the lightweight 6061 aluminum alloy (AL6061). Conditions of the bonding surface of the AL6061 specimen, the temperature of the injection mold, and the content of the glass fiber were considered to be process variables. Bonded specimens were manufactured for different values of these variables. Bonding strength tests were then performed on these specimens and variations were analyzed in their characteristics corresponding to those of the process conditions. Fractures in these specimens were assessed using scanning electron microscopy (SEM) to assess the fracture surface. This was then used to analyze the fracture shape and determine whether anodizing the specimen led to the development of cracks on the joint surface. Results of the above test indicated that while the surface condition of the specimen and the temperature of the injection mold significantly influenced the strength of bonding, the content of the glass fiber did not.