Seungkyu Ahn

Experimental observation of the inductive electric field and related plasma nonuniformity in high frequency capacitive discharge

To elucidate plasma nonuniformity in high frequency capacitive discharges, Langmuir probe and B-dot probe measurements were carried out in the radial direction in a cylindrical capacitive discharge driven at 90MHz with argon pressures of 50 and 400mTorr. Through the measurements, a significant inductive electric field (i.e., time-varying magnetic field) was observed at the radial edge, and it was found that the inductive electric field creates strong plasma nonuniformity at high pressure operation. The plasma nonuniformity at high pressure operation is physically similar to the E-H mode transition typically observed in inductive discharges. This result agrees well with the theories of electromagnetic effects in large area and/or high frequency capacitive discharges.

Driving frequency effect on the electron energy distribution function in capacitive discharge under constant discharge power condition

A modern trend of VHF driven plasma sources in semiconductor processing stimulates a lot of studies concerning the driving frequency effect on plasma parameters in a capacitive discharge. In spite of abundant studies, the validation and application of these results in industrial plasma processing are still questionable because these studies were performed under a fixed rf voltage condition or an assumption of Maxwellian electron energy distribution, while the fixed discharge power condition and non-Maxwellian distribution are typical in industrial plasma processing. To resolve this problem, the authors investigated the driving frequency effect on plasma parameters (electron density and temperature) under the fixed discharge power condition by measuring the electron energy distribution functions, which are the most important factor in chemical reactions during the plasma processing. A remarkable result was observed—as the driving frequency increases, the electron temperature increases and the electron dens...

Cu(In,Ga)Se2 thin films without Ga segregation prepared by the single-step selenization of sputter deposited Cu-In-Ga-Se precursor layers

We report a new approach to fabricating Cu(In,Ga)Se2 (CIGSe) light absorbing layers for thin film solar cells without Ga segregation using a sputtering and single-step selenization process. To mitigate Ga segregation at the CIGSe/back-contact region, which has frequently been observed in the selenization of metal/alloy precursor layers, we used Se-containing precursor layers (Cu-In-Ga-Se) to capture Ga in covalently bonded structures and investigated the effects of Se content in the precursor layers on the properties of the selenized CIGSe films and the devices. As the Se content in the precursor layer increased, Ga segregation was significantly mitigated, resulting in a completely homogenized Ga distribution when the Se/metal ratio of the precursor films is over 0.8. Finally, a thin CIGSe film (∼670 nm) with a uniform Ga distribution was processed to fabricate a solar cell, and the device exhibited a conversion efficiency of 11.7% with an open circuit potential of 0.6 V. An increase of the CIGSe film thickness to 1.55 μm resulted in a device efficiency of up to 13.16%.

Driving frequency effect on electron heating mode transition in capacitive discharge

A study was conducted on the dependence of the electron heating mode transition upon driving frequency in capacitive discharge. The evolution of the electron energy distribution functions (EEDFs) over a wide range of gas pressures was investigated at different driving frequencies. Regardless of the driving frequency, the measured EEDFs exhibited a typical evolution of EEDF from bi-Maxwellian distribution to Druyvesteyn-like distribution with gas pressure, signifying the electron heating mode transition from collisionless to collisional heating. However, the gas pressure, which the heating mode transition takes place, significantly decreased as the driving frequency increased. This result is ascribed to the fact that the collisionless stochastic heating becomes inefficient at high frequency compared with collisional heating.

Establishment of a primary reference solar cell calibration technique in Korea: methods, results and comparison with WPVS qualified laboratories

A primary reference solar cell calibration technique recently established at the Korea Institute of Energy Research in Korea is introduced. This calibration technique is an indoor method that uses a highly collimated continuous-type solar simulator and absolute cavity radiometer traceable to the World Radiometric Reference. The results obtained using this calibration technique are shown with a precise uncertainty analysis, and the system configuration and calibration procedures are introduced. The calibration technique avoids overestimating the short-circuit current of a reference solar cell due to multiple reflections of incident simulator light using a novel method. In addition, the uncertainty analysis indicates that the calibration technique has an expanded uncertainty of approximately 0.7% with a coverage factor of k = 2 for a c-Si reference cell calibration. In addition, the developed primary reference solar cell calibration technique was compared with other techniques established in the World Photovoltaic Scale (WPVS) qualified calibration laboratories to verify its validity and reliability.

Development of semitransparent CIGS thin-film solar cells modified with a sulfurized-AgGa layer for building applications

Different from conventional photovoltaics, building-integrated photovoltaics needs not only high performance but also a high degree of transparency. Nevertheless, the Cu(In,Ga)Se2 (CIGS) solar cell has advantages in terms of the highest conversion efficiency and stability among all thin-film-based solar cells. The semitransparent (ST) CIGS solar cell using an ultrathin CIGS absorber on a transparent conducting oxide (TCO) experiences loss in fill factor and open circuit voltage due to the poor grain morphology and CIGS/TCO interface. Thus, these issues must be addressed to realize ST CIGS solar cells. Wide-bandgap (1.5 eV) submicron CIGS-based solar cells were prepared in this study unlike conventional CIGS with a bandgap of 1.2 eV, in order to enhance see-throughness. But such cells demonstrated low conversion efficiency due to the poor grain morphology and absence of back grading. Therefore, for the sake of improving grain morphology and to create back grading, a sulfurized-AgGa (AGS) layer was deposited between CIGS and the indium tin oxide (ITO) back contact. Ag from the AGS layer diffused throughout the absorber and thus ameliorated the grain morphology. However, Ga and S in the AGS layer remained confined near the back contact, therefore resulting in the creation of back grading. Consequently, a solar cell based on 230 nm thick CIGS modified with a 45 nm thick AGS layer exhibited efficiency of 5.94% with averaged visible transmittance over 25%. This is the highest reported efficiency for a ST CIGS solar cell with over 20% visible transparency. The CIGS solar cell based on this novel approach can be a competent candidate for building-integrated semitransparent photovoltaics applications.

Development of flexible silicon thin-film solar cells on nanotextured Ag/Al:Si bilayers

Highly textured Ag/Al:Si back reflectors were used to improve the short-circuit current density of flexible nanocrystalline silicon thin-film solar cells by means of enhanced light trapping. A nanotextured topography with a root-mean-square (σrms) surface roughness of 60.1 nm was induced at low substrate temperature of 100°C by abnormal grain growth of dc-sputtered Al:Si films. After depositing highly reflective Ag films on nanotextured Al:Si films, effective light scattering of the long wavelength over 500 nm was achieved, resulting in enhanced absorption of weakly absorbing, long-wavelength light in the solar cells. The effects of nanotextured surfaces of the bilayers on the light scattering properties and on the resultant cell performance were investigated.

Time resolved eedf investigation for plasma heating

Summary form only given. Time resolved measurements of EEDF (electron energy distribution function) at various pressure ranges reveal that there are two distinct heating mechanisms as mentioned in previous studies. According to gas pressure, the processes to make high energy electrons which make ionization to sustain the plasma are clearly different. Theoretical calculation based on the non-local kinetics is in a good agreement with experiments

Improvement of Au adhesion on parylene-c and SiO/sub 2/ substrates using oxygen plasma treatment

Summary form only given, as follows. Parylene-c (Pa-c) film is a pinhole-free barrier against moisture, chemical and biofluid. So it is candidate for low dielectric materials for the application of biosensors such as DNA and protein chip Pa-c and SiO/sub 2/ films are deposited on a Si substrate using a vapor deposition method and LPCVD, respectively. Au was then deposited on both substrates using the e-beam evaporation method. For improvement of Au adhesion on Pa-c and SiO/sub 2/ surfaces, surface modification of Pa-c and SiO/sub 2/ was performed using oxygen plasma. Pa-c and SiO/sub 2/ layers were exposed to an oxygen plasma using a reactive ion etcher at various power and times.

Development of hydroxyapatite coating method on titanium using plasma spray and mesh masking

Summary form only given. Titanium and its alloys have been used with some success in several implant applications. However, they can suffer certain disadvantages, such as poor osteoinductive properties and low corrosive-wear resistance. Attempts to overcome the former have involved with coating the metal with bioceramic materials like hydroxyapatite. The hydroxyapatite(HAp, Ca/sub 10/(PO/sub 4/)/sub 6/(OH)/sub 2/) is preferred in general for its ability to interact with living bones, resulting in improvement of implant fixation and faster bone healing. Of the various coating methods, the plasma spraying technique is the primary one currently used commercially to produce HAp coatings on metallic implants with its application increased significantly in recent years due to high tolerance of surgical inaccuracies and fast fixation of the implants in bone as well as firm attachment of implant to bone. Some of the bioactive properties of HAp are degraded during plasma spraying because of high temperature. Control of the composition change is necessary and the crystalline structure of HAp layer will have to increase to eliminate these drawbacks of HAp plasma spray. However, these conditions have opposite relation to each other and are difficult to satisfy at the same time. The purposes of this study are to enhance adhesiveness of initial stage strength and to work out a countermeasure about the absorption of HAp coating layer in human body. Surface porosity is utilized using TiO/sub 2/ blast and HAp plasma spray coating by masking method. The HAp coating layer appears to ensure bonding strength at the initial stage. Separation and/or absorption of HAp may also occur between implant and bone after inserting the implants. The combination between porous surface and grown-up bone can be successfully used to maintain adhesive strength. Screws of pure titanium are made for testing and then HAp coating group is separated using plasma spray, porous surface group using TiO/sub 2/ blast and HAp coating and porous surface group using mesh masking respectively. Screws of each group will be implanted in the femurs of rabbits. After 4 weeks, 8 weeks and 12 weeks, screws of each groups are withdrawn from rabbits femur and examined for the adhesive strength with torque meter and tissue condition between implant and bone.