Yong-Duck Chung

SOA-EAM frequency up/down-converters for 60-GHz bi-directional radio-on-fiber systems

We investigate a frequency up/down-converter based on a single cascaded semiconductor optical amplifier (SOA)-electroabsorption modulator (EAM) configuration for bi-directional 60-GHz-band radio-on-fiber (RoF) system applications. SOA cross-gain modulation and photodetection in EAM are used for frequency up-conversion, and EAM nonlinearity is used for frequency down-conversion. In our scheme, both 60-GHz local-oscillator (LO) signals and IF signals are optically transmitted from a central station to base stations. We characterize the dependence of frequency up/down-conversion efficiencies on EAM bias and optical LO power. For frequency up-conversion, maximum conversion gain of approximately 8 dB was obtained and, for frequency down-conversion, more than approximately 18-dB conversion loss was measured. Utilizing this frequency up/down converter, we demonstrate a 60-GHz bi-directional RoF link. Optically transmitted downlink 10-Mb/s quadrature phase-shift keying (QPSK) data at 100-MHz IF are frequency up-converted to the 60-GHz band at the base station, and uplink 10-Mb/s QPSK data in the 60-GHz band are frequency down-converted to 150-MHz IF and transmitted to the central station. In addition, the dependence of error vector magnitudes on IF signal power and wavelength is investigated.

Adhesion and interface chemical reactions of Cu/polyimide and Cu/TiN by XPS

Abstract The chemical reaction at the interface between Cu and polyimide (PI) and between Cu and TiN at room temperature has been investigated using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). In case of Cu/TiN, there was no interface chemical reaction, but in case of Cu/PI system, there existed marked interface chemical reaction. From XPS core-level spectra, it was found that Cu atoms react mainly to oxygen and nitrogen in the PMDA (pyromellic-dianhybride) part of polyimide. Especially, the initial growth mode of Cu on polyimide was found by Cu LMM Auger spectra as follows; at first CuNO complex is formed, then CuOC complex formation by the weak interaction, and metallic Cu growth occurs simultaneously.

Electronic effect of Na on Cu(In,Ga)Se2 solar cells

We report the effect of Na on the electronic properties of Cu(In,Ga)Se2 (CIGS) thin-film solar cells with a structure of grid/ITO/i-ZnO/CdS/CIGS/Mo/SiOx/soda-lime glass (SLG). The diffusion of Na from the SLG into the CIGS layer was systematically controlled by varying the thickness of SiOx. As the Na content increased, the hole concentration of CIGS was enhanced, while the band-gap was nearly constant, which led to a lower Fermi level in the CIGS towards its valence-band edge. The Na-induced increment in the built-in potential (Vbi) across the n-(ITO/i-ZnO/CdS)/p-CIGS junction yielded an increment of open-circuit voltage that well agreed with the calculated Vbi.

Development and RF characteristics of analog 60-GHz electroabsorption modulator module for RF/optic conversion

We developed an analog 60-GHz module for RF/optic conversion by using an electroabsorption modulator (EAM) device associated with a traveling-wave electrode. The impedance matching of the module was accomplished with the 30-/spl Omega/ termination such that the resonance was precisely located at 60 GHz and the fractional bandwidth at S/sub 11/=-15 dB was as large as 1.0 GHz. The RF/optic conversion gain of the EAM device revealed a strong dependence on the input optical power and was closely related with the slope change in the optical transmission characteristic with reverse bias. From the EAM module with the composite-type multiple-quantum-well electroabsorption core consisting of both 8- and 12-nm quantum wells, we obtained the optimal RF/optic conversion gain at the reverse bias between 1.5-2 V in a wide range of input optical power from -6 to +9 dBm. In the same bias range, the third-order intermodulation distortion was <-49.5 dBc as obtained with RF input of -7.5 dBm.

Incorporation of Cu in Cu(In,Ga)Se2-based Thin-film Solar Cells

of 11.1%. The other prepared with a 3-step process had nearly the same In-Ga and Cu concentrations and showed a conversion eciency of 15.5%. We discuss the incorporation of Cu in the two types of thin-film solar cells.

XPS core-level shifts and XANES studies of Cu–Pt and Co–Pt alloys

The K and L 2,3 absorption edges and core-level binding-energy shifts for pure Co, Cu and Pt and Co-Pt and Cu-Pt alloys are measured to investigate changes in electronic structures of these alloys. Enhancements and depletion of the white lines in the alloys as compared to pure elements were correlated with changes in the unoccupied density of states upon alloying. We find that the Pt atoms lost d 3/2 electrons and gained d 5/2 electrons when alloyed with Cu and Co. However, the total 5d occupancy changes of the Pt were very little in Cu-Pt alloys, and this is consistent with the small shifts of Pt 4f core-level binding energy. From the quantitative analysis of Cu and Co K-edge absorption spectra, it is found that Cu and Co have significant valence occupation changes due to dehybridization of p-d states. From these results, we suggest that the core-level shifts are affected mainly by the d-valence electron changes in these alloys.

A 60-GHz-Band Analog Optical System-on-Package Transmitter for Fiber-Radio Communications

We developed an analog optical system-on-package (SoP) transmitter for a 60-GHz-band radio-over-fiber (RoF) link. The SoP transmitter consisted of an electroabsorption modulator, radio frequency amplifiers, and a bandpass filter. The 60-GHz RoF wireless link was prepared to measure the performance of the SoP transmitter. The transmission characteristics of 64-quadrature amplitude modulation (64-QAM) data of the 60-GHz RoF wireless link, including the SoP transmitter, were investigated by measuring the error vector magnitude (EVM) and signal-to-noise ratio (SNR) with a baseband frequency. The EVM of the 60-GHz RoF wireless link was between 2.25% and 2.80%, and the SNR was between 27.36 and 29.31 dB from 140 and 770 MHz, at input baseband power of -9 dBm. The noise figure had the minimum of 8.44 dB at 500 MHz. We successfully transmitted digital community antenna television (CATV) system signals through the 60-GHz RoF wireless link, including the SoP transmitter. Digital CATV signals of 86 channels could be transmitted through the 60-GHz RoF wireless link, and the total throughput was found to be 2.61 Gb/s.

Titanium oxide films on Si(100) deposited by e-beam evaporation

Titanium oxide films with a thickness of a 400 nm were deposited on p-type Si(100) at room temperature by e-beam evaporation, and titanium dioxide was evaporated in oxygen environment at a pressure of 2×10−6–4×10−5 Torr. Effects of oxygen flow rate (FO2) on the properties of the films, such as surface roughness, composition, and chemical states, have been investigated. The root-mean-square surface roughness of the films increased with increasing FO2 up to 20 sccm, and then decreased over 20 sccm. X-ray diffraction patterns show that the titanium oxide films are amorphous. Oxygen resonance backscattering spectroscopy shows that all films are oxygen rich, i.e., relative atomic ratio (CO/CTi) of the films ranged from 2.05 to 2.25. But the x-ray photoelectron spectroscopy (XPS) analysis shows that the titanium oxide films were oxygen deficient. The ratio of oxygen to titanium (CO2/CTi) of the films calculated by XPS ranged from 1.82 to 1.93. XPS shows that there exist only Ti3+ and Ti4+ charge states in the f...

Photovoltaic Performance and Interface Behaviors of Cu(In,Ga)Se2 Solar Cells with a Sputtered-Zn(O,S) Buffer Layer by High-Temperature Annealing

We selected a sputtered-Zn(O,S) film as a buffer material and fabricated a Cu(In,Ga)Se2 (CIGS) solar cell for use in monolithic tandem solar cells. A thermally stable buffer layer was required because it should withstand heat treatment during processing of top cell. Postannealing treatment was performed on a CIGS solar cell in vacuum at temperatures from 300-500 °C to examine its thermal stability. Serious device degradation particularly in VOC was observed, which was due to the diffusion of thermally activated constituent elements. The elements In and Ga tend to out-diffuse to the top surface of the CIGS, while Zn diffuses into the interface of Zn(O,S)/CIGS. Such rearrangement of atomic fractions modifies the local energy band gap and band alignment at the interface. The notch-shape induced at the interface after postannealing could function as an electrical trap during electron transport, which would result in the reduction of solar cell efficiency.

System-on-packaging with electro-absorption modulator for 60 GHz band radio-over-fiber link

We developed a system-on-packaging (SoP) with an electro-absorption modulator (EAM) for a 60 GHz band radio-over-fiber (RoF) link. It consists of an EAM device, a microstrip filter, and a low noise amplifier (LNA). The microstrip filter was used to achieve the impedance matching between the EAM device and the LNA and to reject the local oscillator (LO) frequency of the heterodyne system. The frequency response and the effect of the EAM bias voltage of a simple RF/optical link were measured. A 60 GHz band RoF link with 2.4 GHz intermediate frequency (IF) was prepared to measure the transmission characteristics of 16 QAM data