• Proceedings of the Korean Vacuum Society Conference
• /
• 2016.02a
• /
• pp.125.1-125.1
• /
• 2016

The excitonic insulator (EI), which is one of fundamental insulators, was theoretically proposed in 1967 but its material realization has not been established well. Only a few materials were proposed as EIs but their experimental evidences were indirect such as the renormalization of band dispersions or an anomaly in electrical resistivity. We conducted scanning tunneling microscopy / spectroscopy measurements and found out that $Ta_2$ $NiSe_5$, which was the most recently proposed as an EI, had a metal-insulator phase transition with the energy gap of 700 meV at 78 K. Moreover, the spatially delocalized excitonic energy level was observed within the energy gap, which could be the direct evidence of the EI ground state. Our theoretical model calculation with the order parameter of 150 meV reproduces the spectral function and the excitonic energy gap very well. In addition, experimental data shows that the band character is inverted at the valence and conduction band edges by the exciton formation, indicating that the mechanism of exciton condensation is similar to the Bardeen-Cooper-Schrieffer (BCS) mechanism of cooper pairs in superconductors.

• New & Renewable Energy
• /
• v.3 no.4
• /
• pp.54-62
• /
• 2007

In this paper, we report the research highlight on the preparation and characterization of Indium-free $Cu_2ZnSnSe_4$ and Indium-reduced $CulnZnSe_2$ thin films in order to seek the viability of these absorber materials to be applied in thin film solar cells. The films of $Cu_2ZnSnSe_4\;and\;CulnZnSe_2$ were prepared using mixed binary chalcogenides powders. It was observed that Cu concentration was a function of substrate temperature as well as CuSe mole ratio in the target. Under an optimized condition, $Cu_2ZnSnSe_4\;and\;CulnZnSe_2$ thin films grew with strong [112]. [220/204] and [312/116] reflections. Both $Cu_2ZnSnSe_4\;and\;CulnZnSe_2$ films were found to exhibit a high absorption coefficient of $104^4cm^{-1}\;Cu_2ZnSnSe_4$ film showed a band gap of 1.5eV which closes to the optimum band gap of an ideal solar absorber for a solar cell. On the other side, an increase of optical band gap from 1.0 to 1.25eV was found to be proportional with an increase of Zn concentration in the $CulnZnSe_2$ film. All films in this study revealed a p-type semiconductor characteristic.

• Proceedings of the Materials Research Society of Korea Conference
• /
• 2003.03a
• /
• pp.207-207
• /
• 2003

최근 광촉매 재료로 각광받고 있는 TiO$_2$는 band gap 에너지가 3.0-3.2eV로 자외선 영역과 일부 가시광선 영역에서 활성을 갖는 것으로 알려져 있다. 따라서 용액 중에 결정화 및 안정화 되어있는 TiO$_2$의 band gap 에너지를 낮춘다면 가시광 영역의 광반응을 얻을 수 있다. 이에 본 연구는 G. Sato등이 제안한 방법으로 TiO$_2$ sol을 제조할 때 band gap 에너지를 낮추고자 천이 금속원소를 첨가하여 복합 및 담지된 TiO$_2$계 복합 sol을 합성하고자 하였다 출발원료는 TiC1$_4$를 가수분해하여 제조한 TiOCl$_2$에 천이금속원소인 V, Cr, Fe, Ni, Nb 등의 chloride 화합물을 첨가하여 중화 및 세척과정을 거친 후, 과산화수소수에 용해하여 전구체 용액인 titania peroxo용액을 제조하였다 제조된 전구체 용액은 온도와 시간을 변수로 각각 열처리하여 TiO$_2$계 복합 sol을 합성하였다. 제조된 시편은 X-선 회절 분석, 투과전자현미경, particle size analyzer, ζ-potential analyzer 및 UV-VIS Spectrometer 통을 이용하여 천이금속 첨가에 따른 TiO$_2$계 복합 sol의 형성과정과 특성변화를 관찰하였다.

• The Transactions of the Korean Institute of Electrical Engineers C
• /
• v.54 no.7
• /
• pp.304-307
• /
• 2005

The $In_2S_3\;and\;In_2S_3:Co^{2+}$ thin films were grown by the spray Pyrolysis method. The thin films crystallized into tetragonal structures. The indirect energy band gap was 2.32ev for $In_2S_3\;and\;1.81eV\;for\;In_2S_3:Co^{2+}$ at 298K. The direct energy band gap was 2.67ev for $In_2S_3:Co^{2+}$ thin films. Impurity optical absorption peaks were observed for the $In_2S_3:Co^{2+}$ thin films. These impurity absorption peaks are assigned, based on the crystal field theory to the electron transitions between the energy levels of the $Co^{2+}$ ion sited in $T_{d}$ symmetry. The electrical conductivity($\sigma$), Hall mobility(${\mu}_H$), and carrier concentration (n) of the $In_2Se_3$ thin film were measured, and their temperature dependence was investigated.

• Korean Journal of Materials Research
• /
• v.15 no.3
• /
• pp.195-201
• /
• 2005

The $SnS_2,\;SnS_2:Cd$, and $SnS_2:Sb$ single crystals were grown by the chemical transport reaction method. The indirect optical energy band gap was found to be 2.348, 2.345, and 2.343 eV for the $SnS_2,\;SnS_2:Cd$, and $SnS_2:Sb$ single crystals, at 6 K respectively. The direct optical energy band gap was found to be 2.511, 2.505, and 2.503 eV f3r the $SnS_2,\;SnS_2:Cd$, and $SnS_2:Sb$ single crystals, at 6 K respectively The temperature dependence of the optical energy band gap was well fitted by the Varshni equation. Two photoluminescence emission peaks with the peak energy of 2.214 and 1.792 eV for $SnS_2$, 2.214 and 1.837 eV for $SnS_2:Cd$, and 2.214 and 1.818 eV the $SnS_2:Sb$ were observed. The emission peaks were described as originating from the donor-acceptor pair recombinations.

• Journal of the Microelectronics and Packaging Society
• /
• v.12 no.1 s.34
• /
• pp.73-76
• /
• 2005

To widen the band gap of ZnO, we have investigated $Zn_{1-x}Mg_xO(ZMO)$ thin films prepared by pulsed laser deposition on c-plane sapphire substrates at $500^{\circ}C$. From X-ray diffraction patterns, ZMO films show only the (0002) and (0004) diffraction peaks. It means that the flints have the wurtzite structure. Segregation of ZnO and MgO phases is found in the films with x=0.59. All the samples are highly transparent in the visible region and have a sharp absorption edge in the UV region. The shift of absorption edge to higher energy is observed in the films with higher Mg composition. The excitonic nature of the films is clearly appeared in the spectra for all alloy compositions. The optical band-gap ($E_g$) of ZMO films is obtained from the ${\alpha}^2$ vs Photon energy plot assuming ${\alpha}^2\;\propto$ (hv - $E_g$), where u is the absorption coefficient and hv is the photon energy. The value of $E_g$ increases up to 3.72 eV for the films with x=0.35. It is important to adjust Mg composition control for controlling the band-gap of ZMO films.

• Carbon letters
• /
• v.9 no.2
• /
• pp.131-136
• /
• 2008

In order to investigate the effect of doping C, N, B and F elements on $TiO_2$ for reducing the band gap, the heat treatment of $TiO_2$ was carried out with tetraethylammonium tetrafluoroborate. Through XRD and XPS analysis, the C, N, B and F doped anatase $TiO_2$ was confirmed. According to the increase of temperature during treatment, the particle size was increased due to aggregation of $TiO_2$ with elements (B, C, N and F). To investigate the capacity of photocatalyst for degradation of dye under solar light, the degradation of acridine orange and methylene blue was conducted. The degradation of dyes was carried out successfully under solar light indicating the effect of doping elements (B, C, N and F) on $TiO_2$ for reducing the band gap effectively.

• Journal of Information Display
• /
• v.9 no.2
• /
• pp.18-21
• /
• 2008

We report on white organic light-emitting diodes (WOLEDs) based on single white dopants, $Ir(pq)_2$($F_2$-ppy) and $Ir(F_2-ppy)_2$(pq), where $F_2$-ppy and pq are 2-(2,4-difluorophenyl) pyridine and 2-phenylquinoline, respectively. The similar phosphorescent lifetime of two ligands lead to luminescence emission in two ligands simultaneously. However, the emission color of the devices was reddish, because the energy was not transferred efficiently from the 4,4,N,N'-dicarbazolebiphenyl (CBP) to the $F_2$-ppy ligand, due to the small band gap of the CBP. Accordingly, we used 1,4-phenylenesis(triphenylsilane) (UGH2) with a large band gap, instead of CBP as the host material. As a result, it was possible to adjust the emission color by the host material. The luminous efficiency of the device with $Ir(F_2-ppy)_2$(pq) doped in UGH2 was about 11 cd/A at 0.06 cd/$m^2$.

• Journal of the Korea Institute of Information and Communication Engineering
• /
• v.16 no.10
• /
• pp.2247-2252
• /
• 2012

In this paper, we produced CIGS thin film by co-evaporation method. During the process, substrate temperature and Ga/(In+Ga) composition ratio was altered to observe the change of resistivity and absorbance spectra measurements. As substrate temperature increased, resistivity decreased and as Ga/(In+Ga) composition ratio increased from 0.30 to 0.72, band gap also increased with the range of 1.26eV, 1.30eV, 1.43eV, 1.47eV. With the constant condition of composition ratio, resistivity decreased with increased thickness of the thin film. On this experiment, we assumed that optical absorbance ratio and optical current will be increased with CIGS thin film fabrication.

• The Transactions of the Korean Institute of Power Electronics
• /
• v.23 no.3
• /
• pp.168-173
• /
• 2018

In this paper, an auxiliary power supply (APS) for railroad cars is proposed. The APS can reduce the number of devices required to supply power through structural modification and operates at a high switching frequency by application of a SiC device. The voltage stress on the device in the proposed circuit can be reduced to less than half of the input voltage of the system; thus, a device with low breakdown voltage can be designed. By adapting a SiC device instead of an IGBT device, the proposed circuit can reduce switching and conduction losses and operate at a high switching frequency, thereby reducing output voltage and inductor current ripples in the proposed circuit. The theoretical analysis results of the proposed APS are verified with a 40 kW computer-based simulation and a 2 kW experiment.