Yonggu Shim - Academia.edu (original) (raw)
Papers by Yonggu Shim
The Japan Society of Applied Physics, Feb 3, 2017
The Japan Society of Applied Physics, Jul 10, 2018
Pisʹma v Žurnal tehničeskoj fiziki, 2018
The Japan Society of Applied Physics, Feb 3, 2017
Japanese Journal of Applied Physics, Oct 17, 2008
The density functional theory (DFT) based ab initio calculations of the phonon spectra of the cha... more The density functional theory (DFT) based ab initio calculations of the phonon spectra of the chain compounds, TlSe and TlInSe2, with analogous crystal structures have been carried out. For both materials, the derived frequencies and symmetry of the phonon modes in the center of Brillouin zone agree well with the experimental data on Raman scattering and infrared absorption. For TlSe, the available phonon dispersion and density-of-states data obtained by neutron scattering are also reproduced well by the results of calculations. In both compounds, the acoustic and low-frequency optic branches greatly overlap, which is a characteristic feature of structurally low-dimensional materials. For both compounds, the calculated and measured heat capacities are in a good agreement up to the temperatures limiting the validity of the used harmonic approximation. Strong anharmonisity of TlInSe2 as compared to TlSe, as well as the probable connection of this anharmonicity with incommensurate phase of TlInSe2 at temperatures above 100 K is figured out.
The Japan Society of Applied Physics, Jul 10, 2018
The Japan Society of Applied Physics, Jul 10, 2018
The Japan Society of Applied Physics, Jul 21, 2017
The Japan Society of Applied Physics, Jul 22, 2016
Journal of Applied Physics, Jan 24, 2018
Ternary thallium compound TlInSe 2 exhibits a giant Seebeck effect below around 410 K, where Tl a... more Ternary thallium compound TlInSe 2 exhibits a giant Seebeck effect below around 410 K, where Tl atoms form one dimensional incommensurate (IC) arrays. To clarify the origin of large thermoelectric power in the IC phase, the electronic properties of Tl-atom super-structured TlInSe 2 were studied using the first-principles calculations. It was shown that the superstructures induce strong binding states between Se-p orbitals in the nearest neighboring layers and produce large density of states near lower conduction bands, which might be one of the origins to produce large thermoelectric power.
The Japan Society of Applied Physics, Jul 10, 2019
Journal of physics, Jun 17, 2015
In this work we examined the phase transitions in the ternary thallium chalcogenide TlGaSe 2 by s... more In this work we examined the phase transitions in the ternary thallium chalcogenide TlGaSe 2 by studying the temperature dependence of the Raman spectra with the aid of confocal microscopy. The unpolarized Raman scattering spectra of TlGaSe 2 single crystals were measured over the temperature range 78-300 K (which includes the range of the successive phase transitions) in the frequency region of 50-300 cm-1. The Raman spectra exhibited 12 lines at 300 K, but the number of lines rose to 17 at 78 K. In the temperature interval between 107 K and 120 K, where the phase transitions take place, the temperature dependence of the phonon frequencies showed discontinuities for several of the Raman lines.
Japanese Journal of Applied Physics
We investigate the effects of several-hundred-micron thick luminescence down-shifting (LDS) films... more We investigate the effects of several-hundred-micron thick luminescence down-shifting (LDS) films composed of sol–gel glass with Zn-based nanoparticles (NPs) dispersed on the characteristics of Si solar cells. Their internal quantum efficiencies (IQEs) are successfully measured by separating the contributions of downshifted photons in measuring reflectance for 300–400 nm, wavelengths of incident photons absorbed by the NPs. We find that IQEs for this wavelength range are more enhanced by employing thicker LDS films, i.e. LDS films with higher optical densities. We also discuss the relationship between the number density of NPs in LDS films, their optical properties, and the IQEs of cells. We observe a discrepancy between the measured and calculated IQEs and note that this is the result of downshifted photons escaping across the sides of the LDS films.
physica status solidi c
A sulfur‐annealing treatment was investigated to control the sulfur content of epitaxial CuInS2 f... more A sulfur‐annealing treatment was investigated to control the sulfur content of epitaxial CuInS2 films grown on GaAs substrate by PLD. The sulfur‐annealing treatment improved the surface roughness and film crystallinity. Photoluminescence measurements obtained using the confocal microspectroscopy demonstrated that the annealed films show band‐edge emissions over a very large area. Electron‐probe microanalysis measurements indicated that the sulfur content of the annealed films was about 50 at.%, and the content of the emission area was more than 50 at.%. Therefore, the sulfur content of epitaxial CuInS2 films was successfully controlled by the sulfur‐annealing treatment.
physica status solidi c
In this work, we examined phase transitions in the layered ternary thallium chalcogenide TlInS2 b... more In this work, we examined phase transitions in the layered ternary thallium chalcogenide TlInS2 by studying the temperature dependence of polarized Raman spectra with the aid of the Raman confocal microscope system. The Raman spectra were measured over the temperature range of 77–320 K (which includes the range of successive phase transitions) in the low‐frequency region of 35–180 cm−1. The optical phonons that showed strong temperature dependence were identified as interlayer vibrations related to phase transitions, while the phonons that showed weak temperature dependence were identified as intralayer vibrations.
The Japan Society of Applied Physics, 2017
三元半導体CuInS2は1.5 eVのバンドギャップエネルギーと大きな吸収係数をもつため太陽電池 材料として期待されている。これまでCuInS2太陽電池は11.4 %の変換効率[1]が達成されて... more 三元半導体CuInS2は1.5 eVのバンドギャップエネルギーと大きな吸収係数をもつため太陽電池 材料として期待されている。これまでCuInS2太陽電池は11.4 %の変換効率[1]が達成されているが、 CuInS2太陽電池の性能の向上には高品質な薄膜を作製する必要がある。CuInS2薄膜の高品質の指 標となるCuInS2薄膜の自由励起子発光についてはこれまでほとんど観測されていない。最近我々 はPLD (Pulse Laser Deposition)法によるGaAs基板上でのCuInS2薄膜エピタキシャル成長について 報告したが、Sの含有量が十分でなかった[2]。 本研究では、CuInS2薄膜をPLD (Pulse Laser Deposition)法によりガラス基板上に成長させ、粉末 原料比の調整とターゲットへのIn2S3、Sの充填により、化学量論比に基づいた薄膜の作製を図った。 図1に成膜した試料のX線回折(XRD)の測定結果を示す。測定結果をJCPDSデータと比較し、作製 した多結晶及び薄膜はカルコパイライト構造のCuInS2であることを確認した。図2にCuInS2多結 晶のPLスペクトルを示す。励起光源にはTi:Sapphireレーザー(第二高調波:波長400 nm)を用い た。PLスペクトルを1.546 eV(ピークA)と1.526 eV(ピークB)のガウス関数を用いて波形分離 した。ピークAおよびBはBinsmaの報告[3]によると自由励起子と束縛励起子にそれぞれ対応してい る。
The Japan Society of Applied Physics, 2016
The Japan Society of Applied Physics, 2017
四元系半導体 Cu2ZnSnS4(以降,CZTS)は構成元素に希少元素および有毒元素を用いないことか ら,環境調和型の半導体といえる。またバンドギャップエネルギーは約 1.5 eV であり,太陽... more 四元系半導体 Cu2ZnSnS4(以降,CZTS)は構成元素に希少元素および有毒元素を用いないことか ら,環境調和型の半導体といえる。またバンドギャップエネルギーは約 1.5 eV であり,太陽電池光 吸収層の最適値に近いことから太陽電池材料として高いポテンシャルを有する。これまでの研究で太 陽電池材料として CZTS における最適組成比率が報告されている 1)。最近,我々は PLD (Pulse laser deposition)法で作製した組成比の均一性の高い CZTS 薄膜を報告したが 2),高い変換効率を有する組 成比率まで十分組成比制御できていなかった。 本研究では PLD 法により SLG 基板上に高品質 CZTS 薄膜を成膜するために使用する高品質 CZTS 多結晶ターゲットの段階から組成制御の検討を行った。まず初めに,薄膜の組成比率を最適組成比に 制御するための固相反応法による CZTS 多結晶の作製と評価を行った。その後,作製した CZTS 多結 晶を用いて,PLD 法による CZTS 薄膜の作製と評価を行った。組成比率を調整した CZTS 多結晶タ ーゲットは,Cu, Zn, Sn, S 粉末を Cu:Zn:Sn:S = 2:1:1:4 のモル比率で混合し作製した CZTS 多結晶 に ZnS, SnS,S 粉末を追加混合し,Cu/Zn/Sn/S = 1.6:1.3:1.1:4 の組成比率になるように作製した。 組成比率調整の際,粉末混合時間を増加することで,EPMA 組成分析の結果から,組成比のばらつき が減少し,組成分布の均一性が向上したことがわかった。また,組成比率調整の際に S 粉末の十分な 添加を行うことで,CZTS 多結晶ターゲットの組成比率を制御できることがわかった。組成比率調整 CZTS 多結晶の組成比率調整の際に混合する ZnS, SnS, S 粉末の量は EPMA による組成分析結果を元 に決定している。CZTS 薄膜は Nd:YAG レーザー(波長:266 nm,パルス幅:10 ns)を 0.5 J/cm2 のパルスエネルギーで CZTS 多結晶ターゲットに照射し,ガラス基板上へ成膜した。作製した CZTS 多結晶および CZTS 薄膜は X 線回折測定,ラマン分光測定,FE-SEM による表面観察,EPMA によ る...
The Japan Society of Applied Physics, 2018
The Japan Society of Applied Physics, Feb 3, 2017
The Japan Society of Applied Physics, Jul 10, 2018
Pisʹma v Žurnal tehničeskoj fiziki, 2018
The Japan Society of Applied Physics, Feb 3, 2017
Japanese Journal of Applied Physics, Oct 17, 2008
The density functional theory (DFT) based ab initio calculations of the phonon spectra of the cha... more The density functional theory (DFT) based ab initio calculations of the phonon spectra of the chain compounds, TlSe and TlInSe2, with analogous crystal structures have been carried out. For both materials, the derived frequencies and symmetry of the phonon modes in the center of Brillouin zone agree well with the experimental data on Raman scattering and infrared absorption. For TlSe, the available phonon dispersion and density-of-states data obtained by neutron scattering are also reproduced well by the results of calculations. In both compounds, the acoustic and low-frequency optic branches greatly overlap, which is a characteristic feature of structurally low-dimensional materials. For both compounds, the calculated and measured heat capacities are in a good agreement up to the temperatures limiting the validity of the used harmonic approximation. Strong anharmonisity of TlInSe2 as compared to TlSe, as well as the probable connection of this anharmonicity with incommensurate phase of TlInSe2 at temperatures above 100 K is figured out.
The Japan Society of Applied Physics, Jul 10, 2018
The Japan Society of Applied Physics, Jul 10, 2018
The Japan Society of Applied Physics, Jul 21, 2017
The Japan Society of Applied Physics, Jul 22, 2016
Journal of Applied Physics, Jan 24, 2018
Ternary thallium compound TlInSe 2 exhibits a giant Seebeck effect below around 410 K, where Tl a... more Ternary thallium compound TlInSe 2 exhibits a giant Seebeck effect below around 410 K, where Tl atoms form one dimensional incommensurate (IC) arrays. To clarify the origin of large thermoelectric power in the IC phase, the electronic properties of Tl-atom super-structured TlInSe 2 were studied using the first-principles calculations. It was shown that the superstructures induce strong binding states between Se-p orbitals in the nearest neighboring layers and produce large density of states near lower conduction bands, which might be one of the origins to produce large thermoelectric power.
The Japan Society of Applied Physics, Jul 10, 2019
Journal of physics, Jun 17, 2015
In this work we examined the phase transitions in the ternary thallium chalcogenide TlGaSe 2 by s... more In this work we examined the phase transitions in the ternary thallium chalcogenide TlGaSe 2 by studying the temperature dependence of the Raman spectra with the aid of confocal microscopy. The unpolarized Raman scattering spectra of TlGaSe 2 single crystals were measured over the temperature range 78-300 K (which includes the range of the successive phase transitions) in the frequency region of 50-300 cm-1. The Raman spectra exhibited 12 lines at 300 K, but the number of lines rose to 17 at 78 K. In the temperature interval between 107 K and 120 K, where the phase transitions take place, the temperature dependence of the phonon frequencies showed discontinuities for several of the Raman lines.
Japanese Journal of Applied Physics
We investigate the effects of several-hundred-micron thick luminescence down-shifting (LDS) films... more We investigate the effects of several-hundred-micron thick luminescence down-shifting (LDS) films composed of sol–gel glass with Zn-based nanoparticles (NPs) dispersed on the characteristics of Si solar cells. Their internal quantum efficiencies (IQEs) are successfully measured by separating the contributions of downshifted photons in measuring reflectance for 300–400 nm, wavelengths of incident photons absorbed by the NPs. We find that IQEs for this wavelength range are more enhanced by employing thicker LDS films, i.e. LDS films with higher optical densities. We also discuss the relationship between the number density of NPs in LDS films, their optical properties, and the IQEs of cells. We observe a discrepancy between the measured and calculated IQEs and note that this is the result of downshifted photons escaping across the sides of the LDS films.
physica status solidi c
A sulfur‐annealing treatment was investigated to control the sulfur content of epitaxial CuInS2 f... more A sulfur‐annealing treatment was investigated to control the sulfur content of epitaxial CuInS2 films grown on GaAs substrate by PLD. The sulfur‐annealing treatment improved the surface roughness and film crystallinity. Photoluminescence measurements obtained using the confocal microspectroscopy demonstrated that the annealed films show band‐edge emissions over a very large area. Electron‐probe microanalysis measurements indicated that the sulfur content of the annealed films was about 50 at.%, and the content of the emission area was more than 50 at.%. Therefore, the sulfur content of epitaxial CuInS2 films was successfully controlled by the sulfur‐annealing treatment.
physica status solidi c
In this work, we examined phase transitions in the layered ternary thallium chalcogenide TlInS2 b... more In this work, we examined phase transitions in the layered ternary thallium chalcogenide TlInS2 by studying the temperature dependence of polarized Raman spectra with the aid of the Raman confocal microscope system. The Raman spectra were measured over the temperature range of 77–320 K (which includes the range of successive phase transitions) in the low‐frequency region of 35–180 cm−1. The optical phonons that showed strong temperature dependence were identified as interlayer vibrations related to phase transitions, while the phonons that showed weak temperature dependence were identified as intralayer vibrations.
The Japan Society of Applied Physics, 2017
三元半導体CuInS2は1.5 eVのバンドギャップエネルギーと大きな吸収係数をもつため太陽電池 材料として期待されている。これまでCuInS2太陽電池は11.4 %の変換効率[1]が達成されて... more 三元半導体CuInS2は1.5 eVのバンドギャップエネルギーと大きな吸収係数をもつため太陽電池 材料として期待されている。これまでCuInS2太陽電池は11.4 %の変換効率[1]が達成されているが、 CuInS2太陽電池の性能の向上には高品質な薄膜を作製する必要がある。CuInS2薄膜の高品質の指 標となるCuInS2薄膜の自由励起子発光についてはこれまでほとんど観測されていない。最近我々 はPLD (Pulse Laser Deposition)法によるGaAs基板上でのCuInS2薄膜エピタキシャル成長について 報告したが、Sの含有量が十分でなかった[2]。 本研究では、CuInS2薄膜をPLD (Pulse Laser Deposition)法によりガラス基板上に成長させ、粉末 原料比の調整とターゲットへのIn2S3、Sの充填により、化学量論比に基づいた薄膜の作製を図った。 図1に成膜した試料のX線回折(XRD)の測定結果を示す。測定結果をJCPDSデータと比較し、作製 した多結晶及び薄膜はカルコパイライト構造のCuInS2であることを確認した。図2にCuInS2多結 晶のPLスペクトルを示す。励起光源にはTi:Sapphireレーザー(第二高調波:波長400 nm)を用い た。PLスペクトルを1.546 eV(ピークA)と1.526 eV(ピークB)のガウス関数を用いて波形分離 した。ピークAおよびBはBinsmaの報告[3]によると自由励起子と束縛励起子にそれぞれ対応してい る。
The Japan Society of Applied Physics, 2016
The Japan Society of Applied Physics, 2017
四元系半導体 Cu2ZnSnS4(以降,CZTS)は構成元素に希少元素および有毒元素を用いないことか ら,環境調和型の半導体といえる。またバンドギャップエネルギーは約 1.5 eV であり,太陽... more 四元系半導体 Cu2ZnSnS4(以降,CZTS)は構成元素に希少元素および有毒元素を用いないことか ら,環境調和型の半導体といえる。またバンドギャップエネルギーは約 1.5 eV であり,太陽電池光 吸収層の最適値に近いことから太陽電池材料として高いポテンシャルを有する。これまでの研究で太 陽電池材料として CZTS における最適組成比率が報告されている 1)。最近,我々は PLD (Pulse laser deposition)法で作製した組成比の均一性の高い CZTS 薄膜を報告したが 2),高い変換効率を有する組 成比率まで十分組成比制御できていなかった。 本研究では PLD 法により SLG 基板上に高品質 CZTS 薄膜を成膜するために使用する高品質 CZTS 多結晶ターゲットの段階から組成制御の検討を行った。まず初めに,薄膜の組成比率を最適組成比に 制御するための固相反応法による CZTS 多結晶の作製と評価を行った。その後,作製した CZTS 多結 晶を用いて,PLD 法による CZTS 薄膜の作製と評価を行った。組成比率を調整した CZTS 多結晶タ ーゲットは,Cu, Zn, Sn, S 粉末を Cu:Zn:Sn:S = 2:1:1:4 のモル比率で混合し作製した CZTS 多結晶 に ZnS, SnS,S 粉末を追加混合し,Cu/Zn/Sn/S = 1.6:1.3:1.1:4 の組成比率になるように作製した。 組成比率調整の際,粉末混合時間を増加することで,EPMA 組成分析の結果から,組成比のばらつき が減少し,組成分布の均一性が向上したことがわかった。また,組成比率調整の際に S 粉末の十分な 添加を行うことで,CZTS 多結晶ターゲットの組成比率を制御できることがわかった。組成比率調整 CZTS 多結晶の組成比率調整の際に混合する ZnS, SnS, S 粉末の量は EPMA による組成分析結果を元 に決定している。CZTS 薄膜は Nd:YAG レーザー(波長:266 nm,パルス幅:10 ns)を 0.5 J/cm2 のパルスエネルギーで CZTS 多結晶ターゲットに照射し,ガラス基板上へ成膜した。作製した CZTS 多結晶および CZTS 薄膜は X 線回折測定,ラマン分光測定,FE-SEM による表面観察,EPMA によ る...
The Japan Society of Applied Physics, 2018