In a long history of photovoltaic innovations, various architectures of crystalline silicon solar... more In a long history of photovoltaic innovations, various architectures of crystalline silicon solar cells have been evolved to result in the highest conversion efficiency (η) of those days as described in Best Research Cell Efficiency by NREL. The cells are characterized by three parameters of the power max (Pmax), the saturation current (Jsc) and open-circuit voltage (Voc), but the fill-factor (FF) can be deduced from those three parameters. The 36 outstanding cells of those days are picked up from the published papers to be plotted on the η-FF and η-Jsc•Voc planes, in which all kinds of the architectures of the cells are included, for instance, PERC, PERL, PERT, HIT, IBC, TopCon, POLO, POLO-IBC, and Hetero-IBC(HJ-IBC). The plots of the η of those outstanding solar cells against their FF or Jsc•Voc clearly reveal a linear relation among them, even they have been independently evolved. The linear relation seems to have some effective meanings. The linear relation being assumable as a straight line, a 87% of FF on the line, which was predicted as the limit of FF by M. Reusch, can meet well with 29% to 30% in efficiency, which have been expected as the efficiency limit of crystalline silicon solar cells by W. Schockley and J. Queisser, and R. Swanson.
The termination of the lower terrace at the atomic steps on the Si(111)7X7surface has been invest... more The termination of the lower terrace at the atomic steps on the Si(111)7X7surface has been investigated with scanning tunneling microscopy (STM). It is proposed that seven positions of the steps relative to a 7X7 unit mesh of the end of the lower terrace are possible for each of two step directions, and ten of them have been observed in STM images. A rule for the termination at the steps is proposed. Structure and restructuring of the steps are described. We report a systematic study on the atomic structure of the single steps (3.1-A height) on the Si(111)7X7surface using scanning tunneling microscopy (STM). It is found that various types of the single steps exist on the surface and they are classified into two groups with seven steps each. In this study we have given a unified view of the atomic steps on Si(111)7X 7. To date, the macroscopic shape of the steps on Si(111)7X7 has been studied with use of refiection electron microscopy (REM) by Yagi and co-workers' and with use of low-energy electron microscopy (I.EEM) by Bauer and co-workers, but atomic resolution of the steps was not achieved by these methods. Recent development
IEEE Translation Journal on Magnetics in Japan, 1989
Co-Cr and Co-Cr-W films were prepared by DC magnetron sputtering on PET film. The dependence of t... more Co-Cr and Co-Cr-W films were prepared by DC magnetron sputtering on PET film. The dependence of the magnetic anisotropy constants Kui and Ku 2 on the film thickness was determined from torque curve measurements. When the film thickness is thin, the Kui is very small, and it increases rapidly as the film thickness increases. On the other hand, the Ku 2 is extremely large for thin films, but drops rapidly with increasing film thickness, approaching a nearly constant value. The ratio Kui/Ku2 is not constant. The dependences of $2 and 04, the tilt angles of the easy magnetization axis from the film normal for Kui and Ku 2 , on the film thickness were measured. The value of $2 was not equal to that of 04. These results suggest that there is another fourfold symmetry besides that of the uniaxial anisotropy.
Extended Abstracts of the 1993 International Conference on Solid State Devices and Materials, 1993
Gate Oxides' thickness of the future LSIs will be less than 6nm. In realizing such devices, it is... more Gate Oxides' thickness of the future LSIs will be less than 6nm. In realizing such devices, it is important to reduce oxide-Si interface roughness[l,2]. At room temperature (native oxide growth), the oxidatiort proceeds layer by laydr. Above ardund 400C or SOOC-(ttrermal oxide growth), the oxide growth is qualitatively different from that at room temperature, because Si-Si bonds are easily broken above this temperature. For example, scannins tunnelins microscopy (STM) and X-ray photoele-ctron spectioscopy (XPS) results suggested thdt 'the initiat therinal oxid6-growth of Si(lll) at 600C proceeds by island oxide formation[3,4]. On the other hand, transmission electron microscopy (TEM) results suggested layer by layer oxidation of Si(ll l) at high temperatures[5]. In this paper, we will show that the initial thermal oxide growth of Si(001).proceeds tgy:tly layer rather than by island oxide formation even at 600L. 2. Experimental The sample was an n-type (0.05-0.09Ocm) Si(001) CZ wafer with a misorientation of 0.4" toward [110]. Similar results were also obtained on a p-type wafer. A clean surface was obtained by heating up io 1200C in ultrahigh vacuum ([IHV). O4ide was formed at 600 C and
The initial stage of growth of Cu on Si(111)7 x 7 surface at room temperature is observed with a ... more The initial stage of growth of Cu on Si(111)7 x 7 surface at room temperature is observed with a field ion-scanning tunneling microscope (FI-STM). Cu atoms adsorb on adatoms, especially center adatoms, as well as on rest atoms and some Cu atoms form small clusters at very low coverage. At submonolayer coverage triangular clusters lay on the center of sub-units of the DAS structure. At a higher coverage, nearly circular shaped islands are observed, which is a different shape than those observed by others.
ABSTRACT It is important to select suitable screen mask for printing fine electrode. In case of f... more ABSTRACT It is important to select suitable screen mask for printing fine electrode. In case of fine line printing with 50 μm width, we demonstrated that the screen mask with 39% opening is the best selection compared with 49% and 60% opening. Variation (max and min) of width and thickness of fine electrode shows lower value when using low opening rate.
IEEE Translation Journal on Magnetics in Japan, 1987
The effect of degassing PET, PEN and PI substrates at different temperatures prior to sputter dep... more The effect of degassing PET, PEN and PI substrates at different temperatures prior to sputter deposition of CoCr films on the film properties, and changes with degassing in the dependence of film properties on the Ar gas pressure during sputtering, were investigated. The effect of the Ar gas pressure on films varies depending on the amount of impurity gases remaining in the substrate, and degassing had a greater effect for PI substrate, which held more impurity gases, than for PET and PEN, in which fewer impurities were present.
We studied the texturization mechanism of mono crystalline silicon wafer using some alternative a... more We studied the texturization mechanism of mono crystalline silicon wafer using some alternative additives. We successfully controlled the texture size with change of concentration of KOH in the etchant. We also proposed one appropriate model of role of additives in texturization process. In our proposed mechanism, the additives should play a role as micro-mask when the micro pyramid is formed and the texturization process should be dominated by the correlation of etching rate and adhesion and desorption rate of additives. We also fabricated solar cells with several texture sizes and evaluated them. The decrease of F.F. with the decrease of texture size was mainly caused by the increase of Rs and this result suggested that we had to optimize the screen printing and firing conditions for the small texture. In addition, there are thick damaged layer on the top and bottom of texture, and these damaged layer cause the deterioration of cell efficiency with texture miniaturization. Reduction of the damage is important to realize high efficiency solar cell using miniaturized texture.
The initial stages of the oxidation of the Si(100)2x 1 surface was studied at room temperature by... more The initial stages of the oxidation of the Si(100)2x 1 surface was studied at room temperature by scanning tunneling microscopy (STM). We found that "type-C defects" which are believed to be two half-dimers have a strong preference for oxidation, compared with areas having no defects. Oxidized type-C defects appear to be depressions in both positive and negative sample bias voltages. We also found two oxidized sites having no defects. Single steps are quite stable against oxidation. The oxidation of the Si(100)2 x 1 surface is discussed in terms of these sites.
The initial stages of the thermal (600° C) oxide growth of Si (001) clean surfaces were studied b... more The initial stages of the thermal (600° C) oxide growth of Si (001) clean surfaces were studied by scanning tunneling microscopy (STM). Oxide growth starts from both step edges and terraces. At 120L, almost the entire surface was covered by oxides. Steps could still be identified, and the apparent roughness had a local minimum at this O2 exposure. These results indicate that the first layer is oxidized quite uniformly. Some local orderings of the initial oxides were observed on the terraces, which may enhance the oxide growth in the lateral (parallel to the surface) direction.
Using scanning lunneling microscopy, ,.~u rt.we;d the atomic slructur¢ of step hunches till v;ein... more Using scanning lunneling microscopy, ,.~u rt.we;d the atomic slructur¢ of step hunches till v;einal Si(ll I) misoriented towards [112] by 4 °. Clusters iff steps, consisting of m~re thao. ten alomic layers, are uhserved to form C?I) facets, where a perkldic structure is I()und. Each bump of the structure is larger Ihan Ihal of an ~ldMom (if Sill 1117 × 7, showing b x 3 reconstruction. We propose a rondel of 6 ~ 3 describing this corrugation as a tlimer ta adatoms to discuss Ihe reduced dangling blinds and the asymmetric structure.
Ultrahigh vacuum (UHV) system equipped with aluminum radiation shields has been developed in orde... more Ultrahigh vacuum (UHV) system equipped with aluminum radiation shields has been developed in order to obtain atomically flat Si(OOl) surfaces in a large area. Aluminum radiation shields were employed to suppress the temperature rise of chamber walls. The UHV system has enabled a 6 inch substrate to be heated up to 700°C under a pressure of less than IO-' Pa range. The surface structure of the substrate was confirmed by the reflection high energy electron diffraction (RHEED). when the 6 inch substrate was heated at 600°C in UHV after a modified RCA cleaning, the RHEED pattern indicated oxide free surface consisted of 2 X I structure. The roughness was evaluated from numerous observations by cross-sectional transmission electron microscopy (XTEM). Root-mean-square roughness of the Si surface was less than 0. I nm for almost all over the 6 inch substrate. This implies that the heating in this system gives rise to the atomic-scale planarization of the 6 inch substrate.
Scanning tunnehng microscopy (STM) is used to investigate the atomic structure of S1(331) The (33... more Scanning tunnehng microscopy (STM) is used to investigate the atomic structure of S1(331) The (331) terraces have large areas of more than 50 × 50 nm 2, and single atomic-height steps of 0 12 nm are observed No facetlng into other planes is observed The reconstructed structure found on the terraces can explain the LEED pattern A umt cell consists of adatom-hke structures, the diameter of which becomes as much as 1 3 nm We call this structure an super adatom Even at domain boundaries, the super adatom seldom breaks into atomic fragments The d~scusslon is focused on the origin of the huge dmmeter and the behavlour of the super adatom at a high-temperature 1 × 1 phase
Scanning tunneling microscopy ~s used to investigate how domain boundaries on Si(111)7×7 are affe... more Scanning tunneling microscopy ~s used to investigate how domain boundaries on Si(111)7×7 are affected by step configuration. For making the configuration of step change, we prepare surfaces having misorientation different from (111). On the one surface misoriented towards [110], steps run in a zigzag pattern, which causes terraces to be constricted. On every constricted site, a domain boundary exists. On the other surface misoriented towards [112], steps run parallel and consist of several atomic layers. Domain boundaries on this surface run in a zigzag pattern. Discussion is focused on surface energy of the stepped surface containing domain boundaries.
In a long history of photovoltaic innovations, various architectures of crystalline silicon solar... more In a long history of photovoltaic innovations, various architectures of crystalline silicon solar cells have been evolved to result in the highest conversion efficiency (η) of those days as described in Best Research Cell Efficiency by NREL. The cells are characterized by three parameters of the power max (Pmax), the saturation current (Jsc) and open-circuit voltage (Voc), but the fill-factor (FF) can be deduced from those three parameters. The 36 outstanding cells of those days are picked up from the published papers to be plotted on the η-FF and η-Jsc•Voc planes, in which all kinds of the architectures of the cells are included, for instance, PERC, PERL, PERT, HIT, IBC, TopCon, POLO, POLO-IBC, and Hetero-IBC(HJ-IBC). The plots of the η of those outstanding solar cells against their FF or Jsc•Voc clearly reveal a linear relation among them, even they have been independently evolved. The linear relation seems to have some effective meanings. The linear relation being assumable as a straight line, a 87% of FF on the line, which was predicted as the limit of FF by M. Reusch, can meet well with 29% to 30% in efficiency, which have been expected as the efficiency limit of crystalline silicon solar cells by W. Schockley and J. Queisser, and R. Swanson.
The termination of the lower terrace at the atomic steps on the Si(111)7X7surface has been invest... more The termination of the lower terrace at the atomic steps on the Si(111)7X7surface has been investigated with scanning tunneling microscopy (STM). It is proposed that seven positions of the steps relative to a 7X7 unit mesh of the end of the lower terrace are possible for each of two step directions, and ten of them have been observed in STM images. A rule for the termination at the steps is proposed. Structure and restructuring of the steps are described. We report a systematic study on the atomic structure of the single steps (3.1-A height) on the Si(111)7X7surface using scanning tunneling microscopy (STM). It is found that various types of the single steps exist on the surface and they are classified into two groups with seven steps each. In this study we have given a unified view of the atomic steps on Si(111)7X 7. To date, the macroscopic shape of the steps on Si(111)7X7 has been studied with use of refiection electron microscopy (REM) by Yagi and co-workers' and with use of low-energy electron microscopy (I.EEM) by Bauer and co-workers, but atomic resolution of the steps was not achieved by these methods. Recent development
IEEE Translation Journal on Magnetics in Japan, 1989
Co-Cr and Co-Cr-W films were prepared by DC magnetron sputtering on PET film. The dependence of t... more Co-Cr and Co-Cr-W films were prepared by DC magnetron sputtering on PET film. The dependence of the magnetic anisotropy constants Kui and Ku 2 on the film thickness was determined from torque curve measurements. When the film thickness is thin, the Kui is very small, and it increases rapidly as the film thickness increases. On the other hand, the Ku 2 is extremely large for thin films, but drops rapidly with increasing film thickness, approaching a nearly constant value. The ratio Kui/Ku2 is not constant. The dependences of $2 and 04, the tilt angles of the easy magnetization axis from the film normal for Kui and Ku 2 , on the film thickness were measured. The value of $2 was not equal to that of 04. These results suggest that there is another fourfold symmetry besides that of the uniaxial anisotropy.
Extended Abstracts of the 1993 International Conference on Solid State Devices and Materials, 1993
Gate Oxides' thickness of the future LSIs will be less than 6nm. In realizing such devices, it is... more Gate Oxides' thickness of the future LSIs will be less than 6nm. In realizing such devices, it is important to reduce oxide-Si interface roughness[l,2]. At room temperature (native oxide growth), the oxidatiort proceeds layer by laydr. Above ardund 400C or SOOC-(ttrermal oxide growth), the oxide growth is qualitatively different from that at room temperature, because Si-Si bonds are easily broken above this temperature. For example, scannins tunnelins microscopy (STM) and X-ray photoele-ctron spectioscopy (XPS) results suggested thdt 'the initiat therinal oxid6-growth of Si(lll) at 600C proceeds by island oxide formation[3,4]. On the other hand, transmission electron microscopy (TEM) results suggested layer by layer oxidation of Si(ll l) at high temperatures[5]. In this paper, we will show that the initial thermal oxide growth of Si(001).proceeds tgy:tly layer rather than by island oxide formation even at 600L. 2. Experimental The sample was an n-type (0.05-0.09Ocm) Si(001) CZ wafer with a misorientation of 0.4" toward [110]. Similar results were also obtained on a p-type wafer. A clean surface was obtained by heating up io 1200C in ultrahigh vacuum ([IHV). O4ide was formed at 600 C and
The initial stage of growth of Cu on Si(111)7 x 7 surface at room temperature is observed with a ... more The initial stage of growth of Cu on Si(111)7 x 7 surface at room temperature is observed with a field ion-scanning tunneling microscope (FI-STM). Cu atoms adsorb on adatoms, especially center adatoms, as well as on rest atoms and some Cu atoms form small clusters at very low coverage. At submonolayer coverage triangular clusters lay on the center of sub-units of the DAS structure. At a higher coverage, nearly circular shaped islands are observed, which is a different shape than those observed by others.
ABSTRACT It is important to select suitable screen mask for printing fine electrode. In case of f... more ABSTRACT It is important to select suitable screen mask for printing fine electrode. In case of fine line printing with 50 μm width, we demonstrated that the screen mask with 39% opening is the best selection compared with 49% and 60% opening. Variation (max and min) of width and thickness of fine electrode shows lower value when using low opening rate.
IEEE Translation Journal on Magnetics in Japan, 1987
The effect of degassing PET, PEN and PI substrates at different temperatures prior to sputter dep... more The effect of degassing PET, PEN and PI substrates at different temperatures prior to sputter deposition of CoCr films on the film properties, and changes with degassing in the dependence of film properties on the Ar gas pressure during sputtering, were investigated. The effect of the Ar gas pressure on films varies depending on the amount of impurity gases remaining in the substrate, and degassing had a greater effect for PI substrate, which held more impurity gases, than for PET and PEN, in which fewer impurities were present.
We studied the texturization mechanism of mono crystalline silicon wafer using some alternative a... more We studied the texturization mechanism of mono crystalline silicon wafer using some alternative additives. We successfully controlled the texture size with change of concentration of KOH in the etchant. We also proposed one appropriate model of role of additives in texturization process. In our proposed mechanism, the additives should play a role as micro-mask when the micro pyramid is formed and the texturization process should be dominated by the correlation of etching rate and adhesion and desorption rate of additives. We also fabricated solar cells with several texture sizes and evaluated them. The decrease of F.F. with the decrease of texture size was mainly caused by the increase of Rs and this result suggested that we had to optimize the screen printing and firing conditions for the small texture. In addition, there are thick damaged layer on the top and bottom of texture, and these damaged layer cause the deterioration of cell efficiency with texture miniaturization. Reduction of the damage is important to realize high efficiency solar cell using miniaturized texture.
The initial stages of the oxidation of the Si(100)2x 1 surface was studied at room temperature by... more The initial stages of the oxidation of the Si(100)2x 1 surface was studied at room temperature by scanning tunneling microscopy (STM). We found that "type-C defects" which are believed to be two half-dimers have a strong preference for oxidation, compared with areas having no defects. Oxidized type-C defects appear to be depressions in both positive and negative sample bias voltages. We also found two oxidized sites having no defects. Single steps are quite stable against oxidation. The oxidation of the Si(100)2 x 1 surface is discussed in terms of these sites.
The initial stages of the thermal (600° C) oxide growth of Si (001) clean surfaces were studied b... more The initial stages of the thermal (600° C) oxide growth of Si (001) clean surfaces were studied by scanning tunneling microscopy (STM). Oxide growth starts from both step edges and terraces. At 120L, almost the entire surface was covered by oxides. Steps could still be identified, and the apparent roughness had a local minimum at this O2 exposure. These results indicate that the first layer is oxidized quite uniformly. Some local orderings of the initial oxides were observed on the terraces, which may enhance the oxide growth in the lateral (parallel to the surface) direction.
Using scanning lunneling microscopy, ,.~u rt.we;d the atomic slructur¢ of step hunches till v;ein... more Using scanning lunneling microscopy, ,.~u rt.we;d the atomic slructur¢ of step hunches till v;einal Si(ll I) misoriented towards [112] by 4 °. Clusters iff steps, consisting of m~re thao. ten alomic layers, are uhserved to form C?I) facets, where a perkldic structure is I()und. Each bump of the structure is larger Ihan Ihal of an ~ldMom (if Sill 1117 × 7, showing b x 3 reconstruction. We propose a rondel of 6 ~ 3 describing this corrugation as a tlimer ta adatoms to discuss Ihe reduced dangling blinds and the asymmetric structure.
Ultrahigh vacuum (UHV) system equipped with aluminum radiation shields has been developed in orde... more Ultrahigh vacuum (UHV) system equipped with aluminum radiation shields has been developed in order to obtain atomically flat Si(OOl) surfaces in a large area. Aluminum radiation shields were employed to suppress the temperature rise of chamber walls. The UHV system has enabled a 6 inch substrate to be heated up to 700°C under a pressure of less than IO-' Pa range. The surface structure of the substrate was confirmed by the reflection high energy electron diffraction (RHEED). when the 6 inch substrate was heated at 600°C in UHV after a modified RCA cleaning, the RHEED pattern indicated oxide free surface consisted of 2 X I structure. The roughness was evaluated from numerous observations by cross-sectional transmission electron microscopy (XTEM). Root-mean-square roughness of the Si surface was less than 0. I nm for almost all over the 6 inch substrate. This implies that the heating in this system gives rise to the atomic-scale planarization of the 6 inch substrate.
Scanning tunnehng microscopy (STM) is used to investigate the atomic structure of S1(331) The (33... more Scanning tunnehng microscopy (STM) is used to investigate the atomic structure of S1(331) The (331) terraces have large areas of more than 50 × 50 nm 2, and single atomic-height steps of 0 12 nm are observed No facetlng into other planes is observed The reconstructed structure found on the terraces can explain the LEED pattern A umt cell consists of adatom-hke structures, the diameter of which becomes as much as 1 3 nm We call this structure an super adatom Even at domain boundaries, the super adatom seldom breaks into atomic fragments The d~scusslon is focused on the origin of the huge dmmeter and the behavlour of the super adatom at a high-temperature 1 × 1 phase
Scanning tunneling microscopy ~s used to investigate how domain boundaries on Si(111)7×7 are affe... more Scanning tunneling microscopy ~s used to investigate how domain boundaries on Si(111)7×7 are affected by step configuration. For making the configuration of step change, we prepare surfaces having misorientation different from (111). On the one surface misoriented towards [110], steps run in a zigzag pattern, which causes terraces to be constricted. On every constricted site, a domain boundary exists. On the other surface misoriented towards [112], steps run parallel and consist of several atomic layers. Domain boundaries on this surface run in a zigzag pattern. Discussion is focused on surface energy of the stepped surface containing domain boundaries.
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Papers by Isao Sumita