高橋 正光; 神津 美和*; 佐々木 拓生
Japanese Journal of Applied Physics, 55(4S), p.04EJ04_1 - 04EJ04_4, 2016/04
The polytypism of GaAs nanowires was investigated by in situ X-ray diffraction using a molecular-beam eppitaxy chamber combined with an X-ray diffractometer. The growth of nanowries was found to start with the formation of zincblende structure, followed by the growth of the wurtzite structure. The wurtzite structure tended to form at a low growth temperature and a high growth rate.
佐々木 拓生; 石川 史太郎*; 高橋 正光
Applied Physics Letters, 108(1), p.012102_1 - 012102_5, 2016/01
We report an anomalous lattice deformation of GaN layers grown on SiC(0001) by molecular beam epitaxy. The evolution of the lattice parameters during the growth of the GaN layers was measured by in situ synchrotron X-ray diffraction. The lattice parameters in the directions parallel and normal to the surface showed significant deviation from the elastic strains expected for lattice-mismatched films on substrates up to a thickness of 10 nm. The observed lattice deformation was well explained by the incorporation of hydrostatic strains due to point defects. The results indicate that the control of point defects in the initial stage of growth is important for fabricating GaN-based optoelectronic devices.
下村 憲一*; 鈴木 秀俊*; 佐々木 拓生; 高橋 正光; 大下 祥雄*; 神谷 格*
Journal of Applied Physics, 118(18), p.185303_1 - 185303_7, 2015/11
Direct measurements on the growth of InAs quantum dots (QDs) and various cap layers during molecular beam epitaxy are performed by X-ray diffraction (XRD). The evolution of strain induced both in the QDs and cap layers during capping is discussed based on the XRD intensity transients obtained at various lattice constants. Transients with different features are observed from those obtained during InGaAs and GaAs capping, attributed to In-Ga intermixing between the QDs and the cap layer. Photoluminescence wavelength can be tuned by controlling the intermixing and the cap layer thickness. It is demonstrated that such information about strain is useful for designing and preparing novel device structures.
高橋 正光; 神津 美和*; 佐々木 拓生; Hu, W.*
Crystal Growth & Design, 15(10), p.4979 - 4985, 2015/10
The evolution of polytypism during GaAs nanowire growth was investigated by in situ X-ray diffraction. The growth of nanowires was found to start with the formation of zincblende structure, followed by the growth of wurtzite structure. The growth process was well reproduced by a simulation based on a layer-by-layer nucleation mode. The good agreement between the measured and simulated results confirms that nucleation costs higher energy for the stackings changing the crystal structure than for those conserving the preceding structure. The transition in prevalent structure can be accounted for by the change of local growth conditions related to the shape of triple phase line rather than by the change in supersaturation level, which quickly reaches equilibrium after starting growth.
佐々木 拓生; 高橋 正光
日本結晶成長学会誌, 42(3), p.210 - 217, 2015/10
Dislocation-mediated strain relaxation during lattice-mismatched InGaAs/GaAs(001) heteroepitaxy was studied through in situ X-ray reciprocal space mapping ( RSM). At the synchrotron facility SPring-8, a hybrid system of molecular beam epitaxy and X-ray diffractometry with a two-dimensional detector enabled us to perform RSM at high-speed and high-resolution. Using this experimental setup, the strain relaxation processes were classified into four thickness ranges with different dislocation behavior. In order to discuss this observation quantitatively, a strain relaxation model was proposed based on the Dodson-Tsao's kinetic model, and its validity was demonstrated by good agreement with the experimental residual strain. In addition to the single InGaAs layer, strain relaxation processes in multi-layer structures are discussed.
佐々木 拓生; 高橋 正光; 鈴木 秀俊*; 大下 祥雄*; 山口 真史*
Journal of Crystal Growth, 425, p.13 - 15, 2015/09
three-dimensional X-ray reciprocal space mapping ( 3D-RSM) was employed for studying molecular beam epitaxial (MBE) growth of InGaAs multilayer structures on GaAs(001). Measuring the symmetric 004 diffraction allowed us to separately obtain film properties of individual layers and to track the real-time evolution of both residual strain and lattice tilting. In two- layer growth of InGaAs, significant plastic relaxation was observed during the upper layer growth, and its critical thickness was experimentally determined. At the same thickness, it was found that the direction of lattice tilting drastically changed. We discuss these features based on the Dunstan model and confirm that strain relaxation in the multilayer structure is induced by two kinds of dislocation motion (dislocation multiplication and the generation of dislocation half-loops).
Biermanns, A.*; Dimakis, E.*; Davydok, A.*; 佐々木 拓生; Geelhaar, L.*; 高橋 正光; Pietsch, U.*
Nano Letters, 14(12), p.6878 - 6883, 2014/12
The dynamic relation between the growth conditions and the structural composition of the catalyst-free InAs nanowires was investigated using time-resolved X-ray scattering and diffraction measurements during the growth by molecular beam epitaxy. A spontaneous build-up of liquid indium is directly observed in the beginning of the growth process and associated with the simultaneous nucleation of InAs nanowires predominantly in the wurtzite phase. After their nucleation, the nanowires grow in the absence of liquid indium, and with a highly defective wurtzite structure. A pathway to pure wurtzite nanowires is presented through this work.
佐々木 拓生; Norman, A. G.*; Romero, M. J.*; Al-Jassim, M. M.*; 高橋 正光; 小島 信晃*; 大下 祥雄*; 山口 真史*
Physica Status Solidi (C), 10(11), p.1640 - 1643, 2013/11
Defect characterization in compositionally step graded InGaAs layers with different thickness of the overshooting (OS) layer was performed using cathodoluminescence (CL) and transmission electron microscopy (TEM). We found that the type and in-plane distribution of defects generated in the top InGaAs layer grown on step graded layers strongly depend on the thickness of the OS layer. In the thin OS layer, a high density of threading dislocations aligned along  was observed. In the thick OS layer, significant line defects associating composition variation were dominantly present. These features on defect type and distribution would relate to strain and configuration of the OS layer.
西 俊明*; 佐々木 拓生; 池田 和磨*; 鈴木 秀俊*; 高橋 正光; 下村 憲一*; 小島 信晃*; 大下 祥雄*; 山口 真史*
AIP Conference Proceedings 1556, p.14 - 17, 2013/09
X-ray reciprocal space mapping during InGaAs/GaAs(001) MBE growth is performed to investigate effects of substrate misorientations on crystallographic tilting. It was found that evolution of the crystallographic tilt for the InGaAs films is strongly dependent on both layer structures and substrate misorientations. We discuss these observations in terms of an asymmetric distribution of dislocations.
Hu, W.; 鈴木 秀俊*; 佐々木 拓生*; 神津 美和*; 高橋 正光
Journal of Applied Crystallography, 45(5), p.1046 - 1053, 2012/10
This paper describes the development of a high-speed three-dimensional reciprocal-space mapping method designed for the real-time monitoring of the strain relaxation process during the growth of heterostructure semiconductors. Each three-dimensional map is obtained by combining a set of consecutive images, which are captured during the continuous rotation of the sample, and calculating the reciprocal-space coordinates from the detector coordinate system. Using this method, the strain relaxation process of InGaAs heteroepitaxial films grown on GaAs(001) has been investigated.
佐々木 拓生*; 下村 憲一*; 鈴木 秀俊*; 高橋 正光; 神谷 格*; 大下 祥雄*; 山口 真史*
Japanese Journal of Applied Physics, 51(2), p.02BP01_1 - 02BP01_3, 2012/02
In-plane asymmetric strain relaxation in lattice-mismatched InGaAs/GaAs(001) heteroepitaxy is studied by three-dimensional X-ray reciprocal space mapping. Repeating crystal growth and growth interruptions during measurements allows us to investigate whether the strain relaxation is limited at a certain thickness or saturated. We find that the degree of relaxation during growth interruption depends on both the film thickness and the in-plane directions. Significant lattice relaxation is observed in rapid relaxation regimes during interruption. This is a clear indication that relaxation is kinetically limited. In addition, relaxation along the  direction can saturate more readily than that along the  direction. We discuss this result in terms of the interaction between orthogonally aligned dislocations.
佐々木 拓生*; 鈴木 秀俊*; 稲垣 充*; 池田 和磨*; 下村 憲一*; 高橋 正光; 神津 美和*; Hu, W.; 神谷 格*; 大下 祥雄*; et al.
IEEE Journal of Photovoltaics, 2(1), p.35 - 40, 2012/01
Compositionally step-graded InGaAs/GaAs(001) buffers with overshooting (OS) layers were evaluated by several characterization techniques for higher efficiency metamorphic III-V multijunction solar cells. By high-resolution X-ray diffraction, we found that fully relaxed or tensile strained top layers can be obtained by choosing appropriate OS layer thickness. Moreover, from real-time structural analysis using X-ray reciprocal space mapping ( RSM), it was proved that the top layer is almost strained to the OS layers, and it is independent of the thicknesses of the OS layers. Dislocations in the vicinity of the OS layers were observed by transmission electron microscopy, and the validity of results of RSM was confirmed from the viewpoint of misfit dislocation behavior. Finally, by photoluminescence measurements, we showed that tensile strained top layers may be suitable for the improvement of minority-carrier lifetime.
佐々木 拓生*; 鈴木 秀俊*; 高橋 正光; 大下 祥雄*; 神谷 格*; 山口 真史*
Journal of Applied Physics, 110(11), p.113502_1 - 113502_7, 2011/12
Dislocation-mediated strain relaxation during lattice-mismatched InGaAs/GaAs(001) heteroepitaxy was studied through X-ray reciprocal space mapping. At the synchrotron radiation facility SPring-8, a hybrid system of molecular beam epitaxy and X-ray diffractometry with a two-dimensional detector enabled us to perform reciprocal space mapping at high-speed and high-resolution. Using this experimental setup, the lattice constants, the diffraction broadenings along in-plane and out-of-plane directions, and the diffuse scattering were investigated. The strain relaxation processes were classified into four thickness ranges with different dislocation behavior. In addition, the existence of transition regimes between the thickness ranges was identified.
佐々木 拓生*; 鈴木 秀俊*; 崔 炳久*; 高橋 正光; 藤川 誠司; 神谷 格*; 大下 祥雄*; 山口 真史*
Journal of Crystal Growth, 323(1), p.13 - 16, 2011/05
InGaAs/GaAs(001)の分子線エピタキシャル成長中のひずみ緩和の様子をその場X線逆格子マッピングにより解析した。成長温度420, 445, 477Cにおける残留ひずみ・結晶性の変化の様子が測定された。Dodson-Tsaoの運動学的モデルは、実験による残留ひずみの測定結果とよく一致することがわかった。さらにひずみ緩和過程における転位の運動の温度依存性の解析から、転位の運動の熱励起を議論することが可能になった。
Elfiky, D.*; 山口 真史*; 佐々木 拓生*; 高本 達也*; 森岡 千晴*; 今泉 充*; 大島 武; 佐藤 真一郎; Elnawawy, M.*; Eldesoky, T.*; et al.
Japanese Journal of Applied Physics, 49(12), p.121201_1 - 121201_7, 2010/12
Elfiky, D.*; 山口 真史*; 佐々木 拓生*; 高本 達也*; 森岡 千晴*; 今泉 充*; 大島 武; 佐藤 真一郎; Elnawawy, M.*; Eldesoky, T.*; et al.
Japanese Journal of Applied Physics, 49(12), p.121202_1 - 121202_5, 2010/12
鈴木 秀俊*; 佐々木 拓生*; 崔 炳久*; 大下 祥雄*; 神谷 格*; 山口 真史*; 高橋 正光; 藤川 誠司
Applied Physics Letters, 97(4), p.041906_1 - 041906_3, 2010/07
Real-time three-dimensional reciprocal space mapping measurement during InGaAs/GaAs(001) molecular beam epitaxial growth has been performed to investigate anisotropy in relaxation processes. Anisotropies, strain relaxation, and crystal quality in  and  directions were simultaneously evaluated via the position and broadness of 022 diffraction. In the small-thickness region, strain relaxation caused by -dislocations is higher than that caused by -dislocations, and therefore crystal quality along  is worse than that along . Rapid relaxation along both  and  directions occurs at almost the same thickness. After rapid relaxation, anisotropy in strain relaxation gradually decreases, whereas crystal quality along  direction, presumably due to -dislocations, becomes better that along  direction and the ratio does not decay with thickness.
Elfiky, D.*; 山口 真史*; 佐々木 拓生*; 高本 達也*; 森岡 千晴*; 今泉 充*; 大島 武; 佐藤 真一郎; Elnawawy, M.*; Eldesuky, T.*; et al.
Proceedings of 35th IEEE Photovoltaic Specialists Conference (PVSC-35) (CD-ROM), p.002528 - 002532, 2010/06
佐々木 拓生*; 鈴木 秀俊*; 崔 炳久*; 高橋 正光; 藤川 誠司; 大下 祥雄*; 山口 真史*
Materials Research Society Symposium Proceedings, Vol.1268, 6 Pages, 2010/05
The in situ X-ray reciprocal space mapping (in situ RSM) of symmetric diffraction measurements during lattice-mismatched InGaAs/GaAs(001) growth were performed to investigate the strain relaxation mechanisms. The evolution of the residual strain and crystal quality were obtained as a function of InGaAs film thickness. Based on the results, the correlation between the strain relaxation and the dislocations during the film growth were evaluated. As a result, film thickness ranges with different relaxation mechanisms were classified, and dominant dislocation behavior in each phase were deduced. From the data obtained in in situ measurements, the quantitative strain relaxation models were proposed based on a dislocation kinetic model developed by Dodson and Tsao. Good agreement between the in situ data and the model ensured the validity of the dominant dislocation behavior deduced from the present study.
奥地 拓生*; 佐々木 重雄*; 長壁 豊隆; 大野 祥希*; 小竹 翔子*; 鍵 裕之*
Journal of Physics; Conference Series, 215, p.012188_1 - 012188_9, 2010/03