Department of Physics, Graduate School of Science, Osaka University Assistant Professor Yutaka Ohno

Transcription

1 Quantitative analysis of optical polarization in semiconductor nanostructures by polarized cathodoluminescence spectroscopy in a transmission electron microscope Microstructure and optical properties of ZnSe nanowires grown on.znse(001) with Fe catalysts Formation mechanism of silicon surface manoholes Department of Physics, Graduate School of Science, Osaka University Assistant Professor Yutaka Ohno Oral Presentations at The 16th International Microscopy Congress September 3 8, 2006 Sapporo Convention Center, Sapporo, Hokkaido, Japan 21 st COE Program Towards a New Basic Science : Depth and Synthesis

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3 Quantitative Analysis of Optical Polarization in Semiconductor Nanostructures by Polarized Cathodoluminescence Spectroscopy in a Transmission Electron Microscope Yutaka Ohno* and Seiji Takeda Department of Physics, Graduate School of Science, Osaka University, 1-1, Machikane-yama, Toyonaka, Osaka , JAPAN Optical properties in nanostructures have been of physical and technological interests since the properties are distinct from those of bulk crystals. Among them, optical polarization is an interesting topic. Nanostructures with anisotropic electronic structures, such as quantum wells (QWs) and nanowires, interact with polarized lights. For example, a QW emits a polarized light whose polarization direction is either parallel to or perpendicular to the well. From a viewpoint of technical interest, nanostructures emitting polarized light have potential application to future photonic nanodevices, such as polarization detectors and polarization threshold switches of nanometer size. Optical polarization in nanostructures was often studied by cathodoluminescence (CL) spectroscopy, since polarized CL light was obtained from a small space where a focused electron beam was illuminated. Polarized CL light was usually collected with a truncated ellipsoidal mirror equipped in an electron microscope, and the polarization of the collected light differed from one of the emitted light due to the effect of reflection on the mirror. So far, the reflection-effect was not fully taken into account. It has been shown that [1] optical polarization can be determined through the simulation of polarized CL intensities taken into account of the reflection-effect [FIG. 1]. By means of the method, and by using an apparatus designed especially for polarized CL spectroscopy in a transmission electron microscope (TEM) [2], optical polarization of semiconductor nanostructures has been examined quantitatively [e.g., 3-5]. Here we show a result concerning the nanostructures in a CuPt-ordered GaInP alloy. Cross-sectional scanning tunnelling microscopy on an alloy revealed that [3, 6] atomic layers of InP on (-111) and (-110), sandwiched between the ordered domains (about 2-10 nm in size), are formed. Polarized CL spectroscopy in a TEM revealed that [3] the InP layers act as QWs oriented on a slant with respect to the substrate and they emit lights linearly polarized parallel to the layers [FIG. 2]. The temperature-dependent intensity, polarization, and photon energy of the CL lights peaking below the band gap energy E g (at E g - 20 and E g - 33 mev) were expounded with the InP QW model. The method is also applied to examine the photo-induced stress in ZnSe due to photo-induced glide of stacking faults [4], anisotropic optical properties of multiple nanotwins in AlGaAs [5], etc. [1] Y. Ohno and S. Takeda, J. Electron Microsc. 51 (2002) 281. [2] Y. Ohno and S. Takeda, Rev. Sci. Instrum. 66 (1995) [3] Y. Ohno, Rhys. Rev. B 72 (2005) (R). [4] Y. Ohno, Appl. Phys. Lett. 87 (2005) [5] Y. Ohno, K. Shoda, S. Takeda, and N. Yamamoto, unpublished. [6] Y. Ohno, Jpn. J. Appl. Phys. 45(3B) (2006) [7] The author is indebted to Dr. K. Fujii of Tokyo University of Science for his supports. This work was partially supported by a Grant-in-Aid for Young Scientist (A) (# , ) from the Ministry of Education, Culture, Sports, Science and Technology. FIG. 1. Simulation of the polarized CL intensity of a (a) twoor (b) one-dimensional nanostructure measured with a truncated ellipsoidal mirror [1]. FIG. 2. (a) Experimental setup for polarized CL spectroscopy [3]. (b) Polarized CL spectra for various. (c) CL intensities vs.. The solid or broken curve is a calculated CL intensity for a QW on (-110) or (-111), respectively.

4 Microstructure and Optical Property of ZnSe Nanowires Grown on ZnSe(001) with Fe Catalysts Yutaka Ohno 1 *, Takeo Shirahama 1, Seiji Takeda 1, Atsushi Ishizumi 2, and Yoshihiko Kanemitsu 2,3 1 Department of Physics, Graduate School of Science, Osaka University, 1-1, Machikane-yama, Toyonaka, Osaka , JAPAN 2 Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara , JAPAN 3 Institute for Chemical Research, Kyoto University, Uji, Kyoto , JAPAN One-dimensional nanocrystals such as nanowires (NWs) have attracted a great deal of interest for applications in optoelectronics, etc. They have been frequently synthesized by the catalytic growth method, since we can grow NWs whose nucleation sites and sizes are precisely decided on a substrate by controlling those of catalysts [e.g., refs. 1 and 2], at temperatures much lower than the usual growth temperatures. By means of the method, ZnSe NWs can be grown on ZnSe(001) at low temperatures of K [3], which are comparable to the usual temperatures for fabrication of ZnSe-base optoelectronic devices [4], with proper catalysts of Fe nanoparticles. In this paper, we have determined in detail the microstructure and the optical property. ZnSe NWs were grown in a VG-80 chamber that is designed especially for growth of II-VI compound semiconductors by molecular-beam epitaxy [5]. A GaAs(001) wafer was installed into the chamber, and a ZnSe epilayer (about 90 nm thick) was grown on the wafer at the temperature of 573 K [5]. The wafer was cooled down to room temperature, and Fe atoms were evaporated on the epilayer, for fear that As and Ga atoms mix with the Fe deposit. The thickness was estimated to be about 1 nm. The deposited wafer was heated up to a growth temperature T g and maintained for a few min in order to generate Fe nanoparticles. Subsequently, NWs were grown at T g by the exposition of Zn and Se fluxes (the flow ratio of 1:2) simultaneously for 1 h. No NW grew at T g without the exposition. The structural nature was characterized by high-resolution transmission electron microscopy (HRTEM) with a JEOL JEM-2010 microscope operated at 160 kv and scanning electron microscopy (SEM) with a FEI Sirion 400 microscope operated at 3 kv [6]. The optical property was characterized with a photoluminescence (PL) spectroscopy system (the spectral resolution of ~3 mev), at temperatures in the range K. A large amount of NWs (the number density of the order of 10 9 cm -2 ) grew at the optimum temperature of T g = 573 K [FIG. 1(a)]. A NW tapered off to the top. The diameter at the top or the bottom was the order of 10 or 30 nm, respectively, and the typical length was about 200 nm. A NW was the zinc-blend structure, and it contained some stacking faults [FIG. 1(b)]. The longitudinal direction was <001>, <111>, <110>, or <112>. A nanoparticle that contains Fe, Zn, and Se atoms existed on the top of a NW, and no Fe atom was detected inside the NW. This result clearly shows that the NW grew via the vapor-liquid-solid mechanism [7]. NWs also grew at T g = 523 or 623 K, even though the number density was small. The NWs emitted excitonic PL light even at room temperature, due to high electronic quality. The temperature dependence of the linewidth, the peak energy, or the peak intensity of the PL light were examined [FIG. 2(a)], and the results were explained in terms of quantum confinement effects [e.g., FIG. 2(b)], as well as effects of structure inhomogeneity such as strain and stacking faults in NWs [8]. [1] J. Kikkawa, Y. Ohno, and S. Takeda, Appl. Phys. Lett. 86 (2005) [2] S. Takeda, K. Ueda, N. Ozaki, and Y. Ohno, Appl. Phys. Lett. 82 (2003) 979. [3] Y. Ohno, T. Shirahama, S. Takeda, A. Ishizumi, and Y. Kanemitsu, Appl. Phys. Lett. 87 (2005) [4] ZnSe nanowires have so far been grown on a substrate at temperatures above about K [3]. [5] Y. Ohno, N. Adachi, and S. Takeda, Appl. Phys. Lett. 83 (2003) 54. [6] SEM images were taken at FEI Company Japan Ltd. [7] R. S. Wagner and W. C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [8] For example, Y. Kanemitsu, T. Nagai, Y. Yamada, and T. Taguchi, Appl. Phys. Lett. 82 (2003) 388. [9] This work was partially supported by a Grant-in-Aid for Young Scientist (A) (# , ) from the Ministry of Education, Culture, Sports, Science and Technology. FIG. 1. (a) SEM and (b) HRTEM images of NW [T g = 573 K]. FIG. 2. (a) PL spectra of a specimen grown at T g = 573 K at different temperatures. (b) Temperature dependence of the linewidth of the excitonic PL, defined as a full-width at the half-maximum (FWHM) of the luminescence. The solid line is the theoretical fit.

5 Formation Mechanism of Silicon Surface Nanoholes Yutaka Ohno 1 *, Seiji Takeda 1, Toshinari Ichihashi 2, and Sumio Iijima 2, 3 1 Department of Physics, Graduate School of Science, Osaka University, 1-1, Machikane-yama, Toyonaka, Osaka , JAPAN. 2 Fundamental Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki , JAPAN. 3 Meijo University, Shiogamaguchi, Tenpaku-ku, Nagoya , JAPAN. Nano-, meso-, and micro-porous materials have recently attracted considerable attention. Among them, allays of nanoholes on surfaces may be applied to optoelectronic devices, such as photonic waveguides and plasmonic integrated circuits, and also they may be used as templates for fabrication of useful nanostructures, catalysts for adsorption of gases, etc. Surface nanohole arrays can be formed by spontaneous fabrication techniques such as electron irradiation and electrochemical etching, etc., which have advantages in comparison with artificial techniques such as electron lithography, even though the formation mechanism is not fully understood. Here, surface nanohole arrays in silicon, formed spontaneously on electron exit surfaces of silicon foils via agglomeration of surface vacancies introduced under electron irradiation [1], are examined. The formation mechanism is studied by transmission electron microscopy (TEM) at an ultrahigh vacuum (UHV) and related techniques, since controlled clean surfaces can be provided and formation of nanostructures on the surfaces, on which an arbitrary number of surface vacancies are introduced intentionally, can be pursued. Nanoholes were formed on a clean deoxidized surface in an UHV microscope (the base pressure P ir ~1x10-8 Pa) or a surface virtually deoxidized in a conventional microscope [2] (P ir ~1x10-5 Pa) by electron irradiation. The electron flux f was the order of cm -2 s -1, and the electron energy was 200 kev. The electron dose D, f multiplied by the irradiation time, was up to about 2x10 24 cm -2. The nominal temperature during irradiation T ir ranged from room temperature to about 680 K in an UHV microscope, and from about 4 to 680 K in a conventional microscope. The irradiated surface was then observed by TEM with electrons of low flux, so that the surface was virtually kept unchanged during observation. The distribution of surface nanoholes is determined at the early stage of irradiation [4], and a nanohole is excavated, parallel to the irradiation direction, without changing the size of the opening under irradiation [1]. Therefore, only the formation process of two-dimensional patterns of nanoholes, which may be discussed by examining the size and spatial distribution of the openings of deep nanoholes after excavation, is taken into account at the stage. The experimental results were quantitatively explained with a classical phase separation model based on the migration of surface vacancies [3]. According to the model, the nearest-neighbor distance l nn [FIG. 1], the number density, and the average opening area of nanoholes followed Arrhenius law at high temperatures, and the activation energy for migration of surface vacancy under electron irradiation was estimated to be about 0.3 ev. After formation of two-dimensional patterns of surface nanoholes, only the excavation process, characterized by the depths, is taken into account. The depths of surface nanoholes can be estimated by three-dimensional TEM [e.g., FIGs. 2(a) and 2(b)], and so the excavation process can virtually be pursued. The typical excavation process [FIG. 2(c)] showed that surface nanoholes are excavated at a constant rate with increasing D [5]. The excavation rate was estimated at different experimental conditions, and the results were reproduced [5] with the model that a part of surface vacancies preferentially flow into nanoholes due to the momentums transferred from irradiating electrons [1]. [1] S. Takeda, K. Koto, S. Iijima, and T. Ichihashi, Phys. Rev. Lett. 79 (1997) [2] On surfaces in a conventional microscope, sputtering of surface atoms, oxidization, and contamination take place simultaneously under electron irradiation. When f is high, the first process is most probably predominant and so the surface is virtually free from oxidation and contamination [3]. [3] Y. Ohno, S. Takeda, T. Ichihashi, and S. Iijima, submitted. [4] N. Ozaki, Y. Ohno, M. Tanbara, D. Hamada, J. Yamasaki, and S. Takeda, Surf. Sci. 493 (2001) 547. [5] Y. Ohno, S. Takeda, T. Ichihashi, and S. Iijima, submitted. [6] This work was partially supported by a Grant-in-Aid for Scientific Research (A)(2) (# , ) and a Grant-in-Aid for Young Scientist (A) (# , ) from the Ministry of Education, Culture, Sports, Science and Technology. FIG. 1. T ir vs. l nn. Triangles, squares and circles mean the results for the nanoholes on {001}, {111} and {110}, respectively [closed triangles: P ir ~1x10-8 Pa, the other marks: P ir ~1x10-5 Pa]. FIG. 2. Surface nanoholes observed (a) parallel to and (b) normal to the irradiation direction. (c) D vs. the depths of nanoholes. The depths at an electron dose are slightly dispersed, and so each depth is plotted.