Research

Minoru FUJII （藤井　稔） : Research on Mesoscopic Materials

RESEARCH ACTIVITIES

Rare-earth Ions and Silicon Nanocrystals Coupled System

		105. I. Izeddin, D. Timmerman, T. Gregorkiewicz, A. S. Moskalenko, A. A. Prokofiev, I. N. Yassievich, and M. Fujii, "Energy Transfer Processes in Er-doped SiO2 Sensitized with Si Nanocrystals,” Physical Revew B 78, 035327, pp.1-14 (2008).
		We present a high-resolution photoluminescence study of Er-doped SiO2 sensitized with Si nanocrystals (Si NCs). Emission bands originating from recombination of excitons confined in Si NCs, internal transitions within the 4f-electron core of Er3+ ions, and a band centered at λ≈1200 nm have been identified. Their kinetics were investigated in detail. Based on these measurements, we present a comprehensive model for energy-transfer mechanisms responsible for light generation in this system. A unique picture of energy flow between the two subsystems was developed, yielding truly microscopic information on the sensitization effect and its limitations. In particular, we show that most of the Er3+ ions available in the system are participating in the energy exchange. The long-standing problem of apparent loss of optical activity in the majority of Er dopants upon sensitization with Si NCs is clarified and assigned to the appearance of a very efficient energy exchange mechanism between Si NCs and Er3+ ions. Application potential of SiO2:Er, sensitized by Si NCs, was discussed in view of the newly acquired microscopic insight.

		87. I. Izeddin, A. S. Moskalenko, I. N. Yassievich, Minoru Fujii, and Tom Gregorkiewicz, "Nanosecond Dynamics of the Near-Infrared Photoluminescence of Er-Doped SiO₂ Sensitized with Si Nanocrystals,” Phys. Rev. Letts. Vol. 97, 207401, pp. 1-4, November (2006).
		We report on an observation of a fast 1.5 μm photoluminescence band from Er3+ ions embedded in an SiO2 matrix doped with Si nanocrystals, which appears and decays within the first microsecond after the laser excitation pulse. We argue that the fast excitation and quenching are facilitated by Auger processes related to transitions of confined electrons or holes between the space-quantized levels of Si nanocrystals dispersed in SiO2. We show that a great part—about 50%—of all Er dopants is involved in these fast processes and contributes to the submicrosecond emission.

		84. Toshihiro Nakamura, Minoru Fujii, Satoru Miura, Masaki Inui, and Shinji Hayashi, "Enhancement and Suppression of Energy Transfer from Si Nanocrystals to Er Ions through a Control of the Photonic Mode Density,” Physical Review B, Vol. 74, pp. 045302-1-6, July (2006).
		The rate of energy transfer from Si nanocrystals to Er ions was modified by placing a Au layer nearby. The distance between the active layer and the Au layer as well as the wavelength were changed systematically to quantitatively discuss how the energy transfer rate is modified by the photonic mode density. It was found that at particular combinations of the spacer thickness and the wavelength the energy transfer was strongly suppressed, while at other combinations it was enhanced. In order to understand the oscillation behavior of the energy transfer rate, a model fitting was performed. By assuming that the energy transfer rate is proportional to the square of the photonic mode density, the observed oscillation behavior could be well reproduced, and from the fitting the energy transfer rate could be estimated. The estimated rates of energy transfer to the 4I11∕2 and 4I9∕2 states of Er3+ were about 31 and 5.7 ks−1, respectively.

		78. Toshihiro Nakamura, Minoru Fujii, Kenji Imakita, Shinji Hayashi, "Modification of Energy Transfer from Si Nanocrystals to Er³⁺ near Au Thin Film,” Physical Review B, Vol. 72, pp. 235412-1-6, December (2005).
		The effects of a Au thin layer on the rate of energy transfer from Si nanocrystals (Si-nc’s) to Er3+ were studied. The energy transfer rate was found to oscillate with increasing the separation between the active layer and the Au thin film. The period of the oscillation agreed well with that of the calculated radiative decay rates of Si-nc’s at the energy transfer wavelength. The present results provide evidence that the rate of energy transfer from Si-nc’s to Er3+ is proportional to the photonic mode density at the energy transfer wavelength and can be controlled by controlling the photonic environment.

		71. Kenji Imakita, Minoru Fujii, and Shinji Hayashi, "Spectrally Resolved Energy Transfer from Excitons in Si Nanocrystals to Er Ions,” Physical Review B Vol. 71, pp. 193301-1-4, May (2005).
		The energy transfer from Si nanocrystals (Si-nc’s) to Er ions was studied spectroscopically. At low temperatures, inhomogeneously broadened photoluminescence bands of Si-nc’s were partially quenched and some dips were observed. A comparison of the quenched spectra with a photoluminescence excitation spectrum revealed that the dips are due to the resonant energy transfer from excitons in Si-nc’s to Er ions. For the energy transfer to the 4I11∕2 state of Er ions, two dips, one very clear and the other indistinctive, were observed, while to the 4I9∕2 state, two dips with comparable depth were observed. Modification of the band structure of Si-nc’s by the quantum size effects is responsible for the different dip structures, depending on to which states the energy transfer is made.

		69. Kenji Imakita, Minoru Fujii, Yasuhiro Yamaguchi, and Shinji Hayashi, "Interaction between Er Ions and Shallow Impurities in Si Nanocrystals,” Physical Review B Vol. 71, 115440-1-7, May (2005).
		The interaction between Er3+ and shallow impurities in Si nanocrystals (nc-Si) is studied for SiO2 films containing Er and nc-Si (Er:nc-Si:SiO2). The luminescence property of Er3+ is strongly modified by shallow impurities in nc-Si. The formation of excess carriers in nc-Si by P or B doping results in the quenching of infrared photoluminescence (PL) of Er3+ and the shortening of the lifetime. When P and B are doped simultaneously and carriers are compensated, the intensity and the lifetime are recovered. It is shown that the mechanism of the interaction is Auger de-excitation of excited Er3+ with the interaction of electrons or holes in nc-Si. The estimated Auger coefficient is found to be two orders of magnitude smaller than that of Er doped bulk Si at low temperatures where carriers are bound to donor or acceptor ions, and four orders of magnitude smaller than that at room temperature. This small Auger coefficient makes nc-Si immune from the impurity Auger de-excitation process compared to Er doped bulk Si and is considered to be responsible for temperature independent efficient PL of Er:nc-Si:SiO2 systems.

		60. Minoru Fujii, Kenji Imakita, Kei Watanabe, Shinji Hayashi, "Coexistence of Two Different Energy Transfer Processes in SiO₂ Films Containing Si Nanocrystals and Er,” Journal of Applied Physics, Vol. 95, No. 1, pp. 272-280, January (2004).
		The mechanism of energy transfer from silicon nanocrystals (nc-Si) to erbium ions (Er3+) in SiO2 films containing nc-Si and Er was studied by analyzing delayed infrared luminescence from Er3+. It was found that, to theoretically reproduce the rising part of the time-dependent luminescence intensity, two different energy transfer processes, i.e., fast and slow processes, should be considered. From the fitting of the delayed luminescence to a model, the ratio of the two energy transfer processes and the energy transfer rate of the slow process were estimated. The ratio exhibited a clear dependence on the luminescence peak energy of Si nanocrystals, which act as photosensitizers for Er3+, indicating that the ratio depends on the size of nc-Si. The ratio of slow to fast processes increased with the decrease in size; this observation is a strong indication that the fast process is the direct inheritance of the process in bulk Si:Er systems, and the slow process is a characteristic process occurring only in nc-Si:Er systems. The energy transfer rate of the slow process was found to depend on the recombination rate of excitons in nc-Si.

		53. Kei Watanabe, Hiroyuki Tamaoka, Minoru Fujii, and Shinji Hayashi, "Excitation of Tm³⁺ by Resonant Energy Transfer from Si Nanocrystals,” Journal of Applied Physics, Vol. 92, No. 7, pp. 4001-4006, October (2002).
		Photoluminescence (PL) properties and PL decay dynamics of SiO2 films containing Si nanocrystals (nc-Si) and Tm3+ were studied. The samples exhibited a broad PL at around 1.5 eV due to the recombination of excitons in nc-Si, and rather sharp PL at 1.58, 0.84, and 0.69 eV corresponding to the intra-4f shell transitions of Tm3+. The correlation between the intensities of nc-Si and Tm3+ related PL was studied as a function of the Tm concentration, the size of nc-Si, and the temperature. It was found that the intensity of Tm3+ related PL depends strongly on the size of nc-Si. At low temperatures, the spectral shape of nc-Si PL was strongly modified by doping Tm. From analysis of the modified spectral shape, a resonant energy transfer from nc-Si to Tm3+ is discussed.

		47. Kei Watanabe, Minoru Fujii and Shinji Hayashi, "Resonant Excitaion of Er³⁺ by the Energy Transfer from Si Nanocrystals,” Journal of Applied Physics, Vol. 90, No. 9, pp. 4761-4767, Nobember (2001).
		Photoluminescence (PL) properties of SiO2 films containing Si nanocrystals (nc-Si) and Er were studied. The average size of nc-Si was changed in a wide range in order to tune the exciton energy of nc-Si to the energy separations between the discrete electronic states of Er3+. PL from exciton recombination in nc-Si and the intra-4f shell transition of Er3+ were observed simultaneously. At low temperatures, periodic features were observed in the PL spectrum of nc-Si. The period agreed well with the optical phonon energy of Si. The appearance of the phonon structure implies that nc-Si which satisfy the energy conservation rule during the energy transfer process can resonantly excite Er3+. For the PL from Er3+, a delay was observed after the pulsed excitation of nc-Si hosts. The rise time of the PL showed strong size dependence. The effects of the quantum confinement of excitons in nc-Si on the energy transfer process are discussed.

		20. Minoru Fujii, Shinji Hayashi and Keiichi Yamamoto, "Excitation of Intra-4f Shell Luminescence of Yb³⁺ by the Energy Transfer from Si Nanocrystals,” Applied Physics Letters, Vol. 73, No. 21, pp.3108-3110, November (1998).
		SiO2 films containing Si nanocrystals (nc-Si) and Yb were prepared and their photoluminescence (PL) properties were studied. For the sample containing nc-Si with an average diameter of 3.1 nm, a weak peak ( ∼ 1.26 eV) attributable to the intra-4f shell transition of Yb3+ could be observed at the low-energy side of a broad peak ( ∼ 1.4 eV) of nc-Si. The intensity of the 1.26 eV peak was found to depend strongly on the size of nc-Si and increase rapidly with decreasing size. The temperature dependence of the PL spectra was studied. It was found that the degree of temperature quenching of the 1.26 eV peak depends on the size of the nc-Si and becomes small as the size decreases. These results suggest that the band-gap widening of nc-Si due to the quantum size effects is essential to efficiently excite Yb3+ by nc-Si.

		19. Minoru Fujii, Masato Yoshida, Shinji Hayashi and Keiichi Yamamoto, "Photoluminescence from SiO₂ Films Containing Si Nanocrystals and Er: Effects of Nanocrystalline Size on the Photoluminescence Efficiency of Er³⁺,” Journal of Applied Physics, Vol. 84, No. 8, pp. 4525 - 4531, October (1998).
		SiO2 films containing Si nanocrystals (nc-Si) and Er were prepared and their photoluminescence (PL) properties were studied. The samples exhibited PL peaks at 0.8 and 1.54 μm, which could be assigned to the electron-hole recombination in nc-Si and the intra-4f transition in Er3+, respectively. Correlation between the intensities of the two PL peaks was studied as functions of the size of nc-Si, Er concentration, excitation power and excitation wavelength. It was found that the 1.54 μm PL of Er3+ is strongly enhanced by incorporating nc-Si in films. Furthermore, the intensity of the 1.54 μm peak was found to depend strongly on the size of the incorporated nc-Si.

		13. Minoru Fujii, Masato Yoshida, Yoshihiko Kanzawa, Shinji Hayashi and Keiichi Yamamoto, "1.54μm Photoluminescence of Er³⁺ Doped into SiO₂ Films Containing Si Nanocrystals: Evidence for Energy Transfer from Si Nanocrystals to Er³⁺,” Applied Physics Letters, Vol. 71, No. 9, pp. 1198-1200, September (1997)
		SiO2 films containing Si nanocrystals (nc-Si) and Er were prepared and their photoluminescence (PL) properties were studied. The samples exhibited luminescence peaks at 0.81 and 1.54 μm, which could be assigned to the electron-hole recombination in nc-Si and the intra-4f transition in Er3+, respectively. Correlation between the intensities of the two luminescence peaks was studied as functions of Er concentration and excitation power. The present results clearly demonstrate that excitation of Er3+ occurs through the recombination of photogenerated carriers spatially confined in nc-Si and the subsequent energy transfer to Er3+.