Research

Minoru FUJII （藤井　稔） : Research on Mesoscopic Materials

RESEARCH ACTIVITIES

Impurity Doping in Group IV (Si, Ge) Semiconductor Nanocrystals

		178.　Hiroshi Sugimoto, Minoru Fujii, Kenji Imakita, Shinji Hayashi, and Kensuke Akamatsu, "Codoping n- and p-Type Impurities in Colloidal Silicon Nanocrystals -Controlling Luminescence Energy from below Bulk Band Gap to Visible Range", The Journal of Physical Chemistry C, Vol. 117, No. 22, pp. 11850–11857 (2013).
		We present a novel synthesis of ligand-free colloidal silicon nanocrystals (Si-NCs) that exhibits efficient photoluminescence (PL) in a wide energy range (0.85− 1.8 eV) overcoming the bulk Si band gap limitation (1.12 eV). The key technology to achieve the wide-range controllable PL is the formation of donor and acceptor states in the band gap of Si-NCs by simultaneous doping of n- and p-type impurities. The colloidal Si-NCs are very stable in an ordinary laboratory atmosphere for more than a year. Furthermore, the PL spectra are very stable and are not at all affected even when the colloids are drop-cast on a substrate and dried in air. The engineering of the all- inorganic colloidal Si-NC and its optical data reported here are important steps for Si- based optoelectronic and biological applications.

		175.　Hiroshi Sugimoto, Minoru Fujii, Kenji Imakita, Shinji Hayashi, and Kensuke Akamatsu, "Phosphorus and Boron Co-doped Colloidal Silicon Nanocrystals with Inorganic Atomic Ligands", The Journal of Physical Chemistry C, Vol. 117, No. 13, pp. 6807−6813 (2013).
		The surface structure of P and B codoped colloidal Si- NCs are studied by photoluminescence (PL) in hydrofluoric acid (HF) solution and X-ray photoelectron spectroscopy (XPS). We find that codoped Si-NCs are much more stable in HF solution than undoped, P- doped, and B-doped Si-NCs. The PL study combined with XPS results reveal that a high B concentration layer is formed on the surface of codoped Si-NCs and the layer acts as a kind of inorganic atomic ligands for Si-NCs. The high B concentration layer makes Si-NCs hydrophilic and dispersible in polar liquids. Furthermore, the layer effectively protects Si-NCs from oxidation in solution and in air.

		164.　Hiroshi Sugimoto, Minoru Fujii, Kenji Imakita, Shinji Hayashi,and Kensuke Akamatsu, All-Inorganic Near-Infrared Luminescent Colloidal Silicon Nanocrystals-High Dispersibility in Polar Liquid by Phosphorus and Boron Co-doping The Journal of Physical Chemistry C., Vol.116, No. 33, pp.17969-17974 (2012). (2012).
		We demonstrate the formation of a new type of surfactant-free colloidal silicon nanocrystal (Si-NC). The characteristic structural feature of the Si-NCs is simultaneous doping of phosphorus (P) and boron (B) in and on the surface of Si-NCs. The codoped Si-NCs are stable in methanol for more than a year and exhibit luminescence in the near-infrared range. We perform comprehensive studies on the structure of codoped colloidal Si-NCs and discuss the mechanism of the high solution dispersibility.

		158.　Toshihiro Nakamura, Sadao Adachi, Minoru Fujii, Kenta Miura, and Shunya Yamamoto, Phosphorus and boron codoping of silicon nanocrystals by ion implantation: Photoluminescence properties　 Physical Rieview B, Vol. 85, 045441, pp.1-7 (2012).
		The photoluminescence (PL) properties of P or B single-doped Si nanocrystals (Si-nc’s) and P and B co-doped Si-nc’s are studied. In the single-doped Si-nc samples, PL quenching occurs as a result of the Auger nonradiative recombination process between the photoexcited excitons and free carriers supplied by doped impurities. In the (P, B) co-doped sample, on the other hand, the donor-acceptor (D-A)-pair recombination emission is clearly observed on the long-wavelength side of the intrinsic Si-nc emission peak at ∼900 nm. The D-A-pair recombination energy is found to be smaller than the band-gap energy of bulk Si and is strongly dependent on the number of P and B impurities doped in a Si-nc. PL spectra are measured at 50 and 300 K and found to indicate that strong thermal quenching occurs in a (P, B) co-doped sample at 300 K. This quenching effect is probably because of carrier migration among the donor and acceptor states. The PL decay rate is determined as a function of the emitted-light wavelength for the pure and (P, B) co-doped Si-nc samples.

		152.　Masatoshi Fukuda, Hiroshi Sugimoto, Minoru Fujii, Kenji Imakita, and Shinji Hayashi, Surfactant-free solution-dispersible Si nanocrystals -surface modification by impurity control　 Optics Letters, Vol. 36, No. 20, pp. 4014-4016 (2011).
		Si nanocrystals (Si-NCs) dispersible in polar liquid without surface functionalization by organic molecules have been realized by simultaneously doping n and p type impurities. We show that the codoped Si-NCs are stable in methanol for more than five months, while intrinsic Si-NCs prepared by the same procedure form large agglom- erates. The different behavior of the intrinsic and codoped Si-NCs in solutions suggests that doped impurities exist on the surface of Si-NCs and the surface potential is large enough to prevent the agglomeration. The colloidal solution of codoped Si-NCs exhibits broad photoluminescence with the maximum in the near infrared range (1:1–1:3 eV).

		150.　Hiroshi Sugimoto, Minoru Fujii, Masatoshi Fukuda, Kenji Imakita, and Shinji Hayashi, Acceptor-related low-energy photoluminescence from boron-doped Si nanocrystals　 Journal of Applied Physics, Vol. 110, 063528, pp. 1-6 (2011).
		Boron (B) doped Si nanocrystals (Si-ncs) dispersed in hydrofluoric (HF) acid solution are prepared by dissolving borosilicate films containing B-doped Si-ncs in HF solution. We find that the etching rate of B-doped Si-ncs is much smaller than that of undoped Si-ncs. The difference of the etching rate allows us to extract only doped Si-ncs in the mixture of doped and undoped Si-ncs and observe the photoluminescence (PL) due to the transition from the conduction band to the acceptor state. The PL was very broad with the maximum around 1.15 eV. From the analysis of the PL data obtained for the samples prepared under different conditions and different etching time, preferential doping sites of B atoms are estimated. The data suggests that B-doped Si-ncs consists of intrinsic cores and heavily B-doped shells.

		143.　Masatoshi Fukuda, Minoru Fujii, and Shinji Hayashi, Room-temperature below Bulk-Si Band Gap Photoluminescence from P and B Co-doped and Compensated Si Nanocrystals with Narrow Size Distributions　 Journal of Luminescence, Vol. 131, Issue 5, pp. 1066-1069 (2011) .
		Thin films consisting of the layers of phosphorus (P) and boron (B) co-doped Si nanocrystals (Si-ncs) and glass spacer layers were prepared and their photoluminescence properties were studied. Cross-sectional TEM observations revealed the growth of Si-ncs with narrow size distributions. The samples exhibited PL below the band gap energy of bulk Si crystal at room temperature. The low-energy PL is considered to arise from the transitions between donor and acceptor states in compensated Si-ncs. The successful formation of narrow size distribution co-doped Si nanocrystals promotes the study of the optical properties of compensated Si nanocrystals.

		138.　Masahiko Ito, Kenji Imakita, Minoru Fujii, and Shinji Hayashi, Nonlinear Optical Properties of Phosphorus Doped Silicon Nanocrystals/Nanoclusters Journal of Physics D: Applied Physics, Vol.43, 505101 pp. 1-5 (2010).
		Comprehensive studies have been performed on the nonlinear optical responses of silica films containing phosphorus (P)-doped Si nanoclusters and/or nanocrystals. In P-doped Si nanocrystals, enhancements of the nonlinear refractive indices (n2) and two photon absorption coefficients (β) relative to those of intrinsic Si nanocrystals were observed. The analysis of electron spin resonance and absorption spectra revealed that P donors are responsible for the large enhancements of n2 and β.

		121. Kenji Imakita, Masahiko Ito, Minoru Fujii, and Shinji Hayashi, "Nonlinear Optical Properties of Phosphorous-doped Si Nanocrystals Embedded in Phosphosilicate Glass Thin Films,” Optics Express, Vol. 17, No. 9 , pp. 7368-7376 (2009).
		Nonlinear optical properties of phosphorus (P) -doped silicon (Si) nanocrystals are studied by z-scan technique in femtosecond regime at around 1.6 eV. The nonlinear refractive index (n2) and nonlinear absorption coefficient (β ) of Si-ncs are significantly enhanced by P-doping. The enhancement of n2 is accompanied by the increase of the linear absorption in the same energy region, suggesting that impurity-related energy states are responsible for the enhancement of the nonlinear optical response.

		104. Kazuyoshi Fujio, Minoru Fujii, Kazuaki Sumida, Shinji Hayashi, Masashi Fujisawa, and Hitoshi Ohta, "Electron spin resonance studies of P and B codoped Si nanocrystals,” Applied Physics Letters, Vol. 93, 021920, pp. 1-3 (2008).
		P- and/or B-doped Si nanocrystals (Si-ncs) embedded in glass matrices were studied by electron spin resonance (ESR) spectroscopy to investigate the origin of strong room-temperature photoluminescence (PL) of n- and p-type impurities codoped Si-ncs below the band-gap energy of bulk Si crystals. It was shown that the intensity and width of the ESR signal depend strongly on impurity concentrations. A clear correlation was observed between the ESR signal width and the PL intensity. The observed correlation suggests that in addition to the geometrical confinement, P and B codoping further localize carriers in Si-ncs, and the strong localization results in the characteristic luminescence properties.

		90. Kazuaki Sumida, Keiichi Ninomiya, Minoru Fujii, Kazuyoshi Fujio, Shinji Hayashi, Masafumi Kodama, and Hitoshi Ohta, "Electron Spin Resonance Studies of Conduction Electrons in Posphorus-doped Silicon Nanocrystals,” Journal of Applied Physics, Vol. 101, pp. 033504-1-5, February (2007).
		The properties of conduction electrons in P-doped Si nanocrystals embedded in insulating glass matrices have been studied by electron spin-resonance spectroscopy. For heavily P-doped samples, a broad conduction electron signal is observed at low temperatures. The width of the signal is found to be much broader than that of P-doped bulk Si crystals. The temperature dependence of the signal intensity obeys the Curie law even when the P concentration is very high. This suggests that in P-doped nanocrystals donor levels do not merge into the conduction band even at very high P concentration, and also provides evidence that Si nanocrystals smaller than a certain threshold size do not become metallic, at least when they are prepared under an equilibrium condition.

		76. Minoru Fujii, Yasuhiro Yamaguchi, Yuji Takase, Kenji Ninomiya and Shinji Hayashi, "Photoluminescence from Impurity Co-doped and Compensated Si Nanocrystals,” Applied Physics Letters, Vol. 87, pp. 211919-1-3, November (2005).
		Photoluminescence (PL) properties of B and P codoped and compensated Si nanocrystals were studied. The compensation of carriers in nanocrystals was confirmed by the annihilation of confined-carrier optical absorption in the infrared region. In the PL spectra obtained under the resonant excitation condition, the codoped samples did not exhibit structures related to momentum-conserving phonons, which were clearly observed for pure Si nanocrystals. The result strongly suggests that in impurity codoped Si nanocrystals, nonphonon quasidirect optical transition is the dominant recombination path for electron-hole pairs, and thus impurity codoping is a possible approach to further improving PL efficiency of Si nanocrystals.

		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.

		65. Minoru Fujii, Yasuhiro Yamaguchi, Yuji Takase, Keiichi Ninomiya, and Shinji Hayashi, "Control of Photoluminescence Properties of Si Nanocrystals by Simultaneously Doping n- and p-type Impurities,” Applied Physics Letters, Vol. 85, No. 7, pp. 1158-1160, August (2004).
		The effects of B and P codoping on photoluminescence (PL) properties of Si nanocrystals (nc-Si) are studied systematically. It is shown that the PL intensity of codoped nc-Si is always higher than that of either P- or B-doped nc-Si. The intensity is sometimes even higher than that of pure nc-Si at relatively low P and B concentrations and low annealing temperatures. By doping P and B simultaneously to very high concentrations, the PL peak shifts below the band gap of bulk Si.

		58. Minoru Fujii, Kimiaki Toshikiyo, Yuji Takase, Yasuhiro Yamaguchi, and Shinji Hayashi, "Below Bulk-bandgap Photoluminescence at Room Temperature from Heavily P and B Doped Si Nanocrystals,” Journal of Applied Physics, Vol. 94, No. 3, pp. 1990 -1995, August (2003).
		Photoluminescence (PL) properties of heavily P- and B-doped Si nanocrystals (nc-Si) are studied. By simultaneously doping two types of impurities, nc-Si exhibit strong PL at around 0.9 eV at room temperature. The temperature quenching of the PL is very small. Although the PL peak energy is very close to that of dangling-bond related PL previously observed, all of the observed properties, i.e., decay dynamics, degree of temperature quenching, etc., are apparently different. The transition between donor and acceptor states in nc-Si is the possible origin of the low-energy PL.

		54. M. Fujii, A. Mimura, Shinji Hayashi, Y. Yamamoto, and K. Murakami, "Hyperfine Structure of Electron Spin Resonance of Phosphorus Doped Si Nanocrystals,” Physical Review Letters, Vol. 89, No. 20, 206805, pp. 1-4, November (2002).
		Electronic states of P donors in Si nanocrystals (nc-Si) embedded in insulating glass matrices have been studied by electron spin resonance. Doping of P donors into nc-Si was demonstrated by the observation of optical absorption in the infrared region due to intraconduction band transitions. P hyperfine structure (hfs) was successfully observed at low temperatures. The observed splitting of the hfs was found to be much larger than that of the bulk Si:P and depended strongly on the size of nc-Si. The observed strong size dependence indicates that the enhancement of the hyperfine splitting is caused by the quantum confinement of P donors in nc-Si.

		48. Kimiaki Toshikiyo, Masakazu Tokunaga, Shinji Takeoka, Minoru Fujii, Shinji Hayashi, and Kazuyuki Moriwaki, "Effects of P doping on Photoluminescence of Si_1-xGe_x Alloy Nanocrystals Embedded in SiO₂ Matrices: Improvement and Degradation of Luminescence Efficiency,” Journal of Applied Physics, Vol. 90, No.10, pp. 5147-5151, November (2001).
		The effects of P doping on photoluminescence (PL) properties of Si1−xGex alloy nanocrystals (nc-Si1−xGex) in SiO2 thin films were studied. P doping drastically decreases the electron spin resonance (ESR) signals that are assigned to the Si and Ge dangling bonds at the interfaces between nc-Si1−xGex and SiO2 matrices (Si and Ge Pb centers). With increasing P concentration, the signal from the Ge Pb centers are first quenched, and then the signal from the Si Pb centers start to be quenched. The quenching of the ESR signals is accompanied by a drastic enhancement of the PL intensity. The PL intensity has a maximum at a certain P concentration, which depends on the Si:Ge ratio. By further increasing the P concentration, the PL intensity becomes weaker. In this P concentration range, optical absorption emerges due to the intravalley transition of free electrons generated by the P doping. The observation of the free-electron absorption provides direct evidence that carriers in nanometer-sized Si1−xGex alloy crystals can be controlled by impurity doping.

		35. Atsushi Mimura, Minoru Fujii, Shinji Hayashi, Dmitri Kovalev, and Frederick Koch, "Photoluminescence and Free-Electron Absorption in Heavily Phosphorous-doped Si Nanocrystals,” Physical Review B, Vol. 62, No. 19, pp. 12625 -12627, November (2000).
		Heavily phosphorus-doped Si nanocrystals several nanometers in diameter are studied by photoluminescence (PL) and optical absorption spectroscopy. It is demonstrated that P doping results in the quenching of the PL. The quenching is accompanied by the appearance of the optical absorption in the infrared range. The absorption was assigned to the intravalley transitions of free electrons generated by P doping (free-electron absorption). The generation of free electrons and the resultant three-body Auger recombination of excitons is considered to be responsible for the observed PL quenching.

		31. Minoru Fujii, Atsushi Mimura, Shinji Hayashi, Keiichi Yamamoto, Chika Urakawa and Hitoshi Ohta, "Improvement in Photoluminescence Efficiency of SiO₂ Films Containing Si Nanocrystals by P Doping: An Electron Spin Resonance Study,” Journal of Applied Physics, Vol. 87, No. 4, pp. 1855-1857, February (2000).
		SiO2 and phosphosilicate glass (PSG) films containing Si nanocrystals (nc-Si) as small as a few nanometers were studied by electron spin resonance (ESR) and photoluminescence (PL), and the correlation between the two measurements was examined. It is shown that the incorporation of nc-Si in SiO2 results in the drastic increase in the ESR signal; the signal is assigned to the Si dangling bonds at the interfaces between nc-Si and matrices (Pb centers). The ESR signal becomes weaker by doping P into SiO2 matrices, i.e., by using PSG as matrices. By increasing the P concentration, the ESR signal decreases further. By decreasing the ESR signal, the low-energy PL peak at 0.9 eV decreases, while the band-edge PL at 1.4 eV increases. These results suggest that the 0.9 eV peak is related to Pb centers, and that the decrease in the density of the Pb centers by P doping brings about an improvement in the band-edge PL efficiency of nc-Si.

		27. Minoru Fujii, Atsushi Mimura, Shinji Hayashi and Keiichi Yamamoto, "Photoluminescence from Si Nanocrystals Dispersed in Phosphosilicate Glass Thin Films: Improvement of Photoluminescence Efficiency,” Applied Physics Letters, Vol. 75, No. 2, pp.184-186, July (1999).
		Photoluminescence (PL) from Si nanocrystals (nc-Si) dispersed in phosphosilicate glass thin films was studied. It was found that, at room temperature, the 1.4 eV PL due to the recombination of electron-hole pairs in nc-Si becomes intense as the P concentration increases. At low temperatures, an additional peak related to defects at interfaces between nc-Si and the matrix was observed at about 0.9 eV. In contrast to the 1.4 eV peak, the 0.9 eV peak became weaker with increasing P concentration and almost disappeared at a P concentration of 1.5 mol %. These results suggest that the number of interface defects decreases with increasing P concentration and that this decrease leads to an improvement of the band-edge PL of nc-Si.

		22. Atsushi Mimura, Minoru Fujii, Shinji Hayashi and Keiichi Yamamoto, "Quenching of Photoluminescence from Si Nanocrystals Caused by Boron Doping,” Solid State Communications, Vol. 109, No. 9, pp. 561-565, February (1999).
		Boron-doped Si nanocrystals (nc-Si) as small as 2.7–3.8 nm in diameter were prepared and their photoluminescence (PL) properties were studied as functions of B concentration, size and excitation power. As the B concentration increased, the PL intensity decreased rapidly. The degree of PL quenching was found to increase with increasing size. In the case of undoped nc-Si, saturation and blue shift of PL peak were observed with increasing excitation power. However, these effects were not observed for heavily B-doped samples. The importance of the nonradiative Auger recombination process in B-doped nc-Si is discussed.

		17. Minoru Fujii, Shinji Hayashi and Keiichi Yamamoto, "Photoluminescence from B-doped Si Nanocrystals,” Journal of Applied Physics, Vol. 83, No. 12, pp.7953 - 7957, June (1998)
		Boron-doped Si nanocrystals as small as 3.5 nm were prepared and their photoluminescence (PL) properties were studied. The PL properties were found to be very sensitive to the B concentration. For the sample without B doping the temperature-dependent shift of the PL peak was almost the same as that of the bulk band gap. As the B concentration increased, the temperature dependence deviated from that of the bulk band gap, and the peak exhibited a low-energy shift as the temperature decreased. The anomalous temperature dependence is considered to be due to the contribution of the PL from excitons bound to the neutral B states.

		11. Yoshihiko Kanzawa, Minoru Fujii, Shinji Hayashi and Keiichi Yamamoto, "Doping of B Atoms into Si Nanocrystals Prepared by Rf Cosputtering,” Solid State Communications, Vol. 100, No. 4, pp. 227-230, October (1996).
		We have succeeded, for the first time, in doping B atoms into Si nanocrystals well dispersed in an oxide matrix. The samples were prepared by cosputtering of Si, SiO2 and B2O3 targets and post-annealing. The growth of Si nanocrystals in the matrix was demonstrated by high-resolution electron microscopy. Raman spectra of the samples exhibited asymmetric line shapes, which showed a strong dependence on the excitation wavelength. The Raman results can be well explained in terms of the Fano-type interference between the phonon scattering and electronic excitations. Since the Fano-type interference is possible only for heavily doped Si crystals, the present Raman results clearly demonstrate the success in B doping into well dispersed Si nanocrystals.