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Hexagonal boron nitride as a new ultraviolet luminescent material and its application—Fluorescence properties of hBN single-crystal powder
Diamond & Related Materials 20 (2011) 849–852
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Hexagonal boron nitride as a new ultraviolet luminescent material and its application—Fluorescence properties of hBN single-crystal powder☆☆ Kenji Watanabe a,⁎, Takashi Taniguchi a, Kenta Miya b, Yoshitaka Sato b, Kazuhito Nakamura b, Takahiro Niiyama b, Masateru Taniguchi b a b
National Institute for Materials Science, Namiki 1–1, Tsukuba Ibaraki 305–0044, Japan Futaba Corporation, Yabutsuka 1080, Chosei, Chiba 299–4395, Japan
a r t i c l e
i n f o
Available online 9 April 2011 Keywords: Boron nitride (BN) High pressure high temperature (HTHP) Optical properties Excitons Light emission
a b s t r a c t Luminous properties of hexagonal boron nitride (hBN) single-crystal powder were studied by cathodoluminescence and photoluminescence spectroscopy. The single crystals grown by a high pressure and high temperature method showed typical excitonic spectra at peak wavelengths of 215, 220, and 227 nm at room temperature. Well-faceted hexagonal shaped single crystals exhibited the dominant 215-nm band. The 220and 227-nm bands were singly or concurrently observed in the case of irregular-shaped crystal powders, though no discernible difference between the morphologies of crystal powders showing differently superimposed luminous spectral profiles of the 220- and 227-nm bands was observed. However, each band showed different spectral and decay features at 8 K, implying that the origins of the two bands are different. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of the far-ultraviolet (FUV) luminous properties of hexagonal boron nitride (hBN) [1,2], hBN has attracted considerable attention regarding its application to an FUV emitter device. A field-emitter display (FED)-type FUV device, which utilizes a field-emitter array as an excitation source for an FUV fluorescence screen based on hBN, has recently been developed [3]. This development is the first step toward the practical use of an FUV light source as an alternative to mercury lamps. However, although this FUV device could be operated on dry batteries (owing to its low current consumption), its FUV output power (namely, 0.2 mW) must be improved for applications such as replacing mercury lamps. To improve the output efficiency of the FUV device, one effective method is to make the crystal grain size of the fluorescence powder printed on the fluorescence screen as small as possible. Decreasing the crystal size increases the surface scattering of the FUV light excited inside the crystals and enhances the external efficiency of the luminescence. However, the minimization of the grain size generally decreases internal quantum efficiency by, for example, increasing the effect of non-radiative centers, because the electronic states of smallgrain-size powder are strongly affected by defects induced near the surface and initial nucleation region, and it leads to the decrease of the
luminescence efficiency. Controlling crystal quality to avoid defectoriginated depletion of internal quantum efficiency is thus one of crucial problems in developing fluorescent materials. In case of the hBN, among the intense near-band-gap luminescence bands, the FUV luminescence at 215 nm is originated from selftrapped exciton luminescence [4]. This self-trapping arises from the strong exciton–phonon interaction of hBN; hence, it is one of the intrinsic characteristics of excitons in the single crystal of hBN. In contrast to the 215-nm luminescence band, the 220- and 227-nm luminescence bands are observed at a defective structure, such as dislocations, around the surface [5,6]. It was argued that the 227-nm luminescence band was originated from one of the four allowed exciton series that were experimentally observed [4,7], and this band was assigned to exciton bound to stacking faults. However, the origin of the 220-nm band, especially in terms of its relation to the 227-nm band, is not yet clearly understood. In light of the above-described circumstances, we studied the bandedge luminescence spectra of the 220 and 227-nm bands from pure single-crystal powder grown by a high pressure and high temperature (HP–HT) method, and we demonstrated the luminescent properties of the 220-nm and 227-nm bands, which are entirely different from each other in terms of their decay and spectral profiles at low temperature. 2. Experimental
☆☆ Presented at the Diamond 2010, 21st European Conference on Diamond, Diamond- Like Materials, Carbon Nanotubes, and Nitrides, Budapest. ⁎ Corresponding author. Tel.: + 81-298604309; fax: + 81-298514005. E-mail address: [email protected] (K. Watanabe). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.04.002
The samples were prepared by the HP–HT method [8]. Commercially available hBN powder as the source material was heat-treated at 2100 °C for 2 h in a flow of nitrogen gas to remove physically adsorbed
850
K. Watanabe et al. / Diamond & Related Materials 20 (2011) 849–852
oxygen impurities. After the heat treatment, the oxygen content of the hBN source was measured to be about 0.06 wt.%. The solvent used was a BaBN compound. A molybdenum sample chamber was used for the sample growth. The assembled cell containing the solvent and the source hBN was compressed to 5.5 GPa and then heated to between 1500 and 1750 °C by employing a belt-type HP–HT apparatus with a 60-mm bore diameter. The holding time was 20 hr. After the HP–HT process, the metal solvent in the molybdenum sample chamber was dissolved with hot aqua regia, and the grown crystals were obtained. The size of the grown crystal was estimated to be around a few hundred microns. The cathodoluminescence spectroscopy system used to measure the luminescence properties of the grown crystals consists of a scanning electron microscope (SEM) (Topcon Co., SM350) as an excitation source, coupled to a single monochromator (Photon Design Co., PDP-320) with a liquid nitrogen-cooled CCD (Roper Scientific, Inc., LN/CCD-400 EB-GI). To deal with the wide wavelength range from the near-infrared to FUV regions, reflective optics, such as concave and plane mirrors, were used to prevent chromatic aberration. This cathodoluminescence system makes it possible to observe a wavelength-dependent luminescence image with spatial resolution of 1 μm over a wide range (i.e., from 200 to 800 nm). The photoluminescence system has two detectors, a CCD for measuring photoluminescence spectra and a microchannel-plate photomultiplier (MCP-PMT) forming a time-correlated single-photoncounting (TCSPC) system with a time-amplitude converter (TAC) for measuring decay profile. These detectors make it possible to measure photoluminescence spectra and their time-dependent profiles with high sensitivity and resolution. The samples for the photoluminescence spectroscopy were excited by the fourth harmonics of a mode-locked Ti:sapphire laser with wavelength of 198 nm. The photoluminescence measurements were carried out from 200 to 800 nm at temperatures from 8 to 300 K with a closed cycle variable temperature optical cyrostat (CRYO Industries of America, Model-469-D-M). In the wavelength range at 230 nm and below, chromatic aberration becomes significant for refracting optical systems; therefore, a reflecting optical system consisting of aluminumcoated UV-grade mirrors was employed to avoid chromatic aberration in the photoluminescence system as well as in the cathodoluminescence system described above. The excitation laser light accompanied by luminescence was removed by a filter monochromator equipped with two prisms in subtractive-dispersion configuration. The output monochrome light from the filter monochromator was sent either to a spectrograph (Photon Design, PDP320) with a CCD (Roper Scientific, Inc., PD-10: 256 E/LN) to obtain a high-resolution spectrum or the MCP-PMT (Hamamatsu, R3809U) with a TAC module (Becker & Hickl GmbH, SPC-630) to measure decay properties. These light paths can be changed over by a flip mirror placed after the prism monochromator. 3. Results and discussion The dominant band-edge luminescence in the FUV region was observed in highly pure hBN single crystals. In case of the luminescence from a faceted single crystal, as shown in Fig. 1(a), the 215-nm band dominates the whole spectral region at room temperature. This luminescence band drastically changes when the sample is deformed [5,6]. The deformation of the sample causing the spectral change can be easily introduced by simply pinching the single crystal by the fingers. Considering the crystal structure of hBN, the coupling between the BN sp2 layers is much weaker than that between boron and nitrogen within a layer; thus, hBN single crystal can be easily deformed by the pressure of fingers. Under this distortion, the stacking layer between c-facets can glide, and the excitons are considered to be localized by the planes that glide, lowering the luminescence energy from 215 to 227 nm as shown in
Fig. 1. Cathodoluminescence spectra from purified powder crystals. The peak wavelengths of luminescence spectra are (a) 215, (b) 220, and (c) 227 nm, respectively. Inset is expanded view around the peaks.
Fig. 1(c). Note that the successive broad bands from 250 to 400 nm in Fig. 2 in Ref. [5] arose from impurities, though it was not clearly described in Ref. [5]; the impure region exhibiting the broad band from 250 to 400 nm [8] was unintentionally mixed with the pure region by the deformation process described in Ref. [5], and the broad impurity band was observed with the 227-nm band. Considering the fact that the data shown in Fig. 2 in Ref. [5] was taken in an early stage of the development of growth technique, and the grown single crystal had both pure and impure regions throughout a single crystal, they could be easily mixed during the grinding process. After that time, by ensuring the purification process of source materials for the HP–HT method, we finally obtained the 215, 220, and 227-nm bands separately, as shown in Fig. 1. Each band dominates the whole spectral range without the broad bands from 250 to 400 nm. Fig. 2 shows the monochromatic cathodoluminescence images at 215, 220, and 227 nm, and corresponding SEM images of the samples. The SEM image in Fig. 2(a) shows a faceted hexagonal crystal morphology, and the whole crystal emits intense exciton luminescence at 215 nm. The intense image at 215 nm fades at 220 nm and almost disappears at 227 nm. On the other hand, the SEM images of the crystals in Figs. 2(b) and (c) show indeterminate crystal morphologies; the intensity of the 220-nm cathodoluminescence image of crystal (b) is the highest, while the 227-nm image of crystal (c) is the brightest. However, no discernible difference between the morphologies of samples (b) and (c) can be seen, even though the spectral profiles of these samples are different. A spatially resolved cathodoluminescence image of hBN was recently reported by Jaffrennou et al. [6]. According to their data, the 220 and 227-nm luminescence bands are localized near dislocations and boundaries and thus, it supports that both bands are produced by the recombination of excitons trapped at structural defects. However, the condition that distinguishes between the crystals showing two luminescence bands at 220 and 227 nm is not evident as described in Ref. [6]. They suggested the phonon replica model for the band assignment, because the energy separation between the 220- and 227-nm wavelengths almost corresponds to the one-phonon energy at the Γ point [9]. However, considering the fact that the 220-nm band is not always accompanied by the 227-nm band as shown in Fig. 1, the phonon-replica model is not plausible in the assignment of these bands. To elucidate the relation of these bands, we measured photoluminescence spectra and decay profiles at low temperature. Fig. 3 shows the low-temperature photoluminescence spectrum from the purified single crystal, which exhibits a dominant 215-nm band, as shown in Fig. 1(a), at room temperature. In terms of the peak position, the 215-nm band and the 227-nm band correspond to the S4 band
K. Watanabe et al. / Diamond & Related Materials 20 (2011) 849–852
851
Fig. 2. SEM images and corresponding cathodoluminescence monochromatic images. The cathodoluminescence images are taken at screened wavelengths of 215, 220, and 227 nm. In the case of the faceted single crystal (a), the intense image at 215 nm fades at 220 nm, and almost disappears at 227 nm. SEM images (b) and (c) show the indeterminate crystal morphology; the intensity of 220-nm cathodoluminescence image is the highest for crystal (b), while the 227-nm image becomes the brightest for crystal (c).
and the D4 band, respectively. The two series of bands in Fig. 3 (indicated by “S” and “D”) are originated from the four optically allowed exciton levels [4]. Thus, the peaks of the 215- and 227-nm bands at room temperature are positioned at the lowest energy levels of S and D series, respectively, as a result of energy relaxation. A weak 220-nm band is observed at the shoulder of the D band series at 8 K. As for the 220-nm band, there seems to be no obvious accompanying band. Since the 220-nm band can exhibit the largest intensity at room temperature, as shown in Fig. 1 (b), the 220-nm band should correspond to the lowest energy level if the band is one of the series components like the S and D series. Although a higher energy than the energy position of the 220-nm band was searched for, a band corresponding to the 220-nm-band series was not found. This band thus does not make an apparent series but seems to almost be a single level. Fig. 4 shows the photoluminescence decay profile for the 215-nm band (S4), the 220-nm band, and the 227-nm band (D4). All the decay profiles for the S and D series are similar to those for S4 and D4, respectively. The decay times (determined by fitting with a single exponential function) are 0.6, 3.0, and 5.0 ns for the 215-, 220-, and 227-nm bands, respectively. The decay time becomes longer with increasing wavelength of the luminescence band. As for the 227-nm band, the time-resolved profile clearly exhibits a slowly rising
Fig. 3. Low-temperature photoluminescence spectrum from hBN single crystal at 8 K.
component after the excitation. This result implies that dynamical relaxation from the higher energy position to the 227-nm band occurs as time elapses. The decay profile of the 220-nm band is completely different from that of the D series including the 227-nm band. The decay time of the 220-nm band is slightly shorter than that of the 227-nm band. In consideration that the 220-nm band forms an isolated band and that the decay profiles of the 220- and 227-nm bands are different, the 220-nm band could be originated from a different exciton state than the optically allowed four exciton levels. The detailed origin of the 220-nm band is not clear at present. It could be related to a triplet state lying below the allowed exciton states [10–14], which becomes partially allowed owing to disorder caused by deformation, strain, dislocations, or defects. 4. Summary The luminescence properties of hexagonal boron nitride (hBN) single-crystal powder were studied by cathodoluminescence, photoluminescence, and time-resolved photoluminescence spectroscopy. The band-edge luminescence at 215 nm from a faceted crystal and the luminescence bands at wavelengths of 220 and 227 nm from
Fig. 4. Decay profiles of photoluminescence spectra for 215, 220, and 227-nm bands at 8 K.
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K. Watanabe et al. / Diamond & Related Materials 20 (2011) 849–852
indeterminate crystals were observed without impurity related bands. No difference between the morphologies of 220-nm-banddominated crystals and 227-nm-band-dominated crystals was found in SEM and cathodoluminescence images. However, the 220- and 227-nm bands exhibit the obvious difference in the photoluminescence spectra and the decay profiles at low temperature. It is thus concluded that the origin of the 220-nm band is different from that of the 227-nm band, which originates from the optically allowed Frenkel exciton states. Acknowledgements This research was partially supported by Grants-in-Aid for Challenging Exploratory Research22651056, Scientific Research(A) 19205026, Priority Areas “Nano Materials Science for Atomic Scale Modification 474,” and World Premier International Research Center (WPI) Initiative, (MANA, NIMS). All these grants were provided by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
References [1] K. Watanabe, T. Taniguchi, H. Kanda, Nat. Mater. 3 (2004) 404. [2] K. Watanabe, T. Taniguchi, H. Kanda, Phys. Status Solidi A 201 (2004) 2561. [3] K. Watanabe, T. Taniguchi, T. Niiyama, K. Miya, M. Taniguchi, Nat. Photonics 3 (2009) 591. [4] K. Watanabe, T. Taniguchi, Phys. Rev. B 79 (2009) 193104. [5] K. Watanabe, T. Taniguchi, T. Kuroda, H. Kanda, Appl. Phys. Lett. 89 (2006) 141902. [6] P. Jaffrennou, J. Barjon, J.-S. Lauret, B. Attal-Trout, F. Ducastelle, A. Loiseau, J. Appl. Phys. 102 (2007) 116102. [7] L. Museur, A. Kanaev, J. Appl. Phys. 103 (2008) 103520. [8] T. Taniguchi, K. Watanabe, J. Cryst. Growth 303 (2007) 525. [9] J. Serrano, A. Bosak, R. Arenal, M. Krisch, K. Watanabe, T. Taniguchi, H. Kanda, A. Rubio, L. Wirtz, Phys. Rev. Lett. 98 (2007) 095503. [10] K. Harigaya, Synth. Met. 135 (2003) 751. [11] K. Harigaya, J Exp Theor Phys+ 97 (2003) 1246. [12] L. Wirtz, A. Marini, M. Grüning, and A. Rubio, arXiv: cond-mat/0508421 (2005). [13] L. Wirtz, A. Marini, M. Grüning, C. Attaccalite, G. Kresse, A. Rubio, Phys. Rev. Lett. 100 (2008) 189701. [14] B. Arnaud, S. Lebègue, P. Rabiller, M. Alouani, Phys. Rev. Lett. 100 (2008) 189702.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Hexagonal boron nitride as a new ultraviolet luminescent material and its application—Fluorescence properties of hBN single-crystal powder☆☆ Kenji Watanabe a,⁎, Takashi Taniguchi a, Kenta Miya b, Yoshitaka Sato b, Kazuhito Nakamura b, Takahiro Niiyama b, Masateru Taniguchi b a b
National Institute for Materials Science, Namiki 1–1, Tsukuba Ibaraki 305–0044, Japan Futaba Corporation, Yabutsuka 1080, Chosei, Chiba 299–4395, Japan
a r t i c l e
i n f o
Available online 9 April 2011 Keywords: Boron nitride (BN) High pressure high temperature (HTHP) Optical properties Excitons Light emission
a b s t r a c t Luminous properties of hexagonal boron nitride (hBN) single-crystal powder were studied by cathodoluminescence and photoluminescence spectroscopy. The single crystals grown by a high pressure and high temperature method showed typical excitonic spectra at peak wavelengths of 215, 220, and 227 nm at room temperature. Well-faceted hexagonal shaped single crystals exhibited the dominant 215-nm band. The 220and 227-nm bands were singly or concurrently observed in the case of irregular-shaped crystal powders, though no discernible difference between the morphologies of crystal powders showing differently superimposed luminous spectral profiles of the 220- and 227-nm bands was observed. However, each band showed different spectral and decay features at 8 K, implying that the origins of the two bands are different. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of the far-ultraviolet (FUV) luminous properties of hexagonal boron nitride (hBN) [1,2], hBN has attracted considerable attention regarding its application to an FUV emitter device. A field-emitter display (FED)-type FUV device, which utilizes a field-emitter array as an excitation source for an FUV fluorescence screen based on hBN, has recently been developed [3]. This development is the first step toward the practical use of an FUV light source as an alternative to mercury lamps. However, although this FUV device could be operated on dry batteries (owing to its low current consumption), its FUV output power (namely, 0.2 mW) must be improved for applications such as replacing mercury lamps. To improve the output efficiency of the FUV device, one effective method is to make the crystal grain size of the fluorescence powder printed on the fluorescence screen as small as possible. Decreasing the crystal size increases the surface scattering of the FUV light excited inside the crystals and enhances the external efficiency of the luminescence. However, the minimization of the grain size generally decreases internal quantum efficiency by, for example, increasing the effect of non-radiative centers, because the electronic states of smallgrain-size powder are strongly affected by defects induced near the surface and initial nucleation region, and it leads to the decrease of the
luminescence efficiency. Controlling crystal quality to avoid defectoriginated depletion of internal quantum efficiency is thus one of crucial problems in developing fluorescent materials. In case of the hBN, among the intense near-band-gap luminescence bands, the FUV luminescence at 215 nm is originated from selftrapped exciton luminescence [4]. This self-trapping arises from the strong exciton–phonon interaction of hBN; hence, it is one of the intrinsic characteristics of excitons in the single crystal of hBN. In contrast to the 215-nm luminescence band, the 220- and 227-nm luminescence bands are observed at a defective structure, such as dislocations, around the surface [5,6]. It was argued that the 227-nm luminescence band was originated from one of the four allowed exciton series that were experimentally observed [4,7], and this band was assigned to exciton bound to stacking faults. However, the origin of the 220-nm band, especially in terms of its relation to the 227-nm band, is not yet clearly understood. In light of the above-described circumstances, we studied the bandedge luminescence spectra of the 220 and 227-nm bands from pure single-crystal powder grown by a high pressure and high temperature (HP–HT) method, and we demonstrated the luminescent properties of the 220-nm and 227-nm bands, which are entirely different from each other in terms of their decay and spectral profiles at low temperature. 2. Experimental
☆☆ Presented at the Diamond 2010, 21st European Conference on Diamond, Diamond- Like Materials, Carbon Nanotubes, and Nitrides, Budapest. ⁎ Corresponding author. Tel.: + 81-298604309; fax: + 81-298514005. E-mail address: [email protected] (K. Watanabe). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.04.002
The samples were prepared by the HP–HT method [8]. Commercially available hBN powder as the source material was heat-treated at 2100 °C for 2 h in a flow of nitrogen gas to remove physically adsorbed
850
K. Watanabe et al. / Diamond & Related Materials 20 (2011) 849–852
oxygen impurities. After the heat treatment, the oxygen content of the hBN source was measured to be about 0.06 wt.%. The solvent used was a BaBN compound. A molybdenum sample chamber was used for the sample growth. The assembled cell containing the solvent and the source hBN was compressed to 5.5 GPa and then heated to between 1500 and 1750 °C by employing a belt-type HP–HT apparatus with a 60-mm bore diameter. The holding time was 20 hr. After the HP–HT process, the metal solvent in the molybdenum sample chamber was dissolved with hot aqua regia, and the grown crystals were obtained. The size of the grown crystal was estimated to be around a few hundred microns. The cathodoluminescence spectroscopy system used to measure the luminescence properties of the grown crystals consists of a scanning electron microscope (SEM) (Topcon Co., SM350) as an excitation source, coupled to a single monochromator (Photon Design Co., PDP-320) with a liquid nitrogen-cooled CCD (Roper Scientific, Inc., LN/CCD-400 EB-GI). To deal with the wide wavelength range from the near-infrared to FUV regions, reflective optics, such as concave and plane mirrors, were used to prevent chromatic aberration. This cathodoluminescence system makes it possible to observe a wavelength-dependent luminescence image with spatial resolution of 1 μm over a wide range (i.e., from 200 to 800 nm). The photoluminescence system has two detectors, a CCD for measuring photoluminescence spectra and a microchannel-plate photomultiplier (MCP-PMT) forming a time-correlated single-photoncounting (TCSPC) system with a time-amplitude converter (TAC) for measuring decay profile. These detectors make it possible to measure photoluminescence spectra and their time-dependent profiles with high sensitivity and resolution. The samples for the photoluminescence spectroscopy were excited by the fourth harmonics of a mode-locked Ti:sapphire laser with wavelength of 198 nm. The photoluminescence measurements were carried out from 200 to 800 nm at temperatures from 8 to 300 K with a closed cycle variable temperature optical cyrostat (CRYO Industries of America, Model-469-D-M). In the wavelength range at 230 nm and below, chromatic aberration becomes significant for refracting optical systems; therefore, a reflecting optical system consisting of aluminumcoated UV-grade mirrors was employed to avoid chromatic aberration in the photoluminescence system as well as in the cathodoluminescence system described above. The excitation laser light accompanied by luminescence was removed by a filter monochromator equipped with two prisms in subtractive-dispersion configuration. The output monochrome light from the filter monochromator was sent either to a spectrograph (Photon Design, PDP320) with a CCD (Roper Scientific, Inc., PD-10: 256 E/LN) to obtain a high-resolution spectrum or the MCP-PMT (Hamamatsu, R3809U) with a TAC module (Becker & Hickl GmbH, SPC-630) to measure decay properties. These light paths can be changed over by a flip mirror placed after the prism monochromator. 3. Results and discussion The dominant band-edge luminescence in the FUV region was observed in highly pure hBN single crystals. In case of the luminescence from a faceted single crystal, as shown in Fig. 1(a), the 215-nm band dominates the whole spectral region at room temperature. This luminescence band drastically changes when the sample is deformed [5,6]. The deformation of the sample causing the spectral change can be easily introduced by simply pinching the single crystal by the fingers. Considering the crystal structure of hBN, the coupling between the BN sp2 layers is much weaker than that between boron and nitrogen within a layer; thus, hBN single crystal can be easily deformed by the pressure of fingers. Under this distortion, the stacking layer between c-facets can glide, and the excitons are considered to be localized by the planes that glide, lowering the luminescence energy from 215 to 227 nm as shown in
Fig. 1. Cathodoluminescence spectra from purified powder crystals. The peak wavelengths of luminescence spectra are (a) 215, (b) 220, and (c) 227 nm, respectively. Inset is expanded view around the peaks.
Fig. 1(c). Note that the successive broad bands from 250 to 400 nm in Fig. 2 in Ref. [5] arose from impurities, though it was not clearly described in Ref. [5]; the impure region exhibiting the broad band from 250 to 400 nm [8] was unintentionally mixed with the pure region by the deformation process described in Ref. [5], and the broad impurity band was observed with the 227-nm band. Considering the fact that the data shown in Fig. 2 in Ref. [5] was taken in an early stage of the development of growth technique, and the grown single crystal had both pure and impure regions throughout a single crystal, they could be easily mixed during the grinding process. After that time, by ensuring the purification process of source materials for the HP–HT method, we finally obtained the 215, 220, and 227-nm bands separately, as shown in Fig. 1. Each band dominates the whole spectral range without the broad bands from 250 to 400 nm. Fig. 2 shows the monochromatic cathodoluminescence images at 215, 220, and 227 nm, and corresponding SEM images of the samples. The SEM image in Fig. 2(a) shows a faceted hexagonal crystal morphology, and the whole crystal emits intense exciton luminescence at 215 nm. The intense image at 215 nm fades at 220 nm and almost disappears at 227 nm. On the other hand, the SEM images of the crystals in Figs. 2(b) and (c) show indeterminate crystal morphologies; the intensity of the 220-nm cathodoluminescence image of crystal (b) is the highest, while the 227-nm image of crystal (c) is the brightest. However, no discernible difference between the morphologies of samples (b) and (c) can be seen, even though the spectral profiles of these samples are different. A spatially resolved cathodoluminescence image of hBN was recently reported by Jaffrennou et al. [6]. According to their data, the 220 and 227-nm luminescence bands are localized near dislocations and boundaries and thus, it supports that both bands are produced by the recombination of excitons trapped at structural defects. However, the condition that distinguishes between the crystals showing two luminescence bands at 220 and 227 nm is not evident as described in Ref. [6]. They suggested the phonon replica model for the band assignment, because the energy separation between the 220- and 227-nm wavelengths almost corresponds to the one-phonon energy at the Γ point [9]. However, considering the fact that the 220-nm band is not always accompanied by the 227-nm band as shown in Fig. 1, the phonon-replica model is not plausible in the assignment of these bands. To elucidate the relation of these bands, we measured photoluminescence spectra and decay profiles at low temperature. Fig. 3 shows the low-temperature photoluminescence spectrum from the purified single crystal, which exhibits a dominant 215-nm band, as shown in Fig. 1(a), at room temperature. In terms of the peak position, the 215-nm band and the 227-nm band correspond to the S4 band
K. Watanabe et al. / Diamond & Related Materials 20 (2011) 849–852
851
Fig. 2. SEM images and corresponding cathodoluminescence monochromatic images. The cathodoluminescence images are taken at screened wavelengths of 215, 220, and 227 nm. In the case of the faceted single crystal (a), the intense image at 215 nm fades at 220 nm, and almost disappears at 227 nm. SEM images (b) and (c) show the indeterminate crystal morphology; the intensity of 220-nm cathodoluminescence image is the highest for crystal (b), while the 227-nm image becomes the brightest for crystal (c).
and the D4 band, respectively. The two series of bands in Fig. 3 (indicated by “S” and “D”) are originated from the four optically allowed exciton levels [4]. Thus, the peaks of the 215- and 227-nm bands at room temperature are positioned at the lowest energy levels of S and D series, respectively, as a result of energy relaxation. A weak 220-nm band is observed at the shoulder of the D band series at 8 K. As for the 220-nm band, there seems to be no obvious accompanying band. Since the 220-nm band can exhibit the largest intensity at room temperature, as shown in Fig. 1 (b), the 220-nm band should correspond to the lowest energy level if the band is one of the series components like the S and D series. Although a higher energy than the energy position of the 220-nm band was searched for, a band corresponding to the 220-nm-band series was not found. This band thus does not make an apparent series but seems to almost be a single level. Fig. 4 shows the photoluminescence decay profile for the 215-nm band (S4), the 220-nm band, and the 227-nm band (D4). All the decay profiles for the S and D series are similar to those for S4 and D4, respectively. The decay times (determined by fitting with a single exponential function) are 0.6, 3.0, and 5.0 ns for the 215-, 220-, and 227-nm bands, respectively. The decay time becomes longer with increasing wavelength of the luminescence band. As for the 227-nm band, the time-resolved profile clearly exhibits a slowly rising
Fig. 3. Low-temperature photoluminescence spectrum from hBN single crystal at 8 K.
component after the excitation. This result implies that dynamical relaxation from the higher energy position to the 227-nm band occurs as time elapses. The decay profile of the 220-nm band is completely different from that of the D series including the 227-nm band. The decay time of the 220-nm band is slightly shorter than that of the 227-nm band. In consideration that the 220-nm band forms an isolated band and that the decay profiles of the 220- and 227-nm bands are different, the 220-nm band could be originated from a different exciton state than the optically allowed four exciton levels. The detailed origin of the 220-nm band is not clear at present. It could be related to a triplet state lying below the allowed exciton states [10–14], which becomes partially allowed owing to disorder caused by deformation, strain, dislocations, or defects. 4. Summary The luminescence properties of hexagonal boron nitride (hBN) single-crystal powder were studied by cathodoluminescence, photoluminescence, and time-resolved photoluminescence spectroscopy. The band-edge luminescence at 215 nm from a faceted crystal and the luminescence bands at wavelengths of 220 and 227 nm from
Fig. 4. Decay profiles of photoluminescence spectra for 215, 220, and 227-nm bands at 8 K.
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indeterminate crystals were observed without impurity related bands. No difference between the morphologies of 220-nm-banddominated crystals and 227-nm-band-dominated crystals was found in SEM and cathodoluminescence images. However, the 220- and 227-nm bands exhibit the obvious difference in the photoluminescence spectra and the decay profiles at low temperature. It is thus concluded that the origin of the 220-nm band is different from that of the 227-nm band, which originates from the optically allowed Frenkel exciton states. Acknowledgements This research was partially supported by Grants-in-Aid for Challenging Exploratory Research22651056, Scientific Research(A) 19205026, Priority Areas “Nano Materials Science for Atomic Scale Modification 474,” and World Premier International Research Center (WPI) Initiative, (MANA, NIMS). All these grants were provided by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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