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Formation of zinc-oxide nanoparticles in SiO2 by ion implantation combined with thermal oxidation
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 242 (2006) 96–99 www.elsevier.com/locate/nimb
Formation of zinc-oxide nanoparticles in SiO2 by ion implantation combined with thermal oxidation H. Amekura a
a,*
, K. Kono a, N. Kishimoto a, Ch. Buchal
b
Nanomaterials Laboratory, National Institute for Materials Science (NIMS), 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan b Institut fuer Schichten und Grenzflaechen (ISG1-IT), Forschungszentrum Juelich GmbH, D-52425 Juelich, Germany Available online 16 September 2005
Abstract Metal-ion implantation followed by thermal oxidation was carried out to fabricate zinc-oxide (ZnO) nanoparticles (NPs) in silica glass (SiO2). A SiO2 substrate was implanted with Zn+ ions of 60 keV up to 1.0 · 1017 ions/cm2. In the as-implanted state, the sample shows a strong absorption peak at 4.8 eV and a weak one at 1.2 eV due to Zn metallic NPs. After annealing in oxygen gas at 700 C for 1 h, the absorption in the visible region disappears and a new absorption edge appears at 3.25 eV. The grazing incidence X-ray diffraction (GXRD) confirms the formation of ZnO NPs. The ZnO NPs show a photoluminescence (PL) peak at 3.32 eV under pulsed nitrogen-laser excitation at 3.68 eV. Annealing at 900 C induces an additional shift of the absorption edge to 5.3 eV. The additional shift indicates the formation of a Zn2SiO4 phase which was confirmed by GXRD. 2005 Elsevier B.V. All rights reserved. PACS: 81.16.Pr; 78.67.Bf; 61.46.+w; 68.60.Dv Keywords: Nanoparticle; ZnO; Ion implantation; Thermal oxidation; Zn2SiO4; PL
1. Introduction Nanometer-size zinc-oxide (ZnO) draws much attention, because of possible applications to optoelectronic devices [1,2]. Up to now, we have fabricated metal-oxide nanoparticles (NPs), such as NiO [3] and CuO [4], in silica glasses (SiO2) using metal-ion implantation and following thermal oxidation. In this study, the method is applied to the fabrication of ZnO NPs in SiO2. Previously, Chen et al. [5] carried out Zn ion implantation of 160 keV to SiO2 up to 1.0 · 1017 ions/cm2 and following annealing at 700 C under oxidizing atmosphere, but they could not obtain clear evidence of ZnO formation. Liu et al. [6] employed a dose of 3.0 · 1017 ions/cm2 at the same energy 160 keV, and observed X-ray diffraction patterns of ZnO and UV photoluminescence (PL) after annealing at 700 C. However,
*
Corresponding author. Tel.: +81 29 863 5479; fax: +81 29 863 5599. E-mail address: [email protected] (H. Amekura).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.019
from X-ray photoelectron spectroscopy (XPS) measurements, they assumed that ZnO NPs were formed at the substrate surface, or in the much shallower region of the SiO2 substrate than expected from the implanted depth profile. On the other hand, the formation processes of NiO NPs in SiO2 have been proposed as follows [7]: Oxygen molecules migrate in SiO2 and react with the Ni NPs when the O2 molecules meet the NPs. Since the oxidation is carried out at the position of the metal NP, the depth profile of NiO NPs is similar to that of Ni NPs [7]. If the assumption of Liu et al. is correct, the formation processes of ZnO NPs are completely different from those of NiO NPs. However, more studies are necessary to assess LiuÕs suggestions. Understanding the formation processes of ZnO NPs is important for applications to optoelectronic devices, e.g. to predict the position (depth) of ZnO NP formation. For this purpose, study of isochronal oxygen-annealing data is valuable. However, past studies [5,6] reported data oxidized at 700 C only. In this paper, as the first step for the whole understanding of the ZnO NP formation
H. Amekura et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 96–99
97
processes, isochronal oxygen-annealing effects in Zn-implanted SiO2 are studied using optical absorption spectroscopy, grazing incidence X-ray diffraction (GXRD) and photoluminescence (PL) spectroscopy. 2. Experimental
3. Results and discussion Fig. 1 shows isochronal annealing effects on the optical absorption spectra of SiO2 sample implanted with Zn+ ions to 1.0 · 1017 ions/cm2. In the as-implanted state, two absorption peaks are observed: a strong and broad one centered at 4.8 eV and a very weak one at 1.2 eV. The former peak was reported in [5] at 5 eV and a similar peak was observed at 4.2 eV in Zn-implanted MgO [11]. Both peaks (5 eV peak in Zn:SiO2 and 4.2 eV peak in Zn:MgO) were ascribed to the surface plasmon resonance (SPR) of Zn NPs. However, the assignments are questionable, because they are based on a simplified formula [12]. Details will be discussed elsewhere. To our knowledge, this
Fig. 1. Optical absorption spectra of silica glass implanted with Zn+ ions of 60 keV up to 1.0 · 1017 ions/cm2, in as-implanted state and annealed in oxygen gas flow at 600, 700, 800 and 900 C for 1 h each, respectively. The spectra at 700 C and higher temperatures are magnified by factors shown.
is the first observation of the weak peak at 1.2 eV. Since this peak is predicted from Mie theory using the refractive index data of bulk Zn [13], the peak is not due to impurities or defects, but of intrinsic nature. GXRD spectrum of the as-implanted sample is shown in Fig. 2. All the diffraction peaks except a broad one from amorphous SiO2 substrate are fitted by a diffraction pattern of Zn metal. These observations indicate that Zn NPs are formed in SiO2 even in the as-implanted state. Up to 600 C, the optical spectrum shows little change. After 700 C annealing, drastic changes are induced. The absorption in the visible region disappears and an absorption edge appears at 3.25 eV. GXRD clearly shows a transformation in the diffraction patterns from Zn metal to ZnO at 700 C, as shown in Fig. 2. As the absorption in the visible region is almost zero, most of Zn NPs transform
S
DIFFRACTION YIELD (a.u.)
Optical-grade silica glasses of KU-1 type (OH 820 ppm) were implanted with Zn+ ions of 60 keV up to a fluence of 1.0 · 1017 ions/cm2. Peak concentration of Zn atoms in SiO2 calculated by TRIDYN (including sputtering effect) [8] and TRIM (neglecting sputtering effect) [9] codes were 25 and 35 at.%, respectively. Since the oxidation processes are governed by diffusional migration of O2 molecules in the substrate, a lower energy such as 60 keV, i.e. a thinner implanted layer, is better for a homogenous oxidation than the implantation energy of 160 keV which was used in past studies [5,6]. The ion flux was kept to less than 2 lA/cm2 to control the sample temperature to less than 100 C during the implantation. Isochronal annealing was carried out from 400 to 900 C with 100 C steps for 1 h each, in a tube furnace under O2 gas flow of 100 sccm. A dual-beam spectrometer with a resolution of 1 nm was used for the transmittance and reflectance measurements in the wavelength range of 190–1700 nm at room temperature (RT). Optical absorption was obtained from the transmittance and the reflectance with correction of the multiple reflection in samples [10]. PL was excited by the 337 nm line (3.68 eV) from a pulsed nitrogen laser (the pulse width of 0.5 ns). The spectra were detected by a monochromator and a streak camera, and were plotted after integration of the signal along the delay time between 0 and 30 ns for band-edge emission and 0–820 ns for sub-gap emission. Rutherford backscattering spectrometry (RBS) was carried out to determine the Zn content and the depth profile both in the as-implanted state and after oxidation, using a 2.06 MeV He+ beam of 1 mm in diameter with a scattering angle of 160. GXRD measurements were performed with an incident angle of 3 using a Cr X-ray source to confirm the transformation of Zn-related NPs (Zn, ZnO and Zn2SiO4).
2
S
+
60 keV Zn => SiO2
S
1.0x1017 ions/cm2 S : Zn2SiO4 O : ZnO m : Zn
S
S SSS O O
1
O
O
SS
S SS
900
O OO
O
O
O
O O
O
O
O O
Substrate
0
800 700
m m m
50
SS
O2 O2 O2
m
m
as-impl.
100
SCATTERING ANGLE 2
150 (deg.)
Fig. 2. Grazing incidence X-ray diffraction (GXRD) spectra of silica glass implanted with Zn+ ions of 60 keV up to 1.0 · 1017 ions/cm2, in asimplanted state and annealed in oxygen gas flow at 700, 800 and 900 C for 1 h each, respectively. Diffraction peaks labeled by ‘‘m’’, ‘‘O’’ and ‘‘S’’ are ascribed to Zn, ZnO and Zn2SiO4, respectively.
98
H. Amekura et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 96–99
to ZnO NPs. Using the Scherrer formula [14], the mean size of ZnO NPs was estimated to be 26 nm from the width of the ZnO(0 0 2) diffraction peak. Only a weak quantum size effect is expected, since the exciton Bohr radius is 1.8 nm [15], i.e. much smaller than the NP diameter. It is consistent with the observed absorption edge at 3.25 eV, because the band gap energy and the exciton binding energy of bulk ZnO at RT are 3.37 eV and 0.060 eV, respectively [15]. It should be noted that a small kink appears at 3.3 eV even after 600 C annealing. A small portion of Zn NPs starts the transformation to ZnO NPs even at 600 C. After the transformation to ZnO NPs, the optical absorption decreases to 1/4 of that of Zn NPs. However, RBS results show that Zn content is almost constant up to 900 C, although the depth profile changes. The decrease of optical absorption after the transformation to ZnO NPs is primarily due to decrease of the oscillator strength in the observed energy region, not due to decrease of Zn content in the sample. It should be noted that the melting points (MP) of bulk Zn is as low as 419.6 C, although that of ZnO is 1970 C [15]. The ZnO NPs may be formed by oxidation of the melting Zn NPs at 700 C. However, in some cases, implanted NPs are subject to high pressures from the surrounding material, which may stabilize the solid phase of the NPs even above the MP for bulk material [16]. More studies are needed to confirm this assumption. After 800 C annealing, the absorption due to ZnO NPs slightly decreases. After 900 C annealing, the absorption edge at 3.25 eV completely disappears, and the edge shifts to 5.3 eV. The new absorption edge of 5.3 eV is close to a literature value of the band gap energy of Zn2SiO4 [17]. The GXRD measurements confirm the transformation of the diffraction patterns from ZnO to Zn2SiO4 at 900 C, as shown in Fig. 2. Isochronal oxygen-annealing effects on PL spectra are shown in Fig. 3. For comparison, the optical absorption
PL INTENSITY (a.u.)
O2 anneal.
Laser exc.
t < 30 ns
t < 820 ns
4. Conclusions Zn-ion implantation followed by thermal oxidation was used to fabricate ZnO NPs in SiO2. A SiO2 substrate was implanted with Zn+ ions of 60 keV up to 1.0 · 1017 ions/ cm2, and was annealed in O2 gas at elevated temperatures. After annealing at 700 C for 1 h, the ZnO NPs which show absorption edge at 3.25 eV are formed in SiO2. Under pulsed nitrogen-laser excitation at 3.68 eV, the ZnO NPs show a PL peak at 3.32 eV. The PL intensity slightly decreases after 800 C annealing. Annealing at 900 C converts the ZnO NPs to the Zn2SiO4 phase which shows an absorption edge at 5.3 eV. Acknowledgements The authors acknowledge support from Dr. G. Kido (NIMS) for visit of H.A. in FZ-Juelich. A part of this study was financially supported by the Budget for Nuclear Research of the MEXT, based on the screening and counseling by the Atomic Energy Commission.
+
60 keV Zn => SiO2
spectrum after 700 C annealing is also shown in Fig. 3. The 3.68 eV line from the nitrogen laser, which is 0.4 eV higher than the absorption edge of ZnO NPs, excites a PL peak at 3.32 eV. Since the peak energy is almost the same as the energy of the absorption edge, the peak is ascribed to the PL from ZnO NPs in SiO2. It should be noted that the signal above 3.4 eV is due to stray light from excitation laser, because the signal appears even without samples. Annealing at 800 C slightly decreases the 3.32 eV PL peak. The decrease may be consistent with the decrease of the optical absorption after 800 C annealing shown in Fig. 1. After 800 C annealing, sub-gap PL signal centered at 2.4 eV, probably due to defects in ZnO NPs, increases. It may indicate that small fractions of ZnO NPs transform to Zn2SiO4 even at 800 C. Such perturbation may increase the sub-gap PL signal at 2.4 eV and decreases the exciton PL at 3.32 eV. It should be noted that clearer exciton peak was observed in PL experiments under CW-mode He–Cd laser excitation [18].
700
References 800 900
0 absorption
2
3 PHOTON ENERGY (eV)
4
Fig. 3. Photoluminescence (PL) spectra of silica glass implanted with Zn+ ions of 60 keV to 1.0 · 1017 ions/cm2 and annealed in oxygen gas flow at 700, 800 and 900 C for 1 h each. The optical absorption spectrum after 700 C annealing is also shown. Signal above 3.4 eV is due to stray light from N2 laser excitation at 3.68 eV. A peak at 3.32 eV is ascribed to exciton PL from ZnO NPs in SiO2.
[1] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459. [2] E.M. Wong, P.C. Searson, Appl. Phys. Lett. 74 (1999) 2939. [3] H. Amekura, N. Umeda, Y. Takeda, J. Lu, N. Kishimoto, Appl. Phys. Lett. 85 (2004) 1015. [4] H. Amekura, Y. Takeda, K. Kono, H. Kitazawa, N. Kishimoto, Rev. Adv. Mater. Sci. 5 (2003) 178. [5] J. Chen, R. Mu, A. Ueda, M.H. Wu, Y.-S. Tung, Z. Gu, D.O. Henderson, C.W. White, J.D. Budai, R.A. Zuhr, J. Vac. Sci. Technol. A16 (1998) 1409. [6] Y.X. Liu, Y.C. Liu, D.Z. Shen, G.Z. Zhong, X.W. Fan, X.G. Kong, R. Mu, D.O. Henderson, Solid State Commun. 121 (2002) 531. [7] H. Amekura, N. Umeda, Y. Takeda, J. Lu, K. Kono, N. Kishimoto, Nucl. Instr. and Meth. B 230 (2005) 193. [8] W. Moeller, W. Eckstein, Nucl. Instr. and Meth. B 2 (1984) 814.
H. Amekura et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 96–99 [9] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985 (Chapter 8). Available from:
99
[14] B.D. Culty, Elements of X-ray Diffractions, Addison-Wesley, Reading, MA, 1978, p. 102. [15] M. Kawasaki, A. Ohtomo, Solid State Phys. 33 (1988) 59 (in Japanese). [16] K. Mitsuishi, M. Song, K. Furuya, C.W. Allen, R.C. Birtcher, U. Dahmen, Nucl. Instr. and Meth. B 206 (2003) 109. [17] H. Chang, H.D. Park, K.S. Sohn, J.D. Lee, J. Korean Phys. Soc. 34 (1999) 545. [18] H. Amekura, N. Umeda, Y. Sakuma, N. Kishimoto, Ch. Buchal, Appl. Phys. Lett. 87 (2005) 013109.
Nuclear Instruments and Methods in Physics Research B 242 (2006) 96–99 www.elsevier.com/locate/nimb
Formation of zinc-oxide nanoparticles in SiO2 by ion implantation combined with thermal oxidation H. Amekura a
a,*
, K. Kono a, N. Kishimoto a, Ch. Buchal
b
Nanomaterials Laboratory, National Institute for Materials Science (NIMS), 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan b Institut fuer Schichten und Grenzflaechen (ISG1-IT), Forschungszentrum Juelich GmbH, D-52425 Juelich, Germany Available online 16 September 2005
Abstract Metal-ion implantation followed by thermal oxidation was carried out to fabricate zinc-oxide (ZnO) nanoparticles (NPs) in silica glass (SiO2). A SiO2 substrate was implanted with Zn+ ions of 60 keV up to 1.0 · 1017 ions/cm2. In the as-implanted state, the sample shows a strong absorption peak at 4.8 eV and a weak one at 1.2 eV due to Zn metallic NPs. After annealing in oxygen gas at 700 C for 1 h, the absorption in the visible region disappears and a new absorption edge appears at 3.25 eV. The grazing incidence X-ray diffraction (GXRD) confirms the formation of ZnO NPs. The ZnO NPs show a photoluminescence (PL) peak at 3.32 eV under pulsed nitrogen-laser excitation at 3.68 eV. Annealing at 900 C induces an additional shift of the absorption edge to 5.3 eV. The additional shift indicates the formation of a Zn2SiO4 phase which was confirmed by GXRD. 2005 Elsevier B.V. All rights reserved. PACS: 81.16.Pr; 78.67.Bf; 61.46.+w; 68.60.Dv Keywords: Nanoparticle; ZnO; Ion implantation; Thermal oxidation; Zn2SiO4; PL
1. Introduction Nanometer-size zinc-oxide (ZnO) draws much attention, because of possible applications to optoelectronic devices [1,2]. Up to now, we have fabricated metal-oxide nanoparticles (NPs), such as NiO [3] and CuO [4], in silica glasses (SiO2) using metal-ion implantation and following thermal oxidation. In this study, the method is applied to the fabrication of ZnO NPs in SiO2. Previously, Chen et al. [5] carried out Zn ion implantation of 160 keV to SiO2 up to 1.0 · 1017 ions/cm2 and following annealing at 700 C under oxidizing atmosphere, but they could not obtain clear evidence of ZnO formation. Liu et al. [6] employed a dose of 3.0 · 1017 ions/cm2 at the same energy 160 keV, and observed X-ray diffraction patterns of ZnO and UV photoluminescence (PL) after annealing at 700 C. However,
*
Corresponding author. Tel.: +81 29 863 5479; fax: +81 29 863 5599. E-mail address: [email protected] (H. Amekura).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.019
from X-ray photoelectron spectroscopy (XPS) measurements, they assumed that ZnO NPs were formed at the substrate surface, or in the much shallower region of the SiO2 substrate than expected from the implanted depth profile. On the other hand, the formation processes of NiO NPs in SiO2 have been proposed as follows [7]: Oxygen molecules migrate in SiO2 and react with the Ni NPs when the O2 molecules meet the NPs. Since the oxidation is carried out at the position of the metal NP, the depth profile of NiO NPs is similar to that of Ni NPs [7]. If the assumption of Liu et al. is correct, the formation processes of ZnO NPs are completely different from those of NiO NPs. However, more studies are necessary to assess LiuÕs suggestions. Understanding the formation processes of ZnO NPs is important for applications to optoelectronic devices, e.g. to predict the position (depth) of ZnO NP formation. For this purpose, study of isochronal oxygen-annealing data is valuable. However, past studies [5,6] reported data oxidized at 700 C only. In this paper, as the first step for the whole understanding of the ZnO NP formation
H. Amekura et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 96–99
97
processes, isochronal oxygen-annealing effects in Zn-implanted SiO2 are studied using optical absorption spectroscopy, grazing incidence X-ray diffraction (GXRD) and photoluminescence (PL) spectroscopy. 2. Experimental
3. Results and discussion Fig. 1 shows isochronal annealing effects on the optical absorption spectra of SiO2 sample implanted with Zn+ ions to 1.0 · 1017 ions/cm2. In the as-implanted state, two absorption peaks are observed: a strong and broad one centered at 4.8 eV and a very weak one at 1.2 eV. The former peak was reported in [5] at 5 eV and a similar peak was observed at 4.2 eV in Zn-implanted MgO [11]. Both peaks (5 eV peak in Zn:SiO2 and 4.2 eV peak in Zn:MgO) were ascribed to the surface plasmon resonance (SPR) of Zn NPs. However, the assignments are questionable, because they are based on a simplified formula [12]. Details will be discussed elsewhere. To our knowledge, this
Fig. 1. Optical absorption spectra of silica glass implanted with Zn+ ions of 60 keV up to 1.0 · 1017 ions/cm2, in as-implanted state and annealed in oxygen gas flow at 600, 700, 800 and 900 C for 1 h each, respectively. The spectra at 700 C and higher temperatures are magnified by factors shown.
is the first observation of the weak peak at 1.2 eV. Since this peak is predicted from Mie theory using the refractive index data of bulk Zn [13], the peak is not due to impurities or defects, but of intrinsic nature. GXRD spectrum of the as-implanted sample is shown in Fig. 2. All the diffraction peaks except a broad one from amorphous SiO2 substrate are fitted by a diffraction pattern of Zn metal. These observations indicate that Zn NPs are formed in SiO2 even in the as-implanted state. Up to 600 C, the optical spectrum shows little change. After 700 C annealing, drastic changes are induced. The absorption in the visible region disappears and an absorption edge appears at 3.25 eV. GXRD clearly shows a transformation in the diffraction patterns from Zn metal to ZnO at 700 C, as shown in Fig. 2. As the absorption in the visible region is almost zero, most of Zn NPs transform
S
DIFFRACTION YIELD (a.u.)
Optical-grade silica glasses of KU-1 type (OH 820 ppm) were implanted with Zn+ ions of 60 keV up to a fluence of 1.0 · 1017 ions/cm2. Peak concentration of Zn atoms in SiO2 calculated by TRIDYN (including sputtering effect) [8] and TRIM (neglecting sputtering effect) [9] codes were 25 and 35 at.%, respectively. Since the oxidation processes are governed by diffusional migration of O2 molecules in the substrate, a lower energy such as 60 keV, i.e. a thinner implanted layer, is better for a homogenous oxidation than the implantation energy of 160 keV which was used in past studies [5,6]. The ion flux was kept to less than 2 lA/cm2 to control the sample temperature to less than 100 C during the implantation. Isochronal annealing was carried out from 400 to 900 C with 100 C steps for 1 h each, in a tube furnace under O2 gas flow of 100 sccm. A dual-beam spectrometer with a resolution of 1 nm was used for the transmittance and reflectance measurements in the wavelength range of 190–1700 nm at room temperature (RT). Optical absorption was obtained from the transmittance and the reflectance with correction of the multiple reflection in samples [10]. PL was excited by the 337 nm line (3.68 eV) from a pulsed nitrogen laser (the pulse width of 0.5 ns). The spectra were detected by a monochromator and a streak camera, and were plotted after integration of the signal along the delay time between 0 and 30 ns for band-edge emission and 0–820 ns for sub-gap emission. Rutherford backscattering spectrometry (RBS) was carried out to determine the Zn content and the depth profile both in the as-implanted state and after oxidation, using a 2.06 MeV He+ beam of 1 mm in diameter with a scattering angle of 160. GXRD measurements were performed with an incident angle of 3 using a Cr X-ray source to confirm the transformation of Zn-related NPs (Zn, ZnO and Zn2SiO4).
2
S
+
60 keV Zn => SiO2
S
1.0x1017 ions/cm2 S : Zn2SiO4 O : ZnO m : Zn
S
S SSS O O
1
O
O
SS
S SS
900
O OO
O
O
O
O O
O
O
O O
Substrate
0
800 700
m m m
50
SS
O2 O2 O2
m
m
as-impl.
100
SCATTERING ANGLE 2
150 (deg.)
Fig. 2. Grazing incidence X-ray diffraction (GXRD) spectra of silica glass implanted with Zn+ ions of 60 keV up to 1.0 · 1017 ions/cm2, in asimplanted state and annealed in oxygen gas flow at 700, 800 and 900 C for 1 h each, respectively. Diffraction peaks labeled by ‘‘m’’, ‘‘O’’ and ‘‘S’’ are ascribed to Zn, ZnO and Zn2SiO4, respectively.
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H. Amekura et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 96–99
to ZnO NPs. Using the Scherrer formula [14], the mean size of ZnO NPs was estimated to be 26 nm from the width of the ZnO(0 0 2) diffraction peak. Only a weak quantum size effect is expected, since the exciton Bohr radius is 1.8 nm [15], i.e. much smaller than the NP diameter. It is consistent with the observed absorption edge at 3.25 eV, because the band gap energy and the exciton binding energy of bulk ZnO at RT are 3.37 eV and 0.060 eV, respectively [15]. It should be noted that a small kink appears at 3.3 eV even after 600 C annealing. A small portion of Zn NPs starts the transformation to ZnO NPs even at 600 C. After the transformation to ZnO NPs, the optical absorption decreases to 1/4 of that of Zn NPs. However, RBS results show that Zn content is almost constant up to 900 C, although the depth profile changes. The decrease of optical absorption after the transformation to ZnO NPs is primarily due to decrease of the oscillator strength in the observed energy region, not due to decrease of Zn content in the sample. It should be noted that the melting points (MP) of bulk Zn is as low as 419.6 C, although that of ZnO is 1970 C [15]. The ZnO NPs may be formed by oxidation of the melting Zn NPs at 700 C. However, in some cases, implanted NPs are subject to high pressures from the surrounding material, which may stabilize the solid phase of the NPs even above the MP for bulk material [16]. More studies are needed to confirm this assumption. After 800 C annealing, the absorption due to ZnO NPs slightly decreases. After 900 C annealing, the absorption edge at 3.25 eV completely disappears, and the edge shifts to 5.3 eV. The new absorption edge of 5.3 eV is close to a literature value of the band gap energy of Zn2SiO4 [17]. The GXRD measurements confirm the transformation of the diffraction patterns from ZnO to Zn2SiO4 at 900 C, as shown in Fig. 2. Isochronal oxygen-annealing effects on PL spectra are shown in Fig. 3. For comparison, the optical absorption
PL INTENSITY (a.u.)
O2 anneal.
Laser exc.
t < 30 ns
t < 820 ns
4. Conclusions Zn-ion implantation followed by thermal oxidation was used to fabricate ZnO NPs in SiO2. A SiO2 substrate was implanted with Zn+ ions of 60 keV up to 1.0 · 1017 ions/ cm2, and was annealed in O2 gas at elevated temperatures. After annealing at 700 C for 1 h, the ZnO NPs which show absorption edge at 3.25 eV are formed in SiO2. Under pulsed nitrogen-laser excitation at 3.68 eV, the ZnO NPs show a PL peak at 3.32 eV. The PL intensity slightly decreases after 800 C annealing. Annealing at 900 C converts the ZnO NPs to the Zn2SiO4 phase which shows an absorption edge at 5.3 eV. Acknowledgements The authors acknowledge support from Dr. G. Kido (NIMS) for visit of H.A. in FZ-Juelich. A part of this study was financially supported by the Budget for Nuclear Research of the MEXT, based on the screening and counseling by the Atomic Energy Commission.
+
60 keV Zn => SiO2
spectrum after 700 C annealing is also shown in Fig. 3. The 3.68 eV line from the nitrogen laser, which is 0.4 eV higher than the absorption edge of ZnO NPs, excites a PL peak at 3.32 eV. Since the peak energy is almost the same as the energy of the absorption edge, the peak is ascribed to the PL from ZnO NPs in SiO2. It should be noted that the signal above 3.4 eV is due to stray light from excitation laser, because the signal appears even without samples. Annealing at 800 C slightly decreases the 3.32 eV PL peak. The decrease may be consistent with the decrease of the optical absorption after 800 C annealing shown in Fig. 1. After 800 C annealing, sub-gap PL signal centered at 2.4 eV, probably due to defects in ZnO NPs, increases. It may indicate that small fractions of ZnO NPs transform to Zn2SiO4 even at 800 C. Such perturbation may increase the sub-gap PL signal at 2.4 eV and decreases the exciton PL at 3.32 eV. It should be noted that clearer exciton peak was observed in PL experiments under CW-mode He–Cd laser excitation [18].
700
References 800 900
0 absorption
2
3 PHOTON ENERGY (eV)
4
Fig. 3. Photoluminescence (PL) spectra of silica glass implanted with Zn+ ions of 60 keV to 1.0 · 1017 ions/cm2 and annealed in oxygen gas flow at 700, 800 and 900 C for 1 h each. The optical absorption spectrum after 700 C annealing is also shown. Signal above 3.4 eV is due to stray light from N2 laser excitation at 3.68 eV. A peak at 3.32 eV is ascribed to exciton PL from ZnO NPs in SiO2.
[1] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459. [2] E.M. Wong, P.C. Searson, Appl. Phys. Lett. 74 (1999) 2939. [3] H. Amekura, N. Umeda, Y. Takeda, J. Lu, N. Kishimoto, Appl. Phys. Lett. 85 (2004) 1015. [4] H. Amekura, Y. Takeda, K. Kono, H. Kitazawa, N. Kishimoto, Rev. Adv. Mater. Sci. 5 (2003) 178. [5] J. Chen, R. Mu, A. Ueda, M.H. Wu, Y.-S. Tung, Z. Gu, D.O. Henderson, C.W. White, J.D. Budai, R.A. Zuhr, J. Vac. Sci. Technol. A16 (1998) 1409. [6] Y.X. Liu, Y.C. Liu, D.Z. Shen, G.Z. Zhong, X.W. Fan, X.G. Kong, R. Mu, D.O. Henderson, Solid State Commun. 121 (2002) 531. [7] H. Amekura, N. Umeda, Y. Takeda, J. Lu, K. Kono, N. Kishimoto, Nucl. Instr. and Meth. B 230 (2005) 193. [8] W. Moeller, W. Eckstein, Nucl. Instr. and Meth. B 2 (1984) 814.
H. Amekura et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 96–99 [9] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985 (Chapter 8). Available from:
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