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Growth of ZnO quantum dots on Si nano ripples
Materials Letters 65 (2011) 1615–1617
Contents lists available at ScienceDirect
Materials Letters 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 / m a t l e t
Growth of ZnO quantum dots on Si nano ripples Liang-Chiun Chao ⁎, Yao-Kai Li, Wan-Chun Chang Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
a r t i c l e
i n f o
Article history: Received 26 November 2010 Accepted 8 March 2011 Available online 11 March 2011 Keywords: Zinc oxide Quantum dot Sputtering
a b s t r a c t Substrates with nano-scale ripples are excellent templates for the deposition of semiconductor nanostructures. We have prepared quasi periodical nano-scale ripples on Si (100) substrates with spatial wavelength λ from ~70 to ~150 nm by ion beam sputtering. ZnO QDs with diameters from 17 to ~30 nm and heights from 2 to ~4 nm have been successfully deposited by reactive ion beam sputter deposition. The QD size and distribution were found to be dependent both on growth conditions and spatial wavelength of the nano-scale ripple. On substrates with λ ~ 150 nm, ZnO QDs were distributed evenly across the wafer, while on substrates with λ ~ 70 nm, ZnO QDs were preferentially located along the crest of the nano-scale ripples. As the QD height decreases from ~4 to 2 nm, room temperature photoluminescence UV emission energy blue shifts by 80 meV. Possible sources of the blue shift are presented. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Recently ZnO has attracted significant attention in optoelectronic device applications [1]. The band gap of ZnO at room temperature is 3.37 eV and its exciton binding energy is 60 meV that efficient UV light emitting devices may be fabricated. As the ZnO dimension becomes compatible to the Bohr exciton diameter, quantum confinement effect becomes eminent that causes increase of band gap, increase of exciton binding energy, increase of exciton–light coupling strength [2] and decrease of exciton–longitudinal optical phonon coupling [3]. Growth of ZnO QDs has been prepared by RF magnetron sputtering [4], sol–gel process [3], metalorganic chemical vapor deposition [5], and pulsed laser ablation [6] that ZnO QD was distributed in a self-assembled fashion or embedded in glassy matrices. In order to control the distribution of ZnO nanostructures, it has been shown that ZnO nanorods can be selectively deposited on FIB patterned nanostructures [7]. An alternative to FIB nano-patterning is utilizing broad ion beam sputtering to form nano-scale ripples. Formation of nano-scale ripple by ion beam sputtering was first observed more than a half century ago [8]. Bradley and Harper successfully described the formation and orientation of ripples by finding curvature dependent sputtering rates [9]. The orientation and spatial wavelength λ of the nano-scale ripple depends on ion beam energy, substrate temperatures and ion beam incident angles [10–12]. In this letter, we report the growth of ZnO QDs on ion beam textured Si substrates with spatial wavelength λ at ~70 and ~150 nm.
Nano-scale ripples on Si substrates were prepared by Ar ion beam sputtering utilizing a capillary gas field ion source [13] at 8 keV. Nano-scale ripples with spatial wavelength from ~ 70 to ~ 150 nm were prepared by adjusting ion beam incident angles. ZnO QD was deposited by reactive ion beam sputter deposition utilizing a metallic zinc target (99.99%) at 200–300 °C in a multi-target ion beam sputter system [13]. The base pressure of the ion beam sputter system was 4 × 10− 6 Torr and the substrate to target distance was 40 mm. The flow rate of argon/oxygen was fixed at 5:3 and the pressure during reactive deposition was 1 mTorr.
⁎ Corresponding author. Tel.: +886 227376369; fax: +886 227376424. E-mail address: [email protected] (L.-C. Chao). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.03.027
3. Results and discussion Fig. 1(a) shows a SEM micrograph of ZnO QDs deposited on Si nano-scale ripples with spatial wavelength λ ~ 150 nm at 250 °C for 20 min. ZnO QDs were found distributed evenly across the substrate, while no QDs were found along the crest of the nano ripple. However, under the same growth condition, ZnO QDs were found preferentially located along the crest of the nano ripples on substrates with spatial wavelength λ ~ 70 nm (Fig. 1(b)). The dependence of QD distribution on spatial wavelength suggests that multi-atomic steps [14] on the Si substrates act as nucleation site that facilitates the formation of ZnO QDs. The diameter and height of ZnO QDs were dependent on growth temperatures. Fig. 2(a) shows that ZnO QDs deposited at 250 °C exhibits the smallest diameter of 23.0 nm, while increasing or decreasing the substrate temperature both results in larger sizes of ZnO QDs. The height of ZnO QD decreases slightly from 2.4 to 1.7 nm as substrate temperature increases from 200 to 300 °C (Fig. 2(b)). Fig. 3 shows the PL properties of ZnO QDs deposited at 250 °C for 15–30 min on
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L.-C. Chao et al. / Materials Letters 65 (2011) 1615–1617
Fig. 1. SEM micrographs of ZnO QDs deposited on Si nano-scale ripples at 250 °C for 20 min with spatial wavelength λ ~ 150 nm (a), and λ ~ 70 nm (b).
substrates with λ ~ 150 nm. For PL measurements, samples were capped with an additional layer of 40 nm SiO2 in situ at room temperature without breaking the vacuum and the emission spectra are shown in Fig. 3(a). The UV emission spectra are fitted with two peaks P1 and P2. Fitting results show that the peak position of P1 remains relatively unchanged at 3.22 eV (385 nm) while P2 increases from 3.33 to 3.41 eV as deposition time decreases from 30 to 15 min. The emission peak P1 is the commonly observed UV peak position of bulk ZnO which is due to donor bound exciton. Possible sources that may cause the blue shift of P2 includes the presence of covalent bond
between ZnO and SiO2 [15], Burstein–Moss effect, strain, surface effects and size dependent quantum confinement effect. As SiO2 was deposited under identical conditions for all samples, this excludes the possibility of covalent bond. To determine the stoichiometry of the ZnO QDs, 100 nm ZnO thin films were also deposited under identical conditions on quartz substrates. XPS analysis indicates that the atomic ratio of Zn/O is ~ 55/45, indicating that ZnO QD is also likely to be zinc rich. The as-deposited ZnO film is semi-insulting, while exposure to
Fig. 2. Diameter (a) and height (b) of ZnO QDs deposited at 200–300 °C, all for 20 min on Si substrates with nano-scale ripples, λ ~ 150 nm.
Fig. 3. (a) PL properties of ZnO QDs. (b) P2 peak position variation due to QD heights. ZnO QDs were deposited on λ ~ 150 nm substrates from 15 to 30 min at 250 °C.
L.-C. Chao et al. / Materials Letters 65 (2011) 1615–1617
air for several days causes the ZnO film to exhibit n-type conductivity. Since SiO2 capping layer was deposited in situ without breaking the vacuum, this indicates that the blue shift of the UV emission peak is not likely due to Burstein–Moss effect. The QD size has an ellipsoidal shape that exciton or individual particles experience strong confinement in the vertical direction [16,17] while weak or no confinement along the horizontal direction. The blue shift of P2 as QD height decreases shows a similar trend as that of ZnO quantum wells [18]. It is thus possible that the P2 is due to confined excitons that ZnO QDs act as quantum wells with a fractional dimension αf ~ 2 [19].
Acknowledgement This research work was supported by the National Science Council of Republic of China under contract number NSC 98-2112-M-011-001. References [1] [2] [3] [4] [5] [6]
4. Conclusion In summary, we have successfully deposited ZnO QDs at 250 °C on Si nano ripples. The size and distribution of the ZnO QDs are dependent on the spatial wavelength of the nano-scale ripples, substrate temperatures and deposition time. Optimized growth condition results ZnO QD with sizes from 17.0 to 31.1 nm. The UV emission peak position of ZnO QD blue shifts by 80 meV as QD height decreases to 2 nm, likely due to strain or quantum confinement effects. These results demonstrate that ion beam textured substrates may be utilized for catalyst-free deposition of ZnO QDs on rigid substrates with controlled size and density distribution which is of great valuable for single photon emitter applications.
1617
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Klingshirn C. Phys Status Solidi B 2007;244:3027–73. Gil B, Kavokin AV. Appl Phys Lett 2002;81:748–50. Hsu WT, Lin KF, Hsieh WF. Appl Phys Lett 2007;91:181913-1-3. Kim KK, Koguchi N, Ok YW, Seong TY, Park SJ. Appl Phys Lett 2004;84:3810–2. Lu JG, Ye ZZ, Zhang YZ, Liang QL, Fujita S, Wang ZL. Appl Phys Lett 2006;89: 023122-1-3. Suzuki K, Kondo H, Inoguchi M, Tanaka N, Kageyama K, Takagi H. Appl Phys Lett 2009;94:223103-1-3. Kim SW, Kotani T, Ueda M, Fujita S, Fujita S. Physica E 2004;21:601–5. Valbusa U, Boragno C, Mongeot FB. J Phys Condens Matter 2002;14:8153–75. Bradley RM, Harper JME. J Vac Sci Technol A 1988;6:2390–5. Chason E, Mayer TM, Kellerman BK, Mcllroy DT, Howard AJ. Phys Rev Lett 1994;72:3040–3. Habenicht S, Bolse W, Lieb KP, Reimann K, Geyer U. Phys Rev B 1999;60:R2200–3. Chini TK, Sanyal MK, Bhattacharyya SR. Phys Rev B 2002;66:153404. Chao LC, Lin SJ, Chang WC. Nucl Instrum Methods B 2010;268:1581–4. Kitamura M, Nishioka M, Oshinowo J, Arakawa Y. Appl Phys Lett 1995;66:3663–5. Yang J, Cao J, Yang L, Zhang Y, Wang Y, Liu X, et al. J Appl Phys 2010;108:0443041-7. Brus LE. J Chem Phys 1984;80:4403–9. Kayanuma Y. Phys Rev B 1988;38:9797–805. Gruber Th, Kirchner C, Kling R, Reuss F, Waag A. Appl Phys Lett 2004;84:5359–61. Nakamura A, Okamatsu K, Tawara T, Gotoh H, Temmyo J, Matsui Y. Jpn J Appl Phys 2008;47:3007–9.
Contents lists available at ScienceDirect
Materials Letters 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 / m a t l e t
Growth of ZnO quantum dots on Si nano ripples Liang-Chiun Chao ⁎, Yao-Kai Li, Wan-Chun Chang Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
a r t i c l e
i n f o
Article history: Received 26 November 2010 Accepted 8 March 2011 Available online 11 March 2011 Keywords: Zinc oxide Quantum dot Sputtering
a b s t r a c t Substrates with nano-scale ripples are excellent templates for the deposition of semiconductor nanostructures. We have prepared quasi periodical nano-scale ripples on Si (100) substrates with spatial wavelength λ from ~70 to ~150 nm by ion beam sputtering. ZnO QDs with diameters from 17 to ~30 nm and heights from 2 to ~4 nm have been successfully deposited by reactive ion beam sputter deposition. The QD size and distribution were found to be dependent both on growth conditions and spatial wavelength of the nano-scale ripple. On substrates with λ ~ 150 nm, ZnO QDs were distributed evenly across the wafer, while on substrates with λ ~ 70 nm, ZnO QDs were preferentially located along the crest of the nano-scale ripples. As the QD height decreases from ~4 to 2 nm, room temperature photoluminescence UV emission energy blue shifts by 80 meV. Possible sources of the blue shift are presented. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Recently ZnO has attracted significant attention in optoelectronic device applications [1]. The band gap of ZnO at room temperature is 3.37 eV and its exciton binding energy is 60 meV that efficient UV light emitting devices may be fabricated. As the ZnO dimension becomes compatible to the Bohr exciton diameter, quantum confinement effect becomes eminent that causes increase of band gap, increase of exciton binding energy, increase of exciton–light coupling strength [2] and decrease of exciton–longitudinal optical phonon coupling [3]. Growth of ZnO QDs has been prepared by RF magnetron sputtering [4], sol–gel process [3], metalorganic chemical vapor deposition [5], and pulsed laser ablation [6] that ZnO QD was distributed in a self-assembled fashion or embedded in glassy matrices. In order to control the distribution of ZnO nanostructures, it has been shown that ZnO nanorods can be selectively deposited on FIB patterned nanostructures [7]. An alternative to FIB nano-patterning is utilizing broad ion beam sputtering to form nano-scale ripples. Formation of nano-scale ripple by ion beam sputtering was first observed more than a half century ago [8]. Bradley and Harper successfully described the formation and orientation of ripples by finding curvature dependent sputtering rates [9]. The orientation and spatial wavelength λ of the nano-scale ripple depends on ion beam energy, substrate temperatures and ion beam incident angles [10–12]. In this letter, we report the growth of ZnO QDs on ion beam textured Si substrates with spatial wavelength λ at ~70 and ~150 nm.
Nano-scale ripples on Si substrates were prepared by Ar ion beam sputtering utilizing a capillary gas field ion source [13] at 8 keV. Nano-scale ripples with spatial wavelength from ~ 70 to ~ 150 nm were prepared by adjusting ion beam incident angles. ZnO QD was deposited by reactive ion beam sputter deposition utilizing a metallic zinc target (99.99%) at 200–300 °C in a multi-target ion beam sputter system [13]. The base pressure of the ion beam sputter system was 4 × 10− 6 Torr and the substrate to target distance was 40 mm. The flow rate of argon/oxygen was fixed at 5:3 and the pressure during reactive deposition was 1 mTorr.
⁎ Corresponding author. Tel.: +886 227376369; fax: +886 227376424. E-mail address: [email protected] (L.-C. Chao). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.03.027
3. Results and discussion Fig. 1(a) shows a SEM micrograph of ZnO QDs deposited on Si nano-scale ripples with spatial wavelength λ ~ 150 nm at 250 °C for 20 min. ZnO QDs were found distributed evenly across the substrate, while no QDs were found along the crest of the nano ripple. However, under the same growth condition, ZnO QDs were found preferentially located along the crest of the nano ripples on substrates with spatial wavelength λ ~ 70 nm (Fig. 1(b)). The dependence of QD distribution on spatial wavelength suggests that multi-atomic steps [14] on the Si substrates act as nucleation site that facilitates the formation of ZnO QDs. The diameter and height of ZnO QDs were dependent on growth temperatures. Fig. 2(a) shows that ZnO QDs deposited at 250 °C exhibits the smallest diameter of 23.0 nm, while increasing or decreasing the substrate temperature both results in larger sizes of ZnO QDs. The height of ZnO QD decreases slightly from 2.4 to 1.7 nm as substrate temperature increases from 200 to 300 °C (Fig. 2(b)). Fig. 3 shows the PL properties of ZnO QDs deposited at 250 °C for 15–30 min on
1616
L.-C. Chao et al. / Materials Letters 65 (2011) 1615–1617
Fig. 1. SEM micrographs of ZnO QDs deposited on Si nano-scale ripples at 250 °C for 20 min with spatial wavelength λ ~ 150 nm (a), and λ ~ 70 nm (b).
substrates with λ ~ 150 nm. For PL measurements, samples were capped with an additional layer of 40 nm SiO2 in situ at room temperature without breaking the vacuum and the emission spectra are shown in Fig. 3(a). The UV emission spectra are fitted with two peaks P1 and P2. Fitting results show that the peak position of P1 remains relatively unchanged at 3.22 eV (385 nm) while P2 increases from 3.33 to 3.41 eV as deposition time decreases from 30 to 15 min. The emission peak P1 is the commonly observed UV peak position of bulk ZnO which is due to donor bound exciton. Possible sources that may cause the blue shift of P2 includes the presence of covalent bond
between ZnO and SiO2 [15], Burstein–Moss effect, strain, surface effects and size dependent quantum confinement effect. As SiO2 was deposited under identical conditions for all samples, this excludes the possibility of covalent bond. To determine the stoichiometry of the ZnO QDs, 100 nm ZnO thin films were also deposited under identical conditions on quartz substrates. XPS analysis indicates that the atomic ratio of Zn/O is ~ 55/45, indicating that ZnO QD is also likely to be zinc rich. The as-deposited ZnO film is semi-insulting, while exposure to
Fig. 2. Diameter (a) and height (b) of ZnO QDs deposited at 200–300 °C, all for 20 min on Si substrates with nano-scale ripples, λ ~ 150 nm.
Fig. 3. (a) PL properties of ZnO QDs. (b) P2 peak position variation due to QD heights. ZnO QDs were deposited on λ ~ 150 nm substrates from 15 to 30 min at 250 °C.
L.-C. Chao et al. / Materials Letters 65 (2011) 1615–1617
air for several days causes the ZnO film to exhibit n-type conductivity. Since SiO2 capping layer was deposited in situ without breaking the vacuum, this indicates that the blue shift of the UV emission peak is not likely due to Burstein–Moss effect. The QD size has an ellipsoidal shape that exciton or individual particles experience strong confinement in the vertical direction [16,17] while weak or no confinement along the horizontal direction. The blue shift of P2 as QD height decreases shows a similar trend as that of ZnO quantum wells [18]. It is thus possible that the P2 is due to confined excitons that ZnO QDs act as quantum wells with a fractional dimension αf ~ 2 [19].
Acknowledgement This research work was supported by the National Science Council of Republic of China under contract number NSC 98-2112-M-011-001. References [1] [2] [3] [4] [5] [6]
4. Conclusion In summary, we have successfully deposited ZnO QDs at 250 °C on Si nano ripples. The size and distribution of the ZnO QDs are dependent on the spatial wavelength of the nano-scale ripples, substrate temperatures and deposition time. Optimized growth condition results ZnO QD with sizes from 17.0 to 31.1 nm. The UV emission peak position of ZnO QD blue shifts by 80 meV as QD height decreases to 2 nm, likely due to strain or quantum confinement effects. These results demonstrate that ion beam textured substrates may be utilized for catalyst-free deposition of ZnO QDs on rigid substrates with controlled size and density distribution which is of great valuable for single photon emitter applications.
1617
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Klingshirn C. Phys Status Solidi B 2007;244:3027–73. Gil B, Kavokin AV. Appl Phys Lett 2002;81:748–50. Hsu WT, Lin KF, Hsieh WF. Appl Phys Lett 2007;91:181913-1-3. Kim KK, Koguchi N, Ok YW, Seong TY, Park SJ. Appl Phys Lett 2004;84:3810–2. Lu JG, Ye ZZ, Zhang YZ, Liang QL, Fujita S, Wang ZL. Appl Phys Lett 2006;89: 023122-1-3. Suzuki K, Kondo H, Inoguchi M, Tanaka N, Kageyama K, Takagi H. Appl Phys Lett 2009;94:223103-1-3. Kim SW, Kotani T, Ueda M, Fujita S, Fujita S. Physica E 2004;21:601–5. Valbusa U, Boragno C, Mongeot FB. J Phys Condens Matter 2002;14:8153–75. Bradley RM, Harper JME. J Vac Sci Technol A 1988;6:2390–5. Chason E, Mayer TM, Kellerman BK, Mcllroy DT, Howard AJ. Phys Rev Lett 1994;72:3040–3. Habenicht S, Bolse W, Lieb KP, Reimann K, Geyer U. Phys Rev B 1999;60:R2200–3. Chini TK, Sanyal MK, Bhattacharyya SR. Phys Rev B 2002;66:153404. Chao LC, Lin SJ, Chang WC. Nucl Instrum Methods B 2010;268:1581–4. Kitamura M, Nishioka M, Oshinowo J, Arakawa Y. Appl Phys Lett 1995;66:3663–5. Yang J, Cao J, Yang L, Zhang Y, Wang Y, Liu X, et al. J Appl Phys 2010;108:0443041-7. Brus LE. J Chem Phys 1984;80:4403–9. Kayanuma Y. Phys Rev B 1988;38:9797–805. Gruber Th, Kirchner C, Kling R, Reuss F, Waag A. Appl Phys Lett 2004;84:5359–61. Nakamura A, Okamatsu K, Tawara T, Gotoh H, Temmyo J, Matsui Y. Jpn J Appl Phys 2008;47:3007–9.