© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

p s scurrent topics in solid state physics

c

sta

tus

so

lid

i

www.pss-c.comph

ysic

aPhys. Status Solidi C 6, No. 4, 894–897 (2009) / DOI 10.1002/pssc.200880656

Cathodoluminescence of ZnSiOx nanocomposite films prepared on Si substrates

Young-Hwan Kim*, Woon-Jo Cho, and Seong-Il Kim

Nano-Devices Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea

Received 6 May 2008, revised 29 May 2008, accepted 28 October 2008

Published online 22 December 2008

PACS 61.46.Hk, 78.60.Hk, 81.05.Dz, 81.15.Cd, 81.40.Gh

* Corresponding author: e-mail youngkim@kist.re.kr, Phone: +82 2 958 5722, Fax: +82 2 958 5739

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Recently various materials with low-dimensional nanostructures, such as nanowire, nanocrystal (or quantum dots), have been widely investigated due to their potential applications for electronic and/or optoelec-tronic devices. ZnO is a II-VI compound semiconductor with a direct band gap structure which is favorable to emit the light. The magnitude of the band gap is ~3.36 eV simi-lar to that of GaN. The exciton binding energy of ZnO is 60 meV, which is large enough to stably emit the light at room temperature. In addition, ZnO has advantages of lower cost and lower processing temperature. So, ZnO has been intensively investigated as a promising alternative to GaN. ZnO has been synthesized in various forms such as single crystal, powder, and thin film. In addition, low-dimensional nanostructures (for example; nanowires, nanocrystals, etc) are widely studied in order to use their unique properties which are different from bulk properties due to the high specific surface area and small size [1-4]. Since nanoparticles have a high surface-to-volume ra-tio, many of their optoelectronic characteristics are related to the nature of the surface and are strongly dependent on

the nature of the host’s matrix as well, leading to offering an effective method to control optoelectronic properties of nanoparticles [5]. Sol-gel derived SiO2 has been exten-sively used as a matrix for synthesizing various nanoparti-cles and SiO2 is transparent in the visible region. Therefore, many researches on ZnO-SiO2 nanocomposites were re-ported in recent years [5-9]. ZnO-SiO2 nanocomposites were fabricated mostly by sol-gel method. However, for their applications to silicon-based optoelectronic devices, it is favorable to fabricate ZnO-SiO2 nanocomposites directly on Si substrates. A few research results about it have been reported [10]; however, photoluminescence (PL) was only measured and discussed in those reports. Therefore, in this work, we attempted to fabricate ZnO-SiO2 nanocomposite films on Si substrates by sputtering ZnO and Si targets simultaneously and to in-vestigate their microstructures and optical properties by us-ing high resolution transmission electron microscopy (HRTEM) and cathodoluminescence (CL).

Optoelectronic materials compatible with silicon substrates

are important from the viewpoint of practical applications for

silicon photonics. ZnO nanocrystals were fabricated directly

on Si substrates by tuning both of the composition and

postannealing conditions of ZnSiOx composite films depos-

ited by sputtering Si and ZnO targets simultaneously. The

high-resolution TEM analysis showed that ZnSiOx nanocom-

posite film containing ZnO nanocrystals could be obtained by

postannealing the ZnSiOx film with Zn/Si ratio = ~1.6 at

700 °C. In this film, ZnO nanocrystals with a size of ~5 nm,

which could be identified as ZnO from electron diffraction

pattern, were densely distributed in the amorphous matrix.

This film exhibited a stronger and broader cathodolumines-

cence emission ranging from 350 nm to 550 nm compared to

that of ZnO film fabricated with the same deposition condi-

tions. These results lead to a practical application of ZnO

nanocrystals to optoelectronic devices using Si substrates.

Phys. Status Solidi (c) 6, No. 4 (2009) 895

www.pss-c.com © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Contributed

Article

2 Experiment ZnSiOx nanocomposite films were de-posited on Si (100) substrates by rf magnetron sputtering of Si and ZnO targets simultaneously. ZnO targets with a diameter of ~10 mm were placed on Si target with a di-ameter of 2 inches. The composition of ZnSiOx nanocom-posite films was controlled by changing area ratio of Si target to ZnO target, that is, changing the number of ZnO target placed on Si target. In this experiment, 2~8 of ZnO target were placed on the Si target to obtain the ZnSiOx films with various compositions. The deposition conditions of the films are as follows; sputtering pressure was sus-tained at ~1.33 Pa by using argon and oxygen gases with a ratio of Ar:O2=1:1. The sputtering power was 100 Watt and the substrate was not heated intentionally. The deposi-tion time was 1 hour and the final film thickness was found to depend on the area ratio of Si and ZnO targets. The compositions of as-deposited ZnSiOx films were measured by using electron dispersive spectroscopy (EDS) equipped with HRTEM. The as-deposited films were post-annealed by using rapid thermal annealing (RTA) system at the temperatures ranging from 500 to 900 oC for 3 min-utes in a nitrogen atmosphere. And then the structural and optical properties were investigated. The microstructures of the films were observed by HRTEM to identify the for-mation of ZnO nanocrystals in the ZnSiOx nanocomposite films. Cathodoluminescence were measured with an accel-eration voltage of electron beam of 6 kV at room tempera-ture. 3 Results and discussion As a result of EDS meas-urement of the compositions of ZnSiOx films, as was ex-pected, the amount of Zn in the film increased with in-creasing the number of ZnO target placed on the Si target, that is, increasing the area ratio of Zn to Si target as shown in Fig. 1. The Zn/Si ratios of ZnSiOx films range from ~0.4

with 2 ZnO to ~11.2 with 8 ZnO. The film thickness, which was measured from cross-sectional image observed by scanning electron microscopy, increased with increas-ing the number of ZnO target placed on Si target as shown in the inset of Fig. 1. The film thicknesses range from ~0.87 µm with 2 ZnO to ~1.62 µm with 8 ZnO. The microstructures of the ZnSiOx films were investi-gated by HRTEM. Figure 2 shows the HRTEM images and selected area electron diffraction (SAED) patterns of the ZnSiOx films. These films were postannealed at 700 oC. The clear difference in microstructure depending on the film composition can be observed. In the case of the film with Zn/Si = ~11.2, it is observed that all part of the film was fully crystallized. From the SAED pattern, this film was found to have ZnO wurtzite structure. On the contrary, the ZnSiOx films with Zn/Si ratios of below 1.6 show very different microstructure from that of the film with Zn/Si = ~11.2. In these films, nanoparticles with a size of ~5 nm are distributed in the SiOx amorphous matrix and density of nanoparticles increases with increasing Zn/Si ratio within this range. From the SAED patterns and HRTEM images of these films, these nanoparticles are found to be crystallized and to have ZnO and Zn2SiO4 phase. From this result, it can be inferred that ZnO nanocrystals surrounded by Zn2SiO4 were formed in the SiOx amorphous matrix. In case of the film with Zn/Si = ~4.0, the nanoparticle net-work structure is developed, that is, all nanoparticles are interconnected with each other as shown in Fig. 2(c). These HRTEM results indicate that some critical value of Zn/Si ratio may exist for the formation of ZnO nanocrys-tals in the SiOx amorphous matrix.

Figure 1 The compositions and thicknesses of the ZnSiOx nano-

composite films depending on the number of ZnO target placed

on Si target.

Figure 2 HRTEM images and SAED patterns of ZnSiOx nano-

composite films depending on Zn/Si ratio: (a) ~0.4, (b) ~1.6, (c)

~4.0, and (d) ~11.2. These films were postannealed at 700 °C.

896 Young-Hwan Kim et al.: Cathodoluminescence of ZnSiOx nanocomposite films

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

ph

ysic

ap s sstat

us

solid

i c

Figure 3 shows CL spectra of the ZnSiOx films depending on the composition, in which the films were an-nealed at 700 oC. As is shown in top panel of Fig. 3, the strong CL emission in UV-blue region is observed from the film with the Zn/Si ratio of ~1.6. In the case that Zn/Si ratio is smaller than 1.6 (that is, Zn/Si = ~0.4), the CL spectrum shows similar trend to that of the film with Zn/Si= ~1.6 although the intensity of CL emission peak is weaker. However, in the case that Zn/Si ratio is larger than ~4.0, quite different CL emission spectra are observed. In these films, CL intensity was greatly decreased and CL emission in the UV-blue regime almost disappeared. Espe-cially, although the CL intensity is very weak, the film with Zn/Si = ~11.2 shows almost the same CL emission characteristics as that of ZnO film with near band edge emission and strong visible emission due to the defects as shown in bottom panel of Fig. 3. These CL emission char-acteristics can be divided into two classes based on the composition of ZnSiOx film. One is the case of the films with Zn/Si ≤ 1.6, in which the CL emission peak in the UV-blue regime exists, and the other is the case of the films with Zn/Si ≥ 4.0, in which the CL emission peak in UV-blue region disappears and CL intensity is very weak. This result may be intimately related to the microstructures

of the ZnSiOx films. As mentioned above, the microstruc-ture of the ZnSiOx film is dependent on the film composi-tion. The microstructures of the films with Zn/Si ≤ 1.6 show that ZnO nanocrystals are embedded in the SiOx amorphous matrix, however, the film with Zn/Si = ~11.2 have ZnO film structure with many structural defects, which is consistent with the CL emission characteristics shown in bottom panel of Fig. 3. From this relationship be-tween the CL emission characteristics and the microstruc-ture, the CL emission peak in the UV-blue regime is thought to be attributed to ZnO nanocrystals embedded in the SiOx amorphous matrix, which is supported by the fact that this UV-blue emission peak increased with increasing the density of ZnO nanocrysals in the amorphous matrix. The ZnSiOx nanocomposite film with Zn/Si = ~1.6, in which ZnO nanocrystals are densely distributed in the SiOx amorphous matrix, shows several CL peaks positioned at 395, 446, 490 and 520 nm indicating that there are various defect in the ZnO nanocrystals. The 395 nm peak is thought to be near band edge emission of ZnO [9], which is a little red-shifted. The other peaks are due to the defects relating to zinc and oxygen [9-11]. More precise emission mechanism is now being analyzed and will be reported elsewhere. Comparing CL emission characteristics of the ZnSiOx nanocomposite film with Zn/Si = ~1.6 with that of ZnO film as shown in Fig. 4, the ZnSiOx nanocomposite film containng ZnO nanocrystals shows very broad CL spec-trum ranging from UV to yellow, which is favorable to fabricate white light emitting devices. 4 Conclusion ZnSiOx composite films with various compositions of Zn/Si = 0.4~11.2 were fabricated on Si substrates by rf sputtering of Si and ZnO targets simulta-neously and postannealed at the temperatures of 400~900 oC. The composition of the film could be controlled by changing the area ratio of Si and ZnO targets. Among these

Figure 4 The CL spectra of the ZnSiOx nanocomposite film

and ZnO film fabricated on Si substrates with the same deposi-

tion conditions. They were annealed at 700 °C.

Figure 3 The CL spectra of the ZnSiOx composite films annealed

at 700 °C depending on the film composition (top panel). The bot-

tom panel shows the CL spectrum of the film with Zn/Si = 5.4.

Phys. Status Solidi (c) 6, No. 4 (2009) 897

www.pss-c.com © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Contributed

Article

films, which were postannealed at 700 oC, we could obtain ZnO nanocrystals embedded in the SiOx amorphous matrix from the film with a composition of Zn/Si = ~1.6. This film showed a strong and broad CL emission characteris-tics ranging from UV to yellow which can be attributed to ZnO nanocrystals. On the contrary, the ZnSiOx film with a larger Zn/Si ratio (for example, Zn/Si = ~11.2) was ob-served to have ZnO film structure with many structural de-fects due to coexistence of Si and showed weaker and al-most the same CL emission spectrum as that of ZnO film. Consequently, ZnO nanocrystals could be fabricated on Si substrate in the form that they were embedded in the amor-phous matrix by adjusting both composition and post-annealing conditions of ZnSiOx composite film, leading a possible application of ZnO nanocrystals to the Si-based optoelectronic devices by using their strong and broad CL emission characteristics ranging from UV to yellow.

Acknowledgements This work was supported by the Ko-

rea Institute of Science and Technology under Contract No.

2E19520.

References [1] G. Zou, D. Yu, D. Wang, W. Zhang, L. Xu, W. Yu, and Y.

Qian, Mater. Chem. Phys. 88, 150 (2004).

[2] Z. Tian, J. A. Voigt, J. Liu, B. McKenzie, M. J. McDermott,

M. A. Rodriguez, H. Konishi, and H. Xu, Nature Mater. 2,

821 (2003).

[3] Z. Wang, H. Zhang, L. Zhang, J. Yuan, S. Yan, and C.

Wang, Nanotechnology 14, 11 (2003).

[4] M. Shim and P. Guyot-Sionnest, J. Am. Chem. Soc. 123,

11651 (2001).

[5] M. Bouguerra, M. Samah, M. A. Belkhir, A. Chergui, L.

Gerbous, G. Nouet, D. Chateigner, and R. Madelon, Chem.

Phys. Lett. 425, 77 (2006).

[6] H. He, Y. Wang, and Y. Zou, J. Phys. D: Appl. Phys. 36,

2972 (2003).

[7] C. M. Mo, Y. H. Li, Y. S. Liu, Y. Zhang, and L. D. Zhang, J.

Appl. Phys. 83, 4389 (1998).

[8] Mikrajuddin, F. Iskandar, K. Okuyama, and F. G. Shi, J.

Appl. Phys. 89, 6431 (2001).

[9] B. Yao, H. Shi, H. Bi, and L. Zhang, J. Phys.: Condens.

Matter. 12, 6265 (2000).

[10] Y.-Y. Peng, T.-E. Hsieh, and C.-H. Hsu, Nanotechnology

17, 174 (2006).

[11] S. Mahamuni, K. Borgohain, B. S. Bendre, V. J. Leppert,

and S. H. Risbud, J. Appl. Phys. 85, 2861 (1999).