Formation of nano-crystalline Si quantum dots in ZnO thin-films using a ZnO/Si multilayer structure

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Formation of nano-crystalline Si quantum dots in ZnO thin-

ﬁlms using a ZnO/Si

multilayer structure

Kuang-Yang Kuo, Shu-Wei Hsu, Wen-Ling Chuang, Po-Tsung Lee

⁎

Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan

a b s t r a c t

a r t i c l e i n f o

Article history:

Received 16 September 2011 Accepted 4 November 2011 Available online 10 November 2011 Keywords:

Si quantum dot ZnO Sputtering Thin-ﬁlms

In this study, we propose to fabricate ZnO thin-ﬁlms with nano-crystalline Si (nc-Si) quantum dots (QDs) embedded using a ZnO/Si multilayer structure by radio-frequency (RF) magnetron sputtering method. Our re-sults show that a high Si sputtering power (PSi) can assist the formation of self-aggregated Si nano-clusters during deposition, which is helpful for the crystallization of nc-Si QDs and ZnO matrix during annealing. Great crystallinity and highly uniform size of nc-Si QDs are obtained for PSiof 110 W. We experimentally dem-onstrate the formation of nc-Si QDs embedded in crystalline ZnO thin-ﬁlms, which has a great potential for various electro-optical device applications.

1. Introduction

Recently, the nano-crystalline Si (nc-Si) quantum dots (QDs) em-bedded in Si-based dielectric materials like SiO2and Si3N4had been

extensively studied and integrated into various electro-optical devices such as light-emitting diodes (LEDs)[1]and photovoltaics (PVs)[2]. LEDs with multi-color light emission properties using nc-Si QDs had been successfully developed by Lai BH et al.[1], and Cho EC et al. had integrated nc-Si QDs into PVs for the third-generation PVs develop-ment[2]. Therefore, the nc-Si QD indeed has a great potential for a variety of electro-optical applications. In this report, we propose to fabricate nc-Si QDs embedded in ZnO thin-films because ZnO has many suitable features such as wide bandgap, high transparency, and tunable conductivity. In addition, ZnO is a well-known semicon-ductor material and its conductivity can be easily tuned to provide de-sirable electrical properties[3]. Therefore, ZnO can serve as the matrix material for nc-Si QDs for bandgap engineering and efficiently de-crease the light absorption loss from matrix material to enhance the performance of electro-optical devices. Besides, ZnO is also a promis-ing material for spintronic applications because of its ferromagnetic behavior, which depends on the nano-structured characteristics of ZnO[4]. Hence, the optical and magnetic properties originating from the interaction of nc-Si QDs with ZnO matrix could also be an interest-ing and meaninterest-ingful research topic. Undoubtedly, there are many ad-vantages to embed nc-Si QDs in ZnO thin-films, and it's important to demonstrate the possibility on the realization of nc-Si QDs embedded ZnO thin-films. In this study, we fabricate the nc-Si QDs in ZnO

thin-ﬁlms using a ZnO/Si multilayer (ML) deposition structure and a post-annealing process, and investigate the formation mechanism and nano-structured properties.

2. Experiment

The ZnO/Si ML thin-ﬁlms with 24-bilayers are deposited on Si(100) wafers by radio-frequency (RF) magnetron sputtering method. The sputtering power of Si (PSi) is varied from 25 (S25) to 110 W (S110)

while that of ZnO isfixed at 75 W. The effective thicknesses of each ZnO and Si layers arefixed at 5 and 3 nm, respectively. After deposition, the ZnO/Si ML thin-films are annealed by a rapid thermal annealing (RTA) process at 1000 °C for 50 s under N2flow since an annealing

tempera-ture higher than 900 °C is generally needed for the nc-Si QDs formation [5]. The Raman spectra are measured using a 488 nm diode-pumped solid-state laser (Horiba LabRam HR). The X-ray diffraction (XRD) pat-terns are examined by a Bede-D1 X-ray diffractometer with Cu Kα

radi-ation. The surface morphologies are analyzed by a Digital Instrument D3100 atomic force microscope (AFM). The high-resolution transmis-sion electron microscope (HRTEM) images are obtained by a JEOL JEM-2010F transmission electron microscope.

3. Results and discussion

To conﬁrm the nc-Si QDs formation, Raman measurement, a well-known and credible technique for examining the nc-Si properties [6,7], is performed.Fig. 1(a) shows the Raman spectra of the annealed ZnO/Si ML thin-_{ﬁlms under different P}Siand its inset shows the

curve-ﬁtting result of Raman spectrum for sample S110. The curve-ﬁtting curve, which is decomposed into four components with peaks located at 436.1, 480.0, 508.3, and 519.7 cm− 1, shows an excellent match with

Materials Letters 68 (2012) 463–465

⁎ Corresponding author. Tel.: +886 3 5712121x31306; fax: +886 3 5735601. E-mail address:[email protected](P.-T. Lee).

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the measured data. The peaks at 480.0, 508.3, and 519.7 cm− 1are usually observed in the nc-Si QD, and they are contributed from the transverse-optical (TO) modes of Si–Si vibrations in the amorphous (a-Si), intermediate (i-Si), and nc-Si phases of Si, where the i-Si phase is caused by grain boundaries or smaller crystallites[6]. The FWHM of nc-Si phase is 7.4 cm− 1, corresponding to nc-Si size about 4 nm[7]. The peak at 436.1 cm− 1comes from the E2(high) mode of

ZnO. The lower peak position than 439 cm− 1of bulk ZnO is attributed to the presence of intrinsic defects in the ZnO nano-clusters[8]. The peak near 520 cm− 1is not observed in sample S25 and signiﬁcantly increases from samples S75 to S110. Besides, sample S110 shows not only the largest Si crystalline intensity but also a great Si crystal vol-ume fraction (fc) of 88%, where fcis estimated from the sum of the

integrated intensities of nc-Si and i-Si phases divided by the total sum of the integrated intensities of nc-Si, i-Si, and a-Si phases[9].

The crystalline property of ZnO matrix has strong influences on the optical and electrical properties of ZnO thin-films[3].Fig. 1(b) shows the XRD patterns of the annealed ZnO/Si ML thin-films under different PSifor the examination of the c-axis (0002) preferred

orien-tation of ZnO matrix. The ZnO/Si ML thin-ﬁlms exhibit a narrower FWHM and higher intensity when increasing PSi. This means a better

crystallization of ZnO matrix can be obtained with a higher PSi. Hence,

the results in Raman spectra and XRD patterns indicate that a high enough PSiis necessary for the formation of nc-Si embedded in ZnO

matrix and the increased PSican improve the crystalline properties

of both nc-Si and ZnO matrix.

In order to understand the formation mechanism, we analyze the AFM images of the ZnO single-layer with a 5 nm thickness and the ZnO/Si single-bilayer under different PSiafter deposition, as shown

inFig. 2. Signiﬁcant variations on the surface morphologies are ob-served. The AFM image of sample S25 shows a smaller root-mean-square (RMS) surface roughness than that of the ZnO single-layer, and the deposited Si layer can be seen as a thin layer-like. However, the AFM images of samples S75, S90, and S110 show larger RMS sur-face roughnesses than that of ZnO single-layer and clear formation of

a-Si nano-clusters. Moreover, the RMS surface roughness increases with increasing PSiand the density of nano-clusters in samples S90

and S110 can be estimated to be 3.1 × 1010_{and 1.9 × 10}10_cm− 2_{. The}

similar results are also obtained in the ZnO/Si double-bilayers. Since an a-Si nano-ﬁlm needs a higher crystallization temperature of 1100 °C than that for a-Si nano-clusters[10], the nc-Si is hard to ef ﬁ-ciently form in sample S25 during annealing. The more obvious forma-tion of a-Si nano-clusters with increasing PSiindicates that a higher PSi

can remarkably assist the sputtered Si atoms gaining more kinetic energy to self-aggregate as a-Si nano-clusters during deposition. Therefore, nc-Si QDs can be formed more easily during annealing. This observation is in good agreement with the Raman results. The peak intensity of the nc-Si is greatly enhanced with increasing PSi.

Fur-thermore, because each ZnO layer is separated by the a-Si thin-layer in sample S25, the crystallization of ZnO matrix is impeded dur-ing annealdur-ing. Hence, a lower quality of ZnO crystallization is obtained in sample S25.

The as-deposited and after-annealing cross-sectional HRTEM images of sample S110 are shown inFig. 3. InFig. 3(a) and (b), we can observe the obvious ML structure with a slightly rough morphol-ogy and the formation of a-Si nano-clusters with a size distribution of 3–5 nm separated by ZnO thin-layers after deposition. The slightly rough morphology is different from the ML structures using Si-based dielectric materials as matrix[11]. This result is reasonable since ZnO is easy to crystallize during deposition[3]. We can adjust the mor-phology of each ZnO thin-layer by tuning the ZnO sputtering power or the working pressure during deposition. The observed size distribu-tion of a-Si nano-clusters is highly consistent with the examined height of a-Si nano-clusters in AFM image and the estimated size of nc-Si about 4 nm in Raman spectrum for sample S110, and such size is suitable for various electro-optical devices using the quantum-conﬁnement effect. FromFig. 3(c), the ML structure can still be clearly seen after annealing and a high-density of nano-crystalline clusters with a size distribution of 2–6 nm can be observed from the zoom-in HRTEM image shown inFig. 3(d). Combined with the Raman and

Fig. 1. (a) Raman spectra and (b) XRD patterns of the annealed ZnO/Si ML thin-ﬁlms under different PSi. Inset of (a) shows the curve-ﬁtting result of Raman spectrum for sample

S110.

Fig. 2. AFM images of (a) the ZnO single-layer with a 5 nm thickness and the ZnO/Si single-bilayer thin-ﬁlms under (b) 25, (c) 75, (d) 90, and (e) 110 W of PSiafter deposition.

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XRD results, these nano-crystalline clusters are the nc-Si QDs embed-ded in crystalline ZnO matrix. Therefore, we can conclude that a high PSican assist the formation of self-aggregated a-Si nano-clusters on

ZnO layers during deposition, and such result is advantageous to form the nc-Si QDs embedded in crystalline ZnO matrix during annealing, as illustrated in Fig. 4. Thus, we demonstrate that the good crystallization of nc-Si QDs and ZnO matrix can be simultaneous-ly achieved with a high enough PSifor the nc-Si QDs embedded ZnO

thin-ﬁlms.

4. Conclusion

In summary, we propose to fabricate nc-Si QDs in ZnO thin-ﬁlms and successfully demonstrate their formation. The sample with PSiof

110 W shows a large fcof 88% and a highly uniform size about 4 nm

of nc-Si QDs. Our results indicate that an obvious self-aggregation of the sputtered Si atoms as nano-clusters with a high enough PSiduring

deposition is essential and helpful for the nc-Si QDs formation and the better crystallization of ZnO matrix during annealing. Therefore, we demonstrate the feasibility of fabricating nc-Si QDs embedded in crys-talline ZnO thin-ﬁlms. We believe this proposed structure has a great potential for the applications in various electro-optical or spintronic devices.

Acknowledgments

This work is supported by Taiwan's National Science Council (NSC) under contract number NSC-100-2120-M-009-005. The authors would like to thank the help from Center for Nano Science and Tech-nology (CNST) of National Chiao Tung University and National Nano Device Laboratories (NDL) in Taiwan.

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Fig. 3. The overall and zoom-in cross-sectional HRTEM images of the ZnO/Si ML thin-ﬁlm. (a) and (b) are as-deposited, and (c) and (d) are after annealing for sample S110.

Fig. 4. Illustration of the formation of nc-Si QDs embedded in the crystalline ZnO matrix with a high enough PSiin a ZnO/Si ML structure.

465 K.-Y. Kuo et al. / Materials Letters 68 (2012) 463–465