Thin Solid Films 520 (2012) 3000–3002

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Thin Solid Films

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Size-dependent effect of energy transfer on photoluminescence from Si nanocrystalsin close proximity with ZnO films

Sung Kim, Dong Hee Shin, Chang Oh Kim, Seung Hui Hong, Suk-Ho Choi ⁎Department of Applied Physics, College of Applied Science, Kyung Hee University, Yongin 446-701, Republic of Korea

⁎ Corresponding author.E-mail address: sukho@khu.ac.kr (S.-H. Choi).

0040-6090/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.tsf.2011.12.026

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 February 2011Received in revised form 7 December 2011Accepted 9 December 2011Available online 14 December 2011

Keywords:SiliconNanocrystalsZinc OxidePhotoluminescence

300 nm SiOx layers grown on p-type (100) Si wafer have been annealed to form Si nanocrystals (NCs) withinSiO2. 100 nm ZnO films have been then deposited on top of the SiO2:Si NC layers and annealed to form hybridstructures of ZnO/Si NCs. The PLSi (photoluminescence from Si NCs) intensity of the hybrid structuresincreases almost linearly with decreasing size of Si NCs (dSi) from 3.8 to 2.0 nm, whilst the PLZnO (PL fromZnO) gradually increases down to dSi=~2.5 nm and then sharply decreases. In the SiO2:Si NC layers withoutZnO, the PLSi intensity sharply increases to a maximum at dSi=~2.5 nm, and by further decrease of dSi, itdecreases. These results suggest that the energy transfer from Si NCs to ZnO occurred in the range ofdSi=3.8 to ~2.5 nm and vice versa below dSi=~2.5 nm.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

In the last decade, enhancement of luminescence from rare earthions by energy transfer has been actively studied by employing vari-ous sensitizing materials such as Si nanocrystals (NCs) [1,2], ZnOfilms/nanoparticles [3,4], and silicon nitride [5]. Upon illumination,incoming photons are predominantly absorbed by band-to-bandtransitions in a sensitizer, which enables energy transfer to rareearth ions located in vicinity of the sensitizer. Recently, evidencehas been provided for nonradiative resonant energy transfer fromquantum dots (QDs) to adjacent quantum wells (QWs) [6] or nano-wires [7], which has supported the feasibility of a solar cell paradigm.In contrast, the energy-transfer-induced outflow of carriers from QWshas resulted in a corresponding increase in the emission of the NCs[8]. Efficient energy transfer has also been demonstrated from zero-dimensional systems to metallic systems such as graphenes [9],single-walled carbon nanotubes [10], and metal films [11]. The fluo-rescence intensity of single NCs is quenched by a factor of ~70 onsingle-layer graphene, in agreement with resonant energy transfertheory [9]. Cooperative electronic interactions between ZnO and Si-based materials are expected to show interesting optical propertiesbased on the energy transfer. Recently, light emission from ZnO nano-particles embedded in SiO2 or SiOx has been characterized by usingcontinuous-wave and time-resolved photoluminescence (PL) [12–14]. The ZnO/SiOx or ZnO/Si nanocomposite materials [14,15] haveshown PL properties that are different from those of either ZnO

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nanoparticles or SiOx (or Si NCs) alone. In this paper, we reportsize-dependent effect of energy transfer on PL from Si NCs in closeproximity with ZnO films.

2. Experimental details

300 nm SiOx layers were prepared on p-type (100) Si wafer usingan Ar+ beamwith an ion energy of 750 eV and 25 mA in a reactive ionbeam sputtering deposition (IBSD) system with a Kaufman type DCion gun and Si target. The deposition chamber was evacuated to apressure of 6.7×10−7 Pa before introducing argon gas into the sys-tem. SiOx layers were deposited by sputtering Si in an oxygen ambi-ent at different partial pressures. The oxygen content (x) of SiOx

was varied from 1.0 to 1.8, which was controlled and determined byin-situ X-ray photoelectron spectroscopy analysis using Al kα line of1486.6 eV. Details of the IBSD system were described elsewhere[16]. The SiOx samples were subsequently annealed at 1100 °C for20 min under N2 ambient to form Si NCs within SiO2 layer, which isnamed as SiO2:Si NC layer.

The ZnO powder was sintered at 650 °C for 2 h under vacuum at apressure of 4.0×10−4 Pa to make ZnO sputtering target. The targetand the SiO2:Si NC layers/Si substrate were then mounted in aradio-frequency magnetron sputtering system to deposit ZnO filmsof 100 nm thickness on top of the SiO2:Si NC layers. After the systemwas evacuated to a base pressure of 4.0×10−4 Pa, the substrateswere pre-sputtered at a power of 50 W for 10 min. The ZnO sputter-ing was then performed at room temperature under a working pres-sure of 1.3×10−1 Pa. Other deposition conditions are as follows: RFpower: 70 W, deposition rate: 2 nm/min, and gas mixing ratio ofO2/Ar: 1:8. Finally, The ZnO/SiO2:Si NC layers were heated at 900 °C

Fig. 1. Cross-sectional HRTEM image of the 100 nm ZnO/300 nm SiO1.2 layers on Siwafer after annealing. The upper and lower insets show higher-resolution HRTEM im-ages of ZnO NC and Si NC, respectively.

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for 3 min under O2 ambient by using rapid thermal annealing to formhybrid structures of ZnO/Si NCs. High-resolution transmission elec-tron microscopy (HRTEM, 1.25 MV, JEM-ARM1300S, Jeol, Japan) im-ages demonstrated the existence of Si NCs whose size depends on xvalue. The specimens for HRTEM were prepared in the usual way,including final Ar ion milling.

For PL measurements, the specimen was mounted on the cold fin-ger in the vacuum chamber (pressureb1.3×10−3 Pa) of a closed-cycle refrigerator. The PL was detected at room temperature byusing a 488 nm line of Ar ion laser or a 325 nm line of He–Cd laseras the excitation source. Emitted light was collected by using a lensand was analyzed using a grating monochromator and a GaAs photo-multiplier tube. Standard lock-in detection techniques were used tomaximize the signal-to-noise ratio. The laser power for the PL excita-tion was about 3 mW.

3. Results and discussion

Fig. 1 shows a cross-sectional HRTEM image of the 100 nm ZnO/300 nm SiO1.2 layers on Si wafer after annealing, which exhibits for-mation of a hybrid structure consisting of SiO2:Si NC layer, polycrys-talline ZnO layer, and 5–10-nm-thick interfacial layer. Theinterfacial layer includes ZnO NCs of about 4–6 nm size within SiO2

matrix, which is proved by the well-defined lattice fringe with inter-planar spacing distance of ~0.25 nm, as shown by a higher-resolution

Fig. 2. (a) PL spectra of hybrid structures for various Si–NC sizes. (b) Normalized PLSi and PLZSi NC single layers as a function of dSi (upper).

image in the upper inset. The formation behaviors of the interfaciallayer were similarly observed in the previous report [15]. Anotherhigher-resolution image in the lower inset confirms the presence ofSi NCs of ~3 nm size within SiO2. The average size (dSi) of Si NCs, asestimated by HRTEM images, decreases from 3.8 to 2.0 nm with in-creasing x from 1.0 to 1.8.

Fig. 2(a) shows PL spectra of hybrid structures with varying sizesof Si NCs, which were excited by a 325 nm laser line. The 2 major PLbands peaked at around 1.5 and 3.3 eV in case of dSi=2.6 nm,which are known to originate from band-to-band emissions from SiNCs and ZnO, respectively [17,18], are referred to as PLSi and PLZnObands, respectively. The broad PL band at around 2.2 eV is attributedto the intrinsic defects of ZnO [18]. As shown in the lower part ofFig. 2(b), the PLSi intensity increases almost linearly with decreasingsize of Si NCs over the full range of dSi. In contrast, the PLZnO intensitygradually increases with the decrease of dSi down to ~2.5 nm, butbelow dSi=~2.5 nm, it sharply decreases. The upper part ofFig. 2(b) summarizes dSi-dependent PLSi intensities for SiO2:Si NC sin-gle layers, which were also excited by a 325 nm laser line. The PLSi in-tensity sharply decreases after it reaches amaximum at dSi=~2.5 nm,as widely observed in the previous reports [16,19], which is very dif-ferent with the dSi-dependent PLSi behaviors in the hybrid structures.

As the size of Si NCs decreases from 3.8 to ~2.5 nm, the PLSi inten-sity increases more greatly in the SiO2:Si NC single layers than in thehybrid structures. Moreover, the PLZnO intensity gradually increaseswith decreasing size of Si NCs in the same range of dSi. These resultssuggest that the energy transfer from Si NCs to ZnO occurs in therange of dSi=3.8 to ~2.5 nm. By decreasing the size of Si NCs below~2.5 nm in the hybrid samples, the PLZnO intensity sharply decreases,whilst the PLSi intensity continues to increase. This is in contrast withthe sharp decrease of the PLSi intensity below dSi=~2.5 nm in theSiO2:Si NC single layers, indicating that the energy transfer occursfrom ZnO to Si NCs below dSi=~2.5 nm. On the other hand, no differ-ence was observed between the dSi-dependent PLSi-intensity behav-iors for both kinds of samples under the excitation of a 488 nm lineof an Ar laser (not shown here). This indicates that no energy transferoccurred from ZnO to Si NCs even below dSi=.2.5 nm because theelectron-hole pairs could not be produced in the ZnO side due tothe excitation energy (488 nm=2.55 eV) less than the bandgap ener-gy of ZnO.

Based on these results, a possible mechanism is proposed to ex-plain the energy transfer in the hybrid structures. Two processes areinvolved to cause the energy transfer in the opposite directions. Oneprocess can be explained based on a schematic of band structure ofZnO NC(or ZnO film)/Si NC interface, as reported before [15]. At largerdSi, the conduction band offset (ΔEC) between ZnO NC(or ZnO film)

nO intensities of hybrid structures as a function of dSi (lower) and PLSi intensity of SiO2:

Fig. 3. (a) PL spectra of hybrid structures with dSi=2.0 nm for various annealing temperatures. (b) PLZnO intensities as functions of annealing temperature for the hybrid structureswith dSi=2.0 and 3.1 nm.

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and Si NC is very small. So, the electrons generated in Si NCs by pho-toionization during illumination can overcome the thin SiO2 barrier atthe ZnO NC(or ZnO film)/Si NC interface to transit to the conductionband of ZnO NC(or ZnO film). As an opposite process, the electron-holes pairs can be excited in Si NCs by luminescence from ZnONC(or ZnO film), thereby reducing the PLZnO intensity. If this effectis smaller than the transition of electrons from Si NC to ZnO NC(orZnO film), then it would reduce the PLSi intensity and enhance thePLZnO intensity, consistent with the PL behaviors in the range ofdSi=3.8 to ~2.5 nm, as shown in Fig. 2(b). The dSi-dependent energychange of the band gap is much larger in Si NCs [15], resulting in theincrease of ΔEC with decreasing size of Si NCs below ~2.5, and thusthe former process becomes increasingly difficult. Therefore, the lat-ter process would be dominant, consistent with the PL behaviorsbelow dSi=~2.5 nm, as shown in Fig. 2(b).

Fig. 3(a) shows PL spectra of hybrid structures with dSi=2.0 nmfor various annealing temperatures (TA). Below TA=1100 °C, no PLSiband is observed and the PLZnO intensity is almost independent ofTA (also see Fig. 3(b)) because no Si NCs were formed. After annealingat TA=1100 °C, the PLSi peak appears, whilst the PLZnO peak is sharplyreduced, resulting from the energy transfer from ZnO to Si NCs.Fig. 3(b) also summarizes the annealing behaviors of the hybridstructures with dSi=3.1 nm. The PLZnO intensity exhibits no suchsharp drop at TA=1100 °C, indicating no energy transfer from ZnOto Si NCs at dSi=3.1 nm. These results are consistent with the dSi-de-pendent PL behaviors shown in Fig. 2.

4. Conclusion

NC-size-dependent behaviors of energy transfer were observed inhybrid structures of ZnO/Si NCs, which were prepared by annealingdouble layers of 100 nm ZnO film/300 nm SiO2:Si NC. The PLSi inten-sity of the hybrid structures increased almost linearly with decreasingsize of Si NCs over the full range of dSi. In contrast, the PLZnO intensitygradually increased with decreasing size of Si NCs down to ~2.5 nm,but below dSi=~2.5 nm, it sharply decreased. The PLSi intensity ofthe SiO2:Si NC single layers sharply increased to a maximum atdSi=~2.5 nm, and by further decrease of dSi, it decreased. These

results suggest that the energy transfer from Si NCs to ZnO occurredin the range of dSi=3.8 to ~2.5 nm and vice versa belowdSi=~2.5 nm. Physical mechanisms to explain the dSi-dependent be-haviors of the energy transfer were discussed based on a band struc-ture at the interface of ZnO NC(or ZnO film)/Si NC.

Acknowledgments

This work was supported by a grant from the Kyung Hee Universityin 2010 (KHU-20100171) and was performed using the high-voltageelectron microscope (1.25 MV, JEM-ARM1300S, Jeol, Japan) installed atKorea Basic Science Institute.

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