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Si multilayers
Optical Materials 96 (2019) 109339
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Effect of the thermal annealing on the structural, morphological and photoluminescent properties of ZnO/Si multilayers
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R. Ambrosioa, F. Galindoa, F. Morales–Moralesb, M. Morenob, A. Torresb, M.A. Vásquez-Aa, S.A. Pérez Garcíac, A. Morales–Sánchezb,∗ a
Electronics Department, Meritorious Autonomous University of Puebla, 72590, Puebla, Mexico Electronics Department, National Institute for Astrophysics Optics and Electronics, 72000, Puebla, Mexico c Centro de Investigación en Materiales Avanzados S.C, Unidad Monterrey-PIIT, 66628, Apodaca, Nuevo León, Mexico b
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO Photoluminescence Multilayered thin films Nanospheres
In this work, multilayers (MLs) thin films composed of ten bilayers of ZnO/Si with high photoluminescence (PL) were deposited by RF-Sputtering and thermally annealed at different temperature. The composition, surface morphology, structural and PL properties of ZnO/Si MLs were studied as a function of the annealing temperature. The ZnO/Si ML structure was corroborated, in the as-deposited sample, by the analysis of its chemical composition in depth profile done by X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM) images show a smooth surface of the ZnO/Si ML before thermal annealing, but the formation of surface nano-spheres with a diameter of about 63.4 ± 1.36 nm and 132.4 ± 2.44 nm after thermal annealing at 900 °C and 1000 °C, respectively. Si:ZnOy nanospheres are grown on a Zn:SiOx film after the thermal annealing, as analyzed by XPS in depth profile. X-ray diffraction (XRD) shows a high crystallinity with the temperature and the XRD peaks observed are related to Si–ZnO and Zn2SiO4 composites, especially at 1000 °C. The luminescent studies reveal that the samples have an intense and broad PL emission after thermal annealing, observed with the naked eye. Moreover, the presence of nanospheres combined with the different defects associated to ZnO and SiOx enhances the PL intensity up to 995-fold as compared to that ZnO/Si ML without thermal annealing. Therefore, this type of multilayer films are potential candidates for their application in light emitting devices.
1. Introduction The development of white advanced luminescent materials has attracted broad interest from scientific and industrial community because of their diverse applications in light-emitting diodes (LEDs), displays and lighting [1–4]. Due to the lack of a material that emit in the whole visible spectrum, this has been solved with systems using components that emit either the two complementary (e.g., blue and yellow) or three primary colors (blue, green and red) [3,5]. Zinc oxide (ZnO) has attracted attention for the development of LEDs [6,7]. The ZnO is an important II-VI compound because it has some superior physical and chemical properties, such as a direct wide band gap energy of ~3.37 eV, a large exciton binding energy of 60 meV at room temperature, and the simple, non-toxic and low-cost fabricating advances [8,9]. On the other hand, since the discovery of the intense luminescence from porous silicon, which contains Si-quantum dots (Si-QDs) and luminescent defects related to the SiOx (x < 2) matrix, many research groups have been working to obtain an intense luminescence [10,11]. Si-QDs have been
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studied in different host matrices, being the most common silicon nitride and silicon oxide [12–15]. In this kind of silicon rich dielectric materials, the band-gap energy is modulated through the Si-QD size allowing new ways to control its photoluminescence (PL) properties [13,16–19]. Moreover, some defects within these silicon rich dielectric matrices also behaves as luminescent centers (LCs) [4,20,21]. As known, the Si-QDs and luminescent defects in SiOx emit visible PL when they are excited with UV light [4,14,20], near the emission energy of ZnO [22–24]. Therefore, it is possible to combine the luminescence properties of ZnO and SiOx-based defects (including Si-QDs) to get an efficient white light emitting material through the energy transfer effect [3,25–27]. Several authors have obtained nanocomposites for their properties and potential applications such as low power consumption, high efficiency, long lifetime, drastically reduced operating costs and environmentally friendly [16–18,22,23,26]. Some works have reported the synthesis of ZnO and Si composites using chemical vapor deposition, co-sputtering, electron beam evaporation among others [27–29]. Other approaches have focused on the surface-modified ZnO
Corresponding author. E-mail address: [email protected] (A. Morales–Sánchez).
https://doi.org/10.1016/j.optmat.2019.109339 Received 4 July 2019; Received in revised form 18 August 2019; Accepted 20 August 2019 Available online 29 August 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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MLs was analyzed by X-ray photoelectron spectroscopy (XPS) using an Escalab 250Xi equipment of Thermo Scientific. Surface and cross-view scanning electron microscopy (SEM) images of the ZnO/Si MLs were obtained with an electron microscope Nova NanoSEM 200 in order to analyze the morphological properties. The crystalline structure of the ZnO/Si MLs was analyzed by X-ray diffraction (XRD) using an Empyrean system from Panalytical with Cu Kα radiation under conventional diffraction geometry (θ−2θ scanning). The PL spectra were measured with a Fluoromax 4 system of Horiba. The samples were excited with UV light (360 nm) radiation and the PL emission signal was collected from 370 to 850 nm with a resolution of 1 nm.
nanoparticles or ZnO nanoparticles embedded in other matrices in order to tailor the luminescent properties by the interaction between ZnO and its surrounding media [29]. Few works have reported the optical properties of multilayer structures; Qian et al. reported the deposition and characterization of a ZnO/Zn2SiO4/SiO2/Si multilayer structure using pulsed electron beam deposition (PED) method; the results showed an enhancement of ultraviolet and visible light emissions due to the improved ZnO crystallinity and the formation of Zn2SiO4 nanoparticles embedded in the amorphous interfacial layer [30]. However, the structure could be seen as a one bilayer of ZnO/SiO with an interfacial layer. Xu et al. have reported a sandwich-structure with four multilayer thin films with ZnO nanoparticles sandwiched in SiOx layers in order to tailor the PL properties of ZnO nanoparticles and improve its luminescent stability [29]. The PL results showed that compared with the pure ZnO thin film, the green emission of the ZnO nanoparticles sandwiched in the SiOx matrix is significantly enhanced. Until now, the films are not of high quality and without repeatable properties to allow the LEDs fabrication. From our knowledge, until the moment the ZnO/Si multilayered films have not been reported using RF sputtering method. Therefore, it is important to investigate the luminescent properties of ZnO/Si multilayer structures for their application as light emitter. In the present work, we have fabricated ZnO/Si ML structures by RF magnetron sputtering which are composed by ten bilayers (ZnO/Si) in a sandwich structure. This multilayer structure shows an intense white light emission after a thermal annealing process. The effect of the thermal annealing process on the composition, surface morphology, structural and PL properties of the ZnO/Si MLs is discussed in detail.
3. Results and discussion The atomic composition in depth profile of the as-deposited ZnO/Si multilayer structure was analyzed by XPS, as shown in Fig. 2(a). Ten maximum and minimum peaks of Si and Zn are observed, indicating the structure of ZnO/Si ML. However, these peaks disappear after the thermal annealing process at 900 °C (dashed line) and 1000 °C (solid line) forming a bilayer structure, as shown in Fig. 2(b). For both temperatures, the absence of the original multilayer structure is notorious, in comparison with the spectrum of Fig. 2(a). In addition, it can be observed that there are two zones where the composition changes. In zone 1, the atomic content (at.%) of Zn shows an increase of ~20% at the surface of the films after annealing, and then gradually decreases its concentration in zone 2, as compared with the film without thermal annealing. For zone 1, the elements of higher concentration are O and Zn, producing a ZnOy film doped with Si (Si:ZnOy), while the elements of higher concentration for zone 2 are O and Si obtaining a SiOx film doped with Zn atoms (Zn:SiOx). This behavior is directly related to the high treatment temperature, which produces a Zn diffusion of the ZnO layers near the Si substrate towards the surface of the ML structure, also causing the formation of surface nano-spheres, as will be shown by the SEM analysis. The atomic content of Zn in the first six layers near to the Si-substrate reduces due to its diffusion towards the upper layers. In contrast, the Si content from upper Si layers reduces by its diffusion towards the first layers near Si-substrate. These Zn and Si diffusion allow the formation of a bilayer composed of a Si-doped ZnO (Si:ZnOy) nanospheres onto a Zn-doped SiOx (Zn:SiOx) film. Fig. 3 shows the surface morphology and cross-section images of the ZnO/Si ML before and after thermal annealing. The SEM images of the as-deposited ZnO/Si ML structure (Fig. 3(a)) shows a smooth surface and a total thickness of ~88.78 nm for the ten layers, which is very close to the expected value of ~85 nm for the total thickness of the ML. Whereas, the films under thermal annealing have a rough surface; the sample annealed at 900 °C exhibits on its surface the formation of a large number of grains in shape of nano-spheres with an average diameter of 63.4 ± 1.36 nm, as observed in the histogram of Fig. 4(a). When the annealing temperature is increased to 1000 °C, the average diameter increases to 132.4 ± 2.44 nm due to the agglomeration of nearby grains and then reducing the number of nano-spheres, as is observed in Figs. 3(c)–4(b) respectively. The density of nanospheres is about 1.3 × 1010 cm2 for the ZnO/Si ML annealed at 900 °C and it reduces to 5.85 × 108 cm2 for that ML thermally annealed at 1000 °C. Also, the cross-section SEM images shown in Fig. 3 indicates that the height of nano-spheres increases from 26.52 ± 3.02 nm to 47.20 ± 13.44 nm as the annealing temperature increases from 900 °C to 1000 °C. The formation of the surface nano-spheres with different sizes suggests that nucleation and growth occur at high temperature. From the analysis of the composition profile by XPS in depth, it can be observed that the ZnO/Si ML forms Si:ZnOy nano-spheres onto a Zn:SiOx film after thermal annealing. In order to analyze the structure of the ZnO/Si ML before and after thermal annealing, XRD patterns of the samples were obtained, as shown in Fig. 5. ZnO/Si MLs were deposited on quartz substrates for this analysis. The XRD pattern of a ZnO film is also shown for
2. Experimental ZnO/Si multilayers were deposited on P-type Si substrates (100) with resistivity of 2–5 Ω-cm by magnetron sputtering (13.56 MHz) at 700 °C. Before deposition, Si substrates were cleaned in ultrasonic bath with acetone, ethanol, and deionized water for 10 min. The native oxide was removed immersing Si wafers in a 10% hydrofluoric acid (HF) aqueous solution and dried with ultra-pure N2 gas, then the substrates were immediately loaded into the sputtering chamber. Ar flow of 60 sccm was introduced into the chamber once a base pressure of ~1 × 10−6 Torr was achieved, to obtain a work pressure of 2.4 mTorr. The deposition of multilayer was as follow: firstly, a ZnO thin layer was deposited onto the silicon substrate followed by a Si thin film to obtain a ZnO/Si bi-layer, then a subsequent deposition of nine ZnO/Si bi-layers were deposited with an additional (upper) ZnO thin film. The final ML structure is depicted in Fig. 1. The thickness of ZnO and Si layers in the ML was around 5 nm and 3 nm, respectively, as measured by reflectance using a Filmetrics F20UV equipment. Therefore, the expected total thickness of the ZnO/Si ML is about 85 nm. After deposition, the samples were thermally annealed at 900 °C and 1000 °C in N2 environment for 2 h. The chemical composition of ZnO/Si
Fig. 1. Representation of deposited ZnO/Si multilayer structures before annealing. 2
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Fig. 2. XPS in depth profile of the a) as-deposited and b) thermally annealed ZnO/Si MLs.
nanoparticles (ZnO/Zn2SiO4/SiO2), where the hexagonal ZnO and willemite Zn2SiO4 may coexist in the composite. In this study and at high temperature, Zn and Si species on the surface appear to be mobile enough to move and diffuse inside the porous body and contribute to the formation of zinc silica phase. In addition, Takuse et al. reported that ZnO and SiO2 react at around 775 °C to form the Zn2SiO4 compound [40]; while Xu et al. reported the formation of Zn2SiO4 for ZnO films deposited by sputtering and thermally annealed at 950 °C [36]. Therefore, this zinc silicate can be obtained when the ZnO/Si MLs, from this work, are thermally annealed at 900 °C and 1000 °C. Fig. 6 shows the PL spectra of as-deposited and thermally annealed ZnO/Si MLs. Two characteristic PL bands are observed, one is located at ~376 nm (UV band), the near band edge emission (NBE), which is observed in all the samples. This PL peak has been reported in different works and is related to a transition of an electron from a level close to the conduction band edge to a deeply trapped hole [36,41–43]. The second one is a broad visible band with a main peak centered at ~555 nm. The ZnO/Si ML annealed at 1000 °C shows an additional emission band at ~428 nm which is associated to transitions related to acceptor levels due to zinc vacancies or excess of oxygen [44]. It is clearly observed that after thermal annealing the PL intensity is greatly increased compared to the multilayer without thermal annealing. The
comparison, which exhibits 5 peaks with a preferential growth (002) crystalline plane indicating a c-axis orientation. The XRD analysis confirms the existence of crystalline Si and ZnO in the as-deposited ZnO/Si ML (without thermal annealing). XRD peaks located at 34.6°, 46°, 63° and 69.2° are related to the (002), (110), (102) and (201) crystalline planes of ZnO, respectively [31–34]. While XRD peaks at 28.7°, 56° and 89° are related to (111), (113) and (113) crystalline planes of Si, respectively, which are in agreement with references [35–37]. Nevertheless, the low intensity and the width of the XRD peaks indicate a low crystallinity in the ZnO/Si ML before the thermal treatment. Based on the XPS results, the thermal annealing process produced a diffusion of the elements that form the different layers in the ZnO/Si ML, especially Zn, producing a compound identified as zinc silicate (Zn2SiO4), as observed by the XRD peaks in Fig. 5, specially that one at 25.79° related to the (220) crystalline plane [33,34,38]. The XRD peaks related to this compound have a higher intensity at 1000 °C, which indicates a nano-crystallinity in the ML. This is attributed to the Zn diffusion which allows more stable Si–O or Zn–O bonds throughout the structure, in accordance to reported by Emir et al. [39]. These Authors synthesized Zn2SiO4/SiO2 nanocomposites by sol-gel technique incorporating Zn2SiO4 in a silica host matrix containing ZnO
Fig. 3. Surface morphology and cross-section of ZnO/Si MLs: a) as-deposited and thermally annealed at b) 900 °C and c) 1000 °C, respectively. 3
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Fig. 4. Histogram of the Si:ZnO nanospheres diameter for ZnO/Si ML thermally annealed at a) 900 °C and (b) 1000 °C.
Fig. 6. PL spectra of as-deposited and thermally annealed ZnO/Si MLs. The asdeposited sample is magnified by × 40 times in order to observe its emission.
which can also be influencing both: the intensity and width of the PL spectra [20]. Luminescent defects such as the weak-oxygen bond (WOB), the neutral oxygen vacancy (NOV) defect, the E’δ defect, and the non-bridging oxygen hole center (NBOHC) defects also emit at the visible region at around 415, 455, 520, and 630 nm, respectively [4]. All the LCs in both the ZnOy and SiOx are indicated in the PL spectra of Fig. 6. Nevertheless, the large quantity of all possible LCs involved in the PL spectra of the ZnO/Si MLs makes their deconvolution complicated [45]. The XPS O1s spectra of the ZnO/Si MLs were analyzed to observe the presence of oxygen-related defects, as is shown in Fig. 7. The O1s spectra of a pure ZnO and SiOx film with a silicon excess of about 6.2 ± 0.2 at.% are also shown for comparison. As is observed in Fig. 7(a), both Si:ZnOy nanospheres obtained at 900 °C and 1000 °C show a similar broad O1s spectra. Therefore, they were deconvoluted to 4 different peaks at about 530.4, 531.4, 532.4 and 533.3 eV, which are related to ZnO, O2− ions surrounded by zinc atoms with the full supplement of nearest-neighbor O2− ions, O2− ions that are in oxygendeficient regions within the ZnO matrix and SiOx components, respectively [28]. As we can see, one of the main contributing peaks is that one related to the oxygen-deficient regions, indicating a large quantity of oxygen defects that contribute to the high PL intensity. By other hand, as observed in Fig. 7(b), the O1s spectra of the Zn:SiOx films shifts towards higher energy bindings near the value of the reference
Fig. 5. XRD pattern of the: ZnO film, as-deposited ZnO/Si ML, and thermally annealed ZnO/Si MLs.
broad PL band in the visible region has been attributed to ZnO-related defects level emissions (DLE), which are associated to either oxygen vacancies or zinc interstitial defects [18,45]. Specifically, the emission defects in these region has been related to the presence of zinc interstitials (Zni), oxygen interstitial (Oi), oxygen vacancies (Vo) and Zn vacancies (VZn) [45–47]. The emission wavelengths of radiative transitions from the conduction band (CB) to VZn, CB to Vo, Zni to Vo, CB to Oi and Zni to Oi are reported at 435, 504, 553, 590 and 602 nm, respectively. The presence of these defects could be the result of the hightemperature of annealing, which consumes a large quantity of O2. Thus, it is inferred that the reduction of the intensity of the UV band and the appearance of a broad visible light emission band in the ZnO/Si ML are related to the different DLE of ZnO within the Si:ZnOy nanospheres, as reported in Refs. [45–47]. In addition, the thermal annealing leads to the formation of Zn2SiO4 which emit at 527 nm and its presence allows an enhancement of the emission in UV and red regions. Also, as analyzed by XPS, the Si:ZnOy nanospheres are grown onto a Zn:SiOx film. It is well known that several luminescent defects exist in a SiOx film, 4
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centers, such as grain boundaries and dislocations improving the PL properties. On the other hand, as we described in previous paragraphs, the thermal annealing applied to the ZnO/Si multilayer structures also promotes the formation of surface nano-spheres. It is known and reported that the use of spherical structures increases the screen brightness and improves the resolution because of its lower scattering of evolved light, and higher packing densities than irregular shaped particles [48]. In fact, dielectric microspheres have been used to improve the UV luminescence of ZnO thin films [49]. However, extra steps in the fabrication process like drop coating and self-assembly are required to deposit these spheres on the ZnO film. Therefore, the formation of Si:ZnO nanospheres and the presence of a large number of luminescent defects related to oxygen vacancies, interstices and Zn vacancies in both the Zn:SiOx film and Si:ZnOy nano-spheres is another factor that allow to enhance the PL intensity making these type of films an alternative for the development of light emitting devices. 4. Conclusions In this study, ZnO/Si MLs were deposited by RF sputtering and thermally annealed to explore their morphological, structural and PL properties for luminescent devices applications. A bilayer formed by Si:ZnOy nano-spheres grown onto a Zn:SiOx film were obtained by the interfacial reaction between the ZnO and Si layers in the ZnO/Si MLs after thermal annealing. The thermal treatment results in the improvement of ZnO crystallinity and the formation of the Zn2SiO4 compound. The formation of Si:ZnOy nanospheres and the presence of a large number of luminescent defects related to oxygen vacancies, interstices and Zn vacancies in both the Zn:SiOx film and Si:ZnOy nanospheres allow an enhanced PL intensity up to 995 times after thermal annealing at 1000 °C, as compared to that as-deposited ML. The large quantity of luminescent defects produces a broad PL band, which is observed from blue to red, practically the complete visible range, especially in the multilayer structure annealed at 1000 °C. Therefore, considering the high intensity obtained and the broad PL spectrum, this study opens the possibility to use these type of ML films in the development of white light emitting devices.
Fig. 7. XPS O1s spectra of the a) Si:ZnOy nanospheres and b) Zn:SiOx films obtained at 900 °C and 1000 °C. The XPS O1s spectra of a pure ZnO (gray line) and SiOx (blue line) films are also shown for comparison. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Fig. 8. PL intensity as a function of the thermal annealing temperature. Inset shows photographs of the visible light emission for comparison under UV excitation.
Authors acknowledge the technical support of Enrique Longoria, Nayely Pineda and Luis Gerardo Silva from the Advanced Material Research Center (CIMAV) for XRD, SEM and XPS measurements, respectively. A Morales–Sánchez is grateful for the support received from CONACYT of Mexico for the sabbatical year. R. Ambrosio thanks for the support through the program VIEP-BUAP.
SiOx film, specially that one obtained at 1000 °C. In these films, the main peaks are related to oxygen-deficient regions and silicon suboxides (SiOx) defects. Therefore, as described above the main LCs involved in the Zn:SiOx films are also related to oxygen defects. Fig. 8 exhibits the maximum intensity of PL peak at 555 nm versus the annealing temperature. It shows that the intensity of visible emission increased 169 times after annealing at 900 °C and up to 995 times at 1000 °C compared with the ZnO/Si ML deposited at 700 °C. Moreover, as shown in the insets of Fig. 8, the PL emission is observed by the naked eye when the samples are stimulated with an UV lamp (360 nm) even when the total thickness of the films, measured by SEM, is around 90 nm. It is possible to see how the emission color changes from yellow to white when the annealing temperature increases to 1000 °C. As analyzed by XRD, the thermal annealing process improves the crystallinity, which reduces the concentration of non-radiative recombination
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Effect of the thermal annealing on the structural, morphological and photoluminescent properties of ZnO/Si multilayers
T
R. Ambrosioa, F. Galindoa, F. Morales–Moralesb, M. Morenob, A. Torresb, M.A. Vásquez-Aa, S.A. Pérez Garcíac, A. Morales–Sánchezb,∗ a
Electronics Department, Meritorious Autonomous University of Puebla, 72590, Puebla, Mexico Electronics Department, National Institute for Astrophysics Optics and Electronics, 72000, Puebla, Mexico c Centro de Investigación en Materiales Avanzados S.C, Unidad Monterrey-PIIT, 66628, Apodaca, Nuevo León, Mexico b
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO Photoluminescence Multilayered thin films Nanospheres
In this work, multilayers (MLs) thin films composed of ten bilayers of ZnO/Si with high photoluminescence (PL) were deposited by RF-Sputtering and thermally annealed at different temperature. The composition, surface morphology, structural and PL properties of ZnO/Si MLs were studied as a function of the annealing temperature. The ZnO/Si ML structure was corroborated, in the as-deposited sample, by the analysis of its chemical composition in depth profile done by X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM) images show a smooth surface of the ZnO/Si ML before thermal annealing, but the formation of surface nano-spheres with a diameter of about 63.4 ± 1.36 nm and 132.4 ± 2.44 nm after thermal annealing at 900 °C and 1000 °C, respectively. Si:ZnOy nanospheres are grown on a Zn:SiOx film after the thermal annealing, as analyzed by XPS in depth profile. X-ray diffraction (XRD) shows a high crystallinity with the temperature and the XRD peaks observed are related to Si–ZnO and Zn2SiO4 composites, especially at 1000 °C. The luminescent studies reveal that the samples have an intense and broad PL emission after thermal annealing, observed with the naked eye. Moreover, the presence of nanospheres combined with the different defects associated to ZnO and SiOx enhances the PL intensity up to 995-fold as compared to that ZnO/Si ML without thermal annealing. Therefore, this type of multilayer films are potential candidates for their application in light emitting devices.
1. Introduction The development of white advanced luminescent materials has attracted broad interest from scientific and industrial community because of their diverse applications in light-emitting diodes (LEDs), displays and lighting [1–4]. Due to the lack of a material that emit in the whole visible spectrum, this has been solved with systems using components that emit either the two complementary (e.g., blue and yellow) or three primary colors (blue, green and red) [3,5]. Zinc oxide (ZnO) has attracted attention for the development of LEDs [6,7]. The ZnO is an important II-VI compound because it has some superior physical and chemical properties, such as a direct wide band gap energy of ~3.37 eV, a large exciton binding energy of 60 meV at room temperature, and the simple, non-toxic and low-cost fabricating advances [8,9]. On the other hand, since the discovery of the intense luminescence from porous silicon, which contains Si-quantum dots (Si-QDs) and luminescent defects related to the SiOx (x < 2) matrix, many research groups have been working to obtain an intense luminescence [10,11]. Si-QDs have been
∗
studied in different host matrices, being the most common silicon nitride and silicon oxide [12–15]. In this kind of silicon rich dielectric materials, the band-gap energy is modulated through the Si-QD size allowing new ways to control its photoluminescence (PL) properties [13,16–19]. Moreover, some defects within these silicon rich dielectric matrices also behaves as luminescent centers (LCs) [4,20,21]. As known, the Si-QDs and luminescent defects in SiOx emit visible PL when they are excited with UV light [4,14,20], near the emission energy of ZnO [22–24]. Therefore, it is possible to combine the luminescence properties of ZnO and SiOx-based defects (including Si-QDs) to get an efficient white light emitting material through the energy transfer effect [3,25–27]. Several authors have obtained nanocomposites for their properties and potential applications such as low power consumption, high efficiency, long lifetime, drastically reduced operating costs and environmentally friendly [16–18,22,23,26]. Some works have reported the synthesis of ZnO and Si composites using chemical vapor deposition, co-sputtering, electron beam evaporation among others [27–29]. Other approaches have focused on the surface-modified ZnO
Corresponding author. E-mail address: [email protected] (A. Morales–Sánchez).
https://doi.org/10.1016/j.optmat.2019.109339 Received 4 July 2019; Received in revised form 18 August 2019; Accepted 20 August 2019 Available online 29 August 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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MLs was analyzed by X-ray photoelectron spectroscopy (XPS) using an Escalab 250Xi equipment of Thermo Scientific. Surface and cross-view scanning electron microscopy (SEM) images of the ZnO/Si MLs were obtained with an electron microscope Nova NanoSEM 200 in order to analyze the morphological properties. The crystalline structure of the ZnO/Si MLs was analyzed by X-ray diffraction (XRD) using an Empyrean system from Panalytical with Cu Kα radiation under conventional diffraction geometry (θ−2θ scanning). The PL spectra were measured with a Fluoromax 4 system of Horiba. The samples were excited with UV light (360 nm) radiation and the PL emission signal was collected from 370 to 850 nm with a resolution of 1 nm.
nanoparticles or ZnO nanoparticles embedded in other matrices in order to tailor the luminescent properties by the interaction between ZnO and its surrounding media [29]. Few works have reported the optical properties of multilayer structures; Qian et al. reported the deposition and characterization of a ZnO/Zn2SiO4/SiO2/Si multilayer structure using pulsed electron beam deposition (PED) method; the results showed an enhancement of ultraviolet and visible light emissions due to the improved ZnO crystallinity and the formation of Zn2SiO4 nanoparticles embedded in the amorphous interfacial layer [30]. However, the structure could be seen as a one bilayer of ZnO/SiO with an interfacial layer. Xu et al. have reported a sandwich-structure with four multilayer thin films with ZnO nanoparticles sandwiched in SiOx layers in order to tailor the PL properties of ZnO nanoparticles and improve its luminescent stability [29]. The PL results showed that compared with the pure ZnO thin film, the green emission of the ZnO nanoparticles sandwiched in the SiOx matrix is significantly enhanced. Until now, the films are not of high quality and without repeatable properties to allow the LEDs fabrication. From our knowledge, until the moment the ZnO/Si multilayered films have not been reported using RF sputtering method. Therefore, it is important to investigate the luminescent properties of ZnO/Si multilayer structures for their application as light emitter. In the present work, we have fabricated ZnO/Si ML structures by RF magnetron sputtering which are composed by ten bilayers (ZnO/Si) in a sandwich structure. This multilayer structure shows an intense white light emission after a thermal annealing process. The effect of the thermal annealing process on the composition, surface morphology, structural and PL properties of the ZnO/Si MLs is discussed in detail.
3. Results and discussion The atomic composition in depth profile of the as-deposited ZnO/Si multilayer structure was analyzed by XPS, as shown in Fig. 2(a). Ten maximum and minimum peaks of Si and Zn are observed, indicating the structure of ZnO/Si ML. However, these peaks disappear after the thermal annealing process at 900 °C (dashed line) and 1000 °C (solid line) forming a bilayer structure, as shown in Fig. 2(b). For both temperatures, the absence of the original multilayer structure is notorious, in comparison with the spectrum of Fig. 2(a). In addition, it can be observed that there are two zones where the composition changes. In zone 1, the atomic content (at.%) of Zn shows an increase of ~20% at the surface of the films after annealing, and then gradually decreases its concentration in zone 2, as compared with the film without thermal annealing. For zone 1, the elements of higher concentration are O and Zn, producing a ZnOy film doped with Si (Si:ZnOy), while the elements of higher concentration for zone 2 are O and Si obtaining a SiOx film doped with Zn atoms (Zn:SiOx). This behavior is directly related to the high treatment temperature, which produces a Zn diffusion of the ZnO layers near the Si substrate towards the surface of the ML structure, also causing the formation of surface nano-spheres, as will be shown by the SEM analysis. The atomic content of Zn in the first six layers near to the Si-substrate reduces due to its diffusion towards the upper layers. In contrast, the Si content from upper Si layers reduces by its diffusion towards the first layers near Si-substrate. These Zn and Si diffusion allow the formation of a bilayer composed of a Si-doped ZnO (Si:ZnOy) nanospheres onto a Zn-doped SiOx (Zn:SiOx) film. Fig. 3 shows the surface morphology and cross-section images of the ZnO/Si ML before and after thermal annealing. The SEM images of the as-deposited ZnO/Si ML structure (Fig. 3(a)) shows a smooth surface and a total thickness of ~88.78 nm for the ten layers, which is very close to the expected value of ~85 nm for the total thickness of the ML. Whereas, the films under thermal annealing have a rough surface; the sample annealed at 900 °C exhibits on its surface the formation of a large number of grains in shape of nano-spheres with an average diameter of 63.4 ± 1.36 nm, as observed in the histogram of Fig. 4(a). When the annealing temperature is increased to 1000 °C, the average diameter increases to 132.4 ± 2.44 nm due to the agglomeration of nearby grains and then reducing the number of nano-spheres, as is observed in Figs. 3(c)–4(b) respectively. The density of nanospheres is about 1.3 × 1010 cm2 for the ZnO/Si ML annealed at 900 °C and it reduces to 5.85 × 108 cm2 for that ML thermally annealed at 1000 °C. Also, the cross-section SEM images shown in Fig. 3 indicates that the height of nano-spheres increases from 26.52 ± 3.02 nm to 47.20 ± 13.44 nm as the annealing temperature increases from 900 °C to 1000 °C. The formation of the surface nano-spheres with different sizes suggests that nucleation and growth occur at high temperature. From the analysis of the composition profile by XPS in depth, it can be observed that the ZnO/Si ML forms Si:ZnOy nano-spheres onto a Zn:SiOx film after thermal annealing. In order to analyze the structure of the ZnO/Si ML before and after thermal annealing, XRD patterns of the samples were obtained, as shown in Fig. 5. ZnO/Si MLs were deposited on quartz substrates for this analysis. The XRD pattern of a ZnO film is also shown for
2. Experimental ZnO/Si multilayers were deposited on P-type Si substrates (100) with resistivity of 2–5 Ω-cm by magnetron sputtering (13.56 MHz) at 700 °C. Before deposition, Si substrates were cleaned in ultrasonic bath with acetone, ethanol, and deionized water for 10 min. The native oxide was removed immersing Si wafers in a 10% hydrofluoric acid (HF) aqueous solution and dried with ultra-pure N2 gas, then the substrates were immediately loaded into the sputtering chamber. Ar flow of 60 sccm was introduced into the chamber once a base pressure of ~1 × 10−6 Torr was achieved, to obtain a work pressure of 2.4 mTorr. The deposition of multilayer was as follow: firstly, a ZnO thin layer was deposited onto the silicon substrate followed by a Si thin film to obtain a ZnO/Si bi-layer, then a subsequent deposition of nine ZnO/Si bi-layers were deposited with an additional (upper) ZnO thin film. The final ML structure is depicted in Fig. 1. The thickness of ZnO and Si layers in the ML was around 5 nm and 3 nm, respectively, as measured by reflectance using a Filmetrics F20UV equipment. Therefore, the expected total thickness of the ZnO/Si ML is about 85 nm. After deposition, the samples were thermally annealed at 900 °C and 1000 °C in N2 environment for 2 h. The chemical composition of ZnO/Si
Fig. 1. Representation of deposited ZnO/Si multilayer structures before annealing. 2
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Fig. 2. XPS in depth profile of the a) as-deposited and b) thermally annealed ZnO/Si MLs.
nanoparticles (ZnO/Zn2SiO4/SiO2), where the hexagonal ZnO and willemite Zn2SiO4 may coexist in the composite. In this study and at high temperature, Zn and Si species on the surface appear to be mobile enough to move and diffuse inside the porous body and contribute to the formation of zinc silica phase. In addition, Takuse et al. reported that ZnO and SiO2 react at around 775 °C to form the Zn2SiO4 compound [40]; while Xu et al. reported the formation of Zn2SiO4 for ZnO films deposited by sputtering and thermally annealed at 950 °C [36]. Therefore, this zinc silicate can be obtained when the ZnO/Si MLs, from this work, are thermally annealed at 900 °C and 1000 °C. Fig. 6 shows the PL spectra of as-deposited and thermally annealed ZnO/Si MLs. Two characteristic PL bands are observed, one is located at ~376 nm (UV band), the near band edge emission (NBE), which is observed in all the samples. This PL peak has been reported in different works and is related to a transition of an electron from a level close to the conduction band edge to a deeply trapped hole [36,41–43]. The second one is a broad visible band with a main peak centered at ~555 nm. The ZnO/Si ML annealed at 1000 °C shows an additional emission band at ~428 nm which is associated to transitions related to acceptor levels due to zinc vacancies or excess of oxygen [44]. It is clearly observed that after thermal annealing the PL intensity is greatly increased compared to the multilayer without thermal annealing. The
comparison, which exhibits 5 peaks with a preferential growth (002) crystalline plane indicating a c-axis orientation. The XRD analysis confirms the existence of crystalline Si and ZnO in the as-deposited ZnO/Si ML (without thermal annealing). XRD peaks located at 34.6°, 46°, 63° and 69.2° are related to the (002), (110), (102) and (201) crystalline planes of ZnO, respectively [31–34]. While XRD peaks at 28.7°, 56° and 89° are related to (111), (113) and (113) crystalline planes of Si, respectively, which are in agreement with references [35–37]. Nevertheless, the low intensity and the width of the XRD peaks indicate a low crystallinity in the ZnO/Si ML before the thermal treatment. Based on the XPS results, the thermal annealing process produced a diffusion of the elements that form the different layers in the ZnO/Si ML, especially Zn, producing a compound identified as zinc silicate (Zn2SiO4), as observed by the XRD peaks in Fig. 5, specially that one at 25.79° related to the (220) crystalline plane [33,34,38]. The XRD peaks related to this compound have a higher intensity at 1000 °C, which indicates a nano-crystallinity in the ML. This is attributed to the Zn diffusion which allows more stable Si–O or Zn–O bonds throughout the structure, in accordance to reported by Emir et al. [39]. These Authors synthesized Zn2SiO4/SiO2 nanocomposites by sol-gel technique incorporating Zn2SiO4 in a silica host matrix containing ZnO
Fig. 3. Surface morphology and cross-section of ZnO/Si MLs: a) as-deposited and thermally annealed at b) 900 °C and c) 1000 °C, respectively. 3
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Fig. 4. Histogram of the Si:ZnO nanospheres diameter for ZnO/Si ML thermally annealed at a) 900 °C and (b) 1000 °C.
Fig. 6. PL spectra of as-deposited and thermally annealed ZnO/Si MLs. The asdeposited sample is magnified by × 40 times in order to observe its emission.
which can also be influencing both: the intensity and width of the PL spectra [20]. Luminescent defects such as the weak-oxygen bond (WOB), the neutral oxygen vacancy (NOV) defect, the E’δ defect, and the non-bridging oxygen hole center (NBOHC) defects also emit at the visible region at around 415, 455, 520, and 630 nm, respectively [4]. All the LCs in both the ZnOy and SiOx are indicated in the PL spectra of Fig. 6. Nevertheless, the large quantity of all possible LCs involved in the PL spectra of the ZnO/Si MLs makes their deconvolution complicated [45]. The XPS O1s spectra of the ZnO/Si MLs were analyzed to observe the presence of oxygen-related defects, as is shown in Fig. 7. The O1s spectra of a pure ZnO and SiOx film with a silicon excess of about 6.2 ± 0.2 at.% are also shown for comparison. As is observed in Fig. 7(a), both Si:ZnOy nanospheres obtained at 900 °C and 1000 °C show a similar broad O1s spectra. Therefore, they were deconvoluted to 4 different peaks at about 530.4, 531.4, 532.4 and 533.3 eV, which are related to ZnO, O2− ions surrounded by zinc atoms with the full supplement of nearest-neighbor O2− ions, O2− ions that are in oxygendeficient regions within the ZnO matrix and SiOx components, respectively [28]. As we can see, one of the main contributing peaks is that one related to the oxygen-deficient regions, indicating a large quantity of oxygen defects that contribute to the high PL intensity. By other hand, as observed in Fig. 7(b), the O1s spectra of the Zn:SiOx films shifts towards higher energy bindings near the value of the reference
Fig. 5. XRD pattern of the: ZnO film, as-deposited ZnO/Si ML, and thermally annealed ZnO/Si MLs.
broad PL band in the visible region has been attributed to ZnO-related defects level emissions (DLE), which are associated to either oxygen vacancies or zinc interstitial defects [18,45]. Specifically, the emission defects in these region has been related to the presence of zinc interstitials (Zni), oxygen interstitial (Oi), oxygen vacancies (Vo) and Zn vacancies (VZn) [45–47]. The emission wavelengths of radiative transitions from the conduction band (CB) to VZn, CB to Vo, Zni to Vo, CB to Oi and Zni to Oi are reported at 435, 504, 553, 590 and 602 nm, respectively. The presence of these defects could be the result of the hightemperature of annealing, which consumes a large quantity of O2. Thus, it is inferred that the reduction of the intensity of the UV band and the appearance of a broad visible light emission band in the ZnO/Si ML are related to the different DLE of ZnO within the Si:ZnOy nanospheres, as reported in Refs. [45–47]. In addition, the thermal annealing leads to the formation of Zn2SiO4 which emit at 527 nm and its presence allows an enhancement of the emission in UV and red regions. Also, as analyzed by XPS, the Si:ZnOy nanospheres are grown onto a Zn:SiOx film. It is well known that several luminescent defects exist in a SiOx film, 4
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centers, such as grain boundaries and dislocations improving the PL properties. On the other hand, as we described in previous paragraphs, the thermal annealing applied to the ZnO/Si multilayer structures also promotes the formation of surface nano-spheres. It is known and reported that the use of spherical structures increases the screen brightness and improves the resolution because of its lower scattering of evolved light, and higher packing densities than irregular shaped particles [48]. In fact, dielectric microspheres have been used to improve the UV luminescence of ZnO thin films [49]. However, extra steps in the fabrication process like drop coating and self-assembly are required to deposit these spheres on the ZnO film. Therefore, the formation of Si:ZnO nanospheres and the presence of a large number of luminescent defects related to oxygen vacancies, interstices and Zn vacancies in both the Zn:SiOx film and Si:ZnOy nano-spheres is another factor that allow to enhance the PL intensity making these type of films an alternative for the development of light emitting devices. 4. Conclusions In this study, ZnO/Si MLs were deposited by RF sputtering and thermally annealed to explore their morphological, structural and PL properties for luminescent devices applications. A bilayer formed by Si:ZnOy nano-spheres grown onto a Zn:SiOx film were obtained by the interfacial reaction between the ZnO and Si layers in the ZnO/Si MLs after thermal annealing. The thermal treatment results in the improvement of ZnO crystallinity and the formation of the Zn2SiO4 compound. The formation of Si:ZnOy nanospheres and the presence of a large number of luminescent defects related to oxygen vacancies, interstices and Zn vacancies in both the Zn:SiOx film and Si:ZnOy nanospheres allow an enhanced PL intensity up to 995 times after thermal annealing at 1000 °C, as compared to that as-deposited ML. The large quantity of luminescent defects produces a broad PL band, which is observed from blue to red, practically the complete visible range, especially in the multilayer structure annealed at 1000 °C. Therefore, considering the high intensity obtained and the broad PL spectrum, this study opens the possibility to use these type of ML films in the development of white light emitting devices.
Fig. 7. XPS O1s spectra of the a) Si:ZnOy nanospheres and b) Zn:SiOx films obtained at 900 °C and 1000 °C. The XPS O1s spectra of a pure ZnO (gray line) and SiOx (blue line) films are also shown for comparison. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Fig. 8. PL intensity as a function of the thermal annealing temperature. Inset shows photographs of the visible light emission for comparison under UV excitation.
Authors acknowledge the technical support of Enrique Longoria, Nayely Pineda and Luis Gerardo Silva from the Advanced Material Research Center (CIMAV) for XRD, SEM and XPS measurements, respectively. A Morales–Sánchez is grateful for the support received from CONACYT of Mexico for the sabbatical year. R. Ambrosio thanks for the support through the program VIEP-BUAP.
SiOx film, specially that one obtained at 1000 °C. In these films, the main peaks are related to oxygen-deficient regions and silicon suboxides (SiOx) defects. Therefore, as described above the main LCs involved in the Zn:SiOx films are also related to oxygen defects. Fig. 8 exhibits the maximum intensity of PL peak at 555 nm versus the annealing temperature. It shows that the intensity of visible emission increased 169 times after annealing at 900 °C and up to 995 times at 1000 °C compared with the ZnO/Si ML deposited at 700 °C. Moreover, as shown in the insets of Fig. 8, the PL emission is observed by the naked eye when the samples are stimulated with an UV lamp (360 nm) even when the total thickness of the films, measured by SEM, is around 90 nm. It is possible to see how the emission color changes from yellow to white when the annealing temperature increases to 1000 °C. As analyzed by XRD, the thermal annealing process improves the crystallinity, which reduces the concentration of non-radiative recombination
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