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Study on interfacial interaction between Si and ZnO
Ceramics International 45 (2019) 21894–21899
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Study on interfacial interaction between Si and ZnO a
a,b
Zhixiang Liu , Yunqing Tang
, Ningbo Liao
c,**
T
a,*
, Ping Yang
a
Laboratory of Advanced Design, Manufacturing & Reliability for MEMS/NEMS/OEDS, School of Mechanical Engineering, Jiangsu University, Zhenjiang, 212013, PR China Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 2V4, Canada c School of Mechanical Engineering, Wenzhou University, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Interfacial reaction Si/ZnO heterojunctions Molecular dynamics First principle
The interfacial reaction and bonding mechanism of Si based undoped and Al-doped ZnO films were investigated experimentally and numerically. Si/ZnO heterojunctions were fabricated onto p-Si(100) wafers by RF magnetron sputtering, then the undoped and Al doped ZnO (AZO) films coated Si substrates were annealed in argon atmosphere (ZnO-Ar and AZO-Ar) and air atmosphere (ZnO-Air and AZO-Air) respectively. X-ray diffraction (XRD) and Fourier transform infrared spectra (FTIR) show the prepared Si-based films have wurtzite structure with (002) preferential growth, and demonstrate several Si oxides exist in both cases. X-ray photoelectron spectroscopy (XPS) finds that oxygen vacancies exist in ZnO-Ar, moreover, the stoichiometry between Zn and O in AZOAr film to be close to 1:1, and high oxygen content in ZnO-Air, which are also supported by Hall measurement. Furthermore, the Si 2p of XPS spectrum shows the width of interfacial interaction between Si and ZnO become weaker with the order of ZnO-Ar, AZO-Ar and ZnO-Air. To elucidate the joining and bonding mechanism between Si substrate and ZnO coating layer, molecular dynamics and first principle simulations were performed in this work, the simulation results show certain interstitial Si atoms coming from Si substrate interact with O atoms related to the Zn–O bonding at the Si/ZnO interface, and resulting in oxygen vacancies in ZnO film. The experiment and simulation show the interfacial interaction between Si and ZnO depends on oxygen content coming from ZnO coating layer, the limited oxygen coming from ZnO film could increase the width of Si/ZnO interfacial interaction and leave oxygen vacancies in ZnO film due to Si robbing O atoms related to Zn–O bonding.
1. Introduction ZnO is a versatile material not only due to its non-toxicity, availability and excellent photoelectric properties but also for the possibility of integrating such devices with mature and advanced Si-based technology [1–17]. The Si/ZnO heterojunction has been widely used in optoelectronic applications, such as photodetectors [9], photodiode [10], terrestrial solar cells [11,12], acoustics materials [13], surfacewave convolver [14], dynamic switching element [15]. Along the process of application and study, the interfacial interactions and formations at Si/ZnO interface that demonstrated by many results [16–23], could profoundly degrade the attaining performance of Si/ ZnO devices, which would limit the applications of Si/ZnO heterojunction [15–27]. In order to understand the interfacial interactions between Si and ZnO, some researches have been made to elaborate the interfacial interaction and bonding mechanism of Si/ZnO junction.
*
Gabás M et al. [17] used spray pyrolysis to grow ZnO films on commercial Si substrates for solar cell, the results showed that the silicon oxide layer is inherent to the growth of ZnO, and oxygen enrichment might be the reason of interface reaction between Si substrate and ZnO film. Kim D K et al. [18] prepared Si-based ZnO films under different substrate temperature by magnetron sputtering, they found that the amorphous SiO2 layer located at Si/ZnO interface could result in oxygen vacancy, which result from oxygen related to the Zn–O bonding, and higher substrate temperature (600 °C) could suppress the Zn–O bonding. Ogata K et al. [19] thought it's difficult to prepare highquality Si based ZnO films because of the easier silicon dioxide formation due to the larger formation enthalpy of SiO2 (−910.7 ± 1.0 kJ/mol) compared to that of ZnO (−350.46 ± 0.27 kJ/mol). Similarly, Wimmer M et al. [20] fabricated Si/ZnO heterostructure by plasma-enhanced chemical vapor deposition (PECVD), they concluded that the silicon oxide exist near Si/ZnO
Corresponding author. Corresponding author. E-mail addresses: [email protected], [email protected] (P. Yang).
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https://doi.org/10.1016/j.ceramint.2019.07.200 Received 4 April 2019; Received in revised form 18 June 2019; Accepted 17 July 2019 Available online 18 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 21894–21899
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Fig. 1. XRD spectra of thin films annealed at 500 °C: (a)ZnO-Air, (b)ZnO-Ar, (c)AZO-Ar.
interface at the expense of Zn–O bonds. Hu Z S et al. [21] grown ZnO films on Si substrate by sputtering method, the results showed that the amorphous silicon oxide located at Si/ZnO interface is caused by Si robbing oxygen atoms from ZnO near the interface, and leave oxygen vacancy in ZnO films. Gerlach D et al. [22] used spatially resolved photoelectron emission microscopy to investigate the interfacial chemical properties of a-Si:H(P)/ZnO(n) prepared by PECVD, the results showed that the silicon oxide formed at the Si/ZnO interface could be explained by reaction correlation (1). However, Si/ZnO junctions were fabricated using spray-pyrolysis method by Kobayashi H et al. [23], they demonstrated that the oxygen atoms in atmosphere penetrate ZnO film and react with Si atoms to form silicon oxide at Si/ZnO interface (reaction correlation 2), not from the reaction of Si with ZnO(reaction correlation 3). Furthermore, the free energy of reaction correlation 3 ( ΔG = −220kJmol−1) is much more positive than that of reaction correlation 2( ΔG = −857kJmol−1), and the progress of reaction correlation 2 is very slow due to the large activation energy of Zn–O bond cleavage. In addition, Baik D G et al. [24] also support the conclusion that silicon oxide located at the Si/ZnO interface was caused by oxygen diffusing through the porous ZnO films. But Yu H K et al. [25] showed that the SiOx at Si/ZnO interface is more thermodynamically favorable (reaction correlation 3), because of Gibbs free energy change of SiO2( ΔGf = −730.256kJ / mol ) is much smaller than that of ZnO ( ΔGf = −248.15kJ / mol ) under 1000 K.
y⋅ZnO + x⋅Si → Zn + Si x Oy
(1)
Si + O2 → SiO2
(2)
Si + 2ZnO → SiO2 + 2Zn
(3)
From the discussion above, it can be inferred that the interfacial interaction and bonding mechanism of Si based ZnO remains unclear, but the Si/ZnO structure is a critical for Si based photoelectronics and
other progressive devices [22,26–30], it is essential to study the mechanism of phase transition between Si and ZnO. In this work, we try to shed light on this controversy, giving argumentation and information about how Si and ZnO interaction with each other. With this aim, the ZnO and AZO films were deposited on p-Si(100) substrates by RF magnetron sputtering, and the prepared Si based films undergo thermal treatment in argon and air atmosphere respectively. The crystal structure, chemical composition and heterogeneous interface properties were investigated via experiment (XRD, FTIR, XPS and Hall measurements) and simulation (molecular dynamics and first principles). 2. Experimental test 2.1. Experimental details ZnO and AZO films were RF sputter-deposited onto p-Si(100) wafer at substrate temperature 100 °C, the ZnO target(99.99% pure ZnO powder) and the AZO target with 2 wt% pure Al2O3 powder were used to grow films, then the prepared films were annealed at 500 °C in argon and air atmosphere (ZnO-Ar and AZO-Ar, ZnO-Air and AZO-Air for short respectively). The film thicknesses of films were measured by TF ProbeTM SR300 thin film analyser. The film crystal structures were characterized by X-ray Diffractometer (Model: D /max 2500VB3 + / PC ). The chemical composition were investigated using Fourier Transform Infrared (FTIR) spectroscopy (Model: NEXUS) with KBr pellet. The electrical properties were carried out by Hall measurements (Model: HALL8800). The elemental composition and chemical state were studied using X-ray photoelectron spectrometer (Model: ESCALABTM 250Xi ), XPS Data treatment was performed using CasaXPS software [31], and the binding energies were calibrated from the C1s peak located at 284.8eV. The X-ray diffraction patterns were analyzed using the MDI Jade 6.5 software in this work.
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Table 1 Influence of annealing atmosphere on lattice parameters. Si based films
Lattice parameter(nm)
ZnO-Air ZnO-Ar AZO-Ar Pure ZnO JCPDS-PDF#36-1451
a
c
0.3244 0.3248 0.3329 0.3249
0.5199 0.5184 0.5170 0.5206
2.2. Results and discussion Fig. 1 a-c show the XRD spectra of ZnO and AZO films respectively. It's find that all Si based films have preferential orientation along (002) direction, and the ZnO-Air has the strongest (002) diffraction peak compared to others, indicating the c-axis oriented growth of grains, and the single crystal ZnO thin films were successfully prepared in this work according to relevant literature [32]. Furthermore, the strongest SiO2(silicon oxide) diffraction peak is observed in ZnO-Air pattern, this illustrates silicon oxide with (111) preferential growth are formed in Sibased ZnO film that annealed in air atmosphere. The crystal sizes of prepared films are obtained by Scherrer formula [33] and Bragg formula [34], which are shown in Table 1. It can been seen that the crystal structures(a and c ) of all films are close to pure ZnO(JCPDS-PDF#36–1451), showing the prepared Si-based ZnO and AZO films have wurtzite structure, moreover, the lattice parameter a of Si based films annealed in argon are larger than that of the films annealed in air, and the lattice parameter c of films annealed in argon are smaller, this indicates the annealing atmosphere has dramatically influence on crystal structure of ZnO films. Fig. 2 shows the three absorption peaks located at 405 cm−1 are strong and sharp, which means all the films have metallic oxide structure related to Zn–O bonding [35], indicating the prepared Si based films have hexagonal structure of ZnO, which are in correspondence with XRD results. The three small bands around 680 cm−1 are ascribed to Si–O–Si stretching vibrations [36], this is consistent with the results of peaks related to Si–O bond in XRD spectrums. There are two absorption peaks located at 1100 cm−1 in ZnO-Air spectra because of coupling between stretching vibrations, the weak band at 880 cm−1 is assigned to Si–O–Si symmetrical stretching frequency. The absorption peaks of ZnO-Air and ZnO-Ar around 1100 cm−1 are attributed to Si–O–Si antisymmetric stretch related to SiO44 −, which may be associated with bonding between Si and O resulting from Si diffusion into
Fig. 2. FTIR of Si-based films.
Fig. 3. XPS spectra of Zn 2p (Solid line: Experimental data, Dotted line: Fitting).
ZnO [3]. Moreover, the peak near 1100 cm−1 is unclear in AZO-Ar, which means it is a very broad and unstructured band due to the perturbation of Si–O–Si and Si–O–Al [37,38]. Fig. 3 shows the Zn 2p XPS spectra of ZnO-Air, ZnO-Ar and AZO-Ar respectively. According to standard spectrum [39], the Zn 2p binding energy of ZnO-Ar is 1020.6eV, indicating the electron binding energy of Zn ion decreases, which caused by the reduction of charge transfer from Zn ions to O ions resulting from the decreasing of O atoms coming from ZnO film, in other words, oxygen vacancies exist in ZnO-Ar. But the Zn 2p binding energy of ZnO-Air shifts to high energy, this indicates more O atoms bond with Zn atoms, illustrating high oxygen content in ZnOAir, and it is consistent with previous results [3]. From the Zn 2p spectra of AZO-Ar, it can been seen that the stoichiometry between Zn and O in AZO-Ar film to be close to 1:1, it is probably because the oxygen vacancies are eliminated by O atoms related to Al2O3 powder, as described by formula (4) [3]. It can be inferred that the oxygen content of AZO-Ar is higher than that of ZnO-Ar but less than ZnO-Air.
Al2 O3 + 2ZnO = 2ZnAlO2 + O + 2e
(4)
The Hall measurements show the ZnO-Air film is p-type, this may due to the O atoms in air diffusion into ZnO film and resulting in acceptor concentration [3,40]. Both ZnO-Ar and AZO-Ar films are n-type, this also support the above conclusion. Fig. 4-a shows a broad and strong Si 2p spectrum between 100.2eV and 103.6eV, which is contributed by Si2O3 (marked as 1), SiO (marked as 2) and SiO44 − (Almand, marked as 3), as the thickness of Si-based ZnO and AZO films both are about 60 nm, this indicates Si atoms diffuse into ZnO film at least 50 nm (or the interface interaction width is at least 50 nm) and resulting in silicon oxide at Si/ZnO interface. Fig. 4-b shows the Si 2p spectra of AZO-Ar, it has two weak peaks that assigned to SiO (marked as 2) and SiO44 − (Almand, marked as 3), implying that the diffusion depth of Si into ZnO film is shallower compared with Si 2p spectra of ZnO-Ar. Furthermore, the Si 2p of ZnO-Air is not detected, but the FTIR results show the silicon oxide between ZnO-Ar and ZnOAir are similar, so it is concluded that certain Si atoms in silicon substrate interact with ZnO coating layer during thermal treatment, Si atoms doesn't need to diffuse deeper into ZnO film due to the sufficient oxygen source when annealed in air atmosphere, but the limited oxygen in ZnO-Ar film drive Si atoms coming from Si substrate near the Si/ZnO interface diffuse deeper across into ZnO film because of the reaction correlation (3) [25], that is to say, less oxygen could increase the width of Si/ZnO interfacial interaction and leave oxygen vacancies in ZnO films due to Si robbing O atoms related to Zn–O bonding. Therefore, it can be obtained that the width of interface interaction between Si and
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Fig. 4. Si 2p XPS spectra of Si-based films: (a)ZnO-Ar, (b)AZO-Ar.
ZnO depends on oxygen content in ZnO films. In order to make the conclusion more convincing and comprehensive, the Si/ZnO model was constructed and analyzed based on molecular dynamics and first principles in the following sections. 3. Calculations 3.1. Calculation by molecular dynamics simulation 3.1.1. Computational details Molecular Dynamics calculations were performed with LAMMPS to simulate the interfacial interaction between Si and ZnO. The MD model, as shown in Fig. 5, which compose of 1372 Zn atoms, 1372 O atoms and 1728 Si atoms. For this, the interface mismatch between Si and ZnO is 0.535% based on calculating formula (5).
f = as − ae / as
(5)
Where f denotes the interface mismatch, as and ae are the lattice constants of Si(100) and ZnO respectively. The TERSOFF potentials were employed for Si-Si [41] and Si–O [42,43], and the response between Zn and O atoms were treated using ReaxFF potential for its good performance in Raymand D's research [44]. After the MD model was equilibrated for 10 ps at 300 K in the starting and ending of NVT ensemble, the temperature was raised to 800 K in the interval of 10ps and keeping for 15 ps, then, the temperature gradually reduced to 300 K in 100 ps?
Si/ZnO model respectively. Compared with initial model, the Si atoms are tightly coupled with Zn and O atoms at Si/ZnO interface and the interface becomes blurry, indicating thermal treatment is conducive to interfacial interaction. In order to clearly study the interface interaction between Si and ZnO, the Si/ZnO interface interaction is shown in Fig. 6. Fig. 6 shows the Si/ZnO interaction situation after 800 K annealing. It can been seen that the upper three Si layers tighter hold on O atoms related to Zn–O bonding, and the 4th Si atoms are parted from O atoms. This illustrates a certain amount of Si atoms interact with O atoms, which is consist with FTIR results. Furthermore, the structural and chemical bond characteristics of Si, Zn and O atoms were studied using first principles in the following process. 3.2. Calculation by first principles 3.2.1. Method In this section, the calculations were carried out using first principles based on density functional theory(DFT), the exchange-correlation function was generalized gradient approximation(GGA) [45,46], ultrasoft pseudo-potential was used for geometric relaxation. 2 × 2 × 2supercells of Zn site replaced by Si atom (ZnO:Si) and Si interstitial atom in ZnO(ZnO–Si-int) were built and as shown in Fig. 7. 3.2.2. Results and discussion Fig. 8 shows the charge density difference of ZnO:Si and ZnO:Si-int, it can be seen that there are little difference between the electron
3.1.2. Results and discussion Fig. 5 shows the thickness of Si and ZnO are 3.26 nm and 3.24 nm in
Fig. 5. The Si/ZnO model: (a) Initial model, (b) Model after thermal treatment.
Fig. 6. The interaction situation of Si/ZnO: (a) 1st layer Si, (b) 2nd layer Si, (c) 3rd layer Si, (d) 4th layer Si. 21897
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Air) atmosphere at 500 °C respectively. The interfacial interaction and bonding mechanism of Si based ZnO were investigated by experiment (XRD, FTIR,XPS and Hall) and simulation(molecular dynamics and first principle). XRD and FTIR show the prepared Si based films have (002) orientation with good crystal structure for ZnO growth. Certain silicon oxide are formed at Si/ZnO interface, which trends to (111) preferred orientations under annealing in air ambience, indicating Si atoms bond with sufficient O atoms. XPS results find that oxygen vacancies exist in ZnO-Ar, high oxygen content in ZnO-Air, and the stoichiometry between Zn and O in AZO-Ar film to be close to 1:1. Hall measurements results also support this conclusion. Moreover, from Si 2p XPS spectra of Si-based films, a broad and strong Si 2p spectra is observed in ZnOAr, and two weak Si 2p peaks exist in AZO-Ar, but Si 2p peaks are not observed in ZnO-Air. From above discussion we can see the increasing oxygen source may lead to the Si diffusion depth gradually diminish, from ZnO-Ar, AZO-Ar to ZnO-Air film. In order to illuminate interfacial interaction and bonding mechanism of Si/ZnO, based on molecular dynamics and first principle, the Si/ZnO and Si diffusion into ZnO models were built and analyzed in this work, the results show a certain amount of Si atoms (as interstitial state) interact with O atoms after thermal treatment, and the SiO32 − (silicate) is formed due to the reaction between Si atoms and O atoms related to Zn–O bonding, furthermore, the Si–O bond is so strong that fracture Zn–O chemical bond that close proximity to Si atom, and leaving oxygen vacancy in ZnO, which is consistent with experimental results. It can be concluded that the interfacial interaction between Si and ZnO depends on oxygen content coming from ZnO film, the limited oxygen could increase the width of Si/ZnO interfacial interaction and leave oxygen vacancies in ZnO films due to Si robbing O atoms related to Zn–O bonding.
Fig. 7. The model of Si diffuse across into ZnO: (a) ZnO:Si, (b) ZnO–Si-int.
Fig. 8. Charge density difference: (a) ZnO:Si, (b)ZnO:Si-int.
Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant 51575246 and 61076098, the support of Six Talent Peaks Project in Jiangsu Province(JXQC_006), the support of A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, the support of Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_1761), the High Performance Computing Platform of Jiangsu University, during the course of this work. References
Fig. 9. The structure and charge properties of ZnO:Si-int: (a)Initial model, (b) the model after geometric relaxation, (c) Charge density difference of (b).
transfer from Si atom to adjacent O atom in ZnO:Si and ZnO:Si-int, showing that Si–O bond is not affect by Si doping type, but Fig. 9(c) shows the Si–O bond is so strong that break apart the Zn–O chemical bond that close proximity to Si atom, and leaving oxygen vacancy in ZnO. Compared with initial model in Fig. 9(a), the length of Si–O is shortened by 0.267 Å and 0.313 Å, as shown in Fig. 9(b), illustrating the Si atom has turned into SiO32 − (silicate), which is consistent with XPS results. It can be concluded that Si atoms diffusion into ZnO as interstitial impurities, and form complexes with Zn and O atoms at Si/ZnO interface. Moreover, the rationality of simulation process is verified by above discussion in this work. 4. Conclusions In summary, ZnO and AZO thin films were successfully deposited on p-Si(100) substrates using magnetron sputtering technique, and then annealed in the argon (ZnO-Ar and AZO-Ar) and air (ZnO-Air and AZO-
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Study on interfacial interaction between Si and ZnO a
a,b
Zhixiang Liu , Yunqing Tang
, Ningbo Liao
c,**
T
a,*
, Ping Yang
a
Laboratory of Advanced Design, Manufacturing & Reliability for MEMS/NEMS/OEDS, School of Mechanical Engineering, Jiangsu University, Zhenjiang, 212013, PR China Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 2V4, Canada c School of Mechanical Engineering, Wenzhou University, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Interfacial reaction Si/ZnO heterojunctions Molecular dynamics First principle
The interfacial reaction and bonding mechanism of Si based undoped and Al-doped ZnO films were investigated experimentally and numerically. Si/ZnO heterojunctions were fabricated onto p-Si(100) wafers by RF magnetron sputtering, then the undoped and Al doped ZnO (AZO) films coated Si substrates were annealed in argon atmosphere (ZnO-Ar and AZO-Ar) and air atmosphere (ZnO-Air and AZO-Air) respectively. X-ray diffraction (XRD) and Fourier transform infrared spectra (FTIR) show the prepared Si-based films have wurtzite structure with (002) preferential growth, and demonstrate several Si oxides exist in both cases. X-ray photoelectron spectroscopy (XPS) finds that oxygen vacancies exist in ZnO-Ar, moreover, the stoichiometry between Zn and O in AZOAr film to be close to 1:1, and high oxygen content in ZnO-Air, which are also supported by Hall measurement. Furthermore, the Si 2p of XPS spectrum shows the width of interfacial interaction between Si and ZnO become weaker with the order of ZnO-Ar, AZO-Ar and ZnO-Air. To elucidate the joining and bonding mechanism between Si substrate and ZnO coating layer, molecular dynamics and first principle simulations were performed in this work, the simulation results show certain interstitial Si atoms coming from Si substrate interact with O atoms related to the Zn–O bonding at the Si/ZnO interface, and resulting in oxygen vacancies in ZnO film. The experiment and simulation show the interfacial interaction between Si and ZnO depends on oxygen content coming from ZnO coating layer, the limited oxygen coming from ZnO film could increase the width of Si/ZnO interfacial interaction and leave oxygen vacancies in ZnO film due to Si robbing O atoms related to Zn–O bonding.
1. Introduction ZnO is a versatile material not only due to its non-toxicity, availability and excellent photoelectric properties but also for the possibility of integrating such devices with mature and advanced Si-based technology [1–17]. The Si/ZnO heterojunction has been widely used in optoelectronic applications, such as photodetectors [9], photodiode [10], terrestrial solar cells [11,12], acoustics materials [13], surfacewave convolver [14], dynamic switching element [15]. Along the process of application and study, the interfacial interactions and formations at Si/ZnO interface that demonstrated by many results [16–23], could profoundly degrade the attaining performance of Si/ ZnO devices, which would limit the applications of Si/ZnO heterojunction [15–27]. In order to understand the interfacial interactions between Si and ZnO, some researches have been made to elaborate the interfacial interaction and bonding mechanism of Si/ZnO junction.
*
Gabás M et al. [17] used spray pyrolysis to grow ZnO films on commercial Si substrates for solar cell, the results showed that the silicon oxide layer is inherent to the growth of ZnO, and oxygen enrichment might be the reason of interface reaction between Si substrate and ZnO film. Kim D K et al. [18] prepared Si-based ZnO films under different substrate temperature by magnetron sputtering, they found that the amorphous SiO2 layer located at Si/ZnO interface could result in oxygen vacancy, which result from oxygen related to the Zn–O bonding, and higher substrate temperature (600 °C) could suppress the Zn–O bonding. Ogata K et al. [19] thought it's difficult to prepare highquality Si based ZnO films because of the easier silicon dioxide formation due to the larger formation enthalpy of SiO2 (−910.7 ± 1.0 kJ/mol) compared to that of ZnO (−350.46 ± 0.27 kJ/mol). Similarly, Wimmer M et al. [20] fabricated Si/ZnO heterostructure by plasma-enhanced chemical vapor deposition (PECVD), they concluded that the silicon oxide exist near Si/ZnO
Corresponding author. Corresponding author. E-mail addresses: [email protected], [email protected] (P. Yang).
**
https://doi.org/10.1016/j.ceramint.2019.07.200 Received 4 April 2019; Received in revised form 18 June 2019; Accepted 17 July 2019 Available online 18 July 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. XRD spectra of thin films annealed at 500 °C: (a)ZnO-Air, (b)ZnO-Ar, (c)AZO-Ar.
interface at the expense of Zn–O bonds. Hu Z S et al. [21] grown ZnO films on Si substrate by sputtering method, the results showed that the amorphous silicon oxide located at Si/ZnO interface is caused by Si robbing oxygen atoms from ZnO near the interface, and leave oxygen vacancy in ZnO films. Gerlach D et al. [22] used spatially resolved photoelectron emission microscopy to investigate the interfacial chemical properties of a-Si:H(P)/ZnO(n) prepared by PECVD, the results showed that the silicon oxide formed at the Si/ZnO interface could be explained by reaction correlation (1). However, Si/ZnO junctions were fabricated using spray-pyrolysis method by Kobayashi H et al. [23], they demonstrated that the oxygen atoms in atmosphere penetrate ZnO film and react with Si atoms to form silicon oxide at Si/ZnO interface (reaction correlation 2), not from the reaction of Si with ZnO(reaction correlation 3). Furthermore, the free energy of reaction correlation 3 ( ΔG = −220kJmol−1) is much more positive than that of reaction correlation 2( ΔG = −857kJmol−1), and the progress of reaction correlation 2 is very slow due to the large activation energy of Zn–O bond cleavage. In addition, Baik D G et al. [24] also support the conclusion that silicon oxide located at the Si/ZnO interface was caused by oxygen diffusing through the porous ZnO films. But Yu H K et al. [25] showed that the SiOx at Si/ZnO interface is more thermodynamically favorable (reaction correlation 3), because of Gibbs free energy change of SiO2( ΔGf = −730.256kJ / mol ) is much smaller than that of ZnO ( ΔGf = −248.15kJ / mol ) under 1000 K.
y⋅ZnO + x⋅Si → Zn + Si x Oy
(1)
Si + O2 → SiO2
(2)
Si + 2ZnO → SiO2 + 2Zn
(3)
From the discussion above, it can be inferred that the interfacial interaction and bonding mechanism of Si based ZnO remains unclear, but the Si/ZnO structure is a critical for Si based photoelectronics and
other progressive devices [22,26–30], it is essential to study the mechanism of phase transition between Si and ZnO. In this work, we try to shed light on this controversy, giving argumentation and information about how Si and ZnO interaction with each other. With this aim, the ZnO and AZO films were deposited on p-Si(100) substrates by RF magnetron sputtering, and the prepared Si based films undergo thermal treatment in argon and air atmosphere respectively. The crystal structure, chemical composition and heterogeneous interface properties were investigated via experiment (XRD, FTIR, XPS and Hall measurements) and simulation (molecular dynamics and first principles). 2. Experimental test 2.1. Experimental details ZnO and AZO films were RF sputter-deposited onto p-Si(100) wafer at substrate temperature 100 °C, the ZnO target(99.99% pure ZnO powder) and the AZO target with 2 wt% pure Al2O3 powder were used to grow films, then the prepared films were annealed at 500 °C in argon and air atmosphere (ZnO-Ar and AZO-Ar, ZnO-Air and AZO-Air for short respectively). The film thicknesses of films were measured by TF ProbeTM SR300 thin film analyser. The film crystal structures were characterized by X-ray Diffractometer (Model: D /max 2500VB3 + / PC ). The chemical composition were investigated using Fourier Transform Infrared (FTIR) spectroscopy (Model: NEXUS) with KBr pellet. The electrical properties were carried out by Hall measurements (Model: HALL8800). The elemental composition and chemical state were studied using X-ray photoelectron spectrometer (Model: ESCALABTM 250Xi ), XPS Data treatment was performed using CasaXPS software [31], and the binding energies were calibrated from the C1s peak located at 284.8eV. The X-ray diffraction patterns were analyzed using the MDI Jade 6.5 software in this work.
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Table 1 Influence of annealing atmosphere on lattice parameters. Si based films
Lattice parameter(nm)
ZnO-Air ZnO-Ar AZO-Ar Pure ZnO JCPDS-PDF#36-1451
a
c
0.3244 0.3248 0.3329 0.3249
0.5199 0.5184 0.5170 0.5206
2.2. Results and discussion Fig. 1 a-c show the XRD spectra of ZnO and AZO films respectively. It's find that all Si based films have preferential orientation along (002) direction, and the ZnO-Air has the strongest (002) diffraction peak compared to others, indicating the c-axis oriented growth of grains, and the single crystal ZnO thin films were successfully prepared in this work according to relevant literature [32]. Furthermore, the strongest SiO2(silicon oxide) diffraction peak is observed in ZnO-Air pattern, this illustrates silicon oxide with (111) preferential growth are formed in Sibased ZnO film that annealed in air atmosphere. The crystal sizes of prepared films are obtained by Scherrer formula [33] and Bragg formula [34], which are shown in Table 1. It can been seen that the crystal structures(a and c ) of all films are close to pure ZnO(JCPDS-PDF#36–1451), showing the prepared Si-based ZnO and AZO films have wurtzite structure, moreover, the lattice parameter a of Si based films annealed in argon are larger than that of the films annealed in air, and the lattice parameter c of films annealed in argon are smaller, this indicates the annealing atmosphere has dramatically influence on crystal structure of ZnO films. Fig. 2 shows the three absorption peaks located at 405 cm−1 are strong and sharp, which means all the films have metallic oxide structure related to Zn–O bonding [35], indicating the prepared Si based films have hexagonal structure of ZnO, which are in correspondence with XRD results. The three small bands around 680 cm−1 are ascribed to Si–O–Si stretching vibrations [36], this is consistent with the results of peaks related to Si–O bond in XRD spectrums. There are two absorption peaks located at 1100 cm−1 in ZnO-Air spectra because of coupling between stretching vibrations, the weak band at 880 cm−1 is assigned to Si–O–Si symmetrical stretching frequency. The absorption peaks of ZnO-Air and ZnO-Ar around 1100 cm−1 are attributed to Si–O–Si antisymmetric stretch related to SiO44 −, which may be associated with bonding between Si and O resulting from Si diffusion into
Fig. 2. FTIR of Si-based films.
Fig. 3. XPS spectra of Zn 2p (Solid line: Experimental data, Dotted line: Fitting).
ZnO [3]. Moreover, the peak near 1100 cm−1 is unclear in AZO-Ar, which means it is a very broad and unstructured band due to the perturbation of Si–O–Si and Si–O–Al [37,38]. Fig. 3 shows the Zn 2p XPS spectra of ZnO-Air, ZnO-Ar and AZO-Ar respectively. According to standard spectrum [39], the Zn 2p binding energy of ZnO-Ar is 1020.6eV, indicating the electron binding energy of Zn ion decreases, which caused by the reduction of charge transfer from Zn ions to O ions resulting from the decreasing of O atoms coming from ZnO film, in other words, oxygen vacancies exist in ZnO-Ar. But the Zn 2p binding energy of ZnO-Air shifts to high energy, this indicates more O atoms bond with Zn atoms, illustrating high oxygen content in ZnOAir, and it is consistent with previous results [3]. From the Zn 2p spectra of AZO-Ar, it can been seen that the stoichiometry between Zn and O in AZO-Ar film to be close to 1:1, it is probably because the oxygen vacancies are eliminated by O atoms related to Al2O3 powder, as described by formula (4) [3]. It can be inferred that the oxygen content of AZO-Ar is higher than that of ZnO-Ar but less than ZnO-Air.
Al2 O3 + 2ZnO = 2ZnAlO2 + O + 2e
(4)
The Hall measurements show the ZnO-Air film is p-type, this may due to the O atoms in air diffusion into ZnO film and resulting in acceptor concentration [3,40]. Both ZnO-Ar and AZO-Ar films are n-type, this also support the above conclusion. Fig. 4-a shows a broad and strong Si 2p spectrum between 100.2eV and 103.6eV, which is contributed by Si2O3 (marked as 1), SiO (marked as 2) and SiO44 − (Almand, marked as 3), as the thickness of Si-based ZnO and AZO films both are about 60 nm, this indicates Si atoms diffuse into ZnO film at least 50 nm (or the interface interaction width is at least 50 nm) and resulting in silicon oxide at Si/ZnO interface. Fig. 4-b shows the Si 2p spectra of AZO-Ar, it has two weak peaks that assigned to SiO (marked as 2) and SiO44 − (Almand, marked as 3), implying that the diffusion depth of Si into ZnO film is shallower compared with Si 2p spectra of ZnO-Ar. Furthermore, the Si 2p of ZnO-Air is not detected, but the FTIR results show the silicon oxide between ZnO-Ar and ZnOAir are similar, so it is concluded that certain Si atoms in silicon substrate interact with ZnO coating layer during thermal treatment, Si atoms doesn't need to diffuse deeper into ZnO film due to the sufficient oxygen source when annealed in air atmosphere, but the limited oxygen in ZnO-Ar film drive Si atoms coming from Si substrate near the Si/ZnO interface diffuse deeper across into ZnO film because of the reaction correlation (3) [25], that is to say, less oxygen could increase the width of Si/ZnO interfacial interaction and leave oxygen vacancies in ZnO films due to Si robbing O atoms related to Zn–O bonding. Therefore, it can be obtained that the width of interface interaction between Si and
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Fig. 4. Si 2p XPS spectra of Si-based films: (a)ZnO-Ar, (b)AZO-Ar.
ZnO depends on oxygen content in ZnO films. In order to make the conclusion more convincing and comprehensive, the Si/ZnO model was constructed and analyzed based on molecular dynamics and first principles in the following sections. 3. Calculations 3.1. Calculation by molecular dynamics simulation 3.1.1. Computational details Molecular Dynamics calculations were performed with LAMMPS to simulate the interfacial interaction between Si and ZnO. The MD model, as shown in Fig. 5, which compose of 1372 Zn atoms, 1372 O atoms and 1728 Si atoms. For this, the interface mismatch between Si and ZnO is 0.535% based on calculating formula (5).
f = as − ae / as
(5)
Where f denotes the interface mismatch, as and ae are the lattice constants of Si(100) and ZnO respectively. The TERSOFF potentials were employed for Si-Si [41] and Si–O [42,43], and the response between Zn and O atoms were treated using ReaxFF potential for its good performance in Raymand D's research [44]. After the MD model was equilibrated for 10 ps at 300 K in the starting and ending of NVT ensemble, the temperature was raised to 800 K in the interval of 10ps and keeping for 15 ps, then, the temperature gradually reduced to 300 K in 100 ps?
Si/ZnO model respectively. Compared with initial model, the Si atoms are tightly coupled with Zn and O atoms at Si/ZnO interface and the interface becomes blurry, indicating thermal treatment is conducive to interfacial interaction. In order to clearly study the interface interaction between Si and ZnO, the Si/ZnO interface interaction is shown in Fig. 6. Fig. 6 shows the Si/ZnO interaction situation after 800 K annealing. It can been seen that the upper three Si layers tighter hold on O atoms related to Zn–O bonding, and the 4th Si atoms are parted from O atoms. This illustrates a certain amount of Si atoms interact with O atoms, which is consist with FTIR results. Furthermore, the structural and chemical bond characteristics of Si, Zn and O atoms were studied using first principles in the following process. 3.2. Calculation by first principles 3.2.1. Method In this section, the calculations were carried out using first principles based on density functional theory(DFT), the exchange-correlation function was generalized gradient approximation(GGA) [45,46], ultrasoft pseudo-potential was used for geometric relaxation. 2 × 2 × 2supercells of Zn site replaced by Si atom (ZnO:Si) and Si interstitial atom in ZnO(ZnO–Si-int) were built and as shown in Fig. 7. 3.2.2. Results and discussion Fig. 8 shows the charge density difference of ZnO:Si and ZnO:Si-int, it can be seen that there are little difference between the electron
3.1.2. Results and discussion Fig. 5 shows the thickness of Si and ZnO are 3.26 nm and 3.24 nm in
Fig. 5. The Si/ZnO model: (a) Initial model, (b) Model after thermal treatment.
Fig. 6. The interaction situation of Si/ZnO: (a) 1st layer Si, (b) 2nd layer Si, (c) 3rd layer Si, (d) 4th layer Si. 21897
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Air) atmosphere at 500 °C respectively. The interfacial interaction and bonding mechanism of Si based ZnO were investigated by experiment (XRD, FTIR,XPS and Hall) and simulation(molecular dynamics and first principle). XRD and FTIR show the prepared Si based films have (002) orientation with good crystal structure for ZnO growth. Certain silicon oxide are formed at Si/ZnO interface, which trends to (111) preferred orientations under annealing in air ambience, indicating Si atoms bond with sufficient O atoms. XPS results find that oxygen vacancies exist in ZnO-Ar, high oxygen content in ZnO-Air, and the stoichiometry between Zn and O in AZO-Ar film to be close to 1:1. Hall measurements results also support this conclusion. Moreover, from Si 2p XPS spectra of Si-based films, a broad and strong Si 2p spectra is observed in ZnOAr, and two weak Si 2p peaks exist in AZO-Ar, but Si 2p peaks are not observed in ZnO-Air. From above discussion we can see the increasing oxygen source may lead to the Si diffusion depth gradually diminish, from ZnO-Ar, AZO-Ar to ZnO-Air film. In order to illuminate interfacial interaction and bonding mechanism of Si/ZnO, based on molecular dynamics and first principle, the Si/ZnO and Si diffusion into ZnO models were built and analyzed in this work, the results show a certain amount of Si atoms (as interstitial state) interact with O atoms after thermal treatment, and the SiO32 − (silicate) is formed due to the reaction between Si atoms and O atoms related to Zn–O bonding, furthermore, the Si–O bond is so strong that fracture Zn–O chemical bond that close proximity to Si atom, and leaving oxygen vacancy in ZnO, which is consistent with experimental results. It can be concluded that the interfacial interaction between Si and ZnO depends on oxygen content coming from ZnO film, the limited oxygen could increase the width of Si/ZnO interfacial interaction and leave oxygen vacancies in ZnO films due to Si robbing O atoms related to Zn–O bonding.
Fig. 7. The model of Si diffuse across into ZnO: (a) ZnO:Si, (b) ZnO–Si-int.
Fig. 8. Charge density difference: (a) ZnO:Si, (b)ZnO:Si-int.
Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant 51575246 and 61076098, the support of Six Talent Peaks Project in Jiangsu Province(JXQC_006), the support of A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, the support of Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_1761), the High Performance Computing Platform of Jiangsu University, during the course of this work. References
Fig. 9. The structure and charge properties of ZnO:Si-int: (a)Initial model, (b) the model after geometric relaxation, (c) Charge density difference of (b).
transfer from Si atom to adjacent O atom in ZnO:Si and ZnO:Si-int, showing that Si–O bond is not affect by Si doping type, but Fig. 9(c) shows the Si–O bond is so strong that break apart the Zn–O chemical bond that close proximity to Si atom, and leaving oxygen vacancy in ZnO. Compared with initial model in Fig. 9(a), the length of Si–O is shortened by 0.267 Å and 0.313 Å, as shown in Fig. 9(b), illustrating the Si atom has turned into SiO32 − (silicate), which is consistent with XPS results. It can be concluded that Si atoms diffusion into ZnO as interstitial impurities, and form complexes with Zn and O atoms at Si/ZnO interface. Moreover, the rationality of simulation process is verified by above discussion in this work. 4. Conclusions In summary, ZnO and AZO thin films were successfully deposited on p-Si(100) substrates using magnetron sputtering technique, and then annealed in the argon (ZnO-Ar and AZO-Ar) and air (ZnO-Air and AZO-
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