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Structural and optical properties of Nb-doped β-Ga2O3 thin films deposited by RF magnetron sputtering
Vacuum 146 (2017) 93e96
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Structural and optical properties of Nb-doped b-Ga2O3 thin films deposited by RF magnetron sputtering Hao Zhang, Jinxiang Deng*, Zhiwei Pan, Zhiying Bai, Le Kong, Jiyou Wang College of Applied Sciences, Beijing University of Technology, Beijing 100124, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 August 2017 Received in revised form 20 September 2017 Accepted 20 September 2017 Available online 21 September 2017
The intrinsic and Nb-doped b-Ga2O3 (b-Ga2O3:Nb) thin films have been deposited on the Si and quartz substrates by radio frequency magnetron technique in argon ambient. The effects of Nb doping on the structural and optical properties of Ga2O3:Nb thin films have been investigated. After Nb-doping, the crystal lattice, surface morphology, optical transmittance and optical energy gap of the b-Ga2O3 films are greatly changed. The crystal lattice of the b-Ga2O3:Nb films is augmented, the energy gap shrinks and the crystalline quality is improved. XPS spectra shows that niobium is incorporated into the oxide matrix and present in the b-Ga2O3 as Nb(Ⅳ). © 2017 Elsevier Ltd. All rights reserved.
Keywords: Nb-doped b-Ga2O3 films RF magnetron sputtering Crystal structure Optical properties
1. Introduction Transparent conducting oxides films (TCOs), as valuable materials, are widely applied in photovoltaics, LED, LCD, ELD, and it has reached the practical stage so far. As a kind of TCO material that possess great application prospects in ultraviolet (UV) optoelectronic devices, b-Ga2O3 shows superior performance as a wide and direct band gap material. Moreover, b-Ga2O3 has a low-cost advantage over other wide band gap TCO materials, and suitable for mass production. Therefore b-Ga2O3 thin film has drawn more and more attention and becomes a hot issue in recent years. Initially, many research groups focus their attention on how to enhance the growth speed and fabrication quality of monoclinic bGa2O3 films [1e5]. However, the conductive properties of pure bGa2O3 thin film is very poor due to the large energy band gap of bGa2O3 [6], and restricting b-Ga2O3 film's application in optoelectronic devices. Considering the above problem, many research groups begin to improve the optical and electrical properties of bGa2O3 thin films by doping technology [7e10]. Besides, H. Peelaers explores the viability of using W, Mo, Re, and Nb impurities as ntype dopants in b-Ga2O3 through first-principles calculations [11]. And it's found that Nb emerges as the best candidate for transitionmetal n-type doping of Ga2O3. As for selecting Nb as the dopant of
* Corresponding author. E-mail address: [email protected] (J. Deng). https://doi.org/10.1016/j.vacuum.2017.09.033 0042-207X/© 2017 Elsevier Ltd. All rights reserved.
Ga2O3, previous researches only focus on the charge-trapping properties of Nb-doped Ga2O3 by using an Al/Al2O3/GaNbO/SiO2/ Si structure [12]. There are still many fundamental properties of bGa2O3:Nb thin films remain insufficiently unknown. Therefore, further systematic study of the structural, optical and electrical properties of b-Ga2O3:Nb thin films is of vital importance for the future applications. In this work, the intrinsic and b-Ga2O3:Nb thin films are grown on Si and quartz substrates by RF magnetron sputtering technique in argon ambient. We report on the influence of Nb-doping on morphology, structural and optical properties of b-Ga2O3:Nb thin films. 2. Experimental The intrinsic and b-Ga2O3:Nb thin films were deposited on the Si and quartz glass substrates by RF magnetron sputtering technique. All the substrates were cleaned in toluene, acetone, ethanol and deionized water ultrasonically for 15 min. Then, the substrates were dried with flowing nitrogen and placed in the sputtering chamber. A high purity Ga2O3 target (purity of 99.99%) and Nb2O5 target (purity of 99.99%) with a diameter of 80 mm were used as Ga2O3 and Nb2O5 source materials, respectively. The RF power applied to the targets was set at ~80 W. Before each deposition, the Ga2O3 target and Nb2O5 target were pre-sputtered for 10 min to remove the contaminants on their surface. The base pressure in the sputtering chamber was 1 103 Pa; then, high purity argon gas
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H. Zhang et al. / Vacuum 146 (2017) 93e96
Fig. 1. Schematic diagram of the as-grown Ga2O3/Nb2O5 multilayer film structure.
(99.999%) was introduced into the chamber through a mass flow controller and the films were deposited at a working pressure of 0.5 Pa. It's worth noting that the two targets are fixed, while the position of substrate can be moved so that the deposition time and species can be controlled by adjusting the substrate position. In this work, Ga2O3 and Nb2O5 were deposited on the substrate alternately. Fig. 1 illustrates the schematic diagram of Ga2O3/Nb2O5 multilayer film structure. The number of repetition periods of Ga2O3 and Nb2O5 were 6 and 5, respectively. As a reference, the intrinsic Ga2O3 film (sample A) with a deposition time of 1800 s was obtained. Sample B and sample C were Ga2O3/Nb2O5 multilayer film structure. And the deposition time of Ga2O3 and Nb2O5 were 300 s and 10 s every period for sample B. The deposition time of Ga2O3 and Nb2O5 were 300 s and 30 s every period for sample C. Subsequently, the as-grown intrinsic and b-Ga2O3:Nb thin films were subjected to annealing in flowing argon gas at 1000 C for 5 h at atmospheric pressure. The doping levels Nb in sample B and sample C were determined to be 0.39 wt% and 3.31 wt% measured by EDS, respectively. The crystalline structure of the films was examined by X-ray diffraction (XRD) using Purkinje D3 diffractometer. Ultravioletvisible (UV-vis) absorption spectrums were carried out using a Shimadzu UV-3600 spectrophotometer. The thickness of the films were measured by a step profiler (Veeco Dektak 150). The surface morphology was characterized using scanning electron microscope
Fig. 3. The top-view SEM images of the b-Ga2O3:Nb films with different Nb concentrations after annealing. Scale bar ¼ 200 nm.
(Supra 55). The valences of chemical ions were analyzed by X-ray photoelectron spectroscopy (VG ESCALAB MK II). 3. Results and discussion
Fig. 2. XRD patterns in a narrow range of the b-Ga2O3:Nb thin films with different doping levels (0 wt%, 0.39 wt%, and 3.31 wt%).
Fig. 2 shows the XRD patterns of intrinsic and Nb-doped Ga2O3 films deposited onto Si substrates post-annealed in Ar ambient at 1000 C for 5 h. It can be observed that polycrystalline b-Ga2O3 films are obtained after post-annealing as shown in Fig. 2. And the diffraction peak gradually shifts to smaller angle with the increase of Nb content, indicates that Nb ions has induced remarkable structural modification of the b-Ga2O3 thin films. Since the radii of Nb4þ ion approximate to that of Ga ions (Nb4þ:0.68 Å, Ga3þ:0.62 Å for octahedral coordination and 0.47 Å for tetrahedral coordination, respectively), the most probable site for the Nb ions substitution is at the octahedral sites of the b-Ga2O3 lattice. And the incorporation of Nb into b-Ga2O3 leads to the increase of lattice constants of bGa2O3, that is, increase in the (110) d-space and resulting in the diffraction peak shifts to the lower angle. Previous researches show that Mg, Pr, Cu, Sn and Nd ions' doping lead to a degradation of the crystallinity of the b-Ga2O3 films [13e15]. However, with the Nb
H. Zhang et al. / Vacuum 146 (2017) 93e96
95
Fig. 4. XPS spectra for the Ga2O3:Nb (3.31 wt%) thin film: survey (a), Ga 2p (b), Nb 3d (c), and O1s (d) core level.
can be used to calculate the average grain size of the Ga2O3:Nb films. In the Scherrer's equation, D is the average grain size, k is a
constant equal to 0.9, l is the x-ray wavelength, and b is the halfpeak width. The average grain sizes of the Ga2O3:Nb thin films increase from 45.6 nm to 68 nm with the increase of Nb content from 0.39 wt% to 3.31 wt%, indicating an improvement in the crystalline quality. So Nb-doping has a great application prospect in enhancing the film properties and improving the crystalline quality. Fig. 3 shows the top-view SEM images of the intrinsic and bGa2O3:Nb films on Si substrates. Some slice-shaped areas can be
Fig. 5. Transmittance spectra of the b-Ga2O3:Nb films with different Nb concentrations and the plot of (ahn)2 versus hn in inset.
Fig. 6. Optical band gaps of the b-Ga2O3:Nb films as a function of Nb concentration.
doping, the Ga2O3 (110) peak get stronger and narrower, indicating an improvement in the crystalline quality. Besides, Scherrer's equation
D¼
kl : b cos q
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H. Zhang et al. / Vacuum 146 (2017) 93e96
observed on the surface of the intrinsic b-Ga2O3 film as shown in Fig. 3a. While the surface of the b-Ga2O3:Nb films show grain-like morphology as shown in Fig. 3b and c, which is quite different from the intrinsic b-Ga2O3 film. Moreover, well-defined grains with clear grain boundaries are observed for the b-Ga2O3:Nb films, and the grain size of b-Ga2O3:Nb films become bigger with the increase of the Nb content, indicating an improvement in the film crystallinity. The SEM images show clearly that the film crystallinity is improved with Nb-doping, which is consistent with the XRD results. The XPS spectra of the survey, Ga 2p, Nb 3d and O 1s core levels for the 3.31 wt% Nb-doped b-Ga2O3 film are shown in Fig. 4a, b, c and d, respectively. The charge-shift spectrum is calibrated using the fortuitous C 1s peak at 284.8 eV. From the survey, photoelectron lines of C 1s, O 1s, Ga 2p, Ga 3s, Ga 3p, Ga 3d, Nb 3d as well as the O KLL, Ga LMM, Ga LMM1 can be observed as shown in Fig. 4a. In Fig. 4b, it is observed that two symmetrical peaks of Ga 2p1/2 located at 1143 eV and Ga 2p3/2 located at 1116 eV, with a separation distance about 27 eV, which is nearly consistent with the binding energy of the Ga 2p (D ¼ 26.8eV) [16]. Fig. 4c reveals the high resolution XPS spectra of Nb 3d3/2 and Nd 3d5/2 peaks, centered at 209.3 eV and 206.7 eV, respectively. Arfaoui use the fact that the 3d core level shift depends linearly on the valence of Nb atoms to identify the different components found in the photoemission spectra. Using that linear variation as an abacus, they deduce the valence of Nb atoms in the oxide layer from the binding energies [17]. Besides, the XP spectra with a doublet peaks at BE of 209.6e209.9 eV for Nb 3d3/2 and of 206.9e207.5 eV for Nb 3d5/2 can indicate that the valence state of niobium is Nb(V) [18]. In our work, the BE of Nb 3d5/2 is lower than 206.9 eV. Furthermore, the BE of Nb 3d5/2 is consistent with the BE of Nb 3d5/2 of NbO2 in Ref [19]. Thus, we could draw a conclusion that niobium is present in the Ga2O3 as Nb(Ⅳ). In addition, based on the results of Fig. 4c analysis, the O 1s peak at ~530.2 eV is split into two peaks located at 530 eV and 531.4 eV. The main peak at 530 eV can be regard as lattice oxygen in the Ga2O3:Nb thin films, while the peak at 531.4 eV could be due to the C/O or OH1 adsorbed on the film surface [19]. The optical transmittance spectra of the intrinsic and bGa2O3:Nb films deposited on the quartz substrates in the wavelength range of 200e800 nm are shown in Fig. 5. The average transmittance of all the films in visible range are over 80%. The spectrum of the pure b-Ga2O3 shows a sharp intrinsic absorption edge at ~250 nm. With the doping of Nb, the absorption edge of bGa2O3:Nb films display a clear red-shift. For the direct band gap transition semiconductors, the absorption coefficient a and optical band gap (Eg) are related by the equation
ahv ¼ A hv Eg
1=2
;
where A is a constant, n is the frequency of the incident photon and h is the Planck's constant. Then the relation curve of (ahn)2 and hn can be plotted, and Eg can be estimated by extrapolating the straight-line portion of this plot to the energy axis, as shown in the insets of Fig. 5. The band gap Eg gradually decreases from 4.86 eV to 4.71eV with increasing the Nb content as shown in Fig. 6. The incorporation of Nb ions on the substitutional sites of Ga2O3 contribute to new unoccupied electron states in the gap below the conduction band edge, leading to the decrease of the band gap. Besides, previous studies [7,8] indicate a larger lattice plane d spacing is correlated to a narrower band gap, and our experiment results is also consistent with the results reported before. 4. Conclusion In summary, monoclinic b-Ga2O3: Nb thin films were grown on
Si substrates by radio frequency magnetron sputtering. The structural and optical properties investigated by XRD, SEM, UV-Vis transmittance spectra and the XPS confirmed the incorporation of Nb into the b-Ga2O3 crystal lattice. Niobium is present in the bGa2O3 as Nb(Ⅳ). The incorporation of Nb into b-Ga2O3 leads to the increase of lattice constants of b-Ga2O3, an improvement of the crystalline quality, and the decrease of the band gap. These results enrich our knowledge of many fundamental properties of bGa2O3:Nb thin films, and provide application guidance for b-Ga2O3: Nb films in optoelectronics area.
Acknowledgements This work was supported by funding for the development project of Beijing Municipal Education Commission of Science and Technology (grant no. KZ201410005008), Natural Science Foundation of Beijing City (no. 4102014), and China Postdoctoral Science Foundation (no. 2015M570020).
References [1] S. Lee, J. Hwang, J. Kim, S. Jeong, C. Cho, Dielectric characterization of transparent epitaxial Ga2O3 thin film on n-GaN/Al2O3 prepared by pulsed laser deposition, Appl. Phys. Lett. 89 (2006) 182906. [2] Y. Oshima, E.G. Vίllora, K. Shimamura, Quasi-heteroepitaxial growth of betaGa2O3 on off-angled sapphire (0001) substrates by halide vapor phase epitaxy, J. Cryst. Growth 410 (2015) 53e58. [3] S. Nakagomi, Y. Kokubun, Crystal orientation of beta-Ga2O3 thin films formed on c-plane and a-plane sapphire substrate, J. Cryst. Growth 349 (2012) 12e18. [4] P. Guo, J. Xiong, X. Zhao, T. Sheng, C. Yue, B. Tao, X. Liu, Growth characteristics and device properties of MOD derived beta-Ga2O3 films, J. Mater. Sci. Mater. Electron 25 (2014) 3629e3632. [5] Y. Cheng, K. Yang, Y. Peng, Y. Yin, J. Chen, B. Jing, H. Liang, G. Du, Research on the structural and optical stability of Ga2O3 films deposited by electron beam evaporation, J. Mater. Sci. Mater. Electron 24 (2013) 5122e5126. [6] M. Passlack, N.E.J. Hunt, E.F. Schubert, G.J. Zydzik, M. Hong, J.P. Mannaerts, R. Opila, R. Fischer, Dielectric-properties of electron-beam deposited GA2O3 films, Appl. Phys. Lett. 64 (1994) 2715e2717. [7] Z. Wu, G. Bai, Q. Hu, D. Guo, C. Sun, L. Ji, M. Lei, L. Li, P. Li, J. Hao, W. Tang, Effects of dopant concentration on structural and near-infrared luminescence of Nd3þ-doped beta-Ga2O3 thin films, Appl. Phys. Lett. 106 (2015) 171910. [8] W. Li, Y. Peng, C. Wang, X. Zhao, Y. Zhi, H. Yan, L. Li, P. Li, H. Yang, Z. Wu, W. Tang, Structural, optical and photoluminescence properties of Pr-doped beta-Ga2O3 thin films, J. Alloy Compd. 697 (2017) 388e391. [9] W. Mi, X. Du, C. Luan, H. Xiao, J. Ma, Electrical and optical characterizations of beta-Ga2O3: Sn films deposited on MgO(110) substrate by MOCVD, RSC Adv. 4 (2014) 30579e30583. [10] E.J. Rubio, C.V. Ramana, Tungsten-incorporation induced red-shift in the bandgap of gallium oxide thin films, Appl. Phys. Lett. 102 (2013) 191913. [11] H. Peelaers, C.G. Van de Walle, Doping of Ga2O3 with transition metals, Phys. Rev. B 94 (2016) 195203. [12] R.P. Shi, X.D. Huang, K.O. Johnny, P.T. Lai, Nb-doped Ga2O3 as charge-trapping layer for nonvolatile memory applications, Microelectron. Reliab 65 (2016) 64e68. [13] X.J. Feng, Z. Li, W. Li, Y. Luo, J. Ma, Mg-doped beta-Ga2O3 films with tunable optical band gap prepared on MgO (110) substrates by metal-organic chemical vapor deposition, Mater. Sci. Semicond. Process 34 (2015) 52e57. [14] Y.J. Zhang, J.L. Yan, Q.S. Li, Optical and structural properties of Cu-doped bGa2O3 films, Mater. Sci. Eng. B 11 (2011) 846e849. [15] W. Mi, X.J. Du, C.N. Luan, Electrical and optical characterizations of b-Ga2O3:Sn films deposited on MgO(110) substrate by MOCVD, RSC Adv. 4 (2014) 30579e30583. [16] H. Xu, Y. Liu, C. Xu, Y. Liu, C. Shao, R. Mu, Room-temperature ferromagnetism in (Mn, N)-codoped ZnO thin films prepared by reactive magnetron cosputtering, Appl. Phys. Lett. 88 (2006) 242502. [17] I. Arfaoui, J. Cousty, C. Guillot, A model of the NbOxz1 nanocrystals tiling a Nb(1 1 0) surface annealed in UHV, Surf. Sci. 557 (2004) 119e128. thivier, M. Che, Incorporation of Nb(V) into BEA [18] S. Dzwigaj, Y. Millot, C. Me zeolite investigated by XRD, NMR, IR, DR UV-vis, and XPS, Microporous Mesoporous Mater 130 (2010) 162e166. [19] W. Mi, J. Ma, Z. Li, C. Luan, H. Xiao, Characterization of Sn-doped beta-Ga2O3 films deposited on MgO (100) substrate by MOCVD, J. Mater Sci. Mater Electron 26 (2015) 7889e7894.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Structural and optical properties of Nb-doped b-Ga2O3 thin films deposited by RF magnetron sputtering Hao Zhang, Jinxiang Deng*, Zhiwei Pan, Zhiying Bai, Le Kong, Jiyou Wang College of Applied Sciences, Beijing University of Technology, Beijing 100124, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 August 2017 Received in revised form 20 September 2017 Accepted 20 September 2017 Available online 21 September 2017
The intrinsic and Nb-doped b-Ga2O3 (b-Ga2O3:Nb) thin films have been deposited on the Si and quartz substrates by radio frequency magnetron technique in argon ambient. The effects of Nb doping on the structural and optical properties of Ga2O3:Nb thin films have been investigated. After Nb-doping, the crystal lattice, surface morphology, optical transmittance and optical energy gap of the b-Ga2O3 films are greatly changed. The crystal lattice of the b-Ga2O3:Nb films is augmented, the energy gap shrinks and the crystalline quality is improved. XPS spectra shows that niobium is incorporated into the oxide matrix and present in the b-Ga2O3 as Nb(Ⅳ). © 2017 Elsevier Ltd. All rights reserved.
Keywords: Nb-doped b-Ga2O3 films RF magnetron sputtering Crystal structure Optical properties
1. Introduction Transparent conducting oxides films (TCOs), as valuable materials, are widely applied in photovoltaics, LED, LCD, ELD, and it has reached the practical stage so far. As a kind of TCO material that possess great application prospects in ultraviolet (UV) optoelectronic devices, b-Ga2O3 shows superior performance as a wide and direct band gap material. Moreover, b-Ga2O3 has a low-cost advantage over other wide band gap TCO materials, and suitable for mass production. Therefore b-Ga2O3 thin film has drawn more and more attention and becomes a hot issue in recent years. Initially, many research groups focus their attention on how to enhance the growth speed and fabrication quality of monoclinic bGa2O3 films [1e5]. However, the conductive properties of pure bGa2O3 thin film is very poor due to the large energy band gap of bGa2O3 [6], and restricting b-Ga2O3 film's application in optoelectronic devices. Considering the above problem, many research groups begin to improve the optical and electrical properties of bGa2O3 thin films by doping technology [7e10]. Besides, H. Peelaers explores the viability of using W, Mo, Re, and Nb impurities as ntype dopants in b-Ga2O3 through first-principles calculations [11]. And it's found that Nb emerges as the best candidate for transitionmetal n-type doping of Ga2O3. As for selecting Nb as the dopant of
* Corresponding author. E-mail address: [email protected] (J. Deng). https://doi.org/10.1016/j.vacuum.2017.09.033 0042-207X/© 2017 Elsevier Ltd. All rights reserved.
Ga2O3, previous researches only focus on the charge-trapping properties of Nb-doped Ga2O3 by using an Al/Al2O3/GaNbO/SiO2/ Si structure [12]. There are still many fundamental properties of bGa2O3:Nb thin films remain insufficiently unknown. Therefore, further systematic study of the structural, optical and electrical properties of b-Ga2O3:Nb thin films is of vital importance for the future applications. In this work, the intrinsic and b-Ga2O3:Nb thin films are grown on Si and quartz substrates by RF magnetron sputtering technique in argon ambient. We report on the influence of Nb-doping on morphology, structural and optical properties of b-Ga2O3:Nb thin films. 2. Experimental The intrinsic and b-Ga2O3:Nb thin films were deposited on the Si and quartz glass substrates by RF magnetron sputtering technique. All the substrates were cleaned in toluene, acetone, ethanol and deionized water ultrasonically for 15 min. Then, the substrates were dried with flowing nitrogen and placed in the sputtering chamber. A high purity Ga2O3 target (purity of 99.99%) and Nb2O5 target (purity of 99.99%) with a diameter of 80 mm were used as Ga2O3 and Nb2O5 source materials, respectively. The RF power applied to the targets was set at ~80 W. Before each deposition, the Ga2O3 target and Nb2O5 target were pre-sputtered for 10 min to remove the contaminants on their surface. The base pressure in the sputtering chamber was 1 103 Pa; then, high purity argon gas
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H. Zhang et al. / Vacuum 146 (2017) 93e96
Fig. 1. Schematic diagram of the as-grown Ga2O3/Nb2O5 multilayer film structure.
(99.999%) was introduced into the chamber through a mass flow controller and the films were deposited at a working pressure of 0.5 Pa. It's worth noting that the two targets are fixed, while the position of substrate can be moved so that the deposition time and species can be controlled by adjusting the substrate position. In this work, Ga2O3 and Nb2O5 were deposited on the substrate alternately. Fig. 1 illustrates the schematic diagram of Ga2O3/Nb2O5 multilayer film structure. The number of repetition periods of Ga2O3 and Nb2O5 were 6 and 5, respectively. As a reference, the intrinsic Ga2O3 film (sample A) with a deposition time of 1800 s was obtained. Sample B and sample C were Ga2O3/Nb2O5 multilayer film structure. And the deposition time of Ga2O3 and Nb2O5 were 300 s and 10 s every period for sample B. The deposition time of Ga2O3 and Nb2O5 were 300 s and 30 s every period for sample C. Subsequently, the as-grown intrinsic and b-Ga2O3:Nb thin films were subjected to annealing in flowing argon gas at 1000 C for 5 h at atmospheric pressure. The doping levels Nb in sample B and sample C were determined to be 0.39 wt% and 3.31 wt% measured by EDS, respectively. The crystalline structure of the films was examined by X-ray diffraction (XRD) using Purkinje D3 diffractometer. Ultravioletvisible (UV-vis) absorption spectrums were carried out using a Shimadzu UV-3600 spectrophotometer. The thickness of the films were measured by a step profiler (Veeco Dektak 150). The surface morphology was characterized using scanning electron microscope
Fig. 3. The top-view SEM images of the b-Ga2O3:Nb films with different Nb concentrations after annealing. Scale bar ¼ 200 nm.
(Supra 55). The valences of chemical ions were analyzed by X-ray photoelectron spectroscopy (VG ESCALAB MK II). 3. Results and discussion
Fig. 2. XRD patterns in a narrow range of the b-Ga2O3:Nb thin films with different doping levels (0 wt%, 0.39 wt%, and 3.31 wt%).
Fig. 2 shows the XRD patterns of intrinsic and Nb-doped Ga2O3 films deposited onto Si substrates post-annealed in Ar ambient at 1000 C for 5 h. It can be observed that polycrystalline b-Ga2O3 films are obtained after post-annealing as shown in Fig. 2. And the diffraction peak gradually shifts to smaller angle with the increase of Nb content, indicates that Nb ions has induced remarkable structural modification of the b-Ga2O3 thin films. Since the radii of Nb4þ ion approximate to that of Ga ions (Nb4þ:0.68 Å, Ga3þ:0.62 Å for octahedral coordination and 0.47 Å for tetrahedral coordination, respectively), the most probable site for the Nb ions substitution is at the octahedral sites of the b-Ga2O3 lattice. And the incorporation of Nb into b-Ga2O3 leads to the increase of lattice constants of bGa2O3, that is, increase in the (110) d-space and resulting in the diffraction peak shifts to the lower angle. Previous researches show that Mg, Pr, Cu, Sn and Nd ions' doping lead to a degradation of the crystallinity of the b-Ga2O3 films [13e15]. However, with the Nb
H. Zhang et al. / Vacuum 146 (2017) 93e96
95
Fig. 4. XPS spectra for the Ga2O3:Nb (3.31 wt%) thin film: survey (a), Ga 2p (b), Nb 3d (c), and O1s (d) core level.
can be used to calculate the average grain size of the Ga2O3:Nb films. In the Scherrer's equation, D is the average grain size, k is a
constant equal to 0.9, l is the x-ray wavelength, and b is the halfpeak width. The average grain sizes of the Ga2O3:Nb thin films increase from 45.6 nm to 68 nm with the increase of Nb content from 0.39 wt% to 3.31 wt%, indicating an improvement in the crystalline quality. So Nb-doping has a great application prospect in enhancing the film properties and improving the crystalline quality. Fig. 3 shows the top-view SEM images of the intrinsic and bGa2O3:Nb films on Si substrates. Some slice-shaped areas can be
Fig. 5. Transmittance spectra of the b-Ga2O3:Nb films with different Nb concentrations and the plot of (ahn)2 versus hn in inset.
Fig. 6. Optical band gaps of the b-Ga2O3:Nb films as a function of Nb concentration.
doping, the Ga2O3 (110) peak get stronger and narrower, indicating an improvement in the crystalline quality. Besides, Scherrer's equation
D¼
kl : b cos q
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H. Zhang et al. / Vacuum 146 (2017) 93e96
observed on the surface of the intrinsic b-Ga2O3 film as shown in Fig. 3a. While the surface of the b-Ga2O3:Nb films show grain-like morphology as shown in Fig. 3b and c, which is quite different from the intrinsic b-Ga2O3 film. Moreover, well-defined grains with clear grain boundaries are observed for the b-Ga2O3:Nb films, and the grain size of b-Ga2O3:Nb films become bigger with the increase of the Nb content, indicating an improvement in the film crystallinity. The SEM images show clearly that the film crystallinity is improved with Nb-doping, which is consistent with the XRD results. The XPS spectra of the survey, Ga 2p, Nb 3d and O 1s core levels for the 3.31 wt% Nb-doped b-Ga2O3 film are shown in Fig. 4a, b, c and d, respectively. The charge-shift spectrum is calibrated using the fortuitous C 1s peak at 284.8 eV. From the survey, photoelectron lines of C 1s, O 1s, Ga 2p, Ga 3s, Ga 3p, Ga 3d, Nb 3d as well as the O KLL, Ga LMM, Ga LMM1 can be observed as shown in Fig. 4a. In Fig. 4b, it is observed that two symmetrical peaks of Ga 2p1/2 located at 1143 eV and Ga 2p3/2 located at 1116 eV, with a separation distance about 27 eV, which is nearly consistent with the binding energy of the Ga 2p (D ¼ 26.8eV) [16]. Fig. 4c reveals the high resolution XPS spectra of Nb 3d3/2 and Nd 3d5/2 peaks, centered at 209.3 eV and 206.7 eV, respectively. Arfaoui use the fact that the 3d core level shift depends linearly on the valence of Nb atoms to identify the different components found in the photoemission spectra. Using that linear variation as an abacus, they deduce the valence of Nb atoms in the oxide layer from the binding energies [17]. Besides, the XP spectra with a doublet peaks at BE of 209.6e209.9 eV for Nb 3d3/2 and of 206.9e207.5 eV for Nb 3d5/2 can indicate that the valence state of niobium is Nb(V) [18]. In our work, the BE of Nb 3d5/2 is lower than 206.9 eV. Furthermore, the BE of Nb 3d5/2 is consistent with the BE of Nb 3d5/2 of NbO2 in Ref [19]. Thus, we could draw a conclusion that niobium is present in the Ga2O3 as Nb(Ⅳ). In addition, based on the results of Fig. 4c analysis, the O 1s peak at ~530.2 eV is split into two peaks located at 530 eV and 531.4 eV. The main peak at 530 eV can be regard as lattice oxygen in the Ga2O3:Nb thin films, while the peak at 531.4 eV could be due to the C/O or OH1 adsorbed on the film surface [19]. The optical transmittance spectra of the intrinsic and bGa2O3:Nb films deposited on the quartz substrates in the wavelength range of 200e800 nm are shown in Fig. 5. The average transmittance of all the films in visible range are over 80%. The spectrum of the pure b-Ga2O3 shows a sharp intrinsic absorption edge at ~250 nm. With the doping of Nb, the absorption edge of bGa2O3:Nb films display a clear red-shift. For the direct band gap transition semiconductors, the absorption coefficient a and optical band gap (Eg) are related by the equation
ahv ¼ A hv Eg
1=2
;
where A is a constant, n is the frequency of the incident photon and h is the Planck's constant. Then the relation curve of (ahn)2 and hn can be plotted, and Eg can be estimated by extrapolating the straight-line portion of this plot to the energy axis, as shown in the insets of Fig. 5. The band gap Eg gradually decreases from 4.86 eV to 4.71eV with increasing the Nb content as shown in Fig. 6. The incorporation of Nb ions on the substitutional sites of Ga2O3 contribute to new unoccupied electron states in the gap below the conduction band edge, leading to the decrease of the band gap. Besides, previous studies [7,8] indicate a larger lattice plane d spacing is correlated to a narrower band gap, and our experiment results is also consistent with the results reported before. 4. Conclusion In summary, monoclinic b-Ga2O3: Nb thin films were grown on
Si substrates by radio frequency magnetron sputtering. The structural and optical properties investigated by XRD, SEM, UV-Vis transmittance spectra and the XPS confirmed the incorporation of Nb into the b-Ga2O3 crystal lattice. Niobium is present in the bGa2O3 as Nb(Ⅳ). The incorporation of Nb into b-Ga2O3 leads to the increase of lattice constants of b-Ga2O3, an improvement of the crystalline quality, and the decrease of the band gap. These results enrich our knowledge of many fundamental properties of bGa2O3:Nb thin films, and provide application guidance for b-Ga2O3: Nb films in optoelectronics area.
Acknowledgements This work was supported by funding for the development project of Beijing Municipal Education Commission of Science and Technology (grant no. KZ201410005008), Natural Science Foundation of Beijing City (no. 4102014), and China Postdoctoral Science Foundation (no. 2015M570020).
References [1] S. Lee, J. Hwang, J. Kim, S. Jeong, C. Cho, Dielectric characterization of transparent epitaxial Ga2O3 thin film on n-GaN/Al2O3 prepared by pulsed laser deposition, Appl. Phys. Lett. 89 (2006) 182906. [2] Y. Oshima, E.G. Vίllora, K. Shimamura, Quasi-heteroepitaxial growth of betaGa2O3 on off-angled sapphire (0001) substrates by halide vapor phase epitaxy, J. Cryst. Growth 410 (2015) 53e58. [3] S. Nakagomi, Y. Kokubun, Crystal orientation of beta-Ga2O3 thin films formed on c-plane and a-plane sapphire substrate, J. Cryst. Growth 349 (2012) 12e18. [4] P. Guo, J. Xiong, X. Zhao, T. Sheng, C. Yue, B. Tao, X. Liu, Growth characteristics and device properties of MOD derived beta-Ga2O3 films, J. Mater. Sci. Mater. Electron 25 (2014) 3629e3632. [5] Y. Cheng, K. Yang, Y. Peng, Y. Yin, J. Chen, B. Jing, H. Liang, G. Du, Research on the structural and optical stability of Ga2O3 films deposited by electron beam evaporation, J. Mater. Sci. Mater. Electron 24 (2013) 5122e5126. [6] M. Passlack, N.E.J. Hunt, E.F. Schubert, G.J. Zydzik, M. Hong, J.P. Mannaerts, R. Opila, R. Fischer, Dielectric-properties of electron-beam deposited GA2O3 films, Appl. Phys. Lett. 64 (1994) 2715e2717. [7] Z. Wu, G. Bai, Q. Hu, D. Guo, C. Sun, L. Ji, M. Lei, L. Li, P. Li, J. Hao, W. Tang, Effects of dopant concentration on structural and near-infrared luminescence of Nd3þ-doped beta-Ga2O3 thin films, Appl. Phys. Lett. 106 (2015) 171910. [8] W. Li, Y. Peng, C. Wang, X. Zhao, Y. Zhi, H. Yan, L. Li, P. Li, H. Yang, Z. Wu, W. Tang, Structural, optical and photoluminescence properties of Pr-doped beta-Ga2O3 thin films, J. Alloy Compd. 697 (2017) 388e391. [9] W. Mi, X. Du, C. Luan, H. Xiao, J. Ma, Electrical and optical characterizations of beta-Ga2O3: Sn films deposited on MgO(110) substrate by MOCVD, RSC Adv. 4 (2014) 30579e30583. [10] E.J. Rubio, C.V. Ramana, Tungsten-incorporation induced red-shift in the bandgap of gallium oxide thin films, Appl. Phys. Lett. 102 (2013) 191913. [11] H. Peelaers, C.G. Van de Walle, Doping of Ga2O3 with transition metals, Phys. Rev. B 94 (2016) 195203. [12] R.P. Shi, X.D. Huang, K.O. Johnny, P.T. Lai, Nb-doped Ga2O3 as charge-trapping layer for nonvolatile memory applications, Microelectron. Reliab 65 (2016) 64e68. [13] X.J. Feng, Z. Li, W. Li, Y. Luo, J. Ma, Mg-doped beta-Ga2O3 films with tunable optical band gap prepared on MgO (110) substrates by metal-organic chemical vapor deposition, Mater. Sci. Semicond. Process 34 (2015) 52e57. [14] Y.J. Zhang, J.L. Yan, Q.S. Li, Optical and structural properties of Cu-doped bGa2O3 films, Mater. Sci. Eng. B 11 (2011) 846e849. [15] W. Mi, X.J. Du, C.N. Luan, Electrical and optical characterizations of b-Ga2O3:Sn films deposited on MgO(110) substrate by MOCVD, RSC Adv. 4 (2014) 30579e30583. [16] H. Xu, Y. Liu, C. Xu, Y. Liu, C. Shao, R. Mu, Room-temperature ferromagnetism in (Mn, N)-codoped ZnO thin films prepared by reactive magnetron cosputtering, Appl. Phys. Lett. 88 (2006) 242502. [17] I. Arfaoui, J. Cousty, C. Guillot, A model of the NbOxz1 nanocrystals tiling a Nb(1 1 0) surface annealed in UHV, Surf. Sci. 557 (2004) 119e128. thivier, M. Che, Incorporation of Nb(V) into BEA [18] S. Dzwigaj, Y. Millot, C. Me zeolite investigated by XRD, NMR, IR, DR UV-vis, and XPS, Microporous Mesoporous Mater 130 (2010) 162e166. [19] W. Mi, J. Ma, Z. Li, C. Luan, H. Xiao, Characterization of Sn-doped beta-Ga2O3 films deposited on MgO (100) substrate by MOCVD, J. Mater Sci. Mater Electron 26 (2015) 7889e7894.