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Catalytic growth and characterization of single crystalline Zn doped p-type β-Ga2O3 nanowires
Journal of Alloys and Compounds 687 (2016) 964e968
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Catalytic growth and characterization of single crystalline Zn doped p-type b-Ga2O3 nanowires Qiuju Feng a, *, Jiayuan Liu a, Yuqi Yang a, Dezhu Pan a, Yan Xing a, Xiaochi Shi a, Xiaochuan Xia b, Hongwei Liang b a b
School of Physics and Electronic Technology, Liaoning Normal University, Dalian, 116029, People’s Republic of China School of Physics and Optoelectronic Engineering, Dalian University of Technology, Dalian, 116024, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2016 Received in revised form 26 June 2016 Accepted 27 June 2016 Available online 30 June 2016
Single crystalline b-Ga2O3 nanowires with different Zn doping contents were grown via catalytic chemical vapor deposition method. The field-emission scanning electron microscopy showed that when the Zn content was 1.3%, the Zn doped b-Ga2O3 nanowires with uniform size and high density were synthesized. The diameter and length of nanowires were about 50 nm and dozens of micron, respectively. The X-ray diffraction measurements indicated that the position of (202) diffraction peak of samples shifted toward lower diffracting angle with increasing Zn content, which was explained by the substitution of Ga3þ by Zn2þ during Zn doping process. Furthermore, the Zn-doped b-Ga2O3 nanowires/ n-type b-Ga2O3 thin film p-n homojunction was fabricated. The current-voltage (I-V) characteristic of the homojunction device showed a good rectifying behavior. This result is suggested that the Zn doped bGa2O3 nanowires showed p-type conductivity. © 2016 Elsevier B.V. All rights reserved.
Keywords: Zn doped b-Ga2O3 Nanowires Chemical vapor deposition p-n homojunction
1. Introduction Monoclinic gallium oxide (b-Ga2O3) nanowires (NWs) is an excellent thermal and chemical stability trivalent metal-oxide semiconductor with a band gap of 4.9 eV. It is a promising candidate for application in high-temperature gas sensors and transparent optoelectronic devices [1e4]. In order to realize b-Ga2O3 based nano-optoelectronic devices, it is necessary to obtain both ntype and p-type b-Ga2O3 nanomaterials. Normally, un-doped bGa2O3 exhibits n-type conductivity due to the donor related to native impurities and defects [5,6]. The p-type doping is still a major obstacle to the device application. There are few reports about growth of p-type b-Ga2O3 NWs. Liu et al. fabricated N-doped p-Ga2O3 nanowires by chemical vapor deposition (CVD) method using NH3 as dopant source [7]. Chang et al. synthesized Zn doped Ga2O3 nanowires by a diffusive doping method, i.e., the Zn diffused in to Ga2O3 nanowires under high temperature in the furnace [8]. Furthermore, as one of group IIB elements, Zn is suggested as a good potential acceptors dopant for b-Ga2O3. The radius of Zn2þ (0.74 Å) is similar to Ga3þ (0.62 Å), thus Zn atom can substitute Ga
* Corresponding author. E-mail address: [email protected] (Q. Feng). http://dx.doi.org/10.1016/j.jallcom.2016.06.274 0925-8388/© 2016 Elsevier B.V. All rights reserved.
atom efficiently [9]. In this paper, we successfully prepared Zndoped b-Ga2O3 NWs on sapphire substrates by CVD method using Au as catalyst. Metallic gallium (Ga) was used as the Ga source for the synthesis of Ga2O3 nanowires. The ZnO and graphite mixture powders were used as the Zn dopant source. In addition, the influence of different Zn doping contents on the surface morphology, structural and optical properties of b-Ga2O3 NWs were also investigated. Moreover, we also fabricated the Zn-doped b-Ga2O3 nanowires/n-type b-Ga2O3 thin film p-n homojunction, which was consisted of n-type b-Ga2O3 thin film grown on sapphire substrates by metaleorganic chemical vapor deposition technology (MOCVD) following Zn doped b-Ga2O3 nanowires grown by CVD. This device exhibits desirable rectifying behavior. 2. Experimental
b-Ga2O3 NWs with different Zn doping contents were grown on sapphire substrates by a traditional CVD apparatus. Sapphire (cplane Al2O3) substrates were cleaned using a standard semiconductor wafer cleaning procedure, and then a 10-nm layer of Au was deposited on the substrate by thermal evaporation technique. High-purity metallic Ga (99.9%) was used as Ga vapor source. The ZnO and graphite mixture powders with a weight ratio 7:1 were used as the Zn dopant source of p-type b-Ga2O3 dopant source.
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Well-mixed of Ga and Zn dopant source with different weight ratios were put in a quartz boat and placed at the center of the tube furnace. The growth parameters of samples with different Zn doping contents are listed in Table 1. The substrate was placed vertical on the above source at a distance of 1 cm. High-purity Ar gas was used as carrier gas. The flow rate of high-purity Ar carrier gas was controlled at 200 sccm. Then, oxygen reactant gas with a flow rate of 2 sccm was carried out into the system. During growth process, the growth temperature and time were maintained at 900 C and 30 min, respectively. The crystal structures of the grown Zn-doped b-Ga2O3 NWs were characterized using X-ray diffraction (XRD) with a Cu Ka radiation (0.15418 nm). Surface morphologies and elemental contents were investigated by field-emission scanning electron microscopy (FE-SEM, Hitachis-S4800), transmission electron microscopy (TEM, Hitachi H-600) and energy-dispersive spectroscopy (EDS) analysis. The optical absorption spectra were measured by spectrophotometer (UV-3600 UV-VIS-NIR, Shimadzu). Furthermore, the currentvoltage (I-V) characteristic of the homojunction device was carried out (Keithley Model 4200-SCS) at room temperature. 3. Results and discussion The surface morphologies of the different Zn contents b-Ga2O3 NWs were measured by FE-SEM. For sample A with low Zn doping content, the b-Ga2O3 nanowires with uniform size and high density were grown on sapphire substrate, as shown in Fig. 1(a). The diameter and length of the Zn doped b-Ga2O3 nanowires are about 50 nm and dozens of micron, respectively. For the sample B and sample C with higher Zn doping contents, the length of nanowires was shorter and surface morphology were composed of some big sheet structures. This indicated that surface morphologies of the samples were become clutter with the increase of Zn contents. EDS analysis was further performed to determine the Zn contents in the as-grown b-Ga2O3 NWs. The Zn contents in samples A, B and C were about 1.3%, 2.5% and 3.6%, respectively. Fig. 2(a) shows the XRD patterns of different Zn doping contents b-Ga2O3 nanowires on sapphire substrate. For comparison, undoped b-Ga2O3 nanowires (sample D) was also investigated. In Fig. 2(a) for all samples, except for the diffraction peak of sapphire (006), other diffraction peaks were fitted to monoclinic structure bGa2O3 with lattice constants of a ¼ 12.23 Å, b ¼ 3.04 Å, c ¼ 5.80 Å, and b ¼ 103.7, which is in agreement with the standard diffraction data (JCPDS 43e1012). No other phase of gallium oxides or impurities related XRD peaks appears, suggesting that the products were b-Ga2O3 phase. In addition, the dependence of the (202) diffraction peak positions of samples (AeD) on the Zn contents was shown in Fig. 2(b). From Fig. 2(b), it is clear that the (202) peak positions of samples shifted toward lower diffracting angle with increasing Zn content. This phenomenon was usually considered that lattice spacing to be changed due to Zn doping. As the ionic radius of Zn2þ of 0.74 Å is larger than that of Ga3þ of 0.62 Å [10], which resulted in the lattice spacing gradually enlarged. The morphology and structure characterizations of Zn-doped bGa2O3 nanowires were further examined by TEM and the selected area electron diffraction (SAED) pattern. The Fig. 3 shows a TEM
Table 1 The growth parameters of the different Zn contents samples. Samples
Ga (g)
Zn dopant source (g)
Growth temperature ( C)
O2 gas (sccm)
Ar carrier gas (sccm)
A B C
0.05 0.05 0.05
0.005 0.007 0.01
900 900 900
2 2 2
200 200 200
Fig. 1. The SEM images of different Zn contents b-Ga2O3 NWs for (a) sample A, (b) sample B and (c) sample C.
image of sample A, it can be clearly seen that the single nanowire with a diameter of about 50 nm, which is in good agreement with SEM images. Moreover, the inset is the SAED pattern of a single nanowires. The bright diffraction spots indicated that Zn-doped bGa2O3 nanowire is single crystal. The optical absorption spectra of un-doped (sample D) and different Zn doping contents b-Ga2O3 nanowires (sample A-C) are
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Fig. 3. TEM image of the sample A. The inset is the SAED pattern.
Fig. 2. (a) XRD patterns of un-doped and different Zn doping b-Ga2O3 nanowires, (b) The peak positions of (202) diffraction of the samples AeD as a function of the Zn contents.
shown in Fig. 4. In the case of parabolic band structure, the optical bandgap (Eg) and absorption coefficient (a) of a semiconductor are related via the well-known equation [11]:
ðahvÞn ¼ A hv Eg
that the fundamental absorption edges of samples A-C were occurred red shift with increasing Zn contents. This phenomenon was usually considered that the Zn doping could reduce the band gap because of lattice mismatch or defect, or stress. To further prove that the Zn doped b-Ga2O3 nanowires may present p-type conductivity. The Zn-doped b-Ga2O3 nanowires/ntype b-Ga2O3 thin film p-n homojunction was also fabricated. The homojunction was consisted of high-quality n-type b-Ga2O3 thin film grown on sapphire substrates by metaleorganic chemical vapor deposition technology (MOCVD) following Zn doped b-Ga2O3
(1)
where a is the absorption coefficient, hv is photon energy, A is a constant, Eg is the optical band gap and n equal to 2 for direct gap and 1/2 for an indirect gap. For b-Ga2O3, the variation of absorption coefficient with photon energy is found to follow the above relation with n ¼ 2, indicating that the transition corresponds to a directly allowed electronic transition [12]. The optical band gaps (Eg) were obtained by extrapolating the linear part of the curves of (ahv)2 as a function of hv to intercept the energy axis [13,14]. We can find that the Eg value of un-doped b-Ga2O3 nanowires (sample D) was determined to be about 4.94 eV. The Eg values of different Zn doping contents b-Ga2O3 nanowires (sample A-C) are about 4.93 eV, 4.91 eV and 4.90 eV, respectively. These value is in agreement with the reported values 4.88 eV for b-Ga2O3 nanowires [15] while a bit higher than that of thin film [12]. A relatively large band gaps of nanostructures were often observed than thin film materials, which is usually attributed to the quantum confinement effect of nanostructures [16]. Furthermore, from Fig. 4, we can see
Fig. 4. The optical absorption spectra of un-doped (sample D) and different Zn doping contents b-Ga2O3 nanowires (sample A-C).
Q. Feng et al. / Journal of Alloys and Compounds 687 (2016) 964e968
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Fig. 7. The IeV curve of the Zn-doped b-Ga2O3 nanowires/n-type b-Ga2O3 thin film pen homojunction.
Fig. 5. The SEM images of (a) n-type b-Ga2O3 thin film and (b) Zn-doped b-Ga2O3 nanowires on the n-type b-Ga2O3 thin film.
nanowires grown (according to growth parameters of sample A) by CVD. The details of growth of b-Ga2O3 thin film grown on sapphire substrates by MOCVD was reported previously [17]. Furthermore, 10-nm Au layer were deposited on n-type b-Ga2O3 thin film to serve as the catalyst of growing Zn doped b-Ga2O3 nanowires. SEM images of the morphology of n-type b-Ga2O3 thin film and Zn-doped b-Ga2O3 nanowires are shown in Fig. 5. The as-deposited Ga2O3 thin film exhibits a relatively smooth surface, as shown in
Fig. 5(a), (b) show the surface morphology for the Zn doped bGa2O3 nanowires. By comparing Figs. 1(a) and 5(b), we found that the Zn-doped b-Ga2O3 nanowires have similar density and size. To fabricate the b-Ga2O3 homojunction device, the ITO glass was clamped on the top end of the nanowires as the positive electrode and Ti/Au was deposited on the un-doped n-type b-Ga2O3 as the negative electrode using the conventional thermal evaporation technique. A schematic diagram of the device is shown on Fig. 6. The I-V characteristics of the Zn-doped b-Ga2O3 nanowires/n-type b-Ga2O3 thin film p-n homojunction was measured at room temperature, as shown in Fig. 7. The I-V curve exhibits typical rectifying nature and the turn-on voltage is about 4.8 V. Therefore, the p-type conductivity b-Ga2O3 nanowires was produced by the Zn-doped process. 4. Conclusions Single crystalline Zn doped p-type b-Ga2O3 nanowires on sapphire substrates by CVD were successfully achieved. The influence of Zn doping on the surface morphology, structural and optical properties of the b-Ga2O3 nanowires was studied. The results were explained that when the Zn content was 1.3%, the b-Ga2O3 nanowires with uniform size and high density were synthesized. And the (202) diffraction peak positions of samples shifted toward lower diffracting angle with increasing Zn content. Moreover, the Zn-doped b-Ga2O3 nanowires/n-type b-Ga2O3 thin film p-n homojunction device showed a good rectifying behavior. This result is suggested that the Zn doped b-Ga2O3 nanowires showed p-type conductivity. Acknowledgements This work was supported by the NSFC (Project No. 11405017, 61574026), and the Liaoning provincial Natural Science Foundation of China (No.2014020004). References
Fig. 6. The schematic diagram of the b-Ga2O3 homojunction device.
[1] Y. Qu, Z. Wu, M. Ai, D. Guo, Y. An, H. Yang, L. Li, W. Tang, J. Alloys Compd. 680 (2016) 247e251. [2] E. Filippo, M. Siciliano, A. Genga, G. Micocci, A. Tepore, T. Siciliano, Mater. Res. Bull. 48 (2013) 1741e1744. [3] Y. Li, T. Tokizono, M. Liao, M. Zhong, Y. Koide, I. Yamada, J.J. Delaunay, Adv.
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Funct. Mater. 20 (2010) 3972e3978. [4] L.W. Chang, T.-Y. Lu, Y.-L. Chen, J.-W. Yeh, H.C. Shih, Mater. Lett. 65 (2011) 2281e2283. [5] N. Suzuki, S. Ohira, M. Tanaka, T. Sugawara, K. Nakajima, T. Shishido, Phys. Status Solidi 4 (2007) 2310. [6] Y. Zhang, J. Yan, Q. Li, C. Qu, L. Zhang, T. Li, Phys. B 406 (2011) 3079e3082. [7] L.L. Liu, M.K. Li, D.Q. Yu, J. Zhang, H. Zhang, C. Qian, Z. Yang, Appl. Phys. A 98 (2010) 831e835. [8] Pai-Chun Chang, Zhiyong Fan, Wei-Yu Tseng, A. Rajagopal, Jia G. Lu, Appl. Phys. Lett. 87 (2005) 222102. [9] W. Yue, J. Yan, J. Wu, L. Zhang, J. Semicond. 33 (2012) 073003. [10] X. Wang, Sh Shen, S. Jin, J. Yang, M. Li, X. Wang, H. Han, C. Li, Phys. Chem. Chem. Phys. 15 (2013) 19380e19386. [11] Sudheer Kumar, G. Sarau, C. Tessarek, Muhammad Y. Bashouti, A. Ha€hnel,
S. Christiansen, R. Singh, J. Phys. D. Appl. Phys. 47 (2014) 435101. [12] X.H. Wang, F.B. Zhang, K. Saito, T. Tanaka, M. Nishio, Q.X. Guo, J. Phys. Chem. Solids 75 (2014) 1201e1204. [13] Y.Z. Zhang, J.G. Lu, Z.Z. Ye, H.P. He, L.P. Zhu, B.H. Zhao, L. Wang, Appl. Surf. Sci. 254 (2008) 1993e1996. [14] Q.J. Feng, D.Z. Shen, J.Y. Zhang, Y.M. Lu, Y.C. Liu, X.W. Fan, Mater. Chem. Phys. 96 (2006) 158e162. [15] P. Feng, X.Y. Xue, Y.G. Liu, Q. Wan, T.H. Wang, Appl. Phys. Lett. 89 (2006) 112114. [16] N. Han, F. Wang, Z. Yang, S.P. Yip, G. Dong, H. Lin, M. Fang, T.F. Hung, J.C. Ho, Nanoscale Res. Lett. 9 (2014) 347. [17] Y.P. Chen, H.W. Liang, X.C. Xia, P.C. Tao, R.S. Shen, Y. Liu, Y.B. Feng, Y.H. Zheng, X.N. Li, G.T. Du, J. Mater. Sci. Mater. Electron. 26 (2015) 3231e3235.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Catalytic growth and characterization of single crystalline Zn doped p-type b-Ga2O3 nanowires Qiuju Feng a, *, Jiayuan Liu a, Yuqi Yang a, Dezhu Pan a, Yan Xing a, Xiaochi Shi a, Xiaochuan Xia b, Hongwei Liang b a b
School of Physics and Electronic Technology, Liaoning Normal University, Dalian, 116029, People’s Republic of China School of Physics and Optoelectronic Engineering, Dalian University of Technology, Dalian, 116024, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2016 Received in revised form 26 June 2016 Accepted 27 June 2016 Available online 30 June 2016
Single crystalline b-Ga2O3 nanowires with different Zn doping contents were grown via catalytic chemical vapor deposition method. The field-emission scanning electron microscopy showed that when the Zn content was 1.3%, the Zn doped b-Ga2O3 nanowires with uniform size and high density were synthesized. The diameter and length of nanowires were about 50 nm and dozens of micron, respectively. The X-ray diffraction measurements indicated that the position of (202) diffraction peak of samples shifted toward lower diffracting angle with increasing Zn content, which was explained by the substitution of Ga3þ by Zn2þ during Zn doping process. Furthermore, the Zn-doped b-Ga2O3 nanowires/ n-type b-Ga2O3 thin film p-n homojunction was fabricated. The current-voltage (I-V) characteristic of the homojunction device showed a good rectifying behavior. This result is suggested that the Zn doped bGa2O3 nanowires showed p-type conductivity. © 2016 Elsevier B.V. All rights reserved.
Keywords: Zn doped b-Ga2O3 Nanowires Chemical vapor deposition p-n homojunction
1. Introduction Monoclinic gallium oxide (b-Ga2O3) nanowires (NWs) is an excellent thermal and chemical stability trivalent metal-oxide semiconductor with a band gap of 4.9 eV. It is a promising candidate for application in high-temperature gas sensors and transparent optoelectronic devices [1e4]. In order to realize b-Ga2O3 based nano-optoelectronic devices, it is necessary to obtain both ntype and p-type b-Ga2O3 nanomaterials. Normally, un-doped bGa2O3 exhibits n-type conductivity due to the donor related to native impurities and defects [5,6]. The p-type doping is still a major obstacle to the device application. There are few reports about growth of p-type b-Ga2O3 NWs. Liu et al. fabricated N-doped p-Ga2O3 nanowires by chemical vapor deposition (CVD) method using NH3 as dopant source [7]. Chang et al. synthesized Zn doped Ga2O3 nanowires by a diffusive doping method, i.e., the Zn diffused in to Ga2O3 nanowires under high temperature in the furnace [8]. Furthermore, as one of group IIB elements, Zn is suggested as a good potential acceptors dopant for b-Ga2O3. The radius of Zn2þ (0.74 Å) is similar to Ga3þ (0.62 Å), thus Zn atom can substitute Ga
* Corresponding author. E-mail address: [email protected] (Q. Feng). http://dx.doi.org/10.1016/j.jallcom.2016.06.274 0925-8388/© 2016 Elsevier B.V. All rights reserved.
atom efficiently [9]. In this paper, we successfully prepared Zndoped b-Ga2O3 NWs on sapphire substrates by CVD method using Au as catalyst. Metallic gallium (Ga) was used as the Ga source for the synthesis of Ga2O3 nanowires. The ZnO and graphite mixture powders were used as the Zn dopant source. In addition, the influence of different Zn doping contents on the surface morphology, structural and optical properties of b-Ga2O3 NWs were also investigated. Moreover, we also fabricated the Zn-doped b-Ga2O3 nanowires/n-type b-Ga2O3 thin film p-n homojunction, which was consisted of n-type b-Ga2O3 thin film grown on sapphire substrates by metaleorganic chemical vapor deposition technology (MOCVD) following Zn doped b-Ga2O3 nanowires grown by CVD. This device exhibits desirable rectifying behavior. 2. Experimental
b-Ga2O3 NWs with different Zn doping contents were grown on sapphire substrates by a traditional CVD apparatus. Sapphire (cplane Al2O3) substrates were cleaned using a standard semiconductor wafer cleaning procedure, and then a 10-nm layer of Au was deposited on the substrate by thermal evaporation technique. High-purity metallic Ga (99.9%) was used as Ga vapor source. The ZnO and graphite mixture powders with a weight ratio 7:1 were used as the Zn dopant source of p-type b-Ga2O3 dopant source.
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Well-mixed of Ga and Zn dopant source with different weight ratios were put in a quartz boat and placed at the center of the tube furnace. The growth parameters of samples with different Zn doping contents are listed in Table 1. The substrate was placed vertical on the above source at a distance of 1 cm. High-purity Ar gas was used as carrier gas. The flow rate of high-purity Ar carrier gas was controlled at 200 sccm. Then, oxygen reactant gas with a flow rate of 2 sccm was carried out into the system. During growth process, the growth temperature and time were maintained at 900 C and 30 min, respectively. The crystal structures of the grown Zn-doped b-Ga2O3 NWs were characterized using X-ray diffraction (XRD) with a Cu Ka radiation (0.15418 nm). Surface morphologies and elemental contents were investigated by field-emission scanning electron microscopy (FE-SEM, Hitachis-S4800), transmission electron microscopy (TEM, Hitachi H-600) and energy-dispersive spectroscopy (EDS) analysis. The optical absorption spectra were measured by spectrophotometer (UV-3600 UV-VIS-NIR, Shimadzu). Furthermore, the currentvoltage (I-V) characteristic of the homojunction device was carried out (Keithley Model 4200-SCS) at room temperature. 3. Results and discussion The surface morphologies of the different Zn contents b-Ga2O3 NWs were measured by FE-SEM. For sample A with low Zn doping content, the b-Ga2O3 nanowires with uniform size and high density were grown on sapphire substrate, as shown in Fig. 1(a). The diameter and length of the Zn doped b-Ga2O3 nanowires are about 50 nm and dozens of micron, respectively. For the sample B and sample C with higher Zn doping contents, the length of nanowires was shorter and surface morphology were composed of some big sheet structures. This indicated that surface morphologies of the samples were become clutter with the increase of Zn contents. EDS analysis was further performed to determine the Zn contents in the as-grown b-Ga2O3 NWs. The Zn contents in samples A, B and C were about 1.3%, 2.5% and 3.6%, respectively. Fig. 2(a) shows the XRD patterns of different Zn doping contents b-Ga2O3 nanowires on sapphire substrate. For comparison, undoped b-Ga2O3 nanowires (sample D) was also investigated. In Fig. 2(a) for all samples, except for the diffraction peak of sapphire (006), other diffraction peaks were fitted to monoclinic structure bGa2O3 with lattice constants of a ¼ 12.23 Å, b ¼ 3.04 Å, c ¼ 5.80 Å, and b ¼ 103.7, which is in agreement with the standard diffraction data (JCPDS 43e1012). No other phase of gallium oxides or impurities related XRD peaks appears, suggesting that the products were b-Ga2O3 phase. In addition, the dependence of the (202) diffraction peak positions of samples (AeD) on the Zn contents was shown in Fig. 2(b). From Fig. 2(b), it is clear that the (202) peak positions of samples shifted toward lower diffracting angle with increasing Zn content. This phenomenon was usually considered that lattice spacing to be changed due to Zn doping. As the ionic radius of Zn2þ of 0.74 Å is larger than that of Ga3þ of 0.62 Å [10], which resulted in the lattice spacing gradually enlarged. The morphology and structure characterizations of Zn-doped bGa2O3 nanowires were further examined by TEM and the selected area electron diffraction (SAED) pattern. The Fig. 3 shows a TEM
Table 1 The growth parameters of the different Zn contents samples. Samples
Ga (g)
Zn dopant source (g)
Growth temperature ( C)
O2 gas (sccm)
Ar carrier gas (sccm)
A B C
0.05 0.05 0.05
0.005 0.007 0.01
900 900 900
2 2 2
200 200 200
Fig. 1. The SEM images of different Zn contents b-Ga2O3 NWs for (a) sample A, (b) sample B and (c) sample C.
image of sample A, it can be clearly seen that the single nanowire with a diameter of about 50 nm, which is in good agreement with SEM images. Moreover, the inset is the SAED pattern of a single nanowires. The bright diffraction spots indicated that Zn-doped bGa2O3 nanowire is single crystal. The optical absorption spectra of un-doped (sample D) and different Zn doping contents b-Ga2O3 nanowires (sample A-C) are
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Fig. 3. TEM image of the sample A. The inset is the SAED pattern.
Fig. 2. (a) XRD patterns of un-doped and different Zn doping b-Ga2O3 nanowires, (b) The peak positions of (202) diffraction of the samples AeD as a function of the Zn contents.
shown in Fig. 4. In the case of parabolic band structure, the optical bandgap (Eg) and absorption coefficient (a) of a semiconductor are related via the well-known equation [11]:
ðahvÞn ¼ A hv Eg
that the fundamental absorption edges of samples A-C were occurred red shift with increasing Zn contents. This phenomenon was usually considered that the Zn doping could reduce the band gap because of lattice mismatch or defect, or stress. To further prove that the Zn doped b-Ga2O3 nanowires may present p-type conductivity. The Zn-doped b-Ga2O3 nanowires/ntype b-Ga2O3 thin film p-n homojunction was also fabricated. The homojunction was consisted of high-quality n-type b-Ga2O3 thin film grown on sapphire substrates by metaleorganic chemical vapor deposition technology (MOCVD) following Zn doped b-Ga2O3
(1)
where a is the absorption coefficient, hv is photon energy, A is a constant, Eg is the optical band gap and n equal to 2 for direct gap and 1/2 for an indirect gap. For b-Ga2O3, the variation of absorption coefficient with photon energy is found to follow the above relation with n ¼ 2, indicating that the transition corresponds to a directly allowed electronic transition [12]. The optical band gaps (Eg) were obtained by extrapolating the linear part of the curves of (ahv)2 as a function of hv to intercept the energy axis [13,14]. We can find that the Eg value of un-doped b-Ga2O3 nanowires (sample D) was determined to be about 4.94 eV. The Eg values of different Zn doping contents b-Ga2O3 nanowires (sample A-C) are about 4.93 eV, 4.91 eV and 4.90 eV, respectively. These value is in agreement with the reported values 4.88 eV for b-Ga2O3 nanowires [15] while a bit higher than that of thin film [12]. A relatively large band gaps of nanostructures were often observed than thin film materials, which is usually attributed to the quantum confinement effect of nanostructures [16]. Furthermore, from Fig. 4, we can see
Fig. 4. The optical absorption spectra of un-doped (sample D) and different Zn doping contents b-Ga2O3 nanowires (sample A-C).
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Fig. 7. The IeV curve of the Zn-doped b-Ga2O3 nanowires/n-type b-Ga2O3 thin film pen homojunction.
Fig. 5. The SEM images of (a) n-type b-Ga2O3 thin film and (b) Zn-doped b-Ga2O3 nanowires on the n-type b-Ga2O3 thin film.
nanowires grown (according to growth parameters of sample A) by CVD. The details of growth of b-Ga2O3 thin film grown on sapphire substrates by MOCVD was reported previously [17]. Furthermore, 10-nm Au layer were deposited on n-type b-Ga2O3 thin film to serve as the catalyst of growing Zn doped b-Ga2O3 nanowires. SEM images of the morphology of n-type b-Ga2O3 thin film and Zn-doped b-Ga2O3 nanowires are shown in Fig. 5. The as-deposited Ga2O3 thin film exhibits a relatively smooth surface, as shown in
Fig. 5(a), (b) show the surface morphology for the Zn doped bGa2O3 nanowires. By comparing Figs. 1(a) and 5(b), we found that the Zn-doped b-Ga2O3 nanowires have similar density and size. To fabricate the b-Ga2O3 homojunction device, the ITO glass was clamped on the top end of the nanowires as the positive electrode and Ti/Au was deposited on the un-doped n-type b-Ga2O3 as the negative electrode using the conventional thermal evaporation technique. A schematic diagram of the device is shown on Fig. 6. The I-V characteristics of the Zn-doped b-Ga2O3 nanowires/n-type b-Ga2O3 thin film p-n homojunction was measured at room temperature, as shown in Fig. 7. The I-V curve exhibits typical rectifying nature and the turn-on voltage is about 4.8 V. Therefore, the p-type conductivity b-Ga2O3 nanowires was produced by the Zn-doped process. 4. Conclusions Single crystalline Zn doped p-type b-Ga2O3 nanowires on sapphire substrates by CVD were successfully achieved. The influence of Zn doping on the surface morphology, structural and optical properties of the b-Ga2O3 nanowires was studied. The results were explained that when the Zn content was 1.3%, the b-Ga2O3 nanowires with uniform size and high density were synthesized. And the (202) diffraction peak positions of samples shifted toward lower diffracting angle with increasing Zn content. Moreover, the Zn-doped b-Ga2O3 nanowires/n-type b-Ga2O3 thin film p-n homojunction device showed a good rectifying behavior. This result is suggested that the Zn doped b-Ga2O3 nanowires showed p-type conductivity. Acknowledgements This work was supported by the NSFC (Project No. 11405017, 61574026), and the Liaoning provincial Natural Science Foundation of China (No.2014020004). References
Fig. 6. The schematic diagram of the b-Ga2O3 homojunction device.
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