- Email: [email protected]
Thermal annealing impact on crystal quality of (GaIn)2O3 alloys
Journal of Alloys and Compounds 614 (2014) 173–176
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Thermal annealing impact on crystal quality of (GaIn)2O3 alloys Fabi Zhang, Katsuhiko Saito, Tooru Tanaka, Mitsuhiro Nishio, Qixin Guo ⇑ Department of Electrical and Electronic Engineering, Synchrotron Light Application Center, Saga University, Saga 840-8502, Japan
a r t i c l e
i n f o
Article history: Received 22 April 2014 Accepted 16 June 2014 Available online 24 June 2014 Keywords: Semiconducting gallium indium oxide Pulsed laser deposition Annealing Phase separation Spectrophotometer
a b s t r a c t In this study, we investigated the phase separation phenomenon and post thermal annealing effects on the (GaIn)2O3 films deposited on sapphire substrates by pulsed laser deposition. Films grown at substrate temperature higher than 300 °C showed phase separation while films grown at substrate temperature lower than 200 °C revealed homogenous elements distributions with amorphous structure. Thermal annealing had no obvious effects on (GaIn)2O3 films grown at substrate temperature higher than 300 °C. The clusters remained on the surface of the films after thermal annealing treatment. On the other hand, however, by thermal annealing the film deposited at room temperature, (GaIn)2O3 film with smooth surface, homogenous element distribution, high orientation crystal and high optical transmittance observed by means of scanning electron microscope, energy dispersive spectrometer, X-ray diffraction and spectrophotometer was successfully obtained, paving a way for obtaining high quality (GaIn)2O3 film without phase separation. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Ga2O3 has been applied in many fields such as ultraviolet photodetector [1,2], deep-ultraviolet transparent electrode [3,4] and field-effect transistors [5] due to its large bandgap and chemical and physical stabilities. Recently, it has been found potential usage in solar water splitting [6–8]. Oshima et al. [8] reported the photoelectrode properties of an n-type b-Ga2O3 single crystal in aqueous solutions. They observed gaseous oxygen and hydrogen successfully from the photoelectrode and Pt counter electrode, respectively. Hwang et al. [9] demonstrated the photoelectrochemical performances of Ga2O3 nanowires grown in situ from GaN. In order to improve the energy conversion efficiency, the bandgap of Ga2O3 should be decreased for broad absorption of the solar spectrum. In2O3 is a candidate to realize the bandgap engineering of Ga2O3 since both indium and gallium belonging to the same elements group have similar electron structures. Recently, we demonstrated the successful growth and bandgap engineering of (GaIn)2O3 films by pulsed laser deposition (PLD) [10]. The bandgap of (GaIn)2O3 films can be tailored between 3.8 eV and 5.1 eV by controlling the indium content. Unfortunately phase separation (the simultaneously existing of both monoclinic and cubic phases) was observed for the films with indium content ranging from 0.16 to 0.33. Similar phenomenon has been reported by Suzuki et al. [11], who observed the phase separation in (GaIn)2O3 films and ⇑ Corresponding author. Tel.: +81 952 28 8662; fax: +81 952 28 8651. E-mail address: [email protected] (Q. Guo). http://dx.doi.org/10.1016/j.jallcom.2014.06.091 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
have tried to solve it by the fabrication of rhombohedral corundum-structured a-(GaIn)2O3 films. However phase separation still appeared when indium content was between 0.08 and 0.67. In order to meet the requirements of water splitting application, it is necessary to suppress phase separation. Here, we report an effective way for suppressing phase separation and obtaining high quality (GaIn)2O3 film.
2. Experiments As-deposited (GaIn)2O3 films were prepared by PLD on (0 0 0 1) sapphire substrates. Details of the growth have been described elsewhere [10]. In brief, the films were deposited with laser energy of 225 mJ at a repetition of 1 Hz for 2 h. The oxygen pressure was kept at 101 Pa. As typically phase separation occurred when indium content of the target is 0.3 for as-deposited (GaIn)2O3 films at substrate temperature of 500 °C in our previously work [10], the indium content of the target in this work is selected as 0.3. The substrate temperature for as-deposition was varied from room temperature (RT) to 500 °C. Since post thermal annealing has been reported to be an effective method to change the properties of semiconductor films [12,13], we carried out here for the purpose of achieving monophase film. The samples were put on a quartz boat which was placed into the center of an electric furnace. The annealing temperature was 800 °C. The samples were held at that temperature for l h in an air atmosphere and then furnace cooled to room temperature. The thickness of all the films determined by a surface step profile analyzer were about 300–400 nm. The structural properties of the films were examined by conventional h–2h X-ray diffraction (XRD) using Cu Ka emission line. The morphology of the samples was examined using scanning electron microscope (SEM) and atomic force microscope (AFM). The element composition was measured by energy dispersive spectroscopy (EDS). Optical transmission spectra were measured with a spectrophotometer.
174
F. Zhang et al. / Journal of Alloys and Compounds 614 (2014) 173–176
30
40
60
70
Δ
ο
(400)
Δ
(222)
(-201)
500 C
50
ο
(-603)
20 o
(-402)
10
Δ
o
400 C
Intensity (arb.units)
o
300 C
The appeared phase separation at high substrate temperatures is ascribed to the nonequilibrium process of PLD [14] because the indium content in the films does not exceed the equilibrium solubility of indium in Ga2O3 [15–18]. It is known that PLD is a process including laser ablation of the target material (creation of a plasma), plasma plume expansion (plasma propagation) and deposition of the ablation material on the substrate. Deposition of ablated species on the substrate surface can be regarded as instantaneous for every pulse in PLD process. The ablated particles exhibit shallow implantation into the upper monolayer of the surface material. During the time interval of the laser pulse, the adatoms rearrange on the surface by migration and subsequent incorporation through nucleation and growth. The surface diffusion coefficient is many orders of magnitude bigger than the volume diffusion [14]. Thus, we believe that the phase separation at high substrate temperatures is due to the different surface
SEM o
(a)
200 C
RT
10
20
30
40
50
60
EDS
(a’)
2µm
2µm
(b)
(b’)
2µm
2µm
70
2θ (°) Fig. 1. XRD patterns of (GaIn)2O3 films deposited on (0 0 0 1) sapphire substrates with different substrate temperatures. Peaks marked by triangle belong to monoclinic structure while that marked by circle belong to cubic structure. Peaks not assigned belong to the sapphire substrate.
(c)
(c’)
3. Results and discussion Fig. 1 shows the XRD diffraction patterns of the as-deposited (GaIn)2O3 films grown at different substrate temperatures by PLD. It is found that the diffraction peak of (GaIn)2O3 films cannot be detected when substrate temperature is lower than 300 °C, indicating the films are of amorphous structure. With increasing the substrate temperature to 400 °C, both the characteristic peaks of monoclinic structure and cubic structure have appeared, suggesting the co-existing of double phases whose parent phases are b-Ga2O3 and cubic In2O3, respectively. At substrate temperature of 500 °C, one more peak belonging to cubic structure appears. The surface morphology together with the indium element distribution of the (GaIn)2O3 films measured by means of SEM and EDS is shown in Fig. 2. When substrate temperature is lower than 200 °C, the surface of the films is very smooth and indium distribution is homogenous as shown in Fig. 2(a), (a0 ) and (b), (b0 ). However for films deposited at substrate temperature higher than 300 °C, it is clear that clusters have formed on the film surface as observed in Fig. 2(c). The morphology of the clusters changes with the substrate temperatures as shown in Fig. 2(d) [400 °C] and (e) [500 °C]. The shapes of indium element distributions detected by EDS shown on the right side of Fig. 2 match well with the results of SEM on the left side of Fig. 2, indicating that the clusters are of indium rich. These clusters are ascribed to In2O3 while the back-body can be assigned as Ga2O3 for the films deposited at substrate temperature higher than 300 °C.
2µm
2µm
(d)
(d’)
2µm
2µm
(e)
(e’)
2µm
2µm
Fig. 2. SEM and indium element distribution morphology of (GaIn)2O3 films deposited at different substrate temperatures. (a) and (a0 ) room temperature, (b) and (b0 ) 200 °C, (c) and (c0 ) 300 °C, (d) and (d0 ) 400 °C and (e) and (e0 ) 500 °C, respectively.
175
Fig. 5 shows the SEM image (a), indium element distribution (b), and AFM image (c) of the film annealed at 800 °C, respectively. The surface of the film is very smooth with the roughness of 1.94 nm. The indium element is uniformly distributed as shown in Fig. 5(b). Both the results from XRD in Fig. 4 and the surface morphology in Fig. 5 indicate that phase separation has been successfully suppressed in (GaIn)2O3 films. Transmittance spectrum of the film annealed at 800 °C is shown in Fig. 6 together with the transmittance of the as-deposited film grown at 500 °C for comparison. The transmittance of the annealed (-603)
(-201)
(-402)
Intensity (arb. unit)
F. Zhang et al. / Journal of Alloys and Compounds 614 (2014) 173–176
(a) 10
20
30
40
50
60
70
2θ (°) Fig. 3. XRD pattern of (GaIn)2O3 film annealed at 800 °C. Film for annealing was prepared at room temperature by PLD. Peaks not assigned belong to the sapphire substrate.
diffusion coefficient between indium and gallium. On the other hand, when the substrate temperature is lower than 200 °C, the adatoms cannot travel far for the low diffusion coefficient which decreases with an exponential function of temperature [19], resulting in the homogenous structure of the films. Next, thermal annealing was carried out on these as-deposited (GaIn)2O3 samples in order to suppress the phase separation and improve the crystal quality of the (GaIn)2O3 films. No obvious effects were observed on (GaIn)2O3 films grown at substrate temperature higher than 300 °C. The clusters remained on the surface of the films after thermal annealing treatment. Thus, we selected the film deposited at room temperature for thermal annealing because it exhibits homogenous element distribution as shown in Fig. 2(a) and (a0 ). Fig. 3 shows the XRD pattern of the (GaIn)2O3 film annealed at 800 °C. It is clear that only peaks belonging to (2 0 1) face of b-(GaIn)2O3 and its higher order have been detected, suggesting that oriented monoclinic structured film without phase separation is successfully obtained by post thermal annealing. The orientation relationship between the substrate and film is: b-(GaIn)2O3 (2 0 1) // sapphire (0 0 0 1). The X-ray rocking curve for b-(GaIn)2O3 (4 0 2) diffraction of the film annealed at 800 °C is presented in Fig. 4, where the full width at half maximum (FWHM) is obtained to be 49 arcmin which is at the same order of the as-deposited high quality b-Ga2O3 [20].
2 µm
(b)
2 µm
(c)
Intensity (arb. unit)
FWHM=49 arcmin
-150
-100
-50
0
50
100
150
ω (arcmin) Fig. 4. X-ray rocking curve for the b-(GaIn)2O3 (4 0 2) diffraction of film annealed at 800 °C.
Fig. 5. SEM (a), indium element distribution (b), and AFM (c) morphology of (GaIn)2O3 film annealed at 800 °C.
176
F. Zhang et al. / Journal of Alloys and Compounds 614 (2014) 173–176
however, by thermal annealing the film deposited at RT, (GaIn)2O3 film with smooth surface, homogenous element distribution, high orientation crystal and high optical transmittance was successfully obtained, paving a way for obtaining high quality (GaIn)2O3 film without phase separation.
100
Transmittance (%)
80
60
Acknowledgement
40
This work was partially supported by the Partnership Project for Fundamental Technology Researches of Ministry of Education, Culture, Sports, Science and Technology, Japan.
As-deposited Annealing
20
References 200
400
600
800
Wavelength (nm) Fig. 6. Transmittance spectra of the annealing film and the one as-deposited at substrate temperature of 500 °C.
sample in infrared region is over 80%, and exhibits clear fringes in the visible and UV regions, which is much higher than the transmittance of the as-deposited film grown at 500 °C. On the other hand, two absorption bands were observed in the transmission spectra for the as-deposited film grown at 500 °C, which are attributed to phase separation of the film. Similar phenomenon has been reported by other groups [21–23]. The improvement of optical properties of the annealed films verifies that thermal annealing is an effect way for obtaining high quality (GaIn)2O3 film without phase separation. 4. Conclusion We have deposited (GaIn)2O3 films on sapphire substrate by PLD at substrate temperatures from RT to 500 °C. The phase separation were observed for the films grown at substrate temperature higher than 300 °C while the films grown at substrate temperature lower than 200 °C revealed homogenous element distributions with amorphous structures. Thermal annealing had no obvious effects on (GaIn)2O3 films grown at substrate temperature higher than 300 °C. The clusters remained on the surface of the films after thermal annealing treatment. On the other hand,
[1] Y. Kokubun, K. Miura, F. Endo, S. Nakagomi, Appl. Phys. Lett. 90 (2007) 031912. [2] T. Oshima, T. Okuno, N. Arai, N. Suzuki, S. Ohira, S. Fujita, Appl. Phys. Express 1 (2008) 011202. [3] M. Orita, H. Ohta, M. Hirano, H. Hosono, Appl. Phys. Lett. 77 (2000) 4166. [4] M. Orita, H. Hiramatsu, H. Ohta, M. Hirano, H. Hosono, Thin Solid Films 411 (2002) 134. [5] M. Higashiwaki, K. Sasaki, T. Kamimura, M.H. Wong, D. Krishnamurthy, A. Kuramata, T. Masui, S. Yamakoshi, Appl. Phys. Lett. 103 (2013) 123511. [6] X. Wang, S. Shen, S. Jin, J. Yang, M. Li, X. Wang, H. Han, C. Li, Phys. Chem. Chem. Phys. 15 (2013) 19380. [7] Y. Sakata, Y. Matsuda, T. Nakagawa, R. Yasunaga, H. Imamura, K. Teramura, ChemSusChem 4 (2011) 181. [8] T. Oshima, K. Kaminaga, H. Mashiko, A. Mukai, K. Sasaki, T. Masui, A. Kuramata, S. Yamakoshi, A. Ohtomo, Jpn. J. Appl. Phys. 52 (2013) 111102. [9] J.S. Hwang, T.Y. Liu, S. Chattopadhyay, G.M. Hsu, A.M. Basilio, H.W. Chen, Y.K. Hsu, W.H. Tu, Y.G. Lin, K.H. Chen, C.C. Li, S.B. Wang, H.Y. Chen, L.C. Chen, Nanotechnology 24 (2013) 055401. [10] F. Zhang, K. Saito, T. Tanaka, M. Nishio, Q. Guo, Solid State Commun. 186 (2014) 28. [11] N. Suzuki, K. Kaneko, S. Fujita, J. Cryst. Growth (2014).. [12] J. Suwardy, Y. Darma, AIP Conf. Proc. 1554 (2013) 112. [13] Q.B. Ma, R. Lieten, G. Borghs, J. Mater. Sci.: Mater Electron. 25 (2014) 1197. [14] R. Eason, Pulsed Laser Deposition of Thin Films, John Wiley & Sons Inc., 2006. [15] L. Binet, G. Gauthier, C. Vigreux, D. Gourier, J. Phys. Chem. Solids 60 (1999) 1755. [16] A. Ratko, O. Babushkin, A. Baran, S. Baran, J. Eur. Ceram. Soc. 18 (1998) 2227. [17] G. Patzke, M. Binnewies, Solid State Sci. 2 (2000) 689. [18] C. Vigreux, L. Binet, D. Gourier, B. Piriou, J. Solid State Chem. 157 (2001) 94. [19] M. Rafique, A. Herklotz, E.J. Guo, R. Roth, L. Schultz, K. Dörr, S. Manzoor, J. Appl. Phys. 114 (2013) 233910. [20] F.B. Zhang, K. Saito, T. Tanaka, M. Nishio, Q.X. Guo, J. Cryst. Growth 387 (2014) 96. [21] C.H. Ahn, J.H. Kim, H.K. Cho, J. Electrochem. Soc. 159 (2012) H384. [22] A. Kaushal, D. Kaur, J. Alloys Comp. 509 (2011) 200. [23] T. Oshima, S. Fujita, Phys. Stat. Sol. (c) 5 (2008) 3113.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Thermal annealing impact on crystal quality of (GaIn)2O3 alloys Fabi Zhang, Katsuhiko Saito, Tooru Tanaka, Mitsuhiro Nishio, Qixin Guo ⇑ Department of Electrical and Electronic Engineering, Synchrotron Light Application Center, Saga University, Saga 840-8502, Japan
a r t i c l e
i n f o
Article history: Received 22 April 2014 Accepted 16 June 2014 Available online 24 June 2014 Keywords: Semiconducting gallium indium oxide Pulsed laser deposition Annealing Phase separation Spectrophotometer
a b s t r a c t In this study, we investigated the phase separation phenomenon and post thermal annealing effects on the (GaIn)2O3 films deposited on sapphire substrates by pulsed laser deposition. Films grown at substrate temperature higher than 300 °C showed phase separation while films grown at substrate temperature lower than 200 °C revealed homogenous elements distributions with amorphous structure. Thermal annealing had no obvious effects on (GaIn)2O3 films grown at substrate temperature higher than 300 °C. The clusters remained on the surface of the films after thermal annealing treatment. On the other hand, however, by thermal annealing the film deposited at room temperature, (GaIn)2O3 film with smooth surface, homogenous element distribution, high orientation crystal and high optical transmittance observed by means of scanning electron microscope, energy dispersive spectrometer, X-ray diffraction and spectrophotometer was successfully obtained, paving a way for obtaining high quality (GaIn)2O3 film without phase separation. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Ga2O3 has been applied in many fields such as ultraviolet photodetector [1,2], deep-ultraviolet transparent electrode [3,4] and field-effect transistors [5] due to its large bandgap and chemical and physical stabilities. Recently, it has been found potential usage in solar water splitting [6–8]. Oshima et al. [8] reported the photoelectrode properties of an n-type b-Ga2O3 single crystal in aqueous solutions. They observed gaseous oxygen and hydrogen successfully from the photoelectrode and Pt counter electrode, respectively. Hwang et al. [9] demonstrated the photoelectrochemical performances of Ga2O3 nanowires grown in situ from GaN. In order to improve the energy conversion efficiency, the bandgap of Ga2O3 should be decreased for broad absorption of the solar spectrum. In2O3 is a candidate to realize the bandgap engineering of Ga2O3 since both indium and gallium belonging to the same elements group have similar electron structures. Recently, we demonstrated the successful growth and bandgap engineering of (GaIn)2O3 films by pulsed laser deposition (PLD) [10]. The bandgap of (GaIn)2O3 films can be tailored between 3.8 eV and 5.1 eV by controlling the indium content. Unfortunately phase separation (the simultaneously existing of both monoclinic and cubic phases) was observed for the films with indium content ranging from 0.16 to 0.33. Similar phenomenon has been reported by Suzuki et al. [11], who observed the phase separation in (GaIn)2O3 films and ⇑ Corresponding author. Tel.: +81 952 28 8662; fax: +81 952 28 8651. E-mail address: [email protected] (Q. Guo). http://dx.doi.org/10.1016/j.jallcom.2014.06.091 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
have tried to solve it by the fabrication of rhombohedral corundum-structured a-(GaIn)2O3 films. However phase separation still appeared when indium content was between 0.08 and 0.67. In order to meet the requirements of water splitting application, it is necessary to suppress phase separation. Here, we report an effective way for suppressing phase separation and obtaining high quality (GaIn)2O3 film.
2. Experiments As-deposited (GaIn)2O3 films were prepared by PLD on (0 0 0 1) sapphire substrates. Details of the growth have been described elsewhere [10]. In brief, the films were deposited with laser energy of 225 mJ at a repetition of 1 Hz for 2 h. The oxygen pressure was kept at 101 Pa. As typically phase separation occurred when indium content of the target is 0.3 for as-deposited (GaIn)2O3 films at substrate temperature of 500 °C in our previously work [10], the indium content of the target in this work is selected as 0.3. The substrate temperature for as-deposition was varied from room temperature (RT) to 500 °C. Since post thermal annealing has been reported to be an effective method to change the properties of semiconductor films [12,13], we carried out here for the purpose of achieving monophase film. The samples were put on a quartz boat which was placed into the center of an electric furnace. The annealing temperature was 800 °C. The samples were held at that temperature for l h in an air atmosphere and then furnace cooled to room temperature. The thickness of all the films determined by a surface step profile analyzer were about 300–400 nm. The structural properties of the films were examined by conventional h–2h X-ray diffraction (XRD) using Cu Ka emission line. The morphology of the samples was examined using scanning electron microscope (SEM) and atomic force microscope (AFM). The element composition was measured by energy dispersive spectroscopy (EDS). Optical transmission spectra were measured with a spectrophotometer.
174
F. Zhang et al. / Journal of Alloys and Compounds 614 (2014) 173–176
30
40
60
70
Δ
ο
(400)
Δ
(222)
(-201)
500 C
50
ο
(-603)
20 o
(-402)
10
Δ
o
400 C
Intensity (arb.units)
o
300 C
The appeared phase separation at high substrate temperatures is ascribed to the nonequilibrium process of PLD [14] because the indium content in the films does not exceed the equilibrium solubility of indium in Ga2O3 [15–18]. It is known that PLD is a process including laser ablation of the target material (creation of a plasma), plasma plume expansion (plasma propagation) and deposition of the ablation material on the substrate. Deposition of ablated species on the substrate surface can be regarded as instantaneous for every pulse in PLD process. The ablated particles exhibit shallow implantation into the upper monolayer of the surface material. During the time interval of the laser pulse, the adatoms rearrange on the surface by migration and subsequent incorporation through nucleation and growth. The surface diffusion coefficient is many orders of magnitude bigger than the volume diffusion [14]. Thus, we believe that the phase separation at high substrate temperatures is due to the different surface
SEM o
(a)
200 C
RT
10
20
30
40
50
60
EDS
(a’)
2µm
2µm
(b)
(b’)
2µm
2µm
70
2θ (°) Fig. 1. XRD patterns of (GaIn)2O3 films deposited on (0 0 0 1) sapphire substrates with different substrate temperatures. Peaks marked by triangle belong to monoclinic structure while that marked by circle belong to cubic structure. Peaks not assigned belong to the sapphire substrate.
(c)
(c’)
3. Results and discussion Fig. 1 shows the XRD diffraction patterns of the as-deposited (GaIn)2O3 films grown at different substrate temperatures by PLD. It is found that the diffraction peak of (GaIn)2O3 films cannot be detected when substrate temperature is lower than 300 °C, indicating the films are of amorphous structure. With increasing the substrate temperature to 400 °C, both the characteristic peaks of monoclinic structure and cubic structure have appeared, suggesting the co-existing of double phases whose parent phases are b-Ga2O3 and cubic In2O3, respectively. At substrate temperature of 500 °C, one more peak belonging to cubic structure appears. The surface morphology together with the indium element distribution of the (GaIn)2O3 films measured by means of SEM and EDS is shown in Fig. 2. When substrate temperature is lower than 200 °C, the surface of the films is very smooth and indium distribution is homogenous as shown in Fig. 2(a), (a0 ) and (b), (b0 ). However for films deposited at substrate temperature higher than 300 °C, it is clear that clusters have formed on the film surface as observed in Fig. 2(c). The morphology of the clusters changes with the substrate temperatures as shown in Fig. 2(d) [400 °C] and (e) [500 °C]. The shapes of indium element distributions detected by EDS shown on the right side of Fig. 2 match well with the results of SEM on the left side of Fig. 2, indicating that the clusters are of indium rich. These clusters are ascribed to In2O3 while the back-body can be assigned as Ga2O3 for the films deposited at substrate temperature higher than 300 °C.
2µm
2µm
(d)
(d’)
2µm
2µm
(e)
(e’)
2µm
2µm
Fig. 2. SEM and indium element distribution morphology of (GaIn)2O3 films deposited at different substrate temperatures. (a) and (a0 ) room temperature, (b) and (b0 ) 200 °C, (c) and (c0 ) 300 °C, (d) and (d0 ) 400 °C and (e) and (e0 ) 500 °C, respectively.
175
Fig. 5 shows the SEM image (a), indium element distribution (b), and AFM image (c) of the film annealed at 800 °C, respectively. The surface of the film is very smooth with the roughness of 1.94 nm. The indium element is uniformly distributed as shown in Fig. 5(b). Both the results from XRD in Fig. 4 and the surface morphology in Fig. 5 indicate that phase separation has been successfully suppressed in (GaIn)2O3 films. Transmittance spectrum of the film annealed at 800 °C is shown in Fig. 6 together with the transmittance of the as-deposited film grown at 500 °C for comparison. The transmittance of the annealed (-603)
(-201)
(-402)
Intensity (arb. unit)
F. Zhang et al. / Journal of Alloys and Compounds 614 (2014) 173–176
(a) 10
20
30
40
50
60
70
2θ (°) Fig. 3. XRD pattern of (GaIn)2O3 film annealed at 800 °C. Film for annealing was prepared at room temperature by PLD. Peaks not assigned belong to the sapphire substrate.
diffusion coefficient between indium and gallium. On the other hand, when the substrate temperature is lower than 200 °C, the adatoms cannot travel far for the low diffusion coefficient which decreases with an exponential function of temperature [19], resulting in the homogenous structure of the films. Next, thermal annealing was carried out on these as-deposited (GaIn)2O3 samples in order to suppress the phase separation and improve the crystal quality of the (GaIn)2O3 films. No obvious effects were observed on (GaIn)2O3 films grown at substrate temperature higher than 300 °C. The clusters remained on the surface of the films after thermal annealing treatment. Thus, we selected the film deposited at room temperature for thermal annealing because it exhibits homogenous element distribution as shown in Fig. 2(a) and (a0 ). Fig. 3 shows the XRD pattern of the (GaIn)2O3 film annealed at 800 °C. It is clear that only peaks belonging to (2 0 1) face of b-(GaIn)2O3 and its higher order have been detected, suggesting that oriented monoclinic structured film without phase separation is successfully obtained by post thermal annealing. The orientation relationship between the substrate and film is: b-(GaIn)2O3 (2 0 1) // sapphire (0 0 0 1). The X-ray rocking curve for b-(GaIn)2O3 (4 0 2) diffraction of the film annealed at 800 °C is presented in Fig. 4, where the full width at half maximum (FWHM) is obtained to be 49 arcmin which is at the same order of the as-deposited high quality b-Ga2O3 [20].
2 µm
(b)
2 µm
(c)
Intensity (arb. unit)
FWHM=49 arcmin
-150
-100
-50
0
50
100
150
ω (arcmin) Fig. 4. X-ray rocking curve for the b-(GaIn)2O3 (4 0 2) diffraction of film annealed at 800 °C.
Fig. 5. SEM (a), indium element distribution (b), and AFM (c) morphology of (GaIn)2O3 film annealed at 800 °C.
176
F. Zhang et al. / Journal of Alloys and Compounds 614 (2014) 173–176
however, by thermal annealing the film deposited at RT, (GaIn)2O3 film with smooth surface, homogenous element distribution, high orientation crystal and high optical transmittance was successfully obtained, paving a way for obtaining high quality (GaIn)2O3 film without phase separation.
100
Transmittance (%)
80
60
Acknowledgement
40
This work was partially supported by the Partnership Project for Fundamental Technology Researches of Ministry of Education, Culture, Sports, Science and Technology, Japan.
As-deposited Annealing
20
References 200
400
600
800
Wavelength (nm) Fig. 6. Transmittance spectra of the annealing film and the one as-deposited at substrate temperature of 500 °C.
sample in infrared region is over 80%, and exhibits clear fringes in the visible and UV regions, which is much higher than the transmittance of the as-deposited film grown at 500 °C. On the other hand, two absorption bands were observed in the transmission spectra for the as-deposited film grown at 500 °C, which are attributed to phase separation of the film. Similar phenomenon has been reported by other groups [21–23]. The improvement of optical properties of the annealed films verifies that thermal annealing is an effect way for obtaining high quality (GaIn)2O3 film without phase separation. 4. Conclusion We have deposited (GaIn)2O3 films on sapphire substrate by PLD at substrate temperatures from RT to 500 °C. The phase separation were observed for the films grown at substrate temperature higher than 300 °C while the films grown at substrate temperature lower than 200 °C revealed homogenous element distributions with amorphous structures. Thermal annealing had no obvious effects on (GaIn)2O3 films grown at substrate temperature higher than 300 °C. The clusters remained on the surface of the films after thermal annealing treatment. On the other hand,
[1] Y. Kokubun, K. Miura, F. Endo, S. Nakagomi, Appl. Phys. Lett. 90 (2007) 031912. [2] T. Oshima, T. Okuno, N. Arai, N. Suzuki, S. Ohira, S. Fujita, Appl. Phys. Express 1 (2008) 011202. [3] M. Orita, H. Ohta, M. Hirano, H. Hosono, Appl. Phys. Lett. 77 (2000) 4166. [4] M. Orita, H. Hiramatsu, H. Ohta, M. Hirano, H. Hosono, Thin Solid Films 411 (2002) 134. [5] M. Higashiwaki, K. Sasaki, T. Kamimura, M.H. Wong, D. Krishnamurthy, A. Kuramata, T. Masui, S. Yamakoshi, Appl. Phys. Lett. 103 (2013) 123511. [6] X. Wang, S. Shen, S. Jin, J. Yang, M. Li, X. Wang, H. Han, C. Li, Phys. Chem. Chem. Phys. 15 (2013) 19380. [7] Y. Sakata, Y. Matsuda, T. Nakagawa, R. Yasunaga, H. Imamura, K. Teramura, ChemSusChem 4 (2011) 181. [8] T. Oshima, K. Kaminaga, H. Mashiko, A. Mukai, K. Sasaki, T. Masui, A. Kuramata, S. Yamakoshi, A. Ohtomo, Jpn. J. Appl. Phys. 52 (2013) 111102. [9] J.S. Hwang, T.Y. Liu, S. Chattopadhyay, G.M. Hsu, A.M. Basilio, H.W. Chen, Y.K. Hsu, W.H. Tu, Y.G. Lin, K.H. Chen, C.C. Li, S.B. Wang, H.Y. Chen, L.C. Chen, Nanotechnology 24 (2013) 055401. [10] F. Zhang, K. Saito, T. Tanaka, M. Nishio, Q. Guo, Solid State Commun. 186 (2014) 28. [11] N. Suzuki, K. Kaneko, S. Fujita, J. Cryst. Growth (2014).