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Epitaxial growth of InN film on intermediate oxide buffer layer by RF-MOMBE
Diamond & Related Materials 20 (2011) 1188–1192
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Epitaxial growth of InN film on intermediate oxide buffer layer by RF-MOMBE☆☆ Shou-Yi Kuo a, b,⁎, Fang-I Lai c, Wei-Chun Chen d, Woei-Tyng Lin c, Chien-Nan Hsiao d, Hsin-I Lin a, Han-Chang Pan e a
Department of Electronic Engineering, Chang Gung University, Taiwan Green Technology Research Center, Chang Gung University, Taiwan Department of Photonics Engineering, Yuan-Ze University, Taiwan d Instrument Technology Research Center, National Applied Research Laboratories, Taiwan e Gintech Energy Corporation, Taiwan b c
a r t i c l e
i n f o
Available online 1 July 2011 Keywords: B1. InN B1. Nitrides A1. Buffer layer B2. Semiconducting III–V materials
a b s t r a c t In this paper, we report the studies on the hetero-epitaxial growth of wurtzite indium nitride (InN) thin films on oxide buffer layer by RF metal-organic molecular beam epitaxy (RF-MOMBE) system. Oxide buffer layer was pre-sputtered using RF sputtering technique. The structural properties and surface morphology were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). We also investigated the optical properties by temperature-dependence photoluminescence (PL). Near-infrared emission peak centered at 0.75 eV was observed from PL measurement. The irregular and asymmetric PL line shape was caused by absorbed moisture and surface electron accumulation in InN films. According to the fitting results of PL spectra measured at 20 K, we could estimate the bandgap and Fermi level is 0.65 eV and 113 meV, which confirm to previous reports. Our results reveal that the oxide thin film could be a suitable buffer layer for engineering the growth of InN on sapphire wafer, and it might be also applicable for other lattice-mismatched III-V hetero-epitaxial systems. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Over the past few years, indium nitride (InN) has been the subject of intensive research. Compared to other III-nitrides, InN is the least studied of the nitride family. Due to the superior physical properties such as the smallest effective mass and the highest electron saturation velocity [1–4], InN is regarded as a promising material for solar cells, high performance high electron mobility transistors and sensors [5–10]. Accordingly, mastering the growth of high-quality InN would allow the emerging of a full nitride device technology, covering the large wavelength range from infrared to ultraviolet. However, InN growth is quite challenging because of its low-dissociation temperature at around 600 °C, small Gibbs' free energy change for InN growth and the lack of lattice-matched substrates for epitaxy. The lack of lattice-matched substrates was also an obstacle for high-quality InN growth. Based on earlier experience on GaN growth, a first approach for InN growth on sapphire was to use GaN or AlN buffer layers [11–13]. Until now, there are several techniques to improve the crystalline quality of the InN layer. Nanishi et al. have reported the high quality InN epitaxial layers by radio-frequency molecular beam epitaxy using low-temperature (LT)
☆☆ Presented at the Diamond 2010, 21st European Conference on Diamond, Diamond- Like Materials, Carbon Nanotubes, and Nitrides, Budapest. ⁎ Corresponding author at: Department of Electronic Engineering, Chang Gung University, Taiwan. E-mail address: [email protected] (S.-Y. Kuo). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.06.006
nitridation sapphire and LT-InN buffer layer techniques [14]. By applying an ArF excimer laser to the metalorganic vapor phase epitaxy, Bhuiyan et al. promote the selective dissociation of ammonia (NH3) gas [15]. Recently, Kumagai et al. reported a thick InN layer on a nitridated sapphire substrate using indium tri-chloride (InCl3) as an indium (In) precursor in HVPE system [16]. Since bulk InN substrates are not available, the InN films must be grown on a foreign substrate. Therefore, it is very important to choose a proper substrate to realize high-quality InN. In general, mainly two substrates, sapphire (Al2O3) and GaN, were well used in recent research. Table 1 shows the lattice mismatch of each substrate for InN growth. Usually, sapphire is used as a substrate material for the hetero-epitaxy of InN because of its wide availability, hexagonal symmetry, and its ease in handling and pre-growth cleaning. However, sapphire has a very large lattice mismatch to InN such that a appropriate buffer-layer is essential to remove the crystal stress. On the other hand, although GaN has relatively small lattice mismatch compared with sapphire, the properties of InN grown on GaN strongly depend on the polarity of the GaN (that is, Ga- or N-face GaN). Matsuda et al. have demonstrated that good crystal quality of InN can be achieved only on a N-face GaN substrate at high temperature [17]. In particular, zinc oxide (ZnO) has shown to be an attractive substrate material for InN growth. In fact, III-V nitrides and ZnO have the same crystal structure, i.e. wurtzite-type lattice, and the lattice mismatch between ZnO(a:3.25 Å, c:5.21 Å) and InN(a:3.54 Å, c:5.70 Å) on the c-plane is about 9%, which is much less than that
S.-Y. Kuo et al. / Diamond & Related Materials 20 (2011) 1188–1192 Table 1 Lattice constants (a-axis) and lattice mismatching with InN (a = 3.54 Å) for each substrate. The physical properties of ZnO is highlighted. Substrate material Sapphire GaN AlN ZnO 3C-SiC (111)
Hexagonal Hexagonal Hexagonal Hexagonal Cubic
Lattice constant (a-axis, Å)
Lattice mismatch (to InN)
4.76 3.19 3.14 3.25 4.358
25% 11% 13% 9% 15%
between InN and sapphire (25.4%). Many studies on GaN/ZnO heterostructures have been reported as well [18–20]. However, to the best of our knowledge, there are only few reports on InN/ZnO heterostructures [21–23]. In this study, we propose ZnO as a suitable substrate for InN growth. As already shown in Table 1, ZnO indeed has a small lattice mismatch to InN to be potential for making high-quality InN. We demonstrated the InN growth grown on ZnO substrates, and investigated the structural and opto-electrical properties of InN films in detail. 2. Experimental procedure N-type ZnO buffer layers with thickness of 250 nm were predeposited with a RF magnetron sputtering system. After deposition of n-type ZnO buffer layers, the substrate was sent into the RF metalorganic molecular beam epitaxy (RF-MOMBE) system. The RF-MOMBE system we employed was equipped with a turbo-molecular pump that evacuates the growth chamber down to 5 × 10 −9 Torr. A radiofrequency plasma discharge cell and trimethylindium (TMIn) are used as the N and In sources, respectively. At the same time, the active nitrogen radicals were supplied by a RF plasma source with fixed power of 350 W and the N2 flow rate was 0.5 sccm. Both flow rates of TMIn vapor and N2 gas were regulated and controlled by mass flow controllers. Finally, InN films were grown on the n-type ZnO buffer layers and commenced at 500 °C for 30 min. Shown in Fig. 1(a) and (b) are the schematic illustration of RF-MOMBE system and a diagram depicting the layered-structure of InN sample. After the growth, the crystalline structure of the InN thin films was studied by X-ray diffraction (XRD, Siemens D5000) using CuKα radiation of 1.54 Å. Surface morphology was measured using a field
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emission scanning electron microscopy (FE-SEM, Hitachi). Electrical resistivity, Hall mobility and carrier concentration were measured at room temperature (model: HL5500, BIO-RAD Co. Ltd.). The PL measurements were obtained using a diode laser operating at a wavelength of 976 nm as the excitation source. The collected luminescence was directly projected into a grating spectrometer and detected with an extended InGaAs detector. 3. Results and discussions For structural characterization, the (0002) ω-scan and ω-2θ scan measurements have been performed on both n-type ZnO buffer (Fig. 2 (a)) and RF-MOMBE InN/n-ZnO/sapphire layer (Fig. 2(b)), respectively. Shown in the inset of Fig. 2(a) is the selected area electron diffraction (SAED) pattern of the n-type ZnO buffer layer. The observation indicates that n-type ZnO buffer layer prepared by RF sputtering have the wurtzite structure with preferential oriented in the c-axis direction and the narrow full-width half maximum (FWHM) of (0002) diffraction peak is about 800 arcsec. Compared with the InN directly grown on sapphire substrate, it is expected that the introducing of single-crystalline n-type ZnO buffer layer might result in better crystallinity. Fig. 2(b) shows XRD ω-2θscan of InN film grown on c-sapphire substrate with n-type ZnO buffer layer. It reveals that there are major diffraction peaks indexed as InN(0002), InN (0004) and Al2O3(0006). At the same time, no other phases were identified such as In-oxide, Zn3N2 and metallic In or Zn [24,25]. This result indicates the InN film is preferentially oriented in the c-axis direction along with the sapphire substrate. A SAED pattern of InN film is shown in the insert of Fig. 2(b) as well, which shows the singlecrystal characteristic. The lattice parameter c was estimated to be 5.7 Å, which was in good agreement with the reported values of hexagonal InN. The crystalline quality of the films was certainly improved by inserting single-crystalline n-type ZnO buffer layer. From the experimental results, it was confirmed that the InN films we obtained are single crystal with wurtzite structure, and epitaxial relationship between InN, ZnO and sapphire is (0001)InN//(0001)ZnO//(0001)sapphire. Oxygen is viewed as a common contaminant of InN and is present at the interface of the substrates. However, the lattice constant estimated for the film was very close to that of high quality InN reported [16]. This suggested that the InN film was not oxidized in the form of indium oxide, which also can be certified in the XRD result as indium oxide related peaks could not be detected by the X-ray spectrometer. In addition, several literatures reported that the In droplets precipitate appeared on
Fig. 1. Schematic diagrams of (a) RF-MOMBE system and (b) sample structure.
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Fig. 2. (a) Rocking curve of RF-sputtered high-quality ZnO buffer layer, and (b) the ω-2θ scan of InN/ZnO/α-sapphire. The insets are the SAED patterns of ZnO and InN films, respectively. Both figures exhibit clear diffraction spotty patterns.
mismatch between InN and ZnO than that between InN and α-sapphire. PL spectroscopy was performed to acquire the energy gap of the InN materials. Shown in Fig. 5(a) is the typical 20 K PL spectrum of InN film grown on ZnO/α-sapphire, in which the near-infrared emission peak is centered at 0.75 eV. Besides, no other PL emissions in the range of 1.0–2.3 eV were detected. It is noteworthy that the PL emission line shape shows asymmetric high energy and low energy wings. The PL emission line shape of InN can be characterized as free-to-bound recombination between the degenerate electrons in the conductionband and the photo-generated holes at the valence-band edge, as schematically shown in Fig. 5(b). According to earlier literatures, the shape of PL was analyze by following expression: [27]. IðħωÞe½ħω−EG ðnÞ
γ
.
the InN films surface and the XRD spectra showed the In(101) phase of In metal at 2θ= 32.93°. Fortunately, we did not see 32.93° peak in our InN samples. In other words, all the InN samples have no In droplets on the surface, and are with high quality. Even so, the absence of the other phases only rules out relatively large crystalline. Binary or ternary compounds might be formed at the interface. For further information, we have taken HRTEM images. Shown in Fig. 3 is the SAED pattern at the interface. This spotty pattern indicated that only wurtzite InN and ZnO exist at the vicinity of interface. The surface morphology of InN thin films deposited on ZnO/αsapphire is shown in Fig. 4. FE-SEM images show the branch-like surface morphology and three hetero-material layers. In addition, we estimated the thickness of oxide layer and InN films are approximately 250 nm and 1.3 μm respectively from the FE-SEM images. Generally, large lattice mismatch will lead to three-dimensional (3D) island growth. Thus the grains are not continuous (Volmer–Weber growth mode) and the size/density of islands depend on the growth parameters. The morphology of InN film on ZnO/α-sapphire substrate seems to be of mosaic structure as shown in Fig. 4. Despite the rough top surface, a flat cleavage indicates a two-dimensional (2D) growth dominates. Such morphology is similar to the GaN thin film grown on α-sapphire substrate by Akasaki et al. [26]. This result reveals that both lateral and vertical growth dominates the InN film growth. From the XRD and SEM measurements, InN thin film was epitaxially grown on ZnO/α-sapphire substrate with the mosaic structure. The n-type ZnO buffer layer transmits the information on the c-axis orientation from (0001) α-sapphire substrate to InN thin film, and the InN thin film grows epitaxially along lateral orientation because of less lattice
2
f ðħω−EG ðnÞ−EF Þ
ð1Þ
where n is the electron carrier concentration, EG(n) is a carrierconcentration-dependent band gap which approaches the band gap EG(n) → EG(n) at vanishing concentration n → 0, γ is a parameter which increases from 2 to 4 with increasing concentration of free carriers for InN, f is the Fermi–Dirac function, EF is the Fermi energy of the degenerate electrons. The solid line in Fig. 5(a) shows the fitted PL spectrum of InN according to Eq. (1), revealing a good agreement between the fitting curves and the experimental data. According to the Eq (1), fitting result of PL spectrum measured at 20 K, we could estimate the bandgap and Fermi level is 0.65 eV and 113 meV, which is consistent with previous reports. Meanwhile, we proposed that the
Fig. 3. (Left) Electron-microscopy cross-sectional image and (Right) the selected area electron diffraction (SAED) pattern at the interface.
S.-Y. Kuo et al. / Diamond & Related Materials 20 (2011) 1188–1192
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Fig. 4. (a) Surface morphology and (b) cross-sectional view of InN thin film. From SEM images, we confirm that the thickness of buffer layers is approximately 250 nm. For the InN films, the surface morphology is branch-like, and the thickness is about 1.3 μm.
no intermediate phases with crystallinity were formed at the interface between InN and ZnO based on the XRD and HRTEM results. Thus the result of Hall measurement can be simplified to bi-layer model. The electron mobility of ZnO buffer is about 10 cm 2/V∙s, which is much less than the value of our InN sample. Moreover, the thicknesses of ZnO and InN are approximately 250 nm and 1300 nm respectively, the contribution of ZnO buffer to the electrical analysis of the InN film can be neglected. The Hall mobility of 165 cm 2/V∙s is obtained, which is close to earlier reported literature [28]. And the carrier concentration of the InN film grown on ZnO/α-sapphire substrate is approximate 2 × 1020 cm-3, which is lower than the carrier concentration of the InN film directly grown on α-sapphire at the same growth condition (mobility: 30 cm 2/V s, carrier concentration: 3.5 × 10 20 cm −3). As expected, inserting ZnO buffer between InN film and Al2O3 substrate effectively reduce the bulk carrier concentration due to the reduction of lattice-mismatch. Though the SAED patterns have revealed the shortrange single-crystalline feature of InN film, disorders still exist in the InN films. The dislocations may be generated because of the difference in lattice constant and thermal expansion coefficient between InN and ZnO. In addition, the possible point defects in InN are interstitials, vacancies, and antisite defects. InN antisite defects and interstitials are unlikely to form due to the presence of ZnO buffer. By contrast, NIn antisite defects and both types of vacancies (VN and VIn) and are expected to relax this compressive stress. Therefore, the potential native point defects in our samples might be NIn, VN and VIn. It is known that the In-rich InN growth leads to a high-quality film, but In droplets are formed on the surface. At the same time, the N-rich growth results in a
poor-quality film with a rough surface. Our experimental observation implies the formation of N-polar InN. Therefore, NIn might be dominant in our samples. Our higher background carrier concentration might result from the presence of structural defects, and optimization is still required for better crystal quality. Compared to our previous studies on InN/sapphire, the density of grain boundaries between InN crystals decreased due to the reduction of defect densities originating from the lattice mismatch. These experimental results indicate that a ZnO buffer is not only useful in reducing the lattice mismatch but also effective in decreasing the carrier-scattering at crystalline boundaries.
4. Conclusion In summary, RF-MOMBE-grown InN film was prepared on α-sapphire substrate incorporating single-crystal n-type ZnO intermediate layer. The electron concentration and mobility of InN film are 2× 1020 cm−3 and 165 cm2/V s, respectively. Strong near-infrared emission peak centered at 0.75 eV was observed from PL measurement. According to the fitting results of PL spectrum measured at 20 K, we could estimate the bandgap and Fermi level is 0.65 eV and 113 meV, which confirm to previous reports. The irregular and asymmetric PL line shape might be attributed to the high surface electron accumulation in InN films, consistent with the Hall results. Our results exhibit that the oxide buffer layer could be a suitable buffer layer for the growth of high-quality InN films. And the mature technology of ZnO epi-film by RF sputtering might be also applicable for other lattice-mismatched III-V hetero-epitaxial systems.
a
b
Intensity (a.u)
InN/ZnO/α-sapphire Measured at 20K
Eg
0.65
EF
0.70
0.75
0.80
0.85
Energy (eV) Fig. 5. (a) Measured (circles) and calculated (solid line) PL of InN/ZnO/α-sapphire measured at 20 K. (b) A schematic diagram for the recombination paths in degenerate InN.
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Acknowledgement The authors would like to gratefully acknowledge partial financial support from the National Science Council of Taiwan under Contract No. NSC97-2112-M-182-004-MY3 and NSC96-2221-E-155-071-MY3. References [1] J. Wu, W. Walukiewicz, W. Shan, K.M. Yu, J.W. Ager, E.E. Haller, H. Lu, W.J. Schaff, Phys. Rev. B 66 (2002) 201403. [2] B.R. Nag, Phys. Status Solidi B 237 (2003) R1. [3] B.E. Foutz, S.K. O'Leary, M.S. Shur, L.F. Eastman, J. Appl. Phys. 85 (1999) 7727–7734. [4] K.T. Tsen, C. Poweleit, D.K. Ferry, H. Lu, W.J. Schaff, Appl. Phys. Lett. 86 (2005) 222103. [5] E. Trybus, O. Jani, S. Burnham, I. Ferguson, C. Honsberg, M. Steiner, W.A. Doolittle, Phys. Stat. Sol.(c) 5 (2008) 1843–1845. [6] C.H. Swartz, R.P. Tomkins, T.H. Myers, H. Lu, W.J. Schaff, Phys. Stat. Sol.(c) 2 (2005) 2250–2253. [7] V.M. Polyakov, F. Schwierz, F. Fuchs, J. Furthmüller, F. Bechstedt, Appl. Phys. Lett. 94 (2009) 022102. [8] J. Wu, W. Walukiewicz, K.M. Yu, W. Shan, J.W. Ager, E.E. Haller, H. Lu, W.J. Schaff, W.K. Metzger, S. Kurtz, J. Appl. Phys. 94 (2003) 6477–6482. [9] H. Lu, W.J. Schaff, L.F. Eastman, J. Wu, W. Walukiewicz, D.C. Look, R.J. Molnar, Mat. Res. Soc. Symp. Proc. 743 (2003) L4.10. [10] H. Lu, W.J. Schaff, L.F. Eastman, J. Appl. Phys. 96 (2004) 3577–3579. [11] O.A. Laboutin, R.E. Welser, Appl. Phys. Lett. 92 (2008) 223103. [12] X. Wang, S.-B. Che, Y. Ishitani, A. Yoshikawa, Appl. Phys. Lett. 92 (2008) 132108. [13] S.Y. Kuo, W.C. Chen, C.C. Kei, C.N. Hsiao, Semicond. Sci. Tech. 23 (2008) 055013–055017.
[14] Y. Nanishi, Y. Saito, T. Yamaguchi, Jpn. J. Appl. Phys. 42 (2003) 2549–2559. [15] A.G. Bhuiyan, T. Tanaka, A. Yamamoto, A. Hashimoto, Phys. Stat. Sol.(a) 194 (2002) 502–505. [16] Y. Kumagai, J. Kikuchi, Y. Nishizawa, H. Murakami, A. Koukitu, J. Crystal Growth 300 (2007) 57–61. [17] F. Matsuda, Y. Saito, T. Muramatsu, T. Yamaguchi, Y. Matsuo, A. Koukitu, T. Araki, Y. Nanishi, Phys. Stat. Sol.(c) 0 (2003) 2810–2813. [18] F. Hamdani, A. Botchkarev, W. Kim, H. Morkoç, M. Yeadon, J.M. Gibson, S.Y. Tsen, D.J. Smith, Appl. Phys. Lett. 70 (1997) 467–469. [19] T. Ohgaki, S. Sugimura, N. Ohashi, I. Sakaguchia, T. Sekiguchia, H. Haneda, J. Crystal Growth 275 (2005) e1143–e1148. [20] Y.I. Alivov, J.E. Van Nostrand, D.C. Look, M.V. Chukichev, B.M. Ataev, Appl. Phys. Lett. 83 (2003) 2943–2945. [21] S.Y. Kuo, W.C. Chen, C.C. Kei, C.N. Hsiao, F.-I. Lai, J. Crystal Growth 310 (2008) 4963–4967. [22] S. Ohuchi, T. Takizawa, J. Electron. Mater. 34 (2005) 424–429. [23] T. Ohgaki, S. Sugimura, H. Ryoken, N. Ohashi, I. Sakaguchi, T. Sekiguchi, H. Haneda, Key Eng. Mater. 301 (2006) 79–82. [24] G.Z. Xing, D.D. Wang, B. Yao, L.F.N. Ah Qune, T. Yang, Q. He, J.H. Yang, L.L. Yang, J. Appl. Phys. 108 (2010) 083710. [25] R. Ayouchi, C. Casteleiro, L. Santos, R. Schwarz, Phys. Status Solidi C 7 (2010) 2294–2297. [26] I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu, N. Sawaki, J.Cryst. Growth 98 (1989) 209–219. [27] V.Y. Davydov, A.A. Klochikhin, V.V. Emtsev, D.A. Kurdyukov, S.V. Ivanov, V.A. Vekshin, F. Bechstedt, J. Furthmuller, F. Aderhold, J. Graul, A.V. Mudryi, H. Harima, A. Hashimoto, A. Yamamoto, E.E. Haller, Phys. Status Solidi (b) 234 (2002) 787–795. [28] A.G. Bhuiyan, T. Tanaka, K. Kasashima, A. Hashimoto, A. Yamamoto, Jpn. J. Appl. Phys. 42 (2003) 7284.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Epitaxial growth of InN film on intermediate oxide buffer layer by RF-MOMBE☆☆ Shou-Yi Kuo a, b,⁎, Fang-I Lai c, Wei-Chun Chen d, Woei-Tyng Lin c, Chien-Nan Hsiao d, Hsin-I Lin a, Han-Chang Pan e a
Department of Electronic Engineering, Chang Gung University, Taiwan Green Technology Research Center, Chang Gung University, Taiwan Department of Photonics Engineering, Yuan-Ze University, Taiwan d Instrument Technology Research Center, National Applied Research Laboratories, Taiwan e Gintech Energy Corporation, Taiwan b c
a r t i c l e
i n f o
Available online 1 July 2011 Keywords: B1. InN B1. Nitrides A1. Buffer layer B2. Semiconducting III–V materials
a b s t r a c t In this paper, we report the studies on the hetero-epitaxial growth of wurtzite indium nitride (InN) thin films on oxide buffer layer by RF metal-organic molecular beam epitaxy (RF-MOMBE) system. Oxide buffer layer was pre-sputtered using RF sputtering technique. The structural properties and surface morphology were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). We also investigated the optical properties by temperature-dependence photoluminescence (PL). Near-infrared emission peak centered at 0.75 eV was observed from PL measurement. The irregular and asymmetric PL line shape was caused by absorbed moisture and surface electron accumulation in InN films. According to the fitting results of PL spectra measured at 20 K, we could estimate the bandgap and Fermi level is 0.65 eV and 113 meV, which confirm to previous reports. Our results reveal that the oxide thin film could be a suitable buffer layer for engineering the growth of InN on sapphire wafer, and it might be also applicable for other lattice-mismatched III-V hetero-epitaxial systems. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Over the past few years, indium nitride (InN) has been the subject of intensive research. Compared to other III-nitrides, InN is the least studied of the nitride family. Due to the superior physical properties such as the smallest effective mass and the highest electron saturation velocity [1–4], InN is regarded as a promising material for solar cells, high performance high electron mobility transistors and sensors [5–10]. Accordingly, mastering the growth of high-quality InN would allow the emerging of a full nitride device technology, covering the large wavelength range from infrared to ultraviolet. However, InN growth is quite challenging because of its low-dissociation temperature at around 600 °C, small Gibbs' free energy change for InN growth and the lack of lattice-matched substrates for epitaxy. The lack of lattice-matched substrates was also an obstacle for high-quality InN growth. Based on earlier experience on GaN growth, a first approach for InN growth on sapphire was to use GaN or AlN buffer layers [11–13]. Until now, there are several techniques to improve the crystalline quality of the InN layer. Nanishi et al. have reported the high quality InN epitaxial layers by radio-frequency molecular beam epitaxy using low-temperature (LT)
☆☆ Presented at the Diamond 2010, 21st European Conference on Diamond, Diamond- Like Materials, Carbon Nanotubes, and Nitrides, Budapest. ⁎ Corresponding author at: Department of Electronic Engineering, Chang Gung University, Taiwan. E-mail address: [email protected] (S.-Y. Kuo). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.06.006
nitridation sapphire and LT-InN buffer layer techniques [14]. By applying an ArF excimer laser to the metalorganic vapor phase epitaxy, Bhuiyan et al. promote the selective dissociation of ammonia (NH3) gas [15]. Recently, Kumagai et al. reported a thick InN layer on a nitridated sapphire substrate using indium tri-chloride (InCl3) as an indium (In) precursor in HVPE system [16]. Since bulk InN substrates are not available, the InN films must be grown on a foreign substrate. Therefore, it is very important to choose a proper substrate to realize high-quality InN. In general, mainly two substrates, sapphire (Al2O3) and GaN, were well used in recent research. Table 1 shows the lattice mismatch of each substrate for InN growth. Usually, sapphire is used as a substrate material for the hetero-epitaxy of InN because of its wide availability, hexagonal symmetry, and its ease in handling and pre-growth cleaning. However, sapphire has a very large lattice mismatch to InN such that a appropriate buffer-layer is essential to remove the crystal stress. On the other hand, although GaN has relatively small lattice mismatch compared with sapphire, the properties of InN grown on GaN strongly depend on the polarity of the GaN (that is, Ga- or N-face GaN). Matsuda et al. have demonstrated that good crystal quality of InN can be achieved only on a N-face GaN substrate at high temperature [17]. In particular, zinc oxide (ZnO) has shown to be an attractive substrate material for InN growth. In fact, III-V nitrides and ZnO have the same crystal structure, i.e. wurtzite-type lattice, and the lattice mismatch between ZnO(a:3.25 Å, c:5.21 Å) and InN(a:3.54 Å, c:5.70 Å) on the c-plane is about 9%, which is much less than that
S.-Y. Kuo et al. / Diamond & Related Materials 20 (2011) 1188–1192 Table 1 Lattice constants (a-axis) and lattice mismatching with InN (a = 3.54 Å) for each substrate. The physical properties of ZnO is highlighted. Substrate material Sapphire GaN AlN ZnO 3C-SiC (111)
Hexagonal Hexagonal Hexagonal Hexagonal Cubic
Lattice constant (a-axis, Å)
Lattice mismatch (to InN)
4.76 3.19 3.14 3.25 4.358
25% 11% 13% 9% 15%
between InN and sapphire (25.4%). Many studies on GaN/ZnO heterostructures have been reported as well [18–20]. However, to the best of our knowledge, there are only few reports on InN/ZnO heterostructures [21–23]. In this study, we propose ZnO as a suitable substrate for InN growth. As already shown in Table 1, ZnO indeed has a small lattice mismatch to InN to be potential for making high-quality InN. We demonstrated the InN growth grown on ZnO substrates, and investigated the structural and opto-electrical properties of InN films in detail. 2. Experimental procedure N-type ZnO buffer layers with thickness of 250 nm were predeposited with a RF magnetron sputtering system. After deposition of n-type ZnO buffer layers, the substrate was sent into the RF metalorganic molecular beam epitaxy (RF-MOMBE) system. The RF-MOMBE system we employed was equipped with a turbo-molecular pump that evacuates the growth chamber down to 5 × 10 −9 Torr. A radiofrequency plasma discharge cell and trimethylindium (TMIn) are used as the N and In sources, respectively. At the same time, the active nitrogen radicals were supplied by a RF plasma source with fixed power of 350 W and the N2 flow rate was 0.5 sccm. Both flow rates of TMIn vapor and N2 gas were regulated and controlled by mass flow controllers. Finally, InN films were grown on the n-type ZnO buffer layers and commenced at 500 °C for 30 min. Shown in Fig. 1(a) and (b) are the schematic illustration of RF-MOMBE system and a diagram depicting the layered-structure of InN sample. After the growth, the crystalline structure of the InN thin films was studied by X-ray diffraction (XRD, Siemens D5000) using CuKα radiation of 1.54 Å. Surface morphology was measured using a field
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emission scanning electron microscopy (FE-SEM, Hitachi). Electrical resistivity, Hall mobility and carrier concentration were measured at room temperature (model: HL5500, BIO-RAD Co. Ltd.). The PL measurements were obtained using a diode laser operating at a wavelength of 976 nm as the excitation source. The collected luminescence was directly projected into a grating spectrometer and detected with an extended InGaAs detector. 3. Results and discussions For structural characterization, the (0002) ω-scan and ω-2θ scan measurements have been performed on both n-type ZnO buffer (Fig. 2 (a)) and RF-MOMBE InN/n-ZnO/sapphire layer (Fig. 2(b)), respectively. Shown in the inset of Fig. 2(a) is the selected area electron diffraction (SAED) pattern of the n-type ZnO buffer layer. The observation indicates that n-type ZnO buffer layer prepared by RF sputtering have the wurtzite structure with preferential oriented in the c-axis direction and the narrow full-width half maximum (FWHM) of (0002) diffraction peak is about 800 arcsec. Compared with the InN directly grown on sapphire substrate, it is expected that the introducing of single-crystalline n-type ZnO buffer layer might result in better crystallinity. Fig. 2(b) shows XRD ω-2θscan of InN film grown on c-sapphire substrate with n-type ZnO buffer layer. It reveals that there are major diffraction peaks indexed as InN(0002), InN (0004) and Al2O3(0006). At the same time, no other phases were identified such as In-oxide, Zn3N2 and metallic In or Zn [24,25]. This result indicates the InN film is preferentially oriented in the c-axis direction along with the sapphire substrate. A SAED pattern of InN film is shown in the insert of Fig. 2(b) as well, which shows the singlecrystal characteristic. The lattice parameter c was estimated to be 5.7 Å, which was in good agreement with the reported values of hexagonal InN. The crystalline quality of the films was certainly improved by inserting single-crystalline n-type ZnO buffer layer. From the experimental results, it was confirmed that the InN films we obtained are single crystal with wurtzite structure, and epitaxial relationship between InN, ZnO and sapphire is (0001)InN//(0001)ZnO//(0001)sapphire. Oxygen is viewed as a common contaminant of InN and is present at the interface of the substrates. However, the lattice constant estimated for the film was very close to that of high quality InN reported [16]. This suggested that the InN film was not oxidized in the form of indium oxide, which also can be certified in the XRD result as indium oxide related peaks could not be detected by the X-ray spectrometer. In addition, several literatures reported that the In droplets precipitate appeared on
Fig. 1. Schematic diagrams of (a) RF-MOMBE system and (b) sample structure.
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Fig. 2. (a) Rocking curve of RF-sputtered high-quality ZnO buffer layer, and (b) the ω-2θ scan of InN/ZnO/α-sapphire. The insets are the SAED patterns of ZnO and InN films, respectively. Both figures exhibit clear diffraction spotty patterns.
mismatch between InN and ZnO than that between InN and α-sapphire. PL spectroscopy was performed to acquire the energy gap of the InN materials. Shown in Fig. 5(a) is the typical 20 K PL spectrum of InN film grown on ZnO/α-sapphire, in which the near-infrared emission peak is centered at 0.75 eV. Besides, no other PL emissions in the range of 1.0–2.3 eV were detected. It is noteworthy that the PL emission line shape shows asymmetric high energy and low energy wings. The PL emission line shape of InN can be characterized as free-to-bound recombination between the degenerate electrons in the conductionband and the photo-generated holes at the valence-band edge, as schematically shown in Fig. 5(b). According to earlier literatures, the shape of PL was analyze by following expression: [27]. IðħωÞe½ħω−EG ðnÞ
γ
.
the InN films surface and the XRD spectra showed the In(101) phase of In metal at 2θ= 32.93°. Fortunately, we did not see 32.93° peak in our InN samples. In other words, all the InN samples have no In droplets on the surface, and are with high quality. Even so, the absence of the other phases only rules out relatively large crystalline. Binary or ternary compounds might be formed at the interface. For further information, we have taken HRTEM images. Shown in Fig. 3 is the SAED pattern at the interface. This spotty pattern indicated that only wurtzite InN and ZnO exist at the vicinity of interface. The surface morphology of InN thin films deposited on ZnO/αsapphire is shown in Fig. 4. FE-SEM images show the branch-like surface morphology and three hetero-material layers. In addition, we estimated the thickness of oxide layer and InN films are approximately 250 nm and 1.3 μm respectively from the FE-SEM images. Generally, large lattice mismatch will lead to three-dimensional (3D) island growth. Thus the grains are not continuous (Volmer–Weber growth mode) and the size/density of islands depend on the growth parameters. The morphology of InN film on ZnO/α-sapphire substrate seems to be of mosaic structure as shown in Fig. 4. Despite the rough top surface, a flat cleavage indicates a two-dimensional (2D) growth dominates. Such morphology is similar to the GaN thin film grown on α-sapphire substrate by Akasaki et al. [26]. This result reveals that both lateral and vertical growth dominates the InN film growth. From the XRD and SEM measurements, InN thin film was epitaxially grown on ZnO/α-sapphire substrate with the mosaic structure. The n-type ZnO buffer layer transmits the information on the c-axis orientation from (0001) α-sapphire substrate to InN thin film, and the InN thin film grows epitaxially along lateral orientation because of less lattice
2
f ðħω−EG ðnÞ−EF Þ
ð1Þ
where n is the electron carrier concentration, EG(n) is a carrierconcentration-dependent band gap which approaches the band gap EG(n) → EG(n) at vanishing concentration n → 0, γ is a parameter which increases from 2 to 4 with increasing concentration of free carriers for InN, f is the Fermi–Dirac function, EF is the Fermi energy of the degenerate electrons. The solid line in Fig. 5(a) shows the fitted PL spectrum of InN according to Eq. (1), revealing a good agreement between the fitting curves and the experimental data. According to the Eq (1), fitting result of PL spectrum measured at 20 K, we could estimate the bandgap and Fermi level is 0.65 eV and 113 meV, which is consistent with previous reports. Meanwhile, we proposed that the
Fig. 3. (Left) Electron-microscopy cross-sectional image and (Right) the selected area electron diffraction (SAED) pattern at the interface.
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Fig. 4. (a) Surface morphology and (b) cross-sectional view of InN thin film. From SEM images, we confirm that the thickness of buffer layers is approximately 250 nm. For the InN films, the surface morphology is branch-like, and the thickness is about 1.3 μm.
no intermediate phases with crystallinity were formed at the interface between InN and ZnO based on the XRD and HRTEM results. Thus the result of Hall measurement can be simplified to bi-layer model. The electron mobility of ZnO buffer is about 10 cm 2/V∙s, which is much less than the value of our InN sample. Moreover, the thicknesses of ZnO and InN are approximately 250 nm and 1300 nm respectively, the contribution of ZnO buffer to the electrical analysis of the InN film can be neglected. The Hall mobility of 165 cm 2/V∙s is obtained, which is close to earlier reported literature [28]. And the carrier concentration of the InN film grown on ZnO/α-sapphire substrate is approximate 2 × 1020 cm-3, which is lower than the carrier concentration of the InN film directly grown on α-sapphire at the same growth condition (mobility: 30 cm 2/V s, carrier concentration: 3.5 × 10 20 cm −3). As expected, inserting ZnO buffer between InN film and Al2O3 substrate effectively reduce the bulk carrier concentration due to the reduction of lattice-mismatch. Though the SAED patterns have revealed the shortrange single-crystalline feature of InN film, disorders still exist in the InN films. The dislocations may be generated because of the difference in lattice constant and thermal expansion coefficient between InN and ZnO. In addition, the possible point defects in InN are interstitials, vacancies, and antisite defects. InN antisite defects and interstitials are unlikely to form due to the presence of ZnO buffer. By contrast, NIn antisite defects and both types of vacancies (VN and VIn) and are expected to relax this compressive stress. Therefore, the potential native point defects in our samples might be NIn, VN and VIn. It is known that the In-rich InN growth leads to a high-quality film, but In droplets are formed on the surface. At the same time, the N-rich growth results in a
poor-quality film with a rough surface. Our experimental observation implies the formation of N-polar InN. Therefore, NIn might be dominant in our samples. Our higher background carrier concentration might result from the presence of structural defects, and optimization is still required for better crystal quality. Compared to our previous studies on InN/sapphire, the density of grain boundaries between InN crystals decreased due to the reduction of defect densities originating from the lattice mismatch. These experimental results indicate that a ZnO buffer is not only useful in reducing the lattice mismatch but also effective in decreasing the carrier-scattering at crystalline boundaries.
4. Conclusion In summary, RF-MOMBE-grown InN film was prepared on α-sapphire substrate incorporating single-crystal n-type ZnO intermediate layer. The electron concentration and mobility of InN film are 2× 1020 cm−3 and 165 cm2/V s, respectively. Strong near-infrared emission peak centered at 0.75 eV was observed from PL measurement. According to the fitting results of PL spectrum measured at 20 K, we could estimate the bandgap and Fermi level is 0.65 eV and 113 meV, which confirm to previous reports. The irregular and asymmetric PL line shape might be attributed to the high surface electron accumulation in InN films, consistent with the Hall results. Our results exhibit that the oxide buffer layer could be a suitable buffer layer for the growth of high-quality InN films. And the mature technology of ZnO epi-film by RF sputtering might be also applicable for other lattice-mismatched III-V hetero-epitaxial systems.
a
b
Intensity (a.u)
InN/ZnO/α-sapphire Measured at 20K
Eg
0.65
EF
0.70
0.75
0.80
0.85
Energy (eV) Fig. 5. (a) Measured (circles) and calculated (solid line) PL of InN/ZnO/α-sapphire measured at 20 K. (b) A schematic diagram for the recombination paths in degenerate InN.
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Acknowledgement The authors would like to gratefully acknowledge partial financial support from the National Science Council of Taiwan under Contract No. NSC97-2112-M-182-004-MY3 and NSC96-2221-E-155-071-MY3. References [1] J. Wu, W. Walukiewicz, W. Shan, K.M. Yu, J.W. Ager, E.E. Haller, H. Lu, W.J. Schaff, Phys. Rev. B 66 (2002) 201403. [2] B.R. Nag, Phys. Status Solidi B 237 (2003) R1. [3] B.E. Foutz, S.K. O'Leary, M.S. Shur, L.F. Eastman, J. Appl. Phys. 85 (1999) 7727–7734. [4] K.T. Tsen, C. Poweleit, D.K. Ferry, H. Lu, W.J. Schaff, Appl. Phys. Lett. 86 (2005) 222103. [5] E. Trybus, O. Jani, S. Burnham, I. Ferguson, C. Honsberg, M. Steiner, W.A. Doolittle, Phys. Stat. Sol.(c) 5 (2008) 1843–1845. [6] C.H. Swartz, R.P. Tomkins, T.H. Myers, H. Lu, W.J. Schaff, Phys. Stat. Sol.(c) 2 (2005) 2250–2253. [7] V.M. Polyakov, F. Schwierz, F. Fuchs, J. Furthmüller, F. Bechstedt, Appl. Phys. Lett. 94 (2009) 022102. [8] J. Wu, W. Walukiewicz, K.M. Yu, W. Shan, J.W. Ager, E.E. Haller, H. Lu, W.J. Schaff, W.K. Metzger, S. Kurtz, J. Appl. Phys. 94 (2003) 6477–6482. [9] H. Lu, W.J. Schaff, L.F. Eastman, J. Wu, W. Walukiewicz, D.C. Look, R.J. Molnar, Mat. Res. Soc. Symp. Proc. 743 (2003) L4.10. [10] H. Lu, W.J. Schaff, L.F. Eastman, J. Appl. Phys. 96 (2004) 3577–3579. [11] O.A. Laboutin, R.E. Welser, Appl. Phys. Lett. 92 (2008) 223103. [12] X. Wang, S.-B. Che, Y. Ishitani, A. Yoshikawa, Appl. Phys. Lett. 92 (2008) 132108. [13] S.Y. Kuo, W.C. Chen, C.C. Kei, C.N. Hsiao, Semicond. Sci. Tech. 23 (2008) 055013–055017.
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