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Structural properties of GaN and related alloys grown by radio-frequency magnetron sputter epitaxy
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Thin Solid Films 516 (2008) 2837 – 2842 www.elsevier.com/locate/tsf
Structural properties of GaN and related alloys grown by radio-frequency magnetron sputter epitaxy Hiroyuki Shinoda ⁎, Nobuki Mutsukura Department of Electronic Engineering, School of Engineering, Tokyo Denki University 2-2 Kanda-Nishiki-cho, Chiyoda-ku, Tokyo 101-8457, Japan Received 19 January 2006; received in revised form 17 March 2007; accepted 18 May 2007 Available online 29 May 2007
Abstract Single-crystalline layers of GaN and related alloys such as AlGaN and InGaN were grown on Al2O3 (0001) substrates by radio-frequency magnetron sputter epitaxy. The crystalline structures of these layers were studied as functions of substrate temperature, N2 composition ratio in N2/Ar mixture source gas and gas pressure during the growth. Surface structure of GaN layer depended on Ga/N ratio in flux density, and nitrogen-rich growth condition resulted in pyramid-type facet structure whereas Ga-rich growth produced flat surface. The crystalline quality of GaN layer improved at relatively low N2 composition ratios, and the GaN layer grown at 30% N2 condition was transparent and colorless. AlxGa1−xN layers with x = 0.06–0.08 and InxGa1−xN layers with x = 0.45–0.5, were obtained at 30–40% and 30–50% N2 composition ratios, respectively. The AlN and InN molar fractions in these layers were considerably different from Al and In molar fractions in starting metal alloys (x = 0.15 in both AlxGa1−x and InxGa1−x alloys). © 2007 Elsevier B.V. All rights reserved. Keywords: Gallium nitride (GaN); Aluminum gallium nitride (AlGaN); Indium gallium nitiride (InGaN); Epitaxy; Sputtering; Structural properties; Surface morphology
1. Introduction The wide band gap III–V nitride semiconductors have been paid significant attention for potential optoelectronic and high frequency devices [1–3]. Epitaxial growth of GaN and related alloys has been mainly performed by two kinds of methods of metalorganic vapor phase epitaxy and molecular beam epitaxy (MBE) for device applications. On the other hand, it has been considered that the growth of high quality GaN epilayer can scarcely be obtained by sputtering technique, and accordingly the sputtering process has not been portrayed as a suitable method for industrial production. Although, GaN films prepared by the sputtering process were predominantly poly-crystal or amorphous [4–8], a few single-crystalline GaN layers could be obtained [9–12]. Recently, a high quality GaN epilayer with a narrow X-ray rocking curve width of about 300 arc sec, has been grown by dc magnetron sputtering [13]. Thus, it has been demonstrated that the sputtering process also can produce high quality GaN single-crystalline layers. ⁎ Corresponding author. E-mail address: [email protected] (H. Shinoda). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.05.035
There is great advantage in the direct epitaxy of GaN layer on Al2O3 substrate without a buffer layer, in order to improve the performances of optoelectronic devices. Recently, pulsed laser deposition has been utilized to grow GaN epilayer directly on Al2O3 substrate [14]. In the sputtering process the energetic ions bombard the substrate surface during the film growth. In the sputter epitaxy of GaN layer using N2 source, a large number of nitrogen ions will bring homogeneous nitridation on Al2O3 substrate surface through ion bombardment at the beginning of the crystal growth, which may allow the subsequent direct growth of GaN c+ [Ga terminated (0001) plane] layer [15]. In this paper, we describe the structural properties of GaN, AlGaN and InGaN epilayers grown by radio-frequency (rf) magnetron sputtering, depending on substrate temperature, N2 composition ratio in a source N2/Ar mixture gas and gas pressure during the growth. 2. Experimental details The crystal growth was carried out in a conventional magnetron sputtering system [12]. A pure Ga target of 6-N grade purity contained in a stainless steel cup was maintained on a
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Fig. 3. The FWHM values of XRD GaN (0002) peaks for 2θ /θ and ω scan modes depending on gas pressure at 30% and 40% N2 composition ratios in N2/Ar mixture source gas. Fig. 1. The XRD patterns of GaN layers grown at various N2 composition ratios in N2/Ar mixture source gas.
lower cathode electrode. Both InGa and AlGa alloys (6-N grade purity for In and 5-N grade purity for Al) were also used as targets for the growth of InGaN and AlGaN epilayers, respectively. A source N2/Ar gas (6-N grade purity) mixture was purified by a liquid N2 trap in order to remove residual oxygen and water vapor. The growth chamber was evacuated to a
pressure less than 1.33 × 10− 7 Pa with a turbomolecular pump just prior to introducing source N2/Ar mixture. A power supply used to generate the plasma was a 13.56 MHz rf oscillator, and rf power input was kept at 70 W. The epilayers were grown on α-Al2O3 (0001) substrates without any buffer layers (InGaN layers were grown on GaN/Al2O3), and the thicknesses of GaN, AlGaN and InGaN layers examined were 1–2 μm. In the X-ray diffraction (XRD, RINT-2000, Rigaku) measurements, CuKα radiation was used as an X-ray source, and 2θ / θ and ω scan
Fig. 2. The XRD patterns of GaN layers grown at various substrate temperatures and (a) 30% and (b) 60% N2 composition ratios in N2/Ar mixture source gas.
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Fig. 4. The surface SEM images of GaN layers grown at gas pressures of (a) 0.66, (b) 0.93, (c) 1.33, (d) 1.99 and (e) 2.66 Pa and 30% N2 composition ratio in N2/Ar mixture source gas.
modes were used to evaluate the structural property. All XRD measurements were performed in a step scan mode with a step of 0.005°, using divergence slit of 0.5°, scattering slit of 0.5° and receiving slit of 0.15 mm. Surface morphology of the grown layer was observed using scanning electron microscope (SEM, JSM-5310LVB, JEOL). The operating voltage was set at 20 keV.
from the presence of inversion domains (IDs), which consist of regions with the opposite polarity to the bulk matrix [15]. The primary bulk matrix of the GaN layer with the pyramidal hillocks is thought to be oriented c− [N terminated (0001) plane] face, because the bulk layer was easily etched away in hot KOH solution (while one with a flat surface was scarcely etched). The IDs associated with the pyramidal hillocks may be oriented c+ face [16]. Thus, the reduction in the crystalline coherency of the
3. Results and discussion 3.1. Growth of GaN layer The X-ray pole-figures indicated that all of the GaN layers on Al2O3 (0001) substrates were epitaxially grown single crystals, and a-axis of the GaN layer rotated by 30° toward that of Al2O3 substrate [12]. The composition ratio of N2 gas in N2/Ar mixture source gas affects the structural property, and the reduction of N2 ratio results in the increase of crystalline coherency, as shown in Fig. 1. At the condition of 30% N2, two diffraction peaks for CuKα1 and CuKα2 radiations as X-ray source, are clearly observed separately. The sputtering rate of Ga target by Ar+ ion is thought to be larger than that by N+ and/ or N2+ ions, and accordingly the reduction of N2 ratio in source gas will cause the increase in the density of Ga flux as compared with that of atomic nitrogen flux reaching onto growing GaN layer surface. Romano and Myers [15] have reported that in the growth of GaN by rf-plasma MBE, nitrogen-rich growth brings a great number of pyramidal hillocks and Ga-rich growth gives a flat surface. Also in our work, surface of GaN layer grown in pure N2 gas represents pyramid-type facet structure, and those in relatively small N2 ratios at a source gas mixture tend to be flat surface. The pyramidal hillock structure may be coming
Fig. 5. The surface SEM images of GaN layers grown at gas pressures of (a) 0.4, (b) 0.66, (c) 0.93 and (d) 1.33 Pa and 40% N2 composition ratio in N2/Ar mixture source gas.
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Fig. 6. The surface structures of GaN layers depending on N2 composition ratio in N2/Ar mixture source gas and gas pressure.
GaN layer with increasing N2 ratio in a source gas would be associated with the number density of the IDs inside the bulk matrix. The GaN layer grown at 30% is transparent and colorless, and one grown at 100% indicates a faint yellow color. Thus, the sputtering process produces the direct epitaxial layer without the buffer layer, and the relatively large Ga/N ratio
Fig. 7. The XRD patterns of AlGaN layers grown at various N2 composition ratios in N2/Ar mixture source gas, together with that of GaN layer grown at 40% N2 composition ratio.
seems to bring a high quality GaN layer as well as in the MBE process [17]. The substrate temperature also affects the crystalline coherency, as shown in Fig. 2. When the substrate temperature was increased, the full-width at half maximum (FWHM) value of diffraction peak for CuKα1 X-ray source was decreased at 60% N2 source gas, whereas that does not depend on the substrate temperature at 30% N2 source gas. The structural property depended on gas pressure during the growth, and at lower pressures the crystalline coherency and mosaicity improved considerably. Fig. 3 shows the FWHM values of XRD peaks for 2θ / θ and ω scan modes depending on gas pressure at 30% and 40% N2 source gases. The FWHM values for both 2θ / θ and ω scan modes decreased with decreasing source gas pressure, and the decreasing rate in the FWHM value was more rapid at 40% N2 source gas. At source gas pressures less than 0.66 Pa the crystalline coherency and mosaicity seem to become high
Fig. 8. The AlN molar fractions of AlxGa1−xN layers depending on N2 composition ratio in N2/Ar mixture source gas.
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Fig. 9. The FWHM values of XRD AlGaN (0002) peaks for 2θ / θ scan mode depending on N2 composition ratios in N2/Ar mixture source gas.
Fig. 11. The InN molar fractions of InxGa1−xN layers depending on N2 composition ratio in N2/Ar mixture source gas.
and low, respectively. Figs. 4 and 5 shows surface SEM images of GaN layers grown at 30% and 40% N2 sources. These images indicate a nanosized grainy or an individually nanosized columnar structures (at lower pressures), a flat surface (at medium pressures) and a pyramid-type facet structures (at relatively high pressures). The surface structures depending on N2 composition ratio and source gas pressure are summarized in Fig. 6. Considering the results described in Figs. 3 and 6, the growth of GaN layer at the condition of 0.66 Pa and 40% N2 source gas is considered to produce a high quality GaN single-crystalline
epilayer with a flat surface maybe oriented c+ face at present stage.
Fig. 10. The XRD patterns of InGaN layers grown at various substrate temperatures.
Fig. 12. The FWHM values of XRD InGaN (0002) peaks for 2θ / θ scan mode depending on N2 composition ratios in N2/Ar mixture source gas.
3.2. Growth of AlGaN layer The growth of AlGaN layer was carried out using a starting material of AlxGa1−x metal alloy with an Al molar fraction x = 0.15. The XRD patterns of grown AlGaN layers are described in Fig. 7. The peak position of AlGaN (0002) diffraction signal depends on N2 composition ratio in N2/Ar source gas. The AlN molar fractions x of the AlxGa1−xN layers were determined by Vegard's law which states that the composition ratio is linearly proportional to the change in the lattice constant. The results are shown in Fig. 8. At 30–40% N2 conditions the x value changes in x = 0.06–0.08 which is almost half of an intentional Al molar fraction in the starting material. When the N2 ratio was increased more than 40%, the x value was abruptly decreased to about zero. On the surface of metal target during the growth, the surface nitridation and the sputtering will be occurred
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simultaneously. If the sputtering overcomes the surface nitridation, the sputtering ratios of both Al and Ga will determine the molar fraction x of the grown layer. When the sputtering rate is comparable to or less than the nitridation rate, the molar fraction x may depend considerably on the sputtering ratios of AlN and GaN. The negligibly low AlN molar fractions at N2 ratios more than 40%, may bring from the very small sputtering ratio of AlN as compared with that of GaN, and accordingly at these N2 composition ratios the nitridation would be considerable. Fig. 9 shows the FWHM values of XRD peaks for 2θ / θ scan mode. When the N2 ratio was increased, the FWHM value was almost constant or somewhat decreased until 40%, and then increased considerably. Thus, the AlxGa1−xN layers with x = 0.06–0.08 were obtained at 30–40% N2 composition ratios. Generally, strong strain remains in the III-nitride epilayer grown on Al2O3 substrate without buffer layers. The fluctuations of AlN molar fraction and the FWHM values of XRD peaks may result from the residual strains, so that the AlN molar fractions in Fig. 8 are quite tentative. 3.3. Growth of InGaN layer The InxGa1−xN layers were grown on GaN/Al2O3 substrates in which the GaN epilayers were grown on Al2O3 substrates prior to the deposition of InGaN layer. A starting material was InxGa1−x metal alloy with an In molar fraction x = 0.15. Fig. 10 shows the XRD patterns of InGaN layers grown at 40% N2 composition ratio. A strong diffraction peak for InGaN (0002) plane was observed at 620 °C of substrate temperature. The InN molar fractions x of the InxGa1−xN layers, determined by Vegard's law, were almost constant at x = 0.45–0.5 for substrate temperatures between 600 °C and 700 °C. The x values are increased by about three times more than the In molar fraction of InGa alloy as a source material. This may be caused by a large sputtering ratio of In as compared with that of Ga. When the substrate temperature was increased up to 830 °C, the InN molar fraction of the InxGa1−xN layer was decreased until x = 0.17 which was almost the same to the In molar fraction of source InGa alloy. These lower InN molar fractions less than those obtained at 600–700 °C, would be coming from the desorption of In atoms from growing film surface due to relatively high substrate temperature. The structural properties of the InGaN layer grown at 830 °C became poor. When the N2 composition ratio in source gas was changed between 30% and 50%, the InN molar fraction x of the InxGa1−xN layers were also almost constant at x = 0.45–0.5, as shown in Fig. 11. At these N2 composition ratios the sputtering ratios of metallic In and Ga may not be changed. Fig. 12 shows the FWHM values of XRD InGaN (0002) peaks for 2θ / θ scan mode, depending on N2 composition ratio. At around 40% N2 condition a minimum FWHM value was obtained, and at N2 ratios less than 40% the FWHM values became considerably large. 4. Summary The epitaxial growths of GaN, AlGaN and InGaN singlecrystalline layers were performed by an ultra high vacuum rf
sputtering method. GaN and AlGaN layers were directly grown on Al2O3(0001) substrate without any kinds of buffer layers, and InGaN layers were grown on GaN/Al2O3 substrates which were prepared in our work. The densities of Ga and nitrogen fluxes reaching onto growing layer surface could be changed with N2 composition ratio in N2/Ar mixture source gas. The nitrogen-rich growth condition produced a great number of pyramidal hillocks on the grown GaN surface, and a transparent and colorless high quality GaN layer with a flat surface could be grown at Ga-rich growth condition (at relatively low N2 composition ratios). The structural properties of GaN layer was also affected by substrate temperature and gas pressure during the growth. The AlxGa1−xN layers with x = 0.06–0.08 were grown at 30–40% N2 composition ratios, and the AlN molar fraction decreased to about zero at N2 composition ratios more than 40%. These AlN molar fractions were about half of an intentional Al molar fraction in a starting Al0.15Ga0.85 material. The InxGa1−xN layers with x = 0.45–0.5 were obtained at 600– 700 °C of substrate temperature and also at 30–50% N2 composition ratios. These InN molar fractions were considerably greater than an In molar fraction of an In0.15Ga0.85 alloy target. Acknowledgements The authors would like to thank Mr. K. Nezu, Mr. J. Yasuhara, Mr. A. Asami and Mr. T. Imoto for their co-operation. This work was partially supported by TDY Co. Ltd. References [1] S. Nakamura, M. Senoh, T. Mukai, Jpn. J. Appl. Phys. 32 (1993) L8. [2] S. Nakamura, M. Senoh, S. Nagahara, N. Iwasa, T. Yamada, T. Matsusita, Y. Sugimoto, H. Kiyoku, Appl. Phys. Lett. 69 (1996) 1477. [3] T. Egawa, K. Nakamura, H. Ishikawa, T. Jimbo, M. Umeno, Jpn. J. Appl. Phys. 38 (1999) 2630. [4] H.J. Hovel, J.J. Cuomo, Appl. Phys. Lett. 20 (1972) 71. [5] T.L. Tansley, R.J. Egan, Thin Solid Films 164 (1988) 441. [6] N. Elkashef, R.S. Srinivasa, S. Major, S.C. Sabharwal, K.P. Muthe, Thin Solid Films 333 (1998) 9. [7] C.-W. Wang, B.-S. Soong, J.-Y. Chen, C.-L. Chen, Y.-K. Su, J. Appl. Phys. 88 (2000) 6355. [8] T. Miyazaki, T. Fujimaki, S. Adachi, K. Ohtsuka, J. Appl. Phys. 89 (2001) 8316. [9] K. Kubota, Y. Kobayashi, K. Fujimoto, J. Appl. Phys. 66 (1989) 2984. [10] W.J. Meng, T.A. Perry, J. Appl. Phys. 76 (1994) 7824. [11] Q.X. Guo, A. Okada, H. Kidera, T. Tanaka, M. Nishio, H. Ogawa, J. Cryst. Growth 237–239 (2002) 1079. [12] Y. Daigo, N. Mutsukura, Thin Solid Films 483 (2005) 38. [13] M. Park, J.-P. Maria, J.J. Cuomo, Y.C. Chang, J.F. Muth, R.M. Kolbas, R.J. Nemanich, E. Carlson, J. Bumgarner, Appl. Phys. Lett. 81 (2002) 1797. [14] T. Nagata, Y.-Z. Yoo, P. Ahmet, T. Chikyow, Jpn. J. Appl. Phys. 44 (2005) 7896. [15] L.T. Romano, T.H. Myers, Appl. Phys. Lett. 71 (1997) 3486. [16] B. Daudin, J.L. Rouviere, M. Arley, Appl. Phys. Lett. 69 (1996) 2480. [17] K. Jegarathan, X.-O. Shen, I. Ide, M. Shimizu, H. Okumura, Jpn. J. Appl. Phys. 41 (2002) 4454.
Thin Solid Films 516 (2008) 2837 – 2842 www.elsevier.com/locate/tsf
Structural properties of GaN and related alloys grown by radio-frequency magnetron sputter epitaxy Hiroyuki Shinoda ⁎, Nobuki Mutsukura Department of Electronic Engineering, School of Engineering, Tokyo Denki University 2-2 Kanda-Nishiki-cho, Chiyoda-ku, Tokyo 101-8457, Japan Received 19 January 2006; received in revised form 17 March 2007; accepted 18 May 2007 Available online 29 May 2007
Abstract Single-crystalline layers of GaN and related alloys such as AlGaN and InGaN were grown on Al2O3 (0001) substrates by radio-frequency magnetron sputter epitaxy. The crystalline structures of these layers were studied as functions of substrate temperature, N2 composition ratio in N2/Ar mixture source gas and gas pressure during the growth. Surface structure of GaN layer depended on Ga/N ratio in flux density, and nitrogen-rich growth condition resulted in pyramid-type facet structure whereas Ga-rich growth produced flat surface. The crystalline quality of GaN layer improved at relatively low N2 composition ratios, and the GaN layer grown at 30% N2 condition was transparent and colorless. AlxGa1−xN layers with x = 0.06–0.08 and InxGa1−xN layers with x = 0.45–0.5, were obtained at 30–40% and 30–50% N2 composition ratios, respectively. The AlN and InN molar fractions in these layers were considerably different from Al and In molar fractions in starting metal alloys (x = 0.15 in both AlxGa1−x and InxGa1−x alloys). © 2007 Elsevier B.V. All rights reserved. Keywords: Gallium nitride (GaN); Aluminum gallium nitride (AlGaN); Indium gallium nitiride (InGaN); Epitaxy; Sputtering; Structural properties; Surface morphology
1. Introduction The wide band gap III–V nitride semiconductors have been paid significant attention for potential optoelectronic and high frequency devices [1–3]. Epitaxial growth of GaN and related alloys has been mainly performed by two kinds of methods of metalorganic vapor phase epitaxy and molecular beam epitaxy (MBE) for device applications. On the other hand, it has been considered that the growth of high quality GaN epilayer can scarcely be obtained by sputtering technique, and accordingly the sputtering process has not been portrayed as a suitable method for industrial production. Although, GaN films prepared by the sputtering process were predominantly poly-crystal or amorphous [4–8], a few single-crystalline GaN layers could be obtained [9–12]. Recently, a high quality GaN epilayer with a narrow X-ray rocking curve width of about 300 arc sec, has been grown by dc magnetron sputtering [13]. Thus, it has been demonstrated that the sputtering process also can produce high quality GaN single-crystalline layers. ⁎ Corresponding author. E-mail address: [email protected] (H. Shinoda). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.05.035
There is great advantage in the direct epitaxy of GaN layer on Al2O3 substrate without a buffer layer, in order to improve the performances of optoelectronic devices. Recently, pulsed laser deposition has been utilized to grow GaN epilayer directly on Al2O3 substrate [14]. In the sputtering process the energetic ions bombard the substrate surface during the film growth. In the sputter epitaxy of GaN layer using N2 source, a large number of nitrogen ions will bring homogeneous nitridation on Al2O3 substrate surface through ion bombardment at the beginning of the crystal growth, which may allow the subsequent direct growth of GaN c+ [Ga terminated (0001) plane] layer [15]. In this paper, we describe the structural properties of GaN, AlGaN and InGaN epilayers grown by radio-frequency (rf) magnetron sputtering, depending on substrate temperature, N2 composition ratio in a source N2/Ar mixture gas and gas pressure during the growth. 2. Experimental details The crystal growth was carried out in a conventional magnetron sputtering system [12]. A pure Ga target of 6-N grade purity contained in a stainless steel cup was maintained on a
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Fig. 3. The FWHM values of XRD GaN (0002) peaks for 2θ /θ and ω scan modes depending on gas pressure at 30% and 40% N2 composition ratios in N2/Ar mixture source gas. Fig. 1. The XRD patterns of GaN layers grown at various N2 composition ratios in N2/Ar mixture source gas.
lower cathode electrode. Both InGa and AlGa alloys (6-N grade purity for In and 5-N grade purity for Al) were also used as targets for the growth of InGaN and AlGaN epilayers, respectively. A source N2/Ar gas (6-N grade purity) mixture was purified by a liquid N2 trap in order to remove residual oxygen and water vapor. The growth chamber was evacuated to a
pressure less than 1.33 × 10− 7 Pa with a turbomolecular pump just prior to introducing source N2/Ar mixture. A power supply used to generate the plasma was a 13.56 MHz rf oscillator, and rf power input was kept at 70 W. The epilayers were grown on α-Al2O3 (0001) substrates without any buffer layers (InGaN layers were grown on GaN/Al2O3), and the thicknesses of GaN, AlGaN and InGaN layers examined were 1–2 μm. In the X-ray diffraction (XRD, RINT-2000, Rigaku) measurements, CuKα radiation was used as an X-ray source, and 2θ / θ and ω scan
Fig. 2. The XRD patterns of GaN layers grown at various substrate temperatures and (a) 30% and (b) 60% N2 composition ratios in N2/Ar mixture source gas.
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Fig. 4. The surface SEM images of GaN layers grown at gas pressures of (a) 0.66, (b) 0.93, (c) 1.33, (d) 1.99 and (e) 2.66 Pa and 30% N2 composition ratio in N2/Ar mixture source gas.
modes were used to evaluate the structural property. All XRD measurements were performed in a step scan mode with a step of 0.005°, using divergence slit of 0.5°, scattering slit of 0.5° and receiving slit of 0.15 mm. Surface morphology of the grown layer was observed using scanning electron microscope (SEM, JSM-5310LVB, JEOL). The operating voltage was set at 20 keV.
from the presence of inversion domains (IDs), which consist of regions with the opposite polarity to the bulk matrix [15]. The primary bulk matrix of the GaN layer with the pyramidal hillocks is thought to be oriented c− [N terminated (0001) plane] face, because the bulk layer was easily etched away in hot KOH solution (while one with a flat surface was scarcely etched). The IDs associated with the pyramidal hillocks may be oriented c+ face [16]. Thus, the reduction in the crystalline coherency of the
3. Results and discussion 3.1. Growth of GaN layer The X-ray pole-figures indicated that all of the GaN layers on Al2O3 (0001) substrates were epitaxially grown single crystals, and a-axis of the GaN layer rotated by 30° toward that of Al2O3 substrate [12]. The composition ratio of N2 gas in N2/Ar mixture source gas affects the structural property, and the reduction of N2 ratio results in the increase of crystalline coherency, as shown in Fig. 1. At the condition of 30% N2, two diffraction peaks for CuKα1 and CuKα2 radiations as X-ray source, are clearly observed separately. The sputtering rate of Ga target by Ar+ ion is thought to be larger than that by N+ and/ or N2+ ions, and accordingly the reduction of N2 ratio in source gas will cause the increase in the density of Ga flux as compared with that of atomic nitrogen flux reaching onto growing GaN layer surface. Romano and Myers [15] have reported that in the growth of GaN by rf-plasma MBE, nitrogen-rich growth brings a great number of pyramidal hillocks and Ga-rich growth gives a flat surface. Also in our work, surface of GaN layer grown in pure N2 gas represents pyramid-type facet structure, and those in relatively small N2 ratios at a source gas mixture tend to be flat surface. The pyramidal hillock structure may be coming
Fig. 5. The surface SEM images of GaN layers grown at gas pressures of (a) 0.4, (b) 0.66, (c) 0.93 and (d) 1.33 Pa and 40% N2 composition ratio in N2/Ar mixture source gas.
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Fig. 6. The surface structures of GaN layers depending on N2 composition ratio in N2/Ar mixture source gas and gas pressure.
GaN layer with increasing N2 ratio in a source gas would be associated with the number density of the IDs inside the bulk matrix. The GaN layer grown at 30% is transparent and colorless, and one grown at 100% indicates a faint yellow color. Thus, the sputtering process produces the direct epitaxial layer without the buffer layer, and the relatively large Ga/N ratio
Fig. 7. The XRD patterns of AlGaN layers grown at various N2 composition ratios in N2/Ar mixture source gas, together with that of GaN layer grown at 40% N2 composition ratio.
seems to bring a high quality GaN layer as well as in the MBE process [17]. The substrate temperature also affects the crystalline coherency, as shown in Fig. 2. When the substrate temperature was increased, the full-width at half maximum (FWHM) value of diffraction peak for CuKα1 X-ray source was decreased at 60% N2 source gas, whereas that does not depend on the substrate temperature at 30% N2 source gas. The structural property depended on gas pressure during the growth, and at lower pressures the crystalline coherency and mosaicity improved considerably. Fig. 3 shows the FWHM values of XRD peaks for 2θ / θ and ω scan modes depending on gas pressure at 30% and 40% N2 source gases. The FWHM values for both 2θ / θ and ω scan modes decreased with decreasing source gas pressure, and the decreasing rate in the FWHM value was more rapid at 40% N2 source gas. At source gas pressures less than 0.66 Pa the crystalline coherency and mosaicity seem to become high
Fig. 8. The AlN molar fractions of AlxGa1−xN layers depending on N2 composition ratio in N2/Ar mixture source gas.
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Fig. 9. The FWHM values of XRD AlGaN (0002) peaks for 2θ / θ scan mode depending on N2 composition ratios in N2/Ar mixture source gas.
Fig. 11. The InN molar fractions of InxGa1−xN layers depending on N2 composition ratio in N2/Ar mixture source gas.
and low, respectively. Figs. 4 and 5 shows surface SEM images of GaN layers grown at 30% and 40% N2 sources. These images indicate a nanosized grainy or an individually nanosized columnar structures (at lower pressures), a flat surface (at medium pressures) and a pyramid-type facet structures (at relatively high pressures). The surface structures depending on N2 composition ratio and source gas pressure are summarized in Fig. 6. Considering the results described in Figs. 3 and 6, the growth of GaN layer at the condition of 0.66 Pa and 40% N2 source gas is considered to produce a high quality GaN single-crystalline
epilayer with a flat surface maybe oriented c+ face at present stage.
Fig. 10. The XRD patterns of InGaN layers grown at various substrate temperatures.
Fig. 12. The FWHM values of XRD InGaN (0002) peaks for 2θ / θ scan mode depending on N2 composition ratios in N2/Ar mixture source gas.
3.2. Growth of AlGaN layer The growth of AlGaN layer was carried out using a starting material of AlxGa1−x metal alloy with an Al molar fraction x = 0.15. The XRD patterns of grown AlGaN layers are described in Fig. 7. The peak position of AlGaN (0002) diffraction signal depends on N2 composition ratio in N2/Ar source gas. The AlN molar fractions x of the AlxGa1−xN layers were determined by Vegard's law which states that the composition ratio is linearly proportional to the change in the lattice constant. The results are shown in Fig. 8. At 30–40% N2 conditions the x value changes in x = 0.06–0.08 which is almost half of an intentional Al molar fraction in the starting material. When the N2 ratio was increased more than 40%, the x value was abruptly decreased to about zero. On the surface of metal target during the growth, the surface nitridation and the sputtering will be occurred
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simultaneously. If the sputtering overcomes the surface nitridation, the sputtering ratios of both Al and Ga will determine the molar fraction x of the grown layer. When the sputtering rate is comparable to or less than the nitridation rate, the molar fraction x may depend considerably on the sputtering ratios of AlN and GaN. The negligibly low AlN molar fractions at N2 ratios more than 40%, may bring from the very small sputtering ratio of AlN as compared with that of GaN, and accordingly at these N2 composition ratios the nitridation would be considerable. Fig. 9 shows the FWHM values of XRD peaks for 2θ / θ scan mode. When the N2 ratio was increased, the FWHM value was almost constant or somewhat decreased until 40%, and then increased considerably. Thus, the AlxGa1−xN layers with x = 0.06–0.08 were obtained at 30–40% N2 composition ratios. Generally, strong strain remains in the III-nitride epilayer grown on Al2O3 substrate without buffer layers. The fluctuations of AlN molar fraction and the FWHM values of XRD peaks may result from the residual strains, so that the AlN molar fractions in Fig. 8 are quite tentative. 3.3. Growth of InGaN layer The InxGa1−xN layers were grown on GaN/Al2O3 substrates in which the GaN epilayers were grown on Al2O3 substrates prior to the deposition of InGaN layer. A starting material was InxGa1−x metal alloy with an In molar fraction x = 0.15. Fig. 10 shows the XRD patterns of InGaN layers grown at 40% N2 composition ratio. A strong diffraction peak for InGaN (0002) plane was observed at 620 °C of substrate temperature. The InN molar fractions x of the InxGa1−xN layers, determined by Vegard's law, were almost constant at x = 0.45–0.5 for substrate temperatures between 600 °C and 700 °C. The x values are increased by about three times more than the In molar fraction of InGa alloy as a source material. This may be caused by a large sputtering ratio of In as compared with that of Ga. When the substrate temperature was increased up to 830 °C, the InN molar fraction of the InxGa1−xN layer was decreased until x = 0.17 which was almost the same to the In molar fraction of source InGa alloy. These lower InN molar fractions less than those obtained at 600–700 °C, would be coming from the desorption of In atoms from growing film surface due to relatively high substrate temperature. The structural properties of the InGaN layer grown at 830 °C became poor. When the N2 composition ratio in source gas was changed between 30% and 50%, the InN molar fraction x of the InxGa1−xN layers were also almost constant at x = 0.45–0.5, as shown in Fig. 11. At these N2 composition ratios the sputtering ratios of metallic In and Ga may not be changed. Fig. 12 shows the FWHM values of XRD InGaN (0002) peaks for 2θ / θ scan mode, depending on N2 composition ratio. At around 40% N2 condition a minimum FWHM value was obtained, and at N2 ratios less than 40% the FWHM values became considerably large. 4. Summary The epitaxial growths of GaN, AlGaN and InGaN singlecrystalline layers were performed by an ultra high vacuum rf
sputtering method. GaN and AlGaN layers were directly grown on Al2O3(0001) substrate without any kinds of buffer layers, and InGaN layers were grown on GaN/Al2O3 substrates which were prepared in our work. The densities of Ga and nitrogen fluxes reaching onto growing layer surface could be changed with N2 composition ratio in N2/Ar mixture source gas. The nitrogen-rich growth condition produced a great number of pyramidal hillocks on the grown GaN surface, and a transparent and colorless high quality GaN layer with a flat surface could be grown at Ga-rich growth condition (at relatively low N2 composition ratios). The structural properties of GaN layer was also affected by substrate temperature and gas pressure during the growth. The AlxGa1−xN layers with x = 0.06–0.08 were grown at 30–40% N2 composition ratios, and the AlN molar fraction decreased to about zero at N2 composition ratios more than 40%. These AlN molar fractions were about half of an intentional Al molar fraction in a starting Al0.15Ga0.85 material. The InxGa1−xN layers with x = 0.45–0.5 were obtained at 600– 700 °C of substrate temperature and also at 30–50% N2 composition ratios. These InN molar fractions were considerably greater than an In molar fraction of an In0.15Ga0.85 alloy target. Acknowledgements The authors would like to thank Mr. K. Nezu, Mr. J. Yasuhara, Mr. A. Asami and Mr. T. Imoto for their co-operation. This work was partially supported by TDY Co. Ltd. References [1] S. Nakamura, M. Senoh, T. Mukai, Jpn. J. Appl. Phys. 32 (1993) L8. [2] S. Nakamura, M. Senoh, S. Nagahara, N. Iwasa, T. Yamada, T. Matsusita, Y. Sugimoto, H. Kiyoku, Appl. Phys. Lett. 69 (1996) 1477. [3] T. Egawa, K. Nakamura, H. Ishikawa, T. Jimbo, M. Umeno, Jpn. J. Appl. Phys. 38 (1999) 2630. [4] H.J. Hovel, J.J. Cuomo, Appl. Phys. Lett. 20 (1972) 71. [5] T.L. Tansley, R.J. Egan, Thin Solid Films 164 (1988) 441. [6] N. Elkashef, R.S. Srinivasa, S. Major, S.C. Sabharwal, K.P. Muthe, Thin Solid Films 333 (1998) 9. [7] C.-W. Wang, B.-S. Soong, J.-Y. Chen, C.-L. Chen, Y.-K. Su, J. Appl. Phys. 88 (2000) 6355. [8] T. Miyazaki, T. Fujimaki, S. Adachi, K. Ohtsuka, J. Appl. Phys. 89 (2001) 8316. [9] K. Kubota, Y. Kobayashi, K. Fujimoto, J. Appl. Phys. 66 (1989) 2984. [10] W.J. Meng, T.A. Perry, J. Appl. Phys. 76 (1994) 7824. [11] Q.X. Guo, A. Okada, H. Kidera, T. Tanaka, M. Nishio, H. Ogawa, J. Cryst. Growth 237–239 (2002) 1079. [12] Y. Daigo, N. Mutsukura, Thin Solid Films 483 (2005) 38. [13] M. Park, J.-P. Maria, J.J. Cuomo, Y.C. Chang, J.F. Muth, R.M. Kolbas, R.J. Nemanich, E. Carlson, J. Bumgarner, Appl. Phys. Lett. 81 (2002) 1797. [14] T. Nagata, Y.-Z. Yoo, P. Ahmet, T. Chikyow, Jpn. J. Appl. Phys. 44 (2005) 7896. [15] L.T. Romano, T.H. Myers, Appl. Phys. Lett. 71 (1997) 3486. [16] B. Daudin, J.L. Rouviere, M. Arley, Appl. Phys. Lett. 69 (1996) 2480. [17] K. Jegarathan, X.-O. Shen, I. Ide, M. Shimizu, H. Okumura, Jpn. J. Appl. Phys. 41 (2002) 4454.