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Structural properties of AlxGa1−xN grown on sapphire by molecular beam epitaxy
Journal of Crystal Growth 208 (2000) 37}41
Structural properties of Al Ga N grown on sapphire by x 1~x molecular beam epitaxy Je Won Kim!,*, Chang-Sik Son!, In-Hoon Choi!, Young K. Park", Yong Tae Kim", O. Ambacher#, M. Stutzmann# !Department of Materials Science, Korea University, Seoul 132-701, South Korea "Semiconductor Materials Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea #Walter Schottky Institut, Technische Universita( t Mu( nchen, D-85748 Garching, Germany Received 10 August 1998; accepted 30 August 1999 Communicated by H. Ohno
Abstract Structural properties of Al Ga N epilayers grown on (0 0 0 1) sapphire substrate by molecular beam epitaxy are x 1~x investigated in the range of AlN molar fraction from 0.16 to 0.76. The AlN molar fraction estimated by X-ray di!raction agrees well with that of Rutherford backscattering spectroscopy, showing a good linear relationship. The uniform Auger electron spectroscopy depth pro"le and linear dependence of average atomic concentration of all the constituents of Al Ga N epilayers on AlN molar fraction imply that the growth of Al Ga N epilayers with variation of AlN molar x 1~x x 1~x fraction is well controlled without the compositional #uctuation in depth of the epilayer. It is observed by atomic force microscopy that the surface grain shape of Al Ga N epilayer changes from roundish to a coalesced one with x 1~x increasing AlN molar fraction. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 68.55.Jk; 68.55.!a Keywords: Al Ga N; Rutherford backscattering spectroscopy; Molecular beam epitaxy; X-ray di!raction; Auger electron microx 1~x scopy; Atomic force microscopy
1. Introduction The recent rapid progress in light-emitting diodes and laser diodes operating in the blue and ultraviolet (UV) spectral regions based on
* Corresponding author. Semiconductor Materials Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea. Tel.: #82-2-9585706; fax: #82-2-958-5739. E-mail address: [email protected] (Je Won Kim)
In Ga N and Al Ga N alloy system, has led x 1~x x 1~x to extensive studies of their properties [1]. The AlGaN alloy has a direct band gap from 3.43 to 6.2 eV. Because the wavelength of radiation from alloys and heterostructures of Al Ga N can be x 1~x tuned over a wide spectral range from the visible to the ultraviolet, it is a promising material for device applications in the visible and UV range. AlGaN alloy is also a very attractive material to use in UV photodetectors, high electron mobility transistors, and "eld-e!ect transistors, based on AlGaN/GaN heterostructures, holding promise for high-power
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 4 8 3 - 2
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Je Won Kim et al. / Journal of Crystal Growth 208 (2000) 37}41
and high-temperature electronics device applications [2}6] due to relatively high mobility, sharp cuto! wavelength, and high quantum e$ciency [7}9]. In addition, due to a small lattice mismatch between AlN and GaN, these ternary materials are suitable for developing heterostructures to improve their device performance. In order to achieve the band gap engineering in the heterostructures of Al Ga N, it is essential to investigate the dex 1~x pendence of structural properties of Al Ga N on x 1~x variation in AlN molar fraction (x) and achieve a better control of AlN composition. Even if much e!ort has been concentrated on the growth of Al Ga N epilayers on sapphire, there are little x 1~x detailed studies on structural properties of Al Ga N epilayers in the wide range of AlN x 1~x molar fraction [10}12]. In this work, the structural properties of Al Ga N epilayers grown on (0 0 0 1) sapphire x 1~x by molecular beam epitaxy (MBE) with variation of AlN molar fraction is characterized by various structural characterization methods. The AlN molar fractions of Al Ga N alloys are measured x 1~x by Rutherford backscattering spectroscopy (RBS). Depth uniformity of constituents and crystallinity of alloys are characterized using Auger electron spectroscopy (AES) and X-ray di!raction (XRD). Atomic force microscopy (AFM) is used to observe the change of the surface morphology with AlN molar fractions.
2. Experimental procedure Al Ga N epilayers were grown on (0 0 0 1) x 1~x c-plane sapphire substrates without bu!er layer by plasma-induced MBE system [13}15]. Akasaki et al. [16] reported that the crystalline quality of AlGaN alloys with AlN bu!er layer was better than that of AlGaN alloys without AlN bu!er layer. We also intend to show the change of the structural properties of Al Ga N epilayers. In this paper, x 1~x however, we mainly studied Al and Ga #uctuations in the AlGaN alloys and the surface morphological change with AlN molar fractions. The compositional variation of constituents and structural change of AlGaN alloys without AlN bu!er layers were principally investigated in this work.
For the nitrogen radicals, radio frequency (RF)plasma source was used and gallium and aluminum sources were conventional Al and Ga e!usion cells, respectively. The RF plasma atomic radical source (Oxford Applied Research CARS-25) contains more nitrogen radicals than the ionic species and has the advantage of a low ion-induced damage during growth. The inductively coupled plasma power was 400 W and the background pressure was 1]10~10 mbar. The pressure in the MBE chamber during growth was 4]10~5 mbar. The Ga and Al #uxes maintained a total #ux of 8]1014 cm~2 s~1, and the growth rate was about 0.6 lm h~1. The growth temperature was 8103C for Al Ga N epilayers. x 1~x AlN molar fraction of Al Ga N epilayers was x 1~x varied from 0.16 to 0.76, which was deduced from RBS. Rutherford backscattering spectroscopy is a quantitative method for the determination of elemental composition and thickness of epilayer. The thickness of all the Al Ga N epilayers dex 1~x termined by scanning electron microscopy and RBS was about 1 lm. The Al Ga N epilayers x 1~x were imaged with contact mode AFM (SFM-BD2, Park Scienti"c Instruments) under ambient conditions. The optical lever de#ection mechanism was used in this AFM. Auger electron spectroscopy (Physical Electronics PHI 680) depth pro"ling was carried out to characterize uniformity of constituents of Al Ga N epilayer, and variation of Al x 1~x and Ga as a function of AlN molar fraction. X-ray di!raction was also performed in order to investigate the structural properties of Al Ga N epix 1~x layers.
3. Results and discussion The lattice constant of AlGaN alloys is measured by XRD from (0 0 0 2) and (0 0 0 4) re#ections in the symmetric h/2h mode, and is used to calculate the AlN molar fraction by Vegard's law [17]. The results are shown in Fig. 1 as a function of the AlN molar fraction determined by RBS. For both cases of (0 0 0 2) and (0 0 0 4) re#ection, the AlN molar fraction of XRD agrees well with that of RBS, showing a good linear relationship. Fig. 2 shows AES depth pro"les of Al Ga N as a function 0.29 0.71
Je Won Kim et al. / Journal of Crystal Growth 208 (2000) 37}41
Fig. 1. AlN molar fractions of Al Ga N epilayers determined x 1~x by RBS and XRD.
Fig. 2. AES depth pro"le of Al Ga N as a function of 0.29 0.71 sputter time.
of sputter time. Solid, dashed, and dotted lines represent the peak of N, Ga, and Al, respectively. The atomic concentration of nitrogen remains almost constant through the sputter time. The atomic concentrations of all the constituents of the epilayer show a linear dependence on the sputter time, which explains that all the constituents of epilayer have good uniformity throughout the sputter time. The concentration of carbon and oxygen as impurities in the epilayer is negligible in the given sputter
39
time range. All the Al Ga N samples studied in x 1~x this work as well as the Al Ga N show 0.29 0.71 a good AES depth pro"le uniformity of all the constituents throughout the sputter time. The uniform depth pro"le implies that there is no compositional #uctuation of constituents in Al Ga N x 1~x epilayer. Inspection of all the Al Ga N samples by x 1~x optical microscope reveals specular surface free of cracks. Fig. 3 shows the AFM image of surface morphology of Al Ga N epilayers as a function x 1~x of AlN molar fraction. The resolution of the AFM is below 10 nm. The "eld of AFM image is 1 lm]1 lm. AFM images show the densely distributed grains on the surface of Al Ga N x 1~x epilayer. With increasing AlN molar fraction up to 0.29, the grain shape changes from roundish to coalesced and the grain size of AlGaN becomes larger. As can be seen in Fig. 3, with increasing AlN molar fraction from 0.29 to 0.76, the grain shape changes to a more faceted and rougher one. The full-width at half-maximum (FWHM) of (0 0 0 4) rocking curve observed in the XRD increases from 12 to 26 min with increasing AlN molar fraction from 0.16 to 0.76. The increase in the XRD rocking curve FWHM is evidence for deterioration, particularly in the structural properties of the AlGaN alloys [18]. The surface morphology of epilayers shows homogeneous grains at low AlN molar fraction. Whereas, with increasing AlN molar fraction, the homogeneity of grains decreases. The variation of surface roughness can be analyzed by the root mean square (RMS) and average roughness. In Fig. 4, the RMS and average roughness of the surface of Al Ga N epilayers obtained from x 1~x AFM images are shown as a function of AlN molar fraction. The RMS roughness is the standard deviation of the data from the best-"t line and the average roughness is the arithmetic mean deviation from the best-"t line. As shown in Fig. 4, the RMS and average roughness of the Al Ga N epilayers x 1~x increase with increment of AlN molar fraction up to 0.6. On the other hand, the RMS and average roughness of the Al Ga N are smaller than 0.76 0.24 those of Al Ga N. The trend may be caused 0.6 0.4 from the grain shape of the Al Ga N epilayers. x 1~x As shown in Fig. 3, the grain shape of the Al Ga N epilayer changes from homogeneous x 1~x
40
Je Won Kim et al. / Journal of Crystal Growth 208 (2000) 37}41
Fig. 3. AFM images of surface morphology of Al Ga N as a function of AlN molar fraction: (a) Al Ga N, (b) Al Ga N, x 1~x 0.16 0.84 0.25 0.75 (c) Al Ga N, (d) Al Ga N, and (e) Al Ga N. 0.29 0.71 0.6 0.4 0.76 0.24
roundish to faceted grain as increasing AlN molar fraction up to 0.6 and then at Al Ga N the 0.76 0.24 grain shape becomes more homogeneous than that of Al Ga N. 0.6 0.4 From the observation of change in grain shape and the analysis of roughness shown in Figs. 3 and 4, there would be the transition point of the change of grain shape at the middle of AlN molar fraction. With increasing AlN molar fraction, the grain shape becomes larger and more coalesced, and also
the roughness increases. After the middle of AlN molar fraction, however, with increasing AlN molar fraction the grain shape becomes more faceted, while the roughness starts to decrease again. It may be attributed to the increase in the constituents due to the change of alloy constituents from binary-toternary alloy. The change of grain shape would be also related to the variation in the amount of mis"tstrain in the Al Ga N epilayers caused by lattice x 1~x mismatch between Al Ga N epilayer and sapx 1~x
Je Won Kim et al. / Journal of Crystal Growth 208 (2000) 37}41
41
This work was supported by the Ministry of Trade, Industry and Energy (Contract No. 2M08380 and 2M08790) and by the Korea Institute of Science and Technology (Contract No. 2E15940). J.W. Kim thanks the Korea Science and Engineering Foundation for "nancial support to visit Walter Schottky Institut, Technische UniversitaK t, MuK nchen.
References
Fig. 4. RMS and average roughness estimated by AFM images as a function of AlN molar fraction.
phire substrate. However, further transmission electron microscopy studies are necessary to clarify the change of grain shape. 4. Conclusions An investigation on the structural properties of Al Ga N epilayers in the range of AlN molar x 1~x fraction from 0.16 to 0.76 has been performed. With increasing AlN molar fraction, it is observed that the surface grain shape of the epilayers changes from roundish to coalesced. There exists a transition point of the change of grain shape at the middle of AlN molar fraction, which corresponds to a peak value of RMS roughness. Based on the comparison of the AlN molar fraction by RBS with that of XRD, the uniform AES depth pro"le, and the linear dependence of average atomic concentration of all the constituents of Al Ga N epilayers, x 1~x it can be concluded that the epitaxial growth of Al Ga N layers with variation of AlN molar x 1~x fraction is well controlled without a compositional #uctuation in depth of the epilayer. Acknowledgements The authors gratefully acknowledge technical assistance of C.K. Hyon for AFM characterization.
[1] S. Nakamura, M. Senoh, S.I. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, K. Chocho, Jpn. J. Appl. Phys. 36 (1997) L1568. [2] M.A. Khan, J.N. Kuznia, D.T. Olson, W.J. Scha!, J.W. Burm, M.S. Shur, Appl. Phys. Lett. 65 (1994) 1121. [3] D. Walker, X. Zhang, P. Kung, A. Saxler, J. Xu, M. Razeghi, Appl. Phys. Lett. 68 (1996) 2100. [4] S. Imanaga, H. Kawai, J. Appl. Phys. 82 (1997) 5843. [5] G.Y. Xu, A. Salvador, W. Kim, Z. Fan, C. Lu, H. Tang, H. Morkoc7 , G. Smith, M. Estes, B. Goldenberg, W. Yang, S. Krishnankutty, Appl. Phys. Lett. 71 (1997) 2154. [6] A. Osinsky, S. Gangopadhyay, B.W. Lim, M.Z. Anwar, M.A. Khan, D.V. Kuksenkov, H. Temkin, Appl. Phys. Lett. 72 (1998) 742. [7] M.A. Khan, A. Bhattarai, J.N. Kuznia, D.T. Olson, Appl. Phys. Lett. 63 (1993) 1214. [8] J.Z. Li, J.Y. Lin, H.X. Jiang, M.A. Khan, Q. Chen, J. Appl. Phys. 82 (1997) 1227. [9] V.E. Bougrov, A.S. Zubrilov, J. Appl. Phys. 81 (1997) 2952. [10] F.A. Ponce, J.S. Major Jr., W.E. Plano, D.F. Welch, Appl. Phys. Lett. 65 (1994) 2302. [11] Y. Ohba, H. Yoshida, R. Sato, Jpn. J. Appl. Phys. 36 (1997) L1565. [12] A.Y. Polyakov, M. Shin, J.A. Freitas, M. Skowronski, D.W. Greve, R.G. Wilson, J. Appl. Phys. 80 (1996) 6349. [13] J.W. Kim, C.S. Son, I.H. Choi, Y.K. Park, Y.T. Kim, J. Korean Vac. Soc. 8 (1999) 153. [14] J.W. Kim, C.S. Son, I.H. Choi, Y.K. Park, Y.T. Kim, Abstracts of the Ninth International Symposium on the Physics of Semiconductors and Applications, Seoul, Korea, 1998, p. 95. J. Korean Phys. Soc. 34 (1999) S378. [15] J.W. Kim, C.S. Son, I.H. Choi, Y.K. Park, Y.T. Kim, O. Ambacher, M. Stutzmann, Abstracts of the Fourth International Conference on Electronic Materials, Cheju, Korea, 1998, p. 94. J. Korean Phys. Soc. 35 (1999) S279. [16] I. Akasaki, H. Amano, Mater. Res. Soc. Symp. Proc. 339 (1994) 443. [17] L. Vegard, Z. Phys. 5 (1921) 17. [18] H. Amano, I. Akasaki, K. Hiramatsu, N. Koide, N. Sawaki, Thin Solid Films 163 (1988) 415.
Structural properties of Al Ga N grown on sapphire by x 1~x molecular beam epitaxy Je Won Kim!,*, Chang-Sik Son!, In-Hoon Choi!, Young K. Park", Yong Tae Kim", O. Ambacher#, M. Stutzmann# !Department of Materials Science, Korea University, Seoul 132-701, South Korea "Semiconductor Materials Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea #Walter Schottky Institut, Technische Universita( t Mu( nchen, D-85748 Garching, Germany Received 10 August 1998; accepted 30 August 1999 Communicated by H. Ohno
Abstract Structural properties of Al Ga N epilayers grown on (0 0 0 1) sapphire substrate by molecular beam epitaxy are x 1~x investigated in the range of AlN molar fraction from 0.16 to 0.76. The AlN molar fraction estimated by X-ray di!raction agrees well with that of Rutherford backscattering spectroscopy, showing a good linear relationship. The uniform Auger electron spectroscopy depth pro"le and linear dependence of average atomic concentration of all the constituents of Al Ga N epilayers on AlN molar fraction imply that the growth of Al Ga N epilayers with variation of AlN molar x 1~x x 1~x fraction is well controlled without the compositional #uctuation in depth of the epilayer. It is observed by atomic force microscopy that the surface grain shape of Al Ga N epilayer changes from roundish to a coalesced one with x 1~x increasing AlN molar fraction. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 68.55.Jk; 68.55.!a Keywords: Al Ga N; Rutherford backscattering spectroscopy; Molecular beam epitaxy; X-ray di!raction; Auger electron microx 1~x scopy; Atomic force microscopy
1. Introduction The recent rapid progress in light-emitting diodes and laser diodes operating in the blue and ultraviolet (UV) spectral regions based on
* Corresponding author. Semiconductor Materials Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea. Tel.: #82-2-9585706; fax: #82-2-958-5739. E-mail address: [email protected] (Je Won Kim)
In Ga N and Al Ga N alloy system, has led x 1~x x 1~x to extensive studies of their properties [1]. The AlGaN alloy has a direct band gap from 3.43 to 6.2 eV. Because the wavelength of radiation from alloys and heterostructures of Al Ga N can be x 1~x tuned over a wide spectral range from the visible to the ultraviolet, it is a promising material for device applications in the visible and UV range. AlGaN alloy is also a very attractive material to use in UV photodetectors, high electron mobility transistors, and "eld-e!ect transistors, based on AlGaN/GaN heterostructures, holding promise for high-power
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 4 8 3 - 2
38
Je Won Kim et al. / Journal of Crystal Growth 208 (2000) 37}41
and high-temperature electronics device applications [2}6] due to relatively high mobility, sharp cuto! wavelength, and high quantum e$ciency [7}9]. In addition, due to a small lattice mismatch between AlN and GaN, these ternary materials are suitable for developing heterostructures to improve their device performance. In order to achieve the band gap engineering in the heterostructures of Al Ga N, it is essential to investigate the dex 1~x pendence of structural properties of Al Ga N on x 1~x variation in AlN molar fraction (x) and achieve a better control of AlN composition. Even if much e!ort has been concentrated on the growth of Al Ga N epilayers on sapphire, there are little x 1~x detailed studies on structural properties of Al Ga N epilayers in the wide range of AlN x 1~x molar fraction [10}12]. In this work, the structural properties of Al Ga N epilayers grown on (0 0 0 1) sapphire x 1~x by molecular beam epitaxy (MBE) with variation of AlN molar fraction is characterized by various structural characterization methods. The AlN molar fractions of Al Ga N alloys are measured x 1~x by Rutherford backscattering spectroscopy (RBS). Depth uniformity of constituents and crystallinity of alloys are characterized using Auger electron spectroscopy (AES) and X-ray di!raction (XRD). Atomic force microscopy (AFM) is used to observe the change of the surface morphology with AlN molar fractions.
2. Experimental procedure Al Ga N epilayers were grown on (0 0 0 1) x 1~x c-plane sapphire substrates without bu!er layer by plasma-induced MBE system [13}15]. Akasaki et al. [16] reported that the crystalline quality of AlGaN alloys with AlN bu!er layer was better than that of AlGaN alloys without AlN bu!er layer. We also intend to show the change of the structural properties of Al Ga N epilayers. In this paper, x 1~x however, we mainly studied Al and Ga #uctuations in the AlGaN alloys and the surface morphological change with AlN molar fractions. The compositional variation of constituents and structural change of AlGaN alloys without AlN bu!er layers were principally investigated in this work.
For the nitrogen radicals, radio frequency (RF)plasma source was used and gallium and aluminum sources were conventional Al and Ga e!usion cells, respectively. The RF plasma atomic radical source (Oxford Applied Research CARS-25) contains more nitrogen radicals than the ionic species and has the advantage of a low ion-induced damage during growth. The inductively coupled plasma power was 400 W and the background pressure was 1]10~10 mbar. The pressure in the MBE chamber during growth was 4]10~5 mbar. The Ga and Al #uxes maintained a total #ux of 8]1014 cm~2 s~1, and the growth rate was about 0.6 lm h~1. The growth temperature was 8103C for Al Ga N epilayers. x 1~x AlN molar fraction of Al Ga N epilayers was x 1~x varied from 0.16 to 0.76, which was deduced from RBS. Rutherford backscattering spectroscopy is a quantitative method for the determination of elemental composition and thickness of epilayer. The thickness of all the Al Ga N epilayers dex 1~x termined by scanning electron microscopy and RBS was about 1 lm. The Al Ga N epilayers x 1~x were imaged with contact mode AFM (SFM-BD2, Park Scienti"c Instruments) under ambient conditions. The optical lever de#ection mechanism was used in this AFM. Auger electron spectroscopy (Physical Electronics PHI 680) depth pro"ling was carried out to characterize uniformity of constituents of Al Ga N epilayer, and variation of Al x 1~x and Ga as a function of AlN molar fraction. X-ray di!raction was also performed in order to investigate the structural properties of Al Ga N epix 1~x layers.
3. Results and discussion The lattice constant of AlGaN alloys is measured by XRD from (0 0 0 2) and (0 0 0 4) re#ections in the symmetric h/2h mode, and is used to calculate the AlN molar fraction by Vegard's law [17]. The results are shown in Fig. 1 as a function of the AlN molar fraction determined by RBS. For both cases of (0 0 0 2) and (0 0 0 4) re#ection, the AlN molar fraction of XRD agrees well with that of RBS, showing a good linear relationship. Fig. 2 shows AES depth pro"les of Al Ga N as a function 0.29 0.71
Je Won Kim et al. / Journal of Crystal Growth 208 (2000) 37}41
Fig. 1. AlN molar fractions of Al Ga N epilayers determined x 1~x by RBS and XRD.
Fig. 2. AES depth pro"le of Al Ga N as a function of 0.29 0.71 sputter time.
of sputter time. Solid, dashed, and dotted lines represent the peak of N, Ga, and Al, respectively. The atomic concentration of nitrogen remains almost constant through the sputter time. The atomic concentrations of all the constituents of the epilayer show a linear dependence on the sputter time, which explains that all the constituents of epilayer have good uniformity throughout the sputter time. The concentration of carbon and oxygen as impurities in the epilayer is negligible in the given sputter
39
time range. All the Al Ga N samples studied in x 1~x this work as well as the Al Ga N show 0.29 0.71 a good AES depth pro"le uniformity of all the constituents throughout the sputter time. The uniform depth pro"le implies that there is no compositional #uctuation of constituents in Al Ga N x 1~x epilayer. Inspection of all the Al Ga N samples by x 1~x optical microscope reveals specular surface free of cracks. Fig. 3 shows the AFM image of surface morphology of Al Ga N epilayers as a function x 1~x of AlN molar fraction. The resolution of the AFM is below 10 nm. The "eld of AFM image is 1 lm]1 lm. AFM images show the densely distributed grains on the surface of Al Ga N x 1~x epilayer. With increasing AlN molar fraction up to 0.29, the grain shape changes from roundish to coalesced and the grain size of AlGaN becomes larger. As can be seen in Fig. 3, with increasing AlN molar fraction from 0.29 to 0.76, the grain shape changes to a more faceted and rougher one. The full-width at half-maximum (FWHM) of (0 0 0 4) rocking curve observed in the XRD increases from 12 to 26 min with increasing AlN molar fraction from 0.16 to 0.76. The increase in the XRD rocking curve FWHM is evidence for deterioration, particularly in the structural properties of the AlGaN alloys [18]. The surface morphology of epilayers shows homogeneous grains at low AlN molar fraction. Whereas, with increasing AlN molar fraction, the homogeneity of grains decreases. The variation of surface roughness can be analyzed by the root mean square (RMS) and average roughness. In Fig. 4, the RMS and average roughness of the surface of Al Ga N epilayers obtained from x 1~x AFM images are shown as a function of AlN molar fraction. The RMS roughness is the standard deviation of the data from the best-"t line and the average roughness is the arithmetic mean deviation from the best-"t line. As shown in Fig. 4, the RMS and average roughness of the Al Ga N epilayers x 1~x increase with increment of AlN molar fraction up to 0.6. On the other hand, the RMS and average roughness of the Al Ga N are smaller than 0.76 0.24 those of Al Ga N. The trend may be caused 0.6 0.4 from the grain shape of the Al Ga N epilayers. x 1~x As shown in Fig. 3, the grain shape of the Al Ga N epilayer changes from homogeneous x 1~x
40
Je Won Kim et al. / Journal of Crystal Growth 208 (2000) 37}41
Fig. 3. AFM images of surface morphology of Al Ga N as a function of AlN molar fraction: (a) Al Ga N, (b) Al Ga N, x 1~x 0.16 0.84 0.25 0.75 (c) Al Ga N, (d) Al Ga N, and (e) Al Ga N. 0.29 0.71 0.6 0.4 0.76 0.24
roundish to faceted grain as increasing AlN molar fraction up to 0.6 and then at Al Ga N the 0.76 0.24 grain shape becomes more homogeneous than that of Al Ga N. 0.6 0.4 From the observation of change in grain shape and the analysis of roughness shown in Figs. 3 and 4, there would be the transition point of the change of grain shape at the middle of AlN molar fraction. With increasing AlN molar fraction, the grain shape becomes larger and more coalesced, and also
the roughness increases. After the middle of AlN molar fraction, however, with increasing AlN molar fraction the grain shape becomes more faceted, while the roughness starts to decrease again. It may be attributed to the increase in the constituents due to the change of alloy constituents from binary-toternary alloy. The change of grain shape would be also related to the variation in the amount of mis"tstrain in the Al Ga N epilayers caused by lattice x 1~x mismatch between Al Ga N epilayer and sapx 1~x
Je Won Kim et al. / Journal of Crystal Growth 208 (2000) 37}41
41
This work was supported by the Ministry of Trade, Industry and Energy (Contract No. 2M08380 and 2M08790) and by the Korea Institute of Science and Technology (Contract No. 2E15940). J.W. Kim thanks the Korea Science and Engineering Foundation for "nancial support to visit Walter Schottky Institut, Technische UniversitaK t, MuK nchen.
References
Fig. 4. RMS and average roughness estimated by AFM images as a function of AlN molar fraction.
phire substrate. However, further transmission electron microscopy studies are necessary to clarify the change of grain shape. 4. Conclusions An investigation on the structural properties of Al Ga N epilayers in the range of AlN molar x 1~x fraction from 0.16 to 0.76 has been performed. With increasing AlN molar fraction, it is observed that the surface grain shape of the epilayers changes from roundish to coalesced. There exists a transition point of the change of grain shape at the middle of AlN molar fraction, which corresponds to a peak value of RMS roughness. Based on the comparison of the AlN molar fraction by RBS with that of XRD, the uniform AES depth pro"le, and the linear dependence of average atomic concentration of all the constituents of Al Ga N epilayers, x 1~x it can be concluded that the epitaxial growth of Al Ga N layers with variation of AlN molar x 1~x fraction is well controlled without a compositional #uctuation in depth of the epilayer. Acknowledgements The authors gratefully acknowledge technical assistance of C.K. Hyon for AFM characterization.
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