sapphire substrates by metalorganic vapor phase epitaxy

CRYSTAL GROWTH ELSEVIER

Journal of Crystal Growth 144 (1994) 133—140

Selective growth of wurtzite GaN and Al~Ga1_~N on GaN/sapphire substrates by metalorganic vapor phase epitaxy Yoshiki Kato, Shota Kitamura, Kazumasa Hiramatsu ~, Nobuhiko Sawaki Department of Electronics, Nagoya Unitersity, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Received 7 March 1994; manuscript received in final form 27 May 1994

Abstract The selective growth of GaN and A1~Ga1~N (x 0.1) by metalorganic vapor phase epitaxy has been carried out on GaN/sapphire substrates using Si02 masks. It is found that the selectivity of GaN is good, but that of Al~Ga1_~N is relatively poor. Ridge growth occurs in the GaN selective growth on linear windows wider than 50 ~ but does not in the Al~Ga1_~Nselective growth. (1101) facets appear in GaN on the 10 j.tm wide linear window in the K1120~direction. On the other hand, a trapezoidal structure with (1101) facets on the side and a (0001) facet on the top appears in the AI~Ga1 ~N growth. Formation mechanisms of these facets in GaN and Al~Ga1_~Nare discussed. =

1. Introduction Selective growth is one of the most important techniques used in the fabrication of field effect transistors (FETs) [1,2] and semiconductor micro-structures such as quantum wires and dots [3—51. Selective growth is also employed as a useful method to understand the growth mechanism [6—81.Therefore, great efforts have been made to realize the selective epitaxy of ordinary Ill—V compound semiconductors such as GaAs, A1GaAs, InP, InGaAs, etc., which have the zincblende structure. Ill—V nitrides like GaN and Al~Ga1_~Nusu-

*

Corresponding author.

ally have the wurtzite structure and are promising wide bandgap materials for photonic device applications such as light emitting devices (LEDs) and laser diodes (LDs) in blue and ultraviolet (UV) regions, since they have direct band gap energies of 3.39 to 6.1 eV. The selective growth of wide bandgap semiconductors is one of the most important methods to realize high performance LEDs and LDs in the short wavelength region as well as to understand the growth mechanism of these materials. Recently, the selective growth of cubic GaN has been successfully carned out on Si02-patterned GaAs substrates using metalorganic vapor phase epitaxy (MOVPE) [9]. However, the mechanisms of the deposition selectivity of the wurtzite 111—V nitrides and the nucleation on the masks have not been clarified. Furthermore, the facet structure of the wurtzite

0022-0248/94/$07.00 © 1994 Elsevier Science By. All rights reserved SSDI 0022-0248(94)00580-X

34

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al. /Journal of (‘ri ‘,tal (,roo th 144 (1994) 131—140

Ill—V nitrides which is formed during the selective growth has not been observed previously, to

In this paper, we investigate the growth selectivity of GaN and Al~Ga1~N(x 0.1) using SiO-, masked GaN (0001) surfaces on sapphire =

the best of our knowledge.

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Fig. 1. SEM images of GaN (left side) and Al ~Ga N (right side) surfaces grown selectivcl~tor 10 mm on different window widths ol (a), (d) 10 ~sm, (b). (e) So jim and (c) (f) 100 jim extending in the K I 120~direction ot the GaN substrate. Markers represent’ (Cm) tO jim. (h) 20 jim; (c) 20 jim’ (d) 5 jim; (e) 20 jim. (f) SO jim.

Y Kato et at /Journal of Crystal Growth 144 (1994) 133—140

substrates by MOVPE. The mechanisms of the

13.5

facet formation during the selective epitaxial growth are discussed.

Al~Ga1 ~N was 1070°C. Under these growth conditions, the Ill/V ratios were 770 and 740 for GaN and Al~Ga1_~N,respectively.

2. Experimental procedure

3. Results and discussion

The selective growth of GaN and Al~Ga1_~N (x 0.1) was performed on 2 ~m thick layers of GaN (0001). This GaN layer was grown on sapphire (a-A1203(1120)) substrates using an A1N buffer layer by MOVPE [10,11]. 5i02 masks were used for the selective growth of GaN and Al~Ga11N. The 100 nm thick Si02 masks were deposited by RF sputtering. Patterning of the masks was carried out by conventional photolithography to form linear windows with lengths extending in the (1100) or (1120) directions of the GaN substrate with widths of 10, 50 and 100 p.m. An MOVPE reactor operated at atmospheric pressure was used in the study of the selective growth of GaN and Al5Ga1 ~N [11,121. The source gases were trimethylgallium (TMG; 50 SCCM at 15°C),trimethylaluminum (TMA; 10 SCCM at 15°C),and ammonia (NH3 1.5 SLM), and the growth temperature of GaN and

3.1. Selective growth of GaN

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—

GaN 10mm

Figs. la, lb and ic show scanning electron microscopy (SEM) images of GaN grown for 10 mm on linear windows of 10, 50 and 100 p.m widths. The Talystep height profiles in Fig. 2a reveal the cross sections of the samples shown in Figs. la—ic. GaN islands are not observed on the 5i02 mask, so the selectivity of GaN growth on the window region is excellent. Ridge growth is observed on the windows of 50 and 100 p.m widths. The ridge growth is due to the lateral vapor phase diffusion of a source gas which flows from the mask region to the window region because of the source gas concentration gradient. A facet structure is formed along (1120> on the 10 p.m wide window. Fig. 3a shows a SEM image in the ridge region of GaN grown for 30 mm on a 400 p.m wide window. Ridge growth is observed

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Fig. 2. Talystep height profiles of cross sections of (a) GaN and (b) Al~Ga1.,,~Ngrown selectively for 10 mm on different window widths: 10, 50 and tOO jim. The origin of the position x indicates the center of the window. The x-axis is arranged in the [11001 direction.

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K 11 OO~direction.

in the range of approximately 50 p.m from the window edge. Therefore, the range in which the ridge structure occurs due to the lateral vapor phase diffusion is estimated to be approximately 50 p.m from the window edge. In order to study the selective growth along different directions, GaN was grown for 10 mm on a 10 p.m wide window along the (1100> direction. Fig. 3b shows a SEM image of GaN grown on the window along (1100). Although we observed mixed facets along the (1100) direction, we could not obtain uniform facets as seen on the window along the (1120) direction. Fig. 4a shows

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a SEM image of GaN grown for 10 mm on the 10 tsm wide window along the (1120) direction. The facets on the sides are inclined from the (0001) interface at an angle of 620, which indicates that the facet on the side is the {llOl} surface. The number of bonds which are cut with this surface is so small that the (1101) surface is quite stable. 3.2. Mechanisms of the facet ftrmation of GaN In order to clarify the mechanisms of the facet formation of GaN, the selective growth was carned out on the 10 p.m wide windows for different

Fig. 5. SEM images of surfaces of GaN (left side) and Al ~Ga — rN (right side) grown on 10 jim wide windows in the K It 2t)) direction for different growth times. GaN: (a), (b). (c) 2 mm and (d) 5 mm, and Al ~Ga N: (e) 2 mm. (f) 5 mm. (g) 10 mm and (h) 20 mm. Markers represent S jim.

Y Kate ci al. /Journal of Crystal Growth 144 (1994) 133 140

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Y Kate et al. /Journal of (‘rvsta/ Growth 144 (1994) 133—140

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growth times. Figs. 5a—5d show SEM images of GaN for different growth times. Figs. 5a—5c show the samples grown for the same growth time of 2 mm, but different morphologies are observed because the supply of the source gases over the substrate surface is nonuniform. Therefore, it is thought that the morphology of GaN changes as seen in Figs. Sa—5c with increasing growth time. In Fig. 5a, the ridge growth is initially observed at the edge of the window. The ridge has sharp (1101) facets about 2 p.m in width. After that, the island growth occurs in the middle region between the ridges as seen in Fig. Sb. During the next stage, the (0001) surface including many pits appears on the_top, while the side surface is composed of (1101) facets, as seen in Fig. Sc. After 5 mm growth, the triangular structure with (1101) facets is formed, as seen in Fig. Sd. The process of the facet formation in GaN is illustrated schematically in Fig. 6. (a) At first the 2 p.m wide ridges are formed at the edges of the window, indicating that the nucleation occurs preferentially at the edges of the window. At the initial growth stage, the source gas concentration

becomes high at the ridge region because of the lateral vapor diffusion and/or the surface diffusion into the ridge region. Furthermore, the SiO~ mask side on the window may serve as the nudeation centers of GaN because the nucleation is easy to occur in the reentrant angles of steps, near and around impurity aggregates, where the free energy of the nucleus is lower than that of a nucleus on the flat surface [13]. Therefore, it is thought that GaN nucleates preferentially at the edge of the window. (b) After the ridge growth, islands of GaN are generated between the two ridges on the window although GaN is grown on the GaN layer homoepitaxially. This nonuniform growth implies that the surface of the GaN substrate is changed before the regrowth. (c) Coalescence occurs among the islands, forming a rough structure with many pits on the top. Then, the rough surface on the top grows at a high growth rate of about 2.0 p.m/mm in the [00011direction, but the (1101) facets on the side do not grow significantly. The pits on the top are composed of (1101) facets. The growth rate of these facets arc thought to be generally slow because they have stable surfaces. However, because the bottom of the pit plays an important role as a nucleation center [13], the growth rate in the pits becomes high. Thus, it is thought that the top surface including the pits grows at a high rate. (d) Finally, GaN has a simple structure composed of two (1101) facets. (e) The facets also grow on the Si02 surface in the lateral direction at a slow rate while maintaining their shape. 3.3. Selective growth of AI~Ga1 ~N Figs. Id, le and if show SEM images of Al0 1Ga~9Ngrown for 10 mm on the linear windows of 10, 50 and 100 p.m widths. Fig. 2b shows the Talystep height profiles of the cross sections measured in those samples. Polycrystals of Al~Ga11Nare observed on the Si02 mask, unlike the case of the selective growth of GaN. although the thickness of the deposited material is small. Therefore, the selectivity of Al~Ga1..,,N growth is not so good as that of GaN growth. This is because the chemical reaction between the Al species and the Si00 mask occurs more easily

Y Kato ci a!. /Journal of Crystal Growth 144 (1994) 133—140

than the reaction of Ga species with the Si02 mask [61. Furthermore, ridge growth is not observed on the wide windows of 50 and 100 p.m widths. Macrosteps appear on the (0001) surface of Al~Ga~.~N, as shown in Figs. le and if. The growth rate of Al~Ga1.~N is one order of magnitude slower than that of GaN. The absence of the ridge growth and the slow growth rate are attributed to the poor selectivity of Al~Ga1~N. The lateral vapor diffusion from the mask region to the window region is reduced because the source gases are also consumed on the Si02 mask; hence, ridge growth does not occur and the growth rate of A1~Ga1_~Non the window is small. Fig. 4b shows a SEM image of the cross section of Al~Ga1..~N grown for 10 mm on the 10 p.m wide window. The typical morphology of the deposited A1~Ga1~N is a trapezoidal structure with two {1101} facets on both sides and a (0001) facet on the top plane. —

.

3.4. Mechanisms of the facet formation of Al~Ga1,,~N In order to clarify the mechanisms of the facet formation of Al~Ga1..~N, the growth of Al~Ga1.,~Nwas carried out on the 10 p.m wide windows in the (1120) direction for different growth times. Figs. 5e—5h show SEM images of A1~Ga1~N for growth times from 2 to 20 mm. For the growth time of 2 mm, a trapezoidal structure with (0001) and (1101) facets appears in contrast with the GaN growth. As the growth time increases, the area of the (0001) facet decreases while polycrystals of Al~Ga1 ~N are deposited on the mask. The process of the facet formation in Al~Ga1.~Nis illustrated schematically in Fig. 7. (a) Since the (0001) facet appears during the early growth stage, Al~Gai ~N nucleates more easily than GaN on the window. This suggests that Al species are adsorbed more easily on the GaN substrate surface than Ga species, even though the GaN substrate surface is changed before the regrowth. Furthermore, Al~Ga1,,~Nis also easily deposited on the Si02 mask, and hence the selec-

139

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Fig. 7. Schematic drawing indicating the facet formation process of Al Ga1 N.

tivity of Al~Ga1~Nbecomes poor. (b) The growth rate of the (0001) facet is approximately 0.2 p.m/mm and is faster than that of the {ilOl} facet; hence the growth of the (0001) facet is dominant. During the initial growth stage, since the quantity of polycrystals deposited on the mask is small, lateral vapor diffusion occurs and the growth on the window is dominant. (c) However, as the mask is covered with polycrystals so as to decrease the lateral vapor diffusion of the source gas, the growth rate of the (0001) facet decreases. (d) Finally, the (0001) and (llOi} facets grow slowly while maintaining their shape.

4. Conclusions The selective growth of GaN and Al~Ga1~N (x = 0.1) by MOVPE was carried out on GaN/sapphire substrates using Si02 masks. Selectivity of GaN was good, but that of Al 1Ga1 N was poor. Ridge growth occurred in the GaN growth on linear windows wider than 50 p.m, but did not in the Al~Ga1 ~N growth. (1101) facets appeared in GaN growth on the 10 p.m wide linear window in the (1120) direction, and {llOi}

140

Y Kate et al. /Journa/ of Crystal Growth 144 (1994) 133—140

facets on the side and a (0001) facet on the top appeared in the Al1Ga1~Ngrowth.

References

In the case of GaN, nucleation occurs initially at the window edge and then in the center of the window. The growth rate in the [0001] direction is very fast because of preferential growth due to pits on the top surface, and finally, a triangular structure covered with the (1101) facets is formed. In the case of Al~Ga1~N,nucleation occurs quite uniformly on the window, and the (0001) facet appears on the top during the first stage of growth. Since the selectivity of Al ~Ga1 5N is poor and the source gas concentration gradient is reduced, the growth rate of the facets decreases with increasing growth time, resulting in a trapezoidal structure with (1101) and (0001) facets.

[I] K. Imamura, N. Yokoyama, T. Ohnishi. S. Suzuki, K Nakai, H. Nishi and A. Shihatomi, Jap. J. AppI. Phys. 23 (1984) L342. [2] H. Asam, S. Adachi, S. Ando and K. Oe, J. AppI. Phys. 55 (1984) 3868. [3] H. Asai, S. Yamada mind T. Fukui. AppI. Phys. Lett. St (1987) 1518. [41T. Fukui. S. Ando mind Y. Fukai, AppI. Phys. Lett. 57 (1990) 1209. [5] T. Fukum and S. Ando. Electron. Lett. 25 (1989) 410. [6] K. Hiruma, T. Hagaand M. Miyazaki. J. Crystal Growth 102 (1990) 717. [7] K. Yamaguchi, M. Ogasmiwara mind K. Okamoto .1 AppI. Phys. 72 (1992) 5919. [8] K. Yamaguchi mind K. Okamoto, Jap. J. AppI. Phys. 32 (1993) 1523. [9] M. Nagahara, S. Miyoshi, H. Yaguchi, K. Onabe. Y. Shiraki and R. Ito. Jap. J. Appl. Phys. 33 (1994) 694. [101 1-1. Amano. N. Sawaki, I. Akasaki and Y. Toyodmm. AppI. Phys. Lett. 48 (1986) 353. [II] 1. Akasaki. H. Amano, Y. Koide, K. Hiramatsu mind N.

Acknowledgements The authors would like to express their thanks to Dr. P. Hacke of Nagoya University and to Professor I. Akasaki of Meijo University for valuable discussions. They are also grateful to Mr. N. Koide of Toyoda Gosei Co. Ltd. for his help throughout these experiments.

Sawaki. J. Crystal Growth 98 (1989) 209. [12] K. Itoh. K. Hiramatsu, H. Amano and I. Akasmmki. J. Growth 104 (1990) 533. [13] Crystal A.A. Chernov. Modern Crystallography Ill’ (‘rvstmml Growth (Springer. Berlin, 1984) p. 79.