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Structural studies on MOCVD grown GaN and AlGaN using atomic force microscopy
Materials Chemistry and Physics 64 (2000) 260–264
Materials Science Communication
Structural studies on MOCVD grown GaN and AlGaN using atomic force microscopy R.S. Qhalid Fareed a,∗ , S. Juodkazis a , S.H. Chung a , T. Sugahara b , S. Sakai a,b a
b
Satellite Venture Business Laboratory, The University of Tokushima, 2-1 Minami josanjima, Tokushima 770, Japan Department of Electrical and Electronic Engineering, The University of Tokushima, 2-1 Minami josanjima, Tokushima 770, Japan Received 8 July 1999; received in revised form 13 September 1999; accepted 14 October 1999
Abstract Surface morphology studies of GaN and AlGaN grown by metalorganic chemical vapor deposition (MOCVD) have been carried out using atomic force microscopy. The open core dislocation and steps connecting two threading dislocations of opposite direction are commonly observed in undoped and doped GaN. Structural studies on AlGaN epitaxial layers grown on undoped GaN revealed the formation of open-core dislocation with width upto 300 nm. The nanopipes originate from the threading dislocation formed due to large lattice mismatch between sapphire and GaN. The mismatch also leads to high strain in the epilayers resulting in cracking effect at the edges of the hexagonal V-type defect. The self organized quantum dots features on the smooth surface of AlGaN epitaxial layer exhibit the Stranski–Krastanov(SK) mode of island growth. © 2000 Elsevier Science S.A. All rights reserved. Keywords: MOCVD; GaN and AlGaN; Atomic force microscopy
1. Introduction Gallium nitride (GaN) and related nitrides have attracted much interest as the most prospective materials for optoelectronic devices, and high power–high temperature device application. The progress of blue laser diodes and blue green light emitting diodes have been remarkable in recent years [1–3]. Due to the lack of GaN substrates, epitaxial films are grown using metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) on conventional substrates such as sapphire (Al2 O3 ) or silicon carbide (SiC). Due to the large mismatch and different thermal coefficient, high density of dislocations on the order of 108 –1010 cm−2 are generated [4,5]. The extended and point defects in GaN films have greatly impacted the performance of many devices. Recently, there have been many reports on the defect studies at different spatial scales. The density and distribution of defects in GaN and related nitride materials differ significantly depending on the growth conditions. Numerous studies have investigated the origin and theory of these defects [6,7] and their effects on the structural [8] and optical properties [9] of the epitaxial layers. The pure edge and mixed-character dislocations are two kinds of thread∗
Corresponding author.
ing dislocation observed in GaN normally. Threading edge ¯ are the most dislocations with burgers vector b = a/3h1120i common line defects found for films grown on sapphire (0001) and 6H-SiC (0001) [10]. The transmission electron microscopy (TEM) studies on the mechanism of formation of nanotubes (V defect) in GaN have also been investigated [11–13]. Atomic force microscopy (AFM) had recently emerged as a useful tool for characterizing the microstructure of the surface of the layers. Although, AFM studies on GaN have been carried out [9,14–16], surface studies on the p-type GaN and AlGaN epitaxial layers have not yet been reported. We note that the formation of self organized quantum dots on GaN has already been published [17] but there is no information on the growth of AlGaN self organized quantum dots. In this paper, we present the surface morphology studies on the step growth and open-core dislocation density in MOCVD grown undoped and p-type GaN. The Stranski–Krastanov (SK) growth mode of three-dimensional island formation in AlGaN epitaxial layers has also been observed by AFM and will be discussed in detail. 2. Experimental The undoped and doped GaN and AlGaN epitaxial layers were grown on sapphire (0001) substrate using MOCVD re-
0254-0584/00/$ – see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 2 3 4 - 5
R.S. Qhalid Fareed et al. / Materials Chemistry and Physics 64 (2000) 260–264
actor employed at atmospheric pressure. The substrate was first cleaned at 1150◦ C in H2 for 10 min. Tri-methyl gallium (TMGa) and ammonia (NH3 ) were used as the source gases. A thin GaN buffer layer (thickness ≈ 450 Å) was first deposited at 450◦ C. Undoped GaN epitaxial layer with thickness of 1.5 m was grown at 1075◦ C. Mg doped GaN was grown at 1050◦ C using bis-cyclopentadienyl magnesium (Cp2 Mg) as the Mg source. First, a 25 nm thin GaN buffer layer was grown at 450◦ C. Then the substrate temperature was increased to 1050◦ C to grow 1.5 m thick Mg-doped GaN films. During the deposition of Mg-doped GaN top layer, the Cp2 Mg flow rate was maintained at 0.3 mol min−1 . The flow rate of TMGa was maintained at 88 mol min−1 for both undoped and doped GaN growth. AlGaN epitaxial layer was grown with tri-methyl aluminium (TMAl) as the Al source over undoped GaN at 1075◦ C. All the samples were cleaned with methanol, acetone and deionised water before surface analysis. AFM studies were carried out using SEIKO Instrument SII (SPA 300) using SiN tips under normal pressure and room temperature to observe the morphology to understand the growth mechanism.
3. Results and discussion Fig. 1a shows the AFM image of an MOCVD grown undoped GaN. The dark regions are small pits that have been observed from our reactor as well as those from several other groups [9,15,16]. The surface is atomically smooth and flat
261
with RMS roughness about 3.055 Å. The lines on the surface represent the steps with the height in atomic scale and this step finally terminates at small pits. This indicates the association of these pits with dislocation terminating at the surface. It can be explained like this: when two screw dislocations of opposite sign emerge on a crystal surface, a ledge will join them. Growth will commence if the supersaturation is raised to such value that the diameter of the critical two-dimensional nucleus becomes less than the distance between the two dislocations [18,19]. The atoms absorbed on the surface will join the step and, hence, the step tends to spiral around both dislocations. Close examination of one of the pits, which is the termination point of steps, shows the open core dislocation (Fig. 1b). The internal surfaces of the open core dislocation are formed with six closed packed planes. AFM studies show the density of nanopipes is about 105 –107 cm−2 . The depth of the dislocation is more than 40 nm and width of the pipes differs from 50 to 200 nm in different places. Surface morphology of the 1.5 m thick as grown Mg-doped GaN is shown in Fig. 2. The p-type GaN (Mg concentration ∼1020 cm−3 ) is rougher than the undoped GaN and the rms roughness is 8.6 Å. Since it is very difficult to control the p-type doping, the surface morphology of the p-type GaN varies with growth conditions. However, the step formation observed in undoped GaN is also found in p-type GaN. There are also hillocks and craters besides the steps in the p-type GaN samples, which are not observed in undoped GaN. The termination of steps at the small pits is not clearly visible and the density of open core dislocation is much less. However, TEM measurement shows that the
Fig. 1. (a) AFM image of 1.5 m thick undoped GaN epitaxial layer. The scanning area is 4 m × 4 m and the rms roughness is 3.055 Å; (b) three-dimensional image of an open core dislocation in GaN with hexagonal feature.
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R.S. Qhalid Fareed et al. / Materials Chemistry and Physics 64 (2000) 260–264
Fig. 2. AFM image of a 1.5 m thick p-type GaN epitaxial layer with steps and hillocks or craters.
dislocation density is in the range of 108 –109 cm−2 in the Mg-doped GaN [20]. Fig. 3 shows AFM image of AlGaN epitaxial layer grown on 2 m thick GaN layer. The step morphology observed in the as-grown GaN samples are not seen on the surface of the AlGaN epitaxial layer. There are two types of surface morphology observed in AlGaN sample; one is large size nanopipes or open core dislocation (V-type defects) and the other is smooth surface. A large number of nanopipes
or V-type defect are observed which are found to originate from the pits or threading dislocation (mixed and pure edge) in the GaN layer due to large lattice mismatch as confirmed by TEM [13]. Besides this open core dislocation, the larger surface area is much more smooth and flat. The overall RMS value with dislocation and smooth surface is 4.488 nm. Fig. 4a and b shows a plane view image of hexagonal open core dislocation and three-dimensional view of a single nanopipe defect in AlGaN epitaxial layer. The depth of the open core dislocation (nanopipe) or V-type defect is about 35–40 nm and the width ranges from 100 to 300 nm. The internal surface of the open core dislocation is formed by ¯ six closed packed {1100} prism plane. Increase in the thickness of the AlGaN epilayer increases the strain induced by lattice mismatch between GaN and AlGaN. This effect appears to be cumulative and it is found that after a certain critical thickness, the strain is released by surface roughening. This roughening leads to the formation of V-type defect and on top of the surface, cracking starts. It is clearly ¯ seen that, at the edge of each of the {1100} plane, cracks start so as to release the high strain existing in the epitaxial layer caused by lattice mismatch and heteroepitaxial growth. AFM scanning on the smooth surface of the AlGaN layer grown on GaN shows self organized quantum dots of AlGaN. The plane view and three dimensional view are shown in Fig. 5 (a) and (b) respectively. The density of the dots is very high (107 –108 cm−2 ) and covers the whole smooth surface. The rms roughness of this smooth surface is found to be 6.29 Å. The dots have an average height of 1.0 ∼ 1.5 nm and a mean diameter of 40–50 nm. The growth on the GaN (0001) surface proceeds via the Stranski–Krastanov growth mechanism in which three-dimensional islands (islands larger than one monolayer in height) emerge after the formation of an Al(Ga)N wetting layer. In this mode of growth, due to minimization of interfacial energy, dots are formed rather than a continuous AlGaN layer. The initial stages of the wetting layer formation substantially depends on the growth technique that is employed, i.e., either an AlGaN alloy or distinct islands which only cover parts of the GaN surface. Also due to the non-stoichiometry of the Ga and Al, self organized quantum dots form on the GaN surface. This self organized quantum dots tend to cover open core dislocation or nanopipes originating from the substrate.
4. Summary
Fig. 3. AFM image of AlGaN epitaxial layer with smooth and open core dislocation surface.
Undoped and doped GaN and AlGaN epitaxial layers have been grown using MOCVD technique. The step growth mechanism with the termination of steps at the pits or dislocation is confirmed by AFM in the undoped GaN. In the Mg : GaN samples, the surface is rougher and seems to be irregular with step formation between the dislocation. How-
R.S. Qhalid Fareed et al. / Materials Chemistry and Physics 64 (2000) 260–264
263
Fig. 4. (a) Hexagonal patterned open core dislocation in AlGaN epitaxial layer with six {1100} planes; (b) three-dimensional view of a V-type defect.
Fig. 5. (a) AFM image of the self organized quantum dots observed on the smooth surface of the AlGaN; (b) three-dimensional view of the self organized quantum dots.
ever, the density of the pits in p-type GaN is less compared to undoped GaN. In the AlGaN epitaxial layer, there are no steps but large open core dislocation or V-type defects with ¯ six {1100} plane facets are found. Self organized quantum dots form the basis of the smooth surface of the AlGaN layer, which may be due to Stranski–Krastanov mode of growth.
References [1] H. Morkoc, S. Strite, G.B. Gao, M.F. Lin, B. Sverdlov, M. Burns, J. Appl. Phys. 76 (1994) 1363. [2] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, T. Yamada, T. Mukai, Jpn. J. Appl. Phys. 34 (1995) L1332. [3] S. Nakamura, T. Mukai, M. Senoh, J. Appl. Phys. 76 (1994) 8189.
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R.S. Qhalid Fareed et al. / Materials Chemistry and Physics 64 (2000) 260–264
[4] I. Akasaki, H. Amano, S. Sota, H. Sakai, T. Tanaka, M. Koike, Jpn. J. Appl. Phys. 34 (1995) L1517. [5] R.P. Vaudo, I.K. Goepfert, T.D. Moustakas, D.M. Geyea, T.J. Frey, K. Meehan, J. Appl. Phys. 79 (1996) 2779. [6] X.H. Wu, P. Fini, S. Keller, E.J. Tarsa, B. Heying, U.K. Mishra, S.P. DenBaars, J.S. Speck, Jpn. J. Appl. Phys., Part. 2, 35 (1996) L1648 [7] J. Elsner, R. Jones, P.K. Sitch, V.D. Porezag, M. Elstner, Th. Fraunheim, M.I. Heggie, S. Oberg, P.R. Briddon, Phys. Rev. Lett. 79 (1997) 3672. [8] B. Heying, X.H. Wu, S. Keller, Y. Li, D. Kapolnek, B.P. Keller, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 68 (1996) 643. [9] S.J. Rosner, E.C. Carr, M.J. Ludowise, G. Girolami, H.I. Erikson, Appl. Phys. Lett. 68 (1996) 1380. [10] L.T. Romano, J.E. Northrup, M.A. O’Keefe, Appl. Phys. Lett. 69 (1996) 2394. [11] Z. Lilental Weber, Y. Chen, S. Ruvimov, J. Washburn, Phys. Rev. Lett. 79 (15) (1997) 2835. [12] W. Qian, G.S. Rohrer, M. Skowronski, K. Doverspike, L.B. Rowland, D.K. Gaskill, Appl. Phys. Lett. 67 (1995) 2284.
[13] S. Mahanty, M. Hao, T. Sugahara, R.S. Qhalid Fareed, Y. Morishima, Y. Naoi, T. Wang, S. Sakai, Mater. Lett. (1999), in press. [14] X.H. Wu, D. Kapolnek, E.J. Tarsa, B. Heying, S. Keller, B.P. Keller, U.K. Mishra, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 68 (1996) 1371. [15] P.J. Hansen, Y.E. Strausserr, A.N. Ericksen, E.J. Tarsa, P. Kozodoy, E.G. Brazel, J.P. Ibbetson, U. Mishra, V. Narayanamurthi, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 72 (1998) 2247. [16] D. Kapolnek, X.H. Wu, B. Heying, S. Keller, B.P. Keller, U.K. Mishra, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 67 (1995) 1541. [17] F. Widmann, D. Daudin, G. Feuillet, Y. Samson, J.L. Rouviere, N. Pelekanos, J. Appl. Phys. 83 (1998) 7618. [18] W.K. Burton, N. Cabrera, F.C. Frank, Philos. Trans. Roy. Soc. A. 243 (1951) 300. [19] K. Sangwal, in: K. Sangwal (Ed.), Elementary Crystal Growth, SAAN Publishers, Lublin, Poland, 1994, p. 356. [20] D.H. Youn, M. Lachab, M. Hao, T. Sugahara, H. Takenaka, Y. Naoi, S. Sakai, Jpn. J. Appl. Phys. 38, Part 1, No. 2A (1999) 631.
Materials Science Communication
Structural studies on MOCVD grown GaN and AlGaN using atomic force microscopy R.S. Qhalid Fareed a,∗ , S. Juodkazis a , S.H. Chung a , T. Sugahara b , S. Sakai a,b a
b
Satellite Venture Business Laboratory, The University of Tokushima, 2-1 Minami josanjima, Tokushima 770, Japan Department of Electrical and Electronic Engineering, The University of Tokushima, 2-1 Minami josanjima, Tokushima 770, Japan Received 8 July 1999; received in revised form 13 September 1999; accepted 14 October 1999
Abstract Surface morphology studies of GaN and AlGaN grown by metalorganic chemical vapor deposition (MOCVD) have been carried out using atomic force microscopy. The open core dislocation and steps connecting two threading dislocations of opposite direction are commonly observed in undoped and doped GaN. Structural studies on AlGaN epitaxial layers grown on undoped GaN revealed the formation of open-core dislocation with width upto 300 nm. The nanopipes originate from the threading dislocation formed due to large lattice mismatch between sapphire and GaN. The mismatch also leads to high strain in the epilayers resulting in cracking effect at the edges of the hexagonal V-type defect. The self organized quantum dots features on the smooth surface of AlGaN epitaxial layer exhibit the Stranski–Krastanov(SK) mode of island growth. © 2000 Elsevier Science S.A. All rights reserved. Keywords: MOCVD; GaN and AlGaN; Atomic force microscopy
1. Introduction Gallium nitride (GaN) and related nitrides have attracted much interest as the most prospective materials for optoelectronic devices, and high power–high temperature device application. The progress of blue laser diodes and blue green light emitting diodes have been remarkable in recent years [1–3]. Due to the lack of GaN substrates, epitaxial films are grown using metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) on conventional substrates such as sapphire (Al2 O3 ) or silicon carbide (SiC). Due to the large mismatch and different thermal coefficient, high density of dislocations on the order of 108 –1010 cm−2 are generated [4,5]. The extended and point defects in GaN films have greatly impacted the performance of many devices. Recently, there have been many reports on the defect studies at different spatial scales. The density and distribution of defects in GaN and related nitride materials differ significantly depending on the growth conditions. Numerous studies have investigated the origin and theory of these defects [6,7] and their effects on the structural [8] and optical properties [9] of the epitaxial layers. The pure edge and mixed-character dislocations are two kinds of thread∗
Corresponding author.
ing dislocation observed in GaN normally. Threading edge ¯ are the most dislocations with burgers vector b = a/3h1120i common line defects found for films grown on sapphire (0001) and 6H-SiC (0001) [10]. The transmission electron microscopy (TEM) studies on the mechanism of formation of nanotubes (V defect) in GaN have also been investigated [11–13]. Atomic force microscopy (AFM) had recently emerged as a useful tool for characterizing the microstructure of the surface of the layers. Although, AFM studies on GaN have been carried out [9,14–16], surface studies on the p-type GaN and AlGaN epitaxial layers have not yet been reported. We note that the formation of self organized quantum dots on GaN has already been published [17] but there is no information on the growth of AlGaN self organized quantum dots. In this paper, we present the surface morphology studies on the step growth and open-core dislocation density in MOCVD grown undoped and p-type GaN. The Stranski–Krastanov (SK) growth mode of three-dimensional island formation in AlGaN epitaxial layers has also been observed by AFM and will be discussed in detail. 2. Experimental The undoped and doped GaN and AlGaN epitaxial layers were grown on sapphire (0001) substrate using MOCVD re-
0254-0584/00/$ – see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 2 3 4 - 5
R.S. Qhalid Fareed et al. / Materials Chemistry and Physics 64 (2000) 260–264
actor employed at atmospheric pressure. The substrate was first cleaned at 1150◦ C in H2 for 10 min. Tri-methyl gallium (TMGa) and ammonia (NH3 ) were used as the source gases. A thin GaN buffer layer (thickness ≈ 450 Å) was first deposited at 450◦ C. Undoped GaN epitaxial layer with thickness of 1.5 m was grown at 1075◦ C. Mg doped GaN was grown at 1050◦ C using bis-cyclopentadienyl magnesium (Cp2 Mg) as the Mg source. First, a 25 nm thin GaN buffer layer was grown at 450◦ C. Then the substrate temperature was increased to 1050◦ C to grow 1.5 m thick Mg-doped GaN films. During the deposition of Mg-doped GaN top layer, the Cp2 Mg flow rate was maintained at 0.3 mol min−1 . The flow rate of TMGa was maintained at 88 mol min−1 for both undoped and doped GaN growth. AlGaN epitaxial layer was grown with tri-methyl aluminium (TMAl) as the Al source over undoped GaN at 1075◦ C. All the samples were cleaned with methanol, acetone and deionised water before surface analysis. AFM studies were carried out using SEIKO Instrument SII (SPA 300) using SiN tips under normal pressure and room temperature to observe the morphology to understand the growth mechanism.
3. Results and discussion Fig. 1a shows the AFM image of an MOCVD grown undoped GaN. The dark regions are small pits that have been observed from our reactor as well as those from several other groups [9,15,16]. The surface is atomically smooth and flat
261
with RMS roughness about 3.055 Å. The lines on the surface represent the steps with the height in atomic scale and this step finally terminates at small pits. This indicates the association of these pits with dislocation terminating at the surface. It can be explained like this: when two screw dislocations of opposite sign emerge on a crystal surface, a ledge will join them. Growth will commence if the supersaturation is raised to such value that the diameter of the critical two-dimensional nucleus becomes less than the distance between the two dislocations [18,19]. The atoms absorbed on the surface will join the step and, hence, the step tends to spiral around both dislocations. Close examination of one of the pits, which is the termination point of steps, shows the open core dislocation (Fig. 1b). The internal surfaces of the open core dislocation are formed with six closed packed planes. AFM studies show the density of nanopipes is about 105 –107 cm−2 . The depth of the dislocation is more than 40 nm and width of the pipes differs from 50 to 200 nm in different places. Surface morphology of the 1.5 m thick as grown Mg-doped GaN is shown in Fig. 2. The p-type GaN (Mg concentration ∼1020 cm−3 ) is rougher than the undoped GaN and the rms roughness is 8.6 Å. Since it is very difficult to control the p-type doping, the surface morphology of the p-type GaN varies with growth conditions. However, the step formation observed in undoped GaN is also found in p-type GaN. There are also hillocks and craters besides the steps in the p-type GaN samples, which are not observed in undoped GaN. The termination of steps at the small pits is not clearly visible and the density of open core dislocation is much less. However, TEM measurement shows that the
Fig. 1. (a) AFM image of 1.5 m thick undoped GaN epitaxial layer. The scanning area is 4 m × 4 m and the rms roughness is 3.055 Å; (b) three-dimensional image of an open core dislocation in GaN with hexagonal feature.
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Fig. 2. AFM image of a 1.5 m thick p-type GaN epitaxial layer with steps and hillocks or craters.
dislocation density is in the range of 108 –109 cm−2 in the Mg-doped GaN [20]. Fig. 3 shows AFM image of AlGaN epitaxial layer grown on 2 m thick GaN layer. The step morphology observed in the as-grown GaN samples are not seen on the surface of the AlGaN epitaxial layer. There are two types of surface morphology observed in AlGaN sample; one is large size nanopipes or open core dislocation (V-type defects) and the other is smooth surface. A large number of nanopipes
or V-type defect are observed which are found to originate from the pits or threading dislocation (mixed and pure edge) in the GaN layer due to large lattice mismatch as confirmed by TEM [13]. Besides this open core dislocation, the larger surface area is much more smooth and flat. The overall RMS value with dislocation and smooth surface is 4.488 nm. Fig. 4a and b shows a plane view image of hexagonal open core dislocation and three-dimensional view of a single nanopipe defect in AlGaN epitaxial layer. The depth of the open core dislocation (nanopipe) or V-type defect is about 35–40 nm and the width ranges from 100 to 300 nm. The internal surface of the open core dislocation is formed by ¯ six closed packed {1100} prism plane. Increase in the thickness of the AlGaN epilayer increases the strain induced by lattice mismatch between GaN and AlGaN. This effect appears to be cumulative and it is found that after a certain critical thickness, the strain is released by surface roughening. This roughening leads to the formation of V-type defect and on top of the surface, cracking starts. It is clearly ¯ seen that, at the edge of each of the {1100} plane, cracks start so as to release the high strain existing in the epitaxial layer caused by lattice mismatch and heteroepitaxial growth. AFM scanning on the smooth surface of the AlGaN layer grown on GaN shows self organized quantum dots of AlGaN. The plane view and three dimensional view are shown in Fig. 5 (a) and (b) respectively. The density of the dots is very high (107 –108 cm−2 ) and covers the whole smooth surface. The rms roughness of this smooth surface is found to be 6.29 Å. The dots have an average height of 1.0 ∼ 1.5 nm and a mean diameter of 40–50 nm. The growth on the GaN (0001) surface proceeds via the Stranski–Krastanov growth mechanism in which three-dimensional islands (islands larger than one monolayer in height) emerge after the formation of an Al(Ga)N wetting layer. In this mode of growth, due to minimization of interfacial energy, dots are formed rather than a continuous AlGaN layer. The initial stages of the wetting layer formation substantially depends on the growth technique that is employed, i.e., either an AlGaN alloy or distinct islands which only cover parts of the GaN surface. Also due to the non-stoichiometry of the Ga and Al, self organized quantum dots form on the GaN surface. This self organized quantum dots tend to cover open core dislocation or nanopipes originating from the substrate.
4. Summary
Fig. 3. AFM image of AlGaN epitaxial layer with smooth and open core dislocation surface.
Undoped and doped GaN and AlGaN epitaxial layers have been grown using MOCVD technique. The step growth mechanism with the termination of steps at the pits or dislocation is confirmed by AFM in the undoped GaN. In the Mg : GaN samples, the surface is rougher and seems to be irregular with step formation between the dislocation. How-
R.S. Qhalid Fareed et al. / Materials Chemistry and Physics 64 (2000) 260–264
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Fig. 4. (a) Hexagonal patterned open core dislocation in AlGaN epitaxial layer with six {1100} planes; (b) three-dimensional view of a V-type defect.
Fig. 5. (a) AFM image of the self organized quantum dots observed on the smooth surface of the AlGaN; (b) three-dimensional view of the self organized quantum dots.
ever, the density of the pits in p-type GaN is less compared to undoped GaN. In the AlGaN epitaxial layer, there are no steps but large open core dislocation or V-type defects with ¯ six {1100} plane facets are found. Self organized quantum dots form the basis of the smooth surface of the AlGaN layer, which may be due to Stranski–Krastanov mode of growth.
References [1] H. Morkoc, S. Strite, G.B. Gao, M.F. Lin, B. Sverdlov, M. Burns, J. Appl. Phys. 76 (1994) 1363. [2] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, T. Yamada, T. Mukai, Jpn. J. Appl. Phys. 34 (1995) L1332. [3] S. Nakamura, T. Mukai, M. Senoh, J. Appl. Phys. 76 (1994) 8189.
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R.S. Qhalid Fareed et al. / Materials Chemistry and Physics 64 (2000) 260–264
[4] I. Akasaki, H. Amano, S. Sota, H. Sakai, T. Tanaka, M. Koike, Jpn. J. Appl. Phys. 34 (1995) L1517. [5] R.P. Vaudo, I.K. Goepfert, T.D. Moustakas, D.M. Geyea, T.J. Frey, K. Meehan, J. Appl. Phys. 79 (1996) 2779. [6] X.H. Wu, P. Fini, S. Keller, E.J. Tarsa, B. Heying, U.K. Mishra, S.P. DenBaars, J.S. Speck, Jpn. J. Appl. Phys., Part. 2, 35 (1996) L1648 [7] J. Elsner, R. Jones, P.K. Sitch, V.D. Porezag, M. Elstner, Th. Fraunheim, M.I. Heggie, S. Oberg, P.R. Briddon, Phys. Rev. Lett. 79 (1997) 3672. [8] B. Heying, X.H. Wu, S. Keller, Y. Li, D. Kapolnek, B.P. Keller, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 68 (1996) 643. [9] S.J. Rosner, E.C. Carr, M.J. Ludowise, G. Girolami, H.I. Erikson, Appl. Phys. Lett. 68 (1996) 1380. [10] L.T. Romano, J.E. Northrup, M.A. O’Keefe, Appl. Phys. Lett. 69 (1996) 2394. [11] Z. Lilental Weber, Y. Chen, S. Ruvimov, J. Washburn, Phys. Rev. Lett. 79 (15) (1997) 2835. [12] W. Qian, G.S. Rohrer, M. Skowronski, K. Doverspike, L.B. Rowland, D.K. Gaskill, Appl. Phys. Lett. 67 (1995) 2284.
[13] S. Mahanty, M. Hao, T. Sugahara, R.S. Qhalid Fareed, Y. Morishima, Y. Naoi, T. Wang, S. Sakai, Mater. Lett. (1999), in press. [14] X.H. Wu, D. Kapolnek, E.J. Tarsa, B. Heying, S. Keller, B.P. Keller, U.K. Mishra, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 68 (1996) 1371. [15] P.J. Hansen, Y.E. Strausserr, A.N. Ericksen, E.J. Tarsa, P. Kozodoy, E.G. Brazel, J.P. Ibbetson, U. Mishra, V. Narayanamurthi, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 72 (1998) 2247. [16] D. Kapolnek, X.H. Wu, B. Heying, S. Keller, B.P. Keller, U.K. Mishra, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 67 (1995) 1541. [17] F. Widmann, D. Daudin, G. Feuillet, Y. Samson, J.L. Rouviere, N. Pelekanos, J. Appl. Phys. 83 (1998) 7618. [18] W.K. Burton, N. Cabrera, F.C. Frank, Philos. Trans. Roy. Soc. A. 243 (1951) 300. [19] K. Sangwal, in: K. Sangwal (Ed.), Elementary Crystal Growth, SAAN Publishers, Lublin, Poland, 1994, p. 356. [20] D.H. Youn, M. Lachab, M. Hao, T. Sugahara, H. Takenaka, Y. Naoi, S. Sakai, Jpn. J. Appl. Phys. 38, Part 1, No. 2A (1999) 631.