ISSN 10637745, Crystallography Reports, 2011, Vol. 56, No. 2, pp. 274–281. © Pleiades Publishing, Inc., 2011. Original Russian Text © A.A. Donskov, L.I. Dyakonov, Yu.P. Kozlova, S.S. Malakhov, M.V. Mezhennyi, V.F. Pavlov, T.G. Yugova, 2011, published in Kristallografiya, 2011, Vol. 56, No. 2, pp. 298–306.
REAL STRUCTURE OF CRYSTALS
Effect of Sapphire Substrate Orientation on the Surface Morphology and Structural Quality of Thick GaN Layers Grown by Hydride Vapor Phase Epitaxy A. A. Donskov, L. I. Dyakonov, Yu. P. Kozlova, S. S. Malakhov, M. V. Mezhennyi, V. F. Pavlov, and T. G. Yugova OAO Giredmet, Moscow, 119017 Russia email:
[email protected] Received September 23, 2010
Abstract—The effect of substrate orientation on the surface orientation of thick GaN layers grown by hydride vapor phase epitaxy (HVPE) has been established. Layers oriented along the (0001), (1 1 2 0 ), and (10 13 ) planes have been obtained on, respectively, c and a, r, and moriented substrates. Depending on the ori entation of the GaN layer surface, surface defects (terraces and growth pits) are faceted by different planes whose intersections with the growth surface are perpendicular to the direction of growth pit faces. It is found that the sapphire substrate surface orientation has an effect on the layer structural quality (which increases with an increase in the layer thickness, regardless of the layer orientation). The directions of crack propaga tion in the GaN layer also depend on the surface orientation of the layer and are mainly determined by the intersections of the {1 100 } planes of the layer with the surface. DOI: 10.1134/S1063774511020052
INTRODUCTION The experimental orientation relationships between sapphire substrates and gallium nitride layers grown on them by molecular beam epitaxy and meta lorganic chemical vapor deposition were reviewed in [1]. According to the data of [1], c and moriented GaN layers were obtained on aoriented sapphire sub strates and layers with semipolar (10 13 ) and (1 212 ) orientations were grown on moriented substrates. Nonpolar GaN layers with the a orientation were obtained on roriented substrates. Layers with the (1 216 ) orientation were also grown on these sub strates. There are no systematic data in the literature on the orientation relationships between sapphire and gallium nitride for thick epitaxial layers grown by hydride vapor phase epitaxy (HVPE); however, it is known that GaN layers with the a orientation were obtained by this method on roriented sapphire sub strates [2]. Concerning moriented GaN layers, they are known to be HVPEgrown on mSiC or mGaN substrates [3]. It is almost impossible to grow GaN layers with specular surfaces by HVPE. The layer surface contains various morphological defects: terraces, pyramids, and pits. The aim of this study was to determine the orienta tions of the {hkjl} crystallographic planes of thick GaN layers grown by HVPE on sapphire substrates with
respect to the substrate crystallographic planes, the relationship between the geometry of the morphologi cal defects formed on the layer surface during epitaxy and the orientation of the GaN layer surface, and the effect that substrate orientation has on the structural quality of epitaxial GaN layers. EXPERIMENTAL GaN with a wurtzite structure was grown on sap phire substrates with the (11 20 ) (a), (10 10 ) (m), and (10 12 ) (r) crystallographic orientations and, for com parison, on standard (0001) (c) substrates. Thick GaN layers were grown by HVPE in a system with a vertical quartz reactor. The growth conditions were described in detail in [4]. The surface morphology and cleavages of the GaN layers grown were studied with a Nomarsky micro scope. The dislocation structure of the layers was determined by etching in a KOH–NaOH eutectic melt at 450°С for 2 min. The structural quality of the GaN layers was esti mated by the halfwidth of the rocking curves (RCs), which were recorded with the GUR8 goniometer of a DRON3 diffractometer (Кα1 radiation, doublecrys tal scheme). The crystallographic planes and directions of the layers parallel to the crystallographic planes and direc
274
EFFECT OF SAPPHIRE SUBSTRATE ORIENTATION
275
Table 1. Mutual orientation of the atomic planes of the GaN layer and sapphire substrate in the epitaxial structures and the crystallographic directions in the grown GaN layers, which specify the coordinate axes in the layer Atomic planes of the substrate surface
Atomic planes (hkjl)l of the GaN layer parallel to the (hkjl)s planes sapphire
Crystallographic directions of the grown GaN layers, specifying the coordinate axes in the layer plane
Designations
(hkjl)s
(hkjl)l
x axis
y axis
c
(0001)s
(0001)l
〈2 110〉
〈01 10〉
a
(11 2 0)s
(0001)l
〈2 110〉
〈01 10〉
r
( 1 012)s
(11 20)l 20 from ( 1012)s of the substrate
〈0001〉 20 relative to the x axis
〈1 100〉
m
(10 10)s
(10 13)
〈 12 10〉
〈10 10〉 320 relative to the x axis
tions in the substrates, which determine the mutual crystallographic orientation of the layer and substrate, were identified (disregarding polarity) on the same system. To this end, a rectangular coordinate system with x and y axes lying in the layer plane was used. The y axis was determined by the layer position on the goniometer at the (01 15 ), (11 20 ), and (20 20 ) reflec tions for the layers with (0001), (10 1 3), and (10 12 ) orientations, respectively. The relation between the geometry of the morphological defects formed on the layer surface during epitaxy and the orientation of the GaN layer surface was determined by their orientation relative to the same orthogonal x – y coordinate sys tem. To this end we used one of the calculated nets of projections onto the (0001), (11 20 ), and (10 13 ) planes corresponding to the crystallographic plane of the grown GaN layer. RESULTS AND DISCUSSION The crystallographic orientations of the GaN layers grown on sapphire substrates relative to those of the substrates are given in Table 1. As can be seen in the table, (0001) layers grow on a oriented sapphire substrates (we failed to obtain the other possible orientation—(10 10 )—of the GaN layer on aoriented substrates) and layers with a semi polar (m) orientation grow on moriented substrates 10 13 . GaN layers with nonpolar (11 20 ) orientation were obtained only on roriented sapphire substrates. During the HVPE growth of GaN layers on sap phire substrates, various morphological defects are formed on the layer surface, such as terraces; growth pyramids; pits; and striations, which are caused by stacking faults in the layers. Depending on the layer surface orientation, the defects observed have dif ferent crystallographic directions in the growth plane. CRYSTALLOGRAPHY REPORTS
Vol. 56
No. 2
The morphology of the GaN layer surface depends strongly on the substrate surface orientation and the layer thickness. The layers with the (0001) orientation, which were grown on the с and аoriented substrates, have a smooth surface with terraces and growth pits. The surface of the GaN layers with the (10 13 ) orien tation is faceted by terraces, making an angle of ~120°; growth pits are also observed. The surface relief dete riorates with an increase in the layer thickness and the shape of the pits changes. In addition, pronounced striations are observed on the layer surface. The surface relief is most developed in the (11 20 ) layers. The surface morphology changes with an increase in the layer thickness (Fig. 1). Ridges extend ing along the 〈1 100 〉 direction are observed on the sur face of thin layers (Fig. 1a); they break with a further increase in the layer thickness (Fig. 1b). There are numerous pyramids with smoothed edges and a vertex, each with a parallelogram base, on the surface of thick (11 20 ) layers (Fig. 1c). The sides of the parallelogram are the intersections of the {20 23 } planes with the (11 20 ) layer surface. A cleavage in the ( 10 10 ) plane of the substrate reveals a sawtooth surface profile, which can be clearly seen in Fig. 1d. This change in the surface morphology of (11 20 ) layers is related to their complex growth. When islands in the form of extended ridges, slightly disoriented relative to one another and to the substrate, grow in, numerous blocks are formed. They grow, absorbing smaller ones, with an increase in the layer thickness. Apparently, this growth results in the formation of pyramids on the layer surface. Stria tions, caused by stacking faults in the layers, are also observed on the surface. Terraces are formed on the surface of с and mori ented layers during growth and are faceted by rapidly growing faces. In addition, terraces are observed on the lateral faces of the pyramids that are formed during spiral growth around a defect (inverse domain, screw dislocation, or nanotube). Typical growth terraces
2011
276
DONSKOV et al.
(a)
20 μm (b)
20 μm
(c)
20 μm (d)
25 μm
Fig. 1. Change in the surface morphology of GaN (1120 ) layers with an increase in the layer thickness to (a) 30, (b) 120, and (c) 250 µm. (d) Cleavage of the structure with an epitaxial 250µmthick GaN(1120 ) layer.
formed during the growth on substrates of different orientations are presented in Figs. 2a and 2b. Figures 2c and 2d show a proposed scheme of the for mation of growth terraces on differently oriented planes of GaN layers. The schematic diagrams in Figs. 2c and 2d were constructed with regard for the shape of the growth islands observed on differently oriented substrates (Figs. 2e, 2f). The islands on the сoriented substrates are shaped like sharp hexagons, while those on the m oriented substrates are in the shape of triangular prisms. As can be seen in Fig. 2c, the terraces on (0001) planes develop from hexagonal growth islands (Fig. 2e). On the (10 13 ) plane, terraces propagate from two adjacent triangular islands (Fig. 2f). In the (0001) layers grown on the с and аoriented substrates (Figs. 2a, 2c), terraces develop in the 〈01 10 〉 direction and the terrace faces are oriented in the 〈2 1 10 〉 direction. The development of terraces along the 〈10 10 directions on the (0001) plane was demonstrated in [5]. During epitaxy, terraces are formed on the (10 13 ) planes (Fig. 2b); the orientations of the terrace faces are determined by the intersection of the ( 12 10 ) planes with the growth plane. The terraces propagate along directions that almost coincide with the projec
tions of the 〈2 1 10 〉 and 〈11 20 〉 directions in the growth plane. In this case the directions of the terrace edges cannot be specified by smallindex directions, but they coincide with the projections of the 〈1 100 〉 and 〈01 10 〉 directions onto the (10 13 ) plane (Fig. 2d). It should also be noted that, when a layer is ori ented along the (0001) plane, terraces can propagate along any of the six 〈0 110 〉 directions and, thus, lead to the formation of hexagonal pyramids around some source of spiral growth (inverse domain, screw dislo cation, or nanotube). These pyramids are often observed on the surface of (0001) layers (Fig. 2a). When the layer surface lies in the (10 13 ) plane, ter races can propagate in only two directions, which coincide with the directions of the 〈2 1 10 〉 and 〈11 20 〉 projections onto the (10 13 ) plane, and pyramids are not formed. Several reasons for the formation of growth pits on the GaN surface have been discussed [6]. They may arise due to the columnar growth of GaN layers on sapphire surface steps [6]. These columns can clearly be seen on the cleavage in Fig. 3c. They can serve as inverse domains (IDs), i.e., Npolarities of islands in the Gapolarity matrix [7]. However, even in the absence of IDs, sufficiently large columns emerging to
CRYSTALLOGRAPHY REPORTS
Vol. 56
No. 2
2011
EFFECT OF SAPPHIRE SUBSTRATE ORIENTATION
277
–
Projection 〈0110〉 ––
〈2110〉
–
〈0110〉
––
Projection 〈2110〉 50 μm (b)
(a)
50 μm
–
[0110]
–
[1120]
–
[0110] ––
––
[2110]
[2110]
(0001) (c)
(d)
25 μm (f)
(e)
–
(1013)
25 μm
Fig. 2. Typical terraces on the surface of thick layers with the (a) (0001) and (b) (10 13 ) orientations; schematic diagrams of the formation of terraces in GaN layers with the (c) (0001) and (d) (10 13 ) orientations; and the shape of the growth islands formed during growth on the substrates with (e) c and (f) m orientations.
the layer surface can form macroscopic pits [8]. Another possible cause of growth pits is the deposition of particles on the growth surface. The main source of particles is the spurious growth on the inlet connection for GaCl into the reactor and on the reactor walls. These particles are transferred to the substrate surface by gas flows supplied to the reactor [8]. The growth pits in the GaN layers oriented in the (0001) and (10 13 ) planes have clear crystallographic faceting. The growth pits in the (0001) layer are shaped like inverse hexagonal pyramids. The edges limiting CRYSTALLOGRAPHY REPORTS
Vol. 56
No. 2
the pits on the surface are oriented along the 〈01 10 〉 directions (Fig. 3a). The pit edge directions are rotated by 90° relative to the orientations of the pyramid faces. An analysis of many structures with a layer thickness from ~40 to 400 µm showed that the average pit size does not increase with an increase in the layer thick ness, and both the average depth and size of pits on the growth surface remain the same. The reason is that pits are laterally healed with an increase in the layer thick ness. Healing traces are clearly seen in Fig. 3c, where a cleavage through the pit center is shown. One can
2011
278
DONSKOV et al. Projection
–– 〈2110〉
– 〈0110〉
Projection –– 〈2110〉
– 〈0110〉 50 μm (b)
(a)
50 μm (c)
50 μm
Fig. 3. Growth pits formed in the layers with the (a) (0001) and (b) (10 13 ) orientations and (c) a growth pit on the cleavage of the structure with an epitaxial GaN(0001) layer.
–– 〈1210〉 〈0001〉 〈0001〉 20 μm (c)
50 μm (b)
(a)
50 μm
Fig. 4. Striations caused by stacking faults on the surface of GaN layers with the (a) (1120 ) and (b) (10 13 ) orientations; (c) the cleavage of the structure with a (10 13 ) layer.
observe a break in the regular columnar cleavage struc ture near the pit. The surface of GaN (10 13 ) layers exhibits triangu lar growth pits at small layer thicknesses; during growth, pits increase in size and change shape from triangular to rhombic (Fig. 3b). The growth pit edges are the intersections of the {01 1 0} planes with the Bh/2, ang. sec.
c a m r
3500 3000 2500 2000 1500 1000 500 0
100
200
300
400
500 h, μm
Fig. 5. Dependence that the RC halfwidth has on the thickness of GaN layers grown on sapphire substrates with different orientations.
(10 13 ) growth plane. The directions of these lines do not coincide with smallindex directions. However, it should be noted that here, like in the case of growth on the (0001) plane, the directions of terraces and growth pit edges are orthogonal (Fig. 3b). The origin of this relation between the directions of terraces and growth pit edges is unclear. No growth pits were observed in the (11 20 ) plane. Stacking faults are rare in GaN(0001) layers [9]. The nonpolar (11 20 ) and semipolar (10 13 ) GaN lay ers grown on r and moriented sapphire substrates have a high density of stacking faults. The formation mechanism of these stacking faults during growth on nonpolar substrates is not quite clear; however, it is believed that stacking faults are formed on islands of N polarity [10]. An intersection between the (10 10 ) plane (which contains stacking faults) and the surface of the (10 13 ) layer yields striations which are oriented in the 〈 12 10 〉 direction (Fig. 4b) [11]. In the layers oriented along the (11 20 ) plane, the intersections of the stacking fault planes with the layer plane have the 〈0001〉 direction (Fig. 4a). The cleavage of the (10 13 ) layer (Fig. 4b) shows that the (10 10 ) planes, which contain stacking faults and form columns, make an angle of ~58° with the heterointerface.
CRYSTALLOGRAPHY REPORTS
Vol. 56
No. 2
2011
EFFECT OF SAPPHIRE SUBSTRATE ORIENTATION ND, cm–2 108 c a m r
107
0
100
200
300
400
500 h, μm
Fig. 6. Dependence that the dislocation density has on the thickness of GaN layers grown on sapphire substrates with different orientations.
The Xray diffraction data in Fig. 5 indicate that the RC halfwidth decreases with an increase in the layer thickness, which implies a higher structural qual ity of the GaN layer structure. With an increase in layer thickness, the RC halfwidth for the (10 13 ) layer decreases most significantly. For the (11 20 ) layer, the RC halfwidth decreases by more than half. For the (0001) layers grown on the с and аoriented sub strates, the RC halfwidth changes only slightly.
Etching GaN layers of different orientations in a KOH–NaOH mixture reveals clearly faceted etching pits of different configurations on the film surface; they are hexagons, acute isosceles triangles, and oblong rhombuses in the layers with (0001), (11 20 ), and (10 13 ) orientations, respectively. Generally, epitaxial (0001) layers grown on the с oriented substrates exhibit a uniform distribution of dislocations over the surface, while, in the layers grown on аoriented substrates, dislocation pileups in the form of lowangle boundaries are most often observed. In the GaN layers with the (11 20 ) and (10 13 ) orien tations, dislocations are nonuniformly distributed over the structure area. The dependence that the dislocation density has on the thickness of the layers grown on differently ori ented substrates is presented in Fig. 6. One can see that the dislocation density in the epitaxial GaN layers decreases with an increase in the layer thickness, regardless of the surface layer orientation. In thick (h ~ 400 µm) layers, the dislocation density is at the same level: ~ 1.0 × 107 cm–2. In thin (~30 µm) layers, the dislocation density is 5–7 × 107 cm–2, except for the GaN (10 13 ) layer, where the dislocation density is much lower: ~2.3 × 107 cm–2. The etching of epitaxial layers with the (11 20 ) and (10 13 ) orientations reveals not only dislocation pits,
10 μm (c)
10 μm (b)
(a)
279
10 μm
Fig. 7. Various shapes of stacking faults revealed in the epitaxial (1120 ) layers.
––
〈2110〉
–
〈0110〉
(a)
––
〈2110〉
–
〈0110〉
–
〈0110〉 〈0001〉
50 μm (b)
50 μm (c)
100 μm
Fig. 8. Cracks in the GaN layers with the (a) (0001) orientation (c substrate), (b) (0001) orientation (a substrate), and (c) (1120 ) orientation. CRYSTALLOGRAPHY REPORTS
Vol. 56
No. 2
2011
280
DONSKOV et al.
Table 2. Crystallographic orientation of different morphological defects observed on the surface of GaN layers with differ ent orientations Surface orientation (hkjl)
(0001)
(11 2 0)
(10 1 3)
Terraces: Terrace propagation Terrace direction
〈0 1 10〉 〈2 11 0〉
no terraces
projections of directions 〈2 11 0〉 and 〈11 2 0〉 projections of directions 〈01 1 0〉 and 〈1 1 00〉
Growth pits: Edge direction
〈01 1 0〉
no pits
projections of directions 〈2 11 0〉 and 〈11 2 0〉
Cracks: Direction
〈2 11 0〉 and 〈01 1 0〉
〈1 1 00〉 и 〈0001〉
Stacking faults Direction of striations
no stacking faults
〈0001〉
but also stacking faults. The calculated linear density of stacking faults in the (10 13 ) layers is, on average, 5 × 104 cm–1, regardless of the layer thickness. In the (11 20 ) layers, the maximum linear density of stacking faults amounts to ~2 × 104 cm–1 and changes from structure to structure within a range of 2 × 103 – 2 × 104 cm–1. In addition, the nonuniform distribution of stacking faults over the area of the epitaxial layers grown on substrates of these two orientations is observed. Some areas reveal only dislocation pits and some contain only stacking faults. Stacking faults in the (11 20 ) epitaxial layers have different shapes, as is clearly seen in Fig. 7. In the (10 13 ) epitaxial layers, stacking faults always form straight lines. In the (11 20 ) layers, as was suggested in [10, 11], prismatic and basal stacking faults are formed. The type of stacking faults formed in the (10 13 ) layers is still being disputed. Some researchers believe that stacking faults lying in prismatic planes are
––
〈2110〉
–
〈0110〉 GaN layer –
〈1120〉 Sapphire substrate
50 μm
Fig. 9. Relative position of the crack directions in a GaN layer grown on a сoriented substrate and the direction of the cleavage of the entire structure.
〈 1 2 1 0〉
formed in GaN layers [9], while others consider them to be in basal planes [12]. The growth of GaN layers on sapphire substrates is accompanied by high stresses due to the lattice mis match and difference in the thermal expansion coeffi cients of the layer and substrate. Therefore, GaN lay ers crack upon cooling. A triangular network of cracks oriented along the 〈2 1 10 〉 direction is formed in the GaN(0001) layers (Fig. 8a). In addition, cracks ori ented along the 〈01 10 〉 direction are observed. According to the data of [13], the cracks in a (0001) layer are parallel to the {10 10 } planes of the layer. As is shown in Fig. 9, the microcracks in the layer are per pendicular to the cleavage line of the entire structure. If the layer is much thinner than the substrate, the entire structure cracks along the planes determined by the substrate crystallography rather than layer crystal lography: for (0001) sapphire substrates, the cracks on the surface are oriented along the 〈11 20 〉 direction in the substrate, which coincides with the 〈01 10 〉 direc tion in the layer. In the structures of the GaN(0001) layers grown on аoriented substrates, cracks oriented along these two directions are also observed; however, the fraction of cracks oriented along the 〈0 110 〉 direction increases (Fig. 8b). The GaN layers oriented in the (11 20 ) plane exhibit an almost orthogonal system of cracks along the 〈1 100 〉 and 〈0001〉 directions (Fig. 8c). All of our data on the crystallographic directions of different morphological defects are listed in Table 2. CONCLUSIONS The close interrelation between the geometry of the morphological defects formed on the layer surface during the HVPE growth of GaN layers was demon strated. It was found that, depending on the orienta tion of the GaN layer surface, the same defects (ter races and growth pits) are faceted by different planes,
CRYSTALLOGRAPHY REPORTS
Vol. 56
No. 2
2011
EFFECT OF SAPPHIRE SUBSTRATE ORIENTATION
which intersect with the growth surface in different directions. However, the terrace directions are always perpendicular to the direction of growth pit faces. It was shown that the directions of cracks also depend on the orientation of the layer surface. With an increase in the thickness of the GaN layers grown on sapphire substrates of all orientations under study, the structural quality of these layers (estimated by the RC halfwidth and dislocation density) consid erably improves. At small layer thicknesses, the struc tural quality of the layers with (0001), (11 20 ), and (10 13 ) orientations deteriorates. At large thicknesses (above 200 µm), the (0001) layers grown on the с and aoriented substrates have nearly the same structural quality. The nonpolar GaN (11 20 ) layers have similar RC halfwidths. The real structure of layers with semi polar (10 13 ) orientation is much worse. At a layer thickness of several hundreds of micrometers, the dis location density approaches 1 × 107 cm–2. REFERENCES 1. L. Liu and J. H. Edgar, Mater. Sci. Eng. 37, 61 (2002). 2. T. Zhu, D. Martin, R. Butte, et al., J. Cryst. Growth 300, 186 (2007).
CRYSTALLOGRAPHY REPORTS
Vol. 56
No. 2
281
3. B. A. Haskel, M. B. McLaurin, S. P. Den Baars, et al., US Patent No. 7, 208, 393 (2007). 4. L. I. Dyakonov, Yu. P. Kozlova, A. V. Markov, et al., Izv. Vyssh. Uchebn. Zaved., Mater. Elektron. Tekh., No. 1, 47 (2008). 5. A. R. A. Zauner, E. Aret, and W. J. P. Enckeyort, J. Cryst. Growth 240, 14 (2002). 6. T. Paskova, E. M. Goldys, and B. Monemar, J. Cryst. Growth 200, 1 (1991). 7. J. Jasinski and Z. LilientalWeber, J. Electron. Mater. 31, 429 (2002). 8. B. Monemar, H. Larsson, C. Hemmingson, et al., J. Cryst. Growth 281, 17 (2005). 9. T. Wei, R. Duan, J. Wang, et al., Jpn. J. Appl. Phys. 47, 3346 (2008). 10. J. Mei, S. Srinivasan, R. Liu, et al., Appl. Phys. Lett. 88, 141 912 (2006). 11. R. Liu, A. Bell, F. A. Ponce, et al., Appl. Phys. Lett. 86, 021 908 (2005). 12. Y. S. Cho, Q. San, I. H. Lee, et al., Appl. Phys. Lett. 93, 111 904 (2008). 13. E. V. Etzkorn and D. V. Clarke, J. Appl. Phys. 89, 1025 (2001).
2011
Translated by E. Bondareva
