DOI: 10.1021/cg1011617
Use of Polytypes to Control Crystallographic Orientation of GaN Hyun-Jae Lee,† T. Yao,*,† Chinkyo Kim,*,‡ and Jiho Chang§
2010, Vol. 10 5307–5311
†
Center for Interdisciplinary Research, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japan, ‡Department of Physics and Research Institute for Basic Sciences, Kyung Hee University, 1 Hoegi-dong Dongdaemun-gu, Seoul 130-701, Korea, and §Major of Semiconductor Physics, Korea Maritime University, Pusan 606-791, Republic of Korea Received September 2, 2010; Revised Manuscript Received October 11, 2010
ABSTRACT: Many characteristics of epitaxially grown films depend sensitively on their orientations. In most cases, orientation is uniquely determined by that of the substrate, a correlation established by energy minimization. Here, we show that substrate-independent control of orientation is possible by paths available in kinetically limited growth regimes for materials exhibiting polytypism. Demonstrated for GaN, our intriguing results provide fundamentally new insight into controlling one of the most important structural parameters of epitaxial films. This approach might be readily extended to other III-nitrides and ZnO, which also exhibit polytypism. Our procedures can be easily applied to tailor characteristics of epitaxial films, allowing investigations of new properties such as unipolarity.
*To whom correspondence should be addressed. E-mail: tyao@ cir.tohoku.ac.jp (T.Y.);
[email protected] (C.K.).
hole wave functions in the active regions is significantly suppressed due to spontaneous polarization and piezoelectric field. Consequently, the quantum efficiency of electron-hole recombination decreases.6 In contrast, polarization effects are greatly reduced in nonpolar [1120]-oriented films grown on (1102) r-plane sapphire substrates.6,7,14,15 In addition to these differences of luminescence characteristics, the polar [0001] and [0001] surfaces exhibit very distinct characteristics regarding the efficiency of indium incorporation16 and chemical reactivity.17 Thus, as with other polar materials, the orientation of IIInitrides is of great significance. However, since at present the orientation of the film is established by the orientation of the substrate, flexibility is severely limited. Although lateral polarity control has been reported,17,18 these films consist of alternating domains of polarization inversion as a result of growth on lithographically patterned templates, not the active control of orientation in the growth direction. Furthermore, the bond angles in these laterally polarization-inverted films are inevitably distorted at the domain boundaries.19,20 One approach to inverting along the growth direction is to insert a layer of centrosymmetric material. For example, the polarities of GaN and ZnO can be inverted using Ga2O3,21 and Zn3N2,22 and Mg3N2,23 respectively. Alternatively, the polarity of GaN films was found to be sensitively influenced by nitridation, a buffer layer, and thermal annealing.24-27 However, polarity inversion within a film already attached to the substrate does not provide much flexibility. On the other hand, localized transitions of orientations can occur in various nanostructures, where orientation is more easily controlled than in films.28-31 We previously reported that polytypism can cause a transition in orientation from [1120] to [0001].32 The driving force from [1120] to [0001] or [0001] is the preferential alignment of the [111] or [111] planes of zincblende GaN along the growth direction within the transition layer. However, up to now, the transition of the orientation of GaN in the reverse direction has not been observed. The key to modifying the orientation is low-temperature-grown (LTG) zincblende GaN. Subsequent high-temperature-grown (HTG) wurtzite GaN then maintains the orientation of the LTG GaN domain on which it is
r 2010 American Chemical Society
Published on Web 10/27/2010
Introduction A substrate is an essential component for epitaxial growth of a film, with the characteristics of the film being strongly dependent on the substrate. Two of the most important physical parameters of the grown film are strain and orientation. In most cases, these two parameters are uniquely determined by the lattice constants and crystallographic orientation, respectively, of the substrate. Many efforts have been made to accommodate lattice mismatch without either surface roughening or the generation of misfit dislocations. Several decades ago, an intriguing new approach involving compliant substrates was introduced.1-3 Here, a thin layer of material was introduced to effectively reduce threading and misfit dislocations previously thought to be inevitable consequences of lattice mismatch.4,5 Control of crystallographic orientation of an epitaxial film independent of that of the substrate is an even more challenging task. Many examples exist of film characteristics that are dependent on orientation. These include luminescence characteristics of III-nitride materials,6,7 mechanical stability of wear-resistant TiN films,8,9 surface-acoustic wave characteristics of ZnO films,10 and photovoltaic characteristics of CuInGaSe11,12 The origin of these orientation-dependent characteristics is closely related to the way in which atoms are arranged in the film. Materials of the non-centrosymmetric space group have no inversion center, so films of these materials naturally exhibit spontaneous polarization. Consequently, the polarities on the top and bottom surfaces in the polar direction are different.13 The opposite surface polarities in these directions are a consequence of charge neutrality and are commonly observed in many crystals including NaCl (rocksalt), GaAs (zincblende), and ZnO (wurtzite). Non-centrosymmetric GaN also exhibits both spontaneous polarization and piezoelectric characteristics. When c-oriented, GaN-based multiple-quantum-well structures are grown on (0001) c-plane sapphire, the most widely used substrate for GaN epitaxy, the overlap of the electron and
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Figure 1. Schematic diagrams of the three sample configurations. Blocks of the same color have the same orientation. In the magnified interfacial regions, the LTG zinc blende GaN layers, which serve to transition orientations, are drawn in graded colors with red and blue representing domains adjacent to [1120] and [0001]-oriented surfaces, respectively. The thicknesses of the transition layers are exaggerated for clarity. Voids spontaneously form in the transition layers so that the HTG GaN overlayers are self-separating.32 Because of the different thicknesses of the LTG GaN layers, self-separation in sample A and in samples B and C takes place in the regions adjacent to the [1120]- and [0001]-oriented domains, respectively. The thickness of the c-oriented domains are 180 μm. Those of the regrown layers are 40, 60, and 80 μm for sample A, B, and C, respectively. Regrowth for all three samples is done on the self-separated regions, which are within the LTG layers.
Figure 2. Atomistic models of the three GaN samples, showing crystallographic relations between wurtzite (WZ) and zinc blende (ZB) domains for the three different cases. The dotted red, dotted black, solid red, and solid cyan lines represent inversion domain boundaries, stacking faults, surface normal directions of the r-plane sapphire substrates before detachment, and [000 ( 1] directions, respectively. The [0001] direction of the as-grown film is 3.7° away, in the clockwise direction, from the normal of the detached substrate. The [0001] and [0001] directions of the regrown films are 3.7° away, in the clockwise and counterclockwise directions, respectively. The a-oriented GaN layer at the top of Figure 2a is drawn as a yellow block instead of the usual ball-and-stick representation because its [0001] direction is tilted 35.3° away from the ZB1 [101] direction, or equivalently, the [1120] direction of WZ2. As a result, the projection of the atoms in this domain onto the plane of the paper does not fall on one another. An unmarked stacking fault parallel to the plane of the paper occurs between ZB1 and the a-oriented GaN. The orientation of the top surface depends on the transition. Note that there are no distorted bonds at any domain boundary. The crystallographic axes of ZB1 and WZ2 are drawn at the top and bottom, respectively. WZ and ZB domains with the same numbers have the same crystallographic orientations.
deposited, simply extending the total thickness. These thick films self-separate from the sapphire substrate due to voids spontaneously formed within the transition layers.33 Thus, selectivity among the intermediate orientations within the
transition layer allows overall controllability of the orientation. The key advances presented here are therefore (1) growth of a zincblende GaN transition layer on a non-c-oriented sapphire substrate in a kinetically limited low-temperature growth regime
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Figure 3. (a) X-ray scattering results for sample A with the beam incident from the top. The (1120) and (0004) Bragg peaks of the a- and c-oriented GaN layers, respectively, are seen. The latter peak is weak since the overlayer is 40 μm thick. (b) The (0002) and (1120) Bragg peaks of the a- and c-oriented GaN films, respectively, with the beam incident on the sides of the films. δ = 10° implies that the [1120] direction of the c-oriented GaN layer is 21.7°(= (57.9° 34.6°)/2 þ 10°) away from the [0002] direction of a-oriented GaN. The insets show the X-ray scattering geometries, with incident and scattered beams denoted by arrows.
and (2) the precise selection of the orientation within the transition layer for the subsequent high-temperature growth of GaN. The precise selection is possible by controlling growth duration of LTG GaN. We demonstrate that this process allows us to actively control orientations even to the extent of fabricating free-standing unipolar films that have the same polarity on both surfaces along the polar direction. New applications based on this novel unipolarity capability can result. This approach might be readily extended to other materials that also exhibit polytypism, for example, other III-nitrides, ZnO and related oxides, and SiC. Experimental Section All the layers in this work were grown in a conventional horizontal hydride vapor phase epitaxy system at atmospheric pressure. The GaCl group-III precursor was generated by passing HCl of a Ga boat with H2 used as the carrier gas. Prior to deposition of GaN, the r-sapphire substrates were nitrided at 1080 °C for 90 min in NH3. Epitaxial GaN layers were grown at 550 °C on nitrided sapphire substrates. The thickness of sample A was 700 nm, and that of samples B and C was 900 nm. The samples were subsequently annealed under a mixture of H2 and NH3 during a 30-min temperature rise to 1040 °C and for an additional 10 min after the temperature reached 1040 °C. The same annealing condition is applied to all three samples. Following this, 180-μm-thick GaN films were grown on the annealed LTG GaN films at 1040 °C. The GaN layers self-separated upon cooling to room temperature due to the voids spontaneously formed during the annealing process.33 Then, 40- and 60-μm-thick GaN films were grown at 1040 °C on samples A and B. On the separated sample C, an additional 600-nm-thick layer of GaN was grown at 550 °C followed by a 100-μmthick GaN layer at 1040 °C. Schematic diagrams of the sample configurations are shown in Figure 1. Note that the orientations change
Figure 4. Scanning electron micrographs of the etched surfaces: (a) etched bottom surface of sample A; (b) etched top surface of sample B; and (c) etched top surface of sample C. The surface morphologies of (a) and (c) exhibit many hexagonal pyramids indicating severe etching . In contrast, that of (b) is nearly intact, with an occasional hexagonal pit. These characteristics are representative of [0001]and [0001]-oriented surfaces, respectively, wet chemical etched with KOH.34 The scale bars are 1, 5, and 1 μm, respectively. within the LTG zincblende GaN layers, as illustrated by the graded colors. The different thicknesses of the LTG GaN layers in samples A and B enable us to select the location within the transition layer where self-separation takes place. Since this occurs at different positions, the orientations of the separated films are consequently different in samples A and B. Crystallographic measurements and the results of surface etching, presented below, show that samples A, B, and C, which were grown on the self-separated templates, are oriented along [1120], [0001], and [0001], respectively. The specific film consists of not a single domain but multiple domains due to the very nature of the way in which polytypism enables the orientation transition to occur. As revealed in our previous report,33 a single domain is as large as 100 μm.
Results and Discussion The ball-and-stick models presented in Figure 2 show that the transitions from [0001] orientation to [1120], [0001], and [0001] are possible with different arrangements of intermediate zincblende domains. Note that the only extended defects involved in these orientation transitions are inversion domain boundaries and stacking faults. Unlike misfit dislocations, these defects themselves do not accommodate strain. Thus, it can be inferred that the polytypism-assisted orientation
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Figure 5. Two-dimensional contour maps of X-ray θ-rocking curves of the (000 ( 2) Bragg peak for samples B and C: (a) top and (b) bottom surface of sample B; (c) top and (d) bottom surface of sample C. The insets show the geometries. The blue and orange arrows represent the predicted 3.7°-tilt from the [000 ( 1] directions. The red arrows denote the surface normal directions about which the samples are rotated. The [000 ( 1] direction lies in the scattering plane for φ = 0 or 180°. The resolution of the detector along the direction perpendicular to the scattering plane picks up at φ = 15 and 165° the shoulders of the peaks. The offset angles of the (0002) planes with respect to the sample surface are -3.3° at φ = 0 and 180 for both top and bottom surfaces of sample B. For sample C, the offsets are -3.3° at φ = 0 and 180 for top surface. However, for bottom surface the offsets are (3.3° at φ = 0 and 180. The magnitudes and signs of these offset angles agree well with predicted values.
transition is not strain-induced. Figure 2a provides an inverted view of the transition layer grown on an r-plane sapphire substrate. The self-separation of sample A takes place in a WZ1 (a-oriented) domain such that the exposed surface is [1120]. The subsequently regrown layer on this separated template retains the same orientation. This model predicts that the [1120] direction of the c-oriented GaN is tilted by 24.7° with respect to the [0001] direction of a-oriented GaN. On the other hand, the self-separation of the 900-nmthick LTG GaN layer takes place in a ZB3 domain. As a result, the orientation of sample B, which was subsequently regrown at 1040 °C on this template, is [0001] as shown in Figure 2b. Our model predicts that the [0001] and [0001] axes are tilted by 3.7° in opposite directions. The LTG GaN of sample C has the same thickness as that of sample B, so the exposed surface of the self-separated GaN template is ZB3. However, the additional low-temperature growth of a 600-nmthick layer of GaN introduces a further orientation transition such that the final orientation is [0001], as shown in Figure 2c. In comparison with sample B, the WZ2 and WZ3 domains of sample C are predicted to be tilted by 3.7° in the same direction with respect to the surface normal. The prediction of the crystallographic orientations of the HTG GaN domains of sample A agrees well with the data. Figure 3a,b shows the X-ray diffraction results of sample A for the X-ray beam incident on the top and side of the sample, respectively. In Figure 3a, the (1120) and (0004) Bragg peaks can be seen in the θ-2θ scan. This implies that the top and bottom layers are a- and c-oriented, respectively. With the X-ray beam incident on the side, the φ angle was scanned with 2θ fixed at 34.6° and 57.9°, which correspond to the Bragg angles for (0002) and (1120), respectively (see Figure 3b). At
2θ = 34.6°, two peaks corresponding to (0002) and (0002) are observed 180° apart. On the other hand, at 2θ = 57.9°, three peaks instead of 6 of the {1120} family are observed because the scattering plane is 3.7° away from the c-plane of the c-oriented film. We note that the (0002) and (1120) peaks of the a- and c-oriented samples, respectively, are 10° apart, which implies that the (1120) plane of the c-oriented GaN sample is tilted by 21.7° (= (57.9° - 34.6°)/2 þ 10°) with respect to the (0002) plane of the a-oriented film. This is in good agreement with the theoretically predicted value of 24.7° given above. The predicted polarities of either [0001] or [0001] surfaces in our samples are also confirmed experimentally. One of the simplest methods to determine the polarity of GaN surfaces is wet chemical etching in KOH. The two polar surfaces exhibit very distinct characteristics in this process. The [0001]oriented surface remains almost intact with sparsely distributed hexagonal pits. In contrast, the [0001]-oriented surface is severely etched and covered by many hexagonal pyramids.34 Figure 4a,c shows the typical morphology of the [0001]oriented surface after wet-chemical etching. The bottom surface morphologies of samples B and C exhibit the same features as that of sample A in Figure 4a. This implies that the bottom surfaces of all samples and the top surface of sample C are [0001] oriented. On the other hand, Figure 4b shows the characteristics representative of a [0001]-oriented surface. This confirms the opposite orientation of the top surface of sample B. The minute crystallographic tilt angles of sample B and C predicted by the atomistic model are experimentally reproduced as well. Figure 5a,b shows two-dimensional contour maps for X-ray θ-rocking curves of the (000 ( 2) Bragg peaks
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with different φ angles for top and bottom surfaces, respectively, of sample B. Figure 5c,d shows the corresponding data for sample C. These contour maps clearly reveal that the crystallographic directions of the [000 ( 1] are tilted 3.3° with respect to both top and bottom normal vectors. In addition, the [000 ( 1] axes of sample B are seen to be tilted in opposite directions, whereas those of sample C are tilted in the same direction. In short, every experimental result regarding the magnitudes and directions of the crystallographic tilts is in excellent agreement with the predictions of the atomistic model. Detailed investigation with transmission electron microscopy is in progress to better understand the mechanism for the control of crystallographic orientations. Conclusions We have shown that the orientations of GaN films in the growth direction can be actively controlled by taking advantage of polytypism and that our atomistic model successfully explains the orientation transitions that we are controlling. Our approach, which capitalizes on polytypism, can be applied to the many other compound semiconductors as well. This provides a new degree of freedom for engineering material properties of epitaxial films independent of substrate orientation and, as we have shown, makes possible the design of films with new properties such as unipolarity. Acknowledgment. H.J.L. would like to thank the Japan Society for the Promotion of Science (JSPS) for providing financial support in the form of a JSPS Research Fellowship for Young Scientists. C.K. was supported in part by National Research Foundation of Korea Grant funded by the Korean Government (2009-0067662) and the grant from the Industrial Source Technology Development Programs (2009-F01401) of the Ministry of Knowledge Economy (MKE) of Korea.
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