APPLIED PHYSICS LETTERS 91, 021914 共2007兲

Dislocation reduction in GaN grown on stripe patterned r-plane sapphire substrates Hou-Guang Chena兲 Department of Materials Science and Engineering, I-Shou University, Kaohsiung 840, Taiwan

Tsung-Shine Ko, Shih-Chun Ling, Tien-Chang Lu, Hao-Chung Kuo, and Shing-Chung Wang Department of Photonics, National Chiao Tung University, Hsinchu 300, Taiwan and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan

Yue-Han Wu and Li Chang Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan

共Received 4 December 2006; accepted 10 June 2007; published online 13 July 2007兲 Extended defect reduction in GaN can be achieved via direct growth on stripe patterned 共1 ¯1 02兲 r-plane sapphire substrates by metal organic chemical vapor deposition. The striped mesa is along 关11 ¯2 0兴 with two etched sides in 兵0001其 and 兵1 ¯1 01其 faces. GaN grown on both etched facets in epitaxy exhibit different crystallographic relationships with sapphire substrate which are 共1 ¯1 02兲sapphire 储 共11 ¯2 0兲GaN and 关11 ¯2 0兴sapphire 储 关 ¯1 100兴GaN, and 共0001兲sapphire 储 共0001兲GaN and 关11 ¯2 0兴sapphire 储 关 ¯1 100兴GaN, respectively. The dislocation densities can be significantly reduced through epitaxial growth on the inclined lateral faces of mesas. Dislocation density in the order of ⬃107 cm−2 can be achieved in the tilted GaN. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2754643兴 GaN and other related III-nitride based semiconductors are promising materials for applications in light-emitting devices covering the ultraviolet and full visible ranges of the electromagnetic spectrum. However, the intrinsic spontaneous and extrinsic piezoelectric polarizations which are always present in GaN-based heterostructure grown with the orientation of basal plane of wurtzite structure can degrade electronic and optical properties.1 Thus, epitaxial growth of nonpolar III-nitride to prevent the polarization effect has attracted intensive attention in recent years and is regarded as one of the important solutions for high efficiency and high performance of nitride-based device applications.2,3 Due to large lattice mismatch between GaN and sapphire substrates, highly defective GaN films are always formed by conventional metal organic chemical vapor deposition process.2,3 In order to reduce defects as demanded for device applications, epitaxial lateral overgrown 共ELOG兲 and Pendeo epitaxy 共PE兲 approaches have been widely proposed to reduce the defect density.4–6 Although the ELOG and PE processes can dramatically eliminate most of dislocations, the two-step growth and regrowth process is too complicated and time consuming. In addition to ELOG and PE, one-step GaN growth on maskless patterned sapphire substrates 共PSSs兲 is a simplified method to enhance device performance and efficiency.7–10 Although PSSs may not reduce the dislocation density to the order of magnitude that ELOG can achieve, the geometrical effect of patterned substrate can effectively enhance light extraction. In this letter, we demonstrate that GaN grown on patterned r-plane sapphire substrates with asymmetric inclined facets can have a very low defect density in GaN epilayers without regrowth process. In our study, fabrication of pattern of sapphire substrates with asymmetric inclined crystallographic facets was illusa兲

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trated as follows. A SiO2 mask was deposited on 共1 ¯1 02兲 r-plane sapphire by plasma-enhanced chemical vapor deposition. The mask pattern consisted of 1 ␮m wide stripe oriented along the 关11 ¯2 0兴 direction and 6 ␮m wide opening that were defined by a standard photolithography process. The sapphire substrates were then wet etched using H3PO4-based solution at 300 ° C for 5 min. Afterward, the sample was dipped into buffered oxide etch solution 共NH4F : HF = 6 : 1兲 to remove the SiO2 mask for the following epitaxial growth. Figure 1共a兲 shows a scanning electron mi-

FIG. 1. 共a兲 SEM image of patterned sapphire substrate showing the etched facets on both sides of the striped mesa which are exhibited in different inclined angles. 关共b兲 and 共c兲兴 Various magnified top-view SEM images of striped GaN grown on patterned sapphire. Based on the variation of image contrast, the striped GaN can be divided into five different regions. 共d兲 Cross-sectional SEM image taken in tilt-view showing that each GaN stripe consists of two crystallites 共as indexed by GaN I and GaN II兲 with facets in different orientations.

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FIG. 2. Bright-field 共BF兲 TEM image. 共a兲 Low-magnification image showing the periodicity of GaN crystallites. 共b兲 High-magnification image showing one complete GaN stripe. 共c兲 Two sets of 关1 ¯1 00兴 zone axis GaN electron diffraction patterns from GaN I and GaN II with a rotation angle of ⬃57.4°. 共d兲 Enlarged image of sapphire striped mesa.

croscope 共SEM兲 image of a patterned sapphire substrate with both etched sides of the striped mesa which exhibit facets in different inclined angles due to the low crystallographic symmetry characteristic on r-face sapphire oriented along the 关11 ¯2 0兴 direction. The patterned substrate was loaded into a low-pressure metal organic chemical vapor deposition system. A 30 nm thick AlN buffer layer was grown at 600 ° C, followed by bulk GaN growth at 1120 ° C with low V / III ratio of ⬃900– 1200 and a pressure of 100 Torr. The morphology observation of GaN was carried out in a SEM 共JEOL-7000F兲. Cross-sectional thin sample was prepared for transmission electron microscopy 共TEM兲 observation using conventional mechanical polishing and Ar+ ion milling at 3.5– 4 kV. TEM was performed on a Philips Tecnai 20 microscope. The top-view SEM images of GaN grown on patterned sapphire are presented in Figs. 1共b兲 and 1共c兲. GaN stripes grown on the striped pattern of sapphire can be observed in Fig. 1共b兲. In the high-magnification image of Fig. 1共c兲, the GaN stripes exhibit periodical image contrast along the direction normal to the stripes. Within each period, the contrast varies from one to another region. According to SEM image contrast, the GaN stripe can be divided into five regions, as designated in Fig. 1共c兲. Also, it can be seen that the density of pits is varied on these five regions. Particularly in region 2, we can see a much larger quantity of pits than in the rest of the regions. In order to investigate the cross-sectional profile of GaN stripes from which one can realize the three dimensional morphology of GaN stripe, a cross-sectional sample was prepared by cleaving along the sapphire 关 ¯1 101兴 direction. A tilt-view SEM image 关Fig. 1共d兲兴 shows that each period of GaN stripe consists of two differently oriented crystallites 共as indexed by GaN I and GaN II兲 terminating with facets. Each terminated facet has the corresponding im-

age contrast and feature shown from the regions in Fig. 1共c兲. The microstructure of the GaN stripe was studied by TEM. A typical low-magnification bright-field 共BF兲 TEM image shown in Fig. 2共a兲 demonstrates the periodicity of GaN crystallites 共GaN I and GaN II兲 which have already been seen in SEM. Figure 2共b兲 in high magnification shows one complete GaN stripe with the corresponding selectedarea electron diffraction pattern in Fig. 2共c兲. The diffraction pattern shows that there are two sets of 关1 ¯1 00兴 zone axis GaN reflections from GaN I and GaN II with a rotation angle of ⬃57.4°. The crystallographic orientation relationships between GaN stripes and sapphire substrate are then determined to be 共1 ¯1 02兲sapphire 储 共11 ¯2 0兲GaN and ¯ ¯ 储 for GaN I, and 关11 2 0兴sapphire 关 1 100兴GaN 共0001兲sapphire 储 共0001兲GaN and 关11 ¯2 0兴sapphire 储 关 ¯1 100兴GaN for GaN II, respectively. Clearly, GaN I has 共11 ¯2 0兲 a plane on the top surface with 共0001兲 c plane at the lateral one. The region 共region 2兲 above the sapphire striped mesa contains a very high density of defects, such as threading dislocations 共TDs兲 and stacking faults 共SFs兲 which are usually observed in epitaxial a-plane GaN grown on r-plane sapphire. However, the defect density in GaN I is much reduced with the distance away from the mesa in the lateral direction. Examination of the neighboring crystallite GaN II shows that the top surface parallel to the r plane of sapphire is 共11 ¯2 2兲 facet with a narrow width, while the remaining two inclined facets are 共11 ¯2 0兲 and 共0001兲 planes. Referring to SEM, we can see in Fig. 2 that regions 3–5 in GaN II contain lower densities of defects. Therefore, the defect density of each region has a close correlation with the number of pits observed in SEM. As previously mentioned, the low symmetry of r-face sapphire along 关11 ¯2 0兴 causes the emergence of asymmetrically etched facets between both sides of the striped mesa. Figure 2共d兲 shows an enlarged image of the sapphire striped mesa, and each etched facet near 兵0001其 and 兵1 ¯1 01其 planes, respectively, deviated from nominal faces by several degrees, based on the measurement of the adjacent angle among the facets. Notably, the GaN II which has an extremely low defect density directly grows on the 兵0001其 etched facets of the mesa. The measured defect densities 共including TDs and SFs兲 corresponding to each GaN facet region are summarized in Table I. In order to further investigate the defect distribution in the GaN stripe, the g-3g weak beam dark field technique was employed. Figures 3共a兲 and 3共b兲 show the dark field images of GaN I taken from g = 11 ¯2 0 and 0002, respectively. A very high density of dislocations and stacking faults can be observed in region 2 where GaN grows directly on part of the top 共1 ¯1 02兲 face and the side 共1 ¯1 01兲 face of the mesa. However, the lateral region 共region 1兲 contains a relatively lower dislocation density than region 2. Based on the g · b

TABLE I. Defect densities in different regions with corresponding terminated facets.

Terminated facet Defect density 共cm−2兲

Region 1

Region 2

Region 3

Region 4

Region 5

共11 ¯2 0兲

共11 ¯2 0兲

共11 ¯2 0兲

共11 ¯2 2兲

共0001兲

⬃4 ⫻ 108

⬎1 ⫻ 1010

共5 ⫻ 107兲 – 108

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FIG. 3. g-3g dark field images of 关共a兲 and 共b兲兴 GaN I, and 关共c兲 and 共d兲兴 GaN II taken from g = 11 ¯2 0 and 0002 conditions, respectively. 共e兲 TEM image of GaN grown on the top face of sapphire mesa 共region 2兲 and neighbor region 共region 1兲 during the initial growth stage 共5 min兲.

rule, where b is the Burgers vector of dislocation, the majority of dislocations in region 1 are in contrast in dark field image with g = 0002 关Fig. 3共b兲兴, indicating that a large fraction of dislocations may be edge and mixed types. The GaN II was also characterized individually by the weak beam dark field imaging technique. It is remarkable that the dislocation density can be found to be extremely low in the dark field image taken from g = 11 ¯2 0 and 0002 conditions, respectively, as shown in Figs. 3共c兲 and 3共d兲. It is noticed that threading dislocations from both sapphire r plane and c plane to the GaN II surfaces are hardly found. From the dark field image in Fig. 3共c兲, dislocation bending toward the 关11 ¯2 0兴 direction can be observed, as indicated by the short white arrow. These bending dislocations terminate finally at the boundary between GaN I and GaN II; therefore, very few dislocations extend to terminated facets 共regions 3–5兲 so that very few pits can be found in the SEM images 关Figs. 1共c兲 and 1共d兲兴. The unique asymmetric sidewall structure leads to two distinct crystallites with different orientations grown on both sides of stripes. From TEM observations of the initial growth stage 共5 min兲 of GaN I, as shown in Fig. 3共e兲, it is noticeable that the GaN nuclei with smaller sizes were grown in region 1 where sapphire surfaces have various degrees of

tilted angles toward the sapphire 关1 ¯1 01兴 direction. Therefore, the off-angle surface might result in the difference between the GaN growth rates of the two regions. Hence, the growth of GaN would originate from the top face of the mesa 共region 2兲, and then the growth front would laterally extend to region 1, due to faster growth rate along the 关0001兴 direction. Thus, the lateral growth mechanism for ELOG and PE growth methods may be applied for the GaN I growth of reduced defect region 共region 1兲. On the other hand, the growth directions of GaN II are likely constrained by GaN I and sapphire substrate. The competing growth mode of the two crystallites 共GaN I and GaN II兲 can be evidenced by the zigzag interface, as shown in Fig. 3共c兲. The growth directions of GaN II are toward parallel and perpendicular to the inclined 共0001兲 face of sapphire mesa. In addition, the inclined 共0001兲 face with a finite area might lead to a special stress gradient field that induces the dislocation bending just above the inclined side face of the mesa. In conclusion, a low dislocation density 共⬃107 cm−2兲 in GaN grown on stripe patterned r-plane sapphire can be achieved through epitaxial growth on inclined step facets without regrowth process. Thus, the development of the appropriate mesa structure and growth condition, in particular, growth rate ratio of 共0001兲 face to 共11 ¯2 0兲 face is critical for obtaining epitaxial nonpolar GaN of low dislocation density with an applicable size. Clearly, understanding the mechanism for dislocation reduction may open a new window for growing high-quality nonpolar crystals of wurtzite structure 共GaN, AlN, ZnO, etc.兲 in epitaxy. This work was supported by the MOE ATU program and in part by the National Science Council of the Republic of China 共R.O.C.兲 in Taiwan under Contract Nos. NSC 952120-M-009-008, NSC 95-2752-E-009-007-PAE, and NSC 95-2218-E-214-003. 1

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