The Optical Characteristics of Epitaxial Lateral and Vertical Overgrowth of GaN on Stripe-Patterned Si Substrate

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1 Journal of the Korean Physical Society, Vol. 50, No. 3, March 2007, pp The Optical Characteristics of Epitaxial Lateral and Vertical Overgrowth of GaN on Stripe-Patterned Si Substrate H. Y. Yeo, S. I. Jung and I. Yun Department of Electrical & Electronic Engineering, Yonsei University, Seoul J. I. Lee Advanced Industrial Metrology Group, Korea Research Institute of Standards and Science, Daejeon I. K. Han Nano Devices Research Center, Korea Institute of Science and Technology, Seoul S. M. Cho Department of Chemical Engineering, Sungkyunkuwan University, Suwon (Received 1 September 2006) GaN thin layers were grown on stripe-patterned silicon (111) substrates with an AlN buffer layer by using metal organic chemical vapor deposition. The Si substrates were structured with various pattern sizes of 2.95, 7.85, and 18.2 µm by using a conventional photolithography and inductively coupled plasma etching process. The GaN layer on the stripe-patterned Si extends vertically and laterally during the growth. Photoluminescence (PL) spectra show a band-edge emission at 3.37 ev corresponding to the exciton recombination and a blue one around 2.8 ev corresponding to the transition associated with intrinsic defects such as oxygen and hydrogenated gallium vacancies. From the µ-pl spectra, the intensity of the band-edge emission from the laterally overgrown region of GaN layer is stronger than that from the vertical one, and the blue emission is not observed for the lateral one. Especially, when the pattern size was 7.85 µm, a 7.02-fold increase in PL intensity for the laterally overgrown region was obtained in comparison with that for the vertical one. We conclude that the enhancement of the optical properties for lateral overgrowth of GaN layers depends on the pattern size of the Si substrate and results from a decrease in the dislocation density. PACS numbers: Eq, Ea, Fy Keywords: ELO, MOCVD, GaN, Patterned Si, Photoluminescence, Lateral overgrowth I. INTRODUCTION A silicon substrate is a good candidate for GaNbased optoelectronic devices such as wide-band-gap light-emitting diodes, lasers, and photodetectors because of it s low cost, large-scale availability, and good thermal and electrical conductivities [1,2]. However, compared to sapphire or silicon-carbide substrates, it s difficult to obtain high-quality GaN on a Si substrate without cracks due to the large tensile strain in GaN coming from the large difference in the lattice constants and the thermal expansion coefficient between GaN and Si. In order to reduce these mismatches, AlN has been used as a buffer Corresponding Author: iyun@yonsei.ac.kr; Fax: ; Corresponding Author: jilee@kriss.re.kr; Fax: layer because it has a smaller lattice (Si: 19 % vs. AlN: 2.4 %) and thermal expansion mismatch (Si: 56 % vs. AlN: 26 %) with GaN than Si [3 6], but a high density of dislocations still results. Common selective growth methods, such as epitaxial lateral overgrowth (ELO) or pendeo-epitaxy, were demonstrated to reduce the density of threading dislocations [7, 8]. Although, these techniques have been reported to improve the crystal quality, the problems related to impurity contamination from the selective growth mask and to the limitations of the mask materials arose. More recently, the maskless epitaxial lateral overgrown GaN on a patterned Si substrate by using metalorganic chemical vapor deposition (MOCVD) has been proposed to reduce the density of threading dislocations and the strain in GaN layer simultaneously [9 13]. In compari

2 -772- Journal of the Korean Physical Society, Vol. 50, No. 3, March 2007 son with planar Si substrates, the growth on patterned Si substrates provides lateral, as well as vertical, overgrowth of GaN layers due different surface conditions. The lateral overgrowth of GaN is an efficient way to achieve high-quality GaN layers when the GaN layers are grown on Si substrates. Moreover, this method enables the Si surface to eliminate potential sources of additional impurities and strain incorporation into the growing layer because no mask is used [9]. We have successfully grown GaN layers on prepatterned Si substrates by using the maskless ELO technique that leads to good crystal quality with a lower threading dislocation density. In this paper, we will demonstrate the structural properties of the overgrown GaN layer, using a scanning electron microscope. Moreover, we report on the differences in the optical properties between the vertically and the laterally overgrown region and on the pattern-size-dependent PL obtained by using µ-pl measurements. II. EXPERIMENTS The GaN layers were grown on Si (111) by using MOCVD with a horizontal quartz reactor. Before GaN layer growth, the phosphorus-doped n-type Si (111) substrates were patterned with periodic stripes by using conventional photolithography and an inductively coupled plasma. The pattern consists of parallel stripes aligned along the Si <1-10> direction with dimensions of about 4 5 µm width and 2 3 µm in depth with 2.95, 7.85, and 18.2 µm spacings. After preparation, the Si (111) substrates were heated to 900 C under a hydrogen ambient for 5 minutes. Pre-deposition of Al was performed for 20 sec just prior to introducing ammonia into the reactor to avoid nitridation of the substrate surface before the AlN growth. A 70-nm-thick HT-AlN buffer layer was deposited at 1020 C by feeding triethylaluminum and ammonia with hydrogen as a carrier gas. Subsequently, a 1.2-µm-thick GaN epilayer was grown at 1020 C for 20 min. The reactor pressures were 100 and 300 Torr for the growth of AlN and GaN, respectively. No dopants were intentionally introduced during the growth. The samples were studied by using scanning electron microscopy (SEM) in order to observe the structural properties of overgrown GaN layers on patterned Si substrates. To investigate the optical properties of the overgrown GaN layer we used, photoluminescence was applied at room temperature with a 325-nm He-Cd laser as the excitation source. The luminescence signal was dispersed by using a 1-m monochromator and was detected by using a photomultiplier tube detector. For the comparison between the vertical and the lateral overgrown regions of GaN, µ-pl measurements were conducted at room temperature. A 325-nm He-Cd laser beam that was focused via an objective lens to 4 5 µm diameters on the sample surface was used. Fig. 1. SEM cross-sectional image of GaN/Si(111) (Terrace Trench): (a) sample A: 2.95 µm 4.35 µm, (b) sample B: 7.85 µm 4.55 µm, and (c) sample C: 18.2 µm 5.15 µm. III. RESULTS & DISCUSSION In order to observe the vertical and the lateral overgrowth of the GaN layer, we performed a SEM analysis. Fig. 1 presents the cross-sectional SEM images of GaN epilayers on patterned Si substrates, including different terrace widths of the patterns: 2.95 µm (sample A), 7.85 µm (sample B) and 18.2 µm (sample C) in size. Due to the different surface conditions with trenches (etched area) and terraces (un-etched area), the lateral overgrowth was formed on top of the trenches while GaN layers were vertically overgrown on top of the terraces. In the initial stage of the GaN growth process, the growth of the GaN layers started both on the bottoms of the trenches and on the tops of the terraces. As the growth proceeded, the GaN layers on the tops of the terraces

3 The Optical Characteristics of Epitaxial Lateral H. Y. Yeo et al Fig. 2. Room-temperature photoluminescence spectra for samples A, B, and C. laterally extended over the trench area with a growth rate higher than that of GaN grown on the trench areas. Finally, the overgrowth of GaN layer on patterned Si resulted in a coalescence of adjacent lateral growth fronts, forming of a continuous layer and leaving voids, as shown in Fig. 1. For the investigation of the optical properties of the overgrown GaN layer, PL measurement was performed at room temperature. In Fig. 2, samples A, B and C exhibit the same results for the three PL bands observed: a strong band-edge emission at 3.37 ev corresponding to the exciton recombination, and weak blue and yellow band emissions around 2.8 ev and 2.2 ev, respectively. Both the blue luminescence and the yellow luminescence are related to the generation of defects acting as a nonradiative centers and provide evidence of poor crystalline quality [14]. These emissions are due to transitions of free electrons in the conduction band or the shallow donor to deep acceptor level associated with intrinsic impurities such as oxygen and hydrogenated gallium vacancies [15]. In the case of the GaN layer on a planar Si (111) substrate, mismatch occurs over the whole interface between the Si substrate and the GaN layer due to the difference in the lattice constants. During the cooling of the samples to room temperature, a large tensile stress caused by a thermal expansion coefficient incompatibility is added. Consequently, these dislocations propagate along the growth direction and destroy the active layer, which contributes to a deterioration of the optical properties such as near band-edge emission. On the other hand, a stripe-patterned Si substrate is very useful for reducing the threading dislocation density because it provides not only vertical overgrowth but also lateral overgrowth of GaN layers. To compare the optical properties between laterally and vertically overgrown regions of GaN/Si (111), we carried out µ-pl at room temper- Fig. 3. Normalized micro-pl spectra taken from different surface areas at room temperature: (a) sample A, (b) sample B, and (c) sample C. In all spectra, the solid lines were measured on the vertical overgrowth of GaN, and the dash lines were measured on the lateral overgrowth of GaN. ature. A 325-nm He-Cd laser was used as an excitation source. The µ-pl measurement enabled us to obtain optical information at different spots on the overgrowth because the size of the beam was estimated to be around 4 5 µm in diameter. Fig. 3 shows the µ-pl spectra measured for lateral and vertical overgrowth of the GaN layer. In all samples (sample A: 2.95-µm terrace, B: 7.85-µm terrace, and C: 18.2-µm terrace), the intensity of the band-edge emission that comes from the lateral overgrowth is stronger than that of the vertical one, and

4 -774- Journal of the Korean Physical Society, Vol. 50, No. 3, March 2007 bending of vertical dislocations observed by Haffouz et al. [10]. The dislocations first propagate vertically, but then bend to adopt a horizontal direction, in the basal plane. This behavior occurs because the surface area is not large enough to relax the thermal strain; thus, the vertical dislocations bend toward the laterally overgrown region. These dislocations are known to influence the crystal quality and to suppress the band-edge emission efficiency of the lateral overgrowth. Consequently, we believe that the low PL intensity of sample A for the laterally overgrown region can be attributed to an increase in the bending of dislocations. IV. CONCLUSION Fig. 4. Dependence of the room-temperature micro-pl spectra on pattern size: Comparison of PL spectra measured on (a) the lateral overgrowth to that measured on (b) the vertical overgrowth of GaN. the intensity ratio between the two regions is 1.87, 7.02, and 5.92 for samples A, B, and C, respectively. Moreover, the blue luminescence from the lateral overgrowth definitely disappeared while PL spectra of the vertically overgrown region included the blue band emission. These results show that the optical properties were improved by reducing the dislocation density and the number of intrinsic impurities in the lateral overgrown region of GaN. The photoluminescence spectra depend on the pattern size, as shown in Fig. 4. Fig. 4(a) and (b) were measured for the lateral and vertical overgrowth of the GaN layers, respectively. In Fig. 4(a), the PL intensity of the 7.85-µm pattern is the most intense in band-edge emission. Conversely, the PL intensity of the 18.5-µm pattern is the lowest in the vertical overgrowth region as well as in the laterally overgrown region. This result is in good agreement with that of Zamir et al. [16], who found that thermal cracks were generated in the overgrown GaN layers and acted as nonradiative recombination centers when the pattern size was more than 14.0 ± 0.3 µm. They explained that a pattern size reduction could lead to enhanced crystal and optical properties. However, when the pattern size is too small, the crystalline quality of the lateral overgrowth declines, as shown in Fig. 4(a). We suppose that this phenomenon is similar to the An epitaxial lateral overgrowth technique was introduced in order to obtain highly crystalline GaN layers with patterned Si substrates. During the growth process of GaN, GaN layers were grown vertically and laterally. From the SEM images, we observed that the bottoms of the trenches were covered with laterally overgrown GaN layers, leaving voids. The band-edge intensity of the photoluminescence from laterally overgrown GaN/Si was strongly enhanced compared to that of the vertically overgrown GaN/Si layers. It can be concluded that this enhancement in the optical properties of the lateral overgrowth resulted from a reduction in the dislocation density. From the µ-pl results, we suggest that PL bandedge intensity of the lateral overgrowth is affected by the pattern size due to a increase in the crack density and a bending of vertical dislocations. ACKNOWLEDGMENTS The authors wish to express their thanks to Dr. Cheul- Ro Lee for sample preparation and to Sookhyun Hwang for the SEM data. REFERENCES [1] S. N. Mohammed, A. A. Salavador and H Morkoc, Proc. IEEE 83, 1306 (1995). [2] S. Nakamura and G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers (Springer, New York, 1997), Chap. 1. [3] E. S. Hellman, D. N. E. Buchanan and C. H. Chen, MRS Internet J. Nitride Semicond. Res. 3, 43 (1998). [4] R. E. Davis, T. Gehrkea, K. J. Linthicuma, E. Preblea, P. Rajagopala, C. Ronning, C. Zorman and M. Mehregany, J. Crystal Growth 231, 335 (2001). [5] D. K. Kim and P. B. Park, J. Korean Phys. Soc. 47, 1006 (2005). [6] P. Chen, R. Zhang, Z. M. Zhao, D. J. Xi, B. Shen, Z. Z. Chen, Y. G. Zhou, S. Y. Xie, W. F. Lu and Y. D. Zheng, J. Crystal Growth. 225, 150 (2001).

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