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Minimization of wafer bowing in GaN-based vertical light-emitting diodes by selective area growth using metal-organic chemical vapor deposition
Journal of Crystal Growth 314 (2011) 66–70
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Minimization of wafer bowing in GaN-based vertical light-emitting diodes by selective area growth using metal-organic chemical vapor deposition Jae Hyoung Ryu a, S. Chandramohan a, Hee Yun Kim a, Hyun Kyu Kim a, Ji Hye Kang a, Chang-Hee Hong a,n, Hyun Kyong Cho b, Hyun Don Song b, Ho-Ki Kwon b a b
School of Semiconductor and Chemical Engineering and Semiconductor Physics Research Center, Chonbuk National University, Chonju 561-756, Republic of Korea. Advanced Technology Laboratory, LED R&D Center, LG Innotek, Seoul 137-724, Republic of Korea.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 July 2010 Received in revised form 20 September 2010 Accepted 18 October 2010 Communicated by T.F. Kuech Available online 27 October 2010
The effect of selective area growth (SAG) on wafer bowing of GaN-based light-emitting diodes (LEDs) is investigated. The SAG of LED structures was carried out on a silicon dioxide (SiO2) mask pattern with periodic 1000 1000 mm openings, along the sapphire /1 1 0 0S and / 1 1 2 0S directions. The morphology of a selectively-grown n-GaN epilayer was examined in relation to various growth parameters such as temperature, pressure, and V/III ratio. Under optimized growth conditions, formation of a ridge-shaped epilayer with a v-pit free smooth surface was realized. Furthermore, the ridge-shaped vertical LED structure, after the removal of the sapphire substrate by laser lift-off (LLO) showed less wafer bowing compared with conventional vertical LED structures. This is attributed to the suppression of lateral strain and dislocations during the site-selective growth process, due to a reduction in the lateral dimensions. & 2010 Elsevier B.V. All rights reserved.
Keywords: A3. Selective epitaxy A3. Metal-organic chemical vapor deposition B1. Nitrides B2. Semiconducting III–V materials B3. Light-emitting diodes
1. Introduction Recently, GaN-based light-emitting diodes (LEDs) have attracted considerable interest due to their applications in optoelectronic devices for illumination, and back-light units in liquid crystal displays, with high efficiency and long life time [1,2]. GaN-based LEDs are most commonly grown on sapphire substrate [2]. However, sapphire being electrical insulator and due to its poor thermal conductivity, the p- and n-pads of GaN-based devices are conventionally grown on the surface of epitaxial layers. The use of a sapphire substrate complicates processing steps, such as the need to dry etch the n-GaN contacts in a p-side-up structure and mirror-cavity facets in a laser diode (LD) structure. The extraction of heat from an operating device through the sapphire substrate is also hampered. Therefore, free-standing GaN optoelectronic devices without sapphire substrate are most desirable. The laser lift-off (LLO) technique has been established as an effective procedure for GaN-based hetero-epitaxial structures, eliminating the constraint of the sapphire substrate [3–7]. Kelly et al. [3] used a frequency-tripled Nd:YAG laser at 355 nm, and Wong et al. [4] used an excimer laser at 248 nm to accomplish GaN film lift-off after irradiation f the structure through the transparent sapphire substrate. However, laser radiation with high energy density often results in
n
Corresponding author. E-mail address: [email protected] (C.-H. Hong).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.10.104
cracked GaN film [8,9]. One of the main sources of cracks in GaN films is bowing-induced large residual strain in the epilayers. Therefore, wafer bowing should be controlled for the production of crack-free LED structures. In this study, we demonstrate selective area growth (SAG) of vertical GaN-based LEDs with a reduced wafer bow by metalorganic chemical vapor deposition (MOCVD). The selective growth of large area (1000 1000 mm) ridge-shaped LED mesa structures on a SiO2 mask-patterned GaN template resulted in a substantial reduction in wafer bow due to the suppression of lateral strain. The optimization of growth conditions in the SAG process is also addressed. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution X-ray diffraction (HR-XRD) measurements were performed to characterize the properties of the SAG LED structures.
2. Experimental GaN epitaxial layers and InGaN/GaN multi-quantum well (MQW) structures were grown on a c-plane (0 0 0 1) sapphire substrate by MOCVD with an AIX200RF horizontal reactor system. Trimethylindium (TMIn), trimethylgallium (TMGa), and ammonia (NH3) were used as the sources of In, Ga, and N, respectively. Biscylopentadienyl magnesium (Cp2Mg) and silane (SiH4) were used as the p-type and the n-type dopant sources, respectively. The
J.H. Ryu et al. / Journal of Crystal Growth 314 (2011) 66–70
detailed processing sequences of SAG of GaN-based vertical LED structures are as follows: first, a thin (30 nm) GaN nucleation layer was deposited at a low temperature of 560 1C. Subsequently, a 1 mm-thick undoped GaN (u-GaN) epilayer at 1100 1C and a 2.5 mm-thick Si-doped n-type GaN epilayer at 1120 1C were deposited. A 100 nm-thick SiO2 layer was deposited onto the n-type GaN template by plasma-enhanced chemical vapor deposition (PEVCD). A mask pattern with periodic 1000 1000 mm mesa structure windows (each separated by 90 mm-wide SiO2 stripes) was created using conventional photolithography and buffered oxide etch solution. After the etching process, the sample was set into the MOCVD chamber for the epilayer re-growing process. The re-growth of n-type GaN was performed by varying the growth parameters such as temperature, pressure, time, and V/III ratio. The re-grown GaN was a 2.5 mm-thick Si-doped n-type GaN epilayer at optimized conditions. After completing the re-growth of the n-type GaN, six period MQWs of InGaN/GaN pairs with emission of about 460 nm were grown at 820 1C, and a 0.25 mm-thick Mg-doped p-GaN layer was grown at 1050 1C. In these experiments, N2 and H2 were used as carrier gas, respectively. Then, a 280 nm-thick transparent indium tin oxide (ITO) film was deposited to serve as a p-contact to the p-GaN. Subsequently, a Cu layer was deposited by e-beam evaporation, which served as a conductive seed layer for the subsequent electroplating process; here, Cu was electroplated for this purpose. Using an ArF excimer laser with a wavelength of 193 nm, the LLO process was performed to separate the LED structure from the sapphire substrate. The schematic representation of the different processes involved in the ridge-shaped vertical LED fabrication is shown in Fig. 1. For comparison, a similar LED structure was fabricated using the conventional MOCVD technique. The crystal quality of the epilayers grown under various growth conditions was examined by SEM, HR-XRD, and TEM. The magnitude of bending of the LED wafer was measured to obtain a quantitative understanding of wafer bowing.
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temperature, pressure, and V/III ratio. Figs. 2–4 show the crosssectional and plan view SEM images of the n-type GaN template and the selectively-grown epilayer under variable growth conditions. First, the n-type GaN epilayer was re-grown at various growth temperatures (1130, 1160, and 1180 1C) by keeping the pressure, V/III ratio, and growth time constant at 50 mbar, 60 min, and 1088 1C, respectively. Fig. 2 illustrates the variation in the surface morphologies as a function of growth temperature. It is evident from the plan view images that an increase in growth temperature reduced the formation of v-pits on the re-grown GaN epilayer. At a low growth temperature (1130 1C), the (0 0 0 1) surface degraded because the layer-by-layer growth was impeded due to poor surface migration of Ga atoms on the (0 0 0 1) surface. As a result, the surface is rich in v-pits. As the temperature increased, however, the surface migration is enhanced and hence the (0 0 0 1) surface become smooth [10]. The differences in center-to-edge heights at 1130, 1160, and 1180 1C were 0.4, 0.8, and 1 mm, respectively. Second, to investigate the re-growth of n-type GaN as a function of pressure, epilayers were grown at various pressure values (100, 50, and 20 mbar) at 1160 1C for 60 min with a V/III ratio of
3. Results and discussion The effect of different growth conditions on the morphology of the re-grown n-GaN epilayer was studied by varying the growth
Fig. 2. SEM images of selective area-grown n-GaN epilayer on GaN template for different growth temperature of 1130, 1160, and 1180 1C.
Fig. 1. A schematic of the fabrication process for ridge-shaped vertical LEDs: (a) formation of creative SiO2 mesa pattern, (b) growth of p–n junction and MQW structures, (c) formation of ITO layer for current spreading and passivation of mesa structures by SiO2 deposition, (d) copper electroplating on the ITO, (e) subsequent removal of sapphire substrate by LLO process, and (f) formation of vertical LED chip.
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1297. Fig. 3 shows the effect of reactor pressure on the morphology of the epilayer. The rough surface with v-pits observed at low pressure could be due to the increased evaporation rate of absorbed source molecules on the (0 0 0 1) surface [11–13]. We note that an
Fig. 3. SEM images of selective area-grown n-GaN epilayer on GaN template for different reactor pressure of 20, 50, and 100 mbar.
Fig. 4. SEM images of selective area-grown n-GaN epilayer on GaN template for different V/III ratios of 777, 1077, 1399, and 2238.
increase in the pressure value above 20 mbar led to considerable reduction in the v-pits, but also increased the center-to-edge height difference. For instance, the measured differences in the center-toedge heights are 0.3, 1, and 2.3 mm at 20, 50, and 100 mbar, respectively. Third, to investigate the re-growth of n-type GaN as a function of the V/III ratio, epilayers were grown under various V/III ratios (777, 1088, and 1399) for a fixed temperature of 1130 1C and a fixed pressure of 50 mbar. This was accomplished by varying the NH3 flow rate. Meanwhile, the total flow was kept constant and the quantity of TMGa flow during the growth was also fixed. The resulting morphologies of the re-grown n-GaN epilayer as a function of V/III ratio are shown in Fig. 4. We observe that the increase in V/III ratio degrades the epilayer quality, and there are many v-pits generated over the epilayer surface for a higher V/III ratio of 1399. In this case, the minimum difference in the center-toedge height was realized for a V/III ratio of 1088. To understand the influence of the gallium (Ga) flow rate, n-type GaN epilayers were grown at V/III ratios of 1399 and 2238 by varying the Ga flow rate. In this case, the quantity of NH3 flow during the growth was fixed at 4500 slm and the total flow was kept constant. As in the previous case, a decrease in the V/III ratio led to a reduction in the GaN growth rate. This is possibly due to the vapor phase reaction being dominant in the reactor under these conditions [14]. However, the surface morphologies of the n-type GaN layer re-grown at a high V/III ratio are found to be smooth. This is clearly shown in Fig. 4, where we did not observe any v-pits for a V/III ratio of 2238. In the preceding cases, as the V/III ratio increased, the surface morphologies improved but the v-pit increased. Alternatively, as the growth pressure increased, the v-pit decreased but the difference in the edge-to-center height increased. As the growth temperature increased, the v-pit decreased. Therefore, we needed to maintain low pressure, high temperature, and a high V/III ratio to reduce the edge-to-center height difference and the number of v-pits, and to obtain smooth surface morphologies. The optimized values of temperature, pressure, and V/III ratio are 1160 1C, 20 mbar, and 2238, respectively. The LED structure was grown on the optimized n-GaN template. Fig. 5 shows the HR-XRD spectra of the n-GaN template and the selectively-grown epilayer for symmetric (0 0 2) and asymmetric (1 0 2) reflection. It can be seen that both the template and the epilayer have almost identical intensities and full-width at half-maximum (FWHM) values. This observation indicates that the deposition of an n-GaN layer over the GaN template by SAG results in good quality epitaxial growth. The n-GaN epilayer growth over the SiO2 mask-patterned GaN template, examined by SEM, is shown in Fig. 6. Under optimized conditions, an n-GaN epilayer with a smooth top surface was
Fig. 5. X-ray rocking curves of n-GaN template and SAG epilayer. (a) (0 0 2) reflection and (b) (1 0 2) reflection.
J.H. Ryu et al. / Journal of Crystal Growth 314 (2011) 66–70
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Fig. 6. SEM images of selective area-grown n-GaN epilayer on GaN template at optimized conditions: (a) top-view, (b)–(d) cross-sectional views. The side walls of the ridge structure are shown in (b) and (d).
Fig. 7. TEM cross-section images of MQW structure at (a) the center and (b) the edge regions.
obtained (see Fig. 6a) simply by changing the crystal growth conditions without using the mesa etching processes, which usually causes severe damage to the device structure. The measured center-to-edge height difference was approximately 0.4 mm on the completed LED structure, which is slightly larger than the value measured after the growth of n-GaN epilayer (0.3 mm). This could be due to the growth of the p-GaN. Fig. 7 shows TEM images of MQW structures at the center and the side wall regions. The measured total thicknesses of six period MQWs at the center and the edge regions were 115 and 190 nm, respectively. Moreover, no growth of MQWs on the side facets of the ridge-shaped LED mesa structure was observed during the selective growth. This is a major advantage of SAG because device reliability or stability remains a concern in the case of conventional vertical LEDs because the side walls of the active region (MQW structure) are affected during dry etching. However, in the case of
SAG, the growth mechanism itself enables the epilayer to grow in the form of ridge-shaped mesa structures, as shown in Fig. 6(b), so that stability-related issues could be overcome by employing this growth process. Nevertheless, obtaining homogeneous growth on different micro-facets is often inhibited by the difference between the growth rates because of the facet-dependent diffusivity of surface adatoms [15]. In other words, the morphological changes were caused by differences in the stability of each surface, which depend mainly on the surface energy and stability of surface atoms. In selective growth, the specific structure and morphology are primarily controlled by adjusting the growth temperature and the reactor pressure [16]. As previously mentioned, an increase in growth temperature (1130–1160 1C) enabled us to obtain a smooth facet growth due to enhanced surface migration, provided the pressure was sufficiently low (20 mbar) to concurrently increase the lateral epitaxial growth rate. This smooth morphology enables the growth of uniform MQWs on the c-plane. Moreover, experiments on side wall growth indicate that the side wall that grows along the { 1 1 2 0} plane becomes a side wall {1 1 2 2} by forming an angle of approximately 571 to the sapphire substrate (Fig. 6). Similarly, the side wall that grows along the {1 1 0 0} plane becomes a side wall {1–1 0 1} by forming an angle of approximately 621 to the sapphire substrate. These observations are in agreement with the results reported by Cho et al. [17], wherein the semipolar {1 1–2 2} and {1–1 0 1} micro-facets were shown to make similar angles with the basal plane. The magnitude of bending of the LED wafer at three different points was measured and the results are shown in Fig. 8. Here, the values represent the height of the wafer top surface from a reference plane for two different configurations; i.e., for the GaN surface facing upward and vice versa. The insets in the figure show a schematic representation of the two different measurement configurations. The results clearly indicate that both growth methods lead to considerable wafer bowing; however, the features of bending distinctly differ from each other. For instance, SAG leads to convex behavior with a large radius of curvature, whereas conventional growth results in concave bending. By considering
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the removal of the sapphire substrate due to strain induced by high wafer bowing. Conversely, we see no such cracking in SAG LED structures with the ridge geometry.
4. Summary
Fig. 8. Bowing measurement of GaN LEDs grown by SAG and conventional methods.
In summary, we studied the effect of selective area growth on the wafer bowing of GaN-based vertical LEDs fabricated using MOCVD and a post deposition laser lift-off process. The siteselective re-growth of the n-GaN epilayer was investigated under different growth conditions. In this experiment, the optimum growth temperature, pressure, and V/III ratio for growing a defect-free epilayer were determined to be 1160 1C, 20 mbar, and 2238, respectively. Initially, the shape of the wafer bow was dissimilar for conventional and SAG LEDs, and the magnitude of bending was large for SAG LEDs. After the removal of sapphire substrate by LLO, a substantial relaxation in wafer bow was observed for SAG vertical LEDs compared with conventional vertical LEDs. These different bowing behaviors are attributed to growth mode-dependent lateral strain in the GaN. Our study provides information that could be useful for the further development of free-standing crack-free GaN-based LEDs.
Acknowledgment This work is financially supported by the Ministry of Knowledge Economy (MKE) of the Korean government and by the Korea Institute for Advancement in Technology (KIAT), through the Workforce Development Program in Strategic Technology and the IT R&D program of the MKE/KEIT (KI002163, Development of Core Technology for High Efficiency Light-Emitting Diode based on New Concepts). References
Fig. 9. Photographs of LED structure after the removal of sapphire substrate by laser lift-off: (a) SAG LED and (b) conventional LED.
that the growth temperatures and substrates are similar in the two cases, the different bowing behaviors observed can be ascribed to growth mode-dependent lateral strain in the GaN. In Fig. 9, the photographs show the LED structures after the removal of the sapphire substrate by LLO. Note that the bowing is significantly relaxed in the SAG LED structure, whereas it is further enlarged in the case of the conventional LED (the photograph was taken by reversing the sample to explicitly show the bowing effect). The low bowing of the LED structure grown by SAG is most likely due to suppression of the lateral strain and dislocations [18,19] because of the reduction in lateral dimensions; these are difficult to control in conventional growth processes. The top-view images of the LED mesa structures are also shown in Fig. 9. We see that the conventional vertical LED structures are prone to cracking after
[1] I. Akasaki, J. Cryst. Growth 300 (2007) 2. [2] H. Jia, L. Guo, W. Wang, H. Chen, Adv. Mater. 21 (2009) 4641. [3] M.K. Kelly, O. Ambacher, B. Dahlheimer, G. Groos, R. Dimitrov, H. Angerer, M. Stutzmann, Appl. Phys. Lett. 69 (1996) 1749. [4] W.S. Wong, T. Sands, N.W. Cheung, Appl. Phys. Lett. 72 (1998) 599. [5] W.S. Wong, M. Kneissl, P. Mei, D.W. Treat, M. Teepe, N.M. Johnson, Appl. Phys. Lett. 78 (2001) 1198. [6] C.F. Chu, C.C. Yu, H.C. Cheng, C.F. Lin, S.C. Wang, Jpn. J. Appl. Phys. 42 (2003) L147. [7] W.S. Wong, Y. Cho, E.R. Weber, T. Sands, K.M. Yu, J. Kruger, A.B. Wengrow, N.W. Cheung, Appl. Phys. Lett. 75 (1999) 1887. [8] J. Xu, R. Zhang, Y.P. Wang, X.Q. Xiu, B. Shen, S.L. Gu, Y. Shi, Z.G. Liu, Y.D. Zheng, Mater. Lett. (2002) 43. [9] J.-H. Cheng, Y.C.S. Wu, W.C. Peng, H. Ouyang, J. Electrochem. Soc. 156 (2009) H640. [10] K. Hiramatsu, K. Nishiyama, A. Motogaito, H. Miyake, Y. Iyechika, T. Maeda, Phys. Status Solidi (a) 176 (1999) 535. [11] H. Miyake, A. Motogaito, K. Hiramatsu, Jpn. J. Appl. Phys. 38 (1999) L1000. ¨ ¨ [12] Z. Bougrioua, M. Laugt, P. Venne´gue s, I. Cestier, T. Guhne, E. Frayssinet, P. Gibart, M. Leroux, Phys. Status Solidi (a) 204 (2007) 282. [13] Philippe Vennegues, Zahia Bougrioua, Tobias Guehne, Jpn. J. Appl. Phys. 46 (2007) 4089. [14] P. Gibart1, M. Leroux1, O. Briot, J.P. Alexis, S. Sanchez, B. Gil, R.L. Aulombard, Solid-State Electron. 41 (1997) 315. [15] M. Funato, T. Kotani, T. Kondou, Y. Kawakami, Y. Narukawa, T. Mukai, Appl. Phys. Lett. 88 (2006) 261920. [16] H. Miyake, A. Motogaito, K. Hiramatsu, Jpn. J. Appl. Phys. 38 (1999) L1000. [17] C.-Y. Cho, I.-K. Park, M.-K. Kwon, J.-Y. Kim, S.-J. Park, D.-R. Jung, K.-W. Kwon, Appl. Phys. Lett. 93 (2008) 241109. [18] K.-M. Chen, H.-H. Huang, Y.-L. Kuo, P.-L. Wu, T.-L. Chu, H.-W. Yu, W.-I Lee, J. Cryst. Growth 311 (2009) 3037. [19] X. Li, A.M. Jones, S.D. Roh, D.A. Turnbull, S.G. Bishop, J.J. Coleman, J. Electron. Mater. 26 (1997) 306.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Minimization of wafer bowing in GaN-based vertical light-emitting diodes by selective area growth using metal-organic chemical vapor deposition Jae Hyoung Ryu a, S. Chandramohan a, Hee Yun Kim a, Hyun Kyu Kim a, Ji Hye Kang a, Chang-Hee Hong a,n, Hyun Kyong Cho b, Hyun Don Song b, Ho-Ki Kwon b a b
School of Semiconductor and Chemical Engineering and Semiconductor Physics Research Center, Chonbuk National University, Chonju 561-756, Republic of Korea. Advanced Technology Laboratory, LED R&D Center, LG Innotek, Seoul 137-724, Republic of Korea.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 July 2010 Received in revised form 20 September 2010 Accepted 18 October 2010 Communicated by T.F. Kuech Available online 27 October 2010
The effect of selective area growth (SAG) on wafer bowing of GaN-based light-emitting diodes (LEDs) is investigated. The SAG of LED structures was carried out on a silicon dioxide (SiO2) mask pattern with periodic 1000 1000 mm openings, along the sapphire /1 1 0 0S and / 1 1 2 0S directions. The morphology of a selectively-grown n-GaN epilayer was examined in relation to various growth parameters such as temperature, pressure, and V/III ratio. Under optimized growth conditions, formation of a ridge-shaped epilayer with a v-pit free smooth surface was realized. Furthermore, the ridge-shaped vertical LED structure, after the removal of the sapphire substrate by laser lift-off (LLO) showed less wafer bowing compared with conventional vertical LED structures. This is attributed to the suppression of lateral strain and dislocations during the site-selective growth process, due to a reduction in the lateral dimensions. & 2010 Elsevier B.V. All rights reserved.
Keywords: A3. Selective epitaxy A3. Metal-organic chemical vapor deposition B1. Nitrides B2. Semiconducting III–V materials B3. Light-emitting diodes
1. Introduction Recently, GaN-based light-emitting diodes (LEDs) have attracted considerable interest due to their applications in optoelectronic devices for illumination, and back-light units in liquid crystal displays, with high efficiency and long life time [1,2]. GaN-based LEDs are most commonly grown on sapphire substrate [2]. However, sapphire being electrical insulator and due to its poor thermal conductivity, the p- and n-pads of GaN-based devices are conventionally grown on the surface of epitaxial layers. The use of a sapphire substrate complicates processing steps, such as the need to dry etch the n-GaN contacts in a p-side-up structure and mirror-cavity facets in a laser diode (LD) structure. The extraction of heat from an operating device through the sapphire substrate is also hampered. Therefore, free-standing GaN optoelectronic devices without sapphire substrate are most desirable. The laser lift-off (LLO) technique has been established as an effective procedure for GaN-based hetero-epitaxial structures, eliminating the constraint of the sapphire substrate [3–7]. Kelly et al. [3] used a frequency-tripled Nd:YAG laser at 355 nm, and Wong et al. [4] used an excimer laser at 248 nm to accomplish GaN film lift-off after irradiation f the structure through the transparent sapphire substrate. However, laser radiation with high energy density often results in
n
Corresponding author. E-mail address: [email protected] (C.-H. Hong).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.10.104
cracked GaN film [8,9]. One of the main sources of cracks in GaN films is bowing-induced large residual strain in the epilayers. Therefore, wafer bowing should be controlled for the production of crack-free LED structures. In this study, we demonstrate selective area growth (SAG) of vertical GaN-based LEDs with a reduced wafer bow by metalorganic chemical vapor deposition (MOCVD). The selective growth of large area (1000 1000 mm) ridge-shaped LED mesa structures on a SiO2 mask-patterned GaN template resulted in a substantial reduction in wafer bow due to the suppression of lateral strain. The optimization of growth conditions in the SAG process is also addressed. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution X-ray diffraction (HR-XRD) measurements were performed to characterize the properties of the SAG LED structures.
2. Experimental GaN epitaxial layers and InGaN/GaN multi-quantum well (MQW) structures were grown on a c-plane (0 0 0 1) sapphire substrate by MOCVD with an AIX200RF horizontal reactor system. Trimethylindium (TMIn), trimethylgallium (TMGa), and ammonia (NH3) were used as the sources of In, Ga, and N, respectively. Biscylopentadienyl magnesium (Cp2Mg) and silane (SiH4) were used as the p-type and the n-type dopant sources, respectively. The
J.H. Ryu et al. / Journal of Crystal Growth 314 (2011) 66–70
detailed processing sequences of SAG of GaN-based vertical LED structures are as follows: first, a thin (30 nm) GaN nucleation layer was deposited at a low temperature of 560 1C. Subsequently, a 1 mm-thick undoped GaN (u-GaN) epilayer at 1100 1C and a 2.5 mm-thick Si-doped n-type GaN epilayer at 1120 1C were deposited. A 100 nm-thick SiO2 layer was deposited onto the n-type GaN template by plasma-enhanced chemical vapor deposition (PEVCD). A mask pattern with periodic 1000 1000 mm mesa structure windows (each separated by 90 mm-wide SiO2 stripes) was created using conventional photolithography and buffered oxide etch solution. After the etching process, the sample was set into the MOCVD chamber for the epilayer re-growing process. The re-growth of n-type GaN was performed by varying the growth parameters such as temperature, pressure, time, and V/III ratio. The re-grown GaN was a 2.5 mm-thick Si-doped n-type GaN epilayer at optimized conditions. After completing the re-growth of the n-type GaN, six period MQWs of InGaN/GaN pairs with emission of about 460 nm were grown at 820 1C, and a 0.25 mm-thick Mg-doped p-GaN layer was grown at 1050 1C. In these experiments, N2 and H2 were used as carrier gas, respectively. Then, a 280 nm-thick transparent indium tin oxide (ITO) film was deposited to serve as a p-contact to the p-GaN. Subsequently, a Cu layer was deposited by e-beam evaporation, which served as a conductive seed layer for the subsequent electroplating process; here, Cu was electroplated for this purpose. Using an ArF excimer laser with a wavelength of 193 nm, the LLO process was performed to separate the LED structure from the sapphire substrate. The schematic representation of the different processes involved in the ridge-shaped vertical LED fabrication is shown in Fig. 1. For comparison, a similar LED structure was fabricated using the conventional MOCVD technique. The crystal quality of the epilayers grown under various growth conditions was examined by SEM, HR-XRD, and TEM. The magnitude of bending of the LED wafer was measured to obtain a quantitative understanding of wafer bowing.
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temperature, pressure, and V/III ratio. Figs. 2–4 show the crosssectional and plan view SEM images of the n-type GaN template and the selectively-grown epilayer under variable growth conditions. First, the n-type GaN epilayer was re-grown at various growth temperatures (1130, 1160, and 1180 1C) by keeping the pressure, V/III ratio, and growth time constant at 50 mbar, 60 min, and 1088 1C, respectively. Fig. 2 illustrates the variation in the surface morphologies as a function of growth temperature. It is evident from the plan view images that an increase in growth temperature reduced the formation of v-pits on the re-grown GaN epilayer. At a low growth temperature (1130 1C), the (0 0 0 1) surface degraded because the layer-by-layer growth was impeded due to poor surface migration of Ga atoms on the (0 0 0 1) surface. As a result, the surface is rich in v-pits. As the temperature increased, however, the surface migration is enhanced and hence the (0 0 0 1) surface become smooth [10]. The differences in center-to-edge heights at 1130, 1160, and 1180 1C were 0.4, 0.8, and 1 mm, respectively. Second, to investigate the re-growth of n-type GaN as a function of pressure, epilayers were grown at various pressure values (100, 50, and 20 mbar) at 1160 1C for 60 min with a V/III ratio of
3. Results and discussion The effect of different growth conditions on the morphology of the re-grown n-GaN epilayer was studied by varying the growth
Fig. 2. SEM images of selective area-grown n-GaN epilayer on GaN template for different growth temperature of 1130, 1160, and 1180 1C.
Fig. 1. A schematic of the fabrication process for ridge-shaped vertical LEDs: (a) formation of creative SiO2 mesa pattern, (b) growth of p–n junction and MQW structures, (c) formation of ITO layer for current spreading and passivation of mesa structures by SiO2 deposition, (d) copper electroplating on the ITO, (e) subsequent removal of sapphire substrate by LLO process, and (f) formation of vertical LED chip.
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1297. Fig. 3 shows the effect of reactor pressure on the morphology of the epilayer. The rough surface with v-pits observed at low pressure could be due to the increased evaporation rate of absorbed source molecules on the (0 0 0 1) surface [11–13]. We note that an
Fig. 3. SEM images of selective area-grown n-GaN epilayer on GaN template for different reactor pressure of 20, 50, and 100 mbar.
Fig. 4. SEM images of selective area-grown n-GaN epilayer on GaN template for different V/III ratios of 777, 1077, 1399, and 2238.
increase in the pressure value above 20 mbar led to considerable reduction in the v-pits, but also increased the center-to-edge height difference. For instance, the measured differences in the center-toedge heights are 0.3, 1, and 2.3 mm at 20, 50, and 100 mbar, respectively. Third, to investigate the re-growth of n-type GaN as a function of the V/III ratio, epilayers were grown under various V/III ratios (777, 1088, and 1399) for a fixed temperature of 1130 1C and a fixed pressure of 50 mbar. This was accomplished by varying the NH3 flow rate. Meanwhile, the total flow was kept constant and the quantity of TMGa flow during the growth was also fixed. The resulting morphologies of the re-grown n-GaN epilayer as a function of V/III ratio are shown in Fig. 4. We observe that the increase in V/III ratio degrades the epilayer quality, and there are many v-pits generated over the epilayer surface for a higher V/III ratio of 1399. In this case, the minimum difference in the center-toedge height was realized for a V/III ratio of 1088. To understand the influence of the gallium (Ga) flow rate, n-type GaN epilayers were grown at V/III ratios of 1399 and 2238 by varying the Ga flow rate. In this case, the quantity of NH3 flow during the growth was fixed at 4500 slm and the total flow was kept constant. As in the previous case, a decrease in the V/III ratio led to a reduction in the GaN growth rate. This is possibly due to the vapor phase reaction being dominant in the reactor under these conditions [14]. However, the surface morphologies of the n-type GaN layer re-grown at a high V/III ratio are found to be smooth. This is clearly shown in Fig. 4, where we did not observe any v-pits for a V/III ratio of 2238. In the preceding cases, as the V/III ratio increased, the surface morphologies improved but the v-pit increased. Alternatively, as the growth pressure increased, the v-pit decreased but the difference in the edge-to-center height increased. As the growth temperature increased, the v-pit decreased. Therefore, we needed to maintain low pressure, high temperature, and a high V/III ratio to reduce the edge-to-center height difference and the number of v-pits, and to obtain smooth surface morphologies. The optimized values of temperature, pressure, and V/III ratio are 1160 1C, 20 mbar, and 2238, respectively. The LED structure was grown on the optimized n-GaN template. Fig. 5 shows the HR-XRD spectra of the n-GaN template and the selectively-grown epilayer for symmetric (0 0 2) and asymmetric (1 0 2) reflection. It can be seen that both the template and the epilayer have almost identical intensities and full-width at half-maximum (FWHM) values. This observation indicates that the deposition of an n-GaN layer over the GaN template by SAG results in good quality epitaxial growth. The n-GaN epilayer growth over the SiO2 mask-patterned GaN template, examined by SEM, is shown in Fig. 6. Under optimized conditions, an n-GaN epilayer with a smooth top surface was
Fig. 5. X-ray rocking curves of n-GaN template and SAG epilayer. (a) (0 0 2) reflection and (b) (1 0 2) reflection.
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Fig. 6. SEM images of selective area-grown n-GaN epilayer on GaN template at optimized conditions: (a) top-view, (b)–(d) cross-sectional views. The side walls of the ridge structure are shown in (b) and (d).
Fig. 7. TEM cross-section images of MQW structure at (a) the center and (b) the edge regions.
obtained (see Fig. 6a) simply by changing the crystal growth conditions without using the mesa etching processes, which usually causes severe damage to the device structure. The measured center-to-edge height difference was approximately 0.4 mm on the completed LED structure, which is slightly larger than the value measured after the growth of n-GaN epilayer (0.3 mm). This could be due to the growth of the p-GaN. Fig. 7 shows TEM images of MQW structures at the center and the side wall regions. The measured total thicknesses of six period MQWs at the center and the edge regions were 115 and 190 nm, respectively. Moreover, no growth of MQWs on the side facets of the ridge-shaped LED mesa structure was observed during the selective growth. This is a major advantage of SAG because device reliability or stability remains a concern in the case of conventional vertical LEDs because the side walls of the active region (MQW structure) are affected during dry etching. However, in the case of
SAG, the growth mechanism itself enables the epilayer to grow in the form of ridge-shaped mesa structures, as shown in Fig. 6(b), so that stability-related issues could be overcome by employing this growth process. Nevertheless, obtaining homogeneous growth on different micro-facets is often inhibited by the difference between the growth rates because of the facet-dependent diffusivity of surface adatoms [15]. In other words, the morphological changes were caused by differences in the stability of each surface, which depend mainly on the surface energy and stability of surface atoms. In selective growth, the specific structure and morphology are primarily controlled by adjusting the growth temperature and the reactor pressure [16]. As previously mentioned, an increase in growth temperature (1130–1160 1C) enabled us to obtain a smooth facet growth due to enhanced surface migration, provided the pressure was sufficiently low (20 mbar) to concurrently increase the lateral epitaxial growth rate. This smooth morphology enables the growth of uniform MQWs on the c-plane. Moreover, experiments on side wall growth indicate that the side wall that grows along the { 1 1 2 0} plane becomes a side wall {1 1 2 2} by forming an angle of approximately 571 to the sapphire substrate (Fig. 6). Similarly, the side wall that grows along the {1 1 0 0} plane becomes a side wall {1–1 0 1} by forming an angle of approximately 621 to the sapphire substrate. These observations are in agreement with the results reported by Cho et al. [17], wherein the semipolar {1 1–2 2} and {1–1 0 1} micro-facets were shown to make similar angles with the basal plane. The magnitude of bending of the LED wafer at three different points was measured and the results are shown in Fig. 8. Here, the values represent the height of the wafer top surface from a reference plane for two different configurations; i.e., for the GaN surface facing upward and vice versa. The insets in the figure show a schematic representation of the two different measurement configurations. The results clearly indicate that both growth methods lead to considerable wafer bowing; however, the features of bending distinctly differ from each other. For instance, SAG leads to convex behavior with a large radius of curvature, whereas conventional growth results in concave bending. By considering
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the removal of the sapphire substrate due to strain induced by high wafer bowing. Conversely, we see no such cracking in SAG LED structures with the ridge geometry.
4. Summary
Fig. 8. Bowing measurement of GaN LEDs grown by SAG and conventional methods.
In summary, we studied the effect of selective area growth on the wafer bowing of GaN-based vertical LEDs fabricated using MOCVD and a post deposition laser lift-off process. The siteselective re-growth of the n-GaN epilayer was investigated under different growth conditions. In this experiment, the optimum growth temperature, pressure, and V/III ratio for growing a defect-free epilayer were determined to be 1160 1C, 20 mbar, and 2238, respectively. Initially, the shape of the wafer bow was dissimilar for conventional and SAG LEDs, and the magnitude of bending was large for SAG LEDs. After the removal of sapphire substrate by LLO, a substantial relaxation in wafer bow was observed for SAG vertical LEDs compared with conventional vertical LEDs. These different bowing behaviors are attributed to growth mode-dependent lateral strain in the GaN. Our study provides information that could be useful for the further development of free-standing crack-free GaN-based LEDs.
Acknowledgment This work is financially supported by the Ministry of Knowledge Economy (MKE) of the Korean government and by the Korea Institute for Advancement in Technology (KIAT), through the Workforce Development Program in Strategic Technology and the IT R&D program of the MKE/KEIT (KI002163, Development of Core Technology for High Efficiency Light-Emitting Diode based on New Concepts). References
Fig. 9. Photographs of LED structure after the removal of sapphire substrate by laser lift-off: (a) SAG LED and (b) conventional LED.
that the growth temperatures and substrates are similar in the two cases, the different bowing behaviors observed can be ascribed to growth mode-dependent lateral strain in the GaN. In Fig. 9, the photographs show the LED structures after the removal of the sapphire substrate by LLO. Note that the bowing is significantly relaxed in the SAG LED structure, whereas it is further enlarged in the case of the conventional LED (the photograph was taken by reversing the sample to explicitly show the bowing effect). The low bowing of the LED structure grown by SAG is most likely due to suppression of the lateral strain and dislocations [18,19] because of the reduction in lateral dimensions; these are difficult to control in conventional growth processes. The top-view images of the LED mesa structures are also shown in Fig. 9. We see that the conventional vertical LED structures are prone to cracking after
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