GaN multi-quantum wells grown by using MOCVD

Materials Chemistry and Physics 113 (2009) 192–195

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Surface morphological studies of green InGaN/GaN multi-quantum wells grown by using MOCVD M. Senthil Kumar, Y.S. Lee, J.Y. Park, S.J. Chung, C.-H. Hong, E.-K. Suh ∗ Semiconductor Physics Research Center and School of Semiconductor and Chemical Engineering, Chonbuk National University, Chonju 561-756, Republic of Korea

a r t i c l e

i n f o

Article history: Received 26 February 2008 Received in revised form 10 July 2008 Accepted 15 July 2008 Keywords: Quantum wells Atomic force microscopy (AFM) Surface properties

a b s t r a c t Surface morphology of green InGaN/GaN multi-quantum wells (MQWs) grown by using metal organic chemical vapor deposition has been analyzed by using atomic force microscopy, scanning electron microscopy, and cathodoluminescence (CL). Effects of barrier growth temperature, indium composition, and background threading dislocation (TD) density on the evolution of the MQW surface morphology have been studied. Low temperature GaN barrier growth generates a high density of V-pits and inclusions embedded within V-pits. Using trimethylindium flow prior to InGaN well growth, In segregation on GaN barrier is shown to be the prime cause for the formation of some inclusions which appear as bright spots in CL mapping while some inclusions are related to low temperature barrier growth. High temperature GaN barrier growth at 910 ◦ C completely suppresses these inclusion defects. In high indium containing InGaN/GaN MQWs, the large lattice mismatch induced strain in the MQW plays the key role in producing V-pits as well as inclusion defects rather than the background TD density of GaN templates. © 2008 Elsevier B.V. All rights reserved.

1. Introduction InGaN based multi-quantum wells (MQWs) are very promising for fabricating opto-electronic devices, such as blue, green and near ultraviolet light emitting diodes (LEDs) and laser diodes (LDs) [1]. In fact, production of InGaN/GaN blue and green LEDs has already been commercialized. However, the optical efficiency of green LEDs is still inferior to the blue LEDs owing to the following reasons. Highly strained InGaN/GaN MQWs with high indium (In) content exhibit a strong built-in piezoelectric field induced quantum-confined Stark effects that results in a reduction in the carrier recombination rate by increasing the spatial separation between the electron and hole wave functions involved in the radiative recombination [2]. In addition, the high In content produces more structural defects due to large lattice mismatch between InN and GaN that deteriorate the optical efficiency of the MQWs [3,4]. On the other hand, the surface morphology of high In containing InGaN/GaN MQWs is also accounted with a high density of surface pits and inclusions embedded within the pits. The surface pits are V-defects, inverted hexagonal pyramids with [10,11] sidewalls, commonly observed morphological feature in InGaN/GaN MQWs. The V-pit formation is closely related to the segregation of In atoms in the strain field around the cores of threading dislocations (TDs)

∗ Corresponding author. Tel.: +82 63 270 3441; fax: +82 63 270 3585. E-mail address: [email protected] (E.-K. Suh). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.07.068

and the growth kinetics of GaN as well [5]. Embedded inclusions are attributed to the presence of In rich clusters that nucleated at some V-defects during InGaN growth and progressively deteriorates the surface morphology of green MQWs [6]. Phase instability within QW related to V-defects and inclusions leads to premature thermal degradation, and restricts further high temperature processing required for LED fabrication. Also, the V-pits cause loss of interface abruptness between MQWs and between MQWs and top p-GaN layers that affect the device performance. Several techniques have been developed towards suppression of embedded inclusions which include GaN barrier growth either at an elevated temperature or in the presence of hydrogen [7–10]. Better understanding of the surface morphological evaluation of green InGaN/GaN MQWs is essential to obtain good quality interfaces and surfaces for constructing high brightness, reliable green LEDs. In this paper, we report the effects of barrier growth temperature, In composition in InGaN well and background TD density of GaN template on the surface morphology of green emitting InGaN/GaN MQWs. 2. Experimental Four or five periods InGaN/GaN green MQWs were grown on c-plane sapphire substrates using a low pressure metal organic chemical vapor deposition (MOCVD) system. Trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia (NH3 ) were used as precursors for Ga, In, and N, respectively. Nitrogen was used as a carrier gas for TMGa and TMIn flow. After high temperature sapphire cleaning, a thin GaN nucleation layer was grown at low temperature followed by 1 ␮m thick un-doped GaN epilayer growth at 1100 ◦ C. InGaN well growth was performed at a temperature between 730 and 745 ◦ C depending upon the designated emission wavelength. To

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analyze the effect of barrier growth temperature, the InGaN well layers were grown at a constant temperature of 740 ◦ C while the GaN barrier growth temperature was varied between 740 and 910 ◦ C. To protect the InGaN well layer, about 2 nm GaN capping layer was subsequently grown during temperature ramping for the barrier growth. A growth interruption of 1 min was given before both well and barrier growth for stabilizing the growth temperature of respective layers. The thickness of InGaN well and GaN barrier were 2.5 and 15 nm, respectively. From the growth conditions employed, In composition of the QW is estimated to be 25–30%. We also prepared high temperature barrier grown MQWs with TMIn pre-flow for InGaN well; i.e., TMIn flow was allowed over GaN barrier along with NH3 for a short duration of 10 s, subsequently added with TMGa flow for InGaN well growth. To analyze the effect of background TD density on the surface morphology, InGaN/GaN MQWs were grown on sapphire and HVPE grown free-standing GaN (FS-GaN) substrates under same growth conditions. Grown samples were characterized using atomic force microscopy (AFM), scanning electron microscopy (SEM), high-resolution X-ray diffraction (HRXRD) and cathodoluminescence (CL) techniques. CL measurements were carried out with an accelerating voltage of 15 kV and the detection wavelength range was 350–650 nm.

3. Results and discussion Surface morphology of four periods InGaN/GaN MQWs with GaN barrier grown at 740 and 910 ◦ C observed using tapping-mode AFM (5 ␮m × 5 ␮m scan height image) is shown in Fig. 1. As seen in Fig. 1(a), the MQWs with the low temperature grown GaN barrier reveal a high density of surface pits and embedded inclusions. The density of V-pits is calculated to be ∼1 × 109 cm−2 with a size of about 100 nm. The density of embedded inclusions is around 7 × 108 cm−2 with diameters varying between 100 and 450 nm. Ting et al. have reported that the inclusions originate at the low temperature InGaN-to-GaN interface when the surface makes a transition from 3D island morphology of the InGaN QW and recovers 2D step flow mode during GaN barrier growth [10]. In case of MQWs, nucleation of new inclusions after each successive InGaNto-GaN interface and the continued growth of previously nucleated inclusions during the GaN barriers account for the more defective surface morphology. Using SEM and CL observation at the same location, we have observed that the inclusion defects are of two types; one of them appears as bright spots in CL mapping while the other resembles the background [marked, respectively, as ‘1’ and ‘2’ in the inset of Fig. 1(a)]. It indicates that these two inclusion defects have different origins. The inclusions observed as CL bright spots are attributed to In rich regions in InGaN well while the growth kinetics of low temperature GaN barrier causes the other [10,11]. The size and the density of inclusion defects gradually decreased with increasing GaN barrier temperature and an inclusion free surface morphology is obtained for the MQWs with GaN barrier grown at 910 ◦ C as shown in Fig. 1(b). The step-flow mode of GaN growth is also well improved by high temperature barrier growth due to enhanced surface diffusion of adatoms. Besides, the high temperature barrier growth also shows a significant reduction of V-pits. In case of 910 ◦ C barrier grown MQWs, a V-pit density of 7–8 × 108 cm−2 has been observed with diameters of 50–100 nm. The elimination of inclusions and the reduction of V-pit size and density at the elevated barrier temperature suggest that the growth kinetics of GaN barrier layer influences the formation of these defects. Re-evaporation of In rich embedded inclusions and superior 2D growth mode of GaN barrier during high temperature growth are thought to be responsible for the elimination of both types of inclusion defects. It should be mentioned that the photoluminescence (PL) peak position shifted from 535 to 515 nm upon high temperature barrier growth due to In out-sourcing [11]. The HRXRD ω–2 scans of InGaN/GaN MQWs with GaN barriers grown at 740 and 910 ◦ C are given in Fig. 2. The sharp, well defined satellite peaks for 910 ◦ C barrier growth represents the improved quality of the MQW interfaces. The HRXRD spectra also revealed that the average In composition of the InGaN QW is not affected by high temperature barrier growth. Hence, the PL red-shift is attributed to

Fig. 1. 5 ␮m × 5 ␮m AFM images of InGaN/GaN MQWs on sapphire substrate with GaN barrier grown at (a) 740 ◦ C (Inset: SEM/CL images observed at the same location), and (b) 910 ◦ C.

Fig. 2. HRXRD spectra of InGaN/GaN MQWs on sapphire substrate with GaN barriers grown at 740 and 910 ◦ C.

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Fig. 3. InGaN/GaN MQWs on sapphire substrate grown with 10 s of TMIn pre-flow for InGaN well: (a) 5 ␮m × 5 ␮m AFM image, and (b) SEM/CL images observed at the same location.

the change in local In concentration, i.e., decomposition of In-rich regions, that weakens the localization of excited carriers. From AFM and HRXRD results, it appears that the high temperature barrier growth reduces the V-pits whose formation mechanism is associated with In-rich regions [12]. Fig. 3 shows the AFM and SEM/CL images of InGaN/GaN MQWs grown with TMIn flow prior to InGaN well growth for 10 s. The well and the barrier growth temperatures were maintained at 745 and 910 ◦ C, respectively. As mentioned earlier, the GaN barrier growth at 910 ◦ C produced an inclusion free smooth MQW surface; however, when TMIn pre-flow was allowed for InGaN growth for a short time, several small-sized inclusion defects reappeared in the AFM surface morphology of the MQWs that were also observed as bright spots in CL mapping as shown in Fig. 3(b). This result indicates that the inclusion defects appeared as bright spots in CL mapping is closely associated with formation of In rich regions at the InGaN–GaN interface. The In pre-flow has not affected the In composition and the interface properties of the InGaN/GaN MQWs but red-shifted the PL emission as reported elsewhere [13]. It is likely that the preflow In atoms form In rich clusters on GaN barrier just before the InGaN well growth. Some In rich clusters nucleated at V-pits act as sink for further In segregation and develop during subsequent InGaN/GaN MQW growth causing the inclusion defects. In this case, In re-evaporation process during high temperature barrier growth does not seem to be effective in completely removing the In rich regions. We have also examined the effect of In composition in the InGaN well and the TD density in GaN template on the surface morpholog-

Fig. 4. 5 ␮m × 5 ␮m AFM images of InGaN/GaN MQWs grown on sapphire substrate with PL emissions at (a) 520 nm, (b) 530 nm, and (c) grown on FS-GaN substrate with PL emission at 530 nm.

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Table 1 Experimental conditions and properties of green InGaN/GaN MQWs grown on sapphire and FS-GaN substrates Substrate Sapphire Sapphire FS-GaN

Barrier growth temperature ( ◦ C) 880 880 880

Well growth temperature ( ◦ C) 740 730 735

ical evaluation of green MQWs. In composition in the InGaN well was altered by varying the well growth temperature to tune the emission wavelength. The GaN barrier growth temperature was kept constant at 880 ◦ C to achieve longer wavelength emission even though small-sized inclusions remained in the MQW surface at this condition. The details of growth conditions are given in Table 1. Surface morphologies of InGaN/GaN MQWs grown on sapphire with PL emission at 520 and 530 nm are shown in Fig. 4(a) and (b), respectively. As can be seen, the V-pits and the inclusion defects increase with increasing In composition in the InGaN well. An inclusion density of 5 × 107 cm−2 and a V-pit density of 3 × 108 cm−2 were observed for the MQW emitting light at 520 nm but both the values strongly increased for the MQWs emitting at 530 nm as shown in Table 1. It implies that the increase of strain in InGaN well with increasing In composition is the main factor to generate these defects since the background TD density is expected to be the same for GaN-on-sapphire. Fig. 4(c) shows the surface morphology of InGaN/GaN MQWs grown on FS-GaN substrate with an emission wavelength of 530 nm. The well growth temperature was slightly different in this case compared to sapphire due to the differences in the substrate thermal conductivity and thermal coupling of the substrates to the susceptors. As given in Table 1, the V-pit density is significantly lesser in the MQWs grown on FS-GaN substrate compared to that grown on sapphire substrate with PL emission at 530 nm, which might be due to the low background TD density of the FS-GaN template (i.e., ∼106 cm−2 ). However, it is noted that the V-pit density value in the homoepitaxially grown MQW is two orders larger than the background TD density of the FS-GaN substrate. Because, in highly strained InGaN/GaN MQWs with high In content, the V-pits originate mainly from the stacking mismatch boundaries induced by stacking faults and the 3D island growth of InGaN well at the initial stage rather than from the TD of GaN templates [14]. Watanabe et al. have reported that the V-pits might also be originated by local lattice expansion due to large In-rich dots in InGaN layer [12]. Interestingly, the inclusion density in the MQWs grown on FS-GaN is comparable with that on sapphire substrate, which clearly shows that the formation of inclusion defect is independent of background TD density but has a strong dependence on In composition in the InGaN well. Hence, we conclude that the large lattice mismatch induced strain plays the key role in originating V-pits as well as inclusion defects in high In containing InGaN/GaN MQWs.

PL peak (nm)

V-pit density (cm−2 )

Inclusion density (cm−2 )

520 530 530

3.0 × 10 1.2 × 109 4.0 × 108

5.0 × 107 3.1 × 108 2.6 × 108

8

4. Conclusions The dependence of surface morphology of MOCVD grown green InGaN/GaN MQWs on barrier growth temperature, In composition, and background TD density have been studied. Barrier growth below 800 ◦ C generates a high density of V-pits and inclusion defects. Some inclusion defects are originated due to In segregation at InGaN/GaN interface which appear as bright spots in CL image while some are caused by low temperature barrier growth. GaN barrier growth at above 900 ◦ C improves the MQW surface morphology with free of inclusion defects and significantly reduced V-pit density. Increase of In content in InGaN well increases the V-pit and inclusion defect densities due to large lattice mismatch induced strain in the MQWs. Acknowledgement This work was supported by the Korea Research Foundation Grant funded by the Korea Government (MOEHRD) (KRF-2005005-J07501). References [1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. [2] T. Takeuchi, S. Sota, M. Katsuragawa, M. Komori, H. Takeuchi, H. Amano, I. Akasaki, Jpn. J. Appl. Phys. 36 (1997) L382. [3] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Appl. Phys. Lett. 67 (1995) 1868. [4] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, T. Yamada, T. Mukai, Jpn. J. Appl. Phys. 34 (1995) L1332. [5] M. Shiojiri, C.C. Chuo, J.T. Hsu, J.R. Yang, J. Appl. Phys. 99 (2006) 073505. [6] D.I. Florescu, S.M. Ting, J.C. Ramer, D.S. Lee, V.N. Merai, A. Parkeh, D. Lu, E.A. Armour, L. Chernyak, Appl. Phys. Lett. 83 (2003) 33. [7] F. Scholz, J. Off, E. Fehrenbacher, O. Gfrorer, G. Brockt, Phys. Status Solidi (a) 180 (2000) 315. [8] Y.-T. Moon, D.-J. Kim, K.-M. Song, C.-J. Choi, S.-H. Han, T.-Y. Seong, S.-J. Park, J. Appl. Phys. 89 (2001) 6514. [9] S. Suihkonen, T. Lang, O. Svensk, J. Sormunen, P.T. Torma, M. Sopanen, H. Lipsanen, M.A. Odnoblyudov, V.E. Bougrov, J. Cryst. Growth 300 (2007) 324. [10] S.M. Ting, J.C. Ramer, D.I. Florescu, V.N. Merai, B.E. Albert, A. Parekh, D.S. Lee, D. Lu, D.V. Christini, L. Liu, E.A. Armour, J. Appl. Phys. 94 (2003) 1461. [11] M. Senthil Kumar, J.Y. Park, Y.S. Lee, S.J. Chung, C.-H. Hong, E.-K. Suh, J. Phys. D: Appl. Phys. 40 (2007) 5050. [12] K. Watanabe, J.R. Yang, S.Y. Huang, K. Inoke, J.T. Hsu, R.C. Tu, T. Yamazaki, N. Nakanishi, M. Shiojiri, Appl. Phys. Lett. 82 (2002) 718. [13] M. Senthil Kumar, J.Y. Park, Y.S. Lee, S.J. Chung, C.-H. Hong, E.-K. Suh, Jpn. J. Appl. Phys. 47 (2008) 839. [14] H.K. Cho, J.Y. Lee, G.M. Yang, Appl. Phys. Lett. 80 (2001) 1370.