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Influence of the Structure Parameters on the Relaxation of Semipolar InGaN/GaN Multi Quantum Wells
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Japanese Journal of Applied Physics 52 (2013) 08JC10 http://dx.doi.org/10.7567/JJAP.52.08JC10
Influence of the Structure Parameters on the Relaxation of Semipolar InGaN/GaN Multi Quantum Wells Stacia Keller1 , Robert M. Farrell2 , Michael Iza2 , Yutaka Terao2 , Nathan Young2 , Umesh K. Mishra1 , Shuji Nakamura2 , Steven P. DenBaars1;2 , and James S. Speck2 1
Electrical and Computer Engineering Department, University of California, Santa Barbara, CA 93106, U.S.A. Materials Department, University of California, Santa Barbara, CA 93106, U.S.A. E-mail:
[email protected] 2
Received October 6, 2012; accepted May 19, 2013; published online July 22, 2013 InGaN/GaN multi quantum well (MQW) structure parameters such as well composition and thickness (dw ), The influence of semipolar (2021) GaN by metal organic barrier thickness, as well as total number of periods on the structural and optical properties of the MQWs grown on (2021) chemical vapor deposition was investigated. At dw < 3 nm, the MQW stacks were very robust with respect to changes in the barrier thickness or the number of periods in the MQW stack, and 30 period (2.5 nm In0:25 Ga0:75 N/8.5 nm GaN) MQWs exhibiting bright luminescence at 465 nm were demonstrated. For all samples with dw < 3 nm in this study, one-dimensional relaxation via misfit dislocations did not lead to any deterioration of the optical properties of the films, and a decrease in the photoluminescence intensity was only observed after the on-set of two-dimensional relaxation via non-basal plane defects. # 2013 The Japan Society of Applied Physics
1. Introduction
Recently (2021) semipolar InGaN/GaN heterostructures have attracted attention because of their reduced polarization related internal electric fields in comparison to structures grown on c-plane GaN in combination with their high indium incorporation efficiency in the metal organic chemical vapor deposition (MOCVD) process.1) Both, high performance light emitting diodes2) and laser diodes3,4) have been demonstrated using bulk (2021) GaN substrates. InGaN quantum Furthermore, green light emitting (2021) wells exhibited significantly higher compositional uniformity compared to their (0001) counterparts.5) Because of the above mentioned advantages semipolar InGaN/GaN heterostructures are also very attractive for solar cell applications. Those, however, require the growth of rather thick InGaN active regions,6) which can be either composed of an InGaN layer (>100 nm) or a thick InGaN/GaN multi quantum well (MQW) stack.7) The combination of high indium content and thickness poses a significant challenge for the solar cell fabrication process as the critical thickness, hcrit , of GaN is rather low: 38 nm for In0:07 Ga0:93 N, InGaN on (2021) 8) for example. Previous investigations have shown that semipolar layers with a thickness in excess of hcrit first relax (0001) via dislocation glide in the basal slip system h1120i under formation of misfit dislocations (MDs) with Burgers vector of (a=3) h1120i-type at the heterointerface, leading to one-dimensional (1D) relaxation and a tilt in the (Al,Ga,In)N layer with respect to GaN, which can be measured by high resolution X-ray diffraction (XRD).9) A good correlation between tilt and the density of defects observed by transmission electron microscopy was found.10) The MDs can also be observed by cathodoluminescence (CL) or fluorescence microscopy (FLM).11) As the layer thickness is further increased, however, additional defects form via prismatic slip on inclined m-planes, allowing two-dimensional (2D) relaxation. The non-basal-plane defects can be observed again by CL or FLM and lead to a general deterioration of the layer quality.12,13) In this study we investigated the influence of the InGaN/GaN MQWs such structure parameters of (2021)
as well composition and thickness, barrier thickness, as well as the total number of periods up to 50 on the relaxation of the MQW stacks grown on bulk GaN substrates. The results establish a design space for high quality MQW stacks and 30 period (2.5 nm In0:25 Ga0:75 N/ (2021) 8.5 nm GaN) MQWs exhibiting bright luminescence were demonstrated. 2. Experiment
All samples in this study were grown by MOCVD using the precursors trimethylgallium, triethylgallium, trimethylindium, and ammonia. InGaN/GaN MQW structure with different Inx Ga1x N well composition, xIn (0.1–0.33), and thickness, dw (2–6 nm), GaN barrier thickness, db (2–11 nm), as well as the total number of periods (10–40) were grown GaN substrates supplied by Mitsubishi Chemical. on (2021) The growth was initiated with a 2-m-thick GaN layer deposited at 1180 C. Afterwards the temperature was lowered to 745–805 C for the growth of the MQW stacks. All samples were evaluated by XRD (Panalytical Materials Research Diffractometer), FLM (Nikon Eclips LV 150), atomic force microscopy (AFM; Digital Instruments Dimension 3100), and room temperature photoluminescence (PL) measurements, using the 325 nm line of a He–Cd laser with an excitation density of 220 mW/cm2 . 3. Results and Discussion
To evaluate the relaxation of the MQWs originating from (0001), dislocation glide in the basal slip system h1120i (2021) reciprocal space maps (RSMs) with X-rays incident direction of the (In,Ga)N crystal were parallel to the [101 4] recorded to determine the lattice tilt of the MQWs with respect to the GaN base as described in detail in Ref. 9. Figure 1 illustrates the measurement results for a 10 period sample with 5.8-nm-thick In0:2 Ga0:8 N wells and 8-nm-thick barriers. The RSM taken with the X-rays incident along the line direction of the MDs present in the sample, [1120], is shown on the left for comparison. When the RSM was direction in the scattering recorded with with the [101 4] plane (defined by the incident and diffracted wavevectors), perpendicular to the MD line direction, a tilt between GaN
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and MQW stack was clearly observed, which was 0.44 for this particular sample. The evolution of tilt with increasing In0:25 Ga:0:75 N well thickness for 10 period MQWs with 8.5-nm-thick GaN barriers is shown in Fig. 2(a). For comparison the average indium composition of the MQW stacks and the total thickness of the stacks were plotted as well. While no tilt was found for the sample with 2-nm-thick wells, a small tilt of 0.03 was determined for a well thickness of 3 nm which further increased to 0.16 and 0.42 for the samples with 4and 6-nm-thick wells, respectively. Contrary to the well thickness, the variations in the barrier thickness for samples with 2.5-nm-thick In0:25 Ga:0:75 N wells had less impact on the tilt: no tilt could be measured for samples with 11- and 8.5-nm-thick barriers and the sample with the thinnest barrier of 3.5 nm exhibited a tilt of only 0.02 [Fig. 2(b)]. Both, the increase in well thickness and the decrease in barrier thickness led to an increase in the average In composition of the MQW. With increasing well thickness, however, the total thickness of the stack slightly increased, whereas the decrease in barrier thickness from 11 to 3.5 nm resulted in a significant decrease in the total thickness of the stack, decreasing the tendency toward relaxation. The impact of the Inx Ga1x N well composition on the tilt for 10 period (2.5 nm Inx Ga1x N/8.5 nm GaN) samples is illustrated on Fig. 3. At x < 0:2 no tilt could be measured. With further increase in the composition the tilt increased and reached 0.18 for the sample with x ¼ 0:35. Note that the data points represented by closed symbols belong to one sample series where all substrates were selected from the same batch whereas the other data points correspond to samples grown on different substrate batches. We speculate that slight differences in threading dislocation density and spatial distribution may affect the relaxation behavior leading to the slight scatter in the tilt values.
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With increasing number of periods in the MQW stack, the tendency to relax and the measured tilt increased, as illustrated in Fig. 4 for 20 and 30 period samples with 2.5-nm-thick In0:25 Ga:0:75 N wells and GaN barriers between 3 and 8.5 nm. The misfit dislocations with Burgers vector of (a=3) h1120i-type which form at the heterointerface and cause the tilt in the MQW with respect to GaN can be directly observed using CL or FLM. As discussed in Ref. 9, the tilt angle measured by XRD and the MD spacing, D, are related via D b? = where b? ¼ b sin is the MD Burgers vector component normal to the layer/substrate interface,
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with b corresponding to the magnitude of the Burgers vector and being the inclination angle of the basal (0001) plane in orientation is a semipolar heterostructure. For the (2021) 8) equal to 75.09 . The amount of strain relaxed by the MD array, "p , is given by "p ; ¼ bk =D ¼ cot , where bk ¼ b cos is the MD burgers vector component parallel to the layer/substrate interface. Using the above equations, the MD spacing can be easily calculated from the tilt angles determined by XRD. Figure 5 shows the FLM images of selected 10 and 20 period samples with 2.5-nm-thick In0:25 Ga:0:75 N wells and GaN barriers of different thickness. Figures 5(a) and 5(b) correspond to the 10 period samples with 8.5- and 6.5-nmthick barriers, respectively. In both FLM images a few dark direction related to the MDs are lines parallel to the h1120i visible. Their density, however, was too low to results in any measurable tilt in the XRD measurements. The dislocation line density was significantly higher in the 20 period sample with 8.5-nm-thick barriers [Fig. 5(c)]. The tilt angle determined in the XRD measurements for this sample was ¼ 0:057 , corresponding to a MD spacing of 1 m, in good agreement with the experimentally observed result. The tilt angles further increased to 0.063, 0.21, and 0.56 when the barrier thickness was decreased to 6.5, 4.5, and 3 nm, respectively, and the density of the MD related dark direction steadily increasline defects parallel to the h1120i ed accordingly [Figs. 5(d) to 5(f )]. The very short line direction in segments about perpendicular to the h1120i Fig. 5(d), the image of the sample with 6.5 nm barriers, mark first signs of the onset of non-basal-plane dark line defect formation.12,13) Caused by slip on inclined prismatic planes these defects have [1 126] (1100) and (0110) and [21 1 6]
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Fig. 5. (Color online) Fluorescence microscopy images of 10 period (In0:25 Ga0:75 N/GaN) MQWs with (a) 8.5- and (b) 6.5-nm-thick barriers, and 20 period (In0:25 Ga0:75 N/GaN) MQWs with (c) 8.5-, (d) 6.5-, (e) 4.5-, and (f ) 3-nm-thick barriers. All samples had 2.5-nm-thick wells.
traces with an angle of 81:5 with respect to the in-plane a-direction as seen in the sample with 4.5-nm-thick barriers, and at a very high density in the sample with 3-nm-thick barriers [Figs. 5(e) and 5(f )]. As observed for bulk InGaN films, relaxation via non-basal-plane defects occurred only after significant relaxation through formation of basal-plane MDs.13) Both, basal and non-basal-plane defects also affect the surface morphology as seen in the AFM images of the corresponding samples. The 10 period samples with 8.5 and 6.5 nm barriers [Figs. 6(a) and 6(b)] are very smooth typical for (2021) films. The with steps parallel to h1120i amplitude of the step undulations slightly increased for the 20 period samples with 8.5 and 6.5 nm barriers [Figs. 6(c) and 6(d)], as the basal-plane line defects lead to a protrusion of the corresponding steps on the surface. In case of the 20 period samples with 4.5- and 3-nm-thick barriers, were additional protruding lines perpendicular to h1120i visible corresponding to the non-basal-plane defects [Figs. 6(e) and 6(f )]. The samples with different barrier thickness and periods also illustrate the relation between defects and optical properties of the MQWs as evaluated by PL. The 10 period samples with 4.5-, 6.5-, and 8.5-nm-thick barriers and tilts of 0, 0.01, and 0.02 , respectively, exhibited bright luminescence at similar intensity [Fig. 7(a)]. Due to the larger number of periods, the luminescence of the 30 period sample with 8.5 nm barriers (tilt: 0.04 ) was even brighter, the PL intensity significantly decreased, however, for the samples with 6.5 and 4.5 nm barriers and tilts of 0.44 and 0.7 ,
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respectively [Fig. 7(b)]. Figure 7(c) summarizes the PL intensities of the sample series with different barrier thickness and periods. In this study all samples with a well thickness 0.2
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The authors would like to acknowledge the support from DARPA (Grant No. HR0011-10-1-0049), the California Advanced Solar Technologies Institute (Grant No. 143188), the Solid State Lighting and Energy Center, and the MRSEC Program of the National Science Foundation under Award No. DMR 1121053. R.M.F. was partially supported by the Center for Energy Efficient Materials at UCSB, funded by the U.S. DOE (Grant No. DE-SC0001009). The trimethylindium metalorganic source used for this study was provided by Sonata LLC.
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1) A. E. Romanov, T. J. Baker, S. Nakamura, and J. S. Speck: J. Appl. Phys.
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DenBaars, and S. Nakamura: Appl. Phys. Express 3 (2010) 122102. 3) Y. Yoshizumi, M. Adachi, Y. Enya, T. Kyono, S. Tokuyama, T.
Fig. 8. (Color online) Tilt versus average MQW composition and total MQW thickness. Samples with no measurable tilt are represented by circles, samples with a tilt below 0.1 with upward triangles, with tilts between 0.1– 0.2 by downward triangles. Samples with a tilt above 0.2 are marked by diamonds (all symbols in black). Also shown are bulk InGaN films represented by red symbols. The red line corresponds to the critical Inx Ga1x N calculated using the Matthews–Blakeslee thickness for (2021) model.
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Sumitomo, K. Akita, T. Ikegami, M. Ueno, K. Katayama, and T. Nakamura: Appl. Phys. Express 2 (2009) 092101. Y.-D. Lin, S. Yamamoto, C.-Y. Huang, C.-L. Hsiung, F. Wu, K. Fujito, H. Ohta, J. S. Speck, S. P. DenBaars, and S. Nakamura: Appl. Phys. Express 3 (2010) 082001. M. Funato, A. Kaneta, Y. Kawakami, Y. Enya, K. Nishizuka, M. Ueno, and T. Nakamura: Appl. Phys. Express 3 (2010) 021002. C. J. Neufeld, N. G. Toledo, S. C. Cruz, M. Iza, S. P. DenBaars, and U. K. Mishra: Appl. Phys. Lett. 93 (2008) 143502. R. M. Farrell, C. J. Neufeld, S. C. Cruz, J. R. Lang, M. Iza, S. Keller, S. Nakamura, S. P. DenBaars, U. K. Mishra, and J. S. Speck: Appl. Phys. Lett. 98 (2011) 201107. A. E. Romanov, E. C. Young, F. Wu, A. Tyagi, C. S. Gallinat, S. Nakamura, S. P. DenBaars, and J. S. Speck: J. Appl. Phys. 109 (2011) 103522. E. C. Young, A. E. Romanov, and J. S. Speck: Appl. Phys. Express 4 (2011) 061001. F. Wu, A. Tyagi, E. C. Young, A. E. Romanov, K. Fujito, S. P. DenBaars, S. Nakamura, and J. S. Speck: J. Appl. Phys. 109 (2011) 033505. P. S. Hsu, E. C. Young, A. E. Romanov, K. Fujito, S. P. DenBaars, S. Nakamura, and J. S. Speck: Appl. Phys. Lett. 99 (2011) 081912. F. Wu, E. C. Young, I. Koslow, M. T. Hardy, P. S. Hsu, A. E. Romanov, S. Nakamura, S. P. DenBaars, and J. S. Speck: Appl. Phys. Lett. 99 (2011) 251909. M. T. Hardy, P. S. Hsu, F. Wu, I. L. Koslow, E. C. Young, S. Nakamura, A. E. Romanov, S. P. DenBaars, and J. S. Speck: Appl. Phys. Lett. 100 (2012) 202103. J. W. Matthews and A. E. Blakeslee: J. Cryst. Growth 27 (1974) 118. X. Chu and S. A. Barnett: J. Appl. Phys. 77 (1995) 4403.
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4. Conclusions
InGaN/GaN MQW A design space for high quality (2021) stacks was established. At dw < 3 nm, the MQW stacks were very robust with respect to changes in the barrier thickness or the number of periods in the stack, and 30 period (2.5 nm In0:25 Ga0:75 N/8.5 nm GaN) MQWs exhibiting bright luminescence were demonstrated. For all samples with dw < 3 nm in this study, 1D relaxation via misfit dislocations did not lead to any deterioration of the optical properties of the films, and a decrease in the luminescence intensity was only observed after the on set of 2D relaxation via non-basal plane defects. As seen in other semiconductor systems, the InGaN/GaN MQWs exhibited delayed relaxation compared to bulk InGaN films.
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