green InGaN light-emitting diodes on low defect density

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Efficient semipolar (11-22) 550 nm yellow/green InGaN light-emitting diodes on low defect density (11-22) GaN/sapphire templates Hongjian Li, Michel Khoury, Bastien Bonef, Abdullah I. Alhassan, Asad Jahangir Mughal, Ezzah Azimah, Muhammad E.A. Samsudin, Philippe De Mierry, Shuji Nakamura, James Speck, and Steven P. DenBaars ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11718 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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ACS Applied Materials & Interfaces 1

Efficient semipolar (11-22) 550 nm yellow/green InGaN light-emitting diodes on low defect density (11-22) GaN/sapphire templates Hongjian Li1 *, Michel Khoury1 , Bastien Bonef1, Abdullah I. Alhassan1, Asad J. †

†

Mughal1, Ezzah Azimah1, 4, Muhammad E.A. Samsudin1, 4, Philippe De Mierry3, Shuji Nakamura1,2, James S. Speck1 and Steven P. DenBaars1,2 1

Materials Department, University of California, Santa Barbara, California 93106, USA

2

Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93117, USA 3 4

CNRS - CRHEA, Rue Bernard Grégory, 06560 Valbonne, France

Institute of Nano Optoelectronics Research and Technology, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia

Abstract: We demonstrate efficient semipolar (11-22) 550 nm yellow/green InGaN light-emitting diodes (LEDs) with In0.03Ga0.97N barriers on low defect density (11-22) GaN/patterned sapphire templates. The In0.03Ga0.97N barriers were clearly identified and no InGaN clusters were observed by atom probe tomography (APT) measurements. The semipolar (11-22) 550 nm InGaN LEDs (0.1 mm2 size) show an output power of 2.4 mW at 100 mA and a peak external quantum efficiency (EQE) of 1.3% with a low efficiency droop. In addition, the LEDs exhibit a small blue-shift of only 11 nm as injection current increases from 5 to 100 mA. These results suggest the potential to produce high efficiency semipolar InGaN LEDs with long emission wavelength on large-area sapphire substrates with economical feasibility. KEYWORDS: Semipolar GaN, MOCVD, light-emitting diodes, InGaN, atom probe tomography.

■ INTRODUCTION

GaN-based light emitting diodes (LEDs) have attracted considerable attentions, and are widely applied in display technologies, back lighting, and general illumination.1-2 Commercially available GaN-based LEDs are grown along the polar c-direction and

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the performance of commercial blue LEDs has been significantly improved over the past few years.3 However, these polar oriented LEDs grown on c-plane sapphire suffer from the quantum confined Stark effect (QCSE) due to the large polarization-related electric fields, which leads to an electron-hole wave function separation within the quantum wells (QWs).4-5 The QCSE becomes more significant for longer-wavelength devices such as green emitters due to the increased lattice mismatch between high-InN-molar-fraction QWs and barriers.6 Consequently, typical c-plane InGaN green LEDs and longer emission wavelength exhibit reduced efficiencies In principle, growing GaN-based devices on alternative orientations such as the semipolar or nonpolar ones can overcome this issue.7-8 High power and high efficiency LEDs have been demonstrated on low defect density semipolar and nonpolar bulk GaN substrates.5–8 Such substrates, however, remain only available in small sizes and are highly priced, which limits the wide applicability of the semipolar or nonpolar LEDs for solid-state lighting. Growing semipolar or nonpolar GaN layers on foreign substrates like sapphire are attractive due to their lower cost and the possibility of scaling up to larger size. Nevertheless, semipolar GaN layers grown on foreign substrates suffer from high densities of basal stacking faults (BSFs) and threading dislocations (TDs), leading to low efficiency devices.9 Efforts have been focused on defect management to improve the crystal quality of semipolar GaN materials grown on foreign ones to improve the device performance.10-18 To date, few papers have been reported on semipolar LEDs with long emission wavelength on sapphire substrates and those LEDs still exhibit low output power and reduced external quantum efficiency (EQE).14-16 In this study, we demonstrate efficient semipolar (11-22) 550 nm yellow/green InGaN LEDs grown on low defect density (11-22) GaN templates on patterned r-plane sapphire. Characterizations by scanning transmission electron microscopy (STEM), atom probe tomography (APT) and X-ray diffraction (XRD) were carried out and the optical and electrical properties of the fabricated devices were discussed.

■ EXPERIMENTS


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Planar 2-inch unintentionally doped (11-22) GaN templates were obtained after coalescing adjacent crystals each grown at an inclined c-axis on patterned r-plane sapphire by employing a three-step growth method. 11 Figure 1(a) shows a photo of the whole 2-inch (11-22) GaN/sapphire template after a chemo-mechanical polishing (CMP) step. The final surface is very smooth, having a roughness of 0.6 nm on a 2×2 µm2 size scanned by atomic force microscope (AFM) measurement.20 A cross sectional scanning electron microscope (SEM) image of the semipolar (11-22) GaN/sapphire template is presented in Figure 1 (b). The nucleation facet over which the inclined growth is carried out using conventional lithography of bands along [1-100] and a wet chemical etch. The nucleation facet size is controlled to a size of around 500 nm and is designed for a reduced area footprint between GaN and sapphire, making it ideal for a reduced number of generated dislocations. Metal-organic chemical vapor deposition (MOCVD) growth conditions were optimized over the three consecutive steps in order to overlap adjacent crystals and produce defect-blocking air-voids (Figure 1 (b)) that create free terminating surfaces for TDs and BSFs, therefore further inhibiting their propagation towards the surface. The XRD rocking curves results are shown in Figure 1(c) and Figure 1(d), with full width at half maximum (FWHM) of 321 and 348 arcsec along [1-100] and [-1-123], respectively, suggesting a high crystal quality of the template. The defect densities for BSFs and TDs are estimated to be 70 cm-1 and 5×107 cm-2, respectively, as measured by cathodoluminescence and transmission electron microscopy (not shown).10, 11 More detailed descriptions on substrate patterning, the 3-step growth process and defect blocking mechanism can be found in Ref. [11]. Semipolar LEDs were then grown on the fabricated templates along the [11-22] direction

by

atmospheric

pressure

MOCVD.

Trimethylgallium

(TMGa),

triethylgallium (TEGa), ammonia (NH3), trimethylindium (TMIn), disilane (Si2H6), and bicyclopentadienyl (Cp2Mg) sources were used as precursors and dopants. Figure 2(a) shows the schematic epitaxial structure, which consists of a 3 µm n-type GaN with a silicon concentration of 7×1018 atoms/cm3, a 150 nm In0.03Ga0.97N strain relaxed layer, 3 pairs In0.29Ga0.71N/In0.03Ga0.97N (2.5 nm/3.5 nm) multiple quantum



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wells (MQWs), a 15 nm p-type AlGaN electron blocking layer (EBL), a 120 nm p-type GaN with Mg concentration of 2×1020 atoms/cm3 and a 20 nm p+GaN with a higher Mg doping concentration. The relaxed In0.03Ga0.97N buffer layer helps to compensate the strain applied on the InGaN QWs grown thereon, which is evidenced by the XRD reciprocal space map (RSM) measurement shown in the supplemental figure. In0.03Ga0.97N quantum barriers (QBs) were used to increase the hole transportation within the active region.20 Finally, LEDs devices with a rectangular 0.1 mm2 size were fabricated using a conventional mesa structure. A 110 nm thick Sn-doped In2O3 (ITO) layer was deposited by electron beam evaporation to form a transparent current spreading layer as well as p-type ohmic contact. Mesa patterns were formed using chlorine-based reactive-ion etching (RIE) after photolithography. A Ti/Al/Ni/Au (10/100/100/100 nm) n-contact was deposited onto the nGaN. Cr/Ni/Au (25/20/500 nm) layers were deposited for the pads. Finally, the LEDs were diced, packaged on silver header, encapsulated with silicone and measured in an integrating sphere.

■ RESULTS AND DISCUSSION The composition of the InGaN QWs and the barrier layers and the alloy distribution in the wells were evaluated by APT.21~23 The pGaN layer was positioned at the top of a tapering tip approximately 40 nm in radius using a FEI Helios 600 dual beam FIB instrument and following standard procedure.24-25 Prior to the APT analysis, the tip shape and the layer thicknesses were examined by STEM. Figure 2(b) is the STEM high angle annular dark field (HAADF) image of the tip. The bright contrast layers are the three QWs whereas the dark contrast layer is the AlGaN EBL. The measured thicknesses for each QW/QB and the AlGaN layer are 11 nm and 15 nm, respectively. These values can be used as references for the reconstruction of the 3D volume. The APT experiment was performed using the Local Electrode Atom Probe (LEAP) 3000X HR instrument manufactured by Cameca. The specimen was cooled to a base temperature of 40 K and a high voltage between 3 and 6 kV was applied. Simultaneous pulsing of a Nd: YAG green laser (13 ps pulse, 532 nm green laser, 10


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µm laser spot size) at 200 kHz with a pulse energy of 0.5 nJ enables controlled field evaporation of the atoms from the tip surface. Atoms were evaporated at a constant rate of 0.02 atoms · pulse-1. APT 3D reconstruction was carried out using commercial software IVASTM. InGaN and AlGaN layer thicknesses measured in STEM were used as references to optimize the reconstruction parameters in the software.26 A 20×20×60 nm3 3D reconstruction of the three QWs and the EBL layers is presented in Figure 3(a). Figure 3(b) reports the corresponding variation of the In/(Al+In+Ga) and Al/(Al+In+Ga) ratios along the growth axis quantifying the amount of In and Al found in each of the layers in the region of interest. The ratios have been obtained by counting In, Al and Ga atoms in sampling boxes of 40×40×0.1 nm3 with a moving step of 0.1 nm. The peak In values measured in the bottom, central and top QWs are respectively 0.29 ± 0.01, 0.23 ± 0.01 and 0.27 ± 0.01, while the average In fraction measured in the bottom, central and top barrier layers are 0.04 ± 0.01, 0.05 ± 0.01 and 0.05 ± 0.01, respectively. These values are in good agreement with the compositions targeted by growth. Figure 4(a) is a 2D In concentration map corresponding to a cylindrical sampling volume extracted in the middle of the second QW and whose radius is 20 nm and depth is 1 nm. The voxel size to draw the concentration map is 0.5×0.5×0.5 nm3 with a Gaussian delocalization with standard deviation equal to 2 nm in each direction.26 The measured In fraction fluctuates between an upper value around 0.35 and a lower value around 0.1. Highly concentrated In clusters are not observed in this 2D map. Statistical distribution analysis (SDA) is used to highlight clustering effects in the wells.22, 26, 27 The same sampling volume was binned in 25-100 atoms bins and the indium fraction in calculated in each bins. Figure 4(b) shows the distribution of bin compositions and a comparison with the binomial distribution, which would be expected in the case of a random alloy. A χ2 test used to quantitatively compare the two distributions gave a value of 15.82 for 13 degrees of freedom and for a bin size of 100 atoms. Neither this example nor any other distribution taken for other bin sizes and in the other two QWs indicated rejection of the homogeneity hypothesis at a p



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value of 0.05 level of significance. Consequently, InGaN is a random alloy in these three QWs and no In rich clusters are observed. The optical and electrical properties of the LEDs devices are summarized in Figures. 5. The electroluminescence (EL) spectra of the semipolar (11-22) yellow/green InGaN LEDs are described in Figure 5(a). The inset is the luminance image of the chip on wafer at a current of 5 mA, which demonstrates a clear yellow/green emission. The blue-shift of the peak emission wavelength and FWHM versus current is plotted in Figure 5(b). At 20 mA, the peak emission wavelength is 551 nm with a FWHM of 45 nm, which indicates a good crystal quality of the In0.29Ga0.71N QWs. As the current increases from 5 to 100 mA, the LEDs shows a blue-shift as small as 11 nm, which is caused by the reduced polarization-related electric fields in QWs grown on semipolar orientation.7 Meanwhile, the FWHM slightly rises from 45 to 49 nm by increasing current from 5 to 100 mA, which is unlike the swift spectrum broadening for the conventional LEDs grown on c-plane with long emission wavelength.28 Furthermore, the current-power and current-voltage characteristics are presented in Figure 5(c). The output power is measured to be 2.4 mW at 100 mA, accompanied by a forward voltage of 3.3 V at 20 mA. It is worth to point out that this is the highest output power reported for semipolar yellow/green LEDs on sapphire substrate, which is attributed to both low defect density of GaN/sapphire template and the light extraction efficiency (LEE) enhancement by the voids emended within the (11-22) GaN template. Ray tracing simulations results show that the LEE can be enhanced by 30% by the (11-22) GaN templates compared to planar structures, as discussed in Ref. [29]. Finally, the dependence of EQE on the driven current is plotted in Figure 5(d). A peak EQE of 1.3% is achieved at 7 mA, followed by a low efficiency droop until 100 mA, which is among the highest reported performances for InGaN-based semipolar long wavelength LEDs on foreign substrate. The EQE remains lower than that of high performance green LEDs on bulk (11-22) GaN30,

31

and c-plane GaN/sapphire

(exceeding 35%)32, which is attributed to the ultrahigh crystal quality of the bulk substrates for semipolar GaN, as well as the device epitaxial maturity of GaN/sapphire given its current application in commercial LEDs. It is therefore


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expected that with further optimization of the growth conditions and chip fabrication and encapsulation, the performance and EQE of semipolar LEDs grown on foreign sapphire will benefit from the high structural quality and continue to improve.

■ CONCLUSION In summary, efficient semipolar (11-22) 550 nm yellow/green InGaN LEDs with In0.05Ga0.95N barriers are realized on 2-inch low defect density (11-22) GaN/patterned sapphire template. No Indium clusters were observed by APT. An output power of 2.4 mW at 100 mA and a peak EQE of 1.3% were achieved. The semipolar (11-22) yellow/green LEDs demonstrate a small blue-shift as well as low efficiency droop. These results indicate that the using low defect density (11-22) GaN templates are promising for achieving high efficiency long emission wavelength semipolar InGaN LEDs at a low cost.

■ AUTHOR INFORMATION Corresponding Author: Hongjian Li *Phone: (805) 280-5423. Fax: (805) 893-8983. *

E-mail: [email protected]

†

Hongjian Li and Michel Khoury contributed equally to this work.

■ ACKNOWLEDGMENTS The authors acknowledge the UCSB-Collaborative Research in Engineering, Science and Technology (CREST) Malaysia project and Solid State Lighting and Energy Center at UCSB for funding. A portion of this work was done in the UCSB nanofabrication facility.

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Figure 1. (a) Image of the 2-inch (11-22) GaN/sapphire template; (b) SEM cross sessional image of the semipolar (11-22) GaN/sapphire template. (c) XRD rocking curve along [1-100] and (d) XRD rocking curve along [-1-123].

Figure 2. (a) Schematic epitaxial structure of LEDs. (b). scanning transmission electron microscopy (STEM) image.

Figure 3. (a) A 20×20×60 nm3 3D reconstruction of the three QWs and the EBL layers. (b) the corresponding variation of the In/(Al+In+Ga) and Al/(Al+In+Ga) ratios along the growth axis quantifying the amount of In and Al found in each of the layers in the region of interest.

Figure 4. (a) A 2D In concentration map corresponding to a cylindrical sampling volume extracted in the middle of the second QW and whose radius is 20 nm and depth is 1 nm. (b) The distribution of bin compositions of In and comparison with the binomial distribution that would be expected in the case of a random alloy.

Figure 5. (a) EL spectra at various current; (b) Blue-shift and FWHM versus injection current; (c) Output power- current-voltage characteristics and (d) dependence of EQE on the injection current of the semipolar (11-22) yellow/green LEDs.


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Figure Figure 1

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Figure 3

Figure 4

Figure 5

Title (a) Image of the 2 inch (11-22) GaN/sapphire template; (b) SEM cross session image of the semipolar (11-22) GaN/sapphire template. (c) XRD rocking curve along [1-100] and (d) XRD rocking curve along [-1-123]. (a) Schematic epitaxial structure of LEDs. (b). scanning transmission electron microscopy (STEM) image. (a) A 20×20×60 nm3 3D reconstruction of the three QWs and the EBL layers. (b) the corresponding variation of the In/(Al+In+Ga) and Al/(Al+In+Ga) ratios along the growth axis quantifying the amount of In and Al found in each of the layers in the region of interest. (a) A 2D In concentration map corresponding to a cylindrical sampling volume extracted in the middle of the second QW and whose radius is 20 nm and depth is 1 nm. (b) The distribution of bin compositions of In and comparison with the binomial distribution that would be expected in the case of a random alloy. (a) EL spectra at various current; (b) Blue-shift and FWHM versus injection current; (c) Output power- current-voltage characteristics and (d) dependence of EQE on the injection current of the semipolar (11-22) yellow/green LEDs.



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Fig. 1 283x196mm (96 x 96 DPI)



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Fig. 2 162x108mm (150 x 150 DPI)


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Fig. 3 170x114mm (150 x 150 DPI)



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Fig. 4 187x81mm (150 x 150 DPI)


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Fig. 5 251x178mm (150 x 150 DPI)


green InGaN light-emitting diodes on low defect density

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