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phys. stat. sol. (c) 4, No. 5, 1605–1608 (2007) / DOI 10.1002/pssc.200674292 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim InGaN/GaN nanopillar-array light emitting diodes C. J. Neufeld*, C. Schaake, M. Grundmann, N. A. Fichtenbaum, S. Keller, and U. K. Mishra Department of Electrical and Computer Engineering, University of California at Santa Barbara, Santa Barbara, CA 93106, USA Received 25 September 2006, revised 6 November 2006, accepted 10 November 2006 Published online 11 April 2007 PACS 78.30.Fs, 78.55.Cr, 78.60.Fi, 78.67.Lt, 85.60.Jb GaN light emitting diodes were fabricated from arrays of nanopillars with embedded InGaN quantum wells. InGaN heterostructures were grown by MOCVD on n-type GaN templates and pillars were fabri- cated by laser interference lithography and subsequent reactive ion etching and annealing. The tops of the pillars were coalesced by lateral growth of p-type GaN by MBE forming a planar contact layer. This structure enables integration with standard planar processing while taking advantage of the nanopillar structure. LEDs were fabricated and characterized by electroluminescence, current-voltage, and output power vs. current measurements. The devices showed rectifying behavior with a turn-on voltage of 3 V and electroluminescence peak at 400 nm. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction The InxGa1–xN material system is of major significance in the field of optoelectronic devices. The band gap energies of this system span the entire visible spectrum from 0.7 eV for InN to 3.4 eV for GaN, ma- king it ideally suited for light emitters in the visible [1]. One-dimensional nanostructured semiconductor materials are very promising for applications in ad- vanced optoelectronic devices and have several advantages over bulk materials. Nanopillar structures have also been shown to relieve strain induced by allowing the material to relax [2] and thus accommo- date large lattice mismatches at hetero-interfaces. Light extraction in nanostructured materials may be enhanced due to increased surface area and reduced reflection at the air-semiconductor interface [3]. Additional enhancements may be had through photonic crystal effects in carefully arranged periodic arrays of nanostructures [4]. Many devices have been reported which take advantage of some of these properties [5, 6], employing such serial processing techniques as focused ion beam deposition [7, 8], electron beam lithography [5] or electrostatic arrangement [9, 10]. In addition, many of these techniques demonstrated, rely on inherently random self-assembled processes which do not allow for exact place- ment of structures and therefore do not take advantage of any photonic crystal effects. In this paper, we report on the fabrication and characterization InGaN/GaN nanopillar array LEDs (NPA-LED) by a top-down etching process which relies solely on scalable, parallel processing tech- niques. Triangular-lattice arrays of GaN nanopillars with embedded InGaN/GaN multiple quantum wells (MQWs) were fabricated and capped by lateral overgrowth of a p-type GaN layer. The devices were characterized by current-voltage, electroluminescence (EL) and output power measurements. * Corresponding author: e-mail: cjn@umail.ucsb.edu 1606 C. J. Neufeld et al.: InGaN/GaN nanopillar-array light emitting diodes © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com Fig. 1 SEM images taken at stage angle of 60o. a) Etched nanopillar array before re-growth. b) Nanopillar array after p-GaN re-growth by MBE. 2 Experimental InGaN multiple quantum well (MQW) samples were grown by metal organic chemical vapor deposition (MOCVD) on a c-plane Sapphire substrate. Growth was initiated with a GaN nucleation layer followed by a 400 nm GaN:Si buffer layer and a five period MQW region consisting of 3 nm InGaN wells and 10 nm GaN:Si barriers and a 10 nm undoped GaN cap. The indium composition in the wells was esti- mated to be 28%. The fabrication of the nanopillars began with the deposition of SiO2 by plasma enhanced chemical vapor deposition as a sacrificial etch mask on the MQW sample. Photoresist dots were then patterned by double-exposure laser interference lithography using a 325 nm HeCd laser in a Lloyd’s mirror configura- tion [11] with a stage angle of 30°. The sample was partially exposed then rotated by 60° for a second exposure. The resulting pattern was resist dots with 100 nm diameter arranged in a triangular lattice with a period of 240 nm. The resist pattern was transferred to the SiO2 sacrificial layer by CHF3 inductively coupled plasma etching, and nanopillars were then formed by Cl2 reactive ion etching (RIE). Figure 1a shows the resulting nanopillar array with an individual pillar height of 220 nm and diameter of 110 nm. After etching, the pillars were annealed at 860 °C for 30 min in a mixture of NH3 and N2 in the MOCVD and characterized by photoluminescence (PL). In order to form a planar contact layer, the pillars were coalesced by lateral overgrowth by plasma-assisted molecular beam epitaxy (MBE). A coalescence layer of 40 nm of undoped GaN was grown to enhance selective lateral growth, and a 200 nm of GaN: Mg was grown to form a p-type contact layer. Figure 1b shows a SEM image of the coalesced pillar array. LED mesas were defined by contact lithography and etched by Cl2 RIE. Transparent contacts were deposited to the p-layer by electron beam evaporation of Ni/Au (5 nm/5 nm) and subsequent annealing at 500 °C for 5 min under atmospheric pressure in a mixture of N2 and O2. Contacts to the n-type material were p-GaN InGaN/GaN MQW n-GaN nanopillars Al/Au n-contact Transparent Ni/Au p-contact Sapphire Substrate Fig. 2 Cross-section diagram of nanopillar array LEDs (NPA-LEDs). phys. stat. sol. (c) 4, No. 5 (2007) 1607 www.pss-c.com © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 380 400 420 440 460 480 500 520 540 560 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 P L I n te n s it y ( A U ) Wavelength (nm) As Grown After Etch After Anneal Fig. 3 RT PL measurements of as-grown QW sample (dashed line), nanopillar array after etching (dotted line), and after anneal (solid line). -4 -2 0 2 4 6 0 10 20 30 40 50 Ni/Au (5 nm/5 nm) p-Contacts Mesa (300 µm) 2 C u rr e n t (m A ) Voltage (V) Fig. 4 Typical current-voltage relationship for NPA-LED at room temperature. 0 20 40 60 80 100 120 140 0.0 0.2 0.4 0.6 0.8 1.0 300 400 500 600 700 800 E L I n te n s it y ( a .u .) Wavelength (nm) P o w e r ( u W ) Current (mA) Fig. 5 Optical output power vs. Current for NPA-LED. Inset: RT EL Spectrum @ 150 mA. formed by deposition of Al/Au (30 nm/ 300 nm). A cross-sectional diagram of the final NPA-LED struc- ture is shown in Fig. 2. 3 Results and discussion Figure 3 shows PL measurements of the as grown sample after pillar fabrication and after annealing. The observed 63% drop in PL peak intensity after the pillars were etched can be explained by etch damage induced by ion bombardment. A blue shift of 9 nm was also observed after etching, from 467 nm for the as-grown sample to 458 nm after etching the pillars. This shift was consistently observed in many sam- ples and we believe it is due to indium loss during the etching process [2]. After annealing the etched nanopillar arrays the PL peak intensity recovered 260% from the etched sample indicating etch dam- age was successfully annealed, and no significant peak shift was observed. More details on the mate- rial growth and pillar fabrication/characterization have been published elsewhere [2]. All electrical characterization was performed on a Tektronix 370A curve tracer at room temperature and optical measurements were made through the substrate. Current-Voltage (I-V) measurements showed rectifying behavior and a turn-on voltage of approximately 3 V, series resistance of 53 Ω, and reverse bias leakage of 2.1 mA @ -5 V. The large reverse bias leakage may be attributed to surface damage during pillar etching as has been reported in the literature [12]. This effect causes considerable leakage in our devices due to the large surface area of our structures, and surface passivation techniques will be used in the future in attempts to reduce the effects of etch damage. Leakage may also be due to the conduction along threading dislocations in the p- type GaN grown by MBE. A typical I-V characteristic is shown in Fig. 4. 1608 C. J. Neufeld et al.: InGaN/GaN nanopillar-array light emitting diodes © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com EL and output power measurements were made on wafer, at room temperature, and through the sub- strate. Figure 5 (inset) shows the EL spectrum with a strong emission peak at 404 nm. A large blue shift in EL peak emission was observed with reference to the PL measurements. The reason for this large shift is believed to be due to recombination in the undoped coalescence layer. This leads us to believe the re- growth interface is causing junction placement to shift away from the quantum wells. This may be due to a Si spike at the re-growth interface that has been reported on in the literature [13]. Figure 5 shows opti- cal power vs. current, a roughly linear P vs. I characteristic is observed and saturates due to self heating at 170 mA. 4 Conclusions Nanopillar Array LEDs were fabricated based on a top-down fabrication process. N-type GaN:Si and InGaN MQW active region was grown by MOCVD. Nanopillar arrays with embedded MWQs were fabricated by laser interference lithography and RIE etching of the MWQ structure. A planar p-type coalescence layer of GaN:Mg was laterally grown by MBE. Measurements showed a shift between PL and EL peak emission wavelengths. This can be attributed to recombination away from active region due to junction placement problems arising from the re-growth interface. The diodes showed rectifying be- havior with a turn-on voltage of 3 V. References [1] V. Yu. Davydov, A. A. Klochikhin, R. P. Seisyan, V. V. Emtsev, S. V. Ivanov, F. Bechstedt, J. Furthmüller, H. Harima, A. V. Mudryi, J. Aderhold, O. Semchinova, and J. Graul, phys. stat. sol. (b) 229(3), R1 (2002). [2] S. Keller, C. Schaake, N. A. Fichtenbaum, C. J. Neufeld, Y. Wu, K. McGroddy, A. David, S. P. DenBaars, C. Weisbuch, J. S. Speck, and U. K. Mishra, J. Appl. Phys. 100, 054314 (2006). [3] J. S. Cabalu, C. Thomidis, T. D. 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