Journal of Science: Advanced Materials and Devices (2017) 150e155 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article High efficiency green/yellow and red InGaN/AlGaN nanowire light-emitting diodes grown by molecular beam epitaxy M.R Philip a, D.D Choudhary a, M Djavid a, K.Q Le b, c, J Piao d, H.P.T Nguyen a, e, * a Department of Electrical and Computer Engineering, New Jersey Institute of Technology, University Heights, Newark, NJ 07102, USA Faculty of Science and Technology, Hoa Sen University, Ho Chi Minh City, Viet Nam c Center for Mesoscopic Sciences, Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan d Epitaxial Laboratory Inc., Tiana Place, Dix Hills, NY 11746, USA e Electronic Imaging Center, New Jersey Institute of Technology, University Heights, Newark, NJ 07102, USA b a r t i c l e i n f o a b s t r a c t Article history: Received 12 May 2017 Received in revised form 19 May 2017 Accepted 19 May 2017 Available online 31 May 2017 We report on the achievement of high efficiency green, yellow, and red InGaN/AlGaN dot-in-a-wire nanowire light-emitting diodes grown on Si(111) by molecular beam epitaxy The peak emission wavelengths were altered by varying the growth conditions, including the substrate temperature, and In/ Ga flux ratio The devices demonstrate relatively high (>40%) internal quantum efficiency at room temperature, relative to that measured at K Moreover, negligible blue-shift in peak emission spectrum associated with no efficiency droop was measured when injection current was driven up to 556 A/cm2 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Light-emitting diodes Molecular beam epitaxy Nanowire Coreeshell III-Nitride Introduction Phosphor-free white light-emitting diodes (LEDs) have been intensively studied and identified as an emerging platform for future solid-state lighting and displays To realize high performance, low cost phosphor-free white LEDs, it is critically important to develop high efficiency LEDs with emission wavelengths in the deep green to red spectral range [1,2] The achievement of such devices using conventional InGaN/GaN quantum well heterostructures has been difficult, due to the presence of large densities of dislocations and polarization fields [3] Moreover, they suffer from efficiency droop, which has been explained by the Auger recombination [4], poor hole transport [5,6], and carrier delocalization [7] in InGaN/GaN heterostructures In this regard, InGaNbased nanowire heterostructures have been intensively investigated, which provides a relatively defect-, strain- and polarization- * Corresponding author Department of Electrical and Computer Engineering, New Jersey Institute of Technology, University Heights, Newark, NJ 07102, USA E-mail addresses: khaidotle@ims.ac.jp (K.Q Le), hieu.p.nguyen@njit.edu (H.P.T Nguyen) Peer review under responsibility of Vietnam National University, Hanoi free platform for realizing high performance LEDs and other nanoscale devices [8e11] Tunable emission has been demonstrated using InGaN/GaN nanowire heterostructures [12e14] In spite of the progress, however, the achievement of high efficiency nanowire LEDs has remained elusive Due to the large surface-to-volume ratios, the surface plays a key role in the operation and electrical and optical properties of IIInitride nanowire LEDs [15,16] Depending on the energy levels of the surface states, as well as the surface stoichiometry, Fermi-level pinning has been theoretically predicted and experimentally observed on the (11-00) plane [17,18], i.e the lateral surfaces of commonly reported GaN nanowire LEDs The resulting lateral electric field, as well as the associated surface nonradiative recombination is highly detrimental to the performance of GaNbased nanowire LEDs In this context, we have recently developed InGaN/AlGaN dot-in-a-wire nanoscale heterostructures, which can provide enhanced carrier confinement and carrier injection, thereby leading to high emission efficiency [19] Moreover, by engineering color emission of such InGaN/AlGaN nanowire heterostructures, we demonstrated full-color emission across the visible spectrum, leading to achievement of phosphor-free white LEDs [19] In this paper, we focus on developing high efficiency green/ yellow, and red LEDs for their applications in high power phosphor- http://dx.doi.org/10.1016/j.jsamd.2017.05.009 2468-2179/Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) M.R Philip et al / Journal of Science: Advanced Materials and Devices (2017) 150e155 151 Fig (a) Schematic illustration of an InGaN/AlGaN dot-in-a-wire LED heterostructure; (b) A 45 tilted scanning electron microscopy image showing the morphology of the InGaN/ AlGaN dot-in-a-wire heterostructures grown on a Si(111) substrate by molecular beam epitaxy Fig Fabrication flow of InGaN/AlGaN nanowire LEDs 152 M.R Philip et al / Journal of Science: Advanced Materials and Devices (2017) 150e155 quantum efficiencies were measured for GaN-based nanowire yellow LEDs No efficiency droop was observed for injection current as high as 556 A/cm2 at room temperature Experimental The InGaN/AlGaN dot-in-a-wire LED heterostructures, illustrated in Fig 1(a), were spontaneously formed on n-type Si(111) substrates under nitrogen rich conditions by a Veeco Gen II molecular beam epitaxial (MBE) system equipped with a radiofrequency plasma-assisted nitrogen source GaN nanowires were doped n- and p-type using Si and Mg, respectively The growth conditions for GaN include a growth temperature of ~750e800 C, nitrogen flow rate of sccm, and forward plasma power of ~350 W The device active region consists of ten vertically aligned InGaN dots, separated by nm AlGaN barrier layers The InGaN/AlGaN quantum dot heterostructures were grown at relatively low temperatures (560e620 C) to enhance the incorporation of In The nanowire diameter and density can be controlled by the substrate temperature and/or In/Ga flux ratios, while the nanowire length can be adjusted by the growth duration [20,21] Using such nanowire structures, have demonstrated that, by varying the growth conditions, high brightness green, yellow and red emissions from the InGaN/AlGaN dot-in-a-wire heterostructures can be achieved Surface morphology of the dot-in-a-wire LED heterostructures was studied by scanning electron microscopy (SEM) A 45 degreetitled SEM image is illustrated in Fig 1(b) The wire areal density is ~1 Â 1010 cmÀ2 Moreover, during the growth of AlGaN barrier layer, an AlGaN shell layer is spontaneously grown due to the diffusion-controlled growth process that fully covers the InGaN dot active region [22] Such coreeshell heterostructures exhibit drastically reduced nonradiative surface recombination, and enhanced carrier injection efficiency More detailed information on the MBE growth and structural properties of this InGaN/AlGaN coreeshell nanowire structures can be found elsewhere [19,22] Fig presents the device fabrication process It involves the use of a polyimide resist for surface planarization, standard photolithography and dry etching processes The nanowire LED sample (Fig 2(a)) was first fully covered with polyimide resist, followed by oxygen plasma dry etching to expose the top of nanowires for metallization, shown in Fig 2(b) Top p-contact with Ni(5 nm)/Au (5 nm)/ITO (200 nm) was then deposited on the nanowire surface, illustrated in Fig 2(c) Subsequently, thick layers of Ni(10 nm)/ (100 nm) and Ti (10 nm)/Au (100 nm) were deposited on top of ITO transparent contact to sever grid-metal contact to facilitate the carrier injection and backside metal contact, respectively, shown in Fig 2(d) The LED devices with areal size of 300 Â 300 mm2 were chosen for characterization Results and discussion Fig Room temperature electroluminescence spectra under different injection currents for (a) green, (b) yellow, (c) red nanowire LEDs The inset of each figure shows the corresponding light emissions from green, yellow and red nanowire LEDs free white LEDs The fabrication and characterization of InGaN/ AlGaN dot-in-a-wire nanowire LEDs on Si(111) substrates were performed We observe that the emission wavelengths can be tuned from green to red spectral range by varying the sizes and/or compositions of the dots Moreover, relatively high (>40%) internal Performance characteristics of the dot-in-a-wire LEDs were measured under pulsed bias conditions with 1% duty cycle to minimize junction heating effect Strong green, yellow, and red emissions were measured from the InGaN/AlGaN dot-in-a-wire LEDs at room temperature, shown in Fig The electroluminescence (EL) spectra of the green, yellow, and red nanowire LEDs under different injection current varied from 30 mA to 400 mA were shown in Fig 3(a)e(c), respectively At injection current of 400 mA, the peak emission wavelengths at ~535, 585, and 645 nm for green, yellow, and red LED devices, respectively It may also be noticed that the spectral linewidths increases progressively with emission wavelengths, varying from 75, 105, to 125 nm for the green, yellow, and red-emitting devices, respectively This is a direct consequence of the enhanced In phase separation with M.R Philip et al / Journal of Science: Advanced Materials and Devices (2017) 150e155 153 Fig 1931 Commission International de l'Eclairage (CIE) chromaticity diagrams of the light emission of green (triangles), yellow (circles), and red (stars) LEDs increasing In compositions, that leads to the formation of In-rich nanoclusters in the dots as well as in the barrier layers The emission properties of LEDs are depended on the compositions, the sizes of the dots, and the diameter of the nanowires as well These parameters can be controlled by adjusting substrate temperature, growth duration, and In/Ga flux ratio The achievement of strong emission in long wavelength from green to red region attributed to the successful usage of coreeshell nanowire heterostructure associated with the embedded quantum dots We have further investigated variations of the peak emission wavelengths with injection currents for the dot-in-a-wire LEDs Illustrated in Fig 3(a) and (b), peak wavelengths of the green and yellow-emitting LEDs are virtually invariant with increasing current, suggesting the presence of a negligible quantum-confined Stark effect [12,23] A very small blue shift (~3e5 nm), on the other hand, was observed for the red-emitting devices, illustrated in Fig 3(c) The highly stable emission characteristics are further illustrated in Fig 4, which shows locations of the light emission from the various LEDs on the 1931 Commission International de l'Eclairage (CIE) chromaticity diagram under injection currents from 100 to 400 mA As expected, the green (triangles) and yellow (circles) LEDs exhibit nearly invariant CIE chromaticity coordinates of (x ¼ 0.22, y ¼ 0.55) and (x ¼ 0.44, y ¼ 0.50) with increasing current, respectively The red (stars) devices show very small variations, with the derived CIE chromaticity coordinates being (x z 0.54e0.55, y z 0.36e0.37) We have further studied the currentevoltage and light-current characteristics of the yellow LEDs The defect density, polarization field, and internal electric field induces quantum-confined Stark effect were minimized also results in perfect diode performances with very low leakage current of ~0.5 mA at À6 V, as presented in Fig 5(a) We have further confirmed that the dot-in-a-wire yellow LEDs exhibit virtually no efficiency droop at room temperature Illustrated in Fig 5(b), the output power increases linearly with current for the entire measurement range (up to ~ 556 A/cm2) This observation is consistent with recent studies that Auger recombination is significantly reduced in InGaN/GaN nanowire heterostructures, due to the reduced defect densities [16] Additionally, hole transport problem may also be minimized with the use of ptype modulation doping in the device active region [14] and the use of self-distributed AlGaN multi-shell electron blocking layer [22,24] Finally, we have investigated the internal quantum efficiencies (IQEs) of the dot-in-a-wire LEDs by comparing the integrated electroluminescence intensity measured at 300 K to that measured at K under the same injection current Shown in Fig 5(c), the IQEs increase with injection currents and reach maximum values of 44.7% for injection currents at ~300 mA (~333 A/cm2) for the yellow-emitting LEDs, suggesting a small, or negligible efficiency droop under relatively high current injection conditions The peak IQE was measured at high injection current (~333 A/cm2) which is significantly higher than that of conventional InGaN/GaN thin-film LEDs which is normally at 10e20 A/cm2 [14,25] The very slow rising trend of the IQE has been commonly reported in nanowire LEDs [14,26,27] and is attributed to the presence of large surface states and defects on nanowire surfaces More detailed information was presented in our previous study [16,28] The negligible efficiency droop is attributed to the strong carrier confinement provided by the quantum dot heterostructures, the enhanced carrier injection, the reduced electron overflow and other higher order effects on the device quantum efficiency [29,30] Compared to previously reported InGaN/GaN nanowires [31] or well/disk-in-awire LED heterostructures [32,33] in the same wavelength range, the unique dot-in-a-wire heterostructures exhibit significantly higher IQE which can be explained due to the higher effective carrier confinement along the wire radial direction, thereby 154 M.R Philip et al / Journal of Science: Advanced Materials and Devices (2017) 150e155 For practical lighting applications, the performances of such nanowire LEDs should be further improved for the enhanced light extraction efficiency We had previously reported that nanowire LEDs with large diameters exhibit higher carrier injection efficiency due to the reduced surface nonradiative recombination, compared to that of a smaller diameter wires However, large diameter nanowires can suffer from large dislocation density in the nanowire structures, resulted in the reduced quantum efficiency Therefore, the optimized nanowire density, height, and diameter play critical roles and lead to further enhanced light extraction efficiency Moreover, the Ni/Au/ITO top metal contact should be further optimized or replaced by a better transparent contact design for higher light extraction efficiency Conclusion In summary, we have demonstrated that, with the use of selforganized InGaN/AlGaN dot-in-a-wire heterostructures, 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(a) green, (b) yellow, (c) red nanowire LEDs The inset of each figure shows the corresponding light emissions from green, yellow and red nanowire LEDs free white LEDs The fabrication and characterization... Materials and Devices (2017) 150e155 [12] W Guo, M Zhang, A Banerjee, P Bhattacharya, Catalyst-free InGaN/ GaN nanowire light emitting diodes grown on (001) silicon by molecular beam epitaxy, Nano... T.T Pham, D Misra, H.P.T Nguyen, Controlling color emission of InGaN/ AlGaN nanowire light- emitting diodes grown by molecular beam epitaxy, J Vac Sci Technol B 35 (2017), 02B108-1e02B108-5 [20]