Simultaneous improvement on light extraction and directional radiation of LEDs by nanocones Jialun He, Version 7, 08/06/2015 Abstract we fabricated cone- and rod-like ZnMgO nanostructures (NCs and NRs) on InGaN/GaN light emitting diodes(LEDs) by a co-evaporated chemical vapor deposition method. Without sacrificing the electrical performances of LEDs ,cone-like NRs can enhance the light extraction efficiency by 10.5% at 20 mA and shape the radiation profile collimated from 136°to 121°. From optical experiments and simulation, we find the waveguiding effect of NCs and accommodation of abrupt index change by the tapered diameter of cone-like ZnMgO nanostructures . Introduction Nitride based LEDs act as promising candidates in displays, solid-state lighting, signaling.The performance is unsatisfactory due to low light extraction efficiency and poor directional emission [1]. The low light extraction efficiency is mainly due to the considerable Fresnel loss and total internal reflection loss caused by the large difference in refractive index (n) at air/GaN(n~2.43) or the air/ITO(n~2.06) interfaces.[2] It is unavoidable to experience a severe Fresnel reflection loss at the surface of the semiconductor, considering the abrupt interface between the two media. As for the total internal reflection, phonons emitted from the MQWs would be reflected from the interface, reabsorbed, and internally confined if the escaped angle is larger than the critical angle. Based on Snell’s law, the critical angles in LEDs are 54.83°and 29.04°at the p-GaN/ITO and ITO/air interface, respectively[3][4]. A small view angle is desired for many LED applications, such as pocket lamps, smart phone cameras, and vehicle head lamps[5]. Briefly speaking, it is important to develop high light-extraction efficiency LEDs with directional radiation profiles for next-generation solid-lighting devices[6]. In this work, ZnMgO nanocones(NCs) and nanorods(NRs) were synthesized on nitride-based LEDs by a low pressure chemical phase deposition(LPCVD) method. The LEDs with ZnMgO NCs(NCLED) exhibit an increased extraction efficiency by 20% at 15 mA with the view angle improved from 150°to 120°, as compared to the bare LEDs(BLED), and LEDs with nanorods (NRLED). We experimentally and theoretically demonstrated that better light extraction efficiency and a more collimated radiation pattern are attributed to the waveguiding effect of ZnMgO nanocones and the mitigation of abrupt index change by the tapered finish of cone-like ZnMgO nanostructure. Based on the optical experiments and theoretical simulations, our findings provide prominent design criteria for the morphology of nanostructures for high-efficiency optoelectronic devices. Experimental The epitaxial layer of the LEDs followed the conventional growth steps. Nitride-based epitaxial layers were grown on sapphire substrates with c-face orientation by the MOCVD technique. The device structure of the InGaN/GaN MQW LEDs were formed by sandwiching nine periods of In0.25Ga0.75N(5nm)/GaN(15nm) MQWs between n-type (2μm) and p-type(0.3μm) GaN layer. ITO was deposited by electron beam evaporation on p-GaN to form transparent ohmic contacts, followed by the deposition of metal electrodes of Ti/Al/Ni/Au on the top surface of ITO and n-GaN. The experimental setup for ZnMgO nanocones growth is depicted schematically in Figure 1a. The precursors used in the CVD synthesis of ZnMgO nanocones were a stoichiometric mixture of Zn and Mg powders. In short, the raw material powders were ground thoroughly and uniformly put in a long fused silica boat. Then the boat was placed into the hot center of the furnace. The LED devices were then placed downstream from the tube reactor separated from the starting materials by 6 in. The distance is vital to the shape of ZnMgO and growth rate. The quartz tube was purged using N2 to drive out air before heating. Then the quartz tube was heated to 500 ℃ with heat rate of 10 ℃ min-1. During this heating period, N2 was introduced with a flow rate of 10 sccm (standard cubic centimeter per minute).Once the temperature reached 350 ℃, O2 was introduced with a flow rate of 2 sccm. After 30 min of reaction, the O2 supply was cut off and the furnace began to cool down. After the reaction, a layer of white product was found deposited on the LED surface. The microstructures of the structures were observed using field emission scanning electron microscope (FE-SEM, Hitachi S-4800, 10 kv) with INCA x-act energy dispersive Spectrometer(EDS). Transmission electron microscopy (TEM) AND high resolution TEM (HRTEM) images were obtained on a JEOL JEM-2100F microscope at 200 kv. The crystal structure of the samples was investigated using an X-ray diffractometer (D/MAX-2500) equipped with a nickel filtered Cu Kα radiation (1.54056Å) at 40 kv and 140 mA. XRD spectra were collected in a 2Ө range of 30-80°at a scanning rate of 0.02°per step and 0.15s per step. Photoluminescence (PL) spectra were detected by a PMT928 PL spectrometer with excitation wavelength of 325nm. Fig. 1 Schematic diagram of the experimental apparatus for growth of ZnMgO nanostructures. Results and discussion Fig. 2 SEM images of ZnMgO nanocones. The inset in (a) highlights the tapered ending on the top of the nanostructures. (b) Schematic of a typical cone-like nanstructure. Fig.2 shows the morphology of the as-grown ZnMgO cone-like nanostructures. The cross-sectional scanning electron microscopy (SEM) presented in Fig.1(a) and (b) is the schematic of an individual cone-like nanostructure. One can see that the cone-like nanostructures consists of two fragments: the rod-like body and the tapered finish at the top. From the high-magnification SEM image shown in the inset of Fig.1(a), the gradual decrease in radius of this tapered part can be perceived. The dimensions of the nanostructures are highlighted in Fig.1(b): the average diameter and height of the rod-like bodies are ~120nm and ~1.75μm, respectively, and the length of the tapered part is 230±200nm. Fig.3 Typical I-V and L-I characteristics of the bare LEDs and LEDs with ZnMgO nanocones and nanorods. The inset depicts the EL spectra of the LEDs under 20 mA injection current. The forward current-voltage(I-V) characteristics of the LEDs are presented in Fig.3. The forward voltage (Vr ) under 20mA injection current are 2.88 and 2.89 V for the LEDs with and without cone-like ZnMgO nanostructures, respectively. The I-V curves are almost identical, indicating that the CVD method for ZnMgO cone-like nanostructures does not degrade the electrical properties of the LEDs. Fig.3 also demonstrates the relative light output intensity as a function of injection current, and the inset in Fig.3 reveals the electroluminescence (EL) spectra of two kinds of LEDs measured at 20mA injection current. After introducing cone-like ZnO nanostructures, the peak intensity of the LED is increased by 10.5% and 16.8% at injection currents of 20 and 100mA, respectively, indicating excellent light extraction characteristics of cone-like ZnMgO nanostructures. Moreover, the external quantum efficiency (EQE) is an important characteristic for revealing the LED performance. The EQE values were obtained by varying the input current and measuring the output light power of the LEDs in an integrating sphere. The EQE of the bare device is 13% at 20mA. By applying cone-like ZnMgO nanostructures, the EQE achieves 14.4%. The enhanced light extraction can be attributed to the waveguiding and the tapered tip morphology of the cone-like ZnMgO nanostructures, which will be discussed with experimental and simulation results in the following paragraphs. Fig.4 OM images of the ZnMgO nanocone before and after laser illumination. The 532nm laser was injected into the nanocone at the bottom end. In order to prove the waveguiding effect of the ZnMgO nanocones, several nanocones were dispersed on a SiO2/Si substrate. Fig. 4(a) shows the optical microscope image of a single ZnMgO nanocone. As shown in Fig. 4(b), When a 532nm laser light was injected into the nanocone at the bottom by the objective lens, it was guided out at the top end, clearly evidencing the waveguiding characteristics of the nanocones. Fig.5 Time-averaged and normalized TE field distribution near the surface of the LEDs with different surface morphologies: (a) bare, (b) flat-end ZnMgO nanorods, and (c) ZnMgO nanocones. To further reveal the light propagation through the cone-like ZnMgO nanostructures, a simulation based on the finite-difference time-domain(FDTD) method is carried out[7]. For simplicity, the nanocones are set to be perpendicular to the ITO surface and equally spaced. The diameter and the height of the nanocone bodies are 130nm and 1.5μm, respectively, and the the length of the tapered part is 300nm. Three different LEDs: the bare LED, the LED with flat-end nanorods and nanocones , are compared in the simulation. To match with peak emission wavelength in the EL spectra, the monochromatic excitation source is chosen to be at 528.5nm. A detector is set up at a distance far enough to detect the time-averaged power emitted from the LEDs. Fig. 5(c)-(e) displays the transverse electric (TE) field intensity distributions inside the three different surface structures. From the coloring at the top of Fig. 5(a)-(c), one can note that the LEDs with nanorods or nanocones exhibit stronger field intensity. The employment of the nanostructures effectively reduces the back reflection and ensures better propagation of light by matching the optical impedance at the interface[8]. Furthermore, the cone-like nanostructure show better light extraction capability than the flat-end nanorods. The electric fields inside the nanostructures reveals the impact of the tapered diameter of the cone-like nanostructures. Due to the mismatch in optical impedance of ZnO and air, some of the light is trapped in the flat-end nanorods. On the other hand, the nanocone ensures a good coupling of the traveling light into the air. The light extraction enhancement via the nanocones can be explained by two steps. Firstly, a major portion of the photons emitted from the MQWs are easily captured by the ZnMgO nanostructure due to the well matched refractive index between ZnMgO and ITO[9]. The captured photons are then guided within the ZnMgO nanocones. Secondly, the traveling light inside the ZnMgO can escape into air because of the improved impedance match between air and and nanocones[10]. However, the light would be trapped in the nanorods owing to the large optical impedance difference between air and the ZnMgO. Conclusions In this paper, we reported for the first time the application of cone-like ZnMgO nanostructures grown on nitride-based LEDs, using a simple, low temperature and etching-free CVD method, for highly enhancing the light extraction by 10.5% at 20mA and shaping the radiation pattern from 136°to 121°without complexly tailoring the configuration of MQW regions. This is attributed to the waveguiding effect and the creation of gradual refractive index profile at the interface between air and NC layers by the tapered diameter of the cone-like ZnMgO nanostructures, confirmed by the experiments and optical simulations. The cone-like ZnMgO nanostructures using scalable, cost-effective fabrication technique demonstrates a new approach to achieve better light extraction and more collimated radiation pattern in the existing solid-state lighting technology. Acknowledgements The authors would like to thank the financial support from the ,,, References 1. J. K. Sheu, C. M. Tsai, M. L. 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