Gallium nitride nanostructures for light-emitting diode applications

Nano Energy (2012) 1, 391–400

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REVIEW

Gallium nitride nanostructures for light-emitting diode applications Moon Sung Kanga,b, Chul-Ho Leea, Jun Beom Parka, Hyobin Yooa,c, Gyu-Chul Yia,n a

National Creative Research Initiative Center for Semiconductor Nanorods, Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Republic of Korea b Department of Chemical Engineering, Soongsil University, Seoul 156-743, Republic of Korea c Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea Received 11 January 2012; received in revised form 7 March 2012; accepted 7 March 2012 Available online 16 March 2012

KEYWORDS

Abstract

Gallium nitride; Nanostructures; Light-emitting diodes

This review summarizes recent research on GaN nanostructures for light-emitting diode (LED) applications. GaN nanostructure fabrication methods are first discussed, followed by a brief explanation of the basic components of the LED structure based on nitride nanostructures. Various device architectures of nanostructured GaN LEDs, as the main focus of the review, are then presented, covering research from the early LEDs based on a single GaN nanostructure to the most advanced LEDs based on GaN nanostructure arrays on flexible substrates. The research discussed in this review will promote novel applications of GaN LEDs that exploit the advantages of nanostructures. & 2012 Elsevier Ltd. All rights reserved.

Introduction Gallium nitride, GaN, features physical characteristics that are highly promising for light-emitting diode (LED) applications [1]. First, the bandgap of GaN is widely tunable in the ultraviolet (UV) and visible ranges by alloying the material with AlN and InN, respectively [2–4]. Second, intrinsically few surface states that act as recombination centers in the bandgap exist in GaN [5]. Third, both p- and n-type GaN can n

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be readily achieved by introducing trace amounts of Mg and Si, respectively, into the material [6–9]. This doping capability enables the formation of an epitaxial p–n homojunction, which leads to highly efficient radiative recombination between electrons and holes. Finally, GaN is stable both chemically and thermally [10–12]. Consequently, substantial efforts have been made to develop GaN into LEDs. In particular, blue and green color GaN LEDs with high efficiency and long-term stability are commercially available [13]. To fabricate GaN LEDs, one must acquire high quality GaN thin films, which are conventionally grown on a crystalline substrate that has an epitaxial relationship with GaN. Otherwise, mismatches in lattice constants and thermal expansion

392 coefficients between GaN and the substrate cause structural defects in the film, resulting in poor LED performance [14]. Thus, a limited type of substrates can be used to prepare highquality GaN thin films, such as silicon carbide or sapphire substrates. However, critical limitations of these substrates prevent even broader applications of LEDs based on GaN. For example, both silicon carbide and sapphire wafers are expensive and are limited in size, which hinders the use of GaN LEDs for large-area displays. The slow heat dissipation in sapphire deteriorates the device stability. Although this issue may be resolved by a lift-off-and-transfer process, this technique is complicated and challenging [15–18] and the substrate is not suitable for flexible-device applications. In this regard, alternative methods to fabricate GaN LEDs are necessary for their universal and economical applications. Many of these concerns originating from the heteroepitaxy between GaN and the substrate can be alleviated by means of nanoepitaxy based on nanoscale contact between the two [19–23]. Because nanoepitaxy uses such a reduced contact dimension, the lattice mismatch between the contacting materials would not play a major role in forming structural defects in GaN, so that single-crystalline GaN nanostructures can be fabricated on a variety of substrates, which are not possible on the macroscale. Accordingly, various one-dimensional GaN nanostructures, such as nanowires, nanorods, and nanotubes, are used in LED fabrications based on nanoepitaxy. The nanoepitaxial approaches provide additional benefits for producing advanced GaN LEDs, which cannot be realized by conventional techniques. First, nanoscale control during device fabrication allows systematic exploitation of quantum confinement effects in LEDs [24–26]. For example, the luminescence wavelength can be tuned precisely by controlling the physical dimension of the nanostructures [27]. Second, employing vertically aligned nanostructures enables the formation of three-dimensional LED architectures, which have a great potential for improved device efficiency due to an enhanced light extraction efficiency [28], an increased lightemitting active area, and reduced quantum Stark effect [29]. Third, GaN LEDs can be fabricated on graphene/supporting substrate using nanoepitaxy, which can then be readily transferred onto various substrates such as flexible, stretchable, and transparent substrates [30,31]. Consequently, LEDs based on GaN nanostructures have considerable potential to open up a whole new field in optoelectronics. This article reviews recent research related to GaN LEDs based on various nanostructures. The first and second sections describe the fabrication methods of GaN nanostructures and the device components of a typical GaN LED. The third section, as the main focus of this review, introduces various GaN LED architectures based on the nanostructures. Finally, conclusions and perspectives on GaN nanostructures for LED applications are presented.

M.S. Kang et al. methods are discussed with a particular emphasis on the metalorganic vapor-phase-epitaxy (MOVPE) followed by a brief introduction to the hydride-vapor-phase-epitaxy (HVPE) and the plasma-assisted molecular-beam-epitaxy (MBE).

Metal-catalyst-assisted vapor–liquid–solid process A typical VLS process, named because it involves vapor, liquid, and solid phases, starts with dissolution of vaporphase reactants into nanosized catalytic liquid-phase metal (typically Ni or Au for the growth of GaN nanostructures) to form a supersaturated liquid alloy [20,32–36]. The supersaturation of the liquid alloy is then relieved via precipitation of the reactants at the liquid/solid interface. Accordingly, a one-dimensional nanostructure growth begins and continues as long as the vapor-phase reactants are supplied, as shown schematically in Fig. 1a. Based on this process, one-dimensional nanostructures with varying diameter and length can be readily achieved by controlling the size of the catalyst and the growth time and temperature [35,37]. In addition, vertically aligned nanostructures – critical for practical LED applications – can be prepared based on a homo- or a heteroepitaxial relationship between the nanostructures and the substrates [36,38,39]. Additionally, forming heterostructures in axial and coaxial configurations is possible, which is highly desirable for advanced LEDs based on nanostructures [40–42]. Consequently, numerous studies have reported LEDs based on GaN and other semiconductor nanostructures fabricated by VLS. Despite these advantages, however, a critical issue remains inherent to nanostructures grown by VLS for LED applications. As-prepared nanostructures are embedded inevitably with metal catalysts, which play an essential role in VLS nanostructure growth but are also detrimental in LED operations. Scanning electron microscope (SEM) images in Fig. 1b and c show the nanostructures prepared by VLS process and a droplet of metal catalyst that sits on top of the as-prepared

GaN nanostructure fabrication methods Various methods for fabricating GaN nanostructures can be classified into two categories: catalyst-assisted and catalystfree methods. This section begins with a description of a representative catalyst-assisted nanostructure growth method known as the vapor–liquid–solid (VLS) process. Then, catalyst-free

Fig. 1 (a) Schematic description of the VLS process. (b) Tilted view and cross-sectional view (inset) of SEM images of GaN nanowires grown by VLS process using a Ni catalyst. (c) SEM image of a single nanowire with metal (Ni) catalysts placed on top.

Gallium nitride nanostructures for light-emitting diode applications nanostructures, respectively. The metals are likely to cause structural defects by diffusing into the nanostructure during the growth process. Because even trace amount of impurities can influence the physical properties of a nanomaterial considerably, accurate and systematic control of the electrical and optical properties of VLS-grown nanostructures is challenging [38]. More importantly for LED applications, the diffused metals in the nanostructure can work as potential defect levels which enhance nonradiative recombination and thereby reduce the internal quantum efficiency of the nanomaterial and device. Accordingly, a method to fabricate GaN nanostructures in a catalyst-free manner is necessary to further promote their LED applications.

Catalyst-free metal-organic vapor-phase-epitaxy One catalyst-free method of particular interest is MOVPE [22,43,44]. A typical MOVPE process, which has been used widely for epitaxial film growth, involves a delivery of reactants in the form of metal-organic precursors onto a heated substrate using a carrier gas, followed by thermal decomposition of the precursors and surface migration of the resulting constituents. A surface chemical reaction takes place between the migrating reactants, resulting in nucleation and subsequent crystal growth, as schematically described in Fig. 2a. The different surface formation energies for different facets of a crystal play an important role in determining the growth mode and the morphology of the nanostructure [45]. In general, nanostructures grow in a direction that minimizes the total surface energy of a crystal. MOVPE is a promising method to produce high quality GaN nanostructures for LED applications. First, MOVPE can grow metal impurity-free single-crystalline nanostructures with negligible defects such as dislocations or stacking faults (Fig. 2b) [22]. Second, their geometric dimensions (e.g. diameter, length, and thickness) are readily tunable by

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varying the growth parameters (e.g. temperature, pressure, and gas flow rates) [22]. Third, the method is capable of large area growth of well-aligned vertical GaN nanostructures with a uniform length distribution on various substrates by employing an intermediate seed layer (e.g. a thin n-GaN layer) [22]. Moreover, position control of the nanostructures is possible by using a patterned mask, which is critical for practical LED applications (Fig. 2c) [22,46,47]. Finally, by adjusting the delivery of reactant gases accurately, MOVPE can produce nanostructures with precise control of composition (or doping) and thickness. This enables both axial and coaxial hetero-nanostructures to be prepared, as well as quantum structures (e.g. multiple quantum wells (MQWs)) with sharply defined interfaces [46–48].

Other catalyst-free methods In addition to MOVPE, HVPE and plasma-assisted MBE have been used to grow GaN nanostructures without metal catalysts. HVPE is also based on vapor-phase-epitaxy, similar to MOVPE, but it uses gaseous metal chlorides (e.g. GaCl typically prepared by a reaction of HCl gas and gallium metal) and hydrides (e.g. NH3) as the gallium and nitrogen precursors, respectively [19,49–52]. In general, HVPE has high growth rates and thus is advantageous for bulk film growth. However, because fine growth control of the nanostructure growth is not trivial at such high rates, fabricating nanostructures via HVPE may not be ideal. For plasma-assisted MBE growth of GaN [53], Ga metal in ultrapure form is heated and slowly sublimated to form the Ga beam source and nitrogen-plasma is used as the nitrogen source. The Ga atomic source and activated-nitrogen gas contact heated substrates in an ultrahigh-vacuum (UHV) environment, where they diffuse and eventually result in nanostructure growth. This technique can produce highquality single-crystalline GaN nanostructures with atomicscale control [54–59]. Moreover, by switching between evaporation sources during growth, doped nanostructures or heterostructures with sharp interfaces can be readily prepared [60]. However, the growth rate of this technique is generally very slow (tens of nanometer per minute), which impedes mass production of GaN nanostructures.

Device components of GaN LEDs based on nanostructures

Fig. 2 (a) Schematic of catalyst-free MOVPE. (b) High-resolution transmission electron microscopy (TEM) image of GaN nanorods prepared by catalyst-free MOVPE. The inset shows the diffraction pattern of the GaN nanorods. (c) Tilted-view SEM image of GaN nanorod arrays.

A typical GaN LED consists of p-type doped GaN (p-GaN) and n-type doped GaN (n-GaN) sandwiched between two electrodes (Fig. 3a) [1]. p-GaN and n-GaN are obtained by introducing trace amounts of Mg and Si, respectively, into GaN [6–9]. Electrons and holes from respective electrodes flow into the depletion region near the p–n junction and recombine to generate light. Different types of metals are used for making good ohmic contacts to inject either electrons or holes selectively. Typically, ohmic contacts on p-GaN and n-GaN are achieved using thermally annealed Ni/Au and Ti/Al (or Ti/Au) electrodes, respectively [61–63]. Various types of p–n junctions can be formed using GaN nanostructures for LED applications. The simplest p–n junction can be made by directly contacting separately-prepared

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M.S. Kang et al. nanostructures for LED applications is introduced. Second, GaN LEDs based on vertically aligned nanostructure arrays are presented, which represent a more suitable device configuration for scalable LED production. Next, methods to control the position of these nanostructure arrays and the resulting GaN LEDs are discussed. Finally, a novel approach for integrating nanostructured GaN LEDs on graphene layers is demonstrated, which enables further applications of GaN LEDs based on nanostructures.

GaN LEDs based on a single nanostructure Fig. 3 (a) Energy diagram of a typical GaN light-emitting diode under positive bias. ECB, EVB, and EF represent the position of conduction band, valence band, and Fermi level. EBG represents the GaN bandgap. Schematics of various types of p–n junctions based on one-dimensional nanostructures (b)–(f). Blue and red segments represent p- and n-GaN, respectively. Navy and gold segments represent p- and n-type electrodes, respectively. Periods of yellow and black segment represent MQWs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

n-GaN nanostructures with respective p-GaN nanostructures (Fig. 3b) [36,64]. However, obtaining a good nanoscale contact between the two can be difficult due to the lack of epitaxy, which results in poor radiative recombination. Alternatively, an epitaxial p–n junction can be realized within a single nanostructure. By controlling the delivery of p- and ntype dopants (e.g. Mg and Si, respectively) during the longitudinal growth of one-dimensional nanostructures, a p– n junction in an axial configuration can be embedded into a single nanostructure (Fig. 3c) [48,65]. Furthermore, achieving a p–n junction in a coaxial configuration within a nanostructure is also possible if the delivery of dopant sources is manipulated during the radial growth of nanostructures (Fig. 3d). The coaxial configuration is more advantageous than the axial configuration for producing efficient LEDs due to the larger active light-emitting area and enhanced current injection through a larger junction area [42,46,66,67]. The emission wavelength of nitride nanostructure LEDs can be tuned by alloying GaN with InN or AlN [3,4,68–70]. The wurtzite structures of both materials have narrower (0.9 eV) or wider (6.2 eV) bandgaps than wurtzite GaN (3.4 eV). Accordingly, by varying the contents of InN or AlN within the GaN alloy, a continuous emission spectrum that covers UV and visible ranges can be achieved. Furthermore, MQW structures can be obtained by placing GaN and InGaN (or AlGaN) periodically, which enhance the local confinement of carriers for recombination and thereby improve the light-emission quantum efficiency [71]. These MQWs can be embedded between the p–n junction of a single GaN nanostructure in both axial and coaxial configurations (p-GaN/MQWs/n-GaN structures) to produce nanostructured GaN LEDs with improved performance (Fig. 3e and f) [21,31,46,47,51,72].

The first demonstration of nanostructured GaN LEDs utilized two single nanowires of an (Si-doped) n-GaN nanowire and a (Mg-doped) p-GaN nanowire that were prepared separately by the catalyst-assisted VLS method [20,36]. In particular, a crossjunction between the two nanowires was formed using a fluidic assembly [73], which yielded a localized emission at the nanowire cross point (Fig. 4a). These prototype LEDs exhibited emission at a wavelength of 415 nm which corresponds to radiative recombination from the conduction band to the acceptor levels associated with Mg [74]. Similarly, LEDs based on a cross-junction between an n-GaN nanowire and a p-Si nanowire yielded UV emission at a wavelength of 365 nm, consistent with GaN band-edge emission [64]. A more advanced type of single GaN nanostructure embedded with a p–n junction could be exploited for LED fabrication. For example, p–n homojunction GaN nanorods were prepared by HVPE by controlling the supply of Mg as a ptype dopant during nanorod growth [65]. Specifically, the first half of HVPE was carried out in the absence of Mg supply, which yielded (nominally undoped) n-GaN nanorod segment due to the presence of nitrogen vacancies and/or oxygen impurities (unintentional n-type dopants), while the second half of the growth was carried out in the presence of Mg to yield a p-GaN nanorod segment on n-GaN in an axial configuration (Fig. 4b). For LED fabrication, the resulting nanorods were dispersed onto a SiO2 substrate. Subsequently,

Various architectures of GaN LEDs based on nanostructures This section describes various device LED architectures based on GaN nanostructures. First, pioneering work on various single-nanostructures demonstrating the potential of GaN

Fig. 4 Schematics of (a) a cross-junction GaN nanowire LED, (b) a single GaN nanowire LED based on axial p–n homojunction structure, (c) a core/shell/shell nanowire heterostructure, and (d) a core/multishell nanowire heterostructure.

Gallium nitride nanostructures for light-emitting diode applications both n- and p-type ohmic contacts (with Ti/Al and Ni/Au layers, respectively) were attached to an individual nanorod using focused ion beam (FIB) lithography. These nanorod LEDs exhibited typical rectifying diode behavior with turnon voltages ranging from 0.5 to 1 V and light emission at a wavelength of 390 nm. In addition to the axial p–n junction nanostructured LED, a single nanostructure containing a p–n junction in a coaxial configuration could be employed in GaN LED applications. For example, LEDs based on a single core (n-GaN)/shell (InGaN)/shell (p-GaN) structured nanowire were fabricated (Fig. 4c) [41]. Inserting an InGaN layer as a shell within this structure generated blue-light emission with an enhanced quantum efficiency. To achieve the coaxial heterostructure, the core nanowire was initially grown by metal-catalystassisted VLS, followed by sequential radial growth of intrinsic InGaN (containing nominally 20% In) and Mg-doped p-GaN as an outer shell. For LED fabrication, simultaneous electrical contacts to both the n-type core and p-type outer shell of a single nanowire were achieved using FIB lithography to etch and expose the core selectively at one end of the nanowire. The LED exhibited sharp rectification behavior with a turn on voltage of 4 V and blue-light emission at 456 nm, corresponding to emission from the inserted InGaN layer. A more sophisticated coaxial structure containing an n-GaN core and sequentially deposited intrinsic-InGaN (i-InGaN), intrinsicGaN (i-GaN), p-AlGaN, and p-GaN shells has been incorporated into a GaN nanowire for LED applications (Fig. 4d) [42]. The resulting LEDs exhibited multicolor emission (from 365 to 600 nm) that was controlled by varying the indium content in the i-InGaN layer. The quantum efficiency was improved due to enhanced confinement of injected carriers and photons in the emissive layer.

GaN LEDs based on vertically aligned nanostructures Despite the exciting opportunities discussed in the previous section, approaches that use a single nanostructure (or two single nanostructures) are not ideal for mass production of LEDs. This is because they require meticulous processes that define the location of the nanomaterial and attach electrical contacts with nanoscale precision. However, the problems inherent to single-nanostructure LEDs can be resolved using vertically aligned nanostructures, which is highly desirable for scalable LED fabrication. LEDs based on vertically-aligned GaN nanorods consisting of n-GaN/MQWs (InGaN/GaN)/p-GaN in an axial configuration were demonstrated by Kim et al. [51]. These nanorods were grown directly on a sapphire substrate covered with a nominally undoped n-type GaN layer by HVPE. To fabricate LEDs, the spaces between individual nanorods were filled with spin-on glass (SOG), and n- and p-type electrodes (Ti/Al and Ni/Au, respectively) were deposited separately onto the n-GaN layer and the SOG layer, respectively, by photolithography and etching (Fig. 5). The resulting LEDs exhibited blue light emission at 470 nm originating from the embedded six-period MQWs. Although the light-emitting active area for the axial nanorod LED is much smaller than that of a conventional thin-film LED, the dislocation-free nature of the as-prepared nanorods would increase the light extrac-

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Fig. 5 Schematic of an LED based on vertically aligned GaN nanorods consisting of n-GaN/MQWs/p-GaN in an axial configuration.

tion efficiency, the internal quantum efficiency, and consequently the overall external quantum efficiency. Plasma-assisted MBE was also employed to fabricate LEDs based on arrays of GaN nanocolumns in an axial configuration, which contain n-GaN/MQWs (InGaN/GaN)/p-GaN, on a (111) or (001) Si substrate [21,23,75–77]. By tuning the indium content in the MQWs, the emission color of the nanocolumn LED could be varied from violet to red [57,75,76]. Moreover, white-light emission was achieved in these LEDs by gradually varying the indium composition within the InGaN segment [23,77,78]. In addition, UV emission (354 nm) was realized by embedding GaN/AlGaN MQWs within the nanocolumns, instead of GaN/InGaN MQWs [79,80]. An approach of using ZnO nanostructures as a template was devised to form vertically aligned nanostructure LEDs, because achieving vertical growth of ZnO nanostructures on various substrates including glass, graphene, and GaN is feasible [81–84]. Note that ZnO has the same wurtzite crystal structure as GaN with small lattice constant misfit of 1.9% and thus the two materials have good epitaxial compatibility. Accordingly, LEDs based on a heterojunction of n-ZnO (nominally undoped) nanorod arrays grown on p-GaN film were demonstrated using catalyst-free MOVPE [85]. Furthermore, LEDs based on GaN homojunctions were obtained by growing coaxial n-GaN/ZnO nanorod arrays on p-GaN thin film (Fig. 6) [86].

Position-controlled nanostructured GaN LED arrays The higher performance nanostructured GaN LEDs require control of the positions and dimensions of the GaN nanostructures. Obviously, position control of LEDs is essential for integrating devices with other components of electrical circuits. Additionally, more uniform geometry (e.g. thickness and height) and controlled density of nanostructures by position-controlled growth lead to highefficiency LEDs, compared to LEDs based on nanostructures grown randomly on the entire substrate with less uniform geometry and poor density control. The position control of vertically aligned nanostructure arrays can be readily achieved by site-selective growth of nanostructures using a hole-patterned mask layer on the substrate of interest; nucleation and subsequent nanostructure growth can only take place on the areas of the substrate exposed by the holes in the mask. By changing the geometric parameters of the mask, the thicknesses of the nanostructures, the interdistances between the nanostructures, and the area density of nanostructure arrays can be readily

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M.S. Kang et al. example, visible-color-tunable LEDs were recently fabricated using a thin-film structure embedded with positioncontrolled arrays of GaN nanorods [87]. Fig. 8a shows the schematic illustration of a nanorod-embedded LED

Fig. 6 (a) Schematic of an LED based on vertically aligned GaN/ZnO coaxial nanorods grown on a p-GaN thin film. (b) SEM image of GaN/ZnO coaxial nanorods. Reproduced with permission from Applied Physics Letters [86].

Fig. 7 SEM image of an LED with position-controlled arrays of GaN nanorods embedded with an axial p–n homojunction. Reproduced with permission from Electronics Letters [48].

controlled. Based on this approach, LEDs with positioncontrolled arrays of GaN nanorods embedded with an axial p–n junction were fabricated by catalyst-free MOVPE (Fig. 7) [48]. Similar to the device structure introduced in Section ‘‘GaN LEDs based on vertically aligned nanostructures’’, the interspaces between position-controlled nanostructures were filled with an insulating SOG layer and respective metal electrodes were deposited separately for LED fabrication. The resulting LEDs exhibited typical rectifying behavior with an electroluminescence (EL) peak at 372 nm. The incorporation of position controlled arrays of GaN nanostructures into novel device structures offers additional functionalities and enhanced LED performance. For

Fig. 8 (a) Schematic of a nanorod-embedded LED comprising position-controlled arrays of MQWs/n-GaN coaxial nanorods and a p-GaN overlayer. (b) Scanning TEM images of InGaN/GaN nanorod MQWs formed on topmost (magnified view in red box) and upright sidewall areas (magnified view in blue box) of a GaN nanorod. (c) Energy-dispersive X-ray (EDX) line profiles of the indium L-characteristic wavelength along the axial (red dotted line in (b)) and radial (blue dotted line in (b)) directions of the nanorod. (d) EL spectra of the nanostructured LEDs taken at various bias voltage from 3.0 to 10.0 V. (e) Illustrations of the change of equipotential planes (white dotted lines) in the p-GaN overlayer of the nanostructured LEDs and paths of hole carriers (white arrows) (i) under a low electric field near the turn-on voltage, (ii) with increasing applied voltage, and (iii) at very high bias voltage. Reproduced with permission from Advanced Materials [87]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Gallium nitride nanostructures for light-emitting diode applications composed of MQWs (InGaN/GaN)/n-GaN coaxial nanorod arrays and a p-GaN overlayer prepared by catalyst-free MOVPE. Fabrication of such a novel structure started from site-selective growth of n-GaN nanorod arrays, followed by heteroepitaxial overgrowth of the MQW layers. Notably, due to the variance in the surface formation energies of different GaN crystal planes, anisotropic MQWs with varying QW thickness and composition were formed on different nanorod facets, as shown in Fig. 8b and c, respectively. Subsequently, a continuous overlayer film of p-GaN was epitaxially coated onto the nanorods with MQWs, similar to the epitaxial lateral overgrowth process. Interestingly, these nanorod-embedded thin-film GaN LEDs exhibited a continuous change in emission color from red to blue with increasing bias voltage, as shown in Fig. 8d. Note that conventional LEDs typically emit a single color with no considerable wavelength shift with varying bias voltage. The highly tunable emission color of these nanorodembedded thin-film LEDs originates from both the anisotropic MQW layers formed on the multifacetted GaN nanorods and the gradual change in electric field distributions in nanorod-embedded thin-film structures upon varying electric bias, as schematically illustrated in Fig. 8e. At low bias voltages with a nearly flat electric field distribution, electrical currents are mostly driven to the topmost MQWs (with the highest indium content), and thus red-light emission is achieved. With increasing bias voltage, the electric field penetrates more deeply into the nanorod stems. Consequently, more electrical current flows into the MQWs that reside on the nanorod sidewalls (with lower indium content), and thus emission color is blueshifted. Furthermore, a monolithic integration of red, green, and blue LEDs that operate at a fixed drive current could be realized from nanorod-embedded LEDs with various active areas. LEDs with position-controlled nanostructures can also be prepared using a ZnO nanostructure template. For example, LEDs based on arrays of GaN/ZnO coaxial nanotube heterostructures, similar to the device configuration introduced in Section ‘‘GaN LEDs based on vertically aligned nanostructures’’ but with position-control, were fabricated on a sapphire substrate by catalyst-free MOVPE (Fig. 9) [46,47]. To prepare the coaxial structure, site-selectively grown ZnO nanotubes were employed as a template, on which subsequent radial growths of n-GaN/MQW/p-GaN coaxial layers were carried out. These LEDs yielded bluish-green emission; the EL spectra exhibited a dominant peak centered at 2.45 eV resulting from the InGaN/GaN MQWs and a broad shoulder around 2.85 eV, which presumably

Fig. 9 Schematic of an LED based position-controlled GaN/ZnO coaxial nanorod heterostructures.

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originated from the nonuniform thickness of the well layers on the side walls of the nanotubes. The approach using position-controlled GaN/ZnO coaxial nanotube heterostructures enabled the fabrication of nanostructure LEDs on Si substrates as well [72]. Assembling LEDs on Si substrates is of particular interest because it allows integration of various novel optoelectronic technologies with the well-established Si-based electronics; however, such integration remains challenging due to the unfavorable heteroepitaxy between Si and optical materials. This issue could be resolved using a crack-free thin GaN buffer layer (1 mm) on a Si substrate and fabricating LEDs on top of the buffer layer based on nanoepitaxy. The resulting LEDs based on position-controlled arrays of n-GaN/ MQW/p-GaN/ZnO coaxial nanotube heterostructures emitted green light (2.35 eV).

GaN LEDs based on nanostructures grown on graphene In the Introduction of this review, we suggested that nanoepitaxy can potentially relieve the constraints of substrates that in general require rigorous heteroepitaxy with GaN layers. However, various examples of nanostructured GaN LEDs that we have presented so far were mostly prepared on sapphire substrates, with a few exceptions which were on Si substrates. Although those examples demonstrating the advanced functionalities of nanostructured GaN LEDs are by themselves meaningful, the advantage of using nanoepitaxial growth of nanostructures, particularly focused on diversifying the substrate selection for LED fabrication, should be emphasized. Indeed, nanoepitaxy can be used to grow various nanostructures on a single graphene sheet or graphene multilayers [30,83,88]. Graphene is a single layer of carbon atoms arranged in a closely packed honeycomb twodimensional crystal lattice structure. In addition to the well-known unique characteristics of graphene, including excellent electronic, mechanical and thermal properties of the materials [89–95], the weak physical interactions between graphene sheets provide novel functionality to nanostructures and the resulting LEDs prepared on graphene layers. Namely, these LEDs can be readily transferred onto various substrates, thus avoiding problems related to the epitaxial relationship between GaN and substrates [30]. Moreover, these LEDs can be transferred onto flexible, stretchable, or/and transparent substrates to further exploit the excellent physical properties of graphene for novel LED applications [30,31]. To grow GaN nanostructures on graphene layers, ZnO nanostructure templates were employed. Specifically, randomly distributed ZnO nanorods were first grown on large graphene films prepared by chemical vapor deposition using Cu foil [94]. Subsequently, heteroepitaxial growths of the GaN layers (n-GaN/MQW/p-GaN) were carried out on the surface of ZnO nanorods. These procedures were performed by catalyst-free MOVPE [31]. For the device fabrication (Fig. 10a), isolated p-type ohmic contacts were first formed on the p-GaN layer of individual nanorods. Subsequently, the gaps between the nanorods were filled with an insulating flexible polymer, and additional metal contact layers were

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M.S. Kang et al. To develop GaN LEDs with enhanced performance, challenges such as forming good ohmic and transparent contacts on GaN nanostructures with non-flat morphology and reducing the significant leakage current relative to conventional thin-film LEDs should be resolved. Then, new opportunities of nanostructured GaN LEDs for future light sources and displays are expected.

Acknowledgment

Fig. 10 (a) Schematic of an LED based GaN/ZnO coaxial nanorod heterostructures grown on graphene films that can be transferred onto a flexible plastic substrate. (b) Optical microscope image of the light emission from the LED at an applied current of 10 mA. (c) Light emission photographs at bending radii of N, 5.5 and 3.9 mm. (d) Plots of integrated emission intensities as a function of bending cycle. The inset shows the corresponding EL spectra. Reproduced with permission from Advanced Materials [31].

deposited on top of the nanostructure-embedded layer to facilitate current spreading. The as-fabricated nanostructured LEDs were readily lifted-off from the original Si substrate with a thin SiO2 layer by wet-etching the sacrificial SiO2 layer underneath the graphene films and transferring it onto metal-coated plastic substrates. Strong blue-light emission (dominant EL peak at 490 nm with a broad shoulder around 465 nm) was observed from the resulting LEDs with the unaided eye, even under room illumination (Fig. 10b). Furthermore, the LEDs demonstrated reliable operation upon substrate bending. At different bending radii, no significant degradation in their EL or electrical characteristics was observed (Fig. 10c). Also, after a few hundred bending cycles, the luminescent and electrical characteristics of the devices did not change noticeably (Fig. 10d). These results indicate that the strain applied to the active region of individual nanorods is negligible, presumably due to nanoscale contact, thus demonstrating the advantage of using nanostructures for flexible electronic applications.

Conclusions and perspectives In this article, we reviewed the recent progresses in GaN nanostructures for LED applications. Various GaN nanostructure fabrication methods were introduced; the methods were categorized into two subgroups based on the use of metal catalyst during nanostructure growth. The concerns related to the use of metal catalyst were discussed, and thus more emphasis was placed on catalyst-free methods. After a brief explanation of the basic structure of a typical GaN LED, various device architectures of nanostructured GaN LEDs were summarized. This review covered technologies from the simplest pioneering device structure based on a single nanostructure to one of the most advanced structures based on nanostructure arrays on transferable and flexible substrates.

This work was financially supported by the National Creative Research Initiative Project (Grant R16-2004-004-01001-0) of the Korea Science and Engineering Foundations (KOSEF) and the Future-based Technology Development Program (Nano Fields, 2010-0029325) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.

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Jun Beom Park received his B.S. degree (2010) in Physics from Seoul National University, Seoul, Korea. He is currently a Ph.D. candidate in Physics at Seoul National University under supervision of Prof. Gyu-Chul Yi.

Hyobin Yoo received his B.S. degree (2010) in Materials Science and Engineering from Seoul National University, Seoul, Korea. He is currently a M.S. candidate in Materials Science and Engineering at Seoul National University under supervision of Prof. Miyoung Kim and Prof. Gyu-Chul Yi.

Gyu-Chul Yi is a professor in the Department of Physics at Seoul National University. He received his Ph.D. degree (1997) from Northwestern University in USA. After working as a post-doc at Oak Ridge National Laboratory (USA) for two years, he joined POSTECH (Korea) in 1999 as an assistant professor. Since 2004, he has been a director of National CRI Center for Semiconductor Nanostructures. He has extensive experience in wide band gap semiconductor nanostructures and has published more than 130 referred articles in the Science, Advanced Materials, and Applied Physics Letters. He has also written a book (entitled ‘Semiconductor Nanostructures for Optoelectronic Devices’, Springer, 2012) as an organizing editor and several book chapters as an author or coauthor. Thanks to his excellent research records, he received several awards including the Chong Yul Lee Chair professorship (POSTECH) in 2004, Song-Gok Science Prize (KIST, 2006), and the Year of POSTECHIAN.