Emission Characteristics of InGaN/GaN Core-Shell Nanorods Embedded in a 3D Light-Emitting Diode
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We report the selective-area growth of a gallium nitride (GaN)-nanorod-based InGaN/GaN multiple-quantum-well (MQW) core-shell structure embedded in a three-dimensional (3D) light-emitting diode (LED) grown by metalorganic chemical vapor deposition (MOCVD) and its optical analysis. High-resolution transmission electron microscopy (HR-TEM) observation revealed the high quality of the GaN nanorods and the position dependence of the structural properties of the InGaN/GaN MQWs on multiple facets. The excitation and temperature dependences of photoluminescence (PL) revealed the m-plane emission behaviors of the InGaN/GaN core-shell nanorods. The electroluminescence (EL) of the InGaN/GaN core-shell-nanorod-embedded 3D LED changed color from green to blue with increasing injection current. This phenomenon was mainly due to the energy gradient and deep localization of the indium in the selectively grown InGaN/GaN core-shell MQWs on the 3D architecture.
KeywordsGallium nitride Nanorod Core-shell structure Light-emitting diodes
III-nitride-based materials are a promising source of energy-saving solid-state lighting, particularly for light-emitting diodes (LEDs). There has been significant technological development in the field of nitride-based solid-state lighting over the past few decades since the discovery of the low-temperature buffer growth technique and a method of magnesium activation [1, 2]. Although considerable improvements in the performance of LED devices have been demonstrated, c-plane polar gallium nitride (GaN)-based LEDs still have unsolved problems that adversely affect their performance, which are caused by piezoelectric and spontaneous polarization [3, 4]. To solve such problems, different crystal planes, particularly nonpolar facets such as the m- and a-planes, can be applied to an InGaN/GaN multiple-quantum-well (MQW) heteroepitaxial structure as basal facets . LED structures based on nonpolar facets are expected to avoid piezoelectric-field-related issues such as efficiency droop. However, the use of an m-plane bulk substrate increases the cost of devices. Moreover, a nonpolar-plane-based heteroepilayer grown on a-plane GaN or r-plane sapphire substrates is still needed to prevent the formation of structural defects such as prismatic and basal stacking faults as well as partial dislocations to ensure sufficient crystal quality [5, 6]. In fact, the results of research on GaN nanorods (NRs) and nanowires (NWs) have produced advances and interesting outcomes in a number of application fields [7, 8, 9]. Among them, the selective-area growth (SAG) of GaN NRs is a particularly promising alternative for producing nonpolar-based LED structures without increasing the production cost of devices or introducing structural defects owing to their unique characteristics. Motivated by these advantages, there has been tremendous effort to realize high-efficiency three-dimensional (3D) LEDs; indeed, 3D LEDs with full-color (or white light) emission have been demonstrated by geometrically emissive color mixing or phosphor-based wavelength conversion [7, 10, 11, 12]. Furthermore, owing to the recent progress in an epilayer transfer technique, the fabrication of flexible displays through the solid-state 3D LEDs is now feasible [13, 14]. However, NR-based 3D LEDs inevitably include several crystal facets, in contrast to a planar epilayer. This strongly affects the light emission properties of core-shell MQWs such as spectrum broadening and indium localization, thereby making it necessary to study their optical properties in more detail [15, 16]. Therefore, in this study, we report the emission characteristics of InGaN/GaN core-shell NRs embedded in a 3D LED structure. To clarify the underlying physics of this core-shell structure, we investigated its structural properties, particularly the well and barrier thicknesses of the MQWs, by Cs-corrected scanning transmission electron microscopy (STEM). We also performed measurements of the temperature (T) and excitation power (P) dependences of the photoluminescence (PL) to reveal their optical properties by analyzing the behaviors of the emission energy, intensity, and linewidth. Additionally, the light emission properties of the core-shell-NR-embedded 3D LEDs are presented in detail.
Preparation of Pattern
A 3-μm-thick GaN epilayer with a (0002) preferential orientation doped with Si was utilized as a basal template for the SAG of GaN NRs by metalorganic chemical vapor deposition (MOCVD). RF magnetron sputtering was used to deposit a 30-nm-thick dielectric SiO2 growth-masking layer. After SiO2 deposition, nanoscale resin patterns with a hole-shaped array (aperture size, 190 nm; center-to-center distance, 920 nm) were produced by thermal nanoimprinting procedure. Then, CF4-based reactive ion etching (RIE) was performed to open the SiO2 selectively. Subsequently, the patterned template was cleaned with an organic solvent (acetone) for 10 min to remove polymer-based residues from the surface.
Synthetic Process and 3D LED Fabrication
For the growth of the GaN NR array and subsequent core-shell layers, the patterned GaN template on sapphire substrates was loaded into a showerhead-type MOCVD chamber. Trimethylgallium (TMGa) and ammonia (NH3) were used as the precursors with flow rates of 15 sccm (78 μmol/min) and 5 slm (223.21 mmol/min), respectively. At the same time, tetramethylsilane (Si(CH3)4) was used as the n-type doping source with a flow rate of 5 sccm (0.0105 μmol/min). The synthesis of the well-defined GaN NR array was carried out in accordance with Hersee’s original pulsed-mode growth procedure with our optimal growth conditions under an ambient of pure hydrogen (H2) as the carrier gas. Details of this pulsed-mode growth process have been reported elsewhere [17, 18]. Subsequently, three pairs of InGaN/GaN MQWs were grown on the GaN NR array in the conventional growth mode at temperatures from 760 to 790 °C depending on the experiment. For MQW growth, trimethylindium (TMI) and triethylgallium (TEG) were used as reactants with nitrogen (N2) as the carrier gas. The reactant flow rates of TMI and TEG were fixed at 400 sccm (40.3 μmol/min) and 100 sccm (29.92 μmol/min), respectively, with a chamber pressure of 200 Torr. Finally, p-type GaN was grown with bis-ethylcyclopentadienyl magnesium (EtCp2Mg) as a precursor at a flow rate in the range of 150–250 sccm (0.24–0.4 μmol/min) for 90 min to ensure full coalescence. To form ohmic contacts, Ni/Au and Ti/Au metal bilayers were deposited by electron beam evaporation onto the p-GaN top surface layer and the n-GaN underlayer, respectively. To decrease the resistance between the metal and the semiconductor (GaN), the metal contacts were treated by rapid thermal annealing (RTA) at 550 and 300 °C after the deposition of the Ti/Au and Ni/Au metal bilayers, respectively.
Morphological and microstructural characterization was carried out by field-emission scanning electron microscopy (FE-SEM; Hitachi S-5200) and Cs-corrected STEM (JEM-ARM200F). The synthesized samples were milled using a dual-beam focused ion beam (DB-FIB; NOVA200) operating in the range of 5–30 kV before TEM investigation to obtain high-resolution (HR), bright-field (BF), and electron diffraction pattern (DP) images. To investigate the emission properties of the InGaN/GaN core-shell NR array, the temperature dependence of the PL was measured as a function of temperature from 12 to 300 K in a closed-cycle helium cryostat. The excitation power dependence of the PL was measured in the power density range of 7.6–764 W/cm2. For all the PL measurements, a 325-nm continuous-wave He-Cd laser (20 mW) was used as the excitation source, and the objective lens (×5) of a reflecting microscope was used to control the beam spot size (~20 μm) and simultaneously collect the signal. In this paper, we define the maximum excitation power density (=764 W/cm2) as 100 %. Moreover, the characterization of the electrical properties (I-V curve) and electroluminescence (EL) spectra of the 3D LED was performed simultaneously.
Results and Discussion
We have demonstrated and analyzed InGaN/GaN core-shell NRs embedded in a 3D LED. The high quality of the core GaN NRs was proved from the dislocation filtering of the structure and the prominent NBE PL emission. Owing to the vapor-phase diffusion and surface diffusion of the selectively grown 3D structures, the InGaN/GaN MQW core-shells that formed on the GaN NRs suffered from an indium compositional gradient and thickness variation, thereby resulting in an energy gradient. Under the excitation-power-dependent PL characterization, the emission on the m-plane surface mainly contributed to the total PL spectra, as observed from the suppressed peak shift. The temperature-dependent PL characterization revealed a monotonic peak shift and linewidth variation with increasing the temperature, indicating the strong indium localization of the m-plane QW layer with the energy gradient. Through the demonstration of an electrically driven 3D LED, the EL properties were also characterized. At a low injection current, the EL spectra were mainly affected by the regions of localized indium concentration such as at the top of m-plane MQWs. Upon increasing the injection current, the m-plane emission became more prominent and the peak shift was suppressed. Further EL peak analysis showed good agreement with the result of PL characterization in terms of the intensity, peak energy, and FWHM. Therefore, the results of this paper are considered to contribute to understanding of the emission properties of the 3D LEDs, thereby helping to achieve highly efficient devices through further control of the growth and fabrication.
This research was funded by the Japan Society for the Promotion of Science (JSPS) through a Grant-in-Aid for Specially Promoted Research (contract number 25000011) and supported by the Institute for Basic Science (IBS) of Korea (IBS-R004-G3-2014-a00). S-YB has been an International Postdoctoral Research Fellow of JSPS, and the other first author (BOJ) was funded by a Monbukagakusho scholarship from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.
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