Phosphor-Free Apple-White LEDs with Embedded Indium-Rich Nanostructures Grown on Strain Relaxed Nano-epitaxy GaN
© The Author(s) 2010
Received: 28 June 2010
Accepted: 19 July 2010
Published: 1 August 2010
Phosphor-free apple-white light emitting diodes have been fabricated using a dual stacked InGaN/GaN multiple quantum wells comprising of a lower set of long wavelength emitting indium-rich nanostructures incorporated in multiple quantum wells with an upper set of cyan-green emitting multiple quantum wells. The light-emitting diodes were grown on nano-epitaxially lateral overgrown GaN template formed by regrowth of GaN over SiO2 film patterned with an anodic aluminum oxide mask with holes of 125 nm diameter and a period of 250 nm. The growth of InGaN/GaN multiple quantum wells on these stress relaxed low defect density templates improves the internal quantum efficiency by 15% for the cyan-green multiple quantum wells. Higher emission intensity with redshift in the PL peak emission wavelength is obtained for the indium-rich nanostructures incorporated in multiple quantum wells. The quantum wells grown on the nano-epitaxially lateral overgrown GaN has a weaker piezoelectric field and hence shows a minimal peak shift with application of higher injection current. An enhancement of external quantum efficiency is achieved for the apple-white light emitting diodes grown on the nano-epitaxially lateral overgrown GaN template based on the light -output power measurement. The improvement in light extraction efficiency, ηextraction, was found to be 34% for the cyan-green emission peak and 15% from the broad long wavelength emission with optimized lattice period.
High efficient group-III nitride-based light emitting diodes (LEDs) have been intensively developed in recent years for various applications such as street and traffic lights, back lighting and for headlights of automobiles. Solid-state lighting would replace conventional light bulbs and will change the way we light the world [1, 2]. InGaN/GaN, multiple quantum wells (MQWs) are often employed as the active layers due to their relatively high recombination efficiency and blue to green III-Nitride LEDs are commercially available. However, III-Nitride LEDs faced severe constraint when we attempt to incorporate high indium content in the materials due to large lattice mismatch (11%) between InN and GaN . This leads to spinodal decomposition once InN content reaches a critical limit (~30%) . The conventional white III-Nitride LEDs generated from phosphor coating have the disadvantages of having poor color rendering index, low yield issues in production and reduced thermal stability . Pioneering work to generate phosphor-free white light was done by Damilano et al.  and Yamada et al.  by having InxGa1−x N/GaN quantum wells (QWs) of different indium compositions. Recent work by Huang et al. made use of prestrained InGaN well layer to generate white light  while Funato et al. has demonstrated a polychromatic emission (inclusive of white) from LEDs using microstructured multifaceted quantum wells . Nanostructures, especially III-As-based quantum dots are a subject of wide variety of studies ranging from fundamental physics, quantum electrodynamics  to quantum information and computing [11, 12]. InGaAs/GaAs, quantum dots system with high uniformity has been generated on AlGaAs, which demonstrated narrow linewidth and improvement in excitons confinement . III-Nitride quantum dots have also been applied by Chua et al. , not to generate narrow linewidth but to have a broad emission spectrum covering from 400 to 700 nm to mimic white (daylight) emission. However, there is still a need to improve on the light extraction efficiency due to total internal reflection, limiting the amount of light which can escape from the LEDs surface . Various patterning techniques have been employed to enhance light extraction from LEDs, which includes surface roughening , geometric modification  and use of photonic crystals [18, 19].
Nanostructures and photonic crystals have been created by deposition into patterned substrates produced by conventional lithographic approaches such as e-beam lithography, interference lithography or X-ray lithography [20, 21]. These lithography approaches enable precise control of the spacing and dimensions of the nanostructures but the techniques are limited by high cost and low throughput. On the other hand, patterning based on self-organization technique such as the self-ordered aluminum oxide template enables the fabrication of arrays of nanostructures over a large area . Earlier work has reported on the advantages of using nano-air bridge GaN template, which includes threading dislocation reduction and strain relaxation in subsequent InGaN/GaN MQWs or InN quantum dots grown [22, 23]. In this paper, we demonstrate the growth of InGaN/GaN LEDs incorporating indium-rich InGaN nanostructures on the nano-epitaxy lateral overgrown GaN template. Photoluminescence show enhanced peak intensity and higher activation energy for the multiple quantum wells grown on the nano-epitaxy lateral overgrown GaN template. Structural analysis of samples with embedded nanostructures and conventional InGaN/GaN well was carried out by scanning electron microscope (SEM).
Preparation of the Nano-ELO GaN Template
Growth of MQWs and LEDs on the Nano-ELO GaN Template
The u-GaN sample used for the preparation of the nano-ELO GaN template was grown by metalorganic chemical vapor depositions (MOCVD) at 1,020°C with trimethylgallium (TMGa) and NH3 gas serving as the precursors for Ga and N, respectively, at a chamber pressure of 500 Torr. A 200 nm thick GaN buffer layer was then regrown on the patterned u-GaN template at 1,020°C as shown in Fig. 1c. To promote lateral overgrowth, the TMGa flow rate was reduced from 90 to 30 sccm, and the chamber pressure was lowered to 200 Torr. The inset of Fig. 1c shows the formation of nucleation islands of GaN over the nanopores of SiO2 with size of ~2.0 μm and a root mean square roughness of ~11.7 nm. The pressure was then increased to 500 Torr to grow the additional ~2 μm thick n GaN as shown in Fig. 1d.
Tabulation of the sample structures and the type of template used
Conventional InGaN/GaN QWs
indium-rich nanostructures incorporated InGaN/GaN QWs
LEDs with Stacked MQWs
Results and Discussion
Determination of the Optical Properties of MQWs
The external quantum efficiency (ηext) of the LEDs is influenced by its internal quantum efficiency (ηint) and light extraction efficiency (ηextr). The internal quantum efficiency of the LEDs is dependent on its materials properties and can be evaluated by temperature-dependent photoluminescence spectra. Figure 2a shows the low temperature PL spectra for the MQWs taken at 10 K. Strong PL emission around 495 nm is observed from the sample B grown on nano-ELO GaN template while emission peak at ~513 nm is due to its LO phonon replica. The dominant PL emission from sample B is about 1.5 times higher in terms of its integrated intensity when compared to sample A grown on the conventional GaN template due to reduction in the density of defects such as the screw and the edge dislocations in the regrown GaN. This is similar to the ELOG GaN growth process where SiO2 is used as the mask. Based on the pits counts from the 5 × 5 μm AFM image of conventional GaN as shown in Fig. 2b, (where no of pits, ns ~18), the density of threading dislocation is estimated to be ~6.8 × 108 cm−2 with a separation of ~600 nm . Since the average spacing between the nanopores is about 100–110 nm, this implies that the SiO2 mask serves as an effective blocking layer for the propagation of threading dislocations. With regrowth of GaN on nanopores, the threading dislocation density has been reduced to 1.2 × 108 cm−2 The surface of the 2.0 μm thick n-GaN on nano-ELO GaN template gives a RMS roughness of 0.32 nm when compared to 0.29 nm for conventional GaN template.
We observed a redshift in the peak emission of sample B from that of sample A due to the stress relaxation of the GaN grown on the nano-ELO GaN, which contributes to the greater ease for incorporation of indium in the InGaN well layer. For the indium-rich InGaN nanostructures incorporated in the multiple quantum wells, a redshift is observed in the peak position of the broadband emission from 573 nm of sample C to 579 nm for sample D. The samples were grown on the conventional GaN and nano-ELO GaN template, respectively. The shoulder emission at the higher wavelength side corresponds to the LO phonon replica . The enhancement in the integrated intensity is only 1.3 times after eliminating the emission from the InGaN wetting layer.
Morphological Studies of the InGaN/GaN MQWs
Electroluminescence Study of the LEDs Grown on Nano-ELO GaN Template
To evaluate the light output performance of the LEDs, Newport Optical Power Meter with a UV detector (of bandpass filtering from 200 to 1,100 nm) is used to measure the relative light output power with injection current. The measurement was carried out at the wavelength of the EL peak, P1 and P2 obtained from the EL spectra of the LEDs in Fig. 5a. Figure 5b shows the light output performance of sample E and F for EL peak, P1 and P2 with increase in injection current. In both cases, LEDs from sample E emits with a higher intensity for both P1 and P2. There is an enhancement in light output power by 1.55 times for P1 and 1.31 times for P2 with an injection current of 80 mA. The difference in the value suggests that the array of GaN nanorods with diameter of 120–130 nm, grown in the nanopores of SiO2 mask with period ~240–250 nm is more effective in enhancing the light extraction for emission wavelength, λP1 ~520 nm than λP2 ~650 nm. The external quantum efficiency, ηext of the multiple quantum wells, is given by ηext = ηextraction x ηint, where ηextraction is the light extraction efficiency with embedded nano-ELO GaN template. Since the improvement in ηint has been approximated to be 15%, this gives an enhancement in light extraction efficiency, ηextraction of ~34% for P1 and 14% for P2. Simulation of light extraction with embedded photonic structures consisting of SiO2 pillar and  emission profile with surface patterning  have been done for GaN-based LEDs sample. The vertical emission profile for light extraction and enhancement is obtained near the second order diffraction condition, a/λ = 0.50 where ‘a’ corresponds to the lattice period and λ is the wavelength of emission. With our emission wavelength peak at λP1 ~520 nm and λP2 ~650 nm, the lattice period of a ~240–250 nm will give a higher enhancement for emission peak P1 with a/λ P1 ~0.47 when compared to broad emission peak, P2 with a/λ P1 ~0.37. As the patterns are not perfectly uniform, several of the neighboring nanopores have merged as shown in Fig. 1c. However, there is a substantial degree of periodicity, and the enhancement would also be due to scattering of light from the embedded SiO2 mask.
In summary, the use of nano-ELO GaN template for growth of LEDs is effective in improving the light output performance of the LEDs through improvement of internal quantum efficiency through the reduction of threading dislocations and stress relaxation. This enhances the indium incorporation in InGaN well layer and also lowers the piezoelectric field in the MQWs layer with a minimal shift in its MQWs emission wavelength with higher injection current for the LEDs grown on the nano-ELO GaN template. Localization of carriers for indium-rich nanostructures incorporated in the MQWs is observed on nano-ELO and conventional GaN templates due to well width fluctuation and localized strain regions at the interface of the GaN barrier/GaN well. The incorporation of the indium-rich nanostructures enables the emission spectrum of the MQWs to be pushed to a longer wavelength which stacked with a conventional cyan emitting InGaN/GaN, MQWs gives the apple-white LEDs. The periodicity of the embedded nano-ELO GaN template enables an enhancement in the light extraction with optimization of the lattice spacing and dimension of the nanopores.
This work was supported by the IMRE, A*STAR funding under the SSL core project, IMRE/09-1P0604. The authors are grateful for the funding and also the support by SNFC group in IMRE for the use of characterization/fabrication tools.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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