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True Yellow Light-emitting Diodes as Phosphor for Tunable Color-Rendering Index Laser-based White Light Bilal Janjua, Tien Khee Ng, Chao Zhao, Aditya Prabaswara, Giuseppe Bernardo Consiglio, Davide Priante, Chao Shen, Rami T. ElAfandy, Dalaver H. Anjum, Abdullah A. Alhamoud, Abdullah A. Alatawi, Yang Yang, Ahmed Y. Alyamani, Munir M. Eldesouki, and Boon S. Ooi ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00457 • Publication Date (Web): 11 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016
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True Yellow Light-emitting Diodes as Phosphor for Tunable Color-Rendering Index Laser-based White Light BILAL JANJUA 1 , TIEN KHEE NG 1 , CHAO ZHAO 1 , ADITYA PRABASWARA 1 , GIUSEPPE BERNARDO CONSIGLIO 1 , DAVIDE PRIANTE 1 , CHAO SHEN 1 , RAMI T . ELAFANDY 1 , DALAVER H . ANJUM 2 , ABDULLAH A . ALHAMOUD 1,3 , ABDULLAH A . ALATAWI 1,3 , YANG YANG 2 , AHMED Y . ALYAMANI 3 , MUNIR M . EL -DESOUKI 3 , BOON S . OOI 1,* 1
King Abdullah University of Science and Technology (KAUST), Photonics Laboratory, Thuwal 23955-6900, Saudi Arabia.
2King
Abdullah University of Science and Technology (KAUST), Imaging and Characterization Core Lab, Thuwal 23955-6900, Saudi Arabia.
3National
Center for Nanotechnology, King Abdulaziz City for Science and Technology (KACST), Riyadh, 11442-6086, Saudi Arabia.
ABSTRACT: An urgent challenge for the lighting research community is the lack of efficient optical devices emitting in between 500 nm and 600 nm resulting in the ‘green-yellow gap’. In particular, the true green (~555 nm) and true yellow (~590 nm), other than blue and red, constitute four technologically important colors. III-nitride material system being the most promising choice of platform to bridge this gap still suffers from high dislocation density and poor crystal quality in realizing high power, efficient devices. Particularly, the high polarization fields in the active region of such 2D quantum confined structures prevent efficient recombination of carriers. Here we demonstrate a true yellow nanowires (NWs) light emitting diode (LED) with peak emission of 588 nm at 29.5 A/cm2 (75 mA in 0.5×0.5 mm2 device), a low turn-on voltage of ~2.5 V while having internal quantum efficiency (IQE) of 39 %, and without ‘efficiency-droop’ up to injection current density of 29.5 A/cm2. By mixing yellow light from NWs LED in reflective configuration, with that of red, green and blue laser diode (LD), white light with correlated color temperature of ~ 6000 K and color rendering index of 87.7 was achieved. The nitride-NWs based device offers a robust, long-term stability for realizing yellow light emitters for tunable color-rendering index solid-state lighting, on scalable, low-cost, foundry-compatible titanium/silicon substrate, suitable for industry uptake.
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KEYWORDS: Nanowire, visible laser, titanium, molecular beam epitaxy, yellow, light emitting diode, solid-state lighting. Optical devices emitting in the yellow regime holds important application such as solid state lighting (SSL),1 visible light communications (VLC),2 flow cytometry3 and optogenetics.4 Current optical devices suffer from low internal quantum efficiency (IQE) in this wavelength regime known as the ‘green-yellow gap’ which has been a subject of intense scientific interest.5 For nitride based planar devices, the strong polarization fields of several MV.cm-1 results in reduced electron-hole wavefunction overlap.6 Severe alloy clustering and strain-induced defects also result in poor crystalline quality. Though planar devices emitting in the ‘green-yellow gap’ have been demonstrated, they suffer from low IQE as well as pronounced ‘efficiencydroop’ which is a decrease in external quantum efficiency (EQE) with an increase in injection current7 as attributed to polarization enhanced carrier overflow or leakage,8 hot electron escape,9 heating,10 Auger recombination,7, 11 and poor hole injection.12,13 The use of AlGaInP-based quantum-well suffers similar efficiency droop due to lattice mismatch and DX-center related defects.14 The InGaN/GaN nanowires (NWs), on the other hand, nucleate via strain relaxation, are dislocation-free in the active region, thus achieving high crystallinity, and considerably lower piezoelectric polarization fields in the active region compared to their planar counterparts, resulting in high internal quantum efficiency and comparatively shorter radiative
recombination lifetimes.15, 16 High-quality NWs based optical devices in the visible regime have been grown on silicon (Si) substrate albeit with shortcomings such as back Si absorption, poor thermal dissipation 17 and higher turn-on voltage.18 Among the main causes are the formation of an insulating amorphous SiNx layer at the nucleation site19 and 2D phonon confinement. The small diameter of the NWs facilitate lateral confinement of acoustic phonons that is restricted to one-dimension transport thus resulting in severe junction heating and even damage to NWs device.20, 21, 22, 23 The thermal-droop commonly observed in such devices hinder their high current operation, thus saturating the radiative recombination, elevating Shockley-Read-Hall (SRH) non-radiative recombination, electron leakage, and Auger recombination.24 Recent efforts by Bhattacharya et al. reported NWs based lasers structures grown on Si emitting at 520 nm,25 610 nm26 and 1.2 μm27 however, the focus on yellow emitting devices are largely not gaining sufficient focus, despite their technological importance in a plethora of applications stated above, in particular for generating white light. For producing white light, most conventional techniques utilize blue lightemitting diode (LED) to excite yellow-emitting YAG:Ce3+ phosphor to achieve broad linewidth white light spectrum.28 Though ‘efficiency-droop’, has been
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Figure 1. (a) Device structure. (b) Visualization of the fabricated yellow NWs LED. (c) The SEM image of the device structure based on 2-temperature-step growth method with TEM of Qdisks embedded in nanowire, as inset. Fill factor and NWs density of 88 % and 9×109 cm-2, respectively were estimated. (d) XRD locked couple scan showing the presence of TiN at the NWs nucleation sites. a bottle neck in such LED-based technology, SSL based on group-III-nitride laser diodes (LDs) have been demonstrated to be ‘droop-free’ and relatively more energy-efficient for eventual implementation of SSL and VLC dualfunction white lamp. The YAG:Ce3+ phosphor when used in conjunction with laser-based SSL, however, is associated with rapid degradation issue.29 In addition, shortcomings, such as limited intensity controllability,30 low efficiency due to down conversion, poor white light characteristics31 and longer carrier relaxation lifetime32 have hindered the progress. This is a pressing issue for unleashing the potential of LD-based white light for SSLVLC dual-functionalities. An alternate platform using InGaN/GaN NWs LED as active yellow phosphor which is capable of sustaining high injection current, while still maintaining junction temperature, and yet intensity-, and bandwidth-tunable for high color-quality constitute an important research gap in the light of LD-based SSL-VLC. Here we report a broad linewidth, and true yellow InGaN quantum-disks (Qdisks)-in-NWs LED operating at room temperature. The peak emission at 588 nm was obtained at 29.5 A/cm2 bias (75 mA over an area of 0.5×0.5 mm2) with an FWHM of ~74 nm. The LED showed a low turn-on voltage of 2.5 V without efficiency droop up to injection current of up to 29.5 A/cm2. The absolute peak optical power measured was 23 μW with an IQE of 39 %. The high IQE is attributed to the high quality of material, strong quantum confinement in Qdisk structures and reduced polarization fields. In addition, a constant peak position with increasing current bias in Raman spectroscopy measurement shows considerably constant temperature within measurement error (0.7 cm-1) at injection current of 100 mA. By implementing the true yellow NWs LED as a source of yellow phosphor light, and as light scattering plane, in a reflective configuration, for color-mixing with red, green and blue (RGB) laser photons, we thus offer a controllable and reliable, laser SSL-VLC platform, with tunable correlated color temperature (CCT) and color rendering index (CRI) close to 90, for overcoming the ‘green-yellow gap’ and efficiency droop. It is noted that
mixed color emission active region, such as the white light emitting nanowires,15 does not offer the flexibility of independent color and intensity tunability, as the white light characteristics change significantly with the ratio of red, green and blue intensity and vary with injection current density. Thus, it is challenging to use the mixed color emission active region approach to produce the desired smart lighting applications, where the color temperature of light is tuned to suit the human circadian rhythm. Hence, the present investigation contributed to a significant conceptual novelty, and our approach demonstrates the practicality of nanowires devices in tunable solid-state lighting applications.
METHODS The yellow NWs p-i-n LED structure was grown catalyst-free using Veeco Gen 930 plasma assisted molecular beam epitaxy system (PAMBE). The native oxide was removed from Si (100) using HF-H2O solution followed by deposition of 100 nm of Ti using electron-beam evaporator. To improve the uniformity and homogeneity of tightly packed NWs, the Ti layer was evaporated using low deposition rate of 0.5 Å/s on a pre-cleaned atomically smooth Si substrate. The GaN seed layer was first nucleated using a 2-temperature-step growth method. In the first step, NWs were nucleated at a much lower temperature of 500 °C, to increase nucleation probability, followed by growth at a higher temperature of 600 °C for improved crystal quality. Conducting titanium nitride (TiN) has been seen to form at nanowire base at the nucleation site which considerably improves current injection and heat dissipation.17 An active region with 5 stacks of ~3 nm InGaN Qdisks separated by ~10 nm quantum barrier (QB) was then grown on GaN nucleation sites. Quantum wells were realized at a lower temperature of 515 °C followed by capping of 2 nm of GaN, to avoid the dissociation or InGaN Qdisks when ramping up the temperature to 565 °C for quantum barrier growth. Indium flux was 2
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fixed at 5×10-8 Torr while Ga flux was varied between 3-6×10-8 Torr. Metal fluxes were measured using the beam flux monitor. The device was completed with a magnesium (Mg) doped 30 nm GaN layer. Nitrogen flow was maintained at 1 sccm with RF power fixed at 350 W. The yellow NWs LED was fabricated using standard UV contact lithography process. The NWs were first planarized with parylene, etched back to reveal the p-GaN contact layers, and then deposited with Ni (5 nm) / ITO (200 nm), which forms an ohmic contact with p-GaN, upon annealing. ITO acts as transparent conductive oxide and a current spreading layer. Ni (10 nm) / Gold (Au) (1 um) was then deposited as the top p-contact. Oxygen plasma and argon treatment were used to expose clean unoxidized Si back surface. For back contact Al (100 nm) / Ti (10 nm) / Au (150 nm) were sputtered as n-pad. Figure 1(b) shows the cross-section 3D visualization of the grown and fabricated yellow NWs LED. The Bruker-D8 diffractometer with Cu Kα radiation was used to examine the orientation of the nanowires via locked coupled 2θ scans. Micro Temperature Dependent Photoluminescence (μTDPL) measurement was performed using a 405 nm excitation laser integrated with a Janis CCSXG-M/204N and Ocean Optics QE6500 spectrometer. The temperature was varied from 10 K to 300 K and the signal was measured to study emission from Qdisks embedded in NWs. Electroluminescent (EL) and white light characteristics were measured on McScience LC5000 LED tester. The light power was captured external to an integrating sphere at a remote distance to facilitate probing. The quality and structure of the NWs were investigated with both scanning electron microscope (SEM) and transmission electron microscope (TEM). The TEM investigations were performed by using a Titan 80-300 ST microscope (FEI Company, Hillsboro, OR). Furthermore, the microscope was operated at 300 kV in high-angle annular dark-field (HAADF) scanning TEM (STEM) mode. Temperature dependent Raman spectroscopy was performed on the asgrown sample on HORIBA LabRAM HR VISIBLE spectroscope. A below bandgap 785nm excitation laser was utilized, and Raman spectra were collected using a 50x objective lens in backscattering geometry. The heating chamber temperature was varied from 20 °C to 200 °C in a step of 20 °C.
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grown on 100 nm Ti coated Si substrates. The titanium layer deposited on crystalline substrate showed a preferential (0002) crystalline orientation because of it having the lowest surface energy.33 According to the Bragg’s law, TiN(111), and GaN(0002) planes are parallel to each other, which confirmed the epitaxial growth of GaN on the TiN nucleation layer. Figure 1(a) shows the structure of the NWs yellow LED with InGaN Qdisks, being nucleated on a 100 nm Ti thin film coated on Si (100) substrate. Such high indium incorporation to achieve yellow emission while maintaining good crystalline quality has only be realized in nano-structures which are seen to be dislocation free.34 TiN formation is favored over insulating Si3N4 layer which is commonly observed when growing similar GaN NWs directly grown on Si. TiN in conjunction with Ti under layer reflects a portion of backemitted light.35 Also being a conductor, it plays a critical role in improving heat dissipation and lateral uniform current injection. Due to the variation in growth temperature as growth proceeds, the NWs are seen to exhibit inverse tapered shape being thinnest at the bottom (~30 nm) and reaching an average diameter of ~55 nm at the top. Growth conditions were optimized to avoid coalescence of the NWs. Parylene which is used as a filling material also acts as surface passivation layer, thus mitigates nonradiative defects.36 To quantify carrier delocalization and defect-related quenching mechanism, the activation energies are calculated by fitting the data of the integrated spectrum to the equation shown in Fig 2(a) inset. From the Arrhenius plot, the activation energies EA1 and EA2 of ~6 meV and ~105 meV were obtained, which are much larger than conventional planar InGaN well structures.37 The larger EA2 correlates to larger delocalization energy, which allows more efficient electron-hole transitions and reduced thermionic emission of photocarriers. Besides higher activation energies, lateral carrier diffusion were inhibited thus reducing their probability of reaching the NWs sidewalls and recombining nonradiatively. With the deep localization, an improved IQE is expected as was obtained in this work. The IQE of the NWs was estimated to be 39 % by measuring the ratio of integrated intensity at room temperature and at 10 K, assuming that at low temperature all the non-radiative recombination centers are frozen and carrier recombine with 100 % efficiency.
Bias dependent Raman was performed on a fabricated LED of 0.5×0.5 mm2 in size using the same Raman spectroscope equipped with tungsten probes and KEITHLEY 2400 source meter. The injected current was varied from 0 to 100 mA in steps of 10 mA. To extract the peak position the raw data was fitted with Lorentzian-Gaussian fitting. For white light generation, red (LP642-SF20), green (LP520-SF15), and blue (LP450-SF15) LDs from Thorlabs, with nominal spectral linewidth / center wavelength of 1 nm / 640 nm, 0.48 nm / 515 nm, 0.9 nm / 447.2 nm, respectively, were utilized. The white light experiment was performed using collimated laser beams. The beams were focused onto the NWs samples using plano-convex optics. The diameter of the spots was adjusted to the scale of the device (0.5×0.5 mm2). The laser-beams were at ~450 incidence angle to the substrate surface to extend laser-beam spot on the NWs device. This in turn produces a projection of homogenous white light on a screen at 50 mm away, and normal to the yellow LED sample. The mixed white light was measured using a GL Opti-probe attachment, fiber-coupled into the GL Spectis 5.0 Touch spectrometer.
RESULTS AND DISCUSSION A top view scanning electron microscope (SEM) image of the InGaN nanowire device sample is shown in Fig 1(c). An average lateral size and length of NWs was measured to be ~55 nm and ~200 nm respectively. It is clear that the nanowires were mostly vertically aligned along c-axis, disjointed with a fill factor of 88 % and an areal density of ~9×109 cm-2 with relatively uniform growth over a large area. TEM image confirms the formation of InGaN Qdisk like structures embedded in GaN matrix as shown in Fig 1(c) inset. Figure 1(d) shows the 2θ scan of Si doped GaN nanowires
Figure 2. (a) Arrhenius plots of the integrated PL intensity at different temperatures for yellow NWs LED. The activation energies of ~6 meV and ~105 meV were fitted using equation in the inset. (b) The PL emission at 300 K can be deconvoluted into three peaks. The insets show the respective change in PL FWHM and peak shift with temperature.
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Figure 3. (a) Top view of the 0.5×0.5 mm2 NWs yellow LED under DC bias of 10 A/cm2. (b) Current-voltage characteristics along with absolute optical power measured. The EL spectra show an emission peak at 588 nm at a bias of 29.5 A/cm2. (c) EL spectrum of nanowire device with changing current bias from 0-29.5 A/cm2 along with peak shift and FWHM variation as an inset. (d) External quantum efficiency as a function of current density. (e) Color correlated temperature and color rendering index with changing current density. To understand strength of the active medium emission, it is important to gain insight into the useful radiative recombination dynamics of the grown structures. A broad linewidth of ~140 nm for integrated PL at 300 K, as shown in Fig 2(b), was obtained which can be due to band filling effect in the presence of inhomogeneous composition and form, and alloy disorder commonly reported in the NWs.38, 39 The strong PL intensity is an indication of the quantum confinement in 3D Qdisk structures with good InGaN crystalline quality.
the yellow region with an increase in current bias. In individual NWs, band filling and possible alloy broadening, seen in PL spectra with high optical power excitation, can cause a blue shift.38 A similar blue shift behavior also occurs in InGaN quantum well based planar devices in the presence of high polarization fields. However, as NWs presented to be a low polarization structure, which forms by strain relaxation at the nucleation site,15, 16 the peak shift can be attributed to band filling effect or a preferable injection of an ensemble of NWs emitting at a shorter wavelength (see Figure S2).
It is shown that the spectrum consists of three Gaussian peaks correlating to ensemble of emissions sites originating due to the inhomogeneity from single NWs and across different NWs. The three peaks emitting at ~576, ~622 and 678 nm at room temperature show a weak blue shift with decreasing temperature, attributed to bandgap expansion (see Table S1). As discussed later the peaks at ~576 and ~622 nm correlate well with the electroluminescence emission at low and high injection. The peak at 696 nm can just be an artifact of the broadening introduced by the inhomogeneity in NWs. The position of the peak emission does not show an apparent Scurve, as shown in Fig 2(b) inset, which is commonly attributed to inhomogeneity and indium clustering within the active region of planar structures.37 Our results indicate a negligible indium clustering effect (see Figure S1).
With an increase in bias, the emission peak linewidth was found to saturate at ~74 nm coinciding with the FWHM of the yellow PL peak seen in the inset of Fig 2(b). At lower bias, the competition of collective emission between ensembles of NWs results in large FWHM. With increase in bias, the ensemble emitting in the shorter wavelengths dominants thus is causing the FWHM to settle at a constant value of ~74 nm.
A homogenous light emission from a 0.5×0.5 mm2 device was shown in Figure 3(a) confirming uniform growth and suitable fabrication process. The NWs yellow LED was characterized by different DC-biases as shown in Fig 3(b). A low turn-on voltage of ~2.5 V was measured. This is due to lower contact resistance in the presence of TiN interlayer during plasma exposure, and improved fabrication process due to uniform height distribution of NWs. Our prior work on Ti/TiN/Mo platform has shown significant improvement in current voltage characteristics compared to devices grown directly on Si thus demonstrating the feasibility of TiN/Ti/Si platform.28 An important feature to be noted is that no saturation in power, reaching 23 μW, is observed up to 29.5 A/cm2 of current injection. Figure 3(c) depicts the measured room temperature EL of the yellow NWs LED. The EL peak at 610 nm at ~5 A/cm2 is close to PL peak of 622 nm at room temperature demonstrating consistency between the two different emission mechanisms and further confirming emission from the active region. Further LED characterization revealed the emission peak dependence on injected current displays a blue shift of 22 nm when the injection current was increased from 0 to 29.5 A/cm2, as shown in the inset of Fig 3(c). This suggests preference injection of NWs ensemble emitting in
The absence of a pronounced peak at 400 nm which is an indication of recombination via trap states in p-GaN Mg acceptor level caused by electron overflow or leakage, shows such effect is absent in our device.40 A stable light output power at high injections levels is necessary for general illumination. The conventional nitride based planar devices suffer from ‘efficiency-droop’ phenomena as discussed earlier. However this is absent in NWs based LED. As shown in Fig 3(d), which depicts the calculated EQE plotted against current density; droop free behavior was observed up to 29.5 A/cm2 injection current with EQE reaching 0.014 %. The considerably lower EQE in contrast to the ~40 % IQE can be caused by reduced photon collection efficiency as the device under test is placed outside and remote to the integrating sphere, poor injection efficiency due to smaller contact area with p-GaN, unpassivated NWs leading to potential surface states recombination, and lower extraction efficiency in the presence of top metal pads shadow (covering 40 % of device area); all of which can be resolved with further engineering design. The external quantum efficiency (EQE) measures the ratio of the number of emitted photons over the number of injected electrons. The optical power and injection current can be derived from the L-I measurement as shown in Fig 3(b) and the emission wavelength can be determined from the spectrum in Fig 3(c). A reduced piezoelectric polarization fields, high doping, and better heat dissipation are some of the reasons which improve the device performance with an increase in current bias. The CCT of the device was measured to be in warm white regime, and going beyond 2200 K with increasing bias, without considerable change in CRI after 10 A/cm2 injection current as shown in Fig 3(e). A similar trend in
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FWHM is also observed, showing positive correlation with the trend in CRI values.
compare white light characteristics. Figure 5(b) shows a bottom-up picture of the mixed white light obtained using the RGB LDs and yellow NWs LED.
To enable LED operation at high injection levels, thermal management becomes very crucial. We evaluated the temperature effect in GaN NWs by comparing the Raman shift of the E2 peak with different levels of injected current. Link et al., related the phonon frequency to the material temperature, taking into account anharmonicity of the crystal lattice potential and the thermal mismatch.17, 41
Corresponding to the intensity ratio between the RGB and yellow, the scattered white light spectral characteristics were measured. The diffused light spectrum along with the white light CRI and CCT based on CIE 1931 standard is shown in Fig 5(c)-(e). In Fig 5(c), the combination of blue LED and yellow spontaneous emission led to CRI similar to conventional blue LEDyellow phosphor configuration. The blue LD and yellow LED mimic the LDphosphor combination as shown in Fig 5(d). Figure 5(e) depicts that further improvement in white light can be obtained by introducing a green and red component which increased CRI to 87.7 with CCT of ~6056 K.
For calibration, E2 phonon peak position was used to derive the calibration curve for temperature ranging from 20 °C to 200°C in step of 20 °C.42 The yellow NWs LED was biased and temperature dependent Raman was performed as shown in Fig 4(a). Figure 4(b) shows the bias dependent Raman study; the variation in the peak shift plot is within the resolution
Figure 4. (a) Raman spectra measured with the device operated at varying biases. (b) E2 peak shift observed with different injection levels for the 0.5×0.5 mm2 device. (0.7 cm-1) of the Raman system limited by the 600 grooves/mm. We were able to operate the device up to 100 mA with insignificant junction temperature change, with in the error limit, showing the sufficient phonon dissipation in presence of Ti thin film and formation of conducting TiN interfacial layer hence highlighting the strength of the process engineering.
In summary, the true yellow NWs LED was used in reflective configuration for generating white light. The vertically aligned NWs were grown using 2temperature-step method on 100 nm Ti coated Si substrate with density, diameter and length of ~9×10-9 cm-2, 55 nm, and 200 nm. An IQE of 39 %, correlating to strong radiative recombination, is obtained due to good crystal quality of material, reduced polarization fields and tightly quantum confined carriers in Qdisks in NWs configuration. The fabricated device showed a peak output power of 23 μW at 29.5 A/cm2 with EQE of 0.014 %. White light based on RGB LDs and yellow NWs LED was demonstrated. In the discussed configuration for the white light generation, the NWs acted as a scattering unit thus mixing RGB and broad yellow component of the emitted light. With just the blue component combined with that of yellow a CRI of more than 65 was obtained which is comparable to the conventional phosphor being utilized. Further improvement in white light was achieved by mixing the red and green component resulting in CRI of 87.7and CCT of ~ 6056 K. Thus, future smart-lighting based on RGB LDs can be made more efficient, reliable and controllable based on combination of RGB LDs with active true yellow NWs based phosphor. Furthermore, the true yellow NWs LED was grown on low cost, CMOS-foundry compatible Ti-thin-film/Si substrate platform, thus favorable for eventual III-V/Si integration as wired backbone communication light source, including potential single-photon source for quantum computing applications.
Associated content Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. TEM, EELS, PL fitting parameters, nextnano3 simulation and EQE calculation.
Authors Information Corresponding author: *E-mail:
[email protected]. Notes: The authors declare no competing financial interest.
Acknowledgement
Figure 5. (a) Schematic of experimental setup for white lighting. (b) The white light generated using RGB LDs and yellow NWs LED in reflective configuration. The spectrum of white light generated using blue LED (c) and LD (d). (e) Mixing RGB LDs and yellow NWs LED. The best CRI for each configuration are shown. The white light experimental setup is shown Fig 5(a). To obtain white light, the collimated beams were directly illuminated onto the NWs sample. The NWs act as both the scattering source and reflective surface thus mixing the RGB LDs beams with the diffused yellow light emitted by the NWs LED. A commercial blue LED with peak emission at 459 nm was also used to
We acknowledge the financial support from King Abdulaziz City for Science and Technology (KACST), Grant No. KACST TIC R2-FP-008. This work is partially supported by King Abdullah University of Science and Technology (KAUST) baseline funding, BAS/1/1614-01-01.
References (1) Neumann, A.; Wierer, J. J.; Davis, W.; Ohno, Y.; Brueck, S. R. J.; Tsao, J. Y., Four-color laser white illuminant demonstrating high color-rendering quality. Opt Express 2011, 19, A982-A990. (2) Janjua, B.; Oubei, H. M.; Retamal, J. R. D.; Ng, T. K.; Tsai, C. T.; Wang, H. Y.; Chi, Y. C.; Kuo, H. C.; Lin, G. R.; He, J. H.; Ooi, B. S., Going beyond 4 Gbps data rate by employing RGB laser diodes for visible light communication. Opt Express 2015, 23, 18746-18753. 5
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