Broadband White-Light Emission from Alumina Nitride Bulk Single

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Broadband White-Light Emission from Alumina Nitride Bulk Single Crystals Ge liu, Chengyuan Yan, Guigang Zhou, Jiamin Wen, Zuoyan Qin, Qin Zhou, Baikui Li, Ruisheng Zheng, Honglei Wu, and Zhenhua Sun ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00601 • Publication Date (Web): 24 Jun 2018 Downloaded from http://pubs.acs.org on June 24, 2018

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Broadband White-Light Emission from Alumina Nitride Bulk Single Crystals Ge Liu, Chengyuan yan, Guigang Zhou, Jiamin Wen, Zuoyan Qin, Qin Zhou, Baikui Li, Ruisheng Zheng, Honglei Wu*, and Zhenhua Sun* College of Optoelectronic Engineering, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, Shenzhen University, Shenzhen, 518000, China.

Abstract Alumina Nitride bulk single crystals (AlN BSCs) were grown using a two-heater Physical Vapor Transport (th-PVT) method. The crystal contains massive lattice defects including aluminum vacancy (VAl) and oxygen substitution (ON). The photoluminescence (PL) spectrum of the crystal demonstrated a broad emission covering from 250 nm to 1000 nm. By study the PL spectrum, abundant mid-gap states in the wide band gap of AlN were nailed down. Based on the crystals, metal-AlN-metal Schottky devices were fabricated. These devices emitted high quality white light with color rendering index (CRI) over 90 under a bias of 60 V. Moreover, it was found that the white light emitting property of AlN BSCs was adjustable through changing impurity density and device structure. This research aims to pave a new way for solid-state white light source. Keywords: nitride crystals, lattice defects, mid-gap states, white-light source, defect induced emissions Solid state white light source is superseding incandescent and fluorescent ACS Paragon Plus Environment

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lamps due to its advantages in cost, efficiency, and stability [1-6]. There are three mainstream technologies used to realize solid state white lighting. First, a blue light-emitting diode (LED) is coated with a yellow phosphor containing rare-earth elements, rendering high luminous efficacy but poor color rendition. Secondly, an ultraviolet LED is coated with a mixture of red, green and blue (RGB) phosphors. The efficiency herein is limited due to the self-absorption. Thirdly, RGB LEDs are used directly to form a white light. Due to the narrow spectrum of a LED device, the light formed by this way is a pseudo-white light with a discontinuous spectrum [7-10]

. Therefore, a high-performance solid state white light source based

on single component (without phosphor) is of increasing interests recently for its wide application potential in illumination, display, visible color communication, as well as full spectrum laser

[1]

. Apparently, a

monolithic semiconductor with wide emission spectrum is pivotal in this device. A series of monolithic semiconductors including CdSe quantum dots [2]

and organic-inorganic hybrids

[3-6]

have been investigated in quest of

broadband white-light emitting materials. Their broadband emissions are attributed to the wide electronic band gap due to the quantum confinement effect, and the existence of mid-gap states due to surface sites and lattice distortion. Nevertheless, these microscale structures need delicate synthesis process and suffer from inferior environment stability.

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Herein, Alumina Nitride bulk single crystals (AlN BSCs) with a good broadband white-light emitting property are presented. These AlN BSCs can be grown massively and possess high stability. AlN has a wide bandgap of ~6 eV. Different lattice defects in the crystal induce plenty of radiative mid-gap energy states. Electron relaxations among the conduction band, valence band, and those mid-gap states lead to a wide spectrum photoluminescence from 210 nm to 1000 nm. What’s more, biased between two electrodes, AlN BSCs emit white light covering broadband spectrum from 300 nm to 1000 nm with a good color rendition. These broadband emission properties make the AlN BSCs a good material candidate for phosphor-free, electricity-driving, solid state white light source. AlN BSCs were grown on tungsten substrates in a Physical Vapor Transport furnace with two resistive heaters (th-PVT) [11, 12]. Crystals with size at millimeter level were obtained. The insets of Figure 1a are the top view SEM image of an AlN BSC on substrate and the schematic of wurtzite AlN crystal lattice structure. The crystal has a regular hexagonal appearance, which matches well with the wurtzite crystal lattice structure. The Figure 1a is the X-ray diffraction (XRD) pattern of the crystal. Only (0002) plane peak could be perceived in parallel with the substrate. This proves the single crystal structure and the c-axis growth orientation of the AlN BSC. The composition of the crystals was characterized by X-ray


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photoelectron spectroscopy (XPS), with the result demonstrated in Figure 1b. The result reveals the existence of O element in the crystals. Quantitively analysis pointed out that the atomic amount of O varied between 0.5% and 0.9% from crystal to crystal. Besides, there is an atomic amount shortage of Al element for about 7.3%. These results imply that the defects in the crystals should be Al vacancy (VAl) and O substitution (ON) mainly. To

gain

insight

into

the

defect

induced

energy

states,

photoluminescence (PL) property of an AlN BSC with O impurity of 0.5% were studied using a series of exciting lasers with different wavelength. Figure 2a shows the PL spectra at low temperature of 8K and room temperature, using excitation laser of 177.5 nm, which has a photon energy above the band gap of AlN. The cryogenic spectrum has a sharp peak at ~210 nm, corresponding to electronic energy of ~6 eV, which can be attribute to the band edge emission of AlN crystals [13]. The band edge emission vanished in the spectrum at room temperature. This is due to that at higher temperature, more defect-related energy states would be activated, the band edge emission was overwhelmed by defect induced nonradiative recombination [11, 14]. It is worth noting that a broad emission cover 250-500 nm is included in the spectra at both low and high temperature. Apparently, these broad PL spectra are the integration of different emission lines, which are mostly assigned to oxygen defect


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related luminescence [14]. The broad emission was deconvoluted to clarify the mid-gap states involved, as shown in Figure 2b. Four emission lines could be obtained by the deconvolution, with peaks at 3.65 eV (A), 3.39 eV (B), 3.16 eV (C) and 2.96 eV (D), respectively. The peak A and C can be assigned to the transitions from conductor band (CB) and shallow donor (SD) to VAl-ON states, respectively [15-18]. The peak C and A can be assigned to the transitions from SD to V3-Al and VAl-ON states, respectively [19-21]

. Furthermore, PL spectra with excitation lasers of 261 nm, 360 nm,

405 nmn and 514 nm, which have photon energies below band gap of AlN, were acquired at room temperature and shown in Figure 2c. The PL spectrum of 177.5 nm excitation at room temperature was put in the figure as well for comparison. The emissions of 360 nm, 405 nm, and 514nm excitation cover a broad range from 500nm to 1000nm. Due to the high excitation photon energy, the PL spectrum of 261 nm demonstrates both broad emissions of 250-500 nm and 500-1000 nm. The former emission is very analogous to the broad emission of 177.5 nm laser and can be resolved into the same four emission lines, as shown in Figure S1. The latter emission is coincident with 360 nm laser. The consistency of the PL spectra with different excitation wavelength corroborates the broad emission property of the AlN BSC. The broad emission of 500-1000 nm demonstrates an apparent red-shift with excitation wavelength increasing. The center position moves from 637 nm (1.95 eV)


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to 693 nm (1.78 eV) with excitation energy reducing from 3.4 eV (360 nm) to 2.4 eV (514 nm). This red-shift suggests a donor acceptor pair (DAP) emission, which was often observed in GaN

[22, 23]

. When the

incident photon has less energy, the excited states should be overall shallower. The PL spectra in 500-1000 nm range of the AlN BSC excited by 261nm, 360 nm, 405 nm, and 514 nm lasers were deconvoluted to specify the included emission lines, as well as the related mid-gap states. The result of 261 nm is shown in Figure 2d, with the others in the Supporting Information. There are four emission lines can be obtained by resolving the spectrum of 261 nm laser, with peak at 2.10 eV (E), 2.03 eV (F), 1.89 eV (G), and 1.60 eV (H), respectively. The peak E and G should originate from the transitions from (VAl-ON)1- and (VAl-ON)2- to valence band (VB), respectively [24]. The peak F can be attributed to the transition from VAl-ON complex to VB [24]. The peak H should be related to a deep acceptor state induced by VAl [25]. The spectra of 360 nm and 405 nm lasers can be resolved well using the above four emission lines, as shown in Figure S2 and S3. The spectrum of 514 nm laser demonstrates an apparent red-shift, so a peak K with energy of 1.40 eV, which can also be attributed to the VAl [25], must be involved to resolve it (Figure S4). In a word, the broadband PL spectra are due to the transitions between conductance band, valence band, and plenty of mid-band gap energy states. The mid-gap energy states and the corresponding transitions to the


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abovementioned emission lines are summarized in Figure 2e. The mid-gap states are derived from VAl, ON and their complex, which is consistent with the XPS result. Because of the broad bandgap and the multiformity of the defect induced energy states, the recombination emission behavior in bulk AlN crystals is very abundant. This make AlN BSCs a good material candidate for electricity-driving solid-state white-light source. A simple prototype of Schottky light emitting device with the AlN crystal sandwiched between metal electrodes was fabricated (vertical-0.5%-device). The device structure is as shown in the inset of Figure 3a. A gold electrode was thermally evaporated on top of an as-grown AlN crystal on W substrate. The shape and area of Au electrode was defined by a shadow mask. The current-voltage (I-V) curve of this Au-AlN-W two terminals device was measured and shown in Figure 3a with the W electrode grounded. Though AlN crystals are intrinsically insulative, the conductance of the AlN crystals herein is very large due to the existence of huge number of doping states. The nonlinear I-V curve indicates a non-ohmic contact between metal and crystal. Moreover, the inset in Figure 3b shows that the crystal could emit bright white light luminescence under a bias of 60 V. The emission spectrum was recorded and shown in Figure 3b. The AlN crystal has a broadband electroluminescence (EL) spectrum ranging from 320 nm to 1000 nm,


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which covers near ultraviolet-visible-near infrared region with center peak at about 558 nm. The color coordinate of the EL spectrum in 1931 CIE chromaticity diagram was calculated to be (0.36, 0.39), as shown in figure 3c. This value is very near to the equi-energy white point (0.33, 0.33) [4, 6]. Its color rendering index (CRI) was calculated to be about 90, which is relatively high in the illumination field. Temperature dependent EL spectrum of this AlN crystal was measured at 300 K, 207 K and 193 K. The results are shown in Figure 3d. These EL spectra have similar coverage but different intensity. The emission intensity increased with the decrease of temperature is because that the non-radiative recombination is less at lower temperature

[11, 14]

. This consistent cryogenic luminescence

spectrum excludes the thermal radiation from possible mechanisms. It is reasonable to attribute this luminescence to the radiative recombination of electrons and holes injected from two electrodes. The AlN BSCs demonstrated a good property of emitting high quality white light under bias in a wide temperature range. And apparently this property would be influenced by the impurity density in the AlN BSCs and the metal electrodes. Another two devices were fabricated and characterized at room temperature to clarify this influence. One is based on an AlN BSC with oxygen atomic amount of 0.9% with the same Au-AlN-W vertical structure (vertical-0.9%-device). And the other is based on an AlN BSC with 0.9% oxygen impurity using two Au pads on


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the top side of the AlN crystal as electrodes (planar-0.9%-device), as demonstrated in the inset of Figure 4a. Their EL spectra under a bias of 60 V were measured and shown in Figure 4a together with the previous vertical-0.5%-device. The emission band of the vertical-0.9%-device is broader than 0.5%, with a little extension the long wavelength side. At the same time, the central peak position of spectrum of vertical-0.9%-device has a little red-shift compared with the 0.5% device. We speculate the reason is that more oxygen dopant may induce larger state density of shallow state. This leads to an increase weight of long wavelength radiative recombination in the overall emission. The emission spectrum of planar-0.9%-device is similar to Au-AlN-W device. This implies that, even though Au and W have different working functions, their difference is small compared with the wide bandgap of AlN. Besides, the center peak position of the EL spectrum of planar-0.9%-device has a slight red-shifting compared with the vertical-0.9%-device. A possible reason is that the planar device suffered less from self-absorption, which happened more in short wavelength region. The color coordinate/CRI of vertical-0.5%-device, vertical-0.9%-device, and planar-0.9%-device are (0.36, 0.39)/90, (0.38, 0.39)/92 and (0.42, 0.41)/93, respectively. These three devices all have pretty good white light emitting property. In conclusion, AlN BSCs with considerable lattice defects of VAl and ON were grown by a PVT technique. These defects induced abundant


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mid-gap states in the wide energy band gap of AlN. As a result, the AlN BSCs demonstrated a PL spectrum including not only the band edge emission at 210 nm, but also a defect-induced broadband emission covering from 250 nm to 1000 nm. Two terminal metal-AlN-metal Schottky devices were fabricated based on the AlN BSCs. These devices could emit high quality white light with CRI above 90 under a voltage bias of 60 V. The broadband EL spectrum ranged from 300 nm to 1000 nm, which were attributed to the defected-induced mid-gap states. The white light emission property could be adjusted through changing impurity density and device structure. This research reveals the promising application potential of AlN BSCs in solid-state white light source.

Methods XRD patterns of AlN crystals were evaluated with Cu Kα radiation in a Philips X-ray diffractometer at 45 kV and 40 mA. SEM image of AlN crystal was performed by a JEOL JSM-5910LV. XPS measurement was performed using a Thermofisher Microlab 350 instrument. PL measurement excited by 177.5 nm was performed via a mode–locked frequency quadrupled Ti–sapphire laser (177.5 nm) with the power of 0.13 mW as an excitation source. The pulse width and repetition rate were 100 fs and 76 MHz, respectively. The emission spectra were obtained by the combination of a 320-mm focal–length monochromator


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(Horiba Jobin Yvon iHR320) equipped with 1200 groves/mm gratings with a spectral resolution of 0.06 nm and a cooled photomultiplier tube. The temperature was controlled by a cryostat from 8 K to 300 K. All setup was installed in an ultrahigh vacuum chamber (RHK technology). PL measurement excited by 261 nm, 360 nm, 405 nm, and 514 nm were performed via four different continuous semiconductor lasers and an Ocean Optic QE pro spectrometer. EL measurement was carried out using a combination of a probe station with a Keithley 42000-SCS and an Ocean Optic QE pro spectrometer. The metal-semiconductor-metal sample was fabricated by thermally evaporating gold electrode on top of the as grown hexagonal AlN crystals through a shadow mask. The color coordinate of the EL spectrum in 1931 CIE chromaticity diagram was calculated using a software of ColorCalculator (version 5.27) from Osram Sylvania Inc.

Acknowledgements. Z.H.S. acknowledges the financial support from the National Natural Science Foundation of China (grant no. 61505108), Natural Science Foundation of Guangdong Province (grant no. 2016A030310055) and the Science and Technology Innovation Commission of Shenzhen (grant no. JCYJ20150625103602228). H.L.W. acknowledges the financial support from the National Natural Science Foundation of China (grant no.


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11447029, 61440028, 61136001) and the Science & Technology Bureau of Shenzhen (grant no. 20160520174438578). The Authors are grateful to Mr. Fuchun Jiang for his assistance in XRD measurement, Ms. Guiwen Xu for her assistance in SEM measurement, and Dr Wenfei Zhang for the helpful discussion with him. The Authors also appreciate Dr Wei Zheng in Sun Yat-Sen University, and Ms. Yali Liu and Prof. Peng Jin in the Institute of Semiconductors, Chinese Academy of Science for their facility and help in PL measurements.

ASSOCIATED CONTENT Supporting Information The deconvolution results of PL spectra of the AlN BSCs excited by 261 nm, 360 nm, 405 nm, and 514 nm lasers (PDF) AUTHOR INFORMATION Corresponding Authors *Email: [email protected]; [email protected]

ORCID Ge Liu: 0000-0001-5774-8093 Honglei Wu: 0000-0002-8852-3951 Zhenhua Sun: 0000-0002-9227-6738 Notes The authors declare no competing financial interest


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References (1). Fan, F.; Turkdogan, S.; Liu, Z. C.; Shelhammer, D.; Ning, C. Z., A monolithic white laser. Nat Nanotechnol 2015, 10, 796-803. (2). Bowers, M. J.; McBride, J. R.; Rosenthal, S. J., White-light emission from magic-sized cadmium selenide nanocrystals. J Am Chem Soc 2005, 127, 15378-15379. (3). Ki, W.; Li, J., A semiconductor bulk material that emits direct white light. J Am Chem Soc 2008, 130, 8114-+. (4). Ki, W.; Li, J.; Eda, G.; Chhowalla, M., Direct white light emission from inorganic-organic hybrid semiconductor bulk materials. J Mater Chem 2010, 20, 10676-10679. (5). Dohner, E. R.; Hoke, E. T.; Karunadasa, H. I., Self-Assembly of Broadband White-Light Emitters. J Am Chem Soc 2014, 136, 1718-1721. (6). Dohner, E. R.; Jaffe, A.; Bradshaw, L. R.; Karunadasa, H. I., Intrinsic White-Light Emission from Layered Hybrid Perovskites. J Am Chem Soc 2014, 136, 13154-13157. (7). Im, W. B.; George, N.; Kurzman, J.; Brinkley, S.; Mikhailovsky, A.; Hu, J.; Chmelka, B. F.; DenBaars, S. P.; Seshadri, R., Efficient and Color-Tunable Oxyfluoride Solid Solution Phosphors for Solid-State White Lighting. Adv Mater 2011, 23, 2300-2305. (8). Wu, J. L.; Gundiah, G.; Cheetham, A. K., Structure-property correlations in Ce-doped garnet phosphors for use in solid state lighting. Chem Phys Lett 2007, 441, 250-254. (9). Xia, Z. G.; Liu, Q. L., Progress in discovery and structural design of color conversion phosphors for LEDs. Prog Mater Sci 2016, 84, 59-117. (10). Ye, S.; Xiao, F.; Pan, Y. X.; Ma, Y. Y.; Zhang, Q. Y., Phosphors in phosphor-converted white light-emitting diodes Recent advances in materials, techniques and properties. Mat Sci Eng R 2010, 71, 1-34. (11). Liu, G.; Zhou, G.; Qin, Z.; Zhou, Q.; Zheng, R.; Wu, H.; Sun, Z., Luminescence characterizations of freestanding bulk single crystalline aluminum nitride towards optoelectronic application. CrystEngComm 2017, 19, 5522-5527. (12). Wu, H.; Zheng, R.; Guo, Y.; Yan, Z., Fabrication and characterisation of non-polar M-plane AlN crystals and LEDs. Mater Res Innov 2015, 19, 1153-1155. (13). Taniyasu, Y.; Kasu, M.; Makimoto, T., An aluminium nitride light-emitting diode with a wavelength of 210 nanometres. Nature 2006, 441, 325-328. (14).Taniyasu, Y.; Kasu, M., Origin of exciton emissions from an AlN p-n junction light-emitting diode. Appl Phys Lett 2011, 98. (15). Wang, W. Y.; Jin, P.; Liu, G. P.; Li, W.; Liu, B.; Liu, X. F.; Wang, Z. G., Effect of high-temperature annealing on AlN thin film grown by metalorganic chemical vapor deposition. Chinese Phys B 2014, 23. (16). Nam, K. B.; Nakarmi, M. L.; Lin, J. Y.; Jiang, H. X., Deep impurity transitions involving cation vacancies and complexes in AlGaN alloys. Appl Phys Lett 2005, 86. (17). Benabdesselam, M.; Iacconi, P.; Lapraz, D.; Grosseau, P.; Guilhot, B., Thermoluminescence Of Aln - Influence Of Synthesis Processes. J Phys Chem-Us 1995, 99, 10319-10323. (18). Freitas, J. A., Optical studies of bulk and homoepitaxial films of III-V nitride semiconductors. J Cryst Growth 2005, 281, 168-182. (19). Sedhain, A.; Du, L.; Edgar, J. H.; Lin, J. Y.; Jiang, H. X., The origin of 2.78 eV emission and yellow coloration in bulk AlN substrates. Appl Phys Lett 2009, 95.


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(20). Pantha, B. N.; Sedhain, A.; Li, J.; Lin, J. Y.; Jiang, H. X., Probing the relationship between structural and optical properties of Si-doped AlN. Appl Phys Lett 2010, 96. (21). Cao, Y. G.; Chen, X. L.; Lan, Y. C.; Li, J. Y.; Xu, Y. P.; Xu, T.; Liu, Q. L.; Liang, J. K., Blue emission and Raman scattering spectrum from AlN nanocrystalline powders. J Cryst Growth 2000, 213, 198-202. (22). Koppe, T.; Hofsass, H.; Vetter, U., Overview of band-edge and defect related luminescence in aluminum nitride. J Lumin 2016, 178, 267-281. (23). Gaddy, B. E.; Bryan, Z.; Bryan, I.; Kirste, R.; Xie, J. Q.; Dalmau, R.; Moody, B.; Kumagai, Y.; Nagashima, T.; Kubota, Y.; Kinoshita, T.; Koukitu, A.; Sitar, Z.; Collazo, R.; Irving, D. L., Vacancy compensation and related donor-acceptor pair recombination in bulk AlN. Appl Phys Lett 2013, 103. (24). Sedhain, A.; Lin, J. Y.; Jiang, H. X., Nature of optical transitions involving cation vacancies and complexes in AlN and AlGaN. Appl Phys Lett 2012, 100. (25). Lamprecht, M.; Jmerik, V. N.; Collazo, R.; Sitar, Z.; Ivanov, S. V.; Thonke, K., Model for the deep defect-related emission bands between 1.4 and 2.4 eV in AlN. Phys Status Solidi B 2017, 254.


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Figure 1. (a) X-ray Diffraction pattern of an AlN crystal. The upper inset is the lattice structure of wurtzite AlN. The lower inset is the Low Magnification Scanning Electron Microscope (SEM) image of a typical AlN crystal with a hexagonal façade, with a scale bar of 1 mm. (b) X-ray Photoelectron Spectrum (XPS) of AlN BSCs showing the binding energies of Al 2p, Al 2s, N 1s and O 1s.


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Figure 2. (a) PL spectra of AlN BSCs with excitation laser of 177.5 nm at 8 K and room temperature. (b) The defect-induced PL spectrum (solid line) of AlN BSCs at room temperature with excitation laser of 177.5 nm and its deconvolution (dash line). (c) Room temperature PL spectra of AlN BSCs with excitation laser of 177.5 nm, 261 nm, 360 nm, 405 nm, and 514 nm. (d) The defect-induced PL spectrum (solid line) of AlN BSCs with excitation laser of 261 nm and its deconvolution (dash line). (e) The summary of defect-induced mid-gap states and the possible transitions between these energy states corresponding to the emission lines obtained by the deconvolution in Figure 2(b), (d), and Supporting Information.


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Figure 3. (a) I-V curve of the vertical-0.5%-device. The inset is the schematic of device structure. (b) EL spectrum of the vertical-0.5%-device under a bias of 60 V. The inset is the photograph of the device emitting white light. (c) The color coordinate in 1931 CIE chromaticity diagram and the color rendering index (CRI) of the EL emission; (d). EL spectra of the vertical-0.5%-device at temperature of 300 K, 207 K and 193 K.


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Figure 4 (a) EL spectra of the vertical-0.5%-device, vertical-0.9%-device, and planar-0.9%-device. The inset is the structure schematic of the planar-0.9%-device. (b) The color coordinate of the EL spectra in 1931 CIE chromaticity diagram and their color rendering index (CRI).


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TABLE OF CONTENTS GRAPHIC


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Figure 1. (a) X-ray Diffraction pattern of an AlN crystal. The upper inset is the lattice structure of wurtzite AlN. The lower inset is the Low Magnification Scanning Electron Microscope (SEM) image of a typical AlN crystal with a hexagonal façade, with a scale bar of 1mm. (b) X-ray Photoelectron Spectrum (XPS) of AlN BSCs showing the binding energies of Al 2p, Al 2s, N 1s and O 1s. 85x121mm (300 x 300 DPI)


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Figure 2. (a) PL spectra of AlN BSCs with excitation laser of 177.5 nm at 8 K and room temperature. (b) The defect-induced PL spectrum (solid line) of AlN BSCs at room temperature with excitation laser of 177.5 nm and its deconvolution (dash line). (c) Room temperature PL spectra of AlN BSCs with excitation laser of 177.5 nm, 261 nm, 360 nm, 405 nm, and 514 nm. (d) The defect-induced PL spectrum (solid line) of AlN BSCs with excitation laser of 261 nm and its deconvolution (dash line). (e) The summary of defect-induced mid-gap states and the possible transitions between these energy states corresponding to the emission lines obtained by the deconvolution in Figure 2(b), (d), and Supporting Information. 85x121mm (300 x 300 DPI)


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Figure 3. (a) I-V curve of the vertical-0.5%-device. The inset is the schematic of device structure. (b) EL spectrum of the vertical-0.5%-device under a bias of 60V. The inset is the photograph of the device emitting white light. (c) The color coordinate in 1931 CIE chromaticity diagram and the color rendering index (CRI) of the EL emission; (d). EL spectra of the vertical-0.5%-device at temperature of 300K, 207K and 193K. 85x70mm (300 x 300 DPI)


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Figure 4 (a) EL spectra of the vertical-0.5%-device, vertical-0.9%-device, and planar-0.9%-device. The inset is the structure schematic of the planar-0.9%-device. (b) The color coordinate of the EL spectra in 1931 CIE chromaticity diagram and their color rendering index (CRI). 85x135mm (300 x 300 DPI)


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ACS Photonics

table of content graphic 88x34mm (300 x 300 DPI)


Broadband White-Light Emission from Alumina Nitride Bulk Single

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