Article pubs.acs.org/journal/apchd5
Monolithic Broadband InGaN Light-Emitting Diode Cong Feng, Jian-an Huang, and H. W. Choi* Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road, Pok Fu Lam, Hong Kong ABSTRACT: A monolithic nonphosphor broadband-emission light-emitting diode is demonstrated, comprising a combination of high-density microstructured and nanostructured InGaN-GaN quantum wells fabricated using a top-down approach. Broadband emission is achieved by taking advantage of low-dimensionalinduced strain-relaxation of highly strained quantum wells, combining light emitted from strain-relaxed nanotips at wavelengths shorter than the as-grown by as much as 80 nm with longer-wavelength light emitted from the larger nonrelaxed microdisks. The localized emission characteristics have been studied by spatially resolved near-field photoluminescence spectroscopy which enabled both the photoluminescence intensity and spectrum from individual nanotips to be distinguished from emission at the larger-dimensioned regions. Distinctive bluegreen-yellow emission can be observed from the electroluminescent device, whose continuous broadband spectrum is characterized by CIE coordinates of (0.39, 0.47) and color rendering index of 41. Emission can be tuned by adjusting the relative densities of nanotips and microdisks along the linear color gamut defined by their respective CIE coordinates. KEYWORDS: light-emitting diodes, broadband emission, strain-relaxation, phosphor-free, near-field spectroscopy
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significant energy losses, negating the efficiencies of the LEDs, not to mention lifetimes and environmental impacts associated with the phosphors.5−7 The ideal LED-based white light source should thus be monolithic and produce a continuous spectrum across the visible band without color-converters. Generally speaking, nonphosphor solutions involve color combination from multiple of a wafer or structure that emit at diversely different wavelengths. The differences between various approaches lie with the method with which these regions are generated. Exemplary reports of such nonphosphor solutions include diodes containing dual sets of quantum wells emitting in the blue and green simultaneously,8 InGaN nanowires or nanopyramids with varying Indium compositions on different facets that emit at different wavelengths,9−11 as well as arrays of nonuniformly sized quantum dots.12,13 InGaN/ GaN quantum wells (QWs) grown on c-plane sapphire substrates experience high levels of strain due to lattice mismatch between the InGaN wells and the GaN barriers, the extent of which depends heavily on the In composition. Such strain-induced piezoelectric field reduces the effective bandgap energy leading to a red-shift of the emission spectrum, a phenomenon known as the quantum-confined Stark effect (QSCE),14 which can be partially relieved by releasing the strain through nanoscale structuring of the QWs.15−17 Previous studies have shown that spectral blue-shifts ranging from 70 to 200 meV can be achieved, dependent on the dimensions of the nanostructures.17 Since 200 meV in energy corresponds to ∼50 nm in wavelength at the visible band, the incorporation of nanostructures in LED structures represents a promising color-
hile sunlight is universally regarded as the perfect fullspectrum light source for our illumination needs, electrical lighting is indispensable for all-day indoor lighting and exterior lighting when sunlight is not available. Although tungsten incandescent lamps generate white light as blackbody radiators in the same way as the sun does, they do so at significantly lower temperatures so that their emission spectra are shifted away from the visible to the infrared regions. Fluorescent lamps, on the other hand, are based on mercury gas discharges that emit predominantly at discrete wavelengths in the ultraviolet. White light is generated through color conversion using color-converters such as phosphors, producing a combined spectrum consisting of a mixture of broad and line emissions. In both cases, the lamps do not emit directly or entirely in the visible region which is useful for illumination, thus compromising on their efficiencies. Such wastage of nonvisible spectral components can be avoided by using lightemitting diodes (LEDs), which inherently are monochromatic sources with spectral line widths in the range of 20−50 nm.1,2 However, such widths are insufficiently broadband to cover the entire visible spectral range. Use of phosphors 3,4 in combination with LEDs, analogous to the role of phosphors in fluorescent lamps, is the primary strategy toward achieving broader band emissions. LEDs based on the wide-bandgap GaN materials are particularly suitable for this purpose due to their abilities to emit at the shorter visible wavelengths with high efficiencies. Together with phosphors such as Ce-doped YAG which typically fluoresce in the yellow color bands, white light can be generated, albeit with spectral incompleteness. Such LED-phosphor solutions have now become commercialized products, but can hardly be regarded ideal white light sources. The large Stokes shifts of nearly 100 nm give rise to © 2016 American Chemical Society
Received: April 15, 2016 Published: June 21, 2016 1294
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Figure 1. (a) Schematic diagram of the proposed monolithic phosphor-free structure comprising arrays of nanostructures of different dimensions. Broadband emission by way of combining monochromatic emissions of different center wavelengths from the differently sized nanostructures is possible due to their different extents of strain-relaxation from the as-grown wafer. Scanning electron microscope (SEM) images of the fabricated structure (b) before and (c) after planarization.
conversion alternative for white-light generation, especially if the blue-shift could be further enhanced. In this work, a monolithic phosphor-free solution toward broadband white light emission is proposed, based on an array of differently sized etched nanopillars emitting at different wavelengths across the visible band, as depicted in the schematic diagram of Figure 1a. The optical properties of the active region has been studied by near-field photoluminescence (nf-PL) and electroluminescence (EL), while the emission characteristics of the devices are evaluated and discussed.
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RESULTS AND DISCUSSION Design of the Light-Emission Region. The design of the light-emission region depends on the maximum extents of blueshifts achievable, which in turn depends on the minimum feature dimensions, the degree of strain of the quantum wells, as well as the emission spectral width of the QWs. To investigate such dimensional effects, arrays of monosized nanopillars have been fabricated onto wafers incorporating InGaN/GaN QWs using a top-down approach, based on a selfassembled nanosphere masking pattern. Figure 2a,b plots the microphotoluminescence (μ-PL) spectra of arrays of monosized nanopillars with individual diameters ranging from 7 μm to 100 nm on blue and yellow wafers with center wavelengths of 470 and 570 nm, respectively. Evidently spectral blue-shifts scale with decreasing dimensions owing to increasing extents of strain relaxation; however, the extent of the shifts differ greatly between the two wafers. As an illustration, when the diameters of nanopillars decrease from 1 μm to 500 nm, a spectral shift of ∼14 nm is observed from the yellow wafer, while the corresponding shift from the blue wafer is limited to ∼3 nm. This is due to the higher strain in the high In content QWs in the yellow wafer. Obviously, a high In content wafer is the preferred choice as platform for the proposed work, offering more room for spectral blue-shifting. Top-down fabrication of nanostructures involving energetic ion bombardments is also employed to fully exploit the strain relaxation effect. While the proposed design involves nanopillars of multiple dimensions, the number of discrete dimensions needed in practice is limited by the maximum extent of blue-shift achievable. Here, the QWs emit at a nominal wavelength of
Figure 2. Microphotoluminescence spectra of differently sized nanopillars fabricated on two wafers with different emission center wavelengths of (a) 470 nm (blue) and (b) 570 nm (yellow). The yellow wafer exhibits significantly larger blue-shifts as the dimensions of the nanopillars shrink, indicating higher levels of strain in the asgrown wafer.
∼575 nm with spectral full width at half-maximum (fwhm) of ∼50 nm; the design consists of a mixture of pillars of two dimensions at 150 nm and 7 μm. The smaller structures would emit at a shorter blue-shifted wavelength, while the larger disks would emit at wavelengths similar to the as-grown. The dualdimension array is fabricated using a top-down approach using 1295
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a monolayer of self-assembled microspheres and nanospheres for masking, an efficient and flexible technique for rapidprototyping of nanostructures. The fabricated structure is shown in the scanning electron microscope (SEM) image of Figure 1b. Near-Field Photoluminescence. In order to study the effects of nanostructuring to the QWs, the fabricated arrays are characterized by near-field photoluminescence (nf-PL), which offers spatial resolutions of better than 100 nm. Figure 3a
Table 1. Nf-PL Spectral Peak Wavelength (PW, nm) and the Spectral Width in FWHM (nm) Measured at Different Points, as Indicated in Figure 3c point d1
point d2
point d3
point t1
excitation power (kW/cm2)
PW
fwhm
PW
fwhm
PW
fwhm
PW
fwhm
2.5 15 46
538 528 522
39 40 42
535 524 515
35 37 40
512 499 491
40 43 45
508 500 496
44 46 45
spectral characteristics is almost identical to that of point d1. Strain-relaxation becomes significant toward the edge of the disk, as evident from the nf-PL spectrum collected at point d3, exhibiting larger blue-shifts compared to points d1 and d2 on the same disk. The nf-PL spectra at t1 is characterized by mild blue-shifts with increasing excitation power, similar to that of point d3, suggesting a high level of strain resemblance in strain conditions between these two points. This is consistent with the postulations that strain relaxation only occurs at the surface regions of nanostructures.15−17 The PL peak wavelength at t1 still exhibits power-dependent blue-shifts. In terms of photon energies, the line width broadening from 2.5 kW/cm2 to 46 kW/cm2 is ∼10 meV, which is relatively small compared to the corresponding peak shift of ∼60 meV. Thus, the blue-shift is believed to be mainly due to the screening of the piezoelectric field over the band-filling effect that give rise to spectral broadening,18,19 indicating incomplete relaxation of strain in the tips. According to the nf-PL intensity map of Figure 3a, the signals at the sidewalls of the disks and at the tips are generally stronger than those from the central regions of the disks; this is attributed to higher internal quantum efficiencies (IQE) at the strain-relaxed regions as a result of increased electron−hole wave function overlap.17 Light extraction of a subwavelength structure is also enhanced due to the presence of leaky modes.20−22 Device Emission Characteristics. The device was fabricated using conventional methods after planarization of the structured active region, shown in the SEM image of Figure 1c. Figure 4a−c shows CCD-captured images of the probed devices from different regions of the sample. Although uniformity within and among devices leaves room for improvement due the randomness in the arrangement of spheres, concurrent blue−green−yellow emission can be clearly resolved from a single device, verifying that the design objectives have been met. Based on this proof-of-concept demonstration, improvements to uniformity can readily be achieved by the use of precise nanopatterning techniques such as electron beam or nanoimprint lithography. The current-dependent EL spectra for the device shown in Figure 4c is plotted in Figure 4d. At currents up to 20 mA, only the disks are turned on to produce yellow emission. As the current increases, blue light with center wavelength of ∼495 nm is also emitted by the tips, while the yellow emission maintains its peak wavelength at 575 nm. Such peak wavelength difference of 80 nm is, while being marginally smaller those of phosphors LEDs, achieved without the unavoidable Stokes shift losses associated with phosphors. With 3 nm Ni/3 nm Au as the current spreading layer, the smaller tips turn on at larger currents, which is attributed to the contact dimensional effect. Several factors make the formation of metal−semiconductor contacts of nanomaterials different from that of bulk
Figure 3. (a) nf-PL intensity map of the nanotip-microdisk structure. (b) Topographical image of the same structure obtained by AFM. (c) nf-PL spectra collected from different points of the structure under varying excitation power densities. PL signals from the edge of a disk (point d3) and the nanotip (point t1) exhibit significant blue-shifts compared to the inner regions of the disk (point d1 and point d2), attributed to strain relaxation.
shows a 15 μm × 15 μm nf-PL intensity map of a typical representative region on the sample comprising a random combination of 7 μm disks and 150 nm tips, while the morphology of the same region as scanned by atomic force microscopy (AFM) is shown in Figure 3b. The nf-PL spectra from four distinct points across the array have been collected: points d1 to d3 being different positions on the 7-μm disk, and point t1 from the 150 nm tips. The nf-PL spectra collected at excitation power densities varying from 2.5 to 46 kW/cm2 are plotted in Figure 3c, while the peak wavelengths (PW) together with their spectral widths (fwhm) from the nf-PL spectra are summarized in Table 1. The strain-relaxation effect on the emission characteristics can be clearly seen by comparing the spectra collected at different locations on a disk. Point d1, being located at the center of the 7 μm disk, experiences hardly any strain relaxation, having the longest peak wavelength among the three data points on the disk. The PL center wavelength shifts to 538 nm at the lowest excitation level of 2.5 kW/cm2 from the nominal center wavelength of 575 nm, due to screening of the piezoelectric field. Further blue-shifts of its peak wavelength are observed as the excitation power increases to 46 kW/cm2. Though point d2 is located nearer to the edge of the disk, its 1296
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Figure 4. (a−c) Emission images from several devices as captured by CCD camera, whereby distinctive blue-green-yellow emission can be seen from a single device. (d) Current-dependent spectra for the device shown in (c). Two spectral peaks centered at 495 and 575 nm can be observed at higher current injections. (e) The chromaticity coordinates of emission from nanotips, microdisks and their mixture (at injection current of 40 mA) on the CIE 1931 xy chromaticity diagram. (f) Comparison between the spectral characteristics of sunlight with the typical incandescent lamp, fluorescent lamp, phosphor-converted white-light LED and the presently demonstrated phosphor-free broadband LED.
materials.23−28 First, the band bending caused by Fermi level pinning should be considered. Also, the band realignment of nanostructures is weaker than that of bulk materials, as the available depletion width is limited by size. Till now, many studies on the formation of contacts to nanostructures have been conducted, with two main experimental strategies developed for achieving improved contacts, namely, through
doping and surface treatments. For example, Cimpoiasu et al.26 reported better Mg-doped GaN nanowires by adopting metalloorganic bis (methylcyclopentadienyl) magnesium vapor and magnesium nitride powders; Lee et al.27 introduced a chlorination surface treatment to reduce the surface density of states of GaN surface; while Ham et al.28 observed ohmic contacts can formed by introducing UV illumination. At a 1297
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on high In content InGaN-GaN QWs structures, long wavelength light from the strained InGaN-GaN QWs is optically mixed with shorter wavelength light from the strainrelaxed nanostructures, effectively producing broadband emission that is tunable according to the dimensions of the nanostructures. The mechanism of emission has been studied using spatially resolved near-field photoluminescence with nanometer-scale resolution, whereby strain-relaxation at the nanotips and at the peripheries of microdisk is evident from spectral blue-shifting. An electroluminescent broadband device is also demonstrated, emitting with CIE coordinates of (0.39, 0.47) and CRI of 41. Emission is also tunable along the color gamut defined by the CIE coordinates of emission from the nanotips and microdisk by adjusting their relative concentrations.
current of 40 mA, the LED emits with a smooth and continuous spectrum with Commission Internationale de ́ lEclairage (CIE) 1931 x−y coordinates of (0.39, 0.47). The emission can be tuned by adjusting the tip-to-disk ratio within the array according to the color triangle rule, while the color gamut is defined by the CIE coordinates of emissions from the tips and disks as indicated in Figure 4e. The tip-disk mixture exhibits a color rendering index (CRI) of 41, which is also continuously tunable along color gamut; calculation predict an optimized CRI value of 46 can be achieved based on the current combination of wafer and dimensions. Emission from the device appears greenish as the color gamut, represented by the green line in Figure 4e, lies slightly north of the white light chromaticity boundary. The color gamut can be translated toward the south by increasing the nominal center wavelength of the starting wafer and reducing the wavelength of emission from the tips. The internal quantum efficiencies (IQE) of indium-rich InGaN/GaN quantum wells are generally low due to poor crystalline quality and strong QCSE effects, a phenomenon commonly described as the “green gap”. Nevertheless, the need for efficient emitters in this spectral region is well acknowledged and has prompted intensive efforts in this direction, leading to optimizations of growth conditions, improvements to structure design and the use of alternative substrates among other techniques. For instance, the crystalline quality of high In-content QWs is improved by growing on nanorod-patterned GaN/Si templates,29 while the IQEs can be increased by controlling the growth temperatures to reduce point defect incorporation.30,31 The introducing of interlayer and electron blocking layer also leads to a smoother band diagram and more uniform carrier distributions which enhances electroluminescence;32,33 on the other hand, light output powers can be boosted with highdensity ultrasmall In-rich quantum dots with good carrier confinement and reduced defects.34 The proposed device structure will benefit significantly from the success of these efforts. On the other hand, the tips will emit at shorter wavelengths at higher IQEs with further dimensional shrinking. As a matter of fact, the IQEs of the present 150 nm nanotips have been determined to be 41% compared to 29% of the asgrown wafer (see Methods). According to the CIE diagram of Figure 4e, the color gamut would overlap with the white light boundary if the tips emit at ∼490 nm, as indicated by the red line in Figure 4e, which is merely 5 nm shorter from its current emission center wavelength. This is perfectly realistic if a precise nanopatterning tool is employed for the definition of the tips. Whether the performances of such devices can catch up with those of phosphor-converted LEDs in the long run remains to be seen; nevertheless what is for certain is that such broadband emission is continuous and balanced as evident from the diagram in Figure 4f comparing spectral characteristics of common electrical lighting sources, all achieved monolithically without the use of phosphors.
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METHODS Fabrication. The dual-dimensioned nanotip/microdisk array is fabricated using a top-down approach as depicted in the schematic diagrams of Figure 5: The metal−organic
Figure 5. Schematic diagrams depicting the process flow for the fabrication of the proposed structure. (a, b) Silica (SiO2) nanospheres with mean diameters of 220 nm are spin-coated into a hexagonal-closepacked (hcp) monolayer on the surface of the initial InGaN wafer shown; silica spheres with mean diameters of 7 μm are sparsely dispersed onto the sample. (c) Inductively coupled plasma (ICP) etch for pattern transfer, followed by KOH solution wash to remove ion damage and further shrink the dimension. The completed structure consists of a combination of randomly distributed nanotips with mean feature sizes of ∼150 nm and microdisks with diameters of ∼7 μm. (d) The structure is planarized with spin-on-glass (SOG) and coated Ni/ Au as the current spreading layer; it is then proceed with conventional LED fabrication method.
chemical vapor deposition (MOCVD) grown epitaxial structure consists of 3 μm of undoped GaN, 2.5 μm of Si-doped GaN, 10 periods of high-In content InGaN/GaN quantum wells capped with a 0.3 μm Mg-doped p-GaN contact layer, on top of a 2 in. c-plane sapphire substrate. Silica (SiO2) nanospheres with mean diameters of 220 nm dispersed in deionized water are dispensed onto a sample, which is then spin-coated into a hexagonal-close-packed (hcp) monolayer; subsequently silica spheres with mean diameters of 7 μm are sparsely dispersed on top of the monolayer of 220 nm nanospheres, as depicted in Figure 5b. The coating consisting of microspheres and nanospheres serves as a hard mask in the subsequent inductively coupled plasma (ICP) etch process for pattern
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CONCLUSION Broadband emission from a monolithic structure without phosphors is demonstrated, based on an array of structured InGaN-GaN QW of two dimensions. The smaller nanotips with feature sizes of around 150 nm emit at wavelengths ∼80 nm shorter than the as-grown, while the larger 7 μm microdisks emit at wavelengths identical to the as-grown. Despite having equal In contents in the QWs, the strain profiles across the nanotips and microdisks are unequal. By fabricating this pattern 1298
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cm2. The PL signal is collected with an optical fiber coupled to the entrance slit of an f = 500 mm spectrograph, dispersed by a 2400 G/mm grating and detected by a cooled open-electrode CCD camera. The Arrhenius plot, together with that from an as-grown wafer, are shown in Figure 7. By assuming the IQE at
transfer to the semiconductor structure. The ICP process is tuned for an etch depth of ∼500 nm with a Cl2 based recipe. After etching the sample is rinsed in 2 mol/L KOH dissolved in developer AZ400, 37% hydrochloric acid and deionized water subsequently. The fabricated structure consists of a combination of randomly distributed nanotips with mean feature sizes of ∼150 nm and microdisks with diameters of ∼7 μm, as illustrated in Figure 5c. For device fabrication, spin-on-glass (SOG) is spin-coated onto the sample for planarization. After baking, the etch-back process is carried out using an SF6 based ICP etch process. A total of 3 nm/3 nm of Ni/Au is then deposited onto the sample by electron-beam evaporation serving as a current spreading layer. The remaining steps follow the regular LED fabrication process. Near-Field Photoluminescence Spectroscopy. A schematic diagram of the near-field photoluminescence (nf-PL) setup is illustrated in Figure 6. The setup is based on an NT-
Figure 7. Arrhenius plots of the PL intensities of the tip-disk structure and the as-grown sample as a function of temperature from ∼15 to 300 K.
15K to be unity, the IQEs at room temperature are estimated to be 29 and 41% for the as-grown sample and the tip-disk structure, respectively. The significant enhancement of IQE is attributed to alleviation of the QCSE effect due to strain relaxation, as well as effective passivation of surface states through chemical treatments.
Figure 6. Near-field photoluminescence setup, consisting of (1) continuous-wave diode laser, (2) dichroic filter, (3) objective, (4) fiber, (5) aperture tip, (6) sample, (7) x−y piezo-driven translation stages, (8) focus lens, (9) beam splitter, (10) photomultiplier tube, and (11) monochromator with photomultiplier tube.
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MTD NTEGRA NSOM microscope platform. The NSOM probe tip with an aperture of ∼50 nm is used for both illumination and collection of the PL signal. The laser beam from a continuous-wave diode laser emitting at 405 nm is coupled to the cleaved end of the SNOM fiber via an objective, exiting from the apertured tip for excitation of the sample with power densities varying from 2.5 to 46 kW/cm2. Note that 2.5 kW/cm2 is by no means low energy excitation, but yet is the lowest possible level for successful collection of nf-PL spectra given the tinyness/weakness of the excitation/luminescent spot. The same tip collects the PL signal directly under the aperture which is coupled to a photomultiplier tube (PMT); a PL intensity map can be generated by scanning across the sample. The same PL signal is coupled to the entrance slit of an f = 150 mm monochromator, which is then dispersed by a 1200 G/mm grating and sensed by a PMT at the exit port. Internal Quantum Efficiency (IQE) Measurement. The internal quantum efficiency (IQE) is known to be lower for high Indium composition InGaN/GaN QWs, widely described as the “green gap”. Additionally, ion damage during etching leading to the formation of surface states on the sidewalls of disks and tips may further degrade IQE. To assess such effects, the tip-disk structure is subjected to temperature-dependent microphotoluminescence (μ-PL). The PL measurements are conducted by probing the QW emission at different temperatures from ∼15 K to RT using a 349 nm diode-pumped solidstate (DPSS) laser with pulse duration of 4 ns at 1 kHz repetition rate, under an excitation energy density of ∼0.2 mJ/
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the Theme-Based Research Scheme (T22-715/12-N) of the Research Grant Council of Hong Kong.
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DOI: 10.1021/acsphotonics.6b00269 ACS Photonics 2016, 3, 1294−1300
