Color-tunablility in GaN LEDs Based on Atomic Emission Manipulation

Apr 17, 2019 - Jiang, Xue, Lu, Fenrich, Morea, Zang, Gao, Cheng, Zhang, Kamins, Harris, and Sun. 2019 6 (4), pp 915–923. Abstract: The full explorat...
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Color-tunablility in GaN LEDs Based on Atomic Emission Manipulation Under Current Injection Brandon Mitchell, Ruoqiao Wei, Junichi Takatsu, Dolf Timmerman, Tom Gregorkiewicz, Wanxin Zhu, Shuhei Ichikawa, Jun Tatebayashi, Yasufumi Fujiwara, and Volkmar Dierolf ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01461 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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Color-tunablility in GaN LEDs Based on Atomic Emission Manipulation Under Current Injection Brandon Mitchell1,2, Ruoqiao Wei3, Junichi Takatsu2, Dolf Timmerman2, Tom Gregorkiewicz2,4, Wanxin Zhu2, Shuhei Ichikawa,2 Jun Tatebayashi2, Yasufumi Fujiwara2, and Volkmar Dierolf3 1 Department

of Physics, West Chester University, West Chester, PA, 19383, USA

2 Division

of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 21 Yamadaoka, Suita, Osaka 565-0871, Japan

3

Department of Physics, Lehigh University, Bethlehem, Pennsylvania 18015, USA

4Van

der Waals-Zeeman Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

ABSTRACT The development of efficient electrically driven color-tunable solid-state light sources will enable new capabilities in lighting and display technologies. Although alternative light sources such as organic light emitting diodes (O-LEDs) have recently gained prominence, GaN-based LEDs remain the most efficient light sources available making GaN the ideal platform for color-tunable devices. In its trivalent form, Europium is well-known for its red emission at ~620nm; however, transitions at ~590nm and ~545nm are also possible if additional excited states are exploited. Using intentional co-doping and energy-transfer engineering, we show that it is possible to attain all three primary colors due to emission originating from two different excited states of the same Eu3+ ion mixed with near band edge emission from GaN centered at ~430nm. The intensity ratios of these transitions can be controlled by choosing the current injection conditions such as injection current density and duty cycle under pulsed current injection.

Keywords: Light-Emitting Diodes, Color-Tunable, Pulsed Current Injection, Re-excitation Dynamics, Europium doped GaN, Spectroscopy

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Artificial lighting is a critical technology that is pervasive in nearly every aspect of modern society. While incandescent lighting dominated this area for over century, there has been, over the last two decades, a shift to more efficient light sources based on light emitting diodes (LEDs) fabricated in solid-state semiconductors and organic materials (O-LEDs). While the energy-saving benefits of these technologies are apparent, their impact on human health remains unclear.[1] One issue with white solid-state LEDs is that they rely on high-intensity blue InGaN/GaN LEDs and secondary excitation of a phosphor mixture to produce white light. To make “warmer” LEDs, a phosphor that decreases the proportion of blue light to red and yellow light is used; however, even in that more elaborate scheme, the blue light is still largely present and can have a significant impact on human sleeping habits and circadian rhythms.[1] Similar concerns arise for displays, such as computer and smartphone screens that utilize the white LED as a backlight. Color-tunable LEDs are of high interest since they would rely on primary color mixing rather than bluelight stimulated phosphors, avoiding the issue mentioned above.[2-11] In addition, this would also allow for smaller pixels in display applications, which would be beneficial for micro-LED displays.[4,12,13] The proposed concept is somewhat similar, but more simple than that presented in several reports on colortunable O-LEDs, which utilize multiple polymer materials. In that case, however, either the polymers are blended into a single pixel where each polymer is activated at a different driving voltage,[2] or multiple polymer layers/devices are used, each being activated under different current injection conditions.[4-6] In terms of solid-state tunable LEDs, there are presently two main approaches. One is to combine three different InGaN layers, where the In concentration is modified to produce red, green or blue emission,[7,8] or where AlGaInP is used as the red emitting layer.[9] Another approach relies on GaN doping with rare earth (RE) ions, whose emissions feature narrow bands that are stable against temperature and current injection.[10,11,14,15] It has been proposed that multiple rare earth ions, each emitting at a different primary color wavelength, could be doped into GaN as individual devices.[10,11,14] In regards to the last approach, substantial progress has been made on red GaN-based LEDs by doping the RE element Eu into GaN by organometallic vapor phase epitaxy (OMVPE).[15] In addition, it was recently demonstrated that the emission spectrum of GaN:Eu could be modified under pulsed laser excitation or current injection.[16] In this report, we demonstrate that it is possible to attain red and green and blue emission originating from a GaN structure doped with a single type of RE ion, making use of different excited states of Eu3+ ions and controlling the behavior of the naturally occurring transitions under current injection without the use of additional phosphors or emitting materials. In particular, red to yellow and red to purple color-tunability are demonstrated under constant current injection or by modifying the duty cycle under pulsed current injection. Using a tunable pulsed laser to perform time-resolved photoluminescence 2 ACS Paragon Plus Environment

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measurements, the properties of this behavior are further elucidated, and a model for the behavior is developed using rate equations.

RESULTS Red to yellow tunability with injection current. Europium is commonly used as a red emitter in phosphors due to its strong emission at ~622 nm, resulting from the 5D0 to 7F2 transition within the 4f shell.[17-20] In GaN:Eu LEDs, this is the standard emission observed under current injection.[21-25] Figure 1 shows other important transitions of Eu 3+ ions that are close in energy to the 622 nm transition. Note that the excitation to higher levels is typically followed by a non-radiative decay to the 5D0 state in many host materials.[17] A GaN:Eu LED (hereafter referred to as the RY-LED) was fabricated with 13 pairs of quantum well structures comprised of alternating Eu-doped GaN layers (1nm thick) surrounded by AlGaN barrier layers (5nm thick), where the Al concentration was ~20%.

Figure 1 | Normalized EL emission spectra as a function of injection current for the RY-LED. (Left) Energy level and transition scheme of a Eu3+ ion. (Right) Normalized EL spectra from a GaN:Eu LED. At low currents, the emission from the 5D1 state is negligible and the emission is dominated by the emission from the 5D0 state. As the injection current is increased, the color of the emitted light changes from red to yellow (inset). This is due to additive mixing of the green emission from the 5D1 state (545 nm) and the red emission from the 5D0 state (622 nm) to produce yellow. The other 5D1 related emission at 595 nm is yellowish-orange, which mixes with the yellow light and increases the brightness of the emitted light, while making it slightly warmer in color.

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Electroluminescence (EL) emission spectra from this LED are shown in Fig. 1 as a function of injection current, where the intensity of the emission was normalized to the 5D0 -7F2 transition. At low injection current, only red emission from the 5D0 state is induced. As the current is increased, emission related to the 5D

1

state appears and increases relative to the 5D0 state related emission. Three regions of emission can be

distinguished, which are due to transitions from the 5D1 state to the 7F3, 7F2, and 7F1 states, yielding green, green-yellow and yellow-orange emission, respectively. The green emission centered at ~545 nm and the red emission at ~622 nm mix additively to produce yellow light. Thus, as the current is increased, the emission from the LED changes continuously from red to orange to yellow, which is shown for three currents in the inset of Fig.1.1

Energy transfer pathways revealed through photoluminescence excitation (PLE) and timeresolved photoluminescence (TR-PL) spectroscopy. To understand the origin and temporal properties of this behavior, PLE and TR-PL experiments were conducted. The results of the PLE measurements are shown in Fig. 2. The Eu3+ related emission lines that were monitored were the 5D0 → 7F1 (600nm) and the 5D0 → 7F2 (622nm) transitions (emission at 696 nm was also collected for background comparison). There is a clear peak for both of these transitions at ~471 nm (2.63 eV), which corresponds to the 7F0 → 5D2 transition.[17,26] There is a very small peak located around 530 nm in Fig. 2, which corresponds to the 7F0 → 5D1 transition. From this, it is inferred that 5D0 emission resulting from absorption directly into the 5D1 state is negligible compared to that of the 5D2 and 5D

0

states.

Figure 2 | PLE measurements around the 5D2 state.

1

A movie of the emission from the LED as the current is increased can be found in Supplemental Information online.

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PLE spectra of the RY-LED active layer while observing emission due to the 5D0 → 7F1 (600nm) and the 5D

0

→ 7F2 (622nm) transitions. There is a clear and strong peak related to absorption into the 5D2 state,

however, the absorption into the 5D1 state is comparatively weak. Emission at 696nm, which does not correspond to a Eu3+transition was also monitored for comparison, and there is no peak from the 7F0 → 5D

2

transition in its PLE spectra.

To understand the temporal evolution of the Eu3+ electronic level populations, TR-PL measurements were taken at an excitation wavelength of 354nm, which excites the GaN above the bandgap, leading to energy transfer from the host to Eu3+ ions. A low repetition frequency of 100 Hz was used to ensure that reexcitation was minimized.[16] The emission due to the 5D0 → 7F2 and 5D1 → 7F1 transitions are shown in Fig. 3(a) and Fig. 3(b), respectively. The TR-PL results on the 5D0 → 7F2 transition exhibits a partial rise that was faster than the temporal resolution of the system followed by a slow rise, with a rise time of ~2 µs. The decay lifetime of the 5D0 → 7F2 transition was found to be ~260 µs, which is in agreement with previously published results on GaN:Eu.[15,26] The TR-PL results on the 5D1 → 7F1 transition only exhibited a rise that was faster than the temporal resolution of the system. The decay lifetime of this transition was found to be ~2.4 µs. The slow (~ 2µs) rise of the 5D0 state is assumed to be due to pumping from the 5D1 state, which has also been suggested elsewhere.[17,26] The low absorption into the 5D1 coupled with the fast rise and shorter lifetime of the 5D2 state suggest that the 5D1 state is primarily pumped from the 5D2 state. Due to fact that there is no rise of the 5D1 state within the time-resolution of our system, the lifetime of the 5D2 state was estimated to be at least 1 order of magnitude shorter than that of the 5D1 state, which was used in the simulations. In addition, only roughly half of the intensity from the 5D0 → 7F2 transition comes from the 5D

1 related

rise, the other half comes faster than the detection limit of the system.

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Figure 3 | TR-PL measurements of the 5D0 → 7F2 and 5D1 → 7F1 transitions. a TR-PL measurements of the 5D0 → 7F2 transition. The inset shows a zoomed-in image, where the slow and fast rise contributions can be observed. Roughly half of the intensity comes shorter than the temporal resolution of the set-up and the other half takes ~2µs to occur. The lifetime of the 5D0 state was determined to be ~ 260 µs from fitting the exponential decay. B TR-PL measurements of the 5D1 → 7F1 transition. The inset shows a zoomed-in image, where only a fast rise can be observed. The full intensity comes shorter than the temporal resolution of the set-up. The lifetime of the 5D1 state was determined to be ~ 2.4 µs from fitting the exponential decay.

Rate equations and simulation for emission during current injection To gain further insight into this process, the system was modeled with a set of rate equations. A schematic figure of the energy levels and important transitions in this model is shown in Fig. 4(a). The energy levels considered in this model are the 5D2, 5D1, 5D0 and 7F(0-6) Stark level manifolds (Note: all 7F levels are taken together. While the relative peak intensities of the individual transitions from a D-state to the different Fstates will be determined by a branching ratio, they will all follow from the D-state occupancy). The transitions from the 5D2 → 5D1 and 5D1 → 5D0 states are primarily a result of non-radiative decay, while the 5D

0

→ 7F transition is dominated by radiative decay.

Figure 4 | Rate equation modeling and comparison with EL intensity as function of injection current. a Energy levels and transitions of a Eu3+ ion in a semiconductor device. The blue arrows indicate the excitation pathways, the black arrows the carrier-induced de-excitation, and the red arrows the radiative and non-radiative transitions. b Simulations of the level occupancy as a function of injection current for a value of the branching parameter B = 0.5. The circles indicate the integrated emission values related to the 5D0 and 5D1 states as measured from the RY-LED. The total number of carriers per unit time, as determined by the current I, govern the excitation to the 5DJ states, as well as the carrier-induced de-excitation. For simplicity, but with no loss of validity, we only consider excitation into the 5D0 and 5D2 states, where the relative contributions of both levels are determined 6 ACS Paragon Plus Environment

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by the branching ratio parameter B. Based on the results shown in Fig. 3, we approximate that ~50% of the Eu ions are excited directly into the 5D2 state and ~50% are excited directly into the 5D0 state, with excitation into the 5D1 state being considered negligible, and thus B was set to 0.5. During device operation, energy is transferred to the Eu ions via local defect traps, the details of this energy transfer have been extensively investigated and are reported elsewhere.[27-32] The transfer time from the host to the Eu ions has been found to be the order of nanoseconds, and the carrier-induced de-excitation takes place on a similar timescale, which are both negligible compared to lifetimes of the 5DJ states.[26] With these considerations, the set of rate equations describing this model take the form: ∂𝑛𝐷2 ∂𝑡 ∂𝑛𝐷1 ∂𝑡 ∂𝑛𝐷0 ∂𝑡 ∂𝑛𝐹 ∂𝑡

𝐼

𝐼

= 𝐵𝑁𝐸𝑢 ― 𝑛𝐷2𝑘21 ― 𝑛𝐷2 𝑁𝐸𝑢 𝐼

= 𝑛𝐷2𝑘21 ― 𝑛𝐷1𝑘10 ― 𝑛𝐷1𝑁𝐸𝑢 𝐼

𝐼

= (1 ― 𝐵) 𝑁𝐸𝑢 + 𝑛𝐷1𝑘10 ― 𝑛𝐷0𝑘0 ― 𝑛𝐷0𝑁𝐸𝑢 𝐼

= 𝑛𝐷0𝑘0 ― 𝑛𝐹𝑁𝐸𝑢

Where nx indicates the fractional population of the 4 levels involved, and k21, k10 and k0 are the transition rates from 5D2 → 5D1, 5D1 → 5D0 and 5D0 → 7F, respectively. NEu is the total number of Eu ions available in the LED structure. The three transition rates k21, k10, and k0 were determined from the TR- PL measurements of the 5D1 and 5D0 related emission under resonant excitation of the 5D2 state. Since all transition rates are fixed, the complete behavior of this system is determined by the factor I/NEu. The evolution of the level populations was calculated by temporal integration of the rate equations with a step size of 100 ns, and running the calculation until a steady-state population has been reached after 1 ms. Under assumption that the radiative rates of the 5D1 state and the 5D0 state are equal,[17] the occupancy of both levels directly reflects the emission intensity. The calculated level occupancy and the integrated EL intensity for both levels as function of injection current are compared in Fig 4(b).

Continuous vs. pulsed current injection. The input and output powers of the LED change as the color changes with varying injection current. However, a constant current is not required for the orange and yellow emission. The only condition for this is to continuously depopulate the 5D0 state while not depopulating the 5D1 state, which can be accomplished by a short pulse at least every 2.5 µs. To explore this, a square-pulsed current injection was used where the percentage of time “on” (duty cycle) was varied (Fig. 5).2 First, the root-mean square (RMS) power (Prms) was held constant at 150 mW with a signal of frequency of 60 Hz. As the duty cycle was reduced from 2

The same LED sample was used as in the EL spectra, however, In contacts with a diameter of ~1mm were used in the duty cycle experiments for visual purposes, as the device with In contacts was significantly larger.

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99.9% to 5.00% the color of the LED changed from red to yellow (Fig. 5). For a 5.00% duty cycle, the peak current is around an order of magnitude higher than the 99.9% duty cycle case. At a frequency of 60 Hz, the “on-pulse” width is ~0.8 ms, which is a sufficient time duration for depopulation to occur. The integrated emission intensity from the LED decreased by ~10% as the duty cycle was decreased. Since the input power was fixed, this means that the overall efficiency of the device only decreased by ~10% as the emission color changed from red to yellow. Thus, it is possible for the color of the RY-LED to be tuned without substantially impacting the efficiency at a standard AC frequency, which opens the door for display applications. To explore to influence of the signal frequency, the peak current was fixed at 200 mA while the duty cycled was varied at frequencies of 60 Hz and 10 kHz. Using a frequency of 60 Hz, as the duty cycle was reduced from 99.9% to 5.00%, the RY-LED became less intense and changed color from yellow to orange. When the frequency was lowered to 10 kHz, the emission from the RY-LED became less intense, and changed color from yellow to orange to red. For both frequencies, the behavior of the LED mimics the emission from incandescent light-bulbs using a dimmer switch, where the light not only dims, but also becomes “warmer,” with the difference being how warm the light can become, which is important for lighting applications. Lastly, the stability of the devices was tested by operating the devices at various injection current conditions for 30 minutes, and comparing the EL spectra after different time intervals. The spectral shape was consistent under all injection current conditions, however, a significant decrease in intensity was observed for non-pulsed injection currents above 100 mA, which is likely due to thermal quenching.

Figure 5 | Properties of the RY-LED under pulsed and continuous current injection. For the fixed Prms of 150 mW, as the duty cycle is decreased the color changes from red to yellow. This is due to the increased current during the “on-time” of the signal combined with its sufficiently long duration at a frequency of 60 Hz. As the duty cycle is decreased with a constant peak current, the emission intensity decreases and changes from yellow to either orange or red depending on the signal frequency.

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Energy transfer engineering and blue emission by co-doping Si and Mg. We have demonstrated that the emission from a GaN:Eu LED can be tuned from red to yellow under current injection by increasing the contribution of the green/yellow 5D1 state emission. There are several different Eu complexes found in GaN:Eu, which have different crystal environments, with different donor and/or acceptor levels being involved in the energy transfer from the GaN host to the Eu3+ ions.[27-34] Co-doping with other elements has also been shown to lead to the formation of additional unique Eu centers with different emission behaviors.[27,28,32-34] For example, when Si was doped in GaN:Eu, an increase in the absorption into and emission from the 5D1 state was observed.[33,35] Additionally, when Si and Mg were co-doped in GaN:Eu, new Si-Mg related Eu complexes were observed with a significantly enhanced energy transfer efficiency, as well as an increase in near-band-edge (NBE) emission at ~420nm at 10K.[33] In addition to facilitating the formation of efficient Eu centers, which should enhance the stimulated deexcitation process, co-doping also enables the realization of blue emission from the same material. Making use of this co-doping approach, we have fabricated a GaN:Eu LED, where a 300nm GaN:Eu active layer was co-doped with Mg and Si during OMVPE growth (hereafter referred to as the RB-LED). The room temperature EL emission spectra from this LED under an injection current of 75 mA is shown in Fig. 6(a). The EL spectra not only contained red emission from the 5D0 state, but also a broader blue emission band centered at ~430 nm, with comparable integrated intensities. A picture of the RB-LED is shown in the center of the figure, and has a near-magenta emission due to the additive mixing of the two colors. A 600 nm short-pass and long-pass filter were used to isolate the blue and red emission, respectively.

Figure 6 | EL spectrum and pulsed vs. continuous current injection properties of the RB-LED. a EL spectra from a GaN:Eu LED that was co-doped with Si and Mg (RB-LED) with an injection current of ~75 mA. The emission from this LED is primarily a mixture of red and blue emission, where the blue and red emission is due to Si/Mg related levels and the 5D0 state of the Eu3+ions, respectively. Note: There is 9 ACS Paragon Plus Environment

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some contribution from the 5D1 state as well. The center image is an unfiltered picture of the LED, while the other two images were obtained using filters. b For nearly-continuous injection currents, as the current is increased the color of the emitted light changes from red to purple to pink, which is due to additive mixing of the blue and red emission. Under pulsed current injection, as the duty cycle is decreased, the emission from the RB-LED changes from pink to purple or pink to red depending on the signal frequency. Similar to the RY-LED, the emission from the RB-LED was found to change as a function of injection current, which is shown in Fig. 6(b). As the current was increased at a duty cycle of 99.9%, the emission changed from red to purple to pink. Under pulsed current injection at 60 Hz, it was found that lowering the duty cycle with a constant 90 mA current changed the LED color from pink to purple. When the frequency was raised to 10 kHz, it was found that lowering the duty cycle changed the LED color from pink to red.

DISCUSSION The 5DJ manifold of Eu3+ ions results in transitions that are centered 545 nm and 595 nm (5D1), and 622 nm (5D0). These transitions can be manipulated either through device design (i.e. AlGaN barriers and layer thickness variation) or defect engineering. The former influences the temporal current density in the GaN:Eu layer, while the latter controls the location of the local trap level with respect to the various levels in the 5DJ manifold and can enhance energy transfer. In addition, co-doping introduces levels within the bandgap that can themselves emit under current injection. Since the 5D0 state remains excited for a relatively long time, it is possible for a subsequent carrier to facilitate a de-excitation of the Eu3+ ion if sufficient carriers are present. Carrier induced de-excitation was shown previously for GaN:Eu and other rare earth ions in semiconductors.[16,36,37] In the case of GaN:Eu, each time the Eu is re-excited, there is a possibility for radiative emission from the 5D1 state, which has a much shorter lifetime than the 5D0 state. The RY-LED is efficient for this process because the AlGaN barriers increase the carrier concentration in the GaN:Eu layers. It is not necessary to have a constant high carrier concentration; de-excitation can also be achieved under pulsed current injection with sufficiently long pulses, where the pulse duration is determined by the duty cycle and frequency. For example, a duty cycle of 5.00% and a frequency of 60 Hz result in an “on-time” of ~0.8 ms, which was found to be long enough for de-excitation to occur. For a duty cycle of 5.00% at 10 kHz, the “on-time” is only ~5 µs. This pulse width was found to be too short, and both the RY-LED and the RB-LED emitted red under these conditions as opposed to the mixed colors that were observed at 60 Hz.3

3In

Ref. 16, the laser pulses were only 200 fs in duration, but did not impact the de-excitation of the Eu ions. A pulsed laser can generate significantly more carriers in each pulse than are available under moderate injection currents (< 400 mA). Thus, de-excitation under current injection requires a combination of sufficient carrier concentrations and pulse durations.

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Mg and Si each introduce levels that are shallower than the native defects in GaN, which can complex with Eu ions to create new Eu emission centers.[27, 28, 33, 38-44] These centers were found to be very efficient and can therefore be excited and de-excited easily, producing both red (5D0 state) and blue emission (SiMg) under low current densities. However, these centers saturate quickly due to their low relative numbers, while the other Eu centers will continue to contribute to red emission. Under high injection currents, the RB-LED emits white-pink. By filtering out the red and blue emission, it was found that the RB-LED was emitting yellow/green due to emission from the 5D1 state. Thus, the RB-LED is capable of emitting all three primary colors under certain current injection conditions. Although, the red emission was still too large to produce white, and the result is a very unsaturated red (whitish-pink). [Additional details can be found in the Supplemental Information, and a movie of the RB-LED emission with different filters can be found online] In Fig. 7, a CIE chromaticity diagram is shown with pictures of the two GaN:Eu LEDs under different current injection conditions. Using a 600 nm long-pass and 600 nm short-pass filter, the NBE emission from the GaN and the 5D1 and 5D0 state emission of the Eu3+ ions could be isolated. These three points in the CIE diagram represent the primary colors of this system, and the gamut of colors that can be produced by their additive mixing lies in the triangle connecting them. The tuning is made possible by exploiting the different time domains of the three primary emission centers. Since the lifetimes of the three emitting states are orders of magnitude apart, it is possible to inhibit the emission from one state and enhance the emission from another state with a shorter lifetime by increasing the current density. Additional control is achieved by changing the timescale of the current through the duty cycle during pulsed current injection. These results open the way for the fabrication of single monolithically grown (or MBE implanted) pixels that are capable of full-color tunability within the CIE triangle limits, as well as the fabrication of light sources that can mimic the emission of incandescent light-bulbs and the natural spectrum of sunlight, i.e. cool white and warm yellow light, without an unnatural contribution of blue light. An additional application, which takes advantage of the sharp and thermally stable emission lines is an alternative excitation source for channelrhodopsins for use in optogenetics, where the wavelength of light that activates or suppresses the channelrhodopsins varies based on the material.[45-48] Color-tunable channelrhodopsins have been achieved with peak absorption wavelengths of 470nm, 545nm, 590nm.[45,46] Other channelrhodopsin have also been found that absorb at between 590nm-630nm and 644nm.[47,48] Although lasers were originally used as excitation sources due to their narrow line widths, LEDs have become commonly used as well, where filters are used to narrow the emission range externally.[45,47,48] The LEDs presented in this work have very sharp emissions that are sufficiently separated in wavelength that multiple 11 ACS Paragon Plus Environment

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channelrhodopsins could be studied simultaneously, where the color-tunability could be exploited to study the channelrhodopsins together or separately. Moreover, given the inherent sharp lines from our LEDs, no external filtering is required and therefore they could be implanted. For these applications, further improvement of the color purity, wavelength and device efficiency are critical. Thus far, defect engineering (RB-LED) and carrier confinement (RY-LED) have been used separately. By combining carrier confinement with the intentional co-doping of defects, it should be possible to increase the efficiency of the RB-LED, and enhance the ability to address the 5D1 state emission electrically. For example, MBE implantation could be used to selectively implant the GaN:Eu layers with Si and Mg post-growth. To improve the color purity, it is imperative that the 5D1 → 7F3 transition, which results in 595 nm emission, is inhibited. This can be achieved by embedding into a DBR microcavity that is resonantly tuned to the 5D1 → 7F1 transition at 545 nm. In that way, the color gamut of the LEDs would be greatly expanded and, in addition, the microcavity would reduce the radiative lifetime of the 5D1 state through the Purcell Effect, and increase the output efficiency.[49] A similar approach could be used on the 622 nm emission. Further improvement on the blue edge of the CIE triangle could be achieved by using In rather than Mg and Si as a co-dopant. Some In would couple to Eu atoms and create new Eu centers, but this will also allow for tunability of the blue emission toward 460 nm.[50] Alternatively, enhancement of the 5D2 state emission, which ranges from 470 nm to 520 nm, could be explored.[26]

Figure 7 | CIE chromaticity diagram. 12 ACS Paragon Plus Environment

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The isolated emission (using 600 nm short-pass and long-pass filters) from the NBE level in GaN, and the 5D

1

and 5D0 states of the Eu3+ions are shown at the top. These three points shape a triangle in the CIE

chromaticity diagram, which represents the gamut of colors that can be achieved by additively mixing the emission from these levels. Pictures of the two LEDs under other current injection and filtering conditions are also shown at their CIE coordinates, which demonstrates the color mixing potential of these emission lines. Filtered images have a yellow border.

CONCLUSION In summary, a color-tunable LED based on GaN:Eu was demonstrated, where the color-tunablility originated from intentionally modified ratios of emission from different levels of the Eu3+ ions and/or intentionally co-doped defect levels. Time-resolved photoluminescence and PLE results indicate that the 5D

2 state

plays a large role in populating the 5D0 state through the 5D1 state, and that a second pathway exists

where the Eu3+ ion is excited directly into the 5D0 state. Under low current densities, the LEDs emit primarily red emission (~622nm) as emission from the 5D0 state dominates. For high carrier densities, a carrier induced de-excitation leads to a back-transfer of energy from the Eu3+ ion back to the GaN host. This energy can either be transferred back to Eu and emit from either the 5D0 or 5D1 state, or it can be emitted from another defect level (Si/Mg), which causes a change in the ratio of the emission lines and the overall color of the LED emission. Control of this behavior is made possible by taking advantage of the various timescales on which the different energy levels emit over. By modeling this process using rate equations, it is shown that the relative population of the various levels changes as a function of injection current, but reaches a steady state at each current value. These simulations were consistent with experimental results and show that the relative ratios of the various emission lines are temporally stable at each current density. Lastly, we show that it is possible to induce a color change in the LEDs by modifying the duty cycle of the injection current at different frequencies, which keeps the excitation power constant while, due to the relatively long lifetime of the Eu emission, the output intensity remains relatively constant as well. Thus, the color-tunability is achieved without significant loss of device efficiency.

METHODOLOGY Growth and fabrication of the GaN:Eu devices. All samples and devices in this work were grown by OMVPE on (0001) sapphire substrates and Trimethylgallium (TMGa) and ammonia (NH3) were used as the gallium and nitrogen sources, respectively. The reactor pressure was maintained at 100 kPa during growth. The AlGaN/GaN:Eu multi-quantum well (MQW) samples consisted of 13 periods of alternating AlxGa1-xN/GaN:Eu layers, where EuCppm2 was used 13 ACS Paragon Plus Environment

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as the Eu source and trimethylaluminum (TMAl) was used as the Al source. The GaN:Eu layers had a thickness of 1 nm, while the thickness of the AlGaN layers was 5 nm. In addition, a 0.5 nm GaN buffer layer was inserted between the two layers to prevent substantial diffusion of the Eu ions into the AlGaN layers. The Eu source and transfer lines were maintained at 125ºC and 135ºC, respectively. The Eu concentration was estimated to be 5.6 x1019 cm-3 from secondary ion mass spectroscopy. The Al concentration in the AlxGa1-xN layers was fixed to be 20%, and the growth temperature of the MQW structure was 960°C, which is the best growth temperature for GaN:Eu.[51,16] An LED structure was grown using the MQW structure as the active layer, which was surrounded by an LED structure that has been extensively explained in Ref. 23 and Ref. 30. Two contact types were used, the first was simply In contacts deposited on the p-type and n-type layers. For the second, Ti/Au and Pt/Au contacts were formed by electron beam deposition on the n-type and p-type layers, respectively. This LED is an optimized version of the active layer reported in Ref. 16. The AlGaN thickness was reduced by half and the Al concentration is 2% higher. The thinner layers and higher barriers are believed to allow higher currents at lower applied voltages and lead to a greater carrier concentration in the GaN:Eu layers. The Mg and Si co-doped active layer was grown using Tris(dipivaroylmethanate)europium [Eu(DPM)3], bis(cyclopentadienyl)magnesium (Cp2Mg), and monomethylsilane (MMSi) for Eu, Mg, and Si precursors, respectively. The concentrations of Eu, Si and Mg are estimated to be 4.8 x 1019 cm-3, 3 x 1018 cm-3, and 1 x 1019 cm-3, respectively. [33,52] The growth temperature was 1030°C. This active layer was 300 nm in thickness and surrounded by the same LED structure as the MQW device, but with 1mm diameter circular Pd/Au electrodes deposited on the p-type layer, and an In electrode deposited on the n-type layer. Note: Mg/Si were used together for two reasons: First, it was found that thermal annealing in N2, which is required to activate the p-layer of the LED, opened up non-radiative channels and substantially reduced the overall Eu emission.[52] When Si was co-doped with Mg it was found to stabilize the emission Eu centers during high temperature annealing and introduced a band of Si-Mg related emission centered at ~420 nm at 10K.[33]

Electroluminescence and Pulsed Current Injection. The total output power and EL emission spectrum of the LEDs were simultaneously measured using an integrating sphere spectrometer (Labsphere/LMS-100).

EL spectral shapes were also confirmed by

collecting the emission into a multi-mode fiber and measuring the spectra using an Ocean Optics USB4000 spectrometer. The pulsed current injection experiments were performed using an arbitrary function generator (Agilent/33220A), where the signal was monitored using a dual channel digital oscilloscope (Tektronix/TDS2000C). The chromaticity coordinates were calculated using the integrated EL spectra from the LED under each current injection condition scaled against the standard CIE color-matching functions. 14 ACS Paragon Plus Environment

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It should be noted that the devices in this manuscript were not designed for optimal efficiency or power output, but to demonstrate and understand the re-excitation phenomena and color-tunability. For example, in the RY-LED, the layers were intentionally made very thin in order to ensure high carrier concentrations and saturation of the Eu ions in the Eu doped layers, but this ultimately limited the output power and EQE of the device. With this in mind, the device efficiencies and output powers were reasonable, with the maximum external quantum efficiency (EQE) of the RY-LED being ~1.7% under continuous current injection and with a maximum emission intensity of ~7 µW. Similarly, the RB-LED had a maximum EQE of ~0.8% and a maximum output of ~5 µW. Improvements of output power and efficiency can be achieved by increasing the Eu concentration, layer thickness and number of MQWs, as well as further tuning of the AlGaN barrier layers. Additional methods are mentioned in the discussion section above.

Tunable Pulsed Laser Photoluminescence and Time-resolved Photoluminescence. Time-dependent PL excitation spectroscopy was performed with an optical parametric oscillator (OPO) system (Solar LS). The 3rd harmonic of a pulsed Nd:YAG laser (100 Hz, 5 ns pulsewidth) was used to pump a tunable OPO for generation of the different photon energies. Emission was dispersed by an M266 (Solar LS) monochromator coupled to either a silicon CCD (Hamamatsu S10141-1108S) or a photo multiplier tube (Hamamatsu R928) combined with a multi-scalar card, for photon detection.

ACKNOWLEDGEMENTS The work at Lehigh University was supported by a CORE grant from Lehigh University. The work at Osaka University was partly supported by a Grant-in-Aid for Scientific Research (A) (JP17H01264) and a Grantin-Aid for Specially Promoted Research (JP18H05212) from Japan Society for the Promotion of Science. B.M., Y.F. and T.G. thank Osaka University for the International Joint Research Promotion Program. B.M. would like to thank Prof. Eric Sweet (West Chester University) for helpful discussions on potential applications of these devices.

AUTHOR CONTRIBUTIONS BM wrote the manuscript with DT, TG, YF and VD. DT and RW performed the PLE and TR-PL measurements. DT performed the simulations. BM and JT performed the electroluminescence and pulsed current injection measurements. WZ and JT grew the LEDs and fabricated the contacts.

SUPPORTING INFORMATION 

Additional information on the RB-LED

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