Re-Excitation of Trivalent Europium Ions Doped into Gallium Nitride

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Re-excitation of Trivalent Europium Ions Doped into Gallium Nitride Revealed Through Photoluminescence under Pulsed Laser Excitation Wanxin Zhu, Ruoqiao Wei, Dolf Timmerman, Tom Gregorkiewicz, Brandon Mitchell, Yasufumi Fujiwara, and Volkmar Dierolf ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01090 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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Re-excitation of Trivalent Europium Ions Doped into Gallium Nitride Revealed Through Photoluminescence under Pulsed Laser Excitation Wanxin Zhu1, Ruoqiao Wei2, Dolf Timmerman1, Tom Gregorkiewicz1,3, Brandon Mitchell1,4*, Yasufumi Fujiwara1, and Volkmar Dierolf2 1

Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan

2

3

Department of Physics, Lehigh University, Bethlehem, Pennsylvania 18015, USA Van der Waals-Zeeman Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

4

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

The behavior of trivalent Europium (Eu3+) ions doped into Gallium Nitride (GaN) was investigated under intense excitation conditions to explore the excitation energy transfer characteristics in the presence of large carrier densities. Under such conditions, strong emission from the higher excited 5D1 and 5D2 states of the Eu3+ ions was observed in highly efficient AlGaN/ Eu-doped GaN multiple quantum wells grown by organometallic vapor phase epitaxy. This behavior was studied using a variety of excitation sources and conditions. Most notably, when a femtosecond-pulse laser was used, the excitation of the Eu3+ ions into the higher energy states became significant only with a second excitation pulse arriving within the lifetime of the 5D0 state. We propose that an already excited Eu3+ ion is promoted from its 5D0 excited state into the higher 5DJ states where it relaxes and can emit from the 5D1 and 5D2 states.

*Email: [email protected]

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KEYWORDS: Pulse Laser Photoluminescence, Re-excitation process, Europium doped Gallium Nitride.


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III-nitride materials have received considerable attention for their potential as efficient and high-intensity visible light emitters.

Although nitride-based blue and green

light-emitting diodes (LEDs) have already been commercialized, red and infrared LEDs based on these materials are still at a premature stage of development due to material limitations.1 A red LED based on the InxGa1-xN/GaN multiple quantum well (MQW) structure was developed; 2-4 however, a special reactor design was needed.

In addition, the spectra

position of the emission shifted, and the full width at half maximum (FWHM) increased upon increased current injection.

There have also been several reports on the luminescent

properties of Eu-doped GaN (GaN:Eu)5-11 and on GaN:Eu based red LEDs.12-18

GaN:Eu

based LED structures can be grown using the conventional organometallic vapor-phase epitaxy (OMVPE) method; their wavelength of the emission is thermally stable, with a FWHM that is < 1nm.14-16,18

These developments make it possible to manufacture blue,

green, and red LEDs on a single substrate, which could allow for the development of a GaN-based active pixel display. Eu ions have been shown to incorporate into GaN with different local defect environments (sites).9 In order to contribute to emission under current injection, a Eu3+ ion must be excited via energy transfer from the GaN host, which strongly depends on its local defect environment.

This energy transfer either directly excites the Eu into its 5D0 state, or

into one of the various higher 5DJ excited states.19-21 If the Eu is excited into a higher 5DJ state, it can relax to the 5D0 state through a (multiple) phonon relaxation process.20,22

It has been

established that one of the Eu sites, referred to as OMVPE7, dominates the emission spectra under current injection while representing only ~4% of the overall Eu incorporation in


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GaN.9,11,23 To a certain degree, the number of OMVPE7 centers that form can be controlled by modifying the defect profile of GaN:Eu through modification of the growth conditions.18 Furthermore, it was reported that the overall energy transfer to all Eu centers can be enhanced by modifying the growth structure from a single active layer to a multilayer structure consisting of alternating GaN/GaN:Eu layers.24 Enhancement in the transfer of energy due to electron-hole recombination leads to higher excitation efficiencies for the Eu ions, which also possess a fairly long excited state lifetime of ~250µs for the 5D0 state.19,20,23 This raises the question of what happens if another carrier mediated energy transfer occurs while the Eu3+ ion is already excited in the 5D0 state. These conditions are present under high excitation intensities due to the resulting high carrier densities. Numerous studies have shown that several emission peaks are observed after exciting Eu in GaN, which originate from transitions within the 4f manifold of the Eu3+ ions.9 The transition from the 5D0 to the 7F2 state, which leads to red emission at ~621 nm, is dominant in GaN:Eu and has been researched most intensively. 8-11,14,19,20,23-28 Transitions from the higher 5DJ levels of Eu3+ are less investigated.14,19,20,28,29 The conditions under which emission from these higher excited levels occur were not determined. For example, Nishikawa et al. noted that the color from GaN:Eu based LEDs changed from red to orange as the injection current was increased, which was attributed to emission from the 5D1 state.14 In a later report, the orange emission was no longer observed in the newer generation of LEDs, but no clear mechanism or explanation was offered.28,29

In this article, the

relationship between the excitation of the 5D0 and higher 5DJ states is investigated. We show that under intense pumping a re-excitation of the Eu3+ ions occurs. Using various pumping conditions, including photoluminescence (PL) under fs pulsed excitation, the details of this


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re-excitation phenomena are explored.

EXPERIMENTAL RESULTS The PL emission spectra due to transitions from the 5D0 to 7F2 state (~1.99 eV) and from the 5D1 to 7F1 state (~2.28 eV) in GaN:Eu under 325 nm continuous-wave (CW) excitation are shown in Fig. 1(a) and Fig. 1(b), respectively. 5

The emission attributed to the

D0 state was dominant for low excitation powers, and its intensity increased for higher

excitation powers. On the other hand, the emission originating from the 5D1 level was barely detectable over the noise level for low laser powers, but once the laser power was raised above 2 mW, this emission began to increase substantially.

Increasing the laser power

increases the photon flux, however the energy of each photon remains constant.

This

suggests that the number of photons rather than their energy contributes to the appearance of the 5D1 related emission. To further explore this observation, electroluminescence (EL) measurements were performed.

Figure 1(c) shows the EL spectra from the GaN:Eu LED under injection

currents of 1 mA and 40 mA, with applied voltages of 3.4 V and 4.8 V, respectively.

The EL

spectra at 1 mA exhibits very little 5D1 related emission, while there is a significant contribution from these transitions in the EL spectra at 40 mA.

Similar to PL results, it

appears that the 5D1 related emission becomes significant when the number rather than the energy of carriers increases, since the applied voltage did not change significantly between the two injection currents.

These observations could be explained as a re-excitation of the

Eu3+ ions, or by a change in nature of the excitation channels under more intense excitation conditions.


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Fig. 1 (a) and (b) show the power-dependent PL emission spectra under CW 325 nm excitation for transitions originating from the 5D0 and 5D1 states, respectively. For low laser powers (< 2.0 mW), the emission from the 5D1 level is barely detectable, but for laser powers above 2.6 mW, emission from the 5D1 becomes quite substantial. (c) EL emission spectra under injections currents of 1 mA and 40 mA.

The EL emission spectra under an injection

current of 1 mA contains very little 5D1 emission; however, under an injection current of 40 mA the 5D1 emission becomes quite strong.


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In order to clarify the mechanism behind this process, PL measurements were performed under fs pulsed laser excitation (342 nm, 200 fs), during which high carrier densities are produced in a short time. Due to the short pulse widths, the carrier generation can be separated from the relatively long Eu emission process, which occurs on a 250 µs time scale. 19,20,23

In this sense, it is possible to separate the influence of the number of instantaneously

generated carriers which is related to the pulse energy, from the availability of carriers while the Eu is excited, which is related to the time interval between pulses.

We investigated two

different conditions.

(1) A 10ms pulse interval, which represents essentially a single pulse experiment as the whole system has sufficient time to completely relax after each pulse. (2) Shorter pulse intervals (down to our instrumentation limit of 5µs) for which additional pulses are present before the Eu3+ ions have had time to relax.

In both cases the energy per pulse was varied from 0.2µJ to 5µJ. Figure 2(a) shows the PL emission spectra at various pulse energies and at a pulse interval of 5 µs. Emission from both the 5D0 and 5D1 states is observed for all pulse energies, unlike for the case of CW laser excitation. The emission from the 5D0 state saturates at a pulse energy of ~0.100 µJ.

On the

other hand, the intensity of the 5D1 related emission increases continuously with higher pulse energies.

Figure 2(b) shows the PL emission spectra at pulse energies higher than the

saturation energy for the 5D0 emission (0. 100 µJ), and a pulse interval of 10 ms.

In this

case, the 5D0 state remains saturated for all pulse energies, and no emission from the 5D1 state is observed, which indicates that within a single pulse, practically all of the Eu3+ ions that are


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excited emit from the 5D0 state. This also clearly demonstrates that the appearance of the 5D1 emission is not related to the carrier density present during the energy transfer process, but rather to the time between successive excitation events (pulses). Moreover, this observation indicates that very little energy transfer takes place during the 200 fs pulse width.

These

results also provide a means to distinguish between which transition peaks originate from the 5

D0 state and the 5D1 state.

Fig. 2. (a) and (b) PL emission spectra at various pulse energies and pulse intervals of 5 µs and 10 ms, respectively. (c) Normalized PL emission spectra at the highest pulse energy and pulse intervals of 10 ms, 250 µs and 5µs.

When the pulse interval is longer than the lifetime

of the 5D0 state, no emission from the 5D1 state is observed, however, the emission becomes significant for pulse intervals much shorter than the lifetime of the 5D0 state.


This also

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allows for a clear distinction between which transition peaks originate from the 5D0 and 5D1 states. Figure 2(c) shows the normalized PL emission intensity at the highest pulse energy (4.8 µJ) and pulse intervals of 5 µs, 250 µs and 10ms, with all known transitions labeled. For pulse intervals longer than the lifetime of the 5D0 state (250 µs), no 5D1 related emission observed even at the highest pulse energy. Only for shorter time intervals, emission from 5D1 appears.

This suggests that the appearance of the 5D1 emission for the short pulse intervals

(e.g. 5µs) results from a re-excitation of already excited Eu3+ ions by a second pulse.

Such

re-excitation can only occur when already excited Eu ion are present during the second pulse. To gain more insight on how the pulse interval influences the emission from the 5

D1 state, PL emission spectra were taken at various pulse intervals by selecting only a subset

of the pulses using the pulse picking capability of the laser set-up.

Figure 2(a) shows that

re-excitation occurs for all values of the pulse energy at the short pulse of 5µs.

To simplify

the interpretation of the pulse interval dependent experiments, the pulse energy was fixed at 0.22 µJ, which is high enough to ensure that a single pulse could saturate the 5D0 excited state population of the Eu3+ ions.

Figure 3(a) shows the wide spectral range of the PL spectra

observed for pulse intervals between 5µs and 500µs. The PL was detected using a 1s integration time for the CCD emission detector such that depending on the pulse interval emission from 2000 (500µs) to 200,000 (5µs) pulses was accumulated.

Figure 3(b) shows

the expanded view of the 5D1 emission range. The 5D1 emission only becomes prominent for pulse intervals shorter than 200 µs.

At very short pulse intervals (< 50 µs), emission

from the 5D2 state can also be observed, as well as a broad emission near the band-edge (NBE) of GaN.

Figures 3(c) shows the expanded view of the 5D2 state and NBE emission


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range. While the emission from the 5D1 – 7F1 transition is already quite strong at a pulse interval of 50 µs, the 5D2 state and NBE emission appear only when the pulse interval has been reduced to 20µs, and increases for even shorter pulse intervals.

Fig. 3. (a) Using a fixed pulse energy of 0.22 µJ, which was shown to saturate the emission from the 5D0 state, the emission spectra as a function pulse interval were measured. (b) The 5

D1 emission is present once the pulse interval becomes shorter than the lifetime of the 5D0

state. (c) Emission from the 5D2 state and a broad near band-edge emission appear at even shorter pulse intervals (< 50 µs).

DISCUSSION To show the results from Fig. 3 more clearly, the integrated emission intensity from


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the 5D0, 5D1, and 5D2 states are plotted against the time interval between pulses in Fig. 4(a). While the emission from the 5D0 state initially increases for shorter pulse intervals, it saturates at a pulse interval of ~10µs and decreases for shorter pulse intervals, despite the larger number of accumulated pulses (i.e.: higher average power). On the other hand, the emission from 5D1 and 5D2 states increases substantially when the interval between pulses is less than 200µs, and continue to increase for even shorter pulse intervals. From Fig. 2(b) it is clear that after the first pulse, the majority of the Eu3+ ions end up in the 5D0 state.

This observation indicates a simple de-excitation path for the Eu ions in

OMVPE grown GaN:Eu, which could explain the high red luminescence intensity of samples grown by this method.18 However, when a second excitation pulse arrives within the lifetime of the 5D0 excited state (250 µs), there is a possibility that it will re-excite the Eu3+ ion into the higher excited 5DJ states. This re-excitation occurs for any pulse energy, so long as the pulse interval is sufficiently short.

The Eu will then relax back to the 5D0 state through the 5D2

and 5D1 states by a multiple phonon relaxtion.20,22 During this relaxation, there is a probability that a radiative transition from the 5D2 or 5D1 states will occur, rather than the full relaxation back to the 5D0 state. If the Eu3+ ion emits from one of these higher states, then that emission will be collected instead of the 5D0 related emission. For shorter pulse intervals, it becomes more unlikely for the Eu to emit from the 5

D0, as the probability for re-excitation becomes increasingly higher. Since the lifetimes of

the 5D1 and 5D1 states are typically orders of magnitude shorter than that of the 5D0 state,20,21,30 it becomes much more probable for a radiative relaxation from the 5D1 and 5D2 state to occur, and thus emission from the 5D1 and 5D2 states becomes significant.

It is also

possible that the re-excitation could lead to a back-transfer of energy to the GaN host, which


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could then excite another Eu ion. Once the pulse interval becomes comparable to the lifetime of the 5D1 state, a third pulse can re-excite Eu ions that ended up in the 5D1 state, and radiative relaxation from the 5

D2 state, which has an even shorter lifetime than the 5D1, becomes more probable.20,21 This

changes the relative strength of 5D1 to 5D2 changes in favor of 5D2. Also, in this short pulse interval regime, emission from the 5D0 state becomes very unlikely and its intensity decreases despite the increase in average power, while the emission from the 5D1 and 5D2 states continues to increase.

In addition, re-excitation from the 5D1 state occurs more frequently

for very short pulse intervals, as does back-transfer to the host, leading to emission from defect levels near the band-edge (NBE).

Figure 4(b) illustrates the proposed model for the

re-excitation process of Eu3+ ions in GaN.

The short pulse width (200 fs) is necessary to

insure that each Eu3+ ion is only excited once per pulse.

This means that re-excitation does

not occur within a single pulse, but rather requires a second pulse to re-excite a Eu3+ ion that was excited by the preceding pulse.


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Fig. 4 (a) The emission intensities from the 5D0, 5D1, and 5D2 states are plotted versus the time interval between pulses. (b) Schematic illustration of the re-excitation process. After the first pulse, almost all of the Eu3+ ions are in the 5D0 state. If the second pulse arrives within the lifetime of the 5D0 state, then the Eu can be excited into the higher 5DJ states, where it can relax back to the 5D0 through the 5D2 and 5D1 states. In this relaxation process, the Eu can emit from the 5D1 state.

It can also transfer energy back to the GaN host.

If the pulse

intervals become comparable to the lifetime of the 5D1, a third pulse can result in another re-excitation from the 5D1 state, resulting in more emission from the 5D2 state and broad NBE emission.


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CONCLUSION The behavior of Eu3+ ions in GaN:Eu under a variety of intense excitation conditions has been investigated. Emission from the higher excited levels of Eu3+ ions was observed for high excitation densities under CW laser excitation and current injection. We were able to exclude a change in the energy transfer process due to high carrier concentrations as an explanation, and demonstrated that the appearance of the higher Eu states can be attributed to a re-excitation of already excited Eu3+ ions. In this process, the Eu3+ ions are re-excited from the 5D0 state into the higher 5DJ states, and can relax back to the 5

D0 state or emit from the 5D1 or 5D2 states.

PL measurements under fs-pulse laser

excitation revealed that re-excitation only occurred when a second excitation pulse was supplied within the lifetime of the 5D0 state, and that the emission from the 5D0 state became less likely as the pulse interval was further reduced.

For very short pulse intervals, the

intensity of the 5D0 emission reduced substantially while the emission from the 5D2 and 5D1 states increased, along with a broad emission near the GaN band-edge.

These results

provide significant insight into the dynamics of trap mediated energy transfer processes, which could be extended to other semiconductor and phosphor systems.

METHODOLOGY For this study, AlGaN/GaN:Eu MQW samples were fabricated using the OMVPE method. The samples consisted of 13 periods of alternating AlxGa1-xN/GaN:Eu layers, with thicknesses of 10nm and 1nm, respectively.

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.11


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A detailed schematic of the sample structures is shown in Fig. 5. Trimethylgallium (TMGa) and ammonia (NH3) were used as starting sources, and EuCppm2 was used as the Eu source.31 The Eu source and transfer lines were maintained at 125ºC and 135ºC, respectively. The Al concentration in AlxGa1-xN was fixed to be 18%, and the growth temperature of MQW structure was 960°C, which is the best growth temperature for GaN:Eu.18 The AlGaN/GaN:Eu structure was chosen because the emission intensity was previously reported to be comparatively high, yet saturate at low injection currents.32 This structure was adopted, but with fewer layers and employing the growth conditions from Ref. 18.

An LED was

fabricated using the MQW structure as the active layer, which was surrounded by an LED structure that has been extensively explained in Ref. 14 and 28. Photoluminescence

(PL)

and

time-resolved

photoluminescence

(TR-PL)

measurements were performed at room temperature to investigate the optical properties under laser excitation. The PL measurements were carried out by exciting the Eu ions indirectly via excitation of the GaN host, using the 325 nm line of a continuous-wave He-Cd laser. The sample emission was focused on a 2400 / mm spectrometer (Acton Research Corporation SpectraPro-2300i) and collected with a Si-CCD (Princeton Instruments PIXIS:256).

For the

TR-PL measurements, the samples were excited at 342nm using a PHAROS frequency tunable pulse laser, which was tuned from 200 KHz to 100 Hz. The sample emission was collected using a spectrometer (Acton Research Corporation SpectraPro-2150i) combined detector Si-CCD (Princeton Instruments PIXIS:400) for high resolution. A combined CCD spectrometer (Ocean Optics USB4000) was used for wide range emission spectra collection (345 nm ~ 1000 nm). The integration times used for both detectors was one second.

Room

temperature electroluminescence (EL) measurements were performed to investigate the


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optical properties of the LED structure under current injection.

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The LED emission detector

system was the same as the one used in the 325 nm He-Cd PL measurements. Two series of EL measurements were performed, where the injection currents were set as 1 mA and 40 mA, with applied voltages of LED of 3.4 V and 4.8 V, respectively.

Fig. 5

Diagram of the MQW sample, which contains 13 periods of AlGaN (10nm), GaN:Eu (1nm), with a 0.5 nm GaN layer between them.

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 (S) (JP24226009) from Japan Society for the Promotion of Science and by the Photonics Center at Osaka University.


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