Current status for light-emitting diode with Eu-doped GaN active layer grown by MBE

Journal of Luminescence 132 (2012) 3113–3117

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Current status for light-emitting diode with Eu-doped GaN active layer grown by MBE Akihiro Wakahara a,n, Hiroto Sekiguchi a, Hiroshi Okada a,b, Yasufumi Takagi c a

Department of Electrical and Electronic Information Engineering, Toyohashi Tech, Toyohashi 441-8580, Japan Electronics Inspired Interdisciplinary Research Institute, Toyohashi Tech, Toyohashi 441-8580, Japan c Central Research Laboratory, Hamamatsu Photonics K. K., Hamamatsu 434-8601, Japan b

a r t i c l e i n f o

abstract

Available online 7 February 2012

Recent progress on light-emitting diode having a Eu-doped GaN active layer is reported. Although the first success on LED using GaN:Eu has been achieved by OMVPE, the factors to be controlled during the crystal growth are not well understood. We found that GaN:Eu co-doped with Mg in NH3-MBE shows a Eu site which is excited only by the above band-gap excitation. The luminescence intensity is enhanced at least 10 times than that without Mg co-doping. The LED operation fabricated using Mg co-doping technique is successfully demonstrated. & 2012 Elsevier B.V. All rights reserved.

Keywords: Eu-doped GaN MBE Mg co-doping LED

1. Introduction Rare earths (REs) doped semiconductors have been developed typically for efficient light-emitting devices, ranging from solidstate lasers to color displays (For a review of photonic applications of rare-earth-doped semiconductors and other materials, see [1]). The advantage of such devices comes from the fact that the luminescence is caused by a transition between a partially filled 4f shell, which is electrically shielded from the surrounding host material by completely filled outer valence electrons. This results in sharp and less temperature sensitive optical emission at wavelengths determined only by the energy diagram in 4f shell of the RE. Generally, REs doping into conventional semiconductors (Si, GaAs, etc.) have suffered from limited solubility and strong temperature quenching of the light emission, which is a key issue for the room temperature operation. The wide-band-gap semiconductors are attractive materials as a host for REs for roomtemperature device applications, because the emission efficiency appears to increase with the increasing band-gap [2]. The III-nitride groups, in particular GaN, have advantages of high solubility of RE dopants and high robustness for thermal and/or chemical stresses. In addition, RE-doped wide-band-gap III-N semiconductors allow light emission from RE ions in the visible wavelength region for fullcolor display applications due to a high transparency. The photoluminescence and electroluminescence from RE-doped GaN have been demonstrated for blue [3–5], green [4,6–8], and red [8–14]

n

Corresponding author. Tel.: þ81 532 44 6742; fax: þ 81 532 44 6757. E-mail address: [email protected] (A. Wakahara).

0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2012.02.001

colors. The argument in favor of RE doped III-N is that integration of the primary colors on a single substrate would allow the development of future generations of flat panel displays. Although light-emitting diodes (LEDs) based on current injection using p–n junction are one of the most popular device for light source, a few publications have been reported for a LED with RE-doped GaN. Zavada et al. demonstrated a p-AlGaN:Mg/GaN:Er, O/n-AlGaN double heterostructure (DH) LEDs [15]. They reported that EL emission lines representative of the GaN:Er system (green: 539 nm, 559 nm, infrared: 1000 nm, 1530 nm) were observed with both forward and reverse bias conditions. Although the EL intensity under the forward bias conditions was five to 10 times more intense than reverse bias condition, the excitation mechanism under forward bias was not fully understood. Dahal et al. reported the electroluminescence from Er doped InGaN p–i–n structure under the forward bias condition [16]. However, their device required forward-bias of about 12.5 V to obtain 20 mA current injection. They suggested that the high series resistance was due to Er doping into InGaN active layer, but the effect of high electric field applied in the active layer on the emission properties was not clear. In case of Eu-doped GaN, molecular beam epitaxy using N radical generated by an rf-plasma (rf-assisted MBE) has been employed. Park and Steckl achieved lasing action using optical pumping [17]. It means that high quality crystal can be obtained, but LED operation was not achieved. MBE growth conditions not only affect the emission capability [18], but also the emission wavelength corresponding to Eu, i.e., the different local arrangements around Eu atoms [19]. Moreover, Okada et al. reported that PL peak from Eu-ion changed by Mg co-doping [20]. However the Eu incorporation processes are very complicated.

A. Wakahara et al. / Journal of Luminescence 132 (2012) 3113–3117

Nishikawa et al. realized red-emission from a LED with GaN:Eu active layer under current injection [21]. They used organometallic vapor phase epitaxy (OMVPE) for growing the layer structure. They achieved optical output of 1.3 mW at DC 20 mA from a LED with a 300 nm-thick GaN:Eu active layer and recently reported 50 mW with 900 nm-thick active layer [22]. However, it is still not well understood that the improvement of emission efficiency by OMVPE and a suitable device structure for realizing a high efficient LEDs. In this paper, Eu doping characteristics of NH3-MBE were investigated in the view point of the effects of growth conditions, nitrogen source, and co-doping of Mg on the luminescence properties. The LED properties fabricated using MBE were also investigated and compared with OMVPE to find the potential of MBE for fabrication of LEDs and/or LDs with Eu doped GaN active layer.

1021 Concentration (Atoms/cm3)

3114

Eu

1020 1019 1018 Mg

1017 1016 10

C

15

1014

0

0.5

1

1.5

2.5

2

Depth (μm)

GaN layers were grown on a 2.5 mm undoped epitaxial GaN template on sapphire (0 0 0 1) substrate by molecular beam epitaxy using NH3 as the nitrogen source. Ga, Mg, and Eu were supplied using conventional effusion cells. The GaN templates were cleaned with organic solvents and loaded into the MBE chamber. The NH3 flow rate was 2.5 sccm, and the working pressure was about 3.6  10  5 Torr during the growth. The beam equivalent pressures (BEPs) of the Ga and Eu fluxes were 1  10  6 and 5–6  10  8 Torr, respectively. Mg doping concentrations were ranged from 7  1016 to 4  1019 cm  3 by varying the Mg cell temperature. The thickness and the Eu concentration of the GaN:(Eu, Mg) layers were 420 nm and 2  1020 cm  3, respectively. In order to avoid the surface roughening exposed by NH3 at high temperature, NH3 supply was stopped after deposition was finished. Both Eu and Mg concentrations in the epi-layer were estimated by secondary ion mass spectroscopy (SIMS) measurement. The luminescence properties of the GaN:(Eu, Mg) were evaluated by photoluminescence (PL) at room temperature. A He–Cd laser (325 nm) and an InGaN laser diode (LD) (402 nm) were used as the excitation source, which correspond to above and below the band-gap excitation, respectively. Time-resolved PL (TRPL) was also measured using a fourth-harmonic-generation YAG (FHG-YAG) pulse laser (266 nm, 20 Hz, and  10 ns) and a streak scope (HAMAMATSU C4334). During the GaN:(Eu, Mg) growth, reflection high energy electron diffraction (RHEED) patterns showed streak pattern, confirming flat surface. Fig. 1 shows SIMS profile of GaN:(Eu, Mg). The Eu BEP was kept at 5  10  8 Torr, while the Mg BEP changed in each layers. As can be seen in the figure, Eu concentration was not affected with the Mg co-doping. H concentration was less than a few percent of Mg concentration of as grown samples. Since the samples were cooled without NH3 exposure, H connected with Mg could be evacuated during the cooling period. Thus hydrogen passivation effect of Mg can be ignored in the present work. Fig. 2 shows the Mg co-doping concentration dependence of the PL spectra observed from the Eu3 þ ion in the GaN:(Eu, Mg) under the above bang gap excitation. It is found that multiple peaks, which are labelled as ‘‘A’’, ‘‘B’’, and ‘‘C’’ in the figure, associated with the 5 D0–7F2 transition in Eu3 þ ions were observed around 620 nm, resulted from multiplicity of Eu3 þ optically active sites in the crystal. The integrated PL intensity initially increases with increasing the Mg concentration up to 3  1018 cm  3 and then decreases, even though the Eu concentration was constant. In addition, peak B (lB ¼622.3 nm) is predominant for a Mg concentration at 1.5  1017 cm  3, but peaks A (lA ¼620.3 nm) and C ( lC ¼633.9 nm )

Fig. 1. SIMS profile of multiple stack of GaN:(Eu, Mg) with different Mg co-doping conditions at constant Eu doping. The Eu doping condition is fixed.

0.4

PL intensity [a. u.]

2. Effects of Mg co-doping on PL properties

A A’

Mg [cm-3] 4x1019

0.3

B

C C’ A”

RT

B’ D

x20

1x1019

x2

3x1018

x1

1x1018

x1

1.5x1017

x5

0.2

0.1

0

600

610

620 630 Wavelength [nm]

640

650

Fig. 2. PL spectra from Eu-doped GaN with different Mg co-doping concentration measured at room temperature. The excitation wavelength is 325 nm and the excitation power is about 1 mW. Labels in the figure ascribe the origin of PL peaks from Eu3 þ ions.

become dominant for the Mg concentration higher than 1018 cm  3. In addition to these major peaks, weak PL peaks are observed in longer wavelength side. Some of these weak peaks, labelled as A0 , B0 , and C0 , have an energy shift of 10 meV from the corresponding major peaks. This energy shift agrees well with the localized phonon energy of Eu in GaN [22–24]. The energy difference between peak A00 and peak A is about 63 meV, which is very close to the phonon energy for A1(TO) and/or E1(TO) in GaN [24]. Thus observed small peaks A0 , B0 , C0 , and A00 are the phonon replicas of peaks A, B, and C, respectively, as reported by other research groups. Moreover, the PL intensity ratio between the peaks A and C was found to be a constant. This suggests that the Eu luminescence centers for peaks A and C are same. Hereafter, the origin of the peaks A and C called as site-A, and for the peak B called as site-B. Fig. 3(a) shows the Mg concentration dependence of the integrated PL intensity related to the Eu3 þ from the GaN:(Eu, Mg). The integrated PL intensity was enhanced by approximately 20 times for a Mg concentration of 3  1018 cm  3. However, the integrated PL intensity decreases with the increasing Mg concentration, of more than 3  1018 cm  3. Therefore, an adequate amount of Mg co-dopant intensified the luminescent efficiency of Eu3 þ in the GaN:(Eu, Mg). An abundance of Eu3 þ sites in the

A. Wakahara et al. / Journal of Luminescence 132 (2012) 3113–3117

3115

∝ NMg

100

without Mg doping ∝ NMg-2 1

NA/Ntotal NB/Ntotal

0.8 NA, NB/ Ntotal

10-1

C

RT λex=325nm λex=402nm

101 Mg:1x1019

100 10-1

Mg:3x1018

10-2

Mg:1x1018

10-3 10-4 600

0.6

Mg:1.5x1017 610

620

630

640

650

Wavelength [nm] without Mg doping

0.4

Fig. 4. Excitation wavelength dependence of PL spectra with different Mg codoping concentration, measured at room temperature. Red lines indicate PL spectra excited with the laser wavelength of 325 nm, corresponding to the above band gap excitation. Blue lines indicate PL spectra excited with the laser wavelength of 402 nm, corresponding to the below band gap excitation. Broken lines exhibit the peak position of peak B for guiding. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

0.2 0 1015

1016

1017

1018

1019

1020

-3

Mg concentration [cm ] Fig. 3. Eu, Mg doped GaN epitaxial film: (a) The integrated PL intensity and (b) ratio of active Eu sites A and B at a fraction of Mg concentration. The abundance of total, site-A and site-B is estimated from the excitation power dependence of PL intensity and a rate-equation analysis.

GaN:(Eu, Mg) can be extracted from the excitation power dependence of the PL spectra, since the integrated PL intensity for each site x (x¼ A or B), Ix, is given by Ix p

A B

PL intensity [a. u.]

RT

Integrated PL intensity [a. u.] (615~640 nm)

101

Nx 1 þðsx tx P ex Þ1

ð1Þ

where Nx is the amount of site x, sx is the effective excitation crosssection of site x, tx is the lifetime of site x, and Pex is the excitation power [25]. Fig. 3(b) shows the estimated result on the NA and NB normalized by a total amount of optical active Eu (Ntotal). Ntotal increases with Mg incorporation up to NMg ¼3  1018 cm  3 and decreases for NMg 43  1018 cm  3, as expected from the Mg concentration dependence on the PL intensity. Although the ratio of NA/Ntotal increases with the increasing Mg concentration of NMg 4 1.5  1017 cm  3, the ratio of NB/Ntotal decreases. Therefore, the Mg co-dopant selectively enhances the formation of site-A. Fig. 4 shows comparison of the PL spectra of GaN:(Eu, Mg) excited by photon having above and below band gap energies. In case of low Mg doping concentration ( o1017 cm  3), PL spectrum shape is almost the same regardless of excitation energy and it is well explained by peaks B and B0 . The fact indicates that the Eu3 þ center corresponding to the peak B (called site-B) can be excited even though the photon energy is lower than the band-gap. As the Mg concentration increases, peaks A and C, which can only be observed by the above-gap excitation, become dominant, and thus site-A has a different local structure from that of site-B. There are at least two types of major optically active Eu3 þ sites as reported previously [23,26–31]. One of them has the emission wavelength of 620 nm and can only be excited by the above bandgap excitation. The other one has the emission wavelength of 622 nm and can excite by both the above and the below band-gap excitation. By comparing the wavelength and the excitation path, site-A and site-B are categorized as a deep trap and shallow trap excitation pathways, respectively [23]. Thus we can conclude that the local structure around optically active Eu3 þ ions is affected by the Mg co-doping. From the TRPL measurement, the PL decay

time of peak A increases with an increase in Mg concentration until the Mg concentration reached to 3  1018 cm  3, which was almost the same as that of peak C (not shown). The increased PL decay time indicates that a nonradiative de-excitation path from the 5D0 state was eliminated by Mg co-doping. This nonradiative path would be related to the native defects in the host GaN near the Eu3 þ ions. We speculate that one possible defect is Ga vacancy in proximity to the Eu3 þ ions since the vacancy type defect was detected in the GaN:Eu [32] and Ga vacancy decreased in bulk GaN by Mg doping [33]. Heavy doping of Mg into GaN:Eu would create an additional nonradiative path since the PL decay time was decreased and the intensity of the Eu3 þ luminescence was diminished. Sanna et al. have theoretically investigated rare-earth defect pairs in GaN using LDAþU total-energy calculation [34]. They pointed out that the REGaVN pair is energetically favored for p-type GaN, and for n-type GaN, the next-nearest-neighbor pair REGaVGa provides the most stable configuration. Even though all Eu-doped samples indicate high resistance, the Fermi level in GaN:(Eu, Mg) could be changed by the Mg co-doping. In the view point of the crystal growth, Wang and Steckl reported that luminescence efficiency, which is defined as PL intensity per Eu atom, for rf-MBE grown GaN:Eu increases with increasing Ga BEP in V-rich regime and becomes almost constant in the III/V r1 regime [35]. Our group reported that the Eu3 þ related PL peak position is affected by the growth conditions and the peak A appears in case of step-flow and/or 2D nucleation growth mode. Moreover they pointed out that the luminescence efficiency has less Ga-BEP dependence for the GaN:Eu films grown in 3D mode, but abruptly increases when the growth mode transfers from 3D to step-flow/2D [19,36]. These results suggest that the local structure of optically active Eu3 þ site as well as defect formation is strongly affected by the growth kinetics, in other words, incorporation process of Eu3 þ . In order to clarify the mechanism of site selection, further investigations, such as positron annihilation, optically detected nuclear magnetic resonance (ODMR), and electron-paramagnetic resonance measurements, are necessary to achieve more detailed information about the exact microscopic structure surrounding the Eu3 þ ions on Ga sub lattice.

A. Wakahara et al. / Journal of Luminescence 132 (2012) 3113–3117

because the GaN:(Eu, Mg) active layer’s high resistance and high electric field may apply in the active layer. In order to check the potential distribution in the device, we simulate the electric properties of p-GaN/ GaN:(Eu, Mg)/n-GaN using SimWindows [37]. Since the electrical properties of GaN:(Eu, Mg), such as mobility and minority carrier lifetime, are not well investigated, we assumed both ND and NA to be zero in the active layer. Fig. 7 shows the simulated results. If the active layer thickness is relatively thin, i.e.,  100 nm, most of the voltage drop occur in the n-GaN layer and the electric field in high-resistance active layer under forward bias of 5 V is low enough to avoid an impact ionization by the injected carriers (right hand side of Fig. 7). While in case of relatively thick active layer, voltage drop occurs in the thick GaN:Eu active layer due to its high resistivity(left hand side of Fig. 7). Position of the quasi-Fermi level suggests that both electron and hole injected into the active layer flow by the

3. Properties of GaN:Eu LEDs Based on the Mg co-doping technique in NH3-MBE growth of GaN:Eu, we try to fabricate a LED with GaN:(Eu, Mg) active layer. The LED structure consists of a 500 nm p-GaN:Mg layer with the Mg doping concentration of NMg  3  1019 cm  3, a 100 nm GaN:Eu active layer with a Mg co-doping concentration of about 1018 cm  3, and a 4-mm-thick n-type GaN:Si template grown by OMVPE on sapphire (0 0 0 1) substrate (see Fig. 5). In order to improve the contact resistance of p-type Ohmic electrode, Mg effusion-cell temperature was gradually increased from 230 1C to 260 1C during last a few tens of nanometers for the growth of GaN p-cladding layer. The Ag and Ti/Al/Ti/Au electrodes were formed on p- and n-type layers, respectively. The diameter of the Ag p-contact electrode was 300 mm. Fig. 6(a) and (b) shows typical current–voltage (I–V) characteristics and a photograph of the fabricated LEDs, respectively. Turn-on voltage was about 3 V, which was close to that of conventional GaN-based LEDs, and the series resistance estimated from the forward bias condition was approximately 125 O. Since the series resistance for a LED having a 100 nm p-GaN cladding layer was about 10 O, most of the series resistance could be caused by the GaN:(Eu, Mg) active layer. A red light emission was observed only under the forward bias condition and the emission spectrum was the same as that obtained by PL measurement, i.e., peak A was the predominant. This red emission was very bright and can be seen even under the room-light condition, as seen in Fig. 6(b). The red EL was observed even at the forward bias higher than 5 V. This situation is similar to the OMVPE grown LEDs [16,22]. Under such high forward bias, question arises, of whether the emission by the electron-hole injection is enabled,

Potential (eV)

p-GaN GaN:Eu

n-GaN

p-GaN GaN:Eu n-GaN 5

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-5 VF= 3 V

Ag contact

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GaN:Mg(500nm) NEu=2×1020 cm-3

GaN:Eu(100nm)

0

Ti/Al/Ti/Au contact GaN:Si template Sapphire(0001)

EL

15

PL

10

VF= 5V

0.2 0.4 0.6 Position (μm)

Intensity [a. u.]

Current [mA]

20

0

1.5

Fig. 7. Calculated potential distribution in p-GaN/GaN: (Eu, Mg)/ n-GaN LEDs under various forward bias conditions. Left hand side corresponds to the LED having a 100 nm p-GaN and 900 nm GaN: (Eu, Mg) active layer, while right hand side is corresponding to a thin active layer (100 nm) case.

Fig. 5. Schematic layer structure of fabricated LED.

25

0.5 1 Position (μm)

600 620 640 660 Wavelength [nm]

5 0 -5

-4

-2

0

2

Voltage [V] I-V characteristics

4

6 Photograph of the fabricated LED

Fig. 6. The properties of the fabricated LED with a GaN:(Eu, Mg) active layer: (a) I–V characteristics and (b) photograph of LED operated under the forward bias at 4 V with 30 mA. The inset in Fig. 6(a) shows both EL and PL spectra for GaN:Eu, Mg active layer for comparison.

A. Wakahara et al. / Journal of Luminescence 132 (2012) 3113–3117

drift-current mode. An expected electric field in the active layer for 1 mm-thick active layer is a few tens of kilovolt per centimeter, which is too small for impact ionization (  2 MV/cm) [38], but it is approaching to the minimum field strength of 200 kV/cm reported for a GaN:Er EL device [39]. Therefore, a part of the excitation could be carried out by a hot carrier under high operation bias conditions, perhaps higher than 10 V. To avoid heating problem by high resistivity of GaN:Eu as well as hot carrier issue, multiple stack of thin GaN:Eu (GaN:(Eu,Mg)) sandwiched by (Al)GaN like a multiple-quantum well is effective.

4. Conclusion We have investigated the effect of Mg co-doping on the luminescence properties of Eu-doped GaN. The Eu3 þ luminescence originating from the 5D0–7F2 transition (  622 nm) in the GaN:(Eu, Mg) has been investigated. Mg co-dopants in GaN:Eu governed the optical site of the Eu3 þ ions and intensified the Eu3 þ luminescence. The optimal doping of Mg in GaN:Eu led to the selective activation of site A (  620.3 nm) and resulted in an increase of about 20 times of the Eu3 þ luminescence. Based on the obtained results, a LED having p-GaN/ GaN:(Eu, Mg) active layer/n-GaN structure has been fabricated. Using optimal Mg codoping condition, bright red emission has been obtained under the forward bias condition. The results indicate that NH3-MBE has high potential for the fabrication of LEDs with rare-earth doped GaN active layer.

Acknowledgments This work was partly supported by the Regional Innovation Cluster Program (Global Type) ‘‘Tokai Region Nanotechnology Manufacturing Cluster’’ and Grants-in-Aid for Young Scientists (B) no. 23760281 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. A part of the work was carried out using device fabrication facilities installed at EIIRIS (Electronics Inspired Interdisciplinary Research Institute), Toyohashi Tech. References [1] A.J. Steckl, J.M. Zavada, MRS Bull., 24 (9) (1999) 16, and articles therein. [2] P.N. Favennec, H. L’Haridon, M. Salvi, D. Moutonnet, Y. Le Guillou, Electron. Lett. 25 (1989) 718. [3] A.J. Steckl, M. Garter, D.S. Lee, J. Heikenfeld, R. Birkhahn, Appl. Phys. Lett. 75 (1999) 2184. [4] D.S. Lee, A.J. Steckl, Appl. Phys. Lett. 81 (2002) 2331. [5] D.S. Lee, A.J. Steckl, Appl. Phys. Lett. 82 (2003) 55. [6] A.J. Steckl, R. Birkhahn, Appl. Phys. Lett. 73 (1998) 1700.

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