Optical Materials 28 (2006) 759–762 www.elsevier.com/locate/optmat
TEM observation of Eu-doped GaN and fabrication of n-GaN/Eu:GaN/p-GaN structure J. Sawahata *, H. Bang 1, J.W. Seo, T. Tsukamoto, K. Akimoto Institute of Applied Physics, University of Tsukuba, 1-1-1 Ten-nodai, Tsukuba 305-8573, Japan Available online 3 November 2005
Abstract Structural properties of Eu-doped GaN with the Eu concentration of about 2 at% were studied by cross sectional transmission electron microscope (TEM) observation, and electrical and optical properties of n-GaN/Eu-doped GaN/p-GaN structure were discussed. Selected area diﬀraction pattern from Eu-doped GaN showed hexagonal structure and no other anomalous pattern was observed. These results suggest that the segregation of Eu and EuN were not formed. From the high resolution TEM observation of Eu-doped GaN, in addition to a small portion of cubic phase of GaN, high density of stacking irregularity which was hardly observed in undoped GaN was detected. Heterostructure of n-GaN/Eu:GaN/p-GaN was grown by molecular beam epitaxy on the p-type GaN template which was prepared by metalorganic chemical vapor deposition. Although the rectiﬁcation behavior was observed, the electroluminescence was obtained not from the Eu-doped layer but from p-GaN layer. These results suggest that the hole mobility in Eu-doped GaN is extremely low probably due to the high density of stacking irregularity. 2005 Elsevier B.V. All rights reserved.
1. Introduction In the last few decades, GaN and related III–N nitride semiconductors have received considerable attention due to their potential applications for blue and ultraviolet opto-electronic devices, and high bright blue and green light-emitting diodes (LEDÕs) and laser diodes (LDÕs) had already been successfully fabricated [1–3]. If nitride based red LEDÕs are realized, attractive applications such as monolithic full color display devices and high eﬃciency white LEDÕs can be expected. However, nitride based red LEDÕs are not realized because the control of the optical properties of InGaN is diﬃcult due to phase separation and segregation [4,5]. Rare earth (RE) dopants in wide-gap semiconductors show sharp and intense luminescence peaks caused by intra-4f optical transitions, and its emission wavelength *
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(J. Sawahata). 1 Present address: Power Electronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST) Central 2, 11-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan. 0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.09.016
extends from infrared to ultraviolet depending on the RE element. Since the room temperature intensity of the RE emission generally depends on the band-gap energy of the host material due to preventing back energy transfer , GaN appears to be one of the promising host materials for optical applications. There are several reports about optical properties of Eu-doped GaN, which shows red emission origination from intra-4f transition [7–10], however, structural properties of Eu-doped GaN are not clear yet, and there are few reports on the fabrication of LED with the active layer of RE-doped materials. In this paper, we report the crystallographic structure of Eu-doped GaN studied by transmission electron microscope (TEM) and fabrication of n-GaN/Eu:GaN/p-GaN structure. 2. Experimental The Eu-doped GaN ﬁlms were grown by gas source molecular beam epitaxy (GSMBE) using uncracked ammonia as nitrogen source. Metallic Ga with 6 N purity, Eu with 3 N purity were evaporated from conventional Knudsen eﬀusion cells, and uncracked ammonia gas with 6 N
J. Sawahata et al. / Optical Materials 28 (2006) 759–762
purity was introduced to the growth surface through a nozzle of stainless steel tube. The Eu-doped GaN was grown on sapphire (0 0 0 1) substrate at 700 C. The ﬁlm thickness was about 1 lm. Eu concentration in GaN was estimated to be 2 at%, in which the luminescence intensity shows maximum, by Rutherford back scattering (RBS) spectroscopy. In order to evaluate the structural properties, X-ray diﬀraction (XRD) measurements were carried out with a h–2h mode using both Cu Ka1 and Ka2 radiations. TEM samples were prepared by using focused ion beams (FIB) method and the TEM observation was performed by JEM-2000 EX system with the acceleration energy of 200 keV. Photoluminescence (PL) measurements were carried out at 77 K and room temperature (RT) using the 325 nm line of a He–Cd laser as an excitation source. The n-GaN/Eu:GaN/p-GaN structure was grown by following procedure. First, the Eu-doped GaN layer was grown at 700 C on p-type GaN ﬁlm that was grown by metalorganic chemical vapor deposition (MOCVD). Subsequently, the substrate temperature was heated up from 700 C to 850 C, and then the n-type GaN layer was grown. Room-temperature hole concentration of the pGaN layer was 4 · 1016 cm 3 and the Eu-doped GaN layer was highly resistive. The electron concentration of the n-type GaN was 6 · 1017 cm 3. The characteristics of the n-GaN/Eu:GaN/p-GaN structure were measured under a direct current at room temperature. 3. Results and discussion Fig. 1 shows the X-ray diﬀraction proﬁles of Eu-doped GaN, compared with that of undoped GaN for reference. The peak position of Eu-doped GaN shifted slightly to a lower angle possibly due to the large atomic radii of Eu. The value of the full width at half maximum (FWHM) of the diﬀraction peak of Eu-doped GaN is almost two times larger than that of undoped GaN. Therefore, the structural property of Eu-doped GaN is inferior to that of undoped one. The diﬀraction peaks from Eu(1 1 0), (2 0 0), (2 1 1),
and EuN(1 1 1), (2 0 0), (2 2 0) should be observed at around 27.5, 39.3, 48.6, and 30.8, 35.7, 51.5, respectively. However, corresponding peaks and other anomalous peaks were not observed, therefore, a possibility of the segregation of Eu and EuN may be excluded. Fig. 2 shows the selected area diﬀraction (SAD) pattern of Eu-doped GaN. As can be seen in the ﬁgure, the diﬀraction pattern consists of spots from hexagonal phase which is almost the same as that of undoped one. Therefore, Eudoped GaN has a hexagonal phase without the formation of secondary phase such as Eu or EuN consistent with the results of XRD. When the Eu concentration is increased more than 2 at%, the SAD pattern began to indicate the formation of cubic and twin structure. Fig. 3(a) shows the bright ﬁeld cross sectional view of Eu-doped GaN with the Eu concentration of 2 at% and undoped GaN for reference (Fig. 3(b)). The inset shows the enlargement of the indicated area. As shown in the inset of Fig. 3(a), the formation of stacking irregularity was observed. Such stacking irregularity was observed all over the place. In addition to the formation of a high density of stacking irregularity, a small portion of cubic phase of GaN were observed. It is hard to ﬁnd such stacking irregularity in undoped GaN as shown in Fig. 3(b), therefore, these stacking irregularitys are induced by the incorporation of Eu atom. The atomic radius of Eu is 1.5 times larger than that of Ga, so the Eu-doped GaN contains large stress. The stress caused by Eu doping may be relaxed by inducing stacking irregularity. Fig. 4 shows the PL spectra of Eu-doped GaN measured at 77 K and RT. The luminescence peak at 622 nm is originating from intra-4f transitions which can be assigned as the 5D0–7F2 transition of the Eu3+ ion. As reported earlier, the optical process in Eu-doped GaN is proposed that the red luminescence at 622 nm is generated through an excitation of GaN, that is, electrons and holes are generated in GaN and they are transferred to Eu ion . If this model
XRD Intensity (a.u.)
Eu -doped GaN
(Eu at 2%) undoped GaN
2θ (Degree) Fig. 1. XRD proﬁles of Eu-doped GaN and undoped GaN.
Fig. 2. Selected area diﬀraction pattern of Eu-doped GaN.
J. Sawahata et al. / Optical Materials 28 (2006) 759–762
8 6 4 2 0 -10 -8
Voltage [V] Fig. 5. I–V characteristics at 300 K of n-GaN/Eu:GaN/p-GaN structure.
PL Intensity (a.u.)
Fig. 3. (a) Bright ﬁeld cross sectional view of Eu-doped GaN. The circled part shows a cubic phase of GaN, and the inset shows enlargement of the stacking irregularity. (b) Bright ﬁeld cross sectional view of undoped GaN.
(a) 77K (b) Room Temp. 580
Wavelength (nm) Fig. 4. PL spectra of Eu-doped GaN at 77 K and room temperature.
is correct, the red luminescence can be obtained by electron and hole injection using p-n junction. The n-GaN/Eu:GaN/p-GaN structures were grown with the thickness of n-GaN and p-GaN to be about 1 lm. The thickness of the Eu:GaN was 0.1 lm. Fig. 5 shows the current–voltage (I–V) characteristics at 300 K
Fig. 6. EL spectrum from n-GaN/Eu:GaN/p-GaN structure (a) and PL spectrum of p-type GaN (b).
of n-GaN/Eu:GaN/p-GaN structure. Electrical rectiﬁcation behavior was obtained with the turn-on voltage of about 3 V for forward bias. The turn-on voltage of 3 V may be reasonable considering the band-gap energy of GaN. The electroluminescence (EL) spectra under forward bias was measured. We could not observe the red emission from Eu-doped GaN although the blue emission considered to be due to the donor–acceptor (D–A) pair transition of p-type GaN was observed as shown in Fig. 6. The spectral proﬁle of the EL is almost the same as that of the PL of p-type GaN. These results suggest that the hole injection into the Eu:GaN is insuﬃcient and electrons are passing through the Eu-doped GaN layer. The diﬃculty of the hole injection may be due to the short life time of hole in Eu:GaN layer and the high density of the stacking irregularity may be responsible for the short life time. 4. Conclusions We have investigated the structural properties of Eudoped GaN using TEM and electrical and optical properties of n-GaN/Eu:GaN/p-GaN structure. In the TEM
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observation of Eu-doped GaN, high density of stacking irregularity and a small portion of cubic phase of GaN were detected. These stacking irregularitys were induced by the incorporation of Eu atom which has large atomic radii. The electrical properties of n-GaN/Eu:GaN/p-GaN structure showed the rectiﬁcation behavior, however, the red emission from Eu-doped layer was not obtained but blue emission from p-GaN layer was observed. These results suggest that the hole mobility in Eu-doped GaN is extremely low probably due to high density of stacking irregularity. References  S.C. Jain, M. Willander, J. Narayan, R. Van Overstraeten, J. Appl. Phys. 87 (2000) 965.
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