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In this work, Pr and Tm co-doped GaN thin films were prepared by ion implantation. After a thermal annealing treatment for the lattice recovery and ions activation, temperature-dependent cathodoluminescence (CL) spectroscopy was applied to investigate the luminescent properties. At room temperature, the intensity ratio of blue emission to infrared emission of Tm ions was decreased as the implantation dose of Pr ions was increased, which is due to the energy transfer between Pr and Tm ions. In addition, the 467 nm blue emission of Tm and the 528 nm green emission of Pr were observed only at low temperature. A model was proposed to illuminate the light emission mechanism of Pr and Tm co-implanted GaN thin films.
© 2016 Optical Society of America
1. Introduction
Rare-earth (RE)-doped materials have been extensively investigated for various applications in the past several years. The applications range from phosphors in flat panel displays (FPDs) to amplifiers and solid state lasers in telecommunications [1]. Previously, doping with RE ions into the conventional semiconductors such as Si, GaAs, etc., has been subjected to solubility limitation and severe temperature quenching of luminescence [2]. For GaN, however, the thermal quenching of luminescence is extremely low due to the wide band gap of 3.4 eV. Investigations of Er ions implanted semiconductors have shown that thermal quenching of Er ions luminescence decreases with the increases of band gap of host materials [3].
It has been shown that Er-doped GaN (GaN:Er) can produce a strong near-infrared emission at 1.5 μm from the lowest excited state of Er ions [4], which is suitable for optic communications. Recently, Eu doped GaN (GaN:Eu) thin film on Si has been reported lasing upon optical pumping by J. H. Park [5]. In addition, Fujiwara and his associates also achieved interesting results on GaN: Eu LEDs [6–8]. Light emission from electroluminescent devices (ELDs) of GaN doped by RE-ions is ideal for the full color FPD due to their very narrow line-width and entire visible spectra: red light emission of GaN: Pr at 650 nm and GaN:Eu at 621 nm [9,10], green light emission of GaN: Er at 537 and 558 nm [11], and blue light emission of GaN: Tm at 477 nm [12]. In general, the emission from RE-doped GaN is strong enough to be observed with naked eyes at room temperature, and match very well to the saturated RGB three primary colors of the US National Television System Committee (NTSC) in the Commission International de l’Eclairage (CIE) chromaticity diagram [13].
Extensive investigations on rare earth doped GaN were focused on the single element doping. There are two color integration methods: vertical integration and lateral integration. A stack layers of GaN:Tm/GaN:Er/GaN:Eu which vertically integrated blue, green and red emissions were designed by Zhang Lei, aiming to obtain direct white light emission from a single GaN chip with two electrodes [14]. Y. Q. Wang investigated a lateral integration method to realize blue, green, and red color integration on RE-doped GaN electroluminescent thin films [15]. However, the rare earth ions co-doped in the same GaN film is still quite lack. I. S. Roqan found that co-implanting with low concentration Er did not have a detrimental effect on the Tm luminescence in GaN [16]. In addition, J. Rodrigues found that both the emission of Eu and the emission of Pr can be observed in Eu and Pr co-doped GaN and the red spectral range was widened [17]. Up to now, there is still no reports about Pr, Tm-codoped GaN.
In this work, we have co-doped GaN layers with Pr and Tm by ion implantion. Cathodoluminescence spectroscopy was applied to investigate the effects of co-doping Pr on Tm emission. The evolution of CL intensity with temperature was also analyzed.
2. Experimental
The unintentionally-doped GaN films were grown by metal-organic chemical vapor deposition (MOCVD) on the c-oriented sapphire substrates. The as-grown GaN films were implanted with Pr and Tm ions. Both the dose of Pr or Tm are 1 × 1014 at/cm2, 5 × 1014 at/cm2, and 1 × 1015 at/cm2, respectively, the energy of implantation is 200 keV. Then the post-implantation annealing was performed at 1040°C for 1h under a flowing NH3 ambience in resistance heating furnace to reduce the lattice damage caused by ions implantation. Finally the cathodoluminescence (CL) measurements were carried out using a Quanta400FEG field emission scanning electron microscope (SEM) and a MonoCL3 + cathodoluminescence spectrometer. The spectra were acquired over a wavelength range of 320–840 nm. CL temperature dependent measurements were carried out in the range of 80–370 K.
3. Results and discussion
3.1 GaN: Pr
The CL spectra of GaN: Pr with a implantation dose of 1 × 1014 at/cm2 measured at 300K and 80K were shown in Fig. 1. At 300 K and 80 K, the sample exhibited GaN band edge emission at 364 nm, and a strong red emission centered at 654nm which corresponds to the 3P0→3F2 transition of Pr ions. A few weaker emission lines were observed at 672 nm (3P1→3F3) and 750–800 nm (3P0→3F4, 1D2→3H6). At 80K, the emission at 662nm was resolved, corresponding to the 1I6→3F3 transition. In addition, shorter wavelength transition was identified in the green spectral region (528 nm) due to the 3P1→3H5 transition of Pr3+ ions. This emission is usually very weak in GaN samples [18] and is more commonly observed in AlN layers and AlxGa1-xN ternary alloys [19]. For temperatures up to 300 K, the emission lines at 662nm and 528nm were not observed as shown in Fig. 1(a). This may be due to the fact that the 3P1 and 1I6 levels are close to the 3P0 level, which was shown in Fig. 2. Therefore, the non-radiative multiphonon transition rate is high.
Fig. 1 CL spectra of Pr implanted GaN with a dose of 1 × 1014 at/cm2 measured at 300 K and 80 K. The annealing temperature is 1040°C.
When the implantation dose of Pr ions was increased to 5 × 1014 at/cm2, or 1 × 1015 at/cm2, the intensity of Pr ions at 654 nm was not significantly increased. With the increase of implantation dose of Pr ions, on the one hand, the concentration of luminescent center was increased to promote the emission; on the other hand, the concentration of defects was also increased to reduce the emission intensity. Therefore, the fluorescence intensity of Pr was not enhanced significantly with the increase of implantation dose.
3.2 GaN: Tm
The CL spectra of GaN: Tm measured at 300 K and 80 K were shown in Fig. 3(a) and Fig. 3(b), respectively. As shown in Fig. 3(a), two major emission lines of the Tm ions at 480nm and 806 nm are attributed to the intra-4f transitions 1G4→3H6 and 3H4→3H6 of Tm ions, respectively [16]. The near-IR line at 806 nm can also be associated with the transition between the 1G4 and 3H5 levels [20]. It reveals that the intensity ratio of the 480 nm to 806 nm peak in GaN: Tm is ~0.30 . When the temperature was decreased to 80 K, a new peak at 467 nm related to a 1D2→3F4 transition was observed in Fig. 3(b). While this peak was not observed at 300K, which may be due to the fact that the 1D2 state (3.44eV) is very close to the band gap of GaN as shown in Fig. 4. The band gap of GaN became narrower as the temperature was increased. At 300 K, the electrons at the 1D2 state can be transported to the conduction band and became free electrons, resulting in emission quenching.
Fig. 3 CL spectra of Tm implanted GaN with a dose of 5 × 1014 at/cm2 measured at 300 K and 80 K.The annealing temperature is 1040°C.
With the increase of implantation dose of Tm ions, the intensity of Tm emission peaks was increased gradually, whereas the intensity ratio of blue light (480 nm) and infrared (806 nm) was still around 0.30 .
3.3 GaN: Pr,Tm
Figure 5 shows the 300 K CL spectra of Pr and Tm co-implanted GaN thin film with a dose of 1 × 1014 at/cm2, 1 × 1015 at/cm2, respectively. Compared with the spectra of Pr or Tm ions implanted GaN, there is no any significant difference in emission lines of Pr and Tm ions. In addition, it was found that when Pr and Tm co-implanted GaN with a dose of 1 × 1014 at/cm2, and 1 × 1015 at/cm2 the difference between the luminescent intensity of blue and red emission is minimum.
Fig. 5 300K CL spectrum of Pr and Tm co-implanted GaN with a dose of 1 × 1014 at/cm2, 1 × 1015 at/cm2, respectively. The annealing temperature is 1040°C.
For Tm-doped GaN, the intensity ratio of the blue light (480 nm) with respect to infrared (806 nm) peak is about 0.30 . The intensity ratio is dependent on the crystal field environment of the Tm atoms [21]. For GaN: Pr,Tm samples, this intensity ratio was decreased due to the doping of Pr ions, as shown in Fig. 6. SRIM is a group of programs which calculate the stopping and range of ions (10 eV-2 GeV/amu) into matter using a full quantum mechanical treatment of ion-atom collisions. According to the SRIM simulations, we calculate the projected range of Pr and Tm under 200 KeV implantation energy, and the value is 73.7 and 64.6 nm, respectively. Therefore, the profile distribution of Tm atoms is nearly the same with Pr atoms in the ion implantation process, which makes it possible to achieve an energy transfer between Pr and Tm ions. For Pr and Tm co-implanted GaN, blue luminescence of Tm ions can excite Pr ions to emit red light. On the other hand, the red light emitted by Pr ions can stimulate Tm ions to emit infrared light. Therefore, for Tm ions, the intensity ratio of blue luminescence (480 nm) with respect to infrared emission (806 nm) is enhanced after Pr ions co-doping. The change in blue/IR intensity ratio is also possible due to the different defect levels in the samples. While the defect levels formed in the growth process of GaN thin film and ion implantation process are various and complicated, the energy transfer mechanism between the blue/IR emission and defect levels need to be investigated further in the near future. The energy transfer mechanism between Pr and Tm was discussed as follows.
Fig. 6 Effects of Pr ions dose on the the intensity ratio of blue light (480 nm) with respect to infrared (806 nm) of Tm ions.
(a) Mechanism for resonance transfer between energy levels of 1G4 (Tm) and 3P0 (Pr);
(b) Mechanism for the 654nm emission of Pr ions reabsorbed by Tm ions.
Figure 7 shows the energy level schemes of Pr and Tm ions. The energy transfer from the 1G4 state (2.6eV) of Tm ions to the 3P0 manifold (2.528eV) of Pr ions can be expected with subsequent 654 nm emission of Pr ions. Therefore, the 480nm luminescence of Tm ions was weakened and the near-IR line at 806 nm would also be reduced slightly. In addition, the Tm ions 3F2 level (1.891eV) can also be populated by the 654 nm (1.899eV) excitation emitted by Pr ions, and followed by phonon decay to 3H4 level, as shown in Fig. 7(b). Then a subsequent transition process 3H4→3H6 would contribute for increasing the infrared emission at 806nm.
Fig. 7 Energy level schemes of Pr and Tm ions. Energy transfer processes were shown by the dashed arrows.
Therefore, after Pr ions are co-doped in GaN: Tm thin films, the intensity of the blue light at 480nm is decreased more than the infrared emission, which results in the declining intensity ratio of the blue light (480 nm) with respect to infrared light (806 nm).
Figure 8 shows the temperature-dependent cathodoluminescence spectra of 1 × 1014 at/cm2 Pr and 1 × 1015 at/cm2 Tm co-implanted GaN thin film from 80 to 370 K. The Tm CL spectra exhibited a number of different narrow lines in the blue region of 467nm, particularly at low temperature. The different emission lines may be due to the different Tm centers formed in GaN or crystal field splitting of the Tm levels [21].
Fig. 8 Temperature-dependent CL spectra of 1 × 1014 at/cm2 Pr and 1 × 1015 at/cm2 Tm co-implanted GaN. The annealing temperature is 1040°C.
As shown in Fig. 8, the 467 nm emission due to the 1D2→3F4 transition of Tm ions was completely quenched and disappeared at temperature above 210 K. The 528 nm emission due to the transition of 3P1→3H5 was also decreased quickly and disappeared as the temperature increases to 170K. However, for 806nm emission, the intensity at 300 K is still about 60% of intensity at 80 K. Different emission peaks have different thermal quenching behavior as indicated in Fig. 9. The emission at 806nm shows remarkably weak thermal quenching. The thermal quenching mechanisms were proposed to be non-radiative recombination of the excited states of a localized rare earth ion center in GaN [22]. If the energy gap of excited state with respect to the next lower state is sufficiently large, the non-radiative multiphonon transition rate is negligible [22].
Fig. 9 Temperature dependence of CL peaks intensity of Pr and Tm ions. The data was extracted from the Fig. 8.
Table 1 shows the energy gap between the excited state and the next lower state of Pr and Tm. As for the peak at 467 nm related to a 1D2→3F4 transition, the 1D2 level is close to the band gap energy of GaN. The value of energy gap is only 0.040eV. And the energy gap of 3P1 with respect to 3P0 is only 0.075eV, as shown in Table 1. Both of the energy gap is close to the LO phonon energy (0.089eV) of GaN [23]. Therefore, as the temperature increases, the non-radiative multi-phonon transition may occur and thermal quenching behavior emerges.
Table 1. The energy gap of the excited state with respect to the next lower state of Pr and Tm.
4. Conclusions
The cathodoluminescence properties of Pr and Tm co-implanted GaN thin films were investigated in this work. It was found that, for Tm ions, the intensity ratio of blue emission (480 nm) with respect to infrared emission (806 nm) was decreased with the increase of the implantation dose of Pr ions. Energy transfer process between Pr and Tm ions was proposed to shed light on this phenomenon, which can be translated by the mechanism for resonance transfer between energy levels of 1G4 (Tm) and 3P0 (Pr), and mechanism for the 654nm emission of Pr ions reabsorbed by Tm ions. Temperature-dependent CL measurements show that different emission peaks have different thermal quenching behavior, which influenced by the energy gap between the excited state to the next lower state.
In summary, a Pr and Tm co-doped GaN thin films were designed to obtain multi-color emission. This method does not need the complicated liftoff process like the lateral color integration, nor require the vertical integration structure.
Acknowledgments
The authors gratefully acknowledge the support of the National Natural Science Foundation of China (NSFC) (grant no. 61306004, 51002179, 11247023, 51272270), the Natural Science Foundation of Jiangsu Province of China (no.BK20130263), the Functional Development Program of the Chinese Academy of Sciences (no. yg2012093), the collaborative Innovation Center of Suzhou Nano Science and Technology, and the PAPD and USTS Cooperative Innovation Center.
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