Optical Materials 28 (2006) 775–779 www.elsevier.com/locate/optmat Comparative optical study of Eu3+ ions doping in InGaN/GaN quantum dots and GaN layer grown by molecular beam epitaxy Thomas Andreev a,*, Nguyen Quang Liem a,b,c, Yuji Hori a,d, Mitsuhiro Tanaka d, Osamu Oda d, Bruno Daudin a, Daniel Le Si Dang e a b CEA/CNRS/UJF Research Group Nanophysique et Semiconducteurs, DRFMC/SP2M/PSC CEA-Grenoble, 17 rue des Martyrs, 38054-Grenoble Cedex 9, France Institute of Materials Science (IMS), Vietnamese Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam c College of Technology, Hanoi National University, 144 Xuan Thuy, Cau Giay, Hanoi, Vietnam d NGK Insulators, LTD 2-24 Sudacho, Mizuhoku, Nagoya, Japan e CEA/CNRS/UJF Research Group Nanophysique et Semiconducteurs, Laboratoire Spectrome´trie Physique (CNRS UMR5588), Universite´ J Fourier, BP 87, 38402 Saint Martin dÕHe`res, France Available online November 2005 Abstract We report on a comparative optical study of InGaN:Eu quantum dots (QDs) and GaN:Eu layer grown by molecular beam epitaxy (MBE) Analysis of the 5D0 ! 7F2 transition as a function of the excitation wavelength shows that Eu3+ ions in InGaN:Eu QDs are located inside InGaN QDs and also in the GaN barrier layer The existence of Eu3+ ions in the GaN barrier layer is explained by Eu segregation/diffusion during growth For Eu3+ ions located inside InGaN QDs the photoluminescence (PL) shows only a slight decrease with temperature from K to 300 K In contrast, the PL from Eu3+ ions in the GaN barrier layer or in GaN thick layer shows a much more pronounced thermal quenching Ó 2005 Elsevier B.V All rights reserved Introduction The combination of rare earth (RE) luminescence with the wide band gap of (In)GaN is a promising solution for full color devices, since efficient energy transfers can occur from carriers of the semiconductor host to the RE excited states Successful RE doping of GaN films was demonstrated by several groups (see for instance Ref [1– 3]) either by implantation or by molecular beam epitaxy (MBE) growth To reduce the nonradiative recombination channels derived from the high dislocation densities in GaN layers we propose to combine the confinement properties of quantum dots (QDs) with the RE luminescence to achieve * Corresponding author Tel.: +33 438 78 5416; fax: +33 438 78 5797 E-mail address: andreev@drfmc.ceng.cea.fr (T Andreev) 0925-3467/$ - see front matter Ó 2005 Elsevier B.V All rights reserved doi:10.1016/j.optmat.2005.09.061 high luminescence even at room temperature We have already demonstrated Eu- (red), and Tm- (blue) doped GaN QDs embedded in an AlN matrix, which showed stable luminescence in the temperature range of 5–300 K Furthermore a strong enhancement of the radiative quantum efficiency, by about one to two orders of magnitude as compared to rare earth doped films, was observed at room temperature [4–6] However the injection of carriers into AlN is hindered by the difficulties in p-type and n-type doping of AlN, so that one solution could be to use RE doped InGaN QDs grown on GaN for current injection devices, since p- and n-doping of GaN are well controlled by MBE Eu was found to act as a surfactant in MBE growth of GaN layers, leading to drastic changes in adatom kinetics, as we demonstrated already in Ref [7] For Tm-doped GaN QDs in AlN, we found that Tm is located inside GaN QDs, but 776 T Andreev et al / Optical Materials 28 (2006) 775–779 also at the GaN/AlN interface [5] Such a situation could happen also in the case of Eu doping in InGaN/GaN QDs due to the complex interaction of RE atoms with the formation of QDs Hence the aim of this article is to address the locations, PL efficiency and thermal quenching property of Eu3+ ions in InGaN/GaN QDs and to compare to those in GaN layer Experimental The growth was performed on lm thick AlN pseudosubstrates deposited by metal organic chemical vapor deposition on c-sapphire [8] After a standard chemical degreasing procedure and acid cleaning with HF, the pseudo-substrate was fixed with an indium bonding on a molybdenum sample holder, and introduced in a MBE chamber equipped with Al, Ga, In and Eu effusion cells and a radio-frequency plasma cell to produce monatomic nitrogen The growth conditions were controlled with reflection high-energy electron diffraction, which allows in situ and real time monitoring the 2D–3D transition corresponding to the formation of self-organized QDs The InGaN QDs were grown at 600 °C following the Stranski-Krastanow (SK) growth mode, i.e the QDs appear after the deposition of a wetting layer of typically two monolayers [9] To allow the QD growth in SK growth mode of InGaN QDs on GaN, a minimal In concentration of around 20% is needed Before the growth of InGaN QDs, a 150 nm thick GaN buffer layer was deposited at 720 °C During growth of InGaN, the shutter of Eu source was opened to dope the material Then, the QDs were capped with about nm of non-intentionally doped (n.i.d.) GaN This process was repeated 165 times to achieve stacks of QD planes sandwiched in GaN barriers From chosen growth conditions we estimate an Eu content of 1% For reference measurements, a GaN:Eu layer was grown on a n.i.d GaN buffer The Eu concentration of the GaN film reference sample was measured by RBS to be about 0.2% Morphology of the grown sample has been studied at room temperature in air with an AFM Dimension 3100 microscope PL spectra were measured using a tunable excitation source consisting of a 500 W high-pressure Xe lamp equipped with a Jobin-Yvon high-resolution double-grating monochromator (Gemini 180) The excitation power density was about 200 lW/cm2 at 350 nm The PL was analyzed by another Jobin-Yvon grating monochromator (Triax 550) and detected by a CCD camera operating at liquid nitrogen temperature The sample was mounted on the cold finger of a micro-cryostat which enabled us to record PL spectra at various temperatures from K to 380 K Results and discussion Fig shows AFM images of InGaN:Eu QDs in different areas from 4000 · 4000 nm2 to 500 · 500 nm2 The surface morphology is characterised by the typical spiral Fig AFM images of InGaN:Eu QDs Areas of images: (a) 4000 · 4000 nm2, (b) 2000 · 2000 nm2, (c) 1000 · 1000 nm2 and (d) 500 · 500 nm2 hillocks of GaN (Fig 1a and b) InGaN:Eu QDs are found aligned on the atomic terraces around the hillocks as visible in Fig 1b–c The higher resolution image of Fig 1d shows the InGaN:Eu QDs, which present diameters between 15 nm and 40 nm and rather small height, between 0.4 nm and nm The quantum dot density was found to be around 1.4 ± 0.2 · 1011 cmÀ2 Fig 2a shows room temperature PL spectra of InGaN:Eu QDs and a GaN:Eu layer excited at 360 nm Stark split lines can be observed, corresponding to the 5D0–7F2 transition of Eu3+ ions The three main lines in the two spectra are located at 620, 622 and 633.5 nm According to the spectroscopic analysis of Brecher et al [10] emission at 633.5 nm is from the 5D1 ! 7F4 transition The line width is slightly broader in the case of InGaN QDs as 618 621 24 627 630 633 636 resulting from internal electric field and strain inside InGaN QDs or poorer crystalline quality A remarkable difference between the two spectra is the larger intensity of the 622 nm line in doped QDs, whereas the 620 and 633.5 nm lines display the same intensity ratios This suggests two contributions of Eu3+ ions to the PL of InGaN:Eu QDs, which come from Eu3+ ions in the InGaN material and in the GaN barriers Note that Eu3+ ions might occupy different sites in the two structures (InGaN and GaN), which can explain the modification of the spectrum [11] It is interesting to note that Eu was observed to exhibit a tendency to segregate on the surface during the growth of GaN and AlN, so that it is quite possible that a small amount of Eu content can be found also in the GaN barrier when doping InGaN QDs [7] We believe that in the PL of InGaN:Eu QDs, both contributions from Eu3+ ions in InGaN QDs and from Eu3+ ions in the GaN barrier layer are superimposed Notice that for Tm doping of GaN QDs grown on AlN the situation is even more complex as Tm3+ ions are found inside QDs and also at the GaN/AlN interface [5] To assess further the origin of emission lines, PLE measurement was carried out at K as shown in Fig 2b The GaN:Eu layer exhibits rather similar PLE spectra for emissions at 620, 622, and 633.5 nm (not shown) The PLE spectra are characterised by band-toband absorption at the GaN band gap and a weak absorption tail below band gap down to 400 nm, which could be due to defect states However, no well-resolved absorption due to a trap level at 400 nm as previously reported in Ref [2] could be observed in our structures, which illustrates the complexity of carrier-mediated energy transfer processes in semiconductors doped with RE ions For InGaN:Eu QDs the PLE spectra measured for the 620 and 633.5 nm emission lines (not shown) are similar, but they differ from that measured for the emission at 622 nm In the later case, the peak absorption is at lower energy and the low energy absorption tail below 380 nm is much stronger This different behaviour is consistent with our assumption that the 622 nm emission observed in InGaN:Eu QDs is to be associated to the Eu3+ ions other than those responsible for the 620 nm and 633.5 nm emissions More precisely we assign the 622 nm line (or the main part of it) to the Eu3+ ions located in InGaN QDs, and the 620 nm and 633.5 nm lines to the Eu3+ ions located in the GaN barrier layer In fact, a recent PL study of Eu doping in GaN/AlN QDs showed that only the 622 nm line could be observed for Eu3+ ions in GaN QDs, by contrast to Eu3+ ions in GaN layer which exhibit three emission lines at 620, 622 and 633.5 nm (Fig 2) At present there is no clear explanation for the very different optical signature of Eu3+ ions at these two types of locations However, it is reasonable to assign them to the different site-symmetry/ligand of Eu3+ ions For InGaN:Eu QDs, the bump marking the beginning of band-to-band absorption in PLE spectra is at lower 778 T Andreev et al / Optical Materials 28 (2006) 775–779 λ exc = 400 nm (a) PL intensity (arb.units) PL intensity (arb.units) 5K InGaN:Eu QDs GaN:Eu layer 618 620 622 624 390 nm x 100 471.5 nm 615 620 625 630 635 640 Wavelength (nm) Wavelength (nm) Fig PL spectra of InGaN:Eu QDs (upper spectrum) and GaN:Eu layer (lower spectrum) with the 400 nm excitation at K The vertical dotted lines guide to the eye 0.1 300 K PL intensity (arb.units) energy than the GaN gap This can be explained by In diffusion of about 1% into the GaN spacing layer or strain effect of the GaN spacer as it was grown onto the InGaN QDs On the other hand the PLE spectrum of the 622 nm line of InGaN QDs shows a remarkable strong absorption at 365 nm with respect to band-to-band absorption and pronounced low energy tail absorption below the band gap All these features can be assigned to absorption in an inhomogeneous ensemble of QDs with a distribution in size and shape (see the AFM images in Fig 1) An excitation wavelength of 400 nm (below the band gap of GaN) was used to obtain the K PL spectra from InGaN:Eu QDs and a GaN:Eu layer shown in Fig The PL integration time was longer by two orders of magnitude in the case of the GaN layer as compared to InGaN QDs This shows that defect related excitation mechanism is very weak in our GaN layer and that a significant part of the InGaN QD distribution can be still excited at 400 nm The 5D0 ! 7F2 transition displays only emission at around 622 nm consisting of two sharp lines with FWHM smaller than 0.3 nm A small blueshift by about 0.1 nm as well as a spectral broadening are observed for emission from InGaN:Eu QDs, which could be induced by electric field effects or the different environment of Eu in the InGaN QDs and in the GaN layer Fig 4a compares the PL of InGaN:Eu QDs at T = 300 K, using different excitations above (360 nm) and below (390 nm and 471.5 nm) the GaN band gap Using 390 nm as excitation wavelength, hardly any emission at 620 nm or 633.5 nm can be detected, since Eu atoms in the GaN spacing layer are not excited, in agreement with the PLE results The total emission intensity is higher in the case of excitation above the barriers (360 nm) due to stronger absorption into the GaN barrier and carrier diffusion from the barriers into the QDs We now discuss the different thermal quenching characteristics originating from different Eu3+ ion locations In 300 K 360 nm 610 626 D0→ F2 0.01 360 nm 390 nm (Eact,1=70 ± meV) (Eact=220 ± 20 meV) 10 (b) 100 1000/T (1/K) Fig (a) PL spectra from InGaN:Eu QDs measured with different excitation wavelengths (indicated at the curves) at 300 K The spectra are normalized by the excitation power density and the accumulation time The 471.5 nm spectrum is multiplied by a factor 100 for clarity (b) Temperature-dependent PL of InGaN:Eu QDs for emissions at 622 nm between K and 380 K in double logarithmic scale The excitation wavelengths were 360 nm (filled dots) and 390 nm (filled squares) The dashed lines are used simulations to get thermal activation energies The corresponding activation energy is also indicated in the figure The vertical line marks 300 K for clarity Fig 4b the temperature dependence between K and 380 K of the 622 nm emission is shown with exciting at 360 nm for InGaN:Eu QDs (above the corresponding InGaN band gap) and 390 nm for the GaN spacing layer (below its band gap) A reduced thermal quenching can be found for excitation below GaN gap This is due to the fact that electron and hole pairs are directly injected and confined in QDs, which should strongly reduce their capture by non-radiative recombination centres at elevated temperatures Furthermore the weak thermal quenching suggests that the carrier mediated energy transfer to RE ions in QDs should be faster than non-radiative recombination channels experienced by carriers in QDs, which T Andreev et al / Optical Materials 28 (2006) 775–779 are found to be on a time scale of ns for carriers in GaN/ AlN QDs as reported by Simon et al [12] Conclusion In conclusion, InGaN:Eu QDs imbedded in GaN barriers have been studied by PL and PLE at various temperatures The analysis of the 5D0 ! 7F2 transition as a function of the excitation wavelength has shown that Eu3+ ions are located inside InGaN QDs and in the GaN barrier layer as well Emission from Eu3+ ions inside InGaN QDs showed only a slight decrease from K up to room temperature In contrast, the PL from Eu3+ ions in the GaN barrier layer or in GaN thick layer shows a much more pronounced thermal quenching Acknowledgements We acknowledge Marle`ne Terrier, Yann Genuist and Yoann Cure´ for their technical assistance One of the authors (NQL) thanks the National Programme for Basic Research (Vietnam) and CNRS (France) for financial supports 779 References [1] H.J Lozykowski, W.M Jadwisienczak, J Han, I.G Brown, Appl Phys Lett 77 (2000) 767 [2] Ei Ei Nyein, U Hoămmerich, J Heikenfeld, D.S Lee, A.J Steckl, J.M Zavada, Appl Phys Lett 82 (2003) 1655 [3] H Bang, S Morishima, J Sawahata, J Seo, M Takiguchi, M Tsunemi, K Akimoto, M Nomura, Appl Phys Lett 85 (2004) 227 [4] Y Hori, X Biquard, E Monroy, F Enjalbert, Le Si Dang, M Tanaka, O Oda, B Daudin, Appl Phys Lett 84 (2004) 206 [5] T Andreev, Y Hori, X Biquard, E Monroy, D Jalabert, A Farchi, M Tanaka, O Oda, Le Si Dang, B Daudin, Phys Rev B 71 (2005) 115310 [6] T Andreev, Y Hori, X Biquard, E Monroy, D Jalabert, A Farchi, M Tanaka, O Oda, Le Si Dang, B Daudin, Superlattices Microstruct 36 (2004) 707 [7] Y Hori, D Jalabert, T Andreev, E Monroy, M Tanaka, O Oda, B Daudin, Appl Phys Lett 84 (2004) 2247 [8] T Shibata, K Asai, T Nagai, S Sumiya, M Tanaka, O Oda, H Miyake, K Hiramatsu, Mater Res Soc Symp Proc 693 (2002) 541 [9] C Adelmann, J Simon, G Feuillet, N.T Pelekanos, B Daudin, G Fishman, Appl Phys Lett 76 (2000) 1570 [10] C Brecher, H Samelson, A Lempicki, R Riley, T Peters, Phys Rev 155 (1967) 178 [11] K.P OÕDonnell et al., Mater Res Soc Symp Proc 831 (2004) 96 [12] J Simon, N.T Pelekanos, C Adelmann, E Martinez-Guerrero, R Andre´, B Daudin, Le Si Dang, Phys Rev B 68 (2003) 035312 ... the main part of it) to the Eu3+ ions located in InGaN QDs, and the 620 nm and 633.5 nm lines to the Eu3+ ions located in the GaN barrier layer In fact, a recent PL study of Eu doping in GaN/ AlN... the same intensity ratios This suggests two contributions of Eu3+ ions to the PL of InGaN: Eu QDs, which come from Eu3+ ions in the InGaN material and in the GaN barriers Note that Eu3+ ions might... contributions from Eu3+ ions in InGaN QDs and from Eu3+ ions in the GaN barrier layer are superimposed Notice that for Tm doping of GaN QDs grown on AlN the situation is even more complex as Tm3+ ions