Cathodoluminescence and its temperature dependence in Tm-doped Al x Ga 1 x N thin films

Transcription

1 phys. stat. sol. (c) 2, No. 7, (25) / DOI.2/pssc Cathodoluminescence and its temperature dependence in Tm-doped Al x N thin films D. S. Lee, A. J. Steckl *, P. D. Rack 2, and J. M. Fitz-Gerald 3 Nanoelectronics Laboratory, University of Cincinnati, Cincinnati, Ohio , USA 2 University of Tennessee, Knoxville, TN 37996, USA 3 University of Virginia, Charlottesville VA, USA Received 9 August 24, accepted 22 December 24 Published online 7 March 25 PACS Ln, 7.55.Eq, 78.6.Hf, Fd Cathodoluminescent (CL) emission from Tm-doped Al x N (Al x N:Tm) has been observed with various Al compositions ( x ). According to CL spectrum from Al.8 N:Tm film D 2 levelrelated CL emissions were observed to dominate: the strongest was blue at 464 nm and the next was UV at 369 nm, corresponding to the Tm D 2 transitions, respectively. CL luminance increases with Al composition up to x =.8, then decreases for AlN:Tm, which is the same as in EL and PL. Luminance value was measured to be 9.4 cd/m 2. The corresponding efficiency was.2 lm/w under 5 kv bias and µa current flow. Temperature dependent CL from the Al.8 N:Tm film shows that D 2 emission increases with temperature up to room temperature (RT) but I 6 emission becomes quenched upon approaching RT. Introduction A major challenge [, 2] for full color displays and corresponding phosphors has been bright blue emission. This is the motivation to seek wider bandgap hosts since blue emission carries the highest energy per photon ( ev) of the three primary colors. We have reported [3] dominant blue electroluminescence (EL) emission from GaN:Tm. Moreover, we have investigated several methods for enhancement of blue emission from GaN:Tm ELDs in both brightness and efficiency, including the photopumping method [4] and optimization [5] of the growth temperature. We have recently reported enhanced blue EL [6] emission and photoluminescence [7] (PL) emission from in-situ Tm-doped Al x N films. This was made possible by using the bandgap engineering method based on the fact that III V alloys of GaN-AlN span a large bandgap energy, ranging from 3.4 to 6.2 ev. Several groups have been studying wider bandgap semiconductors than GaN, primarily AlN: AlN doped with Er, Eu and Tb has resulted in photoluminescence [8] and cathodoluminescence (CL) [9, ], and EL has been reported for AlN:Er []. Very recently, Vetter et al. reported [2] CL from Tm-implanted AlN and Nakanishi et al. reported [3] PL improvement from Tb-doped Al x N (x.5). In this paper, we report on the CL emission from in-situ Tm-doped Al x N films ( x ) and its temperature dependence. 2 Experimental Al x N:Tm films were grown on p-type () Si substrates by molecular beam epitaxy (MBE) with a Ga elemental source and a nitrogen plasma source. Doping was performed in situ during growth from a solid Tm source. The Tm cell temperature was fixed at 6 C resulting in a concentration between ~.2 and ~.5 at.%. Al x N:Tm layers were typically grown for one hour at 55 C with a growth rate of.5.6 µm/hr. Films were grown with various Al compositions ( x ). The * Corresponding author: a.steckl@uc.edu

2 2766 D. S. Lee et al.: CL and its temperature dependence in Tm-doped Al x N thin films growth was performed under slightly N-rich growth conditions:.5 sccm for nitrogen flow rate and 4 W for plasma power. Al composition was controlled by varying the Al cell temperature during growth. The total flux of group III species (Ga and Al) was kept constant since EL properties are reported [4] to be a strong function of V/III ratio during growth. Al composition was determined by the X-ray diffraction (XRD) method. According to Vegard s law [5, 6], the (2) XRD peak of Al x N is a linear function of Al composition between GaN and AlN. Films were grown with various Al compositions, including x = (GaN),.6,.2,.39,.62,.8 and (AlN). 3 Results Fig. shows the CL emission spectrum in log scale from an Al.8 N:Tm film at room temperature (RT). The bias condition was 4 kv and 5 pa. Dominant CL emission peak was observed in the blue region (464 nm) and secondarily in the UV region (369 nm). Both peaks have been reported [6, 7, 2] to come from the same excited state of Tm 3+, namely D nm and 464 nm emissions are due to D 2 transitions, respectively. The nm and 82 nm emissions, not as strong as in EL or PL, are well-known and attributed to G 4 and 3 H 4 transitions, respectively. Note that I 6 related emissions also observed in the spectrum: I 6 (297 nm), I 6 (357 nm) and I 6 (393 nm). Minor peaks were also observed at 527 nm and 684 nm due to D 2 3 H 4 transitions as in EL. Peaks at 594 nm, 738 nm and 928 nm (exactly doubles of 297 nm, 369 nm and 464 nm) are due the spectrometer grating. Note that 3 F 4 and 3 H 4 states are frequently reversed in the literatures, as are I 6 and 3 P. E (ev) 6.5 E (ev) AlN Al.8 N:Tm X=.8 X= CL Intensity (a. u.) (369 2) (297 2) (464 2) X=.4 X= I 6 ( 3 P ) GaN D G H H F 4.5 wavelength (nm) 3 H 6 Fig. CL emission spectrum in log scale from Al.8 N:Tm under 3 kv bias. Strong CL emissions are observed at 369 and 465 nm. Note that weak UV emissions are also seen at 297, 357 and 393 nm which have not been previously observed from EL. Fig. 2 Diagram of energy levels and 4f 4f inner shell transitions of Tm 3+ in Al x Ga -x N:Tm. Strong emission lines at 369 and 464 nm were attributed to the transitions D 2, respectively. The energy levels of 4f 4f inner shell transitions of Tm 3+ in Al x N:Tm are shown in Fig. 2. The bandgap energy in various Al x N alloy compositions from.2 to.8 is shown according to Vegard s law, taking a bowing parameter (b) into account. We have used b =, which is close to the average of many reported [5 7] values. We have attempted an attribution of the transitions for all the emissions based on our current and previous results [6, 7] and other values reported in the literature. Note that I 6 level-related emissions show very good agreement with those from PL results [7] from the same Al x N:Tm films. I 6 level-related emissions were also reported [2] from AlN:Tm by Vetter et al. They also reported dominant D 2 emission at blue region, while Lozykowski et al. reported [8] very weak D 2 emission from GaN:Tm (no D 2 emission and no I 6 level-related emissions).

3 phys. stat. sol. (c) 2, No. 7 (25) / The Al x N:Tm CL emissions were found to change significantly with Al composition. Figure 3 shows a plot of CL luminance for various Al compositions with different current flows at constant bias condition, 5 kv. Curves and dotted lines were drawn for guidance. Overall emission color was yellow from films with lower Al compositions (x.2), which is believed to come from the host, due mainly to defects or dislocations similar to the case of yellow band emission from GaN films. In contrast, films with higher Al composition (x.39) showed blue CL emission and their luminance was proportional to current flow. It is interesting to note that the overall luminance increases with Al composition up to x =.8, and then decreases for AlN:Tm. This is exactly the same trend as in EL and almost the same as in PL. A luminance of 9.4 cd/m 2 and corresponding efficiency of.2 lm/w were measured for the sample with x =.8 under 5 kv bias and µa current flow. This luminance value is comparable to (or better than) reported values from various other Tm host materials under the same bias condition (5 kv): 2.3 cd/m 2 from Ba 2 B 5 O 9 Cl:Tm films [9], cd/m 2 from Y 2 O 3 :Tm on various glass substrates [2] and 3.4 cd/m 2 from Y 2 O 3 :Tm/aluminosilicate [2]. 2 5 kv Bias Blue I = µa Luminance (cd/m 2 ) 5 5 I = 6.4 µa Yellow I = 2.5 µa Fig. 3 Luminance from Al x Ga -x N:Tm films with various Al compositions. Curves and lines are shown for guidance. CL emission looked yellow at lower Al compositions (x.2) and blue at higher Al compositions (x.39). Note that luminance intensity increases with Al composition up to x =.8 and decreases for AlN:Tm, which is the same trend as in EL and PL. Al Composition The CL temperature dependence from Al.8 N:Tm films was investigated from 79 K to 373 K. CL intensity spectra of UV and blue region are shown in Fig. 4. Combined D 2 -related emission (464 nm nm) and combined I 6 -related emission (357 nm nm) are plotted versus temperature in Fig. 5 Al.8 N:Tm Wavelength (nm) CL Intensity (a. u.) 373K 323K 273K 223K 73K 23K 79K Combined CL Intensity (a. u.) Temperature (K) D 2 -related Emission Al.8 N:Tm E A =336 mev I 6 -related Emission E A =6.24 mev Temperature (/K) Intensity Ratio ( D 2 / I 6 ) Fig. 4 CL spectra in UV and blue region from an Al.8 N:Tm film at various temperatures ranging from 79 K to 373 K. Fig. 5 Temperature dependence of combined D 2 - related emission, combined I 6 -related emission and their ratio. Their intensity ratio, D 2 / I 6, is also plotted. The D 2 -related emission experiences an overall increase with temperature up to near RT and then a slight decrease after that. However, over a wide temperature range, from ~5 K to ~35 K, the intensity is essentially constant. The I 6 -related emission is more tem-

4 2768 D. S. Lee et al.: CL and its temperature dependence in Tm-doped Al x N thin films perature-sensitive: it increases with temperature up to 5 K and then decreases drastically reaching a value smaller by almost 2 orders of magnitude at RT. The temperature dependence of the intensity ratio shows two clearly separated regimes with a transition point at ~2 K. While the ratio increases slowly with temperature below 2 K, above 2 K it increases very rapidly. In other words, the I 6 -related emission is becoming quenched upon approaching RT, resulting in a big increase in the ratio. Arrhenius-like thermal activation energies estimated from the fitting were 6.24 mev below ~2 K and 336 mev above ~2K. Vetter et al. reported [2] a temperature trend for the D 2 CL emission similar to which we observed in CL intensity ratio. They concluded, mainly from lifetime measurements, that the increase of the intensity of transitions starting from the D 2 level are due to a population of this level by the upper I 6 level through cross-relaxation. We have reached a similar conclusion. At higher temperatures, the I 6 level could populate the D 2 level by losing energy non-radiatively, either through phonon-assisted transition or cross-relaxation. This is less likely to happen at lower temperatures since the energy difference between two levels seems too large (~.8 ev) to be thermally overcome in that temperature range. However, we believe that more complicated mechanisms are probably involved in the overall process. Additional study is now underway on the CL temperature-dependence from films with various Al compositions. 4 Summary We have obtained dominant blue and UV CL emission from Tm-doped AlGaN. Various CL emission lines from Tm 3+ transitions show very good agreement with those from EL and PL results from the same Al x Ga -x N:Tm films. CL luminance increases with Al composition up to x =.8, but decreases for AlN:Tm, which is the same trend as in EL and PL. Luminance value has been measured to be 9.4 cd/m 2 and corresponding efficiency.2 lm/w. Temperature dependence of CL from the Al.8 N:Tm film shows very interesting behaviour of D 2 and I 6 levels: D 2 emission increases with temperature up to RT, while I 6 emission decreases rapidly upon approaching RT. The quenching of the I 6 emission near RT is believed to be due in non-radiative phonon-assisted or cross-relaxation process. Acknowledgements This work at Cincinnati was supported by ARO and ARL grants. The authors are pleased to acknowledge the support of E. Forsythe, D. Morton and J. Zavada. References [] C. N. King, J. Vac. Sci. Technol. A 4, 729 (996). [2] R. H. Mauch, Appl. Surf. Sci. 92, 589 (996). [3] A. J. Steckl, M. Garter, D. S. Lee, J. Heikenfeld, and R. Birkhahn, Appl. Phys. Lett. 75, 284 (999). [4] D. S. Lee and A. J. Steckl, Appl. Phys. Lett. 8, 233 (22). [5] D. S. Lee and A. J. Steckl, Appl. Phys. Lett. 82, 55 (23). [6] D. S. Lee and A. J. Steckl, Appl. Phys. Lett. 83, 294 (23). [7] U. Hömmerich, E. E. Nyein, D. S. Lee and A. J. Steckl, and J. M. Zavada, Appl. Phys. Lett. 83, 4556 (23). [8] X. Wu, U. Hömmerich, J. D. Mackenzie, C. R. Abernathy, S. J. Pearton, R. N. Schwartz, R. G. Wilson, and J. M. Zavada, Appl. Phys. Lett. 7, 226 (997). [9] W. M. Jadwisienczak, H. J. Lozykowski, I. Berishev, A. Bensaoula, and I. G. Brown, J. Appl. Phys. 89, 4384 (2). [] K. Gurumurugan, H. Chen, G. R. Harp, W. M. Jadwisienczak, and H. J. Lozykowski, Appl. Phys. Lett. 74, 38 (999). [] V. I. Dimitrova, P. G. Van Patten, H. H. Richardson, and M. E. Kordesh, Appl. Phys. Lett. 77, (2). [2] U. Vetter, M. F. Reid, H. Hofsäss, C. Ronning, J. Zenneck, and M. Dietrich, Mater. Res. Soc. Symp. Proc. 743, L6.6. (23). [3] Y. Nakanishi, A. Wakahara, H. Okada, A. Yoshida, T. Ohshima, and H. Itoh, phys. stat. sol. (b) 24, 372 (23). [4] D. S. Lee and A. J. Steckl, Appl. Phys. Lett. 8, 728 (22). [5] M. Stutzmann, O. Ambacher, A. Cros, M. S. Brandt, H. Angerer, R. Dimitrov, N. Reinacher, T. Metzger, R. Hopler, D. Brunner, F. Freudenberg, and R. Handschuh, Mater. Sci. Eng. B 5, 22 (997).

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