Enhanced broadband infrared emission of erbium–thulium codoped ZnO films with Si Nanocrystals as broadband sensitizers

Xiao Wang

1

, Fei Xu

2

, Yongliang Fan

1

, Mingzhu Li

1

, Fang Lu

1

and Zuimin Jiang

1

1 Surface Physics Laboratory (National Key Laboratory), Fudan University, Shanghai 200433, China 2 SHU-SolarE R&D Lab, Department of Physics, College of Sciences, Shanghai University, Shanghai 200444, China

Er Tm Si codoped ZnO bilayer film, consisting of Er-Si-codoped ZnO (Er: Si: ZnO) and Tm-Si-codoped ZnO (Tm: Si: ZnO) layer, has been synthesized by co-sputtering from separated Er, Tm, Si, and ZnO targets. The dependence of Er 3+ Tm 3+ related photoluminescence (PL) properties on annealing temperatures over 800 to 1000 ℃ was studied. A flat and broad emission band in the 1400–1700 nm was achieved, both in the Er-Tm and Er-Tm-Si codoped films. And the observed 1533 and 1800 nm emission bands were attributed to the transitions of Er 3+ : 4 I 13/2 → 4 I 15/2 and Tm 3+ : 3 F 4 → 3 H 6 , respectively. The Tm and Er related photoluminescence (PL) intensities at 1800nm and 1533nm are both enhanced when codoping with Si nanocrystal (Si-NC) as broadband sensitizer thus suggesting the existence of multiple energy transfer processes from Si-NC to Er and Er to Tm in ZnO film.

12 900 0 C 950 0 C 1000 0 C 4 Er: Si: ZnO Er: Tm: Si: ZnO Er: Tm: ZnO 8 2 4 0 1400 1600 1800 Wavelength (nm) 2000

Figure 1

The near-infrared RT PL spectra of the Er: Tm: Si: ZnO bilayer film annealed at different temperatures at the excitation of 795 nm. A flat and broad emission band in the 1400–1700 nm was achieved.With increasing annealing temperature, the intensity of Er 3+ PL from the bilayer film becomes lower, and the intensity of Tm 3+ becomes higher.

8 a 4 0 8 b 4 Er 0 8 c 4 0 460 480 500 Wavelength (nm) 520

Figure 3a-c

The Er 3+ PL peak intensity as a function of excitation energy for the Er: Tm: Si: ZnO bilayer film annealed at 900℃, 950℃ and 1000℃, respectively. The Er 3+ PL peak intensity of the bilayer film anealed at 900 ℃ excited by 488nm is one time higher than the other four wavelengths. It clearly indicates that the excitation of Er 3+ of the bilayer films annealed at 950℃ and 1000℃ could be attributed to the broad absorption by Si-NCs rather than the direct absorption by Er 3+ , while the Er 3+ PL peak intensity of bilayer film annealed at 900℃ was attributed to the direct absorption by Er 3+ .

0 1400 1600 1800 Wavelength (nm) 2000

Figure 2

The room temperature （ RT ） PL spectra of the Er Tm Si codoped ZnO bilayer film and Er Tm codoped ZnO bilayer annealed at 900 ℃ with [Tm]/[Er] concentration ratios of 2. The spectrum for the Er Si codoped ZnO film also shown as reference. It is interesting to note that when codoping with Si, the PL intensity of Er 3+ and Tm 3+ from the bilayer film both increased by 2 times. When codoping with Tm ， the PL intensity of Tm 3+ from the bilayer film is observed, but also the PL intensity of Er 3+ is lower than that from Er Si codoped ZnO film.

4 2 Si 102 Si 321 Si-I 520 805 Si ZnO 979 1000c 900 ℃ 0 200 400 600 800 1000 1200 950c 4 2 0 4 2 0 200 400 Si-I 520 600 A1(LO)574 800 1200 900c 200 400 600 800 1000 E1 （ 2LO ） 1140 1000 1200 850c 4 2 0 200 400 600 800 1000 1200 Raman shift(cm-1)

Figure 4

Raman spectra and TEM images of the Er: Tm: Si: ZnO bilayer film annealed at different temperatures. Two Raman modes of the film annealed at 850 and 900 ℃ are observed at 581 and 1156 cm -1 , and no Raman line of E2 (high) at about 437 cm -1 is observed, which means that the films might be amorphous. When the annealing temperature is above 950 ℃, two peaks located at 581 and 1156 cm -1 disappear, and the peaks located at 102, 520cm -1 which related to Silicon and 982 cm -1 which related to ZnO are observed. The TEM images of the bilayer film also shows that the film annealed at 900 ℃ is amorphous, and the film annealed at 950 is crystallized.

Conclusion

Er Tm codoped ZnO bilayer film has been synthesized by magnetron sputtering. A flat and broad emission band in the 1400–1700 nm was achieved. Upon Er-Tm-Si codoping, the PL intensity of Er 3+ and Tm 3+ from the bilayer film both increased by 2 times. Codoping with Tm leads to a reduction of the intensity of the Er related band at 1533 nm and to an intensity enhancement of the Tm related emission bands at 1800 nm. It indicates that there are multiple energy transfer processes from Si-NC to Er and Er to Tm in ZnO film. With increasing annealing temperature, the intensity of Er 3+ PL from the bilayer film becomes lower, and the intensity of Tm 3+ becomes higher. It may be due to the the phase transition of the films.