Transcript 幻灯片 1 - Fudan University

Enhanced broadband emission from the Er-Tm codoped ZnO film
due to energy transfer processes involving Si nanocrystals
Yu Pu, Zuimin Jiang
State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, PRC.
Introduction
Er-doped semiconductors have drawn considerable attentions since 1980s when Ennen et al. reported the 1.53μm emission at low temperature from Er-implanted III-V semiconductors and
silicon. This wavelength from the transition of Er intra-4f shell is especially important because it locates in the optical communication with minimum loss . As the increasing demand of
information traffic requires the development of wide band integrated amplifiers in order to implement wavelength division multiplexing (WDM) technology, several rare earth (RE) ions with
overlapping emission bands can be used to provide a route to achieve such broadband emission. Since Tm-doped materials show emission in the 1.4-1.5 μm (3H4 → 3F4) and 1.8-2.0 μm (3F4 → 3H6)
regions, it provides an excellent complement to Er3+ ions .
ZnO is chemical and thermo-mechanical stable and it is considered that REs-codoped ZnO films have been proved to be a promising candidate for optoelectronic devices and electrode
materials. However, Er-Tm codoped ZnO has rarely been studied. Meanwhile, due to the low emission efficiency of Er3+ and Tm3+ doped in ZnO, the practical use in the optoelectronic devices has
been hampered. In order to increase the intensity of RE-related emission, Si nanocrystals (Si-NCs) have been chosen to codoped as sensitizers.
We fabricated Er-Tm-Si codoped ZnO (ETSZO) film by magnetron sputtering. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Transmission electron microscope (TEM)and
Electron diffraction pattern (EDP) showed that Si nanoparticles (Si-NPs) have been formed after annealing at 900℃ for 30 minutes under N2 ambient. PL spectra of Er3+ and Tm3+ in this film were
investigated which showed a very broad and flat emission band with ~375 nm bandwidth could be achieved from this film. The Tm- and Er- related PL intensities at 1.8 μm (3F4 → 3H6) and 1.53
μm (4I13/2 → 4I15/2) were both enhanced by an order of magnitude. In order to have a better understand of the properties of this emission at different operating temperature, the temperature
dependence of photoluminescence (PL) spectra from 300 to 20 K has also been investigated which showed that emission from Er3+ had a weak temperature quenching effect in contrast to that of
Tm3+. Three energy transfer (ET) from Si-NCs to Er3+/Tm3+ and from Er3+ to Tm3+ as well as their back transfer (BT) processes are proposed to explain the emission enhancement and temperature
behavior. It should be helpful to understand the interaction among Er3+, Tm3+, and Si-NCs.
Fig.1 (a) Concentration of Si, Er, Tm, O and ZnO at different
depth measured by XPS and XPS spectra of (b) Si 2p, (c) Er 4d
and Tm 4d, (d) O 1s, (e) ZnO 2p3 at the depth of 9 nm from the
surface. It can be seen the Er3+ and Tm3+ are distributed almost
uniformly and the average concentration of Si, Er, Tm, O and Zn
in the ETSZO film are about 6.2, 2.4, 1.7, 47.5 and 42.2 at.%.
Fig.4 Temperature dependence of PL spectra of the
ETSZO and Er-Si codoped ZnO (ESZO) films at
different operating temperature. The red circles and
black squares represent the peak intensity of Tm3+ and
Er3+ from the ETSZO film respectively. The green
triangles represent the peak intensity of Er3+ from the
ESZO film. The IPL of Tm3+ ions versus the
temperature can be well fitted by using an equation
with two activation energy :21 and 91 meV. Moreover,
as we can see the temperature dependence behavior
of peak intensity of Er3+ in the ETSZO film is
different from that in the ESZO film and it nearly
keeps constant as the temperature decreases.
Fig.2 (a) Raman spectra and (b) Cross-section TEM image of
the ETSZO film. Random arranged Si-NCs at the size of 3~6 nm
embedded in the ZnO matrix can be identified in the regions
marked with circles. The inset EDP shows the diffraction
circles of Si-NCs and no existence of the diffraction circles or
spots of Er or Tm precipitates.
Fig.3 Near-infrared PL spectra of the ETSZO and Er-Tm
codoped ZnO (ETZO) films annealed at 900℃. The
intensity of the broadband emission from the ETSZO film
has been enhanced by nearly an order of magnitude
compared to the ETZO film at the excitation line of 795
nm
Fig.5 The temperature dependence of the bandwidth of the
broadband emission from the ETSZO film changes in the
range of 334 - 430 nm which is relatively stable.
Conclusion
A broadband emission with 375 nm bandwidth due to the
1533 nm emission from Er3+ (4I13/2→4I15/2) and the 1800 nm
emission from Tm3+ (3F4→3H6) has been obtained. The
emission intensity is enhanced by an order of magnitude
due to the energy transfer from Si-NCs to Er3+ and Tm3+
ions located around which demonstrates Si-NCs can be
sensitizers for both Er3+ and Tm3+. The temperature
dependence of the PL spectra of the Er-Tm-Si codoped ZnO
film has also been investigated and the possible ET and BT
processes among Er3+, Tm3+ and Si-NCs have been
discussed.
Fig.6 The dominant ET processes among Si-NC, Er3+ and Tm3 are
described as following:
ET1 : Si-NC: a bound exciton, Er3+: 4I15/2  Si-NC: the exciton
recombination, Er3+: 4I13/2 + phonon
ET2 : Si-NC: a bound exciton, Tm3+: 3H6  Si-NC: the exciton
recombination, Tm3+: 3F4 + phonon
ET3: Er3+: 4I13/2, Tm3+: 3H6  Er3+: 4I15/4, Tm3+: 3F4 + phonon
The energy back-transfer of ET1, ET2 and ET3 described as following:
BT1: Er3+: 4I13/2 + phonon  Si-NC: a bound exciton, Er3+: 4I15/2
BT2: Tm3+: 3F4 + phonon  Si-NC: a bound exciton, Tm3+: 3H6
BT3: Er3+: 4I15/2, Tm3+: 3F4 + phonon  Er3+: 4I13/2, Tm3+: 3H6