Er-doped materials are extensively studied in Si-based photonics, owing to the capability of Er ions to emit and absorb photons at discrete wavelengths, extending from the ultraviolet to the infrared. However the maximum Er amount that can be introduced as a dopant in a solid host is limited to about 10^20 at/cm3 owing to the low solid solubility. Such a limit can be overcome in mixed Y-Er compounds: given the strong similarities between Y and Er compounds, the Er amount can be varied with continuity in a wide range, extending from the low values typical of a doping condition to the extreme values of Er compounds (about 10^22 at/cm3). However the Er content increase certainly leads to the occurrence of Er-Er interactions that determine a strong modification of the emitting properties of the Er ions. We have studied the influence of such interactions in both oxide Y2-xErxO3 and disilicate Y2-xErxSi2O7 thin films, grown on c-Si substrates by rf magnetron co-sputtering. In both compounds the existence of two well-defined Er concentration regimes (defined as Er-doping and Er compound) has been demonstrated, with a threshold value of about 10^21 Er/cm3. Above this limit, the interactions between the excited Er ions and the Er population in the ground state lead to a fast depletion of the high-energy levels with a consequent refilling of the low-energy ones. Although the interactions occurring in both materials are exactly the same, their effects are different. Y2-xErxO3 is a low phonon energy host, and then non-radiative phononic decays have low rates: Er-related optical emission both in the visible and in the infrared regions is then observed. In the doping regime a population inversion condition between the first two excited levels is achieved, opening the route for the realization of optical amplifiers, operating at 2.75 um. However when the Er amount is increased (compound regime) the interactions have a detrimental effect, since the condition of population inversion is lost. In the high-phonon energy host Y2-xErxSi2O7 only Er-related emission at 1.54 um is observed for any x value. In this material the Er-Er interactions are demonstrated to produce a quantum cutting process in which it is possible to excite several Er ions with a single incoming excitation photon. In particular, in the Er-richest film (Er2Si2O7) maximum quantum cutting efficiencies of 400% have been reached. In this regime this material can be exploited therefore in Ge solar cells, thanks to the generation of several infrared photons at expenses of only one incident visible photon.

1. R. Lo Savio, M. Miritello, P. Cardile, F. Priolo
Er-Er interactions in
yttrium-erbium compounds
thin films
MATIS CNR-INFM, Catania, Italy
Università di Catania, Catania, Italy
Scuola Superiore di Catania, Catania, Italy

2. Outline
 Optical applications of Er-based materials:
• Si-based optical communications
• Photovoltaics
 Role of Er-Er interactions
 Mixed Y-Er compounds:
• Oxide, Y2-xErxO3
• Disilicate, Y2-xErxSi2O7
 Thin films synthesis
 Structural and optical properties
 Influence of Er-Er interactions on the Er3+ optical emission
 Optical amplification and quantum cutting
 Conclusions

3. Erbium for optical applications
4
I9/2
4
F9/2
4
S3/2
4
F7/2
4
I11/2
4
I15/2
4
I13/2
0.56 mm
0.66 mm
0.98 mm
1.54 mm
0.49 mm
0.80 mm
Optical amplifiers
Optoelectronic devices
Visible phosphors
Photovoltaic
VISIBLE
EMISSION
INFRARED
EMISSION

4. 4I13/2
4I15/2
1.54 μm photon emission
and amplification
Population inversion between
4I13/2 and 4I15/2 states
 High-phonon energy materials
 Er ions excited to 4I13/2 level
 Reduce decay rate of 4I13/2 level
Erbium in optical communications
Fast non-radiative
phononic decays

5. Erbium in photovoltaics (i)
Semiconductor
solar cell
Modify the solar spectrum
Si band gap
Absorption of photons with hn < EG
Transform infrared photons
into visible photons

6. Erbium in photovoltaics (ii)
Ge band gap
Semiconductor solar cell
Carriers thermalization
Transform visible photons
into infrared photons
Modify the solar spectrum

7. Er-Er interactions
Dipole-dipole interactions Strongly depends on the Er content
Cross-relaxation
0
5
10
15
energy(10
3
cm
-1
)
4
I9/24
I11/2
4
I15/2
4
I13/2
 High Er content
Up-conversion
0
5
10
15
energy(10
3
cm
-1
)
4
I9/24
I11/2
4
I15/2
4
I13/2
 High Er content
 High external pumping

8. Er-doping and Er compounds
A. Polman et al., J. Appl. Phys. 70, 3778 (1991)
Er-doping
Er:SiO2
Low solubility
in solid hosts
Er compounds
a-Er2Si2O7 crystalline structure
Fixed [Er]
Erbium content
1020 at/cm3
1022 at/cm3

9.  Same chemical composition
 Same crystalline structures
Y and Er in solid hosts:
 Same trivalent state 3+
 Similar ionic radius
Varying Er content in
mixed Y-Er compounds
[Er] = 1020 – 1022 cm-3
Mixed Y-Er compounds
Er ions are substitutional in Y sites
J. Hoang et al., J. Appl. Phys. 101, 123116 (2007)
RE oxides (RE2O3)
and silicates (RE2O3 + SiO2)

10. Y2O3
SiO2
Er2O3
CONFOCAL CO-SPUTTERING
UHV magnetron sputtering
Er2O3
c-Si
RFRFRF
 ULSI compatible
 Planar thin films
Careful control of films composition
rotating
heated
(400 °C)
Thin films synthesis

11.  Y-Er oxide
 Y-Er disilicate

12. 20 30 40 50 60
0
200
400
600
800
x = 0.01
x = 0.57
Intensity(cps)
2 (degree)
(622)
(541)
(440)
(431)
(422)
(222)
(211)
1.5 1.6 1.7 1.8 1.9
0
10
20
30
40
50
60
Y Er
x = 0.05
x = 0.17
x = 0.57
NormalizedCounts
Energy (MeV)
RBS spectra
th ≈ 120 nm
 [Y]+[Er] is constant
 Sesquioxide composition
 [Er] = 1.2×1020 - 1.03×1022 at/cm3
Y-Er oxide films composition
R. Lo Savio et al., J. Appl. Phys. 106, 043512 (2009)
0.0 0.4 0.8 1.2 1.6 2.0
10.56
10.59
10.62
10.65
10.68
a(Å)
x
a(Y2O3)
a(Er2O3)
 Lattice parameter
linearly decreases
XRD analysis
Y2-xErxO3
x = 0.01 - 0.72
 bcc structure
of RE oxides

13. 0.5 0.6 0.7 1.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
x = 0.05
4
I11/2
PLIntensity(a.u.)
4
F9/2
4
S3/2

exc
= 488 nm
 = 5 10
20
/cm
2
s
1.0 1.4 1.5 1.6
Wavelength (mm)
4
I13/2
RT PL spectra
PL emission in Y2-xErxO3
4
F9/2
4
S3/2
4
F7/2
4
I11/2
4
I15/2
4
I13/2
488 nm
0.56 mm
0.66 mm
0.98 mm
1.54 mm
Low phonon energy
LESS-PROBABLE
NON-RADIATIVE DECAYS

14. 0.5 0.6 0.7 1.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
x = 0.05
4
I11/2
PLIntensity(a.u.)
4
F9/2
4
S3/2

exc
= 488 nm
 = 5 10
20
/cm
2
s
1.0 1.4 1.5 1.6
Wavelength (mm)
4
I13/2
RT PL spectra
PL emission in Y2-xErxO3
10
19
10
20
10
21
10
22
10
23
10
0
10
1
10
2
10
3
 (cm
-2
s
-1
)
0.56 mm
0.66 mm
0.98 mm
1.54 mm
IPL
(a.u.)
slope 1
Linear pumping regime
n
PL excI P
n → number of involved photons

15. 0.5 0.6 0.7 1.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
x = 0.05
x = 0.17
4
I11/2
PLIntensity(a.u.)
4
F9/2
4
S3/2
exc
= 488 nm,  = 5 10
20
/cm
2
s
1.0 1.4 1.5 1.6
Wavelength (mm)
4
I13/2
Er-Er interactions at low fluxes
4S3/2depletion4
I9/2
4
I15/2
4
I13/2
4
S3/2
 Depends only on the Er content
 Occurs also in the linear regime
Er excited + Er in the ground state
Cross-relaxation
CR1
RT PL spectra

16. 0.5 0.6 0.7 1.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
x = 0.05
x = 0.17
x = 0.57 
4
I11/2
PLIntensity(a.u.)
4
F9/2
4
S3/2
exc
= 488 nm,  = 5 10
20
/cm
2
s
1.0 1.4 1.5 1.6
Wavelength (mm)
4
I13/2
Visible PL
is quenched
4
I9/2
4
I15/2
4
I13/2
4
S3/2
4
I11/2
4
I15/2
4
I13/2
4
F9/2
CR1 CR2
Er-Er interactions at low fluxes
 Refilling of low-energy levels
at expenses of high-energy ones
most of
excited Er3+
R. Lo Savio et al., J. Appl. Phys. 106, 043512 (2009)
RT PL spectra

17. 0.0 0.2 0.4 0.6 0.8
0
2
4
6
0 4 8 12 16
Er (at.%)
x
det
= 0.98 mm
det
= 1.54 mm
(ms) Energy migration and quenching
4
I11/2
4
I15/2
4
I13/2
2
1
0
Er3+ -OH
Increase of 4I11/2 and 4I13/2 decay rates
a,b) energy migration;
c) IR photon emission;
d) non-radiative
energy dissipation.
-OH
a
a
b
c
b d
Concentration quenching
c)

18. 0.0 0.2 0.4 0.6 0.8
0
1
2
3
4
5
R=
-1
(10
4
s
-1
)
x
 
 
6
6
6 6
1
2 64Er Er
Er Er
d high
C
d d low


   
1
  Er Er q ErR C N N

E. Snoeks et al., Opt. Mater. 5, 159 (1996)
Er-concentration regimes
Slope increase
by a factor of 64
Er-doping (x < 0.5) → Er-O-Y units
Er compound (x > 0.5) → Er-O-Er units
det = 1.54 mm
Same environment
of Er2O3

19. Infrared PL
Max IPL at 0.98 mmEr-doping
Population inversion
N(4I11/2) > N(4I13/2)
Er compound Max IPL at 1.54 mm
4
I9/2
4
I15/2
4
I13/2
CR3
PLANAR OPTICAL
AMPLIFIERS AT 2.7 mm
4
I11/2
4
I15/2
4
I13/2
0.0 0.2 0.4 0.6 0.8
0
30
60
90
0 4 8 12 16
Er (at.%)
x
det
= 0.98 mm PL
det
= 1.54 mm PL
IPL
(a.u.)
R. Lo Savio et al., J. Appl. Phys. 106, 043512 (2009)

20. 0.50 0.55 0.60 0.65 0.70 0.75
0.00
0.05
0.10
0.15
4
F9/2
exc
= 980 nm
PLIntensity(a.u.)
Wavelength (mm)
x = 0.05
x = 0.17
x = 0.41
x = 0.57
4
S3/2
Visible up-conversion
High up-converted PL
for x < 0.57
0
5
10
15
20
(4)
(3)
(2)
(1)
980 nm
energy(10
3
cm
-1
)
4
F7/2
4
S3/2
4
F9/2
4
I9/2
4
I11/2
4
I15/2
4
I13/2
560 nm
(1)
UP-CONVERSION
exc > emission

21.  Y-Er oxide
 Y-Er disilicate

22. 0.6 0.7 0.8 0.9 1.0 1.1 1.6 1.7 1.8 1.9
0
10
20
30
40
50
ErYSi
x = 0.17
x = 0.65
x = 1.15
NormalizedYield(a.u.)
Energy (MeV)
O
 Total [Y]+[Er] is constant
 Disilicate-like composition
 [Er] = 2×1020 - 1.5×1022 at/cm3
RTA 1200 °C, 30 s, O2
Y-Er disilicate
Y2-xErxSi2O7
x between 0.03 and 2.00
 Stable composition under reactive annealing
 Higher optical emission at 1.54 mm
R. Lo Savio et al., Appl. Phys. Lett. 93, 021919 (2008)

23. 15 20 25 30 35 40 45 50 55 60
0
200
400
600
800
1000
Intensity(cps)
2 (degree)
x = 0.03
x = 1.15
x = 2
a phase
y phase
RE2Si2O7 amorphous y α β γ
Y-Er disilicate
R. Lo Savio et al., Appl. Phys. Lett. 93, 021919 (2008)

24. 1450 1500 1550 1600 1650
0
2
4
6
8
10
12
14
x = 0.03
x = 0.65
x = 2.00

exc
= 488 nm
 = 10
19
cm
-2
s
-1
PLIntensity(a.u.)
Wavelength (nm)
PL emission in a-Y2-xErxSi2O7
Highest PL intensity
for Er disilicate
 PL emission only at 1.54 mm
 Peak shape characteristic of a-phase
 Maximum efficiency is associated to Er in a-Er2Si2O7 structure

25. 10
18
10
19
10
20
10
21
10
22
10
-1
10
0
10
1
10
2
10
3
10
4
IPL
at1.54mm(a.u.)
x = 0.03
x = 0.65
x = 2.00
 (cm
-2
s
-1
)
Up-conversion in a-Y2-xErxSi2O7
Sublinear IPL increase
Rate equation for the 4I13/2 level
 
 1 1 1 1
PL 1
1
1 2 8 N (1 )
I N
4
        
  

up
up
C
C
  
21 1
1 up 1
1
dN N
(N N ) 2C N 0
dt
     

4I15/2
4I13/2
 excitation
 de-excitation
 up-conversion, up = (CupN1)-1
  
Cup
Cup

26. 10
18
10
19
10
20
10
21
10
22
10
-1
10
0
10
1
10
2
10
3
10
4
10
15
10
16
10
17
10
18
10
19
10
20
IPL
at1.54mm(a.u.)
x = 0.03
x = 0.65
x = 2.00
 (cm
-2
s
-1
) N1
(at/cm
3
)
Up-conversion in a-Y2-xErxSi2O7
  = 0 = 2.2 × 10-21 cm2
Cup = (2.3±0.6)×10-17 cm3/s
 Low Cup (similar to Er:SiO2)
 High excited Er fraction (≈ 10%)
PLANAR OPTICAL
AMPLIFIERS AT 1.54 mm
Absorption cross section
at 488 nm for Er:SiO2
4I15/2
4I13/2

27. 10
18
10
19
10
20
10
21
10
22
10
-1
10
0
10
1
10
2
10
3
10
4
10
15
10
16
10
17
10
18
10
19
10
20
IPL
at1.54mm(a.u.)
x = 0.03
x = 0.65
x = 2.00
 (cm
-2
s
-1
) N1
(at/cm
3
)
Cup = (2 ± 1)×10-17 cm3/s
Cup = (6 ± 1)×10-16 cm3/s
Cup = (1.1 ± 0.1)×10-15 cm3/s
Er-Er interactions in the linear regime
  0 = 2.2 × 10-21 cm2
  0 = 4.4 × 10-21 cm2
  30 = 6.6 × 10-21 cm2
Excitation cross section
depends on Er content!!!
Cup and  increase
implies a common root?
4I15/2
4I13/2

28. 0.0 0.4 0.8 1.2 1.6 2.0
0 4 8 12 16
1
2
3
4
NEr
( 10
21
at/cm
3
)
exc
= 488 nm
/0
x
Excitation cross section
Linear pumping regime
PL Er
RAD
I N

 

RAD → independent on NEr
PL
Er RAD
I
N

  
 
x = 0.03  = 0

29. 0.0 0.4 0.8 1.2 1.6 2.0
0 4 8 12 16
1
2
3
4
NEr
( 10
21
at/cm
3
)
exc
= 488 nm
/0
x
Quantum cutting
0.0
0.5
1.0
1.5
2.0
2.5
Energy(eV)
4
F7/2
4
I15/2
4
I13/2
Low NEr(x < 0.65)
1 excitation per photon
pump

30. Quantum cutting
Medium NEr(0.65 ≤ x < 2)
2 excitations per photon
0.0 0.4 0.8 1.2 1.6 2.0
0 4 8 12 16
1
2
3
4
NEr
( 10
21
at/cm
3
)
exc
= 488 nm
/0
x
pump
int <  Cross-relaxations
 = 5 ms
int = 0.5 ms
int = (NErCup)-1
 = 0.2 ms

31. Quantum cutting
4
I15/2
4
I13/2
4
I15/2
4
I13/2
High NEr (x = 2)
3 excitations per photon
2 cross-relaxations
0.0 0.4 0.8 1.2 1.6 2.0
0 4 8 12 16
1
2
3
4
NEr
( 10
21
at/cm
3
)
exc
= 488 nm
/0
x
pump
 = 5 ms
 = 0.2 ms
int = 0.05 ms
Maximum excitation
efficiency of 300 %

32. 0 4 8 12 16
1
2
3
4
0.0 0.4 0.8 1.2 1.6 2.0
/0
NEr
(×10
21
cm
-3
)

exc
= 380 nm
x
Quantum cutting
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Energy(eV)
4
F7/2
4
I9/2
4
I11/2
4
I15/2
4
I13/2
2
G11/2
2
H9/2
4
F5/2
4
S3/2
pump
380 nm
4
I15/2
4
I13/2
4
I15/2
4
I13/2
4
I15/2
4
I13/2
Maximum excitation
efficiency of 400 %

33. 0 4 8 12 16
1
2
3
4
0.0 0.4 0.8 1.2 1.6 2.0
/0
NEr
(×10
21
cm
-3
)

exc
= 380 nm
x
Quantum cutting
Ge

34. Conclusions
Y-Er OXIDE
 Two distinct regimes exist: Er-doping and Er compound.
 In the Er compound regime cross-relaxations deplete the high-energy levels.
Optical amplification at 2.7 mm
0.0 0.2 0.4 0.6 0.8
0
30
60
90
0 4 8 12 16
Er (at.%)
x
det
= 0.98 mm PL
det
= 1.54 mm PL
IPL
(a.u.)
0.5 0.6 0.7 1.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
x = 0.05
x = 0.17
x = 0.57 
4
I11/2
PLIntensity(a.u.)
4
F9/2
4
S3/2

exc
= 488 nm,  = 5 10
20
/cm
2
s
1.0 1.4 1.5 1.6
Wavelength (mm)
4
I13/2
Visible emitter for x < 0.57

35. Conclusions
Y-Er DISILICATE
 Two distinct regimes exist: Er-doping and Er compound.
 In the Er compound regime cross-relaxations deplete the high-energy levels.
Quantum cutting effect with an
excitation efficiency up to 400 %
Correlation between  and Cup increase
10
18
10
19
10
20
10
21
10
22
10
-1
10
0
10
1
10
2
10
3
10
4
10
15
10
16
10
17
10
18
10
19
10
20
IPL
at1.54mm(a.u.)
x = 0.03
x = 0.65
x = 2.00
 (cm
-2
s
-1
)
N1
(at/cm
3
)
0 4 8 12 16
1
2
3
4
/0
NEr
(×10
21
cm
-3
)

exc
= 380 nm