Microcrystalline silicon thin films doped with erbium were produced by RF sputtering and their structural, chemical and optical properties were studied by X-ray diffractometry at grazing incidence, Rutherford back scattering and optical transmission spectroscopy. The samples exhibit a sharp photoluminescence (PL) spectrum from the Er centres with the strongest peak positioned at 1.536 microm with a full width at half maximum of about 8 nm.  When the temperature varies between 5K and 300K the photoluminescence decreases only five fold, in contrast to the behaviour reported for monocrystalline silicon

Cerqueira, M. F.

Ferreira, J. A.

Stepikhova, M.

Materials Science and Engineering B

Universidade do Minho: RepositoriUM

Materials Science and Engineering B81 (2001) 32–35    Photoluminescence of erbium doped microcrystalline silicon thin films produced by reactive magnetron sputtering   M. F. Cerqueira*1, M. V. Stepikhova2 and J. A. Ferreira1  1Physics Department, University of Minho, Largo do Paço 4700-320 Braga Portugal 2Institute for Physics of Microstructures RAS, 603600 Nizhnij Novgorod GSP-105 Russia    Abstract  Microcrystalline silicon thin films doped with erbium were produced by RF sputtering and their structural, chemical and optical properties were studied by X-ray diffractometry at grazing incidence, Rutherford back scattering and optical transmission spectroscopy. The samples exhibit a sharp photoluminescence (PL) spectrum from the Er centres with the strongest peak positioned at 1.536 m with a full width at half maximum of about 8 nm.  When the temperature varies between 5K and 300K the photoluminescence decreases only five fold, in contrast to the behaviour reported for monocrystalline silicon.  Keywords: Photoluminescence, Erbium, nanocrystalline silicon  1. Introduction  During the last 10 years, modern growth techniques have made possible the fabrication of semiconductor nano-structures of typical size < 10nm, with spatial confinement of electrons in all three dimensions. These systems are of considerable current interest because of the new physics involved and potential device applications. One of these applications relates to Si-based optoelectronics. Erbium doped silicon is a recognised candidate to become a material for emitters and detectors to be used in optical communication systems. However, there are problems with efficient energy transfer from the Si matrix to the Er centres at room temperature. The use of Er-doped Si nano-crystallites can help to overcome these problems [1]. Studies of the photoluminescence of Er-doped nanocrystalline silicon constitute an important probe for these applications.   2. Experimental  Erbium doped microcrystalline silicon thin films were grown by reactive magnetron sputtering on ordinary glass substrates using a procedure similar to that used for the preparation of undoped c-Si:H    *Corresponding author: tel: +253 604332; fax: +253678981; e-mail: fcerqueira@fisica.uminho.pt films [2], but adding small pieces of metallic erbium to the c-Si target. The target used was a c-Si with high purity (99.99%). The erbium was placed in a low erosion area of the silicon target, in order to Create PDF files without this message by purchasing novaPDF printer (http://www.novapdf.com)obtain low concentrations. The substrate target distance was fixed at 55 mm. Two series of samples were grown, for different amounts of erbium on the c-Si target. In each series we have changed basically the substrate temperature since it is this parameter that control the crystal grain size [3]. Growth conditions are presented in Table 1. The first series  (Er1,2 and 3) was grown in a argon rich atmosphere and the second series (Er4,5 and 6) in a hydrogen rich atmosphere.  Table 1 Growth conditions  Sample Temperature (ºC) RF power (W) H2 (cm3/min) Ar (cm3/min) Er1 250 40 20 100 Er2 350 40 20 100 Er3 100 40 20 100 Er4 250 80 50 30 Er5 350 80 50 30 Er6 100 80 50 30  For the structural characterisation we used X–ray  diffractometry at the grazing incidence geometry. The voltage was 40 kV (35mA), the incidence angle was fixed at 0.5º, the step was 0.04º and the acquisition time by step was 5.0 s. The range studied was from 23 to 71º. The excitation used was the Cobalt line with  = 1.78896 Ă. The chemical composition was determined by Rutherford Back Scattering (RBS) technique. The thickness was evaluated from the analysis of the near I.R. transmission spectra of the films. The Photoluminescence study was done with a high resolution Fourier set-up (BOMEM spectrometer DA8) using the 514.5 nm line of an argon laser for excitation.   3. Results and discussion  3.1 Structural and chemical characterisation From the interference pattern of the transmission spectra the thicknesses of the films were obtained using the Swanepoel method [4]. Comparing the transmission spectra we verify that the samples grown in a rich argon (Ar) atmosphere show a larger number of interference fringes, compared with the samples grown in a rich hydrogen (H2) atmosphere, indicating that the thickness of those films is higher. This is due to the high sputtering efficiency of the argon atoms compared with the hydrogen atoms. The chemical composition (see Table 2) was determined using the Rutherford Back Scattering technique. The percentage of Er and Si obtained from RBS is very accurate, but the values for oxygen are only approximate. It is nevertheless clear that for the samples grown in a H2 rich atmosphere and high temperatures (Er4 and Er5), the percentage of Si and O are comparable. We know from previous studies by ellipsometry that these nanostructured samples have a SiO top layer of approximately 60-100nm. For the same initial amount of Er in the sputtering target, the concentration of the dopant in the films is highest for the lowest substrate temperature. Our samples have also been studied at room temperature by X-ray diffractometry at grazing incidence. The samples were annealed at 500ºC for 30 minutes in N2 atmosphere. The X-ray experiments were done on the annealed and as-grown samples.  All the samples show the broad band related to the silicon amorphous matrix. The as-grown samples do not exhibit any structure. The (111) diffraction peak of c- Si is only visible, albeit with low intensity, in the Er5 annealed sample. The diffraction peak analysis, by fitting a pseudo-Voigt function [2] to the (111) c-Si diffraction peak gives an average Create PDF files without this message by purchasing novaPDF printer (http://www.novapdf.com)crystal size of 270 Ă. Previous studies have shown that the band gap for crystals of this size is higher than 1.9 eV.  Table 2 Atomic concentration and thickness for different samples  Sample Er (%) Si (%) O (%) Ar (%) Thickness (nm) Er1 4.0 76 19 1 722 Er2 4.1 79 16 1 777 Er3 4.7 78 16.3 - 786 Er4 0.3 50.7 49 - 270 Er5 0.3 42.7 57 - <200 Er6 0.7 77 22 - 407  3.2 Photoluminescence study The photoluminescence of these films was measured using the excitation wavelength 514.5 nm of Ar laser.  The laser power was 200mW and the measurement temperature was 5K, except for the temperature dependence study. a) b) Figure 1 - a) Photoluminescence for the annealed Erbium doped microcrystalline sample. b) Photoluminescence behaviour with temperature for Er5 annealed sample.  Figure 1a) shows the photoluminescence spectrum of the annealed samples. These spectra are characterised by a peak around 1.536 m. This peak is the signature of the erbium centres. The PL intensity of each sample increases by several orders of magnitude if the sample is annealed, especially in the case of Er5 sample. The increase of PL after annealing could be due the formation of Si nanocrystals during the annealing, to the annealing of some structure defects or to the decrease of hydrogen or oxygen concentration during the annealing process. According to the X-ray study we think that the first possibility is indeed true for Er5, which means that with the annealing we promote the crystallisation of some silicon, although for the other samples this is not the case. In Fig. 1b) we present the result of the photoluminescence temperature study for the Er5 annealed sample. Increasing the temperature, from 5K to 300K, the PL intensity decreases only 5 times. This 1 .2 1 .3 1 .4 1 .5 1 .6 1 .702004006008001000120014001600180020002200240026002800300032003400E r1ann.E r2ann .Er3ann.E r6ann .E r4ann .E r5ann .1 .536µmPL intensity (arb. u.)W a ve le n g th  (µ m ) 1.3 1.4 1.5 1.6 1.7 1.8 1.9050010001500200025003000300K280K240K160K80K5KPL intensity (arb. u.)W ave len gth  (µ m)-10 0 10 20 30 40 50 60 70 80 90 1001000PL intensity (arb. u.)1 000/T, K - 1Create PDF files without this message by purchasing novaPDF printer (http://www.novapdf.com)temperature effect is attributed to thermally activated back transfer to the host and is less pronounced than in monocrystalline silicon [5]. This kind of behaviour is promising for applications but the interpretation of the PL origin is difficult. There exist two possibilities, concerning the location of the Er centres, namely the Er ions could be in the amorphous matrix or in the nanocrystals. Since all the samples (except Er3) show photoluminescence and only in the Er5 annealed sample there is evidence of the presence of nanocrystals, it appears that the Er centres are located in the a-Si:H matrix for Er1,2,4,6 samples. The fact that PL quenching in all the samples studied, be lower than in monocrystalline silicon can be explained by the higher energy band gap [5, 6] of the amorphous silicon (around 1.6 eV for sputtered or evaporated material and for a typical hydrogen content – 20 at%) compared with 1.1 eV for c-Si. The back transfer process will then take place only at higher temperatures. This effect is even more remarkable for the nanocrystalline sample, where the energy gap is higher and where some Er can be located in the nanocrystalline phase. Figure 2 shows the correlation between the photoluminescence intensity and the silicon hydrogen ratio content in the samples. It is visible from the figure that the presence of oxygen is fundamental for the PL intensity, as was already shown by others authors [7, 8].  4. Conclusions  The photoluminescence of Er incorporated in thin solid films of amorphous/nanocrystalline silicon was studied as a function of temperature and for different growing conditions. Results show that the Er ions are excited very efficiently through energy transfer from the host. The PL intensity of the sample having Si nanocrystals is much higher than for the others. The effect of increasing temperature on luminescence quenching is much less important than in monocrystalline silicon, and PL is still observed    at room temperature. The PL intensity increases with O/Si ratio of the films. Figure 2- Normalised photoluminescence as a function of Si/O content ration of the samples.   References [1] – Y. Kanzawa, T. Kageyama, S. Takevka, M. Fijii, S. Hayashi, K. Yamamoto, Solid State commun. 102, 533 (1997) [2] – M. F. Cerqueira, M. Andritschky, L. Rebouta, J. A. Ferreira and M. F. da Silva, Vacuum 46, 1385 (1995) [3] – M. F. Cerqueira, J. A. Ferreira, M. Andritschky, Manuel F. M. Costa, Microelectronic Engineering 43-44, 627 (1998) [4] – R. Swanepoel, J. Phys. E 16, 1214 (1983) [5] – H. Przybylinska, W. Jantsch, Yu Suprun-Belevitch, M.Stepikhova, L. Palmetshofer, G. Hendorfer, A. Kozanecki, L. Palmetshofer, G. Hendorfer, A. Kozanecki, R. J. Wilson, B. J. Sealy, Phys. Rev. B 54, 2532 (1996) [6] – A. Polman, J. Appl. Phys. 82, 1 (1997) [7] – F. Priolo, S. Coffa, G. Franzo, C. Spinella, A. Carnera, V. Bellani, J. Appl. Phys. 74, 4936 (1993) [8] – D. C. Adler, D. C. Jacobson, D. J. Eaglesham, M. A. Marcus, J. L. Benton, J. M. Poate, P. H. Citrin, Appl. Phys. Lett. 61,2181, (1992) 0 1 2 3 4 50.000.010.020.030.040.050.060.07  Normalized PL intensity (arb. u.)Ratio Si/O 0 1 2 3 4 5  Create PDF files without this message by purchasing novaPDF printer (http://www.novapdf.com) Create PDF files without this message by purchasing novaPDF printer (http://www.novapdf.com)

Photoluminescence of erbium doped microcrystalline silicon thin films produced by reactive magnetron sputtering

M.F. Cerqueira

M.V. Stepikhova

J.A. Ferreira

Crossref

Materials Science and Engineering B81 (2001) 32–35 
 
 
 
Photoluminescence of erbium doped microcrystalline silicon thin films produced 
by reactive magnetron sputtering 
 
 
M. F. Cerqueira*1, M. V. Stepikhova2 and J. A. Ferreira1 
 
1Physics Department, University of Minho, Largo do Paço 4700-320 Braga Portugal 
2Institute for Physics of Microstructures RAS, 603600 Nizhnij Novgorod GSP-105 Russia 
 
 
 
Abstract 
 
Microcrystalline silicon thin films doped with erbium were produced by RF sputtering and their 
structural, chemical and optical properties were studied by X-ray diffractometry at grazing incidence, 
Rutherford back scattering and optical transmission spectroscopy. The samples exhibit a sharp 
photoluminescence (PL) spectrum from the Er centres with the strongest peak positioned at 1.536 m 
with a full width at half maximum of about 8 nm.  When the temperature varies between 5K and 300K 
the photoluminescence decreases only five fold, in contrast to the behaviour reported for 
monocrystalline silicon. 
 
Keywords: Photoluminescence, Erbium, nanocrystalline silicon 
 
1. Introduction 
 
During the last 10 years, modern growth techniques have made possible the fabrication of 
semiconductor nano-structures of typical size < 10nm, with spatial confinement of electrons in all three 
dimensions. These systems are of considerable current interest because of the new physics involved 
and potential device applications. 
One of these applications relates to Si-based optoelectronics. Erbium doped silicon is a recognised 
candidate to become a material for emitters and detectors to be used in optical communication systems. 
However, there are problems with efficient energy transfer from the Si matrix to the Er centres at room 
temperature. The use of Er-doped Si nano-crystallites can help to overcome these problems [1]. Studies 
of the photoluminescence of Er-doped nanocrystalline silicon constitute an important probe for these 
applications.  
 
2. Experimental 
 
Erbium doped microcrystalline silicon thin films were grown by reactive magnetron sputtering on 
ordinary glass substrates using a procedure similar to that used for the preparation of undoped c-Si:H  
 
 
*Corresponding author: tel: +253 604332; fax: +253678981; e-mail: fcerqueira@fisica.uminho.pt 
films [2], but adding small pieces of metallic erbium to the c-Si target. The target used was a c-Si with 
high purity (99.99%). The erbium was placed in a low erosion area of the silicon target, in order to 
Create PDF files without this message by purchasing novaPDF printer (http://www.novapdf.com)
obtain low concentrations. The substrate target distance was fixed at 55 mm. Two series of samples 
were grown, for different amounts of erbium on the c-Si target. In each series we have changed 
basically the substrate temperature since it is this parameter that control the crystal grain size [3]. 
Growth conditions are presented in Table 1. The first series  (Er1,2 and 3) was grown in a argon rich 
atmosphere and the second series (Er4,5 and 6) in a hydrogen rich atmosphere. 
 
Table 1 
Growth conditions 
 
Sample Temperature 
(ºC) 
RF power 
(W) 
H2 (cm3/min) Ar 
(cm3/min) 
Er1 250 40 20 100 
Er2 350 40 20 100 
Er3 100 40 20 100 
Er4 250 80 50 30 
Er5 350 80 50 30 
Er6 100 80 50 30 
 
For the structural characterisation we used X–ray  diffractometry at the grazing incidence geometry. 
The voltage was 40 kV (35mA), the incidence angle was fixed at 0.5º, the step was 0.04º and the 
acquisition time by step was 5.0 s. The range studied was from 23 to 71º. The excitation used was the 
Cobalt line with  = 1.78896 Ă. The chemical composition was determined by Rutherford Back 
Scattering (RBS) technique. The thickness was evaluated from the analysis of the near I.R. 
transmission spectra of the films. The Photoluminescence study was done with a high resolution 
Fourier set-up (BOMEM spectrometer DA8) using the 514.5 nm line of an argon laser for excitation. 
 
 
3. Results and discussion 
 
3.1 Structural and chemical characterisation 
From the interference pattern of the transmission spectra the thicknesses of the films were obtained 
using the Swanepoel method [4]. Comparing the transmission spectra we verify that the samples grown 
in a rich argon (Ar) atmosphere show a larger number of interference fringes, compared with the 
samples grown in a rich hydrogen (H2) atmosphere, indicating that the thickness of those films is 
higher. This is due to the high sputtering efficiency of the argon atoms compared with the hydrogen 
atoms. 
The chemical composition (see Table 2) was determined using the Rutherford Back Scattering 
technique. The percentage of Er and Si obtained from RBS is very accurate, but the values for oxygen 
are only approximate. It is nevertheless clear that for the samples grown in a H2 rich atmosphere and 
high temperatures (Er4 and Er5), the percentage of Si and O are comparable. We know from previous 
studies by ellipsometry that these nanostructured samples have a SiO top layer of approximately 60-
100nm. For the same initial amount of Er in the sputtering target, the concentration of the dopant in the 
films is highest for the lowest substrate temperature. 
Our samples have also been studied at room temperature by X-ray diffractometry at grazing incidence. 
The samples were annealed at 500ºC for 30 minutes in N2 atmosphere. The X-ray experiments were 
done on the annealed and as-grown samples.  All the samples show the broad band related to the 
silicon amorphous matrix. The as-grown samples do not exhibit any structure. The (111) diffraction 
peak of c- Si is only visible, albeit with low intensity, in the Er5 annealed sample. The diffraction peak 
analysis, by fitting a pseudo-Voigt function [2] to the (111) c-Si diffraction peak gives an average 
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crystal size of 270 Ă. Previous studies have shown that the band gap for crystals of this size is higher 
than 1.9 eV. 
 
Table 2 
Atomic concentration and thickness for different samples 
 
Sample Er 
(%) 
Si 
(%) 
O 
(%) 
Ar 
(%) 
Thickness 
(nm) 
Er1 4.0 76 19 1 722 
Er2 4.1 79 16 1 777 
Er3 4.7 78 16.3 - 786 
Er4 0.3 50.7 49 - 270 
Er5 0.3 42.7 57 - <200 
Er6 0.7 77 22 - 407 
 
3.2 Photoluminescence study 
The photoluminescence of these films was measured using the excitation wavelength 514.5 nm of Ar 
laser.  The laser power was 200mW and the measurement temperature was 5K, except for the 
temperature dependence study. 
a) b) 
Figure 1 - a) Photoluminescence for the annealed Erbium doped microcrystalline sample. b) Photoluminescence behaviour 
with temperature for Er5 annealed sample. 
 
Figure 1a) shows the photoluminescence spectrum of the annealed samples. These spectra are 
characterised by a peak around 1.536 m. This peak is the signature of the erbium centres. The PL 
intensity of each sample increases by several orders of magnitude if the sample is annealed, especially 
in the case of Er5 sample. The increase of PL after annealing could be due the formation of Si 
nanocrystals during the annealing, to the annealing of some structure defects or to the decrease of 
hydrogen or oxygen concentration during the annealing process. According to the X-ray study we 
think that the first possibility is indeed true for Er5, which means that with the annealing we promote 
the crystallisation of some silicon, although for the other samples this is not the case. 
In Fig. 1b) we present the result of the photoluminescence temperature study for the Er5 annealed 
sample. Increasing the temperature, from 5K to 300K, the PL intensity decreases only 5 times. This 
1 .2 1 .3 1 .4 1 .5 1 .6 1 .7
0
200
400
600
800
1000
1200
1400
1600
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2200
2400
2600
2800
3000
3200
3400
E r1ann.
E r2ann .
Er3ann.
E r6ann .
E r4ann .
E r5ann .
1 .536µm
P
L 
in
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ns
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 (a
rb
. u
.)
W a ve le n g th  (µ m ) 1.3 1.4 1.5 1.6 1.7 1.8 1.9
0
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280K
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PL
 in
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W ave len gth  (µ m)
-10 0 10 20 30 40 50 60 70 80 90 100
1000
PL
 in
te
ns
ity
 (a
rb
. u
.)
1 000/T, K - 1
Create PDF files without this message by purchasing novaPDF printer (http://www.novapdf.com)
temperature effect is attributed to thermally activated back transfer to the host and is less pronounced 
than in monocrystalline silicon [5]. This kind of behaviour is promising for applications but the 
interpretation of the PL origin is difficult. There exist two possibilities, concerning the location of the 
Er centres, namely the Er ions could be in the amorphous matrix or in the nanocrystals. Since all the 
samples (except Er3) show photoluminescence and only in the Er5 annealed sample there is evidence 
of the presence of nanocrystals, it appears that the Er centres are located in the a-Si:H matrix for 
Er1,2,4,6 samples. 
The fact that PL quenching in all the samples studied, be lower than in monocrystalline silicon can be 
explained by the higher energy band gap [5, 6] of the amorphous silicon (around 1.6 eV for sputtered 
or evaporated material and for a typical hydrogen content – 20 at%) compared with 1.1 eV for c-Si. 
The back transfer process will then take place only at higher temperatures. This effect is even more 
remarkable for the nanocrystalline sample, where the energy gap is higher and where some Er can be 
located in the nanocrystalline phase. 
Figure 2 shows the correlation between the photoluminescence intensity and the silicon hydrogen ratio 
content in the samples. It is visible from the 
figure that the presence of oxygen is 
fundamental for the PL intensity, as was already 
shown by others authors [7, 8]. 
 
4. Conclusions 
 
The photoluminescence of Er incorporated in 
thin solid films of amorphous/nanocrystalline 
silicon was studied as a function of temperature 
and for different growing conditions. Results 
show that the Er ions are excited very efficiently 
through energy transfer from the host. The PL 
intensity of the sample having Si nanocrystals is 
much higher than for the others. The effect of 
increasing temperature on luminescence 
quenching is much less important than in 
monocrystalline silicon, and PL is still observed    
at room temperature. The PL intensity increases 
with O/Si ratio of the films. 
Figure 2- Normalised photoluminescence as a function of 
Si/O content ration of the samples. 
 
 
References 
[1] – Y. Kanzawa, T. Kageyama, S. Takevka, M. Fijii, S. Hayashi, K. Yamamoto, Solid State commun. 102, 533 (1997) 
[2] – M. F. Cerqueira, M. Andritschky, L. Rebouta, J. A. Ferreira and M. F. da Silva, Vacuum 46, 1385 (1995) 
[3] – M. F. Cerqueira, J. A. Ferreira, M. Andritschky, Manuel F. M. Costa, Microelectronic Engineering 43-44, 627 (1998) 
[4] – R. Swanepoel, J. Phys. E 16, 1214 (1983) 
[5] – H. Przybylinska, W. Jantsch, Yu Suprun-Belevitch, M.Stepikhova, L. Palmetshofer, G. Hendorfer, A. Kozanecki, L. 
Palmetshofer, G. Hendorfer, A. Kozanecki, R. J. Wilson, B. J. Sealy, Phys. Rev. B 54, 2532 (1996) 
[6] – A. Polman, J. Appl. Phys. 82, 1 (1997) 
[7] – F. Priolo, S. Coffa, G. Franzo, C. Spinella, A. Carnera, V. Bellani, J. Appl. Phys. 74, 4936 (1993) 
[8] – D. C. Adler, D. C. Jacobson, D. J. Eaglesham, M. A. Marcus, J. L. Benton, J. M. Poate, P. H. Citrin, Appl. Phys. Lett. 
61,2181, (1992) 
0 1 2 3 4 5
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
 
 
N
or
m
al
iz
ed
 P
L 
in
te
ns
ity
 (a
rb
. u
.)
Ratio Si/O 
0 1 2 3 4 5
 
 
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Photoluminescence of erbium doped microcrystalline silicon thin films produced by reactive magnetron sputtering

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