Electro-optical characterization of Ge–Se–Te glasses

Journal of Non-Crystalline Solids 355 (2009) 2083–2087

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol

Electro-optical characterization of Ge–Se–Te glasses J. Zavadil a,*, P. Kostka b, J. Pedlikova b, K. Zdansky a, M. Kubliha c, V. Labas c, J. Kaluzny c a

Institute of Photonics and Electronics, Academy of Sciences of the Czech Republic, Chaberská 57, 182 51 Praha 8 – Kobylisy, Czech Republic Laboratory of Inorganic Materials, Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 250 68 Rez, Czech Republic c Institute of Materials, Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava, J. Bottu 23, 917 24 Trnava, Slovak Republic b

a r t i c l e

i n f o

Article history: Available online 31 July 2009 PACS: 81.05.Kf 78.20.e 78.55.m Keywords: Conductivity Chalcogenides Optical spectroscopy Absorption Luminescence

a b s t r a c t Chalcogenide bulk glasses Ge20Se80xTex for xe(0,15) have been prepared by systematic replacement of Se by Te. Selected glasses have been doped with Ho, Er and Pr, and samples have been characterized by transmission spectroscopy, measurements of dc electrical conductivity and low-temperature photoluminescence. Absorption coefficients have been derived from measured transmittance and estimated reflectance. Arrhenius plots of dc electrical conductivity, in the measured temperature range 300–460 K, are characterized by single activation energies roughly equal to the half of the optical gap. Activation energies deduced from Arrhenius plots reveal a systematic decrease with increasing Te content. Similarly, both absorption and low-temperature photoluminescence spectra reveal shifts of absorption edge and/ or dominant luminescence band to longer wavelength due to Te ? Se substitution. Samples doped with Ho and Er exhibit a strong luminescence at 1200 and 1540 nm due to 5I6 ? 5I8 and 4I13/2 ? 4I15/2 transitions of Ho3+ and Er3+ ions, respectively. Pr doped samples exhibit only a relatively weak luminescence peak at 1590 nm, which we tentatively assign to 3F3 ? 3H4 transition of Pr3+ ions. Absorption of the base glass luminescence at 1460 and 1520 nm has been observed at low temperature on samples doped with Pr and Er, respectively. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction There is a growing interest in the family of special glasses [1], namely chalcogenide glasses [2,3] and heavy metal oxide glasses (HMO) [4–7] due to their promising properties such as the transparency in middle and far infrared (IR) regions of spectra, higher values of refraction indices and lower values of phonon energy as compared to SiO2 or fluoride glasses. These glasses and optical fibers can be used as passive medium for laser power delivery or as active elements when doped by suitable ions, such as rare earth (RE) ions. Chalcogenide glasses belong to the group of glassy semiconductors having p-type electrical conductivity, are characterized by the value of the forbidden band Eg  2 eV and are transparent in the middle and far IR region of spectra. Their transparency region is determined by a short wavelength absorption edge, related to the electronic band gap, and by fundamental vibrations of chemical bonds at longer wavelengths. Glasses containing tellurium have worse glass forming ability, smaller glass forming region, lower optical transmission due to defects, higher refraction index and the transmission is shifted to longer wavelengths (up to 20 lm) – as compared to sulfide and selenide sys* Corresponding author. Tel.: +420 266 773 436; fax: +420 284 680 222. E-mail address: [email protected] (J. Zavadil). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.12.025

tems. They also have higher tendency to crystallization and phase separation. It should be noted that applications in optoelectronics demand high chemical and physical purity of glass materials. The chemical purity is determined by the concentration of chemical impurities (metals, hydrides, oxides, and carbon); the physical purity is defined by the concentration of physical defects as heterogeneous particles, microcrystalline phase separation, cracks and in-homogeneities. To achieve values of optical losses that approach the theoretical ones, the concentration of impurities should be approximately 105 mol% for hydrides, 106 mol% for oxides, 5  105 mol% for silicon and carbon and the concentration of physical defects should be about 102–103 cm3. For this reason progressive purification and preparation procedures of base glass systems were developed [8– 11]. When doped with RE elements, these glasses, by virtue of their low phonon energy and high refractive index, open up the possibility of 4f–4f transitions and significantly may increase pumping efficiencies. The attention in this paper is focused on the investigation of optical and electrical properties of Ge20Se80xTex glass systems for xe(0,15). Multi-component telluride glasses on the base of Ge have been prepared by systematic substitution of Se with Te. Selected systems have been doped with holmium (Ho), erbium (Er)

J. Zavadil et al. / Journal of Non-Crystalline Solids 355 (2009) 2083–2087

3.2. Absorption spectroscopy Transmission spectra of Ge20Se80xTex system for x(0,15) were measured in detail in reference [12] with emphasis on the position of the long wavelength absorption edge. It follows from the inspection of these spectra that the transmission is around 60% in the wavelength range from 800 nm to 16 lm. At some samples with higher Te concentration the transmission decreases due to partial crystallization and phase separation. In these cases scattering losses increase [12]. Here we concentrate on transmission spectra measured in the range 700–1100 nm and particularly on the behavior of short wavelength absorption edge. To facilitate the correlation of this absorption edge with measured activation energies, deduced from

t [°C] 200

10

3.1. Electrical characterization Currents above 0.5 pA were observable above the noise. The measured conductivity of samples Ge20Se80xTex for x = 0, 10 and

Ge20Se80+ 1000 wt.ppm Ermetal

10-5

Ge20Se80+ 1000 wt.ppm Ermetal

10-6

Ge20Se80+ 1000 wt.ppm Hometal

-7

[S.cm-1]

10

10-8 10-9 10-10 10-11 10-12 10-13 2.5

3.0

1000/T [K-1] Fig. 1. Arrhenius plots of dc electrical conductivity measured on glass samples Ge20Se80 doped with Er and Ho by using various precursors is shown. Plots are described in the inset. Activation energies are in the range 0.93–0.97 eV.

t [°C] 200 10-4

100

10-5 10-6 10-7 10-8 10-9 10-10 -11

10

10-12

3. Results

100

-4

[S.cm ]

Glasses were prepared by direct synthesis from pure starting elements in sealed evacuated quartz ampoules. The major sources of hydride, oxide and carbon related impurities seem to be starting elements (Ge, Se, and Te) and therefore re-purification of these elements must be carried out. The sublimation under vacuum and the reactive atmosphere proved to be very effective for Se and melting under vacuum showed similar effects for Ge. The technological procedures involving the distillation, synthesis of undoped and RE doped glasses and the preparation of preforms for fibers drawing were carried out in a special quartz ampoule [8]. For the removal of oxide impurities a gettering was used by exploiting aluminum (Al) in the amount of 100 wt. ppm as the gettering agent. This procedure lowers the possibility of contamination during preparation steps. The melting temperature of glasses depends on their composition; for glasses containing Ge it is about 850–950 °C. The melting time was 15–20 h and then the ampoule with glass melt was cooled to room temperature. Glasses doped with Ho, Pr and Er are homogenous up to the RE concentration 0.15 wt%. Precursors in the form of metal and oxide have been used for RE addition. Prepared ingots were cylinders with the diameter of 10 mm and the length in the range 50–80 mm. Polished samples with the thickness around 1 mm were used for optical and electrical characterization. Prepared glasses have been examined by absorption spectroscopy, light scattering, low temperature photo-luminescence (PL) spectroscopy and temperature dependent dc current–voltage (I–V) characteristics. The difference in the batch composition and that of the prepared glass was 0.6% on average. The composition has been monitored via quantitative electron diffraction X-ray analysis using Philips XL 30 CP Scanning Electron Microscope equipped with EDX system. Transmission spectra in the visible range have been measured by using Specord 210 Analytic Jena and in the near infrared region by Matson Galaxy 3000. Photoluminescence (PL) spectra were taken at various temperatures and various levels of excitation by He–Ne and Ar ion lasers in an optical He closed cycle cryostat enabling measurements in the range 3.5–300 K. The 1 m focal length monochromator with the cooled high purity Ge detector and/or cooled GaAs photomultiplier enables sensitive and high resolution measurement in the spectral range 400–1800 nm by using the lock-in technique and the computer controlled data collection. Typical spectral resolution in reported experiments was in the range 0.04–0.08 nm and 20 measurements were collected for each wavelength of the spectrum. I–V characteristics were measured manually by pico–ampere– meter Keithley 6485 in the range from 300 to 460 K with the interval of 5 K. The measured current settled down slowly at each voltage step. Sufficient time was awaited at each step before the current was settled down. The random and systematic errors were estimated by repeated measurements of I–V characteristics and they were displayed by symbol sizes on the data in the figures. The I–V characteristics were measured on circular wafers with the diameter of about 10 mm and the widths about 1 mm. The wafers were furnished with metal contacts on both sides by coating with colloidal silver paint and/or conductive layer from colloid graphite.

σdc

2. Experimental

15 are shown in Figs. 1–3, respectively. Four distinct groups of values of activation energies Ei have been observed for Te content equal to x = 0, 5, 10 and 15. The values of measured activation energies are summarized in Table 1. Both used metal contacts yield the same values of activation energies Ei.

-1

and praseodymium (Pr) and inner shell 4f–4f transitions of RE3+ ions have been investigated.

σdc

2084

Ge20Se70Te10 Ge20Se70Te10 + 1000 wt.ppm Proxide Ge20Se70Te10 + 1000 wt.ppm Hooxide Ge20Se70Te10 + 1000 wt.ppm Ermetal Ge20Se70Te10 + 1000 wt.ppm Eroxide Ge20Se70Te10 + 1000 wt.ppm Hometal

-13

10

2.5

3.0

1000/T [K-1] Fig. 2. Arrhenius plots of dc electrical conductivity measured on glass samples Ge20Se70Te10 doped with Er, Ho and Pr by using various precursors is shown. Plots are described in the inset. Activation energies are in the range 0.81–0.84 eV.

2085

J. Zavadil et al. / Journal of Non-Crystalline Solids 355 (2009) 2083–2087

aðkÞ ¼ ð1=dÞ

t [°C] 200 10-4

   1=2   ln ð1  RÞ2 =2TðkÞ 1 þ 1 þ 4R2 TðkÞ2 =ð1  RÞ4

100

10-5

ð2Þ

10-6

in which the multiple reflections are taken into account. Absorption coefficients a(hm), as a function of the energy of incident photons hm are shown in Fig. 4 for four base glass systems Ge20Se80xTex. The curves corresponding to x = 0, 5, 10 and 15 are described as (a), (b), (c) and (d), respectively. Plotted absorption coefficients exhibit a distinct shift towards lower energy with increasing Te content. The data plotted in Fig. 4 have been truncated in the high energy range, i.e. above 1.8, 1.6, 1.48 and 1.43 eV for curves (a), (b), (c) and (d), respectively. The truncation reflects the fact that transmission spectra measured at the short wavelength range limit are meaningless because the spectrometer reached the limit of its dynamic range.

[S.cm-1]

σdc

10-7 10-8 10-9

Ge20Se65Te15

10-10

Ge20Se65Te15 + 1000 wt.ppm Proxide

10-11 10-12

Ge20Se65Te15 + 1000 wt.ppm Ermetal Ge20Se65Te15 + 1000 wt.ppm Eroxide Ge20Se70Te15 + 1000 wt.ppm Prmetal

-13

10

2.5

3.0

1000/T [K-1] Fig. 3. Arrhenius plots of dc electrical conductivity measured on glass samples Ge20Se65Te15 doped with Er and Pr by using various precursors is shown. Plots are described in the inset. Activation energies are in the range 0.72–0.79 eV.

conductivity, we transform the measured transmission data into spectral dependence of the absorption coefficient. When the multiple reflections are taken into account, the optical transmittance can be expressed by reflectance and the absorption coefficient [13], as follows:

h i T ¼ ð1  RÞ2 exp ðadÞ= 1  R2 exp ð2adÞ ;

ð1Þ

where R(k), a(k) and d are reflectance, absorption coefficient, as a function of the wavelength k, and thickness of the sample, respectively. The reflectance of our samples was estimated from the measurements of transmittance as a function of samples thickness. A set of 10 samples with thicknesses in the range 1–6 mm has been used for reflectance estimation. Transmittance follows from the limit of the zero width and was found equal to 70% for the glass Ge20Se70Te10. Thus the reflection on one interface turns out to be in the range 15–17%. The index of refraction determined from the estimated reflectance (R) by using the expression R=[(n1)2]/ [(n + 1)2] is approximately 2.3–2.4 which is a reasonable value for investigated glasses. The absorption coefficient was calculated on the basis of the measured transparency data T(k), the thickness d of the bulk samples, and the estimated reflectance R, by using the relation:

3.3. Photoluminescence spectroscopy PL spectra of Ge20Se80 and multi-component alloys Ge20Se80xTex with various amounts of Te have been measured at different temperatures. The dominant low-temperature luminescence feature of base glass systems is a broad band centered at about half the band gap, and a shift of the dominant low-temperature PL band from 1270 nm (Ge20Se80) to 1520 nm (Ge20Se70Te10) has been observed. The large difference between the PL peak and the optical band gap Eg is the indication that deep states are involved in the recombination. It has been suggested [14] that observed transitions are from band tails to states near the middle of the gap. On some samples this broad band could be observed in the whole measured temperature range (4– 300 K). The band starts to quench above 200 K and a new band, due to transitions between extended states over the optical gap, appears at 900 nm. The used high purity Ge detection system has its response cut offs at 800 and 1700 nm and hence the symmetry of PL bands near these edges is distorted. A comparison of PL spectra for Ge20Se73Te7 systems measured at temperatures 4, 250, and 300 K is given in Fig. 5 by the curves (a), (b) and (c), respectively. The structure at about 1350 nm (designated as A) is due to absorption of emitted luminescence on water vapors in the air. It is seen because there is a strong signal within the water vapors absorption range. Both above mentioned broad bands at 900 and 1550 nm

70

Glass

Rare earth

Form

Activation energy (eV)

Ge20Se80 Ge20Se80 Ge20Se80 Ge20Se75Te5 Ge20Se70Te10 Ge20Se70Te10 Ge20Se70Te10 Ge20Se70Te10 Ge20Se70Te10 Ge20Se70Te10 Ge20Se65Te15 Ge20Se65Te15 Ge20Se65Te15 Ge20Se65Te15 Ge20Se65Te15

– Er Ho Er – Pr Er Er Ho Ho – Pr Pr Er Er

Metal Metal Metal Metal

0.95 0.97 0.93 0.91 0.83 0.83 0.81 0.82 0.84 0.81 0.79 0.72 0.77 0.77 0.75

Oxide Metal Oxide Metal Oxide Metal Oxide Metal Oxide

Ge20Se80-xTex

60

(d)

(b)

-1

Abs. coefficient (cm )

Table 1 Activation energies of Ge20Se80xTex glasses, for x = 0, 5, 10 and 15, deduced from the slope of dc electrical conductivity measured in the temperature range 20–160 °C are shown for base and RE doped glass samples. All doped samples have RE concentration equal to 0.1 wt%.

(c)

50

(a)

40 30 20 10 0 1.0

1.2

1.4

1.6

1.8

Energy (eV) Fig. 4. Absorption coefficients deduced from measured transmission spectra of Ge20Se80xTex base glass systems are shown for x = 0, 5, 10 and 15 by curves (a), (b), (c), and (d), respectively.

2086

J. Zavadil et al. / Journal of Non-Crystalline Solids 355 (2009) 2083–2087

12

Ge20Se75Te5 : RE (1000 ppm) - Er, Ho, Pr

0.8 15

PL intensity (arb. u.)

A

8 (a)

(b)

6 4

4

4

I13/2- I15/2

T=3.5 K

Ge20Se73Te7

10

PL intensity (arb. u.)

Energy (eV) 1.0

1.2

1.4

x50

A

10 B 3 5

5

3

F3- H4

(c)

I6- I8

5

(c)

(b) (a)

2

4

B

4

I11/2- I15/2

x50

0

0

1000 1000

1200

1400

1200

Wavelength (nm) Fig. 5. Broad band PL luminescence spectra of Ge20Se73Te7 base glass are shown for temperatures 4, 250 and 300 K by curves (a), (b) and (c), respectively. The structure described as A corresponds to absorption of luminescence signal on water vapors.

could be seen simultaneously in the temperature range of 200– 250 K. This is demonstrated by the curve (b) in Fig. 5. Low-temperature PL spectra of Ge20Se80xTex doped with Ho, Pr and Er are shown in Figs. 6–8 for x = 0, 5 and 10, respectively. The base glass luminescence due to deep states is seen together with superimposed 4f–4f inner shell radiative transitions of Ho3+, Pr3+ and Er3+ ions. Sharp transitions at 1200, 1600, and 1000 and 1540 nm correspond to (5I6 ? 5I8), (3F3 ? 3H4), and (4I11/2 ? 4I15/2, 4 I13/2 ? 4I15/2) transitions of Ho, Pr and Er ions, respectively. Er3+ emission at 1000 nm is not observed for Te content x > 5. Besides 4f–4f related emission, also the absorption of the base glass luminescence due to Pr3+ and Er3+ ions (described by the letter B) is seen at 1450 and 1530 nm, respectively. The structure described as A corresponds to the absorption due to water vapors in the air. The same applies to Fig. 5.

1400

1600

Wavelength (nm)

1600

Fig. 7. Low temperature PL spectra of Ge20Se85Te5 glasses doped with Er, Ho and Pr are shown by curves (a), (b) and (c), respectively. Inner shell transitions of RE3+ ions, superimposed on the broad base glass luminescence are shown. Structures A and B indicate the absorption of luminescence on water vapors and RE3+ ions, respectively.

6 Ge20Se70Te10 : RE (1000 ppm) - Er, Ho, Pr T=3.5 K

PL intensity (arb. u.)

800

4

4

I13/2- I15/2

A

(a)

4 B

3

3

F3- H4

(c)

2

5

5

I6- I8

(b) B

0 1000

1200

1400

1600

Wavelength (nm)

25

Fig. 8. Low temperature PL spectra of Ge20Se70Te15 glasses doped with Er, Ho and Pr are shown by curves (a), (b) and (c), respectively. Inner shell transitions of RE3+ ions, superimposed on the broad base glass luminescence are shown. Structures A and B indicate the absorption of luminescence on water vapors and RE3+ ions, respectively.

Ge20Se80 : RE (1000 ppm) - Er, Ho, Pr T=3.5 K

5

5

I6- I8

PL intensity (arb. u.)

20 (b)

4

4

I13/2- I15/2

4. Discussion

A

15

(a)

10

(c)

3 4

5

3

F3- H4

4

I11/2- I15/2

B

0 1000

1200 1400 Wavelength (nm)

It has been found that the I–V characteristics fulfilled Ohm’s law at low electric fields yielding a constant value of conductivity. The conductivity of measured samples increased with increasing temperature which indicated the semiconducting character of studied glass materials. It can be suggested that the electrical conductivity r is a consequence of thermally activated processes that can be described by the Arrhenius law:

r¼ 1600

Fig. 6. Low temperature PL spectra of Ge20Se80 glasses doped with Er, Ho and Pr are shown by curves (a), (b) and (c), respectively. Inner shell transitions of RE3+ ions, superimposed on the broad base glass luminescence are shown. Arrows A and B indicate the absorption of luminescence on water vapors and Pr3+ ions, respectively.

X

r0i  exp ½Ei =ðkTÞ;

ð3Þ

i

where r0i is the pre-exponential factor and Ei is the activation energy of the i-th conductivity process, k is Boltzmann’s constant, and T is the absolute temperature. Summation in the relation (3) is performed over possible conduction processes. Parameters r0i could be determined from the intersection of log r with the ordi-

J. Zavadil et al. / Journal of Non-Crystalline Solids 355 (2009) 2083–2087

nate axis (where T ? 1), and the activation energy E is deduced from the slope of log r. The conductivity of investigated glasses Ge20Se80xTex, shown in Figs. 1–3 is characterized by single activation energy. The temperature dependencies of measured I–V characteristics of Ge20Se80xTex bulk glasses are shown in log–log scale. It should be noticed that activation energies of measured glasses exhibit a dependence on Te content, while the influence of RE dopants on activation energies is negligible. With increasing content of Te the activation energies are steadily decreasing, which is in accord with observed shift of optical gap to lower energies with increasing Te content. Activation energies deduced from dc conductivities are shown in Table 1 for Te content x = 0, 5, 10 and 15. Absorption coefficients plotted in Fig. 4 exhibit a distinct shift towards lower energy with increasing Te content. The values of the optical gap (Eg) were estimated from the intersection of the extrapolated straight part of the dependence a = f(hm) with the abscissa and are equal to 1.30, 1.33, 1.45 and 1.70, for x decreasing from 15 to 0, respectively. In view of measured dc electrical conductivities (Figs. 1–3) and absorption coefficients (Fig. 4) it is suggested that the measured activation energies Ei for the temperature range (300–460 K) could be represented by the difference between the edge of the mobility gap and the Fermi level (EC–EF or EF–EV) [15] and correspond to conduction due to carriers excited beyond the mobility tails into extended states. Low temperature PL spectra are overwhelmed by radiative transitions mediated by deep laying states in the middle of the optical gap. These transitions are manifested by a broad band centered at 1270 nm for samples without tellurium (see Fig. 6). Taking into account the slope of the high energy absorption edge, we conclude that the broad band at 900 nm that dominates PL spectra at room temperature, and could be observed on selected samples (see Fig. 5) is due to transitions between extended states over the optical gap. Systems doped with Er exhibit a strong luminescence due to 4 I13/2 ? 4I15/2 transition of Er3+ ion at 1540 nm. Glasses with zero or low tellurium content also exhibit luminescence at 1000 nm due to 4I11/2 ? 4I15/2 transitions of Er3+ ions. Radiative transitions at 1200 nm due to 5I6 ? 5I8 of Ho3+ ions are seen for all four investigated Te contents. However, we did not observe the 1G4 ? 3H5 transition of Pr3+ ion. Instead we observed, on all Pr doped samples, a relatively weak luminescence peak at 1590 nm, which we tentatively assign to 3F3 ? 3H4 transition of Pr3+ ion. Thus we conclude that Ge20Se80xTex systems for x in the range (0,15) do not support radiative transitions of Pr3+ at 1340 nm. We suggest that photons emitted at about 1350 nm due to 1G4 ? 3H5 transitions of Pr3+ ion could be effectively reabsorbed by electronic system of the glass. This reabsorption may be mediated by deep levels in the gap that are manifested by strong luminescence band with maximum around 1300 nm. The presence of Pr3+ and Er3+ ions in the glass matrix is further manifested by the absorption of the base glass luminescence at 1460 and 1520 nm for Pr and Er doped samples, respectively. These absorption bands could be assigned to 1D2 ? 1G4 transition of Pr3+ and to the F21 sub-transition [16] of Er3+ manifold 4I13/2 ? 4I15/2. 5. Conclusions Ge20Se80xTex glasses with considerably reduced concentrations of chemical impurities and physical defects were prepared and

2087

characterized by transmission and PL spectroscopy and by electrical measurements. Selected systems were doped with Er, Pr and Ho. The short wavelength absorption edge shifts towards longer wavelength as a result of Te ? Se substitution. Absorption coefficient a and optical gap Eg have been deduced from measured transmittance and estimated reflectance by using an expression where manifold reflection is fully included. It has been found that Arrhenius plots of dc electrical conductivity, in the temperature range 300–460 K, are characterized by single activation energies roughly equal to the half of the optical gap and correspond to conduction due to carriers excited beyond the mobility tails into extended states. The observed dependence of activation energies Ei on the Te content is in accord with measured transmission spectra, i.e. with the dependence of the band gap on the Te content. The dominant low-temperature luminescence feature is a broad band centered at about half the band gap. This broad band starts to quench above 200 K and a new band at 900 nm appears. The band at 900 nm, due to band to band transitions, overwhelms the spectra at room temperature. Both bands could be seen simultaneously in the 200–250 K temperature range. All systems doped with Er and Ho exhibit a strong luminescence due to 4I13/2 ? 4I15/2 transition of Er3+ ions at 1540 nm and due to 5I6 ? 5I8 transition of Ho3+ ions at 1200 nm. However, we did not observe the 1G4 ? 3H5 transition of Pr3+ ions. Instead we observed, on all Pr doped samples, a relatively weak luminescence peak at 1590 nm, which we tentatively assign to 3F3 ? 3H4 transition of Pr3+ ions. In addition, absorption of the base glass luminescence at 1460 and 1520 nm have been observed at low temperature on Pr and Er doped samples, respectively. These absorption bands are more pronounced for higher Te content.

Acknowledgements This work was supported by the Grant Agency of the Czech Republic, Grant No. 104/08/0734 and by the project APVV 20043505.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

M. Poulain, Ann. Chim. Sci. Mater. 28 (2003) 87. A. Zakery, S.R. Elliott, J. Non-Cryst. Solids 330 (2003) 1. and references therein. V. Lyubin, M. Klebanov, A. Feigel, B. Sfez, Thin Solid Films 459 (2004) 183. K. Tanaka, T. Yoko, H. Yamada, K. Kamiya, J. Non-Cryst. Solids 103 (1988) 250. D. Lezal, J. Pedlikova, P. Kostka, J. Bludska, M. Poulain, J. Zavadil, J. Non-Cryst. Solids 284 (2001) 288. K. Kobayashi, J. Non-Cryst. Solids 316 (2003) 403. P. Kostka, D. Lezal, M. Poulain, J. Pedlikova, M. Novotna, Solid State Phenom. 90&91 (2003) 241. D. Lezal, J. Pedlikova, M. Poulain, Proc. SPIE 3416 (1998) 36. G.C. Devyatykh, M.F. Churbanov, I.V. Scripachev, et al., J. Non-Cryst. Solids 256&257 (1999) 318. X. Liu, B.B. Khan, V.K. Tikhomirov, A. Jha, J. Non-Cryst. Solids 256&257 (1999) 294. M. Scheffler, J. Kirchhof, J. Kobelke, K. Schuster, A. Schwuchow, J. Non-Cryst. Solids 256&257 (1999) 59. D. Lezal, J. Zavadil, J. Kaluzny, M. Poulain, Phys. Chem. Glasses 46 (2005) 249. P. Sharma, M. Vashistha, I.P. Jain, J. Opt. Adv. Mater. 7 (2005) 2647. B.T. Kolomiets, T.N. Mamontova, A.A. Babaev, J. Non-Cryst. Solids 4 (1970) 289. N.F. Mott, E.A. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon, Oxford, 1979. Z.G. Ivanova, J. Opt. Adv. Mater. 9 (2007) 3149.