Infrared luminescence in Bi-doped Ge-S and As-Ge-S chalcogenide glasses and fibers

0.44 MB·

by  V. G. Plotnichenko

Infrared luminescence
in Bi-doped Ge–S and As–Ge–S chalcogenide glasses and fibers
V. G. Plotnichenko,1,2 D. V. Philippovskiy,1 V. O. Sokolov,1
M. V. Sukhanov,3 A. P. Velmuzhov,3 M. F. Churbanov,3 and E. M. Dianov1
1Fiber ics Research Center of the Russian Academy of Sciences
38 Vavilov Street, Moscow, 119333, Russia
2Moscow Institute of Physics and Technology
9 Institutskii per., Dolgoprudny, Moscow Region, 141700, Russia
4 3Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences
1 49 Tropinin Street, Nizhny Novgorod, 603600 Russia
0
2 Experimental and theoretical studies of spectral properties of chalcogenide Ge–S and As–Ge–S
glasses and fibers are performed. A broad infrared (IR) luminescence band which covers the 1.2–
n
2.3µmrangewithalifetimeabout6µsisdiscovered. Similarluminescenceisalsopresentinoptical
a
fibers drawn from these glasses. Arsenic addition to Ge–S glass significantly enhances both its
J
resistancetocrystallizationandtheintensityoftheluminescence. ComputermodelingofBi-related
0 centers shows that interstitial Bi+ ions adjacent to negatively charged S vacancies are most likely
3
responsible for theIR luminescence.

]
i
c I. INTRODUCTION lowingglasssystems: GeS –Ga S –KBr[7],GeS –Ga S
2 2 3 2 2 3
s
[8],andGa S –La S –La O [9]. In[7,8]bismuthinlow-
- 2 3 2 3 2 3
l valencestates,suchasBi+,wassupposedtoberesponsi-
r DuringthelastdecadeBi-dopedbulkglassesandopti-
t bleforthe IRluminescence. Basedoncalculationresults
m calfibershaveattractedalotofinterestduetotheirchar-
[10], bismuth dimers were considered in [9] as possible
acteristic broadband IR luminescence in the 1–1.7 µm
t. range which has a potential for new broadband fiber sources of the IR luminescence.
a
Forourresearchwechosetwoglasssystems: Ge–Sand
m amplifiers and lasers [1]. Despite a successful demon-
As–Ge–S. Thefirstwasusedforstudyingtheinfluenceof
stration of laser generation and optical gain in the 1.15–
- Ge/Sratioonluminescenceintensity. Thesecondsystem
d 1.55 µm range using silica-based optical fibers with dif-
n ferent dopants [2], the origin of active Bi-related centers isfarmoreresistanttocrystallization,andsoitissuitable
o for fiber drawing. We also compared our experimental
in optical materials is still controversial. IR lumines-
c results with Ga–Ge–S glass studied earlier.
cence from such centers has been observed in different
[
types of glasses and crystals, and the spectral proper-

1 ties of infrared (IR) luminescence have similar features.
v TheavailableexperimentaldatasuggestthatBilumines- II. EXPERIMENT
5
centcentershavesimilaroriginindifferenthosts. Besides
1
there are reasons to believe that there are several active
8 Metallic Bi doped bulk glass samples (concentrations
7 centers in the glass sample and the ratio between them 0.05, 0.5, 1 at.%), as well as samples without Bi were
1. depends on the host and on production technology. synthesized from high-purity Ge, Ga, As, S in evacu-
0 Certain properties of chalcogenide glasses [3] make ated quartz ampules with inner diameter 10–12mm in a
4 them, on the one hand, promising materials for various rockingmuffle furnace at800–850◦C. For observationof
1 optical devices and applications including fiber optics, luminescencedependenceonsynthesistemperatureGe–S
v: and on the other, convenient hosts for optical transi- samplesweresynthesizedat700◦C.Thetimesofcooling
i tionsresearch,inparticularfortheluminescencestudies. and annealing of Ge–S and As–Ge–S glasses were 2 and
X
Thesepropertiesincludeawidetransparencyrange(from 15 hours, respectively [11].
ar visible to middle-IR) that depends on glasscomposition, For spectroscopic studies we used 2–3 mm thick opti-
high refractive index, and high nonlinear susceptibility. cally polished bulk samples and 300 µm fibers drawn by
The important features of these glasses are an ability to the crucible method. The transmission spectra of bulk
be doped with rare-earth elements and promising lumi- samples were measured on a Perkin-Elmer Lambda 900
nescence characteristics of rare-earth active centers [4– spectrometer. Semiconductor laser sources with fiber
6]. Low phonon energy allows radiative transitions in outputat975nmand1064nmwavelengthswereusedas
the middle-IR region (with wavelengths beyond 2 µm). excitationsources. Theoutput endofthe fiberwasfixed
A significant number of chalcogenide glasses shows good atafocalpointofthelens,andcollimatedbeamwasused
chemical resistance, especially to atmospheric water. It forluminescence excitationinthe bulk samples. Asetup
is also possible to obtain glasses with a wide variety of consisting of a Hamamatsu P7163 InAs photovoltaic de-
properties depending on their composition. tector, an MDR-2 monochromator, a collimator, and an
Chalcogenideglassesallowonetoresearchtheoriginof SR830 Stanford Research lock-in amplifier was used to
Bi-relatedIRluminescence,previouslystudiedinthefol- measure the luminescence spectra. This equipment was
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c d
FIG. 1. Transmission spectra. a. As–Ge–S glasses with different Bi concentration: (a) As10Ge36S54+0.05% Bi,
(b) As10Ge35S55+0.5% Bi, (c) As10Ge36S54+1% Bi; b. Ge–S glasses: (a) GeS1.35, (b) GeS1.35+1% Bi,
(c) GeS1.5+1% Bi; c. As–Ge–S glasses with different As content: (a) As10Ge35S55+0.5% Bi, (b) As5Ge38S57+0.5% Bi,
(c) As2Ge39.2S58.8+0.5% Bi, (d) AsGe39.6S59.4+0.5% Bi, (e) As0.5Ge39.8S59.7+0.5% Bi; d. Ga–Ge–S glasses:
(a) 80GeS2–20Ga2S3, (b) 80GeS2–20Ga2S3+1% Bi.
able to obtain spectra in the 1–2.4 µm range with a res- short-wave transmission edge depends on the glass com-
olution of up to 4 nm. Samples were measured at 293– positionand Bi content. Increasingthe Bi concentration
298 K temperature. shifts the short-wave edge to the IR region (Fig. 1(a)).
The transmission spectra of glass samples with differ- The GeS1.5:Bi sample has a lower transmission than the
entcompositionareshowninFig.1. Onecanseehowthe GeS1.35 sample without Bi(Fig. 1(b)). This fact may be
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c d
FIG. 2. Luminescence spectra. a. Ge–S glasses (excitation at 1064 nm): (a) GeS1.35+1% Bi, (b) GeS1.5+1% Bi,
(c) As10Ge35S55+0.5% Bi; b.As–Ge–S glasses with variousAscontent(excitation at 975 nm): (a)As10Ge35S55+0.5% Bi,
(b) As5Ge38S57+0.5% Bi, (c) As2Ge39.2S58.8+0.5% Bi, (d) AsGe39.6S59.4+0.5% Bi, (e) As0.5Ge39.8S59.7+0.5% Bi;
c.As–Ge–Sglasses(excitationat975nm): (a)As10Ge35S55+0.5% Bi,(b)As20Ge30S50+0.5% Bi,(c)As10Ge36S54+0.05% Bi,
(d) As10Ge36S54+1% Bi; d. As–Ge–S fiber samples (excitation at 975 nm, the spectra are normalized to 1):
(a) As10Ge35S55+0.5% Bi, (b) As20Ge30S50+0.5% Bi.
due to the refractive index increase and a formation of increase in resistance to crystallization. In the transmis-
inhomogeneities due to a partial crystallization. Addi- sionspectraofthesampleswithBitherearenodistinctly
tion of As expands the transmission range and increases expressed absorption bands.
the transmission (Fig. 1(c)), which may be related to an
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a b
FIG.3. a.Luminescencedecaycurve(As10Ge35S55+0.45% Bi, excitation at975 nm): τ1 ≈0.3µs,τ2 ≈5.7µs. b.Lumines-
cencespectraof(a)80GeS2–20Ga2S3+0.5at.%Biglass(excitationat839nm);(b)As10Ge35S55+0.5at.%Biglass(excitation
at 975 nm,the spectra are normalized to 1).
Ga–Ge–SsystemglassesdopedwithBiwerealsostud- Arsenic presumably affects mainly the resistance to
ied. Bulk samples of such glasses easily crystallize even crystallization and the intensity of luminescence, but
when they are cooled in cold water. Therefore it is ex- does not change its spectral structure. Arsenic unlikely
tremely difficult to make optical fibers using this glass leads to the formation of new centers of luminescence
composition. Ga–Ge–S system glasses are more trans- absent in Ge–S.
parent in the visible range than As–Ge–S and Ge–S Similar luminescence bands were also detected in op-
system glasses, but Bi addition makes them far more ticalfibers madeof the sameglass(Fig. 2(d), to be com-
less transparent. The absorption edge is significantly pared with Fig. 2(c)).
shiftedtowardsshorterwavelengthsinsuchdopedglasses The luminescence lifetime was about 6 µs (Fig. 3(a)).
(Fig. 1(d)). In Ga–Ge–S glasses the IR luminescence band maxi-
The luminescence spectra of glass samples with differ- mum is located near 1.28 µm (Fig. 3(b)), and the band
entcompositionareshowninFig.2. Ge–Sglassesdemon- shape is close to Gaussian with FWHM about 200 nm.
strate a relatively low luminescence band intensity, but BandintensityiscomparabletothatinAs–Ge–Sglasses.
itswidthcoversawidespectralrange(Fig.2(a)). GeS1.5 Disadvantages of these glass systems compared with
glass has a higher luminescence intensity than GeS1.35. As–Ge–Sglassesconsistinmuchhighertendencytocrys-
We couldn’t obtain Bi-doped Ge–S glass with higher S tallizationandarelativelynarrowIRluminescenceband.
content because of too strong a tendency to crystalliza-
tion. Itisknownthatthewell-studiedAs–Sglasssystem
shows no tendency to crystallization,and it was decided III. MODELING OF BI-RELATED
LUMINESCENCE CENTERS
to add As into the Ge–S system to increase the glass-
forming ability of Ge–S system glasses. The S/Ge ratio
wasno higher than2. Also As additionresultedin asig- To understand the origin of Bi-related centers respon-
nificantincreaseintheluminescenceintensity(Figs.2(a) sible for the IRluminescence observedinGeS2−x glasses
and 2(b)). and based on assumptions on the role of subvalent bis-
Measurements of the concentration series of samples muth states [12], we performed computer simulation of
(Figs. 2(b) and 2(c)) with various proportion of As the structure and absorption spectra of several centers
and Bi have shown that the maximum intensity of most probably formed by Bi atoms in GeS2−x glass net-
the IR luminescence band occurs in the As Ge S + work. Namely, trivalent (Bi3+) and divalent (Bi2+) sub-
10 35 55
0.5 at.% Bi glass. Slightly lower intensity is observed in stitutional Bi centers, interstitial Bi+ ions, interstitial
As Ge S +0.5 at.% Bi glass. Bi0 atoms, and complexes formed by an interstitial Bi
20 30 50
5
are very likely to occur in GeS network. In such cen-
2
10.9 ters bismuth is in three- and divalent state, respectively.
Formation energy estimations based on the calculated
total energies of corresponding unit cells show that Bi3+
9.4
centers are favorable. Calculated wavelengths of the ab-
sorption bands of such Bi3+ and Bi2+ centers in GeS
2
network agree well with the experimental data available
7.9
onthe spectralpropertiesofsimilarBi-relatedcentersin
severalhosts[18–22]. ThesecentersarenotrelatedtoIR
luminescence andareofno interestfor ourinvestigation,
6.1 since boththe absorptionandluminescence bands ofthe
centers fall in the host self-absorption range.
5.0 Accordingtothecalculations,bothBi0 atomsandBi+
ions are not stable in interstitial positions of GeS net-
2
work. IntheregularnetworkinterstitialBiatomstendto
0 form bonds with the surrounding S atoms giving rise to
9...2.2 1.5...1.8 1.2...1.3 1.0...1.1 0.9...1. naopnfeetrdswuuBocnhri2ikt+arcrseeeualalbr.rrsratTaintnhgugeetermiemofenoenarnteltcwiiensintetGhsetertisSmh2fe−aotraxembdboeadvtsoe.e-dTbmehegenalatbesionosueensrtegdt0yh.B9egiaeI3iVR+n
1. luminescence is unlikely to be caused by interstitial Bi+
ions. Noticethatthecharacteristicluminescencebandof
0
Bi+ ions is knownfromthe experimentalandtheoretical
data to be located near 1 µm [15, 16, 23].
FIG. 4. Calculated levels and transitions in the complex On the other hand, the calculations show that if an
formed by an interstitial Bi atom and an S vacancy (energy interstitial Bi atom occurs in GeS network in vicinity
in 103 cm−1, transition wavelengths in µm). 2
of such an intrinsic defect as a S vacancy, ≡Ge−Ge≡,
a complex of the interstitial Bi0 atom with the vacancy,
Bi
atom and an S vacancy were studied. The structure of Ge Ge , is formed instead of the Bi3+ and Bi2+
the centers was calculated using network models with substitutional centers. The total energy of the unit cell
periodic boundary conditions. The unit cell of defect- withsuchacomplexisestimatedtobeabout0.6eVlower
free GeS2 network containing 24 GeS2 groups was pre- than that of the unit cell with separated Bi3+ (or Bi2+)
pared by ab initio (Car-Parrinello) molecular dynamics. center and S vacancy. In this complex center bismuth
To study Bi-relatedcenters,a Bi atomwas placedin the turnsouttobeinthestateclosetomonovalentstatedue
central part of the unit cell. S vacancies were formed by to redistribution of electron density. Effective electronic
removalofoneoftheSatoms. Thechargestateofacen- charge ≈−0.8|e| is displaced from Bi and Ge atoms to-
ter was determined by the total unit cell charge. Equi- wardstheareabetweenBiandGe atoms,andtoalesser
librium configurations of Bi-related centers were found extent into the area between both Ge atoms. Thus a
by a complete geometry optimization with the gradient three-centersystemisformedconsistingofaBiatomand
methodintheplanewavebasisusingthegeneralizedgra- twoGeatomswithacoordination-typebondingbetween
dientapproximationofthedensityfunctionaltheoryand threeatoms. Inaroughapproximation,basingonthede-
pseudopotentials. All the structure-related calculations scribed electrondensity redistribution, the complex may
wereperformedwiththeQuantum-Espressopackage[13]. beconsideredasapairofchargedcenters,theinterstitial
TheconfigurationsofBi-relatedcenterswereusedfurther positively charged ion, Bi+, adjacent to the negatively
to calculate the absorption spectra of the centers using chargedSvacancy, ≡Ge−•Ge≡. This makeitpossibleto
the Bethe–Salpeter equation method based on the all- describetheelectronicstructureofthe centerintermsof
electron full-potential linearized augmented plane wave a crystal field model similar to ones used previously for
approach taking into account the spin-orbit interaction Bi-related centers in TlCl:Bi, CsI:Bi [15], SiO :Bi and
2
essential for Bi-containing systems. The Elk code [14] GeO :Bi[16],andinAgCl:Bi[17]. Thegroundstateand
2
was used in spectra calculations. To estimate the lu- the first two excited states of the Bi+ ion are known to
minescence Stokes shift, the configurational coordinate arise from the triplet state, 3P, (electron configuration
diagrams of Bi-related centers were studied in the frame 6s2p2) split by strong intra-atomic spin-orbitinteraction
ofasimplemodel. Themodelingisdescribedindetailin in three components, the ground state, 3P , and excited
0
Refs. [15, 16]. states, 3P and 3P , with the energies of about 13300
1 2
The modeling shows that threefold and twofold coor- and 17000 cm−1 in a free ion, respectively. These states
dinatedBiatomsbondedwith Ge atomsby threeortwo canbe further split and mutually mixed under the influ-
bridging S atoms, respectively (Bi3+ or Bi2+ centers), enceofthecrystalfieldoftheBi+ ionenvironmentinthe
6
glass network. For the ”Bi— vacancy” complex such an ular GeS network mainly as trivalent (Bi3+) substitu-
2
axialcrystalfield causedby a chargedvacancyturns out tional centers, and, probably, as divalent (Bi2+) ones.
to be strong. Inanaxialcrystalfieldthe groundstate of However, centers formed by interstitial Bi atoms and S
Bi+ is not split, and the 3P and 3P excited states are vacancies would be expected to occur in sulfur-deficient
1 2
split into two and three levels, respectively. The dipole network, and only such complexes may give rise to the
transitionsbetweenthegroundandexcitedstatesforbid- bismuth-related IR luminescence. Such a center may be
denin the free ionbecome weaklyalloweddue to mixing considered as an interstitial Bi+ ion adjacent to nega-
of the wave functions. A relatively small (in comparison tively charged S vacancy. Comparison of the calculation
with Bi-related centers in other hosts) IR luminescence results with the experimental data allow us to suggest
lifetime can be explained by a relatively high degree of that these complexes make the main contribution to the
Bi+ ion perturbation by the vacancy crystal field and, IR luminescence in GeS2−x:Bi glasses.
respectively,a significantstate mixing. Forexample,our
Arsenic addition to Ge–S glass significantly enhances
calculations [16] similar to the present ones have shown
its resistance to crystallization and made it possible to
thatinthecomplexcenterformedbyinterstitialBiatom
draw optical fibers. Arsenic also significantly increased
and anion vacancy in the GeS network the bonding is
2 the intensity of luminescence. Such an increase in lumi-
noticeably stronger than in similar centers in SiO or
2 nescence intensity in the arsenic-containing glasses can
GeO networks.
2 be explained by a decrease in concentration of Bi ions
The calculated levels and transitions in the complex
with oxidation degree higher than 1 due to reduction
center formed by the interstitial Bi atom and the S va-
properties of arsenic. Furthermore, arsenic occurs in the
cancy are shown in Fig. 4. The Stokes shift of the lumi-
glass network mainly in the form of threefold atoms and
nescenceisfoundtobesmall,atleastforthelongest-wave
prevents the formation of the above-described substitu-
transitions. Thiscanbeexplainedbyrelativelysmallad- tion centers Bi3+ and Bi2+ centers. Sulfur deficiency in
mixture of Ge electronic states to the wave functions of
As–Ge–S compositions may promote the formation of S
this complex, so that small displacement of the Bi atom
vacancies and complexes with interstitial Bi atoms.
doesnotresultinanoticeablechangeofelectronicstates
of the complex. Thus, one expects the ”Bi — vacancy”
complex in the GeS network to cause IR luminescence
2
bandsnear1.9...2.1µmand1.5...1.8µm,whenexcited
ACKNOWLEDGMENTS
bothatthe sameabsorptionwavelengthandinthreeab-
sorption bands in the 0.9...1.3 µm range.
The authors are grateful to Dr. B. I. Galagan for his
helpinluminescencelifetimemeasurementsandforvalu-
IV. CONCLUSION
able discussions. This work is supported in part by Ba-
sic Research Program of the Presidium of the Russian
Basing on the above-mentioned total energy estima- AcademyofSciences andby RussianFoundationforBa-
tions it may be concluded that bismuth occurs in reg- sic Research (grant 12-02-00907).
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