GexS1−x glasses. II. Synthesis conditions and defect formation

]OURNA L O F

Journal of Non-Crystalline Solids 146 (1992) 285-293 North-Holland

NON-CRYSTALLINE SOLIDS

GexS _ x glasses. II. Synthesis conditions and defect formation * E.A. Z h i l i n s k a y a a, N.Kh. V a l e e v b a n d A.K. O b l a s o v b "Institute of Nuclear Physics, Moscow State University, Moscow 119899, Russia b Politechnical Institute, Sverdlovsk 620072, Russia Received 6 June 1991 Revised manuscript received 17 February 1992

Electron paramagnetic resonance spectroscopy at 9 GHz and at sample temperatures of 77 and 300 K was applied to the investigation defect formation and its dependence on synthesis conditions for GexS t_x glasses with 0.10 < x < 0.45. It is shown that the influence of synthesis conditions on defect formation in glasses depends on the type of intrinsic paramagnetic centers and the glass composition. New intrinsic paramagnetic centers are detected in glasses with 0.10 < x < 0.325. The attribution of two of these with g-values of 2.01 and 2.005 was proposed. It was shown that the influence of the synthesis conditions and -/-irradiation on the defect formation depends on both glass composition and the type of intrinsic paramagnetic center in each of three composition ranges in the glass-forming composition range.

1. Introduction

The G e x S l _ x glasses and multicomponent Geand S-based glass systems, unlike other chalcogenide glasses, contain a number of stable intrinsic paramagnetic centers (IPC) [1-9]. In our previous work [1], we carried out a detailed investigation of GexS a_x glasses in the range of 0.10 < x < 0.44 using electron paramagnetic resonance (EPR) spectroscopy, X-ray diffraction analysis, and density measurements. A complicated relationship between defect formation, glass density and structure was analyzed. It was shown in ref. [1] that the region of compositions in question could be divided into three intervals: 0.10 < x < 0.325, 0.33 < x < 0.39, 0.39 < x < 0.44 according to the types of observed paramagnetic centers (PC) and the specific features of the concentration dependences of the experimental quantities. * Part I of this series appears in J. Non-Cryst. Solids 124 (1990) 48-62. Correspondence to: Dr E.A. Zhilinskaya, Institute of Nuclear Physics, Moscow State University, Moscow 119899, Russia.

It was found that the density, the PC concentration and the features of the radial distribution functions were correlated in each interval. The influence of 7-irradiation on defect formation was also studied in our work [1], where it was shown that 7-irradiation results in a variation (i.e., decrease or increase) of the PC concentration and spin density redistribution among different types of defects. The IPC concentration in G e x S l _ x glasses also depends on other external factors (e.g., photon-irradiation, pressure), and on synthesis conditions (e.g., temperature and rate of quenching and time and temperature of annealing), and on doping elements. The influence of external factors and of the formation of new multicomponent Ge- and S-based systems on defect formation have been studied very intensively [1-3,5,6,8,14-16]. Nevertheless, a systematic investigation of the influence of the synthesis conditions on defect formation has not been undertaken, to our knowledge. In the present paper, we attempt to find the relationship between synthesis conditions and defects over a wide range of glass compositions

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286

E.A. Zhilinskaya et al. / GexS 1 x glasses. II

0.10 _
ref. [18]). The areas under the resonance absorption first derivative curve were measured also by an ordinary method (described for example in ref. [19]). As the standards of spin concentration, we used samples of D P P H or Mn : MgO (attested by third component of Mn 2+ E P R spectrum). The

g6

2. Experimental procedure The glasses in question were prepared in quartz ampoules by direct synthesis from germanium (99.99%) and sulfur (99.98%). The total weight of the ingredients was 10 g in all cases. After synthesis (8 h at T = 850 K) and homogenization (8 h at T = T 1 + 150 K, where T l is the liquidus temperature), the samples were quenched starting from T = T l + 50 K. Peculiarities of the glass synthesis techniques and of the method for control of the homogeneity and glass composition were described earlier [17]. In method A (glasses studied in ref. [1]), samples were moved from the furnace by hand and dropped into cold water (for several seconds). In method B, samples were moved automatically from the furnace into cold water (during the time of free fall - a fraction of a second). Method B gives a faster quenching rate and more reproducible conditions than method A. In method C, samples were moved from the furnace in air (for several minutes). In method B*, samples prepared by the method B were annealed (4 h). The E P R studies were carried out with homodyne, reflection type spectrometer with a high frequency modulation of 100 kHz and a phasesensitive detection system, P ~ - 1 3 0 6 , operating on X-band ( ~ 9 GHz) at Ta = 77 K and Tr = 300 K. The temperature of 77 K was obtained by immersion of the samples in liquid nitrogen. The PC concentration, Ns, was determined by a comparison with spin standards. Calculation of Ns was made by ordinary formulae (see, for example,

# I

/"

-L~I g3

/ Algt*

\c

g5 .......

20oo

c

I-I

f

20 Oe

' H' H (magnetic field) Fig. 1. Typical E P R spectra in X-band at T = 77 K for G e x S I _ x glasses of: (a) IPC IV, (b) IPC ( I + I ' ) , (c) the spectral component due to IPC II', (d) IPC II, (e) IPC III, (f) IPC V and tPC-(t + t ' ) ; ( a - c ) 0.t0 <_ x _< 0.325; (d) 0.33 <_ x <_ 0.45; (e) 0.39 _< x < 0.45; (f) 0.29 < x _< 0.31.

287

E.A. Zhilinskaya et al. / G e x S 1 _ x glasses. H

o~

4~

bO

I0

15

20

25 Atomic %

30

35

~0

~5

Ge

Fig. 2. Graphs of the concentration dependence of the E P R spectral parameters at 77 K for GexS 1 x B*-glasses. o , the value of the g-factors indicated in fig. 1; e, the locations of the E P R spectral extrema shown in fig. l(a,c-e). (The magnetic field intervals between the lowest and the highest E P R spectral extrema are shown by different hatching.)

standards were placed in thin wall quartz ampoules (d ~ 1 mm). Spin concentration in standards was estimated to be _+ 10%. E P R measurements of glasses and standards were carried out in quartz ampoules placed in the resonance cavity at a fixed depth. Glass samples were crushed and weighed (weight of samples was < 100 mg) before measurement. The minimum n u m b e r of m e a s u r e m e n t s was three. While changing samples and standards, the resonance condition changes were taken into account by normalization of the intensity of all E P R signals according to the additional standard signal. This additional signal is due to a standard which is permanently in the resonance cavity. The average relative error of N s determination is estimated to be _+25%. We have not separated strongly overlapped E P R spectra having three-axis anisotropy. The determination of relative concentration of corresponding IPC is described below. Simple E P R spectra were separated by using of characteristic points of the resonance curves [20]. T h e g-factor values were determined from simultaneous measure-

ment of the magnetic field values for D P P H standard line and for the lines for the sample in question.

3. Results

Electron paramagnetic resonance intrinsic paramagnetic center spectra were observed for all of the GexSl_ x glasses with 0.10 < x < 0.44. Figure 1 shows typical E P R spectra, and fig. 2 gives graphs of the E P R spectrum p a r a m e t e r dependence on the glass composition for B*-glasses. Glasses synthesized under other conditions give similar p a r a m e t e r dependencies, and the regions of existence of different types of IPC may be seen from the concentration dependence (see figs. 3 and 5-8). All lines on the figs. 3 - 8 are drawn to guide the eyes. The present p a p e r contains a more detailed (as compared with ref. [1]) investigation of glasses with low germanium concentrations (0.10_< x_< 0.25). Several additional new E P R signals due to

288

E.A. Zhilinskaya et aL

~IO 17

/ GexS 1 x glasses. H xlO 16

I0

9

L!

8

o/ A

7

/ D

? =

;

I

10

5

r

12

i

i

qAt

i

r

I

i

16 '18 Atomic % Ge

r

20

I

i

22

Fig. 5. Graphs of the concentration dependence of the IPC IV concentration for GexSi_ x glass system, for B-, C-, and B *-glasses.

~4 33

0

0

the corresponding IPC on the glass composition were measured. The concentration dependences for the first and second signals are shown in figs. 5 and 6 for B-, C- and B*-glasses.

*

~

/.o----o~

,/ i

I'8'

~'0'

2 ~'''

' 26' 24 Atomic % Ge

'

i

28'

i

i

30

×1015

32

Fig. 3. Graphs of the concentration dependence of the IPC(I + I ' ) concentration for GexSI_ ~ glass system, for B-, C-, and

B*-glasses.

IPC were found in this region, i.e., lines with the effective g-factors, g6 (fig. l(a)), g~ (fig. 1(c)) and g7 (fig. l(f)). The EPR spectral parameters have been determined and the dependences of N s for

~6

~2

"4

°

!2

o

° o

5

1 118

~

L 20

i

~ 22

i

2I ~

i

i 26

i

r 28

i

r 30

t

t 32

Atomic % Ge

Fig. 4. Graphs of the concentration dependence of the relative intensities of the low field EPR spectral components for IPC I and I', I ( g ) / I ( g ~ ) , shown in fig. l(b) for GexSl_ x glass system, for B-, C-, and B*-glasses.

i

11

L

13

i

15

i

I

i

17 Atomic % Ge

19

i

21

23

25

Fig. 6. Graphs of the concentration dependence of the IPC II' concentration for Ge~SI_ x glass system, for B-, C-, and ]~ * - o l ~ * ' ~ e s .

289

E.A. ZhiIinskaya et al. / G e x S 1 _ x glasses. H

tected. The width of the line (i.e., the magnetic field interval between the highest and lowest extrema of the absorption derivative shown by hatching in fig. 2) was also constant. The region of the existence of IPC I and I' (0.18 _
× 1017 6.0

5°o

4.0

3.1.2.

A new EPR signal (see fig. l(a)) with parameters gav = g6 and ~ H 6 (see table 1) was discov~.0

x l O 16 0

g 0

2.0

10

1

1.0

o i 53

i

~ 35

i

I

I

i

37 Atomic

39 %

~

i 41

r

v

.

I

I

~'3

i

v

45

Ge

Fig. 7. Graphs of the concentration dependence of the IPC II concentration for GexS1_ x glass system, for B-, C-, and B*glasses.

To make the reading of this paper easier, all IPC and their average E P R spectral parameters for two measurement temperatures, Tr and Tn, are presented in table 1.

°= I o

6

2

12

3.1. Glasses with Ge concentration 0.1 < x < 0.325 3.1.1.

The E P R lines with the parameters (gl, g2, g3 ) and (g~, g~, g~) (see table 1 and figs. 1 and 2) are due to two IPC denoted as IPC I and IPC I'. The IPC I and I' have the highest concentrations in this interval of the glass-forming region. Their E P R parameters are practically invariant in the whole region in which IPC I and I' were de-

39

41 Atomic

~3 %

1.5

Ge

Fig. 8. Graphs of the concentration dependence of the IPC III concentration for GexSl_ x glass system, for B-, C-, and B *-glasses.

E.A. Zhilinskaya et al. / G e x S l _ x glasses. H

290

ered by us in the glasses with 0.1 _
this glass composition region. This signal has the parameters gay _- g 4 ! and ~H~ (see table 1 and fig. 2). The g-factor g~ is equal (in the limit of accuracy) to the g-factor of the EPR signal of IPC II from another composition region, 0.333 < x < 0.45. The IPC giving rise to this EPR signal is denoted as IPC II'. The concentration dependencies N~(II') for three different synthesis conditions are shown in fig. 6. 3.1.4. A very intense EPR line (fig. l(f)) with a g-factor less than 2.0, geff =g7, and the width ~H7, was recorded for several glass compositions 0.29 < x < 0.31. The concentration of the corre-. sponding IPC (denote as IPC V) is Ns(V)= 4 × 1019 spin/g on average. The largest amount of IPC V was found in B-glasses. After annealing B-glasses (B *-glasses), Ns(V) decreases by approximately a factor of two. IPC V was not ob-

Table 1 EPR spectral parameters for different intrinsic paramagnetic centers EPR

Tmeas

spectral parameters gI a) g2 a) g3t - g3 b) gr a) 2

g~ a) g4 b) ~H4 g~ b) ~H~ e) g5 b) ~H 5 c~ g6 b) 8 H 6 c) g7 a) ~ H 7 f)

Tr Tn Tr Tn Tr, Tn

IPC

~

I

I'

2.038 2.040 2.024 2.025 2.0036

2.0036

Tr

2.024

T~ Tr

2.026 2.046

Tn

2.051

Tn, Tr Tr d) Tn c) Tr, Tn Tr, Tn Tr, Tn Tr, Tn Tr, Tn Tr, Tn Tn Tn

II

II'

III

IV

V

2.096 15 17 2.01 7 2.004 3-10 g~ 2.005 10 1.965 113-145 h)

The linewidths, ~H, are in Oe. The maximal absolute error in the measurement was: a) + 0.002 Oe; b) + 0.0005 Oe; c) + 1 0 e ; d) + 0 . 5 0 e ; ~) + 2 O e ; t ) g) g H 5 increases on average as x increases (see fig. 2) and as temperature decreases. h) g H 7 varies from 113 Oe to 145 Oe for different samples.

+10Oe.

E.A. Zhilinskaya et aL / GexS 1_ x glasses. H

served in the spectra of C-glasses. An interpretation of the IPC V spectra cannot be given at this time. 3.1.5.

The spectra of some glass compositions (x = 0.30, 0.31, 0.325) have weak E P R signals with g-factors close to g5 or g6 that strongly overlap with the intense signal due to IPC (I + I'). The parameters of this signal are shown in fig. 2. Separation of this signal from the line superpositions and its identification cannot presently be given. 3.2. G l a s s e s w i t h G e c o n c e n t r a t i o n s 0.33 < x < 0.44

Spectra of samples from this glass-forming region exhibit E P R signals (see fig. l(d)) with gav = g4 and S H 4 (see fig. 2 and table 1). The shape of this signal, its parameters, and the region of its existence coincide for all of the synthesis conditions investigated here (fig. 2). Figure 7 shows the concentration dependence of the corresponding IPC (denoted as IPC II) for different synthesis conditions. The position and magnitude of the maximum concentrations strictly depend on the synthesis conditions. For B-glass samples, the maximum value of Ns(II) is observed at x = 0.36. The minimal value of the Ns(II) extremum is obtained for B*-glasses at x = 0.39. For C-glasses, the maximum concentration N~(II) is at x = 0.37. 3.3. G l a s s e s w i t h G e c o n c e n t r a t i o n x > 0.39

The spectra of sample from this region of glass composition are characterized by the presence of an E P R signal (fig. l(e)) with gay = g5 and g H 5 (see fig. 2 and table 1) as well as the E P R signal due to IPC II. We denoted the IPC giving rise to this E P R signal with g5 and g H 5 as IPC III. Figure 8 shows the results of the IPC III concentration measurements for samples with 0.39 _
4. Discussion

We consider now the consequences of changes in the glass-synthesis condition and possible in-

291

terpretations of these phenomena. As shown earlier [4,11-13], the IPC I and I' (see section 3.1.1.) are attributed to sulfur-related defects. Namely IPC I' are due to defects in c i s - c o n f i g u r a t i o n of sulfur atoms which are typical for broken sulfur rings. The IPC I are due to defects in the transconfiguration of sulfur which are due to the breaking of sulfur chains. Figures 2 - 4 show that the influence of the synthesis conditions first appears in a variation of the IPC concentration (fig. 3). For example, at x = 0.30, Ns(I + I') for B-glasses is 10 times larger than that for B*-glasses. Second, the synthesis conditions modify the relative concentration of IPC I and I' (fig. 4). It should be noted that the low-field component of the E P R spectrum of IPC (I + I') with g~ is observed for B-glasses if x >_ 0.18, for B*-glasses if x >__0.22, and for C-glasses if x >_ 0.25 due to the g[ line width. Fast quenching (B) and slow cooling (C) result in a maximal formation of IPC (I + I') and preserves the ratio of IPC I and I' in the interval 0 . 1 8 < x < 0 . 2 8 . Annealing (B*) decreases the concentration of IPC I and I' in the glasses. Slow cooling and annealing modify (with respect to the B-glasses) the ratio of the I and I' concentration for the region x < 0.28. They result in higher relative concentrations of IPC I in the low Ge concentration region (x < 0.28) and in a monotonic variation of the concentration ratio of the IPC I and I' as the Ge concentration increases. The concentration ratios of the various sulfur radicals do not depend on the synthesis conditions for the region 0.28
292

E.A. Zhilinskaya et al. / G e x S 1 _ x glasses. H

It is shown in fig. 7 that changing the synthesis conditions does not influence the boundary of the region of existence of IPC II. Also, for all synthesis conditions (at x > 0.39), the amount of IPC II in glass is approximately the same, and its concentration decreases as x increases. ,/-irradiation enables us [1] to show that these defects are only in equilibrium for glasses with 0.32 < x < 0.39. By the equilibrium state of IPC, we mean the following. The paramagnetic state of a structure defect in the glass under normal conditions corresponds to its energy minimum. On the contrary, IPC is in a non-equilibrium state if its energy decreases with the loss or trapping of an electron or hole (i.e., under the disappearance of the paramagnetism). The intensity of the IPC II EPR signal increases under `/-irradiation, and the position of the maximum in Ns(II) (x = 0.35) is not altered. IPC II are not in equilibrium for compositions with x > 0.39. One may conclude that the synthesis conditions do not influence the IPC II existence region, but that they modify the concentration of IPC II for various glass compositions and they lead to a variation of the position of the maximum of Ns(II). These facts distinguish IPC II from IPC (I + I'). The IPC II in the region of the equilibrium state (0.325 < x < 0.39) are more sensitive to the synthesis conditions than that in non-equilibrium state (x > 0.39). For x > 0.39, synthesis conditions modify the value of Ns(II) and the dependence of Ns(II) on composition, within errors of measurement. The next IPC is III (see section 3.3.). It was investigated earlier and it was proposed [1,2] that this IPC be assigned to oxygen related defect centers. In the present work we found (fig. 8) that Ns(III) for B- and C-glasses increases, on average, as x increases (greater than random errors in the data). The result of annealing is (apart from x = 0.39 and x = 0.40 compositions) a monotonic decrease of Ns(III) as x increases (B *-glasses). We conclude that rapid quenching favors formation of oxygen defects in glasses. Annealing reduces concentrations of oxygen defects. `/irradiation of the glasses with x > 0.39 also leads to the disappearance of IPC III [1]. Finally, we consider two new centers, i.e. IPC

IV and IPC II', observed in samples with low Ge concentrations (see sections 3.1.2. and 3.1.3.). Both exist in glasses with compositions which have the liquation structure according to the constitution diagram of the Ge-S system [10]. In other words, these glasses are inhomogeneous. It may be assumed that IPC IV and II' are associated with defects in different glass phases. IPC 1V exists in the glass phase with low germanium concentration, x--0.15, since the maximum IPC IV concentration is observed for samples with x = 0.15 under all synthesis conditions. We attribute IPC 1V to tightly bound holes localized at sulfur with Ge in the nearest coordination sphere. The following facts support this model: (i) the value of the effective g-factor for IPC IV is close to the free electronic g-factor (ge = 2.0023) but is nevertheless higher than ge (g6 > ge); (ii) the signal with g6 is observed only for glasses with high S concentration; (iii) this EPR signal is not observed for pure sulfur without Ge [11-13]. We propose that, by contrast with IPC IV, IPC II' arises in the glass phase with higher Ge concentrations (x close to 0.18). The corresponding value of the effective g-factor indicates that the relationship between the observed EPR signal and localization of the unpaired spin density on GeS4/2 is similar to that for the EPR signal with g4 [1,3-5,7,14-16]. Nevertheless, the EPR signals with g4 and g~ differ in shape (fig. 1(c) and (d)) and width (fig. 2 and table 1). It may be supposed that this difference is due to different surroundings of the corresponding IPC [21-23]. Smaller widths and more symmetrical forms of the EPR signal with g~ indicate that the IPC II' possess more ordered local surroundings and a higher symmetry of the local crystal fields as compared with IPC II. From the comparison of Ns(II') and Ns(IV), it follows that, on average, some type of positive correlation between two components exists. Such behavior is probably related to changing of the volumes of the corresponding liquated glass phase, formed in the presence of stable or metastable liquation. Thus, under our supposition, the new EPR signals with the g-factor g6 and g~ may be used as an indicator of glass liquation and of the

E.A. Zhilinskaya et al. / GexS 1 x glasses. II

presence of low- or higher-germanium glass phases with x ~ 0.15 or x ~- 0.18, respectively.

5. Conclusions

(1) It is shown that the range of compositions for which IPC (I + I') are observed and their concentrations as a function of glass composition do not depend on synthesis conditions. For all synthesis conditions, the maximum concentration of IPC (I + I') is found for glasses with x = 0.30. The amount of IPC (I + I') and the relative concentration of IPC I and IPC I' depend on the synthesis conditions. The maximum of Ns(I + I') observed for B-glasses and minimum for B*glasses. (2) Several new IPC are detected in the spectra of glasses with 0.1 < x < 0.325. Two of these are probably related to the liquation structure of the glasses and are formed in different glass phases. (3) In the region of equilibrium IPC II (0.33 < x < 0.39), the amount of IPC II and its dependence on glass composition depend on synthesis conditions. For glasses with x > 0.39 (non-equilibrium states of IPC II), synthesis conditions do not influence the Ns(II) (within the errors of measurement). (4) It is shown that the formation of IPC III is associated with oxygen trapping and its decrease under changing synthesis conditions from B- to C- and to B*. (5) A comparison of the influence of synthesis conditions and "y-irradiation on glass defect formation was performed for each composition interval of the G e - S glass-forming region. It is shown that these effects strictly depend on the IPC type and glass composition.

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[2] I. Watanabe, U. Inagaki and T. Shimizu, Jpn. J. Appl. Phys. 15 (1976) 1993. [3] I. Watanabe, S. Shiomi and T. Shimizu, Solid State Commun. 39 (1981) 807. [4] K. Arai and M. Namikawa, Solid State Commun. 13 (1970) 1167. [5] N. Kumagai and J. Shirafuji, Solid State Commun. 33 (1980) 1173. [6] T. Shimizu, M. Kumeda, I. Watanabe and Y. Nakagaki, Solid State Commun. 27 (1978) 223. [7] E.A. Zhilinskaya, I.V. Chepeleva, V.N. Lazukin, G.Z. Vinigradova and N.G. Maisashvili, J. Non-Cryst. Solids 38&39 (1980) 317. [8] Y. Watanabe, H. Kawazoe and M. Yamane, Phys. Rev. B38 (1988) 5668. [9] V. (~erny and M. Frumar, J. Non-Cryst. Solids 33 (1979) 23. [10] V. Viane and G. Mob, Neues Jahrb. Miner. Monatsch. 6 (1970) 283. [11] J. Buttet, Helv. Phys. Acta 42 (1969) 65. [12] A. Chatelain, Helv. Phys. Acta 42 (1969) 117. [13] H. Kawazoe, H. Yanagita, Y. Watanabe and M. Yamane, Phys. Rev. B38 (1988) 5661. [14] R. Durn¢, Acta Phys. Slov. 29 (1979) 78. [15] I. Watanabe, Y. Inagaki and T. Shimizu, J. Phys. Soc. Jpn. 41 (1976) 2030. [16] I. Watanabe and T. Shimizu, Solid State Commun. 25 (1978) 705. [17] A.K. Oblasov, N.Kh. Valeev and A.K. Chirkov, Fiz. Khim. Stekla 10 (1984) 634. [18] H.J.M. Slangen, J. Phys. E3 (1970) 775. [19] Ch.P. Poole Jr., Electron Spin Resonance. A Comprehensive Treatise on Experimental Techniques (Interscience, New York, 1967). [20] G.M. Zhidomirov, Ya.S. Lebedev, S.N. Dobryakov, N.Ya. Sheinshneider, A.K. Chirkov and V.A. Gubanov, Interpretation of Complex Spectra (Nauka, Moscow, 1975) (in Russian). [21] S.Ya. Pshezhetskii, A.G. Kotov, V.K. Milinchuk, V.A. Roginskii and V.I. Tupikov, EPR of Free Radicals and Radiation Chemistry (Khimia, Moscow, 1972) (in Russian). [22] S.A. Al'tshuler and B.M. Kozyrev, Electron Paramagnetic Resonance (Academic Press, New York, 1964) (translated from Russian). [23] P.W. Atkins and M.C.R. Symons, The Structure of Inorganic Radicals. An Application of Electron Spin Resonance to the Study of Molecular Structure (Elsevier, Amsterdam, 1967).