The structure of Hg-doped As2Se3 glasses

Journal of Non-Crystalline Solids 23 (1977) 217-228 © North-Holland Publishing Company

THE STRUCTURE OF Hg-DOPED As2Se 3 GLASSES L. CERVINKA Institute of Solid State Physics, Czechoslovak Academy of Science, Prague, Czechoslovakia

and V. TRKAL and D. LEZAL Institute of Radio Engineering and Electronics, Czechoslovak Academy of Science, Prague, Czechoslovakia

Received 18 June 1975 Revised manuscript received 22 January 1976 The structure of the semiconducting glassy As2Se3Hgx system was investigated in a composition range x = 0.005-0.12. An explanation of the anomalous behaviour of the macroscopic density is proposed, based on the analysis of radial electron density distribution curves. A formula is given which correlates quantitatively the magnitude of coordination spheres with the experimental macroscopic density.

1. Introduction Investigation of As2Se 3 glasses doped with various elements is of interest because electrical and optical properties often strongly depend on the concentration of these elements. Investigations have been reported studying the influence of various elements on the qualities and/or structure of As2S 3 and As2Se 3 [ 1 - 5 ] . However, different behaviours were observed, e.g. the influence of Ag or Ge in As2Se 3 [ 2 - 4 ] . Studies of thermal properties of Ge-doped As2Se 3 glasses [3,4] point to a strengthening of chemical bonds between layers by intercalation of Ge atoms between the layers. An extensive structural study of the system Cux(As2Se3)l_ x was made by Liang et al. [6] based on X-ray and ESCA measurements. They conclude that Cu atoms increase the average coordination number from 2.4 for pure As2Se 3 to ~4, and that regions develop with a short-range order similar to that in crystalline CuAsSe2, having a network similar to that associated with amorphous Ge and Si. An anomalous trend (a minimum) in the dependence of macroscopic density (see below, fig. 6), refraction index, absorption edge and dc conductivity in the As2Se3Hgx system for small Hg concentrations x = 0.01 [7] was recently observed. We thought, therefore, that a look at possible structural changes associated with this anomaly could be of interest. 217

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L. Cervinka et al. / Structure o f rig-doped As2Se 3 glasses

2. Preparation o f samples

The samples of As2Se 3 + x at% Hg (As2Se3Hgx) were prepared by adding defined quantities of elemental metallic mercury to purified vitreous As2Se 3. All samples, the basic As2Se 3 as well as the As2Se 3 doped with Hg, were prepared in identical temperature regimes. Temperature of the synthesis was 900°C, for a duration of 8 h using a fused quartz crucible. The melt was cooled by placing the crucible in the oven, where it was annealed at 250°C for 3 h; this was followed by cooling of the oven to room temperature. During the synthesis, elementary Hg reacts with melted As2Se 3 as follows: As2Se 3 + xHg ~ (1 - x) As2Se 3 + xHgSe + xAs2Se 2 .

(1)

Both the actual concentration of Hg and As versus Se stoichiometry of all Hg-doped As2Se 3 samples were checked by X-ray fluorescence analysis. The measurements were performed on a Chirana X-ray spectrometer. The results of the measurements suggest that with the exception of samples doped with x = 0.005 and 0.01 the actual content of Hg was slightly lower than the amount used in synthesis. Values o f x as determined by these analyses are used in this article, i.e. x = 0.0, 0.008, 0.012, 0.056, 0.124. 3. A s 2 S e 3 - crystalline and glassy

Crystalline As2Se 3 is a monoclinic orpiment-type structure with layers distant about 5 A (fig. 1); each As is surrounded by three Se atoms in a form of a triangular pyramid, the bonding angle on As and Se being about 100 °, while the As-Se distance varies from 2.32 to 2.56 [8,9]. Amorphous As2Se 3 was studied in a number of papers [10-14]. It follows from these studies that basic units (AsSe 3 pyramids) as well as their mutual arrangement are nearly the same as in the crystal. Our recent measurements, based on an analysis of radial electron density distribution curves of As2Se 3 and As2Te 3 by pair functions [ 10], confirm the layer character of this amorphous compound.

As2Se 3

o-As •-Se

121

Fig. 1. A schematic projection of the crystalline As2Se3 structure in the (ab) plane (for crystalline As2Se3 [9] a = 12.05, b = 9.57 and c = 4.22 A, angle ~ = 90°28'). Layers perpendicular to the b-axis are clearly visible in tbis projection.

L. Cervinka et al. / Structure of rig-doped As2Se 3 glasses

219

4. The glassy As2Se3Hgx system Results of our intensity measurements and of the determination of the radial electron density distribution curves in the composition interval of interest, are given in figs. 2 and 3 and tables 1 and 2. We see that the anomalous course of macroscopic density in the region of small

As~Se3Hgx

>.

x : 0.124

Z LLI Z m

-~_

x : 0.056

.......... _x_oo12

x : 0.008

x:O.O

0 0

2

4

6

8

40 10

50 12

60 14

16

70 ® S

Fig. 2. Corrected and scaled diffracted intensity as a function o f the Bragg angle 0 (s = 4~r sin 0/k) in the As2Se 3 system for samp|es with x : 0.00, 0.008, 0.012, 0.056 and 0.124.

220

L. Cervinka et al. / Structure o f rig-doped As2Se 3 glasses I

I

I

I

I

I

I

I

I

I

1

I

As2Se,Hg

20

10

\ ~

z

~

J

/

x = 0.008 "'/ ~..// /

I\

fir"

/

~-

x=

/ f

0.00"j

/

/

I I I I I

o.¢-

1,0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

s.s

6.o

~.5

Rill Fig. 3. Radial electron density distribution curves in the As2Se3Hgx system for Hg concentrations x = 0.0, 0.008, 0.012, 0.056 and 0.124.

Table 1 Values of positions of coordination spheres R i (As2 Se3) and Rix (As2 Se3 Hg,x). x

RI

RII

Rii I

RIV

A

0.0 0.008 0.012 0.056 0.124 Error

2.44 2.43 2.40 2.43 2.45 +-0.01

3.68 3.69 3.70 3.70 3.75 +-0.03

4.47 4.61 (4.50) +-0.05

5.85 5.90 5.80 5.90 5.8s ±0.10

Ri

R~

L. Cervinka et al. / Structure of rig-doped As2Se 3 glasses

221

Table 2 Values of the areas under the first (A I) and second (AII) peak characterizing first and second coordination spheres. A symmetric shape was assumed for the second peak (fig. 3).

0.0 0.008 0.012 0.056 0.124 Error

AI (e 2)

AII (e 2)

2960 2940 2900 2960 3450 ±100

9270 9050 9200 9760 11300 ±250

Hg concentrations is reflected partly by an anomalous value of the first coordination sphere which suddenly decreases to 2.40 .~. at x = 0.012, partly by an increase of the RIII value, until the concentration x = 0.012 where the RIII peak begins to be washed out. The value of the second coordination sphere has a tendency to increase slightly. The areas of the first (AI) and second (AII) coordination spheres were measured (table 2) in order to estimate possible changes in coordination number. The area AII was measured assuming that the RII peak has a symmetric shape, see fig. 3. We observe that the area of the first peak does not change up to the concentration x = 0.124, having a value A I = 2960 e 2 (giving an average coordination number of 2.6 - slightly higher than that reported by Liang et al. [14] who had A I = 2725 with an average coordination number of 2.4). This result may indicate that Hg atoms do not influence the first coordination sphere until the concentration reaches x = 0.124 for which A I increases to a value A I -- 3450 e 2, thus giving an average coordination number of 2.9. The second peak of the radial distribution curve is bound mainly with A s - A s and S e - S e distances. It is therefore interesting that here the area increases already from x = 0.056 and that this increase is accompanied by a broadening of the peak. The error limits for positions R and areas A of coordination spheres (tables 1 and 2) were obtained as follows. Every sample was measured six times, two sets of intensity data I = I(O) were joined together in order to obtain three radial electron density distributions. Positions and areas of coordination spheres were then measured on special large-scale plots of these electron density distributions. Greater differences were observed only for the positions and heights of higher coordination spheres. These discrepancies were then used to estimate error limits both of positions R i and A i (table 2).

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L. Cervinka et al. / Structure of rig-doped As2Se3 glasses

5. Discussion of the measurements It is the aim of this paper to contribute to a structural explanation of the minimum in the density versus concentration [XHg] curve in the region of small Hg concentrations. In this region we observed two effects - the first is an increase in the Ri11 value accompanied by a gradual washing out of the R l l I peak, the second effect is an immediate decrease of the R I position for the sample with x = 0.012.

5.1. A model o f low-density regions We now deal with the first fact. Imagine that the increase of the R i i I distance is caused by decoupling of AsSe 3 motives by Hg atoms (fig. 4). However, it seems that an unambigous interpretation of whether the Hg atoms enter the As2Se 3 layer network or prefer interstitial positions between layers, is not possible. We assume that Hg atoms could form either tetrahedral HgSe 4 configurations as in the zincblende structure of HgSe (with the shortest HgSe distance equal 2.64 A), or they may occupy positions like those in the high-pressure cinnabar-type structure of HgSe (a highly distorted octahedral arrangement in which the number of nearest neighbours of the Hg atoms is reduced to two Se atoms [15]). To illustrate the idea o f the decoupling of AsSe 3 motives quantitatively, we shall demonstrate how, by a mere increase o f the R i i I distance Qeaving the structure of elementary AsSe 3 units unchanged), it is possible to explain development of local volumes with low density. Low-density local volumes could then contribute to an overall decrease of the macroscopic density in the material.

•

-

Se

0 -As Fig. 4. A schematic two-dimensional representation of an As2 Se3 layer. A model of decoupling of AsSe3 basic units due to Hg atoms is shown on the left-hand side of the figure. On the righthand side are given quantities, used for the calculation of local density in a local volume (dashed lines) of the layer, i.e. a = RII = 3.68 A, o = 3.19 A and R~II = 4.25 A. See text for details.

L. Cervinka et al. / Structure of rig.doped As2Se 3 glasses

223

Fig. 5. The calculation of the height h of the layer. The height of the layer is given as h = 2 rse + 1 + rAs, the quantity 1 is calculated from the triangle [R I, (g-)v, 1].

Let us take a region of an As2Se 3 layer marked with dashed lines o n fig. 4. First, we calculate the volume of this section based on values RI, RII and RIII for x = 0.0 (table 1). Because a = RII = 3.68 A, we have for v the value o = (a/2)31/2 = 3.19 A. The projection o f R i i I into the plane of fig. 4 is R~II = (3)v = 4.25,8, and the projection angle a is characterized as cos a = R~II/RII I = 4.25/4.47 = 0.951. The height of the layer h (fig. 5) is easily calculated as h = rAs + 1 + rse, (rAs and rse is the covalent bond radius) where 1 = [R 2 - (30)2] 1/2-. Thus 1 = (2.442 - 2.132) 1/2 = 1.19 A and h = 1.21 + 1.19 + 1.17 = 3.57 3,. The volume of this section of layer is therefore V = 3a X 30 × h = 11.04 × 9.57 X 3.57 = 377 A 3. The local Pl density of this section can be calculated according the formula

Pl = n i A i / N V ,

(2)

where n i is the number of atoms, A i their atomic weight, N the Avogadro number and V the volume. Because there are six arsenic and nine selenium atoms in this part o f the layer we have 6 × 74.92 au + 9 × 78.96 au

Pl =

=

6.024 × 1023 au X g-1 × 377 × 10 -24 cm 3

5.27 g cm - 3 .

We obtained a value which is too high as we had not calculated the space between layers. Therefore, we calculate the total volume in this model of a layer leading to the value of density Pl = P A s , z S e 3 = 4.60 g cm - 3 . This volume is equal to V=

niAi O1N

-

1160 au

= 419 A 3 ,

4.60 g c m - 3 × 6.023 X 1023 au × g-1

The space between layers A V i s therefore A V = 419 A 3 - 377 A 3 = 42 A 3, and the hypothetical distance between layers is therefore A V/P = A V/(3a X 30) = 42/105.65 = 0.40 A. We now calculate the change o f the local density in this local volume, inserting one Hg atom (for this calculation the Hg atom may be situated either in the layer or in the space between the layers) and taking the R i i I value for x = 0.008, i.e.

224

L. Cervinka et aL /Structure of Hg-dopedAs2Se3 glasses

Rii I = 4.61 A. With this value we calculate effective values of a and o, aef and Oef, contributing to the increase of this local volume to a value Vincr. Because Ri11 characterizes the mutual removal of AsSe 3 motives in the layer we assume that it does not contribute to a change of the height of the layer h. Thus, Vincr = 3 a e f X 3 0 e f X h, where Oef = 3RIII cos o~= 3.29 A (for simplicity we assume that cos a has the same value as for RtI t = 4.47 A, i.e. cos c~= 0.951) and aef = 2Oef/31/2 = 3.80 A. For Vincr w e obtain Vincr = 11.40 X 9.87 X 3.57 = 402 A 3. Adding to this value the contribution of the space A V between layers which we calculate from the layer distance 0.40 A and the area Pef = 3 a e f X 3Oef = 113 A 2 as AV = P e f X 0.40 = 45 A 3, we obtain for Vtota I VtotaI = Vincr + A V = 402 + 45 = 447/~3 . We again use the density formula (2) with the value Vtota1 and put in the area one Hg atom, assuming its weight as fulfilling the composition condition. Hence _ (3 X 2) × 74.92 + (3 X 3) X 78.96 + (3 × 0.008) × 200.59 au O16.024 X 1023 au g-1 × 447 X 10 -24 cm 3 = 4.34 g cm -3 . Thus, we have a local volume characterized by a value of density slightly smaller than the value of the minimum in the density versus Hg concentration curve (4.40 g cm-3). Therefore, it is reasonable to assume that such local volume may contribute to an overall decrease of the macroscopic density. A remark should be made here concerning the distance between layers (0.4 A), as calculated above. It is hard to estimate what may be the interlayer distance, even in the crystalline structure (fig. 1). In the amorphous phase our analysis indicates that atomic pairs from different layers contribute mostly to the RIV peak (5.9 A). However, we have not observed any essential changes in this distance with increasing Hg content. Assuming that the amorphous interlayer configuration is the same as in the crystal (fig. 1) there could be room for interstitial Hg atoms to form zincblende- or cinnabar-like HgSe bonds without moving the layers one from another. 5.2. The R l anomaly The second fact - a sudden decrease o f R I for x = 0.012 to a value R I = 2.40 accompanied by a washing out of the Rii I maximum is hard to explain - especially bearing in mind that this sample is still in the region of the anomalous low density (fig. 6), and that at the same time the areasA I and AII show no changes (table 2). It is possible that due to the decoupling of AsSe 3 units a shortening of the As-Se bond can occur, bound up with an increase of the S e - A s - S e bonding angle.

L. ~ervinka et al. / Structure of rig-dopedAs2Se3 glasses

225

5.3. Coordination numbers The third experimental fact left for discussion is the increase of areas A I and AII, starting at x = 0.056 and x = 0.124. However, with these concentrations we are already too far from the minimum in the density versus Hg concentration dependence. Calculating now for x = 0.124 the coordination number CNHg on Hg atoms contributing to the increase of the average coordination number from 2.6 to 2.9 according to the equation (2 × 3 + 3 X 2 + 0.124 X CNHg)/5.124 = 2.9, we obtain CNHg = 23. This is an unusually high coordination number for Hg atoms. We assume, therefore, a mutual increase of the coordination number on Se atoms ACN~. This increase can be easily calculated assuming further that the Hg atoms are built in as HgSe4 (zincblende-like motive), i.e. CNHg = 4. Thus, [2 X 3 + 3 X (2 + ACNse ) + 0.124 × 4]/5.t24 = 2.9 and/XCNse = 0.8, i.e. the coordination number on Se increases to the value 2.8. This could mean that here the As2Se 3 amorphous layer structure is greatly disturbed, and that with higher Hg content it passes to a continuous random network. Furthermore, the observed increase of the average coordination number to 2.9 indicates that for concentrations x > 0.1 there is a similarity in the behaviour of As2Se3Hgx and (As2Se3)l_xCu x [6] systems, because both admixtures contribute to an increase in the average coordination number.

5.4. Correlation between macroscopic density and magnitude of coordination spheres In order to find a quantitative correlation between macroscopic density and local ordering characterized by the diameters R i of coordination spheres, we put the following consideration. We assume first that contributing to the overall density is not only the density of the basic As2Se 3 but also the densities of HgSe and As2Se 2 which may develop according to reaction (1). Thus, the resulting macroscopic density in the As2Se3Hgx system will be the sum of the densities of individual components multiplied by their molecular ratios mi,

p(x) = mlPAs2Se3 + m2PHgSe + m3PAs2Se2 .

(3)

Taking PAs2Se3 as the density of amorphous As2Se 3 (4.60 g cm-3), PHgSe as the density of crystalline HgSe (8.26 g cm -3) [ 16] and'PAs2 Sea as the density of glassy As2Se 2 (4.45 g cm - 3 ) [17], we obtain the linear course of density p(x) (dashed line) in fig. 6. Secondly, we shall correct the course of density P, eq. (3), by data characterizing local ordering (Rix values, table 1), i.e. we shall take into account only the influence of volume changes. We express therefore

p = M/V

(4)

226

L. Cervinka et al. / Structure o f rig-doped As2Se 3 glasses .

.

.

S

~

.

.

.

.

.

.

.

.

.

.

.

.

.

:..-

5.0 4.9 &8 4.7 4.6 4,5

1 ~

Q

As2Se3H ×g

g 4.4 43 ~

4.2

o

4.1 2

3

4

5

6

7

8

9

1

11 12 13

14 15 16 17 18 19 20

Fig. 6. Density in the amorphous As2Se3 Hgx system. Experimental macroscopic density Pexp measured by the pycnometric method - full circles and full line; density P calculated according to eq. (3) - dashed line; values of density Ox calculated according to eq. (11) - open squares.

and

px = M/Vx,

(5)

where M is the mass and V can be expressed assuming isotropic volume changes as V = (~-)TrR3 .

(6)

Here Vis defined as a local volume (sphere-like) in As2Se 3, and R is its radius. We consider now the change of volume A V leading to a local volume V x in As2Se3Hgx, i.e. (7)

Vx = V + A V .

The volume V x is characterized by the radius R x (8)

R x = R + ~.

The relation between fix and fi is, combining eqs. (4), (5) and (7) Px = p(1 + A V / V ) -1

(9)

Because AV = 4TrR2&R, we can write, using eqs. (6) and (8) 1 + (AV/V) = 3(Rx/R ) - 2.

(10)

fix = fi R / R x [3 - 2 ( R / R x ) ] -1 .

(11)

Thus,

We point out here that both fi and fix are functions of x, i.e. fi(x) and fix(X), but that the correction offi is undertaken for a certain value ofx. In eq. (11) we now take the values for fi expressing its linear course, according to eq. (3), and for the

L. ~ervinka et al. / Structure of rig-doped As2Se3 glasses

227

ratio R/Rx, the expression R 1 Rx 4

RI + R I I + R I I I +R_~_~_

R Ii x

R illx

(12)

IV x

where R i are the positions of coordination spheres in As2Se 3, and Rix the positions of these spheres for samples with x ~ 0. Because we were not able to measure Riiix for x = 0.056 and 0.124, in these two cases for R/R x we have taken the value R _l[gI

Rx

3 ~

+ R I I + RIV

RII x

(13)

RWx]"

We see that the values o f p x (open squares in fig. 6) calculated according to eqs. (11) and (12) or (13) give a sufficient agreement with experimental density, bearing in mind that we took into account really only local changes (<6 A).

6. Conclusions We sought a structural explanation for an anomaly (minimum) in the course of macroscopic density in the region of small Hg concentrations (about 1%). The experimental phenomena accompanying this anomaly points to a decoupling mechanism of elementary AsSe 3 motives evoked by Hg atoms. The structure of elementary AsSe 3 configurations as well as the average coordination number remains unchanged in this section. For higher Hg content (x > 10%)we observed an increase in the average coordination number from 2.6 to 2.9. Assuming that Hg atoms are embedded in the material in zincblende-like HgSe4 configurations, we interpreted this greater value as an increase in the coordination number on Se atoms reaching a value of 2.8. This fact may further point to a gradual transition of the As2Se3 amorphous layer structure to a continuous random network structure.

Acknowledgement Our thanks are due to V. Vorlf~ek (Institute of Solid State Physics, Prague) for suggestions leading to formula (11).

References [1] R.E. Andreichin, M.S. Nikiforova, E.R. Skordeva and L.S. Yurikova, Commun. Conf. on Amorphous,Liquid and Vitreous Semiconductors, Sofia, 1972. [2] R. Manaila and M. Popescu, Commun. Conf. on Amorphous, Liquid and Vitreous Semiconductors, Sofia, 1972.

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L. Cervinka et al. / Structure of rig-doped As2Se3 glasses

[3] L. Stoura6, B.T. Kolomiets and V.P. Silo, Czech. J. Phys. B18 (1968) 92. [4] B.T. Kolomiets, L. Stoura~ and L. Pajasov~, Fiz. Tverd. Tela 7 (1965) 1588. [5] J. 13urEek, L. Hrivn~k, S. Kolnik, C. Musil and F. Strba, Proc. Int. Conf. on Amorphous and Liquid Semiconductors, Cambridge, 1969, p. 69. [6] K.S. Liang, A. Bienenstock and C.W. Bates, Phys. Rev. B10 (1974) 1528. [7] I. Srb, D. Legal, J. Mi~ek, L. Krat6na and V. Trkal~, in: Proc. Conf. on Amorphous Semiconductors, ed. P. Sfiptitz (Akad. Wissensch. DDR, Reinhardsbrunn, 1974) p. 232. [8] A.L. Renninger and B.L. Averbach, Acta Cryst. B29 (1973) 1583. [9] A.A. Vaipolin, Kristallogr. 10 (1965) 596. [10] L. Cervinka, in: Proc. Conf. on Amorphous Semiconductors, ed. P. Siiptitz (Akad. Wissensch. DDR, Reinhardsbrunn, 1974) p. 167. [11] A.A. Vaipolin and E.A. Poraj-Ko~ic, Fiz. Tverd. Tela 2 (1960) 1656. [12] A.A. Vaipolin and E.A. Poraj-Ko~ic, Fiz. Tverd. Tela 5 (1963) 246. [13] Ju.G. Poltavcev, V.M. Pozdfiakova and V.P. Rubcov, Ukr. Fiz. Z. 18 (1973) 915. [ 14] D.E. Sayers, F.W. Lytle and E.A. Stern, Proc. Int. Conf. on Amorphous Semiconductors, Garmisch-Partenkirchen 1973, p. 403. [15] H. Krebs, Fundamentals of inorganic crystal chemistry, (McGraw-Hill, London, 1968). [16] Handbook of Chemistry and Physics, 51 ed. (The Chemical Rubber Co., Ohio 1970-71). [17] R.L. Mjuller, Chimija Tverd. Tela, Izd. Leningrad University (1965), p. 30.