Nonstoichiometric Compounds

19 Nonstoichiometry in Some Group IV Tellurides ROBERT MAZELSKY and M. S. LUBELL

Downloaded by NORTH CAROLINA STATE UNIV on November 6, 2013 | http://pubs.acs.org Publication Date: January 1, 1963 | doi: 10.1021/ba-1964-0039.ch019

Westinghouse

Research Laboratories,

Pittsburgh

35, Pa.

Germanium telluride is a nonstoichiometric semiconductor with a narrow phase field. Changes in crystallographic rhombohedral angle, Seebeck coefficient, and Hall constant have been studied for the nominal compositions GeTe and GeTe with additions of TlSbTe , TlBiTe , and Bi Te . The solubility of germanium in GeTe is shown to be dependent on total vacancy concentration. A dependence of the lattice parameter on vacancy concentration was observed for tin telluride. However, no simple relationship of either carrier concentration or Seebeck coefficient to vacancy concentration was found. 1.025

2

2

2

3

1.025

M major portion of the effort on semiconductors has been expended on the binary compounds having the zinc blende or wurtzite structure. These are commonly classified b y the A group numbers such as I I I - V (InAs) and I I - V I ( C d T e ) and have what may loosely be described as a 1 to 1 cation-anion ratio. H o w ever, another series of compounds that has become of increased interest can be generally classified as the I V - V I compounds. Specifically, these are the chalcogenides of germanium, t i n , and lead. I n this discussion, we present some experimental observations on the tellurides of these group I V A elements. The series of germanium, t i n , and lead tellurides is found to have the rocksalt structure, SnTe and P b T e being cubic and G e T e being rhombohedrally distorted from the cube. I n the series, P b T e appears to be the only material that can be prepared w i t h nearly a 1 to 1 ratio of the metal to tellurium. G e T e has recently been investigated by M c H u g h and Tiller (4) and shown to be congruently melting at a composition corresponding to GeTe . Phase diagram data on SnTe are not available, but indications are that it too is a nonstoichiometric m a terial. This discussion is confined to the germanium and t i n tellurides. F o r clarity the experimental results a n d discussions of these materials are treated separately. 1025

Experimental A l l the samples under discussion were prepared b y a modified powder metallurgical technique. T h e elements (99.999% pure w i t h respect to metallic i m purities) were weighed i n the proper proportions and placed i n a V y c o r tube, which was then evacuated and sealed. T h e elements were melted and manually 210 In Nonstoichiometric Compounds; Ward, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1963.

19. MAZELSKY AND LU BELL

Group IV Tellurides

211

mixed w h i l e molten. T h e samples were air-quenched and the resulting ingot was ground into powder, pressed, and sintered at around 500° C . for 16 hours. Three analytical tools were used to characterize the compounds. T h e first is powder x-ray diffraction methods using a 114.6-mm. diameter camera. T h e photographs were i n general poor for germanium telluride a n d , as a result, the parameters were determined from low-angle reflections only. T h e second pro cedure involved room temperature Seebeck coefficient data (taken versus copper and converted to absolute values) w h i c h qualitatively vary inversely as the log of the carrier concentration. F i n a l l y , H a l l measurements were taken on 1.6 X 0.5 X 0.1 c m . plates i n a manner already described ( 6 ) . Carrier concentrations were calculated from the H a l l measurements on the basis of a degenerate, single-band model using the expression ρ = R \ \ »


H

e

w

n

e

r

e

\e\ is the absolute value of the electric charge. Because some of the H a l l measure ments showed an anomalous rise w i t h temperature (presumably due to two-phase material), a l l the calculations used H a l l measurements taken at l i q u i d nitrogen temperature. I n this way, we obtain a self-consistent set of values for carrier con centrations, although not necessarily the absolute values. A l l the samples dis cussed i n this paper are p-type. Germanium

Telluride

Results

T h e nominal compositions G e T e and GeTe 025 were prepared i n the manner described. T h e composition G e T e b y itself a n d i n solid solutions is not a singlephase material, about 2 atom % germanium metal being present. Small amounts of other components were added to both G e T e a n d GeTe!.025 order to inves tigate the changes i n rhombohedral angle, Seebeck coefficient, a n d H a l l constant. T h e materials used were T l B i T e , T l S b T e , and B i T e . T h e first two materials were reported b y Hockings and W h i t e ( 3 ) , w h o characterized them as having a rhombohedral structure w i t h the cations ordered i n layers perpendicular to the c direction of the hexagonal cell. T h e space group was reported to be R3m-D for both materials. B i T e is reported to have the same space group. 1

m

2

2

2

3

5

M

2

3

The solid solutions prepared w i t h G e T e were of the following general type: (1 - *)GeTe + £ TI (Sb, Bi) Te Δ (1 — #)GeTe + #Bi Te 2

Gei-^TL (Sb, Bi)xTe

2

2 3

2

—» Ge i _ a;Bi2a:Tei 2x +

F o r simplicity i n comparing the systems, the atom per cent of the cations was recalculated on the basis of one tellurium atom per molecule—i.e., G e IZSL B i l+2x

Te.

l+2x

Therefore, the solid solution of G e T e w i t h B i T e was considered to result i n vacancies on the cation sites. These are introduced v i a an essentially stoichio metric material and do not i n themselves affect the carrier concentrations. ( B i T e is nearly stoichiometric, the deviation from 2 to 3 ratio being so small that its carrier concentration is negligible compared to that of G e T e 2 5 - ) Similar solu tions were prepared using G e T e i as a base material. Figure 1 shows the variation of the rhombohedral angle of G e T e a n d G e T e ! 025 w i t h varying concentrations of the solute materials. U p to around 4 atomic % bismuth, the rhombohedral angle w i t h G e T e and G e T e i 2 5 I the same. Beyond this amount, the rhombohedral angle for G e T e continues to increase whereas that for GeTei.025 does not. T l S b T e a n d T l B i T e , on the other h a n d , seem to affect the germanium telluride distortion to the same extent, whether G e T e or GeTei.025 is used as the host. A t 8% T l S b T e or T l B i T e , the distortion seems to be less for G e T e than for G e T e 2 5 - Beyond 8 % the x-rays indicate that the materials are no longer single phase. 2

3

2

1 0

0 2 5

-0

2

2

2

2

1 0

In Nonstoichiometric Compounds; Ward, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1963.

s

3

212


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0

02

.04 .06 .08 Atom Fraction of the Solute Cation

CHEMISTRY SERIES

.10

Figure 1. Effect of solute concentration rhombohedral angle

0

.02

Figure 2.

.04 .06 .08 Atom Fraction of the Solute Cation

Effect of solute concentration coefficient

.12

on

.10

.14

on Seebeck

μν. per degree

O n Figure 2, the Seebeck coefficients of G e T e and G e T e j 5 w i t h T l S b T e and T l B i T e are drawn versus composition of the solute compound. W i t h both T l S b T e and T l B i T e , the Seebeck coefficient increases significantly w h e n G e T e rather than G e T e ! 025 is used as the solvent, and a maximum Seebeck voltage of + 8 0 to 90 μν. per degree is obtained. O n F i g u r e 3, the Seebeck coefficients of B i T e dissolved i n G e T e and G e T e ! 025 e shown. The difference between the 02

2

2

2

3

2

a r


2


19. MAZELSKY AND

LU BELL

0

Group IV Tellurides

0.02

0.04

0.06

213

0.08

Atom Fraction of Bismuth

Figure 3. Effect of bismuth concentration on Seebeck coefficient • GeTe ο GeTei .025

Atom Fraction of Solute

Figure 4. Relation of solute concentrations

to hole carrier

concentration

two is appreciable and the maximum Seebeck value is +140 μν. per d e g r e e higher than that observed w i t h T l B i T e or T l S b T e . O n Figure 4, carrier concentrations calculated from H a l l data are plotted against solute concentration. The difference i n carrier concentrations between G e T e and G e T e 5 w i t h T l S b T e and T l B i T e seems to be fairly constant over the single-phase range—i.e., to χ — 0.08—and is small. The difference i n carrier concentrations seems to be of the same order of magnitude as that between G e T e 2

1 - 0 2

2

2

2


ADVANCES IN

214

CHEMISTRY SERIES

and G e T e ! 025- In accord with the Seebeck coefficient, the carrier concentration w i t h B i T e doping i n G e T e shows a marked difference from that i n G e T e i 2

3

02rj

Germanium

Telluride

Discussion

T h e H a l l measurements show that there is a significant difference i n carrier concentration between the nominal compositions of G e T e and GeTei 25- Assum ing two carriers per vacancy and the composition G e T e ! 025 (equivalent to Ge T e ) , a carrier concentration of 9.0 X 1 0 c m . is expected. A value of 10 to 11 X 1 0 c m . was measured. T h e material w i t h a nominal composition G e T e has a measured carrier concentration of 7.0 X 1 0 c m . " , indicating a composition Ge .98oTe, resulting i n 1.97 atomic % vacancies (and an equal amount of germanium metal present as a second phase). W e have interpreted the difference i n the carrier concentration of the two compositions to mean that there is a small w i d t h i n the phase field at the sintering temperature. There is, then, a reasonable dependence between the vacancies i n the G e ^ T e lattice (here after referred to as "empty Ge sites") and the carrier concentrations calculated from H a l l measurements. It is, on the other hand, possible to change the vacancy concentration without changing carrier concentration. B i T e serves as an ex ample. If it is dissolved i n G e T e , it is convenient to assume no change i n the tellurium sublattice. (Other models assuming G e + , B i + , and T e yield analogous results.) However, for every three tellurium ions introduced only two cations are placed i n the cation sites, resulting i n a single vacancy i n the cation sublattice. A l t h o u g h these may be physically indistinguishable from the empty germanium sites already present i n the lattice, they do not contribute any charge imbalance upon their introduction, and therefore do not affect the carrier concen tration. F o r convenience, the vacancies introduced i n such a manner are called "neutral vacancies." W e can see from F i g u r e 4 that there is little change i n carrier concentration as greater amounts of B i T e are dissolved i n G e T e i 25 that is, no antidoping -0

0

2 0

9 7 6

2 0

- 3

- 3


2 0

3

0

2

2

2

3

0

3

3

- 2

—

2.4

2.2 2. οΛ— 1.8

α> ι/ι

1.6

f

1.4

a

0.8

ΙΟ

0.6

0.4

0.2 0

0

2

3 4 5 Atom % Neutral Vacancies

6

Figure 5. Effect of neutral vacancies introduced on germanium second phase

8

via

Bi Te 2

3



215

Group IV Tellurides


6.330r-

0

0.005

0.010

0.015

0.020

0.025

0.030

0.035

X Figure 6. Refotion of lattice parameter to vacancy concentration in Sn _ Te (1

x)

effect is observed as w i t h T l S b T e or T l B i T e . W h e n B i T e is dissolved i n G e T e t G e ^ T e + G e ( m ) ], there is a decided diminution i n the number of carriers as calculated from H a l l measurements. Since the only difference i n the two preparations is the presence of excess germanium v i a G e T e , it seems probable that when bismuth telluride dissolves i n germanium telluride, the germanium present as a second phase dissolves into the lattice, thereby filling u p vacancies a n d reducing the carrier concentration. The amount of solution of germanium appears to be governed b y the number of "neutral vacancies" introduced b y B i T e . Figure 5 is a plot of the atom per cent of neutral vacancies introduced b y B i T e versus the atomic per cent of germanium second phase. Since germanium telluride was prepared as G e T e , the amount of germanium left as a second phase is equivalent to the empty germanium sites, w h i c h we have assumed yields two carriers per vacancy. T h e calculations of both the neutral vacancies and the germanium second phase (empty germanium sites) are based on one formula weight of the congruently melting composition, Ge T e . W h e n no neutral vacancies are added, about 0.47 atom % germanium in the second phase dissolves. This leaves 1.97 atom % germanium as a second phase. T h e amount of germanium second phase decreases as the vacancies i n crease. Since the shape of the curve appears exponential, it does not seem that stoichiometry can be reached. 2

2

2

2

3

3

2

3

0 9 7 6

Tin Telluride

Results

A somewhat different situation exists for SnTe. F r o m the x-ray data the phase field of SnTe appears to be wide, at least at the 500° C . sintering temperaIn Nonstoichiometric Compounds; Ward, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1963.

ADVANCES IN CHEMISTRY SERIES


216

0

. 02

. 04

. 06

. 08

.10

X Figure

7. Effect of vacancy concentration Sn _ Te on Seebeck coefficient 1

Figure 8.

in

x

Relation of vacancy concentration in Sn _ TE hole carrier concentration 1

A •

x

to

experimental

Liquid nitrogen Room temperature

ture. It can be seen on Figure 6 that the lattice parameter of S n ^ T e varies monotonically from χ = 0.0 to approximately χ — 0.03. As the defect concentra tion increases, the lattice contracts. O n the other hand, the Seebeck coefficient does not show a smooth trend. O n Figure 7 it can be seen that the Seebeck co efficient remains constant at about +24 μν. per degree i n the two-phase region, until the single-phase region, as indicated by the change i n slope of the lattice parameter, is reached. T h e n the Seebeck coefficient begins to increase as ex pected, since the number of vacancies and consequently charge carriers are de creasing. However, at near S n T e , the Seebeck coefficient shows a sharp de09 9



Group IV Tellurides

217

crease, although the vacancy concentration must still be decreasing, since the lattice parameter keeps increasing.


Tin Telluride

Discussion

O n F i g u r e 8 the hole concentration is plotted against vacancy concentration. A theoretical curve calculated on the basis of t w o carriers per vacancy is also shown. There is a very poor agreement between the two curves. T o get the theo retical curve to fit the experimental points, it is necessary to assume more than t w o carriers per vacancy. This is difficult to reconcile. It is apparent that SnTe is not a simple semiconductor like G e T e . A t present single crystals of SnTe are being investigated at our laboratory, at the U . S. N a v a l Ordnance Laboratory, and at L i n c o l n Laboratory. Preliminary results indicate a complex band structure for this compound (1,2,5). Acknowledgment T h e authors acknowledge the technical assistance of W . E . Kramer, D . A . Zupon, and R . Jones. T h e y also appreciate the helpful criticism of R . R . Heikes and R . C . M i l l e r . Literature Cited (1) (2) (3) (4) (5) (6)

Allgaier, R. S., Scheie, P. O., Bull. Am. Phys. Soc., Ser. II, 6, 436 (1961). Brebrick, R. F., Strauss, A. J., Ibid., 7, 203 (1962). Hockings, E. F., White, J. F., Acta Cryst. 14, 328 (1961). McHugh, J. R., Tiller, W. Α., Trans AIME 218, 187 (1960). Sagar, Α., Miller, R. C., Bull. Am. Phys. Soc., Ser. II, 7, 203 (1962). Ure, R. W., Jr., Rev. Sci. Instr. 28, 836 (1957).

RECEIVED September 6, 1962. tract NOBS-84317.

Work supported by the U. S. Navy, Bureau of Ships, Con


Nonstoichiometric Compounds

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