- Email: [email protected]
Thermoelectric power of bulk specimens of the system Bi1 − xTex
THERMOELECTRIC POWER OF BULK SPECIMENS OF THE SYSTEM Bi,_,Te, M. M. IBRAHIM,~ N. AFIFY,$M. M. WAPZ$ and M. A. MARMOUD~ tPhysics Department, Fact&y of Science, As&t University, Sohag, Egypt SPhysics Department, Faculty of Science, Assiut University, Assiut, Egypt (Received IS September 1989; accepted in revised form 20 October 1989)
Abstract-The thermoelectric power of bulk specimens of the system Bi, _,Te,V (0.3 G x iE 0.6 at.) was studied. The effects of Te content, temperature and time of annealing were considered. For most of the compositions considered, the activation of holes seemed predominant. The electron activations, as well as mixed activations, seemed dependent on Te content and the conditions of annealing. Compensation semiconductor behaviour could also be observed. However, both the value and polarity of the Seebeck coefficient seemed dependent on Te content, conditions of annealing and the temperature of measurement. Keywordsr Bi, _ %Te,*.thermoelectric power, Seebeck coefficient, X-ray diffraction, microstructure.
1. lNTRODUCTION In recent years, a number of compound semiconductors and their alloys have been the subject of intensive investigation as possible materials for thermoelectric devices. Among these materials, the binaries such as Bi,Te, and Bi,Se, have recently been of considerable interest. The annealing process has been found to affect both the polarity and values of the Seebeck coefficient (S). Yokota and Katayama (11 found that the P-type Bi:Te, compound changes to n-type after a short time of annealing at temperatures of 400 and 500°C. Ukllinov et al. [2] noted that the increase in thermoelectric power, resulting from the opposite effects of the contact potential on the nature of the surface scattering of electrons and holes in the Bi layers, can be considered an improvement in the~oelect~c properties. Detailed measurements of S were made on different samples of the system Bi,TeJ+,%with x ranging from -0.06 to +0.48 at. The magnitude of S at temperatures above room temperature appeared to require a value of b (electron-to-hole mobility ratio) of about 2 ]3]. For single crystals of several alloys of Bi, _.TTe, (0.1 < x Q 0.4 at.), Smith and Wolfe (41 found that the thermoelectric properties were anisotropic. The present paper aims to investigate the thermoelectric power of bulk specimens of the system Bi,_,Te, (0.3
2. EXPERIMENTAL WORK A study of the preparation of Bi,Te, has been reported [S-7]. fn the present work, the method
used for the preparation of Bi-Te alloys is basically that previously suggested by Brown and Lewis [7], in which the component elements are simultaneously heated and agitated in evacuated silica capsules. The working materials: Bi(99.999%, Gold Label) and Te(99.999%, Gold Label) were supplied by Aldrich Chemical Co. The different syntheses were attempted at temperatures only slightly in excess of their solidus temperature predicted from the phase diagram 171.To promote the reaction between liquid Bi and solid Te, and to homogenize the contents, the assembly was first heated to the melting point of Bi and immediately shaken. The furnace temperature was immediately raised above the soiidus temperature of the desired composition. Shaking of the assembly at the maximum temperature was continued for about 10 h after which the assembly was cooled in steps to 250°C over a period of 6 h and the shaking was then stopped. Quenching at O’C was then immediately carried out. Samples with the required thickness were then prepared by automatic cutting. Bulk specimens with parallel and optically flat surfaces could then be obtained by mechanical polishing. Cleaning of specimen surfaces was performed by using the extractor condenser technique with acetone as a solvent [S]. For the thermoelectric measurement, a modified Shen et al. method [9] was ,used. The thermoelectric power was measured using platinum probes (4 = 0.13 mm), and was corrected for platinum. The difference between the temperatures of the working surfaces, AT = (T: - T, ), was adjusted to be as small as 2°C. The temperature of the measurement (T) was adjusted to equal gr, + T2) when measured very near the specimen and at the mid-point of the plane parallel to it. The
253
254
M. M.
IBRAHIM et al.
60-
Fig. 3. Effect of annealing time on the Seebeck coefficient measured at room temperature for specimens with different at.% Te annealed at 480°C. : -6O-
3. RESULTS AND DISCUSSION Fig. 1. Dependence of the Seebeck coefficient measured at room temperature on composition of untreated bulk specimens.
thermovoltages of the specimens as well as the thermocouples (chromel-alumel) were all measured by a digital multimeter, type Keithley 177, with resolution as small as I pV. X-Ray diffraction was carried out for the powders (54pm) of both as-prepared and thermally treated specimens using a Schematzu-X-ray diffractometer with Cu K target, giving a monochromatic beam with wavelength 1.54 A. For scanning electron microscopy examinations, sample surfaces were etched using the etcher suggested by Sagar and Faust [lo] which is a 10% solution of bromine in methanol. The etching time was usually 2min. The surface microstructure was revealed by the scanning electron microscope, type JEOL-Japan.
As shown in Fig. I, the as-prepared specimens of the system Bi-Te seem to behave as n-type and/or p-type with the activation of electron or holes and/or mixed activation of both electrons and holes depending on the Te content in the compositions. However, the phase diagram of this system showed the possibility of forming different phases [7]. Figure 2 shows that annealing at 270°C which is almost the melting point of Bi(27lS’C), does not change the polarity. For x = 0.4,0.45 and 0.6 at., the activation of holes remained dominant and S increased with prolonged time of annealing (t,) up to 240 min, after this it possessed values almost independent of I,,. For x = 0.3 and 0.5 at., S remained negative. Longer periods of annealing had almost no affect on the values of S. As shown in Fig. 3 the polarity of S could be changed by annealing even for a short time at 480°C which is slightly higher than the melting point of Te (~4509 In general, the activation of electrons
100
ry*
X-X
60
at. Y.Te I-
t
Fig. 2. Effect of annealing time on the Seebeck coefficient measured at room temperature for specimens with different at.% Te annealed at 270°C.
Bulk specimens of the system Bi, _,=Te,
Fig. 4. S-1000/7’relation for specimens with different at.% Te anneated at 270°C for 480 min.
diminished and that of holes increased with prolongation of fGafor both compositions with x = 0.3 and 0.5 at. As shown in Fig. 4, for different composidons annealed at 270°C for 480 min, the relation between S and l/7’ (T is the ambient temperature) was linear, consisting of more than one stage. In the range near room temperature, S of compositions 0.3, 0.35 and 0.55 at.Te showed negative polarity. Compensation took place at 7’ = 60, 115 and 48°C for x = 0.3,0.35 and 0.55 at., respectively. The activation of holes increased continuously with increasing T. Except for
,o
x = 0.55 at. and within the whole range of T, the S values are positive. Figure 5 shows linear S vs l/T plots for different compositions annealed at 500°C for 480 min. Transition from positive to negative polarity of S took place as T exceeded 75°C for x = 0.35 at. and 72°C for x = 0.55 at. Exceeding these two temperatures, the (-S) increased continuously with increasing T. For the other two compositions with x = 0.40 and 0.60 at., S showed only positive polarity over the whole range of T. The slopes of the S-I/T relations reversed their signs as T exceeded 222 and 185°C for x = 0.40 and 0.60 at., respectively, showing that the contribution of electrons to the activation was increased with increasing T, but compensation was never reached. Regarding the results in Figs 4 and 5, it may be concluded that both the value and the polarity of S could be changed with changing Te content, annealing conditions and the temperature of measurement. This might be attributed to corresponding phase changes and transfo~ations as revealed by microstructural analysis. Such behaviour of S with 7’ indicates the instability of the system (Bi-Te) even after being heat-treated at temperatures near the melting point for long periods of time. However,
(a 1 60 at. %Te
1.
Bi
(5)
55 at.%Te
2.
Te
(cf
45at.%Te
3.
Bi,Te,
Id)
35 ot.%Te
4.
lhiderrtif ied
Ce)
30ot.%Te
-240 *(a) 3sut.%T*
L rr
z.55
.(b)4Oat.%TI Q (5) $0 ot.'*T*
-360
2
~1(d) 60 at.%T*
v? -480
-72o-P 0
0
Fig. 5. S-lOOO/Trctation for specimens with different at.% Te anneaied at SCWC for 480min.
t
I
I
I
I
I
I
I
I
54
46
42
36
30
24
I8
12
6
28 Fig. 6. X-Ray diffractograms for untreated specimens with different at.% Te.
256
M. M.
IBRAHIM
those compositions exhibiting one mode of polarity over wide ranges of the ambient temperature show stability. Results shown in Figs 4 and 5 fit well the equation of Edmond [l I] and Cutler and Mott [12]. S=
+ S+21ie+/4), (
where A is constant. Thus the activation energy (E,) of the thermoelectric power could be obtained from the slope. In addition a positive sign means that A& represents the electrochemical potential for P-type materials, similarly a negative sign indicates n-type materials, where E, represents the Fermi energy. Figures 4 and 5 show that the activation energy (from the slope) varied unsequentially with x and T. In the case of specimens annealed at 270’C for 480 min (Fig. 4) the higher range of ambient temperature is, in general, characterized by p-type behaviour (+ S). For the specimens annealed at 5OO’C for 480min Fig. 5 shows the promotion of electron activation by
(a)
600t
%Te
(b)
55at
%Te
(cl
45ot
%Te
(d)
35at
%Te
et al.
annealing, and that the lower range of ambient temperature is, in general, characterized by P-type behaviour (-t 5). The X-ray diffractograms shown in Fig. 6 confirm the possibility of the existence of different crystalline phases for compositions which have not been thermally heated. As denoted with the corresponding marks on the diffractograms, Bi,Te, and both Te and Bi in their elemental crystalline phases could be identified. These results confirm those obtained by Abrikosov and Bankina [ 131. Except for .x = 0.60 at., the Te phase could not be identified. The most intense phase was that of Bi,Te,. Comparing the diffractograms plotted in Figs 6 and 7 it may be concluded that: phase transformation and formation of new phases and/or dissolution of existing phases are all possible with annealing. With annealing at 27OC for 480 min for .‘c= 0.35 and 0.55 at., the elemental Te phase is dissolved and could not be identified, and the intensity of crystallization of both Bi and Bi,Te, is changed, for all the
I
4
Unldentlf
4
led
I
3
33 3
(0)
3 J 3
(b) 3 $L.?t_$i
3
A
3
3 2
3 Cc) A
I
60
32
I 54
I 46
I 42
I 36
I 30
I 24
I 18
I I2
I 6
28
Fig. 7. X-Ray
diflractograms
for specimens
with different
at.%
Te annealed
at 27O’C for 480min.
25-l
Bulk specimens of the system Bi, _ ,Te,
(b)
(500 X)
Cd) (750X)
Fig. 8. Photomicrographs for specimens annealed for 480 min at: (a) 48O‘C with 35 at.% Te; (b) 480-C with 58 at.% Te; (c) 38O’C with 55 at.% Te; and (d) SOO^Cwith 55 at.% Te.
compositions, by annealing. In addition, the Te elemental phase could not be identified for x = 0.60
at. as before annealing. and the most intense phase no longer corresponded to Bi. The photomicrographs shown in Fig. 8(a,b) are those of two specimens with .x = 0.35 and 0.58 at. The features of the surface microstructure seemed quite different. For .Y= 0.35 at. the structure is absolutely discontinuous. The different separated phases occupied certain areas, and the microcrystallites seemed small in size and oriented in almost the same direction. The microstructure of the other composition-Fig. 8(b)--seemed more homogeneous, randomly oriented sticks formed crystallites which were larger in size. Furthermore. the different separated phases appear clearly but more inter-diffused. The photomicrographs in Fig. 8(c,d) show the surface microstructure of two specimens with x = 0.55 at. annealed at 380 and 5OO’C for 480 min, respectively. Two different structures appear, confirming the effect of the annealing temperature. However, both show the existence of microcrystallites. Larger crystallites could be observed in the photomicrograph [Fig. 8(c)] although the temperature of annealing is relatively low.
4. COSCI.xSION (i) Both the value and polarity of S could be changed depending on: Te content, annealing conditions and the temperature of measurement. (ii) For the compositions Bi,,Te,, and Bi,,Team, compensation could not be held and both showed positive polarity over the whole considered range of the ambient temperature. indicating stability. (iii) Annealing carried out at 270 and 5OO’C was, in general, characterized by the promotion of hole and electron activation, respectively, and each activation was greatly enhanced by elevation of the ambient temperature. (iv) With both as-prepared and annealed specimens, there is the possibility of the existence of the three phases, Bi, Te and Bi?Te,, together [13]. The intensity of microcrystalline phases embedded into the amorphous matrix depends on Te content and annealing conditions. REFERESCES 1. Yokota K. and Katayama S., Technology Report of Kansai University, No. 16 (1975). 2. Ukhlirtov G. A., Gamin V. P. and Vigdovoich V. N., Socier Phys. Srmicond 13, 1360 (1979).
258
M. M.
IBRAHIM
3. Champness C. H. and Kipling A. L., Can. J. Phyr. 44, 769 (1966). 4. Smith G. E. and Wolfe R., J. appl. Phys. 33,841 (1962). 5. Ainsworth L.. Proc. phyx Sm. (Land.) B69.606 (1956). 6. Satterthwaite C. B. and Ure R. W., Phys. Rev. 10&1164 (1957). 7. Brown A. and Lewis B., J. Phys. Chem. Solids 23, 1597 (1962). 8. Abd El-Rahiem M. N., Structure and Electrical Prop-
9. 10.
Il. 12.
13.
et al. erties of Bi-Ge-Se, Semi-conductor Alloys, Ph.D. thesis, Assiut University (1985). Shen Y. R., Leonard W. F. and Yu H. Y., Rec. scimt. Instrum. 48, 688 (1977). Sagar A. and Faust J. W., J. appl. Phys. 38,482 (1967). Edmond J. T., Br. J. appi. Phys. 17, 979 (1966). Cutler M. and Mott N. F.. Phys. Rev. 181.1336 (1969). Abrikosov N. Kh. and Bankina V. F., Zh. neorg. Khim. 3, 659 (1958) (in Russian).
Abstract-The thermoelectric power of bulk specimens of the system Bi, _,Te,V (0.3 G x iE 0.6 at.) was studied. The effects of Te content, temperature and time of annealing were considered. For most of the compositions considered, the activation of holes seemed predominant. The electron activations, as well as mixed activations, seemed dependent on Te content and the conditions of annealing. Compensation semiconductor behaviour could also be observed. However, both the value and polarity of the Seebeck coefficient seemed dependent on Te content, conditions of annealing and the temperature of measurement. Keywordsr Bi, _ %Te,*.thermoelectric power, Seebeck coefficient, X-ray diffraction, microstructure.
1. lNTRODUCTION In recent years, a number of compound semiconductors and their alloys have been the subject of intensive investigation as possible materials for thermoelectric devices. Among these materials, the binaries such as Bi,Te, and Bi,Se, have recently been of considerable interest. The annealing process has been found to affect both the polarity and values of the Seebeck coefficient (S). Yokota and Katayama (11 found that the P-type Bi:Te, compound changes to n-type after a short time of annealing at temperatures of 400 and 500°C. Ukllinov et al. [2] noted that the increase in thermoelectric power, resulting from the opposite effects of the contact potential on the nature of the surface scattering of electrons and holes in the Bi layers, can be considered an improvement in the~oelect~c properties. Detailed measurements of S were made on different samples of the system Bi,TeJ+,%with x ranging from -0.06 to +0.48 at. The magnitude of S at temperatures above room temperature appeared to require a value of b (electron-to-hole mobility ratio) of about 2 ]3]. For single crystals of several alloys of Bi, _.TTe, (0.1 < x Q 0.4 at.), Smith and Wolfe (41 found that the thermoelectric properties were anisotropic. The present paper aims to investigate the thermoelectric power of bulk specimens of the system Bi,_,Te, (0.3
2. EXPERIMENTAL WORK A study of the preparation of Bi,Te, has been reported [S-7]. fn the present work, the method
used for the preparation of Bi-Te alloys is basically that previously suggested by Brown and Lewis [7], in which the component elements are simultaneously heated and agitated in evacuated silica capsules. The working materials: Bi(99.999%, Gold Label) and Te(99.999%, Gold Label) were supplied by Aldrich Chemical Co. The different syntheses were attempted at temperatures only slightly in excess of their solidus temperature predicted from the phase diagram 171.To promote the reaction between liquid Bi and solid Te, and to homogenize the contents, the assembly was first heated to the melting point of Bi and immediately shaken. The furnace temperature was immediately raised above the soiidus temperature of the desired composition. Shaking of the assembly at the maximum temperature was continued for about 10 h after which the assembly was cooled in steps to 250°C over a period of 6 h and the shaking was then stopped. Quenching at O’C was then immediately carried out. Samples with the required thickness were then prepared by automatic cutting. Bulk specimens with parallel and optically flat surfaces could then be obtained by mechanical polishing. Cleaning of specimen surfaces was performed by using the extractor condenser technique with acetone as a solvent [S]. For the thermoelectric measurement, a modified Shen et al. method [9] was ,used. The thermoelectric power was measured using platinum probes (4 = 0.13 mm), and was corrected for platinum. The difference between the temperatures of the working surfaces, AT = (T: - T, ), was adjusted to be as small as 2°C. The temperature of the measurement (T) was adjusted to equal gr, + T2) when measured very near the specimen and at the mid-point of the plane parallel to it. The
253
254
M. M.
IBRAHIM et al.
60-
Fig. 3. Effect of annealing time on the Seebeck coefficient measured at room temperature for specimens with different at.% Te annealed at 480°C. : -6O-
3. RESULTS AND DISCUSSION Fig. 1. Dependence of the Seebeck coefficient measured at room temperature on composition of untreated bulk specimens.
thermovoltages of the specimens as well as the thermocouples (chromel-alumel) were all measured by a digital multimeter, type Keithley 177, with resolution as small as I pV. X-Ray diffraction was carried out for the powders (54pm) of both as-prepared and thermally treated specimens using a Schematzu-X-ray diffractometer with Cu K target, giving a monochromatic beam with wavelength 1.54 A. For scanning electron microscopy examinations, sample surfaces were etched using the etcher suggested by Sagar and Faust [lo] which is a 10% solution of bromine in methanol. The etching time was usually 2min. The surface microstructure was revealed by the scanning electron microscope, type JEOL-Japan.
As shown in Fig. I, the as-prepared specimens of the system Bi-Te seem to behave as n-type and/or p-type with the activation of electron or holes and/or mixed activation of both electrons and holes depending on the Te content in the compositions. However, the phase diagram of this system showed the possibility of forming different phases [7]. Figure 2 shows that annealing at 270°C which is almost the melting point of Bi(27lS’C), does not change the polarity. For x = 0.4,0.45 and 0.6 at., the activation of holes remained dominant and S increased with prolonged time of annealing (t,) up to 240 min, after this it possessed values almost independent of I,,. For x = 0.3 and 0.5 at., S remained negative. Longer periods of annealing had almost no affect on the values of S. As shown in Fig. 3 the polarity of S could be changed by annealing even for a short time at 480°C which is slightly higher than the melting point of Te (~4509 In general, the activation of electrons
100
ry*
X-X
60
at. Y.Te I-
t
Fig. 2. Effect of annealing time on the Seebeck coefficient measured at room temperature for specimens with different at.% Te annealed at 270°C.
Bulk specimens of the system Bi, _,=Te,
Fig. 4. S-1000/7’relation for specimens with different at.% Te anneated at 270°C for 480 min.
diminished and that of holes increased with prolongation of fGafor both compositions with x = 0.3 and 0.5 at. As shown in Fig. 4, for different composidons annealed at 270°C for 480 min, the relation between S and l/7’ (T is the ambient temperature) was linear, consisting of more than one stage. In the range near room temperature, S of compositions 0.3, 0.35 and 0.55 at.Te showed negative polarity. Compensation took place at 7’ = 60, 115 and 48°C for x = 0.3,0.35 and 0.55 at., respectively. The activation of holes increased continuously with increasing T. Except for
,o
x = 0.55 at. and within the whole range of T, the S values are positive. Figure 5 shows linear S vs l/T plots for different compositions annealed at 500°C for 480 min. Transition from positive to negative polarity of S took place as T exceeded 75°C for x = 0.35 at. and 72°C for x = 0.55 at. Exceeding these two temperatures, the (-S) increased continuously with increasing T. For the other two compositions with x = 0.40 and 0.60 at., S showed only positive polarity over the whole range of T. The slopes of the S-I/T relations reversed their signs as T exceeded 222 and 185°C for x = 0.40 and 0.60 at., respectively, showing that the contribution of electrons to the activation was increased with increasing T, but compensation was never reached. Regarding the results in Figs 4 and 5, it may be concluded that both the value and the polarity of S could be changed with changing Te content, annealing conditions and the temperature of measurement. This might be attributed to corresponding phase changes and transfo~ations as revealed by microstructural analysis. Such behaviour of S with 7’ indicates the instability of the system (Bi-Te) even after being heat-treated at temperatures near the melting point for long periods of time. However,
(a 1 60 at. %Te
1.
Bi
(5)
55 at.%Te
2.
Te
(cf
45at.%Te
3.
Bi,Te,
Id)
35 ot.%Te
4.
lhiderrtif ied
Ce)
30ot.%Te
-240 *(a) 3sut.%T*
L rr
z.55
.(b)4Oat.%TI Q (5) $0 ot.'*T*
-360
2
~1(d) 60 at.%T*
v? -480
-72o-P 0
0
Fig. 5. S-lOOO/Trctation for specimens with different at.% Te anneaied at SCWC for 480min.
t
I
I
I
I
I
I
I
I
54
46
42
36
30
24
I8
12
6
28 Fig. 6. X-Ray diffractograms for untreated specimens with different at.% Te.
256
M. M.
IBRAHIM
those compositions exhibiting one mode of polarity over wide ranges of the ambient temperature show stability. Results shown in Figs 4 and 5 fit well the equation of Edmond [l I] and Cutler and Mott [12]. S=
+ S+21ie+/4), (
where A is constant. Thus the activation energy (E,) of the thermoelectric power could be obtained from the slope. In addition a positive sign means that A& represents the electrochemical potential for P-type materials, similarly a negative sign indicates n-type materials, where E, represents the Fermi energy. Figures 4 and 5 show that the activation energy (from the slope) varied unsequentially with x and T. In the case of specimens annealed at 270’C for 480 min (Fig. 4) the higher range of ambient temperature is, in general, characterized by p-type behaviour (+ S). For the specimens annealed at 5OO’C for 480min Fig. 5 shows the promotion of electron activation by
(a)
600t
%Te
(b)
55at
%Te
(cl
45ot
%Te
(d)
35at
%Te
et al.
annealing, and that the lower range of ambient temperature is, in general, characterized by P-type behaviour (-t 5). The X-ray diffractograms shown in Fig. 6 confirm the possibility of the existence of different crystalline phases for compositions which have not been thermally heated. As denoted with the corresponding marks on the diffractograms, Bi,Te, and both Te and Bi in their elemental crystalline phases could be identified. These results confirm those obtained by Abrikosov and Bankina [ 131. Except for .x = 0.60 at., the Te phase could not be identified. The most intense phase was that of Bi,Te,. Comparing the diffractograms plotted in Figs 6 and 7 it may be concluded that: phase transformation and formation of new phases and/or dissolution of existing phases are all possible with annealing. With annealing at 27OC for 480 min for .‘c= 0.35 and 0.55 at., the elemental Te phase is dissolved and could not be identified, and the intensity of crystallization of both Bi and Bi,Te, is changed, for all the
I
4
Unldentlf
4
led
I
3
33 3
(0)
3 J 3
(b) 3 $L.?t_$i
3
A
3
3 2
3 Cc) A
I
60
32
I 54
I 46
I 42
I 36
I 30
I 24
I 18
I I2
I 6
28
Fig. 7. X-Ray
diflractograms
for specimens
with different
at.%
Te annealed
at 27O’C for 480min.
25-l
Bulk specimens of the system Bi, _ ,Te,
(b)
(500 X)
Cd) (750X)
Fig. 8. Photomicrographs for specimens annealed for 480 min at: (a) 48O‘C with 35 at.% Te; (b) 480-C with 58 at.% Te; (c) 38O’C with 55 at.% Te; and (d) SOO^Cwith 55 at.% Te.
compositions, by annealing. In addition, the Te elemental phase could not be identified for x = 0.60
at. as before annealing. and the most intense phase no longer corresponded to Bi. The photomicrographs shown in Fig. 8(a,b) are those of two specimens with .x = 0.35 and 0.58 at. The features of the surface microstructure seemed quite different. For .Y= 0.35 at. the structure is absolutely discontinuous. The different separated phases occupied certain areas, and the microcrystallites seemed small in size and oriented in almost the same direction. The microstructure of the other composition-Fig. 8(b)--seemed more homogeneous, randomly oriented sticks formed crystallites which were larger in size. Furthermore. the different separated phases appear clearly but more inter-diffused. The photomicrographs in Fig. 8(c,d) show the surface microstructure of two specimens with x = 0.55 at. annealed at 380 and 5OO’C for 480 min, respectively. Two different structures appear, confirming the effect of the annealing temperature. However, both show the existence of microcrystallites. Larger crystallites could be observed in the photomicrograph [Fig. 8(c)] although the temperature of annealing is relatively low.
4. COSCI.xSION (i) Both the value and polarity of S could be changed depending on: Te content, annealing conditions and the temperature of measurement. (ii) For the compositions Bi,,Te,, and Bi,,Team, compensation could not be held and both showed positive polarity over the whole considered range of the ambient temperature. indicating stability. (iii) Annealing carried out at 270 and 5OO’C was, in general, characterized by the promotion of hole and electron activation, respectively, and each activation was greatly enhanced by elevation of the ambient temperature. (iv) With both as-prepared and annealed specimens, there is the possibility of the existence of the three phases, Bi, Te and Bi?Te,, together [13]. The intensity of microcrystalline phases embedded into the amorphous matrix depends on Te content and annealing conditions. REFERESCES 1. Yokota K. and Katayama S., Technology Report of Kansai University, No. 16 (1975). 2. Ukhlirtov G. A., Gamin V. P. and Vigdovoich V. N., Socier Phys. Srmicond 13, 1360 (1979).
258
M. M.
IBRAHIM
3. Champness C. H. and Kipling A. L., Can. J. Phyr. 44, 769 (1966). 4. Smith G. E. and Wolfe R., J. appl. Phys. 33,841 (1962). 5. Ainsworth L.. Proc. phyx Sm. (Land.) B69.606 (1956). 6. Satterthwaite C. B. and Ure R. W., Phys. Rev. 10&1164 (1957). 7. Brown A. and Lewis B., J. Phys. Chem. Solids 23, 1597 (1962). 8. Abd El-Rahiem M. N., Structure and Electrical Prop-
9. 10.
Il. 12.
13.
et al. erties of Bi-Ge-Se, Semi-conductor Alloys, Ph.D. thesis, Assiut University (1985). Shen Y. R., Leonard W. F. and Yu H. Y., Rec. scimt. Instrum. 48, 688 (1977). Sagar A. and Faust J. W., J. appl. Phys. 38,482 (1967). Edmond J. T., Br. J. appi. Phys. 17, 979 (1966). Cutler M. and Mott N. F.. Phys. Rev. 181.1336 (1969). Abrikosov N. Kh. and Bankina V. F., Zh. neorg. Khim. 3, 659 (1958) (in Russian).