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Annealing behavior of quench-deposited amorphous GeTe and SnTe films
Journal of Non-Crystalline Solids 24 (1977) 131-136 © North-Holland Publishing Company
LETTERS TO THE EDITOR
ANNEALING BEHAVIOR OF QUENCH-DEPOSITED AMORPHOUS GeTe AND SnTe FILMS R.W. BROWN * Naval Surface Weapons Center, White Oak, Silver Spring, Maryland 20910, USA
Received 16 February 1977
The main purposes of this letter are to present data on the resistivity of quenchdeposited amorphous GeTe f'rims and to compare the annealing effects observed with those found earlier in amorphous SnTe. Both GeTe and SnTe are amorphous when evaporated onto substrates held well below their crystallization temperatures. For GeTe, this temperature is roughly 400 K, and it is well established that CreTe films sputtered or vapor-deposited onto room temperature substrates are amorphous. The electrical and optical properties of such films have been studied extensively [ 1-10]. The crystallization temperature of amorphous SnTe films is about 180 K. We previously reported the resistivity as a function of temperature for amorphous SnTe f'dms quench-deposited on substrates at 60 K. Temperature cycling in the 6 0 - 1 8 0 K range produced irreversible annealing to lower resistivities [11]. We have now extended these measurements to amorphous GeTe. The f'dms were vapor deposited at 60 K and temperature cycled between 60 and 400 K. The annealing behavior which resulted supplements that published earlier on room-temperature deposited amorphous GeTe [ 1 - 6 , 8 - 9 ] , and exhibits some features which are similar to and others which are different from those of our earlier SnTe measurements [11 ]. The experiments were carried out using the improved version of the apparatus with which some of the measurements on the amorphous SnTe films were made [11]. The starting material was polycrystalline powder, originally prepared from high-purity constituents by R.F. Brebrick. The GeTe films were deposited on crystalline quartz under a vacuum of about 10 - 7 torr. During the subsequent temperature cycling, the pressure was considerably lower. Film thicknesses of 1000 ,~ were produced at deposition rates of about 20 A/s. The resistivity of a GeTe film as a function of temperature is shown in fig. 1 ; the circled numbers from 1 to 10 indicate the sequence of the measurements. Sections of the curves along which irreversible annealing took place are identified by dashed lines; no annealing was detected along those sections marked with solid lines. The * Deceased; correspondence should be addressed to Dr. R.S. Allgaier at above address. 131
R.W. Brown / Annealing behaviour of amorphous GeTe and SnTe films
132
333
286
250
I
222
200
182
167
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I
I
I
I
"=54
143
133
I
I
(~1
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108
,~E .28
107 .28
i ~
N
106
g
¢_
/
.31
105 --
104
~ /
.37e~1(~/~ •//e/
0])~/ 10 3
103/T(OK-l) 1
3.5
|
4
I
4.5
1
5
l
5.5
I
6
]
6.5
l
7
1
7.5
Fig. 1. Annealing history of GeTe film quench-deposited on a substrate held at 60 K. The circled integers, solid and dashed portions of the curve, and AE values are explained in the text.
gaps between the solid segments do not indicate any interruptions in the datataking process. They merely delineate temperature intervals over which average slope values were calculated for the data, as discussed below. As deposited at 60 K, the resistivity of the film could not be determined. As in the case of the earlier SnTe work [11], the highest resistance that could be measured accurately was 5 X 10: l ~2, corresponding to a film resistivity o f about 108
R. 14I.Brown /Annealing behaviour of amorphous GeTe and SnTe films
133
~2-cm. Thus the data in fig. 1 begin at about 140 K, considerably higher than the 65 K minimum temperature resistivity measurement in the amorphous SnTe films [11 ]. But the temperature range covered by the GeTe data is actually larger than for SnTe, because of the higher crystallization temperature of GeTe. Figure 1 shows that reversibility occurs only if the sample remains at temperatures below the highest one previously achieved, and also that the irreversible annealing of amorphous GeTe produces higher resistivities. The numbers printed alongside the reversible segments of the data in fig. 1 are the values of AE obtained by fitting the resistivity to the expression (1)
P = Pl e x p ( A E / K T ) .
There is a gradual increase of AE with increasing temperature, and fig. 1 also shows that two different reversible sections covering the same temperature interval have the same AE values. A similar plot for an amorphous SnTe film deposited on a substrate at 60 K [11] is shown in fig. 2, with AE values assigned to the reversible sections, as in fig. 167
143
125
111
100
91
83
77
71.5
66.5
I
I
I
I
l
I
I
I
I
T'°K
~ E = . O 9 J
(~)
*(~)
107
/ / 106
105
g 104
lO 3
102
/" I 7
210 6
I 8
I 9
1 10
1 11
I 12
I 13
t 14
L 15
103/T (oK 1)
]Fig. 2. Annealing history of SnTe film quench-deposited on a substrate held at 60 K.
16
134
R.W. Brown /Annealing behaviour of amorphous GeTe and SnTe films
1. Again, there is a gradual increase in AE with increasing temperature, and AE for different reversible sections within a given temperature interval remains the same. However, the resistivity of amorphous SnTe anneals to lower, rather than higher, values. The parallel shifting (i.e. constant AE') of the resistivity-temperature data seen in both figs. 1 and 2 is reminiscent of the annealing behavior of many metals, e.g. tin and lead, when deposited on a substrate held at low temperature, typically that of liquid helium [ 12,13 ]. The initial deposit has a high resistivity, attributed to defects on a microscopic scale frozen into what is otherwise the normal vibrating crystalline lattice [14]. In this case, annealing removes these defects, lowering the residual resistivity Po and thus the total resistivity p, in accordance with Matthiessen's rule, p = po + p ( ~ .
(2)
If the annealing is interrupted, and the temperature is cycled below the highest value previously reached, the resistivities are reversible. Such annealing records for lead and tin f'rims are shown in fig. 3 [15]. A number of the previously published studies of the resistivity in amorphous GeTe have investigated the possibility of annealing effects [4-10]. Pronounced changes in resistivity were observed in sputtered fdms [5,10] and almost always in vapor-deposited f'dms [4-8]. The notable exception in the latter category is the work of Nath et al. [9]. Resistivity changes, when observed after a vacuum anneal, were always towards larger values, as in the present work. However, most of the previous studies were made on films deposited on substrates held at room temperature or above. And no changes in resistivity were reported previously unless the annealing was carried out above room temperature [4-8]. Some previous studies have warned of spurious annealing effects caused by diffusion of metallic contacts into the amorphous materials [4,10]. But again, such effects were observed only when the annealing takes place well above room temperature. It is intriguing that the resistivity changes in opposite directions when amorphous GeTe and SnTe f'dms are annealed. Although some controversy remains concerning the GeTe2 case [5,9], studies of the composition dependence of various properties in the amorphous Ge-Te system suggest that at the 50-50 composition, it is appropriate to regard the material as a random alloy of Ge and Te, rather than as an amorphous form of the compound GeTe [5,9]. Presumably, a similar statement could be made concerning amorphous SnTe. Thus it should be mentioned that the opposite annealing effects in amorphous GeTe and SnTe do correspond to the resistivity increases and decreases which occur when amorphous Ge and Sn are annealed. The last sentence of the preceding paragraph is an observation, not an explanation. We therefore hope that the presentation of these results on the low-temperature annealing of amorphous GeTe and their comparison with the earlier data on amorphous SnTe will stimulate further work which will ultimately lead to a better
R.W. Brown /Annealing behaviour of amorphous GeTe and SnTe films
135
TIN FILM
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I
LEAO F I L M
90
I
120 1 50 180 210 TEMPERATURE,°K
120
150
180
~
210
I
240
270
~
240
, 1
300
)
270
300
TEMPERATURE,°K
Fig. 3. Annealing histories of Sn and Pb films quench-deposited at substrates held at 4,2 K. understanding of the transport mechanisms in amorphous material and their relation to structural disorder.
Note. The above letter constitutes the last work carried out by the late Dr. Brown. It was originally submitted for publication in 1975. What appears above is a slightly revised version, with updated references, prepared by the undersigned to whom correspondence should be addressed. The paper is dedicated to the memory of an outstanding scientist with whom it was a privilege to have collaborated. R.S. Allgaier Naval Surface Weapons Center, White Oak, Silver Spring, Maryland 20910, USA
136
R. W. Brown /Annealing behaviour o f amorphous GeTe and Sn Te films
References [1] [2] [3] [4] [5 ]
[6] [7] [81 [9] [10] [11 ] [12] [13] [14] [ 15 ]
S,K. Bahl and K.L. Chopra, J. Vac. Sci. Technol. 6 (1969) 561. K.L. Chopra and S.K. Bahl, J. Appl. Phys. 40 (1969) 4171. S.K. Bahl and K.L. Chopra, J. Appl. Phys. 41 (1970) 2196. W.E. Howard and R. Tsu, Phys. Rev. B1 (1970) 4709. H.K. Rockstad and J.P. deNeufville, Proceedings of the 11 th International Conference on the Physics of Semiconductors, Warsaw (PWN-Polish Scientific Publishers, Warsaw, 1972) p. 542. C. Wood, L.R. Gilbert, R. Mueller and C.M. Garner, J. Vac. Sci. Technol. 10 (1973) 739. H,R. Riedl, K.P. Scharnhorst and D.G. Simons, Infrared Phys. 14 (1974) 139. K.P. Scharnhorst and H.R. Riedl, J. Appl. Phys. 45 (1974) 2971. P. Nath, S.K. Suri and K.L. Chopra, Phys. Stat. Solidi (a) 30 (1975) 771. A. Deneuville, P. Gerard and J. Devenyi, J. Non-Crystalline Solids 22 (1976) 77. R.W. Brown, A.R. Millner and R.S. Allgaier, Thin Solid Films 5 (1970) 157. R. Hilsch, Conference on Non-Crystalline Solids, Alfred, New York, 1958 (Wiley, New York, 1960) p. 348. W. Buckel and R. Hilsch, Z. Phys. 138 (1954) 109. W. Riihl, Z. Phys. 138 (1954) 121. R.W. Brown, PhD. thesis (University of Maryland, College Park, Md, 1967) p. 41.
LETTERS TO THE EDITOR
ANNEALING BEHAVIOR OF QUENCH-DEPOSITED AMORPHOUS GeTe AND SnTe FILMS R.W. BROWN * Naval Surface Weapons Center, White Oak, Silver Spring, Maryland 20910, USA
Received 16 February 1977
The main purposes of this letter are to present data on the resistivity of quenchdeposited amorphous GeTe f'rims and to compare the annealing effects observed with those found earlier in amorphous SnTe. Both GeTe and SnTe are amorphous when evaporated onto substrates held well below their crystallization temperatures. For GeTe, this temperature is roughly 400 K, and it is well established that CreTe films sputtered or vapor-deposited onto room temperature substrates are amorphous. The electrical and optical properties of such films have been studied extensively [ 1-10]. The crystallization temperature of amorphous SnTe films is about 180 K. We previously reported the resistivity as a function of temperature for amorphous SnTe f'dms quench-deposited on substrates at 60 K. Temperature cycling in the 6 0 - 1 8 0 K range produced irreversible annealing to lower resistivities [11]. We have now extended these measurements to amorphous GeTe. The f'dms were vapor deposited at 60 K and temperature cycled between 60 and 400 K. The annealing behavior which resulted supplements that published earlier on room-temperature deposited amorphous GeTe [ 1 - 6 , 8 - 9 ] , and exhibits some features which are similar to and others which are different from those of our earlier SnTe measurements [11 ]. The experiments were carried out using the improved version of the apparatus with which some of the measurements on the amorphous SnTe films were made [11]. The starting material was polycrystalline powder, originally prepared from high-purity constituents by R.F. Brebrick. The GeTe films were deposited on crystalline quartz under a vacuum of about 10 - 7 torr. During the subsequent temperature cycling, the pressure was considerably lower. Film thicknesses of 1000 ,~ were produced at deposition rates of about 20 A/s. The resistivity of a GeTe film as a function of temperature is shown in fig. 1 ; the circled numbers from 1 to 10 indicate the sequence of the measurements. Sections of the curves along which irreversible annealing took place are identified by dashed lines; no annealing was detected along those sections marked with solid lines. The * Deceased; correspondence should be addressed to Dr. R.S. Allgaier at above address. 131
R.W. Brown / Annealing behaviour of amorphous GeTe and SnTe films
132
333
286
250
I
222
200
182
167
I
I
I
I
I
"=54
143
133
I
I
(~1
e~
108
,~E .28
107 .28
i ~
N
106
g
¢_
/
.31
105 --
104
~ /
.37e~1(~/~ •//e/
0])~/ 10 3
103/T(OK-l) 1
3.5
|
4
I
4.5
1
5
l
5.5
I
6
]
6.5
l
7
1
7.5
Fig. 1. Annealing history of GeTe film quench-deposited on a substrate held at 60 K. The circled integers, solid and dashed portions of the curve, and AE values are explained in the text.
gaps between the solid segments do not indicate any interruptions in the datataking process. They merely delineate temperature intervals over which average slope values were calculated for the data, as discussed below. As deposited at 60 K, the resistivity of the film could not be determined. As in the case of the earlier SnTe work [11], the highest resistance that could be measured accurately was 5 X 10: l ~2, corresponding to a film resistivity o f about 108
R. 14I.Brown /Annealing behaviour of amorphous GeTe and SnTe films
133
~2-cm. Thus the data in fig. 1 begin at about 140 K, considerably higher than the 65 K minimum temperature resistivity measurement in the amorphous SnTe films [11 ]. But the temperature range covered by the GeTe data is actually larger than for SnTe, because of the higher crystallization temperature of GeTe. Figure 1 shows that reversibility occurs only if the sample remains at temperatures below the highest one previously achieved, and also that the irreversible annealing of amorphous GeTe produces higher resistivities. The numbers printed alongside the reversible segments of the data in fig. 1 are the values of AE obtained by fitting the resistivity to the expression (1)
P = Pl e x p ( A E / K T ) .
There is a gradual increase of AE with increasing temperature, and fig. 1 also shows that two different reversible sections covering the same temperature interval have the same AE values. A similar plot for an amorphous SnTe film deposited on a substrate at 60 K [11] is shown in fig. 2, with AE values assigned to the reversible sections, as in fig. 167
143
125
111
100
91
83
77
71.5
66.5
I
I
I
I
l
I
I
I
I
T'°K
~ E = . O 9 J
(~)
*(~)
107
/ / 106
105
g 104
lO 3
102
/" I 7
210 6
I 8
I 9
1 10
1 11
I 12
I 13
t 14
L 15
103/T (oK 1)
]Fig. 2. Annealing history of SnTe film quench-deposited on a substrate held at 60 K.
16
134
R.W. Brown /Annealing behaviour of amorphous GeTe and SnTe films
1. Again, there is a gradual increase in AE with increasing temperature, and AE for different reversible sections within a given temperature interval remains the same. However, the resistivity of amorphous SnTe anneals to lower, rather than higher, values. The parallel shifting (i.e. constant AE') of the resistivity-temperature data seen in both figs. 1 and 2 is reminiscent of the annealing behavior of many metals, e.g. tin and lead, when deposited on a substrate held at low temperature, typically that of liquid helium [ 12,13 ]. The initial deposit has a high resistivity, attributed to defects on a microscopic scale frozen into what is otherwise the normal vibrating crystalline lattice [14]. In this case, annealing removes these defects, lowering the residual resistivity Po and thus the total resistivity p, in accordance with Matthiessen's rule, p = po + p ( ~ .
(2)
If the annealing is interrupted, and the temperature is cycled below the highest value previously reached, the resistivities are reversible. Such annealing records for lead and tin f'rims are shown in fig. 3 [15]. A number of the previously published studies of the resistivity in amorphous GeTe have investigated the possibility of annealing effects [4-10]. Pronounced changes in resistivity were observed in sputtered fdms [5,10] and almost always in vapor-deposited f'dms [4-8]. The notable exception in the latter category is the work of Nath et al. [9]. Resistivity changes, when observed after a vacuum anneal, were always towards larger values, as in the present work. However, most of the previous studies were made on films deposited on substrates held at room temperature or above. And no changes in resistivity were reported previously unless the annealing was carried out above room temperature [4-8]. Some previous studies have warned of spurious annealing effects caused by diffusion of metallic contacts into the amorphous materials [4,10]. But again, such effects were observed only when the annealing takes place well above room temperature. It is intriguing that the resistivity changes in opposite directions when amorphous GeTe and SnTe f'dms are annealed. Although some controversy remains concerning the GeTe2 case [5,9], studies of the composition dependence of various properties in the amorphous Ge-Te system suggest that at the 50-50 composition, it is appropriate to regard the material as a random alloy of Ge and Te, rather than as an amorphous form of the compound GeTe [5,9]. Presumably, a similar statement could be made concerning amorphous SnTe. Thus it should be mentioned that the opposite annealing effects in amorphous GeTe and SnTe do correspond to the resistivity increases and decreases which occur when amorphous Ge and Sn are annealed. The last sentence of the preceding paragraph is an observation, not an explanation. We therefore hope that the presentation of these results on the low-temperature annealing of amorphous GeTe and their comparison with the earlier data on amorphous SnTe will stimulate further work which will ultimately lead to a better
R.W. Brown /Annealing behaviour of amorphous GeTe and SnTe films
135
TIN FILM
®
a::
p.
Z
I
30
i~)
0
1
60
~
90
i
t
-- ~
30
E0
L
I
LEAO F I L M
90
I
120 1 50 180 210 TEMPERATURE,°K
120
150
180
~
210
I
240
270
~
240
, 1
300
)
270
300
TEMPERATURE,°K
Fig. 3. Annealing histories of Sn and Pb films quench-deposited at substrates held at 4,2 K. understanding of the transport mechanisms in amorphous material and their relation to structural disorder.
Note. The above letter constitutes the last work carried out by the late Dr. Brown. It was originally submitted for publication in 1975. What appears above is a slightly revised version, with updated references, prepared by the undersigned to whom correspondence should be addressed. The paper is dedicated to the memory of an outstanding scientist with whom it was a privilege to have collaborated. R.S. Allgaier Naval Surface Weapons Center, White Oak, Silver Spring, Maryland 20910, USA
136
R. W. Brown /Annealing behaviour o f amorphous GeTe and Sn Te films
References [1] [2] [3] [4] [5 ]
[6] [7] [81 [9] [10] [11 ] [12] [13] [14] [ 15 ]
S,K. Bahl and K.L. Chopra, J. Vac. Sci. Technol. 6 (1969) 561. K.L. Chopra and S.K. Bahl, J. Appl. Phys. 40 (1969) 4171. S.K. Bahl and K.L. Chopra, J. Appl. Phys. 41 (1970) 2196. W.E. Howard and R. Tsu, Phys. Rev. B1 (1970) 4709. H.K. Rockstad and J.P. deNeufville, Proceedings of the 11 th International Conference on the Physics of Semiconductors, Warsaw (PWN-Polish Scientific Publishers, Warsaw, 1972) p. 542. C. Wood, L.R. Gilbert, R. Mueller and C.M. Garner, J. Vac. Sci. Technol. 10 (1973) 739. H,R. Riedl, K.P. Scharnhorst and D.G. Simons, Infrared Phys. 14 (1974) 139. K.P. Scharnhorst and H.R. Riedl, J. Appl. Phys. 45 (1974) 2971. P. Nath, S.K. Suri and K.L. Chopra, Phys. Stat. Solidi (a) 30 (1975) 771. A. Deneuville, P. Gerard and J. Devenyi, J. Non-Crystalline Solids 22 (1976) 77. R.W. Brown, A.R. Millner and R.S. Allgaier, Thin Solid Films 5 (1970) 157. R. Hilsch, Conference on Non-Crystalline Solids, Alfred, New York, 1958 (Wiley, New York, 1960) p. 348. W. Buckel and R. Hilsch, Z. Phys. 138 (1954) 109. W. Riihl, Z. Phys. 138 (1954) 121. R.W. Brown, PhD. thesis (University of Maryland, College Park, Md, 1967) p. 41.