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Electrical resistivity and hall effect of single crystals of GaTe and GaSe
J. Phys. Chem. Soli&
Pergamon
Press 1962. Vol. 23, pp. 1363-1370.
Printed in Great Britain.
ELECTRICAL RESISTIVITY AND HALL EFFECT SINGLE CRYSTALS OF GaTe AND GaSe G. FISCHER*
OF
and J. L. BREBNERt
Division of Pure Physics, National Research Council, (Received 8 March 1962
Ottawa, Canada
; revised 13 April 1962)
Abstract-The resistivity and Hall effect of single crystals of GaTe have been investigated in the temperature range from 80” to 1000°K. The influences of etching and cold-working have also been studied. All samples investigated are p-type in the whole temperature range. Acids appear to diffuse between the layers characteristic of the structure, contributing an ionic conductivity. Upon coldworking resistivity, p, as well as Hall coefficient, AH, increase, Ap/p is somewhat larger than AAH/AH, indicating that both concentration and mobility of the holes decrease. We believe that cold-working creates electronic levels near the conduction and valence bands. These levels, which are probably associated with dangling bonds of dislocations, have also been observed optically. Two samples of GaSe have been investigated, between about 150” and 330°K. One sample is p-type with an activation energy of 0.15 + 0.02 eV. No Hall effect was measured on the second sample and the resistivity curve suggests an activation energy of 0.37 + 0.01 eV. 1. INTRODUCTION THE
THREE compounds
GaTe, GaSe and GaS have in common a distinctive structural property. They crystallize in layer structures which are extremely easy to cleave. The atomic arrangement within each layer is known only for GaSe and Gas, and is the same for both substances. Figure 1 is an illustration of one of the layers, which, as may be seen, is itself composed of four close-packed sub-layers in the order Se Ga Ga Se. The figure also suggests that the bonds within the four-fold layer are between Ga-1 and Se+1 ions, implying an electron transfer; the bond configuration of Ga-1 then being of the tetrahedral sp3 type, and that of Se+1 of the trigonal ps type with a saturated ss sub-shell. This bond scheme is thus basically covalent with an appreciable ionic contribution. Whereas the trigonal structure of each four-fold layer, as illustrated in Fig. 1, has been generally accepted for both GaSe and Gas, (iAs4) confusion existing in literature about the way these layers are stacked in the actual crystal has been resolved only recently. In an electron micrograph investigation BASINSKI et aZ.(5) have shown that the stacking, * Now at Laboratories
RCA,
Zurich,
as well as the possible stacking faults, is different in the two compounds, and that GaSe is likely to occur in several modifications. It is possible that GaTe is built from the same basic layers, with yet another stacking pattern, but so far little is known about the structure of this substance.(s)$ The striking ease with which the crystals cleave along the layers is a consequence of the nature of the bonds and of the large separation between the layers. This separation is similar in magnitude to the thickness of a layer. To simplify the description we shall call the direction perpendicular to the layers the c-axis for all three compounds. For GaS and GaSe this is also the trigonal axis of a single four-fold layer. If we describe a four-fold layer of GaSe by the sequence Se Ga Ga Se, whereby each symbol refers to the projection of atomic positions onto the c-axis, then in the following pattern -SeGaGaSeSeGaGaSe. . .-e-b+
.
Switzerland.
+ National Research Council Post-doctorate Fellow, now at Cyanamid European Research Institute, Geneva, Switzerland.
.ta+.tb+.
. -_j
. c axis
$ Note added in proof The structure of GaTe has now been determined J. H. BRYJXN (in press, Acta tryst., Camb.). 1363
by
1364
G.
FISCHER
and
a = 3.177 A and b = 4.776 A. The bonds between the layers are believed to be essentially of the Van der Waals type. However, if the suggested electron transfer takes place, and if one considers the stacking described by BASINSKI et a&(5) there may also be a small ionic part due to the stacking cohesion.
J. L.
BREBNER
the c-axis of all the crystallites, was briefly investigated for pr . All measurements of resistance and Hall voltage were made with two bridges described elsewhere.@a) The discussion of our results is divided into three parts in view of the observed influence of
FIG. 1. Structure of one of the four-fold layers of GaS and GaSe; atoms, 0 S or Se Atoms. 2. EXPERIMENTAL
METHOD
Large single crystals and a few polycrystalline samples were prepared by MOOSER and BECK, following an improved Czochralski technique.(‘) From the single crystals plate-like samples were cut with a diamond wheel. Thinner samples were obtained by cleaving either a thick sample or the whole crystal; in the latter case the desired contour of the sample was achieved by use of scissors. Electrodes were attached by evaporation of gold and in some instances by electrolytic deposition of copper. At first the samples were etched before the contacts were applied, but it was found later that this had a large influence on the measurements and etching was then avoided. In order to obtain clean surfaces for deposition of gold a few layers were removed by cleaving them from the samples. The single crystals at our disposal were particularly suitable for measuring the component of resistivity, p1 perpendicular to the c-axis, but were not very convenient for measurement of the resistivity component, p ,,, p arallel to the c-axis. A polycrystal, GaTe-101, obtained by zonerefining, with a highly preferential orientation of
l
Ga
acids and mechanical working. Section III describes the properties of samples that have not come in contact with acids and have not been cold-worked. Section IV is a discussion of the results from measurements on GaTe single crystals which have been dipped into acids for various lengths of time. In view of the dependence of the optical absorption of GaTe upon cold-working reported by BREBNERet aZ.,QO)we have investigated the corresponding effect on the galvanomagnetic properties. These experiments are described in Section V. 3. RESISTIVM’Y AND HALL EFFECT OF UN’ETCHED AND UNSTRAINED GaTe AND GaSe
Resistivity, p, Hall coefficient, AH, and mobility, pP, of various samples of GaTe and GaSe are shown in Figs. 2, 3 and 4 respectively. A single crystal of GaS was also investigated; only the room temperature resistivity was measured, near the limit of sensitivity of the bridge. GaTe is the material which we have most extensively investigated and several interesting features were observed :
ELECTRICAL
RESISTIVITY
AND
HALL
EFFECT
OF SINGLE
CRYSTALS
/ 0 -
!
0
I
2
I
3
4
Ga Te-Z-ETCHED
LIGHTLY
v -
Ga TO-~-ETCHED
LIGHTLY
x -
Ga To-6-CU-DOPED
+ -Go
To-En-ETCHED
0 -Go
Ta-IO
l -Go
Te-IO’-COLD
v -Go
Ta-IOI-ETCHED
A -
Ge Se-I-ETCHED
. -
Ga Se-2
HEAVILY WORKED
n -Gas-l
:
IL
Ga To-I-ETCHED
A -
I 5
6
IOOO/TEMPERATURE
7
8
I 9
1 IO
II
1 I2
(OK-‘)
FIG. 2. Electrical resistivity of various samples of GaTe, GaSe, and Gas. For GaTe-101 the data gives pn, and for all other samples pI.
13
1365
G.
1366
FISCHER
0 -
Go Te -l-ETCHED
JZ,-
Ga Ta -2-
D -
GaTe -3-ETCHED
and
ETCHED
J.
L.
BREBNER
LIGHTLY LIGHTLY
x-GoTa-S’-CU-DOPED + -
Go To-S”-
ETCHED
HEAVILY
o-GaTo-IO 0-
Ga Ts -IO’-COLD
WORKED
*--GaSa-
I’
0
I 1
I 2
3
4
5
6
7
IOOOITEMPERATURE FIG.
I
I
6
9
I
I
1
II
I2
13
(“K-‘)
3. Hall coefficient of various samples of GaTe
(a) All samples investigated were p-type at all temperatures, including a zone refined sample investigated previously (11) which was erroneously described as n-type. Sample No. 10 was in-
IO
and GaSe.
vestigated by BASINSKI et d.(5) for crystalline perfection and found to be of extremely high quality in this respect, having less than 104 dislocations per cm2.
ELECTRICAL
RESISTIVITY
AND
HALL
EFFECT
OF SINGLE
CRYSTALS
40( I-
30( )-
-
\
\ q -GaTe-I-ETCHED A -GoTo-2-ETCHED + -
LIGHTLY
GOTO-8”-ETCHED
HEAVILY
0 -GaTe-10
2oc I-
+
-
Go Te -IO’-
COLD
WORKED
l -Goss-2
)-
I-
-
I-
-
I-
_
I :
I
I : IO I : I
I 5
d
4
3
A
i + I
5( 1
75
100
150
200
TEMPERATURE
m
I1111 300
I 500
III
I 001 0
(‘K)
FIG. 4. Mobility of holes, or numerical values of expression (l), against temperature for various samples of GaTe and GaSe.
1367
1368
G.
FISCHER
and J. L.
(b) All GaTe samples, regardless of their low temperature properties, exhibited a high temperature intrinsic behaviour above 600-700°K. Entry into this intrinsic range was always accompanied by small anomalous changes in both resistivity and Hall effect, as shown in Figs. 2 and 3. (c) It is difficult to obtain a reliable figure for the energy gap at the absolute zero from galvanomagnetic measurements. Above 700°K the composition of the crystals is not stable, suffering probably from sublimation of tellurium and diffusion of electrode material. The energy gap at various temperatures has been determined more accurately by optical absorption experiments.(le) (d) Considering Fig. 4, it appears that below 300°K the mobility of sample No. 10 is determined by thermal scattering. The mobility of holes, iu,, given in Fig. 4 has been calculated in the following way : 8 &)=---‘--3r
AH
(1)
P_L
(e) If we assume a certain amount of compensation, that is NA acceptors and Nn donors such that NA
>
ND,
then the hole concentration, low temperatures to i
-
(2)
p, is proportional SEA
kT 1
at
(3)
likely that if the two resistivity components were measured on the same single crystal different ratios would obtain. (g> The persistence of p-type behaviour into the intrinsic range indicates that the mobility of holes is larger than that of electrons at high temperatures. GaSe has been investigated less extensively than GaTe and very few resistivity measurements were obtained from Gas. The activation energies observed in the present experiments do not correspond to the optical absorption edge of GaSeul) at about 2 eV. It is interesting to note, in Fig, 2, the extremely high resistivity of the zone-refined sample No. 1 and the relatively low resistivity of the pulled single crystal sample No. 2. We did not succeed in measuring the Hall effect of sample No. 1. Sample No. 2 is P-type (Fig. 3) and the scattering of holes above 160°K appears to be thermal (Fig. 4). We then deduce the activation energy from the resistivity curve above that temperature, assuming again partial compensation of acceptors and donors. This yields an acceptor level at 0.15 f 0.02 eV above the valence band. If an activation energy is derived in similar fashion from the resistivity curve of sample No. 1, a value of 0.37 f 0.01 eV is found. The resistivity measurements of GaS given in Fig. 2 are not very reliable, as they are obtained near the working limit of 4.
where
is the ionization energy. magnitude and dependence of resistivity and coefficient are readily explained with density of lO1s crnd3 acceptors an energy 0.145 k eV above the valence This concentration in agreement with spectrographic purity the material, as as with high crystalline perfection above. (f) It interesting to that pI displays very nearly the same activation energies as fL both in the high temperature intrinsic range and at low temperatures. intrinsic range it seems that
BREBNER
OF ETCHED
CRYSTALS
2 and show the of various of treatment acids. Typical of are 10 for sample 3, 20 No. 1, 2 hr No. 8”. No. 8’ the as No. but after up to an atmosphere helium, it the effect copper from the set of acids cause resistivity and coefficient to We believe the acids very easily into the samples by diffusion in the large spacing between the layers. The decrease in produced by the ionic conductivity
CRYSTALS
with a pulled single crystal (pJ
and a
In view of the sensitivity measured
the
ELECTRICAL
RESISTIVITY
AND
HALL
resistivity of sample No. 4 after subjecting it to increasing degrees of cold-working. No change in sample dimensions could be detected as a result of working, and the same gold electrodes were preserved throughout the experiment, eliminating
EFFECT
OF SINGLE
CRYSTALS
1369
is probably true for AH. The changes in pL are larger than those in AH at all temperatures. In particular, the relative change in pI becomes very large below 120°K. The behaviour of AH is somewhat more difficult to assess at low tem-
8 . PL
-
4 -
Game
- 4’-
WORKED
LIGHTLY
4”-
WORKED
MORE
Ga Te-
a--GaTe-IO’-WORKED A,
l
0-
Ga Te-
HEAVILY
IO’-
WORKED
HEAVILY
y:
1
I
I
100
150
I
I
200
250
TEMPERATURE
-.
.-,300
35 0
(“K)
FIG. 5. Percentage increase of resistivity and Hall coefficient after mechanical working of GaTe.
any error related to geometry. Sample No. 10 was investigated for changes in the Hall coefficient AH and resistivity py Figure 5 shows the percentage increase in pl and AH observed. It is interesting to note that both pI and AH increase upon working, indicating that we do not have a pure “mobility effect”. We see that pL increases progressively upon repeated working and the same
peratures because of the scatter of our results, but it seems that the relative change in AH reaches a maximum near 100°K and decreases rapidly below it. Such a decrease, if sustained, might result in a change of sign of AH at some lower temperature. Mechanical working has the effect of producing dislocations in the crystals. With certain kinds of
1370
G.
FISCHER
and
dislocations unsaturated dangling bonds are introduced. We believe that levels associated with dangling bonds are created in the energy gap, some of which probably lie close to the conduction and valence bands.(ls) The result would be an increase in the number of electrons and a related decrease in the number of holes. At the same time there is a slight decrease in the mobilities which explains the larger change in pI. The relative carrier concentration changes are most important at low temperatures and could readily account for the observed changes of pL and AH. The postulated level creation by cold-working is at least in qualitative agreement with the line structure observed in optical absorption experiments(lO) and more particularly with the dependence of this structure upon mechanical working and annealing. Acknowledgements-We would like to express our gratitude to Dr. E. MOOSER and Mr. A. BECK who supplied us with all the crystals investigated, and to Dr. E. MOOSER for discussions.
J.
L.
BREBNER REFERENCES
1. SCHUBERT K.
and
D&IRE E.,
Naturwissenschajten
40, 604 (1953). 2. SCHUBERT K., DGRRE E. and KLVGE, Z. Metallk. 46, 216 (1955). 3. HAHN H. and FRANK G., Z. anorg. Chem. 278, 340 (1955). 4. SEMILETOV S. A., Kristallografiia, 3, 288 (1958). 5. BAXNSKI 2. S., DOVE D. B. and MOOSER E., Helv. Pkys. Acta 34, 373 (1961). 6. SCHUBERTK., D~RRE E. and G~~NZELE., Naturwissenschajten 41, 448 (1954). 7. BECK A. and MOOSER E., Helv. Phys. Acta, 34, 370 (1961). 8. DAVPHINEE T. M. and MOOSER E., Rev. sci. Instrum. 26, 660 (1955). 9. FISCHER G., GREIC D. and MOOSER E., Rev. sci. Instrum. 32, 842 (1961). 10. BREBNLXR J. L., FISCHERG. and MOOSER E., _I. Phys. Chem. Solids 23, 1417 (1962). 11. FIELDING P., FISCHER G. and MOOSER E., _I. Phys. Chem. Solids 8.434 (1959). 12. CELLI V., GOLD .A. and THOMPSON R., Phys. Rev. Letters 8, 96 (1962). See also BAFUISLEY W., Progr. in Semiconductors, 4, 155 (1960).
Pergamon
Press 1962. Vol. 23, pp. 1363-1370.
Printed in Great Britain.
ELECTRICAL RESISTIVITY AND HALL EFFECT SINGLE CRYSTALS OF GaTe AND GaSe G. FISCHER*
OF
and J. L. BREBNERt
Division of Pure Physics, National Research Council, (Received 8 March 1962
Ottawa, Canada
; revised 13 April 1962)
Abstract-The resistivity and Hall effect of single crystals of GaTe have been investigated in the temperature range from 80” to 1000°K. The influences of etching and cold-working have also been studied. All samples investigated are p-type in the whole temperature range. Acids appear to diffuse between the layers characteristic of the structure, contributing an ionic conductivity. Upon coldworking resistivity, p, as well as Hall coefficient, AH, increase, Ap/p is somewhat larger than AAH/AH, indicating that both concentration and mobility of the holes decrease. We believe that cold-working creates electronic levels near the conduction and valence bands. These levels, which are probably associated with dangling bonds of dislocations, have also been observed optically. Two samples of GaSe have been investigated, between about 150” and 330°K. One sample is p-type with an activation energy of 0.15 + 0.02 eV. No Hall effect was measured on the second sample and the resistivity curve suggests an activation energy of 0.37 + 0.01 eV. 1. INTRODUCTION THE
THREE compounds
GaTe, GaSe and GaS have in common a distinctive structural property. They crystallize in layer structures which are extremely easy to cleave. The atomic arrangement within each layer is known only for GaSe and Gas, and is the same for both substances. Figure 1 is an illustration of one of the layers, which, as may be seen, is itself composed of four close-packed sub-layers in the order Se Ga Ga Se. The figure also suggests that the bonds within the four-fold layer are between Ga-1 and Se+1 ions, implying an electron transfer; the bond configuration of Ga-1 then being of the tetrahedral sp3 type, and that of Se+1 of the trigonal ps type with a saturated ss sub-shell. This bond scheme is thus basically covalent with an appreciable ionic contribution. Whereas the trigonal structure of each four-fold layer, as illustrated in Fig. 1, has been generally accepted for both GaSe and Gas, (iAs4) confusion existing in literature about the way these layers are stacked in the actual crystal has been resolved only recently. In an electron micrograph investigation BASINSKI et aZ.(5) have shown that the stacking, * Now at Laboratories
RCA,
Zurich,
as well as the possible stacking faults, is different in the two compounds, and that GaSe is likely to occur in several modifications. It is possible that GaTe is built from the same basic layers, with yet another stacking pattern, but so far little is known about the structure of this substance.(s)$ The striking ease with which the crystals cleave along the layers is a consequence of the nature of the bonds and of the large separation between the layers. This separation is similar in magnitude to the thickness of a layer. To simplify the description we shall call the direction perpendicular to the layers the c-axis for all three compounds. For GaS and GaSe this is also the trigonal axis of a single four-fold layer. If we describe a four-fold layer of GaSe by the sequence Se Ga Ga Se, whereby each symbol refers to the projection of atomic positions onto the c-axis, then in the following pattern -SeGaGaSeSeGaGaSe. . .-e-b+
.
Switzerland.
+ National Research Council Post-doctorate Fellow, now at Cyanamid European Research Institute, Geneva, Switzerland.
.ta+.tb+.
. -_j
. c axis
$ Note added in proof The structure of GaTe has now been determined J. H. BRYJXN (in press, Acta tryst., Camb.). 1363
by
1364
G.
FISCHER
and
a = 3.177 A and b = 4.776 A. The bonds between the layers are believed to be essentially of the Van der Waals type. However, if the suggested electron transfer takes place, and if one considers the stacking described by BASINSKI et a&(5) there may also be a small ionic part due to the stacking cohesion.
J. L.
BREBNER
the c-axis of all the crystallites, was briefly investigated for pr . All measurements of resistance and Hall voltage were made with two bridges described elsewhere.@a) The discussion of our results is divided into three parts in view of the observed influence of
FIG. 1. Structure of one of the four-fold layers of GaS and GaSe; atoms, 0 S or Se Atoms. 2. EXPERIMENTAL
METHOD
Large single crystals and a few polycrystalline samples were prepared by MOOSER and BECK, following an improved Czochralski technique.(‘) From the single crystals plate-like samples were cut with a diamond wheel. Thinner samples were obtained by cleaving either a thick sample or the whole crystal; in the latter case the desired contour of the sample was achieved by use of scissors. Electrodes were attached by evaporation of gold and in some instances by electrolytic deposition of copper. At first the samples were etched before the contacts were applied, but it was found later that this had a large influence on the measurements and etching was then avoided. In order to obtain clean surfaces for deposition of gold a few layers were removed by cleaving them from the samples. The single crystals at our disposal were particularly suitable for measuring the component of resistivity, p1 perpendicular to the c-axis, but were not very convenient for measurement of the resistivity component, p ,,, p arallel to the c-axis. A polycrystal, GaTe-101, obtained by zonerefining, with a highly preferential orientation of
l
Ga
acids and mechanical working. Section III describes the properties of samples that have not come in contact with acids and have not been cold-worked. Section IV is a discussion of the results from measurements on GaTe single crystals which have been dipped into acids for various lengths of time. In view of the dependence of the optical absorption of GaTe upon cold-working reported by BREBNERet aZ.,QO)we have investigated the corresponding effect on the galvanomagnetic properties. These experiments are described in Section V. 3. RESISTIVM’Y AND HALL EFFECT OF UN’ETCHED AND UNSTRAINED GaTe AND GaSe
Resistivity, p, Hall coefficient, AH, and mobility, pP, of various samples of GaTe and GaSe are shown in Figs. 2, 3 and 4 respectively. A single crystal of GaS was also investigated; only the room temperature resistivity was measured, near the limit of sensitivity of the bridge. GaTe is the material which we have most extensively investigated and several interesting features were observed :
ELECTRICAL
RESISTIVITY
AND
HALL
EFFECT
OF SINGLE
CRYSTALS
/ 0 -
!
0
I
2
I
3
4
Ga Te-Z-ETCHED
LIGHTLY
v -
Ga TO-~-ETCHED
LIGHTLY
x -
Ga To-6-CU-DOPED
+ -Go
To-En-ETCHED
0 -Go
Ta-IO
l -Go
Te-IO’-COLD
v -Go
Ta-IOI-ETCHED
A -
Ge Se-I-ETCHED
. -
Ga Se-2
HEAVILY WORKED
n -Gas-l
:
IL
Ga To-I-ETCHED
A -
I 5
6
IOOO/TEMPERATURE
7
8
I 9
1 IO
II
1 I2
(OK-‘)
FIG. 2. Electrical resistivity of various samples of GaTe, GaSe, and Gas. For GaTe-101 the data gives pn, and for all other samples pI.
13
1365
G.
1366
FISCHER
0 -
Go Te -l-ETCHED
JZ,-
Ga Ta -2-
D -
GaTe -3-ETCHED
and
ETCHED
J.
L.
BREBNER
LIGHTLY LIGHTLY
x-GoTa-S’-CU-DOPED + -
Go To-S”-
ETCHED
HEAVILY
o-GaTo-IO 0-
Ga Ts -IO’-COLD
WORKED
*--GaSa-
I’
0
I 1
I 2
3
4
5
6
7
IOOOITEMPERATURE FIG.
I
I
6
9
I
I
1
II
I2
13
(“K-‘)
3. Hall coefficient of various samples of GaTe
(a) All samples investigated were p-type at all temperatures, including a zone refined sample investigated previously (11) which was erroneously described as n-type. Sample No. 10 was in-
IO
and GaSe.
vestigated by BASINSKI et d.(5) for crystalline perfection and found to be of extremely high quality in this respect, having less than 104 dislocations per cm2.
ELECTRICAL
RESISTIVITY
AND
HALL
EFFECT
OF SINGLE
CRYSTALS
40( I-
30( )-
-
\
\ q -GaTe-I-ETCHED A -GoTo-2-ETCHED + -
LIGHTLY
GOTO-8”-ETCHED
HEAVILY
0 -GaTe-10
2oc I-
+
-
Go Te -IO’-
COLD
WORKED
l -Goss-2
)-
I-
-
I-
-
I-
_
I :
I
I : IO I : I
I 5
d
4
3
A
i + I
5( 1
75
100
150
200
TEMPERATURE
m
I1111 300
I 500
III
I 001 0
(‘K)
FIG. 4. Mobility of holes, or numerical values of expression (l), against temperature for various samples of GaTe and GaSe.
1367
1368
G.
FISCHER
and J. L.
(b) All GaTe samples, regardless of their low temperature properties, exhibited a high temperature intrinsic behaviour above 600-700°K. Entry into this intrinsic range was always accompanied by small anomalous changes in both resistivity and Hall effect, as shown in Figs. 2 and 3. (c) It is difficult to obtain a reliable figure for the energy gap at the absolute zero from galvanomagnetic measurements. Above 700°K the composition of the crystals is not stable, suffering probably from sublimation of tellurium and diffusion of electrode material. The energy gap at various temperatures has been determined more accurately by optical absorption experiments.(le) (d) Considering Fig. 4, it appears that below 300°K the mobility of sample No. 10 is determined by thermal scattering. The mobility of holes, iu,, given in Fig. 4 has been calculated in the following way : 8 &)=---‘--3r
AH
(1)
P_L
(e) If we assume a certain amount of compensation, that is NA acceptors and Nn donors such that NA
>
ND,
then the hole concentration, low temperatures to i
-
(2)
p, is proportional SEA
kT 1
at
(3)
likely that if the two resistivity components were measured on the same single crystal different ratios would obtain. (g> The persistence of p-type behaviour into the intrinsic range indicates that the mobility of holes is larger than that of electrons at high temperatures. GaSe has been investigated less extensively than GaTe and very few resistivity measurements were obtained from Gas. The activation energies observed in the present experiments do not correspond to the optical absorption edge of GaSeul) at about 2 eV. It is interesting to note, in Fig, 2, the extremely high resistivity of the zone-refined sample No. 1 and the relatively low resistivity of the pulled single crystal sample No. 2. We did not succeed in measuring the Hall effect of sample No. 1. Sample No. 2 is P-type (Fig. 3) and the scattering of holes above 160°K appears to be thermal (Fig. 4). We then deduce the activation energy from the resistivity curve above that temperature, assuming again partial compensation of acceptors and donors. This yields an acceptor level at 0.15 f 0.02 eV above the valence band. If an activation energy is derived in similar fashion from the resistivity curve of sample No. 1, a value of 0.37 f 0.01 eV is found. The resistivity measurements of GaS given in Fig. 2 are not very reliable, as they are obtained near the working limit of 4.
where
is the ionization energy. magnitude and dependence of resistivity and coefficient are readily explained with density of lO1s crnd3 acceptors an energy 0.145 k eV above the valence This concentration in agreement with spectrographic purity the material, as as with high crystalline perfection above. (f) It interesting to that pI displays very nearly the same activation energies as fL both in the high temperature intrinsic range and at low temperatures. intrinsic range it seems that
BREBNER
OF ETCHED
CRYSTALS
2 and show the of various of treatment acids. Typical of are 10 for sample 3, 20 No. 1, 2 hr No. 8”. No. 8’ the as No. but after up to an atmosphere helium, it the effect copper from the set of acids cause resistivity and coefficient to We believe the acids very easily into the samples by diffusion in the large spacing between the layers. The decrease in produced by the ionic conductivity
CRYSTALS
with a pulled single crystal (pJ
and a
In view of the sensitivity measured
the
ELECTRICAL
RESISTIVITY
AND
HALL
resistivity of sample No. 4 after subjecting it to increasing degrees of cold-working. No change in sample dimensions could be detected as a result of working, and the same gold electrodes were preserved throughout the experiment, eliminating
EFFECT
OF SINGLE
CRYSTALS
1369
is probably true for AH. The changes in pL are larger than those in AH at all temperatures. In particular, the relative change in pI becomes very large below 120°K. The behaviour of AH is somewhat more difficult to assess at low tem-
8 . PL
-
4 -
Game
- 4’-
WORKED
LIGHTLY
4”-
WORKED
MORE
Ga Te-
a--GaTe-IO’-WORKED A,
l
0-
Ga Te-
HEAVILY
IO’-
WORKED
HEAVILY
y:
1
I
I
100
150
I
I
200
250
TEMPERATURE
-.
.-,300
35 0
(“K)
FIG. 5. Percentage increase of resistivity and Hall coefficient after mechanical working of GaTe.
any error related to geometry. Sample No. 10 was investigated for changes in the Hall coefficient AH and resistivity py Figure 5 shows the percentage increase in pl and AH observed. It is interesting to note that both pI and AH increase upon working, indicating that we do not have a pure “mobility effect”. We see that pL increases progressively upon repeated working and the same
peratures because of the scatter of our results, but it seems that the relative change in AH reaches a maximum near 100°K and decreases rapidly below it. Such a decrease, if sustained, might result in a change of sign of AH at some lower temperature. Mechanical working has the effect of producing dislocations in the crystals. With certain kinds of
1370
G.
FISCHER
and
dislocations unsaturated dangling bonds are introduced. We believe that levels associated with dangling bonds are created in the energy gap, some of which probably lie close to the conduction and valence bands.(ls) The result would be an increase in the number of electrons and a related decrease in the number of holes. At the same time there is a slight decrease in the mobilities which explains the larger change in pI. The relative carrier concentration changes are most important at low temperatures and could readily account for the observed changes of pL and AH. The postulated level creation by cold-working is at least in qualitative agreement with the line structure observed in optical absorption experiments(lO) and more particularly with the dependence of this structure upon mechanical working and annealing. Acknowledgements-We would like to express our gratitude to Dr. E. MOOSER and Mr. A. BECK who supplied us with all the crystals investigated, and to Dr. E. MOOSER for discussions.
J.
L.
BREBNER REFERENCES
1. SCHUBERT K.
and
D&IRE E.,
Naturwissenschajten
40, 604 (1953). 2. SCHUBERT K., DGRRE E. and KLVGE, Z. Metallk. 46, 216 (1955). 3. HAHN H. and FRANK G., Z. anorg. Chem. 278, 340 (1955). 4. SEMILETOV S. A., Kristallografiia, 3, 288 (1958). 5. BAXNSKI 2. S., DOVE D. B. and MOOSER E., Helv. Pkys. Acta 34, 373 (1961). 6. SCHUBERTK., D~RRE E. and G~~NZELE., Naturwissenschajten 41, 448 (1954). 7. BECK A. and MOOSER E., Helv. Phys. Acta, 34, 370 (1961). 8. DAVPHINEE T. M. and MOOSER E., Rev. sci. Instrum. 26, 660 (1955). 9. FISCHER G., GREIC D. and MOOSER E., Rev. sci. Instrum. 32, 842 (1961). 10. BREBNLXR J. L., FISCHERG. and MOOSER E., _I. Phys. Chem. Solids 23, 1417 (1962). 11. FIELDING P., FISCHER G. and MOOSER E., _I. Phys. Chem. Solids 8.434 (1959). 12. CELLI V., GOLD .A. and THOMPSON R., Phys. Rev. Letters 8, 96 (1962). See also BAFUISLEY W., Progr. in Semiconductors, 4, 155 (1960).