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Electrical and optical properties of bias sputtered ZnO thin films
Solar Energy Materials 7 (1982) 65-73 North-Holland Publishing Company
65
ELECTRICAL AND OPTICAL PROPERTIES OF BIAS S P U T T E R E D ZnO T H I N F I L M S O. C A P O R A L E T T I Xerox Research Centre of Canada, 2480 Dunwin Drive, Mississauoa, Ontario L5L 1J9, Canada
Received 5 February 1982;in revised form 15 April 1982
Bias sputtered ZnO films were characterized by resistivity, Hall effect and optical transmission measurements. The carrier concentration is 1019 cm 3 for film resistivities in the range 10-310-1 f~ cm and the mobility, ~ 10 cmZ/Vs,is comparable to that of sintered samples. The films are degenerate semiconductors with the Fermi level 50-200 meV above the conduction band edge. The resistivity is strongly controlled by the carrier concentration and an analysis of various scattering mechanisms shows that the mobility is limited by highly resistive intergrain regions. The band gap is stronglydependent on the carrier concentration and agrees approximately with the Burstein Moss model although a sudden drop of the energygap is observedat 3 x 1019cm- 3carriers similarly to that reported for films grown by other methods.
!. Introduction Zinc oxide has the potential to become one of the standard barrier electrode semiconductors for photovoltaic solar cells. Its bandgap, 3.2 eV, is large enough to be transparent to most of the solar spectrum and its resistivity can be adjusted by the fabrication process to sufficiently low values to have a negligible contribution to the device series resistance. These characteristics together with the abundance, nontoxicity and low cost of the constituent elements make Z n O very attractive as a component of SIS and heterojunction solar ceils. Zinc oxide is a I I - V I n-type defect semiconductor where deviations from stoichiometry, in the form of Zn interstitials are electrically active. Past studies have shown that the properties of Z n O are strongly influenced by the preparation method, thermal history and oxygen chemisorption [1-4]. In contrast to the extensive literature on single crystals and sintered specimens, few studies have been reported on sputtered Z n O thin films. In general the structural, electrical and optical characteristics of sputtered Z n O can be adjusted by a suitable choice of deposition parameters and thermal treatment [5-8-]. Recent measurements of the optical absorption edge on samples produced by organo-metallic chemical vapour deposition and reactive rf magnetron sputtering have shown a very strong anomaly at a carrier concentration of ~3 x 1019 cm -3. These results were interpreted by assuming the merging of the d o n o r band with the conduction band following a semiconductormetal transition [9]. 0165-1633/82/0000-0000/$02.75 © 1982 North-Holland
O. Caporaletti / Properties of bias sputtered ZnO thin films
66
Table 1 Sample preparation
Sample ZNO-047 ZNO-075 ZNO-088 ZNO-096
Sputtering conditions (in 5~,, H2/Ar) - 150 V bias 5000 )~ thick - 2 0 0 V bias 5600 )~ thick - 150 V bias 5000 ,/~ thick - 100 V bias 5000 ,~ thick
Heat treatment 400'C, 5 rain in hydrogen atmosphere
375-400°C 7 rain in hydrogen atmosphere
This paper discusses the electrical and optical properties of ZnO films prepared by a substrate biased rf sputtering technique and indicates how the resistivity and energy configuration of these films can be modified and controlled.
2. Experimental The resistivity of bias sputtered ZnO can be decreased by several orders of magnitude by varying the bias voltage applied to the substrate and the composition of the plasma [5]. It can be reduced further, if required, by a short post-deposition heat treatment in a reducing atmosphere. Zinc oxide films were deposited on Corning 7059 glass substrates in a conventional rf diode sputtering system. The bias voltage to the substrate was obtained by means of a voltage divider network, from the same power supply used for the target voltage. Films were sputtered from a 15 cm ZnO target with a purity of 99.999~o in a-plasma consisting of Ar/5~o H2. The dc bias voltage applied to the substrate varied between - 100 and - 2 0 0 V and some samples were heat treated afterwards at ~,400°C for about 5 min in a hydrogen atmosphere. The samples employed in this study are described in table 1. They were cut in a sixarm configuration by etching masked films in a solution of 7~o HCI for 5-10 s. The sample and a copper-constantan thermocouple were in contact with a copper block inside a dewar filled with LN2. The resistivity and dc Hall effect were determined by standard techniques. The electric and temperature data were recorded continuously, as the temperature was allowed to increase slowly, by a HP-3467A data logger. No field dependence of the Hall coefficient was observed and the data were obtained at a constant field of 3.1 kG. The optical transmission spectra of these samples were obtained in the range 300-450 nm in order to study the effect of carrier concentration on the optical absorption edge. A computer controlled HP 8450 spectrophotometer, with a resolution of 1 nm for wavelengths smaller than 400 nm was employed for the measurements. A clean glass substrate served as the reference and the reproducibility with respect to positioning of the samples was checked several times during the data acquisition.
O. Caporaletti/Propertiesofbiassputtered ZnO thinfilms
67
T(K) 300
10-1
200 i
150 i
100 i
~
80 i
ZnO- 096
>.
-088
_>
D-
-047
t/') i,iJ R. 1 0 - 2 -
-075
10 -3
I 4
I 6
I 8 1 0 3 / T ( K -1 )
I 10
I 12
Fig. 1. R e s i s t i v i t y vs. t e m p e r a t u r e for s e v e r a l bias s p u t t e r e d Z n O t h i n films.
3. Results and discussion
3.1. Electrical transport properties The resistivity p and Hall coefficient R n were measured in the temperature range 77-300 K. The resistivity is plotted as a function of the reciprocal of the temperature in fig. 1. The temperature response is quite featureless and the resistivity values are consistent with the bias voltage applied to the substrate during sputtering, i.e. samples deposited with larger bias voltage present the lowest values of resistivity if the plasma composition is kept constant [5]. The interpretation of resistivity data of polycrystalline semiconductors, in a thin film geometry, is in general complicated by the presence of grain boundaries, localized space charges, inhomogeneities in the films, etc. Hall measurements, on the other hand, are characteristic of the bulk material when the grain boundaries are highly resistive and are much thinner than the grains themselves [3]. The carrier concentration is shown in fig. 2 as a function of the reciprocal of the temperature. In all cases N is in the range of 1019 cm-3. The carrier concentration, like the resistivity, is also determined by the sputtering conditions; samples prepared with the highest bias voltage exhibit the largest values of N. The heat treatment affects the results only in the case of identical bias voltage, i.e. ZnO-047 has a larger carrier density than ZnO-088. The Hall mobility, #=RH/P, approximately 10 cm2/Vs is comparable to that of sintered samples but an order of magnitude smaller than single crystal ZnO. As seen
68
O. Caporaletti ,," Properties of bias sputtered ZnO thin films T(K) 102c
300
200 ~
150 I
100 ,
80 , - -
~" E
ZnO-075
-047 I
~
_
~
-088
Z
o ,~ 1019
~
-
0
9
6
I.Z UJ (3'
Lu
n-. ¢,,-
(") 1018
Nc
1017
Fig. 2.
I 4
I 6
I I 8 10 1 0 3 / T ( K -1 )
I 12
Carrier concentrationvs. temperaure for the same samples of
I 14
fig. 1.
The conduction band density
o f s t a t e s N c is a l s o s h o w n .
in fig. 3, only the heat treated samples, ZnO-047 and -096 exhibit a pronounced temperature dependence. The conduction band density of states Nc is also shown in fig. 2 for a density of states mass m* = 0.3too [10]. The carrier concentration data indicate therefore, that the bias sputtered films are degenerate semiconductors with the Fermi level EF inside the conduction band. The position of Ev in the degenerate limit can be estimated by means of numerical methods [11]. For these samples, EF is 50-200 meV above the conduction band edge, depending on the carrier concentration. The interpretation of the mobility results is complicated by the possible simultaneous action of various scattering mechanisms. The lack of temperature dependence of the resistivity suggests that lattice scattering is not significant in the temperature range of these measurements. Moreover, a model of neutral impurity scattering [12] gives a mobility of about 500 cm2/Vs for a concentration of neutral impurities 1020 cm - 3 and a dielectric constant e = 8 which is two orders of magnitude larger than the measured values. Scattering of carriers by ionized impurities is most important at low temperatures. However, sufficiently large doping levels, as in the present films, may extend this regime to room temperature. The mobility limited by ionized impurity scattering
O. Caporaletti / Properties of bias sputtered ZnO thin films
69
T(K) 102300
~"
200
150
100
80
1
I
I
I
ZnO-047
_-J 10 °m
- 088
-096
1
I 4
I 6
I 8
I 10
I 12
I 14
1 0 3 / T ( K -1 )
Fig. 3. Calculated Hall mobility vs. temperature for bias sputtered ZnO films.
decreases with decreasing temperature since the low temperature carriers have smaller thermal velocities. ZNO-047 and -096 exhibit stronger temperature dependences of the mobility which may be related to the post-sputtering heat treatment on these samples. However, other effects must be present besides ionized impurity scattering, since this mechanism should be of comparable or larger magnitude in the more heavily doped ZNO-075 which exhibits a less pronounced temperature dependence. The behaviour of the mobility of the heat treated samples ZNO-047 and -096 may be indicative of the polycrystalline nature of the films. In the case of not very large grains, the effective carrier concentration may become smaller than the doping concentration due to trapping of carriers by grain boundaries defects [7]. These defects, acting as acceptor states, give rise to potential barriers which reduce the carrier mobility to values smaller than those of single crystals. The barrier controlled mobility is expressed as [ 13]: C~ # = NBkT
exp (-q~B/kT),
(1)
where ~ is the mean thermal carrier velocity, NB is the number of barriers per unit length along the current path and tPB is the potential barrier height. The barrier heights calculated from fig. 3 for ZnO-047 and -096 are about 5 meV. These values
70
O. Caporaletti / Properties of bias sputtered ZnO thin films
are lower than those reported for sputtered films of carrier concentration 1018 c m -~ [7]. This may be an indication that the donors are not uniformly distributed among the grains giving rise to an electric field in opposition to the field of the grain boundary charge. The origin of this intergrain charge is not clear but low-lying oxygen acceptor levels may well have an effect since oxygen chemisorption can become very strong in these polycrystalline films. In this respect, Hahn [3] has discussed the effect of a granular structure on the measured resistivity of sintered samples. Sputtered ZnO is, in this sense, similar to sintered ZnO and the same analysis could be applied to it. In first approximation then, the sputtered films would consist of cubical grains of characteristic dimension L and conductivity a separated by intergrain regions of length W and conductivity go. If the grains are assumed to be Zn-rich and the grain boundaries Oz-rich the resistance of the grains would be much lower than that of the intergrain regions and the measured conductivity is: L aMEAS= O'o~ <2a.
121
On the other hand, the Hall voltage in the intergrains is shorted out by the better conducting faces of the grains, and the measured Hall coefficient will be more representative of the grain itself, R MEAs ~ RH aR AIN.Therefore the Hall mobility is expressed as
/~M E A S = K~ G. R
AIN
a o ~L ~/~RAIN
(3)
and the apparent mobility is in this approximation, much lower than that of single crystals. All the electrical transport data are combined in fig. 4, where the carrier concentration and mobility are plotted vs. resistivity at 77 K. As seen, the resistivity is strongly 1 0 2o
i
E Z 1019
10 2 u~
E
1o
I
10-3
a
i
,
i ,,,,I
,
,
,,~,,i
10 -2
' 1 0 -1
pcft
'
'
''"
1 10 0
cm~
Fig. 4. Carrier concentration and mobilityvs. resistivityof bias sputtered ZnO films, T= 77 K.
O. Caporaletti / Properties of bias spurtered ZnO thin films
71
100
ZNO-103-1 80
-103-7
v
,,,
jj/;,7
z
60
z
40
2O
0 300
340
I
l
380
420
WAVELENGTH
(nm)
Optical transmission near the absorption edge of the samples employed in the electrical transport measurements and also of samples ZnO-103-1 and -103-7 with known carrier concentration. The observed "jump" at ~400 nm is not intrinsic to the samples but is due to a spectrophotometer artifact produced by the switching of different light sources. F i g . 5.
1.O
AEn (eV) 0.1
0.01
J
1018
,
,
, ,
,
1019
,
,
, =
,
,
,
,
10 20
N (cm -3)
Fig. 6. Shift of the energy bandgap with carrier concentration. Circles correspond to bias sputtered films and triangles are data of ref. 1-9] on organo-metallic vapour deposited and magnetron sputtered films. The full line, AE,o~ N 2/3, is given by the Burstein Moss model.
O. Caporaletti / Properties of bias sputtered ZnO thin jilms
72
controlled by the donor density with its mobility playing a secondary role. This finding indicates that to increase the conductivity of ZnO films, the carrier concentration must be increased, either by a post-sputtering heat treatment or by doping with column IIl elements. 3.2. Optical transmission near the absorption edge
The effect of thee carrier concentration on the optical band gap, was studied by measuring the transmission spectra of all the samples employed in the electrical measurements and two other samples, ZnO-103-1 and -103-7, of known carrier density. A sharp absorption edge was observed in all cases at 340-380 nm as shown in fig. 5. The films thickness varied between 3000-5600/~ and interference effects are already noticeable in the ZnO-047 spectrum at 420 .,am. The transmittance spectra were used to calculate the shift of the optical gap AEn= E n - E o with increasing carrier concentration. The optical gap Eo = 3.3 eV in insulating ZnO was taken as the reference and the results, together with those of Roth et al. [9] are plotted in fig. 6. A drop of AE, at ~ 3 x 1019 cm-3 carriers is observed, independently of the preparation method. However, bias sputtered films of density greater than 6 × 1019 c m - 3 agree better with the Burstein-Moss band filling model [ 14, 15] in which the absorption edge shifts with the Fermi level as: - -
-
AE, = \8m*l \n/
N 2/3.
(4)
Magnetron sputtered and organo metallic vapour deposited films exhibit smaller values of AE, at higher concentrations and Roth et al. [9] have ascribed its abrupt decrease to the merging of the donor and conduction band due to a semiconductor metal transition. In samples deposited by bias sputtering, the better fit of eq. (4t and the lack of a strong temperature dependence of the resistivity gives support to the interpretation based on degenerate semiconductors rather than metallic behaviour. The anomalous decrease of AEo is, in any case, puzzling and its origin is unknown. The presence of a donor band. rather than a discreet donor level, may produce a narrowing of the band gap [9]. However, it is not clear how this mechanism might result in a drop of AEn at a particular carrier density.
4. Conclusions
Electrical transport and optical data indicate that bias sputtered ZnO are degenerate semiconductors, with the Fermi level 50-200 meV above the conduction band edge. The transport properties are apparently strongly influenced by highly resistive intergrain regions and higher conductivity films can be obtained by increasing the grain size and the carrier concentration. The mobility of the films is comparable to that of sintered specimens. The shift of the optical gap with carrier concentration is approximately given by the band filling model of Burstein-Moss except for films of carrier concentration
O. Caporaletti / Properties of bias sputtered ZnO thin films
73
~, 3 x 1019 cm - 3. For these samples, an anomalous decrease of the band gap has been observed similarly to that reported for films grown by other methods.
Acknowledgements This work was supported in part by the National Research Council of Canada through IRAP Grant No. 568. The author is very grateful to Prof. S. Zukotynsky of the University of Toronto who kindly allowed the use of his low temperature Hall apparatus. I also thank Dr. D. K. Murti and Mr. D. Polk for preparing the ZnO films and Dr. A. Paine for the transmission spectra. Thanks are also due to Dr. R. O. Loutfy for his support and interest in this work.
References [1] [-2] [-3] [-4] [-5] [6] [-7] [-8] [9] [-10] [-11] [,12] [13] [14] [15]
A. R. Hutson, Phys. Rev. 108 (1957) 222. P. W. Li and K. I. Hagemark, J. Solid State Chem. 12 (1975) 371. E. E. Hahn, J. Appl. Phys. 22 (1951) 855. S. E. Harrison, Phys. Rev. 93 (1954) 52. D. K. Murti and T. L. Bluhm, Thin Solid Films, in press. D. K. Murti, J. Appl. Surface Sci., in press. J. O. Barnes, D. J. Leary and A. G. Jordan, J. Electrochem. Soc. 7 (1980) 1636. D. E. Brodie, R. Singh, J. H. Morgan, J. D. Leslie, C. J. Moore and A. E. Dixon, Proc. 14th IEEE Photovoltaic Specialists Conf. (1980) p. 468. A. P. Roth, J. B. Webb and D. F. Williams, Solid State Commun. 39 t1981) 1269. E. Ziegler, A. Heinrich, H. Oppermann and G. Stover, Phys. Stat. Sol. (a) 66 11981 t 635. J. S. Blakemore, Semiconductor Statistics (Pergamon, New York, 19621. C. Erginsoy, Phys. Rev. 79 (1950) 1013. A. Waxman, V. E. Henrich, F. V. Shallcross, H. Borkan and P. K. Weiner, J. Appl. Phys 36 (1965~ 168. E. Burstein, Phys. Rev. 93 (1954) 632. T. S. Moss, Proc. Phys. Soc. London Ser. B67 11964) 775.
65
ELECTRICAL AND OPTICAL PROPERTIES OF BIAS S P U T T E R E D ZnO T H I N F I L M S O. C A P O R A L E T T I Xerox Research Centre of Canada, 2480 Dunwin Drive, Mississauoa, Ontario L5L 1J9, Canada
Received 5 February 1982;in revised form 15 April 1982
Bias sputtered ZnO films were characterized by resistivity, Hall effect and optical transmission measurements. The carrier concentration is 1019 cm 3 for film resistivities in the range 10-310-1 f~ cm and the mobility, ~ 10 cmZ/Vs,is comparable to that of sintered samples. The films are degenerate semiconductors with the Fermi level 50-200 meV above the conduction band edge. The resistivity is strongly controlled by the carrier concentration and an analysis of various scattering mechanisms shows that the mobility is limited by highly resistive intergrain regions. The band gap is stronglydependent on the carrier concentration and agrees approximately with the Burstein Moss model although a sudden drop of the energygap is observedat 3 x 1019cm- 3carriers similarly to that reported for films grown by other methods.
!. Introduction Zinc oxide has the potential to become one of the standard barrier electrode semiconductors for photovoltaic solar cells. Its bandgap, 3.2 eV, is large enough to be transparent to most of the solar spectrum and its resistivity can be adjusted by the fabrication process to sufficiently low values to have a negligible contribution to the device series resistance. These characteristics together with the abundance, nontoxicity and low cost of the constituent elements make Z n O very attractive as a component of SIS and heterojunction solar ceils. Zinc oxide is a I I - V I n-type defect semiconductor where deviations from stoichiometry, in the form of Zn interstitials are electrically active. Past studies have shown that the properties of Z n O are strongly influenced by the preparation method, thermal history and oxygen chemisorption [1-4]. In contrast to the extensive literature on single crystals and sintered specimens, few studies have been reported on sputtered Z n O thin films. In general the structural, electrical and optical characteristics of sputtered Z n O can be adjusted by a suitable choice of deposition parameters and thermal treatment [5-8-]. Recent measurements of the optical absorption edge on samples produced by organo-metallic chemical vapour deposition and reactive rf magnetron sputtering have shown a very strong anomaly at a carrier concentration of ~3 x 1019 cm -3. These results were interpreted by assuming the merging of the d o n o r band with the conduction band following a semiconductormetal transition [9]. 0165-1633/82/0000-0000/$02.75 © 1982 North-Holland
O. Caporaletti / Properties of bias sputtered ZnO thin films
66
Table 1 Sample preparation
Sample ZNO-047 ZNO-075 ZNO-088 ZNO-096
Sputtering conditions (in 5~,, H2/Ar) - 150 V bias 5000 )~ thick - 2 0 0 V bias 5600 )~ thick - 150 V bias 5000 ,/~ thick - 100 V bias 5000 ,~ thick
Heat treatment 400'C, 5 rain in hydrogen atmosphere
375-400°C 7 rain in hydrogen atmosphere
This paper discusses the electrical and optical properties of ZnO films prepared by a substrate biased rf sputtering technique and indicates how the resistivity and energy configuration of these films can be modified and controlled.
2. Experimental The resistivity of bias sputtered ZnO can be decreased by several orders of magnitude by varying the bias voltage applied to the substrate and the composition of the plasma [5]. It can be reduced further, if required, by a short post-deposition heat treatment in a reducing atmosphere. Zinc oxide films were deposited on Corning 7059 glass substrates in a conventional rf diode sputtering system. The bias voltage to the substrate was obtained by means of a voltage divider network, from the same power supply used for the target voltage. Films were sputtered from a 15 cm ZnO target with a purity of 99.999~o in a-plasma consisting of Ar/5~o H2. The dc bias voltage applied to the substrate varied between - 100 and - 2 0 0 V and some samples were heat treated afterwards at ~,400°C for about 5 min in a hydrogen atmosphere. The samples employed in this study are described in table 1. They were cut in a sixarm configuration by etching masked films in a solution of 7~o HCI for 5-10 s. The sample and a copper-constantan thermocouple were in contact with a copper block inside a dewar filled with LN2. The resistivity and dc Hall effect were determined by standard techniques. The electric and temperature data were recorded continuously, as the temperature was allowed to increase slowly, by a HP-3467A data logger. No field dependence of the Hall coefficient was observed and the data were obtained at a constant field of 3.1 kG. The optical transmission spectra of these samples were obtained in the range 300-450 nm in order to study the effect of carrier concentration on the optical absorption edge. A computer controlled HP 8450 spectrophotometer, with a resolution of 1 nm for wavelengths smaller than 400 nm was employed for the measurements. A clean glass substrate served as the reference and the reproducibility with respect to positioning of the samples was checked several times during the data acquisition.
O. Caporaletti/Propertiesofbiassputtered ZnO thinfilms
67
T(K) 300
10-1
200 i
150 i
100 i
~
80 i
ZnO- 096
>.
-088
_>
D-
-047
t/') i,iJ R. 1 0 - 2 -
-075
10 -3
I 4
I 6
I 8 1 0 3 / T ( K -1 )
I 10
I 12
Fig. 1. R e s i s t i v i t y vs. t e m p e r a t u r e for s e v e r a l bias s p u t t e r e d Z n O t h i n films.
3. Results and discussion
3.1. Electrical transport properties The resistivity p and Hall coefficient R n were measured in the temperature range 77-300 K. The resistivity is plotted as a function of the reciprocal of the temperature in fig. 1. The temperature response is quite featureless and the resistivity values are consistent with the bias voltage applied to the substrate during sputtering, i.e. samples deposited with larger bias voltage present the lowest values of resistivity if the plasma composition is kept constant [5]. The interpretation of resistivity data of polycrystalline semiconductors, in a thin film geometry, is in general complicated by the presence of grain boundaries, localized space charges, inhomogeneities in the films, etc. Hall measurements, on the other hand, are characteristic of the bulk material when the grain boundaries are highly resistive and are much thinner than the grains themselves [3]. The carrier concentration is shown in fig. 2 as a function of the reciprocal of the temperature. In all cases N is in the range of 1019 cm-3. The carrier concentration, like the resistivity, is also determined by the sputtering conditions; samples prepared with the highest bias voltage exhibit the largest values of N. The heat treatment affects the results only in the case of identical bias voltage, i.e. ZnO-047 has a larger carrier density than ZnO-088. The Hall mobility, #=RH/P, approximately 10 cm2/Vs is comparable to that of sintered samples but an order of magnitude smaller than single crystal ZnO. As seen
68
O. Caporaletti ,," Properties of bias sputtered ZnO thin films T(K) 102c
300
200 ~
150 I
100 ,
80 , - -
~" E
ZnO-075
-047 I
~
_
~
-088
Z
o ,~ 1019
~
-
0
9
6
I.Z UJ (3'
Lu
n-. ¢,,-
(") 1018
Nc
1017
Fig. 2.
I 4
I 6
I I 8 10 1 0 3 / T ( K -1 )
I 12
Carrier concentrationvs. temperaure for the same samples of
I 14
fig. 1.
The conduction band density
o f s t a t e s N c is a l s o s h o w n .
in fig. 3, only the heat treated samples, ZnO-047 and -096 exhibit a pronounced temperature dependence. The conduction band density of states Nc is also shown in fig. 2 for a density of states mass m* = 0.3too [10]. The carrier concentration data indicate therefore, that the bias sputtered films are degenerate semiconductors with the Fermi level EF inside the conduction band. The position of Ev in the degenerate limit can be estimated by means of numerical methods [11]. For these samples, EF is 50-200 meV above the conduction band edge, depending on the carrier concentration. The interpretation of the mobility results is complicated by the possible simultaneous action of various scattering mechanisms. The lack of temperature dependence of the resistivity suggests that lattice scattering is not significant in the temperature range of these measurements. Moreover, a model of neutral impurity scattering [12] gives a mobility of about 500 cm2/Vs for a concentration of neutral impurities 1020 cm - 3 and a dielectric constant e = 8 which is two orders of magnitude larger than the measured values. Scattering of carriers by ionized impurities is most important at low temperatures. However, sufficiently large doping levels, as in the present films, may extend this regime to room temperature. The mobility limited by ionized impurity scattering
O. Caporaletti / Properties of bias sputtered ZnO thin films
69
T(K) 102300
~"
200
150
100
80
1
I
I
I
ZnO-047
_-J 10 °m
- 088
-096
1
I 4
I 6
I 8
I 10
I 12
I 14
1 0 3 / T ( K -1 )
Fig. 3. Calculated Hall mobility vs. temperature for bias sputtered ZnO films.
decreases with decreasing temperature since the low temperature carriers have smaller thermal velocities. ZNO-047 and -096 exhibit stronger temperature dependences of the mobility which may be related to the post-sputtering heat treatment on these samples. However, other effects must be present besides ionized impurity scattering, since this mechanism should be of comparable or larger magnitude in the more heavily doped ZNO-075 which exhibits a less pronounced temperature dependence. The behaviour of the mobility of the heat treated samples ZNO-047 and -096 may be indicative of the polycrystalline nature of the films. In the case of not very large grains, the effective carrier concentration may become smaller than the doping concentration due to trapping of carriers by grain boundaries defects [7]. These defects, acting as acceptor states, give rise to potential barriers which reduce the carrier mobility to values smaller than those of single crystals. The barrier controlled mobility is expressed as [ 13]: C~ # = NBkT
exp (-q~B/kT),
(1)
where ~ is the mean thermal carrier velocity, NB is the number of barriers per unit length along the current path and tPB is the potential barrier height. The barrier heights calculated from fig. 3 for ZnO-047 and -096 are about 5 meV. These values
70
O. Caporaletti / Properties of bias sputtered ZnO thin films
are lower than those reported for sputtered films of carrier concentration 1018 c m -~ [7]. This may be an indication that the donors are not uniformly distributed among the grains giving rise to an electric field in opposition to the field of the grain boundary charge. The origin of this intergrain charge is not clear but low-lying oxygen acceptor levels may well have an effect since oxygen chemisorption can become very strong in these polycrystalline films. In this respect, Hahn [3] has discussed the effect of a granular structure on the measured resistivity of sintered samples. Sputtered ZnO is, in this sense, similar to sintered ZnO and the same analysis could be applied to it. In first approximation then, the sputtered films would consist of cubical grains of characteristic dimension L and conductivity a separated by intergrain regions of length W and conductivity go. If the grains are assumed to be Zn-rich and the grain boundaries Oz-rich the resistance of the grains would be much lower than that of the intergrain regions and the measured conductivity is: L aMEAS= O'o~ <2a.
121
On the other hand, the Hall voltage in the intergrains is shorted out by the better conducting faces of the grains, and the measured Hall coefficient will be more representative of the grain itself, R MEAs ~ RH aR AIN.Therefore the Hall mobility is expressed as
/~M E A S = K~ G. R
AIN
a o ~L ~/~RAIN
(3)
and the apparent mobility is in this approximation, much lower than that of single crystals. All the electrical transport data are combined in fig. 4, where the carrier concentration and mobility are plotted vs. resistivity at 77 K. As seen, the resistivity is strongly 1 0 2o
i
E Z 1019
10 2 u~
E
1o
I
10-3
a
i
,
i ,,,,I
,
,
,,~,,i
10 -2
' 1 0 -1
pcft
'
'
''"
1 10 0
cm~
Fig. 4. Carrier concentration and mobilityvs. resistivityof bias sputtered ZnO films, T= 77 K.
O. Caporaletti / Properties of bias spurtered ZnO thin films
71
100
ZNO-103-1 80
-103-7
v
,,,
jj/;,7
z
60
z
40
2O
0 300
340
I
l
380
420
WAVELENGTH
(nm)
Optical transmission near the absorption edge of the samples employed in the electrical transport measurements and also of samples ZnO-103-1 and -103-7 with known carrier concentration. The observed "jump" at ~400 nm is not intrinsic to the samples but is due to a spectrophotometer artifact produced by the switching of different light sources. F i g . 5.
1.O
AEn (eV) 0.1
0.01
J
1018
,
,
, ,
,
1019
,
,
, =
,
,
,
,
10 20
N (cm -3)
Fig. 6. Shift of the energy bandgap with carrier concentration. Circles correspond to bias sputtered films and triangles are data of ref. 1-9] on organo-metallic vapour deposited and magnetron sputtered films. The full line, AE,o~ N 2/3, is given by the Burstein Moss model.
O. Caporaletti / Properties of bias sputtered ZnO thin jilms
72
controlled by the donor density with its mobility playing a secondary role. This finding indicates that to increase the conductivity of ZnO films, the carrier concentration must be increased, either by a post-sputtering heat treatment or by doping with column IIl elements. 3.2. Optical transmission near the absorption edge
The effect of thee carrier concentration on the optical band gap, was studied by measuring the transmission spectra of all the samples employed in the electrical measurements and two other samples, ZnO-103-1 and -103-7, of known carrier density. A sharp absorption edge was observed in all cases at 340-380 nm as shown in fig. 5. The films thickness varied between 3000-5600/~ and interference effects are already noticeable in the ZnO-047 spectrum at 420 .,am. The transmittance spectra were used to calculate the shift of the optical gap AEn= E n - E o with increasing carrier concentration. The optical gap Eo = 3.3 eV in insulating ZnO was taken as the reference and the results, together with those of Roth et al. [9] are plotted in fig. 6. A drop of AE, at ~ 3 x 1019 cm-3 carriers is observed, independently of the preparation method. However, bias sputtered films of density greater than 6 × 1019 c m - 3 agree better with the Burstein-Moss band filling model [ 14, 15] in which the absorption edge shifts with the Fermi level as: - -
-
AE, = \8m*l \n/
N 2/3.
(4)
Magnetron sputtered and organo metallic vapour deposited films exhibit smaller values of AE, at higher concentrations and Roth et al. [9] have ascribed its abrupt decrease to the merging of the donor and conduction band due to a semiconductor metal transition. In samples deposited by bias sputtering, the better fit of eq. (4t and the lack of a strong temperature dependence of the resistivity gives support to the interpretation based on degenerate semiconductors rather than metallic behaviour. The anomalous decrease of AEo is, in any case, puzzling and its origin is unknown. The presence of a donor band. rather than a discreet donor level, may produce a narrowing of the band gap [9]. However, it is not clear how this mechanism might result in a drop of AEn at a particular carrier density.
4. Conclusions
Electrical transport and optical data indicate that bias sputtered ZnO are degenerate semiconductors, with the Fermi level 50-200 meV above the conduction band edge. The transport properties are apparently strongly influenced by highly resistive intergrain regions and higher conductivity films can be obtained by increasing the grain size and the carrier concentration. The mobility of the films is comparable to that of sintered specimens. The shift of the optical gap with carrier concentration is approximately given by the band filling model of Burstein-Moss except for films of carrier concentration
O. Caporaletti / Properties of bias sputtered ZnO thin films
73
~, 3 x 1019 cm - 3. For these samples, an anomalous decrease of the band gap has been observed similarly to that reported for films grown by other methods.
Acknowledgements This work was supported in part by the National Research Council of Canada through IRAP Grant No. 568. The author is very grateful to Prof. S. Zukotynsky of the University of Toronto who kindly allowed the use of his low temperature Hall apparatus. I also thank Dr. D. K. Murti and Mr. D. Polk for preparing the ZnO films and Dr. A. Paine for the transmission spectra. Thanks are also due to Dr. R. O. Loutfy for his support and interest in this work.
References [1] [-2] [-3] [-4] [-5] [6] [-7] [-8] [9] [-10] [-11] [,12] [13] [14] [15]
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