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THE INFLUENCE OF THE pH ON THE SURFACE STATE DENSITY AT THE SiO2-Si INTERFACE
433
THE INFLUENCE OF THE pH ON THE SURFACE STATE DENSITY AT THE Si02-Si INTERFACE
N.F. de Rooij and P. Bergveld Twente University of Technology, P.O. Box 217, Enschede, The Netherlands ABSTRACT Quasistatic C-V measurements have been carried out on Si-Si02-electrolyte structures. The C-V curves show a shift in voltage when the pH of the electrolyte is varied. This is due to a variation in the potential difference across the electrolyte-Si02 interface. The shape of the C-V curves also changes; from the variation in the minimum of these curves the change in the surface state density near midgap is derived. It is found that a decrease in pH leads to a decrease in the surface state density at the Si-Si0 2 interface. This decrease is believed to be caused by a hydrogen bearing species released at the Si02-electrolyte interface and transported to the Si-Si0 2 interface. The possibility of application in ISFETs is discussed. INTRODUCTION Recently the system Si-thermally grown Si02-electrolyte has gained interest . as a consequence of its application in unreferenced Ion Sensitive Field Effect Transistors (ISFETs) (1,2,3). Especially, the influence of variations in the electrolyte composition on the surface state density at the Si-Si0 2 interface has been discussed (2,3). In order to investigate whether and how the surface state density depends on changes in the electrolyte composition, e.g. a pH change, quasistatic capacitance-voltage (C-V) measurements (4) were carried out on Si-Si0 2 electrolyte structures. In this paper the results of the C-V measurements will be presented and the consequences of the application of surface state modulation in ISFETs will be discussed. EXPERIMENTAL P-type (111) silicon wafers of 10 Ωαη resistivity were oxidized at a temperature of 1200°C. The oxide thickness was about 500 Â. Afterwards aluminum was evaporated on the oxide and on the back of the wafer, followed by a heat treatment at 450°C with wet N 2 during 10 minutes. The aluminum on the oxide was etched away just before the experiment. The Si-Si0 2 structures were mounted in a sample holder, in such a way that an electrolyte could be brought in contact with the oxide and also with the reference electrode. The exposed area of the oxide was in the order of 0.2 cm 2 . The quasistatic C-V measurements were carried out in accordance with Ref. 4. After recording a C-V curve with an electrolyte of a given pH, the electrolyte was replaced by one with a different pH value and a C-V recording repeated. The pH was varied in the range from 1 to 13 and also in the reverse direction.
434 RESULTS The measured C-V curves at three different pH values are shown in Fig. 1.
11
-L
5
-3
-2
A P P L I E D VOLTAGE ON THE ELECTRODE
Fig.
-1
0
(SCE)/V
1. Measured C-V curves at different pH values (exposed oxide area 1.96x1ο"1 cm2;.oxide thickness 500 Ä; N A = 1.5xl015 cm
The shift of these curves along the voltage axis is very pronounced. This observed shift can be explained by a change in the electric potential difference across the Si02-electrolyte interface, due to a change in the net charge, which exists in a layer of hydrolysed material near the Si02-electrolyte interface. The silanol groups of this layer respond to the proton activity in the electrolyte according to SiOH Ϊ Sio"
H
(1)
An increase in the proton activity leads to a higher hydrogen content in the Si0 2 . The total voltage shift between the curves at pH 1 and pH 13 is about 400 mV, which is considerably less than predicted by the Nernst equation (59 mV per pH unit at room temperature). The deviation from the Nernst equation has been discussed by several authors (5,6). It can also be seen from the curves of Fig. 2 that the minimum in the capacitance ratio (C/C o x ) m £ n depends on the pH. This dependency manifested itself after exposing the Si0 2 to the electrolyte for at least a few hours. It appears that the observed shift in (C/C o x ) m £ n is reversible. The small change in (C/C o x ) m £ n can be explained by a change in the surface state density N S S ( E ) . According to the method outlined by Van Overstraeten et al. (7), N S S (E) near midgap is obtained from (C/C o x ) m £ n using the following formula «Nss(E)
(C/C
) . ox m m "(C/C ) . ox m m
Si
(2)
Here q is the elementary charge, C o x the oxide capacitance, C the measured capacitance and Cg^ the theoretical semiconductor capacitance. All capacitances are per unit area. A value for (Cg^/C ) m ^ n is obtained from Fig. 3 of Ref. 8 for a given oxide thickness and doping concentration. Values of N S S (E) at three pH values are given in Table 1.
435 TABLE 1
Surface state density N S S (E) near midgap at different pH values using ( C S i / C 0 X ) m i n = 0 . 2 5 PH
(C/C
1 8.6 13
ox>
N (E)/I0ncm ss
. mm
2
eV_1
2.1 2.4 2.7
.43 .45 .47
It is found, that N S S (E) near midgap increases about 30 percent in the pH range from 1 to 13. The C-V measurements show that the surface state density is influenced by changes in the electrolyte composition. Which processes actually take place at the two interfaces is uncertain at the present time. As pointed out by Revesz (4) the most realistic process will be the transport of (1) hydrogen in the form of hydroxyl, (2) a fast moving H-related species, (3) H 2 molecules. Also complexes involving Na + ions are amongst the possibilities . These species are thought to be either released or captured at the Si0 2 electrolyte interface. They are transported through the Si0 2 and will then interact with the surface states at the Si-Si02 interface. EFFECT OF VARIATIONS IN THE SURFACE STATE DENSITY ON THE CHANNEL CONDUCTIVITY OF ISFETs In order to find a value for the change in the channel conductivity of an ISFET as a function of a variation in the surface state density the following calculations have been carried out. Without an external gate the semiconductor space charge (Q sc ) is exactly compensated by the fixed oxide charge (Q ss ) and the mobile oxide charge (0o) as well as the charge in the surface, states (Q st ) or Q + Q + Q + x o ^ss ^sc *st
0.
(3)
All charges are per unit area. The charge Q s t in the surface states is equal N ÎE) to ss (4) dE = q ίΕ"*Ετ? %t " + exp (*nrn I——*kT while Q
can be written as Q
sc
=
2
(5)
^niLDF(us'Ub)
Here n^ is the intrinsic carrier concentration, L D the intrinsic Debye length and F(u s ,u^) the Kingston-Neustadter function (8). Only acceptor type states are considered because these states can interact with the conduction band in the inverted silicon, as a consequence of their position near the conduction band edge. It is assumed that the distribution of N S S ( E ) around the maximum state density N g s ( E s s ) is Gaussian as expressed in the following equation:
N
ss
(E) = IT -4a- exp {ss σ/2π
ss 2σ2
(6)
436 where N s s denotes the total number of acceptor states per unit area and σ denotes the standard deviation of the surface state density. Combining equations (4) and (6) gives: (E-E ss ) 2 r
N
Q
st=
_q
ss
2σ E-E
-mπ 1^
where u
F ss
l
exp - {
/
2
} dE
(7)
SS
1+exp {-j——
ss kT
Now Q s t can be solved numerically as a function of u s s for given values of N^ s and σ, resulting in Q s ç(u s s , N^ g , σ ) . Moreover, because E g s is fixed with respect to the intrinsic level E^, it follows that u -u = n (8) ss s where n is constant. We will consider the case in which only the number of acceptor states changes. The gaussian distribution (σ) and also the position of the maximum in N S S (E) are not influenced by any chemical process. By combining equations (3), (5), (7) and (8) we obtain %+%s
+ 2
<>ηΛ> F ( V V
=
- ^ s t ( n + u s ' Nss«
σ)
(9)
The left and right-hand part of equation (9) can be represented graphically as a function of u s . The values of n (-16) and σ (100 mV) are based on the data as given in Fig. 9 of Ref. 9 and the value of u^ (-11.51) follows from the doping concentration (N^ = 1.5xl0 15 c m " 3 ) . The intersections of the curves representing the right-hand part of equation (9) for different values of N| s with the curves representing the left-hand part with Q 0 +Q S s a s a parameter, yield values of u s from which Q s c can be calculated according to equation (5). N° is varied between 2x1ο 1 l and 2xl0 1 2 states cm"2 and (Q +Q ) oo
io
Ί« Λ
o
'-'so
between lxl0 lz and 2x10 1 Z elementary charges cm" z . For a certain value of u s the inversion charge Q n can be calculated. The conductivity G of the inversion channel is given by G = -p n Q n _ (.0) where y n is the electron mobility (yn = 600 cii^V^s" 1 ) (1). It is found that the change in the conductivity as a function of a change in the total number of states ( ) is approximately -5xl0~~17 S cm 2 . For an ISFET, the change dN° ss in the drain current (ΔΙ^) due to a change in N s S (A N ss^ ^s e^ua^ t o AT
- £ (4£_) Δ Ν ° V, . (11) L dNo ss ds ss If N g s (E) increases with the same percentage over the whole energy range as near midgap and we take lxlO 12 states cm"2 for N^ g at pH 1, ΔΝ| 8 is about 3x1ο 11 states cm -2 . According to equation (11) the decrease in drain current (ΔΙ^) for an ISFET with a γW value of 50 and a V d g of 0.1 Volt, is about 75μΑ in the pH range 1 to d
13, which is a measurable quantity.
437 CONCLUSION It can be concluded from the C-V measurements that the surface state density at the Si-Si0 2 interface is influenced by a change in the electrolyte composition, although the mechanism involved is uncertain. Modulation of the surface state density at the Si-Si02 interface due to variations in the composition at the ambient (liquid or gas) will be an interesting subject for further investigation in order to find applications in Chemically Sensitive Semiconductor Devices (3). ACKNOWLEDGEMENT This work has been supported by the Netherlands Organization for the Advancement at Pure Research (Z.W.O.) and the Hogeschoolfonds of the Twente University of Technology. REFERENCES (1)
(2) (3) (4) (5) (6) (7)
(8)
(9)
P. Bergveld, Development, operation and application of the Ion Sensitive Field Effect Transistor as a tool for electrophysiology, IEEE Trans. on Biomed. Eng. BME-19, 342 (1972). A.G. Revesz, On the mechanism of the ion sensitive field effect transistor, Thin Solid Films 41, L43 (1977). Jay N. Zemel, Chemically sensitive semiconductor devices, Research/Development 28, 38 (1977). M. Kuhn, A quasi-static technique for MOS C-V and surface state measurements, Solid-St. Electron. 13, 873 (1970). S. Levine and A.L. Smith, Theory of the Differential Capacity of the Oxide/ Aqueous Electrolyte Interface, Discuss. Faraday Soc. 52, 290 (1971). J.W. Perram, Structure of the Double Layer at the Oxide/Water Interface, J. Chem. Soc. Faraday II, 993 (1973). R. van Overstraeten, G. Declerck and G. Broux, Graphical Technique to Determine the Density of Surface States at the Si-SiU2 Interface of MOS Devices Using the quasistatic C-V Method, J. Electrochem. Soc. 120, 1785 (1973). R.H. Kingston, S.F. Neustadter, Calculation of the Space Charge, Electric Field and Free Carrier Concentration at the Surface of a Semiconductor, Journ. of Appl. Physics 26, 718 (1955). B.E. Deal, The Current Understanding of Charges in the Thermally Oxidized Silicon Structure, J. Electrochem. Soc. 121, 198c (1974).
THE INFLUENCE OF THE pH ON THE SURFACE STATE DENSITY AT THE Si02-Si INTERFACE
N.F. de Rooij and P. Bergveld Twente University of Technology, P.O. Box 217, Enschede, The Netherlands ABSTRACT Quasistatic C-V measurements have been carried out on Si-Si02-electrolyte structures. The C-V curves show a shift in voltage when the pH of the electrolyte is varied. This is due to a variation in the potential difference across the electrolyte-Si02 interface. The shape of the C-V curves also changes; from the variation in the minimum of these curves the change in the surface state density near midgap is derived. It is found that a decrease in pH leads to a decrease in the surface state density at the Si-Si0 2 interface. This decrease is believed to be caused by a hydrogen bearing species released at the Si02-electrolyte interface and transported to the Si-Si0 2 interface. The possibility of application in ISFETs is discussed. INTRODUCTION Recently the system Si-thermally grown Si02-electrolyte has gained interest . as a consequence of its application in unreferenced Ion Sensitive Field Effect Transistors (ISFETs) (1,2,3). Especially, the influence of variations in the electrolyte composition on the surface state density at the Si-Si0 2 interface has been discussed (2,3). In order to investigate whether and how the surface state density depends on changes in the electrolyte composition, e.g. a pH change, quasistatic capacitance-voltage (C-V) measurements (4) were carried out on Si-Si0 2 electrolyte structures. In this paper the results of the C-V measurements will be presented and the consequences of the application of surface state modulation in ISFETs will be discussed. EXPERIMENTAL P-type (111) silicon wafers of 10 Ωαη resistivity were oxidized at a temperature of 1200°C. The oxide thickness was about 500 Â. Afterwards aluminum was evaporated on the oxide and on the back of the wafer, followed by a heat treatment at 450°C with wet N 2 during 10 minutes. The aluminum on the oxide was etched away just before the experiment. The Si-Si0 2 structures were mounted in a sample holder, in such a way that an electrolyte could be brought in contact with the oxide and also with the reference electrode. The exposed area of the oxide was in the order of 0.2 cm 2 . The quasistatic C-V measurements were carried out in accordance with Ref. 4. After recording a C-V curve with an electrolyte of a given pH, the electrolyte was replaced by one with a different pH value and a C-V recording repeated. The pH was varied in the range from 1 to 13 and also in the reverse direction.
434 RESULTS The measured C-V curves at three different pH values are shown in Fig. 1.
11
-L
5
-3
-2
A P P L I E D VOLTAGE ON THE ELECTRODE
Fig.
-1
0
(SCE)/V
1. Measured C-V curves at different pH values (exposed oxide area 1.96x1ο"1 cm2;.oxide thickness 500 Ä; N A = 1.5xl015 cm
The shift of these curves along the voltage axis is very pronounced. This observed shift can be explained by a change in the electric potential difference across the Si02-electrolyte interface, due to a change in the net charge, which exists in a layer of hydrolysed material near the Si02-electrolyte interface. The silanol groups of this layer respond to the proton activity in the electrolyte according to SiOH Ϊ Sio"
H
(1)
An increase in the proton activity leads to a higher hydrogen content in the Si0 2 . The total voltage shift between the curves at pH 1 and pH 13 is about 400 mV, which is considerably less than predicted by the Nernst equation (59 mV per pH unit at room temperature). The deviation from the Nernst equation has been discussed by several authors (5,6). It can also be seen from the curves of Fig. 2 that the minimum in the capacitance ratio (C/C o x ) m £ n depends on the pH. This dependency manifested itself after exposing the Si0 2 to the electrolyte for at least a few hours. It appears that the observed shift in (C/C o x ) m £ n is reversible. The small change in (C/C o x ) m £ n can be explained by a change in the surface state density N S S ( E ) . According to the method outlined by Van Overstraeten et al. (7), N S S (E) near midgap is obtained from (C/C o x ) m £ n using the following formula «Nss(E)
(C/C
) . ox m m "(C/C ) . ox m m
Si
(2)
Here q is the elementary charge, C o x the oxide capacitance, C the measured capacitance and Cg^ the theoretical semiconductor capacitance. All capacitances are per unit area. A value for (Cg^/C ) m ^ n is obtained from Fig. 3 of Ref. 8 for a given oxide thickness and doping concentration. Values of N S S (E) at three pH values are given in Table 1.
435 TABLE 1
Surface state density N S S (E) near midgap at different pH values using ( C S i / C 0 X ) m i n = 0 . 2 5 PH
(C/C
1 8.6 13
ox>
N (E)/I0ncm ss
. mm
2
eV_1
2.1 2.4 2.7
.43 .45 .47
It is found, that N S S (E) near midgap increases about 30 percent in the pH range from 1 to 13. The C-V measurements show that the surface state density is influenced by changes in the electrolyte composition. Which processes actually take place at the two interfaces is uncertain at the present time. As pointed out by Revesz (4) the most realistic process will be the transport of (1) hydrogen in the form of hydroxyl, (2) a fast moving H-related species, (3) H 2 molecules. Also complexes involving Na + ions are amongst the possibilities . These species are thought to be either released or captured at the Si0 2 electrolyte interface. They are transported through the Si0 2 and will then interact with the surface states at the Si-Si02 interface. EFFECT OF VARIATIONS IN THE SURFACE STATE DENSITY ON THE CHANNEL CONDUCTIVITY OF ISFETs In order to find a value for the change in the channel conductivity of an ISFET as a function of a variation in the surface state density the following calculations have been carried out. Without an external gate the semiconductor space charge (Q sc ) is exactly compensated by the fixed oxide charge (Q ss ) and the mobile oxide charge (0o) as well as the charge in the surface, states (Q st ) or Q + Q + Q + x o ^ss ^sc *st
0.
(3)
All charges are per unit area. The charge Q s t in the surface states is equal N ÎE) to ss (4) dE = q ίΕ"*Ετ? %t " + exp (*nrn I——*kT while Q
can be written as Q
sc
=
2
(5)
^niLDF(us'Ub)
Here n^ is the intrinsic carrier concentration, L D the intrinsic Debye length and F(u s ,u^) the Kingston-Neustadter function (8). Only acceptor type states are considered because these states can interact with the conduction band in the inverted silicon, as a consequence of their position near the conduction band edge. It is assumed that the distribution of N S S ( E ) around the maximum state density N g s ( E s s ) is Gaussian as expressed in the following equation:
N
ss
(E) = IT -4a- exp {ss σ/2π
ss 2σ2
(6)
436 where N s s denotes the total number of acceptor states per unit area and σ denotes the standard deviation of the surface state density. Combining equations (4) and (6) gives: (E-E ss ) 2 r
N
Q
st=
_q
ss
2σ E-E
-mπ 1^
where u
F ss
l
exp - {
/
2
} dE
(7)
SS
1+exp {-j——
ss kT
Now Q s t can be solved numerically as a function of u s s for given values of N^ s and σ, resulting in Q s ç(u s s , N^ g , σ ) . Moreover, because E g s is fixed with respect to the intrinsic level E^, it follows that u -u = n (8) ss s where n is constant. We will consider the case in which only the number of acceptor states changes. The gaussian distribution (σ) and also the position of the maximum in N S S (E) are not influenced by any chemical process. By combining equations (3), (5), (7) and (8) we obtain %+%s
+ 2
<>ηΛ> F ( V V
=
- ^ s t ( n + u s ' Nss«
σ)
(9)
The left and right-hand part of equation (9) can be represented graphically as a function of u s . The values of n (-16) and σ (100 mV) are based on the data as given in Fig. 9 of Ref. 9 and the value of u^ (-11.51) follows from the doping concentration (N^ = 1.5xl0 15 c m " 3 ) . The intersections of the curves representing the right-hand part of equation (9) for different values of N| s with the curves representing the left-hand part with Q 0 +Q S s a s a parameter, yield values of u s from which Q s c can be calculated according to equation (5). N° is varied between 2x1ο 1 l and 2xl0 1 2 states cm"2 and (Q +Q ) oo
io
Ί« Λ
o
'-'so
between lxl0 lz and 2x10 1 Z elementary charges cm" z . For a certain value of u s the inversion charge Q n can be calculated. The conductivity G of the inversion channel is given by G = -p n Q n _ (.0) where y n is the electron mobility (yn = 600 cii^V^s" 1 ) (1). It is found that the change in the conductivity as a function of a change in the total number of states ( ) is approximately -5xl0~~17 S cm 2 . For an ISFET, the change dN° ss in the drain current (ΔΙ^) due to a change in N s S (A N ss^ ^s e^ua^ t o AT
- £ (4£_) Δ Ν ° V, . (11) L dNo ss ds ss If N g s (E) increases with the same percentage over the whole energy range as near midgap and we take lxlO 12 states cm"2 for N^ g at pH 1, ΔΝ| 8 is about 3x1ο 11 states cm -2 . According to equation (11) the decrease in drain current (ΔΙ^) for an ISFET with a γW value of 50 and a V d g of 0.1 Volt, is about 75μΑ in the pH range 1 to d
13, which is a measurable quantity.
437 CONCLUSION It can be concluded from the C-V measurements that the surface state density at the Si-Si0 2 interface is influenced by a change in the electrolyte composition, although the mechanism involved is uncertain. Modulation of the surface state density at the Si-Si02 interface due to variations in the composition at the ambient (liquid or gas) will be an interesting subject for further investigation in order to find applications in Chemically Sensitive Semiconductor Devices (3). ACKNOWLEDGEMENT This work has been supported by the Netherlands Organization for the Advancement at Pure Research (Z.W.O.) and the Hogeschoolfonds of the Twente University of Technology. REFERENCES (1)
(2) (3) (4) (5) (6) (7)
(8)
(9)
P. Bergveld, Development, operation and application of the Ion Sensitive Field Effect Transistor as a tool for electrophysiology, IEEE Trans. on Biomed. Eng. BME-19, 342 (1972). A.G. Revesz, On the mechanism of the ion sensitive field effect transistor, Thin Solid Films 41, L43 (1977). Jay N. Zemel, Chemically sensitive semiconductor devices, Research/Development 28, 38 (1977). M. Kuhn, A quasi-static technique for MOS C-V and surface state measurements, Solid-St. Electron. 13, 873 (1970). S. Levine and A.L. Smith, Theory of the Differential Capacity of the Oxide/ Aqueous Electrolyte Interface, Discuss. Faraday Soc. 52, 290 (1971). J.W. Perram, Structure of the Double Layer at the Oxide/Water Interface, J. Chem. Soc. Faraday II, 993 (1973). R. van Overstraeten, G. Declerck and G. Broux, Graphical Technique to Determine the Density of Surface States at the Si-SiU2 Interface of MOS Devices Using the quasistatic C-V Method, J. Electrochem. Soc. 120, 1785 (1973). R.H. Kingston, S.F. Neustadter, Calculation of the Space Charge, Electric Field and Free Carrier Concentration at the Surface of a Semiconductor, Journ. of Appl. Physics 26, 718 (1955). B.E. Deal, The Current Understanding of Charges in the Thermally Oxidized Silicon Structure, J. Electrochem. Soc. 121, 198c (1974).