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On the open-circuit voltage of a Schottky-barrier MIS solar cell
Solid-Sfole Elecfronics Vol. 25, No. 12, pp. 1205-1206, 1982 Printed in Great Britain.
0038-1101/82/121205-02$03.00/0 0 1982 Pergamon Press Ltd.
NOTES ON THE OPEN-CIRCUIT VOLTAGE OF A SCHOTTKY-BARRIER
MIS SOLAR CELL
(Received 28 April 1982) Schottky-barrier MIS (Metal-Insulator-Semiconductor) solar cells are receiving increasing attention because of a number of inherent advantages such as low cost, high yield, fabrication at low substrate temperature, etc. One drawback of such cells is that their open-circuit voltage (V,,) is somewhat low. In this communication, a number of design guidelines for improving V,,, of this type of cells will be suggested. It is well-known [l] that V,, of a Schottky barrier MIS solar cell can be expressed by a relation of the form
where n is the ideality factor, S the thickness of the interfacial layer, & the barrier height in volts, J,, the short circuit photogenerated current density in A/cm’, A** the effective Richardson constant in A/cm*l”K*and x the mean barrier height presented by the interfacial layer to the flow of majority carriers, and the other terms have their usual significance. Now, the expression for barrier height for n- and p-type substrates are respectively given by [2] &9,=Y(dJ-x,)+(l-Y)(E,-9”)-~$$$
(2)
and
where y=(1t9)’
QJq represents the amount of fixed charge lying within the interfacial layer very close to the oxide-semiconductor interface and the other symbols have their usual significance. If values of barrier height are calculated for p-type silicon assuming D, = 3 x 10” states/cmL/eV and Q,, = 2 x IO” charges/cm*, and are plotted as a function of I$,,,(Fig. 1) with S as parameter, it is seen that the barrier height increases as 4, decreases. Regarding the dependence on S, it is interesting to note that there is a critical value of work function &, = &,,,(say) above which barrier height increases as S increases while the opposite is true for values of 4, lower than &,,. An expression for &,,, has been derived by the authors[3] and is of the form
oxide layer, the value of &, would be higher than &,,, if the oxide charge in the interfacial layer is negative and lower than 4 mooif the charge is positive. Further, the difference in the value of &,, and &,,,, depends on the values of both Q,,, and D,. For a chemically cleaned surface, assuming (QJq = 10” charges/cm’ and D, =3 x 10” states/cm’/eV, (QJqD,) comes out to be 0.33eV. For vacuum cleaved surfaces, however, having much higher value of surface state density, the term QJqD, becomes negligible. Comparing the values of b,,, for both p- and nsilicon with the work function of some typical metals which are or may be used for deposition, e.g. Cr(4.6eV ), Al (4.2 eV) and Mg (3.7eV) for p- Si and Au (4.70eV), Ni (4.74 eV) and Pd (4.82eV) for n- Si, it is clearly seen that the desired condition discussed above, viz., 4, > 4, ,,,, for p-silicon is not satisfied by most of the metals. However, as already stated, by a proper choice of Q,, and D,, it may be possible to satisfy the condition vis-a-vis $,, in most of the cases. Figure I enables us to make an appropriate choice of the values of I$,,, D, and S for a MIS SBSC. If the work function of the metal (4,) is so chosen that &, > drno,it is evident from Fig. 1 that an increae in the value of 6 would lead to an increase in &p which in turn would lead to an increase in Voc provided /,, is not adversely affected. Thus; for a p-type substrate, since the barrier height increases as the metal work function decreases, it is obviously desirable that &,, should be lower than the work function of the metal selected for deposition. Referring to the expression f$,,, it follows that a higher value of Q,,, or a lower value of D, will give us the desired low value of r#~,,,, provided Q,,, is positive as in silicon. For materials like GaAs, GaP, etc., where Q,,, is usually negative and n-type substrates are commonly used, a similar argument shows that here also, a higher value of Q,, or a lower value of D, would lead to higher values of V,,. It is to be noted that presence of donor or acceptor type surface states at the oxide-semiconductor interface may also cause a similar increase of V,,. It may be possible to introduce such surface states by developing suitable process schedules for chemical treatment of semiconductor surfaces and the growth of oxide thereon[b61. For metals with work function equal or near to &,,, the barrier height becomes more or less independent of both S and D, and the variation of open-circuit voltage in such cases should be primarily determined by n. Referring to eqn (l), it is evident that V,, would increase with increase of n, provided n and ~$a are independent quantities. In practice however, they are not so. The parameter n is ideally unity, but for non-ideal cells, with an interfacial layer of thickness S, n is given by [7]
Table 1. Table for &,,
for both p- and n-type substrates. Writing &,, = &,, for the zero oxide charge case (Q,, = 0), it follows from eqn (5) that Semiconductor rPmoo= Es + ,G - 40
(6) Si
In Table I, values of &,, for different semiconductors using eqn (6) and the data shown in the said table are given. Also, it is evident from eqns (5) and (6) that in the presence of charge in the SW. Vol. 25, No. 12-D
+
GaP GaAs
1205
1.12 2.24 1.43
4.05 4.0 4.07
0.30 0.66 0.53
0.27 0.294 0.38
4.87 5.58 4.97
1206
Notes
n=l+(6iq)[$+qD.] It is obvious from the above expression that n will increase as fi increases. Hence a larger value of S will lead to a higher value of V,,,,. It is interesting to note that in the discussion on the effect of $RD on V,,,, also, it was concluded that a higher value of S is desirable provided however the value of J,, is not adversely affected. In the case of n-type silicon substrate however, since the work function of the Schottky metal will usually be higher than the value of &,,, increase of fi will lead to a decrease of m,,,, which is undesirable. In the case of n-GaAs. since the oxide charge Q,,, is negative, it follows from eqn (5) &,,,, is higher. Therefore if the work function of the Schottky metal is chosen lower than b,,,,. an increase of S will lead to an increase of both &,, and n. Thus we arrive at the conclusion that for semiconductors having positive charge in the oxide layer, the substrate should be of p-type, the value of &,<, should be kept low and 4, should be greater than &~,, while for semiconductors with negative charge in the oxide layer, the substrate should be of n-type, &,, should be high and 4, < d,,,,,. As already mentioned, a higher value of the charge density in the oxide layer leads to the desired value of 4 n/o as stated above for both p- and n-type substrates. Also, the interfacial layer thickness should be high without affecting J,,.
METAL
WORK
FUNCTION,
‘$,,,
(PY)
Fig. 1. Showing variation of barrier height (&, or &,) with metal work function 4, for different values of interfacial layer thickness S, assuming the latter to be charge-free or to contain charge, xs = 4.05 eV, D, = 3 x IO” states/cm*/eV, &&t-type) = 0.27 eV and &@-type) = 0.33 eV.
where D,,, and Drh are densities of interface states in equilibrium with the metal and the semiconductor respectively and the other terms have their usual significance. For the sake of simplicity, if we assume that all the surface states are in equilibrium with the semiconductor, i.e., D,, = D, and D,, =0 we get the following expression for n:
Institute of Radiophysics and Electronics 92, Acharya Prafulla Chandra Road Calcutta-700009 India
A. N. D.+w A. K. DATT~ M. C. ASH
REFERENCES 1. H. C. Card, Solid-St. Electron. 20,971 (1977). 2. J. T. Lue, Solid-St. Electron. 23, 263 (1980). 3. A. N. Daw, A. K. Datta and M. C. Ash, Solid-St. Electron. 25,431 (1982). 4. D. L. Pulfrey, IEEE Trans. Electron. Deu. ED-25, 1308 (1978). 5. J. Majhi. P. R. Vaya and V. Ramachandran. J. Instn. Electron. and Telecom. Engrs. 27,318 (1981). 6. S. F. Cagina and E. H. Snow, J. Electrochem. Sot. 114, II65 (1967). 7. H. C. Card and E. H. Rhoderick. J. Phys. D: Appl. Pnyu. 4. 1589 (1971).
Solid-Sfore Elecwonio Vol 25, No I?. pp. l2@%1209. 1982 Printed in Great Britain.
003R-i101/82/l?l2~$0300/1~ 0 1982 Pergamon Prrc\ Ltd
THE EFFECT OF Al-GaAs INTERACTION ON THE TECHNOLOGY OF SELF-ALIGNED GALLIUM ARSENIDE MESFETS (Receioed 23 December 1981; in revised form I5 April 1982)
Aluminium is commonly used as a Schottky contact metal in GaAs devices. Recent studies indicate arsenic migration from the GaAs surface during the initial stages of deposition of insulators or metals[l]. Auger studies of such an interface strongly suggest arsenic migration during the aluminium deposition, where arsenic is observed to be accumulated on the deposited Al surface [2]. In addition, comparative studies of reliability under
temperature stress of GaAs MESFETS having Al and Au based Schottky contacts show considerable deterioration in saturation current of devices having Al Sckottky contacts. This is attributed primarily to a decrease in effective channel height caused by Al interaction with the GaAs active layer[3]. It seems from the above findings that the interaction of Al with GaAs results in a change in the stoichiometry of the GaAs surface and this results
0038-1101/82/121205-02$03.00/0 0 1982 Pergamon Press Ltd.
NOTES ON THE OPEN-CIRCUIT VOLTAGE OF A SCHOTTKY-BARRIER
MIS SOLAR CELL
(Received 28 April 1982) Schottky-barrier MIS (Metal-Insulator-Semiconductor) solar cells are receiving increasing attention because of a number of inherent advantages such as low cost, high yield, fabrication at low substrate temperature, etc. One drawback of such cells is that their open-circuit voltage (V,,) is somewhat low. In this communication, a number of design guidelines for improving V,,, of this type of cells will be suggested. It is well-known [l] that V,, of a Schottky barrier MIS solar cell can be expressed by a relation of the form
where n is the ideality factor, S the thickness of the interfacial layer, & the barrier height in volts, J,, the short circuit photogenerated current density in A/cm’, A** the effective Richardson constant in A/cm*l”K*and x the mean barrier height presented by the interfacial layer to the flow of majority carriers, and the other terms have their usual significance. Now, the expression for barrier height for n- and p-type substrates are respectively given by [2] &9,=Y(dJ-x,)+(l-Y)(E,-9”)-~$$$
(2)
and
where y=(1t9)’
QJq represents the amount of fixed charge lying within the interfacial layer very close to the oxide-semiconductor interface and the other symbols have their usual significance. If values of barrier height are calculated for p-type silicon assuming D, = 3 x 10” states/cmL/eV and Q,, = 2 x IO” charges/cm*, and are plotted as a function of I$,,,(Fig. 1) with S as parameter, it is seen that the barrier height increases as 4, decreases. Regarding the dependence on S, it is interesting to note that there is a critical value of work function &, = &,,,(say) above which barrier height increases as S increases while the opposite is true for values of 4, lower than &,,. An expression for &,,, has been derived by the authors[3] and is of the form
oxide layer, the value of &, would be higher than &,,, if the oxide charge in the interfacial layer is negative and lower than 4 mooif the charge is positive. Further, the difference in the value of &,, and &,,,, depends on the values of both Q,,, and D,. For a chemically cleaned surface, assuming (QJq = 10” charges/cm’ and D, =3 x 10” states/cm’/eV, (QJqD,) comes out to be 0.33eV. For vacuum cleaved surfaces, however, having much higher value of surface state density, the term QJqD, becomes negligible. Comparing the values of b,,, for both p- and nsilicon with the work function of some typical metals which are or may be used for deposition, e.g. Cr(4.6eV ), Al (4.2 eV) and Mg (3.7eV) for p- Si and Au (4.70eV), Ni (4.74 eV) and Pd (4.82eV) for n- Si, it is clearly seen that the desired condition discussed above, viz., 4, > 4, ,,,, for p-silicon is not satisfied by most of the metals. However, as already stated, by a proper choice of Q,, and D,, it may be possible to satisfy the condition vis-a-vis $,, in most of the cases. Figure I enables us to make an appropriate choice of the values of I$,,, D, and S for a MIS SBSC. If the work function of the metal (4,) is so chosen that &, > drno,it is evident from Fig. 1 that an increae in the value of 6 would lead to an increase in &p which in turn would lead to an increase in Voc provided /,, is not adversely affected. Thus; for a p-type substrate, since the barrier height increases as the metal work function decreases, it is obviously desirable that &,, should be lower than the work function of the metal selected for deposition. Referring to the expression f$,,, it follows that a higher value of Q,,, or a lower value of D, will give us the desired low value of r#~,,,, provided Q,,, is positive as in silicon. For materials like GaAs, GaP, etc., where Q,,, is usually negative and n-type substrates are commonly used, a similar argument shows that here also, a higher value of Q,, or a lower value of D, would lead to higher values of V,,. It is to be noted that presence of donor or acceptor type surface states at the oxide-semiconductor interface may also cause a similar increase of V,,. It may be possible to introduce such surface states by developing suitable process schedules for chemical treatment of semiconductor surfaces and the growth of oxide thereon[b61. For metals with work function equal or near to &,,, the barrier height becomes more or less independent of both S and D, and the variation of open-circuit voltage in such cases should be primarily determined by n. Referring to eqn (l), it is evident that V,, would increase with increase of n, provided n and ~$a are independent quantities. In practice however, they are not so. The parameter n is ideally unity, but for non-ideal cells, with an interfacial layer of thickness S, n is given by [7]
Table 1. Table for &,,
for both p- and n-type substrates. Writing &,, = &,, for the zero oxide charge case (Q,, = 0), it follows from eqn (5) that Semiconductor rPmoo= Es + ,G - 40
(6) Si
In Table I, values of &,, for different semiconductors using eqn (6) and the data shown in the said table are given. Also, it is evident from eqns (5) and (6) that in the presence of charge in the SW. Vol. 25, No. 12-D
+
GaP GaAs
1205
1.12 2.24 1.43
4.05 4.0 4.07
0.30 0.66 0.53
0.27 0.294 0.38
4.87 5.58 4.97
1206
Notes
n=l+(6iq)[$+qD.] It is obvious from the above expression that n will increase as fi increases. Hence a larger value of S will lead to a higher value of V,,,,. It is interesting to note that in the discussion on the effect of $RD on V,,,, also, it was concluded that a higher value of S is desirable provided however the value of J,, is not adversely affected. In the case of n-type silicon substrate however, since the work function of the Schottky metal will usually be higher than the value of &,,, increase of fi will lead to a decrease of m,,,, which is undesirable. In the case of n-GaAs. since the oxide charge Q,,, is negative, it follows from eqn (5) &,,,, is higher. Therefore if the work function of the Schottky metal is chosen lower than b,,,,. an increase of S will lead to an increase of both &,, and n. Thus we arrive at the conclusion that for semiconductors having positive charge in the oxide layer, the substrate should be of p-type, the value of &,<, should be kept low and 4, should be greater than &~,, while for semiconductors with negative charge in the oxide layer, the substrate should be of n-type, &,, should be high and 4, < d,,,,,. As already mentioned, a higher value of the charge density in the oxide layer leads to the desired value of 4 n/o as stated above for both p- and n-type substrates. Also, the interfacial layer thickness should be high without affecting J,,.
METAL
WORK
FUNCTION,
‘$,,,
(PY)
Fig. 1. Showing variation of barrier height (&, or &,) with metal work function 4, for different values of interfacial layer thickness S, assuming the latter to be charge-free or to contain charge, xs = 4.05 eV, D, = 3 x IO” states/cm*/eV, &&t-type) = 0.27 eV and &@-type) = 0.33 eV.
where D,,, and Drh are densities of interface states in equilibrium with the metal and the semiconductor respectively and the other terms have their usual significance. For the sake of simplicity, if we assume that all the surface states are in equilibrium with the semiconductor, i.e., D,, = D, and D,, =0 we get the following expression for n:
Institute of Radiophysics and Electronics 92, Acharya Prafulla Chandra Road Calcutta-700009 India
A. N. D.+w A. K. DATT~ M. C. ASH
REFERENCES 1. H. C. Card, Solid-St. Electron. 20,971 (1977). 2. J. T. Lue, Solid-St. Electron. 23, 263 (1980). 3. A. N. Daw, A. K. Datta and M. C. Ash, Solid-St. Electron. 25,431 (1982). 4. D. L. Pulfrey, IEEE Trans. Electron. Deu. ED-25, 1308 (1978). 5. J. Majhi. P. R. Vaya and V. Ramachandran. J. Instn. Electron. and Telecom. Engrs. 27,318 (1981). 6. S. F. Cagina and E. H. Snow, J. Electrochem. Sot. 114, II65 (1967). 7. H. C. Card and E. H. Rhoderick. J. Phys. D: Appl. Pnyu. 4. 1589 (1971).
Solid-Sfore Elecwonio Vol 25, No I?. pp. l2@%1209. 1982 Printed in Great Britain.
003R-i101/82/l?l2~$0300/1~ 0 1982 Pergamon Prrc\ Ltd
THE EFFECT OF Al-GaAs INTERACTION ON THE TECHNOLOGY OF SELF-ALIGNED GALLIUM ARSENIDE MESFETS (Receioed 23 December 1981; in revised form I5 April 1982)
Aluminium is commonly used as a Schottky contact metal in GaAs devices. Recent studies indicate arsenic migration from the GaAs surface during the initial stages of deposition of insulators or metals[l]. Auger studies of such an interface strongly suggest arsenic migration during the aluminium deposition, where arsenic is observed to be accumulated on the deposited Al surface [2]. In addition, comparative studies of reliability under
temperature stress of GaAs MESFETS having Al and Au based Schottky contacts show considerable deterioration in saturation current of devices having Al Sckottky contacts. This is attributed primarily to a decrease in effective channel height caused by Al interaction with the GaAs active layer[3]. It seems from the above findings that the interaction of Al with GaAs results in a change in the stoichiometry of the GaAs surface and this results