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room temperature chloroaluminate molten-salt interfaces
ANALYSES ON CHLOROALUMINATE
AND p-GaAs/ROOM MOLTEN-SALT
R. THAPAR and K. RAJESHWAR* Department
of Electrical Engineering,-Colorado (Received 4 January
State University, Fort Collins, CO 80523, U.S.A.
1982; in reuisedform 19 July 1981)
Abstract-Mott-Schottky analyses were performed on n- and p_GaAs electrodes in room temperature molten-salt electrolytes comprising mixtures of AICI, and n-butyl pyridinium chloride (BPC) in approx. the 2:Lmolar ratio. These analyses revealed that the two interfaces, n-GaAs/AICI,-BPC and pGaAs/AlCI,-BPC, conformed with the behavior expected from the simple depletion layer model after suitable etching of the electrode surfaces. The flat-hand potentials (k$J thus determined for interfaces were: - 0.20 + 0.03 V (us Al‘IX’) for n-GaAs and + 1.22 + 0.01 V (us AI”” ) for p-GaAs, respectively. The doping densities obtained from the slope of the Mott-Schottky plots, however, were 2-3 times higher than those predicted from the Hall data supplied by the manufacturer. Possible reasons for these discrepancies are discussed. The difference between the VI, values for the n- and p-type GaAs correspond to the energy bandgap of the semiconductor-a bchavior consistent with the absence of significant band-edge unpinning effects in the two cases. Alternatively, the data were seen to be consistent with a behavior such that the band-edges are fixed at the surface for n- and p-GaAs electrodes and all or most of the applied voltage drops across the depletion layer in the semiconductor. The present data arc compared with results from cyclic voltammetry obtained in a previous study. In this study, photocurrents were sustained on n-GaAs at potentials significantly positive of the valence band-edge in the presence of certain eleclroative species in the AICl,-BPC electrolyte. This behaviat may be understood in terms of charge accumulation at the interface such that the semiconductor band-edges no longer remain fixed, unlike in the situation above in the absence of redox species. The mechanisms underlying charge accumulation could involve the presence of either a high density of surface states or an inverted g-tvoe surface region caused by hole injection from highly oxidizing redox species as discussed by previous abthors. INTRODUCTION
at the interface.* Therefore, the difference between the band-edge positions determined in the two cases may not correspond to E,. This presupposes, ofcourse, that the unpinning effect occurs tp different degrees at the n- and p-type semiconductor/ electrolyte interfaces respectively. Such a possibility was tested on the model GaAs/AICl,-BPC interface by differential capacitance measurements on n- and p-type samples. Our findings are detailed in this paper. Finally, our previous observations of supra-bandgap photoeffects on the model interface in contact with highly oxidizing redox couples[2] are discussed in the light of the data presented in this paper.
Room temperature molten-salts have certain properties which favor their choice as candidate nonaqueous electrolytes in photoelectrochemical (PEC) systems[l]. Chloroaluminate mixtures, in particular, offer the combination of good electrical conductivity (necessary for minimisation of iR losses) and apotic characteristics (necessary for electrode stability)[2]. Furthermore, we[2] and others[3] have demonstrated that cation radicals have useful life-times which are significantly longer in chloroaluminate electrolytes than in more conventional non-aqueous systems such as acctonitrile. This behaviour implies that a wide range of redox couples are potentially available for use in PEC devices based on chloroaluminate electrolytes. In previous papers from this laboratory, the use of chloroaluminate mixtures in PEC devices was demonstrated on a model system comprising the nGaAs/aluminum chloride-n--butyl pyridinium chloride (AlCl,-BPC) interface[l]. For an ideal semiconductor/electrolyte interface, the difference between the flat-band potentials (V,d for an n-type and p-type electrode material should correspond to its energy band-gap (E.J after correction is made for the difference between the band-edges and the respective Fermi levels[4]. On the other hand, if an appreciable fraction of the applied voltage drops across the interfacial layer, the band-edges may become unpinned * To whom correspondence
EXPERIMENTAL The n- and p-GaAs wafers were obtained from commercial sources. Electrodes were fabricated from these wafers as described previously[l]. They had (100) faces exposed to the electrolyte. The doping densities as quoted by the manufacturer were 4.5 x lOI cm- 3 and 7.9 x 10” cm- 3, respectively for the n- and p-type samples. The nominal electrode area
* Charge accumulation at the semiconductor/electrolyte interface is responsible for band-edge unpinning effects. Either a high density of surface states151 or the formation of an inversion layer[6] on the semiconductor surface can account for this accumulation of charge.
should be addressed. 195
196
R.
THAPAR AND K. RAIESHWAR
(uncorrected for surface roughness) was: 0.24cm’ for n-type electrodes and 0.3hcm’ for p-type samples. The AICl,-BPC electrolyte was prepared according to procedures described previously[ la]. The electrolyte composition was adjusted to be acidic (- 2: 1 AlCl,-BPC mole ratio composition) for all the measurements reported herein, All experiments were conducted under an argon atmosphere in a commercial dry box. Capacitance measurements were carried out in the frequency range l-10 kHz using a lock-in techniquecla]. The instrumentation and procedures for these measurements were identical to those described in a previous paper[ la]. A conventional three-electrode electrochemical cell was employed for the capacitance measurements. The counterelectrode was a large sheet (area: _ 10cm’) of vitreous carbon. The reference electrode was an Al wire immersed in a 2~1 AlCl,-BPC electrolyte which was separated from the main compartment electrolyte by a glass tube fitted with a finely fritted ceramic disk. All potentials below are quoted with reference to this Al”j3 + clcctrode. RESULTS
AND
DISCUSSION
Figure 1 illustrates representative Mott&Schottky data on n-and p-type GaAs electrodes. These measurements were obtained at an ac signal frequency of 2 kHz. Similar bebavior with varying degrees of frequency dispersion was noted at other frequencies up to 1OOkH.z. These dispersion effects could be eliminated by appropriate etching of the electrode surface with a 3: 1: 1 H2S04/H20Z/HZ0 solution, followed by a rinse with deionized (1X MR) water and a final 6 M HCl etch. The beneficial effects of etching confirm the interpretation of previous authors[7] for frequency dispersion effects in terms of damaged surface layers. In some instances an increase in capacitance values with increasing frequency was noted which disappeared after the etching sequence described above. We note that similar anomalous effects have
Fig. 1. Representative Mott&Schottky data on n-GaAs (filled circles) and p-GaAs (open circles) electrodes in 2:l AlC13 BPC electrolyte. The LV signal frequency was 2 kHz.
been observed by previous authors for nGaAs/aqueous electrolyte interface@]. From data such as those shown in Fig. 1, a V,, value of - 0.20 f 0.03 V is deduced for the n-type sample and + 1.22 + 0.01 V for the p-type electrode respectively.* These data were used to calculate the band-edge locations by means of (1) and (2)[4]: E;(n) = E;,--kTln(N,/N,),
(1)
E;(p) = ETb+kTIn(NdN,)
(2)
El. and
EC are the energies corresponding to the conduction and valence band-edges at the surface, N ,, = donor density, N, = acceptor density, N, and N, are the effective density of states in the conduction and valence bands respectively (N, = 4.7 x 10” cmv3, N, = 7.0 x IO” cm- 3 for GaAs[9]) and Enfband E&are the Fermi energies at flat-band corresponding to the nand p-type samples (ie, E,, = -q V,,J. The other symbols in (1) and (2) have their usual meaning. For determination of Ef and Et from MottSchottky data, knowledge of ND and N, is necessary except in the case of heavily-doped semiconductors. Although, in principle, one can use manufacturer-quoted values for this purpose, experience in this laboratory[ 101, as well as in others[8], indicates that Hall data may not be truly reflective of the situation at the semiconductor/electrolyte interface. Serious discrepancies may arise because of differences in the sensitivity of Hall and capacitance techniques to the degree of ionization of the impurities, ie, whereas the former measures only the free (dissociated) charge carriers, much higher doping densities may be measured in the latter because the high electric fields across the depletion layer at the interface often result in more complete ionization of the donor or acceptor impurities. For example, the slopes in Fig. 1 correspond to ND = 1.0 x lOI cmm3 and NA = 2.3 which are 2-3 times higher than the Hall x 10’8cm-3 values quoted by the manufacturer (uide supra). Apart from the above factor, another possible reason for the discrepancy between the two sets of values is the error arising from the neglect of surface roughness in electrode area computations. An estimate for this effect is obtained from the surface roughness factor. The No and N, values obtained from the present MottSchottky data would agree with quoted values if surface roughness factors of 1.5 and 1.7 are assumed for n- and p-GaAs respectively. Both sets ofvalues were used for computation of EEand Ezoia (1) and (2). The results are assembled in Table 1. These data show the following: (a) The error introduced by the discrepancy between the Motf-Schottky slopes and those predicted by the manufacturer-quoted ND and N, values, is negligible for the computation of E:. and E:, in the present case. (b) The doping levels seem high enough that the approximations V;, = -E& and J+ = ~ E?,,4 are valid within the limits of experimental error. In this respect, it is noted that the difference q Y”$,- qY$, (Table 1) is very close to E, (1.43eV) of GaAs. The
* These values include the correction for the thermal voltage term which IS - 0.03 Vat room temperature (qf[ 1 l] ).
MotttSehottky
197
analyses
Table 1.
0.20 + 0.03
-
Vfb
(V as Alo’3 ‘1 Doping density (cm- 3)
1.0 X 101’ (4.5 X 1016)
J? (eVusAI”/3’)
2.3 X 10’8 (7.9 X 10”)
0.24
0.18
(0.26) - 1.19
(0.15 - 1.25
E: (eV vs Al”i3’)
+ 1.22 _tO.Ol
(-
1.17)
(-
1.28)
The values in parentheses are those obtained or determined from manufacturer’s data (see text).
surface energy levels in Table 1 were determined employing (3) in addition to (1) and (2): E:-E:=
E,
by (3)
(c) The locations of Ef and E:, are relatively independent of the type of conductivity (n or p) of the semiconductor. If the semiconductor surface had significantly different characteristics (eg, surface charge), these surface levels would have been expected to lie at different energies in the two cases. Stated in a somewhat different manner, the present MottSchottky data are considered to indicate “ideal” behavior in that a negligible portion of the applied voltage is dropped across the interfacial layer. It is emphasized, however, that the Yr, values (and there-
Figure 2 presents a comparison of the surface energy levels for n- and p-GaAs with selected redox couples, plotted on a common energy scale. The redox levels in Fig. 2 were taken from cyclic voltammetry data obtained from previous studies[la,2]. In the case of decamethyl ferrocene [ (q5-CSHJZFe], the redox level seems to be dependent on electrolyte acidity[2], while in all the other cases, the behavior is independent of electrolyte composition[la]. The Et and E; levels, however, are shifted upward with an increase in the basicity (ie, BPC content) of the AlCl,-BPC electrolyte[l]. In accordance with the simple model for semiconductor/electrolyte interface@1 I], both (q5C,H,),Fe and ferrocene should give maximum photooutput (ie, band-bending) with p-GaAs, whereas benzopyrene (BP) should be ideal for n-GaAs for solar energy conversion. On the other hand, photooxidation of 9-10, diphenyl anthraccne (DPA) and anthracene (A) is expected to be suppressed on n-GaAs because of the exponential decrease in the carrier transfer (or thermal excitation) probability from the filled redox level with increasing E:-E,dOJ1l]. (However. BP. DPA and A are all expected to undereo reversible electrochemical charge transfer with p-GaAs in terms of the simple model). Recent experiments in this laboratory, however, show that the simple model is or& aartiallv aoulicable to the n-GaAs/AICI,-BPC int&face[l, 23. S’pkcifically, although ferrdcene yieldsa higher photovoltage than decamethyl ferrocene (cf Fig. 2) in agreement with the simple model, photo-
E, eV vs AI”=+ I
I-
,-E: Efn O-
-I
-
_-_____ WE: n- GaAs
p-GaAs
EfF
-BP -
DPA
----_A
Fig 2. Comparison of surface energy levels for n- and p-GaAs in 2: 1 AICI,-BPC electrolyte with redox levels as determined by cyclic vo)tammetry[la, 21. See text for explanation of symbols for redox systems. For the determination of the surface energy levels, values of N, = 1.0 x 10” cm- 3 and N, = 2.3 x lo’.’ cm-s were assumed (Table 1). Em and Err denote Fermi energy levels in a- and p-GaAs respecttve~y.
198
R. THAPAR AND K. RMESHWAR
oxidation of both DPA and A isjacile on n-GaAs-a result which is contrary to the expectation from an “ideal” interface. We interpret this behavior as follows. While the band-edges at the n-GaAs/AlCl,-BPC interface are fixed in the absence of certain redox species and in the dark, the surface energy levels of the semiconductor and redox species (cfFig. 2) are shifted relative to each other when the device is in operation or when the redox species is capable of injecting charge in the dark. As mentioned above, this shift which is a consequence of charge accumulation in the interfacial layer may be caused by two mechanisms: (a) hole injection from the redox species into the n-type semiconductor and creation of a p-type inversion layer at the surface[6], or (h) interaction of the redox species with the semiconductor surface and creation of a high density of surface states (cf[12]) which causes the interface to behave like a Schottky barrier in contact with the electrolyte[5]. That there is a voltage drop in the interfacial layer when the illuminated nGaAs/AICI,-BPC interface contacts the A and DPA species is also shown by the fact that the onset of photocurrents occurs at potentials well positive of the Vr,value as measured for this interface in the dark[Z]. Unfortunately, unequivocal identification of the precise mechanisms whereby the band-edges become unpinned at the n-GaAs/AICl,&3PC interface is not possible on the basis of data available at present. It is pertinent, however, to note that the Mott-Schottky plots for this interface in the presence of A and DPA did not conform to a simple depletion layer model nor did they show a parallel shift along the voltage axis relative to the control case. Such a shift would have been consistent with the interpretation[5] based on high surface state densities.
REFERENCES 1.
2. 3.
4. 5. 6.
7.
8. 9. 10.
Acknuwledgemenrs-This research was partially supported by a grant from the Solar Energy Research Institute (Grant XS&9272-l). The authors also thank the referee for his comments on the possible origin of supra-band-edge effects at the n-GaAs/AlCl,-BPC interface.
(a) P. Singh, K. Rajeshwar, J. DuElow and R. Job, J. Am. Sot. 102, 4676 (1980); (b) P. Singh, R. Singh, K. Rajeshwar and 3. Dubow, J. e[ectrochem. Sor. 128, 1145 (1981); (c) P. Singh, R. Singh, R. Gale, K. Rajeshwar and J. DuBow, J. appf. Pkys. 51, 6286 (1980); (d) R. Gale, P. Smith, P. Singh, K. Rajeshwar and J. DuBow, ACS Svmoosium Series. Vol. 146 (Edited by A. J. Nozik), Ch. 2i, 6.343. American Chemi&l Society(l981); (e) P. Smgh and K. Raieshwar. J. elecrrochrm Ser. 128, 1724 (1981). R. Thaparand K. Rajeshwar.J. electrockem. Sot. 129,560 (1982). (a) H. L. Jones, L. G. Boxall and R. A. Osteryoung, J. e!ecrroanaL Ckem. 38,426 (1972); (b) H. L. Jonesand R.A. Osteryoung ibid. 49, 281 (1974); (c) H. L. Chum, V. R. Koch, L. L. Miller and R. A. Osteryoung, J. Am. them. Sot. 97,3X4 (1975); (d) V. R. Koch, L. L. Miller and R. A. Osteryoung, d. Am. them. Sot. 98, 5227 (1976); (e) J. Robinson and R. A. Osteryoung, J. Am. chrm. Sot. 101, 323 (1979). W. P. Games and F. Cardon, 2. phys. Chem. NF 86,330 (1973). A. J. Bard, A. B. Bocarsly, F, R. Fan, E. G. Walton and M. S. Wrighton, .!. Am. them. Sot. 102, 3671 (1980). C. D. Jaeger, H. Gerischer and W. Kautek, Ber. Bunsenges. Phys. Ckem. 86, 20 (1982), see also referenms therein. (al E. C. Dutoit. R. L. Van Meirhaeehe. F. Cardon and W. P. Games, Ber’Bunsenges. Pkys. chehem.79. 1206 (1975); (b) W. H. Lafltre. R. L. Van Meirhaeghe, F. Cardon and in;. P. Games, Surj: Sci. 59,401 (1976);-(c) M. J. Madou,F. Cardon and W. P. Gomes, J. eleclrurkrm. Sot. 124, 1625 (1977). P. Janietz, R. Weiche, I. Westfahl. R. Landsberg and R. Dehmlow, J. e/ectroanaL Ckem. 106, 23 (1980). S. M. Sze, Physics ofSemiconductor Devices, Ch. 1, p. 57. John Wiley and Sons, New York (1969). K. Rajeshwar. L. Thompson, P. Singh, R. C. Kainthla and K. L. Chopra, .I. electrockem. Sot. 128. 1744 (1981). H. Gerischer in Physical Ckemistrq--An Advanced Treatise, Vol. 9A (Edited by H. Eying), Ch. 5. Academic Press, New York (1970). K. Rajeshwar, J. elertrochrm. Sot. 129, 1003 (1982). them.
11.
12.
II-
AND p-GaAs/ROOM MOLTEN-SALT
R. THAPAR and K. RAJESHWAR* Department
of Electrical Engineering,-Colorado (Received 4 January
State University, Fort Collins, CO 80523, U.S.A.
1982; in reuisedform 19 July 1981)
Abstract-Mott-Schottky analyses were performed on n- and p_GaAs electrodes in room temperature molten-salt electrolytes comprising mixtures of AICI, and n-butyl pyridinium chloride (BPC) in approx. the 2:Lmolar ratio. These analyses revealed that the two interfaces, n-GaAs/AICI,-BPC and pGaAs/AlCI,-BPC, conformed with the behavior expected from the simple depletion layer model after suitable etching of the electrode surfaces. The flat-hand potentials (k$J thus determined for interfaces were: - 0.20 + 0.03 V (us Al‘IX’) for n-GaAs and + 1.22 + 0.01 V (us AI”” ) for p-GaAs, respectively. The doping densities obtained from the slope of the Mott-Schottky plots, however, were 2-3 times higher than those predicted from the Hall data supplied by the manufacturer. Possible reasons for these discrepancies are discussed. The difference between the VI, values for the n- and p-type GaAs correspond to the energy bandgap of the semiconductor-a bchavior consistent with the absence of significant band-edge unpinning effects in the two cases. Alternatively, the data were seen to be consistent with a behavior such that the band-edges are fixed at the surface for n- and p-GaAs electrodes and all or most of the applied voltage drops across the depletion layer in the semiconductor. The present data arc compared with results from cyclic voltammetry obtained in a previous study. In this study, photocurrents were sustained on n-GaAs at potentials significantly positive of the valence band-edge in the presence of certain eleclroative species in the AICl,-BPC electrolyte. This behaviat may be understood in terms of charge accumulation at the interface such that the semiconductor band-edges no longer remain fixed, unlike in the situation above in the absence of redox species. The mechanisms underlying charge accumulation could involve the presence of either a high density of surface states or an inverted g-tvoe surface region caused by hole injection from highly oxidizing redox species as discussed by previous abthors. INTRODUCTION
at the interface.* Therefore, the difference between the band-edge positions determined in the two cases may not correspond to E,. This presupposes, ofcourse, that the unpinning effect occurs tp different degrees at the n- and p-type semiconductor/ electrolyte interfaces respectively. Such a possibility was tested on the model GaAs/AICl,-BPC interface by differential capacitance measurements on n- and p-type samples. Our findings are detailed in this paper. Finally, our previous observations of supra-bandgap photoeffects on the model interface in contact with highly oxidizing redox couples[2] are discussed in the light of the data presented in this paper.
Room temperature molten-salts have certain properties which favor their choice as candidate nonaqueous electrolytes in photoelectrochemical (PEC) systems[l]. Chloroaluminate mixtures, in particular, offer the combination of good electrical conductivity (necessary for minimisation of iR losses) and apotic characteristics (necessary for electrode stability)[2]. Furthermore, we[2] and others[3] have demonstrated that cation radicals have useful life-times which are significantly longer in chloroaluminate electrolytes than in more conventional non-aqueous systems such as acctonitrile. This behaviour implies that a wide range of redox couples are potentially available for use in PEC devices based on chloroaluminate electrolytes. In previous papers from this laboratory, the use of chloroaluminate mixtures in PEC devices was demonstrated on a model system comprising the nGaAs/aluminum chloride-n--butyl pyridinium chloride (AlCl,-BPC) interface[l]. For an ideal semiconductor/electrolyte interface, the difference between the flat-band potentials (V,d for an n-type and p-type electrode material should correspond to its energy band-gap (E.J after correction is made for the difference between the band-edges and the respective Fermi levels[4]. On the other hand, if an appreciable fraction of the applied voltage drops across the interfacial layer, the band-edges may become unpinned * To whom correspondence
EXPERIMENTAL The n- and p-GaAs wafers were obtained from commercial sources. Electrodes were fabricated from these wafers as described previously[l]. They had (100) faces exposed to the electrolyte. The doping densities as quoted by the manufacturer were 4.5 x lOI cm- 3 and 7.9 x 10” cm- 3, respectively for the n- and p-type samples. The nominal electrode area
* Charge accumulation at the semiconductor/electrolyte interface is responsible for band-edge unpinning effects. Either a high density of surface states151 or the formation of an inversion layer[6] on the semiconductor surface can account for this accumulation of charge.
should be addressed. 195
196
R.
THAPAR AND K. RAIESHWAR
(uncorrected for surface roughness) was: 0.24cm’ for n-type electrodes and 0.3hcm’ for p-type samples. The AICl,-BPC electrolyte was prepared according to procedures described previously[ la]. The electrolyte composition was adjusted to be acidic (- 2: 1 AlCl,-BPC mole ratio composition) for all the measurements reported herein, All experiments were conducted under an argon atmosphere in a commercial dry box. Capacitance measurements were carried out in the frequency range l-10 kHz using a lock-in techniquecla]. The instrumentation and procedures for these measurements were identical to those described in a previous paper[ la]. A conventional three-electrode electrochemical cell was employed for the capacitance measurements. The counterelectrode was a large sheet (area: _ 10cm’) of vitreous carbon. The reference electrode was an Al wire immersed in a 2~1 AlCl,-BPC electrolyte which was separated from the main compartment electrolyte by a glass tube fitted with a finely fritted ceramic disk. All potentials below are quoted with reference to this Al”j3 + clcctrode. RESULTS
AND
DISCUSSION
Figure 1 illustrates representative Mott&Schottky data on n-and p-type GaAs electrodes. These measurements were obtained at an ac signal frequency of 2 kHz. Similar bebavior with varying degrees of frequency dispersion was noted at other frequencies up to 1OOkH.z. These dispersion effects could be eliminated by appropriate etching of the electrode surface with a 3: 1: 1 H2S04/H20Z/HZ0 solution, followed by a rinse with deionized (1X MR) water and a final 6 M HCl etch. The beneficial effects of etching confirm the interpretation of previous authors[7] for frequency dispersion effects in terms of damaged surface layers. In some instances an increase in capacitance values with increasing frequency was noted which disappeared after the etching sequence described above. We note that similar anomalous effects have
Fig. 1. Representative Mott&Schottky data on n-GaAs (filled circles) and p-GaAs (open circles) electrodes in 2:l AlC13 BPC electrolyte. The LV signal frequency was 2 kHz.
been observed by previous authors for nGaAs/aqueous electrolyte interface@]. From data such as those shown in Fig. 1, a V,, value of - 0.20 f 0.03 V is deduced for the n-type sample and + 1.22 + 0.01 V for the p-type electrode respectively.* These data were used to calculate the band-edge locations by means of (1) and (2)[4]: E;(n) = E;,--kTln(N,/N,),
(1)
E;(p) = ETb+kTIn(NdN,)
(2)
El. and
EC are the energies corresponding to the conduction and valence band-edges at the surface, N ,, = donor density, N, = acceptor density, N, and N, are the effective density of states in the conduction and valence bands respectively (N, = 4.7 x 10” cmv3, N, = 7.0 x IO” cm- 3 for GaAs[9]) and Enfband E&are the Fermi energies at flat-band corresponding to the nand p-type samples (ie, E,, = -q V,,J. The other symbols in (1) and (2) have their usual meaning. For determination of Ef and Et from MottSchottky data, knowledge of ND and N, is necessary except in the case of heavily-doped semiconductors. Although, in principle, one can use manufacturer-quoted values for this purpose, experience in this laboratory[ 101, as well as in others[8], indicates that Hall data may not be truly reflective of the situation at the semiconductor/electrolyte interface. Serious discrepancies may arise because of differences in the sensitivity of Hall and capacitance techniques to the degree of ionization of the impurities, ie, whereas the former measures only the free (dissociated) charge carriers, much higher doping densities may be measured in the latter because the high electric fields across the depletion layer at the interface often result in more complete ionization of the donor or acceptor impurities. For example, the slopes in Fig. 1 correspond to ND = 1.0 x lOI cmm3 and NA = 2.3 which are 2-3 times higher than the Hall x 10’8cm-3 values quoted by the manufacturer (uide supra). Apart from the above factor, another possible reason for the discrepancy between the two sets of values is the error arising from the neglect of surface roughness in electrode area computations. An estimate for this effect is obtained from the surface roughness factor. The No and N, values obtained from the present MottSchottky data would agree with quoted values if surface roughness factors of 1.5 and 1.7 are assumed for n- and p-GaAs respectively. Both sets ofvalues were used for computation of EEand Ezoia (1) and (2). The results are assembled in Table 1. These data show the following: (a) The error introduced by the discrepancy between the Motf-Schottky slopes and those predicted by the manufacturer-quoted ND and N, values, is negligible for the computation of E:. and E:, in the present case. (b) The doping levels seem high enough that the approximations V;, = -E& and J+ = ~ E?,,4 are valid within the limits of experimental error. In this respect, it is noted that the difference q Y”$,- qY$, (Table 1) is very close to E, (1.43eV) of GaAs. The
* These values include the correction for the thermal voltage term which IS - 0.03 Vat room temperature (qf[ 1 l] ).
MotttSehottky
197
analyses
Table 1.
0.20 + 0.03
-
Vfb
(V as Alo’3 ‘1 Doping density (cm- 3)
1.0 X 101’ (4.5 X 1016)
J? (eVusAI”/3’)
2.3 X 10’8 (7.9 X 10”)
0.24
0.18
(0.26) - 1.19
(0.15 - 1.25
E: (eV vs Al”i3’)
+ 1.22 _tO.Ol
(-
1.17)
(-
1.28)
The values in parentheses are those obtained or determined from manufacturer’s data (see text).
surface energy levels in Table 1 were determined employing (3) in addition to (1) and (2): E:-E:=
E,
by (3)
(c) The locations of Ef and E:, are relatively independent of the type of conductivity (n or p) of the semiconductor. If the semiconductor surface had significantly different characteristics (eg, surface charge), these surface levels would have been expected to lie at different energies in the two cases. Stated in a somewhat different manner, the present MottSchottky data are considered to indicate “ideal” behavior in that a negligible portion of the applied voltage is dropped across the interfacial layer. It is emphasized, however, that the Yr, values (and there-
Figure 2 presents a comparison of the surface energy levels for n- and p-GaAs with selected redox couples, plotted on a common energy scale. The redox levels in Fig. 2 were taken from cyclic voltammetry data obtained from previous studies[la,2]. In the case of decamethyl ferrocene [ (q5-CSHJZFe], the redox level seems to be dependent on electrolyte acidity[2], while in all the other cases, the behavior is independent of electrolyte composition[la]. The Et and E; levels, however, are shifted upward with an increase in the basicity (ie, BPC content) of the AlCl,-BPC electrolyte[l]. In accordance with the simple model for semiconductor/electrolyte interface@1 I], both (q5C,H,),Fe and ferrocene should give maximum photooutput (ie, band-bending) with p-GaAs, whereas benzopyrene (BP) should be ideal for n-GaAs for solar energy conversion. On the other hand, photooxidation of 9-10, diphenyl anthraccne (DPA) and anthracene (A) is expected to be suppressed on n-GaAs because of the exponential decrease in the carrier transfer (or thermal excitation) probability from the filled redox level with increasing E:-E,dOJ1l]. (However. BP. DPA and A are all expected to undereo reversible electrochemical charge transfer with p-GaAs in terms of the simple model). Recent experiments in this laboratory, however, show that the simple model is or& aartiallv aoulicable to the n-GaAs/AICI,-BPC int&face[l, 23. S’pkcifically, although ferrdcene yieldsa higher photovoltage than decamethyl ferrocene (cf Fig. 2) in agreement with the simple model, photo-
E, eV vs AI”=+ I
I-
,-E: Efn O-
-I
-
_-_____ WE: n- GaAs
p-GaAs
EfF
-BP -
DPA
----_A
Fig 2. Comparison of surface energy levels for n- and p-GaAs in 2: 1 AICI,-BPC electrolyte with redox levels as determined by cyclic vo)tammetry[la, 21. See text for explanation of symbols for redox systems. For the determination of the surface energy levels, values of N, = 1.0 x 10” cm- 3 and N, = 2.3 x lo’.’ cm-s were assumed (Table 1). Em and Err denote Fermi energy levels in a- and p-GaAs respecttve~y.
198
R. THAPAR AND K. RMESHWAR
oxidation of both DPA and A isjacile on n-GaAs-a result which is contrary to the expectation from an “ideal” interface. We interpret this behavior as follows. While the band-edges at the n-GaAs/AlCl,-BPC interface are fixed in the absence of certain redox species and in the dark, the surface energy levels of the semiconductor and redox species (cfFig. 2) are shifted relative to each other when the device is in operation or when the redox species is capable of injecting charge in the dark. As mentioned above, this shift which is a consequence of charge accumulation in the interfacial layer may be caused by two mechanisms: (a) hole injection from the redox species into the n-type semiconductor and creation of a p-type inversion layer at the surface[6], or (h) interaction of the redox species with the semiconductor surface and creation of a high density of surface states (cf[12]) which causes the interface to behave like a Schottky barrier in contact with the electrolyte[5]. That there is a voltage drop in the interfacial layer when the illuminated nGaAs/AICI,-BPC interface contacts the A and DPA species is also shown by the fact that the onset of photocurrents occurs at potentials well positive of the Vr,value as measured for this interface in the dark[Z]. Unfortunately, unequivocal identification of the precise mechanisms whereby the band-edges become unpinned at the n-GaAs/AICl,&3PC interface is not possible on the basis of data available at present. It is pertinent, however, to note that the Mott-Schottky plots for this interface in the presence of A and DPA did not conform to a simple depletion layer model nor did they show a parallel shift along the voltage axis relative to the control case. Such a shift would have been consistent with the interpretation[5] based on high surface state densities.
REFERENCES 1.
2. 3.
4. 5. 6.
7.
8. 9. 10.
Acknuwledgemenrs-This research was partially supported by a grant from the Solar Energy Research Institute (Grant XS&9272-l). The authors also thank the referee for his comments on the possible origin of supra-band-edge effects at the n-GaAs/AlCl,-BPC interface.
(a) P. Singh, K. Rajeshwar, J. DuElow and R. Job, J. Am. Sot. 102, 4676 (1980); (b) P. Singh, R. Singh, K. Rajeshwar and 3. Dubow, J. e[ectrochem. Sor. 128, 1145 (1981); (c) P. Singh, R. Singh, R. Gale, K. Rajeshwar and J. DuBow, J. appf. Pkys. 51, 6286 (1980); (d) R. Gale, P. Smith, P. Singh, K. Rajeshwar and J. DuBow, ACS Svmoosium Series. Vol. 146 (Edited by A. J. Nozik), Ch. 2i, 6.343. American Chemi&l Society(l981); (e) P. Smgh and K. Raieshwar. J. elecrrochrm Ser. 128, 1724 (1981). R. Thaparand K. Rajeshwar.J. electrockem. Sot. 129,560 (1982). (a) H. L. Jones, L. G. Boxall and R. A. Osteryoung, J. e!ecrroanaL Ckem. 38,426 (1972); (b) H. L. Jonesand R.A. Osteryoung ibid. 49, 281 (1974); (c) H. L. Chum, V. R. Koch, L. L. Miller and R. A. Osteryoung, J. Am. them. Sot. 97,3X4 (1975); (d) V. R. Koch, L. L. Miller and R. A. Osteryoung, d. Am. them. Sot. 98, 5227 (1976); (e) J. Robinson and R. A. Osteryoung, J. Am. chrm. Sot. 101, 323 (1979). W. P. Games and F. Cardon, 2. phys. Chem. NF 86,330 (1973). A. J. Bard, A. B. Bocarsly, F, R. Fan, E. G. Walton and M. S. Wrighton, .!. Am. them. Sot. 102, 3671 (1980). C. D. Jaeger, H. Gerischer and W. Kautek, Ber. Bunsenges. Phys. Ckem. 86, 20 (1982), see also referenms therein. (al E. C. Dutoit. R. L. Van Meirhaeehe. F. Cardon and W. P. Games, Ber’Bunsenges. Pkys. chehem.79. 1206 (1975); (b) W. H. Lafltre. R. L. Van Meirhaeghe, F. Cardon and in;. P. Games, Surj: Sci. 59,401 (1976);-(c) M. J. Madou,F. Cardon and W. P. Gomes, J. eleclrurkrm. Sot. 124, 1625 (1977). P. Janietz, R. Weiche, I. Westfahl. R. Landsberg and R. Dehmlow, J. e/ectroanaL Ckem. 106, 23 (1980). S. M. Sze, Physics ofSemiconductor Devices, Ch. 1, p. 57. John Wiley and Sons, New York (1969). K. Rajeshwar. L. Thompson, P. Singh, R. C. Kainthla and K. L. Chopra, .I. electrockem. Sot. 128. 1744 (1981). H. Gerischer in Physical Ckemistrq--An Advanced Treatise, Vol. 9A (Edited by H. Eying), Ch. 5. Academic Press, New York (1970). K. Rajeshwar, J. elertrochrm. Sot. 129, 1003 (1982). them.
11.
12.
II-