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First observation of photoconductivity in the semiconducting phase of VO2
Solid State Communications,Vol. 15, pp. 1645—1649, 1974.
Pergamon Press.
Printed in Great Britain
FIRST OBSERVATION OF PHOTOCONDUCTIVITY IN THE SEMICONDUCTING PHASE OF VO2 G. von Schuithess and P. Wachter Laboratorium für Festkörperphysik, ETH Zurich, 8049 Zurich, Hanggerberg, Switzerland (Received 11 July 1974 by J.L. Olsen)
We report in this paper the first successful measurements of photoconductivity in the semiconducting phase of VO2 single crystals. At low temperatures the frequency dependence of the photosensitivity Qs) exhibits an edge which gradually flattens at higher temperatures. The resulting mobility gap at 4.2 K amounts to 0.95 eV 1and These thusmeasurements, differs from the together optically with deterother mined band known data suggest gap (0.65—0.75 a “quasi-amorphous” eV). electronic behavior of crystalline and semiconducting VO 2.
1. INTRODUCTION WITH to DECREASING VO2 transition exhibits a at metal semiconductortemperature first order phase 7; = 340 K. Several models for the driving mechanism of this transition have been proposed.25 However, it is not the purpose of this paper to discuss these aspects since our investigations are only concerned with the semiconducting phase. A number of proposals exist for an energy level scheme of the semiconducting state.3’59 All authors agree that the highest occupied state (ground state) is a 3d,~2_~2 bonding level, but different opinions are offered for the degree of localization of this 3d level, Berglund et. al.6 and Guntersdorfer5 assume a 3d band, 6 On the other hand Rice et. aL4 and about 1 eV wide. Goodenough3 propose a more or less localized 3d state, subject to crystal field effects. Here the question arises whether the lowest excited state is also localized or extended. The latter case includes the possibility of a hybridization with a ir” antibonding orbital as postulated by Goodenough.3 An answer to this problem can be given by the simultaneous observation of photoconductivity and optical absorption. First absorption data on V0 films byet.Verleur 10 were later explained by 2Berglund al.6 with et. al. a large density of localized states in the gap of the
semiconductor. More recent absorption measurements 1 showed that in on single crystals by the Laddnumber and Paul high quality samples of states in the gap is drastically reduced, as judged by the excellent transparency of the material. (Absorption coefficient 100 cm’). One is tempted to conclude that a per. fect material has no defect or impurity induced localized states in the gap. In this case the low temperature energy gap, as determined by optical absorption and photoconductivity, should coincide if at least the final optical state ends in a band. However, there has been no experimental verification of the existence of photoconductivity up to now.6 We report in this paper the first successful measurements of photoconductivity in the semiconducting phase of V0 2 single crystals as a function of frequency and temperature. 2. EXPERIMENTAL RESULTS Throughout our measurements we used single crystals in the shape of rods with typical dimensions of about 0.4 X 0.4 X 4 mm3. These crystals were grown by Guntersdorfer with a method described elsewhere.5 Our measurements of photosensitivity (ps) i.e. photocurrent per incident light intensity, were made (13 (Preliminary Hz), monochromatic light and a using lock-inchopped technique. measurements have been published in Helv. Phys. Acta).” In addition
1645
1646
SEMICONDUCTING PHASE OF VO2
we have measured the temperature dependence of the electrical conductivity in order to estimate the crystal2 quality by comparison with data fr9m With a conductivity ratio of more than the 1 04literature.’ at 7; and a thermal activation enrgy of 0.6 eV near 7; our crystals
Vol. 15, No. 10
au
300
58102
150K 125K
I 50K;
range among the best ones (Fig. 1).
a 0
______
_____
____
____
.0
10~
i04 100
—
~
101
101
1
5K
3
2
V
inn
10~ 100/T
10/T—~
i~6
\
0
FIG.
-
1.0
aoPhoton
Energy30 eV _—
2. Photosensitivity of V0 2 vs photon energy.
i~
—
N 114
108 0.26
0.28
I / T1’—O030 K
032
ton energy of 1.4 eV as function of bOlT. The insert shows the photosensitivity for a fixed phothe optical absorption edge, following quantively even a temperature or magnetic dependence of the 14 In contrast to this,field in VO latter. 2 an optical absorption gap of only EG = 0.75 eV at 93 K has been measured by we Ladd and Paul.’from (As reference a minor correction to this value extrapolate 1 an energy
FIG. 1. Electrical conductivity of V0 4 The insert shows the electrical conductivity 2 vs versus T” lO~/7’.
gap of 0.78 eV at 5 K). Thus it can be seen from Fig. 2 that up to 125 K the edge of the ps always is at higher energies than the absorption edge.
Fig. 2 shows the ps as a function of incident photon energy for several temperatures. (The vertical displacement of the curves is in the correct sequence with temperature, but in an arbitrary scale). The cirves exhibit two striking features. First, the drop of the ps between 1.5 and 0.5 eV becomes larger with decreasing temperature. (Below about 75 K this trend is ob-
In the insert of Fig. 2 we have plotted the ps obtamed with 1.4 eV photon energy as a function of reciprocal temperature. From the straight section of the curve an activation energy of 0.1 eV can be determined.
scured by the limits of the experimental detectability of the ps signal). Second, the edge of the ps at 5 K is found at 0.95 ±0.05 eV. To obtain this value we have made use of a derivation of Moss,13 showing that the edge of the ps corresponds to the energy for which the ps has dropped to half its maximum value. For quite a variety of perfect and crystalline materials, including Si and Ge’3 and also for the Eu chalcogenides14 the so determined edge of the ps is in exact agreement with
3. DISCUSSION OF RESULTS The first point to be discussed is the relative position in energy of the photosensitivity edge and the optical absorption edge. Concentrating on the results at 5 K the optical gap is found at 0.78 eV and the ps edge at 0.95 eV. At these low temperatures the ps is only detectable when we have a band mobility. The edge of the ps, therefore, is a measure of the mobility gap.
Vol. 15, No. 10
SEMICONDUCTING PHASE OF V02 >~
~a.
a— ~
E
E
1647
300K
5K --
--
-
a ci
~‘
d
>>
~‘D(E)
~
d °
~D(E)
FIG. 3. Proposed energy levels of VO2 with a mobility edge and localized states, tailing intQ the band gap. The fact that one observes optical absorption at appreciably the lower energies than mobility gap, exnecessitates assumption of a the density of states tending within the mobility gap. Since optical absorption and photoconductivity measurements have been made on different crystals, the question may arise, whether the crystals were of comparable perfection. However, the mobility gap at T 0 is rather independent of impurity ceiiters, as long as they do not form an impurity band. On the other hand, the absorption coefficient at 0.95 eV (the edge of the ps) is about io~cm’,so the absorption -~
edge definitely is at lower energies. The conclusion reached in the last paragraph thus remains valid, A density of states within the mobility gap may arise e.g. fromHowever, a localized level would as a first excited empty state. this3d~2_~2 assumption most probably result in a weak (lOs —l0~cm’) optical absorption peak corresponding to the transition energy between ground and state. of This is not observed experimentally.’ Thusexcited the density localized states is rather a continuous function of the energy similar as in amorphous materials. Here we are not implying that V0 2 has no we long range acrystal periodicity, but in the following collect few physical facts which stress the electronically amorphous behavior of VO 2. First, the absorption coefficient versus photon 1 for V0 energy 2 shows an exponential type behavior, Such a relation is amongst other cases typical for amorphous semiconductors with a density of localized 15 states tailing into the mobility gap (Si, Ge, GeTe, CdSb 0,3 As2 16) and it has been derived theoretically
for the case of semiconductors by Bonch— Second, 7 amorphous the logarithm of the electrical Bruevich~~ in a lIT diagram is not a linear function conductivity for VO 1’5 have 2 (see insert Fig. 1). Various authors derived thermal activation energies from such a plot, assuming that the mobility is only a weakly varying function of temperature. On the other hand, such a concave ln a vs l/T behavior has been observed also in e.g. amorphous InSb18 and GeTe.19 It has been] proposed by Mott20 that in amorphous semiconductors the conduction at low temperatures may be due to a variable-range hopping and then ln a is proportional to— liT114. In fact, also in V0 2 such a plot of our conductivity measurements yield straight lines as shown in Fig. 1. Third, measurements of the a.c. electrical conductivity in VO2 indicate a high frequency hopping, which can be interpreted in terms of a “very 21 Fourth, the dense distribution ofof localized levels”. spectral dependence the ps of V0 2 and especially the rise of the low energy tail with increasing temperature (Fig. 2) is very to ps measurements on 15 similar For materials with a distribution amorphous of localized GeTe. levels in the gap and where the photocurrent is governed by trapping processes, Howard and Tsu15 have derived the following formula for the 1ph exp {— ~ [(1 /kT) photocurrent temperatures: — i3} /2) whereat~ low is the depth to which the localized levels are thermalized and ~3is a measure theshould distri1ph vsoflIT bution of states the gap. Thusis ln be a straight lineinwhich indeed observed in VO 2 in the region between 100 and 60 K. From the insert of Fig. 2 one derives ~ to be 0.1 eV. One is therefore driven to the conclusion that also -~
VO2 exhibits some “quasi amorphous” behavior. This
1648
SEMICONDUCTING PHASE OF V02
Vol. 15, No. 10
in spite of the fact that the material has excellent X-ray diagrams. Of course, if the density of20/cm3 states inthe the mobility gap does not exceed about l0 X-ray pattern would be expected to be quite sharp.
secutive thermal excitation of this electron, resulting in is probably thewith mainincresprocessphotoconductivity. for~theincrease ofThis the low energy ps ing temperature.
In the following we propose a model, generally accepted for amorphous materials, which is consistent with the observed experimental results, before we discuss the problem why VU 2 behaves similar as an amorphous material. accept For theGoodenough’s relative energymodel3 level with scheme we basically a highest occupied ground state of 3d~z~2symmetry. The conduction band is formed by the (d~~ : Px)~ anti-bonding band, being jiybridized with the anti-bonding portion of the dx2_~2 band. This conduction band
Now the question arises whether the density of states within the energy gap is due to imperfections or whether it is intrinsic in nature. Of course, off stoichi-
is assumed much wider than the bonding part of the valence which eventails mayofbelocalized a localized state. To this basicstate, model we add states extending throughout the band gap (Fig. 3). Near the center of the gap the density of states must be quite low in order to explain the excellent transparency of the material.1 For T-÷0 the states are occupied up to about 0.1 eV below the mobility edge of the conduction band. This explains the difference between the onset of pptical absorption (into the localized states above EF) and the ps edge, which amounts to about 0.15 eV. At higher temperatures the uppermost localized valence states will become more depleted and instead the higher lying empty localized states and the conduction band will become populated. On the other hand we can now have a photo absorption process from the valence band (due to the high density of electrons, the initial state for photon absorption will always be the valence band) into some empty localized state, e.g. even below the middle of the gap with a con-
,
ometry such as random vacancies in the cation or V5~ anion 3~and sublattice resultlead in the formationstates of Vwithin the ions, whichwill in turn to localized gap. In the following we propose that also stoichiometric VU 3~and Vs+ ions 2 can statistically distributed V which canhave be the cause of the “quasi amorphous” behavior of V0 2. The formation of these pairs needs an energy of4~ the order ionization energy minus affinity ions, which can be positive or negative. of the22Vhas suggested that this energy in oxides may Mott be very small due to screening effects of the 2p electrons. If we take the existence of about iO’9 to 1020 cm3 (i.e. only about 1 per cent) V3~and V5~pairs as granted, the Coulomb interaction between these pairs will result in bound states. Due to the statistical distance of the V3~and V5~ions forming the pairs, we obtain a distribution in energy of the localized states. Once V3~and V5~ions have been created the accompanying lattice distortion will stabilize these states.
Acknowledgements The authors are very grateful to Dr. M. Guntersdorfer for providing the VU 2 single crystal also wish to thank Mr.of M.the Fischerspecimens. for makingThey preliminary measurements photosensitivity. The technical assistance of Mr. H.P. Staub is gratefully acknowledged.
REFERENCES 1.
LADD L.A. and PAUL W., Solid State Commun. 7, 425 (1969).
2.
ADLER D., FEINLEIB J., BROOKS H. and PAUL W.,Phys. Rev. 155, 851 (1967).
3.
GOODENOUGH J.B., J. Solid State Chem. 3, 490,(1 971).
4. 5.
RICE T.M., MCWHAN D.B. and BRINKMAN W.F., Proc. of the Xth mt. Conf Phys. Semicond. p.293, Cambridge, Massachussetts (1970). GIJNTERSDORFER M., Solid State Electronics 13, 355 (1970).
6.
BERGLUND C.N. and GUGGENHEIM H.J.,Phys. Rev. 185,3 (1969).
7. 8.
FORD C.J., SEYMOUR E.F.W. and HYLAND G.J., Phys. Kondens. Mat. 14, 111(1972). HEARN C.J. and HYLAND G,J., Phys. Lett. 43A, 87(1973).
Vol. 15, No. 10
9.
SEMICONDUCTING PHASE OF VO2
CARUTHERS E. and KLEINMAN L.,Phys. Rev. B 7,3760(1973).
10. 11.
VERLEUR H.W., BARKER A.S., Jr. and BERGLUND C.N., Phys. Rev. 172, 788 (1968). FISCHER M., VON SCHULTHESS G. and WACHTER P., Helv. Phys. Acta (in print)
12.
PAUL W., Mat. Res. Bull. 5, 691(1970).
13.
MOSS T.S., Photoconductivily in the Elements, Butterworth, London (1952).
14.
WACHTER P., Z. Angew. Phys. 32, 171 (1971); CRCCrit. Rev. Solid State Sci. 3, 189 (1972).
15.
HOWARDW.E. and TSU R.,Phys. Rev. B, 1,4709 (1970).
16.
ABRAHAM A. et. a!., Proc. Xth mt. Conf Phys. Semicond., Cambridge, Massachusetts (1970).
17.
BONCH-BRUEVICH Y.L., Phys. Status Solidi 42,35 (1970).
18.
STUKE J,,.&oc. Xth mnr. Conf Phys. Semicbnd., Cambridge, Massachussetts (1970).
19.
HOWARD W.E. and TSU R., Proc. Xth mt. ConfPhys. Semicond., Cambridge, Massachussetts (1970).
20.
MUTT N.F.,Phys. Rev. Lett. 31,7(1973).
21.
PALMIER J.F. and BALLINI Y., Solid State Commun. 14, 575 (1974).
22.
MOTTN.F.,Adv. Phys. 16, 49 (1967).
Wir berichten in dieser Publikation über die ersten erfolgreich durchgefuhrten Messungen der Photoleitfahigkeit von halbleitenden V02 -Einkristallen. Bei tiefen Temperaturen zeigt die Frequenzabhangigkeit der Photoempfmdlichkeit (ps) eine Kante, die mit steigender Temperatur allmählich verschwindet. Die aus der Kante bestimmte Beweglichkeits-Energie lücke betragt bei 4,2°K 0,95 eV und unterscheidet 1 Unsere sichMessungen damit von zusammen der optischmit zu 0,65—0,75 den anderneV bekanbestimmten nten Daten Energie deuten lUcke. auf em “quasi-amorphes” Verhalten von kristallinem und halbleitendem V0 2 hin.
1649
Pergamon Press.
Printed in Great Britain
FIRST OBSERVATION OF PHOTOCONDUCTIVITY IN THE SEMICONDUCTING PHASE OF VO2 G. von Schuithess and P. Wachter Laboratorium für Festkörperphysik, ETH Zurich, 8049 Zurich, Hanggerberg, Switzerland (Received 11 July 1974 by J.L. Olsen)
We report in this paper the first successful measurements of photoconductivity in the semiconducting phase of VO2 single crystals. At low temperatures the frequency dependence of the photosensitivity Qs) exhibits an edge which gradually flattens at higher temperatures. The resulting mobility gap at 4.2 K amounts to 0.95 eV 1and These thusmeasurements, differs from the together optically with deterother mined band known data suggest gap (0.65—0.75 a “quasi-amorphous” eV). electronic behavior of crystalline and semiconducting VO 2.
1. INTRODUCTION WITH to DECREASING VO2 transition exhibits a at metal semiconductortemperature first order phase 7; = 340 K. Several models for the driving mechanism of this transition have been proposed.25 However, it is not the purpose of this paper to discuss these aspects since our investigations are only concerned with the semiconducting phase. A number of proposals exist for an energy level scheme of the semiconducting state.3’59 All authors agree that the highest occupied state (ground state) is a 3d,~2_~2 bonding level, but different opinions are offered for the degree of localization of this 3d level, Berglund et. al.6 and Guntersdorfer5 assume a 3d band, 6 On the other hand Rice et. aL4 and about 1 eV wide. Goodenough3 propose a more or less localized 3d state, subject to crystal field effects. Here the question arises whether the lowest excited state is also localized or extended. The latter case includes the possibility of a hybridization with a ir” antibonding orbital as postulated by Goodenough.3 An answer to this problem can be given by the simultaneous observation of photoconductivity and optical absorption. First absorption data on V0 films byet.Verleur 10 were later explained by 2Berglund al.6 with et. al. a large density of localized states in the gap of the
semiconductor. More recent absorption measurements 1 showed that in on single crystals by the Laddnumber and Paul high quality samples of states in the gap is drastically reduced, as judged by the excellent transparency of the material. (Absorption coefficient 100 cm’). One is tempted to conclude that a per. fect material has no defect or impurity induced localized states in the gap. In this case the low temperature energy gap, as determined by optical absorption and photoconductivity, should coincide if at least the final optical state ends in a band. However, there has been no experimental verification of the existence of photoconductivity up to now.6 We report in this paper the first successful measurements of photoconductivity in the semiconducting phase of V0 2 single crystals as a function of frequency and temperature. 2. EXPERIMENTAL RESULTS Throughout our measurements we used single crystals in the shape of rods with typical dimensions of about 0.4 X 0.4 X 4 mm3. These crystals were grown by Guntersdorfer with a method described elsewhere.5 Our measurements of photosensitivity (ps) i.e. photocurrent per incident light intensity, were made (13 (Preliminary Hz), monochromatic light and a using lock-inchopped technique. measurements have been published in Helv. Phys. Acta).” In addition
1645
1646
SEMICONDUCTING PHASE OF VO2
we have measured the temperature dependence of the electrical conductivity in order to estimate the crystal2 quality by comparison with data fr9m With a conductivity ratio of more than the 1 04literature.’ at 7; and a thermal activation enrgy of 0.6 eV near 7; our crystals
Vol. 15, No. 10
au
300
58102
150K 125K
I 50K;
range among the best ones (Fig. 1).
a 0
______
_____
____
____
.0
10~
i04 100
—
~
101
101
1
5K
3
2
V
inn
10~ 100/T
10/T—~
i~6
\
0
FIG.
-
1.0
aoPhoton
Energy30 eV _—
2. Photosensitivity of V0 2 vs photon energy.
i~
—
N 114
108 0.26
0.28
I / T1’—O030 K
032
ton energy of 1.4 eV as function of bOlT. The insert shows the photosensitivity for a fixed phothe optical absorption edge, following quantively even a temperature or magnetic dependence of the 14 In contrast to this,field in VO latter. 2 an optical absorption gap of only EG = 0.75 eV at 93 K has been measured by we Ladd and Paul.’from (As reference a minor correction to this value extrapolate 1 an energy
FIG. 1. Electrical conductivity of V0 4 The insert shows the electrical conductivity 2 vs versus T” lO~/7’.
gap of 0.78 eV at 5 K). Thus it can be seen from Fig. 2 that up to 125 K the edge of the ps always is at higher energies than the absorption edge.
Fig. 2 shows the ps as a function of incident photon energy for several temperatures. (The vertical displacement of the curves is in the correct sequence with temperature, but in an arbitrary scale). The cirves exhibit two striking features. First, the drop of the ps between 1.5 and 0.5 eV becomes larger with decreasing temperature. (Below about 75 K this trend is ob-
In the insert of Fig. 2 we have plotted the ps obtamed with 1.4 eV photon energy as a function of reciprocal temperature. From the straight section of the curve an activation energy of 0.1 eV can be determined.
scured by the limits of the experimental detectability of the ps signal). Second, the edge of the ps at 5 K is found at 0.95 ±0.05 eV. To obtain this value we have made use of a derivation of Moss,13 showing that the edge of the ps corresponds to the energy for which the ps has dropped to half its maximum value. For quite a variety of perfect and crystalline materials, including Si and Ge’3 and also for the Eu chalcogenides14 the so determined edge of the ps is in exact agreement with
3. DISCUSSION OF RESULTS The first point to be discussed is the relative position in energy of the photosensitivity edge and the optical absorption edge. Concentrating on the results at 5 K the optical gap is found at 0.78 eV and the ps edge at 0.95 eV. At these low temperatures the ps is only detectable when we have a band mobility. The edge of the ps, therefore, is a measure of the mobility gap.
Vol. 15, No. 10
SEMICONDUCTING PHASE OF V02 >~
~a.
a— ~
E
E
1647
300K
5K --
--
-
a ci
~‘
d
>>
~‘D(E)
~
d °
~D(E)
FIG. 3. Proposed energy levels of VO2 with a mobility edge and localized states, tailing intQ the band gap. The fact that one observes optical absorption at appreciably the lower energies than mobility gap, exnecessitates assumption of a the density of states tending within the mobility gap. Since optical absorption and photoconductivity measurements have been made on different crystals, the question may arise, whether the crystals were of comparable perfection. However, the mobility gap at T 0 is rather independent of impurity ceiiters, as long as they do not form an impurity band. On the other hand, the absorption coefficient at 0.95 eV (the edge of the ps) is about io~cm’,so the absorption -~
edge definitely is at lower energies. The conclusion reached in the last paragraph thus remains valid, A density of states within the mobility gap may arise e.g. fromHowever, a localized level would as a first excited empty state. this3d~2_~2 assumption most probably result in a weak (lOs —l0~cm’) optical absorption peak corresponding to the transition energy between ground and state. of This is not observed experimentally.’ Thusexcited the density localized states is rather a continuous function of the energy similar as in amorphous materials. Here we are not implying that V0 2 has no we long range acrystal periodicity, but in the following collect few physical facts which stress the electronically amorphous behavior of VO 2. First, the absorption coefficient versus photon 1 for V0 energy 2 shows an exponential type behavior, Such a relation is amongst other cases typical for amorphous semiconductors with a density of localized 15 states tailing into the mobility gap (Si, Ge, GeTe, CdSb 0,3 As2 16) and it has been derived theoretically
for the case of semiconductors by Bonch— Second, 7 amorphous the logarithm of the electrical Bruevich~~ in a lIT diagram is not a linear function conductivity for VO 1’5 have 2 (see insert Fig. 1). Various authors derived thermal activation energies from such a plot, assuming that the mobility is only a weakly varying function of temperature. On the other hand, such a concave ln a vs l/T behavior has been observed also in e.g. amorphous InSb18 and GeTe.19 It has been] proposed by Mott20 that in amorphous semiconductors the conduction at low temperatures may be due to a variable-range hopping and then ln a is proportional to— liT114. In fact, also in V0 2 such a plot of our conductivity measurements yield straight lines as shown in Fig. 1. Third, measurements of the a.c. electrical conductivity in VO2 indicate a high frequency hopping, which can be interpreted in terms of a “very 21 Fourth, the dense distribution ofof localized levels”. spectral dependence the ps of V0 2 and especially the rise of the low energy tail with increasing temperature (Fig. 2) is very to ps measurements on 15 similar For materials with a distribution amorphous of localized GeTe. levels in the gap and where the photocurrent is governed by trapping processes, Howard and Tsu15 have derived the following formula for the 1ph exp {— ~ [(1 /kT) photocurrent temperatures: — i3} /2) whereat~ low is the depth to which the localized levels are thermalized and ~3is a measure theshould distri1ph vsoflIT bution of states the gap. Thusis ln be a straight lineinwhich indeed observed in VO 2 in the region between 100 and 60 K. From the insert of Fig. 2 one derives ~ to be 0.1 eV. One is therefore driven to the conclusion that also -~
VO2 exhibits some “quasi amorphous” behavior. This
1648
SEMICONDUCTING PHASE OF V02
Vol. 15, No. 10
in spite of the fact that the material has excellent X-ray diagrams. Of course, if the density of20/cm3 states inthe the mobility gap does not exceed about l0 X-ray pattern would be expected to be quite sharp.
secutive thermal excitation of this electron, resulting in is probably thewith mainincresprocessphotoconductivity. for~theincrease ofThis the low energy ps ing temperature.
In the following we propose a model, generally accepted for amorphous materials, which is consistent with the observed experimental results, before we discuss the problem why VU 2 behaves similar as an amorphous material. accept For theGoodenough’s relative energymodel3 level with scheme we basically a highest occupied ground state of 3d~z~2symmetry. The conduction band is formed by the (d~~ : Px)~ anti-bonding band, being jiybridized with the anti-bonding portion of the dx2_~2 band. This conduction band
Now the question arises whether the density of states within the energy gap is due to imperfections or whether it is intrinsic in nature. Of course, off stoichi-
is assumed much wider than the bonding part of the valence which eventails mayofbelocalized a localized state. To this basicstate, model we add states extending throughout the band gap (Fig. 3). Near the center of the gap the density of states must be quite low in order to explain the excellent transparency of the material.1 For T-÷0 the states are occupied up to about 0.1 eV below the mobility edge of the conduction band. This explains the difference between the onset of pptical absorption (into the localized states above EF) and the ps edge, which amounts to about 0.15 eV. At higher temperatures the uppermost localized valence states will become more depleted and instead the higher lying empty localized states and the conduction band will become populated. On the other hand we can now have a photo absorption process from the valence band (due to the high density of electrons, the initial state for photon absorption will always be the valence band) into some empty localized state, e.g. even below the middle of the gap with a con-
,
ometry such as random vacancies in the cation or V5~ anion 3~and sublattice resultlead in the formationstates of Vwithin the ions, whichwill in turn to localized gap. In the following we propose that also stoichiometric VU 3~and Vs+ ions 2 can statistically distributed V which canhave be the cause of the “quasi amorphous” behavior of V0 2. The formation of these pairs needs an energy of4~ the order ionization energy minus affinity ions, which can be positive or negative. of the22Vhas suggested that this energy in oxides may Mott be very small due to screening effects of the 2p electrons. If we take the existence of about iO’9 to 1020 cm3 (i.e. only about 1 per cent) V3~and V5~pairs as granted, the Coulomb interaction between these pairs will result in bound states. Due to the statistical distance of the V3~and V5~ions forming the pairs, we obtain a distribution in energy of the localized states. Once V3~and V5~ions have been created the accompanying lattice distortion will stabilize these states.
Acknowledgements The authors are very grateful to Dr. M. Guntersdorfer for providing the VU 2 single crystal also wish to thank Mr.of M.the Fischerspecimens. for makingThey preliminary measurements photosensitivity. The technical assistance of Mr. H.P. Staub is gratefully acknowledged.
REFERENCES 1.
LADD L.A. and PAUL W., Solid State Commun. 7, 425 (1969).
2.
ADLER D., FEINLEIB J., BROOKS H. and PAUL W.,Phys. Rev. 155, 851 (1967).
3.
GOODENOUGH J.B., J. Solid State Chem. 3, 490,(1 971).
4. 5.
RICE T.M., MCWHAN D.B. and BRINKMAN W.F., Proc. of the Xth mt. Conf Phys. Semicond. p.293, Cambridge, Massachussetts (1970). GIJNTERSDORFER M., Solid State Electronics 13, 355 (1970).
6.
BERGLUND C.N. and GUGGENHEIM H.J.,Phys. Rev. 185,3 (1969).
7. 8.
FORD C.J., SEYMOUR E.F.W. and HYLAND G.J., Phys. Kondens. Mat. 14, 111(1972). HEARN C.J. and HYLAND G,J., Phys. Lett. 43A, 87(1973).
Vol. 15, No. 10
9.
SEMICONDUCTING PHASE OF VO2
CARUTHERS E. and KLEINMAN L.,Phys. Rev. B 7,3760(1973).
10. 11.
VERLEUR H.W., BARKER A.S., Jr. and BERGLUND C.N., Phys. Rev. 172, 788 (1968). FISCHER M., VON SCHULTHESS G. and WACHTER P., Helv. Phys. Acta (in print)
12.
PAUL W., Mat. Res. Bull. 5, 691(1970).
13.
MOSS T.S., Photoconductivily in the Elements, Butterworth, London (1952).
14.
WACHTER P., Z. Angew. Phys. 32, 171 (1971); CRCCrit. Rev. Solid State Sci. 3, 189 (1972).
15.
HOWARDW.E. and TSU R.,Phys. Rev. B, 1,4709 (1970).
16.
ABRAHAM A. et. a!., Proc. Xth mt. Conf Phys. Semicond., Cambridge, Massachusetts (1970).
17.
BONCH-BRUEVICH Y.L., Phys. Status Solidi 42,35 (1970).
18.
STUKE J,,.&oc. Xth mnr. Conf Phys. Semicbnd., Cambridge, Massachussetts (1970).
19.
HOWARD W.E. and TSU R., Proc. Xth mt. ConfPhys. Semicond., Cambridge, Massachussetts (1970).
20.
MUTT N.F.,Phys. Rev. Lett. 31,7(1973).
21.
PALMIER J.F. and BALLINI Y., Solid State Commun. 14, 575 (1974).
22.
MOTTN.F.,Adv. Phys. 16, 49 (1967).
Wir berichten in dieser Publikation über die ersten erfolgreich durchgefuhrten Messungen der Photoleitfahigkeit von halbleitenden V02 -Einkristallen. Bei tiefen Temperaturen zeigt die Frequenzabhangigkeit der Photoempfmdlichkeit (ps) eine Kante, die mit steigender Temperatur allmählich verschwindet. Die aus der Kante bestimmte Beweglichkeits-Energie lücke betragt bei 4,2°K 0,95 eV und unterscheidet 1 Unsere sichMessungen damit von zusammen der optischmit zu 0,65—0,75 den anderneV bekanbestimmten nten Daten Energie deuten lUcke. auf em “quasi-amorphes” Verhalten von kristallinem und halbleitendem V0 2 hin.
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