- Email: [email protected]
Energy gaps in bismuth trioxide∗
J. Phys. Chem. Solos
Pergamon Press 1961. Vol. 18, Nos. 213, PP. 21.5-217.
ENERGY
GAPS IN BISMUTH D.
M.
MAnOX
and
L.
Printed in Great Britain.
TRIOXIDE*
GILDART
University of Kentucky, Lexington, Kentucky (Received
1 August
1960)
unique method of preparing polycrystalline films of BiaOa having thicknesses in the range 10 to 100~ is presented. Using these films temperature vs. resistance measurements give a thermal energy gap of 2.07 + 0.07 eV in the temperature range 400 to 550°K. Optical transmission studies at ro& temperature and liquid nitrogen temperature give optical energy gaps of 3.1 eV at 77’K and 2.85 eV at 300°K. This gives an apparent temperature dependence of -l*l~lO-~ eV/“K
Abstract-A
in this temperature range assuminga linear &ation.
difference between the melting points of bismuth metal, 271°C and the trioxide, 828”C.(Q When bismuth metal is melted in a graphite crucible, open to the air and the temperature is raised toward the melting point of the trioxide, the metal begins to oxidize. If the temperature is then raised slightly above the melting point of the trioxide, the graphite of the crucible reduces the trioxide back to bismuth and a molten pool of pure bismuth fills the crucible. If the temperature is then lowered slightly below the melting point of the trioxide a uniform film of BisOs forms rapidly and covers the surface of the melt. This film may be lifted from the surface using a nichrome wire loop. Films formed in this way are pale yellow in appearance, microcrystalline and have fairly uniform thicknesses. Since the thickness of the film depends mainly on the temperature of the melt, and only slightly on the time, films of desired thickness can be prepared by adjusting the temperature. Satisfactory films can be obtained in thicknesses ranging from 10 to 100~. It should be noted that if the melt is not heated initially above the melting point of the trioxide, the resulting films turn out to be too granular, and contain inclusions of slag. There may be scattered inclusions of bismuth metal but these can be avoided by visual inspection and uniform sections of film as large as four square centimeters in area can easily be selected from the main pieces. For temperature studies, films are carefully mounted on glass slides, and fine nichrome
INTRODUCTION
A REVIEW of the literature on semiconducting compounds of the Group Vb-Group VIb elements of the form AsBs reveals comparatively little published work on bismuth trioxide, BisOs. The reason for this is not hard to find : Molten bismuth trixode cannot be contained in crucibles of commonly available materials. That this is so has already been observed,(l) and sILLENc2’ states that molten bismuth trioxide attacks glass. The authors found that molten bismuth trioxide dissolves its way through a Vycor test tube (96 per cent silica) in about fifteen minutes’ time; and that crucibles of alumina, magnesia, zirconia, platinum and stainless steel are likewise unsatisfactory. Graphite crucibles have a reducing action on molten bismuth trioxide, and rapidly reduce it to metallic bismuth. Surface melting of sintered samples of the trioxide would therefore seem to be a better approach to the problem.(s) PREPARATION
OF BiaOa FILMS
Despite the inherent difficulties a method has been developed by the authors for preparing polycrystalline films of bismuth trioxide using a graphite crucible, and is described below. Films formed in this manner are quite fragile but can be mounted in sample holders if carefully handled. The method takes advantage of the large *This work was supported in part by the United States Air Force through the Air Research and Development Command. 215
216
D. femperature, 555
M.
MATTOX
and
L.
GILDART
OK
600
Photon energy. 455
eV
416
4
0
2.0.
I
_L
K
tOOU/T,
OK-’
FKG. 1. Resistance of a typical polycrystalline BiaOs as a function of temperature. photofl energy.
%%&?nQth,
film of
r-k-5
1
,a
FXG, 2. Optical transmission of a polycrystailine film 30~ thick.
BisOs
ev
008
03-
0.5
1 60
FIG. 3. Absorption
edge of 30~ thick Slm of SisOs at 77% and 30W’K.
I 50
I 40
Z
FIG. 4. Infrared transmission of two 30~ thick filma of Bis0s at 3UO”K.
ENERGY
GAPS
IN
wires are attached as electrodes by means of silver paste. For optical transmission studies, the films are mounted on suitable frames. Portions of various BiaOs films were powdered and an X-ray powder diffraction study was made. Without exception the patterns were those of the normal orthorhombic form of BiaOs, with no indication that other phases or large proportions of impurity were present. THERMAL
AND
OPTICAL
MEASUREMENTS
For thermal measurements a film, mounted as described, is put into an electric oven and a voltage applied to it. The temperature is slowly raised at a rate of about S”C/min. The small current through the sample is fed into the Y-axis input of an Autograf X-Y recorder. The temperature of the oven is monitored by a thermocouple and its output fed into the X-axis input. Table 1 gives the thermal energy gap for a series of BisOa films in the region of 400-550°K as determined from the plots of the X-Y recorder. Table 1. Thermal energy gaps for polycrystalline jilms of BiaOs in the temperature range 400-550°K Sample 1 2 3 4 5 6
E Blh 1.99 2.05 2.20 2.07 1.99 2.12 Average El,, = 2.07 +0*07 eV
Figure 1 shows the resistance of a typical sample as a function of temperature. From this graph and crude measurements on the width and thickness of the sample, we have made an order of magnitude measurement for the resistivity of 104 ohm-cm at 500°K. This agrees with the results given by MANsFIELDc5)on sintered samples of BisOs powder at this temperature. Figure’2 shows the optical transmission at 300°K
BISMUTH
TRIOXIDE
217
of a typical film of BisOa through the visible range as measured on a Cary model 14R spectrophotometer. Figure 3 shows the absorption edge for this film at two temperatures, 300°K and 77°K. The optical energy gap for these films is found to be 3.1 eV at 77°K and 2.85 eV at 300°K giving an apparent temperature dependence of - 1 *l x 10-a eV/“K, on the assumption that a linear variation holds over this temperature range. This is to be compared with the activation energy of 3.2 eV, as obtained by DOYLE(~), on films 300 A thick, who prepared his films by oxidizing metallic bismuth layers deposited on glass substrates. The authors prepared films by the method of DOYLE, but found that as the bismuth oxidized, the film broke up into a granular structure resembling a smoke deposit. Figure 4 shows the infrared transmission for two samples as determined by Perkin-Elmer model 21 infrared spectrophotometer. Curve 1 is for the sample used for the above optical transmission studies and curve 2 is for a sample which showed maximum transmission. Other samples lay between these two extremes. All the samples show a maximum infrared transmission in the 10-12~ region. Acknowledgements-The authors are indebted to Dr. C. NELSON and the Solid State Division of the Oak Ridge National Laboratory for the use of their Cary spectrophotometer.
REFERENCES 1. MELLOR J. W., A Comprehensive Treatise on Inorganic and Theoretical Chemistry Vol. IX, Chapter 53, p. 646. Longmans, Green and Co., New York (1922). 2. SILLEN L. G. Ark., Kemi 12A, 18.1 (1937). 3. RAYMONDR. J. and STERLING H.- F.; described by WILSON I. M.. Birminaham Svmbosium on the Metallur&zl Aspects if Semz~o&ctors, Feb. 25 (1958). 4. VANARKEL A. E., FLOOD E. A. and BRIGHTN. F. H.. Canad. J. Phys. 31, 1009 (1953). 5. MANSFIELDP.,P~OC.Phys. Soc.Lmd. 62B, 476 (1949). 6. DOYLE W. P., J. Phys. Chem. Solids 4, 144 (1958).
Pergamon Press 1961. Vol. 18, Nos. 213, PP. 21.5-217.
ENERGY
GAPS IN BISMUTH D.
M.
MAnOX
and
L.
Printed in Great Britain.
TRIOXIDE*
GILDART
University of Kentucky, Lexington, Kentucky (Received
1 August
1960)
unique method of preparing polycrystalline films of BiaOa having thicknesses in the range 10 to 100~ is presented. Using these films temperature vs. resistance measurements give a thermal energy gap of 2.07 + 0.07 eV in the temperature range 400 to 550°K. Optical transmission studies at ro& temperature and liquid nitrogen temperature give optical energy gaps of 3.1 eV at 77’K and 2.85 eV at 300°K. This gives an apparent temperature dependence of -l*l~lO-~ eV/“K
Abstract-A
in this temperature range assuminga linear &ation.
difference between the melting points of bismuth metal, 271°C and the trioxide, 828”C.(Q When bismuth metal is melted in a graphite crucible, open to the air and the temperature is raised toward the melting point of the trioxide, the metal begins to oxidize. If the temperature is then raised slightly above the melting point of the trioxide, the graphite of the crucible reduces the trioxide back to bismuth and a molten pool of pure bismuth fills the crucible. If the temperature is then lowered slightly below the melting point of the trioxide a uniform film of BisOs forms rapidly and covers the surface of the melt. This film may be lifted from the surface using a nichrome wire loop. Films formed in this way are pale yellow in appearance, microcrystalline and have fairly uniform thicknesses. Since the thickness of the film depends mainly on the temperature of the melt, and only slightly on the time, films of desired thickness can be prepared by adjusting the temperature. Satisfactory films can be obtained in thicknesses ranging from 10 to 100~. It should be noted that if the melt is not heated initially above the melting point of the trioxide, the resulting films turn out to be too granular, and contain inclusions of slag. There may be scattered inclusions of bismuth metal but these can be avoided by visual inspection and uniform sections of film as large as four square centimeters in area can easily be selected from the main pieces. For temperature studies, films are carefully mounted on glass slides, and fine nichrome
INTRODUCTION
A REVIEW of the literature on semiconducting compounds of the Group Vb-Group VIb elements of the form AsBs reveals comparatively little published work on bismuth trioxide, BisOs. The reason for this is not hard to find : Molten bismuth trixode cannot be contained in crucibles of commonly available materials. That this is so has already been observed,(l) and sILLENc2’ states that molten bismuth trioxide attacks glass. The authors found that molten bismuth trioxide dissolves its way through a Vycor test tube (96 per cent silica) in about fifteen minutes’ time; and that crucibles of alumina, magnesia, zirconia, platinum and stainless steel are likewise unsatisfactory. Graphite crucibles have a reducing action on molten bismuth trioxide, and rapidly reduce it to metallic bismuth. Surface melting of sintered samples of the trioxide would therefore seem to be a better approach to the problem.(s) PREPARATION
OF BiaOa FILMS
Despite the inherent difficulties a method has been developed by the authors for preparing polycrystalline films of bismuth trioxide using a graphite crucible, and is described below. Films formed in this manner are quite fragile but can be mounted in sample holders if carefully handled. The method takes advantage of the large *This work was supported in part by the United States Air Force through the Air Research and Development Command. 215
216
D. femperature, 555
M.
MATTOX
and
L.
GILDART
OK
600
Photon energy. 455
eV
416
4
0
2.0.
I
_L
K
tOOU/T,
OK-’
FKG. 1. Resistance of a typical polycrystalline BiaOs as a function of temperature. photofl energy.
%%&?nQth,
film of
r-k-5
1
,a
FXG, 2. Optical transmission of a polycrystailine film 30~ thick.
BisOs
ev
008
03-
0.5
1 60
FIG. 3. Absorption
edge of 30~ thick Slm of SisOs at 77% and 30W’K.
I 50
I 40
Z
FIG. 4. Infrared transmission of two 30~ thick filma of Bis0s at 3UO”K.
ENERGY
GAPS
IN
wires are attached as electrodes by means of silver paste. For optical transmission studies, the films are mounted on suitable frames. Portions of various BiaOs films were powdered and an X-ray powder diffraction study was made. Without exception the patterns were those of the normal orthorhombic form of BiaOs, with no indication that other phases or large proportions of impurity were present. THERMAL
AND
OPTICAL
MEASUREMENTS
For thermal measurements a film, mounted as described, is put into an electric oven and a voltage applied to it. The temperature is slowly raised at a rate of about S”C/min. The small current through the sample is fed into the Y-axis input of an Autograf X-Y recorder. The temperature of the oven is monitored by a thermocouple and its output fed into the X-axis input. Table 1 gives the thermal energy gap for a series of BisOa films in the region of 400-550°K as determined from the plots of the X-Y recorder. Table 1. Thermal energy gaps for polycrystalline jilms of BiaOs in the temperature range 400-550°K Sample 1 2 3 4 5 6
E Blh 1.99 2.05 2.20 2.07 1.99 2.12 Average El,, = 2.07 +0*07 eV
Figure 1 shows the resistance of a typical sample as a function of temperature. From this graph and crude measurements on the width and thickness of the sample, we have made an order of magnitude measurement for the resistivity of 104 ohm-cm at 500°K. This agrees with the results given by MANsFIELDc5)on sintered samples of BisOs powder at this temperature. Figure’2 shows the optical transmission at 300°K
BISMUTH
TRIOXIDE
217
of a typical film of BisOa through the visible range as measured on a Cary model 14R spectrophotometer. Figure 3 shows the absorption edge for this film at two temperatures, 300°K and 77°K. The optical energy gap for these films is found to be 3.1 eV at 77°K and 2.85 eV at 300°K giving an apparent temperature dependence of - 1 *l x 10-a eV/“K, on the assumption that a linear variation holds over this temperature range. This is to be compared with the activation energy of 3.2 eV, as obtained by DOYLE(~), on films 300 A thick, who prepared his films by oxidizing metallic bismuth layers deposited on glass substrates. The authors prepared films by the method of DOYLE, but found that as the bismuth oxidized, the film broke up into a granular structure resembling a smoke deposit. Figure 4 shows the infrared transmission for two samples as determined by Perkin-Elmer model 21 infrared spectrophotometer. Curve 1 is for the sample used for the above optical transmission studies and curve 2 is for a sample which showed maximum transmission. Other samples lay between these two extremes. All the samples show a maximum infrared transmission in the 10-12~ region. Acknowledgements-The authors are indebted to Dr. C. NELSON and the Solid State Division of the Oak Ridge National Laboratory for the use of their Cary spectrophotometer.
REFERENCES 1. MELLOR J. W., A Comprehensive Treatise on Inorganic and Theoretical Chemistry Vol. IX, Chapter 53, p. 646. Longmans, Green and Co., New York (1922). 2. SILLEN L. G. Ark., Kemi 12A, 18.1 (1937). 3. RAYMONDR. J. and STERLING H.- F.; described by WILSON I. M.. Birminaham Svmbosium on the Metallur&zl Aspects if Semz~o&ctors, Feb. 25 (1958). 4. VANARKEL A. E., FLOOD E. A. and BRIGHTN. F. H.. Canad. J. Phys. 31, 1009 (1953). 5. MANSFIELDP.,P~OC.Phys. Soc.Lmd. 62B, 476 (1949). 6. DOYLE W. P., J. Phys. Chem. Solids 4, 144 (1958).