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Comment on proposed barriers in thin film TiO2 diodes
Solid-State
Electronics
Pergamon Press 1963. Vol. 6, pp. 531-546.
Printed in Great Britain
NOTES Comment
on proposed barriers diodes
in thin film TiOa
there have RECENTLY, in the literatureo-4) appeared three separate proposals concerning the nature of the rectifying barriers in TiOa diodes. These have been a p-n junction,(lJ) a Mott-type barrier,(s) and a compound barrier.@) The purpose of this note is to show that all could conceivably be compound barriers or a limiting case of a compound barrier. This type of a barrier is proposed in spite of the fact that all diodes concerned were either prepared differently or utilized a different structure. Huber and Rottersman utilized anodized evaporated films. From a linear plot of l/C’s vs. V, they concluded that a graded p-n junction exists. That p-n junction theory supports this linear relationship is well known. However, it is interesting to review this work in terms of l/C2 vs. P’. This is done in Fig. 1. Here again a linear relationship exists except for a deviation near the origin. This same deviation is noted in the original work.(l) 0.8
x /Of6
1
0.7 0.6 ./’ ./
0.5 0.4 i_ .
l /
.Y
%_iI , (
0
FIG. 1.
.,
,
,
1234567 V-VOLTS Characteristic of (l/@) vs. S’.
,
(
An analysis of the linear plot (Fig. 1) may be made in terms of a compound barrier. This type of a barrier has been theoretically discussed by BILLIG and LANDSBERG and recently utilized by this writer.(d) It predicts an intercept (l/C2 = 0) value equal to vn+(%-fre/~))d$ where vo is the diffusion potential, n the carrier concentration, K the dielectric constant, and da the insulating film of the linear plot thickness. By extrapolation (Fig. 1) this value is found to be -8.0 V. The value of at, calculated from the slope, is 7.9 x lOIs/ ems. Using a value of 40 for K,(s) and assuming vo = 0.5, da is calculated to be 206 A. This is practically identical to that stated by Huber as the thickness of his anodized film. Using the intercept value (V = 0) of the linear graph one finds that an extremely narrow region (N 10 A) exists as a space-charge region. Upon application of the reverse bias voltage this space charge region extends in width and accounts for incremental change in capacitance. One serious discrepancy does, however, exist. This is realized if one compares the intercept value (V = 0) to the measured value of capacitance. The latter value (N 1.8 pF/cma) is approximately 10 per cent higher than the intercept value and consequently prevents any subsequent analysis of a space-charge region. As noted previously, the deviation near the origin occurs in both cases. This may be due to a carrier concentration, adjacent to the insulating layer, that is smaller than the given calculated value. In spite of this, the ability to predict an insulation thickness comparable to experimental value lends credence to the compound barrier theory. DAVIS and GRANNEMANN@)studied the v-1 characteristics of point contacts to hydrogen reduced, single-crystal rutile. Their results were similar to other investigator#*s) as well as those investigated by this writer. One of the previous investigators@) anodized their films after the reduction process. Davis and Grannemann concluded that the rectification mechanism was most likely attributed to a Mott-type barrier (ho = lo-scm). 531
532
NOTES
secondary electrons from specimens with and \vithout an applied d.c. bias. Use \vas made of :I scanning electron microscope being developed by the Cambridge Instrument Co. Briefly, a beam of 15 kV electrons (in this case the beam \\idth was 500 L% with a total beam current lW1’1~~10 1’ :\) was scanned in a square raster across the specimen. The secondary electrons with energies up to a few tens of electron v-olts were collected with a biased collector. The resultant current, after amplification, was used to modulate a cathode-ray tube being scanned in synchronism \vith the primary beam. In the absence of an applied bias on the specimen, the secondary emission gives information IBM General Products Division, P. J. MAGILL about the topography of the spccimene?) (See Development Laboratory, Figs 1 and 2.) It was thought that by applying a Endicott, N. Y. bias the secondary emission could be affected by local variations in electric field. Two possible References mechanisms were envisaged : (I) local variations 1. F. HUBEH and 1111.ROTTERSMAN, J. Applied Physics of the effective work function could be caused by 33, 3385 (1962). small (a few percent) local changes in the free2. 1’. HIUBER,Solid-State Electron. 5, 410 (1962). carrier velocities in the crystal, and (2) localized 3. G. DAVIS, JR. and IV. W. GRANXEMAS, J. Applied irregularities in crystal potential could perturb Physics 34, 228 (1963). 4. P. J. MAGILL, Proc. I.E.E.E. 51, 223 (1963). the trajectories followed by the emitted second5. E. BILLIC and P. T. LAIWSRERG, Pm. Phys. SW., aries. To test this idea we examined the secondary I,ond. 63A, 101 (1950). emission from specimens (containing resistivity 6. F. HUBEH and J. BLoxAnf, Ttms. I.R.E. CP-8, 80 variations associated with crystal defects) under (1961). 7. R. G. BRECKENRIDCEand W. R. HOSLER, J. Res. both biased and unbiased conditions. Some results N.B.S. 49, 65 (1954). are shown in Figs 1 and 2. 8. T. 1. KOMOLOVA atld D. N. NASLEDOV, Soa. P/z~T. In each figure there is a lo\\-magl~i~ication Solid State 3, 2469 (1962). optical picture (a) of the structural defects in the region studied, a v-oltage map (b) shovving the lines of cquipotential, and photographs (c), (d) and (e) taken on the scanning microscope. In these latter photographs it is the changes in intensity at different parts of the field of view that are sigDirect observation of the high-field regions in GaAs nificant. The absolute brightnesses are not significant, because the darkness level was altered (Received 3 May 1963) from photograph to photograph in order to stay IN A recent paper(r) it was shown that the diswithin the working range of the recording film. Account must also be taken of a slight forelocation distribution can affect the electrical shortening in the scanning-microscopy pictures as properties of high-resistivity GaAs that has been zone-refined in vitreous carbon. The voltagethe specimen was inclined at 45 to the primary probing experiments previously described were beam. An examination of these and similar figures limited in the spatial resolution that could be obtained ( w&C mm) and were laborious and timeleads to the following conclusions: (1) In general the shapes and locations of the consuming. This Note describes preliminary bright regions are largely independent of the experiments in which the high-field regions in applied bias. The bright regions follow reasonably GaAs were observed by studying the emission of Unfortunately, they did not perform any capacitance measurements. However, their data is similar to others and the Mott-type barrier is a limiting case of a compound barrier. Therefore, it is reasonable to ask if the latter type of barrier does not, in reality, exist in this case. In retrospect, only supporting evidence has been presented for the existence of compound barriers in TiOp diodes. Certainly further investigative work is required to substantiate or negate this theory. One final note in support of the theory has been the lack of experimental evidence of any minority carrier injection in TiOs diodes.
~ .-*0. ‘I
'b
FIG. 1. Showing the relationship between structural defects, changes in electrical resistivity and the variations in secondary emission resulting from a d.c. bias. (a) Optical micrograph of the grain and dislocation boundaries in the area studied (X 33). (b) Voltage map of the region in (a), 15 V across specimen (1 cm long) (X 33). The dotted lines indicate the positions of some of the salient crystal defects shown in the optical and electron micrographs. (c) Electron optical micrograph of the small area approximately within the dashed lines in (a), no bias applied (X 65). (d) Same area as in (c) with 15 T. bias across specimen (x 65). (e) Same area as in (c) with 60 V across specimen (x 65).
Lfacingp.
532
Y ._.
m
“.’
b
Electronics
Pergamon Press 1963. Vol. 6, pp. 531-546.
Printed in Great Britain
NOTES Comment
on proposed barriers diodes
in thin film TiOa
there have RECENTLY, in the literatureo-4) appeared three separate proposals concerning the nature of the rectifying barriers in TiOa diodes. These have been a p-n junction,(lJ) a Mott-type barrier,(s) and a compound barrier.@) The purpose of this note is to show that all could conceivably be compound barriers or a limiting case of a compound barrier. This type of a barrier is proposed in spite of the fact that all diodes concerned were either prepared differently or utilized a different structure. Huber and Rottersman utilized anodized evaporated films. From a linear plot of l/C’s vs. V, they concluded that a graded p-n junction exists. That p-n junction theory supports this linear relationship is well known. However, it is interesting to review this work in terms of l/C2 vs. P’. This is done in Fig. 1. Here again a linear relationship exists except for a deviation near the origin. This same deviation is noted in the original work.(l) 0.8
x /Of6
1
0.7 0.6 ./’ ./
0.5 0.4 i_ .
l /
.Y
%_iI , (
0
FIG. 1.
.,
,
,
1234567 V-VOLTS Characteristic of (l/@) vs. S’.
,
(
An analysis of the linear plot (Fig. 1) may be made in terms of a compound barrier. This type of a barrier has been theoretically discussed by BILLIG and LANDSBERG and recently utilized by this writer.(d) It predicts an intercept (l/C2 = 0) value equal to vn+(%-fre/~))d$ where vo is the diffusion potential, n the carrier concentration, K the dielectric constant, and da the insulating film of the linear plot thickness. By extrapolation (Fig. 1) this value is found to be -8.0 V. The value of at, calculated from the slope, is 7.9 x lOIs/ ems. Using a value of 40 for K,(s) and assuming vo = 0.5, da is calculated to be 206 A. This is practically identical to that stated by Huber as the thickness of his anodized film. Using the intercept value (V = 0) of the linear graph one finds that an extremely narrow region (N 10 A) exists as a space-charge region. Upon application of the reverse bias voltage this space charge region extends in width and accounts for incremental change in capacitance. One serious discrepancy does, however, exist. This is realized if one compares the intercept value (V = 0) to the measured value of capacitance. The latter value (N 1.8 pF/cma) is approximately 10 per cent higher than the intercept value and consequently prevents any subsequent analysis of a space-charge region. As noted previously, the deviation near the origin occurs in both cases. This may be due to a carrier concentration, adjacent to the insulating layer, that is smaller than the given calculated value. In spite of this, the ability to predict an insulation thickness comparable to experimental value lends credence to the compound barrier theory. DAVIS and GRANNEMANN@)studied the v-1 characteristics of point contacts to hydrogen reduced, single-crystal rutile. Their results were similar to other investigator#*s) as well as those investigated by this writer. One of the previous investigators@) anodized their films after the reduction process. Davis and Grannemann concluded that the rectification mechanism was most likely attributed to a Mott-type barrier (ho = lo-scm). 531
532
NOTES
secondary electrons from specimens with and \vithout an applied d.c. bias. Use \vas made of :I scanning electron microscope being developed by the Cambridge Instrument Co. Briefly, a beam of 15 kV electrons (in this case the beam \\idth was 500 L% with a total beam current lW1’1~~10 1’ :\) was scanned in a square raster across the specimen. The secondary electrons with energies up to a few tens of electron v-olts were collected with a biased collector. The resultant current, after amplification, was used to modulate a cathode-ray tube being scanned in synchronism \vith the primary beam. In the absence of an applied bias on the specimen, the secondary emission gives information IBM General Products Division, P. J. MAGILL about the topography of the spccimene?) (See Development Laboratory, Figs 1 and 2.) It was thought that by applying a Endicott, N. Y. bias the secondary emission could be affected by local variations in electric field. Two possible References mechanisms were envisaged : (I) local variations 1. F. HUBEH and 1111.ROTTERSMAN, J. Applied Physics of the effective work function could be caused by 33, 3385 (1962). small (a few percent) local changes in the free2. 1’. HIUBER,Solid-State Electron. 5, 410 (1962). carrier velocities in the crystal, and (2) localized 3. G. DAVIS, JR. and IV. W. GRANXEMAS, J. Applied irregularities in crystal potential could perturb Physics 34, 228 (1963). 4. P. J. MAGILL, Proc. I.E.E.E. 51, 223 (1963). the trajectories followed by the emitted second5. E. BILLIC and P. T. LAIWSRERG, Pm. Phys. SW., aries. To test this idea we examined the secondary I,ond. 63A, 101 (1950). emission from specimens (containing resistivity 6. F. HUBEH and J. BLoxAnf, Ttms. I.R.E. CP-8, 80 variations associated with crystal defects) under (1961). 7. R. G. BRECKENRIDCEand W. R. HOSLER, J. Res. both biased and unbiased conditions. Some results N.B.S. 49, 65 (1954). are shown in Figs 1 and 2. 8. T. 1. KOMOLOVA atld D. N. NASLEDOV, Soa. P/z~T. In each figure there is a lo\\-magl~i~ication Solid State 3, 2469 (1962). optical picture (a) of the structural defects in the region studied, a v-oltage map (b) shovving the lines of cquipotential, and photographs (c), (d) and (e) taken on the scanning microscope. In these latter photographs it is the changes in intensity at different parts of the field of view that are sigDirect observation of the high-field regions in GaAs nificant. The absolute brightnesses are not significant, because the darkness level was altered (Received 3 May 1963) from photograph to photograph in order to stay IN A recent paper(r) it was shown that the diswithin the working range of the recording film. Account must also be taken of a slight forelocation distribution can affect the electrical shortening in the scanning-microscopy pictures as properties of high-resistivity GaAs that has been zone-refined in vitreous carbon. The voltagethe specimen was inclined at 45 to the primary probing experiments previously described were beam. An examination of these and similar figures limited in the spatial resolution that could be obtained ( w&C mm) and were laborious and timeleads to the following conclusions: (1) In general the shapes and locations of the consuming. This Note describes preliminary bright regions are largely independent of the experiments in which the high-field regions in applied bias. The bright regions follow reasonably GaAs were observed by studying the emission of Unfortunately, they did not perform any capacitance measurements. However, their data is similar to others and the Mott-type barrier is a limiting case of a compound barrier. Therefore, it is reasonable to ask if the latter type of barrier does not, in reality, exist in this case. In retrospect, only supporting evidence has been presented for the existence of compound barriers in TiOp diodes. Certainly further investigative work is required to substantiate or negate this theory. One final note in support of the theory has been the lack of experimental evidence of any minority carrier injection in TiOs diodes.
~ .-*0. ‘I
'b
FIG. 1. Showing the relationship between structural defects, changes in electrical resistivity and the variations in secondary emission resulting from a d.c. bias. (a) Optical micrograph of the grain and dislocation boundaries in the area studied (X 33). (b) Voltage map of the region in (a), 15 V across specimen (1 cm long) (X 33). The dotted lines indicate the positions of some of the salient crystal defects shown in the optical and electron micrographs. (c) Electron optical micrograph of the small area approximately within the dashed lines in (a), no bias applied (X 65). (d) Same area as in (c) with 15 T. bias across specimen (x 65). (e) Same area as in (c) with 60 V across specimen (x 65).
Lfacingp.
532
Y ._.
m
“.’
b