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Contact studies on semiconductor cleavage surfaces under liquids
SURFACE
SCIENCE
4 (1966) 486-493 0 North-Holland
CONTACT
STUDIES
SEMICONDUCTOR
H. K. HENISCH
Co., Amsterdam
ON
CLEAVAGE
UNDER
Materials
Publishing
SURFACES
LIQUIDS and W. P. NOBLE, Jr.
Research Laboratory, Pennsylvania State University Park, Pa., U.S.A.
University,
Received 7 February 1966 Experiments are described on contacts between a mercury jet and surfaces of germanium andsilicon, cleaved under dielectric liquids. The results show that cleaved surfaces are non-homogeneous, giving rise to substantial localized variations of barrier height. These variations are ascribed to electronic states associated with dislocations parallel to the cleavage surface. The similarities between surfaces cleaved under liquids and those cleaved in ultra-high vacuum are discussed.
1. Introduction One of the interesting unsolved, states
is that
is changed
problems
of ascertaining when
a metallic
in semiconductor whether layer
surface
the position is applied.
physics,
and density
This would
as yet
of surface
involve
com-
in the two cases, and each of these problems is beset with difficulties. Barrier heights can be evaluated from measurements of voltage-current characteristics, but the results are much less reliable than one would wish, and internal consistency is rarely achieved. As far as is known, there is only one recorded case in which satisfactory agreement was obtained between barrier heights evaluated from contact characteristics by three independent methodsl). On free surfaces, the measurement of barrier height demands measurements of the longitudinal conductance as a function of an external, capacitively applied field. This procedure involves an area of substantial dimensions and its interpretation is based on the assumption that the surface is homogeneous. In the past, cleaved semiconductor surfaces have frequently been so regarded 2p4). Visible surface features have indeed been noted, but their role in determining electrical surface properties has been regarded as small 5). However, Frank16) has given results, obtained by means of transmission electron microscopy, which could be interpreted by postulating the existence of non-uniformly
parative
measurements
of barrier
height
SEMICONDUCTOR
distributed
dislocations
CLEAVAGE
SURFACES
UNDER
LIQUIDS
parallel to the cleavage surfaces of silicon.
487
Moreover,
Donald’) has reported localized variations of contact properties which, if generally present, would throw all area (averaging) measurements on cleaved surfaces into varying degrees of doubt. The work here described had the aim of checking
and clarifying
this point.
2. Experimental specimens and procedure The test surfaces were produced by cleavage under (relatively) oxygenfree liquids in order to delay oxidation. For this purpose, rod-shaped specimens were embedded in epoxy resin which was, in turn, molded into a split Teflon block. This was later pulled apart by means of an accurately made screw-and-capstan tensioning system. This procedure was adopted in order to achieve a pure tensile stress and thus to avoid, as far as possible, the appearance of compression regions on the cleavage surface and the transmission of torque. It was found to give much more repeatable results than simpler procedures. For localized contacting, all conventional methods are obviously ruled out. In particular, wire ‘point’ contacts cannot be used because they involve contact pressures (approximately equal to the yield pressure) which must be expected to damage the surface. It was nesecsary, instead, to use a very thin (30pm diameter) mercury jet, the impact point of which could be moved over the test surface by means of a micromanipulator. After impact, the mercury globules disperse and play no further part in the proceedings. With a mercury pressure of 30 psi, a sufficiently stable contact was achieved and its electrical characteristics were displayed on a CRT. The following specimens were used in the investigation: germanium:
1R*cm and lOR*cm
silicon:
0.60.cm and lOR.cm 5 R. cm (n-type), 30R.cm and 300R.cm
(n-type), (p-type); (p-type).
Cleaved germanium surfaces were frequently characterized by two or more distinct regions. These are illustrated by the example in fig. 1. The cleavage region was seen to be one of flat cleavage planes, bordered by step lines radiating from a nucleation center, a very small area of smoothly curved fracture at which cleavage initiates. A second region was characterized by a slightly curved surface with many nearly parallel lines resembling and sometimes originating from the step lines of the flat cleavage planes. In fig. 1, these regions are designated by A and B, respectively. On very poorly cleaved surfaces there would be a third type of region over which there was no orientated cleavage, but random shattering. The cleavages obtained exhibited only
(4 Fig. 1.
(b)
(a) General cleavage surface illustrating regions A and B as well as random shattering. (b) Best cleavage obtained exhibiting only A region.
the nucleation center and the flat cleavage planes radiating from it. Poorer surfaces could usually be ascribed to deviation from purely tensile stress during cleavage. Cleaved silicon surfaces were observed to exhibit the same features as germanium surfaces, but the quality of cleavage was generally poorer. In A regions, the density of cleavage steps was usually greater and B regions were generally found to occupy a larger portion of the surface are.
Electrical measurements on freshly cleaved surfaces Over germanium surfaces of the kind shown in fig. 1, the electrical properties can vary widely. An attempt was made to correlate electrical properties with physical features of the cleaved surfaces. Not only was there a difference between A regions and B regions, but the electrical properties within the A regions varied in a systematic manner. Using the zero-voltage resistance (slope of the V-Z characteristic at the origin as a criterion, ratios up to 17 : 1 have been observed for different positions of the mercury jet on A regions, even between cleavage steps. The geometrical spreading resistance, which is common to all the contacts on a given specimen, is small (4Ofi on 1SZ.cm material). When it is subtracted from the zero-voltage resistance, the ratio increases, but only to 19 : 1. The most pronounced inhomogeneities were observed on IQ-cm (n-type) and the least on 0.6n.cm (p-type) material, B regions having systematically higher zero-voltage resistances than A regions. In traversing a smooth plane between cleavage steps, the zero-voltage resistance was found to increase as a B region was approached. On surfaces exhibiting no B region, variations in zero-voltage resistance were also ob-
SEMKONDUCTOR
CLEAVAGE
SURFACES
UNDER
LIQUlDS
489
served, but no definite correlation with visuaf surface features could be made. It follows that visual features atone are insufficient to characterize the surface; the contact properties at any point can be influenced by the presence of disordered regions elsewhere on the cleavage surface. The presence of B regions is evidently not the only consequence of disordered fracture. On the other hand, the etching procedures used in this laboratory (ferricyan~de etchants)) as well as those reported by othersg), have failed to reveal any systematic variation in the density of etch pits within the A regions or, for that matter, anywhere on the cleaved surfaces. On the silicon surfaces, the variation in zero-voltage resistance was neither as pronounced nor as systematic as on germanium surfaces. On surfaces of 5Q*cm (n-type) silicon, no inhomogeneities were observed and the presence of a B region was found to have no effect. Inhomogeneities were observed on the p-type samples of both doping levels. Thus, 3tXZZ~crn (p-type) material, a ratio of 30 : 1 was found between the extremes of measured zero-voltage resistance values in the A region. The lowest resistance was found in regions of dense cleavage steps, and the highest resistance in smooth regions, again without any marked dependence on the presence of a B region. Ratios of only 8: f were measured on surfaces of 3oDO~cm (p-type) materials, increased to 9: 1 by the subtraction of the spreading resistance. 4. Character of surfaces prepared under liquids inevitably, the question arises as to how the surfaces prepared by cleavage under liquids compare with those cleaved in ultra-high vacuum. This is partly a matter which concerns the speed of contamination under the two sets of conditions and partly one which concerns the difference between surface states and interface states. A full and final answer is not yet available and will have to await the performance of experiments on capacitively modulated surface conduction. However, the present results indicate that the similarities may be considerable. Thus, photovoltaic tests on n-type as well as on p-type germanium yield positive photo-voltages, showing that the curvature of the potential profile is the same in the two cases. This is in harmony wit,h the findings of Gobeli and Allen ‘0) to the effect that the average position of the Fermi level on vacuum cleaved surfaces is firmly locked near the valence band. ft is here envisaged that there are relatively minor and focali,zed variations from this average position. These show themselves as variations of contact resistance, which is known to be a very sensitive function of barrier height. Because of variations in effective lifetime, the magnitude of the photovoltaic effect does not yield information which can be usefully analyzed. As expected, the photo-voltages tend to zero in the most disordered
K.
490
K.
HENISCH
AND
W.
P. NOBLE,
JR.
regions. Contacts on the present silicon specimens yielded photo-voltages of opposite sign for n-type and p-type material. This is also consistent with the findings of Allen and Gobelill).
mA
mA
(a)
Cc)
Cd) : I
n-type 1 iX.cm
;
//
-4’
-:
/ -’
/’ 1v
:
0.2
I
I /
p-type Ifi.cm
I
0.2
I -
i
/
_-
/
/
/
f
3 _/’ 1v
/
II
t (e) n-type 50n.cm
Fig. 2.
il
o,2
Effects of contamination
(f) p-type 3_CL,cm
’IlZ 0.2
ii
/ I
on V-I characteristics of metal contacts on cleaved Ge surfaces. (a)-(d) Present work, cleavage under decane, Hg contacts. (e)-(f) After Banbury, Davies and Green12), cleavage in ultra-high vacuum, tungsten contacts. 1 ~~~~ Immediately after cleavage. 2 ----- After approximately one hour. 3 - --- After exposure to air.
SEMICONDUCTOR
After
cleavage,
the surfaces
CLEAVAGE
SURFACES
become
UNDER
gradually
491
LIQUIDS
contaminated,
and
the
corresponding changes in the rectification characteristics are very similar to those observed after cleavage in ultra-high vacuum12). This is shown in fig. 2. These measurements must be made at low currents since there is some evidence of ‘jet-etching’ when currents exceed about 0.5 mA. Its consequence is to restore the surface under the mercury impact point to the state observed immediately after cleavage. This is in broad agreement with Donald’s7) results on the electro-forming of mercury contacts, which yielded properties similar to those resulting from ion bombardment of the surface. In ultra-high vacuum, the generally agreed contamination period is of the order of 20-30 min. Similar periods can be achieved under liquids, though in general, the observed times are somewhat shorter. To some extent, the time estimate depends on the criterion used. It seems likely that the free oxygen content of liquids is considerably higher, but the mean free path is many orders of magnitude lower than in vacuum. In initial attempts to find an organic liquid which would retard the oxidation of freshly cleaved surfaces, several hydrocarbons which do not have oxygen as a constituent were tried: xylene, toluene, benzene, tetralin and a series of aliphatic hydrocarbons. In each case, the diode characteristics of a fresh contact under the liquid was observed as a function of time. The most reproducible behavior was observed with the aliphatic compounds. Reference to available solubility datal3) indicates that these compounds do have a relatively low solubility for oxygen. Surfaces cleaved under hexane (CsH,,) showed evidence of contamination after 2 min, whereas decane (C,,H,,) was found to give times of 7 min or more. Bubbling of nitrogen through the decane for several hours prior to cleavage was found to extend the time by several minutes. 5. Discussion
and conclusions
The present results confirm, in general terms, the existence of major difficulties in the interpretation of area rectification experiments on cleaved semiconductor surfaces. Unless some new and subtle method of cleaving can be devised which produces homogeneous surfaces, these difficulties would appear to be permanent and inescapable. The experiment which could contribute most to the development of contact theory would be one on an ultrapure surface on which a single crystal metallic layer has been epitaxially deposited. The present results make the use of cleaved surfaces for such purposes questionable and suspect. On the other hand, it does not follow that all surface experiments suffer equally from the observed surface heterogeneity. The precise meaning of ‘average’ barrier height, as used above, is not clear, but it is plausible that experiments involving the modulation of
492
surface conductance barrier height than
H. K. HENISCH
AND
W.
P. NOBLE,
JR.
may be less disturbed by highly localized variations those involving the voltage-current characteristics
of of
contacts. A plausible model for the variations in electrical properties of cleaved surfaces as a function of surface or sub-surface damage can be constructed which is compatible with the observed data on germanium. It is necessary in formulating such a model to utilize available data on electrical properties of dislocations as well as surface potential of well ordered clean surfaces. A discussion of the available data on dislocations in germanium has been given by Read14). It is concluded that dislocations give rise to a line of electron acceptor states 0.2 eV below the conduction band. This, together with the work function measurements on apparently well ordered clean surfaces of germanium by Gobeli and AllenlO) leads to the expectation that dislocations will contribute to the upward bending of the energy bands at the surface in the same manner as surface states. Applying this model to n-type germanium, one would observe the effect of disorder as in increased surface barrier height and thus as an increase in zero-voltage resistance. On p-type material the effect should be much less pronounced, since the effects of an upward bending of the bands would not impede the flow of majority carriers. It cannot be remotely expected that cleavage under liquids can replace cleavage under vacuum in all contexts, but the results show that for certain purposes the simpler method will suffice, since it leads to surfaces which are somewhat similar to those produced in ultra-high vacuum. Detailed evaluations of the contact barrier height from the rectification characteristics of n-type germanium cleaved under liquids will be described on another occasion. The preliminary indications are that interface barriers between germanium and decane are lower than those expected for clean germanium surfaces in ultra-high vacuum.
Acknowledgments The work here described was carried out under Grant No. 3232 issued by the National Science Foundation, Engineering Branch. The authors are also indebted to Professor D. R. Frank1 for valuable discussions.
References 1) D. Kahng, Solid-State Electron. 6 (1963) 281. 2) P. Handler, Appl. Phys. Letters 3 (1963) 96. 3) D. R. Palmer, S. R. Morrison and C. E. Dauenbaugh, J. Phys. Chem. Solids 14 (1960) 2-l. 4) R. J. Archer and M. M. Atalla, Ann. N. Y. Acad. Sci. 101 (1963) 697.
SEMICONDUCTOR
5) 6) 7) 8) 9) 10) 11) 12) 13) 14)
CLEAVAGE
SURFACES
UNDER
LIQUIDS
493
D. R. Palmer, S. R. Morrison and C. E. Dauenbaugh, Phys. Rev. 129 (1963) 608. D. R. Frank], J. Appl. Phys. 34 (1963) 3514. D. K. Donald, J. Appl. Phys. 34 (1963) 1758. P. J. Holmes, The Electrochemistry of Semiconductors(Academic Press, London and New York, 1962) p. 370. 0. W. Johnson and P. Gibbs, in: Proc. Met. Sot. ConJ, Vol. 20 (Interscience, New York, 1963) p. 31.5. G. W. Gobeli and F. G. Allen, Surface Sci. 2 (1964) 402. F. G. Allen and G. W. Allen, Phys. Rev. 127 (1962) 150. P. C. Banbury, E. A. Davies and G. W. Green, Proc. Intern. Conf. Physics of Semiconductors (The Inst. of Phys. and the Phys. Sot., London, 1962) p. 813. International Critical Tables, Vol. 3 (McGraw-Hill, New York, 1928) p. 262. W. T. Read, Phil. Mag. 45 (1954) 775.
SCIENCE
4 (1966) 486-493 0 North-Holland
CONTACT
STUDIES
SEMICONDUCTOR
H. K. HENISCH
Co., Amsterdam
ON
CLEAVAGE
UNDER
Materials
Publishing
SURFACES
LIQUIDS and W. P. NOBLE, Jr.
Research Laboratory, Pennsylvania State University Park, Pa., U.S.A.
University,
Received 7 February 1966 Experiments are described on contacts between a mercury jet and surfaces of germanium andsilicon, cleaved under dielectric liquids. The results show that cleaved surfaces are non-homogeneous, giving rise to substantial localized variations of barrier height. These variations are ascribed to electronic states associated with dislocations parallel to the cleavage surface. The similarities between surfaces cleaved under liquids and those cleaved in ultra-high vacuum are discussed.
1. Introduction One of the interesting unsolved, states
is that
is changed
problems
of ascertaining when
a metallic
in semiconductor whether layer
surface
the position is applied.
physics,
and density
This would
as yet
of surface
involve
com-
in the two cases, and each of these problems is beset with difficulties. Barrier heights can be evaluated from measurements of voltage-current characteristics, but the results are much less reliable than one would wish, and internal consistency is rarely achieved. As far as is known, there is only one recorded case in which satisfactory agreement was obtained between barrier heights evaluated from contact characteristics by three independent methodsl). On free surfaces, the measurement of barrier height demands measurements of the longitudinal conductance as a function of an external, capacitively applied field. This procedure involves an area of substantial dimensions and its interpretation is based on the assumption that the surface is homogeneous. In the past, cleaved semiconductor surfaces have frequently been so regarded 2p4). Visible surface features have indeed been noted, but their role in determining electrical surface properties has been regarded as small 5). However, Frank16) has given results, obtained by means of transmission electron microscopy, which could be interpreted by postulating the existence of non-uniformly
parative
measurements
of barrier
height
SEMICONDUCTOR
distributed
dislocations
CLEAVAGE
SURFACES
UNDER
LIQUIDS
parallel to the cleavage surfaces of silicon.
487
Moreover,
Donald’) has reported localized variations of contact properties which, if generally present, would throw all area (averaging) measurements on cleaved surfaces into varying degrees of doubt. The work here described had the aim of checking
and clarifying
this point.
2. Experimental specimens and procedure The test surfaces were produced by cleavage under (relatively) oxygenfree liquids in order to delay oxidation. For this purpose, rod-shaped specimens were embedded in epoxy resin which was, in turn, molded into a split Teflon block. This was later pulled apart by means of an accurately made screw-and-capstan tensioning system. This procedure was adopted in order to achieve a pure tensile stress and thus to avoid, as far as possible, the appearance of compression regions on the cleavage surface and the transmission of torque. It was found to give much more repeatable results than simpler procedures. For localized contacting, all conventional methods are obviously ruled out. In particular, wire ‘point’ contacts cannot be used because they involve contact pressures (approximately equal to the yield pressure) which must be expected to damage the surface. It was nesecsary, instead, to use a very thin (30pm diameter) mercury jet, the impact point of which could be moved over the test surface by means of a micromanipulator. After impact, the mercury globules disperse and play no further part in the proceedings. With a mercury pressure of 30 psi, a sufficiently stable contact was achieved and its electrical characteristics were displayed on a CRT. The following specimens were used in the investigation: germanium:
1R*cm and lOR*cm
silicon:
0.60.cm and lOR.cm 5 R. cm (n-type), 30R.cm and 300R.cm
(n-type), (p-type); (p-type).
Cleaved germanium surfaces were frequently characterized by two or more distinct regions. These are illustrated by the example in fig. 1. The cleavage region was seen to be one of flat cleavage planes, bordered by step lines radiating from a nucleation center, a very small area of smoothly curved fracture at which cleavage initiates. A second region was characterized by a slightly curved surface with many nearly parallel lines resembling and sometimes originating from the step lines of the flat cleavage planes. In fig. 1, these regions are designated by A and B, respectively. On very poorly cleaved surfaces there would be a third type of region over which there was no orientated cleavage, but random shattering. The cleavages obtained exhibited only
(4 Fig. 1.
(b)
(a) General cleavage surface illustrating regions A and B as well as random shattering. (b) Best cleavage obtained exhibiting only A region.
the nucleation center and the flat cleavage planes radiating from it. Poorer surfaces could usually be ascribed to deviation from purely tensile stress during cleavage. Cleaved silicon surfaces were observed to exhibit the same features as germanium surfaces, but the quality of cleavage was generally poorer. In A regions, the density of cleavage steps was usually greater and B regions were generally found to occupy a larger portion of the surface are.
Electrical measurements on freshly cleaved surfaces Over germanium surfaces of the kind shown in fig. 1, the electrical properties can vary widely. An attempt was made to correlate electrical properties with physical features of the cleaved surfaces. Not only was there a difference between A regions and B regions, but the electrical properties within the A regions varied in a systematic manner. Using the zero-voltage resistance (slope of the V-Z characteristic at the origin as a criterion, ratios up to 17 : 1 have been observed for different positions of the mercury jet on A regions, even between cleavage steps. The geometrical spreading resistance, which is common to all the contacts on a given specimen, is small (4Ofi on 1SZ.cm material). When it is subtracted from the zero-voltage resistance, the ratio increases, but only to 19 : 1. The most pronounced inhomogeneities were observed on IQ-cm (n-type) and the least on 0.6n.cm (p-type) material, B regions having systematically higher zero-voltage resistances than A regions. In traversing a smooth plane between cleavage steps, the zero-voltage resistance was found to increase as a B region was approached. On surfaces exhibiting no B region, variations in zero-voltage resistance were also ob-
SEMKONDUCTOR
CLEAVAGE
SURFACES
UNDER
LIQUlDS
489
served, but no definite correlation with visuaf surface features could be made. It follows that visual features atone are insufficient to characterize the surface; the contact properties at any point can be influenced by the presence of disordered regions elsewhere on the cleavage surface. The presence of B regions is evidently not the only consequence of disordered fracture. On the other hand, the etching procedures used in this laboratory (ferricyan~de etchants)) as well as those reported by othersg), have failed to reveal any systematic variation in the density of etch pits within the A regions or, for that matter, anywhere on the cleaved surfaces. On the silicon surfaces, the variation in zero-voltage resistance was neither as pronounced nor as systematic as on germanium surfaces. On surfaces of 5Q*cm (n-type) silicon, no inhomogeneities were observed and the presence of a B region was found to have no effect. Inhomogeneities were observed on the p-type samples of both doping levels. Thus, 3tXZZ~crn (p-type) material, a ratio of 30 : 1 was found between the extremes of measured zero-voltage resistance values in the A region. The lowest resistance was found in regions of dense cleavage steps, and the highest resistance in smooth regions, again without any marked dependence on the presence of a B region. Ratios of only 8: f were measured on surfaces of 3oDO~cm (p-type) materials, increased to 9: 1 by the subtraction of the spreading resistance. 4. Character of surfaces prepared under liquids inevitably, the question arises as to how the surfaces prepared by cleavage under liquids compare with those cleaved in ultra-high vacuum. This is partly a matter which concerns the speed of contamination under the two sets of conditions and partly one which concerns the difference between surface states and interface states. A full and final answer is not yet available and will have to await the performance of experiments on capacitively modulated surface conduction. However, the present results indicate that the similarities may be considerable. Thus, photovoltaic tests on n-type as well as on p-type germanium yield positive photo-voltages, showing that the curvature of the potential profile is the same in the two cases. This is in harmony wit,h the findings of Gobeli and Allen ‘0) to the effect that the average position of the Fermi level on vacuum cleaved surfaces is firmly locked near the valence band. ft is here envisaged that there are relatively minor and focali,zed variations from this average position. These show themselves as variations of contact resistance, which is known to be a very sensitive function of barrier height. Because of variations in effective lifetime, the magnitude of the photovoltaic effect does not yield information which can be usefully analyzed. As expected, the photo-voltages tend to zero in the most disordered
K.
490
K.
HENISCH
AND
W.
P. NOBLE,
JR.
regions. Contacts on the present silicon specimens yielded photo-voltages of opposite sign for n-type and p-type material. This is also consistent with the findings of Allen and Gobelill).
mA
mA
(a)
Cc)
Cd) : I
n-type 1 iX.cm
;
//
-4’
-:
/ -’
/’ 1v
:
0.2
I
I /
p-type Ifi.cm
I
0.2
I -
i
/
_-
/
/
/
f
3 _/’ 1v
/
II
t (e) n-type 50n.cm
Fig. 2.
il
o,2
Effects of contamination
(f) p-type 3_CL,cm
’IlZ 0.2
ii
/ I
on V-I characteristics of metal contacts on cleaved Ge surfaces. (a)-(d) Present work, cleavage under decane, Hg contacts. (e)-(f) After Banbury, Davies and Green12), cleavage in ultra-high vacuum, tungsten contacts. 1 ~~~~ Immediately after cleavage. 2 ----- After approximately one hour. 3 - --- After exposure to air.
SEMICONDUCTOR
After
cleavage,
the surfaces
CLEAVAGE
SURFACES
become
UNDER
gradually
491
LIQUIDS
contaminated,
and
the
corresponding changes in the rectification characteristics are very similar to those observed after cleavage in ultra-high vacuum12). This is shown in fig. 2. These measurements must be made at low currents since there is some evidence of ‘jet-etching’ when currents exceed about 0.5 mA. Its consequence is to restore the surface under the mercury impact point to the state observed immediately after cleavage. This is in broad agreement with Donald’s7) results on the electro-forming of mercury contacts, which yielded properties similar to those resulting from ion bombardment of the surface. In ultra-high vacuum, the generally agreed contamination period is of the order of 20-30 min. Similar periods can be achieved under liquids, though in general, the observed times are somewhat shorter. To some extent, the time estimate depends on the criterion used. It seems likely that the free oxygen content of liquids is considerably higher, but the mean free path is many orders of magnitude lower than in vacuum. In initial attempts to find an organic liquid which would retard the oxidation of freshly cleaved surfaces, several hydrocarbons which do not have oxygen as a constituent were tried: xylene, toluene, benzene, tetralin and a series of aliphatic hydrocarbons. In each case, the diode characteristics of a fresh contact under the liquid was observed as a function of time. The most reproducible behavior was observed with the aliphatic compounds. Reference to available solubility datal3) indicates that these compounds do have a relatively low solubility for oxygen. Surfaces cleaved under hexane (CsH,,) showed evidence of contamination after 2 min, whereas decane (C,,H,,) was found to give times of 7 min or more. Bubbling of nitrogen through the decane for several hours prior to cleavage was found to extend the time by several minutes. 5. Discussion
and conclusions
The present results confirm, in general terms, the existence of major difficulties in the interpretation of area rectification experiments on cleaved semiconductor surfaces. Unless some new and subtle method of cleaving can be devised which produces homogeneous surfaces, these difficulties would appear to be permanent and inescapable. The experiment which could contribute most to the development of contact theory would be one on an ultrapure surface on which a single crystal metallic layer has been epitaxially deposited. The present results make the use of cleaved surfaces for such purposes questionable and suspect. On the other hand, it does not follow that all surface experiments suffer equally from the observed surface heterogeneity. The precise meaning of ‘average’ barrier height, as used above, is not clear, but it is plausible that experiments involving the modulation of
492
surface conductance barrier height than
H. K. HENISCH
AND
W.
P. NOBLE,
JR.
may be less disturbed by highly localized variations those involving the voltage-current characteristics
of of
contacts. A plausible model for the variations in electrical properties of cleaved surfaces as a function of surface or sub-surface damage can be constructed which is compatible with the observed data on germanium. It is necessary in formulating such a model to utilize available data on electrical properties of dislocations as well as surface potential of well ordered clean surfaces. A discussion of the available data on dislocations in germanium has been given by Read14). It is concluded that dislocations give rise to a line of electron acceptor states 0.2 eV below the conduction band. This, together with the work function measurements on apparently well ordered clean surfaces of germanium by Gobeli and AllenlO) leads to the expectation that dislocations will contribute to the upward bending of the energy bands at the surface in the same manner as surface states. Applying this model to n-type germanium, one would observe the effect of disorder as in increased surface barrier height and thus as an increase in zero-voltage resistance. On p-type material the effect should be much less pronounced, since the effects of an upward bending of the bands would not impede the flow of majority carriers. It cannot be remotely expected that cleavage under liquids can replace cleavage under vacuum in all contexts, but the results show that for certain purposes the simpler method will suffice, since it leads to surfaces which are somewhat similar to those produced in ultra-high vacuum. Detailed evaluations of the contact barrier height from the rectification characteristics of n-type germanium cleaved under liquids will be described on another occasion. The preliminary indications are that interface barriers between germanium and decane are lower than those expected for clean germanium surfaces in ultra-high vacuum.
Acknowledgments The work here described was carried out under Grant No. 3232 issued by the National Science Foundation, Engineering Branch. The authors are also indebted to Professor D. R. Frank1 for valuable discussions.
References 1) D. Kahng, Solid-State Electron. 6 (1963) 281. 2) P. Handler, Appl. Phys. Letters 3 (1963) 96. 3) D. R. Palmer, S. R. Morrison and C. E. Dauenbaugh, J. Phys. Chem. Solids 14 (1960) 2-l. 4) R. J. Archer and M. M. Atalla, Ann. N. Y. Acad. Sci. 101 (1963) 697.
SEMICONDUCTOR
5) 6) 7) 8) 9) 10) 11) 12) 13) 14)
CLEAVAGE
SURFACES
UNDER
LIQUIDS
493
D. R. Palmer, S. R. Morrison and C. E. Dauenbaugh, Phys. Rev. 129 (1963) 608. D. R. Frank], J. Appl. Phys. 34 (1963) 3514. D. K. Donald, J. Appl. Phys. 34 (1963) 1758. P. J. Holmes, The Electrochemistry of Semiconductors(Academic Press, London and New York, 1962) p. 370. 0. W. Johnson and P. Gibbs, in: Proc. Met. Sot. ConJ, Vol. 20 (Interscience, New York, 1963) p. 31.5. G. W. Gobeli and F. G. Allen, Surface Sci. 2 (1964) 402. F. G. Allen and G. W. Allen, Phys. Rev. 127 (1962) 150. P. C. Banbury, E. A. Davies and G. W. Green, Proc. Intern. Conf. Physics of Semiconductors (The Inst. of Phys. and the Phys. Sot., London, 1962) p. 813. International Critical Tables, Vol. 3 (McGraw-Hill, New York, 1928) p. 262. W. T. Read, Phil. Mag. 45 (1954) 775.