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Electrical properties of clean germanium surfaces
SESSION
D
of pns/c~pBas a function of q&/kT, for three different oxygen admissions. The mobility is definitely below its bulk value. The solid line represents the value of the reduced mobility based on Schrieffer’s theory. The fit is good considering the approximations used in interpreting the data and strongly supports the model of diffuse scattering at the surface. However, since some joints lie
f
Ill
SURFACES
above the curve, some specular scattering may occur. Further interpretation would include the effect of fast holes and quantum effects. For the extreme degenerate situation shown in Fig. 3 the width of the well at the Fermi energy is approximately 10 A. This represents a few atom layers and one wonders whether the band representation or Poisson’s equation can be applied to such narrow space charge regions. Moreover, the Heisenberg uncertainty principle indicates that states should not exist at the bottom of the well. REFERENCES
2
FIG. 4. The ratio of the effective mobility of the p-type surface carriers to the bulk mobility as a function of the depth of the space charge potential well.
J. Phys. Chew. Solids
D.7
Pergamon
J. R., Phys. Rev. 97, 641 (1955). I. SCHRIEFFER 2. ZEMEL J. N., Bull. z&w. Phys. Sot. 3, 105 (1958). 3. PETRITZR. L., Phys. Rev. 110, 1254 (1958). 4. ZEMEL J. N. and PETRITZ R. L., Phys. Rev. 110, 1263 (1958). 5. HANDLERP., Semiconductor Surface Physics p. 39. Universitv of Pennsvlvania Press. Philadelphia (1957). . 6. SELWATZR. and GREEN M., J. Appl. Phys. 29, 1034 (1958).
Press 1959. Vol. 8. pp. 111-113.
Printed in Great Britain
ELECTRICAL PROPERTIES OF CLEAN GERMANIUM SURFACES G. A. BARNES Department
and P. C. BANBURY
of Physics, University
1. INTRODUCTION
THE technique developed by FARNSWORTHfor the cleaning of surfaces by ion bombardment, has recently been applied in investigations of the electrical properties of semiconductor surfaces,(l) in an attempt to improve understanding of the nature and origin of surface states. The procedure involves bombarding the surface with positive ions from an inert gas discharge, evacuating to a high vacuum, and annealing at a moderate tem-
of Reading
perature to restore crystalline order at the surface. The bombardment process itself may be regarded as a means of removing a part of the crystal by sputtering; an alternative approach to the problem of obtaining a clean surface is to expose a new face by fracture. Contamination of a fresh surface may, in principle, arise from the presence of foreign atoms previously located in the region now exposed, and by their arrival by surface diffusion, bulk
112
SESSION
D:
diffusion, or adsorption from the gaseous ambient. The removal of the vacuum limitation imposed by the discharge, and of the annealing process, will tend to reduce the effect of the last two sources. At a pressure of lo-10 mm Hg the time taken to establish a tenth of a monolayer of contaminant by adsorption, even assuming a sticking coefficient of unity, is -30 min. It has been assumed that the other sources of progressive contamination are not more rapid than this at room temperature. We have measured the field effect on conductance, and the photoconductance, in specimens of germanium with freshly cleaved surfaces under these conditions. 2. EXPERIMENTAL
PROCEDURE
The nature of the field effect measurement (the effective transverse mobility of induced surface charge) requires that the surface shall be plane. The measurements reported here have been taken on surfaces exposed by cleavage on a (111) plane. For convenience the final specimen should be in the form of a rectangular filament, say of length, 1, breadth b, and thickness t, where the thickness r of the specimen beneath the freshly exposed surface should be small. In practice, to obtain cleavage it has been found necessary to make it greater than b; values of t = O-2 cm and 6 = 0.1 cm have been attained. The system for supporting and cleaving the specimen has been described elsewhere.(s) An oriented crystal of size about 2.5 x 1.0~ 0.1 cm, with welded low-resistance molybdenum and contacts near one side is mounted by this side in close-fitting jaws of glass. A magnetically operated lever carrying a diamond at the tip, is arranged to provide a short scratch at one end of the specimen. The cleaving stress is applied by a glass lever hung on to the main body of the specimen near the end where the scratch is applied; this lever is again magnetically operated. The cleavage thus commences at the scratched end of the specimen, and is allowed to run freely along the length of the crystal, leaving a filament of approximate dimensions 25 x O-2 x O-1 cm, with a freshly exposed surface of 2.5x0.1 cm. After cleavage a field electrode of metal with bead spacers of glass or mica with a metal film on the reverse side may
SURFACES
be allowed to fall into position near the fresh surface for the application of electric fields; alternatively, the surface may be illuminated through a nearby slit. The best surfaces obtained by this means have shown little irregularity to the naked eye except in the region beneath the diamond scratch, which is not used in the field effect measurement for this reason. Microscopic examination down to ~1 p generally fails to reveal any sudden steps, but irregular deviations from the (111) plane over the 2 cm length are sufficient to make examination by interference techniques difficult. Typically these deviations are estimated as of height a few microns. Spacings between this surface and the field electrode of -0.3 mm have been employed. It may be remarked that all attempts to perform cleavage on specimens to which the diamond scratch had been applied before assembly and evacuation were unsuccessful. The failure was associated with the prolonged baking. The whole system occupies a volume of less than one litre, and contains little metal apart from the specimen itself. By continuous pumping with a mercury diffusion pump with two cold traps, and the use of an Alpert ion pump, pressures in the region of lo-10 mm Hg, are obtained after many hours baking at 420°C. The gauge is not normally operated during the cleavage process, nor while the electrical measurements are being taken An initial set of measurements of either kind can be completed within a few minutes of cleavage. 3. RESULTS
Measurements have been taken of the d.c. field effect on the clean surface, and of the 50 c/s a.c. field effect and the low-level photoconductance, both on the fresh surface and, in preliminary measurements, during contamination by air. An approximate calibration for the lifetime was obtained by a comparison, after contamination, of the photoconductance and photoconductive decay. In the measurements of d.c. field effect induced conductance changes were observed over periods up to 1 mm and found to remain constant (to better than 30 per cent), indicating that slow surface states with relaxation times of less than 100 set, if present, must have much lower densities than occur on oxidized surfaces.
SESSION
D:
The photoconductance measurements have been carried out on 20 Q cm n-type material. Values of surface recombination velocity obtained were in the region of 100 to 300 cm/set. During progressive contamination by air to atmospheric pressure the s-value was found to rise monotonically by a factor of ~6. Attempts to observe the large signal surface conductance curve in the a.c. field effect measurement involved the application of high voltages (~10~ V) to the field electrode, and spurious conductances invalidated the measurements. It was not practicable to reduce the Ge to electrode spacing by a large factor, and observations were therefore limited to small-signal field effect mobility, with a consequent inability to evaluate a barrier height in the general case. The results of such measurements are shown in Fig. 1. In the case of the near intrinsic material,
--==F (9)
2
1
/c
2 (b)
/
3
/
FIG. 1. Small-signal ax. field effect curves before and during contamination of surfaces, (a) near-intrinsic material, (b) -20 R cm n-type material.
113
SURFACES
The results on intrinsic material might be interpreted as indicating a near-intrinsic surface with little or no barrier in the clean condition, with the surface becoming first P-type and then more n-type on contamination. In the results on the n-type material, since the slope does not change sign it is not possible to distinguish between changes of barrier height and of surface state density, but the similarity in behaviour between the two sets of results suggests that the surface again moves during contamination first towards a p-type and then a more strongly n-type condition. The initial barrier height appears to be less than 4kT, since the conductivity is n-type. It might be supposed, for example, that in all cases the fresh surface is near the intrinsic condition. Alternatively, as pointed out by HANDLER (private communication), the results might be reconcilable with the model established from measurements on bombarded surfaces in which the fresh surface is supposed to be highly p-type with a high surface state density, if the observed effects are attributed to a balance between a low p-type effective mobility on the fresh surface and a high n-type mobility associated with the stray capacity to the uncleaved side surfaces. It is hoped that further measurements will give more decisive information as to barrier heights. Acknowledgments-Thanks are due to Professor R. W. DITCHBURN for his support of the work, and to the Admiralty for permission to publish this paper. REFERENCES
the field effect mobility on the fresh surface was very low. In one case a minimum appeared detectable, indicating a zero field barrier height of approximately 2kT. During contamination an initial swing towards a p-type mobility was followed by a much larger change to an n-type condition. On n-type material an initial n-type mobility was seen, which underwent first a small reduction, and then a large increase, during contamination.
E., SemiconPhysics, Proceedings of Conference on Physics of Semiconductor Surfaces, June 1956. University of Pennsylvania Press, Philadelphia. 2. HANDLER P., Semiconductor Surface Physics, Proceedings of Conference in Physics of Semi-
1.
SCHLIER R. E. and FARNSWORTH H.
ductor Surface
conductor Surface, June 1956. University Pennsylvania Press, Philadelphia. 3.
BARNES G. A.
First
and BANBURY P. C.,
International
Congress
on
of
Proceedings of Vacuum Tech-
n&es, June, 1958. Pergamon Press, London.
D
of pns/c~pBas a function of q&/kT, for three different oxygen admissions. The mobility is definitely below its bulk value. The solid line represents the value of the reduced mobility based on Schrieffer’s theory. The fit is good considering the approximations used in interpreting the data and strongly supports the model of diffuse scattering at the surface. However, since some joints lie
f
Ill
SURFACES
above the curve, some specular scattering may occur. Further interpretation would include the effect of fast holes and quantum effects. For the extreme degenerate situation shown in Fig. 3 the width of the well at the Fermi energy is approximately 10 A. This represents a few atom layers and one wonders whether the band representation or Poisson’s equation can be applied to such narrow space charge regions. Moreover, the Heisenberg uncertainty principle indicates that states should not exist at the bottom of the well. REFERENCES
2
FIG. 4. The ratio of the effective mobility of the p-type surface carriers to the bulk mobility as a function of the depth of the space charge potential well.
J. Phys. Chew. Solids
D.7
Pergamon
J. R., Phys. Rev. 97, 641 (1955). I. SCHRIEFFER 2. ZEMEL J. N., Bull. z&w. Phys. Sot. 3, 105 (1958). 3. PETRITZR. L., Phys. Rev. 110, 1254 (1958). 4. ZEMEL J. N. and PETRITZ R. L., Phys. Rev. 110, 1263 (1958). 5. HANDLERP., Semiconductor Surface Physics p. 39. Universitv of Pennsvlvania Press. Philadelphia (1957). . 6. SELWATZR. and GREEN M., J. Appl. Phys. 29, 1034 (1958).
Press 1959. Vol. 8. pp. 111-113.
Printed in Great Britain
ELECTRICAL PROPERTIES OF CLEAN GERMANIUM SURFACES G. A. BARNES Department
and P. C. BANBURY
of Physics, University
1. INTRODUCTION
THE technique developed by FARNSWORTHfor the cleaning of surfaces by ion bombardment, has recently been applied in investigations of the electrical properties of semiconductor surfaces,(l) in an attempt to improve understanding of the nature and origin of surface states. The procedure involves bombarding the surface with positive ions from an inert gas discharge, evacuating to a high vacuum, and annealing at a moderate tem-
of Reading
perature to restore crystalline order at the surface. The bombardment process itself may be regarded as a means of removing a part of the crystal by sputtering; an alternative approach to the problem of obtaining a clean surface is to expose a new face by fracture. Contamination of a fresh surface may, in principle, arise from the presence of foreign atoms previously located in the region now exposed, and by their arrival by surface diffusion, bulk
112
SESSION
D:
diffusion, or adsorption from the gaseous ambient. The removal of the vacuum limitation imposed by the discharge, and of the annealing process, will tend to reduce the effect of the last two sources. At a pressure of lo-10 mm Hg the time taken to establish a tenth of a monolayer of contaminant by adsorption, even assuming a sticking coefficient of unity, is -30 min. It has been assumed that the other sources of progressive contamination are not more rapid than this at room temperature. We have measured the field effect on conductance, and the photoconductance, in specimens of germanium with freshly cleaved surfaces under these conditions. 2. EXPERIMENTAL
PROCEDURE
The nature of the field effect measurement (the effective transverse mobility of induced surface charge) requires that the surface shall be plane. The measurements reported here have been taken on surfaces exposed by cleavage on a (111) plane. For convenience the final specimen should be in the form of a rectangular filament, say of length, 1, breadth b, and thickness t, where the thickness r of the specimen beneath the freshly exposed surface should be small. In practice, to obtain cleavage it has been found necessary to make it greater than b; values of t = O-2 cm and 6 = 0.1 cm have been attained. The system for supporting and cleaving the specimen has been described elsewhere.(s) An oriented crystal of size about 2.5 x 1.0~ 0.1 cm, with welded low-resistance molybdenum and contacts near one side is mounted by this side in close-fitting jaws of glass. A magnetically operated lever carrying a diamond at the tip, is arranged to provide a short scratch at one end of the specimen. The cleaving stress is applied by a glass lever hung on to the main body of the specimen near the end where the scratch is applied; this lever is again magnetically operated. The cleavage thus commences at the scratched end of the specimen, and is allowed to run freely along the length of the crystal, leaving a filament of approximate dimensions 25 x O-2 x O-1 cm, with a freshly exposed surface of 2.5x0.1 cm. After cleavage a field electrode of metal with bead spacers of glass or mica with a metal film on the reverse side may
SURFACES
be allowed to fall into position near the fresh surface for the application of electric fields; alternatively, the surface may be illuminated through a nearby slit. The best surfaces obtained by this means have shown little irregularity to the naked eye except in the region beneath the diamond scratch, which is not used in the field effect measurement for this reason. Microscopic examination down to ~1 p generally fails to reveal any sudden steps, but irregular deviations from the (111) plane over the 2 cm length are sufficient to make examination by interference techniques difficult. Typically these deviations are estimated as of height a few microns. Spacings between this surface and the field electrode of -0.3 mm have been employed. It may be remarked that all attempts to perform cleavage on specimens to which the diamond scratch had been applied before assembly and evacuation were unsuccessful. The failure was associated with the prolonged baking. The whole system occupies a volume of less than one litre, and contains little metal apart from the specimen itself. By continuous pumping with a mercury diffusion pump with two cold traps, and the use of an Alpert ion pump, pressures in the region of lo-10 mm Hg, are obtained after many hours baking at 420°C. The gauge is not normally operated during the cleavage process, nor while the electrical measurements are being taken An initial set of measurements of either kind can be completed within a few minutes of cleavage. 3. RESULTS
Measurements have been taken of the d.c. field effect on the clean surface, and of the 50 c/s a.c. field effect and the low-level photoconductance, both on the fresh surface and, in preliminary measurements, during contamination by air. An approximate calibration for the lifetime was obtained by a comparison, after contamination, of the photoconductance and photoconductive decay. In the measurements of d.c. field effect induced conductance changes were observed over periods up to 1 mm and found to remain constant (to better than 30 per cent), indicating that slow surface states with relaxation times of less than 100 set, if present, must have much lower densities than occur on oxidized surfaces.
SESSION
D:
The photoconductance measurements have been carried out on 20 Q cm n-type material. Values of surface recombination velocity obtained were in the region of 100 to 300 cm/set. During progressive contamination by air to atmospheric pressure the s-value was found to rise monotonically by a factor of ~6. Attempts to observe the large signal surface conductance curve in the a.c. field effect measurement involved the application of high voltages (~10~ V) to the field electrode, and spurious conductances invalidated the measurements. It was not practicable to reduce the Ge to electrode spacing by a large factor, and observations were therefore limited to small-signal field effect mobility, with a consequent inability to evaluate a barrier height in the general case. The results of such measurements are shown in Fig. 1. In the case of the near intrinsic material,
--==F (9)
2
1
/c
2 (b)
/
3
/
FIG. 1. Small-signal ax. field effect curves before and during contamination of surfaces, (a) near-intrinsic material, (b) -20 R cm n-type material.
113
SURFACES
The results on intrinsic material might be interpreted as indicating a near-intrinsic surface with little or no barrier in the clean condition, with the surface becoming first P-type and then more n-type on contamination. In the results on the n-type material, since the slope does not change sign it is not possible to distinguish between changes of barrier height and of surface state density, but the similarity in behaviour between the two sets of results suggests that the surface again moves during contamination first towards a p-type and then a more strongly n-type condition. The initial barrier height appears to be less than 4kT, since the conductivity is n-type. It might be supposed, for example, that in all cases the fresh surface is near the intrinsic condition. Alternatively, as pointed out by HANDLER (private communication), the results might be reconcilable with the model established from measurements on bombarded surfaces in which the fresh surface is supposed to be highly p-type with a high surface state density, if the observed effects are attributed to a balance between a low p-type effective mobility on the fresh surface and a high n-type mobility associated with the stray capacity to the uncleaved side surfaces. It is hoped that further measurements will give more decisive information as to barrier heights. Acknowledgments-Thanks are due to Professor R. W. DITCHBURN for his support of the work, and to the Admiralty for permission to publish this paper. REFERENCES
the field effect mobility on the fresh surface was very low. In one case a minimum appeared detectable, indicating a zero field barrier height of approximately 2kT. During contamination an initial swing towards a p-type mobility was followed by a much larger change to an n-type condition. On n-type material an initial n-type mobility was seen, which underwent first a small reduction, and then a large increase, during contamination.
E., SemiconPhysics, Proceedings of Conference on Physics of Semiconductor Surfaces, June 1956. University of Pennsylvania Press, Philadelphia. 2. HANDLER P., Semiconductor Surface Physics, Proceedings of Conference in Physics of Semi-
1.
SCHLIER R. E. and FARNSWORTH H.
ductor Surface
conductor Surface, June 1956. University Pennsylvania Press, Philadelphia. 3.
BARNES G. A.
First
and BANBURY P. C.,
International
Congress
on
of
Proceedings of Vacuum Tech-
n&es, June, 1958. Pergamon Press, London.