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The effect of thermal desorption of water on surface states on germanium in ultrahigh vacuum
SURFACE
SCIENCE
THE EFFECT SURFACE
13 (1969) 393-400 0 North-Holland
OF THERMAL
STATES
JERZY
DESORPTION
ON GERMANIUM
SOCHANSKI*
Publishing Co., Amsterdam
OF WATER ON
IN ULTRAHIGH
and HARRY
VACUUM
C. GATOS
Department of Metallurgy and Materials Science Massachussett Institute of Technology Cambridge, Massachusetts 02139, U.S.A.
Received 12 August 1968 Large signal alternating current field effect experiments in the dark and under illumination were carried out on “real” germanium surfaces following a heat treatment in ultrahigh vacuum. After prolonged heating (about 10h at 520”K, the surfaces were slightly p-type, exhibiting 3 to 5 times higher density of traps than prior to heating. Higher temperatures (560”K, 600°K and 640°K) rendered the surfaces more p-type, but caused essentially no change in the density of traps. The observed changes of the electrical properties were correlated to the results of desorption experiments in which a mass spectrograph was employed. The total surface charge was found to increase linearly with the amount of desorbed water (2.5 x 1O-4 negative elementary charges per desorbed water molecule).
1. Introduction The origin and properties of the semiconductor surface states are still not completely understood although they have been extensively investigated in the last twenty years. The early studies were performed primarily on germanium and were directed to either atomically “clean” surfaces or “real” surfaces as commonly prepared. Subsequent studies attempted to methodically expose the “clean” surfaces to carefully controlled (idealized) ambients in order to understand the phenomena and processes associated with the transition from “clean” to “real” surfaces l). More recently a complementary approach has been taken whereby “real” surfaces are gradually submitted to cleaning processes in high vacuum while the changes in their properties are studiedz). It is this type of an approach that was adopted in the present investigation. Germanium surfaces were chosen as a starting point since there is by far more information available on these surfaces than on any others. * Visiting Scientist from the Institute of Physics, Polish Academy of Science, Warsaw, Poland. 393
394
J. SOCHANSKI
2. Experimental
AND
H. C. GATOS
techniques and procedures
The experiments were carried out in a vacuum system capable of achieving pressures down to 10-l’ mm Hg and equipped with a mass spectrometer and appropriate “feed-throughs” for electrical measurements. Desorption experiments were performed by heating the germanium samples while under high vacuum and monitoring the desorbed gases with the mass spectrometer. With the present experimental arrangement it was found impossible to perform continuous analysis for all the gases being desorbed. Since water was found to be main constituent of the desorbed gases it was decided to monitor quantitatively only the desorbed water (main peak, m/e = 18). The electrical surface characteristics were determined by sinusoidal (60 c/s) field effect experiments in the dark and under illumination. Such experiments allowed the simultaneous determination of the surface conductance and the surface recombination velocity as a function of the capacitively applied electric fields). For these measurements the sample was mounted on a specially designed holder which allowed precise adjustment of the spacing between the sample and the field electrode. With a sample-field plate capacitance of the order of 30 to 50 pF/cm’ and a voltage amplitude exceeding 1000 V it was possible to induce surface charges up to about 4 x lo’* elementary charges per cm’. The desorption and electrical measurements were not carried out simultaneously and on the identical samples. However, the samples employed for both types of measurements were taken from the same single crystal of germanium; their surfaces were prepared by the same techniques and were submitted to the same high vacuum heat treatment processes. The samples were cut from single crystal high resistivity (about 50 ohmcm) n-type germanium. Their large faces were of the (111) orientation. All samples were mechanically polished, then etched in CP-4 and finally etched in boiling 30% H,O,. Before employing them in the experiments the samples were kept in the room atmosphere for several days to achieve a steady state chemical configuration. Prior to the measurements the samples were mounted in the vacuum apparatus which was then evacuated to 10e7 - 10e8 mm Hg and baked out at 520°K for about 10 hours. Subsequently the system was pumped down to about 3 x lo-” mm Hg. Desorption was brought about by resistively heating the samples in the high vacuum. The desired temperature was achieved within a few seconds. The sample resistance served as a measure of the temperature. Both current and voltage across the sample were displayed on an oscilloscope. The samples were held at the desired temperature for one minute or longer and then
THERMAL
DESORPTIOb
OF WATER
ON Ge
SURFACES
395
cooled down to room temperature. This cooling took approximately one hour. The samples could subsequently be heated at higher temperatures. The samples employed for electrical characterization were placed in the large metal chamber (about 200 liters) of the system. Because of the large pumping speed of the system (about 2000 liters per set) ultrahigh vacuum was maintained during the desorption process. The actual measurements were carried out after cooling to room temperature. Chopped light required for the surface recombjnation measurements was introduced into the chamber through a glass window. The samples employed for determining the extent of desorption were pfaced in a small chamber attached to the mass spectrometer and connected to the main chamber with a cylinder 4.5 cm diameter and 20 cm long. Due to the high pumping speed (pumping time was less than 0.05 set) and low rate of desorption (time constant of the order of one minute), the partial pressure of the gases as determined with the mass spectrometer was proportional to the desorption rate. The total amount of desorbed gases could be evaluated from the partial pressure versus time curve and the conductance of the cyiinder. 3. Results Measurements of the extent of desorption were performed on three different samples all of which gave similar results. Typical data obtained
1.0
2.0
4.0
Time of Heating imin 1
Fig. 1, Number of desorbed water molecules as a function of time.
396
J. SGCHANSKI
AND
H. C. GATOS
are shown in fig. 1, Apparently the desorption kinetics can be expressed by: Ndes= a log t + b
(1)
where Ndes is the number of desorbed molecules, t is the time and a and b are constants for a given temperature. Five different samples were employed in the field effect experiment and here again all gave similar results. Typical field effect data obtained in the dark are shown in figs. 2 and 3. Surface recombination velocities as a function of surface potential at different stages of the desorption process are shown in fig. 4. The values of surface potential were calculated assuming that current carrier mobiIity was reduced by diffusive surface scattering3). It can be seen (fig. 2) that the slope of the trapped surface charge versus surface potential is significantly higher in the case of the heat treated surfaces. This slope is essentially the same for the various stages of desorption (fig. 2). With each consecutive heat treatment the surface becomes more r
Fig. 2.
Trapped charge as a function of barrier height:
fl before bake out, vacuum 1O-7 mm Hg. In all following cases the measurements were @after 0 after ‘(I after v after
performed under a vacuum of 3 10 h of bake out heating for 1 min at 560°K heating for 7 min at 560°K heating for 30 min at 560°K.
x
to-11 mm Hg.
397
Fig. 3.
Fig. 4.
Trapped charge + after heating 0 after heating @after heating Oafter heating wafter heating v after heating
Dependence
as a function of barrier height
at 560°K (from fig. 2f for for for for for
1 min 6 min 1 min 3 min 8 min
of surface recombination
at at at at at
600°K 600°K 640°K 640°K 640°K.
velocity on surface potential.
p-type as is clearly seen in fig. 3. The surface recombination velocity increases during the initial stages of heating at 56U”K (fig. 41, but decreases upon heating at higher temperatures (600% and 640°K). It was not possible to determine the parameters of the recombination centers since upon heating
398
J. SOCHANSKI
AND
H. C. GATOS
the surface became strongly p-type and the capacitively applied field could no longer shift the surface potential to the regions where the surface recombination velocity becomes maximum. It is well known that the total surface trapped changed can be unambiguously evaluated in a field effect experiment only when a minimum in surface conductance can be reached. This was the case for the experiments summarized in fig. 2. No minimum in conductance, however, could be achieved after heating at 600°K and 640°K. The results shown in fig. 3 for U, < -4 were evaluated by assuming that the experimental data for the 560°K heat treatment of fig. 2 could be extrapolated to more negative values of surface potential. Thus, by integrating the experimentally obtained quantities dQ,,/dV, versus V,, the uncertainty of the charge trapped in the slow surface states (resulting from the absence of a minimum in the surface conductance) was approximated and the results of fig. 3 (for US< -4) were obtained. In the absence of externally applied electric field the charge in slow surface the charge the fast surface states Q,, and space charge Q,, are states, Q,,, related as follows:
Q,,= Qfs+ Qsc.
(2)
Assuming that QfS is equal to trapped surface charge, Q,,, determined in the present experiments than from eq. (2), Q,, was determined at different
Number of Desorbed Molecules of H,O/cm*
Fig. 5.
Induced surface charge as a function of the amount of desorbed water.
THERMALDESORPTIONOFWATERON
stages of the desorption
process
Ge
399
SURFACES
since Q,, is a single valued
function
of U,
as defined by the Poisson equation. The results obtained are plotted in fig. 5. It is seen that each water molecule contributes 2.5 x 10e4 negative charge to the slow surface states. The total amount of desorbed water (8 x 1Ol5 molecules/cm2) corresponds to approximately 10 monolayers. The activation energy for desorption determined from the results of fig. 1 was found to be 0.15 eV. This value is in the range usually attributed to chemical reactions. 4. Discussion The results of the present investigation are in part in agreement with those of Rzhanov and coworkerss). In both cases the density of germanium surface traps was found to increase considerably upon heating in vacuum. Furthermore the maximum in surface recombination velocity (whenever a maximum could be observed) was found to increase after heat treatment. The activation energy for the desorption process was found also to be similar in the two cases (of the order of 0.15 eV). In the present study, after the first heat treatment at 520°K (or at any higher temperature), the density of surface traps did not undergo further changes when submitted to subsequent heat treatments, as it did in the studies of Rzhanov. Unlike Rzhanov’s finging, it was presently found that with each consecutive heat treatment the surfaces became more p-type. Consequently, after heating at 600” and 640°K no minimum in conductance and maximum in recombination velocity could be reached. These differences are most likely associated with some differences in the two experimental techniques employed. Thus, in this study the experimental system, including the samples, were baked out at 520°K for many hours, so that physically adsorbed species were removed and the surface film (oxide) on the samples was stabilized. This treatment is likely responsible for observing no change in the density in surface traps after the first heat treatment at 560°K. The high vacuum employed in this study (about 3 x lo-l1 mm Hg) and the absence of a spacer between the sample and the field electrode made readsorption of desorbed species unlikely. Such was not the case in Rzhanov’s experiments where the vacuum was of the order of 10e7 mm Hg and a mica spacer was employed which is known to adsorb tenaciously molecular and ionic species. Charged species taken up by mica during the desorption process would tend to maintain the surface potential of the sample relatively unchanges. These differences in experimental conditions can account for the present finding that the samples became more p-type with each consecutive heating. No such behavior was observed in Rzhanov’s experiments.
400
J. SOCHANSKI
AND H. C. GATOS
The total amount of desorbed water following the bake-out step was found to be rather extensive corresponding to about 10 mono-molecular layers; it was found to desorb at a very slow (logarithmic) rate. These results suggest that the desorbed water does not originate in the outermost surface layer. It seems likely that it diffuses to the surface through the oxide film where it is in some combination with germanium oxide (GeO,.xH,O). The details of the diffusion process and the reasons for the observation that each desorbed water molecule induces only 2.5 x 10e4 negative elementary charges on the surface are not clear at present.
Acknowledgement This work was supported Nonr-3963 (05).
by the Office of Naval Research
under Contract
References 1) See for example, D. R. Frank& Electrical Properties of Semiconductor Surfaces (Pergamon Press, New York, 1967). 2) For a summary and particularly for an account of the work of Rzhanov and coworkers see: A. Many, Y. Goldstein and N. B. Grover, Semiconductor Surfaces (NorthHolland Publishing Company, Amsterdam, 1965). 3) Y. Goldstein, N. B. Grover and A. Many, J. Applied Phys. 32 (1961) 2540.
SCIENCE
THE EFFECT SURFACE
13 (1969) 393-400 0 North-Holland
OF THERMAL
STATES
JERZY
DESORPTION
ON GERMANIUM
SOCHANSKI*
Publishing Co., Amsterdam
OF WATER ON
IN ULTRAHIGH
and HARRY
VACUUM
C. GATOS
Department of Metallurgy and Materials Science Massachussett Institute of Technology Cambridge, Massachusetts 02139, U.S.A.
Received 12 August 1968 Large signal alternating current field effect experiments in the dark and under illumination were carried out on “real” germanium surfaces following a heat treatment in ultrahigh vacuum. After prolonged heating (about 10h at 520”K, the surfaces were slightly p-type, exhibiting 3 to 5 times higher density of traps than prior to heating. Higher temperatures (560”K, 600°K and 640°K) rendered the surfaces more p-type, but caused essentially no change in the density of traps. The observed changes of the electrical properties were correlated to the results of desorption experiments in which a mass spectrograph was employed. The total surface charge was found to increase linearly with the amount of desorbed water (2.5 x 1O-4 negative elementary charges per desorbed water molecule).
1. Introduction The origin and properties of the semiconductor surface states are still not completely understood although they have been extensively investigated in the last twenty years. The early studies were performed primarily on germanium and were directed to either atomically “clean” surfaces or “real” surfaces as commonly prepared. Subsequent studies attempted to methodically expose the “clean” surfaces to carefully controlled (idealized) ambients in order to understand the phenomena and processes associated with the transition from “clean” to “real” surfaces l). More recently a complementary approach has been taken whereby “real” surfaces are gradually submitted to cleaning processes in high vacuum while the changes in their properties are studiedz). It is this type of an approach that was adopted in the present investigation. Germanium surfaces were chosen as a starting point since there is by far more information available on these surfaces than on any others. * Visiting Scientist from the Institute of Physics, Polish Academy of Science, Warsaw, Poland. 393
394
J. SOCHANSKI
2. Experimental
AND
H. C. GATOS
techniques and procedures
The experiments were carried out in a vacuum system capable of achieving pressures down to 10-l’ mm Hg and equipped with a mass spectrometer and appropriate “feed-throughs” for electrical measurements. Desorption experiments were performed by heating the germanium samples while under high vacuum and monitoring the desorbed gases with the mass spectrometer. With the present experimental arrangement it was found impossible to perform continuous analysis for all the gases being desorbed. Since water was found to be main constituent of the desorbed gases it was decided to monitor quantitatively only the desorbed water (main peak, m/e = 18). The electrical surface characteristics were determined by sinusoidal (60 c/s) field effect experiments in the dark and under illumination. Such experiments allowed the simultaneous determination of the surface conductance and the surface recombination velocity as a function of the capacitively applied electric fields). For these measurements the sample was mounted on a specially designed holder which allowed precise adjustment of the spacing between the sample and the field electrode. With a sample-field plate capacitance of the order of 30 to 50 pF/cm’ and a voltage amplitude exceeding 1000 V it was possible to induce surface charges up to about 4 x lo’* elementary charges per cm’. The desorption and electrical measurements were not carried out simultaneously and on the identical samples. However, the samples employed for both types of measurements were taken from the same single crystal of germanium; their surfaces were prepared by the same techniques and were submitted to the same high vacuum heat treatment processes. The samples were cut from single crystal high resistivity (about 50 ohmcm) n-type germanium. Their large faces were of the (111) orientation. All samples were mechanically polished, then etched in CP-4 and finally etched in boiling 30% H,O,. Before employing them in the experiments the samples were kept in the room atmosphere for several days to achieve a steady state chemical configuration. Prior to the measurements the samples were mounted in the vacuum apparatus which was then evacuated to 10e7 - 10e8 mm Hg and baked out at 520°K for about 10 hours. Subsequently the system was pumped down to about 3 x lo-” mm Hg. Desorption was brought about by resistively heating the samples in the high vacuum. The desired temperature was achieved within a few seconds. The sample resistance served as a measure of the temperature. Both current and voltage across the sample were displayed on an oscilloscope. The samples were held at the desired temperature for one minute or longer and then
THERMAL
DESORPTIOb
OF WATER
ON Ge
SURFACES
395
cooled down to room temperature. This cooling took approximately one hour. The samples could subsequently be heated at higher temperatures. The samples employed for electrical characterization were placed in the large metal chamber (about 200 liters) of the system. Because of the large pumping speed of the system (about 2000 liters per set) ultrahigh vacuum was maintained during the desorption process. The actual measurements were carried out after cooling to room temperature. Chopped light required for the surface recombjnation measurements was introduced into the chamber through a glass window. The samples employed for determining the extent of desorption were pfaced in a small chamber attached to the mass spectrometer and connected to the main chamber with a cylinder 4.5 cm diameter and 20 cm long. Due to the high pumping speed (pumping time was less than 0.05 set) and low rate of desorption (time constant of the order of one minute), the partial pressure of the gases as determined with the mass spectrometer was proportional to the desorption rate. The total amount of desorbed gases could be evaluated from the partial pressure versus time curve and the conductance of the cyiinder. 3. Results Measurements of the extent of desorption were performed on three different samples all of which gave similar results. Typical data obtained
1.0
2.0
4.0
Time of Heating imin 1
Fig. 1, Number of desorbed water molecules as a function of time.
396
J. SGCHANSKI
AND
H. C. GATOS
are shown in fig. 1, Apparently the desorption kinetics can be expressed by: Ndes= a log t + b
(1)
where Ndes is the number of desorbed molecules, t is the time and a and b are constants for a given temperature. Five different samples were employed in the field effect experiment and here again all gave similar results. Typical field effect data obtained in the dark are shown in figs. 2 and 3. Surface recombination velocities as a function of surface potential at different stages of the desorption process are shown in fig. 4. The values of surface potential were calculated assuming that current carrier mobiIity was reduced by diffusive surface scattering3). It can be seen (fig. 2) that the slope of the trapped surface charge versus surface potential is significantly higher in the case of the heat treated surfaces. This slope is essentially the same for the various stages of desorption (fig. 2). With each consecutive heat treatment the surface becomes more r
Fig. 2.
Trapped charge as a function of barrier height:
fl before bake out, vacuum 1O-7 mm Hg. In all following cases the measurements were @after 0 after ‘(I after v after
performed under a vacuum of 3 10 h of bake out heating for 1 min at 560°K heating for 7 min at 560°K heating for 30 min at 560°K.
x
to-11 mm Hg.
397
Fig. 3.
Fig. 4.
Trapped charge + after heating 0 after heating @after heating Oafter heating wafter heating v after heating
Dependence
as a function of barrier height
at 560°K (from fig. 2f for for for for for
1 min 6 min 1 min 3 min 8 min
of surface recombination
at at at at at
600°K 600°K 640°K 640°K 640°K.
velocity on surface potential.
p-type as is clearly seen in fig. 3. The surface recombination velocity increases during the initial stages of heating at 56U”K (fig. 41, but decreases upon heating at higher temperatures (600% and 640°K). It was not possible to determine the parameters of the recombination centers since upon heating
398
J. SOCHANSKI
AND
H. C. GATOS
the surface became strongly p-type and the capacitively applied field could no longer shift the surface potential to the regions where the surface recombination velocity becomes maximum. It is well known that the total surface trapped changed can be unambiguously evaluated in a field effect experiment only when a minimum in surface conductance can be reached. This was the case for the experiments summarized in fig. 2. No minimum in conductance, however, could be achieved after heating at 600°K and 640°K. The results shown in fig. 3 for U, < -4 were evaluated by assuming that the experimental data for the 560°K heat treatment of fig. 2 could be extrapolated to more negative values of surface potential. Thus, by integrating the experimentally obtained quantities dQ,,/dV, versus V,, the uncertainty of the charge trapped in the slow surface states (resulting from the absence of a minimum in the surface conductance) was approximated and the results of fig. 3 (for US< -4) were obtained. In the absence of externally applied electric field the charge in slow surface the charge the fast surface states Q,, and space charge Q,, are states, Q,,, related as follows:
Q,,= Qfs+ Qsc.
(2)
Assuming that QfS is equal to trapped surface charge, Q,,, determined in the present experiments than from eq. (2), Q,, was determined at different
Number of Desorbed Molecules of H,O/cm*
Fig. 5.
Induced surface charge as a function of the amount of desorbed water.
THERMALDESORPTIONOFWATERON
stages of the desorption
process
Ge
399
SURFACES
since Q,, is a single valued
function
of U,
as defined by the Poisson equation. The results obtained are plotted in fig. 5. It is seen that each water molecule contributes 2.5 x 10e4 negative charge to the slow surface states. The total amount of desorbed water (8 x 1Ol5 molecules/cm2) corresponds to approximately 10 monolayers. The activation energy for desorption determined from the results of fig. 1 was found to be 0.15 eV. This value is in the range usually attributed to chemical reactions. 4. Discussion The results of the present investigation are in part in agreement with those of Rzhanov and coworkerss). In both cases the density of germanium surface traps was found to increase considerably upon heating in vacuum. Furthermore the maximum in surface recombination velocity (whenever a maximum could be observed) was found to increase after heat treatment. The activation energy for the desorption process was found also to be similar in the two cases (of the order of 0.15 eV). In the present study, after the first heat treatment at 520°K (or at any higher temperature), the density of surface traps did not undergo further changes when submitted to subsequent heat treatments, as it did in the studies of Rzhanov. Unlike Rzhanov’s finging, it was presently found that with each consecutive heat treatment the surfaces became more p-type. Consequently, after heating at 600” and 640°K no minimum in conductance and maximum in recombination velocity could be reached. These differences are most likely associated with some differences in the two experimental techniques employed. Thus, in this study the experimental system, including the samples, were baked out at 520°K for many hours, so that physically adsorbed species were removed and the surface film (oxide) on the samples was stabilized. This treatment is likely responsible for observing no change in the density in surface traps after the first heat treatment at 560°K. The high vacuum employed in this study (about 3 x lo-l1 mm Hg) and the absence of a spacer between the sample and the field electrode made readsorption of desorbed species unlikely. Such was not the case in Rzhanov’s experiments where the vacuum was of the order of 10e7 mm Hg and a mica spacer was employed which is known to adsorb tenaciously molecular and ionic species. Charged species taken up by mica during the desorption process would tend to maintain the surface potential of the sample relatively unchanges. These differences in experimental conditions can account for the present finding that the samples became more p-type with each consecutive heating. No such behavior was observed in Rzhanov’s experiments.
400
J. SOCHANSKI
AND H. C. GATOS
The total amount of desorbed water following the bake-out step was found to be rather extensive corresponding to about 10 mono-molecular layers; it was found to desorb at a very slow (logarithmic) rate. These results suggest that the desorbed water does not originate in the outermost surface layer. It seems likely that it diffuses to the surface through the oxide film where it is in some combination with germanium oxide (GeO,.xH,O). The details of the diffusion process and the reasons for the observation that each desorbed water molecule induces only 2.5 x 10e4 negative elementary charges on the surface are not clear at present.
Acknowledgement This work was supported Nonr-3963 (05).
by the Office of Naval Research
under Contract
References 1) See for example, D. R. Frank& Electrical Properties of Semiconductor Surfaces (Pergamon Press, New York, 1967). 2) For a summary and particularly for an account of the work of Rzhanov and coworkers see: A. Many, Y. Goldstein and N. B. Grover, Semiconductor Surfaces (NorthHolland Publishing Company, Amsterdam, 1965). 3) Y. Goldstein, N. B. Grover and A. Many, J. Applied Phys. 32 (1961) 2540.