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Characterization of electron trapping defects on silicon by scanning tunneling microscopy
Surface Science 181 (1987) 333-339 North-Holland, Amsterdam
333
CHARACTERIZATION OF ELECTRON TRAPPING DEFECTS ON SILICON BY SCANNING TUNNELING MICROSCOPY R.H. KOCH IBM
and R.J. HAMERS
Thomas J. Watson Research Center, Yorktown
Heights, NY IO.598, USA
Received 15 July 1986; accepted for publication 25 September 1986
We have used scanning tunneling microscopy to locate and characterize electron trapping defects on in-situ oxidized Si(100) surfaces. When the tunneling tip is held stationary over a trap, the tunnel current switches between two well defined values. By changing the voltage on the tip we can establish the location of the trap in the tunneling direction and the trapping energies.
Following the initial observation of electron trapping using scanning tunneling microscopy (STM) [l] of Welland and Koch [2], we have made a more thorough study on atomically clean surfaces using an STM consisting of three orthogonally mounted transducers with the sample mounted on an adjustable foot [3]. Well characterized and clean samples were oxidized in-situ resulting in cleaner and thinner oxide layers when compared to previous work [2]. The oxidation and all measurements were made at room temperature. We measured the variance of the tunnel current while scanning to characterize noise on the surface. With the tip held stationary over a single trapping site, time traces of the tunnel current digitally switching were recorded. Most of the trapping and un-trapping times were found to be Poisson distributed and analysis of these times allowed us to measure the activation and state energy of the traps. Fig. 1 is a topograph of an oxidized Si(100) surface. A clean surface, that displayed the 2 x 1 reconstruction of Si(lOO), was oxidized by exposing the silicon to molecular 0, at room temperature to produce approximately one monolayer of oxygen on the surface. The exposure was for about 20 s at 3 x 10e7 Torr. The circular shaped “depressions” in the surface seen in the topograph are characteristic of all the oxidized surfaces we measured. The clean surfaces measured before the oxidation, while rough in some places, did not show these type of circular defects. These “depressions” were - 10 A across and - 5 A deep. The depth of the “depressions” were not correlated with the “depression” area or the specific scan. We do not know whether the “depressions” or the rest of the surface is the more highly oxidized region of the sample. The scanning direction was from left to right on this figure, and 0039-6028/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
334
Fig. 1. Topology
R. H. Koch, R.J. Hamers
/ Electron
rrqpng
defects on SI
of oxidized Si(100) surface. The area scanned was 65 X 65 2 range was 10 p\. There were 100 x 100 pixels in the scan.
and the gray-scale
several tip instabilities, which are thought to occur when the configuration of the tip changes during scanning, can be seen on the topograph. These instabilities become much more prevalent after oxidation, possibly because both substrate and tip are exposed. The number of instabilities decrease with time as the tip is scanned. In order to study the surface dependent noise, we waited until the instabilities almost disappeared, typically about 1 h after the exposure. Fig. 2 is a scan of the variance of the tunnel current versus position on an oxidized surface. The variance scan is taken simultaneously with the topological scan. At each individual pixel position, the tip is stopped and 10 samples of the instantaneous tunnel current, measured at the output of the tip preamplifier, and 10 samples of the normal feedback voltage are measured with a A/D converter at a rate of 4 kHz. The feedback voltage is averaged and the variance of the 10 samples of the tunneling current is computed. Four types of noise are present. The sources identified are from system noise, room noise, tip instabilities, and, what we are interested in, surface related noise. Fig. 2 shows large spikes of noise that were recorded over a background of smaller fluctuations in the baseline of the variance image. The smaller fluctuations are caused by the STM system noise that is independent of the tip position and dominated by electronic noise from the preamplifier and the
R. H. Koch, R.J. Hamers / Electron trapping defects on Si
Fig. 2. Variance scan of oxidized surface. The area scanned was 65 x 65 A2.
feedback circuits. The spectrum of this noise is flat with sharp lines at frequencies of 60 Hz and its harmonics and wider lines associated with resonances in the microscope. On fig. 2, the large spike in the bottom center of the image represents a single pixel of the scan having a much larger than normal value for the variance. This is an external noise pulse not produced by the microscope that most likely occurred when the microscope responded to an external vibration. These two types of noise, system and room, are seen on all surfaces, but the following types of noise, tip instabilities are surface related, and primarily seen on oxidized surfaces. Although not present in fig. 2, tip instabilities often are seen in variance scans. A tip instability will show up as a large variance increase on a single scan line and not on previous and subsequent scans. Not all tip instabilities show up in the variance scan. The large spikes in the right center of fig. 2 result from surface specific noise. These are large increases in the variance of the tunnel current that occur on repeated scans across the surface at the same place. The size of the increase
336
R.H. Koch, R.J. Humers / Electron trupping defects on 5.1
in variance depends on the voltage of the tip during the scan and the trapping energy of the electronic state. The patch of noisy variance values in the lower left of fig. 2 is a trap that is not switching rapidly at the tip voltage selected during the scan. The typical variance scan on an oxidized surface may show 1 to 5 large peaks associated with surface trapping states. The peaks in the variance scan from surface specific noise usually occurred around the edge of the “depression” seen on the topographs, which suggests that unsatisfied bonds of oxygen atoms near the edge of the oxidized region may be responsible for the trapping defects. On a clean Si(100) surface these variance peaks can occasionally be seen (1 peak per 10 scans) and probably result from a defect or impurity on the surface. Large topological steps and features do not in themselves produce structure in the variance scan. When the microscope is stopped from scanning and the tip is held over a location on the surface where large surface dependent noise was seen, the instantaneous tunneling current versus time is often observed to be very noisy as shown in fig. 3. These time traces were recorded using an A/D converter at a rate of 20 kHz, after low pass filtering at 20 kHz. The computer can move the tip in sub-A sized steps under manual control, and only when the tip was directly over the defect position was this noise seen. The time traces shows a digital switching behavior characteristic of a two-level system [4]. The duty cycle of the tunnel current is very voltage dependent, as seen in fig. 3. At low voltages the tunnel current is high with occasional fast pulses to lower values. Here the trap is mostly empty and occasionally capturing an electron and quickly releasing it, which produces the downward going spikes. As the voltage is increased the trap on average becomes more occupied so the duty cycle changes. At high voltages the trap is filled most of the time and the current is mainly low, with occasional quick pulses to much higher values as the electron quickly leaves and re-enters the trap. The average current in both cases for all tip voltages was fixed by the feedback circuit to be 2 nA. Fig. 4 plots the time constant of the filling and emptying times. These values of the time constant were obtained by making a histogram of the time the current was at the upper value and a histogram for the lower value at each value of applied tip voltage. The probability of an up or down pulse with a given time duration was fit to a Poisson distribution and the time constant was thus obtained. The measured distributions in most cases fit the Poisson distribution very well. If the trap is nearer the silicon than to the tip, as usually is the case, increasing the positive voltages on the tip lowers the energy of the trapped electron, and hence decreases the time for the trap to fill, as seen in the upper part of fig. 4. At the same time this increases the average time the electron takes to leave the trap, as shown by the upward sloping curve in the lower part of fig. 4. The observed behavior can be understood as a single electron trapping state on the silicon surface. An electron hops in and out of the state on the surface.
RX
Koch, R..J. Hamers / Electron trapping dejects on Si
,
6.9,
1.1 ’ 0.0
337
1
I
, 0.01
0.02
0.03
0.04
0.05
Time (s)
Fig. 3. Time traces of tunneling current for six values of applied tip voltage, V,+,, when the tip was positioned over a trap on the surface. The average tunneling current was set to be 2 nA, and the y-scale is in nA.
When the trapping state is occupied, the tunneling probability of all the other electrons (which make up essentially all of the measured tunnel current) is strongly suppressed by the Coulomb repulsion of the trapped electron. The spatial extent of the suppression is roughly the distance from the silicon to the tip, say 5 A, and the smallest measured width of the variance noise is consistent with this value. This contrasts with the earlier work [2] where the oxide was 1.5 to 2 nm thick and the width of the noise peak was about 3 nm. Larger widths may be observed when the trapping sites are clustered together on the surface. When the instantaneous current value is low the electron is occupying the trap and when the electron leaves the trap, the total tunnel current jumps up rapidly. An electron re-entering the trap will immediately suppress the tunneling current. If the switching times are faster than the
338
2.5
3.0
3.5
4.0
4.5
Voltage on f&l WI
Fig. 4. Time constant for trap filling (up time constant) function of tip voltage. Instrumental limitations prevent
and emptying (down time constant) a.\ a resolving times much faster than 10 ~’ s
feedback response time, the tunnel current will reflect the occupation of the trap. We have no direct experimental measure of the number of hopping electrons involved, but this data is consistent with a single electron changing its state. We have also observed more complicated trapping behavior which may represent multiple electronic transitions. The measured voltage dependence of the trapping and un-trapping rates can be understood with a single electron trapping model and can be used to extract the state and activation energies. Define E,“, the state energy, as the energy of the occupied state with no tip voltage applied. Let E: and Eg be the activation energies for the electron to fill and empty the state at zero applied tip voltage. If the state is nearer the silicon than to the tip the filling constant is rr =‘ra exp(E,/k,T) where r0 is the attempt time, typically 10P9 to lOPI2 s (see below), and E, is the activation energy with tip voltage applied, E, = EE - aeVtlP. a is the (electrical) distance fraction the trapping state is across total tunnel barrier. For surface related states this fraction is typically 0.1. The time to empty the trap is rE = r0 exp( E,/k,T). E, is the activation energy to empty the state with applied voltage, E, = Ez + aeV&,. Using the data of fig. 4, we have determined the average value of (Y to be s. When the filling and emptying time 0.14 + 0.04 and T” to be lo-i0 constants are equal (at about 3.6 V in fig. 4) the state is at the Fermi energy of the silicon and E,! is easily obtained using E, = E,” - aeV& = 0. The measured energies are E’ = 0.49 + 0.05 V, EE = 0.69 f 0.05 eV, and Eg = 0.25 f 0.05 eV. For a trap near the silicon, it is easy to show that the sum of E,” and
R. H. Koch, R.J. Hamers / Electron trapping defects on Si
339
EE must be equal to Ei. Because the measured values of Ei and Eg depend on To and Es0 does not, this relation can be used to determine the value of TV. Most of the determined values of ~a are nearer to lo-l2 s than to lop9 s. This is a very simplified model of the dynamics and a better model would be specific as to the type of transition occurring. We have done this for about 20 traps. We have only verified the voltage dependence of these relations because at this time it is not feasible to vary the temperature over a wide range while keeping the tip above the exact same spot on the surface. The activation energies we have determined are similar to those found by other methods for trapping sites on the Si/SiO, interface [5]. Besides thermally activated hopping, electrons in the silicon can enter states in the oxide via elastic tunneling, but these times would be far more rapid that observed here and have a different voltage dependence. The requirement that a trapping or un-trapping electron mount an atomic configuration barrier slows the process down enough to allow measurement with essentia!ly audio electronics. The voltage dependence and size of the current fluctuations, their spatial localization on the surface, and the area over which they are seen together are strong evidence that we have observed electron capture and emission from surface traps. It is unlikely that the switching seen arises from an configuration change in the atomic positions without a corresponding electronic transition because of the large voltage dependence observed and that fact that oxygen atoms bind very strongly to the silicon surface. The exact atomic detail of the trapping sites remains to be determined. The activation energy is most probably that required for a local atomic rearrangement to occur as the electron enters the state [6]. Polaron or image charge related energies may also be important in determining the various energies of the trapping process. The tremendous potential of the scanning tunneling microscope gives us hope that these atomic rearrangements associated with electron trapping may someday by observed. The authors thank Joe Demuth, Sokrates Pantelides, Jim Stathis, and Mark Welland for useful suggestions and encouragement.
References [l] G. BiMig, H. Rohrer, Ch. Gerber and W. Weibel, Phys. Rev. Letters 50 (1983) 120. [2] M.E. Welland and R.H. Koch, Appl. Phys. Letters 48 (1986) 724. [3] J.E. Demuth, R.J. Hamers, R. Tromp and M.E. Welland, J. Vacuum Sci. Technol. A4 (1986) 1320. [4] K.S. Rails, W.J. Skocpol, L.D. Jackel, R.E. Howard, L.A. Fetter, R.W. Epworth and D.M. Tennant, Phys. Rev. Letters 52 (1984) 228. [S] M.J. Kirton and M.J. Uren, Appl. Phys. Letters 48 (1986) 1270; D.K. Biegelsen, N.M. Johnson, M. Stutzmann, E.H. Poindexter and P.J. Caplan. Appl. Surface Sci. 22/23 (1985) 879. [6] R.D. Black, P.J. Snow and M.B. Weissman, Phys. Rev. Letters 28 (1983) 1935.
333
CHARACTERIZATION OF ELECTRON TRAPPING DEFECTS ON SILICON BY SCANNING TUNNELING MICROSCOPY R.H. KOCH IBM
and R.J. HAMERS
Thomas J. Watson Research Center, Yorktown
Heights, NY IO.598, USA
Received 15 July 1986; accepted for publication 25 September 1986
We have used scanning tunneling microscopy to locate and characterize electron trapping defects on in-situ oxidized Si(100) surfaces. When the tunneling tip is held stationary over a trap, the tunnel current switches between two well defined values. By changing the voltage on the tip we can establish the location of the trap in the tunneling direction and the trapping energies.
Following the initial observation of electron trapping using scanning tunneling microscopy (STM) [l] of Welland and Koch [2], we have made a more thorough study on atomically clean surfaces using an STM consisting of three orthogonally mounted transducers with the sample mounted on an adjustable foot [3]. Well characterized and clean samples were oxidized in-situ resulting in cleaner and thinner oxide layers when compared to previous work [2]. The oxidation and all measurements were made at room temperature. We measured the variance of the tunnel current while scanning to characterize noise on the surface. With the tip held stationary over a single trapping site, time traces of the tunnel current digitally switching were recorded. Most of the trapping and un-trapping times were found to be Poisson distributed and analysis of these times allowed us to measure the activation and state energy of the traps. Fig. 1 is a topograph of an oxidized Si(100) surface. A clean surface, that displayed the 2 x 1 reconstruction of Si(lOO), was oxidized by exposing the silicon to molecular 0, at room temperature to produce approximately one monolayer of oxygen on the surface. The exposure was for about 20 s at 3 x 10e7 Torr. The circular shaped “depressions” in the surface seen in the topograph are characteristic of all the oxidized surfaces we measured. The clean surfaces measured before the oxidation, while rough in some places, did not show these type of circular defects. These “depressions” were - 10 A across and - 5 A deep. The depth of the “depressions” were not correlated with the “depression” area or the specific scan. We do not know whether the “depressions” or the rest of the surface is the more highly oxidized region of the sample. The scanning direction was from left to right on this figure, and 0039-6028/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
334
Fig. 1. Topology
R. H. Koch, R.J. Hamers
/ Electron
rrqpng
defects on SI
of oxidized Si(100) surface. The area scanned was 65 X 65 2 range was 10 p\. There were 100 x 100 pixels in the scan.
and the gray-scale
several tip instabilities, which are thought to occur when the configuration of the tip changes during scanning, can be seen on the topograph. These instabilities become much more prevalent after oxidation, possibly because both substrate and tip are exposed. The number of instabilities decrease with time as the tip is scanned. In order to study the surface dependent noise, we waited until the instabilities almost disappeared, typically about 1 h after the exposure. Fig. 2 is a scan of the variance of the tunnel current versus position on an oxidized surface. The variance scan is taken simultaneously with the topological scan. At each individual pixel position, the tip is stopped and 10 samples of the instantaneous tunnel current, measured at the output of the tip preamplifier, and 10 samples of the normal feedback voltage are measured with a A/D converter at a rate of 4 kHz. The feedback voltage is averaged and the variance of the 10 samples of the tunneling current is computed. Four types of noise are present. The sources identified are from system noise, room noise, tip instabilities, and, what we are interested in, surface related noise. Fig. 2 shows large spikes of noise that were recorded over a background of smaller fluctuations in the baseline of the variance image. The smaller fluctuations are caused by the STM system noise that is independent of the tip position and dominated by electronic noise from the preamplifier and the
R. H. Koch, R.J. Hamers / Electron trapping defects on Si
Fig. 2. Variance scan of oxidized surface. The area scanned was 65 x 65 A2.
feedback circuits. The spectrum of this noise is flat with sharp lines at frequencies of 60 Hz and its harmonics and wider lines associated with resonances in the microscope. On fig. 2, the large spike in the bottom center of the image represents a single pixel of the scan having a much larger than normal value for the variance. This is an external noise pulse not produced by the microscope that most likely occurred when the microscope responded to an external vibration. These two types of noise, system and room, are seen on all surfaces, but the following types of noise, tip instabilities are surface related, and primarily seen on oxidized surfaces. Although not present in fig. 2, tip instabilities often are seen in variance scans. A tip instability will show up as a large variance increase on a single scan line and not on previous and subsequent scans. Not all tip instabilities show up in the variance scan. The large spikes in the right center of fig. 2 result from surface specific noise. These are large increases in the variance of the tunnel current that occur on repeated scans across the surface at the same place. The size of the increase
336
R.H. Koch, R.J. Humers / Electron trupping defects on 5.1
in variance depends on the voltage of the tip during the scan and the trapping energy of the electronic state. The patch of noisy variance values in the lower left of fig. 2 is a trap that is not switching rapidly at the tip voltage selected during the scan. The typical variance scan on an oxidized surface may show 1 to 5 large peaks associated with surface trapping states. The peaks in the variance scan from surface specific noise usually occurred around the edge of the “depression” seen on the topographs, which suggests that unsatisfied bonds of oxygen atoms near the edge of the oxidized region may be responsible for the trapping defects. On a clean Si(100) surface these variance peaks can occasionally be seen (1 peak per 10 scans) and probably result from a defect or impurity on the surface. Large topological steps and features do not in themselves produce structure in the variance scan. When the microscope is stopped from scanning and the tip is held over a location on the surface where large surface dependent noise was seen, the instantaneous tunneling current versus time is often observed to be very noisy as shown in fig. 3. These time traces were recorded using an A/D converter at a rate of 20 kHz, after low pass filtering at 20 kHz. The computer can move the tip in sub-A sized steps under manual control, and only when the tip was directly over the defect position was this noise seen. The time traces shows a digital switching behavior characteristic of a two-level system [4]. The duty cycle of the tunnel current is very voltage dependent, as seen in fig. 3. At low voltages the tunnel current is high with occasional fast pulses to lower values. Here the trap is mostly empty and occasionally capturing an electron and quickly releasing it, which produces the downward going spikes. As the voltage is increased the trap on average becomes more occupied so the duty cycle changes. At high voltages the trap is filled most of the time and the current is mainly low, with occasional quick pulses to much higher values as the electron quickly leaves and re-enters the trap. The average current in both cases for all tip voltages was fixed by the feedback circuit to be 2 nA. Fig. 4 plots the time constant of the filling and emptying times. These values of the time constant were obtained by making a histogram of the time the current was at the upper value and a histogram for the lower value at each value of applied tip voltage. The probability of an up or down pulse with a given time duration was fit to a Poisson distribution and the time constant was thus obtained. The measured distributions in most cases fit the Poisson distribution very well. If the trap is nearer the silicon than to the tip, as usually is the case, increasing the positive voltages on the tip lowers the energy of the trapped electron, and hence decreases the time for the trap to fill, as seen in the upper part of fig. 4. At the same time this increases the average time the electron takes to leave the trap, as shown by the upward sloping curve in the lower part of fig. 4. The observed behavior can be understood as a single electron trapping state on the silicon surface. An electron hops in and out of the state on the surface.
RX
Koch, R..J. Hamers / Electron trapping dejects on Si
,
6.9,
1.1 ’ 0.0
337
1
I
, 0.01
0.02
0.03
0.04
0.05
Time (s)
Fig. 3. Time traces of tunneling current for six values of applied tip voltage, V,+,, when the tip was positioned over a trap on the surface. The average tunneling current was set to be 2 nA, and the y-scale is in nA.
When the trapping state is occupied, the tunneling probability of all the other electrons (which make up essentially all of the measured tunnel current) is strongly suppressed by the Coulomb repulsion of the trapped electron. The spatial extent of the suppression is roughly the distance from the silicon to the tip, say 5 A, and the smallest measured width of the variance noise is consistent with this value. This contrasts with the earlier work [2] where the oxide was 1.5 to 2 nm thick and the width of the noise peak was about 3 nm. Larger widths may be observed when the trapping sites are clustered together on the surface. When the instantaneous current value is low the electron is occupying the trap and when the electron leaves the trap, the total tunnel current jumps up rapidly. An electron re-entering the trap will immediately suppress the tunneling current. If the switching times are faster than the
338
2.5
3.0
3.5
4.0
4.5
Voltage on f&l WI
Fig. 4. Time constant for trap filling (up time constant) function of tip voltage. Instrumental limitations prevent
and emptying (down time constant) a.\ a resolving times much faster than 10 ~’ s
feedback response time, the tunnel current will reflect the occupation of the trap. We have no direct experimental measure of the number of hopping electrons involved, but this data is consistent with a single electron changing its state. We have also observed more complicated trapping behavior which may represent multiple electronic transitions. The measured voltage dependence of the trapping and un-trapping rates can be understood with a single electron trapping model and can be used to extract the state and activation energies. Define E,“, the state energy, as the energy of the occupied state with no tip voltage applied. Let E: and Eg be the activation energies for the electron to fill and empty the state at zero applied tip voltage. If the state is nearer the silicon than to the tip the filling constant is rr =‘ra exp(E,/k,T) where r0 is the attempt time, typically 10P9 to lOPI2 s (see below), and E, is the activation energy with tip voltage applied, E, = EE - aeVtlP. a is the (electrical) distance fraction the trapping state is across total tunnel barrier. For surface related states this fraction is typically 0.1. The time to empty the trap is rE = r0 exp( E,/k,T). E, is the activation energy to empty the state with applied voltage, E, = Ez + aeV&,. Using the data of fig. 4, we have determined the average value of (Y to be s. When the filling and emptying time 0.14 + 0.04 and T” to be lo-i0 constants are equal (at about 3.6 V in fig. 4) the state is at the Fermi energy of the silicon and E,! is easily obtained using E, = E,” - aeV& = 0. The measured energies are E’ = 0.49 + 0.05 V, EE = 0.69 f 0.05 eV, and Eg = 0.25 f 0.05 eV. For a trap near the silicon, it is easy to show that the sum of E,” and
R. H. Koch, R.J. Hamers / Electron trapping defects on Si
339
EE must be equal to Ei. Because the measured values of Ei and Eg depend on To and Es0 does not, this relation can be used to determine the value of TV. Most of the determined values of ~a are nearer to lo-l2 s than to lop9 s. This is a very simplified model of the dynamics and a better model would be specific as to the type of transition occurring. We have done this for about 20 traps. We have only verified the voltage dependence of these relations because at this time it is not feasible to vary the temperature over a wide range while keeping the tip above the exact same spot on the surface. The activation energies we have determined are similar to those found by other methods for trapping sites on the Si/SiO, interface [5]. Besides thermally activated hopping, electrons in the silicon can enter states in the oxide via elastic tunneling, but these times would be far more rapid that observed here and have a different voltage dependence. The requirement that a trapping or un-trapping electron mount an atomic configuration barrier slows the process down enough to allow measurement with essentia!ly audio electronics. The voltage dependence and size of the current fluctuations, their spatial localization on the surface, and the area over which they are seen together are strong evidence that we have observed electron capture and emission from surface traps. It is unlikely that the switching seen arises from an configuration change in the atomic positions without a corresponding electronic transition because of the large voltage dependence observed and that fact that oxygen atoms bind very strongly to the silicon surface. The exact atomic detail of the trapping sites remains to be determined. The activation energy is most probably that required for a local atomic rearrangement to occur as the electron enters the state [6]. Polaron or image charge related energies may also be important in determining the various energies of the trapping process. The tremendous potential of the scanning tunneling microscope gives us hope that these atomic rearrangements associated with electron trapping may someday by observed. The authors thank Joe Demuth, Sokrates Pantelides, Jim Stathis, and Mark Welland for useful suggestions and encouragement.
References [l] G. BiMig, H. Rohrer, Ch. Gerber and W. Weibel, Phys. Rev. Letters 50 (1983) 120. [2] M.E. Welland and R.H. Koch, Appl. Phys. Letters 48 (1986) 724. [3] J.E. Demuth, R.J. Hamers, R. Tromp and M.E. Welland, J. Vacuum Sci. Technol. A4 (1986) 1320. [4] K.S. Rails, W.J. Skocpol, L.D. Jackel, R.E. Howard, L.A. Fetter, R.W. Epworth and D.M. Tennant, Phys. Rev. Letters 52 (1984) 228. [S] M.J. Kirton and M.J. Uren, Appl. Phys. Letters 48 (1986) 1270; D.K. Biegelsen, N.M. Johnson, M. Stutzmann, E.H. Poindexter and P.J. Caplan. Appl. Surface Sci. 22/23 (1985) 879. [6] R.D. Black, P.J. Snow and M.B. Weissman, Phys. Rev. Letters 28 (1983) 1935.