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Competition between desorption and diffusion at a GaAs(110) surface
Volume 203, number 2,3
CHEMICAL PHYSICS LETTERS
19 February 1993
Competition between desorption and diffusion at a GaAs ( 110) surface A. vom Felde *, C.C. Bahr and M.J. Cardillo AT&T Bell Laboratories, Murray Hill, NJ 07974, USA Received 6 October 1992; in final form 23 November 1992
We directly demonstrate for NOr on GaAs( 1lo), using time-resolved modulated molecular beam techniques, that desorption is a competitive rate process with surface diffusion. Using defects as markers, we observe a transition from defect-dominated low coverage chemistry to defect insensitive adsorption/desorption near room temperature. We argue that this competition is generic to adsorbates on semiconductors, in distinct contrast to adsorbates on metal surfaces where diffusion lengths are often large ( z 10’A) for almost all accessible temperatures.
1. Introduction The diffusion of atoms and molecules at a crystal surface is a central dynamical step in surface chemistry and new materials development. There are several techniques which have traditionally contributed to our knowledge of transport at surfaces [ 11. The more recent exploitation of current fluctuations in field emission microscopy [ 21, laser-induced desorption [ 3,4 ], and Fourier transform infrared [ 5 ] techniques have greatly extended the range of times and nature of materials that can be studied. One important goal of this research is to characterize the associated experimental activation energies and to relate them to the energy barriers which determine the topography of the adsorbate surface interaction potential. Of comparable importance is the characterization of surface diffusion as a rate process, which may often be in competition with other parallel rates such as reaction or desorption. Recently, a result which related the competition between desorption and diffusion was demonstrated, and argued to be generic to smooth metal surfaces, based on the analysis of strong non-linearities in desorption kinetics at low coverage [ 61, It was specifically shown for the case of NO on Pt ( 111) ’ Siemens AG, ZFE/SPT22, Munich 83, Germany.
104
Otto-Hahn-Ring 6, W-8000
that surface diffusion on close-packed metal surfaces is sufficiently fast that all defects are sampled prior to desorption, up to the highest of surface temperatures. In simplest terms, the determining parameter in this competition is the ratio of the energy barrier to surface diffusion to the energy of desorption, which ranges from 0.1-0.2 for a great variety of systems. One important consequence of this result is that at sufficiently low covcragc, chemistry on close-packed metal surfaces is totally dominated by defects and steps, as these features are generally of higher binding energy than the single crystal terrace sites, and are easily accessed. In this Letter, we demonstrate a dramatically contrasting situation for the case of NO* at the GaAs ( 1IO) surface. Near room temperature a transition occurs in the probability that an adsorbate (NO,) finds a defect prior to desorption. Alternatively stated, desorption effectively competes with diffusion to defects at relatively low temperatures. This transition, from defect-dominated to defect-insensitive low coverage surface chemistry, occurs over the temperature range 250-350 K. This transition occurs because of the corrugated nature of the GaAs( 110) surface which leads to high barriers to surface diffusion. Inasmuch as all semiconductor surfaces are expected to be significantly more corrugated than close-packed metal surfaces, we argue that this competition in rate processes enters imElsevier Science Publishers B.V.
Volume 203,
CHEMICAL PHYSICS LETTERS
number 2,3
portantly into all semiconductor growth and surface
processing kinetics. Specifically, molecular beam techniques are used to show that at low coverage defects dominate the surface chemistry for NOz on GaAs( 110) at T,=250 K. The time- and angle-resolved scattering distributions show that the NO2 sticking probability is close to unity and the mean residence times x 1O-' to 10e3 s over the temperature range explored. Yet the onset of molecular NO2 desorption is delayed by several seconds, which corresponds to the time for which a small fraction (r lo-‘) of the surface has been oxidized by the dissociative deposition of 0. We argue, in analogy to NO on Pt ( 111) [ 61, that this temporal delay is due to the requirement that nearly all adsorbates sample defects at this temperature and thus the defects must be nearly saturated prior to the onset of desorption. Here, the role of defects is confirmed by creating additional defects through lightly sputtering without annealing and observing an extension of the delay time for the onset of desorption. At T,= 250 K the initial desorption rate during the delay period is almost zero. However, as the surface temperature is raised from 250-350 K, the initial desorption rate at zero coverage increases, i.e. the delay in desorption applies to only a fraction of the incident NO* ( z f ). Thus a substantial fraction of the NO, desorbs without having sampled defects. Increasing the temperature to T,= 450 K at z zero coverage, the initial desorption rate is approximately the same as the steady-state value at long time, i.e. desorption compared tively
has
become
the
to diffusion,
with diffusion
dominant
in that
rate
it competes
process, effec-
to defects.
2. Experimental The experiments are conducted in an ultra-highvacuum (UHV) molecular beam scattering apparatus which has been described in detail previously [ 7 1. The molecular beam source is a 100 nm aperture in a heatable Pt nozzle. For these experiments the nozzle is operated at room temperature at a driving pressure of x 2000 Torr. The reactive NO* gas is diluted to 1% in Ar in order to keep the surface oxidation at a low rate t and to reduce dimerization of
19 February la93
NO* in the beam). The scattered or desorbed NO2 is detected by a rotatable differentially pumped quadrupole mass spectrometer (QMS) operated in pulse mode. The ionizer of the QMS is located 5.5 cm away from the crystal surface with an angular acceptance of t- 2”. The beam is square-wave modulated at typically 90 Hz. The time-resolved waveforms are collected digitally by a multichannel analyzer.,In order to approach the limit of zero coverage, most of the experiments have been carried out with averaging of no more than 100 waveforms corresponding to a total beam exposure time of = 1 s at an incident flux corresponding to 0.01 ML/s (monolayer/s). At such low exposures the statistics for the scattered waveforms are poor. However, by integrating the waveforms and subtracting the total of background counts, we are able to routinely monitor the desorption signal at most angles for exposures as low as 0.01 ML. The high resistance (p= lo7 n cm) GaAs ( 110) crystal, 5.5 mm by 11 mm in size and 0.38 mm thick, supplied by Nimic Corp., Japan, was polished by grinding under a flow of trimethylbromine until about 100 pm was removed from the surface. The sample was cleaned only a few times by Ar+-sputtering after mounting in UHV. Radiative heating of the sample up to 850 K for 10 min often proved to be sufficient to regain a clear surface after exposure to N02, The surface chemical composition, including the Ga/As ratio, was routinely checked with AES, and the surface crystallinity was monitored using He specular scattering. The crystal was cleaned prior to each set of data we show. The build-up of oxygen on the surface due to dissociation of NO2 was quantitatively monitored by means of specular He attenuation after elemental confirmation by AES. The He scattering cross section for oxygen atoms adsorbed on GaAs( 110) was calibrated previously using an oxygen atom source [ 81. For our incident beam conditions [9] we determined an 0 adatom scattering cross section for attenuation of the specular He beam of 400 A’.
2. Results In fig. 1 we show polar plots of the evolution of the angular distribution of NOz, coming from the 105
Volume 203, number 2,3
I9 February 1993
CHEMICAL PHYSICS LETTERS
O0
/-YR -90" Fig. 1. Polar plot of the angular distribution for emission of NOa from the GaAs( 110) surface for different beam exposure in monolayer (ML). The incident flux corresponds to 0.01 ML/s. The crystal temperature T,=250 K, the angleofincidence 6i=70°.
GaAs( 110) surface, with increasing time of exposure. The angle of incidence (0,) of the incoming NOz is 70”. The scattered NO, has been monitored at a series of angles (0,): -2O”, O”, 30”, 50”, 65”, 70”, 75”, and 80” measured from the surface normal. As can be seen in fig. I, during the first 1.0 s of exposure, corresponding to = 1.3~ 10e2 ML of NOz, only a small amount of quasi-specularly scattered NOz molecules evolves with no detectable desorbing NO2 molecules. This small amount of initially scattered NOz corresponds to less than 1% of the incident beam flux and indicates that most ( N 99%) NOz initially sticks to the surface and further that it disappears. In the course of a few seconds of additional exposure the desorption signal develops. The small scattering contribution broadens and can no longer be observed in the rapidly growing desorption signal. We note that the mean residence time for NOz, which we discuss in detail elsewhere [9], over the range of temperatures we consider is typically of the order of x lo-’ to 10e3 s. Thus the delay of the order of seconds in the onset of NO, desorption is not to be associated with the intrinsic residence time of NOz on the single crystal GaAs( 110) surface. In fig. 2 (circles) we show the evolution of the desorption signal with increasing exposure for a fixed angle of reflection (&=O), i.e. a cut along the normal in fig. 1, together with the developing oxygen coverage as a function of exposure. The oxygen coverage was determined separately by measuring the He attenuation after a series of exposures to the NO,/ 106
0.2
0.4 NO, exposure
0.6 (ML)
Fig. 2. The emission of NOa at t7,=0° (see fig. I ) versus NO2 exposure at T,=250 K. The open circles (0) correspond to a sputtered and annealed clean surface. The squares (0 ) correspond to a surface which has been sputtered, annealed, and then resputtered at 250 K with a dose of 2.9X 10” Ar+/cm* (~0.66 Ar+/surface unit cell). The two curves saturate outside the range of this graph, at 2-3 ML. The upper panel shows the estimated oxygen coverage taken for a series of exposures corresponding to the NO1 fluxes plotted below.
Ar beam, confirmed by AES. From a comparison of these two curves, as well as with the NOz angular distributions, we conclude that by the time the NOz desorption distribution accounts for most of the incident NO*, i.e. the shape becomes cos 6’type, the rate of oxygen deposition has nearly saturated or slowed considerably, i.e. dissociation of NOz has decreased to a low steady-state probability (of the order of 2%3% at several ML exposure as determined in separate experiments [ 9 ] ). In figs. 1 and 2 the sudden onset of desorption corresponds to a rapid transition between two extremes, from 100% dissociation at very low exposures to a very small ( x 3%) dissociation probability after moderate exposures. In fig. 2 we also plot the temporal evolution of the NOz desorption signal for the same GaAs( 110) surface after having been sputtered with x2.9X lOI Ar+ ion/cm’ (x0.66 Ar+/ surface unit cell) without annealing. The delay in the evolution of the NOz desorption rate is displaced toward higher exposures, compared to that obtained from the annealed (ordered) surface. This directly implies that NOz dissociation occurs at defects, such
Volume203, number 2,3
CHEMICAL.PHYSICS LETTERS
as those caused by (unannealed)
sputter damage, and
that the NOz dissociation product (0 atoms) inhibits further NOz dissociation by blocking defect sites. We note here that we have also varied the sputter dose (at 350 K) and have observed a continual increase in the delay for desorption. In summary we note that at T,=250 K, NOz has a nearly unit sticking probability and that at zero coverage all adsorbed NO2 find defects and dissociate. As the defects fill with oxygen atoms we observe the onset of NO2 desorption, which based on the angular distribution indicates approximately unit sticking probability. This sequence of experiments has been carried out at different surface temperatures, the results of which are plotted in fig. 3. In contrast to the data at T,= 250 K, where the initial desorption rate is ~0, at T,= 350 K the initial NOz desorption rate is about 4 of its steady state value, and at T,=450 K the initial desorption rate is nearly the same as the steady-state value. We interpret this change as an effective competition between the desorption rate and the rate for diffusion to defects. Under most circumstances, this competition is governed by the relative activation energies for desorption and diffusion, Edes and EdiKeAS discussed by Gomer [ 11, if we describe desorption and dif-
Annealed
01.““““” 0.0
0.4
0.2 NO,
exposure
0.6
’
(monolayers)
Fig. 3. The NO1 desorption signal versus exposure,as plotted in fig. 2, for three surface temperatures, r, ( (0 ) T,=250 K, ( X ) TS=350 K; (A) T,=450 K). ei=70~ for each case. The relative signals are on the same scale although the absolute scale is arbitrary.
19 February 1993
fusion as simple thermally activated processes, we can simply compare their associated time scales. We write an inequality describing the probability P for finding a particle on the surface after a time t. The relevant time scales are &,, the residence time before desorption, and tdiff,the time after which we find the particle at mean squared distance (x2>, away from its origin. After cancelling terms [ I], we write
(1) where x/a is the mean diffusion length normalized to the mean jump distance, and v and E are the attempt frequencies and energy barriers for diffusion and desorption, respectively. We consider only the temperature dependence of P in eq. ( 1). The probability that the adsorbed NO* will react at a defect is found by normalizing the initial NO1 desorption flux and subtracting from one. We assume that the probability P of finding a defect is proportional to the reaction probability. The value of Edes-Edin determined in this way is 2.8 kcal/mol, in good agreement with our molecular beam scattering results [91. Based on our determination of Edes=9 kcal/mol in ref. [ 91, we estimate a diffusion energy barrier Ed,-+ 6 kcal/mol. These results are in distinct contrast to the behaviour of adsorbates on low Miller index surfaces of metals, where diffusion barriers are known to be small in general compared to heats of desorption (typically lo%-20%). The consequence of this small ratio for metals was shown to be very large migration distances, often of the order of 1O-3 cm, by adsorbates prior to desorption, for almost all accessible temperatures [ lo]. In contrast, the similar diffusion and desorption barriers as found in this experiment confine migration distances to be on the order of defect separations or less at room temperature and above. We suggest that this result should be quite general for semiconductor surfaces. Interatomic spacings at the low Miller index surfaces of semiconductors are considerably greater than on metals due to the different nature of the crystallographic lattices. In addition the electronic structure of semiconductor surfaces may be qualitatively described as less delocalized than at metals. The net result is that contours 107
Volume 203, number 2,3
CHEMICAL PHYSICS LETTERS
of constant interaction energy are very much more corrugated on semiconductors. This corrugation may be visualized at both van der Waals distances and at chemical bonding distances associated with higher electron densities, by considering the substrate electron densities alone as a crude approximation for the topography of the interaction energy contours, such as calculated in ref. [ 111. Thus for both weakly (physisorption) and strongly (chemisorption) interacting adsorbate systems, one expects a similar competition to arise between adparticle desorption and diffusion to specific sites on semiconductors. The consequences of this competition are of particular importance in the determination of growth and processing parameters for electronic materials. High substrate temperatures are often required to insure defect free epitaxial growth, and to have admolecules react at or adatoms diffuse to steps. These high temperatures are limited however by the requirements of maintaining the artificially highly structured layers of the material and narrow doping profiles. In effect the conclusion of this work is that an important additional aspect of high temperature processing is the competition from desorption which may kinetically restrict adparticles from reaching the appropriate growth or reaction sites.
108
19 February 1993
Acknowledgement The authors wish to acknowledge helpful discussions with J. Tully and expert technical assistance of E. Chaban.
References [ 11R. Gomer, Rept. Progr. Phys. 53 (1990) 917. [2] R. Gomer, Surface Sci. 38 (1973) 373. [ 31 SM. George, A. DeSantolo and R.B. Hall, Surface Sci. 159 (1985) L425. [ 41 G.A. Reider, U. Hofer and T.F. Heinz, Phys. Rev. Letters 66 (1991) 1994. [5] I.E. Reutt-Robey, D.S. Doren, Y.J. Chabal and S.B. Christman, Phys. Rev. Letters 61 (1988) 2883. [6] J.A. Serri, G. Becker and M.J. Cardillo, J. Chem. Phys. 77 (1982) 2175; J.A. Serri, J.C. Tully and M.J. Cardillo, J. Chem. Phys. 79 (1983) 1530. [ 71 M.J. Cardillo, C.C. Ching, E.F. Greene and G.E. Becker, J. Vacuum Sci. Technol. 15 ( 1978) 423. [8] A. vom Felde, K. Kern, G.S. Higashi, Y.J. Chabal, S.B. Christman, C.C. Bahr and M.J. Cardillo, Phys. Rev. B 42 ( 1990) 5240. [ 9 ] CC. Bahr, A. vom Felde and M.J. Cardillo, to be published. [lo] J.C. Tully, Surface Sci. II 1 (1981) 461. [ 1I ] D.R. Hamann, Phys. Rev. Letters 46 ( 1981) 1227.
CHEMICAL PHYSICS LETTERS
19 February 1993
Competition between desorption and diffusion at a GaAs ( 110) surface A. vom Felde *, C.C. Bahr and M.J. Cardillo AT&T Bell Laboratories, Murray Hill, NJ 07974, USA Received 6 October 1992; in final form 23 November 1992
We directly demonstrate for NOr on GaAs( 1lo), using time-resolved modulated molecular beam techniques, that desorption is a competitive rate process with surface diffusion. Using defects as markers, we observe a transition from defect-dominated low coverage chemistry to defect insensitive adsorption/desorption near room temperature. We argue that this competition is generic to adsorbates on semiconductors, in distinct contrast to adsorbates on metal surfaces where diffusion lengths are often large ( z 10’A) for almost all accessible temperatures.
1. Introduction The diffusion of atoms and molecules at a crystal surface is a central dynamical step in surface chemistry and new materials development. There are several techniques which have traditionally contributed to our knowledge of transport at surfaces [ 11. The more recent exploitation of current fluctuations in field emission microscopy [ 21, laser-induced desorption [ 3,4 ], and Fourier transform infrared [ 5 ] techniques have greatly extended the range of times and nature of materials that can be studied. One important goal of this research is to characterize the associated experimental activation energies and to relate them to the energy barriers which determine the topography of the adsorbate surface interaction potential. Of comparable importance is the characterization of surface diffusion as a rate process, which may often be in competition with other parallel rates such as reaction or desorption. Recently, a result which related the competition between desorption and diffusion was demonstrated, and argued to be generic to smooth metal surfaces, based on the analysis of strong non-linearities in desorption kinetics at low coverage [ 61, It was specifically shown for the case of NO on Pt ( 111) ’ Siemens AG, ZFE/SPT22, Munich 83, Germany.
104
Otto-Hahn-Ring 6, W-8000
that surface diffusion on close-packed metal surfaces is sufficiently fast that all defects are sampled prior to desorption, up to the highest of surface temperatures. In simplest terms, the determining parameter in this competition is the ratio of the energy barrier to surface diffusion to the energy of desorption, which ranges from 0.1-0.2 for a great variety of systems. One important consequence of this result is that at sufficiently low covcragc, chemistry on close-packed metal surfaces is totally dominated by defects and steps, as these features are generally of higher binding energy than the single crystal terrace sites, and are easily accessed. In this Letter, we demonstrate a dramatically contrasting situation for the case of NO* at the GaAs ( 1IO) surface. Near room temperature a transition occurs in the probability that an adsorbate (NO,) finds a defect prior to desorption. Alternatively stated, desorption effectively competes with diffusion to defects at relatively low temperatures. This transition, from defect-dominated to defect-insensitive low coverage surface chemistry, occurs over the temperature range 250-350 K. This transition occurs because of the corrugated nature of the GaAs( 110) surface which leads to high barriers to surface diffusion. Inasmuch as all semiconductor surfaces are expected to be significantly more corrugated than close-packed metal surfaces, we argue that this competition in rate processes enters imElsevier Science Publishers B.V.
Volume 203,
CHEMICAL PHYSICS LETTERS
number 2,3
portantly into all semiconductor growth and surface
processing kinetics. Specifically, molecular beam techniques are used to show that at low coverage defects dominate the surface chemistry for NOz on GaAs( 110) at T,=250 K. The time- and angle-resolved scattering distributions show that the NO2 sticking probability is close to unity and the mean residence times x 1O-' to 10e3 s over the temperature range explored. Yet the onset of molecular NO2 desorption is delayed by several seconds, which corresponds to the time for which a small fraction (r lo-‘) of the surface has been oxidized by the dissociative deposition of 0. We argue, in analogy to NO on Pt ( 111) [ 61, that this temporal delay is due to the requirement that nearly all adsorbates sample defects at this temperature and thus the defects must be nearly saturated prior to the onset of desorption. Here, the role of defects is confirmed by creating additional defects through lightly sputtering without annealing and observing an extension of the delay time for the onset of desorption. At T,= 250 K the initial desorption rate during the delay period is almost zero. However, as the surface temperature is raised from 250-350 K, the initial desorption rate at zero coverage increases, i.e. the delay in desorption applies to only a fraction of the incident NO* ( z f ). Thus a substantial fraction of the NO, desorbs without having sampled defects. Increasing the temperature to T,= 450 K at z zero coverage, the initial desorption rate is approximately the same as the steady-state value at long time, i.e. desorption compared tively
has
become
the
to diffusion,
with diffusion
dominant
in that
rate
it competes
process, effec-
to defects.
2. Experimental The experiments are conducted in an ultra-highvacuum (UHV) molecular beam scattering apparatus which has been described in detail previously [ 7 1. The molecular beam source is a 100 nm aperture in a heatable Pt nozzle. For these experiments the nozzle is operated at room temperature at a driving pressure of x 2000 Torr. The reactive NO* gas is diluted to 1% in Ar in order to keep the surface oxidation at a low rate t and to reduce dimerization of
19 February la93
NO* in the beam). The scattered or desorbed NO2 is detected by a rotatable differentially pumped quadrupole mass spectrometer (QMS) operated in pulse mode. The ionizer of the QMS is located 5.5 cm away from the crystal surface with an angular acceptance of t- 2”. The beam is square-wave modulated at typically 90 Hz. The time-resolved waveforms are collected digitally by a multichannel analyzer.,In order to approach the limit of zero coverage, most of the experiments have been carried out with averaging of no more than 100 waveforms corresponding to a total beam exposure time of = 1 s at an incident flux corresponding to 0.01 ML/s (monolayer/s). At such low exposures the statistics for the scattered waveforms are poor. However, by integrating the waveforms and subtracting the total of background counts, we are able to routinely monitor the desorption signal at most angles for exposures as low as 0.01 ML. The high resistance (p= lo7 n cm) GaAs ( 110) crystal, 5.5 mm by 11 mm in size and 0.38 mm thick, supplied by Nimic Corp., Japan, was polished by grinding under a flow of trimethylbromine until about 100 pm was removed from the surface. The sample was cleaned only a few times by Ar+-sputtering after mounting in UHV. Radiative heating of the sample up to 850 K for 10 min often proved to be sufficient to regain a clear surface after exposure to N02, The surface chemical composition, including the Ga/As ratio, was routinely checked with AES, and the surface crystallinity was monitored using He specular scattering. The crystal was cleaned prior to each set of data we show. The build-up of oxygen on the surface due to dissociation of NO2 was quantitatively monitored by means of specular He attenuation after elemental confirmation by AES. The He scattering cross section for oxygen atoms adsorbed on GaAs( 110) was calibrated previously using an oxygen atom source [ 81. For our incident beam conditions [9] we determined an 0 adatom scattering cross section for attenuation of the specular He beam of 400 A’.
2. Results In fig. 1 we show polar plots of the evolution of the angular distribution of NOz, coming from the 105
Volume 203, number 2,3
I9 February 1993
CHEMICAL PHYSICS LETTERS
O0
/-YR -90" Fig. 1. Polar plot of the angular distribution for emission of NOa from the GaAs( 110) surface for different beam exposure in monolayer (ML). The incident flux corresponds to 0.01 ML/s. The crystal temperature T,=250 K, the angleofincidence 6i=70°.
GaAs( 110) surface, with increasing time of exposure. The angle of incidence (0,) of the incoming NOz is 70”. The scattered NO, has been monitored at a series of angles (0,): -2O”, O”, 30”, 50”, 65”, 70”, 75”, and 80” measured from the surface normal. As can be seen in fig. I, during the first 1.0 s of exposure, corresponding to = 1.3~ 10e2 ML of NOz, only a small amount of quasi-specularly scattered NOz molecules evolves with no detectable desorbing NO2 molecules. This small amount of initially scattered NOz corresponds to less than 1% of the incident beam flux and indicates that most ( N 99%) NOz initially sticks to the surface and further that it disappears. In the course of a few seconds of additional exposure the desorption signal develops. The small scattering contribution broadens and can no longer be observed in the rapidly growing desorption signal. We note that the mean residence time for NOz, which we discuss in detail elsewhere [9], over the range of temperatures we consider is typically of the order of x lo-’ to 10e3 s. Thus the delay of the order of seconds in the onset of NO, desorption is not to be associated with the intrinsic residence time of NOz on the single crystal GaAs( 110) surface. In fig. 2 (circles) we show the evolution of the desorption signal with increasing exposure for a fixed angle of reflection (&=O), i.e. a cut along the normal in fig. 1, together with the developing oxygen coverage as a function of exposure. The oxygen coverage was determined separately by measuring the He attenuation after a series of exposures to the NO,/ 106
0.2
0.4 NO, exposure
0.6 (ML)
Fig. 2. The emission of NOa at t7,=0° (see fig. I ) versus NO2 exposure at T,=250 K. The open circles (0) correspond to a sputtered and annealed clean surface. The squares (0 ) correspond to a surface which has been sputtered, annealed, and then resputtered at 250 K with a dose of 2.9X 10” Ar+/cm* (~0.66 Ar+/surface unit cell). The two curves saturate outside the range of this graph, at 2-3 ML. The upper panel shows the estimated oxygen coverage taken for a series of exposures corresponding to the NO1 fluxes plotted below.
Ar beam, confirmed by AES. From a comparison of these two curves, as well as with the NOz angular distributions, we conclude that by the time the NOz desorption distribution accounts for most of the incident NO*, i.e. the shape becomes cos 6’type, the rate of oxygen deposition has nearly saturated or slowed considerably, i.e. dissociation of NOz has decreased to a low steady-state probability (of the order of 2%3% at several ML exposure as determined in separate experiments [ 9 ] ). In figs. 1 and 2 the sudden onset of desorption corresponds to a rapid transition between two extremes, from 100% dissociation at very low exposures to a very small ( x 3%) dissociation probability after moderate exposures. In fig. 2 we also plot the temporal evolution of the NOz desorption signal for the same GaAs( 110) surface after having been sputtered with x2.9X lOI Ar+ ion/cm’ (x0.66 Ar+/ surface unit cell) without annealing. The delay in the evolution of the NOz desorption rate is displaced toward higher exposures, compared to that obtained from the annealed (ordered) surface. This directly implies that NOz dissociation occurs at defects, such
Volume203, number 2,3
CHEMICAL.PHYSICS LETTERS
as those caused by (unannealed)
sputter damage, and
that the NOz dissociation product (0 atoms) inhibits further NOz dissociation by blocking defect sites. We note here that we have also varied the sputter dose (at 350 K) and have observed a continual increase in the delay for desorption. In summary we note that at T,=250 K, NOz has a nearly unit sticking probability and that at zero coverage all adsorbed NO2 find defects and dissociate. As the defects fill with oxygen atoms we observe the onset of NO2 desorption, which based on the angular distribution indicates approximately unit sticking probability. This sequence of experiments has been carried out at different surface temperatures, the results of which are plotted in fig. 3. In contrast to the data at T,= 250 K, where the initial desorption rate is ~0, at T,= 350 K the initial NOz desorption rate is about 4 of its steady state value, and at T,=450 K the initial desorption rate is nearly the same as the steady-state value. We interpret this change as an effective competition between the desorption rate and the rate for diffusion to defects. Under most circumstances, this competition is governed by the relative activation energies for desorption and diffusion, Edes and EdiKeAS discussed by Gomer [ 11, if we describe desorption and dif-
Annealed
01.““““” 0.0
0.4
0.2 NO,
exposure
0.6
’
(monolayers)
Fig. 3. The NO1 desorption signal versus exposure,as plotted in fig. 2, for three surface temperatures, r, ( (0 ) T,=250 K, ( X ) TS=350 K; (A) T,=450 K). ei=70~ for each case. The relative signals are on the same scale although the absolute scale is arbitrary.
19 February 1993
fusion as simple thermally activated processes, we can simply compare their associated time scales. We write an inequality describing the probability P for finding a particle on the surface after a time t. The relevant time scales are &,, the residence time before desorption, and tdiff,the time after which we find the particle at mean squared distance (x2>, away from its origin. After cancelling terms [ I], we write
(1) where x/a is the mean diffusion length normalized to the mean jump distance, and v and E are the attempt frequencies and energy barriers for diffusion and desorption, respectively. We consider only the temperature dependence of P in eq. ( 1). The probability that the adsorbed NO* will react at a defect is found by normalizing the initial NO1 desorption flux and subtracting from one. We assume that the probability P of finding a defect is proportional to the reaction probability. The value of Edes-Edin determined in this way is 2.8 kcal/mol, in good agreement with our molecular beam scattering results [91. Based on our determination of Edes=9 kcal/mol in ref. [ 91, we estimate a diffusion energy barrier Ed,-+ 6 kcal/mol. These results are in distinct contrast to the behaviour of adsorbates on low Miller index surfaces of metals, where diffusion barriers are known to be small in general compared to heats of desorption (typically lo%-20%). The consequence of this small ratio for metals was shown to be very large migration distances, often of the order of 1O-3 cm, by adsorbates prior to desorption, for almost all accessible temperatures [ lo]. In contrast, the similar diffusion and desorption barriers as found in this experiment confine migration distances to be on the order of defect separations or less at room temperature and above. We suggest that this result should be quite general for semiconductor surfaces. Interatomic spacings at the low Miller index surfaces of semiconductors are considerably greater than on metals due to the different nature of the crystallographic lattices. In addition the electronic structure of semiconductor surfaces may be qualitatively described as less delocalized than at metals. The net result is that contours 107
Volume 203, number 2,3
CHEMICAL PHYSICS LETTERS
of constant interaction energy are very much more corrugated on semiconductors. This corrugation may be visualized at both van der Waals distances and at chemical bonding distances associated with higher electron densities, by considering the substrate electron densities alone as a crude approximation for the topography of the interaction energy contours, such as calculated in ref. [ 111. Thus for both weakly (physisorption) and strongly (chemisorption) interacting adsorbate systems, one expects a similar competition to arise between adparticle desorption and diffusion to specific sites on semiconductors. The consequences of this competition are of particular importance in the determination of growth and processing parameters for electronic materials. High substrate temperatures are often required to insure defect free epitaxial growth, and to have admolecules react at or adatoms diffuse to steps. These high temperatures are limited however by the requirements of maintaining the artificially highly structured layers of the material and narrow doping profiles. In effect the conclusion of this work is that an important additional aspect of high temperature processing is the competition from desorption which may kinetically restrict adparticles from reaching the appropriate growth or reaction sites.
108
19 February 1993
Acknowledgement The authors wish to acknowledge helpful discussions with J. Tully and expert technical assistance of E. Chaban.
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