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Phosphorus vacancies and adatoms on GaP(110) surfaces studied by scanning tunneling microscopy
UItramicroscopy 49 (1993) 344-353 North-Holland
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Phosphorus vacancies and adatoms on GaP(110) surfaces studied by scanning tunneling microscopy Ph. E b e r t a n d K. U r b a n Institut )=firFestk6rperforschung, Forschungszentrum Jiilich GmbH, Postfach 1913, W-5170 Jiilich, Germany Received 12 July 1992; at Editorial Office 24 August 1992
Atomic defects were investigated on p-doped G a P ( l l 0 ) surfaces by scanning tunneling microscopy. The defects were identified as positively charged phosphorus vacancies and uncharged phosphorus adatoms. They are found to migrate on the phosphorus sublattice. Their motion was observed with atomic resolution and the dependence of the j u m p probability on the tunneling voltage was measured. The position changes of the defects were found to be induced by tip-specimen interactions.
1. Introduction Controlled by thermodynamics, atomic defects are expected to occur on semiconductor surfaces in a similar manner as in the bulk. In addition, it is expected that such defects are formed by evaporation during high-temperature treatments or during reactions with adatoms. Also during cleavage of semiconductor samples, surfaces are created on which atomic defects are observed [1-3]. Scanning tunneling microscopy (STM) provides the only tool for studying individual defects on semiconductor surfaces with atomic resolution. In the present paper, we report on the observation of single atomic defects and their migration by STM on cleaved GaP surfaces. The experimental results are interpreted in terms of tipstimulated migration.
image of, e.g., an 8 x 8 nm 2 area every 8 s. Up to 180 frames were recorded in one individual run. The investigations were performed on clean (110) surfaces of Zn-doped, p-type, LEC-grown G a P obtained from MCP, London. The surfaces were prepared by in-situ cleavage along the [001] direction under ultra-high vacuum. The Zndopant concentration ranged between 1.7 × l(117 and 5.8 x 1017 cm s. All experiments were performed at room temperature at a pressure below 8 x 10 9 Pa. The images were recorded in the constant-current mode and displayed with a gray scale, which is proportional to the height of the tip. Thus, the values on the gray scale represent a measure of the height changes of the surface as well as of changes in the density of electronic surface states.
3. Static properties of atomic defects 2. Experiment
3.1. General
A particularly fast scanning mode of the scanning tunneling microscope was used for the investigation. Due to the small size and the high resonance frequency of the microscope, it was possible to acquire a complete constant-current
The images of the freshly cleaved surfaces revealed a very low number of terrace steps on the cleavage surfaces, which were essentially flat (fig. 1). In addition, a low density of atomic defects, occurring mainly in the vicinity of the
0304-3991/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
Ph. Ebert, I¢2 Urban / Phosphorus vacancies and adatoms on GaP(l lO)
terrace steps, was present. In fig. 1 two terraces are visible separated by a step one atomic layer high. Two different types of atomic defects can be recognized on the terraces. They display dark (A) or bright (B) contrast with respect to the background.
3.2. Results and discussion of type-A defects The type-A defects shown in fig. 2 were found inhomogeneously distributed with a highest area density close to 1011 cm -2 occurring near the steps. They were observed directly after cleavage of the sample as well as several days after cleavage with no change in their density. However, at elevated values of negative tunneling voltage, this type of defect could also be produced by scanning a particular surface area for a prolonged period. Fig. 2 shows a series of constant-current images of five type-A defects acquired at three
345
different negative tunneling voltages (the sample is at negative potential relative to the tip). From figs. 2a to 2c the values of the negative tunneling voltage decrease. All images show the density of occupied surface states localized at the phosphorus (P) atoms [1,4-7]. In fig. 2a, at a high negative tunneling voltage, the defects are imaged as small, up to 0.07 nm deep depressions. The lateral extension of the depressions depends on the tunneling voltage and increases with decreasing negative tunneling voltage. In fig. 2c the diameter of the depressions reaches about 4 nm. The dark areas as a whole do not originate from real geometrical depressions in the surface. The change of the lateral extension, rather, indicates a positive charge of the atomic defects [8,9]. A positive charge center induces a local downward band bending which reduces the number of occupied states near the valence-band maximum contributing to the tunnel current. In the con-
Fig. 1. Image of two terraces separated by a step one atomic layer high. Two different types of point defects, labeled A and B, can be recognized. The atomic rows along the [110] direction are separated by 0.545 nm.
346
Ph. Ebert, K. Urban / Phosphorus uacancies and adatoms on GaP( l lO)
Fig. 2. Series of images of type-A defects measured at different tunneling voltages. The voltages are (a) - 3.5 V, (b) - 2.7 V, and (c) - 2 . 4 V with a tunnel current of 0.4 nA. The corrugation of the unperturbed lattice is 0.05 nm along the [001] direction. The figurcs have the same crystallographic orientation as fig. 1.
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Ph. Ebert, K. Urban / Phosphorus vacancies and adatoms on GaP(l lO)
stant-current mode this results in a decrease of the tunnel current in those surface areas where band bending is present and the electronic feedback loop of the STM responds to this by reducing the tip-to-sample distance. At moderately low negative tunneling voltages the fraction of states is large, which cannot contribute to the current due to band bending. This leads to a deep and wide depression (fig. 2c). With increasing negative tunneling voltage, more surface states within a larger interval of energy in the valence band contribute to the tunnel current. Thus the fraction of states inhibited from participating in the tunnel process by band bending decreases and, consequently, a small and narrow depression is observed (fig. 2a). Close inspection of fig. 2a shows that the defect constrast consists of two basically different parts. The first is the charge-related depression already described, which is dependent on the tunneling voltage. The second is due to a "hole" which has a lateral dimension of one danglingbond state. It is localized at the position of a P atom along the [110] rows of the occupied P dangling-bond states. Only a single occupied dangling bond is missing. The neighboring dangling bonds are maintained. However, they appear darker as in unperturbed areas. Fig. 3 shows the height of the tunneling tip along the [1-10] direction on both sides of a defect. We recognize the atomic corrugation and the effect of the chargeinduced band bending. At the center of fig. 3 the deep additional depression representing the core of the atomic defect is visible. The slight asymmetry of the curves from left to right is due to a weak electric A C coupling of the height signal, used to compensate, to a certain extent, a uniform sample inclination that otherwise would produce a trivial but disturbing contrast gradient along the slope of the surface. Various possibilities have to be considered in order to explain the nature of the defects. We can rule out adsorbate atoms or molecules as being responsible for the observed image contrast. Occasionally observed adsorbates displayed a different contrast and their density increased with time, irrespective of whether the sample was scanned or not. We also rule out interstitial atoms,
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distance from defect (nm) Fig. 3. Height profiles along the [110] direction through one of the defects of fig. 2. The upper profile corresponds to fig. 2a, the middle to fig. 2b, and the lower to fig. 2c. The positioning of height 0 is arbitrary.
since the defects are centered on P dangling-bond locations. Neither can any type of antisite defects explain our observations of a positively charged defect on a P-sublattice site. The antisite defect Gap consists of a gallium (Ga) atom on a P-sublattice site, but it is not positively charged. Gap on surfaces has only three valence electrons in its neutral charge state, two less than the missing P atom. It can therefore act as an acceptor and not as a donor and is expected to be on p-doped G a P ( l l 0 ) surfaces either electrically neutral or negatively charged, in analogy to the Gaa~ antisite defect on GaAs(110) surfaces [10]. The reconstruction of the G a P ( l l 0 ) surface induces a charge transfer from the Ga to the P atoms resulting in empty dangling bonds above the Ga atoms and filled dangling bonds above the P atoms. Therefore, the structure of valences of the reconstructed surface is comparable with that of the bulk. Indeed, for Gap in the bulk also an acceptor character was found [11]. The antisite defect PGa consists of a P atom on a Ga site and is predicted to act only as a donor on surfaces [10]. Although it is expected to be positively charged, it cannot explain the nature of the observed defects, because PGa is localized on the Ga sublattice.
348
Ph. Ebert, K. Urban / Phosphorus z'acancies and adatoms on GaP(l lO)
Pig. 4. ~and-bending-induced elevations due to the negative charge originating from Zn-dopant atoms. Three positively charged defects (P vacancies) are visible as dark depressions. The image of an area of 130× 130 nm 2 was measured at 2.5 V tunneling voltage. The figure has the same crystallographic orientation as fig. 1.
Zn-dopant atoms can also be excluded because they are negatively charged and are localized on the Ga sublattice. In fact, we observed in other STM images elevations about 5 nm in diameter (fig. 4) which can be explained as due to band bending induced by negatively charged centers as expected for Zn-dopant atoms. Their area density was measured as 2 × l01~ cm -2 in agreement with the dopant concentration. Although the density of other impurities in the GaP crystals is not known, impurities are unlikely to be the defects imaged in fig. 2. They should occur uniformly distributed in the crystal and should not be produced by scanning. We only very rarely observed defects which can be attributed to impurity atoms. The possibility remains that the observed defects are Ga or P vacancies. Since only the P sublattice is imaged in fig. 2 they cannot be Ga vacancies. The following arguments can be developed to support the interpretation of the defects as P vacancies. In a simple P surface vacancy, one atom is missing in the surface layer. The dangling-bond
state localized at that P atom on the defect-free surface cannot exist. Instead, the bonds that were broken for the formation of the vacancy are expected to rehybridize and to develop one or more defect states. Spatially, these defect states lie below the surface, as the three broken bonds necessary for the vacancy formation are already deeper in the crystal than the neighboring dangling bonds. Therefore, the defect states themselves cannot be imaged with the STM because of the greater distance to the tip and the exponential dependence of the tunnel current on distance. Thus at negative voltages, where the dangling-bond states localized at the P atoms are imaged, the P vacancy should appear as a small hole with the lateral dimension of one dangling bond. The positive charge of the defects observed in the STM images allows us to conclude that at least one defect state is situated well above the valence-band edge and is not occupied. Since the crystal is p-doped, it can be expected that the states in the band gap are unoccupied. From the observation that the defects mainly occur near steps it can be concluded that they are formed by the fracture during the cleavage pro-
Fig. 5. Image of a type-13 detect (P adatom) measured at - 3 . 3 V tunneling voltage. The figure has the same crystallographic orientation as fig. 1.
Ph. Ebert, K. Urban / Phosphorus vacancies and adatoms on GaP(l lO)
cess. Thermal equilibrium defects quenched into the sample during cooling of the crystal from its preparation temperature should have no spatial relation to cleavage steps. The appearance and morphology of the defects in fig. 2 are very similar to that of P vacancies observed on the p-doped I n P ( l l 0 ) surface [3]. Here, too, the neighboring dangling bonds of the defects appeared darker and the defects were positively charged and occurred mainly near steps. Arsenic vacancies were also reported on p-doped G a A s ( l l 0 ) and were found to be positively charged [12].
349
3.3. Results and discussion of type-B defects Fig. 5 shows an image of a type-B defect in greater detail. It consists of an elevation extending over four P atoms. The vertical maximum lies in between these four P atoms and reaches a height of 0.1 nm above the surface layer. Type-B defects were only observed near steps where their density reached about 1012 c m - 2 . We found that the images were not dependent on the tunneling voltage. From this it can be concluded that the defects are uncharged.
Fig. 6. Sequences of constant-current images of three type-A defects (P vacancies) selected from a series extending over 24 rain. The defects changed their positions. The tunneling voltages ranged from - 3 . 1 to - 3 . 5 V. The tunnel current was 0.4 nA. The corrugation of the lattice was 0.05 nm. Each image was measured in 8 s. Scanning direction from the upper left corner to the lower right corner. The figures have the same crystallographic orientation as fig. 1.
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Ph. Ebert, K. Urban / Phosphorus t,acancies and adatoms on GaP(I lO)
The type-B defects cannot be P vacancies because their morphology is completely different from that of the vacancies in fig. 2. PGa antisite defects are expected to be positively charged and the Zn-dopant atoms as shown in fig. 4 are negatively charged. Adsorbates and impurities can again be excluded for the same reasons as mentioned above. We presume that the defect shown in fig. 5 consists of a single P adatom. A P adatom is expected to be bonded to a Ga surface atom, which is itself placed in between four P atoms because this would be its place in the next surface layer. Such a P adatom has three dangling bonds of which at least two can be easily recon-
structed since their distance to neighboring P dangling bonds is small. Upon reconstruction the dangling bonds form a new defect state which can explain the lateral extension of the defect in fig. 5 over four P dangling bonds. The view that the type-B defects are P adatoms is corroborated by the occasional observation that these defects, if localized close to a P vacancy, suddenly disappeared together with the vacancy. This can be considered as a recombination process. It can be understood that the type-B defects are only found near steps if it is assumed that, as discussed for the P vacancies, they were created in the cleavage process.
Fig. 7. Sequences of constant-current images of three type-B defects (P adatoms). The tunneling voltage ranged from - 2.6 to - 3.5 V. The tunnel current was 0.5 nA and the corrugation 0.015 nm. The images have been selected from a series extending over a time of 24 min. Scanning direction from the upper left corner to the lower right corner. The crystallographic orientation is rotated by 180 ° as compared to fig. 1.
Ph. Ebert, K. Urban / Phosphorus vacancies and adatoms on GaP(l lO)
We note that P adatoms can also be formed by interstitial P atoms created during the cleavage process in the bulk but sufficiently close to the surface so that, under the action of image forces, they were able to relax to the surface.
4. Defect migration
4.1. Obser~,ation of migration of P cacancies and P adatoms Both the P vacancies and the P adatoms were found to migrate on the surface. Fig. 6 shows images of P vacancies selected from a series measured over a total period of 24 min at tunneling voltages between - 3 . 1 and - 3 . 5 V. The defects changed their positions regularly between consecutive images. Similar effects were observed for the P adatoms at tunneling voltages in the range o f - 2 . 6 to - 3 . 5 V (fig. 7). Below these values no migration could be observed. At negative voltages of an absolute value higher than about - 4 V the surface was destroyed. The position changes could be measured quantitatively when two or more defects were visible simultaneously. If one defect changed its position while the others remained stable, these latter could be taken as a reference together with the atomically resolved lattice to determine the jump length and direction. This allows jumps produced by tip instabilities to be excluded, e.g., jumps of the tunneling tip state to another place on the tip. They should affect all defects simultaneously. A special characteristic of defect migration was the frequent observation of defects which were only partially imaged. These images can be explained by a position change of a defect occurring while the tip is crossing the defect. This results in a partially imaged defect since only that part of the defect is recorded which was scanned prior to the jump. The defect can be imaged again in the same STM image if it jumped into a position situated in the part of the STM image still to be scanned. For example in fig. 6a the defect marked by an arrow moves by one lattice spacing in [001] direction (fig. 6b). In fig. 6b a partially imaged defect (upper arrow) is visible
351
and just below the defect in its new position (lower arrow). If the defect moved in the opposite direction its new position cannot be imaged in the same image, but only in the following one. For example, in fig. 6g the partially imaged defect marked by an arrow moved in [110] direction to its new position visible in the following frame (fig. 6h).
4.2. Analysis of defect migration A statistical evaluation of the images containing defects which were only partially imaged shows that about 90% of all position changes of the P adatoms occurred at the moment when the tip was positioned in a scan line passing over the defect. This value can be compared with an estimate of the probability of obtaining an image with a partial defect, assuming jumps with no correlation to the position of the tip. This is equal to the ratio of the number of scan lines crossing the defect to the total number of scan lines per image and is about 15%. For the P vacancies the measured value was about 50% as compared to a calculated probability of only 10% without a correlation. This can be taken as an indication that the migration of the defects is tip-induced. The jumps which could not be directly observed can nevertheless be explained as induced by the tip if it is assumed that the jump was triggered just before or after the defect was scanned. The site changes were studied quantitatively by examining the distribution of the jump lengths. For this, the components of a given jump along the [110] and the [001] directions were measured and an effective jump length was calculated as the total number of lattice spacings along both directions. The data are presented in fig. 8. The number of observed jumps decreases with increasing jump length both for the P vacancies and for the P adatoms. In both cases the proportion of long jumps is high. We find that only jump lengths of multiples of lattice spacings occur. This is equivalent to the fact that the defects migrated only on the P sublattice. This holds for the P vacancies and for the P adatoms as well. The latter migrate on a set of adatom sites which are equivalent to the P sublattice of an extra GaP
352
Ph. Ebert, K. Urban / Phosphorus l,acancies and adatoms on GaP(l lO)
layer if this were added to the existing surface. We note that a jump to the next P atom position is not a jump to the nearest-neighboring atom position, which is occupied by a Ga atom. Measurements of the jump probability per image as a function of the tunneling voltage V were limited to relatively small usable tunneling voltage ranges ( - 3 . 5 < V < - 3 . 1 V for the P vacancies and - 3 . 5 < V < - 2 . 6 V for the P adatoms). At smaller absolute values of voltage the defects remained immobile and at larger values the surface was destroyed. The results are shown in fig. 9. The solid squares are the data points of jump components along the [001] direction while solid triangles are those along the [110] direction. The dashed line refers to the P vacancies and the solid line to the P adatoms. We find an exponential dependence of the jump probability from the tunneling voltage for both types of defects and no differences between the two selected directions. The results show that the migration of defects is connected to the presence of the tip. The tip induces a localized electric field of approximately 109 V / m due to the tunneling voltage and the small tip-to-surface separations. This field follows the movement of the tip, penetrates into the
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jump length n Fig. 8. Relative number of jumps as a function of the jump length n (number of lattice spacings) for type-A defects (P vacancies) (empty squares) and for type-B defects (P adatoms) (asterisks). N ( n ) is the number of jumps of length n. N is the number of scanned defects (568 adatoms and 411 vacancies).
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V (volt) Fig. 9. Dependence of the jump probability per image for type-A defects (P vacancies) (dashed line) and for type-B defects (P adatoms) (solid line) from the tunneling voltage V. Solid squares are the data measured in [001] direction and solid triangles in [130] directions (the two directions with the lowest crystallographic indices).
semiconductor and changes the potential of the surface. The tip-field gradient can in principle exert a force on the atoms strong enough to stimulate the jumps of the defects. An independent observation corroborating this interpretation is the effect of the corrugation amplitude on the migration. With a tip having a low resolution, i.e. a low corrugation amplitude, only the migration of the more easily migrating P adatoms was observed, while a tip with a high corrugation was necessary to induce migration of P vacancies. Tips with a better resolution induce a stronger lateral decay of the electric field since the decay is aproximately scaled by the effective tip radius [13]. Therefore, it is to be expected that a tip with a high corrugation exerts a stronger force on the defects than a tip with a low corrugation. Vacancy migration similar to that observed in our work was found for As vacancies on G a A s ( l l 0 ) surfaces [14]. In this case, too, jumps over more than one lattice spacing were observed and migration increased with tunneling voltage. Moreover, thermal adatom migration was reported for Pb adatoms on G e ( l l l ) 2 × 8 surfaces [151.
Ph. Ebert, I~ Urban / Phosphorus uacancies and adatoms on GaP(l lO)
5. Conclusions Atomic defects were studied on p-doped G a P ( l l 0 ) surfaces by scanning tunneling microscopy. From the observations it was concluded that the defects were positively charged P vacancies and uncharged P adatoms. The observation of the migration of single defects on the G a P ( l l 0 ) surface revealed that the defects only migrated on the P sublattice. A substantial number of jumps occurred over more than one lattice spacing. It was concluded that migration is tip-stimulated. The jump probability was found to be higher for P adatoms than for P vacancies. It increases with increasing negative tunneling voltage. No differences between the [110] and the [001] directions were observed.
Acknowledgements The authors would like to express their thanks to L. Koenders, PTB Braunschweig, Germany, for many helpful discussions and for providing the GaP samples and to K.H. G r a f for technical assistance.
353
References [1] L.J. Whitman, J.A. Stroscio, R.A. Dragoset and R.J. Celotta, J. Vac. Sci. Technol. B 9 (1991) 770. [2] G. Cox, K.H. Graf, D. Szynka, U. Poppe and K. Urban, Vacuum 41 (1990) 591. [3] Ph. Ebert, G. Cox, U. Poppe and K. Urban, Ultramicroscopy 42-44 (1992) 871. [4] R.M. Feenstra, J.A. Stroscio, J. Tersoff and A.P. Fein, Phys. Rev. Lett. 58 (1987) 1192. [5] Ph. Ebert, G. Cox, U. Poppe and K. Urban, Surf. Sci. 271 (1992) 587. [6] J.L.A. Alves, J. Hebenstreit and M. Scheffier, Phys. Rev. B 44 (1991) 6188. [7] L.J. Whitman, J.A. Stroscio, R.A. Dragoset and R.J. Celotta, Phys. Rev. B 42 (1990) 7288. [8] R.J. Hamers, J. Vac. Sci. Technol. B 6 (1988) 1462. [9] J.A. Stroscio, R.M. Feenstra and A.P. Fein, Phys. Rev. Lett. 58 (1987) 1668. [10] R.E. Allen and J.D. Dow, Phys. Rev. B 25 (1982) 1423. [11] R.W. Jansen and O.F. Sankey, Phys. Rev. B 39 (1989) 3192. [12] G. Cox, Ph. Ebert and K. Urban, Inst. Phys. Conf. Ser. 117 (1991) 347. [13] L.J. Whitman, J.A. Stroscio, R.A. Dragoset and R.J. Celotta, Science 251 (1991) 1206. [14] G. Cox, 1990, unpublished. [15] E. Ganz, S.K. Theiss, I.-S. Hwang and J. Golovchenko, Phys. Rev. Lett. 68 (1992) 1567.
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Phosphorus vacancies and adatoms on GaP(110) surfaces studied by scanning tunneling microscopy Ph. E b e r t a n d K. U r b a n Institut )=firFestk6rperforschung, Forschungszentrum Jiilich GmbH, Postfach 1913, W-5170 Jiilich, Germany Received 12 July 1992; at Editorial Office 24 August 1992
Atomic defects were investigated on p-doped G a P ( l l 0 ) surfaces by scanning tunneling microscopy. The defects were identified as positively charged phosphorus vacancies and uncharged phosphorus adatoms. They are found to migrate on the phosphorus sublattice. Their motion was observed with atomic resolution and the dependence of the j u m p probability on the tunneling voltage was measured. The position changes of the defects were found to be induced by tip-specimen interactions.
1. Introduction Controlled by thermodynamics, atomic defects are expected to occur on semiconductor surfaces in a similar manner as in the bulk. In addition, it is expected that such defects are formed by evaporation during high-temperature treatments or during reactions with adatoms. Also during cleavage of semiconductor samples, surfaces are created on which atomic defects are observed [1-3]. Scanning tunneling microscopy (STM) provides the only tool for studying individual defects on semiconductor surfaces with atomic resolution. In the present paper, we report on the observation of single atomic defects and their migration by STM on cleaved GaP surfaces. The experimental results are interpreted in terms of tipstimulated migration.
image of, e.g., an 8 x 8 nm 2 area every 8 s. Up to 180 frames were recorded in one individual run. The investigations were performed on clean (110) surfaces of Zn-doped, p-type, LEC-grown G a P obtained from MCP, London. The surfaces were prepared by in-situ cleavage along the [001] direction under ultra-high vacuum. The Zndopant concentration ranged between 1.7 × l(117 and 5.8 x 1017 cm s. All experiments were performed at room temperature at a pressure below 8 x 10 9 Pa. The images were recorded in the constant-current mode and displayed with a gray scale, which is proportional to the height of the tip. Thus, the values on the gray scale represent a measure of the height changes of the surface as well as of changes in the density of electronic surface states.
3. Static properties of atomic defects 2. Experiment
3.1. General
A particularly fast scanning mode of the scanning tunneling microscope was used for the investigation. Due to the small size and the high resonance frequency of the microscope, it was possible to acquire a complete constant-current
The images of the freshly cleaved surfaces revealed a very low number of terrace steps on the cleavage surfaces, which were essentially flat (fig. 1). In addition, a low density of atomic defects, occurring mainly in the vicinity of the
0304-3991/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
Ph. Ebert, I¢2 Urban / Phosphorus vacancies and adatoms on GaP(l lO)
terrace steps, was present. In fig. 1 two terraces are visible separated by a step one atomic layer high. Two different types of atomic defects can be recognized on the terraces. They display dark (A) or bright (B) contrast with respect to the background.
3.2. Results and discussion of type-A defects The type-A defects shown in fig. 2 were found inhomogeneously distributed with a highest area density close to 1011 cm -2 occurring near the steps. They were observed directly after cleavage of the sample as well as several days after cleavage with no change in their density. However, at elevated values of negative tunneling voltage, this type of defect could also be produced by scanning a particular surface area for a prolonged period. Fig. 2 shows a series of constant-current images of five type-A defects acquired at three
345
different negative tunneling voltages (the sample is at negative potential relative to the tip). From figs. 2a to 2c the values of the negative tunneling voltage decrease. All images show the density of occupied surface states localized at the phosphorus (P) atoms [1,4-7]. In fig. 2a, at a high negative tunneling voltage, the defects are imaged as small, up to 0.07 nm deep depressions. The lateral extension of the depressions depends on the tunneling voltage and increases with decreasing negative tunneling voltage. In fig. 2c the diameter of the depressions reaches about 4 nm. The dark areas as a whole do not originate from real geometrical depressions in the surface. The change of the lateral extension, rather, indicates a positive charge of the atomic defects [8,9]. A positive charge center induces a local downward band bending which reduces the number of occupied states near the valence-band maximum contributing to the tunnel current. In the con-
Fig. 1. Image of two terraces separated by a step one atomic layer high. Two different types of point defects, labeled A and B, can be recognized. The atomic rows along the [110] direction are separated by 0.545 nm.
346
Ph. Ebert, K. Urban / Phosphorus uacancies and adatoms on GaP( l lO)
Fig. 2. Series of images of type-A defects measured at different tunneling voltages. The voltages are (a) - 3.5 V, (b) - 2.7 V, and (c) - 2 . 4 V with a tunnel current of 0.4 nA. The corrugation of the unperturbed lattice is 0.05 nm along the [001] direction. The figurcs have the same crystallographic orientation as fig. 1.
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Ph. Ebert, K. Urban / Phosphorus vacancies and adatoms on GaP(l lO)
stant-current mode this results in a decrease of the tunnel current in those surface areas where band bending is present and the electronic feedback loop of the STM responds to this by reducing the tip-to-sample distance. At moderately low negative tunneling voltages the fraction of states is large, which cannot contribute to the current due to band bending. This leads to a deep and wide depression (fig. 2c). With increasing negative tunneling voltage, more surface states within a larger interval of energy in the valence band contribute to the tunnel current. Thus the fraction of states inhibited from participating in the tunnel process by band bending decreases and, consequently, a small and narrow depression is observed (fig. 2a). Close inspection of fig. 2a shows that the defect constrast consists of two basically different parts. The first is the charge-related depression already described, which is dependent on the tunneling voltage. The second is due to a "hole" which has a lateral dimension of one danglingbond state. It is localized at the position of a P atom along the [110] rows of the occupied P dangling-bond states. Only a single occupied dangling bond is missing. The neighboring dangling bonds are maintained. However, they appear darker as in unperturbed areas. Fig. 3 shows the height of the tunneling tip along the [1-10] direction on both sides of a defect. We recognize the atomic corrugation and the effect of the chargeinduced band bending. At the center of fig. 3 the deep additional depression representing the core of the atomic defect is visible. The slight asymmetry of the curves from left to right is due to a weak electric A C coupling of the height signal, used to compensate, to a certain extent, a uniform sample inclination that otherwise would produce a trivial but disturbing contrast gradient along the slope of the surface. Various possibilities have to be considered in order to explain the nature of the defects. We can rule out adsorbate atoms or molecules as being responsible for the observed image contrast. Occasionally observed adsorbates displayed a different contrast and their density increased with time, irrespective of whether the sample was scanned or not. We also rule out interstitial atoms,
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distance from defect (nm) Fig. 3. Height profiles along the [110] direction through one of the defects of fig. 2. The upper profile corresponds to fig. 2a, the middle to fig. 2b, and the lower to fig. 2c. The positioning of height 0 is arbitrary.
since the defects are centered on P dangling-bond locations. Neither can any type of antisite defects explain our observations of a positively charged defect on a P-sublattice site. The antisite defect Gap consists of a gallium (Ga) atom on a P-sublattice site, but it is not positively charged. Gap on surfaces has only three valence electrons in its neutral charge state, two less than the missing P atom. It can therefore act as an acceptor and not as a donor and is expected to be on p-doped G a P ( l l 0 ) surfaces either electrically neutral or negatively charged, in analogy to the Gaa~ antisite defect on GaAs(110) surfaces [10]. The reconstruction of the G a P ( l l 0 ) surface induces a charge transfer from the Ga to the P atoms resulting in empty dangling bonds above the Ga atoms and filled dangling bonds above the P atoms. Therefore, the structure of valences of the reconstructed surface is comparable with that of the bulk. Indeed, for Gap in the bulk also an acceptor character was found [11]. The antisite defect PGa consists of a P atom on a Ga site and is predicted to act only as a donor on surfaces [10]. Although it is expected to be positively charged, it cannot explain the nature of the observed defects, because PGa is localized on the Ga sublattice.
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Ph. Ebert, K. Urban / Phosphorus z'acancies and adatoms on GaP(l lO)
Pig. 4. ~and-bending-induced elevations due to the negative charge originating from Zn-dopant atoms. Three positively charged defects (P vacancies) are visible as dark depressions. The image of an area of 130× 130 nm 2 was measured at 2.5 V tunneling voltage. The figure has the same crystallographic orientation as fig. 1.
Zn-dopant atoms can also be excluded because they are negatively charged and are localized on the Ga sublattice. In fact, we observed in other STM images elevations about 5 nm in diameter (fig. 4) which can be explained as due to band bending induced by negatively charged centers as expected for Zn-dopant atoms. Their area density was measured as 2 × l01~ cm -2 in agreement with the dopant concentration. Although the density of other impurities in the GaP crystals is not known, impurities are unlikely to be the defects imaged in fig. 2. They should occur uniformly distributed in the crystal and should not be produced by scanning. We only very rarely observed defects which can be attributed to impurity atoms. The possibility remains that the observed defects are Ga or P vacancies. Since only the P sublattice is imaged in fig. 2 they cannot be Ga vacancies. The following arguments can be developed to support the interpretation of the defects as P vacancies. In a simple P surface vacancy, one atom is missing in the surface layer. The dangling-bond
state localized at that P atom on the defect-free surface cannot exist. Instead, the bonds that were broken for the formation of the vacancy are expected to rehybridize and to develop one or more defect states. Spatially, these defect states lie below the surface, as the three broken bonds necessary for the vacancy formation are already deeper in the crystal than the neighboring dangling bonds. Therefore, the defect states themselves cannot be imaged with the STM because of the greater distance to the tip and the exponential dependence of the tunnel current on distance. Thus at negative voltages, where the dangling-bond states localized at the P atoms are imaged, the P vacancy should appear as a small hole with the lateral dimension of one dangling bond. The positive charge of the defects observed in the STM images allows us to conclude that at least one defect state is situated well above the valence-band edge and is not occupied. Since the crystal is p-doped, it can be expected that the states in the band gap are unoccupied. From the observation that the defects mainly occur near steps it can be concluded that they are formed by the fracture during the cleavage pro-
Fig. 5. Image of a type-13 detect (P adatom) measured at - 3 . 3 V tunneling voltage. The figure has the same crystallographic orientation as fig. 1.
Ph. Ebert, K. Urban / Phosphorus vacancies and adatoms on GaP(l lO)
cess. Thermal equilibrium defects quenched into the sample during cooling of the crystal from its preparation temperature should have no spatial relation to cleavage steps. The appearance and morphology of the defects in fig. 2 are very similar to that of P vacancies observed on the p-doped I n P ( l l 0 ) surface [3]. Here, too, the neighboring dangling bonds of the defects appeared darker and the defects were positively charged and occurred mainly near steps. Arsenic vacancies were also reported on p-doped G a A s ( l l 0 ) and were found to be positively charged [12].
349
3.3. Results and discussion of type-B defects Fig. 5 shows an image of a type-B defect in greater detail. It consists of an elevation extending over four P atoms. The vertical maximum lies in between these four P atoms and reaches a height of 0.1 nm above the surface layer. Type-B defects were only observed near steps where their density reached about 1012 c m - 2 . We found that the images were not dependent on the tunneling voltage. From this it can be concluded that the defects are uncharged.
Fig. 6. Sequences of constant-current images of three type-A defects (P vacancies) selected from a series extending over 24 rain. The defects changed their positions. The tunneling voltages ranged from - 3 . 1 to - 3 . 5 V. The tunnel current was 0.4 nA. The corrugation of the lattice was 0.05 nm. Each image was measured in 8 s. Scanning direction from the upper left corner to the lower right corner. The figures have the same crystallographic orientation as fig. 1.
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Ph. Ebert, K. Urban / Phosphorus t,acancies and adatoms on GaP(I lO)
The type-B defects cannot be P vacancies because their morphology is completely different from that of the vacancies in fig. 2. PGa antisite defects are expected to be positively charged and the Zn-dopant atoms as shown in fig. 4 are negatively charged. Adsorbates and impurities can again be excluded for the same reasons as mentioned above. We presume that the defect shown in fig. 5 consists of a single P adatom. A P adatom is expected to be bonded to a Ga surface atom, which is itself placed in between four P atoms because this would be its place in the next surface layer. Such a P adatom has three dangling bonds of which at least two can be easily recon-
structed since their distance to neighboring P dangling bonds is small. Upon reconstruction the dangling bonds form a new defect state which can explain the lateral extension of the defect in fig. 5 over four P dangling bonds. The view that the type-B defects are P adatoms is corroborated by the occasional observation that these defects, if localized close to a P vacancy, suddenly disappeared together with the vacancy. This can be considered as a recombination process. It can be understood that the type-B defects are only found near steps if it is assumed that, as discussed for the P vacancies, they were created in the cleavage process.
Fig. 7. Sequences of constant-current images of three type-B defects (P adatoms). The tunneling voltage ranged from - 2.6 to - 3.5 V. The tunnel current was 0.5 nA and the corrugation 0.015 nm. The images have been selected from a series extending over a time of 24 min. Scanning direction from the upper left corner to the lower right corner. The crystallographic orientation is rotated by 180 ° as compared to fig. 1.
Ph. Ebert, K. Urban / Phosphorus vacancies and adatoms on GaP(l lO)
We note that P adatoms can also be formed by interstitial P atoms created during the cleavage process in the bulk but sufficiently close to the surface so that, under the action of image forces, they were able to relax to the surface.
4. Defect migration
4.1. Obser~,ation of migration of P cacancies and P adatoms Both the P vacancies and the P adatoms were found to migrate on the surface. Fig. 6 shows images of P vacancies selected from a series measured over a total period of 24 min at tunneling voltages between - 3 . 1 and - 3 . 5 V. The defects changed their positions regularly between consecutive images. Similar effects were observed for the P adatoms at tunneling voltages in the range o f - 2 . 6 to - 3 . 5 V (fig. 7). Below these values no migration could be observed. At negative voltages of an absolute value higher than about - 4 V the surface was destroyed. The position changes could be measured quantitatively when two or more defects were visible simultaneously. If one defect changed its position while the others remained stable, these latter could be taken as a reference together with the atomically resolved lattice to determine the jump length and direction. This allows jumps produced by tip instabilities to be excluded, e.g., jumps of the tunneling tip state to another place on the tip. They should affect all defects simultaneously. A special characteristic of defect migration was the frequent observation of defects which were only partially imaged. These images can be explained by a position change of a defect occurring while the tip is crossing the defect. This results in a partially imaged defect since only that part of the defect is recorded which was scanned prior to the jump. The defect can be imaged again in the same STM image if it jumped into a position situated in the part of the STM image still to be scanned. For example in fig. 6a the defect marked by an arrow moves by one lattice spacing in [001] direction (fig. 6b). In fig. 6b a partially imaged defect (upper arrow) is visible
351
and just below the defect in its new position (lower arrow). If the defect moved in the opposite direction its new position cannot be imaged in the same image, but only in the following one. For example, in fig. 6g the partially imaged defect marked by an arrow moved in [110] direction to its new position visible in the following frame (fig. 6h).
4.2. Analysis of defect migration A statistical evaluation of the images containing defects which were only partially imaged shows that about 90% of all position changes of the P adatoms occurred at the moment when the tip was positioned in a scan line passing over the defect. This value can be compared with an estimate of the probability of obtaining an image with a partial defect, assuming jumps with no correlation to the position of the tip. This is equal to the ratio of the number of scan lines crossing the defect to the total number of scan lines per image and is about 15%. For the P vacancies the measured value was about 50% as compared to a calculated probability of only 10% without a correlation. This can be taken as an indication that the migration of the defects is tip-induced. The jumps which could not be directly observed can nevertheless be explained as induced by the tip if it is assumed that the jump was triggered just before or after the defect was scanned. The site changes were studied quantitatively by examining the distribution of the jump lengths. For this, the components of a given jump along the [110] and the [001] directions were measured and an effective jump length was calculated as the total number of lattice spacings along both directions. The data are presented in fig. 8. The number of observed jumps decreases with increasing jump length both for the P vacancies and for the P adatoms. In both cases the proportion of long jumps is high. We find that only jump lengths of multiples of lattice spacings occur. This is equivalent to the fact that the defects migrated only on the P sublattice. This holds for the P vacancies and for the P adatoms as well. The latter migrate on a set of adatom sites which are equivalent to the P sublattice of an extra GaP
352
Ph. Ebert, K. Urban / Phosphorus l,acancies and adatoms on GaP(l lO)
layer if this were added to the existing surface. We note that a jump to the next P atom position is not a jump to the nearest-neighboring atom position, which is occupied by a Ga atom. Measurements of the jump probability per image as a function of the tunneling voltage V were limited to relatively small usable tunneling voltage ranges ( - 3 . 5 < V < - 3 . 1 V for the P vacancies and - 3 . 5 < V < - 2 . 6 V for the P adatoms). At smaller absolute values of voltage the defects remained immobile and at larger values the surface was destroyed. The results are shown in fig. 9. The solid squares are the data points of jump components along the [001] direction while solid triangles are those along the [110] direction. The dashed line refers to the P vacancies and the solid line to the P adatoms. We find an exponential dependence of the jump probability from the tunneling voltage for both types of defects and no differences between the two selected directions. The results show that the migration of defects is connected to the presence of the tip. The tip induces a localized electric field of approximately 109 V / m due to the tunneling voltage and the small tip-to-surface separations. This field follows the movement of the tip, penetrates into the
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semiconductor and changes the potential of the surface. The tip-field gradient can in principle exert a force on the atoms strong enough to stimulate the jumps of the defects. An independent observation corroborating this interpretation is the effect of the corrugation amplitude on the migration. With a tip having a low resolution, i.e. a low corrugation amplitude, only the migration of the more easily migrating P adatoms was observed, while a tip with a high corrugation was necessary to induce migration of P vacancies. Tips with a better resolution induce a stronger lateral decay of the electric field since the decay is aproximately scaled by the effective tip radius [13]. Therefore, it is to be expected that a tip with a high corrugation exerts a stronger force on the defects than a tip with a low corrugation. Vacancy migration similar to that observed in our work was found for As vacancies on G a A s ( l l 0 ) surfaces [14]. In this case, too, jumps over more than one lattice spacing were observed and migration increased with tunneling voltage. Moreover, thermal adatom migration was reported for Pb adatoms on G e ( l l l ) 2 × 8 surfaces [151.
Ph. Ebert, I~ Urban / Phosphorus uacancies and adatoms on GaP(l lO)
5. Conclusions Atomic defects were studied on p-doped G a P ( l l 0 ) surfaces by scanning tunneling microscopy. From the observations it was concluded that the defects were positively charged P vacancies and uncharged P adatoms. The observation of the migration of single defects on the G a P ( l l 0 ) surface revealed that the defects only migrated on the P sublattice. A substantial number of jumps occurred over more than one lattice spacing. It was concluded that migration is tip-stimulated. The jump probability was found to be higher for P adatoms than for P vacancies. It increases with increasing negative tunneling voltage. No differences between the [110] and the [001] directions were observed.
Acknowledgements The authors would like to express their thanks to L. Koenders, PTB Braunschweig, Germany, for many helpful discussions and for providing the GaP samples and to K.H. G r a f for technical assistance.
353
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