- Email: [email protected]
In-situ BEEM study of interfacial dislocations and point defects
s __
-_ BB
,ouwuop CRYSTAL GROWTH Journal of Crystal Growth 175/l 76 (I 997) 340-345
ELSEVIER
In-situ BEEM study of interfacial dislocations and point defects H. von K%nel*, T. Meyer, H. Sirringhaus Laboratorium fiir Festkcrperphysik,
ETH Ziirich, CH-8093 Ziirich. Switzerland
Abstract Epitaxial CoSi,/n-Si(1 1 1) interfaces have been studied by in-situ ballistic-electron-emission microscopy (BEEM) at 77 K. The scattering of hot electrons by individual misfit dislocations and point defects leads to significant contrast in the BEEM images, proving that interfacial defects of atomic dimensions can be probed by BEEM. The Schottky barrier height is not measurably affected by these defects. When present, grains of a metastable CoSi, phase with a defect-CsCI structure do, however, lower the barrier appreciably.
1. Introduction The invention of ballistic-electron-emission microscopy (BEEM) by Kaiser and Bell [l] has made it possible to study electric transport across interfaces with a spatial resolution unattainable before. In BEEM the tip of a scanning tunneling microscope (STM) acts as the emitter injecting hot charge carriers into a metallic base. The base layer is kept thin, typically < 10 nm, in order to allow a fraction of the injected current I, to enter the semiconducting collector where it is measured as the BEEM or collector current, I,. The configuration resembles thus the one of a point contact transistor with the exception that here the emitter can be moved across the sample just as in the usual STM operation. In the simplest case the metal base
*Corresponding author: [email protected].
Fax:
0022-0248/97/$17.00 Copyright PII SOO22-0248(96)00867-6
+ 41
1 633 10 72; e-mail:
rc 1997 Published
is deposited directly onto a uniform semiconductor. such that the injected carriers just have to surmount the Schottky barrier at the metal-semiconductor (m-s) interface in order to contribute to the collector current. More complicated heterostructures have been studied by BEEM as well, however, such as resonant tunneling diodes [2], metal-insulator-semiconductor (MIS) [3] and MOS structures [4]. Even though BEEM was designed to be a technique for studying buried interfaces, surface effects may contribute significantly to the variation of the collector current [.5], such that operation in UHV is mandatory in order to attain the highest possible spatial resolution. Here we shall focus on epitaxial CoSi,/Si(l 1 1) heterostructures fabricated by MBE. Misfit dislocations are shown to affect hot electron transport by leading to elastic scattering. They do not lead to a measurable change of the Schottky barrier height at the m-s interface. Barrier fluctuations do occur, however, when there are
by Elsevier Science B.V. All rights reserved
regions in which the crystal structure of the silicide deviates from the usual CaFz type. It has been shown recently that under certain conditions CoSi, may indeed assume a defected CsCl structure with vacancies on the cation sublattice [6. 71.
2. Experimental
partially relaxed. In order to avoid the (2 x 1) surface reconstruction present on strained films [l 11 the number of dislocations was maximized for the BEEM studies by adjusting the growth procedure accordingly. All BEEM experiments were carried out at 77 K. in a low-temperature STM situated in a UHV chamber attached to the MBE system.
results and discussion 2.1. Scattering by dislocations and point &f&s
All Co%, films were grown by standard MBE on heavily As doped procedures [S, 91 (<0.004 R cm) 3 in Si substrates, after depositing first an undoped Si buffer layer with a thickness of typically 300 nm. Before growing the base for the BEEM measurements, the wafer was flipped and a thick (> 10 nm) CoSi, layer was deposited on its back side. serving as an ohmic collector contact. The base thickness was chosen to be in the range 2-4 nm in order to ensure ballistic electrons to reach the interface [lo]. In the case of CoSi,Si( 111) films of this thickness can be grown coherently or
The defect structure at CoSi,,‘Si( 1 1 1) interfaces has been studied extensively by transmission electron microscopy (TEM) 1121. The main defects of films annealed above 5OO’C are partial dislocations with Burgers vectors b = &z( 1 1 2) associated with interfacial steps. In topographic STM images these dislocations appear as faint surface corrugations resulting from their strain field (Fig. la). The simultaneously acquired BEEM image, taken at a tunneling voltage of I/, = ~ 1.4 V and a tunneling current of I, = 20 nA, is displayed in Fig. I b. It is
Fig. 1. (a) Topographic STM image obtained on a 3.2 nm thick CoSiz film on Si(I 1 1). The height of the dislocation induced corrugation IS 0.06 nm. (b) BEEM image acquired simultaneously with the topography. The gray scale ranges from 0 to 160 pA. Tunneling parameters were: 1/, = - 1.4 V, I, = 20 nA. S and P are surface and interface point defects. respectively
evident that the contrast in the BEEM image is far more pronounced than the topographic contrast, the collector current being enhanced by as much as 40% above the dislocation. In addition, the BEEM current exhibits the same kind of increase at certain point-like features which are not associated with surface defects, as can be seen by comparing with the topography image (Fig. la). It follows also from Fig. 1 that the dislocation appears much narrower in the BEEM image than in the topography image. This could be corroborated by taking cross-sections perpendicular to the dislocation line 1131. The smallest FWHM of a dislocation ever observed in a BEEM image was -0.8 nm, whereas in the topography it amounts to twice the film thickness in accordance with an elastic continuum model [ll]. The width of the point-like features in Fig. 1b is of the same order of magnitude as the one of the dislocation. Let us now discuss the origin of the BEEM contrast visible in Fig. 1b. As outlined in the Introduction. the nonuniformities of the BEEM current can be caused either by variations of the Schottky barrier height or by scattering processes. The barrier height follows immediately from the variation of the collector current I, with the tunneling voltage V, (ballistic-electron-emission spectroscopy or BEES). The Schottky barrier height at the CoSiZSi(l 1 1) interface was found to be @,, = 0.66 + 0.03 eV both in the vicinity of a dislocation as well as far away from it, as long as the silicide has its normal fluorite structure everywhere. The contrast variations observed on a scale of nanometers must hence be due to elastic scattering at dislocation cores and at point defects. It has to be kept in mind that the angular distribution of the tunneling electrons is strongly forward focused, which should lead to low transmission across a perfect CoSi,/Si(l 1 1) interface. The reason for this is indicated in Fig. 2 showing a projection of the constant energy surfaces of Si onto the interface Brillouin zone. Whereas hot electrons with a small parallel momentum kit are primarily reflected back into the metal, this is no longer true if they undergo a scattering event, yielding the necessary large kll. While it is certain that the bright spots in Fig. lb are due to scattering at point defects, the exact location of these cannot be determined
CB
Fig. 2. Pictorial k-space representation of a scattering process allowing a hot electron to enter the off-centered conduction band minima at the CoSiz.Si(l I I) interface.
experimentally at present. On the other hand, the dislocation line, which is known to lie at the interface, has an internal structure looking very much like a linear chain of similar point defects. It is likely therefore that the isolated spots are of the same kind. Of course. the point defects could also be trapped by the strain held of the dislocation. One argument in favour of interfacial point defects might be the electrical resistivity which was found to rise due to interfacial scattering only in films below -5 nm [14]. The ability to observe single point defects by BEEM depends very much on the structure and quality of the surface. Not only do surface defects give rise to contrast variations of the same size (Fig. lb). Surface reconstructions were found to modify the tunneling distribution locally, again leading to large contrast changes and therefore masking any scattering effects taking place at the interface [S, 151. On the other hand, variations of the Schottky barrier height are affected much less by these surface scattering effects. 2.2. Variations of the Schotthy barrier height As pointed out above, the Schottky barrier (Ph at the interface between CoSi, with the fluorite structure and Si(1 1 1) was found to be unaffected by the commonly observed misfit dislocations of type b = ba( 1 1 2). In the past few years it has become increasingly evident, however, that, apart from its bulk stable fluorite structure, CoSiZ can assume
other crystal structures which are stabilized by epitaxy [6,7, 161. The growth of CoSi, usually involve :s an annealing step typically at 600°C. which impr’ oves its electrical properties [S]. The anneal also (:auses most of the material to transform to the bulk stable phase. Grains of the defect-CsCl phase have. , however, been observed in some films even after such an annealing [7]. A detailed account of
this phase transition between epitaxially stabilized and bulk stable CoSi, phases will be given elsewhere [17]. Here we just discuss the BEEM results obtained on a film kno\~n from high-resolution transmission electron microscopy (HRTEM) to contain CsCl grains. The topography and corresponding BEEM image of such a film are shown in Fig. 3a and Fig. 3b, respectively. In the middle of
Fig. 3. (a) Topographic STM image of a 2.4 nm thick CoSi, film on Si(1 1 I). containing grains of the epitaxlally acquired BEEM image. Tunneling CsCl ]phase in addition to the bulk stable CaF, phase. (b) Simultaneously v, = - 2 V, I, = 5 nA.
stabilized defected parameters were:
the topography image a large terrace can be seen (region 2) in which the BEEM current is nearly uniform. In a few nearly circular patches it is larger, however, by more than a factor of two (regions 1.3). From spatially resolved BEES spectra it follows that the barrier height in region 2 is close to the normal value for this interface (0.66 + 0.03 eV) (see Fig. 4). In the analysis we assumed a square law for the collector current I,, according to Ref. [1X]. In regions I and 3 the barrier is lowered to 0.4 and 0.5 eV. respectively. It is not possible to prove directly that the regions with a lower Schottky barrier really consist of material with a different crystal structure. We consider it to be very likely. however, that the CsCl grains found by cross-section HRTEM do correspond to those regions, since they could never be found in any of the samples exhibiting a uniform barrier. The regions with a low barrier are large enough in order to analyse the I-L’ curves obtained on macroscopic diodes by the parallel conduction model [ 191. The temperature dependence of the saturation current density I, (not shown) revealed
a lowering of the Schottky barrier height to Qr, = 0.43 & 0.03 eV in 11% of the active diode area. in good agreement with the BEEM experiments.
3. Conclusions In-situ BEEM experiments carried out at 77 K on epitaxial CoSi,!n-Si(1 1 1) films have shown that the Schottky barrier height at this interface remains uniform as long as the CoSi, crystallizes with the bulk stable fluorite structure. In contrast, lower barriers are found in films containing grains of a metastable phase with the CsCl structure, characterized by random vacancies on the cation sites. The hot electron current across the silicide/Si interface exhibits large spatial variations even when the Schottky barrier is completely uniform. In the case of defect-free, unreconstructed surfaces these variations are entirely due to elastic scattering at dislocation cores and point defects. In favourable cases a spatial resolution below 1 nm has been attained, allowing isolated point defects to be imaged.
Acknowledgements Financial Foundation
support by Swiss National is gratefully acknowledged.
Science
References -1.8
-1.4
-1
-0.6
-0.2
[l] [2] [3] [4] [S] 0 -1.8
[6] -1.4
-0.6
-0.2
b)
Fig. 4. (a) BEES spectra taken in the rqions labeled I 3 in Fig. 3. (b) Determination of the barrter height from the square root of the BEEM current.
[7] [8] [9]
W.J. Kaiser and L.D. Bell. Phys. Rev. Lett. 60 (1988) 1406. T. Sajoto. J.J. O’Shea, S. Bhargava, D. Leonar-d, M.A. Chin and V. Narayanamurti, Phys. Rev. Lett. 74 (I 995) 3427. M.T. Cuberes, A. Batter, H.J. Wen. M. Prietsch and G Kaindl. Appl. Phys. Lett. 64 (1994) 2300. R. Ludeke, A. Bauer and E. Cartier, Appl. Phys. Lctt. 66 (I 995) 730. H. Sirringhaus, E.Y. Lee and H. van Kanel. Surf. Sci. 331-333 (1995) 1277. C. Pirri. S. Hong, M.H. Tuilier. P. Wetzel. G. Gewmner and R. Corti-s, Phys. Rev. B 53 (1996) 1368. H. van Kanel, E. Miiller, S. Goncalves-Conto. C. Schwarl and N. Onda, Appl. Surf. Sci. 00 (1996) 000. H. van Kanel, Mater. Sci. Rep. 8 (1992) 193. R. Staider. C. Schwarz. H. Sirrmghaus and H. \on Kane). Surf. Sci. 271 (1992) 355.
H. aon Kiinel et al. f Journal of Crystal Growth 175/l 76 (1997) 340-345
[lo] [ll] [12] [13] [14]
E.Y. Lee, H. Sirringhaus. U. Kafader and H. von Kane], Phys. Rev. B 52 (1995) 1816. R. Stalder, H. Sirringhaus, N. Onda and H. von Kanel, Surf. Sci. 258 (1991) 153. C.W.T. Bulle-Lieuwma, Appl. Surf. Sci. 68 (1993) 1. H. Sirringhaus. E.Y. Lee and H. von Kanel, Phys. Rev. Lett. 73 (1994) 577. H. von Kane1 and G. Fishman, Phys. Rev. B 45 (1992) 3929.
[15]
345
H. Sirringhaus, E.Y. Lee and H. von Kanel, Phys. Rev. Lett. 74 (1995) 3999. 1161 S.-L. Zhang, J. Cardenas, F.M. d’Heurle, B.G. Svensson and C.S. Petersson, Appl. Phys. Lett. 66 (1995) 58. [17] S. Goncalves-Conto, U. Scharer, E. Miiller, H. von Kanel, L. Miglio and F. Tavazza, unpublished. [ 181 L.D. Bell and W.J. Kaiser, Phys. Rev. Lett. 61 (1988) 2368. [19] I. Ohdomari and K.N. Tu, J. Appl. Phys. 51 (1980) 3735.
-_ BB
,ouwuop CRYSTAL GROWTH Journal of Crystal Growth 175/l 76 (I 997) 340-345
ELSEVIER
In-situ BEEM study of interfacial dislocations and point defects H. von K%nel*, T. Meyer, H. Sirringhaus Laboratorium fiir Festkcrperphysik,
ETH Ziirich, CH-8093 Ziirich. Switzerland
Abstract Epitaxial CoSi,/n-Si(1 1 1) interfaces have been studied by in-situ ballistic-electron-emission microscopy (BEEM) at 77 K. The scattering of hot electrons by individual misfit dislocations and point defects leads to significant contrast in the BEEM images, proving that interfacial defects of atomic dimensions can be probed by BEEM. The Schottky barrier height is not measurably affected by these defects. When present, grains of a metastable CoSi, phase with a defect-CsCI structure do, however, lower the barrier appreciably.
1. Introduction The invention of ballistic-electron-emission microscopy (BEEM) by Kaiser and Bell [l] has made it possible to study electric transport across interfaces with a spatial resolution unattainable before. In BEEM the tip of a scanning tunneling microscope (STM) acts as the emitter injecting hot charge carriers into a metallic base. The base layer is kept thin, typically < 10 nm, in order to allow a fraction of the injected current I, to enter the semiconducting collector where it is measured as the BEEM or collector current, I,. The configuration resembles thus the one of a point contact transistor with the exception that here the emitter can be moved across the sample just as in the usual STM operation. In the simplest case the metal base
*Corresponding author: [email protected].
Fax:
0022-0248/97/$17.00 Copyright PII SOO22-0248(96)00867-6
+ 41
1 633 10 72; e-mail:
rc 1997 Published
is deposited directly onto a uniform semiconductor. such that the injected carriers just have to surmount the Schottky barrier at the metal-semiconductor (m-s) interface in order to contribute to the collector current. More complicated heterostructures have been studied by BEEM as well, however, such as resonant tunneling diodes [2], metal-insulator-semiconductor (MIS) [3] and MOS structures [4]. Even though BEEM was designed to be a technique for studying buried interfaces, surface effects may contribute significantly to the variation of the collector current [.5], such that operation in UHV is mandatory in order to attain the highest possible spatial resolution. Here we shall focus on epitaxial CoSi,/Si(l 1 1) heterostructures fabricated by MBE. Misfit dislocations are shown to affect hot electron transport by leading to elastic scattering. They do not lead to a measurable change of the Schottky barrier height at the m-s interface. Barrier fluctuations do occur, however, when there are
by Elsevier Science B.V. All rights reserved
regions in which the crystal structure of the silicide deviates from the usual CaFz type. It has been shown recently that under certain conditions CoSi, may indeed assume a defected CsCl structure with vacancies on the cation sublattice [6. 71.
2. Experimental
partially relaxed. In order to avoid the (2 x 1) surface reconstruction present on strained films [l 11 the number of dislocations was maximized for the BEEM studies by adjusting the growth procedure accordingly. All BEEM experiments were carried out at 77 K. in a low-temperature STM situated in a UHV chamber attached to the MBE system.
results and discussion 2.1. Scattering by dislocations and point &f&s
All Co%, films were grown by standard MBE on heavily As doped procedures [S, 91 (<0.004 R cm) 3 in Si substrates, after depositing first an undoped Si buffer layer with a thickness of typically 300 nm. Before growing the base for the BEEM measurements, the wafer was flipped and a thick (> 10 nm) CoSi, layer was deposited on its back side. serving as an ohmic collector contact. The base thickness was chosen to be in the range 2-4 nm in order to ensure ballistic electrons to reach the interface [lo]. In the case of CoSi,Si( 111) films of this thickness can be grown coherently or
The defect structure at CoSi,,‘Si( 1 1 1) interfaces has been studied extensively by transmission electron microscopy (TEM) 1121. The main defects of films annealed above 5OO’C are partial dislocations with Burgers vectors b = &z( 1 1 2) associated with interfacial steps. In topographic STM images these dislocations appear as faint surface corrugations resulting from their strain field (Fig. la). The simultaneously acquired BEEM image, taken at a tunneling voltage of I/, = ~ 1.4 V and a tunneling current of I, = 20 nA, is displayed in Fig. I b. It is
Fig. 1. (a) Topographic STM image obtained on a 3.2 nm thick CoSiz film on Si(I 1 1). The height of the dislocation induced corrugation IS 0.06 nm. (b) BEEM image acquired simultaneously with the topography. The gray scale ranges from 0 to 160 pA. Tunneling parameters were: 1/, = - 1.4 V, I, = 20 nA. S and P are surface and interface point defects. respectively
evident that the contrast in the BEEM image is far more pronounced than the topographic contrast, the collector current being enhanced by as much as 40% above the dislocation. In addition, the BEEM current exhibits the same kind of increase at certain point-like features which are not associated with surface defects, as can be seen by comparing with the topography image (Fig. la). It follows also from Fig. 1 that the dislocation appears much narrower in the BEEM image than in the topography image. This could be corroborated by taking cross-sections perpendicular to the dislocation line 1131. The smallest FWHM of a dislocation ever observed in a BEEM image was -0.8 nm, whereas in the topography it amounts to twice the film thickness in accordance with an elastic continuum model [ll]. The width of the point-like features in Fig. 1b is of the same order of magnitude as the one of the dislocation. Let us now discuss the origin of the BEEM contrast visible in Fig. 1b. As outlined in the Introduction. the nonuniformities of the BEEM current can be caused either by variations of the Schottky barrier height or by scattering processes. The barrier height follows immediately from the variation of the collector current I, with the tunneling voltage V, (ballistic-electron-emission spectroscopy or BEES). The Schottky barrier height at the CoSiZSi(l 1 1) interface was found to be @,, = 0.66 + 0.03 eV both in the vicinity of a dislocation as well as far away from it, as long as the silicide has its normal fluorite structure everywhere. The contrast variations observed on a scale of nanometers must hence be due to elastic scattering at dislocation cores and at point defects. It has to be kept in mind that the angular distribution of the tunneling electrons is strongly forward focused, which should lead to low transmission across a perfect CoSi,/Si(l 1 1) interface. The reason for this is indicated in Fig. 2 showing a projection of the constant energy surfaces of Si onto the interface Brillouin zone. Whereas hot electrons with a small parallel momentum kit are primarily reflected back into the metal, this is no longer true if they undergo a scattering event, yielding the necessary large kll. While it is certain that the bright spots in Fig. lb are due to scattering at point defects, the exact location of these cannot be determined
CB
Fig. 2. Pictorial k-space representation of a scattering process allowing a hot electron to enter the off-centered conduction band minima at the CoSiz.Si(l I I) interface.
experimentally at present. On the other hand, the dislocation line, which is known to lie at the interface, has an internal structure looking very much like a linear chain of similar point defects. It is likely therefore that the isolated spots are of the same kind. Of course. the point defects could also be trapped by the strain held of the dislocation. One argument in favour of interfacial point defects might be the electrical resistivity which was found to rise due to interfacial scattering only in films below -5 nm [14]. The ability to observe single point defects by BEEM depends very much on the structure and quality of the surface. Not only do surface defects give rise to contrast variations of the same size (Fig. lb). Surface reconstructions were found to modify the tunneling distribution locally, again leading to large contrast changes and therefore masking any scattering effects taking place at the interface [S, 151. On the other hand, variations of the Schottky barrier height are affected much less by these surface scattering effects. 2.2. Variations of the Schotthy barrier height As pointed out above, the Schottky barrier (Ph at the interface between CoSi, with the fluorite structure and Si(1 1 1) was found to be unaffected by the commonly observed misfit dislocations of type b = ba( 1 1 2). In the past few years it has become increasingly evident, however, that, apart from its bulk stable fluorite structure, CoSiZ can assume
other crystal structures which are stabilized by epitaxy [6,7, 161. The growth of CoSi, usually involve :s an annealing step typically at 600°C. which impr’ oves its electrical properties [S]. The anneal also (:auses most of the material to transform to the bulk stable phase. Grains of the defect-CsCl phase have. , however, been observed in some films even after such an annealing [7]. A detailed account of
this phase transition between epitaxially stabilized and bulk stable CoSi, phases will be given elsewhere [17]. Here we just discuss the BEEM results obtained on a film kno\~n from high-resolution transmission electron microscopy (HRTEM) to contain CsCl grains. The topography and corresponding BEEM image of such a film are shown in Fig. 3a and Fig. 3b, respectively. In the middle of
Fig. 3. (a) Topographic STM image of a 2.4 nm thick CoSi, film on Si(1 1 I). containing grains of the epitaxlally acquired BEEM image. Tunneling CsCl ]phase in addition to the bulk stable CaF, phase. (b) Simultaneously v, = - 2 V, I, = 5 nA.
stabilized defected parameters were:
the topography image a large terrace can be seen (region 2) in which the BEEM current is nearly uniform. In a few nearly circular patches it is larger, however, by more than a factor of two (regions 1.3). From spatially resolved BEES spectra it follows that the barrier height in region 2 is close to the normal value for this interface (0.66 + 0.03 eV) (see Fig. 4). In the analysis we assumed a square law for the collector current I,, according to Ref. [1X]. In regions I and 3 the barrier is lowered to 0.4 and 0.5 eV. respectively. It is not possible to prove directly that the regions with a lower Schottky barrier really consist of material with a different crystal structure. We consider it to be very likely. however, that the CsCl grains found by cross-section HRTEM do correspond to those regions, since they could never be found in any of the samples exhibiting a uniform barrier. The regions with a low barrier are large enough in order to analyse the I-L’ curves obtained on macroscopic diodes by the parallel conduction model [ 191. The temperature dependence of the saturation current density I, (not shown) revealed
a lowering of the Schottky barrier height to Qr, = 0.43 & 0.03 eV in 11% of the active diode area. in good agreement with the BEEM experiments.
3. Conclusions In-situ BEEM experiments carried out at 77 K on epitaxial CoSi,!n-Si(1 1 1) films have shown that the Schottky barrier height at this interface remains uniform as long as the CoSi, crystallizes with the bulk stable fluorite structure. In contrast, lower barriers are found in films containing grains of a metastable phase with the CsCl structure, characterized by random vacancies on the cation sites. The hot electron current across the silicide/Si interface exhibits large spatial variations even when the Schottky barrier is completely uniform. In the case of defect-free, unreconstructed surfaces these variations are entirely due to elastic scattering at dislocation cores and point defects. In favourable cases a spatial resolution below 1 nm has been attained, allowing isolated point defects to be imaged.
Acknowledgements Financial Foundation
support by Swiss National is gratefully acknowledged.
Science
References -1.8
-1.4
-1
-0.6
-0.2
[l] [2] [3] [4] [S] 0 -1.8
[6] -1.4
-0.6
-0.2
b)
Fig. 4. (a) BEES spectra taken in the rqions labeled I 3 in Fig. 3. (b) Determination of the barrter height from the square root of the BEEM current.
[7] [8] [9]
W.J. Kaiser and L.D. Bell. Phys. Rev. Lett. 60 (1988) 1406. T. Sajoto. J.J. O’Shea, S. Bhargava, D. Leonar-d, M.A. Chin and V. Narayanamurti, Phys. Rev. Lett. 74 (I 995) 3427. M.T. Cuberes, A. Batter, H.J. Wen. M. Prietsch and G Kaindl. Appl. Phys. Lett. 64 (1994) 2300. R. Ludeke, A. Bauer and E. Cartier, Appl. Phys. Lctt. 66 (I 995) 730. H. Sirringhaus, E.Y. Lee and H. van Kanel. Surf. Sci. 331-333 (1995) 1277. C. Pirri. S. Hong, M.H. Tuilier. P. Wetzel. G. Gewmner and R. Corti-s, Phys. Rev. B 53 (1996) 1368. H. van Kanel, E. Miiller, S. Goncalves-Conto. C. Schwarl and N. Onda, Appl. Surf. Sci. 00 (1996) 000. H. van Kanel, Mater. Sci. Rep. 8 (1992) 193. R. Staider. C. Schwarz. H. Sirrmghaus and H. \on Kane). Surf. Sci. 271 (1992) 355.
H. aon Kiinel et al. f Journal of Crystal Growth 175/l 76 (1997) 340-345
[lo] [ll] [12] [13] [14]
E.Y. Lee, H. Sirringhaus. U. Kafader and H. von Kane], Phys. Rev. B 52 (1995) 1816. R. Stalder, H. Sirringhaus, N. Onda and H. von Kanel, Surf. Sci. 258 (1991) 153. C.W.T. Bulle-Lieuwma, Appl. Surf. Sci. 68 (1993) 1. H. Sirringhaus. E.Y. Lee and H. von Kanel, Phys. Rev. Lett. 73 (1994) 577. H. von Kane1 and G. Fishman, Phys. Rev. B 45 (1992) 3929.
[15]
345
H. Sirringhaus, E.Y. Lee and H. von Kanel, Phys. Rev. Lett. 74 (1995) 3999. 1161 S.-L. Zhang, J. Cardenas, F.M. d’Heurle, B.G. Svensson and C.S. Petersson, Appl. Phys. Lett. 66 (1995) 58. [17] S. Goncalves-Conto, U. Scharer, E. Miiller, H. von Kanel, L. Miglio and F. Tavazza, unpublished. [ 181 L.D. Bell and W.J. Kaiser, Phys. Rev. Lett. 61 (1988) 2368. [19] I. Ohdomari and K.N. Tu, J. Appl. Phys. 51 (1980) 3735.