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Ultra accurate measurements of interface parameters with free-electron laser
ap¢~o surface science ELSEVIER
Applied Surface Science 92 (1996) 267-272
Ultra accurate measurements of interface parameters with free-electron laser C. C o l u z z a
* ,1
lnstitut de Physique Appliqu~e, Ecole Polytechnique Fdd~rale, CH-IOI5 Lausanne, Switzerland Received 13 December 1994; accepted for publication 4 March 1995
Abstract
We used optical pumping by the Vanderbilt free-electron laser (FEL) and the technique of internal photoemission (IPE) to measure with an accuracy of nearly 5 meV the conduction-band discontinuity of semiconductor heterojunction interfaces as GaA1As-GaAs and Ge-GaAs. Very recently we demonstrated, using a titanium-sapphire pumped laser, that spatially resolved internal photoemission measurements could be performed on a Pt/n-GaP Schottky barrier by a scanning near-field optics microscope within a spatial resolution of 100 nm. Shear-force and photocurrent X-Y images at different photon energies enable us to map the topography and the Schottky barrier height on the same surface. The topography's images, compared with the internal photoemission images, revealed zones where the morphology of the metallic layer was homogeneous, whereas the photocurrent was varying from place to place. Both results opened the possibility of measuring, in a simple and direct way, the local interface properties of real devices. A novel technique with submicrometric spatial resolution could be implemented: the spatially analyzed FEL-IPE (SAN FEL-IPE).
1. Introduction
The band lineup at the heterostrnctures interface has been the subject of intense experimental and theoretical research for many years [1,2]. Unfortunately, progress in this field is adverse affected by the limitations of the experimental methods to measure band discontinuities [1]. The conduction-band and valence-band discontinuities at the interface in heterojunctions can be determined by a number of experimental techniques. In most cases, the accuracy
* Corresponding author. Fax: +39 6 49914535; e-mail: coluzza@romal .infn.it. l Present address: Dipartimento di Fisica, Universith di Roma "La Sapienza", P.le A. Moro 2, 00185 Roma, Italy.
of the measurements is not sufficient to discriminate one theory from another; in the few cases in which high accuracy is obtained, one must work with specialized preparation techniques a n d / o r one has questionable reliability [1]. Internal photoemission (IPE) is a simple and direct method for determining the band discontinuities at semiconductor interfaces [37]. Free-electron laser internal photoemission (FELIPE) is an optical technique that achieves very high accuracy and complex modeling is not required to interpret the experimental data [8-12]. The conduction band discontinuity A E c is given by the lowest photon energy that can induce electronic transitions from the conduction-band minimum of one material to the conduction-band minimum of the other, so as to produce a photocurrent at the interface. FEL-IPE has important advantages over other techniques for
0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 1 6 9 - 4 3 3 2 ( 9 5 ) 0 0 2 4 0 - 5
C. Coluzza /Applied Surface Science 92 (1996) 267-272
268
measuring heterojunction band discontinuities: it can be used on m a t u r e interfaces buried deep beneath a surface, making it possible to study device quality interfaces without the requirement of in situ growth and measurement. Furthermore, the discontinuities can be measured while the interface is under bias, which is necessarily the case for any interface used in a " r e a l " device. FEL-IPE can also measure the positions of localized interface states and interface quantum well states not accessible to conventional photoemission. A scheme of the internal photoemission processes induced by FEL-IR (A) and by visible (B) sources are reported in Fig. 1.
Ge(lp.m)/n-GaAs reverse bias 77I( light on Ga
[.(..)vv2,-n,E
1
ift
1 0.31
0.33
0.35
0.37
0.39
Photon Energy leVI
2. E x a m p l e s o f s o m e F E L - I P E
results
Fig. 2 shows a typical FEL-IPE spectrum (curve (a) in the upper part) near the photocurrent threshold region for the GaAs/amorphous-Ge heterojunction, A E¢ is given directly by the photon energy at the threshold (E). The Fowler fit gave 0.334 eV. In the same frame it is also shown the spectrum (curve (b) in the upper part) of the FEL-photoconductivity measurements made on the GaAs. The photoconductivity measurements were performed on the same sample locking the photocurrent across two parallel strips of ohmic contacts on the back of the crystalline semiconductor. The photoconductivity did not show any onset on the same photon energy interval ruling out other contributions to the onset in curve (a) different
Na_ __h i .....~
" (B)
Fig. 1. Scheme of the internal photoemission processes induced in heterojunctions by FEL-IR (A) and by visible (B) sources. Similar photocurrent processes are expected for Sehottky barriers.
r~
1
1.1
1.2
1.3
Plmtou Energy leVI Fig. 2. Typical FEL-IPE spectrum (curve (a) in the upper part) near the photocurrent threshold region for the GaAs/amorphousGe heterojunction at 77 K. The curve (b) in the upper part is the GaAs FEL-photoconductivity measured on the substrate of the same sample. In the lower part is reported the curve corresponding to the visible internal photoemission made on the same sample.
from conduction-to-conduction band intemal photoemission. Our discontinuity measurements of the GaAs/amorphous-Ge heterojunction was accurate at least to + 5 meV. This level of accuracy is required to correctly distinguish between the rival theoretical models of the heterojunction band lineup. Therefore, systematic use of the FEL-IPE technique over wide classes of interfaces potentially could resolve longstanding theoretical debates. FEL-IPE gives complementary information to the earlier visible lamp-IPE experiments [3-7] which used monochromatic visible light to induce transitions from the valence-band maximum of the smaller gap semiconductor (or from metallic contact) to the conduction-band minimum of the larger gap semi-
269
C. Coluzza / Applied Surface Science 92 (1996) 267-272
3. Spatially resolved IPE measurements
conductor. In this case, A E c (or the barrier height ~b in the Schottky junctions) is given by the threshold photon energy minus the value of the smaller band gap (or directly from the onset in the square root of the yield curve versus the photon energy). If flat-band conditions exist at the interface, the comparison of the visible-IPE with FEL-IPE is a direct measurement of the interfacial band gap of the small gap material. This is important because processes such as microdiffusion and interfacial strain can introduce uncertainty about the value of the near-interface band gap relative to the bulk gap. In the lower part of Fig. 2 it is reported the curve corresponding to the visible internal photoemission made on the same sample. The value of conduction-band discontinuity is given by the relation A E c = Eg(Ge) - E', where E' is the measured onset and Es(G-e) is the Ge band gap at the temperature of the measurement (77 K). The comparison between the A E c measured by FEL-IPE and by visible-IPE resulted in a Eg(Ge)= 0.76 eV in good agreement with the value reported in the literature. Unfortunately must of such interfaces processes are not laterally homogeneous and a complete understanding demands for spatially resolved measurements.
turnable Ti-Sapphire laser
Spatially resolved measurements of heterostructures is a very actual subject in todays research. The combination of non-destructive with high spatial resolution techniques gives an interesting analysis possibility [13-17]. Among high spatial resolution techniques as ballistic electron emission microscopy, the scanning near-field optics microscope showed their possibilities for surface microanalysis [18,19]. High lateral resolution is achieved by illuminating the sample by a small light source, and an extension of the optical characterization is used to locally study a metal-semiconductor barrier. Another major research thrust will be IPE microscopy which can check the spatial uniformity of interfaces. To do this, microscopy with lateral resolution less than the Debye length (typically 100-1000 ,~) must be achieved. Using near-field microscopy techniques (SNOM), this level of spatial resolution should be possible. This technique demands for coherent and tunable sources: the case of a FEL. This is an important step since spatial averaging over a large interface with nonuniformity in the discontinuity can introduce error by broadening the photocurrent threshold. In Fig.
reflection
photomultiplier 1 lock-in amplifier
~---~wave lenght ] laseromatrol filter &
shear force signal
oscillator []
opticalfibe:
, plezoVZ"brati°n z piezo
x-y scanner
photocurrent signal lock-in amplifier
computer Fig. 3. Scheme of the spatially analyzed internalphotoemissionapparatus.
C. Coluzza / Applied Surface Science 92 (1996) 267-272
270
3 is reported a scheme of the apparatus used for the spatially analyzed IPE measurements. The spatially resolved measurements for a P t / G a P Schottky barrier is shown in Fig. 4. The mapping scanned an area of 40 /.tm x 40 /xm, and it was obtained by a conventional Ti: sapphire laser, tuned at a photon energy of 1.42 eV, that is greater than the Schottky barrier height ~b" The image in the left is the shear-force result giving the topological homogeneity. The image in the center is the reflection measurement result. Finally, in the right, the image shows the results of IPE mapping: there are some features that represents the local properties of the interface and that are not evident in the previous two images. Fig. 5 shows photocurrent mapping of the P t / G a P sample at a photon energy of 1.5 eV (by> qbb). In the lower part, are reported the Fowler plots of the spatially resolved IPE measurements taken at two different places of the P t / G a P sample: at a zone (A) were the measured photocurrent reached a higher value and at a zone (B) presenting a very low photon yield. The two obtained values for qbb (1.413 eV at the " d a r k " zone and 1.409 eV at the "bright" one) both are in good agreement with the value obtained by spatially integrated IPE (1.412 eV) and with the
value quoted in the literature. Using the Fowler power law Y or ( h v - ~ b ) 2, where Y is the yield and hv is the photon energy, we can estimate the ratio between the intensity at the two different places. The result is a value close to 1.1 for a photon energy of h v = 1.5 eV. This value has to be compared with the experimental value (of the order of 3) for the ratio between the pixel intensity at the two zones of the photocurrent image obtained with the same photon energy. The clear difference between the two ratio values demonstrates that the contrast in the photocurrent image is determined by other effects rather than a local difference in the barrier height. A more probable one could be a spatially inhomogenous recombination at the metal-semiconductor interface that locally killed the photocurrent. This explanation is also supported by the I - V characteristics of our P t / G a P Schottky diodes that presents a bad rectification ratio, ideality factor near to n = 2, and high saturation current: all indicating recombination contribution to the current across the junction. This preliminary result demonstrates the feasibility of this laterally resolved technique and it can be easily applied to FEL-IPE experiments: the spatially analyzed FEL-IPE (SAN FEL-IPE). It is important to stress that this technique could be applied to real
Pt/GaP
Shear-Force
Reflection
IPE Imaging
Fig. 4. The image in the left is the shear-force result giving the topological homogeneity over an area of 4 0 / ~ m x 40/.tm. The image in the center is the reflection measurement result on the same area. Finally, in the right, the image shows the results of 1PE mapping.
C. Coluzza / Applied Surface Science 92 (1996) 267-272
A
S
I I l~m
B
References
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/ .'"oo °°°°
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~
~"
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C. Dupuy, A. Rudra, M. Ilegems, O. Bergossi, M. Spajer and D. Courjon for their collaboration. This work was supported by the Fonds National Suisse de la Recherche Scientifique, by the Ecole Polytechnique F6d6rale de Lausanne, and by the Office of Naval Research under Contract N00014-91-C-0109 and Grant N00014-91-J-4040.
a)
I
271
," o
,/,f j°
//
• B=1.409 eV \ L ~ OB=I'413eV
1.40 1.42 1.44 Photon Energy (eV)
I
1.46
Fig. 5. In the upper part of the figure is reported the photocurrent mapping of the Pt/GaP sample with a photon energy of 1.5 eV. In the bottom part are shown the Fowler plots of the spatially resolved IPE measurements measured at two different places: at a zone (A) were the measured photocurrent reached a higher value and at a zone (B) presenting a very low photon yield.
heterostructures opening the opportunity of checking and optimizing in line the industrial production of optoelectronic devices. Acknowledgements We thank J. Almeida, Tiziana dell'Orto, F. Gozzo, E. Tuncel, J.L. Staelhi, P.A. Baudat, G. Margaritondo, J.T. McKinley, A. Ueda, V. Barnes, R.G. Albridge, A.N.H. Tolk, D. Martin, F. Morier-Genoud,
[1] See, for example: F. Capasso and G. Margaritondo, Eds., Heterojunction Band Discontinuities: Physics and Device Applications (North-Holland, Amsterdam 1987); G. Margaritondo, Ed., Electronic Structure of Semiconductor Heterojunctions (Kiuwer, Dordrecht, 1988); P. Perfetti, G. Margaritondo and O. Bisi, Eds; Proc. 3rd Int. Conf. on the Formation of Semiconductor Interfaces, Appl. Surf. Sci. 56-58 (1992). [2] G. Margaritondo, J.T. McKinley, D. Rioux and D.W. Niles, Appl. Surf. Sci. 56-58 (1992) 713. [3] C. Coluzza, F. Lama, A. Frova, P. Perfetti, C. Quaresima and M. Capozi, J. Appl. Phys. 64 (1988) 3304. [4] C. Coluzza, G. Margaritondo, A. Neglia and R. Carluccio, J. Vac. Sci. Technoi. A 10 (1992) 744. [5] C. Coluzza, A. Neglia, A. Bennouna, M. Capizzi, R. Carluccio, A. Frova and P.C. Srivastava, Appl. Surf. Sci. 56-58 (1992) 733. [6] S. Perfetti, R. Carluccio, C. Lanzieri, A. Cetronio and C. Coluzza, Appl. Surf. Sci. 56-58 (1992) 335. [7] C. Coluzza, Phys. Scr. T 45 (1992) 192. [8] C. Coluzza, E. Tuncel, J.-L. Staehli, P.A. Baudat, G. Margaritondo, J.T. McKinley, A. Ueda, A.V. Barnes, N. Tolk, D. Martin, F. Morier-Genoud, C. Dupuy, A. Rudra and M. llegems, Phys. Rev. B 46 (1992) 12834. [9] C. Coluzza, J. Almeida, E. Tuncel, J.L. Staelhi, P.A. Baudat, G. Margaritondo, J.T. McKinley, A. Ueda, V. Barnes, R.G. Albridge, A.N.H. Tolk, D. Martin, F. Morier-Genoud, C. Dupuy, A. Rudra and M. Ilegems, SPIE - The International Society for Optical Engineering 1985 (1993) 442. [10] J.T. McKinley, R.G. Albridge, A.V. Barnes, A. Ueda, N.H. Tolk, C. Coluzza, F. Gozzo, G. Margaritondo, D. Martin, F. Morier-Genoud, C. Dupuy, A. Rudra and M. llegems, J. Vac. Sci. Technol. B 11 (1993) 1614. [1 I] J.T. McKinley, R.G. Albridge, A.V. Barnes, G.C. Chen, J.L. Davidson, M.L. Languell, P.L. Polavarapu, J.F. Smith, X. Yang, A. Ueda, N. Tolk, C, Coluzza, P.A. Baudat, C. Dupuy, F. Gozzo, M. llegems, D. Martin, F. Morier-Genoud, A. Rudra, E. Tuneel and G. Margaritondo, J. Vac. Sci. Technol. A 12 (1994) 2323. [12] J.T. McKinley, R.G. Albridge, A.V. Barnes, P.A. Baudat, G.C. Chert, C. Coluzza, J.L. Davidson, C. Dupuy, F. Gozzo, M. Ilegems, M.L. Languell, D. Martin, F. Morier-Genoud, P.L. Polavarapu, A. Rudra, J.F. Smith, E. Tuneel, X. Yang,
272
C. Coluzza / Applied Surface Science 92 (1996) 267-272
A. Ueda, G. Margaritondo and N. Tolk, Nucl. Instr.Meth. Phys. Res. A 341 (1994) 156. [13] R.H. Williams, Appl. Surf. Sci. 70/71 (1993) 386. [14] H. Palm, M. Arbes and M. Schulz, Phys. Rev. Lett. 71 (1993) 2224. [15] K.K. Shih, G.D. Pcttitand M.R. Lorenz, J. Appl. Phys. 39 (1968). [16] A.A. Talin, R.S. Williams, B.A. Morgan, K.M. Ring and K.L. Kavanagh, Phys. Rev. B 49 (1994) 16474.
[17] R. Ludeke, M. Prietsch and A. Samsavar, J. Vac. Sci Technol. B 9 (1991) 2342. [18] D. Courjon, J.M. Vigoreux, M. Spajer, K. Sarayeddine and S. Leblanc, Appi. Opt. 29 (1990) 3734. [19] C. Bainier, S. Leblanc and D. Courjon, Proc. NATO Advanced Research Workshop on Near Field Optics, Arc-etSenans, October 1992, Eds. D.W. Polh and D. Courjon NATO Set. E (Kluwer, Dordrecht, 1993).
Applied Surface Science 92 (1996) 267-272
Ultra accurate measurements of interface parameters with free-electron laser C. C o l u z z a
* ,1
lnstitut de Physique Appliqu~e, Ecole Polytechnique Fdd~rale, CH-IOI5 Lausanne, Switzerland Received 13 December 1994; accepted for publication 4 March 1995
Abstract
We used optical pumping by the Vanderbilt free-electron laser (FEL) and the technique of internal photoemission (IPE) to measure with an accuracy of nearly 5 meV the conduction-band discontinuity of semiconductor heterojunction interfaces as GaA1As-GaAs and Ge-GaAs. Very recently we demonstrated, using a titanium-sapphire pumped laser, that spatially resolved internal photoemission measurements could be performed on a Pt/n-GaP Schottky barrier by a scanning near-field optics microscope within a spatial resolution of 100 nm. Shear-force and photocurrent X-Y images at different photon energies enable us to map the topography and the Schottky barrier height on the same surface. The topography's images, compared with the internal photoemission images, revealed zones where the morphology of the metallic layer was homogeneous, whereas the photocurrent was varying from place to place. Both results opened the possibility of measuring, in a simple and direct way, the local interface properties of real devices. A novel technique with submicrometric spatial resolution could be implemented: the spatially analyzed FEL-IPE (SAN FEL-IPE).
1. Introduction
The band lineup at the heterostrnctures interface has been the subject of intense experimental and theoretical research for many years [1,2]. Unfortunately, progress in this field is adverse affected by the limitations of the experimental methods to measure band discontinuities [1]. The conduction-band and valence-band discontinuities at the interface in heterojunctions can be determined by a number of experimental techniques. In most cases, the accuracy
* Corresponding author. Fax: +39 6 49914535; e-mail: coluzza@romal .infn.it. l Present address: Dipartimento di Fisica, Universith di Roma "La Sapienza", P.le A. Moro 2, 00185 Roma, Italy.
of the measurements is not sufficient to discriminate one theory from another; in the few cases in which high accuracy is obtained, one must work with specialized preparation techniques a n d / o r one has questionable reliability [1]. Internal photoemission (IPE) is a simple and direct method for determining the band discontinuities at semiconductor interfaces [37]. Free-electron laser internal photoemission (FELIPE) is an optical technique that achieves very high accuracy and complex modeling is not required to interpret the experimental data [8-12]. The conduction band discontinuity A E c is given by the lowest photon energy that can induce electronic transitions from the conduction-band minimum of one material to the conduction-band minimum of the other, so as to produce a photocurrent at the interface. FEL-IPE has important advantages over other techniques for
0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 1 6 9 - 4 3 3 2 ( 9 5 ) 0 0 2 4 0 - 5
C. Coluzza /Applied Surface Science 92 (1996) 267-272
268
measuring heterojunction band discontinuities: it can be used on m a t u r e interfaces buried deep beneath a surface, making it possible to study device quality interfaces without the requirement of in situ growth and measurement. Furthermore, the discontinuities can be measured while the interface is under bias, which is necessarily the case for any interface used in a " r e a l " device. FEL-IPE can also measure the positions of localized interface states and interface quantum well states not accessible to conventional photoemission. A scheme of the internal photoemission processes induced by FEL-IR (A) and by visible (B) sources are reported in Fig. 1.
Ge(lp.m)/n-GaAs reverse bias 77I( light on Ga
[.(..)vv2,-n,E
1
ift
1 0.31
0.33
0.35
0.37
0.39
Photon Energy leVI
2. E x a m p l e s o f s o m e F E L - I P E
results
Fig. 2 shows a typical FEL-IPE spectrum (curve (a) in the upper part) near the photocurrent threshold region for the GaAs/amorphous-Ge heterojunction, A E¢ is given directly by the photon energy at the threshold (E). The Fowler fit gave 0.334 eV. In the same frame it is also shown the spectrum (curve (b) in the upper part) of the FEL-photoconductivity measurements made on the GaAs. The photoconductivity measurements were performed on the same sample locking the photocurrent across two parallel strips of ohmic contacts on the back of the crystalline semiconductor. The photoconductivity did not show any onset on the same photon energy interval ruling out other contributions to the onset in curve (a) different
Na_ __h i .....~
" (B)
Fig. 1. Scheme of the internal photoemission processes induced in heterojunctions by FEL-IR (A) and by visible (B) sources. Similar photocurrent processes are expected for Sehottky barriers.
r~
1
1.1
1.2
1.3
Plmtou Energy leVI Fig. 2. Typical FEL-IPE spectrum (curve (a) in the upper part) near the photocurrent threshold region for the GaAs/amorphousGe heterojunction at 77 K. The curve (b) in the upper part is the GaAs FEL-photoconductivity measured on the substrate of the same sample. In the lower part is reported the curve corresponding to the visible internal photoemission made on the same sample.
from conduction-to-conduction band intemal photoemission. Our discontinuity measurements of the GaAs/amorphous-Ge heterojunction was accurate at least to + 5 meV. This level of accuracy is required to correctly distinguish between the rival theoretical models of the heterojunction band lineup. Therefore, systematic use of the FEL-IPE technique over wide classes of interfaces potentially could resolve longstanding theoretical debates. FEL-IPE gives complementary information to the earlier visible lamp-IPE experiments [3-7] which used monochromatic visible light to induce transitions from the valence-band maximum of the smaller gap semiconductor (or from metallic contact) to the conduction-band minimum of the larger gap semi-
269
C. Coluzza / Applied Surface Science 92 (1996) 267-272
3. Spatially resolved IPE measurements
conductor. In this case, A E c (or the barrier height ~b in the Schottky junctions) is given by the threshold photon energy minus the value of the smaller band gap (or directly from the onset in the square root of the yield curve versus the photon energy). If flat-band conditions exist at the interface, the comparison of the visible-IPE with FEL-IPE is a direct measurement of the interfacial band gap of the small gap material. This is important because processes such as microdiffusion and interfacial strain can introduce uncertainty about the value of the near-interface band gap relative to the bulk gap. In the lower part of Fig. 2 it is reported the curve corresponding to the visible internal photoemission made on the same sample. The value of conduction-band discontinuity is given by the relation A E c = Eg(Ge) - E', where E' is the measured onset and Es(G-e) is the Ge band gap at the temperature of the measurement (77 K). The comparison between the A E c measured by FEL-IPE and by visible-IPE resulted in a Eg(Ge)= 0.76 eV in good agreement with the value reported in the literature. Unfortunately must of such interfaces processes are not laterally homogeneous and a complete understanding demands for spatially resolved measurements.
turnable Ti-Sapphire laser
Spatially resolved measurements of heterostructures is a very actual subject in todays research. The combination of non-destructive with high spatial resolution techniques gives an interesting analysis possibility [13-17]. Among high spatial resolution techniques as ballistic electron emission microscopy, the scanning near-field optics microscope showed their possibilities for surface microanalysis [18,19]. High lateral resolution is achieved by illuminating the sample by a small light source, and an extension of the optical characterization is used to locally study a metal-semiconductor barrier. Another major research thrust will be IPE microscopy which can check the spatial uniformity of interfaces. To do this, microscopy with lateral resolution less than the Debye length (typically 100-1000 ,~) must be achieved. Using near-field microscopy techniques (SNOM), this level of spatial resolution should be possible. This technique demands for coherent and tunable sources: the case of a FEL. This is an important step since spatial averaging over a large interface with nonuniformity in the discontinuity can introduce error by broadening the photocurrent threshold. In Fig.
reflection
photomultiplier 1 lock-in amplifier
~---~wave lenght ] laseromatrol filter &
shear force signal
oscillator []
opticalfibe:
, plezoVZ"brati°n z piezo
x-y scanner
photocurrent signal lock-in amplifier
computer Fig. 3. Scheme of the spatially analyzed internalphotoemissionapparatus.
C. Coluzza / Applied Surface Science 92 (1996) 267-272
270
3 is reported a scheme of the apparatus used for the spatially analyzed IPE measurements. The spatially resolved measurements for a P t / G a P Schottky barrier is shown in Fig. 4. The mapping scanned an area of 40 /.tm x 40 /xm, and it was obtained by a conventional Ti: sapphire laser, tuned at a photon energy of 1.42 eV, that is greater than the Schottky barrier height ~b" The image in the left is the shear-force result giving the topological homogeneity. The image in the center is the reflection measurement result. Finally, in the right, the image shows the results of IPE mapping: there are some features that represents the local properties of the interface and that are not evident in the previous two images. Fig. 5 shows photocurrent mapping of the P t / G a P sample at a photon energy of 1.5 eV (by> qbb). In the lower part, are reported the Fowler plots of the spatially resolved IPE measurements taken at two different places of the P t / G a P sample: at a zone (A) were the measured photocurrent reached a higher value and at a zone (B) presenting a very low photon yield. The two obtained values for qbb (1.413 eV at the " d a r k " zone and 1.409 eV at the "bright" one) both are in good agreement with the value obtained by spatially integrated IPE (1.412 eV) and with the
value quoted in the literature. Using the Fowler power law Y or ( h v - ~ b ) 2, where Y is the yield and hv is the photon energy, we can estimate the ratio between the intensity at the two different places. The result is a value close to 1.1 for a photon energy of h v = 1.5 eV. This value has to be compared with the experimental value (of the order of 3) for the ratio between the pixel intensity at the two zones of the photocurrent image obtained with the same photon energy. The clear difference between the two ratio values demonstrates that the contrast in the photocurrent image is determined by other effects rather than a local difference in the barrier height. A more probable one could be a spatially inhomogenous recombination at the metal-semiconductor interface that locally killed the photocurrent. This explanation is also supported by the I - V characteristics of our P t / G a P Schottky diodes that presents a bad rectification ratio, ideality factor near to n = 2, and high saturation current: all indicating recombination contribution to the current across the junction. This preliminary result demonstrates the feasibility of this laterally resolved technique and it can be easily applied to FEL-IPE experiments: the spatially analyzed FEL-IPE (SAN FEL-IPE). It is important to stress that this technique could be applied to real
Pt/GaP
Shear-Force
Reflection
IPE Imaging
Fig. 4. The image in the left is the shear-force result giving the topological homogeneity over an area of 4 0 / ~ m x 40/.tm. The image in the center is the reflection measurement result on the same area. Finally, in the right, the image shows the results of 1PE mapping.
C. Coluzza / Applied Surface Science 92 (1996) 267-272
A
S
I I l~m
B
References
b)
iiiii1:0o o°° o
/ .'"oo °°°°
B
~
~"
°
C. Dupuy, A. Rudra, M. Ilegems, O. Bergossi, M. Spajer and D. Courjon for their collaboration. This work was supported by the Fonds National Suisse de la Recherche Scientifique, by the Ecole Polytechnique F6d6rale de Lausanne, and by the Office of Naval Research under Contract N00014-91-C-0109 and Grant N00014-91-J-4040.
a)
I
271
," o
,/,f j°
//
• B=1.409 eV \ L ~ OB=I'413eV
1.40 1.42 1.44 Photon Energy (eV)
I
1.46
Fig. 5. In the upper part of the figure is reported the photocurrent mapping of the Pt/GaP sample with a photon energy of 1.5 eV. In the bottom part are shown the Fowler plots of the spatially resolved IPE measurements measured at two different places: at a zone (A) were the measured photocurrent reached a higher value and at a zone (B) presenting a very low photon yield.
heterostructures opening the opportunity of checking and optimizing in line the industrial production of optoelectronic devices. Acknowledgements We thank J. Almeida, Tiziana dell'Orto, F. Gozzo, E. Tuncel, J.L. Staelhi, P.A. Baudat, G. Margaritondo, J.T. McKinley, A. Ueda, V. Barnes, R.G. Albridge, A.N.H. Tolk, D. Martin, F. Morier-Genoud,
[1] See, for example: F. Capasso and G. Margaritondo, Eds., Heterojunction Band Discontinuities: Physics and Device Applications (North-Holland, Amsterdam 1987); G. Margaritondo, Ed., Electronic Structure of Semiconductor Heterojunctions (Kiuwer, Dordrecht, 1988); P. Perfetti, G. Margaritondo and O. Bisi, Eds; Proc. 3rd Int. Conf. on the Formation of Semiconductor Interfaces, Appl. Surf. Sci. 56-58 (1992). [2] G. Margaritondo, J.T. McKinley, D. Rioux and D.W. Niles, Appl. Surf. Sci. 56-58 (1992) 713. [3] C. Coluzza, F. Lama, A. Frova, P. Perfetti, C. Quaresima and M. Capozi, J. Appl. Phys. 64 (1988) 3304. [4] C. Coluzza, G. Margaritondo, A. Neglia and R. Carluccio, J. Vac. Sci. Technoi. A 10 (1992) 744. [5] C. Coluzza, A. Neglia, A. Bennouna, M. Capizzi, R. Carluccio, A. Frova and P.C. Srivastava, Appl. Surf. Sci. 56-58 (1992) 733. [6] S. Perfetti, R. Carluccio, C. Lanzieri, A. Cetronio and C. Coluzza, Appl. Surf. Sci. 56-58 (1992) 335. [7] C. Coluzza, Phys. Scr. T 45 (1992) 192. [8] C. Coluzza, E. Tuncel, J.-L. Staehli, P.A. Baudat, G. Margaritondo, J.T. McKinley, A. Ueda, A.V. Barnes, N. Tolk, D. Martin, F. Morier-Genoud, C. Dupuy, A. Rudra and M. llegems, Phys. Rev. B 46 (1992) 12834. [9] C. Coluzza, J. Almeida, E. Tuncel, J.L. Staelhi, P.A. Baudat, G. Margaritondo, J.T. McKinley, A. Ueda, V. Barnes, R.G. Albridge, A.N.H. Tolk, D. Martin, F. Morier-Genoud, C. Dupuy, A. Rudra and M. Ilegems, SPIE - The International Society for Optical Engineering 1985 (1993) 442. [10] J.T. McKinley, R.G. Albridge, A.V. Barnes, A. Ueda, N.H. Tolk, C. Coluzza, F. Gozzo, G. Margaritondo, D. Martin, F. Morier-Genoud, C. Dupuy, A. Rudra and M. llegems, J. Vac. Sci. Technol. B 11 (1993) 1614. [1 I] J.T. McKinley, R.G. Albridge, A.V. Barnes, G.C. Chen, J.L. Davidson, M.L. Languell, P.L. Polavarapu, J.F. Smith, X. Yang, A. Ueda, N. Tolk, C, Coluzza, P.A. Baudat, C. Dupuy, F. Gozzo, M. llegems, D. Martin, F. Morier-Genoud, A. Rudra, E. Tuneel and G. Margaritondo, J. Vac. Sci. Technol. A 12 (1994) 2323. [12] J.T. McKinley, R.G. Albridge, A.V. Barnes, P.A. Baudat, G.C. Chert, C. Coluzza, J.L. Davidson, C. Dupuy, F. Gozzo, M. Ilegems, M.L. Languell, D. Martin, F. Morier-Genoud, P.L. Polavarapu, A. Rudra, J.F. Smith, E. Tuneel, X. Yang,
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C. Coluzza / Applied Surface Science 92 (1996) 267-272
A. Ueda, G. Margaritondo and N. Tolk, Nucl. Instr.Meth. Phys. Res. A 341 (1994) 156. [13] R.H. Williams, Appl. Surf. Sci. 70/71 (1993) 386. [14] H. Palm, M. Arbes and M. Schulz, Phys. Rev. Lett. 71 (1993) 2224. [15] K.K. Shih, G.D. Pcttitand M.R. Lorenz, J. Appl. Phys. 39 (1968). [16] A.A. Talin, R.S. Williams, B.A. Morgan, K.M. Ring and K.L. Kavanagh, Phys. Rev. B 49 (1994) 16474.
[17] R. Ludeke, M. Prietsch and A. Samsavar, J. Vac. Sci Technol. B 9 (1991) 2342. [18] D. Courjon, J.M. Vigoreux, M. Spajer, K. Sarayeddine and S. Leblanc, Appi. Opt. 29 (1990) 3734. [19] C. Bainier, S. Leblanc and D. Courjon, Proc. NATO Advanced Research Workshop on Near Field Optics, Arc-etSenans, October 1992, Eds. D.W. Polh and D. Courjon NATO Set. E (Kluwer, Dordrecht, 1993).