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Temperature dependent dechanneling of 1.4 MeV He+ and optical near-edge absorption in B+ implanted GaP
NININI B
Nuclear Instruments and Methods in Physics Research B 85 (1994) 533-536
Beam Interactions with Materials&Atoms
North-Holland
Temperature dependent dechanneling of 1.4 MeV He+ and optical near-edge absorption in B+ implanted GaP W. Wesch a, E. Wendler a,*, J. Kaczanowski b, A. Turos b a Friedrich-Schiller-Universitiit Jena, Institut fiir Festksperphysik, Max-wien-Platz I,
D-07743 Jena, Germany
b Soltan Institute for Nuclear Studies, Hoza 69, 00-681 Warsaw, Poland
Weakly damaged boron implanted GaP has been investigated by means of temperature dependent RBS-channeling measurements in combination with Monte Carlo channeling simulation as well as by optical transmission measurements. In the dose region between 5X 1013 cm-* and 2X1014 cm-’ displaced atoms with displacement distances r, = 0.3 A up to 0.5 A exist, the concentration of which increases with the ion fluence. The near-edge optical properties correspond to those expected for material which contains mainly point defects and point defect complexes. The nearly linear dependence of the absorption coefficient K on the concentration Npd of displaced atoms confirms the correlation between the near-edge optical properties and the defects produced. Further, both K and Npd increase linearly with the primarily produced vacancy concentration N,,,, indicating the stability of the defects at room temperature.
1. Introduction
2. Experimental
It has been shown previously that the analysis of the temperature dependence of the dechanneling of ions provides information about the concentration and the position of displaced atoms in weakly damaged crystal layers [l]. By measuring the minimum yield xmin of perfect and damaged crystals at two temperatures and evaluating the spectra in the framework of the discontinuous model of dechanneling the displacement distance ra and concentration profiles of atoms perpendicular to the crystalline direction are obtained. This method has been successfully applied to the analysis of weakly damaged ion implanted GaAs layers where two distinct positions vf displaced atoms around ra = 0.18 A and ra = 0.65 A were found, the concentration of which depends on the implantation parameters [2,3]. By Monte Carlo channeling simulations the results obtained with the use of the analytical model could be verified [4]. In the present work we have investigated weakly damaged ion implanted GaP layers by means of temperature dependent RBS/channeling measurements in combination with Monte Carlo channeling simulation and with optical transmission measurements.
(100) GaP samples were implanted with 150 keV B+ ions at room temperature (ion doses 5 X 1013 cmp2, 1 x 1014 cm-’ and 2 x lOi cmp2). Implanted and virgin crystals were analyzed by means of RBS-channeling measurements at 293 K and 105 K using 1.4 MeV He+ ions and a backscattering angle of 177”. The defect concentration n,,(z) =&/No (Npd absolute defect concentration, No - atomic density) and the displacement distance ra (perpendicular to the (100) direction) of the displaced atoms were determined with the help of Monte Carlo channeling simulation of the energy spectra of backscattered ions. Additionally, the depth profile and the frequency dependence of the absorption coefficient K were obtained by means of optical transmission measurements (frequency range 1.4 eV Q ho Q 2.2 eV) in combination with successive chemical etching of the implanted layers [5]. For comparison, the depth distribution of primarily produced vacancies N,,, was calculated by Monte Carlo TRIM-87 simulation [6].
3. Results and discussion
* Corresponding 3641 23 843.
author, phone +49 3641 635 870, fax +49
0168-583X/94/$07.00
As a first step the channeling spectra of virgin (100) GaP were simulated by means of Monte Carlo channeling simulations for the two measurement tem-
0 1994 - Elsevier Science B.V. All rights reserved
SSDZ 0168-583X(93)E0506-C
VIII. LATTICE SITES
534
W. Wesch et al. /Nucl.
Instr. and Meth. in Phys. Res. B 85 (1994) 533-536
peratures 293 K and 105 K assuming different values of thermal vibrational amplitudes of the lattice atoms. In Fig. 1 the minimum yield xmin = Y,i/Y,,(Y,,, Y,, backscattering yields for aligned and random incidence, respectively) of crystalline GaP measured at the two temperatures is depicted versus the energy of points). The backscattered He+ ions (measurement best agreement of the experimental ~,i,(El-curves with those calculated with the Monte Carlo channeling simulation was obtaited assuming thermal vibrational amplitudes of 0.086 A for room temperature which is in good agreement with the previously published mean value of 0.085 A 171, and of 0.067 A for 105 K (solid lines in Fig. 1). Considering the differences in the minimum yield of damaged and perfect GaP, Axmin = ~2: - xgif measured at the two temperatures, a different behaviour from GaAs is found. Whereas in GaAs at sufficiently small defect concentrations a so-called negative temperature dependence of Axmin, i.e. Axmin (105 K) > Axmin (293 K), is found which points to high concentrations of atoms with small displacement distance, the temperature dependence in GaP for comparable defect concentrations is always positive (i.e. Axmin (293 K) > Ax,~,, (105 K); for details see ref. [5]). This indicates that in GaP such high concentrations of atoms with small displacement as in GaAs do not seem to exist. Because an evaluation of the spectra of materials with a large mass difference of the constituents in the framework of the discontinuous model of dechanneling is not possible at present, the Monte Carlo channeling simulation was applied to simulate the energy spectra of the implanted GaP samples. The quantity considered in this case is the minimum yield xmin for the two measurement temperatures used as a function of the energy of backscattered He+ ions (which corresponds
E (keV) Fig. 1. Minimum yield xmin of crystalline (100) GaP versus the energy E of backscattered He+ ions. The symbols represent the results of measurement at 293 K (0) and 105 K (0). The solid curves represent the results of Monte Carlo channeling simulation with thermal vibrational amplitudes of 0.086 A (293 K) and 0.067 w (105 K).
lo
E &W Fig. 2. Minimum yield xmin versus the energy E of backscattered He+ ions of GaP implanted with 5 X 1013 Bf cm-’ at room temperature and measured at 293 K (0) and 105 K (0). The solid curves are the result of the simulations with different displacement distances ra perpendicular to the (100) direction.
to the depth z). For the simulations uncorrelated displaced atoms with displacement distances ra in the range between 0.2 and 0.8 A have been assumed. Furthermore, the thermal vibrational amplitudes determined from the crystalline spectra were also used for the displaced atoms. For the depth distribution the profiles calculated with TRIM 87 had to be shifted slightly towards the surface to fit the shape of the ,ymin curves. As an example, Fig. 2 shows the minimum yield xmin as a function of the energy of backscattered ions for GaP implanted with 5 x lOi B+ cm-’ and measured at 293 K and 105 K (measurement points). The comparison with the calculated spectra (full lines) clearly shows that within the error of the methods the experimental spectra can be fittedowith displacement distances ra between 0.3 and 0.5 A. The same result was obtained for the samples implanted with 1 X 1014 Bf cm-’ and 2 X 1014 B+ cmp2 (not shown). That means, in weakly damaged GaP in the dose region investigated, and independent of tFe ion fluence, atoms displaced between 0.3 and 0.5 A from their regular positions exist. The corresponding ;elative concentration n,,(z) of atoms with r, = 0.4 A determined with the simulations are represented in Fig. 3 for the three ion fluences. Additional information about the concentration and the kind of defects can be obtained by analyzing the near-edge optical properties of the material [8]. Optical transmission measurements in the near-edge spectral region performed on the B+ implanted GaP samples yield an exponential dependence of the absorption coefficient on the photon energy with characteristic tailing energies 0.44 eV Q E, Q 0.54 eV and an only slight increase of the refractive index (An/n < 0.01). These results are a hint to the existence of weakly damaged layers containing mainly point defects and point defect complexes.
W. Wesch et al. /Nucl.
Instr. and Meth. in Phys. Res. B 85 (1994) 533-536
7 18
0.4
GaP:B+
(15OkeV)
0.1
2
0.0 0.0
0.2
0.4
0.6
0 0.8
Fig. 3. RFlative concentrations n,,(z) of atoms displaced to ra = 0.4 A in GaP implanted with various ion fluences. For comparison the profiles of the near-edge absorption coefficient K at hw = 2.1 eV for two ion fluences are included.
The profiles of the absorption coefficient K determined from the transmission measurements in combination with successive chemical etching are included in Fig. 3 for comparison. As can be seen, good agreement between the concentration of displaced lattice atoms npd and the absorption coefficient K is found with respect to the position of the maxima of the profiles. However, in the transition region to the crystalline substrate the profiles deviate from each other. One possible reason for that may be the existence of a thin layer containing defects which cannot be described with the assumption of uncorrelated displaced lattice atoms (e.g. small dislocation loops). To obtain further information about the correlation of the absorption coefficient K with the defect concentration, represented by the absolute concentration of displaced atoms Npd, K (at Aw = 2.1 eV) and the tailing energy E, are depicted versus Npd (see Fig. 4). The values of K and E, are determined for each etched layer, and for ND,, the mean value within the
lo-‘-103 ld’
Id* Nr,,,(cm-3)
ld3
Fig. 4. Absorption coefficient K at ho = 2.1 eV and tailing energy E, versus the absolute concentration Npd of displaced lattice atoms. Curves 1 and 2 for EO(Npd) were calculated using theoretical considerations of Unger [13] and BonchBruevich [14].
535
corresponding layer is taken. Fig. 4 shows that over a wide range K is linearly proportional to the concentration of displaced lattice atoms. This supports the previously drawn conclusion [9] that the absorption coefficient in the near-edge region can be taken as a measure of the defect concentration (see also ref. [lo]). The absorption coefficient in the B+ implanted GaP layers increases linearly with the concentration of primarily produced vacancies N,,, (for details see refs. [5,11]). Because of the approximate proportionality between Npd and K (see Figs. 3 and 4) Npd also depends linearly on N,,,, confirming the assumption that during room temperature implantation in GaP the primarily produced defect complexes are almost stable and defect transformation and annealing are less important than in GaAs, for instance ref. [9]. The dominating process of defect accumulation therefore seems to be heterogeneous defect nucleation [12]. The tailing energy E, increases only weakly as a function of the concentration of displaced atoms Npd (see Fig. 4). However, taking into account theoretical estimations of E, as a function of the concentration of absorbing centres, which were derived for highly doped semiconductors [13,14], the agreement with the experimental results is rather good (see also ref. [lo]). This can be taken as a further confirmation that the near-edge optical absorption is strongly correlated to the concentration of defects in the weakly damaged ion implanted semiconductor layers.
4. Summary Using temperature dependent RBS-channeling measurements in combination with the Monte Carlo channeling simulation the concentration and position of displaced lattice atoms in weakly damaged boron implanted GaP has been determined. The results show that in the investigated region of the ion fluence atoms with displacement distances ra perpendicular to the (100) direction between 0.3 _& and 0.5 A exist. The relative concentration of the displaced atoms in the maximum of the depth distribution varies between about 8% and 28% for ion fluences between 5 x 1013 cm-* and 2 X 1Or4 cm-‘. The near-edge optical parameters indicate that the damaged layers contain mainly point defects and/or point defect complexes. The absorption coefficient K is approximately linearly proportional to the concentration Npd of displaced lattice atoms indicating the strong correlation between the defects produced and the near-edge optical properties. Because K as well as Npd increase nearly proportionally with the primarily produced vacancy concentration N,,, the previously drawn conclusion is confirmed that the defects produced in GaP at room temperature are rather stable. VIII. LA-RICE SITES
536
W. Wesch et al. /Nucl. Znstr. and Meth. in Phys. Res. B 85 (1994) 533-536
However, detailed investigations are necessary to get more accurate information about the displacement distance and the possible defect configuration connected with the displaced atoms.
Acknowledgement
The authors wish to thank K. GIrtner discussions.
for helpful
References [l] K. Gartner, K. Hehl and G. Schlotzhauer, Nucl. Instr. and Meth. 216 (1983) 275; B 4 (1984) 55 and B 4 (19841 63. [2] W. Wesch, A. Jordanov, K. Gartner and G. GGtz, Nucl. Instr. and Meth. B 39 (1989) 445.
[3] W. Wesch, K. GHrtner, A. Jordanov, and G. Giitz, Nucl. Instr. and Meth. B 45 (1990) 446. [4] A. Dygo, J. Kaczanowski, A. Turos, W. Wesch, K. GIrtner and G. Gatz, Nucl. Instr. and Meth. B 64 (1992) 721. [5] W. Wesch, E. Wendler, and K. Glirtner, Nucl. Instr. and Meth. B 63 (1992) 52. [6] J.P. Biersack and L.C. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [7] A. Dygo and A. Turos, Phys. Rev. B 40 (1989) 7704. [S] W. Wesch, E. Wendler and G. GGtz, J. Appl. Phys. 65 (1989) 519. [9] E. Wendler and W. Wesch, Nucl. Instr. and Meth. B 73 (1993) 489. [lo] E. Wendler, K. Glrtner, W. Wesch, U. Zammit and K.N. Madhusoodanan, these Proceedings (IBA-11, Balatoufired, Hungary, 1993) Nucl. Instr. and Meth. B 85 (1994) 528. [ll] W. Wesch, Proc. MRS Spring Meeting, San Francisco 1993, MRS Symp. Proc. vol. 300 (1993). [12] L.T. Chadderton, Radiat. Eff. 8 (1971) 77. 1131K. Unger, in: Semiconductor Optoelectronics, ed. M.A. Herman (PWN, Warsaw, 1980) p. 13. [14] V.L. Bench-Bruevich, Phys. Status Solidi 42 (1979) 35.
Nuclear Instruments and Methods in Physics Research B 85 (1994) 533-536
Beam Interactions with Materials&Atoms
North-Holland
Temperature dependent dechanneling of 1.4 MeV He+ and optical near-edge absorption in B+ implanted GaP W. Wesch a, E. Wendler a,*, J. Kaczanowski b, A. Turos b a Friedrich-Schiller-Universitiit Jena, Institut fiir Festksperphysik, Max-wien-Platz I,
D-07743 Jena, Germany
b Soltan Institute for Nuclear Studies, Hoza 69, 00-681 Warsaw, Poland
Weakly damaged boron implanted GaP has been investigated by means of temperature dependent RBS-channeling measurements in combination with Monte Carlo channeling simulation as well as by optical transmission measurements. In the dose region between 5X 1013 cm-* and 2X1014 cm-’ displaced atoms with displacement distances r, = 0.3 A up to 0.5 A exist, the concentration of which increases with the ion fluence. The near-edge optical properties correspond to those expected for material which contains mainly point defects and point defect complexes. The nearly linear dependence of the absorption coefficient K on the concentration Npd of displaced atoms confirms the correlation between the near-edge optical properties and the defects produced. Further, both K and Npd increase linearly with the primarily produced vacancy concentration N,,,, indicating the stability of the defects at room temperature.
1. Introduction
2. Experimental
It has been shown previously that the analysis of the temperature dependence of the dechanneling of ions provides information about the concentration and the position of displaced atoms in weakly damaged crystal layers [l]. By measuring the minimum yield xmin of perfect and damaged crystals at two temperatures and evaluating the spectra in the framework of the discontinuous model of dechanneling the displacement distance ra and concentration profiles of atoms perpendicular to the crystalline direction are obtained. This method has been successfully applied to the analysis of weakly damaged ion implanted GaAs layers where two distinct positions vf displaced atoms around ra = 0.18 A and ra = 0.65 A were found, the concentration of which depends on the implantation parameters [2,3]. By Monte Carlo channeling simulations the results obtained with the use of the analytical model could be verified [4]. In the present work we have investigated weakly damaged ion implanted GaP layers by means of temperature dependent RBS/channeling measurements in combination with Monte Carlo channeling simulation and with optical transmission measurements.
(100) GaP samples were implanted with 150 keV B+ ions at room temperature (ion doses 5 X 1013 cmp2, 1 x 1014 cm-’ and 2 x lOi cmp2). Implanted and virgin crystals were analyzed by means of RBS-channeling measurements at 293 K and 105 K using 1.4 MeV He+ ions and a backscattering angle of 177”. The defect concentration n,,(z) =&/No (Npd absolute defect concentration, No - atomic density) and the displacement distance ra (perpendicular to the (100) direction) of the displaced atoms were determined with the help of Monte Carlo channeling simulation of the energy spectra of backscattered ions. Additionally, the depth profile and the frequency dependence of the absorption coefficient K were obtained by means of optical transmission measurements (frequency range 1.4 eV Q ho Q 2.2 eV) in combination with successive chemical etching of the implanted layers [5]. For comparison, the depth distribution of primarily produced vacancies N,,, was calculated by Monte Carlo TRIM-87 simulation [6].
3. Results and discussion
* Corresponding 3641 23 843.
author, phone +49 3641 635 870, fax +49
0168-583X/94/$07.00
As a first step the channeling spectra of virgin (100) GaP were simulated by means of Monte Carlo channeling simulations for the two measurement tem-
0 1994 - Elsevier Science B.V. All rights reserved
SSDZ 0168-583X(93)E0506-C
VIII. LATTICE SITES
534
W. Wesch et al. /Nucl.
Instr. and Meth. in Phys. Res. B 85 (1994) 533-536
peratures 293 K and 105 K assuming different values of thermal vibrational amplitudes of the lattice atoms. In Fig. 1 the minimum yield xmin = Y,i/Y,,(Y,,, Y,, backscattering yields for aligned and random incidence, respectively) of crystalline GaP measured at the two temperatures is depicted versus the energy of points). The backscattered He+ ions (measurement best agreement of the experimental ~,i,(El-curves with those calculated with the Monte Carlo channeling simulation was obtaited assuming thermal vibrational amplitudes of 0.086 A for room temperature which is in good agreement with the previously published mean value of 0.085 A 171, and of 0.067 A for 105 K (solid lines in Fig. 1). Considering the differences in the minimum yield of damaged and perfect GaP, Axmin = ~2: - xgif measured at the two temperatures, a different behaviour from GaAs is found. Whereas in GaAs at sufficiently small defect concentrations a so-called negative temperature dependence of Axmin, i.e. Axmin (105 K) > Axmin (293 K), is found which points to high concentrations of atoms with small displacement distance, the temperature dependence in GaP for comparable defect concentrations is always positive (i.e. Axmin (293 K) > Ax,~,, (105 K); for details see ref. [5]). This indicates that in GaP such high concentrations of atoms with small displacement as in GaAs do not seem to exist. Because an evaluation of the spectra of materials with a large mass difference of the constituents in the framework of the discontinuous model of dechanneling is not possible at present, the Monte Carlo channeling simulation was applied to simulate the energy spectra of the implanted GaP samples. The quantity considered in this case is the minimum yield xmin for the two measurement temperatures used as a function of the energy of backscattered He+ ions (which corresponds
E (keV) Fig. 1. Minimum yield xmin of crystalline (100) GaP versus the energy E of backscattered He+ ions. The symbols represent the results of measurement at 293 K (0) and 105 K (0). The solid curves represent the results of Monte Carlo channeling simulation with thermal vibrational amplitudes of 0.086 A (293 K) and 0.067 w (105 K).
lo
E &W Fig. 2. Minimum yield xmin versus the energy E of backscattered He+ ions of GaP implanted with 5 X 1013 Bf cm-’ at room temperature and measured at 293 K (0) and 105 K (0). The solid curves are the result of the simulations with different displacement distances ra perpendicular to the (100) direction.
to the depth z). For the simulations uncorrelated displaced atoms with displacement distances ra in the range between 0.2 and 0.8 A have been assumed. Furthermore, the thermal vibrational amplitudes determined from the crystalline spectra were also used for the displaced atoms. For the depth distribution the profiles calculated with TRIM 87 had to be shifted slightly towards the surface to fit the shape of the ,ymin curves. As an example, Fig. 2 shows the minimum yield xmin as a function of the energy of backscattered ions for GaP implanted with 5 x lOi B+ cm-’ and measured at 293 K and 105 K (measurement points). The comparison with the calculated spectra (full lines) clearly shows that within the error of the methods the experimental spectra can be fittedowith displacement distances ra between 0.3 and 0.5 A. The same result was obtained for the samples implanted with 1 X 1014 Bf cm-’ and 2 X 1014 B+ cmp2 (not shown). That means, in weakly damaged GaP in the dose region investigated, and independent of tFe ion fluence, atoms displaced between 0.3 and 0.5 A from their regular positions exist. The corresponding ;elative concentration n,,(z) of atoms with r, = 0.4 A determined with the simulations are represented in Fig. 3 for the three ion fluences. Additional information about the concentration and the kind of defects can be obtained by analyzing the near-edge optical properties of the material [8]. Optical transmission measurements in the near-edge spectral region performed on the B+ implanted GaP samples yield an exponential dependence of the absorption coefficient on the photon energy with characteristic tailing energies 0.44 eV Q E, Q 0.54 eV and an only slight increase of the refractive index (An/n < 0.01). These results are a hint to the existence of weakly damaged layers containing mainly point defects and point defect complexes.
W. Wesch et al. /Nucl.
Instr. and Meth. in Phys. Res. B 85 (1994) 533-536
7 18
0.4
GaP:B+
(15OkeV)
0.1
2
0.0 0.0
0.2
0.4
0.6
0 0.8
Fig. 3. RFlative concentrations n,,(z) of atoms displaced to ra = 0.4 A in GaP implanted with various ion fluences. For comparison the profiles of the near-edge absorption coefficient K at hw = 2.1 eV for two ion fluences are included.
The profiles of the absorption coefficient K determined from the transmission measurements in combination with successive chemical etching are included in Fig. 3 for comparison. As can be seen, good agreement between the concentration of displaced lattice atoms npd and the absorption coefficient K is found with respect to the position of the maxima of the profiles. However, in the transition region to the crystalline substrate the profiles deviate from each other. One possible reason for that may be the existence of a thin layer containing defects which cannot be described with the assumption of uncorrelated displaced lattice atoms (e.g. small dislocation loops). To obtain further information about the correlation of the absorption coefficient K with the defect concentration, represented by the absolute concentration of displaced atoms Npd, K (at Aw = 2.1 eV) and the tailing energy E, are depicted versus Npd (see Fig. 4). The values of K and E, are determined for each etched layer, and for ND,, the mean value within the
lo-‘-103 ld’
Id* Nr,,,(cm-3)
ld3
Fig. 4. Absorption coefficient K at ho = 2.1 eV and tailing energy E, versus the absolute concentration Npd of displaced lattice atoms. Curves 1 and 2 for EO(Npd) were calculated using theoretical considerations of Unger [13] and BonchBruevich [14].
535
corresponding layer is taken. Fig. 4 shows that over a wide range K is linearly proportional to the concentration of displaced lattice atoms. This supports the previously drawn conclusion [9] that the absorption coefficient in the near-edge region can be taken as a measure of the defect concentration (see also ref. [lo]). The absorption coefficient in the B+ implanted GaP layers increases linearly with the concentration of primarily produced vacancies N,,, (for details see refs. [5,11]). Because of the approximate proportionality between Npd and K (see Figs. 3 and 4) Npd also depends linearly on N,,,, confirming the assumption that during room temperature implantation in GaP the primarily produced defect complexes are almost stable and defect transformation and annealing are less important than in GaAs, for instance ref. [9]. The dominating process of defect accumulation therefore seems to be heterogeneous defect nucleation [12]. The tailing energy E, increases only weakly as a function of the concentration of displaced atoms Npd (see Fig. 4). However, taking into account theoretical estimations of E, as a function of the concentration of absorbing centres, which were derived for highly doped semiconductors [13,14], the agreement with the experimental results is rather good (see also ref. [lo]). This can be taken as a further confirmation that the near-edge optical absorption is strongly correlated to the concentration of defects in the weakly damaged ion implanted semiconductor layers.
4. Summary Using temperature dependent RBS-channeling measurements in combination with the Monte Carlo channeling simulation the concentration and position of displaced lattice atoms in weakly damaged boron implanted GaP has been determined. The results show that in the investigated region of the ion fluence atoms with displacement distances ra perpendicular to the (100) direction between 0.3 _& and 0.5 A exist. The relative concentration of the displaced atoms in the maximum of the depth distribution varies between about 8% and 28% for ion fluences between 5 x 1013 cm-* and 2 X 1Or4 cm-‘. The near-edge optical parameters indicate that the damaged layers contain mainly point defects and/or point defect complexes. The absorption coefficient K is approximately linearly proportional to the concentration Npd of displaced lattice atoms indicating the strong correlation between the defects produced and the near-edge optical properties. Because K as well as Npd increase nearly proportionally with the primarily produced vacancy concentration N,,, the previously drawn conclusion is confirmed that the defects produced in GaP at room temperature are rather stable. VIII. LA-RICE SITES
536
W. Wesch et al. /Nucl. Znstr. and Meth. in Phys. Res. B 85 (1994) 533-536
However, detailed investigations are necessary to get more accurate information about the displacement distance and the possible defect configuration connected with the displaced atoms.
Acknowledgement
The authors wish to thank K. GIrtner discussions.
for helpful
References [l] K. Gartner, K. Hehl and G. Schlotzhauer, Nucl. Instr. and Meth. 216 (1983) 275; B 4 (1984) 55 and B 4 (19841 63. [2] W. Wesch, A. Jordanov, K. Gartner and G. GGtz, Nucl. Instr. and Meth. B 39 (1989) 445.
[3] W. Wesch, K. GHrtner, A. Jordanov, and G. Giitz, Nucl. Instr. and Meth. B 45 (1990) 446. [4] A. Dygo, J. Kaczanowski, A. Turos, W. Wesch, K. GIrtner and G. Gatz, Nucl. Instr. and Meth. B 64 (1992) 721. [5] W. Wesch, E. Wendler, and K. Glirtner, Nucl. Instr. and Meth. B 63 (1992) 52. [6] J.P. Biersack and L.C. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [7] A. Dygo and A. Turos, Phys. Rev. B 40 (1989) 7704. [S] W. Wesch, E. Wendler and G. GGtz, J. Appl. Phys. 65 (1989) 519. [9] E. Wendler and W. Wesch, Nucl. Instr. and Meth. B 73 (1993) 489. [lo] E. Wendler, K. Glrtner, W. Wesch, U. Zammit and K.N. Madhusoodanan, these Proceedings (IBA-11, Balatoufired, Hungary, 1993) Nucl. Instr. and Meth. B 85 (1994) 528. [ll] W. Wesch, Proc. MRS Spring Meeting, San Francisco 1993, MRS Symp. Proc. vol. 300 (1993). [12] L.T. Chadderton, Radiat. Eff. 8 (1971) 77. 1131K. Unger, in: Semiconductor Optoelectronics, ed. M.A. Herman (PWN, Warsaw, 1980) p. 13. [14] V.L. Bench-Bruevich, Phys. Status Solidi 42 (1979) 35.