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Investigation of point defects by temperature-dependent dechanneling
131
Nuclear Instrumentsand Methods in Physics Research B50 (1990) 131-134 North-Holland
INVESTIGATION G. GijTZ,
OF POINT DEFECTS
K. G;iRTNER
Friedrich-Schiller-Universitiit
BY TEMPERATURE-DEPENDENT
DECHANNELING
and W. WESCH
Jena, Sektion Physik, Max- Wien-Platz
1, Jena, DDR 6900, GDR
With respect to decharmeling,point defects are characterizedby a relativenumberof displacedatoms, npd, and by a distribution of the displacementdistances r, perpendicularto the channelingdirection. Within the theoreticaldescriptionof axial dechanneling, the Rutherford backscatteringminimum yield can be calculated numericallyai a function of depth and target temperature. The resultsshow that the temperaturedependenceof the minimumyield for a givendepth stronglydependson the distance. In the case of point defects giving rise to preferredpositionsof the displacedatoms, temperature-dependentdechannelingmeasurementsallow to determineboth the point defect density n@(r) and the displacementdistance ra_The efficiencyof the method is demonstratedfor weaklydamaged GaAs layers.
1. Introduction Channeling of energetic light ions is a useful method for defect studies in crystals [l-9]. If a crystal contains defects (displaced lattice atoms), the relative Rutherford backscattering yield for ions incident parallel to a crystal axis (RBS minimum yield X&) is increased. The enhancement of the minimum yield is caused by direct backscattering and dechanneling of the ions due to the interaction with the displaced lattice atoms. The dechanneling mechanisms are different for different kinds of defects which are characterized by different correlations between the displacements of the lattice atoms. This paper deals with uncorrelated displaced lattice atoms (point defects, small defect clusters). With respect to dechanneling, point defects (and small clusters) are characterized by the relative number npd of lattice atoms which are displaced from their lattice sites and by their displacement distance ra perpendicular to the channeling direction. Only in the case where r,, can be assumed to be randomly distributed within the area belonging to one atomic string (heavily damaged crystals) the defect density npd( z) can be calculated directly from the minimum yield X&(z) measured as a function of the depth z [l-3,7]. Otherwise, additional information is necessary for the determination of both the defect density n@(z) and the distribution of the displacement distances r,. Diffusion model calculations of Matsunami et al. [lO,ll] proved the temperature and energy dependence of dechanneling to depend on the value of r,. This offered the possibility of a quantitative defect analysis by performing RBS measurements at different temperatures and different energies. More detailed investigations using a discontinuous model [7] showed that only the temperature dependence of dechanneling provides a useful information about the distribution of r,. Therefore, the temperature depen0168-583X/90/$03.50 (North-Holland)
0 EIsevierScience PublishersB.V.
dence of axial dechanneling in crystals with nonwrrelated displaced lattice atoms is of special interest in this paper. The investigations are performed assuming all displaced lattice atoms to have the same value of the displacement distance r, (ra s-distributed). This wrresponds to point defects with preferred positions in the lattice cell which appear mainly in weakly damaged crystals. In this paper the principal procedure for the evaluation of RBS spectra measured at different temperatures is described and the application of the method is discussed for the analysis of defects in ion-implanted GaAs.
2. Method
The discontinuous model of the dechanneling used for the analysis, which is based on the general description of dechanneling given by Lindhard [12], is described in detail in refs. [7,13]. For the practical analysis the minimum yield Xmin for the sample to be analyzed as well as the minimum yield of the corresponding perfect crystal have to be measured at two different temperatures T,. Because above room temperature defect annealing and transformation may occur, the measurements are usually carried out at room temperature and at a lower temperature (the temperature difference should be 2 100 K to realize a sufficiently large difference in minimum yield). From the measured energy spectra of backscattered ions (fig. la) the minimum yields xmin = Y,/Y,* of perfect and damaged crystals are calculated as a function of the depth z (see ref. [14]). In a second step, from these curves the differences in minimum yield of damaged and perfect crystals, Axmin =xmin - xmin,pert (fig. lb), are determined for the corresponding measurement temperature Tt, as well as the second difference in minimum yield, AzXmin= II. ELASTIC SCATTERING
G. Giiiz et al. / Investigation
132
of point defects
equal to the value obtained for r, randomly distributed within the area belonging to one atomic string. Therefore, the method is especially sensitive for point defects with small and medium displacement distances of the atoms (ra < 0.07 nm for (lll)Si [13,15]). The procedure for the determination of ra and n pd (z) is the following. In the first step, from the difference of the minimum yield Axtin measured at the first temperature T,, defect densities npd( z) are calculated assuming a set of different values r,. The result is a set of (ra, npd(z)). In the second step, with the set of (rar n,,(z)), Axtin is calculated for the second temperature used, T,. The calculated temperature dependence of Axmin is quite different for different (r,, npd( z)). The comparison with Axmin(z, T,) measured provides the correct ra and
channel number
depth z
Q(Z). The method is applied to the defect analysis in (1OO)GaAs implanted at room temperature with different fluences (7 X 1013-1 X 1016 cmP2) of nitrogen ions (energy 280 keV). The RBS analysis was carried out with 1.4 MeV He+ ions in standard geometry (backscattering angle 0 = 170 o ), the measurement temperatures were 125 and 295 K [16].
depth z
Fig. 1. Schematic illustration of the analysis of the temperature dependence of dechanneling:(a) xmin= YA/r.; (b) AX~,, = x,,,i,,-~,,,in,~rr; (c) A2x,,,ii,= Axtin (q)- Axmin (T,).
3. Results The results obtained for the sample implanted with 7 x 1013 N’/cm’ are depicted in figs. 3 and 4a. As can
Axmin(T,) - Axmin(T2) (fig. lc). By theoretical considerations it has been previously shown [7] that the temperature dependence is strongly influenced by the position r, of displaced latice atoms. This can clearly be seen from the dependence of A2xti on r,, schematically illustrated in fig. 2. The second difference ax,, depends heavily on the displacement distance for small r. and becomes even negative at very small r,-values. At changes weakly and is approximately large r,, A2xh
l.4MeV He+ -N 0,06.E 0.04!:
implanted (100) GaAs
exp. data D 125 K
./o-a I
_*_"_,_o__so
.
talc.(r,=O.O22nml ---
125K
-
295K a
0
I
I b
randomly dlstrlbutedr.
P
positlvetemperature dependence of aXmln
0 1 1
/ negative temperature /
dependence
of AX,,,
1 1 depth z IFurn)
-a
position ra Inm)
Fig. 2. Schematic illustration of the temperature dependence of A2x,,,t,, at a fried depth I as a function of the position ra.
Fig. 3. Depth dependence of the measured and calculated Axtin at temperatures of 125 and 295 K for 1.4 MeV He+ incident on (1OO)GaAs implanted at room temperature with 7~10’~ N’/cm’ (a) and the depth dependence of nti calculated for r, = 0.022 mn (b).
G. Giitzet al. / Investigationof point defects
0
100
200
3lY.l
0
temperature TCKI
100
200
133
300
temperatureT(K)
Fig. 4. Temperature dependence of Axmin calculated for different values of r, measured with 1.4 MeV He+ incident on (1OO)GaAs implanted at room temperature with (a) 7 X lOi N+/cm’ and (b) 1016 N+/cm2.
be seen in fig. 3a, Axmin increases if the measurement temperature is decreased from 295 to 125 K, indicating the existence of slightly displaced lattice atoms (negative temperature dependence of the dechanneling). Fig. 4a shows AX& at the depth z = 0.6 pm, calculated for different positions r,. For ra= 0.022 nm and the point defect concentration npd( I) shown in fig. 3b, a good accordance with the experiment is achieved (compare measurement points and lines in fig. 3a).
1.4MeV Hei+
0.4 -
o
1
N implanted (100) GaAs
*-WA
exp.data
F*_s_O-=*,o-0
125 K
calc.(r,-0.065 nm --- 125K -
I
295K
a
0 b
0.6 r,-0.065nm
01
I
0
0.2
I
0.4
0.6
depth z (,umI
Fig. 5. Temperature dependence of the measured and calculated Axmin at temperatures of 125 and 295 K for 1.4 MeV He+ incident on (1OO)GaAs implanted at room temperature with lOi N’/cm’ (a) and the depth dependence of npd calculated for r, = 0.065 nm (b).
Fig. 6. Integrated concentrations rr@, i and npd,2 of lattice atoms displaced to about r,, , = 0.018 nm and ra. 2 = 0.065 nm from their regular positions as a function of the nitrogen ion fluence Ni.
For the ion fluence 1016 N+/cm’ a positive temperature dependence of the dechanneling is observed ( Axtii. decreases with decreasing temperature; fig. 5a), indicating larger displacement distances r,. The comparison of the calculated Axmin in fig. 4b shows that the experimental data are fitted with the point defect distribution npd( z ) in fig. 5b and r, = 0.065 nm. These results indicate that the N+ implantations with different doses cause different point defects or point defect complexes. A more precise analysis of the experimental data has shown that all curves measured for different fluences can be fitted assuming a double-peaked distribution of r, characterized by two fixed values ra,l = 0.018 nm and ra,2= 0.065 nm (for details see ref. [16]). The values of ra,~ and ~a.2 are equal for all fluences. Only the defect densities of the two groups depend on the fluence. The variation of the integrated defect densities npd,l and npd,z of the two groups of displaced atoms with the ion fluence is shown in fig. 6. The monotonic change of both contributions with the ion fluence indicates a dose-dependent change of the internal structure of the weakly damaged layers. From these results and the near-edge optical properties of these layers it was previously concluded [16] that the initial production of Frenkel defects by the ion beam results in the formation of complexes containing vacancies, antisite defects and interstitials, the concentration of which varies with the ion fluence. The final defect structure is the result of a stationary process of formation, transformation and annealing of the defect complexes. II. ELASTIC SCATTERING
134
G. Giitz et al. / Investigation
References
[l] E. Be& Can. J. Phys. 46 (1968) 653. [2] J.E. Westmoreland, J.W. Mayer, F.H. Eisen and B. Welch, Radiat. Eff. 6 (1970) 161. [3] S.T. Picraux, J. Appl. Phys. 44 (1973) 587. [4] B.R. Appleton, Defects in Semiconductors, Proc. Mater. Res. Sot. Annual Meeting, Boston, 1980 (North-Holland, Amsterdam, 1981) p. 97. [5] W.K. Chu, ibid., p. 117. [6] S.T. Picraux, ibid., p. 135. [7] K. Glner, K. Hehl and G. Schlotzhauer, Nucl. Instr. and Meth. 216 (1983) 275; B4 (1984) 55 and B4 (1984) 63. [8] K. G%rtner and K. HehI, Nucl. Instr. and Meth. B12 (1985) 205.
of point defects
[9] K. G&tner and A. Uguzzoni, Nucl. Instr. and Meth. B33 (1988) 607. [lo] N. Matsunami, T. Gotoh and N. Itoh, Radiat. Eff. 33 (1977) 209. [ll] N. Matsunami, T. Gotoh and N. Itoh, Nucl. Instr. and Meth. 149 (1978) 430. [12] J. Lindhard, K. Dan. Vidensk. Selsk. Mat. Phys. Medd. 34 no. 14 (1965). (131 K. Glrtner, W. Wesch and G. G&z, Nucl. Instr. and Meth. (in press). (141 G. Gbtz and K. Gartner, eds., High Energy Ion Beam Analysis of Solids (Akademie, Berlin, 1988). [15] K. G&rtner, W. Wesch and A. Jordanov, Mater. Sci. Form 38-41 (1989) 1307. [16] W. Wesch, A. Jordanov, K. G%rtner and G. Gotz, Nucl. Instr. and Meth. B39 (1989) 445.
Nuclear Instrumentsand Methods in Physics Research B50 (1990) 131-134 North-Holland
INVESTIGATION G. GijTZ,
OF POINT DEFECTS
K. G;iRTNER
Friedrich-Schiller-Universitiit
BY TEMPERATURE-DEPENDENT
DECHANNELING
and W. WESCH
Jena, Sektion Physik, Max- Wien-Platz
1, Jena, DDR 6900, GDR
With respect to decharmeling,point defects are characterizedby a relativenumberof displacedatoms, npd, and by a distribution of the displacementdistances r, perpendicularto the channelingdirection. Within the theoreticaldescriptionof axial dechanneling, the Rutherford backscatteringminimum yield can be calculated numericallyai a function of depth and target temperature. The resultsshow that the temperaturedependenceof the minimumyield for a givendepth stronglydependson the distance. In the case of point defects giving rise to preferredpositionsof the displacedatoms, temperature-dependentdechannelingmeasurementsallow to determineboth the point defect density n@(r) and the displacementdistance ra_The efficiencyof the method is demonstratedfor weaklydamaged GaAs layers.
1. Introduction Channeling of energetic light ions is a useful method for defect studies in crystals [l-9]. If a crystal contains defects (displaced lattice atoms), the relative Rutherford backscattering yield for ions incident parallel to a crystal axis (RBS minimum yield X&) is increased. The enhancement of the minimum yield is caused by direct backscattering and dechanneling of the ions due to the interaction with the displaced lattice atoms. The dechanneling mechanisms are different for different kinds of defects which are characterized by different correlations between the displacements of the lattice atoms. This paper deals with uncorrelated displaced lattice atoms (point defects, small defect clusters). With respect to dechanneling, point defects (and small clusters) are characterized by the relative number npd of lattice atoms which are displaced from their lattice sites and by their displacement distance ra perpendicular to the channeling direction. Only in the case where r,, can be assumed to be randomly distributed within the area belonging to one atomic string (heavily damaged crystals) the defect density npd( z) can be calculated directly from the minimum yield X&(z) measured as a function of the depth z [l-3,7]. Otherwise, additional information is necessary for the determination of both the defect density n@(z) and the distribution of the displacement distances r,. Diffusion model calculations of Matsunami et al. [lO,ll] proved the temperature and energy dependence of dechanneling to depend on the value of r,. This offered the possibility of a quantitative defect analysis by performing RBS measurements at different temperatures and different energies. More detailed investigations using a discontinuous model [7] showed that only the temperature dependence of dechanneling provides a useful information about the distribution of r,. Therefore, the temperature depen0168-583X/90/$03.50 (North-Holland)
0 EIsevierScience PublishersB.V.
dence of axial dechanneling in crystals with nonwrrelated displaced lattice atoms is of special interest in this paper. The investigations are performed assuming all displaced lattice atoms to have the same value of the displacement distance r, (ra s-distributed). This wrresponds to point defects with preferred positions in the lattice cell which appear mainly in weakly damaged crystals. In this paper the principal procedure for the evaluation of RBS spectra measured at different temperatures is described and the application of the method is discussed for the analysis of defects in ion-implanted GaAs.
2. Method
The discontinuous model of the dechanneling used for the analysis, which is based on the general description of dechanneling given by Lindhard [12], is described in detail in refs. [7,13]. For the practical analysis the minimum yield Xmin for the sample to be analyzed as well as the minimum yield of the corresponding perfect crystal have to be measured at two different temperatures T,. Because above room temperature defect annealing and transformation may occur, the measurements are usually carried out at room temperature and at a lower temperature (the temperature difference should be 2 100 K to realize a sufficiently large difference in minimum yield). From the measured energy spectra of backscattered ions (fig. la) the minimum yields xmin = Y,/Y,* of perfect and damaged crystals are calculated as a function of the depth z (see ref. [14]). In a second step, from these curves the differences in minimum yield of damaged and perfect crystals, Axmin =xmin - xmin,pert (fig. lb), are determined for the corresponding measurement temperature Tt, as well as the second difference in minimum yield, AzXmin= II. ELASTIC SCATTERING
G. Giiiz et al. / Investigation
132
of point defects
equal to the value obtained for r, randomly distributed within the area belonging to one atomic string. Therefore, the method is especially sensitive for point defects with small and medium displacement distances of the atoms (ra < 0.07 nm for (lll)Si [13,15]). The procedure for the determination of ra and n pd (z) is the following. In the first step, from the difference of the minimum yield Axtin measured at the first temperature T,, defect densities npd( z) are calculated assuming a set of different values r,. The result is a set of (ra, npd(z)). In the second step, with the set of (rar n,,(z)), Axtin is calculated for the second temperature used, T,. The calculated temperature dependence of Axmin is quite different for different (r,, npd( z)). The comparison with Axmin(z, T,) measured provides the correct ra and
channel number
depth z
Q(Z). The method is applied to the defect analysis in (1OO)GaAs implanted at room temperature with different fluences (7 X 1013-1 X 1016 cmP2) of nitrogen ions (energy 280 keV). The RBS analysis was carried out with 1.4 MeV He+ ions in standard geometry (backscattering angle 0 = 170 o ), the measurement temperatures were 125 and 295 K [16].
depth z
Fig. 1. Schematic illustration of the analysis of the temperature dependence of dechanneling:(a) xmin= YA/r.; (b) AX~,, = x,,,i,,-~,,,in,~rr; (c) A2x,,,ii,= Axtin (q)- Axmin (T,).
3. Results The results obtained for the sample implanted with 7 x 1013 N’/cm’ are depicted in figs. 3 and 4a. As can
Axmin(T,) - Axmin(T2) (fig. lc). By theoretical considerations it has been previously shown [7] that the temperature dependence is strongly influenced by the position r, of displaced latice atoms. This can clearly be seen from the dependence of A2xti on r,, schematically illustrated in fig. 2. The second difference ax,, depends heavily on the displacement distance for small r. and becomes even negative at very small r,-values. At changes weakly and is approximately large r,, A2xh
l.4MeV He+ -N 0,06.E 0.04!:
implanted (100) GaAs
exp. data D 125 K
./o-a I
_*_"_,_o__so
.
talc.(r,=O.O22nml ---
125K
-
295K a
0
I
I b
randomly dlstrlbutedr.
P
positlvetemperature dependence of aXmln
0 1 1
/ negative temperature /
dependence
of AX,,,
1 1 depth z IFurn)
-a
position ra Inm)
Fig. 2. Schematic illustration of the temperature dependence of A2x,,,t,, at a fried depth I as a function of the position ra.
Fig. 3. Depth dependence of the measured and calculated Axtin at temperatures of 125 and 295 K for 1.4 MeV He+ incident on (1OO)GaAs implanted at room temperature with 7~10’~ N’/cm’ (a) and the depth dependence of nti calculated for r, = 0.022 mn (b).
G. Giitzet al. / Investigationof point defects
0
100
200
3lY.l
0
temperature TCKI
100
200
133
300
temperatureT(K)
Fig. 4. Temperature dependence of Axmin calculated for different values of r, measured with 1.4 MeV He+ incident on (1OO)GaAs implanted at room temperature with (a) 7 X lOi N+/cm’ and (b) 1016 N+/cm2.
be seen in fig. 3a, Axmin increases if the measurement temperature is decreased from 295 to 125 K, indicating the existence of slightly displaced lattice atoms (negative temperature dependence of the dechanneling). Fig. 4a shows AX& at the depth z = 0.6 pm, calculated for different positions r,. For ra= 0.022 nm and the point defect concentration npd( I) shown in fig. 3b, a good accordance with the experiment is achieved (compare measurement points and lines in fig. 3a).
1.4MeV Hei+
0.4 -
o
1
N implanted (100) GaAs
*-WA
exp.data
F*_s_O-=*,o-0
125 K
calc.(r,-0.065 nm --- 125K -
I
295K
a
0 b
0.6 r,-0.065nm
01
I
0
0.2
I
0.4
0.6
depth z (,umI
Fig. 5. Temperature dependence of the measured and calculated Axmin at temperatures of 125 and 295 K for 1.4 MeV He+ incident on (1OO)GaAs implanted at room temperature with lOi N’/cm’ (a) and the depth dependence of npd calculated for r, = 0.065 nm (b).
Fig. 6. Integrated concentrations rr@, i and npd,2 of lattice atoms displaced to about r,, , = 0.018 nm and ra. 2 = 0.065 nm from their regular positions as a function of the nitrogen ion fluence Ni.
For the ion fluence 1016 N+/cm’ a positive temperature dependence of the dechanneling is observed ( Axtii. decreases with decreasing temperature; fig. 5a), indicating larger displacement distances r,. The comparison of the calculated Axmin in fig. 4b shows that the experimental data are fitted with the point defect distribution npd( z ) in fig. 5b and r, = 0.065 nm. These results indicate that the N+ implantations with different doses cause different point defects or point defect complexes. A more precise analysis of the experimental data has shown that all curves measured for different fluences can be fitted assuming a double-peaked distribution of r, characterized by two fixed values ra,l = 0.018 nm and ra,2= 0.065 nm (for details see ref. [16]). The values of ra,~ and ~a.2 are equal for all fluences. Only the defect densities of the two groups depend on the fluence. The variation of the integrated defect densities npd,l and npd,z of the two groups of displaced atoms with the ion fluence is shown in fig. 6. The monotonic change of both contributions with the ion fluence indicates a dose-dependent change of the internal structure of the weakly damaged layers. From these results and the near-edge optical properties of these layers it was previously concluded [16] that the initial production of Frenkel defects by the ion beam results in the formation of complexes containing vacancies, antisite defects and interstitials, the concentration of which varies with the ion fluence. The final defect structure is the result of a stationary process of formation, transformation and annealing of the defect complexes. II. ELASTIC SCATTERING
134
G. Giitz et al. / Investigation
References
[l] E. Be& Can. J. Phys. 46 (1968) 653. [2] J.E. Westmoreland, J.W. Mayer, F.H. Eisen and B. Welch, Radiat. Eff. 6 (1970) 161. [3] S.T. Picraux, J. Appl. Phys. 44 (1973) 587. [4] B.R. Appleton, Defects in Semiconductors, Proc. Mater. Res. Sot. Annual Meeting, Boston, 1980 (North-Holland, Amsterdam, 1981) p. 97. [5] W.K. Chu, ibid., p. 117. [6] S.T. Picraux, ibid., p. 135. [7] K. Glner, K. Hehl and G. Schlotzhauer, Nucl. Instr. and Meth. 216 (1983) 275; B4 (1984) 55 and B4 (1984) 63. [8] K. G%rtner and K. HehI, Nucl. Instr. and Meth. B12 (1985) 205.
of point defects
[9] K. G&tner and A. Uguzzoni, Nucl. Instr. and Meth. B33 (1988) 607. [lo] N. Matsunami, T. Gotoh and N. Itoh, Radiat. Eff. 33 (1977) 209. [ll] N. Matsunami, T. Gotoh and N. Itoh, Nucl. Instr. and Meth. 149 (1978) 430. [12] J. Lindhard, K. Dan. Vidensk. Selsk. Mat. Phys. Medd. 34 no. 14 (1965). (131 K. Glrtner, W. Wesch and G. G&z, Nucl. Instr. and Meth. (in press). (141 G. Gbtz and K. Gartner, eds., High Energy Ion Beam Analysis of Solids (Akademie, Berlin, 1988). [15] K. G&rtner, W. Wesch and A. Jordanov, Mater. Sci. Form 38-41 (1989) 1307. [16] W. Wesch, A. Jordanov, K. G%rtner and G. Gotz, Nucl. Instr. and Meth. B39 (1989) 445.