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Structural studies of amorphous Cd59As41 and Cd26As74 films by anomalous X-ray scattering
Journal of Non-Crystalline Solids 164--166 (1993) 151-154 North-Holland
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Structural studies of amorphous Cd59As41 and Cd~As74 films by anomalous X-ray scattering A. Burian 8, p. Lecante b, A. Mosset b, j. Galy b, j. M. Tonnerre °,~ and D. Raoux d
aZaktad Fizyki Ciata Stalego PAN, ul. Wandy3, 41-800 Zabrze, Poland bCEMES-LOE, CNRS, 29, rue Jeanne Marvig, BP 4347, 31055 Toulouse Cedex, France CLURE, UPR CNRS 8, B. 209D, UPS, 91405 Orsay Cedex, France ~Laboratoire de Cristallographie, UPR CNRS 503, BP 166X, 38042 Grenoble Cedex, France The atomic-scale structure of vacuum evaporated Cd-As (containing 41 and 74 at.% As) has been studied using a differential X-ray scattering technique. The changes in the differential radial distribution functions with composition are related to the differences in the local bonding configurations. The coordination numbers and the interatomic distances have been estimated from the experimental distribution functions. 1. INTRODUCTION
The electronic properties of amorphous semiconductors are basically determined by their topological and chemical disorder and by the defects as dangling bonds, which can occur in these materials. Most of previous investigations of amorphous binary semiconductors have concentrated on III-V compounds owing to their potential technological applications and their contribution to understanding the fundamental properties of non-crystalline semiconductors [1-3]. This paper describes studies of the structure and the local bonding configuration in amorphous CdsgAs41 and Cd28As~4 by means of differential AXS. These materials have been previously investigated by the conventional X-ray scattering and EXAFS techniques [4,5]
2. EXPERIMENTAL DETAILS AND DATA ANALYSIS The Cd59AS41and Cd28As74 amorphous films were prepared by thermal evaporation in va-
cuum of approximately 10-3 Pa onto glass substrates held at room temperature using polycrystalline Cd3 As 2 and CdAs2 as source materials. Several evaporation runs were necessary until the thicknesses of about 60-100/zm were obtained. The compositions of the samples were determined by chemical analysis. The X-ray scattering data were collected on the DCI positron ring at LURE (Orsay, France), which is equipped with a five-pole superconducting wiggler, a Si=0 double crystal monochromator and a diffractometer with a Si:Li multidetector. [6] The scattered intensities were measured using incident photon energies immediately and further below the As (11861, 11103 eV) and Cd (26704 or 26698, 25711 eV) absorption edges in a reflection and transmission geometry, respectively. The measured absorption profiles served as an energy calibration criterion. The energies of the experiments were taken 6 and 7 eV below the midpoint of the rise in the absorption at the edge for the As and Cd edges. The values of the real and imaginary parts of the anomalous scattering factors f' and f " for the energy close to the As edge were calcula-
This work was supported in part by the Committee for Scientific Research under Grant No 2 P302 195 04. One of us (A. B.) acknowledges CNRS for financial support. 0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.
A. Buric;', et .7l. / Structural studies of amorphous Cd59As41 and Cd26As74rims
152
'
L
'
c°,,,:,:
O3
O32
~
0 0
J 4.
As
,
I 8
,
K=4~sinO//
(~ 2
I , , , I 12 16 20 X [A - 1 ]
As
Cd
0 0
4-
8
12
K=4~sinO//X
16
20
[A - t ]
Fig. 1. The differential structure factors (DSFs) for Cds9As41 from the experiments near the As and Cd absorption edges,
Fig. 2. The diferential structure factors (DSFs) for Cd26As74 from the experiments near the As and Cd absorption edges.
ted from the previously recorded EXAFS data, extended to the energy range from 2 to 600 keV, using the optical theorem and the Kramers-Kronig relation, according to [7,8]. At the Cd edge, the tabulated values, as given by Sasaki, were taken for further calculations [9]. The f' values of about -8 and -7.5 electrons were achieved at the As and Cd edges. After correction of the AXS data for absorption, the Compton scattering, the K# flourescence and normalisation, as described in [10], the total structure factors (TSF) were computed,
that obtained from an analysis of extended X-ray absorption fine structure data but includes long-range correlations. The DSFs for the investigated films are shown in figs. 1 and 2. The DSFs were then Fourier transformed leading to the differential radial distribution functions (DRDFs). The transforms were taken over a range from K=0.6, 0.9 A -1 to 9, 22 ,~,-1 after multiplication by a convergence term exp(-(xK2 ) with (x = 0.02, 0.0075 for the Asand Cd-rich compositions, respectively. The results are presented in figs. 3 and 4. The differential AXS technique requires the high quality experimental data which are subjected to complicated data reduction and analysis procedures. A precise knowledge of the atomic scattering factors is also necessary. The estimated errors in coordination numbers for carefully performed experiment and data treatment are of about _+0.25 [11,12]. However the evaluation of the coordination numbers, based on the approximation that the atomic scattering factors are simply proportional to each other may lead to larger errors (about +_1)
3. RESULTS AND DISCUSSION The differences of the intensities scattered at two energies close to the As (11861 and 11103 eV) and to the Cd absorption edge (26704 eV for Cds9As, l, 26698 eV for Cd~ A s T , and 25711 eV) yielded the differential structure factors (DSFs), which contain information about As-As, As-Cd and Cd-Cd, Cd-As correlations. This information is qualitatively similar to
A. Burian et al. / Structural studies of amorphous Cd59As41 and
O ~1"
Cd26As74films
153
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Cd5g As41
Cd26 As74
O
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Lu 121
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,
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~1
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6
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8
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0
2
4.
6
8
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Fig. 3. DRDFs calculated from DSFs presented in fig. 1.
Fig. 4. DRDFs calculated from DSFs presented in fig. 2.
especially in the case of the complicated coordination sphere. Of note in the curves shown in figs. 1 and 2 is that the peak at ~ 2 A -1 disappears in the As edge DSF for CdsgAS41. On the contrary, such a peak is observed in the Cd edge DSF for Cds~As41 , suggesting that it is due primarily to Cd-Cd correlations. The previously performed modeling studies suggest that the correlations between layers with interlayer spacing of ~ 3.14A, occupied by the Cd atoms, may be the origin of this diffraction peak in CdsgAs41 [4]. From each DRDF obtained, the first peak positions and areas have been estimated and are listed in table 1. For CdsgAs41 the first-neighbour peak positions, obtained at the AS and Cd edges are approximately equal within an estimated experimental error. Instead, an apparent difference in the first peak position is observed for Cd26 As74 . The shorter near neighbour distance at the As edge, close to the sum of the covalent radii for As (2.5 ,&, [13] ), is consistent with an expectation that the As-As correlations are predominant forthe Cd26As~, film. Assuming that in Cd26As~, only As is the
near neighbour to Cd, the coordination number Nod_As, calculated from the Cd edge peak area is 3.5. This value is slightly lower than that resulting from tetrahedral coordination. The peak area at the As edge also suggests that the coordination number of the As atoms is smaller than four. This result could be explained by the presence of the threefold coordinated As sites. However, the large proportion of three-coordinated As atoms is incompatible with the number of the Nc~_As bonds, estimated from the Cd edge DRDF. The coordination numbers NOd_As and NAs_Od a r e correlated by the bond-consistencycondition: Ccd Nod_As= CAsNAs_Cd, where Cod and CAs are the atomic concentrations. Assuming NA~_Cd= 3 one obtains NOd_As = 8.5, which is clearly unrealistic. The obtained peak areas can be satisfactorily explained by the occurrence of the As-As bonds, which are expected at 74 at. % As. The model containing, on the average, the 2.5 (As-As) and 1.5 (As-Cd) neighbours leads to the peak area of
A. Burian et al. / Structural studies of amorphous Cd59As4/ and Cd26As74 fdms
154
Table 1 Peak positions and areas from DRDFs. sample As edge Cd edge r [A] area r [A] area ~d~9As4~
2.71
5.2
2.68
4.1
3d26As74 2.55 3.6 2.71 3.5 The estimated errors for the peak positions and areas are _+0.02 and _+0.25.
ded that As and Cd remain almost tetrahedrally coordinated. The results for the As and Cd environments provide evidence that both amorphous alloys are chemically ordered. The bonds between atoms of the same kind are characteristic of their crystalline counterparts [4] and are not interpreted in terms of chemical disorder or wrong bonds. REFERENCES
about four. These numbers of the near neighbours satisfy the bond-consisting condition. The changes in the experimental DRDFs for Cd59As41 at the As and Cd edges, which can be seen in fig. 3, suggest that the Cd-Cd contribution is involved in the first peak of the Cd DRDF. Several models have been tested in order to decide whether this suggestion leads to a reasonable interpretation of the first peak in the As and Cd DRDFs. Assuming that only heteropolar bonds are formed in Cds9As4~ ,one gets the coordination numbers NAs_cd= 4.1 and NCd_A~ = 3.9. These values do not satisfy the bond-consisting relationship, The calculated first-peak areas, resulting from the models in which Nc~_cd =1.2(1.5), NCd_A =2.8(2.5) and NAs_As=0(0.5), NAs_Cd=4(3.5) are 4.1(4.1) and 5.1(4.8) for the Cd and As edge DRDFs, respectively. These coordination numbers account reasonably for the experimental peak areas and are fully compatible with expectations based on the previously described bond condition; nevertheless the EXAFS study [5] suggests the presence of the As-As bonds. In the absence of definitive separation of the Cd-Cd, Cd-As and As-As contributions these explanations are adopted as being the most consistent with the experimental data.
4. CONCLUSIONS The differential anomalous X-ray scattering technique has been used to obtain the DRDFs around the both components of the amorphous Cd59 As4~ and Cd26 As74 films. Allowing for the estimated experimental errors it can be conclu-
1. D. Udron, A. M. Flank, P. Lagarde, D. Raoux and M. L. Theye, J. Non-Cryst. Solids 150 (1992) 361. 2. S. H. Baker, M. J. Manssor, S. J. Gurman, S.C. Bayliss and E. A. Davis, J. Non-Cryst. Solids 144 (1992)63. 3. N. Elgun, S. J. Gurman and E. A. Davis, J. Phys. Condens. Matter 4 (1992) 7759. 4. A. Burian, P. Lecante, A. Mosset and J.Galy, Z. Kristallogr. 193 (1990) 199. 5. A. Burian, P. Lecante, A. Mosset and J.Galy, Proc. 1st European East-West Symposium on Materials and Product Technology, M. H. Dorgham (ed.)(Helsinki 1990) in Int. J. Materials and Product Tech. vol.1 (191)625. 6. G. Nicoli, R. Andouart, C. Barbier, D. Dangeneaux, M. de Santis and D. Raoux, Sec. Ital. Fis. Conf. Prec. 25 (1990) 353. 7. P. Dreier, P. Rabe, W. Malzfeldt and W. Niemann, J. Phys. C: Solid State Phys. 17 (1984) 3123. 8. J. J. Hoyt, D. de Fontaine and W. K. Warburton, J. Appl. Cryst. 17 (1984) 344. 9. S. Sasaki, KEK Report 83-22, Nat. Lab. for High Energy Phys., Tsukuba, Japan (1984). 10. J. M. Tonnerre, Phd thesis, Universit~ de Paris-Sud, Centre d'Orsay, France (1989). 11. P. H. Fuoss, P. Eisenberger, W. K. Warburton and A. Bienenstock, Phys. Rev. Lett. 46 (1981) 1537. 12. K. F. Ludwig, W. K. Warburton, L. Wilson and A. Bienensteck, J. Chem. Phys. 87 (1987) 604. 13. J. P.Suchet, J.Phys.Chem.16 (1960) 265.
~
.'OUR,~ALor U
Structural studies of amorphous Cd59As41 and Cd~As74 films by anomalous X-ray scattering A. Burian 8, p. Lecante b, A. Mosset b, j. Galy b, j. M. Tonnerre °,~ and D. Raoux d
aZaktad Fizyki Ciata Stalego PAN, ul. Wandy3, 41-800 Zabrze, Poland bCEMES-LOE, CNRS, 29, rue Jeanne Marvig, BP 4347, 31055 Toulouse Cedex, France CLURE, UPR CNRS 8, B. 209D, UPS, 91405 Orsay Cedex, France ~Laboratoire de Cristallographie, UPR CNRS 503, BP 166X, 38042 Grenoble Cedex, France The atomic-scale structure of vacuum evaporated Cd-As (containing 41 and 74 at.% As) has been studied using a differential X-ray scattering technique. The changes in the differential radial distribution functions with composition are related to the differences in the local bonding configurations. The coordination numbers and the interatomic distances have been estimated from the experimental distribution functions. 1. INTRODUCTION
The electronic properties of amorphous semiconductors are basically determined by their topological and chemical disorder and by the defects as dangling bonds, which can occur in these materials. Most of previous investigations of amorphous binary semiconductors have concentrated on III-V compounds owing to their potential technological applications and their contribution to understanding the fundamental properties of non-crystalline semiconductors [1-3]. This paper describes studies of the structure and the local bonding configuration in amorphous CdsgAs41 and Cd28As~4 by means of differential AXS. These materials have been previously investigated by the conventional X-ray scattering and EXAFS techniques [4,5]
2. EXPERIMENTAL DETAILS AND DATA ANALYSIS The Cd59AS41and Cd28As74 amorphous films were prepared by thermal evaporation in va-
cuum of approximately 10-3 Pa onto glass substrates held at room temperature using polycrystalline Cd3 As 2 and CdAs2 as source materials. Several evaporation runs were necessary until the thicknesses of about 60-100/zm were obtained. The compositions of the samples were determined by chemical analysis. The X-ray scattering data were collected on the DCI positron ring at LURE (Orsay, France), which is equipped with a five-pole superconducting wiggler, a Si=0 double crystal monochromator and a diffractometer with a Si:Li multidetector. [6] The scattered intensities were measured using incident photon energies immediately and further below the As (11861, 11103 eV) and Cd (26704 or 26698, 25711 eV) absorption edges in a reflection and transmission geometry, respectively. The measured absorption profiles served as an energy calibration criterion. The energies of the experiments were taken 6 and 7 eV below the midpoint of the rise in the absorption at the edge for the As and Cd edges. The values of the real and imaginary parts of the anomalous scattering factors f' and f " for the energy close to the As edge were calcula-
This work was supported in part by the Committee for Scientific Research under Grant No 2 P302 195 04. One of us (A. B.) acknowledges CNRS for financial support. 0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.
A. Buric;', et .7l. / Structural studies of amorphous Cd59As41 and Cd26As74rims
152
'
L
'
c°,,,:,:
O3
O32
~
0 0
J 4.
As
,
I 8
,
K=4~sinO//
(~ 2
I , , , I 12 16 20 X [A - 1 ]
As
Cd
0 0
4-
8
12
K=4~sinO//X
16
20
[A - t ]
Fig. 1. The differential structure factors (DSFs) for Cds9As41 from the experiments near the As and Cd absorption edges,
Fig. 2. The diferential structure factors (DSFs) for Cd26As74 from the experiments near the As and Cd absorption edges.
ted from the previously recorded EXAFS data, extended to the energy range from 2 to 600 keV, using the optical theorem and the Kramers-Kronig relation, according to [7,8]. At the Cd edge, the tabulated values, as given by Sasaki, were taken for further calculations [9]. The f' values of about -8 and -7.5 electrons were achieved at the As and Cd edges. After correction of the AXS data for absorption, the Compton scattering, the K# flourescence and normalisation, as described in [10], the total structure factors (TSF) were computed,
that obtained from an analysis of extended X-ray absorption fine structure data but includes long-range correlations. The DSFs for the investigated films are shown in figs. 1 and 2. The DSFs were then Fourier transformed leading to the differential radial distribution functions (DRDFs). The transforms were taken over a range from K=0.6, 0.9 A -1 to 9, 22 ,~,-1 after multiplication by a convergence term exp(-(xK2 ) with (x = 0.02, 0.0075 for the Asand Cd-rich compositions, respectively. The results are presented in figs. 3 and 4. The differential AXS technique requires the high quality experimental data which are subjected to complicated data reduction and analysis procedures. A precise knowledge of the atomic scattering factors is also necessary. The estimated errors in coordination numbers for carefully performed experiment and data treatment are of about _+0.25 [11,12]. However the evaluation of the coordination numbers, based on the approximation that the atomic scattering factors are simply proportional to each other may lead to larger errors (about +_1)
3. RESULTS AND DISCUSSION The differences of the intensities scattered at two energies close to the As (11861 and 11103 eV) and to the Cd absorption edge (26704 eV for Cds9As, l, 26698 eV for Cd~ A s T , and 25711 eV) yielded the differential structure factors (DSFs), which contain information about As-As, As-Cd and Cd-Cd, Cd-As correlations. This information is qualitatively similar to
A. Burian et al. / Structural studies of amorphous Cd59As41 and
O ~1"
Cd26As74films
153
O ~'
I
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i
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Cd5g As41
Cd26 As74
O
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Lu 121
r'~ °
r..~o
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,
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~1
2
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I
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6
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0
8
r [AI
~r~
0
2
4.
6
8
r [A]
Fig. 3. DRDFs calculated from DSFs presented in fig. 1.
Fig. 4. DRDFs calculated from DSFs presented in fig. 2.
especially in the case of the complicated coordination sphere. Of note in the curves shown in figs. 1 and 2 is that the peak at ~ 2 A -1 disappears in the As edge DSF for CdsgAS41. On the contrary, such a peak is observed in the Cd edge DSF for Cds~As41 , suggesting that it is due primarily to Cd-Cd correlations. The previously performed modeling studies suggest that the correlations between layers with interlayer spacing of ~ 3.14A, occupied by the Cd atoms, may be the origin of this diffraction peak in CdsgAs41 [4]. From each DRDF obtained, the first peak positions and areas have been estimated and are listed in table 1. For CdsgAs41 the first-neighbour peak positions, obtained at the AS and Cd edges are approximately equal within an estimated experimental error. Instead, an apparent difference in the first peak position is observed for Cd26 As74 . The shorter near neighbour distance at the As edge, close to the sum of the covalent radii for As (2.5 ,&, [13] ), is consistent with an expectation that the As-As correlations are predominant forthe Cd26As~, film. Assuming that in Cd26As~, only As is the
near neighbour to Cd, the coordination number Nod_As, calculated from the Cd edge peak area is 3.5. This value is slightly lower than that resulting from tetrahedral coordination. The peak area at the As edge also suggests that the coordination number of the As atoms is smaller than four. This result could be explained by the presence of the threefold coordinated As sites. However, the large proportion of three-coordinated As atoms is incompatible with the number of the Nc~_As bonds, estimated from the Cd edge DRDF. The coordination numbers NOd_As and NAs_Od a r e correlated by the bond-consistencycondition: Ccd Nod_As= CAsNAs_Cd, where Cod and CAs are the atomic concentrations. Assuming NA~_Cd= 3 one obtains NOd_As = 8.5, which is clearly unrealistic. The obtained peak areas can be satisfactorily explained by the occurrence of the As-As bonds, which are expected at 74 at. % As. The model containing, on the average, the 2.5 (As-As) and 1.5 (As-Cd) neighbours leads to the peak area of
A. Burian et al. / Structural studies of amorphous Cd59As4/ and Cd26As74 fdms
154
Table 1 Peak positions and areas from DRDFs. sample As edge Cd edge r [A] area r [A] area ~d~9As4~
2.71
5.2
2.68
4.1
3d26As74 2.55 3.6 2.71 3.5 The estimated errors for the peak positions and areas are _+0.02 and _+0.25.
ded that As and Cd remain almost tetrahedrally coordinated. The results for the As and Cd environments provide evidence that both amorphous alloys are chemically ordered. The bonds between atoms of the same kind are characteristic of their crystalline counterparts [4] and are not interpreted in terms of chemical disorder or wrong bonds. REFERENCES
about four. These numbers of the near neighbours satisfy the bond-consisting condition. The changes in the experimental DRDFs for Cd59As41 at the As and Cd edges, which can be seen in fig. 3, suggest that the Cd-Cd contribution is involved in the first peak of the Cd DRDF. Several models have been tested in order to decide whether this suggestion leads to a reasonable interpretation of the first peak in the As and Cd DRDFs. Assuming that only heteropolar bonds are formed in Cds9As4~ ,one gets the coordination numbers NAs_cd= 4.1 and NCd_A~ = 3.9. These values do not satisfy the bond-consisting relationship, The calculated first-peak areas, resulting from the models in which Nc~_cd =1.2(1.5), NCd_A =2.8(2.5) and NAs_As=0(0.5), NAs_Cd=4(3.5) are 4.1(4.1) and 5.1(4.8) for the Cd and As edge DRDFs, respectively. These coordination numbers account reasonably for the experimental peak areas and are fully compatible with expectations based on the previously described bond condition; nevertheless the EXAFS study [5] suggests the presence of the As-As bonds. In the absence of definitive separation of the Cd-Cd, Cd-As and As-As contributions these explanations are adopted as being the most consistent with the experimental data.
4. CONCLUSIONS The differential anomalous X-ray scattering technique has been used to obtain the DRDFs around the both components of the amorphous Cd59 As4~ and Cd26 As74 films. Allowing for the estimated experimental errors it can be conclu-
1. D. Udron, A. M. Flank, P. Lagarde, D. Raoux and M. L. Theye, J. Non-Cryst. Solids 150 (1992) 361. 2. S. H. Baker, M. J. Manssor, S. J. Gurman, S.C. Bayliss and E. A. Davis, J. Non-Cryst. Solids 144 (1992)63. 3. N. Elgun, S. J. Gurman and E. A. Davis, J. Phys. Condens. Matter 4 (1992) 7759. 4. A. Burian, P. Lecante, A. Mosset and J.Galy, Z. Kristallogr. 193 (1990) 199. 5. A. Burian, P. Lecante, A. Mosset and J.Galy, Proc. 1st European East-West Symposium on Materials and Product Technology, M. H. Dorgham (ed.)(Helsinki 1990) in Int. J. Materials and Product Tech. vol.1 (191)625. 6. G. Nicoli, R. Andouart, C. Barbier, D. Dangeneaux, M. de Santis and D. Raoux, Sec. Ital. Fis. Conf. Prec. 25 (1990) 353. 7. P. Dreier, P. Rabe, W. Malzfeldt and W. Niemann, J. Phys. C: Solid State Phys. 17 (1984) 3123. 8. J. J. Hoyt, D. de Fontaine and W. K. Warburton, J. Appl. Cryst. 17 (1984) 344. 9. S. Sasaki, KEK Report 83-22, Nat. Lab. for High Energy Phys., Tsukuba, Japan (1984). 10. J. M. Tonnerre, Phd thesis, Universit~ de Paris-Sud, Centre d'Orsay, France (1989). 11. P. H. Fuoss, P. Eisenberger, W. K. Warburton and A. Bienenstock, Phys. Rev. Lett. 46 (1981) 1537. 12. K. F. Ludwig, W. K. Warburton, L. Wilson and A. Bienensteck, J. Chem. Phys. 87 (1987) 604. 13. J. P.Suchet, J.Phys.Chem.16 (1960) 265.