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Electron paramagnetic resonance of InP:Co2+
Physica 116B (1983) 467-469 North-Holland PublishingCompany
Paper presented at ICDS-12 Amsterdam, August 31 - September 3, 1982
ELECTRON PARAMAGNETIC RESONANCE OF InP :Co2+ B. LAMBERT*, B. DEVEAUD and Y. TOUDIC
C.N.E.T. LANNION, LAB/1CM/MPA, 22301 Lannion, France * L U.T. LANNION, Universitd de Rennes, France B. CLERJAUD
Laboratoire de Luminescence*, Universitg P. et M. Otrie Tour 13, 4 place Jussieu, 75230 Paris Cedex 05, France * Equipe de recherche associg au C.N.R.S.
Electron paramagnetic resonance (E.P.R.) experiments on n type and semi-insulating cobalt doped InP are reported. In all samples, an isotropic E.P.R. spectrum characterized by g = 2.192 ± 0.001 and a line width of 150 Gauss is observed. This signal is attributed to Co 2~ (3d 7) substitutional to indium.
I - INTRODUCTION
II - CRYSTAL GROWTH
Indium phosphide is a quite promising material for technological applications, in the fields of fast electronics or optoelectronics. Semiinsulating (SI) substrates are needed for devices ; the role of 3d impurities as compensation centers is now well established. For InP, chromium and iron have successively been used and semi-insulating substrates doped with these impurities are eommercialy available. However, technological problems such as redistribution in the substrate during annealing have not been solved yet [I].
Polycrystalline indium phosphide is used to study the cobalt doping. The ingots are grown in sealed silica tubes placed in a conventional two zones gradient freeze furnace, by direct reaction of indium and phosphrus above the melting point of InP (1062°C) and at 27.5 bar.
Cobalt has recently [2] been proposed as compensating center and cobalt doped InP has been grown and widely studied in the United-Kingdom by several techniques : photoluminescence and photoluminescence excitation [3], E.P.R. [4], electrical measurements and space charge techniques [5]. In [4], Baker et al reported an isotropic E . P . ~ spectrum characterized by g = 2.059 + O.O01 and a line width which can be as samll+as 40 Gauss. They attributed this ~ignal to Co 2 substitutional to indium ( C o ~ ) . In the present paper, we report an E.P.R. spectrum, obtained in any Co doped InP, characterized by g = 2.192 + 0.001 and a line width equal to 150 Gauss. We give some arguments which show that the g = 2.192 signal is more e+ than the g = 2.059 likely to be related to COin spectrum is.
0378-4363/83/0000-0000/$03.00 © 1983 North-Holland
Indium (6N) and cobalt are placed in a PBN boat, vacuum baked, and transferred at one end of the silica tube containing red phosphorus (6N) at the other end. The temperature of the phosphorus end is 540°C in order to obtain the 27.5 bars. Undoped materials grown in the same way have the following characteristics : residual carrier concentration 3 to 9.10 IS cm -3 and Hall mobility 1.5-2.5 10~ cm 2 V -I sec -I at 77 K. The main impurities, as determined by Spark Source Mass Spectrometry (SSMS) are silicon, aluminum, magnesium and copper. They are not homogeneously distributed. The cobalt content used for doping the ingot, determined following the work of Cockayne et al [2], is ].9mgg -]. After complete reaction of indium and phosphorus a cooling rate equal to 2°C/hour has been applied. In the first to freeze part of the ingot (A and B zones), the material is semiinsulating (whereas in the last to freeze (C and D) the polycristal is n type. The following characteristics have to be pointed out : - the cobalt concentration (0.25-2.5 ppma : SSMS) is very inhomogeneous except in the B zone (0.25 ppma) seemingly due to segregation at the grain boundaries or to the precipation of cobalt phosphorus [2]. If we consi-
468
B. Lambert et al.
/ Electron
paramagnetic resonance o f lnP:Co 2+
der the results of zone B, the segregation • -5 . coefficient that we deduce ms 5.10 zn accordance with Cockayne et al results [2] ; - the silicon concentration is anomalously high (i to 60 ppma) that is perhaps due to silica deposition during the sealing of the ampoule ;
In the n type samples a background signal is present which is due to the cyclotron resonance of the electrons (or transverse magnetoresistance). This kind of signal quite common in the study of n type III-V compounds [6],is probably responsible of the large 115 kHz background observed by Baker et al [4].
- the single crystals sizes are smaller than in the case of the undoped polycrystalline InP. IV - DISCUSSION IIl-
E.P.R.
IV.I - Discussion of the E.P.R. results ................................
EXPERIMENTS
We have performed E.P.R. measurements on several n type and SI crystals extracted from the ingot; we have choosen these single crystals in all parts of the ingot. We have also studied several samples extracted f r o m a L.E.C. cobalt doped boule grown in Lannion and a semi-insulating sample (L 940) grown at the R.S.R.E. and very kindly provided to us by Dr M.S. Skolnick. The experiments have been performed at liquid helium temperature using an X band E-line Varian spectrometer The magnetic field was precisely measured with an N.M.R. Gaussmeter and the microwave frequency with a frequency counter. In all the cobalt doped samples that we have studied, we observed the signal shown in fig. i. It consists of an intense single line at g = 2.192 + O.OO1 having a peak to peak line width of 150 Gauss. This line is isotropic and not significantly photosensitive in the studied samples (semi-insulating and n type). In addition, we observe a small signal at g = 2 probably due to a residual impurity or a lattice defect.
i
9.23
f
GI4;'
InP : Co
3.5 K
26 i
Fig.
22
28
29
3
3il
32
313
i. E.P.R. spectrum of Co doped InP (sample P.32 B).
34
:
Since the g = 2.192 signal is observed in all cobalt doped samples and not in other sample~, we attribute it to cobalt. Substitutional Co 3 (3d 6) and Co (3d 8) are not expected to be paramagnetic. Co2+(3d 7) has a F8 ground state expected to be paramagnetic, it should be slightly ~nisotropic because of the u ~ E S x H x + S.y3 H y + .S z H-] zj term in spin hamiltonian. As the u factor zs not expected to be large, the anisotropy might be smaller than the experimental error. Natural cobalt (59Co) has a nuclear spin 7/2; one expects a eight lines spectrum because of the hyperfine interaction, but in III-V compounds this effect can be hidden by the ligands hyperfine interaction. A single unresolved line has also been observed in the E.P.R. spectrum of Co 2+ in GaP [7] and GaAs [8]. It is to be noted that in all studied samples, we also observed the Co 2+ luminescence spectrum [3,5]. Axial Co 2+ has to be rejected because its EPR spectrum would be very strongly snisotropic [i0]. In all the II-VI and+III-V tetrahedral compounds the g factor of Co 2 is larger than the g factor of Ni 3 by an amount which is the range O.O7-O.1 this should also be true for InP. This is the case for the g = 2.192 signal, but not for the 2.059 one for which g is smaller than the Ni 3+ value (2.098) [7]. The g value can be approximated by using the expressional, obtained from a simple theory : g = 2 - ~ (see for instance ref. [9]) where k is a ~ovalency reduction factor, ~ the spin orbit coupling constant and A the 4T2 ÷ ~A2 splitting. If one takes k = 0.7 and ~ = - 135 cm I as for GaAs and GaP [9], the free ion value being ~ = - 178 cm -I and A = 3850 cm -I obtained from the optical experiments [3,5] one obtains g = 2.20 in good agreement with the value presently reported. It is to be noted that the g = 2.059 value leads to ~ = - 42 c m i I [5] and an inconsistency between the values of ~ = 0.24 and the chosen value of k = 0.7. From the linewidths analysis one can get an other argument which favour~the attribution of 2 the g = 2.192 signal to COin . The unresolved line width is due to the hyperfine and ligands hyperfine interactions. The hyperfine interaction should be about the same in the case of GaP and InP (but slightly larger in the case of InP), but the ligands hyperfine interaction should be quite different. The contribution of
B. Lambert et al.
/ Electron paramagnetic resonance o f lnP.'Co 2+
the first anion and cation shells to (AH) 2 are dominant and amount to [7] : 4 a 2 (p31) + 46.2 a 2 (Ga 71 ) for GaP and 4 a 2 (p31) + 396 a 2 (In lls) for InP. The p31 contributions are about the same for GaP and inP, but a scaling factor for a (In115)/a (Ga 71) = 1.213 [7] has to be used. This e~sures that the line width of substitution~l Co 2 in InP must be larger than that of Co 2 in GaP which is 70 Gauss [7]. This is satisfied for the g = 2.]92 signal but not for the g = 2.059 one. IV.2 - ~ ! ~ ! ~ _ ~ _ ~ _ ~ ~ $ - ~ Some problems remain unresolved concerning the acceptor role of cobalt. The first one has already been pointed out in ~5]. The ionisation energy Ei (Co 2- + Ei ÷ Co 3 + eC.B.) has been found to be 0.5 eV [5]. This is rather surprising because the semi-emplrical laws (for instance [ii]) predict that for Co 2+, the value of E i should be larger than that of Fe 2+ (0.65eV). The second one is raised by some preliminary D.L.T.S. measurements performed in Lannion by G. Pelous [12]. He observes the Co related peak [5]~ but with a very low concentration ~ i0~3 cm (the IEI electron trap due to iron is observed with a concentration of ~ 1014 cm-3). The g = 2.192 E.P.R. signal observed in the same sample is very intense and the lowest concentration of cobalt determined by S.S.M.S. in the ingot is 1016 cm -3. Such a discreapancy gives rise to a doubt about the attribution to substitutional cobalt of the 0.53 eV D.L.T.S. peak. It is to be noted that, in [5], the electron trap ~oncentration measured by D.L.T.S. (i x I0 Is cm 3) is lower than the estimated Co concentration > 3.3 x 1015 cm-3). These two points make questionnable whether the level located at 0.5 eV below the conduction ban~ minimum is due to the 4A 2 ground state of Co 2 substitutional to indium. V - CONCLUSION We have shown that the C o ~ E.P.R0 spectrum in InP is characterized by g = 2.192 ~ O.O01 and a line width of 150 Gauss. Further studies are needed in order to understand the compensating behaviour of cobalt in InP and we believe that the E.P.R. results presently reported are a safe basis for future work. ACKNOWLEDGEMENTS We wish to thank G. Moisan and L. Le Mar~chal for their technical assistance in sample growth, R. Coquill~ who grow the L.E.C. InP:Co and M. Gauneau who performed the S.S.M.S. measurements.
469
We are grateful to G. Pelous for very stimulative discussions related to D.L.T.S. measurements which he performed. We enjoyed the information exchange with the Malvern team and we wish to thank particulary Dr Skolnick who furnished us an InP:Co sample and the manuscript (of ref.[5]) prior to publication. NOTE ADDED IN PROOF We have recently learnt that M.S. Skolnick et al [13] have obtained E.P.R. results in agreement with those reported here. REFERENCES [i] M. Gauneau, J.C. Paris To be published [2] B. Cockayne, W.R. Mac Ewan, G.T. Brown J. of Crystal Growth, 55, 263 (1981) [3] M.S. Skolnick, P.J. Dean, P.R. Tapster, D.J. Robbins, B. Cockayne, W.R. Mac Ewan J. of Luminescence, 24/25, 241 (1981) [4] J.M. Baker, L.J.C. Bluck, B. Cockayne, W.R. Mac Ewan J. Phys. C, 14, 3953 (1981) [5] M.S. Skolnick, P.R. Tapster, P.J. Dean, R.G. Humphreys, B. Cockayne, W.R. Mac Ewan J.M. Noras J. Phys. C, 15, 3333 (1982) [6] B. Clerjaud, A.M. Hennel, G. Martinez Solid State Commun., 33, 983 (1980) [7] U. Kaufmann, J. Schneider Solid State Com~un., 25, 1113 (1978) [8] M. Goldlewski, A.M. Hennel Phys. Status Solidi, (b)88, KI1 (1978) [9] J. Weber, H. Ennen, U. Kaufmann, J. Schneider Phys. Rev., B21, 2394 (1980) [IO] P. Koidl, A. Rauber J. Phys. Chem. Solids, 35,
1061 (1974)
[11] J.W. Allen Proc. Semi-insulating III-V Materials Nottingham 1982 - Ed. by G.J. Ress Shiva Pub., p. 261 (1981) [12] G. Pelous Private communication [13] M.S. Skolnick, L. Eaves, S. Clough, B. Cokayne, P.J. Dean Private communication.
Paper presented at ICDS-12 Amsterdam, August 31 - September 3, 1982
ELECTRON PARAMAGNETIC RESONANCE OF InP :Co2+ B. LAMBERT*, B. DEVEAUD and Y. TOUDIC
C.N.E.T. LANNION, LAB/1CM/MPA, 22301 Lannion, France * L U.T. LANNION, Universitd de Rennes, France B. CLERJAUD
Laboratoire de Luminescence*, Universitg P. et M. Otrie Tour 13, 4 place Jussieu, 75230 Paris Cedex 05, France * Equipe de recherche associg au C.N.R.S.
Electron paramagnetic resonance (E.P.R.) experiments on n type and semi-insulating cobalt doped InP are reported. In all samples, an isotropic E.P.R. spectrum characterized by g = 2.192 ± 0.001 and a line width of 150 Gauss is observed. This signal is attributed to Co 2~ (3d 7) substitutional to indium.
I - INTRODUCTION
II - CRYSTAL GROWTH
Indium phosphide is a quite promising material for technological applications, in the fields of fast electronics or optoelectronics. Semiinsulating (SI) substrates are needed for devices ; the role of 3d impurities as compensation centers is now well established. For InP, chromium and iron have successively been used and semi-insulating substrates doped with these impurities are eommercialy available. However, technological problems such as redistribution in the substrate during annealing have not been solved yet [I].
Polycrystalline indium phosphide is used to study the cobalt doping. The ingots are grown in sealed silica tubes placed in a conventional two zones gradient freeze furnace, by direct reaction of indium and phosphrus above the melting point of InP (1062°C) and at 27.5 bar.
Cobalt has recently [2] been proposed as compensating center and cobalt doped InP has been grown and widely studied in the United-Kingdom by several techniques : photoluminescence and photoluminescence excitation [3], E.P.R. [4], electrical measurements and space charge techniques [5]. In [4], Baker et al reported an isotropic E . P . ~ spectrum characterized by g = 2.059 + O.O01 and a line width which can be as samll+as 40 Gauss. They attributed this ~ignal to Co 2 substitutional to indium ( C o ~ ) . In the present paper, we report an E.P.R. spectrum, obtained in any Co doped InP, characterized by g = 2.192 + 0.001 and a line width equal to 150 Gauss. We give some arguments which show that the g = 2.192 signal is more e+ than the g = 2.059 likely to be related to COin spectrum is.
0378-4363/83/0000-0000/$03.00 © 1983 North-Holland
Indium (6N) and cobalt are placed in a PBN boat, vacuum baked, and transferred at one end of the silica tube containing red phosphorus (6N) at the other end. The temperature of the phosphorus end is 540°C in order to obtain the 27.5 bars. Undoped materials grown in the same way have the following characteristics : residual carrier concentration 3 to 9.10 IS cm -3 and Hall mobility 1.5-2.5 10~ cm 2 V -I sec -I at 77 K. The main impurities, as determined by Spark Source Mass Spectrometry (SSMS) are silicon, aluminum, magnesium and copper. They are not homogeneously distributed. The cobalt content used for doping the ingot, determined following the work of Cockayne et al [2], is ].9mgg -]. After complete reaction of indium and phosphorus a cooling rate equal to 2°C/hour has been applied. In the first to freeze part of the ingot (A and B zones), the material is semiinsulating (whereas in the last to freeze (C and D) the polycristal is n type. The following characteristics have to be pointed out : - the cobalt concentration (0.25-2.5 ppma : SSMS) is very inhomogeneous except in the B zone (0.25 ppma) seemingly due to segregation at the grain boundaries or to the precipation of cobalt phosphorus [2]. If we consi-
468
B. Lambert et al.
/ Electron
paramagnetic resonance o f lnP:Co 2+
der the results of zone B, the segregation • -5 . coefficient that we deduce ms 5.10 zn accordance with Cockayne et al results [2] ; - the silicon concentration is anomalously high (i to 60 ppma) that is perhaps due to silica deposition during the sealing of the ampoule ;
In the n type samples a background signal is present which is due to the cyclotron resonance of the electrons (or transverse magnetoresistance). This kind of signal quite common in the study of n type III-V compounds [6],is probably responsible of the large 115 kHz background observed by Baker et al [4].
- the single crystals sizes are smaller than in the case of the undoped polycrystalline InP. IV - DISCUSSION IIl-
E.P.R.
IV.I - Discussion of the E.P.R. results ................................
EXPERIMENTS
We have performed E.P.R. measurements on several n type and SI crystals extracted from the ingot; we have choosen these single crystals in all parts of the ingot. We have also studied several samples extracted f r o m a L.E.C. cobalt doped boule grown in Lannion and a semi-insulating sample (L 940) grown at the R.S.R.E. and very kindly provided to us by Dr M.S. Skolnick. The experiments have been performed at liquid helium temperature using an X band E-line Varian spectrometer The magnetic field was precisely measured with an N.M.R. Gaussmeter and the microwave frequency with a frequency counter. In all the cobalt doped samples that we have studied, we observed the signal shown in fig. i. It consists of an intense single line at g = 2.192 + O.OO1 having a peak to peak line width of 150 Gauss. This line is isotropic and not significantly photosensitive in the studied samples (semi-insulating and n type). In addition, we observe a small signal at g = 2 probably due to a residual impurity or a lattice defect.
i
9.23
f
GI4;'
InP : Co
3.5 K
26 i
Fig.
22
28
29
3
3il
32
313
i. E.P.R. spectrum of Co doped InP (sample P.32 B).
34
:
Since the g = 2.192 signal is observed in all cobalt doped samples and not in other sample~, we attribute it to cobalt. Substitutional Co 3 (3d 6) and Co (3d 8) are not expected to be paramagnetic. Co2+(3d 7) has a F8 ground state expected to be paramagnetic, it should be slightly ~nisotropic because of the u ~ E S x H x + S.y3 H y + .S z H-] zj term in spin hamiltonian. As the u factor zs not expected to be large, the anisotropy might be smaller than the experimental error. Natural cobalt (59Co) has a nuclear spin 7/2; one expects a eight lines spectrum because of the hyperfine interaction, but in III-V compounds this effect can be hidden by the ligands hyperfine interaction. A single unresolved line has also been observed in the E.P.R. spectrum of Co 2+ in GaP [7] and GaAs [8]. It is to be noted that in all studied samples, we also observed the Co 2+ luminescence spectrum [3,5]. Axial Co 2+ has to be rejected because its EPR spectrum would be very strongly snisotropic [i0]. In all the II-VI and+III-V tetrahedral compounds the g factor of Co 2 is larger than the g factor of Ni 3 by an amount which is the range O.O7-O.1 this should also be true for InP. This is the case for the g = 2.192 signal, but not for the 2.059 one for which g is smaller than the Ni 3+ value (2.098) [7]. The g value can be approximated by using the expressional, obtained from a simple theory : g = 2 - ~ (see for instance ref. [9]) where k is a ~ovalency reduction factor, ~ the spin orbit coupling constant and A the 4T2 ÷ ~A2 splitting. If one takes k = 0.7 and ~ = - 135 cm I as for GaAs and GaP [9], the free ion value being ~ = - 178 cm -I and A = 3850 cm -I obtained from the optical experiments [3,5] one obtains g = 2.20 in good agreement with the value presently reported. It is to be noted that the g = 2.059 value leads to ~ = - 42 c m i I [5] and an inconsistency between the values of ~ = 0.24 and the chosen value of k = 0.7. From the linewidths analysis one can get an other argument which favour~the attribution of 2 the g = 2.192 signal to COin . The unresolved line width is due to the hyperfine and ligands hyperfine interactions. The hyperfine interaction should be about the same in the case of GaP and InP (but slightly larger in the case of InP), but the ligands hyperfine interaction should be quite different. The contribution of
B. Lambert et al.
/ Electron paramagnetic resonance o f lnP.'Co 2+
the first anion and cation shells to (AH) 2 are dominant and amount to [7] : 4 a 2 (p31) + 46.2 a 2 (Ga 71 ) for GaP and 4 a 2 (p31) + 396 a 2 (In lls) for InP. The p31 contributions are about the same for GaP and inP, but a scaling factor for a (In115)/a (Ga 71) = 1.213 [7] has to be used. This e~sures that the line width of substitution~l Co 2 in InP must be larger than that of Co 2 in GaP which is 70 Gauss [7]. This is satisfied for the g = 2.]92 signal but not for the g = 2.059 one. IV.2 - ~ ! ~ ! ~ _ ~ _ ~ _ ~ ~ $ - ~ Some problems remain unresolved concerning the acceptor role of cobalt. The first one has already been pointed out in ~5]. The ionisation energy Ei (Co 2- + Ei ÷ Co 3 + eC.B.) has been found to be 0.5 eV [5]. This is rather surprising because the semi-emplrical laws (for instance [ii]) predict that for Co 2+, the value of E i should be larger than that of Fe 2+ (0.65eV). The second one is raised by some preliminary D.L.T.S. measurements performed in Lannion by G. Pelous [12]. He observes the Co related peak [5]~ but with a very low concentration ~ i0~3 cm (the IEI electron trap due to iron is observed with a concentration of ~ 1014 cm-3). The g = 2.192 E.P.R. signal observed in the same sample is very intense and the lowest concentration of cobalt determined by S.S.M.S. in the ingot is 1016 cm -3. Such a discreapancy gives rise to a doubt about the attribution to substitutional cobalt of the 0.53 eV D.L.T.S. peak. It is to be noted that, in [5], the electron trap ~oncentration measured by D.L.T.S. (i x I0 Is cm 3) is lower than the estimated Co concentration > 3.3 x 1015 cm-3). These two points make questionnable whether the level located at 0.5 eV below the conduction ban~ minimum is due to the 4A 2 ground state of Co 2 substitutional to indium. V - CONCLUSION We have shown that the C o ~ E.P.R0 spectrum in InP is characterized by g = 2.192 ~ O.O01 and a line width of 150 Gauss. Further studies are needed in order to understand the compensating behaviour of cobalt in InP and we believe that the E.P.R. results presently reported are a safe basis for future work. ACKNOWLEDGEMENTS We wish to thank G. Moisan and L. Le Mar~chal for their technical assistance in sample growth, R. Coquill~ who grow the L.E.C. InP:Co and M. Gauneau who performed the S.S.M.S. measurements.
469
We are grateful to G. Pelous for very stimulative discussions related to D.L.T.S. measurements which he performed. We enjoyed the information exchange with the Malvern team and we wish to thank particulary Dr Skolnick who furnished us an InP:Co sample and the manuscript (of ref.[5]) prior to publication. NOTE ADDED IN PROOF We have recently learnt that M.S. Skolnick et al [13] have obtained E.P.R. results in agreement with those reported here. REFERENCES [i] M. Gauneau, J.C. Paris To be published [2] B. Cockayne, W.R. Mac Ewan, G.T. Brown J. of Crystal Growth, 55, 263 (1981) [3] M.S. Skolnick, P.J. Dean, P.R. Tapster, D.J. Robbins, B. Cockayne, W.R. Mac Ewan J. of Luminescence, 24/25, 241 (1981) [4] J.M. Baker, L.J.C. Bluck, B. Cockayne, W.R. Mac Ewan J. Phys. C, 14, 3953 (1981) [5] M.S. Skolnick, P.R. Tapster, P.J. Dean, R.G. Humphreys, B. Cockayne, W.R. Mac Ewan J.M. Noras J. Phys. C, 15, 3333 (1982) [6] B. Clerjaud, A.M. Hennel, G. Martinez Solid State Commun., 33, 983 (1980) [7] U. Kaufmann, J. Schneider Solid State Com~un., 25, 1113 (1978) [8] M. Goldlewski, A.M. Hennel Phys. Status Solidi, (b)88, KI1 (1978) [9] J. Weber, H. Ennen, U. Kaufmann, J. Schneider Phys. Rev., B21, 2394 (1980) [IO] P. Koidl, A. Rauber J. Phys. Chem. Solids, 35,
1061 (1974)
[11] J.W. Allen Proc. Semi-insulating III-V Materials Nottingham 1982 - Ed. by G.J. Ress Shiva Pub., p. 261 (1981) [12] G. Pelous Private communication [13] M.S. Skolnick, L. Eaves, S. Clough, B. Cokayne, P.J. Dean Private communication.