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Laser pulse energy dependence of annealing in ion implanted Si and GaAs semiconductors
Volume 65A, number 2
PHYSICS LETTERS
20 February 1978
LASER PULSE ENERGY DEPENDENCE OF ANNEALING IN ION IMPLANTED Si AND GaAs SEMICONDUCTORS E. RIM1NI, P. BAERI and G. FOTI fatituto di Struttura della Materia, Università di Catania, 95129 Catania, Italy Received 14 November 1977 Revised manuscript received 19 December 1977
Crystallization of amorphous layers in ion implanted Si and GaAs samples has been obtained by single pulse ruby laser irradiation. The channeling effect with MeV He backscattering was adopted to investigate the structure of the irradiated layer. Using 50 and 20 ns duration time of the Q-switched laser pulse it has been found that the transition to single crystal for a given material amorphous layer thickness requires nearly the same energy density. The energy density to induce the single crystal transition in Si amorphous layer is about 50% higher than that in GaAs layer of the same thickness. The threshold energy value, the absence of partial regrowth and the excellent agreement of the energy density difference between Si and GaAs with estimated value from thermal properties support the formation of a liquid—solid system during laser irradiation.
Laser irradiation has been used recently [1—4]to anneal out the disorder induced by ion implantation in semiconductors. A threshold laser power density for a fixed duration of the pulse induces the transition of the amorphous region into a single crystal [5]. Lower values of the power density produces polycrystalline materials [6] or heavily disordered crystalline regions [7]. With the aim to clarify the physical mechanisms involved in the transition to single crystal the effect of the irradiation has been investigated by varying the duration of the laser pulses. Si and GaAs were used as substrate materials. The amorphous layer in both substrates was obtained by ion implantation. Si (111) substrates were implanted with 50 keY P, 1 015/cm2 and GaAs (100> substrates with 400 keY Te, 1015/cm2. The implantations were performed at room temperature and along a nonchanneled direction. The channeling effect technique with MeV He backscattering was used to study the structure of the implanted layer before and after laser irradiation. Q-switched ruby (X = 0.69 pm) laser pulses of duration 50 ns and 20 ns were adopted for the annealing. Typical pulse shapes are shown in fig. 1. The area under the signal is proportional to the total energy density for each stroke. The dimension of the
Laser p~i~ ~
t 20 nsec
1
~ 0.5
0 0
/t=50 nsec
~o
i~o
-
t me (n sec) Fig. 1. Laser pulse shapes adopted in the experiments. The scale of the power intensity is in arbitrary units.
irradiated spot was a few cm2. Fig. 2 shows random and (111) aligned backscattering spectra for a 1.0 MeV He beam on Si implanted with P ions. The aligned yield of the as-implanted sample reaches the random level for an energy width of 70 keY corresponding to a Si layer 1000 A thick. After irradiation with a power density of 80 MW/cm2 and of 20 ns pulse duration the aligned yield decreases noticeable and has a minimum yield near the surface 153
Volume 65A, number 2
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C
PHYSICS LETTERS
20 February 1978
2.0 MeV He on GaAs
1.0MeV Heon S~
random random ~ 6
~
augned 4
2
~
~-
ec
A
aa-imptantad ~o~ ~c m~ 20 nsec
A
8OM~/220n~~
~
6 altgned spectra
spectra <111>
3OMW/crn2,50nseC__06
Energy (MeV)
~
~
2
A
25MW/cm25OcS
Energy(Mev)
Fig. 2. Energy spectra of1.0 MeV He + ions backscattered from a Si (111) sample implanted with 50 keV P, 1015 ions! 2. The <111) aligned spectra are recorded for the ascm implanted sample (A), after irradiation with a 80 MW/cm2, 20 ns (.) and 35 MW/cm2, 50 ns (0) laser pulse.
Fig. 3. Energy spectra of 2.0 MeV He + ions backscattered from a GaAs <100) sample implanted with 400 keV Te, 1015 ions/cm2. The <100) aligned spectra are recorded for the asimplanted sample (h), after irradiation with a 70 MW/cm2, 20 ns (,) and 25 MW/cm2, SOns (0) laser pulse.
of about 5%. This spectrum is comparable with that of an unimplanted sample, indicating the good quality of the crystallized layer. Similar results were obtained after irradiation with 35 MW/cm2, 50 ns pulse duration. The corresponding aligned spectrum is shown in fig. 2 as open squares. The energy density of the two laser pulses inducing crystallization of the amorphous layer is 1.6 J/cm2 and 1.75 i/cm2, respectively, Within the experimental accuracy the same energy density is required for both laser pulse durations to induce the transition to single crystal. Residual disorder as seen by channeling effect measurements is still present after irradiation with 1.2 i/cm2 (20 ns) or with 1 .5 J/cm2 (50 ns). This indicates that the threshold for crystallization of 1000 A amorphous layer is between 1.5 and 1 .6 i/cm2. A similar experiment was performed with a GaAs amorphous layer. Fig. 3 shows random and (100> aligned backscattering spectra obtained with 2.0 MeV He, of GaAs implanted with Te ions. The aligned yield of the as-implanted sample reaches the random level for an energy width corresponding to a 2200 A thick layer. After irradiation with a 70 MW/cm2, 20 ns or 25 MW/cm2, 50 ns laser pulse the aligned yields
are nearly the same and coincide with that of an unimplanted GaAs crystal sample. The minimum yield near the surface is about 4%. Irradiation with a 50 MW/cm2, 20 ns or 15 MW/cm2, 50 ns laser pulse produces a polycrystalline layer. The crystallization occurs at 1 .4 J/cm2 or 1.25 i/cm2 for the 20 and 50 ns duration of the laser pulse, respectively. The laser annealing effects are also for GaAs well described in terms of energy density at least in the range of times used in this work. The energy density involved in the amorphous to single crystal transition depends on the thickness of the amorphous layer [8]. A 2500 A thick amorphous layer on Si (111> substrate requires for the transition about 2.5 i/cm2 energy density. Si layers need an energy density 50% higher than that for GaAs layers of the same thickness. Variations of less importance are associated with the substrate crystallographic orientation. In thermal annealing the crystallization of the Si amorphous layer is epitaxial and it is initiated at the amorphous to crystal interface with an activation energy of 2.3 eV [9]. The maximum growth rate in the solid state at the melting point ‘~1700K is about
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PHYSICS LETTERS
2 X 10~A/s. In laser irradiation the specimens are at “high” temperatures for times of about 1 0~ s [101. To grow a 1000 A thick amorphous layer in 1 0~s a growth rate of 1010 A/s is required. This value is orders of magnitude higher than the maximum rate in solid state. In GaAs implanted samples high residual disorder is always present in thermally regrown amorphous layer [11]. These considerations and the threshold energy density values found in the amorphous to single crystal transition suggest that in laser annealing the processes involved could be related more to liquid-state than to solid-state epitaxy. The energy deposited by the laser pulse in the absorbing layer produces mainly a thermal effect by increasing the layer temperature up to the melting point and melting then the layer. Assuming for Si a heat capacity of 0.84 J/g°C,a latent heat of fusion of 1800 JIg, a melting point of 1420°Cand for GaAs 0.31 i/g°C,600 J/g and 1230°C, the amount of heat required for the melting of a Si layer initially at room temperature is about 40% larger than that required for GaAs of the same thickness. This value compares very well with the threshold energy density percentage difference between Si and GaAs, thus supporting the hypothesis of the formation of a liquid—solid system during laser irradiation.
20 February 1978
The authors wish to thank F.H. Eisen, J.W. Mayer, A. Cingolani and I. Catalano for useful discussions. References [1] E.I. Shtyrkov et al., Soy. Phys. Semicond. 9 (1976) 1309. [2] G.A. Kachurin, N.B. Pridachin and L.S. Smirnov, Soy. Phys. Semicond. 9 (1976) 946. [3] A.Kh. Antonenko et al., Sov. Phys. Semicond. 10 (1976) 81. [4] G. Vitali, M. Bertolotti, G. Foti and E. Rimini, 7th Intern. Conf. on Amorphous and liquid semiconductors (Institute of Physics, London, 1977). [5] G. Foti, S.U. Campisano, E. Rimini and G. Vitali, Lattice location of Te in laser annealed Te-implanted silicon, J. Appi. Phys. (March 1978). [6] G. Foti, E. Rimini, G. Vitali and M. Bertolotti, Appl. Phys. 14 (1977) 189. [7) G. Foti, E. Rimini, W.F. Tseng and J.W. Mayer, Crystallization of amorphous Si layers by Q-mode laser annealing, submitted to Appi. Phys. [8] G. Foti, E. Rimini, G. Vitali and M. Bertolotti, submitted to Appi. Phys. Lett. [9] L. Csepreghi, J.W. Mayer and T.W. Sigmon, Appi. Phys. 29 (1976)et92.al., On some peculiarities of laser an1101 Lett. l.B. Khaibullin nealing of implanted silicon layers, First USSR—USA Seminar on Ion implantation (Albany, NY, 1977). [111 K. Gamo, T. Inada, J.W. Mayer, F.H. Eisen and C.G. Rhodes, Rad. Effects 33 (1977) 85.
155
PHYSICS LETTERS
20 February 1978
LASER PULSE ENERGY DEPENDENCE OF ANNEALING IN ION IMPLANTED Si AND GaAs SEMICONDUCTORS E. RIM1NI, P. BAERI and G. FOTI fatituto di Struttura della Materia, Università di Catania, 95129 Catania, Italy Received 14 November 1977 Revised manuscript received 19 December 1977
Crystallization of amorphous layers in ion implanted Si and GaAs samples has been obtained by single pulse ruby laser irradiation. The channeling effect with MeV He backscattering was adopted to investigate the structure of the irradiated layer. Using 50 and 20 ns duration time of the Q-switched laser pulse it has been found that the transition to single crystal for a given material amorphous layer thickness requires nearly the same energy density. The energy density to induce the single crystal transition in Si amorphous layer is about 50% higher than that in GaAs layer of the same thickness. The threshold energy value, the absence of partial regrowth and the excellent agreement of the energy density difference between Si and GaAs with estimated value from thermal properties support the formation of a liquid—solid system during laser irradiation.
Laser irradiation has been used recently [1—4]to anneal out the disorder induced by ion implantation in semiconductors. A threshold laser power density for a fixed duration of the pulse induces the transition of the amorphous region into a single crystal [5]. Lower values of the power density produces polycrystalline materials [6] or heavily disordered crystalline regions [7]. With the aim to clarify the physical mechanisms involved in the transition to single crystal the effect of the irradiation has been investigated by varying the duration of the laser pulses. Si and GaAs were used as substrate materials. The amorphous layer in both substrates was obtained by ion implantation. Si (111) substrates were implanted with 50 keY P, 1 015/cm2 and GaAs (100> substrates with 400 keY Te, 1015/cm2. The implantations were performed at room temperature and along a nonchanneled direction. The channeling effect technique with MeV He backscattering was used to study the structure of the implanted layer before and after laser irradiation. Q-switched ruby (X = 0.69 pm) laser pulses of duration 50 ns and 20 ns were adopted for the annealing. Typical pulse shapes are shown in fig. 1. The area under the signal is proportional to the total energy density for each stroke. The dimension of the
Laser p~i~ ~
t 20 nsec
1
~ 0.5
0 0
/t=50 nsec
~o
i~o
-
t me (n sec) Fig. 1. Laser pulse shapes adopted in the experiments. The scale of the power intensity is in arbitrary units.
irradiated spot was a few cm2. Fig. 2 shows random and (111) aligned backscattering spectra for a 1.0 MeV He beam on Si implanted with P ions. The aligned yield of the as-implanted sample reaches the random level for an energy width of 70 keY corresponding to a Si layer 1000 A thick. After irradiation with a power density of 80 MW/cm2 and of 20 ns pulse duration the aligned yield decreases noticeable and has a minimum yield near the surface 153
Volume 65A, number 2
N
C
PHYSICS LETTERS
20 February 1978
2.0 MeV He on GaAs
1.0MeV Heon S~
random random ~ 6
~
augned 4
2
~
~-
ec
A
aa-imptantad ~o~ ~c m~ 20 nsec
A
8OM~/220n~~
~
6 altgned spectra
spectra <111>
3OMW/crn2,50nseC__06
Energy (MeV)
~
~
2
A
25MW/cm25OcS
Energy(Mev)
Fig. 2. Energy spectra of1.0 MeV He + ions backscattered from a Si (111) sample implanted with 50 keV P, 1015 ions! 2. The <111) aligned spectra are recorded for the ascm implanted sample (A), after irradiation with a 80 MW/cm2, 20 ns (.) and 35 MW/cm2, 50 ns (0) laser pulse.
Fig. 3. Energy spectra of 2.0 MeV He + ions backscattered from a GaAs <100) sample implanted with 400 keV Te, 1015 ions/cm2. The <100) aligned spectra are recorded for the asimplanted sample (h), after irradiation with a 70 MW/cm2, 20 ns (,) and 25 MW/cm2, SOns (0) laser pulse.
of about 5%. This spectrum is comparable with that of an unimplanted sample, indicating the good quality of the crystallized layer. Similar results were obtained after irradiation with 35 MW/cm2, 50 ns pulse duration. The corresponding aligned spectrum is shown in fig. 2 as open squares. The energy density of the two laser pulses inducing crystallization of the amorphous layer is 1.6 J/cm2 and 1.75 i/cm2, respectively, Within the experimental accuracy the same energy density is required for both laser pulse durations to induce the transition to single crystal. Residual disorder as seen by channeling effect measurements is still present after irradiation with 1.2 i/cm2 (20 ns) or with 1 .5 J/cm2 (50 ns). This indicates that the threshold for crystallization of 1000 A amorphous layer is between 1.5 and 1 .6 i/cm2. A similar experiment was performed with a GaAs amorphous layer. Fig. 3 shows random and (100> aligned backscattering spectra obtained with 2.0 MeV He, of GaAs implanted with Te ions. The aligned yield of the as-implanted sample reaches the random level for an energy width corresponding to a 2200 A thick layer. After irradiation with a 70 MW/cm2, 20 ns or 25 MW/cm2, 50 ns laser pulse the aligned yields
are nearly the same and coincide with that of an unimplanted GaAs crystal sample. The minimum yield near the surface is about 4%. Irradiation with a 50 MW/cm2, 20 ns or 15 MW/cm2, 50 ns laser pulse produces a polycrystalline layer. The crystallization occurs at 1 .4 J/cm2 or 1.25 i/cm2 for the 20 and 50 ns duration of the laser pulse, respectively. The laser annealing effects are also for GaAs well described in terms of energy density at least in the range of times used in this work. The energy density involved in the amorphous to single crystal transition depends on the thickness of the amorphous layer [8]. A 2500 A thick amorphous layer on Si (111> substrate requires for the transition about 2.5 i/cm2 energy density. Si layers need an energy density 50% higher than that for GaAs layers of the same thickness. Variations of less importance are associated with the substrate crystallographic orientation. In thermal annealing the crystallization of the Si amorphous layer is epitaxial and it is initiated at the amorphous to crystal interface with an activation energy of 2.3 eV [9]. The maximum growth rate in the solid state at the melting point ‘~1700K is about
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Volume 65A, number 2
PHYSICS LETTERS
2 X 10~A/s. In laser irradiation the specimens are at “high” temperatures for times of about 1 0~ s [101. To grow a 1000 A thick amorphous layer in 1 0~s a growth rate of 1010 A/s is required. This value is orders of magnitude higher than the maximum rate in solid state. In GaAs implanted samples high residual disorder is always present in thermally regrown amorphous layer [11]. These considerations and the threshold energy density values found in the amorphous to single crystal transition suggest that in laser annealing the processes involved could be related more to liquid-state than to solid-state epitaxy. The energy deposited by the laser pulse in the absorbing layer produces mainly a thermal effect by increasing the layer temperature up to the melting point and melting then the layer. Assuming for Si a heat capacity of 0.84 J/g°C,a latent heat of fusion of 1800 JIg, a melting point of 1420°Cand for GaAs 0.31 i/g°C,600 J/g and 1230°C, the amount of heat required for the melting of a Si layer initially at room temperature is about 40% larger than that required for GaAs of the same thickness. This value compares very well with the threshold energy density percentage difference between Si and GaAs, thus supporting the hypothesis of the formation of a liquid—solid system during laser irradiation.
20 February 1978
The authors wish to thank F.H. Eisen, J.W. Mayer, A. Cingolani and I. Catalano for useful discussions. References [1] E.I. Shtyrkov et al., Soy. Phys. Semicond. 9 (1976) 1309. [2] G.A. Kachurin, N.B. Pridachin and L.S. Smirnov, Soy. Phys. Semicond. 9 (1976) 946. [3] A.Kh. Antonenko et al., Sov. Phys. Semicond. 10 (1976) 81. [4] G. Vitali, M. Bertolotti, G. Foti and E. Rimini, 7th Intern. Conf. on Amorphous and liquid semiconductors (Institute of Physics, London, 1977). [5] G. Foti, S.U. Campisano, E. Rimini and G. Vitali, Lattice location of Te in laser annealed Te-implanted silicon, J. Appi. Phys. (March 1978). [6] G. Foti, E. Rimini, G. Vitali and M. Bertolotti, Appl. Phys. 14 (1977) 189. [7) G. Foti, E. Rimini, W.F. Tseng and J.W. Mayer, Crystallization of amorphous Si layers by Q-mode laser annealing, submitted to Appi. Phys. [8] G. Foti, E. Rimini, G. Vitali and M. Bertolotti, submitted to Appi. Phys. Lett. [9] L. Csepreghi, J.W. Mayer and T.W. Sigmon, Appi. Phys. 29 (1976)et92.al., On some peculiarities of laser an1101 Lett. l.B. Khaibullin nealing of implanted silicon layers, First USSR—USA Seminar on Ion implantation (Albany, NY, 1977). [111 K. Gamo, T. Inada, J.W. Mayer, F.H. Eisen and C.G. Rhodes, Rad. Effects 33 (1977) 85.
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