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Pulsed ion beam formation of highly doped GaAs layers
Nuclear Instruments and Methods in Physics Research B 139 (1998) 418±421
Pulsed ion beam formation of highly doped GaAs layers Rustem M. Bayazitov a
a,*
, Landish Kh. Antonova a, Ildus B. Khaibullin a, Gennadii E. Remnev b
Physical-Technical Institute, Sibirsky Trakt 10/7, Kazan 420029, Russian Federation b Institute of Nuclear Physics, Lenin str.2a, Tomsk 634050, Russian Federation
Abstract The formation of heavily doped n-GaAs layers using continuous ion implantation and subsequent treatment by powerful pulsed ion beams has been investigated. Using Auger electron spectroscopy (AES), electrical measurements and computer simulations, correlation between donor distributions and electrical activation was established. It is shown that the n -GaAs layers (n 1019 ±1020 cmÿ3 ) are formed in the deep tail of the impurity atom distributions. Thermal stability of formed supersaturated layers was investigated. Ó 1998 Elsevier Science B.V. PACS: 72.80.Ey; 81.40.Wx; 61.72.Vv Keywords: Implantation; GaAs; Carrier concentration
1. Introduction It is very dicult to produce high levels of carrier concentration in n-GaAs by conventional methods including ion implantation and subsequent thermal or laser annealing. For high doses (>1015 cmÿ2 ) electrical activation is very poor (<15%) [1,2]. Thermal annealing of implanted GaAs does not cause considerable increase in the concentration of donor atoms in substitutional positions or in the carrier concentration. Nanosecond laser annealing does enable to increase the concen-
* Corresponding author. Fax: +8432-765-075; tel.: +8432761-241; e-mail: [email protected].
tration of atoms in substitutional positions but the concentration of carriers is low due to decomposition of GaAs. It is known [1] that the electrical properties are strongly in¯uenced by the location of the dopant atoms in the matrix and by the presence of defects which may result in donor compensation. In [3] it was shown that using continuous ion implantation and subsequent powerful ion beam treatment heavily doped (8 ´ 1019 cmÿ3 ) layers may be formed in a subsurface region (0.1±0.5 lm). Unfortunately, the distributions of implanted atoms were unknown. In this paper we investigate and simulate the distribution of donor atoms after implantation, ion beam pulse treatment and nanosecond laser annealing. The thermal stability of the formed supersaturated n -GaAs layers has been investigated.
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 0 3 0 - 5
R.M. Bayazitov et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 418±421
419
2. Experimental methods and computer simulations
3. Results and discussion
GaAs (1 0 0) wafers were implanted using Te and Si ions with energies of 60 and 80 keV respectively to form the n-type GaAs layer. The implantation was carried out at room temperature resulting in total amorphization of the GaAs up to a thickness of 0.1 lm. For the recrystallization and electrical activation of the dopants after implantation the samples were treated with a single pulse of an ion beam or laser irradiation. The pulsed ion accelerator [4] allowed us to obtain wide-aperture ion beams with a pulse duration (FWHM) of tp 5 ´ 10ÿ8 s, an ion energy in the maximum of pulse Emax 300 keV and a current density j 10±150 A/cm2 . The ion beam generally contained Cn (n 1,. . .,3) ± 75±80%, H ± 15± 20% and heavy metal ions (Fe , Cu ) ± 5±10%. The total dose of these dopants per pulse does not exceed 3 ´ 1013 cmÿ2 . Laser annealing was performed with a ruby laser which generates pulses of light (wavelength 0.69 lm) with a duration tp 50±70 ns and energy density W 2.0 J/cm2 . The depth distribution of impurities and their electrical activation (carrier concentration and mobility) were investigated using AES and Hall measurements Simulations of the heating process were carried out by solving a non-linear one-dimensional heat equation by the method of ®nite dierences. Standard thermophysical and optical parameters of GaAs and their temperature dependencies were used. The main dierence between the pulsed ion beam and the laser treatment is in the spatial-temporal distribution of the absorbed energy [3]. The ion energy loss distribution depends only slightly on the phase state of the material, but it depends mainly on the variation of ion energy E(t) during annealing. For pulsed ion beam treatment the distribution of energy is more uniform than in case of laser annealing. In order to calculate atom distributions after pulse treatment we used results of the heating simulations and solved the diusion equation with moving boundary [5]. The condition on the moving boundary is k Cs /Cl , where k, Cs , Cl are the segregation coecient, atom concentration in solid and liquid phases, respectively.
Results of the heating calculations (Fig. 1) show, that for the same pulse duration in the case of ion pulse treatment, the thickness of the melt is considerably higher than in case of laser annealing. This fact is connected to the spatial distribution of ion energy during pulsed ion beam treatment. Both for ion beam and laser annealing we observe a signi®cant redistribution of Si and Te atoms (Figs. 2 and 3). For Te, segregation of atoms to the surface is observed. Results of computer simulations show good agreement for the parameters D 5 ´ 10ÿ4 cm2 /s, k 1 for Si and k 0.5 for Te atoms in GaAs. This diusion coecient D agrees with data obtained in [6] for liquid GaAs. High values of carrier concentration were obtained: up to 1020 cmÿ3 for pulsed ion treatment and 1±3 ´ 1019 cmÿ3 for pulsed laser treatment with the carrier mobility about 100 and 300 cm2 Vÿ1 sÿ1 . These oversaturated solutions of GaAs are formed at the subsurface (0.05±0.2 lm) by diusion of Te and Si atoms in the melt with subsequent rapid epitaxial recrystallization. Near the surface, the concentration of Si and Te atoms essentially exceeds the carrier concentration. This fact can be attributed to the decomposition of
Fig. 1. Calculated dependence of the melt thickness of GaAs versus time during treatment by laser and ion pulses. Pulse duration is 50 ns.
420
R.M. Bayazitov et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 418±421
thermal annealing of the implanted crystal (for Si )3 ´ 1018 cmÿ3 [1] and for Te )8 ´ 1018 cmÿ3 [2]). These supersaturated GaAs solutions are unstable with respect to subsequent thermal treatments above 250°C (Fig. 4). However, after this annealing treatment at 250±600°C the sheet concentration level is about one order of magnitude higher than that obtained after conventional annealing [2]. 4. Conclusions
GaAs and formation of neutral pairs or impurityvacancy complexes. The carrier concentrations in the n-GaAs layers exceed the electrical activation limit obtained by
Ion implantation and subsequent treatment by powerful nanosecond beams of light ions are an effective means to form n -GasAs layers. The depth distribution of Si and Te dopants is de®ned by diffusion in the melt and segregation to the surface (for Te). Despite of this process heavily doped layers are formed in the subsurface part of the distribution. These supersaturated GaAs solutions are thermally unstable. However, after thermal treatment electron concentration levels are signi®cantly higher than that obtained by conventional annealing without pulse treatment.
Fig. 3. Te-concentration pro®les obtained from AES spectra for Te implanted GaAs (60 keV, 1016 cmÿ2 ) before, after laser and after ion pulse treatment. The dashed curves represent calculated distributions of Te after treatments (D 5 ´ 10ÿ4 cm2 /c, k 0.5). The dotted curve in the right bottom corner represents electrical activity of Te after ion pulse treatment.
Fig. 4. Sheet electron densities in Te (60 keV) implanted GaAs after ion (1,3) and laser (2) treatment as a function of subsequent thermal annealing (5 min). 1,2 ± Dose 1016 cmÿ2 ; 3 ± Dose 1015 cmÿ2 . The arrow points to the maximum density obtained after only thermal annealing of Te implanted GaAs in an As±H2 ambient atmosphere [2].
Fig. 2. Si-concentration pro®les obtained from AES spectra for Si implanted GaAs (80 keV, 1016 cmÿ2 ) before, after laser and after ion pulse treatment. The dashed curves represent calculated distributions of Si after treatments (D 5 ´ 10ÿ4 cm2 /c, k 1). The dotted curve represents electrical activity of Si after ion pulse treatment.
R.M. Bayazitov et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 418±421
Acknowledgements This work is supported by the Russian Technology Research Program ``Advanced technology of micro- and nanoelectronics'', grant N 141/57/2 and by the Research Program of Tatar Academy of Science, grant N16.3. References [1] R.S. Brattacharia, A.K. Rai, Y. Yeo, P.P. Pronko, S.C. Ling, R.S. Wilson, Y.S. Park, J. Appl. Phys. 54 (1983) 2329.
421
[2] S.J. Pearton, J.S. Williams, K.T. Short, S.T. Johnson, D.C. Jacobsen, J.M. Poate, J.M. Gibson, D.O. Boerma, J. Appl. Phys. 65 (1989) 1089. [3] R.M. Bayazitov, L.Kh. Zakirzyanova, I.B. Khaibullin, G.E. Remnev, Nucl. Instr. and Meth. B 122 (1997) 35. [4] I.F. Isakov, N.V. Kollodii, M.S. Opekunov, V.M. Matvienko, S.A. Pechenkin, G.E. Remnev, Yu.P. Usov, Vacuum 42 (1991) 619. [5] J.M. Poate, G. Foti, D.C. Jacobson (Eds.), Surface Modi®cation, Alloying by Laser, Ion and Electron Beams, Plenum Press, New York, 1987. [6] W. Wesch, G. Gotz, Phys. Status Solidi 94 (1980) 745.
Pulsed ion beam formation of highly doped GaAs layers Rustem M. Bayazitov a
a,*
, Landish Kh. Antonova a, Ildus B. Khaibullin a, Gennadii E. Remnev b
Physical-Technical Institute, Sibirsky Trakt 10/7, Kazan 420029, Russian Federation b Institute of Nuclear Physics, Lenin str.2a, Tomsk 634050, Russian Federation
Abstract The formation of heavily doped n-GaAs layers using continuous ion implantation and subsequent treatment by powerful pulsed ion beams has been investigated. Using Auger electron spectroscopy (AES), electrical measurements and computer simulations, correlation between donor distributions and electrical activation was established. It is shown that the n -GaAs layers (n 1019 ±1020 cmÿ3 ) are formed in the deep tail of the impurity atom distributions. Thermal stability of formed supersaturated layers was investigated. Ó 1998 Elsevier Science B.V. PACS: 72.80.Ey; 81.40.Wx; 61.72.Vv Keywords: Implantation; GaAs; Carrier concentration
1. Introduction It is very dicult to produce high levels of carrier concentration in n-GaAs by conventional methods including ion implantation and subsequent thermal or laser annealing. For high doses (>1015 cmÿ2 ) electrical activation is very poor (<15%) [1,2]. Thermal annealing of implanted GaAs does not cause considerable increase in the concentration of donor atoms in substitutional positions or in the carrier concentration. Nanosecond laser annealing does enable to increase the concen-
* Corresponding author. Fax: +8432-765-075; tel.: +8432761-241; e-mail: [email protected].
tration of atoms in substitutional positions but the concentration of carriers is low due to decomposition of GaAs. It is known [1] that the electrical properties are strongly in¯uenced by the location of the dopant atoms in the matrix and by the presence of defects which may result in donor compensation. In [3] it was shown that using continuous ion implantation and subsequent powerful ion beam treatment heavily doped (8 ´ 1019 cmÿ3 ) layers may be formed in a subsurface region (0.1±0.5 lm). Unfortunately, the distributions of implanted atoms were unknown. In this paper we investigate and simulate the distribution of donor atoms after implantation, ion beam pulse treatment and nanosecond laser annealing. The thermal stability of the formed supersaturated n -GaAs layers has been investigated.
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 0 3 0 - 5
R.M. Bayazitov et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 418±421
419
2. Experimental methods and computer simulations
3. Results and discussion
GaAs (1 0 0) wafers were implanted using Te and Si ions with energies of 60 and 80 keV respectively to form the n-type GaAs layer. The implantation was carried out at room temperature resulting in total amorphization of the GaAs up to a thickness of 0.1 lm. For the recrystallization and electrical activation of the dopants after implantation the samples were treated with a single pulse of an ion beam or laser irradiation. The pulsed ion accelerator [4] allowed us to obtain wide-aperture ion beams with a pulse duration (FWHM) of tp 5 ´ 10ÿ8 s, an ion energy in the maximum of pulse Emax 300 keV and a current density j 10±150 A/cm2 . The ion beam generally contained Cn (n 1,. . .,3) ± 75±80%, H ± 15± 20% and heavy metal ions (Fe , Cu ) ± 5±10%. The total dose of these dopants per pulse does not exceed 3 ´ 1013 cmÿ2 . Laser annealing was performed with a ruby laser which generates pulses of light (wavelength 0.69 lm) with a duration tp 50±70 ns and energy density W 2.0 J/cm2 . The depth distribution of impurities and their electrical activation (carrier concentration and mobility) were investigated using AES and Hall measurements Simulations of the heating process were carried out by solving a non-linear one-dimensional heat equation by the method of ®nite dierences. Standard thermophysical and optical parameters of GaAs and their temperature dependencies were used. The main dierence between the pulsed ion beam and the laser treatment is in the spatial-temporal distribution of the absorbed energy [3]. The ion energy loss distribution depends only slightly on the phase state of the material, but it depends mainly on the variation of ion energy E(t) during annealing. For pulsed ion beam treatment the distribution of energy is more uniform than in case of laser annealing. In order to calculate atom distributions after pulse treatment we used results of the heating simulations and solved the diusion equation with moving boundary [5]. The condition on the moving boundary is k Cs /Cl , where k, Cs , Cl are the segregation coecient, atom concentration in solid and liquid phases, respectively.
Results of the heating calculations (Fig. 1) show, that for the same pulse duration in the case of ion pulse treatment, the thickness of the melt is considerably higher than in case of laser annealing. This fact is connected to the spatial distribution of ion energy during pulsed ion beam treatment. Both for ion beam and laser annealing we observe a signi®cant redistribution of Si and Te atoms (Figs. 2 and 3). For Te, segregation of atoms to the surface is observed. Results of computer simulations show good agreement for the parameters D 5 ´ 10ÿ4 cm2 /s, k 1 for Si and k 0.5 for Te atoms in GaAs. This diusion coecient D agrees with data obtained in [6] for liquid GaAs. High values of carrier concentration were obtained: up to 1020 cmÿ3 for pulsed ion treatment and 1±3 ´ 1019 cmÿ3 for pulsed laser treatment with the carrier mobility about 100 and 300 cm2 Vÿ1 sÿ1 . These oversaturated solutions of GaAs are formed at the subsurface (0.05±0.2 lm) by diusion of Te and Si atoms in the melt with subsequent rapid epitaxial recrystallization. Near the surface, the concentration of Si and Te atoms essentially exceeds the carrier concentration. This fact can be attributed to the decomposition of
Fig. 1. Calculated dependence of the melt thickness of GaAs versus time during treatment by laser and ion pulses. Pulse duration is 50 ns.
420
R.M. Bayazitov et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 418±421
thermal annealing of the implanted crystal (for Si )3 ´ 1018 cmÿ3 [1] and for Te )8 ´ 1018 cmÿ3 [2]). These supersaturated GaAs solutions are unstable with respect to subsequent thermal treatments above 250°C (Fig. 4). However, after this annealing treatment at 250±600°C the sheet concentration level is about one order of magnitude higher than that obtained after conventional annealing [2]. 4. Conclusions
GaAs and formation of neutral pairs or impurityvacancy complexes. The carrier concentrations in the n-GaAs layers exceed the electrical activation limit obtained by
Ion implantation and subsequent treatment by powerful nanosecond beams of light ions are an effective means to form n -GasAs layers. The depth distribution of Si and Te dopants is de®ned by diffusion in the melt and segregation to the surface (for Te). Despite of this process heavily doped layers are formed in the subsurface part of the distribution. These supersaturated GaAs solutions are thermally unstable. However, after thermal treatment electron concentration levels are signi®cantly higher than that obtained by conventional annealing without pulse treatment.
Fig. 3. Te-concentration pro®les obtained from AES spectra for Te implanted GaAs (60 keV, 1016 cmÿ2 ) before, after laser and after ion pulse treatment. The dashed curves represent calculated distributions of Te after treatments (D 5 ´ 10ÿ4 cm2 /c, k 0.5). The dotted curve in the right bottom corner represents electrical activity of Te after ion pulse treatment.
Fig. 4. Sheet electron densities in Te (60 keV) implanted GaAs after ion (1,3) and laser (2) treatment as a function of subsequent thermal annealing (5 min). 1,2 ± Dose 1016 cmÿ2 ; 3 ± Dose 1015 cmÿ2 . The arrow points to the maximum density obtained after only thermal annealing of Te implanted GaAs in an As±H2 ambient atmosphere [2].
Fig. 2. Si-concentration pro®les obtained from AES spectra for Si implanted GaAs (80 keV, 1016 cmÿ2 ) before, after laser and after ion pulse treatment. The dashed curves represent calculated distributions of Si after treatments (D 5 ´ 10ÿ4 cm2 /c, k 1). The dotted curve represents electrical activity of Si after ion pulse treatment.
R.M. Bayazitov et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 418±421
Acknowledgements This work is supported by the Russian Technology Research Program ``Advanced technology of micro- and nanoelectronics'', grant N 141/57/2 and by the Research Program of Tatar Academy of Science, grant N16.3. References [1] R.S. Brattacharia, A.K. Rai, Y. Yeo, P.P. Pronko, S.C. Ling, R.S. Wilson, Y.S. Park, J. Appl. Phys. 54 (1983) 2329.
421
[2] S.J. Pearton, J.S. Williams, K.T. Short, S.T. Johnson, D.C. Jacobsen, J.M. Poate, J.M. Gibson, D.O. Boerma, J. Appl. Phys. 65 (1989) 1089. [3] R.M. Bayazitov, L.Kh. Zakirzyanova, I.B. Khaibullin, G.E. Remnev, Nucl. Instr. and Meth. B 122 (1997) 35. [4] I.F. Isakov, N.V. Kollodii, M.S. Opekunov, V.M. Matvienko, S.A. Pechenkin, G.E. Remnev, Yu.P. Usov, Vacuum 42 (1991) 619. [5] J.M. Poate, G. Foti, D.C. Jacobson (Eds.), Surface Modi®cation, Alloying by Laser, Ion and Electron Beams, Plenum Press, New York, 1987. [6] W. Wesch, G. Gotz, Phys. Status Solidi 94 (1980) 745.