- Email: [email protected]
Rapid thermal annealing of arsenic implanted Si1−xGex epilayers
Nuclear Instruments
2s__ _-
and
Methods in
Physics Research B I22 (1997)639-642 NONil
B
Beamlnteractions with Materials&Atoms
!!I ELSEVIER
Rapid thermal annealing of arsenic implanted Si , _xGen epilayers Lyu-fan Zou * , Z.G. Wang, D.Z. Sun, T.W. Fan, X.F. Liu, J.W. Zhang Laboratory
of Semiconducror
Materials
Science, Institute
of Semiconductors.
Chinese Academy
ofSciences.
P.O. Bar
912. Beijing
lOOO83.
China
Received 2 1 August 1996; revised form received 4 November
1996
Abstract Rapid thermal annealing of arsenic implanted Si , _ ,Ge, was studied by secondary ion-mass spectroscopy (SIMS) and spreading resistance probe (SRP) over a wide range of Ge fractions (O-43%). Redistribution of the implanted arsenic was followed as a function of Ge content and annealing temperature. Arsenic concentration profiles from SIMS indicated that the behavior of implanted arsenic in Si, _,Ge, after RTA was different from that in Si, and the Si, _,Ge., samples exhibited box-shaped, concentration-dependent diffusion profiles with increasing Ge content. The maximum concentrations of electrically active arsenic in Si, _,Ge, was found to decrease with increasing Ge content. Experimental results showed that the arsenic diffusion is enhanced with increasing temperature for certain Ge content and strongly dependent on Ge content, and the higher Ge content, the faster As diffusion.
Si , _lGe, alloys grown epitaxially on Si substrates have become the subject of scientific and technological interest in recent years primarily because of the enormous potential for fabricating novel devices compatible with existing Si-based processing technology [ 1,2]. From a technological viewpoint it is important that physics phenomena in Si, _xGe., related to processing, such as impurity diffusion, should be characterized for integrated circuit processing. In pure silicon, the diffusion and segregation of arsenic in arsenic-implanted Si have been investigated for more than two decades, because silicon layers doped with arsenic at high concentrations are often required in very large scale integrated (VLSI) circuit technology [3-51. However, the arsenic diffusion in Si, _,Ge 1, although very important for the control of the dopant concentration depth profiles during integrated circuit processing, has received little attention [6]. Recently, Hu et al. [7] studied that arsenic was diffused from polycrystalline silicon layer into the Si,,,Ge,, 1 alloy layer and reported that arsenic tends to segregate away from Si, _.,Ge, layer. In this letter, the redistribution of ion implanted arsenic in fully relaxed Si , _.Ge., epilayers during RTA is reported as a function of Ge content over a wide range of Ge contents (O-43%). The redistribution has been studied by measurement of chemical profiles as well as carrier density profiles.
* Corresponding 0168-583X/97/$17.00
author. Email: [email protected]. 0
P/I SOl68-583X(96)00827-0
1997 Elsevier
The Si and Si,_.,Ge, epitaxial layers were grown by gas source molecular beam epitaxy (GSMBE) using Si,H6 and GeH,. Standard wafer preparation and growth procedures were followed and have been reported previously [8,9]. The Si , _XGe, epilayers and the 200 nm Si control epilayers were grown on CZ p-Si substrates (5-10 ficm) with (100) orientation at 620°C and 750°C. respectively. The SiGe layers were grown on compositionally graded SiGe buffer layers to ensure maximum strain relaxation and low defect concentration in the layer itself [IO]. Ge composition was determined by Auger electron spectroscopy (AES). In our work, the Ge contents of three Si , m,rGe,, samples were 9%, 27% and 43%. The four samples were implanted simultaneously at room temperature with 100 keV As ions to a dose of 2 X 10 I6 cm-*. During implantation the samples were tilted 7” with respect to the incoming beam to minimize channeling effects, and the beam current density was kept low to avoid heating of the samples. After implantation, the samples were cleaved and divided into two groups. Each group included four Si , _ ,Gel ( x = 0, 0.09, 0.27 and 0.43) specimens, and subsequently was rapid thermally annealed (RTA) together at 950°C and 105O”C, respectively, for 18 s (including the sample-temperature rise time of 3 s), in a nitrogen atmosphere. Arsenic depth profiles for the as-implanted and annealed samples were acquired sequentially under identical analysis conditions using a Cameca IMS4f SIMS instrument. The 14.5 keV, 39 nA,
Science B.V. All rights reserved
L.-f: Zm et al./Nucl.
640
Instr. and Meth. in Phys. Res. 3 122 (19971639442
--SIMS,as-implanted -StMS,sxl’C
/“t. :7
.;’
l
SIMS,1owC Cerrier,950'C
- Carrier,lOWC
Fig. 1. Chemical arsenic profiles measured by SIMS and carrier density profiles measured by SRP in Si. Samples were implanted at100keVwithadoseof2X10’6cm~‘andRTAfor18s.The annealed chemical profiles are only slightly broadened by diffusion and the peak concentrations decrease with increasing annealing temperature during RTA.
Cs+ primary sputter beam was rastered over a 250 X 250 pm’. The analyzed area was 60 X 60 pm’. Ge profiles were also obtained for the Si , _ ,Ge r samples. Crater depths were measured on the Si and Si, _ ,Ge., samples after SIMS profiling with a Tencor Alpha-Step 250 profilometer and were used to determine the depth scale. The electrically active profiles of implanted-arsenic were measured by Solid State Measurements ASR-lOOC/2 spreading resistance probe instrument. Figs. l-4 show chemical depth profiles (from SIMS) and electrically active profiles (from SRP) of arsenic in Si, Si,,,,Ge,.,, Si0.73Ge0,27 and Si,,,,Ge,,,,. respectively. Comparing the figures, the depth of the as-implanted chemical peak concentration decreases from 66 nm (Fig. I> to 37 nm (Fig. 4) as the Ge contents increase from 0% to 43%. This can be well understood if it is taken into account that the main energy loss mechanism of 100 keV As+ implantation is due to nuclear stopping and the projected range is inversely proportional to the ratio of the target atom mass to the incident atom mass [ 11,121. It can be readily seen from Fig. 1 to Fig. 4 that an arsenic diffusion occurs during RTA for four samples, but the annealed chemical profiles of arsenic for these samples are different. For silicon the annealed chemical profiles are oniy slightly broadened by diffusion and the peak concentrations decrease with increasing annealing temperature. The most interesting feature of Figs. 2-4 is that the behavior of arsenic in Si , _ .,Ge., is different from that in Si with increasing Ge content x, and the chemical concentration profiles from SIMS in Si, -,Ge, samples are also different with different Ge content. For the sample of .Y= 0.09, the implanted arsenic profile is slightly broadened by diffusion at 950°C annealing and obviously broad-
..&Mi$i&&c
lo
. carrlw.sso’C
./“.
lb’
Fig. 2. Chemical
. carrw.1050'C
;: ,..,. ....
arsenic
density profiles measured
profiles measured by SIMS and carrier by SRP in Si, ,,Ge,,,. Implantation
was done at 100 keV with a dose of 2 X 10’” cm-* before RTA for 18 s. The profile is slightly broadened by diffusion at 950°C annealing and remarkably broadened at 1050°C annealing.
ened when the annealing temperature rise to 1050°C. For Ge content I = 0.27, the annealed chemical concentration profile is remarkably broadened by diffusion at 950°C annealing and box-shaped at 1050°C annealing (Fig. 3). For x = 0.43, at both 950°C and 1050°C annealing, the annealed chemical profiles are all box-shaped (Fig. 4). indicating that the diffusion is concentration dependent [13]. Diffusion is obviously enhanced in Si, _,YGe., with increasing Ge content. The electrically active profiles of arsenic in Si, Si,,a,Ge,,,, Si,,,,Ge,.,, and Si,,,Geo.,3 from SRP are also shown in Figs. 1-4, respectively. The carrier peak
0
0.1
0.2
Depth WN
Fig. 3. Chemical arsenic profiles measured by SIMS and carrier density profiles measured by SRP in Si, ,,Ge, 27. Samples were implanted at 100 keV with a dose of 2X lOI cm-’ and RTA for 18 s. The annealed profile is remarkably broadened by diffusion at 950°C and box-shaped at 1050°C.
L.-f: Zou et al./Nucl.
In&. and Meth. in Phys. Rex B 122 (1997) 639-642
concentrations in Si (Fig. 1) are 2.2 X 102’ and 3.0 X lo*’ annealing temperatures are 950°C cme3, corresponding and 105O*C, respectively, which is very close to the equilibrium solubility limit of arsenic as an electrically active dopant in Si [14]. With increasing Ge content, the peak concentration of electrically active arsenic decreases (Fig. 5) and the profiles are flat in the depth region (Fig. 3 and 4). In particular, the maximum concentrations of electrically active arsenic in Sio.57Geo,4j (Fig. 4) are 3.0 X lOi and 4.2 X lOI cmd3 for 18 s anneals at 950°C and 105o”C, respectively, which is about one order of magnitude lower than that in Si. These results correlate with that the maximum solubility of arsenic in Si is higher than that in Si,_,Ge,Y [15]. After 2 X lOI As/cm* implantation in Si and Si , _,Ge ~ alloys the implanted layers become amorphous. The redistribution of dopant is related to the segregation and diffusion which occurs during the solid phase epitaxial regrowth of Si, _IGe, alloys amorphized by ion implantation [16]. If arsenic concentration is below the electrical solubility, during the RTA arsenic atoms are incorporated into the lattice site along with the host atoms and activated fully with minimal redistribution. However, for heavily implanted samples, there is a large fraction of electrically inactive arsenic, besides electrically active arsenic. The behavior of this fraction can account for the different redistribution of dopant during RTA in Si from that in Si , _,Ge, alloys. For Si, one can see from Fig. 1 that there are a large fraction of electrically inactive arsenic which are in the form of clusters [ 171. Since the annealing temperature in this work is lower than declustering temperature (2 1100°C [18]), the possibility of clustered arsenic moving without rupture of bonds is small; thus, the clustered As atoms are believed to be immobile [5,17]. Consequently, the redistri-
lo”o..““““““““‘l”“““’
0.1
0.2
wfm (urn) Fig. 4. Chemical arsenic profiles measured by SIMS and carrier density profiles measured by SRP in Si,,,Ge,,,. At both 950°C and 1050°C annealing, the annealed profiles are all box-shaped. Diffusion is obviously enhanced in Si, *Ge,.
I
”
0.1
”
”
641
”
“’
I””
0.2 0.3 0.4 Germanium Fraction x
I ”
0.5
“1
0.6
Fig. 5. Solubility of arsenic as an electrically active dopant in Si , _ xGei as a function of germanium fraction. The solid lines are a least squares fit to the data, and are given by N,,,,, = 2.2X
IO’* exp[ - (4.721 x +0.478 eV/kT)].
bution is minor, as confirmed by the chemical concentration profiles. Since the saturation concentration of arsenic in pure Si (m 3 X 102’ cm- 3, is much higher than that in pure Ge (y 2 X lo*’ cmm3) [5,19], and precipitation of arsenic was found at an arsenic concentration atoms in Si,,,Ge,,, of 9 X 1020 cmm3 which is at least one order of magnitude lower than that in Si [15]. Furthermore, it can be readily seen from Fig. 5 that the carrier peak concentrations decrease with increasing Ge content. One can reasonably infer that electrically inactive arsenic atoms increase with increasing Ge content for a same dose of 2 X lOI As/cm2 implantation in Si, .Ge, alloys with different Ge content. Since the formation of arsenic precipitates in Si, _*Ge_ alloys can occur at a much lower As-peak concentration [15,20] and much lower annealing temperature [6] than those in Si, and the dopant which is not involved in GeAs precipitates may diffuse at the annealing temperature, which is similar to that in Si [5], the inactive mobile dopants increase with increasing Ge content. Thus, the annealed arsenic profiles are obviously broadened by diffusion of the inactive mobile dopants (compared to that in Si). Moreover, since the annealing temperature in this work is higher than that required to form monoclinic GeAs phase ( 5 900°C [6,15]), formation and dissolution of GeAs precipitates coexist during RTA, which leads to that the inactive mobile dopant penetrates deeper into Si,-,Ge, alloys. As a result, the box-shaped profiles of arsenic in Si , _.,Ge )( alloys occur, which shows the characteristic of concentration dependent diffusion behavior. As Fig. 4 shows, the total arsenic concentrations at 1050°C annealing are lower than that at 950°C which may originate from exodiffusion at higher annealing temperature. The details of the mechanism of diffusion of ion implanted As in Si, _,Ge,r alloys are under investigation. In summary, the behavior of implanted As in Si, _,Ge, is different from that in Si, and the concentration profiles
642
L.,f: Zou et d./Nucl.
fnstr. and Meth. in Phys. Res. B 122 (1997) 639-642
are also different for Si ,_,Ge, samples with different x. With increasing the Ge content, the samples exhibit boxshaped, concentration-dependent diffusion profiles. Arsenic diffusion is enhanced with increasing temperature for certain Ge content and strongly dependent on Ge content and the larger Ge content, the faster As in Si,_,Ge,, diffusion. The maximum concentrations of electrically active arsenic in Si, _.,Ge. are found to decrease with increasing Ge content.
Acknowledgement The authors are grateful for the support of the National Natural Science Foundation of China and exchange program between the Chinese Academy of Sciences and the Royal Society of UK.
References [I] B.S. Meyerson, IBM J. Res. Dev. 34 (1990) 806. [2] M. Arienzo, J. Comfort, E.F. Crabbe, D.L. Harame,
S.S. lyer, V.P. Kesan, B.S. Meyerson, G.L. Patton, J.M.C. Stork and Y.-C. Sun, Microelectron. Eng. 19 (I 992) 5 19. [3] A. Nylandsted Larsen, S.Yu. Shiryaev, E. Schwartz Sorensen and P. Tidemand-Petersson, Appl. Phys. Len. 48 (1986) I80.5. [4] A. Armigliato and A. Parisini, J. Mater. Res. 6 (1991) 1701. [s] D. Nobili, S. Solmi, A. Parisini, M. Derdour and A. Armigliato, Phys. Rev. B 49 (1994) 2477. [6] T.W. Fan, A. Nejim. J.P. Zhang, Z.G. Wang, P.L.F. Hemment and D. Chescoe, Appl. Phys. Lett. 66 (1995) 1117.
[71 S.M. Hu, D.C. Ahlgtcn, P.A. Ronsheim and J.O. Chu, Phys. Rev. Lett. 67 (1991) 1450.
[81 Lyu-fan Zou, Z.G. Wang, D.Z. Sun, J.W. Zhang, X.F. Liu, M.Y. Kong and L.Y. Lin, in: Proc. Third Int. Conf. on Nonlinear Optical Physics and Applications, Harbin (1995) p. 12. [91 Lyu-fan Zou, Z.G. Wang, D.Z. Sun, J.W. Zhang, X.F. Liu, S.R. Zhu, J.P. Li, M.Y. Kong and L.Y. Lin, in: Proc. Third National Conf. on Molecular Beam Epitaxy, Jiande (1995) p. 118. II01 E.A. Fitzgerald, Y.-H. Xie, D. Monroe, D.J. Silverman, J.M. Kuo, A.R. Kortan, F.A. Thiel and B.E. Weir, J. Vat. Sci. Technol. IO (1992) 1807. [I II S.M. Sze, Semiconductor Device, Physics and Technology (Wiley, New York, 1985) chap. 10, p. 409. [I21 H. Maes, W. Vandervorst and R. Van Overstraeten, in: Impurity Doping Processes in Silicon, ed. F.F.Y. Wang (North-Holland, Amsterdam, 1981) p. 474. [I31 C.C. Lee, M.D. Deal and C. Bravman. Appl. Phys. Lett. 66 (1995) 355. [141 J.L. Hoyt and J.F. Gibbons, Mater. Res. Sot. Symp. Proc. 52 (1986) 15. [ISI V.S. Tishkov, PI. Gaiduk, S.Yu. Shiryaev and A. Nylandsted Larsen, Appl. Phys. Lett. 68 (1996) 655. [I61 B.T. Chilton, B.J. Robinson, D.A. Thompson, T.E. Jachman and J.-M. Baribeau, Appl. Phys. Lett. 54 (1989) 42. [I71 M.Y. Tsai, F.F. Morehead, J.E.E. Baglin and A.E. Michel, J. Appl. Phys. 51 (1980) 3230. iI81 T.E. Seidel, C.S. Pai, D.J. Lischner, D.M. Maher, R.V. Knoell, J.S. Williams, B.R. Penumalh and D.C. Jacobson, Mater. Res. Sot. Symp. Proc. 35 ( 1985) 329. [I91 F.A. Trumbore, Bell Syst. Tech. J. 38 (1960) 205. DOI Lyu-fan Zou, Z.G. Wang, D.Z. Sun, T.W. Fan, X.F. Liu and J.W. Zhang, Characterization of strain relaxation in As ion implanted Si, _ xGel epilayers grown by gas source molecular beam epitaxy, Appl. Phys. Lett., submitted.
2s__ _-
and
Methods in
Physics Research B I22 (1997)639-642 NONil
B
Beamlnteractions with Materials&Atoms
!!I ELSEVIER
Rapid thermal annealing of arsenic implanted Si , _xGen epilayers Lyu-fan Zou * , Z.G. Wang, D.Z. Sun, T.W. Fan, X.F. Liu, J.W. Zhang Laboratory
of Semiconducror
Materials
Science, Institute
of Semiconductors.
Chinese Academy
ofSciences.
P.O. Bar
912. Beijing
lOOO83.
China
Received 2 1 August 1996; revised form received 4 November
1996
Abstract Rapid thermal annealing of arsenic implanted Si , _ ,Ge, was studied by secondary ion-mass spectroscopy (SIMS) and spreading resistance probe (SRP) over a wide range of Ge fractions (O-43%). Redistribution of the implanted arsenic was followed as a function of Ge content and annealing temperature. Arsenic concentration profiles from SIMS indicated that the behavior of implanted arsenic in Si, _,Ge, after RTA was different from that in Si, and the Si, _,Ge., samples exhibited box-shaped, concentration-dependent diffusion profiles with increasing Ge content. The maximum concentrations of electrically active arsenic in Si, _,Ge, was found to decrease with increasing Ge content. Experimental results showed that the arsenic diffusion is enhanced with increasing temperature for certain Ge content and strongly dependent on Ge content, and the higher Ge content, the faster As diffusion.
Si , _lGe, alloys grown epitaxially on Si substrates have become the subject of scientific and technological interest in recent years primarily because of the enormous potential for fabricating novel devices compatible with existing Si-based processing technology [ 1,2]. From a technological viewpoint it is important that physics phenomena in Si, _xGe., related to processing, such as impurity diffusion, should be characterized for integrated circuit processing. In pure silicon, the diffusion and segregation of arsenic in arsenic-implanted Si have been investigated for more than two decades, because silicon layers doped with arsenic at high concentrations are often required in very large scale integrated (VLSI) circuit technology [3-51. However, the arsenic diffusion in Si, _,Ge 1, although very important for the control of the dopant concentration depth profiles during integrated circuit processing, has received little attention [6]. Recently, Hu et al. [7] studied that arsenic was diffused from polycrystalline silicon layer into the Si,,,Ge,, 1 alloy layer and reported that arsenic tends to segregate away from Si, _.,Ge, layer. In this letter, the redistribution of ion implanted arsenic in fully relaxed Si , _.Ge., epilayers during RTA is reported as a function of Ge content over a wide range of Ge contents (O-43%). The redistribution has been studied by measurement of chemical profiles as well as carrier density profiles.
* Corresponding 0168-583X/97/$17.00
author. Email: [email protected]. 0
P/I SOl68-583X(96)00827-0
1997 Elsevier
The Si and Si,_.,Ge, epitaxial layers were grown by gas source molecular beam epitaxy (GSMBE) using Si,H6 and GeH,. Standard wafer preparation and growth procedures were followed and have been reported previously [8,9]. The Si , _XGe, epilayers and the 200 nm Si control epilayers were grown on CZ p-Si substrates (5-10 ficm) with (100) orientation at 620°C and 750°C. respectively. The SiGe layers were grown on compositionally graded SiGe buffer layers to ensure maximum strain relaxation and low defect concentration in the layer itself [IO]. Ge composition was determined by Auger electron spectroscopy (AES). In our work, the Ge contents of three Si , m,rGe,, samples were 9%, 27% and 43%. The four samples were implanted simultaneously at room temperature with 100 keV As ions to a dose of 2 X 10 I6 cm-*. During implantation the samples were tilted 7” with respect to the incoming beam to minimize channeling effects, and the beam current density was kept low to avoid heating of the samples. After implantation, the samples were cleaved and divided into two groups. Each group included four Si , _ ,Gel ( x = 0, 0.09, 0.27 and 0.43) specimens, and subsequently was rapid thermally annealed (RTA) together at 950°C and 105O”C, respectively, for 18 s (including the sample-temperature rise time of 3 s), in a nitrogen atmosphere. Arsenic depth profiles for the as-implanted and annealed samples were acquired sequentially under identical analysis conditions using a Cameca IMS4f SIMS instrument. The 14.5 keV, 39 nA,
Science B.V. All rights reserved
L.-f: Zm et al./Nucl.
640
Instr. and Meth. in Phys. Res. 3 122 (19971639442
--SIMS,as-implanted -StMS,sxl’C
/“t. :7
.;’
l
SIMS,1owC Cerrier,950'C
- Carrier,lOWC
Fig. 1. Chemical arsenic profiles measured by SIMS and carrier density profiles measured by SRP in Si. Samples were implanted at100keVwithadoseof2X10’6cm~‘andRTAfor18s.The annealed chemical profiles are only slightly broadened by diffusion and the peak concentrations decrease with increasing annealing temperature during RTA.
Cs+ primary sputter beam was rastered over a 250 X 250 pm’. The analyzed area was 60 X 60 pm’. Ge profiles were also obtained for the Si , _ ,Ge r samples. Crater depths were measured on the Si and Si, _ ,Ge., samples after SIMS profiling with a Tencor Alpha-Step 250 profilometer and were used to determine the depth scale. The electrically active profiles of implanted-arsenic were measured by Solid State Measurements ASR-lOOC/2 spreading resistance probe instrument. Figs. l-4 show chemical depth profiles (from SIMS) and electrically active profiles (from SRP) of arsenic in Si, Si,,,,Ge,.,, Si0.73Ge0,27 and Si,,,,Ge,,,,. respectively. Comparing the figures, the depth of the as-implanted chemical peak concentration decreases from 66 nm (Fig. I> to 37 nm (Fig. 4) as the Ge contents increase from 0% to 43%. This can be well understood if it is taken into account that the main energy loss mechanism of 100 keV As+ implantation is due to nuclear stopping and the projected range is inversely proportional to the ratio of the target atom mass to the incident atom mass [ 11,121. It can be readily seen from Fig. 1 to Fig. 4 that an arsenic diffusion occurs during RTA for four samples, but the annealed chemical profiles of arsenic for these samples are different. For silicon the annealed chemical profiles are oniy slightly broadened by diffusion and the peak concentrations decrease with increasing annealing temperature. The most interesting feature of Figs. 2-4 is that the behavior of arsenic in Si , _ .,Ge., is different from that in Si with increasing Ge content x, and the chemical concentration profiles from SIMS in Si, -,Ge, samples are also different with different Ge content. For the sample of .Y= 0.09, the implanted arsenic profile is slightly broadened by diffusion at 950°C annealing and obviously broad-
..&Mi$i&&c
lo
. carrlw.sso’C
./“.
lb’
Fig. 2. Chemical
. carrw.1050'C
;: ,..,. ....
arsenic
density profiles measured
profiles measured by SIMS and carrier by SRP in Si, ,,Ge,,,. Implantation
was done at 100 keV with a dose of 2 X 10’” cm-* before RTA for 18 s. The profile is slightly broadened by diffusion at 950°C annealing and remarkably broadened at 1050°C annealing.
ened when the annealing temperature rise to 1050°C. For Ge content I = 0.27, the annealed chemical concentration profile is remarkably broadened by diffusion at 950°C annealing and box-shaped at 1050°C annealing (Fig. 3). For x = 0.43, at both 950°C and 1050°C annealing, the annealed chemical profiles are all box-shaped (Fig. 4). indicating that the diffusion is concentration dependent [13]. Diffusion is obviously enhanced in Si, _,YGe., with increasing Ge content. The electrically active profiles of arsenic in Si, Si,,a,Ge,,,, Si,,,,Ge,.,, and Si,,,Geo.,3 from SRP are also shown in Figs. 1-4, respectively. The carrier peak
0
0.1
0.2
Depth WN
Fig. 3. Chemical arsenic profiles measured by SIMS and carrier density profiles measured by SRP in Si, ,,Ge, 27. Samples were implanted at 100 keV with a dose of 2X lOI cm-’ and RTA for 18 s. The annealed profile is remarkably broadened by diffusion at 950°C and box-shaped at 1050°C.
L.-f: Zou et al./Nucl.
In&. and Meth. in Phys. Rex B 122 (1997) 639-642
concentrations in Si (Fig. 1) are 2.2 X 102’ and 3.0 X lo*’ annealing temperatures are 950°C cme3, corresponding and 105O*C, respectively, which is very close to the equilibrium solubility limit of arsenic as an electrically active dopant in Si [14]. With increasing Ge content, the peak concentration of electrically active arsenic decreases (Fig. 5) and the profiles are flat in the depth region (Fig. 3 and 4). In particular, the maximum concentrations of electrically active arsenic in Sio.57Geo,4j (Fig. 4) are 3.0 X lOi and 4.2 X lOI cmd3 for 18 s anneals at 950°C and 105o”C, respectively, which is about one order of magnitude lower than that in Si. These results correlate with that the maximum solubility of arsenic in Si is higher than that in Si,_,Ge,Y [15]. After 2 X lOI As/cm* implantation in Si and Si , _,Ge ~ alloys the implanted layers become amorphous. The redistribution of dopant is related to the segregation and diffusion which occurs during the solid phase epitaxial regrowth of Si, _IGe, alloys amorphized by ion implantation [16]. If arsenic concentration is below the electrical solubility, during the RTA arsenic atoms are incorporated into the lattice site along with the host atoms and activated fully with minimal redistribution. However, for heavily implanted samples, there is a large fraction of electrically inactive arsenic, besides electrically active arsenic. The behavior of this fraction can account for the different redistribution of dopant during RTA in Si from that in Si , _,Ge, alloys. For Si, one can see from Fig. 1 that there are a large fraction of electrically inactive arsenic which are in the form of clusters [ 171. Since the annealing temperature in this work is lower than declustering temperature (2 1100°C [18]), the possibility of clustered arsenic moving without rupture of bonds is small; thus, the clustered As atoms are believed to be immobile [5,17]. Consequently, the redistri-
lo”o..““““““““‘l”“““’
0.1
0.2
wfm (urn) Fig. 4. Chemical arsenic profiles measured by SIMS and carrier density profiles measured by SRP in Si,,,Ge,,,. At both 950°C and 1050°C annealing, the annealed profiles are all box-shaped. Diffusion is obviously enhanced in Si, *Ge,.
I
”
0.1
”
”
641
”
“’
I””
0.2 0.3 0.4 Germanium Fraction x
I ”
0.5
“1
0.6
Fig. 5. Solubility of arsenic as an electrically active dopant in Si , _ xGei as a function of germanium fraction. The solid lines are a least squares fit to the data, and are given by N,,,,, = 2.2X
IO’* exp[ - (4.721 x +0.478 eV/kT)].
bution is minor, as confirmed by the chemical concentration profiles. Since the saturation concentration of arsenic in pure Si (m 3 X 102’ cm- 3, is much higher than that in pure Ge (y 2 X lo*’ cmm3) [5,19], and precipitation of arsenic was found at an arsenic concentration atoms in Si,,,Ge,,, of 9 X 1020 cmm3 which is at least one order of magnitude lower than that in Si [15]. Furthermore, it can be readily seen from Fig. 5 that the carrier peak concentrations decrease with increasing Ge content. One can reasonably infer that electrically inactive arsenic atoms increase with increasing Ge content for a same dose of 2 X lOI As/cm2 implantation in Si, .Ge, alloys with different Ge content. Since the formation of arsenic precipitates in Si, _*Ge_ alloys can occur at a much lower As-peak concentration [15,20] and much lower annealing temperature [6] than those in Si, and the dopant which is not involved in GeAs precipitates may diffuse at the annealing temperature, which is similar to that in Si [5], the inactive mobile dopants increase with increasing Ge content. Thus, the annealed arsenic profiles are obviously broadened by diffusion of the inactive mobile dopants (compared to that in Si). Moreover, since the annealing temperature in this work is higher than that required to form monoclinic GeAs phase ( 5 900°C [6,15]), formation and dissolution of GeAs precipitates coexist during RTA, which leads to that the inactive mobile dopant penetrates deeper into Si,-,Ge, alloys. As a result, the box-shaped profiles of arsenic in Si , _.,Ge )( alloys occur, which shows the characteristic of concentration dependent diffusion behavior. As Fig. 4 shows, the total arsenic concentrations at 1050°C annealing are lower than that at 950°C which may originate from exodiffusion at higher annealing temperature. The details of the mechanism of diffusion of ion implanted As in Si, _,Ge,r alloys are under investigation. In summary, the behavior of implanted As in Si, _,Ge, is different from that in Si, and the concentration profiles
642
L.,f: Zou et d./Nucl.
fnstr. and Meth. in Phys. Res. B 122 (1997) 639-642
are also different for Si ,_,Ge, samples with different x. With increasing the Ge content, the samples exhibit boxshaped, concentration-dependent diffusion profiles. Arsenic diffusion is enhanced with increasing temperature for certain Ge content and strongly dependent on Ge content and the larger Ge content, the faster As in Si,_,Ge,, diffusion. The maximum concentrations of electrically active arsenic in Si, _.,Ge. are found to decrease with increasing Ge content.
Acknowledgement The authors are grateful for the support of the National Natural Science Foundation of China and exchange program between the Chinese Academy of Sciences and the Royal Society of UK.
References [I] B.S. Meyerson, IBM J. Res. Dev. 34 (1990) 806. [2] M. Arienzo, J. Comfort, E.F. Crabbe, D.L. Harame,
S.S. lyer, V.P. Kesan, B.S. Meyerson, G.L. Patton, J.M.C. Stork and Y.-C. Sun, Microelectron. Eng. 19 (I 992) 5 19. [3] A. Nylandsted Larsen, S.Yu. Shiryaev, E. Schwartz Sorensen and P. Tidemand-Petersson, Appl. Phys. Len. 48 (1986) I80.5. [4] A. Armigliato and A. Parisini, J. Mater. Res. 6 (1991) 1701. [s] D. Nobili, S. Solmi, A. Parisini, M. Derdour and A. Armigliato, Phys. Rev. B 49 (1994) 2477. [6] T.W. Fan, A. Nejim. J.P. Zhang, Z.G. Wang, P.L.F. Hemment and D. Chescoe, Appl. Phys. Lett. 66 (1995) 1117.
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