Nuclear Instruments and Methods in Physics Research B55 (1991) 633-636 North-Holland

633

Anomalous redistributions of As and Sb atoms in As-implanted Sb-doped Si and Sb-implanted As-doped Si during annealing

Katsuhiro Yokota a, Hiroshi Furuta a, Shinji Ishihara b and Itsuro Kimura b ’ Faculty of Engineering, Kansai University, Suita, Osaka 564, Japan b Research Reactor Institute, Kyoto University, Kumatori, Sennan-gun, Osaka 590-04, Japan

Sb+ ions were implanted into heavily As-doped bulk silicon and As + ions were implanted into heavily Sb-doped bulk silicon. Subsequent high-temperature annealing indicated the loss of Sb atoms. l[jor the samples implanted with antimony the amount of lost Sb depends on the implanted dose. In comparison, the arsenic concentration in both samples remained constant and no abnormal loss of arsenic was evident during the high-temperature anneal.

1. In~~uction

Ion implantation as a conventional doping technique requires high-temperature annealing to restore the de- stroyed crystal lattice and to electrically activate the implanted dopants. The implanted dopants will redis- tribute to a profile which can be determined by the diffusion of a limited-source during annealing. How- ever, the dopants induce stress in the Si crystal since the atomic radius of the dopant atoms differs from that of Si [l]. This stress results in a significant effect on the dopant diffusion and the solid solubility of the impuri- ties [2].

In many cases the impurities are implanted with low doses. The differences in the atomic radii are not a serious problem during annealing since the concentra- tion of the dopant is low. However, the behavior of B, P and As in heavily doped Si during annealing is com- plicated, as the diffusion coefficient of the dopant de- pends on its concentration.

The interest in this subject is whether or not the differences in the atomic sizes of Si and impurities really affect the redistributions of the dopants in Si during annealing. We reveal in this paper that the redistributions of the dopants in As-implanted Sb-doped Si and Sb-implanted As-doped Si during annealing de- pend highly on the radius of the implanted ion and the solid solubility.

2. Experiments

Heavily Sb-doped Czochralski (100) Si wafers with a concentration of 0.8 X IO” cmm3 and heavily As-doped Czochralski (100) Si wafers with a concentration of 2 X 10” cm-l were used for the experiments. 50 keV

As’ ions were impl~ted into the Sb-doped Si wafers at doses in the range 1 x 1014-1 X 1016 As+ cm-‘. This sample is referred to as Si(Sb-As). Also, 70 keV Sb+ ions were implanted into the As-doped Si wafers at doses in the range 1 X 1014-1 X 10” Sb’ cm-‘. This sample is referred to as Si(As-Sb). The incident direc- tion of the ion beam was misaligned by 7O from the (100) crystal axis to minimize the channeling effect. The implanted samples were annealed in flowing Ar gas at a temperature of 1OOO’C for 30 min. The ellipsometric thickness of native oxide films grown on silicon wafers was about 2.5 nm.

To confirm the results for Si(As-Sb) and Si(Sb-As), the following experiments were performed. First, 70 keV Sb+ ions were implanted into B-doped Czochralski (lOO)Si wafers with a concentration of 3 X 1019 cmM3, at a dose of 1 X 1015-1 Sbf cmM2. The sample is referred to as Si(B-As). Secondly, 50 keV As+ ions were im- planted into P-doped Czochralski (lOO)Si wafers with a ~ncentration of 3 X 101s crK3, at a dose of 1 X 101’-1 As+ cmp2. The sample is referred to as Si(P-As). Thirdly, 40 keV Ar+ ions were implanted into Sb-doped Czochralski (lOO)Si wafers with a concentration of 0.8 X 1019 crnm3 and As-doped Czochralski (lOO)Si wafers with a concentration of 2 X 10” cmm3, at a dose of 1 X lOi* Ar+ cm-‘. These samples are referred to as Si(Sb-Ar) and Si(As-Ar), respectively. The Si wafers were annealed in flowing Ar gas at 1000°C for 30 min.

The annealed samples were activated in a neutron flux of 2.8 X 1013 cm-’ s-l for 1 h. Some of the As and Sb converted into the radio-isotopes of 76As and i2*Sb, respectively. A thickness of 10 nm was removed succes- sively from the sample surfaces by repeating anodic oxidation and oxide removal by HP diluted with water. Depth profiles of the As and Sb concentrations in Si were obtained by measuring the radiation emitted from

0168-583X/91/$03.50 0 1991 - Elsevier Science Publishers B.V. forth-polled) VI. MATERIALS SCIENCE

634 K. Yokota et al. / Anomalous redistributions of As and Sb atoms

the radioactive isotopes of ‘“As and lz2Sb in the HF solutions. The radiation was measured using an NaI(T1) scintillation counter and a 1024-channel pulse-height analyzer. The system was calibrated by measuring the radiation emitted from standard samples of 76As and lz2Sb.

The y-ray spectrum from 76As was in the same energy region as that from lz2Sb. The energy of the dominant radiation from 76As is 0.56 MeV and that from lZ2Sb is 0.564 MeV. Time decay curves of the radiations were measured to separate the y-ray spec- trum. The half-life of 76As is 26.4 h and that of 12’Sb is 64.3 h.

3. Results and discussion

Fig. 1 shows the concentration profiles of Sb and As in Si(Sb-As) annealed at a temperature of 1000°C for 30 min as a function of the implanted dose. The larger the arsenic dose, the deeper the As has diffused in the Sb-doped sample. The depth where the concentration of the As is reduced by an order of magnitude from its peak concentration became deeper with the increase of implanted dose. It was 85 nm for a dose of 1 X 1014

cmP2, 115 nm for a dose of 1 X 1015 cm-‘, and 265 nm for a dose of 1 x 1016 cm-‘. The diffusion coefficient of As in Si at 1000” C increases with the increase of As concentration. It reaches a maximum value of 5 X lo-l4

cm2 s-l at an As concentration of 3 x 10” cm-3, and then decreases [3]. At high concentrations of As, i.e. above 10” cme3, it is believed that arsenic can form clusters [4], which are immobile below 1000°C. How- ever, the diffusion of As in Si(Sb-As) implanted with As at a dose of 1 x 1016 cmp2 after annealing at 1000 o C for 30 min was compared with that in As-implanted p-type Si after annealing at 1000 o C for 20 min [5]. The distribution of As in Si(Sb-As) was shallower than that in both Si(B-As) and Si(P-As) shown in fig. 3b. The

redistribution profile of As in the Si(B-As) was the same as that in the Si(P-As). This indicates that the n-type or p-type nature of the substrate had no effect on the redistribution of the implanted arsenic. Thus, the anomalous redistribution profiles of As in Si(Sb-As) and Si(As-Sb) may be caused by the differences in the atomic sizes of impurities and Si atoms. A similar profile of antimony has been obtained in Ga-implanted Sb-doped Si covered by native oxide films after being annealing at 950 o C for 30 min [6]. However, the profile differs from that of antimony in silicon with a capping layer [7]. This seems to be because the native oxide films are much thinner than the capping layer and their physical and chemical properties differ considerably from those of thermally grown silicon oxide.

A large amount of Sb in Si(Sb-As) with the As concentrations of 1 X 1014 and 1 X 1015 cm-’ was lost from the surface into the atmosphere during annealing. The number of lost Sb atoms increased proportionally to the implanted dose. The profile of the Sb is described by the equation NSb = 8 X 10” (cm-3) - O.O43N,, for the sample implanted with arsenic at a dose of 1 X 1Ol4

cm-‘. The profile of antimony in all regions in silicon, except the near surface of the sample implanted with arsenic at a dose of 1 X 1015 cm-‘, is described by this equation. Here, NSb is the Sb concentration and NAs is the As concentration. The Sb concentration in Si(Sb-As) is 8 X 1018 cmm3. The value of 0.043 agrees with the value calculated from the relation (1 - (dA,/dsi)3)/(1 - (~&,/ds~)~) [l], where d is the atomic size. Thus, this latter equation implies that the stress induced by the As+ implantation is released by losing Sb from Si into the atmosphere. However, the redistribution of Sb in Si(Sb-As) with an As concentration of 1 X 1Ol6 cm-’ did not follow the former equation. The stress release caused by the Sb loss seems not to occur in this sample since the stress due to As incorporation is much larger than the stresses induced by the Sb.

Fig. 2 shows the concentration profiles of As and Sb

Distance from Surface (urn)

Fig. 1. Concentration profiles of Sb and As in Si(Sb-As) annealed at a temperature of 1000 o C for 30 min as a function of .the implanted dose. Solid lines show the concentration of Sb remaining in Si after annealing. This is represented by the equation

Nsb = 8 x 10” (cm-3) - O.O43N,,, approximately, in silicon implanted with doses of 1 X 1Cl14 and 1 X 1015 As+ cmm2.

K. Yokota et al. /Anomalous red&rib&ions of As and Sb atoms 635

Fig.

Distance from Surface (pm)

2. Concentration profiles of As and Sb in Si(As-Sb) annealed at a temperature of 1000 o C for 30 min as a function of implanted dose of As ions. The solid line represents the profile calculated from a solution for limited-source diffusion [4].

the

in Si(As-Sb) annealed at a temperature of 1000 o C for 30 min as a function of the implanted dose. The solid line in this figure represents the profile calculated from a solution for limited-source diffusion [8]. The diffusion of Sb is retarded in Si(As-Sb) with the increase of the Sb dose. A large loss of Sb atoms during annealing must be noted in addition to the retarded diffusion, com- pared to the calculated profile. The number of lost Sb atoms increased with the increase of dose.

The experimental results for the Sb+-implanted sam- ples are shown in fig. 3a. The diffusion of Sb in an- nealed Si(P-Sb) is retarded slightly compared to the curve calculated from the solution for limited-source diffusion [8]. However, the retardation of the diffusion was smaller than that in Si(As-Sb). The atomic size of P is smaller than that of Si by 0.07 nm [9]. Thus, the stress induced by the incorporation of Sb ions is com- pensated by that induced by P doped into Si and the

redistribution profile of Sb in Si(P-Sb) compares with the calculated curve.

The experimental results for As+ implanted in B- and P-doped silicon are shown in fig. 3b. The redistri- bution profile of As in Si(P-As) agrees with that of As in Si(B-As). The redistribution profile of As is indepen- dent of the background impurity in Si although the atomic size of B differs slightly from that of P. This implies that the n-type or p-type nature of the substrate had no effect on the redistribution of the implanted arsenic. The stress induced by the As ion implant is compensated by that induced by boron and phos- phorus. Thus, the larger stress induce by ion implanta- tion causes the anomalous redistributions of As and Sb in both Si(Sb-As) and Si(As-Sb).

The experimental results for Art-implanted silicon are shown in fig. 3c. The redistribution of As in Si(As- Ar) and of Sb in Si(Sb-Ar) occurred even after high-

(a) (b) Cc)

lo*’ ! r’mblantation 7OkeVSb’l I,II,,,I I

% ’ 11,#1111111 IIIIIIIIIIII

lx10’5cm-2 :: Implantation 5OkeVAs’ %Implantation 40keVAr’ g

lx1015cm-* :I lxlO%m-* I -Annealing 10CKl°C 30min --Annealing 1000°C 30min -‘Annealing lOOO”c 3Omin -

l Bdoped Si l Asdoped Si 0 Sbdoped Sk

““” “““““” ““““t’L’ 0.2 0 OS 0.2O 0 0.’ 0.2

Distance from Surface (pm)

Fig. 3. (a) Concentration profiles of Sb in Si(P-Sb) annealed at a temperature of 1000 o C for 30 min. The solid line represents the profile calculated from a solution for limited-source diffusion [4]. (b) Concentration profiles of As in Si(B-As) and Si(P-As) annealed at a temperature of 1000 o C for 30 min. (c) Concentration profiles of As in Si(As-Ar) and Sb in Si(Sb-Ar) annealed at a

temperature of 1000 o C for 30 min

VI. MATERIALS SCIENCE

636 K. Yokota et al. / Anomalous redistributions of As and Sb atoms

temperature annealing. The implantation of argon by the As ion implantation. The majority of the Sb

damages the Si at this high dose and destroys the Si atoms in Si(Sb-As) and Si(As-Sb) was lost into the

lattice. The destroyed Si lattices recover at temperatures atmosphere during annealing. To release the stress in-

as low as 500” C [lo]. The annealing temperatures in duced by ion implantation an unusual redistribution of

these experiments were much higher. That is, the the Sb atoms took place, aided by the low solid solubil-

anomalous redistributions in figs. 1 and 3 are not caused ity of antimony in silicon. In Si(Sb-As) with an As

by the mechanism of impurity segregation at the mov- concentration of 1 x 1016 cme3, the redistribution of

ing amorphous-crystalline boundary as it moves to- the Sb atoms did not occur because the stress induced

wards the surface during regrowth [ll]. by As is much larger than that induced by Sb.

However, the anomalous redistribution of implanted antimony in Si(As-Sb) cannot be considered to be caused only by the stress induced by the incorporation of impurities. That is, the concentration of Sb remain- ing in Si(As-Sb) after annealing was limited to about

1.4 X 102’ cmW3 for samples implanted with a dose of 1 x 1015-1 X 1016 Sb+ cme2 as shown in fig. 2. This

concentration compares approximately with a solid solubility of 8 X 1019 cmm3 [12] for antimony-implanted silicon. On the other hand, the distribution of As in Si(As-Sb) was scarcely modified by annealing in com- parison to the anomalous redistribution of Sb in Si(Sb- As) during annealing. The solid solubility of As in Si is 2 x 10” cmm3 [8]. The difference in the distributions of the Sb profile in Si(Sb-As) and the As profile in Si(As- Sb) may be explained by the difference in the solid solubilities of As and Sb in silicon.

References

[l] K. Yagi, N. Miyamoto and J. Nishizawa, Jpn. J. Appl. Phys. 9 (1970) 246.

[2] C.P. Flynn, Point Deffects and Diffusion (Clarendon, Oxford, 1972) chaps. 3 and 9.

[3] T.L. Chiu and H.N. Ghosh, IBM J. Res. Dev. 15 (1971) 472.

[4] J.C.C. Tsai, in: Diffusion in VLSI Technology, ed. S.M. Sze (Wiley, New York, 1983).

[5] K. Tsukamoto, Y. Akasaka and K. Kijima, Jpn. J. Appl. Phys. 19 (1980) 87.

[6] K. Yokota, H.l Furuta, S. Ishihara and I. Kimura, J. Appl. Phys. 68 (1990) 5385.

[7] T.E. Seidei and A.U. MacRae, Trans. Metall. Sot. AIME 25 (1969) 491.

5. Conclusion

The diffusion of As in Si(Sb-As) was retarded com- pared to that in both Si(B-As) and Si(P-As) wafers. In both Si(Sb-As) and Si(As-Sb) no significant loss of arsenic occurred during annealing. The retarded diffu- sion of arsenic was caused by the larger stress induced

[S] S.M. Sze, Semiconductor Devices Physics and Technology (Wiley, New York, 1985) p. 418.

[9] H.F. Wolf, Silicon Semiconductor Data (Pergamon, Ox- ford, 1969) p. 150.

[lo] L. Czepregi, E.F. Kennedy, J.W. Mayer and T.W. S&non, J. Appl. Phys. 49 (1978) 3906.

(111 J.S. Williams and K.T. Short, Metastable Phases by Ion Implantation (North-Holland, Amsterdam, 1982) p. 131.

[12] F.A. Trumbore, Bell Syst. Tech. J. 39 (1960) 205.