phys. stat. sol. (c) 0, No. 2, 788–794 (2003) / DOI 10.1002/pssc.200306213

© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1610-1634/03/00002-0788 $ 17.50+.50/0

Electron beam doping of impurity atoms into semiconductors by superdiffusion

Takao Wada*, 1 and Hiroshi Fujimoto**, 2 1 Nagoya Sangyo University, 3255-5 Arai, Owariasahi, Aichi 488–8711, Japan 2 Daido Institute of Technology, 10-3 Takiharu, Mimami, Nagoya 4578530, Japan

Received 24 July 2002, revised 19 August 2002, accepted 4 September 2002 Published online 30 January 2003

PACS 61.72.Tt, 61.72.Ww, 621.80.Fe, 61.82.Fk, 66.30.Jt

Thin diffusion layers (50–1000 Å) in semiconductors can be introduced by electron beam doping (EBD) processes. In this technique, an evaporate film or impurity layer is irradiated with an electron beam to in-duce superdiffusion into the underlying semiconductor substrate. In this study, two- and three-layer struc-tures are examined without annealing, and the EBD of P, B, N and Al into Si, diamond and SiC substrates is investigated.

1. Introduction A point defect in a crystal can be described as any interruption in lattice periodicity. Various types of defects are possible, including intrinsic or extrinsic defects, and substitutional or inter-stitial defects [1]. In the fabrication of semiconductor devices, the controlled introduction of defects such as implanted ions is critical to the production of functional devices. Ion implantation is one common method of introducing defects, however such treatment is usually accompanied by severe radiation dam-age [1]. In contrast, electron beam irradiation creates charged interstitial–vacancy pairs known as Fren-kel defects and avoids the complications of complex damage regions. Electron beam doping (EBD), or superdiffusion, at room temperature has been proposed and investigated as an alternative doping tech-nique by Wada [2–8]. EBD is expected to have many advantages over other doping techniques in that it is a non-damaging, room-temperature process that may also allow for solid-phase epitaxy [4]. The en-ergy spectra of primary displaced atoms has previously been calculated by Monte Carlo simulation [5], and the mechanism of EBD has been investigated both theoretically and experimentally, taking into account the migration of interstitial atoms and surface and volume diffusion. In the present study, the authors examine the use of EBD for doping common impurities into Si, and conduct EBD experiments involving diamond and SiC. 2. Experimental details 2.1 Electron beam doping of silicon Powders of B6Si and P were pressed lightly into sheets as dopant source layers of ~1.0 and 0.8 mm thickness, respectively, and sandwiched between two ~0.525 mm thick Si wafers to give systems of Si//P//Si and Si//B6Si//Si. A schematic illustration for set up is shown in Fig. 1. In the case of B6Si, although both components of the binary compound are expected to migrate into the underlying Si wafer, the change in Si concentration in the wafer is considered to have scarcely effect on B-doping by EBD. These composite samples were then irradiated with an electron

** Corresponding author: e-mail: takawada@nagoya-su.ac.jp ** e-mail: fujimoto@daido-it.ac.jp

phys. stat. sol. (c) 0, No. 2 (2003) 789

Cu container

Cu cover

bolt

Si wafersP or B6Si powder

Electron beam

beam at a fluence of 5 × 1017 electrons/cm2 at 750 keV and about 100 °C in N2 gas with a mean current density of 8.1 µA/cm2 using a Van de Graaff accelerator (Nissin High Voltage Co., Japan) [6]. The surface of the substrate was often rinsed in water using a supersonic cleaner to remove residual powder used as a dopant source. The Al film was carefully removed using a solution of HNO3, H2O, and HOCH2CH2OH (ethylene glycol) at a volume ratio of 3:2:95 at 20 °C. No residual impurity dopant was detected from every surface by the Auger electron spectroscopy (AES). The diffusion of B and P atoms into the Si substrate after EBD was examined by secondary-ion mass spectrometry (SIMS) using a 2 keV primary-ion beam. 2.2 Electron beam doping of diamond and silicon carbide Two synthetic single-crystal diamond wafers (⟨100⟩, dimensions 5 × 5 × 0.5 mm3 and 2 × 3 × 0.5 mm3) and two single-crystal SiC wafers (6H(0001), Si-face 8 × 5 × 0.35 mm3; 4H(0001), Si-face 8 × 5 × 0.3 mm3) were used in the experiments. Powders of B4C, P3N5 and B6Si were pressed lightly into ~0.5 mm thick sheets. Four systems were examined B4C//diamond, diamond // P3N5, SiC//B6Si//SiC and Al/SiC. The 850 Å thick Al layer was deposited on SiC by vacuum evaporation. These systems were wrapped in Al foil and irradiated at a fluence of ~5 × 1017 electrons/cm2 at 750 keV and 100 °C in N2 gas using a Van de Graaff accelerator. The diffusion profiles of impurities in the diamond and SiC were determined by SIMS. 3. Results and discussion 3.1 Electron beam doping of silicon The impurity concentration profiles of P and B atoms in Si are shown in Fig. 2a and b as a function of depth from upper interfaces, respectively. The measured SIMS counts were converted to concentrations using relative sensitivity factors obtained by measurement of an ion-implanted reference sample at the same time. The depth scale is accurate to around 20%. The data is summarized in Table 1. The pn junction is one of the most fundamentals for semiconductor devices. To fabricate a pn junction diode by EBD method is attempted. Phosphorus doped n-type Si wafer was used as a substrate. An impu-rity sheet of B4C powder was formed into a plate of ~0.8 mm in thickness by slightly pressing. It was

Table 1 Electron beam doping of P and B into Si with a fluence of 5 × 1017 electrons/cm2 at 750 keV.

sample Si//P//Si Si//B6Si//Si

impurity concentration at interface (cm–3) ~5 × 1020 ~1021 diffusion depth from interface (Å) ~1000 ~400

Fig. 1 (online colour at: www.interscience.wiley.com) Cross-sectional view for sample set up to irradiate with electron beams on the surface of the three-layer struc-ture.

790 Takao Wada and Hiroshi Fujimoto: Electron beam doping of impurity atoms

1022

1021

1020

1019

1018

1017

1016

1015

0 0.5 1 1.5 2 2.5 3

Depth (µm)

Con

cent

ratio

n(a

tom

s/cm

3 )

Sec

onda

ryIo

nC

ount

s

Si →

P

TRC-2710A, Sample 3, Si003-1

Si//P//SiSi(100B doped p-typeNA=(5.5--9.5)×1015 cm--3

e- e-e-

layer 3

layer 1

layer 2

Si

Si 0.525 mm

0.525 mm

0.8 mmP

107

106

105

104

103

(a)

Fig. 2 (online colour at: www.interscience.wiley.com) a) SIMS concentration profiles of P, and b) B diffusion into Si as a function of depth from the upper interface, together with insets of a schematic diagram of EBD. placed on the substrate, then the surface of it was irradiated with a total fluence of 5 × 1017 electrons/cm2 at 750 keV. B doped Si substrate was polished at an angle of 5°, then provided for the measurement by a two-point probe method. Figure 3 shows the V–I characteristics of a fabricated diode before annealing. It is obvious that B atoms diffused into the n-type Si substrate and the pn junction is performed in the substrate. 3.2 Electron beam doping of diamond and silicon carbide The concentration profiles of B atoms in diamond, and Al atoms in SiC are shown in Fig. 4 as functions of depth from the substrate surface. The conversion of measured SIMS counts to concentration was performed using relative sensitivity factors as mentioned above. The SIMS measurements for these samples are summarized in Table 2.

Table 2 Electron beam doping into diamond and silicon carbide with a fluence of 5 × 1017 electrons cm–2 at 750 keV.

diamond SiC substrate impurity sheet ~B4C ~P3N5 ~B6Si ~Al (evaporated)

impurity concentration at interface (cm–3)

~1019 (B) ~1018 (P) ~1017 (N) ~1021 (B) ~1019 (Al)

diffusion depth from interface (Å)

~50 ~800 ~500 ~600 ~1000

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Depth (µm)

Con

cent

ratio

n(a

tom

s/cm

3 )

Sec

onda

ryIo

nIn

tens

ity(c

ount

s/s)

11B

11B

11B

Si(Raw Ion Counts) →

TRC-2710, Sample 1, ID: Si002-1

e- e-e-

layer 3

layer 1

layer 2

Si

Si 0.525 mm

0.525 mm

1 mmB6Si

Si//B6Si//Si

Si (n-type, ND=5.5×1015 cm-3, t ≈ 0.525 mm)

B6Si (t ≈ 1 mm)

1021

1020

1019

1018

1017

1016

1015

1014

109

108

107

106

105

104

103

102

101

100

(b)

phys. stat. sol. (c) 0, No. 2 (2003) 791

--6 --4 --2 0 2 4

Voltage (V)

1 mm 0.9 mm

14

12

10

8

6

4

2

0

Cur

rent

(A)

750 keV, f = 5 × 1017 e/cm2

B4C (t ≈ 0.8 mm)

n--Si (t ≈ 0.6 mm)

ND = 1015 ≈ --5 × 1015 cm--3

4. Discussion 4.1 Electron beam doping of Si The present experiments clarified that the incident electrons penetrate only marginally into the underlying Si substrate. In fact, at 750 keV, electrons penetrate 1.2 mm into Si, 1.08 mm into B6Si and 1.22 mm into P. However, B and P atoms were successfully doped into the Si substrate. In EBD, four major processes are involved; (i) migration of displaced atoms to the interface, (ii) sur-face diffusion of atoms at the interface, (iii) creation of impurity atoms by one kick-out mechanism at the interface, and (iv) impurity doping into the Si substrate by another kick-out mechanism. These processes are described in detail below.

0 0.01 0.02 0.03

Depth (µm)

Con

cent

ratio

n(a

tom

s/cm

3 )

Sec

onda

ryIo

nIn

tens

ity(c

ount

s/s)

C (counts)→

B

1015

1016

1017

1018

1019

1020

1021 107

102

103

104

105

106

101

750 keV, 5×1017 ecm-2,

diamond (100, 5×5×0.5 t mm3 ),B4C (t ≈0.5 mm)

B4Cdiamond

e- e-e-

0.5mm0.5mm

(a)

Fig. 4 (online colourat: www.interscience.wiley.com) SIMS concentration profiles of a) B into diamond and b) Al into SiC as a function of depth from the substrate surface, together with insets of schematic diagram of EBD.

Fig. 3 (online colour at: www.interscience.wiley.com) Voltage–current characteristics for a B4Si//n-Si diode.

Al

SiC

e- e- e-

Al/SiCSiC (6H(0001) n-type ND = 2 × 1016 cm--3

1021

1020

1019

1018

1017

1016

1015

1014

109

108

107

106

105

104

103

102

0 500 1000 1500 2000 2500 3000

Depth (A)

Con

cent

ratio

n(a

tom

s/cm

--3)

Sec

onda

ry Io

n In

tens

ity (

coun

ts/s

)

(b)

Si (Raw Ion Counts)

850 A

0.35 mm

792 Takao Wada and Hiroshi Fujimoto: Electron beam doping of impurity atoms

(i) Migration of displaced atoms: The production rate of Frenkel defects (η) in Si for 750 keV elec-trons is approximately 2.8 cm–1 [11], and the concentration of Frenkel defects introduced at an electron fluence of 5 × 1017 electrons/cm2 ~1018 cm–3 [7]. If 90% of vacancies recombine with the interstitials in Frenkel defects in the bulk, a certain concentration (10%) will remain as interstitials in the crystal [7]. As these displaced atoms lose energy along the migration path [12] and the migration energy of interstitials is very small (0–0.22 eV) [13], the displaced atoms will become fixed at or near the interface. Atoms sticking at the interface may behave similar to a free atom with an energy of <1 eV and diffuse along the surface. (ii) Surface diffusion: The surface diffusivity (Ds) was estimated to be 10–5 cm2/s from the concentra-tion profile. This value is 1010 times larger than the volume diffusivities in this system [7]. (iii) First kick-out mechanism: In previous papers [8, 9] it was shown that the kick-out mechanism at the interface of three-layer systems for atoms migrating under electron irradiation plays an important role in EBD. For P doping, the thermal equilibrium between Sii and Sip may be established via self-interstitial P(Pi) at the interface between the P sheet and Si wafer by [14]

Sii Sip + Pi (Sii SiB + Bi) ,

where Sip represents a Si atom in a P site, and Sii is a Si atom in an interstitial site. (iv) Second kick-out mechanism: A number of Pi migrate by surface diffusion along the interface and then substitute into sites in the substrate as a result of volume diffusion (~10–17 cm2/s) and another kick-out mechanism, as follows [14]

Pi PSi + Sii (Bi BSi + Sii) .

Electron irradiation also produces high concentrations of electron–hole pairs in Si. If conduction electrons and/or holes recombine or are trapped at defects via nonradiative transitions, the defects may be displaced with the aid of the energy released in these processes and the reaction of the kick-out mecha-nism will occur [15, 16]. Accordingly, surface diffusion plays an important role in superdiffusion, and the main effect of EBD is the reaction of the kick-out mechanism at the interface. Table 3 is a summary of the calculated and experimental results of EBD for B and P doping in Si//B6Si//Si and Si//P//Si. The total number of Frenkel defects in Si layers under electron irradiation is estimated using Sugiyama’s program [17], which is a Monte Carlo simulation implementing the theory of fast electron penetration in matter [7]. If 10% (90%) of the unrecombined interstitials migrate to the interface between the Si substrate and the overlying dopant layer, Sii atoms create P or B impurities by the first kick-out mechanism. The SIMS measurement results indicate that the total number of doped impurities was estimated with reasonable accuracy (see Table 3). Roughly 2 ~ 80% of high concentration impurities of atoms arriving at the surface layers induce the reactions of the kick-out mechanism, then these impurities are doped into

Table 3 Effects for EBD of impurity (Boron and Phosphorus) atoms created by kick-out mechanism of displaced Si atoms.

sample ~Si//P//Si ~Si//B6Si//Si conduction type of substrate ~p-type (1 ~ 10 Ω cm) ~n-type (2.5 ~ 15 Ω cm)

total number of Frenkel defects (displaced Si atoms) in top layer (cm–2)

9.55 × 1014

9.55 × 1014

cal

cula

tion

unrecombined interstitials migrating to the interface (cm–2)

8.9 × 1014

(~90% ) 9.55 × 1013

(~10% )

total number of doped atoms in sub- strate (cm–2)

7.15 × 1014 1.17 × 1012

SIM

S

rates for reaction of kick-out mechanism ~80% ~1.23%

phys. stat. sol. (c) 0, No. 2 (2003) 793

the substrate. In the case of P, the P atoms are effectively doped into the Si wafer. As the substrate is p-type, the kick-out mechanism for P atoms can be expected to be enhanced by hole carriers. 4.2 Electron beam doping of diamond and silicon carbide The experiments on 750 keV EBD of B, P and N atoms into diamond wafers, and B and Al atoms into SiC wafers clarified that incident electrons do not penetrate all the way into the diamond or SiC substrate. The penetration depth for electrons at 750 keV is 0.76 mm in diamond, 0.86 mm in SiC, and 1.08 mm in B4C [9]. However, impurity atoms are successfully doped into both diamond and SiC. In this case, the four major processes involved are as follows. (i) Migration of displaced atoms to the interface [7]: (ii) Surface diffusion (Ds ~ 10–5 cm2/s) at the interface [5, 6]: (iii) Impurity doping by the kick-out mechanism [10] or creation of impurity atoms by the first kick-out mechanism at the interface [5, 8] and subsequent surface diffusion, i.e.,

Bi BC + Ci (B4C//diamond) Ci CP + Pi (diamond//P3N5) ,

Sii SiB + Bi (SiC//B6Si//SiC) Ali AlSi + Sii (Al/SiC) .

(iv) Impurity doping (Dv ~ 10–17 cm2/s) into the substrate by a second kick-out mechanism, given by

Pi PC + Ci (diamond//P3N5) Bi BSi + Sii (SiC//B6Si//SiC) .

Electron irradiation also introduces high concentrations of electron–hole pairs. When the introduced conduction electrons and/or holes recombine or are trapped at defects via nonradiative transitions, the defects may be displaced with the aid of the energy released in these processes and the kick-out mecha-nism reactions will occur [11, 12]. 5. Conclusion Two- and three-layer structures of Si, SiC and diamond with B, P, N and Al dopant layers were treated by electron beam doping (EBD). Doping was confirmed by SIMS measurements, and the mechanism of superdiffusion was discussed. High concentration impurities of atoms arriving at the surface layers induce the reaction of kick-out mechanism. Superdiffusion processes would be useful with advantages over alternative doping techniques, because even in the unirradiated region without damage and at room temperature, doping processes for shallow depth from the surface may be possible.

Acknowledgements The authors wish to thank Mr. K. Mizusawa of Nissin-High Voltage Co. Ltd. for use of the Van de Graaff accelerator, and Professor E. Arai of the Nagoya Institute of Technology for invaluable assistance. Gratitude is also extended to Mr. T. Kato of Daido Steel Co. Ltd. and Dr. M. Nagakubo, Dr. M. Katayama, Dr. S. Sobue and Mr. Y. Yamada of Denso Co. Ltd.

References

[1] J. W. Mayer and O. J. March, in: Appl. Solid State Science, edited by C. D. Kriesman and R. Wolf (Academic Press, New York, 1968).

[2] T. Wada, Proc. 3rd Int. Conf. Neutron Transmutation Doped Silicon, Copenhagen, 1981, edited by J. Guldberg (Plenum Press, New York, 1981), p. 447.

[3] T. Wada and H. Hada, Phys. Rev. B 30, 3384 (1984). [4] T. Wada, and Y. Maeda, Appl. Phys. Lett. 51, 2130 (1987). [5] T. Wada and Y. Maeda, Appl. Phys. Lett. 52, 60 (1988). [6] T. Wada, Defect and Diffusion Forum 117–118, 13 (1995). [7] T. Wada and K. Yasuda, Phys. Rev. B 53, 4770 (1996). [8] T. Wada, H. Fujimoto, D. Toyota, H. Masuda, Y. Yamada, and K. Mizusawa, Jpn. J. Appl. Phys. 36, 7669

(1997). [9] T. Wada and H. Fujimoto, Jpn. J. Appl. Phys. 37, 6998 (1998). [10] T. Wada and H. Fujimoto, Jpn. J. Appl. Phys. 38, 7144 (1999).

794 Takao Wada and Hiroshi Fujimoto: Electron beam doping of impurity atoms

[11] J. H. Cahn, J. Appl. Phys. 30, 1310 (1959). [12] Stopping Power for Electron and Positrons, ICRU Rep 37 (Int. Commission on Radiation Units and Measure-

ments, Bethesda, MD1984). [13] R. R. Hasiguti, J. Phys. Soc. Jpn. 21, 617 (1996). [14] W. Frank, A. Seeger, and U. Gosele, Defects in Semiconductors (North-Holland, Amsterdam, 1981), p. 31. [15] J. D. Weeks, J. C. Tully, and L. C. Kimerling, Phys. Rev. 12, 3286 (1975). [16] Y. Shinozuka, Mater. Sci. Forum 83–87, 527 (1992). [17] H. Sugiyama, Bull. Electrotech. Lab. 34, 573 (1970).