Proceedings of 6th Thin Film Materials & Devices Meeting
November 2-3, 2009, Kyoto, Japan
http://www.tfmd.jp/

Analysis of Impurity doping Using Free Carrier Absorption
T. Sameshima(1), K. Ukawa(1), N. Sano(2), M.Naito(3) and N. Hamamoto(3)Tokyo University of Agriculture and Technology, 2-24-16, Naka-cho, Koganei, Tokyo,
184-8588(2)
Hightec Systems Corporation, 3-9-15, Shin Yokohama, Kohoku-ku, Yokohama, Kanagawa,
222-0033(3)
Nissin Ion Equipment Co., Ltd. Koka, Shiga 528-0068, Japan
E-mail :tsamesim@cc.tuat.ac.jp(1)

Keywords: free carrier absorption, carrier density, activation, laser annealing, opticalinterference, FTIR, ion implantation

Abstract.
We report analysis of activation of phosphorus atoms implanted into silicon. Phosphorus ionswith 1×1015 cm-2 were implanted at 100 and 500 keV into p-type silicon samples. The samples
were annealed by 976 nm semiconductor laser irradiation at 500 kW/cm2 for 19~42 μs. These
processes resulted in buried layers with a high density of electron free carriers. Measurements
of infrared transmissivity showed oscillation spectra between 1000 and 7000 cm-1 because of
optical interference effect due to change in refractive indexes induced by electron free carriersin the buried layers. Analysis of transmissivity spectra was conducted using a numericalprogram constructed with Fresnel coefficient and free carrier photo absorption theories.
Activation initiated at deep region and proceeded to the surface as the laser dwell timeincreased in the case of implantation at 100 keV. On the other hand, activation occurred
uniformly in depth direction for the every laser dwell time in the case of implantation at 500
keV.

Introduction
Free carrier photo absorption is useful for nondestructive investigation of electricalproperties of semiconductor materials [1]. Free carriers change the effective dielectric constantas,⎛ε = ε s ⎜1 −⎝

ω p2 ω 2τ 2 + iωτ ⎞⎟ω 2 ω 2τ 2 + 1 ⎠(1),where εs is the intrinsic dielectric constant of materials, ω is the angular frequency of incident
electromagnetic wave, τ is the lifetime of free carrier motion, and ωp is the plasma frequency
given as,
ωp =

e2 N
m*ε s

(2),

Where, e is the elemental charge, N is the free carrier volume density, m* is the effective massof free carriers. When the effective dielectric constant is complexly expressed by ε = εr - iεi, therefractive index and extinction coefficient are respectively given as,n=ε r + ε r2 + ε i2
2ε 0

(3),

100228044-1

k=

εi

(4),

2ε 0 ε r + ε r2 + ε i2

where ε0 is the dielectric constant in vacuum. Eqs.(3) and (4) mean that the refractive index and
extinction coefficient depend on real and imaginary parts of the effective dielectric constant.
They can be changed by exist of free carriers. Changes in n and k cause change in opticalreflectivity and transmissivity of materials.
This paper reports analysis of activation of impurities formed by ion implantation followed
by laser annealing using the free carrier absorption effect [2, 3]. When impurities implanted indeep regions are activated, buried layers with free carriers are formed in the deep regions. Free
carriers cause change in the refractive index and extinction coefficient in the buried layers. On
the other hand, the surface region keeps initial refractive index and extinction coefficientbecause of low impurity concentration. A layered structure with different refractive indexes andextinction coefficients is therefore formed by impurity ion implantation and its activation.
Optical interference effect is caused by the layered structure of refractive index and extinction
coefficient. We discuss analysis of the optical interference effect in the infrared range forsamples with phosphorus atoms implanted at 100 and 500 keV and annealed by infrared
semiconductor laser with different conditions.
Analysis
Free carrier optical absorption effect was calculated in the case of electron free carriers
which distributed like Gaussian shape in depth with different peaks and widths in crystalline
silicon. The carrier mobility μ was given according to the carrier density [4]. The carrier
lifetime τ was defined as τ=m*μ/e. We constructed a numerical analysis program combining
finite element Fresnel coefficient method with free carrier effect shown in Eqs. (1)~(4) in orderto calculate transmittance spectra in the infrared range [5]. Figure 1 shows transmissivity
spectra calculated with a total electron carrier density of 1x1015 cm-2, a peak width of 50 nm,
and different depths of carrier density peak ranging from 50 to 600 nm (a), and difference intransmissivity spectra obtained by subtracting spectrum of undoped silicon from the spectra
shown in Fig. 2(a), (b). Every spectrum in Fig. 1(a) showed marked decrease in transmissivity
in wavelength lower than 1000 cm-1 because of substantial free carrier optical absorption.
Oscillating line shapes owing to optical interference effect were observed in wavelength
between 1000 and 7000 cm-1, as shown in Fig. 1(b). When free carriers located in shallow

Fig. 1 Transmissivity spectra calculated with a total electron carrier density of 1x1015 cm-2, apeak width of 50 nm, and different depths of carrier density peak ranging from 50 to 600 nm(a), and difference in transmissivity spectra obtained by subtracting spectrum of undoped
silicon from spectra (b).

100228044-2

regions, valleys and peaks appeared only in high wavenumbers because the thin region at thesurface with intrinsic refractive index allowed the optical resonance condition only for lightwith high wavenumbers and low wavelengths. On the other hand, when free carriers located indeep regions, many valleys and peaks appeared because the thick surface region with intrinsic
refractive index allowed the opticalresonance condition even for infrared lightwith low wavenumbers. Figure 2 shows
calculated difference in transmissivity
spectra obtained by subtracting spectrum of
undoped silicon from spectra calculated witha total electron carrier density of 1x1015 cm-2,
a peak depth of 600 nm, and different widths
ranging from 50 to 500 nm. Three valleys
and peaks appeared in spectra between 1000
and 7000 cm-1 for every condition. Height ofvalleys and peaks in high wavenumber
regions markedly decreased as the peakwidth increased. Moreover, the wavenumberof valleys and peaks in low wavenumber Fig. 2 calculated difference in transmissivity
regions slightly decreased as the peak width spectra obtained by subtracting spectrum of
increased. Those calculations indicate that undoped silicon from spectra calculated with a15-2transmissivity spectra sensitively change of 1x10 cm carrier density, a peak depth ofwith distribution of free carriers in buried 600 nm, and different widths.
regions.
Experimental
Phosphorus atoms with a concentration of 1x1015 cm-2 were implanted at 100 and 500 keV to
20 Ωcm P-type silicon substrates. Measurements of optical reflectivity spectra in ultra violet
and visible ranges revealed that the top 140 nm surface region was amorphized in the case of
implantation at 100 keV, and that there was no substantial amorphous region in the case of
implantation at 500 keV [6]. 200 nm thick carbon photo absorption layers were formed on thesilicon surface by sputtering method. Continuous wave 976 nm semiconductor laser were
irradiated to the carbon surface at 500 kW/cm2 for 19~42 μs [7]. Figure 3 shows phosphorus
in-depth profiles measured by secondary ion
mass spectroscopy (SIMS) for samples
implanted at 100 and 500 keV and samples
laser annealed for 42 μs. SIMS showedalmost the same in-depth profiles between
samples as-implanted at 100 and 500 keV
and samples laser annealed, respectively.
There was no substantial diffusion ofphosphorus atoms induced by laser annealing.
The phosphorus concentration had a peak of
125 nm and a width of 135 nm in the case of
implantation at 100 keV. It had a peak of 600
nm and a width of 360 nm in the case of
implantation at 500 keV. Carbon layers were Fig. 3 phosphorus in-depth profiles forremoved by oxygen plasma treatment after samples implanted at 100 and 500 keV andlaser annealing. The rear surface was samples laser annealed for 42 μs.

100228044-3

polished to a mirror surface for optical transmissivity measurement using Fourier transfer
infrared spectroscopy.
Results and discussion
Figure 4 shows transmissivity spectra for samples implanted at 100 keV (a) and 500 keV (b),which were heated by laser irradiation for 25~42 μs. Sheet resistivity was also presented in the
figures. The transmissivity lower than 2500 cm-1 decreased as laser dwell time increased forsamples implanted at 100 and 500 keV. In addition, decrease in transmissiviy around 2000 cm-1
was observed for samples implanted at 500 keV. The sheet resistivity decreased as the laserdwell time increased for the both implantation conditions. Figure 5 shows difference intransmissivity spectra obtained by subtracting spectrum of undoped silicon from spectra shownin Fig. 4. In the case of implantation at 100 keV, difference in transmissivity below 2500 cm-1
was low and a valley and peak appeared at 2700 and 5500 cm-1 for laser annealing for 25 μs.
Difference in transmissivity below 2500 cm-1 increased as the laser dwell time increased
because of substantial free carrier absorption. The valley and peak shifted to higher
wavenumber as the laser dwell time increased. Those shifts mean that the free carrier region
moved near the surface as the laser dwell time increased. On the other hand, in the case of
implantation at 500 keV, large transmissivity difference was observed for every laser dwelltime. Moreover, samples annealed with different laser dwell times showed similar spectral line
shape. There were valleys at 1000, 3000 and 5600 cm-1, and peaks at 1800 and 4300 cm-1. The
peak height at 1800 cm-1 increased. It means that the carrier density increased as the laserdwell time increased.

Fig. 4 Transmissivity spectra for samples
implanted at 100 keV (a) and 500 keV (b),which were heated by laser irradiation for
25~42 μs.

100228044-4

Fig. 5 Differences in transmissivity spectra
obtained by subtracting spectrum of
undoped silicon from spectra shown in Fig.
4 for samples implanted at 100 keV (a) and500 keV (b).

We analyzed experimental spectra shown in Fig. 5 with our numerical calculation program.
Figure 6 shows carrier in-depth profiles prepared for fitting experimental spectra for implanted
100 keV (a) and 500 keV (b). And figure 7 shows deference in tansmissivity spectra calculatedusing carrier in-depth profiles shown in Fig. 6. Shallow carrier localization shown in Fig. 6(a)resulted in broad spectra as shown in Fig. 7(a). Laser dwell time for 25 μs resulted in carrier
location around 180 nm in deep, which gave a valley at low wavenumber in difference intransimissivity spectra as shown in Fig. 7(a). Laser dwell time for 42 μs resulted in carrier
distribution from surface to 200 nm, which gave a valley at high wavenumber in difference intransimissivity spectra as shown in Fig. 7(a). This means that impurity activation initiated fromdeep regions, and that it proceeded to the surface as the laser dwell time increased. The surface
140 nm region was amorphized by implantation at 100 keV. Recrystallization was necessary to
generate high density free carrier. The results of Figs. 6 and 7 indicate that recrystallization
proceeded from the deep region to the surface region with increasing the laser dwell time. On
the other hand, similar carrier in-depth profiles were used for fitting experimental spectra forevery laser dwell time in the case of implantation at 500 keV, as shown in Fig. 6(b). This
indicates that uniform activation in-depth direction occurred for every laser dwell time.
Crystalline state was kept after phosphorus ion implantation at 500 keV. Uniform activation
would be possible because silicon substrate is heated to high temperature in 30 μm deep
regions at least during laser irradiation longer than 20 μs. Thermal energy for moving
phosphorus atoms into silicon lattice sites was only necessary for activation. Long laser dwell
time gave high enough energy to activate phosphorus atoms uniformly. Similar and deep
carrier density profiles gave similar difference spectra in transmissivity with oscillation line
shapes as shown in Fig. 7(b), which well explained the experimental spectra shown in Fig.

Fig. 6 Carrier density in-depth profiles used
for fitting experimental spectra in the casesof 100 keV (a) and 500 keV (b) withdifferent laser dwell times. Inset in (a)presents image of carrier in-depth profiles.

Fig. 7 Difference in transmissivity spectra
obtained by subtracting spectrum of undoped
silicon calculated using carrier density
in-depth profile shown in Fig. 6.

100228044-5

5(b).
Figure 8 shows the total carrier density as afunction of laser dwell time. In the case of
implantation at 100 keV, the carrier density
was very low of 1.4×1014 cm-2 for a laserdwell time of 19 μs. It increased to 1×1015
cm-2, which was the same value of doping
concentration. On the other hand, a carrier
density of 4×1014 cm-2 was already observed
for a laser dwell time of 19 μs in the case of
implantation at 500 keV. It increased to 8×
1014 cm-2, which was lower than theimplantation concentration, as the laser dwell
time increased 42 μs

Fig. 8 carrier density as a function of laserdwell time for samples implanted at 300 and
500 keV.

Summary
We investigated activation behavior of phosphorus atoms implanted into silicon using
numerical calculation program of transmissivity with finite element free carrier opticalabsorption and Fresnel coefficient interference models. Free carriers change refractive index
and extinction coefficient. When they locate in deep regions, optical interference effect occurs
among layers with different refractive indexes and extinction coefficients. Phosphorus ionswith 1×1015 cm-2 were implanted at 100 and 500 keV into 20 Ωcm p-type silicon samples.
200 nm thick carbon optical absorption layers were then formed on the surface by sputtering.
Samples were annealed by 976 nm semiconductor laser irradiation at 500 kW/cm2 for 19~42
μs. Infrared transmissivity spectra of samples showed oscillation spectra between 1000 and
7000 cm-1 because of optical interference effect due to change in refractive indexes induced byelectron free carriers. In the case of implantation at 100 keV, a valley and peak appeared at2700 and 5500 cm-1 for laser annealing for 25 μs. The valley and peak shifted to higher
wavenumber as the laser dwell time increased. Those shifts mean that the free carrier region
moved near the surface as the laser dwell time increased. This revealed that activation
proceeded from the deep region to the surface according to recrystallization. On the other hand,in the case of implantation at 500 keV, there were valleys at 1000, 3000 and 5600 cm-1, andpeaks at 1800 and 4300 cm-1 for every dwell time condition. This means that activation
occurred uniformly in depth direction for the every laser dwell time in the case of implantation
at 500 keV.
Acknowledgments
This work was partially supported by NEDO foundation P07026.
REFERENCES[1] T. Sameshima, tutorial text in 6th Thin Film Materials & Devices Meeting (Kyoto, 2009).
[2] H. Engstrom: J. Appl. Phys. 51 (1980) 5245.
[3] T. Sameshima, K. Saitoh, N. Aoyama, S. Higashi, M. Kondo, and A. Matsuda: Jpn. J. Appl.
Phys. 38(1999)1892.
[3] M. J. Kerr and A. Cuevas: Semicond. Sci. Technol. 17, (2002) 166.
[4] J. C. Irvin: Bell Syst. Tech. J. 41 (1962) 387.
[5] T. Sameshima, H. Hayasaka, and T. Haba, Jpn. J.Appl. Phys. 48 (2009) 021204-1-6.
[6] T. Sameshima, Y. Matsuda, Y.Andoh, and N. Sano, Jpn. J.Appl. Phys. 47 (2008) 1871.
[7] N. Sano, M. Maki, N. Andoh and T. Sameshima, Jpn. J. Appl. Phys. 46 (2007) 1254.

100228044-6