Download Structural Characterization of Photoluminescent Porous Silicon with

158
Brazilian Journal of Physics, vol. 27, no. 4, december, 1997
Structural Characterization of Photoluminescent
Porous Silicon with FTIR Spectroscopy
Walter Jaimes Salcedo, Francisco J. Ramirez Fernandez, Elisabete Galeazzo
Laboratorio de Microeletr^onica LME - DEE - EPUSP
Caixa postal 8174, CEP 01065-970, S~ao Paulo, SP
Fax (011) 818-5718 e-mail [email protected]
Received February 2, 1997
This paper presents an analytical model which is proposed to determine morphological
parameters of porous silicon (PS) using results obtained with the Fourier Transform Infrared
(FTIR) characterization techniques. Using the proposed model, it was found that porous
silicon layers have porous size of 6.6 nm, specic surface of 600 m2/cm3 and the residual
crystal size 3.7 nm. The PS and oxidized PS were characterized with Photoluminescence
(PL) and FTIR techniques. These techniques give some evidences of the possible quantum
connement eect in PS layer. However it is necessary to have caution at this point.
Introduction
Signicant attention has recently been directed towards porous silicon due to its visible photoluminescence (PL). The strong visible light emission in porous
silicon is quite surprising, in view of the fact that bulk
crystalline silicon has an indirect gap at 1.1 eV at room
temperature, which results in a very inecient radiative
recombination producing light in the infrared[1].
Porous silicon can exhibit a large variety of morphologies and particles sizes. In nanostructures, the
weak emission problem could be overcome by the removal of local or global periodicity, with energies that
scale inversely with the size of the connement region[2]. In the case of silicon, particles sizes with dimensions below 5 nm would enhance the oscillator strength,
as well as produce a sizable blue shift of the optical gap
from 1.1eV into the range of 1.5 to 1.9 eV due to quantum connement[3].
Experimental results
Porous silicon layers (PS) were formed by electrochemical anodization on P - type silicon wafer (100)
with resistivity of 20 cm, in 40% (w/o) HF solution
at current density of 20 mA/cm2 for 5, 10 and 15 min.
The back face of wafers were doped with boron followed by Al metalization to improve the uniformity of
current ow during anodization and to obtain homogeneous porous layers. The porosity and thickness of
the layers were determined gravimetrically by weight
loss and etching in KOH solution [4]. After anodization, some layers were anodically oxidized in aqueous
solution of KNO3 at a constant current density of 1
mA/cm2. Oxidation proceeds from the interface between the porous layer and silicon substrate towards
the surface.[5]
Experimental measurements
The infrared spectra of PS were measured in absorvance mode with BIO - RAD FTIR Spectrometer
and they were referenced against a single silicon crystal
sample. Fig. 1 shows spectral curves of PS and oxidized
PS.
W. J. Salcedo et. al.
159
In the infrared spectra of PS, the surface bonding
are showed in accordance with the interpretation of table 16;8.
A noticeable blueshitf of the whole luminescence
spectra was observed. The PL peaks emission of PS
and oxidized PS were at = 725 nm and = 650nm
respectively.
Discussion
The frequency of bulk bonding vibrations of Si-Hn
in the silicon relative to surface bonding vibrations,
downshift. This fact can be calculated from a model
where the downshift is caused by a depolarizing produced by a vibrating dipole within a spherical cavity of
radius R[9].
2
! = 2s ;+11 Re 3!
s
The Si-Hn vibrationals bonding were considered to
analyze structural characteristic of PS because they
have absorption peaks in region with few density of absorption peaks. The absorption intensity which belong
the Si-Hn bonding presents linear dependency with anodization time.
I=
Z
(!)d! = a + bt
(1)
Where, is the absorvance.
The room temperature PL spectra of PS and oxidized PS are showed in Fig. 2. The PL emission was
excited by an Ar+ laser line of 450nm. The emitted
light was detected by a photomultiplier tube.
(2)
With: e : apparent charge associated with the dipole
moment per unit of displacement, es: dielectric constant of Si, !w: center frequency for an absorption
band. Applying equation, (2) with s = 12, e = 0:4e
and R the covalent radius of Si to ! = 2100 cm;1, it
is obtained ! = -80 cm;1. Absorption peaks relevant
to the Si-Hn bonds of PS layers evidence dislocations
of 4 cm;1 when compared with results of Si-Hn surface bonds reported[9,10]. Then, the Si-Hn bonds of PS
samples are surface bonds.
In order to develop a model to obtain the structural
parameters of PS layer, it is dened the geometrical
parameters for PS layer. The graphic representation of
these parameters are shown in the Fig. 3. Another consideration is that all PS surface is covered with Si-Hn
bonds.
The parameters are dened as follow: d is the average pores size, q is the average size of the residual
160
Brazilian Journal of Physics, vol. 27, no. 4, december, 1997
silicon crystal between pores. The average size of residual silicon crystallite between pores is obtained using
the following relationship:
p
q = 2 3 3 1 + md d ; d
Then, the density of PS layer is given by:
(3)
!
1
PSL = 1 ; p
;
2 3 1 + md 2 Si
(4)
The porosity of PS is dened as the quantity of silicon
removed during anodization compared with the silicon
concentration before anodization evaluated in the same
volume. The porosity is obtained by the relation.
P = Si ; PSL
Si
(5)
Where P is the porosity, Si is the density of silicon.
Then, the porosity as function of geometrical parameters is written as:
2
1
P = p 1+ m
(6)
2 3
d
The other important parameter is the specic surface.
The following equation results of the geometric model
utilized:
1
P
3
Aesp = X (m) + 8:10 d(nm) (m2=cm3) (7)
p
A complete description of the PS layer structure is given
by Aesp , P, d parameters. The equation (7) shows a relation between these parameters.
Using this geometrical parameters, the surface concentration of the Si-Hn bonds in the PS layer is determined. This concentration is obtained from the following equation:
NSi;Hn = 6:75 1013 + 6:10 1014Aesp Xp
(8)
Where, NSi;Hn is the Si ; Hn bonds concentration
on PS surface (cm;2), Aesp . The Specic surface
(m2/cm3), and Xp is the PS layer thickness (mm).
The silicon dilution rate is approached for 1 mm/min
[11,12]. Then, it means that Xp = t (m).
Using the proposed model, the Si ; Hn bonds concentration is linearly dependant of anodization time.
This result agrees with experimental results obtained
in FTIR techniques. The Si ; Hn bonds concentration
can be obtained by evaluating the infrared absorption
band by the mean of the following equation [10,13]
Z
NSi;Hn K!0;1 (!)d!
(9)
Where K is the absorption force. It is considered the
mean value[13] of K = 9:0 1019. The geometric parameters of PS are obtained using the proposed model
relating with experimental results of infrared spectra
(eq. 9) and porosity of 50% which was determined by
gravimetric techniques. It was obtained the following
values: d = (6:65 0:15) nm, Aesp = (601:8 13:6)
m2/cm3 and q = (3:69 0:08) nm. These values showed
be independent of anodization time. Quantum connement for charge carries due to the very small dimensions
of the wires of silicon in PS is hoped. The quantum
connement leads to an increase in the eective band
gap energy. The eective mass model leads to following
expression:
2 2 1
1
h
(10)
E = q2 m + m
e
n
Where, me is the eective mass of electron and mn is
the eective mass of holes. For our PS it was obtained
the following value: E = (0:64 0:03) eV. The PS
layer, without oxidation, has the PL peak at = 725
nm, with a eective band gap of 1.711ev. The increase
of band gap was E = 0:650 eV. In the quantum connement frame work the correspondently residual crystallite size value is q = 3.65 nm.
The oxidized PS presents a PL emission peak at = 650 nm with a eective band gap energy of EBG =
1:908 eV. The increase of band gap is of E = 0:79
eV. In the quantum connement frame work the correspondently residual crystallite size value is q = 3.31
nm.
The comparison of `qs' values, that they were estimated for PS and PSoxidized from the PL spectra, give
the following ratio:
q = 13:4%
q
The result obtained, using infrared spectroscopy on preceding two samples, shows variation of Si ; Hn bonds
concentration corresponding to:
W. J. Salcedo et. al.
IPSL ; IPSLoxid = I = (14:1 2:6)%
IPSL
IPSL
IPS AespPS and IPSoxid
I = Aesp = (14:1 2=6)%
I
Aesp
It can be observed that the q=q and I=I ratios
are similar. These results give some evidences that
there is a possible quantum connement eect in the
PS layer. However the literature report a possible surface eect [14-18] of the PL phenomena in the PS layer.
At this point it is necessary to have caution.
Conclusions
The model proposed is an eective auxiliary tool
to evaluate the parameters related to the porous structure. The PL spectra are consistent with evaluation obtained using this model. Our experimental observation
pointed out the quantum connement as possible cause
of the PL emission in the PS. Even though many group
have reported the existence of quantum connement
eect, the origin and mechanics of the luminescence in
the PS layer remain controversial.
Acknowledgements
We acknowledge to CNPq and Grant-PADCT
620118/94-6 for the nancial support.
161
References
[1] W. Michaelis and M. H. Pilkuhn, Phys. Stat. Sol. 36,
311, (1969).
[2] V. Lehmann, U. Gosele, Appl. Phys. Lett. V. 58, 8,
856 (1991).
[3] D. J. Lockwood, Solid State Commun. V 92, 1-2, 101
(1994).
[4] E. Galeazzo, et al. 11o CBECIMAT, A guas de S~ao Pedro, SP, 11 a 14 de Dezembro de 1994.
[5] Holimaoui A., et al., Appl. Phys. Lett., No 59, V. 3, 15
July 1991.
[6] L.M. Peter; Phys Rev. Lett. V. 62, No 3, pp. 308, 16
Jan 1989.
[7] P. Gupta, et al; Phys. Rev. B, V. 37, No 14, pp. 8234,
15 May 1988-I.
[8] P. Gupta, et al; Surf. Sci. V. 245, 360 (1991).
[9] M. Cardona; Phys. Stat. Sol. (b), V. 118, 463 (1983).
[10] R. C. Anderson, et al; J. Electrochem. Soc., V. 140,
No 5, 1393, May 1993.
[11] V. Lehmann, H. Foll; J. Electrochem. Soc. V. 137, 653
(1990).
[12] X. G. Zhang; J. Electrochem. Soc. V. 138, No 12, 3750
(1991).
[13] H. J. Stein, S. M. Myers, D. M. Follstaedt; J. Appl.
Phys. V. 73, No 6, 2755, 15 Mar. 1993.
[14] H. D. Fuchs, M. Stuzmann, M.S. Brandt, M. Rosenbauer, cJ. Weber, A. Breitsschwerdt, P. Deak, and M.
Cardona, Phys. Rev. B 48(11), 8172 (1993).
[15] P. Deak, M. Rosenbauer, M. Stuzmann, J. Weber, M.
S. Brandt, Phys. Rev. B, 69 (17), 2531 (1992).
[16] M. Stuzmann, Phys. Stat. Sol. (b)192, 273 (1995).
[17] F. Koch, V. Petrova,-Koch and T. Muschik, J. Lumin.
57, 271 (1993).
[18] M. Stuzmann, M.S. Brandt, M. Rosenbauer, H. D.
Fuchs, S. Finkbeiner, H. D. Fuchs and P. Deak, J. Lumin. 57, 321 (1993).