Structural and Optical Characterization of Silicon Nanocrystals

Obtained by Ion Implantation

S. N. M. Mestanza1, G. O. Dias1, J. E. C. Queiroz1, I. Doi1 and J. W. Swart1

E. Rodriguez2, A. A. R. Neves2, H. Martinho2

1Centro de Componentes Semicondutores CCS/FEEC-UNICAMP, CP6165, 13083-970 Campinas, SP, Brazil

2Instituto de Física Gleb Wataghin IFGW/DEQ-UNICAMP, CP6165, 13083-970 Campinas, SP, Brazil

e-mail: nilo@led.unicamp.br

Abstract

Photoluminescence (PL) was used to investigate the luminescence mechanism in Si

nanocrystals. Si ions were implanted in SiO2 films at 100 keV to a dose of 5x1016 / cm2. An

intense photoluminescence band in (1.2-1.6) eV was observed after implanted films were

annealed at 1000oC in nitrogen. Deconvolution of photoluminescence suggests that others

recombination centers possibly are present into the samples. Red PL band (1.5 eV) was

attributed to emission from Si nanocrystals because gaussian peak in the 1.5 eV increase

continuously with annealing temperature became narrower. Raman spectra for all samples

show a band approximately at 521 cm-1, related to typical nanocrystalline silicon. From the

X-ray diffraction (XRD) results the clusters are shown crystalline and with plane

orientation (311). Scanning electron microscopy (SEM) and atomic force microscopy

(AFM) images showed the presence of Si nanocrystals with diameter ~ 10 nm on etched

annealed samples.

Keywords: Clusters, silicon, ion implantation, Raman spectroscopy, silicon oxide.

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I - Introduction

In spite of high efficiency emitter devices can be obtained from III-V

semiconductor compounds; its integration in the current Si planar technology is very

complex and often ineffective. On the other hand silicon is a very poor emitter due to its

indirect band gap. However, in nanocrystal form (Si-nano), silicon properties change

drastically presenting a direct band gap with emission in the wavelength range of [600-

1000] nm, depending on the nanocrystal size distribution. Samples obtained by

implantation of silicon into silicon oxide matrix present strong PL in the range cited above

[1]. These silicon nanoclusters have been studied along several years as a promised

structure to be applicable in photonic technology. Many techniques have been used to

produce silicon nanoparticles into silicon oxide, however the most advantageous technique

is the ion implantation, because, among other advantages, presents the possibility of

controls the depth of distribution of ions.

Mechanism of optical gain into these nanocrystalline structures remains not well

explained. Some works report the decreasing of photoluminescence intensity after

annealing [2] and others report just the contrary, i.e., the increasing of determined

photoluminescence band intensities after annealing time, up to the saturation [3]. Still isn't

well understood what is the real contribution of the nanoclusters on the new observed

luminescence. Ion implantation technique itself causes defects that may be related to this

luminescence [4]. Many experiments have been carried out in order to clarify the origin of

photoluminescence bands in structures created by ion implantation of Si into SiO2 [4].

2

Silicon is a very desirable candidate for photonic technology, because it is found in

abundance in all the Earth and the technology of silicon is well known and developed,

which result in low cost processes. Indeed, as the technology related to microelectronics is,

nowadays, based in 98% on silicon, development of optoelectronics technology also based

in silicon is of great interest, because this could lead to create a bridge between

microelectronics and optoelectronics.

It is well known that photoluminescence bands in structures created by implantation

of Si into SiO2 are dependent on ion dose (concentration of ion implanted), annealing time

and annealing temperature.

Thermal silicon oxides with ~ 300 nm, were grown on (100) p-Si wafers. 28Si+

implantation was performed with energies of 100 keV. The implantation energies were

chosen from Monte Carlo simulation, using the TRIM code. Conventional PL spectra were

obtained at room temperature, showing a broad luminescence band in the red region for all

the samples. We have studied the behavior of PL emission peak and full width at half

maximum (FWHM) with the annealing time. We have estimated the size of nanocrystals by

PL and Raman spectroscopy analysis.

II – Experimental

Our samples were prepared by implantation of Si into thermal oxidized p-doped

(100) silicon wafers. The p-doped Si substrate had a thickness of 393 µm, V/I ratio of

3

Ω± )36.085.25( and the measured resistivity was 4.60 Ωcm, with a dopant concentration

of 3.0x1015 cm-3. These silicon wafers were cleaned following the standard RCA cleaning

process. After, the thermal oxidation was carried out in a conventional furnace at

temperature of 1000oC.

The thickness (measured by 632.8 nm He/Ne laser Rudolph Ellipsometer, model

AUTO EL) of grown oxide was 310 nm. The dose of implanted silicon was 5.0x1016 cm-2

at energy of 100 keV. Annealing was realized at 1000oC on these silicon-implanted samples

at times of 1h, 2h and 3h.

The PL spectra were obtained by an argon laser excitation source, with excitation

lines of 488 nm and 514 nm, at powers between mW)5100( ± , at room temperature. The

PL measurements were performed using a MG252 SPECTRAL ENERGY monochromator

with a blaze grid of 1 micra of 590 lines/mm. The detector used was a PIN Si photodiode

model 818-BB-40 of the Newport.

Raman spectroscopy was carried out to investigate the possible formation of silicon

nanocrystals into SiO2 matrix. We aim to observe emission about 521cm-1, typical of

monocrystaline silicon. The excitation light source at 488 nm was emitted from a Spectra

Physics argon laser with an incident power of around 10 mW.

For the morphological analysis we use a Field Emission Gun Microscope (FEG)

JSM 6330F Jeol. Micrographs were also obtained with a Topometrix AFM Microscope,

model DISCOVERER, by non-contact technique.

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In order to observe the formation of clusters inside the oxide we carried out an

etching with buffer of HF to discover the clusters formed. The ratio of corrosion was nearly

1000 Å/min. We did 1.20 minutes corrosion, and after the oxide thickness was measured,

which result approximately 150 nm.

III - Analysis and Discussion

Fig. 1 shows time evolution of the photoluminescence spectra of the three annealed

samples studied (B, C, D) at 1000oC for several hours. It’s clear from the figure that the

photoluminescence intensity and annealing time are directly dependent, increasing

altogether. It’s also clear that peak energies of the luminescence spectra are independent of

the annealing time in all the samples. These results are in accordance to those reported in

literature [3]. The fact that not well defined PL peak was observed we attribute basically to

factors related with stequiometric composition of the SiO2 matrix. In one first

approximation we could say that in fig. 1 the PL peak stayed in the same position, so it is

related with the same energy band. This fact could be showing that clusters size is not

increasing with the increase of the annealing time. Indeed the clusters are always increasing

with the annealing time. However, we observe an apparently unmovable energy peak of PL

and we suppose that it is basically due to emission from a new generation of clusters,

originating from silicon dispersed in SiO2 matrix. Another curious fact is that spectra show

a broad PL band, centered between 1.4-1.5 eV, which seems a superposition of different

band contributions.

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To better understand these results, we carried out gaussian adjusts for each PL

spectrum (fig. 2). We can observe from these adjusts that each spectrum may be very well

fit by three contributions (gaussians α, β, γ) with the following characteristics: for all

instant α, β and γ show the same energy peak; the intensity for β and γ contributions stay

constant. Only for α contribution one observe a continuous increasing in intensity, a

narrowing of bandwidth with annealing time and independence of energy peak position

(from the literature, this is a typical behavior of a nanocrystal that satisfy the quantum

confinement model). These results lead us think that β e γ emissions can be attributed to

recombination processes in cluster-Si/SiO2 interface or yet to defects in SiO2 matrix.

Concerning the theorist-experimental curves of diameter vs. Energy reported [5],

[6], [7], we have estimated the diameter of our nanocrystals utilizing for this the peak of

contribution α in fig. 2. The peak of this curve (α) is localized in approximately 1.5eV and

clusters associated with this peak would have ~ 9nm in size.

Raman spectroscopy is an efficient technique to evaluate quantitatively

measurements of stress, presence of nanocrystals and homogeneity. Si crystals have typical

characteristic of strength emission and narrow band at 521cm-1, which corresponds to

transverse acoustic mode in first order. However in the case of amorphous silicon, the lack

of order induce to modifications in the vibrational density of states and in this case the

Raman spectrum is characterized for two faint and broad bands at 150cm-1 and 480cm-1.

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We measured the Raman spectra of sample B and C (3 and 2 hours annealing,

respectively). The results are shown in the fig.3. A few observations can be easily deduced

from these results. With respect to crystalline silicon (Wo), the relative Raman frequency

shift (Wc-Wo, Wo=521.0 cm-1) is ~ 4.2 cm-1 and ~ -4.5 cm-1 for the samples C and B

respectively. In some works [8], [9] the shifts in Si characteristic emission around 521cm-1

are utilized to estimate the size of silicon clusters. Using the equation below (I) we made an

estimative of size of our clusters, through the qualitatively model in literature (this model is

applicable more adequately to porous silicon and do not make phenomenological

considerations).

γωωω )()( 0 LaAL −=−=∆ (I)

ω(L) is the frequency of Raman phonon in a nanocrystal with size L, ω0 is the frequency of

the optical phonon at the zone center, a is the lattice constant of silicon (a=543.1x10-12m),

A=47.41cm-1 and γ=1.44 are parameters used to describe the vibrational confinement due to

the finite size in a nanocrystal and your values were assumed for spherical nanoclusters.

Then considering equation (I) we obtain ~ 3 nm for the clusters size.

In the fig. 4 we show the Glancing Angle X-ray Diffraction (GAXRD) for the sample

B (3 hours annealing). As seen in this figure, the peak in ~ 57 degrees show the presence of

crystalline features of silicon in the plane (311) inside SiO2 [10] (the spectrum of substrate

silicon (100) was considered as a baseline and was not showed in fig. 4). The distance

between planes may be easily calculated by equation (II).

2

2

2

2

2

2

1

cl

bk

ah

dhkl

++

= (II)

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For silicon we have a=b=c=543.1x10-12m. So, we obtain dhkl=1,64Å. This plane

orientation must be related to other two not observed directly, since silicon has a fcc

symmetry. This will be confirmed by further experimental analysis.

Fig. 5 show SEM image of D sample (3h anneal.), after etching of 1.2 minutes with

HF buffer (~150nm). In this image we can observe an almost uniform distribution of Si

clusters, spanned on an area of ~ 2.5x1.9 µm2. By this image we can observe clusters of

approximately 10nm in size, this is compatible with that value found by PL estimative for

α contribution.

Fig. 6 shows the AFM micrograph of p-type sample after etching. An approximately

homogeneous, high distribution of clusters, over all 5µm x 5µm area scanned with AFM,

can be observed. The same feature can be observed from the image obtained with FEG (fig.

5). The mean clusters size observed from these images is ~ 10 nm.

The discrepancy in relation to mean clusters size obtained from Raman shift may be

explained by the fact that the values were obtained by a not phenomenological model. It has

been used to describe quantitatively the Raman spectra from porous Si [10].

IV – Conclusion

SEM, AFM and PL results confirm the existence of Si clusters into a SiO2 matrix.

Furthermore the sizes obtained by these methods are compatible. Raman and XRD results

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confirm that these clusters are really nanocrystals of Si. Increasing annealing time the

clusters also increase in size, but we can observe, from immobility of PL energy peak

associated to clusters, that the predominant effect is the originating of new generations of

clusters. By the narrowing of PL band we are leaded to think that exist a preferential

tendency to a unique size.

V – Acknowledgements

The authors are grateful to Prof. Dr. C. Lenz from IFGW/DEQ/UNICAMP, for the

PL measurements, to Prof. Dr. L. P. Cardoso from IFQW/DFA /UNICAMP, for the XDR

measurements, and to Msc. Eng. C. Costa EESC/USP/São Carlos – SP, for AFM

measurements. This work was supported by the Fundação de Amparo à Pesquisa do Estado

de São Paulo (FAPESP). The support is gratefully acknowledged.

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Figure Caption

1.- Photoluminescence spectra of 100 KeV Si+-implanted thermal oxide film grown on

crystalline p-Si wafer and implanted to a fluence of 5x1016 ions/cm2 at room

temperature and subsequent annealing at 1000 oC for (D) 1h, (C) 2h, (B) 3h and

(A) 0h reference substrate (from bottom to top).

2.- Deconvolution of PL time evolution for various annealing time.

3.- First-order Raman spectra measured at room temperature for the samples B and

C for different annealing time. For these measurements the excitation wavelength

was 488.0 nm and laser power 10mW with a spot diameter of about 50 µm.

4.- Glancing angle X-ray diffraction (GAXRD) showing typical peak for (311) silicon.

5.- SEM image of a SiO2 film containing Si clusters, after etched in HF buffer.

6.- AFM image of a SiO2 film containing Si clusters, after a 1.2 minute etched in HF

buffer. Note the many particles and few bare spots seen, despite the fact the oxide

has been fully etched.

Figure 1 Figure 2

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Figure 3 Figure 4

Figure 5 Figure 6

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