emission spectroscopy of laser-ablated si plasma related to nanoparticle formation

Emission spectroscopy of laser-ablated Si plasmarelated to nanoparticle formation

V. Narayanan, R.K. Thareja*

Department of Physics and Center for Laser Technology, Indian Institute of Technology Kanpur, Kanpur 208 016, UP, India

Received 28 June 2003; accepted 6 September 2003

Abstract

We report on the laser ablation of Si in vacuum, and in the presence of helium ambient at 1 and 10 Torr, respectively. The

silicon nanoparticles were deposited on silicon substrate at room temperature by ablating silicon wafer in ambient atmosphere of

helium at 1 Torr. The mean cluster size ranging from 1.8 to 4.4 nm is observed depending on the laser intensity. Optical emission

spectroscopy and images of the plume are used to study the spatial and temporal variation of the silicon plasma. The electron

density, measured by the Stark-broadening of Si I transition 3p2 1S�4s 1P0 at 390.55 nm and temperature, assuming thermal

equilibrium, were found to be 1:2 � 1018 cm�3 and 2 eV, respectively. The temporal variation of Si I transition 3p2 1S�4s 1P0 at

390.55 nm showed a shift in peak position attributed to collisions at an early stage of plasma formation. The relative

concentration of Si II/Si I estimated by using the Saha–Boltzmann relation showed abundance of Si I. Time resolved images of

the plume were used to investigate the dynamics of the expanding plasma plume, estimating the vapor pressure, vapor

temperature, velocity, and stopping distance of the plume. The photoluminescent spectra of the Si thin films showed three

distinct emission bands at 2.7, 2.2 and 1.69 eV, the origin of these bands is attributed to defects and quantum confinement.

# 2003 Elsevier B.V. All rights reserved.

PACS: 42.62.Fi; 52.70.Kz; 81.15.Fg

1. Introduction

Pulsed laser ablation and deposition has been exten-

sively exploited for several practical applications such

as laser-induced mass analysis, laser-induced break-

down spectroscopy, light sources including X-ray laser

mediums, synthesis of nanoclusters, and thin film

deposition [1,2]. Various materials like semiconduc-

tors, metals, and dielectric can be deposited by pulsed

laser deposition technique [1,3]. The interesting optical

and electrical properties exhibited by nanocrystalline

materials different from the bulk material, have

attracted attention of the field. The laser ablation

technique has been widely used to produce functional

films consisting of particles whose diameter is on the

nanometer scale. The characteristics of the synthesized

particles largely depend on fabrication conditions such

as the distance between an ablated target and collecting

substrate, the ambient gas, the pressure of the ambient

gas, etc. On the other hand ablation dynamics depends

on factors like wavelength, pulse width, laser energy

and presence of ambient gas [3–5].

The laser-ablated plasma consists of neutral and

ionized species along with clusters of the target

material. The leading part of the expanding plume

with high kinetic energy of the particles pushes and

Applied Surface Science 222 (2004) 382–393

* Corresponding author. Tel.: þ91-512-2597143;

fax: þ91-512-2590914.

E-mail address: [email protected] (R.K. Thareja).

0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2003.09.038

compresses the background gas. Various species,

atoms, ions and aggregates of a few atoms, make

several collisions with the background gas where high

kinetic energy particles lose their energy and get

thermalized with the surroundings, resulting in con-

densation and formation of clusters. Besides initial

conditions, the laser intensity, pulse width, and back-

ground gas pressure, the size distribution of the clusters

is determined by the hydro-dynamical expansion of the

plume. The clusters are further cooled owing to the

collisions with ambient gas molecules. The small

clusters on collisions with background gas atoms are

scattered in background gas at higher angles and

consequently are deposited on substrate placed at

larger distances. Thus, the nanoparticle formation,

growth, and deposition on to the substrate involves

the initial process of nucleation determined by thermo-

dynamic parameters of the material and initial condi-

tions, like temperature and density of the vapor ejected

after ablation. The ablation dynamics of the plume in

the ambient gas is quite different from its expansion in

vacuum. Since the cooling due to collisions takes place

when the plume expands in an ambient gas, the effi-

ciency of the cooling strongly depends on the ambient

gas parameters determined by the hydrodynamics of

the plume expansion. Increasing the background gas

pressure results in the confinement and slowing of the

plume relative to the propagation in vacuum. The

increase in fluorescence from all species due to colli-

sions on the expansion front and subsequent inter-

plume collisions and formation of a shock front has

also been reported. To optimize the synthesis of quan-

tum-confined nanomaterials by laser ablation in the

background gas it is imperative to know the temporal

and spatial scales for nanoparticles formation, and how

the nanoparticles are transported and deposited [6].

Nanoscaled structures play a major role in optoe-

lectronic and semiconductor research with many

potential applications. Recent observations of visible

photoluminescence (PL) in porous Si by Canham [7]

and from silicon ultrafine particles by Takagi et al. [8]

makes Si nanoclusters (nc-Si) a promising material for

optical applications requiring room temperature

photoluminescence. A large part of the present silicon

nanocrystals research is focused on the preparation of

nc-Si embedded in an oxide host. Since the properties

of nc-Si are quite different from bulk-silicon, fabrica-

tion of nanocrystalline silicon (nc-Si) by various

methods has acquired more interest due to technolo-

gical applications. Recently, electroluminescent light

emitting diodes have been fabricated from pulsed laser

deposited nc-Si films [9]. The observed visible PL has

been attributed to quantum confinement effects in the

Si nanostructures, as amorphous Si silixones (Si–O–

H) or surface hydrides/polysilanes being possible

radiative recombination centers in silicon nanostruc-

tures and oxygen related defect centers [10]. The

decrease in nanocrystal size has shown a blue shift

of the luminescence. The size of nanocrystals is con-

trolled by the chemical stoichiometry of the films [11].

This amounts to reducing the implanted Si dose or the

O enrichment. However, decreasing the nanocrystal

size into the desired range may reduce the density of

the nanocrystals also. Moreover, there is only a limited

control of the size distribution [12]. Thus for device

applications accurate engineering of spatial position,

size, and density of the nc-Si is mandatory.

In this paper, we report a study of laser ablation of

silicon in helium atmosphere and photoluminescence

(PL) from the deposited nc-Si films. The nc-Si was

deposited on silicon substrate at room temperature by

ablating silicon wafer in ambient atmosphere of

helium at 1 Torr. Earlier works reported on Si ablation

has been at atmospheric pressure [13,14]. Several

diagnostics such as optical emission spectroscopy

(OES), imaging (fast photography), ion probes, inter-

ferometry (pump-probe) and time of flight mass spec-

troscopy (TOFMS) tools have been routinely used to

characterize the dynamics of the plasma plume [2]. We

have used OES and imaging techniques in the present

work. The electron density is estimated using Stark-

broadening, and the ionic temperature is derived using

neutral intensity data from and singly ionized species.

The dynamic parameters of the plume are studied by

using a intensified charge coupled device (ICCD)

imaging the expanding plume. PL spectra from the

nanocluster films show various luminescence bands.

The surface morphology and particle size distribution

of nanoclusters is ascertained through atomic force

microscopy (AFM) images.

2. Experimental details

The experimental set up used in the present study

is similar to the one described elsewhere [15] for

V. Narayanan, R.K. Thareja / Applied Surface Science 222 (2004) 382–393 383

the OES. A third harmonic (355 nm, pulse width

5 ns FWHM) at pulsed repetition rate of 10 Hz of a

Q-switched Nd:YAG laser (DCR-4G, Spectra phy-

sics) was used for creating silicon plasma both in

vacuum (pressure better than 10�5 Torr) and ambient

helium gas. The vacuum chamber was evacuated to a

pressure better than 10�5 Torr and was flushed with

helium many times before introduction of the gas in

a controlled manner using a needle valve. Laser

radiation was focused using a spherical lens to a

spot size of diameter 250 mm onto a silicon target

mounted in the vacuum chamber. The laser radiation

falls normal to the target surface. The silicon target

was continuously rotated with an external motor so

that each laser pulse falls on a fresh target surface. A

high-purity silicon wafer (The Nilaco Corp., Tokyo)

free of native SiO2 was used as a target. The emis-

sion spectra were recorded at different spatial

positions of the plasma in vacuum, 1 and 10 Torr

of helium of high purity (99.99%) at constant laser

intensity of 2:45 � 109 W/cm2. Plasma was imaged

onto a slit of a monochromator (HRS-2, Jobin Yvon)

with a lens so as to have one-to-one correspondence

with the plasma and its image on the slit. The output

of the monochromator was detected with a photo-

multiplier tube (PMT) and recorded on strip-chart

recorder or by an ICCD and the data were taken to

computer for further analysis. The dynamics of the

laser-ablated plume in an ambient atmosphere of

helium was studied by taking images of the plume at

different delay times with respect to the ablating

pulse, by the ICCD camera, using fast (5 ns or

longer) gate pulse [16]. For the film deposition

silicon and quartz substrates were kept close to

the target and the helium gas was continuously flown

through the chamber. Helium ambient was used for

initiating of the condensation of the nanoclusters in

the gas phase. The particle size distribution in the

deposited films was analyzed using atomic force

microscope (AFM). To investigate PL properties

of the Si nanoparticle films, the films were optically

pumped by third harmonic (355 nm, pulse width 5 ns

FWHM) of the Nd:YAG laser and argon ion laser.

The laser was focused on to a spot size of 3 mm2 in

the films and photoluminescence spectra were col-

lected using a fiber-coupled monochromator (Jobin

Yvon, HRS-320) [17]. The system has resolution of

0.15 nm.

3. Results and discussion

3.1. Plasma emission

The plasma emission was recorded at several dis-

tances normal and parallel to the target surface. At

distances (z < 2 mm) close to the target surface an

intense continuum emission was observed. The spec-

tra were recorded at constant gate width of 20 ns and

delay of 30 ns with respect to the ablating pulse. The

emission spectrum is attributed to both the elastic

collision of the electrons with the ions and atoms

(free–free emission) and recombination of the elec-

trons with the ions (free-bound emission). The spec-

trum recorded close to the target surface shows

continuum intensity exhibiting a maximum around

400 nm and gradual reduction in the intensity on either

side of the spectrum. Fig. 1 shows the optical emission

spectrum of silicon in vacuum, at 1 and 10 Torr helium

ambient at laser intensity of 2:45 � 109 W/cm2 at

2 mm away and parallel to the target surface. Various

observed transitions marked in figure were identified

using the standard table [18]. The lines marked are

those used for the estimation of electron and ionic

temperature. Further, emission lines from helium were

not observed under our experimental conditions of

laser fluence and ambient helium pressure. As the

plume expands further the principal constituents of the

line emissions are derived from the neutral and singly

ionized silicon. In general, the plasma emission was

predominantly due to the electronic transitions of the

neutral and singly ionized silicon. Because of the

transitory nature of laser-produced plasmas, the atomic

and ionic population present in the plume rapidly

evolved with time and position. The line intensity of

the observed transitions is higher in the presence of

ambient gas than that in vacuum, Fig. 1. In vacuum the

plasma expands freely whereas the ambient gas con-

fines it to a smaller region resulting in reduced expan-

sion rate and hence an enhanced cooling rate due to

collisions. The rate at which temperature changes

depend on the elastic collisions, electron heating due

to collisional de-excitation of metastable ions and the

recombination effects [19]. The elastic collision term

depends among various other factors on mass of the

ambient gas, lighter gases are efficient for rapid cool-

ing. The ionic recombination processes are much more

likely to occur due to the presence of low energy

384 V. Narayanan, R.K. Thareja / Applied Surface Science 222 (2004) 382–393

Fig. 1. Optical emission spectrum of Si in vacuum at 1, and 10 Torr of helium ambient recorded at 2 mm from the target surface.


electrons in the plume. The recombination can either

be due to radiative or the three-body recombination

process. The functional dependence of the recombina-

tion rate for the radiative and three-body recombination

are neni�z2T

�3=4e and n2

eni�z3T

�9=2e ln

ffiffiffiffiffiffiffiffiffiffiffiffiz2 þ 1

p, respec-

tively, where�z is the ionic charge, ne, and ni the electron

and ion density, respectively, and Te is the electron

temperature. It follows from the dependence that

radiative processes are important close to the target,

whereas the three-body recombination is a dominant

process away from the target surface. The density of

the plasma decreases on expansion but still a significant

rate of three-body recombination can exist at distances

of a few millimeters away due to simultaneous reduc-

tion of temperature of the expanding plasma. However,

the three-body recombination continues at much larger

distances when the recombination heating becomes

important. This results in a slower decrease of tem-

perature during expansion. The background gas essen-

tially provides a heat sink so that recombination

process can continue for a longer period. The excitation

of molecules of the background gas, if any reduces the

electron energies thereby increasing collisional cool-

ing. Thus the role of background gas is essentially to

increase the cooling rate of the plasma in the expanding

region and hence of increasing the three-body recom-

bination rate, resulting in increase in excited neutral

and ionic species and hence their line intensity.

3.2. Electron temperature

Assuming local thermodynamics equilibrium (LTE)

resulting from collisions in the plasma, the popula-

tions of the bound states follow the Boltzmann dis-

tribution. The relative line intensities from a particular

state can be used to calculate the electron temperature

of the plasma [20]:

lnImnlmn

Amngmn

� �¼ ln

N

U

� �� Em

kTe

� �(1)

where Imn is intensity of the observed transition line,

Amn the transition probability, lmn the transition wave-

length, gmn degeneracy of the upper level, and Em the

energy of the upper level, k the Boltzmann constant, N

the total number of states, U the partition function,

and Te the electron temperature. For the transition the

upper state is labeled as m and lower state by n. The

slope (�1/kTe) of the plot of lnðImnlmn=AmngmnÞ)

versus Em gives temperature. The spectroscopic lines

whose transition probabilities [18] and the other

parameters are known were taken from the recorded

spectra at different spatial distances from the target.

We obtain temperature of 1.96 eV at z ¼ 2 mm at a

laser intensity of 2:45 � 109 W/cm2 in 1 Torr of

helium using the transitions Si II 3d 2D�4f 2F0 at

413.09 nm, 4p 2P0�4d 2D at 505.59 nm, 4p 2P0�4d 2D at 504.1 nm, 3p2 2D�4p 2P0 at 386.26 nm,

and 3p2 2D�4p 2P0 at 385.6 nm, respectively. Fig. 2

shows the electron temperature at various distances

from the target surface. It decreases with axial distance

from the target. The inset in the figure shows the

Boltzmann plot used for the estimation of Te at

10 mm from the target.

3.3. Ionic temperature

The relative populations of excited levels of differ-

ent stages of ionization are given by the combined

Saha–Boltzmann equation [20,21]. Therefore, for the

optically thin plasma the ratios of line intensities of

successive ionization of the same element can be used

to determine the ionization temperature given by

I0

I¼ f 0g0l3

fgl03

� �ð4p3=2a3

0neÞ�1 kT

EH

� �3=2

� exp �E0 þ E1 � E � DE1kT

� �(2)

Fig. 2. Electron temperature (Te) in 1 Torr of helium ambient

determined using Eq. (1) at various distances from the target surface.

Inset: Boltzmann plot at a distance of 10 mm from the target surface.


where the prime symbol represents the line of the atom

with higher ionization stage, f is the oscillator strength,

g the statistical weight, a0 the Bohr radius, EH the

ionization energy of the hydrogen atom, E the excita-

tion energy, T the ionic temperature and DE1 the

correction to the ionization energy E1 of lower ioni-

zation stage due to plasma interactions. The correction

factor in the ionization energy is given by

DE1 ¼ 3zae2

4pe0

4pne

3

� �1=3

(3)

where za ¼ 2 for the lower ionization state. We used

the atomic lines of Si I (3p2 1S�4s 1P0) at 390.5 nm

and Si II (3d 2D�4f 2F0) at 413.09 nm in our experi-

ment. To get ionic temperature, Eq. (2) is solved by

iterative method using Mathematica software [22]. We

obtained ionic temperature of 1.07 eV at 2 mm.

3.4. Electron density

There are three likely broadening mechanisms that

can occur in the laser-ablated plume. The Doppler

broadening, resulting from motion of the atoms, the

Stark-broadening from collisions with charged spe-

cies, and the resonance broadening arising from colli-

sions between the neutral species exciting strong

resonance lines [20,21]. The high expansion velocities

and the angular spread of the plasma plumes results in

a change in the observed wavelengths for a given

emission line due to the Doppler shift created by the

different velocity components of the emitting particles

along the viewing axis. Doppler broadening is esti-

mated using the formula ðDlÞ1=2 ¼ 7:16 � 10�7l(Te/

M) where Te is the electron temperature in Kelvin and

M the atomic mass [21]. The Doppler effect causes a

broadening of 0.0078 nm for Si I transition

3p2 1S�4s 1P0 at 390.55 nm with Te ¼ 21560 K at

z ¼ 2 mm much less than the minimum observed

FWHM of 0.2 nm in our experiments. Resonance

broadening is neglected because atomic state of Si I

line is not related to the resonance state. The electrons

in the plasma can perturb the energy levels of the

individual ions that broaden the emission lines origi-

nating from these excited levels. Stark-broadening

profile of an isolated atom or singly charged ion

[20,21,23] provides the estimation of the electron

density in the plasma. We have chosen Si I transition

3p2 1S�4s 1P0 at 390.55 nm line for electron density

measurements. The Stark-broadened profiles were

recorded at various distances from the target and fitted

to the Lorentzian profile. The observed line shape was

corrected by subtracting the contribution of instru-

mental line broadening at its minimum 0.04 nm. The

true line broadening profile is given by

Dltrue ¼ Dlobserved � Dlintstrument (4)

where Dl is the full width at half maximum (FWHM).

The FWHM of the Stark-broadened lines is related to

the electron density by the expression [21]:

Dl1=2 ¼ 2Wne

1016

� �þ 3:5A

ne

1016

� �1=4

� 1 � 3

4N

�1=3D

� �W

ne

1016

� �A0 (5)

It is the sum of two (electron impact and ion-impact)

terms. W is the electron impact width parameter and A

is the ion broadening parameter. The coefficients W

and A are independent of the density and vary slowly

with temperature. ND is the number of particles in

Debye sphere at a plasma temperature and Te is given

by

ND ¼ 1:72 � 109½Te ðeVÞ3=2

½ne ðcm�3Þ1=2(6)

Using typical values of our experiment, we find par-

ticle density in Debye sphere ND ¼ 4. Using broad-

ening coefficients [20] the line width due to ionic

contribution is estimated to be 0.02 nm, much less

than the observed width in our experiments. The

contribution is almost entirely due to the electron

impact and hence the half width of Stark-broadened

transition can be estimated by the first term in Eq. (5)

viz; Dl1=2 ¼ 2Wðne=1016ÞA0. Fig. 3 shows the spatial

variation of electron density for plasma produced at

1 Torr of helium. Inset in figure shows a typical

broadened profile, FWHM of 0.34 nm, fitted with a

Lorentzian line obtained at 50 ns after the irradiating

laser pulse at 2 mm away from the target.

In order to have an estimate the number densities of

two consecutive ionization stages for locally equili-

brated plasma, we used Saha equation [20]:

nine

na¼ 2

Ui

U

2mpkTe

h2

� �3=2

exp � Ei

kTe

� �(7)


where ni, ne and na are the number density of singly

charged ions, electrons and neutral atoms, respec-

tively, Ui and U are partition functions of the ion

and the atom, and Ei is the ionization potential. For

silicon vapor Eq. (7) can be rewritten as

ni

na¼ 1:98 � 1021

neðkTeÞ3=2

exp � 8:151

kTe

� �

Using the measured values of electron temperature

and the number density, we find that the relative

concentration of Si II/Si I decreased from 70 at

2 mm to 7 at 10 mm from the target surface.

3.5. Time resolved spectroscopy

The temporal variation of the ablated plume was

studied by recording the spectra by an ICCD (Prince-

ton Inc. ICCD-576/G2, with 576 � 384 pixels) that

gives us a spectral window of 25 nm and resolution

of 0.051 nm in accordance with the dispersion of

the monochromator. The resolution was ascertained

using a neon hallow cathode lamp (Westinghouse,

USA). The spectra were recorded in ambient helium

pressure of 1 Torr at 2 mm from the target at various

delays with respect to the ablating pulse. The tempo-

ral evolution of Si I transition 3p2 1S�4s 1P0 at

390.55 nm, Si II transitions 3p2 2D�4p 2P0 and

3p2 2D�4p 2P0 at 386.26 and 385.6 nm, respectively

is shown Fig. 4. Both the electron temperature and

density measured for Si II, as mentioned earlier,

showed a rapid decrease in the initial stages and a

relatively much slower fall at later stages of expansion.

The electron temperature and density of 1.3 eV and

1:14 � 1018 cm�3, respectively, is estimated at 2 mm

at a delay of 50 ns with respect to the ablating pulse.

Since the plasma is expanding it is expected that

temperature will be less than that calculated in

Section 3.2 which corresponds to time integrated

value. Fig. 5 shows temporal evolution of Si I transition

Fig. 3. Variation of plasma electron density (ne), calculated from

the Stark-broadening, with axial distance from the target surface at

laser intensity of 2:45 � 109 W/cm2. Inset: Stark-broadened profile

for Si I transition (3p2 1S�4s 1P0) at 390.55 nm at a distance of

2 mm from the target. The dotted line represents the Lorentzian fit.

Fig. 4. Temporal evolution of Si I transition 3p2 1S�4s 1P0 at

390.55 nm, and Si II transitions 3p2 2D�4p 2P0 at 386.26 nm and

3p2 2D�4p 2P0 at 385.6 nm at 1 Torr.

Fig. 5. Temporal variation of intensity of Si I transition

3p2 1S�4s 1P0 at 390.55 nm in 1 Torr of helium.


3p2 1S�4s 1P0 at 390.5 nm. As the time progresses the

spectral width (and hence density) decreases and the

position of the peak shifts towards shorter wavelength

as shown in Fig. 5. It is attributed to the dynamical

change of density and temperature of plasma with time.

In addition, there exists a steep spatial variation in the

density and other physical parameters of the plasma due

to the nature of the expanding plasma. The Si I lines are

present for a longer delay times as compared to the Si II

lines. From Figs. 4 and 5, it follows that the ratio of life-

time in presence of helium for neutral silicon species

(450 ns) to first ionized species (150 ns) of Si I/Si II� 3

as compared to 2 reported earlier in vacuum [24].

3.6. Dynamics of the laser-ablated Si-plasma

To investigate the dynamics of Si-plume in an

ambient atmosphere we have recorded ICCD images

of the plume at 1 and 10 Torr of helium at different

delay times with respect to the ablating pulse using

fast (5 ns or longer) gate pulse which acts as a shutter

for the camera. The presence of an ambient gas slows

down the plume relative to propagation in vacuum. In

order to model the dynamics of expansion it is

assumed that the plume expands in the ambient gas

in the form of a cone at earlier times, as in vacuum,

getting spherical shape at later times. Fig. 6 shows the

variation of velocity of the plume with time at laser

intensity of 2:45 � 109 W/cm2 and helium ambient of

1 and 10 Torr. The inset shows distance (position of

plume front)–time plot in helium ambient of 10 Torr at

the same intensity. The images showed that plume is

unaffected by the background gas in the used pressure

range in the initial stages of the expansion and then

slows down progressively at later times, justifying our

assumption of conical expansion in the initial stages.

The measured velocity of the plume front V is used to

calculate Si vapor density rv, pressure Pv, and tem-

perature Tv of the plume using hydrodynamic relations

of adiabatic shock expansion and mass, density, and

momentum conservation equations given by [25]:

rv ¼ rg 1 þ 1

gv

� �(8a)

Pv ¼rgV2

1 þ gv

(8b)

Tv ¼ Pv

rvRv

(8c)

where the subscripts v and g denote vapor and gas,

respectively, gv ¼ ð1=2Þðg� 1Þ, g is the ratio of the

specific heat, R (Rv ¼ R=M) is the gas constant, and M

is the molecular weight of the vapor. Eqs. (8a) and (8b)

are derived with the conditions of Pv @ Pg. Assuming

the vapor pressure far exceeds the ambient gas pres-

sure, the maximum velocity attainable is given by

Vmax ¼ 2a=ðg� 1Þ where a is the speed of the sound

(a ¼ ðgkTs=mÞ1=2). Vmax is used to estimate the surface

temperature Ts of the target, g is the specific heat, m the

mass of the solid and k the Boltzmann constant.

Fig. 7 shows the temporal variation of vapor pres-

sure with ambient helium gas pressures. To estimate

temporal variation of the vapor pressure Pv, by using

Eq. (8b), we used the velocity of the expanding plume

front at different delay times with respect to the

ablating pulse from Fig. 6. The expansion of plasma

in ambient gas environment can be explained on the

basis of blast wave and drag models [26]. The shock

model seems to hold for higher pressures but it fails to

explain the deceleration of the expanding plume. The

deceleration can be explained using the drag model; a

resistive force due to collisions with the background

gas slows the plume eventually stopping it at later

times. The force is proportional to the expansion

velocity and is given by �bV, where b slowing co-

efficient is defined by V0/zmax. V0 is the initial velocity

and zmax is the distance where plume pressure equals

Fig. 6. Variation of velocity of the plume front with time in 1 (*)

and 10 Torr (&) of helium ambient. Inset shows distance–time plot

in 10 Torr ambient.


the ambient gas pressure. The plume front at time

t follows the relation of the type z ¼ zmaxð1 � exp

ð�btÞÞ. We obtain initial velocity equal to 1:3 � 106

and 1:84 � 106 cm/s for 10 and 1 Torr, respectively,

from the fit of z � t plot. The stopping distance

decreases with increase in pressure from 1.05 cm at

1 Torr to 0.65 cm at 10 Torr. On increasing the ambient

gas pressure, the backward pressure on the plume

increases and as a result its velocity decreases. The

variation of vapor temperature with time is shown in

Fig. 8. The vapor temperature Tv was obtained by using

Eq. (8c) by substituting the values of vapor pressure Pv

from Fig. 7. Our measured plasma electron density,

Fig. 3, of 1:2 � 1018 cm�3 obtained at 2 mm from the

target is much higher than that of the ambient helium

density 3:2 � 1016 cm�3 in 1 Torr. This implies that

near the target the plume is hardly affected by the

presence of the ambient gas and its expansion is similar

to the free expansion in vacuum at the pressure range

used in this work. With the increase in axial distance

from the target surface the plasma density decreases as

ne ¼ n0ðtÞð1 � z=zðtÞÞ, where n0 is the density at the

center of the laser irradiated spot (z ¼ 0) at time t, the z

coordinate is directed perpendicular to the target and

zðtÞ refer to the position of the leading edge of the

plasma. At larger distances the density of the plasma

falls sharply.

3.7. Local thermodynamic equilibrium

To assure the validity of LTE approximation, colli-

sions must dominate the energy-transfer processes and

establish a Boltzmann distribution among the bound

energy levels, the electron density, ne must satisfy the

following condition [21]:

ne � 1:6 � 1012½Te ðKÞ1=2½DE3 (9)

The termDE is the energy difference between the upper

and lower states, Te (K) the electron temperature, ne the

lower limit for the electron number density. The esti-

mated value of the lower limit of ne ¼ 4:8 � 1016 cm�3

is obtained by using DE ¼ 3 eV for the transition at

413.09 nm while the Stark-broadened profile gives

a value of ne ¼ 1:2 � 1018 cm�3, two orders of mag-

nitude higher that the lower limit justifies our use of

LTE.

Since photon energy, hn ¼ 3:5 eV of the ablating

radiation is much less than the first ionization potential

of Si (8.151 eV), the photoionization via three steps

might be a dominant ionization mechanism. Once the

plasma is initiated in the hot vapor, the absorption of

laser radiation generally occurs via electron–neutral

inverse bremsstrahlung and photoionization of the

excited state atoms, for the UV radiation. However,

when the sufficient ionization stage is reached (>1%),

the dominant laser absorption mechanism makes a

transition to electron–ion inverse bremsstrahlung [20].

It involves the absorption of a photon by a free electron

Fig. 7. Temporal variation of vapor pressure (Pv) of the plume in 1

(5) and 10 Torr (&) of helium ambient.

Fig. 8. Temporal variation of vapor temperature (Tv) of plume in 1

(*) and 10 Torr (&) of helium ambient.


during electron–ion collisions. The absorption coeffi-

cient is given by [14]:

ap ðcm�1Þ ¼ ð3:69 � 108Þ z3ffiffiffiffiT

p n2e

n3

� 1 � exp � hnKT

� �� (10)

where Z, ne and T are the atomic number, electron

density in cm�3, and electron temperature, respec-

tively. Since hn > KT , the term f1 � expð�hn=KTÞgcan be approximated to unity. The absorption coeffi-

cient of ap ¼ 8:8 � 10�2 cm�1 is obtained for the

electron–ion inverse bremsstrahlung.

Absorption via photoionization can be estimated

from [25]:

api ðcm�1Þ ¼X

n

7:9 � 1018 En

hn

� �3I

En

� �1=2

Nn (10)

where En and Nn are ionization energy and number

density of the excited state ‘n’, respectively, and h is

the Planck constant, n the laser frequency and I the

ionization potential of the ground state atom. The

absorption coefficient of the photoionization is

obtained by summing up all, the excited states whose

ionization energy is smaller than laser photon energy.

The excitation potentials of atomic transitions of

silicon atoms involved in the temperature calculations

are higher than the laser photon energy (3.5 eV) used

in this experiment, therefore the direct photoionization

will not contribute to the absorption. Hence the pos-

sible mechanism for photoionization may be simulta-

neous absorption of more than one photon.

3.8. Film deposition and photoluminescence

Pulsed laser ablation technique was used to deposit

nanoparticles on the substrates (silicon and quartz)

positioned at a distance of 1.5–3 cm from the target

surface and on substrates placed just close to the target

(4 mm parallel/below the target) surface at varying

laser intensity on the target. The size of nanocrystal-

lites was ascertained using atomic force microscopy.

The AFM images showed the clusters fairly spherical

in shape. Since the lateral dimensions of the clusters

were usually enlarged due to tip-object convolution

effect, the cluster size was determined by measuring

the height of the clusters from AFM images. The

size of the clusters varied from 4.4 to 1.76 nm as

the laser intensity decreased from 2:45 � 109 to

1:16 � 109 W/cm2. The optical properties of the nano-

particle were studied using photoluminescence of

crystallites excited by 457 nm of argon ion laser

and 355 nm radiation. The PL spectra of the samples

prepared using varying laser intensity conditions at

1 Torr of He and in vacuum (10�5 Torr) showed three

distinct emission bands at 2.7, 2.2, 1.69 eV. It is

generally accepted that the physical mechanism

underlying the visible and near infrared light emission

in silicon materials is essentially that of the quantum

confinement of carriers in a nanometer-scale crystal-

line structure, and the Si/SiO2 interface is also thought

to play an active role in both the formation of radiative

states and the passivation of nonradiative states

[27,28]. Therefore, it is essential to have a stoichio-

metric SiO2 matrix and perfect Si–SiO2 interfaces.

However, at least in the vicinity of Si crystals the oxide

matrix is not completely stoichiometric. The latter are

actually surrounded by some substoichiometric SiOx

(x < 2) transition layers with, possibly, changing x

[29]. The existence of such transition layers degrades

the PL efficiency. It may cause many carriers to

recombine via the nonradiative rather than radiative

centers and impedes to a certain extent the quantum

confinement. The presence of oxygen induces the

passivation of the silicon nanoparticle surface and

hence reduces the role of nonradiative processes and

as a consequence increases the luminescence intensity.

The oxidation leads to increase in the amount of silicon

oxide in the films resulting in the appearance of defect

bands. These bands are mainly in the region of violet–

ultra violet spectral region. Fig. 9 shows the PL spec-

trum from a sample deposited on silicon substrate at

Fig. 9. Photoluminescence spectra of a deposited nc-Si film

deposited under 1 Torr of helium and excited by 355 nm.


laser intensity of 2:45 � 109 W/cm2 under 1 Torr of

helium when pumped with 355 nm radiation. The PL

spectrum at 2.2 eV was observed for films deposited on

quartz substrate kept at a distance 15 mm from the

target in vacuum when pumped with 457.9 nm of Arþ

ion laser (not shown in the figure). The appearance of

luminescence band at 2.7 eV (460 nm) is attributed to

point defects in SiO2 such as neutral oxygen vacancies

in silica glass [27,28]. The band at 1.69 eV (735.7 nm)

is from a film deposited on silicon substrate placed

close to the target surface (4 mm parallel/below the

target). The band originates [30] from the Si nanocrys-

tallites covered with amorphous oxide layers and the

light emission is caused by exciton confinement in the

interfacial region between the silicon oxide and the Si

nanocrystalline core. We did not observe PL in the red

region of the spectrum. Our results are similar to that of

Patrone et al. [31] in agreement with the effect of

quantum confinement of electrons and holes in nano-

sized silicon clusters. Contrary to our experiment they

used pulsed ArF� (l ¼ 193 nm, pulse width 15 ns,

repetition rate of 3 Hz), a UV laser in helium ambient

of 4 Torr for deposition on highly oriented pyrolitic

graphite and a broad band high pressure Xe lamp for

PL studies.

4. Conclusions

We have characterized the laser-ablated Si plasma

in vacuum, and in the presence of helium ambient at 1

and 10 Torr, respectively. OES of the ablated silicon

showed the presence of mostly Si I and Si II. The

relative concentration of Si II/Si I decrease away from

the target surface. The optimization of neutral con-

centration found at large distances from the target may

be helpful in the formation of nanocrystalline silicon.

The electron density and temperature were found to be

1:2 � 1018 cm�3 and 2 eV in 1 Torr of helium at 2 mm

from the target surface. The spatial dependence

showed the decrease in temperature and density with

increase in distance. The temporal variation of Si I

(3p2 1S–4s 1P0) at 390.55 nm showed a shift in peak

position attributed to collisions at an early stage of

plasma formation. In earlier reports on laser ablation

of silicon [13,14] second and fourth harmonic of

Nd:YAG (266 and 532 nm, FWHM of 3 ns) were used

at atmospheric pressure. It is worthwhile to state the

differences between our and their experiment. The

ambient environment being atmospheric air an absorp-

tion layer of high electron–ion density [32] may be

formed above the target surface resulting in a high

plasma pressure during the laser pulse. The high

plasma pressure results in lateral plasma compression

wave leading to change in energy distribution on the

target. Laser irradiated spots have shown the appear-

ance of highly porous layer consisting of holes and

cracks under the irradiated spot on the silicon surface

[33]. Since the plasma is confined to small spatial

dimensions three-body recombination is predominant

and the higher ionic states of Si are not observed, even

the Si I transitions were observed up to 2 mm only,

though the recombination heating increases the dura-

tion of emission. However, we observed Si I transitions

as far as 22 mm from the target surface with the relative

concentration of Si II/Si I being 70 at 2 mm in 1 Torr of

helium compared to 6 in atmospheric pressure. The Si I

species was observed at late times compared to Si II

species. The observed spectrum in Fig. 3 shows the

formation of Si I after 50 ns and lasting till 450 ns

whereas Si II has maximum intensity at 50 ns which

disappears after 150 ns implying that Si I originates

from recombination of Si II. This suggest that the

nucleation of cluster starts with Si I, which, after

condensation, gets deposited onto the suitably placed

substrate. The images of the expanding plume reveal

that the stopping distance decreases with increase in

ambient pressure from 1.05 cm at 1 Torr to 0.65 cm at

10 Torr and initial velocity increases from 1:3 � 106 to

1:84 � 106 cm/s as the pressure decreases from 10 to

1 Torr. The PL spectra of the samples prepared using

laser intensity of 2:45 � 109 W/cm2 at 1 Torr of helium

showed three distinct emission bands at 2.7, 2.2,

1.69 eV. The surface morphology and the size distribu-

tion of cluster studied using AFM revealed the depen-

dence of size of clusters on incident laser radiation. The

size of the clusters decreases with the decrease in

intensity. Thus, in principle it should be possible to

get desired cluster size by varying the incident intensity

at a given ambient pressure.

Acknowledgements

Partial support by Department of Science and Tech-

nology (New Delhi) for this work is acknowledged.


References

[1] D.B. Chrisey, G.K. Hubler, Pulsed Laser Deposition of Thin

Films, Wiley, New York, 1994.

[2] J.C. Miller, D.B. Geohegan (Eds.), in: AIP Conference

Proceedings on Laser Ablation: Mechanisms and Application,

vol. II, AIP, New York, 1994.

[3] T. Yoshida, S. Takeyama, Y. Yamada, M. Mutoh, Appl. Phys.

Lett. 68 (1996) 1772.

[4] E. Narumi, J. Lee, C. Li, S. Horokawa, S. Patel, D.T. Shaw,

Appl. Phys. Lett. 59 (1991) 3180.

[5] D.H. Lowndes, D.B. Geohegan, A.A. Puretzky, D.P. Norton,

Science 273 (1996) 898.

[6] D.B. Geohegan, A.A. Puretzky, G. Duscher, S.J. Pennycook,

Appl. Phys. Lett. 72 (1998) 2987.

[7] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046.

[8] H. Takagi, U. Ogawa, Y. Yamazaki, A. Ishizaki, T. Nakagiri,

Appl. Phys. Lett. 56 (1990) 2379.

[9] T. Yoshida, Y. Yamada, T. Orii, J. Appl. Phys. 83 (1998) 5427.

[10] S. Sato, H. Ono, S. Nozaki, H. Morisaki, Nanostruct. Mater. 5

(1995) 589.

[11] T. Makimura, Y. Kunii, N. Ono, K. Murakami, Jpn. J. Appl.

Phys. 35 (1996) L1703.

[12] D. Bauerle, Laser Processing and Chemistry, 3rd ed.,

Springer, New York, 2000.

[13] M. Milan, J.J. Laserna, Spectrochim. Acta B 56 (2001) 275.

[14] H.C. Liu, X.L. Mao, J.H. Yoo, R.E. Russo, Spectrochim. Acta

B 54 (1999) 1607.

[15] A.K. Sharma, R.K. Thareja, J. Appl. Phys. 88 (2000) 7334.

[16] A. Neogi, A. Misra, R.K. Thareja, J. Appl. Phys. 83 (1998)

2831.

[17] A. Mitra, R.K. Thareja, J. Appl. Phys. 89 (2001) 2025.

[18] W.L. Wiese, G.A. Martin, Wavelengths and Transition

Probabilities for Atoms and Atomic Transitions NSRDS,

NBS 68, Department of Commerce, US Government Printing

Office, Washington, DC, 1980.

[19] A. Khare, R.K. Thareja, IEEE J. Quant. Electr. 24 (1988)

2525.

[20] H.R. Griem, Plasma Spectroscopy, Cambridge University

Press, Cambridge, 1964.

[21] G. Bekefi, Principles of Laser Plasmas, Wiley–Interscience,

New York, 1976.

[22] Mathematica version 3.0 (Silicon Graphics 3.0 (September

16, 1997)), Release Number: 2, License ID: L2786-0318.

[23] A. Neogi, R.K. Thareja, Phys. Plasm. 6 (1999) 365.

[24] M. Hanabusha, S. Moriyama, H. Kikuchi, Thin Solid Films

107 (1983) 227.

[25] J.J. Chang, B.E. Warner, Appl. Phys. Lett. 69 (1996) 473.

[26] Y. Zel’dovich, Y. Raizer, Physics of Shock Waves and High-

Temperature Hydrodynamics, Academic Press, New York,

1966.

[27] S. Tong, X. Liu, T. Gao, X. Bao, Appl. Phys. Lett. 71 (1997)

698.

[28] R. Tohmon, Y. Shimogaichi, H. Mizuno, Y. Ohki, K.

Nagasawa, Y. Hama, Phys. Rev. Lett. 62 (1989) 1388.

[29] T. Makimura, Y. Kunii, K. Murakami, Jpn. J. Appl. Phys. 35

(1996) 4780.

[30] Y. Kanemitsu, T. Ogawa, K. Shiraishi, K. Tadeka, Phys. Rev.

B 48 (1993) 4883.

[31] L. Patrone, D. Nelson, V.I. Safarov, M. Sentis, W. Marine, S.

Giorgio, J. Appl. Phys. 87 (2000) 3829.

[32] J.R. Ho, C.P. Grigoropoulos, J.A.C. Humphrey, J. Appl. Phys.

79 (1996) 156.

[33] A.V. Kabashin, M. Neunier, Appl. Phys. Lett. 82 (2003) 1619.


emission spectroscopy of laser-ablated si plasma related to nanoparticle formation

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