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An alternative approach to understand the photoluminescence and thephotoluminescence peak shift with excitation in porous siliconJ. Anto Pradeep and Pratima AgarwalCitation: J. Appl. Phys. 104, 123515 (2008); doi: 10.1063/1.3043626View online: http://dx.doi.org/10.1063/1.3043626View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v104/i12Published by theAmerican Institute of Physics.Related ArticlesLuminescent LaF3:Ce-doped organically modified nanoporous silica xerogelsJ. Appl. Phys. 113, 013111 (2013)Temperature dependent photoluminescence from porous silicon nanostructures: Quantum confinement and

oxide related transitionsJ. Appl. Phys. 110, 094309 (2011)Micropipe absorption mechanism of pore growth at foreign polytype boundaries in SiC crystalsJ. Appl. Phys. 106, 123515 (2009)Photo-oxidation effects of light-emitting porous SiJ. Appl. Phys. 105, 113518 (2009)Vapor-phase silanization of oxidized porous silicon for stabilizing composition and photoluminescenceJ. Appl. Phys. 105, 114307 (2009)Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/Journal Information: http://jap.aip.org/about/about_the_journalTop downloads: http://jap.aip.org/features/most_downloadedInformation for Authors: http://jap.aip.org/authors

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An alternative approach to understand the photoluminescence and thephotoluminescence peak shift with excitation in porous silicon

J. Anto Pradeep and Pratima Agarwala

Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781 039, India

Received 16 July 2008; accepted 3 November 2008; published online 17 December 2008

There have been many different models proposed for the luminescence in porous silicon PS , yet

it is believed that the quantum confinement effect persists at the absorption. However, from ourinvestigation on both constant and pulsed electrochemically etched silicon PS , the absence of

quantum confinement effect at the absorption has been identified from the close correspondence of

photoluminescence excitation PLE spectra of PS to the simulated absorption spectrum of an

ultrathin silicon film with the bulk optical constants. In the simulation of absorption spectrum, the

spectral dependence of reflectivity of the solid, which had been omitted in the traditional analysis of

PLE, is considered. Further, although nanocrystallites of silicon are present in the PS matrix, the

absence of quantum confinement is explained on the basis of structural characteristics of PS.

Following that, many common observations in the luminescence of PS are attributed to the surface

states. The blueshift of the PL peak with the increase in excitation energy is explained with the idea

of quasithermal equilibrium and the probability of occupation of the carriers at the surface states.

2008 American Institute of Physics. DOI: 10.1063/1.3043626

I. INTRODUCTION

Although porous Si PS is known since 1956,1

the dis-

covery of efficient photoluminescence PL at room tempera-

ture by 1990 Ref. 2 has stimulated the interest of many

researchers on this new form of Si. Apart from the techno-

logical interest,3,4

many extensive studies have been devoted

from the basic research viewpoint for the understanding of

the mechanism of PL. Consequently, many models have been

suggested for the explanation of the phenomenon.5

Still it is

not clear what could be the actual mechanism causing the

strong visible luminescence at room temperature. This ques-

tion is yet a topic of intense debate.Canham,

2upon observation, was the first to suggest the

quantum confinement model QCM to the visible lumines-

cence at room temperature. Independently, Lehmann and

Gsele6

ascribed the upshift of fundamental absorption edge

of Si in PS to the effect of quantum confinement. This was

the very first convincing experimental evidence for the

QCM. Following that, many theoretical calculations have

been reported79

based on the QCM that could exactly re-

trace the experimental PL spectrum. However, the QCM

model, other than explaining the upshift of fundamental ab-

sorption edge of Si, is not found useful in understanding any

other experimental observation in PS.1014

In fact, in the light

of QCM, the experimental observation such as the tempera-

ture dependence of PL is found anomalous.10,11

Subsequently

many other models have been proposed. The other proposed

models suggest the visible luminescence to the presence of

siloxene,15

hydride species,16

or hydrogenated amorphous

Si.17,18

Of these, siloxene model gained appreciation due to

the popularity of the visible luminescence in the material

since 1922.19

However, only recently,20

it has been found

that the conception of the visible luminescence in siloxene is

the chemical quantum confinement due to oxygen in a Si

matrix. So the siloxene model for the luminescence in PS, in

fact, adds support to the QCM. The other models, although,

have no clear correspondence to the QCM, are supported

only by certain specific observations.18,21

In the current article, we present our extensive studies on

PL excitation PLE spectroscopy along with PL for the un-

derstanding of the luminescence mechanism in PS. Although

primarily PLE is an alternative to the absorption spectros-

copy on opaque samples, an absorption spectrum is simu-

lated for scrutiny. The simulation is done for an ultrathin Si

film with the bulk optical constants. Under the frame of ex-isting models, the simulated spectrum is expected to bear no

similarity to the PLE of PS. However interestingly, the spec-

tra have shown very similar features. This intriguing obser-

vation is explained on the basis of structural characteristics

of PS. Further, the blueshift of PL peak energy with the

increase in excitation energy is understood from the quasith-

ermal equilibrium and the probability of occupation of the

carriers at the surface states.

II. EXPERIMENTAL SECTION

A. Sample preparation

PS is prepared by both pulsed22

and constant23

current

electrochemical etching of boron-doped p-type Si of 100

orientation, with a resistivity of 110 cm at the anode,

under normal room lighting condition. A thick layer of alu-

minum is deposited on the textured surface of the c-Si

wafer and annealed for 30 min at 450 C under vacuum

6106 mbar for good Ohmic contact. The anodization of

all samples reported in this work is carried out in an electro-

lyte of HF 48% aqueous and ethanol with 1:1 by volume on

a vertical Teflon anodization cell. The constant anodization isa Electronic mail: [email protected], [email protected].

JOURNAL OF APPLIED PHYSICS 104, 123515 2008

0021-8979/2008/104 12 /123515/8/$23.00 2008 American Institute of Physics104, 123515-1

http://dx.doi.org/10.1063/1.3043626http://dx.doi.org/10.1063/1.3043626http://dx.doi.org/10.1063/1.3043626http://dx.doi.org/10.1063/1.3043626http://dx.doi.org/10.1063/1.3043626http://dx.doi.org/10.1063/1.3043626


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performed at 50 and 80 mA/ cm2 current density for 15 and

5 min, respectively, whereas, the pulsed anodization is per-

formed at a frequency of 20 Hz and 60% duty cycle at

80 mA / cm2 current density with the total duration of the

active cycle in the pulsed anodization as 5 min. The anod-

ization cell is set to a mild vibration during the process so as

to facilitate the bubble removal at the pores and also to avoid

the accumulation of the bubbles at the cathode. The PS thus

obtained is rinsed with de-ionized water instantly.

B. Optical measurements

All the measurements on PS have been performed with

the c-Si substrate attached at its back not a free standing

film on both as prepared and stored in ambient conditions

for a couple of months. PL and PLE spectra are recorded on

AMINCO-Bowman Series 2 Luminescence spectrometer

with 150 W Xenon arc lamp followed by a monochromator

as the excitation source, under identical conditions. In the

present investigation, a PL spectrum is the one that has been

recorded at fixed excitation energy with the spectrum as a

function of the emission photon energy and the PLE spec-

trum is the one that has been recorded at fixed emission

photon energy with the spectrum as a function of the excita-

tion energy. The Fourier transform infrared FTIR transmis-

sion spectra are recorded with a resolution of 4 cm1 on

PerkinElmer Spectrum BX in order to monitor the change in

the surface constituents of PS with aging. Spectroscopic el-

lipsometry UVSEL, Jobin Yvon Horiba is performed on the

polished surface of the c-Si substrate at 70 fixed angle of

incident for the optical constants measurement.

III. RESULTS AND DISCUSSIONS

A. Luminescence data

Figure 1 shows the PL spectra of PS prepared under

different anodization conditions at various excitations from

2.92 to 4.13 eV. The dependent parameters of a PL spectrum,

provided the spectral response of the instrument is a con-

stant, can be given as24

(a) (b)

1.4 1.6 1.8 2.0 2.2 2.4 2.6

0.0

0.4

0.8

1.2

1.6

2.0As-prepared (Pulse)

NormalizedPLSignal

Emission Photon Energy (eV)

3.82 eV

3.54 eV

3.31 eV

3.10 eV2.92 eV

Excitation energy:

1.4 1.6 1.8 2.0 2.2 2.4 2.6

0

1

2

3

4

5

6

7

Aged (Pulse)

NormalizedPLSignal


4.13 eV

3.82 eV

3.54 eV

3.31 eV

3.10 eV

2.92 eV

Excitation energy:

(c) (d)

1.4 1.6 1.8 2.0 2.2 2.4 2.6

0

1

2

3

4

5

6

7

Aged (Constant) Excitation energy:

NormalizedPLSig

nal


3.94 eV3.54 eV

3.31 eV3.10 eV2.92 eV

1.4 1.6 1.8 2.0 2.2 2.4 2.6

0

1

2

3

4

5

6

Aged (Constant) Excitation energy:

NormalizedPLSig

nal


3.82 eV3.54 eV

3.31 eV3.10 eV2.92 eV

FIG. 1. PL spectra of PS at various excitation energies normalized with the excitation photon flux prepared by pulsed current electrochemical etching with

80 mA/cm2 current density for 5 min: a as prepared and b stored in ambient condition for 5 months from preparation. Also shown are the PL spectra of

PS prepared using constant current density and stored in ambient for 10 months: c 50 mA/cm2 current density for 15 min and d 80 mA/cm2 current

density for 5 min.

123515-2 J. A. Pradeep and P. Agarwal J. Appl. Phys. 104, 123515 2008



4/9

IEm , A PES , PEm IEx , 1

where IEm, A, PES, PEm, and IEx are the emission intensity,

the exciton creation absorption probability, the probability

that the exciton traps at the emission state, the probability

that the exciton in the emission state returns back to the

ground state by the emission of a photon of , and the

excitation intensity, respectively. For a given PL spectrum,

frequency of excitation is fixed and IEm is only a functionof frequency of emission .

Each spectrum in Fig. 1 has been normalized with the

excitation photon flux. The spectra are found to be broad and

encompass approximately 1 eV. A small peak at 1.55 eV is

identified to be the spectral response of the instrument,

which has not been corrected, and is believed not to change

the spectrum as a whole. It is observed in Fig. 1 that the PL

efficiency is maximum neither at lower nor at higher excita-

tion energies but at some intermediate energies. However,

the excitation energy at which the PL efficiency is greater

could be found with more certainty only in PLE. In addition

to the change in PL efficiencies, the normalized PL peak

shows blueshift with the increase in excitation energy. Fig-

ures 1 a and 1 b show the PL spectra at various excitation

energies of as-prepared and aged PS, respectively, prepared

by the pulsed current electrochemical etching. It is found

from these spectra that apart from the increase in PL effi-

ciency, the PL peak at every excitation shows a blueshift

with aging. Figures 1 c and 1 d show the PL spectra at

various excitation energies of aged PS prepared by the con-

stant current electrochemical etching, under different current

densities to different lengths of time. From these spectra it is

found that although there is a slight redshift in the PL peak

position, the PL efficiency increases with the current density.

These PL features are very similar to those observed by

others.2527

Since the PL from PS is a broad spectrum, it is possible

to record the PLE spectra at various emission energies with-

out much overlap. The PLE spectra of PS prepared under

different anodization conditions at various emission energies

from 1.65 to 2.25 eV are shown in Fig. 2. Like the PL spec-

tra, the PLE spectra of PS prepared under different anodiza-

(a) (b)

2.0 2.4 2.8 3.2 3.6 4.0

0.0

0.4

0.8

1.2

1.6

2.0

2.4As-prepared

(Pulse)

Norm

alizedPLE

Signal

Excitation Photon Energy (eV)

2.25 eV

2.06 eV

1.91 eV

1.77 eV

1.65 eV

Emission energy:

2.0 2.4 2.8 3.2 3.6 4.0

0

1

2

3

4

5

6

7

Aged (Pulse)

Norm

alizedPLE

Signal


2.25 eV2.06 eV1.91 eV1.77 eV1.65 eV

Emission energy:

(c) (d)

2.0 2.4 2.8 3.2 3.6 4.0

0

1

2

3

4

5

6Aged (Constant)Emission energy:

NormalizedPLES

ignal


2.25 eV2.06 eV

1.91 eV1.77 eV1.65 eV

2.0 2.4 2.8 3.2 3.6 4.0

0

1

2

3

4

5

6

7

Aged

(Constant)

Emission energy:

NormalizedPLES

ignal


2.25 eV

2.06 eV

1.91 eV1.77 eV

1.65 eV

FIG. 2. PLE spectra of PS at various emission energies normalized with the excitation photon flux prepared by pulsed current electrochemical etching with80 mA/cm2 current density for 5 min: a as prepared and b stored in ambient condition for 5 months from preparation. Also shown are the PL spectra of

PS prepared using constant current density and stored in ambient for 10 months: c 50 mA/cm2 current density for 15 min and d 80 mA/cm2 current

density for 5 min.




5/9

tion conditions have shown remarkably similar spectral fea-

tures. The dependent parameters for a PLE spectrum are the

very same as given by Eq. 1 . The only difference being that

the spectrum is a function of rather than . Each spec-trum in Fig. 2 has been normalized with the excitation pho-

ton flux. Every PLE spectrum shows a peak at 3.4 eV. The

significance of the peak energy in PLE would be discussed

later with its correspondence to the absorption spectrum.

Above 3.4 eV, the spectra show negative slopes. However,

the negative slope increases as we increase .

It would seem at first sight that each PLE spectrum is

very distinct. However, it should be remembered that the

spectra are not normalized to the same value of peak inten-

sity. If they are so normalized, though they are recorded at

different emission energies, every PLE spectrum would show

similar features. This assures that there could be only one

underlying mechanism for the broad PL from PS.

B. Simulation of optical absorption

Since the PLE spectrum is related but not identical to an

absorption spectrum, for the complete analysis of PLE, the

absorption of PS has to be measured or found by some other

means. Here, we have simulated the absorption spectrum as

shown in Fig. 3 for a Si film of 3 nm thickness with the bulk

optical constants obtained from the spectroscopy ellipsom-

etry data of a c-Si substrate. In the calculations of opticalconstants, although no corrections have been made for the

possible presence of oxides on the crystal surface, the results

are identical in their functional form as reported in the

literature.28,29

The dispersion of real and imaginary part of

the refractive index of bulk silicon is shown in Fig. 4 for

quick reference.

For the simulation of absorption A spectrum, the fol-

lowing equations have been used:

A = 1 R 1 ed , 2

where

R = n 1 2 + k2

n + 1 2 + k2and = 4k/.

Here, n and k are the real and imaginary parts of a refractive

index, respectively, R is the reflectivity, d is the thickness of

the film, and is the wavelength of light.

Although only the normal incidence of light is consid-

ered in the simulation, the spectral features are not changed

even when it is averaged over all oblique angles of incidence

say, 0 to / 2 . So Eq. 2 suffices in its simplest form for

the simulation of the absorption spectrum. However, in the

literature, for the analysis of PLE spectrum, the absorption is

taken to be 1 ed .30,31

The 1 R factor is generally

omitted. It is evident from Eq. 2 that it is possible to omit

the 1 R factor only if R =0 or at least spectrally indepen-dent of the incident light. In the case of Si, at least in the

region of our interest, R is very much spectral dependent. So,

in general, Eq. 2 must be used for absorption in the analysis

of PLE spectrum for any solid.

C. Comparison of simulated optical absorption withPLE

Although the absorption spectrum has been simulated

with the bulk optical constants of Si, it has shown very simi-

lar features as the measured PLE spectrum of PS below 3.4eV. It must be noted that the similarity between the spectra

is observed only on the absorption spectrum simulated with

thickness 5 nm; greater than this value, the absorption

spectrum shows a peak centered at 3.7 eV where the disper-

sion of k has a minimum as shown in Fig. 4. The 3.4 eV is

the least of all direct band gaps of bulk Si and lies at the L

point of the Brillouin zone. The electronic transition at the L

point is generally denoted as the E1 transition.32

It is from the

close correspondence between the simulated absorption

spectrum and the measured PLE spectrum that we would

remark that the physical quantum confinement is less signifi-

cant on our samples. Moreover, if there is a significant quan-

2.0 2.4 2.8 3.2 3.6 4.0

0

5

10

15

20

25

Absorption%

Photon Energy (eV)

FIG. 3. Theoretical simulation of normal incidence absorption spectrum for

a Si film of 3 nm thickness with the bulk optical constants of Si.

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

0

1

2

3

4

5

Refractive

Index

Energy (eV)

n

k

FIG. 4. The dispersion of real and imaginary part of refractive index ob-

tained from the spectroscopic ellipsometry data of the c-Si substrate. In the

calculation, no corrections have been made for the possible presence of

oxides on the crystal surface.




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tum confinement effect, the L point absorption in PLE spec-

tra would have been shifted to much higher energies than

what is observed.

D. Structure of PS

For the understanding of these intriguing results, know-

ing the structural characteristics of PS is vital. Here, a brief

description on the PS structure is given for clarification. The

PS under investigation can be described as a single crystal-line Si with labyrinth of permeating pores, or alternatively,

islands of Si interconnected through Si channels. The shape

of islands/channels is very random due to the labyrinthine

pores. However, their physical size depends on the doping

concentration.33

Further, it is believed, since the PS has been

prepared from a prime grade Si wafer, the core of islands/

channels is defect free. Nevertheless, due to the labyrinthine

pores, the surfaces are very defective. From the vibrational

spectroscopic studies, it has been found that the PS not only

contains Si with incorporated voids but also contains some

forms of hydrides and oxides in the Si matrix.34

The possible

site for the hydrides/oxides to be found in PS is at the surface

of Si islands/channels. As a result, in PS, the Si dangling

bond density is reduced due to the passivation of the dan-

gling bonds by oxygen/hydrogen.

E. Understanding the experimental data

Henceforth, we would stick to this picture of PS, and in

the following, our observations on PS are explained.

1. Loss of quantum confinement due to collision

In Si, the excitons are MottWannier type35,36

the big

radius exciton . The estimated excitonic effective Bohr ra-

dius of Si is around 5.25 nm.37

Transmission electron mi-

croscopy TEM measurements on PS revealed that the crys-

tallites are of few nanometers across. In the light of QCM,

this would fall under the category of strong confinement.38,39

However, for the applicability of the model, the nanostruc-

ture must be extremely defect free so that the excitonic co-

herency would not be lost within a short length span. In fact,

as already pointed out, the islands/channels in PS are perfect

at the core, so the excitons seldom collide as they move

within the core before they have any interaction with the

surface/boundary. The interaction with the surface often re-

sults in inelastic collision due to the different environments

both structurally and chemically at the surface. In addition,

the excitons after their first encounter with the surface alsoexperience collisions among themselves under the electro-

magnetic field. As a result, the excitonic coherencies are lost,

and consequently, the confinement effects are also lost alto-

gether. Hence the quantum confinement effects have not

been observed on our PS sample.

2. PS with the bulk Si absorption properties

Although TEM measurements have revealed the pres-

ence of nanocrystallites in PS, from the analysis of PLE, it is

observed that optical absorption properties of PS have not

changed from the bulk. This could presumably be due to the

interconnecting channels present in the PS. At this point it

must be noted that no isolated crystallite could ever be a part

of PS. Henceforth, the crystallite would be referred to as

island in this paragraph. Every island is interconnected

through Si channels, as aforementioned. If an island is con-

nected to more channels or the channels by themselves have

many connections, the atomic wave functions of Si atoms in

an island could have overlaps with the Si atoms in the chan-

nels. Due to these interconnections and the atomic wave

functions overlaps, the absorption in PS has shown very

similar features to that of the bulk Si.

3. Broad PL spectrum of PS

The PL in PS is a broad spectrum and encompasses ap-

proximately 1 eV as can be inferred from Fig. 1. This broad

feature is ascribed to the surface states formed by the pres-

ence of oxides and hydrides in PS. The FTIR spectrum of PSis shown in Fig. 5 for reference. The IR absorption of the

surface constituents of PS is found to be broad due to the

varying bond lengths or the different chemical environment

at the surface of PS. Consequently, the electronic density of

states at the surface is also broadened and that has been

reflected in the PL spectrum of PS.

4. Excitation energy dependent peak shift

In Eq. 1 , of the dependent parameters of PL intensity,

the PES is a function of both and . It is due to the fact

that PES , is itself proportional to the product of the

available density of states and the probability of occupationof carriers at those states. Since the available density of

states, in the case of PS, is the surface density of states and

the carriers occupy those states from the conduction band,

the probability of occupation of carriers follows the Boltz-

mann distribution. In the PL study of PS, the excitation en-

ergies are so chosen that they are close to the E1 transition of

Si, which causes the creation of excess carriers in the con-

fined region of Si island/channel. As a result, the system

would lie in a quasithermal equilibrium,40

where the carriers

are in thermal equilibrium among themselves but not with

the lattice. The carriers temperature i.e., the kinetic energy

of the carriers is due to the excitation energy. The higher the

500 1000 1500 2000 2500 3000 3500 4000 4500

0

4

8

12

16

20

4

Transmission%

Wavenumber (cm-1

)

As-prepared (Pulse)

FIG. 5. Regular normal incidence FTIR transmission spectrum of as-

prepared PS by pulsed current electrochemical etching with 80 mA/cm2

current density for 5 min.




7/9

excitation energy, the higher is the carriers temperature, and

thereby, the occupation probability of higher energetic statesis favored. Hence, the blueshift with the increase in excita-

tion energy is seen in Fig. 1.

5. Dissimilarities between PLE and absorption

As already pointed out and could also be inferred from

Eq. 1 , the PLE is not identical but only related to the ab-

sorption spectrum. This is due to the fact that the parameter

PES in Eq. 1 is also a function of. So, on this ground, it

is not expected for a PLE to bear every minute detail of an

absorption spectrum. However, the measured PLE spectra

have shown almost similar features of the absorption spec-

trum below 3.4 eV, and above this energy, they are very

much dissimilar. The absorption above 3.4 eV shows almost

a linear increase with , whereas the PLE spectra have

shown a steep decrease with . However, it could be inferred

from Fig. 2 that the steepness above 3.4 eV gradually flattens

as is increased. This is because the occupation probability

for higher energetic states is favored at higher energetic ex-

citations as we have seen in the shift in the PL peak with .

Another major dissimilarity is the peak at 3.4 eV in the PLE

spectra, which is neither found nor is there any signature for

its appearance in the absorption spectrum. This can be un-

derstood as follows. The PLE spectrum is fundamentally

governed by the product of the absorption spectrum with the

occupation probability. The absorption above 3.4 eV, al-

though it shows a linear increase with , has a very small

slope, whereas the occupation probability would have had a

much steeper decrease. It is because of this product that the

PLE spectra have shown a decreasing trend with above 3.4

eV, and as a result, a peak has appeared in the PLE spectra.

6. Aging effect on PS

Figure 6 shows, in addition to the blueshift of the spec-

trum, the rise in luminescence efficiency of PS with aging.

The FTIR spectra of Figs. 7 and 8, recorded at the same time

as the luminescence measurements, show that certain SiHxbonds are being replaced by certain other hydrides and ox-

ides with aging. A similar observation has been reported in

the literature.41,42

As a result, the shift in the luminescence

peak and the rise in luminescence efficiency with aging are

attributed to the change in surface constituents and the

change in surface density of states, respectively. Although we

have observed an enhancement in the luminescence, there

are reports on its quenching with the aging.43

We would be-

lieve that this is due to the different ambient conditions those

samples are exposed. It is the formation of chemical bonds

under certain ambient condition that decides whether an en-

hancement or a quenching in the luminescence would resultwith the aging.

1.4 1.6 1.8 2.0 2.2 2.4 2.6

0

1

2

3

4

5

6

7

NormalizedPLSignal


As-prepared (Pulse)

Aged (Pulse)

FIG. 6. PL spectra recorded at 3.31 eV excitation energy normalized with

the excitation photon flux of both as prepared and stored in ambient condi-

tion for 5 months prepared by pulsed current electrochemical etching with

80 mA/cm2 current density for 5 min.

600 800 1000 1200 1400

0

5

10

15

20

Transmission%

Wavenumber (cm-1

)

As-prepared (Pulse)

Aged (Pulse)

FIG. 7. Regular normal incidence FTIR transmission spectra of both as

prepared and stored in ambient condition for 5 months prepared by pulsed

current electrochemical etching with 80 mA/cm2 current density for 5 min

in the wavenumber range from 400 to 1600 cm1. The vibrational absorp-

tion modes are attributed Ref. 34 as follows: 624 cm1 to SiH bending in

Si3 SiH, 663 cm1 to SiH wagging, 827 to SiO cm1 bending in OSiO,

856 cm1 to SiH2 wagging, 906 cm1 to SiH2 scissor, 948 cm

1 to SiH

bending in Si2 H SiH, 979 cm1 to SiH bending in Si2 H SiH, and

10341182 cm1 to SiO stretching in OSiO and CSiO.

2000 2100 2200 2300 2400

2

4

6

8

10

12

ransmsson%

Wavenumber (cm-1

)

As-prepared (Pulse)

Aged (Pulse)

FIG. 8. Regular normal incidence FTIR transmission spectra of both as

prepared and stored in ambient condition for 5 months prepared by pulsed

current electrochemical etching with 80 mA/cm2 current density for 5 min

in the wavenumber range from 1975 to 2350 cm1. The vibrational absorp-

tion modes are attributed Ref. 34 as follows: 2090 cm1 to SiH stretching

in Si3 SiH, 2116 cm1 to SiH stretching in Si2H SiH, 2197 cm

1 to SiH

stretching in SiO2 SiH, and 2248 cm1 to SiH stretching in O3 SiH.




8/9

7. Luminescence enhancement with porosity

From Fig. 1, it could be inferred that the increase in

current density, thereby the porosity, enhances the PL effi-

ciency in PS. This observation has already been explained by

the QCM elsewhere.44

However, from the analysis of PLE,

we have found that the quantum confinement effects are less

significant in PS. Following that, this observation can be

explained as follows. The increase in current density widens

the pore diameter.2 The pore widening would result in threemajor changes in PS: i the decrease in the effective refrac-

tive index, ii the increase in the internal surface area, and

iii the decrease in the scattering losses of the emitted light

as it traverses the PS before reaching the detector. Of these

three changes, the changes in the internal surface area and

the scattering are directly related to the PL extraction effi-

ciency. The increase in the internal surface area would facili-

tate to accommodate more oxides/hydrides on the pore walls,

and thereby, the surface density of states would increase,

which would ultimately result in the PL enhancement. How-

ever, the internal surface area does not increase forever with

the current density. It reaches a maximum and then de-

creases. On the contrary, the scattering always decreases with

the current density. It is these two competing effects that

would decide whether the luminescence increases or de-

creases with the current density once the internal surface area

reaches a maximum.

IV. CONCLUSIONS

In summary, the traditional approach to the analysis of

PLE has been corrected to incorporate the spectral depen-

dence of the reflectance of the material in the case of solids.

The corrected formula for the absorption better reveals thesimilarity between the PLE and absorption of the material

over a broad spectral range. In addition, since the absorption

is simulated for an ultrathin Si film with the bulk optical

constants of Si, the similarity also confirms that the quantum

confinement does not have a pronounced effect on our

samples. This is one of the primary conclusions we make out

in this article. The reasons for the absence of quantum con-

finement have also been given from structural characteristics

of PS. Following that, the luminescence is ascribed to the

surface states formed by the presence of oxides and hydrides

at the pore walls. Of these, the presence of certain hydrides/

oxides facilitates the intense luminescence in PS has been

confirmed from the aging studies. Although the ascription ofsurface states and oxides for the luminescence in PS has

already been reported,26,45,46

we differ from them by explic-

itly showing the absence of quantum confinement also in the

absorption and hence we are consistent with our claim. Fol-

lowing that, many common observations in the luminescence

of PS have been explained under the same framework. Fi-

nally, so far, the blueshift of the PL peak energy with the

increase in excitation energy has been explained from the

quantum confinement point of view8,24

and is explained from

the ideas of quasithermal equilibrium and the probability of

occupation of the carriers at the surface states, without any

loss of consistency.

ACKNOWLEDGMENTS

The authors would like to thank Debjit Datta, Indian

Institute of Technology Kanpur, India, for performing the

spectroscopic ellipsometry measurement on the c-Si wafer,

and we also thank the Department of Science and Technol-

ogy DST Contract Nos. SR/S5/NM and 01/2005 , Govern-

ment of India, for supporting the TEM facility at Central

Instruments Facility CIF , Indian Institute of TechnologyGuwahati, India.

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An alternative approach to understand the photoluminescence and the photoluminescence peak shift with excitation in porous silicon.pdf