Photoluminescence of Por-Si with High-ordered Mosaic Structure Received at Long Anodic Etching p-Si (100) in the Electrolyte with an Internal Current Source

doi:10.4236/opj.2013.32B051

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Optics and Photonics Journal, 2013, 3, 217-221

doi:10.4236/opj.2013.32B051 Published Online June 2013 (http://www.scirp.org/journal/opj)

Photoluminescence of Por-Si with High-ordered Mosaic

Structure Received at Long Anodic Etching p-Si (100) in

the Electrolyte with an Internal Current Source

K. B. Tynyshtykbaev, V. B. Glazman, M. A. Yeleuov, A. T. Isova,

B. A. Rakymetov, D. Muratov, S.Z.Tokmoldin

Institute of Physics and Technology, Almaty, Kazakhstan

Email: [email protected]

Received 2013

ABSTRACT

Photoluminescence spectra and nature of light-emitting centers of a po rous silicon (por-Si) samples are given. The por-

Si samples had high-ordered mosaic structure, which was received under long anodic etching p-Si(100) in electrolyte

with an internal current source. The photoluminescence spectra were monitored at room temperature before and after

annealing in air and vacuum. Comparative analysis of photoluminescence spectra of the por-Si samples annealed at

different temperatures in air and vacuum shows that t he thermal annealing conditions has significant effect on the int e n-

sity and spectral content of the photoluminescence spectra. The nature of the luminescence emission centers at different

temperatures and annealing conditions was discussed.

Keywords: Photoluminescence Spectra; T hermal Annealing; Emitting Centershydrides; Oxides; Mosaic Structure;

Nanocr ystallite; P or-Si

1. Introduction

Today, a large number of works dedicated to the study of

photoluminescence (PL) properties of por-Si, promising

for practical applications, such as LEDs. The por-Si in-

cludes silicon nanocrystallites (Si-NCs) in the form of

nano wire s on t he s urfac e l aye r o f monocr ystalline silico n

with different phases of crystalline c-Si and amorphous

a-Si, covered with oxides (SiOy) and hydrides (SiHx).

The nature of light-e mitting c enters (LEC) P L is still not

a fully established and different models are offered for its

explanation [1]. One of the earliest and the most widely

used models is quantum model PL in which the lumine-

scence is determined by the recombination of excitons in

the Si-NCs. Another model suggests that the lumines-

cence related to hydride (Si-Hx) bonds on the surface of

Si-NCs po r-Si. T here is also a model of the PL related to

the presence of defect centers in oxides (SiOy) at the

interface of Si-NCs por-Si/SiO y. The most widely accepted

model for explaining the maximum intensity of PL at

λmax = 640 nm associated with the defective levels of

complexes hydrides and oxides on the surface of nanocry-

stallites (NCs), such as SiHx or SiOy (x, y = 1-4). Max-

imum PL intensity at λmax = 440 nm related to radiative

rec ombi natio n of excitons in Si-NCs por Si [2].

In our report, it considers the nature of the PL and

light- emitting ce nters in por-Si with high-ordered mosaic

struct ure (MS) , received at l ong ano dic e tching p -Si(100)

in electrolyte with an internal current source [3].

2. Experimental Results and Discussion

The PhL spectra of samples with MS of por-Si received

at room temperature before and after annealing on air and

in vacuum are presented. Samples Si (B ), ρ = 0.01 Ω∙cm

and with plane of crystallographic orientation (100), were

carried out etching in the electrolyte HF (49%): H2O2

(40 %) = 1:1. Ohmic In-contacts were created on the

back side of the samples by annealing at 300℃ during

30 min. Anode was p-Si, cathode - Ni. The densities of

anodic current were ja = 3 . 0 mA/ cm2. T he porous surfaces

of silicon were researched by the scanning electron

(JSM-6490LA) and atomic force (JSPM5200) micro-

scop es.

PhL excitatio n was carried out He -Cd laser at a wave-

length of λexc = 325 nm with output power 15 mW, fo-

cused on the sample surface by patch with diameter of

1.0 mm. Thermal annealing of freshly prepared samples

of por-Si was carried out both on air and under controlled

conditions of a vacuum 10-4 Torr (sputtering unit ARC

200 0) in the te mper ature r ange 50℃ - 500℃ at steps of

50o. It was fo und, that the P hL obser ved at room te mper-

ature as a red-orange glow in the place of incidence of

K. B. TYNYSHTYKBAEV ET AL.

218

the exciting laser radiation on the islets of NCs por-Si,

while the silico n led ge s and c ell- t he free sites fro m islets

no show light-emitting properties.

Islets o f NCs po r-Si is a n ens emble o f cluste rs o xi- di-

zed Si-NCs with size about 10 nm - 20 nm [4], and free

cell and ledges are the pure silicon [5]. Comparative

analysis of the PhL spectra of samples of por-Si, an-

nealed on air and in vacuum shows a significant the an-

nealing effect on parameters PhL. It is seen, that the PhL

spectra, received under these conditions have similarities

and im- portant differences. The PhL spectra of freshly

prepared samples of por-Si, received at room temperature,

repre- sent broad band and have two characteristic re-

gions of maximum intensity of the light emitting at wa-

velengths of λmax = 640 nm and λmax = 440 nm (Figure

1).

The dominantly long-wavelength maximum of the

PhL spectrum λma x = at 640 nm usually associated with

the recombination of charge carriers at the defect centers

of the surface of hydrides and oxides coatings of Si-NCs

por-Si, while the short-wave maximum of the PL at λmax

= 440 nm with recombination of excitons in the itself of

Si-NCs [2].

In our case, in the long-wave part of spectra, received

both in air and vacuum, on the short-wave their wings i n

the region of λ = 600 nm, the inflection is observed. This

indicates on the presence of two different emitting cen-

ters. The superposition of their spectra and gives this

inflection. It can be assumed, that the emitting center

responsible for the inflection in the λ = 600 nm, due to

the presence of containing carbon-silicon complexes [6].

The presence of carbon in the samples of the por-Si ma y

be due to the adsorption of carbon-containing molecules

from the air, which is confirmed by our results of ele-

mental analysis [5]. The nature of short-wave part of the

PhL spectrum at λmax = 440 nm coincides with p ublished

data [2].

For a more complete understanding of the nature emit-

ting centers the PhL was performed thermal annealing of

Figure 1. The PhL spectrum of the samples por-Si, the re-

ceived in electrolyte HF (49%):H2O2 (40%), tetch.=4 h.

the samples por-Si on air and in vacuum, when at an-

nealing gradually removes the adsorbate, which is a reac-

tion product of etching and adsorption from the environ-

ment, and at this possibly transformation of their struc-

tures [6].

Thermal anneal ing of the samples freshly etchi ng por-

Si showed, that the maximum PhL intensity at a wave-

length of λmax = 640 nm observed at 100℃ annealing

both on air and in vacuum (F ig ures 2, 3). The increase

intensity of the Ph L at Tann = 100℃, is usually, attributed

to desorption of water molecules in this temperature

range [7,8].

In both cases are observed a broadening of the long-

wavelength wing of the PhL spectrum by shifting the

wavelength of maximum emitting and the significant

gro wth of the Ph L signal in t he highl y-energy part of the

spectrum. While its the short-wavelength wing of the

PhL spectrum is practically unchanged.

This experimental fact differs from the published re-

sults, when there is a broadening of the highly-energetic

wing the PL spectrum with shift of the maximum inten-

sity to highly-energetic region [9], or broadening of the

low-energy wing of the PL spectrum with shift of the

maximum intensity, but at this case is observed the de-

crease in PL intensity [10].

Figure 2. The PL spectrum as a function of annealing tem-

perature sample in vacuum 10 - 4 T orr.

Figure 3. Dependence of the maximum intensity of the PL

spectrum at λmax = 640 nm (a) and λ

max = 440 nm (b) as a

function of annealing temperature (■-on the air, ♦ - in

K. B. TYNYSHTYKB AEV ET AL.

219

vacuum 10 -4 Torr).

Typically, according to quantum-size model the PL

maxi mum shifts to high-energy side as a result of reduc-

ing the sizes of Si-NCs. The broadening of the

low-energy wing of the PhL spectrum are explained in-

creasing the sizes Si-NCs at additional processes of hy-

drogenation and oxidation the surface Si-NCs at or after

the desorption of water. At the sa me time possible r econ-

struction of structure of hydride SiNy and oxide SiOx (x,

y = 1-3) complexes on the surface of Si-NCs in more

complex structures of various sizes. Low-energy maxi-

mum of the PhL spectrum of our samples is determined

by irradiative recombination in non-quantum-size NCs

por-Si [11], due to fluctuations potential in the highly

developed surface of por-Si, leading to a localization of

charge carriers. In our case, a significant increase in the

intensity of the low- energy maximum PL by compared

with the high-energy maximum of intensity of PhL spec-

trum is determined by the size of Si-NCs por-Si, which

are mostly of the order of 10 nm - 20 nm. It should be

noted the follo wing interes ting exp eri mental re sult, when

a small increase in vacuum of temperature annealing to

Tann. = 150℃, only on 50°, maximum intensity reduces to

the initial state and returns the original form of the

long-wavelength p art of the spectr um (Figure 3).

Recovery of intensity and line shape of the PhL maxi-

mum at λmax = 640 nm for small change in the annealing

temperature suggests, that the irreversibly structural

changes in the volume of Si-NCs does not occur, but

only changes the surface structure. That is, at this an-

nealing temperature take place the reconstruction of the

surf ace coating with out s ign ifica nt chan ges in thei r ch emic al

and structural state of the surface of Si-NCs por-Si. This

is confirmed by the fact that the decrease in PL intensity

occurs without a significant change in the shape of the

spectrum. All this only shows the transformation of

chemical bonding and structure the surface hydrides and

oxide complexes. That is, as a result of desorption of

water molecules from the surface of NCs por-Si take

place the transformation of complex systems at chemi-

sorptions of H and O, and thus decreases the PhL inten-

sity due to shielding of the emitting centers with these

complexes. In the case of annealing at Tann= 150℃ on

air maximum intensity decreases to lesser extent (Figure

3(a)) than during annealing in a vacuum, retaining the

original shape of the spectrum before annealing. The fact,

that at annealing on air the maximum intensity of the

PhL spectrum is reduced to lesser extent t han in the case

of annealing in a vacuum, can be explained the possible

formation of more complex systems, involving oxides. It

is po ssible , that they a re eit her give t he wea kly the shie l-

ding the emitting centers compared with complexes,

formed at vacuum, or in these complexes are formed

more defective emitting centers. Figure 4(a) shows, that

a further increases in air the temperature annealing to

Tann. = 2 50℃ the PhL i ntensit y mono tonicall y decreases,

and in range Tann. = 300℃ - 450℃, its intensity bec o mes

minimum and practically almost constant, and disappears

at Tann.= 500℃. This well-known experimental fact that

the decrease and disappearance of the PhL signal upon

annealing in the range Tann. = 250℃ - 500℃ due to the

formation of surface stable oxide layer, which can either

to shield emitting centers [6] or to replace hydrides on

oxid es [7]. I n addition, the PhL inte nsity can be reduced,

as we believe, due to annealing emitting defects centers

the surface complexes.

It is interesting to note, that the intensity of the PhL

signal samples por-Si, annealed at Tann= 250℃ - 350℃

in a vacuum, more higher (8 times), than signal PhL in-

tensity of the samples por-Si, annealed at the same tem-

peratures on air.

In samples annealed in a vacuum, the high PhL inten-

sity co ntinue s obser ved up to Tann.= 400℃, in contrast to

the samples annealed in air. Sufficiently intense PhL

signal, observed during annealing in vacuum at Tann. =

250℃ - 350℃ is explained the presence of hydride

bonds of SixHy, which were formed during etching of

λmax = 440 nm

5000

10000

15000

20000

25000

0200 400600 8001000

Annealing temperature , oC

Intensivity

(a)

λmax = 640 nm

20000

40000

60000

80000

100000

120000

140000

160000

0500 1000

Annealing temperature , oC

Intensivity

(b)

Figure 4. Dependence of the maximum intensity of the PL

K. B. TYNYSHTYKBAEV ET AL.

220

spectrum at λmax = 440 nm (a) and λ

max = 640 nm (b) as a

function of annealing temperat ure on the air.

crystalline silicon and is dominated by these annealing

conditions [6]. The sharp drop in PhL intensity at Tann.

above 350℃ is due to the loss of the hydrogen coverage

[12] as a result of dehydration of the surface emitting

NCs centers and growth the concentration of the centers

of non-emit- ting recombination due to the formation of

dangling bonds in silicon [13].

Previously, according to IR Fourier spectroscopy has

been installed complete correlation between the change

in signal PhL intensity and absorption intensity in all

types of bonds SiH [14]. About this is also evidenced by

recent results of the study the influence of aging on the

PhL of por-Si, obtained by ultrasoft x-ray emissi on spec-

troscopy USXES [1,15], which shows the influence of

phase composition of samples of the por-Si on the inte n-

sity and position of the PhL peak. In fresh samples of

por-Si dominated the amorphous hydrogenated phase of

silicon a-Si:H (48%) and crystalline phase c-Si (42%),

and only 10% are oxide phases SiO x + SiO 2.

Thus, the lo ng -wave PhL spectrum at λmax = 640 nm is

changed of due to formation the hydride complexes

(SixHy)n, in the process of porous formation.

For the PhL sig nal in t he shor t-wave range with λmax =

440 nm (Figure 3(b)) in samples annealed in air, there

are two maxima at Tann. = 100℃, and more intense at

Tann.= 200℃. Upon fur t her annealing them on air, ther e is

a monotonic decrease of PhL signal with a small splash

at Tann. = 350℃.

In samples annealed in a vacuum, apart from the peak

intensity o f the P hL signal at Tann = 100℃ has increase

in sig nal i n the r egio n 2 00 ℃ - 250℃, with access on the

plateau at Tann. = 250℃ - 300℃. At Tann. = 350℃ has a

sharp increase in the PhL signal, which at further at

Tann.= 400℃ rapidly decreases and disappears at 500℃.

This ambiguous behavior of photoluminescent proper-

ties of por-Si during annealing in air and vacuum in the

short-wave side of the PhL spectrum can be explained by

the manifestation of e mitting center s in Si-N Cs due to t he

change of the phase and elemental composition of a mul-

ticomponent, complex structure of the surface of NCs

por-Si. The increase the PhL signal at 100℃ both in air

and in vacuum due to the removal of water, which

screened emitting NCs centers. There is a second maxi-

mum PhL at Tann. = 200℃ in air, which is more intense,

than the first at 10 0℃. This maximum may be due to the

formation of siloxane compounds during a nnealin g in air

[8]. At this temperature annealing in vacuum formation

of siloxane complexes does not occur, therefore the PhL

signal don’t observed.

Decreasing the PhL signal at 200℃ and above on air

take place due to oxidation of the surface of Si-NCs. T his

may be as a result of the formation of stable oxides at

removing hydrogen from the complexes of the type

Si6O3H6.

The growth of the signal PhL intensity at Tann. above

200℃ in vacuum is explained due to the formation of

different hydride complexes (SiyHx)n. At Tann. above

350 ℃ take place dehydrogenation surface emitting

centers Si-NCs and as is observed a sharp decrease of

PhL i ntensit y. T he ob se r ve s mal l inc r ea se o f t he intensi ty

of the PhL signal at Ta nn.= 350℃ on air has a similar

nature, as for por-Si samples a nneale d i n vac uum.

The intensity of the long-wave part of the PhL spec-

trum for 1 hour exposure of laser radiation decreases

more strongly (8 times), than in the short-wave part (2

times). A further increase of exposure time practically

does not change the intensity of the PhL spectra.

The change of PhL intensity is reversible, after stop

of illumination is observed the restoration of the original

PhL intensity in during several hours. Such a reversible

change signal PhL can be explained by changes in the

dielectric constant of the medium surrounding the Si-

NCs [6] through of photo-stimulated reactions on the

surface of por-Si at illumination [8]. At this forms of

dangling bonds and as a consequence non-emitting re-

combination centers. At exposed on air take place re-

versible “healing” of dangling bonds in the process of

natural oxid atio n of t he surface o f N C s por-Si.

Finally, the high-temperature annealing air at T =

800 ℃ leads to the appearance the PL signal (Figure 4

(a), (b)).

This fact we associate with the internal of oxidized of

silicon at these annealing temperatures on the air, when

the formation of light-emitting centers in the form of

oxides of silicon. Such is not observed during annealing

in a vacuu m.

3. Conclusions

Thus, 1) maximum intensity of PhL spectra of por-Si

samples a t Tann. = 100 ℃ on air and in vacuum is changed

of due to thermal desorption of H2O molecules from the

surface of light-emitting centers in the por-Si; 2) long-

wave part of the PhL spectrum of samples por-Si in the

field λmax = 640 nm for freshly prepared samples is due to

emitti ng rec ombinat ion of exc itons i n non-quan-tum-size

crystallites por-Si dimensions, mainly of 10 nm - 20 nm

and surface hydride complexes of the type (SixNy)n,

formed in the process a pore formation; 3) low intensity

short-wave part of the PhL spectrum of for freshly pre-

pared samples in the region λmax = 440 nm due to of

quantum-sized crystallites (1 nm to 1.5 nm); 4) thermal

variation of the maxima of the intensity of the short and

long-wave part of the PhL spectrum is due to the trans-

formation of hydride SiH-bonds in the porous surface

duri ng a n nea li n g in va c uu m a nd o n ai r i n t he tempe ra t ur e

range Tann. = 200 ℃ - 400℃; 5) d egradation the emitting

K. B. TYNYSHTYKB AEV ET AL.

221

centers at Tann.> 400℃ and complete absence of PhL at

Tann.> 500℃ are observed due to the delete of hydride

SiH-bonds on the porous surface; 6) long excitation of

PL leads to heating that transforms of hydride complexes.

This fact confirms, that the light emitting centers are

formed due to hydride coverages of surface NCs por-Si;

7) a nnealing at Tann = 8 00℃ of por-Si on air leads to the

formation of light-emitting centers in the form of oxides

of silicon.

4. Acknowledgements

This work was supported financially by the Ministry of

Science and Education of Republic of Kazakhstan.

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