PL Spectra and Structure of Porous Si

7/28/2019 PL Spectra and Structure of Porous Si

1/6

Semicond. Sci. Technol. 11 (1996) 18151820. Printed in the UK

Correlation of photoluminescence

spectra and structure of porous

silicon

B Bessas, H Ezzaouia, H Elhouichet, M Oueslati andR Bennaceur

Institut National de Recherche Scientifique et Technique, USI, Laboratoire dePhotovoltaque et des Materiaux Semiconducteurs, BP 95, 2050 Hammam-Lif,Tunisia Laboratoire de Spectroscopie Raman, Departement de Physique, Faculte desSciences de Tunis, 1006 Le Belvedere, Tunis, Tunisia Laboratoire de Physique de la Matiere Condensee, Department de Physique,Faculte des Sciences de Tunis, 1006 Le Belvedere, Tunis, Tunisia

Received 23 April 1996, accepted for publication 5 August 1996

Abstract. Porous silicon (PS) layers emitting red photoluminescence (PL) have

been prepared by anodization of p-type (100) monocrystalline silicon substrate inaqueous HF solutions. PS layers oxidized in free air exhibit under UVphotoirradiation an intense yellow-orange PL, whilst as-prepared samples emit redPL. Our aim is to explain the PL behaviour and its origin in both unetched and HFetched as-prepared and oxidized PS layers according to calculated PL based onquantum confinement formalism and to infrared spectroscopy (IRS). It was foundthat the PL behaviour is associated with a quantum size effect and concentrationchange in quantum dots and wires. It was observed that HF etching of oxidized PSmay induce a preponderance of dots or wires in the PS structure, depending on theoxidation degree, and produce a PL blueshift or redshift respectively. By correlatingPL spectra of unetched and HF-etched oxidized PS, we found that highly oxidizedPS transforms into an SiO2 matrix in which photoluminescent nanocrystalline Siquantum dots are embedded.

1. Introduction

Recently, significant attention has been focused on porous

silicon (PS) owing to its visible photoluminescence (PL)

at room temperature [1]. The first visible PL in porous

silicon was obtained by Pickering et al [2], but it was not

until 1990 that Canham pointed out the importance of this

surprising phenomenon [1].

The mechanism of PL in PS remains unclear, and as

yet there has been no universal agreement on its origin.

PS is formed by electrochemical etching of single-crystal

silicon in hydrofluoric acid (HF), and it was found to

be composed of silicon nanocrystallites [3] covered by

hydrides or polysilanes [4, 5], amorphous silicon and silicondioxide [6]. Thus, the disagreement about the origin of the

PL may be due to the structure and the composition of

PS, which are difficult to control and thus difficult to know

with certainty. However, the quantum size effect in silicon

nanocrystallites [1, 7] is the hypothesis most often used to

explain such PL from an indirect-bandgap semiconductor.

HF chemical etching has been widely used [1, 4, 8] to

determine the effect of the different structures existing in

PS on its emission properties. Indeed, concentrated HF

attacks bulk 111 n-type Si at a rate of 18 A h1 [9]

and easily removes amorphous hydrogenated silicon at a

relatively high rate ( 100 A h1 for unannealed films and40 A h1 for partially annealed films [10]). Furthermore,silicon oxides are easily removed by HF etching.

Several experiments using different oxidation meth-

ods [8, 11] have been used to passivate the highly reactive

PS surface and to determine the origin of the PL. Both HF

etching and oxidation produce a structural change in PS,

inducing a PL blueshift [11] or redshift [4], and this raised

many controversies regarding the PL origins. In this paper

we try to clear up these controversies. We report results

concerning the effect of HF etching and natural oxidation

assisted by UV photoirradiation on the PL behaviour and

structure of PS. The PL behaviour is correlated with the

PS structure, based on calculation of PL spectra and on

infrared spectroscopy (IRS).

2. Experiment

The PS layers are obtained by anodization in aqueous

HF solution of (100)-oriented, boron-doped p-type silicon

substrates of resistivity 11.5 cm. The samples have

a polished mirror-like surface. Screen-printed Al/Ag thick

0268-1242/96/121815+06$19.50 c 1996 IOP Publishing Ltd 1815


2/6

B Bessas et al

film or eutectic GaIn were used as the contact on the back

side of the Si wafers to ensure uniform current distribution.

In order to control the experimental conditions, and hence

the porosity of the PS layers, we used the diagram given by

Zhang et al [12] which depicts the critical current densities

as a function of HF concentration. The PS samples were

anodized in the dark in 3% HF solution (prepared from

49% HF and deionized water) at a constant current density

of 5 mA cm2 for a period of 20 min; the resultant porosity

(the ratio of pore volume to the total volume in the anodizedSi region) of the PS layer, determined by the gravimetric

method [13], is about 80%. As-prepared PS layers of 80%

porosity present a yellow-gold colour. After anodization,

the PS layers were rinsed with deionized water, dried

under nitrogen flux and stored in a dry, quite dark box,

in order to preserve their characteristics and to avoid PL

degradation [14]. Oxidized PS layers were obtained by

performing an experimental cycle consisting of exposing

PS to UV irradiation (light from a 30 W deuterium lamp

focused on 1 cm2) for 5 min a day and to normal aging in

free air and ambient light.

The PL spectra were monitored using a set-up

consisting of the 514.5 nm line of the Ar ion laser,a series of lenses, a triple-grating spectrometer and a

photomultiplier. As-prepared PS samples to be studied

must be stored in an inert ambient (such as N2) prior to

exposure to light, in order to avoid PL evolution in the

presence of O2 [15]. In order to study the evolution of the

terminal chemical bonds in the PS layer and their effects

on the PL, IRS measurements were performed on a Perkin-

Elmer spectrophotometer.

3. Theoretical approach

To calculate the PL lineshape, we assume that PS is formed

by Si crystallites in the form of quantum dots and wires.Also, we assume that both dots and wires have a Gaussian

distribution of diameter d centred around a mean d0. The

Gaussian distribution is given as

Gd =1

2exp

(d d0)

2

22

(1)

where is the root mean square value.

We use the same calculations and assumptions as those

made by John and Singh [16], but vary both dot and wire

diameter and take a dependence of d1.39 for the energygap versus the crystallite diameter d [17] (deduced from the

LCAO method) rather than a d2 dependence [16] (deduced

from effective mass theory). The total PL emitted fromxw wires and xd dots (xw and xd being the concentration

of wires and dots respectively in the PS layer) is It =Iw + Id, where Iw and Id represent the PL spectra ofwires and dots respectively. The PL energy is given by

h = Eg Eb + E, where Eg is the bulk silicon gap,Eb the exciton binding energy and E the confinement

energy of electrons in quantum dots or wires. Since Egranges from 1.14 to 1.17 eV, depending on the temperature

and the exciton binding energy Eb 0.15 eV [16], one maytake EgEb = 1 eV. Thus the PL energy is h = E+1.

By introducing a d1.39 dependence of the confinementenergy, E = cd/w/d1.39 where cw and cd are appropriatelydimensioned constants. The contributions of wires and dots

to the PL lineshape are

Iw = kwxwE3.16

exp1

2

d0w

w

2 c0.72w

d0wE0.72 1

2(2)

Id=

kdxdE3.88

exp1

2

d0d

d

2 c0.72dd0d

E0.72 12

(3)

kw and kd being suitable normalization constants.

4. Results and discussion

4.1. Effect of HF etching on the PL and structure of

as-prepared PS

Figure 1 shows experimental and calculated PL spectra

of unetched and HF-etched as-prepared PS at room

temperature. The effect of HF etching on as-prepared

PS is well illustrated: when an as-prepared PS layerundergoes etching in a 30% HF solution, for 60 min, its PL

peak increases in intensity and blueshifts from 1.78 eV to

1.94 eV. Calculated PL spectra were obtained by computing

equations (2) and (3) with the optimized parameters shown

in table 1. One can notice that there is a good agreement

between theoretical and experimental curves. It seems

that we have obtained better agreement than John and

Singh [16], who used a d2 dependence of the energygap versus the crystallite diameter. On the other hand,

for as-prepared PS, our calculation has given dot and wire

concentrations of 70% and 30% respectively and a dot mean

diameter slightly larger than that for wires (cf table 1).

For as-prepared PS made by Cullis and Canham [3] theconcentration of dots and wires (calculation made by John

and Singh [16]) is about 10% and 90% respectively. These

Figure 1. Room-temperature photoluminescence spectraof as-prepared PS of 80% porosity: (a) unetched PS,(b) etched in a 30% HF solution. Broken curves aretheoretically calculated PL spectra.

1816


3/6

Correlation of photoluminescence and structure of PS

Table 1. Optimized fitting parameters giving the theoreticalcurves shown in figure 1.

xd xw d0d d0w d w(%) (%) (nm) (nm) (nm) (nm)

As-prepared 70 30 3.3 2.83 0.35 0.3As-prepared 85 15 2.96 2.5 0.305 0.26

+60 min HF

contradictory results are probably due to the difference in

the substrate resistivity and the preparation conditions.

Transmission electron microscopy (TEM) analysis

[18, 19] shows that as-prepared PS of 80% porosity(obtained from p-type silicon) has a microporous structure

with a pore size almost identical to crystallite size; the latter

is about 3 nm and is in good agreement with our results(table 1). On the other hand, due to agressive anodization

(i.e. high porosity), TEM analysis [18, 19] revealed many

wire sections undulating in width [18] or detached [19],

leading to the formation of interconnected dots or free dots

respectively embedded in an amorphous phase. Thus, from

a simple undulating wire may originate many dots having aslightly larger mean diameter than that of the original wire.

Indeed, the dot mean diameter corresponds approximately

to the maximum diameter of the original undulating wire,

which is greater than the wire mean diameter. According

to root mean square values (table 1), it is evident that some

of the wires may have a larger diameter than some of the

dots. Therefore, according to TEM analysis [18,19], the

assumption that dots originate from many undulating wire

sections could explain the preponderance of quantum dots

in PS of 80% porosity (obtained from anodization of p-type

silicon of 11.5 cm resistivity).

HF etching has changed the concentration of dots and

wires from 70% to 85% and from 30% to 15% respectively

(cf table 1), probably by amplifying undulation in partiallyundulating wire sections. This may explain, in part, the

observed PL blueshift, but the size reduction in both dots

and wires may make a large contribution to this behaviour.

Hence, by taking just the PL calculation into account, the

PL blueshift depicted in figure 1 is due to an enlargement

of pores by chemical dissolution, hence to quantum sized

crystallites as described by Canham [1], and to a non-

negligible change in the population of crystallites that

luminesce (cf table 1). Although HF attacks bulk Si and

removes all amorphous phases, as previously described,

in PS it leaves a hydrogen-passivated Si skeleton. It is

therefore important to quantitatively evaluate the variation

of hydrogen content produced by HF etching in thePS layer, and to see whether it influences the emission

properties of PS. Figure 2, curves (2-a) and (2-b), shows

IR spectra of as-prepared and HF-etched PS, corresponding

to the PL spectra of figure 1. It was found that in both

cases the surface of the Si crystallites is surrounded by

SiHx bonds, exhibiting different vibrational modes [20].

It is important to note the existence of SiO bonds, even

after 60 min of HF etching, proof that PS instantaneously

becomes oxidized when it comes into contact with air. It

should be noted that all SiO bonds originate from the PS

structure, since IRS of the underlying Si substrate does not

exhibit any SiO peak (figure 2 curve (2-c)).

One can notice that SiHx bonds slightly decrease after

HF etching, except for SiH bonds corresponding to the

wagging mode at 661 cm1, and HF-etched PS has nogreater hydrogen content than as-prepared PS. This result

is contrary to what has been reported elsewhere [4, 21].

By correlating PL and IR spectra (figures 1 and 2), we

can deduce that the SiHx bonds and their content have

no influence on the observed PL blueshift. This is indisagreement with some authors [4, 21] who have argued

that SiHx bonds may play a more important role than

mere passivation. Just the variation in the intensity of

the wagging mode is not sufficient to explain such a PL

blueshift from 1.78 eV to 1.94 eV. The slight variation

of hydrogen content in HF-etched PS may be explained

by a new distribution of the SiHx bonds on smaller

crystallites. According to PL calculation, HF etching

has simply modified the size and the population of the

crystallites that luminesce, leading to a PL blueshift.

4.2. Effect of aging assisted by UV photoirradiation on

the PL and structure of PS

UV photoirradiation was performed to accelerate the

oxidation mechanism and to replace unstable SiHx bonds

by stable SiO bonds [22]. The UV photoirradiation

treatment begins 10 min after PS preparation, the IR

spectrum for which is shown in figure 2 curve (2-a).

From the first moments of UV irradiation, in ambient

air, we observe (qualitatively with the naked eye) a

relatively important reduction in the PL intensity. The

photodegradation can occur even on a time-scale of

milliseconds [23]. This could be related to photoinduced

structural changes such as hydrogen photodesorption from

the Si crystallites. However, after rapid photodegradation in

the initial stage of the photoirradiation, a photoenhancementeffect was observed. Thus, the photocreated Si dangling

bonds are being made passive by photo-oxidation [24].

Curve b in figure 3 shows that after 5 min of UV

photoirradiation, the PL peak of as-prepared PS has

blueshifted and increased in intensity. Curves c, d, e

in figure 3 depict the variation of the PL spectrum of

PS for different oxidation times. Under such oxidation

conditions, the PL peak blueshifts day after day (figure 3),

but its intensity seems to be dependent on the degree of

oxidation. Indeed, one can notice that the PL intensity

of PS oxidized over 15 days is lower than that oxidized

over 5 days. It should be noted that this result is not

specific to the oxidation conditions applied, since it hasbeen observed under other oxidation conditions [25]. In

fact, degradation appears when PS becomes oxidized (the

spectra are measured when the intensity stabilizes). The

origin of this degradation is detailed elsewhere [26].

Calculated PL spectra show that during oxidation the

concentration of dots increases and that of wires decreases

(cf table 2). This is accompanied by a reduction in the mean

diameter of both dot and wire crystallites. It is well known

that oxidation produces a thinning of the Si crystallites;

this thinning occurs day after day and should be the main

1817


4/6

B Bessas et al

Figure 2. Infrared absorbance spectra of: (2-a) freshly prepared PS layer of 80% porosity, (2-b) as-prepared PS of 80%porosity etched during 60 min in a 30% HF solution, (2-c) silicon substrate.

Figure 3. Evolution of room-temperaturephotoluminescence of PS versus post-anodizationtreatment: (a) as-prepared PS, (b) as-prepared + 5 min UV,(c) oxidized over 3 days at room temperature + 5 minUV/day, (d) oxidized over 5 days at roomtemperature + 5 min UV/day, (e) oxidized over 15 days atroom temperature + 5 min UV/day. Broken curves aretheoretically calculated PL spectra.

factor responsible for the PL blueshift. On the other hand,

apart from the fact that thinning affects both dot and wire

diameter, it would also transform, via oxide growth, manyundulating wire sections into oxide-covered interconnected

dots and/or free dots embedded in the oxide. In assuming

that the thinning rate, throughout oxide growth, is the same

for dots and wires, the dot mean diameter should remain

slightly greater than that of wire, in oxidized PS (cf table 2).

The PS samples oxidized for 5 and 15 days have been

submitted to 1 min of HF etching. A PL blueshift was

observed for the 5-day oxidized PS and a redshift for the

15-day oxidized PS (figure 4). The decrease of the PL

intensity in HF-etched oxidized PS (figure 5) is due to SiO2

Figure 4. Room-temperature photoluminescence spectraof oxidized PS, before (a and b) and after (c and d) 1 minetching in a 30% HF solution: (a) oxidized over 5 days atroom temperature + 5 min UV/day, (b) oxidized over 15days at room temperature + 5 min UV/day, (c) = (a) + 1 minof HF etching, (d) = (b) + 1 min of HF etching. Brokencurves are theoretically calculated PL spectra.

dissolution and consequently to the removal of many of the

Si crystallites embedded in the oxide. Also, an important

degradation phenomenon has been observed in oxidized

PS [26], before stabilization and measurement of the PLspectra. In both cases, the concentration of dots decreased

and that of wires increased (see table 2).

The decrease in the concentration of dots, obtained after

HF etching of 5-day oxidized PS, is probably due to the

fact that many dots have been trapped in the growing oxide

and then removed. This may also contribute to the large

downshift of the PL intensity (figure 4). Thus, the PL

blueshift produced in 5-day oxidized PS, after such HF

treatment, is due to the large reduction in diameter of both

dots and wires (see table 2).

1818


5/6

Correlation of photoluminescence and structure of PS

Table 2. Optimized fitting parameters giving the theoretical curves shown in figures 3, 4 and 6.

Oxidation time xd xw d0d d0w d wand treatment (%) (%) (nm) (nm) (nm) (nm)

3 days 76.5 23.5 3.15 2.7 0.36 0.285 days 83 17 3.10 2.62 0.35 0.3115 days 91 9 2.75 2.39 0.3 0.1565 days + 1 min HF 69 31 2.74 2.26 0.25 0.2715 days + 1 min HF 0 100 2.2 0.4

Figure 5. Infrared absorbance spectra of 15-day oxidized PS: (a) unetched, (b) etched for 1 min in a 30% HF solution.

PL calculation (see table 2) shows that for 15-day

oxidized PS, HF etching has removed all Si quantum

dots leaving only 100% wires, evidently linked to the

underlying Si substrate. At first sight this means that

at this oxidation stage all dots are trapped in the oxide.Indeed, IRS shows that in 15-day oxidized PS (figure 5,

curve (5-a)) all Si crystallites are surrounded by SiO

bonds. Nevertheless, SiH bonds in Si3SiH having a

bending mode at 624 cm1 [20] persist. The persistence ofthese bonds is not surprising, since it has been reported that

considerable numbers of H atoms still remain in oxidized

PS, even after thermal oxidation at high temperature [27].

Figure 5, curve (5-b), shows IRS of 15-day oxidized PS

after 1 min of HF etching. The presence of the bending

mode at 624 cm1, before and after HF treatment, suggeststhat the corresponding SiH bonds could be related to the

surface of the underlying Si substrate. The non-existence

of any SiO peak or SiHx peak proves that all dots areremoved and that the existing luminescing wires linked

to the underlying Si substrate are scarce. This confirms

our calculation result, where only 9% of wire crystallites

exist in 15-day oxidized PS (table 2). The number of wire

crystallites is best conserved after HF etching. Regarding

the PL behaviour of 15-day oxidized PS, after HF etching

(figure 4), we are in the presence of two competing

phenomena, a thinning effect (table 2) and a quasicomplete

presence of quantum wires which would induce a blueshift

and a redshift respectively. Indeed, the PL contributions of

Figure 6. Theoretically calculated PL contributions of dots(dotted curves) and wires (broken lines) to the total (fullcurves) in 5-day (a) and 15-day (b) oxidized PS.

dots and wires in the total PL spectrum, depicted in figure

6, show that the disappearance of quantum dots induces a

redshift, since the PL contribution of quantum dots is more

energetic than that of wires (figure 6).

The oxide dissolution mostly results in a large PL

redshift [11], suggesting that the observed PL blueshift

during oxidation does not result from a thinning effect of

the Si crystallites. In fact, the result of oxide dissolution

depends on the conditions of preparation of PS and on the

1819


6/6

B Bessas et al

oxidation methods. For oxidized PS, John and Singh [16]

have estimated the concentration of dots and wires to

be 15% and 85% respectively (experimental data taken

from Vial et al [28]); these values are reversed in our

5-day oxidized PS (table 2). When quantum wires are

preponderant, oxide dissolution may easily remove many

of the trapped minority dots, inducing a redshift; this is the

case in the experiments of Muller et al [11] and Vial et al

[28].

One may remark that 30% HF solution attacks oxidizedPS at a relatively high rate of about 3.5 A min1 (seetable 2). This rate is excessively high compared with that

reported for monocrystalline silicon [9]. In fact, when Si

quantum dots and wires are oxidized, an amorphous silicon

phase is found at the interface between the Si crystallite core

and the oxide [5, 29]. The dissolution rate of amorphous

silicon is rather high [10], so it should make a large

contribution to the rate of HF attack on oxidized PS, and

hence on the important reduction in the diameter of dots

and wires. As we have no data concerning the HF attack

rate in PS, it appears that the amorphous phase existing

at the SiO2/Si interface would be responsible for a such

reduction in the diameter of dots and wires.Since the SiO2 growing layer is accompanied by an

expansion of about 56% [30], and referring to table 2 and

figure 5, we can affirm that highly oxidized PS transforms

into an SiO2 matrix in which photoluminesent Si quantum

dots are embedded.

Techniques other than PS preparation [31] have been

used to obtain a luminescing SiO2 matrix in which Si

crystallites are implanted. A similarity in the emission

properties has been pointed out: it has been shown that

the PL energy depends on crystallite size, which in our

case depends on SiO2 growth.

Passivation and thinning of the Si crystallites are

the main responsibilities of SiO2

, although in other

works [28, 32] the existence of possible luminescing centres

at the SiO2/Si interface has been reported.

5. Conclusion

The effect of HF etching on the PL behaviour of as-

prepared and oxidized PS has been studied. The change

in the emission properties has been explained based on

confinement in Si quantum dots and wires and on infrared

spectroscopy. In as-prepared PS, HF etching produces a PL

blueshift, due to a change in concentration and reduction

in size of dot and wire Si crystallites. It was found

that in as-prepared PS, HF etching does not produce anyincrease in hydrogen content, as has always been reported,

but it simply modifies the size and the population of the

crystallites that luminesce. In oxidized PS, it has been

shown that oxidation induces a PL blueshift which becomes

all the more important as SiO2 grows; HF etching may

produce a PL blueshift or redshift depending on the degree

of oxidation and consequently on the evolution of dot and

wire concentration. Highly oxidized PS transforms into

an SiO2 matrix in which luminescing Si quantum dots of

reduced size are confined.

Acknowledgment

This work is supported by the Secretariat dEtat a la

Recherche Scientifique et a la Technologie (PNM 92).

References

[1] Canham L T 1990 Appl. Phys. Lett. 57 1046[2] Pickering C, Beale M I J, Collins D J, Pearson P J and

Greef R 1984 J. Phys. C: Solid State Phys. 17 6535

[3] Cullis A G and Canham L T 1992 Nature 353 335[4] Prokes S M, Carlos W E and Bermudez V M 1992 Appl.

Phys. Lett. 61 1447[5] Takeda Y, Hyodo S, Suzuki N, Motohiro T, Hioki T and

Noda S 1993 J. Appl. Phys. 73 1924Prokes S M and Glembocki O S 1994 Phys. Rev. B 49 2238

[6] Perez J M, Villalobos J, McNeill P, Prasad J, Cheek R,Kelber J, Estrera J P, Stevens P D and Glosser R 1992Appl. Phys. Lett. 61 563

[7] Bsiesy A, Vial J C, Gaspard F, Herino R, Muller F,Romestain R, Wasiela A, Halimaoui A and Bromchil G1991 Surf. Sci. 254 195

[8] Prokes S M 1993 Appl. Phys. Lett. 62 3246[9] Hu S M, and Kerr D R 1967 J. Electrochem. Soc. 114 414

[10] Liou L, Spitzer W G and Prussin S 1984 J. Electrochem.Soc. 131 672

[11] Muller F, Herino R, Ligeon M, Gaspard F, Romestain R,Vial J C and Bsiesy A 1993 J. Luminesc. 57 283[12] Zhang X G, Collins S D and Smith R L 1989

J. Electrochem. Soc. 136 1561Canham L T, Houlton M R, Leong W Y, Pickering C and

Keen J M 1991 J. Appl. Phys. 70 422[13] Herino R, Bromchil G, Barla K and Bertrand C 1987

J. Electrochem. Soc. 134 1994[14] Collins R T, Tischler M A and Stathis J H 1992 Appl.

Phys. Lett. 61 1649[15] Chang J M, Chuo G S, Chang D C and Chen Y F 1995

J. Appl. Phys. 77 5365[16] John G C and Singh V A 1994 Phys. Rev. B 50 5329[17] Delerue C, Allan G and Lannoo M 1993 Phys. Rev. B 48

11024[18] Cullis A G, Canham L T and Dosser O D 1992 Mater. Res.

Soc. Symp. Proc. 256 7[19] Berbezier I and Halimaoui A 1993 J. Appl. Phys. 74 5421[20] Borghesi A, Sassela A, Pivac B and Pavesi L 1993 Solid

State Commun. 87 1[21] Prokes S M, Glembocki O J, Bermudez V M, Kaplan R,

Friedersdorf L E and Pearson P C 1992 Phys. Rev. B 4513788

[22] Weng Y M, Fan Z N and Zong X F 1993 Appl. Phys. Lett.60 168

[23] Takai M, Indou S and Murase H 1993 Mater. Res. Soc.Symp. Proc. 283 257

[24] Xu Z Y, Gal M and Gross M 1992 Appl. Phys. Lett. 601375

[25] Ito T, Motoi K, Arakaki O, Hatta A and Hiraki A 1994Japan. J. Appl. Phys. 33 L941

[26] Elhouichet H, Oueslati M, Bessas B and Ezzaouia H 1996J. Luminesc. at press

[27] Ito T, Otha T, Motoi K, Arakaki O and Hiraki A 1993Mater. Res. Soc. Symp. Proc. 283 263

[28] Vial J C, Bsiesy A, Gaspard F, Herino R, Ligeon M,Muller F, Romestain R and MacFarlane R M 1992 Phys.Rev. B 45 147

[29] Kanemitsu Y, Ogawa T, Shiraichi K and Takeda T 1993Phys. Rev. B 48 4883

[30] Yvon J J, Barla K, Herino R and Bromchil G 1987 J. Appl.Phys. 63 1042

[31] Takagi H, Ogawa H, Yamazaki Y, Ishizaki A and NakagiriT 1990 Appl. Phys. Lett. 56 2379

[32] Prokes S M and Glembocki O S 1994 Phys. Rev. B 49 2238

1820

Documents

PL Spectra and Structure of Porous Si