Age-induced phase transitions on mechanically alloyed amorphous GaSe

Article abstract of DOI:10.1016/j.ssc.2007.02.029

After aging it for four years at room temperature, a mechanically alloyed amorphous GaSe powder was transformed to a multi-phase crystalline alloy, where major phase is the trigonal Se one. The structural, thermal and optical properties of this aged amorphous GaSe were investigated through systematic X-ray diffraction, differential scanning calorimetry and Raman scattering measurements. The X-ray diffraction results on the aged GaSe powder suggest the presence of oxides, and X-ray absorption spectroscopy was employed to further investigate it.

Full text of DOI:10.1016/j.ssc.2007.02.029

Solid State Communications 142 (2007) 270–275
www.elsevier.com/locate/ssc
Age-induced phase transitions on mechanically alloyed amorphous GaSe
a,∗
a
a
a
b
C.E.M. Campos , J.C. de Lima , T.A. Grandi , S.M. Souza , P.S. Pizani
a
Departamento de F ´ı sica, Universidade Federal de Santa Catarina, Trindade, C.P. 476, 88040-900 Florian o´ polis, SC, Brazil
b
Departamento de F ´ı sica, Universidade Federal de S a˜ o Carlos, C.P. 676, 13565-905 S a˜ o Carlos, SP, Brazil
Received 1 November 2006; received in revised form 14 February 2007; accepted 20 February 2007 by J. Fontcuberta
Available online 28 February 2007
Abstract
After aging it for four years at room temperature, a mechanically alloyed amorphous GaSe powder was transformed to a multi-phase crystalline
alloy, where major phase is the trigonal Se one. The structural, thermal and optical properties of this aged amorphous GaSe were investigated
through systematic X-ray diffraction, differential scanning calorimetry and Raman scattering measurements. The X-ray diffraction results on the
aged GaSe powder suggest the presence of oxides, and X-ray absorption spectroscopy was employed to further investigate it.
ꢀ
c 2007 Elsevier Ltd. All rights reserved.
PACS: 81.20.Ev; 61.10.-i; 87.64.Je; 61.82.Rx; 61.10.Ht
Keywords: A. Semiconductors; A. Nanostructures; B. Nanofabrications; E. X-ray and γ -ray spectroscopies
1
. Introduction
temperature, the layers are bound by weak van der Waals-
type interactions. The weakness of this interaction explains
the existence of a number of polytypes [3]. According to the
phase diagram [4] of the Ga–Se system, there are two congruent
Gallium–selenium (Ga–Se) alloys have a number of
interesting optical and electronic properties. When produced
in the crystalline form, they have been employed to design
several devices with direct technological applications, such as
MOSFET type transistors, photodiodes, phototransistors, IR
detectors, solar cells, X-ray detectors, photoelectric analyzers
of polarized light, high-energy muon detectors, etc. [1,2]. When
produced in amorphous form, it is a potential candidate for
optical memory type applications, large area flexible solar
cells, etc. [1,2]. Faced with a huge interesting technological
applications for such Ga–Se alloys, it is very important to find
alternative ways to produce them and to study their stability
with respect to temperature, pressure, radiation, aging and so
on.
GaSe is a layered semiconductor of the III–VI family like
GaS and InSe, and its basic units, two dimensional layers, are
formed by two hexagonal sheets with gallium and selenium
atoms at alternate corners of the hexagons; the hexagonal
layers undergo “chairlike” deformations through the bonding
of the gallium atoms by strong ionicovalent bonds. At ambient
◦
compounds Ga2Se3 and GaSe, which melt at 1005 C and
◦
◦
9
38 C, respectively. A monotectic transition occurs at 915 C
in the lower composition region of Se. There is a eutectic
between GaSe and Ga2Se3. There are some controversial
reports [4] on the existence of peritectics.
GaSe and Ga2Se3 samples can be prepared by fusion,
vapor deposition and molecular beam epitaxy techniques [5–
8
]. In the case of Ga2Se3, there is another difficulty: its native
surface oxidizes and is thus not stable in air [9]. Recently, our
group has successfully used mechanical alloying (MA) [10]
to produce amorphous GaSe [11] and nanocrystalline Ga2Se3
alloys [12], starting from their elemental powder mixtures with
nominal compositions Ga50Se50 and Ga40Se60, respectively.
More detailed studies on the local atomic structure of the
amorphous phases were performed by using techniques related
to synchrotron radiation sources (EXAFS, XRD and RMCA
modeling) [13].
In 2001, de Lima et al. [14] reported for the first time
results on the influence of aging in the structural properties
of an amorphous selenium sample (a-Se) produced by MA.
They have observed that after some years of aging, the
∗
Corresponding author. Tel.: +55 48 3331 6832; fax: +55 48 3331 9068.
E-mail address: pcemc@fisica.ufsc.br (C.E.M. Campos).
0
038-1098/$ - see front matter ꢀc 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2007.02.029

C.E.M. Campos et al. / Solid State Communications 142 (2007) 270–275
271
atomic structure of a-Se produced by MA partially transforms
from the ordinary Sen chain-like form to the Se8 ring-
like one, which is commonly found in a-Se produced by
the rapid quenching or vapor deposition methods. Based on
this interesting observation, more recently, an aged ZnSe
sample, also produced by MA, was re-examined [15]. Our
main observations in that paper pointed out that the presence
of a small quantity of c-Se seeds in the as-milled ZnSe
sample, probably in the interfacial region, were responsible
for promoting the growth of c-Se clusters/agglomerates, due
to migration of Se atoms located on this component with just
room temperature energy (< 0.025 eV). The Se diffusion and c-
Se re-crystallization due to aging in ambient conditions was not
observed even in a pure mechanically alloyed a-Se sample [14].
Very recent investigation of the degradation in chalcogenide
II–VI semiconductors grown by the high-pressure Bridgman
method [16] showed that in order to improve the degradation
and short lifetimes of II–VI based devices, the incorporation
of beryllium content in II–VI compounds should be ensured
(
less than 10%), since it is believed that the degradation can
be attributed to the recombination-enhanced defect motion, and
the beryllium-based bonding can be used to enhance the crystal
elastic rigidity in II–VI alloys.
Fig. 1. XRD patterns of the c-Se, as-milled-GaSe (with different periods of
∗
time) and aged-GaSe (named 15 ). The circles represent the 2θ positions of the
In the present paper, we followed our preliminary
studies reporting results on the aging effects of the
amorphous GaSe alloy produced by MA in 2002 (named
as aged-GaSe). X-ray diffraction (XRD) results show a
spontaneous amorphous–crystalline transformation four years
after mechanical amorphous alloy production. Extended X-ray
absorption spectroscopy (EXAFS) was carried out to study the
local atomic disorder due to aging and oxidation processes.
Differential scanning calorimetry (DSC) and Raman scattering
most intense peaks of the hexagonal GaSe phase.
rejections are irrelevant for the D04B beamline. The energy
and average current of the storage ring were 4.4 GeV and
110 mA. All DSC measurements were carried out from 25 C
up to 600 C, at a heating rate of 10 C min , in a
2010 DSC cell manufactured by TA Instruments, with flowing
nitrogen gas. The temperature of the cell was calibrated with
an In standard sample with a temperature accuracy better than
◦
◦
◦
−1
◦
(
RS) were employed to verify the amorphous GaSe phase
deterioration and to track changes in the vibrational spectra due
to aging.
±0.2 C. Micro-Raman measurements were performed with
a T64000 Jobin–Yvon triple monochromator coupled to an
˚
optical microscope and a cooled CCD detector. The 5145 A and
470 A lines of an Ar–Kr laser were used as exciting light,
˚
6
2
. Experimental procedure
always in backscattering geometry. The output power of the
˚
laser was selected at about 3 mW and 0.3 mW for 5145 A
˚
Sample preparation details can be found elsewhere [11].
and 6470 A lines, respectively, in order to avoid overheating
The aged-GaSe sample was stored in a sub-micrometer powder
form and as small pellets for Raman measurements. The
powder was analyzed by energy dispersive X-ray (EDX)
measurements in a scanning electron microscope, and the mean
values obtained were: 41.0 at.% Se, 31.0 at.% Ga and 28.0 at.%
O, while in 2002 the results were 48.9 at.% Se and 51.1 at.%
Ga. The absence of bulk-Fe or Fe compounds, already reported
in 2002, was confirmed in the M o¨ ssbauer spectrum of the aged
sample. XRD patterns were recorded using a Rigaku powder
diffractometer, Miniflex model, with Cu Kα radiation (λ =
samples, and consequently avoiding photo-induced structural
modifications. All Raman measurements were performed at
room temperature.
3. Results and discussion
3.1. XRD measurements
As it is reported in Ref. [11], the hexagonal GaSe phase was
formed for a short milling time (3 h), and when increasing the
milling time to greater than 10 h its amorphization occurred.
Hereafter, the as-milled amorphous sample will be called the
as-milled-GaSe, and the same sample aged for four years will
be called the aged-GaSe.
˚
.5418 A), in an air atmosphere. The EXAFS measurements
1
were performed at room temperature in transmission mode on
the D04B-XAFS beamline of an LNLS (Campinas, Brazil),
using a channel cut monochromator (Si 111) and two ionization
chambers filled with air as detectors, working at 10% and
Fig. 1 shows the XRD patterns for the as-milled-GaSe
measured in 2002 (patterns named as 1/2 h, 3 h and 15 h), and
7
0% efficiency, respectively, and the beam size at the sample
2
∗
was about 1 × 3 mm . This yielded a resolution of about 3
for the aged-GaSe measured in 2006 (pattern named 15 h ).
eV on the Ga and Se K edges. At these energies, harmonic
In order to clarify the comparison and discussions that will

2
72
C.E.M. Campos et al. / Solid State Communications 142 (2007) 270–275
The attempted identification these new phases in Fig. 2 is
shown the aged-GaSe pattern together with the main diffraction
lines of the possible candidate phases (see symbols). The
Rietveld method [19] was applied using the GSAS package
[20], and with exception of the c-Se phase, no conclusive
identification was reached for the aged-GaSe pattern. The
structural refinement for the c-Se was reached for a lattice
˚
˚
parameter of a = 4.372 A and c = 4.955 A, which is 0.1%
greater than that given in ICSD Database, card 22251 (a =
˚
˚
4
.366 A and c = 4.954 A). The best-fitted pattern obtained
is shown in Fig. 2.
3.2. EXAFS measurements
Fig. 3(a) shows EXAFS spectra of the aged-GaSe (solid
Fig. 2. Experimental XRD pattern of the aged-GaSe sample (noisy curve)
overlapped by the best fitting achieved by the Rietveld method using the GSAS
package. The symbols represent the 2θ positions of the most intense peaks of
possible minority phases: (ꢀ) Ga O , (ꢀ) SeO , (♦ e H) high-pressure Se
lines) and as-milled GaSe samples (dashed line) at both Ga
and Se K-edges obtained using the iFeffFit package [21]. A
comparison of these EXAFS signals reveals important changes
with aging; for example, the EXAFS oscillations measured at
2
3
2
phases.
the Ga K-edge for the aged-GaSe shows a huge attenuation for
be made, the XRD pattern of trigonal selenium (c-Se) is also
shown in this figure. A comparison between the XRD patterns
for the as-milled-GaSe and aged-GaSe samples reveals a huge
structural evolution from the amorphous pattern to a multi-
crystalline-phase pattern, where the main phase corresponds to
the trigonal Se (c-Se). In addition, several other diffraction lines
arise, and they remain unexplained. An exhaustive comparison
of these diffraction lines with the many XRD patterns given
in the crystallographic databases [17] and [18] (including all
compounds of the Ga–Se, Ga–O, Se–O and Ga–Se–O systems)
suggests that these new other phases can be the trigonal
Ga2O3 (ICSD [17] card no 27431), tetragonal SeO2 (ICSD
card no 24022) and some high-pressure Se phases (JCPDS [18]
cards no 471516 and 471515). The XRD patterns for some
Ga–Se phases, such as GaSe polytypes or Ga2Se3, were also
compared, and they were discarded.
−1
˚
k > 7.5 A , which indicates destructive interferences due to
phase coexistence. At the Se K edge, the EXAFS signal for this
sample shows only a shift to smaller k values, and practically
no attenuation with aging.
Fig. 3(b) shows the Fourier Transformed functions of the
aged-GaSe (solid lines) and as-milled-GaSe samples (dashed
line) at both Ga and Se K-edges. For Fourier transformations,
the EXAFS oscillations were performed over k ranges
−1
˚
delimited by Kaiser–Bessel windows from 2 to 16 A and
−1
˚
from 5 to 17 A for Se and Ga K-edges, respectively.
Based on these XRD results, several theoretical models
were proposed to fit the EXAFS signals of aged-GaSe in
direct d-space. At the Se K-edge, the best fit was reached by
considering the contributions from the an amorphous GaSe
phase (fixing their values as those obtained in 2004 [13]) and
Fig. 3. (a) EXAFS spectra of the aged-GaSe (solid lines) and as-milled samples (dashed line) at both Ga and Se K-edges. (b) Fourier Transformed functions of the
aged-GaSe (solid lines) and as-milled samples (dashed line) at both Ga and Se K-edge. The symbols represent the best fits achieved by EXAFS fittings obtained
using iFeffFit package (see numerical results in Table 1). The dotted lines indicate Kaiser–Bessel windows used in the fitting processes.

C.E.M. Campos et al. / Solid State Communications 142 (2007) 270–275
273
Table 1
Numerical results from EXAFS best fits of as-milled and aged-GaSe samples
Model
Bond type
Ga K edge
Ga–Ga
Model
Se K edge
Se–Ga
Ga–Se
Se–Se
N
d
σ (10
1.6
2.4
2.46
0.62
Ga–O
3/3/1
1.91/2.17/2.82
1.52/0.57/0.92
2.4
2.46
0.62
1.5
a-GaSe
2.39
1.26
Ga–Ga
3
2.98
1.11
a-GaSe
+
2.40
1.78
Se–Se
2
2.36
0.37
2
−2
−2
)
)
+
N
d
–
–
–
Ga O
Se
2
3
2
σ (10
N is the coordination number, d is the interatomic distances in A˚ and σ2 is the Debye–Waller parameter in A˚ . Sequences of three numbers on the Ga–O column
2
represent the values obtained for the nearest O shells.
c-Se, while at the Ga K-edge, the best fit was reached by
considering the contributions from the amorphous GaSe and
trigonal Ga2O3 phases. We tried to consider a contribution from
the SeO2 phase for the EXAFS signal at the Se K-edge, but
the fitting shows no realistic numerical results. The symbols
in Fig. 3(b) represent the best fits achieved, whose numerical
results are listed in Table 1.
3
.3. DSC measurements
Fig. 4 shows the measured DSC curves for the as-milled-
GaSe and aged-GaSe (1st run and 2nd run — dashed line)
samples. The first DSC run on the aged-GaSe sample shows
◦
several endothermic peaks between 50 and 230 C, and an
◦
exothermic broad band between 300 and 600 C, which can
be decomposed into, at least, three sub-peaks centered at 390,
◦
◦
4
55 and 520 C. The endothermic peak located at about 215 C
is attributed to the c-Se melting, whose calculated enthalpy
−1
−1
change values are 3.4 J g and 0.6 J g (g of the sample)
in the first and second runs, respectively.
The second DSC run, besides confirming the c-Se melting,
allowed the association of the endothermic peak with the
elimination of intra-molecular water molecules. However, a
comparison between the measured DSC curves during the first
and second runs reveals the arising of a small endothermic peak
at 209 C, which does not fit precisely the melting points of
any selenium oxides in bulk form. The melting point of some
Fig. 4. DSC curves of as-milled GaSe sample (3 h and 15 h) and DSC curves
of aged-GaSe sample (1st and 2nd runs, solid and dashed lines, respectively).
All DSC measurements were carried out with a heating rate of 10 C min
◦
−1
.
◦
Raman line remains easily visible in the spectrum, which is
◦
located at the same spectral range of c-Se features (at about
selenium oxides are: Se2O5 (Tm = 224 C), SeO2 (Tm =
−
1
−1
◦
◦
237 cm ); however, a very weak line at about 140 cm can
be seen in the magnified region of the spectrum shown in the
inset of Fig. 5. As well as shown by XRD, no features from
the Ga–Se phases are observed in the Raman spectrum for the
aged-GaSe sample.
3
16 C) and SeO3 (Tm = 394 C) [22]. Based on these melting
◦
point values, one could attribute the endothermic peak at 209 C
to a probable presence of Se2O5, but XRD measurements
disagree and suggest the SeO2 as the main Se oxide candidate.
The absence of most of those exothermic features (observed
in the first run) in the second run corroborates the structural
relaxation and amorphous-crystalline transformation already
suggested for the as-milled GaSe sample in the DSC results
obtained in 2002.
The Raman spectrum of the c-Se displays two lines located
−
1
−1
at about 140 cm and 235 cm , which can be identified as the
0
E and A1 optical modes of the [Se] chains, respectively [23–
n
0
0
2
2
2
7]. There is also some contribution from the E mode to the
−
1
35 cm Raman line, as its frequency has been reported to be
−
1
31 cm [24].
3
.4. RS measurements
The Raman spectrum of the as-milled-GaSe 3 h shows
Fig. 5 shows the micro-Raman spectra of the as-milled GaSe
named 3 h and 15 h), aged-GaSe (named 15 h ), as well as the
spectra of polycrystalline trigonal selenium (c-Se) and gallium
c-Ga). The results show that after aging, just one intense
at least four broad lines located at 130, 155, 235 and
∗
−1
(
280 cm , and several other lines at the lower frequency
−1
range (<100 cm ). For longer milling times, between 10
(
and 15 h, the Raman spectra obtained in 2002 were very

2
74
C.E.M. Campos et al. / Solid State Communications 142 (2007) 270–275
epitaxial Ga2Se3 layers in Ref. [9], where two other lines (at
−
1
2
50 and 290 cm ) are also reported as Raman active modes.
Thus, as already mentioned, only Raman lines at about
−
1
1
40 and 237 cm were observed for the aged-GaSe sample,
and this corroborates all attributions done for this sample
using the XRD and DSC results, which suggests the c-Se as
the major crystalline phase. This is a very interesting result
that shows the enormous atomic mobility of the Se atoms
in the as-milled-GaSe, even after it forms amorphous alloys.
In some way, this result alerts us to the metastable character
of the amorphous GaSe phase produced by MA. Further
investigations are underway to discover from where the Se
atoms come, and how they get the energy to recuperate their
trigonal structure. Our first suggestion is that they are in the
interfacial component, where there is sufficient free-energy
stored in the defects to promote that structural regeneration.
4. Conclusions
The structural evolution of an amorphous GaSe alloy
produced by MA in 2002 was investigated in order to detect
aging effects in ambient conditions. The main conclusions of
this study are the following:
Fig. 5. Micro-Raman spectra of c-Se, c-Ga, as-milled GaSe (3 h and 15 h) and
aged-GaSe (15 h ) samples.
∗
(
i) The XRD measurements of the aged-GaSe alloy showed
that the major crystalline phase is now the trigonal-Se one.
Several indications of oxides (Ga2O3) and high-pressure
Se phases can be also pointed out based on its pattern.
An amorphous halo can also be seen, and suggests an
amorphous phase coexistence.
Table 2
Raman lines observed and their suggested assignments according to GaSe and
Ge Se modes
2
3
−
1
(ii) The EXAFS results supported almost all the XRD results,
except those about the selenium dioxide (SeO2).
Frequency (cm
)
Mode symmetry assignment
GaSe [3]
Ga Se [30]
2
3
(
iii) The DSC measurements for the aged-GaSe are not
conclusive, but corroborate a residual amorphous phase
coexistence as well as several structural relaxation
features. Moreover, a second-run measurement revealed
evidence of a hydrophilic character for the aged-GaSe
sample, and confirmed one of the endothermic features as
been from c-Se melting.
0
0
∼
60
05
30
E
1
1
1
2
2
TA
LA
0
A
1
0
0
0
55
35
80
ZB: A and/or E
2
0
0
0
E
A
1
(
iv) The RS measurements corroborated an age-induced
amorphous-polycrystalline phase transition in the as-
milled-GaSe, also discarding any possibility of the
nucleation of Ga–Se crystalline polytypes.
similar in shape and quite different to those for 3 h of milling,
which was explained based on the crystalline–amorphous phase
transition detected by the XRD and DSC measurements. As a
consequence of the breakdown of the selection rules caused
by disorder, Raman scattering can access phonons out of the
Brillouin zone center.
Acknowledgements
The authors wish to thank the agencies CAPES, CNPq and
FAPESP for their financial support. We are indebted to Dr.
V. Drago for the M o¨ ssbauer measurements presented here. Part
of this paper was performed using the LNLS’s facilities and
financial support under project D04B-XAFS 4163/04.
The Raman active modes observed in the spectra of the
as-milled-GaSe were compared to those given in the literature
for both crystalline GaSe and Ga2Se3 [9,28], and also with the
Phonon Frequency Distribution Function (PFDF) for the GaSe
phase [3] (see Table 2).
−1
Two interpretations for the 155 cm
line were pointed
References
0
0
0
out: (i) zone-boundary (ZB) A and/or E modes activated by
2
selection rule relaxation due to defects and/or vacancies in the
[1] N.B. Singh, D.R. Suhre, W. Rosch, R. Meyer, M. Marable, N.C. Fernelius,
F.K. Hopkins, D.E. Zelmon, R. Narayanan, J. Cryst. Growth 198/199
(1999) 588.
−
1
crystalline structures [28–31], and (ii) LA (155 cm ) and TA
−
1
(
105 cm ) modes of the X point of the large Brillouin zone
[
2] N.B. Singh, D.R. Suhre, V. Balakrishna, M. Marable, R. Meyer,
N.C. Fernelius, F.K. Hopkins, D. Zelmon, Progr. Cryst. Growth Char. 37
(1998) 47.
of the Ga2Se3 zinc-blend structure [28]. Moreover, both 155
−
1
and 105 cm lines have been attributed to phonons from the

C.E.M. Campos et al. / Solid State Communications 142 (2007) 270–275
275
[
[
3] S. Jandl, J.L. Brebner, B.M. Powell, Phys. Rev. B 13 (1976) 686.
4] M. Hansen, Constitution of Binary Alloys, Geminuim Publ. Co., New
York, 1985, p. 756.
5] A. Ludviksson, L.E. Rumaner, J.W. Rogers Jr., F.S. Ohuchi, J. Cryst.
Growth 151 (1995) 114.
H. MJczy n´ ska, A. Marasek, S. ŁJgowski, K. Strzakołwski, Appl. Phys.
Lett. 89 (2006) 121918.
[17] Inorganic Crystal Structure Database (ICSD), Gmelin-Institut f u¨ r
Anorganische Chemie and Fachinformationszentrum FIZ Karlsruhe,
1995.
[
[
6] K. Fujita, T. Izumi, K. Ohsaki, T. Tambo, H. Ueba, C. Tatsuyama, Thin
Film Solids 247 (1994) 134.
[18] JCPDS — Powder Diffraction File Search Manual, International Centre
for Diffraction Data, Pennsylvania, USA, 1981.
[
[
7] S.L. Stoll, E.G. Gillan, A.R. Barron, Adv. Mater. 8 (1996) 182.
8] T.L. Ng, N. Maung, G. Fan, I.B. Poole, J.O. Willians, A.C. Wright, D.F.
Foster, D.J. Cole-Hamilton, Chem. Vapor Deposition 2 (1996) 185.
9] A. M a¨ rkl, M. von der Emde, C. Nowak, W. Richter, D.R.T. Zahn, Surf.
Sci. 331–333 (1995) 631.
[19] H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65.
[20] A.C. Larson, R.B. Von Dreele, General Structure Analysis System
(GSAS), Los Alamos National Laboratory Report LAUR 86-748, 1994.
[21] M. Newville, J. Synchrotron Radiat. 8 (2001) 322;
S. Pascarelli, O. Mathon, G. Aquilanti, J. Alloys Compd. 362 (2004) 33.
[22] TAPP, Version 2.2, E. S. Microwave Inc., Wade Court, Hamilton, OH.
[23] J.B. Renucci, Ph.D. Thesis, Universite Paul Sabatier, Toulouse, France,
1974.
[
[
[
10] C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1.
11] C.E.M. Campos, J.C. de Lima, T.A. Grandi, K.D. Machado, P.S. Pizani,
Solid State Commun. 126 (2003) 611.
[
[
[
[
[
12] S.M. Souza, C.E.M. Campos, J.C. de Lima, T.A. Grandi, P.S. Pizani, Solid
State Commun. 139 (2006) 70.
13] K.D. Machado, J.C. de Lima, C.E.M. Campos, P. Jovari, J. Phys.:
Condens. Matter 16 (2004) 581.
[24] F.Q. Guo, K. Lu, Phys. Rev. B 57 (1998) 10414.
[25] M. Rajalakshmi, A.K. Arora, Nanostruct. Matter 11 (1999) 399.
[26] D. Nesheva, I.P. Kotsalas, C. Raptis, E. Vateva, J. Non-Cryst. Solids 224
(1998) 283.
[27] F.Q. Guo, K. Lu, Philos. Mag. Lett. 77 (1998) 181.
[28] E. Finkman, J. Tauc, R. Kershaw, A. Wold, Phys. Rev. B 11 (1975) 3785.
[29] C.E.M. Campos, P.S. Pizani, Appl. Surf. Sci. 200 (2002) 111.
[30] L.A. Falkosky, J. Camassel, Physica B 284–288 (2000) 1145.
[31] A. Ingale, K.C. Rustagi, Phys. Rev. B 58 (1998) 7197.
14] J.C. de Lima, T.A. Grandi, R.S. de Biasi, J. Non-Cryst. Solids 286 (2001)
9
3.
15] K.D. Machado, J.C. de Lima, C.E.M. Campos, A.A.M. Gasperini, Solid
State Commun. 127 (2003) 477.
16] W.C. Tsai, C.L. Cheng, T.T. Chen, Y.F. Chen, Y.S. Huang, F. Firszt,