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2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
p s scurrent topics in solid statephysics
c
status
solidi
www.pss-c.comphysicaPhys. Status Solidi C 6, No. 5, 1203 1206
(2009) / DOI 10.1002/pssc.200881183
The optical properties
of Cd1x
MnxTe (0 < x< 0.55) solid solutions
in monocrystals and thin polycrystalline films
Iuliana Caraman*, Gabriel Lazar Luminita Bibire, Iuliana Lazar,
Marius Stamate, and Dragos Rusu
The Bacau University, Calea Marasesti 157, 600115 Bacau,
Romania
Received 1 September 2008, revised 3 November 2008, accepted 8
December 2008Published online 11 March 2009
PACS 73.50.Gr, 73.50.Pz, 78.20.Ci, 78.40.Ha, 78.66.Li
*Corresponding author: e-mail [email protected], Phone:
+40234542411/161, Fax: +40234580170
2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction CdTe and MnTe crystals have at-tracted
considerable attention of scientists working in bothfundamental and
applied research, thanks to a number ofsignificant advantages. They
are able to form solid solu-tions overall concentration range [1],
are used as parentsubstances in semimagnetic superlattices
preparation, be-sides they are suitable in investigating the
effects related toMn2+ centres, magnetic polarons characteristics
etc. [2, 3].Cd1xMnxTe solid solutions crystallise based on the
CdTezincblende lattice in a wide concentration range; the
latticeparameter, a, decreases linearly with concentrationx, up tox
= 0.8.
The present paper reports on the study of the opticalabsorption
of Cd1xMnxTe single crystals and thin films, inorder to improve
their efficiency in a series of technologi-cal applications
concerning the quantum structures andphotosensitive heterojunctions
for the VIS spectral range.
2 Experimental Cd1xMnxTe monocrystals withcomposition x = 0.00;
0.01; 0.05; 0.13; 0.40; 0.40 and 0.50were synthesized from
elementary compounds Cd, Mn andTe (99.999 %) and were grown by
Bridgman method.
By using the emission spectral analyze of original crystals,the
following impurities have been identified: Cu(~106 %), Fe, Cr, Ni
(~105 %).
The films under study have been prepared by twomethods: discrete
evaporation of Cd0.5Mn0.5Te single crys-tals with grain dimensions
d< 15 m, and physical vapordeposition. In the case of physical
vapor deposition, singlecrystalline blocks of Cd0.5Mn0.5Te with
dimensions from 3mm to 5 mm have been used as source materials.
Thesource temperature laid in the range (1020-1050) K, whilethe
substrate was maintained at temperatures between 860K and 930 K. In
this temperature range, single crystallinethin films with mirrored
faces have been obtained. In bothmethods, substrate temperature
(T
s) has been varied, so that
single crystalline films have been obtained. For
discreteevaporation of Cd0.5Mn0.5Te single crystals, a wolfram
cru-cible with temperature ~1700-1750 K has been used. Thesubstrate
temperature was maintained at ~ 680 K.
The quantity of Mn, and the composition of films ob-tained by
both methods were determined by Atomic Emis-sion Spectral Analysis
method with Cu atoms. The analyti-cal spectral line for Mn was the
line which wavelength wasMn = 2801.084 and it was set beside the
wavelength of
The optical absorption spectra at the fundamental absorption
edge of Cd1xMnxTe (0 x 0.5) at 78-300 K have been
studied. Single crystals were grown by Bridgman method. At
293 K, the fundamental absorption red edge in crystals withx
0.5 is determined by the exciton absorption. The fundamen-
tal absorption edge has the specific shape of crystals with
high impurity levels density. The absorption spectra in the
domain of the fundamental absorption band of Cd1xMnxTesolid
solutions in thin films prepared by discrete (flash)
evaporation of synthesized single crystals and by physical
va-
por deposition (using Cd0.5Mn0.5Te as source material) are
in-
vestigated. The optical gap of Cd1xMnxTe films prepared by
discrete evaporation is equal to ~2.0 eV and 2.13 eV at tem-
peratures 293 K and 78 K, respectively.
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1204 I. Caraman et al.: The optical properties of Cd1xMnxTe
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2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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physica
p s sstatus
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c
Cu (Cu = 2824.369 ). The spectra were recorded byPGSZ
spectrometer that use 600 mm1 diffraction grating.As determined by
an interferometric method, the film
thickness ranged between 0.10 m and 8.00 m.The absorption
coefficient, , at wavelength has beencalculated by taking into
account the multiple reflexion ofincident radiation on sample
faces, as well as the interfer-ence in plan-parallel layers, using
the equation [4]:
( )2
2
1
d d
RT
e R e
=
(1)
where d is the sample thickness, T and R represent
thetransmission and reflexion factors respectively.
The optical transmission and reflexion spectra, T(h)and R(h)
respectively, at temperature 78 K and 293 K,have been recorded by a
spectrophotometric equipmentbased on a diffraction grill
monochromator (600 mm1),with a spectral resolution ~ 0.5 meV, in
the domain (1.30-2.50) eV.
3 Results and discussion Figures 1 and 2 illustratethe spectral
dependences of the optical absorption for thesolid solutions in the
range 0.0 x 0.5 at temperatures78 K and 293 K, respectively. As a
characteristic feature ofthe absorption spectrum of respective
solid solutions at lowtemperatures, an outlined absorption edge is
present, whichdisplaces in function of sample composition, x. In
thehigher wavelengths domain, a series of maxima clearlyevidences,
that can be associated to the absorption accom-panied by formation
of free excitons. The exciton absorp-tion at 78 K is manifested by
a narrow band localised inthe vicinity of the fundamental
absorption threshold. Ascan be observed in Fig. 1, the absorption
coefficient in theexciton line maximum (n = 1) is weakly decreasing
at in-creased amount of MnTe in CdTe. At the same time, an
in-crease of the average width of the exciton line contour
forcompositions 0.00 x 0.13 is registered. When theamount of MnTe
in CdTe is further increased, the dynam-ics of the exciton line
change is seen to reverse: the peakabsorption increases, and the
width of the line n = 1 de-crease.
1.50 1.75 2.00 2.25
105
104
103
102
(
cm
-1
)
h (eV)
x=0.00
x=0.01
x=0.05
x=0.13
x=0.30
x=0.40
x=0.45
x=0.50
Figure 1 Spectral dependence of the absorption coefficient
of
Cd1xMnxTe at T= 78 K.
With a precision up to the exciton binding energy,equal to ~ 7
meV, one can admit that the energy position ofthe exciton (n = 1)
line peak indicates the bandgap of the
respective solid solution.
1.50 1.75 2.00 2.25
104
103
102
(
cm-
1
)
h (eV)
x=0.00
x=0.01
x=0.05
x=0.13
x=0.3
x=0.45
x=0.50
Figure 2 Spectral dependence of the absorption coefficient
of
Cd1xMnxTe at T= 293 K.
In Fig. 3 the dependence of this energy on the
samplecomposition,x, is presented. As can be found in this
figure,the width of the forbidden band in solid solutions
ofCd1xMnxTe linearly increases with the amount of MnTe inCdTe. This
dependence is also characteristic to other solidsolutions of
different binary compounds [5]. A small de-viation from the Vegard
law is registered in the solutions
with compositions in the vicinity of CdTe, when
MnTeconcentration is under 5 %.
0.0 0.1 0.2 0.3 0.4 0.5 0.6
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
E
(eV)
x-1
Figure 3 Energy of the exciton line as a function of x for
Cd1xMnxTe at T= 78 K.
Probably, in these solutions a part of Mn and Te ionsfills up
the Cd and Te native defects in CdTe, while an-other part
participates in the formation of the solid solutionitself. At
increasing sample temperature from 78 K to293 K, the thermal
coefficient of the fundamental absorp-
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Phys. Status Solidi C 6, No. 5(2009) 1205
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Contributed
Article
tion edge of Cd1xMnxTe solid solutions varies from2.8104 K1,
forx = 0.01, to 4.6104 K1, forx = 0.13.
We have examined the fundamental absorption edge of
as-prepared Cd1xMnxTe thin films, as well as of theirsource
material.
1.95 2.00 2.05 2.10 2.15 2.20 2.250
10
20
30
40
50
60
70 Cd0.5Mn0.5Te
293 (K)
78 (K)
(
cm
-1
)
h (eV)
Figure 4 The absorption spectrum of Cd1xMnxTe single
crystals
at 293 K and 78 K.
Figure 4 shows the absorption spectra, at temperatures78 K and
293 K, of Cd0.5Mn0.5Tesingle crystals. The opti-
cal energy gap at temperatures 293 K and 78 K, as deter-mined by
extrapolation to zero ( = 0) of the linear portionof(h) dependence,
was found as 2.10 eV and 2.18 eV,respectively. At the same time,
for absorption coefficients < 8 cm1 at 78 K and < 20 cm1 at
293 K clearly evi-dences the presence of an absorption band which
may beof impurity nature (A and A, respectively). Since holes
arethe majority charge carriers in these crystals [6], one canadmit
that respective absorption particularities are pro-duced by
electron transitions from the acceptor level intothe conduction
band.
1.4 1.6 1.8 2.0 2.20
400
800
1200
1600
2000
2400
2800
3200
3600
78(K)
293 (K)
(
cm-
1
)
h (eV)
A'
A
Cd1-x
MnxTe
Figure 5 The absorption spectrum for Cd1xMnxTe film on mica.
Because the studied samples have the hole conduction,
theacceptor levels which are negative ionized are active in
theoptical absorption. The acceptor level is localized at ~30
meV forT= 293 K (difference between 2.10 and 2.07 eV),and at ~40
meV forT= 78 K (difference between 2.18 and2.14 eV), above the
valence band.
The absorption spectra of films deposited onto micasubstrate at
temperature 680 K, by discrete evaporation ofCd1xMnxTe crystals,
are presented in Fig. 5. The absorp-tion coefficient for < 2400
cm1 decreases monotonouslywith the energy. This shape of absorption
coefficient in therange of fundamental band edge can be caused by
the pres-ence of the big concentration of structural defects in
thesample. The thermal treatment in vacuum at the tempera-ture of
620 K for 10 hours accentuate the edge of absorp-tion band (Fig. 6)
which indicate the decrease of defects
concentration in studied films.The absorption edge, approximated
by extrapolation oflinear portion of (h) dependence to = 0, is
localized at1.99 eV and 2.13 eV for temperatures 293 K and 78 K,
re-spectively.
As can be ascertained from Fig. 5, in the absorptionspectrum of
Cd1xMnxTe solid solutions in thinfilms pre- pared by discrete
evaporation, a particularity in form ofthreshold is evidenced,
located at ~1.70 eV (the point A, T= 293 K) and 1.84 eV (the point
A, T= 78 K).
1.4 1.6 1.8 2.0 2.2 2.4
0
400
800
1200
1600
2000
2400
2800
3200
3600
78 (K)
293 (K)
(
cm
-1
)
h (eV)
Cd1-x
MnxTe
A'
A
Figure 6 The absorption spectrum of Cd1xMnxTe thin film ob-
tained by discrete evaporation on mica and thermal treated at
the
temperature of 620 K for 10 hours.
It can be admitted that the picks A and A are deter-mined by the
optical transitions from the negative ionizedacceptor levels in the
conduction band because the electri-cal conductivity through holes
take place in theCd1xMnxTe films obtained by discrete
evaporation.
The impurity acceptor levels responsible for these ab-sorption
bands are located at ~0.30 eV above the top of thevalence band. At
decreasing temperature from 293 K to78 K, the width of the
forbidden band increases by ~0.13eV, while the position of the
acceptor level is practically
unchanged.
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1206 I. Caraman et al.: The optical properties of Cd1xMnxTe
solid solutions
2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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physica
p s sstatus
solidi
c
1 .36 1 .4 0 1 .4 4 1 .4 8 1 .5 2 1.5 60
1
2
3
4
5
6
(
103
cm
-1)
h (eV)
Cd0.5
Mn0.5
Te
Sample 1:
293 K; 1.45 eV
Sample 2:
293 K; 1.44 eVSample 1:
78K; 1.51 eV
Sample 2:
78K; 1.508 eV
Figure 7 Absorption spectra for Cd1xMnxTe films prepared by
physical vapor deposition for substrate temperatures Ts = 883
K
(sample 1) and 893 K (sample 2). In legend temperature,
T(K),
and optical gap Ego(eV) are indicated.
In Fig. 7 the absorption spectra of Cd1xMnxTe films prepared by
physical vapor deposition are presented. Forsubstrate (mica)
temperatures between 870 K and 930 K,single crystalline films with
mirrored faces are obtained.
As can be seen in the last figure, the substrate tempera-ture
weakly influences the shape of the absorption spec-trum in the
vicinity of the fundamental absorption edge.
600 625 650 675 700
1.450
1.475
1.500
1.525
1.550
Eg
(eV)
Ts
(0C)
Cd0.5
Mn0.5
Te
293 K
78K
Figure 8 Dependence of the optical gap on the vapor phase
deposition temperature for films prepared from Cd1xMnxTe
sin-
gle crystals.
The absorption coefficient is seen to change moremarkedly with
temperature only for values less than 2000cm1. At increasing
substrate temperature, the slope of(h) curve slowly decreases at
high temperatures and isseen to increase at temperatures above 910
K. The shift ofthe absorption curves from Fig. 7 is mainly due to
the de-crease of the absorption coefficient at increased
depositiontemperatures. Since the optical band gap is determined
by
the linear segment of(h) curve (Fig. 7) for absorption
coefficients in the range (2-5)103 cm1, one can admit thatthe
fundamental absorption edge is formed by direct elec-tron
transitions between the valence and the conduction
bands, and it is not of impurity nature. By
extrapolating(h)curve to = 0, the optical gap was found as ~1.45
eVat 293 K and ~1.51 eV at 78 K. Since the slope of(h)depends on
the crystallization temperature of respectivecompounds, the
dependence of the optical gap, Eg, on thesubstrate temperature (Ts)
can be determined, which is il-lustrated in Fig. 8.
4 Conclusions Cd1xMnxTe solid solutions havebeen synthesized
from their chemical components. Theconcentration of the impurity
atoms (Cu, Fe, Ni, Mg, Cr)was under 105 % at. The single crystals
with compositionin the range 0 x 0.50 have been grown by
Bridgman
method, from which parallelepipedic samples have beenprepared
for optical measurements.The fundamental absorption of Cd1xMnxTe
crystals
(0.00 x 0.50) is formed by free excitons, whose energyas a
function ofx comply the Vegards rule. When thetemperature is
increased from 78 K to 293 K, the red edgeof the fundamental
absorption band displaces toward lowerenergies. The thermal
coefficient of the bandgap lines inthe range from 2.8104 K1 to
4.6104 K1.
The optical gap of Cd1xMnxTe films prepared by dis-crete
evaporation of Cd0.5Mn0.5Te fragments, is equal to~2.0 eV and 2.13
eV at temperatures 293 K and 78 K, re-spectively. Based on the
linear dependence of the optical
gap on temperature of Cd1x
Mnx
Te solid solutions, was es-tablished that by discrete
evaporation of compound with x= 0.50, the obtained composition of
deposited films wasnotx = 0.50, butx 0.45. The optical gap of
single crystal-line Cd1xMnxTe films, grown onto mica from vapor
phase,depends on the substrate temperature and reaches its mini-mum
at Ts ~ 890 K at both temperatures, 78 K and 293 K.
Acknowledgements This work is supported by grant
ANCS CEEX 89/2006.
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