Optical and electrical studies of vapour phase grown Cd1-xCoxTe crystals

Article abstract of DOI:10.1016/j.saa.2007.11.025

The optical properties of vapour phase grown crystals of diluted magnetic semiconductor (DMS) Cd_1-xCo_xTe were investigated for x = 0.001, 0.003, 0.005, 0.007 and 0.009. The reflectivity spectra exhibited a regular shift in the fundamental absorption edge (E₀) with composition x. The reflectivity spectra for the present samples of Cd_1-xCo_xTe of all compositions exhibit broad bands at 5300 and 11,100 cm^-1 due to the transitions of ⁴A₂ → ⁴T₁ (F) and ⁴A₂ → ⁴T₁ (P), respectively. Temperature dependence of resistivity revealed semiconducting behaviour of the samples. Hot probe test revealed that all the samples exhibited p-type conductivity.

Full text of DOI:10.1016/j.saa.2007.11.025

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Spectrochimica Acta Part A 70 (2008) 934–938
Short communication
Optical and electrical studies of vapour phase grown Cd1−xCoxTe crystals
Y. Dwarakanadha Reddy∗, B.K. Reddy, D. Sreekantha Reddy, D.R. Reddy
Department of Physics, Sri Venkateswara University, Tirupati, Andhra Pradesh 517502, India
Received 24 June 2007; accepted 26 November 2007
Abstract
The optical properties of vapour phase grown crystals of diluted magnetic semiconductor (DMS) Cd1−xCoxTe were investigated for x = 0.001,
0.003, 0.005, 0.007 and 0.009. The reflectivity spectra exhibited a regular shift in the fundamental absorption edge (E0) with composition x. The
reflectivity spectra for the present samples of Cd1−xCoxTe of all compositions exhibit broad bands at 5300 and 11,100 cm−1 due to the transitions
4
4
of 4A2 → T1 (F) and 4A2 → T1 (P), respectively. Temperature dependence of resistivity revealed semiconducting behaviour of the samples. Hot
probe test revealed that all the samples exhibited p-type conductivity.
© 2008 Published by Elsevier B.V.
Keywords: Vapour phase growth technique; DMS; Cd1−xCoxTe crystals; Reflectivity studies; Band gap; Electrical properties
1. Introduction
spin-polarized electrons. Though II–VI DMS have been studied
extensively, the experimental data published on Cd1−xCoxTe
to date are very limited and sporadic. This could perhaps be
due to the difficulties in preparing homogeneous crystals with
sufficient amounts of substitutionally incorporated Co due to
its very low solubility. Also it is obvious from literature that
no single paper reports a systematic study on preparation and
complete characterization of this system. The dispersed results
reported in literature and the interesting properties exhibited by
Cd1−xCoxTe system motivated the present authors to take up the
present study. This paper reports the results of the investigations
on diffuse reflectance spectra and electrical conductivity.
Diluted magnetic semiconductors (DMS) are a class of
semiconducting materials formed by randomly replacing some
fraction of the cation in compound semiconductors with a mag-
netic ion like Mn2+, Fe2+or Co2+. The presence of the magnetic
ion leads to a number of unusual electronic and magnetic prop-
erties, including a magnetically tunable band gap, resulting
from the large sp–d exchange interaction between the magnetic
ions and the band electrons. These interactions are strongly
influenced by the ground state of the particular substitutional
magnetic ion used [1,2]. DMS are expected to play an important
role in interdisciplinary materials science and future electron-
ics because charge and spin degrees of freedom accommodated
into a single material exhibit interesting magnetic, magneto-
optical and magneto-electronic and other properties [3]. It has
been attempted to make use of not only the charge but also the
spin degree of freedom in modern semiconductor electronics for
information processing. This new developing field is called spin-
tronics [4]. The possibility of using electron spins in electronic
devices known as spintronic devices has attracted a growing
interest in DMS. In these devices both charge and spin of the
electrons are utilized as carriers of information. The interest
greatly increased after the discovery of ferromagnetic DMS as
they can be used in spintronic devices as an effective source of
2. Experimental
Cd1−xCoxTe crystals with x = 0, 0.001, 0.003, 0.005, 0.00
7and 0.009 were grown from CdTe and CoTe (99.99%, M/s
Sigma–Aldrich) by the modified vapour phase growth technique
[5]. The required cubic phase of CoTe was prepared afresh
and was used immediately as cubic CoTe is not stable. Pure
Co and Te (99.99%, M/S Sigma–Aldrich) were used to prepare
CoTe. Appropriate quantities of Co and Te were weighed, mixed
and ground thoroughly and heated in vacuum (∼10−6 Torr)
sealed graphitized quartz tube at 1000 ◦C for about 5 min. It
was then quickly withdrawn to yield the desired poly crystalline
CoTe. Appropriate quantities of freshly prepared CoTe and CdTe
(99.99%, M/S Sigma–Aldrich) were mixed and ground thor-
oughly to ensure homogeneity. The powder was packed tightly
∗
Corresponding author. Tel.: +91 9885532989.
E-mail address: ydreddy64@yahoomail.com (Y.D. Reddy).
1386-1425/$ – see front matter © 2008 Published by Elsevier B.V.
doi:10.1016/j.saa.2007.11.025

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Fig. 1. Cd1−xCoxTe crystals with x = 0.001, 0.003, 0.005, 0.007 and 0.009 pre-
pared using vapour phase growth technique.
Fig. 2. EDAX spectrum of Cd0.997Co0.003Te crystal.
in the quartz growth tube and was used for crystal growth in a
vapour phase growth setup described else where [5]. Good qual-
ity crystals of Cd1−xCoxTe were obtained after trial and error
method. A growth temperature of 1100 ◦C, a temperature gradi-
entof 30–40 ◦C/cm and pulling ratesof 0.4–0.6 mm/h wereused.
Each growth run lasted for about 5–7 days. Good quality crystals
of 1–2 cm long were grown in the present study and the pho-
tographs of the as-grown boules are shown in Fig. 1. Samples of
appropriate dimensions cut from the as-grown boules were used
for chemical analysis and structural studies using EDAX (Model
JSM-840A), and XRD (Seifert 3003 TT) techniques The room
temperature diffuse reflectance spectra of the powdered sam-
ples of all compositions were recorded using optical Carey-5E
Spectrophotometer in the wavelength range 200–2000 nm. Elec-
trical conductivity measurements were made by the two-probe
technique.
3.2. Structural analysis
X-ray diffractograms of Cd1−xCoxTe powdered samples of
all compositions obtained using “Seifert Rayflex” and “Pow-
der” X-ray powder pattern indexing softwares. A typical XRD
spectrum for Cd0.995Co0.005Te crystal is shown in Fig. 3. The
diffraction spectra of Cd1−xCoxTe samples exhibit sharp peaks
at 23.45◦, 39.18◦, 46.25◦, 56.49◦, 62.20◦ and 71.21◦, which cor-
respond to diffraction from (1 0 0), (2 2 0), (3 1 1), (4 0 0), (3 3 1)
and (4 2 2) cubic planes, respectively. The diffracting planes
were indexed using JCPDS data of pure CdTe and CoTe. It was
found that crystals of all compositions exhibited zinc blende
structure. That there is no appearance of new X-ray diffraction
peaks indicates that Co is substituted in CdTe. A gradual shift
in the angular positions of the XRD peaks corresponding to the
host CdTe lattice is observed, which indicates changes in the
lattice parameters of the host because of the incorporation of the
dopant ions into the basic unit cell.
3. Results and discussion
3.1. Chemical analysis
The typical EDAX spectrum for Cd0.997Co0.003Te crystal
is shown in Fig. 2. The average compositions obtained from
EDAX were within 0.03 at.% of the target compositions for
all the samples. Incongruent melting and the differences in the
vapourization of various chemical species present in the samples
might have resulted in the observed microscopic deviations in
compositions. This could also be due to the differences in the
saturated vapour pressures of transition metal dopants and those
of the II–VI compounds. Thus slight inhomogeneities and sto-
ichiometric deviations are common in II–VI based DMS. Such
minor deviations in chemical compositions have been reported
by earlier workers also [6–9] in Bridgman grown Cd1−xCoxTe
crystals. There are no reports on the vapour phase growth of
Cd1−xCoxTe crystals for comparison. In view of the minor dif-
ferences between the nominal and the estimated compositions,
in all the future discussions only the nominal compositions are
used.
Fig. 3. XRD spectrum of Cd0.995Co0.005Te crystal.

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Y.D. Reddy et al. / Spectrochimica Acta Part A 70 (2008) 934–938
Fig. 6. Variation of band gap energy with composition of Cd1−xCoxTe crystals.
Fig. 4. Reflectivity spectrum of pure CdTe at room temperature.
with x in the present samples and is shown in Fig. 6. Similar
linear dependence of band gap on x was reported by Kim et
al. [11,12] in Cd1−xCoxTe crystals grown by vertical Bridgman
technique with x = 0.001, 0.004 and 0.008. Such decrease in
band gap energies with ‘Co’ content has also been reported by
some other authors [13,14] also in similar other II–VI DMS. The
present linear dependence of band gap (Eg) on x is expressed by
a straight line fit of the form:
3.3. Reflectivity studies
The diffuse Reflectance spectrum of undoped CdTe is as
shown in Fig. 4. The observed transitions are labeled as
Γ 15 → Γ 1 (E0), Λ3 → Λ1 (E1) and X5 → X1 (E2). These are
due to the transitions from the highest valence band (Γ 15) to the
lowest and second lowest conduction bands (Γ 1), respectively.
Transitions at the fundamental absorption edge (E0) mark the
beginning of this structure. The pronounced structures found
in the energy region 1.4–6 eV are due to E0, E0 + Δ0, E1,
E1 + Δ1 and E2 transitions. The present experimental values of
E0, E0 + Δ0, E1, E1 + Δ1 and E2 are in good agreement with the
values reported by Greenway [10] in CdTe. Fig. 5 shows the
reflectance spectra of Cd1−xCoxTe samples of all compositions
in the wavelength range 300–1300 nm. It is obvious from Fig. 5
that there is a regular shifting of the fundamental absorption
edge (E0) to lower energies (higher wavelengths) with increas-
ing Co concentration. The band gap energy decreases linearly
E0(x) = 1.487 − 4.947x (eV)
(1)
This is in good agreement with the best fit.
E0(x) = 1.48 − 4.23x (eV)
(2)
reported earlier by Kim et al. [12]. This variation is dependent
on the quality of the crystals and the preparation of the reflec-
tive surfaces. The electronic configuration of Co2+ion is 3d7
4
so that, the ground state of the free Co2+ ion is F and the
4
4
first excited state is P. The lowest energy term F splits into
4
4
an upper T1 orbital triplet, a middle T2 orbital triplet and a
lower 4A2 orbital singlet ground state under a crystal potential of
tetrahedral symmetry (Td). Introduction of spin-orbital coupling
shifts 4A2 level, which becomes Γ 8 state. The orbital triplet 4T1
and 4T2 are split according to Γ 6 + Γ 7 + 2Γ 8 [7]. Sugano et al.
[15] have shown band positions of each of the above-mentioned
transitions.
The reflectivity spectra for the present samples of
Cd1−xCoxTe of all compositions exhibit broad bands around
4
4
4
4
5300 and 11,100 cm−1 due to A2 → T1 (F) and A2 → T1
(P) transitions, respectively. These are shown in Fig. 7. Hom-
merich et al. [16] have observed the same type of transitions at
∼5347 cm−1 (1870 nm) due to 4A2 → T1 (F) transition and at
4
∼11,111 cm−1(900 nm) due to 4A2 → T1 (P) transition. Simi-
4
lar types of bands were also observed by Kim et al. [7] at room
temperature. Therefore it is concluded that when cobalt ions
are incorporated in CdTe lattice, the Cobalt ions enter the lat-
Fig. 5. Shift in the position of absorption edge (E0) with composition (x).

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937
Fig. 7. Reflectance spectra of Cd1−xCoxTe crystals with wave number.
Fig. 9. Variation of resistivity with Co concentration at 300 K.
tice substitutionally in place of divalent metal ions and the site
symmetry is tetrahedral.
in temperature showing semiconducting behaviour. Further the
resistivity showed a definite decrease with increasing Co con-
centration. Fig. 9 shows the variation of resistivity with Co
concentration at room temperature. There are no reports on elec-
trical studies of Cd1−xCoxTe system for comparison. Ueda et al.
[17] measured the temperature dependence of resistivity of Co
doped ZnO samples and observed semiconducting behaviour.
The resistance measured by Ko and Blamire [18] for x = 0.04
and 0.06 of Cd1−xCrxTe in the temperature range 77–320 K
also exhibited semiconducting behaviour. Saito et al. [19] also
observed similar semiconducting nature in Cr doped ZnTe with
x = 0.035.
The decrease in resistivity with increasing Co concentra-
tion in the present samples can be explained as follows. The
magnetic impurity‘d’ states in DMS generally appear near the
Fermi level (here Co is magnetic impurity in CdTe). The states
may exhibit splitting resulting in high spin configuration of ‘d’
electrons. These impurity states undergo hybridization with ‘p’
states. The ‘d’ states of Co split under the influence of tetra-
hedral crystal field of CdTe. This results in lower doublet eg
state and a higher energy t2g state. This doublet hybridizes with
‘p’ orbital of the valence band further splitting into t-bonding
and t-antibonding states. The t-bonding states participate in
the bonding and hence are localized. However, the antibonding
states have higher energy level and contain itinerant electrons.
The energy of bonding states lies in the close vicinity of the
conduction band. Hence with the increase in temperature the
electrons in the impurity states jump into the conduction band
on thermal activation. This explains the observed decrease in
resistivity with increase of temperature that is the semiconduct-
ing behaviour in Cd1−xCoxTe samples (Fig. 8). Also since CdTe
is a semiconductor by itself and small dopant concentrations
which is the case in the present samples may not affect the semi-
conducting nature of the host CdTe which is a wide bandgap
material. Upon increase in Co concentration more number of
electrons are promoted to conduction band, resulting in further
decrease in resistivity. This explains the observed decrease in
resistivity with increase in Co content (Fig. 9).
3.4. Electrical studies
All the samples exhibited ‘p’ type conductivity as revealed
by the “hot probe test”. Electrical resistance measurements were
carried out using the two probe technique on samples of dimen-
sions 4 mm × 3 mm × 2 mm cut from the as-grown boules of
Cd1−xCoxTe in the temperature range 190–450 K. Keithley 2000
DMM along with scanner cord facility was used to measure
the resistance of the crystals. The measuring instruments were
integrated to a computer through RS 232 cables. The scanner
chord of the instrument scanned the data in required intervals
of time/temperature and stored the data in ASCII files. At room
temperature, the resistivities of the present Cd1−xCoxTe sam-
ples were in the range 1–4 Mꢀ-cm. Fig. 8 shows the variation
of resistivity ‘␳’ with temperature ‘T’ for samples of all composi-
tions studied. A decrease in resistivity is observed with increase
Fig. 8. Variation of electrical resistivity with temperature of Cd1−xCoxTe crys-
tals.