APPLIED PHYSICS LETTERS 92, 102507 共2008兲
Properties of „Zn,Cr…Te semiconductor deposited at room temperature
by magnetron sputtering
W. G. Wang,1,a兲 K. J. Han,2 K. J. Yee,2 C. Ni,3 Q. Wen,4 H. W. Zhang,4 Y. Zhang,1,b兲
L. Shah,1 and John Q. Xiao1,c兲
1
Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA
Department of Physics, Chungnam National University, Daejeon 305-764, Republic of Korea
3
Department of Materials Sciences, University of Delaware, Newark, Delaware 19716, USA
4
University of Electronic Science and Technology of China, Chengdu, 610054, People’s Republic of China
2
共Received 4 January 2008; accepted 11 February 2008; published online 11 March 2008兲
We report the fabrication of 共Zn,Cr兲Te films at room temperature by magnetron sputtering. Various
structural and elemental characterizations revealed there was only a zinc blende phase from the
ZnTe host and Cr atoms were distributed uniformly in these films. The magnetization measurement
by superconducting quantum interference device magnetometer clearly showed that the samples
were ferromagnetic at low temperatures with Curie temperature around 150 K. The magnetic
circular dichroism measurements confirmed that the observed ferromagnetism was originated from
the interaction of substitutional Cr ions and ZnTe host. Transport measurement revealed typical
semiconductor behaviors with the large negative magnetoresistance observed. © 2008 American
Institute of Physics. 关DOI: 10.1063/1.2890087兴
Recent intensive studies on diluted magnetic semiconductors 共DMSs兲 have opened more avenues in spintronics
applications such as spin injection, optically or electrically
controlled ferromagnetism.1–4 Besides many reports in the
pioneering systems such as Ga1−xMnxAs and In1−xMnxAs,2–4
various efforts have also been focused on transition metals
doped II-VI compounds.5–13 Especially for Cr or V doped
ZnTe, first principle calculation based on density functional
theory predicted an energetic favorable ferromagnetic ground
state.14 Indeed, it was found that Curie temperature 共TC兲 in
共Zn,Cr兲Te could be as high as 300 K and the ferromagnetism
was proved by the magnetic circular dichroism 共MCD兲 to be
originated from the s, p-d interaction between Cr ions and
the ZnTe host.5–7
In all previous reports on 共Zn,Cr兲Te and most other crystalline DMS systems, the samples were fabricated at elevated
substrate temperature higher than 200 ° C.3,5–13,15–17 However, it is important to investigate the properties of the
DMS materials grown at ambient temperature. Firstly, multilayers are often needed to exploit the spintronic functionalities of a DMS material. It is known that the properties of
multilayer samples are strongly dependent on the interdiffusion among the layers.18–22 Due to the exponential dependence of the diffusion coefficient on temperature as D共T兲
⬀ exp共−1 / KBT兲,23 it is often necessary to reduce the substrate temperature to achieve better performance. For example, it was found that in the Co/ Cu multilayers the magnetoresistance 共MR兲 ratio was increased from 40% to 80%
when the substrate temperature was lowered form 105 ° C to
room temperature20 and in the ZnO / Cu multilayers, the
roughness was greatly reduced when samples were fabricated at lower substrate temperatures.21 Therefore, it would
be interesting to study the properties of the DMS materials
fabricated at room temperature. Secondly, it was well known
that high substrate temperature could induce the formation of
a兲
Electronic mail: weigang@udel.edu.
Present address: MIT Center for Materials Science and Engineering.
c兲
Electronic mail: jqx@udel.edu.
b兲
magnetic precipitates such as Cr1−xTex and MnxAs1−x.2,3,6
Thus, the deposition of 共Zn,Cr兲Te at room temperature could
potentially suppress the development of Cr1−xTex phase during growth.
In this letter, we report the fabrication of 共Zn,Cr兲Te thin
films at ambient substrate temperature by magnetron sputtering. Besides the advantages discussed above for the room
temperature growth, it is also imperative to investigate the
properties of 共Zn,Cr兲Te films fabricated by sputtering
method, which is an economically variable process for practical applications. The polycrystalline samples grown in this
way clearly showed ferromagnetic behavior at low temperature. Subsequent structural and elemental characterizations
show that the films are free of Cr1−xTex impurities and Cr
atoms are uniformly incorporated in the ZnTe matrix. The
MCD measurements confirmed that the observed ferromagnetism originated from the interaction of substitutional Cr
ions and ZnTe host lattice.
Samples in this report were fabricated at room temperature by rf magnetron sputtering in a chamber with a base
pressure of 5 ⫻ 10−8 Torr. The films were deposited on thermally oxidized silicon substrates. The targets used have a
purity of 99.99%. The Cr concentration was controlled by
placing different amounts of Cr chips on the ZnTe target. In
the present study, sputtering under different Ar pressures and
rf power was carried out, with the Cr concentration fixed at
14%. The structures of the Cr doped ZnTe films were characterized by x-ray diffraction 共XRD兲 and transmission electron microscopy 共TEM兲. The Cr concentration of the films
was determined by energy dispersive x-ray 共EDX兲 spectroscopy. The magnetic properties of the films were characterized by a superconducting quantum interference device
共SQUID兲 magnetometer. The MCD measurement was performed in the reflection mode with magnetic field perpendicular to the film. The transport properties were tested in a
physical property measurement system with the standard
four-probe configuration.
Figure 1 shows the XRD patterns for the samples sputtered under the Ar pressures of 2 and 4 mTorr. The samples
exhibit a very strong 关111兴 orientation under these fabrication
0003-6951/2008/92共10兲/102507/3/$23.00
92, 102507-1
© 2008 American Institute of Physics
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102507-2
Wang et al.
Appl. Phys. Lett. 92, 102507 共2008兲
FIG. 3. 共Color online兲 共a兲 Hysteresis loop of the sample at 7 K with applied
magnetic field in the film plane. Inset shows the hysteresis loop in the 5 T
scale, 共b兲 FC magnetization as a function of the temperature under the applied field of 2000 Oe, and ZFC magnetization measurement at various
applied fields.
FIG. 1. 共Color online兲 XRD pattern of 共Zn,Cr兲Te film with the standard
ZnTe reference pattern.
conditions. All diffraction peaks can be indexed from a zinc
blende structure. The structure of the films remains the same
when the sputtering power was varied from 80 to 120 W
共not shown兲. Thus, all the discussions below will be made on
the sample fabricated at 120 W and 4 mTorr. It is known
that chromium and tellurium form a wide range of magnetic
compounds Cr1−xTex with Curie temperature ranging from
100 to about 350 K.24 To confirm, there is no Cr1−xTex phase
or clustering of Cr atoms, we performed high resolution
TEM 共HRTEM兲 and EDX elemental mapping studies. Figure
2共a兲 shows the plan-view HRTEM picture of the sample. No
Cr1−xTex clusters can be detected over a wide range of film
areas. Figure 2共b兲 shows the mapping of the Cr K␣ emission
intensity, demonstrating that the Cr atoms are homogeneously distributed in the film. Detailed selected area diffraction 共SAD兲 experiment was also performed. Again, the diffraction rings in all the SAD patterns from different locations
of the sample can be attributed to the zinc blende structure
only.
Figure 3共a兲 shows the M-H curve of the sample measured at 7 K by SQUID. Obvious hysteretic behavior with
coercivity about 700 Oe was observed. The irreversible behavior between the zero-field-cool 共ZFC兲 and field-cool 共FC兲
measurements is shown in Fig. 3共b兲. The fit of the FC magnetization versus temperature by Curie–Weise law yields the
Curie temperature of 150 K for the sample. This characteristic behavior of ZFC measurement is an indication of the
existence of mictomagentic/spin-glass interactions25,26 or superparamagnetism in the samples.25,27 Very recently, it is reported that Cr atoms are uniformly distributed when 共Zn,Cr兲Te films are doped with N as acceptor or fabricated under
Te-rich condition.27 However, the clustering of Cr atoms appears, although the crystallographic phase is unchanged, in
the presence of n-type doping, which significantly increases
the TC of 共Zn,Cr兲Te. The mapping of the Cr K␣ emission
intensity from those samples confirms inhomogeneous Cr
distribution in the EDX experiment. The size of Cr-rich
nanocrystals measured by the EDX mapping matches well
with the size of the superparamagnetic particles estimated
from the blocking temperature of the ZFC magnetization
measurement.27 If we would assume that the ZFC behavior
in our samples was also due to the superparamagnetic clusters of Cr-rich region, the lower-limit size of the clusters can
be estimated by KV = 25KBTB,27 where KB is the Bolzman
constant, TB is the blocking temperature from ZFC curve, V
is the volume of the nanocrystals, and K is the anisotropy
energy, which can be expressed as K = M SHA / 2. Substituting
720 emu/ cc for the saturation magnetization M S, 500 Oe for
the anisotropy field HA 共Refs. 5 and 27兲, and 30 K for the TB
as shown in Fig. 3共b兲, we have estimated that the lower limit
of the cluster size is about 10 nm. However, unlike what was
observed in Ref. 27, the Cr atoms are distributed homogonously in the scale of 10 nm, as evident from the EDX
mapping shown in Fig. 2共b兲. Therefore, the irreversible behavior in ZFC and FC in our sample is more likely due to the
spin-glass or mictomagnetic interaction instead of clustering
of Cr. A Te-rich growth condition in sputtering is probably
achieved due to the much higher vapor pressure of Zn, thus
help to form a homogenous Cr distribution.
The origin of ferromagnetism in 共Zn,Cr兲Te is probably
the double-exchange interaction between the d electrons of
Cr atoms instead of the carrier-mediate mechanism as in
共Ga,Mn兲As, since the samples showing strong magnetism is
highly resistive at low temperature5,7,27 and the ferromagnetism was not enhanced by hole doping.16 Besides ferromagnetic double-exchange interaction, anitferromagnetic
coupling between Cr ions could also exist in our samples.
It is likely that substitutional Cr ions also interact antiferromagnetically with some of interstitial Cr ions, similar to
the antiferromagnetic coupling in 共Ga,Mn兲As due to Mn
interstitial.3,28 The frustration created by the competition between the ferromagnetic and antiferromagnetic interactions,
plus the disorder due to the random substitution of Cr atoms
and the polycrystalline nature of the films, result in the mictomagentic or spin-glass phase, as reported previously in
共Zn,Co兲O,29 共Zn,Mn兲Te,26 and MnxCe1−x 共Ref. 30兲 systems.
In a DMS material, the strong hybridization of conduction s-p carrier and localized d spins leads to large Zeeman
splitting ⌬E. The MCD signal, which is the reflection difference between ␴+ and ␴-polarized photons, is directly pro-
FIG. 2. 共Color online兲 共a兲 Plan-view HRTEM picture and 共b兲 EDX mapping
of the Cr K␣ emission intensity for the 共Zn,Cr兲Te film.
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102507-3
Appl. Phys. Lett. 92, 102507 共2008兲
Wang et al.
Transport measurement showed typical semiconductor behaviors in these samples, with the large negative magnetoresistance observed. These results demonstrate the promising
application of 共Zn,Cr兲Te films in a multilayer spintronics
device.
The authors are grateful to Dr. H. Saito, B. McCandless,
K. Dobson, and K. Hart for valuable discussions. This work
was supported by NSF DMR Grant No. 0405136.
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1
2
FIG. 4. 共Color online兲 共a兲 MCD spectrum at 17 K, 共b兲 normalized MCD
intensity and M-H curve of the 共Zn,Cr兲Te film, 共c兲 temperature dependent
resistance, and 共d兲 MR curves measured with magnetic field perpendicular
to current direction at different temperatures. Inset shows the MR measured
with magnetic field parallel to current direction at 5 K.
portional to ⌬E as well as dR / dE, where the latter is the
derivative of reflectivity with respect to photon energy.31 The
dR/dE term will have an extremum near the critical points of
each material depending on the band structure. Indeed, we
observed the enhanced MCD signal at the G and L critical
points of ZnTe, as shown in Fig. 4共a兲, indicating the large
Zeeman splitting of the host semiconductor band structure
induced by the strong exchange interaction between the d
electrons of doped Cr atoms and the s-p band electrons of
ZnTe host.5,32 Figure 4共b兲 shows the magnetic field dependence of MCD intensity for the sample at 17 K. The normalized M-H curve measured by SQUID at 17 K is also plotted.
The superposition of these two curves demonstrates further
the observed magnetic signals by SQUID comes from the
共Zn,Cr兲Te DMS as does the MCD intensity.
The resistance of the sample monotonically increases
when temperature is lowered, as shown in Fig. 4共c兲. Negative MR larger than −30% is observed at 2 K. The MR is
negative in the entire temperature range from 2 to 300 K, as
evident in Fig. 4共d兲, unlike what was observed in the MBE
共Zn,Cr兲Te samples where the MR changed sign around TC.7
The shape of the MR only changes a little when the applied
external field changes from perpendicular-to-current to
parallel-to-current configuration, as shown in the inset of
Fig. 4共d兲, indicating this MR is not anisotropic magnetoresistance observed in traditional magnetic materials. Detailed
studies shows the large magnetoresistance is governed by
variable range hopping at low temperatures and will be discussed elsewhere.13
In summary, we have fabricated highly 关111兴 oriented
共Zn,Cr兲Te films at room temperature by magnetron sputtering. The deposition at ambient substrate temperature helped
to suppress the clustering of Cr atoms, which often existing
in higher growth temperatures6,16,17,27 The observed ferromagnetism was proven by the MCD to be originated from
the s, p-d interaction between doped Cr ions and ZnTe host.
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