Sensitivity of Zn1-xCdxTe to tera-hertz signals enhanced by structural
orderings
S.-W. Han*a, S.-H. Kangb
Division of Science Education, Chonbuk National University, Jeonju 561-756,
Korea
aE-mail address: swhan@chonbuk.ac.kr
bE-mail address: kangh3@chonbuk.ac.kr
Keywords: structure, disorder, crystal, ZnCdTe, semiconductor, x-ray
diffraction, XAFS, tera hertz
Abstract. A better efficiency of sensing tera-hertz signals (T-rays) depends on a better
crystalline structure which is obtained by replacing a small amount of Cd on Zn sites of
ZnTe crystals. Zn1-xCdxTe (ZCT) crystals were fabricated with an advanced vertical
Bridgeman method and the structural properties of the ZCT crystals were studied with
high resolution x-ray diffraction (HRXRD) and x-ray absorption fine structure (XAFS)
measurements. The Rietveld refinement to the HRXRD data showed that the ZCT
crystals at x ~ 0.05 were slightly less disordered than the ZnTe crystals. These results
were confirmed by the XAFS data analysis. The better structural orderings can be
directly related to the sensing and emitting efficiencies of T-ray by the ZCT crystals.
Introduction
Tera-hertz waves (T-rays) have attracted large attentions for their practical applications
as well as academic research [1,2]. As detectors or sources of T-rays, oriented (<110>)
Zn1-xCdxTe (ZCT) single crystals have been widely used [2]. Liu et al. [2] have reported
a very interesting observation that the optimized efficiency of detecting and emitting Trays could be obtained by the ZCT crystals with the Cd composition ratio of 0.05. As
Cd (4d105s0) cations are replaced on the Zn (3d104s0) sites of the ZCT crystals, the band
gaps of the ZCT ternary alloys are approximately linear with the Cd composition ratio
[3]. Therefore, the efficiency of the ZCT crystals to T-ray is expected to be linear.
However, Liu et al. reported that the efficiency was maximized at the Cd composition
of 0.05. The wavelengths of T-rays induced with a femto second pulsed laser are in the
range of ~10-4 m with the energies of several meV. It has been known that the optical
rectification of T-rays is related to the refractive index and energy band gaps of the
crystals [4]. Therefore, it should be related to the structural properties of the crystals.
We explored the possibility of structural changes in the ZCT crystals with highresolution x-ray diffraction (HRXRD) and x-ray absorption fine structure (XAFS)
measurements.
Experimental Details
Zn1-xCdxTe (ZCT) (x = 0.0, 0.05, 0.1, 0.15, 0.2, 0.95, 1.0) single crystals were fabricated
with an advanced vertical Bridgeman method [2]. In the single crystal fabrication, the
highly pure shots of Zn(6N), Cd(6N) and Te(6N) were used. First, the shots were fully
melted in a regular vertical Bridgman furnace and moved to the advanced vertical
Bridgman furnace where the ampule was kept at 1200 oC for a day and then slowly
lowered with a stepper motor for ampule cooling and crystallization. This process
Figure 1 HRXRD from Zn1-xCdxTe crystals as a function of momentum transfer. From the top x = 0,
0.05, 0.1, 0.15, 0.2, 0.95 and 1.0.
considerably improved the quality of the ZCT crystals because the temperature can be
kept uniformly over whole crystallizing region of the ampule. The ingots are cut along
the {110} plane into disks with thickness of a few mm and measured the emitting and
detecting efficiencies of the T-rays [2]. The disks were confirmed to be high quality
single crystals by conventional XRD measurements. The single crystal ingots were
ground and sieved with a 25 m sieve. High-resolution x-ray diffraction (HRXRD)
measurements from the powder ZCT crystals at the atmosphere were performed on 8C2
beamline of Pohang Light Source (PLS) with the incident x-ray energy of 7500 eV. The
incident x-ray angle to the sample was fixed at 7 degree and the sample rotates with the
speed of 2 rotations per minute to average over all faces. The analyzer of Si(111) double
crystals and a collimator with the acceptance angle of 0.2 degree were used to eliminate
inelastic and diffuse scatterings. For XAFS measurements, the sieved powder was
uniformly spread over an adhesive tape and fold to achieve the x-ray absorption step at
each absorption edge to be about 1.0. XAFS measurements were performed on ID line
of PNC-CAT at Advanced Photon Source (APS). The incident x-ray energy was
selected with three fourth tuned Si(111) and Si(311) double monochromator for Zn and
Cd K-edges, and Te K-edge measurements, respectively. The specimens were cooled to
20 K with a closed-cycle refrigerator for the XAFS measurements.
Results and Discussion
The HRXRD data measured from the ZCT series show a cubic-zinc-blende structure
Figure 2 Magnitude of Fourier transformed XAFS (k3χ) at the (a) Te, (b) and (c) Cd K-edges from
Zn1-xCdxTe for different Cd compositions of 0.0, 0.05, 0.1 and 0.15 measured at 20 K. The XAFS
data obtained after the atomic background was removed from raw data were Fourier transformed
to r-space. Data in the k-range of 3.0 - 12.0, 3.0 - 16.0, and 3.0 - 14.0 Å-1 for Te, Zn and Cd K-edges,
respectively, were used for the Fourier transform with the Hanning windowsills of 0.5 Å-1.
(space group of f -4 3 m) without an extra phase, as shown in Fig. 1. The width of the
diffraction peak was abruptly broad as x ≥ 0.1. To obtain more accurate structural
information, the data was analyzed with a Rietveld refinement. The details of the
refinements of the ZCT crystals can be found else where [5]. From the refinements, we
found that the lattice constants of the ZCT crystals were linearly increased from
6.11566(2) to 6.49558(2) Å, as the Cd composition ratio was changed from 0 to 1.
The HRXRD studies showed an very interesting feature that the displacement factors of
both sites of Te and Zn/Cd had a minimum value at the Cd composition ratio of 0.05
and that the displacement factors had a disorder transition at the Cd composition ratio of
x = 0.1. This implied that the quality of the ZCT crystals with the Cd composition ratio
of 0.05 was better than that of ZnTe crystals. This result agrees with the previous
studies which showed the possibility of defects in ZCT crystals experimentally [6] as
well as theoretically [7].
Environmental structures around Te, Zn and Cd atoms of the ZCT crystals were studied
with XAFS as shown in Fig 2. These XAFS data were analyzed with the UWXAFS
package [8] using standard procedures [9] and photoelectron back-scattering functions
calculated with the FEFF8 code [10]. After the atomic background absorption μ0 was
determined using AUTOBK (part of the UWXAFS package), the XAFS function χ(k) =
μ /μ0 - 1 was obtained. Figure 2 shows the magnitude of Fourier transformed χ as a
function of distance from the probe atoms. In ZnTe crystals with the zinc-blende
structure each Te atom has 4 Zn neighboring atoms and 12 Te atoms as the second
neighboring atom. At Te K-edge, the Fourier transformed XAFS shows that the
magnitude of XAFS from Te-Te pairs (second peak) is gradually diminished with Cd
composition ratio. At Zn K-edge, the second peak and third peaks corresponding with
12 Zn-Zn/Cd pairs and 12 Zn-Te pairs are more affected by the Cd substitution than the
first neighbor atoms, Zn-Te pairs. Note that the peaks are shifted on the ř axis from
Figure 3 Magnitude of Fourier transformed XAFS of Zn0.95Cd0.05Te crystals. Dotted lines are data
measured at (a) Te, (b) Zn and (c) Cd K-edges. The solid line is the best fit with the XAFS theory
[10].
their true bond lengths due to the phase shift of the back scattered photoelectron.
Detailed fits are therefore necessary to obtain quantitative information.
Fits to the XAFS data with the XAFS theory [10] including single and multi-scattering
paths start from the zinc-blende structure, allowing the bond lengths and Debye-Waller
factor (σ2, includes thermal vibrations and static disorder) for each shell below 5.5 Å to
vary. Figure 3 shows the XAFS data from Zn0.95Cd0.05Te and the best fits at Te, Zn and
Cd K-edges. For the ZCT samples, the total XAFS was obtained by the summation of
XAFS of each species atom with a weight: χ(r) = χTe + χZn (1 - x) + χCd(x), where x is the
Cd composition ratio, assuming that the Cd atom is randomly distributed over the Zn
sites. In the fit, the σ2s of Te-Cd (Zn-Cd) pairs were fixed as those of Te-Zn (Zn-Zn)
pairs while the bond lengths of Te-Cd (Zn-Cd) pairs were fixed as those of Te-Zn (ZnZn) pairs, added an extra distance determined at Cd K-edge. The fit results are
summarized in Table I.
As shown in the Table I, all bond lengths were gradually increased from ZnTe with Cd
composition ratio except the pairs of Te-Zn/Cd(1) and Te-Te. From the fits, we found
that the bond length of Zn-Te(1) pairs determined at Zn K-edge is 0.13 Å shorter than
that of Cd-Te(1) pairs. 0.13 Å is about the difference of atomic radii of the Zn and Cd
atoms. For the higher shells, the bond length differences of neighboring atoms from the
Zn and Cd atoms were reduced. This implied that the structural distortion due to the Cd
substitution on the Zn atoms was diminished at a distance from the Cd atoms. The
distance where the structural distortion was negligible was estimated to be ~6 Å which
is very comparable to the lattice constant of the ZCT crystals ~6.10 Å. At Cd K-edge,
the possibility of existing Cd on the Te site was determined to be negligible within
uncertainty of 3 %. The σ2s of Zn-Te(1) and Cd-Te(1) were not changed much with Cd
composition ratio. However, The σ2 of Te-Zn/Cd(1) pairs was decreased in the Cd
composites. This implied that the substitution of Cd in Zn assisted a tight bond of
Table I. Results from the XAFS data analysis. d(Å) is the bond length of pairs, σ2(Å2) is
Debye-Waller factor. S02s of 0.88(6) and 0.85(4), 0.91(5) at Te, Zn and Cd K-edges,
respectively, were determined from the best fits to the ZnTe and CdTe data and fixed in
the fits for all specimens.
Specimen
ZnTe
Zn0.95Cd0.05Te
Zn0.90Cd0.10Te
Zn0.85Cd0.15Te
Zn0.80Cd0.20Te
dTe-Zn/Cd(1)
2.644(3)
2.653(3)
2.659(4)
2.641(3)
2.644(4)
σ Te-Zn/Cd(1)
0.0034(2)
0.0022(2)
0.0024(2)
0.0015(2)
0.0015(2)
Specimen
ZnTe
Zn0.95Cd0.05Te
Zn0.90Cd0.10Te
Zn0.85Cd0.15Te
Zn0.80Cd0.20Te
dTe-Zn/Cd(1)
2.641(2)
2.641(1)
2.642(1)
2.646(2)
2.646(2)
σ2Te-Zn/Cd(1)
0.0030(2)
0.0031(1)
0.0030(1)
0.0030(1)
0.0030(1)
Specimen
Zn0.95Cd0.05Te
Zn0.90Cd0.10Te
Zn0.85Cd0.15Te
Zn0.80Cd0.20Te
dTe-Zn/Cd(1)
2.775(2)
2.778(1)
2.784(1)
2.786(1)
σ2Te-Zn/Cd(1)
0.0019(1)
0.0019(1)
0.0021(1)
0.0021(1)
2
Te K-edge
dTe-Te
4.326(7)
4.330(4)
4.337(6)
4.326(4)
4.326(6)
Zn K-edge
dTe-Te
4.320(6)
4.335(4)
4.341(4)
4.338(7)
4.356(7)
Cd K-edge
dTe-Te
4.376(4)
4.384(4)
4.399(4)
4.405(4)
σ2Te-Te
0.0038(2)
0.0033(2)
0.0047(3)
0.0039(2)
0.0059(2)
dTe-Zn/Cd(2)
5.069(9)
5.097(7)
5.098(11)
5.098(6)
5.121(12)
σ2Te-Zn/Cd(2)
0.0052(6)
0.0046(6)
0.0050(8)
0.0037(5)
0.0059(10)
σ2Te-Te
0.0058(2)
0.0060(3)
0.0062(3)
0.0049(5)
0.0052(6)
dTe-Zn/Cd(2)
5.065(3)
5.076(4)
5.081(6)
5.086(9)
5.101(12)
σ2Te-Zn/Cd(2)
0.0053(2)
0.0063(3)
0.0078(4)
0.0068(6)
0.0093(10)
σ2Te-Te
0.0048(3)
0.0056(3)
0.0053(3)
0.0050(3)
dTe-Zn/Cd(2)
5.081(5)
5.092(4)
5.109(5)
5.119(7)
σ2Te-Zn/Cd(2)
0.0050(3)
0.0065(3)
0.0075(4)
0.0088(6)
Te-Zn/Cd pairs. In the farther shells, σ2s were gradually increased with the Cd amount
in the ZCT crystals. We found an interesting feature that the σ2s of the shells from the
Te atoms had minimum values at the Cd composition ratio of 0.05. From these XAFS
analysis, we concluded that the lattice constant was elongated and bond length
distributions were increased with the Cd composition ratio. However, the structural
disorders around the Te atoms was slightly reduced at x = 0.05.
Summary
We have studied the structural properties of Zn1-xCdxTe crystals with the measurements
of long- and short-range orderings. From HRXRD measurements, we found that the
lattice constant was linearly increased and the structural disorder transition was
observed at the Cd composition ratio of 0.05. The XAFS studies showed that the bonds
and σ2s of the nearest neighboring atoms from Zn and Cd are not affected by the Cd
substitution on the Zn site. However, the distance and σ2s of the farther shells were
gradually increased with the Cd composition ratio. At the Te K-edge measurements, we
found that the σ2s of the neighboring atoms around the Te atoms were minimized at x =
0.05. These structural changes due to the Cd replacement on the Zn atoms should be
directly related to the efficiencies of absorbing and emitting T-rays by the ZCT crystals.
Acknowledgments
This work was supported by the Koran MOST through Proton Accelerator User
Program (No. M202AK010021-04A1101-02110) of Proton Engineering R&D Project
from the Nuclear R&D Program and the 21th Century Frontier R&D Program. The
HRXRD data were collected at the 8C2 beamline of the Pohang Light Source, Korea; a
facility which was partially supported by MOST and POSCO. XAFS measurements
were performed at the ID line of PNC-CAT at APS; a U.S. national facility which was
supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy
Sciences, under Contract No. W-31-109-Eng-38.
*
Author to whom correspondence should be addressed.
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