Raman Spectroscopy Study of Zn
1
−x
Fe
x
Se under
High Pressure
Chih-Ming Lin1
* and Der-San Chuu2
1National Hsinchu Teachers’ College, Hsinchu, Taiwan, R.O.C.
2Department of Electro-Physics, National Chiao Tung University, Hsinchu, Taiwan, R.O.C.
Zn1−xFexSe, x = 0, 0.018, 0.035 and 0.16, were studied by Raman scattering spectroscopy up to 35.0 GPa.
It was found that the semiconductor – metal phase transition pressures for these samples are 14.4, 12.8, 12.0 and 10.9 GPa, respectively. Before the semiconductor– metal phase transition, a visible anomaly of the TO Raman mode splitting was observed at 4.7 and 9.1 GPa for ZnSe, at 3.3 and 5.9 GPa for Zn0.982Fe0.018Se, and at 4.5 and 7.2 GPa for Zn0.965Fe0.035Se, respectively, while Zn0.84Fe0.16Se showed mode splitting at 4.7 GPa only. For these samples, one of the TO splitting modes exhibits phonon softening (red shift), while the other manifests wavenumber increasing (blue shift) with pressure. For x = 0.018, 0.035 and 0.16, a new Raman mode, which was identified as Fe local mode, was observed between the pure ZnSe LO and TO modes. Fe local mode exhibits blue shift behaviour before metallization and disappears as the pressure is higher beyond the metallization pressure. PACS numbers: 62.50. Y p, 64.60.− i, 78.30.Fs. Copyright 1999 John Wiley & Sons, Ltd.
INTRODUCTION
Zn1 xFexSe ternary compound crystals represent a giant
interesting sub-class of semi-magnetic semiconductors (SMSC),1 – 3 which contain interesting properties of
elec-tronic structure, far-IR absorption and Raman scattering spectroscopy, high field magnetization, exchange effects between ions and band carriers, and so on. In the past decade, the high-pressure behaviours of Zn1 xFexSe have
attracted much attention since the high-pressure-induced phase transition of the Zn0.83Fe0.17Se crystal was
dis-cussed by Qudri et al.4
by using energy-dispersive x-ray-diffraction (EDXD) measurement. Qudri et al. found that the existence of Fe in the crystal results in a reduc-tion in the transireduc-tion pressure due to hybridizareduc-tion of 3d orbitals into tetrahedral bonds. The role played by Fe in the observation of the reduction in the transition pres-sure of Zn1 xFexSe is similar to the role that Mn plays
in the Zn1 xMnxSe5 – 7 crystal. Lin et al.8 – 10 also found
by using micro-Raman and EDXD measurements that the Zn0.84Fe0.16Se crystal underwent structural transformation
from a four-coordinated B3 structure to a six-coordinated B1 structure under high pressure. For theoretical stud-ies, no report on the phase transformation of Zn1 xFexSe
under high pressure has been published. But for ZnSe, Andreoni et al. used a soft-core self-consistent pseu-dopotential plane wave method11 to predict that ZnSe
formed a new semiconductor phase at transition pressure. Smelyansky et al.12showed that the high-pressure phase of
ZnSe was metallized by using the full potential linearly augmented plane wave approach and numerical atomic orbital band structure calculations within the local-density * Correspondence to: C.-M. Lin, National Hsinchu Teachers’ College, Hsinchu, Taiwan, R.O.C.
Contract/grant sponsor: National Science Council, Taiwan; Contract/ grant numbers: NSC88-2112-M-134-001; NSC87-2112-M-009-009.
approximation.12Smelyansky et al.12and Itkin et al.13
sug-gested that the metallization occurs when ZnSe transforms from a four-coordinated zinc-blende (ZB) phase into a six-coordinated rock salt (RS) phase. Recently, Michel
et al.14 reported that ZnSe follows the structural transition
sequence: zinc-blende ! cinnabar ! NaCl ! Cmcn with increasing pressure by using the ab initio pseudopo-tential calculation. Similarly, Ahuji et al.15 also indicated
that CdTe follows the structural sequence: zinc-blende ! cinnabar ! NaCl ! orthorhombic with increasing pres-sure by using the full-potential linear muffin-tin-orbital (FPLMTO) calculation in the study of the structural phase transition of CdTe under high pressure. A similar study of the ZB structure of CdTe16 showed that one more phase
exists below the structure transformation from ZB to RS structure.
In this work, Raman scattering measurements were performed to investigate the pressure effect on the phase transition of Zn1 xFexSe crystals with x D 0, 0.018, 0.035
and 0.16, under high pressure up to around 35.0 GPa. It was found that these samples exhibit four phase regions and two unidentified transitions occur respectively at 4.7 and 9.1 GPa, 3.3 and 5.9 GPa, 4.5 and 7.2 GPa, and 4.7 GPa for x D 0, 0.018, 0.035 and 0.16. In addition, three split transverse optical (TO) phonon modes can be observed for pressure even higher than the metallization pressure. These split TO modes are found to persist up to 35.0 GPa. The effect of the Fe element on the phase transition will also be discussed.
EXPERIMENTAL
Zn1 xFexSe crystals were grown by the modified
Bridg-man method, and crystallized in the zinc-blende structure which is similar to ZnSe in the range of Fe composi-tion 0 x 0.30.17 Energy-dispersive x-ray-diffraction
CCC 0377 – 0486/99/100951–05 $17.50 Received 31 March 1999
(EDXD) measurement has been employed to characterize the structure phase of Zn1 xFexSe crystals at ambient
pres-sure. The source of EDXD is the superconductor wiggler synchrotron beam line X17C of the National Synchrotron Light Source (NSLS) of Brookhaven National Labora-tory, U.S.A. Figure 1 shows the EDXD pattern of the Zn1 xFexSe samples. One can note from Fig. 1 that the
composition of the Zn1 xFexSe samples is homogeneous.
In Fig. 1, the B3 (zinc-blende) phase but not the wurtzite phase can be observed. Our samples were then ground to 1µm size for micro-Raman measurement. The crystals and ruby chips were sealed with the pressure transmitting medium (deionized water) in the sample chamber which is a hole of 165 µm diameter and 50 µm deep drilled in the stainless steel 301 gasket as used in the previous works.8 The pressure was calibrated by the fluorescence
scale method.18,19
In each run, the applied pressure was homogeneous by using the pressure value which was obtained from averaging the values determined from four ruby chips in the sample chamber. Ruby fluorescence and Raman scattering measurements were performed in a Ren-ishaw 2000 micro-Raman system. The 5145 radiation with power of 0.6 W from the Coherent INNOVA 5.0 W Argon ion laser was focused to about 5µm on the sample surface. Most of the laser power is lost on the way and only 10% of the intensity reaches the sample. The back-scattered sig-nal was collected by a microscopic system and recorded with a Peltier cooled CCD detector. The recording time for each ruby fluorescence was of the order of 1 s, and Raman spectrum, 10 min. After the experiments, the spectra were processed under a Peakfit program as in previous work.10
The precision in the frequency determination was in the range of 1 cm 1. The corresponding error of pressure
val-ues was within š1.0 GPa at the highest pressure obtained because a good signal-to-noise ratio could be achieved in the system even for broad peaks.
Figure 1. A series of spectra of Zn1 xFexSe at ambient pressure. There is only the B3 (zinc-blende) phase, which also contains the standard identified pressure lines of internal gold.
RESULTS AND DISCUSSION
The pressure dependence of Raman scattering spec-troscopy at room temperature for Zn1 xFexSe, x D 0.018
and 0.035, is shown in Figs 2 and 3, respectively. Simi-lar results of Raman spectra of ZnSe and Zn0.84Fe0.16Se
were reported by Lin et al.8,10
At atmospheric pres-sure, two peaks identified as LO and TO phonons were observed at 250 and 200 cm 1
, 253 and 206 cm 1
for Zn1 xFexSe x D 0.018 and 0.035, respectively, as reported
previously.8,10,20,21Between these two peaks, a weak
struc-ture attributed to the Fe local (impurity) phonon mode can be labelled through the deconvolution process.8,10The
labelled Fe local phonon mode arises from the introduc-tion of the local electric field resulting from the substi-tution of a Zn atom by an Fe atom.21
At around 1.8 and 2.5 GPa, for x D 0.018 and 0.035, the Fe local mode becomes more intense at higher pressure and the Raman shift energy increases with the pressure. The pres-sure effects on the LO and TO phonon modes exhibit similar blue-shift behaviour as the Fe local phonon mode. Two new phase transitions before metallization occur at 3.3 and 5.9 GPa for Zn0.982Fe0.018Se and 4.5 and 7.2 GPa
for Zn0.965Fe0.035Se, respectively. This is similar to the
case of ZnSe at 4.7 and 9.1 GPa. But for Zn0.84Fe0.16Se,
two new modes appeared as the pressure was increased to 4.7 GPa.8,10 The peaks of Raman phonons located at
204.6 and 208.3 cm 1
(labelled as TO split mode I) for
x D 0.018 and 0.035 exhibited red shift, whilst the peaks
of Raman modes located at 220.5 and 225.4 cm 1
(labelled as TO split mode II) exhibited blue shift for x D 0.018 and 0.035, respectively. Note that in the recent x-ray work of Greene et al.22
on Zn0.83Fe0.17Se, an anomaly was also
found at a pressure around 5.0 GPa, although no structure transitions were identified by Quadri et al.4
for pressure lower than 10.0 GPa from the EDXD work. In contrast to the case of Zn1 xFexSe, the new phase transitions were
ignored in the previous studies of pure ZnSe, although these two phase transitions were reported in the high-pressure Raman study of Zn1 xMnxSe.7 In the case of
Zn1 xMnxSe, one more mode, the Mn impurity mode, was
also observed. Arora et al.6
reported that the splitting of the impurity mode at 4.0 GPa was caused by the lower-ing of the crystal symmetry. Later, Arora and Sakuntala7
found one more phase transition occurred at 8.0 GPa. At 8.0 GPa, the sample became opaque. This transition was considered as a transformation from the direct to indirect band gap. However, if one refers to the study of the sim-ilar cubic structure of CdTe16
and ZnTe,23,24
more phases (cinnabar for CdTe and cinnabar and orthorhombic for ZnTe) were found before they underwent the structure transformation from B3 phase to B1 phase. Therefore, we suspect that Zn1 xFexSe might also undergo a
simi-lar structure transformation from the B3 through cinnabar and orthorhombic to the B1 structure for x D 0, (4.7, 9.1, and 14.4 GPa), 0.018 (3.3, 5.9, and 12.8 GPa), 0.035 (4.5, 7.2, and 12.0 GPa), and 0.16 (4.7 and 10.9 GPa), respec-tively. As the pressure was increased further to 14.4, 12.8, 12.0, and 10.9 GPa which were the semiconductor–metal transition pressures for x D 0, 0.018, 0.035, and 0.16, respectively, both LO and Fe local modes disappeared.13
The higher the Fe ion concentration the more the reduction in semiconductor–metal transition pressure value. The disappearance of the LO phonon and Fe local phonon
Figure 2. Pressure dependence of phonon wavenumbers of
Zn0.982Fe0.018Se. Note the lowest frequency component was softened at high pressure and was continuous to 34.9 GPa.
Figure 3. Pressure dependence of phonon wavenumbers of
Zn0.965Fe0.035Se. Note the lowest frequency component was softened at high pressure and was continuous to 33.5 GPa.
modes can be understood as a semiconductor–metallic transition from the high-pressure resistivity25
and Raman spectroscopy measurements on ZnSe powder.8,10 Three
Table 1. Phase transition presents (including those of two new phases) of Zn1−xFexSe are listed and compared
with ZnSe
New phase (I) New phase (II)
pressure (GPa): pressure (GPa): Metallization
Sample red shift blue shift pressure (GPa)
ZnSe 4.7 9.1 14.4 Zn0.982Fe0.018Se 3.3 5.9 12.8 Zn0.965Fe0.035Se 4.5 7.2 12.0 Zn0.84Fe0.16Se 4.7 4.7 10.9
phase transition pressures of Zn1 xFexSe are listed in
Table 1.
The variations of Raman mode energies of ZnSe, Zn0.982Fe0.018Se, Zn0.965Fe0.035Se, and Zn0.84Fe0.16Se as
func-tions of the pressure are shown in Fig. 4. The open, dark shaded, light shaded, and solid symbols correspond to the samples with ZnSe, Zn0.982Fe0.018Se, Zn0.965Fe0.035
Se, and Zn0.84Fe0.16Se, respectively. From these plots,
one can reconfirm that the phase transition occurred at 14.4, 12.8, 12.0 and 10.9 GPa for ZnSe, Zn0.982Fe0.018Se,
Zn0.965Fe0.035Se, and Zn0.84Fe0.16Se, respectively. These
phase transitions relate structure transformation from the four-coordinated B3 structure (semiconductor) to a six-coordinated B1 structure(metal). Our results are consistent with the results obtained by Itkin et al.13 and Greene et al.22
The Gr¨uneisen parameter ( i) for a quasi-harmonic
mode i of frequency, ωi, the wavenumber in cm 1, was
defined by Cadona et al.,26
B0 is the bulk modulus at
zero pressure, and is taken as 62.4 GPa27 for all samples.
Figure 4. Pressure dependence of Raman peaks in the Zn0.84Fe0.16 Se (solid symbols), Zn0.982Fe0.018Se (dark shaded symbols), Zn0.965Fe0.035Se crystals (light shaded symbols) and ZnSe pow-der (open symbols). The arrows at 10.9, 12.0, 12.8, and 14.4 represent the semiconductor metal phase transition pres-sures of Zn0.84Fe0.16Se, Zn0.982Fe0.018Se, Zn0.965Fe0.035Se, and ZnSe, respectively.
Table 2. Effect of pressure on various Raman vibrational modes of Zn0.982Fe0.018Se, Zn0.965Fe0.035Se, and Zn0.84Fe0.16Se, respectively, at room temperature (298K). The values of mode wavenumbers ˜ni,
pressure dependence (d˜n/dp), mode Gr ¨uneisen parameter gi and (dgi/dp) were extrapolated at
ambient conditions i Sample Mode Qi/cm1 dQi dp cm1 Gpa1 K 0 Q i dQ i dP d i dp GPa 1 Zn0.982Fe0.018Se LO 252.1 5.01 0.17p 1.24 0.11 C 3.23 ð 10 3p Fe local 222.5 5.89 0.23p 1.65 0.15 C 5.43 ð 10 3p TO 197.2 5.56 0.7p 1.76 0.05 C 1.14 ð 10 3p TO split (I) 209.5 1.44 0.003p 0.43 0.0015 1.04 ð 10 5p TO split (II) 213.0 2.43 C 0.05p 0.71 0.014 C 1.58 ð 103p Zn0.965Fe0.035Se LO 252.0 4.35 0.199p 0.91 0.064 C 1.66 ð 103p Fe local 239.0 0.9 C 0.324p 0.23 0.088 C 2.67 ð 103p TO 204.7 5.43 0.181p 1.65 0.08 C 1.49 ð 10 3p TO split (I) 215.4 1.21 0.005p 0.35 0.0033 C 3.02 ð 10 5p TO split (II) 219.5 2.79 C 0.11p 2.51 0.03 C 3.66 ð 10 4p Zn0.84Fe0.16Se LO 254.4 3.37 0.266p 0.83 0.08 C 1.74 ð 10 3p Fe local 228.3 6.14 0.626p 1.73 0.21 C 8.19 ð 10 3p TO 202.6 5.60 0.222p 1.73 0.10 C 2.4740 ð 10 3p TO split (I) 216.5 0.15 C 0.042p 0.04 0.0121 C 1.61 ð 10 5p TO split (II) 216.0 3.57 C 0.198p 1.03 0.01 C 4.12 ð 10 3p
The pressure effects on Raman vibrational modes of Zn1 xFexSe at room temperature (298 K) are listed in
Table 2, respectively. As a comparison with previous work,20 some conclusions can be drawn: (i) the
LO
val-ues of Zn0.982Fe0.018Se, Zn0.965Fe0.035Se, and Zn0.84Fe0.16Se
are very close to one; (ii) TO > LO for all systems (for
any value of x); (iii) the ratio TO/ LO for x D 0.16 is
the highest among these three compounds. This shows that Zn0.84Fe0.16Se has higher ionicity than Zn0.965Fe0.035Se,
Zn0.982Fe0.018Se, or ZnSe. The higher ionicity is
conjec-tured to result from the Fe impurity. In Fig. 4, one can also find that the transition pressure of the split TO phonon mode which exhibited blue shift decreased as the impu-rity concentration was increased. But for the other mode which manifested a red shift, the transition pressure was almost a constant value of 4.7 GPa. One can also note that the TO mode split into three components which were still visible for pressures even up to 35.0 GPa in our work. This was not observed in other previous Zn1 xFexSe work.
Furthermore, it is found that the semiconductor –metal transition pressures for Zn0.982Fe0.018Se, Zn0.965Fe0.035Se,
and Zn0.84Fe0.16Se are 12.8, 12.0 and 10.9 GPa,
respec-tively, which are 1.6, 2.4, and 3.5 GPa lower than that of ZnSe, respectively, and the higher the Fe ion concen-tration the more the reduction in semiconductor –metal transition pressure value. The reduction in the transition pressure is due to the existence of Fe which may result in the hybridization of 3d orbitals into the tetrahedral bonds.5
Our result is consistent with the study of pressure-induced phase transition of Zn0.83Fe0.17Se by energy-dispersive
x-ray-diffraction measurement.4
CONCLUSIONS
High pressure Raman scattering spectroscopy of ZnSe powder, Zn0.982Fe0.018Se, Zn0.965Fe0.035Se, and Zn0.84Fe0.16Se
crystals up to 35.0 GPa has been investigated. The exis-tence of the Fe element was found to result in a reduction in the semiconductor–metal phase transition pressure. The disappearance of the LO and Fe local phonons is attributed to the metallization of the Zn0.982Fe0.18Se, Zn0.965Fe0.035Se,
and Zn0.84Fe0.16Se crystals. Three visible TO phonon
split-ting components in the Zn1 xFexSe system were observed
up to 35.0 GPa. The semiconductor–metal transition pressures for the Zn0.982Fe0.018Se, Zn0.965Fe0.035Se, and
Zn0.84Fe0.16Se are 1.6, 2.4, and 3.5 GPa lower than that
of the ZnSe, respectively. It was found that the transition pressure of the blue-shifted TO split mode decreased as the impurity concentration was increased. The calculated Gr¨uneisen parameter implied that Zn0.84Fe0.16Se has higher
ionicity than Zn0.965Fe0.035Se, Zn0.982Fe0.018Se, or ZnSe. The
reason for the observation of Raman peaks at pressure above the metallization pressure may be the existence of TO modes in the thin surface of the high-pressure metallic phase as in previous works.8
Acknowledgements
We would like to thank Professor W. C. Chou for providing the samples used in this study. This work was supported by the National Science Council, Taiwan by the grant number NSC88-2112-M-134-001 at NHCTC and NSC 87-2112-M-009-009 at NCTU.
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