Pressure-induced phase transitions in bulk Zn1-xMnxSe

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Pressure-induced phase transitions in bulk Zn

1 @x

Mn

x

Se

Chih-Ming Lin

a,

*, Der-San Chuu

b

a_{Department of Science Education, National Hsinchu Teacher’s College, Hsinchu, Taiwan} b_{Department of Electro-Physics, National Chiao Tung University, Hsinchu, Taiwan}

Received 16 November 2000; received in revised form 22 February 2001; accepted 15 March 2001

Abstract

Energy-dispersive X-ray-diﬀraction (EDXD) is used to study the pressure-induced transitions of Zn_1@xMnxSe bulk

samples, x ¼ 0:016, 0.026, 0.053, 0.07, and 0.24, below 30 GPa. The EDXD results show that possible structure transitions from the zinc blende (B3) to the sodium chloride phase (B1) for Zn0:984Mn0:016Se, Zn0:974Mn0:026Se, Zn0:947

Mn0:053Se, Zn0:93Mn0:07Se, and Zn0:76Mn0:24Se occur at 13.1, 12.4, 12.0, 11.8, and 9:6 GPa, respectively. The unloading

run (the measurement with decreasing pressure) reveals that a reversible phase transition exists in the bulk Zn_1@xMnxSe . In this work, our EDXD data show that the larger the increase of the fractional volume change at the

PACS: 62.50.þp; 64.60.@i; 78.30.Fs

Keywords: Pressure-induced phase transitions; Zn1@xMnxSe; EDXD

ZnSe-based ternary compound semiconductors have attracted much attention due to their poten-tial applications in optical devices, such as blue semiconductor injection lasers, blue-light emitting diodes, and ﬂat-panel displays, and so on. The last two devices are fabricated by some layered ZnSe to form a number of multiple quantum wells and superlattice structures. The semimagnetic

semi-conductor Zn1@xMnxSe is a direct band-gap

semiconductor with a band gap of 2:67 eV at room temperature. It has the ability to be one of the candidates for optoelectronic devices in the blue-light region of visible spectrum. The presence of the Mn ions has been reported to have many

interesting physical properties, such as tunable band gap [1,2] and can provide a highly useful feature to the superlattice and multiple quantum well [1]. Recently, the crystal structures, electronic and phonon properties of wide band gap II–VI compound dilute magnetic semiconductors (DMS) under high pressure have been extensively investi-gated by using the powerful photoluminescence tools [3], absorption spectra [4], dielectric permit-tivity and conducpermit-tivity [5], micro-raman [6], and energy dispersive X-ray diﬀraction (EDXD) [7,8]. By measuring the transmission spectroscopy, Ves et al. [9] reported that the reduction of the phase transition pressure of a Zn1@xMnxSe bulk crystal

occurs as the manganese concentration x is increased. It was found that the optical absorption edges of Zn_1@xMnxSe manifest blue shift of the *Corresponding author. Fax: +886-3-5257178.

E-mail address:[email protected] (C.-M. Lin).

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direct interband gap when the pressure is applied. In the present work, we show that by using energy dispersive X-ray diﬀraction (EDXD), the same phenomenon exists in Zn1@xMnxSe bulk crystals,

with manganese concentrations x ¼ 0:016, 0.026, 0.053, 0.07, and 0.24 under high pressure.

Our Zn1@xMnxSe bulk crystals were grown by

modiﬁed Bridgman method. Due to the high

melting point (B15001C) of Zn1@xMnxSe

crys-tals, high-purity argon gas at about 100 atm was used as the pressurizing gas during the crystal growth to prevent the quartz container from becoming too soft to be useful. The EDXD measurement was used to characterize the

struc-ture of Zn_1@xMnxSe crystals. Experimental

de-tails of EDXD measurements and the dede-tails in the data fitting of the equation of state (EOS) were described in earlier work [6–8,10–12]. EDXD was performed by using the superconductor wiggler synchrotron beam line X17C of the National Synchrotron Light Source of Brookhaven Na-tional Laboratory. The diamond cell was mounted on a sample stage which has x; y; z; w; o, and y movements. The stage was controlled by a micro-VAX computer. Alignment of the incident beam, sample, and detector can be obtained with a posi-tional accuracy of 1 mm and an angular accuracy of 0:0011. The energy-dispersive Germanium detector was set in the position where the diffracted angle (y) was changed to 61. So, the relation of the energy of reflection, E, versus d-spacings, d, was Ed ¼ 59:317 KeV (AA.

A series of spectra of Zn0:984Mn0:016Se and

Zn0:974Mn0:026Se bulk crystals for diﬀerent loading

run and the process of decompressing to ambient pressure are shown in Figs. 1 and 2. The standard pressure is identiﬁed by the gold chips put in the sample chamber. Lines of internal gold ð1 1 1Þ, ð2 0 0Þ, ð2 2 0Þ, ð3 1 1Þ, and ð2 2 2Þ are also mani-fested in Figs. 1 and 2, respectively, for standard pressure identiﬁcation. The peak positions are read out by a peak search program provided by the VAX computer in the beam line X-17C of Brookhaven National Lab [7,8]. For the loading run, the lattice parameters of Zn0:984Mn0:016Se and

Zn0:974Mn0:026Se bulk samples are 5:66670:001

and 5:66870:001 (AA obtained from EDXD

mea-surements at ambient pressure, respectively, whilst

Figs. 1 and 2 show that there are six reﬂections ð1 1 1Þ, ð2 2 0Þ, ð3 1 1Þ, ð4 0 0Þ, ð3 3 1Þ, and ð4 2 2Þ of B3 (zinc-blende, ZB) phase. By the relation Ed ¼59:317 KeV (AA, the d-spacings of those re-ﬂections appearing in Figs. 1 and 2 are 3.271,

2.003, 1.709, 1.416, 1.300 and 1:157 (AA for

Zn0:984Mn0:016Se and 3.269, 2.018, 1.708, 1.413,

1.300 and 1:156 (AA for Zn0:974Mn0:026Se,

respec-tively. Because the diﬀracted energies are very close to each other, the peaks Auð2 0 0Þ and B3ð2 2 0Þ; Auð2 2 0Þ and B3ð4 0 0Þ; and Auð2 2 2Þ and B3ð4 2 2Þ, respectively, are overlapping. At high pressure, all the peaks can be observed clearly. The B3 peaks appearing at the high-energy side of the reﬂection of gold peaks shows that bulk Zn1@xMnxSe crystals are more compressible than

gold. When the pressure is increased to the onset pressure of 13:1 and 12:4 GPa, in Figs. 1 and 2,

Fig. 1. A series spectra of bulk Zn0:984Mn0:016Se at various

pressure recorded in a loading run and process of decompres-sion to ambient pressure. The spectra contain the X-ray emission lines of the standard identiﬁed pressure lines of internal gold peaks.

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respectively, the reflections ð2 0 0Þ, ð2 2 0Þ, ð2 2 2Þ, and ð4 0 0Þ of the B1 (rock salt, RS) phase appear at high-energy side of the reflections ð1 1 1Þ, ð2 2 0Þ, ð3 1 1Þ, and ð4 0 0Þ of the B3 phase, respectively, while the ð4 0 0Þ reflection of the B3 (zinc-blende, ZB) phase disappears in Fig. 1. The d-spacings at 13:1 GPa in Fig. 1 are 3.124, 1.904, and 1:634 (AA for ð1 1 1Þ, ð2 2 0Þ, and ð3 1 1Þ of the B3 phase and

the lattice parameter is 5:40570:001 (AA. The

d-spacings at 13:1 GPa in Fig. 1 are 2.561, 1.814, 1.482 and 1:276 (AA for ð2 0 0Þ, ð2 2 0Þ, ð2 2 2Þ, and ð4 0 0Þ of the B1 (rock salt, RS) phase and the lattice parameter is 5:12970:001 (AA. Similarly, the d-spacings at 12:4 GPa in Fig. 2 are 3.130, 1.916, 1.634, 1.353, 1.243, and 1:106 (AA for ð1 1 1Þ, ð2 2 0Þ, ð3 1 1Þ, ð4 0 0Þ, ð3 3 1Þ, and ð4 2 2Þ of B3 phase and

the lattice parameter is 5:41870:001 (AA. The

d-spacings at 12:4 GPa in Fig. 2 are 2.560, 1.814,

and 1:484 (AA for ð2 0 0Þ, ð2 2 0Þ, and ð2 2 2Þ of the B1 (rock salt, RS) phase and the lattice parameter is

5:13070:001 (AA. The reﬂections of the B3 phase

of Zn0:984Mn0:016Se and Zn0:974Mn0:026Se bulk

crystals disappear completely and only reﬂections of the B1 phase appear apparently above 14.7 and 12:9 GPa, respectively. The transition pres-sure, Pt, of B3 to B1 for Zn0:984Mn0:016Se and

Zn0:974Mn0:026Se bulk crystals are assigned as the

onset pressure of 13.1 and 12:4 GPa, respectively. The ambiguous region in which the B3 and the B1 phases mixed together, exists from 13.1 to

14:7 GPa and from 12.1 to 12:9 GPa for Zn0:984

Mn0:016Se and Zn0:974Mn0:026Se, respectively. The

B1 reﬂections, ð2 0 0Þ, ð2 2 0Þ, ð2 2 2Þ, and ð4 0 0Þ are found to exist up to 19.0 and 18:3 GPa for Zn0:984Mn0:016Se and Zn0:974Mn0:026Se bulk

sam-ples, respectively. The unloading run reveals that bulk Zn1@xMnxSe samples have reversible

pres-sure property.

The variations of the interplanar distances dhkl ð (AAÞ for loading run of bulk Zn1@xMnxSe

crystals, x ¼ 0:016, 0.026, 0.053, 0.07, and 0.24, are shown in Figs. 3–7. The lattice parameters for Zn0:984Mn0:016Se, Zn0:974Mn0:026Se, Zn0:947

Mn0:053Se, Zn0:93Mn0:07Se, and Zn0:76Mn0:24Se

are 5:66670:001, 5:66870:001, 5:67270:001,

5:67770:001 and 5:70870:001 (AA at ambient

pressure, respectively. This is consistent with the results obtained previously from measuring the lattice parameters of Zn1@xMnxSe in which the

lattice parameters are found to increase with the increasing of Mn concentration [13–16]. All the interplanar distances decrease as the pressure is increased for both the B3 and the B1 phases. The results indicate that the decrease of the interplanar distances with pressure is due to a decrease of the lattice parameter.

Fig. 8 shows the equation of state relations as a function of pressure for bulk Zn_1@xMnxSe

crys-tals. V0 is the volume at ambient pressure. The

data for the B3 and the B1 phases are ﬁtted to the Murnaghan equation by a ﬁtting process as

reported previously [7,8,12]. The values of K0,

the isothermal bulk modulus at ambient pressure, and K₀0, the pressure derivative of the isothermal bulk modulus evaluated at ambient pressure, of

the Murnaghan equation for Zn1@xMnxSe bulk

Fig. 2. A series spectra of bulk Zn0:974Mn0:026Se at various

pressure recorded in a loading run and during the process of decompression to ambient pressure. The spectra contain the X-ray emission lines of the standard identiﬁed pressure lines of internal gold peaks.

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crystals in the B3 and the B1 phase transitions obtained from the ﬁtting process are listed in Table 1. The values of K0

0 are consistent with the

slopes of the d-spacings below and above the phase transitions (the B3 and the B1 phases) in the loading run spectra. In general, the pressure derivative of the B3 phase is larger than that of the B1 phase in both bulk and thin ﬁlm [7,8]. It also shows that Zn1@xMnxSe bulk crystals in the

pressure region (B1) above the phase transition are less compressible than that in the pressure region (B3) below the phase transition. The isothermal bulk moduli evaluated at low- and high-pressure regions at ambient pressure slightly decrease with increasing the Mn concentration which indicates that the softening of the lattice is due to the

substitution of Zn by Mn. The V=V0 versus

pressures of Zn0:984Mn0:016Se, Zn0:974Mn0:026Se,

Zn0:947Mn0:053Se, Zn0:93Mn0:07Se, and Zn0:76

Mn0:24Se crystals are shown by using hollow

circles, hollow squares, hollow triangles up, hollow triangles down, and hollow diamonds, respec-tively. The EDXD results show that possible zinc blende(B3) to sodium chloride phase (B1) structure transition for Zn0:984Mn0:016Se, Zn0:974

Mn0:026Se, Zn0:947Mn0:053Se, Zn0:93Mn0:07Se, and

Zn0:76Mn0:24Se occur at 13.1, 12.4, 12.0, 11.8,

9:6 GPa, respectively. Similar to previous work [7], we can explain the reduction of the phase transition pressure of the Mn impurity mixing ZnSe semiconductor by considering the volume change of the unit cell for phase transition from the B3 to the B1 phase. One can note from Table 2 that the increasing of the percentage of the reduction of phase transition pressures with respect to 14:4 GPa of ZnSe relates prominently with the increasing percentage of the reduction of

the volume changes for our Zn_1@xMnxSe bulk

Fig. 3. The variation of dhklð (AAÞ of bulk Zn0:984Mn0:016Se with

pressure (GPa) for the B3 and the B1 phases. Fig. 4. The variation of dhklð (AAÞ of bulk Zn0:974Mn0:026Se with

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crystals while phase transition from B3 to the B1 occurs. The percentage of the reduction of the volume changes is the ratio of volume changes

from B3 to B1 with respect to B3 volume at Pt

times 100% at Pt. Above measurements indicate

that decreasing in the phase transition pressure Pt

for phase transition from B3 to B1 phase can be related to the increasing percentage of the

re-duction of volume changes, (DV=V0), by the

expression Pt ¼ ½14:4024 þ 0:5053ðDV=V0Þ@

0:0492ðDV=V0Þ2 in GPa for our cases of the

Zn1@xMnxSe bulk system. Therefore, the larger

the decrease of the relative reduction of the volume of B3 to B1 phase transition, the larger is the relative reduction of the phase transition pressure that can be obtained in ZnSe-based ternary compound semiconductors. Hence, the relative

change of the volume, (DV=V0), in the B3 to B1

phase transition may play an important role in the

reduction of the phase transition from B3 to B1 in ZnSe compound semiconductors with Mn impur-ity ions.

In summary, our EDXD data show that the

bulk modulus of the Zn1@xMnxSe sample

de-creases as the Mn concentration is increased, e.g. for Zn0:984Mn0:016Se, Zn0:974Mn0:026Se,

Zn0:947Mn0:053Se, Zn0:93Mn0:07Se, and Zn0:76

Mn0:24Se; K0 are 62:2670:31, 62:1770:21,

61:9370:29, 61:8270:81, and 60:4870:26 GPa

before phase transition and the pressure

deriva-tives are 4:3970:18, 4:4370:19, 4:2970:21,

4:3270:19, and 4:3770:16, respectively. The

relative volume change from B3 to B1 phase is

12.6, 13.3, 13.6, 13.7, and 16.3% for Zn0:984

Mn0:016Se, Zn0:974Mn0:026Se, Zn0:947Mn0:053Se,

Zn0:93Mn0:07Se, and Zn0:76Mn0:24Se, respectively.

The B3 to B1 phase transition pressure is 13.1, 12.4, 12.0, 11.8, and 9:6 GPa for Zn0:984Mn0:016Se, Fig. 5. The variation of dhklð (AAÞ of bulk Zn0:947Mn0:053Se with

pressure (GPa) for the B3 and the B1 phases.

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pressure (GPa) for the B3 and the B1 phases. Fig. 8. V=V_{bulk Zn} 0 versus pressure for the B3 and the B1 phases of

1@xMnxSe.

Table 1

The values of K0and K00for bulk Zn1@xMnxSe under and above the phase transition (B3 and the B1 phase) obtained from the ﬁtting

process by Xu et al.a_[12]

Sample Phase K0(GPa) K00

Zn0:984Mn0:016S e B3 62:2670:31 4:3970:18 B1 80:86_71:73 3:79_70:83 Zn0:974Mn0:026S e B3 62:1770:21 4:4370:19 B1 79:6771:53 3:8270:79 Zn0:947Mn0:053S e B3 61:9370:29 4:2970:21 B1 76:5871:81 3:9470:93 Zn0:93Mn0:07S e B3 61:8270:81 4:3270:19 B1 73:6771:79 3:8770:91 Zn0:76Mn0:24S e B3 60:4870:26 4:3770:16 B1 70.8671.61 3.6470.87 a

K0is the isothermal bulk modulus at zero pressure, and K00 is the pressure derivative of the isothermal bulk modulus evaluated at

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Zn0:974Mn0:026Se, Zn0:947Mn0:053Se, Zn0:93Mn0:07Se,

and Zn0:76Mn0:24Se bulk crystals, respectively. So,

the larger the increase of the relative volume change at the phase transition from B3 to B1, the larger is the decrease of the reduction in the semiconductor-metal phase transition pressure. We conclude that the eﬀect of increasing the relative volume change of ZnSe-based ternary semiconductors, as conjec-tured in our previous paper [7], may be the main reason for the reduction of the stability of the B3 phase under the application of pressure.

Acknowledgements

This work was supported by the National Science Council, Taiwan by the grant number NSC 88-2112-M-134-001 at NHCTC, NSC 89-2112-M-009-038 and NSC90-2112-M-009-018 at NCTU.

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The phase transition pressure decreased with increasing the volume changed of Zn0:984Fe0:016Se, Zn0:974Fe0:026Se, Zn0:947Fe0:053Se,

Zn0:93Fe0:07Se, and Zn0:76Mn0:24Se, respectively

Sample (DV=V0) : the relative volume

change from the B3 to the B1 phase (with respect to B3 volume while at Pt) (%)

ðDPt=14:4Þ : reduction of the

transition pressure (GPa) from the B3 to the B1 phase (with respect to 14:4 GPa of ZnSeÞ (%)

Pt: B3–B1 phase transition pressure (GPa) Zn0:984Mn0:016Se 12.6 9.0 13:170:5 Zn0:974Mn0:026Se 13.3 13.9 12:470:5 Zn0:947Mn0:053Se 13.6 16.7 12:070:5 Zn0:93Mn0:07Se 13.7 18.1 11:871:5 Zn0:76Mn0:24Se 16.3 33.3 9:670:5