High-pressure phase transitions in Zn0.83Mn0.17Se thin film

Ž .

Physics Letters A 266 2000 435–440

www.elsevier.nlrlocaterphysleta

High-pressure phase transitions in Zn

_0.83

Mn

_0.17

Se thin film

Chih-Ming Lin

a,)

_{, Der-San Chuu}

b

a

National Hsinchu Teacher’s College, Hsinchu, Taiwan, ROC

b

Department of Electro-Physics, National Chiao Tung UniÕersity, Hsinchu, Taiwan, ROC

Received 27 July 1999; accepted 11 January 2000 Communicated by A. Lagendijk

Abstract

Ž .

The energy-dispersive X-ray-diffraction EDXD and Raman spectroscopy are used to study phase transitions of

Ž .

Zn0.83Mn0.17Se thin film up to 17.5 and 16.1 GPa, respectively. The EDXD results show that possible zinc blende B3 to

Ž .

sodium chloride phase B1 structure transition for Zn_0.83Mn_0.17Se thin film occurs at 10.0 GPa. The unloading run reveals a reversible phase transition existed in the Zn0.83Mn0.17Se thin film. For micro-Raman spectra at ambient pressure, three Raman peaks are distinct as LO, TO, and Mn local modes in Zn1yxMn Se bulk. As the pressure is increased to 10.9 GPa,x metallization occurs, the LO and Mn local phonon peaks disappear while the two unidentified Raman peaks of TO mode are still observable above the metallization pressure till 17.5 GPa. The phase transition pressure P obtained from the results oft micro-Raman spectra seems to be in good agreement with that obtained by EDXD measurements. q 2000 Published by Elsevier Science B.V. All rights reserved.

PACS: 62.50.q p; 64.60.-i; 78.30.Fs

The recent developments of diluted magnetic

Ž .

semiconductors DMS in crystal growth and

particu-Ž . w x

larly in molecular beam epitaxy MBE 1–3 and

w x

hot wall epitaxial growth 4 now allow zinc blende structure crystals of Zn1y xMn Se to be grown onx

Ž .

GaAs 100 wafer. As a result of the tunability of the lattice parameters, these materials are excellent can-didates for growing quantum wells and superlattices

w x_{5 . The existence of magnetic interactions within}

quantum wells and superlattices may result in new

w x

effects related to low dimensional phenomena 6 and the large Zeeman splitting of the electronic bands. This fact allows the formation of quantum

)

Corresponding author. Fax: q886-3-5257178.

Ž .

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

wells whose depth can easily be tuned by using a

w x

magnetic field 7 . It is also possible to obtain differ-ent spin superlattices with field-induced type transi-tions. But, the study of the lattice vibration of the DMS thin film under high-pressure is still absent. To our knowledge, this is the first characterization of Zn1y xMn Se thin film by using high-pressure tech-x

nical measurements.

The thickness of Zn0.83Mn0.17Se thin film was

˚

about 7000 A. It was grown by the EPI 620

molecu-Ž . Ž .

lar beam epitaxy MBE system on GaAs 100

sub-w x _{Ž .}

strate 8 . The GaAs 100 substrate was then re-moved by etching it in a mixed solution of

H O :NaOH:H O s 1:5:52 2 2

in gramme before the thin film can be used in the micro-Raman and energy-dispersive

X-ray-diffrac-0375-9601r00r$ - see front matter q 2000 Published by Elsevier Science B.V. All rights reserved.

Ž .

and the Jandel Scientific Peakfit computer programs

w x

were described earlier 9–14 . The Germanium en-ergy dispersive detector was set in the position where

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the diffracted angle u was changed to 68. So, the relation of the energy of reflection, E, versus

d-spac-˚

ings, d, was Ed s 59.317 keV A.

A series of spectra of substrate free Zn0.83Mn0.17Se thin film for different loading run and the process of decompressing to ambient pressure are shown in Fig. 1. The standard identified pressure lines of internal

Ž . Ž . Ž . Ž . Ž .

gold 111 , 200 , 220 , 311 , and 222 and the

Ž .

escape peaks of B3 220 are also manifested in Fig. 1. The peak positions are read out by a peak search program provided by the VAX computer in the beam

w x

line X-17C of Brookhaven National Lab 11,12 . The

Fig. 1. A series spectra of Zn0.83Mn0.17Se thin film at various pressure recorded in a loading run and process of decompressing to ambient pressure. The spectra contain the X-ray emission lines of the standard identified pressure lines of internal gold and the

Ž .

escape peaks of B3 220 .

the loading run at ambient pressure, Fig. 1 shows

Ž . Ž . Ž .

that there are six reflections 111 , 220 , 311 ,

Ž400 ,. Ž331 , and. Ž422. of B3 Žzinc-blende, ZB.

˚

phase. By the relation Ed s 59.317 keV A the d-spacings of those reflections appeared in Fig. 1 are

˚

3.293, 2.016, 1.720, 1.426, 1.309 and 1.164 A, re-spectively. Because the diffracted energy is very

Ž . Ž .

close to each other, the peaks Au 200 and B3 220 ;

Ž . Ž . Ž . Ž .

Au 220 and B3 400 ; and Au 222 and B3 422 respectively are overlapping together. No stress in-duced or GaAs diffraction peaks appeared in Fig. 1, which manifests that the removal of the GaAs sub-strate is able to establish a strain and subsub-strate free experimental environment. At 7.8 GPa, all the peaks can be observed clearly. The fact that the B3 peaks appear at high energy side of the reflection of gold peaks exhibits that Zn0.83Mn0.17Se thin film is more compressible than gold. When the pressure is

in-Ž . Ž .

creased to 10.0 GPa, the reflections 200 , 220 , and

Ž222 of B1 rock salt, RS phase appear at high. Ž .

Ž . Ž . Ž .

energy side of the reflections 111 , 220 , and 311

Ž .

of B3 phase, respectively. While the reflection 400

Ž .

of B3 zinc-blende, ZB phase disappears. The d-spacings at 10.0 GPa are 3.179, 1.944, 1.665, 1.261,

˚

Ž . Ž . Ž . Ž . Ž .

and 1.124 A for 111 , 220 , 311 , 331 , and 422 of B3 phase and the lattice parameter is 5.505 "

˚

0.001 A. While the d-spacings at 10.0 GPa are

˚

Ž . Ž . Ž .

2.587, 1.830, and 1.492 A for 200 , 220 , and 222

Ž .

of B1 rock salt, RS phase and the lattice parameter

˚

Ž . Ž

is 5.173 " 0.001 A. The reflection 400 of B1 rock

.

salt, RS phase appears at high energy side of the

Ž .

reflection 400 of B3 phase at 10.5 GPa. The reflec-tions of the B3 phase of Zn0.83Mn0.17Se disappear completely and only reflections of B1 phase appear apparently above 12.4 GPa. The transition pressure of B3 to B1 for Zn0.83Mn0.17Se is assigned as 10.0 GPa. Actually, the exact phase transition pressure must exist between 10.0 to 12.4 GPa. The B1

reflec-Ž . Ž . Ž . Ž .

tions, 200 , 220 , 222 , and 400 are found to exist up to 16.1 GPa. The unloading run reveals that Zn_0.83Mn_0.17Se thin film has reversible pressure property.

The variations of the interplanar distances dh k l

˚

ŽA for loading run of Zn. 0.83Mn0.17Se thin film are shown in Fig. 2. All the interplanar distances

de-˚

Ž .

Fig. 2. The variation of dh k l A of Zn0.83Mn0.17Se thin film with

Ž .

pressure GPa for the B3 and B1 phases.

crease as the pressure is increased for both B3 and B1 phases. Fig. 3 shows the equation of state rela-tions as a function of pressure for Zn0.83Mn0.17Se thin film. V is the volume at ambient pressure. The0

data for B3 and B1 phases are fitted to the Mur-naghan equation by a fitting process of the previous

w x

reports 11,12,14 . The values of K , the isothermal₀ bulk modulus at ambient pressure, and K₀X, the pressure derivative of the isothermal bulk modulus evaluated at ambient pressure, of the Murnaghan equation for Zn_0.83Mn_0.17Se thin film in the B3 and B1 phase transitions obtained from the fitting pro-cess are listed in Table 1. The values of K0X values are consistent with the slops of d-spacings for below

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and above the phase transitions B3 and B1 phases in the loading run spectra. In general, the pres-sure derivative of B3 is larger than B1 phase in both

w x

bulk and thin film 11,12 . It also shows that

Ž .

Zn0.83Mn0.17Se thin film in the pressure region B1 above phase transition is less compressible than that

Fig. 3. The V r V versus pressure and Murnaghan equation used0 to fit for the B3 and B1 phases of Zn0.83Mn0.17Se thin film.

Ž .

in the pressure region B3 below the phase transi-tion. This result is the same as that exhibited in the case of bulk.

Raman spectra of Zn0.83Mn0.17Se thin film at various pressure for different loading run are shown in Fig. 4. At ambient pressure, two peaks identified as the LO and weak TO phonons are observed at 255.5 and 207.3 cmy1

, respectively. This is

consis-w x

tent with the bulk work reported previously 10 .

Table 1

The values of K and K0 X0 for Zn0.83Mn0.17Se thin film under and

Ž .

above phase transition B3 and B1 phase obtained from the

w x

fitting process by Xu et al. 14 . K0 is the isothermal bulk modulus at zero pressure, and K0X is the pressure derivative of the isothermal bulk modulus evaluated at zero pressure.

X

Ž .

sample phase K0 GPa K0

Zn0.83Mn0.17Se thin film B3 62.77"5.38 5.43"2.56 B1 67.18"6.69 5.19"2.67

Fig. 4. Pressure dependence of phonon frequencies of Zn0.83Mn0.17Se thin film. Note the lowest frequency component was softened at high pressure and was continuous to 17.5 GPa.

Between these two peaks, a weak structure being able to be labelled through the Jandel Scientific Peakfit deconvolution process is attributed to the

Ž .

existence of the Mn local impurity phonon mode at

y1 _{w x}

236.9 cm 10,15,16 . Especially, there are none

w x

GaAs peaks in Fig. 1 4,17 , which agrees with the EDXD works. The Mn local phonon is caused from the local electric field resulting from the substituting

w x

Zn atom by the Mn atom 10,15,16 . The pressure effects on the LO and TO phonons exhibit similar

w x

blue shift behavior as the Mn local phonon 10,15,16 . As the pressure is increased to 2.4 GPa, one new mode which is labelled as TO split mode I appears at 218.4 cmy₁

. This TO split mode I exhibits red shift and can be observed as the pressure is increased up

w x

to around 17.5 GPa 10 . As the pressure is increased further to 10.9 GPa, the semiconductor-metal transi-tion of Zn0.83Mn0.17Se thin film occurs, and both the LO and the Mn local modes disappear. Consistent with the EDXD works, the exact phase transition

Fig. 5. Pressure dependence of Raman peaks in the Zn0.83Mn0.17Se thin film. The solid lines are quadaratic polynomial fitting curves for Zn0.83Mn0.17Se thin film.

pressure must exist between 10.1 to 10.9 GPa. Hence, we may conclude that the measured results of semi-conductor-metal transition pressure of Zn_0.83 -Mn_0.17Se thin film obtained in EDXD and micro-Raman works are in good agreement. Below the metallization pressure, only one phase transition pressure can be observed. After the metallization pressure, only two TO peaks can be observed up to 17.5 GPa. The previous work of Zn0.76Mn0.24Se

Table 2

The quadratic polynomial fitting equation of the Zn0.83Mn0.17Se thin film

Raman modes Quadratic polynomial fitting equation 2 LO 257.4q2.24 py0.142 p 2 Mn local 241.1q1.68 py0.104 p 2 TO 217.2q2.16 py0.071 p 2 Ž . TO split I 225.2y3.37pq0.074 p

w x

bulk 18 showed that three TO peaks can exist after the metallization pressure. We suggest that the ab-sence of one TO peak in the thin film after the metallization transition is attributed to the lower dimensional behavior of the thin film. There are no structure transitions identified by experimental re-ports below metallization until now. Accroding to

w x

the theoretical works of Cote et al. for ZnSe 19 and

ˆ ´

w x

Ahuji et al. and Nelmes et al. for CdTe 20,21 . We therefore attribute the appearance of the new phonon mode to the broken symmetry of the structure trans-formation. The splitting of TO phonon at 2.4 GPa could be also due to the structure transition as

zinc-w x

blende phase transforms into cinnabar phase 18 . The variation of the mode energies as a function of the pressure are shown in Fig. 5. The solid circle, solid square, solid triangle up, and solid triangle down symbols correspond to the LO, Mn local, TO and TO split Raman modes of Zn0.83Mn0.17Se thin film, respectively. The relationships of the mode frequencies versus pressure of Zn0.83Mn0.17Se thin film can be obtained by the quadratic polynomial fitting using the equations listed in Table 2, where v_i is the wave number in cmy₁

and p is the pressure in GPa. The effects of pressure on various Raman vibrational modes of Zn0.83Mn0.17Se thin film at

Ž .

room temperature 298 K are listed in Table 3. The

Ž .

Gruneisen parameter g

¨

i for a quasi-harmonic mode

w x

i of frequency v was defined by Cardona et al. 22 .i

As a comparison with the previous works with bulk

w_{10,23 , some conclusions can be obtained: i the}x _{Ž .} Ž . g_{LO value of Zn0.83Mn0.17Se thin film is 0.546; ii}

the gTO value of Zn0.83Mn0.17Se thin film is 0.624,

Ž .

and is larger than the gLO value; iii the ratio

g_TOrg_{LO for Zn0.83Mn0.17Se thin film is 1.143.}

This manifests that Zn0.83Mn0.17Se thin film also has higher ionicity from the Mn impurity as the previous

w x

works with bulk 10–12,23 .

In summary, we have carried out high-pressure micro-Raman scattering and EDXD experiments on Zn_0.83Mn_0.17Se thin film up to 17.5 and 16.1 GPa, respectively. The existence of the Mn element causes a reduction in the semiconductor-metal phase transi-tion pressure that is in similar to the behavior of bulk. The disappearance of the LO and Mn local phonons is attributed to the metallization of the Zn0.83Mn0.17Se thin film. Two components of visi-ble TO phonon splitting in Zn0.83Mn0.17Se thin film

Table 3

Effect of pressure on various Raman vibrational modes of

Ž .

Zn0.83Mn0.17Se thin film at room temperature 298K . The values of mode frequencies v , pressure dependence dv r dp, modei i

Gruneisen parameter g are extrapolated at ambient conditions¨ i K0 d vi y1 y1 y1 Ž . wŽ .Ž .x mode v_i cm dv r dp cm_i GPa g_i vi dP LO 257.4 2.24-0.142 p 0.546 Mn local 241.1 1.68-0.104 p 0.437 TO 217.2 2.16-0.071 p 0.624 Ž . TO split I 225.2 -3.37q0.074 p -0.939

system were observed up to 17.5 GPa. The cal-culated Gruneisen

¨

param eter im plied that Zn0.83Mn0.17Se thin film has higher ionicity. Our EDXD data showed that the bulk modulus for Zn0.83Mn0.17Se thin film was 62.77 " 5.38 GPa be-fore phase transition and the pressure derivative was 5.43 " 2.56. We conclude that the measured results of the semiconductor-metal transition pressure of Zn0.83Mn0.17Se thin film in EDXD and micro-Ra-man works are in good agreement with each other.

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 Grant numbers NSC 88-2112-M-134-001 at NHCTC and NSC 87-2112-M-009-009 at NCTU.

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