Materials Chemistry and Physics 92 (2005) 274–277

Effect of substrate temperature on the phase change ofZn0.40Mn0.60Se thin films

D.-J. Kima, Y.-M. Yu a, S.-H. Eoma, T.-H. Kimb, C.-S. Goc, Y.D. Choia, ∗a Department of Optical& Electronic Physics, Mokwon University, Daejeon 302-729, Republic of Korea

b Department of Physics, Chungnam National University, Daejeon 305-764, Republic of Koreac Division of Physics and Semiconductor Science, Wonkwang University, Iksan 570-749, Republic of Korea

Received 6 August 2004; received in revised form 8 January 2005; accepted 18 January 2005

Abstract

Zn0.40Mn0.60Se thin films were grown on GaAs (1 0 0) substrate by hot-wall epitaxy. The grown films had both NaCl structure and zincblendestructure. Energy-dispersive X-ray analysis revealed that the Mn composition ratio of the grown ZnMnSe thin films was approximately 0.60and this Mn composition ratio had no relation to the increase of the substrate temperature. However, it was found that the surface state andt d throughe cf©

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he crystal structure of Zn0.40Mn0.60Se thin films were changed with increasing substrate temperature. These results were confirmexamination of the surface morphology using atomic force microscopy and the change of the imaginary partε2(E) peak of the dielectriunction measured by spectroscopic ellipsometry.

2005 Elsevier B.V. All rights reserved.

ACS: 61.10.−i; 61.50.Ks; 68.55.Jk

eywords:Hot-wall epitaxy; Zn0.40Mn0.60Se thin film; Substrate temperature

. Introduction

Zn1−xMnxSe has become a well-known diluted magneticemiconductor (DMS), in which the Zn2+ cations of the hostrystal are replaced with Mn2+ ions [1,2]. Since the energyand gap of DMS can be varied by the composition ratiof the transition metal, DMS exhibits very interesting char-cteristics such as magnetic, optical, and transport properties

3,4]. In general, when Mn composition is added to ZnSe, therystal structure of Zn1−xMnxSe is formed into three crystaltructures, cubic, zincblende, and wurtzite. These DMS ma-erials have the characteristic that the lattice constant andhe magnetic strength-change increase with increasing of then composition ratio[5,6]. To date, Zn1−xMnxSe has beenainly studied on optical properties, and there have been

ew reported studies on the phase transition of the crystaltructure. Wang et al. and Ro et al. reported that the crystal

∗ Corresponding author. Tel.: +82 42 829 7552; fax: +82 42 823 0639.E-mail address:ydchoi@mokwon.ac.kr (Y.D. Choi).

structure of Zn1−xMnxSe thin films could be changed byincrease of the Mn composition ratio and various deposconditions, respectively[7,8].

In this study, Zn0.40Mn0.60Se thin films were grown by howall epitaxy (HWE), and the phase change on the strucand optical properties of these thin films were investigatea function of substrate temperature.

2. Experimental

Zn0.40Mn0.60Se thin films were grown on GaAs (1 0substrates by HWE. The source materials used5N Mn powder and 5N ZnSe powder (Furuuchi CJapan). The GaAs substrates were ultrasonically cleantrichloroethylene, acetone, and methanol, in sequenc5 min. They were then chemically etched at 50–60◦C in a3H2SO4:H2O2:H2O solution for 1 min and rinsed by flusing deionized water. After being dried with high puritygas, they were put on a substrate holder in the HWE sy

254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2005.01.026

D.-J. Kim et al. / Materials Chemistry and Physics 92 (2005) 274–277 275

Just before the growth, thermal etching at 590◦C for 20 minunder∼10−7 Torr was carried out to remove the remainingimpurities and oxidized layers, and then slowly cooled downto growth temperature. During the crystal growth, the HWEsystem was maintained at∼1× 10−7 Torr. The temperaturesfor the ZnSe source and Mn source were kept at 700◦C,and the substrate temperature was controlled from 240◦C to400◦C.

The thickness of the grown thin films was calculated usingthe Franz–Keldysh oscillation of the reflectance spectra. X-ray diffraction (XRD) and atomic force microscopy (AFM)were used to measure the crystal structure and the surfacemorphology of Zn0.40Mn0.60Se thin films, respectively.Also, the composition ratio of thin films was measured usingenergy-dispersive X-ray spectrometry (EDX). The dielectricfunction spectra were measured at room temperature usingan automatic spectroscopic rotating analyzer ellipsometer(Woollam VUV–VASE system) with 300 W xenon and 70 Wdeuterium lamps at an incident angle of 70◦.

3. Results and discussion

The thickness of the grown Zn0.40Mn0.60Se thin films mea-sured from the reflectance spectrum was almost 1�m.

a stratet est e in-c withi rateb 40a eat -t asd ionr new

F es t showst Xa

Fig. 2. XRD patterns according to the substrate temperature ofZn0.40Mn0.60Se thin films.

that the composition ratio was scarcely affected by the changeof the substrate temperature[11,12].

Fig. 2shows the XRD patterns according to substrate tem-perature during the growth of Zn0.40Mn0.60Se thin films. Itis observed that the thin films grown in this work have NaClstructure of�-phase and zincblende structure of�-phase si-multaneously, as Tomasini et al. reported[13]. As shown inFig. 2, the (4 0 0) peaks of�- and�-phase of thin films arelocated at the right side and at the left side of the GaAs (4 0 0)substrate peaks, respectively. The (4 0 0) peak of�-phase thinfilms at the substrate temperature of 260◦C is smaller thanthat of �-phase. However, the (4 0 0) peak intensities of�-phase thin films become remarkably stronger with increasingsubstrate temperature and, conversely, those of�-phase be-come weaker. The (4 0 0) peak intensity of�-phase thin filmsat the substrate temperature of 380◦C nearly disappears, andthe peak is no longer separated. These results indicate that thecrystal structure of Zn0.40Mn0.60Se thin film with increasingsubstrate temperature is changed from the unstable crystalstructure of�-phase into the stable crystal structure of�-phase[14]. Therefore, it can be concluded that the crystalstructure of Zn0.40Mn0.60Se thin film is changed by variationof the substrate temperature.

Fig. 3shows the substrate temperature dependence of thelattice constant for�- and�-Zn0.40Mn0.60Se thin films. It iswNz[i -et -c reater.T�

Fig. 1shows the growth rate of Zn0.40Mn0.60Se thin filmss a function of the substrate temperature. When the sub

emperature increases from 240◦C to 340◦C, the growth rathows a tendency to increase from 0.5A s−1 to 0.9A s−1 forhe most part. This result is considered to arise from threase of the thermal energy for chemical adsorptionncreasing substrate temperature. However, the growthegins to decrease at substrate temperatures above 3◦C,nd it becomes 0.5A s−1 at 400◦C. This is considered to bresult of re-evaporation[9,10]. The inset ofFig. 1 shows

he Mn composition ratio of Zn0.40Mn0.60Se thin films deermined by EDX analysis. The Mn composition ratio wetermined to be approximately 0.60. All Mn compositatios were included in error range. From this result, we k

ig. 1. The growth rate of Zn0.40Mn0.60Se thin films as a function of thubstrate temperature. The dotted line represents a guideline. The insehe Mn composition ratio of Zn0.40Mn0.60Se thin films determined by EDnalysis.

ell known that the lattice constant of bulk�-MnSe withaCl structure and the lattice constant of bulk�-MnSe withincblende structure are 5.462A and 5.894A, respectively15]. The lattice constant of�-phase Zn0.40Mn0.60Se thin films generally larger than that of bulk�-MnSe, and the differnce between the lattice constant of�-phase Zn0.40Mn0.60Se

hin film and the lattice constant of bulk�-MnSe with dereasing substrate temperature becomes increasingly ghis is thought to be due to the increase of Zn2+ ions in-phase Zn0.40Mn0.60Se thin film. As shown inFig. 3, the

276 D.-J. Kim et al. / Materials Chemistry and Physics 92 (2005) 274–277

Fig. 3. The substrate temperature dependence of the lattice constant for�-and�-Zn0.40Mn0.60Se thin films. The dotted line represents a guideline.

lattice constants of�-phase Zn0.40Mn0.60Se thin film withincreasing substrate temperature continuously decrease, andthey finally approach the lattice constant of bulk�-MnSeof 5.462A at above 340◦C. This indicates that the crys-tal structure of�-phase Zn0.40Mn0.60Se thin film becomesstabilized at a substrate temperature of above 340◦C. Also,the lattice constant of the�-phase Zn0.40Mn0.60Se thin filmwith increasing substrate temperature has a tendency to in-crease, but that of�-phase rapidly decreases at above 340◦C.This means that, with increasing substrate temperature, theMn2+ ions in Zn0.40Mn0.60Se thin film have a preference for�-phase of a stable crystal structure rather than the unsta-ble �-phase[13]. Therefore, as a result of the decrease ofMn2+ ions in �-phase Zn0.40Mn0.60Se thin film, the latticeconstant of�-phase decreases abruptly above 340◦C. In thesame manner, as a consequence of the increase of Mn2+ ions

in �-phase Zn0.40Mn0.60Se thin film, the lattice constant of�-phase Zn0.40Mn0.60Se thin film decreases, and approachesthe lattice constant of bulk�-MnSe[15].

Fig. 4 shows AFM surface morphologies ofZn0.40Mn0.60Se thin films grown with increasing substratetemperature. As shown inFig. 4(a), the Zn0.40Mn0.60Sethin film grown at a substrate temperature of 260◦C, wherezincblende structure is dominant, is characterized by aflat surface with small islands. However, increase of theNaCl structure of�-phase Zn0.40Mn0.60Se thin film withincreasing substrate temperature is indicated by observationsof numerous quadrangular shapes and small dots on thesurface, as shown inFig. 4(b) and (c), respectively. Thesurface of a thin film grown at a substrate temperature of380◦C, as shown inFig. 4(d), reveals that quadrangularshape structures are predominant. It is estimated that thesequadrangular shapes arise from the NaCl structure.

To determine the substrate temperature dependence ofphase change,Fig. 5 compares the dielectric constantε2(E)of Zn0.40Mn0.60Se thin films fabricated in this study withε2(E) of �-MnSe and�-Zn0.37Mn0.63Se[16]. The inset ofFig. 5 shows theE1 peak position energy shift of the imag-inary partε2(E) in a dielectric function of Zn0.40Mn0.60Sethin films due to the influence of the variation of substratetemperature. The peak position energyE1 is the critical pointe eEs and4 rZ hina red-

F follow

ig. 4. AFM surface morphology of Zn0.40Mn0.60Se thin films grown at the

nergy of the imaginary partε2(E) in dielectric functions. Th1 peak position energies of�-MnSe and�-Zn0.37Mn0.63Sehown inFig. 5 were found to be located near 3.36 eV.63 eV, respectively. TheE1 peak position energy for foun0.40Mn0.60Se samples grown in this study exists witn energy range of 3.36–4.63 eV, and they gradually

ing substrate temperatures: (a) 260◦C, (b) 300◦C, (c) 340◦C, and (d) 380◦C.

D.-J. Kim et al. / Materials Chemistry and Physics 92 (2005) 274–277 277

Fig. 5. Comparison of the dielectric constantε2(E) of Zn0.40Mn0.60Sethin films fabricated in this study withε2(E) of �-MnSe and the�-Zn0.37Mn0.63Se to determine the substrate temperature dependence of phasechange. The inset shows theE1 peak position energy shift of the imaginarypartε2(E) in a dielectric function of Zn0.40Mn0.60Se thin films.

shift from theE1 peak energy of�-Zn0.37Mn0.63Se towardthat of �-MnSe with increasing substrate temperature, asshown in the inset ofFig. 5. This means that the crystalstructure of our four Zn0.40Mn0.60Se samples is changed withincreasing substrate temperature from the unstable�-phaseinto the stable�-phase. Therefore, it can be determined thatour Zn0.40Mn0.60Se thin films possess the crystal structureof �- and�-phase simultaneously, as indicated by the previ-ous XRD results ofFig. 2. The red-shift ofE1 peak positionenergy is thought to be closely related with the substrate tem-perature, which plays an important role in the phase change.

4. Conclusions

Zn0.40Mn0.60Se thin films were grown on GaAs (1 0 0)substrates by HWE. The Mn composition ratio of the thinfilms from EDX analysis was measured to be approximately0.60. With increasing substrate temperature, the growthrate of the Zn0.40Mn0.60Se thin film increased linearly, butdecreased owing to re-evaporation at temperature above340◦C. From XRD measurements, we confirmed that thecrystal structure of Zn0.40Mn0.60Se thin films was grown into�-phase thin films of NaCl structure and�-phase thin filmsof zincblende structure simultaneously. With increasings Di Theq nant

on the surface of Zn0.40Mn0.60Se thin film grown at asubstrate temperature of 380◦C than at that of 260◦C. It isinterpreted that this is due to the influence of NaCl structureof �-phase Zn0.40Mn0.60Se thin film. The red-shift ofE1peak position energy of the imaginary partε2(E) obtainedfrom SE measurement with increasing substrate temperatureenergy is closely connected with the substrate temperature,which plays an important role in the phase change. In thiswork, we found that the crystal structure of Zn0.40Mn0.60Sethin film could be changed from unstable�-phase structureto stable�-phase structure through control of the substratetemperature during the growth.

Acknowledgements

This work was supported by a Korea Research Foundationgrant (KRF-2002-070-C00036).

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