Optical properties of Zn1-xMnxO thin films grown by molecular beam epitaxy

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Optical properties of Zn

1 x

Mn

x

O thin ﬁlms grown by molecular

beam epitaxy

K.F. Chien

a

, Y.L. Yang

a

, A.J. Tzou

a

, W.C. Chou

a,b,n

a

Department of Electrophysics, National Chiao Tung University, Hsin-Chu 30010, Taiwan

b_{NSC Taiwan Consortium of Emergent Crystalline Materials, Taiwan}

a r t i c l e

i n f o

Available online 4 December 2012 Keywords:

A3. Molecular beam epitaxy B1. Oxides

B1. Zinc compounds B2. Magneto-optic materials B2. Semiconducting II–VI materials

a b s t r a c t

Zn1 xMnxO (x ¼0–0.061) thin films were grown by molecular beam epitaxy (MBE) system. Transmit-tance shows an increase of the band gap with the increasing Mn concentration. Resonant Raman scattering (RRS) spectra showed 11 longitudinal optical phonon lines for the Zn1 xMnxO samples. For the Zn0.997Mn0.003O sample, circular polarization degree of 9% was observed at magnetic field B ¼5 T. The dependence of circular polarization rate on the magnetic field intensity exhibits Brillouin type para-magnetism.

1. Introduction

Zn1 xMnxO was theoretically predicted[1]and experimentally

observed [2] to exhibit ferromagnetism at room temperature. Spin dependent tunneling properties were observed in the metal-oxide-semiconductor diode consisting of ferromagnetic Zn1 xMnxO nanocrystals [3]. However, there was no

ferromag-netism observed for the Zn1 xMnxO thin ﬁlms grown by pulsed

laser ablation and rf magnetron sputtering methods [4]. Mixed magnetic phases (paramagnetic and ferromagnetic) were reported for the Zn1 xMnxO nanostructures synthesized by chemical vapor

deposition[5]. This controversy of paramagnetism or ferromagnet-ism in Zn1 xMnxO motivates intensive studies of magnetism in ZnO

related materials in recent years [6,7]. In this study, Zn1 xMnxO

thin ﬁlms were grown by plasma-assisted MBE. The magneto-optical properties were investigated to study the magnetism of Zn1 xMnxO thin ﬁlms and to explore the potential application of

this material in the spintronic devices.

2. Experiments

Zn1 xMnxO thin ﬁlm was grown on c-plane Al2O3substrate by

a SVT Associates MBE system equipped with conventional effu-sion cells for evaporation of elemental Zn (6N), Mg (6N), and Mn (5N). Oxygen (5N5) ﬂow rate of 0.6 SCCM with plasma power 250 W is supplied via a rf-plasma source after additional

gas puriﬁcation. The substrates were degreased in acetone, metha-nol, and then chemically etched in H2SO4:H3PO4¼3

7

1 at 160 1C for

15 min followed by deionized water rinse and spin drying. Before growth, the substrates were desorbed at 850 1C and treated in oxygen plasma, which is expected to produce an oxygen terminated Al2O3 surface. In order to reduce the lattice mismatch, MgO was

grown at 650 1C as a buffer layer. Because the lattice misﬁt between the MgO and Al2O3is 8%, that is lower than that between ZnO and

Al2O3(18%)[8]. A 325 nm He–Cd laser was used as the excitation

source to obtain the PL and RRS spectra and a 800 W xenon lamp was the light source for the transmittance measurements. The RRS, PL and transmittance spectra were analyzed using a 0.55 m single-grating spectrometer and photomultiplier tube.

3. Results and discussion

Fig. 1(a) shows room temperature transmittance spectra of Zn1 xMnxO thin ﬁlms with Mn concentration x ¼0, 0.009, 0.030

and 0.061. The absorption edge energy increasing with Mn concentration can be observed. Fig. 1(b) shows the absorption edge energy of the Zn1 xMnxO versus Mn concentration. The blue

shift of the absorption energy is due to MnO having a larger band gap than ZnO [9]. The shift of the absorption edge can be expressed by the following equation.

EðxÞ ¼ 3:337 þ 3:056x ðeVÞ ð1Þ

The experimental results are in good agreement with reference

[9]. Furthermore, the broadening of the absorption edge increases with the Mn concentration. The broadening is mainly due to the increasing disorder with increasing Mn concentration in Zn1 xMnxO.

There are also obvious mid-gap absorption around 3 eV for higher Contents lists available atSciVerse ScienceDirect

journal homepage:www.elsevier.com/locate/jcrysgro

Journal of Crystal Growth

n

Corresponding author at: Department of Electrophysics, National Chiao Tung University, Hsin-Chu 30010, Taiwan. Tel.: þ 886 3 571 2121;

fax: þ 886 3 572 5230.

E-mail address: wuchingchou@mail.nctu.edu.tw (W.C. Chou).

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Mn concentration samples. This effect has been ascribed to the d–d transitions of the Mn2 þ_ion_[10]_.

Fig. 2shows low temperature (10 K) resonant Raman scatter-ing (RRS) spectra of ZnO and Zn0.97Mn0.03O thin ﬁlms with the

He–Cd laser (l¼325 nm) excitation. RRS experiment is performed under the excitation laser energy higher than the band gap, and the incident photon energy will be in resonance with the electronic interband transition. The peak at 578 cm1 _{is the}

ﬁrst-order longitudinal optical (LO) phonon mode[11], in which both O and Zn atoms vibrate in the same direction. The weak peak around 457 cm1 _{is ascribed to the E}

2 (high) mode. Compared

with 440 cm1 in bulk ZnO single crystal[11], the frequency of the E2(high) mode in our sample is slightly larger, it is mainly due

to strain effect in the thin ﬁlm. Under RRS condition, some intense peaks at frequency positions of approximately integer times 578 cm1 _{contribute to the nth-order LO phonon scattering}

processes. These are intense LO phonon lines because of the Frohlich interaction, which is the interaction between electrons and the longitudinal electrical ﬁeld induced by the LO phonons

[12]. In addition, there are also some relatively weak peaks at frequency positions next to these LO phonon modes. Interestingly, the intervals of these weak peaks are also close to the frequency of LO phonon mode. Considering the frequency positions of these

peaks, they are probably caused by the combination of E2(high)

mode and multiple LO phonon scattering.

From the RRS spectra, we ﬁnd 5 and 11 LO phonon modes for ZnO and Zn0.97Mn0.03O samples, respectively. In previous studies [13], Scott et al. reported that the LO phonon numbers (n) in RRS spectra varies proportionally with the electron–phonon coupling coefﬁcient (

a

), which is given as the ratio of the Frohlich interaction energy to the LO phonon energy. They also predicted the number of LO phonon modes in ZnO is more than n ¼9 in CdS. However, they only found n ¼8 in their ZnO sample. From our results, we could not ﬁnd LO phonon lines for n 45 in ZnO due to the strong near band edge PL emission. However, for Zn0.97Mn0.03O, the near band edge emission is weak and due to

the large electron–phonon coupling coefﬁcient

a

¼0.9 (is assumed to be the same as ZnO), the observation of large amount of LO phonon lines (n ¼11) in RRS spectra can be understood.

Fig. 3shows the RRS spectra of Zn1 xMnxO (x¼ 0.003–0.030)

thin ﬁlms. Besides some intense LO phonon lines, there is an extra peak at 3632 cm1 _{for Zn}

0.997Mn0.003O sample. This peak is

ascribed to the neutral donor bound exciton (D0_{X) emission. As}

shown in the spectra, the LO phonon mode intensity at the frequency position of around 3500 cm1_{, which is assigned to}

the sixth-order LO phonon mode, is always the largest in each of

Fig. 1. (a) Room temperature transmittance spectra of Zn1 xMnxO thin ﬁlms with Mn concentration x¼ 0, 0.009, 0.030 and 0.061. (b) The absorption edge energy of

Zn1 xMnxO thin ﬁlms as a function of the Mn concentrations (x). The experimental results of ours (&) and reference[9](’) are plotted.

Fig. 2. Resonant Raman scatterings of ZnO and Zn0.97Mn0.03O thin ﬁlms, using the

He–Cd laser (l¼325 nm).

Fig. 3. Resonant Raman scatterings of Zn1 xMnxO thin ﬁlms, using the He–Cd

laser (l¼325 nm).

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Zn1 xMnxO samples, and the intensity decreases with increasing

Mn concentration. The behavior of intensity variation is mainly related to the band gap position, and it can be explained by using the Raman cross section for the nth-order LO phonon mode which is given as[14]

s

nð

o

Þ ¼

m

4 X1 j ¼ 0 X1 m ¼ 0 g,n þj9e,m e,m9g,j EexþðmjÞ_

o

LO_

o

iþ

i

_

G

2 exp j_

o

LO kBT ð2Þ where

m

is the electronic transition dipole moment, Eex is the

electronic transition energy._

o

iand_

o

LOare the energies of the incident photon and the LO phonon, respectively.

G

is the homogeneous line width. g,n þ j and g,j are the (n þj)th-order and jth-order LO phonon states in the electronic ground state, respectively. e,mj i is the mth-order LO phonon state in the electronic excited e state. kB is Boltzmann’s constant and T is

the temperature. From this equation, the nth-order LO phonon mode intensity will become larger if Egﬃ_

o

in_

o

LO. The band gap of Zn1 xMnxO shifts to higher energy when Mn concentration

increases, and it tends to be away from the frequency position of around 3500 cm1_{. Therefore, the intensity of sixth-order LO}

phonon mode decreases.

To investigate the dependence of RRS intensity on the band gap energy, temperature dependent RRS spectra of Zn0.991Mn0.009O is

shown inFig. 4(a). At 10 K, the intensity of sixth-order LO phonon mode around 3500 cm1 is the largest. However, when the temperature increases to 160 K, the seventh-order LO phonon mode around 4100 cm1_{becomes the largest in intensity. The behavior}

can be explained by considering the temperature dependence of the photoluminescence (PL). Fig. 4(b) shows the PL peak position of Zn0.991Mn0.009O as a function of the temperature, and the curve can

be ﬁtted by considering the Bose–Einstein statistical factor for phonons[15]

E Tð Þ ¼E 0ð Þ 2aB

exp

Y=T

1 ð3Þ

where E(T) and E(0) are the energies at T K and 0 K, respectively, aBis

the strength of the electron–phonon interaction, and

Y

is associated with the mean frequency of the phonons. FromFig. 4(a) and (b), the shift of PL position results in the LO phonon line intensity variation. To summarize, multiple LO phonon scattering in RRS spectra can be explained by using the ‘‘cascade model’’ [16,17], the scattered

photons will have energy _

o

ﬃ_

o

in_

o

LO or _

o

ﬃ_

o

i n_

o

E2ðhighÞn_

o

LO. Moreover, by studying RRS spectra, we ﬁnd that when the scattered photon energy is close to the band gap, the LO phonon intensity will be resonantly enhanced.

Fig. 5 shows the low temperature (10 K) PL spectra of Zn0.997Mn0.003O analyzed by (

s

þ) and (

s

) circular polarization

at magnetic ﬁeld B ¼0 and B ¼5 T. At B¼0, no difference was observed between two circular polarization. The D0_{X (at 3.356}

and 3.363 eV) and RRS (at 3.306 and 3.378 eV) intensities for (

s

þ)

and (

s

) components are approximately the same. However, at

B¼5 T, a slight difference is observed between the two circular

Fig. 4. (a) Resonant Raman scatterings of Zn0.991Mn0.009O thin ﬁlms with variable temperature, using the He–Cd laser (l¼325 nm). The arrows show the PL positions.

(b) Temperature dependent photoluminescence (PL) position of Zn0.991Mn0.009O. The solid curve describes the ﬁt of these data by using the Bose–Einstein statistical factor

for phonons. The dashed lines represents the energy positions of the scattered photons with energy_oﬃ_oin_oLO, n¼ 6 or 7.

Fig. 5. Low temperature (10 K) PL spectra of Zn0.997Mn0.003O for B ¼0 and B¼ 5 T.

K.F. Chien et al. / Journal of Crystal Growth 378 (2013) 218–221 220

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polarization components of the D0_{X. While the intensities of the}

two circular polarization components of the RRS remain the same. The degree of circular polarization can be deﬁned as

P ¼ IsþIs IsþþIs

ð4Þ where Isþ and Isare the intensities of the right and left circular

polarization, respectively. For RRS, P¼0 at B ¼0 and 5 T. Whereas, for D0_{X emission, P¼0 at 0 T and P¼ 9% at 5 T. The non-zero}

circular polarization is due to the energy splitting of the two spin components of the D0_{X, (electron 1/2 and hole 3/2) and}

(electron þ1/2 and hole þ3/2). Although, the energy separation is too small to be resolved, due the energy relaxation from the higher energy spin state to the lower energy spin state, P ¼9% is observed. The dependence of circular polarization on the mag-netic ﬁeld shows Brillouin-type para-magnetism. No hysteresis is observed. It implies that the Zn0.997Mn0.003O exhibits

para-magnetism due sp–d exchange interaction between conduction band s electrons/valence band p electron and d electrons of the Mn atoms.

4. Conclusion

We have grown Zn1 xMnxO (x ¼0–0.061) thin ﬁlms by MBE.

Transmittance measurement shows an increase of the band gap with the increasing Mn concentration. From RRS spectra, we observe LO phonon lines up to 5 and 11 order for ZnO and ZnMnO samples, respectively. From the temperature dependent RRS experiment, we ﬁnd the intensities of these LO phonon lines are sensitive to the band gap position. Low temperature PL spectra of Zn0.997Mn0.003O at magnetic ﬁeld B ¼0 T and 5 T were

investi-gated to calculate the degrees of circular polarization of P¼0% and 9%, respectively.

Acknowledgments

This work was supported by the National Science Council and the Ministry of Education under the Grant numbers NSC100-2119-M-009-003 and MOE-ATU 101W961 respectively.

References

[1] T. Dietl, H. Ohno, F. Matsukara, J. Gilbert, D. Ferrand, Zener model description of ferromagnetism in zinc-blende magnetic semiconductors, Science 287 (2000) 1019.

[2] Parmanand Sharma, Amita Gupta, K.V. Rao, J. Owens, Renu Sharma, Rajeev Ahuja, J.M. Osorio Guillen, B ¨orje Johansson, G.A. Gehring, Ferromag-netism above room temperature in bulk and transparent thin ﬁlms of Mn-doped ZnO, Nature Materials 2 (2003) 673.

[3] Sejoon Lee, Youngmin Lee, Yoon Shon, Deuk Young Kim, Tae Won Kang, Tunneling transport properties for metal-oxide-semiconductor diode con-sisting of ferromagnetic ZnMnO nanocrystals, Applied Physics Letters 97 (2010) 182103.

[4] A.I. Savchuk, V.I. Fediv, G.I. Kleto, S.V. Krychun, S.A. Savchuk, Optical and magneto-optical properties of ZnMnO and ZnMnFeO single crystals and thin ﬁlms, Physica Status Solidi A 204 (2007) 106.

[5] V.K. Sharma, B.K. Gupta, G.D. Varma, Optical and magnetic properties of (Zn,Mn)O nanostructures synthesized by CVD method, Crystal Research and Technology 46 (2011) 523.

[6] Yufeng Tian, Yongfeng Li, Mi He, Irwan Ade Putra, Haiyang Peng, Bin Yao, Siew Ann Cheong, Tom Wu, Bound magnetic polarons and p–d exchange interaction in ferromagnetic insulating Cu-doped ZnO, Applied Physics Letters 98 (2011) 162503.

[7] The-Long Phan, P. Zhang, D.S. Yang, N.X. Nghia, S.C. Yu, Local structure and paramagnetic properties of Zn1 xMnxO, Journal of Applied Physics 110

(2011) 063912.

[8] Y.F. Chen, H.J. Ko, S.K. Hong, T. Yao, Layer-by-layer growth of ZnO epilayer on Al2O3(0001) by using a MgO buffer layer, Applied Physics Letters 76 (2000)

559.

[9] C.A. Johnson, K.R. Kittilstved, T.C. Kaspar, T.C. Droubay, S.A. Chambers, G.M. Salley, D.R. Gamelin, Mid-gap electronic states in Zn1 xMnxO, Physical

Review B 82 (2010) 115202.

[10] T. Mizokawa, T. Nambu, A. Fujimori, T. Fukumura, M. Kawasaki, Electronic structure of the oxide-diluted magnetic semiconductor Zn1 xMnxO, Physical

Review B 65 (2002) 085209.

[11] R. Cusco, E. Alarcon-Llado, J. Ibanez, L. Artus, J. Jimenez, B. Wang, M.J. Callahan, Temperature dependence of Raman scattering in ZnO, Physical Review B 75 (2007) 165202.

[12] K.W. Boer, Survey of Semiconductor Physics: Electrons and Other Particles in Bulk Semiconductors, Van Nostrand Reinhold, New York, (1990), Chapter 28. [13] J.F. Scott, T.C. Damen, W.T. Silfvast, R.C.C. Leite, L.E. Cheesman, Resonant raman scattering in zns and znse with the cadmium laser, Optics Commu-nications 1 (1970) 397.

[14] M.C. Klein, F. Hache, D. Ricard, C. Flytzanis, Size dependence of electron– phonon coupling in semiconductor nanospheres: the case of CdSe, Physical Review B 42 (1990) 11123.

[15] P. Lautenschlager, M. Garriga, M. Cardona, Temperature dependence of the interband critical-point parameters of InP, Physical Review B 36 (1987) 4813. [16] R.M. Martin, C.M. Varma, Cascade theory of inelastic scattering of light,

Physical Review Letters 26 (1971) 1241.

[17] W.H. Sun, S.J. Chua, L.S. Wang, X.H. Zhang, Outgoing multiphonon resonant Raman scattering and luminescence in Be- and C-implanted GaN, Journal of Applied Physics 91 (2002) 4917.