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Raman study for E2 phonon of ZnO in Zn1-xMnxO nanoparticles

May 2005
Journal of Applied Physics 97(8):086105 - 086105-3

May 2005
97(8):086105 - 086105-3

DOI:10.1063/1.1865340

Source
IEEE Xplore

Authors:

Z. -F. Li

Chinese Academy of Sciences

Wei Lu

Renmin University of China

Raman scattering at room temperature is reported in Zn 1-x Mn x O nanoparticles for the range of 0≤x≤0.15 . The effect of compositional disorder is obtained by analyzing the broadening and asymmetry of the first-order E2( high ) phonon mode. It is found that the Raman line shapes for the ZnO E2( high ) phonon in Zn 1-x Mn x O alloys can be well described by the spatial correlation model. It is shown that the substitutional disorder can introduce changes in the linewidth, line center position, and asymmetry of the first-order E2( high ) phonon mode in Zn 1-x Mn x O nanoparticles.

X-ray diffraction pattern of a ZnO and b Zn 1−x Mn x O nanoparticles grown by the chemical precipitation method. Miller indices of ZnO crystal lattice planes are given for each diffraction line. The arrows indicate the diffraction lines originating from the Zn 2 MnO 4 phase. FIG. 2. Raman spectra of both Zn 1−x Mn x O and ZnO nanoparticles at the range of 280-850 cm −1 .

…

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Raman study for

2phonon of ZnO in Zn1−

O nanoparticles

J. B. Wang

National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of

Science, Shanghai 200083, China and Department of Physics, Xiangtan University, Xiangtan 411105, Hunan

Province, China

H. M. Zhong, Z. F. Li, and Wei Lua兲

National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of

Science, Shanghai 200083, China

共Received 7 September 2004; accepted 4 January 2005; published online 8 April 2005兲

Raman scattering at room temperature is reported in Zn1−xMnxO nanoparticles for the range of 0

艋x艋0.15. The effect of compositional disorder is obtained by analyzing the broadening and

asymmetry of the ﬁrst-order E2共high兲phonon mode. It is found that the Raman line shapes for the

ZnO E2共high兲phonon in Zn1−xMnxO alloys can be well described by the spatial correlation model.

It is shown that the substitutional disorder can introduce changes in the linewidth, line center

position, and asymmetry of the ﬁrst-order E2共high兲phonon mode in Zn1−xMnxO nanoparticles. ©

2005 American Institute of Physics.关DOI: 10.1063/1.1865340兴

The use of electron spin, in addition to its more com-

monly used charge, holds great promise for a new class of

semiconductor memory and signal processing devices with

new functionality.1–3 Theoretical predictions of room-

temperature 共RT兲ferromagnetism 共FM兲in diluted magnetic

semiconductors4共DMSs兲recently focused attention on

magnetic-ion-substituted ZnO with a wurtzite structure simi-

lar to GaAs. According to these calculations, RT FM can

exist in p-type doped Zn1−xMnxO with 5% Mn and 3.5

⫻1020 holes/cm3.Ab initio band calculations5predict the

stability of FM in p-type doped Zn1−xMnxO, and antiferro-

magnetism 共AF兲in n-type doped Zn1−xMnxO. Therefore, the

structural and magnetic properties of Mn-doped and/or im-

planted ZnO have attracted signiﬁcant coverage from the

researchers.6–8

One of the most important aspects of substitutional semi-

conductor alloys is the nature of the alloy potential ﬂuctua-

tions 共APFs兲.9Since Raman scattering can yield important

information about the nature of the solid on a scale of the

order of a few lattice constants, it can be used to study the

microscopic nature of structural and/or topological disorder.

Raman scattering thus has been widely used to study the

structural properties of alloy semiconductors, such as

Ga1−xAlxAS 共Refs. 9 and 10兲and ZnMnxSe1−x.11 Recently,

considerable attention has been devoted to the dependence of

the widths of the E2phonons of ZnO on isotopic mass.12,13

The isotopic disorder effect on the full width at half maxi-

mum 共FWHM兲of the E2共high兲phonons of ZnO has been

detailed studied with Raman scattering and perturbation

theory.12 However, the relation between alloy disorder and

the line shape 共i.e., linewidth and asymmetry兲of E2phonons

of ZnO in Zn1−xMnxO nanoparticles has not been fully stud-

ied. In this paper, we will provide a detailed investigation on

the inﬂuence of the ﬁrst-order Raman spectra caused by

APFs in Zn1−xMnxO nanoparticles. We concentrate on the

upper E2phonon, E2共high兲, locating at 437 cm−1 at room

temperature. In ternary semiconductor alloys, the Raman

spectra show changes of various phonon modes with compo-

sitional disorder, including a shift in phonon frequency and

changes of the linewidth, asymmetry, and emergence of

disorder-activated modes. The broadening in linewidth and

asymmetry can be investigated in terms of the spatial corre-

lation 共SC兲model9based on the ﬁnite correlation length of a

propagating phonon due to the APFs. In this paper, it is

found that the Raman line shapes for the ZnO E2共high兲in

Zn1−xMnxO nanoparticles in the range of 0艋x艋0.15 can be

well described by the SC model.

The synthesis of Zn1−xMnxO precursor was carried out

using the chemical precipitation method which was similar

to that published elsewhere.14 Stoichiometric ZnCl2and

MnCl2·4H2O were added into the solution of NH4HCO3

mixed with a given surfactant. A white precipitate occurred

immediately when the two solutions mixed each other but it

was dissolved with stirring. A stable state slowly occurred

due to the concentration of Zn2+ ions which was high enough

in the mixed solution, so that it reached the state of super-

saturation. The solid was collected by ﬁltration, repeatedly

rinsed with ethanol for several times, then dried at 50–60 °C

for 3–4 h. At last, the Zn1−xMnxO powders were obtained

after annealing of the precipitates at the temperature of

700°Cfor1hinair.Thesizes and micrographs of

Zn1−xMnxO nanoparticles were measured by transmission

electron microscopy 共TEM兲, which was shown that the aver-

age crystallite size of the Zn1−xMnxO was ⬃50 nm.

The crystalline structure was studied by means of an

x-ray diffraction. Diffraction patterns of the microcrystallites

were characteristic of hexagonal ZnO with a small contribu-

tion of other compounds, which was shown in Fig. 1. How-

ever, this contribution was less than 1% of that of ZnO, as

estimated from the x-ray line intensity, and most probably

resulted from Zn2MnO4precipitates. In fact, for some

samples, the Zn2MnO4phase was clearly identiﬁed 共Fig. 1兲.

a兲Author to whom correspondence should be addressed; electronic mail:

luwei@mail.sitp.ac.cn

JOURNAL OF APPLIED PHYSICS 97, 086105 共2005兲

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Precipitation of Mn-based compounds is a rather common

problem encountered in the various growth techniques of

diluted magnetic II-VI semiconductors.7

The Raman-scattering experiments were carried out us-

ing a Jobin–Yvon LabRam-INFINITY micro-Raman system

at room temperature. The 514.53-nm line of an Ar+laser was

used for excitation. The Raman spectra of both Zn1−xMnxO

and Zno nanoparticles at the range of 280–850 cm−1 are

shown in Fig. 2. Our Raman study has shown that the Raman

spectra of ZnO nanoparticles is similar to that of bulk ZnO

共not shown in this paper兲, which indicates that the ZnO nano-

particles keep the overall crystal structure of the bulk ZnO.

This result is consistent with that of ZnO nanotube.15 The

assignments of the Raman peaks of ZnO nanoparticles are

summarized in Table I according to the previous Raman

study of ZnO.16 From Fig. 2, it is seen that although most of

the Raman peaks from the Zn1−xMnxO nanoparticles corre-

spond well to those of the ZnO nanoparticles, the Raman

spectra of Zn1−xMnxO nanoparticles have shown their own

characteristics. First, the E2共high兲phonon lines of

Zn1−xMnxO nanoparticles have 共a兲shifted to lower frequen-

cies and 共b兲broadened asymmetrically 共⌫a⬎⌫b兲, where

⌫a共⌫b兲represents the low-energy 共high-energy兲half width of

half maximum in the phonon line. The line shape of E2共high兲

phonons of Zn1−xMnxO nanoparticles will be detailed studied

in this paper. Second, the Raman peak at about 663 cm−1

becomes stronger and stronger with increasing the Mn con-

centration in Zn1−xMnxO nanoparticles. The enhancement of

the 663-cm−1 Raman peak may result from 共a兲the two pho-

non processes 关A1共LO兲+E2共low兲兴 and 共b兲the increment of

the Mn-based compounds, such as Zn2MnO4precipitates

shown in Fig. 1.

In alloy semiconductors, atom substitution induces not

only topological disorder but often also structural disorder.

For allowed phonons, these disorders result basically in the

breaking of the translational symmetry, leading to the contri-

bution of q⫽0 phonons to the Raman line shape, corre-

sponding to so-called ﬁnite-size effects. This greatly drives

the line-shape asymmetry and the line center shift, which can

be investigated in terms of the SC model9based on the ﬁnite

correlation length of a propagating phonon due to the APFs.

Here we discuss the effects of disorders in Zn1−xMnxO nano-

particles using the SC model.

In an ideal crystal, because of the momentum conserva-

tion, only phonons at the center of the Brillouin zone 共q

=0兲can be observed by Raman scattering. As the crystal is

alloying, the phonons can be conﬁned in space owing to the

potential ﬂuctuations of the alloy disorder, which gives rise

to a relaxation of the q=0 selection rule in Raman

scattering.11 So, the spatial correlation length of phonon in

alloys becomes ﬁnite. The ﬁnite phonon mode will lead to

the broadening and asymmetry of the Raman line shape.

With the SC model, we can evaluate the asymmetric broad-

ening of Raman scattering by the theoretical calculation. The

assumption of a Gaussian attenuation factor exp共−2r2/L2兲,

where Lis the diameter of the correlation region, has been

successfully used to account for q-vector relaxation related

to ﬁnite-size effects17 and structural disorder.9Then, the Ra-

man intensity I共

␻

兲at a frequency

␻

can be written as

I共

␻

兲⬵

冕

exp共−q2L2/4兲d3q

关

␻

−

␻

共q兲兴2+关⌫0/2兴2,共1兲

where qis expressed in units of 2

␲

/a,ais the lattice con-

stant, and ⌫0is the natural linewidth. Assuming that there is

a spherical region associated with the ﬁnite size of the cor-

relation regions in the alloys, Eq. 共1兲can be written as

TABLE I. The assignments of the Raman peaks of ZnO nanoparticles according to the study of Ref. 9 at the

range of 280–850 cm−1.

␻

共cm−1兲331 387 437 540 577 663

Assignments 2E2共low兲A1共TO兲E2共high兲2LA A1共LO兲A1共LO兲+E2共low兲

FIG. 1. X-ray diffraction pattern of 共a兲ZnO and 共b兲Zn1−xMnxO nanopar-

ticles grown by the chemical precipitation method. Miller indices of ZnO

crystal lattice planes are given for each diffraction line. The arrows indicate

the diffraction lines originating from the Zn2MnO4phase. FIG. 2. Raman spectra of both Zn1−xMnxO and ZnO nanoparticles at the

range of 280–850 cm−1.

086105-2 Wang

et al.

J. Appl. Phys. 97, 086105 共2005兲

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I共

␻

兲⬵

冕

␲

q2exp共−q2L2/4兲d3q

关

␻

−

␻

共q兲兴2+关⌫0/2兴2.共2兲

For the calculation of Zn1−xMnxO, ⌫0is determined to be

9.0 cm−1 by the linewidth of E2共high兲phonon in the ZnO

powder prepared by the same method as that of Zn1−xMnxO

powders. As for the dispersion

␻

共q兲, it is usually determined

by the neutron-scattering. But for the hexagonal structure

ZnO, the neutron-scattering data are sparse. We take the ana-

lytical model relationship,

␻

共q兲=A+Bcos共

␲

q兲,共3兲

which with A=424.5 cm−1 and B=12.5 cm−1 for the

E2共high兲phonon dispersion according to the ab initio

phonon-dispersion relations calculated for ZnO.13 Setting the

correlation length Las an adjustable parameter, one can get

the value of Lby ﬁtting the Raman line-shape of E2共high兲

band. The Lvalues corresponding to x=1%, 2%, 5%, 10%,

and 15% are 6.37, 6.19, 4.01, 3.65, and 2.92 nm, respec-

tively.Accordingly, for Zn1−xMnxO nanoparticles with differ-

ent Mn concentrations, the values of frequency shift ⌬

␻

linewidth ⌫, and the asymmetry ⌫a/⌫bcan be obtained by

the ﬁtting, which will be analyzed in the following para-

graph.

Figure 3 shows the frequency shift ⌬

␻

from 437 cm−1

and the ⌫of E2共high兲phonon line as a function of Las

evaluated from Eqs. 共2兲and 共3兲plotted by solid line. Also

shown are the experimental values of ⌬

␻

and ⌫for various

Mn concentrations, which are obtained by ﬁtting the experi-

mental Raman spectra using the SC model. From Eqs. 共2兲

and 共3兲, we are also able to evaluate the asymmetry ⌫a/⌫bin

relation to ⌬

␻

for various L. This is shown as the solid line

in Fig. 4 together with the experimental values. The agree-

ment in both cases is quite good. Thus from Figs. 3 and 4 it

is possible to relate the shift, broadening, and asymmetry of

the E2共high兲phonon for a given Mn concentration. For ex-

ample, a Mn concentration of 5% corresponds to an L

⬇4.01 nm. In Zn1−xMnxO alloy semiconductors, the

E2共high兲phonon correlation length Lcan be physically in-

terpreted as the average size of the localized region.11 The

value of Ldecreases with increasing Mn concentration,

which indicates that the phonon-extended region becomes

very small. This is caused by the compositional disorder in

alloys. Therefore, the Lvalue is a very appropriate parameter

accounting for the disorder of Zn1−xMnxO alloys.

In summary, Raman scattering at room temperature is

reported in Zn1−xMnxO nanoparticles for the range of 0艋x

艋0.15. The microscopic nature of the alloy disorder is dis-

cussed by investigating the compositional dependence of

E2共high兲phonon mode. It is shown that the Raman line

shapes 共asymmetric broadening兲induced by the substitu-

tional disorder can be quantitatively explained in terms of a

SC model.

This work is supported in part by the One-hundred-

person Project of the Chinese Academy of Science 共Grant

No. 200012兲, Chinese National Key Research Special Fund,

Key fund of Nation Science Foundation 共Grant No.

10234040兲, Key Fund of Shanghai Science and Technology

Foundation 共Grant No. 02DJ14066兲, and Shanghai Informa-

tion Special Foundation.

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FIG. 3. E2共high兲phonon Raman shift ⌬

␻

and broadening ⌫as a function of

L. Also shown are the experimental values of ⌬

␻

and ⌫for various Mn

concentrations. FIG. 4. Relationship between the E2共high兲phonon Raman shift ⌬

␻

and

asymmetry, ⌫a/⌫b, as a function of L. Also shown are the experimental

values of ⌬

␻

and ⌫a/⌫bfor various Mn concentrations.

086105-3 Wang

et al.

J. Appl. Phys. 97, 086105 共2005兲

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Sep 2003

A large quantity of nanosized ZnO tubular structures was prepared using a very simple thermal evaporation of mixed Zn-ZnO powders under a wet oxidation condition. The ZnO nanotubes have a hollow core with crystalline wall of 8-20 nm in thickness. Optical properties of ZnO nanotubes were studied at room temperature. Raman peaks arising from the ZnO nanotubes were analyzed, which correspond well to that of the bulk ZnO sample. The photoluminescence measurements of ZnO nanotubes revealed an intensive UV peak at 377 nm corresponding to the free exciton emission, and a broad peak at about 500 nm arising from defect-related emission. (C) 2003 American Institute of Physics.

Raman Scattering in Alloy Semiconductors: "Spatial Correlation" Model

Article

May 1984

Using a "spatial correlation" model with a Gaussian correlation function we have for the first time quantitatively explained the broadening and asymmetry of the first-order longitudinal-optic phonon Raman spectrum induced by alloy potential fluctuations. The systems studied were the representative alloy semiconductors ${\mathrm{Ga}}_{1$-${}x}{\mathrm{Al}}_{x}\mathrm{As}/\mathrm{GaAs}$ and ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/InP. This analysis provides important insights into the microscopic nature of the alloy potential fluctuations.

Lattice dynamics of ZnO and BeO

Article

Feb 1970

A. W. Hewat

Phonon dispersion in ZnO has been mapped for frequencies up to 4 × 1012c/sec. A postulated low frequency mode does not exist, and the results are in good agreement with the predictions of a simple shell model. Mean square vibrational amplitudes in ZnO and BeO are calculated.

Synthesis and characterization of ZnO nanoparticles assembled in one-dimensional order

Article

Jul 2003
INORG CHEM COMMUN

ZnO nanoparticles assembled in one-dimensional order were synthesized by a template-free method. The synthesized ZnO powder have a hexagonal zincite structure. The ZnO aggregates with rod-like shapes are typically ∼1.2 μm in length and 100 nm in diameter with an aspect ratio as high as ∼12, which consist of many small nanocrystals with diameters of ∼15 nm. Longer wires connected by many hexahedral ZnO nanocrystals were obtained after calcination at the temperature over 500 °C. The formation of the rod-shaped ZnO powders is related to the existence of [Zn(NH3)x]2+ complex cations.

Material Design for Transparent Ferromagnets With ZnO-Based Magnetic Semiconductors

Article

Jun 2000

Ferromagnetism of ZnO-based magnetic semiconductors was investigated by ab initio calculations based on the local density approximation. In a system of Mn atom doped ZnO, the ferromagnetic ordering of Mn magnetic moments was induced by hole doping. It was also found that 3d transition metal atoms of V, Cr, Fe, Co and Ni showed the ferromagnetic ordering of their magnetic moments in ZnO without any additional carrier doping treatments. Appearance of the ferromagnetism in these systems suggests possibility for a fabrication of a transparent ferromagnet which will have great impact on industrial applications in magneto optical devices.

Raman investigation of anharmonicity and disorder-induced effects in Zn_ {1-x} Be_ {x} Se epifilms

Article

Feb 2004

Raman scattering of Zn1-xBexSe epifilms grown by molecular beam epitaxy on GaAs (001) substrates has been investigated. The effect of compositional disorder was obtained by analyzing the broadening and asymmetry of the first-order LO mode. It is found that the Raman line shapes for the ZnSe-like LO mode in Zn1-xBexSe alloys can be well described by the spatial correlation model. We have also analyzed comparatively the anharmonic effects due to temperature and compositional fluctuations in ZnSe and ZnBeSe systems, using temperature-dependent Raman-scattering measurements. It is found that the anharmonicity is higher for ZnBeSe alloys than ZnSe and it increases with compositional disorder. Therefore, both temperature- and compositional-fluctuation-induced anharmonicities can introduce changes in the linewidth, line center position, and anharmonic decay time of the first-order optical phonons in Zn1-xBexSe alloys.

Disorder effects in Ga1−xAlxAs

Article

Sep 1986

Disorder effects of Raman scattering of Ga1−xAlxAs are studied using the various fixed lines of an Ar ion laser. We analyzed the spectra and the properties of polarization of Raman scattering from 50 cm−1 to 400 cm−1, and found some new Raman scattering peaks which were considered to be optical phonons are non-Γ-point. We report the broadening and asymmetry profile of Raman lines and present the formulas describing the linewidth of two mode behavior mixed crystals in this paper.

Magnetic properties of Mn doped ZnO tetrapod structures

Article

Feb 2004

ZnO tetrapod nanostructures were prepared by evaporating Zn metal under humid argon flow. After the fabrication, Mn diffusion doping was performed at two different temperatures (600 and 800 degreesC). The samples were characterized by scanning electron microscopy, transmission electron microscopy, x-ray fluorescence, x-ray diffraction (XRD), superconducting quantum interference device magnetometer, and photoluminescence. Diffusion doping resulted in the increase of the size of tetrapods, but no new peaks were found in XRD spectrum. Mn doped ZnO tetrapod structures were found to be ferromagnetic with Curie temperature similar to50 K, and showed large coercive field (similar to3500 Oe for 800 degreesC sample, similar to5500 Oe for 600 degreesC sample). (C) 2004 American Institute of Physics.

Dependence of Phonon Widths on Pressure and Isotopic Mass

Article

Feb 2003
PHYS STATUS SOLIDI B

Considerable attention has been devoted recently to the dependence of the widths of the Raman phonons of semiconductors on pressure and on isotopic mass. The dependence on pressure is usually small and monotonic unless the phonon happens to be close to a singularity of the two-phonon density of states (DOS) which determines its width. In the latter case, strong nonmonotonic dependences of the phonon width on pressure and on isotopic mass can appear. We have investigated the E2high phonons of ZnO crystals with different isotopes and observed a wide range of FWHM depending on isotopic masses. Ab initio calculations of the two-phonon DOS provide an explanation for this variation of the FWHM: the E2high frequency falls on a sharp ridge of the 2-DOS corresponding to combinations of TA and LA phonons. Changes in isotopic mass result in a motion of the E2high frequency up and down that ridge which produces the changes in FWHM. These phenomena suggest a decrease of the FWHM with pressure which seems to be present in existing data obtained at 300 K. Similar phenomena are discussed for the E2low phonons. Applications of the isotope and pressure techniques to the elucidation of two-phonon spectra will be presented.

Raman study for E2 phonon of ZnO in Zn1-xMnxO nanoparticles

Abstract and Figures

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