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4 (2008) 5034–5038

Applied Surface Science 25

A spectroscopic ellipsometric investigation of new critical

points of Zn1�xMnxS epilayers

D.-J. Kim a, J.-W. Lee b, Y.-M. Yu c, Y.D. Choi d,*a Institute of Science & Technology, Mokwon University, Daejeon 302-729, Republic of Korea

b Department of Materials Engineering, Hanbat National University, Daejeon 305-719, Republic of Koreac Process Reengineering Team, National Archives and Records Service, Daejeon 302-701, Republic of Korea

d Department of Optical & Electronic Physics, Mokwon University, Daejeon 302-729, Republic of Korea

Received 19 July 2007; received in revised form 30 January 2008; accepted 31 January 2008

Available online 8 February 2008

Abstract

Zn1�xMnxS epilayers were grown on GaAs (1 0 0) substrates by hot-wall epitaxy. X-ray diffraction (XRD) patterns revealed that all the

epilayers have a zincblende structure. The optical properties were investigated using spectroscopic ellipsometry at 300 K from 3.0 to 8.5 eV. The

obtained data were analyzed for determining the critical points of pseudodielectric function spectra, he(E)i = he1(E)i + ihe2(E)i, such as E0,

E0 + D0, and E1, and three E2 (S, D, G) structures at a lower Mn composition range. These critical points were determined by analytical line-shapes

fitted to numerically calculated derivatives of their pseudodielectric functions. The observation of new peaks, as well as the shifting and broadening

of the critical points of Zn1�xMnxS epilayers, were investigated as a function of Mn composition by ellipsometric measurements for the first time.

The characteristics of the peaks changed with increasing Mn composition. In particular, four new peaks were observed between 4.0 and 8.0 eV for

Zn1�xMnxS epilayers, and their characteristics were investigated in this study.

# 2008 Elsevier B.V. All rights reserved.

PACS : 78.20.�e; 78.40.Fy

Keywords: Zn1�xMnxS epilayer; Spectroscopic ellipsometry; Critical points

1. Introduction

Zn1�xMnxS has a wide band gap energy and hence is

considered a promising material for applications related to

optoelectronic devices and thin film electroluminescent devices

[1–4]. For the realization of such applications, it is essential to

grow high quality single crystal epilayers and to attain an

extensive understanding of the optical and electrical properties

of Zn1�xMnxS. However, the optical properties of this material

have yet to be thoroughly investigated. While some results have

been reported for the photon energy range below 6 eV [5–7],

few investigations have been conducted for photon energy

above 6 eV, which belongs to the high-energy range, for pure

ZnS without a Mn component [8]. Therefore, extensive study

on the optical response of Zn1�xMnxS at high photon energy is

* Corresponding author. Tel.: +82 42 829 7552; fax: +82 42 823 0639.

E-mail address: ydchoi@mokwon.ac.kr (Y.D. Choi).

0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.01.153

required. In particular, studies on the S, D, G-transitions and

newly observed transitions originating in the Brillouin zone

need to be carried out.

Spectroscopic ellipsometery (SE) is a well-known, powerful

tool to study the optical dielectric properties of semiconductors.

Using SE, the critical points can be easily obtained from a

spectroscopic analysis of the pseudodielectric functions, which

are related closely with the energy band structure [5]. The

purpose of the present study is to determine the optical

properties of Zn1�xMnxS/GaAs (1 0 0) epilayers by SE

measurement for a wide photon energy range of 3.0–8.5 eV.

In particular, in this work, for the first time the spectral

dependence of pseudodielectric function spectra he(E)i= he1(E)i + ihe2(E)i of Zn1�xMnxS is described as a function

of the Mn composition. Shifting and broadening of the critical

points, such as E0/E0 + D0, E1, and E2 with increasing Mn

composition were also investigated. Further, the second

derivative spectra, d2e(E)/dE2, of the pseudodielectric function

of Zn1�xMnxS epilayers have been studied on the basis of

Fig. 1. XRD patterns as a function of the Mn composition of Zn1�xMnxS

epilayers.

D.-J. Kim et al. / Applied Surface Science 254 (2008) 5034–5038 5035

numerically calculated analytical line-shapes with two-dimen-

sional critical structures. Four new peaks were observed and

investigated for the energy range between 4.0 and 8.0 eV in

conjunction with increasing Mn composition of Zn1�xMnxS

epilayers.

2. Experiments

All Zn1�xMnxS epilayers investigated in this work were

grown on GaAs (1 0 0) substrates by hot-wall epitaxy. The

optimum substrate temperature, wall temperature, and

source temperature for the growth of the Zn1�xMnxS epilayers

were found to be 200, 580, and 750 8C, respectively. The

temperature for Mn was controlled from 720 to 790 8C.

The thickness of the Zn1�xMnxS epilayers was determined to

be approximately 1 mm from reflectance measurements

carried out using a spectrophotometer, and the growth rate

was identified as 1–3 A/s. The Mn composition x was

determined to range from 0 to 0.59 by energy dispersive X-

ray spectrometry measurements. The crystal structure and

lattice constant were examined by X-ray diffraction (XRD).

Prior to the SE measurements, the samples were rinsed via

flushing with methanol. During the SE measurements, dried

nitrogen gas of high purity was flowed continuously onto the

sample surface in order to prevent oxidation and contamination

by air. In order to investigate the optical properties of

Zn1�xMnxS epilayers, the pseudodielectric function spectra

were measured at 300 K between 3.0 and 8.5 eV using an

automatic spectroscopic rotating analyzer ellipsometer (Wool-

lam VUV-VASE system) with 300 W xenon and 70 W

deuterium lamps at an incident angle of 708. In this SE

experiment, the elliptical azimuth C and phase angle D

determined with respect to the polarized components, which

vibrate in directions perpendicular (s) and parallel (p) to the

incident plane, can be measured precisely. Therefore, the

complex pseudodielectric he(E)iof the epilayer can be

determined in the two-phase model by

heðEÞi ¼ easin2f

�1þ tan2f

�1� r

1þ r

�2�; (1)

where r = tan CeiD; ea = 1 and f are the pseudodielectric

function of the ambient medium and the incident angle of

the probing light, respectively. Since corrections to the over-

layers and surface roughness have not been made, the dielectric

spectra derived from the ellipsometric data can be treated as

‘pseudodielectric’, he(E)i.

3. Results and discussion

From Fig. 1, it has seen that the XRD patterns vary according

to the Mn composition x of the Zn1�xMnxS epilayers. All the

epilayers have a zincblende structure. Zn1�xMnxS (2 0 0) and

(4 0 0) peaks with GaAs (2 0 0) and (4 0 0) peaks can be clearly

observed. The separation between the Zn1�xMnxS and GaAs

peaks decreases with increasing Mn composition and the XRD

peak intensities of Zn1�xMnxS are also gradually weakened

with increasing Mn composition. The XRD spectra show that,

through the entire range of Mn composition x, no other peaks

exist except for a homogeneous zincblende ZnMnS phase. It

should be noted that the peak observed at 218 appears to be due

to an error originating in the measurement. For all the samples,

if Vegard’s law is assumed and the lattice constants of ZnS and

MnS are taken to be 5.410 and 5.559 A, respectively, as

indicated in our previous report, the positions of the X-ray peak

for the epilayers correspond to those of the fully relaxed film

[9].

Fig. 2 shows the real part of the pseudodielectric function

spectra he1(E)i of Zn1�xMnxS epilayers obtained from SE

measurements with increasing Mn composition x. The

structures in Fig. 2 are represented using the notation of

Cardona and Greenaway [10]. Although there have been many

reports on the optical properties of Zn1�xMnxS [11–13,3], data

measured by SE as a function of Mn composition x have not

been reported to date.

The strong interference patterns appearing at energy below

the energy band gap shown in Fig. 2, E0/E0 + D0, are due to

multiple internal reflections of the light beam in the transparent

epilayer. Kim and Sivananthan, and Dahmani et al. reported

that the E0 peak appears at the end of the right side of the

interference patterns for ZnSe [14,15]. As shown in Fig. 2, the

E0/E0 + D0 peaks of Zn1�xMnxS in this study also distinctly

appear at the end of the right side of the interference patterns.

The E0 and E0 + D0 peaks caused by the Gv15!Gc

1 transition

could not be readily distinguished, because the intrinsic spin

splitting energy D0 of ZnS is very small (�70 meV) [5]. Note

that this value is slightly smaller than that of other II–VI

semiconductors. Thus, the E0 and E0 + D0 peaks appeared as a

single peak in all the Zn1�xMnxS epilayers investigated in this

Fig. 2. Real parts he1(E)i of the pseudodielectric function spectra of

Zn1�xMnxS epilayers obtained from SE measurement as a function of Mn

composition. Spectra have been offset by amounts indicated in the parentheses.

Fig. 3. Imaginary parts he2(E)i of the pseudodielectric function spectra of

Zn1�xMnxS epilayers obtained from SE measurement as a function of Mn

composition. Spectra have been offset by amounts indicated in the parentheses.

D.-J. Kim et al. / Applied Surface Science 254 (2008) 5034–50385036

study. As shown in Fig. 2, the E0/E0 + D0 peak energy at x = 0

was obtained at about 3.81 eV, and thereafter it slowly

decreased with increasing Mn composition and approached

3.65 eV at x = 0.59.

The E1 (Lv3!Lc

1) peak due to the contribution of 2D

excitons at x = 0 was observed at 5.74 eV [16]. Note that the E1

peaks also red-shifted to lower energies with increasing Mn

composition. The E2 structure at x = 0 was divided into three

peaks according to the transition direction of the Brillouin zone

(BZ). These structures appeared at approximately 7.00 eV

(Sv2!S

c1), 7.25 eV (Dv

5!Dc1), and 7.80 eV (Gv

15!Gc15) [8].

The present experimental data on the E2 peak positions (7.00,

7.25, and 7.80 eV) are in good agreement with results reported

by Ghong et al. [17].

Note that a new peak positioned near 7.20 eV begins to

appear at x = 0.21, and becomes increasingly apparent with

increasing Mn composition. Thus, this peak can be ascribed to

the increase of Mn composition and, more specifically, may be

caused by Mn 3d+, as explained in our previous report on

Zn1�xMnxSe [18]. The peaks located at 8.35 eV will be

discussed in detail with the numerically calculated second

derivative spectra of Fig. 4.

Fig. 3 shows the imaginary part he2(E)i of the pseudodi-

electric function of Zn1�xMnxS epilayers. The E0/E0 + D0 peak

caused by the G-transition can be considered a 3D M0CP type

[5,16]. The E0/E0 + D0 peak is represented as a single peak, as

shown in Fig. 2. The E1 peak is located at 5.74 eV at lower Mn

composition, and it is known to be due to the 2D excitons. In

general, the 2D excitons in II–VI semiconductors act strongly

[5,19–21]. The excitonic effects are reported to be much

stronger in II–VI semiconductors than in III–V semiconductors

[19–21]. Therefore, the E1 peak is stronger and more distinct

than any other peaks, as shown in Fig. 3. As the Mn

composition was increased, the E1 peak showed red-shifting. In

our experimental results, all E1 peaks were weakened and

broadened because of the small contribution of 2D excitons [5],

which were strongly localized at high Mn composition. These

results could be attributed to an increase of the misfit

dislocation density in ternary compounds and the overlap of

some peaks, respectively. The overlapped peaks are comprised

of two peaks, which are further described in Fig. 5. Three E2

peaks clearly appeared at lower Mn composition (x � 0.10), but

these peaks completely disappeared at higher Mn composition.

Unlike the transition at the S-point, the transitions at the D and

G-points are relatively weak. When the Mn composition was

larger than x = 0.21, a new peak, attributed to Mn 3d+, was

generated at approximately 7.20 eV. This peak is also further

described in Fig. 5.

Fig. 4 illustrates the numerically calculated second

derivatives spectra, d2he(E)i/dE2, of the pseudodielectric

function spectra he(E)i of Zn1�xMnxS epilayers and data fitted

by the standard analytic critical points line shape for further

analysis of the critical points. The open squares and circles in

Fig. 4 denote the data obtained from SE measurement in the

present study and the solid and dotted curves represent the best

fits of e1(E) and e2(E), respectively. The fitted data are expressed

Fig. 4. The second derivative spectra d2 he(E)i/dE2 of the pseudodielectric

function spectra of Zn1�xMnxS epilayers. The open squares and circles repre-

sent the data measured by SE, and the solid and dotted lines represent the data

fitted by Eq. (2). Spectra have been offset by amounts indicated in the

parentheses.

Fig. 5. Shift of the peak position energy of Zn1�xMnxS epilayers obtained from

the results of Fig. 4. Open symbols and dotted lines represent the obtained

structures and guidelines, respectively, and filled symbols and dot-dashed lines

denote the four structures newly observed and the guidelines, respectively.

D.-J. Kim et al. / Applied Surface Science 254 (2008) 5034–5038 5037

as analytical line-shapes with two-dimensional critical points

[22,23],

eðEÞ ¼ C � A lnðE � Ei � iG Þ expðiFÞ: (2)

This numerical formula consists of four parameters for each

point: energy E, broadening G, amplitude A, and phase angle F.

We determined the critical points of the epilayers by taking the

zero-crossing of the second derivative spectra of the imaginary

parts of their pseudodielectric function spectra he(E)i. As

shown in Fig. 4, the second derivative spectra have a clearer

structure than no differentiated spectra shown in Figs. 2 and 3.

In particular, the structure located at 8.35 eV was reported as a

Lv3!Lc

3 transition by Walter and Cohen [24]. Note that they

obtained this structure at 8.35 eV, from reflectance spectra

measurement. We observed this structure via SE measurement,

for the first time, as a function of Mn composition. In Figs. 2

and 3, this peak is not clearly revealed because of its very weak

intensity. However, it is clearly shown in the numerically

calculated second derivative spectra of Fig. 4. Note that four

new peaks, (E(L), E(X), E(G), and Mn 3d+), are observed above

x = 0.21 for the Zn1�xMnxS epilayers investigated in this study.

It can be inferred that the respective peaks are generated with

broadening of critical point structures, as noted above. Detailed

descriptions of these peaks are provided in Fig. 5.

Fig. 5 shows the change of the respective peak position

energies obtained from the fitted second derivative spectra

shown in Fig. 4. Open symbols (v, ", D, *, &) represent the

structures obtained in this study, i.e., E0/E0 + D0, E1, and three

E2 (S, D, G) structures. The dotted lines are guidelines. Filled

symbols (!, ~, &, *) and dot-dashed lines designate the four

structures newly observed in this study with increasing Mn

composition and the guidelines, respectively. Through the

entire range of Mn composition, the peak position energies of

E0/E0 + D0 changed weakly. This finding is consistent with the

energy band gap of zincblende MnS (b-MnS), i.e., 3.6–3.8 eV

[25–27], as indicated by the guideline in Fig. 5. Lu et al.

reported that the absorption edge of zincblende MnS is near

340 nm (3.647 eV) [26]. The peak position energy of the E1

structure was not changed. However, with increasing Mn

composition, the peak position energy E2 (S) structures were

red-shifted and the peak position energies of the E2 (D) and E2

(G) structures were slightly blue-shifted, as indicated in Fig. 5.

As noted above, we observed four new peaks E(L), E(X), E(G),

and Mn 3d+, at Mn composition above x = 0.21. These peaks are

located near 4.43, 5.68, 6.53, and 7.20 eV, respectively, and

originate from the transition caused by interaction of the

excitons. Huffman and Wild suggested that these structures could

be generated by excitation from the Mn 3d2+ band to the Mn 3d

band [28]. In general, the dielectric behavior of compound

semiconductors is known to be strongly connected with their

electronic energy band structures. This study has discussed the

transition that is generated according to the respective transition

points. We have obtained an abundance of information from the

results of Huffman et al. Also, from the electronic energy band

structure of ZnS and MnS, it was determined that the Mn 3d band

is affected by increasing Mn composition.

D.-J. Kim et al. / Applied Surface Science 254 (2008) 5034–50385038

In II-Mn-VI semiconductors, the peak position energies of the

respective critical point structures decreased with increasing Mn

composition. A qualitative explanation of this behavior in the

presence of Mn 3d electrons in semiconductors, such as

Zn1�xMnxS, Zn1�xMnxSe, Zn1�xMnxTe, Cd1�xMnxTe, is as

follows. The relative locations of d states are very important,

because they determine the direction of the repulsion effect of

hybridization. The d states of ZnS exist at�10 eV, but the d states

of MnS are located between the 3 and 4 eV with increasing Mn

composition. Thus, the valence bands that contribute to the

respective critical point structures are repelled to higher energies

by interaction with the Mn 3d levels, while the conduction band is

pushed downward [29]. Consequently, the peak position energies

of the respective critical points are decreased.

The peak positioned at 4.43 eV is classified as an E(L)

structure, and is caused by a transition in the L-point of the

energy band structure. Also, the peak located at 5.68 eV is

considered to be an E(X) structure induced by a transition in the

X-point. Note that the E(L) (4.43 eV) and E(X) (5.68 eV)

structures positioned in the L-point and X-point are newly

observed at Mn composition x = 0.21 for the Zn1�xMnxS

epilayers investigated here. These two structures arise due to an

intraband transition with increasing Mn composition. They red-

shift to the lower energy side with increasing Mn composition,

as indicated by the dot-dashed lines in Fig. 5. As noted above,

two structures were observed between 6.0 and 8.0 eV. The

structure located at 6.53 eV is due to the contributions of the

excitons in the G-position between the valence band and

conduction band, and can be designated as E(G). This structure

was red-shifted, as represented by the guideline in Fig. 5. Also,

the structure positioned near 7.20 eV is due to Mn 3d+. These

peaks, which are red-shifted, were explained in detail in our two

previous reports [18,30]. As described above, in most cases, the

peak position energies of the critical point structures observed

in this study decreased with increasing Mn composition.

4. Conclusions

Zn1�xMnxS epilayers were grown on GaAs (1 0 0) substrates

by a hot-wall epitaxy method. From the XRD patterns, the

epilayers were found to have a homogeneous zincblende

structure that does not contain wurtzite or any other structures.

The optical properties of the epilayers were determined by

ellipsometric measurements in a range of 3.0–8.5 eV at 300 K.

The E0/E0 + D0 peak of the Zn1�xMnxS epilayers was observed

as a single peak at the edge of the oscillation region in the

pseudodielectric function spectra he(E)i. The E1 peak

originated from the contribution of 2D excitons and was not

changed. The E2 structure at lower Mn composition (x < 0.10)

was separated into three peaks according to the transition

positions, which were near 7.0 eV (S), 7.4 eV (D), and 7.8 eV

(G). Note that we observed four new peaks in the second

derivative spectra of the Zn1�xMnxS epilayers for Mn

composition above x = 0.21; these are located near 4.43,

5.68, 6.53, and 7.20 eV. It is also emphasized that E(L)

(4.43 eV) and E(X) (5.68 eV) structures positioned in the L-

point and X-point are newly observed for Zn1�xMnxS epilayers.

The second derivative spectra of the pseudodielectric functions

d2he(E)i/dE2 of Zn1�xMnxS are expressed as analytical line-

shapes with two-dimensional critical points. These peaks show

clearer structures than no differentiated spectra. The change of

the respective peak position energies in Zn1�xMnxS is closely

related to the increase of Mn composition.

Acknowledgement

This work was supported by a Korea Research Foundation

(KRF-2005-075-C00012).

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