www.elsevier.com/locate/apsusc

Applied Surface Science 244 (2005) 385–388

Growth of MgxZn1 � xO films using remote plasma MOCVD

Atsushi Nakamura a,*, Junji Ishihara b, Satoshi Shigemori b,Toru Aoki a,b, Jiro Temmyo a,b

a Graduate School of Electronic Science and Technology, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8011, Japanb Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8011, Japan

Received 31 May 2004; accepted 6 October 2004

Abstract

MgxZn1 � xO films were successfully grown on a-plane sapphire (1 1 2 0) substrates by remote plasma enhanced metalor-

ganic chemical vapor deposition (RPE-MOCVD). Diethyl zinc (DEZn), bis-ethylcyclopentadienyl magnesium (EtCp2Mg) and

oxygen plasma were used as source materials. By increasing Mg content in the films, the crystal structure was shifted through a

mixed state from wurtzite to rock-salt with no significant segregation. Both optical absorption edges and emission peaks of

MgxZn1 � xO films shifted to the higher energy by increasing the Mg content at room temperature, showing an alloy broadening.

The Stokes’ shift of wurtzite MgxZn1 � xO alloy films was quantitatively evaluated, resulting in a linear dependence on the

absorption edge energy.

# 2004 Elsevier B.V. All rights reserved.

PACS: 81.05.Dz; 81.15.Gh

Keywords: Remote plasma enhanced MOCVD; MgxZn1 � xO; Diethyl zinc (DEZn); bis-Ethylcyclopentadienyl magnesium (EtCp2Mg);

Stokes shift

1. Introduction

Recently, ZnO attracted much attention to short

wavelength optoelectronic devices [1]. It has a large

excitonic binding energy (60 meV) in comparison

with GaN (25 meV) and an efficient exciton lumines-

cence at room temperature. In applications to UV

* Corresponding author. Tel.: +81 53 478 1321;

fax: +81 53 478 1321.

E-mail address: nakamura@nvrc.rie.shizuoka.ac.jp

(A. Nakamura).

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2004.10.095

detectors or UV emitters, one of the key issues is the

growth of heterostructures and quantum wells.

MgxZn1 � xO alloy is suitable for the barrier layers

of ZnO/MgxZn1 � xO heterostructures due to its wider

band gap.

There have been several reports on MgxZn1 � xO

films grown by pulsed-laser deposition, laser mole-

cular-beam epitaxy (L-MBE), and MBE techniques

[2–4]. Only limited information is available on the

growth of MgxZn1 � xO thin films grown by MOCVD

[5], though MOCVD technique has an advantage of

large-area deposition for industrial applications. In

.

A. Nakamura et al. / Applied Surface Science 244 (2005) 385–388386

addition, while the Stokes’ shift of laser-MBE-grown

MgxZn1 � xO films, or a measure of the quality of the

film, has been qualitatively discussed [6], even its

experimental data on MOCVD-grown MgxZn1 � xO

films has not been reported. In this study, we report

on the growth of MgxZn1 � xO films using remote

plasma enhanced MOCVD (RPE-MOCVD) and

discuss the Stokes’ shift of the MgxZn1 � xO films

quantitatively.

2. Experiment

ZnO and MgxZn1 � xO films were fabricated by

RPE-MOCVD [6]. The plasma was generated by a

radio frequency discharge in O2. Diethyl zinc (DEZn)

and bis-ethylcyclopentadienyl magnesium (EtCp2Mg)

were used as group-II sources. MgxZn1 � xO films

were grown on a-plane sapphire (1 1 2 0) and quartz

substrates. In our study, MgxZn1 � xO films were

directly grown on sapphire substrates. The films were

Fig. 1. XRD spectra of the MgxZn

grown at a total gas pressure of 0.01 Torr, substrate

temperature of 450 8C, oxygen flux of 5 sccm, RF

power of 30 W, and hydrogen carrier flux of 5 sccm.

The content x in MgxZn1 � xO films was controlled by

a flow ratio of group-II sources. The chemical

composition ratios of MgxZn1 � xO were estimated

by electron probe micro analysis (EPMA). The phase

and crystallography were characterized by X-ray

diffraction (XRD). The optical transmission spectra

were recorded on an UV–vis-NIR scanning spectro-

meter at a wavelength from 200 to 700 nm. The optical

band gap energies were determined from a plot of

(ahn)2 as a function of photon energy (hn). Photo-

luminescence (PL) measurements were carried out at

room temperature using a He–Cd laser (325 nm).

3. Results and discussion

Fig. 1 shows the XRD patterns of MgxZn1 � xO

films. ZnO has a wurtzite structure and MgO has a

1 � xO films with different x.

A. Nakamura et al. / Applied Surface Science 244 (2005) 385–388 387

Fig. 2. (a) Transmittance spectra and PL spectra measured at room

temperature and (b) stokes shift energy and FWHM of PL peak as a

function of an absorption edge.

rock-salt type cubic structure. Considering the

similarity in radii of Zn2+ (0.60 A) and Mg2+

(0.57 A) [7], the incorporated Mg may easily

substitute the Zn site up to certain amount. The

lattice constant of MgxZn1 � xO (wurtzite structure)

is close to that of ZnO. MgxZn1 � xO films exhibit

two types of the crystal structure such as wurtzite

and rock-salt. Single-phase films having wurtzite

(0 0 0 2) could be prepared with x up to 0.065. With

an increase in the Mg content, the (0 0 0 2) peak

slightly shifted to a high diffraction angle, indicating

the decreases in the c-axis lattice constant of the

films due to Mg incorporation. At x = 0.084–0.149,

there are (0 0 0 2), (1 1 1) and (4 0 0) peaks. In this

region, MgxZn1 � xO films have a mixed state. The

(1 1 1) and the (4 0 0) peaks indicate the rock-salt

structure. It is noted that the peaks of ZnO and MgO

were not observed, indicating no phase segregation.

Single-phase films having rock-salt could be pre-

pared at x � 0.174. The (1 1 1) and (4 0 0) peaks

shifted to a high diffraction angle as the Mg content

increased.

Fig. 2(a) shows the typical transmittance spectra

and PL spectra measured at room temperature. The

broadening of the luminescence peak was observed in

the alloy films. The PL spectra of the alloy films in the

near-band gap spectral region are probably considered

to be dominated by the broad emission associated with

the recombination of localized excitons and/or trapped

in the near-band gap states. The slope of the PL low-

energy tail provides some qualitative measure of the

random fluctuations in the alloy composition, respon-

sible for the exciton localization. The absorption edge

shifted to the higher energy with an increase of the Mg

content. Tailing of the absorption edge is also found to

occur around x = 0.149, probably due to fluctuation of

the Mg content and due to grain boundaries. Fig. 2(b)

shows the Stokes’ shift and the FWHM of the PL peak

as a function of an absorption edge energy. The ZnO

film showed an emission peak close to the absorption

edge. The luminescence peaks of the alloy films

showed the Stokes shift to the low-energy side of the

absorption edge. The Stokes shift in MgxZn1 � xO

spectra is increased with Mg content. It seems to be

due to an increasing localization with an increase of

Mg content. As for the FWHM of the PL peak, a

similar tendency of increasing is observed, except for

the data at an absorption edge of 3.7 eV. Here, the

FWHM value at 3.7 eV is relatively small due to the

mixed state of wurtzite and rock-salt. The actual

mechanism is not clear yet.

It was found that the Stokes’ shift of the wurtzite

MgxZn1 � xO films exhibits a linear dependence on the

absorption edge energy.

4. Conclusions

The MgxZn1 � xO alloy films have been grown

successfully on (1 1 2 0), a-plane sapphire substrates

using RPE-MOCVD. MgxZn1 � xO alloy films exhibit

two types of the crystal structure such as wurtzite and

rock-salt. Single-phase films having wurtzite structure

could be prepared with x up to 0.065, as verified by

XRD analysis. PL and transmittance measurements

A. Nakamura et al. / Applied Surface Science 244 (2005) 385–388388

showed that each spectral peak shifts to the higher

energy with increasing a Mg content. The Stokes’ shift

between the absorption edge and the PL peak seems to

be caused by the exciton localization.

Acknowledgements

The authors would like to thank Prof. Y. Nakanishi,

Prof. A. Tanaka, and H. Katsuno of the Research

Institute of Electronics, Shizuoka University, for

assistance in optical measurement, EPMA measure-

ment, and analysis, and for useful discussions. This

research project was supported in part by The Murata

Science Foundation.

References

[1] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason,

G. Cantwell, Appl. Phys. Lett. 81 (2002) 1830.

[2] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K.

Tamura, T. Yasuda, H. Koinuma, Appl. Phys. Lett. 78 (2001) 1237.

[3] S. Choopum, R.D. Vispute, W. Yang, R.P. Sharma, T. Venka-

tesan, Appl. Phys. Lett. 80 (2002) 1529.

[4] K. Ogata, K. Koike, T. Tanite, T. Komuro, F. Yah, S. Sasa, M.

Inoue, M. Yano, J. Cryst. Growth 251 (2003) 623.

[5] S. Muthkumar, J. Zhong, Y. Chen, Y. Lu, T. Siegrist, Appl. Phys.

Lett. 82 (2003) 742.

[6] A. Nakamura, Y. Shigemori, T. Shimizu, A. Aoki, J. Tanaka,

Temmyo, Jpn. J. Appl. Phys. 42 (2003) 775–777.

[7] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koi-

numa, Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys.

Lett. 72 (1998) 2466.