Current Applied Physics 4 (2004) 637–639

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Reduction of dislocation density and improvement of opticalquality in ZnO layers by MgO-buffer annealing q

Hiroki Goto a,*, Hisao Makino b, Agus Setiawan a, Takuma Suzuki a, Chihiro Harada a,Tsutomu Minegishi a, Meoung-Whan Cho b, Takafumi Yao a,b

a Center for Interdisciplinary Research, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japanb Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

Received 20 November 2003; accepted 30 January 2004

Available online 9 April 2004

Abstract

Optical properties of ZnO thin films with/without MgO-buffer annealing were investigated by low and room temperature

photoluminescence measurements. The ZnO films were grown on c-sapphire substrates by plasma-assisted molecular-beam epitaxy

employing a thin MgO-buffer layer. Dislocation density of ZnO layer was reduced from 5.3 · 109 to 1.9· 109 cm�2 by annealing

MgO-buffer prior to the growth of ZnO. The intensity of free exciton emission from the sample with MgO-buffer annealing was

almost twice of that from the sample without annealing, while the deep level emission from the sample with MgO-buffer annealing

was about 1/3 of that without annealing. The MgO-buffer annealing improves optical quality of overgrown ZnO films.

� 2004 Elsevier B.V. All rights reserved.

PACS: 61.72.Hh; 71.55.Gs; 78.55.Et; 81.40.Tv

Keywords: ZnO; Photoluminescence; Molecular beam epitaxy; MgO buffer; Annealing

1. Introduction

ZnO has a direct band gap of in the ultra-violet re-

gion with a large exciton binding energy of 60 meV [1].

This value is about three times larger than GaN.

Therefore, ZnO is a good candidate for exciton-related

optical devices since the excitons are stable even at room

temperature. ZnO films were usually grown on sapphiresubstrates since suitable materials were absent. How-

ever, large residual strain and structural defects were

remained due to large lattice mismatch of 18.3% in the

ZnO/c-sapphire heterointerface [1]. Lots of efforts have

been done to grow high quality ZnO so far. Usage of

suitable substrates such as GaN [2], and CaF2 [3], and

introducing low temperature (LT) ZnO-buffer [4] were

reported. Concerning growth of ZnO on c-sapphire, our

qOriginal version presented at QTSM&QFS 2003 (Quantum

Transport Synthetic Metals & Quantum Functional Semiconductors),

Seoul National University, Seoul, Korea, 20–22 November 2003.* Corresponding author. Tel.: +81-22-217-4404; fax: +81-22-217-

7810.

E-mail address: hgoto@cir.tohoku.ac.jp (H. Goto).

1567-1739/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.cap.2004.01.038

group proposed growth of high quality ZnO film

introducing a MgO-buffer layer [5]. The lattice mismatch

reduced from 18.3% to 9% (in the ZnO/MgO interface)

and 8% (in the MgO/Al2O3 interface) in this method. In

addition, we have found annealing of MgO-buffer layer

reduces dislocation density and improves surface mor-

phology in overgrown ZnO layer, recently [6]. In this

study, we report the effects of MgO-buffer annealing onoptical property of overgrown ZnO films.

2. Experiment

ZnO thin films were grown on sapphire (0 0 0 1)

substrate by plasma-assisted molecular-beam epitaxy (P-

MBE) with employing a MgO-buffer. The sapphiresubstrates were degreased in acetone, ethanol, and de-

ionized water, then it was etched in 3:1 of H2SO4:

H3PO4 solution for 15 min at 160 �C. The substrates

were treated by thermal cleaning for 1 h in buffer

chamber at 750 �C. Oxygen plasma is pre-exposured for

30 min in main chamber. High temperature (HT) MgO-

buffer layer was grown at 700 �C, and low temperature

638 H. Goto et al. / Current Applied Physics 4 (2004) 637–639

(LT) MgO-buffer was grown at 490 �C. Prior to growthof ZnO, LT-MgO-buffer was in situ annealed at 800 �Cfor 25 min. LT-ZnO buffer was grown at 490 �C sup-

ported by annealing at 750 �C for 5 min. HT-ZnO

overlayer grown at 700 �C. In order to examine effects of

MgO-buffer annealing, we prepared another sample

which was grown without MgO-buffer annealing.

The dislocation density of grown ZnO films was

determined from cross-sectional TEM measurement.The dislocation density is 5.3 · 10�9cm�2 at ZnO

without MgO-buffer annealing, and 1.9 · 10�9cm�2 at

ZnO with MgO-buffer annealing. Details of growth

and structural characterization are described in Ref.

[6].

Low and room temperature photoluminescence (PL)

spectra were measured using 325 nm line of He-Cd laser

as an exciting source. Samples were fixed in an closed-cycle He cryostat with varying temperature from 10 to

300K. PL signal was dispersed by a 32 cm mono-

chrometer with 1200 g/mm grating and detected by a

charge coupled devices (CCD) camera.

Fig. 1. Photoluminescence spectra at low temperature. (a) T ¼ 10 K (b)

T ¼ 77 K Solid line is ZnO with MgO-buffer annealing, and dotted line

is ZnO without MgO-buffer annealing. The spectra are normalized

with FX(A) emission.

3. Results and discussion

Fig. 1 shows low temperature PL spectra of the

samples with and without MgO-buffer annealing in near

band edge emission region measured at 10 K (a) and 77

K (b). The spectra were normalized at free exciton-A

emission intensity for clarity. The emission line located

at 3.368 eV dominates the spectrum in both samples at

10 K. This emission can be divided into two peaks, 3.367

eV and 3.363 eV and assigned as I2 (ionized donorbound excitons emission) and I4 (hydrogen donor re-

lated emission), respectively [7]. Note that these lines are

clearly resolved to two lines at 77 K in the sample with

annealing as shown in Fig. 1(b). The intensity of I2 is

greater than that of I4 in both samples. In donor bound

excitons region [8], another relatively weaker emission

which assigned as I8 or I9 (Gallium or Indium donor

related emission) [7] was also observed at about 3.358eV. It can be seen that relative intensity of these donor

bound excitons to free exciton emission of the sample

without MgO-buffer annealing is greater than that of the

sample with MgO-buffer annealing. It might be related

to different impurity incorporation or formation of na-

tive defects in the samples.

At lower energy region, a sharp emission located at

3.335 eV and longitudinal optical (LO) phonon assistedbound exciton emission were observed in both samples.

In the temperature dependence of PL spectra, the

intensity of emission line at 3.335 eV reduces like donor

bound excitons with the increase of temperature.

Accordingly this line can be attributed to excitonic

nature. Kato et al. reported origin of this emission is

neutral acceptor bound exciton (Ia) emission from PL

spectra of epitaxial layers on a-sapphire grown by MBE

[9]. On the other hand, Alves et al. assigned the excitonic

emission line at 3.335 eV as exciton bound to structural

defects (DBX) emission from CL experiments on as-

grown and annealed bulk samples [10]. In the present

case, the intensity of the 3.335 eV line in ZnO film withMgO-buffer annealing is greater than that of without

annealing, that is, the intensity is increasing although

dislocation density is lower. Considering this point, it is

appropriate for assigning this line to neutral acceptor

bound excitons in this case although the origin of

acceptor is unknown.

At 77 K, free exciton-A emission located at 3.374 eV

became dominant instead of donor bound excitons. Thefree exciton-B emission is appeared at 3.380 eV with

6meV energy spacing to free exciton-A emission. LO-

phonon replica of free exciton emission are clearly ap-

peared at 3.312, 3.239, and 3.166 eV. As the each

spectrum of excitonic emission lines can be resolved, the

emission lines from the sample with MgO-buffer

annealing have narrower spectral width compared to

those of ZnO without MgO-buffer annealing. This

Fig. 2. Photoluminescence spectra of ZnO films at room temperature.

H. Goto et al. / Current Applied Physics 4 (2004) 637–639 639

indicates better crystal quality of ZnO film with MgO-buffer annealing.

Fig. 2 shows PL spectra of ZnO films with and

without MgO-buffer annealing measured at room tem-

perature. The free exciton emission at 3.31 eV and broad

deep-level emission at around 1.75 eV were observed in

both samples. Comparing PL spectra of ZnO with and

without annealing, we can find the intensity of free

exciton emission from the sample with MgO-bufferannealing is twice of that from the sample without

annealing. We can also find the intensity of deep-level

broad emission is reduced about 1/3 by MgO-buffer

annealing. Generally, the deep level luminescence of

ZnO originates in oxygen vacancy and some impurities

related complexes in ZnO films [11]. The decrease of

deep level emission intensity by MgO-buffer annealing

means reduction of defects in ZnO film, i.e., improve-ment of crystal quality. This result is in agreement with

structural characterization of reduction in dislocation

density with MgO-buffer annealing.

4. Conclusions

In summary, we studied optical properties of ZnO

thin films with and without MgO-buffer annealing. The

radiative intensity of bound exciton emission to free

exciton emission in ZnO film were reduced by MgO-

buffer annealing in low temperature spectra. In room

temperature PL spectra, the intensity of free exciton

emission from ZnO with MgO-buffer annealing is twotimes greater than that of without MgO-buffer

annealing, while deep level emission was about 1/3 of

that of without annealing. These results indicate that

MgO-buffer annealing improves not only structural

quality but also optical quality of overgrown ZnO

films.

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