phys. stat. sol. (c) 1, No. 4, 880–883 (2004) / DOI 10.1002/pssc.200304178

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ZnO growth using remote plasma metalorganic chemical vapor deposition

Atsushi Nakamura*, 1, Satoshi Shigemori2, Yoshimi Shimizu2, Toru Aoki2, and Jiro Temmyo1,2 1 Graduate School of Electronic Science and Technology, Shizuoka University, 3-5-1 Johoku,

Hamamatsu, Japan 2 Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Japan

Received 22 September 2003, revised 25 September 2003, accepted 20 November 2003 Published online 23 February 2004

PACS 34.90.+q, 78.55.Et, 81.15.Gh

ZnO layers were grown on a-plane sapphire by remote plasma metalorganic chemical vapor deposition (MOCVD) using oxygen plasma and Diethyl Zinc (DEZn) as a source material with hydrogen carrier. Oxygen radicals and OH radicals, which were observed by optical spectroscopy, promoted the film growth rate and suppressed the deep level emission in the photoluminescence (PL) spectra. To prove the effect of O and OH radicals, hydrogen plasma and helium plasma techniques were used in film growth.

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Recently, wide-gap semiconductors have attracted much attention for their applications as blue and UV light emitters or UV detectors [1]. Compared with GaN and SiC [2, 3], ZnO has some advantages. It has a large excitonic binding energy (60meV) than GaN (25meV) and an efficient exciton luminescence at room temperature. Moreover, it is abundant in nature and is an environmental friendly material. We report on ZnO film growth by remote plasma metalorganic chemical vapor deposition using oxy-gen as a VI group source and oxygen radicals as a decomposition agent source of DEZn. For the same DEZn flux, growth rate was seven times higher in the case of hydrogen carrier than in the case of nitro-gen carrier. In the case of ZnO film growth by metalorganic molecular beam epitaxy by using DEZn, which used H2O as an oxygen source, there is a report of a remarkably large growth rate [4, 5]. For tita-nium oxide growth by remote plasma chemical vapor deposition, a change in growth rate by changing the plasma of a mixture gas of oxygen and hydrogen has been reported [6]. The effects of oxygen and OH radicals have been also considered in the case of diamond film growth [7]. The effect of OH radicals on ZnO film growth rate was studied. OH radicals were formed by the reac-tion of hydrogen with the O radicals produced in a hollow cathode discharge plasma, and it is thought that they have an important role in the growth of the ZnO film. The role of OH radical in the ZnO growth mechanism is discussed in Ref. [8]. The OH radical effect on film growth is proved by a drastic change in growth rate by changing the carrier gas from H2 to N2. In order to investigate the effect of OH radicals related to O radicals on oxygen remote plasma with hydrogen carrier gas, another two ways of plasma sources were approached. One is hydrogen plasma with hydrogen carrier gas and oxygen gas as a VI group source. Another one is helium plasma with he-lium carrier gas and oxygen gas. Active chemical species were observed in light emission from the reac-tion region by plasma emission spectra.

* Corresponding author: e-mail: atsushi@vc.gsest.shizuoka.ac.jp, Phone: +81 53 478 1321, Fax: +81 53 478 1321

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© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 Experiment Remote plasma MOCVD was used for ZnO film deposition on a-plane sapphire. In this study, we used three kinds of plasma condition as shown in Table 1. We defined the oxygen plasma, hydrogen plasma, and helium plasma from the different plasma sources. A plasma generator was placed 20 cm away from the reaction region and radicals were transported through a Pyrex tube to the substrate. A capacity coupled discharge was applied to the Pyrex tube to produce radicals. The tilted tube was attached to the chamber and extended to 10 cm distance from the top of the substrates. The nozzle tips, which were DEZn with carrier gas line and oxygen line (when hydrogen plasma and helium plasma were used), were positioned downstream of the plasma. The chamber was designed so that a reaction can progress on the substrate. We can observe the emission of the reaction region on the substrate surface through a quartz window from the top of the chamber. An optical spectrometer (C6670 HAMAMATSU) was used to monitor the light emission in a reaction region close to the deposition substrate. The PL measurements were carried out at low and room temperature using He–Cd laser (325 nm).

Table 1 Three kinds of the plasma condition and experiment conditions.

Plasma name Plasma source Carrier gas VI group source

Oxygen plasma O2 H2, N2 O radicals Hydrogen plasma H2 H2 O2 Helium plasma He He O2

3 Result and discussion Figure 1(1) shows the observed growth rate dependence on discharge power and DEZn gas carrier. The films were grown at a total gas pressure of 0.01 Torr, DEZn flow rate of 12 µmol/min, substrate temperature of 400 oC, oxygen flux of 2 sccm, and hydrogen or nitrogen flux of 5 sccm. Growth rate increased markedly by the increase in oxygen related OH radical production in discharge plasmas. This proves that hydrogen promotes growth, when oxygen radicals are transported to the reaction region on the substrate. For the case of the hydrogen carrier, atomic oxygen radicals, atomic hydrogen radicals and also OH radicals coexisted (Fig. 1(2a)). For the case of the nitrogen carrier, the main light emission was found to be N2 (Fig. 1(2b)). Oxygen was observed mainly in molecular state. The nature of the carrier gas determined the occurrence of atomic radicals.

0

0.5

1

1.5

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3

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0 10 20 30 40 50 60 70 80

Hydrogen carrier

Nitrogen carrier

OH radicals

Gro

wth

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in)

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RF power (W)

(1)

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arb. unit

s)

Wavelength (nm)

NO

N2

N2

N2

O2

OH

OH Hα

O

(a)

(b)

(2)

Fig. 1 (1) Growth rate of ZnO films and light emission intensity of OH radicals as a function of RF power. Solid circles show ZnO growth rate with hydrogen carrier gas and solid squares show ZnO growth rate with nitrogen carrier gas. Open circles show light emission intensity of OH radicals. DEZn flow rate is 12 µmol/min with 5 sccm of H2 or N2 carrier gas flow rate. The substrate temperature is 400 oC. (2) Plasma emission spectra of oxygen and hydrogen or nitrogen mixture conditions: (a) in the case of hydrogen carrier gas, and (b) in the case of nitrogen carrier gas.

882 A. Nakamura et al.: ZnO growth using remote plasma metalorganic chemical vapor deposition

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 2 shows the plasma light emission spectra and the PL spectra of ZnO films at room temperature by changing the flux of the plasma gas and oxygen (VI group source). An oxygen line was 10 cm posi-tioned from the substrate and downstream of the plasma. In Fig. 2(1a), the main light emission was found to be Hα from the optical spectroscopy. The deep level emission (around 500 nm) and the weak band edge emission were observed in the PL spectrum. As hydrogen flux (plasma) decreased and oxygen flux (line) increased, atomic O radicals, Hα, and OH radicals were observed by optical spectroscopy in Fig. 2(1b, 1c). Especially, the emission intensity of O radicals and OH radicals were larger than that of Hα in Fig. 2(1c). The structure of spectroscopy in Fig. 2(1c) was close to the oxygen plasma with hydro-gen carrier gas in Fig. 1(2a). When the oxygen plasma was used, large O radicals and OH radicals were efficiently existing on the reaction region even though the plasma oxygen was a low flux. In the PL spec-trum, the band edge emission increased and the deep level emission decreased. This result suggests that the chemical species of O radicals and OH radicals have an effect of suppressing the deep level emission.

200 400 600 800

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Hα

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He-Cd 325 nm

room temperature

(a)

(b)

(c)

(1)

plasma H2 10sccm

line O2 5sccm

plasma H2 5sccm

line O2 10sccm

plasma H2 1sccm

line O2 14sccm

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x10

x5

plasma He 1sccm

line O2 14sccm

plasma He 5sccm

line O2 10sccm

plasma He 10sccmline O

2 5sccm

Fig. 2 Plasma light emission spectra and PL spectra of ZnO films at room temperature by changing the flux of the plasma gas and oxygen (VI group source). Oxygen line was positioned downstream of the plasma. The films were grown at 0.01 Torr, DEZn flow rate 3 µmol/min, DEZn carrier gas flux of 5 sccm, substrate temperature of 400 oC, plasma RF power of 50W. (1) Hydrogen plasma conditions. (2) Helium plasma conditions.

2 2.4 2.8 3.2 3.6

Inte

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ty (

arb.

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s)

Photon energy (eV)

He-Cd 325 nm

room temperature

3.27 eV

3.15 3.2 3.25 3.3 3.35 3.4 3.45

Inte

nsi

ty (

arb.

unit

s)

Photon energy (eV)

D0X 3.361 eV

FX(A) ?

3.371 eV

20 K

3.352 eV

FX(A)-1LO ?

3.299 eV

FX(A)-2LO ?

3.227 eV

72 meV

72 meV

Fig. 3 PL spectra of the ZnO film at room temperature and near-band gap luminescence at 20 K. The ZnO film was prepared by using oxygen plasma.

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© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

On the other hand, the helium plasma was also investigated. When oxygen flux increased and plasma helium flux decreased (Fig. 2(2e, 2f)), large O radicals emission was observed by spectroscopy. OH radicals were not existing there. The deep level emission was not observed in the PL spectrum (Fig. 2(2e)). But, the band edge emission intensity was weak. Depending on the intensity of the O radicals emission in spectroscopy, the intensity of the band edge emission increased in the PL spectrum (Fig. 2(2f)). The deep level emission was not enough suppressed by only the existence of O radicals. OH radi-cals and O radicals are needed to coexist in the film growth of ZnO for suppressing the deep level emis-sion. In order to assess the optical quality of the ZnO layer, which had been prepared by using oxygen plasma, the PL measurements were carried out at room and low temperature (20 K). The room tempera-ture PL spectrum has a strong peak of band edge emission at 3.27 eV as shown in Fig. 3. It should be noted that the deep level emission was not observed. This means that using oxygen plasma system is suitable for ZnO growth, because O and OH radicals are effectively produced by oxygen plasma with hydrogen carrier. At the low temperature PL spectrum, a neutral donor-bound exciton appears at 3.361 eV [9,10]. The origin of the emission observed at 3.352 eV is not clear. The peak interval between the peak around 3.227 eV and 3.299 eV was 72 meV, which is LO-phonon energy. But free exciton (3.371 eV) peak was not observed clearly. 4 Conclusion ZnO layers were grown on a-plane sapphire by remote plasma MOCVD technique using DEZn as a source material and an oxygen plasma. By using hydrogen as a DEZn carrier, we demon-strated an activation effect of the OH radicals in film growth of ZnO. To prove the effect, we changed the carrier gas to nitrogen, and plasma gas to hydrogen or helium. OH radicals and O radicals not only promoted growth rate but also suppressed the deep level emission in the PL properties. Using oxygen plasma system is suitable for ZnO growth, because O and OH radicals are effectively produced by oxy-gen plasma with hydrogen carrier.

Acknowledgements This research project was supported in part by “ The Murata Science Foundation”.

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