Growth of the Single-Crystalline ZnO Films on Si (111) Substrates by Plasma-Assisted Molecular-Beam Epitaxy
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Growth of the Single-Crystalline ZnO Films on Si (111) Substrates by Plasma-Assisted Molecular-Beam Epitaxy Kazuto Koike1, Takanori Tanite2, Shigehiko Sasa1,2, Masataka Inoue1,2, and Mitsuaki Yano1,2 Bio Venture Center1, New Materials Research Center2, Osaka Institute of Technology, Asahi-ku Ohmiya, Osaka 535-8585, Japan ABSTRACT This report describes the growth of single-crystalline ZnO films on Si (111) substrates by plasma-assisted molecular-beam epitaxy. X-ray diffraction measurement shows that c-axis oriented ZnO films are easily grown on Si (111) substrates. However, in-plane random rotational domains are included in the ZnO films due to the inevitable oxidation of substrate surface at the initial stage of ZnO growth. By employing a thin CaF2 buffer layer between the ZnO films and Si substrates, we have succeeded in suppressing the generation of rotational domains and in obtaining an intense ultraviolet photoluminescence even at room temperature. These results indicate that the use of CaF2 buffer layer is promising for the growth of device-quality ZnO films on Si (111) substrates.
INTRODUCTION Wide and direct band-gap semiconductor ZnO has attracted much attention to the room-temperature operating ultraviolet lasers and sensors because of its large exciton binding energy of about 60 meV [1]. To integrate these optoelectronic devices with Si-base electrical circuitry, however, high-quality ZnO growth on Si substrates is a key issue although the direct growth is apt to yield in-plane rotational domains by the oxidation of substrate surface at the initial stage of ZnO growth [2]. Recently, Iwata et al. demonstrated that the plasma-assisted nitridation of Si substrate prior to ZnO growth was effective to suppress the oxidation of Si surface although in-plane 30° rotational domains still remained in the ZnO films [2]. We notice the use of CaF2 buffer layer as an alternative way to suppress the oxidation of Si surface. This idea is promoted by the recent work of Ko et al. that reports the growth of single-crystalline ZnO (0001) films on bulk CaF2 (111) substrates using plasma-assisted molecular-beam epitaxy (MBE) _ _ _ [3]. In this heterosystem, the epitaxial relationship is reported to be [ 2 11 0 ] ZnO (0001) // [ 110 ] CaF2 (111) [3]. Due to the close lattice matching of about 0.6 % between CaF2 (111) and Si (111), we can expect single crystalline MBE growth for the CaF2 buffer layer [4,5]. In this paper, we report the effect of CaF2 buffer layer on the ZnO films grown on Si (111) substrates.
EXPERIMENTAL PROCEDURE The growth in this experiment was carried out using an Epiquest-MBE system equipped with effusion K-cells for Zn and CaF2 sources and an rf-plasma cell for oxygen radical source. H11.9.1
The growth process was in-situ monitored by a reflection high-energy electron diffraction (RHEED) system operated at 15 KV. Compound CaF2 with a purity of 4 N was used for the CaF2 growth, and elemental Zn with a purity of 7 N and oxygen gas with a purity of 6 N were used for the ZnO growth. (111)-oriented p-type (
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