ZnO and ZnMgO Growth by Molecular Beam Epitaxy
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ZnO and ZnMgO Growth by Molecular Beam Epitaxy Mitsuaki Yano,1,2 Ken-ichi Ogata,2 FengPing Yan,1 Kazuto Koike,1 Shigehiko Sasa,1,2 and Masataka Inoue1,2 1) New Materials Research Center, Osaka Institute of Technology, Asahi-ku Ohmiya, Osaka 535-8585, Japan 2) Bio Venture Center, Osaka Institute of Technology, Asahi-ku Ohmiya, Osaka 535-8585, Japan
ABSTRACT Characteristics of the ZnO and ZnMgO films grown by radical-source molecular beam epitaxy are reported. The ZnO films on a-plane sapphire substrates had a superior quality in crystallographic, optical and electrical properties, and n-type doping with Al was successfully performed up to 1020 cm–3. The Mg-content of ZnMgO alloys was found to be controlled by a simple growth mechanism as a function of Mg-cell temperature. The alloying in the ZnO-rich region resulted in single-crystalline growth although the photoluminescence characteristics at x = 0.22 suggested the presence of microscopic phase separation. Single-crystalline growth was also achieved on Si (111) substrates by using a CaF2 buffer layer to protect the Si-surface from oxidation. INTRODUCTION Recent progress on the growth techniques of single-crystalline ZnO film promotes much attention to ZnO-related materials for electronic and optoelectronic applications. ZnO has many fascinating properties such as a large bandgap energy corresponding to the UV region, a large exciton binding energy treble of GaN, and a possibility to magnetic semiconductors. Low toxicity to human body is another importance of this material since ZnO has a long history in the use of cosmetics and skin-cares such as “baby-powder” and both Zn and O are ubiquitous in environment. We notice this material to the application for bioelectronics. One target is ZnMgO/ZnO FET type sensors to detect chemical species such as glucose, oxygen and urea in blood [1]; a specific enzyme on the gate electrodes is bonded to the S or Se atoms replaced to the uppermost O atoms of ZnMgO. The other is wearable-size photosensors in the UV region to detect selectively the specific wavelength harmful to the skin. Both applications require ZnMgO/ZnO heterosystem with the doping control, at least, in n-type region. Devices on Si substrates are more favorable to system integration. From this point of view, we have studied the n-type doping and alloying with MgO for ZnO films on sapphire substrates, as
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well as the growth on Si substrates, using molecular beam epitaxy (MBE). The state of the art characteristics are reported in this paper. EXPERIMENTAL We used an EpiQuest-made MBE apparatus equipped with Knudsen cells (K-cells) for Zn and Mg and a plasma cell for oxygen radicals operated at 350 W. The purities of these sources were 99.99999% (7N) for Zn, 4N for Mg, and 6N for oxygen. The base-pressure of the MBE growth chamber was less than 5×10–8 Pa, and a typical working-pressure was 1×10–4 Pa using beam-equivalent pressures (BEPs) of PZn = 3.1×10–4 Pa and PMg = 10–6-10–5 Pa and an oxygen flow rate of FO2 = 0.4 ccm. We also used 6N Al in K-cell f
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