Cubic Zn x Mg 1-x O thin films grown by plasma-assisted molecular-beam epitaxy for optoelectronic applications
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Heteroepitaxial ZnxMg1xO thin films were grown on lattice-matched MgO (100) substrates using radiofrequency plasma-assisted molecular-beam epitaxy. High-quality epilayers with zinc concentrations ranging from x ¼ 0 (MgO) to x ¼ 0.65 were grown and characterized optically, structurally, and electrically. The ZnxMg1xO films were found to maintain the rocksalt cubic (B1) crystal structure for concentrations z < 0.65, with a linear dependence of lattice constant on Zn concentration. X-ray diffraction (XRD) also revealed the emergence of phase segregation into wurtzite (B4) phase for the highest concentration film. The band gap energy of the films was successfully varied from 4.9 to 6.2 eV (253– 200 nm), showing a linear relationship with Zn concentration. The strictly cubic films ˚ and resistivities of approximately 106 Ocm. exhibit roughness on the order of 10 A
I. INTRODUCTION
Binary and ternary wide band gap oxide compounds have gained increased attention recently for optoelectronic device engineering in the ultraviolet (UV) and deep ultraviolet (DUV) spectral regions. There are a great number of applications in the DUV (l < 300 nm) that are motivating extensive investigations into a variety of wide band gap materials. Semiconductors with a band gap greater than approximately 4.35 eV, for example, find application in important solar blind detection technologies in which the device is sensitive only to those UV wavelengths strongly attenuated by the Earth’s atmosphere. In medicine, the germicidal/viricidal application of UVC light is well known and exploited as is its use in the treatment of skin conditions including psoriasis, vitiligo, and eczema. Likewise, in manufacturing and industry DUV light is exploited in numerous varied ways such as photochemistry processing, UV curing, and transistor erasure in erasable programmable read-only memory (EPROM) chips. Of the wide band gap oxides, certainly some of the most significant successes have been demonstrated in the growth and application of ZnO (Eg ¼ 3.37 eV) to near ultraviolet photonics.1–8 The success with ZnO has naturally motivated investigations into alloying this binary with alternative cations such as magnesium and beryla)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr_policy DOI: 10.1557/JMR.2010.0150 1072
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J. Mater. Res., Vol. 25, No. 6, Jun 2010 Downloaded: 13 Mar 2015
lium to increase the band gap of the resultant ternary,9–15 or cadmium to reduce the band gap.11,16–19 Magnesium incorporation into ZnO (ZnxMg1xO) by various growth techniques and in recent years has been intently investigated for DUV optoelectronics.12,13,20–30 ZnxMg1xO is a particularly promising candidate for DUV optoelectronics for a variety of reasons including its band gap tunability, high visible transparency, and a propen
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