High quality w-MgxZn1-xO thin films were grown epitaxially on c-plane sapphire substrates by plasma-assisted Molecular Beam Epitaxy. ZnO thin films with high crystalline quality, low defect and dislocation densities, and subnanometer surface roughness were achieved by applying a low temperature nucleation layer. By tuning Mg/Zn flux ratio, wurtzite MgxZn1-xO thin films with Mg composition as high as x=0.46 were obtained without phase segregation. Metal- Semiconductor-Metal (MSM) photoconductive and Schottky barrier devices with interdigitated electrode geometry and active surface area of 1 mm2 were fabricated and characterized. Resultant devices showed ~100 A/W peak responsivity at wavelength of ~260nm. We also report on cubic rock salt c-MgxZn1-xO thin films, following a non-traditional approach on MgO substrates, to demonstrate solar-blind photoresponse in MSM photodetectors, realizing a peak responsivity of 460 A/W (@ 250 nm) and 12.6 mA/W (@ 240nm) for mixed phase and single crystal films, respectively. A specific focus of the work is on identifying the impact of various growth parameters on the performance of the c- MgZnO detectors.
ZnO thin films were epitaxially grown on Zn-polar (0001) ZnO substrates by plasma-assisted molecular beam epitaxy. Surface root mean square (rms) roughness below 0.3 nm was achieved on a large range of growth temperatures by growing on ZnO substrates with 0.5 degree miscut angle toward [11¯00] axis. Surface treatment with acid etching and ozone exposure was required to remove contamination such as silica residual and carboxyl and carbonate groups on the surface. Removal of these surface impurities reduces the likelihood of extrinsic defect migration into the epitaxial films. High growth temperature (> 640°C) and oxygen rich conditions were required for films with terrace steps, but resulted in a very low growth rate (~30nm/h) and low photoluminescence (PL) lifetimes of lower than 50 ps. With moderate growth temperature (~610°C), higher growth rate and higher PL lifetime with up to 380 ps were achieved. EIT was used for the oxygen plasma to reduce reactive oxygen species etching of the surface, resulting in a higher growth rate and fewer defects in the films. Good crystalline quality was evident in Xray rocking curves with consistent narrow full width at half maximum (FWHM) of (0002), (101¯2) and (202¯1) peaks, indicating low threading dislocations. Both room-temperature and low-temperature photoluminescence indicated high optical quality of the resultant films with few non-radiative recombination centers.
We will discuss the epitaxial growth, characterization, and application of a new set of ternary cubic oxide
semiconductor compounds, ZnxMg1-xO and NiyMg1-yO, offering a new route towards deep-UV optical devices.
Results demonstrating bandgap tunability and excellent thin film quality will be presented validating the potential of
these alloys in the 200 - 350 nm region. Significantly, we have successfully fabricated MSM solar blind detectors
using both ternary alloys, demonstrating operation in the solar blind region without external opal filters.
Oxide based compounds have been of increasing interest for wide bandgap, deep ultraviolet optoelectronics. While high
Al content AlGaN has enabled many UV-DUV technologies, it suffers inherent drawbacks including difficulty achieving
increasing Al incorporation, high threading dislocation densities and challenges in bandgap engineering due to
polarization and piezoelectric effects. Here we present two wide bandgap cubic oxide compounds, ZnMgO and NiMgO,
that offer advantages over AlGaN for deep ultraviolet (DUV) applications. NixMg1-xO and ZnxMg1-xO are both direct
band gap, cubic rocksalt (B1) semiconductors with bandgaps in the UV-DUV spectral regions, offering alternatives
without the aforementioned drawbacks associated with AlGaN. Here we present NixMg1-xO and ZnxMg1-xO thin films
grown by plasma-assisted MBE on lattice matched MgO substrates as a novel means by which to realize DUV detection
devices. In both systems we have shown the films to exhibit abrupt, continuously tunable absorptions edges over their
respective bandgap ranges. NixMg1-xO films were varied compositionally from x=0 to 1, realizing bandgaps from 3.5 to
7.8 eV. ZnxMg1-xO films were similarly varied over the entire B1 range of the ternary (0<x<0.42) and show bandgap
tunability from ~5 to 7.8 eV. All films are characterized through Rutherford backscattering (RBS), x-ray diffraction
(XRD), atomic force microscopy (AFM) measurements and optical transmission. Significantly, we have successfully
fabricated solar blind detectors in both categories and highlight the results from NixMg1-xO here.
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