Temperature dependencies of optical reflectance and cathodoluminescence (CL) spectra were measured for the MgO single crystal using a custom-built vacuum ultraviolet (VUV) spectroscopic system. Simultaneous observation enabled us to identify free exciton (FE) and bound exciton (BE) emissions by comparing the CL emission with the exciton resonance structures. The results indicated that the BE emission dominates the near-band edge emission, and the FE emission was observed as a shoulder at 300 K. The results ensure strong excitonic nature and potential of a rock salt-structured MgO-based material system for an active element in the VUV light emitter.

Temperature-dependent cathodoluminescence spectra were measured for rock salt-structured MgxZn1−xO films with x = 0.95–0.61. The Mg0.95Zn0.05O film exhibited the shortest deep UV peak wavelength of 199 nm (6.24 eV) at 6 K. Relatively high equivalent internal quantum efficiencies of 0.9%–11% were obtained. The Tauc plots, which were obtained from temperature-dependent optical transmittance measurements, exhibited large Stokes-like shifts of 0.7–0.9 eV at 6–300 K. Time-resolved photoluminescence (PL) signals at 7 K exhibited fast and slow decay components. The fast decay component had PL lifetimes of 2.59–3.08 ns, and the slow decay component far exceeded the measurement time range of 12.5 ns. The fast decay constant reflected the transfer lifetime of the photoexcited carriers to certain trapping centers. These centers were tentatively ascribed to Zn-related isoelectronic trapped-hole centers and may be a cause of the large Stokes-like shifts. The signals at 300 K exhibited very short PL lifetimes of 120–180 ps. The PL lifetimes were mainly attributed to the nonradiative recombination lifetime. Simultaneous decreases in the Zn-related isoelectronic trapped-hole centers and the nonradiative recombination centers were found to be necessary to improve the DUV emission properties of RS-MgxZn1−xO films.

Deep ultraviolet (DUV) and vacuum ultraviolet (VUV) lights are expected to be utilized in various fields; such as virus inactivation, [1] ozone generator, [2] and nanofabrication. [3] Especially, DUV lights in an energy range from 5.6 to 6.0 eV (207 to 222 nm) are attracting special attention because of their capability for sterilization without harm to human tissues. [4,5] Furthermore, VUV lights in an energy range above 6.2 eV (below 200 nm) can effectively produce photochemical reactions. [6,7] So far, most of commercialized DUV and VUV light sources consist of either by discharge type lamps or AlGaNbased light emitting diodes (LEDs). [8,9] However, they still have disadvantages, that is, the discharge type lamps exhibit short lifetime, high energy consumption, and large size. For AlGaN-based LEDs, their bandgap energies E g are in a range 3.4 to 6.0 eV, and they cannot produce VUV lights. [10,11] Demands are thus increasing for developing semiconductor-based light emitters in a sub-200 nm range, because of their advantages such as miniaturization, long lifetime, and low energy consumption.To develop the light emitters covering widely in DUV and VUV spectral regions, we are focusing on rocksalt-structured (RS) MgZnO alloys. RS-Mg x Zn 1Àx O alloys can vary their E g by changing x in a range from 2.45 eV of RS-ZnO to 7.8 eV of RS-MgO, [12][13][14] though the rocksalt-structure is thermally stable for x > 0.6. The E g values for the RS-Mg x Zn 1Àx O have been determined by optical transmittance measurements [14,15] to be in a range from 5.73 eV for x ¼ 0.61 to 6.53 eV for x ¼ 0.92 at 300 K. According to the first-principles calculations, [15,16] the E g values for x > 0.6 correspond to the direct Γ-Γ transition. Recently, we have succeeded in growth of atomically flat RS-MgZnO thin films on MgO substrates by the mist chemical vapor deposition (mist CVD) method. [17][18][19] The epitaxial films exhibited cathodoluminescence (CL) peaks located in a range from 5.09 eV (4.91 eV) at 6 K (300 K) for x ¼ 0.61 to 6.24 eV (6.05 eV) at 6 K (300 K) for x ¼ 0.95. [15,[17][18][19][20] The energy difference between the E g and CL peak positions is defined as the Stokes-like shift. The origin of the relatively large Stokes-like shift of 0.8-0.9 eV is a debatable issue. Possible origin was attributed to the differences in the local arrangement of Mg and Zn atoms in the Mg x Zn 1Àx O alloy by virtue of the electronic structure calculation. [15] However, Gorczyca and coworkers have recently pointed out that the clustered Zn-O-Zn configurations result in an indirect bandgap for x > 0.5 by evaluating the unfolded electronic structures for different-sized supercells. [16] Though

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