Amorphous Al2O3 thin films were deposited on a Si (1 1 1) substrate at 150 °C in oxygen-rich conditions by atomic layer deposition. Rapid thermal annealing was performed at high temperatures, ranging from 1000 to 1200 °C, to study the crystallization characteristics of the Al2O3 films. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy, atomic force microscopy and frequency-dependent capacitance measurements were used to characterize the structural and electrical properties before and after the annealing. It was found that the best crystallization of the Al2O3 film was achieved after annealing at 1150 °C, corresponding to a minimum of full width at half maximum (FWHM) of the Al2O3 (012) XRD peak and the maximum of surface roughness and the capacitances. This suggests that 1150 °C is the optimal transition temperature from amorphous to crystalline and to get the best insulating property for Al2O3 thin film. The formation of the SiO2 interfacial layer after annealing has been observed by HRTEM for the annealed sample, accompanied by a decrease in the thickness of Al2O3 films.

We report on p-n junction light-emitting diodes fabricated from MgZnO / ZnO / AlGaN / GaN triple heterostructures. Energy band diagrams of the light-emitting diode structure incorporating piezoelectric and spontaneous polarization fields were simulated, revealing a strong hole confinement near the n-ZnO / p-AlGaN interface with a hole sheet density as large as 1.82 ϫ 10 13 cm −2 for strained structures. The measured current-voltage ͑IV͒ characteristics of the triple heterostructure p-n junctions have rectifying characteristics with a turn-on voltage of ϳ3.2 V. Electron-beam-induced current measurements confirmed the presence of a p-n junction located at the n-ZnO / p-AlGaN interface. Strong optical emission was observed at ϳ390 nm as expected for excitonic optical transitions in these structures. Experimental spectral dependence of the photocurrent confirmed the excitonic origin of the optical transition at 390 nm. Light emission was measured up to 650 K, providing additional confirmation of the excitonic nature of the optical transitions in the devices.

Optical absorption and reflectance measurements are performed to evaluate compositional and temperature dependences of band gap energies of Zn1−xCdxO alloys grown by molecular-beam epitaxy. The compositional dependence of the band gap energy, determined by taking into account excitonic contributions, is shown to follow the trend Eg(x)=3.37−2.82x+0.95x2. Incorporation of Cd was also shown to somewhat slow down thermal variation of the band gap energies, beneficial for future device applications.

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We report on the fabrication and characterization of an ultraviolet (UV) lightemitting diode (LED) based on a p-n junction MgZnO/ZnO/AlGaN/GaN semiconductor triple-heterostructure (THS). Radio-frequency (RF) plasma-assisted molecular-beam epitaxy (MBE) has been employed to grow individual epitaxial layers of ZnO, Mg x Zn 1-x O, and the complete heterostructure on c-plane GaN/sapphire templates. Various growth strategies have been used to optimize the quality of the ZnO layers as well as to precisely control the composition of the Mg x Zn 1-x O compound. Cross-sectional transmission electron microscopy (TEM) study shows the excellent crystalline quality of the pseudomorphically grown ZnO active region of the device. A strong electroluminescence (EL) emission associated with ZnO excitonic transition was observed up to 650 K. The results shown in this paper strongly suggest the viability of RF plasma-assisted MBE in the development of next-generation UV emitters using ZnO-based materials.