Properties Of ZnMgO Films And ZnMgO/ZnO Hetero-structures Grown By P-MBE

ZnO has been extensively studied for potential applications in optoelectronic devices owing to its unique properties, including high electron mobility, wide direct band gap (3.37eV at room temperature), and large exciton binding energy (60meV). One of the important capabilities involved in constructing optical and electrical confinement structures is band gap engineering. The ternary alloy semiconductor, Zn1-xMgxO, is considered as one of the best candidates for increasing the band gap energy of ZnO because of its large band gap and small lattice-mismatch with ZnO. In order to develop ZnO-based heterostructures and deep ultraviolet devices, the most important issues are to fabricate high quality Zn1-xMgxO films and ZnO/Zn1-xMgxO multiple quantum wells (MQWs).It is well known that heterojunctions consisting of the fundamental quantum wells and superlattice structures have been playing an important role in fabricating optoelectronic devices because of the carrier confinement effect and high carrier injection ratio properties. Furthermore, the valence band maximum (VBM) strongly influences the capability of p-type doping in Zn1_-MgxO films. Therefore, it is necessary to design the ideal valence and conduction band offsets of Zn1-xMgxO/ZnO heterojunctions.In this work, we focus on the growth and characterization of high quality ZnMgO thin films using intervening epitaxial MgO buffer layers; on the basis of this, we investigate the Mg composition and polarity orientation dependent band offsets of Zn1-xMgxO/ZnO heterojunctions, in an attempt to reveal the p-type doing mechanism. After that, ZnO/Zn1-xMgxO MQWs were fabricated and the relative optical properties were analyzed by photoluminesence (PL) spectra. The work includes:1. High quality epitaxial Zn1-xMgxO films were fabricated on c-plane sapphire substrates using MgO buffer layer by plasma-assisted molecular beam epitaxy. The quality of the Zn1-xMgxO epilayers is manifested by a Hall mobility of more than60cmV-1S-1at room temperature, and x-ray diffraction rocking curve full-width at half-maximum of47arcsec. A screw dislocation density of4x10cm-2is estimated by x-ray diffraction.2. The valence band offsets (AEv) of Zn1-xMgxO/ZnO heterojunctions grown by plasma-assisted molecular beam epitaxy were measured by photoelectron spectroscopy. From the directly obtained AEv values, the related conduction band offsets (ΔEC) were deduced. All the Zn1-xMgxO/ZnO heterojunctions exhibit a type-Ⅰ band alignment with the ΔEC/ΔEV estimated to be1.5,1.8,2.0for x=0.10,0.15and0.20, respectively. The band offsets of Zn1-xMgxO/ZnO heterojunctions depend on Mg composition.3. Polar and non-polar ZnMgO:Na films that were fabricated on c-plane and r-plane sapphire substrates using intervened ZnO layers by pulsed laser deposition. Hall-effect measurements indicate that the a-plane ZnMgO:Na film exhibits p-type conductivity with a carrier concentration of about3.5×1016cm-3, while the polar film shows a compensatory conductivity. Meanwhile, the dependence of the band alignment on the orientation of the ZnMgO/ZnO heterojunctions has been investigated using photoelectron spectroscopy. The heterojunctions form in the type-Ⅰ straddling alignment with valence band offsets of0.07(0.02) eV for the (non-)polar heterojunction. The difference in valence band offsets is primarily attributed to the spontaneous polarization effect. We propose that the smaller valence band offsets and larger conduction band offsets would reduce the NaZn acceptor level and enhance the relative intrinsic donor levels. Such effects consequently lead to p-type conductivity in non-polar ZnMgO:Na films.4. A series of5-period ZnO/Zn0.9Mg0.1O multiple quantum wells (MQWs) with different well layer thicknesses in the range of3-10nm have been fabricated on (0001) sapphire substrates by plasma-assisted molecular beam epitaxy (MBE) with combined MgO and low-temperature ZnO thin film as buffer layers. The good quality of ZnO/Zn0.9Mg0.1O MQWs is evidenced by the observation of readily resolved Pendellosung fringes and the small full-width at half-maximum (FWHM) value of exciton emission as low as8.3meV, as well as the observation of high order phonon replicas. The dominated photoluminescence (PL) peak in the MQWs shows a systematic blueshift with decreasing well width, which is consistent with a quantum confinement effect.5. In addition, we investigaed the temperature-dependent photoluminescence (PL) measurements of ZnO/Zn1-xMgxO MQWs with different barrier compositions. The PL band in the well layers is dominated by localized excitons (LEs), free excitons (FEs), and two longitudinal optical (LO) phonon replicas of the LE emission. The LE emission of higher Mg compositional barrier exhibits a significant blue shift of about70meV with respect to the lower one. The mechanisms of carrier dynamics and localization are investigated within the temperature range14-300K. As the temperature increases, luminescence from the excitons localized in the well layers shows an’S-shaped’shift in the high-barrier MQWs, whereas a monotonic red shift is observed in the low-barrier MQWs. The’S-shaped’shift behavior is associated with delocalization of the excitons in the potential minima induced by interface fluctuations or alloy disorder. Large exciton binding energies of64meV and76meV were deduced, demonstrating efficient quantum confinement in the ZnO/ZnMgO MQWs.