Two-step growth regimes were applied to realize a homoepitaxial growth of ZnO films on freestanding diamond substrates by radio-frequency(RF)reactive magnetron sputtering method.ZnO buer layers were deposited on freestanding diamond substrates at a low sputtering power of 50 W,and then ZnO main layers were prepared on this buffer layer at a high sputtering power of 150 W.For comparison,a sample was also deposited directly on freestanding diamond substrate at a power of 150 W.The effects of ZnO buffer layers on the structural, optical,electrical and morphological properties of the ZnO main layer were studied by X-ray diffraction(XRD), scanning electron microscopy(SEM),Raman spectroscopy,semiconductor characterization system and atomic force microscopy(AFM)respectively.The experimental results suggested that homo-buffer layer was helpful to improve the crystalline quality of ZnO/diamond heteroepitaxial films.
Jian Huang Linjun Wang Run Xu Weimin Shi Yiben Xia
HfO2 films on silicon substrates have been prepared by electron beam evaporation in ultrahigh vacuum using atomic oxygen.Synchrotron radiation photon-electron spectroscopy was used to investigate the thermal stability of HfO2 films under an ultrahigh vacuum environment.At the temperature of 750℃,HfO2 films begin to decompose.After being further annealed at 850℃ for 3 min,HfO2 films decomposes completely,partially to form Hf-silicide and partially to form gaseous HfO.Two chemical reactions are responsible for this decomposition process.A small amount of Hf-silicide,which is formed at the very beginning of growth,may result in the films grown subsequently to be loosened,and thereby leads to a relatively low decomposition temperature.
The valence band offsets of the strained and longitudinally relaxed diamond/cubic boron-nitride (c-BN) (110) superlattice are investigated by the plane wave density functional theory approach and using the on-site core electron as a reference energy level. For the strained diamond/c-BN superlattice, the valence band offset of around 1.50 eV is in good agreement with those using all the electrons methods. As for the longitudinally relaxed superlattice, the valence band offset of around 1.28 eV is smaller than that of the strained superlattice. The reason for this is mainly due to the split of the valence band maximum caused by the anisotropic strain.
A plane wave density functional theory method was used to investigate the adsorption properties of isolated alkali metal atoms, including Li, Na, K, Rb and Cs on-top of the F 0 s defective center of MgO(001) surface. Among all the alkali metals, the lithium atom binds most strongly with the highest adsorption energy of 0.67 eV and the shortest distance of about 0.257 nm between metal and the surface, the binding energy for the sodium atom comes second, and just half of this value for the other alkali metal atoms. The relatively strong interaction of Li with the F 0 s center can be explained by a more covalent bonding involved, evidenced by results of both the projected density of states and the projected charge density. The bonding mechanism is discussed in detail.
Highly c-axis-oriented ZnO films were deposited successfully on the nucleation sidesof free-standing diamond (FD) films by the direct current (DC) magnetron sputtering method. Theeffect of the sputtering parameters, such as power, gas pressure and sputtering plasma compositionof Ar-to-O_2, on the properties of ZnO thin films was investigated in detail. X-ray diffraction (XRD)measurements showed that, at a sputtering power of 200 W, gas pressure of 0.5 Pa and an Ar-to-O_2 composition of 1:1, a higher intensity of the (002) diffraction peak and a narrower full widthat half maximum (FWHM) were detected which meant high c-axis orientation and high qualityof the ZnO films. To improve the quality of the ZnO film, a thin ZnO layer was pre-grown as ahomo-buffer layer. XRD measurements showed that this buffer layer had a beneficial effect on thestructural and morphological properties of the post-grown ZnO film.
P-type undoped freestanding diamond(FSD)films were grown by the microwaveplasma chemical vapor deposition(MPCVD)method.The effects of the hydrogen plasma treatmentand annealing process on the p-type behavior of FSD films were investigated by the Halleffect method.The results revealed that the sheet carrier concentration increased and the sheetresistivity decreased with the treating time and a stable value was achieved after a period of time.Up to an annealing temperature of 250℃,the sheet resistivity and sheet carrier concentrationremained in a relatively stable range but changed dramatically after annealing at 300℃.A heterojunctionwas also fabricated by the growth of an n-type ZnO film on the p-type FSD film.Current-voltage(I-V)characterization of the heterojunction at room temperature indicated thatthis structure was rectifying in nature with a turn-on voltage of about 0.6 V.
