Huahan Zhan

Tailoring of polar and nonpolar ZnO planes on MgO (001) substrates through molecular beam epitaxy

Polar and nonpolar ZnO thin films were deposited on MgO (001) substrates under different deposition parameters using oxygen plasma-assisted molecular beam epitaxy (MBE). The orientations of ZnO thin films were investigated by in situ reflection high-energy electron diffraction and ex situ X-ray diffraction (XRD). The film roughness measured by atomic force microscopy evolved as a function of substrate temperature and was correlated with the grain sizes determined by XRD. Synchrotron-based X-ray absorption spectroscopy (XAS) was performed to study the conduction band structures of the ZnO films. The fine structures of the XAS spectra, which were consistent with the results of density functional theory calculation, indicated that the polar and nonpolar ZnO films had different electronic structures. Our work suggests that it is possible to vary ZnO film structures from polar to nonpolar using the MBE growth technique and hence tailoring the electronic structures of the ZnO films.PACS: 81; 81.05.Dz; 81.15.Hi.

Atomic structure and formation mechanism of identically sized Au clusters grown on Si(111)-(7×7) surface

Identically sized Au clusters are grown on the Si(111)-(7×7) surface by room temperature deposition of Au atoms and subsequent annealing at low-temperature. The topographical images investigated by in situ scanning tunneling microscopy show a bias-dependent feature. The current-voltage properties measured by scanning tunneling spectroscopy indicate some semiconducting characteristics of the Au adsorbed surface, which is attributable to the saturation of Si dangling bonds. These experimental results, combined with the simulated scanning tunneling microscopy images and the first-principles adsorption energy calculations, show that the Au cluster is most likely to have a Au(6)Si(3) structure. In the Au(6)Si(3) cluster, three adsorbed Au atoms replace the three Si center adatoms, forming a hollow triangle, while the replaced Si atoms and other three Au atoms connect into a hexagon locating within the triangle. The formation mechanism of this atomic configuration is intimately associated with the complicated chemical valences of Au and the specific annealing conditions.

Wurtzite ZnO (001) films grown on cubic MgO (001) with bulk-like opto-electronic properties

We report the growth of ZnO (001) wurtzite thin films with bulk-like opto-electronic properties on MgO (001) cubic substrates using plasma-assisted molecular beam epitaxy. In situ reflection high-energy electron diffraction patterns and ex situ high resolution transmission electron microscopy images indicate that the structure transition from the cubic MgO substrates to the hexagonal films involves 6 ZnO variants that have the same structure but different orientations. This work demonstrates the possibility of integrating wurtzite ZnO films and functional cubic substrates while maintaining their bulk-like properties.

Synthesis of ZnO/Si Hierarchical Nanowire Arrays for Photocatalyst Application

ZnO/Si nanowire arrays with hierarchical architecture were synthesized by solution method with ZnO seed layer grown by atomic layer deposition and magnetron sputtering, respectively. The photocatalytic activity of the as-grown tree-like arrays was evaluated by the degradation of methylene blue under ultraviolet light at ambient temperature. The comparison of morphology, crystal structure, optical properties, and photocatalysis efficiency of the two samples in different seeding processes was conducted. It was found that the ZnO/Si nanowire arrays presented a larger surface area with better crystalline and more uniform ZnO branches on the whole sidewall of Si backbones for the seed layer by atomic layer deposition, which gained a strong light absorption as high as 98% in the ultraviolet and visible range. The samples were proven to have a potential use in photocatalyst, but suffered from photodissolution and memory effect. The mechanism of the photocatalysis was analyzed, and the stability and recycling ability were also evaluated and enhanced.

Structural and electronic properties of identical-sized Zn nanoclusters grown on Si(111)-(7x7) surfaces

Identical-sized Zn nanoclusters have been grown on Si(111)-(7x7) surfaces at room temperature. In situ scanning tunneling microscopy (STM) studies and first-principles total energy calculations show that room-temperature grown Zn nanoclusters tend to form the seven-Zn-atom structure with one excess Zn atom occupying characteristically the center of the cluster. The evolution of the surface electronic structures measured by scanning tunneling spectroscopy reveals that the formation of Zn nanoclusters is responsible for the saturation of the metallic Si adatom dangling bond states at about -0.3 and +0.5 V and causes the semiconducting characteristics of the nanoclusters. Furthermore, the Zn nanocluster in a faulted half unit cell empties the filled surface dangling bond state of the closest edge Si adatoms in the nearest neighboring uncovered unfaulted half unit cells at about -0.3 V, leading to the suppressed height of the closest edge Si adatoms in the filled-state STM images.

Quality improvement of ZnO thin layers overgrown on Si(100) substrates at room temperature by nitridation pretreatment

Chinese National Foundation of Natural Science [61076084]; Foundation of Natural Science, Fujian Province, China [2009J01267]

Laplace defect spectroscopy for recognition of deep-level fine structures

A Laplace defect spectrometer (LDS) was investigated for use in decomposition of non-exponential transients. The system was tested by measuring known multi-exponential transients generated by RC circuits and applied to the study of non-exponential transients resulting from electron emission from Sn-related DX centers and hole emission from Fe-related deep acceptors. The non-exponential transients were investigated under different conditions and related to the alloy random effect. Their LDS spectra exhibited several well-resolved sharp peaks that were assigned to the fine structures of the two DX centers and the Fe-related deep acceptors, respectively, after comparison with DLTS observations. The activation energies of the fine structures were determined by linear fitting of the slopes of temperature dependences of electron and hole emission rates. The results show that the LDS is useful for investigation of deep-level fine structures.

Tuning the Surface Morphologies and Properties of ZnO Films by the Design of Interfacial Layer

Wurtzite ZnO films were grown on MgO(111) substrates by plasma-assisted molecular beam epitaxy (MBE). Different initial growth conditions were designed to monitor the film quality. All the grown ZnO films show highly (0001)-oriented textures without in-plane rotation, as illustrated by in situ reflection high-energy electron diffraction (RHEED) and ex situ X-ray diffraction (XRD). As demonstrated by atomic force microscopy (AFM) images, “ridge-like” and “particle-like” surface morphologies are observed for the ZnO films grown in a molecular O2 atmosphere with and without an initial deposition of Zn adatoms, respectively, before ZnO growth with oxygen plasma. This artificially designed interfacial layer deeply influences the final surface morphology and optical properties of the ZnO film. From room-temperature photoluminescence (PL) measurements, a strong defect-related green luminescence band appears for the ZnO film with a “particle-like” morphology but was hardly observed in the films with flat “ridge-like” surface morphologies. Our work suggests that the ZnO crystallinity can be improved and defect luminescence can be reduced by designing interfacial layers between substrates and epilayers.

Influence of nitrogen and magnesium doping on the properties of ZnO films

Undoped ZnO and doped ZnO films were deposited on the MgO(111) substrates using oxygen plasma-assisted molecular beam expitaxy. The orientations of the grown ZnO thin film were investigated by in situ reflection high-energy electron diffraction and ex situ x-ray diffraction (XRD). The film roughness was measured by atomic force microscopy, which was correlated with the grain sizes determined by XRD. Synchrotron-based x-ray absorption spectroscopy was performed to study the doping effect on the electronic properties of the ZnO films, compared with density functional theory calculations. It is found that, nitrogen doping would hinder the growth of thin film, and generate the NO defect, while magnesium doping promotes the quality of nitrogen-doped ZnO films, inhibiting (N2)O production and increasing nitrogen content.

Initial stages of Mg adsorption on the Si(111)-7x7 surface

First-principles calculations are performed to determine the configurations of adsorbed Mg atoms on the Si(111)-7×7 surface. It is shown that Mg atoms prefer to occupy the higher coordination sites of the faulted half unit cell and that they may diffuse laterally to the nearby ones around the same Si center adatom by overcoming a low diffusion barrier of 0.1325 eV. These characteristics are attributed to weak interactions among Mg and nearby Si atoms. The initial adsorption processes of Mg atoms on the Si(111)-7×7 surface are investigated by scanning tunneling microscopy imaging at room temperature. Three types of adsorption sites are identified, which, by comparing with theoretical simulations, are found to be associated with the adsorbed configurations of one, two, and three Mg atoms, respectively.