Zhu Qin-Sheng

Investigation of oxygen vacancy and interstitial oxygen defects in ZnO films by photoluminescence and x-ray photoelectron spectroscopy

ZnO films prepared at different temperatures and annealed at 900 degrees C in oxygen are studied by photoluminescence (PL) and x-ray photoelection spectroscopy (XPS). It is observed that in the PL of the as-grown films the green luminescence (GL) and the yellow luminescence (YL) are related, and after annealing the GL is restrained and the YL is enhanced. The O 1s XPS results also show the coexistence of oxygen vacancy (Vo) and interstitial oxygen (O-i) before annealing and the quenching of the V-o after annealing. By combining the two results it is deduced that the GL and YL are related to the V-o and O-i defects, respectively.

Measurement of GaN/Ge(001) Heterojunction Valence Band Offset by X-Ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy has been used to measure the valence band offset (VBO) at the GaN/Ge heterostructure interface. The VBO is directly determined to be 1.13 +/- 0.19 eV, according to the relationship between the conduction band offset Delta E-C and the valence band offset Delta E-V : Delta E-C = E-g(GaN) - E-g(Ge) - Delta E-V, and taking the room-temperature band-gaps as 3.4 and 0.67 eV for GaN and Ge, respectively. The conduction band offset is deduced to be 1.6 +/- 0.19 eV, which indicates a type-I band alignment for GaN/Ge. Accurate determination of the valence and conduction band offsets is important for the use of GaN/Ge based devices.

Electron mobility limited by surface and interface roughness scattering in AlxGa1−xN/GaN quantum wells

The electron mobility limited by the interface and surface roughness scatterings of the two-dimensional electron gas in AlxGa1−xN/GaN quantum wells is studied. The newly proposed surface roughness scattering in the AlGaN/GaN quantum wells becomes effective when an electric field exists in the AlxGa1−xN barrier. For the AlGaN/GaN potential well, the ground subband energy is governed by the spontaneous and the piezoelectric polarization fields which are determined by the barrier and the well thicknesses. The thickness fluctuation of the AlGaN barrier and the GaN well due to the roughnesses cause the local fluctuation of the ground subband energy, which will reduce the 2DEG mobility.

ZnO nanostructures grown on AlN/sapphire substrates by MOCVD

ZnO nanorods and nanotubes are successful synthesized on AlN/sapphire substrates by metal-organic chemical vapour deposition (MOCVD). The different morphology and structure properties of ZnO nanorods and nanotubes are found to be allected by the AlN under-layer. The photoluminescence spectra show the optical properties of the ZnO nanorods and nanotubes, in which a blueshift of UV emission is observed and is attributed to the surface effect.

Photoluminescence Investigation of Two-Dimensional Electron Gas in an Undoped AlxGa1−xN/GaN Heterostructure

Low-temperature photoluminescence measurement is performed on an undoped AlxGa1-xN/GaN heterostructure. Temperature-dependent Hall mobility confirms the formation of two-dimensional electron gas (2DEG) near the heterointerface. A weak photoluminescence (PL) peak with the energy of similar to 79meV lower than the free exciton (FE) emission of bulk GaN is related to the radiative recombination between electrons confined in the triangular well and the holes near the flat-band region of GaN. Its identification is supported by the solution of coupled one-dimensional Poisson and Schrodinger equations. When the temperature increases, the red shift of the 2DEG related emission peak is slower than that of the FE peak. The enhanced screening effect coming from the increasing 2DEG concentration and the varying electron distribution at two lowest subbands as a function of temperature account for such behaviour.

Growth and Characterization of an a-Plane InxGa1-xN on a r-Plane Sapphire

The non-polar a-plane (110) InxGa1−xN alloys with different indium compositions (0.074 ≤ x ≤ 0.555) were grown on r-plane (102) sapphire substrates by metalorganic chemical vapor deposition, and the indium compositions x are estimated from x-ray diffraction measurements. The in-plane orientation of the InxGa1−xN with respect to the r-plane substrate is confirmed to be [100]sapphire|| [110]InxGa1−xN and [101]sapphire|| [0001]InxGa1−xN. The effects of substrate temperature, reactor pressure and trimethylindium input flow on the indium incorporation and growth rate are investigated. The morphology of the a-plane InxGa1−xN is found to be significantly improved with the decreasing indium composition x and growth rate. Moreover, the in-plane anisotropic structural characteristics are revealed by high resolution x-ray diffraction employing azimuthal dependence, and the degree of anisotropy decreases with the increase of indium composition.

The Growth of Semi-Polar ZnO (101̄1) on Si (111) Substrates Using a Methanol Oxidant by Metalorganic Chemical Vapor Deposition

Semi-polar ZnO (1011) epitaxial films are demonstrated using a methanol oxidant by metalorganic chemical vapor deposition on Si (111) substrates at 500°C. X-ray scanning indicates that there are six kinds of in-plane domain growths, with the ZnO [1012] parallel to the Si (112) direction families. The crystallographic orientation of ZnO is supposed to be caused by surface passivation. The methanol, as a polar molecule, may be adsorbed on the Si (111) surface to form a passivation layer, which inhibits the (0001) ZnO plane deposition on the substrate surface, and as a result the ZnO (1011) plane becomes preferred. The optical properties, examined by a room-temperature photoluminescence spectrum, exhibit a strong near-band-edge emission peak at 379 nm, indicating that the (1011) ZnO film has good crystal quality. These results are significant for research into and for the applications of semi-polar ZnO films.

A simple route of morphology control and structural and optical properties of ZnO grown by metal-organic chemical vapour deposition

Employing the metal-organic chemical vapour deposition (MOCVD) technique, we prepare ZnO samples with different morphologies from the film to nanorods through conveniently changing the bubbled diethylzinc flux (BDF) and the carrier gas flux of oxygen (OCGF). The scanning electron microscope images indicate that small BDF and OCGF induce two-dimensional growth while the large ones avail quasi-one-dimensional growth. X-ray diffraction (XRD) and Raman scattering analyses show that all of the morphology-dependent ZnO samples are of high crystal quality with a c-axis orientation. From the precise shifts of the 2 theta. locations of ZnO (002) face in the XRD patterns and the E-2(high) locations in the Raman spectra, we deduce that the compressive stress forms in the ZnO samples and is strengthened with the increasing BDF and OCGF. Photoluminescence spectroscopy results show all the samples have a sharp ultraviolet luminescent band without any defects-related emission. Upon the experiments a possible growth mechanism is proposed.