We report on the properties of nonpolar (Zn,Mg)O/ZnO quantum wells (QW) homoepitaxially grown by molecular beam epitaxy on a-plane ZnO substrates. We demonstrate a drastic improvement of the structural properties. We compare the photoluminescence properties of nonpolar homoepitaxial QWs and nonpolar heteroepitaxial QWs grown on sapphire and show that the reduction in structural defects and the improvement of surface morphology are correlated with a strong enhancement of the photoluminescence properties: reduction in full width at half maximum, strong increase in the luminescence intensities and their thermal stability. The comparison convincingly demonstrates the interest of homoepitaxial nonpolar QWs for bright UV emission applications.

No abstract

No abstract

We report on the properties of nonpolar a-plane (Zn,Mg)O/ZnO quantum wells (QW) grown by molecular beam epitaxy on r plane sapphire and a plane ZnO substrates. For the QWs grown on sapphire, the anisotropy of the lattice parameters of the (Zn,Mg)O barrier gives rise to an unusual in-plane strain state in the ZnO QWs, which induces a strong blue-shift of the excitonic transitions, in addition to the confinement effects. We observe this blue-shift in photoluminescence excitation experiments. The photoluminescence excitation energies of the QWs are satisfactorily simulated when taking into account the variation of the exciton binding energy with the QW width and the residual anisotropic strain. Then we compare the photoluminescence properties of homoepitaxial QWs grown on ZnO bulk substrate and heteroepitaxial QWs grown on sapphire. We show that the reduction of structural defects and the improvement of surface morphology are correlated with a strong enhancement of the photoluminescence properties: reduction of full width at half maximum, strong increase of the luminescence intensities. The comparison convincingly demonstrates the interest of homoepitaxial nonpolar QWs for bright UV emission applications.

Mahbub; Maaskant, Pleun P.; Frayssinet, E.; de Mierry, P.; Draeger, A. Rights

We report on the structural in-plane anisotropy of GaN films grown on A-plane sapphire substrates by metal organic chemical vapor deposition. It is found that GaN:Si grown on A-face sapphire exhibits a strongly anisotropic wafer bending in the two orthogonal in-plane directions, with a ∼24% larger curvature along the c-axis of sapphire than along the m-axis. Using a model developed for an elastically anisotropic bilayer structure and using our curvature data, the anisotropic biaxial stresses in the two in-plane directions have been estimated as σ1x≅−1.3 GPa and σ1y≅−1.1 GPa along parallel and perpendicular to c-axis of sapphire, respectively. This anisotropic stress is also responsible for the distortion of the GaN hexagonal basal plane, as evidenced by x-ray diffraction measurements. The broadening of full width at half maximum of the GaN (0002) x-ray reflections varies with different azimuthal angles, correlated with the tilt of the c-axis of GaN. The in-plane epitaxial relationships between the GaN (0001) and A-face sapphire are found as a-axis of GaN aligned with c-axis of sapphire and m-axis of GaN aligned with m-axis of sapphire.

The difference of growth temperatures between InGaN quantum wells and GaN barriers has detrimental effects on the properties of the wells. Different capping processes of InGaN quantum well with a thin AlGaN layer have been investigated to prevent these effects. Both structural and optical properties of the samples, grown on c-plane sapphire substrates by metalorganic vapor phase epitaxy, were studied through transmission electron microscopy (TEM), x-ray diffraction and room temperature photoluminescence. The average quantum well thickness and its indium composition were determined by digital processing of lattice fringes in cross-sectional TEM images. From the analysis of the well thickness distribution, it is shown that AlGaN as a capping layer helps to compensate an unwanted undulation at the upper InGaN QW-barrier interface. Moreover, when deposited at the same temperature as InGaN, the AlGaN layer is effective in avoiding or reducing the evaporation and/or diffusion of indium from InGaN wells, which results in the thinning of the well. It therefore helps to extend the emission wavelength up to 540 nm with a reduced degradation of the room temperature photoluminescence efficiency.

Monolithic InGaN-based light-emitting diodes (LEDs) using a light converter fully grown by metal organic vapor phase epitaxy are demonstrated. The light converter, consisting of 10–40 InGaN/GaN quantum wells, is grown first, followed by a violet pump LED. The structure and growth conditions of the pump LED are specifically adapted to avoid thermal degradation of the light converter. Electroluminescence analysis shows that part of the pump light is absorbed by the light converter and reemitted at longer wavelength. Depending on the emission wavelength of the light converter, different LED colors are achieved. In particular, for red-emitting light converters, a color temperature of 2100 K corresponding to a tint between warm white and candle light is demonstrated.