J.-M. Chauveau

Benefits of homoepitaxy on the properties of nonpolar (Zn,Mg)O/ZnO quantum wells on a-plane ZnO substrates

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.

Interface structure and anisotropic strain relaxation of nonpolar wurtzite (1120) and (1010) orientations: ZnO epilayers grown on sapphire

The interface properties between nonpolar ZnO and sapphire have been studied using high resolution transmission electron microscopy. Two nonpolar orientations are investigated: a- and m-orientations corresponding to [112¯0] and [101¯0] crystallographic directions. After the definition of the epitaxial relationships and the resulting initial lattice mismatch, we show that nonpolar ZnO can be grown on sapphire with perfectly flat interfaces. Geometrical misfit dislocations are observed at the interface ZnO/sapphire and their density gives the residual strain in the layer. A strong anisotropy in the strain relaxation is found along the two perpendicular in-plane directions. This anisotropy may be explained in terms of initial anisotropic mismatch yielding different relaxation processes. A domain matching epitaxy is observed in m- and a-oriented layers for mismatches larger than 9% while a lattice matching epitaxy, in which the relaxation is driven by nucleation and glide of dislocations, is observed in a-ori...

Annealing effects on the crystal structure of GaInNAs quantum wells with large In and N content grown by molecular beam epitaxy

The impact of rapid thermal annealing on the optical emission of GaInNAs/GaAs quantum wells (QWs) grown by molecular beam epitaxy with high In and N content is shown to be highly dependent on the crystal structure of the QWs, as determined by transmission electron microscopy. Due to the presence of higher concentrations of nonradiative recombination centers, the annealing temperature required to obtain maximum photoluminescence emission is higher for the QW with strong structural modulation of the upper interface [at the onset of three-dimensional (3D) growth], intermediate for the two-dimensional (2D) grown QW with compositional fluctuations, and lower for the homogeneous 2D grown QW. Moreover, the transition from homogeneous 2D growth, to 2D growth with compositional fluctuations, and finally 3D growth, leads to progressively deeper carrier localization states below the conduction-band edge. Increasing annealing temperatures gradually shifts the localization states closer to the conduction-band edge, pr...

Nanoscale analysis of the In and N spatial redistributions upon annealing of GaInNAs quantum wells

By using transmission electron microscopy on as-grown and annealed GaInNAs/GaAs heterostructures, we demonstrate that annealing induces a correlated behavior of both In and N species within the GaInNAs quantum well. While no intermixing occurs, the analysis of the strain situation reveals that the main driving force for the observed inward diffusion is not composition gradients at the interfaces, but local strain fields. This mechanism leads to the improvement of the photoluminescence (PL) properties and to the blueshift of the PL peak.

Interfacial structure and defect analysis of nonpolar ZnO films grown on R-plane sapphire by molecular beam epitaxy

The interfacial relationship and the microstructure of nonpolar (11−20) ZnO films epitaxially grown on (1−102) R-plane sapphire by molecular beam epitaxy are investigated by transmission electron microscopy. The already-reported epitaxial relationships [1−100]ZnO∥[11−20]sapphire and ⟨0001⟩ZnO∥[−1101]sapphire are confirmed, and we have determined the orientation of the Zn–O (cation-anion) bond along [0001]ZnO in the films as being uniquely defined with respect to a reference surface Al–O bond on the sapphire substrate. The microstructure of the films is dominated by the presence of I1 basal stacking faults [density=(1–2)×105cm−1] and related partial dislocations [density=(4–7)×1010cm−2]. It is shown that I1 basal stacking faults correspond to dissociated perfect dislocations, either c or a+c type.

Residual and nitrogen doping of homoepitaxial nonpolar m-plane ZnO films grown by molecular beam epitaxy

We report the homoepitaxial growth by molecular beam epitaxy of high quality nonpolar m-plane ZnO and ZnO:N films over a large temperature range. The nonintentionally doped ZnO layers exhibit a residual doping as low as ∼1014 cm−3. Despite an effective incorporation of nitrogen, p-type doping was not achieved, ZnO:N films becoming insulating. The high purity of the layers and their low residual n-type doping evidence compensation mechanisms in ZnO:N films.

Indium content measurements in metamorphic high electron mobility transistor structures by combination of x-ray reciprocal space mapping and transmission electron microscopy

We propose a method to determine the indium concentrations x and y in the InyAl1−yAs/InxGa1−xAs metamorphic structures. This approach is based on the combination of two experimental techniques: (i) reciprocal space mapping (RSM) to determine the average In composition in the InAlAs layers and (ii) transmission electron microscopy (TEM) using the intensity measurements of the chemically sensitive (002) reflection from dark-field images to determine the composition in the InGaAs quantum well. We apply this method to a InyAl1−yAs/InxGa1−xAs metamorphic high electron mobility transistor, with x and y approximately equal to 0.35. Furthermore, we present an original and straightforward way to evaluate experimental errors in the determination of composition and strain with the RSM procedure. The influence of these errors on the TEM results is discussed. For In concentrations in the 30%–40% range, the accuracy of this simple method is about 0.5% on the In composition in the InGaAs quantum well.

GaInNAs/GaAs quantum wells grown by molecular-beam epitaxy emitting above 1.5 μm

We demonstrate that a careful optimization of the molecular-beam-epitaxy growth conditions allows us to obtain high-quality GaInNAs/GaAs quantum-well (QW) heterostructures exhibiting a perfect two-dimensional microstructure at high In and N contents. Room-temperature emission is achieved up to 1.61 and 1.51 μm for as-grown and annealed samples, respectively. High-resolution x-ray diffraction and transmission electron microscopy reveal that post-growth annealing does not affect the QW composition and width. This confirms that the GaInNAs semiconducting material is well suited for emission in the telecommunication wavelength range near 1.55 μm.

Interplay between the growth temperature, microstructure, and optical properties of GaInNAs quantum wells

We investigated the influence of the growth temperature (Tgr) on the microstructure and on the optical properties of GaInNAs quantum wells (QWs). By comparing the structural information (transmission electron microscopy) with the optical properties (photoluminescence spectroscopy), we demonstrate that high photoluminescence efficiency of GaInNAs QWs is achieved only when the two-dimensional growth mode is preserved, which can be obtained at a low Tgr even for high In content. We also show composition modulations in the GaInNAs QWs, which can lead to the interface roughness.

Correlations between structural and optical properties of GaInNAs quantum wells grown by MBE

Abstract In this work, the structural properties of Ga1−xInxNyAs1−y (GINA) quantum wells (QW) are investigated in terms of interface roughness and chemical composition variations by using conventional and high-resolution transmission electron microscopy (HRTEM). The structural behaviors of these heterostructures are systematically compared with the optical properties investigated by photoluminescence (PL). Our results demonstrate that high PL efficiency of GINA material, irrespective of composition, can be obtained only when the epitaxy is performed under conditions preserving a 2D growth mode. Moreover, composition variations are shown at a local scale using HRTEM and strain mapping.