C. Deparis

Surface stoichiometry variation associated with GaAs (001) reconstruction transitions

Abstract GaAs (001) surface structural transitions induced by Ga deposition have been studied through the correlation between RHEED specular beam intensity variations and observed reconstruction changes. Surface stoichiometry variations associated with the onset of each surface reconstruction have been measured. From the best defined 2×4 reconstruction to the onset of the 3×1 and 4×6 reconstructions, As surface coverage variations of 0.31 and 0.53 ML are found, respectively. Results are also obtained for other surface reconstructions: c(4×4), 4×8 (reported for the first time in MBE) and 4x2. In addition, our study clearly shows that each surface reconstruction exists over a rather wide surface stoichiometry range.

Effect of the s , p − d exchange interaction on the excitons in Zn 1 − x Co x O epilayers

We present a spectroscopic study of ZnCoO layers grown by molecular beam epitaxy on sapphire substrates. ZnCoO is commonly considered as a promising candidate for being a Diluted Magnetic Semiconductor ferromagnetic at room temperature. We performed magneto-optical spectroscopy in the Faraday configuration, by applying a magnetic field up to 11T, at temperatures down to 1.5K. For very dilute samples (x<0.5%), the giant Zeeman splitting of the A and B excitons is observed at low temperature. It is proportional to the magnetization of isolated Co ions, as calculated using the anisotropy and g-factor deduced from the spectroscopy of the d-d transitions. This demonstrates the existence of spin-carrier coupling. Electron-hole exchange within the exciton has a strong effect on the giant Zeeman splitting observed on the excitons. From the effective spin-exciton coupling, =0.4eV, we estimate the difference of the exchange integrals for free carriers, N0 |alpha - beta|=0.8=eV. The magnetic circular dichroism observed near the energy gap was found to be proportional to the paramagnetic magnetization of anisotropic Co ions even for higher Co contents.

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...

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.

Growth modes and microstructures of ZnO layers deposited by plasma-assisted molecular-beam epitaxy on (0001) sapphire

Transmission electron microscopy and high resolution x-ray diffraction are used to characterize defects in ZnO layers grown by plasma-assisted molecular-beam epitaxy on (0001) sapphire. Two- and three-dimensional types of growth modes are described and the observed mosaic structure is analyzed in each case. It is found that two-dimensional layers exhibit a roughness as low as 6 nm. Their subdomains have small lateral coherence lengths and a mean in-plane misorientation of ±0.4°, leading to an important dislocation density of 1–4×1010 cm−2. On the contrary, it is demonstrated that, through numerous interactions between dislocations, the three-dimensional growth mode leads to a better structural quality with a larger lateral coherence length and a smaller in-plane mosaic spread of ±0.07°. The total dislocation density is consequently reduced by 1 order of magnitude down to 3–5×109 cm−2 and the radical modification of the structure results in a change of the dislocation distribution. Our results thus demonst...

Residual strain in nonpolar a-plane Zn1−xMgxO (0<x<0.55) and its effect on the band structure of (Zn,Mg)O/ZnO quantum wells

We investigate the dependence on Mg content of the lattice parameters and the surface morphology of nonpolar a-(112¯0) Zn1−xMgxO (x≤0.55) grown by molecular beam epitaxy. The anisotropy of the lattice parameters gives rise to an unusual in-plane strain state in the ZnO QWs: tensile strain along [11¯00] and compressive strain along [0001]. For a Zn0.6Mg0.4O barrier, the strain in a ZnO QW reaches −1.3% along [0001] and +0.3% along [11¯00]. This induces a strong blueshift of the excitonic transitions, in addition to the confinement effects, which we observe in photoluminescence excitation experiments.

Temperature Dependence of Exciton Lifetimes in GaAs/AlGaAs Quantum Well Structures

The photoluminescence decay times in GaAs/AlGaAs single quantum well structures with layer thickness between 20 A and 80 A have been investigated in the temperature range from 4 K up to room temperature. It is shown that the increase in the PL decay time usually ascribed to radiative free-excitons recombination is present even in conditions where other mechanisms such as recombination of localized excitons and free carriers, exciton ionization and thermal activation of nonradiative processes are effective.

Smooth, pseudosmooth, and rough GaAs/AlxGa1−xAs interfaces: Effect of substrate misorientation

GaAs/AlxGa1−xAs quantum well and superlattice structures have been grown at 600 °C by molecular beam epitaxy on GaAs (001) substrates exactly oriented or slightly misoriented towards (111) Ga and (111) As. It is shown that for the growth conditions used, the step orientation (nature) has a striking influence on the photoluminescence (PL) features of the grown structures: steps along [110] (Ga‐type steps) lead to sharp PL lines (pseudosmooth interfaces), while a considerable broadening of the PL (rough interfaces) is obtained when the steps are along [110] (As‐type steps).