Thickness For Strain Relaxation Research Articles

X-RAY DIFFRACTION OF FERROELECTRIC NANODOMAINS IN PBTIO3 THIN FILMS

ABSTRACT X-ray diffraction constitutes a powerful technique with which to characterise ferroelectric domains. Here we describe the principles of ferroelectric nanodomain diffraction and present some results for PbTiO3 thin films grown under tensile strain on two different substrates, with thicknesses below and above the critical thickness for strain relaxation. The combination of conventional and grazing incidence diffraction and the analysis of the scattering between Bragg peaks allowed the identification of a new polar symmetry in ultra-thin films with only anti-parallel 180° domains. Thick films showed tetragonal 90° ferroelectric/ferroelastic domains instead, with a depressed TC and a domain periodicity largely independent of temperature.

Growth and nonlinear characterization of AlN/GaN structures

In this work we present a preliminary study on the influence of the epitaxial growth mode and defect density on second harmonic generation carried out on AlN/GaN structures. The analysed AlN/GaN structures represent the basic unitary cell of a AlN/GaN multilayer structure designed for the enhancement of second harmonic generation. The individual layer thickness is beyond the critical thickness for strain relaxation. The structures were grown on sapphire (Al2O3) substrates using an HT-AlN crystallization layer and morphologically characterized by AFM measurements. Second harmonic generation was observed only in a structure presenting a 3D-like growth mode, obtained for an intermediate value of the V/III ratio and a crystallization layer growth rate of 500 nm h−1.

Effect of above band-gap illumination on structure of free exciton reflection spectra of ZnSe

The fine structure of free exciton reflection spectra is often observed in bulk and heteroepitaxial ZnSe epilayers of thickness higher than the strain relaxation thickness. This modification of excitonic lines gives no way of direct correlation between optical spectra and energetic structure of semiconductors. We present results on well-resolved measurements and theoretical treatment of reflectance spectra in the free-exciton region of MOVPE-grown ZnSe/GaAs epilayers under additional above band-gap illumination. The measurements were performed at T=77 K, in order to eliminate the polariton phenomena from consideration. A radical change was observed, even the total reversion of the spectral line contour after external illumination of the power of P=1 mW. A multilayer model of the near-surface region was used to analyze the mechanism of lineshape formation, the resonance energy and the decay parameter of free excitons. They were estimated by fitting procedure. It was shown that the spike structure of the ZnSe reflection spectra at high values of damping parameter of excitons is mainly due to the surface electric field and excitonic Stark effect. The characteristic of the electric field distribution in the space-charge region was evaluated for various illumination conditions. The nature of illumination induced surface charged states is discussed.

Structure of free exciton luminescence spectra in heteroepitaxial ZnSe/GaAs

Free exciton photoluminescence (PL) and reflection spectra of MOVPE-grown ZnSe/GaAs epilayers with thickness higher than the strain relaxation thickness were studied experimentally and theoretically at T = 10−120 K. The calculations were performed using a single- and a two-oscillator model including spatial dispersion, in the frame of the dead layer model. The results allow us to rule out the explanation of the fine structure of the free exciton spectra by the thermal strain-splitting and by polariton effects. It was shown that this structure appears mainly due to the light interference caused by the presence of a dead layer in the near-surface region. The dead layer thickness depends on the illumination intensity of the light used for PL excitation.

Strain relaxation during the surfactant modified epitaxial growth of Ge/Si(001)

The initial strain relaxation of Ge on Si(001) has been investigated during epitaxial growth modified by a “surfactant” layer of Sb. Grazing-incidence X-ray diffraction was used to measure the strain relaxation due to its high sensitivity to the in-plane distribution of lattice spacings. We have observed the critical thickness for strain relaxation in the Ge overlayer to be ~ 11 monolayers (ML), with further relaxation developing gradually and in stages. A meta-stable, partially relaxed layer forms up to a coverage of ~ 30 ML, after which a more pronounced relaxation takes place. Even at a coverage of ~ 55 ML, complete strain relief has not been reached, and was only achieved after a 700°C thermal anneal. Concurrent specular reflectivity measurements also reveal that the Ge overlayer grows in a layer-by-layer fashion, and not in the Stranski-Krastanow mode expected for the Ge/Si system. These data show dramatically how the modification of surface energies through the presence of a “surfactant” can affect the morphology of, and strain in, a lattice-mismatched heteroepitaxial system.