Structural characterization of highly strained InAs N monolayer lasers and quantum well structures by X-ray diffraction and transmission electron microscopy

X-ray interference effect and transmission electron microscopy are used to study the relaxation process in a series of laser structures as a function of InAs content in the quantum well. It is shown that the X-ray interference effect is a powerful, fast and non-destructive method to assess the strain status in samples of this kind. A set of strained layer laser structures containing N monolayers of InAs (Nx(InAs)1(GaAs)3 with (N = 1, 3, 5, 7) in an 8 nm quantum well active region and a set of strained layer quantum wells consisting of P monolayers of InAs (Px(InAs)1(GaAs)Q with (P = 2, 4 and (Q = 2, 4) were grown [Dotor et al., J. Crystal Growth 127 (1993) 46] by atomic layer molecular beam epitaxy. X-ray interference effect and cross-section transmission electron microscopy analysis of the samples show that in the series of lasers with N monolayers of InAs the whole laser structure is coherent with the substrate (and consequently dislocation free) for 1 and 3 monolayers of InAs, while a sample with 5 monolayers of InAs is in a certain stage of relaxation (dislocation density nd {all equal to}107 cm-2) and a sample with 7 monolayers of InAs is almost completely relaxed (nd{all equal to}108 cm-2). In strained layer quantum well samples, the influence of the InAs/GaAs thickness ratio (P/Q) on the critical thickness has also been studied. These results are compared with those predicted by theoretical critical thickness models. Optical characterization as well as threshold current measurements of the lasers are correlated with X-ray diffraction and transmission electron microscopy relaxation status results. © 1993.Peer Reviewe

Fluorescein angiography (FA) images of all study groups.

<p>Left-image: Baseline, right-image: 4 weeks post-treatment. A: Intravenous control group (IV-Control). B: Intravenous P17 group (IV-17). C: Intravenous P144 group (IV-144). D: Intravitreal P17 group (IVT-17). E: Intravitreal P144 group (IVT-144). F: Intravitreal P17+P144 group (IVT-17+144).</p

Western blotting for vascular endothelial growth factor (VEGF) expression in RPE in intravenous groups (left) and intravitreal groups (right).

<p>Bars represent mean ± SEM of percentage of arbitrary units (AU) vs. control (Total protein loaded: 8 µg; 5–6 eyes/group). (* p<0.05).</p

VEGF protein expression, pSMAD-2 protein levels and MMP-2 activity in RPE homogenates - Intravitreal groups.

<p>Data are shown as mean ± SEM of percentage of arbitrary units (AU) vs control eye. (*p<0.05 and **p<0.01).</p

Western blotting for phosphorylated SMAD-2 (pSMAD-2) expression in RPE in intravenous groups (left) and intravitreal groups (right).

<p>Bars represent mean ± SEM of percentage of arbitrary units (AU) vs control (Total protein loaded: 8 µg; 5–6 eyes/group).(* p<0.05; ** p<0.01).</p

VEGF protein expression, pSMAD-2 protein levels and MMP-2 activity in RPE homogenates - Intravenous groups.

<p>Data are shown as mean ± SEM of percentage of arbitrary units (AU) vs control group. (*p<0.05).</p

Real time Polymerase Chain Reaction.

<p>Results are expressed as specific gene Relative Quantity (RQ) and p-value, β-Actin was selected as a control gene (5 eyes/group). Significance level: p<0.05. (NS: Not significant; *:p<0.05 ; **:p:<0.01; NA: Not analyzed).</p

Mean CNV areas assessed by fluorescein angiographies in all study groups (measured in pixels).

<p>Differences between treated and untreated eyes. Top left: Intravenous Controls vs Intravenous P17 vs Intravenous P144 (IV-Control vs IV-17 vs IV-144); top right: Intravitreal P17 (IVT-17); bottom left: Intravitreal P144 (IVT-144); bottom right: Intravitreal P17+P144 (IVT-17+144). (NS: Not significant; * p<0.05; ** p<0.01).</p

Real time-Polymerase Chain Reaction (Rt-PCR) results.

<p>Differences between treated and untreated eyes in all study groups (Mean+/−SD; 5 eyes/study group). Top left: IV-Control vs IV-17. Top right: IV-Control vs IV-144. Middle left: IVT-17. Middle right: IVT-144. Bottom left: IVT-17+144. (NS: Not significant; * p<0.05; ** p<0.01).</p

Gelatin zymography assay for matrix metalloproteinase-2 (MMP-2) activity in RPE homogenates from intravenous groups (left) and intravitreal groups (right).

<p>Bars represent mean ± SEM of percentage of arbitrary units (AU) vs. control (Total protein loaded: 8 µg; 5–6 eyes/group). (* p<0.05).</p