Page - 191 - in Photovoltaic Materials and Electronic Devices

GaAssubstrate, theHRXRDspectrashowtheappearanceofotherpeaksappointed satellite peaks, due to the periodicity introduced by the bilayers repetition and the angular period of this peak is related to the thickness of the bilayer [28,29]. In our case, the absence of satellite peaks could be explained by the existence of defects produced in the interfaces layers. Indeed, as shown by the Figure 4, the cross section transmission electron microscopy image unambiguously shows that the GaAs buffer layer was not sufficiently smooth. The surface roughness greatly influencedthemultiple layerQDs, resulting indistorted layers. Consequently, the grown InAs/InGaAs QDs display a non-uniform thickness which in turn provokes plastic strainrelaxationviadefectsandthreadingdislocations. Materials 2015, 8 4548 section transmission electron microscopy image unambiguously shows that the GaAs buffer layer was not sufficiently smooth. The surface roughness greatly influenced the multiple layer QDs, resulting in distorted layers. Consequently, the grown InAs/InGaAs QDs display a non-uniform thickness which in turn provokes plastic strain relaxation via defects and threading dislocations. Figure 4. Cross section TEM image of the InAs/InGaAs multilayer QDs. Additional details can be given by PL characterization. Figure 5 shows the 11 K PL spectra of the pin-GaAs/n+-Si structures with and without QDs. A peak centered at 842 nm appear in both structures and are attributed to the GaAs’ emission. The red shift of GaAs emission peak is a consequence of the lattice mismatch between GaAs and Si, the polar/nonpolar character and of the strong tensile stress, since the thermal expansion coefficient of GaAs is about twice that of the silicon value. The low intensity of these peaks is directly linked to the subsistence of non-radiative recombination channels due to the defects in the structure. Figure 5. PL spectra recorded at 11 k from the pin-GaAs/Si structure (red line) and from pin-GaAs/Si with InAs/InGaAs multilayer QDs (blue line). For the structure containing multiple layer QDs, the PL measurement reveals a broad band centered at 1100 nm. This band is likely to arise from the luminescence of the InAs QDs. Although this result Figure4. Cros sectionTEMimageof the InAs/ ltilayer s. Additional details can be given by PL characterization. Figure 5 shows the 11 K PL spectra of the pin-GaAs/n+-Si structures with and without QDs. A peak centered at842nmappear inbothstructuresandareattributedto theGaAs’emission. The red shift of GaAs emission peak is a consequence of the lattice mismatch between GaAs and Si, the polar/nonpolar character and of the strong tensile stress, since the thermalexpansioncoefficientofGaAsisabout twice thatof thesiliconvalue. The low intensity of these peaks is directly linked to the subsistence of non-radiative recombinationchannelsdueto thedefects in thestructure. For the structure containing multiple layer QDs, the PL measurement reveals a broadbandcenteredat1100nm. Thisbandis likely toarise fromthe luminescence of the InAs QDs. Although this result confirms the formation of InAs/GaAs QDs, the broadening of the PL band with relatively weak intensity confirms that the QDs structuralpropertiesarealtered. 191