We investigated structural and optical properties of type-II InP/GaAs quantum dots using reflection high energy electron diffraction, transmission electron microscopy, atomic force microscopy, grazing incidence x-ray diffraction, and photoluminescence techniques. The InP dots present an efficient optical emission even when they are uncapped, which is attributed to the low surface recombination velocity in InP. We compare the difference in the optical properties between surface free dots, which are not covered by any material, with dots covered by a GaAs capping layer. We observed a bimodal dispersion of the dot size distribution, giving rise to two distinct emission bands. The results also revealed that the strain accumulated in the InP islands is slightly relieved for samples with large InP amounts. An unexpected result is the relatively large blue shift of the emission band from uncapped samples as compared to capped dots.

Three-dimensional electronic properties of multiple vertically stacked In As ∕ Ga As self-assembled quantum dots

Strain relaxed Si 1-x Ge x layers are attractive virtual substrates for the epitaxial growth of strained Si. Tensile strained Si has attracted a lot of attention due its superior electronic properties. In this study, the strain relaxation of pseudomorphic Si 1-x Ge x layers grown by chemical vapor deposition (CVD) on Si(100) substrates was investigated after He + ion implantation and thermal annealing. The implantation induced defects underneath the SiGe/Si interface promote strain relaxation during annealing via preferred nucleation of dislocation loops which form misfit dislocations at the interface to the substrate. The amount of strain relaxation as well as the final threading dislocation density depend on the implantation dose and energy. Si 1x Ge x layers with thicknesses between 75 and 420 nm and Ge concentrations between 19 and 29 at% were investigated. The strain relaxation strongly depends on the layer thickness. Typically the structures show ≈70 % strain relaxation and threading dislocation densities in the low 10 6 cm -2 range. AFM investigations proved excellent surface morphology with an rms roughness of 0.6 nm. The samples were investigated by Rutherford backscattering spectrometry, ion channeling, transmission electron microscopy and atomic force microscopy.

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The strain relaxation of pseudomorphic Si 1−x Ge x layers ͑x = 0.21, . . . , 0.33͒ was investigated after low-dose Si + ion implantation and annealing. The layers were grown by molecular-beam epitaxy or chemical vapor deposition on Si͑100͒ or silicon-on-insulator. Strain relaxation of up to 75% of the initial strain was observed at temperatures as low as 850°C after implantation of Si ions with doses below 2 ϫ 10 14 cm −2 . We suggest that the Si implantation generates primarily dislocation loops in the SiGe layer and in the underlying Si which convert to strain relaxing misfit segments. Strained Si films grown on strain-relaxed SiGe buffer layers will soon be applied for advanced microelectronic devices since strained Si exhibits significantly enhanced carrier mobilities and therefore, yields to transistors with higher transconductance and drive currents.1,2 However, a key problem is the fabrication of thin strain relaxed SiGe buffer layers, which serve as virtual substrates for the growth of strained silicon. The standard approach is the growth of several micrometer thick, compositionally graded buffer layers. 3Recently, it was shown that He + ion implantation and subsequent thermal annealing can successfully be employed to relax the strain of thin pseudomorphic SiGe layer grown on Si͑100͒.4-6 The required He + implantation doses to achieve substantial strain relaxation during the subsequent annealing are in the range of 5 ϫ 10 15 cm −2 to 2 ϫ 10 16 cm −2 . These relatively high doses lead to long implantation times and limit throughput of wafers in production and form voids in the underlying silicon. Previously it was shown that strain relaxation can also be achieved by H + implantation and annealing.4 However, this method was only successful forSiGe layers with Ge concentrations below 25 at. %. Enhanced strain relaxation due to the introduction of point defects by Ge + ion implantation at 400°C or the implantation of dopants ͑B,As͒ was reported, however, without revealing the resulting microstructure. 7,8 The use of ion implantation for strain relaxation has several advantages: It is easy to use, is highly reproducible, is area-selective by the use of masks, and is fully compatible with existing Si technology. Therefore, there is interest in the development of an ion implantation method, which allows efficient strain relaxation after annealing at moderate temperatures while maintaining a high sample quality. It is important to use ions, which require only a low dose and avoid contamination or unintended doping by the implanted ions.In this work, we have investigated the strain relaxation of pseudomorphic Si 1−x Ge x layers induced by low dose Si + ion implantation at room temperature and subsequent annealing. We also provide a preliminary explanation for the mechanism of strain relaxation. The layers were grown by chemical vapor deposition (CVD) and solid-source molecular-beam-epitaxy (MBE) on Si͑100͒ and on siliconon-insulator (SOI) wafers. The samples were characterized using transmission electron microsco...