H. Trinkaus

Effect of helium ion implantation and annealing on the relaxation behavior of pseudomorphic Si1−xGex buffer layers on Si (100) substrates

The influence of He implantation and annealing on the relaxation of Si0.7Ge0.3 layers on Si (100) substrates is investigated. Proper choice of the implantation energy results in a narrow defect band ≈100 nm underneath the substrate/epilayer interface. During annealing at 700–1000 °C, He-filled bubbles are created, which act as sources for misfit dislocations. Efficient annihilation of the threading dislocations is theoretically predicted, if a certain He bubble density with respect to the buffer layer thickness is maintained. The variation of the implantation dose and the annealing conditions changes density and size of spherical He bubbles, resulting in characteristic differences of the dislocation structure. Si1−xGex layers with Ge fractions up to 30 at. % relax the initial strain by 70% at an implantation dose of 2×1016 cm−2 and an annealing temperature as low as 850 °C. Simultaneously, a low threading dislocation density of 107 cm−2 is achieved. The strain relaxation mechanism in the presence of He fi...

Asymmetric strain relaxation in patterned SiGe layers: A means to enhance carrier mobilities in Si cap layers

Strain relaxation in patterned Si0.77Ge0.23 stripes grown on Si(001) by chemical vapour deposition was investigated after He+ ion implantation and annealing. Ion channeling measurements indicate asymmetric strain relaxation with a significantly higher residual strain parallel to the stripes than perpendicular to the stripes. These results are confirmed by plan view transmission electron microscopy showing a much higher density of misfit dislocations running along the stripes than across the stripes. Estimates based on a piezoresistivity model indicate significant enhancements of electron and hole mobilities for asymmetrically strained Si cap layers on such SiGe stripes.

Si+ ion implantation for strain relaxation of pseudomorphic Si1−xGex/Si(100) heterostructures

A mechanism of strain relief of pseudomorphic Si1−xGex/Si(100) heterostructures by Si+ ion implantation and annealing is proposed and analytically modeled. The degree of strain relaxation is presented as a function of Ge content and implantation and annealing parameters. Rutherford backscattering spectrometry/channeling, Raman spectroscopy, and transmission electron microscopy are employed to quantify the efficiency of the relaxation process and to examine the quality of the samples, respectively. The mechanism and the conditions for strain relaxation are discussed in terms of dislocation loop formation in the implanted range with emphasis on loop formation in the compressively strained SiGe layer. The detrimental effect of local amorphization of the SiGe layer on its relaxation and on strain transfer to the Si-cap layer is also addressed.

Anisotropy of strain relaxation in (100) and (110) Si/SiGe heterostructures

Plastic strain relaxation of SiGe layers of different crystal orientations is analytically analyzed and compared with experimental results. First, strain relaxation induced by ion implantation and annealing, considering dislocation loop punching and loop interactions with interfaces/surfaces is discussed. A flexible curved dislocation model is used to determine the relation of critical layer thickness with strain/stress. Specific critical conditions to be fulfilled, at both the start and end of the relaxation, are discussed by introducing a quality parameter for efficient strain relaxation, defined as the ratio of real to ideal critical thickness versus strain/stress. The anisotropy of the resolved shear stress is discussed for (001) and (011) crystal orientations in comparison with the experimentally observed anisotropy of strain relaxation for Si/SiGe heterostructures.

Substrate engineering by hydrogen or helium implantation for epitaxial growth of lattice mismatched Si/sub 1-x/Ge/sub x/ films on silicon

Strain relaxed Si/sub 1-x/Ge/sub x/ layers on Si[100] are used as virtual substrates for the growth of, for example, Si/Si/sub 1-x/Ge/sub x/ quantum well structures. We investigated the effects of He/sup +/ ion implantation and subsequent annealing on pseudomorphic Si/sub 1-x/Ge/sub x//Si[100] heterostructures grown by molecular beam epitaxy (MBE). A narrow defect band was generated by ion implantation slightly underneath the interface inducing the formation of strain relieving misfit dislocations during subsequent thermal annealing. Nearly complete strain relaxation of Si/sub 1-x/Ge/sub x/ layers with Ge fractions up to 30 at.% was obtained at temperatures as low as 850/spl deg/C and the samples appeared free of threading dislocations within the SiGe layer to the limit of transmission cross-sectional electron microscopy. We thus have developed a method for producing high-quality, thin, relaxed Si/sub 1-x/Ge/sub x/ films on Si[100] with threading dislocation densities below 10/sup 7/ cm/sup -2/ by standard techniques as molecular beam epitaxy and ion implantation. The heterostructures were analyzed using X-ray diffraction, Rutherford backscattering/channeling spectrometry and transmission electron microscopy.

Nucleation and Movement of Dislocations during Relaxation of He Implanted SixGe1-x/Six Heterostructures

Substantial improvement of the performance of Si-based microelectronic devices can be achieved by the implementation of strained silicon as high-mobility channel material [1]. Since strained silicon is generally realized by epitaxial deposition of Si on relaxed SiGe layers, relaxed buffers in an adequate quality are required. Efficient relaxation of thin SiGe films with thicknesses of 100 200 nm and Ge contents of up to 30 % was demonstrated by ion implantation of H or He and subsequent annealing [24]. The platelet-shaped He precipitates, which constitute the initial stages of He precipitation, contain a large He pressure of up to 13 MPa. Thus, the critical shear stress for dislocation formation is reached at the edge of the platelets [5]. This is in line with a model [6], which predicts that the He precipitates located below the heterostructure interface within the Si substrate act as internal dislocation sources. In order to witness the role of the He precipitates for the relaxation mechanism we focus on the formation and movement of dislocations in He implanted SiGe/Si heterostructure during in-situ annealing within a transmission electron microscope. Si81Ge19 layers with a thickness of 170 nm were deposited onto Si (001) substrates by chemical vapor deposition. Helium was implanted with an energy of 37 keV at a dose of 1x10 cm resulting in a helium concentration profile with its maximum in a depth about double the layer thickness. In order to start the in-situ experiment with a defined defect structure, the formation of overpressurized He filled platelets was provided by an exsitu annealing step at 420 °C for one minute [5,7]. These precipitates are located about 170 nm below the SiGe/Si interface, i.e. within the Si substrate. To monitor the formation and evolution of the dislocation structure a temperature of 720 °C was adjusted. According to the (400) two-beam imaging condition chosen in figure 1 local strain fields surrounding the platelets and the dislocations show dark contrast. The projection of features located in different depth of the heterostructure into the image plane is inherent to the plan view geometry of the TEM specimen. At the He precipitate marked by an arrow in frame 2 a dislocation is emitted. In frame 3 the threading arms terminating the misfit dislocation segment, are separated by about 500 nm (marked by arrows). Finally the threading dislocation have almost moved out of the field of view (frame 4) leaving the misfit dislocation behind. For this sequence the dislocation velocity of the threading dislocations is 7 μm/s. The role of He precipitates as internal dislocation sources is clearly proven by the in-situ annealing experiments showing the emission of a dislocations by a He platelet. Detailed analyses of the dislocation types revealed 60° misfit dislocations with Burgers vectors a/2 and an extrinsic nature of dislocation loops. Figure 2 illustrates the relaxation mechanism, which is deduced from the experimental results and which is predicted by the model mentioned above. A prismatic, extrinsic dislocation loop is emitted by the He-precipitates. These loops glide to the interface. When a loop comes into contact with the SiGe layer it experiences an asymmetric force. As a consequence, one side of the loop is pinned at the interface where it forms a strain relieving misfit segment. The other side is driven by the mismatch stress to the surface where an atomic step is generated. Relaxation proceeds as the threading arms run apart and thus elongate the misfit segment. The process depicted in Fig. 2 complies with the experimental observation of the misfit dislocation formation shown in Fig. 1, which is governed by glide processes. However, climb processes could be observed, too [7]. This is of particular importance regarding the efficiency of annihilation of threading dislocations, which requires climb processes to allow the reaction between dislocations on adjacent glide planes. Therefore, climb processes may be essential to achieve relaxed buffer layers with low threading dislocation densities.

Strain Relaxation of He + Implanted, Pseudomorphic Si 1−x Ge x Layers on Si(100)

Strain relaxed Si1-xGex buffer layers are of great importance as virtual substrates for Si1-xGex/Si quantum well structures and devices. We apply He + ion implantation and subsequent annealing on pseudomorphic, MBE-grown Si1-xGex/Si(100) heterostructures with an implantation depth of about 100 nm below the Si1-xGex/Si interface. A narrow defect band is generated inducing the formation of strain relieving misfit dislocations during subsequent thermal annealing. Efficient strain relaxation was demonstrated for Si1-xGex layers with Ge fractions up to 30 at. %. The variation of the implantation dose and the annealing conditions changes the dislocation configuration and the He bubble structure. At a dose of 2x10 cm a high degree of relaxation is accompanied by a low density of threading dislocations of about 10 cm for a Ge content of 30%. An additional increase of the Ge content can be achieved by annealing in oxygen. The oxidation of Si1-xGex leads to the formation of SiO2 while the Ge atoms are rejected from the oxide leading to a pile-up of Ge below the oxidation front. The heterostructures were analyzed using X-ray diffraction, Rutherford backscattering/channeling spectrometry and transmission electron microscopy.