Synthesis of ordered Ge-Si heterostructures containing ultrasmall germanium nanoclusters

Different techniques for the fabrication of structures containing ensembles of ultrasmall germanium nanoclusters distributed with a high density over the substrate surface are discussed. How to control the morphology and ordering of these ensembles is also discussed.     

Edge misfit dislocations in the GeSi/Si(001) pair: Conditions and specific features of high-quantity generation

Dislocation structure of Ge_xSi_1?x films (x=0.4?0.8) grown by molecular-beam epitaxy on Si(001) substrates was studied by means of transmission electron microscopy. It was found that the density of edge MDs formed at the early stage of plastic strain relaxation in the films could exceed the density of 60° MDs. In our previous publications, a predominant mechanism underlying the early formation of edge misfit dislocations (MD) in Ge_xSi_1?x/Si films with x>0.4 was identified; this mechanism involves the following processes. A 60° glissile MD provokes nucleation of a complementary 60° MD gliding on a mirror-like tilted plane (111). A new edge MD forms as a result of interaction of the two complementary 60° MDs, and the length of the newly formed edge MD can then be increased following the motion of the “arms” of the complementary 60° MDs. Based on this scenario of the edge MD generation process, we have calculated the critical thickness of insertion of an edge MD into GeSi layers of different compositions using the force balance model. The obtained values were found to be more than twice lower than the similar values for 60° MDs. This result suggests that a promising strategy towards obtaining dislocation arrays dominated by 90° dislocations in MBE-grown Ge_xSi_1?x/Si films can be implemented through preliminary growth on the substrate of a thin, slightly relaxed buffer layer with 60° MDs present in this layer. The dislocated buffer layer, acting as a source of threading dislocations, promotes the strain relaxation in the main growing film through nucleation of edge MDs in the film/buffer interface. It was shown that in the presence of threading dislocations penetrating from the relaxed buffer into the film nucleation of edge MDs in the stressed film can be initiated even if the film thickness remains small in comparison with the critical thickness for insertion of 60° MDs. Examples of such unusual MD generation processes are found in the literature.     

Edge misfit dislocations in GexSi1 ? x/Si(001) (x ～ 1) heterostructures: role of buffer GeySi1 ? y (y < x) interlayer in their formation

The structure of dislocations in Ge _x Si_{1 − x} (x ∼ 0.4–0.8) films grown by molecular beam epitaxy on Si(001) substrates tilted by 6° toward the nearest (111) plane has been studied. The epitaxy of GeSi films on substrates deviating from the exact (001) orientation has allowed us to establish the main mechanism of formation of edge misfit dislocations (MDs), which most effectively (for heterostructures of the given composition) relieve stresses caused by the mismatch between lattice parameters of the film and substrate. Despite the edge MDs being defined as immobile (sessile) dislocations, their formation proceeds according to the gliding mechanism proposed by Kvam et al. [J. Mater. Res. 5, 1900 (1990)]. A comparative estimation of the propagation velocities of the primary and induced 60° dislocations, as well as the resulting 90° MDs, has been performed. It has been established that the condition providing for the most effective edge MD formation by the induced nucleation mechanism is the appearance of 60° MDs in a stressed film immediately after it reached a critical thickness. A source of these dislocations can be provided by a preliminarily grown buffer GeSi layer that occurs in a metastable state at the initial stage of plastic relaxation.     

Strained germanium films in Ge/InGaAs/GaAs heterostructures: Formation of edge misfit dislocations at the Ge/InGaAs interface

Heterostructures of the “strained Ge film/artificial InGaAs layer/GaAs substrate” type have been grown by molecular beam epitaxy. A specific feature of these structures is that the plastically relaxed (buffer) InGaAs layer has the density of threading dislocations on a level of 10⁵–10⁶ cm⁻². These dislocations penetrate into the strained Ge layer to become sources of both 60° and 90° (edge) misfit dislocations (MDs). Using the transmission electron microscopy, both MD types have been found at the Ge/InGaAs interface. It has been shown that the presence of threading dislocations inherited from the buffer layer in a tensile-strained Ge film favors the formation of edge dislocations at the Ge/InGaAs interface even in the case of small elastic deformations in the strained film. Possible mechanisms of the formation of edge MDs have been considered, including (i) accidental collision of complementary parallel 60° MDs propagating in the mirror-tilted {111} planes, (ii) induced nucleation of a second 60° MD and its interaction with the primary 60° MD, and (iii) interaction of two complementary MDs after a cross-slip of one of them. Calculations have demonstrated that a critical layer thickness (h _c ) for the appearance of edge MDs is considerably smaller than h _c for 60° MDs.     

Specific features of formation and propagation of 60° and 90° misfit dislocations in GexSi1−x/Si films with x>0.4

The dislocation structure at the initial stage of relaxation of Ge_xSi_1−x films (x∼0.4–0.8) grown on Si (0 0 1) substrates tilted at 6° to the nearest (1 1 1) plane is studied. The use of Si substrates tilted away from the exact (0 0 1) orientation for epitaxial growth of Ge_xSi_1−x films (x≥0.4) allowed finding the basic mechanism of formation of edge dislocations that eliminate the mismatch stresses. Though the edge dislocations are defined as sessile dislocations, they are formed in accordance with the slipping mechanism proposed previously by Kvam et al. (1990). It is highly probable that a 60° misfit dislocation (MD) propagating by the slipping mechanism provokes the nucleation of a complementary 60° MD slipping in a mirror-like tilted plane (1 1 1). The reaction between these dislocations leads to the formation of an edge MD that ensures more effective reconciliation of the discrepancy. Comparative estimation of the slip velocities of the primary and induced 60° MDs and also of the resultant 90° MD is fulfilled. The slip velocity of the induced 60° MD is appreciably greater than the velocity of the primary 60° MD. Therefore, the induced MD “catches up” with the second front of the primary MD, thus forming a 90° MD propagating to both sides due to slipping of the 60° MDs forming it. The propagation velocity of the 90° MD is also greater than the slip velocity of a single 60° MD. For these reasons, 90° MDs under certain conditions that favor their formation and propagation can become the main defects responsible for plastic relaxation of GeSi films close to Ge in terms of their composition.     

Specific features of plastic relaxation of a metastable Ge x Si1 − x layer buried between a silicon substrate and a relaxed germanium layer

Heterostructures Ge/Ge _x Si_{1 ? x} /Si(001) grown by molecular beam epitaxy have been investigated using atomic scale high-resolution electron microscopy. A germanium film (with a thickness of 0.5–1.0 μm) grown at a temperature of 500°C is completely relaxed. An intermediate Ge_0.5Si_0.5 layer remains in a strained metastable state, even though its thickness is 2–4 times larger than the critical value for the introduction of 60° misfit dislocations. It is assumed that the Ge/GeSi interface is a barrier for the penetration of dislocations from a relaxed Ge layer into the GeSi layer. This barrier is overcome during annealing of the heterostructures for 30 min at a temperature of 700°C, after which dislocation networks having different degrees of ordering and consisting predominantly of edge misfit dislocations are observed in the Ge/GeSi and GeSi/Si(001) heteroboundaries.     

On Automatic Reconnects in the Control Systems of Large Experimental Facilities

Physics of Particles and Nuclei Letters - All modern control systems (CS’s) of large experimental facilities are distributed. Nodes are connected via Ethernet or via specific links such as...