A. S. Deryabin

Potentialities and basic principles of controlling the plastic relaxation of GeSi/Si and Ge/Si films with stepwise variation in the composition

GeSi/Si heterostructures consisting of a plastically relaxed layer that includes various fractions of Ge and which is grown on Si (001) span the values of the lattice parameter from equal to that in silicon to equal to that in germanium. The corresponding substrates are conventionally referred to as artificial. A number of methods exist for growing high-quality GeSi layers with as large as 100% of Ge on Si (001) substrates through an intermediate GeSi layer with a varying composition. However, it is desirable in a number of cases to have ultrathin (<1 μm) GeSi and Ge layers directly on the Si (001) substrate for practical applications. The results of new methods such as the use of a buffer Si layer grown at a comparatively low temperature (300–400°C) in plastic relaxation of the GeSi/Si(001) heterostructures and also the use of surfactants (antimony and hydrogen) are analyzed. The examples of artificial introduction of centers for origination of misfit dislocations as an alternative to their introduction from the rough surface are considered. It can be concluded that, in order to expand the range of potentialities of growing perfect plastically relaxed GeSi (001) films, it is necessary to (i) make it possible to form in a controlled manner the centers for origination of the misfit dislocations and (ii) retard or completely suppress the transition of the growth mechanism from two-to three-dimensional in order to prevent the formation of additional misfit dislocations from the surface of the stressed film and, correspondingly, additional threading dislocations.

Origination of misfit dislocations at the surface during the growth of GeSi/Si(001) films by low-temperature (300–400°C) molecular-beam epitaxy

The method of the molecular-beam epitaxy, at comparatively low temperatures (300–400°C), was used to grow GexSi1 − x/Si(001) films with a constant composition (x = 0.19–0.32) across a film and as well as two-layer heterostructures with the Ge content at the upper layer no lower than 0.41. Using transmission electron microscopy, it is shown that the main cause of an increase in the density of threading dislocations with increasing Ge fraction in the plastically relaxed films is the origination of the dislocation half-loops at the film surface; in turn, these dislocation half-loops are generated owing to the formation of a three-dimensional profile at the surface of the growing or annealed film.

Plastic relaxation of GeSi/Si(001) films grown by molecular-beam epitaxy in the presence of the Sb surfactant

Plastically relaxed GeSi films with the Ge fraction equal to 0.29–0.42 and thickness as large as 0.5 μm were grown on Si (001) substrates using the low-temperature (350°C) buffer Si layer and Sb as a surfactant. It is shown that introduction of Sb that smoothens the film surface at the stage of pseudomorphic growth lowers the density of threading dislocations in the plastically relaxed heterostructure by 1–1.5 orders of magnitude and also reduces the final roughness of the surface. The root-mean-square value of roughness smaller than 1 nm was obtained for a film with the Ge content of 0.29 and the density of threading dislocations of about 106 cm−2. It is assumed that the effect of surfactant is based on the fact that the activity of surface sources of dislocations is reduced in the presence of Sb.

Structure and electrical properties of polycrystalline SiGe films grown by molecular beam deposition

The structural and electrical properties of polycrystalline Si0.5Ge0.5 films 150 nm thick grown by molecular beam deposition at temperatures of 200–550°C on silicon substrates coated with amorphous layers of silicon oxynitride were studied. It is shown that the films consist of a mixture of amorphous and polycrystalline phases. The amorphous phase fraction decreases from ∼50% in films deposited at 200°C to zero in films grown at 550°C. Subsequent 1-h annealing at a temperature of 550°C results in complete solid-phase crystallization of all films. The electron transport of charge carriers in polycrystalline films occurs by the thermally activated mechanism associated with the energy barrier of ∼0.2 eV at grain boundaries. Barrier lowering upon additional annealing of SiGe films correlates with an increase in the average grain size.

Optimization of the plastic relaxation of misfit stresses in GexSi1−x/Si(001) (x≤0.61) heterostructures

High-quality GexSi1−x/Si(001) (x=0.38–0.61) heterostructures (600–750 nm thick) with plastically relaxed mechanical stresses were grown by low-temperature molecular beam epitaxy. The total film thickness was reduced by using low-temperature growth and two-step variation of the GexSi1−x layer composition. High quality of the heterostructure is achieved due to the fact that misfit stresses between layers are released by dislocations passing from the first layer to the second.

Formation of a Stepped Si(100) Surface and Its Effect on the Growth of Ge Islands

The transition from a two-domain to one-domain surface on a Si(100) substrate is investigated. It is demonstrated using reflection high-energy electron diffraction that at a temperature of 600°C and a deposition rate of 0.652 Å/s onto a Si(100) substrate pre-heated to 1000°C and inclined at an angle of 0.35°C to the plane, a series of reflections from the 1 × 2 superstructure completely vanishes at a constant flow of Si. This is attributed to the transition of the surface from monoatomic to diatomic steps. At growth rates lower than 0.652 Å/s, the transition from a two-domain to one-domain surface is also observed; with a decrease in the growth rate, the intensity ratio I2 × 1/I1 × 2 decreases and the maximum of the dependences shifts toward lower temperatures. The complete vanishing of the series of superstructural reflections after preliminary annealing at a temperature of 700°C is not observed; this series only vanishes after annealing at 900 and 1000°C. The growth of Ge islands on a Si(100) surface preliminary annealed at a temperature of 800°C is studied. It is shown that the islands tend to nucleate at the step edges. A mechanism of Ge island ordering on the Si(100) surface is proposed.