In Situ Study of Isothermal Strain Relaxation in Si-Ge Heteroepitaxial Films Using Substrate Curvature Measurements
- PDF / 428,818 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 55 Downloads / 214 Views
IN SITU STUDY OF ISOTHERMAL STRAIN RELAXATION IN Si-Ge HETEROEPITAXIAL FILMS USING SUBSTRATE CURVATURE MEASUREMENTS Vdronique T. GiUard*, David B. Noble* and William D. Nix* *Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305
ABSTRACT Understanding the kinetics and mechanisms of strain relaxation in Si-Ge heteroepitaxial films is pertinent to several device applications. In this paper we present a method for determining the evolution of the mobile dislocation density with time during the course of strain relaxation taking place in an isothermal annealing experiment. Wafer curvature measurements using a laser scanning technique are used to determine the elastic strain after growth in films of variable thickness and to follow the strain relaxation during isothermal annealing experiments. By coupling the strain relaxation measurements with previous TEM measurements of dislocation velocities in this system, the mobile threading dislocation density and its evolution with time are determined.
INTRODUCTION Lattice mismatch between a heteroepitaxial Si-Ge film and a Si substrate causes elastic misfit strain to exist in the film. When the film thickness is higher than a critical value [1], the biaxial stress associated with this elastic strain can serve as driving force for the movement of threading dislocations. As they move through the film, these threading dislocations lay down misfit dislocations at the film/substrate interface and this causes the elastic strain to relax. Several device applications of Si-Ge heteroepitaxial films are based on the significant reduction of the indirect bandgap caused by the misfit strain [2]. For these applications, relaxation of the misfit strain is not desirable. There exist other applications for which there is a need for a high density of misfit dislocations in conjunction with a low density of threading dislocations [3]. In both cases, understanding of the kinetics of relaxation, as well as the mechanisms involved is important to the development of reliable devices. The biaxial stress in the film causes elastic bending of the substrate. The resulting substrate curvature is related to the elastic strain in the film through the following equation. =Mshh2 AK Mf 6hf
(1)
where M represents biaxial modulus, h represents thickness and AK is the difference in curvature between bare substrate and substrate with film. The subscripts "s"and "f" refer to substrate and film, respectively. During isothermal annealing experiments, the change in substrate curvature with time is measured with a laser scanning system [4]. The relaxation of elastic strain and the corresponding evolution of plastic strain are found using the above formula. The kinetics of formation of misfit dislocations can be studied by considering that they are deposited at the film/substrate interface by threading dislocations moving through the film. Since each threading dislocation creates a misfit dislocation as it moves, the mean spacing, S, between misfit dislocations varies with time a
Data Loading...
