Substrate Engineering for Microelectronic Devices: Molecular Beam Epitaxial Growth of Gallium Arsenide on Calcium FLUORIDE/SILICON(111) Substrates
Abstract
This thesis addresses the issue of residual strain in heteroepitaxial layers due to lattice mismatch and thermal expansion coefficient mismatch. In particular, we have tried to solve this problem in desirable heteroepitaxial layers by introducing a CaF_2 strain relaxation buffer. My research focuses on molecular beam epitaxial (MBE) growth of GaAs on CaF_2 /Si(111) substrates. In the thesis, three topics are chosen to be discussed in detail: (1) strain relaxation behavior in epitaxial group-IIa fluoride films, (2) GaAs nucleation on the (111) CaF_2 surface, and (3) optimization of epitaxial growth conditions for GaAs on CaF_2/Si(111) substrates. Residual tensile strain in SrF_2 films has been measured and plotted versus film thickness. An equilibrium force-balance model with a frictional force was proposed to interpret the experimental data. Stress in the CaF_2/Si(111) and SrF _2/Si(111) systems was also measured during thermal cycling from RT to 650^circ C. Both the residual strain versus film thickness and the stress versus temperature show that CaF _2/Si(111) system behaves very differently in strain relaxation than the SrF_2/Si(111) system. In SrF_2 films, residual strain remains even if the film is thicker than 6000A unlike CaF_2 on Si(111) for which CaF _2 films thicker than 3000A are observed to have no strain. It has been observed that, under certain conditions, a chemical reaction between As adatoms and CaF _2 layers can be induced, by which a stable As layer forms on the CaF_2 surface. This As layer modifies the CaF_2 surface free energy and, if properly controlled, leads to 2D nucleation of GaAs on the CaF_2/Si(111) surface as opposed to the more commonly observed 3D growth. A nucleation mode has been proposed to understand the phenomenon. Two kinds of twins have been observed in GaAs films grown on CaF_2/Si(111) substrates: micro-twins and rotational twins. The methods to suppress the formation of twins have been demonstrated. Three growth regimes have been distinguished for epitaxial growth of GaAs on CaF_2/Si(111) substrates. MBE growth conditions for GaAs on CaF _2/Si(111) were optimized. Under the optimal growth condition, excellent crystal quality of GaAs films was achieved (a minimum ion channeling yield of 3.5% is achieved at the GaAs surface) along with specular surface morphology. These GaAs films also exhibit fairly high Hall mobility (mu_{rm n } = 1640 rm cm^2/V{cdot }sec with doping level equal to 3 times 10^{17} cm^{-3}).
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1994
- Bibcode:
- 1994PhDT.......176L
- Keywords:
-
- GALLIUM ARSENIDE;
- CALCIUM FLUORIDE;
- SILICON(111);
- Engineering: Materials Science; Engineering: Electronics and Electrical; Physics: Condensed Matter