A Study Of Epitaxial Growth Of Caf2 On Silicon

The alkaline earth fluorides are good insulators at room temperature and have received significant attention as epitaxial dielectrics on semiconductors, their crystal structure and lattice parameters resembling those of common semiconductors. Such dielectrics enable passivation of semiconductors lacking stable oxides, isolation of devices on one substrate, and fabrication of 3-dimensional epitaxial heterostructures. The CaF, /Si system was the structure investigated since the room temperature lattice mismatch is only 0.6% .A vacuum system was therefore developed for the deposition of CaF, onto silicon, and an RBS system, incorporating detector cooling, developed to establish the dependence of epitaxy on substrate temperature using channelling of 340 keV protons (giving an enhanced depth resolution and improved sensitivity to light elements compared to 2 Mev Het analysis). Epitaxial growth was obtained on n-type $i(111) and 98i(100) substrates at 400-750°C and 575-675°C respectively. A reaction between the CaF, and silicon occurred at higher temperatures producing non-uniform films. The epitaxy was also dependent on film thickness, the optimum de-channelled fractions obtained in the film of 8.25% and 15.2% for Si(111) and Si(100) substrates respectively were unobtainable for films under 1200 A . The insulator surface morphology was examined using Scanning Electron Microscopy. Epitaxial films on S8i(111) were generally smooth, while preferential growth along directions was observed for epitaxial insulators on 8i(100), possibly due to slip along the {111} fluorite cleavage planes resulting from differences in the thermal expansion coefficients of CaF, and silicon and an increase in lattice mismatch with substrate temperature, although no cracking of the insulator was observed. The insulation and electrical properties of the films were investigated. Film resistivities upto 5E8 Jicm and breakdown voltages of approximately 1E5 V/cm were obtained from I-V characteristics. The C-V characteristics of several films exhibited large flat-band voltage shifts and significant hysteresis consistent with poor insulator- semiconductor interfaces and contaminated insulators, possibly caused by a comparatively poor deposition vacuum (1E-7 torr rather than 1E-10 torr or better for MBE-grown films), insufficient control over deposition rate, and/or inadequate substrate preparation. Thus good epitaxy of CaF, was readily obtainable on Si(111) substrates, but proved more difficult on Si(100). The CaF, insulation properties were poor, however, suggesting modifications to the deposition technique and use of higher quality (better than 99%) source material may be necessary to produce satisfactory insulators.