L. J. Schowalter

Epitaxial growth and characterization of CaF2 on Si

CaF2 films have been grown epitaxially on (100) and (111) Si substrates by molecular beam epitaxy. These films have been characterized by electron microscopy, reflection high‐energy electron diffraction, Rutherford backscattering ion channeling, and back‐reflection Laue x‐ray diffraction. In addition, chemical etching has been used to reveal dislocations and to delineate cracks. Film cracking appears to be related to crystalline perfection through misfit dislocation mobility. It is possible to grow high quality, (xmin=3.0%) single‐crystal films on (111) Si which are free of cracks and atomically flat. However, the high free energy of the (100) surface in an ionic fluorite crystal prevents the growth of comparable CaF2 films on the (100) Si surface.

Strain measurement of epitaxial CaF2 on Si (111) by MeV ion channeling

Planar strain in CaF2 films, grown by molecular beam epitaxy at 700 °C on (111) Si substrates, has been measured by MeV 4He+ ion channeling. For CaF2 films thinner than 200 nm, the planar strain was found to be tensile. No strain was observed for films thicker than 200 nm. The observed tensile strain cannot be explained by a simple pseudomorphic growth model since the planar strain would be compressive due to the larger lattice constant of CaF2 relative to Si. A plausible explanation of the results is that defects are nucleated at the growth temperature to relieve stress. These defects then result in a tensile planar strain as the sample is cooled down after growth due to the large difference in thermal expansion coefficients between CaF2 and Si.

Surface morphology of epitaxial CaF2 films on Si substrates

The surfaces of epitaxial CaF2 layers grown on (100) and (111) Si by molecular beam epitaxy have been studied using scanning electron microscopy. The (111) surface exhibits small triangular hillocks, while the (100) surface exhibits a columnar structure. This latter structure can be accounted for by the prohibitively large free energy of the (100) surface. A dipole moment exists perpendicular to this surface which causes the electrostatic energy to diverge. This phenomenon explains the inferior (100) growth as compared to (111) and has important implications for possible applications of group II‐A fluoride/semiconductor epitaxial structures.

Steering effect at a strained NiSi2/Si (001) interface

Abstract In a 700 A thick NiSi 2 , epitaxial film grown on Si (001), tensile strain due to the lattice mismatch was observed by means of 1.0–3.5 MeV He ion channeling angular scans about three different off-normal channeling axes in major planes, i.e. [Ill] in (110), [112] in (110) and [Oil] in (100). Because of angular misalignment between the NiSi 2 ; and the Si substrate (Si sub ) off-normal channeling axes, peculiar behaviors of channeled ions were observed in the Si sub angular scan profiles. Namely, at ion incident angles corresponding to the NiSi 2 axes, the Si sub yield changed dramatically with the incident ion energy, while at incident angles corresponding to the Si Sub , axes, the Si Sub yield showed no energy dependence. Considering a relation between the angular misalignment and the channeling critical angle (which depends on the ion energy and the channeling axis), the energy dependent behavior is explained by the contribution of channeled ions steered into the Si Sub axial channels, i.e. the steering effect. The nature of the channeling minimum when ions are incident along the Si Sub axial direction suggests that a transition from planar-like motion to axial motion occurs.

Schottky barrier height measurements of epitaxial NiSi2 on Si

Photoresponse measurements of the Schottky barrier heights of epitaxial NiSi2 layers on nondegenerate n‐(111) Si, for type‐A and type‐B orientations, have been performed. The type‐A and type‐B cases are consistently observed to differ in barrier height by greater than 0.1 eV. We obtain measured values for φB0 (at T=300 K) of 0.62±0.01 eV and 0.77±0.05 eV for type A and B, respectively.

Heteroepitaxy of semiconductor‐on‐insulator structures: Si and Ge on CaF2/Si(111)

Si and Ge layers have been grown on CaF2/Si(111) by molecular‐beam epitaxy. Both Ge and Si grow as islands, and both the island size and the spacing between nucleation sites are considerably larger for Ge (∼300 nm) than for Si (∼100 nm). In addition, Ge and Si layers are found to be a mixture of type‐A (aligned with the underlying CaF2) and type‐B (rotated 180° about the surface normal with respect to the underlying CaF2) regions. The crystalline quality and surface morphology of the Ge layers are much better than those of the Si layers. This is thought to be due to the larger island size of the Ge deposits and to a greater ease of movement of the boundaries between type‐A and type‐B regions in Ge. Hall measurements show electron mobilities of up to 664 cm2/V s in Si layers and hole mobilities of 234 cm2/V s for Ge layers. Finally, the use of a GexSi1−x‐Si superlattice, when grown on a GexSi1−x buffer layer which is lattice matched to the CaF2 at the growth temperature, is shown to improve Si heteroepitaxy.

GROWTH AND CHARACTERIZATION OF EPITAXIAL Si/CoSi 2 AND Si/CoSi 2 /Si HETEROSTRUCTURES

Thin ( 2 films have been grown on (111) Siwafers in a UHV system using a variety of growth techniques including solid phase epitaxy (SPE), reactive deposition epitaxy (RDE), and molecular beam epitaxy (MBE). SEN and TEN studies reveal significant variations in the epitaxial silicide surface morphology as a function of the sillciqd formation method. Pinhole densities are generally greater than 10 7 cm -2 , although some reduction can be achieved by utilizing proper growth techniques. Si epilayers were deposited over the CoSi 2 films inthe temperature range from 550oC to 800oC, and the reesuulttinng structures have been characterized using SEM, cross—sectional TEN, and ion channeling measurements. These measurements show that the Si epitaxial quality increases with growth temperature, although the average Si surface roughness and the CoSi 2 pinhole density also increase as the growth temperature is raised.

Heteroepitaxy of insulator/metal/silicon structures: CaF2/NiSi2/Si(111) and CaF2/CoSi2/Si(111)

Epitaxial insulator (CaF2) layers have been grown on epitaxial metal (CoSi2 and NiSi2) layers on Si(111) by molecular beam epitaxy. The surface morphology and bulk crystallinity are much better for growth on NiSi2, with scanning electron microscopy revealing only small triangular hillocks, and channeling minimum yields as low as 3% measured in the CaF2 using 2.5 MeV 4He+ ions. CaF2 layers grown at 650 °C on CoSi2 consist of a mixture of regions either aligned or rotated 180° with respect to the CoSi2 lattice, while CaF2 layers grown at 550 °C on NiSi2 are of a single orientation, regardless of the orientation of the NiSi2 with respect to the Si substrate.

ELECTRICAL PROPERTIES AND STRUCTURAL DEFECTS IN EPITAXIAL CaF//2 ON Si.

In this paper, a study is made of the electrical properties and of the types of structual defects which can occur in epitaxial CaF 2 grown on Si(111) substrates by molecular beam epitaxy (MBE). High-resolution transmission electron microscopy (HRTEM) and high-energy (≥2 MeV) He + ion channeling techniques are used to characterize defects in the epitaxial layer and at the CaF 2 /Si interface while current-voltage (I–V) and capacitance-voltage (C–V) 3 measurements are used to characterize the electrical properties. The HRTEM images show an atomically abrupt interface between the CaF 2 and Si. The most common defect we have been able to identify is associated with an atomic step at the interface and is similar to a Shockley partial dislocation at a (111) twin interface in an fcc crystal structure. The ion channeling measurements also indicate the presence of defects at the CaF 2 /Si interface. The apparent defect density measured by ion channeling decreases in the CaF layer as one moves away from the interface at a rate which depends on ihe final thickness of the epitaxial film. Ion channeling has also been used to measure strain and it is found that, while thin layers of epitaxial CaF 2 on Si(111) have large strain, the strain becomes vanishingly small as the layers exceed 200 nm in thickness. This result can be adequately explained using an equilibrium model for the introduction of strain relieving misfit dislocations and indicates that epitaxial fluoride layers may be useful as thermal mismatch buffers in heteroepitaxial structures. In C–V measurements of epitaxial CaF 2 layers on Si(111) which have been fabricated into metal-insulator-semiconductor structures, the capacitance is observed to remain constant over a large variation in applied voltage. This constant capacitance region can be explained as due to a high density of interface states in the band gap. In addition, I–V measurements indicate that, at low fields, the CaF 2 resistivity exceeds 10 14 Ω-cm. At high fields, the CaF 2 starts to conduct in a manner which we speculate is due to defects at the CaF 2 /Si interface. The field at which this conduction takes place has been ovserved to exceed 1 MV/cm for a 42-nm-thick CaF 2 film with the device geometry used for this work.

Linear energy dependence of the interface peak intensity

Abstract The MeV He ion channeling technique was utilized for characterizing the interfacial defects at MBE grown CaF2/Si (111) interfaces. The Ca interface peak (IP) intensity was measured as a function of the incident energy (E). The energy dependence of the IP intensity was observed to be E1 .This E1 dependence cannot be explained by scattering from random defects or by dechanneling. The E1 dependence is explained by considering direct scattering of channeled ions from curved atomic strings in misfit dislocation network regions.