J. M. Gibson

Ion implantation in GaAs

Recent advances in the understanding of the relationship between implanted dopant solubility and electrical activity in GaAs are reviewed and direct lattice configuration measurements explaining these results are presented. The nature of residual defects remaining after activation annealing of GaAs, in particular rapid thermal annealing, are discussed, along with a review of the application of ion beams in promoting compositional disordering of GaAs-AlAs superlattices. The outstanding problems remaining in the use of ion implantation technology for GaAs are also detailed.

Hydrogen surface coverage: raising the silicon epitaxial growth temperature

The chemisorption of molecular hydrogen onto the Siu2009(100) surface is shown to disrupt the epitaxial growth of silicon and silicon/germanium alloys grown by molecular beam epitaxy. It is only after the substrate temperature is raised above the hydrogen desorption temperature, or the deposition rate is lowered, that high quality single‐crystal films can be grown. The results also suggest the surface segregation of hydrogen during growth.

Plan‐view transmission electron diffraction measurement of roughness at buried Si/SiO2 interfaces

We have developed a novel technique for determining interfacial roughness from plan‐view transmission electron diffraction. Certain bulk forbidden Bragg reflections can occur due to crystal termination at surfaces and are very sensitive to steps on crystal boundaries. We demonstrate the technique in the study of Si/SiO2 interfaces and observe that interfaces appear to be significantly flatter than previously found, especially after post‐oxidation annealing. The technique is simply quantified and is more reliable than those which require stripping of the oxide to expose the interface.

Comparison of conductivity produced in polymers and carbon films by pyrolysis and high energy ion irradiation

We compare the effects of pyrolysis and 2 MeV Ar+ ion irradiation in modifying the conductivity of polymeric and carbon films. Chemical degradation (in the polymer films) and structural rearrangement (in both polymer and carbon films) are introduced by either process. Metallic carrier densities (1022 − 1023cm−3) have been observed in these films by Hall measurements. Transmission electron microscope (TEM) images show the formation of graphitic ordering with correlation lengths ∼ 20 A in the highly conductive films, with planes oriented parallel to the film surface. Crystalline graphite has a small overlap of the conduction and valence bands (≲ 30 meV), and consequently a low carrier density (∼ 1018 cm−3). We believe the modified carbon films to have larger overlap of the bands (∼1 eV) at the Fermi level, due to significant smearing of the carrier energy associated with the short scattering (correlation) lengths. While pyrolysis and ion irradiation produce similar end results, ion irradiation can yield ultimately higher carrier densities in these films as the ion-annealing process is self-limiting in terms of the extent of crystalline order produced in the film.

Characterization of GaAs layers grown directly on Si substrates by metalorganic chemical vapor deposition

The electrical and structural properties of GaAs layers grown directly by metalorganic chemical vapor deposition on Si substrates oriented 2° off (100) toward [011] are reported. The uniformity of minority‐carrier lifetime in the 2 in.‐ diam heteroepitaxial wafers is comparable to that in bulk GaAs of the same doping density (2×1016 cm−3). Selective etching of the GaAs layer reveals an etch pit density of ∼108 cm−2, consistent with plan view transmission electron microscopy which shows a defect density of ∼108 cm−2. Rapid annealing at 900u2009°C for 10 s does not significantly alter the heterointerface abruptness, and at the same time the crystalline quality of the GaAs improves slightly. The deep level concentration in the as‐grown layer is ∼1013 cm−3 as determined by capacitance spectroscopy. Finally, the activation characteristics of low dose Si implants (3×1012 cm−2 at 60 keV) are similar to those in high quality bulk GaAs.

Trapping of oxygen at homoepitaxial Si‐Si interfaces

Microstructural studies of Si‐Si interfaces grown by molecular beam epitaxy reveal pockets of oxygen‐rich material for certain substrate preparation conditions. For Czochralski substrates which are cleaned using an argon ion sputtering technique, a high density (about 1011/cm2) of oxygen‐rich pockets about 30 A in size is observed using cross‐sectional transmission electron microscopy and x‐ray microanalysis for short (less than about 15 s) sputtering times. For longer sputtering times, no significant defect density is observed. Post‐deposition thermal annealing causes a dramatic increase in the density of oxygen‐rich defects, and it is suggested that this is due to trapping of oxygen which has diffused from the bulk of the substrate wafer. For (100) interfaces, no significant dislocation activity is associated with the defects, even for densities up to 1012/cm2. On (111) interfaces, however, a large planar fault density is observed. Ramifications for silicon homoepitaxy are discussed.

Activation characteristics and defect structure in Si-implanted GaAs-on-Si

Undoped metalorganic chemical vapor deposited GaAs layers on Si substrates were implanted with 29Si ions (5×1012 cm−2 dose at 100 keV energy) to form a shallow n‐type region. The net donor activation (74%) and electron mobility (3014 cm2u2009V−1u2009s−1) after rapid thermal annealing (900u2009°C, 10 s) were compared to those obtained for similar implants into bulk GaAs. There was a slight improvement in the proton backscattering yield from the GaAs‐Si interface region after the annealing cycle, consistent with cross‐sectional transmission electron microscopy data showing an alignment of defects in annealed samples.

Imaging of defects and recrystallization studies in ion implanted graphite

This work reports the study of ion-implantation induced damage and annealing in the highly anisotropic semimetal graphite using high resolution transmission electron microscopy. The recrystallization process and regrowth kinetics are compared for highly oriented pyrolytic graphite (HOPG) and benzene derived graphite fibers as host materials. We obtain activation energies of ∼ 0.7 eVatom for grain growth processes along both the c-axis and in the graphite basal plane for the fiber host. Post-ion implantation annealing of HOPG shows two competing processes; epitaxial regrowth and regrowth with random orientation of the crystallites. For both host materials, time dependences of ∼ t12 and t14 are obtained for the regrowth kinetics in the basal plane and along the c-axis, respectively. The regrowth process is explained by the annealing of dislocations.

Trapping of Oxygen at Homoepitaxial Si-Si Interfaces Grown by Molecular Beam Epitaxy

Interfaces between Si substrates and epitaxial Si buffer layers grown by Molecular Beam Epitaxy (MBE) are shown to contain a high density of SiO x pockets for certain sustrate preparation conditions. It is also shown that post-deposition thermal annealing of these structures grown upon Czochralski wafers can lead to a greatly increased defect density at the interface. The primary model proposed for this increase is trapping of background oxygen diffusing from the bulk of the Czochralski substrate wafers.