Yong‐Fen Hsieh

Mechanically and thermally stable Si‐Ge films and heterojunction bipolar transistors grown by rapid thermal chemical vapor deposition at 900 °C

Traditional techniques for growing Si‐Ge layers have centered around low‐temperature growth methods such as molecular‐beam epitaxy and ultrahigh vacuum chemical vapor deposition in order to achieve strain metastability and good growth control. Recognizing that metastable films are probably undesirable in state‐of‐the‐art devices on the basis of reliability considerations, and that in general, crystal perfection increases with increasing deposition temperatures, we have grown mechanically stable Si‐Ge films (i.e., films whose composition and thickness places them on or below the Matthews–Blakeslee mechanical equilibrium curve) at 900 °C by rapid thermal chemical vapor deposition. Although this limits the thickness and the Ge composition range, such films are exactly those required for high‐speed heterojunction bipolar transistors and Si/Si‐Ge superlattices, for example. The 900 °C films contain three orders of magnitude less oxygen than their limited reaction processing counterparts grown at 625 °C. The fi...

Magnetic properties and microstructure of Fe4N and (Fe,Ni)4N

Single‐phase Fe4N and (Fe1−xNix)4N compounds have been synthesized in a continuous form by heat treating iron and iron‐nickel alloy sheets at various temperatures under NH3/H2 atmospheres. The Fe4N sheet has a high room‐temperature magnetization value of 179 emu/g (2.14 μB/Fe), which is only slightly less than 218 emu/g (2.19 μB/Fe) observed in pure iron. The magnetic moments of the Fe‐Ni alloy nitrides decreased monotonically as x was increased, in contrast to those for the starting alloys Fe1−xNix which exhibited a peak value around x=0.05. The decrease in magnetic moment with nickel content in the alloy nitrides was close to the value anticipated by magnetic dilution from nickel. The coercive force is about 5 Oe and is slightly decreased by the Ni substitution. The Fe‐nitride offers a significantly improved corrosion resistance over pure iron. Even further improvement is obtained in the (Fe1−xNix)4N system with only slight sacrifice in magnetic moment. The addition of nickel has been found to noticeabl...

Growth of epitaxial Ba2YCu3O7−x films on LaAlO3 (001)

Abstract We report the ex situ growth of 1000 and 2000 A epitaxial Ba2YCu3O7−x (BYCO) filmsonLaAlO3 (001 with surface morphologies and crystallinity generally associated with high quality in situ films. Films are grown by co-depositing BaF2, Y and Cu in a stoichiometric ratio within 1% of 2:1:3, followed by annealing in a two-stage process in a tube furnace. By optimizing the annealing conditions, excellent crystallinity is obtained, with χmin∼ 2–4] from Rutherford backscattering/channeling. These films have sharp superconducting resistance transitions at 90–91 K. Critical current densities at 77 K are ∼ 106 A/cm2 in zero magnetic field and ⪅ 105 A/cm2 in H =0.9 T oriented perpendicular to the ab plane of the films.

Exploiting Si/CoSi2/Si heterostructures grown by mesotaxy

Abstract Buried single-crystal silicide layers in silicon formed by ion implantation and annealing have many potential applications (some more practical than others!) including metal base transistors (operating at 77 K), buried collector contacts for bipolar transistors and buried groundplanes for ultrahigh-speed electronics. Fabrication of prototype devices is complicated by the presence of defects in the overlayer silicon, but preliminary results are reported.

Formation of cobalt silicide in Co+ implanted Si(111)

The microstructural variation of CoSi2 buried layers formed by 100 keV Co+ implantation at 350 °C into Si (111) is systematically studied. The critical dose dc of Co+ implantation at 100 keV required to form a continuous CoSi2 buried layer after annealing is the same in both Si (111) and (001), ≊1.1×1017 cm−2, corresponding to a threshold peak concentration of 18.5 at. % Co. In addition, we observe continuous buried layers consisting of both A‐(fully aligned) and B‐(twinned) CoSi2 grains in the (111) samples implanted at doses ≊ dc. The relative fractions of A and B are found to vary with the implanted doses, current densities of the ion beam, and annealing conditions with the B fractions varying from 0% to 100%. Continuous A‐type layers are formed only in the samples implanted to doses ≥1.6×1017 cm−2.

Microstructure of epitaxial YBa2Cu3O7−x thin films grown on LaAlO3 (001)

We report a microstructural investigation of the epitaxial growth of YBa2Cu3O7−x (YBCO) thin films on LaAlO3 (001) substrates using transmission electron microscopy (TEM). Epitaxial films grow with two distinct modes: c epitaxy (YBCO) single crystal with the c axis normal to the surface) and a epitaxy (YBCO) single crystal with the c axis in the interfacial plane), where c epitaxy is the dominant mode grown in all samples 35–200 nm thick. In 35 nm YBCO films annealed at 850 °C, 97±1% of the surface area is covered by c epitaxy with embedded anisotropic a‐epitaxial grains. Quantitative analysis reveals the effect of film thickness and annealing temperature on the density, grain sizes, areal coverages, and anisotropic growth of a epitaxy.

Amorphization and regrowth in si cosi2 si heterostructures

Reduction of the defect density in the Si overlayer of Si/CoSi2/Si heterostructures fabricated by mesotaxy has been achieved by selective amorphization and regrowth of the Si. Layer‐by‐layer regrowth of the silicide with an activation energy of 1.14 eV has also been clearly shown.

Evolution of buried compound layers formed by ion implantation

Abstract The coalescence of buried silicide layers formed by high implantation in silicon and high temperature annealing occurs via a precipitate coarsening mechanism that is different in (100) and (111) silicon. The results of an extensive study of these phenomena are summarized and compared with the formation of amorphous SiO2 by implantation.

Coalescence of buried CoSi2 layers formed by mesotaxy in Si(111)

Evolution and microstructure of buried CoSi2 layers formed by 100‐keV Co+ implantation at 350 °C into Si(111) are systematically studied in this work. Implant doses ranged from 1 × 1016 to 1.6 × 1017 cm−2. Three types of CoSi2 precipitates are observed during the mesotaxial process: A type (fully aligned with the host lattice), B0 type (twinned on the (111) plane parallel to the surface), and B1,2,3 type (twinned on one of the three {111} planes inclined to the surface). The fraction (probability of occurrence) of each type varies with both the implantation and annealing conditions. The critical dose required to form a continuous layer after 600+1000 °C annealing is found to be the same, ≊ 1.1 × 1017 cm−2, in both (111) and (001) substrates, despite pronounced differences in precipitate morphology. Formation of a continuous, twinned (B0 type), buried layer after 600+1000 °C annealing is shown to be possible in (111) samples implanted at the critical dose.

Interfacial structure and its effect on nucleation and growth energetics in mesotaxial Si/CoSi2/Si structures

We show how analysis of the stacking sequences at CoSi2/Si interfaces formed by 100 kV Co+ implantation into Si (001) or (111) predicts the formation of partial dislocations or stacking faults at precipitate corners. The presence and nature of the stacking fault can uniquely identify the bonding coordination at the CoSi2/Si(111) interface. Consideration of the interfacial structure for twinned (B) and untwinned (A) {111} interfaces helps explain the competitive nucleation and growth of A vs B precipitates.