S. H. Li

Carrier velocity‐field characteristics and alloy scattering potential in Si1−xGex/Si

The alloy scattering potential is an important parameter in SiGe alloys since it not only affects the velocity‐field characteristics for carrier transport, but also allows increased optical transitions by relaxing k‐selection rules. In this letter, we report on the velocity‐field measurements for relaxed and coherently strained SiGe alloys. The alloy scattering potential is obtained from a careful fit to the data. The hole velocity at any field is found to have a bowing behavior as a function of alloy composition. This reflects a strong alloy scattering potential which is calculated to be 0.6 eV for the valence band.

Low-temperature photoluminescence of SiGe/Si disordered multiple quantum wells and quantum well wires

Low-temperature photoluminescence from disordered SiGe/Si quantum wells and quantum wires made from periodic quantum wells by electron beam lithography and reactive ion etching has been measured. No enhancement in luminescence is seen, compared to that in periodic quantum wells, in the disordered wells or quantum wires. New transitions are observed in the wire luminescence, including a possible no-phonon transition exhibiting a 32 meV blue shift compared to the same transition in the wells.

Direct measurement of the Hall factor for holes in relaxed Si1−xGex (0<x<1)

The Hall factor for holes in relaxed p‐type Si1−xGex alloys has been determined from mobility measurements at magnetic fields up to 7 T at 290 K. Our data together with previously published values for Si and Ge suggest that r for holes in SiGe varies between 0.73 and 1.7 with a possible strong bowing.

Low temperature silicon epitaxy using supersonic molecular beams

Epitaxy of silicon using a non-Maxwellian beam source is demonstrated. Pulses of monoenergetic, high kinetic energy supersonic beams of a Si 2 H 6 -H 6 mixture are directed at the substrate leading to epitaxial growth. Reflection high energy electron diffraction (RHEED) intensity variations are observed for low temperature growth (< 500°C) of silicon on Si(100). Arrhenius plots from these data indicate surface hydrogen desorption processes. Cross sectional transmission electron microscopy has been used to characterize the thin films for thickness and crystallinity. Results show that good quality crystalline films can be obtained at temperatures as low as 300°C with this novel technique

Gas‐source molecular‐beam epitaxy using Si2H6 and GeH4 and x‐ray characterization of Si1−xGex (0≤x≤0.33) alloys

Gas‐source molecular‐beam epitaxy (MBE) has been used to grow SiGe alloys with Si2H6 and GeH4 as sources on (100) Si substrates. Single‐crystalline epilayers with Ge composition as high as 33% have been produced at 610 °C, the lowest temperature hitherto used for gas‐source SiGe MBE. Growth parameters, growth modes, and the structural characteristics have been studied by a variety of in situ and ex situ techniques. Double‐crystal x‐ray diffraction data for the alloys have been obtained for the first time in thin mismatched layers.

Molecular beam epitaxial growth of Si 1-x Ge x /Si pseudomorphic layers using disilane and germanium

Molecular beam epitaxial growth of pseudomorphic Si1−xGex/Si layers using disilane (Si2H6) and elemental germanium has been studied for the first time. It is found that at a fixed flow rate of Si2H6, the germanium content in the Si1−xGex alloys is a function of the germanium cell temperature. Heterostructures and multi-quantum wells with good surface morphology, excellent crystalline quality, and abrupt interfaces are demonstrated, indicating little or no sourcerelated transient effects.

Modeling growth of Si1−xGex epitaxial films from disilane and germane

A Langmuir–Hinshelwood‐type kinetic model is developed for modeling growth of silicon–germanium alloys from disilane and germane on Si substrates. Gas source molecular beam epitaxy was employed to grow Si1−xGex films at various germanium fractions, x, in the alloy and at different temperatures. The model correctly predicts experimentally observed and previously reported behavior; a monotonic decrease with germanium fraction at higher substrate temperatures (700 °C) and a maximum in the growth rate for lower temperatures (550 °C and 610 °C).

Impact ionization coefficients in Si1−xGex

We have measured the electron and hole impact ionization coefficients in Si1−xGex alloys. Carrier multiplication measurements were made on relaxed Si1−xGex/Si diodes grown by gas source molecular beam epitaxy. The hole to electron impact ionization coefficient ratio, β/α, varies from 0.3 to 4 in the composition range of x=0.08–1.0.

Deep levels in undoped Si 1-x Ge x grown by gas-source molecular beam epitaxy

Deep levels have been identified and characterized in undoped Si1−xGex alloys grown on silicon substrates by gas-source molecular beam epitaxy. Hole traps in the p-type layers have activation energies ranging from 0.029–0.45 eV and capture cross sections (σ∞ ranging from 10−9 to 10−20 cm2. Possible origins of these centers are discussed.

Donor‐doping characteristics of gas‐source molecular beam epitaxial Si and Si1−xGex using phosphine

Well‐behaved and reproducible n‐type doping of Si and Si1−xGex by phosphine during gas‐source molecular beam epitaxy is demonstrated. No significant reduction of growth rate of these materials in the presence of phosphine is recorded in the doping range of 1017–1019 cm−3 and perfect surface morphologies are observed. The incorporated P atoms are fully activated without ex situ annealing. The doping profiles are well defined in both Si and Si1−xGex layers. A p‐Si0.9Ge0.1/n‐Si heterojunction diode made with boron and phosphine doping has demonstrated excellent rectifying characteristics.