B. A. Orner

Growth of germanium-carbon alloys on silicon substrates by molecular beam epitaxy

Metastable Ge1−yCy alloys were grown by molecular beam epitaxy as homogeneous solid solutions having a diamond lattice structure. The substrates were (100) oriented Si wafers and the growth temperature was 600 °C. We report on measurements of the composition, structure, lattice constant, and optical absorption of the alloy layers. In thick relaxed layers, C atomic fractions up to 0.03 were obtained with a corresponding band gap of 0.875 eV. These alloys offer new opportunities for fundamental studies, and for the development of silicon‐based heterostructure devices.

Optical and electronic properties of SiGeC alloys grown on Si substrates

Abstract Metastable Si 1 − x − y Ge x C y alloys were grown by molecular beam epitaxy on (100) Si substrates. Solid elemental sources were used for the Si and Ge beams, and a resistively heated graphite filament was used for the C beam. Up to 3 at% of C was incorporated in the alloy layers. Optical transmission measurements showed that the absorption edge of thick layers increased to higher energies with increasing C fraction, and revealed the presence of SiC and GeC vibrational modes in the infrared. At low temperatures, the alloys showed significant photoluminescence. The bandgap energies of thick layers increased linearly with the C fraction and followed a linear dependence of the bandgap on composition. Measurements of the valence band density of states using X-ray photoelectron spectroscopy indicated that the valence band energy maximum increased with the C fraction relative to that of SiGe alloys of similar composition. Our results indicated that SiGeC alloys are promising materials for Si-based heterostructure devices.

Si1−x−yGexCy alloy band structures by linear combination of atomic orbitals

We have applied a virtual crystal approximation to the linear combination of atomic orbitals method to calculate critical point energies of unstrained Si1−x−yGexCy alloys spanning the composition parameter space. Additionally, we have calculated the band structure across the Brillouin zone for a series of alloy compositions. We found the band energies had significant bowing departures from linearity throughout the system. In some cases, the energy band gap was not monotonically dependent on composition. Our theoretical results are compared with recent experimental results, and good agreement was found overall.

Band gap of Ge rich Si1−x−yGexCy alloys

Si1−x−yGexCy films ( x≊0.90, y⩽0.02) were grown by molecular beam epitaxy on Si substrates. Infrared optical absorption was used to obtain the band gap energy at room temperature. Biaxial strain obtained from x‐ray diffraction measurements verified the presence of nearly relaxed films, and the total and substitutional C contents were obtained from channeling C‐resonance backscattering spectrometry. We show by direct measurements that interstitial C had a negligible impact on the band gap, but substitutional C was found to increase the band gap with respect to equivalently strained Si1−xGex alloys. While strain decreases the band gap, the effect of substitutional C on the band gap depends on the Si and Ge fractions.

1.3 μm photoresponsivity in Si-based Ge1−xCx photodiodes

Ge1−xCx/Si heterostructure photodiodes with nominal carbon percentages (0⩽x⩽0.02), which exceed the solubility limit, were grown by solid source molecular beam epitaxy on n-type (100) Si substrates. The p-Ge1−xCx/n-Si photodiodes were fabricated and tested. The p-Ge1−xCx/n-Si junction exhibits diode rectification with a reverse saturation current of about 10 pA/μm2 at −1 V and high reverse breakdown voltage, up to −80 V. A significant reduction in diode reverse leakage current was observed by adding C to Ge, but these effects saturated with more C. Photoresponsivity was observed from these Si-based p-Ge1−xCx/n-Si photodiodes at a wavelength of ⩾1.3 μm, compatible with fiber optic wavelengths. External quantum efficiency of these thin surface-normal photodetectors was measured up to 2.2%, which decreased as the carbon percentage was increased.

Optical properties of Ge 1-y C y alloys

The Ge1-yCy semiconductor alloy system offers promise as a material for use in heterostructure devices based on Si as well as other materials. We have grown Ge1-y Cy alloys by solid source molecular beam epitaxy on Si substrates. Layer thicknesses ranged from 0.01 to 3 µm, and Auger electron spectroscopy and secondary ion mass spectrometry indicated C fractions up to 3 at. %. Optical absorption in the near-infrared region indicated a shift in the energy bandgap from that of Ge which was attributed to the effects of alloying. The dependence of the bandgap on composition was consistent with linear interpolations of the Ge and C conduction band minimums. We observed a fundamental absorption edge characteristic of an indirect bandgap material. Photoluminescence spectra at 11K of thick, relaxed layers indicated single broad peaks near the expected bandgap energy.

Optical absorption in alloys of Si, Ge, C, and Sn

Group IV semiconductor alloy systems offer promise as variable band gap alloys compatible with Si technology. Binary, ternary, and quaternary group IV alloys were grown by molecular beam epitaxy on Si substrates. The fundamental absorption edge was measured by Fourier transform infrared spectroscopy to obtain the optical band gap of the alloys, and the position of the fundamental absorption edge was observed to depend on the experimentally measured alloy composition. Our results indicate a variety of Si‐rich group IV alloys with various band gaps are experimentally producible.

Dielectric response of thick low dislocation-density Ge epilayers grown on (001) Si

Spectroscopic ellipsometry was used to measure the dielectric functions of epitaxial and bulk Ge at photon energies from 1.5 to 5.2 eV. The epitaxial Ge was grown at 400 °C by molecular beam epitaxy on (001) Si substrates. The optical response and the interband critical‐point parameters of Ge on Si were found to be indistinguishable from that of bulk single crystal Ge, indicating high optical quality. Dislocation density measurements using an iodine etch verified low surface defect densities. We conclude that epitaxial Ge grown on Si at relatively low temperatures is suitable for optical device applications.

Current transport characteristics of SiGeC/Si heterojunction diode

The characteristics of heterojunction diodes fabricated from p-type epitaxial Si/sub 0.07/Ge/sub 0.91/C/sub 0.02/ alloy grown by molecular beam epitaxy on n-type Si

STRAIN MODIFICATION IN THIN SI1-X-YGEXCY ALLOYS ON (100) SI FOR FORMATION OF HIGH DENSITY AND UNIFORMLY SIZED QUANTUM DOTS

The effects of alloying C with Ge and Si and varying the C/Ge ratio during the growth of very thin layers of the ternary alloy SiGeC grown on Si (100) substrates and the resulting strain modification on self-assembled and self-organized quantum dots are examined. During coherent islanded growth, where dislocations are not formed yet to relieve the strain, higher strain energy produced by greater lattice mismatch acts to reduce the island size, increase the density of islands, and significantly narrow the distribution of island sizes to nearly uniformly sized quantum dots. Strain energy can also control the critical thickness for dislocation generation within the three-dimensional islands, which then limits the maximum height which coherent islands can achieve. After the islands relax by misfit dislocations, the island sizes increase and the island size distribution becomes broader with the increase of misfit and strain. The optimal growth for a high density of uniform coherent islands occurred for the Si0...