K. Roe

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.

Silicon carbide and silicon carbide:germanium heterostructure bipolar transistors

In this letter, we report on heterostructure bipolar transistors (HBTs) based on silicon carbide (SiC) and a silicon carbide:germanium (SiC:Ge) alloy. The SiC:Ge base alloy was formed by the ion implantation of Ge into p-type 4H–SiC and subsequent annealing. HBT mesa structures were fabricated using a reactive ion etching process. The incorporation of Ge was found to increase the gain and the Early voltage of the devices. A common-emitter current gain (β) of greater than 3 was measured for the SiC:Ge HBTs. Homojunction SiC transistors were fabricated as a reference using the same process (except no Ge in the base region) and exhibited a β of 2.2. The transistors exhibited high breakdown voltages (>50 V without passivation), that typify SiC-based devices. These results indicate that SiC:Ge is a promising material for use in SiC-based heterostructure devices.

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.

The electrical effects of DNA as the gate electrode of MOS transistors

The gate conductor material affects the threshold voltage of metal-oxide-semiconductor (MOS) transistors through the influence of the electrochemical work function and electric charge. Measurements of the threshold voltage from current voltage characteristics may therefore provide a method to estimate the electronic properties of biomolecules located on the gate electrode. We have deposited DNA from the corn genome onto the gate oxide of Si nMOS transistors and measured the effects on the current-voltage characteristics. We found that the DNA decreased the drain-source current compared to devices with clean gate oxides and pure buffer solutions. The threshold voltage was extracted by current-voltage measurements in the linear operating region and was found to increase by +1.9 volts after application of the DNA specimen, a value consistent with the expected negative charge density. This large change suggests that MOS devices may be useful as sensitive bioelectronic detectors.

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...

Ge incorporation in SiC and the effects on device performance

Silicon carbide has been given much attention as a promising material for use in high-voltage and high-power devices. The absence of closely lattice-matched materials precludes the existence of heterostructure devices with good properties. The availability of a lattice-matched heterojunction partner should allow for new SiC-based devices that can exploit the heterojunction band offsets to enhance device properties. Silicon-carbide:germanium (SiC:Ge) alloys were formed by ion implantation of Ge into 4H-SiC wafers at 1000/spl deg/C. We have observed the resultant SiC:Ge material to have favorable properties, such as good crystal structure, interface quality and electrical characteristics. Diodes and bipolar transistors have been fabricated using these layers. These devices have been characterized for properties including forward current density and transistor gain. In this paper we report on the effects of Ge incorporation on devices formed using SiC:Ge layers.

Electrical and optical properties of phosphorus doped Ge1−yCy

In situ n-type doping was investigated for Ge1 - yCy/Si heteroepitaxial layers (y~0.001) for a potential optoelectronic material compatible with Si. Using a solid GaP sublimation source for phosphorus doping, epitaxial Ge1 - yCy films were in situ doped on Si(100) substrates during solid source molecular beam epitaxy and we compare their electrical and optical properties with those of epitaxial Ge on Si. Infrared absorption revealed red shifts in the absorption of visible light with increasing P doping for both Ge 1 - yCy and Ge. The index of refraction decreases for Ge1 - yCy layers compared with Ge. Free carrier absorption increased with increasing phosphorus concentrations, following a wavelength dependence of ~l 3.1 in the region of 10‐20 mm for heavily doped material. Addition of C did not affect the incorporation of P donors in the grown layers or the electrical activation of the donors. An increase in the electron mobility for heteroepitaxial Ge 1 - yCy layers compared with Ge was observed for the doping levels studied.