C. P. Swann

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 600u2009°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.

Band gap bowing and refractive index spectra of polycrystalline AlxIn1−xN films deposited by sputtering

The AlGaInN semiconductor system is currently of high interest for applications in blue light emitting devices. AlInN is a prospective material for lattice matched confinement layers. We measure the refractive index as well as the band gap across the entire compositional range of high-quality polycrystalline AlInN samples. Strong band gap bowing is observed.

The electrical properties of MIS capacitors with ALN gate dielectrics

Abstract We report on the characteristics of metal–insulator–semiconductor (MIS) capacitors with aluminum nitride (AlN) as the dielectric material. Using reactive magnetron sputtering, we deposited layers of AlN on 1–10xa0Ωxa0cm p-type (1xa00xa00) silicon wafers. The deposition rates were investigated as a function of sputter pressure, power, gas composition, and substrate temperature. On films deposited over a range of sputter parameters, X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) were performed indicating that optimal deposition conditions for best crystal quality and stoichiometry were a total pressure between 4 and 10xa0mT, a gas mixture of 85% nitrogen and 15% argon, and a substrate temperature ≈200°C. The films had a weak microcrystalline structure with the c -axis preferentially orientated parallel to the substrate normal. MIS capacitors were fabricated on silicon substrates with Ti/Au contacts. Current–voltage (IV) and capacitance–voltage (CV) measurements revealed breakdown fields of 4–12xa0MV/cm. Depending on the thickness, leakage current densities were between 10 −10 and 10 −3 xa0A/cm 2 at 1xa0V reverse bias, the interface charge density was ≤10 13 xa0cm 2 , and flat band voltages were from −10 to 2xa0V. The dielectric permittivity was between 4 and 11 for thick layers (≥100xa0A) and decreased to values between 2 and 6 for thicknesses below 100xa0A.

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 Siue5f8C and Geue5f8C 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.

The effects of oxidation temperature on the capacitance–voltage characteristics of oxidized AlN films on Si

The thermal oxidation of AlN thin films produces a high quality insulator which exhibits the gate voltage-controlled charge regimes of accumulation, depletion, and inversion on Si surfaces. The temperature dependence of oxidation is important for device processing. We report on the composition, structure, and electrical properties of the AlN versus the oxidization temperature. AlN layers 500 nm thick were deposited by rf sputtering on p-type Si (100) substrates, followed by oxidation in a furnace at temperatures from 800 to 1100u2009°C with O2 flow. An oxidation time of 1 h produced layers of Al2O3 with small amounts of N having a thickness of 33 nm at 800u2009°C, and 524 nm at 1000u2009°C. Electrical measurements of metal-oxide-semiconductor capacitors indicated that the dielectric constant of the oxidized AlN was near 12. The best layer had a flatband voltage near zero with a net oxide trapped charge density less than 1011u2009cm−2. These results show that oxidized AlN has device-grade characteristics for the gate regi...

THERMALLY OXIDIZED ALN THIN FILMS FOR DEVICE INSULATORS

The structural, optical, and electronic properties of an insulating material prepared by the thermal oxidation of AlN thin films on Si have been studied by a number of different experimental techniques. The thermal oxidation at 1100u2009°C of reactively sputtered AlN films on Si wafers was found to result in the formation of an oxide with a relative Al to O concentration near Al2O3 with small amounts of incorporated N. The structure of the AlO:N oxide could be varied between amorphous and polycrystalline, depending on the preparation conditions, and the oxide surface was found to be approximately three time smoother than the as-sputtered AlN films. Metal–oxide–silicon capacitors had an oxide charge density of about 1011u2009cm−2, capacitance–voltage characteristics similar to pure SiO2, and a dielectric constant of 12.4. Infrared measurements yielded a refractive index of 3.9. These results indicate that thermally oxidized AlN films show promise as insulating structures for many integrated circuit applications, p...

Electrical and optical properties of Ge–implanted 4H–SiC

The structural, electronic, and optical properties of single crystalline n-type 4H–SiC implanted with Ge atoms have been investigated through x-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), Raman spectroscopy, and sheet resistivity measurements. Ge atoms are implanted under the conditions of a 300 keV ion beam energy with a dose of 2×1016u200acm−2. X-ray diffraction of the Ge-implanted sample showed broadening of the Bragg peaks. A shoulder on the (0004) reflection indicated an increase in the lattice constant corresponding to substitutional Ge and implantation induced lattice damage, which was repaired through thermal annealing at 1000u200a°C. The diffraction pattern after annealing indicated improved crystal structure and a peak shift to a lower reflection angle of 35.2°. The composition of Ge detected through XRD was reasonably consistent with RBS measurements that indicated 1.2% Ge in a 1600-A-thick layer near the SiC surface. Raman spectroscopy also showed fundamental differences in th...

The electrical characteristics of silicon carbide alloyed with germanium

Abstract As an electronic material for high power, high voltage applications, silicon carbide (SiC) would be more versatile if suitable heterojunction partners were available. Using ion implantation, we have formed alloys of SiC with a few atomic percent of germanium (Ge). The Ge was implanted at 346xa0keV and a dose of 1.67×10 16 xa0cm −2 into a p-type 4H SiC wafer at room temperature, and followed by subsequent annealing up to 1700°C. Through X-ray diffraction (XRD) measurements it was determined that the Ge implanted SiC had a larger lattice constant which implies that some of the Ge is substitutional. The X-ray measurements indicated that a secondary peak attributable to substitutional Ge increases in intensity and shifted toward a lower Bragg angle as the implanted Ge dose was increased. SiC/SiC:Ge heterojunction devices were formed using titanium/gold (Ti/Au) as electrical contacts. Current–voltage ( I – V ) and capacitance measurements confirmed a reduction of the forward voltage drop and built-in voltage in SiC:Ge, compared to similar SiC devices without Ge. Other p-type substrates implanted with Ge that used chromium/nickel (Cr/Ni) metalization as electrical contacts were shown to have a significantly lower contact resistance compared to SiC. These results indicate that the SiC/SiC:Ge material system may be promising for SiC heterojunction device applications.

The electrical and optical properties of thin film diamond implanted with silicon

Abstract The superb mechanical and electrical properties of diamond make it an attractive material for use in extreme conditions. Diamond devices have been fabricated, but the combination of diamond with other materials to form alloys is not yet well-understood. We have investigated the electrical and optical properties of diamond implanted with Si, which is in principle isoelectronic in diamond. The diamond layers were 23xa0μm thick p-type layers grown on Si(1xa00xa00) substrates. Silicon was implanted at room temperature, at energy of 300xa0keV with doses of up to 8.0×10 16 xa0cm −3 . A two-stage post-implant anneal process was then performed at 500°C for 30xa0min, then 800°C for 30xa0min. Rutherford backscattering spectrometry (RBS) measurements indicated Si concentrations up to 2xa0at.%, in agreement with simulations of implant profiles. X-ray measurements indicated that about 60–70% of the Si was incorporated substitutionally, corresponding to up to 1.5xa0at.% from linear interpolation of lattice constants. Raman spectroscopy measurements confirmed that the diamond structure was reconstructed with the post-implant anneal treatment. NEXAFS measurements confirmed the reconstruction of the sp 3 diamond bonds from sp 2 graphite after annealing. Electrical measurements indicated an increase in diode current density with increased Si dose and a decrease in contact resistance with contact anneals. These novel diamond alloys may be useful for electronic and optical devices. We report on the preparation and properties of these alloys.

Magnetic Hysteresis of CoPt Films

Co-Pt bulk alloys have been of interest due to their high coercivity and anisot-ropy.(1,2) Several uses of these alloys have been suggested in magnetic recording, such as longitudinal recording(3) and magnetooptical recording.(4) Of particular interest are the alloys between 25 and 60 at.%. Within this composition range the alloys undergo an ordering phase transition.(5,6) Direct evidence of this ordering was first reported by Gebhardt and Koster(7) who showed that the face centered cubic (fcc) structure of equiatomic alloys transforms into a face centered tetragonal (fct) at temperatures below 835°C. The fct phase is highly anisotropic with K ≃ 4 × 107erg/cc.(8) The coercivity of these films is small for the disordered fcc Co50-Pt50 alloys, but increases with annealing because of the transformation to fct and then decreases in overaged state because of coarsening of the microstructure. Higher Pt content alloys ~ 75%, transform from disordered fcc to ordered fcc. Although there are several reports on the magnetic properties of Co-Pt thin films, most of the work has been on films with high Co concentrations. Yanagisawa et al(3) reported a peak in coercivity as high as 1700 Oe for a 350A thick film with 20% Pt. The coercivity decreased on either side of this composition. Kitada and Shimizu(9) related this anomaly in coercivity to the microstructure of the films. It was assumed that the increase in coercivity was due to both grain growth and the tendency for the c-axis of α-Co (hep structure) to collapse into the film surface. Aboaf and co-workers(10,11) reported the results of annealing in Co50-Pt50 films. The coercivity was reported to increase from 90 Oe for the as-grown films to “very high” values after annealing at 700°C for one hour. They also related this increase in coercivity to the solid state transformation from fcc to ordered fct.