Ayaz Muhammad Khan

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.

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.

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

Long Wavelength Metal-semiconductor-metal Photodiodes With Transparent Cadmium Tin Oxide Schottky Contacts

A metal-semiconductor-metal (MSM) Ino.53G~.47As photodiode using a transparent cadmium tin oxide (CTO) layer for the interdigitated electrodes was investigated. The CTO functions as a Schottky contact, an optical window and an anti-reflection (AR) coating. The transparent contact prevents shadowing of the active layer by the top electrode, thus allowing greater collection of incident light. The 2000 8, CTO electrodes were reactively sputtered w i t h an argon/oxygen plasma. Transmittance of the I T 0 and CTO was measured. CTO shows a gradually decaying transmittance over the spectral range starting at 94% at 1 pm and falling to 87% at 1.55 pm. However, the I T 0 film which does start off at 87% transmission at 1 pm drops rapidly starting around 1.2 pm, and falls to rjO% at 1.35 pm. Clearly, CTO is a better choice for transparent conductors at wavelengths suitable for long-haul optical fibers (1.3 pm 5 X 5 1.55pm). The device is first grown by molecular beam epitaxy (MBE). The MBE structure consists of a superlattice buffer on a semi-insulating InP substrate, a 0.75 pm i-Ino.53G%.47As active region ( 1 . 3 ~ 10” ~ m ~ ) , and a 200 A z ’ I ~ O . ~ ~ A ~ O , ~ ~ A S Schottky barrier enhancement layer. The barrier height ( 4 ~ ) of the CTO versus Ti/Au was investigated. Using the value for the effective Richardson’s constant (A* * ) of 18 A/cm2.K2 for InAlAs, we can find the barrier height. By extrapolating the current on the I-V characteristics to zero bias and estimating the contacted area to be 1875 pm2 for 75x75 pm2 area MSM photodiodes with 1 pm wide electrodes and 2 pm wide spacings. Js, the saturation current density at zero bias is found. The barrier height, dBn, for CTO and T i /Au were estimated to be 0.47 eV and 0.595 eV, respectively, by using zero bias currents of 400 nA and 3.2 nA, respectively. The low Schottky barrier height for CTO is probably due to damage of the thin 200 8, Ino.52Alo.48As layer created during the sputtering process. Defect related tunneling through the barrier can lead to an effective lowering of the barrier height. MSM photodiodes with CTO electrodes and active area 75x75 pm2 exliibited leakage currents of 4.8 pA at 10 V and soft breakdown voltages of 2 10 V. Similar MSM photodiodes with Ti/Au electrodes had leakage currents of 270 nA at 10 V and soft breakdown voltages of 2 13 V. Responsivity for 1.3 pm incident light was 0.49 A/W and 0.28 A/W, respectively, for the CTO and Ti/Au MSM photodiodes. No AR coating was utilized over the bare semiconductor surface. The CTO MSM photodiode shows a factor of almost two improvement in responsivity over conventional Ti/Au MSM photodiodes. The