Tomoko Borsa

6H and 4H-SiC Avalanche Photodiodes

We report on the fabrication and testing of SiC p-i-n avalanche photodiodes. APDs of 0.25 mm2 area on a-plane (1120) 6H-SiC as well as off-axis Si face 6H and 4H-SiC were successfully fabricated. A beveled mesa was used as edge termination. Recessed windows were formed using reactive ion etching to enhance low-wavelength UV performance. We performed current-voltage tests with and without UV illumination to determine dark current, photocurrent, and gain on selected devices. Dark current was less than 1 pA at 0.5Vbr on multiple devices. Quantum efficiency of 40% or greater was observed for all orientations and polytypes.

High-Contrast Visualization of Anti-Phase Domains and Screw Dislocations in 3C-SiC

Silicon carbide (SiC) is an important wide bandgap semiconductor with many polytypes. 3C-SiC is the only cubic polytype, to be distinguished from the hexagonal polytypes such as 4H-SiC and 6H-SiC. Crystalline 3C-SiC, grown by heteroepitaxy, often contains twin structures, also called anti-phase domains (APDs), that are separated by double positioning boundaries (DPBs). This naturally occurs when growing 3C-SiC, since there are two possible and equally-likely stacking sequences with a relative in-plane rotation of 180 degrees as shown in Figure 1. The challenge faced when imaging these APDs, is that neither optical or scanning electron microscopy provides a clear contrast between the rotated domains, as they are composed of the same material with identical properties. This paper demonstrates high-contrast imaging of such APDs, enabled by channeling of electrons or ions.

6.4: Fabrication of 4H-SiC cold field emitter arrays

A new fabrication method of SiC field emitter arrays has been developed. The new method involves the formation of three-dimensional silicon dioxide masks, dry etching, and tip sharpening. The field emitter arrays were successfully fabricated on 4H-SiC substrates with tip diameter less than 10 nm.

Novel Nano-structured Metal–Semiconductor–Metal Photodetector with High Peak Voltage

A novel nano-structured metal–semiconductor–metal photodetector consisting of interdigitated metal fingers and nanodots is successfully fabricated on a semi-insulating GaAs substrate by electron beam lithography, and integrated with an on-chip ground–signal–ground coplanar transmission line for pulse response measurements. The fabricated nano-structured metal–semiconductor–metal photodetector can be operated at 5 V, more than three times higher than the operating voltage of the regular metal–semiconductor–metal photodetector composed of narrowly spaced interdigitated electrodes only. Its dark current is lower than 0.5 nA until the bias voltage approaches the breakdown voltage. More importantly, it demonstrates a more than three times higher peak voltage output than that of the regular metal–semiconductor–metal photodetector while maintaining approximately a 10 ps pulse width that is limited by the bandwidth of the measurement setup, not by the speed of the photodetector. The transit model simulation indicates that the amplitude of the pulse response is strongly influenced by the voltage collapse across the photodetector.

Graphene-Silicon Heterojunction Infrared Photodiode at 1.3/1.55 μm

A novel infrared photodiode based on a graphene/n-type silicon heterojunction is explored. The heterojunction photodiode of interest has a large Schottky barrier that results in a low dark current. Graphene serves as the absorbing medium at a wavelength for which silicon is transparent. Under infrared illumination, photo-excited electrons in the graphene gain energy and thus have a greater probability to overcome the barrier and contribute to the photocurrent. We have demonstrated photodiode operation of a graphene/n-Si heterojunction at 1.3 and 1.55 μm wavelength, with 14% internal quantum efficiency and 1.5 pW/Hz1/2 noise-equivalent power, for potential use in silicon photonics.

Photo-response of integrated photonic crystal-photodiode micro-electro-optic filters

A novel micro-electro-optic filter formed by integrating photonic crystals with photodiodes on a silicon substrate is demonstrated in this paper. P-n diodes were fabricated on a Silicon wafer using standard processes. Reactive ion etching (RIE) was used to form trenches into the diodes to contain and position photonic crystals. The wafer was then immersed vertically into a slowly evaporating colloidal suspension of silica mircrospheres to assemble the photonic crystal over the photodiodes. Spectral measurements using a grating monochrometer confirmed that a dip exists in the photocurrent response of the photonic crystal filter-photodetectors at the predicted wavelength of 600 nm. We performed a series of measurements using several different sphere sizes and light incidence angles to further characterize the filters, and evaluated the use of device as a wavelength selective detector. Since silica has a low coefficient of thermal expansion, the wavelength selective characteristics of the device are expected to be insensitive to ambient temperatures.