Date of Award

2018

Document Type

Open Access Dissertation

Department

Electrical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

MVS Chandrashekhar

Abstract

Ultraviolet (UV) detectors are important for their applications in flame detection, furnace gas control systems, plume monitoring etc. Due to their wide band-gap, 4H-SiC (Eg=3.26eV) and III-V semiconductors such as GaN (Eg=3.4eV), AlxGa1-xN (Eg=3.4eV to 6.2eV) etc., are excellent candidates for visible(𝜆cut-0ff=400nm) and solar blind(𝜆cut-0ff=290nm) UV detectors. Conventional SiC UV detectors suffer from poor UV responsivities due to reflection/absorption/transmission losses caused by the metal electrodes used in those detectors.

In the first part, a novel bipolar transistor with epitaxial graphene(EG)/p-SiC (30μm)/n+-SiC substrate was fabricated and characterized. The 2-3 ML thick, transparent and conducting, EG used in this work was grown by using thermal sublimation of SiC. Under 0.43 μW 365nm UV illumination, this device showed a responsivity(R) of 7.1A/W better than or comparable to the state of the art SiC Schottky and PiN diodes, and a bipolar current gain of 113, when operated in the Schottky emitter(SE) mode. Further, a UV-visible rejection (R365: R444) >103 is estimated for this device.

In the second part, EG/p-SiC(13μm)/ n+-SiC bipolar transistor device structures were fabricated, where EG was grown by selectively etching Si from SiC using a novel Tetrafluorosilane(SiF4) precursor. The photo-transistor showed responsivity as high as 25 A/W at 250 nm in the SE mode. The SC mode showed a responsivity of 17A/W at 270nm with a visible rejection (R270: R400)>103. The fastest response was seen in the SC-mode, with 10ms turn-on and 47ms turn-off, with a noise equivalent power(NEP) of 2.3fW at 20 Hz and a specific detectivity of 4.4x1013 Jones.

In the final part, HEMT devices with Al0.85Ga0.15N/Al0.65Ga0.35N as barrier and channel layers, were fabricated and characterized. These devices showed a photo-responsivity ~1×106A/W at 220nm, with solar and visible rejection ~102 limited by sub-bandgap states in the AlGaN. The lowest NEP was observed near the threshold voltage, 4.7fW at 220nm and ~4.4fW at 260nm, with a responsivity of ~103A/W. A measured slow response time of ~20s is attributed to trapping at the AlN/AlGaN growth interface. Potential solutions to reduce the trapping responsible for the increased response times are discussed.

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