Fabrication and Measurement of 4H-Silicon Carbide Avalanche Photodiodes
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Fabrication and Measurement of 4H-Silicon Carbide Avalanche Photodiodes Kent Burr, Peter Sandvik, Stephen Arthur, Dale Brown and Kevin Matocha General Electric Global Research Center, Niskayuna, NY 12309, U.S.A. ABSTRACT Avalanche photodiodes (APDs) that are sensitive in the ultraviolet (UV) from approximately 250 – 350 nm have been fabricated from 4H-SiC. The SiC APDs, which use hole-initiated avalanche multiplication, were produced using n-type SiC epitaxial layers grown on a p-type substrate. In order to achieve avalanche breakdown in the bulk of the material, a dry ion-based etching technique was used to form sloped sidewalls on the devices. The devices had an area of approximately 1 mm2, and they had maximum breakdown voltages of approximately 500 V. The APDs had a positive temperature coefficient for avalanche breakdown voltage and showed excellent stability for multiplication factors in excess of several hundred. Dark current, photo responsivity, and multiplication measurements from room temperature to 150°C will be presented. The dark noise performance of the APDs has also been characterized using a standard nuclear spectroscopy system consisting of a charge sensitive preamplifier, a shaping amplifier, and a multichannel analyzer. The input equivalent dark noise charge and excess noise factor for the dark current was measured over a range of shaping times, temperatures, and bias voltages. The noise performance of SiC APDs in applications such as gamma ray or x-ray spectroscopy will be highly dependent on the achievement of low bulk leakage current at the operating point. Here, an ionization coefficient ratio (k=α/β) of 0.078 was found. INTRODUCTION SiC photodiodes have been the subject of study for several years [1,2]. More recently, avalanche photodiodes fabricated from SiC have been investigated [3] for potential use in very low light settings. With the avalanche aspect of operation, however, these devices become increasingly complex to characterize and manipulate. Both device signal and noise aspects must be considered in order to validate their potential use for most applications. Previous efforts to characterize 4H SiC APDs have demonstrated their potential due to a positive temperature coefficient with breakdown voltage [4] and a favorable ionization coefficient (α/β) ratio of approximately 0.1 [5]. Here, we present the signal and noise analysis of 4H SiC avalanche photodiodes, and for the first time, estimate their compatibility with a scintillator material for use in high energy spectroscopy applications. EXPERIMENTAL DETAILS SiC APDs were fabricated using 2 inch diameter 4H substrates and epitaxial material. These wafers and epitaxial materials were oriented 8° off axis from the (0001) crystal plane, and were purchased from Cree Research (Durham, NC). The epitaxial layers were grown using metalorganic chemical vapor deposition. The substrate and epitaxial layer doping levels and thicknesses are illustrated in figure 1.
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Cathode metal (Ni/Ti/Mo/Au) Passivation N+ SiC epi layer [5e18
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