High performance ultraviolet 4H-SiC avalanche photodiodes
High performance 4H-SiC avalanche photodiodes (APDs) were investigated as a potential candidate to replace photomultiplier tubes that are currently being used in low-level ultraviolet detection such as laser-induced fluorescence biological-agent detection and non-line-of-sight covert communications. A simple p-n junction APD structure was studied first, followed by the pi-n structures with i-layer thicknesses from 180nm to 960nm. With increasing ilayer thickness, the dark current of a 100µm-diameter APD, at a gain, M, of 1000, was reduced from 2nA (25µA/cm2 ) to 1.5pA (19nA/cm2 ). This was primarily the result of the reduced tunneling current in APDs with thicker i-layers. At room temperature, the peak responsivity of the APDs with 960nm-thick i-layer was 122mA/W (QE=52.7% at λ=268nm). Low excess noise, corresponding to k=0.1, was observed. At high temperature, the breakdown voltage increased with a linear coefficient of ~10mV/o C. Gain as high as 108 was demonstrated. To improve the sensitivity and speed, separate absorption and multiplication (SAM) APD structures were studied. The first SAM structure showed high quantum efficiency 85% at 274nm after reach-through, but edge breakdown was observed. Edge breakdown was eliminated in the second SAM structure, by utilizing a positive bevel that effectively reduced the edge electric field. The second SAM structure, which was also optimized for solar blind performance, exhibited high external quantum efficiency of 83% (187mA/W) at 278nm after reach-through. In parallel with the performance characterization, two important issues were addressed: reverse dark current and spatial uniformity of photoresponse at high gain. Passivation studies showed that a sacrificial thermal oxide could reduce the surface leakage. Bulk-leakage studies showed that trap-assisted tunneling current was significant in APDs with thin i-layers but was no longer a primary leakage mechanism for APDs with i-layer thicker than 480nm. Spatial uniformity studies showed that a uniform spatial response was achieved for gain<1000. At higher gain (M>1000), the response became nonuniform, with a steady drop in photoresponse from one side of the APD to the other side. This variation was associated with graduation of the doping density. The nonuniform doping characteristic was observed for all APD structures in this work.