Enhanced high performance 4H-silicon carbide avalanche photodiodes

4H-SiC avalanche photodiodes are candidates to replace photomultiplier tubes in some applications for low-level ultraviolet signals. These applications include biological-agent detection and gamma detection. This dissertation mainly describes enhancements of high-performance 4H-SiC avalanche photodiodes (APDs).;The external quantum efficiency of a previously characterized 4H-SiC PIN-structure APD with 480 nm of intrinsic layer was increased by incorporating a recessed-window structure. The recessed window greatly improved the PIN APD's external quantum efficiency from ∼40% to ∼60%, roughly a 50% enhancement. The dark current remained low (90 pA, ∼0.18 muA/cm 2) at a photocurrent gain of 1000.;A second APD enhancement was targeted to improve the fill factor (FF) for APD arrays by reducing the device area lost to the beveled sidewalls of earlier SiC APDs. Successful incorporation of the double mesa sidewall eliminated premature edge breakdown and increased the avalanche breakdown voltage by ∼1 V compared to 4H-SiC PIN APDs with a ∼10° beveled sidewall termination. The dark current was ∼33 pA at a gain of 1000 and the primary multiplied dark current was ∼0.8 nA/cm-2. A fill-factor improvement of ∼20% can be gained by moving from a 10° beveled sidewall device 1.2 mum in height to a double mesa sidewall termination with a 5 mum step width of the bottom mesa.;6H-SiC PIN APDs were investigated due to their photoresponse to longer wavelengths of light. Relative to 4H-SiC APDs, the responsivity was red-shifted ∼ 30 nm; the peak responsivity was 80 mA/W at lambda = 290 nm. High gains, low dark current (Jdark = 9.2 muA/cm2 at gain of 1000), and low excess noise (k = ∼ 0.1) were achieved.;4H-SiC reach-through SACM APDs were also studied. The charge layer provided a more uniform electric field distribution in the multiplication layer. The one parameter fitting based on the comparisons of the external quantum efficiencies of my SACM APD and an earlier SAM_II APD fabricated in our group were conducted. This yielded a punch-through gain of ∼ 6.2. Taking this punch-through gain into account, an excess noise k value of ∼0.2, external QE of ∼80% and a dark current of ∼ 5 nA (∼ 60 mum/cm2) was calculated.