Study On Fabrication And Characterization Of Novel 4H-SiC Ultraviolet Avalanche Photodiodes

As a typical wide bandgap semiconductor,silicon carbide(SiC)has drawn worldwide attention recently.Due to its wide bandgap,good thermal conductivity,high electron saturation drift velocity and good chemical stability,4H-SiC is ideal material for UV detectors.On the other side,UV avalanche photodiode(APD)which can detect extreme low level UV light signal is required in numerous applications,such as fire alarm,space astronomy,quantum communication,ballistic missile early-warning,corona detection,environmental monitoring,down-hole oil exploration,etc.Having high internal gain,low dark current and dark count rate(DCR),high quantum efficiency,good radiation-resistance,no need to work under cooling conditions,make 4H-SiC APD an excellent candidate for the above-mentioned application fields.Although 4H-SiC APDs with high performance have been successfully fabricated by many research groups,only a few of them could achieve 4H-SiC APDs with single photon detection performance.There are many problems remaining to be solved for SiC single photon counting avalanche photodiodes(SPADs),such as high dark count rate(DCR),low single photon detection efficiency(SPDE),low fill factor,poor breakdown voltage uniformity and poor reliability.In this work,in order to understand and solve the above-mentioned problems,we designed and fabricated several novel SiC APDs,and analyzed their performance.The main contents of this thesis are shown as below:1.In order to improve detection efficiency,lower excess noise and avoid poor-quality p-type ohmic contact in traditional pin-type 4H-SiC APDs,as well as to avoid using p-type SiC substrate,vertical 4H-SiC n-i-p-n APDs are designed and fabricated,in which avalanche current flows through a forward-biased pn junction formed between the p epi-layer and the n-type substrate.Compared with a traditional n-i-p APD,no obvious difference of gain-voltage and photo-response characteristics is observed,indicating the feasibility of the proposed n-i-p-n device structure.When a partial trench isolation scheme is applied,the vertical n-i-p-n APD shows a high fill factor of 78.3%and a low-bias quantum efficiency of 66%.2.In order to improve quantum efficiency and avalanche gain uniformity among pixels in focal plane array,as well as to reduce dark current and operation voltage,4H-SiC separated-absorption-charge-multiplication UV APDs with low breakdown voltage are designed and fabricated.The room temperature dark current of the APD remains at?0.1 pA level(?29 pA/cm2)when reverse bias is below 50 V.An avalanche gain up to 106 is achieved at 67 V.The RT maximum quantum efficiency is?80%at 270 nm with a UV/visible rejection ratio more than 104.3.In order to understand the mechanism of high DCR and low SPDE of some SiC APD devices,four 4H-SiC p-i-n APDs with different intrinsic layer thickness are designed and fabricated.SPDE performance becomes better as intrinsic layer thickness increases,which can be attributed to inhabitation of tunneling.Dark count origin is also investigated.An activation energy as small as 0.22 eV of DCR suggests that trap-assisted tunneling(TAT)process is a main source of DCR.Temperature coefficient of breakdown voltage ranges from-2.6mV/0C to 18.3mV/? as intrinsic layer thickness increases,demonstrating that TAT process is dominant in APDs with thinner intrinsic layer.Additionally,the maximum quantum efficiency at 280 nn rises from 48%to 65%under-10 V bias with UV/visible rejection ratio more than 104.4.In order to understand the low SPDE phenomenon of some SPAD devices,a mapping system based on a scanning near-field optical microscope(SNOM)is built to characterize the single photon counting(SPC)spatial uniformity of SPADs,which is important for device physics study and process optimization.The system comprises of a passive quenching circuit module and a mapping module,which are used to record SPC signals and to scan over the SPAD photo-sensitive area,respectively.For the first time,two-dimensional mapping of SPC uniformity of 4H-SiC SPADs is carried out at different over-bias voltages.The possible reasons for the observed SPC non-uniformity across the active area of the 4H-SiC SPAD are discussed.Meanwhile,the suggested physical mechanism of SPC non-uniformity is supported by high sensitivity CCD imaging.