Fabrication Of 4H-SiC Ultraviolet Avalanche Single Photon Detectors And Device Physics Study

Detection of ultra-weak ultraviolet(UV)light is imperative in many applications,such as corona discharge detection,UV communication,missile plume detection and scientific research.Avalanche photodiodes(APDs)working in Geiger mode are attractive candidates for weak UV detection due to their high multiplication gain,small size,low power consumption and high quantum efficiency(QE).Compared with other semiconductors for UV APD development,4H-SiC is very competitive owing to its wide bandgap energy,high critical electric field,superior radiation hardness,lower defect density and relatively mature processing technologies.Although 4H-SiC APDs with single photon counting capability have been reported by several research groups,there are many remained problems should be solved for 4H-SiC APDs,such as low single photon detection efficiency(SPDE),non-uniform avalanche multiplication gain across device active area and breakdown voltage fluctuation among pixels in focal plane arrays.In order to understand and solve these problems,the fabrication process,structural optimization and avalanche mechanisms of 4H-SiC APDs are studied in this thesis.In order to ensure reliable operation and to reduce dark current of SiC APDs,bevel mesa structure is adopted and realized to suppress peak electrical field around mesa edge;Si O2 layer with optimized deposition conditions has been used for device passivation to repair etching damage;and the ohmic contact technology is also optimized to enhance deviceperformance.The main results and conclusions of this thesis are summarized as below.1.In order to reduce absorption of UV light by the top contact layer,and to ensure sufficient thickness of contact layer to achieve good ohmic contact,recessed-window 4H-SiC APDs are fabricated by partial etching of the top contact layer.For the fabricated recessed-window 4H-SiC APD,its SPDE is increased by approximately 14 %.However,hot carrier luminescence images of the recessed-window 4H-SiC APD in avalanche regime indicate that a large etching depth could result in electric field crowding under the p-type contact electrode but a weaker electric field under the recessed-window,which is related to the lateral resistance of top contact layer.This spatial non-uniform electric field distribution across the device mesa is a great obstacle to improve the SPDE of SiC APDs.Thus,the depth of recessed-window should be carefully chosen to ensure effective UV light absorption and uniform electric field in recessed-window SiC APDs.This work provides an effective scheme for improving the single photon detection efficiency of SiC APDs.2.To improve the SPDE of 4H-SiC APDs,it is important to realize uniform avalanche multiplication across the device active area.The degree of avalanche uniformity across device mesa of 4H-SiC APDs is studied by imaging hot carrier luminescence.It is observed that luminescence emission from the pin APDs under avalanche always first appears in the [1120] side of the top contact electrode.As over-bias increases,the luminescence pattern gradually extends toward the [1120] direction.A physical model is proposed to explain the observed non-uniform avalanche phenomenon.In the model,the substrate off-orientation and anisotropy of carrier mobility related carrier lateral drift along the [1120] direction would cause asymmetric carrier collection efficiency between [1120] and [1120] sides of the electrode,which further induces asymmetric carrier accumulation and screening of the junction electric field.Then,the asymmetric screening of junctionelectric field results in non-uniform hot carrier luminescence along the [1120] direction in the 4H-SiC APD.This work should provide insights into guidelines for optimizing design of SiC APD.3.Vertical 4H-SiC nip and pin APDs are fabricated,and the avalanche characteristics and single photon counting performance of two types of APDs are compared.By studying the evolution of breakdown voltage as a function of incident light wavelength,it is confirmed that at deep UV wavelength region,the avalanche events in SiC nip APDs are mainly induced by hole-initiated ionization,while electron-initiated ionization causes the main avalanche events in SiC pin APDs.Due to the fact that the impact ionization coefficient of holes is higher than that of electrons in 4H-SiC,4H-SiC nip APDs have lower critical electric field and lower dark count rate.Moreover,since the impact ionization coefficient of holes is less sensitive to electric field strength for 4H-SiC,the nip APD exhibits smaller gain-voltage slope,which is advantageous for deep UV focal plane array application.This study of avalanche mechanisms is helpful for optimizing design of SiC APDs,and is highly constructive to improve the detection performance of SiC APDs.4.SiC APDs have to continuously operate under critical electric field(~3.3 MV/cm),which makes the performance of SiC APDs more sensitive to structural defects within the device active region.In this work,1×128 SiC APD linear arrays have been fabricated to investigate the effect of threading dislocations on the performance of SiC APDs.After current-voltage and single photon detection characteristics,APD pixels are etched by molten KOH to expose the etch pits of structural defects to study the relationship between threading dislocations and the performance of SiC APDs.Based on a statistical study of 650 APD devices,it is determined with high confidence level that even a single threading dislocation within APD active region would cause apparent device performance degradation,including increase of dark current near breakdown voltage,premature breakdown,increase of dark count rate,and reduction of SPDE.Trap-assisted tunneling effectis determined as the main reason for device performance degradation.Thus,for the future of SiC APD development and UV imaging applications,defect density within the SiC APD epi-structure should be further considerably reduced.In this work,the physical model is proposed for the first time to explain the non-uniform avalanche phenomenon across device mesa of SiC pin APDs.This study is important to future design optimization of SiC APDs.The avalanche mechanisms of SiC nip and pin APDs are compared for the first time,and SiC APDs with dominant hole-initiated avalanche would feature relatively lower dark counts and higher gain consistency among each pixels.This work is important for future focal plane array applications.We also systematically investigated the effect of threading dislocations on electrical and single photon detection characteristics of SiC APDs,which is important for SiC material quality optimization and high-quality weak UV imaging.