Research And Fabrication Of 4H-SiC Ultraviolet P-i-n Photodiodes And Linear Array

As a promising semiconductor material for high temperature, high frequency, and high power devices, silicon carbide (SiC) has received more remarkable attentions now. Due to the wide bandgap of 4H-SiC ( 3.26 eV), photodetectors utilising this material should have the advantage of a good visible-blind/solar-blind performance and a very low dark current which can be many orders of magnitude lower than that of conventional Si detectors even at high temperature (theoretically up to 600℃). Moreover, the 4H-SiC ultraviolet (UV) detectors have many applications in chemical sensing, flame detection, ozone-hole sensing, short-range communication, and missile plume sensing in harsh and severe radiation environment. The p-i-n photodiode fabricated with 4H-SiC material delivers low noise, high-speed response and high photosensitivity at low reverse bias due to a low terminal capacitance and a large shunt resistance. It can work insensibly to visible/IR backgrounds without visible-blind interference filter. Recently several research groups have demonstrated high-performance UV photodetectors using 4H-SiC material system, such as Schottky, MSM, p-i-n, and avalanche photodiodes. The 6H-SiC UV p-i-n photodiodes have already been fabricated and are commercially available. But up to now there are few reports on the fabrication of 4H-SiC p-i-n photodiodes. Based on the above statements, the following results were obtained:1. In order to obtain a high-performance 4H-SiC UV p-i-n photodiode, the structure of device was designed. Considering the structure properties of p-i-n photodiode, the impurity concentration of each epilayer was discussed at first. Considering the effect of different junction-depth on the spectral responsivity, the continuity equations of photo-generated minority carriers were then used to determine the widths of intrinsic layer (I-layer) and p⁺-layer. As a result, the peak responsivity of a p⁺-n-(i)-n-n+ detector, as high as 0.16 A/W, was achieved at 260-nm wavelength when the widths of I-layer and p⁺-layer were 0.5μm and 0.2μm, simultaneously the impurity concentrations for I-layer and p⁺-layer were ¹.0×10¹⁶/cm³ and >1.0×10¹⁹/cm³, respectively.2. A high-performance 4H-SiC p-i-n photodiode with the photoactive window area of 200×200μm² has been fabricated. The electrical and optical characteristics were measured at room temperature. The photodiode suffered from significant dark current density of 2.5 pA/mm² at the reverse bias of 5 V, and the ratio of the UV (@ 270 nm) photocurrent to the dark current was > 104^. The built-in potential ( 2.97 V) and the unintentional i-layer doping concentration ( 4.5×10¹⁵/cm³) were obtained from capacitance-voltage (C-V) measurement. The spectral peak responsivity of the detector reached 0.13 A/W at the wavelength of 270 nm, corresponding to a maximum external quantum efficiency of 61%. And the ratio of responsivity at 270 nm to that at 380 nm was >103. The peak of the spectral detectivity D* was 3.6×10¹⁴ cm·Hz^1/2/W, and the corresponding noise equivalent power (NEP) was 5.5×10^-17 W. D^* over the wavelength range from 240 nm to 320 nm was above 10¹⁴ cm·Hz^1/2/W. The characteristics imply that the photodiode has a great improved ultraviolet-visible rejection ratio which is needed for ultraviolet signal detection.3. Aδ-doping 4H-SiC UV p-i-n photodiode with the photoactive window area of 200×200μm² was fabricated. The I-V characteristic results revealed that the detector suffered from low dark current density of 12.5 pA/mm² at a low reverse bias of 5 V. The photodiode exhibited a broad spectral response of wavelengths from 250 nm to 320 nm. At the reverse bias range from 0 V to 5 V, the peak responsivity of detector reached 0.1 A/W at the wavelength of 270 nm, corresponding to a maximum external quantum efficiency of 49.7%. The peak of D^* was found to be 1.75×10¹⁴ cm·Hz^1/2/W at 270 nm, and the corresponding NEP was 1.15×10^-16 W. D* over the wavelength range from 260 nm to 310 nm was above 10¹⁴ cm·Hz^1/2/W. At the reverse bias range of 10 V～30 V, the peak responsivity of detector reached 0.12 A/W at the wavelength of 280 nm, with a maximum external quantum efficiency of 52%. The measured wavelengths of peak responsivity showed a slight red-shift with the increasing reverse bias. Furthermore, the UV-visible rejection ratio of detector was nearly 103, which indicated an improved visible-blind performance.4. 4H-SiC UV p-i-n photodiode linear arrays have been designed and fabricated. A 30-pixel 4H-SiC p-i-n photodiode linear array containing only one bad pixel has been achieved. The linear arrays showed good integrality and uniform dark current characteristic, but the average value of the dark current was abnormally high. The reason was found out to be the destruction of the passivation layer during the wet etching, and the suggestions for further improvement and potential applications were also presented.