Design, Fabrication And Application Of 4H-SiC Based Ultraviolet Antireflection Coatings For Photodetectors

Silicon carbide(SiC) has been performing considerable potential for ultraviolet(UV) photodetectors due to its properties such as wide band gap(3.26 eV for 4H-SiC),high break down electric field and high thermal stability.4H-SiC based UV photodetectors such as Schottky,metal-semiconductor-metal(MSM),p-i-n and avalanche have been presenting excellent performance for UV detection application in flame detection,ozone-hole sensing, short-range communication,etc.Generally,the most widely used antireflection coating and passivation layer for 4H-SiC based photodetectors are native SiO₂ layer grown by heating 4H-SiC atmosphere in order to improve absorption of photodetectors.Nevertheless,the SiO₂ single layer suffers from high reflection,large absorption and inaccurate film thickness.Therefore,in this dissertation,UV antireflection coatings were designed,fabricated and applied in order to reduce optical losses and improve the quantum efficiency(QE) of 4H-SiC based photodetectors.The important results were obtained as follows:1.According to transparent range,extinction coefficient,refractive index,mechanical properties and chemical reliability,Al₂O₃ and SiO₂ films were selected in tens of optical film materials as antireflection coatings on 4H-SiC based UV photodetectors.SiO₂ film was designed between Al₂O₃ film and 4H-SiC substrate and Al₂O₃ film was deposited on SiO₂ film according to its reliability.The optical thicknesses of Al₂O₃ and SiO₂ film were designed according to the vector method and admittance matching technology.Errors of refractive index,thickness,etc were simulated to evaluate error effects on reflectance of Al₂O₃/SiO₂ films.Thickness error was the main factor.However,the effects of refractive index,extinction coefficient and surface roughness could not be ignored.2.Al₂O₃/SiO₂ films were deposited on 4H-SiC substrates by using electron-beam evaporation according to above film design.The minimum reflectance of the films was 0.25% at 276nm,which is the minimum attained so far.The Al₂O₃/SiO₂ films were annealed in N₂ at 550,950 and 1100℃,respectively,to examine film performance.The minimum reflectance shifted to shorter wavelength with the increase of annealing temperature due to reduction of film thickness.The surface grains appeared to get larger in size and the root mean square (RMS) roughness of the annealed films increased with the annealing temperature but was less than that of the as-deposited.Although the Al₂O₃/SiO₂ films kept amorphous,there were diffusion,Al-silicates and Si-suboxides at the interface between films and 4H-SiC substrate.3.4H-SiC based MSM UV photodetectors with Al₂O₃/SiO₂ films had been fabricated and compared with SiO₂/4H-SiC MSM detectors.The photocurrent of former was twice as large as the latter,while the dark current was also larger.The Al₂O₃/SiO₂/4H-SiC devices showed a peak responsivity of 0.12 A/W at 290 nm under 20 V,which was twice as much as that of MSM detectors.The internal and external QE of the Al₂O₃/SiO₂/4H-SiC devices were 50%and 77%at 280 nm respectively,which are the highest attained so far for 4H-SiC based MSM photodetectors.The responsivity of the Al₂O₃/SiO₂/4H-SiC devices agreed well with their surface reflectance in 240-300nm.4.The Al₂O₃/SiO₂ films prepared by oxidation and electron-beam evaporation were applied on 4H-SiC based p-i-n photodiodes.The dark current of the devices was 1 pA,which was larger than that of SiO₂/4H-SiC detectors due to undercut of mesa sidewall.But the photocurrent of the former was 2.8 nA,which 2.8 times as larger as that of the latter.There were slight gains in these two devices with the increase of backward bias voltage.The peak responsivities of Al₂O₃/SiO₂/4H-SiC and SiO₂/4H-SiC devices were 49 mA/W at 270 nm and 23 mA/W at 260 nm,respectively,corresponding to external QEs of 23%and 15%.The low external QEs were due to incomplete depletion in i layer.The peak responsivities of these two devices agreed well with their minimum surface reflectances.