High-responsivity SiC Ultraviolet Photodetectors with SiO2 and Al2O3 Films

Abstract

Silicon carbide (SiC) has shown considerable potential for ultraviolet (UV) photode-tectors 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), metal-insulator-semiconductor (MIS) 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 SiO 2 grown by heating 4H-SiC in O 2 in order to improve the absorption and passivation of photodetectors. Nevertheless, the thermally grown SiO 2 single layer suffers from high reflection, large absorption and inaccurate thickness. Therefore, in this chapter, 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 will be introduced as follows: According to transparent range, extinction coefficient, refractive index, mechanical properties and chemical reliability, Al 2 O 3 and SiO 2 films were selected in tens of optical film materials as antireflection coatings on 4H-SiC-based UV photodetectors. SiO 2 film was designed between Al 2 O 3 film and 4H-SiC substrate and Al 2 O 3 film was deposited on SiO 2 film according to its reliability. The optical thicknesses of Al 2 O 3 and SiO 2 film were designed according to the admittance matching technology. Al 2 O 3 /SiO 2 films were deposited on 4H-SiC substrates by using electron-beam evaporation according to the film's design. The minimum reflectance of the films was 0.25% at 276 nm, which is the minimum attained so far. The minimum reflectance shifted to shorter wavelengths 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 annealing temperature but was less than that of the as-deposited. Although the Al 2 O 3 /SiO 2 film was kept amorphous, there were diffusion that Al silicates and Si suboxides were formed at the interface between films and 4H-SiC substrate. 4H-SiC-based MSM UV photodetectors with Al 2 O 3 /SiO 2 films have been fabricated and compared with SiO 2 /4H-SiC MSM detectors. The photocurrent of the former was twice as large as the latter, while the dark current was also larger. The Al 2 O 3 /SiO 2 /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 Al 2 O 3 / SiO 2 /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 Al 2 O 3 / SiO 2 /4H-SiC devices agreed well with their surface reflectance of 240-300 nm. The Al 2 O 3 /SiO 2 films prepared by oxidation and electron-beam evaporation were applied on 4H-SiC-based MIS photodiodes. The dark current of the devices was 1 pA, which was larger than that of SiO 2 /4H-SiC detectors due to undercutting of the mesa sidewall. But the photocurrent of the former was 2.8 nA, which is 2.8 times as large as that of the latter. There were slight gains in these two devices with the increase of backward bias voltage. The peak responsivities of Al 2 O 3 /SiO 2 /4H-SiC and SiO 2 /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 peak responsivities of these two devices agreed well with their minimum surface reflectances.