Material defects are one of the keys limiting factors for semiconductor device performances and reliability. In this work, the impact of surface bump defects in 4H-SiC epi-material on the performance of Avalanche Photodiodes (APDs) are investigated for ultraviolet (UV) detection. Based on the 4H-SiC epi-material with a concentration of ~104 cm-2 for surface bump defects, 4H-SiC APDs were fabricated and characterized. The results demonstrate that surface bump defects have no effect with the leakage current of devices at low reverse voltage. However, premature breakdown or even device failure will be caused due to the surface bump defects. It is proved that surface bump defects can be ignored for low-voltage photodiodes but must be taken into account for APDs. The physical mechanism and suppression of surface bump defects in 4H-SiC epi-material are further analyzed. Optimization of C/Si ratio for source gas during the growth process of epilayer will help to suppress the formation of surface bump defects. This work will be useful for researchers in the related fields of 4H-SiC photodiode UV detectors.
In this work, 4H-SiC avalanche photodiodes (APDs) were fabricated and investigated both in linear and Geiger modes for high-temperature ultraviolet (UV) detection applications. With the temperature varying from 300 K to 425 K, the avalanche breakdown voltage of our 4H-SiC APDs keeps very stable with a small temperature coefficient of <8 mV/K. In the Geiger mode, the impact of temperature on the output signal pulse height, dark count rate (DCR) and single-photon-detection efficiency (SPDE) is analyzed from the aspect of device physics. At a fixed bias voltage of 166.5 V, the DCR and SPDE at 300 K, 375 K and 425 K are 5.3 Hz∙μm-2 /15.6%, 11.8 Hz∙μm-2 /17% and 16.5 Hz∙μm-2 /15.7%, respectively. The results in this work demonstrate that our fabricated 4H-SiC APDs can operate stably and reliably under the conditions with a high temperature.
Anti-counterfeiting technologies of banknotes are essential to avoid financial fraud and maintain the stability of the socio-economic system. In recent years, the technologies used by counterfeiters have become more and more advanced, bringing great challenges to traditional methods of authenticity check. Therefore, the identification technologies of banknotes urgently need to be improved. Terahertz spectroscopy is widely used for analysis and material identification due to its properties (such as biomolecular fingerprinting spectrum, ultra-weak photon emission, non-destructive and time-resolution detection). In this paper, we apply the transmission terahertz imaging to the identification of anti-counterfeit labels of RMB. We designed and built a THz-TDS confocal imaging system, which has higher image resolution than traditional THz-TDS imaging systems. We used this system to perform terahertz imaging on anti-counterfeit labels of banknotes. The results show that our designed and built THz-TDS confocal imaging system not only provides higher image quality, but also can identify more details of the anti-counterfeit labels of banknotes.
In this paper, we reported the design, fabricate, and characterize of an electronically controlled terahertz (THz) amplitude modulator composed of a four-open ring metamaterial structure and a high electron mobility transistor (HEMT) structure, HEMT area is located at the openings of the four-opening ring metamaterial structure. The concentration of 2D electron gas of the HEMT is adjusted by applying an external electrical field and results in the resonance shifts. Both static and dynamic characterizations of the modulator were carried out by using an optical fiber-coupled terahertz time-domain spectroscopy (THz-TDS) system in the transmission type. Experimental results show that the amplitude modulator can modulate the incident THz waves with two orthogonal polarization directions. The modulation depths are 56% at 0.32 THz for vertical polarized beam and 40% at 0.34 for horizontal polarized beam respectively. The modulator has potential application in wireless communication and real-time imaging.
In this letter, an avalanche photodiode (APD) for ultraviolet detection was fabricated on a 4H-SiC epi-layer with a radius of 150 µm. By adopting passive quenching method, the impact of quenching resistor on single photon detection performance of the fabricated APD was investigated for the first time. It is found that both dark count rate (DCR) and single photon detect efficiency (SPDE) were reduced with the increasing quenching resistance. When the DCR fixed at 5 Hz/μm2 , the SPDE is 7.1% /6.7%/5.4%/5.2% corresponding to the quenching resistance of 10/20/50/100 kΩ. Variation of the SPDEs can be ascribed to the changing death time by comparing the photon counting spectra with various resistors. The obtained results have built up a good basis for the design of SiC APD single photon detection.
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