Structure Optimization And Temperature Dependent Characteristics Study Of SiC MSM Ultraviolet Detector

As a third generation semiconductor materials, silicon carbide （SiC） demonstratesexcellent potential for high-frequency, high-power electronic devices and hightemperature resistant optoelectronic devices due to its outstanding properties of widebandgap, high break down electric field, low dielectric constant and high thermalstability. With the rapid development of ultraviolet （UV） detecting technology, high-performance UV photodetectors have been regarded as a research hotspot. In recentyears, owing to great superiority of a low dark current, a fast response and easy tointegration, SiC based metal-semiconductor-metal （MSM） UV detectors have been ofgreat interest for a wide range of commercial and military applications, includingmissile tracking, flame detection, ozone monitoring and UV-astrophysics, etc.The electrode distribution of MSM UV detector exerts shadow effect for normalincidence, which definitely has a negative influence on photoelectric properties of thedetectors. In order to obtain the optimum performance, material quality and fabricationprocess should be further improved. Simultaneously, proper device parameters andnovel structures are considered as effective solutions to enhance the UV incidence. Inview of the domestic research status of photodetectors, this dissertation reveals asystemic investigation of photoelectric and temperature characteristics of the SiC basedMSM UV detectors by means of physical modeling, numerical simulation, theoreticalanalysis and structure optimization. The main studies and conclusive results are asfollows.1. Based on thermionic emission theory, a model of a6H-SiC MSM UV detector isestablished. A device with3μm electrode width （W） and3μm electrode spacing （L） issimulated. The findings show that the detector has quite a low dark current of15pA at10V bias and the photocurrent is two orders of magnitude higher than the dark current.Simulation and optimization results indicate that the maximum photocurrent and thehighest ratio of photocurrent to dark current are obtained with device parameters of W=6μm, L=3μm and W=3μm, L=6μm, respectively.2. A two-dimensional model of a4H-SiC MSM UV detector has been establishedusing a self-consistent numerical calculation method. The spectral response of thephotodetector is calculated by solving Poisson’s equation, the current continuityequation and the current density equation. With consideration of the reflection andabsorption on the metal contacts, a detailed study involving various electrode heights （H）, spacings （S） and widths （W） reveals conclusive results in device design. Theinfluences of those parameters on responsivity and relevant mechanisms are analyzed.The findings show that responsivity is inversely proportional to electrode height and isenhanced with an increase of electrode spacing and width. In addition, all UV-to-visiblerejection ratios are>103and the findings show that a responsivity as high as180.056mA/W, a comparable quantum efficiency of77.93%and a maximum UV-to-visible ratioof1875are achieved at10V bias with a detector size of H=50nm, S=9μm and W=3μm.3. A model of novel triangular electrode MSM （TEMSM） and conventionalelectrode MSM （CEMSM） detectors is established. The TEMSM detector shows asuperiority of a113%photocurrent increase and similar low dark current over theCEMSM device. Furthermore, the electrode angle α, width W and spacing S play a vitalrole to enhance the effect of incidence. Under30V bias, the maximum UV-to-visiblerejection ratio, comparable responsivity and external quantum efficiency at310nm are13049,0.1712A/W and68.48%for a TEMSM detector with device parameters ofα=60°, W=3μm and S=4μm, respectively.4. A model of novel semicircular electrode MSM （SEMSM） UV detector andspectral response features are investigated and characterized based on a series ofphysical models. To achieve the enhanced device performance, structure parametershave been adjusted appropriately to optimize the detector. The findings show that adevice with2μm finger radius and3μm spacing exhibits outstanding characteristics interms of a peak responsivity of0.177A/W at290nm, a maximum external quantumefficiency of over75%and a comparable normalized photocurrent to dark current ratio（NPRD） of1.192×10¹¹1/W under0.3V bias.5. Temperature-dependent characteristics of a4H-SiC MSM UV photodetectorhave been investigated based on thermodynamic, hydrodynamic and absorption modelrelated with temperature. It is found that the dark current and photocurrent increase withthe increasing temperature and the former is larger than the latter at higher temperature,which leads to a decrease to photodetector current ratio （PDCR）. Nevertheless, thePDCR obtained under5V bias and300nm UV illumination remains2orders ofmagnitude even at800K. As temperature rises, quantum efficiency increases and allpeak values occur at280nm. In addition, the responsivity exhibits an unsymmetricaltrend. It is clear that a remarkable12nm red-shift of peak responsivity occurs andoverall spectrum response is enhanced for longer wavelength. While the shortwavelength response remains relatively independent of the temperature.