Abstract
Ultraviolet (UV) radiation covers the wavelength range from 10 nm to 400 nm. The detection of UV radiation presents a wide range of civil and military applications, such as chemical and biological analysis, flame detection, inter-satellite communications and astronomical studies. For many decades, the detection of UV light has been accomplished primarily with photomultiplier tubes (PMT), which have the beneficial characteristics of high sensitivity with low noise. But they are also fragile, need large power sources, which make systems bulky and heavy, and are relatively expensive. For some application, avalanche photodiodes, which are semiconductor detectors, are potential replacements for PMTs. Among all the material candidates, 4H-SiC has the most promising material properties and APDs based on SiC have achieved excellent performance with high sensitivity and good robustness. However their relatively narrow spectral response has limited their use for deep-UV and near-UV detection. This dissertation describes my work on the deep-UV enhancement of SiC-based APDs and a study of GaAs/AlGaAs-based APDs to achieve comparable dark current, multiplication gain and excess noise performance to SiC but with enhanced responsivity for near-UV detection.
From my study of the spectral response of SiC PIN-structure APDs, the weak deep-UV response was found to be related to the high surface recombination velocity at the top surface and the short diffusion length in the top highly doped layer. Two different approaches were investigated to enhance the deep-UV response of these SiC APDs. First, a SiC metal-n--p structure was examined. By replacing the highly doped top layer with a semitransparent metal contact in the active area, deep-UV photons are absorbed within the high electric field depletion region. Photodiodes with external quantum efficiency (EQE) greater than ~40% at wavelength between 200 nm and 235 nm were demonstrated with this structure. Second, an NIP structure with a graded doped top layer was studied. The high-low junction formed due to the graded doping in the top layer is expected to provide an electric field that pushes carriers away from the surface and, thus, suppresses surface recombination. However no significant improvement in the deep-UV response was observed. The possible reason is that the electric field generated by the high-low junction is not large enough to compensate the electric field in the opposite direction induced by the surface band bending.
As an alternative method to achieve high deep-UV response, while keeping SiC as multiplication layer material, an AlGaN/SiC heterogeneous structure photodiode was explored. As early effort, a separate absorption and multiplication (SAM) structure was utilized so that the spectral response could be tuned by changing the Al mode fraction in the AlGaN absorption layer. However, it was found that the electric field is confined in the SiC multiplication region by the polarization-induced charge at the AlGaN/SiC interface. Since there is a lattice mismatch between AlGaN and SiC, high defect densities were found in the AlGaN layers. Consequently, an electric field is needed in this layer to achieve good collection efficiency. An AlN inter-layer was simulated to be effective in introducing an electric field in the AlGaN layer, but experimental results showed that the AlN acted as a barrier that prevented carriers in the AlGaN absorption layer from entering the SiC multiplication layer. To overcome this problem, an AlGaN top layer with higher Al fraction was used. With a bandgap of over 5.38 eV, this layer is transparent to the deep-UV photons. The photodiodes with this structure achieved high peak external quantum efficiency of ~76% at 242 nm.
The second part of my work focused on near-UV detection using the GaAs/AlGaAs material system. Al0.8Ga0.2As, with a band gap of 2.23 eV, is a promising candidate to replace Si for near-UV detection. An Al0.8Ga0.2As PIN-structure APD was designed, fabricated, and characterized. These APDs have exhibited very low dark current of 179 nA/cm2 at gain as high as 200. Also the measured excess noise factor was very low. The k factor, which is the ratio of the electron ionization coefficient, , to that of the holes, , is about 0.15, which is comparable to Si. The peak external quantum efficiency was ~26% at 470 nm. The response in the near-UV was lower than the expectation. Secondary ion mass spectrometry (SIMS) measurements showed that the doping in the p- layer is significantly higher than the specification that was sent to the foundry. The relatively low near-UV response is due to the fact that a large fraction of the carriers were generated in the undepleted p- layer.
My final research project was to utilize recessed window and surface texturing techniques to enhance the near-UV response of GaAs/AlGaAs photodiodes. The surface texturing using nanosphere natural lithography provided a photodiode surface with very low reflectivity (<5%) across a wide spectrum. Combined with a recessed window structure, the GaAs/AlGaAs photodiode exhibited an external quantum efficiency in the range 45% to 55% from 300 nm to 850 nm. These photodiodes were fabricated into arrays and sent to NASA for a project to reduce the number of cameras in satellites.
