High Power Laser Processing of Semiconductor Germanium for Optoelectronic Devices

Recently germanium semiconductor material is receiving renewed interest both in microelectronic and optoelectronic applications. Due to its higher carrier mobilities than silicon, germanium is a candidate for high speed metal oxide semiconductor field effect transistors. Also germanium can effectively absorb near infrared light, so it has applications in near infrared optoelectronic devices. The germanium-based photodetectors, imaging sensors and thermophotovoltaic devices have been widely investigated.
Conventional fabrication steps of germanium-based optoelectronic and microelectronic devices include surface microtexturing, doping, surface antireflection coating and metallization. Unlike silicon, the process steps for germanium are neither well established nor fully understood. Among them, two critical steps are surface microtexturing and doping. The special material properties of germanium result in lots of challenges in these two steps. Cost-effective methods for microtexturing of germanium surface for reflection reduction do not exist. Problems such as dopant out-diffusion, substrate loss and shallow junction formation occur for conventional doping methods. These challenges encountered by conventional fabrication methods can be effectively overcome by laser-based methods.
In addition, the performance of germanium-based optoelectronic devices still needs to be improved further in order to stand out among devices based on other materials. Chalcogen hyperdoping has been proven to bring high photo-gain and sub-bandgap photoresponse to silicon-based photodiodes. Since germanium and silicon are similar materials, chalcogen hyperdoping can be expected to be a potential way to improve the performance of germanium-based optoelectronic devices. This hyperdoping can only be achieved by means of short pulsed lasers due to the fast heating and cooling process which results in super-saturation of chalcogen atoms in the semiconductor material.
In this work, laser microtexturing method was developed to reduce the optical reflection over broad spectral and angular range and good performance of germanium photodetector was demonstrated. Secondly, the formation of germanium p-n junction was demonstrated by means of pulsed laser doping process and the performance was characterized. Lastly, sulfur as a deep level dopant was incorporated into germanium by short pulsed laser and from which a photodiode was fabricated. The novel photodiode based on sulfur-doped germanium showed higher external quantum efficiency than conventional germanium photodetectors over broad spectral range. We have demonstrated that pulsed laser can be effectively used to enhance performance of Ge optoelectronic devices and for the development of novel device concepts.