Design And Fabrication Of Microwave Power SiGe HBT And Novel SiGe HPT Based On Virtual Substrate

The communication market has experienced a substantial and rapid growth over the past few years, which will require higher speed and low cost semiconductor devices in the future communication units. One of the solutions to meet these requirements is the concept of a complete communication system integrating electron devices and optoelectronic devices on a single chip. The successful development of high-speed SiGe heterojunction bipolar transistor (HBT) has provided the opportunity to integrate RF/microwave circuits and CMOS low-power circuit on a single chip in the wireless communication units. At the same time, SiGe heteroj unction bipolar phototransistor (HPT) with high external quantum efficiency, low noise and compatibility of the device's epitaxial structure and fabrication with that of the HBT becomes more attractive for high-speed optoelectronic integrated circuit applications. However, it is a great challenge to obtain higher responsivity in near-infrared communication band for the traditional SiGe HPT due to the lower Ge content and the limitation of the critical thickness of SiGe layer on Si substrates. In this thesis, in order to improve the responsivity and extend the detection wavelength of SiGe HPT as well as the power and frequency of SiGe HBT, comprehensive theoretical designs and systemic experiments on SiGe HBT and HPT have been carried out. The main works and results are summarized as follows:(1) A homemade double-chamber chemical vapor deposition system was built and tested. Based on the growth kinetics of SiGe alloy, the growth conditions including source gas flux and substrate temperature were optimized for reaching certain Ge contents, growth rates and doping concentrations of SiGe layer, which pave the way for the following study of SiGe HBT and HPT.(2) A microwave power SiGe HBT with cut-off frequency of 2GHz have been designed and fabricated. We found that a little increase of Ge content in SiGe base region such as from 0.20 to 0.23 could lead to significant increase in current gain from 60 to 158. It was observed that the SiGe HBTs present positive and negative thermal-electric feed back when the base was biased with voltage and current source respectively. Those opposite trends of thermal-electric feed back were successfully utilized to compensate the self-heating effect of SiGe HBT by inserting a certain ballast resistance in the bias circuit.(3) High quality SiGe relaxed substrate based on Si and SOI substrate were prepared by dry oxidation. With lots of oxidation data, a SiGe oxidation kinetic model including the stress in the oxide was proposed for the first time. We found that the growth rate enhancement in SiGe oxidation process was induced by the lower activation energy of oxygen diffusivity rather than the weaker Ge-Ge bond energy reported by other group. More important, the self-limiting oxidation of SiGe alloy, which was still under debate, can be better understood in this model.(4) Thermal cleaning of SiGe surface under ultra-high vacuum has been systematically investigated. We found that Ge islands preferentially formed in the process of decomposition of native oxide covering the SiGe layer. The size and density of Ge islands could be controlled by the initial Ge content of the SiGe layer, which offered a simple route to fabricated small Ge quantun dots. However, for the SiGe layer passivated with hydrogen, lots of pits formed in the SiGe film during vacuum thermal annealing. When the temperature decreased to 550℃, a planar surface of SiGe substrate was obtained, on which a high quality SiGe epitaxy layer with the same content was also successfully achieved.(5) A novel SiGe HPT based on SiGe virtual substrate has been proposed and fabricated for the first time. The SiGe HPT showed a break voltage BV_ceo of about 14V and a low dark current density in the order of 4mA/cm² at 5V. The responsivity of 1.94mA/W was achieved at 1.55μm for normal incidence, which was about 20 times higher than that of previous SiGe HPT with Ge dot absorption layer reported by other group. It was also the first time to extent the response wavelength up to 1.55μm for SiGe HPT with multi-quantum-well absorption region.