Fabrication and Characterisation of SiGe Heterojunction Bipolar Transistors Formed by Germanium Implantation

Mitchell, Michele Joan (2001) Fabrication and Characterisation of SiGe Heterojunction Bipolar Transistors Formed by Germanium Implantation. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

A batch of pnp transistors is made to investigate methods of eliminating defects created by Ge implantation. Hairpin dislocations are not seen in this batch, and it is proposed that they nucleate in other batches because of stress introduced by deep trench processing. A high energy Si implant (EPIFAB) is shown to move the 'horizontal' EoR defects deeper into the substrate, out of the active area of the transistors and qualitatively reduces the defect density seen by TEM. Mask edge defects and 'vertical' EoR defects are removed by performing a blanket Ge implant at the start of the fabrication process. These defects are found to be the dominant source of reverse biased collector-base leakage. Surface defects are seen in transistors implanted with 130keV 3.09 x 10¹⁶cm^-2Ge⁺, but are removed by reducing the implant dose to 1.55 x 10¹⁶cm^-2, which significantly reduces emitter-base diode leakage currents. Collector current modelling using the generalised Moll-Ross equation and measured doping profiles, shows excellent agreement with control devices, but poorer agreement with Ge implanted transistors. It is suggested that this poor agreement may be due to reduced mobility due to defects.

Diffusion studies are carried out to further investigate and quantify the effects of the Ge implant seen in transistor batches. Values of As diffusion coefficients in SiGe are determined at a temperature of 1025°C, these being 5.16 x 10^-15cm²/s and 3.45 x 10^-15cm²/s for 4.57at.% and 2.16at.% SiGe, respectively. The effect of implantation damage on As diffusion is studied by implanting Si prior to an As implant. A comparison of simulated and measured As profiles shows that the damage from a Si or Ge implant increases As diffusion. However, a regrowth anneal after the Ge implant, but before the As implant, can be used to eliminate this enhanced As diffusion. Measured Ge profiles from the transistors show diffusion of the Ge into the emitter polysilicon, which is not expected. A detailed study is therefore carried out of Ge diffusion in polycrystalline silicon at temperatures between 800°C and 900°C. The diffusion coefficient in polysilicon is found to be a factor of �10⁴ higher than reported values in single crystal silicon at 900°C. This is explained by Ge diffusion along grain boundaries. Detailed analysis gives an activation energy of E=2.39±0.18eV and a pre-exponential factor of D_o=0.0012±0.0003cm²/s.