Strained high percentage (60%) boron doped silicon-germanium alloys - Strain, dopant substitionality, carrier concentration, resistivity, and microstructure development

Abstract

Future generations of silicon based integrated circuit technology require carrier concentrations in excess of the equilibrium dopant concentrations. Implantation is not the dopant method of choice anymore, since the structures get smaller and thinner with each technology node, additional three-dimensional structures being introduced with FinFET as one option for the 22 nm node [1]. Implantation into thin semiconductor structures will amorphize them and relax eventual strain in them. In-situ doping during the epitaxial growth is an alternate way to deposit dopants where you need them in concentrations desired and avoid the problems associated with implantation. In this work we study the incorporation of boron into fully strained high percentage (60%) boron doped Silicon-Germanium (SiGe). We will discuss the epitaxial growth, dopant incorporation, and strain compensation due to the dopant atoms, defect generation in highly strained doped SiGe layers as well as dopant activation, free carrier concentration and mobilities. We will look at band structure effects due to high doping in strained SiGe layers. The studied SiGe films were epitaxially deposited at low temperature to achieve high doping levels and avoid thermally induced relaxation. At those low deposition temperatures we observed high dopant incorporation with perfect crystal quality. A thickness range of 20 to 27 nm was chosen to retain a fully strained layer while allowing sufficient material for physical and electrical analyses. Doping levels reach from 3·10 19cm -3 to 1.5·10 21cm -3 and higher. At the higher doping levels three-dimensional growth is observed, which leads to pronounced relaxation (Figure 1). From highly n-doped silicon it is known, that not all substitutional incorporated phosphorus is electrically active, since it forms donor dimers [2]. These dimers electrically deactivate the phosphorus, but still contribute to the lattice contraction. We also observe a high lattice contraction, measured here as strain compensation, from the added boron atoms. The existence of similar boron acceptor dimers is imaginable, which would lead to less electrical active dopants than the total substitutional incorporated dopant. Hall measurements on boron doped 60% SiGe showed that the carrier density is significantly higher than the boron doping concentration determined by SIMS. Mobilities are very low with 30 cm 2/V sec as example for the 1×10 20 cm -3 boron doped sample. We will explain these effects and draw conclusions for the band structure and density of states in doped 60% SiGe. This work was performed by the Research Alliance Teams at various IBM Research and Development Facilities. © 2012 IEEE.