Carrier mobility and strain effect in heavily doped p-type Si

Although carrier mobility (μ) in Si is a fundamental property deeply investigated since 40 years, a complete understanding of its characteristics over a large range of carrier concentration is still lacking. For example, the effect of strain was largely debated and μ enhancement was demonstrated in strained Si channels where the carrier concentration is 10^20 cm−3), which is actually of fundamental interest for USJ applications, many questions are still open about μ: why is μ lower in presence of some dopants with respect to other chemical species? The relevant point is that high μ in either n- or p-type Si is observed when the dopant has a covalent radius smaller than Si and that, at the concentration at which the chemical effect on the μ is visible, an appreciable strain is generated in the doped layer. We present here an experiment to study the dependence of the hole μ on the dopant species in heavily doped p-type Si under low electrical field. The Hall carrier concentration and μ has been measured in Si co-implanted with B and Ga in the (0.1–2) × 10^20 cm−3 range. The strain induced by substitutional dopants, detected by high resolution X-ray diffraction (HRXRD), has been varied by changing the B and Ga concentration. The effect of strain on μ has been disentangled and a linear dependence of 1/μ on the perpendicular strain has been found. Using this relationship we demonstrate that the strain induced by the substitutional dopant can account for the effect of chemical species on charge carrier μ