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 <10^17 cm−3. On the other hand, in heavily doped Si (>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.