Dopant electrical activity and majority-carrier mobility in B- and Sb-δ-doped Si thin films

The electrical characteristics of Si films grown by low-temperature molecular-beam epitaxy and doped with Sb or B in single or multiple {δ}-like doping profiles have been investigated. Two-dimensional concentrations ranging from {}10¹³ cm^-2 to 5×{}10¹⁴ cm^-2 have been realized. For single doping spikes Hall measurements indicate an apparent reduction in electrical activity to about 50%. This is linked to the confinement of the carriers in the potential well of the doping spike. The reduced electrical activity is not due to growth imperfections. The effect of the confinement on the Hall measurements can be reduced in {δ}-doping superlattices, provided the distance between doping spikes is sufficiently small. For NSb2D=10¹³ cm^-2 the mobility in a {δ}-doping superlattice drops exponentially as a function of inverse spike spacing until, at about 5 nm, a value close to the mobility in an equivalent, uniformly doped film (NSb{}3×{}10¹⁹ cm^-3) is reached. Carrier mobilities in {δ}-doped layers far exceed those of uniformly doped layers of equivalent concentration. Electron mobilities reach 400 cm²/V s for a sheet concentration of 1×{}10¹³ cm^-2 and are still at {}40 cm²/V s at NSb2D=5×{}10¹⁴ cm^-2. For p-type doping these numbers are 180 and 20 cm²/V s, respectively.