Boron doping of Si molecular-beam epitaxy layers: A new high-temperature effusion cell

Abstract

The high-temperature effusion cell (HTEC) presented in this paper is designed for materials requiring evaporation temperatures above 1000 °C. It employs a novel concept of charge heating, indirectly, through a graphite crucible, by electron bombardment combined with thermal radiation. The cell is particularly well suited to molecular-beam epitaxy (MBE) systems and we have used it as a source of B in the MBE growth and doping of Si, obtaining the resulting B concentration NB between 1015/cm3 and 5×1019/cm3. Some characteristics of Si:B layers thus fabricated are presented and it is shown that NB is: (i) an exponential function of Tcell ; (ii) inversely proportional to the growth rate; and (iii) does not depend on the substrate temperature. We conclude from (ii) and (iii) that B is totally incorporated in the crystal when coevaporated with Si. Because of the total incorporation of B and the absence of its surface segregation very abrupt doping profiles can be obtained. NB gradients as steep as one decade per 100 Å have been obtained. A comparison of the secondary ion mass spectroscopy (SIMS) data and electrical measurements results indicates that B impurities are all electrically active. The values of resistivity ρ, and the Hall mobility μ, at room temperature are both bulklike and both ρ(NB) and μ(NB) follow the standard bulk Si dependences. The MBE layers grown without intentional doping are n-type, and have an electron concentration of 5×1013/cm3 and μ=1450 cm2/V s at room temperature, like pure n-type Si.