Ge(001):B gas-source molecular beam epitaxy: B surface segregation, hydrogen desorption, and film growth kinetics

Abstract

Ultrahigh B-doped Ge(001) layers, with concentrations CB up to 8×1021 cm−3, were grown by gas-source molecular beam epitaxy from Ge2H6 and B2H6 at temperatures Ts=325 °C (in the surface-reaction-limited regime) and 600 °C (in the flux-limited regime). The samples were quenched, D site exchanged for H, and D2 temperature-programed desorption (TPD) used to determine B coverages θB as a function of CB and Ts by comparison with B-adsorbed Ge(001) reference samples with known θB values. During Ge(001):B film growth, strong surface B segregation to the second layer was observed with surface-to-bulk B concentration ratios ranging up to 6000. The TPD spectra exhibited α2 and α1 peaks associated with dideuteride and monodeuteride desorption as well as lower-temperature B-induced α2* and α1* peaks associated with deuterium desorption from Ge* surface atoms with B backbonds. Increasing θB expanded the area under α2* and α1* at the expense of α2 and α1 and decreased the total D coverage θD. The TPD results were used to determine the B segregation enthalpy, −0.64 eV, and to explain and model the effects of high B coverages on Ge(001) growth kinetics. At Ts=325 °C, where B segregation is kinetically hindered, film deposition rates RGe are not a strong function of CB, exhibiting only a small decrease at CB≳5×1018 cm−3. However, at Ts=600 °C, RGe decreases by up to 40% with increasing CB≳1×1018 cm−3. This is due primarily to the combination of B-induced Ge dimer vacancies and the deactivation of surface dangling bonds caused by charge transfer to Ge–B backbonds. Calculated RGe(CB,Ts) curves, based upon equilibrium segregation, exhibit very good agreement with deposition rate data at Ts=600 °C and overestimate the effect of B on RGe at 325 °C.