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

Ultrahigh B-doped Ge(001) layers, with concentrations C_B up to 8 × 10²¹ cm^-3, were grown by gas-source molecular beam epitaxy from Ge₂H₆ and B₂H₆ at temperatures T_s = 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 θ_B as a function of C_B and T_s by comparison with B-adsorbed Ge(001) reference samples with known θ_B 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 α*₂ and α*₁ peaks associated with deuterium desorption from Ge* surface atoms with B backbonds. Increasing θ_B expanded the area under α*₂ and α*₁ at the expense of α₂ and α₁ and decreased the total D coverage θ_D. 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 T_s = 325°C, where B segregation is kinetically hindered, film deposition rates R_Ge are not a strong function of C_B, exhibiting only a small decrease at C_B≳5×10¹⁸ cm^-3 However, at T_s = 600°C, R_Ge decreases by up to 40% with increasing C_B≳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 R_Ge(C_B,T_s) curves, based upon equilibrium segregation, exhibit very good agreement with deposition rate data at T_s = 600°C and overestimate the effect of B on R_Ge at 325°C.