Modulation doping in GexSi1−x/Si strained layer heterostructures

Abstract

We report the first observation of the modulation doping effect in Si/Ge_0.2Si_0.8 heterojunctions grown by molecular beam epitaxy. Peak hole mobilities of ∼3300 cm² V⁻¹ s⁻¹ have been observed at 4.2 K. These values, although nonoptimum, are comparable to the best reported values for holes in Si/SiO₂ inversion layers. Low temperature, angular dependent, Shubnikov–de Haas measurements have demonstrated the two‐dimensional nature of the hole gas and yield a surface carrier density of 3.5×10¹¹ cm⁻². From the temperature dependence of the Shubnikov–de Haas amplitudes a hole effective mass of 0.30±0.02m_o has been derived. Identical measurements on n‐type heterojunctions having the same Ge content (x=0.2) have failed to show a sustained enhancement of mobility at low temperatures, indicating that ΔE_v≫ΔE_c.