Modulation doping in Ge(x)Si(1 - x)/Si strained layer heterostructures: Effects of alloy layer thickness, doping setback, and cladding layer dopant concentration

Selectively doped Gex Si , _ x lSi strained layer heterostructures have been grown in a single quantum well configuration on (OOl)-Si substrate using molecular beam epitaxy. The modulation doping effect has been observed inp-type structures only; although both nandp-type double heterostructures were grown. We have investigated the effects of: (i) alloy layer thickness (well width), (ii) doping setback, and (iii) cladding layer dopant concentration, on the hole mobilities. At present, optimum structures show peak hole mobilities in excess of 3300 cmZy-ls1 at 4.2 K, sheet charge densities of 3.0--14 X 10" cm -z, and hole effective mass mt = 0.32 ± 0.03 mo. I t is found that the low temperature (T -10 K) hole mobility, in structure having x = 0.2 is relatively insensitive to alloy layer (well) thickness in the range 100 A S Lw S Lc; where Lc is the critical thickness marking the transition from strained layer to relaxed, misfit accommodated, alloy growth. For Lw > Lc' misfit dislocation scattering dramatically decreases J-lh at TS 100 K; whereas for Lw S 100 A the hole mobility is uniformly lowered over the entire range of measurement temperatures. Doping setback and cladding layer dopant concentration affect the mobility via changes in sheet charge density, in a manner consistent with theoretical expectations. The absence of the modulation doping effect in n-type heterostructure is taken as an indication that £lEv >£lEc .