Suppression of surface segregation of silicon dopants during molecular beam epitaxy of (411)A In_0.75Ga_0.25As/In_0.52Al_0.48As pseudomorphic high electron mobility transistor structures

The authors achieved considerable suppression of surface segregation of Si dopants in In_0.75Ga_0.25As/In_0.52Al_0.48As high electron mobility transistor (HEMT) structures grown on (411)A InP substrates by molecular beam epitaxy (MBE). The (411)A HEMT structures were conventionally grown at a high substrate temperature (T_s) of 540 °C in order to form the extremely flat (411)A heterointerfaces. This results in considerable surface segregation of Si dopants. Surface segregation of Si dopants was suppressed by lowering growth temperature of the top InAlAs barrier layer (T_B) down to 450 °C with keeping T_s of 540 °C for other parts. Sheet carrier concentration (N_s) of two-dimensional electron gas (2DEG) was measured as a function of thickness (L_b) of the top InAlAs barrier for T_B=450 and 540 °C samples. Observed N_s of the T_B=540 °C sample vanished when L_b approached to 7 nm, while 2DEG with of N_s∼2×10¹² cm^-2 remained for the T_B=450 °C sample. Surface segregation of Si-sheet-doped (411)A InAlAs layers grown at T_B was also characterized by secondary ion m- - ass spectrometry measurements. Si segregation length (λ_Si:1/e decay length of Si concentration) observed for T_B=450 °C sample was 2.3 nm, which is 56% smaller than that for T_B=540 °C (λ_Si=5.2 nm).