MODULATION DOPED INVERTED AND NORMAL GAAS/ALXGA1-XAS HETEROSTRUCTURES - INFLUENCE OF SI-SEGREGATION ON THE 2-DIMENSIONAL ELECTRON-GAS

Modulation doped GaAs/Al(x)Ga1-xAs heterostructures have been grown by molecular beam epitaxy in the temperature range of 350-850-degrees-C. Special emphasis is laid on the two different types of heterostructures: doped Al(x)Ga1-xAs on top of undoped GaAs (normal heterostructure) and undoped GaAs on top of doped Al(x)Ga1-xAs (inverted heterostructure). The electrical properties, determined by Hall effect measurements at 77 and 300 K, were investigated as a function of growth temperature. At 77 K the normal heterostructures exhibit a maximum mobility of 120,000 cm2/V.s at a substrate temperature of 750-degrees-C. In contrast, the maximum mobility of 45,000 cm2/V.s for inverted heterostructures is found at a substrate temperature of 500-degrees-C, whereas at a temperature of 750-degrees-C a drastic reduction of mobility of 3500 cm2/V.s is observed which is attributed to the segregation of Si atoms into the channel. SIMS depth profiles indeed show Si segregation in the spacer layer at a growth temperature of 700-degrees-C. Based upon these results we have grown inverted heterostructures at 750-degrees-C with suppressed Si segregation by growing the doped Al(x)Ga1-xAs layer at 500-degrees-C.