TEM characterization of MBE-grown interfaces for inverted-HEMT structures

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With the advent of the high electron mobility transistor (HEMT), device mobilities in excess of 130,000 cm2/V-sec (77K) for an MBE-grown GaAs/AlxGa1-xAs (100) (0.25>x>0.30) heterojunction have become commonplace. However, the corresponding growth of high quality AlGaAs/GaAs inverted HEMT structures (Figure 1) has been difficult due to the high degree of disorder at the inverted heterointerface (maximum 77K mobility 80,000 cm2/V-sec). The AlGaAs growth front is subject to: 1) slower migration kinetics of Al with respect to Ga, 2) high reactivity of Al to the ambient impurities, and 3) outdiffusion of the silicon dopant along the growth front, which further compromises the uniformity and planarity of the growth. Past attempts to grow inverted HEMTs have been limited by the competing parameters of high substrate temperatures (T) prefered for achieving high migration of Al for smoother interfaces, and lower Ts to minimize silicon outdiffusion toward the critical interface. In this presentation we introduce TEM characterization of a novel MBE technique which optimizes interface growth conditions, achieving mobilities >120,000 cm2/V-sec, for the inverted HEMT structure.