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ELECTRON ACCUMULATION IN AIGaSb/InAs OW SYSTEM, - EVIDENCE FOR COEXISTENCE OF DEEP ACCEPTOR AND DONOR -

S. Ideshita, A. Furukawa, Y. Mochizuki and M. Mizuta Fundamental Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305, Japan ABSTRACT We investigated the reason of the (imbalanced) accumulation of electrons in AlGaSb/InAs/AlGaSb QW system in spite of the p-type conduction of undoped AlGaSb. It was found that the concentration of the accumulated electrons negligibly depended on the number of the interfaces, but increased linearly with the effective AlSb thickness. These results indicate that donor levels in AIGaSb are the dominant electron sources. We propose a model that the deep acceptors with larger concentration and donors coexist, and the electron accumulation depends on the energy position of the acceptor in AIGaSb with respect to the quantum level in the InAs well. Acceptor levels obtained experimentally are about 100 meV higher than the bottom of the InAs conduction band, and we succeeded in eliminating the electron accumulation by making the quantum level of the InAs well higher than this acceptor level. The origins of the donors and acceptors are also discussed. 1.INTRODUCTION The AlGaSb/InAs/AlGaSb QW system is very interesting because of their enormous potential for ultra-fast electronic device applications, especially low noise FETs[1 -3]. The fabrication technique of this QW structure has not been established yet. One of the most serious problems is the uncontrollable electron accumulation discussed inthis report. Electrons with the areal concentration of the order of 1012 cm 2 are accumulated in the InAs well for Al composition between 0.5 and 1 even if AIGaSb barrier layers are not intentionally doped[4]. For the semi-metallic GaSb/InAs/GaSb QW system, the imbalanced electron accumulation was also reported[5-7]. There have been a few reports about origins of the electron accumulation in AlGaSb/InAs/AlGaSb QW structures[4-6]. Most of these works, so far reported, attributed the electron source to the trapping levels at InAs/AIGaSb hetero-interfaces [4-6]. The only exception is the report by Altarelli et al. explaining the electron accumulation by surface levels[7]. The systematic study by Tuttle et al.[4] has revealed that the areal electron concentration and electron mobility are dependent on the growth conditions. They proposed the As-on-Al (hetero-antisite) defect as the source of the electron accumulation. In the case of the AlGaSb/InAs/AlGaSb QW system, three types of electron sources should be taken into account: surface levels[7] at AlGaSb, donors at the AIGaSb/InAs interface (interface levels) and traps inthe AlGaSb (bulk levels). In the following, we first demonstrate that the effect of surface levels can be experimentally separated and made negligible. Next, the dominant contribution by the bulk levels over the interface levels is clarified. We further propose a model for the accumulation mechanism and provide experimental evidence, and discuss origins of defects.