dc.description.abstract | Along with the increase in device density of Si-based integrated circuits, power consumption and heat dissipation have become key issues that cannot be ignored any more. Replacing Si with high mobility materials to realize high performance transistors with low operating voltage and power consumption is thus a subject under extensive investigations. Among the materials under investigations, III-V compounds are considered one of the most promising candidates. Sb-based compounds have therefore received a lot of attention due to their high intrinsic carrier mobility and low bandgap. However, high performance p-channel transistors and the integration of Sb-based devices on Si substrates remain challenging. To explore the feasibility of the approach above, this work aims at the growth of high quality n-channel InAs/AlSb quantum-well (QW) and p-channel InGaSb/AlSb QW heterostructure field-effect transistors (HFETs) on GaAs and Si substrates by molecular beam epitaxy.
Through the adjustments in layer structures and growth parameters, InAs/AlSb QW HFETs grown on GaAs substrates show electron mobility greater than 27,000 cm2/V•s at room temperature. As for InGaSb/AlSb QW HFETs grown on GaAs substrates, hole mobility higher than 1,000 cm2/V•s at room temperature has also been achieved.
It is found that the buffer layers employed for the growth of InAs/AlSb QW HFETs on GaAs substrates do not give satisfactory results when used for the growth on Si substrates. In this study, AlSb/GaSb and GaAs/GaAsSb/GaSb are used as the initial buffer layer for the growth on Si substrates and how the buffer layer affect the properties of QW HFETs is explored. In the case of AlSb/GaSb buffer, there occur numerous edge dislocations, which do not propagate to the channel, at the AlSb/Si interface to accommodate the lattice mismatch. This buffer also generates numerous planar defects, i.e. twins, which deteriorate the growth of the InAs channel and the electrical properties of the transistors. Besides, parallel conduction of this buffer layer due to its low resistivity is observed. In contrast, the GaAs/GaAsSb/GaSb buffer layer can effectively block the planar defects at GaAsSb/GaSb interface and suppress parallel conduction of the buffer. As a result, the anisotropic transport behavior is greatly reduced. With an approximately 2 m-thick buffer layer, electron mobility as high as 18,100 cm2/V•s has been achieved on InAs/AlSb QWs grown on Si.
P-channel InGaSb/AlSb QW HFETs grown on Si with GaAs/GaAsSb/GaSb buffer layers are also investigated. The growth temperature of InGaSb/AlSb QW is found to be a key parameter for obtaining high hole mobility. By optimizing the growth temperature and layer structure, room-temperature hole mobility of 838 cm2/V•s with sheet carrier density of 9.5×1011 cm-2 has been reached. It is also found that the effects of twins on the electrical properties of InGaSb/AlSb QW HFETs are unobvious. In collaboration with Professor Yu-Ming Hsin, devices with a gate length of 0.25 m are fabricated and exhibit a maximum drain current of 81 mA/mm and a peak transconductance of 75 mS/mm. | en_US |