Modulation Doping of GaAs/AlGaAs Core-Shell Nanowires With Effective Defect Passivation and High Electron Mobility
Reliable doping is required to realize many devices based on semiconductor nanowires. Group IIIV nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 +/- 0.06 x 10(16) cm(3) was extracted, demonstrating the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was also found to be high at 2200 +/- 300 cm(2) V-1 s(-1) and importantly minimal degradation was observed compared with undoped reference nanowires at similar electron densities. In addition, modulation doping significantly enhanced the room-temperature photoconductivity and photoluminescence lifetimes to 3.9 +/- 0.3 and 2.4 +/- 0.1 ns respectively, revealing that modulation doping can passivate interfacial trap states.