Role of quantum confinement in giving rise to high electron mobility in GaN nanowall networks

•Recently, µ in random networks of GaN nanowalls is estimated to be several orders of magnitude larger than that is observed in GaN bulk [Appl. Phys. Lett. 101, 132109 (2012)]. The origin of this effect is not very clear yet. Here, our study reveals that the high µ mobility region extends down to several hundreds of nanometer below the tip of the walls.•In longer time grown samples, walls are found to be flat-topped with the top surface containing certain secondary tip structures. These additional features run along the length of the walls to form a well-connected network. Our study reveals that the carriers are quantum mechanically confined not only in the secondary tip structures but also in the wider part of the walls.•Secondary tip structures, which are identified as the regions of stronger confinement than the remaining parts of the walls, have been found to offer higher µ than the rest of the network. The effect of µ enhancement observed in these samples has been attributed to the quantum confinement of electrons.

Origin of unprecedentedly high electron mobility observed in the c-axis oriented GaN nanowall networks is investigated by studying the depth distribution of structural, electrical and optical properties of several such high mobility samples grown by molecular beam epitaxy (MBE) technique for different time durations. It has been found that in two hour grown samples, walls are tapered continuously from the bottom to the top. While in four hour grown samples, walls are flat-topped with the top surface containing certain secondary tip structures. These additional features run along the length of the walls to form a well-connected network. Our study reveals that the carriers are quantum mechanically confined not only in the secondary tip structures but also in the wider part of the walls. The secondary tip structures, which are found to offer higher mobility than the rest of the network, are also identified as the regions of stronger confinement. The effect of mobility enhancement observed in these samples has been attributed to a 2D quantum confinement of electrons in the central vertical plane of the walls.