Electronic transport through GaN quantum dots in nanowires

Friday 30 April 2010

Nanowires are considered both as promising building blocks for nano-scale devices and as an alternative route to access the physics of low dimensional systems. In III-N materials, the strain relaxation offered by nanowires overcomes the typical problem of high dislocation density. Furthermore, III-N nanowire heterostructures have opened a new pathway to create III-N quantum dots with a flexibility to tune the dot height and to adjust the material composition without the requirement of lattice mismatch as the quantum dot grown by in Stranski Krastanow growth mode. Currently, most of the studies on III-N quantum dots are oriented towards photonic aspects, while there are very few works focusing on probing such structures via electrical means.

Here, we fabricate GaN quantum dots by utilizing III-N nanowire heterostructures. We focus on the electronic transport through single GaN quantum dots in nanowires. Our results indicate that GaN quantum dots formed in a nanowire are promising nano-objects for accessing the electronic properties of III-N based nanostructures. The GaN nanowires were grown on Si(111) substrates by using plasma-assisted molecular beam epitaxy without using any catalysts [Fig.1 (a)]. The GaN quantum dots were fabricated by introducing two AlN tunnel barriers into the GaN nanowires [high resolution TEM image in Fig. 1(b)]. After growth, the nanowires were dispersed on a SiO2 layer on top of a heavily doped Si substrate.

Figure 1: Transmission electron microscopy image of a GaN nanowire ensemble grown on a Si (111) substrate (b) High resolution transmission electron microscopy image of a GaN quantum dot formed by AlN double tunnel barriers (bright contrast) grown inside GaN nanowires.

For the electronic transport measurements the nanowires are electrically contacted by metal electrodes written by electron beam lithography [Fig. 2(a)]. The stability diagram of the differential conductance dI/dVSD versus source-drain voltage (VSD) and gate voltage (VG) at 4.2 K shows well-defined diamond structures [Fig. 2(b)]. Coulomb diamonds are clearly separated from each other and have all their edges fully defined, revealing Coulomb-blockaded transport through a single electronic island. The periodicity along the VG axis suggests that the length of the dot through which transport is taking place is corresponding to the GaN dot size in between the two tunnel barriers.

Even though such a study has already been done in several types of nano-scale islands such as Si, III-As and metals, there are very few reports in III-N so far. The present results show for the first time, single electron transport through single GaN/AlN quantum dots based on nanowire heterostructures. This work is not only valuable for the application of III-N nanowires but also for a deeper understanding of quantum transport in nitride materials which is mostly unexplored.

Figure 2(a): Schematic illustration of a nanowire with double tunnel barriers with metal contacts on an oxidized Si substrate (b) Differential conductance dI/dVSD versus VSD and VG for a GaN quantum dot formed in a nanowire (measurement is taken at 4.2K).

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