Quantum dot to quantum wire transition of $m$-plane GaN islands

Structural studies of $m$-plane GaN quantum dots and quantum wires are presented. A shape transition responsible for the evolution of quantum dots into quantum wires is put in evidence and shown to depend on the amount of material deposited. In addition, it is established that vertical correlation of successive nanostructure planes may also induce the dot-wire shape transition. Consistent with theoretical predictions, it is proposed that the shape transition results from an elastic energy minimization process made possible by an easy adatom diffusion along [11-20] direction.

Figure 1

(Color online) (a) AFM image of a 300-nm-thick AlN buffer layer on $m$-plane SiC. (b) Profile of the undulations along $c$ axis.

Figure 2

(Color online) $1\times 1\mu {\text{m}}^2$ AFM images showing GaN islands deposited on a 300-nm-thick AlN buffer layer, as a function of GaN amount. (a) 5, (b) 7, (c) 15, and (d) 20 ML.

Figure 3

High-resolution transmission microscopy images of $m$-plane GaN quantum wires corresponding to 15 ML of GaN deposited. (a) HREM of a GaN QWire taken along the [11-20] zone axis, showing the anisotropic wire shape correlated with the direction of $c$ polar axis. The sharp edge corresponds to $+c$. (b) HREM taken along the [0001] zone axis, showing the needle shape morphology of a GaN quantum wire.

Figure 4

Statistics of (a) length $\left(L\right)$, (b) width $\left(w\right)$, and (c) height $\left(h\right)$ of $m$-plane GaN quantum dots and quantum wires extracted from AFM images shown in Fig. 2. Solid squares: QDs; open triangles: QWires. Note that height does not depend on island morphology and saturates to a value of about 2.5 nm. In (b), the dash-dotted line corresponds to $W_1$, the top width of AlN stripes shown in Fig. 1, and the dotted line corresponds to $W_2$, the mean width of AlN stripes.

Figure 5

Width $w$ (open squares and open triangles for QDs and QWires, respectively) and length $L$ (solid squares and solid triangles for QDs and QWires, respectively) of GaN islands as a function of their area, from data in Fig. 4. The vertical dotted line at $A=1500{\text{nm}}^2$ corresponds to the critical area for the transition from dots to wires.

Figure 6

(Color online) $1\times 1\mu {\text{m}}^2$ AFM images of the surface plane of a superlattice consisting of 25 planes of GaN islands separated by an AlN spacer (a) 3, (b) 15, and (c) 22 nm thick. The amount of GaN deposited corresponds to 5 ML.

Figure 7

(Color online) $1\times 1\mu {\text{m}}^2$ AFM images of the surface plane of a superlattice of correlated GaN islands (AlN spacer thickness equal to 3 nm) as a function of the number of planes of the superlattice, $N$: (a) $N=2$, (b) $N=3$, and (c) $N=5$.

Figure 8

(a) Evolution of the density (open circles) and area $A$ (solid diamonds) of the GaN nanostructures with the number of correlated planes, $N$. (b) Evolution of $w$ (open squares) and $L$ (solid squares) of GaN nanostructures as a function of their area.