Exciton fine-structure splitting in GaN/AlN quantum dots

Exciton bright-state fine-structure splitting (FSS) in single GaN/AlN quantum dots (QDs) is reported, presenting an important step toward the realization of room temperature single-qubit emitters for quantum cryptography and communication. The FSS in nitride QDs is up to 7 meV and thus much larger than for other QD systems. We find also a surprising dependence of FSS on the QD size, inverse to that of arsenide QDs. Now we are able to explain why FSS can only be observed in small QDs of high-emission energies. Our calculations reveal a shape/strain anisotropy as origin of the large FSS allowing different approaches to control FSS in nitrides.

Figure 1

(Color online) Linear polarization of luminescence from a large QD (a) showing identical polarization of exciton and biexciton, Ref. 14, and a small QD (b) with clear orthogonal polarization of excitonic peaks $X_1$ and $X_2$ ($\Delta E_{X_{1,2}}=3.9\text{meV}$, $P_{X_1}=0.95$, $P_{X_2}=0.96$, and $\Delta {\phi }_{X_{1,2}}=98°$).

Figure 2

(Color online) Overview of luminescence properties from different QDs. (a) Energy splitting between peaks $X_1$ and $X_2$; linear regression (dashed line) shows clear tendency to larger splitting for higher emission energies. (b) Difference of linear-polarization angles between peaks $X_2$ and $X_1$ (cubes) and lower side peaks and $X_1$ (circles). (c) Degree of polarization of peaks $X_1$ and $X_2$. Data points from QDs with $P<0.8$ for peak $X_1$ or $X_2$ are shaded in gray.

Figure 3

(Color online) Calculated FSS of QDs with a height of $h=1.2\text{nm}$ and diameter of $d=7.5\text{nm}$. (a) Absorption spectrum of the A band (A-X) and B band (B-X) excitonic states in a QD with an in-plane elongation of 20% along the $y$ axis. [(b) and (c)] Fine-structure splitting of the $A$ exciton as a function of (b) in-plane elongation or (c) uniaxial stress.

Figure 4

(Color online) FSS of A-exciton bright states as function of QD size. Solid circles are calculated, empty squares are experimental values. The model QDs have a fixed aspect ratio $a{r}_{\text{vert}}=h/d=0.2$ but varying height $h$ (0.8–2.0 nm). Inset shows FSS for QDs with constant height $h=1.2\text{nm}$ but varying aspect ratio $a{r}_{\text{vert}}$ (0.14–0.22).