Single photon emitters in thin GaAsN nanowire tubes grown on Si

TL;DR

This study demonstrates the monolithic integration of thin GaAs/GaAsN/GaAs core–multishell nanowires on Si(111) using plasma-assisted MBE, achieving a GaAsN shell with ~2.7% N and 10 nm thickness that reduces the bandgap by ~400 meV. The NWs exhibit high crystalline quality with defect-free zincblende cores and a short WZ tip, and they host tightly localized excitons in the thin GaAsN shell, producing sharp low-temperature emission around 1.09 eV. Importantly, a spectrally isolated GaAsN-shell line shows true single-photon emission with $g^{(2)}(0)=0.056\pm0.027$ and a lifetime of ~16 ns, demonstrating quantum-light performance without photonic cavities. The results establish a path toward fiber-coupled, on-chip quantum photonic devices based on site-controlled, nitrogen-dilute GaAsN NW emitters integrated on silicon.

Abstract

III-V nanowire heterostructures can act as sources of single and entangled photons and are enabling technologies for on-chip applications in future quantum photonic devices. The unique geometry of nanowires allows to integrate lattice-mismatched components beyond the limits of planar epilayers and to create radially and axially confined quantum structures. Here, we report the plasma-assisted molecular beam epitaxy growth of thin GaAs/GaAsN/GaAs core-multishell nanowires monolithically integrated on Si (111) substrates, overcoming the challenges caused by the low solubility of N and a high lattice mismatch. The nanowires have a GaAsN shell of 10 nm containing 2.7% N, which reduces the GaAs bandgap drastically by 400 meV. They have a symmetric core-shell structure with sharp boundaries and a defect-free zincblende phase. The high structural quality reflects in their excellent opto-electroinic properties, including remarkable single photon emission from quantum confined states in the thin GaAsN shell with a second-order autocorrelation function at zero time delay as low as 0.056.

Figures (5)

• Figure 1: Growth and design of the GaAs/GaAsN/GaAs core-shell-shell heterostructured NWs. (a) illustrates the NW growth scheme: First, the nucleation of Ga droplets, second, the VLS growth of the thin GaAs core that is terminated with the controlled crystallization of the Ga droplet, third, the VS growth of the thin GaAsN shell at reduced temperature, fourth, the VS growth of the GaAs outer layer. (b) SEM image of the NWs on sample A. (c) $\upmu$-PL spectrum at room temperature showing the bandgap emission of the GaAs-core at 1.46 eV and the emission of the GaAsN-shell at 1.02 eV. The inset shows the reduction of the GaAsN bandgap energy as a function of the N concentration according to the band-anti-crossing model at room temperature for bulk (orange line), while the horizontal purple line marks the GaAs bandgap energy value as a reference.
• Figure 2: Structural analysis of a single NW by TEM. (a) BF-TEM image of an entire NW with an inset of the SAED pattern. (b)-(d) HR-TEM images from the middle region with a pure defect-free ZB phase and (e)-(g) HR-TEM images from the tip of the NW with a short WZ segment. The insets in (d) and (g) display the fast Fourier transform showing the ZB phase in (d) and the WZ phase in (g). These images are taken from the <110> zone axis on a NW transferred from sample A.
• Figure 3: Structural and compositional analysis of the multi-shell cross-section of sample A. (a)-(b) Atomic-resolution ADF-STEM images from the <111> zone axis. The Fourier transform in the inset of (a) evidences the ZB crystal structure. A brighter contrast is observed in the GaAsN shell along three out of the six 112 planes laying in the symmetry axes linking the corners of the hexagonal section. (c) EDX maps of the atomic concentrations of Ga, and As, N; measuring a N concentration of 2.9% in the shell.
• Figure 4: Optical properties at low temperature. (a) $\upmu$-PL spectrum from the growth chip of sample B at T = 6 K. The emission from the 10 nm thick GaAsN shell is colored orange, while the emission from the 40 nm thick GaAs core is colored purple and multiplied by a factor of 100 for better visibility. (b) shows the power and (c) the temperature study of a different point on sample B. They are measured with the CCD detector, which cuts off the emitted PL signal at the low energy side of the GaAsN band. (d) shows the thermal quenching of the quantum dot emission at 1.29 eV with respect to the thermal quenching of the GaAsN shell emission in an Arrhenius plot.
• Figure 5: Single photon emission by a strongly localized exciton in the GaAsN shell. (a) Three spectra from a power study of a narrow, spectrally isolated peak from a point close to the one of the spectrum shown in Figure 4. The linewidth broadens for increasing laser power. (b) shows the integrated area of the sharp emission peak as a function of excitation power, saturating like a typical two-level system. (c) Second-order autocorrelation measurement of the line in (a). The normalized coincidence counts are shown as a function of the time delay $t$. Single-photon emission is confirmed by a value of $g^{2}\left(0\right)$ equal to $0.056\pm 0.027$. (d) shows the time-resolved PL measurement from which a decay time of $\tau=16$ ns is extracted.