Chu Hsiang Teng

Single photon emission from site-controlled InGaN/GaN quantum dots

We report single-photon emission from electrically driven site-controlled InGaN/GaN dot-in-nanowires, fabricated from a planar single InGaN quantum well LED using a top-down approach. Each dot-in-nanowires formation site, diameter, height and material compositions were precisely controlled.

Carrier dynamics in site- and structure-controlled InGaN/GaN quantum dots

We report on the carrier dynamics in InGaN/GaN disk-in-a-wire quantum dots with precisely controlled location and structural parameters, including diameter, thickness and material composition. We measured the time-integrated and time-resolved spectra and the second-order correlation function of the photoluminescence from quantum dots with diameters ranging from 19 nm to 33 nm at temperatures of 10 K to 120 K. The influence of the small fluctuations in structural parameters, most importantly the quantum dot thickness, on the optical properties are also investigated through statistical correlations among multiple optical properties of many individual quantum dots. We found that in a single dot the strain-induced polarization field and the strain relaxation at the sidewall form a potential barrier to protect the exciton from reaching the sidewall surface. However, the exciton can overcome this potential barrier and recombine nonradiatively at the surface through two mechanisms: tunnelling through the barrier quantum mechanically and hopping over the barrier by attaining sufficient thermal energy. The former (latter) mechanism is temperature insensitive (sensitive) and dominates nonradiaitve exciton decay at low (high) temperatures. We also found that despite the good uniformities in structural parameters, all optical properties still exhibit inhomogeneities from dot to dot. However, all these inhomogeneities can be modeled by simply varying the potential barrier height, which also explains the observed correlation curves among all optical properties. Finally, we found that the biexciton-to-exciton quantum efficiency ratio, which determines the probability of multi-photon emission, can be tuned by adjusting the potential barrier height and the temperature, suggesting a new way to achieve single photon emission at high temperatures.

Elliptical quantum dots as on-demand single photons sources with deterministic polarization states

In quantum information, control of the single photons polarization is essential. Here, we demonstrate single photon generation in a pre-programmed and deterministic polarization state, on a chip-scale platform, utilizing site-controlled elliptical quantum dots (QDs) synthesized by a top-down approach. The polarization from the QD emission is found to be linear with a high degree of linear polarization and parallel to the long axis of the ellipse. Single photon emission with orthogonal polarizations is achieved, and the dependence of the degree of linear polarization on the QD geometry is analyzed.

How much better are InGaN/GaN nanodisks than quantum wells—Oscillator strength enhancement and changes in optical properties

We show over 100-fold enhancement of the exciton oscillator strength as the diameter of an InGaN nanodisk in a GaN nanopillar is reduced from a few micrometers to less than 40 nm, corresponding to the quantum dot limit. The enhancement results from significant strain relaxation in nanodisks less than 100 nm in diameter. Meanwhile, the radiative decay rate is only improved by 10 folds due to strong reduction of the local density of photon states in small nanodisks. Further increase in the radiative decay rate can be achieved by engineering the local density of photon states, such as adding a dielectric coating.

Site-controlled InGaN/GaN single-photon-emitting diode

We report single-photon emission from electrically driven site-controlled InGaN/GaN quantum dots, fabricated from a planar light-emitting diode structure containing a single InGaN quantum well using a top-down approach. The location, dimension, and height of each single-photon-emitting diode are controlled lithographically, providing great flexibility for chip-scale integration.

Strain-induced red-green-blue wavelength tuning in InGaN quantum wells

Monolithically integrating multi-color pixels from a standard InGaN quantum well active region was demonstrated with a wavelength tuning range of 178 nm. Nanopillar structures were fabricated to enable the wavelength tuning. Strain induced wavelength shift was investigated both experimentally and theoretically. A simple one-dimensional strain relaxation model was shown to accurately predict the wavelength shift as a function of the nanopillar diameter. The strain relaxation was found to depend on the indium composition in the quantum well. No noticeable increase of the defect density was observed after the strain relaxation process.

Monolithic integration of individually addressable light-emitting diode color pixels

Monolithic integration of individually addressable light-emitting diode (LED) color pixels is reported. The integration is enabled by local strain engineering. The use of a nanostructured active region comprising one or more nanopillars allows color tuning across the visible spectrum. In the current work, integration of amber, green, and blue pixels is demonstrated. The nanopillar LEDs exhibit an electrical performance comparable to that of a conventional thin-film LED fabricated on the same wafer. The proposed platform uses only standard epitaxy and a similar process flow as a conventional LED. It is also shown that the emission intensity can be linearly tuned without shifting the color coordinate of individual pixels.

Impact of carrier localization on recombination in InGaN quantum wells and the efficiency of nitride light-emitting diodes: Insights from theory and numerical simulations

We examine the effect of carrier localization due to random alloy fluctuations on the radiative and Auger recombination rates in InGaN quantum wells as a function of alloy composition, crystal orientation, carrier density, and temperature. Our results show that alloy fluctuations reduce individual transition matrix elements by the separate localization of electrons and holes, but this effect is overcompensated by the additional transitions enabled by translational symmetry breaking and the resulting lack of momentum conservation. Hence, we find that localization increases both radiative and Auger recombination rates, but that Auger recombination rates increase by one order of magnitude more than radiative rates. Furthermore, we demonstrate that localization has an overall detrimental effect on the efficiency-droop and green-gap problems of InGaN LEDs.

Charge-tunable indium gallium nitride quantum dots

III-Nitride quantum dots have emerged as a new chip-scale system for quantum information science, which combines electrical and optical interfaces on a semiconductor chip that is compatible with non-cryogenic operating temperatures. Yet most work has been limited to optical excitations. To enable single-spin based quantum optical and quantum information research, we demonstrate here quantized charging in optically active, site-controlled III-Nitride quantum dots. Single-electron charging was confirmed by the voltage dependence of the energy, dipole moment, fine structures and polarization properties of the exciton states in the quantum dots. The fundamental energy structures of the quantum dots were identified, including neutral and charged excitons, fine structures of excitons, and A and B excitons. The results lay the ground for coherent control of single charges in III-Nitride QDs, opening a door to III-Nitride based spintronics and spin-qubit quantum information processing.

Fabrication of nanoscale zero-mode waveguides using microlithography for single molecule sensing

We present a novel approach to the fabrication of zero-mode waveguides (ZMWs) using inexpensive processing techniques. Our method is capable of rapid fabrication of circular nanoapertures with diameters ranging from 70 nm to 2 μm, allowing us to perform a detailed characterization of the dependence of the fluorescence emission on the waveguide diameter. We also validated the use of the fabricated ZMWs by detecting single molecule binding events with a signal-to-noise ratio of ten.