Proc. SPIE. 10527, Physics, Simulation, and Photonic Engineering of Photovoltaic Devices VII
KEYWORDS: Solar concentrators, Solar cells, Gallium arsenide, Image resolution, Electron microscopes, Control systems, Quantum dots, Scanning electron microscopy, Molecular beams, Semiconducting wafers
Recent efforts to include a quantum dot array within the intrinsic region of a pin GaAs solar cell have focused on minimising the open circuit voltage (Voc) loss relative a control device without quantum dots . Strategies include the addition of strain balancing (e.g. GaP, GaAsP)  or high energy (e.g. AlGaAs) barrier layers . In this work we demonstrate a significant improvement of up to 260 mV in Voc by controlling only the size of the quantum dots at the nanometre scale using precise molecular beam epitaxial wafer growth. High resolution scanning transmission electron microscope (STEM) imaging is used to determine the dimensions of individual quantum dots, providing valuable input to a theoretical model. The modelling suggests that the performance improvement is a direct consequence of opening a clear energy gap between the conduction band and the quantum dot ensemble ground state energy e0. With appropriate quantum mechanical design this energy gap can be up to ~90 meV, giving rise to intermediate band behaviour rather than quantum dot solar cell behaviour at room temperature. Current-voltage measurements under air mass 1.5 conditions indicate an efficiency (active area) of 18.4% (19.7%) at 5-suns concentrations. Higher concentration measurements confirm the quality of the material with diode ideality factors as low as 1.16 and Voc ≈ 1 V at 500 suns.
References:  Y. Okada et al., Appl. Phys. Rev. 2, 021302 (2015),  C. G. Bailey et al., Appl. Phys. Lett. 98, 163105 (2011),  A. Varghese et al., Nanoscale 8, 7248 (2016).
We present the first demonstration of telecom fiber-based quantum key distribution using single photons from
a quantum dot in a pillar microcavity. The source offers both telecommunication wavelength operation at 1.3
microns and Purcell enhancement of the spontaneous emission rate. Several emission lines from the InAs/GaAs
quantum dot are identified, including the exciton-biexciton cascade and charged excitonic emission. We show an
order of magnitude increase in the collected intensity of the emission from a charged excitonic state when temperature
tuned onto resonance with the HE11 mode of the pillar microcavity, as compared to the off-resonance
intensity. Above- and below-GaAs-bandgap optical excitation was used and the effect of the excitation energy
on the photoluminescence investigated. Exciting below the GaAs-bandgap offers significant improvement in the
quality of the single photon emission and a reduction of the multi-photon probability to 0.1 times the value for
Poissonian light was measured, before subtraction of detector dark counts, the lowest value recorded to date
for a quantum dot source at a fibre wavelength. We observe also the first evidence of Purcell enhancement of
the spontaneous emission rate for a single telecommunication wavelength quantum dot in a pillar microcavity.
We have incorporated the source into a phase encoded interferometric scheme implementing the BB84 quantum
cryptography protocol and distributed a key, secure from the pulse splitting attack, over standard telecommunication
optical fibre. We show a transmission distance advantage over that possible with (length-optimized)
uniform intensity weak coherent pulses at 1310 nm in the same system.
Single photon sources are important components for future quantum communication networks. Lights emitting diodes with emission from an embedded self-organized quantum dot offer compact semiconductor sources that can be easily fabricated using standard photolithographic techniques. In this paper, progress towards an electrically driven 1300 nm quantum dot single photon emitter for fiber optic based applications are addressed. Low density longer wavelength emissions were achieved by exploiting the second critical growth threshold for large self-assembled InAs quantum dots on GaAs. The single photon collection efficiency was improved by incorporating the quantum dots between GaAs/AlxGa1-xAs distributed Bragg reflector mirror stacks and laterally confined inside etched micropillars. Resonance of the microcavity mode with the InAs quantum dot emission leads to an enhancement in the collection intensity. Emission from an active quantum dot was collected using a confocal microscope and coupled directly into a single mode fiber. Strong suppression in the multiphoton emission rate was verified by a custom Hanbury-Brown and Twiss interferometer set-up with optical fibers and InGaAs single photon avalanche photodetectors. Integration of electrical contacts with a planar resonant microcavity structure for a single photon light emitting diode is also discussed. Electroluminescence spectra recorded on such a device revealed sharp lines due to the charge recombination in a quantum dot. Correlation measurements on a single quantum dot line showed the suppression of multiphoton emission for an electrically driven source near 1300 nm for the first time.
We review progress in realizing a semiconductor source of single photons and photon pairs based on the emission of individual self-assembled quantum dots. Integration of the quantum dot into a pillar microcavity produces a strong Purcell enhancement of the radiative recombination rate, resulting in photon collection efficiencies into a lens of ~10%. The residual multi-photon emission is found to derive from the emission of other layers within the structure, such that under resonant laser excitation of the dot a greater than 50-fold reduction in the 2-photon rate can be achieved compared to a laser of the same average intensity. The polarization of the emitted photons can be controlled and selected in appropriately designed cavities. Through careful control of the dot growth conditions, we realize a single photon source at the fiber compatible wavelength of 1300nm. This is achieved by utilizing a second critical InAs coverage to produce a low density of large, long wavelength InAs quantum dots. We demonstrate also an electrically driven planar cavity structure with photon collection efficiencies into a lens of ~5%, corresponding to an order of magnitude enhancement in the photon collection compared to dots in a bulk semiconductor LED. Single photon emission is demonstrated for both the biexciton and exciton state of the quantum dot.