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Dynamics of semiconductor nanoscale lasers

Periodic Reporting for period 1 - NANOLASER (Dynamics of semiconductor nanoscale lasers)

Reporting period: 2015-04-01 to 2017-03-31

Electronic data connections are increasingly becoming a bottleneck in the exponential growth of data traffic worldwide. Future optical interconnects are the obvious successors but will require ultrasmall light sources with sub-micrometer sizes to achieve low energy consumption and ultrafast speeds. In both nanoscale light-emitting diodes (LEDs) and lasers radiative and nonradiative recombination rates play a key role in the efficiency. Specifically, radiative recombination (both spontaneous and stimulated) is affected by the small mode volume in nanoLEDs and nanolasers, potentially leading to strong Purcell enhancements and higher speed. On the other hand, nonradiative recombination rates are also very high due to the high surface-to-volume ratios, typically leading to low radiative efficiencies.

In this project, using advanced nanofabrication, characterization and modelling methods, we have investigated novel nanoscale light sources consisting of a waveguide-coupled metal-dielectric cavity nanopillar LED on silicon. These devices work at telecommunications wavelengths featuring more than 20 nW waveguide-coupled powers and GHz-range modulation bandwidths at room-temperature (RT). The efficiency of the reported nanoLEDs currently lies between 0.01 and 1 percent, at RT and at 10 K, respectively, mostly limited by nonradiative recombination effects. We have developed a passivation method using sulfur treatment, followed by silicon oxide capping deposited by plasma-enhanced chemical vapor deposition,that strongly suppresses the surface recombination at the InGaAs surfaces of nanopillars from a few hundred picoseconds to more than 20 nanoseconds. These results will ensure substantial improvements in the efficiency of future nanoLEDs and reduce the threshold current in nanolasers, which are of crucial importance for their application in optical interconnects.

The performance of the experimental nanoLEDs was analyzed using a rate equations model which properly takes into account the nanocavity effects in the spontaneous emission rate and the spatial and spectral overlap between carriers and photons. The model was extended to describe the stimulate emission processes occurring in nanolasers. Using this model, the ultimate limits of scaling down these nanoscale lasers and LEDs leading to Purcell enhancement of the emission and higher speeds was theoretically analyzed.
The work performed can be summarized in the following two main points:

1. Purcell enhancement of the emission in nanoscale light sources
The Researcher addressed the quantum effects in nanoscale laser devices, specifically, the modification of the emission rate in a cavity and its effect in the threshold of a nanolaser. Finite-difference time domain (FDTD) simulations were carried out to perform a preliminary study on the Purcell effect in metal-dielectric cavities. Simulations were performed in an initial design of a metallo-dielectric nanopillar cavity using a bulk InGaAs as the semiconductor active gain material.

In order to take overcome a few limitations identified using finite-element analysis in the calculations of the Purcell effect for bulk active materials, a detailed single-mode rate equation model was implemented that takes into account i) the stimulated and spontaneous emission rates for a homogeneously broadened two-level atom in a resonant cavity using the Fermi’s golden rule; and ii) the inhomogeneous broadening of the carriers and the carriers’ spatial distribution in the volume of the active region in the case of nanocavity lasers. Using this model, the ultimate limits of scaling down nanoscale light sources based on metallo-dielectric nanopillar cavities leading to Purcell enhancement of the emission and substantially higher modulation speeds were investigated in detail.

2. High-speed dynamics of nanoscale semiconductor light sources
The researcher performed current-voltage, light-current, and spectral analysis of fabricated electrically pumped waveguide-coupled nanopillar metal cavity nanoscale LEDs on silicon. The measurements were carried out using electro-optical and micro-electroluminescence experimental techniques. Emission at the target telecommunications wavelength (~1550 nm) was achieved featuring more than 20 nW waveguide-coupled optical powers at room temperature. The estimated on-chip external quantum efficiency of measured nanoLEDs was between 0.01 and 1 percent, at room temperature and at 10 K, respectively.

In order to investigate the high-speed dynamic characteristics of nanoLEDs and their modulation speed capabilities, time-correlated single photon counting measurements at low- and room-temperature were performed. Dynamic characterization revealed sub-nanosecond electro-optical response at room temperature. Such fast modulation was possible due to a strong non-radiative recombination effect in the sidewalls of the fabricated nanopillars. We concluded that while the non-radiative recombination affected the radiative efficiency at room temperature (0.01 per cent), it can be used to achieve fast on–off switching and high-speed direct electrical modulation for future low-power interconnects operating at Gb/s data rates.

Lastly, a new passivation method which strongly suppresses the surface recombination at InGaAs surfaces was investigated in detail. Record-low surface recombination velocities around 260 cm/s were obtained using an optimized treatment with ammonium sulfide combined with a SiOx coating. The SiOx coating revealed to play a key role in the surface passivation, a key result not reported in the literature of surface passivation. The very long carrier lifetimes (tens of nanosecond) obtained in passivated nanopillars are expected to enable high efficiency in nanoLEDs and low threshold current in nanolasers, which are key requirements for their application in optical interconnects.
The development of photonic/nanophotonic integrated circuits will play a major role in our society in the coming years in ultra-high speed information and communication technologies. The energy of interconnects inside information processing systems is growing extremely rapidly and without a change in technology, the performance of the circuit communications will become bounded by these interconnects. Despite all these major challenges, it is becoming impressively frustrating that the fundamental building block of a photonic/nanophotonic integrated circuit, which is to demonstrate an efficient and high speed light source with an ultra-small footprint on a silicon substrate, is still missing. The results of this project show for the first time the possibility of achieving such a nanoscale light source. The demonstration relies on a novel concept for coupling light efficiently from a nanophotonic cavity to a waveguide on a silicon wafer and is therefore the first demonstration of a nanoscale light source on silicon. Foremost, a 1,000-fold increase of the efficiency when compared with the best (III-V based) integrated nanoscale light-emitting diodes in the literature was achieved. It is also expected that the results achieved on a new passivation method will enable us to go well above that figure soon. These record results clearly show the prospect of operating nanophotonic integrated circuits at much lower energy/bit budgets than today’s light sources, paving the way to solve the challenges of energy efficiency and moving our society to a low-carbon economy.