Semiconductor Nanolasers

Recent progress in the field of semiconductor nanolasers is discussed. New designs have emerged that eliminate the need for a conventional Fabry-Perot cavity, bringing down the physical dimensions of the lasers below the diffraction limit. Semiconductor nanolasers are critical components for nanophotonics and offer possible integration with Si nanoelectronics

Detecting dopant diffusion enhancement at grain boundaries in multicrystalline silicon wafers with microphotoluminescence spectroscopy

Employing microphotoluminescence spectroscopy at low temperatures, we are able to detect dopant diffusion enhancement along various grain boundaries and subgrain boundaries in multicrystalline silicon wafers. We find an enhancement of phosphorus diffusion at all investigated grain boundary types. In addition, the subgrain boundaries are demonstrated to contain a relatively high density of defects and impurities, suggesting that their presence does not significantly hinder the preferential diffusion of dopant atoms along the subgrain boundaries. Finally, we demonstrate that the technique can be applied to different diffused layers for solar cell applications, even at room temperature if an appropriate excitation wavelength is used. The results are validated with secondary electron dopant contrast images, which confirm the higher dopant concentration along the grain boundaries and subgrain boundaries

Integration of an InGaAs Quantum-Dot Laser with a Low-Loss Passive Waveguide using Selective-Area Epitaxy

An InGaAs quantum-dot (QD) laser integrated with a low- losswaveguideisdemonstrated. Selective-areaepitaxy isusedto simultaneously form the QDs that form the active region of the laser and quantum wells (QWs) that form the waveguide section of the integrated devices.Thelosses in the activeand passive sections of the integrated devices are 6 and 3 cm-1, respectively. Very low losses in the waveguide section are due to a large difference of 200 meV in the bandgap energies of the selectively grown QDs and QWs

Review on photonic properties of nanowires for photovoltaics

III-V semiconductor nanowires behave as optical antennae because of their shape anisotropy and high refractive index. The antennae like behavior modifies the absorption and emission properties of nanowires compared to planar materials. Nanowires absorb light more efficiently compared to an equivalent volume planar material, leading to higher short circuit current densities. The modified emission from the nanowires has the potential to increase the open circuit voltage from nanowire solar cells compared to planar solar cells. In order to achieve high efficiency nanowire solar cells it is essential to control the surface state density and doping in nanowires. We review the physics of nanowire solar cells and progress made in addressing the surface recombination and doping of nanowires, with emphasis on GaAs and InP materials

Plasmonic quantum dot solar cells for enhanced infrared response

Enhanced near infrared photoresponse in plasmonic InGaAs/GaAs quantum dotsolar cells (QDSC) is demonstrated. Long wavelength light absorption in the wetting-layer and quantum-dot region of the quantum dotsolar cell is enhanced through scattering of light by silver nanoparticles deposited on the solar cellsurface.Plasmonic light trapping results in simultaneous increase in short-circuit current density by 5.3% and open circuit voltage by 0.9% in the QDSC, leading to an overall efficiency enhancement of 7.6%.This work was supported by the Australian Research Council (Grant No. DP1096361)

Analytical expression for the quantum dot contribution to the quasistatic capacitance for conduction band characterization

This paper demonstrates an analytical expression for the quasistatic capacitance of a quantum dot layer embedded in a junction, where the reverse bias is used to discharge the initially occupied energy levels. This analysis can be used to determine the position and the Gaussian homogeneous broadening of the energy levels in the conduction band, and is applied for an InGaAs/GaAs quantum dot structure grown by metal organic chemical vapor deposition. It is shown that the Gaussian broadening of the conduction band levels is significantly larger than the broadening of the interband photoluminescence (PL) transitions involving both conduction and hole states. The analysis also reveals a contribution from the wetting layer both in PL and modeled C-V profiles which is much stronger than in typical molecular beam epitaxy grown dots. The presence of a built-in local field oriented from the apex of the dot toward its base, contrary to the direction expected for a strained dot with uniform composition (negative dipole), is also derived from fitting of the C-V experimental data

Enhanced luminescence from GaN nanopillar arrays fabricated using a top-down process

We report the fabrication of GaN nanopillar arrays with good structural uniformity using the top-down approach. The photoluminescence intensity from the nanopillar arrays is enhanced compared to the epilayer. We use finite difference time domain simulations to show that the enhancement in photoluminescence intensity from the nanopillar arrays is a result of anti-reflection properties of the arrays that result in enhanced light absorption and increase light extraction efficiency compared to the epilayer. The measured quantum efficiency of the nanopillars is comparable to that of an epitaxially grown GaN epilayer.ARC grant DP140103278 (2014-2016) - H.H. Tan, Nitride-based Compound Semiconductors for Solar Water Splittin

Indium phosphide based solar cell using ultra-thin ZnO as an electron selective layer

According to the Shockley–Queisser limit, the maximum achievable efficiency for a single junction solar cell is ~33.2% which corresponds to a bandgap (E g) of 1.35 eV (InP). However, the maximum reported efficiency for InP solar cells remain at 24.2%  ±  0.5%, that is  >25% below the standard Shockley–Queisser limit. Through a wide range of simulations, we propose a new device structure, ITO/ ZnO/i-InP/p+ InP (p-i-ZnO-ITO) which might be able to fill this efficiency gap. Our simulation shows that the use of a thin ZnO layer improves passivation of the underlying i-InP layer and provides electron selectivity leading to significantly higher efficiency when compared to their n+/i/p+ homojunction counterpart. As a proof-of-concept, we fabricated ITO/ZnO/i-InP solar cell on a p+ InP substrate and achieved an open-circuit voltage (V oc) and efficiency as high as 819 mV and 18.12%, respectively, along with ~90% internal quantum efficiency. The entire device fabrication process consists of four simple steps which are highly controllable and reproducible. This work lays the foundation for a new generation of thin film InP solar cells based solely on carrier selective heterojunctions without the requirement of extrinsic doping and can be particularly useful when p- and n-doping are challenging as in the case of III–V nanostructures.This research is supported by the Australian Research Council

Ultrathin Ta2O5 electron-selective contacts for high efficiency InP solar cells

Heterojunction solar cells with transition-metal-oxide-based carrier-selective contacts have been gaining considerable research interest owing to their amenability to low-cost fabrication methods and elimination of parasitic absorption and complex semiconductor doping process. In this work, we propose tantalum oxide (Ta2O5) as a novel electron-selective contact layer for photo-generated carrier separation in InP solar cells. We confirm the electron-selective properties of Ta2O5 by investigating band energetics at the InP-Ta2O5 interface using X-ray photoelectron spectroscopy. Time-resolved photoluminescence and power dependent photoluminescence reveal that the Ta2O5 inter-layer also mitigates parasitic recombination at the InP/transparent conducting oxide interface. With an 8 nm Ta2O5 layer deposited using an atomic layer deposition (ALD) system, we demonstrate a planar InP solar cell with an open circuit voltage, Voc, of 822 mV, a short circuit current density, Jsc, of 30.1 mA/cm2, and a fill factor of 0.77, resulting in an overall device efficiency of 19.1%. The Voc is the highest reported value to date for an InP heterojunction solar cells with carrier-selective contacts. The proposed Ta2O5 material may be of interest not only for other solar cell architectures including perovskite cells and organic solar cells, but also across a wide range of optoelectronics applications including solid state emitting devices, photonic crystals, planar light wave circuits etc

Excited state biexcitons in atomically thin MoSe2

The tightly bound biexcitons found in atomically thin semiconductors have very promising applications for optoelectronic and quantum devices. However, there is a discrepancy between theory and experiment regarding the fundamental structure of these biexcitons. Therefore, the exploration of a biexciton formation mechanism by further experiments is of great importance. Here, we successfully triggered the emission of biexcitons in atomically thin MoSe2, via the engineering of three critical parameters: dielectric screening, density of trions, and excitation power. The observed binding energy and formation dynamics of these biexcitons strongly support the model that the biexciton consists of a charge attached to a trion (excited state biexciton) instead of four spatially symmetric particles (ground state biexciton). More importantly, we found that the excited state biexcitons not only can exist at cryogenic temperatures but also can be triggered at room temperature in a freestanding bilayer MoSe2. The demonstrated capability of biexciton engineering in atomically thin MoSe2 provides a route for exploring fundamental many-body interactions and enabling device applications, such as bright entangled photon sources operating at room temperature