Jacques Levrat

High quality factor two dimensional GaN photonic crystal cavity membranes grown on silicon substrate

We report on the achievement of freestanding GaN photonic crystal L7 nanocavities with embedded InGaN/GaN quantum wells grown by metal organic vapor phase epitaxy on Si (111). GaN was patterned by e-beam lithography, using a SiO2 layer as a hard mask, and usual dry etching techniques. The membrane was released by underetching the Si (111) substrate. Micro-photoluminescence measurements performed at low temperature exhibit a quality factor as high as 5200 at ∼420 nm, a value suitable to expand cavity quantum electrodynamics to the near UV and the visible range and to develop nanophotonic platforms for biofluorescence spectroscopy.

Polariton lasing in a hybrid bulk ZnO microcavity

We demonstrate polariton lasing in a bulk ZnO planar microcavity under non-resonant optical pumping at a small negative detuning (δ ∼ −1/6 the 130 meV vacuum Rabi splitting) and a temperature of 120 K. The strong coupling regime is maintained at lasing threshold since the coherent nonlinear emission from the lower polariton branch occurs at zero in-plane wavevector well below the uncoupled cavity mode. The contribution of multiple localized polariton modes above threshold and the non-thermal polariton statistics show that the system is in a far-from-equilibrium regime, likely related to the moderate photon lifetime and in-plane photonic disorder in the cavity.

Exciton localization on basal stacking faults in a-plane epitaxial lateral overgrown GaN grown by hydride vapor phase epitaxy

We present a detailed study of the luminescence at 3.42 eV usually observed in a-plane epitaxial lateral overgrowth (ELO) GaN grown by hydride vapor phase epitaxy on r-plane sapphire. This band is related to radiative recombination of excitons in a commonly encountered extended defect of a-plane GaN: I1 basal stacking fault. Cathodoluminescence measurements show that these stacking faults are essentially located in the windows and the N-face wings of the ELO-GaN and that they can appear isolated as well as organized into bundles. Time-integrated and time-resolved photoluminescence, supported by a qualitative model, evidence not only the efficient trapping of free excitons (FXs) by basal plane stacking faults but also some localization inside I1 stacking faults themselves. Measurements at room temperature show that FXs recombine efficiently with rather long luminescence decay times (360 ps), comparable to those encountered in high-quality GaN epilayers. We discuss the possible role of I1 stacking faults in...

Realization of GaInP/Si Dual-Junction Solar Cells With 29.8% 1-Sun Efficiency

Combining a Si solar cell with a high-bandgap top cell reduces the thermalization losses in the short wavelength and enables theoretical 1-sun efficiencies far over 30%. We have investigated the fabrication and optimization of Si-based tandem solar cells with 1.8-eV rear-heterojunction GaInP top cells. The III–V and Si heterojunction subcells were fabricated separately and joined by mechanical stacking using electrically insulating optically transparent interlayers. Our GaInP/Si dual-junction solar cells have achieved a certified cumulative 1-sun efficiency of 29.8% ± 0.6% (AM1.5g) in four-terminal operation conditions, which exceeds the record 1-sun efficiencies achieved with both III–V and Si single-junction solar cells. The effect of luminescent coupling between the subcells has been investigated, and optical losses in the solar cell structure have been addressed.

High reflectivity airgap distributed Bragg reflectors realized by wet etching of AlInN sacrificial layers

The authors report on the achievement of a vertically oriented three pair airgap/GaN distributed Bragg reflector realized by controlled oxidation and wet-chemical etching of AlInN sacrificial layers. Microreflectivity measurements exhibit high peak reflectivity values of 87% around 500 nm after the oxidation process and 90% around 600 nm after the etching process in overall good agreement with simulations. The broad stopband of airgap/GaN mirrors, about 250 nm wide, results from the strong refractive index contrast between air and GaN layers.

Room temperature polariton lasing in III-nitride microcavities: a comparison with blue GaN-based vertical cavity surface emitting lasers

The authors report on room temperature (RT) lasing action in two different types of nitride-based microcavities (MCs): vertical cavity surface emitting lasers (VCSELs) and polariton lasers which operate in the weak and in the strong coupling regime, respectively. Following a brief description of these two operating regimes, an analysis of lasing action at RT is reported for a crack-free planar VCSEL structure based on a bottom lattice-matched AlInN/GaN distributed Bragg reflector (DBR) and a top dielectric DBR. The cavity region, formed by n- and p-type GaN layers surrounding only three InGaN/GaN quantum wells, corresponds to a typical active region suitable for an electrically driven VCSEL. Processing issues of such planar VCSEL structures and electroluminescence characteristics of processed devices are also reported. Then, an alternative approach relying on the realization of coherent GaN-MC light sources based on the spontaneous decay of a macroscopic polariton population, the so-called polariton laser, is described. It is shown that this kind of devices could work at RT with a potentially much lower threshold current density than VCSELs as it does not necessitate reaching population inversion conditions. As for VCSELs, we demonstrate laser-like properties above threshold, i.e. an intense polarized emission and a strong spectral narrowing. Differences of polariton lasers with conventional lasers are also highlighted.

One-dimensional exciton luminescence induced by extended defects in nonpolar GaN/(Al,Ga)N quantum wells

In this study, we present the optical properties of nonpolar GaN/(Al,Ga)N single quantum wells (QWs) grown on either a- or m-plane GaN templates for Al contents set below 15%. In order to reduce the density of extended defects, the templates have been processed using the epitaxial lateral overgrowth technique. As expected for polarization-free heterostructures, the larger the QW width for a given Al content, the narrower the QW emission line. In structures with an Al content set to 5 or 10%, we also observe emission from excitons bound to the intersection of I1-type basal plane stacking faults (BSFs) with the QW. Similarly to what is seen in bulk material, the temperature dependence of BSF-bound QW exciton luminescence reveals intra-BSF localization. A qualitative model evidences the large spatial extension of the wavefunction of these BSF-bound QW excitons, making them extremely sensitive to potential fluctuations located in and away from BSF. Finally, polarization-dependent measurements show a strong emission anisotropy for BSF-bound QW excitons, which is related to their one-dimensional character and that confirms that the intersection between a BSF and a GaN/(Al,Ga)N QW can be described as a quantum wire.

The impact of silicon solar cell architecture and cell interconnection on energy yield in hot & sunny climates

Extensive knowledge of the dependence of solar cell and module performance on temperature and irradiance is essential for their optimal application in the field. Here we study such dependencies in the most common high-efficiency silicon solar cell architectures, including so-called Aluminum back-surface-field (BSF), passivated emitter and rear cell (PERC), passivated emitter rear totally diffused (PERT), and silicon heterojunction (SHJ) solar cells. We compare measured temperature coefficients (TC) of the different electrical parameters with values collected from commercial module data sheets. While similar TC values of the open-circuit voltage and the short circuit current density are obtained for cells and modules of a given technology, we systematically find that the TC under maximum power-point (MPP) conditions is lower in the modules. We attribute this discrepancy to additional series resistance in the modules from solar cell interconnections. This detrimental effect can be reduced by using a cell design that exhibits a high characteristic load resistance (defined by its voltage-over-current ratio at MPP), such as the SHJ architecture. We calculate the energy yield for moderate and hot climate conditions for each cell architecture, taking into account ohmic cell-to-module losses caused by cell interconnections. Our calculations allow us to conclude that maximizing energy production in hot and sunny environments requires not only a high open-circuit voltage, but also a minimal series-to-load-resistance ratio.

Light-induced performance increase of silicon heterojunction solar cells

Silicon heterojunction solar cells consist of crystalline silicon (c-Si) wafers coated with doped/intrinsic hydrogenated amorphous silicon (a-Si:H) bilayers for passivating-contact formation. Here, we unambiguously demonstrate that carrier injection either due to light soaking or (dark) forward-voltage bias increases the open circuit voltage and fill factor of finished cells, leading to a conversion efficiency gain of up to 0.3% absolute. This phenomenon contrasts markedly with the light-induced degradation known for thin-film a-Si:H solar cells. We associate our performance gain with an increase in surface passivation, which we find is specific to doped a-Si:H/c-Si structures. Our experiments suggest that this improvement originates from a reduced density of recombination-active interface states. To understand the time dependence of the observed phenomena, a kinetic model is presented.

Q-factor of (In,Ga)N containing III-nitride microcavity grown by multiple deposition techniques

A 3λ/2 (In,Ga)N/GaN resonant cavity, designed for ∼415 nm operation, is grown by molecular beam epitaxy and is sandwiched between a 39.5-period (In,Al)N/GaN distributed Bragg reflector (DBR), grown on c-plane GaN-on-sapphire pseudo-substrate by metal-organic vapor phase epitaxy and an 8-period SiO2/ZrO2 DBR, deposited by electron beam evaporation. Optical characterization reveals an improvement in the cavity emission spectral purity of approximately one order of magnitude due to resonance effects. The combination of spectrophotometric and micro-reflectivity measurements confirms the strong quality (Q)-factor dependence on the excitation spot size. We derive simple analytical formulas to estimate leak and residual absorption losses and propose a simple approach to model the Q-factor and to give a quantitative estimation of the weight of cavity disorder. The model is in good agreement with both transfer-matrix simulation and the experimental findings. We point out that the realization of high Q-factor (In,G...