C. Levallois

Evaluation of InGaPN and GaAsPN materials lattice-matched to Si for multi-junction solar cells

We compare the potentiality of bulk InGaPN and GaAsPN materials quasi-lattice-matched to silicon (Si), for multi-junction solar cells application. Bandgaps of both bulk alloys are first studied by a tight-binding model modified for nitrogen incorporation in diluted regimes. The critical thicknesses of those alloys are then calculated for various compositions. For the same lattice-mismatch and nitrogen amount, the bandgap of bulk GaAsPN is found to be closer to the targeted gap value of 1.7u2009eV for high efficiency tandem solar cell. GaPN and GaAsPN epilayers are then grown by molecular beam epitaxy on GaP substrate and studied by photoluminescence and X-ray diffraction. A GaAsPN bulk alloy emitting light at 1.77u2009eV at room temperature is obtained, demonstrating promising properties for further use in III-V/Si photovoltaic multijunction solar cells.

Si wafer bonded of a‐Si∕a‐SiNx distributed Bragg reflectors for 1.55‐μm-wavelength vertical cavity surface emitting lasers

Amorphous silicon (a-Si) and amorphous silicon nitride (a-SiNx) layers deposited by magnetron sputtering have been analyzed in order to determine their optical and surface properties. A large value of ~1.9 of index difference is found between these materials. Distributed Bragg reflectors (DBR) based on these dielectric materials quarter wave layers have been studied by optical measurements and confronted to theoretical calculations based on the transfer matrix method. A good agreement has been obtained between the experimental and expected reflectivity. A maximum reflectivity of 99.5% at 1.55 µm and a large spectral bandwidth of 800 nm are reached with only four and a half periods of a-Si/a-SiNx. No variation of the DBR reflectivity has been observed with the time nor when annealed above 240°C and stored during few months. This result allows to use this DBR in a metallic bonding process to realize a vertical cavity surface emitting laser (VCSEL) with two dielectric a-Si/a-SiNx DBR. This bonding method using AuIn2 as the bonding medium and Si substrate can be performed at a low temperature of 240°C without damaging the optical properties of the microcavity. The active region used for this VCSEL is based on lattice-matched InGaAs/InGaAsP quantum wells and a laser emission has been obtained at room-temperature on an optically pumped device.

Nano-polymer-dispersed liquid crystal as phase modulator for a tunable vertical-cavity surface-emitting laser at 1.55 μm

We demonstrate what we believe is the first nonmechanical tunable vertical-cavity surface-emitting laser operating in the C band. This was achieved as a result of the combination of an InGaAs quantum well structure with a 6lambda thickness tunable index nano-polymer-dispersed liquid-crystal material. Experimental results exhibited a potential tunable range close to 10 nm, in the preliminary version, and excellent single mode locking due to the side-mode suppression ratio (more than 20 dB) over the whole spectral range. Another decisive advantage, compared to mechanical solutions, was the tuning response time of a few tens of microseconds (>30 micros) to scan the full spectral range (10 nm), making this device appropriate for some access network functions, as well as being robust and low cost. The voltage values are the main limitation to wavelength range extension. We present a first version of the device optically pumped. The next version will be electrically pumped as required for the access network applications targeted here.

1.55-µm optically pumped tunable VCSEL based on a nano-polymer dispersive liquid crystal phase modulator

We present a new approach to achieve tunability on a 1.55 μm vertical cavity surface emitting laser (VCSEL). Tunability is achieved thanks to an electro-optic index modulator. This electro-optic material consists in a n-PDLC phase layer introduced inside the VCSEL cavity. N-PDLC comprises nematic liquid crystal dispersed in a polymer material. This first VCSEL exhibits a 10 nm tuning range and an excellent side-mode suppression ratio higher than 20 dB over the whole spectral range. The device is formed by a conventional InP-based active region with an epitaxial and a dielectric Bragg mirror. The n-PDLC layer length, close to 6 μm, is in agreement with a tunable laser emission without mode-hopping. Another decisive advantage, compared to mechanical solutions, is the tuning response time which is close to a few 10 μs to scan the full spectral range, making this device appropriate for some access network functions. Voltage values are the main limiting factor, 170 Volts have been required to obtain 10 nm tunability, but material engineering is in progress to improve this point. We presented a first version of the device optically pumped, the next version will be electrically pumped as required for access network applications targeted here.

Substrate bonding using electroplated copper through silicon vias for VCSEL fabrication

We present a novel approach to bound any substrate on a silicon host platform, in the particular case of the realization of InP based vertical cavity surface emitting lasers (VCSEL). This process is based on a mechanical bonding, using electroplated copper through silicon vias. It enables a cost effective bonding with a low induced stress, and a significant improvement of the device thermal properties. Preliminary results are presented on the realization of light emitting diodes.

Electrical injection in GaP-based laser waveguides and active areas

This paper presents the recent advances in device engineering towards the fabrication of an electrically pumped laser on GaP substrate for photonic integration on silicon. The letter first presents the electrical properties of a GaP-based PIN diode, in particular the reduction of the characteristic resistance of the p-contact, thanks to a judicious combination of metal choice and thermal annealing. Secondly, carrier injection in the active area is investigated by use of time-resolved photoluminescence, regarding particularly the nature and composition of the barrier and quantum wells materials, with a focus on the nitrogen incorporation issues.

1540 to 1645 nm continuous VCSEL emission based on quantum dashes

We report on an optically excited InAs quantum dash vertical cavity surface emitting lasers (VCSEL) on InP substrate. By introducing a wedge microcavity design, we obtain a spatial dependence of the resonant wavelength along the wafer, enabling us to monitor the gain material bandwidth. In this paper we show a continuously variable VCSEL emission from 1645 down to 1540 nm all across the wafer, a consequence of the important and wide gain afforded by the use of optimized quantum dashes.

Structural and optical properties of (In,Ga)As/GaP quantum dots and (GaAsPN/GaPN) diluted-nitride nanolayers coherently grown onto GaP and Si substrates for photonics and photovoltaics applications

Lattice-matched GaP-based nanostructures grown on silicon substrates is a highly rewarded route for coherent integration of photonics and high-efficiency photovoltaic devices onto silicon substrates. We report on the structural and optical properties of selected MBE-grown nanostructures on both GaP substrates and GaP/Si pseudo-substrates. As a first stumbling block, the GaP/Si interface growth has been optimised thanks to a complementary set of thorough structural analyses. Photoluminescence and time-resolved photoluminescence studies of self-assembled (In,Ga)As quantum dots grown on GaP substrate demonstrate a proximity of two different types of optical transitions interpreted as a competition between conduction band states in X and Γ valleys. Structural properties and optical studies of GaAsP(N)/GaP(N) quantum wells coherently grown on GaP substrates and GaP/Si pseudo substrates are reported. Our results are found to be suitable for light emission applications in the datacom segment. Then, possible routes are drawn for larger wavelengths applications, in order to address the chip-to-chip and within-a-chip optical interconnects and the optical telecom segments. Finally, results on GaAsPN/GaP heterostructures and diodes, suitable for PV applications are reported.

Design and Fabrication of a Tunable InP-based VCSEL using a Electro-Optic Index Modulator

We present the first vertical surface emitting lase r (VCSEL) operating at 1.55-µm comprising a electro-optic modulator inside its cavity. This material consists of nematic liquid crystal dispersed in a polymer m aterial (nano-PDLC). This first VCSEL exhibits a 10 nm tuning range and an excellent side-mode suppression rat io higher than 20 dB over the whole spectral range. Th e device is formed by a conventional InP-based acti ve region with an epitaxial and a dielectric Bragg mir ror. The nano-PDLC layer length, close to 6 µm, is in agreement with a tunable laser emission without mode-hopping. Another decisive advantage, compared to mechanical solutions, is the tuning response time w hich is close to a few 10 µs to scan the full spect ral range, making this device appropriate for some access netw ork functions. This first version is optically pump ed and requires 170 volts to obtain a 10 nm tunability.