Jean-Claude Guillaume

Below Band-Gap IR Response of Substrate-Free GaAs Solar Cells Using Two-Photon Up-Conversion

We have developed a device based upon the concept of two-photon up-conversion to use a part of the IR photons otherwise lost by transparency in a GaAs cell. An ultra thin GaAs cell has been fabricated using the technique of epitaxial lift-off (ELO). This thin cell is placed on top of a 100 µ m thick vitroceramic doped with Yb3+ and Er3+. The two photon upconversion process involved here is based on sequential absorption and energy transfer of two IR photons from Yb3+ to Er3+, which then emit one photon in the green. This green light then produces a photoresponse in the GaAs cell. This cell coupled to the vitroceramic was lighted by an Ti-sapphire IR laser at 1.391 eV, a photon energy below the band gap of GaAs, with an input power able to reach ~1 W. The GaAs cell photoresponse increases quadratically with the input excitation. For an input excitation of 1 W at 1.39 eV on a 0.039 cm2 substrate-free GaAs cell, the measured efficiency was 2.5%.

Mg-enhanced lateral overgrowth of GaN on patterned GaN/sapphire substrate by selective Metal Organic Vapor Phase Epitaxy

Selective and lateral overgrowth by Metal Organics Vapour Phase Epitaxy (MOVPE) was carried out until coalescence to produce smooth and optically flat thick GaN layers. A GaN epitaxial layer is first grown using atmospheric pressure Metalorganic Vapour Phase Epitaxy on a {0001} Al 2O3. substrate. Then a 30Å silicon nitride dielectric film is deposited in-situ by reaction of silane and ammonia to form a selective mask. Afterwards, the openings and the figures in the dielectric films are achieved using standard photolithographic technology. Stripes openings in the mask, revealing free GaN surface, are aligned in the 〈1010〉 direction. Typical stripes spacing and width are 10 μm and 5 μm respectively. These patterned layers are further on used for epitaxial regrowth of GaN by MOVPE. The growth anisotropy and therefore the coalescence process is achieved by introducing (MeCp)2Mg in the vapour phase. A two-step process is reported which allows a dramatic reduction of threading dislocations density not only above the masked areas but also above the windows opened in the mask. With this process, very sharp bound exciton luminescence peaks are measured at low temperature in the overgrown GaN.

High efficiency conversion of laser energy and its application to optical power transmission

The authors address several aspects of the optical power transmission concept: the modeling of monochromatic conversion efficiency versus matched bandgap of a photovoltaic (PV) cell, parameterized as a function of illumination intensity; the selection criteria of the suitable semiconductors which are expected to have the highest PV conversion efficiency for the various applications; laboratory experimental study using an (Al,Ga)As laser beam transmitted through an optical fiber: a GaAs-based PV cell, designed for being set up at the optical fiber extremity, has been grown by MOVPE and has demonstrated an efficiency of 52.8% under 14-W/cm/sup 2/ illumination. Several future applications, concerning transmission of optical power in space and from space, or through optical fiber in terrestrial utilizations, are discussed.<<ETX>>

Low temperature photoluminescence of n‐type GaInAsP layers grown on InP by liquid phase epitaxy

Layers of the quaternary alloy GaxIn1−xAsyP1−y (x∼0.1, y∼0.3) were grown on InP (100) substrates by super‐cooling liquid phase epitaxy. Tellurium was used for n‐type doping in the range 8×1017–5×1019 cm−3. A good homogeneity of the composition of the layers was obtained at the surface. No microprecipitates of tellurium were detected by scanning electron microscope. Systematic studies of luminescence at low temperature were used to characterize the samples, with a particular attention focused on the degeneracy of the conduction band. Calculations of the line shapes were performed based on a recombination mechanism involving acceptor‐like states of the valence band and conduction band filling effects. Good agreement was obtained between calculated and experimental line shapes. The Fermi level in quaternary alloys is found to be lower than predicted by theoretical calculations using band structure properties developed in the past for definite III‐V compounds such as GaAs and InP. It is suggested that this co...

High-Efficiency Al0.2Ga0.8As/Si Stacked Tandem Solar Cells Using Epitaxial Lift-Off

In this paper, the potential of high conversion efficiency of solar energy through the combination of Al0.2Ga0.8As and Si cells is illustrated by a mechanically stacked tandem using, for the first time, the epitaxial lift-off (ELO) technique to remove the top cell from its substrate. The selective etching of the GaAs substrate provides efficient light transmission to the bottom cell. Photoluminescence decay experiments show that substrate removal also enhances photon recycling effects. The measured efficiency of the Al0.2Ga0.8As/Si tandem reaches 21% AM1.5.

Ga0.47In0.53As photovoltaic booster cells for tandem solar energy conversion

Abstract Ga0.47In0.53As photovoltaic structures are grown lattice matched on InP[100] substrates. The cells that are fabricated from these structures are intended to convert the part of the solar spectrum that is lost by transparency of GaAs cells (i.e. photons with energy of less than 1.43 eV). An average collection efficiency of 60% in the 0.75-1.43 eV range is measured without antireflection coating (ARC). A double-layer ARC is expected to improve the collection efficiency to about 90% in this small band of the solar spectrum. In this work, particular attention has been paid to conversion efficiency measurements under concentration. The projected Ga0.47In0.53As cell contribution to the GaAs/(Ga,In)As stacked tandem conversion reaches 6% with concentration ratio C = 170 (AM 1), including grid obscuration and optical losses due to GaAs spectrum filtering. Recent efficiency values of GaAs under concentration (28%, C = 500 , AM 1), measured by Sandia Laboratories, suggest that a GaAs/Ga0.47In0.53As tandem photovoltaic structure might have a combined efficiency as high as 34% under concentration ( C = 500 ).

Mechanically stacked two-tandem concentrator solar cell concept

Modeling results and some data from experiments currently under study for a new photovoltaic (PV) structure representing the tractable device adaptation of a quadrispectral concept proposed some years ago are presented. Based on a four-band solar spectrum division, it combines a couple of two-bandgap monolithic tandem cells grown, respectively, on GaAs and InP in a mechanical stacking configuration. In the PV concentrator approach, the objective of this rainbow cell design is to demonstrate the feasibility of efficiencies on the order of 40% (100*AM0, BOL) and 45% (500*AM1.5), utilizing III-V semiconductor materials currently used in optical fiber communication systems. In studying the influence of airmass values, the InP substrate-based booster tandem of universal use, either in space or in terrestrial applications, is found. Considering the GaAs-based tandem structure, an Am1-to-AM3 analysis leads to specification of the optimal top bandgap value. Projections of expected experimental efficiencies of this stacked two-tandem concentrator cell show that the objectives are realistic.<<ETX>>

Epitaxial lift-off in photovoltaics:ultra thin Al/sub 0.2/Ga/sub 0.8/As cell in a mechanically stacked (Al,Ga)As/Si tandem

The potential of high conversion efficiency of solar energy through the combination of Al/sub 0.2/Ga/sub 0.8/As and Si cells is illustrated by a mechanically stacked tandem using for the first time the epitaxial lift-off (ELO) technique to remove the top cell from its substrate. The selective etching of the GaAs substrate provides efficient light transmission to the bottom cell. Photoluminescence decay experiments show also that substrate removal enhances photon recycling effects. The measured efficiency of the Al/sub 0.2/Ga/sub 0.8/As/Si tandem reaches 21% AM1.5.

Anisotropic Growth of GaAs in MOVPE

In chloride vapour phase epitaxy of GaAs, it is known that the growth velocity depends on the cr stallographic face. Two orders of magnitude_difference in growth rate could occur between {111}Ga and {111}Aa. SO far, these features are strongly supported by the analysis of orientational dependence of the growth rate associated with surface kinetics. It is assumed that a dynamic equilibrium between the growing surfaces and the gas phase occurs, and the rate limiting process was shown to be the desorption of chlorine via hydrogen at the steps (1).

A InP Lattice-Matched Ga0.47.In0.53As Cell for Multispectral Photovoltaics

GaInAs (0.75 eV) photovoltaic cells with a GaInAsP (0.94 eV) window layer are lattice-matched grown by LPE on InP substrates. These cells are devised for the conversion of the solar energy spectrum with a cut-off chosen at the GaAs cell bandgap. Combining published results on GaAs with our results on GaInAs cells, a maximum efficiency of 34% is projected at 750 AM 1.5, assuming no loss in the optical concentration system.