Eric Guiot

New Efficiency Frontiers with Wafer-Bonded Multi-Junction Solar Cells.

Triple-junction (3J) solar cells will soon be history. The next generation of multi-junction (MJ) devices are now reaching efficiencies far beyond the record levels of 3J cells on Germanium. In this paper we present results of a 4J wafer-bonded solar cell with bandgaps 1.88 / 1.45 // 1.10 / 0.73 eV measured with an improved efficiency of 46.5% at 324x by Fraunhofer ISE. Design for cost has, from the outset, been a priority with the development of engineered substrates to replace costly and low yielding InP substrates, a product building on Soitec’s proprietary SmartCut technology. Wafer bonding enables the electro-mechanical combination of lattice and thermal expansion mismatched materials with electrically conductive, transparent bonds, enabling concentrator solar cells to be built from high quality 2J GaInP/GaAs absorbers lattice-matched to GaAs bonded to high quality 2J GaInPAs/GaInAs absorbers lattice matched to InP that operate well at high concentration.

Mechanism of the Smart Cut™ layer transfer in silicon by hydrogen and helium coimplantation in the medium dose range

We investigate the mechanism of the Si layer transfer in the Smart Cut™ technology for H and He coimplantation in the dose range of (2.5–5)×1016cm−2. Using infrared spectroscopy and cross-section transmission electron microscopy we study the microstructure of defects formed in Si in the as-implanted state. With H preimplant we observe significant enhancement of damage production as compared to the case where He is implanted first. At higher coimplant doses a buried nonuniform amorphouslike layer is formed. The structure of the layer resembles “swiss cheese” with highly damaged but still crystalline pockets embedded into amorphous material. The effect of coimplantation parameters on the thickness and crystal quality of transferred layer is discussed in the framework of a simple phenomenological model.

Four-Junction Wafer-Bonded Concentrator Solar Cells

The highest solar cell conversion efficiencies are achieved with four-junction devices under concentrated sunlight illumination. Different cell architectures are under development, all targeting an ideal bandgap combination close to 1.9, 1.4, 1.0, and 0.7 eV. Wafer bonding is used in this work to combine materials with a significant lattice mismatch. Three cell architectures are presented using the same two top junctions of GaInP/GaAs but different infrared absorbers based on Germanium, GaSb, or GaInAs on InP. The modeled efficiency potential at 500 suns is in the range of 49-54% for all three devices, but the highest efficiency is expected for the InP-based cell. An efficiency of 46% at 508 suns was already measured by AIST in Japan for a GaInP/GaAs//GaInAsP/GaInAs solar cell and represents the highest independently confirmed efficiency today. Solar cells on Ge and GaSb are in the development phase at Fraunhofer ISE, and the first demonstration of functional devices is presented in this paper.

Challenges and Progress in Germanium-on-Insulator Materials and Device Development towards ULSI Integration

SOITEC, Parc Technologique des Fontaines, F38190, Bernin, France The recent progress in the fabrication of GeOI substrates and devices is reviewed. Improvements have been made in threading dislocation density, Ge-buried oxide interface passivation, device performance. The potential of various co-integration schemes (lateral and vertical) has been illustrated as alternatives to the fabrication of n-type germanium channel devices. GeOI is also shown to be a versatile platform for the monolithic integration of Si and III-V devices and tunneling field effect transistors.

Development of high efficiency wafer bonded 4-junction solar cells for concentrator photovoltaic applications

The next generation of multi-junction concentrator solar cells will have to reach higher efficiencies than todays devices. At the same time these solar cells must be reliable in the field, be manufacturable with good yield and at sufficiently low cost. Inevitably the request of higher efficiency requires four or even more junction devices. A four-junction solar cell combination of GaInP/GaAs//GaInAsP/GaInAs with bandgap energies of 1.9, 1.4, 1.1, 0.7 eV is developed in a close collaboration between the Fraunhofer ISE, Soitec, CEA-LETI and HZB. This 4-junction cell hits close to the optimum of theoretical efficiency contour plots and has the potential to reach efficiencies up to 50 % under concentration. Challenges are associated with lattice-mismatch between GaAs and InP which is overcome by direct wafer-bonding. The high cost of the InP is addressed by the use of engineered substrates which only require a 500 nm thin mono-crystalline InP layer instead of several hundred μm. Excellent solar cell results up to 44.7 % efficiency have been obtained under concentration for devices manufactured on InP bulk substrates. The high cell efficiency is also supported by out-door characterization of one cell below a Fresnel lens with 16 cm2 aperture area. 38.5 % conversion efficiency has been measured for this mono-module in Freiburg under real operating conditions without any corrections.

Wafer bonded 4-junction GaInP/GaAs//GaInAsP/GaInAs concentrator solar cells

Multiple-junction solar cells made from III-V compound semiconductors are delivering the highest solar-electric conversion efficiencies. Increasing the number of junctions offers the potential to reach higher efficiencies. Direct wafer bonding offers a unique opportunity to combine lattice mismatched materials through a permanent, electrically conductive and optically transparent interface. In addition, the use of Smart Cut ™ technology, associated with its material recycling capabilities allows from a cost perspective the use of expensive bulk material such as InP. Combination of both technologies opens new opportunities to deliver cost effective high efficiency solar cells. In this respect, we have been able to demonstrate a record efficiency of 44,7% with a wafer bonded 4-junction GaInP/GaAs//GaInAsP/GaInAs concentrator solar cell with bandgap energies of 1.88/1.44//1.11/0.70 eV respectively. The bandgaps are chosen to be close to optimal for conversion under concentrated sunlight [1]. This paper prese...

SiGe and Ge on Insulator Wafers

Since 90nm technology Germanium (Ge) element has become increasingly popular in the CMOS processing for enhancing transistor performance, especially enhancing hole mobility for P-type transistors. The main driver has been the embedded SiGe in the source/drain region and its extraordinary boost on PFET drive current [1]. More recently Ge has enabled band engineering with respect to Silicon for Vt tuning [2,3] in addition to channel engineering for mobility enhancement. Looking into the future the need for SiGe alloys or pure Ge is increasing, as it is contemplated as a seed for IIIV material growth [4], or even the replacement of Si by Ge in the channel to take advantage of the high electron and hole mobilities [5]. Today the manufacturing reality shows us that all Ge needs can be fulfilled by epitaxy during the processing of the devices [6]. In this paper we will introduce the Dual Channel substrate having a strained SiGe layer grown on top of a SOI substrate. Starting with this wafer and thru condensation process one can produce a uniform SiGe layer suitable for Fully Depleted applications.

InP-based composite substrates for four junction concentrator solar cells

A photovoltaics conversion efficiency of 46% at 508 suns concentration was recently demonstrated with a four-junction solar cell consisting in a GaAs-based top tandem cell transferred onto an InP-based bottom tandem cell, by means of wafer bonding. We have successfully produced and characterized different InPOS (for InP-On-Substrate) composite substrates, that could advantageously replace fragile and expensive InP bulk wafers for the growth of the bottom tandem cell. The InPOS composite substrates include a thin top InP layer with thickness below 1µm, transferred onto a host substrate using the Smart Cut™ layer transfer technology. We developed InP-On-GaAs, InP-On-Ge and InP-On-Sapphire substrates with surface and crystal qualities similar to the InP bulk ones. A low electrical resistance of 1.4mΩ.cm² was measured along the InP transferred layer and the bonding interface. An epitaxial bottom tandem cell was grown on an InPOS substrate, and the corresponding PL behavior was found identical to that of cells...

Fabrication of compressively-strained GeOI substrates using the Smart Cut TM technology

Compressively-strained Germanium-on-Insulator (c-GeOI) substrates have been fabricated using the Smart CutTM technology. The technique is based on the optimized epitaxial growth process that reduces the threading dislocation density (TDD) in the strained Ge layer to the levels of 8·105 /cm2. Pseudo-MOSFET characterization showed 67% hole mobility enhancement with respect to conventional GeOI (150% relative to SOI).

High throughput ion implanter for environmentally beneficial products with III–V compound semiconductor

We have developed an ultra-high current ion implantation system for high throughput wafer processing. The ion source is designed based on implanters used in the flat panel display industry to produce 80 cm high beams which exceed 80 mA. Multiple wafers are processed at a time with the large-sized beam. The implantation tool exhibited throughput of 107 and 38 wafers per hour for 1E16 and 1E17 ions/cm2, respectively. Successful InP layer transfer with smooth surface was demonstrated using Smart Cut™ technology.