Epitaxy Of InP/GaAs And Design Of GaAsP/Si Dual-junction Solar Cells

High-performance and low-cost are the main objectives in the field of the integrated devices.According to the Moore's law,with the improvement of integration level,traditional electric interconnection technology which relies on electronics as the carrier will face some problems,such as energy coinsumption and delay.In contrast,optical interconnection shows a great potential in the fabrication of integrated devices,because the photon has the characteristics of non-electricity and non-interference.The integration of GaAs and InP can not only realize the aforesaid optical interconnection,but also show the advantages of low-cost and large-size due to the GaAs substrates.However,because of the mismatch in the properties between GaAs and InP,the growth of such heterostructure face some challenges.Epitaxial lateral overgrowth(ELO)which relies on the mechanism of dislocation blocking is an effective approach for the large lattice mismatched epitaxy.In this method,an essential mask that is patterned with growth windows is adopted to limit the initial growth only into the opening areas until protruding out of the windows,at which point it is able to coalesce with the neighboring windows to finally form a continuous film.However,coalescence of different growth regions induces new defects and a rough surface.One way to overcome the aforementioned problems is to adjust seed line orientation,the effect of which on the ELO GaAs/Si layer has been investigated in our previous work.However,the similar issue in the InP/GaAs material growth is still not carried out.In this paper,we prepared several nanopatterned GaAs substrates with different seed line orientations and analyzed both the morphology and crystalline quality of the as-grown InP/GaAs epilayers.On the other hand,as with the environmental pollution and energy shortage being more serious,solar cells have made an important progress in recent years due to their characteristics of renewable and pollution-free.As in the integrated circuits shown above,high conversion efficiency and low cost are the main targets of this industry.Due to the limitation of spectral absorption,there is a limited value in conversion efficiency of the traditional single-junction solar cell.In contrast,the multi-junction solar cell shows a great potential for the development.GaAsP is the most suitable III-V material for Si-based single-junction and dual-junction solar cells because of its direct and tunable bandgap.For that,many experiments regarding the GaAsP/Si solar cells have been performed in recent years.But only a few certain As compositions in GaAsP/Si are realized and the simulations about which are still hardly reported at present.Hence,a further study in design and optimization for GaAsP/Si solar cells is urgently needed.Based on the above analysis,our work mainly includes the following contents:1.Design and preparation of nanopattemed substrates.The geometric parameters of nanopatterned substrates,such as the window width,the mask thickness and the seed line orientation,were designed.Subsequently,the nanopatterned substrates were fabricated by deep-uv lithography and dry etching processes.Before the growth,we analyzed the types of pollutants and the corresponding cleaning treatments of the patterned substrates.The parameters of the prepared substrates are as follows:window width is 170 nm,SiO2 mask thickness is 250 nm,the seed line orientations are[110],[410],[010]and[4-10].2.Optimization of InP growth on nonpatterned GaAs substrate.Growth conditions of InP on nonpatterned GaAs substrates were first optimized by metal organic chemical vapor deposition,the optimized parameters mainly include the growth temperature of GaAs buffer layer and thickness of InP nucleation layer.The optimized results are as follows:a 30-nm GaAs buffer layer was first grown at 720?.Then the temperature was ramped to 450? to grow an 18-nm InP nucleation layer.Finally the temperature was ramped to 655? to grow the high-temperature layer.3.Growth and optimization of InP layer on nanopatterned GaAs substrates.By employing the above optimized conditions,1.8-?m InP epilayers were grown on the nanopattemed GaAs substrates.Both the morphology and crystalline quality of InP films were investigated by scanning electron microscopy,double crystal X-ray diffraction and atom force microscopy.In order to observe the morphology of initial coalescence stage,500-nm InP epilayers were also grown on nanopattemed substrates.Results reveal that lateral overgrowth rate is largest under the seed line orientation of[4-10],of which the surface roughness and crystalline quality are also optimized.4.Design and optimization of GaAsP/Si dual-junction solar cells.By using AMPS software,the single-junction crystalline sillicon solar cell structures were first optimized.Based on the results,the simulation calculation of GaAsP/Si dual-junction solar cells was carried out.We propose a new method to evaluate the properties of dual-junction solar cells from the single-junction subcells,the effect of As-composition and base layer thickness of the top cell on the short-circuit current and efficiency is systematically optimized.By calculating,the respective optimal efficiency of 33.2%for GaAs0.65P0.35 with base layer thickness of 2000 nm,35.8%for GaAs0.75P0.25 with base layer thickness of 1200 nm and 33.9%for GaAs0.85P0.15 with base layer thickness of 500 nm dual-junction solar cells are obtained.