Epitaxial Growth Of In_0.3Ga_0.7As Film On GaAs By Using A Large Lattice Mismatched Buffer Layer And Fabrication Of Its Solar Cell

GaAs-based III-V multi-junction solar cells have the advantages of high photoelectric conversion efficiency, superior radiation resistance and long lifetime, etc. The Ga_0.5In_0.5P/Ga As/In_0.3Ga_0.7As 3-junction solar cells, compared with the conventional GaInP/InGa As/Ge tandem solar cells, are of band-gap matched structure which results in a higher theoretical power conversion efficiency. However, due to a relatively large lattice mismatch（2.3%） of In_0.3Ga_0.7As with the GaAs substrate, one possible way to obtain high-quality In_0.3Ga_0.7As films on Ga As is to use buffer layers to release the lattice mismatch-caused stress between them. So far, compositionally graded buffer layers which usually contain more than four thin sublayers with various compositions have been applied. However, it is a big challenge to accurately control the composition and thickness of each sublayer. As a result, the crystal quality of as-grown In_0.3Ga_0.7As films on GaAs with the compositionally graded buffer layers is deteriorated. In order to solve this problem, a single amorphous buffer layer is proposed in this dissertation to replace the compositionally graded buffer layers. On the one hand, the ultra-thin amorphous buffer layer can effectively release the misfit strain between the GaAs and In_0.3Ga_0.7As, leading to high-quality In_0.3Ga_0.7As films grown on Ga As substrates. On the other hand, the conventional buffer layer has a series of problems such as complicated structure configuration, complexed fabrication processes and high costs. These problems can be effectively solved by incorporation of the ultra-thin amorphous buffer layer, laying a solid foundation for the fabrication of high efficiency tandem solar cells. The main contents and conclusions of this dissertation are as follows:Firstly, we investigate the growth temperature dependence of the amorphous In_0.6Ga_0.4As films grown on GaAs substrates by low temperature molecular beam epitaxy（MBE）. The crystalline states of In_0.6Ga_0.4As films are very sensitive to the growth temperature. When the growth temperature is 410 °C, the as-grown In_0.6Ga_0.4As film is polycrystalline. The uniform surface and amorphous In_0.6Ga_0.4As films is obtained at a substrate temperature lower than 380 °C. When the growth temperature is reduced to 350 °C, the as-grown In_0.6Ga_0.4As amorphous film exhibits a very rough surface, including the ridges-like surface grains and clusters. We also understand the forming mechanism of the large-mismatched amorphous In_0.6Ga_0.4As films grown on GaAs substrates at low growth temperature. On the one hand, the atoms migrate slowly on surface of the substrate at a lower growth temperature. Moreover, the In has a larger atomic radius and average mass than Ga and As, a large proportion of In atoms cannot obtain high enough kinetic energy to freely migrate on GaAs surface. On the other hand, the lattice mismatch between In_0.6Ga_0.4As and Ga As is as large as 4.3%, and thus a severe lattice distortion is generated at the In_0.6Ga_0.4A/Ga As interface. In this regard, the amorphous In_0.6Ga_0.4As can be formed on the GaAs substrate at low growth temperature.Secondly, effects of the thickness of the amorphous In_0.6Ga_0.4As buffer layer on In_0.3Ga_0.7As films grown on a GaAs substrates have been systematically investigated. When the amorphous In_0.6Ga_0.4As buffer is higher than 4 nm, the following growth of In_0.3Ga_0.7As film on GaAs is actually equvalent to growth on an amorphous In_0.6Ga_0.4As substrate, resulting in the amorphous In_0.3Ga_0.7As film. When the thickness of amorphous In_0.6Ga_0.4As buffer layer is too thin（< 2 nm）, it cannot efficiently release the misfit strain between the In_0.3Ga_0.7As epilayer and the substrate, leading to a poor-quality In_0.3Ga_0.7As film. Given an appropriate thickness of 2 nm, this ultrathin amorphous In_0.6Ga_0.4As buffer layer is acting like an “elastic layer” between In_0.3Ga_0.7As epilayer and GaAs substrate. Clearly, this amorphous elastic layer can efficiently release misfit strain and trap dislocations, effectively preventing them from penetrating into the following In_0.3Ga_0.7As epilayer.As a result, the high-crystallinity In_0.3Ga_0.7As film is obtained with 2-nm-thick amorphous In_0.6Ga_0.4As buffer layer, and the FWHM of In_0.3Ga_0.7As XRD rocking curve is 108 arcsec, which can completely meet the requirement of fabrication of high-efficiency solar cells.At last, the In_0.3Ga_0.7As single-junction solar cell structure with 2-nm-thick amorphous In_0.6Ga_0.4As buffer layer is designed and simulated by using the Crosslight APSYS. The optimized structure of the In_0.3Ga_0.7As single-junction solar cells is confirmed via simulating and optimizing parameters of each individual layer of the solar cell. then, the stable and efficiently doped n- and p-type In_0.3Ga_0.7As films with 2-nm-thick amorphous In_0.6Ga_0.4As buffer layer are obtained after systematically investigating the doping conditions. Finally, we fabricate the In_0.3Ga_0.7As single-juntion solar cells with an open-circuit voltage of 0.63 V under AM0 solar spectrum, which is very close to the theoretical value of 0.65 V.In short, large-mismatched In_0.6Ga_0.4As amorphous films grown on GaAs substrates have been achieved. And we also understand the forming mechanism of the large-mismatched In_0.6Ga_0.4As amorphous films on GaAs substrates. Meanwhile, the high-quality In_0.3Ga_0.7As film has been obtained with a 2-nm-thick In_0.6Ga_0.4As amorphous buffer layer on GaAs substrate, and the mechanism of the release of misfit stress between In_0.3Ga_0.7As and GaAs is discussed in details. Finally, the In_0.3Ga_0.7As single-juntion solar cells is achieved successfully in this thesis, which provides a new approach for fabrication of GaAs-based high-efficiency solar cells.