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A method of growing highly oriented MoS

A method of growing highly oriented MoS2 is presented. First, a Mo film is deposited on a graphene/SiC(0001) substrate and the subsequent annealing of it at 750 °C leads to intercalation of Mo underneath the graphene layer, which is confirmed by secondary ion mass spectrometry (SIMS) measurements. Formation of highly oriented MoS2 layers is then achieved by sulfurization of the graphene/Mo/SiC system using H2S gas. X-ray diffraction reveals that the MoS2 layers are highly oriented and parallel to the underlying SiC substrate surface. Further SIMS experiments reveal that the intercalation process occurs via the atomic step edges of SiC and Mo and S atoms gradually diffuse along SiC atomic terraces leading to the creation of the MoS2 layer. This observation can be explained by a mechanism of highly oriented growth of MoS2: nucleation of the crystalline MoS2 phase occurs underneath the graphene planes covering the flat parts of SiC steps and Mo and S atoms create crystallization fronts moving along terraces.

Further development and optimization of modern optoelectronic devices requires fast and reliable procedures that may evaluate the quality of interfaces. For thick multilayer devices, mixing effect may significantly prevent proper interpretation of secondary ion mass spectrometry depth profiles especially if a region of interest is located far from the sample surface. In this work, we present how to overcome this problem with a so-called a-crater-within-a-crater approach. In this notion, a high energetic primary ion beam is used to rapidly remove most of the material forming a large crater. Then, the energy is significantly reduced and a new smaller crater is formed at the bottom of the previous one. Close to the region of interest, the impact energy is decreased to 150 eV and thus an interface can be analyzed with minimal mixing effect and thus its quality can be adequately assessed. Usefulness of this approach is tested on an epitaxial structure of a triple-junction solar cell and reliable information about the structure imperfection has been obtained: p and n dopants in the tunnel junction overlapped, deteriorating the operation of the device.

Epitaxial lift-off (ELO) is a process which enables the removal of solar cell structures (one junction GaAs, two junction GaAs/InGaP or three junction GaAs/InGaAs/InGaP) from the substrate on which they are grown and their transfer onto lightweight carriers such as metal or polymeric insulator films. The said solar cells exhibit superior power conversion efficiency compared with alternative single-junction photovoltaic cell designs such as those based on crystalline Si, copper indium gallium sulfide (CIGS) or CdTe. The major advantage of ELO solar cells is the potential for wafer reuse, which can enable significant manufacturing cost reduction by minimizing the consumption of expensive wafers. Here in this work we have grown one junction GaAs solar cells on GaAs (100) substrates. A 10 nm thick AlAs layer has been used as a release layer, which has been selectively etched in HF solution. We have investigated different methods of transferring thin films onto polymer and copper foils, including the usage of temporary mounting adhesives and electro-conductive pastes. Lift-off has been demonstrated to be a very promising technique for producing affordable solar cells with a very high efficiency of up to 30%.