Multijunction solar cells have shown their ability to achieve very high solar energy conversion efficiencies, making them optimal for energy production on Earth and in space. Many multijunction solar cell architectures use III–V semiconductor materials, like gallium arsenide, on top of a germanium bottom cell. Germanium has a narrow band gap making it suitable for harvesting long wavelengths up to 1800 nm and it can simplify the solar cell fabrication process. One challenge with germanium is, that there is inevitable cross-diffusion between the germanium and III–V layers, which can lead to a formation of a compensated region or an unintentional pn-junction. This problem lowers the efficiency of the multijunction solar cell. There are many possible fabrication methods to minimize the unintentional diffusion between germanium and III–V layers, but to evaluate their effectiveness we need to obtain the depth profile of the solar cell composition.
In this thesis, inductively coupled plasma mass spectrometry (ICP-MS) was used to measure the composition depth profiles of germanium bottom cells and III–V layers on top of them. In ICP-MS the sample material (in liquid or gas form) is turned to a plasma and then analysed in a mass spectrometer. For depth profiling semiconductor samples, a custom-made wet etching flow cell was integrated to the unit. In the flow cell, a flowing etchant is used to etch the sample surface and the dissolved material is then fed to the ICP-MS. This method is compared to secondary ion mass spectrometry (SIMS) and electrochemical capacitance-voltage profiling (ECV), that are commonly used for depth profiling. A series of samples of gallium arsenide, germanium and III–V/Ge heterostructures were measured with ICP-MS. A working measurement procedure and a post-analysis program were developed.
Comparing the results measured with ICP-MS to SIMS and ECV results showed that ICP-MS with the wet etching flow cell does not produce as accurate diffusion profiles as SIMS for gallium and arsenic in germanium. The shape of etched crater was irregular instead of preferred flat profile. The shape of the etch crater also prevented the measurement of germanium diffusion in gallium arsenide layer. The interference caused by germanium isotopes also affected substantially the measurement of gallium and arsenic compositions. The depth profiles were attempted to reconstruct computationally with an evolution algorithm from the ICP-MS measurement signal and etch crater shape, but the method did not produce reliable results except for one measurement. The SIMS and ECV results were in good agreement and showed the junction formation and cross-diffusion of gallium that was less than 100 nm deep in germanium. In some cases gallium diffused more than arsenic resulting an unintentional junction. It could be possible to obtain similar results with the ICP-MS if its wet etching process would be more uniform, which would require redesigning the flow cell
