Surface studies of LaAlO3 substrates for TiO2 thin film deposition
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Inanattempttoincreasetheefﬁciencyofsolarcells, the one proposed solution is intermediate band solar cells (IBSCs). IBSCs are based on so-called intermediate band materials, that have an intermediate band within the bandgap. It is a rather new concept and there are still challenges with the fabrication of materials with an intermediate band (IB). At NTNU, our group is currently exploring TiO2 as the basis for IB materials, and the group aims to fabricate TiO2 ﬁlms on LaAlO3 (LAO) substrates. The main focus of this work was to characterize the LAO substrates, but also a set of TiO2 ﬁlms on LAO were characterized. To get high-quality ﬁlms, the substrate should meet some speciﬁcations. First, the lattice constant of the substrates should be closely matched with those of the ﬁlms. Secondly, the substrate should not undergo phase transformationthatcancausedefectsintheﬁlms. Thirdly, there should only minimal chemical reaction occurring at the interface between the substrate and ﬁlms. Unfortunately, LAO does not obey the second requirement of a good substrate. However, it’s lattice mismatch with TiO2 is only 0.2 % and therefore it worth exploring. AnotheradvantageisthatLAOcanbemadewithatomicallyﬂatterraces, with a single(either AlO- or LaO-) termination. In this work, we studied two batches of LAO annealed at either 1025 ◦C for 2 hours or 1200 ◦C for 3 hours for deposition of TiO2 ﬁlms and seven (TiO2) ﬁlms deposited on LAO substrates prepared as in batch 1 at 1025 ◦C. We used a 3D-optical proﬁler, optical microscope, atomic force microscope (AFM) and Raman spectroscopy to characterized substrates and ﬁlms. We found that LAO has twin domains in all substrates. Two different annealing times and temperatures were used to prepare the substrates. We have seen sharp steps and wide terraces in batch 2 at higher temperature and time as comparedtobatch1. ThedepositedﬁlmsonLAOsubstrateshavealsoadoptedfeaturesof twindomainsseenbythe3D-opticalproﬁler. Thethicknessoftheﬁlmsismeasuredusing the 3D-optical proﬁler and compared with thicknesses obtained using VASE and RBS. The Raman spectroscopy is used to determine the phase of (TiO2) that is deposited at two different ﬂuences in different oxygen and argon background using Pulsed laser deposition (PLD). The phases obtained using Raman spectroscopy matches the results from prior XPS analysis in the group except for two ﬁlms.