For the growth of a single layer FeSe on Nb-STO, molecular beam epitaxy (MBE) was applied. Within 1-UC FeSe films, superconductivity can only be achieved under cer- tain preparation conditions. In order to learn how to grow a superconducting single layer FeSe on STO, I had the opportunity to visit the group of Prof. Chunlei Gao at Jiao Tong University in Shanghai, funded by the Karlsruher House of Young Scientists (KHYS). Critical temperatures of single FeSe layers of up to 100 K were reported at the same institute [12]. In the group of Prof. Chunlei Gao, it was possible to grow the layers within an MBE chamber attached to a commercial LT-Unisoku STM. The latter was used in order to check the quality of the samples after growth and to perform first measurements, before the samples were covered with a protection layer (thick Se-layer). The coverage was necessary for the transportation of the samples back to Karlsruhe for further investigations.
1Note that all iron-based superconductors and cuprates are rather two-dimensional systems with su- perconductivity appearing in the FeSe(As)/CuO2-layers
9 FeSe monolayer on SrTiO3
9.3.1 Sample Preparation
For the successful growth of superconducting single layer FeSe on Nb-STO, a clean Nb-STO surface has to be ensured. Thus, the Nb-STO substrates were carefully de- gassed under UHV up to temperatures of about 1100◦C. The purpose is to flatten the samples and to eliminate some contaminations. Special care must be taken at temper- atures between 600◦C and 900◦C. Within this temperature range, the STO substrates can be easily cracked due to thermal expansion. Once the temperature of 1100◦C was reached, the sample was etched via a selenium flux for about 20 min. This should ac- celerate the desorption of carbon and oxygen contaminations from the STO surface. Subsequently, the sample was annealed at the same temperature for another half an hour before it was slowly cooled down to 480◦C. In general, different surface recon-
struction can form at the STO surface, e.g., (1×1), (2×1), (2×2), c(4×2), (√5 ×√5R26.6◦) [339]. Thereby, the reconstructions are usually formed by oxygen vacancies of the TiO2-
terminated STO surface [340]. No significant differences in surface conditions could be observed between pure STO and Nb-doped STO [340]. Furthermore, at annealing temperatures of around 800◦C, (1×1)-patterns were predominant, whereas at higher annealing temperatures (T>1250◦C) [340], (√5 ×√5R26.6◦)-reconstructions were ex- pected [340]. In Fig. 9.2a, a RHEED diffraction pattern, of one of our prepared Nb- doped STO-substrates, is displayed, showing a (1×1)-pattern.
Figure 9.2:a) The RHEED diffraction pattern of a clean Nb-doped STO sub- strate is shown. The pattern be- longs to a (1×1)-pattern of the TiO2- terminated substrate. b) The sharp diffraction spots become elongated due to the increasing amount of FeSe on top.
a)
b)
In literature, it is reported that in the case of a single layer of FeSe, superconductivity
9.3 Growth Mechanism and Surface Properties of a Single Layer of FeSe on Nb-Doped STO occurs at a growth temperature in the range of 400◦C-500◦C [325]. For temperatures below 400◦C, results show a rather semiconducting behavior [325]. For this reason, the growth temperature was set to 480◦C in our studies. Once the substrate temper-
ature was stabilized, Fe and Se were evaporated simultaneously from conventional MBE sources with a growth rate of 0.059 ML/min. In order to estimate the grown amount of layers, RHEED (see Sec. 3.2.3) was applied. The intensity of certain spots of the diffraction pattern was measured over time while growing. For a layer-by-layer growth mode, the intensity of the (0,0) spot shows oscillations over time. Once the first period is displayed, the growth of the first layer is finished. At the beginning of the growth process, sharp spots originating from the crystalline order of the sub- strate were visible (see Fig. 9.2a). With deposition of additional material, where the impinging electrons are scattered off, the spots became smeared out (see Fig. 9.2b). Once a flat layer was completed, the roughness of the surface was again reduced to a minimum and sharp reflection spots appeared again. The measurement of the spot intensity helps to determine the time needed for the completion of a complete layer FeSe on STO. Thus, this method serves as a growth control.
While growing, the Se-flux was usually considerably higher than that of Fe (≈ 1:10 turned out to be a good ratio [315]). In order to assure a Se-flux that is high enough, the Se-source was calibrated in UHV by using a Nb-STO-substrate at room-temperature, before starting the actual growth process. The flux was adjusted in the following way: The intensity of the (0,0) spot of the diffraction pattern of Nb-STO was mea- sured over time. At a certain time, the Se-flux was added by opening the shutter of the Se-evaporator. The amount of the flux was assumed to be suitable if the intensity of the measured lattice spot dropped to 1/3 of its original value within 15 s. The Se-covered STO can easily be cleaned by heating the substrate to about 300◦C for a few minutes.
Once a complete layer of FeSe had been grown on STO, the sample was post-annealed at 500◦C. The sample quality and superconducting properties improved with time. Thus, the samples were usually post-annealed for several hours.
9.3.2 Surface Topography
Topographic results for an FeSe monolayer are shown in Fig. 9.3. The measurements were carried out with the commercial Unisoku STM. Fig. 9.3a is an overview picture showing STO terraces with an almost complete FeSe monolayer on top of it. The height of a single layer of FeSe corresponds well to the value of 0.55 nm reported in literature [13]. The dark areas within one terrace correspond to an uncovered STO surface. By looking at the areas in Fig. 9.3a, covered by FeSe, it can be recognized that the contrast is not completely constant within an FeSe-layer of the same Nb-STO terrace and slight streaks are visible. The magnified part of Fig. 9.3a is presented in Fig. 9.3c, showing more details. These streaks have a corrugation of ≈ 60 pm. In Ref. [325], it is argued that such stripes have a considerably larger corrugation compared to twin boundaries (≈ 10 pm) [325, 341]. Furthermore, these stripes trace tortuous ways compared to twin boundaries which usually appear as rather straight lines. In contrast to a twin bound- ary, which is the mirror plane of two adjacent lattices, the angle of the crystalline lat-
9 FeSe monolayer on SrTiO3
tice changes only by a few degrees across these stripes. They most likely occur due to strain-induced effects within the layer while growing [325]. Fig. 9.3b shows the surface of an FeSe monolayer within a smaller area of 33 nm×33 nm. Some intrinsic adatoms remained on the surface after the growth. According to Ref. [242, 315], these are Se adatoms. One of the impurities is shown in detail in Fig. 9.3e. The atomically resolved Se-lattice is visible underneath both in Fig. 9.3e and Fig. 9.3b. For reasons of clarity it is shown separately in Fig. 9.3d.
40 nm 2.40 nm 0.00 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20 3 nm 52.2 pm 0.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 10 nm 174 pm 0 40 60 80 100 120 140 500 pm 137 pm 0 40 60 80 100
a)
b)
c)
d)
e)
Figure 9.3:a) Overview scan showing STO terraces covered by an almost complete single layer of FeSe. At some places (darker areas), pure STO is still visible. (I=180 pA, U=1 V). b) Topog- raphy taken in a smaller area (33 nm×33 nm). Intrinsic impurities are visible. They are most likely Se adatoms. One is shown in more detail in e). Furthermore, an atomically resolved lattice is visible in b). The lattice shows the Se-atoms of the upper Se-layer within an Se-Fe-Se trilayer. It is illustrated in d) in for more detail. In some areas of an FeSe-layer, stripe-like fea- tures are visible as shown in c). These stripes have a corrugation of 60 pm and are most likely induced by strain within the film.
Results of quasiparticle interference (QPI) measurement that could be achieved on the in-situ grown FeSe monolayer shown in Fig. 9.3, will be discussed in Sec. 9.5. Before that, results obtained with the JT-STM will be discussed first, for a better understand- ing of the QPI results. For the investigations of the FeSe monolayers with our home