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1 UvA-DARE (Digital Academic Repository) When X-rays and oxide heterointerfaces collide Slooten, E. Link to publication Citation for published version (APA): Slooten, E. (2013). When X-rays and oxide heterointerfaces collide General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam ( Download date: 20 Jan 2019

2 Chapter 6 The effect of the STO substrate: STO/LAO interfaces grown on In the dark, see past our eyes. Pursuit of truth no matter where it lies Metallica Abstract The role of the SrTiO 3 thickness on the electronic properties of NdGaO 3 (110)/SrTiO 3 /LaAlO 3 (NGO/STO/LAO) quantum wells is investigated using X-ray Absorption Spectroscopy (XAS) and Hard X-ray Photoemission Spectroscopy (HAXPES). The strain induced by using NdGaO 3 (NGO) as a substrate and the variation of the STO thickness have a clear effect on the (transport) properties of the system. Photoemission shows a large offset between the STO and LAO valence bands for STO thicknesses showing strong localization ( 6 unit cells). In the regime showing only weak localization in transport high carrier concentrations approaching half an electron per unit cell are deduced from Hall measurements, and confirmed from the HAXPES data. In addition, X-ray linear dichroism experiments show that for all STO thicknesses an orbital inversion 97

3 6. The effect of the STO substrate: STO/LAO interfaces grown on occurs in these systems: the lowest energy Ti 3d-orbitals in the NGO/STO/LAO systems are of xz/yz character, wholly unlike the case for regular STO/LAO interfaces and STO/LAO/SCO/STO systems, in which the xy orbitals are lowest in energy. This means that the combination of strain engineering in the STO, and interfacing STO with an NGO substrate leads to a crystal field that dictates an orbitally-degenerate ground state, potentially opening the path to novel orbital orderings or orbital liquid physics in these systems. Finally, the HAXPES data display the confinement of the carriers within a depth-dependent potential in the STO for all samples with n ST O < 20 in an unprecedentedly clear manner. Firstly, there is carrier confinement near the NGO/STO interface irrespective of the STO thickness. Interestingly, an additional, second potential well appears for the larger STO thicknesses that show weakly localized transport, thus proving that it is the carriers confined at the LAO side of the STO that are transport active. For these same samples the XAS dichroic data also show that the crystal field splitting between the degenerate xz/yz and the xy orbital levels doubles. 6.1 Introduction After having examined STO/LAO heterointerfaces in chapter 4, and studying the effect of altering the LAO/vacuum interface using STO and SCO/STO caps in chapter 5, we now turn to engineering STO/LAO properties from the other side of the sample: the STO substrate. As mentioned in chapter 3 there is a lattice parameter mismatch between bulk STO and LAO of 3 %. Altering the STO lattice parameter could influence this mismatch and in doing so affect the properties of the STO/LAO interface. This can be achieved by growing an STO/LAO interface on a different substrate. The lattice parameter of NdGaO 3 (110) (NGO) is Å. So by growing STO/LAO interfaces on NGO substrates one can reduce the strain at the STO/LAO interface and additionally vary the thickness of the STO layer. The effect of strain on the lattice parameters of the constituent materials of the oxide heterointerface is illustrated in Fig In the case of STO bulk /LAO the greater lattice parameter of the STO forces the LAO to adopt the same in-plane lattice parameter (128, 129). Because the strained film always conserves its unit cell volume (127) the strain leads to a reduction of the LAO c-axis lattice parameter. Because NGO has a lattice parameter smaller than the STO lattice parameter now, the STO film is squeezed and its c-axis lattice parameter 98

4 6.1 Introduction Figure 6.1: Illustration of the effect of strain on the lattice parameters of thin films. Each small square represents a unit cell. Not to scale. increases in the case of NGO/STO/LAO. The NGO lattice parameter is still larger than the corresponding value for LAO, so like in STO bulk /LAO the LAO in plane lattice parameter is increased and its out of plane lattice parameter is reduced. The difference with STO bulk /LAO is that the strain at the STO/LAO interface is, however, less strong. A more rigorous approach to determine the relationship between parallel and perpendicular strain, compared to these somewhat hand waving arguments, involves determining the Poisson ratio, which is determined by the elastic constants of the involved materials (127). As explained in chapter 3.4 the transport properties of NGO/STO/LAO systems depend critically on the STO thickness. A low thickness ( 6 unit cells (UC)) leads to strongly localized behavior whereas for larger STO thicknesses only weak localization is observed. Also the critical LAO thickness for conductivity increases from 4 UC for LAO films grown on STO single crystals to 8-10 UC for NGO/STO/LAO interfaces for 12 UC of STO. In order to uncover the microscopic mechanisms behind these differences in electronic behavior the NGO/STO/LAO samples have been investigated using both X-ray Absorption Spectroscopy (XAS) and Hard X-ray Photoemission Spectroscopy 99

5 6. The effect of the STO substrate: STO/LAO interfaces grown on (HAXPES) experiments. It will be shown that due to a combination of the strain and presence of the NGO substrate an orbital inversion occurs in the energy level scheme of the Ti d-orbitals, making the Ti 3d xz and 3d yz the lowest energy electron addition orbitals. This orbital degeneracy could possibly open the way for novel orbital orderings or orbital liquid physics in these systems. In addition, a realignment of the STO and LAO bands takes place as the STO thickness is increased from below to above the strong/weak localization threshold value. Finally the high carrier concentration deduced for these systems from Hall experiments will be confirmed and it will be shown that the majority of these transport active carriers are located near the STO/LAO interface. As discussed in chapter 2.3, XAS is a very powerful tool to probe local distortions (47, 103) in the crystal lattice and in this case to determine the Ti 3d energy level scheme. In order to measure this energy level scheme as a function of STO thickness, polarization dependent XAS experiments were performed on NGO/STO n /LAO 15 samples, grown for us using PLD by our collaborators at the National University of Singapore. Four different samples were investigated: n ST O = 4, 6, 8 and 12. By comparing the linear dichroism signal obtained from this experiment to simulations based on Charge Transfer Multiplet theory (10) one can determine the energy spacing between d-orbitals as a function of n ST O. The same set of samples plus one additional n ST O = 20 sample have then been studied using HAXPES, with particular attention being paid to the energy band alignment across the buried interfaces. By carefully monitoring the binding energies of core levels from the STO and LAO layer as a function of STO thickness one can determine both the band alignment across the interface and any band bending present in either the STO or the LAO. 6.2 Experiment The experiments were performed on NGO(110) substrates with STO overlayer thicknesses 4 n 12 UC, where n ST O is the number of STO layers. An LAO layer of 15 UCs was deposited on top of the STO layer. The samples are grown using pulsed laser deposition by our collaborators at the National University of Singapore, in an oxygen partial pressure of Torr. 100

6 6.3 XAS: orbital inversion Polarization dependent XAS experiments were performed at the undulator beamline UE52-SGM at Helmholtz Zentrum Berlin, using the ALICE end-station (130). During the experiment the sample was held in a grazing incidence geometry in such a way that the incidence angle of the photon beam on the sample was 20. In this way a horizontally polarized photon beam probes states out-of-plane and a vertically polarized light probes in-plane states, see equation 2.20 and table 2.3. The energy bandwidth of the beamline was set to 120 mev and the spot-size was 1 mm horizontal by 60 µm vertical. The Ti L 2,3 -edge was measured in Total Electron Yield (TEY) mode as a function of polarization for all samples. A smooth background was subtracted from the spectra and were normalized to peak intensity near 466 ev. XAS spectra were simulated using the CTM4XAS package (10). The HAXPES experiments were performed on the same sample set at the double crystal monochromatized KMC-1 beamline (21) using the HiKE endstation (23) at Helmholtz Zentrum Berlin. Unless stated otherwise, a photon energy of hν = 4 kev was used, and the total energy resolution was set to be 300 mev 1. The electron inelastic mean free path λ at this photon energy amounts to 5.5 nm for the Sr 3d, Al 2s and La 4d core levels and 5.1 nm for Ti 2p, as calculated by the TPP-2M relation (equation 2.8)(33). All experiments were carried out at room temperature. Further details of the methodology of the HAXPES experiments have been reported in chapter XAS: orbital inversion As explained in chapter 2.3, in XAS the photon beam excites core electrons into unoccupied states. The Ti L 2,3 -edge is dominated by transitions from the Ti 2p to the Ti 3d states. For Ti 4+ this gives a transition from a 2p 6 3d 0 to a 2p 5 3d 1 state. The crystal field splits the 3d levels into the t 2g (xz, yz and xy) and e g (x 2 y 2 and z 2 ) levels with z along the sample surface normal. In Fig. 6.2a the E a, b and E c Ti L 2,3 -edge spectra for n ST O = 12 are shown. E a, b favors transitions in-plane (xy and x 2 y 2 ) and E c favors transitions out of plane (xz, yz and z 2 ). All main features observed in STO single crystals (48) and STO/LAO interfaces (103) (see also chapter 4.3) are also present in these XAS spectra. The first major peak at ev and the second peak 1 This is sufficient to enables the detection of core level shifts of the order of 30 mev. 101

7 6. The effect of the STO substrate: STO/LAO interfaces grown on Figure 6.2: a) Total electron yield, Ti L 2,3 -edge, polarization dependent, soft X-ray absorption spectrum for the n ST O = 12 sample. Linear dichroism is calculated by subtracting the E a, b spectrum from the E c spectrum. b) Linear dichroism for NGO/STO/LAO samples and a STO/LAO 4 for reference. The spectra are offset vertically for clarity. A clear increase in the dichroic signal can be seen between n ST O = 6 and n ST O = 8. at 461 ev together form the L 3 -edge and correspond to transitions into the t 2g and e g levels respectively (48). The third and fourth major peak at 464 ev and 466 ev form the L 2 -edge and, like the L 3 edge, correspond to transitions into the t 2g and e g levels. By taking the difference between the E c and E a, b spectra (after careful normalization) the linear dichroism spectrum (green curve) can be calculated, which is very sensitive to any distortion of the TiO 6 octahedra and the concomitant alteration of the degeneracy and energy order of the 3d orbitals, in this case, of the Ti ions (47). One surprising feature that is immediately clear, is that the sign of the dichroism is in fact opposite to what has been reported for STO/LAO interfaces in the literature (103) and for the STO/LAO interfaces reported in chapter 4.3 and STO/LAO/SCO/STO systems reported in chapter 5.4 of this thesis. For comparison, a linear dichroism spectrum for STO/LAO 4 (from chapter 4.3) is also shown in Fig. 6.2b. This leads us to conclude that, unlike in STO/LAO, in these systems the lowest energy electron addition states are the xz and yz orbitals: an orbital inversion. As can be seen from Fig. 6.2b this is true for all samples grown on NGO. This orbitally degenerate ground state could possibly lead to novel orbital orderings. The linear dichroism spectra for n ST O = 12 and n ST O = 8 are exactly the same in sign and magnitude. This indicates that there is no change in order and energy separation of the Ti 3d orbitals between these two samples. The magnitude of the linear dichroism for n ST O = 4 and n ST O = 6 is smaller however. This shows that, 102

8 6.3 XAS: orbital inversion Figure 6.3: a) Linear dichroism of the Ti L-edge of NGO/STO/LAO samples for n ST O = 4 and n ST O = 12 plotted together with simulated data (see text). b) Ti 3d energy level scheme for n ST O = 4 and n ST O = 12 based on the simulations shown in panel a). although the energy order of the Ti 3d orbitals is still the same, the energy separation between the energy levels is lower when compared to the thicker STO layers. This increase in the magnitude of the linear dichroism between n ST O = 6 and n ST O = 8 coincides with the transition from variable range hopping to weak localization observed in transport (discussed in chapter 3.4). In Fig. 6.3a the dichroism signals for n ST O = 4 and n ST O = 12 are compared to atomic multiplet calculations, see chapters and In these calculations 10Dq = 1.6 ev, =2.9 ev, V (e g ) = 3.4 ev, U dd = 4.0 ev, U dc = 6.0 ev in accordance with Ref For n ST O = 4 values of Ds = 15 ± 3 mev and Dt = 8 ± 2 mev were found, which gives an energy level splitting of the t 2g levels of 5 ± 10 mev and 100 ± 20 mev for the e g levels. For n ST O = 12, Ds = 20 ± 3 mev and Dt = 8 ± 2 mev, yielding 20 ± 10 mev for the t 2g level splitting and 120 ± 20 mev for the e g levels. The linear dichroism is much stronger for e g than for the t 2g levels. This is possibly due to long distance metal-metal interactions between the NGO and STO. DFT calculations have shown the xy orbitals to be more prone to localization than the xz and yz orbitals (92). Because the xz and yz orbitals are the lowest energy electron addition states it is to be expected that transport is dominated by these two orbitals. But for n ST O = 4 and n ST O = 6 the splitting within the t 2g manifold is a mere 5 mev, making partial occupation of the xy states via overlap with the xz and yz bands due to the finite bandwidth, not unlikely. When the t 2g energy splitting increases 103

9 6. The effect of the STO substrate: STO/LAO interfaces grown on Figure 6.4: a) Ti 2p HAXPES spectra for all NGO/STO/LAO samples reported here. A strong n ST O -dependence of the intensity of the Ti 3+ shoulder can be seen. b) Red: the Ti 3+ /Ti 4+ ratio as a function of STO layer thickness. Green: the corresponding carrier concentration in electrons/cm 2 and electrons/uc. to 20 mev between n ST O = 6 and n ST O = 8 the overlap reduces and the system shows a transition from localized to weakly localized behavior. As was the case in Ref. (103) no sign of Ti 3+ states in the XAS spectra can be found in these data, which suggests that the carriers in this system are delocalized. In order to examine the issue of the carriers -presumably electrons in Ti 3d related states - from a different perspective, HAXPES investigations have been carried out on the same samples plus one additional n ST O = 20 sample. In these experiments delocalized carriers of Ti 3d character will be localized by the core-hole potential and therefore become visible as a Ti 3d 1 shoulder in a Ti 2p HAXPES spectrum, as discussed earlier in the context of chapter 4.3. Because of this, Ti 2p HAXPES is sensitive to the sum of both localized and delocalized carriers, thus giving complementary information to transport experiments. 6.4 HAXPES: carriers and their distribution Fig. 6.4a shows the Ti 2p HAXPES spectra measured from these samples. All samples on NGO substrates, except for n ST O = 4 and n ST O = 20, show a remarkably large Ti 3+ shoulder, indicating a large Ti 3d 1 population in these systems. The shoulder intensity observed for n ST O = 4 is similar to what is observed for the first generation samples discussed in chapter 4.3. For n ST O = 8 and n ST O = 12 the number of 3d electrons from HAXPES is in accordance with the high carrier concentration measured in transport (see chapter 3.4). For n ST O = 6, although HAXPES provides clear evidence 104

10 6.4 HAXPES: carriers and their distribution of significant electronic charge in Ti 3d states, no such mobile carrier concentration is measured in transport. This point will be returned to later. For n ST O = 20 no clear Ti 3+ shoulder was discernible, and simulations give an upper limit for a hypothetical 2D carrier layer of < 0.05 e/uc. The strong downward step in shoulder intensity between n ST O = 12 and n ST O = 20 is in agreement with transport experiments where at the same n ST O interval a large decrease in carrier concentration was observed, see chapter 3.4. In Fig. 6.4b the carrier concentration calculated from the Ti 3+ /Ti 4+ intensity ratio is shown. This carrier concentration is based on the assumption that the Ti 3d 1 states are spatially distributed equally across the entire STO layer. Additionally, it is assumed contributions from the entire STO layer are measured in the experiment, which, given the value of λ = 5.1 nm and the fact that Ti 2p HAXPES spectra measured at different excitation energies showed unchanging main peak/shoulder intensity ratios (not shown), is a valid assumption. The electron concentration measured in photoemission is higher than the carrier concentration observed in transport measurements due to the fact that HAXPES measures all Ti 3d electrons, not only those active in transport. The highest carrier concentrations observed in transport approach the value of 0.5 e/uc, the charge concentration required to avert the polar catastrophe (see chapter 3.1.) The highest carrier concentrations observed in HAXPES, however, reach up to 0.7 e/uc, exceeding the polar catastrophe value, a point that will be returend to later on. The fact that at n ST O = 4 and 20 the Ti 3+ shoulder is much lower is in keeping with the transport data, thus only for n = 6 there is a remarkable discrepency between the transport and spectroscopic measurements. Another important feature of the data from the NGO/STO/LAO system is the width of the Ti 2p 3/2 HAXPES main peak at ev ( 1.0 ev), which for all STO thicknesses but the n ST O = 20 is significantly larger than that of a bare STO substrate ( 0.8 ev) or regular STO/LAO ( 0.9 ev) measured at these resolution settings. This is shown in Fig 6.5a. The same trend is observed in the Sr 3d core levels. That this is seen in both Sr and Ti core levels is a strong indication that this is related to band bending near either one or both of the STO interfaces (i.e. STO/LAO and NGO/STO). A spatial variation in the energy of the STO bands will cause the core level spectra emitted from each individual STO layer to be shifted with respect to one another. In a photoemission experiment, all contributions from the individual layers 105

11 6. The effect of the STO substrate: STO/LAO interfaces grown on Figure 6.5: a) Ti 2p 3/2 HAXPES spectrum for NGO/STO 12 /LAO 15 compared to the same spectrum for bare STO and STO/LAO 10 (also provided by the NUS group). A distinctly larger width can be seen for the NGO/STO 12 /LAO 15 sample. The inset shows the Ti 3+ shoulder for all three samples. b) and c) The same spectrum as shown in panel a) for NGO/STO 12 /LAO 15 now with a simulation of that spectrum based on a potential well confining all electrons to the LAO side (b) and NGO side (c) of the STO. are measured, scaled by the inelastic mean free path λ = 5.1 nm at 4 kev. The result of such band bending is then a broader spectrum (121). The width of the n ST O = 20 sample is very similar to that of a bare STO single crystal, indicating that the bands in STO layer of this sample are relatively flat compared to the other samples. Careful analysis of the Ti 2p line shape (for n ST O < 20, see for instance Figs. 6.5a) shows that the main line is not only wider than usual, but also slightly asymmetric. In order to determine the exact nature of the band bending, the data were compared to simulations, based on different spatial band profiles in the STO. The band profiles were simulated by using a Ti 2p spectrum from a bare STO substrate as the basic building block. Taking the spectrum of a clean STO substrate as the basic model means we make no attempt to simulate the 3d 1 -related low binding energy shoulder, which has no consequence for the conclusions regarding the band bending profile. By shifting this building block spectrum in energy and scaling its intensity with λ, a contribution for each STO layer is generated. The simulations in Fig. 6.5b and 6.5c clearly show that for n ST O = 12 a potential well at either only the STO/LAO or the NGO/STO interface does not agree with the experimental data. In order to get good agreement with the data a potential well at both interfaces must be considered. Fig. 6.6 shows the simulation which gave best agreement with the data for n ST O = 12. The inset of panel a) shows the band profile within the STO. There are two potential wells: one deep well ( 0.7 ev) at the LAO side and one shallow well ( 0.4 ev) 106

12 6.4 HAXPES: carriers and their distribution Figure 6.6: a) Simulated Ti 2p spectrum for n ST O = 12. The contributions from the individual layers are shown (green and orange curves), with the orange curve originating from the STO layer closest to the LAO. The inset shows the band profile on which this simulation was based. b) Sketch of the sample illustrating what is meant by the term STO layer index. c) Values for the potential well depth and d) electron concentration at both interfaces for all simulated samples. The colored bands indicate the general trend. The detection limits are indicated and the upper limits for potential well depth and electron concentration for n ST O = 20 are included as open data points. at the NGO side. As mention above, these simulations apply only to Ti 4+ states and hence do not reproduce the Ti 3+ shoulder observed in the experimental data. Similar double-well profiles also give the best results for n ST O = 8 and n ST O = 6. Thus for 6 n ST O 12, the NGO/STO/LAO system has -two- potential wells: one at each interface. For n ST O = 4, the sample with much lower carrier concentration, both in transport and as 3d 1 states in HAXPES, the data are only compatible with the existence of a single potential well, situated in the STO at the NGO/STO interface. For n ST O = 20 such a potential well at the NGO/STO interface is also possible, as the contribution to the total spectrum from such a well would be almost negligible for a sample with such a thick STO layer (which in turn means these n ST O = 20 data cannot deliver a firm conclusion about the presence/absence of a potential well at the NGO/STO interface). Band bending near the STO/LAO interface for the n ST O = 20 would give a clear increase in the peak width as this interface is much closer to the surface. The fact that such broadening is not apparent limits the possible band bending near this interface to < 100 mev. The correlation between electron concentration and potential well structure can be 107

13 6. The effect of the STO substrate: STO/LAO interfaces grown on further explored by filling the simulated wells with electrons up to the point where the simulation matches the area ratio of the Ti 3+ /Ti 4+ features in the experimental Ti 2p 3/2 HAXPES data. Doing so gives 0.2 electrons at the NGO side and 0.4 electrons at the LAO side for n ST O = 6, 8 and 12. For n ST O = 4 the situation is quite different, with a potential well only at the NGO side, which holds 0.1 electrons. Figs. 6.6c and 6.6d show the well-depth values and electron concentrations obtained from the simulations for all samples. Note that both the well depths and electron concentrations are more or less constant after the transition between n ST O = 4 and n ST O = 6, as illustrated by the thick guides drawn over the data points. The electron concentrations observed at the STO/LAO interface are very close to the value of 0.5 e/uc, as predicted by the electronic reconstruction model (chapter 3.1). This also agrees with the carrier concentrations observed in Hall experiments, shown in chapter 3.4. The fact that the total Ti 3d 1 concentrations observed in HAXPES mentioned earlier this chapter exceed the value of 0.5 e/uc is now seen to be due to the localized carriers at the NGO/STO interface. Although the simulations are robust, giving meaningful results at the level discussed until now, the data do not support conclusions on the fine structure of the n ST O dependence of the well depths and elated electron occupancies between n ST O = 6, 8 and HAXPES: Band alignment Having determined the band profile within the STO layer, we now turn to the STO/LAO interface. Using a core level from the LAO and STO films, together with the valence band maxima for two reference samples (an STO single crystal and a 20 UC thick LAO film grown on STO) the valence band offset (VBO) between the two films can be determined (18). As a consistency check, the binding energy of two different core levels from each film was taken: Ti 2p and Sr 3d on the STO side and La 4d and Al 2s on the LAO side. The spectra from last three aforementioned core levels are shown in Fig. 6.7a. For the Ti 2p and Sr 3d core levels, the component originating from the middle of the STO layer as determined by the simulations shown in Fig. 6.6 was taken as the relevant STO energy level. The four core levels used give four different combinations that can result in the determination the VBO, and each combination gave the same result within the experimental error bar. The resulting VBOs are shown in Fig. 6.7b. 108

14 6.5 HAXPES: Band alignment Figure 6.7: a) Sr 3d, Al 2s and La 4d spectra used for valence band offset (VBO) determination. All core levels are indicated. b) Upper panel: valence band offsets. The red datapoints are VBO determinations using a single set of core levels, the blue data points indicate the average values. Lower panel: electron concentration at the STO/LAO interface from simulations (also shown in Fig. 6.6d.) The values of the VBO for n ST O = 6, 8 and 12 are quite similar to those found in regular STO/LAO interfaces, (97, 121) reported in chapter 4.4, where values between 50 and 400 mev are typically observed. The VBO for n ST O = 4 however is much larger at value of 0.67 ev. This is the largest VBO ever recorded for a system with a bare LAO/vacuum interface terminating the thin film stack. At this stage, the reader is reminded that a valence band offset the size of the STO band gap (= 3.2 ev) would be required in the simplest picture for charge transfer as part of an electronic reconstruction to occur. This implies that the carriers probably originate from a different source, such as oxygen vacancies. As was the case in the previous chapter (5.5), the low values of the VBO for 6 n ST O 12 correspond to charges being present at the STO/LAO interface, as evident from Fig. 6.7b. Likewise, the high VBO values for n ST O = 4 and 20 correspond to cases in which there are little or no charges at the STO/LAO interface. This is consistent with the idea that these charges screen the polar discontinuity, thereby reducing the valence band offset. Using the VBO and the band profile obtained in the Ti 2p width simulations, a band alignment scheme for all samples can be constructed. These schemes are shown for the n ST O = 12 and n ST O = 4 samples in Fig For n ST O = 8 and n ST O = 6 the picture looks very similar to n ST O =

15 6. The effect of the STO substrate: STO/LAO interfaces grown on Figure 6.8: Band alignment diagrams derived from the VBO determination and Ti 2p HAXPES peak width simulation for NGO/STO/LAO systems with n ST O = 4 (fully localized in transport) and n ST O = 12 (weak localization in transport). From this combination of data the following picture emerges: for all samples (although this is a moot point for n ST O = 20) there are Ti 3+ centers in a potential well at the NGO/STO interface. These carriers are not transport active and may well originate from a combination of oxygen defects and Nd indiffusion into the STO film. At n ST O = 6, the appearance of a second potential well at the STO/LAO interface potentially opens up a new channel for transport, and leads to a sharp increase in the number of charges located in the STO/LAO interfacial region, and thus to the concomitant drop in the VBO between n ST O = 4 and 6. Meanwhile the original well at the NGO/STO interface remains. At n ST O = 20 the potential well at the STO/LAO interface vanishes or at least has a strongly reduced depth, possibly due to oxygen vacancies and strain. This severe reduction in the number of charges in the STO/LAO interfacial region means the polar discontinuity is far less effectively screened and the VBO returns to a large value, reminiscent of that for n ST O = 4. The XAS experiments have shown that in all cases the lowest energy Ti 3d orbitals are the xz and yz orbitals, which are degenerate. However, the increase in t 2g energy splitting between n ST O = 6 and n ST O = 8 is the key to allowing the system to switch from strongly localized to weakly localized behavior. According to DFT calculations (93), in systems with strong hopping 1 along the z direction, the xz and yz 3d orbitals are lower in energy than the xy orbital. Since 1 A term originating from the tight binding model. The hopping amplitude gives the probability of an electron to hop from site to site along a certain direction. 110

16 6.6 Conclusions this orbital energy scheme is exactly that observed in NGO/STO/LAO systems, it is possible that hopping along the z direction is indeed strong in these systems. So if the weakly localized behavior observed in transport is driven by this strong c-axis hopping, one could theorize that for n ST O = 6 the STO film is too thin, constraining this hopping, and thus forcing the transport behavior of the system to remain localized. Thus, even though the additional well at the STO/LAO interface is present for n ST O = 6, the low film thickness keeps the carriers we observe in Ti 2p HAXPES localized. When the STO thickness is increased to 8 UC the transport data tell us that the hopping is no longer constrained and the samples show only weak localization. There is evidently still effective scattering of the charge carriers, even after the transition to weak localization, as the mobility of the whole system is low. Whether this has its origins at the STO/LAO interface or from additional scattering from the NGO/STO interface (the latter is strong enough to localize all the charges we identify as occupying the NGO/STO potential well for all n ST O < 20) remains an open question at this stage. 6.6 Conclusions In conclusion, detailed XAS and HAXPES experiments on NGO/STO/LAO systems have been conducted. The data show that by interfacing STO with NGO it is possible to tune the energy of the Ti 3d-orbitals in such a way that not the xy, but the xz and yz 3d-orbitals are lowest in energy. This orbital inversion has a significant effect on the transport behavior, via modification of the relationship between quasi-2d (Ti 3d xy -related) hopping and stronger c-axis hopping in systems with yz and xz as lowest lying Ti 3d orbitals. Additionally there is shown to be not one, but two potential wells hosting electrons in the Ti 3d orbitals: one at the NGO side of the STO and a second one at the LAO side for thicker STO films. The former states are strongly localized, whereas the latter are more mobile, displaying transport behavior consistent with weak localization. If the polar discontinuity at the STO/LAO interface is not screened by electrons in a potential well at the STO/LAO interface, this discontinuity leads to high values for the valence band offset between STO and LAO, thereby confirming the trend identified in systems based upon bulk STO substrates between low charge density at the STO/LAO interface and large energy offsets between the neutral and polar oxide blocks in these oxide heterostructures. Finally, the spectroscopic determination of the 111

17 6. The effect of the STO substrate: STO/LAO interfaces grown on potential profile in the STO presented here is of unique clarity and definition in the field of oxide heterointerfaces. 112

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