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Electronic properties of buried hetero-interfaces of LaAlO3 on SrTiO3

We have made very thin films of LaAlO3 on TiO2 terminated SrTiO3 and have measured the properties of the resulting interface in various ways. Transport measurements show a maximum sheet carrier density of 1016 cm-2 and a mobility around 104 cm2 V-1 s-1. In situ ultraviolet photoelectron spectroscopy (UPS) indicates that for these samples a finite density of states exists at the Fermi level. From the oxygen pressure dependence measured in both transport as well as the UPS, we detail, as reported previously by us, that oxygen vacancies play an important role in the creation of the charge carriers and that these vacancies are introduced by the pulsed laser deposition process used to make the heterointerfaces. Under the conditions studied the effect of LaAlO3 on the carrier density is found to be minimal.


Introduction
Recently, Ohtomo and coworkers [ 1,2 ] have reported the existence of a conducting electron layer at the hetero-interface between two nominal insulators. This is a remarkable result and has intrigued many researchers in the field [3][4][5][6]. It suggests, at a minimum, a very interesting charge transfer system due to valence mismatch of insulators. Specifically, in the work of reference [ 1,2 ], the hetero-interfaces were formed from very thin films of LaAlO 3 grown on SrTiO 3 substrates. found for a TiO 2 /LaO interface that this charge layer is conducting and has a sheet carrier density of ~10 17 electrons/cm 2 , as inferred from Hall-effect measurements. This carrier density exceeds by two orders of magnitude what one would expect based on simple considerations. The surface polarization expected by simply terminating bulk SrTiO 3 with LaAlO 3 is half an electron per unit cell, which amounts to 3.2×10 14 charges/cm 2 . Even though the original work [ 1,2 ] does mention that 10 17 carriers/cm 2 is rather unphysical to be explained by an electronic reconstruction, a very natural explanation is found through oxygen vacancies, which was dismissed by those authors.
We have already discussed this discrepancy in an earlier paper, where we compared with a variety of physical measurements (near edge x-ray absorption spectroscopy (NEXAS), visible to 3 vacuum UV-spectroscopic ellipsometry (vis-VUV-SE), annealing experiments, transport, and ultraviolet photoelectron spectroscopy (UPS)) samples made under different deposition conditions, and we proposed a model to explain the results [ 7 ]. However, in that paper only some of the results of the transport and UPS measurements were presented.
Here we present a fuller account of our results, particularly on the transport and ultraviolet photoelectron spectroscopy (UPS) properties at this interface. In part one, we will show results from resistivity measurements. We show that the magnitude of the sheet density and the mobility of the electrons are a sensitive function of the deposition conditions in ways that suggest that the origin of this large sheet charge density is oxygen vacancies (donating electrons) in the SrTiO 3 substrate. Further, we argue that these vacancies are introduced by the pulsed laser deposition (PLD) process used by us and all previous authors to grow these hetero-structures. We will close this part by discussing localization effects at low temperatures and hysteretic behavior in the resistivity measurements. In part two we will discuss the UPS spectra of the buried interfaces with the highest sheet carrier densities, i.e. the films made at the lowest pressures. We will look at the number of states that appear at the Fermi level and at their dependence on deposition conditions, where several interesting trends are observed. Before we discuss the results we turn to the experimental parameters.

Experiment
All of the films reported here were grown using PLD in a stainless steel vacuum chamber with a background pressure of 10 -9 Torr. A KrF excimer laser produces a 248 nm wavelength beam with typical pulse lengths of 20-30 ns. A rectangular mask shapes the beam, and a variable 4 attenuator permits variation of the pulse energy. The energy density on the target has been kept at approximately 1.2 J/cm 2 unless specified otherwise. Before each run, the rotating LaAlO 3 target was pre-ablated for two minutes at 4 Hertz. The LaAlO 3 layers were grown on TiO 2terminated SrTiO 3 substrates at 815 °C. The HF chemical treatment needed to form TiO 2terminated SrTiO 3 substrates is described elsewhere [ 8 ].
Various parameters were varied from run to run. These included laser repetition rate, number of laser pulses, mask size and post-deposition treatment. The effects of these variations are important and will be discussed below. In addition, to obtain consistent results, we found it necessary to use a standard post-deposition treatment for samples grown at low oxygen pressures (10 -6 Torr) in which both the oxygen flow and the substrate temperature were quenched immediately. This treatment is believed to prevent the very thin LaAlO 3 layer from diffusing oxygen during cooling; this way it is possible to look at the interface as grown. At higher oxygen pressures (10 -5 Torr), such treatment was not necessary and samples were cooled down in deposition pressure. During growth, RHEED was used both to determine the amount of material deposited (by counting the periods of intensity oscillations in the standard way) and to monitor the morphology of the sample. RHEED intensity oscillations reveal that typically 120 laser pulses are required to grow a monolayer of LaAlO 3 . After deposition, the samples were moved in situ into an adjacent photoemission analysis chamber (<5×10 -10 Torr base pressure). Electrical transport properties were measured ex situ with a Quantum Design Physical Property Measurement System using the Van der Pauw geometry, taking appropriate precautions to avoid photo-induced carriers.

Part one: Transport
Here, we discuss transport properties of two classes of samples. The first was prepared under relatively low oxidation conditions (10 -6 Torr, as measured with a hot cathode ion gauge), resulting in a high number of carriers. The second was deposited and cooled at higher oxidation conditions (2×10 -5 Torr), resulting in a reduced carrier density. In Figure 1 we show the temperature dependence of the resistivity for representative samples of the two classes. Together with accompanying Hall measurements, we deduce a sheet carrier density of ~10 16 cm -2 and a mobility of 10 4 cm 2 V -1 s -1 (at 4 K) for the samples that are made at low pressure and a sheet carrier density of ~10 13 cm -2 and a mobility of 10 3 cm 2 V -1 s -1 (at 4 K) for the samples made at higher pressure. These findings are very much in line with what has been reported by others and demonstrates directly that the oxygen pressure during deposition clearly affects the transport properties of these hetero-structures.
Now we focus on the transport data for films made at low oxygen pressure (10 -6 Torr), since these display the most extraordinary densities and mobilities. Note that these samples are also discussed in the next section on UPS measurements. The sheet resistance and Hall effect of the films were measured as a function of temperature. The results for the 1, 3 and 5 monolayer (ML) samples are shown in Figure 2. Note that the 1 ML thick sample was additionally oxidized after deposition whereas the other samples were not. In any event, the sheet resistance (see inset of Next, we turn to the interfaces made under high oxidation conditions. From resistivity and Hall measurements we arrive at a sheet carrier density of ~10 13 cm -2 and a mobility of 10 3 cm 2 V -1 s -1 (at 4 K), as expected lower than the previous case, this is shown also in Figure 1. In addition, an interesting temperature hysteresis effect is measured for the films with low carrier densities, this is shown in Figure 3. The amplitude of these features increases for thinner LaAlO 3 overlayers: so far a maximum effect has been observed for 4 unit cell layers for as deposited films (fewer than 4 layers deposited resulted in insulating samples for these deposition conditions, which has also been observed by Thiel et al. [ 4 ]); and they also increase with magnetic field. The temperature at which they occur is always the same though. These features appear not to be stable over time: when during warm up the temperature is fixed to the maximum of one of the features, the resistance of the sample slowly drops over time until it reached the resistance the sample had during cool down. The time over which this happens is in the order of hours.

Part two: Photoemission
Of the samples made under low oxygen pressure, of which the transport properties were discussed in the previous section, UPS spectra were taken. The angle-integrated UPS data for hetero-structures with different LaAlO 3 layer thicknesses are shown in Figure 4. These UPS spectra were taken for hetero-structures with LaAlO 3 films ranging in thickness from a half to three unit cell layers, as determined by RHEED. Note that as the thickness of LaAlO 3 is increased, the relative intensities of the oxygen 2p double-peak structure (at about 5 eV and 7 eV, respectively) change in a systematic fashion. Unfortunately, excessive charging has prevented us from taking a useful '0 ML case' spectrum, i.e., undoped SrTiO 3 (instead we have added a spectrum of a Nb doped SrTiO 3 substrate). On the other hand, this charging confirms directly that the SrTiO 3 is initially insulating. In addition, by extrapolation, our data seems compatible with known fully oxidized SrTiO 3 UPS data (see for example Henrich and Cox's book [ 9 ]).
To check the conductivity of the sample one has to measure carefully around the Fermi level to check for states being present. As an example, an expanded version of the UPS data near the Fermi energy is presented in Figure 5 for a representative sample, both as deposited and after additional oxidation. This particular sample had a 1 ML thick LaAlO 3 layer and was deposited at a repetition rate of 1 Hz in an atmosphere of 10 -6 Torr molecular oxygen. The additional oxidation was carried out by exposing the sample to 6000L of oxygen at 150 °C. The oxidation of the sample had no discernable effect on the UPS spectrum outside the region shown in Figure   5.
Most importantly, the UPS data show the existence of electron states extending up to the Fermi energy, indicating the existence of a conducting charge layer at the interface (the conducting nature of this interface was confirmed directly by transport measurements presented below).
Moreover, the size of this density of states is sensitive to the oxidation history of the sample, being lowered by further oxidation. At a more quantitative level, the photoemission signal at the In order to compare the resulting density of states produced using various deposition conditions, we subtracted from the UPS data a background function given by the overall trend in the data from -3 eV to +2 eV (see the green dotted line in Figure 5 for the sample shown). An example of this difference spectrum is shown as the inset in Figure 5 for the case of one monolayer deposited at 1 Hz in 10 -6 Torr of oxygen. Finally, in order to better statistically average the data, we integrated the area under the difference spectra. Physically, this integrated density of states corresponds to the number of carriers per unit volume in the layer. These values for various samples are compared in Figure 6.
In Figure 6a, we see that the integrated density of states decreases monotonically with the thickness of the LaAlO 3 for otherwise identical samples. This is consistent with the states existing at the interface and the usual escape depth arguments, i.e., the fall off is roughly exponential with a characteristic mean free path of ~8 Å. Samples with thicknesses greater than 5 ML yielded no appreciable photoelectron signal, and for an order of magnitude greater thicknesses, insulating behavior and its associated charging effects prevented UPS study. Figure 6b shows the effect of varying the PLD laser repetition rate. The data suggest an optimal repetition rate of ~1 Hz for the number of interface states generated. Repetition rates over 5 Hz did not show any discernable UPS signal.
Finally we discuss some special cases. When exposing the 1 ML film deposited at 1 Hz to 10 -5 Torr oxygen for ten minutes at 150 °C the spectral weight at the Fermi energy is reduced, see Figure 5 and Figure 6a (red open triangle). Signal reduction is also observed when the pressure quench is not applied and thus the sample is cooled in 10 -6 Torr of oxygen (Figure 6b, open circle) or similarly when the laser energy per pulse is lowered from 1.2 to 0.8 J cm -2 (Figure 6b, solid triangle). Samples grown at higher oxygen pressure  show no discernable signal at the Fermi level.

Discussion
Summarizing the results in part two: as we argued in [ 7 ], the experimental results presented above strongly suggest that the origin of this high charge density is associated with oxygen vacancies in the SrTiO 3 , despite the fact that prior to deposition such vacancies are not present.
The existence of states at the Fermi level is shown explicitly by the UPS data, as is the fact that the number of states can be reduced by oxidation. The various dependencies of the results on deposition conditions are consistent with this view. The data demonstrate clearly that the number of states is sensitive to deposition conditions. A natural interpretation of these data is that competing processes are present. For example, deposited material at the surface may need some time to crystallize, which we estimate to be on the order of one tenth of a second [ 12 ], and for the lowest repetition rates re-oxidation occurs. Another is the sputtering of oxygen from the surface being balanced by growth of the LaAlO 3 over-layer and anion vacancies formed deeper in the SrTiO 3 [ 13 ] through oxygen diffusion. Further evidence for oxygen vacancies and a discussion on suppressing them was presented by ourselves in a recent paper [ 7 ] as concluded from NEXAS and vis-VUV-SE measurements and others [ 3 ] based on cathode-luminescence. Similarities in transport between the LaAlO 3 /SrTiO 3 interface and low energy Ar sputtering were found by Reagor and Butko [ 14 ] and suggest that particles generated by the PLD process can indeed cause the SrTiO 3 to become conducting.
The effect of oxygen vacancies is also clearly reflected in the transport measurements, where sample properties depend on deposition conditions. For samples made at low oxygen pressures we showed remarkably high sheet densities. The inferred mobilities (~10 4 cm 2 V -1 s -1 ) at 4 K are also high, suggesting that the charge donors of the interface carriers are well removed from the interface region itself, as in modulation doping in semiconductor hetero-structures, which was described elsewhere [ 7 ]. For samples made at higher oxygen pressure we find temperature hysteresis. Extra features were shown to appear at fixed temperatures, which tend to relax slowly In that same paper a critical thickness was observed for a sample to be conductive: samples with a LaAlO 3 layer thickness of fewer than 4 ML were found to be insulating. Interestingly, the hysteresis also appears to be maximal for samples with an over-layer thickness of 4 ML. What causes this critical thickness has not been determined yet. If one assumes an intrinsically doped 11 interface though (due to the polarized nature of the LaAlO 3 [ 1,2 ]) the potential that is created can put electrons in the conduction band. For this effect to occur it needs to be energetically favorable to put an electron in the conduction band and for that to happen the potential over the LaAlO 3 layer needs to be larger than its bandgap. We calculated that the jump in potential per unit cell is ~3.8 eV, from ΔV=8πe 2 /εa, which is quite large compared to the bandgap of 5.6 eV.
This would mean that after two unit cells there is enough of a potential to put electrons in the conduction band. For real films more layers are probably needed due to the presence of defects.
Also note that if some oxygen vacancies are present in the SrTiO 3 the diverging potential in the   The dotted line is a guide to the eye. These spectra were given an arbitrary offset for clarity.