Properties of the alloy/SrTiO 3 interface

Sample preparation Two independent series of LASTO:xfilms were prepared by PLD using different sets of growth parameters. The first set of films, produced at the Paul Scherrer Institut, was grown on (001) STO substrates using two different PLD sintered targets, withx= 0.50 and 0.75. Neither target (> 85 % dense) was conducting.

Growth conditions using 266 nm Nd:YAG laser radiation were: pulse energy = 16 mJ (≈2 J cm⁻²), 10 Hz;T= 750^◦C,pO2 = 2.5×10⁻⁸mbar; sample cooled after growth at 25^◦C min⁻¹ and post-annealed for one hour in 1 atm. O2at 550^◦C. The second set of films using a target withx= 0.5was grown in Geneva using standard growth conditions, as described in Chapter1: KrF laser (248 nm) with a pulse energy of 50 mJ (≈0.6 J cm⁻²), 1 Hz;T = 800^◦C,pO2 = 1×10⁻⁴mbar; sample cooled after growth to 550^◦C in 200 mbar O₂and maintained at this temperature and pressure for one hour before being cooled to room temperature in the same atmosphere. The stoichiometry of the films from both sets were shown by Rutherford backscattering (RBS) to be equal to the nominal PLD target compositions of 0.5 and 0.75 within the experimental accuracy

Figure 2.8 – Structural and transport characterisations:(a) X-ray diffraction patterns recorded as a function of reciprocal lattice units (r.l.u.) around the (002) peaks of the SrTiO3substrate (sharp feature) and the LASTO:0.5 films; (b) Sheet carrier densitynsas a function of temperature for different film thickness for pure LaAlO3and LASTO:0.5.

of 1.5 %.

Structural characterisations The crystallographic quality of the films is shown in Fig.2.8(a). Growth was layer-by-layer, as also seen from clear RHEED oscillations and the atomic flatness observed in AFM. All films were perfectly strained. The out-of-plane lattice constant of the LASTO:0.5 films for conducting interfaces (no electrostriction effect) was determined to be 3.83±0.01 Å, in agreement withab-initio calculations for strained growth on SrTiO3(the pseudo cubic lattice parameter of the bulk PLD target was 3.85±0.01 Å). No evidence could be found either in plane or out-of-plane of superstructure peaks showing that there was not any obvious spontaneous ordering of the cations, at least for superstructures with a periodicity, an even multiple of that of the normal unit cell.

Transport properties Neither of the mixed-composition film stoichiometries inves-tigated produces layers whose conductance increases with thickness, as one might otherwise expect for intermixed materials which were intrinsically electrically con-ducting. In addition, none of the films are conducting at the top surface, but instead require careful bonding at the interface to exhibit conductivity. These metallic inter-faces were characterised for their transport properties using the van der Pauw method.

All samples remained metallic down to the lowest measured temperature of 1.5 K.

The sheet carrier densitiesnsare in the range 3 to 15×10¹³cm⁻², though with no obvious dependence on composition or thickness of the layers [Fig.2.8(b)]. According to the "polar-catastrophe" model, we should expect LASTO:x samples to exhibit a lower carrier density, as the screening charge scales as x·e/2S, whereby S is the unit-cell surface area. However, as already observed for LaAlO₃/SrTiO₃interfaces, the

Figure 2.9 – The conductivity threshold thickness for different film compositions:

Room-temperature conductance of LASTO:xfilms for (a)x= 0.50, (b)x= 0.75, and (c)x

= 1. The dashed vertical lines forx= 1.0 and 0.75 indicate the experimentally determined threshold thicknessestc, which forx= 0.5, is represented by a band for the more gradual transition. The blue triangles are from samples belonging to the first set, and red points denote samples from second set.

estimation of the carrier density from the Hall effect yields values up to one order of magnitude smaller than those predicted from theory, possibly suggesting a large amount of trapped interface charges [97]. This can explain the known variation of data from one sample to another, irrespective of composition or layer thickness, and precludes the possibility of testing the "polar-catastrophe" scenario simply from inspection of ns. We will further test "polar-catastrophe" scenario by investigating the confinement at the interface (in Section2.5). The Hall mobilityµ(∼500 cm² V⁻¹s⁻¹ at 1.5 K) and sheet resistance (∼5 - 20 kΩ/at room temperature) of the LASTO:xinterfaces measured as a function of temperature exhibit essentially the same dependence as those for the interfaces with pure LaAlO₃films [136].

0 50 100 150 200 250 300

Figure 2.10 – Dielectric constants as a function of temperature: for a 20 u.c.

LASTO:0.5/SrTiO3sample (violet open circles) with gold electrodes and for a 10 u.c.

LaAlO3/SrTiO3sample (blue open square) with platinum electrodes

Dielectric constant To probe experimentally the dielectric constant, capacitors were fabricated with different thickness of pure LaAlO₃ and of LASTO:0.5 films, using different metals as the top electrodes. A serious complication in measurements of such ultra-thin films is the significant contribution of the electrode-oxide interface on the capacitance [137], which means the results can only be viewed semi-quantitatively. We observe that the dielectric constants of the LASTO:0.5 and LaAlO3at room temperature display values in the range of 5 to 30 depending on the metal electrodes. This is in good agreement with previous reports on ceramic solid solutions, where no large enhancement of the relative permittivity was observed for the solid solution up to 80 % [138]. Figure2.10shows a temperature dependent dielectric constant measured on a 20 u.c. LASTO:0.5/SrTiO3sample using gold top electrodes. As can be seen in the figure, cooling the sample to 4 K produces a small change in the dielectric constant, in sharp contrast with the low-temperature divergence of the dielectric constant of SrTiO3. The comparison with the measured dielectric constant as a function of temperature for a LaAlO₃/SrTiO₃sample confirms that LASTO:xbehaves like LaAlO₃rather than SrTiO3. The relatively small dielectric constants measured are probably due to the contribution from metal-oxide interface [137] (especially the gold-LASTO:0.5 film interface). These experimental results demonstrate that there is no large enhancement of the dielectric constant in LASTO:0.5 with respect to pure LaAlO3, as predicted by thefirst principlescalculations.

Critical thickness Figure2.9is the central result of this work. The conductance of the interface as a function of the LASTO:xfilm thickness is shown forx= 0.5 [Fig.2.9

(a)], x = 0.75 [Fig.2.9 (b)], andx = 1 [pure LAO, Fig. 2.9(c)]. The conductivity is given in sheet conductance (left axis) and/or conductance (right axis). As seen in Section2.3.1, the step in conductance forx= 1.0 is observed between 3 and 4 u.c..

Forx= 0.75 and 0.50, the data unambiguously demonstrate that the critical thickness increases with SrTiO₃ content in the solid solution, witht^LASTO:0.75_c close to 5 u.c., andt^LASTO:0.5_c between 6 and 7 u.c.