**4.2 The LSMO/STO interfaces**

Incorporating a few uc of combinatorial LSMOx (0 ≤ x ≤ 1) at the LSMO/STO interface to modulate the chemical bonding, the carrier density and the polar discontinuity could potentially induce STO lattice polar distortion, SBH modulation, as well as restoring interface ferromagnetism.

#### *4.2.1 Ferromagnetism at STO/LSMO interface*

Before producing the described ICPLD heterostructures, we first optimized the LSMO physical properties, composition and thickness uniformity. The magnetic properties of the film were then characterized versus temperature using a

**13**

tion over a 9x9 mm2

tively tight.

*Interface Combinatorial Pulsed Laser Deposition to Enhance Heterostructures Functional…*

commercial Kerr magnetometer equipped with a cryostat (NanoMoke II, Durham Magneto Optics). As the magnetism at the LSMO/STO interface is weakened, the Curie temperature will depend on LSMO thickness for very thin films. To avoid this regime, we worked with 30 nm thick LSMO films (~80uc). Several films were deposited on TiO2 terminated (100) STO substrates with high-pressure RHEED monitoring (Staib/TSST) at various fluence, temperature and oxygen pressure. The optimized deposition conditions leading to a TC = 341 K were PO2 = 0.2 mbar,

visible during all the deposition process implying a layer by layer growth. X-ray diffraction patterns (Θ-2Θ) showed only (00 l)pc reflections with thickness fringes attesting for the crystalline quality and the surface and interface smoothness. RSM confirmed epitaxial "cube on cube" growth of LSMO on STO. The homogeneity of

strate was verified for thinner films, in the range where TC is thickness dependent. A 20uc thick sample was deposited with vertical and horizontal scanning of the laser, staying in focus at the target position, and of the substrate respectively.

The Kerr magnetometer laser spot (diameter < 5 μm) was scanned on the sample surface at fixed temperatures to measure magnetic hysteresis curves in 311 points spread across the sample surface. This (x,y) scan was repeated every 2.5 K from room temperature to 350 K after thermal stabilization. Each hysteresis curve was processed in order to extract saturation and remnant magnetization (Msat and Mr respectively). M(T) curves can then be reconstructed for each point on the sample surface, allowing to assess for Tc in each location. Maps reporting FM and paramagnetic (PM) areas of the sample are reproduced in **Figure 6a** (top) for various temperatures, the measurement points being indicated with black dots. The distribution of Tc is reported as a FM area percentage in **Figure 6a** bottom. Over 91% of the surface transit from FM to PM states on a temperature range less than 5 K wide (325 K < T < 330 K) and 100% inside a 10 K range. As LSMO's TC is very thickness and composition sensitive, the tight TC distribution indicates a good composition and thickness uniformity. We confirmed this uniformity with a WDS characteriza-

area (25 x 25 = 625 points) of the same sample for La, Sr and

Mn (JEOL 8530F). The small film thickness conjugated to the presence of Sr in the substrate did not allow to compute the composition with cationic ratios of the film. However, the WDS sensitivity is high enough to provide maps of relative variations for each element (see **Figure 6b**). The Sr map, with signal originating mostly from STO substrate, illustrates the electron beam stability (σSr ~ 1%) which is crucial for point to point comparison. Note that the drop in the corner, corresponding to silver paste contact to evacuate the charges, was excluded from the statistical analysis. On La and Mn maps a similar slight slope is visible with a corresponding standard deviation of 4.6% and 4.9% respectively. WDS signal is strongly correlated to the thickness, therefore we can conclude that the thickness distribution is rela-

The relative interface contribution to the overall magnetic signal increases as the LSMO thickness decreases. It is however difficult to predict the optimal LSMO thickness leading to an improved interface contribution detection as the overall magnetic signal also decreases with thickness. A powerful aspect of CPLD is the possibility to deposit wedge-shaped layers with continuous thickness variation using shadow-masking (see **Figure 7a**). Before inserting the ICPLD layer, we checked the thickness control on two LSMO wedges, spanning from 8 uc to 76 uc, by measuring TC versus (x,y) and temperature. The obtained TC are represented **Figure 7b** with standard deviation represented as error bars. Tc noticeably decreases below 30 uc with an acceleration below 20 uc. In the inset of **Figure 7b** is represented a TC map of wedge#2, with the measured points represented as

the films in term of composition and magnetic properties over a 1 cm2

. RHEED oscillations were clearly

STO sub-

*DOI: http://dx.doi.org/10.5772/intechopen.94415*

Tsub = 850°C, f = 5 Hz and a fluence of 0.83 J/cm<sup>2</sup>

#### *Interface Combinatorial Pulsed Laser Deposition to Enhance Heterostructures Functional… DOI: http://dx.doi.org/10.5772/intechopen.94415*

commercial Kerr magnetometer equipped with a cryostat (NanoMoke II, Durham Magneto Optics). As the magnetism at the LSMO/STO interface is weakened, the Curie temperature will depend on LSMO thickness for very thin films. To avoid this regime, we worked with 30 nm thick LSMO films (~80uc). Several films were deposited on TiO2 terminated (100) STO substrates with high-pressure RHEED monitoring (Staib/TSST) at various fluence, temperature and oxygen pressure. The optimized deposition conditions leading to a TC = 341 K were PO2 = 0.2 mbar, Tsub = 850°C, f = 5 Hz and a fluence of 0.83 J/cm<sup>2</sup> . RHEED oscillations were clearly visible during all the deposition process implying a layer by layer growth. X-ray diffraction patterns (Θ-2Θ) showed only (00 l)pc reflections with thickness fringes attesting for the crystalline quality and the surface and interface smoothness. RSM confirmed epitaxial "cube on cube" growth of LSMO on STO. The homogeneity of the films in term of composition and magnetic properties over a 1 cm2 STO substrate was verified for thinner films, in the range where TC is thickness dependent. A 20uc thick sample was deposited with vertical and horizontal scanning of the laser, staying in focus at the target position, and of the substrate respectively.

The Kerr magnetometer laser spot (diameter < 5 μm) was scanned on the sample surface at fixed temperatures to measure magnetic hysteresis curves in 311 points spread across the sample surface. This (x,y) scan was repeated every 2.5 K from room temperature to 350 K after thermal stabilization. Each hysteresis curve was processed in order to extract saturation and remnant magnetization (Msat and Mr respectively). M(T) curves can then be reconstructed for each point on the sample surface, allowing to assess for Tc in each location. Maps reporting FM and paramagnetic (PM) areas of the sample are reproduced in **Figure 6a** (top) for various temperatures, the measurement points being indicated with black dots. The distribution of Tc is reported as a FM area percentage in **Figure 6a** bottom. Over 91% of the surface transit from FM to PM states on a temperature range less than 5 K wide (325 K < T < 330 K) and 100% inside a 10 K range. As LSMO's TC is very thickness and composition sensitive, the tight TC distribution indicates a good composition and thickness uniformity. We confirmed this uniformity with a WDS characterization over a 9x9 mm2 area (25 x 25 = 625 points) of the same sample for La, Sr and Mn (JEOL 8530F). The small film thickness conjugated to the presence of Sr in the substrate did not allow to compute the composition with cationic ratios of the film. However, the WDS sensitivity is high enough to provide maps of relative variations for each element (see **Figure 6b**). The Sr map, with signal originating mostly from STO substrate, illustrates the electron beam stability (σSr ~ 1%) which is crucial for point to point comparison. Note that the drop in the corner, corresponding to silver paste contact to evacuate the charges, was excluded from the statistical analysis. On La and Mn maps a similar slight slope is visible with a corresponding standard deviation of 4.6% and 4.9% respectively. WDS signal is strongly correlated to the thickness, therefore we can conclude that the thickness distribution is relatively tight.

The relative interface contribution to the overall magnetic signal increases as the LSMO thickness decreases. It is however difficult to predict the optimal LSMO thickness leading to an improved interface contribution detection as the overall magnetic signal also decreases with thickness. A powerful aspect of CPLD is the possibility to deposit wedge-shaped layers with continuous thickness variation using shadow-masking (see **Figure 7a**). Before inserting the ICPLD layer, we checked the thickness control on two LSMO wedges, spanning from 8 uc to 76 uc, by measuring TC versus (x,y) and temperature. The obtained TC are represented **Figure 7b** with standard deviation represented as error bars. Tc noticeably decreases below 30 uc with an acceleration below 20 uc. In the inset of **Figure 7b** is represented a TC map of wedge#2, with the measured points represented as

*Practical Applications of Laser Ablation*

Interface engineering can be used to tailor band alignment and interface polarizability. The insertion of a thin layer with different atomic element(s) at the interface allows to manipulate the chemical bonding and promotes atomic rearrangement. Let us consider for instance the anti-displacement of anions and cations predicted at Ba2+O2−/M and Sr2+O2−/M interfaces and quantified by a rumpling parameter R [30]. R depends on the chemical bonding and is responsible for an interface dipole, which in turn modulate the SBH. Interestingly, the insertion of e.g. a single Al atomic plane at the BaO/M interface strongly affects R and SBH. Indeed, for M = Pd the SBH goes from 1.4 eV to 2.6 eV [30]. Significant rumpling has been experimentally shown for SrTiO3 (STO) in contact with La2/3Sr1/3MnO3 (LSMO), a metallic perovskite electrode, inducing a polarization in the non-ferroelectric STO [31]. The continuity of the perovskite structure through the LSMO/STO interface and its ionic character offer new ways to control electronic properties. In La1-xSrxMnO3 (LSMOx), the B-site cation ratio Mn3+/Mn4+ is determined by the A-site ratio La3+/Sr2+. Along [100], successive AO and BO2 planes are polar for LSMOx and charge neutral for BST. Interfacing LSMOx with BST leads to tunable interfacial polar discontinuity which can induce lattice polar distortion and result in SBH modulation [32–34].

LSMO is a ferromagnetic (FM) half-metal, i.e. having a 100% spin-polarization at the Fermi level. For the latter reason it has been intensively studied as a spin-polarized electrode in LSMO/STO/LSMO magnetic tunnel junction (MTJ). MTJs are used e.g. as memory bits in magnetic MRAMs. The tunnel resistance depends on electrode spinpolarization and on the relative orientation of the electrode magnetic moments, with high resistance RAP (resp. low resistance RP) for antiparallel (resp. parallel) states. A 100% spin polarized electrode leads to a theoretical infinite RAP which is ideal for the cited application. In LSMO/STO/LSMO, a record tunnel magneto-resistance (TMR = (RAP-RP)/RP) of about 2000% was reported, but unfortunately for temperature far below the Curie temperature TC [35]. The vast majority of the electrons tunnel from the interfaces, their spin-polarization being affected by the nature of the chemical bonding. FM correlations at manganite interfaces are known to be weaker than in bulk, causing a magnetic "dead layer" which probably explains the diminution of TMR close to TC [36–38]. Attempts have been reported at creating a doping profile at the interfaces by inserting a 2 uc thick LaMnO3 layer [39, 40] or a single uc thick La0.33Sr0.67MnO3 [41] layer to overcome this problem with some improvement of interface magnetism but still not a full recovery of bulk properties. As for SBH and interface polarizability, multiple factors might participate to interface magnetism weakening, like charge discontinuity driven intermixing, octahedral tilt induced in the first LSMO layers by octahedral connectivity at the interface, substrate strain and so on. A combinatorial heuristic approach to the definition of interface composition is a powerful tool to help understanding all these factors interplay and to enhance the

Incorporating a few uc of combinatorial LSMOx (0 ≤ x ≤ 1) at the LSMO/STO interface to modulate the chemical bonding, the carrier density and the polar discontinuity could potentially induce STO lattice polar distortion, SBH modulation,

Before producing the described ICPLD heterostructures, we first optimized the LSMO physical properties, composition and thickness uniformity. The magnetic properties of the film were then characterized versus temperature using a

interface magnetism, SBH or interface polarization.

as well as restoring interface ferromagnetism.

*4.2.1 Ferromagnetism at STO/LSMO interface*

**4.2 The LSMO/STO interfaces**

**12**

#### **Figure 6.**

*(a) Magnetic state maps of LSMO 20 uc film at various temperature (top) and TC distribution at the sample surface (bottom). (b) WDS signal for La, Sr and Mn over 9x9 mm2 of the same sample.*

**Figure 7.** *(a) Schematic of LSMO wedge#2. (b) TC versus thickness and (inset) TC map for wedge #2.*

black dots. Constant nominal thickness levels are vertical, with thickness variation along x. A 10 K color increment is used and one can see that the lines separating the adjacent areas are almost vertical, attesting the good control of thickness variation in the wedge.

To synthesize the ICPLD LSMOx layer we used LaMnO3 (LMO) and SrMnO3 (SMO) targets with the deposition parameters identified for LSMO, including laser and substrate stage scans. Deposition rate was evaluated using RHEED oscillations. A 3 uc thick LSMOx layer (0 ≤ x ≤ 1) was deposited onto TiO2-terminated STO substrate, followed by a LSMO wedge with thickness variation direction perpendicular to LSMOx composition gradient. A schematic representation of this sample is represented **Figure 8a**.

M(H) cycles were acquired versus position (512 sites) and temperature (120 temperatures with 90 K < T < 340 K) automatically during a few days. Then Msat and Mr were extracted for each loop, M(T) curves reconstructed and TC estimated for each (composition x, thickness t) doublet. **Figure 8b** presents the TC curves plotted versus tLSMO for various Sr content x. One can see that the variation of Tc versus tLSMO depends on x, and in particular for tLSMO > 7 uc, the less Sr the more rapid is the TC decrease. Going from LMO to SMO at tLSMO = 10 uc, TC is increased by 60 K (blue arrows **Figure 8b**). Furthermore, to reach a given Curie temperature

**15**

described above.

**Figure 8.**

*Interface Combinatorial Pulsed Laser Deposition to Enhance Heterostructures Functional…*

of 240 K, one needs 8 uc of LSMO on top of SMO and more than 11 uc of LSMO on top of LMO (red arrows in **Figure 8b**). One can compare these results to the one obtained for x = 0.29 (pink curve in **Figure 8**) where the heterostructure is similar to a simple LSMO/STO interface. Inserting a 3 uc SMO layer at the LSMO/STO interface proves to be beneficial in terms of Tc for tLSMO > 7 uc. However we observe a cross-over for tLSMO ≤ 7 uc. The Tc decrease with tLSMO accelerates for Sr rich compositions, and no magnetism could be detected at tLSMO = 5 uc for 0.29 ≤ x ≤ 1. On the contrary, the lower the Sr content the higher the TC for 0 ≤ x ≤ 0.21 at tLSMO = 5 uc. This reinforcement of FM for LMO coincides with an important increase of the coercive field to values higher than usually observed for LSMO (Hc > 300 Oe at T = 100 K). This is compatible with a second FM phase, harder than LSMO and in contact with it. LMO is antiferromagnetic (AFM) in bulk form. However, several studies reported FM LMO films on STO substrate down to 6 uc (e.g. [42]). In this article, the transition from AFM to FM has been attributed to an electronic reconstruction at the interface originating from the polar nature of the LMO. In our case the LMO layer is topped by LSMO, and it is quite possible that by proximity effect

*(a) Schematic representation of the ICPLD LSMOx / wedge LSMO sample. (b) Curie temperature curves* 

We now turn to interface issues arising in tunable capacitors with thinned FE film i.e. the increased influence of dead ferroelectric layer on tunability and the increased leakage current. As discussed previously the insertion of a LSMOx ICPLD layer at LSMO/BST interface may increase interface polarizability and modulate SBH. In order to easily disentangle spontaneous and chemically induced polarizations we choose to work with a non-polar composition of BST i.e. STO. We deposited onto TiO2-terminated STO substrate 38 uc of LSMO followed by 3 uc of LSMOx (0 ≤ x ≤ 1) and in the direction perpendicular to the gradient a STO wedge (3-15 uc) keeping an access to both LSMO and LSMOx with the deposition parameters

A schematic of the sample structure is represented **Figure 9a**. The sample was transferred into an ultra-high vacuum atomic force microscope chamber (UHV-AFM Omicron) without breaking the vacuum. The AFM image presented in **Figure 9b** was taken about the red dot in **Figure 9a** with a total thickness of 56 uc. Terraces separated by steps of about 4 Å, i.e. one perovskite cell parameter, are clearly visible (see profile in **Figure 9c**) attesting of the layer by layer growth up to 56 uc. There exists however some 2 Å height features on the terraces indicating the

probable existence of two terminations at the surface (SrO and TiO2).

*DOI: http://dx.doi.org/10.5772/intechopen.94415*

*versus tLSMO for various Sr content x in LSMOx layer.*

and/or stress LMO becomes FM at 3 uc thick.

*4.2.2 Band alignment at LSMO/STO interface*

*Interface Combinatorial Pulsed Laser Deposition to Enhance Heterostructures Functional… DOI: http://dx.doi.org/10.5772/intechopen.94415*

**Figure 8.**

*Practical Applications of Laser Ablation*

black dots. Constant nominal thickness levels are vertical, with thickness variation along x. A 10 K color increment is used and one can see that the lines separating the adjacent areas are almost vertical, attesting the good control of thickness variation

*(a) Schematic of LSMO wedge#2. (b) TC versus thickness and (inset) TC map for wedge #2.*

*(a) Magnetic state maps of LSMO 20 uc film at various temperature (top) and TC distribution at the sample* 

 *of the same sample.*

*surface (bottom). (b) WDS signal for La, Sr and Mn over 9x9 mm2*

To synthesize the ICPLD LSMOx layer we used LaMnO3 (LMO) and SrMnO3 (SMO) targets with the deposition parameters identified for LSMO, including laser and substrate stage scans. Deposition rate was evaluated using RHEED oscillations. A 3 uc thick LSMOx layer (0 ≤ x ≤ 1) was deposited onto TiO2-terminated STO substrate, followed by a LSMO wedge with thickness variation direction perpendicular to LSMOx composition gradient. A schematic representation of this sample

M(H) cycles were acquired versus position (512 sites) and temperature (120 temperatures with 90 K < T < 340 K) automatically during a few days. Then Msat and Mr were extracted for each loop, M(T) curves reconstructed and TC estimated for each (composition x, thickness t) doublet. **Figure 8b** presents the TC curves plotted versus tLSMO for various Sr content x. One can see that the variation of Tc versus tLSMO depends on x, and in particular for tLSMO > 7 uc, the less Sr the more rapid is the TC decrease. Going from LMO to SMO at tLSMO = 10 uc, TC is increased by 60 K (blue arrows **Figure 8b**). Furthermore, to reach a given Curie temperature

**14**

in the wedge.

**Figure 7.**

**Figure 6.**

is represented **Figure 8a**.

*(a) Schematic representation of the ICPLD LSMOx / wedge LSMO sample. (b) Curie temperature curves versus tLSMO for various Sr content x in LSMOx layer.*

of 240 K, one needs 8 uc of LSMO on top of SMO and more than 11 uc of LSMO on top of LMO (red arrows in **Figure 8b**). One can compare these results to the one obtained for x = 0.29 (pink curve in **Figure 8**) where the heterostructure is similar to a simple LSMO/STO interface. Inserting a 3 uc SMO layer at the LSMO/STO interface proves to be beneficial in terms of Tc for tLSMO > 7 uc. However we observe a cross-over for tLSMO ≤ 7 uc. The Tc decrease with tLSMO accelerates for Sr rich compositions, and no magnetism could be detected at tLSMO = 5 uc for 0.29 ≤ x ≤ 1. On the contrary, the lower the Sr content the higher the TC for 0 ≤ x ≤ 0.21 at tLSMO = 5 uc. This reinforcement of FM for LMO coincides with an important increase of the coercive field to values higher than usually observed for LSMO (Hc > 300 Oe at T = 100 K). This is compatible with a second FM phase, harder than LSMO and in contact with it. LMO is antiferromagnetic (AFM) in bulk form. However, several studies reported FM LMO films on STO substrate down to 6 uc (e.g. [42]). In this article, the transition from AFM to FM has been attributed to an electronic reconstruction at the interface originating from the polar nature of the LMO. In our case the LMO layer is topped by LSMO, and it is quite possible that by proximity effect and/or stress LMO becomes FM at 3 uc thick.

#### *4.2.2 Band alignment at LSMO/STO interface*

We now turn to interface issues arising in tunable capacitors with thinned FE film i.e. the increased influence of dead ferroelectric layer on tunability and the increased leakage current. As discussed previously the insertion of a LSMOx ICPLD layer at LSMO/BST interface may increase interface polarizability and modulate SBH. In order to easily disentangle spontaneous and chemically induced polarizations we choose to work with a non-polar composition of BST i.e. STO. We deposited onto TiO2-terminated STO substrate 38 uc of LSMO followed by 3 uc of LSMOx (0 ≤ x ≤ 1) and in the direction perpendicular to the gradient a STO wedge (3-15 uc) keeping an access to both LSMO and LSMOx with the deposition parameters described above.

A schematic of the sample structure is represented **Figure 9a**. The sample was transferred into an ultra-high vacuum atomic force microscope chamber (UHV-AFM Omicron) without breaking the vacuum. The AFM image presented in **Figure 9b** was taken about the red dot in **Figure 9a** with a total thickness of 56 uc. Terraces separated by steps of about 4 Å, i.e. one perovskite cell parameter, are clearly visible (see profile in **Figure 9c**) attesting of the layer by layer growth up to 56 uc. There exists however some 2 Å height features on the terraces indicating the probable existence of two terminations at the surface (SrO and TiO2).

#### *Practical Applications of Laser Ablation*

The sample was then air-exposed and inserted into a UV photoelectron spectroscopy chamber (UPS ESCALAB 250Xi Thermo Fisher) to evaluate the work function WF as a function of position. Although the surface contamination due to air exposure prevented to extract absolute WF values, the relative variations of WF with Sr content and STO thickness could be determined assuming a "uniform" surface contamination. UPS spectra were taken at various x content for STO thickness ranging from 0 to 9 uc. A zoom around the emission threshold of the He II UPS spectra (He II energy 40.8 V, bias 4 V) is shown in **Figure 10a** for LSMOx (tSTO = 0 uc). From the thresholds one can estimated the WF reported in **Figure 10b**. A clear continuous decrease of the work function is observed as the Sr content increases. This trend is

#### **Figure 9.**

*(a) Schematic of the STO (sub.)/ LSMO / LSMOx / STO heterostructure. (b) UHV-AFM image. (c) Profile from the white arrow in b).*

#### **Figure 10.**

*(a) HeII UPS threshold for various x of air-exposed LSMOX Ubias = 4 V. (b) Corresponding extracted WF versus Sr content x.*

#### **Figure 11.**

*(a) HeII UPS threshold for various tSTO at x = 0.2 Ubias = 4 V. (b) Corresponding extracted WF versus tSTO for various Sr content x.*

**17**

**5. Conclusion**

properties enhancement.

**Acknowledgements**

*Interface Combinatorial Pulsed Laser Deposition to Enhance Heterostructures Functional…*

function of x reported in the literature [43] and seen by us (not shown).

opposite to the downward Fermi level shift inferred from core-level XPS shift as a

charge per unit cell going from −1 to 0. The more negatively charged a surface is, the harder for an electron to escape from the surface, the higher the WF [44]. The electrical nature of the contact between a metal and a semiconductor directly depends on the relative values of the metal WF and semiconductor electronic affinity Ea for ionic semiconductor [45]. For Ea > WF an ohmic contact forms, while for Ea < WF a Schottky barrier is created. STO is generally considered an n-type ionic semiconductor with a Fermi level very close to the conduction band (i.e. Ea ~ WF). As the LSMOx WF varies the LSMOx/STO contact nature might be affected. UPS spectra were acquired for various LSMOx Sr content every 200 μm along the STO wedge. A zoom of the corresponding UPS emission thresholds obtained for x = 0.2 and 0 ≤ tSTO ≤ 9 uc is shown in **Figure 11a**. The threshold position varies rapidly with tSTO for thin STO layers then stabilizes. The WF estimated from the UPS thresholds for various (x, tSTO) doublet are reported in **Figure 11b**. One can see the curves folding together toward a WF value of about 3.58 eV (rela-

Interestingly this value is inside the range of WF spanned within LSMOx.(see pink part of **Figure 11b**). Looking at the evolution of WF vs. tSTO for thicknesses up to 3 uc, there is a clear transition from a downward to an upward bending as x increases. This reflects the band bending that occurs at the LSMOx / STO interface and implies that the contact is modified from Ohmic type to Schottky type.

This result is of importance regarding the optimization of the SBH in BST FE tunable capacitor in particular, but more generally for any metal/semiconductor contacts.

Authors would like to thanks J-L Longuet from CEA Le Ripault (France) for the WDS characterizations presented here and Xavier Wallart from IEMN, UMR CNRS

In this chapter we reviewed the qualities and limitations of PLD for the synthesis of oxides in general and for its use in combinatorial PLD synthesis (CPLD) in particular. We listed some counter-actions to mitigate the PLD limitations together with the mandatory steps to take before attempting reliable CPLD synthesis, i.e. demonstrating the control of both thickness and composition over the whole sample surface. We then detailed a statistical characterization approach to reliably interpret results from CPLD libraries of compounds. An example of this approach is presented, regarding the exploration of lead-free Ga-doped BiFeO3 solid solution for MPB-related piezoelectric properties enhancement. Finally we described a new interface CPLD development (ICPLD) for the exploration of functional interface libraries. This combinatorial interface synthesis approach, with continuous lateral chemical modulation of a few atomic layers, is unique to the best of our knowledge. The effectiveness of ICPLD regarding the control of interface magnetism for magnetic tunnel junctions and energy band and Schottky barrier height tuning in ferroelectric tunable capacitors was demonstrated. This shows that ICPLD is a powerful tool to accelerate heterostructures functional

The counter-intuitive decrease of WF while EF decreases too is due to the LSMOx induced charge discontinuity variation at the surface. Going from LMO to SMO, the

+ − to 4 2 *Mn O*<sup>2</sup>

+ − , i.e. with a surface

*DOI: http://dx.doi.org/10.5772/intechopen.94415*

LSMOx terminal plane changes from 3 2 *Mn O*<sup>2</sup>

tive) corresponding to the intrinsic STO work function.

#### *Interface Combinatorial Pulsed Laser Deposition to Enhance Heterostructures Functional… DOI: http://dx.doi.org/10.5772/intechopen.94415*

opposite to the downward Fermi level shift inferred from core-level XPS shift as a function of x reported in the literature [43] and seen by us (not shown).

The counter-intuitive decrease of WF while EF decreases too is due to the LSMOx induced charge discontinuity variation at the surface. Going from LMO to SMO, the LSMOx terminal plane changes from 3 2 *Mn O*<sup>2</sup> + − to 4 2 *Mn O*<sup>2</sup> + − , i.e. with a surface charge per unit cell going from −1 to 0. The more negatively charged a surface is, the harder for an electron to escape from the surface, the higher the WF [44].

The electrical nature of the contact between a metal and a semiconductor directly depends on the relative values of the metal WF and semiconductor electronic affinity Ea for ionic semiconductor [45]. For Ea > WF an ohmic contact forms, while for Ea < WF a Schottky barrier is created. STO is generally considered an n-type ionic semiconductor with a Fermi level very close to the conduction band (i.e. Ea ~ WF). As the LSMOx WF varies the LSMOx/STO contact nature might be affected. UPS spectra were acquired for various LSMOx Sr content every 200 μm along the STO wedge. A zoom of the corresponding UPS emission thresholds obtained for x = 0.2 and 0 ≤ tSTO ≤ 9 uc is shown in **Figure 11a**. The threshold position varies rapidly with tSTO for thin STO layers then stabilizes. The WF estimated from the UPS thresholds for various (x, tSTO) doublet are reported in **Figure 11b**. One can see the curves folding together toward a WF value of about 3.58 eV (relative) corresponding to the intrinsic STO work function.

Interestingly this value is inside the range of WF spanned within LSMOx.(see pink part of **Figure 11b**). Looking at the evolution of WF vs. tSTO for thicknesses up to 3 uc, there is a clear transition from a downward to an upward bending as x increases. This reflects the band bending that occurs at the LSMOx / STO interface and implies that the contact is modified from Ohmic type to Schottky type.

This result is of importance regarding the optimization of the SBH in BST FE tunable capacitor in particular, but more generally for any metal/semiconductor contacts.
