Study of Interfaces in Metal/ C60 Bilayer under X-ray Standing Wave Condition; Theoretical Simulations

Sonia Kaushik, Md. Shahid Jamal, Avinash G. Khanderao and Dileep Kumar a)

(UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore - 452001, India)

a) Corresponding author: dkumar@csr.res.in

Abstract. In the present work, we demonstrated the depth selectivity of FeCoB/C60 interface on basis of theoretical simulations by generating x-ray standing wave (XSW) using a waveguide structure Si/Pt/C60/FeCoB (interface)/FeCoB (bulk)/C60/Pt. Fe-k

fluorescence (XRF) spectra were simulated theoretically for increasing angle of incidence for the different values of electron density and thickness of interface FeCoB layer. It is found that the fluorescence peaks corresponding to the XSW antinodes crossing the interface layer is highly sensitive to the electron density and thickness of interface FeCoB layers. Present study demonstrates that by measuring electron density profiles through XRF measurement under XSW, interface resolved structural information can be achieved by moving antinode region and selecting appropriate incident angle.

Introduction

Fullerene (C60) based ferromagnetic (FM) multilayered (ML) nanostructures are the imperative candidates for organic spintronics (OS) devices because of the low spin-orbit coupling due to lower atomic weight, longer spin lifetime, good thermal stability and reasonably good mobility [1-2]. In such devices, organic semiconductor (OSC- such as C60, Alq3 etc, sandwiched between two ferromagnetic (FM) layers are used to mediate or control a spin-polarized electron transport but mechanical softness of OSC create challenges owing to penetration, diffusion and possible chemical reaction of metal atoms at the interfaces, which creates complicated magnetism and transport in these structures. Recently, combination of different FM layers such as FeCoB, Co have been grown on C60 [1, 2, 3] and unusual magnetic properties at the interfaces, significantly different from their constituent layers were obtained. In view of this, a clear picture of how effective organic spin valve (OSV) could be achieved for an efficient spin injection has not been experimentally realized till date [3-4]. The main difficulty is to get genuine and depth resolved interface structure using conventional lab-based characterization techniques such as x-ray reflectivity (XRR), secondary ion mass spectroscopy (SIMS) etc., either do not have enough depth resolution so as to resolve the interfaces or may not be probing the real interfaces. Furthermore, techniques like XRR is sensitive to all the individual parameters of the layers (thickness, roughness, and electron density etc). Hence to get genuine information from a particular interface is difficult. On the other hand, after creating XSW in the wave guide structure and placing the metal layer in the wave guide cavity, element sensitive fluorescence depends on mainly illuminated area by XSW antinodes and very sensitive to the particular layer [5,6].

In the present work, understanding of interface diffusion and deep penetration of FeCoB atom into C 60 is understood through theoretical simulation, where use of XRF under XSW will be demonstrated to be a suitable method for FM-C 60

systems otherwise it is difficult in the systems where deep penetration of the FM atom and spread over up to hundreds of angstroms in OSC layer. Present simulation has been used to demonstrate increased sensitivity from the interface by generating x-ray standing wave (XSW) using a waveguide structure (WGS). Interface selectivity could be used to follow the evolution of the interfaces with thermal annealing to get insight the interface structure and diffusion. Reported XRF simulations under XSW condition seems to be the promising in order to develop better understanding at FM/C 60 interface and to tune the properties for the desired functionality.

Formation of XSW in Wave Guide Structure

In case of wave guide structure, the XSW can be generated by total reflection of x-rays from an under-layer of high Z element (e.g., Au, Pt). Interference between the incident and the reflected wave-fronts form a standing wave above the surface of the reflecting layer, with separation between the successive antinodes given by D=1/q, where q is the scattering vector. Figure 1(a) gives a waveguide structure, where a low density layer (Fullerene- C60) is sandwiched between two dense layers (say Pt). Simulated electric field intensity (EFI) profile inside the cavity (along the depth), as shown in fig. 1(b-c), clearly confirm formation of XSW in wave guide structure. It may be noted that, when angle of incidence changes, the position of antinode shifts and formation of different standing wave modes (TE01, TE02, TE03 etc.,) takes place along the depth of the cavity. X-ray measurements such as XRF, under XSW condition would provide selective information from the region of antinodes, when it crosses the layer inside the cavity [5,7].

(a) (b) (c)

FIGURE 1. (a) Schematic of wave guide structure: where one low density layer is sandwiched between two high density layers, (b) Rearrangement of x-ray field intensity (EFI) inside the C60 cavity as function of the scattering vector q; simulated for Si substrate /Pt(30nm)/C60/(35nm)/Pt (2nm) structure (c) Variation of EFI along the depth at q1 = 0.0304 Å-1 (TE0 mode), q2 = 0.04 Å-1 (TE1 mode) and q3 = 0.0536 Å-1 (TE2 mode).

Interface Selectivity of Metal Layer in The C60 Waveguide Cavity:

In order to show the formation of XSW, even in the presence of metal layer inside the cavity, EFI profile is theoretically generated for the same wave guide structure in the presence of 50 Å thick FeCoB. Simulated EFI profiles are shown in fig. 2 (a-d), which were generated by placing FeCoB layer at the different depth in the cavity; (a)140 Å, (b) 120 Å, (c) 170 Å, (d) 220 Å. As the angle of incidence is changed, the formation of the different XSW modes takes place at the different fixed q values. In the present case, the position of antinodes (TE1 and TE2) crosses top and bottom side of the FeCoB layer at different q values. X-ray measurements such as fluorescence, XRD, XAFS etc. understanding wave condition would provide selective information from the region of antinodes. It can clearly be observed that on introducing a FeCoB layer, the field inside the waveguide cavity gets perturbed and hence, formation of XSW waves takes place in an asymmetric way as compared to Fig. 1(b).

It is important to note that for positions 140 Å (depth), TE1 and TE2 XSW antinodes coincide with the FeCoB layer at the top and bottom side of the interface at different q values (q2 = 0.0432 Å-1 and q3 = 0.0552 Å-1). It suggests that interface sensitivity for both interfaces can be achieved by performing other X-ray based measurements at q2 = 0.0432 Å-1

and q3 = 0.0552 Å-1.

FIGURE 2. Simulated x-ray intensity profile inside the cavity with the presence of a 50Å thick FM (FeCoB) layer at different positions of the wave guide.

In case of XRF measurement fluorescence intensity at any given value of q is obtained by integrating the concentration

profile ρ( z ) of FeCoB layer weighted with the x-ray intensity I (q , z ) at that depth [6,8]:

0 100 200 300 400 5000.02

0.03

0.04

0.05

0.06

0.07

Depth (Å)q(

Å-1

)

C60

Pt bu

ffer

Ptca

p

TE0

TE1

TE2

TE3

0 100 200 300 400

flour

esce

nce

inte

nsity

(a.u

.)

Depth (Å)

q=0.0304 q=0.04q=0.0536

TE0

TE1

TE2

F (q )∝∫ I (q , z ) ρ( z )dz , (1)

Therefore, obtained fluorescence peaks at q2 = 0.0432 Å-1 and q3 = 0.0552 Å-1 (corresponding to the TE1 and TE2 modes) from FeCoB layer in the C60 cavity, will be very much sensitive to any modification in the top and bottom side of the FeCoB interface. Thus, the q dependence of the fluorescence from a given FeCoB layer can be used to get highly sensitive interface selective information.

Fluorescence under XSW; Results and Discussion

FeK fluorescence patterns are calculated by taking an incident x-ray of energy 8.045 keV, where the sensitivity of the XRF peaks on interfaces of the FeCoB layer is demonstrated for (a) different combinations for densities of the bulk and interface FeCoB layer by keeping ρbulk = 7.45 g/cc and varying ρinterface (3.19 g/cc , 2.11 g/cc, 6.35 g/cc, 5.63 g/cc, 3.72 g/cc) .

For this purpose, FeCoB layer is divided in two sections; one with FeCoBbulk (dbulk = 40Å) and FeCoBinterface (dint = 120Å), and (b) increasing thickness of interface layer (d int = 20 Å, 30 Å, 40 Å, 60 Å and 80 Å) but keeping bulk FeCoB layer fixed (dbulk = 40 Å). From the contour plots of EFI 2D profile, as shown in Fig. 3a & 3b, the position of the FeCoB bulk and interface layer is marked in the presence of XSW modes inside the waveguide cavity.

(a) (b)

(c) (d)

FIGURE 3. (a) & (b) shows simulated contour plot for the Si/Pt/ C60/ FeCoB (interface)/ FeCoB(bulk)/ C60/Pt. The shaded region represents the bulk and interface FeCoB layers. (c) Simulated XRF for ρbulk = 7.45 g/cc and different values of ρinterface.(i) ρinterface = 3.19 g/cc (ii) ρinterface = 2.11 g/cc (iii) ρinterface = 6.35 g/cc (iv) ρinterface = 5.63 g/cc (v) ρinterface = 3.72 g/cc d) Simulated XRF for varying thickness of interface layer (i) tinterface= 80 Å (ii) tinterface = 60 Å (iii) tinterface = 40Å (iv) tinterface = 30 Å (v) tinterface = 20 Å

Figures 3 (c) & 3 (d) show FeK fluorescence patterns for the case of (a) and (b) as shown in Fig. 3(a) and (3b). In case of Fig. 3(c), XRF curves for different values of ρinterface (3.19 g/cc, 2.11 g/cc, 6.35 g/cc, 5.63 g/cc, 3.72 g/cc) for FeCoBinterface

clearly show drastic change in the relative peak intensities corresponding to TE1, TE2 and TE3 modes. The intensity of the second peak mainly depends upon the FeCoB bulk layer, while the shape and intensity of the third peak depends upon the concentration profile of FeCoB interface layer and sensitive to the subtle change in interface layer. In the case of Fig.3d, where FeK fluorescence from FeCoB layer is plotted for increasing thickness of interface layer. It is clear from the contour plot that interface layer is near to the bottom side of TE1 mode, therefore XRF second peak is sensitive to the interface.

Hence, increasing intensity of the second peak is mainly due to the decrease in thickness of the interface layer. In view of the fact, the fluorescence intensities peaks corresponding to FeCoBbulk and FeCoBinterface layer are very sensitive to the subtle change in the concentration, thickness, and roughness of FeCoBbulk and FeCoBinterface layer. Even a small variation in the depth and electron density of the FeCoB interface layer would result in a significant variation in the relative intensities of fluorescence peaks corresponding to TE1 and TE2 modes.

CONCLUSION

Fe- K fluorescence (XRF) patterns are calculated with an aim to demonstrate generation of node and antinodes inside the wave guide structure and to make them useful for the precise study of interfaces in metal-organic systems. It is made possible through XRF data simulated under XSW from FeCoB/C60 system by taking different electron densities and thicknesses of the interface FeCoB (diffused) layer. It is found that high resolution with depth selectivity of XSW based x-ray fluorescence (XRF) makes it possible to measure subtle change at the interface. In case of the metal organic interfaces (FeCoB-C60), the situation is complicated due to deep penetration of the FeCoB atoms into soft C60 layer, where a few angstroms thick FeCoB layer is spread over up to hundreds of angstroms in C60 layer. Therefore, along with depth selectivity of XRF measurement, enhanced contribution from interface part is achieved for the genuine information, which is being fulfilled through coinciding the antinode part of XSW with FeCoB layer at interface and by moving antinode of the XSW along the depth. Detail theoretical simulations have demonstrated usefulness of XSW method to study the interfaces of Metal /organic layered structures.

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