Preliminary Investigations of Ferroelectric Tunneling Junctions November 4, 2014 Department Mannhart: Solid State Quantum Electronics Max Planck Institute for Solid State Research Brandon Stuart

Outline of Talk Introduction and motivation. What is a ferroelectric tunneling junction? How are the samples we use fabricated and analyzed? Current stages of the project. Future goals.

Introduction - What is a Ferroelectric Material? Has a spontaneous electric polarization. Electric polarization direction can be manipulated/switched through the application of an external electric field. First discovered in 1920 by Joseph Valasek in Rochelle salt [1]. Many types of ferroelectrics discovered since, such as BaTiO 3 and PbTiO 3. In our samples, the ferroelectric material is Ba 0.7 Sr 0.3 TiO 3 P P Energy Polarization U0U0 [1] Physical Review 15: 537 (1921).

Introduction - Motivation In , it was reported that barium titanate (BaTiO 3 ) films thinner than 100nm did not show ferroelectric switching. In 1971, films down to 40nm were created which exhibited ferroelectric properties, and down to 23nm by By 1999, films as thin as 40Å, 10 unit cells, showed ferroelectric switching properties. Recently, we have been able to grow Ba 0.7 Sr 0.3 TiO 3 films down to 4 unit cells which were switchable. C. Lichtensteiger et. al. “Physics of Ferroelectrics: A Modern Perspective (Topics in Applied Physics)”, 105, (2007)

E I What is a Ferroelectric Tunneling Junction? Standard Tunneling Junction 2 metals with a thin insulating barrier between them. Classically impossible for electron to pass through the barrier. Tunneling current mainly affected by: ◦Properties of the materials used ◦Thickness of the insulating barrier

What is a Ferroelectric Tunneling Junction? Ferroelectric Tunneling Junction Insulating barrier replaced with a ferroelectric thin film. Tunneling current mainly affected by: ◦Direction of the electric polarization of the ferroelectric ◦Properties of the materials used ◦Thickness of the ferroelectric material Low Current State High Current State I EE I

How are the Samples Fabricated? RHEED Electron Source Pulsed Laser Beam Target

How are the Samples Fabricated? Phosphorus Screen RHEED Electron Source Pulsed Laser Beam Target

How are the Samples Fabricated? Intensity Time Phosphorus Screen RHEED Electron Source Pulsed Laser Beam Target

How are the Samples Fabricated? Intensity Time Phosphorus Screen RHEED Electron Source Pulsed Laser Beam Target

How are the Samples Fabricated? SrRuO 3 8 u.c. BST Nb:SrTiO 3 (100) SrRuO 3 Ba 0.7 Sr 0.3 TiO 3

How are the Samples Fabricated? 8 u.c. BSTSrRuO 3 Nb:SrTiO 3 (100) SrRuO 3 Ba 0.7 Sr 0.3 TiO 3

How are the Samples Fabricated? 8 u.c. BSTSrRuO 3 Nb:SrTiO 3 (100) SrRuO 3 Ba 0.7 Sr 0.3 TiO 3

How do we Analyze our Samples? Piezoresponse force microscopy (PFM) Apply an AC voltage signal to the cantilever tip. Material deforms due to the inverse piezoelectric effect. Can determine amplitude and phase of these deformations. Conductive atomic force microscopy (cAFM) Apply a DC voltage signal to the conducting cantilever tip. Measure current from bottom electrode of sample to cantilever tip. Useful to show tunneling through the ferroelectric barrier. Source: Courtesy of Dr. Brian Smith

Testing the Ferroelectric Properties – Piezoresponse Loops A bias voltage is applied through the PFM tip in a pulsed triangular waveform. The amplitude and phase of the ionic displacements are measured at each pulse. Data plotted as amplitude/phase vs. applied voltage. 5 seconds

Testing the Ferroelectric Properties – Piezoresponse Loops Nb:SrTiO 3 (100) SrRuO 3 Ba 0.7 Sr 0.3 TiO 3 Pt coated cantilever

Testing the Ferroelectric Properties – Piezoresponse Loops Nb:SrTiO 3 (100) La 0.7 Ba 0.3 MnO 3 Ba 0.7 Sr 0.3 TiO 3 Pt coated cantilever Nb:SrTiO 3 (100) SrRuO 3 Ba 0.7 Sr 0.3 TiO 3 Pt coated cantilever

Testing the Ferroelectric Properties – Writing Domains

BST [8 u.c.] / SRO / Nb:STO(100)BST [8 u.c.] / LBMO / Nb:STO(100) BST [4 u.c.] / SRO / Nb:STO(100) Phase Amplitude Scan top to bottom: 0 – 17 minutes

Testing the Ferroelectric Properties – Writing Domains BST [8 u.c.] / SRO / Nb:STO(100)BST [8 u.c.] / LBMO / Nb:STO(100) BST [4 u.c.] / SRO / Nb:STO(100) Phase Amplitude Scan top to bottom: minutes

Testing the Ferroelectric Properties – Writing Domains BST [8 u.c.] / SRO / Nb:STO(100)BST [8 u.c.] / LBMO / Nb:STO(100) BST [4 u.c.] / SRO / Nb:STO(100) Phase Amplitude Scan top to bottom: 34 – 51 minutes -4

Moving Forward – The Future Goals Use cAFM to analyze the tunneling characteristics of the samples. Perform electro-resistance measurements of the ferroelectric tunneling junctions. Deposit top electrodes onto the sample to perform macroscopic ferroelectric measurements. Solid: P > 0 Dashed: P < 0

Thank you

Why BST? Z. G. Ban, S. P. Alpay, Journal of Applied Physics 91, No. 11, 9288 (2002)

How are the Samples Grown? – 4 u.c. BST 4 u.c. BSTSrRuO 3

DART PFM Dual amplitude resonance tracking Apply driving AC voltage near sample resonant frequency for best signal Resonant frequency shifts during scanning and measuring, need to track peak

DART PFM Dual amplitude resonance tracking Apply driving AC voltage near sample resonant frequency for best signal Resonant frequency shifts during scanning and measuring, need to track peak

DART PFM Dual amplitude resonance tracking Apply driving AC voltage near sample resonant frequency for best signal Resonant frequency shifts during scanning and measuring, need to track peak

Testing the Ferroelectric Properties – Writing Domains BST [8 u.c.] / SRO / Nb:STO(100)BST [8 u.c.] / LBMO / Nb:STO(100) BST [4 u.c.] / SRO / Nb:STO(100) Phase Amplitude

Testing the Ferroelectric Properties – Writing Domains BST [8 u.c.] / SRO / Nb:STO(100)BST [8 u.c.] / LBMO / Nb:STO(100) BST [4 u.c.] / SRO / Nb:STO(100) Phase Amplitude

Testing the Ferroelectric Properties – Writing Domains BST [8 u.c.] / SRO / Nb:STO(100)BST [8 u.c.] / LBMO / Nb:STO(100) BST [4 u.c.] / SRO / Nb:STO(100) Phase Amplitude