Barium Strontium Titanate

(Ba,Sr)TiO3 70/30 ferroelectric thin films 199030202 Experimental Project

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Temperature dependent permittivity and phase transitions in Ba0.7Sr0.3TiO3 ferroelectric thin film oxides Abstract Phase transitions in barium strontium titanate (BST 70/30) thin films were inferred using dielectric [ε(T)∼110-180] and X-ray diffraction (XRD) thermal expansion measurements. A sandwiched Au/BST/Pt/BST/Au capacitor pair was electrically characterised from 30 to 300K under vacuum (<30 mTorr) over 2.7×102 hours: recording zero-bias capacitance [C(T)∼2-5nF] and loss tangent [D(T)∼2-5%] at 100mV (ƒ=105 Hz) by impedance analyzer, and remanent polarisation (P-E) [Pr(T)] (ƒ=1 kHz, E <103 kV cm-1) by computer-controlled hysteresis analyzer1. The wafer was produced by a propionate-based chemical solution deposition (CSD) with ex situ gold sputtered (≈0.95mm2) top electrodes on a polycrystalline single-phase Ba0.7Sr0.3TiO3 175nm perovskite layer, with a platinum [111] bottom electrode on a SiO2/Si [111] substrate. The film demonstrated low leakage (I<10-8A at 300kV cm-1, ≈290K), low dielectric loss (tan δ<0.05), symmetric capacitance-voltage (C/V) and current-voltage (I/V) profiles, ferroelectric switching at cryogenic temperatures and a breakdown field (EB) >103 kV cm-1 at ≈290K. We report for the first time that all three phase transitions expected in bulk BST 70/30 were observed in thin film on heating, with comparable or lower transition temperatures. In particular, the presence (or otherwise) of certain transitions depended upon thermal history. This was not exhaustively investigated and is worthy of further study. 1.0 Introduction Ferroelectric thin films are of special importance due to the vast array of applications that could benefit from their unique dielectric, ferroelectric, pyroelectric, piezo-electric, acousto-optic and electro-optic properties. One of the major driving forces behind research in this area stems from the two hundred billion dollar global semiconductor industry, and their desire to utilise ferroelectric thin films in dynamic random access memory (DRAM) as a high permittivity dielectric, and in non-volatile ferroelectric random access memory (NV-FRAM) as the active memory element [1]—relying upon ferroelectricity to retain information without a power source. Ferroelectric materials have a high permittivity compared to conventional dielectrics such as silicon dioxide (ε ≈ 3.9 – 4.5). Bulk BST 70/30 has a dielectric constant of approximately 5000 at 300K, although it is more than an order of magnitude smaller in thin film solid solution [2]. Nevertheless, BST is one of the most promising ferroelectric materials available due to its high permittivity and composition-dependent Curie temperature (Tc from 30 to 400K) [3]. It is an excellent candidate for future DRAM dielectrics where its ferroelectric properties are not utilised. Familiarity with the temperature dependent characteristics, ferroelectric and dielectric response of BST thin film is therefore of commercial and theoretical importance. We present our findings with an emphasis on features that have not been reported elsewhere to date.

1 Radiant Technologies, Inc. Precision Premier materials analyser Windows NT 4™ workstation (Vision software Version 3.1.0)

Supervisor: Prof. James F. Scott May 2003


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2.0 Background Due to the nature of the ferroelectric physics involved, a summary of relevant introductory theory is provided—and should be referred to at the reader’s discretion2. Introduction to ferroelectrics Figure 2.0.1 (below) outlines the basic displacement behaviour of a perovskite structure in its ferroelectric state. Ferroelectric materials3 are a class of low symmetry crystals, which exhibit a reversible spontaneous electric polarisation along one or more crystal axes [4]. Such a polarisation arises due to the non-centrosymmetric arrangement of ions in a unit cell, resulting in a permanent electric dipole moment. This phenomenon is commonly observed in ABO3 crystal structures, where an octahedron, consisting of six oxygen atoms, contains a smaller metallic element at its centre. ABO3 type ferroelectric crystals are categorised as perovskite, ilmenite and tungsten-bronze types. Most ferroelectrics that are candidates for storing charges in non-volatile memory applications are of the perovskite type. 4 Ferroelectric cells (such as in figure 2.0.1 above) collectively orientate with adjacent cells in a common polarisation direction, forming ferroelectric domains. Applying an external electric field causes domains orientated favourably with the field to grow at the expense of others, until total domain growth and reorientation is complete. The material then possesses its maximum saturated electrical polarisation (Psat). Removing the field does not, however, restore all domains to a random orientation, such that the ferroelectric retains a stable remanent polarisation (Pr). The electric field strength required to return this remanent to zero polarisation is termed the coercive field (Ec) or coercive voltage (Vc). The response of ferroelectric materials to an applied electric field is entirely non-linear (see figure 2.0.2 overleaf). Their ability to switch robustly between one of two stable polarisation states and exhibit hysteretic character is the basic property used in non-volatile FRAM semiconductor data storage technology5 [5].

2 The reader is referred to a comprehensive introductory text in reference [1]: Ferroelectric Memories, J. F. Scott. 3 Unlike ferromagnetic materials, few ferroelectric materials actually contain any iron 4 Adapted from figure 1.3, page 5, reference [1] and others 5 Thin-film perovskite oxides are starting to see commercial production in random access memories (RAMs) for non-volatile computer memories

A B O

c-axis (001)

a-axis (100)

b-axis (010)

Electric field

Octahedron of Oxygen atoms

Cubic lattice of A

[Extended orientation]

Two stable state displacement of B

Ba / Sr O Ti E.g. BST 70%/30% solid solution

Figure 2.0.1 — Displacement in a non-centrosymmetric ABO3 cubic perovskite cell (e.g. barium strontium titanate (Ba,Sr)TiO3) The tetragonal crystal system of a perovskite structure consists of a cubic lattice with one lattice face extended in the 001 orientation, whilst the other two orientations are shrunk respectively. In the extended direction, the displacement of plus ions (A and B), minus ions and their valence electrons separates the cell’s centre of positive and negative electronic charge, resulting in a permanent electrical dipole moment. The dipole moment per unit area is measured by the electrical polarisation (µC/cm2). The extended orientation is in the c-axis [001], while the a-axis [100] and b-axis [010] directions are the shrunken orientations. Electrical polarisation occurs along the c-axis orientation. The two stable polarisation states can be used to correspond to, for example, logic states 1 and 0.


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As the temperature is raised, the bulk polarisation decreases and vanishes abruptly at the Curie temperature (Tc). This phase transition7 is due to the perovskite atomic structure reverting to a centrosymmetric state and therefore lacking the ability to spontaneous polarise. It is referred to as the paraelectric phase. As the temperature is lowered below the Curie point, a phase transition from the paraelectric to ferroelectric state occurs. The central ion is displaced from its body centre position and the cubic atomic structure deforms into a non-centrosymmetric form, such as tetragonal, orthorhombic or rhombohedral. 2.1 Barium Strontium Titanate Overview The main purpose for researching ferroelectric thin films has been to reliably reproduce the properties of bulk ceramics or macroscopic single crystals in thin-film form, for use in device applications. For example, ferroelectric materials require very high coercive fields (of the order of kV/cm) in order to be able to switch their domains from one orientation to another. It is therefore necessary and highly desirable to utilise these materials as thin films. The fundamental problem limiting their use is that ferroelectric thin films invariably have inferior dielectric, piezoelectric and pyroelectric properties compared to their bulk counterparts. In some cases, there may be an order of magnitude difference in the electrical and electromechanical properties of the bulk ferroelectric and its thin-film form [6]. This difference is usually

6 Adapted from figure 2, page 24, reference [5] 7 The Landau theory of phase transitions is a macroscopic theory that can be used to describe the behaviour of many ferroelectrics. See Ferroelectric Crystals, F. Jona and G. Shirane, Dover 1993

Logic state “1”

Figure 2.0.2 — An example of a hysteresis loop of polarisation against voltage with two stable states in a ferroelectric capacitor Psat – Saturation polarisation Pr – Remanent polarisation

Vc – Coercive voltage Vmax – Maximum voltage Qs – Polarisation charge

Polarisation charge (Q) (µµµµC/cm2)

Applied Voltage (Vf) (V)

+Vc -Vc

+Pr

-Pr

+Psat

-Psat

Logic state “0”

Qs

-Qs

-Vmax +Vmax

Vf=0, P=-Pr – logic 0 Vf=+Vc, P=0 – Zero polarisation Vf=+Vmax, P=+Psat – Saturated Vf=0, P=+Pr – logic 1 Vf=-Vc, P=0 – Zero polarisation Vf=-Vmax, P=-Psat – Saturated

Non-volatile memory applications are based on the polarisation hysteresis behaviour shown above. When an external voltage is applied to a ferroelectric capacitor (1→3 or 4→6), there is a net ionic displacement in the unit cells of the ferroelectric material. The individual cells interact constructively with their neighbours to produce domains within the material. As the voltage is removed (3→4 or 6→1) a majority of the domains remain in the direction of the previously applied field, requiring compensation charge (Q) to remain on the plates of the capacitor. At zero applied field (1 or 4), there are two states of polarisation (±Pr), which are equally stable. Either of these two states could be used to represent logic “1” or “0”, and since no external field is require to maintain these states, the memory is non-volatile.


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attributed to compositional and microstructural inhomogeneities, internal stresses and defects. Internal stresses in thin film can arise due to numerous reasons, including lattice mismatch between the film and substrate8, differences in the thermal expansion coefficients of the film and substrate, and phase transformations [7]. Bulk (Ba,Sr)TiO3 Figure 2.1.1 (below) is the phase diagram for bulk single crystal and ceramic barium strontium titanate, composed from numerous established sources [8]. The composition dependent Curie temperature of bulk BST ranges from 30 to 400K for x = 0 to x = 1, respectively [9]. For the 70/30 composition of BST of interest (x = 0.7), we can identify all three expected bulk phase transition temperatures, as a comparison reference to thin-film properties. Bulk BST 70/30 possesses four distinct phase states, three of which are ferroelectric, with three distinct phase transitions. 9

The properties of thin-film ferroelectrics compared to bulk are invariably different. In particular, shifted phase transition temperatures are expected from experimental work to date, due to the aforementioned reasons10. Thin film (Ba,Sr)TiO3 A theoretical analysis [7] of epitaxal 70/30 barium strontium titanate thin films predicts a cubic-tetragonal transition temperature (Tc) of ≈ 307K (compared to 290K found experimentally in bulk BST). Phase transitions and Curie temperatures can also be identified in thin film samples from measurements made of permittivity with temperature. Figure 2.1.2 (overleaf) shows a plot of permittivity as a function of temperature for six different barium strontium titanate 70%/30% film thicknesses, composed from recent experimental data published by an independent group (Parker, Maria and Kingon) [10]—and represents a typical example of the mainstream capacitance-temperature behaviour reported for BST thin films to date11.

8 In the case of epitaxal films 9 Adapted from Landolt-Börnstein, Springer-Verlag Heidelberg, http://www.landolt-boernstein.com 10 The origin of the differences between the dielectric behaviour of thin film and bulk BST has been attributed to three phenomena: an intrinsic size effect that results in a drop in permittivity with decreasing crystal dimensions, a reduction in the polarisability due to nonstoichiometry, and changes in polarisability due to biaxial strain arising from the thermal expansion mismatch with the substrate [10] 11 For example, Figure 2. L. A. Knauss, J. M. Pond, J. S. Horwitz and D. B. Chrisey, Appl. Phys. Lett. 69, 25 (1996)

Figure 2.1.1 — A phase diagram for composition-dependent barium strontium titanate bulk (with crystal diagrams) – Temperature vs. fractional barium content Full and open circle data points represent single crystal data sources, and full and open triangular points represent bulk ceramic BST data The three phase transition temperatures for bulk BST 70/30 are: P (cub) ⇔ F (tetr) Tc ≈ 290 K F (tetr) ⇔ F (orth) ≈ 235 K F (orth) ⇔ F (rh) ≈ 180 K

Ferroelectric phases (F)

Paraelectric state (P)

Cubic

Tetragonal

Orthorhombic

Rhombohedral

BST 70/30

1

2

3

4


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13 Equations 2.1.0 and 2.1.1 were used to fit the experimental ε vs. T data in figure 2.1.2, to quantify the diffuseness of the dielectric anomaly14. 3.0 Method: overview of apparatus and experimental aspects (Ba,Sr)TiO3 70/30 thin film characterisation Film and sample handling

Figure 3.0.1 (below) summarises the basic BST 70/30 sample configuration used for twice-through (top-to-top)15 electrical and dielectric measurements. 12 Source: Figure 1, C. B. Parker, J.-P. Maria and A. I. Kingon, Appl. Phys. Lett. 81, 341 (2002) 13 T is the temperature in K, ε is the dielectric constant, εmax is the maximum dielectric constant, Tmax is the temperature at the permittivity maximum, and δ is the diffuseness coefficient 14 Equation 2.1.0 is a second-degree polynomial where the coefficient β2 describes the diffuseness of the parabolic maxima. Equation 2.1.1 corresponds to a function commonly used to describe the temperature dependence of ε in relaxor ferroelectrics [11] 15 The electrical measurements were arranged as twice through, as the platinum bottom electrode was not accessible for contact, despite attempts to abrasively remove adjacent layers.

Figure 2.1.2 — Permittivity as a function of temperature for six (Ba,Sr)TiO3 70/30 thin film thicknesses [Parker et al.] Closed circles mark the data; a second-order fit [equation 2.1.1] by open circles. Each film thicknesses are marked by several pieces of data. An arrow marks the maximum value of permittivity. Tabulated with each arrow is the film thickness, the maximum permittivity, and the temperature at which the maximum permittivity occurs. Data obtained from warming at 0.2K/s To the right are two diffuseness measures δ and β2, corresponding with each respective thickness, as a result of equation fitting (below). Larger values of δ and smaller values of β2 indicate more diffuse transitions.

221 ** TTo βββε ++= Equation 2.1.0

2max

max

max 2)(11

δεεεTT −+= Equation 2.1.1

A 175nm BST 70/30 thin film should have a peak permittivity εmax in this region of approximately 400 at T ≈230K

Si (111) 4” diameter wafer substrate

SiO2 layer

Ex-situ sputtered Au top electrodes (Circular – area 0.95mm2)

Ag (silver paint) [Cold applied wire bond solder]

D = 175nm

≈ 1mm

Electrical copper wire contact

Ba0.7Sr0.3TiO3 175nm thin film layer

Pt (111) bottom electrode (confirmed by XRD)

Figure 3.0.1 — [Left] A cross sectional representation of the BST 70/30 twice-through capacitor set-up (not to scale) — [Below] The equivalent electrical circuit All layers were grown in situ except for the Au / Ag electrodes The twice-through (top-to-top) electrical contact is equivalent to two (theoretically identical) asymmetric capacitors in series (with representative leakage resistors). The circuit should exhibit overall symmetric behaviour.

[Not to scale]

≡

BST capacitor

BST capacitor

≈ 4mm

≈ 1.1mm

Equivalent circuit


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The Ba0.7Sr0.3TiO3 175nm thin film was grown on a 4-inch platinum-coated silicon wafer by chemical solution deposition (CSD) using a propionate-based solution, and was obtained from Dr. S. Hoffman and Prof. R. Waser of RWTH Univ. Aachen [12/13]16. The single-phase BST film was reported to exhibit a polycrystalline structure with columnar morphology. The state of the film’s surface was also verified by scanning electron microscope (SEM). When not in use, the wafer and samples were stored at room temperature and pressure (RTP) in a dehumidifying environment. Postage-stamp sized samples of the BST/Pt/SiO2/Si wafer were clean fractured and air dusted in preparation for ex situ gold electrode deposition. A regular matrix of up to 48 circular gold electrodes (with a circular diameter of ≈ 1.1mm) was deposited on the BST surface using a copper contact mask. Four (three minute) gold deposition cycles were subsequently performed using an EMITech K550 sputter coater unit17. The Au/BST/Pt samples were then encased in a supportive putty matrix and cold wire bonded, in order to provide stable electrical contact with the delicate gold surface electrodes. Short copper wires were bonded to the electrodes using solvent blend conductive silver paint18 applied by hand, and were supported by the putty in such a way as to minimise physical stress at the wire-gold electrode interface. Prepared samples were then stored for several hours at RTP to allow the silver paint solvent to evaporate19 and for the putty to harden. Figure 3.0.2 (below) shows an example of a prepared specimen. Alternatively, some samples were directly contacted using spring-mounted needle pins, which were positioned above the specimen’s gold electrodes and achieved contact by exerting a downward pressure. However, this was found to be inferior and much less reliable than the wire-bonding method outlined previously20. The prepared samples were mounted onto the cold head of a CTI-Cryogenics cryostat unit, ensuring good thermal conduction through the putty matrix (and, where necessary, aided by heat sink compound) between the head and thin film sample. Wires running from within the cryostat unit were used to provide electrical contact to the sample’s wire bonds externally. A standard passive temperature probe was also mounted on the cryostat cold head using putty, to provide an indication of the thin-film temperature on a Fluke 52 K/J digital thermometer, to within an upper

16 For further processing details see Hasenkox et al. 17 http://www.emitech.co.uk/K550.htm 18 Electrolube silver conductive paint (SCP): Surface Resistance @ 0.6 to 2 g/100cm2 = 0.01 to 0.03 Ω/sq 19 There are, however, some interesting questions over whether there is still solvent present in some of the samples tested, which could account for some unexpected behaviour 20 In part, the physical force exerted by the contact needles resulted in erosion of the gold electrodes and potentially great stress on the thin film. As the temperature was altered, this invariably resulted in imperceptible needle movement which resulted in loss of electrical contact

Figure 3.0.2 — A photograph of a typical Au/BST/Pt sample after preparation (as outlined in figure 3.0.1, page 5)

Supportive putty matrix (bluetack) encasing the BST sample and supporting wires

Bare copper wires – each cold bonded to a single Au electrode

Silver paint bonding on the surface applied by hand

≈ 1cm


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(predicted) error limit of ±5K. The cryostat unit was evacuated using an Edmunds vacuum pump and maintained a vacuum pressure of lower than 30 milliTorr (< 4 Pascals) at room temperature and below. Cryogenic cooling was achieved using a closed-cycle CTI-Cryogenics helium cooler, and was capable of achieving temperatures as low as 30K over a couple of hours operation. It was not, however, possible to automatically pre-select a cryostat temperature or maintain a constant temperature ramp rate (∆T/∆t). This was achieved manually using a filament-heating element positioned at the base of the cold head, and controlled by an external variable power supply unit (PSU). A warming / cooling rate of approximately 0.2 K/s was typically achieved. Figure 3.0.3 (below) illustrates the basic apparatus. The BST samples were studied in a carefully controlled environment under vacuum, to eliminate problems associated with low temperature experiments, such as water or other atmospheric gas condensation. During this set-up process, it was especially important to continuously monitor the quality and integrity of the electrical contacts with the sample surface, due to the very real risk of contact deterioration caused by any of the aforementioned steps21.

Measurements of the BST capacitor sandwich were taken by connecting the sample to an impedance analyzer or Radiant Precision Premier Workstation. It was not possible to do this concurrently. Dielectric measurements (Impedance analyzer) Zero-bias permittivity measurements of the BST capacitor with temperature were taken using a Hewlett-Packard 4192A LF impedance analyzer (5Hz – 13Mhz), working with an oscillation signal voltage of 100 mV at a spot frequency of 100 MHz. Capacitance (C)(in nF to 3.d.p) and loss tangent (D)(tan δ to 4.d.p) were manually recorded with temperature. Treating the twice-through electrical configuration as the equivalent of a parallel plate capacitor, capacitances values were converted into permittivity values using equation 3.0.1 (overleaf), assuming a parallel plate area

21 Indeed, this was a very common problem with attempting to produce a BST capacitor in this way. See the discussion for further details

Figure 3.0.2 — A photograph of the basic cryostat apparatus and cold head (inset)

Fluke 52 K/J digital thermometer

≈ 5cm

To the Edmunds 8 vacuum pump (<30 millitorr)

Cryostat cold head surface

Inlet and return for the closed-cycle CTI-cryogenics He-cooler

Sealed CTI-cryogenics cryostat chamber (evacuated)

Sample mounting stage for the BST 70/30 specimen

Electrical contacts to sample (shown here with spring mounted needle contacts), connected to an impedance analyzer or Radiant tester

Connections for temperature probe, sample contacts and heating filament


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(0.0095cm2) equivalent to the circular gold electrode area (where C is the capacitance in F, A is the area in m2, d is the film thickness in m, εo is the permittivity of free space and εr is the relative permittivity of the sample)22. C = (εoεrA)/d [Equation 3.0.1] C and D measurements were concurrently taken by hand using the impedance analyzer on demand (manual trigger), recording values at specific temperature (1 or 0.5K) intervals. It could also provide continuous readings at approximately two samples per second (internal trigger), which was used to monitor capacitor contact quality during set-up and configuration. The unit was operated in parallel circuit mode and was calibrated for an open and short circuit before use at 100Mhz. A computer system connected to the impedance analyzer by GPIB interface (using Oxford Objectbench 3.2.8a software) was also used to record the impedance spectrographs for specimens, sampling from 5Hz to 13Mhz at 100mV in parallel circuit mode23. Electrical measurements (Radiant Precision Premier Workstation) A Radiant Technologies Inc. Precision Premier NT Workstation computer system (operating Vision software version 3.1.0), designed solely for providing a range of highly specialised hysteresis and ferroelectric related measurements, was used to acquire electrical data on the BST capacitor sandwich. The method, operation and specifications of this equipment are well documented elsewhere by the manufacturer [14]. Capacitance-voltage (C/V), current-voltage (I/V), leakage, hysteresis, remanent hysteresis24, charge and PUND measurements were all taken, applying up to 40V to the specimen (E ≈ 103kV cm-1)25. X-Ray diffraction

XRD measurements were carried out using Cu K\α1 radiation and a one-dimensional position-sensitive detector, from T ≈ 160 to 250K, in order to observe the thermal expansion of the BST thin-film. The rate of temperature change (0.0003 K/s) was such that each data run took several days continuously to complete. By comparison, the ramp rate used in the aforementioned dielectric and electrical measurements (typically ≈ 0.2K/s) was many orders of magnitude faster, involving a typical timescale of several hours. 4.0 Results and Discussion 4.1 General characterisation: Results Examining the film surface in SEM revealed the presence of a large number of randomly distributed ‘specks’, appearing as dark points. It was not possible to identify these more conclusively at a higher resolution.

22 As the set up is ‘twice-through’, there are effectively two 175nm BST capacitors in series, which can be effectively treated as a single 350nm BST capacitor for calculation and measurement purposes 23 A geometric factor of d/A = 0.0037 cm-1 was supplied to the application. Datasets were view using ZView software, version 2.4a 24 See the Appendix for a general introduction to this measurement 25 The unit is capable of applying up to 100V. Not all data recorded is presented in this paper


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Practical difficulties were often encountered26 in attempting to obtain a satisfactory electrical contact with the BST capacitor sandwich. The nature of these difficulties appeared to fall into two main categories: 1) The inability to maintain an electrical contact due to a physical loss of contact (e.g. failure of the wire-bond) 2) Electrical contacts with unsatisfactory behaviour in capacitance/loss values and/or radiant hysteresis measurements (e.g. poorly formed, erratic or nonsensical hysteresis loops). Figures 4.1.0 and 4.1.1 (below) illustrate an example of such hysteresis and I/V data, which were commonly recorded. On average, of the 48 top electrodes sputtered onto a typical sample, only 2 - 3 electrode pairs were successfully bonded and provided sensible results. Many contact pairs (regardless of physical separation distance) were found to represent electrical shorts (and thus behave as a linear resister). This was especially true for a sample sputtered with gold contacts 25 times larger (circular area of 0.25cm2). Figures 4.1.2 and 4.1.3 (below) illustrate a hysteresis and I/V plot for a successful contact pair27.

26 Typically 70% of the time 27 Figures 4.1.2 and 4.1.3 are characteristic of a ‘successful’ contact providing sensible results – in comparison to the expected theoretical behaviour and based on observations elsewhere to date

-1110

-1010

-910

-810

-710

-610

-510

-410

-300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300

Cur

rent

(Am

ps)

Field (kV/cm)

-25

-20

-15

-10

-5

0

5

10

15

20

25

-400 -300 -200 -100 0 100 200 300 400

Pol

ariz

atio

n (µ

C/c

m2)

Field (kV/cm)

Figure 4.1.0 — A common example of a distorted hysteresis measurement (Vmax = 15V, f = 1 kHz, T = 290K)

Figure 4.1.1 — An example of an asymmetric I/V plot, reaching 1mA leakage (Vmax = 10V, T = 180K)

-1010

-910

-810

-300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300

Cur

rent

(Am

ps)

Field (kV/cm)

-5.0

-2.5

0.0

2.5

5.0

-500 -400 -300 -200 -100 0 100 200 300 400

Pol

ariz

atio

n (µ

C/c

m2)

Field (kV/cm)

Hysteresis loop failing to close

Asymmetric I/V profile

Figure 4.1.2 — An example of a paraelectric hysteresis measurement (Vmax = 15V, f = 1 kHz, T = 290K, Pmax = 6.648 µC/cm2, Pr = 1.044 µC/cm2)

Figure 4.1.3 — An example of a typical symmetric I/V measurement (Vmax = 10V, T = 290K)

A symmetric I/V profile – Current in the order of nA

Dielectric loss loop for BST


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Good contact pairs were observed to have a breakdown field of EB > 103kV cm-1 at room temperature, although some pairs did breakdown at lower fields. Inevitably, all good contact pairs were lost after some time, regardless of the measurements taken. Again, two categories of causation were encountered: 1) Physical contact was lost with the sample due to wire-bonding failure (especially with temperature cycling) 2) The BST capacitor sandwich for the corresponding pair of electrodes broke down and was reduced to a short circuit. The process of obtaining and maintaining a pair of contacts for enough time to take dielectric and electrical measurements was therefore an arduous and highly repetitive process.

Figure 4.1.4, 4.1.5 and 4.1.6 (below) demonstrates a range of electrical measurements taken using the Radiant tester, for good contact pairs. In particular, 4.1.5 and 4.1.6 clearly demonstrate the ferroelectric behaviour of the BST 70/30 thin film at low temperature (T = 35K).

28 4.2 General characterisation: Discussion The results of the SEM inspection together with the general difficulties with contacting and shorted electrodes, appears to suggest that the film has quite a number of dispersed pin-hole shorts throughout its surface—and raises some interesting questions about the homogeneity of the film. Given the practical difficulties already inherent in wire-bonding small thin film samples by hand, coupled with the less than

28 See the appendix for an explanation of remanent hysteresis measurements

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

-300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300

Cap

acita

nce

(nF)

Field (kV/cm)

-15

-10

-5

0

5

10

15

-750 -500 -250 0 250 500 750

Pol

ariz

atio

n (µ

C/c

m)

Field (kV/cm)

-5

-4

-3

-2

-1

0

1

2

3

4

5

-10.0 -7.5 -5.0 -2.5 0.0 2.5 5.0 7.5 10.0

Pol

ariz

atio

n (µ

C/c

m2)

Voltage (V)

Unswitched - Logic 0 Switched - Logic 1 Remanent

Symmetric C/V at T = 290K

Ferroelectric hysteresis at T = 35K

A distinct remanent ferroelectric polarisation at T = 35K

Figure 4.1.4 [top left] — A capacitance-voltage plot for BST 70/30 at room temperature (T=290K, Vmax = 10V) Figure 4.1.5 [top right] — A hysteresis plot at T = 35K (Vmax = 30V, f = 1 kHz, Pmax = 14.128 µC/cm2, Pr = 4.990 µC/cm2) Figure 4.1.6 [bottom left] — A remanent hysteresis plot at T = 35K (Vmax = 10V, f = 1 kHz, Pmax (remanent) = 0.412 µC/cm2, Pr (remanent) = 0.360 µC/cm2)

Switched

Remanent

Unswitched


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ideal film quality, it is unsurprising that the success rate for suitable contact pairs has been so low. Furthermore, the attempt to use larger gold electrodes illustrates this point well: the larger contacts, although easier to work with by hand, are more likely to cover pinhole shorts and therefore fail. Figures 4.1.0 and 4.1.1 (page 9) showing a distorted hysteresis and I/V are therefore typical of one electrode contacting a poor section of the film. Indeed these figures are good examples of where causation can be easily identified. In many cases, entirely nonsensical data plots were all too frequently obtained29. Many of the physical difficulties experienced with maintaining surface electrode contact, are likely due to trivial practical issues: such as silver paint solder failing at low temperatures, and the effects of thermal expansion with temperature cycling dislodging contacts or straining wire-bonds. These difficulties can, however, be eliminated by designing a different approach to establishing a BST sandwich.

Figures 4.1.2 to 4.1.6 (page 9 and 10) illustrate a range of measurements taken from suitable contact pairs, and demonstrates predictable electrical characteristics (dielectric and ferroelectric behaviour) at room and cryogenic temperatures. Results of this nature are in line with expectations, have been reported and explained elsewhere, and shall not be discussed here further. They are, however, to be taken as a baseline for what constitutes an acceptable contact pair for subsequent C-T and P-T measurements. 4.3 Phase transitions: Results and Discussion Figure 4.3.1 (below) represents permittivity with temperature data for a virgin sample of BST 70/30 cooled and then subsequently warmed up. As expected, compared to the work of Parker et al. (see figure 2.1.2, page 5), we observe a broad diffuse transition in permittivity on cooling for our BST 70/30 thin film—although at a transition temperature of around 185K, compared to the expected 230K from the work of Parker. Using equation 2.1.0 (page 5), the cooling permittivity data has been fitted to a quadratic equation, to obtain a measure of the

29 Not shown in this paper

150

155

160

165

170

175

180

50 100 150 200 250 300

Perm

ittiv

ity

Temperature / K

Figure 4.3.1 — A plot of permittivity with temperature for a virgin sample of BST 70/30 thin film – cooled down and sequentially warmed up between 300 and 30K The permittivity on cooling exhibits a diffuse transition, with a peak permittivity at 183K For comparison, β2 = -8.0 x 10-4, fitting the cooling permittivity data to quadratic equation 2.1.0 (page 5) 0.2K/s warming / cooling rate

1

2 Cooling: εmax at T ≈ 183K Heating: discontinuities at T ≈ 186K, 212K and 238K

A single broad transition on cooling

Three apparent transitions on warming


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diffusiveness of the transition. The β2 value obtained is reasonable compared to the values shown in figure 2.1.2. Of particular interest is the anomaly present in the permittivity plot on heating, which has not been reported before, and is not seen in our cooling permittivity data. We can report that this anomaly is only ever seen in our permittivity data on warming—whilst permittivity with cooling produces broad transitions, as expected and reported in work elsewhere. Figure 4.3.2 (below) presents the associated loss tangent with temperature data, for figure 4.3.1. In the above figure, two distinct ‘kinks’ can be seen in the warming loss data at 212K and 238K, corresponding identically to two ‘kinks’ observed in the warming anomaly in ε(T). Figure 4.3.3 (below) summarises the results for XRD data taken on the BST sample. This provides a method of analysing the phase transitions in the thin-film independently of electrical measurements, and at a temperature ramp rate, which is vastly different. The 1st heating data line in the above XRD plot shows three clear transitions at approximately 190K, 210K and 227K. This corresponds well with the three transitions that appear in ε(T) warming data, which occur at approximately 186K, 212K and 238K respectively. The XRD data therefore lends independent credibility to

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Figure 4.3.2 — A plot of dielectric loss tangent with temperature for a virgin sample of BST 70/30 thin film – sequentially cooled down and warmed up between 300 and 30K 0.2K/s warming / cooling rate

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Two ‘kinks’ are observed in the warming loss data at 212K and 238K, matching the ε(T) data

Figure 4.3.3 — XRD data for the BST 70/30 thin film Channel position (y-axis) is a measure of the lattice position of the specimen. As the temperature is varied, thermal expansion of the sample will occur—and would result in a smooth baseline curve. Deviations from a smooth baseline curve can be interpreted as a change of phase state in the sample. The sample was warmed from 160K to 250K, and subsequently cooled and warmed again. 0.0003K/s warming / cooling rate Error in temperature ± 5K

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the notion that three phase transitions are being observed in the dielectric data for the sample: corresponding to the rhombohedral to orthogonal, orthogonal to tetragonal, and tetragonal to cubic phase transitions. It is, however, interesting to note that the behaviour in the subsequent warming and cooling XRD is not as distinct. It is not possible to clearly identify any other transitions, other than perhaps a single diffuse phase transition in the sample. Figure 4.3.4 (above) shows sequential permittivity data for a non-virgin sample of BST 70/30, which was cooled and warmed between 300 and 30K, and then cooled and warmed between 300 and 175K. Of particular interest is the fact that a warming anomaly is only observed on the 1st warming—and then, only with two distinct phase transitions in the anomaly, compared to the three seen in figure 4.3.1. Once again, the lack of a distinct anomaly is well supported by the XRD data, which also lacks any distinct transitions in its second heating run. Clearly, as there is a lack of systematic data investigating this phenomenon, we can only speculate about the feature that is being observed. One possibility is that we are witnessing some sort of re-entrant phase transition, as is found in potassium nitrate (albeit in reverse), where on cooling we are moving directly from the cubic to the rhombohedral state—but on heating, we pass through all four phase states expected in bulk BST. There is some evidence to suggest that a thermal hysteresis coupled with a re-entrant phase transition is responsible. The fact that subsequent heating cycles suggest a reduction in the number of observed phase transitions (three, to two, to one) could be due to some kind of release of interfacial stress in the sample—a stress built into the film from the fabrication and post-deposition anneal process. Finally, one further thought is to consider the asymmetry of the experiment: the thin film has been kept at room temperature in the cubic state for a long time, relative to being held at 30K in the rhombohedral state.

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Figure 4.3.4 — A plot of permittivity with temperature for a non-virgin sample of BST 70/30 thin film – cooled down and sequentially warmed up between 300 and 30K, and then cooled down and warmed up between 300 and 175K 0.2K/s warming / cooling rate

No anomaly is found on warming for a sample which is only cooled to 175K

1st cooling

1st warming

2nd cooling and warming


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5.0 Conclusions We have observed, for the first time, in barium strontium titanate 70/30 thin film all three phase transitions known to exist in bulk BST. These were found at the expected bulk transition temperatures, given the inherent experimental error, except for the cubic-tetragonal transition, which is approximately 50K lower in thin film. Of particular interest is the disappearance of phase transitions in subsequent sample heating cycles, for which we have speculated upon several possible explanations. This phenomenon remains unexplained and is worthy of further research. 6.0 References

1. Scott, J. F. Ferroelectric Memories (Springer-Verlag, Berlin, 2000). ISBN 3-540-66387-8

2. A. D. Hilton and B. W. Ricketts, J. Phys. D 39, 1321 (1996) 3. J. G. Chen, X. J. Meng, J. Tang, S. L. Guo, J. H. Chu, Appl. Phys. A 70 411

(2000) 4. M. E. Lines and A. M. Glass, Principles and Applications of Ferroelectrics

and Related Materials (Clarendon Press, Oxford, 1977) 5. O. Auciello, J. F. Scott and R. Ramesh. The Physics of Ferroelectric

Memories, Physics Today (1998) 6. T. M. Shaw, S. Trolier-McKinstry, and P.C. McIntyre, Annu. Rev. Mater. Sci.

30, 263 (2000) 7. Z.G. Ban and S. P. Alpay, J. Appl. Phys. 91, 9288 (2002) 8. Landolt-Börnstein, Springer-Verlag Heidelberg, http://www.landolt-

boernstein.com 9. G. A. Smolenskii and K. I. Rozgachev, Zh. Tekh. Fiz. 24, 1751 (1954) 10. C. B. Parker, J.-P. Maria and A. I. Kingon, Appl. Phys. Lett. 81, 341 (2002) 11. G. A. Smolenskii, J. Phys. Soc. Jpn. S28, 26 (1970) 12. U. Hasenkox, S. Hoffmann, R. Waser, J. Sol-Gel Sci. Tech. 12, 67 (1998) 13. S. Hoffmann and R. Waser, J. Eur. Ceram. Soc. 19, 1339 (1999) 14. Radiant Technologies, 2021 Girard SE #100 Albuquerque, New Mexico.

87106. USA. http://www.ferrodevices.com 7.0 Acknowledgements Special thanks go to Prof. James F. Scott, Matthew Dawber, Dr. Andreas Rüdiger, Dong Jin Jung and Dr. Finlay D Morrison (Symetrix Centre for Ferroics, Department of Earth Sciences, Cambridge, England) for their tireless insight and assistance, and Dr. Susanna Ríos (Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ) for the low temperature X-Ray diffraction measurements made on the Ba0.7Sr0.3TiO3 thin film.


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8.0 Appendix A summary of methodology behind remanent hysteresis radiant measurements

The primary tool for the ferroelectric memory researcher is the hysteresis loop. The loop shows the non-linear polarization response of a ferroelectric memory sample to a bipolar triangular voltage stimulus waveform, where polarization is the non-volatile memory property of a ferroelectric sample. The hysteretic nonlinearities revealed by the measurement characterize the sample as a memory device. The basic hysteresis measurement is a particularly effective tool as it shows the total sample response to the stimulus voltage. This response can be modelled as the superposition of a number of parasitic linear elements along with the non-linear ferroelectric mechanism. The linear components of the ferroelectric model include linear capacitance, resistance and diode effects. In a quality sample, these response components will be very small with respect to the non-linear signal that is of most interest to the ferroelectric memory researcher. But the non-linear polarization response of the sample can also be modelled as two or more non-linear components. The two components of primary concern are the remanent polarization and the non-remanent polarization. Remanent polarization is the polarization of most interest. It is that bistate ferroelectric parameter that maintains its state once switched and serves as the memory characteristic of the sample. Non-remanent polarization also switches with the remanent. However, once the sample has returned to a quiescent voltage, the non-remanent polarization does not maintain its switched state, but re-randomizes. This polarization does not contribute to the memory properties of the sample.

A number of sample properties are of interest: at what voltage does polarization switch states; how fast does polarization switch states; and the problem addressed by remanent hysteresis radiant measurements: how much non-remanent polarization is available. The hysteresis measurement normally captures the super positive response of all of the linear and non-linear components. Normally the linear components may be ignored. As a result: Hysteresis Response = Resistance + Linear Capacitance + Diode Effects + Remanent

Polarization + Non-Remanent Polarization ≈ Remanent Polarization + Non-Remanent Polarization

This applies to the measurement of a sample’s response to the second leg of the standard bipolar triangular stimulus voltage in all hysteresis measurements. It applies to the first leg only if the sample is preswitched into a state that ensures that the first leg will switch the sample into the opposite state. If the sample is preswitched into a polarization state that ensures that the first leg reinforces (does not switch) the preset state, then only non-remanent polarization will be switched and measured, as the remanent polarization is already switched into the particular state. This leaves a tool for directly measuring both the combined remanent and non-remanent polarization values and the non-remanent only. Subtracting the non-remanent measurement from the remanent plus non-remanent (non-switching from switching) allows the remanent-only hysteresis loop to be derived. Repeating the experiment at the opposite polarization state allows both halves of the bipolar non-remanent sample response to be independently derived. Recombining the two halves results in a full non-remanent hysteresis loop being constructed.

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Barium Strontium Titanate