Eindhoven University of Technology

MASTER

Electroforming of alkali halide-organic semiconductor memory diodes

Wang, J.

Award date:2014

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I

Electroforming of Alkali halide-Organic

semiconductor memory diodes

Graduation report of

Jingxin Wang

Supervisors

Dr. S. C. J. Meskers

Dr. Benjamin Bory

Supervising professor

Prof. Dr. Ir. R. A. J. Janssen

Advising committee

Dr. Ir. M. M. Wienk

Dr. M.G. Debije

Eindhoven, June 2014

Molecular Material and Nanosystems

Laboratory of Macromolecular and Organic Chemistry

Department of Chemical Engineering and Chemistry

Eindhoven University of Technology

II

Contents Abstract ............................................................................................................. IV

Chapter 1 Introduction ......................................................................................... 1

1.1 Challenge and Development of Memory technology .................................................... 1

1.2 ReRAM technology .......................................................................................................... 2

1.3 Metal Oxide as Insulator layer ........................................................................................ 3

1.4 Reproducible electroforming and resistive switching .................................................. 5

1.5 Alkali halides ................................................................................................................... 6

1.5.1 Defects and F-centers in Alkali halides .......................................................................... 7

1.6 Organic semiconductors....................................................................................................... 9

1.7 Aim of this project and hypothesis ......................................................................................10

Chapter 2 Experiments and Characterizations ....................................................... 11

2.1 Materials .........................................................................................................................11

2.2 Device fabrication ..........................................................................................................11

2.3 Characterizations ...........................................................................................................12

Chapter 3 Optical Characterization of Diodes under Bias Stress ............................... 14

3.1 Reflection measurement ...............................................................................................14

3.2 Time dependence ..........................................................................................................16

3.3 Influence of Alkali Halides on Coloration .....................................................................18

3.4 Determination of the coloration voltage ......................................................................20

3.5 Influence of thickness of alkali halide layer. .................................................................21

3.6 Influence of the organic semiconductor .......................................................................22

3.7 Correlation with electrical properties ...........................................................................25

3.8 Summary ........................................................................................................................27

Chapter 4 Electrical Characterization ................................................................... 28

III

4.1 Electroforming process .................................................................................................28

4.2 Negative differential resistance and resistive switching ..............................................29

4.3 Electronic Memory Effects .............................................................................................30

4.4 Influence of alkali halide on the electroforming ..........................................................31

Chapter 5 Discussion .......................................................................................... 33

5.1 Storage of charges in diodes .........................................................................................33

5.2 Defect formation in alkali halide ...................................................................................35

5.3 Electroforming and defects formation .........................................................................37

Chapter 6 Conclusion ......................................................................................... 39

References ........................................................................................................ 40

Acknowledgements ............................................................................................ 42

IV

Abstract

A layer of polymer on a layer of alkali halide shows an electronic memory effect. The electrical

resistance of the stacked layers can be switched between two different values by applying

voltage pulses. The memory can be read without modifying the contained information and no

power is required to keep the information. Before realizing memory effect, electroforming is

required by applying a high voltage. Electroforming of ITO/alkali halide/semiconductor

polymer/Ba/Al diodes have been investigated for nine different alkali halides and three

different semiconductor polymers. Optical reflection measurements indicates defects

formation when the bias excesses the coloration voltage. Electroforming and coloration

voltage correlate with the valence band energy of alkali halide indicating that electroforming

involves defects formation.

1

Chapter 1 Introduction

1.1 Challenge and Development of Memory technology

In today’s computers, every component has gotten so much faster in comparison with ten

years ago. The speed of operation of hard disk drives (HDD) has however not increased as

much as the speed of the other components. The most common storage media nowadays in

PC is the HDD which stores digital information by using rapidly rotating disks coated with

magnetic material. The capacity of a single HDD increased from 0.01GB in 1985 to around

several TB in 2014. The rate of read and write operation is the bottleneck of HDD. The spin

speed of the coated disk in HDD has stayed at 7200 rpm for a long time and it is difficult to

improve it. Moreover, the emergence of mobile devices in the last few years such as

smartphones, ultra-books and tablets, has raised a demand for require smaller, faster and

lower energy consumption. Current HDDs cannot fulfill these requirements especially in term

of data access time and downscaling.

The current solution for mobile devices is the so called flash memory technology which is

based on floating-gate transistors. Recently also computers, such as MacBook Air laptops, use

flash storage instead of HDD. The advantage of flash memory is the absence of mechanical

limitations, higher speed and lower power consumption in comparison with the HDD. The

main disadvantage of flash memory is that the price of per gigabyte is significantly higher than

for hard disks. Also flash memory has a finite number of program/erase cycles. A new

challenge for flash memory technology is its scalability. As the storage area shrinks, also the

amount of stored charge required to store the information decreases, making the operation of

less reliable.

Many new memory technologies are developed such as Ferroelectric random access memory

(FeRAM), Magneto resistive random access memory (MRAM) and phase-change random access

memory (PRAM). Resistive random access memory (ReRAM) is a two-terminal electronic

device also referred as “memristor” that retain internal resistance state according to the

history of applied voltage and current [3.5]. The ReRAM has drawn extensive research interests

as a leading candidate for next generation non-volatile memory applications due to its simple

structure, excellent scalability, fast switching speed and high endurance [4]. Table 1.1

2

summarizes the key parameters of existing and emerging memory technologies which can

work as nonvolatile working memory.

Table 1.1: key parameters of existing and emerging memory technologies. [2]

DRAM NAND MRAM PRAM ReRAM

Feature size ~8F2 ~5F2 ~8F2 ~4F2 ~8F2 (transistor)

<F2 (stack)

Latency ~50 ns 20~200 s ~50 ns ~200 ns ~50 ns to 1s

Bandwidth ~1 GBps ~100 MBps ~1 GBps ~100 MBps ~100 MBps

Volatility volatile Nonvolatile Nonvolatile Nonvolatile Nonvolatile

Endurance >1015 >105 >1015 >106 to 1012 >105

Retention >64 ms > 10 years > 10 years > 10 years > 10 years

Application Working

memory

Data storage

memory

Working

nonvolatile RAM

Code memory and

buffer memory

Data storage

memory

In this introduction chapter, we will first discuss the device architecture of ReRAM and the

various materials used in this type of device.

1.2 ReRAM technology

A typical ReRAM memory has a capacitor-like metal-insulator-metal (MIM) structure. The

insulator or resistive switching material “I” is sandwiched between two conductive electrodes

“M”. See Fig. 1.1.

Figure 1.1: MIM structure of ReRAM memory.

Electrode

Insulator layer

Electrode

Bi

Bias

3

These MIM cells can be electrically switched between at least two different resistance states,

after an initial electroforming cycle which is usually required to activate the switching

property [6]. Switching can be induced by application of bias voltage. There are two switching

modes in ReRAM respect to the polarity of external electrical field. One is unipolar switching

where switching of device can be achieved in same polarity. The other is bipolar switching

which requires different polarities to switch device. In unipolar switching, the current of

device drops with increasing voltage after the reset operation. The decrease of current with

increasing voltage also is referred as negative differential resistance (NDR).

Resistive switching in MIM structure has been achieved in a wide range of materials. Metal

oxides are typical materials for the insulator layer, such as NiO,[7,8] TiO2,[9] HfOx,[10] SrTiO3, [11]

and ZnO [12]. More recently, the ReRAM based on oxide heterostructures offers superior

performance over single-layer based devices. [4] The memory based on asymmetric Ta2O5-

x/TaO2-x bilayer structures possesses cycling endurance of over 1012, and fast switching time

with 10 ns. [14]

Also organic materials have been used as insulators in resistive switching MIM diodes.

Polymers like polystyrene (PS), acrylates and poly(vinylcarbazole) (PVK) and small organic

molecules such as tetracene and nanoparticles in a polymer matrix can be used to make MIM

structure and show resistive switching. [13]

Although various materials were found to have resistive switching behavior and people can

fabricate excellent ReRAM devices in the lab, the switching mechanism still remains unclear. [4]

One popular explanation is filamentary conduction. The migration of ions in the device leads

to formation of a conductive region in the insulator, known as filament. [4, 10] People reported

direct observation of the conductive filaments by transmission electron microscopy (TEM). [4]

1.3 Metal Oxide as Insulator layer

Currently, metal oxides with wide bandgap are the most popular materials which are used as

the insulator layer in MIM structure. Many studies show that MIM diodes require an

electroforming procedure before resistive switching. The electroforming involves applying a

high bias voltage to the MIM diode such that the insulator is close to the dielectric breakdown.

4

[5] Electroforming is believed to involve formation of defects in the metal oxide. [5, 6] The

resistive switching is related to these defects.

Figure 1.3: Figure 4. Schematic diagram for the mechanism of resistive switching in Pt/ZnO/Pt devices. Retrieved from Ref.4.

A schematic diagram for a possible mechanism of resistive switching in Pt/ZnO/Pt device is

shown in Fig. 1.3. The formation of oxygen vacancies is accompanied with the release of

molecular oxygen (O2). When applying a bias to the device the vacancies move toward the

anode (Fig. 1.3a). When the concentration of oxygen vacancies at the cathode is high, the zinc

atoms rearrange their position and the structure of the oxygen deficient ZnO1−x changes

(Fig.1.3b). The precipitation of Zn atoms leads to formation of a conductive filament this can

be observed as electroforming (Fig. 1.3c). Joule heating of the conductive filaments provides

to thermal energy that can induce rupture of the filaments and the Zn metal changes back to

ZnO. Owing to the migration of oxygen atoms, the diodes switches back to the off state (Fig.

1.3d). [4]

M-J Lee etal. [14] have shown that memristors with excellent properties can be made by using a

bilayer of TaO2-x and TaO5-x (A 30nm thick medium-resistivity TaO2-x base layer ranging from 107

to 108 Ω/cm and a localized 10nm highly insulating (~1013 Ω/cm) TaO5-x above it). They found

the properties of devices (programing current and stability) strongly depend on the ratio of

resistance between the base layer and insulating layer. They argue that, in a proper ratio

5

window, the filaments form in the insulator layer and the oxygen ions are trapped at the

interface of the base layer and insulator. See Fig. 1.9.

Figure1.4: Schematic representation of the TaOx device consisting of a thin Ta2O5-x insulating layer and a TaO2-x

base layer. The movement of internal oxygen ions or vacancies is used to model the switching. Retrieved from ref. 14.

1.4 Reproducible electroforming and resistive switching

Reliable and reproducible electroforming and resistive switching behavior can be obtained by

using an organic-inorganic junction as an insulator layer in the MIM structure. [5, 17] See Fig. 1.5.

Figure 1.5: Layer-out of an MIM structure with an organic-inorganic junction.

The organic semiconductor layer acts as a current limiting series resistance to prevent hard

breakdown. [5, 17]

6

1.5 Alkali halides

Alkali halides are inorganic compounds with general chemical formula MX, where M is an

alkali metal and X is a halogen. Alkali halides are usually insulators because of the high band

gap in these materials. See Fig. 1.6. Defects in alkali halides have been studied intensively

because defect formation can be probed conveniently using optical spectroscopy.

Electroforming also is observed in alkali halides. [5] However, the MIM structure device with

alkali halide insulator layer is less studied than the metal oxide one. [5, 15] Previous studies have

shown that MIM diodes based on LiF insulator layer can be switched > 30 times and have a

retention time > 1000 s. [15]

-8

0

4

En

erg

y (

eV

)

2

-2

-4

-6

CsIKICsBrKBrCsClCsFLiClKFRbFNaF

-10

LiF

Figure 1.6: band energy of LiF, NaF, RbF, KF, LiCL, CsF, CsCl, KBr, CsBr, KI and CsI. [18] The bottom bars represent the valence band energy levels. The top bars represent the conduction band energy levels.

7

1.5.1 Defects and F-centers in Alkali halides

As we know, there is no perfect crystal in nature. Different defects exist in a crystal, including

point defects, line defects, two-dimensional defects and volume defects. In this research we

focus on the point defects. A point defect affect isolated site in the crystal structure and there

are two types of point defects: intrinsic defects and extrinsic defects. The extrinsic point

defects are foreign atoms which is added in the material. The doping process in

semiconductor industry typically involves extrinsic defects. There are two main types of

intrinsic point defects in a crystal. One is Schottky defect and the other is Frenkel defect. A

schottky defect is an equal number of cations and anions are missing from their lattice sites.

Frenkel defects involve the displacement of an atom from its lattice position to an interstitial

site, creating a vacancy at the original site and an interstitial defect at the new location. The

two types of point defects are shown in Fig. 1.7.

Figure 1.7: Illustration of two types of point defects. a): Schottky defect. b): Frenkel defect.

In alkali halide materials, Schottky defects predominate at elevated temperatures. Two or

more vacancies sign may aggregate to form clusters of vacancies. Various vacancies and

vacancy clusters are shown in Fig. 1.8. [16]

8

Fig. 1.8: Schematic of different vacancies and vacancy clusters in alkali halides. As displayed from upper left to

lower right and reading across, the units with square are: negative-ion vacancy. Positive-ion vacancy. Coupled pair of vacancies of opposite sign. Cluster of two positive-ion vacancies and one negative-ion vacancy. [16]

Alkali halides can be colored by introducing F-centers by e.g. irradiation wth X-rays. [16] F-

center is the anion vacancy consisting a trapped electron. The name F-center comes from the

German word for color, farbe, as crystals containing these point defects are highly colored.

These color centers have been investigated by various spectroscopic techniques. The color is

the result of the absorption of a photon by the trapped electron inducing a transition from the

ground state to an excited state of the F-center. The color center has been studied

systematically in last century. The color centers give rise to prominent absorption bands in the

alkali halides. The negative-ion vacancy gives rise to the band. A negative-ion vacancy with

an electron is F-center and gives rise to for F or band. A negative-ion vacancy accompanied

with two electrons is F’ center. R1 and R2 centers are two aggregated F-centers with one and

two trapped electrons respectively. The M center consists of a pair of anion vacancies joined

with a cation vacancy. [16] See Fig. 1.7.

9

Fig. 1.6: Schematic of different vacancies and vacancy clusters in alkali halides. As displayed from upper left to

lower right and reading across, the units with square are: negative-ion vacancy; F-center; F’-center; R1-center; R2-

center and M-center (F2 center). The blue cycle represents a trapped electron.

1.6 Organic semiconductors

Organic semiconductors are -conjugated molecular materials. The molecules contain carbon

atoms with sp2-hybridisation. -bonds can form between two carbon atoms with sp2-

hybridisation. The electrons in the -orbitals are delocalized. There are two important energy

levels in organic semiconductors, one is the highest occupied molecular orbital (HOMO) and

the lowest unoccupied molecular orbital (LUMO). Similar to the conduction band and valence

band in inorganic semiconductors, the HOMO is involved in hole transport and LUMO in

electron transport. Recombination of an electron and a hole in organic semiconductors leads

to formation of an exciton which can decay rediatively and emit light. Similar to inorganic

semiconductors, electrons and holes can be injected in organic semiconductors. An important

difference between organic and inorganic semiconductors is the large number trap sites for

electrons due to impurities. [5]

In this thesis, we use three organic semiconductor. The Poly(3-hexylthiophene) has band gap

of Eg=1.9 eV. The band gap of phenyl-C60-butyric acid methyl ester (PCBM) is around 2.4 eV.

Poly(spirofluorene) has an energy gap between the HOMO and LUMO of 2.8 eV. [5] The chemical

structure of these polymers is shown in Fig. 1.7.

10

Figure 1.7: Chemical structure of a) Poly(3-hexylthiophene), b) PCBM and c) Poly(spirofluorene).

1.7 Aim of this project and hypothesis

The aim of this project is to study the electroforming step in metal/alkali halide/organic

semiconductor/metal diodes.

There are some benefits to use alkali halide as insulator layer. As shown in Fig. 1.6, the energy

of the conduction and valence bands can be varied by the choice of alkali halides. Additionally,

defect formation in alkali halide, as depicted in section 1.4.1, is well documented in literatures.

Considering that vacancies play an important role in metal oxides based MIM device, we

assume that the mechanisms are similar in both alkali halide and metal oxide based MIM

devices. The charged defects, F-center, can be traced easily by spectroscopy.

Different alkali halides and organic semiconductors will be used in this research. The

thickness of the layers will be varied. The changes in conductivity, absorption,

electroluminescence after electroforming will be investigated by using electrical and optical

methods. We find experimental evidence for formation of defects during application of bias

voltage. Defect formation seems to involve injection of charge carriers into the alkali halide.

11

Chapter 2 Experiments and Characterizations

2.1 Materials

Table 2.1: Materials supplier, purity and type of boat or crucible.

Material Supplier and purity Type of boat or crucible

Lithium fluoride (LiF) Sigma Aldrich 99.9% Alumina crucible

Cesium fluoride (CsF) Sigma Aldrich 99.9% Ta baffled or W

Cesium chloride (CsCl) Sigma Aldrich 99.9% Ta baffled

Cesium bromide (CsBr) Alfa Aesar 99.999% Ta baffled

Cesium iodide (CsI) Sigma Aldrich 99.999% Ta baffled or W

Sodium fluoride (NaF) Alfa Aesar 99.99% Ta baffled

Lithium chloride (LiCl) Sigma Aldrich, 99.9% Ta baffled

Rubidium fluoride (RbF) Sigma Aldrich 99.8% Ta baffled

Poly(spirofluorene) Merck

2.2 Device fabrication

All the devices are fabricated in a similar way. Glass substrates coated with a layer of indium

tinoxide (ITO) were used as the transparent bottom electrode for each device. Before making

device, each glass substrate has to be cleaned to remove any impurities that left on it. The

cleaning steps are: 15 minutes sonification in acetone, rubbing with soap, which is made in

ultra-pure water, and after this sonification in the soap for 15 minutes, rinsing with streaming

de-ionized water for 15 minutes and sonfication in isopropanol for 15 minutes.

Preparation of poly(spirofluorene) solution: 10mg poly(spirofluorene) and 1ml toluene

solution are mixed to get a polymer solution with 10mg/mL. The polymer solution was stirred

overnight at 60°C-65°C under dark environment.

After the cleaning step above, we put the glass substrates in a through and a boat filled with

alkali halide in a metal evaporator. The alkali halide was deposited by thermal sublimation

under high vacuum (less than 2*10-6 mbar normally) onto the entire substrate area. The

thickness and evaporation rate can be controlled by the control panel. Normally, the

evaporation rate is from 0.2 nm/s to 0.5 nm/s. The through with glass substrates rotates

during evaporation to make a uniform layer.

12

The 80nm poly(spirofluorene) is spin coated on top of the alkali halide covered glass substrate

in a N2 filled glove box by spin coating at 3500 rpm for 60 seconds. After the spin coating step,

we use acetone and cotton stick to remove the polymer from the area where the top electrode

will be contacted by the external circuit.

Top electrode is made by thermal sublimation under high vacuum (<2*10-6 mbar normally),

and the glass substrates with alkali halide and polymer are encapsulated with a special steel

mask. The top electrode consist two layers: a 5nm thick Barium followed by a 100nm

aluminum.

All the devices were stored and characterized in N2 atmosphere at all time because of the

hydroscopic properties of alkali halides.

2.3 Characterizations

2.3.1 Electrical properties and Electroluminescence

An Agilent 4155C semiconductor analyzer is used for measuring electrical properties and a

silicon photon diode sensor is used for detecting electroluminescence. To avoid the noise

from outside, the characterization is measured in dark environment. The bias is applied in a

cyclic manner and swept by 50 mV per step from zero volt. The silicon photon diode is put

under the diode to measure the electroluminescence from diode.

2.3.2 Thickness measurement

The thickness of each layer was measured with a “Veeco Dektak 150” surface profiler. We

scratch the area of device containing both polymer and alkali halide layers by knife, because

the stiffness difference between polymer and alkali halide, more polymer can be removed

than the alkali halide and forms a “step”. See Fig. 2.1. The thickness of both layers can be

determined by sweeping the probe. The problem of thickness measurement is the

hygroscopic property of alkali halides. When we take the diode out of the glove box, the

transparent device will turn to semi-transparent and the surface turns to rough in several

13

minutes and we can not get the acuurate thickness of alkali halide any more. Thus, the

thickness of device need to be measured as soon as possible.

Figure 2.1: Schematic graph of a scratched device.

2.3.3 Reflection measurement

With the Ba/Al device top electrode being highly reflective and the ITO bottom electrode being

transparent, the investigation of the absorption of the alkali halide/polymer layers can be

studied via specular reflection. Reflection measurements are carried out with a Lamda 900

spectrophotometer. We use the reflection configuration shown in Fig. 2.2a. The incoming light

goes through both the alkali halide layer and polymer layer, and being reflected on the Ba/Al

electrode and is again transmitted through the two active layers. The measurement method is

shown in Fig. 2.2b, the light coming from emitter 2 is used as reference light. The absorption of

our device can be obtained by taking the signal difference between detector 1 and detector 2.

The reflection measurement is characterized by Perkin Elmer Lambda 900. The diode is kept in

N2 atmosphere, using a closed box with electrical access.

Figure 2.2. a): optical path diagram in device. b): The structure of reflection measurement.

Scratch by knife

Reflected light

Emitter 1

Polymer

Incoming light

Emitter2

Alkali halide

ITO

Device

Glass substrate

Alkali halides

Detector1

poly(spirofluoren

e)

Detector2

Ba/Al

b a

14

Chapter 3 Optical Characterization of Diodes under

Bias Stress

3.1 Reflection measurement

A typical reflectivity-photon energy spectrum is shown in Fig.1.The diode structure is ITO/CsI

(100 nm)/poly(spirofluorene) (80 nm)/Ba/Al. When we apply +4 V bias to the diode, the

reflectivity of the diode decreases in comparison with the unbiased diode (red curves and

black curves in Fig 3.1) for certain ranges of photon energies. The higher applied bias, the

larger the decrease in reflectivity.

0.5 1.0 1.5 2.0 2.5 3.0 3.5

5

10

15

20

25

30

Re

fle

ctivity (

%)

Photon energy (eV)

Figure 3.1 Reflectivity of a ITO/CsI (100 nm)/poly(spirofluorene) (80 nm)/Ba/Al diode as a function of photon energy. Black curve: reflectivity of the pristine device. Red curve: reflectivity of device with applying 4v bias.

The differential reflectivity R is defined as: R=R0-R(x), the R(x) is the diode reflectivity under x

V bias voltage, and R0 is the reflectivity of a pristine diode under zero bias voltage. Then we

calculate 0/R R and in Fig.3.2, we get the

0/R R - photon energy relationship of Fig.3.1

15

1.0 1.5 2.0 2.5 3.0 3.5

0.00

0.02

0.04

0.06

0.08

0.10

R

/R0

Photon energy (eV)

R/R0 (2v)

R/R0 (3v)

R/R0 (4V)

Figure 3.2: ∆𝑅/𝑅0-photon energy characteristic of a ITO/CsI (100 nm)/poly(spirofluorene) (80 nm)/Ba/Al diode under different applied bias. R0 is the reflectivity of pristine diode.

Light travelling through the diode may be absorbed by defects introduced by voltage stress.

From the Beer-Lambert law, we have:

0

logI

AI

Equation 3.1

with I is the intensity of reflected light and I0 is the intensity of incoming light. Assuming that

application of bias leads to a change I in the intensity of reflected light. We get:

V 0

0 0 0

log( ) ( log ) log( )V

I I I I I I RA A A

I I I I R

Equation 3.2

The relative change in reflection ∆𝑅/𝑅0 can be interpreted as a voltage induced change in

absorbance. As shown in Fig. 3.2, the device does not show differential reflectivity at 2 V. When

the bias exceeds 3 V, there are 2 absorption bands, at 1.0 eV and at 2.0 eV respectively and the

absorption bands intensity increase with increasing bias.

16

3.2 Time dependence

As shown in Fig.3.3, we measure the reflectivity (R) of the ITO/CsI (100 nm)/poly(spirofluorene)

(80 nm)/Ba/Al diode at 1.0eV as function of time and apply a time dependent voltage. The

reflectivity is normalized to 100% before we apply bias. At 80s, a 4 V bias is applied to the

diode for 100s. Upon application of the bias, the reflectivity drops quickly in first 10 seconds,

then the rate of decrease slows down. At 180s the bias is withdrawn and the reflectivity

recovers in the following 50s. At long times, the reflectivity approaches the initial value before

application of bias. This shows that the voltage induced changes in reflectivity are reversible

and arise from a relatively slow process taking place on the time scale of seconds.

0 50 100 150 200 250 300 350 400

0

1

2

3

4

0 50 100 150 200 250 300 350 400

90

92

94

96

98

100B

Bia

s (

v)

A

R(%

)

time (s)

Figure 3.3. Time dependence of the differential reflectivity of a ITO/CsI (100 nm)/poly(spirofluorene) (80 nm)/Ba/Al

diode at 1.0eV. A): From 80s to 180s, 4v bias is applied on the diode, and in the rest time, the bias keeps at zero volt. B): Normalized reflection of the diode as function of time.

Based on the results above, we measure the time dependence of differential reflectivity of

ITO/CsI (100nm) /poly(spirofluorene) (80 nm)/Ba/Al diode at 1.0eV for various applied bias

voltages. As shown in Fig. 3.4a, the minimal R values for each applied bias are different. For

clarity, we normalized all the results from 0 to 1 in one picture (Fig. 3.4b).

The results in Fig 3.4b show that the rate of decrease of R becomes higher with increasing bias.

At higher bias, the R tends to saturate towards the end of the bias period. For the recovery, R

17

corresponding the lowest bias (4v) recovers fastest. The rate of recovery decreases with higher

bias.

0 200 400 600 800

96

97

98

99

100

0 200 400 600 800

0.0

0.2

0.4

0.6

0.8

1.0

R(%

)

Time(s)

4v

5v

7v

A

B

norm

aliz

ed

R

Time (s)

normalized 4v

normalized 5v

normalized 7v

Figure 3.4. Time dependence of reflectivity of a ITO/CsI (100 nm)/poly(spirofluorene) (80 nm)/Ba/Al diode at 1.0eV with varied bias (4v,5v and 7v).

To investigate the recovery and repeatability in more detail, we have studied the reflectivity

behavior from the diode while sweeping the applied bias in a cyclic manner. In each sweep the

bias is scanned starting at -2 V in 20 mV steps to 4V and then scanned back to the zero volt.

The reflectivity is measured at 1.0 eV and the current density is recorded at same time. In

forward sweep, the reflectivity only changes above certain bias voltage. In backward sweep,

the negative bias speeds up the recovery of reflectivity. Three voltage cycles were induced in

the measurement, see Fig.3.5, it is clear that the voltage induced change in reflectivity is

practically fully reversible.

18

0 500 1000 1500 2000

-2-101234

0 500 1000 1500 2000

95

100

0 500 1000 1500 2000

1E-101E-81E-61E-40.01

0 500 1000 1500 2000

-0.00009

-0.00006

-0.00003

0.00000

Bia

s (

v)

R(%

)

vcolor

curr

ent density (

mA

/cm

2)

Time (s)

Figure 3.5 Bias voltage dependence of reflectivity R and J-V characteristics of ITO/CsI (100 nm)/poly(spirofluorene)

(80 nm)/Ba/Al diode. From top to bottom: applied bias; Voltage dependence of R probed at a photon energy of 1eV;

positive part of J-V characteristics; negative part of J-V characteristics. The bias is sweep from -2v to 4v and start at

100s.

3.3 Influence of Alkali Halides on Coloration

ITO/alkali halide (100nm)/poly(spirofluorene) (80 nm)/Ba/Al diodes with 9 different alkali

halides were investigated. The differential reflection-photon energy characteristics are shown

in Fig.3.6. Data for LiF, KBr and KI are taken from the thesis of Benjamin Bory [23]. All the diodes

show similar behavior: differential reflectivity 0/R R is induced above a certain bias and then

increase with increasing voltage.

We discern two main bands in the voltage induced reflectivity spectra of the alkali halide-

polymer diodes. The first is around 2 eV, the second at 1 eV. For alkali halides with high band

gap, the 2 eV band has the highest intensity. For alkali halides with smaller band gap the 1 eV

band is the predominant feature in the differential reflection spectra. For alkali halides with

intermediate band gap, the intensity of the 1 eV band increases relative to the 2 eV band at

higher voltage. We take the ITO/KF (100nm)/poly(spirofluorene) (80 nm)/Ba/Al diode as an

example. The intensity of absorption band around 1 eV increases relative to the intensity of

the band at 1.9 eV with increasing voltage. This is illustrated in Fig. 3.7, by showing the

19

absorption normalized to unity at the 2 eV band. Thus at high voltage, the absorption band at

1 eV rises more steeply with increasing voltage than the 2 eV band.

1.0 1.5 2.0 2.5 3.0 3.5

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

1.0 1.5 2.0 2.5 3.0 3.5

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

1.0 1.5 2.0 2.5 3.0 3.5

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

1.0 1.5 2.0 2.5 3.0 3.5

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

1.0 1.5 2.0 2.5 3.0 3.5

-0.04

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

0.05

1.0 1.5 2.0 2.5 3.0 3.5

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

R

/R0

Photon energy (eV)

R/R0 6v

R/R0 8v

R/R0 10v

R/R0 12v

LiF

LiCl

NaF

R

/R0

Photon energy (eV)

R/R0 1v

R/R0 9v

R/R0 10.5v

R/R0 11v

R

/R0

Photon energy (eV)

R/R0 2v

R/R0 6v

R/R0 8v

KF

R

/R0

Photon energy (eV)

R/R0 5v

R/R0 6v

R/R0 7v

R/R0 8v

CsFRbF

R

/R0

Photon energy (eV)

R/R0 2v

R/R0 6v

R/R0 8v

R

/R0

Photon energy (eV)

R/R0 2v

R/R0 8v

R/R0 10v

R/R0 11v

Figure 3.6 Differential reflectance-photon energy characteristics of ITO/Alkali halide (100nm)/poly(spirofluorene)

(80nm)/Ba/Al diodes with different alkali halides layers under bias, the alkali halide is indicated in the upper left corner. For LiCl the layer thickness is 50nm.

1.0 1.5 2.0 2.5 3.0 3.5

-0.04

-0.02

0.00

0.02

0.04

0.06

1.0 1.5 2.0 2.5 3.0 3.5

-0.05

0.00

0.05

0.10

0.15

0.20

1.0 1.5 2.0 2.5 3.0 3.5

0.00

0.05

0.10

1.0 1.5 2.0 2.5 3.0 3.5

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

1.0 1.5 2.0 2.5 3.0 3.5

0.00

0.02

0.04

0.06

0.08

0.10

R

/R0

Photon energy (eV)

R/R0 2V

R/R0 3V

R/R0 4V

KI

KBr

R

/R0

Photon energy (eV)

R/R0 2V

R/R0 4V

R/R0 6V

R/R0 8V

R

/R0

Photon energy (eV)

R/R0 2V

R/R0 4V

R/R0 6V

R/R0 8V

CsBr

R

/R0

Photon energy (eV)

R/R0 4V

R/R0 5V

R/R0 6V

R/R0 7V

CsI

R

/R0

Photon energy (eV)

R/R0 2V

R/R0 3V

R/R0 4V

CsCl

20

1.0 1.5 2.0 2.5 3.0 3.5

No

maliz

ed

R

/R0

Photon energy (eV)

R/R0 (5V)

R/R0 (6V)

R/R0 (7V)

R/R0 (8V)

0

1

Figure 3.7 Normalized Differential reflectance-photon energy characteristics of ITO/KF (100nm)/poly(spirofluorene) (80 nm)/Ba/Al diodes. Spectra are normalized such that the 2 eV absorption band has unit intensity.

3.4 Determination of the coloration voltage

As discussed in the sections above, the reflectivity of ITO/alkali halide/poly(spirofluorene) (80

nm)/Ba/Al diodes will change only for bias voltages above a certain threshold. We will call this

bias the coloration voltage Vcolor. We introduce two methods to determine the coloration

voltage.

The first method involves determining the bias voltage for which 0/R R exceeds 0.04.

The second method involves measuring differential reflectivity at a certain wavelength during

a voltage sweep (around 10 mV). As illustrated in Fig. 3.5, a value for the coloration voltage can

obtained by extrapolating the baseline of the reflectivity at 100% during zero bias and also

extrapolating the linear part of the decrease of the reflectivity with voltage. The voltage where

the two extrapolated lines cross is taken as the value for the coloration voltage.

The two values for the coloration voltage obtained by the two different methods are shown in

Fig. 3.8 for ITO/alkali halide/poly(spirofluorene) (80 nm)/Ba/Al diodes with different alkali

21

halides. Vcolor for different alkali halides is plotted versus the energy of the valence band edge

EVB of the alkali halide.

The coloration voltages determined by the two methods are not exactly same. Yet with an

experimental error on the order of 1V in the 0/R R ≥ 0.04 method, the results of the two

methods are consistent. For alkali halides with EVB≤8 eV, the coloration seems independent of

EVB. For alkali halides with EVB≥8 eV, we notice a correlation between coloration voltage and

valence band energy.

5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

2

4

6

8

10

12

14

Rb

F

CsC

lKB

r

KI CsB

r Method R/R

0>0.04

Method intersect

co

lora

tio

n v

olta

ge

(v)

valence band energy (eV)

CsI

CsF

LiC

l

KF

Na

F LiF

Figure 3.8 Different coloration voltages of ITO/Alkali halides (100nm)/poly(spirofluorene) (80 nm)/Ba/Al diodes as

function of valence band energy.

3.5 Influence of thickness of alkali halide layer.

We use the second method involving the intersection point to determine the coloration

voltages of ITO/CsI/poly(spirofluorene) (80 nm)/Ba/Al diodes with different thickness of CsI

(50nm, 100nm, 200nm and 500nm). Results are shown in Fig.3.9. The coloration voltages for

diodes with different thickness various between 3 V and 4.5 V. We conclude that within the

22

experimental uncertainty, there is no systematic variation of the coloration voltage with the

thickness of the alkali halide.

0 100 200 300 400 500

0

1

2

3

4

5

Vco

lor (V

)

CsI thickness (nm)

Figure 3.9 Coloration voltages of ITO/CsI/poly(spirofluorene) (80 nm)/Ba/Al diodes with different thickness of CsI layer.

3.6 Influence of the organic semiconductor

The chemical nature of the organic semiconductor in the ITO/CsI/polymer/Ba/Al diodes was

varied. Apart from the poly(spirofluorene) polymer also poly(3-hexylthiophene) (P3HT) and

(6,6) phenyl-C61-butyric acid methyl ester (PCBM) were used as organic semiconductor.

-0.02

0.00

0.02

0.04

0.06

-0.02

0.00

0.02

0.04

1.0 1.5 2.0 2.5 3.0 3.5

0.00

0.02

0.04

0.06

0.08

0.10

Photon energy (eV)

R

/R0

P3HT 4V

R

/R0

PCBM 4V

R

/R0

poly(spirofluorene) 4 V

Figure 3.10 Differential reflectance-photon energy spectra characteristics for ITO/CsI/organic polymer/Ba/Al diodes.

From top to bottom, red line: P3HT(80nm), blue line: PCBM(80nm), black line: poly(spirofluorene) (80nm).The vertical arrows indicate the optical bandgap of the organic semiconductor

23

As shown in Fig.3.10, all three organic semiconductors give rise to voltage induced differential

reflectivity in the region from 0.75 to 1.5 eV. The sign of R/R0 corresponds to reduced

transmission, indicating voltage induced absorption bands in this spectral region. Data for

P3HT are shown in the top panel of Fig. 3.10. P3HT has an optical bandgap of 1.9 eV and the

voltage induced absorption band at 0.9 eV is in the subgap region where the neutral polymer

does not absorb. In this subgap region absorption could arise from charge carriers on the

polymer, resulting in polaron and/or bipolaron absorption bands [14]. Also defects in the alkali

halide such as F centers and aggregated F centers, could contribute to the induced absorption

[15]. In the spectral region above the bandgap where the neutral P3HT polymer absorbs light

with a maximum around 2.2.eV, we find also voltage enhanced absorption. The fact that no

bleaching of the absorption (R/R0 <0) of the neutral polymer is observed, indicates that the

number of polaronic charges on the polymer is too low to reduce the absorption of the neutral

polymer. Positive polaronic charge carriers on P3HT induce two main subgap absorption

bands at 1.2 and 0.35 eV [31]. In the region 0.8-1.0 eV the polaron absorption is at a minimum.

Hence, the voltage induced absorption band in the P3HT/CsI diodes at 0.8 eV cannot be

assigned to positively charged polarons. Considering the polarity of the applied bias, one

would expect negatively charged polarons of the P3HT polymer. The subgap absorption bands

of the negative polarons are not well characterized. Based on electron-hole symmetry one

expects that negative polaron should have absorption bands similar to positive polarons.

Experimental studies on poly(paraphenylenevinylene) support approximate electron-hole

symmetry [31,32]. Based on the electron-hole symmetry argument, assignment of the voltage

induced absorption band at 0.8 eV to negative polarons on the polymer seems questionable.

The middle panel of Fig. 3.10 shows the voltage induced differential reflectivity of diodes

incorporating PCBM as organic semiconductor. PCBM has an optical gap of 1.74 eV. The

voltage induced differential reflectivity, shows two weak bands at 1.3 and 0.9 eV. The 1.3 eV

band may indicate the presence of PCBM-. The PCBM anion has an absorption band of

moderate intensity at 1.2 eV [34]. The feature at 0.9 eV can however not be attributed to the

anion of the fullerene derivative.

24

Finally, the lower panel in Fig 3.10 shows the differential reflectivity for the

ITO/CsI/polyspirofluorene/Ba/Al diodes. The absorption bands of the negative polaron on

polyfluorene polymers have not been characterized; the positive polaron features two broad

absorption bands of roughly equal intensity at 2.0 and 1.0 eV [35].

The voltage induced reflectivity of the CsI/polyfluorene diode features a main band at 0.8 eV.

Because a corresponding equally intense feature around 2 eV is missing, assignment of the

voltage induced reflectivity to negative polarons is problematic. We also note that the

differential reflection for CsI/polyfluorene diode does not show bleaching in the region of

polymer absorption (3.2 eV).

Incidentally, we note that for ITO/LiCl/polyfluorene/Ba/Al diodes, very intense voltage

induced reflectance can be measured with relative change in reflection approaching 50 % (see

Fig 3.11). This indicates extensive defect formation in this type of diode. For this particular

diode, negative differential reflectance in the spectral region corresponding to absorption by

the neutral polymer at 3.0 eV can be observed. This bleaching indicates that, especially at high

defect concentrations, the polymer may become charged.

1.0 1.5 2.0 2.5 3.0 3.5

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

R

/R0

ev

2 V

4 V

5 V

6 V

7 V

8 V

8.5 V

9 V

9.5 V

10 V

ITO/LiCl(50 nm)/polyfluoren/Ba/Al

Figure 3.11 Differential reflectance-photon energy spectra for a ITO/LiCl(50nm)/polyfluorene/Ba/Al diode as function of applied bias voltage.

In summary, variation of the organic semiconductor in the ITO/CsI/organic

semiconductor/Ba/Al diodes shows that voltage induced changes in reflectivity are a general

phenomenon and are not associated with a specific type of organic semiconductor.

25

Assignments of the voltage induced differential reflection bands to specific voltage induced

defects, can presently not be made with certainty. We conclude that the voltage induced

changes in reflectivity are probably due to a variety of defects and charges in the halide and

on the organic semiconductor.

3.7 Correlation with electrical properties

We have seen that the reflectivity decreases when applying bias voltage to the diode, and that

the differential reflectivity recovers to the original state after withdrawal of the bias. We now

investigate any correlation between the bias voltage induced optical response and the

current-voltage characteristics of the diode. To realize this goal, the diode is subjected to a

cyclic bias voltage sweep. In each sweep the bias is scanned between-2 V and 4 V in steps of 20

mV/sec. The bias voltage applied is shown in the top panel of Fig.3.5. The differential

reflectivity is measured at 1.0 eV and current density is recorded simultaneously.

From Fig.3.5, the differential reflectivity starts to decrease at the coloration voltage (3 V). The

current density shows a marked upturn for voltages above the coloration voltage and follows

exp( )J V . In Fig.3.12, we show results for an of ITO/CsBr (100 nm)/poly(spirofluorene) (80

nm)/Ba/Al diode. Also for this diode, the correlation between induced differential reflectance

and charge transport.

From both Fig.3.5 and Fig. 3.12, we find that the current density reaches its largest negative

value close to zero volt. When the bias voltage crosses over to negative voltages the rate of

recovery of the reflectance shows an acceleration. We conclude that application of negative

bias speeds up the recovery.

When the bias is lowered to about 3 V at 700 sec, the current changes sign and becomes

negative. This shows that the diode acts as a battery. We notice that a voltage of 3 V

corresponds to the polymer (3 eV).

26

200 300 400 500 600 700 800 900 1000

-4

-2

0

2

4

6

200 300 400 500 600 700 800 900 1000

94

95

96

97

98

99

100

101vo

ltag

e (

V)

J<0

R

(%)

Time (s)

J>0

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

cu

rren

t d

esity (

mA

/cm

2)

Figure 3.12: Voltage dependence of R and J-V characteristics of ITO/CsBr (100 nm)/poly(spirofluorene) (80

nm)/Ba/Al diode; blue curve: Voltage dependence of R probed at a photon energy of 1eV; red curve: positive part

of J-V characteristics, green curve: absolute value of negative part of J-V characteristics. The bias voltage is swept from -3 V to 5V.

The negative part of current density is shown as green line in Fig.3.12. In absolute magnitude,

the negative current density first increases and then decreases. We interpret this as extraction

of charge carriers that were accumulated in the diode under forward bias. We can calculate

the amount of carriers in before electroforming. We choose the a ITO/CsI (100

nm)/poly(spirofluorene) (80 nm)/Ba/Al diode in Fig. 3.5 as an example.

We first integrate the negative current part over time, and get a surface density of electric

charge: (C)f

i

t

tQ Jdt

We use this method to calculate the charge densities for Al/alkali halide /poly(spirofluorene)

(80 nm)/Ba/Al diodes with different CsI thickness and with different alkali halides (CsCl and

CsBr 100nm). The results are listed in table 3.1. The diodes with very thin CsI layers

electroform very readily and the current density cannot be analyzed in the same way.

27

Table 3.1

Surface charge density of Al/alkali halide/poly(spirofluorene) (80 nm)/Ba/Al diodes with different alkali halides and different thickness associated with storage of charge.

Alkali

halide

Thickness of alkali

halide (nm)

Bias voltage (V) Sweep rate

(mV/sec)

Surface charge density

(C/cm2)

CsI 100 -2 to 4 20 7×10-6

CsI 200 -3 to 5 20 5×10-6

CsI 500 -3 to 5 20 12×10-6

CsCl 100 -3 to 4.5 20 22×10-6

CsBr 100 -3 to 5 20 11×10-6

As can be seen in table 3.1, the typical surface charge density in a Al/alkali

halide/poly(spirofluorene) (80 nm)/Ba/Al diode is around 10-5 (C/cm2).

3.8 Summary

Al/alkali halide/polymer/Ba/Al structure diodes with different alkali halides and polymers are

investigated optically and electrically. The coloration voltage Vcol varies with the chemical

nature of the alkali halide and the valence band energy. The recovery of the original reflection

is associated with the extraction of charge out of the diode.

28

Chapter 4 Electrical Characterization

In this chapter, we investigate the electrical properties of the alkali halide-polymer diodes and

use results for CsI / poly (spirofluorene) diodes as example.

4.1 Electroforming process

Fig 4.1 shows the current density through an ITO/CsI (100 nm)/poly(spirofluorene) (80

nm)/Ba/Al diode during a cyclic voltage sweep. The pristine diode is an insulating diode with

low current density in the low bias voltage region.

When the bias reaches 6.3 V, the current density of the device increases dramatically. In the

backward sweep, the current density at low bias is high indicating high conductivity. In

subsequent sweeps the current density at low bias remains high.

The sudden increase in current that occurs in pristine and highly insulating diodes is known as

electroforming. The voltage at which the surge in conductivity occurs, 6.3 V in this particular

case, is defined as the electroforming voltage, Vform .

During the electroforming measurement illustrated in Fig 4.1 we simultaneously recorded the

electroluminescence intensity using a silicon photodiode. The signal is shown as black trace in

Fig.4.1. Above the electroforming voltage (6.3 V in Fig. 4.1), the diode emits light, indicting

radiative recombination of electrically injected electrons and holes.

-1 0 1 2 3 4 5 6 7 8 9

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

10

Curr

ent density (

mA

/cm

2)

bias (V)

Vform

2E-12

4E-12

6E-12

Ele

ctr

olu

min

escence inre

nsity

Figure 4.1: Current density-voltage (blue trace) and electroluminescence intensity-voltage characteristics (black

trace) for a pristine ITO/CsI (100 nm)/poly(spirofluorene) (80 nm)/Ba/Al diode. Bias sweep to 8 volt.

29

The electroforming voltages were determined using diodes with general structure ITO/alkali

halide (100nm)/poly(spirofluorene) (80 nm)/Ba/Al. The electroforming voltage Vform is taken as

the voltage at which an irreversible rise in current density occurs. When measuring different

diodes with the same nominal structure, we find a relatively broad distribution for Vform and

statistical averaging was performed. Diodes that show high conductivity already in pristine

state due to e.g. shorts are not included in the determination of the electroforming voltage. A

further criterion in determining Vform is that the current density after the electroforming has to

exceed ~ 10-3 mA/cm2 Electroforming voltages show only a moderate dependence on the

thickness of the alkali halide layer (in Fig. 4.2), consistent with earlier studies on LiF polymer

diodes [Benjamin Thesis]

40 60 80 100 120 140 160 180 200 220

0

2

4

6

8

10

Ele

ctr

ofo

rmin

g V

olta

ge

(V

)

Thickness (nm)

Figure 4.2: A): Electroforming voltage of Al/CsI/poly(spirofluorene) (80 nm)/Ba/Al diodes with different Cesium

iodide thickness devices.

4.2 Negative differential resistance and resistive switching

The J-V characteristic of an electroformed ITO/CsI (100 nm)/poly(spirofluorene) (80 nm)/Ba/Al

diode is shown in Fig.4.3. We distinguish three different regions (figure 4.3). Firstly, the current

density starts with a high value (0.2 mA/cm2 at 1V) compared with the one before

electroforming (~10-7mA/cm2 at 1 V). In the bias region from 0 V to 3 V, the current density rises

approximately linear with bias. From 3 V to 7 V, the current density decreases with increasing

30

voltage and the diode thus shows negative differential resistance (NDR) [4]. In this intermediate

voltage range the diodes shows eratic electroluminescence. The luminescence occurs in

bursts. Thirdly, if we increase the voltage from 7 V to 10 V in Fig.4.3, the current density

increases again with bias voltage. In this third high voltage range, the diode shows steady

electroluminescence. When the bias is swept back to 0 V, the current and electroluminescence

approximately show the same behavior as during the outward scan for 0 to +8V.

0 2 4 6 8 10

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

10

Cu

rre

nt

de

nsity(m

A/c

m2)

bias (volt)

1E-12

1E-11

1E-10

1E-9

Ele

ctr

olu

min

escence in

ten

sity (

A)

Figure 4.3: Current density-voltage (blue line) and electroluminescence current-voltage characteristics (black line)

for a ITO/CsI (100 nm)/poly(spirofluorene) (80 nm)/Ba/Al diode. Bias sweep to 10 V after electroforming step (figure

4.1).

4.3 Electronic Memory Effects

The electroformed CsI/polymer diodes show non-volatile memory effects. The resitance of the

diode at 0.5 V can be programed reversibly into high resistance “off” states and low resistance

“on” states by application of voltage pulses. A voltage programing cycle to measure the

memory effects is illustrated in Fig. 4.4a. A “write” voltage with 4.5 V is applied on the diode for

100 ms to switch the diode to the “on” state; then the current density of diode is probed twice

with two 0.5 V “read” pulses with 100 ms duration. After the read pules, an “erase” pules with

7 V is applied on the diode for 100 ms to switch the diode to “off” state. Finally, the current

density of diode is probed twice with two 0.5 V “read” voltages for 100 ms respectively. There

is a 50 ms time interval between each operation during which the bias is kept at zero bias. The

current density measured using the 0.5 V read pulses is shown in the Fig. 4.4b. The diode can

31

be switched repeatedly and the difference of current density between “on” and “off” states is

around one order of magnitude.

A volatile memory will lose all the information when no bias voltage is applied. In our case, the

resistance of a particular state survives in the 50ms time interval of zero bias voltage. Thus we

conclude our diode is a non-volatile memory and can be read out nondestructively.

0 2 4 6 8 10 12

0.01

0.1

2 4

0

1

2

3

4

5

6

7

vo

ltag

e (

V)

time (s)

...

a 100ms

b on state

off state

cu

rren

t d

en

sity (

mA

/cm

2)

time (s)

Figure 4.4. Memory effect characterization of an electroformed Al/CsI (100 nm)/poly(spirofluorene) (80 nm)/Ba/Al

diode .a): Write-read-read-erase-read-read voltage cycle for testing memory effects in alkali halide/polymer diodes. b): Current density of ITO/CsI/ poly(spirofluorene)/Ba/Al diode measured during the 0.5 V read pulses.

In summary, like many other metal/insulator/metal diodes incorporating metal oxides as

insulators, our metal/CsI/polyfluorene/metal diodes show electroforming. After

electroforming the diodes show resistive switching and display non-volatile electronic

memory effects.

4.4 Influence of alkali halide on the electroforming

Also for ITO/alkali halide/polymer/Ba/Al diodes with alkali halide other than CsI, we find

electroforming, negative differential resistance and electronic memory effects. We

investigated 10 different alkali halides and determined the electroforming voltage for diodes

32

with structure ITO/Alkali halide(100nm)/poly(spirofluorene) (80 nm)/Ba/Al. Criteria for

determining Vform were the same as mentioned in section 4.2.

The electroforming voltage determined for diodes incorporating different alkali halides are

plotted in Fig.4.4 versus the energy of the valence band edge of the alkali halide included in

the diode.

5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

2

4

6

8

10

12

14

16

18

Rb

F

CsI

Vfo

rm (V

)

EVB

(eV)

CsB

r

KI

CsC

l

LiC

l

CsF

KF

Na

F

LiF

Figure 4.4: Average electroforming voltage with standard deviation as a function of the valence band energy level

of the alkali halides studied. The electroforming voltage of LiF and KI are taken from [Benjamin’s thesis].

From Fig. 4.4, we conclude that the electroforming voltage of the diodes shows a correlation

with the valence band energy of the alkali halide which is used. For the alkali halides with

valence band edge energy EVB < 8.5 eV, the electroforming voltage seems independent of the

valence band energy. However for the alkali halide with deep valence bands (EVB 9 V) there is

clearly an increase of the electroforming voltage with high band edge energy. We note that the

dependence of electroforming voltage on valence band energy is similar to the dependence of

coloration voltage on valence band energy. This will be discussed in more detail in the

following section.

33

Chapter 5 Discussion

In chapter 4, optical and electrical properties of ITO/alkali halide/polymer/Ba/Al diodes were

investigated. It was found that for bias voltages higher than coloration voltage Vcol, defects are

induced in the diode. Furthermore, for bias voltage exceeding the electroforming voltage Vform,

electroforming is induced. Comparing different alkali halides it was found that both Vcol and

Vform correlate with the energy of valence band edge of the alkali halide. In this section, we

discuss a mechanism for defects formation in the alkali halide that can account for the

correlations of electroforming and coloration voltage with the valence band edge.

5.1 Storage of charges in diodes

From Fig. 3.5, we inferred that charges are stored in the diode. The surface density of stored

charge in the CsI/polymer diode after applying 4 V bias is estimated at 7×10-6 (C/cm2) in table

3.1. The area of this diode is 0.09 cm2, and the charge stored in the diode is Q= 6×10-7 (C). Using

the relation Q=CV we can calculate a capacitance for the Al/CsI (100 nm)/poly(spirofluorene)

(80 nm)/Ba/Al diode. This yields:

7 7 / 6 10 / 4 1.5 10 .realC Q V C V F

If we regard the diode as a simple metal/insulator/metal capacitor with both alkali halide and

polymer as insulator, we can calculate the geometrical capacitance of the diode, using the

relative dielectric constant 6.59CsI , 3.2polymer [4]. Using:

0 /rC A d

for each layer and

1

1 1 1

geo MX polymerC C C

for the bilayer stack of alkali halide (MX) and polymer we get9

geo1 2 10 (F)C . This value

for the capacitance is much smaller than 71.5 10 (F)realC which is estimated above.

34

In the diode, the poly(spirofluorene)/Ba/Al is a ohmic contact for injection of electrons into the

organic semiconductors [4,17]. The alkali halide is nominally a high band gap insulator.

Assuming fast transport of electrons from the Ba/Al contact to the alkali halide/polymer

interface, we make a new estimate of the geometrical capacitance. Using:

2geo MXC C we get9

2 5 10 (F)geoC .

This value is higher than 1geoC , but still much smaller than realC . This shows that much more

charge can be stored in the diode than expected for an insulating alkali halide layer.

Earlier experiments have shown that electrons can be stored at the alkali halide/polymer

interface. To explain the high capacitance for the diode, we propose that positive charges can

be stored in the alkali halide layer. This would lead to formation of a charge double layer at

the alkali/halide interface, see Fig. 5.1.The distance between the two charge layers may be

estimated using the relation: 0 /r reald A C , this yields 93 10 (m)d . This indicates that

the positive charges in the alkali halide are stored close to the alkali halide/polymer interface.

Looking at the band diagram for the alkali halide-polymer junction, Fig.5.1. We note that

because of the band offset of alkali halide/polymer valence bands, holes in the valence band

of alkali halide are not stable but would recombine immediately with electrons in the

conduction band. To explain the stability of the holes in the alkali halide, we argue that the

holes are trapped at defect sites in the alkali halide.

35

Figure 5.1: Schematic band structure of a charged diode which V>Vcol. The squares indicate trap levels for hole and electrons.

5.2 Defect formation in alkali halide

In the previous section, it was concluded that defects that can trap hole must be present in the

alkali halide. From the optical measurements we found evidence for defects formation in the

alkali halide/polymer diodes upon application of bias. We now argue that the defects in the

alkali halide that can trap positive charges are formed during application of bias and can be

detected optically. This explanation accounts for the correlation between optical and

electrical transients of alkali halide/polymer diode recorded during application of bias.

Formation of defects in alkali halide has been studied intensely and it was found that optical

excitation of the alkali halide can results in formation of defects [4,16]. The mechanism for

defect formation proposed in literature is as follows. Optical excitation leads to formation of

an exciton in the alkali halide. The exciton can relax further in energy to form so called a self-

trapped exciton. This involves displacement of a halide atom away from its equilibrium

position. See Fig. 5.2.

36

Figure 5.2: Schematic of defects formation in alkali halides, retrieved for Ref. 4. From left to right: e-h

recombination; Self-trapped exciton; F-H defect pair.

Further separation of the halide atom away from the vacancy it has left behind, results in

formation of a defect pair, consisting of an F-center or anion vacancy and an H-center or

halide interstitial. In the charge neutral state, the F-center itself accommodates an electron.

This electron gives arise to optical transitions, from which the name of the defect center

derives (F is Farbe). Upon the removal of the electron, the defect becomes positive charged. F-

centers can occur as isolated defects but can also aggregate to form larger clusters of defects.

The optical transitions of these clusters are in the visible and near infrared spectral.

To explain the voltage induced coloration and charge trapping in the alkali halide/polymer

diodes we propose that injection of electrons and holes into the alkali halides leads to

formation of excitons that can form defects.

Applying positive bias to the diodes, electrons are injected in the polymer and drift towards

the alkali halide/polymer interface. At higher bias the electrons might tunnel into the alkali

halide. This causes electrical current. If now also a hole is injected into the alkali halide via the

ITO electrode, electron-hole recombination can occur and defects can be induced in the alkali

halide. We propose that F-type defects forms and can become positively charged. The

formation of defects induces optical absorption and enhance the tunneling of electrons form

the polymer into the alkali halide.

A key step in this reaction sequence is the injection of holes in the alkali halide. Given the large

offset between the work function of the ITO electrode and the valence band edge of the alkali

halide, we expect a considerable barrier for injection of holes. Therefore the hole injection

might be the rate limiting step in the defects formation.

e

-

e

- e-

h+

37

The proposed injection of holes into the alkali halide can explain the dependence of the

coloration voltage on the energy of valence band energy edge of the alkali halide. For alkali

halides with deep valence band, hole injection will be difficult and is expected to occur at only

high voltages. Because the injection of holes is required for defects formation and because the

defects color the material, the coloration voltage is expected to depend on the valence band

energy of alkali halide.

5.3 Electroforming and defects formation

Electroforming happens at the voltage higher than the coloration but the difference can be

small. Coloration involves defect formation and each trapped charge is located on a defect.

The surface charge defects density has been calculate above. And so a lower limit for the

defect concentration can be estimated form density of trapped charge, determined in Section

3.7.

The relationship between total number of electrons (N) and surface charge density (Qs) is:

191.602 10

sQ AN

with area of diode A=0.09 cm2. The defects density before electroforming is:

/volumeD N V withV A d , d is the thickness of alkali halide which is 100nm. In table 3.1,

the surface charge density of CsI (100nm)/polymer diode Q= 7×10-7 (C), and the density of

defects before electroforming Dvol= 5×1024 (N/m3).

The metal-insulator transitions (MIT) phenomenon has been studied intensively in the past

decades. A huge changes in resistivity, even over tens of orders of magnitude can be achieved

in a semiconductor by increasing the doping above a certain density Nc. [36,37] According to

previous study, the critical dopant concentration Nc for GaN which leads MIT is from 1024 to

1026 (N/m3) [36]. Thus we argue the critical defect density for insulator-metal transition of alkali

halides Nc is around 1024 (N/m3).

A schematic microscopic view on the electroforming is shown in Fig. 5.3. When the defect

density reaches the Nc, a highly conductive path is formed at the electroforming voltage for

transport of the charge carriers. Because a defect can accommodate a charge, the

transportation of charges though whole alkali halide layer can be realized via hopping

38

between defects. And the high conduction state at electroforming voltage involves

percolation paths also referred as filaments.

Figure 5.3: Schematic of percolation pathway formation by defects in alkali halide layer. The transport pathways of

carriers are indicated by orange arrows.

The electroforming process depends on the concentration of defects, and the formation of

defects depends on the injection of holes as discussed in previous section. This establishes a

correlation between the electroforming voltage and the valence band edge of alkali halide

shown in Fig. 4.4.

Before electroforming At electroforming

Alkali halide layer Alkali halide layer

Defects Defects

39

Chapter 6 Conclusion

Resistive switching is a phenomenon in which the resistance of a material suddenly changes

under the application of voltage or current. Resistive switching can induce a non-volatile

change in the resistivity which is a candidate for next generation information storage

technology. People realize resistive switching via different materials, e.g. metal oxides and

polymers, and people find that a step named electroforming is required to induce resistive

switching.

The aim of this report was to investigate the dependence of coloration and electroforming on

the nature of alkali halide based on the polymer-alkali halide diode structure.

We investigated different alkali halide-polymer diodes by varying alkali halides and polymers.

All diodes show electroforming and coloration. The correlation between

electroforming/coloration voltage and the valence band energy of alkali halide. Indicates that

injection of holes and electrons leads to formation of defects. When the defect concentration

exceeds a threshold. Conductive pathway could established leading to a dramatic change in

resistivity.

40

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42

Acknowledgements

I’d like to use the last pages to thank people who accompanied me in this ten months

graduation project. Without their help, I cannot finish this interesting thesis.

First and foremost I wish to thank my supervisor, Dr. Stefan Meskers. Thank you for giving me

the opportunity to finish my Mater graduation project in this excellent group. You always are

patient to answer my numerous (maybe stupid) questions since the beginning of my project.

The way you explain difficult theories and the acute perception in research have left a deep

impression on me. I still remember that you, Benjamin and me sited around the UV-Vis

machine and expected the experimental results. It is your encouragement that gave me the

power to finish this project. Except the research, I appreciate a lot for your guidance in my

future choice. I really enjoyed the talk between us.

Dr. Benjamin Bory: Sorry for bothering you almost every day. Thanks for your daily guidance

in my research. Your patience and suggestions made my project having fruitful results. I really

appreciate that you can involve in my project. It is an unforgettable experience in your PhD

defence and the celebration party. Do you like the painting I sent it to you? Do you understand

the painting process now? I wish you lots of prosperity and good luck in your future.

M2N members: It is my honor to be a member in your family. I really enjoyed my life in the

group in the past ten months. Prof. Rene. Janssen, thanks for your suggestions and

comments in the lunch meetings. I really admire your broad knowledge in different fields.

Martijn, I sincerely acknowledge you for taking part in my committee. Alice, Dhritiman, Tom,

Sandra, Koen, Hans and Serkan, thanks for your guidance and patience during my project.

Especially for Serkan, sorry for using all the LiF in the boat and I guess you spend a lot time on

filling it. I would like to sincerely thank our group secretary Jolanda for arranging everything

very well.

I would like to thank my officemates in SMO. Martijn and Luuk. You both are especially

acknowledged for the help of my Dutch class. I really enjoyed our daily life in STO 4.80. We

share our stories in the fields of culture, food and sport. I am looking forward to meeting with

you in the future.

43

I’d like to use the rest pages to thank my friends who live in China. They gave me a lot of help

in my foreign life.

Finally, I would like to acknowledge my parents and family that support me studying abroad

and always give the faith to go forward.

Jingxin Wang

08-10-2014, Eindhoven, The Netherlands.