printed schottky diodes based upon zinc oxide materials694402/fulltext01.pdfschottky diode is used...

Department of Science and Technology Institutionen för teknik och naturvetenskap Linköping University Linköpings universitet

gnipökrroN 47 106 nedewS ,gnipökrroN 47 106-ES

LiU-ITN-TEK-A--13/073--SE

Printed Schottky Diodes basedupon Zinc Oxide Materials

Emma Persson

2013-12-18

LiU-ITN-TEK-A--13/073--SE

Printed Schottky Diodes basedupon Zinc Oxide Materials

Examensarbete utfört i Teknisk fysikvid Tekniska högskolan vid

Linköpings universitet

Emma Persson

Handledare Negar Abdollahi SaniExaminator Isak Engquist

Norrköping 2013-12-18

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© Emma Persson

Printed Schottky Diodes Based Upon Zinc Oxide Materials

Master’s Thesis 30 hp

Emma Persson

December 19 2013 ver. 1.0

Contents

1 Introduction 5

2 Theory and background 6

2.1 Printed electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.2 ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.1 The tetrapod shaped nanocrystal ZnO . . . . . . . . . . . . . . . . . . . . 72.3 Schottky diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3.1 Schottky junctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.3.2 Ohmic contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.3.3 I-V characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.3.4 C-V characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.4 Rectifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.4.1 Half wave rectifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.4.2 Full wave rectifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3 Experimental details 17

3.1 ZnO mix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.2 Electrode materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.3 Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.4 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4 Results and discussion 20

4.1 The effect of different ZnO concentration . . . . . . . . . . . . . . . . . . . . . . 204.2 Screening of suitable electrode materials. . . . . . . . . . . . . . . . . . . . . . . . 224.3 Adhesion problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.4 The effect of different UV-doses with and without annealing . . . . . . . . . . . . 254.5 The effect on saturation current with a thicker mesh . . . . . . . . . . . . . . . . 274.6 The effect on forward current with surface treatment . . . . . . . . . . . . . . . . 274.7 Frequency measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.8 Physical characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5 Conclusions and future work 37

5.1 Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

1

List of Figures

2.1 a) a screen printed image of Marilyn Monroe by Andy Warhol. b) A mesh with afinished pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Images showing ZnO the difference in electron transportation for a) nanorods andb) nanotetrapods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3 Isolated metal and n-type semiconductor . . . . . . . . . . . . . . . . . . . . . . . 92.4 Schottky junction in equlibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.5 A schottky junction in forward bias and reverse bias. a) Forward bias. b) Reversed

bias. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.6 Ohmic contacts a) Metal work function close to or smaller than the electron affinity

of the semiconductor. b) High doping. . . . . . . . . . . . . . . . . . . . . . . . . 112.7 Surface rougness which limits surface-to-surface contact. The more conctact points,

the lower conctact resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.8 Images showing ZnO nanotetrapods ability to pierce materials and increase the

amount of contactpoints. a) Increasing the contact points b) Piercing of Teflon,taken from ref. [15] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.9 I-V characteristics of a typical Schottky diode . . . . . . . . . . . . . . . . . . . . 132.10 C-V characteristics for a Schottky diode . . . . . . . . . . . . . . . . . . . . . . . 132.11 A half wave rectifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.12 A capacitor has been added to a half wave rectifier decreasing the ripples and

increasing the effective DC voltage . . . . . . . . . . . . . . . . . . . . . . . . . . 152.13 a) a full wave rectifier and the current in b) the positive half cycle and c) the

negative half cycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.14 A capacitor has been added to a full wave rectifier to decrease the ripples and

increase the effective DC voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.1 a) A Roku Print prepared and ready for printing and b) a fully printed samplewith Carbon as top electrode and Silver as bottom electrode . . . . . . . . . . . . 18

3.2 A rectifier bridge with Aluminum as ohmic contact and Silver as Schottky contact 193.3 How to probe the diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.1 Profilometer measurements (conf20x magnification) of mixes in table 4.1. a)EP010, b) EP011, c) EP012, and d) EP013. . . . . . . . . . . . . . . . . . . . . . 21

4.2 Picture of Mix 5 taken with an optical microscope at 100x magnification. TheZnO tetrapods can easily be seen with a magnification of 100x. . . . . . . . . . . 21

4.3 Ohmic to ohmic vs. Schottky to Schottky contacts. a) C/ZnO/C and b) Ag/ZnO/Ag.23

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4.4 Ag/binder 1/Ag printed and measured to see how much leakage the binder isresponsible for. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.5 I-V measurements of EP030. A Schottky diode with a bottom electrode of Al(ohmic), 3,68 percent ZnO in binder 1 and a top electrode of Ag1 (schottky)annealed in 120 °C for 15 min) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.6 Diode A4 from sample EP079 with a rectification ratio of 2,6*10ˆ5 at +/- 2V . . 274.7 Sample EP110 diode A7. It has a high rectification ratio at 2 V but the forward

current is too low to reach the target of 0.1 mA at 2 V. . . . . . . . . . . . . . . 284.8 Sample EP110 diode B4. It has a lower rectification ratio at 2 V (7000 times)

with a bit higher saturation current. The forward current is higher (30 kOhm)and is almost reaching the goal of 20 kOhm. . . . . . . . . . . . . . . . . . . . . . 29

4.9 Sample EP114 diode A9. It has a high rectification ratio at 2 V (100 000 times)with a low saturation current but the forward current is only in the uA range. . . 29

4.10 Diode EP112A4 measured 1 h after print and 3 days after print. Note that thesaturation current has decreased after 3 days. . . . . . . . . . . . . . . . . . . . . 30

4.11 Frequency measurements of diode EP110A7 at a) 100 Hz, b) 200 Hz, c) 1 kHz, d)10 kHz, e) 100 kHz and f) 1 MHz. . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.12 Frequency measurements of diode EP110B4 at a) 100 Hz, b) 200 Hz, c) 1 kHz, d)10 kHz, e) 100 kHz and f) 1 MHz. . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.13 Frequency measurements of diode EP119D7 at a) 1 kHz, b) 10 kHz, c) 100 kHzand d) 1 MHz. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4.14 Frequency measurements of a rectifier bridge with sample nr EP132B1. The blackdotted line shows measurements from a commercial rectifying bridge. a) 100 Hz,b) 1 kHz, c) 10 kHz and d) 100 kHz. . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.15 Non-anneald EP033A9 a) before and b) after tape-test. Ag as top and bottomelectrode with only the binder without ZnO. . . . . . . . . . . . . . . . . . . . . . 35

4.16 Non-anneald EP040A6 a) before and b) after tape-test. Ag as bottom electrodeand C as top electrode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.17 A SEM picture of a crack in the EP013 binder. EDX tells there is a lot of mag-nesium in it . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.18 a) An SEM picture of a ZnO nanotetrapod inside the EP012 binder, one can seethe legs point in different directions. 20k magnification, 20 kV. b) An SEM pictureof a ZnO nanotetrapod inside the EP013 binder, one can see the sharp peak. . . 36

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Abstract

The aim of this master thesis was to develop a process for fabricating Schottky diodes, usingtechniques that are suitable for cheap large volume mass production e.g. printing, with tetrapodstructured ZnO as the semiconductor. Part of the work involved selecting suitable metals forohmic and Schottky contact and identification of a binder that can be used for dispersion of theZinc Oxide (ZnO). ZnO is a II-VI compound semiconductor with a wide band gap (3.4 eV). TheSchottky diode is used as a rectifier. A rectifier serves the purpose to turn Alternating Current(AC) to Direct Current (DC). The Schottky diode should only conduct current in the forwarddirection, in the reverse direction the current should be blocked. In this thesis printed diodeswere used to construct different types of rectifiers for example half wave rectifiers and full waverectifiers. Aside from electrical properties, adhesion properties have also been investigated. Ad-hesion was showed to depend on not only the choice of binder, but also UV-dose and annealingtemperature. Aluminum and silver together with ZnO proved to be the best materials combina-tion with a rectification ratio up to 105 −106. Different sizes of Schottky diodes were printed andthe smaller diodes with an area of 0, 5x0, 5mm2 performed best as a half wave rectifiers whilethe larger size,1x1mm2, performed best as a full wave rectifier.

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Chapter 1

Introduction

Thin Film Electronics ASA is a company that produce electronics with the printed electronicstechniques. It is a Norweigan company with its headquarters in Oslo and product developmentin Linkoping. They were the first to commercialize printed rewritable memory and are nowcreating system products that will include memory, sensing, display and wireless communicationthat will be part of integrated systems making the way for electronic intelligence in applicationswhere they have never been affordable before[1]. Some integrated systems need printable diodesthat for example can convert a DC signal from an AC input. In this project nano tetrapods ofZinc Oxide are used as the semiconductor in a Schottky diode.

Zinc Oxide (ZnO) is a II-VI compound semiconductor with a wide band gap (3.4 eV) [2]. Thetetrapod ZnO nanostructure material that are used is one of several ZnO nanostructures and iscommercially manufactured and is called the "Pana-Tetra" [3]. One of the advantages with thetetrapod structure is that the tetrapods may spontaneously orientate with one of the four armsdirected normal to the substrate [4].

Metal contacts are required for almost any electrical device ZnO can be used for, but thereis a lack of information about printable Schottky diodes and Schottky diodes based on ZnO nanostructures even though studies about Schottky barriers of metals on ZnO began in the mid-1960s[5]. A better understanding of how the ZnO surfaces, ZnO-metal interfaces and the processesinvolved during contact formation affect the electronic properties is needed [5].

The aim of this master thesis is to develop a process for fabricating printable Schottky diodesbased upon ZnO materials for Thin Film Electronics. The work is focused on formulating screenprintable ZnO inks using commercially available ZnO particles with a tetrapod structure and tomix these formulations with commercially available pre-formulated binders used in the printedelectronic industry. The effect of different surface preparations is a relatively unexplored areaand hence it’s hard to predict which methods are suitable. Surface preparations may be of impor-tance, but for Roll-to-Roll production in air at ambient pressure the number of preparation stepsare limited. therefore this project is focused on the use of different contact materials, binders,junction areas and junction thicknesses, annealing temperatures and UV-doses.

5

Chapter 2

Theory and background

2.1 Printed electronics

When people hear the word "Screen printing" they might think of Andy Warhol, the Americanartist responsible for pop art paintings like Marilyn. Others might think of T-shirt prints orsomething totally different.

(a) (b)

Figure 2.1: a) a screen printed image of Marilyn Monroe by Andy Warhol. b) A mesh with afinished pattern

Today this printing technique has evolved and is not only used in the art industry, but inseveral other areas such as printed electronics. Printing of functional materials such as conductivepolymers, inorganic semiconductors etc. can be used as a low-cost and flexible technique thatin some aspects can compete with traditional electronics for example cost per tag [6], [7]. Thebasic technique itself is the same as before. To make a pattern a woven mesh is used and thenfilled with a layer of photo emulsion and dried. The pattern is placed on the screen and thenexposed to light. The emulsion rigidifies and binds to the woven mesh except for the area wherethe pattern covers the emulsion [8]. When the mesh is finished one can print the pattern on asubstrate by pressing ink against the mesh. The most important rule in screen printing is toadjust the mesh to the ink instead of adjusting the ink to the mesh. The ink should only beformulated to get the conductivity and other desirable properties. On the other hand the amountof ink deposited should be dependent on the mesh [9].

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2.2 ZnO

Zinc Oxide (ZnO) is a II-VI compound semiconductor with a direct wide band gap (3.37 eV). Ithas a stable wurtzite structure and a lattice spacing a=0.325 nm and c=0.521 nm [2]. Becauseof intrinsic defects such as O vacancies and Zn interstitials, ZnO is an n-type semiconductor bynature [10]. Because of its wide band gap and unique properties it is suitable for a broad rangeof applications such as UV-light emitters, chemical sensors and transparent electronics. Becauseof this ZnO is attracting more attention now than ever before [5]. ZnO is even suitable forspace applications due to its high resistivity to high-energy radiation [11]. Several types of nanostructured ZnO exist and can be easily synthesised such as nanohelixes, nanorings, nanosprings,nanorods, nanobelts and nanotetrapods [12]. In this project a ZnO powder of nanotetrapodsmixed with an organic binder is used.

2.2.1 The tetrapod shaped nanocrystal ZnO

The tetrapod structured ZnO material is one of several ZnO nanostructures and has been com-mercially manufactured by Panasonic and is called the "Pana-Tetra".[3] The actual size of thetetrapods in this project is not in the nanometer scale but in the micrometer scale. Some of thephysical properties of semiconductor materials undergo changes when one shrink the dimensionsdown to nanometer scales and the smaller tetrapods might behave differently than the ones usedin this project [2].

There is little information about how the ZnO tetrapod differs from bulk ZnO and other struc-tures but one of the known advantages with the tetrapod structure is that the tetrapods mayspontaneously orientate with one of its four arms directed normal to the substrate [4]. The nan-otetrapod consist of four nanorods joined at tetrahedral angles to a central core. This gives it theadvantage of higher vertical conduction as shown in figure 2.8b. The nanorods usually lie downhorisontally on its substrate and can not use the nanorods one dimensional electron transport inthe vertical direction in comparison to the tetrapods that can use the one dimensional electrontransport in a direction perpendicular to the conductive substrate giving the electrons a shorterdistance to travel [13], [14].

(a) (b)

Figure 2.2: Images showing ZnO the difference in electron transportation for a) nanorods andb) nanotetrapods

ZnO may also be used to modify adhesion properties. At the Zoological Institute of KielUniversity ZnO tetrapods have succesfully been used to stick polymer Teflon to Silicone [15].Both Teflon and Silicone are non-sticky materials that are joint together by using the tetrapodsas linkers as the needle shaped nanoparticles. A SEM picture of this can be seen in figure 2.8b.

Wei Chen et al achieved high performance solar energy conversion photoelectrodes based on

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ZnO nanotetrapods [16]. They showed that ZnO is superior for electron transport and collectionwhen assembled into a network.

The properties of the commercially manufactured "Pana-Tetra" is given in table 2.1.

Table 2.1: Properties of the ZnO tetrapods used in this project

Material name Zinc OxideChemical formula ZnO

Structure Single crystal (Needle shape)Shape Tetrapod Shape

Average length of leg ∼ 10 − 20µmSpecific gravity 5.78

Relative density ∼ 0.1Melting point under pressure 2000C

Sublimation point 1720CSpecific heat 0.1248cal/g · deg

Thermal conductivity 25.3W/m · KThermal expansion coefficient 3.18 · 10−6/K

Refractive index 1.9-2.0Electricity induction (2.4 · 1010Hz) ǫ = 8.5

Volume resistance ∼ 10Ω

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2.3 Schottky diodes

Metal contacts are required for almost any electrical device ZnO can be used for, but there isa lack of information about printable Schottky diodes and Schottky diodes based on ZnO eventhough studies about Schottky barriers of metals on ZnO began in the mid-1960s. A betterunderstanding of how the ZnO surfaces, ZnO-metal interfaces and how the processes involvedduring contact formation affect the electronic properties is needed [5]. The effects of differentsurface preparations is a relatively untouched area, consequently there is not enough informationabout the properties of the contact between ZnO and different metals. Two forms of contactscan be formed between a metal and a semiconductor, Schottky barriers and the ohmic contacts.Schottky barriers act as rectifiers that allows current in only one direction and blocks it inthe other direction while ohmic contact is a symmetric type of contact that conducts in bothdirections. The goal of this project is to produce a printed Schottky diode with an series resistanceless than 20 kOhm, a saturation current larger than 10−15 and a zero bias junction capacitanceless than 100 pF. Different electrode materials, binders and the effect of the thickness of thebinder and semiconductor layer are studied.

2.3.1 Schottky junctions

Schottky barriers act as rectifiers and are suitable for high-speed applications due to their highswitching speed. Theoretically a Schottky junction is formed if the metal work function is largerthan the n-type semiconductor work function, for a p-type semiconductor it’s the other wayaround. Consider a metal and a n-type semiconductor at thermal equlibrium with the energystates shown in figure 2.3. The metal has a workfunction φm and the semiconductor has aworkfunction φs. The Fermi level for the metal is EF m and for the semiconductor EF s [17],[18].

EV ac

EF m

EV

ECEF s

qφm

Eg

qχs qφs

Figure 2.3: Isolated metal and n-type semiconductor

When the metal and the semiconductor are brought to contact the Fermi levels align and aband bending in the energy diagram is observed (figure 2.4). By calculating the difference inwork function across the interface the junction built-in-potential, φi, for the semiconductor isobtained as seen in figure 2.4.

φi = φm − φs

The barrier potential φb can be calculated as

φb = φm − χs

where χs is the electron affinity in the semiconductor [17].

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EV ac

EV

ECEF

qφm

Eg

qχs

qφs

W

qφbqφi

Figure 2.4: Schottky junction in equlibrium

Applying a bias, VA, causes the Fermi levels of the semiconductor and the metal to split upwith the energy difference of qV AeV . In a forward bias the band bending is less significant,the depletion region is reduced and a large current flows. In reverse bias on the other hand thedepletion region width is increased and the current is consequently very small. This means thatif an alternating voltage is applied across the junction the charges can flow in only one directionand therefore the current is rectified. This is explained more thoroughly in section 2.4.

Electronflow

−+V

EV

ECEF s

qφbqV A

q(φi − VA)

(a)

+−V

EV

ECEF s

qφb

|qV A|

q(φi − VA)

(b)

Figure 2.5: A schottky junction in forward bias and reverse bias. a) Forward bias. b) Reversedbias.

ZnO has an electron affinity of qχs ≈ 4.2eV [5]. Metals with workfunctions a bit higherthan qφm = 4.2eV are candidates to form Schottky contact with ZnO. In table 2.2 materialswith work functions close to the desirable value are listed.

Table 2.2: Suitable metals for ohmic contacts on ZnO

Material Work function qφm(eV )

Ag 4.52 - 4.74 [19]PEDOT:PSS 5.1 [20]

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2.3.2 Ohmic contacts

The ohmic contact is a linear type of contact in which voltage- current relation follows the Ohm’slaw [23]. Ideally, the ohmic contact should have a linear I-V characteristic for both forward andreversed bias, it does not perturb device performance significantly due to a small voltage dropacross the contact and the contact resistance. An ohmic contact is formed between an n-typesemiconductor and a metal if φm is close to or smaller than the semiconductors electron affinity,χs, the barrier height is reduced so that the electrons can flow in both directions, see figure 2.6a.

electronflow

EV ac

EV

ECEF s

qφm

qχs

qφb

(a)

tunneling

EV

ECEF s

qφb

(b)

Figure 2.6: Ohmic contacts a) Metal work function close to or smaller than the electron affinityof the semiconductor. b) High doping.

High resistance in metal-semiconductor ohmic contacts are often caused by contact failure orhigh thermal stress, which leads to a major loss of device performance [23]. (See figure 2.7)

Figure 2.7: Surface rougness which limits surface-to-surface contact. The more conctact points,the lower conctact resistance.

Our hypothesis is that the nanotetrapod ZnO pierce the contact material, figure 2.8a, andform more contact points resulting in a low contact resistance, as seen in figure 2.8a. Moreover,we believe that this leads to a junction with good mechanical properties and contact strength.

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(a) (b)

Figure 2.8: Images showing ZnO nanotetrapods ability to pierce materials and increase theamount of contactpoints. a) Increasing the contact points b) Piercing of Teflon, taken from ref.[15]

As discussed before one of the contacts in a diode should be ohmic. The desirable propertiesof such contact can be summarized as below:

• The metal work function should be close to or smaller than the semiconductor electronaffinity φm . χs.

• The surfaces should have as many contact points as possible in order to have a smallercontact resistance

A survey of the literature presented in the field does not give a clear answer of which metalsare suitable to form an ohmic contacts with ZnO. ZnO has an electron affinity of qχs ≈ 4.2eV[5]. Metals with workfunctions qφm . 4.2eV are candidates to form ohmic contact with ZnO.The binder we are using with the ZnO powder might change the electron affinity but we assumethe work function to be constant. In table 2.1 metals with work functions close to the desirablevalue are listed.

Table 2.3: Suitable metals for ohmic contacts on ZnO

Metal Work function qφm(eV )

C 5 [19]Sn 4.42 [19]Al 4.06-4.41 [19]

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2.3.3 I-V characteristics

According to the Schockley diode equation the current running through the diode can be de-scribed as [21]:

I = Isat(eq(VA−φi)

ηkT − 1)

where Isat is the reverse saturation current, φi is the barrier height, VA is the applied voltage, qis the elementary charge, K is the Boltzmann constant, T is the diode temperature and η is theideality factor, 1 ≤ η ≤ 2.

In reversed bias the applied voltage VA is negative leading to a negligably small current Isat.The I-V characteristic of a typical Schottky diode is shown in figure 2.9.

I

V

Figure 2.9: I-V characteristics of a typical Schottky diode

2.3.4 C-V characteristics

The Schottky junction act as a parallel plate capacitor due to the depletion region width betweenthe metal and the doped semiconductor that acts as an insulator. The depletion region decreasewith forward bias and increase with reversed bias. The junction capacitance-voltage relation canbe expressed as [22]:

CJ(VA) = CJ (0)

(1−VAφi

)1/2

V

C

Figure 2.10: C-V characteristics for a Schottky diode

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2.4 Rectifiers

The Schottky diode is used as a rectifier. A rectifier serves the purpose of turning AlternatingCurrent (AC) to Direct Current (DC). The Schottky diode should only conduct current in theforward direction from its anode to its cathode. In the revers direction, from cathode to anode,the current is blocked. In an n-type diode the metal part acts as the anode and the semiconductoracts as the cathode. In a p-type diode the metal part acts as the cathode and the semiconductoras the anode. Diodes can be used to make different types of rectifier circuits for example halfwave rectifiers and full wave rectifiers. The half wave rectifier has only one diode and the fullwave rectifier can have two diodes, or four diodes as in a diode bridge.

2.4.1 Half wave rectifiers

The simplest rectifier is the half wave rectifier which contains only one diode. In the ideal case,when an ac voltage is applied across the diode the current is blocked during the negative halfcycles but in the positive half cycles it lets current through making it unidirectional, i.e. it turnsAC to DC.

Figure 2.11: A half wave rectifier

The load resistor in figure 2.11 gives a proportional relationship, U = RI, between thecurrent and the voltage across it in the forward direction. The voltage measured in the forwarddirection is equal to the supply voltage, VS , minus the voltage drop through the diode in forwardbias, VF .

Vout = VS − VF

The effective DC voltage, i.e. the mean value of the output voltage, can be written as:

VDC = Vmax/π

where Vmax is the maximum value of the output voltage. The effective DC current and theoutput power can be written as:

IDC = VDC/R P = I2DCVDC

where R is the load resistance. Since the output voltage is zero in the negative half cycle theoutput voltage contains ripples with the same frequency as the input signal. By adding a shuntcapacitance to the load this ripples are diminished and the effective DC voltage increases. During

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forward bias the capacitor is charged and during reversed bias the capacitor is slowly dischargeduntil the next half cycle of forward bias, keeping the output voltage more stable (see figure 2.12).This gives a larger effective DC voltage and smaller ripples. But for a half wave rectifier thiscapacitor needs to be relatively large because of the large ripples and low ripple frequency dueto the cancelled out half cycles, therefore half wave rectifiers are used in low-power applications.For high-power applications a full wave rectifier is more suitable.

Figure 2.12: A capacitor has been added to a half wave rectifier decreasing the ripples andincreasing the effective DC voltage

2.4.2 Full wave rectifiers

With four diodes one can create a diode bridge. This kind of rectifiers passes the full wavefrom input to output but they invert the negative half cycle. During the positive half cyclescurrent goes through diode D1 and D2 while diode D3 and D4 are off (figure 2.13b). Duringthe negative half cycles current goes through D3 and D4 while diode D1 and D2 are off instead(figure 2.13c)[25].

(a)

(b) (c)

Figure 2.13: a) a full wave rectifier and the current in b) the positive half cycle and c) thenegative half cycle.

In the ideal condition, since both negative and positive half cycles contribute to the output

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voltage, the DC output of a full wave rectifier is twice as large as a half wave rectifier. But dueto the fact that the current passes two diodes in series the voltage drop is also twice as largecompared to half wave rectifier case. The voltage measured in the forward direction should bethe same as the supply voltage, VS , minus the forward voltage drop in the diodes, VF , that istypically 0.7 V for one Schottky diode.

Vout = VS − 2VF

The effective DC voltage, i.e. the mean value of the output voltage, can now be written as:

VDC = 2Vmax/π

The effective DC current and output power can be calculated in the same way as for the halfwave rectifier. A capacitor can be added in parallel with the resistive load to diminish the ripplesand achieve a higher dc output (see figure 2.14).

Figure 2.14: A capacitor has been added to a full wave rectifier to decrease the ripples andincrease the effective DC voltage

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Chapter 3

Experimental details

In this project several parameters are optimized by examining the effect of different ZnO con-centrations, mesh sizes, electrode materials, different UV-doses, annealing time and annealingtemperatures.

3.1 ZnO mix

The ZnO nanotetrapods were delivered as a powder. Since it is not possible to print a powder,it had to be mixed in a binder. Seven different UV-curable binder materials were examined.Different concentrations of ZnO in different binders were prepared as seen in table 3.1.

Table 3.1: List of the different mixes used in the project

Mix Binder ZnO mass conc.

#1 Binder 1 1,02%#2 Binder 2 0,88%#3 Binder 1 3,68%#4 Binder 3 3,65%#5 Binder 1 3,70%#6 Binder 4 1,70%#7 Binder 5 3,10%#8 Binder 6 3,70%#9 Binder 7 5,20%

#10 Binder 1 5,70%#11 Binder 1 + retarder 9,34%

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3.2 Electrode materials

Several electrode materials were used as listed in table 3.2 to find the best combination ofSchottky and ohmic contact for the diode. Two different types of Ag ink were used which arereferred as Ag ink (1) and Ag ink (2) in the table.

Table 3.2: Combinations of electrode materials that were tested

Ohmic contact Schottky contact

C Ag ink (1)C PEDOT:PSSC Ag2

Al Ag ink (2)Al PEDOT:PSSAl Ag2

3.3 Printing

The printing equipment used was a Roku Print screen printing machine RP 2.2. This equipmentlets the user save settings like positions and speed in a program that makes the printing easierto replicate. A velocity of 25% was used for all samples.

The Schottky diodes were printed on a PEN substrate in a pattern where ZnO is sandwichedbetween a top electrode and a bottom electrode. To do this three meshes were needed. One forthe bottom electrode, one for the ZnO layer and one for the top electrode. During the project thesize of the bottom and top electrode meshes were 140-34 and 100-34 respectively for all samples.Two different sizes were used for the ZnO layer. Most of the samples were printed with 120-34size but the mesh size 61-64 was tested as well. The Al electrodes were patterned in another waysince screen printing Al is not possible. A pattern of four different surface areas, 0.5x0.5mm2,1x1mm2, 2x2mm2 and 4x4mm2 was designed as shown in figure 3.1b. Since the diode partlyworks as a parallell-plate capacitor the contact area affects the performance of the diode.

(a) (b)

Figure 3.1: a) A Roku Print prepared and ready for printing and b) a fully printed sample withCarbon as top electrode and Silver as bottom electrode

Two diode bridge patterns with different diode surface areas, 0.5x0.5mm2 and 1x1mm2, werealso designed and tested with Al and Ag bottom electrodes and carbon top electrode.

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Figure 3.2: A rectifier bridge with Aluminum as ohmic contact and Silver as Schottky contact

3.4 Measurements

The I-V-measurements were done with a Keithley 4200-SCS. Three to five measurements weredone before saving the plots to make sure that the I-V characteristic of the device was stable.

Figure 3.3: How to probe the diode

The half- and full-wave measurements were performed using an input AC voltage from anarbitrary waveform generator (National Instruments PXI-5412) and the output voltage was readwith a digitizer (National Instruments PXI-5114) with a load resistance of 1 MOhm. The thick-ness and surface roughness of the samples were characterized with a dektak profilometer.

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Chapter 4

Results and discussion

4.1 The effect of different ZnO concentration

One advantage of using a ZnO powder is that the concentration of ZnO in the binder does nothave to be fixed. This gives a possibility to change the electrical properties while keeping thesize of the diode fixed. The concentration is an important parameter that affects the electricalproperties and viscosity of the mix. A low concentration of ZnO gives lower currents and allowsthe diode to work at higher voltages. A high concentration of ZnO gives a rougher surface,higher currents but does not allow high voltages. Screen printing inks with ZnO concentrationsof 0,88%, 1% and 3% were prepared and printed on Si wafers and characterized with a Dektakprofilometer (Figure 4.1). At 6% ZnO binder 1 the viscosity becomes to high for screen printing,therefore the concentration has to be lower than 6% for binder 1, if no retarder is added. Theconcentrations and sample names are listed in table 4.1.

Table 4.1: Samples with different ZnO concentrations prepared for roughness characterization

Sample Binder ZnO mass conc.

EP010 mix#1 1,02%EP011 mix#2 0,88%EP012 mix#3 3,68%EP013 Binder 1 0%

A significant difference in topography is observed between the different concentrations listedin table 4.1. The binder without ZnO (EP013) gives a smooth surface if compared with the mixeswith 1% ZnO concentration (EP010 and EP011) that has peaks on the surface. In the samplewith higher ZnO concentration (EP012) the peaks are more concentrated, proving that the ZnOconcentration affects the surface topography. The ZnO nanotetrapod’s legs have a length of10-20 um and the height of the standing pods is 18-37 um. therefore the thickness of the printedbinder should be slightly less than 18-37 um for the pods to point out from the binder and makecontact with the electrodes. Thickness measurements indicate that a screen size of 120-34 issuitable for printing of one layer ZnO nanotetrapods.

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(a) (b)

(c) (d)

Figure 4.1: Profilometer measurements (conf20x magnification) of mixes in table 4.1. a) EP010,b) EP011, c) EP012, and d) EP013.

A characterization performed with an optical microscope (figure 4.2) shows that the ZnOtetrapods are in the um-scale and can easily be seen with a magnification of 100x due to thetransparency of the binder.

Figure 4.2: Picture of Mix 5 taken with an optical microscope at 100x magnification. The ZnOtetrapods can easily be seen with a magnification of 100x.

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4.2 Screening of suitable electrode materials.

The choice of materials for the ohmic and the Schottky contacts is one of the most importantsteps in this project. Empirical tests with different combinations of materials and binders showthat Ag ink 1 as a Schottky contact and Al as an ohmic contact are suitable for fabrication of aSchottky diode with ZnO as a semiconductor. Carbon can also be used for an ohmic contact, butit is not ideal. Three samples were printed in the orders of C/ZnO/C, Ag/ZnO/Ag and Ag/onlybinder/Ag to examine the contact properties. In figure 4.3a the I-V curve of a sample with topand bottom carbon electrode is shown. The four different plots are measurements from the fourdifferent sizes, A, B, C and D. Higher currents can be expected from the largest size, D. The I-Vcurve of all the samples are symmetric, but not fully linear. This indicates that carbon is not agood choice to provide an ohmic contact with ZnO. In figure 4.3b the sample with the printedsilver against silver is plotted. Three of the sizes (B, C and D) are plotted. These plots have aslightly less symmetrical appearance, probably due to a better contact in either of the electrodes.The current is low indicating that Ag ink 1 is working as a Schottky contact since a low leakagecurrent is the goal. It is observed that the current is blocked differently in the forward and thereversed region and that the size of the diode current does not seem to scale with the surface area.The hysteresis is very large. The difference between the materials is later proved to be significantwith controlled parameters. The measurement of one sample with a Ag/only binder 1/Ag print(figure 4.4) shows that the leakage current in binder 1 is in the pA-scale meaning that it is theZnO, and not the binder, that is responsible for almost all the leakage. The samples preparedwith Ag ink 1 have a better performance compared to the ones prepared with PEDOT:PSS. Agink 2 have an extremely poor adhesion to the binder and PEDOT:PSS is too thin due to thescreen size that is not suitable for the low viscosity of PEDOT:PSS.

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(a)

(b)

Figure 4.3: Ohmic to ohmic vs. Schottky to Schottky contacts. a) C/ZnO/C and b) Ag/ZnO/Ag.

−5 0 510

−14

10−13

10−12

10−11

EP033

Voltage [V]

Cur

rent

[A]

Figure 4.4: Ag/binder 1/Ag printed and measured to see how much leakage the binder is re-sponsible for.

A sample (EP030, see fig 4.5) with Al instead of C as a bottom electrode is printed to increasethe current going through the diode. This sample conducts more current which is the purpose ofthe ohmic contact, but the leakage current is very high as seen in the negative voltage range in

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figure 4.5. The four different plots are measurements from the four different sizes, A, B, C andD. The largest size (D) has the highest leakage and forward current as expected. The leakage hasto do with the Schottky contact and controlled parameters can reduce the leakage significantly.Carbon has a high resistivity and the work function does not fit with the theoretical value as itdoes for Al. This makes Al a better ohmic contact as earlier expected. A decision to continuewith Aluminum as an ohmic contact and Ag ink1 as a Schottky contact is made according tothe experimental results from examining different electrodes.

Figure 4.5: I-V measurements of EP030. A Schottky diode with a bottom electrode of Al (ohmic),3,68 percent ZnO in binder 1 and a top electrode of Ag1 (schottky) annealed in 120 °C for 15min)

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4.3 Adhesion problems

Adhesion problems are observed in all samples from the first batches. The mix does not stick tothe substrate and the top electrodes crumble like sponges. A solution to the substrate adhesionis to anneal binder 1 for 10 min at 120 °C in after the UV treatment and before printing the topelectrode. This is observed when printing mix #4 and mix #5 to test the annealing of bindersprior to the printing of the top electrode. Humidity and UV-dose have to be controlled to solvethe the adhesion problem of the top electrode.

Binder 4 shows a poor adhesion to Ag and Al electrodes, therefore it was discarded. Binder6 has a good adhesion but the use of this material in the ZnO mix results in a lot of pinholes inthe top electrodes. Despite the pinholes the two binders, binder 6 and binder 1 seem promisingto continue with, by optimizing and controlling the UV dose.

4.4 The effect of different UV-doses with and without an-

nealing

The UV-dose the binder is exposed for affect the adhesion between binder and the substrate andelectrodes. Deionization of the binder before printing the top electrode improves the adhesion ofthe Ag ink 1 to the binder.The print trials continues with samples (EP060-EP063) with binder 6, mix #8, and Ag ink 1 astop electrode (table 4.2). The sample with the lowest UV-dose seems to have the best adhesionof Ag ink 1 on top of the binder, but the binder have poor adhesion to the substrate which makesboth binder and top electrode fall of together.

Table 4.2: Prints to test UV-dose importance. All samples are printed with mix #8, mesh size120-34 and Ag ink 1 as top electrode.

UV

EP060 120W 580 Jx1 (+ 20 s deionization)EP062 120W 580 Jx1EP063 80W 430 Jx1EP066 120W 530 Jx1EP067 120W 530 Jx1 (+ 20 s deionization)EP068 120W 413 Jx1EP069 120W 413 Jx1 (+ 20 s deionization)

When deionizing the surface of the binder after UV curing the surface of the top electrodeseems to be smoother. For sample EP066-69 it is clear that deionization improves the adhesionfor Ag ink 1 on the binder as observed before. The binder has poor adhesion to the substrateand it is possible that this is a result of too low UV-dose.

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Table 4.3: Prints to test UV-dose with and without annealing. The samples are printed withmix #5 and mix #8 with mesh size 120-34 and Ag1 as top and bottom electrode

ZnO UV speed Anneal Rest after anneal Comments

EP071-1 mix #8 580 Jx2 10 min 120 °C 6 minEP072-1 mix #8 580 Jx1 no annealEP073-1 mix #8 580 Jx1 10 min 120 °C 6 minEP074-1 mix #8 740 Jx1 no anneal smooth surfaceEP075-1 mix #5 580 Jx3 10 min 120 °C 5 minEP076-1 mix #5 580 Jx2 10 min 120 °C 5 minEP077-1 mix #5 1086 Jx2 10 min 120 °C 5 minEP078-1 mix #5 1086 Jx1 10 min 120 °C 5 min smooth surface

The humidity during print is 40 % for samples EP071-1 - EP078-1. All samples with binder 1mix #5 look good and adhere well to the substrate. The samples with binder 6 mix #8 preparedwith a low UV-dose (EP076-1 and EP078-1) have a smooth surface without crumbles in thetop electrode and adheres well to the substrate. No electrical measurements are done since thesamples do not have any bottom electrodes.

Sample EP075-2 - EP078-2 are printed in the same way as EP0xx-1 but with an added Agink 1 bottom electrode. All samples crumbles just as they did in the beginning of the projectand this might be due to lower humidity on the day of print.The samples from the two mixes that seem to have the best adhesion is EP078-1 and EP074-1.EP079 is printed with the same parameters as for EP078-1 but with Al bottom instead of Agink 1 and EP080 is processed in the same way as EP074-1 also using an Al bottom instead ofthe Ag ink (see table 4.4). The Al bottom is added to enable electrical characterization.

Table 4.4: Prints with the same parameters as EP078-1 and EP074-1 with Al bottom instead ofAg ink 1

EP079 EP078-1 with Al bottom instead of Ag

EP080 EP074-1 with Al bottom instead of Ag

During processing the humidity is 40 %. Some of the samples have poor adhesion to the Alelectrodes but overall these samples have a good smooth appearance. The rectification ratio ofsample EP079 is measured (see fig 4.6) and relative to the first samples (fig 4.5) for example)a good rectification is observed. The rectification ratio is ca. 104 for all diodes at +/- 2 V butfor some of the diodes the rectification ratio gets as high as 105. The leakage current is low butunstable (10−10 −10−8) and the forward current is not stable either. In order to control (achievea better stability) the forward current and saturation current attempts are made to surface treatAl.

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−3 −2 −1 0 1 2 3

10−10

10−5

EP079A4

Voltage [V]

Cur

rent

[A]

rect ratio=2.6e+05mA at 2V=0.013

Figure 4.6: Diode A4 from sample EP079 with a rectification ratio of 2,6*10ˆ5 at +/- 2V

A low UV-dose gives good results with binder 1. Due to very poor adhesion between binder6 and Al in sample EP080 the binder 6 track is terminated. The samples with mix #5 is nowhaving smooth surfaces without crumbled top electrodes and a good adhesion but almost all ofthe samples have electrical short circuits. It might have to do with mix #5 at this point is a fewweeks old. A new mix with the same binder is made to continue with, mix #10.

4.5 The effect on saturation current with a thicker mesh

A low saturation current is a key performance indicator for the Schottky diode. By varying theUV-dose and mesh thickness the saturation current can be decreased. Several samples printedusing mix #10 with different UV-doses (single exposure with 310 J to 740 J (80W)), with 10min annealing in 120 °C and the same canvas thickness as before, 120-34, were examined. Singledose exposure of 580 J (120W) gives the best measurement data but the difference from theother samples is not significant. A similar test with a thicker canvas (61-64) in 65% humidityshows that the thickness of the binder affects the saturation current too. Significantly lowersaturation current are observed for thicker canvas, but the forward current is also affected andthe rectification ratio is constant, which implies that the difference is that the amount of contactpoints was decreased for thicker prints. The number of electrical shorts is lower for thicker prints,which is an advantage that likely can be explained by less pinholes.

4.6 The effect on forward current with surface treatment

A good diode does not only need a low saturation current, the forward current needs to be highas well. To increase the forward current the ohmic contact needs to have a low ohmic resistance.Al oxidizes in air and give a pinned layer to the semiconductor, increasing the ohmic resistance.With a quick surface treatment with Acetone, IPA and some distilled water, prior to printing ofthe ZnO layer, the forward current increase slightly. The two different thicknesses were used inthe same sample batch, with mix #10, one without annealing and one annealed for 10 min in120 °C. The two thicknesses show almost the same rectification ratio but the I-V plots have anoffset of two decades. The I-V curve of a diode with the surface area of (0.5x0.5 (mm2), printedwith frame with the mesh size of 120-43 is illustrated in figure 4.7. The rectification factor at 2V is 6.9 ∗ 104, the saturation current is the tenth of nA-range at -2 V and the forward current

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is slightly above 10 uA at 2V. The target is an even higher forward current (0.1 mA at 2 V) toreach 20 kOhm resistance of the diode.

−3 −2 −1 0 1 2 3

10−12

10−10

10−8

10−6

10−4

EP110A7

Voltage [V]

Cur

rent

[A]

rect ratio=6.9e+04

Figure 4.7: Sample EP110 diode A7. It has a high rectification ratio at 2 V but the forwardcurrent is too low to reach the target of 0.1 mA at 2 V.

In figure 4.8 the next smallest size, B (1x1 mm2), from the same sample show a less stablesaturation current and a lower rectification ratio (1.4 ∗ 104). The forward current is higher (30kOhm) and is almost reaching the goal of 20 kOhm.

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−3 −2 −1 0 1 2 3

10−12

10−10

10−8

10−6

10−4

EP110B4

Voltage [V]

Cur

rent

[A]

rect ratio=1.4e+04

Figure 4.8: Sample EP110 diode B4. It has a lower rectification ratio at 2 V (7000 times) with abit higher saturation current. The forward current is higher (30 kOhm) and is almost reachingthe goal of 20 kOhm.

A sample with the thicker mesh (61-64) but surface treated and annealed as sample EP110shows excellent diode characteristics (see figure 4.9). The saturation current is as low as pA at -2V and the forward current is almost reaching the uA level at 2 V giving it a rectification ratio of(1.3 ∗ 105). The forward current is too low to reach the goal, but with an offset of slightly morethan two decades this diode would be reaching the goal of 20 kOhm and still having a saturationcurrent under 1 nA.

−3 −2 −1 0 1 2 3

10−12

10−10

10−8

10−6

EP114A9

Voltage [V]

Cur

rent

[A]

rect ratio=1.3e+05

Figure 4.9: Sample EP114 diode A9. It has a high rectification ratio at 2 V (100 000 times) witha low saturation current but the forward current is only in the uA range.

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The analysis of the electric characteristics of devices prepared using mix 1-10 shows thenecessity to increase forward current, yield and stability. The increase in forward current islikely governed by increasing concentration of the ZnO. However increasing ZnO concentration isnot trivial since it increases viscosity. Therefore additional retarder was added to lower viscosityand possibly increase the levelling which could reduce loss yield caused by various sorts of printdefects.

A new mix of binder 1 with a higher concentration of ZnO and an extra retarder was printedusing two different canvases. The diodes were printed in the same way as the diodes in figure 4.7and 4.9. Surprisingly none of the diodes are shorted, probably due to the retarder helping thebinder not to crack when cured and annealed. A problem with these samples are that since theconcentrations of ZnO is too high leading to higher currents going through the diode the diodesare shorted during measurements with a high voltage range.

Some samples show an aging effect that is the saturation current decreases during few daysafter printing. This effect, illustrated in figure 4.10, might have affected decisions on whichsamples were the best since some samples are measured the same day as they are printed andsome of them are measured a couple of days later. Most of the later samples are measured aftera couple of days. The red plot in figure 4.10 corresponds to a measurement 1 h after the printand the blue plot correspond to a measurement 3 days after print. The saturation current islower but the forward current is unaffected by time.

Figure 4.10: Diode EP112A4 measured 1 h after print and 3 days after print. Note that thesaturation current has decreased after 3 days.

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4.7 Frequency measurements

Frequency measurements are carried out on some of the best diodes of three different sizes to seehow the saturation current and forward current affect the rectifying behaviour. In diode EP110A7the saturation current is really low and this can be observed in the frequency measurements(figure 4.11)as well as the I-V measurements in figure 4.7. At 100 Hz and 200 Hz the outputcurrent has an almost ideal appearance. The reversed current is almost blocked and the voltagedrop is ca. 0.7 V as expected. At 1 kHz there is not enough time for the diode to discharge duringthe negative half cycle. When the frequency gets higher this behaviour is more pronounced andat 1 MHz the output voltage is almost a stable 2 V signal. At 1 kHz the voltage drop increasedleading to a higher effective voltage.

Type Al Treatment ZnO Anneal

EP110A7 Al/ZnO/Ag Aceton+IPA+dest.vatten 120-34 mix #10 (5,20%) 10 min (120 C)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

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Figure 4.11: Frequency measurements of diode EP110A7 at a) 100 Hz, b) 200 Hz, c) 1 kHz, d)10 kHz, e) 100 kHz and f) 1 MHz.

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In diode EP110B4 the leakage and the forward current are higher than the device with sizeA (0.5x0.5mm2) from the same sample. The mean value of the output voltage remains constantuntil 10 kHz but at 100 kHz starts to decrease. At 10 kHz an extra high leakage current can beobserved. It might be due to the high frequencies heat contribution giving thermal excitation.At 100 kHz the built in capacitance in the diode helps by not letting the reverse current gothrough but this leads to a low forward current as well. At 1 MHz the output voltage still has abit of a wave shape but even lowered.


EP110B4 Al/ZnO/Ag Aceton+IPA+dest.wat 120-34 mix #10 (5,20%) 10 min (120 C)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

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Figure 4.12: Frequency measurements of diode EP110B4 at a) 100 Hz, b) 200 Hz, c) 1 kHz, d)10 kHz, e) 100 kHz and f) 1 MHz.

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Figure 4.13 shows an example of a diode that is not optimal. The leakage is too high and itcannot operate in voltages higher than 2 V without burning off. Since the wave shape is almostfollowing the input voltage even at 1 MHz it might work better at even higher frequencies butthe measurement equipment could not measure at higher frequencies. It could be possible thatdiodes with this area and ZnO concentration could work better in series in a diode bridge ratherthan working alone.


EP119D7 Al/ZnO/Ag Aceton+IPA+dest.wat 120-34 mix #11 (9,34%) 10 min (120 C)

0 1000 2000 3000 4000 5000−2.5

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Vol

tage

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(b)

0 5 10 15 20 25 30 35 40 45 50−2.5

−2

−1.5

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(d)

Figure 4.13: Frequency measurements of diode EP119D7 at a) 1 kHz, b) 10 kHz, c) 100 kHz andd) 1 MHz.

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Full wave rectification

In figure 4.14 measurements from one of the diode bridges that was printed can be seen. Thebridges were printed with diodes of the two smallest sizes: 0,5x0,5 mm2 (A) and 1x1mm2 (B).

(a) (b)

(c) (d)

Figure 4.14: Frequency measurements of a rectifier bridge with sample nr EP132B1. The blackdotted line shows measurements from a commercial rectifying bridge. a) 100 Hz, b) 1 kHz, c) 10kHz and d) 100 kHz.

The smallest size did not work as good for the rectifier bridge as when used as a half waverectifier. That might have been because of the high series resistance which is twice higher in adiode bridge compared to a half wave rectifier. The larger size (B) worked better in the rectifierbridge than as a half wave rectifier. When used alone the saturation current was to high but whenconnected in series the saturation current was decreased to almost zero as seen in figure 4.14a.In figure 4.14 a commercial diode bridge is compared to the sample EP132B1. The shape ofEP132B1 signal is following the commercial bridge quite well except for a higher voltage drop.It can also be observed that in 100 kHz, in the negative half cycles the voltage drop is slightlylower than for the positive half cycles.

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4.8 Physical characterization

When testing adhesion standard office-tape was used to see if the materials were stuck to eachother or not. The two samples below, EP033 and EP040, are two of the first samples which showpoor adhesion. The adhesion was good when an identical sample was annealed before tape-test.

(a) (b)

Figure 4.15: Non-anneald EP033A9 a) before and b) after tape-test. Ag as top and bottomelectrode with only the binder without ZnO.

(a) (b)

Figure 4.16: Non-anneald EP040A6 a) before and b) after tape-test. Ag as bottom electrodeand C as top electrode.

The samples EP012 and EP013 were gold plated using Chemical Vapour Deposition (CVD)ca. 10 Angstrom to be able to characterize them with a Scanning Electron Microscope (SEM).All that could be seen were cracks in the binder as seen in figure 4.17. When measuring withan Energy-dispersive X-ray spectroscopy in the cracks there were no trace of Zn but a lot ofmagnesium, silicon and oxygen. The magnesium probably derived from the binder and thesilicon from the substrate since the crack opened up a hole down to the substrate. Cracks likethese might have caused electrical shortages in some of the diodes. When increasing the electronbeam voltage it was possible to see the tetrapods below the Au (see figure 4.18). An EDXmeasurement showed that there were both Zn and O in the tetrapods. The ZnO tetrapods donot point out of the binder as expected which might have to do with the screen being the rightsize in combination with the Au evaporation to cover all of the pods.

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Figure 4.17: A SEM picture of a crack in the EP013 binder. EDX tells there is a lot of magnesiumin it

(a) (b)

Figure 4.18: a) An SEM picture of a ZnO nanotetrapod inside the EP012 binder, one can seethe legs point in different directions. 20k magnification, 20 kV. b) An SEM picture of a ZnOnanotetrapod inside the EP013 binder, one can see the sharp peak.

Emma Persson 36

Chapter 5

Conclusions and future work

Different sizes of Schottky diodes has been printed with good performance and a rectificationratio of 105 − 106 for the best ones. These diodes have been proved to work both as half waverectifiers and full wave rectifiers. Identification of suitable materials for the Schottky and theohmic contact was a fairly straightforward process. Aluminum and silver together with ZnOmakes, with the printing technique, a relatively good Schottky diode. Carbon did not work aswell as aluminum and this was expected due to the theoretical values of the work functions. It alsohad a high resistivity. The measurements of PEDOT:PSS was a bit unstable, possibly caused bythe mesh making it too thin. The second silver-ink gave a poor printing result, probably becauseof the age of the ink.

The binders were a bigger obstacle than expected. They crumbled during the annealing of thetop electrode and had a poor adhesion to the substrate. In the end binder 1 was chosen to be thebinder in the final products since it adhered better and the I-V curves became more stable whenannealing the binder before printing the top electrode. Short circuits, high saturation currentsand low forward currents have been a large problem. Adding a retarder in the mix to lowerthe viscosity solved the yield problem. Lowering the UV-dose led to a reduction in saturationcurrent. The experimental results showed that the best results came from a UV-dose that waslow, but not too low. There are two possible explanations for these observations. The highUV-dose can affect ZnO polarity and affect the interface [23]. The high dose also lead to a highertemperature in the binder matrix, this often affects some mechanical properties of the bindersuch as an increase in the transition temperature.

The results of every diode vary a lot for most of the samples but usually the results are quitesimilar when the diodes are close to each other on the substrate. This indicates that it might haveto do with the printing technique. Non-even pressure during the prints, air bubbles below thesubstrate or meshes that were poorly cleaned that might have caused this behaviour. The con-centration of ZnO in the binder is an important parameter. In mix 11 the concentration is reallyhigh ca 9.5 %. These diodes could not handle as high voltages as the other diodes could. Diodesprepared from mixes of lower ZnO concentration or with higher thickness could be operated athigher voltages without suffering shorts. It was not possible to find an optimal concentrationor say which one was better than the others, since different concentrations and different sizessuit different purposes. This project suggests a method to tune the diode characteristics such asrectification, series resistance and the saturation current to fit different applications.

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5.1 Goals

• Which is the best combination of the contact materials available in my project?

Aluminum as an ohmic contact and the silver Ag1 as a Schottky contact proved to be thebest contact material combination of the ones tested.

• Which is the best binders available in my project to use with the contact

materials and the ZnO?

After some problems with adhesion binder 1 proved to be the best binder.

• How does the device stand outer strains?

This has not been tested in a large range, only in the production and measurement stage.The diodes’ saturation currents get lower when letting it rest in room temp for a coupleof days. The binder does behave different depending on humidity when printing. In themeasurements it was observed that the diodes wasn’t significantly light sensitive. Whenthe currents reached 1 mA some of the diodes got shortened.

• Is the goal achieved? Is it a product that works good enough for commercial

use and Roll-to-Roll production?

The diodes were tested alone as a half wave rectifier and in series as a full wave rectifier.The smallest size performed well as a half-wave rectifier at 4V due to the low saturationcurrent but had a too low forward current exceed the voltage drop in the next diode whenconnected in series. At higher voltages and ZnO concentrations this would probably notbe a problem, which could be tested in future research. The next smallest size (B) did notwork good alone due to high saturation currents but in series this was cancelled out and itperformed quite good except for a high voltage drop. The diode is unfortunately not fullyprintable since the Aluminum bottom has been ordered and manufactured using a methodthat is not a printing method.

5.2 Future work

If performance requirements are set one could try to produce a diode with the right ZnO con-centrations and diode size to match a specific purpose. The forward currents of the small diodesare quite low but if one surface treats the aluminum with phosphoric acid, for a longer time thantried in the project, to remove the aluminum oxide the forward current might be higher. Alsoreal capacitance vs. voltage measurements could be performed. The goal was to make a fullyprintable diode that was not reached, so new inks as a substitute for aluminum should be tried,for example tin-ink. Even though the diodes are not fully printable the diodes can still be massproduced with roll-to-roll production, but the process might have to be optimized for this typeof production.

Emma Persson 38

Bibliography

[1] www.thinfilm.no 2013-04-15

[2] Zinc Oxide Nanostructure: Synthesis and Properties Zhiyong Fan and Jia G. Lu, 22 April2005.

[3] Whisker ultra-high strength single crystal published from SANGYO TOSHO

[4] ZnO tetrapod nano crystals Marcus C. Newton and Paul A. Warburton Materials today,2007, 10, number 5.

[5] ZnO Schottky barriers and Ohmic contacts Leonard J. Brillson and Yicheng Lu Journal ofApplied Physics, 2011, 109, 121301.

[6] Printed electronics: The challenges involved in printing devices, interconnections and con-tacts based on inorganic materials. J. Perelaer, P. J. Smith, D. Mager, D. Soltman, S. K.Volkman, V Subramanian, J. G. Korvink and U. Schubert J. Mater. Chem,, 2010, 20,8446-8453.

[7] http://www.lope-c.com, accessed 20th January 2010.

[8] http://www.instructables.com/id/Photo-emulsion-Screen-Printing/ , , ,

[9] How to be a great screen printer Professor Steven Abbot.

[10] Group III Impurity Doped Zinc Oxide Thin Films Prepared by RF Magnetron SputteringLook, Reynolds Japanese Journal of Applied Physics, 1985, 24, L781

[11] Production and annealing of electron irridation damage in ZnO Look, Reynolds AppliedPhysics Letter, 1999, 75, 811

[12] Zinc Oxide Bulk, Thin Films and Nanostructures: Processing, Properties and ... ChennupatiJagadish, Stephen Pearton.

[13] Photovoltqaic Devices Using Blends of Branched CdSe Nanoparticles and Conjugated Poly-mers B. Sun, E. Marx and N. C. Greenham Nano Lett., 2003, 3, 961

[14] Hybrid Solar Cells with Prescribed Nanoscale Morphologies Based on Hyperbranched Semi-conductor Nanocrystals I. Gur, N. A. Fromer, C.-P. Chen, A. G. Kanaras and A. P. AlivisatosNano Lett, 2007, 7, 409

[15] Joining the un-joinable: Adhesion between low surface energy polymers using tetrapodal ZnOlinkers X. Jin, J. Strueben, L. Heepe, A. Kovalev, Y.K. Mishra, R. Adelung, S.N. Gorb, A.Staubitz, Advances Materials, 2012

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[16] Branched ZnO nanostructures as building blocks of photoelectrodes for efficient solar energyconversion Wei Chen, Yoingcai Qui and Shihe Yang Phys. Chem. Chem. Phys., 2012, 14,10872-10881

[17] Semiconductor Device Physics and Simulation J. S. Yuan, Juin Jei Liou.

[18] Introduction to solid state physics Charles Kittel.

[19] Handbook of Chemistry and Physics 66th edition CRS PRESS

[20] Conductivity, work function, and environmental stability of PEDOT:PSS thin films treatedwith sorbitol A.M. Nardesa, M. Kemerinka, M.M. de Kokb, E. Vinkenc, K. Maturovaa,R.A.J. Janssena

[21] http://ece-www.colorado.edu/ bart/book/ , , ,

[22] Schottky diodes Rick Cory, Skyworks Solutions Inc.

[23] A comprehensive review of ZnO materials and devices U. Ozgur, Ya. I. Alivov, C. Liu, A.Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S.-J. Cho and H. Morkoc Journal of AppliedPhysics, 2005, 98, 041301.

[24] Selection of non-alloyed ohmic contacts for ZnO nanostructure based devices N. KoteeswaraReddy, q. Ahsanulhaq, J. H. Kim, M. Devika and Y. B. Hahn Nanotechnology, 2007, 18,445710

[25] Basic Electronics Tutorials Wayne Stor

[26] Fundamentals of Zinc Oxide as a Semiconductor Anderson Janotti and Chris G Van deWalle Rep. Prog. Phys., 2009 72 126501.

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printed schottky diodes based upon zinc oxide materials694402/fulltext01.pdfschottky diode is used...

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