Progress Report

SCHOOL OF ELECTRICAL AND ELECTRONIC ENGINEERING

Progress Report

Temperature dependent characteristics of IGZO SBTFTs

Yuxin Ji 8573039

Supervised by: Prof. Aimin Song

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Nomenclature *All notations used in this report are listed here. Explanations of notations following each formula are thus omitted.

Abbreviation

Quantity

Magnitude/Unit

Cc

Depletion layer capacitance

F

Cox

Gate-oxide capacitance

F

Ea

Activation energy

eV

EF

Fermi Level

eV

Eg

Bandgap

eV

ε0

Electric constant

8.854x10-12 F/m

F

Electric field strength

N/C

h

Planck’s constant

4.136x10-15

ħ

Reduced Planck’s constant

h/2π

IDS

Drain source current

A

IDSO

Prefactor of drain current

/

J

Current density

A/m2

k

Boltzmann Constant

1.3806 × 10-23 m2 kg s-2 K-1

L

Gate Length

m

m*

Effective mass (of a carrier)

9.11×10−31 kg

N

Dopant concentration

m-3

q

Elementary charge

1.602x10-19 C

S

Surface area

m2

T

Temperature

K

VT H

Threshold voltage

V

Vs-th

Subthreshold swing

V

VDS

Drain source voltage

V

VGS

Gate source voltage

V

Von

Turn-on voltage

V

W

Gate width

m

X

Electron affinity

V

μ

Carrier mobility

m2 /Vs

μeff

Effective mobility

m2 /Vs

σ

Conductivity

S/m

Φ

Work function

V

ΦB

Schottky barrier

V

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Table of Contents Nomenclature ....................................................................................................... 2 1. Background ....................................................................................................... 4 1.1 Aims ............................................................................................................................................. 4 1.2 Objectives .................................................................................................................................... 4 1.3 Motivation ................................................................................................................................... 4 1.3.1 a-IGZO and Flat-panel Display................................................................................................. 4 1.3.2 SBTFT and Temperature.......................................................................................................... 5 1.4 Literature Review ......................................................................................................................... 5

2. Progress to Date ................................................................................................ 5 2.1 Project Plan .................................................................................................................................. 5 2.2 Health and Safety Risk Assessment ............................................................................................... 6 2.3 Technical Risk Analysis.................................................................................................................. 6 2.4 Practical Progress ......................................................................................................................... 6 2.4.1 The Discovery of a-IGZO ......................................................................................................... 6 2.4.2 Characteristics of a-IGZO ........................................................................................................ 7 2.4.3 TFT......................................................................................................................................... 9 2.4.4 Metal-Semiconductor Junction ..............................................................................................11

References .......................................................................................................... 14 Appendices ......................................................................................................... 16 1. Gantt Chart of Project Plans ...........................................................................................................16 2. Health and Safety Risk Assessment ................................................................................................17 3. Technical Risk Analysis...................................................................................................................20

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1. Background 1.1 Aims The aim of this project is to investigate characteristics of an amorphous Indium-Gallium-Zinc-Oxide Schottky Barrier Thin-Film Transistor (a-IGZO SBTFT) which particularly relate to and vary with temperature alterations by inspecting and revealing the temperature dependence of properties of a-IGZO SBTFTs via a series of measurements and analysis.

1.2 Objectives There are a series of tasks that need to be undertaken in order to fully understand the effects of temperature on the performance of a SBTFT. These include:  Intensive literature review on relevant key concepts  Simulated calculation on the output and transfer characteristics of a SBTFT  Technical risk assessment and familiarization of equipment  Experiment design on  Determination of Schottky barrier height  Measurement of IDS with varying temperature (T-1/T-(1/4))  Measurement of IDS vs. VGS and IDS vs. VDS  Analysis of data collected from the experiments, especially on  Dominance of different conduction mechanisms  Determination of transfer and output characteristics  Effects of varying temperature on IDS and thus the performance

1.3 Motivation 1.3.1 a-IGZO and Flat-panel Display The 21st century has witnessed an increasingly prosperous paperless media, and consequently the demand for flat-panel display has been augmenting in recent years. Transparent electronic components therefore come to the fore in the research field. With a wide bandgap and an oxide channel which is able to function as both an active and a passive element, TFT is anticipated as one of the most auspicious material for future generation flat-panel display. Here, what really makes TFT excel in such applications is the composition of the oxide layer (Fortunato, Barquinha and Martins, 2012). The oxide active layer discussed in this report is composed of a-IGZO which has been intensively investigated, as it exhibits a higher mobility (10~25cm2/Vs) compared to that of hydrogenated amorphous silicon (0.1~0.5cm2/Vs) and can be fabricated under low temperature (200°C). Hence, aIGZO TFT appears to be an extraordinarily suitable material to construct active matrix display panels on plastic substrates with large size, high resolution and low power consumption (Fortunato, Barquinha and Martins, 2012). So far, there are many successful commercial digital devices whose display panels are made of IGZO TFTs. Sharp Electronics started to mass-produce IGZO TFT LCD panels and introduced the panels in 4

smartphones, tablets and TVs (Androidauthority, 2014). iPad Air, which was announced by Apple in 2013, received an overwhelming welcome in the market (extremetech.com, 2013). Overall, statistics from the consumer’s market prove that IGZO TFT will be a mainstream constructing material for matrix display in the near future. Therefore, its novel characteristics and commercial value are the first motivation of this project.

1.3.2 SBTFT and Temperature The Schottky barrier (SB), formed from deformation of energy bands in the metal-semiconductor interface, is a unique rectifying property that is put to good use in many electronic devices such as Schottky diode and Schottky transistor. These devices work differently from conventional PNjunction based devices in carrier transport mechanisms, fabrication, application and many other aspects (Chandra and Prasad, 1983). Till now, researchers have reported very few works on SBTFTs, let alone SBTFTs with a-IGZO active channel. One possible reason for this is that grain boundaries present in the channel may deteriorate junction characteristics, which consequently results in devices with intolerable performance (Lin et al., 2001). Therefore, the second motivation of the project is to investigate the characteristics of a IGZO SBTFT, a relatively new device, so as to replace state-of-the-art SB poly-Si TFT for an even better performance.

1.4 Literature Review In this section, reviewed literatures are categorized here in accordance with the key questions discussed in each weekly meeting. These categories are listed as follows.  The chemical structure of IGZO  Composition and fabrication of a TFT  Mobility and factors that determine the mobility of IGZO  Main conduction mechanisms of IGZO  Conduction mechanism dominance under different conditions  The concept of threshold voltage, turn-on voltage and subthreshold swing  Transfer and output characteristics of TFT  The concept of Schottky barrier  The concept of thermionic emission and Schottky effect  I-V characteristic of a Schottky diode Since literature review is of paramount importance in earlier stages of the project, time and effort allocated to the project was devoted entirely to literature review until Week 6, and will be continuously allotted to more intensive literature review process in the following weeks for a higher level of solidity and profundity in understanding relevant concepts. Therefore, a detailed elaboration of findings from literature review will be discussed in Section 2.4.

2. Progress to Date 2.1 Project Plan This project is a research-based project which requires intensive research and literature review processes at preparation stage, which will last for around 9 to 12 weeks. Amongst the 12 weeks, 2 5

weeks will be spent on generating a progress report, illustrating the progress up to Week 6. In the rest of the weeks, students undertaking the project are given specific questions to do research on, and are expected to present their answers to the group members and the supervisor during weekly meetings in a logical manner. The next stage is experimental stage, which will be started with technical risk assessment and handson familiarization of equipment. Lasting for about 2 weeks, this procedure is crucial because the experiments will be conducted in D13a/b laboratory/cleanroom specifically designed for microelectronics and nanostructures, which will be a relatively unfamiliar experience. From February 2016 onwards, 3 to 4 afternoons will be spent to perform a series of experiments so as to gather data for relevant analysis later. The rest of the time of Semester 2 will be used to generate a final report and prepare for the oral examination afterwards. If there are abundant time left, fabrication of new devices can be considered as an optional exploration of the topic. A detailed Gantt Chart of the project plan is provided in Appendices 1. Note that this project plan is not finalized and always subject to change.

2.2 Health and Safety Risk Assessment Please refer to Appendices 2.

2.3 Technical Risk Analysis Please refer to Appendices 3.

2.4 Practical Progress Following Section 1.4, this section is aimed at elaborating and corroborating relevant theories and findings from various literatures.

2.4.1 The Discovery of a-IGZO The discovery of a-IGZO started and advanced with commercial flat-panel display industry. In days of yore, the semiconductors inside display medium were fabricated using traditionally accepted channel materials such as Si, GaAs and GaN. In late 1990s, novel and unconventional metal oxide such as ZnO was then well recognized as a better substitute due to its extraordinary intrinsic properties at heterojuctions. More importantly, polycrystalline ZnO (poly-ZnO) still possesses its active characteristics when the device is fabricated below 300°C, making it a more promising channel layer than hydrogenated amorphous silicon (a-Si:H) which was previously prevailing in all flat-panel displays (Kamiya, Nomura and Hosono, 2010). Since 2000 onwards, there has been an increasingly research interest in flexible electronics, and more characteristics of ZnO was revealed at that time. What makes ZnO desirable is the remarkable low fabrication temperature below 300°C, but it suffers from instability and unrilability brought by grain boundaries. Even worse, threshold voltage of TFT fabricated using poly-ZnO cannot be managed easily because of a great amount of residual free electrons (>1017 cm−3) in its native defects (Kamiya, Nomura and Hosono, 2010). In order to avoid these disadvantages, researchers started to fabricate TFTs with single-crystalline InGaZnO4 whose residual carrier density was lower. Accidentally, it was discovered that amorphous 6

IGZO compound had even lower residual carrier density (Von, electrons are introduced from the source to the channel to form an accumulation layer, and extra electrons will travel through this layer and eventually be withdrawn at drain. a) Now if VDS