Eindhoven University of Technology

MASTER

Opto-electrical characterization of plasma-deposited zinc oxide thin films

van de Loo, B.W.H.

Award date:2012

DisclaimerThis document contains a student thesis (bachelor's or master's), as authored by a student at Eindhoven University of Technology. Studenttheses are made available in the TU/e repository upon obtaining the required degree. The grade received is not published on the documentas presented in the repository. The required complexity or quality of research of student theses may vary by program, and the requiredminimum study period may vary in duration.

General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.

Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain

Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.

Download date: 11. May. 2018

Eindhoven University of Technology,

Department of Applied Physics,

Plasma & Materials Processing.

the color of the line of the text box to No line before printing:

Color: No Line

A master thesis,

under the supervision of:

Dr. ir. H.C.M. Knoops,

Dr. M. Creatore,

Prof. dr. ir. W.M.M. Kessels.

Opto-Electrical Characterization

of Plasma-Deposited Zinc Oxide

Thin Films

B.W.H. van de Loo, BSc

August 2012 PMP 12-14

ee

Hall, FPP

FTIR

II

III

Abstract

Due to its high transparency and low resistivity, polycrystalline zinc oxide (ZnO) is used as a

transparent conductive oxide (TCO) in thin film photovoltaics. In this research, an expanding thermal

plasma (ETP) is used to deposit (doped) ZnO films (70-600 nm), using metal-organic precursors, i.e.

diethylzinc and trimethylaluminum, and oxygen. Process conditions of high pressure and high

diethylzinc flow rate allowed for dense films, with resistivity values as low as 44 10 cm at 300 nm

film thickness.

The conductivity of ZnO is determined by its carrier density and electron mobility. Intrinsic ZnO is an

n-type semiconductor, and by doping with aluminum, the carrier density can be increased. However,

free-carrier absorption reduces the transmission of light in the infrared (IR) spectral region. By

studying the behavior of this free-carrier absorption by means of optical diagnostic tools,

information can be inferred on the ingrain electrical properties.

In this work, a novel approach was used to further evaluate the ZnO films, based on extensive optical

modeling by means of spectroscopic ellipsometry and reflection Fourier transform infrared

spectroscopy. A Drude model was used to model the free-carrier absorption, and was extended by

taking a frequency-dependent broadening term, to correctly account for the decreased ionized

impurity scattering of the free electrons at high frequencies. By combining the Drude, Psemi-M0 and

Tauc-Lorentz oscillator models, the SE and FTIR data of ZnO films could be modeled. The optical

model was suitable to characterize ZnO films in terms of ingrain resistivity, i.e. optical mobility and

carrier density, thickness and transparency, and therefore provided a link between the optical and

electrical properties of ZnO.

Since optical diagnostic tools are not sensitive in probing grain boundary scattering, from a

combination of optical tools and direct-current measurements (i.e. Hall and four-point probe), the

grain boundary mobility could be derived. By following this approach, it was concluded that the grain

boundary scattering was only significant when the average grain size was small, i.e. at low film

thicknesses, and was more pronounced in aluminum-doped ZnO films than in intrinsic ZnO films. The

ingrain scattering mechanisms were further evaluated by applying Masetti model fits, to disclose the

limiting factor of the electron mobility in ZnO layers.

IV

V

Table of contents

Chapter 1, Introduction to Zinc Oxide ................................................................................. 7

1.1 Zinc oxide and its applications ...................................................................................................................7

ZnO as a TCO in photovoltaics ......................................................................................... 8

1.2 Deposition of ZnO films .............................................................................................................................. 10

1.3 Conductivity of ZnO ...................................................................................................................................... 11

The origin of free carriers in ZnO ................................................................................... 12

The origin of the mobility in ZnO .................................................................................... 12

Discriminating between scattering mechanisms in ZnO ................................................ 15

1.4 Optical properties of ZnO .......................................................................................................................... 17

1.5 ETP-MOCVD of AZO ...................................................................................................................................... 19

1.6 Routes to improve the properties of ZnO .......................................................................................... 20

Towards larger grains .................................................................................................... 21

Passivation of the grain boundaries .............................................................................. 21

Optimization of the doping ............................................................................................ 22

1.7 Goals of this work .......................................................................................................................................... 22

1.8 Thesis outline .................................................................................................................................................. 23

Chapter 2, Experimental Details ....................................................................................... 24

2.1 ETP-MOCVD setup ........................................................................................................................................ 24

2.2 Structural analysis techniques ................................................................................................................ 26

X-ray diffraction (XRD) ................................................................................................... 26

Atomic force microscopy (AFM) ..................................................................................... 27

Scanning electron microscopy (SEM) ............................................................................. 28

2.3 Optical analysis techniques ...................................................................................................................... 28

Spectroscopic ellipsometry (SE) ..................................................................................... 28

Reflectance Fourier transform infrared spectroscopy (FTIR) ......................................... 29

Integrating sphere ......................................................................................................... 29

2.4 Electrical analysis techniques ................................................................................................................. 30

Four-point probe (FPP) ................................................................................................... 30

Hall ................................................................................................................................. 30

VI

Chapter 3, Growth of ETP-MOCVD ZnO ............................................................................ 32

Chapter 4, Optical Modeling of ZnO .................................................................................. 38

4.1 Spectroscopic ellipsometry ...................................................................................................................... 38

Modeling the substrates (SE) ......................................................................................... 39

Modeling the ZnO films (SE) ........................................................................................... 40

4.2 Optical models of ZnO ................................................................................................................................. 41

Psemi-M0 model ............................................................................................................ 41

Tauc-Lorentz model ..........................................................