原子層沉積法成長單晶氧化鋅薄膜之光學與晶體結構特性研究

國 立 交 通 大 學

光 電 工 程 學 系 博 士 班

博 士 論 文

原子層沉積法成長單晶氧化鋅薄膜

之光學與晶體結構特性研究

Optical and Structural Properties of ZnO Epitaxial Films Grown

by Atomic Layer Deposition

研 究 生：楊松

指導教授：謝文峰 教授

原子層沉積法成長單晶氧化鋅薄膜

之光學與晶體結構特性研究

Optical and Structural Properties ZnO Epitaxial Films Grown by

Atomic Layer Deposition

研 究 生：

楊松

Student：Song Yang

指導教授：

謝文峰

Advisor：Dr. Weng-Feng Hsieh

國 立 交 通 大 學

光電工程學系博士班

博 士 論 文

A Dissertation

Submitted to Department of Photonics and Institute of Electro-Optical Engineering College of Electrical Engineering and Computer Science

National Chiao Tung University In Partial Fulfillment of the Requirements

For the Degree of Doctor of Philosophy

in

Electro-Optical Engineering

December 2012

Hsinchu, Taiwan, Republic of China

I

原子層沉積法成長單晶氧化鋅薄膜之光學與晶體結構特性研究

研究生: 楊松

指導教授: 謝文峰

國立交通大學光電工程學系博士班

摘要

我們成功地利用原子層沉積法成長纖維鋅礦結構單晶氧化鋅薄膜於 c-plane 與 m-plane 藍 寶石碁板，並且利用 X-ray 繞射與穿遂式電子顯微鏡研究其晶體結構特性。 經過高溫退 火處理之後的氧化鋅單晶薄膜，其晶體品質顯示明顯的改進，並發現基面堆疊缺陷(basal stacking fault)為其主要的晶體結構缺陷。我們在 c-plane 藍寶石碁板上成長出單晶氧化鋅薄 膜，其磊晶面關係為 3 2 } 0 1 10 ){ 0001 ( || } 0 1 10 ){ 0001 ( _{ZnO Al O} ；而在 m-plane 藍寶石碁板上之磊晶 面關係為 3 2 0 1 2 1 ) 0 1 10 ( || 0001 ) 0 1 10 (  !ZnO  !Al O 。以光激螢光光譜觀察到以3.325 eV 為中 心的主要發光波段，可能來自於基面堆疊缺陷。由於基面堆疊缺陷的原子結構可視為一層 非常薄的閃鋅礦晶體內嵌在纖維鋅礦結構之中，形成的基面堆疊缺陷量子井結構。我們也 研究了熱退火處理對單晶氧化鋅薄膜的晶體結構特性和發光特性所造成的影響。另外，我 們也透過時間解析光激螢光實驗來測定此基面堆疊缺陷量子井結構的發光機制與近能帶間 隙發光的特性。我們觀察到光激載子被基面堆疊缺陷量子井結構局限後，形成量子局限激 子。而此量子局限激子更持續地受到由許多局限能態所形成的局限效應所束縛。這些局限

II

能態可能是由隨機分佈的基面堆疊缺陷之間的量子耦合效應所形成；量子耦合效應是由於 局限在不同量子井中的電子之間發生波函數交疊的關係，此現象亦形成了局限能態。由於 測得的氧化鋅薄膜的低施子濃度以及近能帶能隙發光並無激子遷移的現象，所以我們排除 了靠近基面堆疊缺陷的施子以及離子成份濃度擾動這兩個因素形成局限能態的可能性。

III

Optical and Structural Properties of ZnO Epitaxial Films Grown by

Atomic Layer Deposition

Student: Song Yang

Advisor: Wen-Feng Hsieh

Department of Photonics and Institute of Electro-Optical Engineering

National Chiao Tung University

Abstract

We have successfully grown mono-crystalline ZnO epitaxial films on c-plane and m-plane sapphire substrates by using the atomic layer deposition. X-ray diffraction and transmission electron microscopy were employed to verify the structural properties of the ZnO thin films. The structure of the ZnO epi-films exhibits significantly improvement upon thermal annealing and intrinsic types of basal plane stacking faults (BSFs) are the predominant structural defects in the ZnO films after thermal treatment. The ZnO epi-films grown on the c-plane and m-plane sapphires have the epitaxial relationships of

3 2 } 0 1 10 ){ 0001 ( || } 0 1 10 ){ 0001 ( _{ZnO Al O} and 3 2 0 1 2 1 ) 0 1 10 ( || 0001 ) 0 1 10 (  !_ZnO  !_{Al O} , respectively. The BSF is found to contribute to the emission at 3.325 eV in the photoluminescence (PL) spectra of the annealed ZnO films. This is attributed to quantum-well (QW) structure formed by

IV

the BSF, which has the thin layer of zinc blend structure embedded in the wurtzite structure ZnO layer. The influence of thermal annealing to the structural and optical properties of the ZnO epi-films was also investigated. Through the time-resolved PL, we determined the decay times of the BSF related emission and the near-band-edge (NBE) emission. The QWs formed by the BSFs are found to trap the carriers to form BSF-bound excitons. The PL measurements revel that the BSF-bound excitons are influenced by the localization effect, which consists of localization states, and these bound excitons migrate among these localization states. Such localization states are attributed to the quantum coupling effect among the random distributed BSFs; the quantum coupling effect results from the wave function overlapping of the electrons bound in QWs and leads to the localization states. Because of the obtained low donor concentration and near band emission without the phenomenon of exciton migration, we exclude the donors in the vicinity of BSFs and the alloy density fluctuation from the origins of these localization states.

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Acknowledgement

在學校攻讀碩博士學位的這一段歷程裡，感謝我的指導教授謝文峰老師這一路上的耐 心支援與指導，讓我在尋找題目、實驗與理論探討的過程當中學習到很多。也感謝徐嘉鴻 老師在同步輻射資源上的支援與X-ray 實驗與理論上的指導，讓學生得以在晶體材料的研 究上有著非常大的幫助。也感謝口試委員們對的指正與建議，使本論文更臻完善，也讓我 對於研究的未來的方向有更進一步的想法與做法。也感謝實驗室裡面的伙伴們長久以來的 互相支持，並一起渡過了大大小小像是工安環安的難關。感謝阿政學長的大力幫忙，還在 關鍵時刻從瑞典帶回來了珍貴的量測資料，幫助我通過最後的畢業門檻。這一段艱辛的過 程之中，也非常感謝黃董、維仁、至賢、智章學長們從碩士班以來這一段時間裡無數的討 論、大餐、咖啡、是非與八卦，讓我在課業繁忙之餘培養了許多興趣與嗜好，並讓我不負 菸酒生的威名。小郭、碧軒、厚仁、智雅，一起攻讀博士班的同伴們，在眼花撩亂的實驗 室事務和實驗操作上給了我很多的幫忙。 我要感謝我的雙親，在這一路上無私的支持與鼓勵，讓我能夠在這漫長的求學過程中 能夠得以持續的走下去。 最後感謝國科會的計劃，讓此論文得以順利的完成。

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Table of Contents

Abstract in Chinese«««««««««««««««««««««««««««, Abstract in English««««««««««««««««««««««««««,,, Acknowledgement««««««««««««««««««««««««««««««V Table of Contents««««««««««««««««««««««««««««9I List of Figure CaptiRQV««««««««««««««««««««««««« X Chapter 1 Introduction«««««««««««««««««««««««««  2YHUYLHZ R I =Q2«««««««««««««««« ««««««« 1.1.1 ZnO Properties««««««««««««««««««««««« 1.1.2 Growth of ZnO Thin Film««««««««««««««««« 1.2 Atomic Layer Deposition«««««««««««««««««««««« 1.2.1 App lic at io n o f ALD ««« ««« «««« ««« ««« ««« 1.2.2 ZnO grown by ALD««««««««««««««««««««5 1.3 Mot ivat io n« «««««« «««« «««««««« «««««« « 1.3.1 The Growth Mechanism of the ALD Reaction««««««««««6 1.3.2 Epitaxial Growth of Polar and Non-Polar ZnO Films««««««« 1.3.3 Research of the Basal Stacking Faults in Wurtzite ZnO««««««8 1.4 Organization of this Dissertation««««««««««««««««««

Chapter 2 Theoretical Background«««««««««««««««««««2 2.1 Atomic Layer Deposition Growth Mechanism««««««««««««2 2.2 Photoluminescence of ZnO««««««««««««««««««««4 2.2.1 Free Excito n (FX) ««««««««««««««««««««4

VII

2.2.2 Bound Exciton Complex««««««««««««««««««5 2.2.3 Basal Stacking Faults Emission«««««««««««««««6 2.3 XRD Measurement «««««««««««««««««««««««7 2.3.1 X-ray Diffraction Theory««««««««««««««««««7 2.3.2 Radial Scan«««««««««««««««««««««««« 2.3.3 Ro ck ing C ur ve ««« ««« «« «««« «««« ««« ««20 2.3.4 Azimuthal Scan««««««««««««««««««««««1 2.3.5 X-ray Reflect ivit y«««««««««««««««««««« 2 2.4 Transmission Electron Microscope«««««««««««««««««4 2.4.1 TEM setup«««««««««««««««««««««««« 5 2.4.2 Dark Field Imaginat io n««««««««««««««««««6 2.4.3 Basal Stacking Fault Analysis by TEM««««««««««««7 2.5 AF0««««««««««««««««««««««««««««« 30 C hapt er 3 Exp e U LPH QW V« ««« «« « ««« ««« «« ««« «« «««  3 3.1 Atomic Layer Deposition System and Growth Procedure«««««««««3 3.2 X-ray Diffraction««««««««««««««««««««««««3 3.3 Atomic Force Microscope««««««««««««««««««««4 3.4 Transmission Electron Microscopy«««««««««««««««««5 3.5 Photoluminescence«««««««««««««««««««««««6 3.5.1 Low Temperature Photoluminescence ««««««««««««6 3.5.2 Time Resolved Photoluminescence Measurement ««««««««7 Chapter 4 C-plane ZnO grown on c-plane Sapphire by ALD««««««««««9 4.1 XRD Measurement s «««««««««««««««««««««« 9  șș 6FDQ«««««««««««««««««««««««« 9

VIII

 Ȧ-rocking Curve««««««««««««««««««««««41 4.1.3 Azimuthal Cone Scans«««««««««««««««««««3 4.2 TEM Analysis«««««««««««««««««««««««««6 4.3 AFM Measurements«««««««««««««««««««««««50  3KRWROXPLQHVFHQFH««««««««« ««««««««««««««1 4.4.1 Temperature DependeQW 3KRWROXPLQHVFHQFH««««««««««3 4.4.2 Power Dependent Photoluminescence ««««««««««««6 4.5 Conclusio ns«««««««««««««««««««««««««« 8 Chapter 5 m-plane ZnO grown on m-plane Sapphire by ALD ««««««««««5 5.1 XRD measurements««««««««««««««««««««««« 6 5.1.1 șș VFDQ DQG Ȧ-rocking curve«««««««««««««««6 5.1.2 Azimuthal cone scans««««««««««««««««««««8 5.1.3 X-ray Reflect ivit y««««««««««««««««««««70 5 . 2 T E M « « « « « « « « « « « « « «« « « « « « « « « « « « « « 7 1 5.2.1 Cross Section Images with Zone Axis [1210]ZnO ««««««««71

5.2.2 Cross Section Images with Zone Axis [0001 ]ZnO «««««««««4

5.2.3 Fourier Filtered Images Analysis«««««««««««««««6 5.3 AFM««««««««««««««««««««««««««««« 8 5.4 Photoluminescence«««««««««««««««««««««««9 5.4.1 Low-Temperature Photoluminesce nce««««««««««««9 5.4.2 Time-Resolved Photoluminescence««««««««««««««81 5.5 Conclusio ns«««««««««««««««««««««««««« 82 Chapter 6 Recombination dynamics of localized exciton in Basal Stacking Faults«6

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6.1 Temperature Dependent and Power Dependent Photoluminescence«««««7 6.2 Time-Resolved Photoluminescence«««««««««««««««««9 6.3 Spectra Dependent Decay Times««««««««««««««««««91 6.4 Conclusio ns«««««««««««««««««««««««««« 7 Chapter 7 Conclusions and Prospective«««««««««««««««««100 7.1 Conclusions««««««««««««««««««««««««««100 7.2 Prospective«««««««««««««««««««««««««««102

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List of Figures

Figure 1-1 Stick and ball representations of three ZnO crystal structures«««««««««2 Figure 2-1 Schematics of one cycle of ALD growth steps««««««««««..««««3 Figure 2-2 The schematic plot of the band alignment of ZnO at WZ/ZB/WZ regions««««7 Figure 2-3 Bragg diffraction condition in real space and reciprocal space«««««««««8 Figure 2-4 The radial scan situation in the real space and reciprocal space««««««««20 Figure 2-5 T -rocking scan situations in the real and reciprocal spaces for ideal and non-ideal lattice structures««««««««««««««««««««««««««««««« Figure 2-6 Azimuthal angle scan situations in the real and reciprocal spaces for ideal and non-ideal lattice structures«««««««««««««««««««««««««««2 Figure 2-7 X-ray reflectivity measurement of the thin film structure and the analysis of the obtained data««««««««««««««««««««««««««««««««3 Figure 2-8 Simulated XRR curves of the ZnO films on sapphire substrate with different Rrms

values««««««««««««««««««««««««««««««««««..24 Figure 2-9 The measurement setup of TEM in bright field imaging and selected area diffraction modes«««««««««««««««««««««««««««««««««««6 Figure 2-10 The measurement setup of dark filed imaging with the selected spots with specific diffraction vector g««««««««««««««««««««««««««««««7 Figure 2-11 Three kinds of planes with different atom stacking arrangements in ZnO material marked as A-, B-, and C- planes«««««««««««««««««««««««««8 Figure 2-12 Four types of stacking faults in ZnO««««««««...«««««««««9 Figure 2-13 The measurement setup of the atomic force microscope«««««««««««31 Figure 3-1 The four-circle diffractometer in NSRRC at beamline BL13A«««««««««4

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Figure 3-2 Veeco Dimension 5006FDQQLQJ3UREH0LFURVFRSH««««...««««5 Figure 3-3 The TEM measurement instruments: the focused ion beam and Philips TECNAI-20 field emission gun type TEM««««««««««««««««««««««.««««6 Figure 3-4 The sketch of the photoluminescence measurement setup««««««««««7 Figure 3-5 The sketch of the TRPL measurement setup««««««««««««««««8 Figure 4-1 The radial scans (ș-2ș scan) along surface normal of the as-deposited and annealed ZnO ILOPV««««««««««««««««««««««««««««««««««40 Figure 4-2 The ș-rocking curves of the as-deposited and annealed ZnO layers and the variation of the FWHM of PS and PB as a function of annealing temperature««««««««««««...42

Figure 4-3 The I -scan across the off-normal ZnO {10 11} reflection of as-deposited and

annealed ZnO layers, together with the I -scan across sapphire {1126}«««««««««44

Figure 4-4 The cross sectional TEM image of the as-deposited ZnO epi-layer along the [1120]

zone axis, the SAED pattern, and the image of 800°C annealed sample taken under the same pole«««««««««««««««««««««««««««««««««««7 Figure 4-5 The bright-filed images of the ZnO layer annealed at 800°C with diffraction vector g set to (10 11) , (0002), and (10 10) «««««««««««««««««««««««

Figure 4-6 The AFM images of the as-deposited and annealed ZnO layers with the scan image profiles of the as-deposited and annealed samples«««««««««««««««.«««51 Figure 4-7 Low temperature PL spectra taken at 10 K of the ZnO films grown by ALD and PLD methods««««««««««««««««««««««««««««««««««2 Figure 4-8 Temperature dependent PL spectra of the ZnO film taken between 10 and 280 K and the energy versus temperature plot of the BSF and NBE emissions««««««««««5 Figure 4-9 The power dependent PL spectra recorded at 10 K of the ZnO film grown by ALD and annealed at 800°C««««««««««««««««««««««««««««««6

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Figure 5-1 XRD radial scans along the surface normal (ș-2ș scan) of an as-deposited ZnO film and a 800ºC annealed sample, and the Ȧ rocking curves across the (10 10)_ZnO specular

reflection««««««««««««««««««««««««««««««««««7 Figure 5-2 The azimut hal reflect io n scan across the {10 12}ZnO and {3030}Al₂O₃

planes«««««««««««««««««..««««««««««««««««««9 Figure 5-3 X-ray reflectivity of the as-deposited ZnO films and those after annealed at 400, 600, and 800 ºC«««««««««««««««««««««««««««««««««71 Figure 5-4 The cross sectional TEM image and the SAED pattern taken along the

ZnO ] 0 1 2 1

[ zone axis, and the bright field images recorded with diffraction vector g equal to ZnO ) 0 1 10 ( and (0002 )ZnO «««««««««««««««««««««««««««3

Figure 5-5 The cross sectional TEM image and the selected area electron diffraction (SAED) pattern taken along the zone axis of [0001 ]ZnO , and the bright field images taken with diffraction

vector g equal to (10 10)_ZnO and (1210)_ZnO ««««««««««««««««««5

Figure 5-6 The high resolution images of the interface with the zone axis of [1210]ZnO and

ZnO

] 0001

[ , and the inset is the corresponding Fourier filtered image«««««.««««««

Figure 5-7 AFM images of the as-deposited and annealed m-ZnO films with surface roughness (Rrms) of 1.62 and 1.38 nm, respectively««««««««««««««««««««««7

Figure 5-8 The time integrated photoluminescence at 5 K of the as-deposited sample and sample annealed at 400 and 800°C with the inset of the annealing temperature dependent intensity ratio of NBE band to BSF band«««««««««««««««««««««««««««81 Figure 5-9 The time resolved photoluminescence at 5 K of the NBE and B band in the sample annealed at 800°C««««««««««««««««««««««««««««««81

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Figure 6-1 The power dependent TI-PL spectra recorded at 5 K of the ZnO film grown by ALD and annealed at 600°C and the intensity versus the excitation power of the BSF and NBE emissions««««««««««««««««««««««««««««««««««7 Figure 6-2 The temperature dependent TI-PL spectra of the ZnO film taken between 10 and 200 K, the energy versus temperature plot of the BSF and NBE emissions««««««««««.88 Figure 6-3 The TR-PL spectra at temperature 10 K with excLWDWLRQSRZHURIȝ:DQGHDFK curve is integrated with time interval of 6 ps after pulsed excitation«««««««««««90 Figure 6-4 The decay curves of the intensity integrated from the BSF and NBE emissions in energy ranges of 3.280 ~ 3.345 eV and 3.345 ~ 3.H9UHVSHFWLYHO\««««««««««91 Figure 6-5 The spectra dependent decay times are the plots with the same excitation power ȝ:DWGLIIHUHQWWHPSHUDWXUHDQG.DQGZLWKGLIIHUHQWH[FLWDWLRQSRZHUDQGȝ: at the same temperature 5 K««««««««««««««««««««««««««92 Figure 6-6 The potential fluctuation due to the fluctuation of alloy density««««««««4 Figure 6-7 The wave functions of the lowest two energy levels of the symmetric QWs with a central barrier width of 4 nm, and the confinement energies of the lowest two states as function of the central barrier width«««««««««««««««««««««««««««5 Figure 6-8 Band profiles (black curve), electron envelope functions (red curve) and energy (dotted red line) for electron confined on a D0-BSF complex with distance d of 2.5 nm between donor and BSF plane««««««««««««««««««««««««««««.«6

1

Chapter 1 Introduction

1.1 Overview of ZnO

The ZnO material is II-VI semiconductor and attracts much research attention for decades because of its wide direct band gap about 3.374 eV and large exciton binding energy about 60 meV, which leads the high emission efficiency and the lasing action based on exciton recombination at room temperature. In addition, the other properties of the ZnO such as the non-toxicity, high physical stability, and piezoelectricity also reveal the potential for industrial and advanced optics-electro applications [1-9]. Over the past decade, in order to perform the properties of the quantum physics many efforts are put into the topic of fabricating the nano-scale structures of ZnO such as nano-rods [10], quantum-dots [11] and quantum-wells. [12]

1.1.1 Properties of ZnO

The ZnO has the crystal structures in either hexagonal wurtzite, cubic zinc-blende, or cubic rocksalt structure, they are shown in Fig. 1-1. [13] At the ambient conditions, the thermodynamically stable ZnO structure is wurtzite structure, which belongs to the space group 4

6 v

C and has the lattice constants a = 3.243 and c = 5.203 Å. The ZnO with zinc-blende crystal structure could be formed on cubic substrates and the estimated lattice constant ranges from 4.37 ~ 4.47 Å. The basal stacking faults can be considered as a thin layer of zinc-blende structure being embedded in the wurtzite ZnO. According to the arrangement of atoms in basal stacking fault, the lattice constant of zinc-blende could be estimated as about 4.586 Å from the lattice constants of wurtzite ZnO. Besides, the rocksalt ZnO structure could be obtained at relatively high pressures from the wurtzite

2

structure due to the phase transition, and the estimated lattice constant ranges from 4.271 to 4.294 Å. [13]

Fig. 1-1 Stick and ball representation of ZnO crystal structures: (a) cubic rocksalt (B1), (b) cubic zinc blend (B3), and (c) hexagonal wurtzite (B4). The shaded gray and black spheres denote Zn and O atoms, respectively.

As the structures shown in Fig. 1-1, the atomic arrangement of zinc-blende structure is similar to that of the wurtzite structure, but the zinc-blende ZnO structure is metastable and is reported to be grown on cubic substrates such as ZnS. [31] Many efforts have been put onto the research of wurtzite ZnO epitaxy, while a few experimental and theoretical literatures had reported on zinc-blende ZnO growth and its fundamental properties. The zinc-blende structure has the highest symmetry compatible with the existence of piezoelectric polarization under the strain in the [001] directions, which offers an attractive platform for exploring the excitonic systems without the perturbation field. [14]

3

planes, which determine the optical and electrical properties of the films. Strong spontaneous polarization and piezoelectric polarization at the interfaces of ZnO based heterostructures induce a large internal electric field along the c-axis that results in the quantum-confined Stark effect leading to a spectral red-shift and a dramatic decrease in the luminous efficiency. In order to avoid this problem, many efforts have been put to enhance the luminous efficiency by taking advantage of the non-polar m-plane orientated ZnO layers [15].

1.1.2 Growth of ZnO Thin Films

The traditional methods of fabricating the ZnO films include the metal-organic chemical deposition (MOCVD), molecular beam epitaxial (MBE), radio-frequency sputter (RF-sputter), and pulse-laser deposition (PLD), which usually demand high growth temperature and high vacuum environment. In addition, many kinds of substrates are used to fabricate these ZnO structures. The c-plane sapphire substrate is usually adopted for fabricating the c-plane ZnO films. For reducing the strains, diminishing dislocation density, and obtaining high quality of epitaxial crystalline, ZnO are also fabricated on other substrates, such as Si, SiC, GaAs, CaF2, and ScAlMgO4. [13] Nonetheless, in

order to fabricate the non-polar orientated ZnO layers the non-polar sapphire such as r- and m-plane sapphires are adopted as the substrates. [16]

1.2 Atomic Layer Deposition

Atomic layer deposition (ALD) is a chemical vapor deposition technique, which is used to manufacture inorganic material layer with thickness down to a fraction of a

4

monolayer. ALD is able to deposit a conformal layer with high quality on extremely complex shapes that is the unique feature among various deposition techniques. ALD based on sequential self-terminating gas-solid reactions has been applied for about four decades. In recent years, the increasing research interest in use of ALD is attributed to the scaling down microelectronic devices.

1.2.1 Application of ALD

Because of the ability of controlling thickness, ALD has been studied for depositing the gate insulator layer of the transistor. The gate insulator layer, traditionally made by silicon dioxide to separate the gate terminal of a transistor from the source and drain terminals as well as the conductive channel that connects source and drain when the transistor is turned on. The gate oxide layer sustaining electric field to modulate the conductance of the channel should be made as thin as possible to increase performance. However, while the thickness of the gate layer is down to few nanometers the quantum mechanical phenomenon of electron tunneling occurs between the gate and channel and leads to increase power consumption. In order to overcome the increase in power consumption, the high-ț GLHOHFWULF PDWHULDOV VXFK DV $O2O3, ZrO2, and HfO2 are used

instead of silicon dioxide for higher gate capacitance and lower leakage effects. [17,18] Besides, in the development of DRAM capacitor, the high conformal depositing requirement leads ALD to the most promising depositing technique. [19] In order to assure sufficient data retention time of DRAM, high charge storage capacitance is required. Three-dimensional capacitor structures with high aspect ratios are therefore commonly used in the DRAM industry for small cell size and thus reduce chip cost. For the three-dimensional structures of nano-rods, and nano-particles, ALD also shows excellent conformal depositing ability. [20,21]

5

1.2.2 ZnO grown by ALD

Recently years, the research interest in fabricating the ZnO thin film by ALD is increasing, and it is able to obtain the epitaxial quality. Depending on the different substrates, the epitaxial ZnO thin films have been grown on different surface planes. There has been reported that the c-plane ZnO thin films could be grown on the c-plane sapphire, c-plane GaN, or yttria-stabilized zirconia with (111) layer; and the m-plane ZnO has been grown on the m-plane sapphire. [22,23,24] For the precursors, the sources of zinc atoms could be the diethyl-zinc (DEZ) or zinc acetate, and the sources of oxygen could be the water or nitrous oxide. [25,26]

The ALD method is also able to deposit the functional ZnO layer by doping other material such as Mg, Al, Ga, P, and N. For doping Mg, Al, Ga, and P, the cyclopentadienylmagnesium, trimethy-aluminum, triethylgallium, and trimethylphosphite are used as the precursors, respectively, to provide the dopants. Controlling the doping ratio has been implemented by manipulating the number ratio of ZnO and dopant-oxide layers, or by using the mixed precursor of DEZ and dopant sources. [32-35] For N doping, the ammonium hydroxide and ammonia have been used as the precursor, and the doping ratio controlling is implemented by the number ratio of depositing layers or mixing the dopant precursor with the carrier gas. [36,37]

1.3 Motivation

The ALD has usually been used for the deposition of the insulator layer of the DRAM and transistors. These growth features of the ALD are very powerful for fabricating electronic devices; especially for the dimension downscales to the few

6

nanometers in the complex three-dimensional structures. However, the ALD on the respect of fabricating the electro-optic device is still very lack of understanding and needs more efforts on research.

1.3.1 The Growth Mechanism of the ALD Reaction

ALD growth mechanism bases on the sequential self-terminating chemical reactions that is a kind of chemical style/way. Such a growth technique is different from the deposition techniques based on the physical style/way such as PLD, MBE, and RF-sputtering, which decompose the material into particles or plasma in high temperature for following deposition. The demand of depositing particles and/or plasma is very high vacuum to prevent from colliding with the undesired gas molecules that raises the difficulty of deposition. Besides, the dissolving problem is usually the problem for depositing the alloy material with high concentration by using the techniques in physical ways. Therefore, the depositing with sequential self-terminating chemical reactions is reasonably considered to make sure that the different materials of alloy would be deposited layer-by-layer along growing direction. The concentration of the alloy material could be expected to closely 50 %, in theory, and in the meanwhile the demand of the growth temperature and vacuum condition would be rather low. In addition, the controlling of depositing thickness in few atoms and the conformal depositing layer on complex structure could be reached. Such features of ALD technique are reasonably expected and worthy of investing in effort to research.

1.3.2 Epitaxial Growth of Polar and Non-Polar ZnO Films

7

MBE, PLD, MOCVD, and RF-sputter require high growth temperature, high vacuum, high precursor consuming, or high electrical power. In ALD, the growth temperature depends on the reaction temperature of the precursors. For growing the ZnO thin film, diethyl-zinc (DEZ) and water are used as the precursors for providing the source of zinc and oxygen atoms with suitable growth temperature at 200 oC. The precursors reacting can occur at the pressure of about few Torrs, which means that the low requirement of vacuum. The consumption of the precursors depends on the reaction surface area and the capacity of reaction chamber.

In addition to the requirement of growth conditions, to fabricate m-plane ZnO epi-films with high structural perfection and smooth surface morphology is still difficult for these tradition deposition techniques. One common problem is the coexistence of minor domains with (10 13) which are difficult to eliminate. [27,28,29] Moreover,

surface morphologies of the deposited m-plane ZnO films on foreign substrates often exhibit stripe features elongated along a direction either parallel or normal to the ZnO c-axis depending on the growth conditions. These features strongly hamper the applications of ZnO-based heterostructures. For the ALD technique, the sequential self-terminating gas-solid reactions are expected to lead to the layer-by-layer growth mechanism, which is a two-dimensional growth mode. It is expected to grow the polar and non-polar ZnO thin films with epitaxial quality, high flatness, high uniform thickness, and without the co-existing domains. Therefore, to investigate the fabrication process, chemical reaction situation, influence of lattice mismatch, and crystalline structure property are very import issues for the developing the fabrication technique.

8

1.3.3 Research of the Basal Stacking Faults in Wurtzite ZnO

The growth mechanism of ALD is considered to influence the structure of the as-grown ZnO thin films and leads to specific crystalline features such as the high-density basal stacking fault (BSF). Theoretical study showed that the BSF structure embedded in the ZnO thin film leads to the band alignment in the form of type-II quantum wells, and influences the photoluminescence properties. [30] Such a specific band alignment is attributed to that BSFs in the material of wrutzite (WZ) structure has the lattice structure of zinc blend (ZB). For the research of ZnO material, the ZB structure is very difficult to be grown, which leads to that the understanding of ZB ZnO material is still insufficient and desired. As the development of the ab initio technique, many properties the ZB ZnO are calculated such as the lattice constants, and band gaps. The ZB ZnO is also predicated to have lower carrier scattering and higher doping efficiencies than the WZ ZnO. [14] Therefore, such high density of the BSFs in the ZnO thin films makes measuring and observing the properties of ZB ZnO thin film possible. Through the photoluminescence measurements, we investigate the ZB ZnO thin film by observing the dynamic of the excitons in the BSFs.

1.4 Organization of this Dissertation

In this thesis, we present the growth of ZnO thin films by ALD, and investigate the influence of the thermal annealing treatment. And we study the structural properties of these ZnO films through the measurements of X-ray diffraction (XRD), transmission electron microscope (TEM), and atomic force microscope (AFM). In Chapters 4 and 5, we present the results of the c- and m-plane ZnO thin films, and discuss how the growth

9

mechanism of ALD influences the crystalline structure. As high density of BSFs are observed in the m-plane ZnO, in Chapter 6 we take the advantages of the temperature dependent, power dependent, and time-resolved photoluminescence measurements to observe the excitons dynamics in BSFs²ZB ZnO thin films.

References

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[2] J. H. Lim, C. K. Kang, K. K. Kim, I. K. Park, D. K. Hwang, and S. J. Park, Adv. Mater. 18, 2720 (2006)

[3] A. Smith, Thin Solid Films 376, 47 (2000)

[4] X. T. Hao, L. W. Tan, K. S. Ong, and F. R. Zhu, J. Cryst. Growth 287, 44 (2006) [5] R. K. Shukla, A. Srivastava, A. Srivastava, K. C. Dubey, J. Cryst. Growth 294, 427 (2006)

[6] J. Jo, O. Seo, H. Choi, B. Lee, Appl. Phys. Express 1, 041202 ( 2008)

[7] I. T. Tang, Y. C. Wang, W. C. Hwang, C. C. Hwang, N. C. Wu, M. P. Houng, Y. H. Wang, J. Cryst. Growth 252, 190(2003)

[8] R. Tena-Zaera, A. Katty, S. Bastide, C. Levy-Clement, Chem. Mater. 19, 1626 (2007) [9] C. Platzer-Bjorkman, I. Torndahl, A. Hultqvist, J. Kessler, M. Edoff, Thin Solid Films 515, 6024 (2007)

[10] J. C. Johnson, H. Yan, R. D. Schaller, L. H. Haber, R. J. Saykally, and P. D. Yang, J. Phys. Chem. B 105, 11387 (2001)

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[11] S. Monticone, R. Tufeu, and A. V. Kanaev, J. Phys. Chem. B 102, 2854 (1998) [12] Z. L. Wang, J. Phys.: Condens. Matter 16, R829 (2004)

[13] U. Ozgur, Ya. I. Alivov, C. Liu, M. A. Reshchikov, S. Dogan, V. Avrutin, S.-J. Cho, and H. Morkoc, J. Appl. Phys. 98, 041301 (2005)

[14] A. Ashrafi, C. Jagadish, J. Appl. Phys. 102, 071101 (2007)

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[17] P. D. Ye, G. D. Wilk, B. Yang, J. Kwo, S. N. G. Chu, S. Nakahara, H.-J. L. Gossmann, J. P. Mannaerts, M. Hong, K. K. Ng, and J. Bude, Appl. Phys. Lett. 83, 180 (2003)

[18] G. D. Wilk, R. M. Wallace, and J. M. Anthony, J. Appl. Phys. 89, 5234 (2001) [19] M. Rinkiö, A. Johansson, G. S. Paraoanu, and P. Törmä, Nano Lett. 9, 643 (2009) [20] D. M. King, X. Liang, C. S. Carney, L. F. Hakin, P. Li, and A. W. Weimer, Adv. Funct. Mater. 18, 607 (2008)

[21] B. Min, J. S. Lee, J. W. Hwang, K. H. Keem, M. I. Kang, K. Cho, M. Y. Sung, S. Kim, M.-S. Lee, S. O. Park, and J. T. Moon, J. Crystal Growth 252, 565 (2003)

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[23] S. Yang, B. H. Lin, C. C. Kuo, H. C. Hsu, W.-R. Liu, M. O. Eriksson, P.-O. Hotz, C.-S. Chang, C.-H. Hsu, and W. F. Hsieh, Cryst. Growth Des. 12, 4745 (2009)

[24] C.-W. Lin, D.-J. Ke, Y.-C. Chao, L. Chang, M.-H. Liang, and Y.-T. Ho, J. Crystal Growth 298, 472 (2007)

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K.-J. Chang, and W.-J. Lin, J. Crystal Growth 310, 3024 (2008)

[26] W. WóJicik, M. Godlewski, E. Guziewicz, R. Minikayev, and W. Paszkowicz, J. Crystal Growth 310, 284 (2008)

[27] J. W. Lee, J. H. Kim, S. K. Han, S. K. Hong, J. Y. Lee, S. I. Hong, T. Yao, J. Cryst. Growth 312, 238±244 (2010)

[28] J. H. Kim, S. K. Han, S. I. Hong, S. K. Hong, J. W. Lee, J. Y. Lee, J. H. Song, J. S. Park, T. Yao, J. Vac. Sci. Technol. B 27, 1625±1630 (2009)

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Chapter 2 Theoretical Background

2.1 Growth Mechanism of Atomic Layer Deposition

Atomic Layer Deposition (ALD) is a chemical vapor deposition technique with layer-by-layer growth mechanism and the features of low growth temperature, high uniformity, low vacuum demand, and thickness controlling in the nanometer range. Such features are the results of the sequential and self-terminating chemical reactions of the precursors on substrate surface. For depositing the epitaxial ZnO thin film, the reacting temperature of the chemical reaction is ranged from room temperature to about 250 ºC, and the most suitable growing temperature is about 200 ºC. The precursor for providing zinc is the diethyl-zinc (DEZ), which is the colorless liquid contained in the bubbler and has vapor pressure about 15 torr at 25 ºC and boiling point at 118 ºC under 1 atm.

The sequential self-terminating gas-solid reactions for growing ZnO layers by ALD consist of the following four steps:

(a) The introducing and the self-terminating reaction of the first precursor A -- DEZ. (b) Purging the residual precursor and the by-products, and then evacuating the

reaction chamber.

(c) The introducing and self-terminating reaction of the second precursor B -- water (H2O).

(d) Purging the residual precursor and the by-products, and then evacuating the reaction chamber.

One reaction cycle consists of the steps 1 to 4, which would deposit amount of ZnO material to the surface.

13

Fig. 2-1 One cycle of ALD growth consists of the four steps: (a) introducing DEZ, (b) purging by-products, (c) introducing water (H2O), and (d) purging

by-products.

The illustration of one ALD reaction cycle is sketched in Fig. 2-1. Each reaction cycle adds a given amount of ZnO onto the surface. To grow a ZnO layer, reaction cycles are repeated until the desired thickness of layer achieved. The ALD process starts when the substrate surface is in a stabilized state with the desired temperature and hydroxyl group on the substrate surface. The ALD growth mechanism bases on the self-terminating reactions, which means the dominating factor is the surface-state control while the other process parameters (such as the precursor flow rate and chamber pressure) have little or no influence.

14

2.2 Photoluminescence of ZnO

The wurtzite ZnO material has the wide direct band gap of about 3.37 eV and the high binding energy of the free-exciton (FX) about 60 meV. The properties of the emission transitions in ZnO attract variety of research attention. The main intrinsic excitonic emission transition is free exciton (FX), which is usually bound by the donors and/or acceptors to form donor-bound exciton (D0X) and/or acceptor-bound exciton (A0X), respectively. In certain conditions, the transitions of the two-electron satellites (TES) related to the D0X, LO-phonon replica of the main excitonic transitions, and the donor-acceptor-pair (DAP) transitions could be observed. Recently, the basal stacking fault (BSF) is found to form the type-II quantum well in the wurtzite structure and trap the excitons. Such excitons bound by the BSF is possibly influenced by intrinsic defects such donor and acceptor, which leads to a complex transition mechanism. [2] Through the Mg-doping technique, the energy band gap and band alignment could be engineered to fabricate the quantum wells (QWs) structure. By taking advantage of the designed micro-cavity to enhance the coupling of excitons and photons, the transition of excion-polaritons was observed. [3]

2.2.1 Free Exciton (FX)

The FX is a bound state of an electron and a hole, which attract to each other by the Coulomb interaction. It is taken as an electrically neutral quasi-particle. An FX could result from an excited electron from the valence band into the conduction band and leaves a positively charged hole. Such electron and hole pair is in the form of hydrogenic system, which provides a stabilizing energy state slightly less than the energy of

15

unbounded electron and hole. The binding energy is the difference of energy between the stabilizing energy of the hydrogenic system and the unbounded electron and hole. The binding energy of the FX in ZnO is about 60 meV that prevents the FX from thermal ionization at room temperature while the thermal energy at room temperature is about 25 meV.

2.2.2 Bound Exciton Complex

The exciton as a quasi-particle is usually bound by the dopants or native defects, which results in the bound exciton complex and discrete electronic states in the band gap. The exciton bound by the neutral or charged donors and acceptors forms the neutral donor bound exciton (D0X), charged donor bound exciton (D+X), neutral acceptor bound exciton (A0_{X), and charged acceptor bound exciton (A}-_{X). In high quality bulk ZnO, the D}0_X

often dominates because of the unintentional impurities and/or shallow donor-like defects, and the A0_{X is usually observed in ZnO containing acceptors due to the intentionally}

p-type doping technique. Therefore, the transition of D0X is usually observed to dominate the PL spectra at low temperature, at which the thermal energy is able to ionize into FX. At low temperature under 10 K, the transition energy of the D0X is in the range of 3.360~3.368 eV and the transition energy of the A0X is in the range of 3.348~3.374 eV.

Another characteristic of the neutral D0X is the two electron satellites (TES). For the ZnO material, the transition energy of TES is in the range of 3.32~3.34 eV. The transition of TES results from an exciton bound to a neutral donor in excited state, and is usually observed in the samples with extreme crystalline quality, and has the intensity weaker than the D0X.

16

When an electron on a donor has the wave function overlap with a hole on an acceptor, the transition of DAP would occur. The energy of the transition of the DAP is given by

LO A D g D AP m r e E E E E Z H SH  !    0 2 4 ) ( _,

where r is the separation value of donor-acceptor pair, ED and EA are the respective

ionization energies of the donor and the acceptor as isolated impurities. Eg is energy

band gap of the ZnO and the last term presents the LO phonon replica. Therefore, the energy of the DAP transition EDAP depending on the separation value r of the

donor-acceptor pair. With increasing excitation power, the number of occupied donor and acceptor centers increases and their average distance necessarily decreases, which lead to the DAP blue-shift of the DAP transition energy EDAP.

2.2.3 Basal Stacking Faults Emission

According to the atomic stacking sequence of the BSFs, each BSF embedded in wurtzite structures can be taken as a thin zinc-blende layer surrounded by the wurtzite barriers. The band structure of a zinc-blende ZnO is still under debate since a few papers have been reported. Based on the ab initio calculation [4], the BSFs as the zinc blend layer leads to the QW-like region act as type-II QWs with 147 and 37 meV negative band offsets of the conduction band minimum and the valance band maximum to those of the barriers, respectively. The sketch of the band alignment is plot in Fig. 2-2. This means that the BSF structure would be a potential barrier at the valence band and a potential well in the conduction band to capture the electrons, which attract the holes in wurtzite structure via Coulomb interaction to form the BSF confined exciton [5].

17

Fig. 2-2 The schematic plot of the band alignment of ZnO at WZ/ZB/WZ regions.

2.3 XRD Measurement

The X-ray measurements were conducted using a four-circle diffractometer at beamline BL13A of National Synchrotron Radiation Research Center (NSRRC) Taiwan with incident wavelength 1.02473 Å. Two pairs of slits located between the sample and a NaI scintillation detector were employed and yielded a typical resolution of better than 1u10-3 _Å-1_{. The X-ray Diffraction measurements are the powerful technique to observe}

and determine the structural properties of the deposited crystalline film in the theory of diffraction.

2.3.1 X-ray Diffraction Theory

For the crystalline material, the periodical structural information can be determined by analyzing the reflected X-ray from the material. In Fig. 2-3, the incident X-rays are

18

reflected from the adjacent atomic planes with spacing of d in the crystalline material, and the reflected rays result in the constructive interference while match the condition of WKH%UDJJ¶VODZ nO 2d sin T , where the O is the wavelength of X-ray and T is the angle between incident X-rays and reflecting planes.

Fig. 2-3 Bragg diffraction condition in (a) real space and (b) reciprocal space.

In the elastic scattering process, the incident and emergent X-rays with wave vectors

k and kc, both of which have the magnitude of 2S /O , and the scattering vector q can

be obtained by the equation: q { kck 2ksinT . In reciprocal space, every periodical plane with spacing d in real space is transformed as a point with a lattice spacing of

d

/

2S , while q 2S /d WKH GLIIUDFWLRQ FRQGLWLRQ PDWFK WKH %UDJJ¶V /DZ  7KHUHIRUH each set of periodical parallel planes in real space can be represented by the Miller indices

)

(hk l , and be expressed by corresponding lattice vector: ghkl , where the |ghkl | value is

equal to 2S /dhkl . The ZnO crystal structure is hexagonal with lattice parameters

243 . 3

a Å and c 5.203 Å (D E 90q,J 120 q). The reciprocal lattice vector

hkl

19 2 2 2 2 2 3 4 2 | | c l a k hk h d g hk l hk l ¸¸ ¹ · ¨¨ © §   S S .

Conventionally, for crystals with hexagonal and rhombohedral symmetry, crystallographic planes are denoted using the four indices based on a four-axis Miller-Bravias coordinate system, consisting of three basal plane axes (a1, a2, a3) at 120 q

angles to each other and the fourth axis c perpendicular to the basal plane. The Miller-Bravias indices (hk il) satisfy the conditions i (h k). In this thesis, 4-digit Miller-Bravias indices are used for materials with hexagonal and rhombohedral symmetries including ZnO and sapphire to distinguish them from those with cubic symmetry.

For determining the crystalline structure properties, the four-circle diffractormeter is used which consists of four rotatable circles: T, 2T, F and I. The 2T circle is the detector axis controlling the magnitude of scattering vector q. The T, F, and I circles control the sample orientation. When the q vector coincides with the specific reciprocal lattice vector g, the Laue condition is satisfied. The I angle is equivalent to the azimuthal angle and the F angle is related to the polar angle of the crystal film. Different scan methods could perform the macro observation with different respect. The T/2T scan could observe the coherent length along the surface normal. The orientation of the deposited film could be observed through the T rocking curve scan, and the Iscan could determine the symmetry. The XRR method could confirm the roughness and thickness of the deposited layers.

2.3.2 Radial Scan

20

shown in Fig. 2-4, to vary the q vector to scan the reciprocal space in the surface normal direction. The most commonly performed radial scan is the one along sample surface normal, which is often known as T 2T or Z 2T scan as shown Fig. 2-4. From the positions of diffraction peaks we can determine the corresponding interplanar spacing along the direction of q and the line width of the diffraction peak can yield the structural coherence length (grain size) and inhomogeneous strain along the same direction.

Fig. 2-4 The radial scan situation in the real space and reciprocal space.

2.3.3 Rocking Curve

As shown in Fig. 2-5 for a given incident x-ray direction, a detector is placed at the position of a diffraction spot with certain lattice vector, the scattered x-ray collected while the crystal is rotated by means of scanning the T DQJOHZKLFKLVDOVRFDOOHG³T rocking FXUYH´ 7KH ZLGWK RI D URFNLQJ FXUYH 'T is a direct measurement of the width of the diffraction spot in the reciprocal space. The 'T also presents the distribution of the sub-JUDLQV¶RULHQWDWLRQLQWKHILOPWKHZLGHGLVWULEXWLRQOHDGVWRWKHODUJHU 'T .

21

Fig. 2-5 T -rocking scan situations in the real space and reciprocal for ideal and non-ideal lattice structures.

2.3.4 Azimuthal Scan

Azimuthal scan means measuring the diffraction intensity as a function of azimuthal angle I by rotation the sample along an axis, which is usually parallel to surface normal or, in some cases, to a specific crystallographic axis. Figure 2-6 illustrates the scheme of the Azimuthal scan which can used to study the symmetry and crystal quality of the grown film and determine its relative orientation with substrate in epitaxy.

22

Fig. 2-6 Azimuthal angle scan situations in the real space and reciprocal for ideal and non-ideal lattice structures.

2.3.5 X-ray Reflectivity

X-ray reflectivity (XRR) is a surface-sensitive and non-destructive analytical technique to estimate the density, thickness and roughness of thin film structures by analyze the reflection of the X-rays from the surface and interfaces among layers. The setup of the XRR is sketched in Fig. 2-7(a). [6] Through scanning of the incident X-ray, the reflected X-rays result in interference that leads to the periodic interference stripes in Fig. 2-7(b). For the X-ray beam, the index of refraction n is defined as:

x e e i r i n P S O U S O E G 4 2 1 1 2     ,

where _{is the x-ray wavelength,} re is the classical electron radius, Ue is the electron

density of the material, and Px is the absorption length. According to the Fresnel

reflection equation, the total external reflection of X-ray occurs at the angle of incidence smaller than the critical angle, Tc 2G , which depends on the electron density of the

23

(a)

(b)

Fig. 2-7 X-ray reflectivity measurement situation of (a) the thin film structure and (b) the analysis of the obtained data.

For an incident angle T , which is half of the scattering angle 2T in the reflectivity

measurements, the X-ray momentum transfers along the surface normal could be presents as _q 4_OS _sinT _{. Hence the period of interference fringes of the reflected X-ray beams is}

related to the thickness d of the film via 2S /d . And the roughness of the interfaces, which results in the dampling of reflevitity intensity, can be taken into account. [17] Fig.

24

2-8 shows the simulation results of reflectivity with different situations: ideal surface and film/substrate interface, surface roughness of 1 nm and ideal film/substrate interface, and ideal film surface with interfacial roughness of 1 nm. The simulated results reveal that the surface roughness of ZnO film has negative influence on the decay rate of the reflectivity curve, and the substrate roughness manily affects the amplitude of interference fringes.

Fig. 2-8 Simulated XRR curves of the ZnO films on sapphire substrate with different Rrms values: ideal interface and surface, film roughness of 1 nm with

ideal substrate roughness, and ideal surface with substrate roughness of 1 nm.

2.4 Transmission Electron Microscope

25

crystalline material in the atomic level, which make implementation of the direct observation of crystal defect possible.

2.4.1 TEM setup

A schematic presentation of the microscope is shown in Fig. 2-9. [7] The TEM instrument consists of an electron gun connecting to a high voltage (typically about 100-300 kV) accelerating electronic filed to emit electrons. By using condensor lenses (magnetic lens), the electron beam is focused to a spot of the order of 1 mm on the specimen. The image is magnified more than 106 times in the bright filed image mode (Fig. 2-9 (a)). In selected area diffraction mode (Fig. 2-9 (b)), the electron diffraction patterns are formed on the final image screen. In bright field imaging, the image of a thin sample is formed by the electrons, which pass the film without diffraction, the diffracted electrons being stopped by a diaphragm. In the corresponding dark-field-imaging mode, a diffracted beam is used for imaging.

26

Fig. 2-9 The measurement setup of TEM in (a) bright field imaging and (b) selected area diffraction modes.

2.4.2 Dark Field Image

Selected area electron diffraction (SAED), is a crystallographic experimental technique, which selects certain area of the specimen to obtain the diffraction pattern. The crystalline specimen is subjected to a parallel beam of high-energy electrons. Because the wavelength of high-energy electrons is close to the spacing of atoms in a crystal, the periodic atoms act as a diffraction grating to the electrons. As a result, the image on the screen of the TEM will be a series of diffraction spots (diffraction pattern), and each diffraction spot is corresponding to a satisfied diffraction condition of the crystal structure.

27

As a diffraction technique, SAED can be used to identify crystal structures and examine crystal defects. As shown in Fig. 2-10 below, the specimen holder is an objective aperture, which can be inserted into the beam path to block the electron beam except for the selected diffraction spot. [8] The diffraction spot, which is selected by the objective aperture can form the image on the screen is called dark-field (DF) images. Such an imaging mode is called the DF mode. In addition, the image formed by the unblocked spots including the direct beam and specific selected diffraction spots is called the bright filed (BF) image and the imaging mode is called the BF mode. Through these modes the TEM image can be formed by only the selected spot with a specific diffraction vector g.

Fig. 2-10 The measurement setup of dark filed imaging with the selected spots with specific diffraction vector g.

2.4.3 Basal Stacking Fault Analysis by TEM

The most common crystal structure of ZnO epitaxial film is wurtzite structure, which KDVWKHDWRPVVWDFNLQJVHTXHQFH«AaBbAaBbAaBb«DORQJWKH [0001 ] direction (c-axis

28

layer and the capital letters represent the zinc atoms and the lowercase letters represent the oxygen atoms, the positions are shown in Fig. 2-11. [9] The stacking fault means that WKHLGHDOVWDFNLQJVHTXHQFHRIZXUW]LWHVWUXFWXUH«AaBbAaBb«KDVWKHIDXOWVVXFKDV WKH VHTXHQFH «AaBbAaBbCcBbCc« ZKLFK LV WHUPHG DV WKH type-I stacking fault. In DGGLWLRQWKHVWDFNLQJVHTXHQFHRIWKH]LQFEOHQGVWUXFWXUHRI=Q2LV«AaBbCcAaBbCc« along the [111 ] direction.

Fig. 2-11 Three kinds of planes with different atom stacking arrangements in ZnO material marked as A-, B-, and C- planes.

The BSF can be classified into four types: type-I, type-II, type-III, and Extrinsic SF. The scheme of the four types of stacking sequences are shown in Fig. 2-12. [10]

(a) Type-I stacking fault (I1):

The type-I BSF is shown in Fig. 2-12(a). It is commonly expected to have the lowest formation energy. For the type-I SF, two stacking sequences are FRQVLGHUHG «AaBbCcBbCc« DQG «AaBbAaCcAaCc« ERWK W\SHV RI 6) VWDFNLQJ sequences have the same energy.

29

7KH%6)KDVWKHVWDFNLQJVHTXHQFHRI«AaBbAaBbCcAaCcAaCc«ZKLFKKDVWKH second lowest formation energy.

(c) Type-III stacking fault:

These are intrinsic BSFs, in which one of the Aa or Bb layers occupied by the Cc SRVLWLRQ«$D%E$D&F$D%E«

(d) Extrinsic stacking fault:

These SFs have the additional Cc layer inserted in the midst of the normal stacking VHTXHQFH«$D%E&F$D%E«

Fig. 2-12 Four types of stacking faults in ZnO: (a) type I, (b) type II, (c) type III, and (d) extrinsic. The arrows indicate the position of the stacking faults and the black and white circles denote zinc and oxygen atoms, respectively.

Depending on the type of error in stacking sequence or equivalently the displacement vector R defines the relative displacement between the unfaulted lattices above and below the fault. In wurtzite crystal structure the displacement vectors associated with the I1, I2

and Extrinsic type of BSFs are 2203 6 1 , 1100 3 1 , and 0001 2 1 , respectively; [11-13] but the type-III BSF has no displacement vector. From the TEM image of a crystal, the intensity of the electron beam diffraction could be described as following:

30

³

 ˜  t isz R g i i g _g

e

e

dz

0 2 2S S [ S

I

& & . In this formula, the part of the phase factor is as the following:

R g i

e

2S & ˜&

.

According to this formula, a stacking fault will be out of contrast if the dot product of its displacement vector R with the diffraction vector g used for imaging equals to 2Sn, where n = 0, r1, r«[14, 15] Consequently, all three types of stacking faults are visible in image with g = (10 11) and out of contrast as g = (0002). In the case of g = (10 10) ,

only intrinsic stacking faults of types I1 and I2 are in contrast. Table 1 shows the

visibility of the stacking faults with different g vectors.

(0002) 0} 2 {11 1) 1 (10 (10 10)

Type-I Invisible Visible Visible Type-II Invisible Visible Visible Extrinsic Invisible Invisible Visible

Table. 1 The visibility of the BSFs in TEM dark-field imaging with different selected g vectors.

2.5 Atomic force microscopy

Atomic force microscopy (AFM) is a very high-resolution type of scanning probe microscopy with demonstrated resolution on the order of fractions of a nanometer. The

31

brief setup of the AFM is shown in Fig. 2-13. The AFM consists of a cantilever with a sharp tip (the radius of curvature is on the order of nanometers), which is used to scan the specimen surface. When the tip is brought into the vicinity of the specimen surface, the forces between the tip and the surface deflects the cantilever according to Hooke's law. Depending on the scanning modes, the forces that are measured in AFM include mechanical contact force, van der Waals force, electrostatic force, magnetic force, etc. Along with force, additional quantities may simultaneously be measured through the use of specialized types of probe. Typically, the deflection of the cantilever is measured by using the photodiode to detect the laser spot reflected from the surface of the cantilever. The sample is mounted on a piezoelectric device to move the sample in the z direction for maintaining a constant force, and in the x and y directions for scanning the sample.

Fig. 2-13 The measurement setup of the atomic force microscope.

References

[1] U. Ozgur, Ya. I. Alivov, C. Liu, M. A. Reshchikov, S. Dogan, V. Avrutin, S.-J. Cho, and H. Morkoc, J. Appl. Phys. 98, 041301 (2005)

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[2] S. Yang, C. C. Kuo, W.-R. Liu, B. H. Lin, H.-C. Hsu, C.-H. Hsu, and W. F. Hsieh, Appl. Phys. Lett. 100, 101907 (2012)

[3] J.-R. Chen, T.-C. Lu, Y.-C. Wu, S.-C. Lin, W.-F. Hsieh, S.-C. Wang, and H. Deng, Opt. Express 19, 4101 (2011)

[4] Y. Yan, G. M. Dalpian, M. M. Al-Jassim, S.-H. Wei, Phys. Rev. B. 70, 193206 (2004). [5] Y. T. Rebane, Y. G. Shreter, M. Albrecht, Phys. Status Solidi A 164, 141 (1997). [6] Elza Bontempi's Science Page from http://dimgruppi.ing.unibs.it/

[7]http://inano.au.dk/research/competences-and-facilities/nanotools/transmission-and-scan ning-electron-microscopy/

[8] http://www.microscopy.ethz.ch/

[9] http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_5/backbone/r5_4_1.html [10] C. Stampfl and Chris G. Van de Walle, Phys. Rev. B. 57, R15052 (1998). [11] Stampfl, C.; Van de Walle, C. G., Phys. Rev. B 1998, 57, R15052.

[12] Gerthsen, D.; Litvinov, D.; Gruber, T.; Kirchner, C.; Waag, A., Appl. Phys. Lett. 2002, 81, 3972.

[13] Vennegues, P.; Chauveau, J. M.; Korytov, M.; Deparis, C.; Zuniga-Perez, J.; Morhain, C., J. Appl. Phys. 2008, 103, 083525.

[14] Wang, X. Q.; Iwaki, H.; Murakami, M.; Du, X. L.; Ishitani, Y.; Yoshikawa, A., Jpn. J. Appl. Phys. 2003, 42, L99.

[15] Chen, Y. F.; Bagnall, D. M.; Koh, H. J.; Park, K. T.; Hiraga, K.; Zhu, Z. Q.; Yao, T., J. Appl. Phys. 1998, 84, 3912.

[16] A. Ashrafi and C. Jagadish, J. Appl. Phys. 102, 071101 (2007)

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Chapter 3 Experiments

3.1 Atomic Layer Deposition System and Growth Procedure

The substrates are cleaned by sequential D.I. water/acetone/D.I. water rinse for 5min/5min/5min and then blew dry with N2 gas. The cleaned substrates are then loaded

into the ALD reactor (SYSKEY Ltd., Taiwan), heated to 200°C, and held at this temperature for 30 min. Diethylzinc (DEZn with chemical formula of Zn(C2H5)2 and

purity 4N8) and H22',ZDWHURIUHVLVWLYLW\0ȍÂFPNHSWLQUHVHUYRLUVDWƒ&DUH

used as zinc and oxygen precursors, respectively. The growth cycle consists of precursor exposures and N2 purge, which follows the sequence of DEZn/N2/H2O/N2 with

corresponding duration of 5s/15s/5s/15s. After each N2 purging, the reactor is pumped

down to ~ 1×10-2 torr by a mechanical pump. Precursor introduction is done by opening the inlet valve between the reservoir and reactor chamber while the outlet valve is closed; no carrier gas was employed. The pressures of the DEZn and H2O in the reactor chamber

were approximately 7 and 17 torr, respectively, monitored by a vacuum gauge. The substrate temperature was maintained at 200°C under the vacuum of 1~2 torr during the deposition. The reaction repeated 200 ~ 400 times for all the studied samples. Ideally, two cycles of reaction yield a unit cell of ZnO along the c-axis which means the 200 cycles would lead to 100 unit cells along the growth direction equivalent to a thickness of about 52 nm. Thermal annealing was performed at temperatures varying from 300 to 800°C for 1.5 hrs in pure oxygen gas at 1 atm.

3.2 X-ray Diffraction

34

beamline BL13A of National Synchrotron Radiation Research Center (NSRRC) Taiwan with incident wavelength 1.02473 Å, which is shown in Fig. 3-1. Two pairs of slits located between the sample and a NaI scintillation detector were employed and yielded a typical resolution of better than 1u10-3 _Å-1_.

Fig. 3-1 The four-circle diffractometer in NSRRC at beamline BL13A.

3.3 Atomic Force Microscope

Surface morphology and roughness of the ZnO layer were measured by the commercial Scanning Probe Microscope (Veeco Dimension 5000), which is shown in Fig. 3-2. The surface morphology images of the ZnO layers were scanned by the tapping mode, and the roughness values were estimated by analyzing the images of morphology.

35

Fig. 3-2 Veeco Dimension 5000 Scanning Probe Microscope

3.4 Transmission Electron Microscopy

Cross sectional TEM specimens with thickness of about 90±10 nm were prepared by focused ion beam (FIB) shown in Fig. 3-3(a). TEM images were taken with a Philips TECNAI-20 field emission gun type TEM, which is shown in Fig. 3-3(b). The structural defects of the ZnO thin films were analyzed using transmission electron microscopy (TEM). TEM specimens with a thickness ~80 nm were prepared by using a focused ion beam (FEI Helios 400S). High resolution TEM (HR-TEM) images were captured by using the JEOL JEM-2100F TEM with accelerating voltage 200 kV and the selected area electron diffraction (SAED) patterns were taken with a camera length of 300 mm and electron beam focused at the interface between ZnO film and sapphire substrate.

36

Fig. 3-3 The TEM measurement instruments: (a) the focused ion beam and (b) Philips TECNAI-20 field emission gun type TEM.

3.5 Photoluminescence

3.5.1 Low Temperature Photoluminescence

The PL measurements were carried out by using a He-Cd laser at 325 nm as the excitation source, and the sample is placed in a closed cycle cryogenic system. The laser beam is introduced onto the sample by using the mirror while the emission light is collected by using a convex lens. The emission is then conducted into a spectrometer (TRIAX 320) equipped with a photo-multiplier tube. The sketch of this setup is shown in Fig. 3-4.

37

Fig. 3-4 The sketch of the photoluminescence measurement setup.

3.5.2 Time Resolved Photoluminescence Measurement

Stationary PL spectra were measured with the fourth harmonic (Ȝexc = 266 nm) of a

continuous wave Nd:YVO4 laser as the excitation source. The PL signal was dispersed by

a 0.55 m monochromator and detected by a UV enhanced liquid-nitrogen-cooled charge coupled device camera. For the transient PL measurements the third harmonic (Ȝexc = 266

nm) from a Ti:sapphire femtosecond pulsed laser (pulse length 150 fs) was employed. The PL transients were detected by a UV sensitive Hamamatsu streak camera system with a temporal resolution better than 20 ps. The samples were placed in a variable-temperature cryostat for measurements in the temperature range 2±300 K. The setup of the TRPL system is shown in Fig. 3-5.

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