Growth and p doping of zinc oxide nanostructures and films in aqueous solution

This work studies the growth of ZnO nanorods and films in aqueous solution using zinc acetate and ammonium hydroxide in detail.. Regardless of the type of substrate used, the solubility

GROWTH OF ZINC OXIDE NANOSTRUCTURES AND FILMS

AND P-DOPING OF FILMS

IN AQUEOUS SOLUTION

TAY CHUAN BENG

B Eng (Hons.), M Eng

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2009

i

ACKNOWLEDGEMENTS

First and foremost, I would like to express my sincere appreciation to both of my

supervisors, Prof Chua Soo Jin and Prof Loh Kian Ping, whose patience, guidance and insights are crucial to this body of work

I would also like to express my thanks to:

• Dr S Tripathy, Dr C.B Soh, Dr H.Q Le and Dr H.F Liu from IMRE, whose instructions and guidance were important lifelines during the early stages of my research,

• H Musni and B H Tan from Centre for Optoelectronics, NUS whose experience, skill and time helped to keep the lab equipments and experiments running

smoothly and properly,

• Wang Miao and Haryono from Singapore MIT-Alliance, as well as Lin Fen, Huang Leihua, Tian Feng, Mantavya Sinha, Vivek Dixit for all the good memories,

• Liu Minghui, Deng Suzi, Zhong Yulin from Chemistry Dept, NUS, for opening up the world of chemistry to me,

• and all the others at NUS and IMRE that have helped me one way or another Finally, and most importantly, my profound gratitute goes to my Dad, Mom, Chuan Hock, MIchelle, Benjamin and Matthew Without your constant support, motivation and love, I would not have been able to finish this work Thank you for everything

ii

TABLE OF CONTENTS

1 Introduction 1

1.1 Introduction 1

1.2 Background 1

1.3 Crystal Structure 2

1.4 ZnO Growth Techniques 3

1.4.1 Vapor phase transport 3

1.4.2 Chemical vapor deposition (CVD) and metal-organic chemical vapor deposition (MOCVD) 4

1.4.3 Molecular beam epitaxy (MBE) 4

1.4.4 Aqueous solution-based synthesis 5

1.4.5 Comparison of gas and solution phase growth methods 5

1.5 Doping in ZnO 8

1.6 Motivation and objectives 13

1.7 Organization of the thesis 14

2 Aqueous solution growth of ZnO 16

2.1 Introduction 16

2.2 Basic terminologies and concepts 16

2.3 Temperature-dependent ionic equilibrium of ZnAc2 and NH3 19

2.4 Nucleation and growth 26

2.4.1 Homogeneous nucleation 26

2.4.2 Heterogeneous nucleation 28

2.4.3 Crystal growth 29

2.5 Effect of pH on ZnO surface 33

2.6 Conclusion 35

iii

3 Experimental methods for growth and characterization of ZnO 36

3.1 Introduction 36

3.2 Growth procedure and apparatus 36

3.2.1 Pre-coating of substrate with ZnO seeds 36

3.2.2 ZnO growth in solution 38

3.3 Characterization tools 40

3.4 Field-emission scanning electron microscopy (FESEM) 40

3.5 Photoluminescence spectroscopy 41

3.6 Raman spectroscopy 45

3.7 Secondary ion mass spectrometry (SIMS) 49

3.8 Hall effect measurement 50

3.9 Conclusion 54

4 Prediction of Length and Density of ZnO Nanorods on GaN Substrate 55

4.1 Introduction 55

4.2 Experimental Procedure 57

4.3 Results 58

4.4 Discussion 61

4.5 Effect of Solubility of Zinc on Density and Length of ZnO Nanorod Arrays 62

4.6 Effect of Temperature on Density and Length 66

4.7 ZnO Nanorod Length and Density Maps 68

4.8 Limitations of Model 69

4.9 Conclusion 71

5 Growth and Defects of ZnO Nanorods Grown from a ZnO Seed Layer 72

5.1 Introduction 72

5.2 Experimental Procedure 73

5.3 Results 74

iv

5.4 Discussion 80

5.4.1 Role of solubility in growth morphology 80

5.5 Role of interfacial properties in aqueous solution 85

5.6 Defects and the growth mechanism 87

5.7 Conclusion 88

6 Growth of p-ZnO film using multiple growth cycles 90

6.1 Introduction 90

6.2 Experiment 92

6.3 Results and discussion 94

6.3.1 Evolution of film morphology using a multi-step growth approach 94

6.4 Role of K as a dopant for ZnO films 96

6.5 Effect of electric field on the growth and doping of ZnO films in solution 98

6.6 Effect of annealing in nitrogen ambient on p-type doping by K 103

6.7 Fabrication of p-ZnO / n-GaN LED 105

6.8 Conclusion 106

7 Conclusions and Recommendations 108

7.1 Conclusions 108

7.2 Recommendations 111

8 Bibliography 113

v

SUMMARY

ZnO is a wide bandgap material with a large exciton binding energy (60 meV) and highly polar surfaces which promote anisotropic growth of many interesting nanostructures Due to its multifunctional properties, ZnO has been proposed for a wide variety of applications such as transparent conducting electrodes, gas sensors, piezoelectric sensors and generator, acoustic wave devices, light emitting diodes and solar cells This work studies the growth of ZnO nanorods and films in aqueous solution using zinc acetate and ammonium hydroxide in detail Regardless of the type of substrate used, the solubility of zinc (SZn), interface properties of the substrate and growth

temperature emerged as the main factors determining the growth rate and

morphology of the nanorods For GaN substrates, the activation energies for density and length of nanorods are -2.11 and 0.77 eV respectively An empirical growth map for growth prediction of the density and length of nanorods is generated For

substrates with a pre-coated layer of ZnO nanoparticles, a uniform coverage of

nanorods is obtained when SZn < 0.88 mM, and large clustered rods are obtained when

SZn > 1.56 mM For values of SZn that lies in between, both nanorods and large clustered rods can be obtained

Using photoluminescence and Raman spectroscopy, the native defects were identified and associated with the growth conditions When growth pH < PZC, the growth rate is very slow and hydrogen defects are the major defects with very strong UV emissions When growth pH > PZC, the growth rate is fast and the major defects are interstitial oxygen, interstitial zinc and zinc vacancies with strong visible emissions Interstitial zinc and zinc vacancies contributes to the green emission while interstitial oxygen, the red component

Next, ZnO films were grown and doped with potassium using a new growth strategy which can be applied to any substrate, regardless of its lattice matching The p-type conductivity in ZnO:K films is confirmed using Hall effect, SIMS and XPS measurements

An optimum hole concentration of 3.8 x 1017 cm-3 is obtained at 0.07 M KAc without any applied bias and 3.98 x 1017 cm-3 when -0.4 V is applied To the best of our

vii

LIST OF TABLES

Table 1.1 Summary of intrinsic doping levels of undoped ZnO polycrystalline films and single crystals which have been grown using various methods 8 Table 1.2 Summary of various group III elements as well as their corresponding growth methods and levels of n-doping 9 Table 1.3 Calculated bond lengths and the defect energy levels in ZnO for group I and

V dopants Ideal ZnO bond length (ro) is 1.93 Å Taken from [32] 10 Table 1.4 Summary of p-type mono-doping of ZnO using group V elements 11 Table 2.1 List of Enthalpy Values [58-60]. Enthalpy alues with an asterisk * denotes calculated values of enthalpy of formation from tabulated enthalpy of reaction 21 Table 3.1 Lattice parameters of various substrate materials for ZnO growth [69] 37 Table 3.2 Frequency and symmetry of the fundamental optical modes in ZnO 48 Table 4.1 Summary of different results and methods for aqueous solution growth 56 Table 4.2 Summary of effects of temperature and reactant concentrations on density and length of ZnO nanorods 62 Table 5.1 Summary of observed growth behavior with solution pH 81 Table 6.1 Summary of reported investigators, precursors, growth temperature and substrates for epitaxial ZnO growth in aqueous solution 91 Table 6.2 Summary of carrier parameters obtained from Hall effect measurements for samples grown without KAc and with 0.07 and 0.24 M KAc The film thickness is

obtained from the SEM image of the cross-section of the film 97 Table 6.3 Summary of carrier parameters obtained from Hall effect measurements for samples grown with 0.24 M KAc at different bias voltages The film thickness is

obtained from the SEM image of the cross-section of the film 99 Table 6.4 Summary of percentage atomic concentrations from quantifation of the fitted components of Zn 2p, O 1s and K 2s in the XPS survey spectra The relative

viii sensitivity factors (RSF) that were used for quantification are indicated beside the element in parenthesis 101

ix

LIST OF FIGURES

Figure 1.1 Schematic diagram of wurtzite crystal structure of ZnO and its common surface planes 2 Figure 1.2 Schematic showing the free energy of the precursors in gaseous and

hydrated states and the final ZnO product 6 Figure 1.3 Carrier concentrations as a function of the preservation period after

deposition A very stable p-type conductivity is obtained when Li-N codoping method is used Graph was taken from [45] 12 Figure 2.1 Equilibrium complex concentrations and solubility of zinc as a function of pH

at 300K The pH is increased by adding more NH3 while keeping the mass of ZnAc2constant at 0.016 M Curves show the equilibrium concentrations of (a) zinc acetate complexes, (b) Zn2+ ions, (c) zinc ammine complexes, (d) zinc hydroxide complexes and (e) total zinc ion concentration respectively 22 Figure 2.2 Variation of solubility of zinc with pH The solubility of zinc was calculated using Eq (2.15) The data for each curve is obtained by keeping the concentration of ZnAc2 fixed while varying the concentration of NH3 The concentrations of ZnAc2 are indicated on each curve 25 Figure 2.3 Variation of solubility of zinc and pH when the concentration of NH3 is varied while ZnAc2 is kept constant at 0.02 M The solubility of zinc was calculated using

Eq (2.15) 25 Figure 2.4 The Gibbs free energy of nucleation with respect to embryo radius The critical radius r* and energy ∆G* depends on the balance between the surface and volume energy of the growing embryo 28 Figure 2.5 Processes involved in heterogeneous nucleation on a substrate surface 28 Figure 2.6 Hydrolysis of hydrated metal ions in aqueous solution The positively

charged metal ion attracts the electrons away from the O-H bond, leading to the

breakage of the O-H bond and release of the H+ ion into the solution 30 Figure 2.7 (A) Aggregation and (B) coalescence of individual particles 32

x

Figure 2.8 A model for adsorption of Zn2+ on ZnO surface 34 Figure 2.9 Adsorption of Zn2+ ions depends on the pH of the solution Highest rate of adsorption when the pH is higher than the PZC of ZnO 34 Figure 3.1 TEM image of the ZnO nanoparticles that are grown by refluxing 0.02 M KOH and 0.01 M ZnAc2 in methanol for 2 h The diameter of the nanoparticles range from 10

to 20 nm Agglomeration of the nanoparticles can be clearly seen 38 Figure 3.2 Apparatus for growth of ZnO on a substrate 39 Figure 3.3 (a) Band structure and symmetries of wurtzite ZnO The splitting into three valence bands (A, B and C) is caused by field and spin-orbit splitting [75] (b) Schematic drawing of the exciton states (c) Summary of various optical transitions near the band gap and their corresponding energy and wavelength ranges 43 Figure 3.4 Comparison of the low-temperature PL (4 K) spectra from (a) a bulk single crystal ZnO grown by VPT [77] and (b) ZnO nanorods grown using a solution containing zinc nitrate, HMT and PEI [78] on a pre-coated Si substrate which had been pre-coated using ZnAc2 solution Both samples have been annealed in forming gas at 600°C 44 Figure 3.5 Rayleigh and Raman scattering 46 Figure 3.6 Depth profiling using a dual beam technique 49 Figure 3.7 Schematic of the Hall effect in a long, thin bar of semiconductor with four ohmic contacts The direction of the magnetic field B is along the z-axis and the sample has a finite thickness d 50 Figure 3.8 Schematic of a van der Pauw configuration used in the determination of the two characteristic resistances RA and RB 52 Figure 3.9 Schematic of a van der Pauw configuration used in the determination of the Hall voltage VH 53 Figure 4.1 SEM images of ZnO nanorods grown at temperatures (a) 60°C, (b) 80°C, (c) 100°C and (d) 150°C in solutions containing 0.016 M Zn(Ac)2 and 0.173 M NH4OH 59 Figure 4.2 The effect of growth temperature on (a) length and (b) density of ZnO nanorods 59

xi

Figure 4.3 SEM images of ZnO nanorods with different molar ratios: (a) 0.016 M

Zn(Ac)2, 0.1 M NH4OH, (b) 0.016 M Zn(Ac)2, 0.143 M NH4OH, (c) 0.016 M Zn(Ac)2, 0.204

M NH4OH and (d) 0.016 M Zn(Ac)2, 0.306 M NH4OH 60 Figure 4.4 Effect of molar ratio on (a) length and (b) density of ZnO nanorods The concentration of Zn(Ac)2 is kept constant at 0.016 M and the concentration of NH4OH is varied from 0.1 M to 0.4 M to increase the molar ratio 60 Figure 4.5 Effect of increasing concentration of precursors while maintaining a

constant molar ratio on (a) length and (b) density of ZnO nanorods Zn(Ac)2 is increased from 0.01 to 0.03 M , and concentration of ammonia by a proportional amount to maintain a constant molar ratio of 6.27 61 Figure 4.6 Logarithm of ZnO nanorods lengths plotted against the total concentration

of zinc ions in the precursor solution
represents the data points when the Zn(Ac)2 concentration is kept constant at 0.016 M and the NH4OH concentration is varied from 0.1 to 0.4 M while  represents the data points when concentration of Zn(Ac)2 is increased from 0.01 to 0.033 M with a constant molar ratio [NH4+]/[Zn2+] The growth temperature is kept constant at 373 K 63 Figure 4.7 Logarithm of rod density (cm-2) plotted against the total concentration of zinc ions in the precursor solution The inset shows the corresponding initial degree of supersaturation of zinc in the precursor solution at the growth temperature 373K The data points for varying the ratio of reactant concentrations are represented by
, while the increasing reactant concentrations with a constant ratio by  64Figure 4.8 Plot of (a) Y = [ ( n ) ]

Zn

B S A

B 1

Zn

L S A

L 1

ln against 1/T  and
represent the density and length data points respectively when temperature is varied from 60 to 150°C The inset shows the degree of supersaturation of zinc, S, against temperature for a precursor solution containing 0.016 M Zn(Ac)2 and 0.173 M NH4OH 67 Figure 4.9 Black lines show the contour plot of (a) log[B(cm-2)] and (b) length (nm) for various concentrations of ZnAc2 and NH4OH The validity limits for pH between 9.7 and 10.6, and degree of supersaturation of zinc between 20 and 60 are shown in red and blue lines respectively 70

xii

Figure 5.1 SEM morphology of ZnO nanorods grown on Si substrates with a pre-coat of ZnO nanoparticles using growth solutions with 0.02 M ZnAc2 and (a) 0.02 M, (b) 0.04 M, (c) 0.1 M, (d) 0.3 M, (e) 0.4 M and (f) 1.1 M NH4OH The concentration of NH4OH and the corresponding initial solution pH values in square parentheses are indicated on the top left corner Scale bar shows 1 µm 75 Figure 5.2 SEM image showing the morphologies of ZnO nanorods grown in various concentrations of ZnAc2 and NH4OH (a), (b) and (c) were grown with 0.4 M, 0.8 M and 1.1 M NH4OH respectively while keeping ZnAc2 fixed at 0.01 M (d), (e) and (f) were grown with 0.4 M, 0.8 M and 1.1 M NH4OH respectively while keeping ZnAc2 fixed at 0.02 M (g), (h) and (i) were grown with 0.4 M, 0.8 M and 1.1 M NH4OH respectively while keeping ZnAc2 fixed at 0.03 M The scale bar is 1 µm and all images were taken with the same magnification 75 Figure 5.3 The Raman spectra measured from samples grown with 0.4, 0.8 and 1.1 M

NH4OH on a glass substrate Inset shows the shift of the E2H peak to higher frequencies

as concentration of NH4OH is increased 77 Figure 5.4 Photoluminescence spectra recorded from samples grown in 0.02 M (black line), 0.04 M (blue line), 0.3 M (green line) and 1.1 M NH4OH (red line) while the

concentration of ZnAc2 is kept constant at 0.02 M 77 Figure 5.5 PL spectra of sample grown in high pH (10.7) after annealing at various temperatures in (a) air and (b) nitrogen ambient, as well as low pH sample (7) annealed

in (c) air and (d) nitrogen ambient The sharp peak at 650 nm is due to the doubling of the 325 nm laser line and should be ignored 78 Figure 5.6 Plot showing the solubility of zinc, SZn, against the concentration of NH4OH for 0.006 M (black dotted line), 0.01 M (blue line), 0.02 M (green line) and 0.03 M (red line) of ZnAc2 The SZn data points which are labeled (a) to (i) corresponds to the SEM images in Figs 5.2 (a) to (i) respectively which have been reproduced here for ease of comparison The value of SZn when 0.006 M ZnAc2 and 0.4 M NH4OH is marked with a square (
) and the corresponding SEM image is shown in Fig 5.7 Growth in region 1 produces uniform nanorods, region II a mixed morphology of nanorods and large rods and region III only large rods 82

positions of Zn, O, K and C have been marked Au peaks are from the calibration

reference 100 Figure 6.9 Typical component fitting of Zn, O and K using Zn 2p3/2, O 1s and K 2s peaks for atomic concentration quantification Synthetic peaks were fitted to the measured

xiv

peaks using a Shirley background and Gaussian-Lorentian distributions Quantification was performed using the fitted synthetic peaks to improve estimation accuracy 100 Figure 6.10 XPS valence band spectra of Zn 3d for samples grown with (a) -0.9V, (b) -0.1V and (c) -0.4V The corresponding hole concentrations in cm-3 has been indication

in the legend A larger core level shift is observed for a higher hole concentration 101 Figure 6.11 Plot of core energy level Zn 2p3/2 against the hole concentration as

measured using Hall effect The as-measured as well as the C 1s and O 1s calibrated peak values are shown A line is fitted to show the increasing binding energy with hole concentration 102 Figure 6.12 Effect of anneal temperatures on the carrier concentration and mobility for ZnO films grown (a) without any KAc, and with (b) 0.07 and (c) 0.24 M KAc (d) The effect of anneal duration at 800°C for sample grown in 0.24 M KAc Annealing for all samples were done in a nitrogen ambient Data points for as-grown samples were represented at 100 °C The electron concentrations and mobilities are marked by ● and

● respecQvely, while the hole concentraQon and mobility by ○ and ○ respecQvely 103 Figure 6.13 I-V characteristic plotted in (a) logarithmic and (b) linear scale Each line shows the I-V from measured from a different device Inset of (a) shows a schematic diagram of the device while the inset of (b) confirm the ohmic behavior of the top and bottom contacts after annealing at 700°C 1 h 107 Figure 6.14 The electroluminescence spectra at various current injection levels from

20 mA to 70 mA 107

luminescence, high excitation effects and lasing Several reviews [1-3] cover the work done during this early period The research interest faded for several reasons: difficulty

in obtaining a p-type ZnO and a shift in interest to lower dimension structures such as quantum wells which were exclusively based on GaAs/Al1-xGaxAs A revival in ZnO

research began in mid-1990s based on the possibility to grow epitaxial layers, quantum wells, nanorods or quantum dots and its possible applications in blue/UV

optoelectronics, radiation hard electronic devices, visible

semiconductor spintronics and transparent conducting oxides

reviews covering the current progress have been

1.3 Crystal Structure

ZnO has a hexagonal wurtzite structure Part of its

The Zn2+ and O2- sublattices exhibit

other along the c-axis The lattice parameters are

density of 5.605 g cm-3 [7]

Figure 1.1 Schematic diagram of wurtzite crystal structure of ZnO and its common surface planes

In the wurtzite structure, each

versa This tetrahedral coordination characterizes covalent bonds with sp

It is known that when moving from the group IV over the III

semiconductors, the bonds will show an increasing amount of ionic bon

ZnO shows a substantial amount of ionic bonding and lies at the borderline between being classed as a covalent and ionic compound The bottom of the conduction band is formed essentially from the 4s levels of

2p levels of O2- The band gap between the conduction and valence bands is about 3.437 eV at low temperatures of about 4 K

adiation hard electronic devices, visible-blind electronic circuits, semiconductor spintronics and transparent conducting oxides Several excellent

current progress have been published [4-6]

Crystal Structure

ZnO has a hexagonal wurtzite structure Part of its wurtzite structure is shown in

sublattices exhibit hexagonal close packing and interpenaxis The lattice parameters are a = 3.2495 Å and c = 5.2069 Å with a [7]

Schematic diagram of wurtzite crystal structure of ZnO and its common surface planes

In the wurtzite structure, each Zn2+ is surrounded tetrahedrally by four O

versa This tetrahedral coordination characterizes covalent bonds with sp

It is known that when moving from the group IV over the III-V and II-VI to the I

semiconductors, the bonds will show an increasing amount of ionic bon

ZnO shows a substantial amount of ionic bonding and lies at the borderline between being classed as a covalent and ionic compound The bottom of the conduction band is formed essentially from the 4s levels of Zn2+ and the top of the valence

The band gap between the conduction and valence bands is about 3.437 eV at low temperatures of about 4 K [8]

Schematic diagram of wurtzite crystal structure of ZnO and its

is surrounded tetrahedrally by four O2- and vice versa This tetrahedral coordination characterizes covalent bonds with sp3 hybridisation

VI to the I-VII semiconductors, the bonds will show an increasing amount of ionic bonding As such, ZnO shows a substantial amount of ionic bonding and lies at the borderline between being classed as a covalent and ionic compound The bottom of the conduction band is

and the top of the valence band from the The band gap between the conduction and valence bands is about

3

Furthermore, the tetrahedral coordination gives a polar symmetry along the c-axis This polarity is responsible for its piezoelectricity, spontaneous polarization, anisotropic crystal growth habit, etching behaviour and defect generation

Fig 1.1 also shows the common polar and non-polar planes in the wurtzite structure Common polar face terminations of wurtzite ZnO are the Zn-terminated (0001) and O-terminated (000-1) faces which are both c-axis oriented The common non-polar faces are (11-20) which is a-axis oriented, (10-10) and (1-102) faces, which both have equal number of Zn and O atoms

1.4 ZnO Growth Techniques

ZnO is a versatile material with a rich chemistry It can be grown using a wide variety of methods, ranging from simple thermal evaporation to more sophisticated state-of-the-art epitaxial growth techniques Vapor phase transport growth methods are most commonly used They consist of thermal evaporation, ion sputtering, pulsed laser deposition, CVD, MOCVD and MBE An alternative method which has not gained wide spread adoption is the aqueous chemical growth method These techniques will be described briefly in the sections below

1.4.1 Vapor phase transport

In vapor phase transport, material is vaporized from a ZnO solid source, typically in powder form, and transported onto a substrate where it condenses and deposits ZnO source can be vaporized by thermal evaporation, laser ablation, sputtering, or electron beam

High temperatures are required for vaporization of ZnO powder as its melting point is about 1975°C In thermal evaporation, for example, ZnO powders are heated to a temperature range of 1100 to 1400°C in order produce Zn vapors The Zn vapors are transported by a carrier gas and deposited as ZnO on a substrate placed downstream of the carrier gas [9, 10]

4

Lower growth temperatures can be achieved by using sub-oxides of zinc (ZnOx, 0 ≤ x < 1) which have a melting point of about 419°C ZnOx can be obtained by reduction of ZnO using graphite [10, 11] as shown in the reactions (1.1) and (1.2) below:

2 2

1 2

1

CO Zn

C

2

)1()

1( x CO ZnO x CO ZnO+ − → x+ − , where 0 ≤ x < 1 (1.2)

Reduction can also be achieved using hydrogen [12], or reduction of zinc salts such as ZnS [13]

1.4.2 Chemical vapor deposition (CVD) and metal-organic chemical vapor

deposition (MOCVD)

The use of volatile Zn sources in CVD and MOCVD methods allows even lower

vaporization temperatures to be applied In CVD, zinc acetylacetonate hydrate (hereon denoted as Zn(acac)2), with vaporization temperatures between 130 and 140°C, is typically used as a source Upon vaporization, Zn2+ vapor is transported by nitrogen for reaction with oxgen at temperatures ranging from 500 to 600°C

ZnO Zn

O H acac

Zn ⋅ 160 →°C 2+ O, 500−600°C→

2

)(

In MOCVD, a metal-organic source, typically dimethyl zinc or diethyl zinc with

vaporization temperatures ranging from 117 to 130°C, is used The metal-organic source is decomposed to form Zn vapor and then transported into the reaction

chamber using inert gas argon where it reacts with oxygen to form ZnO This reaction typically takes place at temperatures ranging from 300 to 500°C [14, 15]

ZnO Zn

DeZn  117 − 130 °C→ 2+ O 2 , 300  −500 °C→

1.4.3 Molecular beam epitaxy (MBE)

In MBE, high purity Zn metal (melting point 420°C) is thermally evaporated in a

Knudsen effusion cell Under ultrahigh vacuum conditions (< 10-8 Pa), Zn vapor is

directed onto the substrate which typically has a thin layer of Ag as a catalyst In the

5

presence of O2 and a growth temperature of 300 to 500°C, growth of ZnO on the

substrate can be achieved [16]

1.4.4 Aqueous solution-based synthesis

In general, oxides are particularly suited for growth in solution Literature is rich with reports of nanostructures fabricated in chemical solutions The ease of ZnO growth in solution is reflected in the low growth temperatures of 60 to 90°C Growth precursors

in aqueous solution generally consists of a zinc salt, such as zinc acetate, zinc nitrate or zinc chloride, and a base such as sodium hydroxide and aqueous ammonia

Occasionally a surfactant is added to influence the growth habit In water, hydration of the zinc salt leads to free Zn2+ ions which undergoes hydrolysis and condensation to give ZnO The growth method and mechanisms will be explored in detail in Chapter 2 Growth of ZnO in aqueous solution is an attractive alternative to MOCVD because it is a simple, cheap, non-toxic and low temperature method Large-scale processing has also been demonstrated [17]

1.4.5 Comparison of gas and solution phase growth methods

Growth of ZnO is more readily achieved with precursors in gaseous state than in

aqueous state Obviously, the higher free energy of growth units in gaseous state results in a large driving force and a lower activation energy barrier as shown in Fig 1.2 Since the growth units have sufficient energy for diffusion, adsorption, surface

reactions, nucleation and growth, growth can be achieved over a wide range of

conditions and precursor concentrations

The opposite is true for aqueous chemical growth methods which have a small driving force and high activation energy barrier as shown in Fig 1.2 While formation of ZnO can be encouraged by shifting the chemical equilibrium to favor hydrolysis and

condensation of ZnO, the growth species have much lower free energy due to low growth temperatures Careful control of precursor concentrations and zinc solubility is needed to achieve growth of ZnO As such, understanding of chemical equilibriums is essential in controlling the growth process

6

Natural growth processes in nature has shown that it is possible to grow perfectly

crystalline structures at ambient temperatures and pressure One example is the

growth of single crystal calcium carbonate by sea urchins A recent paper described the conversion of amorphous calcium carbonate to single crystal calcium carbonate

through a secondary nucleation mechanism at an ambient temperature of 15°C [18] It

is possible that an organic catalyst exists to lower the activation energy and aid the dissolution and secondary nucleation process Unraveling this process and applying it

to the case of ZnO will help to establish aqueous chemical growth as a viable

alternative to gas phase methods

Figure 1.2 Schematic showing the free energy of the precursors in gaseous and hydrated states and the final ZnO product

When compared in terms of energy, material and processing, aqueous chemical growth methods have clear advantages over gas phase methods Gas phase methods generally waste a large amount of energy and material A huge amount of energy is needed to convert the solid state Zn source to free Zn2+ ions in vapor state as growth precursors

Zn, O precursor atoms,

ions cluster molecules

or complexes in fluid (air

or solution)

ZnO in solid phase Diffusion

Adsorption Surface reaction Nucleation and Growth

7

Upon condensation of the solid ZnO, this excess energy is simply discarded into the environment Furthermore, recycling waste of material is uneconomical because the exhaust gases are emitted in large diluted volumes, especially when high vacuum systems are used

In contrast, very little extra energy is needed in aqueous chemical growth methods to break the lattice bonds of the solid Zn source to form free Zn2+ ions This is because the energy needed to dissolve the zinc salt and break the lattice bonds are provided by the hydration energy in water at room temperature Upon dissolution, growth proceeds by hydrolysis and condensation which can be induced by manipulating the chemical

equilibrium of precursors The growth system is typically a closed system which allows easy separation and recycling of materials As such, wastage of energy and material are minimized

In solution phase, growth precursors has higher concentration and better homogeneity than the gas phase, especially when high vacuum growth conditions are used It follows that aqueous chemical growth methods should give high homogeneity and faster growth rates than that of the gas phase However, it is noted that due to much lower growth temperatures typically less than 100 °C in solution methods, growth units may not have enough kinetic energy to diffuse across the surface to obtain a smooth film layer growth

Finally, growing in solution is a low cost, safe and simple process Basic equipment consists of a growth vessel, water bath or convection oven is sufficient In comparison, gas phase methods will need a special setup in order to operate at high temperatures and vacuum conditions In the case of CVD or MOCVD, the growth precursors are hazardous and additional safety systems are needed

In summary, the additional complexity in understanding the growth process and

mechanism of aqueous chemical growth methods are more than compensated by its energy, materials and processing advantages over gas phase methods

Table 1.1 Summary of intrinsic doping levels of undoped ZnO polycrystalline films and single crystals which have been grown using various methods

Type of film Growth

method

Intrinsic electron conc (cm-3)

Substrate Ref

Polycrystalline PLD 1018 - 1020 fused silica and Si [19] Polycrystalline Magnetron

sputtering

1019 glass and sapphire [20]

• Firstly, the intrinsic doping concentrations in undoped ZnO single crystals is about five orders of magnitude less than those in polycrystalline ZnO This large difference of intrinsic doping concentrations shows that there is plenty of room

to reduce the concentration of intrinsic defects

to minimize energy and material wastages, solution phase methods are indeed

a promising growth method for ZnO

By substituting Zn with group III elements such as Ga [25, 26], Al [27, 28] and In [29], high levels of n-doping beyond 1020 cm-3 have been achieved Success in n-doping was not limited to gas phase methods Aqueous chemical growth methods have also

achieved electron concentrations of approximately 1019 cm-3 using Al [28] and In [30] Table 1.2 summarizes the various dopants and methods that have been reported

Table 1.2 Summary of various group III elements as well as their corresponding growth methods and levels of n-doping

Dopant Electron conc (cm-3) Growth method Substrate Ref

In contrast, p-type doping is more difficult to achieve This difficulty is due to the high densities of intrinsic defects as shown in table 1.1 as well as low solubility of dopant species in ZnO and tendencies of dopants to form deep level instead of shallow level acceptor states [8] There are two groups of candidates for p-type dopants: group I elements which substitute Zn atoms and group V elements which substitute O atoms

Table 1.3 Calculated bond lengths and the defect energy levels in ZnO for group I and V dopants Ideal ZnO bond length (ro) is 1.93 Å Taken from [32]

r r

However, experimental results show otherwise Hydrothermally-grown bulk ZnO

crystals are typically grown in a high concentrations of KOH or LiOH bases as

mineralizers As such, they typically have a high concentration of K or Li incorporated The high doping concentration of Li did not give a good p-type conductivity and the ZnO crystals were highly resistive This contradiction is explained by Li occupation of interstitial sites where it acts as a donor [33] and compensates the acceptor

contributions This may also be the reason why hydrothermally-grown LEO films were n-type instead of p-type, despite the presence of Na in the growth solution [34]

Contrary to theoretical predictions, reports have shown that group V elements are more promising in achieving p-type doping Among group V elements, N is considered

as the most ideal p-type dopant since its bond length is closest to the ideal Zn-O bond length and it has the shallowest acceptor energy level as seen from Table 1.3

is unstable and may disappear with time [35, 36]

Table 1.4 Summary of p-type mono-doping of ZnO using group V elements Dopant Hole concentration (cm-3) Method Substrate Reference

Another successful method was the dual-acceptor codoping method using Li and N [45] This combination has achieved very reproducible and stable hole concentrations of about 1019 cm-3 As shown in Fig 1.3, the hole concentrations arising from Li or N

monodoping schemes deteriorate and disappear completely after 3 months In contrast, the Li-N scheme gives a very stable hole concentration The mechanism leading to the enhanced p-type stability is still unclear

12

Figure 1.3 Carrier concentrations as a function of the preservation period after deposition A very stable p-type conductivity is obtained when Li-N codoping method is used Graph was taken from [45]

It is interesting to note in Fig 1.3 that in a mono-doping scheme, Li does give a

comparable, or better, doping results than N, although it is unstable and disappears after 3 months Very recently, Lin et al successfully fabricated Na-doped ZnO film using pulsed laser deposition on glass and quartz substrates and obtained stable p-type conductivity in the range of 1016 to 1018 cm-3 Although the doping levels are low, they appear to be stable Fig 1.3, Lin’s result and theoretical calculations point to the

possibility of using group I elements as p-dopants despite earlier difficulties

Looking back at Tables 1.2 and 1.4, gas phase methods, such as magnetron sputtering and pulsed laser deposition, appear to be the method of choice for growth and in-situ p- and n-type doping There are only a few reports employing solution methods for n-type ZnO and none for p-type ZnO This is surprising because:

• aqueous chemical growth methods offer a comparable intrinsic defect density

to other gas phase methods

13

• the dopant concentrations in aqueous solution are very much higher than that

in gaseous state, and this leads to a more homogeneous dopant distribution and a higher level of dopant incorporation in ZnO

In fact, Chapter 6 of this thesis will describe how the aqueous solution growth method

is employed to grow and dope a type ZnO film with potassium from group I as the dopant To the best of our knowledge, this is the first report of extrinsic p-type doping using aqueous solution growth methods This successful demonstration shows that aqueous chemical growth methods have an important role to play in the growth and doping of ZnO

p-1.6 Motivation and objectives

Gas phase growth methods have emerged as the preferred growth method due to ability to grow and dope ZnO films and nanostructures although these methods are expensive and not environmentally-friendly Solution methods offer an alternative processing route that is environmentally-friendly, are low cost, non-toxic and suitable for large scale processing Due to lack of understanding of underlying growth

mechanisms as well as difficulty in growing and doping ZnO epitaxial layers, solution methods have not been accepted as one of the mainstream growth methods

The understanding of growth mechanisms of ZnO in solution has been lacking because

of the wide variety of precursors and growth methodology that are available in the literature The current focus on achieving functional devices based on ZnO has not helped to encourage further research on growth mechanism in aqueous solution Therefore, it is an important objective of this thesis to understand the growth

mechanisms for one particular growth solution, namely the aqueous system based on zinc acetate and ammonia This system is chosen because the materials are readily available in the laboratory It is also believed that other growth systems, as long as they consist of a zinc salt and base, will behave in a similar manner

A further objective of this thesis is to study how ZnO thin films, with a thickness range

of 1 to 5 µm, can be formed from ZnO nanostructures in aqueous solution This

14

objective is motivated by the ease of growth of nanostructures in solution and the ready application of ZnO films in device fabrication, such as transparent conductive oxides for LEDs and optoelectronic devices

As mentioned earlier, doping is important challenge to be overcome There are only a few reports of n-type doping and none of p-type using aqueous chemical growth

methods Current achievements in p-type doping have mainly focused on group V elements as well as codoping using group I and V elements using gas phase methods Considering that theory favors group I elements as acceptors, and the recent report of successful p-type doping using Na using PLD [46] and the processing advantages of doping in solution phase, it is another objective of this thesis to investigate p-doping of ZnO using group I elements using aqueous chemical growth methods

1.7 Organization of the thesis

In this section, the layout of this thesis is described

The first chapter introduces the material properties of ZnO, its growth methods and doping while the second chapter covers the aqueous solution growth method in more detail The third chapter describes the experimental setup and characterization

methods that will be regularly referenced by the experimental chapters that follows Chapters four and five presents the experimental results and discussions on the growth

of nanostructures on various substrates In particular, chapter four looks into the growth factors of ZnO nanorods on GaN substrates and chapter five extends this

understanding to other substrates that have a larger lattice mismatch compared to ZnO, where growth is initiated from ZnO nanoparticles that have been spincoated onto the substrate

Chapter six focuses on the growth and p-type doping of the ZnO film A new growth film growth strategy is presented for substrates that are closely lattice-matched, such

as GaN, as well as substrates with large lattice mismatches, such as glass, sapphire and

Si The p-doping of ZnO film using potassium from group I is explored using two setups: the traditional closed vessel without any applied voltage bias, and a new growth setup

15

with an applied voltage bias The electrical properties of the unintentionally-doped and potassium-doped ZnO films are studied and compared The effects of thermal

annealing on the electrical properties are also presented This is followed by the

description of the fabrication and characterization of a p-ZnO / n-GaN light emitting diode

Finally, chapter eight summarizes and concludes the work done and presents the future directions for further work

al [48] used this method to grow nanorods on conducting glass and Si substrates using

a seed layer Mende et al[49] and Govender et al [50] has reviewed the various growth precursors in solution and the resulting nanostructures In addition, Le et al has

studied the growth of ZnO nanorods on GaN substrates using zinc acetate (ZnAc2) and ammonium hydroxide (NH3) [51] This thesis will focus mainly on the growth solution consisting of ZnAc2 and NH3

In this chapter, a brief introduction to the chemical principles related to aqueous

solution growth of ZnO is provided First, the basic terminologies and concepts such as concentration, supersaturation, pH, solubility product and complexation are introduced Then the calculation ionic equilibrium of the ZnAc2 and NH3 system is discussed Finally, nucleation and growth processes are described in terms of homogeneous and

heterogeneous nucleation and crystal growth Further details on the principles

described herein can be readily obtained from several excellent authors [52-54]

2.2 Basic terminologies and concepts

Growth solutions usually consist of at least two components such as zinc acetate

dihydrate (Zn(CH3COO)2.2H2O), hereon denoted as ZnAc2 for brevity, and aqueous ammonia (NH3) The concentration of any component in the solution is typically

expressed in molar (M), i.e the number of moles of solute per litre of the solution, and

17

is denoted by square brackets For example, [Zn2+] represents the concentration of Zn2+ions When 0.3 g of ZnAc2 powder is dissolved in 42 ml of water, and assuming all the powder dissolves in the water, then the number of moles of ZnAc2 in water is

mmol 1.367

or mol 001367 0 5 219

3 0 ZnAc of mass molar

added ZnAc of mass )

in water of volume

ZnAc of moles of number ]

The growth solution is supersaturated when ZnAc2 concentration exceeds its solubility concentration The degree of supersaturation is defined as

Zn

S

C

where C is the actual concentration of ZnAc2 added, and SZn is the solubility

concentration When S < 1, no growth or nucleation will occur For low to moderate

18

values of S greater than 1, heterogeneous nucleation on a substrate will occur When S

is very large, precipitation via homogenous nucleation in the solution will occur in the solution

For example, consider the sparingly soluble Zn(OH)2 in equilibrium with its saturated aqueous solution:

)(2)()

()(OH 2 s Zn2 aq OH aq

Zn(OH)2 dissolves in water to give a small concentration of Zn2+ and OH- This

concentration is defined by the solubility product Ksp, which is the product of the

concentrations of the dissolved ions:

Zn2+ such as Zn(OH)42- Formation of Zn complexes removes Zn2+ ions from the solution, shifts the balance in equation (2.5) to the right, and thus reduces the degree of

precipitation With sufficiently high concentrations of OH-, Zn(OH)2 precipitate can be completely dissolved As such, it is misleading to rely solely on the solubility product of Zn(OH)2 in equation (2.5) and (2.6) to estimate of the amount of zinc acetate that can

be dissolved in the growth solution before Zn(OH)2 is precipitated The presence of zinc complexes in the solution should also be taken into consideration

As such, a better way to capture the true solubility of zinc in aqueous solution is to calculate the temperature-dependent ionic equilibrium of the solution Such a model will account for all the possible zinc complex species and is very useful in understanding aqueous solution growth One such model for the growth system using ZnAc2 and NH3will be described in next section

account the various hydroxide, ammine and acetate complex species are formed when ZnAc2 and NH3 is mixed in an aqueous solution The reaction equations and the

corresponding rate constants at 298 K are given below

Hydroxide complex formation [55]

4 4 2

1 2

10]][

[

])([)

OH Zn K

OH Zn OH

71 11 2

]][

)([

1)

()

OH OH

Zn K

OH Zn OH

OH

5 4 2

3 2

4 3

][

])([)

()

OH Zn OH

OH

61 0 2

2 4 5

2 4

][

])([)

(2

OH Zn OH

OH

Acetate complex formation [55]

3 1 2

6 2

10]][

[

])([)

+ +

−

Ac Zn

Ac Zn K

Ac Zn Ac

8 0 2

7

]][

)([

])([)

()

Ac Ac Zn

Ac Zn K

Ac Zn Ac

Ac

Ammine complex formation [56]

20

59 2 3 2

2 3 8

2 3 3

2

10]][

[

])([)

↔

+ +

+

NH Zn

NH Zn K

NH Zn NH

91 4 2 3 2

2 2 3 9

2 2 3 3

2

10]][

[

])([)

(

+ +

+

NH Zn

NH Zn K

NH Zn NH

92 6 3 3 2

2 3 3 10

2 3 3 3

2

10]][

[

])([)

(

+ +

+

NH Zn

NH Zn K

NH Zn NH

62 8 4 3 2

2 4 3 11

2 4 3 3

2

10]][

[

])([)

(

+ +

+

NH Zn

NH Zn K

NH Zn NH

39 4 4

3 12

2 3

]][

[

][

=

=+

↔

+

OH NH

NH K

O H NH OH

The ionic equilibrium for the aqueous solution can be obtained by solving

simultaneously the reaction equations, the mass and charge balance The mass balance equation can be written as

])([2])([]

where cAc and cN are the total concentrations of acetate and ammine ions

The charge balance equation equates the positive and negative charges in the aqueous solution and can be written as

][][])([2])(

[

])([2][])([])([][

2

2 4 3

4

1

2 3 4

+ +

+

++

+

=

++

+

OH Ac

OH Zn OH

Zn

NH Zn NH

Ac Zn OH

Zn Zn

log

T T R

H K

T

T

(2.13)

where the ideal gas constant R = 8.314 × 10-3 kJ.mol-1K-1, KT1 and KT2 are rate constants

at temperature T1 and T2 respectively, and ∆rH0 is the standard enthalpy of reaction and is given by

f j

where ∆fH0 is the standard enthalpy of formation, i and j specify reactants and products respectively, and ni and nj are the amounts in moles of each substance in the chemical reaction The standard enthalpy values for the product and reactants for reaction equations (2.7), (2.8) and (2.9) are summarized in Table 2.1

Table 2.1 List of Enthalpy Values [58-60]. Enthalpy alues with an asterisk * denotes calculated values of enthalpy of formation from tabulated enthalpy

22

Figure 2.1 Equilibrium complex concentrations and solubility of zinc as a function of pH at 300K The pH is increased by adding more NH3 while keeping the mass of ZnAc2 constant at 0.016 M Curves show the equilibrium concentrations of (a) zinc acetate complexes, (b) Zn2+ ions, (c) zinc ammine complexes, (d) zinc hydroxide complexes and (e) total zinc ion concentration respectively

In the calculation of the ionic equilibrium, it is assumed that the equilibrium

concentrations of zinc acetate complex species are very small compared to the other species Thus, in the range of 0 to 150°C, the temperature dependence of K6 and K7 in equations (2.8a) and (2.8b) is neglected

Using the model described above, the ionic equilibrium of the ZnAc2 and NH3 in an aqueous solution can be calculated for various ZnAc2 and NH3 precursor concentrations and temperature A detailed description of the procedure to calculate the ionic

equilibrium is provided in the Appendix Fig 2.1 shows the solubility of zinc and

concentration of the major zinc complexes as a function of pH at 300 K respectively The pH is increased by adding more NH3 while keeping the mass of ZnAc2 constant at 0.016 M

Several important points can be seen from Fig 2.1:

• At pH values greater than 9.7, the increase in zinc solubility is contributed mainly by the increasing concentration of zinc ammine complexes On the other hand, at pH values lower than 8, the increase is due to Zn2+ ions In chapter 4, we will see how these different dominant species play an important role in determining the growth rate and the structural defects of ZnO

Proceeding from the equilibrium concentrations, the “true” solubility of zinc and the degree of supersaturation of zinc, which are related to the nucleation density, can be calculated:

The solubility of zinc, or the total concentration of zinc ions in the precursor solution is given by the sum of all the zinc species in the solution:

+

− +

3 )

2 ( 2

p p n

m m

24

the dominant growth control variable This is reflected in the reported growth

procedures where a final adjustment to the pH range of 10 to 11 is usually practiced Using pH as the primary growth control variable presents several drawbacks

• Firstly, for the same pH value, the solubility of zinc can vary with concentration

of ZnAc2 that is added This is illustrated in Fig 2.2 which shows the variation of the zinc solubility with pH Each curve in Fig 2.2 is obtained by keeping the ZnAc2 concentration fixed while varying the concentration of NH3 to change the solution pH This sequence is similar to typical experimental procedures for adjustment of growth pH The variation of the solubility of zinc, due to different concentrations of ZnAc2, is smallest at pH 7 and increases with pH At pH 10, the difference in the solubility of zinc for initial ZnAc2 concentrations of 0.03 M and 0.06 M is large enough to cause differences in growth morphology as will be shown later in chapter 4

• Secondly, for a fixed concentration of ZnAc2, large increases in NH3

concentration shifts the pH only slightly while affecting the solubility of zinc significantly This is shown in Fig 2.3 where the solubility of zinc and pH is

plotted against the concentration of NH3 In the absence of an accurate pH titration and measurement, this variation in the solubility of zinc in different growth batches may produce an inconsistent growth morphologies between batches

By employing the calculated solubility of zinc as the primary growth control variable, these drawbacks can be minimized In fact, it will be shown in chapters 3 and 4 that the solubility of zinc can be used a predictor of ZnO nanorod density and length on GaN substrates, as well as its growth morphology on substrates

25

Figure 2.2 Variation of solubility of zinc with pH The solubility of zinc was calculated using Eq (2.15) The data for each curve is obtained by keeping the concentration of ZnAc2 fixed while varying the concentration of NH3 The concentrations of ZnAc2 are indicated on each curve

Figure 2.3 Variation of solubility of zinc and pH when the concentration of

NH3 is varied while ZnAc2 is kept constant at 0.02 M The solubility of zinc was calculated using Eq (2.15)

0.02 M ZnAc 2

Solubility of Zn

pH