FABRICATION and CHARACTERIZATION
ELECTRONIC GAS (2DEG)
HETEROSTRUCTURES
A Thesis Presented to the University of Genova
in Partial Fulfilment of the Requirements for
the Degree of Doctor
in Materials Science & Technology
Supervisor: Prof.
CHARACTERIZATION of BIDIMENSIONAL
ELECTRONIC GAS (2DEG) in ZnO BASED
HETEROSTRUCTURES
by
Alejandro Plaza
A Thesis Presented to the University of Genova
n Partial Fulfilment of the Requirements for
he Degree of Doctor of Philosophy
n Materials Science & Technology
(Ciclo XXXI)
March/2020
Genova, Italy
Supervisor: Prof. Daniele Marrè
BIDIMENSIONAL
O BASED
A Thesis Presented to the University of Genova
n Partial Fulfilment of the Requirements for
Tutto ignoro di te fuor del messaggio muto che mi sostenta sulla via: se forma esisti o ubbia nella fumea d’un sogno t’alimenta
la riviera che infebbra, torba, e scroscia incontro alla marea.
Acknowledgment
I would like to thank my supervisor, Professor Daniele Marré for his support and trust. In particular I’m grateful for his ‘letting me do’ that allowed me to experiment the difficulties and gratifications of facing and solving the problems that show up in the realm of the research endeavour.
I am also indebted to Dott. Emilio Bellingeri who was the infallible last resource any time everything went wrong with the MBE or the XRD setups. Likewise, I like to thank Dott. Alessandro Leveratto who was always at hand to solve the frequent setup nightmares. I must also thank Dott. Andrea Gerbi and Dott. Renato Buzzio for the many discussions held and the readily help at the laboratory, especially – but not only - with the AFM, but not only. I would also say like to say thanks to Dott.ssa Ilaria Pallechi who was always available to deal with the difficulties of transport measurement issues. Of course I am also grateful to all the people of both CNR-SPIN and the Physics Department of University of Genova that warmly received and helped me.
I thank to the University of Genova for the research and study opportunity and for the support provided during my PhD. I’m grateful to the CNR-SPIN for the opportunity to use the laboratory facilities at Genova.
Last but not least, I would like to thank my family, friends and some former professors because all of them contributed in many different ways to make this journey possible.
General Index
1. List of Tables ... 6 2. List of Figures ... 6 3. List of Equations ... 9 4. Abstract ... 10 5. Summary ... 11 5.1. Overall perspective ... 11 5.2. Specific results ... 13 6. Introduction ... 147. Components of the studied films and heterostructures ... 14
7.1. Zinc Oxide (ZnO) ... 14
7.1.1. ZnO Crystal structures and alloys ... 14
7.1.2. Band structure, electrical transport and doping ... 17
7.1.3. ZnO applications ... 21
7.2. Bidimensional electron gas (2DEG) ... 21
7.2.1. 2DEG behaviour ... 23
8. Experimental techniques ... 29
8.1. Pulsed Laser Deposition (PLD) ... 29
8.2. Molecular Beam Epitaxy (MBE) ... 29
8.3. Reflection High Energy Electron Diffraction (RHEED) & Azimuthal RHEED (ARHEED) 29 8.4. Atomic Force Microscopy (AFM) ... 30
8.5. X-Ray Diffraction Techniques (XRD & XRR) ... 30
8.6. Transport Properties Measurements ... 31
9. Results ... 32
9.1. MBE deposition of structures based in ZnO/MgO ... 32
9.1.1. Substrates: Al2O3 001 & ZnO 001 – properties ... 32
9.1.2. Preparation and pre-treatment ... 33
9.1.3. MBE ZnO Films – Deposition ... 44
9.1.4. Structural Characterization ... 45
9.1.5. Phases and the growth process ... 46
9.1.6. Film Thickness and the scaling exponents ... 54
9.1.7. Roughness, relaxation and mound formation ... 58
9.1.8. Morphology and evolution of the growth front ... 63
9.1.9. Surface crystallography of the structures ... 74
9.1.10. Strain, micro-strain & crystallite size ... 88
9.1.11. MgO layer, epitaxial relationship and twist disorder ... 90
9.1.1. Nucleation and patches ... 98
9.1.2. Percolation and charge transport ... 103
9.1.3. Growth mechanism summary ... 107
9.1.4. A note on the relationship between structural features and deposition conditions 108 9.1.5. Conclusions ... 109
9.2. PLD ZnO Based Heterostructures – Study of Field Effect Magnetoresistance of ZnO110 9.2.1. Experiments and modelization ... 110
9.2.2. Results & discussion ... 114
9.2.1. Conclusion ... 114
9.3. PLD ZnO Based Heterostructures – Study of Magnetic Field enhanced 2DEG Effective Mass 115 9.3.1. Experiments and modelling ... 115
9.3.2. The fitting approach ... 120
9.3.1. Discussion ... 126
9.3.2. Conclusions ... 127
10. General conclusions and perspectives ... 128
10.1. Conclusions... 128
10.2. Prospectives ... 128
11. Bibliography ... 130
12. Annexes ... 138
1. List of Tables
Table 1- Substrate pre-treatments ... 42
Table 2- Steps of a typical MBE deposition ... 44
Table 3- Sequence of characterization steps ... 46
Table 4- XRD main peaks considered ... 47
Table 5- Distribution of samples (number) ... 47
Table 6- Fitting model for the peaks ... 48
Table 7- Scaling exponents of Density & Thickness ... 58
Table 8- Morphology at different stages of growth ... 63
Table 9- Slope of the Weibull-linearized AFM height distributions ... 74
Table 10- RHEED analyzed samples ... 75
Table 11- Summary of samples analyzed by Extended reflectivity ... 99
Table 12- Electric properties of the samples ... 106
2. List of Figures
Figure 1- Relative stability of ZnO crystalline forms ... 15Figure 2- Crystal structure of wurtzite ZnO... 15
Figure 3- Energy gap vs. lattice constant ... 16
Figure 4- Band gap vs. x in MgxZ1-xO ... 17
Figure 5- Energy level diagram of ZnO – Crystal field & Spin-orbit coupling ... 18
Figure 6- Band alignment of semiconductors and interstitial hydrogen energy position ... 19
Figure 7- Mobility vs. Temperature ... 20
Figure 8- Mobility vs. carrier concentration ... 20
Figure 9- 2DEG hosting heterostructure, wave function and potential ... 22
Figure 10- Varieties of two dimensional systems ... 23
Figure 11- Polarization vs. Mg content in MgxZn1-xO ... 23
Figure 12- 2DEG mobility vs. temperature ... 24
Figure 13- 2DEG mobility vs. electron density ... 24
Figure 14- Methods to study the 2DEG behaviour ... 25
Figure 15- Quantum Hall Effect Characteristic curve and measurement schema ... 27
Figure 16- 2DEG mobility vs. Temperature and Hall resistances vs. Magnetic field ... 27
Figure 17- EFG Sapphire Growth Method ... 32
Figure 18- Miscut () & Azimuth () angles ... 32
Figure 19- Sapphire substrates diffractograms ... 33
Figure 20- AFM scan of the ‘As received’ Sapphire substrates ... 34
Figure 21- AFM scan of annealed sapphire substrates at different temperatures ... 35
Figure 22- AFM scan of annealed sapphire substrates for different periods of time ... 35
Figure 23- XRD Sapphire 006 peak FWHM ... 36
Figure 24- Sapphire substrates diffractogram detail peak [003] (=20.52º) ... 37
Figure 25- Sapphire substrates diffractogram detail peak [009] (2θ=64.47º) ... 37
Figure 26- Rocking curve of sapphire forbidden reflection [003] (=20.52º) ... 38
Figure 27- Rocking curve of sapphire forbidden reflection [009] (2θ=64.47º) ... 38
Figure 28- Pre-treatment duration and temperature vs. Intensity of the pre-treatment XRD peak .... 39
Figure 30- -scan of sapphire peak [113] of substrate... 40
Figure 31- -scan of sapphire peak 113 of substrate (detail) ... 41
Figure 32- FWHM of sapphire substrates -scan of [116] peaks & shift between sub-peaks ... 41
Figure 33- Sapphire substrates blocks observed under polarized light ... 42
Figure 34- Sapphire surface terminations ... 43
Figure 35- Sapphire termination stability maps ... 43
Figure 36- Typical MBE deposition run characteristic curves ... 45
Figure 37- XRD diffractogram of a complete sample ... 46
Figure 38- Detail of the fingerprint zone of a typical sample diffractogram ... 47
Figure 39- Fitting of XRD diffraction peaks (Sample Mbe3301)... 50
Figure 40- Normalized XRD -2 & rocking curve peaks ... 51
Figure 41- XRD general correlation ... 53
Figure 42- XRD general correlation (Intermediate thickness, detail) ... 53
Figure 43- Spatially confined oriented growth mechanism ... 54
Figure 44- AFM thickness measurement ... 55
Figure 45- SEM cross section measurement ... 56
Figure 46- Thickness vs. Integrated Intensity of XRD ZnO002 peak ... 57
Figure 47- Film thickness & density vs. XRD ZnO002 integrated intensity ... 57
Figure 48- Roughness vs. XRD ZNO002 integrated intensity ... 59
Figure 49- Nozzle & Sample holder alignment ... 59
Figure 50- Smoothed roughness (RMS) vs. XRD -2 ZnO002 Integrated intensity... 60
Figure 51- Mound growth mechanisms ... 61
Figure 52- AFM Grain diameter vs. Grain height ... 62
Figure 53- RMS & thickness vs. deposited mass (XRD ZnO002 Integrated intensity) ... 62
Figure 54- Concentrated small deposits (Mbe1801) ... 64
Figure 55- Development of columnar or globular structure (Mbe1904) ... 65
Figure 56- Development of non relaxed film of globular structure (Mbe2603) ... 66
Figure 57- AFM & XRD surface interaction indicators vs. surface thickness ... 67
Figure 58- Development of a relaxed film of globular structure (Mbe3407) - I ... 68
Figure 59- Development of a relaxed film of globular structure (Mbe3407) - II ... 69
Figure 60- Development of a dense film (Mbe3301) ... 70
Figure 61- Average EDS Zn/Al vs. thickness ... 71
Figure 62- Weibull-linearized plots of selected samples AFM height distributions ... 73
Figure 63- Weibull parameter vs. Thickness for each growth front zone ... 74
Figure 64- RHEED pattern of sample Mbe2606 ... 76
Figure 65- RHEED pattern of sample Mbe2801 ... 77
Figure 66- RHEED pattern of sample Mbe3203 ... 78
Figure 67- RHEED pattern of sample Mbe2601 ... 78
Figure 68- RHEED pattern of sample Mbe3110 ... 79
Figure 69- RHEED pattern of sample Mbe3003 ... 80
Figure 70- RHEED pattern of sample Mbe3402 ... 80
Figure 71- RHEED pattern of sample Mbe3003 at direction (110) vs. time ... 81
Figure 72- RHEED spot Intensity evolution for sample Mbe3003 ... 82
Figure 73- Relative position shift of Spot 1 & Spot 2 sample Mbe3003 ... 82
Figure 74- RHEED pattern of sample Mbe3408 ... 83
Figure 75- RHEED pattern of sample Mbe3015 ... 83
Figure 76- RHEED pattern of sample Mbe3301 ... 84
Figure 77- RHEED pattern of sample Mbe3401 ... 85
Figure 78- RHEED pattern of sample Mbe3407 ... 85
Figure 80- RHEED patterns vs. ZnO & MgO thickness ... 87
Figure 81- Plain view of ARHEED map (Sample Mbe3309) ... 87
Figure 82- ZnO layer strain vs. Film Thickness ... 88
Figure 83- MgO layer strain vs. Thickness ... 89
Figure 84- Williamson-Hall plot of ZnO samples ... 89
Figure 85- Williamson Hall Micro-strain & Size vs. Film thickness ... 90
Figure 86- Main XRD parameters of MgO111 layer ... 91
Figure 87- MgO films thickness & roughness ... 92
Figure 88- MgO films grain diameter & AFM phase RMS ... 92
Figure 89- XRD -scan of MgO200 and other peaks (sample Mbe3103) ... 93
Figure 90- XRD -2 scan of MgO200 in two different ... 93
Figure 91- Distorted MgO200 and layers of MgO111 ... 94
Figure 92- XRD -scan of Sapphire113 & MgO200 ... 95
Figure 93- XRD -scan of distorted MgO200 (peak splitting) ... 95
Figure 94- Rotational disorder (-scan FWHM) vs. Film thickness ... 96
Figure 95- Rotational disorder of the ZnO vs. Pre-treatment duration ... 97
Figure 96- General correlation II ... 98
Figure 97- Extended reflectivity of selected samples ... 99
Figure 98- XRR rocking curves of sample Mbe2602 ... 100
Figure 99- Extended reflectivity fitting ... 101
Figure 100- Extended reflectivity peak fitting I ... 101
Figure 101- Extended reflectivity peak fitting II ... 102
Figure 102- General correlation III ... 102
Figure 103- Bonded samples... 103
Figure 104- Curves I-V ... 103
Figure 105- Curves R(T) ... 104
Figure 106- R(T): cooling, heating & UV light application ... 105
Figure 107- Arrhenius plot ... 105
Figure 108- Sheet specific conductivity & Eg vs. thickness ... 106
Figure 109- Crystallographic quality and Eg ... 107
Figure 110- Growth mechanism summary ... 108
Figure 111- Field effect modulated resistivity vs. temperature ... 111
Figure 112- MR vs. T & M for undoped and Co doped Field Effect devices ... 111
Figure 113- Factor ‘g’ & ’h’ vs. Electric field and temperature ... 113
Figure 114- Polaron percolation scheme ... 114
Figure 115- Schema of 2DEG device ... 115
Figure 116- Magnetoresistance vs. Magnetic field at T=1.25K ... 117
Figure 117- Magnetoresistance vs. Reciprocal of magnetic field at T=1.25K ... 117
Figure 118- Experimental magnetoresistance vs. Reciprocal of magnetic field for different temperatures ... 118
Figure 119- Fitted magnetoresistance vs. Reciprocal of magnetic field for different temperatures 121 Figure 120- Fitted magnetoresistance vs. Reciprocal of magnetic field for T = 1.25K ... 122
Figure 121- Fitted magnetoresistance vs. Reciprocal of magnetic field for T = 6K ... 122
Figure 122- Fitted magnetoresistance vs. Reciprocal of magnetic field for T = 8K ... 123
Figure 123- Fitted magnetoresistance vs. Reciprocal of magnetic field for T = 12K ... 123
Figure 124- Relative effective mass (m*/mo*) vs. Magnetic field ... 124
3. List of Equations
Equation 1 – Shubnikov de Haas oscillations model ... 25
Equation 2- 2DEG Energy quantization ... 26
Equation 3- XRD Integrated intensity decomposition ... 52
Equation 4- Weibull distribution & ‘failure rate’ ... 72
Equation 5- Weibull linearizing transformation ... 72
Equation 6- Removed background ... 100
Equation 7- Khosla & Fischer magnetoresistance model (positive part)... 112
Equation 8- Khosla & Fischer magnetoresistance model (negative part) ... 112
Equation 9- Lifshitz-Kosevic Model... 118
Equation 10 - Electron-electron correlation: Wigner-Seitz parameter ... 126
4. Abstract
Spintronics and High Electronic Mobility Transistors (HEMTs) are active frontiers in the development of electronic devices. For their development, Diluted Magnetic Semiconductors (DMS) and heterostructures hosting Two Dimensional Electronic Gas (2DEG) are required.
MBE has become into the state of the art technique to prepare ZnO based heterostructures, surpassing PLD limitations. However, the former has a much more complex operation and higher costs. In order to improve the quality of ZnO based heterostructures at bounded costs, a low oxidative MBE process was studied.
The aim of the present thesis is double: first, it intends to make a contribution to the knowledge of the non conventional low oxidative MBE production of ZnO heterostructures. In particular we wanted to assess its feasibility and understand the growth mechanism. Second, this thesis intends to make specific contributions to the physics of electric transport in such heterostructures, namely Electric Field modulated Magnetoresistance and 2DEG Magnetic Field Effective Mass Enhancement.
For the first objective, 142 MgO/ZnO heterostructures were grown on sapphire on a home-made MBE in low oxidative conditions. An extensive characterization of the sample set by RHEED, AFM, XRD, SEM, EDS and Transport measurement was carried out. The analysis of the data gathered made it possible to identify different growth stages of the ZnO structures. They were mainly discussed in terms of misfit accommodation. The results suggest a complex process with interlinked steps, in which subtle transformations take place.
For the second objective, Field Effect devices were produced by depositing Co:ZnO/STO heterostructures by PLD. After XRD, AFM, TEM and magnetic characterization, transport measurements were carried out. The magnetic scattering was studied by appropriate model fitting and discussed in terms of percolation of Bounded Magnetic Polarons (BMP). The results suggest free charge carrier mediated magnetic polarization of Co:ZnO.
Next, a 2DEG hosting Mg:ZnO/ZnO heterostructure was deposited by PLD. Transport measurements showed complex quantum oscillations. The simultaneous model fitting for all temperatures showed magnetic field effective mass enhancement. The results suggest band non-parabolicity and electron-electron correlation as possible causes.
5. Summary
5.1.
Overall perspective
In this study I have followed two parallel paths:
In the first one, I have produced high quality ZnO based heterostructures by molecular beam epitaxy (MBE) under low oxidative conditions. I have characterized them from a structural and electric transport point of view to shed light on the growth process and electric transport.
In the second one, I have gone one step further in the study of the electric transport behaviour of some ZnO based heterostructures fabricated by pulsed laser deposition (PLD). The MBE path encompassed most of my PhD research time, where many challenges were faced and solved: from practicalities of the operation of the home made MBE setup (i.e. very difficult troubleshooting, stability and control issues, (not so) simple maintenance, etc.) to the fundamental understanding of the complexities involved in the growth and behaviour of ZnO heterostructures. The undertaking of this research path and the assembling of the MBE setup itself were intended to produce better quality heterostructures. So, a precise understanding of both the MBE setup and the produced samples was required to move closer to the intended quality, or at least recognize its limits. All this had to be carried out at the same time: MBE troubleshooting, optimization of the produced heterostructures and the study of its (intended superior) behaviour.
The PLD path encompassed most of the PhD work time as a part of a research team, where research is not done alone, but by collaborating and sharing efforts and knowledge with the rest of the research team. I was fortunate to work with the CNR-SPIN Oxides team where I was kindly received.
In the MBE path I have carried out an extensive experimental campaign of long trials to produce ZnO and MgO films consistently under low oxidative conditions with the home made MBE setup. So, two competing objectives had to be achieved at the same time:
a) Make our home made MBE setup produce oxide films for the first time and do it in a controlled manner. This objective required to explore the parameter space to find the right conditions to get the ZnO deposited under low oxidative conditions. Since the parameter space is too big to be easily explored in a reasonable time, the trials were not designed to obtain a precise view of particular details but to catch the most significant trends of the growth process.
b) Produce films and heterostructures better than those fabricated by PLD. This objective implies an optimization phase of the quality of the samples until a significant improvement is obtained. However, such phase could not be postponed till the troubleshoot of the MBE was finished due to time constrictions. So, the experiments should be carried out with the simultaneous scope of troubleshooting the MBE setup and provide measurements useful to understand and optimize the produced samples.
The result of this mix mode effort produced a quite big sample set (142 samples, where each sample takes two days of previous preparation, a complete dedicated full day of deposition and more than a week to make a complete AFM and XRD characterization). Different kind of outputs were reached at the end of the third year:
a) The troubleshoot of the MBE setup: ZnO and MgO layers got deposited on the sapphire substrates, the quality was improved, maintenance stops and run failures reduced.
b) The accumulation of a database with all the characterization measurements carried out and the learning of the behaviour of the complex system under study: substrate, film, effusion cells, vacuum, XRD, AFM, RHEED, etc.
c) The fact-based knowledge of the ZnO film growth mechanism and features obtained as the main content of the thesis:
i. Substrate features (roughness, surface quality, morphology) obtained as a result of the transformations induced during different treatments (cleaning, annealing, thermal pre-treatment, chemical pre-treatment, hydroxylation, relaxation, terracing, roughening, etc.)
ii. Film deposition: wetting, nucleation, growth, coalescence, annealing, straining, relaxation, roughening, mound development, crystallographic transformations, etc.
iii. Film/heterostructure features: phases, thickness, roughness, tilt disorder, twist disorder, roughness, strain, micro-strain, crystallite size, heteroepitaxial
relationships, electric charge transport percolation, band gap,
photoconductivity, etc.
iv. Mechanism of growth: coordinated sequence of steps and transformations that go from the ‘as received’ substrate to the grown film or heterostructure. In particular I have been able to connect details of the fine heteroepitaxial relationship, independence of the film features from the deposition conditions, the connected sequence of nucleation evolution and role of strain and dislocations - rotational reorganization – electric coalescence threshold, etc. I was also able to differentiate different parts of the buffer layer with different characteristics, distortion, their mutual relationships, which dislocations they probably create and how.
In the PLD path, I have collaborated with the research group on mainly two endeavours:
a) Strengthen the study of the field effect modulated magnetoresistance of Co doped heterostructures based in ZnO/STO: my specific contribution in this front was the independent production of samples by PLD, with an independent criteria analysis. The completely free production and characterization of these samples yielded an independent control and validation of an important part of the study. This strengthened the experimental and analytical component of the study that finally led to the publication of the results in Scientific Reports of the article “Influence of free charge carrier density on the magnetic behaviour of (Zn, Co)O thin film studied by Field Effect modulation of magnetotransport” (Sci Rep 9, 149, 2019). The contribution consisted in the production of samples of similar
but different characteristics of ZnO by PLD, the realization of the charge transport characterization, the independent search of the analytical approach and then the analysis of the gathered measurements. It was verified, in this way, the reasonability and robustness of the estimated parameters of the study of the positive and negative magnetoresistance carried out by the research team. In particular this personal elaboration yielded productive discussions regarding the estimation and meaning of the parameters of positive and negative magnetoresistance of the model of Khosla & Fischer applied to the studied Co:ZnO heterostructure.
b) The complete resolution of the data analysis of the study of the field effect effective mass enhancement in a 2DEG hosted in a ZnO heterostructure: my specific contribution in this front was the ideation and realization of a methodology to obtain a robust fit of a complex model to the quantum oscillations of the longitudinal magnetoresistance of the 2DEG. The mathematical problem is ill posed. To solve it, it was necessary to find a creative solution (i.e. find reliable fitting parameters). The solution proposed to the problem improved the quality and reliability of the fitted parameters and a better interpretation of the undergoing phenomena. The solution consisted in a transformation of the fitting problem into another, better posed, easier to solve, with solutions more robust and improved indicators of the quality of the solution found. The interpretation of the phenomena is based on non-parabolicity of the conduction band and the electron-electron correlation arguments.
5.2.
Specific results
The following is a list of specific aspects of the structure or the ZnO growth process that were addresses at least partially
Understanding of the sequence in the growth phases
Epitaxial relationship between elements stacked in the heterostructure Epitaxial relationship between elements of the buffer layer
A proposed growth model of ZnO films
MgO layer nucleation relationship with hydroxylation and pre-treatment Differentiation of mechanisms of strain relaxation
Growth front statistics
Dynamic growth analysis by statistical analysis of AFM images Mound and morphology formation
Dominant dislocation Burgers vector directions
The following is a list of specific contributions to the research group:
Independent PLD ZnO sample preparation, characterization, model fitting and interpretation of negative magnetoresistance
Robust fitting procedure to identify the Lifshitz-Kosevich model parameters of a ZnO based heterostructure hosted 2DEG
6. Introduction
ZnO is a compound semiconductor with a high potential for many applications based in some strong properties: eco-friendly, nontoxicicity, chemical and photo-thermal stability, semiconducting behaviour of wide band gap, high exciton binding energy, piezoelectricity, wide availability and low price. All these properties make of ZnO a very promising material for many mono and multifunctional applications like laser diodes, solar detectors, spintronics, transparent conductors and electronics, hydrogen storage (Bashar, 2016; Azam, 2013; Ahmad, 2010; Du, 2009; Ogale, 2006). Some other properties, like thermochromism, were also reported.
Besides applications, ZnO is also a sort of sandbox to study many physics phenomena due to the numerous properties listed before and the variety of morphologies and nanostructures that can be built with it (Dien, 2019; Emil, 2018; Wang, 2011).
Since properties of such devices non only depend on the materials used to build them, but also in the heterostructure/device configuration itself, a brief review of the main features of both ZnO properties and heterostructures physics is presented.
7. Components of the studied films and heterostructures
7.1.
Zinc Oxide (ZnO)
Zinc oxide is a widespread inorganic compound used in many non high tech applications ranging from etc. Pure ZnO, usually obtained from synthetic methods, presents itself as white powder or transparent crystals. When some impurities or doping is present, it may become slightly red or yellowish (Klingshirn, 2010). ZnO is an amphoteric oxide, insoluble in water that reacts readily with inorganic acids or bases, but very slowly with fatty acids.
7.1.1. ZnO Crystal structures and alloys
ZnO belongs to the II-IV compound semiconductor group (Combinations of Zn, Cd and Hg with O, S, Se, Te). As most of the members of this group ZnO has tetrahedral coordination. Band gaps of these binary compounds, in the other hand, span from 3.94 eV (ZnS) to 0 (Hg semimetallic compounds). ZnO places itself near the former, with 3.34 eV in the near ultraviolet. The tetrahedral coordination of ZnO occurs in the wurtzite form, except in extremely particular cases (i.e. very high pressures). This coordination is based in the sp3 hybridization that constitutes the valence band. This coordination and hybridization produces the two main crystallographic forms of the ZnO: Zinc blende-type and wurtzite-type, being the latter the most stable in normal conditions and the most commonly found (Klingshirn, 2010; Jaffe, 1993) (see Figure 1)
Figure 1- Relative stability of ZnO crystalline forms
(After Jaffe, 1993)
The lattice symmetry of wurtzite ZnO has point group 6mm (C6v). Lattice parameters are
a=0.325nm & c=0.52nm, being c the direction of the C6 rotation axis, usually denoted as [001] (see
Figure 2).
Figure 2- Crystal structure of wurtzite ZnO
(Left panel: grey & black spheres denote Zn & O atoms respectively (Morkoç, 2009)) (Right panel: black & blue spheres denote Zn & O atoms respectively (Saito, 2011))
The bonding character of ZnO is strongly ionic (O=3.5 ; Zn=0.91) that yields a high thermal
stability of ZnO (Tm=2250K) and induces a big polarity of the crystal in the c-axis direction. Zn terminated faces (001) are positive charged respect to the oxygen terminated faces (00-1) faces.
This polarity induce many differences between both faces: different etching and growth rate, morphology during etching and growth, stability and generation of defects, plasticity, etc. Other crystallographic planes are associated to a different distribution of atoms between successive planes that induce different grades of polarity (e.g. [101]) or perfect neutrality (e.g. [100]). Moreover, the big induced electrostatic force between anionic and cationic planes distort the ideal wurtzite crystalline structure reducing 2% the c/a. Still, the polarity of the Zn-O bond plus the lack of centro-symmetry of the structure are responsible for the wurtzite ZnO piezoelectricity (Klingshirn, 2010; Morkoç, 2008; Wander, 2001; Lawaetz, 1972).
The need for precisely tuned band gaps in applications can usually be tackled by means of alloying. This is easily implemented in many systems, since alloying produces semiconducting materials with an intermediate behaviour. In the case of ZnO, this is not straightforward since there is the risk of crystal structure change (wurtzite to rock-salt) that should be avoided. However, under moderated Zn substitution, the crystal structure is preserved and the band gap can be varied to a great extent (i.e. Eg from 2 to 5 eV, see Figure 3 and Figure 4) (Huso, 2015; Makino, 2001; Ohtomo, 2001). In the case of heteroepitaxial films, additional requirements shall be meet in order to maintain the effective misfit between acceptable limits. In these cases, depending on the substrate, additional constrictions are put on the alloy composition due to Vegard’s law. In any case, the band gap is much more sensible to a percentage change in the alloy composition than in the cell parameters. In the case of cation substitution (Mg, Cd, Be), Mg seem to be the preferred alloying element, since Cd reduces the band gap and Be use has the downside of its toxicity. In the case of anion substitution (S, Se) the big difference in electronegativity between them introduces too much big changes in the system parameters that are not suitable for fine tuning.
Figure 3- Energy gap vs. lattice constant
(Williander, 2013, after Makino, 2001 & Ohtomo, 2001)
Figure 4- Band gap vs. x in MgxZ1-xO
(Left panel: grey area is the wurtzite to rock-salt transition zone)
7.1.2. Band structure, electrical transport and doping
In spite the structure of the bands of ZnO has been investigated and discussed for more than 50 years now, many details are still not clear. The ZnO (3d10 4s2) conduction band edge has s-like character, while the highest occupied valence band orbital have p-like character. The energy degeneracy of the valence bands are claimed to be split due to both the crystal field and spin-orbit coupling (cf & so in Figure 5). The combined effect of both interactions has been discussed extensively in the literature without a consensus about the right ordering of them (see Klingshirn, 2007 for a review). It is assumed that this degeneracy breaking produces three states (A, B, C) that differ by less than 100 meV, giving rise to three exciton types of slightly different binding energy. In any case, when dealing with alloyed ZnO, the band gap dependence on the amount of the substituted ion (Mg, Cd, Be) tends to be fairly linear (see Figure 3 & Figure 4) (Klingshirn, 2010)
Figure 5- Energy level diagram of ZnO
ZnO may be doped by both impurities
like Al, Ga, In. On the acceptor side, the situation is much complex since p doping has been an elusive search for a long time.
targets for obtaining p doping. Even though
claimed, there are still active discussions on going there seem to doping can be primarily done by the introduction of complex 2017; Tzukazaki, 2005).
Among the doping defects, the most important are zinc interstitials (Zn
(VO). However, oxygen vacancies, being deep donors are not able to influence the n doping but can
just neutralize any p doping. Zn interstitials, in the other hand, being sh
unstable. So they are better avoided than a tuning resource (McKluskey, 2012; Janotti, 2007; Van der Walle, 2003). There are a lot of other doping defects partially addressed by the l
of p type (e.g. basal stacking faults, Schirra, 2007) Other important class of doping
interest due to its intended potential to produce Diluted Magnetic Semiconductor (DSC) spintronic devices. In a DSC, free ca
leading to the stabilization of ferromagnetic states (see Liu, 2005 for an extensive review on
GaN based DSC). So, TM ions do not only supply the charge carriers needed for conduction but also for the ferromagnetism enhancement
(Sing, 2007; Sato, 2001; Dietl, 2000)
led to segregation of the dopant (secondary phase) and change compl magnetic behaviour (i.e. ferromagnetism of the secondary phase itself) Another critical aspect of the band structure of ZnO is the band alignment key insight is provided by the fact that different
different band alignment, have all almost the same hydrogen transition energy level (i.e. the energy Energy level diagram of ZnO – Crystal field & Spin-orbit coupling
(Laskowski, 2006)
impurities and defects. Typical donors impurities
On the acceptor side, the situation is much complex since p doping has been an elusive search for a long time. Nitrogen, arsenic, phosphorous, antimony on oxygen sites are
ven though a consistent and stable p doping cannot
there are still active discussions on going there seem to point to a consensus that doping can be primarily done by the introduction of complexes of defects and
ping defects, the most important are zinc interstitials (Zni) and oxygen vacancies
xygen vacancies, being deep donors are not able to influence the n doping but can neutralize any p doping. Zn interstitials, in the other hand, being shallow mobile defects, are unstable. So they are better avoided than a tuning resource (McKluskey, 2012; Janotti, 2007; Van
There are a lot of other doping defects partially addressed by the l faults, Schirra, 2007).
Other important class of doping impurities are transition metals (TM). They concentrate great potential to produce Diluted Magnetic Semiconductor (DSC)
In a DSC, free carriers mediate the magnetic interaction between TM ion spins, leading to the stabilization of ferromagnetic states (see Liu, 2005 for an extensive review on
TM ions do not only supply the charge carriers needed for conduction but also for the ferromagnetism enhancement, coupling both behaviours, even at room temperature
Sato, 2001; Dietl, 2000). Limited solubility of TM in the semiconductive matrix led to segregation of the dopant (secondary phase) and change completely the nature of the overall
(i.e. ferromagnetism of the secondary phase itself).
Another critical aspect of the band structure of ZnO is the band alignment and hydrogen doping the fact that different semiconductors with different
band alignment, have all almost the same hydrogen transition energy level (i.e. the energy orbit coupling
impurities are group III metals On the acceptor side, the situation is much complex since p doping has been an Nitrogen, arsenic, phosphorous, antimony on oxygen sites are a consistent and stable p doping cannot still be reliably point to a consensus that stable p of defects and/or impurities (Tang,
) and oxygen vacancies xygen vacancies, being deep donors are not able to influence the n doping but can allow mobile defects, are unstable. So they are better avoided than a tuning resource (McKluskey, 2012; Janotti, 2007; Van There are a lot of other doping defects partially addressed by the literature, even
. They concentrate great potential to produce Diluted Magnetic Semiconductor (DSC), pillars for interaction between TM ion spins, leading to the stabilization of ferromagnetic states (see Liu, 2005 for an extensive review on ZnO & TM ions do not only supply the charge carriers needed for conduction but , coupling both behaviours, even at room temperature in the semiconductive matrix may etely the nature of the overall
and hydrogen doping. A semiconductors with different band gaps and band alignment, have all almost the same hydrogen transition energy level (i.e. the energy
at which H+ and H- ions are in equilibrium (Van der Walle, 2003) (see Figure 6). The Fermi energy level, lying very close to the conduction band edge of ZnO, induces a very difficult p doping and enhances the critical role of hydrogen. Since its presence is frequently unavoidable, involuntary n doping cannot be ruled out. In this semiconductor, it acts as a shallow donor (i.e. its energy level lies inside the conduction band) with high molecular mobility that allows him to interact with almost any other impurity or defect neutralizing it or forming more stable complexes (Nahm, 2014; Janotti, 2007; Van der Walle, 2003). Many potential mechanisms are refereed by the literature as a source for the stabilization of hydrogen in ZnO: (i) substitutional hydrogen as HO creating a
multicentrum with bonds with all four first Zn neighbours (Janotti, 2007), (ii) reconstructed hydroxylated surface of ZnO can form shallow donors with hydrogen (Meyer, 2004), (iii) plain diffusion in the interstitial sites of ZnO (as demonstrated by deuteration ZnO films (Li, 2008), etc. It must be noted that even though hydrogen involuntary presence cannot be avoided, hydrogen doping can be ruled out when a more strong doping is on place (e.g. Al, Ga, In) because of the reasons presented.
Figure 6- Band alignment of semiconductors and interstitial hydrogen energy position
(Van der Walle, 2003)
Mobility has been studied mostly by temperature dependent Hall effect and reported in the literature in the order of 500 cm2/Vs. at room temperature, but reaching some multiples of this value for highly optimized ZnO films (see Figure 7). The scattering processes considered are polar optical scattering, acoustic phonon scattering, ionized impurity scattering and piezoelectric scattering. The first seem to be the dominating scattering process at room temperature, while piezoelectric-phonon scattering at low temperature. However, for high doping ionized impurity scattering becomes the dominant one (see Figure 8) (see Look, 1998 and Makino, 2005 for a review)
Figure 7- Mobility vs. Temperature
(After Makino, 2005)
Figure 8- Mobility vs. carrier concentration
(After Makino, 2005)
ZnO is also rich of surface states. In particular there were found surface states of metallic character that can create additional channels for charge conduction (Silva, 2018; Wander, 2001). Photoconductivity is part of the wealth of behaviours that derive from the rich surface chemistry of ZnO: Under UV illumination holes and electrons are separated. The former can diffuse to the surface to get oxygen molecules desorbed (or other gases, mainly H2O, etc). Differently from other
heavily dependent on the environment composition. This is probably due to different transport dynamics of the charge carriers under illumination or darkness. From this variety of behaviours, many opportunities for a diversity of gas sensing applications can be potentially derived (Schenkler, 2008, Studenikin, 2000).
7.1.3. ZnO applications
Since ZnO is a wide band gap material, ZnO is quite a firm candidate for transparent electronics (transparent conductive oxides & Transparent thin film transistors), solar cells and detectors, gas sensors, near UV LEDs and UV photodetectors. Due to the possibility of modulate its magnetic properties, it is also considered valuable for applications in spintronics. The possibility of widening the band gap by alloying with Mg opens also the opportunity to produce deep ultraviolet optoelectronic devices. The improvement in the fabrication techniques and its natural piezoelectricity suggest a wide array of applications to nano and micro electromechanical systems (MEMS). Other application include photocatalysts, windows coating, ferroelectric memories, acoustic-optodevices, etc.
In particular the applications of ZnO to the field of electronics, is rather hindered due to the difficulties of obtaining p doping that was signalled in previous sections. The great advances indicated regarding nitrogen based complexes n doping is still in development, since the stability, tunability and flexibility of the involved structures is still under study and much more is required to be understood (Ng, 2018; Pearton, 2004)
Extensive reviews can be found in Klingshirn, 2010; Ozgur, 2005; Janotti, 2009; Sing, 2007 and Schenkler, 2018.
7.2.
Bidimensional electron gas (2DEG)
The formation of a 2DEG is a medium to demonstrate the crystalline high quality obtained and the basis for the HEMTs (high electron mobility transistors). The 2DEG characterizations is a sort of probe of the structural quality of the hosting heterostructure, since the stringent quality requirements it imposes. In the case of a polar semiconductor like ZnO (or GaN), the 2DEG can be formed by both modulation doping or by polarization effects. In a typical 2DEG hosting device or heterostructure (see inset Figure 9), the charge carriers are confined to a channel where the ionized impurity scattering power has a reduced influence, hence enhancing a high electron mobility. Structural quality of the channel and its surrounding are important for obtaining high mobilities since structural disorder can activate more phonon modes increasing the phonon scattering (Falson, 2016; Klingshirn, 2010). Moreover, state of the art MBE of ZnO heterostructures 2DEG is directly concerned with reducing different contamination sources as we briefly review in the next paragraphs. Typically material sources are of the highest grade purity (e.g. Zn, ozone, heating cermets, substrate holders, etc.) (see Falson, 2016 for a review)
Figure 9- 2DEG hosting heterostructure, wave function and potential
(Ozdemir, 2015)
A 2DEG of good quality can have mobilities that largely exceed those of ZnO films or ZnO bulk by many order of magnitude (see typical metal-like behaviour of 2DEG in Figure 12). The creation of a 2DEG in an oxide heterostructure is based in the production of a potential well where electrons are confined. The well may be created and filled with electrons in different modes (see Figure 10) (Falson, 2018). In the case of modulated doping, the well is created by the band bending due to the two different band gap materials in contact in a perfect crystalline interface. The filling of the well is produced by the diffusion of the charge carriers coming from a doped layer close to the well. The mobility of this kind of 2DEG may be hampered by the slight disorder introduced by the ionized doped layer. In the case of the MgO:ZnO based 2DEGs, the well is created by the change in the polarization field of the interface (Mg is isovalent with Zn so it cannot act as a dopant of ZnO). Differently, the Mg content modulate the ZnO c unit cell parameter (Kozuka, 2012) thus increasing the electric dipole. All this is done without introducing any sort of doping layer, thus preventing ionized impurity scattering that would limit electron mobility. The polarization introduced by this procedure is proportional to the Mg content (see Figure 11). MgZnO/ZnO out of plane thickness of the 2DEG (i.e. 2DEG electron wavelength axial characteristic length) was estimated by means of a combination of state of the art heterostructures PL measurements and with first principles calculations to yield 10 nm thick (Falson, 2018)
Figure 10- Varieties of two dimensional systems
(Falson, 2018)
Figure 11- Polarization vs. Mg content in MgxZn1-xO
(Falson, 2018)
7.2.1. 2DEG behaviour
A good quality 2DEG can exhibit Quantum Hall Effect (QHE), as was demonstrated in the case of ZnO based heterostructures (Tzukazaki, 2007). The significance of this is the fact that 2DEG QHE was accepted as a phenomena only observable in extremely ordered crystalline structures. So oxides were deemed not appropriated for them till the development of oxide MBE (see notes of Figure 13 for MBE mobilities development milestones).
Figure 12- 2DEG mobility vs. temperature
(Ozdemir, 2015)
In the best cases reported in the literature the 2DEG mobility can reach mobilities in the order of 106 cm2V/s, large effective mass (m* 0.3 m0) and small dielectric constant. Figure 13 presents a
summary of the evolution mobilities reported over the last decade (Falson, 2018).
Figure 13- 2DEG mobility vs. electron density
(2007 curve corresponds to state of the art PLD 2DEGs; 2009 curve corresponds to first MBE report; 2010 corresponds to improved MBE in which fractional QHE was observed; 2011 corresponds to the introduction of distilled ozone as oxidizing species; 2016 corresponds to actual state of the art
The active scattering processes in high mobility 2DEGs may be produced even by weak forms of disorder: remote ionized dopants can create smooth weak fields able to remove a significant part of the mobility (Gold, 2011; Amirabbasi, 2018). The hosting interface roughness may also induce important scattering effects (Thongnum, 2011). The study of the mobility of the 2DEGs is usually based in the analysis of the Shubnikov-de Haas oscillations in the magnetoresistance (see panel ‘a’ in Figure 14 and Equation 1). Based in this mathematical representation, fitting procedures like the Ding plot can be used to estimate the relaxation time and hence the 2DEG mobility (see panel ‘c’ in Figure 14 where the slope is proportional to the effective mass).
Equation 1 – Shubnikov de Haas oscillations model
When a strong magnetic field is applied perpendicular to the plane of a 2DEG, an additional restriction is applied to the behaviour of the system and that constraint quantizes the its energy. These new allowed quantized energy levels are separated by gaps proportional to semi-integer multiples of the synchrotron frequency (see Equation 2) and are termed Landau levels (LL). As a result of this quantization, band-gaps are open in the energy spectra. As the magnetic field is increased, the available states close the Fermi energy level change, creating the SdH oscillations.
Equation 2- 2DEG Energy quantization
(The first terms stand for the energy level reference. The second term proportional to the semi-integer N+1/2; The third term is proportional to the magnetic field and stands for the Zeeman spin splitting)
In spite of what has been said, in order to have energy quantization of the charge carriers, the system should comply with some requirements: the energy split of the system should be larger than the thermal energy of the system (i.e. KBT), so thermal fluctuations cannot blur it. As advanced in
the previous section, structural perfection is a requirement of first order to reduce the scattering process and maintain the s << 1. Heisenberg uncertainty also poses a lower limit to the energy
gap opening to be observed (E > h/(4s)). Ideally, electron density should be very low to hinder
electron-electron interaction scattering, a mobility reducing mechanism. In summary to make quantum oscillations, landmark of 2DEG transport behaviour, high structural quality, high mobility and low temperatures are needed.
The variety of Quantum Hall Effects available in MgZnO/ZnO based heterostructures includes both Integer Quantum Hall effect (IQH) and Fractional Quantum Hall effect (FQH). They were demonstrated in 2007 and 2010 respectively, as described before, due to the steady and long term improvement in the quality of the heterostructures produced (Falson, 2016).
IQH effect manifests as plateaus in the transversal Hall coefficients as function of the magnetic field (von Klitzing, 1980). Figure 15 reports the first reference of this phenomena and Figure 16 a recent sharp observation of it in a ZnO based heterostructure.
FQH effect presents when the perfection of the heterostructure is extreme, the applied magnetic fields are still very high and the temperature extremely low. In these conditions the filling of the bands is never complete, so due to the low carrier density the electron-electron interaction becomes non negligible anymore. With the incorporation of electron-electron interaction the Hamiltonian of the system changes and a wealth of new physical phenomena can be expected and have been already discovered.
Figure 15- Quantum Hall Effect Characteristic curve and measurement schema
(UH stands for the transversal Hall tension, Vg stands for the gate polarization, After von Klitzing, 1980)
Figure 16- 2DEG mobility vs. Temperature and Hall resistances vs. Magnetic field
(Panel a: maximum 2DEG mobility; Panel b: very high mobility (106cm2/Vs) sample magnetotransport curves)
At very high LL energy, the SdH oscillations can be observed. When them reduce, a point arrives when rotational symmetry is broken at half filling, so strong anisotropic behaviour is observed and charge density waves are present. At the lowest LL FQH effect is observed due to the prevalence of
Coulomb repulsion. In this state, electron are highly correlated and the extreme case would be a Wigner crystal (Ashcroft, 1977).
The FQH arises when a LL is partially filled and the ratio of electron and magnetic flux quanta can be expressed as p/q where p and q are mutual primes (see Figure 16). Advanced theory, proves this state can be understood as a composed fermion (i.e. an electron bounded to an even number of magnetic flux quanta), so acting in a non magnetic local environment. A wealth of these states can be studied by the ‘coincidence method’ (see Falson, 2018 for an overview)
8. Experimental techniques
In the present work, films and heterostructures were produced by Pulsed Laser Deposition and Molecular Beam Epitaxy (MBE) and characterized by RHEED, AFM, XRD/R and transport measurements. Following, we present a brief summary of the application of all of them.
8.1.
Pulsed Laser Deposition (PLD)
PLD is a technique that has been used for a widespread number of systems. An excimer pulsing laser is focused in a sintered target of the material that it is intended to be deposited. The target flash evaporates and after that a shock wave transport the material to the substrate where it is condensed. Is a technique highly out of equilibrium with the particularity of being able to be stoichiometrically conservative. However, among the drawbacks of this technique can be found it is very difficult to control contamination that enters by many channels but principally by the sintered targets since each step in their production entails contamination risks. Other important drawback resulting from its out of equilibrium nature is the difficulty to control crystallographic defects.
This technique was used to produce the samples of the second part of the thesis in the PLD at CNR-SPIN laboratory at Genova.
8.2.
Molecular Beam Epitaxy (MBE)
MBE is a technique to grow material over a substrate in which the material is effused from an effusion cell with high flexibility and reaching to the most extreme control over the deposition process. This technique can explore the parameter space with much more flexibility at the cost of much more difficult operation. It operates in UHV and the contamination is be controlled to the extreme. Oxidizing species in MBE can be ozone (taken from a distiller), oxygen radicals (produced by a RF plasma generator) or hydrogen peroxide. The chamber is usually equipped with a RHEED, QCM, cryogenic N2 panels and independent effusion cells (Klingshirn, 2010). This technique was
originally developed for pure semiconductors but was adapted for the deposition of oxides where additional requirements should be meet leading to the ‘reactive MBE’, nowadays the state of the art standard for best quality oxide heterostructures production (Schlom, 2015).
This technique was used to produce all the samples of the first part of thesis in the home made MBE at CNR-SPIN of Genova.
8.3.
Reflection High Energy Electron Diffraction (RHEED) &
Azimuthal RHEED (ARHEED)
RHEED is a technique able to give crystallographic information of the outer layers of a film (surface), even at the same time the film is growing. It is based in the diffraction of a grazing angle incident beam of electrons. Depending on the structure of the studied film, the technique can
operate in two modes: transmission or reflection mode, each one producing characteristic patterns associated to part of the bulk material or the more superficial cells of the film.
ARHEED is a variant of the standard RHEED technique. It allows a complete mapping of the reciprocal space (RS) by means of a controlled azimuthal rotation of the sample (and hence the associated RS lattice). With this technique it is possible to explore in situ the symmetry and structure of the RS of the film surface crystalline structure.
This technique was used to characterize the growing front of the MBE deposited samples.
8.4.
Atomic Force Microscopy (AFM)
AFM is a scanning microscopy technique that is sensitive to both topography and specific weak chemical interaction of surfaces. It is based in the interaction of the oscillating scanning tip of the instrument with the sample surface. This interaction produces two main signals: height and phase. The first encodes mainly topography information (i.e. it provides local measurements of height distribution, and hence roughness and other surface features like terraces, steps, pits, mounds, segregations, etc.). The second signal is related to the dissipative aspect of the surface-tip interaction and encodes mainly chemical information.
In this study we have used AFM to study transformations of the surface of both substrates and films/heterostructures and statistical analysis of the height distribution of the growing front.
8.5.
X-Ray Diffraction Techniques (XRD & XRR)
XRD (X-Ray diffraction) is one of the most powerful tools for the study of structural properties of crystalline materials. X-rays interact with the structure of matter producing interference patterns every time the material has some grade of crystallinity. So crystalline planes distances and hence phases can be easily determined.
In this study this technique was used to control phases, and structural features like orientation, structural disorder, strain, symmetry, etc. We use both a two circle Xpert diffractometer and a Siemens four circle diffractometer. The former was mainly used for high speed screening, while the second was employed for more precise crystallographic determinations. In the four circles diffractometer the scanning modes used were:
scanning: out of plane scan of the reciprocal space that permits identification of different phases crystal interplanar distances.
scanning (rocking curve): scan complementary of the previous one, that explore reciprocal space in the transverse direction (i.e. parallel to the crystallographic plane) that allows to measure orientation distribution of the crystallographic planes.
scanning: azimuthal angle scanning of the reciprocal space, that is useful to study epitaxial films to recognize epitaxial relationships and measure rotational disorder.
Peak mapping or Reciprocal Space Map (RSM) & mapping: 2D techniques useful to explore combination of 2 conditions of diffraction. The first one is used to recognize fine details of the crystalline structure by crossing scans and scans. The second one is used to analyze texture (i.e. orientation distribution of the crystalline planes).
XRR (X-Ray reflectivity) is a technique very useful to study surface specific properties or measure thickness of smooth thin films. We employed it to scan thickness with limited profit due to the high roughness of most of our films.
8.6.
Transport Properties Measurements
Transport properties measurement is carried out at different temperatures, in particular cryogenic temperatures. The measurement is usually carried out under the four probe schema. The setup consist of four basic components: the helium based thermal set; the electric measurements set and the associated registration and control system. In its basic configuration it is a Dewar reservoir where the sample is placed to fix its temperature during the apparent resistivity measurement. By sliding the sample over the spatial temperature gradient that develops inside the reservoir a fine tuning of the temperature is obtained at the same time the conductivity is be being measured. By this way the so called R(T) curves can be collected. We have used this technique for basic electric transport characterization of the MBE samples.
A second more elaborated technique of transport measurements is the so called PPMS (Physical Properties Measurement System) in which simultaneously with the registered thermal stabilization of the sample, a magnetic field is applied. The range of the applied field may spans some teslas (i.e. 10 T) in laboratory setups to still higher fields in special facilities (i.e. 40 T). We have used this technique to measure the quantum oscillations of the 2DEG of a MgZnO/ZnO based heterostructure.
9. Results
9.1.
MBE deposition of structures based in ZnO/MgO
9.1.1. Substrates: Al
2O
3001 & ZnO 001 – properties
The substrates employed to deposit these samples were mainly chemo-mechanically polished c-sapphire substrates, as is custom in the field of heterostructures of ZnO. However, Silicon (Si111), Glass and ZnO substrates were used to troubleshoot the MBE system. This part of the study is focused on ZnO MBE produced heterostructures on Sapphire substrates.
For the massive part of the samples produced and as it is customary in the field, sapphire substrates were used. C-Plane (001) oriented substrates cut from EFG (Edge-Defined Film-Fed Growth) produced crystals provided by Crystech were employed. The nominal miscut is very small (circa 0.01º)
Figure 17- EFG Sapphire Growth Method
(Balint, 2008)
Figure 18- Miscut () & Azimuth () angles
The substrates are made of C-sapphire, the corundum form of Al2O3, a highly stable oxide. In spite
of the scientific and technological importance of sapphire, after more than 40 years of study, sapphire surface behaviour and properties are still elusive (Barth, 2002). However, its choice as the preferred substrate for ZnO deposition is dominant. It has an extremely high etching resistance (attackable with a low rate with strong acids mixtures at elevated temperatures, like H2SO4+H3PO4
at 160ºC) (Williams, 2003; Marasina, 1982). This resistance renders etching very difficult, cumbersome, and highly variable from sample to sample. So some trials were done, but finally abandoned. Sapphire is also very hard (9 in the Mohs scale). This property was useful for the AFM scratching thickness measurement method, since the substrate remains unaltered when the deposited film is scratched. Sapphire has also a high resistivity (1019 cm at RT) and a moderated dielectric constant (r=9.35 at RT) making it very convenient for electronic devices fabrication (Dobrovinskaya, 2009).
9.1.2. Preparation and pre-treatment
Sapphire substrates were prepared by three successive operations: cut and cleaning, annealing and pre-treatment. 5 mm x 5 mm x 0.5 mm substrates were used for main samples and 10 mm x 10 mm x 0.5 mm were cut to made replicas. Cleaning was done with 3 to 5 cycles of 5 to 15 minutes of degreasing in ultrasonic bath in acetone and ethanol. Ethanol drying was done under pure nitrogen flow. Different sets of parameters were tested to optimize both cleaning and annealing. It was find that 1000ºC for 2 hours in air was good enough for our purposes, rather weaker than suggested by the literature (Cuccureddu, 2010; Wang, 2006; Tsuda, 2003; Yoshimoto, 1995). Most samples were prepared on substrates cleaned and annealed under these optimized conditions. Stronger annealing trials produced lower quality surfaces due to a diversity of reasons: step bunching, coalescence, surface contamination (i.e. secondary reactions stabilized contamination or segregation).
Figure 19- Sapphire substrates diffractograms
(Sapphire substrates XRD -2 scan, background removed)
0,1000 0,6000 1,1000 1,6000 2,1000 2,6000 3,1000 3,6000 4,1000 4,6000 10 20 30 40 50 60 70 80 In te ns ity [a .u .] 2 Just cleaned
Annealed at 1000ºC for 75 min Annealed at 1000ºC for 120 min
XRD and AFM analysis were performed to assess the quality and consistency of the prepared sapphire substrates and to have a reference per subsequent analysis.
The AFM analysis reveals an irregular, but quite smooth surface of the ‘as received’ substrates. After the annealing procedure an atomic scale ultra-smooth defective surface is obtained: there are not real parallel terraces as reported by the literature, but very wide zones circa 2000 nm wide with many holes and meanders with a characteristic dimension of 100-800nm (reference terrace width from literature is in the order of 250 nm). Note in the figures, that the lowest annealing temperature produces a colander morphology. Increasing the annealing temperature, holes tend to grow and merge, meanders tend to thin and both of them tend to reduce in number. At the highest annealing temperature analyzed (1050ºC) meanders have completely disappeared, just small holes remain and steps have straightened out. However, at these higher temperatures some surface defects, segregation or contamination may arise.
Annealing at the same temperature for different periods of time were also tested. No significant differences were found, although it was expected. AFM scans show that most described morphology motives can be found at almost all times. This suggest a complex global evolution path of reorganization of the surface that consist of an ensemble of different local paths conditioned by the starting configuration of the surface. The characteristic length is too high to be covered for the moving species, even during the longest tested annealing time. So, they tend to jump from one motive to the other many times before a proper terrace step is reached (see Figure 22).
The described morphology evolution is probably produced by the very small miscut (0.01º) that induce very wide equilibrium terraces, wider than the diffusive characteristic length at reasonable annealing times and temperatures. The created holes, islands and meanders are just the metastable structures that can be accessed by the system in reasonable annealing times that allows the system to reduce the average distance between reforming steps. Perhaps, a little higher miscut would have produce not so wide terraces, but better oriented. In any case, the obtained surface is satisfactory because terraces are wide, steps have the minimum attainable height of 0.22 nm (one Al-O sapphire interplanar distance, the third part of a unitary cell) and there is no evidence of step bunching or surface reconstructions in accordance with the literature (Cuccureddu, 2010; Kurnosikov, 2000; Yoshimoto, 1995) (see Figure 21 & Figure 22).
Figure 20- AFM scan of the ‘As received’ Sapphire substrates
Figure 21- AFM scan of annealed sapphire substrates at different temperatures
(AFM can 4 x 4: From left to right and up-down: 120 minutes annealing in air at 950ºC, 975ºC, 1000ºC and 1050ºC,)
Figure 22- AFM scan of annealed sapphire substrates for different periods of time
(AFM scan 2 x 2: From left to right: 75, 90 and 120 minutes annealing in air at 1000ºC)
From the analysis of XR diffractograms it was found that the sapphire 006 crystallographic plane (2=41.644º) of the “as received” (or ‘Just cleaned’) substrates are very well oriented and almost free of significant micro-strain as expected: the rocking curve peak shows a typical FWHM of
0.025º and the -2 scan peak a FWHM of 0.045º. However, after 75 minutes of annealing at 1000ºC in air, both of them shrink by approximately -6%. After a total accumulated period of 120 minutes of annealing at 1000ºC in air, the FWHM of the rocking curve remains almost constant but the FWHM of the θ-2θ scan is still reduced -12% (see Figure 23). This suggest that the structural defects that perturb the out of plane interplanar distances (point defects and/or vertical Burgers vector threading dislocations ; i.e. screw dislocations) must overcome a higher activation barrier than those that induce tilt disorder (dislocations in the basal plane with vertical Burgers vectors). Moreover, by means of this way of reasoning, and taking into account that both defect populations have a significant vertical component of Burgers vectors, it could be inferred that an important part of defects that can be eliminated by thermal treatment are point defects or screw like dislocations. In other words, annealing has a stronger effect in normalizing interplanar distances than normalizing orientation of crystallites by means of removing the geometrical required defects.
Figure 23- XRD Sapphire 006 peak FWHM
Another interesting feature of the annealed sapphire substrates revealed by the XR diffractograms is the presence of the sapphire forbidden reflections [003] (2θ=20.52º) and [009] (2θ=64.47º). These features are found only in sapphire substrates. The intensity of these two features seem to follow no clear trend from sample to sample. They are not attributable to persistent surface contamination because they correspond to highly oriented planes with a FWHM = 0.05º in the rocking curve. They neither seem to be associated to intermediate stages of the annealing process because do not decrease with longer annealing times, but seem to be the result of the final surface morphology (Martinez-Boubeta, 2013). As expected they remain unaltered after MBE depositions (carried out at lower temperatures) or successive annealing periods at higher temperatures for longer times.
Sap006 Sap006 Sap006 0.0000 0.0050 0.0100 0.0150 0.0200 0.0250 0.0300 0.0350 0.0400 0.0450 0.0500 0 20 40 60 80 100 120 140 Anneal t [min] roc [º] t2t [º] Sample P e a k w id th [º ] A n n e a lin g ti m e [m in ]
Figure 24- Sapphire substrates diffractogram detail peak [003] (=20.52º)
Figure 25- Sapphire substrates diffractogram detail peak [009] (2θ=64.47º)
0,00 0,20 0,40 0,60 0,80 1,00 1,20 1,40 1,60 20 20,2 20,4 20,6 20,8 21 In te ns ity [a .u .] 2 Just cleaned
Annealed at 1000ºC for 75 min Annealed at 1000ºC for 120 min
0,00 0,10 0,20 0,30 0,40 0,50 0,60 63,5 64 64,5 65 65,5 66 In te ns ity [a .u .] 2 Just cleaned
Annealed at 1000ºC for 75 min Annealed at 1000ºC for 120 min
Figure 26- Rocking curve of sapphire forbidden reflection [003] (=20.52º)
Figure 27- Rocking curve of sapphire forbidden reflection [009] (2θ=64.47º)
During the annealing surface reorganization of very low miscut substrate surface, “incomplete layers” are created between terraces (‘ultra-smooth surfaces’). Each step has a height close to 0.2 nm, the distance between successive Al-O planes. Since in a complete primitive cell of sapphire there are three Al-O planes, this imply that the annealed surface of the sapphire substrate is covered by terraces that follow the next pattern:
0 200 400 600 800 1000 1200 1400 1600 1800 9,9 10 10,1 10,2 10,3 10,4 10,5 10,6 In te ns ity [a .u .] 0 100 200 300 400 500 600 31,9 32 32,1 32,2 32,3 32,4 32,5 32,6 In te ns ity [a .u ]
