Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review
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Review Journal of Nanoscience and Nanotechnology
Copyright © 2014 American Scientific Publishers All rights reserved Printed in the United States of America
Vol. 14, 1911–1930, 2014 www.aspbs.com/jnn
Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review Rajesh Kumar1 , Girish Kumar1 ∗ , and Ahmad Umar2 3 ∗ 1
PG Department of Chemistry, JCDAV College, Dasuya 144205, Punjab, India Promising Centre for Sensors and Electronic Devices, Najran University, Najran 11001, Kingdom of Saudi Arabia 3 Faculty of Arts and Sciences, Department of Chemistry, Najran University, Najran 11001, Kingdom of Saudi Arabia 2
This review summarizes the work principles of pulse laser deposition (PLD) apparatus, physical processes like ablation, and plasma plume formation accompanying the deposition of un-doped ZnO from target to substrate material. Various modes of deposition and factors influencing the properties of thin films such as substrate temperature, background gas pressure, laser energy density (laser fluence), target to substrate distance, repetition rate, oxygen partial pressure in deposition chamber, deposition time and post growth annealing which control deposition parameters such as adsorption, desorption, surface diffusion, nucleation, and crystallization/re-crystallization are also discussed in this review. Moreover, various film properties such as morphology, roughness of the film surface, film thickness, grain size, optical transmittance, sensitivity, electrical conductivity, uniformity and electrical resistivity of the deposited ZnO thin films have also been enumerated in the present review.
Keywords: ZnO Nanostructures, Thin Films, Pulsed Laser Deposition (PLD), Ablation.
CONTENTS 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. PLD Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Pulsed UV Laser Sources . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Ultrahigh Vacuum (UHV) Chamber . . . . . . . . . . . . . . . . . . 3. Ablation, Target, Plasma Plume and Substrate . . . . . . . . . . . . . . 3.1. Laser Ablation and Plasma Plume . . . . . . . . . . . . . . . . . . . 3.2. The Target and Substrate . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Thin Film Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Nucleation and Growth Mechanism . . . . . . . . . . . . . . . . . . 5. Morphological Control and Properties of ZnO Thin Films . . . . 5.1. Effect of Substrate Temperature . . . . . . . . . . . . . . . . . . . . . 5.2. Effect of Oxygen Partial Pressure . . . . . . . . . . . . . . . . . . . 5.3. Effect of Post Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Influence of Laser Fluence . . . . . . . . . . . . . . . . . . . . . . . . . 5.5. Effect of Deposition Time and Film Thickness . . . . . . . . . 5.6. Effect of Pulse Repetition Rate . . . . . . . . . . . . . . . . . . . . . 5.7. Effect of Target to Substrate Distance . . . . . . . . . . . . . . . . 6. Crystallographic Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 2
1. INTRODUCTION 1911 1913 1913 1913 1914 1914 1915 1915 1915 1916 1916 1917 1919 1920 1920 1921 1921 1926 1927 1927 1927
In the recent past, transparent and highly conducting oxide films have attracted many researchers due to their wide range of applications in industry as well as in research. Thin films are layers of a material whose thickness ranges from fractions of a nanometer to several micrometers. They are deposited on the substrates to achieve better properties than that of bulk materials. Deposition of thin films increases the contact area, resulting in a high fraction of reactants. Thin films are especially appropriate for applications in microelectronics and integrated optics. However the physical properties of the films like electrical resistivity do not substantially differ from the properties of the bulk material. For a thin film the limit of thickness is considered between tenths of nanometer and several micrometers. Thin film materials are the key elements of continued technological advances made in the fields of optoelectronic, photonic, and magnetic devices. The processing of materials into thin films allows easy integration into various types of devices. The properties of material significantly differ when analyzed in the form of thin films. Most of the functional
1533-4880/2014/14/1911/020
doi:10.1166/jnn.2014.9120
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Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review
materials are applied in thin film form due to their specific electrical, magnetic, and optical properties, or wear resistance. Among them, there has been a surge of interest in the growth of high quality thin films of ZnO for its wide applications. ZnO has been considered a promising material for short-wavelength optoelectronic devices like light emitting diode (LED) and flat panel display (FPD) because it has a direct band gap of 3.37 eV and a low threshold
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voltage with high mechanical and thermal stabilities. ZnO deposition as thin films increases the contact area of the cell components, resulting in a high fraction of reactants. Thin films result in higher current densities and cell efficiencies because the transport of ions is easier and faster through thin-film layers than in bulk ZnO. Large exciton binding energy of 60 meV at 300 K of ZnO has also paved the way for efficient room-temperature excitonbased emitters. As is well known, exciton is a pair of
Rajesh Kumar has been working as Assistant Professor of Inorganic Chemistry at JCDAV College, Dasuya, Punjab, India since 2004. He obtained his B.Sc. Degree from Himachal Pradesh University, Shimla and M.Sc. degree from Guru Nanak Dev Univeristy, Punjab in 2003. His present areas of research are Nanotechnology and Polymer chemistry.
Girish Kumar is an Assistant Professor of Physical Chemistry at JCDAV College, Dasuya, Punjab, India since 2004. He obtained B.Sc. (1993) and Ph.D. degrees (2001) from the Himachal Pradesh University, Shimla, and M.Sc. degree (1995) from Kurukshetra University. He was awarded Research Associateship in a CSIR sponsored research project from 2001–2004 at Himachal Pradesh University, Shimla. His present areas of research are Nanotechnology and solution chemistry of surfactants.
Ahmad Umar received his B.Sc. in biosciences and M.Sc. in Inorganic Chemistry from Aligarh Muslim University (AMU), Aligarh, India, and Ph.D. in Semiconductor and Chemical Engineering from Chonbuk National University, South Korea. He worked as a Research Scientist in Brain Korea 21, Centre for Future Energy Materials and Devices, Chonbuk National University, South Korea during 2007–2008. In December, 2008, he joined the Department of Chemistry in Najran University, Najran, Saudi Arabia. He is the deputy director for the Promising Centre for Sensors and Electronic Devices (PCSED), Najran University, Najran, Saudi Arabia. Professor Ahmad Umar specializes in growth, properties and applications of semiconductor nanostructures especially doped and undoped metal oxide and sulphide nanostructures. He has authored 20 book chapters, over 175 research articles in peer-reviewed international journals, and more than 161 proceedings, abstracts and technical reports. He is referee for many scientific journals and foreign expert for Ph.D. He has 6 patents either issued or applied for on metal oxide nanostructures and their based sensors and electronics devices. Professor Ahmad Umar serves as the Editor-in-Chief and founding editor for Science of Advanced Materials (www.aspbs.com/sam), Journal of Nanoengineering and Nanomanufacturing (www.aspbs.com/jnan),” “Reviews in Advanced Sciences and Engineering (www.aspbs.com/rase), Energy and Environment Focus (www.aspbs.com/efocus) and Materials Focus (www.aspbs.com/mat), published by American Scientific Publishers (ASP; www.aspbs.com). He is Asian Editor of Advanced Science Focus (www.aspbs.com/asfo), an editor of ‘Advanced Science, Engineering and Medicine (ASEM; www.aspbs.com/asem), and serves as associate editors for Journal of Nanoscience and Nanotechnology (www.aspbs.com/jnn)” and Advanced Science Letters (www.aspbs.com/science), published by American Scientific Publishers. He edited the world’s first handbook series on Metal Oxide Nanostructures and Their Applications (5volume set) and forthcoming Encyclopedia of Semiconductor Nanotechnology (7-volume set) published by American Scientific Publishers. 1912
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electron and hole bound due to columbic interaction and plays an important role near the band edge for the resulting optical characteristics of the semiconductor. Due to these properties, ZnO thin films present optical, acoustical and electrical properties which have applications in the fields of solid state devices including electronics, optoelectronics and sensors. ZnO thin films are applied to the transparent conductive films and the solar cell windows because of the high optical transmittance in the visible region.1–7 A variety of methods are reported in literature by researchers throughout the globe in order to deposit ZnO thin film by controlling the important parameters and usage of various techniques. These methods are mainly divided into three main categories; physical vapor deposition (PVD), chemical vapor deposition (CVD) and chemical solution deposition. These categories involve Molecular beam epitaxy (MBE),8–11 metal-organic chemical vapor deposition (MOCVD),12 sputtering,13 cathodic magnetron sputtering and reactive electron beam evaporation,14–18 spray pyrolysis,19–21 electrodeposition22 23 and sol–gel method.24–27 Technical realization of the first optical laser using a rod of ruby in 1960 by Maiman “the father of the electrooptics industry,” opened the new channels for the laserassisted thin film growth. In 1962, Breech and Cross used ruby laser for vaporization and excitation of atoms from solid surfaces. In 1965 Smith and Turner used a ruby laser to deposit thin films which marked the very beginning of the development of the pulsed laser deposition technique.28 In 1987 Dijkkamp and Venkatesan were able to laser deposit a thin film of YBa2 Cu3 O7 , a high temperature superconductive material, which was of more superior quality than films deposited with alternative techniques.29 30a Since then, the technique of PLD has been utilized to fabricate high quality crystalline films. With more technological development, lasers with high repetition rate and short pulse durations, high efficient harmonic generator and excimer lasers delivering UV radiation made PLD a very competitive tool for the growth of thin, well defined films with complex stoichiometry. The processes during PLD deposition can mainly be divided into four regimes involving, the interaction of the laser beam with the target resulting in evaporation of the surface layers (evaporation regime), the interaction of the laser beam with the evaporated materials causing the formation of isothermal expanding plasma (isothermal regime), the anisotropic three-dimensional adiabatic expansion of the laser induced plasma with a rapid transfer of thermal energy of the species in the plasma into kinetic energy and thin film growth.30b
2. PLD APPARATUS In this section basic apparatus used for the majority of PLD techniques, its major components and their uses are J. Nanosci. Nanotechnol. 14, 1911–1930, 2014
Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review
illustrated. Basic set up of commonly used PLD is demonstrated in Ref. [31]. 2.1. Pulsed UV Laser Sources The most commonly used pulsed UV laser sources for the deposition of ZnO nano thin layer are Neodymium Doped Yttrium–Aluminum–Garnet (Nd:YAG) and Excimer lasers. Nd:YAG Laser has a laser wavelength range of 255–1064 nm (fundamental and second harmonic outputs at 1064 nm and 532 nm), Pulse energy of 100–1000 mJ, pulse repetition rates of 6–100 ns, repetition frequency range of 1–10 Hz and an inner circulation water cooling unit. The energy density or laser fluence values range from 2 J/cm2 to 7 J/cm2 . The laser energy density impinged on the target depends on the set laser pulse energy and beam area recorded. Laser fluence of a laser beam is calculated by applying the Eq. (1): Laser fluence J/cm2 = Laser energy J/Beam area cm2
(1)
The most common excimer laser used in the PLD system is KrF which operates at a wavelength of 248 nm with pulse energy of 150–300 mJ and 6–10 pulses per second. The pulse duration range for KrF excimer laser is 20–30 ns. Other excimer lasers, which operate at a number of different UV wavelengths include, 308 nm (XeCl),106 107 193 nm (ArF)65 136 137 and 157 nm (F2 which are also used to ablate target materials to form a plasma plume. Excimer laser have pivotal advantages over Nd:YAG lasers in ZnO thin film manufacturing. Excimer laser has superior ablation characteristics and much better energy stability than that of Nd:YAG lasers. Nd:YAG lasers inherently have inappropriate gaussian beam profile instead of a flat-top profile as well as temperature-induced polarization and thermal lensing effect which create donut-shaped beam profile and lateral distortions. Excimer lasers operate at shortest wavelength, highest photon energy, UV pulse stability and flat-top beam homogeneity. As a result transparent conductive layers of Zinc oxide can be evenly ablated without fractionation of the constituents and with reasonable ablation rate.32 The laser evaporation process is wholly controlled and monitored by a computer program through a specially designed interface. The computer program triggers the laser pulses and the target switching.33 2.2. Ultrahigh Vacuum (UHV) Chamber In an ultrahigh vacuum (UHV) chamber, the target materials are struck at an angle generally of 45 by a pulsed and focused UV laser beam. Vacuum chamber is equipped with an ultrahigh vacuum gauge (turbo molecular pump) connected to a pressure meter, RHEED port or piezoelectric quartz unit connected to measure thin film deposition rate and thickness meter, a quadrupole mass spectrometer 1913
Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review
head connected to the mass spectrometer, gas inlets, target and substrate holders, substrate heater and laser beam slit. The mass spectrometer is used both for the analysis of the residual gas composition and the chemical composition of the vapor stream (up to 300 amu) generated by the laser radiation.33 2.2.1. Target and Substrate Holder To place the target at fixed position a stainless steel target is used. It keeps the target in the vertical orientation during the deposition process and can be rotated so as to form a uniform layer of the material on the substrate surface. Proper distance between target-substrate holder and the angle between target surface and laser beam play an important role in the film thickness of the material.128 To place the substrate at fixed position a substrate holder is used. Substrate holder can be moved relative to the target and plasma plume and can be heated up to 1000 C. The adjustment of the target-substrate distance provides an opportunity to control the deposition rate thin film thickness, as well as the energy of the particles hitting the substrate. The substrate holder and heater are composed of copper caps having the form of a ring through which the laser beam passes. There is room between the caps for a small resistive heater. Substrate heater is provided with a thermocouple to stabilize the substrate holder temperature. The holder is suspended on a rod-like outrigger that allows the holder to rotate.33 2.2.2. Reflected High Energy Electron Diffraction RHEED It is an in-situ reflected high energy electron diffraction (RHEED) tool which assists in monitoring thin film crystalline structure in real time and thin film growth rates. It gathers information from the surface layer of the sample. A RHEED system requires an electron source gun to generate a beam of electrons which strike the sample at a very small angle relative to the sample surface and a photoluminescent detector screen.34 35 Incident electrons diffract from atoms at the surface of the sample, and a small fraction of the diffracted electrons interfere constructively at specific angles and form regular patterns on the detector. The electrons interfere according to the position of atoms on the sample surface, so the diffraction pattern at the detector is a function of the sample surface. The use of RHEED makes it possible to track the number of layers deposited in real-time, as the intensity oscillates with a periodicity equal to the time required for the deposition of a monolayer.36 Kaidashev et al.84 observed streaky RHEED patterns of four-step PLD grown ZnO nano thin films with 1 to 2 m thickness which indicate the excellent surface flatness of the multistep-grown ZnO films on c-plane sapphire substrates at reduced temperatures. However RHEED is not an ideal tool for growth kinetics studies because the strong interaction of the electrons with the 1914
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surface causes multiple scattering and requires dynamical theory for rigorous interpretation of the intensities.34 37
3. ABLATION, TARGET, PLASMA PLUME AND SUBSTRATE Pulsed laser deposition (PLD) method is the most promising and probably the simplest technique, among all thin film growth techniques. It uses short and intensive laser pulses to evaporate target material. The target should be as dense and homogenous as possible to ensure a good quality of the deposit. The evaporated particles escape from the target and condense on the substrate. The deposition process occurs in vacuum chamber to minimize the scattering of the particles. In some cases, however, reactive gases like O2 , N2 , Ar etc. are used to vary the stoichiometry of the deposit. 3.1. Laser Ablation and Plasma Plume Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. In PLD technique a high power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. Laser ablation is also referred to as photon induced sputtering. In this the incident laser pulse penetrates into the surface of the material within the penetration depth of about 1,000 Å. This dimension is dependent on the laser wavelength and the index of refraction of the target material at the applied laser wavelength and is typically in the region of 10 nm for most materials. The strong electrical field generated by the laser light is sufficiently strong to remove the electrons from the bulk material of the penetrated volume. This process occurs within 10 pico second of a nano second laser pulse and is caused by non-linear processes such as multi photon ionization which are enhanced by microscopic cracks at the surface, voids, and nodules, which increase the electric field.38 At low laser fluence, low kinetic energy of the plasma plume species leads to island growth whereas, too high laser beam fluence, through the bombardment of the growing film by energetic species causes a degradation of the crystallinity of ZnO films. This indicates that at the lower fluence values the target surface melts and the quantity of ablated material formed is too low to form dense plasma. At high laser fluence matter clusters are relatively more stable and are therefore likely to be deposited in the thin films or other quantum structures. At higher laser fluence plasma contains a significant amount of highly energetic species. The high energy plum species after colliding with substrate may penetrate the substrate surface and get embedded in it. These immobile atoms then act as additional nucleation centers and promote an island type of growth. These islands along with naturally formed nucleation centers then grow in size and coalesce to form a continuous film. Very high laser plasma fluence can induce explosive boiling of the target material leading to phase explosion. J. Nanosci. Nanotechnol. 14, 1911–1930, 2014
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Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review
The electric field produced by the laser beam inside an absorbing medium under can be estimated from the following relation:47 E = 2/C0 n1/2
(2)
Where E is the electric field, the power density, 0 the dielectric constant in vacuum, n the refractive index of the medium and C the speed of light. The free electrons oscillate within the electromagnetic field of the laser light and can collide with the atoms of the bulk material thus transferring some of their energy to the lattice of the target material within the surface region. The surface of the target is then heated up and the material is vaporized forming gas plasma with the characteristic shape of a plume called plasma plume. Besides atoms, electrons and ions, the material plume also consists of particulates, with dimensions ranging from nanometer to micrometer. The smallest particles (∼ nm size) are probably formed in the expanding vapor plume, by condensation of vapor atoms. The larger particles (∼ m size) are likely created by direct ejection from the solid target.39 The ejected species have kinetic energies in the range 10–100 eV.46 Adiabatic collision-less expansion transfers the concentration of the plasma plume parallel to the normal vector of the target surface towards the substrate. The spatial distribution of the plume is dependent on the background pressure inside the PLD chamber. Heating rates as high as 1011 K/s and instantaneous gas pressures of 10–500 atm are observed at the target surface.40 Usually, the laser ablation process is divided in two time dependent stages: Target evaporation into plasma plume and plasma plume expansion.41 Plasma plume expansion dynamics play a crucial role in PLD and are not yet been totally understood, even in the simpler case of propagation in vacuum or in presence of an ambient gas.42 45 3.2. The Target and Substrate For the formation of ZnO thin film applying PLD, highly pure crystalline ZnO powder is first converted into a cylindrical or disc shape pallet of variable diameters and thicknesses using high pressure palletizers by hot or cold pressing followed by sintering or annealing in vacuum or in ambient atmosphere of O2 . Laser beam bombardment over large diameter target can produce uniform, large surface area thin films with predictable and reproducible growth rates and properties.43 Pulsed laser deposition (PLD) proved to be a favorable technique for the deposition of Zinc oxides at different technological conditions on different substrates resulting in the different structural and micro structural properties, different surface morphology of the nanostructured thin films. Laser ablation produced a transient, highly luminous plasma plume that expands rapidly away from the target surface and is condensed on an appropriately placed substrate surface to nucleate and grow a thin ZnO film.48 J. Nanosci. Nanotechnol. 14, 1911–1930, 2014
Optical properties of ZnO are known to be sensitive for substrate structural qualities; hence the choice of an appropriate substrate material in PLD is very important.44 A variety of substrate materials are used for the thin film deposition of ZnO semiconductors which include quartz glass,78–81 c-Sapphire,69 71 84 85 94 95 Indium Titanium Oxide (ITO),90 silicon (100) and (111),96–99 132 134 135 -Ga2 O3 (100),116 MgO (001),118 Calcite (CaCO3 ,121 a, c, and r-plane Al2 O3 ,122 Corning glass 7059,127 129 ZnO (0001) and Al2 O3 (1120),138 GaAs,65 130 137 Indium phosphide,141 metal foils like that of Tungsten and Rhenium113 etc.
4. THIN FILM FORMATION PLD is a growth method for thin films by condensation of laser plasma ablated from a single target, excited by the high-energy laser pulses far from equilibrium. The energy efficiency of high power lasers is only a few percent, thus the overall efficiency of PLD is low. As a result volume deposition rate of PLD is only about 10−5 cm3 s−1 , and is much lower than that of other physical vapor deposition techniques such as electron beam evaporation, magnetron sputtering, and vacuum arc deposition.49 Several works were devoted to the study of the parameters affecting the particle size of zinc oxide. In this section we will discuss possible growth and nucleation mechanisms proposed by some authors regarding ZnO nano thin films. 4.1. Nucleation and Growth Mechanism In PLD, condensation of the plasma plume leads to the accumulation of plume particles by lowering of their energies on the substrate surface. Lattice mismatch has a marked effect on film morphology. The strain resulting from lattice mismatch contributes to the interface energy, a key parameter in determining the growth mode. For heteroepitaxial growth, depending upon the thermodynamics that relates the surface energies of the film and substrate and the film substrate interface energy, three types of growth modes, Volmer–Weber or island growth, Frank– Van der Merwe or layer growth and Stranski–Krastanov or intermediate growth are described in literature. 4.1.1. The Volmer–Weber Growth It is also referred to as “3D Island Growth” which occurs when the intermolecular forces of attractions between the atoms and molecules attaching to the substrate surface are stronger than the forces they will form with the substrate material. In this epitaxial growth mechanism, as soon as the nucleation sites of the first layer are formed the second layer begins to form on top. Growth of these islands causes rough multi-layer films to grow on the substrate surface. Different orientations of island to island result as most of the nucleation takes place on substrate surface defects, and islands of the deposited films grow out of 1915
Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review
these nuclei.50 53 Hence the role of substrate, molecular interaction, surface roughness and crystal symmetries play important roles in 3D island growth mechanism. Nucleation rate in Volmer–Weber growth can be increased by increasing the deposition rate or decreasing the substrate temperature. 4.1.2. Frank–Van der Merwe or Layer Growth Frank–Van der Merwe or layer growth occurs when the bonds to the substrate are stronger than the intermolecular forces of attractions between the atoms and molecules attaching to the substrate surface. Nucleation sites are formed as before but as new material arrives it is incorporated into the first layer exclusively. Full monolayer growth involves the nucleation and growth of islands that are only one monolayer thick and grow to essentially complete coalescence before significant clusters are developed on the next film layer. In this case there is no free energy barrier to nucleation. If the substrate material is different from the film material, full monolayer nucleation will be promoted by strong film-substrate bonding, low film surface energy and high substrate surface energy.51 4.1.3. Stranski-Krastinov Nucleation and Growth A layer-by-layer growth takes place in the first stage. Then, the thicker the film becomes, the higher is the elastic energy due to the strain. Such large strain energy can be lowered by forming islands in which strain is relaxed. Hence it is an intermediate or hybrid of the previous two cases involving a transition from the Frank and Van der Merwe to the Volmer–Weber growth mode can be observed. The mismatch between film and substrate, inducing a strain on the growing film plays a crucial role in the Stranski-Krastinov mechanism. This mechanism results in a continuous film of one or two monolayers onto which successively discrete islands are formed.52–54 All the three types of growth modes, discussed above are diagrammatically represented in Figure 1.
Figure 1. Three classical thin film growth modes.
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5. MORPHOLOGICAL CONTROL AND PROPERTIES OF ZnO THIN FILMS Thermally activated processes such as adsorption, desorption, surface diffusion, nucleation, and crystallization/recrystallization, roughness of the substrate surface, film thickness and morphology of the deposited thin films of ZnO can be controlled by a number of factors which include substrate temperature, background gas pressure, laser energy density (laser fluence), target to substrate distance, repetition rate, oxygen partial pressure in deposition chamber and post growth annealing of the ZnO thin films formed leading to various property modifications. 5.1. Effect of Substrate Temperature Substrate temperature directly affects mobility rate, reevaporation, and crystallization of deposited species on the substrate surface, and thus affects the microstructure and properties of thin films. Activation of the surface mobility of the plasma plume species deposited onto the substrate surface and increase in kinetic energy of ablated species in the PLD process are temperature dependent; as a result substrate temperature plays a crucial role in thin film deposition. A crystalline ZnO film with low surface roughness is easily obtained at low temperature.55 56 However for Zn targets, a high substrate temperature favors rapid and defect free growth of crystallites due to full oxidation of Zn atoms and optimum surface diffusion of the species, whereas a low substrate temperature results in the growth of a disordered or a poorly crystallized structure.57 At low temperature, the zinc and oxygen atoms have no energy to migrate to the normal lattice site which leads to small grain size whereas at high temperature, small grains have enough energy to combine together to from large grains. The surfaces of ZnO thin films are more planar and compact as the mobility of adsorbent atoms increase at higher temperatures.60 Grain size for the ZnO nano thin films as determined by XRD and FESEM was found to increase with an increase in surface temperature and reported by Zhao et al.79 Film thickness increases from 67–89 nm with increase in surface temperature from 100–250 C. It is clearly seen that an increase of the substrate temperature leads to the enhancement of (002) diffraction peak intensity. Ramamoorthy et al.80 observed c-axis orientated degradation of ZnO thin film at substrate temperature (< 200 C) due to reduced surface mobility and migration of species on the substrate surface, and the texture quality normally improves with increasing substrate temperatures. Re-evaporation of atoms from the surface of a deposited film at very high substrate temperature (> 600 C) has been observed resulting in the formation of an off-stoichiometry thin film. Higher substrate temperature decreases film thickness (Fig. 2), enhances the transparency and anti-reflection property of ZnO thin films, decreases the photoluminescence intensity of ZnO thin films and thereby increases the electrical conductivity. J. Nanosci. Nanotechnol. 14, 1911–1930, 2014
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Figure 2. The plot of thickness of ZnO thin films deposited at RT, 200 C and 300 C versus substrate temperature. Reprinted with permission from [80], K. Ramamoorthy, et al., Current Applied Physics 6, 103 (2006). © 2006, Elsevier.
Optical transmittance (T ) in the visible range (400–700 nm) is also found to increases slightly with increasing the substrate temperature from room temperature to 300 C, and is related to the increase in grain size of the films with increasing substrate temperature.58 79 81 150 Increased optical transmission of the ZnO thin films is due to high mobility of the charge carriers, reduction of free carrier absorption, ability of the film surface roughness to reduce reflectivity, increase of a structural homogeneity, fine texturing and decrease of the diffuse scattering.80 Columnar structured c-axis oriented ZnO thin films grown on Si (111) and Si (100) substrates at growth temperatures from 500 to 675 C and oxygen pressures from 3 × 10−2 to 10 Pa, as confirmed by XRD, AFM, and TEM investigations, show diametric growth on the increasing growth temperature without any explicit dependence of the grain size on the oxygen pressure.82–84 Zeng et al.59 85 in temperature range of 200–600 C studied deposition of ZnO films on (0001) sapphire -Al2 O3 substrate and found decrease in deposition rate above 400 C but better crystalline qualities as compared to low temperature. At higher temperatures Zn and O2 molecules have too much thermal energy and O2 molecules begin to resputter. Similar results were obtained by Liu et al.61 86 for the deposition of ZnO on glass and silicon substrates between substrate temperature 200–500 C. However, good crystalline nature of ZnO thin films grown on Si (111) substrate at a temperature of 600 C is observed by Jianting.62 A close correlation is observed between surface roughness and substrate temperature. The surface roughness, related to grain size generally increases as the substrate temperature increases.63–65 96 115 123 125 129 132–134 140 The intensity of UV PL peaks increases markedly with the increases of substrate temperature. Strong UV luminescence obtained by increasing the substrate temperatures of ZnO films make them possible for using in light emission device applications. Ultraviolet (UV) sensitive films deposited by Ayouchi et al.69 exhibit maximum J. Nanosci. Nanotechnol. 14, 1911–1930, 2014
Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review
sensitivity at lower temperature than the films deposited at higher temperatures which show crystallite sizes of typically 500 nm, a high dark current and minimum photo response. More densely packed crystallites and a faster decay in photocurrent is observed for films deposited at lower temperature. To conclude, the ZnO thin films exhibit low surface roughness, more thickness, small grain size, low optical transmittance, maximum sensitivity, high surface conductivity, good conductive uniformity, increased band gap, more oxygen vacancies, low mobility of the charge carriers, low reflectivity, decreased structural homogeneity and increased diffuse scattering with lower substrate temperature. 5.2. Effect of Oxygen Partial Pressure After initial free expansion from the target surface, the mean free path of the ablated particles is reduced in the presence of gas. At higher ambient pressure, the more collisions and scatterings occur; particles lose energy to form ionic complexes or molecules. These clusters reach the substrate surface and small grain particles start to grow as they become the nucleus. On the other hand, most of the ablated particles can reach the substrate in the state near the single atoms if the ambient pressure is extremely low.41 Hence, Oxygen pressure in the deposition chamber of PLD apparatus influences both the deposition rate and the kinetic energy of ejected ablated species. The thickness of the ZnO films also increases with increasing the oxygen partial pressure.87 With the increase of O2 partial pressure, the kinetic energy of the ablated species reduces due to a large number of collisions with background gas molecules and the size of ablated plume decreases.53 The higher the O2 pressure in the growth chamber, the more oxygen is incorporated into the ZnO film lattice. When oxygen is excessively incorporated due to a high O2 pressure, lattice expansions are likely to happen. At lower oxygen pressure, the ablated species have sufficient kinetic energy for diffusion and can thus diffuse to the right crystallographic sites. However, at higher oxygen pressure, the ablated species have less kinetic energy which results in less diffusion resulting in poor crystalline quality of the films. Films deposited under very low pressure are often oxygen deficient, and thereby affect the micro structural and electrical properties.66 88 Oxygen pressure also influences the mean free path of the ablated species, which increases with decreasing O2 partial pressure. The processing of film at very low oxygen pressures is generally known as laser molecular-beam epitaxy (LMBE) where the mean free path of the plume species is much larger than the target to substrate distance.67 Chupoon et al.68 deposited ZnO thin films at low oxygen pressure of 10−5 to 10−4 Torr to form compressive strained c-axis lattice parameter larger than the bulk value. However, this strain was reduced for higher oxygen pressures 1917
Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review
(10−2 Torr) deposition with minimal surface roughness. The ZnO thin films exhibit low surface roughness, high surface conductivity, and good conductive uniformity with lower oxygen pressure. Zhaoyang et al.70 studied the oxidation rates of ZnO thin films grown on sapphire (001) substrate between 1 to 500 mTorr O2 pressures at 400 C. ZnO grain growth rate retards and fine ZnO grain structure forms at low oxygen partial pressures. As the oxygen partial pressure increases, the oxidation rate increases, leading to large ZnO grains. Moreover, as the oxygen partial pressure increases, the formation of ZnO gas is enhanced by the collisions between the Zn and oxygen in this nonthermal equilibrium case of PLD. Very high pressure oxygen treatment of 5 atm on ZnO thin films deposited on Sapphire (0001) substrate at 500 C by the pulsed laser deposition for 24, 72, 240 and 480 h, respectively enhanced the intensities of defect peaks at 2.1 and 2.5 eV without affecting near band edge (NBE). A decreased carrier concentration and increased mobility has been observed by the high pressure oxygen treatment.71 104 Optical transmission of the ZnO films at high oxygen pressure is lower than for low oxygen pressure, and is due to the grain size. The lower the oxygen pressure, the smaller the grain size.41 78 The oxygen pressure also affects the direct band gap of the ZnO thin films. The decrease in the direct band gap with an increase in oxygen pressure is due to a decrease in carrier concentration as explained by the Burstien—Moss effect.53 89 At higher oxygen pressure transparency of the deposited ZnO thin films is enhanced as shown in Figure 3. The superior optical transmission of the ZnO thin films could be dominantly due to the ability of the film surface roughness to reduce
Figure 3. Transmission spectra for the ZnO films deposited with various oxygen pressures. Reprinted with permission from [89], C. F. Yu, et al., Appl. Surf. Sci. 256, 792 (2009). © 2009, Elsevier.
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Rajesh Kumar et al.
reflectivity, an increase in the structural homogeneity, fine texturing and the approach of the film composition to the stoichiometry.89 90 117 126–128 146 Two emission bands in PL UV were apparently observed for all the thin films: one is the near-band-edge emission (NBE) peak at the UV region, which is due to the carrier’s recombination from band to band, and the other was the deep-level-emission (DLE) peak around the green–yellow band that corresponds to the transition of the excited optical center from the deep level to the valence band. With the increase in oxygen pressure, the intensity of the DLE peak decreases remarkably and that of the NBE peak increases rapidly as shown in Figure 4. The intensity of DLE peak is generally proportional to the conductivity. The strong DLE peak is found to be strongly dependent on the oxygen pressure. This is probably because the stoichiometry of oxygen-deficient ZnO film with less oxygen vacancies and radiative transitions between shallow donors (O vacancy and Zn interstitial) and deep acceptor (Zn vacancy), is improved by increasing oxygen pressure.89 91 The PL intensity of ultra-violet (UV) luminescence and the dependent electrical resistivity generally increase as the oxygen pressure for the PLD of ZnO increases. At the lower oxygen pressure with the largest number of oxygen vacancies, correspondingly the electron concentration and the conductivity is thus higher than other samples grown at the higher oxygen pressure.132 137 139
Figure 4. The PL spectra of the ZnO films grown under the conditions of oxygen pressure of 50, 300, 400 and 500 mTorr at the substrate temperature of 400 C. Reprinted with permission from [91], B. J. Jin, et al., Appl. Surf. Sci. 169–170, 521 (2001). © 2001, Elsevier.
J. Nanosci. Nanotechnol. 14, 1911–1930, 2014
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To conclude the ZnO thin films exhibit low surface roughness, more thickness, high surface conductivity, lower electrical resistivity, good conductive uniformity, increased band gap, more oxygen vacancies, high electron concentration and increased DLE peak intensity at lower oxygen pressure. 5.3. Effect of Post Annealing After the thin films are prepared under certain ambient conditions, crystallinity, stoichiometric ratio, surface morphology and hence, the transmittance and electrical properties of the thin films can be further modified by annealing treatment. From literature most ZnO films were deposited on crystal substrates at substrate temperature from room temperature (RT) to about 400 C temperature.92 Substrate materials, such as glass and polymer cannot be subjected to a higher temperature annealing, thus low temperature annealing; also referred to as aging of thin films under RT and air atmosphere for a long time may be an alternate approach to further improve the properties of thin films. Post annealing temperature is usually higher 200–800 C,93 800 C,94 95 400–800 C,96–98 132 400–700 C,99 300–800 C.100 101 500–700 C,102 1000 C:145 than that of substrate temperature. From reports it is revealed that 400–600 C temperature is necessary for relieving accumulated strain energy, diminishing defects, and enlarging grain size to get strong UV emission (Fig. 5).92 94–98 Quality of ZnO thin films improves by the recrystallization and obviously due to reduction in extrinsic and intrinsic defects of ZnO thin film by supplying
Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review
sufficient thermal energy in the presence of an ambient supply of air or O2 . The surface roughness of the annealed thin film is much smaller than that of the as-deposited thin film. The surface is flattened by annealing. However, the annealing gases N2 and O2 make little difference on the surface morphology of the thin film.94 95 A strong correlation is observed between substrate temperature during deposition of thin films and post annealing temperature. For ZnO thin film deposited at RT, annealing treatment improves the density of thin films and grain size with no obvious change in stress state. For ZnO thin film, deposited at higher temperature, annealing treatment not only improves the crystal quality but also the stoichiometric ratio with strain relaxation and smaller grain size.92–97 Annealing at higher temperatures favors the diffusion of atoms absorbed on the substrate and accelerates the migration of atoms to the energy favorable positions, resulting in the enhancement of the crystallinity on the ZnO thin films.93 Similar results are obtained for transparent zinc oxide thin films grown by reactive pulsed laser deposition on glass substrates at 200 C by Stamatakia et al.72 Post-deposition heat treatment promotes crystallization, and reduces oxygen deficiency and crystallinity of the zinc oxide films. Transmittance of the thin film annealed at higher temperature up to 800 C decreases due to rearrangement of the thin film and the changes of the interface, between the thin film and the substrate. High-temperature annealing flattens the surface and decreases the transmittance. Concurrently, the annealing makes the thin film denser. With increasing annealing temperature, the decrease of band gap
Figure 5. The SEM morphology of ZnO thin films deposited at RT and annealed for different temperatures for 2 h: (a) un-annealed (b) 150 C (c) 300 C (d) 450 C. Reprinted with permission from [92], B. L. Zhu, et al., Vacuum 84, 1280 (2010). © 2010, Elsevier.
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Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review
Eg is attributed to the increase of grain size and decrease of carrier concentration.92 94 95 145 146 However, the ZnO films grown by Rusop et al.93 show increased transmittance with annealing temperature up to 600 C in the visible region, indicating increased crystallinity, less defects and good optical quality. Above 600 C, transmittance decreases with higher annealing temperature may be due to decreasing optical scattering caused by the densification of grains followed by grain growth and the reduction of grain boundary density. Annealing in O2 ambient atmosphere, causes the decrease of ZnO intrinsic defects such as oxygen vacancies and Zn interstitials, which results in increase of radiation transition and decrease of non-radiative transition owing to the improvement of ZnO crystal quality. ZnO thin films annealed in N2 ambient shows large grain boundaries defects.96 Figure 6 shows XRD spectra of ZnO thin films annealed at annealing temperature 600 C in different ambient conditions: (a) vacuum, (b) nitrogen gas, and (c) oxygen gas. The intensity of XRD (0002) peaks for the annealing sample in N2 and O2 shows a notable increasing contrast to that of sample annealed in vacuum in deposition chamber. The narrow FWHM and the increasing XRD intensity imply the crystal quality is best for ZnO film annealed in O2 ambient. As the bond dissociation energy of N2 is much larger than that of O2 , the activation barrier for N2 bond breaking on the surface of the reactants is very high. The annealing temperature of 800 C is insufficient for N2 to break the bond and diffuse into the lattice to fill the oxygen vacancies. So the oxygen vacancies were enhanced during annealing in N2 .95 Low resistivity at lower annealing temperature which increases dramatically at higher temperatures, is attributing to the decline of the carrier concentration as the annealing
Figure 6. XRD spectra of ZnO thin films annealed in different ambient at annealing temperature 600 C in: (a) vacuum, (b) nitrogen gas and (c) oxygen gas. Reprinted with permission from [96], X. Q. Wei, et al., Physica B: Cond. Matter 388, 145 (2007). © 2007, Elsevier.
1920
Rajesh Kumar et al.
temperature increases. Kang et al. and Stamataki et al. believe that this is because the number of Zn interstitials decreases probably due to Zn evaporation by increasing annealing temperature, or the oxygen vacancies, which contribute the free carriers, declined by oxygen diffusion into the film with annealing treatment.72 100 101 5.4. Influence of Laser Fluence As stated earlier in Eq. (1), laser fluence of a laser beam is the ratio of Laser energy (J) to laser beam area (cm2 ). To ablate an atom from a solid surface by a laser pulse, the laser fluence should exceed the binding energy of the constituents of the target material. As a result the ablation rate is a function of fluence. Laser beam fluence affects the surface morphology, electrical, optical and transmittance properties of the ZnO thin films by directly affecting the nature of species formed in the plasma plume. The density and the size of particulates on the deposited film surface tend to increase with increasing laser fluence and laser wavelength.59 Studies carried out by Naszalyi et al.73 103 show increased O/Zn ratio due to increased crystallite grain size, surface roughness and optical band gap with an increase in laser energy density from 1.5 to 3 J/cm2 for zinc oxide thin films deposited on glass and silicon substrates. This further confirms that the increase of crystallite size with an increase in laser energy density leads to an increase of O/Zn atomic ratio. Films deposited at low laser fluence have defects and impurities of interstitial zinc atoms whose concentration decreases with an increase in laser energy density. Thickness, crystallite size, and compactness of ZnO films increase with the increasing laser energy for metallic Zn target as that of ZnO targets.81 Texturecontrolled growth of ZnO films on different substrate materials at room temperature by pulsed laser deposition demonstrate the texture of the film changed progressively from (001) to (110) to (100) as the laser fluence increased from 2 J cm−2 up to 45 J cm−2 .105 Such texture controlled ZnO film are important not only for the application as seed layers but also for the application of the textured film itself in surface acoustic wave (SAW) devices.74 The peak intensity of the (002) diffraction of ZnO films deposited at various laser fluence, increases with increase of laser fluence as shown in Figure 7.81 103 105 5.5. Effect of Deposition Time and Film Thickness PLD results in the deposition of films with thickness ranging from a few nanometers to micrometers. Film thickness is an important factor to control the structural, optical and electrical properties of the ZnO films and is deposition time, pulse laser fluence, O2 pressure and growth temperature dependent. Longer deposition time leads to the deposition of ZnO thin films with enhanced thickness.87 Morphology of the film grain particles can change with the thickness of the films. ZnO thin films grown at a temperature of 350 C displayed a granular, polycrystalline J. Nanosci. Nanotechnol. 14, 1911–1930, 2014
Rajesh Kumar et al.
Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review
interstitials which are donor type point defects. Hence the enhancement of the defects leads to the rise of carrier concentration.5 On the contrary, the decrease of defects with increase of the thickness of ZnO thin films causes the decline of carrier concentration, or the increase of resistivity. For very thick films auto-doped carrier concentration becomes constant.109
Figure 7. XRD spectra of ZnO films deposited on glass substrates at various laser energies. Reprinted with permission from [81], B. L. Zhu, et al., Physica B 396, 95 (2007). © 2007, Elsevier.
morphology, which transforms to nano or micro rod as the thickness of the film increases.106 107 Films deposited for a small 5-minute duration show no characteristic wurtzite structures for ZnO due to the strain originating from the lattice mismatch between the film and the substrate. The ratio of oxygen to zinc (O/Zn) is larger than unity and becomes closer to unity for the films with longer deposition times thereby improving crystallinity.108 The effect of the thickness variation on the surface morphology and structural, electrical, and optical properties of ZnO thin films grown on c-plane sapphire by PLD reveals that, as the film thickness increases, the crystallinity and optical and electrical properties of ZnO thin films improves. Furthermore, ZnO films with thickness 400 nm or more exhibit the near-bulk properties. ZnO thin films with a thickness of 40 nm show a smooth surface with several islands indicating the film layer-plus-island growth mode (Stranski-Krastinov mode) and for the films with a thickness of 170 and 400 nm, a rough surface is observed with 3D-island growth mode (Volmer–Weber growth) as discussed in Section 4.3.1. The tensile strain in the films decreases with increasing film thickness resulting in better crystalline quality as indicated by a more intense peak for (002) diffraction of ZnO films deposited with various film thicknesses.109 The increase in ZnO film thickness also results in the improvement of the optical properties, and may be due to the decrease of strain at the interface between ZnO film and sapphire substrate by the increase of film thickness. Park et al. found that the films thinner than 400 nm are under a severe misfit strain, which decreases as the film thickness increases further.75 Electrical resistivity decreases significantly with increasing thickness due to the highest carrier concentration and fewer point defects like oxygen vacancies. The conductivity of ZnO thin films can probably be attributed to oxygen vacancies and Zn J. Nanosci. Nanotechnol. 14, 1911–1930, 2014
5.6. Effect of Pulse Repetition Rate The amount of ablated species arriving on a substrate per laser shot increases as pulse repetition rate is increased. King et al.110 studied the effect of laser repetition rates of 10, 5 and 2 Hz for ZnO thin films grown on Si substrates at 300 C. The film crystallinity was found to be sensitive to the pulse repetition rate with constant number of pulses and yielded film thicknesses of 272 nm, 289 nm and 255 nm respectively, as characterized by ellipsometry. The grain size of ZnO thin film deposited at 5 Hz was larger than that of 10 Hz. The variation of repetition rates does not have an effect on the optical property of ZnO thin films as UV emission intensity/visible emission intensity ratios of ZnO thin films deposited at 5 Hz and 10 Hz are almost identical. Lee have reported the degradation of the crystalline quality and surface morphology in ZnO thin film deposited at 10 Hz results from super saturation effect by decrease of time interval between a ZnO particle arriving on a substrate by laser shot and a ZnO particle arriving on a substrate by next laser shot.111 During the study of effect of pulse repetition rate other experimental parameters, such as substrate temperature, energy density of the laser, ambient gas pressure and the distance between a target and substrate are kept constant. 5.7. Effect of Target to Substrate Distance The oxygen partial pressure in the chamber, the target-tosubstrate distance and the laser energy density has synergic effect in determining the energy of the ablated particles when they reach the substrate. As the plume particles can thermalize completely at high O2 pressure, their kinetic energy reduces due to large number of collisions with background gas molecules and hence a decrease in the size of ablated plume is observed.12 However, at high vacuum or low O2 pressure the mean free path of the energized ions and particulates is longer; so, the particles bombard the substrate to form a crystalline or polycrystalline film. Therefore it is better to reduce the distance between the target and substrate while working at higher pressures to maintain the optimum energy of the ablated species.30 At low pressure working conditions, however, samples grown at shorter target–substrate distance may develop defects, cracks or peeling off. On the other hand the films grown at longer distance are homogenous, smooth, adherent, and without cracks.112 In the deposition chamber with the smaller target to the substrate distance, a strong dependence of carrier concentration on oxygen partial pressure is observed.77 The Hall 1921
1922 ZnO ZnO ZnO
W. Zhaoyang et al. [70]
B. I. Kim et al. [71] K. Shrinivasrao et al. [78]
ZnO
Kaidashev et al. [84]
ZnO ZnO ZnO ZnO
ZnO
C. F. Yu et al. [89]
J. B. Franklin et al. [90]
B. J. Jin et al. [91]
B. L. Zhu et al. [92]
Zn Metal
Liu et al. [88]
S. Fiat et al. [87]
ZnO ZnO
ZnO
S. Heitsch et al. [82]
S. H. Bae et al. [85] V. Craciun et al. [86]
Zn Metal
ZnO
B. L. Zhu et al. [81]
K. Ramamoorthy et al. [80]
Zn Metal
ZnO ZnO
Y. R. Ryu et al. [65] R. Ayouchi et al. [69]
L. Zhao et al. [79]
ZnO ZnO ZnO
Precursor
S. Cho [58] S. J. Kang et al. [63] C. S. Son et al. [64]
References
Glass
Sapphire (001)
ITO
Glass
Sapphire, Silicon
Sapphire (0001) Silicon (100), Corning glass Soda lime glass
Silicon (100), (111) c-Plane sapphire
Glass
Glass
Sapphire (0001) Quartz glass, Si(100) Quartz glass
Si(111)
GaAs c-Sapphire
c-Sapphire Si(100) Si(100)
Substrate
KrF 248 25 ns
KrF 248 10 ns KrF 248 25 ns Nd:YAG 532 7 ns KrF 248 25 ns Nd:YAG 355
KrF 248 25 ns KrF 248 25 ns Nd:YAG 355 KrF 248
ArF 193 Nd:YAG 355 5 ns KrF 248 25 ns KrF 248 KrF 248 25 ns Nd:YAG 1064 100 ns Nd:YAG 355 6 ns KrF 248 25 ns
KrF 248 KrF 248 KrF 248
Laser source, wavelength (nm), pulse duration
5
5
8
–
–
10
5 –
1
1
5
10
10
5 5
5
20 –
5 5 5
Pulse rate (Hz)
150
18.5∗
2.78
2.5
200–500
400
50–650
550–700
250∗
0.85
300
200–600 200–500
600–750
RT, 200, 300 RT 200 500 500–675
100–250
500 200–600
650
400–700 100–700 400 500 600 300–450 400, 500
Ts ( C)
2.6
2.5 0.5–5
–
–
100– 325∗
5
7
2.1 156∗
2.5
2 –
2 2 2
Laser energy/ fluence (J/cm2 )
1–2a
2.5 × 10−2 , 5 × 10−4 mbar 10−6 –400 mTorr 5 × 10−6 – 2 × 10−2 Torr 10, 15 Pa
50 mTorr 200 mTorr 300 mTorr 400 mTorr 500 mTorr 12 Pa
5 or 50 mTorr
40–150 mTorr
0.25–20 Torr
300–700
3 × 10−2 –10 Pa
5 5
7
3 × 10−5 Torr 5 × 10−5 Torr
3 × 10−3 Pa
120–250 0.6–1.8 a
150–200
2.5
2–4.2
5
10−4 Pa. 5–20 Torr
5 4
5.5
5.5
1 × 10−6 Torr 10−6 –10−7 Torr
–
–
10−6 Torr –
–
275–306
1a 632
60–400
100 Pa
0.66, 8, 13a
10−5 Torr
5
2.3
5 × 10−4 Pa
67–89
3 × 10−3 Pa
– –
10−8 Pa 10−7 mBar
450–603 456
6
5
10−8 Pa
10−6 Torr
7 5
8 × 10−7 –
0.7–1.1a – –
6 – 6
Td (cm)
– 5 × 10−6 Torr –
Background chamber pressure
– – –
Film thickness (nm)
11 Pa
5 atm 0.05–0.5 mbar
0–80 Pa
20–50 mTorr 0.25 m bar
– 250 mTorr 350 mTorr
Oxygen partial pressure
Table I. Physical parameters for un-doped ZnO thin films deposited by PLD technique and crystallographic orientation.
150–450
400
–
–
–
–
– –
–
–
–
–
–
– –
–
– –
– – –
Post annealing Temp. ( C)
(0 0 2)
Un-annealed (002), (101), (100) Annealed at 400 mTorr (002)
(0 0 2)
(0 0 2)
(002) (004) (006)
(002), (004)
(0 0 2) (002), (004)
–
Amorphous (002) (002) (0002), (0004)
–
(002), (103) weak
(0002), (0004) (0 0 2)
(0 0 2)
(0 0 2) – (002), (004) (002), (103) (101), (103) (0 0 0 2) (0 0 2)
Cryslallographic data
Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review Rajesh Kumar et al.
J. Nanosci. Nanotechnol. 14, 1911–1930, 2014
J. Nanosci. Nanotechnol. 14, 1911–1930, 2014 ZnO ZnO ZnO
ZnO ZnO ZnO ZnO ZnO ZnO
Y. R. Jang et al. [108] J. M. Myoung et al. [109] King et al. [110]
J. W. Kim et al. [111] J. Dattatray et al. [113]
A. O. Dikovska et al. [114]
H von et al. [115] J. Zhang et al. [116]
K. J. Saji et al. [117]
Zn Metal
L. Zhao et al. [102]
ZnO
ZnO ZnO
S. H. Bae et al. [99] Kang et al. [100]
S. Christoulakis et al. [106], [107]
ZnO
F. K. Shan et al. [98]
ZnO
ZnO
X. Q. Wei et al. [96]
J. I. Hong et al. [105]
ZnO
F. K. Shan et al. [95]
ZnO ZnO
ZnO
F. K. Shan et al. [94]
Venkatachalam et al. [103] B. L. Zhu et al. [104]
ZnO
Precursor
M. Rusop et al. [93]
References
Table I. Continued.
Quartz, Silicon, and Polymer, Al2 O3 (0001)
ZnO wafers -Ga2 O3 (100)
SiO2 (001)
Silicon wafers, Kapton films, Glass plates Corning 1737F glass Si(100) Sapphire (0001) Coming 7059 glass, pure quartz, Si(100) Sapphire (001) W, Re foil
Glass and silicon Amorphous glass
Quartz glass
Si(100) Sapphire (001)
Si(100)
Si(111)
Sapphire (0001)
Sapphire (0001)
Si(100)
Substrate
Nd:YAG 355 Nd:YAG 355 6 ns XeCl 308 30 ns KrF 248 KrF 248 25 ns Nd:YAG 355 9 ns
Not specified Nd:YAG 355 KrF 248 25 ns
XeCl 308
Nd:YAG 1064 100 ns KrF 248 KrF 248 25 ns KrF 248
KrF 248 10 ns KrF 248 25 ns Nd:YAG 1064 10 ns KrF 248 25 ns Nd:YAG 355 Nd:YAG 355
XeCl 308
10
5 –
2
5, 10 10
– 5 2, 5, 10
–
30
10 5
10
5 5
5
10
5
5
10
3
– 5
–
2.5 1
– 2.5 5
–
5–45
1.5–3 348∗
7, 7.6
2.5 2.5
1
200∗
1
2
5.2
Laser Laser source, Pulse energy/ wavelength (nm), rate fluence pulse duration (Hz) (J/cm2 )
RT
470–700 600
25–500
400 600
600 400 200–500
350
850
30 –
100–250
400 400
100–600
100–300 400–600 400
400
RT
Ts ( C)
0.007–0.003 mbar
0.016 mbar 150 mTorr
0.05–0.3 mbar
350 mTorr 10−4 mBar
– 350 mTorr I .3 mTorr
560 Torr
–
0.025 mbar 0.2, 24, 150 Pa
50 pa
350 mTorr 350 mTorr
200 mTorr
0.13 pa
200 mTorr
200 mTorr
6 Torr
Oxygen partial pressure
– 3.5 5 4
5 × 10−4 Pa 4 × 10−5 mbar 3 × 10−3 Pa 10−6 Torr
–
700
10 nm−1 a
5 5 4
4 4
10−6 –10−7 Torr 10−6 mBar 10−5 mbar – – 10−6 mbar
450–650 1a 200 200
1000 –
– 5 – 1 × 10−6 Torr –
100–400 40–2400 289–255
244 140
–
–
5 5
– 700–800
1 × 10−6 Torr –
4
50
5
5.6 × 10−5 Pa
–
50
45
2 × 10−5 Torr
–
Td (cm)
Background chamber pressure
–
–
600
–
–
–
Film thickness (nm)
Cryslallographic data
–
– –
–
– –
– – –
–
–
– –
400–700 400 800 500−700
400–600
(0 0 2)
– (0 0 2)
(0 0 2)
– (0 0 2)
(0 0 2) (0 0 2) (0 0 2)
(002), (004)
(100) (002) (101)
(100) (002) (101) (0 0 2)
(0 0 2) (002) (100) (002) (101) (0 0 2)
–
RT (100)(002) (101) 200 (100) (002) (101) 300–800 (002) 800 – N2 , O2 Atm. 800 (0 0 2) (002), (004) 400–800 (0 0 2)
Post annealing Temp. ( C)
Rajesh Kumar et al. Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review
1923
1924
ZnO, Al2 O3 ZnO
ZnO
W. S. Hu et al. [131]
X. Q wie et al. [132]
X. Sun et al. [133]
Zn Metal
ZnO
V. Craciun et al. [130]
X. M. Fan et al. [134]
ZnO
ZnO
S. S. Kim et al. [126]
T. Ohshima et al. [129]
ZnO
P. Pant et al. [125]
ZnO
ZnO
G. Cramer et al. [124]
S. Karamat et al. [128]
ZnO
M. Z. Lin et al. [123]
ZnO
ZnO
M. Khalid et al. [122]
Amirhaghi et al. [127]
Zn Metal
ZnO
L. Z. Fu et al. [119] M. Novot´ny et al. [120]
Rahman et al. [121]
Zn
Precursor
L. C. Nistor et al. [118]
References
Table I. Continued.
Si(111)
c-sapphire
Fused Silica, Al2 O3 (001) c-sapphire
Si(100), silica glass, -Al2 O3 (0001), corning 7059 glass GaAs
Corning glass 7059, Si(100) sapphire (001)
Si(001)
r-Plane sapphire
Pt/SiO2 /Si
Al2 O3 (0001)
a-plane Al2 O3 c-plane Al2 O3 r-plane Al2 O3
c-Sapphire Sapphire (0001), MgO (100), fused silica Calcite (CaCO3 )
MgO (001)
Substrate
Nd:YAG 1064 100 ns
Nd:YAG 1064 10 ns ArF 193
KrF 248 20 ns KrF248
Nd:YAG 532 4 ns Nd:YAG 532 8 ns KrF 248 25 ns
Nd:YAG 355 15 ns KrF 248 20 ns KrF 248 25 ns KrF 248 30 ns
Nd:YAG 1064 9–14 ns KrF 248
KrF 248 Nd:YAG 266 6 ns
Nd:YAG 355
10
10
10
2–5
–
10
10
5
5
5
6
10
–
–
5 10
10
31
300–550
500–800
400–800
200∗ 6
375
250
RT-700
400
300
600
–
2.1
2
47
2.5–3
3
200–700
350
350∗ 2–3
600–900
2
300–400
RT
10∗ 2
100–500 300
500
Ts ( C)
1 2.8
20
Laser Laser source, Pulse energy/ wavelength (nm), rate fluence pulse duration (Hz) (J/cm2 )
300
2 × 10−4 torr
–
1.33 × 10−3 Torr
11 Pa
10 mTorr
Various
−3
Pa
5
7 × 10−5 mbar
5 2.5
10−7 Torr 5 × 10−4 Pa
650–420 0.8–1.3a
–
4
4
–
–
–
–
10−7 torr
–
4
–
–
5
–
5
5 5.5
5.5
–
–
–
–
–
–
–
–
–
–
–
–
–
–
– –
–
Post Td annealing (cm) Temp. ( C)
10−6 Torr
7 × 10−7 torr
10−6 Torr
3 × 10−8 Torr
–
10−3 torr
– 2 × 10−4 Pa.
10
Background chamber pressure
5 × 10−6 pa 800
–
242–1520
6.67 × 10−3 – 26.7 Pa
27 pa
60
0.1–1 mbar
250–355 5 × 10−4 Torr 5 × 10−3 Torr 5 × 10−2 Torr 5 × 10−1 Torr 2 × 10−6 –2 × 10−2 torr 300–1000
240
–
–
– 80
180
Film thickness (nm)
–
10−5 –3 × 10−4 Torr
–
10−3 torr
200 mTorr 10 Pa
–
Oxygen partial pressure
(0001)ZnO/(0001) (1010)ZnO/(1120) (100) (002) (101)
(0 0 2)
(0 0 2)
(002), (004)
(002), (004)
(002), (004)
(002), (004) (002), (004) (002), (004) (002), (101), (102) (0 0 2)
–
ZnO (0002)/Al2 O3 (1120) ZnO (0002)/Al2 O3 (0001) ZnO (1120)/ Al2 O3 (0112) ZnO[1010]/ sapphire[1120] –
(100), (101), (102)
(1100), (1101), (1120), (0002) weak (0002), (0004) (002), (103)
Cryslallographic data
Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review Rajesh Kumar et al.
J. Nanosci. Nanotechnol. 14, 1911–1930, 2014
J. Nanosci. Nanotechnol. 14, 1911–1930, 2014
ZnO
Zn ZnO
ZnO
ZnO
ZnO ZnO
ZnO
ZnO
Y. Inoue et al. [136]
Y. R. Ryu et al. [137] Z. Q. Chen et al. [138]
S. Im et al. [139]
F. K. Shan et al. [140]
E. S. Shim et al. [141] J. Duclere et al. [142]
J. H. Jo et al. [146]
Y. W. Sun et al. [147]
Si
Indium phosphide Pt film on c-sapphire Sapphire (001)
Glass
R-face (0112) sapphire GaAs (001) ZnO (0001), Al2 O3 (1120) Sapphire (001)
Si(111)
Substrate
KrF248 20 ns KrF248 25 ns
KrF 248 25 ns Nd:YAG 355 Nd:YAG 266
ArF 193 KrF 248 nm 25 ns Nd:YAG 1064
ArF 193
KrF 248
Laser source, wavelength (nm), pulse duration
–
5
2 2
5
5
20 10
10
5
Pulse rate (Hz)
3
–
2.5 1.4
1
2.5
1.3–1.9 –
1
320∗
Laser energy/ fluence (J/cm2 )
–
500
RT 500 400 450
400
350 500
350 450 –
Ts ( C)
Notes: (∗ ) Laser energy in mJ/pulse, (a) Film thichness in m units, (Ts ) Substrate Temperature, (Td ) Target-Substrate distance.
ZnO
Precursor
Y. Zhang et al. [135]
References
Table I. Continued.
100 mTorr
5–150 mTorr
350 mTorr –
1–200 mTorr 500 mTorr 100 mTorr
35, 50 mTorr 10−2 Torr
–
1 mTorr
Oxygen partial pressure
–
–
320 500
4
2 × 10−6 Torr
5
5 –
1 × 10−6 Torr –
–
5
5
5 × 10−5 Torr 0.6–1.8a
–
7.5 –
8 × 10−7 Torr –
100 1–2a
–
3.5
10−5 Pa
Td (cm) –
Background chamber pressure –
97 212 150
Film thickness (nm)
600
900
400–600 –
400–500
–
– –
–
–
Post annealing Temp. ( C)
(100) (002) (101)
(0 0 2)
(0 0 2) (002), (004) Amorphous (0 0 2) (0 0 0 2) ZnO (0001)/Pt (111)
(0 0 2) (0002), (0004)
(002), (004) (002) –
Cryslallographic data
Rajesh Kumar et al. Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review
1925
Pulse Laser Deposited Nanostructured ZnO Thin Films: A Review
Rajesh Kumar et al.
mobility at room temperature of ZnO films on c-plane sapphire grown by a multistep PLD process with lowtemperature nucleation layers depicts highest mobility values in a narrow carrier concentration range. A much weaker dependence of carrier concentration and Hall mobility on the oxygen partial pressure was found for PLD growth of ZnO thin films at larger 100 mm target to substrate distance.84 From the discussion, target-substrate distance is found to influence electron mobility, roughness of the surface and resistivity of the deposited crystalline ZnO films. Transparent conductive ZnO films grown at a larger target to substrate distance have a low surface roughness and electrical resistance which have a wide range of applications to develop sensors, optoelectronic devices, solar cells, photovoltaic inverters, transistor structures based on transparent conductive films of ZnO.
6. CRYSTALLOGRAPHIC STUDIES XRD patterns of ZnO thin films deposited at different substrate temperature, oxygen partial pressures and annealing temperatures can be compared from Table I. A distinct characteristic diffraction peak (002), along with weak (100) and (101) peaks are detected for the films corresponding to c-axis orientation perpendicular to the substrate. These films consist of an epitaxially ordered array of hexagonal micro crystallites; and the facets of all hexagons are parallel to those of the others, forming natural Fabry–Pérot lasing cavities.143 144 With the increase of the substrate temperature the intensity of ZnO (100) diffraction peaks decreases, and the intensity of the ZnO (002) diffraction peaks becomes more intense and sharper.58 For ZnO thin film deposited at low substrate temperature a relatively low (002) diffraction peak was observed due to low atomic mobility, which limits the crystal growth during the crystallization process. However, as the substrate temperature increases, there is enough thermal energy to supply to atoms on the substrate, which increases the surface mobility. As a result, it leads to an increased peak intensity and peak angles (2 in the (002) plane orientation showing high crystallinity with c-axis orientation.58 69 78 79 81 90 93 132–134 149 150 Unannealed ZnO thin films prepared at room temperature on Si(100) substrate by Rusop et al. exhibit three diffraction peaks for planes (100), (002) and (101) with (002) peak being most intense, even at low temperature indicating that ZnO films grown by PLD are strongly c-axis oriented.93 Figure 8 shows the relative intensity of the peaks corresponding to (002) plane increases with increasing annealing temperatures as compared with (100) and (101) planes, indicating the preferred grain growth along the (002) plane for the films. The increasing of annealing temperatures favors the diffusion of atoms absorbed on the substrate and accelerates the migration of atoms to the 1926
Figure 8. XRD spectra of as-grown and annealed ZnO thin films with different annealing temperatures. Reprinted with permission from [67], S. Cho, et al., Appl. Phys. Lett. 75, 2761 (1999), © 1999, American Institute of Physics.
energy favorable positions, resulting in the enhancement of the crystallinity and c-axis orientation of ZnO films.67 91–96 98–100 It is revealed in Figure 9 that the structure of ZnO films is sensitive to the partial pressure of oxygen gas in the deposition chamber. The relative peak intensity of
Figure 9. XRD patterns of ZnO thin films as a function of the oxygen pressure. Reprinted with permission from [89], C. F. Yu, et al., Appl. Surf. Sci. 256, 792 (2009). © 2009, Elsevier.
J. Nanosci. Nanotechnol. 14, 1911–1930, 2014
Rajesh Kumar et al.
the diffraction peaks decreases with increasing oxygen pressure. The (002) peak becomes sharper, undoubtedly due to increased crystallinity and almost unit stoichiometric ratio of Zn/O for the ZnO thin films with increasing oxygen pressure.82 84–91 126 However, above a certain O2 pressure the c-axis orientation almost disappears and the film crystallinity becomes much lower, suggesting that O2 pressure in a PLD process should be adjusted below some threshold in order to obtain a c-axis oriented ZnO film.126 ZnO samples are found to be epitaxially grown and ¯ and (000l) oriented, when deposited on r-Al2 O3 (1120) and c-Al2 O3 , respectively. ZnO c-axis either lies in the substrate plane or is perpendicular to it. In the case of r-Al2 O3 , the sample is completely striated and oriented along one direction where as columnar growth is observed in the case of c-Al2 O3 .148
7. CONCLUSION The PLD technique is very effective and well suited for developing epitaxial films, and allows fabrication of multilayers, hetero-structures and super lattices on lattice matched substrates. The major part leads to growth of films oriented along the (002) plane. ZnO crystallites with preferential orientation are desirable for applications where crystallographic anisotropy is a prerequisite e.g., UV diode lasers, piezoelectric surface acoustic wave or acousto-optic devices. The technique of PLD has significant benefits over other film deposition methods, including: the capability for stoichiometric transfer of material from target to substrate. The optimized value of growth parameters like substrate temperature, background gas pressure, laser energy density (laser fluence), target to substrate distance, repetition rate, oxygen partial pressure in deposition chamber, deposition time and post growth annealing which control deposition parameters such as adsorption, desorption, surface diffusion, nucleation, crystallization/recrystallization as well as properties like morphology, roughness of the film surface, film thickness, grain size, optical transmittance, sensitivity, electrical conductivity, uniformity, and electrical resistivity can be extracted from the review to develop sensors, optoelectronic devices, solar cells, photovoltaic inverters and transistor structures based on transparent conductive films of un-doped ZnO. Acknowledgments: Ahmad Umar would like to acknowledge the support of the Ministry of Higher Education, Kingdom of Saudi Arabia for this research through a grant (PCSED-002-11) under the Promising Centre for Sensors and Electronic Devices (PCSED) at Najran University, Kingdom of Saudi Arabia.
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Received: 11 April 2013. Accepted: 21 October 2013.
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