Tunable Infrared Absorption and Visible Transparency of Colloidal Aluminum-Doped Zinc Oxide Nanocrystals

Metal oxide nanostructures are an important class of thin film electronic materials well-suited for applications, which can exhibit large changes in their electrical and optical properties upon exposure to different external stimuli, gas and/or chemical environments at their surfaces [1, 2]. Zinc oxide (ZnO) in particular, has been garnering increased interest due to its unique combination of properties that are advantageous for electronics and optoelectronics such as wide band gap, large exciton binding energy, low-cost, ease of processing and availability [3]. This has led to the development of ZnO nanoparticles and thin films wherein the large surface areas and tunable properties of ZnO nanostructures make them particularly suitable for applications requiring enhanced performance with low-power and decreased cost [4].ZnO nanostructures have been used to demonstrate light-emitting diodes, photodetectors, gas sensors, transparent conductors, and memristors, among other applications [5]. Many of the desirable properties of ZnO arise because of surface states that can be tuned by various fabrication and processing steps and are further enhanced at nanoscale dimensions [4]. Recently, these attributes have shown that ZnO nanomaterials are promising for applications in optoelectronic neuromorphic computing [6]: ZnO nanostructured thin films have shown desirable properties analogous to long-term/short-term memory and synaptic plasticity in biological systems [7], which could allow for efficient analog computing devices that emulate brain functions and overcome performance limitations of conventional computing based on the von Neumann architecture [8]. Such neuromorphic materials have great potential for artificial neural networks, ultra-low power biologically inspired computing and vision [6].In this work, we present results on solution-processed ZnO nanoparticle thin films that display optoelectronic neuromorphic or synaptic behavior based on persistent photoconductivity [9]. The thin films were fabricated using nanoparticle inks (nanoinks) obtained via planetary ball milling (PBM) of bulk ZnO powders [10]. PBM is a solution-based nanofabrication approach that can quickly produce nanoink suspensions via colloidal grinding in a given solvent for low-cost thin film coatings without requiring complex processing.The thin film optoelectronic neuromorphic devices were fabricated by depositing ZnO nanoink onto flat insulating glass substrates followed by contact formation (see Fig. 1a for overall device structure). PBM was performed using ZnO powder in ethylene glycol or deionized water solvent with zirconia grinding beads. The grinding speed was varied between 200 and 1000 rpm. Analysis of the ZnO films after deposition showed they consist of nanostructured particles with sizes reaching below 100 nm, as displayed in the atomic force microscopy (AFM) images in Fig. 1b, depending on grinding conditions.The optoelectronic properties of the ZnO thin films were evaluated via two-terminal photoconductance measurements using a probe station connected to a precision source-measure unit, under ambient atmosphere and temperature. In order to determine long-term memory (LTM) and short-term memory (STM) properties for artificial optoelectronic synapses, the ZnO films were exposed to different width broadband light pulses: Fig. 1c shows photocurrent vs. time plots for a typical ZnO thin film device subjected to long pulses with large intervals, which exhibits the rise and decay of photoconductance associated desorption/adsorption of surface oxygen species in ZnO (i.e., oxygen surface vacancies that act as donors) [9]. On the other hand, Fig. 1d shows the photoconductive behavior for short pulses separated by short intervals – here the current continues to increase between pulses, which is indicative of an optoelectronic neuromorphic memory effect, or LTM. In other words, the system “remembers” the previous pulse state, in contrast to losing that information in the case of complete photocurrent decay between pulses (Fig. 1c), or STM. Thus, the ZnO films display optoelectronic memory properties that mimic brain functions/learning in order to recognize different optical signals, promising for synaptic devices and systems in neuromorphic memory and computing.[1] N. Nasiri, D. Jin, A. Tricoli, Adv. Opt. Mat., 7, 1800580 (2019).[2] C. Wang et al., Sensors, 10, 2088 (2010).[3] C. Klingshirn, Phys. Status Solidi B, 244, 3027 (2007).[4] R. Khokhra, B. Bharti, H.-N. Lee, R. Kumar, Sci. Rep., 7, 15032 (2017).[5] M. Laurenti, S. Porro, C. F. Pirri, C. Ricciardi, and A. Chiolerio, Crit. Rev. Solid State Mater. Sci., 42, 153 (2017).[6] Y. Wang et al., Adv. Intell. Syst., 3, 2000099 (2021).[7] R. D. Chandra, K. G. Gopchandran, ACS Appl. Electron. Mater., 3, 3846 (2021).[8] M. A. Zidan, J. P. Strachan, W. D. Lu, Nat. Electron., 1, 22 (2018).[9] S.-L. Gao et al., ACS Appl. Electron. Mater., 6, 1542 (2024).[10] R. Sapkota, P. Duan, T. Kumar, A. Venkataraman, C. Papadopoulos, Appl. Sci., 11, 9676 (2021). Figure 1

Zinc oxide (ZnO) is currently one of the most significant wide-bandgap semiconductor materials, attracting extensive research across diverse fields including materials science, chemistry, physics, medicine, electronics, and power engineering. Its exceptional properties, such as high optical transparency, high electron mobility, chemical stability, and compatibility with low-cost fabrication techniques, have established ZnO as a versatile material with immense application potential. A critical application for ZnO is its role as a transparent conducting oxide (TCO) in modern optoelectronic and photovoltaic devices, as well as in sensors, transparent electronics, and spintronics. To meet the requirements of these advanced applications, precise control over the structural, optical, and electrical properties of ZnO thin films is essential. This is effectively achieved through the selection of specific synthesis methods and intentional modification techniques, such as doping. This review provides a comprehensive overview of the synthesis and modification of ZnO thin films, with a particular focus on how various dopants influence their fundamental characteristics. The work discusses a range of deposition techniques, including physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), sol–gel methods, spray pyrolysis, and other solution-based approaches. The novelty of this review lies in its comparative analysis of different doping strategies combined with various thin-film deposition techniques, highlighting how specific synthesis routes influence dopant incorporation and ultimately determine functional properties. Furthermore, recent advances in tailoring ZnO thin films are summarized, alongside the identification of key challenges and future research directions. Ultimately, this work aims to provide researchers with a systematic perspective on the synthesis–structure–property relationships in doped ZnO thin films to support the development of optimized materials for next-generation electronic and optoelectronic devices. This review, thus, serves as a comprehensive reference for researchers and engineers seeking to optimize the functionality of ZnO-based thin films for emerging technological applications.

Zinc oxide (ZnO) has been widely deployed as electron conducting layer in emerging photovoltaics including quantum dot, perovskite and organic solar cells. Reducing the curing temperature of ZnO layer to below 200 oC is an essential requirement to reduce the cell fabrication cost enabled by large-scale processes such as ink-jet printing, spin coating or roll-roll printing. Herein, we present a novel water-based ZnO precursor stabilized with labile NH3, which allow us to spin coat crystalline ZnO thin films with temperatures below 200 oC. Thin film transistors (TFTs) and diode-type quantum dot solar cells (QD SCs) were fabricated using ZnO as electron conduction layer. In the QD SCs, a p-type 1,2-ethylenedithiol treated PbS QDs with a bandgap of 1.4 eV was spin-coated on top of ZnO layer by a layer-by-layer solid state ligand exchange process. Electron mobility of ZnO was about 0.1 cm2V-1s-1 as determined from TFT measurements. Power conversion efficiency of solar cells: FTO/ZnO/PbS/Au-Ag was 3.0% under AM1.5 irradiation conditions. The possibility of deposition of ZnO at low temperatures demonstrated herein is of important for solution processed electronic and optoelectronic devices. 
 Keywords
 ZnO, low-temperature, quantum dots, solar cells, TFTs
 References
 [1] A. Janotti, A. Janotti, C.G. Van De Walle-fundamental of ZnO as a semiconductor, Reports on Progress in Physics, 72 (2009) 126501.[2] H. You, Y. Lin-investigation of the sol-gel method on the flexible ZnO device, International Journal of Electrochemical Science, 7 (2012) 9085–9094.[3] Y. Lin, C. Hsu, M. Tseng, J. Shyue, F. Tsai-stable and high-performance flexible ZnO thin-film transistors by atomic layer deposition, Applied Materials &Interfaces, 7(40) (2015) 22610–22617.[4] C. Lin, S. Tsai, M. Chang-Spontaneous growth by sol-gel process of low temperature ZnO as cathode buffer layer in flexible inverted organic solar cells, Organic Electronics, 46 (2017) 218-255.[5] H. Park, I. Ryu, J. Kim, S. Jeong, S. Yim, S. Jang-PbS quantum dot solar cells integrated with sol−gel-derived ZnO as an n‑type charge-selective layer, Journal of Physical Chemistry C, 118(2014) 17374−17382.[6] Y. Sun, J.H. Seo, C.J. Takacs, J. Seifter, A.J. Heeger-inverted polymer solar cells integrated with a low- temperature-annealed sol-gel-derived ZnO film as an electron transport layer Advanced Materials, 23(2011) 1679–1683.[7] V.A. Online, R. Suriano, C. Bianchi, M. Levi, S. Turri, G. Griffini-the role of sol-gel chemistry in low-temperature formation of ZnO buffer layers for polymer solar cells with improved performance, RSC Advances, 6(2016) 46915-46924.[8] X. D. Mai, J. An, H. Song, J. Jang-inverted Schottky quantum dot solar cells with enhanced carrier extraction and air-stability, Journal of Materials Chemistry A, 2 (2014) 20799–20805.[9] H. Choi, J. Lee, X.D. Mai, M.C. Beard, S.S. Yoon, S. Jeong - supersonically spray-coated colloidal quantum dot ink solar cells, Scientific Report, 7(2017) 622.[10] C.R. Newman, C.D. Frisbie, A. Demetrio, S. Filho, J. Bre- introduction to organic thin film transistors and design of n-channel organic semiconductors, Chemistry Materials, 16(2004) 4436-4451.[11] M. Asad, N. Abdul, Chapter 9: Sol-Gel-Derived Doped ZnO Thin Films: Processing, Properties, and Applications, in Recent Applications in Sol-Gel Synthesis, Edt:C. Usha. InTech, Rijeka, Croatia, 2017. [12] D. Guo, K. Sato, S. Hibino, T. Takeuchi, H. Bessho, K. Kato, Low-temperature preparation of (002)-oriented ZnO thin films by sol–gel method, Thin Solid Films, 550 (2014), 250-258. [13] S. T. Meyers, J. T. Anderson, C. M. Hung, J. Thompson, J. F. Wager, D. A. Keszler, Aqueous Inorganic Inks for Low-Temperature Fabrication of ZnO TFTs, J. Am. Chem. Soc, 130 (2008), 17603-17609.

Sputtered transparent electrodes for optoelectronic devices: Induced damage and mitigation strategies

Zinc oxide (ZnO), an attractive functional material having fascinating properties like large band gap (~3.37 eV), large exciton binding energy (~60 meV), high transparency, high thermal, mechanical and chemical stability, easy tailoring of structural, optical and electrical properties, has drawn a lot of attention for its optoelectronic applications including energy harvesting. Some of the promising applications are solar cells, ultraviolet light emitting diodes, photodiodes, ultraviolet lasers, high-temperature electronics, and spintronics devices. ZnO is a very versatile material vindicating itself with different access such as nanostructures, epitaxial structures, composite, and thin films. The ZnO nanostructures exist in various shapes and sizes including 0-D (nanoparticles), 1-D (nanowires, nanorods), 2-D (nanopetals, sheets), and 3-D (nanoflowers, tetrapods) structures with its tunable band gap energy, nature malleable behavior, and its potential application in optoelectronic devices. The ZnO being naturally an n-type inorganic semiconductor has been used in various types of solar cells such as conventional Si wafer solar cells, thin film solar cells, organic solar cells (OPVs), dye-sensitized solar cells (DSSCs), perovskite solar cells, hybrid solar cells (HSCs), and in several organic/inorganic as well as inorganic/inorganic heterojunction solar cell concepts. The ZnO acts as electron transport material, thereby it plays a major role in all the emerging third-generation PV devices. The ZnO thin films have manifold properties to make it interesting in photovoltaic applications. The ZnO thin film, owing to its easy synthesis and simple deposition techniques, reliability, cost effectiveness, non-toxicity, high stability, and good optoelectronic properties, has been studied extensively in several PV devices including the conventional silicon wafer-based solar cells as an antireflection and surface passivation layer. Here, a short review on ZnO nanostructures and thin films is presented in the perspective of their photovoltaic applications in different roles which include, as capping layer, electron selective layer, window layer, buffer layer, antireflection and passivation layer, as well as active layer for different types of solar cells. A brief overview of the synthesis methods of ZnO nanostructures and different deposition techniques of ZnO thin films via physical methods, cost-effective chemical routes and green methods is discussed. A brief discussion on the structural, optical, and electrical properties of the ZnO nanostructures and thin films is also included which are important for their PV applications. Finally, the chapter briefly outlines the different types of solar cells’ structures employing ZnO (nanostructures and thin films) in different roles, progress so far, their state-of-art-performance, and the challenges associated with different ZnO-based photovoltaic devices are critically discussed. At last, chapter closes with a summary including a remark indicating the future prospects of ZnO-based PV devices.

A low-cost spin coating technique and rapid thermal annealing method have been utilized for the preparation of aluminum doped zinc oxide with Si dots thin film on the quartz substrates compared with zinc oxide doped bismuth thin film with low-temperature annealing. The dispersed Si dots particle in the film showed that these additional Si dots can be tunable material for light absorption applied in photovoltaic work. X-ray diffraction was used to study the nanocrystalline silicon and zinc oxide with the crystal grain size of ~100 and ~20 nm presented in the films, respectively. The optical result of the thin films shows that <20% of reflectance and >80% transmittance were presented in the film. The absorption edge of thin films that have occupied Si dots was expanded from the original zinc oxide which strongly absorbs UV wavelength (~380 nm) to near-infrared wavelength (~800 nm) depending on the amount of Si dots layer. For the electrical properties of the film, the two-probe method was used to observe the photocurrent gain under the illumination compared with the dark test and showed that the film can greatly respond to the light and provided a higher current density. Additionally, the increase in annealing temperature and amount of zinc oxide layer leads to the enchantment of the current density. Hence, these zinc oxide thin films can be an alternative candidate for the tunable absorption emitter layer of solar cells and can be applied for other optoelectronic devices.

Structural, optical and morphological properties of pure and silver doped zinc oxide thin films by attuning the deposition layer

Many physical processes such as exciton interfacial dissociation, exciton interfacial recombination, and exciton-electron and exciton-hole interactions coexist at the interface of organic solar cells (OSC). In this study, the direction of free charge generation is defined as the direction from the interface to the side where free charges are left. For a p-n type device, the direction of free electron (hole) generation from exciton dissociation at the donor/accepter (D/A) interface is the same as the subsequent transportation direction under the built-in electric field. However, the direction of free electron (hole) generation from exciton-exciton recombination across the D/A interface is opposite to the direction of free charge transportation. Both free charges generated from exciton interfacial dissociation and recombination are contributed to the photocurrent for a p-n type device. In a device with a heterojunction formed by two n-type materials (here it is defined as an n-n type device), the direction of free electron (hole) generation from exciton recombination across the interface is also the same as the subsequent free charge transportation. At the same time, there are also some free electrons (free holes) generated by exciton interfacial dissociation. The direction of free charge generation from exciton dissociation for this n-n type device is also opposite to the direction of free charge transportation. However, only free charges generated from exciton interfacial recombination are contributed to the photocurrent for an n-n type device. But so far there has been no direct experimental evidence to prove the above theories. In this work, an NPB interfacial layer with a high LUMO was introduced in an n-n type OSC to inhibit the backflow of electrons, which are generated from exciton dissociation at the heterojunction formed by two n-type materials, enhancing the device performance accordingly. This work is conducive to interfacial engineering in an OSC to further improve its performance.

Several reports state that it is crucial to analyze nanoscale semiconductor materials and devices with potential benefits to meet the need for next-generation nanoelectronics, bio, and nanosensors. The progress in the electronics field is as significant now, with modern technology constantly evolving and a greater focus on more efficient robust optoelectronic applications. This dissertation focuses on the study and examination of the practicality of Electrophoretic Deposition (EPD) of zinc oxide (ZnO) nanoparticles (NPs) for use in semiconductor applications. The feasibility of several synthesized electrolytes, with and without surfactants and APTES surface functionalization, is discussed. The primary objective of this study is to demonstrate that the electrophoretic method for depositing ZnO NPs can also be used to produce ZnO films onto p-type silicon, functionalized p-type silicon, and aluminum substrates. This investigation uses ZnO NPs deposited at room temperature onto silicon, functionalized silicon, and aluminum substrates via EPD. The experimental work examines EPD solution formulations, EPD optimization for ZnO NP coverage, imaging of the surfaces, and electrical characterizations. Thin films produced were examined using Scanning Electron Microscopy (SEM), Raman Spectroscopy (RS), Ultraviolent Photoelectron Spectroscopy (UPS), and Atomic Force Microscopy (AFM), electrical impedance, and current-voltage (I-V) measurements. The results obtained are viewed in the context of providing valuable information in the ongoing search for reproducible and robust yet economical means of NP and thin-film deposition. This work fabricates a proof-of-concept pn junction composed of n-type ZnO NPs electrophoretically deposited onto p-type Si. This investigation presents a potential opportunity for integrating this deposition method into applications where ZnO contributes to the reliability, affordability, and highly increased sensitivity needed for the next generation of nanoscale devices and systems. The EPD of ZnO nanoscale thin films is important to several research areas, including biosensors, photophilic dye-sensitized solar cells, optoelectronic devices, and thin-film transistors. ZnO NPs have recently attracted attention due to their excellent optoelectronic performance and low cost of production [1-11]. Combining EPD with ZnO NPs will result in a more accessible technique of nanomaterial deposition, research tools for thin films and nanostructures, and improved materials for next-generation electronics

Degenerately-doped semiconductor nanocrystals exhibiting tunable localized surface plasmon resonance (LSPR) have attracted significant attention in recent years due to their unique optoelectronic properties. Unlike noble metal nanoparticles, colloidal plasmonic semiconductor nanocrystals have LSPR frequencies tunable in the infrared region, which makes them appealing for terahertz imaging, heat-responsive devices, and surface-enhanced infrared spectroscopic measurements. Besides expanding the LSPR frequency range, plasmonic semiconductor nanocrystals could potentially bring about numerous other opportunities related to single-phase plasmon-exciton interactions. However, non-resonant nature of the LSPR and exciton in semiconductors has been a major obstacle toward realizing such opportunities. In this talk I will discuss the results of our recent work on the structure and composition dependent plasmonic properties of colloidal semiconductor nanostructures. I will particularly focus on generating robust excitonic splitting in degenerately-doped transparent metal oxide nanocrystals, enabled by non-resonant plasmon-exciton coupling in an external magnetic field. This phenomenon allows for controlling carrier polarization in semiconductor nanocrystals using circularly polarized light. Furthermore, the ability to control carrier localization and nanocrystal morphology allows for further manipulation of the excitonic splitting pattern and quantum states in these non-magnetic semiconductor nanostructures. Possible applications of this emerging class of multifunctional materials for new electronic and quantum information technologies will also be discussed.

Thin film hetero-structures (TFHSs) involving metal oxide thin films and noble metal nanoparticles are very important for many optoelectronics based device applications. This work reports the growth, characterization, and tuning of photoluminescence and I–V properties of TFHSs involving zinc oxide (ZnO) and gold nanoparticles (GNPs). ZnO thin films and GNPs were respectively deposited by the Pulsed Laser Deposition (PLD) and DC sputtering with subsequent annealing. Three different TFHSs were prepared by varying the relative positions of ZnO and GNPs, namely Si-GNPs-ZnO, Si-ZnO-GNPs, and Si-ZnO-GNPs-ZnO. X-ray diffraction results confirmed the high crystallinity of the films, with single phase nature of the ZnO and GNPs. Scanning electron microscopy micrograph analysis confirmed that the morphology of structures containing both GNPs and ZnO is influenced by the bottom layer. Diffuse reflectance spectroscopy results also indicated that the position of GNPs relative to ZnO affects the plasmon resonance of GNPs as well as the overall optical properties of the TFHSs. Photoluminescence studies revealed that the presence of GNPs affects the defect concentration in the TFHSs. The I–V characteristics showed that the TFHSs where ZnO contains GNPs in embedded form are better suited for photodiode application. This study adds a new dimension to the research on optoelectronics devices.

Zinc oxide (ZnO) is a wide bandgap semiconducting oxide with many potential applications in optoelectronic devices such as light emitting diodes (LEDs) and field effect transistors (FETs). Controlling the conductivity of ZnO is a foremost concern in the optoelectronic industry. Understanding the nature of ZnO native defects is vital for the application of ZnO in the semiconductor industry. Many electronic properties depend significantly on the intrinsic defect concentration as it affects the doping efficiency and the growth of the material. These properties also depend on the type and amount of impurities and degree of crystallinity. There is a need to understand the function of native point defects and the incorporation of impurities, with the intention for a better control of the conductivity of ZnO. However, one major issue in determining whether it is zinc or oxygen deficiency that provides ZnO its unique properties remains. In this thesis, ZnO films and powders were synthesized via a low temperature aqueous solution chemical bath deposition route and its short to medium range structure order characterized. In addition, Ga-doped ZnO films and powders were also synthesized as Ga has been found to be an excellent donor, increasing the conductivity of ZnO films. All samples were post-growth thermally annealed in air in order to improve the electrical and optical properties, and correlate the annealing temperature with the defect structure. The epitaxial nature of the ZnO films (undoped and Ga-doped) were determined via high resolution X-ray diffraction (HRXRD). The morphology of the films was studied via field emission gun scanning electron microscopy (FEGSEM) showed that the films were smooth and continuous on the plane-view. Cross-sectional images showed that the films were about 2 µm thick after a growth time of 4 hours at 90°C. Visible pores were also observed in the films after post-growth thermal annealing at temperatures of above 200°C. Electrical properties were obtained via Hall measurements showed characteristic n-type conductivity and a carrier concentration of the order of 1020 cm-3 after post-growth thermal annealing at 400°C in air for 1 h for the Ga-doped ZnO films. For the ZnO powders, phase identification was determined by conventional powder XRD methods. The morphologies were obtained via FEGSEM and showed different nanorods morphologies for the different precursors used for the syntheses. Internal pores were observed in the ZnO nanorods upon thermal annealing at >300 °C and were examined using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). Three-dimensional electron tomography reconstruction shows that pores exist within the nanorods. These pores coalesce and also increase in size as the ZnO nanorods were annealed at higher temperatures or for extended periods of time, as observed by an in situ TEM study. As such, this pore formation phenomenon will significantly affect the utilisation of solution-synthesized ZnO for optoelectronic devices, but new application fronts such as gas sensors and photo/electro catalysis will benefit due to its high light absorption properties and high surface areas. Further defect characterization was performed for the all the ZnO samples, largely using synchrotron-based characterization techniques, namely X-ray absorption spectroscopy (XAS) and X-ray pair distribution function (PDF) techniques. XAS is an ideal, atom specific characterization technique that is able to probe defect structure in many materials, including ZnO. Comparative studies of commercially available ZnO and aqueous solution grown (≤ 90°C) ZnO powders using XAS and PDF has allowed for the study of short to medium range structure order. The ZnO films were studied via ex situ XAS and the ZnO powder samples were studied via ex situ and in situ XAS and in situ PDF experiments. The local structure order of the ZnO films and powders were determined. From XAS of the Ga-doped ZnO samples, it was evidently shown that Ga3+ resides tetrahedrally in the Zn2+ site within the ZnO lattice. The understanding of the role of defects and the short to medium range order and structure within the ZnO system has provided useful insight for further exploitation of the material and is invaluable for the exploitation of ZnO for further development of optoelectronic device applications.

Development of a novel high optical quality ZnO thin films by PLD for III–V opto-electronic devices

Zinc oxide (ZnO) nanoparticles is an attractive candidate and gain attention for their novel properties and promising applications such as optoelectronic devices by virtue of their high surface to volume ratio and molecular absorption and desorption characteristics. Ultraviolet light emmiters, gas sensor (particularly for hydrogen) etc. are some of the potential applications of this wide direct band gap (Eg ~ 3.37 eV) semiconductor. Currently, there are considerable amount of research attention on the growth of p-type ZnO thin film and nanoparticles. Thus, in this paper we will discuss recent development in the growth of p-type and ZnO thin film in nanoparticles by magnetron sputtering. There are several important parameters (e.g. RF power/DC voltage, working pressure, substrates temperature and target to substrates distance) that we have to consider in order to attain small grain size (<100 nm). Grain size that are peculiarly small can be achieved by reducing kinetic energy of the sputtered material and lowering the growth rates either by increasing the working pressure or reducing RF power/ DC voltage. The temperature of the substrate is also believed to have some influence in slowing the kinetic energy of the sputtered atoms. For ZnO, n- type conductivity is easy to realize via excess zinc or with trivalent dopants such as Aluminium (Al), Gallium (Ga) or Indium (In) by substituting Zn2+ ions with Al3+ ions. Meanwhile, p-type doping has recently been achieved by doping with the group V elements. Most of the attempts have employed Nitrogen (N) as the acceptor to substitute Oxygen (O) by introducing either Nitrous Oxide (N2O), Nitric Oxide (NO), Ammonia (NH3) or Nitrogen (N2) gas. Other elements of group V are Phosphorus (P), Arsenic (As) and Antimony (Sb). Recently, Phosphorus oxide (P2O5) has been doped in ZnO thin film and post deposition of rapid thermal annealing (RTA) has been used to activate the dopants. Furthermore, a new method called codoping has been proposed to produce p-type ZnO, which was achieved using acceptors and reactive donors simultaneously to increase the solubility of Nitrogen in ZnO.

Zinc oxide (ZnO) is a wide band gap (~3.37 eV) semiconductor. Thin film ZnO has many attractive applications in optoelectronics and sensors. Recently, nanostructured ZnO (e.g. ZnO quantum dot) has been demonstrated as a hyperbolic material; its dielectric function has opposite signs along different crystal axes within the mid-infrared, making it an interesting material for metamaterials and nanophotonics. Conventional sputtering deposition usually leads to the formation of polycrystalline ZnO films with randomly oriented grains and rough surface. This work demonstrated a solution-based process to grow ZnO thin films with highly oriented nanocrystals. Low-temperature plasmas were employed to modulate the microstructure and optical properties of the films. Such highly anisotropic nanostructured transparent semiconductor films may lead to interesting material properties in developing new optoelectronic devices.