Metal Oxide Nanowires as Building Blocks for Optoelectronic Devices

*Andreea Costas, Nicoleta Preda, Camelia Florica and Ionut Enculescu*

## **Abstract**

Metal oxide nanowires have become the new building blocks for the next generation optoelectronic devices due to their specific features such as quantum confinement and high aspect ratio. Thus, they can be integrated as active components in diodes, field effect transistors, photodetectors, sensors, solar cells and so on. ZnO, a n-type semiconductor with a direct wide band gap (3.3 eV) and CuO, a p-type semiconductor with a narrow band gap (1.2–1.5 eV), are two metal oxides which were recently in the spotlight of the researchers for applications in the optoelectronic devices area. Therefore, in this chapter we focused on ZnO and CuO nanowires, the metal oxides nanowire arrays being prepared by straightforward wet and dry methods. Further, in order to emphasize their intrinsic transport properties, lithographic and thin films deposition techniques were used to integrate single ZnO and CuO nanowires into diodes and field effect transistors.

**Keywords:** metal oxide nanowire arrays, single ZnO and CuO nanowires, lithographic techniques, diodes, field effect transistors

#### **1. Introduction**

Over the last decades, metal oxide nanowires, one dimensional nanostructures characterized by a high aspect ratio [1], have gained a special interest owed among others to their large specific area given by the size effects. This feature is responsible for their high sensitivity that is very important in a wide range of applications in optoelectronics [2], electrochemical sensors [3], spintronics [4], photocatalysis [5], noninvasive medical diagnosis [6], drug delivery [7], etc. Thus, due to their high sensitivity, metal oxide nanowires can detect even a single molecule, or even mechanical, optical or electrical signals [8–10]. For example, the size of biological molecules, such as proteins and nucleic acids, is comparable to the size of nanostructures, therefore any interaction between these molecules should induce major changes in the properties of the nanowires. Consequently, the metal oxide nanowires can be regarded as the perfect candidates for integration as single components in diodes [11], field effect transistors [12, 13], advanced biosensors [14], photodetectors [15], light emitting diodes [16], solar cells [17], magnetoresistive sensors [18], etc.

The preparation methods represent the key factor in order to obtain metal oxide nanowires with tunable dimensions and tailored physico-chemical properties. To date, many preparation approaches were used for preparing arrays of metal oxide nanowires, such as template electrodeposition [19], electroless deposition [20], sol–gel [21], chemical bath deposition [22], hydrothermal growth [23], pulsed laser deposition [24], chemical vapor deposition [25], atomic layer deposition [26], lithographic techniques [27], etc. Among them, chemical synthesis carried out in water as a reaction medium is a simple wet preparation route, easy to process and suitable for large-scale synthesis of metal oxide powders consisting in micro- and nano-structures with different morphologies featured by a good crystallinity [13, 28, 29]. Another preparation method, thermal oxidation in air is a relatively facile dry, low-cost, nonhazardous and high throughput approach that can be used at a large-scale to obtain metal oxides nanostructures of a high purity and crystallinity [13]. Using thermal oxidation in air to obtain arrays of metal oxide nanowires, the nanowires length, diameter and density can be easily controlled by modifying the parameters involved in the thermal oxidation in air process, such as: the heating rate, the annealing temperature and the time of the treatment [30].

Zinc oxide (ZnO) is an interesting eco-friendly and versatile metal oxide, suitable for many applications due to its remarkable physico-chemical properties. Being an n-type semiconductor with a direct band gap (3.37 eV), with an excitonic binding energy of 60 meV, it can be easily integrated in optoelectronic devices such as photodetectors [15], light emitting diodes [31], solar cells [32]. Additionally, its flexibility in terms of nanostructure morphology (nanowires, nanotubes, nanofibers, nanorods, nanoneedles, hexagonal nanoprisms, nanoflowers, rings, etc.) [33, 34] offers another important advantage for applications in sensors [35], photocatalysis [36], as well as in surfaces with self-cleaning properties [37].

Copper oxide (CuO) is a p-type semiconductor easy to prepare, with a high stability, having an indirect narrow band gap (1.2–1.8 eV). This metal oxide can be implemented in various applications such as: solar cells [38], field effect transistor [11], gas sensors [39], photocatalysis [40], water purification [41], etc. Being also an antiferromagnetic material below 220 K with a local magnetic moment of about 0.6 μB, CuO was also investigated for application in magnetic storage units [42].

In this chapter, we present our research regarding the preparation and complex characterization of metal oxide nanowire arrays by wet (chemical synthesis in aqueous solution) and dry (thermal oxidation in air) approaches. In addition, electronic devices based on single metal oxide nanowires were developed and analyzed in terms of electrical characterization. Further, lithographic techniques such as photolithography, electron beam lithography and focused ion beam induced deposition, combined with radio-frequency magnetron sputtering and thermal vacuum evaporation were used for fabricating electronic devices like diodes and field effect transistors based on single metal oxide (ZnO or CuO) nanowires. In addition, the electrical properties of the electronic devices based on single metal oxide nanowires prepared by wet and dry methods were analyzed and discussed.

#### **2. Lithographic techniques**

Lithographic techniques are mainly used in modern semiconductor manufacturing industry, in micro- and nano-fabrication to pattern thin films with specific geometries integrated into electronic devices. Lithographic techniques are subsequently divided according to the targeted application into: photolithography, electron beam lithography, focused ion beam induced deposition, extreme UV lithography, nanoimprint lithography, colloidal lithography, soft lithography, and

**27**

implanted with Ga+

are obtained.

*Metal Oxide Nanowires as Building Blocks for Optoelectronic Devices*

many others. In the following, the techniques involved in the fabrication of our electronic devices based on single metal oxide nanowires are briefly described.

Photolithography is a conventional lithographic process in which specific geometric shapes drawn on a photomask are transferred to the desired substrate by means of light in the UV range, using a lamp that emits UV light of a certain wavelength and a polymer that has a photoactive component sensitive to the UV light named photoresist. In this case, the resolution is limited by the wavelength used and by the type of the aligner. In our case, the photolithography was involved in the fabrication of Ti/Au metallic interdigitated electrodes onto Si/SiO2 wafers. The main steps of the process being: cleaning of the Si/SiO2 wafers, deposition of the primer and the photoresist by centrifugation, baking of the deposited layer, the mask alignment, the UV exposure, baking after UV irradiation, the UV exposure through clear mask, the development process and the deposition of Ti and Au thin films using radio-frequency (RF) magnetron sputtering and thermal vacuum

Electron beam lithography (EBL) is a lithographic technique that uses a highvoltage accelerated electron beam to directly pattern a substrate without the need of a mask. In this process, the electron beam irradiates a thin layer of electron-sensitive polymer which was previously deposited on the substrate. During the exposure, the polymer (electron beam resist) bonds break and thus after the lift-off process, the appropriate geometric configuration is obtained. This technique has a high resolution, that can be used in nanoscale electronics, optoelectronics and photonics. In our case, in order to contact single nanowires using EBL, the first step is to transfer the nanowires between the metallic interdigitated electrodes. Further, a layer of electron beam resist –polymethyl methacrylate (PMMA) is deposited on the sample. Then, the desired contacts are designed into a CAD program and exposed with the electron beam. After the development process, the Ti and Au or Pt thin films are deposited by RF magnetron sputtering and thermal vacuum evaporation, respectively. Finally, the lift-off process removes the excess metal and the Ti/Au contacts

Focused ion beam induced deposition (FIBID) is a lithographic technique that

organometallic precursor gas to deposit a metallic thin film without the need of a mask onto a substrate. In our case, similar to the EBL lithographic process, to contact single metal oxide nanowires by FIBID, the first step is to place the nanowires between the metallic interdigitated electrodes. Afterwards, the future Pt contacts are designed into a CAD program. During the deposition of the Pt contacts, an injector needle is placed very near to the substrate and upon the interaction between the organometallic compound with the ion beam, the precursor molecules are decomposed into a platinum layer and a volatile organic compound exhausted into the vacuum system. FIBID deposition is limited by the organometallic precursor gas and by the delivery rate of the gas. The deposition of a Pt contact by FIBID leads to the deposition of a carbon amorphous matrix that incorporates Pt nanoparticles

), a gas injection system and an

*DOI: http://dx.doi.org/10.5772/intechopen.94011*

**2.1 Photolithography**

evaporation, respectively.

**2.2 Electron beam lithography**

**2.3 Focused ion beam induced deposition**

ions.

uses a highly focused ion beam of gallium ions (Ga+

many others. In the following, the techniques involved in the fabrication of our electronic devices based on single metal oxide nanowires are briefly described.
