Mtech Thesis New(ZnO)

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Chapter 1Introduction


1. IntroductionThis report provides a comprehensive review of the current research activities that focus on the ZnO based materials and their physical property and Characterizations. It begins with the different methods that have been exploited to grow ZnO thin films. A range of remarkable characteristics are then presented, organized into sections describing the electrical, magnetic and chemical sensing properties. These studies constitute the basis for developing versatile applications of ZnO nanostructures. The attraction can simply be attributed to the large exciton binding energy of 60 meV of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the physical, chemical and electrical properties of ZnO in addition to the technological issues such as growth. ZnO is not really a newly discovered material. Research on ZnO has continued for many decades with interest following a roller-coaster pattern. Interest in this material at the time of this writing is again at a high point. ZnO is easily etched in all acids and alkalis, and this provides an opportunity for fabrication of small-size devices. In addition, ZnO has the same crystal structure. In this paper we collate the properties of ZnO as well as review the recent progress in ZnO research. This present review is distinguishable from the other reviews which is focused mainly on material processing, doping, and transport properties. In recent years there has been an extensive research towards introducing ferromagnetic property at room temperature in semiconductors to realize a new class of spintronic devices such as spin valves, transistors, spins light emitting diodes, magnetic sensors, non-volatile memory, logic devices, optical isolators and ultra-fast optical switches. The potential advantages of spintronic devices will be higher speed, greater efficiency, and better2

stability, in addition to the low energy required to flip a spin. A ZnO based DMS would be very promising because of its widespread applications in electronic devices, such as transparent conductors, gas sensors, varistors, surface acoustic wave devices, optical wave guides, acousto-optic modulators/deflectors, ultra violet laser source, and detectors. Despite uncertainty in the mechanism of ferromagnetism in doped semiconductors, and the fact that the obtained magnetization is lower than the theoretically predicted value in most of the reports appearing in literature, the results reported thus far, provide a pathway for exploring the transition metal doped DMS. It is however, imperative to understand the phenomenon and the factors affecting the magnetization value in order to realize commercially applicable devices.

The main aim of the present thesis is to show that it is indeed possible to obtain room temperature ferromagnetic semiconductors by controlling the process parameters. Theoretical prediction of room temperature ferromagnetism in transition metal doped ZnO could be realized experimentally. It is shown that the properties of precursors used for making of DMS have a great influence on the final properties of the material. Use of various experimental techniques to verify the physical properties, and to understand the mechanism is demonstrated. Methods to improve the magnetic moment are also described.


ORGANIZATION OF THESISAfter giving the brief introduction about the spintronic devices, now work has been focused on ZnO thin film based samples deposited by CWD (Chemically Wet and Dry) technique. ZnO based semiconductor films are deposited on to the glass, silver, copper, aluminum substrates using CWD technique. The deposited films are characterized using X-Ray Diffraction (XRD), Four-probe method, Hotprobe method, and Hall Effect. The remaining part of the thesis is organized in the following manner:

Chapter-2:- Literature review. Chapter-3:- Spintronic devices and semiconductors materials. Chapter-4:- Experimental works. Chapter-5:- Results and analysis. Chapter-6:- Conclusion and scope for further work.


Chapter-2Literature review


Literature review has been carried out in view of my project work. The project is focused on ZnO and transition metal doped ZnO thin film based samples deposited by CWD (Chemically Wet and Dry) technique on different types of conducting and non conducting substrates. In the following sections review of literature has been reported. Dr. E. SENTHIL KUMAR who got his doctor of philosophy from IIT, Madras has out lined a thisis about ZnO based thin films, nanostructures and hetro structures for optoelectronics and spintronic applications in the year 2010 has followed in this report. Dr. Wei Guo and got his PHD from University of Michigan has presented a work over Epitaxial growth and properties of zinc oxide thin films on silicon substrates in the year 2010 were studied. Transition metal implanted ZnO:a correlation between structure and magnetism is a report which was presented by Doctor Shengqiang Zhou, Doctor rerum naturalium , Prof. Dr. Manfred Helm. Prof. along with oter group members of Institute of structural physics and Material Science Dresden-Rossendorf in the year 2007 . Dr Ngwashi Divine Khan who has got his doctorate degree from De Montfort University has a deep investigation of the Performance and Stability of Zinc Oxide Thin-film Transistors and the Role of High-k Dielectrics in the year 2010 which has a very important application in this field Fe implanted ferromagnetic ZnO. which was published in Appl. Phys. Lett., 88:052508, 2006 by the authors K. Potzger, S. Q. Zhou, H. Reuther, A. Mcklich, F. Eichhorn and co. Microstructure and electronic structure of transparent ferromagnetic ZnO-Spinel iron oxide composite films was published Chem. Mater., 18:763770, 2006 by T. Shinagawa, M. Izaki, H. Inui, K. Murase etc. and carried out a results about the ZnO thin films magnetic properties.


Chapter-3Spintronic devices and semiconductor materials.


3.1 Spintronic devices

Spintronic devices came into action after the discovery of powerful effect called Giant magneto resistance (GMR) in 1988 by French and German physicists . It results from subtle electron-spin effects in ultra-thin 'multilayer' of magnetic materials, which cause huge changes in their electrical resistance when a magnetic field is applied. It is a sandwich structure with alternating layers of magnetic and nonmagnetic metal (Fig-1). Depending upon the relative orientations of the magnetizations in the magnetic layers, the electrical resistance changes from small (parallel magnetizations) to large (antiparallel magnetizations). The magnitude of this change is two order of magnitude larger than is possible with conventional materials, hence the name giant magneto resistance.

Fig-1 A GMR sandwich structure. Consisting of alternating magnetic and nonmagnetic metal layers.

Physicists have been quick to see the further possibilities of spin valves. Not only they are highly sensitive magnetic field sensors, they can also be made to act as switches by flipping the magnetization in one of the layers. This allows information to be stored as 0s and 1s8

(magnetizations of the layers parallel or antiparallel) conventional transistor memory device.

as in a

MRAM is rapidly developing as a technology because it can allow quantum computing by use of spin of individual particles to process information. Such an information bearing particle is known as a quantum bit or qubit. It is amazing to know that only 34 qubits are required to represent the total amount of information stored in a 10 GB hardrive. With the exception of MRAM, none of the spintronic devices such as spin based light emitting diodes (spin LEDs), resonant tunneling diodes (spin RTDs), field effect transistors (spin FETs), and spin based single electron devices based on quantum dot arrays can be fabricated without the ability to generate, maintain, and propagate long lived spins in a semiconductor.

All spintronic or more specifically semiconductor spintronic devices act according to the simple scheme: (1) information is stored (written) into spins as a particular spin orientation (up or down), (2) the spins, being attached to mobile electrons, carry the information along a wire, and (3) the information is read at a terminal . The basic idea behind semiconductor based spintronics is to add the characteristics of magnetic devices to existing devices such as light-emitting diodes and field effect transistors. This would lead to technologies such as memory and microprocessor functions integrated on the same chip, magnetic devices with gain and integrated sensors with on-chip signal processing and off-chip optical communications. A technology tree which summarizes the spin-based devices is shown is shown in Fig.2.


Fig. 2 a technology tree for Spin based device


3.2. Ferromagnetic semiconductors 3.2.1. Rare-earth chalcogenidesSince the 1960s rare-earth chalcogenides (e.g. EuO [16]) were found to possess both ferromagnetic and semiconducting properties. In these magnetic semiconductors, all magnetic atoms are periodically arranged in the crystal lattice as shown in Figure 2.1(a). Such ferromagnetic semiconductors show low transition temperatures usually not exceeding 70 K, i.e. far below room temperature. Moreover, the crystal structure of the rare-earth chalcogenides is quite different from that of technologically relevant semiconductors such as GaAs or Si, therefore these materials are rat