Growth and Characterization of ZnO based Heterojunction diodes and ZnO Nanostructuresby Pulsed Laser Ablation

Transparent conducting oxides (TCO’s) have been known and used for technologically important applications for more than 50 years. The oxide materials such as In2O3, SnO2 and impurity doped SnO2: Sb, SnO2: F and In2O3: Sn (indium tin oxide) were primarily used as TCO’s. Indium based oxides had been widely used as TCO’s for the past few decades. But the current increase in the cost of indium and scarcity of this material created the difficulty in obtaining low cost TCO’s. Hence the search for alternative TCO material has been a topic of active research for the last few decades. This resulted in the development of various binary and ternary compounds. But the advantages of using binary oxides are the easiness to control the composition and deposition parameters. ZnO has been identified as the one of the promising candidate for transparent electronic applications owing to its exciting optoelectronic properties. Some optoelectronics applications of ZnO overlap with that of GaN, another wide band gap semiconductor which is widely used for the production of green, blue-violet and white light emitting devices. However ZnO has some advantages over GaN among which are the availability of fairly high quality ZnO bulk single crystals and large excitonic binding energy. ZnO also has much simpler crystal-growth technology, resulting in a potentially lower cost for ZnO based devices. Most of the TCO’s are n-type semiconductors and are utilized as transparent electrodes in variety of commercial applications such as photovoltaics, electrochromic windows, flat panel displays. TCO’s provide a great potential for realizing diverse range of active functions, novel functions can be integrated into the materials according to the requirement. However the application of TCO’s has been restricted to transparent electrodes, ii notwithstanding the fact that TCO’s are n-type semiconductors. The basic reason is the lack of p-type TCO, many of the active functions in semiconductor originate from the nature of pn-junction. In 1997, H. Kawazoe et al reported the CuAlO2 as the first p-type TCO along with the chemical design concept for the exploration of other p-type TCO’s. This has led to the fabrication of all transparent diode and transistors. Fabrication of nanostructures of TCO has been a focus of an ever-increasing number of researchers world wide, mainly due to their unique optical and electronic properties which makes them ideal for a wide spectrum of applications ranging from flexible displays, quantum well lasers to in vivo biological imaging and therapeutic agents. ZnO is a highly multifunctional material system with highly promising application potential for UV light emitting diodes, diode lasers, sensors, etc. ZnO nanocrystals and nanorods doped with transition metal impurities have also attracted great interest, recently, for their spin-electronic applications This thesis summarizes the results on the growth and characterization of ZnO based diodes and nanostructures by pulsed laser ablation. Various ZnO based heterojunction diodes have been fabricated using pulsed laser deposition (PLD) and their electrical characteristics were interpreted using existing models. Pulsed laser ablation has been employed to fabricate ZnO quantum dots, ZnO nanorods and ZnMgO/ZnO multiple quantum well structures with the aim of studying the luminescent properties.

Fast imaging of laser-blow-off plume:Lateral confinement in ambient environment

The dynamics of plasma plume, formed by the laser-blow-off of multicomponent LiF-C thin film under various ambient pressures ranging from high vacuum to argon pressure of 3 Torr, has been studied using fast imaging technique. In vacuum, the plume has ellipsoidal shape. With the increase in the ambient pressure, sharp plume boundary is developed showing a focusing-like confinement in the lateral space behavior in the front end, which persists for long times. At higher ambient pressure (> 10−1 Torr ), structures are developed in the plasma plume due to hydrodynamic instability/turbulences.

Amorphous Oxide Transparent Thin Films: Growth, Characterisation and Application to Thin Film Transistors

This work mainly concentrate to understand the optical and electrical properties of amorphous zinc tin oxide and amorphous zinc indium tin oxide thin films for TFT applications. Amorphous materials are promising in achieving better device performance on temperature sensitive substrates compared to polycrystalline materials. Most of these amorphous oxides are multicomponent and as such there exists the need for an optimized chemical composition. For this we have to make individual targets with required chemical composition to use it in conventional thin film deposition techniques like PLD and sputtering. Instead, if we use separate targets for each of the cationic element and if separately control the power during the simultaneous sputtering process, then we can change the chemical composition by simply adjusting the sputtering power. This is what is done in co-sputtering technique. Eventhough there had some reports about thin film deposition using this technique, there was no reports about the use of this technique in TFT fabrication until very recent time. Hence in this work, co-sputtering has performed as a major technique for thin film deposition and TFT fabrication. PLD were also performed as it is a relatively new technique and allows the use high oxygen pressure during deposition. This helps to control the carrier density in the channel and also favours the smooth film surface. Both these properties are crucial in TFT.Zinc tin oxide material is interesting in the sense that it does not contain costly indium. Eventhough some works were already reported in ZTO based TFTs, there was no systematic study about ZTO thin film's various optoelectronic properties from a TFT manufacturing perspective. Attempts have made to analyse the ZTO films prepared by PLD and co-sputtering. As more type of cations present in the film, chances are high to form an amorphous phase. Zinc indium tin oxide is studied as a multicomponent oxide material suitable for TFT fabrication.

Metastable States and Irregular Behaviour in a Josephson Junction with Nonlinear Resistance

We consider a resistively shunted Josephson junction with a resistance that depends inversely on voltage. It is shown that such a junction in the underdamped case can give rise to extremely long-lived metastable states even in the absence of external noise. We investigate numerically this metastable state and its transition to a chaotic state. The junction voltages corresponding to these states are studied.

Creation and Annihilation of Fluxons in ac‐driven Semi-annular Josephson Junction

A new geometry (semiannular) for Josephson junction has been proposed and theoretical studies have shown that the new geometry is useful for electronic applications [1, 2]. In this work we study the voltage‐current response of the junction with a periodic modulation. The fluxon experiences an oscillating potential in the presence of the ac‐bias which increases the depinning current value. We show that in a system with periodic boundary conditions, average progressive motion of fluxon commences after the amplitude of the ac drive exceeds a certain threshold value. The analytic studies are justified by simulating the equation using finite‐difference method. We observe creation and annihilation of fluxons in semiannular Josephson junction with an ac‐bias in the presence of an external magnetic field.