tion band derived electrons. Achieving high performance p-type oxide TFTswilldefinitelypromoteaneweraforelectronicsinrigidandflexible substrate away

Transcription

1 Preface Thin film transistor (TFT) is a combination of thin films necessary to create the function of a transistor. It consists of a thin film of a semiconducting material which forms the conducting channel between metallic source and drain electrodes. There is a dielectric layer between the semiconducting material and gate electrode. The most important application of TFT is in liquid crystal displays (LCD) wherein TFTs are integrated to each of the sub pixels to modulate the amount of light reaching observers eyes. In addition to this, TFTs are widely used in medical imaging equipments, organic light emitting diode displays, flat panel displays, field emission displays, imagers, scanners and various types of sensors etc. Presently, all these applications make use of n channel oxide TFTs. The enormous success of n type oxide semiconductors and its applications in the field of TFTs motivated the interest in p-type semiconductorbased TFTs. But still there is no report on p-type oxide TFTs with performance similar to that of n-type oxide TFTs. p-type TFTs are generally limited by the low hole mobilities since the mobility of valance band derived carriers generally lower than the conducvii

2 tion band derived electrons. Achieving high performance p-type oxide TFTswilldefinitelypromoteaneweraforelectronicsinrigidandflexible substrate away from silicon. Moreover it will shape the electronics of tomorrow by allowing the production of complementary metal oxide semiconductors, a key device to fuel the microelectronics revolution in the so called technologies of information and communication. Scientists had been looking p-type material for the application of TFTs. The simple binary oxides like SnO have also been explored as a promising p-type semiconductor for TFT fabrication. In the present study a systematic synthesis of tin oxide thin films using RF magnetron sputtering was carried out. and of Recently much attention has been given to Cu based semiconductors (Cu 2 O, CuAlO 2, CuCrO 2, CuBO 2 etc.). Recently,afewgroupreportedp-typeoxidesemiconductor based TFTs using the CuO and Cu 2 O as active channel materials. The maximum mobility obtained for Cu 2 O channel TFT fabricated using high temperature PLD technique was 4.3 cm 2 V 1 s 1. Sung et al fabricated TFT on p-type silicon substrate using post annealed CuO channel layer with field effect mobility 0.4 cm 2 V 1 s 1 and on off ratio 1.1 x In the present study we have tried room temperature deposited CuO and Cu 2 O channel layer for the fabrication of TFTs. Co-sputtering is an effective technique for a multicomponent film to control the film chemical composition in a systematic and easy way. We have used co sputtering technique to develop p type copper boron oxide and copper chromium oxide thin films. The optoelectronic properties of amorphous and transparent Cu 1 x B x O 2 δ and Cu 1 x Cr x O 2 δ films examined as a function of the chemical composition. The room temperature deposited copper chromium oxide channel layer was also employed for the fabrication TFTs.

3 Chapter 1 presents a brief description about the fundamentals of amorphous semiconductors, challenges in p-type oxide semiconductors and a brief review of oxide thin film transistors. The optical absorption and electronic transport in amorphous materials are discussed. This chapter also provide the basic working hypothesis of transparent amorphous oxide semiconductors and their electronic structure. The literature review on thin film transistors using transparent oxide semiconductor material channel is given in the last part of this chapter. Chapter 2 describes in detail the thin film growth techniques and characterization tools employed in the present work. Thin film growth was accomplished using pulsed laser deposition(pld) and co-sputtering techniques. In this chapter a brief description of the basic mechanism of RF magnetron sputtering is given. Thickness of the films was measured using stylus profilometer. Structural characterization was performed by glancing angle x-ray diffraction method and atomic force microscopy (AFM). Compositional analysis was carried out by x-ray photo electron spectroscopy (XPS). Band gap of the grown thin films were estimated by optical transmittance studies using a UV-Vis- NIR spectrophotometer. The electrical characterisation of the films were carried out at room temperature by a Hall effect measurement system. The basic operation and specifications of all these characterisation tools are described in this chapter. A detailed description about TFTs is given towards the end of this chapter. This section includes thin film transistor structures, the basic device operation, modes of operation, dielectrics, amorphous oxide channels, TFT characterisation etc. Chapter 3 deals with the room temperature deposition and characterization of tin oxide thin films using RF magnetron sputtering. p-

4 Type and n-type tin oxide thin films were deposited by RF magnetron sputtering at different oxygen partial pressure. Structural, optical and electrical properties of these films were studied as a function of oxygen partial pressure and annealing temperature. Amorphous structure of as deposited SnO x films and polycrystalline tetragonal structure of annealed SnO films were confirmed by GXRD. Surface morphology of synthesized samples was characterised by atomic force microscopy. Atomic percentage of the SnO thin film according to the intensity of the XPS peak was about 48.85% and 51.15% for tin and oxygen respectively. Band gap of as deposited SnO x thin films was varying from 1.6 ev to 3.2 ev as the oxygen partial pressure varies from 3-30% which indicates the oxidization of metallic phase tin to SnO and SnO 2. Type of the charge carrier and conductivity of the tin oxide thin films were varied with oxygen partial pressure. Transparent pn hetero junction fabricated in the structure glass/ito/n-zno/p-sno show rectifying behaviour. Chapter 4 describes the deposition of copper oxide thin films using RF magnetron sputtering. This chapter is divided in to three sections viz., the systematic synthesis of copper oxide thin films using Rf magnetron sputtering by varying oxygen partial pressure to grow thin films with pure Cu 2 O and CuO phase at room temperature, fabrication of transparent flexible pn hetero junction in the structure PET/ITO/n- ZnO/p-CuO/Au, fabrication of room temperature deposited transparent p-type CuO and Cu 2 O thin film transistors. p-type copper oxide thin films were deposited by RF magnetron sputtering at different oxygen partial pressure. Structural, optical and electrical properties of these films were studied as a function of oxygen partial pressure. Transparent flexible pn hetero junction fabricated in the structure

5 PET/ITO/n-ZnO/p-CuO show rectifying behaviour. Transparent p- type TFTs with CuO channel layer were fabricated at room temperature. The bottom gate structured p-channel CuO TFTs exhibited an on-off ratio of 10 4 and the field-effect mobility of 0.01 cm 2 V 1 s 1. Cu 2 O layer was also employed for the fabrication of TFT in which field effect mobility obtained was 1.34 cm 2 V 1 s 1 and on-off ratio Chapter 6 is divided into two sections. First section deals with the growth and characterisation of stable p-type copper boron oxide thin films at room temperature by RF magnetron co-sputtering and the fabrication of a pn hetero junction using p-type Cu 1 x B x O 2 δ and n- type silicon. The second section deals with the development of p-type Cu 1 x Cr x O 2 δ thinfilmsat room temperaturebyrfmagnetronsputteringandfabricationoftftsusingcu 1 x Cr x O 2 δ asachannellayer. Transparent p-type amorphous Cu 1 x B x O 2 δ thin films were grown on the glass substrate by RF magnetron co- sputtering of copper and boron at room temperature in an oxygen atmosphere. Structural, optical and electrical properties of these films were studied as a function of boron content in the film. Amorphous structure of as deposited films was confirmed by GXRD. Compositional analysis was carried out by XPS measurement and surface morphology of synthesized samples was characterised by atomic force microscopy. Band gap of as deposited Cu 1 x B x O 2 δ thin films varies from 1.8 ev to 3.2 ev as boron content increases in the film. Positive Hall coefficient obtained for all the film confirms the p-type conductivity in Cu 1 x B x O 2 δ. pn Hetero junction fabricated in the structure n-si/p-cu 1 x B x O 2 δ /Au show rectifying behaviour. Transparent p-type amorphous Cu 1 x Cr x O 2 δ thin films were grown on the glass substrate by RF magnetron co-sputtering

6 at room temperature. Structural, optical and electrical properties of these films were studied as a function of chromium content in the film. Amorphous structure of as deposited films was confirmed by GXRD. Compositional analysis was carried out by XPS measurement and surface morphology of synthesized samples was characterised by atomic force microscopy. Positive Hall coefficient obtained for all the film confirms the p-type conductivity in Cu 1 x Cr x O 2 δ. The Bottom gate structured TFTs fabricated using p-type Cu 1 x Cr x O 2 δ operated in enhancement mode with an on-off ratio of 10 4 and field effect mobility 0.3 cm 2 V 1 s 1. Chapter 6 summarizes the major contributions of the present investigations and recommends the scope for future works. Part of the work presented in the thesis has been published in various journals Journal papers 1. Growth and characterisation of tin oxide thin films and fabrication of transparent p-sno/n-zno pn hetero junction K. C. Sanal, M. K. Jayaraj Material Science and Engineering B (2013) Room temperature deposited transparent p-channel CuO thin film transistors K. C. Sanal, and M. K. Jayaraj, Journal of Applied Surface Science (Accepted, 2014). 3. Development of p-type amorphous Cu 1 x B x O 2 δ thin films and fabricationofpnheterojunctionk. C. Sanal, andm.k.jayaraj

7 (under review-mater. Sci. Eng. B). 4. Fabrication of room temperature deposited p-channel copper oxide thin film transistors K. C. Sanal, M. K. Jayaraj (under review- Royal Soci. Chem). 5. Developmentofp-TypeAmorphousCu 1 x Cr x O 2 δ thinfilmsand fabrication of thin film transistors, K. C. Sanal, and M. K. Jayaraj (under review-royal Soci. Chem). Conference papers 1. Growth and characterisation of p-type amorphous CuCrO x thin films by RF mangetron co-sputtering, K. C. Sanal and M. K. Jayaraj, National seminar on emerging trends in growth and characterization of single crystals and nanomaterials, 2013 march, SH college chalakudy. 2. Growth of p-type Amorphous Transparent Oxide Thin Films by Co-sputtering. K. C. Sanal and M. K Jayaraj, ICMATT-2013, Singapore. 3. Growth of transparent conducting copper boron oxide thin films and fabrication of pn hetero junction. K. C. Sanal and M. K. Jayaraj, National conference on Nanotechnology and Renewable energy (NSNRE-2013) October, K. E college, Mannanam, Kottayam. 4. Room temperature deposited copper chromium oxide thin film transistors, K. C. Sanal and M. K. Jayaraj, National conference on nanotechnology and renewable energy (NSNRE-2013) October, K. E college, Mannanam, Kottayam.

8 5. Growth of copper chrmium oxide thin films and fabrication of thin film transistors K. C. Sanal and M.K.Jayaraj, IUMRS, ICA 2013, Dec Other publication to which author has contributed Journal papers 1. Growth of silver nanoparticles in SiO 2 matrix by co-sputtering technique, K. C. Sanal, R. Sreeja, K.Anlin Lazar and M.K.Jayaraj, Nanophotonicmaterials, Proc. of SPIE 7393 (2009) 73930J Growth of ITO thin films on polyimide substrate by bias sputtering M. Nisha, K. A. Vanaja, K.C. Sanal, K. J. Saji, P. M. Aneesh and M. K. Jayaraj, Materials Science in Semiconductor Processing 13 (2010) Growth of IGZO thin films and fabrication of transparent thin film transistors by RF magnetron sputtering. K. C. Sanal, Majeesh and M. K. Jayaraj, Proc. SPIE 8818, Nanostructured Thin Films VI, (2013). Conference papers 1. Growth of silver nanoparticles in SiO 2 matrix by co-sputtering technique, K. C. Sanal, R. Sreeja, K. Anlin Lazar and M. K. Jayaraj, SPIE Optics and Photonics 2009, San Diego, USA.

9 2. Electrical Conductivity Measurement of a Single ZnO Nanorod, L. S. Vikas, M. K. Jayaraj, P. P. Subha, J. Puigdollers, T. Trifonov, S. Galindo, C. Voz, Aldrin Antony, K. C. sanal, ICMATT 2013, Singapore.