SiC BASED THIN FILMS FOR ELECTRONIC DEVICE APPLICATIONS

Abstract

In recent years SiC and SiCN based wide band gap semiconductors have attracted much attention due to their exotic and versatile properties such as high thermal conductivity, low intrinsic carrier concentration, high breakdown field and high electron saturation velocity etc. These superior properties make them potential candidate for various devices and sensors application in harsh environment. These include excellent operation of tunable devices, thermal sensors, and memory devices in high temperature and harsh environment conditions. In addition gas sensing in nuclear reactor, oil refineries as well as detection of fuel leaks in automobiles and aircraft respectively. Amorphous thin films of SiC and SiCN have been extensively analyzed for basic studies. Although, crystalline or nanocrystalline thin films are quit demanded for applications in microelectronics, tunable devices, thermal sensors and memory device etc., but fabrication of crystalline thin films needed very high temperature, which deteriorates device performance. In last few years hierarchical nanostructured thin films have been recognized for gas sensing applications due to their large specific area which provides sufficient adsorbing sites to gas molecules. The main objective of this thesis is to fabricate nanostructured thin films of SiC and SiCN by RF magnetron sputtering on Pt coated silicon and porous silicon substrates in order to: (i) Study structural and electrical properties of these thin films, (ii) Hydrogen gas sensing properties of SiC nanoballs deposited on porous Si prepared by metal assisted chemical etching, (iii) Resistive switching properties of SiCN thin films. A chapter wise summary of this thesis is given below: Chapter 1 gives an overview of SiC and SiCN materials. The structural and other characteristic properties have been also discussed. The application of these materials for resistive switching memory device and hydrogen gas sensor has also been included in this chapter. Chapter 2 presents the details of experimental techniques, which we have used for the synthesis and characterization of SiC and SiCN thin film. The synthesis of thin films in present thesis has been carried out by DC/RF magnetron sputtering technique. The first measurement that is usually carried out after synthesis is to record the X-ray diffraction pattern of the as-deposited material. Analysis of the position and width of the Bragg reflections gives an idea of the crystallographic phase, presence of impurities, particle size etc. Further, the structural properties were studied using Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) respectively. Surface morphology and microstructure of thin films were studied iv using atomic force microscopy (AFM) and field emission electron microscopy (FESEM). The film thickness was measured using cross-sectional FESEM. Electrical properties such as temperature dependent resistivity (R-T), dielectric, leakage current, and resistive switching properties were studied using four/two probe resistivity set up, impedance analyzer and semiconducting characterizing setup, respectively. Further, the hydrogen gas sensing properties were studied in-situ by two-probe resistivity technique using a source meter (Keithley 2400) and nanovoltmeter (Keithley 2181A). The sensing measurement setup consists of a custom made chamber of volume 500 cm3 equipped with a PID controlled electric heater and mass flow controller for monitoring the target gases. Chapter 3 describes the growth and characterization of nanocrystalline silicon carbide (nc- SiC) and silicon carbide nitride (nc-SiCN) thin films. This chapter is divided into two sections. The first section (Section 3.1) mainly describes the influence of substrate temperature on structural and electrical properties of nc-SiC thin films deposited on Pt coated Si substrate. The substrate temperature was varied from 750 C to 1100 C. X-ray diffraction (XRD) pattern revealed that the intensity of highly oriented (105) reflection is increasing with increment in substrate temperature. Atomic force microscope (AFM) images shows reduction in micropipe density with increasing crystallinity. Dielectric measurement shows an improvement in dielectric properties with increment in substrate temperature. Leakage current density (J) reduces by an order of 103 with increase in substrate temperature or crystallinity which may be ascribed to reduced micropipe density. The conduction mechanism shows ohmic conduction in low field region and interface limited Schottky emission at high field (E>60 kV/cm) regime for all nc-SiC thin films. Section 3.2 describes effect of measurement temperature on dielectric and leakage current properties of nanocrystalline silicon carbide nitride (nc-SiCN) thin films. In addition, temperature dependence resistivity of nc-SiCN thin film was also studied. Resistivity measurement in temperature range 300-773 K, reveals large negative temperature coefficient of resistance (TCR) from 6200 to 2300 ppmK-1, which indicates nc-SiCN thin films could be a potential candidate for futuristic thermal-based sensors in harsh environment. The temperature dependent (300-673 K) leakage current studies reveals an ohmic conduction at the low applied electric field (<65 kV/cm). However, in higher electric field (>65 kV/cm) region, the conduction mechanism was found to be make a transition from space charge limited conduction (SCLC) at low temperature (<473 K) to Poole-Frenkel mechanism above 473 K. The observed stability in dielectric constant r (r -T curve) and temperature invariant dielectric v tunability nr 10% (r- E curve) along with low leakage current density J  10-5 A/cm2 (at 100 kV/cm, 673 K temperature), implies possibility of nc-SiCN thin films in tunable device applications at hightemperature. Chapter 4 describes fabrication of platinum decorated silicon carbide (Pt/SiC) nanoballs (NBs) on Ag coated porous Si substrates via DC/RF magnetron sputtering. Porous Si substrates were prepared by the metal-assisted chemical etching process at room temperature. The structural and morphological properties of as-grown SiC nanoballs (NBs) were studied using various techniques such as X-ray diffraction (XRD), Raman spectroscopy and field scanning electron microscopy (FESEM) respectively. Hydrogen gas sensing properties along with the sensing mechanism of Pt/SiC nanoball-based sensor within low detection limit (5-1000 ppm) at the high operating temperature range (30-480 C) were investigated in detail. The sensor exhibits high sensing response (44.48 %) with very fast response time (15 s) toward 100 ppm hydrogen gas at 330 C. These above results suggest the feasibility of Pt decorated SiC nanoballs for highly sensitive and selective hydrogen gas sensor at the high operating temperature and harsh environment conditions. Chapter 5 this chapter has been divided into two sections: section 5.1 describes fabrication of Cu/SiCN/Pt-based capacitor structure by dc/RF sputtering for resistive switching-based nonvolatile memory device applications. The device exhibit uniform and stable bipolar resistive switching behavior. A thorough current-voltage (I-V) analysis suggests an Ohmic conduction mechanism within the low resistance state (LRS), whereas within the high resistance state (HRS) trap-controlled modified space charge limited conduction (SCLC) mechanism was found to be dominated. The resistance vs. temperature measurement (R-T curve) within LRS and HRS along with a model analysis indicates an interesting result that the formation of conduction path during LRS is not due to Cu filament but may be formed by trap-to-trap hopping of electrons via nitriderelated traps between the top and bottom electrodes. The resistive switching in Cu/SiCN/Pt device was operated via electron transport path formation/rupture by electron trapping/de-trapping. The reliability of device was measured in terms of endurance and retention, which exhibits good endurance over 105 cycles and long retention time of 104 s at room-temperature as well as at 200 C. The above result suggests the feasibility of Cu/SiCN/Pt devices for futuristic nonvolatile memory application at high temperature and harsh environment. Section 5.2 describes resistive switching (RS) properties of Ag/SiCN/Pt and W/SiCN/Pt devices having electrochemically active (Ag) and inactive (W) top electrodes. Both devices revealed stable and reproducible bipolar resistive vi switching characteristics. The W/SiCN/Pt device exhibits two state resistive switching behavior i.e. low resistance state (LRS) and high resistance state (HRS) whereas, Ag/SiCN/Pt device shows tristate RS characteristics [LRS, intermediate resistance state (IRS) and HRS)]. The two resistance state RS characteristics of W/SiCN/Pt device were ascribed to conduction path formation/rupture via electron trapping/de-trapping in nitride-related traps. However, tri-state RS behavior of Ag/SiCN/Pt device could be attributed to conduction path formation via electron trapping in nitriderelated traps followed by an additional Ag filament growth between the top and bottom electrodes. The origin of tri-state switching in Ag/SiCN/Pt device and Ag filament formation was well explained by a conceptual model as well as temperature and cell area dependence of resistance measurement. Ag/SiCN/Pt device exhibits good reliable properties such as endurance of 105 cycles and long retention time 105 s at the high temperature of 200 C, respectively. This comprehensive study suggests that nonvolatile multi-level (three-level) resistive switching in the SiCN-based device can be achieved by the formation of different types of conducting filaments sequentially and Ag/SiCN/Pt device could be capable of futuristic multi-bit storage ReRAM which can operate at high temperature. Chapter 6 presents the summary and conclusions of the entire work presented in the thesis and also proposes the future prospects in which these studies can be extended