III-NITRIDE BASED NANOSTRUCTURED THIN FILMS AND MULTILAYERS: GROWTH, CHARACTERIZATION, AND APPLICATIONS

Abstract

From the last two decades, wide bandgap semiconductors significantly become an important materials in the semiconductor research groups worldwide. Out of them, III-Nitride materials i.e. aluminium nitride (AlN), gallium nitride (GaN) and their alloys have attracted much attention due to their exotic and versatile properties such as wide bandgap, high piezoelectricity, high thermal conductivity, high stability, high breakdown field, and high electron saturation velocity etc. The bandgap of III-Nitrides AlN, GaN and InN are 6.2, 3.4 and 0.7eV respectively. By alloying one of the materials with another one by keeping nitrogen element constant (bandgap engineering) one can get a wide range of bandgap material from 0.7eV to 6.2 eV (IR to deep-UV). These materials (AlN, GaN and their alloys) now days are very popular in the field of semiconductor research. The main reason their popularity may be the fabricated devices and sensors from these materials are high piezoelectricity, capable to operate at high temperature and high power levels in harsh environments. Additionally, due to high stability, it becomes resistive to chemical attack and can be used for their choice for electronic, optoelectronic and sensing applications. III-Nitride materials, AlN based thin films can be widely used as actuators and vibration damping devices in micro-electro-mechanical systems (MEMS). Also due to fast acoustic velocity in AlN, it is a promising material for surface acoustic wave (SAW) devices. The c-axis oriented AlN films on Si substrate gives better piezoelectric behaviour for MEMS devices. It is well known that the residual stress in the film affects the piezoelectric properties as well as the stability of the MEMS structure. Therefore, the detailed study on residual stress is essential for the films before fabricating devices to ensure the performance and reliability of the device. Hence, it is planned to study the films residual stress on AlN thin films on Si substrate grown by DC sputtering. Also, design the AlN based MEMS accelerometer devices and analyze the effect of residual stress in the device performance by modeling. The other III-Nitride material is GaN. GaN-based semiconductor and its alloys have been extensively used for the development of light-emitting diodes (LEDs), UV-detectors and high frequency, and high power electronic devices. The defects in the GaN layer affect the device performance drastically. Therefore the estimation of threading dislocation density (TDDs) of the GaN epitaxial layer is very important before fabricating the GaN-based devices. AlGaN/GaN multilayer High Electron Mobility Transistor (HEMT) structures on Silicon carbide (SiC) provide better device performance due to less lattice mismatch (3.5%). Growth and characterization of AlGaN/GaN multilayer on SiC are fewer studied due to expensive SiC ii substrate. Therefore it is planned to investigate structural and electrical parameters of the HEMT structure grown by metal organic chemical vapour deposition (MOCVD). In the present thesis, we have studied AlN, GaN and AlGaN/GaN multilayer structures. The main objectives of the thesis are divided into two parts: A. (i) To synthesize AlN thin films by DC magnetron sputtering technique on various Si substrates to achieve grain refined oriented films (ii) To study the structural, morphological, optical and electrical properties of AlN thin films on differently oriented Si substrates and (iii) To determine the residual stress of AlN thin film as a function of film thickness and study the effect of residual stress on the AlN/Si (100) film for MEMS accelerometer device structure by theoretical modelling. B. (i) To establish the threading dislocation density (TDDs) determination methods for GaN epitaxial layers on sapphire substrate by MOCVD, by destructive and non-destructive procedure (iii) Investigate various structural, morphological, optical and electrical parameters for AlGaN/GaN multilayer structures grown by MOCVD method for HEMT devices. A chapter wise summary of this thesis is given below: Chapter 1 gives an overview of AlN and GaN materials. The structural, optical, electrical, piezoelectric and other properties have been discussed thoroughly for AlN and GaN. The application of these materials for MEMS device, SAW Device, LEDs, Lasers and AlGaN/GaN multilayer heterostructure (HEMT), etc have been included in this chapter. Chapter 2 presents the details of experimental techniques. In the chapter (i) synthesis and characterization of AlN thin films on Si substrates (ii) Growth and characterization of GaN and AlGaN epitaxial layers have been explained. The synthesis of AlN thin films in the present thesis has been carried out by DC magnetron sputtering technique. Various characterization techniques, such as X-rays diffraction (XRD), Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscopy (FE-SEM) and Optical characterization techniques (FTIR, UV-visible spectroscopy) have been discussed in detail. Electrical characterization Capacitance-frequency(C-f) and Capacitance-Voltage(C-V), I-V and breakdown field determination methods by two-probe setup, impedance analyzer and semiconducting characterizing setup is also described. In this chapter MOCVD, epitaxial growth method for GaN and based multilayer has also been explained. Characterization techniques for epitaxial samples High-resolution X-ray diffraction (HRXRD), TEM, etc for structural and morphological and Hall Effect for electrical characterization techniques have been described. iii Chapter 3 describes the growth optimization of AlN thin films on Si substrates by DC sputtering and its characterization. This chapter is divided into three sections. The first section (Section 3.1) mainly describes the AlN film growth optimization procedure. The influence of substrate temperature from 300 C to 600 C and N2 to Ar gas flow ratio [N2 (in sccm): Ar (in sccm)] 16:4 to 8:12 are varied as main deposition parameter for AlN thin films on Si substrate. For highly oriented AlN films for the best-optimized conditions, i.e. growth temperature ~5500C and gas flow ratio of (N2: Ar) is 10:10 for our DC sputtering system. After optimization four AlN thin films on Si (100) substrate by varying the film thickness (300nm to 830nm) have been deposited. The degree of crystallinity of the AlN films is found to increases from 61 % to 97 %. Films Texture Coefficient (orientation parameter) along (002) and (100) direction were determined and compared as a function of the thickness of the films. Highest TC (~3.2) was obtained for the 830nm thick film along (002) direction. Section 3.2 presents the influence of substrate orientation Si (100), Si (110) and Si (111) in the growth and characterization of AlN by keeping the films thickness constant (~0.9μm). Grown films are showing columnar structure. Films texture coefficient along (002) direction were found to be 3.12, 2.90 and 3.87 corresponding to the silicon (100), (110) and (111) substrates respectively. AlN film along (002) direction on Si (111) is more preferred due to same in-plane atom atomic arrangements. AlN films on Si (100) can be used as a wide variety of MEMS device applications due to ease in micro-machinability. Therefore integration of (001) oriented wurtzite AlN films on Si substrates is very essential for superior piezoelectric MEMS structures. Section 3.3 describes the fabrication of Metal-Insulator-Semiconductor (MIS) Au/AlN/Si-based capacitor structure for the AlN films on Si (100) and Si (111) substrates. A thorough C-f and C-V analysis have been done at room temperature. The dielectric constant values at low frequency are found to be 7.4 and 8.8 corresponding to the films on Si (100) and Si (110) substrates respectively. Interface trap density was determined using C-f/C-V characteristics for both the samples and found in the range ~1012 cm-2 eV-1. The leakage current density 2x10-5 A/cm2 and 1x10-4 A/cm2 for AlN on Si (100) and Si (110) respectively were determined by I-V characteristics. AlN/Si (110) shows the higher breakdown field ~6.6 MV as compare to AlN/Si(100)~ 4.4 MV/ cm. The variation in the electrical parameter values was explained by physical interface trap model for AlN and Si. The electrical properties of the MIS capacitor provide valuable information about dielectric behavior for MEMS structures of AlN films grown on Si (100) and Si (110) substrates. Chapter 4, In chapter 4, Section 4.1 expresses the overview of residual stress for thin films. Section 4.2 presents the residual stress of AlN thin films on Si (100) by varying the film iv thickness (300nm to 830nm), determined by three different techniques i.e. XRD (modified Sin2Ψ method), FTIR (shift in E1 (TO)) and wafer curvature (by stoney’s method). The good correlations in the residual stress value of the AlN layer as a function of thickness have been obtained. The residual stresses are found from -2.1GPa to -0.6GPa with the increase in films thickness. The change in microstructure, as well as the increase in grain size and annihilation of grain boundaries, seems to be responsible for the reduction of the compressive stress as thickness increases. Section 4.3 demonstrates the influence of substrate orientation in the residual stress values of AlN thin films on Si (100), Si(110) and Si(111) determined by three different techniques i.e. XRD, FTIR and wafer curvature. The determined average residual stress in AlN films are found to be in the range of – (620-650) MPa, – (730-750) MPa and – (260-315) MPa corresponding to the AlN films on Si (100), (110) and (111) substrates respectively. The minimum residual stress on AlN film on Si (111) is due to reduced grain boundary density and matched in-plane hexagonal lattice arrangement of Si (111) and AlN (002) oriented films. In Section 4.4 MEMS accelerometer structure has been designed for AlN films on Si (100) substrate. MEMS accelerometer structures have been modeled by finite element method. The structure was simulated using the residual stress ~- 600MPa for 830nm thick AlN film on Si (100) and compared with the simulated unstressed value for MEMS accelerometer structures. The simulated result shows the normalized stress sensitivity of the accelerometer structure is found to be altered from 1.33x10-2/ g (at no residual stress) to 1.26x10-4/ g (at -600 MPa residual stresses). Results explained that to get the improved performance of the piezoelectric MEMS accelerometer structure the residual stress of the AlN film should be minimized. Chapter 5 divided into two parts. Section 5.1 describes the growth of the GaN epitaxial layer on the sapphire substrate by MOCVD method. GaN samples are used to study the threading dislocation density (TDDs) estimation using wet chemical defect etching, dry etching, HRXRD, AFM and TEM methods. Determined screw and edge threading dislocation density for GaN layers were ~4x108/cm2 and ~5x109/cm2 respectively. TDDs have been compared with all the methods mentioned above and found an internal consistency among them. The electronic/optoelectronic device's performance on GaN-based samples will be affected by defects; therefore, the determination of TDDs is very important for the GaN-based device structure. Section 5.2 describes the MOCVD growth of three multilayers AlGaN/GaN HEMT structures on SiC samples by varying GaN buffer growth parameters. In this section, AlGaN/GaN HEMT structures structural, morphological, optical and electrical properties have investigated for suitability of HEMT devices. Various parameters i.e. threading dislocation v density (TDDs), surface and interface roughnesses, the role of the cap layer on the HEMT device structure, are determined and studied. Two-step GaN buffer is grown sample shows the low surface roughness (~.28nm), low dislocation density (1.6x109/cm2) and high electron mobility ~1835 cm2/V-Sec at room temperature. These structural and electrical parameters are very advantageous for high frequency and high power HEMT device. Chapter 6 presents the summary and conclusion of the entire work presented in the thesis and also proposes the future directions in which these studies can be extended.