Characterization And Properties Of Hf-based And Rare Earth High K Films Epitaxially Grown On Si, Ge, InP Substrates

With the rapid development of Complementary Metal-Oxide-Semiconductor (CMOS) integrated circuit according to Moore's law, the device size will scale down continuously. On the one hand, decreasing of thickness of SiO2 gate dielectric causes high leakage current. It is necessary to find new gate materials with high dielectric constant to replace SiO2. On the other hand, the new channel materials with higher carrier mobility have been demanded, which will overcome problems of Si based channel materials for high speed devices. Research study of high k gate dielectric and compatibility of high-k materials on the new channel materials is of high academically value for the applications of high-k dielectrics in advanced nano CMOS devices.Hf-based and rare earth oxide amorphous films as gate dielectrics have been widely studied. However, there are problems in application, due to their metastable states of amorphous structure. In this thesis, we have systematically researched on experiments and theoretical analysis of high-k materials and channel materials. Crystalline epitaxial oxide films brought about prominent advantages in improving dielectric constant, reducing the leakage current and impurity diffusion, which can avoid the amorphous stability problems. Considering the cost and operability, pulsed laser deposition (PLD) method is employed to grow epitaxial films. High k gate dielectrics including Gd2O3-HfO2 (GHO) solid solution oxide, pure HfO2 and La2O3 were epitaxially grown on different substrates (Si, Ge and InP). We studied the growth characteristics and basic performances of the high k films. In addition, structure characterizations and property measurements of these films grown on the substrates were discussed. The main contents of the present work are as follows:(1) The cubic single crystalline HfO2 (001) films have been epitaxially grown on Si (001) substrates at 820℃using PLD method which presents a cube-on-cube growth mode with the orientation relationship (001)HfO2‖(001)Si and [011]HfO2‖[011]Si. RHEED studies indicate without formation of an amorphous interface layer during the deposition process. HRTEM reveals final existence of the interface layer which is due to oxygen diffusion from the HfO2 films toward Si substrates. Oxygen diffusion in cubic HfO2 has been confirmed by experiments. The leakage current density of Ru/HfO2/Si/Ag MOS capacitors have been improved by six orders of magnitude through annealing at 900℃in N2. The HfO2 gate insulators show excellent electrical properties with a k value of～26 and leakage current of 5×10-6 A/cm2 at-1V (a field of 1.4 MV/cm). Our results indicate that cubic HfO2 epitaxial films as high k dielectrics have broad prospects applied in the Si based integrate current.(2) Gd2O3-HfO2 solid solution (GHO-10) oxide high k dielectrics have been designed and fabricated to prevent generation of interface layer and inhibit oxygen diffusion in the films. The GHO-10 target with purity of 99.995% was prepared using a conventional sintering method. GHO-10 thin films have been cube-on-cube epitaxially grown on Si (100) substrates by PLD method. Highly crystalline GHO-10 films with orientation relationships of (100)GHO-10‖(100)Si and [011]GHO-10‖[011]Si and good quality interface are confirmed with RHEED and HRTEM studies. No chemical reaction of SiO2 or silicide at interface of GHO-10/Si inter have been proved by XPS, It will be beneficial to improve electrical properties of the thin oxides as high-k dielectrics without generating silicates during the deposition process. Owing to the acceptable electrical properties for 6-nm thick GHO-10 films on silicon substrates, the epitaxial GHO-10 layer grown on Si by PLD method can be a promising high-k gate dielectric candidate. Cubic GHO-35 epitaxial films possessing good thermal stability and suppression ability of oxygen diffusion has been revealed by XPS. Our findings provide reference or evidence for improving thermal stability of high-k gate dielectrics, suppressing oxygen diffusion and interface layer.(3) According to existing problems of lattice mismatch during depositing, we have studied microstructure of HfO2-Gd2O3 solid solution (GHO) oxides and films. The targets with different molar ratios of Gd2O3:HfO2 (10:90,20:80,35:65,50:50, and 60:40) were fabricated by optimized conventional sintering process (see table 5.1). The effect of Gd2O3 content on lattice constant of GHO oxide was researched. The GHO films (GHO-20, GHO-35 and GHO-60) have been cube-on-cube epitaxially grown on Si (100) substrates by PLD method. The relationship between the lattice constant of the epitaxial films and Gd2O3 content was obtained through experiments and theoretical calculation. Also, the reasons for change of GHO film lattice constant have been discussed. Selection basis of GHO high k materials used for epitaxial growth has been obtained in this chapter.(4) The GHO-35 films have been epitaxially grown on Ge (001) substrates at 660℃using PLD method. The GHO-35/Ge stack presents a cube-on-cube growth mode with the orientation relationship (001)GHO-35‖(001)Ge and [011]GHO-35‖[011]Ge. RHEED studies indicate GHO-35 films having good crystallinity and surfaces flatness and HRTEM results reveal GHO-35/Ge stack structure with a zero interface layer.The influence of growth conditions on microstructure of GHO-35 film has been analyzed. We studied the growth kinetics of GHO-35 in the deposition process. Band offsets for the GHO-35/Ge are determined to be～3.92 eV for VB and～1.38 eV for CB by XPS. Experimental evidence of a sharp interface between GHO-35 high k film and Ge substrate has been presented. The Ge surface passivated by Hf/Gd-covering and two incomplete planes named GeOx-Hf/Gd and GHO-Gex atomic layer at the interface between GHO-35 film and Ge substrate have been revealed. An epitaxial growth mode and the atomic configuration of the interface in GHO/Ge stack have been proposed. A small 0.59 nm EOT, Dit-7×1011 cm-2 and k-28 have been obtained from Au/Ti/GHO-35/Ge/Al MOS capacitors by C-V, I-V and G-V measurements, respectively. These experimental findings indicate that epitaxial GHO-35 films on Ge substrates with zero interface layer is promising structure for the next generation CMOS devices.(5) We have epitaxially grown La2O3 thin film as a higher k dielectric on InP substrates as a high mobility channel material by PLD. The growth model of cube-on-cube and a good lattice matching without intermediate layers between La2O3 film and InP substrate have been revealed by RHEED and HRTEM measurements. The energy band offsets between La2O3 dielectric and InP substrates are 1.62 eV in VB edge and 2.61 eV in CB edge, respectively. Capacitors with W/2.5 nm-La2O3/InP/Al stack have been fabricated for electrical tests. A dielectric constant of 31, a small EOT of 0.3 nm and a low leakage current of 2×10-4 A/cm2 at a bias voltage of 1V have been calculated from the results of electrical measurements. The present experimental findings conclude that it is feasible to epitaxially grow La2O3 high k dielectric on InP channel materials for advanced nano integrated circuit.