title: Characterization of high-κ dielectrics on Germanium
Creators: name: Althobaiti, Mohammed
Publisher: name: University of Liverpool Repository
Year: 2016

description: This study explores and describes the interface properties of various high-k materials deposited on the Ge substrate. Deposition/ growth of these material films has been achieved using multiple techniques such as atomic layer deposition (ALD), molecular beam epitaxy and thermal growth. High dielectrics (k) materials based on metal (4d and 5d) such as Y2O3, ZrO2, HfO2, Ta2O5, and from the lanthanide series, La2O3 and Tm2O3 were deposited on germanium and characterized to find out interface quality and band offset between Ge substrate and the oxides. Additionally, Al2O3 was considered, both as an interface barrier layer and as a high –k layer. Material and interface characterization was done using atomic force microscopy (AFM), capacitance-voltage (C-V), current-voltage (I-V), Variable Angle Spectral Ellipsometry (VASE), X-Ray diffraction (XRD), and X-ray photoelectron Spectroscopy (XPS) including the post growth micro-structural and compositional analysis using high resolution transmission electron microscope (HRTEM). Various physical and electrical studies were performed based on the above mentioned characterization techniques. The high-k material / Ge interface has been studied systematically using XPS and VASE characterization, considering the effects of temperature and thickness during deposition. Two germanium interface engineering methods were developed and discussed: (i) germanate formation using La2O3 and Y2O3, and (ii) using Al2O3 and Tm2O3 as barrier layers, and S passivation for Ta2O5 films. Based on the physical and electrical characterization carried out in this work, Ge interface engineering using rare-earth material inclusion happens to be a promising route to fabricate Ge CMOS devices with high performance. This statement is supported by the fact that these high-k materials provide a defect free interface and reduce the possibility of unstable GeOx formation at the interface, hence improving the interface quality. Post deposition annealing effects on Tm2O3 has been analysed using XPS and VUV-VASE. The stack prepared for the purpose was of EOT (equivalent oxide thickness) ~5 nm Tm2O3/epi-Ge/Si. Study with Tm2O3 presented 3 main findings, i) Valence band offset estimation using Kraut’s method was consistent within the experimental error, and found to be 3.05 ± 0.2 eV, ii) the VBO for thermal GeO2/Ge stack was found to be matching with the recently reported value by Toriumi’s group. The value of conduction band offset was estimated to be higher than 1 eV, indicating the favorability of GeO2 as a passivation layer for Ge, iii) the reactivity of Tm2O3 on Ge was found to be even lower than that of Si, indicating the possibility of a desirable interface. This thesis further explores the use of hafnia and alumina with Sulphur (S) passivated and un-passivated Ge samples. For this purpose HfO2/Ge and Al2O3/Ge stacks were prepared using ALD technique. It was observed that using H2O with O plasma, reduces the purge time and gives low carbon incorporation from metals. Hence O plasma and H2O were used as oxidizing agents and the interface properties were studied systematically, which is a new contribution by this work. Further the effects of adding TiO2 contents to HfO2 layer on interface properties were studied, using Al2O3 (0.3 nm) as surface passivation.In this work the achieved EOT of HfO2 with the controlled introduction of TiO2 was ~ 1.3 nm, giving a leakage current as low as10-7 A/cm-2 at ±1 V, which is in the acceptable limits. Finally, Ta2O5 films were characterized on Ge for band line up with respect to Ge. The deposition of the films was done by ALD technique at 250 °C. The analysis was done on both S passivated and un-passivated samples. The band line up parameters were estimated using XPS and it was observed that the valence band offset for S passivated sample was 2.67 eV whereas it was 2.84 eV for un-passivated Ge sample. Ta2O5 reflected a band gap of 4.44 eV (estimated from the energy loss spectrum of O1 s core level) for a 20 nm thick film deposited by ALD. Hence this thesis will cover the high-k materials and their application as a gate oxide and also the passivation layer for Ge substrates for Ge CMOS devices.