Characterization of defects at the interface between germanium and oxides by electrically detected magnetic resonance and admittance spectroscopy

Germanium has been the subject of a renewed interest as a promising material for future electronic devices. In particular its integration as channel material in CMOS architectures would be beneficial to operation speed, thanks to one of the highest hole mobility among semiconductors. Nevertheless, in real devices it is essential that the excellent property of the bulk are retained at the interfaces, where the devices are actually built. Despite many decades of efforts, the realization of the performance promises has been hindered by the poor quality of the interface and the lack of effective passivation strategies. Therefore, motivated by the necessity of a deeper understanding of the physical nature of the interface traps that are responsible for the performance degradation, several germanium - oxides interfaces have been investigated by Electrically Detected Magnetic Resonance (EDMR) spectroscopy. After the recent observation of germanium dangling bonds (DBs) at the (001) oriented Ge/GeO2 interface, the (111) oriented Ge/GeO2 interface was studied. That is, indeed, the most interesting for MOSFET application due to the inherent higher mobility according to band structure theory and experimental observations. EDMR measurements led to the unprecedented identification of a Ge DB with trigonal symmetry and principal axis oriented along the <111> direction, analogous to the well characterized Pb center observed at the (111) Si/SiO2 interface. The (001) Ge/GeO2 interface does not replicate the (001) Si/SiO2 interface, where a trigonal center, termed Pb0, is observed. In contrast with the (111) Ge/GeO2 interface, a dangling bond with a lower point symmetry is observed. This fact is rationalized in terms of suboxide interface rearrangement and oxide viscoelasticity. The suboxide rearrangement at the (001) interface promotes the generation of the non-axial centers at distorted dimers, while the viscoelasticity of the germanium oxide impedes the occurrence of the trigonal centers, which are located at monolayer steps. Moving from the identification of the microstructure features, the complementary study of the electrical properties in both surface orientations was undertaken. Accurate characterizations require the fabrication of Metal-Oxide-Semiconductor (MOS) capacitors with low gate leakage. The gate stack comprising a thermal grown GeO2 layer and a Atomic Layer Deposited Al2O3 capping was chosen as the better compromise in terms of performance and simplicity. X-ray Photoemission Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) attested the chemical and compositional mutual consistency between the as grown and the capped GeO2. EDMR confirmed that the structure of the trigonal Ge DBs and local stress distribution are not affected by the capping. p- and n- type MOS capacitors comprising the Ge/GeO2/Al2O3 gate stack were fabricated on (001) and (111) substrates and characterized by admittance spectroscopy measurement in the temperature range 300 - 110 K. Consistently with the current literature, the density of interface traps (Dit) as a function of energy results in a smooth U-shaped curve. It is not possible to recognize the specific features of energy localized levels, such as in the case of the Si DBs. Other germanium interfaces have been investigated. EDMR spectroscopy detected the trigonal Ge DB at the untreated (111)Ge/Al2O3 interface, possibly due to the presence of a suboxide GeOx transition layer. The DB signature has been found also when the Ge surface was treated with ammonium sulfide prior to the Al2O3 deposition. Admittance spectroscopy demonstrated the good properties of thermal GeO2 in terms of Dit, unparalleled in the direct Ge/Al2O3 interface, especially in the upper half region of the bandgap. Ammonium sulfide treatment is effective in reducing the defect density at the interface, but the GeO2 performance are, on the whole, superior. As a general trend, it is observed a slightly higher Dit in the (111)-oriented substrates with respect to the (001)-oriented substrates. Analyzing in detail the admittance curves of p-type Ge/GeO2 MOS capacitors, an anomalous behavior has been recognized. The characteristics of the response time of interface traps, as the chemical potential is swept from the valence band to midgap, hints at an exchange of two carriers in a single charging event of the electrically active defects. A negative-U model (where U indicates the electron-electron correlation energy) is invoked to explain the experimental facts that emerged from the EDMR and admittance spectroscopy investigations, together with the information available in the literature. The evidence of a negative-U nature of the interface traps has relevant consequences both on the procedures adopted in the characterization of interface traps and on the techniques employed in their passivation.