Eun Ji Kim's Dissertation Defense

Interface and Defect Study of High Permittivity Dielectrics on Si and III-V semiconductors

Eun Ji Kim

Advisors: Prof. Paul C. McIntyre in Materials Science and Engineering
Prof. Krishna C. Saraswat in Electrical Engineering
 
Date: Thursday, Mar. 19, 2009
Time: 2:00 PM (Refreshments served at 1:45 PM)
Location: Packard Bldg. Rm. 202
 
     In an effort to decrease electronic device dimensions and improve device performance, high permittivity dielectrics have been introduced to metal-oxide-semiconductor field effect transistors (MOSFETs). Even though replacing SiO2 with high permittivity dielectrics enabled aggressive device scaling, however, the introduction of new materials gave rise to fundamental problems that could lead to device performance degradation, such as reduction of the effective carrier channel mobility and threshold voltage instabilities. Unsatisfied dangling bonds at the interface of the high permittivity dielectrics and Si, intrinsic and extrinsic point defects in high permittivity dielectrics, and remote phonon scattering are believed to cause degradation of device performance. To understand the limitations to the performance of MOSFETs with high permittivity dielectrics, it is critical to probe phonon modes and defect states directly in high-k dielectrics.
     Inelastic electron tunneling spectroscopy (IETS) is employed to study soft phonon modes and defect states in HfO2 grown by atomic layer deposition (ALD) on Si. Observed spectral features suggest that monoclinic- and tetragonal- HfO2 vibrational modes exist in the annealed HfO2 while crystalline HfO2 vibrational modes are not detected in the as-deposited samples, consistent with selective area electron diffraction analysis. In addition to soft phonon modes of HfO2, changes in amplitude and energy of spectral features were observed as the bias condition changes. We attribute these features to defect-related states in HfO2 and analyze them in terms of electron energy states in the HfO2 bandgap and reported oxygen vacancy states in HfO2.
     For further device scaling, III-V compound semiconductors are receiving increasing attention for channel replacement in the metal-oxide-semiconductor (MOS) technology beyond 22 nm node because of their high intrinsic electron mobility. Unlike SiO2 that exhibits excellent passivating properties on Si with low interface state densities, there typically exists a large density of defect states at the interface of III-V semiconductors and their native oxides. Previous research on GaAs showed that less than 1 % of a monolayer of chemisorbed O2 can pin the Fermi level at the semiconductor surface. Therefore, suppressing oxidation of the III-V semiconductors' surface prior to and during gate dielectric deposition could be essential to achieving device performance superior to that of silicon in nanoscale devices. Several different approaches have been demonstrated to prepare III-V semiconductor-based MOS devices. However, previously attempted methods resulted in frequency-dependent flat band voltage (Vfb) shift, charge trapping in the dielectrics and a relatively high density of interface trap states, possibly from unintentional oxidation of the III-V channel.
     We used In0.53Ga0.47As (100) channels that were capped with an arsenic layer after channel epitaxial growth to avoid III-V oxidation during exposure of the samples to air. The As capping layer was thermally desorbed in-situ in a load-locked ALD reactor prior to Al2O3 gate dielectric deposition. By preventing subcutaneous oxidation of the channel surface, we obtained unpinned Al2O3/In0.53Ga0.47As interfaces with a low density of interface states. The C-V characteristics show a hysteresis of less than 40 mV and relatively small frequency dispersion in accumulation. The surface potential swing (0.44~1.2 eV) calculated using the Berglund integral suggests the absence of a high density of midgap interface states. The observation of near-ideal flat band voltage values for Pt- and Al-electroded MOS capacitors indicates the absence of a significant interface dipole. The temperature-independence of the frequency dispersion of the accumulation capacitance and its scaling with measurement frequency are consistent with tunneling of carriers into defects in the Al2O3 layer, border traps. This also indicates a low interface state density for these devices.