AVS2011 Session EM-TuA: High-k Dielectrics for MOSFETs Part 2

The introduction of high-k dielectrics and metal gates in advanced CMOS has opened the door to Ge and III-V compounds as potential replacements for Si to further increase the device performance. Using MOCVD the selective area growth of low-defect InP and InGaAs layers in submicron trenches on Si was demonstrated. These virtual Ge-III/V substrates can be processed in a standard CMOS line. Short channel Ge pMOS devices with high drive currents were fabricated. Strain engineering using GeSn source/drain areas allows to boost the performance of these devices so that they can outperform their strained Si counterparts. One of the key problems in developing III/V devices is the near midgap Fermi level pinning associated with the high density of defect states present at the high-k/III-V interface. The origin of these states is still under debate but there are clear indications that there exists a strong relationship with native antisite point defects. Various sulfide and other treatments were investigated to passivate the surface. The measured distribution of interface states and border traps on typical III/V MOS structures has some special consequences on the electrostatic operation of different transistor designs. Since inversion mode devices do not seem to be the appropriate choice for III/V based logic applications, other device types have been explored. The Implant-Free Quantum Well (IF-QW) device enables VLSI-compatible processing by self-aligned source/drain definition. Strained Implant Free Quantum Well Ge-based pFETs show excellent short channel control and record drive currents. The concept was also used to demonstrate high mobility n-channel InGaAs devices. For III/V pMOS devices GaSb is at present the material of choice. Very encouraging results have been obtained on direct heteroepitaxy of GaSb epilayers on InP(001) combined with in-situ deposition of an Al₂O₃ high-κ gate dielectric. The introduction of these advanced materials also allows the development of new device concepts that can fully exploit the properties of these new materials. Tunnel-FETs, where the III/V material may be either introduced only in the source or in the complete device, can provide superior performance at lower power consumption by virtue of their improved subthreshold behavior, allowing to reduce the supply voltages. Vertical surround gate devices can be produced from III/V nanowires directly grown on silicon, allowing the introduction of a wide range of III/V materials and functionalities on Si. This illustrates some of the possibilities that are created by the combination of new materials and devices to allow scaling of nanoelectronics beyond the Si roadmap.

Ge-based metal-oxide-semiconductor (MOS) devices are focused as complementary-MOS devices for the next-generation in the post-Si technology. However, the dielectric properties of GeO₂ gate ultrathin films have not been clarified yet in detail. Our purpose is to clarify the spatial variation of the local dielectric constant for the GeO₂ thin films using first-principles ground-state calculations in external electric fields [1,2]. In particular, we reveal the local profile of the dielectric constant near the oxygen vacancy in the film, focusing on the crystal phase dependence.

We have adopted quartz (0001) and rutile (001) films with/without oxygen vacancies, in which Ge atoms at the topmost surfaces are terminated with H atoms. We have evaluated the optical and the static dielectric constants that are attributed to the electronic polarization and both the lattice and electronic polarizations, respectively.

From the local profile of the dielectric constants for the ideal films, it has been clarified that the dielectric constants change gradually from the surface and approach constant values at the center of the film. Such features have also been confirmed for the Si and SiO₂ films [1,2]. The dielectric constant for the defective model of the quartz film becomes larger locally “at” the oxygen vacancy site compared with that for the ideal model, but at “adjoining” oxygen sites to the vacancy for the rutile film. Such features stem from the difference in the fashion of the chemical bonding between Ge and O atoms: The dielectric constant for the defective quartz model becomes large at the vacancy site where the covalent Ge-Ge bonding is formed. For the rutile, on the other hand, the Ge-O bondings surrounding the vacancy site are softened because of their less ionic character, which results in the larger displacement in external electric fields, leading to the larger lattice polarization around the vacancy.

[1] J. Nakamura et al., J. Appl. Phys. 99, 054309 (2006); Appl. Phys. Lett. 89, 053118 (2006)

[2] S. Wakui et al., J. Vac. Sci. Technol. B 26, 1579 (2008); ibid 27, 2020 (2009).

Germanium is a promising channel material for next generation MOSFET. The best method to passivate Ge(100) is to form a layer of GeO₂, free of Ge suboxides, using high pressure O₂ or O₃. However, there are three challenges: (1) it is difficult to keep a stoichiometric GeO₂ monolayer (ML) at elevated temperatures, (2) the thermal oxidation process creates a rough interface degrading mobility at high field, and (3) scaling the passivation layer to only 1 ML is a challenge. This study presents a process to form a ½ ML of Ge-H and ½ ML of Ge-OH bonds without disrupting the Ge(100) surface. In-situ scanning tunneling microscopy (STM), in-situ scanning tunneling spectroscopy (STS), and in-situ X-ray photoelectron spectroscopy (XPS) were employed to determine the atomic and electronic structure of the passivation monolayer.

Using a differentially-pumped H₂O dosing system, an ordered, flat monolayer of H₂O chemisorption sites on Ge(100) was formed with a low density of unreacted dangling bonds at 300K. STS data showed that the Ge-H and Ge-OH sites removed the bandgap states from the Ge(100) dangling bonds. Annealing the surface between 20°C and 250°C gradually decreased the coverage of H₂O sites. However, even at 300K, the H₂O surface is highly reactive to trimethyl aluminum (TMA) since it contains a half monolayer of Ge–OH which catalyzes the breaking of Al-CH₃ bonds thereby inducing the formation of Al-O bonds, and the Ge-H sites block ALD ligand chemisorptions. STM experiments showed that the H₂O chemisorbed Ge surface provides a half monolayer of nucleation centers with approximately 0.5 nm spacing for TMA dissociative chemisorption at 300K. High resolution XPS experiments indicated that thermally unstable Ge-OH bonds were converted to thermally stable Al-O bonds. Furthermore, passivating the surface with H₂O prior to TMA dosing doubles the aluminum coverage compared to the TMA only dosed Ge(100) surface. The higher nucleation density from the two step functionalization process, TMA + H₂O, should be favorable for pinhole reduction. DFT calculations are consistent with the data showing TMA reaction with either –Ge-H and –Ge-OH is exothermic, but the reaction of TMA on the –Ge-OH site has both a low activation barrier and higher exothermicity (-41.4 kcal/mol) compared to TMA reaction on the –Ge-H site (-10.8 kcal/mol). The calculation is consistent with the key to full monolayer nucleation, the formation of a full monolayer of Ge-OH chemisorption sites, which is being studied with HOOH dosing.

Research on novel channel materials such as Ge and InGaAs for high performance MOSFETs prompts interest in alternative high permittivity gate dielectrics because the thermal stability requirements that led to the adoption of HfO₂-based high-k dielectrics on silicon are relaxed for such channels. We have focused recently on ALD-grown TiO₂/Al₂O₃ bilayer dielectrics that combine large band gap Al₂O₃ interfacial layers (1-2 nm in thickness) with physically thicker but very high k TiO₂ gate dielectrics as a means of scaling gate capacitance and gate leakage current. This bilayer structure relies on the electrical passivity of the Al₂O₃/channel interface to achieve high performance. This presentation will summarize the scaling potential of these bilayer dielectric structures and methods by which low defect density Al₂O₃/Ge and Al₂O₃/InGaAs interfaces can be prepared.

Memresistive devices operate by changing resistance from high (Roff) to low (Ron) values in response to an applied voltage. Despite a tremendous amount of work in the scientific literature, the actual underlying switching mechanism has yet to be fully explained. Few studies, most of which lack measurement details, have reported on the transient current response and high speed switching characteristics of memresistive devices. The papers suggest the presence of the variety of active current and thermal dissolution of the low resistance filament while switching OFF. The switching ON has been attributed to the movement of the metal ions towards the cathode and being neutralized by the electrons to form the metal filament. Transient current measurements for switching ON have not yet been analyzed in detail in the literature. Another crucial problem in measurements of these devices is due to the preferred values of Ron(<1kΩ) and Roff(>1GΩ). Thus the key obstacle preventing fundamental understanding has been the lack of reliable and accurate measurements of the transient response while switching ON (from Roff to Ron). In this work, we developed a new measurement capability that enables reliable and accurate investigation of the transient switching response, monitoring change from low Ron to high Roff.

In order to address the need to reliably and accurately monitor the device I-V transient response extremely fast, we have designed an amplifier with low-gain and high bandwidth (1.7 GHz) to accommodate much faster (and realistic) “program” voltage pulses. For switching purposes, this amplifier allows for fast transient current monitoring during programming with proficiency but not limited to follow 2 nsec of pulse rise time. The high gain bandwidth of the amplifier allows us to monitor change from low current (Roff) to high current (Ron) accurately.

1. R. Waser, R. Dittmann, G. Staikov, K. Szot, Advanced Materials 21, 2632 (2009).