The scaling of complementary metal oxide semiconductor (CMOS) transistors has led to the silicon dioxide layer used as a gate dielectric becoming so thin that the gate leakage current becomes too large. This led to the replacement of SiO2 by a physically thicker layer of a higher dielectric constant or ‘high-K’ oxide such as hafnium oxide. Intensive research was carried out to develop these oxides into high quality electronic materials. In addition, the incorporation of Ge in the CMOS transistor structure has been employed to enable higher carrier mobility and performance. This review covers both scientific and technological issues related to the high-K gate stack – the choice of oxides, their deposition, their structural and metallurgical behaviour, atomic diffusion, interface structure, their electronic structure, band offsets, electronic defects, charge trapping and conduction mechanisms, reliability, mobility degradation and oxygen scavenging to achieve the thinnest oxide thicknesses. The high K oxides were implemented in conjunction with a replacement of polycrystalline Si gate electrodes with metal gates. The strong metallurgical interactions between the gate electrodes and the HfO2 which resulted an unstable gate threshold voltage resulted in the use of the lower temperature ‘gate last’ process flow, in addition to the standard ‘gate first’ approach. Work function control by metal gate electrodes and by oxide dipole layers is discussed. The problems associated with high K oxides on Ge channels are also discussed.

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High-K materials and metal gates for CMOS applications

The move to implement metal oxide based gate dielectrics in a metal-oxide-semiconductor field effect transistor is considered one of the most dramatic advances in materials science since the invention of silicon based transistors. Metal oxides are superior to SiO 2 in terms of their higher dielectric constants that enable the required continuous down-scaling of the electrical thickness of the dielectric layer while providing a physically thicker layer to suppress the quantum mechanical tunneling through the dielectric layer. Over the last decade, hafnium based materials have emerged as the designated dielectrics for future generation of nano-electronics with a gate length less than 45 nm, though there exists no consensus on the exact composition of these materials, as evolving device architectures dictate different considerations when optimizing a gate dielectric material. In addition, the implementation of a non-silicon based gate dielectric means a paradigm shift from diffusion based thermal processes to atomic layer deposition processes. In this report, we review how HfO 2 emerges from all likely candidates to become the new gold standard in the microelectronics industry, its different phases, reported electrical properties, and materials processing techniques. Then we use specific examples to discuss the evolution in designing hafnium based materials, from binary to complex oxides and to non-oxide forms as gate dielectric, metal gates and diffusion barriers. To address the impact of these hafnium based materials, their interfaces with silicon as well as a variety of semiconductors are discussed. Finally, the integration issues are highlighted, including carrier scattering, interface state passivation, phonon engineering, and nano-scale patterning, which are essential to realize future generations of devices using hafnium-based high- k materials.

Development of hafnium based high-k materials—A review

In search of a proper passivation for high-k Ge metal-oxide-semiconductor devices, the authors have deposited high-k dielectric layers on GeO2, grown at 350–450°C in O2. ZrO2, HfO2, and Al2O3 were deposited by atomic layer deposition (ALD). GeO2 and ZrO2 or HfO2 intermix during ALD, together with partial reduction of Ge4+. Almost no intermixing or reduction occurs during Al2O3 ALD. Capacitors show well-behaved capacitance-voltage characteristics on both n- and p-Ge, indicating efficient passivation of the Ge∕GeOx interface. The density of interface states is typically in the low to mid-1011cm−2eV−1 range, approaching state-of-the-art Si∕HfO2∕matal gate devices.

Effective electrical passivation of Ge(100) for high-k gate dielectric layers using germanium oxide

Internal photoemission spectroscopy provides the most straightforward way to characterize the relative energies of electron states at interfaces of insulators with metals and semiconductors by measuring the spectral onset of electron/hole photoemission from one solid into another. The article reviews the application of this technique for characterization of advanced nanometer-thin insulators prospected to be used in microelectronic devices. Fundamental aspects and technical features of the internal photoemission experiments are discussed together with basic electronic properties of a number of investigated high-permittivity insulating films and their interfaces in semiconductor heterostructures. Significant differences are found in the electronic properties of nanometer-thin amorphous insulating layers as compared to the known bulk phase characteristics. The band alignment at the interfaces of these insulators with metals is found to be highly sensitive to the surface preparation procedures. By contrast, ...

Internal photoemission at interfaces of high-κ insulators with semiconductors and metals

We review oxygen K-edge X-ray absorption spectra of both molecules and solids. We start with an overview of the main experimental aspects of oxygen K-edge X-ray absorption measurements including X-ray sources, monochromators, and detection schemes. Many recent oxygen K-edge studies combine X-ray absorption with time and spatially resolved measurements and/or operando conditions. The main theoretical and conceptual approximations for the simulation of oxygen K-edges are discussed in the Theory section. We subsequently discuss oxygen atoms and ions, binary molecules, water, and larger molecules containing oxygen, including biomolecular systems. The largest part of the review deals with the experimental results for solid oxides, starting from s- and p-electron oxides. Examples of theoretical simulations for these oxides are introduced in order to show how accurate a DFT description can be in the case of s and p electron overlap. We discuss the general analysis of the 3d transition metal oxides including discussions of the crystal field effect and the effects and trends in oxidation state and covalency. In addition to the general concepts, we give a systematic overview of the oxygen K-edges element by element, for the s-, p-, d-, and f-electron systems.

Oxygen K-edge X-ray Absorption Spectra

Thin insulator films of the high-κ dielectric HfO2 are deposited on Ge(100) substrates by evaporating Hf in atomic oxygen beams after in situ thermal desorption of the native oxide in ultrahigh vacuum and subsequent treatment of the clean Ge surface in oxygen and nitrogen. It is shown that HfO2 forms atomically sharp interfaces with Ge and behaves as an excellent insulator with dielectric permittivity κ∼25, which is close to the expected bulk value. Very low equivalent oxide thickness of 0.75 (±0.1) nm with a low gate leakage current of ∼4.5×10−4A∕cm2 at 1 V in accumulation is achieved. Strong frequency dispersion of the inversion capacitance and low frequency behavior of the high frequency capacitance–voltage curves is observed. This is attributed to a combined effect of a high generation rate of minority carriers due to impurity traps and the high intrinsic carrier concentration in Ge, which result in a short minority carrier response time.

HfO2 high-κ gate dielectrics on Ge (100) by atomic oxygen beam deposition

We show that at deposition temperature in the 750–770°C range, the La2Hf2O7 (LHO) compound can be grown crystalline on Si(001). The predominant orientation is (001)LHO∕∕(001)Si and [110]LHO∕∕[110]Si and results in ultimately clean interfaces, indicating a strong tendency for cube-on-cube epitaxy. The ordered pyrochlore and random fluorite phases coexist in the dielectric. Acceptable gate leakage current, negligible hysteresis and high dielectric permittivity κ∼23 were obtained from electrical characterization of metal–insulator–semiconductor capacitors. The quality of interfaces and the good electrical characteristics make crystalline LHO a promising high-κ candidate for the replacement of SiO2 in the gate of future aggressively scaled transistors.

La2Hf2O7 high-κ gate dielectric grown directly on Si(001) by molecular-beam epitaxy

In this work, we investigate La-based complex oxides such as perovskite-like LaAlO 3 and pyrochlore La 2 Hf 2 O 7 , deposited on rapid thermal SiO 2 (RTO)/Si substrates using rf plasma source of atomic oxygen in an MBE chamber. The C – V curves measured in MIS capacitors were analyzed solving self-consistently the Poisson and Schrodinger equations to take into account quantum mechanical effects. From the dependence of the equivalent oxide thickness (EOT) as a function of physical thickness t ph , the dielectric permittivity was estimated to be 14 and 18 for perovskite-like LaAlO 3 (LAO) and pyrochlore La 2 Hf 2 O 7 (LHO), respectively, which are lower than expected probably due to slight deviations from the stoichiometric compositions. Values of EOT∼1.1 nm and J g ∼2×10 −3  A/cm 2 at 1 V for the LAO, and EOT∼1.14 nm and J g ∼4×10 −5  A/cm 2 at 1 V for the LHO were obtained.

MBE lanthanum-based high-k gate dielectrics as candidates for SiO2 gate oxide replacement

Stabilization of a very high-k tetragonal ZrO2 phase by direct doping with germanium

In the perspective of exploring alternative gate dielectrics for the future generation of microelectronic devices, we investigated experimentally and theoretically the interface energy barriers induced on (001) silicon by La2Hf2O7, whose growth has been recently attained by molecular-beam epitaxy. Experimental results show that the 5.6±0.1eV band gap of La2Hf2O7 is aligned to the band gap of silicon with a valence band offset of 2.4±0.1eV and a conduction band offset of 2.1±0.1eV. Density functional theory calculations yield valence band offset values ranging between 1.8 and 2.4eV.

G. Mavrou

Papers

HfO2 high-κ gate dielectrics on Ge (100) by atomic oxygen beam deposition

La2Hf2O7 high-κ gate dielectric grown directly on Si(001) by molecular-beam epitaxy

MBE lanthanum-based high-k gate dielectrics as candidates for SiO2 gate oxide replacement

Stabilization of a very high-k tetragonal ZrO2 phase by direct doping with germanium

Band alignment at the La2Hf2O7∕(001)Si interface