Many materials systems are currently under consideration as potential replacements for SiO2 as the gate dielectric material for sub-0.1 μm complementary metal–oxide–semiconductor (CMOS) technology. A systematic consideration of the required properties of gate dielectrics indicates that the key guidelines for selecting an alternative gate dielectric are (a) permittivity, band gap, and band alignment to silicon, (b) thermodynamic stability, (c) film morphology, (d) interface quality, (e) compatibility with the current or expected materials to be used in processing for CMOS devices, (f) process compatibility, and (g) reliability. Many dielectrics appear favorable in some of these areas, but very few materials are promising with respect to all of these guidelines. A review of current work and literature in the area of alternate gate dielectrics is given. Based on reported results and fundamental considerations, the pseudobinary materials systems offer large flexibility and show the most promise toward successful integration into the expected processing conditions for future CMOS technologies, especially due to their tendency to form at interfaces with Si (e.g. silicates). These pseudobinary systems also thereby enable the use of other high-κ materials by serving as an interfacial high-κ layer. While work is ongoing, much research is still required, as it is clear that any material which is to replace SiO2 as the gate dielectric faces a formidable challenge. The requirements for process integration compatibility are remarkably demanding, and any serious candidates will emerge only through continued, intensive investigation.

Articles you may be interested inElectrical and interfacial characteristics of ultrathin ZrO 2 gate dielectrics on strain compensated SiGeC/Si heterostructure Appl. Phys. Lett. 82, 2320 (2003); 10.1063/1.1566480 Ultrathin zirconium silicate gate dielectrics with compositional gradation formed by self-organized reactions Stable zirconium silicate gate dielectrics deposited directly on silicon Electrical properties of hafnium silicate gate dielectrics deposited directly on siliconHafnium and zirconium silicate ͑HfSi x O y and ZrSi x O y , respectively͒ gate dielectric films with metal contents ranging from ϳ3 to 30 at. % Hf, or 2 to 27 at. % Zr ͑Ϯ1 at. % for Hf and Zr, respectively, within a given film͒, have been investigated, and films with ϳ2-8 at. % Hf or Zr exhibit excellent electrical properties and high thermal stability in direct contact with Si. Capacitance-voltage measurements show an equivalent oxide thickness t ox of about 18 Å ͑21 Å͒ for a 50 Å HfSi x O y ͑50 Å ZrSi x O y ͒ film deposited directly on a Si substrate. Current-voltage measurements show for the same films a leakage current of less than 2ϫ10 Ϫ6 A/cm 2 at 1.0 V bias. Hysteresis in these films is measured to be less than 10 mV, the breakdown field is measured to be E BD ϳ10 MV/cm, and the midgap interface state density is estimated to be D it ϳ1 -5ϫ10 11 cm Ϫ2 eV Ϫ1 . Au electrodes produce excellent electrical properties, while Al electrodes produce very good electrical results, but also react with the silicates, creating a lower ⑀ layer at the metal interface. Transmission electron microscopy ͑TEM͒ and x-ray photoelectron spectroscopy indicate that the dielectric films are amorphous silicates, rather than crystalline or phase-separated silicide and oxide structures. TEM shows that these films remain amorphous and stable up to at least 1050°C in direct contact with Si substrates.

We report on a GaN metal-oxide-semiconductor high-electron-mobility-transistor ͑MOS-HEMT͒ using atomic-layer-deposited ͑ALD͒ Al 2 O 3 as the gate dielectric. Compared to a conventional GaN high-electron-mobility-transistor ͑HEMT͒ of similar design, the MOS-HEMT exhibits several orders of magnitude lower gate leakage and several times higher breakdown voltage and channel current. This implies that the ALD Al 2 O 3 /AlGaN interface is of high quality and the ALD Al 2 O 3 /AlGaN/GaN MOS-HEMT is of high potential for high-power rf applications. In addition, the high-quality ALD Al 2 O 3 gate dielectric allows the effective two-dimensional ͑2D͒ electron mobility at the AlGaN/GaN heterojunction to be measured under a high transverse field. The resulting effective 2D electron mobility is much higher than that typical of Si, GaAs or InGaAs metal-oxide-semiconductor field-effect-transistors ͑MOSFETs͒.One of the major factors that limit the performance and reliability of GaN high-electron-mobility-transistors ͑HEMTs͒ for high-power radio-frequency ͑rf͒ applications is their relatively high gate leakage. The gate leakage reduces the breakdown voltage and the power-added efficiency while increasing the noise figure. To help solve the problem, significant progress has been made on metal-insulatorsemiconductor high-electron-mobility-transistors ͑MIS-HEMTs͒ and metal-oxide-semiconductor high-electronmobility-transistors ͑MOS-HEMTs͒ using SiO 2 , 1-5 Si 3 N 4 , 6,7 Al 2 O 3 8,9 ͑formed by electron cyclotron resonance plasma oxidation of Al͒, and other oxides. 10 However these gate dielectrics and their associated processes may not be readily scalable for low-cost and high-yield manufacture. Atomic layer deposition ͑ALD͒ is a surface controlled layer-by-layer process for the deposition of thin films with atomic layer accuracy. Each atomic layer formed in the sequential process is a result of saturated surface controlled chemical reactions. The thickness control of the ALD films thus scalability is much superior than those of the plasma-enhanced-chemicalvapor-deposition ͑PECVD͒ grown SiO 2 and Si 3 N 4 . The quality of the ALD Al 2 O 3 is also much higher than those deposited by other methods, i.e., sputtering and electron-beam deposition, in terms of uniformity, defect density and stoichiometric ratio of the films.In this letter, we report on a GaN MOS-HEMT with atomic-layer-deposited Al 2 O 3 as the gate dielectric. Similar to SiO

Hafnium silicate (HfSixOy) gate dielectric films with ∼6 at. % Hf exhibit significantly improved leakage properties over SiO2 in the ultrathin regime while remaining thermally stable in direct contact with Si. Capacitance–voltage measurements show an equivalent oxide thickness (tox) of less than 18 Å for a 50 Å HfSixOy film deposited directly on a Si substrate, with no significant dispersion of the capacitance for frequencies ranging from 10 kHz to 1 MHz. Current–voltage measurements show for the same film a leakage current of 1.2×10−6 A/cm2 at 1 V bias. Hysteresis in these films is measured to be less than 20 mV, the breakdown field is measured to be EBD∼10 MV/cm, and the midgap interface state density is Dit∼1011 cm−2 eV−1. Cross-sectional transmission electron microscopy shows no signs of reaction or crystallization in HfSixOy films on Si after being annealed at 800 °C for 30 min.

A study was undertaken to determine the efficacy of various underlayers for the nucleation and growth of atomic layer deposited HfO2 films. These were compared to films grown on hydrogen terminated Si. The use of a chemical oxide underlayer results in almost no barrier to film nucleation, enables linear and predictable growth at constant film density, and the most two-dimensionally continuous HfO2 films. The ease of nucleation is due to the large concentration of OH groups in the hydrous, chemical oxide. HfO2 grows on chemical oxide at a coverage rate of about 14% of a monolayer per cycle, and films are about 90% of the theoretical density of crystalline HfO2. Growth on hydrogen terminated Si is characterized by a large barrier to nucleation and growth, resulting in three-dimensional, rough, and nonlinear growth. Thermal oxide/oxynitride underlayers result in a small nucleation barrier, and nonlinear growth at low HfO2 coverages. The use of chemical oxide underlayers clearly results in the best HfO2 layers. Further, the potential to minimize the chemical oxide thickness provides an important research opportunity for high-κ gate dielectric scaling below 1.0 nm effective oxide thickness.