Theoretical Evaluation of Zirconia and Hafnia as Gate Oxides for Si Microelectronics

Abstract

Parameters determining the performance of the crystalline oxides zirconia (ZrO2) and hafnia (HfO2) as gate insulators in nanometric Si electronics are estimated via ab initio calculations of the energetics, dielectric properties, and band alignment of bulk and thin-film oxides on Si (001). With their large dielectric constants, stable and low-formation-energy interfaces, large valence offsets, and reasonable (though not optimal) conduction offsets (electron injection barriers), zirconia and hafnia appear to have a considerable potential as gate oxides for Si electronics.PACS numbers: 68.35.-p, 77.22.-d, 85.30.-z, 61.66.-p The performance needs of modern information technology are forcing Si-based ultra-large-scale-integrated (ULSI) devices into the domain of nanometric dimensions. This downscaling implies, among others, the effective continuing reduction of the physical thickness of insulating gate oxide layers in CMOS (Complementary Metal-Oxide-Semiconductor) devices. Amorphous SiO 2 , the natural oxide of Si technology, is now nearing its fundamental size limits, with physical thicknesses currently down to 2 unit cells [1]. This leads to uncomfortably large (> 1 A/cm 2 ) leakage currents and increased failure probabilities. The main reason for the strong reduction of gate-oxide thickness in device downscaling is the need for increasing capacitances in the CMOS conducting channel. In a CMOS, the gate oxide layer dominates the series capacitance of the channel. An increase in capacitance can be obtained reducing the dielectric thickness d/ε of the oxide layer, having physical thickness d and relative dielectric constant ε. Given its small dielectric constant, it is understandable that SiO 2 as a gate oxide has emerged as one of the key bottlenecks in device donwscaling [1,2].It thus appears that, if Moore's law [3] on ULSI circuit component density -and hence circuit performance -is to remain valid in the next decade, a replacement will have to be found for silica as a gate insulator. The basic selection criteria for such a replacement are i) larger dielectric constant ("high-κ"), ii) interface band offsets to Si as large as or comparable to those of silica (especially the electron injection barrier), iii) epitaxy on Si energetically not too costly, iv) thermodynamical stability in contact with Si. In this work we address the expected performance, in terms of the above criteria, for the two important current candidates [1, 2, 4] hafnia (HfO 2 ) and zirconia (ZrO 2 ) through first-principles density-functional calculations of the structure, energetics, thermodynamical stability, dielectric constants, and band offsets of crystalline hafnia and zirconia thin films epitaxially grown on the (001) face of crystalline Si. We find stable and moderate-cost interfaces, large dielectric constants, and large band offsets, except for the electron injection barrier, estimated at 1 eV at most, appreciably lower than the Si/silica barrier.Our density functional theory calculations in the generalized gradient approximation [5...

Advances in computational studies of energy materials

“…Owing to the increased miniaturization of transistors, the silica gates are reaching the lower size limit at which quantum tunnelling effects will cause failure, and therefore there is a strong incentive to find substitutes for SiO 2 . The high dielectric constant of ZrO 2 and HfO 2 makes them the most likely replacement (Wilk et al 2001;Fiorentini & Gulleri 2002;Pasquarello & Stoneham 2005;Stoneham et al 2005). We now review our work on the three compounds.…”

Impurity Centers in Oxides Investigated by γ-γ Perturbed Angular Correlation Spectroscopy and <i>Ab Initio</i> Calculations

“…HfO 2 presents great technological importance because of its high melting point, high chemical stability, and large dielectric constant. In various forms and with the addition of small amounts of impurities, it has applications ranging from solid oxide fuel cell electrolytes to catalyst substrates and protective coatings [107][108][109][110][111]. HfO 2 is of particular interest because of its applications for electronic devices.…”

Structure and dielectric properties of amorphous high-κoxides: HfO2, ZrO2, and their alloys

“…Major research efforts are continuously being carried out in search for a suitable replacement of the SiO 2 as a gate material. 1 Several promising candidates are metal oxides such as HfO 2 , 2,3 ZrO 2 3-5 and Al 2 O 3 , 6 all of which have high value of dielectric constant κ. High-κ metal oxides in their amorphous (a-) form are more preferable as a gate oxide over their crystalline form due to several important advantages they provide: (i) isotropic physical properties; (ii) no crystalline domain boundary which leads to less defects at the interface with the Si substrate; and (iii) good compatibility with the conventional CMOS fabrication process. Some alloy structures of these high-κ metal oxides are also being studied extensively [7][8][9][10][11][12] and, in fact, a Hf based alloy material has already been in its third generation of production as a gate oxide in the semiconductor industry and further improvements of thermal stability, dielectric constant and material preparations are underway.…”