Oxides, Silicides, and Silicates of Zirconium and Hafnium; Density Functional Theory Study
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Oxides, Silicides, and Silicates of Zirconium and Hafnium; Density Functional Theory Study Maciej Gutowskia,b, John E. Jaffea a
Pacific Northwest National Laboratory
Environmental Molecular Sciences Laboratory Theory, Modeling & Simulations Richland, WA 99352, USA b
Department of Chemistry, University of Gdańsk
80-952 Gdańsk, Poland Chun-Li Liu, Matt Stoker, Anatoli Korkin Advanced Process Development and External Research Laboratory, Motorola, Mesa, AZ 85202, USA ABSTRACT It is known that the chemistries of hafnium and zirconium are more nearly identical than for any other two congeneric elements. Thus, both zirconia and hafnia, with the dielectric constant K > 20, have emerged as potential replacements for silica (K = 3.9) as a gate dielectric. We report an important difference between the zirconia/Si and hafnia/Si interfaces based on density functional theory calculations with the PerdewWang 91 exchange-correlation functional on the oxides, silicides, and silicates of Zr and Hf. The zirconia/Si interface has been found to be unstable with respect to formation of silicides whereas the hafnia/Si interface is stable. The difference between the two interfaces results from the fact that HfO2 is more stable than ZrO2 (i.e. has a larger heat of formation from its constituent elements) by more than 53 kJ/mol. The hafnium silicides, on the other hand, are less stable than zirconium silicides by ca. 20 kJ/mol.
INTRODUCTION The microelectronics industry depends on continual miniaturization of all components, especially metal-oxide-silicon field-effect transistors. However, the industry is approaching the useful limits of SiO2 gate dielectrics, due to the simultaneous need for B6.5.1 Downloaded from https://www.cambridge.org/core. Access paid by the UCSB Libraries, on 09 Mar 2018 at 18:02:37, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-716-B6.5
sufficient resistance (proportional to gate thickness) and capacitance (inversely proportional to thickness). Alternate gate dielectrics with a dielectric constant K higher than that of SiO2 are needed since then a thicker gate can be used.1,2,3 In addition to a high K, a large band gap, and low density of electrically active defects, an alternate material needs to be thermodynamically stable in contact with silicon to withstand the 900-1000 °C dopant drive-in anneal.2,3
Table I. The enthalpy (∆H) and Gibbs energy (∆G) changes (kJ/mol) for the potential chemical reactions across ZrO2/Si, ZrO2/SiO2, HfO2/Si, and HfO2/SiO2 interfaces. Reaction
∆Ha 0K
∆Hb
∆Hb
∆Gb
298 K 1000 K 1000 K
(1) Si + ZrO2 → Zr + SiO2
165.8
186.6
185.6
177.7
(2) 3Si + ZrO2 →ZrSi2 + SiO2
-13.5
5.7
1.9
3.3
(3) 2Si + ZrO2 →ZrSi + SiO2
-8.5
-2.4
-4.9
-11.0
(4) 3Si + 2ZrO2 → ZrSi2 + ZrSiO4
-30.3
-19.8
-15.2
-3.2
(5) 2Si + 2ZrO2 → ZrSi + ZrSiO4
-25.3
-27.9
-22.0
-17.5
(6) Si + 2ZrO2 → Zr + ZrSiO4
149.0
161.1
168.5
171.2
(7) SiO2 + ZrO2 → ZrSiO4
-16.8
-25.5
-17.1
-6.5
(1) Si + HfO2 → Hf + SiO
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