Thermodynamics and Kinetics for Suppression of GeO Desorption by High Pressure Oxidation of Ge
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1155-C06-02
Thermodynamics and Kinetics for Suppression of GeO Desorption by High Pressure Oxidation of Ge K. Nagashio1,2, C. H. Lee1, T. Nishimura1,2, K. Kita1,2 and A. Toriumi1,2 1 2
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan JST-CREST, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
ABSTRACT We analyze a main scheme for the suppression of GeO desorption by the high pressure oxidation which drastically improve the electrical quality of Ge/GeO2 capacitors. The inherent driving force for GeO to form at the Ge/GeO2 interface and to diffuse toward the GeO2 surface was realized by the concentration gradient in the GeO2 film, which was obtained from the thermodynamic calculation. Kinetic consideration based on the comparison with Si/SiO2 stacks suggests that GeO desorption at the GeO2 surface is the rate-limiting process under passive oxidation conditions. When O2 pressure is increased by high pressure oxidation, the vapor pressure of GeO at the GeO2 surface is reduced, restricting GeO desorption at the GeO2 surface. 1. INTRODUCTION The introduction of high-k dielectrics into the Si-CMOS process makes Ge very attractive for applications in field effect transistors, since Ge possesses an intrinsically higher carrier mobility than that of Si and the historical deterioration in quality of the Ge/GeO2 interface due to GeO desorption [1,2] can be overcome by replacing GeO2 to high-k dielectrics [3]. Although several techniques (e.g., surface nitridation [4] and passivation with several Si layers [5]) have been proposed for Ge/high-k stacks, the best intrinsic quality of Ge/GeO2 interface should be realized since the interface layer composed of GeOx between Ge and high-k dielectrics are quite important [6]. Recently, we have shown that C-V hysteresis in Ge/GeO2 MIS capacitors was dramatically improved from ~1V to ~0.1V by high pressure oxidation (HPO) at PO2 ~70 atm at 550 °C [7]. Although we suggested that GeO desorption was suppressed by high pressure oxidation, a detailed mechanism was not presented yet. The oxidation of Si has been extensively investigated, and the volatility of SiO has been noted well [8-10]. At low oxygen pressures (i.e., ultrahigh vacuum conditions) and high temperatures, the interaction of O2 with a clean Si surface results in a rapid flux of volatile SiO molecules away from the surface via the reaction 2Si(s) + O2(g) = 2SiO(g) and the subsequent formation of a non-protective SiO2 smoke [9,10]. This active oxidation is a promising way to produce high quality and atomically clean Si surfaces [11]. On the other hand, at high oxygen pressures and high temperatures, a continuous SiO2 layer is formed on the Si surface via the reaction Si(s) + O2(g) = SiO2(s), which is known as passive oxidation. The active-passive transition occurs if the oxygen partial pressure PO2 is equal to PSiO, (e.g., PO2=PSiO=10-5.48 atm at 900 °C) [9,10]. Once a Si surface is covered with a thin SiO2 film under typical oxidation conditions, the SiO2 film is very uniform and stable, creating extremely hi
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