Electron Photoinjection from Silicon to Ultrathin SiO2Films via Ambient Oxygen

Abstract

Hot electrons are generated in Si(001) at 295 K via linear absorption of .4.3 eV photons or by threephoton processes using 270 fs, 800 nm (1.55 eV) optical pulses. Electron trapping in oxide films is observed via time-dependent optical second harmonic generation induced by the electric field associated with charge transfer. For anodically oxidized samples and constant beam irradiance, the transfer rate decreases to zero with increasing oxide thickness with a characteristic length of 3.5 nm, comparable to the electron scattering length; the rate increases with ambient oxygen pressure (P) as P 0.035 . These results indicate that oxygen is essential to hot electron transfer in ultrathin oxides and serves at least as a trapping catalyst. [S0031-9007(96)00817-4] PACS numbers: 73.50. Gr, 42.65.Ky, 73.40.Qv The physical characteristics of the amorphous oxide which forms on crystalline silicon have played a major role in establishing the dominance of silicon in semiconductor technology. Since electrical properties related to charge trapping are especially relevant to the performance of many semiconductor devices, carrier trapping has long been an important field of study. Charge transfer from Si to SiO 2 via thermionic or photoemission apparently is also important in thermal oxidation of Si [1,2]. For thin (,20 nm) oxides [3] and for clean Si [4] there have also been suggestions that electrons from Si may attach to gaseous O 2 in "harpooning" reactions [5] at the surface, although many details are unclear. As dimensions of Si-based devices continue to shrink, it is becoming increasingly important to better understand how thin oxide films influence electron transfer. However, traditional electronic methods for observing trapped charge require metal contacts which prevent oxygen access to the SiO 2 ͞ambient interface and alter the properties of thin oxides through in-diffusion of metal atoms. Noncontact methods such as electron photoemission spectroscopy can only be used in vacuo. Since electron activation from the Si valence band into the oxide requires [6] at least DV 4.25 eV, nanosecond ultraviolet (UV) laser pulses [7] are often used to investigate electron transport and even oxide growth. However, since the fluence typically exceeds 10 mJ cm 22 and the optical absorption depth in Si is ,5 nm [8], transient surface temperature increases of ¿100 K make the separation of photonic and thermal effects difficult. Evidence for electron transfer or trapping is based on post facto measurements of irreversible oxide growth.Optical second harmonic generation (SHG) has proven to be a useful, noninvasive probe of interfaces between centrosymmetric materials [9-18] such as the Si͞SiO 2 buried interface which cannot be accessed by other techniques. As an optical technique SHG can be used in situ, in the presence of ambient gases. When high repetition rate ϳ100 fs laser pulses are used, high second harmonic (SH) signal sensitivity is obtained with negligible sample heating [11]. SHG can also detect changes in electric fields [15 ...

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