Universal Mechanism for Gas Adsorption and Electron Trapping on Oxidized Silicon

Abstract

We report that common gases (such as He, Ar, H 2 , O 2 , N 2 , CO) experience adsorption at oxidized silicon surfaces at 300 K via electrostatic coupling. This is deduced using contact potential measurements of the work function for gas pressure in the range 10 23 , P , 10 2 Torr. The adsorption can be enhanced through surface charging via internal photoemission of electrons leading to mutual electron-gas transient trapping. A simple electrostatic model based on monopole-dipole coupling results in an isotherm in agreement with the data. [S0031-9007(98)08233-7] PACS numbers: 68.45. Da, 79.60.Dp, 82.30.Fi, 82.65.Pa Surface electronic states are crucial in determining the characteristics of one of the most important technological systems, Si͞SiO 2 . Not only do electrons assist oxide growth [1] but they also have a profound effect on MOS (metal-oxide-semiconductor) technology. Knowledge of the SiO 2 surface is rather limited in comparison to the much studied surfaces of single crystal metals, semiconductors, and crystalline oxides [2]. SiO 2 is an amorphous material, the structure of which depends to some extent on growth conditions. The major interest in the properties of such surfaces corresponds to conditions in which the ambient gas pressure is close to atmospheric. Here we demonstrate that a wide variety of gases, including rare gases, have a "universal" adsorption mechanism involving purely electrostatic forces resulting also in a universal photoinduced gas-assisted charging of the surface. The key experimental results can be explained by a model in which an electrostatic field generated by the presence of naturally occurring as well as photoinduced negative charge on the surface polarizes the adsorbed gas particles leading to binding energies in the range of a few hundred meV.From electric-field-induced second harmonic generation [3] measurements, it has previously been demonstrated that, for thin (1-10 nm) oxide Si͞SiO 2 systems, surface charging can occur in the presence of ambient O 2 . Since other gases produced much less gas-assisted surface charging over a 10 22 760 Torr pressure range, oxygen was presumed to be unique and a possible mechanism of electron attraction via a "harpooning" reaction [4], possibly to form O 2 2 on the surface [5] was proposed. Using contact potential difference (CPD) and multiphoton-photoemission techniques, we now show that a variety of gases (He, Ar, H 2 , N 2 , O 2 , and CO) can be adsorbed nondissociatively on the oxide surface at room temperature and close to atmospheric pressure. We also demonstrate that such adsorption also assists transient charge trapping on the surface. Oxygen adsorption is special in that accumulation effects can occur as O 2 2 forms at a later stage [3,5]. The other gases give rise to transient behavior as dictated by adsorption, effectively serving as catalysts for trapping.The sample used was a polished, optically smooth ntype Si(001) wafer (resistivity 20 100 V cm), having a surface area of ϳ2 mm 3 12 mm and thickness 0.3 mm. It was covered w...

Electric Fields on Oxidized Silicon Surfaces: Static Polarization of PbSe Nanocrystals

“…However, the electric field on real oxide surfaces is not completely screened due to (a) the discrete nature of the interface charge, (b) the possible presence of additional discrete static charges in the oxide, and (c) the fundamental fact that the oxide is amorphous with varying local electrical properties. [1][2][3][4][5][6] We now study these electric fields, and their effect on adsorbed PbSe nanocrystals, using a direct imaging method: electrostatic force microscopy (EFM). We also photoexcite the samples in order to vary the surface charge distribution.…”

The Role of the Dipole Interaction of Molecules with Charged Particles in the Polar Stratosphere

“…3, the increased concentration of the molecules on the surface of the supercharged particles is strongly dependent on the dipole moment of the molecules. In this case, the calculated concentration of molecules with large dipole moments exceeds the maximum permissible concentration n lim ( [20,33,34] the adsorption of the non-polar molecules and atoms on the thin films charged by electrons was observed experimentally. Thus, in the work of Shamir et al [33,34] such effect was observed on the silicon surfaces for Не, Ar, N 2 , O 2 , etc., and Lu [20] showed the similar effect on a thin ice surface for CCl 4 molecules.…”

“…In this case, the calculated concentration of molecules with large dipole moments exceeds the maximum permissible concentration n lim ( [20,33,34] the adsorption of the non-polar molecules and atoms on the thin films charged by electrons was observed experimentally. Thus, in the work of Shamir et al [33,34] such effect was observed on the silicon surfaces for Не, Ar, N 2 , O 2 , etc., and Lu [20] showed the similar effect on a thin ice surface for CCl 4 molecules. Also, in the Lu's experiment [20] the simultaneous decomposition of a part of the CCl 4 molecules was observed as a result of the electron capture by these molecules.…”

Harmonic generation in thin films and multilayers

“…2,[36][37][38] Heating effects can alter the efficiency with which a surface generates pp-SHG, 2 but this does not account for all observations. The mechanism proposed by Mihaychuk and coworkers involves electron transport through the thin oxide overlayer, combined with trapping at the free surface mediated by oxygen in the ambient atmosphere.…”