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 ...

We have observed that the second-harmonic signal generated from oxidized Si(001) varies on a time scale of several seconds in experiments involving a fundamental beam of lambda = 770 nm, 110-fs pulses at 76 MHz. We suggest that the temporal behavior arises from absorption of weak (<100-fW average power) third-harmonic light generated in air or in the sample, inducing charge transfer across the Si-SiO(2) interface and trapping in the oxide layer. Detrapping has been determined to take several minutes.

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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...

Transient trapping/detrapping of electrons at the Si͑100͒/SiO 2 outer surface is studied studied in vacuum or with an O 2 ambient ͑between 10 Ϫ3 and 30 Torr͒ following internal electron photoemission from Si. Photoemission-current ͑produced by a 150 fs, 800 nm laser source͒ and contact-potential-difference techniques were used to investigate a wide variety of n-and p-doped samples at 300 K with thermally grown, steam grown, and dry oxides with thickness р5 nm as well as samples with the oxide layers removed. Characteristics of the steam grown oxide were also studied at 400 and 200 K. For samples in vacuum charging is attributed to direct filling of at least two families of traps, one related to the oxide and the other the Si/SiO 2 interface. For samples in O 2 , details of oxygen-assisted surface charging as reported previously ͓Phys. Rev. Lett. 77, 920 ͑1996͔͒ are given. A fast, Coulomb-repulsion driven spillover of surface charge from the irradiated spot to the rest of the surface was detected. Oxygen aids trap filling of the in-vacuum filled and gas-sensitive traps and also detrapping ͑the efficacy of which increases strongly from 400 to 200 K͒ when the optical excitation source is removed. Surface transient charging and charge trapping efficacy for the oxidized samples are not very sensitive to sample preparation. A mobility of the trapped charges, probably hopping between traps and also Coulomb-repulsion driven, was measured.

Surface charging and electron trapping in ultrathin (1.6 nm) SiO2 films on n-type silicon during bombardment by 350–600 eV electrons are observed by electric-field-induced optical second harmonic generation (SHG). Transient surface charging by fast dissipating electrons (&lt;1 ms charge/discharge time) can be distinguished from oxide electron trapping occurring over hundreds of seconds. The maximum SHG enhancement corresponds to an areal density of trapped electrons of ∼6×1012 cm−2. The gradual recovery of the SHG following electron bombardment suggests the trap sites are “slow traps”, i.e., oxide traps which discharge via tunneling to the Si/SiO2 interface. The effective trap lifetime is about 500 s.

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In addition to Si band-edge electroluminescence ͑EL͒ near 1.1 eV, we observe hot-electron EL in metal-insulator-silicon tunnel diodes that can span a detector-limited range from 0.7 to 2.6 eV ͑1780-480 nm͒. The maximum photon energy increases with increasing forward bias. In one implementation, sub-micron-sized EL sites appear during the forward-bias stress. The number of sites grows linearly with the current, consistent with the dielectric breakdown of the insulator. We compare the poststress current-voltage data with the quantum-point-contact model. Results are presented for various p-type Si͑100͒ devices having 2-8-nm-thick SiO 2 , Al 2 O 3 , and HfO x N y insulators. We also describe devices in which electron-beam lithography of an 18-nm-thick SiO 2 is used to define EL sites.