S. P. Frigo

Observations on the surface and bulk luminescence of porous silicon

Using the x‐ray excited optical luminescence technique, we have investigated the soft x‐ray induced photoluminescence of porous silicon in the optical region (200–900 nm) and the Si K‐edge x‐ray absorption fine structures of porous silicon in the near edge region. It is found that while porous silicon prepared at low current density (20 mA/cm2 for 20 min) exhibits a single broad luminescence band, porous silicon prepared at high current density (200 mA/cm2 for 20 min) exhibits three optical luminescence channels; i.e., in addition to the broad peak characteristic of all porous silicon, there are at least two additional optical luminescence channels at shorter wavelengths, one with modest intensity at ∼460 nm and the other a weak and very broad peak at ∼350 nm. These optical channels have been used to monitor the Si K‐edge absorption of porous silicon in the near edge structure region. Analysis of the data shows that while the band at ∼627.5 nm corresponds to the bulk emission, the other channels are of a ...

Adsorption of saturated hydrocarbons on the Si(111)-7 × 7 surface studied by photoelectron and photon stimulated desorption spectroscopies

Abstract Photoelectron spectroscopy (PES) and photon stimulated desorption (PSD) experiments were carried out to follow the thermal chemistry of methane- (CH 4 ), neopentane- (C 5 H 12 ), and adamantane- (C 10 H 16 )-dosed-Si(111)-7 × 7 surfaces. Both methane and adamantane adsorb molecularly on the 7 × 7 reconstructed Si surface at temperatures of 30 and 85 K, respectively. In contrast, at low coverages ( 15 molecules/cm 2 , 85 K; 0.4 ML), a fraction of the neopentane adlayer adsorbs dissociatively; at higher coverages, neopentane adsorption is predominantly molecular. Conversely, the adamantane and neopentane adlayers desorb at temperatures of ∼200 and 115 K, respectively, for heating rates of ∼1 K/min. No desorption temperature was determined for methane, but the methane adlayer was observed to desorb below 100 K. As determined by both PES and PSD, annealed adamantane- and neopentane-dosed surfaces react to form nearly identical surfaces. Si 2p core-level spectra show chemically shifted components of 0.48±0.05, 1.00±0.05, and 1.50±0.05 eV with respect to the bulk component. This demonstrates formation of Si(CH y ) x ( x =1-3; y =0-3) type surface species. The H + PSD spectra of the thermally reacted surfaces were measured and show chemical shifts of ∼0.7 eV with respect to bulk Si and two sharp resonances at 100.7 and 101.3 eV. The edge shift and associated structure highlights the chemical specificity of PSD and demonstrates its utility for following surface chemical reactions.

Soft x‐ray photoemission characterization of the H2S exposed surface of p‐InP

Synchrotron radiation soft x‐ray photoemission spectroscopy was used to characterize the surface chemistry of p‐InP before and after exposure to H2S gas at ambient temperature. The effect of the H2S dosing was determined by in situ photoemission measurements which were acquired after each gas exposure in order to observe changes in the valence band electronic structure as well as changes in the In 4d, P 2p, and S2p core lines. The results were used to correlate the surface chemistry to the electronic properties. These measurements indicate that the H2S exposure type converts the p‐type InP surface to an n‐type surface and that the magnitude of the band bending is 0.6 eV resulting in a homojunction interface.

Synchrotron radiation induced reactions on surfaces: mechanisms and applications

Abstract In this paper we tabulate recent research that delves into the application of synchrotron radiation for inducing surface chemical reactions. We then present results from recent experiments designed to test whether surface photochemical reactions can be brought about by direct core-level excitation of the adsorbate, as opposed to an indirect mechanism where photoabsorption by bulk atoms produces secondary electrons that cause the reaction. Experiments were performed on SiF 4 adsorbed on Ge(100) at 30 K. It was found that direct excitation of Si2p core levels causes the reaction to occur. Finally we show the application of X-ray dependent surface photochemistry in two systems of technological relevance. The first is adamantane (C 10 H 16 ) adsorbed on Si(111) at 85 K. X-rays with energies greater than the CK edge (284 eV) cause significant bond breaking and the formation of SiC. Lower energy X-rays appear to create adamantyl radicals which bond strongly with the substrate and are stable to 625 K. The second is Teflon where irradiation with photons with energies lower than 12 eV induces the desorption of relatively large fluorocarbon fragments, as compared to photolysis with photons of energies greater than 22 eV (C2s excitation).

Synchrotron radiation (5–50 eV) induced degradation of fluorinated polymers

The photochemistry of solid, free‐standing films of poly(tetrafluoroethylene) (PTFE) and poly(hexafluoropropylene–tetrafluoroethylene) (FEP) was investigated by monitoring the neutral photodesorption products produced upon irradiation with synchrotron radiation (5–50 eV). CF3, C2F5, C3F5, C3F7, and C4F7 were the predominant species observed in the mass spectra of the irradiated polymers. Oligomers at least as large as 293 amu (C7F12) were also detected. Distinct differences are observed in the photodesorption yields and time responses of the yields based on the energy of the initial excitation. An edge jump at 22 eV in the photon‐stimulated desorption spectra of both PTFE and FEP films is assigned to the ionization of the C(2s) states of the polymers. The energy dependence of the photodesorption yields is accounted for by considering two separate reaction pathways.

X‐ray energy dependent photochemistry of the adamantane (C10H16)/Si(111)‐7×7 surface

We have studied the synchrotron radiation (SR) induced photochemistry of the adamantane (C10H16) dosed Si(111)‐7×7 surface using photoelectron spectroscopy. The adamantane/Si surfaces were characterized before and after they were exposed to broadband SR of varying energies and then annealed. When annealed to 525 K, the unexposed adamantane layers desorbed leaving only a layer of Si–H and hydrated Si–C species on the surface. When the adamantane adlayer was irradiated with broadband SR that contained energies greater than the C(1s) binding energy (284 eV), the surface reacted to form a layer similar to SiC. In contrast, when the surface was irradiated with broadband SR with energies below 284 eV, only the adamantane in direct contact with the Si surface dissociated. The remaining adamantane adlayer reacted with the surface in a manner which left the molecular structure of the reacted adamantane intact. The reacted adamantane layer was stable up to temperatures of at least 675 K.

Photoemission characterization of the H2 plasma etched surface of InP

Synchrotron radiation soft x‐ray photoemission spectroscopy was used to characterize the surface chemistry of InP before and after exposure to a H2 plasma. The low‐power H2 plasma was generated with a commercial electron cyclotron resonance plasma source using a mixture of H2 and Ar with the plasma exposure being performed at ambient temperature. Plasma species were identified with quadrupole based mass spectrometry and optical emission spectroscopy. Photoemission measurements were acquired after each plasma exposure in order to observe changes in the valence‐band electronic structure as well as changes in the In 4d and P 2p core lines. The results were correlated in order to relate the plasma species and characteristics to changes in surface chemistry.

Surface type conversion of InP by H2S plasma exposure : a photoemission investigation

The surface chemistry and the electronic structure of InP(100) before and after exposure to a H2S plasma was investigated by synchrotron radiation soft x‐ray photoemission spectroscopy. The low power H2S plasma was generated with a commercial electron cyclotron resonance plasma source using H2S with the plasma exposure being performed as a function of incident angle and temperature. Plasma species were identified with optical emission spectroscopy. In situ photoemission measurements were acquired after each plasma exposure in order to observe changes in the valence band electronic structure as well as changes in the In 4d, P 2p, and S 2p core lines. The results were correlated in order to relate the plasma species and characteristics to changes in surface chemistry and electronic properties. These measurements indicate that the H2S plasma exposure type converts the p‐type InP(100) surface to an n‐type surface resulting in a shallow homojunction interface.

The soft x‐ray photochemistry of physisorbed SiF4. I. Reactions of the molecular species through desorption and dissociation

We present evidence that demonstrates photolysis of SiF4 adsorbed on Ge(100) at 30 K. Silicon 2p soft x‐ray photoemission spectroscopy (PES) indicates that upon irradiation, the molecularly adsorbed SiF4 dissociates into SiFn species (where n=0,1,2,3) and desorbs as molecular SiF4. Also, the Si 2p PES from undissociated molecules exhibits a number of distinct kinetic‐energy shifts. These are attributed to anisotropic adsorption in which different molecular sites have different apparent Si 2p binding energies. A structure of the adsorbate layer is proposed to account for the varying core hole screening. Examination of the gas phase during irradiation confirms molecular desorption and shows the system to have a significant neutral molecular desorption yield. Changes in the valence‐level photoemission structure and signal intensity are consistent with the observed fragmentation and desorption, both of which lead to disappearance of the molecularly adsorbed species.

THE SOFT X-RAY PHOTOCHEMISTRY OF PHYSISORBED SIF4. II. MECHANISMS AND KINETICS

We present an analysis of the extensive photolysis of an adsorbate resulting from adsorbate core‐level excitation. The system studied was SiF4 adsorbed on Ge(100) at 30 K. Photolysis fragments and molecular species (identified with Si 2p soft x‐ray photoemission spectroscopy) were measured as a function of monochromatic (140‐eV) photon exposure and adsorbate coverage. The photolysis cross sections for 55–140‐eV photons were determined and the neutral photon‐stimulated desorption cross section for a selected SiF4 excitation is also presented. In the Si 2p absorption region, it was found that the photolysis cross section was one to three times the preedge value and comparable in magnitude to that of gas phase photoabsorption, while the total yield increased at most by a factor of 1.4. Both of these observations indicate that direct core excitation of the adsorbate is a major path by which photolysis occurs as opposed to an indirect, substrate‐driven one.