Jory A. Yarmoff

The chemisorption of chlorosilanes and chlorine on Si(111)7 × 7

Abstract The chemisorption of SiCl4, Si2Cl6, and chlorine on Si(111)7 × 7 has been characterized using soft X-ray photoemission with synchrotron radiation, thermal desorption spectroscopy, and Auger electron spectroscopy. SiCl4 dissociatively chemisorbs on room temperature Si(111)7 × 7 with an extremely low sticking coefficient, with only SiCl remaining on the surface. In contrast, Si2Cl6 chemisorbs with ∼ 500 times greater probability and then partly dissociates into SiClx (x = 1, 2, 3) fragments. A monolayer of Cl deposited directly also contains SiCl, SiCl2, and SiCl3 surface species, but they are created via reaction with substrate Si atoms and have lower Si2p core level binding energies. Upon heating the surface all the adsorbed Cl is removed via desorption of silicon chlorides, primarily SiCl2, indicating that SiCl4, Si2Cl6, and chlorine will etch Si(111)7 × 7 if an additional reactant is not avail to remove the surface Cl. Interestingly, the different reactivities of SiCl4 and Si2Cl6 upon adsorption can be explained by the dynamics of different adsorption mechanisms.

Resonant neutralization of 7Li+ scattered from alkali/Al(100) as a probe of the local electrostatic potential

Abstract Time-of-flight detection is used to measure the neutralization probabilities for 1.2 and 2.0 keV 7 Li + ions scattered from alkali-adsorbate-covered Al(100) surfaces. At low coverages, the neutral fractions for single scattering from adsorbate and substrate sites differ significantly while, at larger coverages, they are nearly equal. This change denotes a transition in the surface local electrostatic potential (LEP) from being spatially inhomogeneous to becoming homogeneous. The measured neutral fractions are compared to a quantum mechanical multi-state model of resonant charge transfer that includes the inhomogeneous adsorbate-induced part of the LEP. When the adsorbate-induced potential is modeled as the sum of isolated dipoles, good agreement between theory and experiment is obtained for single scattering from substrate atoms.

Atomic layer epitaxy of silicon by dichlorosilane studied with core level spectroscopy

The chemisorption and reaction of dichlorosilane (SiH2Cl2) with Si(111) and Si(100) surfaces is investigated with core‐level soft x‐ray photoelectron spectroscopy employing synchrotron radiation, in order to ascertain the surface chemistry involved in atomic layer epitaxy (ALE). Exposures to 8 kL of SiH2Cl2 were performed as a function of sample temperature in the range from room temperature to 800 °C. At all temperatures, SiH2Cl2 chemisorbs dissociatively forming silicon monochloride surface species. The coverage of monochloride displays a maximum for exposures at ∼600 °C. Under all conditions studied, larger chlorine coverages are observed on Si(100) than on Si(111). A Si surface that was first saturated with SiH2Cl2 at 600 °C was subsequently exposed to H2 at 600 °C, and no reaction occurred. These results indicate that recent models for silicon ALE are incorrect. An alternative method for low‐temperature ALE of Si is proposed, in which SiH2Cl2 is adsorbed onto Si at 600 °C and Cl is removed via reaction with atomic H.

Ge∕Si heteronanocrystal floating gate memory

Metal oxide semiconductor field effect transistor memories with Ge∕Si heteronanocrystals (HNCs) as floating gate were fabricated and characterized. Ge∕Si HNCs with density of 5×1011cm−2 were grown on n-type Si (100) substrate with thin tunnel oxide on the top. Enhanced device performances including longer retention time, faster programming speed, and higher charge storage capability are demonstrated compared with Si nanocrystal (NC) memories. The erasing speed and endurance performance of Ge∕Si HNC memories are similar to that of Si NC devices. The results suggest that Ge∕Si HNCs may be an alternative to make further floating gate memory scaling down possible.

Bi-induced acceptor states in ZnO by molecular-beam epitaxy

Bi-doped ZnO films were grown on Si (111) substrates by molecular-beam epitaxy. X-ray photoelectron spectroscopy and diffraction measurements reveal that Bi was incorporated into ZnO films without any phase separation. Room-temperature Hall effect measurements show a significant reduction of electron concentration and an increase of resistivity for Bi-doped ZnO films. In addition, a 3.222eV photoluminescence emission was observed particularly in the Bi-doped ZnO films and was identified as a donor-acceptor pair transition by temperature-dependent and excitation power-dependent photoluminescence measurements. These results indicate the possibility of generating acceptor states by Bi doping.

TiSi2∕Si heteronanocrystal metal-oxide-semiconductor-field-effect-transistor memory

TiSi2∕Si heteronanocrystals with a density of 5×1011cm−2 were formed on a thermally oxidized p-type Si substrate by using self-aligned silicide technique. Metal-oxide-semiconductor-field-effect-transistor (MOSFET) memory devices were fabricated using these heteronanocrystals as floating gates. As compared to Si nanocrystal MOSFET memory, TiSi2∕Si heteronanocrystal memories exhibit higher charge storage capacity, longer retention, better writing efficiency, less writing saturation, and faster erasing speed.

Chlorine chemisorption on and the onset of etching of cleaved GaAs(110) at room temperature

Cleaved GaAs(110) surfaces are exposed to Cl2 at room temperature and then examined by surface-sensitive soft X-ray photoelectron spectroscopy of the valence, As 3d, and Ga 3d levels. The results show that AsCl, AsCl2, and GaCl all form on the surface after small exposures, while GaCl2 forms after larger exposures. A detailed analysis of the data shows that, after the initial Cl2 exposures, Ga and As atoms are anisotropically etched from the surface, with Ga being removed first. As a consequence of the etching process, a number of Ga and As atoms are stabilized in sub-surface three-fold sites. After larger exposures, the surface is considerably roughened.

Desorption Induced by Electronic Transitions

Bombardment of a surface by electrons or photons can cause rupture of surface bonds and desorption from the surface, by inducing transitions to repulsive electronic states. The phenomenon of desorption induced by electronic transitions (DIET) includes both electron stimulated desorption (ESD) and photon stimulated desorption (PSD) [4.1–3]. DIET processes are of widespread importance in many areas of science and technology, including beam damage in surface analysis using x-rays or electrons, in electron and photon beam lithography, and in radiation physics of interstellar space, to name a few.

Chemical effects of Ne+ bombardment on the MoS2(0001) surface studied by high-resolution photoelectron spectroscopy

The effect of 1 keV Ne+ bombardment on the clean MoS2(0001)-1 × 1 surface with fluences between 4 × 1014 and 4 × 1016 Ne+/cm2 was studied using high-resolution photoelectron spectroscopy excited with synchrotron radiation. Spectra of the Mo 3d and S 2p core levels were measured with photon energies that ensured that the kinetic energy of the photoelectrons was the same, resulting in the same depth being probed for both core levels. For lower fluences (i.e., ≲2 × 1015 Ne+/cm2), S vacancy defect formation occurs in the MoS2 lattice, with the concurrent formation of a small amount (< 10%) of dispersed elemental molybdenum [Mo(0)]. For fluences greater than ∼l × 1016 Ne+/cm2, the Mo(0) is the predominant species in the surface region, while the remaining species consist of amorphous MoS2−x and polysulfide species. Valence band spectra taken with photon energies of 152 and 225 eV were consistent with the core level results. The movement of the valence band maximum toward the Fermi level indicated the formation of a metallic surface region. Annealing the sample to temperatures up to 1000 K resulted in the formation of metallic Mo coexisting, in approximately equal amounts, with reformed MoS2 in a surface with no long-range order as determined by LEED. Finally, a qualitative depth distribution of the chemical species present after Ne+ bombardment was determined by varying the photon energies used for the core level spectra. The results indicate that the preferential sputtering of sulfur over molybdenum occurs predominantly through a mechanism involving chemical bonding effects, specifically, through the preferential emission of polysulfide ions over other species in the bombarded region.

Photon- and electron-stimulated desorption of O+ from zirconia

We present a study of the photon- and electron-simulated desorption (PSD and ESD) of cations from yttriastabilized cubic ZrO 2 (100) and undoped amorphous ZrO 2 surfaces. For both types of zirconia, O+ is the primary ionic desorption product. A weak threshold for ESD of O+ from yttria-stabilized cubic zirconia (YSZ ) is observed at ~26‐27 eV with a rapid rise above ~30 eV. The PSD threshold from YSZ is ~31±1 eV. This is essentially the same as the O+ ESD and PSD thresholds (~30±1 eV ) from undoped amorphous ZrO 2 surfaces. The PSD O+ kinetic energy distributions extend from 0 to ~7 eV with a peak at ~2 eV and are similar from both surfaces. A comparison of the ion threshold data with photoelectron spectra indicates that desorption of O+ is primarily initiated by excitation of the Zr(4p) core level. All of the evidence is consistent with a desorption mechanism, in which the O+ ions are produced and ejected from the surface via a multi-electron Auger decay process.