Frank J. Grunthaner

The localization and crystallographic dependence of Si suboxide species at the SiO2/Si interface

X‐ray photoemission spectroscopy has been used to examine the localization and crystallographic dependence of Si+1, Si+2, and Si+3 suboxide states at the SiO2/Si interface for (100)‐ and (111)‐oriented substrates with gate oxide quality thermal oxides. The Si+1 and Si+2 states are localized within 6–10 A of the interface while the Si+3 state extends ∼30 A into the bulk SiO2. The distribution of Si+1 and Si+2 states shows a strong crystallographic dependence with Si+2 dominating on (100) substrates and Si+1 dominating on (111) substrates. This crystallographic dependence is anticipated from consideration of ideal unreconstructed (100) and (111) Si surfaces, suggesting that (1) the Si+1 and Si+2 states are localized immediately within the first monolayer at the interface and (2) the first few monolayers of substrate Si atoms are not significantly displaced from the bulk. The total number of suboxide states observed at the SiO2/Si interface corresponds to 94% and 83% of a monolayer for these (100) and (111) ...

Sulfate minerals and organic compounds on Mars

Strong evidence for evaporitic sulfate minerals such as gypsum and jarosite has recently been found on Mars. Although organic molecules are often codeposited with terrestrial evaporitic minerals, there have been no systematic investigations of organic components in sulfate minerals. We report here the detection of organic material, including amino acids and their amine degradation products, in ancient terrestrial sulfate minerals. Amino acids and amines appear to be preserved for geologically long periods in sulfate mineral matrices. This suggests that sulfate minerals should be prime targets in the search for organic compounds, including those of biological origin, on Mars.

Hydrogen-terminated silicon substrates for low-temperature molecular beam epitaxy

Abstract The preparation of hydrogen-terminated silicon surfaces for use as starting substrates for low-temperature epitaxial growth by molecular beam epitaxy is examined in detail. The procedure involves the ex-situ removal under nitrogen of residual oxide from a silicon substrate using a spin-clean with HF in ethanol, followed by the in-situ low-temperature desorption (150°C) of physisorbed etch residues. The critical steps and the chemical basis for these steps are examined using X-ray photoelectron spectroscopy. Impurity residues at the epilayer-substrate interface following subsequent homoepitaxial growth are studied using Auger spectroscopy, secondary ion mass spectrometry, and transmission electron microscopy. Finally, scanning tunneling microscopy is used to examine the effect of cleaning methods on substrate morphology.

A novel x‐ray photoelectron spectroscopy study of the Al/SiO2 interface

We report on the first nondestructive measurement of the chemical and physical characteristics of the interface between bulk SiO2 and thick aluminum films. Both x‐ray photoelectron spectroscopy (XPS) and electrical measurements of unannealed, resistively evaporated Al films on thermal SiO2 indicate an atomically abrupt interface. Post metallization annealing (PMA) at 450 °C induces reduction of the SiO2 by the aluminum, at a rate consistent with the bulk reaction rate. The XPS measurement is performed from the SiO2 side after the removal of the Si substrate with XeF2 gas and thinning of the SiO2 layer with HF:ETOH. This represents a powerful new approach to the study of metal‐insulator and related interfaces.

X-ray photoelectron spectroscopy study of the chemical structure of thermally nitrided SiO2

X‐ray photoelectron spectroscopy has been used to study the composition of 100‐A thermally grown SiO2 films that have been thermally nitrided in ammonia. The SiOxNy/Si interface was studied both by chemical depth profiling of the oxynitride and by removal of the Si substrate with XeF2. It is found that N is distributed throughout the film, but with the concentration higher at the surface and in a region centered 25 A from the film/substrate interface. The interface region itself is found to be oxygen‐rich relative to the rest of the film. Possible models which can explain these results are discussed.

Enhanced Amine and Amino Acid Analysis Using Pacific Blue and the Mars Organic Analyzer Microchip Capillary Electrophoresis System

The fluorescent amine reactive probe Pacific Blue succinimidyl ester (PB) is used for the detection of trace amounts of amines and amino acids by microchip capillary electrophoresis on the Mars Organic Analyzer (MOA). The spectral and chemical properties of PB provide a 200-fold increase in sensitivity and improved resolution compared to fluorescamine derivatization. With the use of cross injection and PB labeling, the MOA detected amino acids at concentrations as low as 75 pM (sub-parts-per-trillion). Micellar electrokinetic chromatography (MEKC) which separates PB-labeled amino acids by their hydrophobicity is also demonstrated. The optimized MEKC conditions (45 mM CHAPSO, pH 6 at 5 degrees C) effectively separated amines and 25 amino acids with enantiomeric resolution of alanine, serine, and citrulline. Samples from the Yungay Hills region in the Atacama Desert, Chile, and from the Murchison meteorite are successfully analyzed using both techniques, and amino acids are found in the parts-per-billion range. Abiotic amino acids such as beta-alanine and epsilon-aminocaprioc acid are detected along with several neutral and acidic amino acids in the Murchison sample. The Atacama Desert sample is found to contain homochiral L-alanine and L-serine indicating the presence of extant or recently extinct life.

Perchlorate radiolysis on Mars and the origin of martian soil reactivity.

Results from the Viking biology experiments indicate the presence of reactive oxidants in martian soils that have previously been attributed to peroxide and superoxide. Instruments on the Mars Phoenix Lander and the Mars Science Laboratory detected perchlorate in martian soil, which is nonreactive under the conditions of the Viking biology experiments. We show that calcium perchlorate exposed to gamma rays decomposes in a CO2 atmosphere to form hypochlorite (ClO(-)), trapped oxygen (O2), and chlorine dioxide (ClO2). Our results show that the release of trapped O2 (g) from radiation-damaged perchlorate salts and the reaction of ClO(-) with amino acids that were added to the martian soils can explain the results of the Viking biology experiments. We conclude that neither hydrogen peroxide nor superoxide is required to explain the results of the Viking biology experiments.

X‐ray photoelectron spectroscopic study of the oxide removal mechanism of GaAs (100) molecular beam epitaxial substrates in in situ heating

A standard cleaning procedure for GaAs (100) molecular beam epitaxial (MBE) substrates is a chemical treatment with a solution of H2SO4/H2O2/H2O, followed by in situ heating prior to MBE growth. X‐ray photoelectron spectroscopic (XPS) studies of the surface following the chemical treatment show that the oxidized As is primarily As+5. Upon heating to low temperatures (<350 °C) the As+5 oxidizes the substrate to form Ga2O3 and elemental As (As0), and the As+5 is reduced to As+3 in the process. At higher temperatures (500 °C), the As+3 and As0 desorb, while the Ga+3 begins desorbing at ∼600 °C.

Growth of a delta‐doped silicon layer by molecular beam epitaxy on a charge‐coupled device for reflection‐limited ultraviolet quantum efficiency

We have used low‐temperature silicon molecular beam epitaxy to grow a δ‐doped silicon layer on a fully processed charge‐coupled device (CCD). The measured quantum efficiency of the δ‐doped backside‐thinned EG&G Reticon CCD is in agreement with the reflection limit for light incident on the back surface in the spectral range of 260–600 nm. The 2.5 nm silicon layer, grown at 450 °C, contained a boron δ‐layer with surface density ∼2×1014 cm−2. Passivation of the surface was done by steam oxidation of a nominally undoped 1.5 nm Si cap layer. The UV quantum efficiency was found to be uniform and stable with respect to thermal cycling and illumination conditions.

Direct control and characterization of a Schottky barrier by scanning tunneling microscopy

Scanning tunneling microscopy (STM) methods are used to directly control the barrier height of a metal tunnel tip‐semiconductor tunnel junction. Barrier behavior is measured by tunnel current‐voltage spectroscopy and compared to theory. A unique surface preparation method is used to prepare a low surface state density Si surface. Control of band bending with this method enables STM investigation of semiconductor subsurface properties.