Paula 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) ...

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.

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.

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.

Room‐temperature codeposition growth technique for pinhole reduction in epitaxial CoSi2 on Si (111)

A solid phase epitaxy technique has been developed for the growth of CoSi2 films on Si (111) with no observable pinholes (103 cm−2 detection limit). The technique utilizes room‐temperature codeposition of Co and Si in stoichiometric ratio, followed by the deposition of an amorphous Si capping layer and subsequent in situ annealing at 550–600 °C. CoSi2 films grown without the Si cap are found to have pinhole densities of 107–108 cm−2 when annealed at similar temperatures. A CF4 plasma etching technique was used to increase the visibility of the pinholes in the silicide layer. This plasma technique extends the pinhole detection resolution to 103 cm−2 and is independent of the pinhole size.

Transmission electron microscopy study of the formation of epitaxial CoSi2/Si (111) by a room‐temperature codeposition technique

Co and Si have been codeposited on Si (111) substrates near room temperature in a stoichiometric 1:2 ratio in a molecular beam epitaxy system. Annealing of these deposits yields high‐quality single‐crystal CoSi2 layers. Transmission electron microscopy has been used to examine as‐deposited layers and layers annealed at 300, 500, and 600 °C. Single‐crystal epitaxial grains of CoSi2 embedded in a matrix of amorphous Co/Si are observed in as‐deposited samples, while the layer is predominantly single‐crystal, inhomogeneously strained CoSi2 at 300 °C. At 600 °C, a homogeneously strained single‐crystal layer with a high density of pinholes is observed. In contrast to other solid phase epitaxy techniques used to grow CoSi2 on Si (111), no intermediate silicide phases are observed prior to the formation of CoSi2.

Optical properties of epitaxial CoSi2 and NiSi2 films on silicon

We have measured the optical constants of epitaxial films of CoSi2 and NiSi2, grown by molecular beam epitaxy on Si(111), in the energy range of 0.9–4.0 eV. The behavior of the optical constants is characteristic of metals: Drudelike in the low energy region and deviating from Drude behavior as interband transitions set in. Interband transitions are found to have already set in at 1 eV. The absorption varies significantly with energy, which has implications for photoresponse studies of internal photoemission in these material systems.

Heavily boron-doped Si layers grown below 700˚C by molecular beam epitaxy using a HBO2 source

Boron doping in Si layers grown by molecular beam epitaxy (MBE) at 500–700 °C using a HBO2 source has been studied. The maximum boron concentration without detectable oxygen incorporation for a given substrate temperature and Si growth rate has been determined using secondary‐ion mass spectrometry analysis. Boron present in the Si MBE layers grown at 550–700 °C was found to be electrically active, independent of the amount of oxygen incorporation. By reducing the Si growth rate, highly boron‐doped layers have been grown at 600 °C without detectable oxygen incorporation.

ULTRAHIGH VACUUM ARCJET NITROGEN SOURCE FOR SELECTED ENERGY EPITAXY OF GROUP III NITRIDES BY MOLECULAR BEAM EPITAXY

The key technical challenge in the molecular beam epitaxial (MBE) growth of group III nitrides is the lack of a suitable source of incorporatable nitrogen. In contrast with the growth of the other III–V compound semiconductors by MBE, direct reaction of N2 with excess group III metal is not feasible, because of the high bond strength of dinitrogen. An incorporatable MBE nitrogen source must excite N2 forming a beam of atomic nitrogen, active nitrogen (N2*), or nitrogen ions. rf and electron cyclotron resonance sources use electron impact excitation to obtain atomic nitrogen and in the process generate a wide variety of excited ions and neutrals. Experiments have shown that ionic species in the beam degrade the morphology of the epitaxial layer and generate electrically active defects. Recent theoretical studies have predicted that ground state atomic nitrogen will successfully incorporate into the growing GaN surface, while atomic nitrogen in either of the excited doublet states will lead to etching. In t...

Diffusion of Si in thin CoSi2 layers

We present evidence of Si diffusion in 100 A layers of CoSi2 grown by room‐temperature codeposition and annealing on Si(111) substrates. By monitoring the intensity of the Co MVV and Si LVV Auger peaks, we find a Si‐rich surface layer after annealing, in agreement with the results of others. We find that this layer can be removed by chemical etching and re‐formed by subsequent annealing. By measuring the intensity of the plasmon energy loss peak associated with the Co L23 VV Auger peak, we conclude that the Si must exist on top of the CoSi2 and we obtain the effective Si overlayer thickness as a function of annealing temperature by calibrating the plasmon loss data against known overlayer thicknesses on unannealed samples. We find similar results on samples grown both with and without the addition of a 10 A Si cap to prevent pinhole formation in the CoSi2 and we have indications that the same type of diffusion occurs also beneath the native oxide layer on samples that have not had the surface Si removed b...