C. R. Helms

Analyzing atomic force microscopy images using spectral methods

Various statistical quantities (such as average, peak-to-valley, and root-mean-square roughness) have been applied to characterize surface topography. However, they provide only vertical information. Because spectral analysis provides both lateral and longitudinal information, it is a more informative measurement than all these commonly used statistical quantities. Unfortunately, a standard method to calculate power spectral density (PSD) is not available. For example, the dimensions of PSD are often denoted as either (length)3 or (length)4. This may lead to confusion when utilizing spectral analysis to study surface morphology. In this paper, we will first compare the definitions of PSD commonly used by various authors. Using silicon surface roughness measurements as examples, we will demonstrate the advantages of spectral methods on atomic force microscopic (AFM) image analysis. In this context, we study the effects of typical AFM imaging distortions such as image bow, drift, tip-shape effects, and acou...

Experimental investigation of a PtSi source and drain field emission transistor

A p‐type PtSi source and drain, no ‘‘gap,’’ metal oxide semiconductor field effect transistor (MOSFET) has been successfully fabricated and experimentally investigated in detail down to 4.2 K. Gate curves (source current versus gate voltage) clearly show that, in the ‘‘on’’ state, the current flow mechanism from the source metal into the channel gradually changes from primarily thermal emission over the small ∼0.2 eV Schottky barrier to holes to completely field emission through the triangular Schottky barrier as the temperature is lowered below ∼100 K. Gate curves for different channel lengths also show minimal short channel effects down to 1.0 μm, in agreement with previous simulations. Drain curves (source current versus drain voltage) demonstrate that the drive current is comparable to that of a conventional MOSFET, and that the Schottky barrier is rendered transparent to the flow of holes when the device is strongly ‘‘on.’’

Comparison of surface properties of sodium sulfide and ammonium sulfide passivation of GaAs

Sodium sulfide has been demonstrated to significantly improve the electrical properties of the ambient‐exposed (100) GaAs surface. It has also shown usefulness in several practical device structures. Ammonium sulfide has been proposed to have similar properties and has already been demonstrated to improve InP and GaAs devices. We compare the photoluminescence behavior and band bending of GaAs samples treated with these two different sulfide species. We also compare the chemical nature of the surfaces treated using these two processes by Auger electron spectroscopy. The surface recombination velocity was found to be similarly affected by the two treatments and the band bending found to be essentially identical. Sodium sulfide produces a film and interfacial region rich in oxygen due to hydration. However, the ammonium sulfide was found to leave very little oxygen on the surface, even compared to conventional GaAs cleaning and etching procedures.

Comparison of Si surface roughness measured by atomic force microscopy and ellipsometry

Measurements of Si surface roughness by atomic force microscopy and ellipsometry have been performed over a wide range of conditions. Advanced methods of data analysis have been applied to both techniques leading to a quantitative comparison of root‐mean‐square (rms) roughness to the ellipsometric paramter Δ. Differences in Δ are observed for surfaces with the same rms roughness, but different roughness spectral densities, as expected from theory.

Effect of sodium sulfide treatment on band bending in GaAs

Recent evidence suggests that sodium sulfide treatment of GaAs surfaces results in the reduction of surface recombination and in Fermi level unpinning. We have used a surface conductivity technique to measure the Fermi level position of samples with and without sodium sulfide treatment. This method has specific advantages over spectroscopic techniques for studying thin films on GaAs. Photoluminescence (PL) measurements were used to qualitatively evaluate surface recombination. We conclude that while the reduction of surface recombination is attained, the surface is not unpinned.

Mechanisms of the HF/H2O vapor phase etching of SiO2

In this paper we report on studies of the mechanisms responsible for HF/H2O etching and cleaning of Si surfaces. From these studies we have clearly established that the role of water is to provide a condensed solvent medium for the HF on the surface. Our results, as well as those in the literature, show that if the partial pressures of HF and H2O are too low (or sample temperature too high) condensation does not occur and little or no oxide etching is observed. Based on these ideas and vapor pressure data from the literature, we have developed a detailed model that provides for the calculation of the onset of condensation as a function of wafer temperature and reactant partial pressures. In addition, the model allows determination of the HF concentration in the condensed H2O film. Comparison of etch rates obtained for aqueous etching as a function of HF concentration and vapor phase etching as a function of calculated HF concentration show good agreement.

Comparison of the oxidation process in strontium and cerium by ultraviolet photoelectron spectroscopy

The ultraviolet photoemission technique has been applied to the study of the oxidation of strontium and cerium. It has been found that the photoemission measurement provides a method to determine the oxide growth rate perpendicular to the surface, the composition of the surface, and the stable oxide phases formed during the oxidation process.

Interface effects in titanium and hafnium Schottky barriers on silicon

The effect of Si surface contaminants present prior to metal deposition, and that of post‐metalization anneals has been investigated for Ti and Hf Schottky barriers on Si. These diodes have been prepared in ultrahigh vacuum, characterized with Auger spectroscopy and measured in situ using internal photoemission, and ex situ using current‐voltage measurements. Although barriers to p‐type Si as high as 0.9 eV have been reported in the literature for these metals, barriers of 0.72 eV were the highest observed in this investigation, for surfaces contaminated with significant amounts of oxygen.

Photoemission study of the band bending and chemistry of sodium sulfide on GaAs (100)

Recently, there has been a great deal of interest in Na2S⋅9H2O as a passivating chemical treatment for GaAs surfaces. It has been shown that it reduces the high surface recombination velocity characteristic of GaAs surfaces, and may offer hope for ‘‘unpinning’’ the surface Fermi level. We have used photoemission spectroscopy to study the band bending and chemistry of these overlayers on n‐type GaAs (100). Identically prepared samples show the characteristic increase in the photoluminescence signal, and have also been characterized using surface conductivity measurements. We find using photoemission that the surface Fermi level of the treated wafer is still near midgap. We also observe the chemistry at the interface, and offer a possible explanation of the photoluminescence and surface conductivity data in terms of it and the advanced unified defect model.

Time‐dependent compositional variation in SiO2 films nitrided in ammonia

We report on selected results of an extensive study of SiO2 nitridation in an NH3 ambient obtained primarily using Auger sputter profiling. By investigating a large matrix of samples, we have sorted out the discrepancies present in the literature concerning the kinetics of the nitridation process. Two major new results obtained are the quantitative observation of oxygen depletion in the nitroxide ‘‘bulk’’ region and, consistent with a recent report by R. Vasquez, M. Hecht, F. Grunthaner, and M. Naiman [Appl. Phys. Lett. 44, 969 (1984)], the reoxidation of the interface by oxygen presumably liberated by the bulk exchange reaction.