Rolf E. Hummel

Novel technique for preparing porous silicon

We have performed photoluminescence studies on porous, p‐type as well as n‐type silicon wafers which have been prepared in air or in a dry nitrogen atmosphere, utilizing a spark‐erosion technique. This sample preparation, which does not involve aqueous solutions or fluorine contaminants, yields similar photoluminescence spectra as those obtained by anodic etching in HF or unbiased etching in various HF‐containing reagents. The wavelength of the photoluminescence peaks are somewhat shifted into the blue region compared to porous silicon obtained by anodic etching. We have also taken photoluminescence spectra on amorphous silicon, SiO2, and oxidized, annealed porous silicon. Our results are interpreted in the light of the presently suggested theories.

Activation energy for electrotransport in thin silver and gold films

Abstract The activation energy Δ H for electrotransport in thin silver and gold films was measured by a resistometric technique involving several individual resistance measurements along a stripe. The temperature of the center of the films was held constant during the entire experiment using a thin film thermocouple. For silver films, two distinct temperature regions with different activation energies were observed. Between 225° and 280°C Δ H was found to be 0.95 eV, which suggests predominance of ion movement in the grain boundaries, whereas at temperatures between 160° and 225°C Δ H was found to be 0.3 eV, which suggests surface-dominated electrotransport. In gold films a uniform activation energy of 0.98 eV was observed throughout the entire temperature range (260°–380°C).

Activation energy for electrotransport in thin aluminum films by resistance measurements

Abstract The activation energy for electrotransport in thin aluminum films was measured by a resistometric technique involving several individual resistance measurements along the stripe. An equation was derived which relates the rate of resistance change to the ion velocity. A thin film thermocouple which is free of any loss of heat was used to monitor the temperature of the stripe. This thermocouple was calibrated by the melting points of pure metals placed on the film. The activation energy for electrotransport in thin aluminum films was found to be temperature dependent and to vary between 0.45 and 0.72 eV in a temperature range between 220 and 360°C. The average ion velocity in the grain boundaries due to electrotransport was found to be around 10 −7 cm sec in agreement with the literature.

The passivation of nickel in aqueous solutions—I. The identification of insoluble corrosion products on nickel electrodes using optical and ESCA techniques

Abstract In situ differential reflectometry in conjunction with ESCA techniques are used to identify various surface films which form electrochemically on nickel. The film observed on pure Ni in a 0.15 N Na 2 SO 4 electrolyte has been identified to be primarily Ni(OH) 2 . At different values of pH and potential, NiO has been found to form simultaneously with Ni(OH) 2 . At solutions with pH > 8, the Ni(OH) 2 film partially transforms into a third oxide, presumably NiOOH, which can only be observed in situ .

Electromigration in thin silver, copper, gold, indium, tin, lead and magnesium films

Abstract An experimental study of electromigration in thin films of silver, copper, gold, magnesium, indium, tin and lead has been carried out. Various experimental methods have been employed, including resistance measurements, observations with the scanning electron microscope, determination of stress with X-rays and the measurement of the displacement of inert markers under the influence of the electric field. It was found that the migration of ions is directed towards the cathode in films of silver, copper and gold, and towards the anode in magnesium, indium, tin and lead films. The direction of electrotransport in small-grained and large-grained silver films was found to be alike. Electrotransport in single-crystalline silver films appears to be small. Evidence is given that under the experimental conditions used here grain boundary electrotransport predominates in silver films over surface and volume electrotransport.

On the origin of photoluminescence in spark‐eroded (porous) silicon

Photoluminescence measurements and high‐resolution transmission electron microscopy studies on spark‐treated (porous) silicon have been performed. Contrary to suggestions put forward by others, it has been found that spark erosion does not yield structures comparable to those obtained for irradiated, that is, damaged silica. Instead, evidence is given that spark treatment of single crystalline silicon wafers produces randomly oriented nanometer‐sized silicon crystallites surrounded by a SiO2 matrix. This configuration is believed to be responsible for the observed room temperature visible photoluminescence.

ON THE DIRECTION OF ELECTROMIGRATION IN THIN SILVER, GOLD, AND COPPER FILMS

Migration of ions in thin silver, copper, and gold films under the influence of a direct current has been found to be directed towards the cathode, contrary to observations in bulk metals. This was concluded from resistance and radioactive tracer measurements as well as scanning electron micrographs. The results can be explained by using the concept that electromigration in thin films is confined to grain boundaries, and the electron concentration in the grain boundary is different from that in the bulk.

BRIGHT VISIBLE PHOTOLUMINESCENCE OF SPARK-PROCESSED GE, GAAS, AND SI

High‐frequency spark discharges were applied to single‐crystalline wafers of Ge, GaAs, and Si. The spark‐processed (sp‐) samples were characterized by photoluminescence (PL) and Raman measurements. Strong and stable luminescence with wavelengths centered at 416 and 525 nm was observed in sp‐Ge and sp‐Si layers, respectively, when excited with a 325 nm laser beam. A considerable blue shift of the PL (compared to the unsparked specimen) was also detected for sp‐GaAs with an average peak wavelength around 500 nm. The Raman shifts of the spark‐processed materials indicate that nanocrystals were formed, having diameters of 3.5–4 nm for Si and about 6 nm for Ge. A correlation between the PL wavelengths, the nanocrystal sizes, and the different semiconductor materials has been established based on the effective‐mass approximation. Making use of this model the nanocrystallite sizes have been found to range between ∼3 nm for Si and ∼5 nm for Ge. The related wavelengths for optical transitions confirm the PL result...

Thermal grooving, thermotransport and electrotransport in doped and undoped thin gold films

Abstract Thin gold films, undoped and doped with sodium or indium, were deposited onto quartz glass substrates. In order to separate the contributions of thermal grooving, thermomigration and electromigration to structural evolution and hole formation, series of samples were subjected to three treatments: (1) an isothermal anneal which produces only thermal grooving; (2) application of an a.c. which sets up temperature gradients and potentially produces thermal grooving and thermotransport; (3) application of a d.c. which potentially induces thermal grooving, thermotransport and electrotransport. Examination of the resulting samples with transmission optical, transmission electron and scanning electron microscopy revealed that (1) all three mechanisms may contribute significantly to hole formation in undoped films (2) addition of indium stabilizes the as-deposited grain size, minimizes thermal grooving but promotes hole formation by the other two mechanisms and (3) addition of sodium greatly enhances grain boundary grooving, promotes early failure in a.c.-stressed samples and results in hole formation at the anode under a d.c. These results do not provide a rationalization of the reversal in the site of hole formation in d.c.-stressed sodium-doped gold films in comparison with the “normal” behavior exemplified in undoped gold films. Three possible mechanisms are suggested which might explain this observed reversal in failure site.

Optical investigations of ion implant damage in silicon

Ion implantation damage in silicon has been studied utilizing a new optical technique (differential reflectometry). It has been demonstrated that differential reflectometry can be used to identify whether an implanted layer is crystalline, damaged crystalline, or amorphous. The intensity of interband transitions can be used to determine the thickness of a damaged crystalline layer over a submerged amorphous layer. Interference effects were utilized to determine the thickness of an amorphous layer. Thus, differential optical reflectance has far‐reaching potential for characterizing implanted substrates.