P. A. Psaras

Crystallization of amorphous silicon during thin‐film gold reaction

The crystallization of a‐Si in a‐Si (50‐nm) and Au (5‐nm) thin‐film bilayers has been investigated during heat treatment in a transmission electron microscope. When crystallization of a‐Si first begins at 130 °C, the Au‐Si alloy (Au and a precursor phase) reflections observed at lower temperatures vanish, and several new reflections from metastable Au‐Si compounds occur. Dendritically growing islands of poly‐Si are observed after heating at 175 °C. If the samples are held at a constant temperature of 175 °C for 10 min, the poly‐Si islands coalesce. The formation of poly‐Si depends on the diffusion of Au into a‐Si and the formation of metastable Au‐Si compounds, which act as transport phases for both Si and Au. After crystallization Au segregates to the front and back surfaces of the poly‐Si film. The result of this work and earlier diffraction investigations are interpreted in terms of superlattices based on a sublattice. A fundamental body‐centered‐cubic structure with a=5.52 A and composition Au4Si is s...

Effect of a substrate on the phase transformations of amorphous TiSi2 thin films

The crystallization of an amorphous TiSi2 evaporated film has been studied on both poly‐Si and SiO2 substrates. A metastable form of TiSi2 (base‐centered orthorhombic; a=3.62 A, b=13.76 A, and c=3.605 A) [P. G. Cotter, J. H. Kohn, and R. A. Potter, J. Am. Ceram. Soc. 39, 11 (1956)] is formed first at a temperature of approximately 350 °C on both substrates. This phase consumes the entire amorphous layer before undergoing a polymorphic transformation to face‐centered orthorhombic TiSi2 (a=8.24 A, b=4.78 A, and c=8.54 A) [F. Laves and H. J. Wallbaum, Z. Kristallogr. 101, 78 (1979)] at 600 and 800 °C on poly‐Si and SiO2, respectively. These transformations were investigated using in situ resistivity, x‐ray diffraction, and transmission electron microscopy. The room‐temperature resistivities observed were 96 and 20 μΩ cm for the base‐centered and face‐centered TiSi2, respectively. The enhanced polymorphic transformation on poly‐Si over SiO2 is explained by a lowering of surface energy barrier to nucleation.

Formation of aluminum silicide between two layers of amorphous silicon

Thin‐film structures of amorphous Si/Al/amorphous Si were deposited consecutively without breaking the vacuum. During annealing to 440 K, Al reacts with Si to form a homogeneous compound layer between the two a‐Si layers. This compound has a unique and well‐defined structure, different from both Al and Si although some similarities exist. The Al silicide observed is stable up to 575 K, at which temperature it dissociates when a‐Si crystallizes.

Intermetallic compound formations in titanium‐copper thin‐film couples

Formation of intermetallic compounds in Ti‐Cu thin film bilayer samples has been studied between 300–475 °C by Rutherford backscattering and glancing‐angle x‐ray diffraction techniques. The first intermetallic compound to be formed was TiCu and was followed by TiCu3, the latter showed detectable deviations from the stoichiometry. The thickening of both compounds obeyed a parabolic relationship with time. The interdiffusion coefficient derived from compound formation can be described by the following expressions: TiCu:D=4.85×10−4 exp(—1.48eV/kT) cm2/sec, TiCu3:D=7.73×10−2 exp(—1.82eV/kT) cm2/sec.

Metal induced crystallization of amorphous silicon

The reactions between a thin metal film of Al or Ag and a layer of amorphous Si have been investigated using transmission electron microscopy and Auger electron spectroscopy. The samples were analyzed after deposition and after different heat treatments. Al and Ag were found to reduce the crystallization temperature of amorphous Si. A crystallization temperature of 325 °C was observed for the Al–Si structure and of 525 °C for the Ag–Si structure. A new phase between Al and Si, which has not previously been reported, has been detected at a temperature of 150 °C. Some evidence for a formation of a metastable phase between Ag and Si during the crystallization process has also been found.

In situ resistivity measurement of cobalt silicide formation

In situ resistivity measurements have been utilized to study the reaction and silicide formation between cobalt and amorphous silicon thin films from room temperature to 800 °C. In conjunction, structure and composition changes were analyzed by x‐ray diffraction and Rutherford backscattering spectrometry. Formation of Co2Si, CoSi, and CoSi2 were observed. Interfacial reaction to form Co2Si occurs at approximately 400 °C. In bilayers of excess silicon, CoSi forms at approximately 520 °C and, if free silicon is still present, CoSi2 forms at about 550 °C. In the case of excess cobalt, Co2Si forms first and is followed by a cobalt‐rich solid solution. Co3Si silicide was not observed.

Effects of substrate crystallinity and dopant on the growth kinetics of platinum silicides

The growth kinetics of platinum silicides have been studied on four substrate categories: single‐crystal, amorphous, undoped polycrystalline, and phosphorus‐doped (8×1020 at./cm3) polycrystalline silicon. The sequential growth of Pt2Si and PtSi were analyzed by Rutherford backscattering spectroscopy (RBS), Seeman–Bohlin x‐ray diffraction, and cross‐section transmission electron microscopy. Phosphorus depth profiles were measured by secondary ion mass spectroscopy (SIMS). Our results conclude that the activation energies for the growth of Pt2Si and PtSi are not affected by substrate crystallinity and doping of phosphorus. Analysis of the phosphorus profile by SIMS clearly showed that phosphorus atoms are segregated near the interface between PtSi and polycrystalline silicon, but not at the Pt2Si/polycrystalline silicon interface.

Phase transformations in alloy and bilayer thin films of vanadium and silicon

Phase transformations in coevaporated amorphous vanadium‐silicon thin alloy films and bilayer vandium/silicon films have been studied as a function of heat treatment by in situ electrical resistivity measurement together with Rutherford backscattering spectrometry, Seeman–Bohlin glancing angle incidence x‐ray diffraction, and scanning and transmission electron microscopy. In the as‐deposited state the amorphous alloy films were silicon rich, having an atomic ratio of 1:3 for vanadium and silicon, respectively. Upon heat treatment a sharp decrease in resistivity occurs at approximately 250 °C, which has been determined to be a transformation from the amorphous to crystalline VSi2 phase. The kinetics of the transformation have been obtained by isothermal treatment over the temperature range of 184–220 °C. The transformation is described by a Johnson–Mehl–Avrami‐type equation with an apparent activation energy of 1.30±0.06 eV. Subsequent heat treatment causes a gradual decrease in resistivity up to 850 °C. U...

Morphology and kinetics of crystallization of amorphous V75Si25 thin‐alloy films

Electrical and microstructural changes of coevaporated V75Si25 alloy thin films have been studied as a function of temperature from room temperature to 830 °C. In situ resistivity measurements, hot‐stage transmission electron microscopy, Rutherford backscattering spectroscopy and the Seeman–Bohlin glancing angle incidence x‐ray diffraction technique were applied. Upon heat treatment at a heating rate of 8 °C/min, a sharp decrease in resistivity occurs at ∼670 °C which results from an amorphous to crystalline phase transformation. The crystallized phase was identified as V3Si. The mechanism of transformation is random nucleation at a rapidly decreasing rate and a fast quasi‐isotropic growth. The kinetics of crystallization have been studied by utilizing electrical resistivity measurements during isothermal heat treatment. Six different temperatures between 570 °C and 630 °C were adopted. The apparent activation energy (∼3.6 eV) obtained from isothermal measurements was found to be in agreement with that ob...

Sequential silicide formation between vanadium and amorphous silicon thin‐film bilayers

Solid‐state reactions between bilayer thin films of vanadium and amorphous silicon with an excess amount of vanadium have been studied by Rutherford backscattering spectroscopy and Seemann–Bohlin x‐ray diffraction. In prior studies of the interaction between vanadium thin films and single‐crystal silicon, VSi2 has been the only compound observed. In the present study a sequence of compounds was observed. The first silicide, VSi2, was observed to form at 475 °C. At higher temperatures the compounds V5Si3 and V3Si formed in sequence. The growth of VSi2 is linear in time with an activation energy of 2.3±0.4 eV. The growth of V5Si3 is also linear with an activation energy of 2.5±0.1 eV.