F.W. Saris

Prediction of phase formation sequence and phase stability in binary metal-aluminum thin-film systems using the effective heat of formation rule

The effective heat of formation (ΔH’) concept allows heats of formation to be calculated as a function of concentration. In this work the effective heat of formation rule is used to predict first phase formation in metal‐aluminum thin‐film systems and to predict subsequent phase sequence for thin metal films on thick aluminum or thin aluminum on thick metal substrates. The effective concentration at the growth interface is taken to be that of the lowest temperature eutectic (liquidus) for the binary system. Although the effective heat of formation rule may predict that formation of a certain phase would lead to the largest free energy change, this phase does not necessarily form at the moving reaction interface if it has difficulty to nucleate. By excluding phases with a large number of atoms per unit cell and which thus have difficulty to nucleate, the effective heat of formation rule successfully predicts first phase aluminide formation for all 15 metal‐aluminum binary systems for which experimental data could be found. It is also shown how the effective heat of formation rule can be used to predict formation and decomposition of aluminide phases in contact with each other or in contact with their component metals.

Low temperature epitaxial NiSi 2 formation on Si(111) by diffusing Ni through amorphous Ni–Zr

Although amorphous alloys are known to be good diffusion barriers, amorphous nickel-zirconium is shown to react with Si at relatively low temperatures. Diffusion of Ni at 350°C through an amorphous Ni–Zr buffer layer leads to the formation of epitaxial NiSi 2 on single crystal silicon substrates. Interplay of mobility and thermodynamics is applicable for epitaxial silicide nucleation and growth. Also, a one-step annealing process in oxygen ambient leads to bilayer formation of NiSi 2 /ZrO 2 structures on silicon substrates.

The role of diffusion in amorphous-phase formation and crystallization of amorphous Ni–Zr

The Ni--Zr system is examined as a representative system for the formation of an amorphous phase by diffusion and for the crystallization of an amorphous phase by diffusion. High-resolution electron microscopy (HREM) is used to show that the amorphous phase grows by bulk diffusion through the amorphous material rather than by short-circuit diffusion. Also, the HREM shows that the amorphous phase formed by diffusion appears to be the same as the vapor-deposited amorphous phase. A correlation between crystallization temperatures (T/sub x/) and the enthalpy of large-atom hole formation is given. This correlation predicts values of T/sub x/ that are lower than those predicted from the small-atom hole-formation model. The difference in hole-formation enthalpies for the large and small atoms is given as a criterion for amorphous-phase formation via diffusion.

Critical temperatures for radiation enhanced diffusion and metastable alloy formation in ion beam mixing

Abstract Phase formation, stability, and mixing behavior in the metallic systems NiZr, FeZr, AuZr, and PdTa have been investigated under influence of high energy heavy ion beams as a function of temperature. Our results and data from the literature support a simple model which correlates onset temperatures for radiation enhanced diffusion and equilibrium phase formation in ion beam mixing with hole (or vacancy) formation enthalpies. The model offers the possibility to predict temperature ranges in which efficient mixing occurs by radiation enhanced diffusion without formation of stable alloys.

Thermal stability of thin‐film amorphous W‐Ru, W‐Re, and Ta‐Ir alloys

The crystallization behavior of self‐supporting thin‐film amorphous W‐Ru, W‐Re, and Ta‐Ir alloys has been studied with transmission electron microscopy. Crystallization temperatures have been observed which are much lower than the temperatures predicted by a semiempirical model: the highest observed temperatures are 775 °C for W‐Ru and W‐Re alloys, and 900 °C for the Ta‐Ir alloys. All three systems show maximum thermal stability at a composition expected using enthalpy considerations.

Buffer layers for superconducting YBaCuO thin films on Si and SiO2.

By direct deposition onto hot substrates, using laser ablation, crystalline YBa2Cu3O7?? (123) was obtained at 650°C on SiO2, but not on (100) Si substrates. The 123 film did not show a superconducting transition due to interfacial reactions. The failure temperature of insulating buffer layers, such as tantalum oxide and hafnium oxide, is around 500 °C. Although MgO and BaZrO3 show a high stability in contact with 123 at 900 °C, they fail as a diffusion barrier at much lower temperatures. Below 400 °C barium diffuses through MgO, which itself remains unaffected. Using BaZrO3 the same happens around 700 °C. BaF2 fails as a diffusion barrier below 400 °C. Using laser ablation, high quality 123 films were grown on ZrO2 buffer layers above 650 °C. For the first time we report superconducting transitions of 123 deposited at 650 °C onto an amorphous metal alloy, Ir45Ta55. The problems encountered using conducting buffer layers are either a low reaction temperature with 123 (HfB2 and HfN) or oxidation of the metal alloy (Ir45Ta55) around 400 °C. Intermediate noble metal layers silver and Ag/Au/Ag could not prevent oxygen diffusion towards the underlying buffer layer.

Stability of amorphous IrTa diffusion barriers between Cu and Si

Abstract Thin-film amorphous IrTa (aIrTa) has been tested as a diffusion barrier between (100) Si and Cu. Sandwich structures of (100) Si/aIrTa/Cu/aIrTa were found to be stable during annealing in vacuum up to 700°C. At 750°C interdiffusion of Cu and Si took place as observed by Rutherford backscattering spectrometry. Using free-standing aIrTa/Cu/aIrTa samples for transmission electron microscopy, it was determined that the crystallization temperature of thin-film aIr45Ta55 is reduced from 900 to 750°C in the presence of Cu.

The crystallization temperature of amorphous transition-metal alloys

Comparison of the formation enthalpy of the amorphous phase (ΔHam) to the formation enthalpy of simple solid solutions (ΔHss) shows that diffusionless polymorphic crystallization occurs at low temperatures if ΔHam >ΔHss. In the composition range where ΔHam < ΔHss, crystallization occurs via diffusion and the crystallization temperature is roughly proportional to the formation enthalpy of holes the size of the larger constituent. This model is supported by data from literature of 74 binary alloys. Formation enthalpies are calculated using Miedemas macroscopic atom approach. To calculate ΔHam the amorphous alloy should not be regarded as a random alloy, but a certain degree of chemical short-range order should be accounted for.

Structure and magnetism of metastable BiFe alloy films

Abstract Metastable alloy thin films in the insoluble BiFe system were formed by means of electron beam co-evaporation at substrate temperatures of 140 K. Backscattering of 2.0 MeV helium ions and transmission electron microscopy showed that films with 45–60 at.% Fe were of a uniform amorphous phase. The mutual solute concentrations of iron and bismuth in the Bi 65 Fe 35 , Bi 60 Fe 40 and Bi 35 Fe 65 films were determined by X-ray diffraction and amounted to 0.2% Bi in b.c.c. iron and 1%–2% Fe in rhombohedral bismuth depending on composition. Magnetic measurements were performed using a Faraday balance. Some samples showed minor ferromagnetic effects at measuring temperatures of 77–290 K. The characteristics of the metastable BiFe alloy films obtained were compared with those observed in previous experiments with ion mixing and magnetron co-sputtering and with a model of the magnetic properties of amorphous 3d-base alloys.

Thermal stability of amorphous TaIr between YBaCuO and silicon

Abstract Amorphous buffer layers of TaIr, sandwiched between YBaCuO thin films and Si(100) substrates, prevent interface reactions during annealing in oxygen ambient up to 800 °C. Above 650 °C, amorphous TaIrO, which is formed during annealing, crystallizes into Ta 2 O 5 and IrO 2 , and pinholes in the YBaCuO film and buffer layer are observed. Reaction with silicon does not occur after annealing up to 900 °C.