B. X. Liu

Structural difference rule for amorphous alloy formation by ion mixing

We formulate a rule which establishes a sufficient condition that an amorphous binary alloy will be formed by ion mixing of multilayered samples when the two constituent metals are of different crystalline structure, regardless of their atomic sizes and electronegativities. The rule is supported by the experimental results we have obtained on six selected binary metal systems, as well as by the previous data reported in the literature. The amorphization mechanism is discussed in terms of the competition between two different structures resulting in frustration of the crystallization process.

Amorphous film formation by ion mixing in binary metal systems

Abstract A structural difference rule is formulated according to which an amorphous binary alloy film is formed by ion mixing of multilayered sample when the two constituent metals are of different structure, apparently independently of their atomic sizes and electronegativities. The rule is supported by the experimental results we have obtained on eight selected binary metal systems, which cover all possible combinations between b.c.c., f.c.c. and h.c.p. structures. The previous data reported in the literature also support this rule. The available experimental data indicate that the validity of the rule may extend to other structures and to production techniques other than ion mixing. The amorphization mechanism is discussed in terms of the competition between two different structures resulting in frustration of the crystallization process.

Ion mixing and phase diagrams

Abstract Interactions induced by ion irradiation are generally considered to be non-equilibrium processes, whereas phase diagrams are determined by phase equilibria. These two entities are seemingly unrelated. However, if one assumes that quasi-equilibrium conditions prevail after the prompt events, subsequent reactions are driven toward equilibrium by thermodynamical forces. Under this assumption, ion-induced reactions are related to equilibrium and therefore to phase diagrams. This relationship can be seen in the similarity that exists in thin films between reactions induced by ion irradiation and reactions induced by thermal annealing. In the latter case, phase diagrams have been used to predict the phase sequence ofstable compound formation, notably so in cases of silicide formation. Ion-induced mixing not only can lead to stable compound formation, but also to metastable alloy formation. In some metal-metal systems, terminal solubilities can be greatly extended by ion mixing. In other cases, where the two constituents of the system have different crystal structures, extension of terminal solubility from both sides of the phase diagram eventually becomes structurally incompatible and a glassy (amorphous) mixture can form. The composition range where this bifurcation is likely to occur is in the two-phase regions of the phase diagram. These concepts are potentially useful guides in selecting metal pairs that from metallic glasses by ion mixing. In this report, phenomenological correlation between stable (and metastable) phase formation and phase diagram is discussed in terms of recent experimental data.

Surface morphology of erbium silicide

The surface of rare‐earth silicides (Er, Tb, etc.), formed by the reaction of thin‐film metal layers with a silicon substrate, is typically dominated by deep penetrating, regularly shaped pits. These pits may have a detrimental effect on the electronic performance of low Schottky barrier height diodes utilizing such silicides on n‐type Si. This study suggests that contamination at the metal‐Si or silicide‐Si interface is the primary cause of surface pitting. Surface pits may be reduced in density or eliminated entirely through either the use of Si substrate surfaces prepared under ultrahigh vacuum conditions prior to metal deposition and silicide formation or by means of ion irradiation techniques. Silicide layers formed by these techniques possess an almost planar morphology.

Chemical effects in ion mixing of transition metals on SiO2

Abstract The mixing of thin evaporated films of Ti, Cr and Ni of various thicknesses on thick (≅ 650 nm) thermally grown SiO2 layers on 〈111〉 Si wafers has been investigated for 290 KeV Xe irradiations over the range of Xe fluence, Φ, of 1015 to 2 × 1016 Xe/cm2 and of irradiation temperature, T, of 77 to about 750 K. For the analysis of the irradiated samples, mainly backscattering spectrometry was used after the free metal had been removed by chemical etching. The amount of mixed metal contained in the SiO2 increases as Φβ, where β ≅ 0.56, to 0.73, at room temperature and below, for all metals and thicknesses, suggesting that cascade mixing dominates the transport process in this temperature region. The contribution of recoil mixing at the deep tail of the metal profiles is probable and may account for the values of β > 1 2 . At 750 K, enhanced mixing occurs for the thermally reactive systems Ti/SiO2 and Cr/SiO2; however, mixing is suppressed for Ni/SiO2. The fact that the deep tail of the metal profiles is not altered suggests that only the cascade mixing process is affected by chemical driving forces.

Surface morphology and electronic properties of ErSi2

Abstract The surface of ErSi 2 , formed by the reaction of thin erbium layers with a single-crystal silicon substrate, is typically dominated by deeply penetrating regularly shaped pits. These pits are shown to have detrimental effects on the electronic performance of Schottky barrier diodes. Surface pits may be reduced in density or eliminated entirely (i) by the use of silicon substrate surfaces prepared under ultrahigh vacuum conditions prior to metal deposition, (ii) by means of ion irradiation techniques or (iii) by reacting erbium with an amorphous silicon (a-Si) layer. In this investigation, planar ErSi 2 layers (pit free) are made using the third approach with a sample structure of a-Si/Er/c-Si where c-Si denotes crystalline silicon. The fast reaction between a-Si and erbium leads to a planar sample structure of ErSi 2 /c-Si with little or no reaction between erbium and the c-Si substrate. The electronic performance of pit-free ErSi 2 diodes made in this manner is shown to be much superior to that of diodes made by reacting erbium with silicon substrates.

Ion mixing to produce amorphous Mo‐Ru superconducting films

Amorphous Mo55Ru45 alloy films were formed by ion mixing of multilayered samples. The ion mixed films, which contain no metalloid element, show excellent superconducting properties. The measured properties are correlated with the microstructure obtained by both x-ray diffraction and transmission electron microscopy.

Metastable phases in the AuCo system formed by ion mixing

Abstract Two AuCo metastable crystalline phases (MX1 and MX2) were formed by 300 keV Xe+ irradiation at 250 °C of multilayered samples with overall composition around Au2Co8. At an early stage of irradiation, MX1 is formed, which transforms to MX2 when the whole multilayer is uniformly mixed. At even higher doses, the overall composition of the mixed film changes as a result of sputtering and MX2 transforms back to MX1. The mechanism of metastable phase formation is discussed.