Some titanium germanium and silicon compounds: Reaction and properties O. Thomas, a) F. M. d'Heurle, and S. Delage b) IBM T. J. Watson Research Center, P.O. 218, Yorktown Heights, New York 10598 (Received 27 December 1989; accepted 20 March 1990) Titanium reacts with pure Ge in two different ways: At low temperatures one observes the formation of Ti 6 Ge 5 with some characteristics typical of diffusion-controlled reaction. Upon completion of this first stage Ti 6 Ge 5 reacts with remaining Ge to form TiGe 2 , isomorphous with C54 TiSi 2 , in a process which is clearly controlled by nucleation. The same observations apply to reactions with a Ge alloy containing 25 at.% Si. With an alloy containing 50 at.% Si the two stages become merged, so that while remaining identifiable, they are much less distinct than with the previous conditions. The reaction behavior observed with a Ge alloy containing 80 at.% Si resembles that generally obtained with pure Si: there are no easily identifiable steps between the initial Si-Ti sample and the final one, Si-TiSi 2 . With both the 50-50 and 80-20 Si-Ge alloys the formation of the C54 structure is preceded by that of the C49 structure (ZrSi 2 type), as with pure Si. The gradual merging of the diffusion-controlled reaction and that controlled by nucleation as the concentration of Si in the substrate increases implies that nucleation plays a significant role in the formation of TiSi 2 , even if that role cannot be easily isolated. Effects due to gas impurities on the path of the metal-substrate reaction have been analyzed. The resistivities of several pure and alloyed phases have been measured. Alloy scattering in the system TiSi 2 -TiGe 2 is briefly discussed. I. INTRODUCTION The subject of this paper has already been the ob- ject of two short publications 1 ' 2 that focused on the be- havior of Ti with layers of pure Ge and 50-50 Si-Ge alloys on (100) Si substrates. Thus for the sake of being brief, both this introduction and the usual description of the experimental details will be kept to a minimum. In this ternary system the phases with the highest metalloid content, TiGe 2 and TiSi 2 , have the same structure, orthorhombic (C54) with unit cell dimensions that are very close: a = 8.594 A, b = 5.030 A, c = 8.864 A, and a = 8.252 A, b = 4.783 A, c = 8.540 A, respectively. 3 (One notes here some ambiguity in the literature about the respective definitions of the a and c axes so that it is not exactly clear whether the a lattice parameters are smaller than c parameters for both com- pounds. It will be assumed here that the axes are prop- erly defined as presently given.) Thus with lattice parameter differences of the order of 6%, well within the limits of 15% given by the Hume-Rothery rule for "'Permanent address: Laboratoire des Materiaux et de Genie Physique, ENSPG, BP 46, 38402 Saint Martin d'Heres, France. b) Permanent address: Thomson-CSF, Domaine de Corbeville, 91401 Orsay, France. extended solubilities between elements 4 (and pre- sumably between chemically equivalent compounds also), the two compounds should be mutually soluble, as verified 2 for samples containing 50 at.% Si (and 50 at.% Ge, ignoring Ti). On the other hand, in the middle of the diagram there exists 3 ' 5 ' 6 no exact equivalent in the Si-Ti system to the phase 35 ' 7 Ti 6 Ge 5 . Most of the investigations were carried out with layers of Ge and Si-Ge alloys deposited on (100) Si sub- strates via molecular beam (MBE) deposition tech- niques. Although these were not specifically intended to be epitaxial, they were in fact found to be so. In the results reported thus far one notes the two-stage reac- tion of Ti with the epitaxial layers of pure Ge: at low temperatures the formation of Ti 6 Ge 5 with features that strongly imply a diffusion-controlled mode of growth, and quite distinctly at higher temperatures the nucle- ation-controlled 8 formation of TiGe 2 . With the 50-50 Si-Ge alloy, two stages can still be recognized but the overall picture is much less clear than with pure Ge. Here attention will be focused on the behaviors of the 25-75 and 80-20 alloys, and on the electrical properties of the final disilicide-germanide solid solutions. Some relevant observations made on the reaction of Ti with single crystal Ge and on bilayers of Ti and Ge on Si will also be reported. J. Mater. Res., Vol. 5, No. 7, Jul 1990 © 1990 Materials Research Society 1453