The present paper deals with a systemic investigation of the interface electronic
structure of as-deposited as well as annealed Ti/Ni multilayer (ML) samples up to
400 °C
using core level and valence band (VB) photoemission techniques. For this purpose
[Ti(50 Å)/Ni(50 Å)] × 10
ML samples have been prepared by employing an electron beam evaporation technique
under ultrahigh vacuum conditions.
The depth profile core level photoemission investigation carried out on annealed ML
samples indicates a gradual change in the nature of the electronic bonding at the
interface with temperature. In particular the ML samples annealed at 300 and
400 °C clearly show
shifts in the Ni 2p3/2
and Ti 2p3/2
core levels towards the higher binding energy side as compared to as-deposited samples,
suggesting the formation of a TiNi alloy phase at the interface. The corresponding VB spectra
also show appreciable changes and provide strong evidence for TiNi alloy formation. Further
confirmation of this alloy phase formation is clearly reflected in the x-ray diffraction
measurements carried out on these samples. The recorded x-ray diffraction patterns show a
solid state reaction leading to amorphization when the ML sample is annealed at
300 °C
and recrystallization to a TiNi alloy phase at the annealing temperature of
400 °C.
In order to determine the charge transfer between Ti and Ni atoms in the formation of the TiNi alloy
phase, the 2p3/2
core levels and the x-ray excited Auger regions of Ti and Ni were carefully investigated.
The experimentally measured core level shifts for Ti and Ni were both found to be
positive, leading to the conclusion that electronegativity criteria cannot be used to
decide the direction of charge transfer in this case. The observed shifts in modified
Auger parameters determined from recorded experimental data show a positive
value for Ti and a negative one for Ni. This provides clear evidence that the
direction of charge transfer is from Ni to Ti atoms during the formation of the TiNi
alloy at the interface. The charge on ionized atoms calculated by using a simple
electrostatic model indicates similar trends for the charge transfer deduced from Auger
parameters and chemical shifts. In addition to this, areas under the core level peaks
have been calculated by employing Shirley and Touggard background methods.
The difference between the backgrounds, when normalized with respect to the
elemental values, provides information about the density of states at the Fermi level
(EF). The density
of states at EF
calculated in this way shows reductions in values for both Ti and Ni when the ML sample
is annealed at different temperatures. This is in complete agreement with corresponding
theoretically calculated densities of states.