Abstract:
X-ray photoelectron spectroscopy (XPS) data obtained experimentally on
phosphorous doped carbon films are analyzed using a specialized free software
XPSPEAK 4. I to find out the atomic structure of doped carbon films. The carbon
films were grown by pulsed laser deposition technique on p-type silicon using
camphoric carbon target containing 0%, 1%, 3%, 5% and 7% of phosphorous by
mass. The spectra for the whole energy range show the presence of carbon (C),
phosphorous (P) and oxygen (0) in the grown films. Atomic percentage (at. %) of P
content in the film varies approximately linearly with the phosphorous content in the
target by mass. The analysis of the spectra for CIs, P 2p and 0 Is regions is
performed after providing appropriate background subtraction to account for
inelastic scattering and other energy loss processes. Curve fitting technique using
Gaussian-Lorentzian lineshapes is used in Cis, P 2p and 0 Is regions of the spectra
to isolate the presence of different elements and compounds in the grown films. Also
to find the chemical structure at various depths of the film, the spectra in CIs, P 2p
and 0 Is regions of argon ion etched carbon films are analyzed. The analysis of CIs
region shows the presence of Sp2 and Sp3hybridized carbon and a Sp2 satellite peak
due to x-x* shake up. The Sp2content is seen to remain almost constant for the films
grown from target containing different amount of P. The FWHM of C-C Sp2 peak
increases up to 5% P but decreases for 7% P probably due to clustering of Sp2chains
and this clustering in Sp2phase probably decreases the bandgap for 7% P film. The P
2p region shows the presence of isolated phosphorous, P-C (C Sp2hybridized), P-C
(C sp3 hybridized) and P-O bonds. The 0 Is region shows the presence of C-O and
P-O bonds. The difference in peak position between the P-C (C Sp2) and P-C (C
Sp3) peaks is found to be around 3 eV, though in the carbon region it is around 1 eV.
The amount of phosphorous in P-C bond is seen to be much greater than donor
concentration. A large amount of phosphorous in P-C bonds go into nondoping
configurations and even some of the donors from doping configurations are used to
convert the paramagnetic defects into diamagnetic. From the analysis of titted
spectra in C, P and 0 regions and correlating the peak parameters for all the peaks, a
simplified overall atomic structure of the phosphorous doped carbon film has been
suggested containing Sp2 and Sp3 hybridized carbon, P-C bond (in both Sp2 and Sp3
network), P-O bond and C-O bond. Carbon film parameters like intrinsic carrier
concentration, donor concentration, conductivity, minority carrier mobility and
minority carrier diffusion length have been correlated with findings in the XPS
analysis.
