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Abstract:
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In the present studies, various copper delafossite materials viz; CuAlO2,
CuGaO2, CuFeO2 , CuGa1-xFexO2, CuYO2 and CuCaxY1-xO2 were synthesised by
solid state reaction technique. These copper delafossite materials were grown in
thin film form by rf magnetron sputtering technique. In general copper
delafossites exhibit good optical transparency. The conductivity of the CuYO2
could be improved by Ca doping or by oxygen intercalation by annealing the film in oxygen atmosphere. It has so far been impossible to improve the p-type
conductivity of CuGaO2 significantly by doping Mg or Ca on the Ga site. The ptype
conductivity is presumed to be due to oxygen doping or Cu Vacancies [6].
Reports in literature show, oxygen intercalation or divalent ion doping on Ga
site is not possible for CuGaO2 thin films to improve the p-type conductivity.
Sintered powder and crystals of CuFeO2 have been reported as the materials
having the highest p-type conductivity [14, 15] among the copper and silver
delafossites. However the CuFeO2 films are found to be less transparent in the
visible region compared to CuGaO2. Hence in the present work, the solid
solution between the CuGaO2 and CuFeO2 was effected by solid state reaction,
varying the Fe content. The CuGa1-xFexO2 with Fe content, x=0.5 shows an
increase in conductivity by two orders, compared to CuGaO2 but the
transparency is only about 50% in the visible region which is less than that of
CuGaO2 The synthesis of α−AgGaO2 was carried out by two step process which
involves the synthesis of β-AgGaO2 by ion exchange reaction followed by the
hydrothermal conversion of the β-AgGaO2 into α-AgGaO2. The trace amount of
Ag has been reduced substantially in the two step synthesis compared to the
direct hydrothermal synthesis. Thin films of α-AgGaO2 were prepared on silicon
and Al2O3 substrates by pulsed laser deposition. These studies indicate the
possibility of using this material as p-type material in thin film form for
transparent electronics. The room temperature conductivity of α-AgGaO2 was
measured as 3.17 x 10-4 Scm-1and the optical band gap was estimated as 4.12 eV.
A transparent p-n junction thin film diode on glass substrate was fabricated
using p-type α-AgGaO2 and n-ZnO.AgCoO2 thin films with 50% transparency in the visible region were deposited
on single crystalline Al2O3 and amorphous silica substrates by RF magnetron
sputtering and p type conductivity of AgCoO2 was demonstrated by fabricating
transparent p-n junction diode with AgCoO2 as p-side and ZnO: Al as n-side
using sputtering. The junction thus obtained was found to be rectifying with a
forward to reverse current of about 10 at an applied voltage of 3 V.The present
study shows that silver delafossite thin films with p-type conductivity can be
used for the fabrication of active devices for transparent electronics applications. |