Crystallization of sputtered In
2
O
3
and In
2
O
3
:H
Alexander Steigert
1*
, Iver Lauermann
1
, Stefan Körner
2
, Ruslan Muydinov
2
, Bernd Szyszka
2
,
Rutger Schlatmann
1
and Reiner Klenk
1
1) Helmholtz-Zentrum Berlin für Materialien und Energie, D 12489 Berlin, Germany
2) TU Berlin, D 10587 Berlin, Germany
Contact
The main advantage of In
2
O
3
:H over industry standard TCO materials is the high
electron mobility that can be achieved. This promises lower optical absorption losses (free
carrier absorption) in the transparent front contact for chalcopyrite based solar cells and
modules. While the direct preparation with high mobility is possible (at least by atomic layer
deposition), the standard procedure for sputtered films involves a crystallization step. In order
to fully exploit the potential of In
2
O
3
:H, the crystallization procedure has to optimized. Goals
of the optimization are a robust process with a wide process window as well as highest
transparency and mobility at the sheet resistance required for the specific application.
Furthermore, annealing conditions have to be such that they do not degrade the chalcopyrite/-
buffer heterojunction. Here we will report on the fundamental parameters influencing the
crystallization and the structural, electrical, optical, and surface properties before and after
crystallization. The films were prepared in a sputtering system directly connected to a surface
analysis system.
Films sputtered in different gas mixtures (Ar/Ar-H
2
/Ar-O
2
) without water were
(partially) crystalline with poor electrical and/or optical properties. The lattice constants in
these films were generally larger than those of the powder reference. The difference in lattice
constants was reduced at higher working gas pressures. However, higher pressures, except for
the Ar-O
2
mixture, also lead to films with oxygen deficiency and poor transparency. Metal
phases were indicated by XPS and XRD in extreme cases. With the addition of water partial
pressure, the ratio of amorphous vs. crystalline phases increased and the metallic character
was reduced, but many of the general trends described above were unchanged. Very thin
films (in the order of 100 nm) were completely amorphous. The best trade-off between
electrical and optical properties before annealing was achieved at low Ar pressure, no
oxygen, and a water partial pressure of 10
-5
mbar. XPS suggested that part of the In is bound
as hydroxide. Hydroxide was transformed to oxide by annealing in vacuum, but films did not
crystallize under these conditions. Instead, films had to be annealed in air to trigger
crystallization. This is in contrast to literature data where films were sputtered in Ar/O
2
/H
2
O
and annealed in vacuum. Together with other observations, this suggests a critical role of the
oxygen stoichiometry for crystallization. For both, In
2
O
3
and In
2
O
3
:H, there was a distinct
difference in lattice constants of those parts of the film that crystallized already during
sputtering and the part crystallized by annealing. For the latter the lattice constant is in good
agreement with the powder reference. A time series carried out with a 100 nm film suggested
that crystallization in this case is almost complete after 5-10 minutes @ 180°, which is
compatible with the chalcopyrite cell. Crystallization of In
2
O
3
:H deposited on an actual cell
has been verified. The crystalline fraction of as-deposited In
2
O
3
:H films increased with
thickness. Nevertheless, the mobility was almost constant at 50 cm
2
/Vs between 100 nm and
1 µm. With annealing, the mobility increases to 120 cm
2
/Vs in the thin film but only to 90
cm
2
/Vs in thicker films. This suggests that the highest mobility is only achieved within the
relaxed crystals grown from the amorphous phase by annealing, not in the stressed crystals
growing already during sputtering.
PS1 25
-181-
