a Laboratory for Thin Film Technology, Federal Institute for
Materials
Research and Testing
Unter den Eichen 87, D-12205 Berlin, Germany
b Laboratoire de Physique des Solides de Bellevue,
Centre National de la Recherche Scientifique
1, Place Aristide Briand, F-92195 Meudon Cedex, France
It is demonstrated that Grazing Incidence X-Ray Diffractometry (GIXD) canbe
used for
depth profiling and phase characterization of multilayer thin film devices. A
Bragg-Soller
x-ray diffractometer system with a flat secondary monochromator and a fixed
CuKα tube (Seifert XRD 3000TT) was employed at varying
low incidence
with fixed θ scans (Fig. 1).
As an example, semiconducting (SC) layers on transparent electrically
conducting
substrates (TCO) have been characterized. A typical structure was
The semiconducting layers, SC1 and SC2 of
II-VI
compounds, were synthesized by electrochemical deposition from aqueous
electrolyte
solutions. In such optoelectronically active rectifying devices (PV cells),
SC1
was the window layer and SC2 the absorber layer. ZnTe or ZnSe
served as
window material, SC-1, and CdTe as absorber, SC2.
The aim of this study was to show whether subsequent growth of layers by this
technique led to (i) attack (partial dissolution) of underlying layers including TCO, or
(ii)
interdiffusion (mixing) of layers. Finally, it served to identify very thin window layers
which
cannot be detected at all, or cannot be distinguished against the substrate
background
(TCO) by conventional XRD (θ - 2θ scans).
The GIXD investigation of a glass/SnO2/ZnSe/CdTe device is
depicted in
Fig. 2. The upper CdTe absorber layer was about 300 nm thick. The ZnSe window
was 100
nm and the transparent conductive oxide 400 nm. A variation of the incident angle
ω
between 0.3ø and 3.0ø allowed to discriminate between the layer phases on the
nanometer
scale. At ω ~ 0.9ø, the ZnSe (111) reflex and the SnO2 (211)
reflex
emerged. The penetration depth of the x-ray beam was about 190 nm at this
ω. The
CdTe absorber signals, which show (111), (220) and (311) texture, increase and level
off
above ω ~ 1.5ø. At ω > 0.9, diffraction peaks due to the ZnSe layer
emerged,
and around ω > 2, the SnO2 substrate diffraction peaks
became visible
in addition. Below ω ~ 0.9, only the topmost layer consisting of CdTe leads to
diffraction peaks.
Such nanometer profile diagnostic results can be correlated with data relevant
for
photovoltaic applications.
| |
| Fig.1 Bragg-Soller x-ray diffractometer system with a flat secondary monochromator and a fixed CuKα tube (Seifert XRD 3000TT) | Fig.2 GIXD of glass / SnO2 / ZnSe / CdTe thin film device |
