Abstract
An organic-III-V hybrid semiconductor interface has been studied using real-time photoelectron spectroscopy and x-ray absorption spectroscopy to reveal the evolving morphology and molecular structure within the organic layer during thin film growth. This new approach to in situ characterization has been enabled by electron detection using a direct electron-counting array detector coupled to a hemispherical electron analyzer. The nonplanar tin phthalocyanine (SnPc) molecules initially form a uniform layer within which they have a distinct molecular orientation relative to the S-passivated gallium arsenide substrate surface [GaAs:S(001)]. The critical thickness of 0.9 nm that marks the transition between layered and clustered growth, determined from the photoemission measurements, corresponds to a single molecular layer with the molecules oriented at an angle of (39±2)° to the substrate plane. This value is confirmed by angle-resolved near-edge x-ray absorption fine structure measurements in the same experimental environment. However, the angle is less for the thicker films as the molecule-molecule interaction dominates over the molecule-substrate interaction and the structure is close to that of the bulk triclinic SnPc crystal.
Highlights
Organic semiconductors offer low cost and low toxicity options for optoelectronic and electronic applications where these factors offset their lower inherent performance compared to crystalline inorganic optoelectronic materials
We have developed a multichannel array detector that fulfills these requirements and that has been applied with both laboratory and synchrotron x-ray sources to study surface processes in real time,14,15 and this has been incorporated into an organic molecular beam deposition system as a real-time in situ probe of the growth of multilayer organic-inorganic device structures
Cally inactive when exposed to metal phthalocyanines, and this is reflected in the absence of significant line-shape changes in the substrate core levels during thin film growth
Summary
Organic semiconductors offer low cost and low toxicity options for optoelectronic and electronic applications where these factors offset their lower inherent performance compared to crystalline inorganic optoelectronic materials. Of the organic semiconductors currently in use, the highest structural and electronic quality is found in ordered crystalline films of small conjugated molecules.1 They can be grown with high purity and structural quality, most commonly by vacuum deposition, but they exhibit a range of bulk and thin film crystal structures that are very process-sensitive. There have been many attempts to improve data collection efficiency using multichannel detection combined with bright x-ray and UV sources These have been successfully applied to study surface processes for inorganic substrates but have not been applied to date as an in situ tool for monitoring the growth of organic semiconductor thin films and interfaces. The SnPc-GaAs system is a useful model system for this new experimental approach in that it provides an applications-relevant system where high-purity, controlled thin films can be grown on a well-characterized, passivated substrate surface in an environment that enables fast photoelectron spectroscopy to be applied in situ and in real time
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