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Title: BEEM studies of metal-organic interfaces
Authors: ZHENG YI
Keywords: Metal-organic interfaces, BEEM/STM, Charge Injection, Pentacene
Issue Date: 10-Feb-2009
Citation: ZHENG YI (2009-02-10). BEEM studies of metal-organic interfaces. ScholarBank@NUS Repository.
Abstract: This thesis deals with charge injection at metal-organic (MO) interfaces and electrical transport in organic semiconductor thin films using ballistic electron emission microscopy (BEEM) and other complementary techniques.Unlike metal-inorganic semiconductor (MIS) junctions, charge transport in metal-organic diodes is interplay between charge injection at metal-organic interface and space-charge conduction in semiconductor thin film. The space-charge effect leads to a transition from injection-limited current to space-charge-limited current at low temperature. For BEEM experiments, the space-charge effect requires a voltage on the collector (VBEEM) to tilt the energy band of organic thin film and to assist charge carriers in hopping towards the collector.Using conventional current-voltage characteristics and BEEM spectroscopy with different polarity of VBEEM, we show that metal diffusion modifies metal-on-organic (M/O) interface by creating low Schottky barrier (SB) patches. BEEM spectroscopy reveals that, in these low SB patches, both hole and electron injection barriers are reduced, instead of simply shifting the Fermi level (EF) of organic thin film inside the band gap.We have also compared ballistic charge injection studies on MIS, M/O, and metal-oxide interfaces. We show that unlike the conventional MIS interfaces, both the valence band and the conduction band structures of organic semiconductors and oxides can be characterized by BEEM. Such a finding greatly improves the versatility of BEEM in studying interfacial electronic structures. Based on these results, we point out that the current knowledge of BEEM, derived mainly from metal-inorganic semiconductor interfaces, is insufficient for understanding ballistic charge injection at M/O and metal-oxide interfaces.
Appears in Collections:Ph.D Theses (Open)

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