Interface formation, hole injection

For homogeneously doped silicon samples free of metals the identification of cathodic and anodic sites is difficult. In the frame of the quantum size formation model for micro PS, as discussed in Section 7.1, it can be speculated that hole injection by an oxidizing species, according to Eq. (2.2), predominantly occurs into the bulk silicon, because a quantum-confined feature shows an increased VB energy. As a result, hole injection is expected to occur predominantly at the bulk-porous interface and into the bulk Si. The divalent dissolution reaction according to Eq. (4.4) then consumes these holes under formation of micro PS. In this model the limited thickness of stain films can be explained by a reduced rate of hole injection caused by a diffusional limitation for the oxidizing species with increasing film thickness. 

The interface between the quasi-metal PEDOT PSS and organic semiconductors has been investigated in numerous experiments. As illustrated in Figure 14.16, the energy barrier for hole injection (AE) is not simply determined by the difference between the PEDOT PSS work function (4>) and the ionization potential (/ ) of the semiconductor as predicted by the Schottky-Mott model dipole layer formation at the interface will lead to a vacuum level shift A [158-161]. 

Figure 14.16 Energy level alignment at the PEDOT PSS-semiconductor interface. The two layers are (a) separated and (b) in contact. The formation of an interface dipole (ID) might significantly determine the energy barrier for hole injection AE...

Fig. 12 Schematic energy level diagram of the impact of the formation of an interface dipole at an ITO/organic semiconductor (OS) interface (a) the barrier in the absence of the dipole layer (b) and (c) reduction of the hole injection and the electron injection barrier, respectively, in the presence of the interface dipole layer on the surfaces of ITO. Here, the interface dipoles with opposite sign are directed toward the ITO surfaces (b) increasing and (c) decreasing the work function of ITO.

Kolasinski KW, Hartline JD, Kelly BT, Yadlovskiy J (2010) Dynamics of porous silicon formation by etching in HF + V2O5 solutions. Mol Phys 108 1033-1043 Kolasinski KW, Gogola JW, Barclay WB (2012) A test of Marcus theory predictions for electroless etching of silicon. J Phys Chem C 116 21472-21481 Kooij ES, Butter K, Kelly JJ (1998) Hole injection at the silicon/aqueous electrolyte interface a possible mechanism for chemiluminescence from porous silicon. J Electrochem Soc 145 1232-1238... 

This is the regime of anodic current densities below JPS. A hole approaching the interface initiates the divalent electrochemical dissolution of a silicon surface atom at the emitter. The dissolution proceeds under formation of H2 and electron injection, as shown in Fig. 4.3. The formation of PS structures is confined to this region. 

The magnitude of the injection barrier is open to conjecture. Meanwhile there is consensus that energy barriers can deviate significantly from the values estimated from vacuum values of the work-function of the electrode and from the center of the hole and electron transporting states, respectively. The reason is related to the possible formation of interfacial dipole layers that are specific for the kind of material. Photoelectron spectroscopy indicates that injection barriers can differ by more than 1 eV from values that assume vacuum level alignment [176, 177]. Photoemission studies can also delineate band bending close to the interface [178]. 

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