Solids semiconductor interfaces

Similar results have recently been reported by Aspnes and Heller. They proposed an autocatalytic model for photoactive systems involving metal/compound semiconductor interfaces. To explain induction times in CdS systems (.9), they suggest that hydrogen incorporated in the solid lowers the barrier to charge transfer across the interface and thereby accelerates H2 production rates. 

A number of solid compounds have been examined with this time-domain method since the first report of coherent phonons in GaAs [10]. Coherent phonons were created at the metal/semiconductor interface of a GaP photodiode [29] and stacked GaInP/GaAs/GalnP layers [30]. Cesium-deposited [31-33] and potassium-deposited [34] Pt surfaces were extensively studied. Manipulation of vibrational coherence was further demonstrated on Cs/Pt using pump pulse trains [35-37]. Magnetic properties were studied on Gd films [38, 39]. 

Fig. 15. Schematic energy diagram of the n-type semiconductor electronic bands at the solid/liquid interface modulated by discontinuous metal coating ...

The term photovoltaic effect is further used to denote non-electrochemical photoprocesses in solid-state metal/semiconductor interfaces (Schottky barrier contacts) and semiconductor/semiconductor pin) junctions. Analogously, the term photogalvanic effect is used more generally to denote any photoexcitation of the d.c. current in a material (e.g. in solid ferroelectrics). Although confusion is not usual, electrochemical reactions initiated by light absorption in electrolyte solutions should be termed electrochemical photogalvanic effect , and reactions at photoexcited semiconductor electodes electrochemical photovoltaic effect . 

Most of the experiments for detecting charged macromolecules with FEDs, reported in literature, have been realized using a transistor structure [11-36], Recent successful experiments on the detection of charged biomolecules as well as polyelectrolytes with other types of FEDs, namely semiconductor thin him resistors [39 11], capacitive MIS [42] and EIS structures [43-50], have demonstrated the potential of these structures - more simple in layout, easy, and cost effective in fabrication - for studying the molecular interactions at the solid-liquid interface. A summary of results for the DNA detection with different types of FEDs is given in Table 7.1. 

For instance, the more efficiently the photoholes are trapped from the valence band of an n-type semiconductor, the higher is the probability that the photoelectrons in the conduction band reach the surface and can reduce a thermodynamically suitable electron acceptor at the solid-liquid interface. This is illustrated with an example taken from a paper by Frei et al, 1990. In this example methylviologen, MV2+, acts as the electron acceptor and TiC>2 as the photocatalyst. Upon absorption of light with energy equal or higher than the band-gap energy of Ti02, a photoelectron is formed in the conduction band and a photohole in the valence band ... 

Fig. 6.3 Schematic picture of the electrochemical potential ( > as a function of distance x in an oxide semiconductor electrolyte system a) bulk semiconductor potential b) solid/solution interface potential c) space charge potential d) flat band potential e) potential in the double layer (White, 1990, with permission.

Structural information on the atomic arrangements at the early stage of formation of metal-metal, metal-semiconductor interfaces and semiconductor-semiconductor heterojunctions is needed along with the determination of the structure of the electron states in order to put on a complete experimental ground the discussion of the formation of solid-solid junctions. Amongst the structural tools that have been applied to the interface formation problem Surface-EXAFS is probably the best... 

The advances made since 1970 start with the fact that the solid/solution interface can now be studied at an atomic level. Single-crystal surfaces turn out to manifest radically different properties, depending on the orientation exposed to the solution. Potentiodynamic techniques that were raw and quasi-empirical in 1970 are now sophisticated experimental methods. The theory of interfacial electron transfer has attracted the attention of physicists, who have taken the beginnings of quantum electrochemistry due to Gurney in 1932 and brought that early initiative to a 1990 level. Much else has happened, but one thing must be said here. Since 1972, the use of semiconductors as electrodes has come into much closer focus, and this has enormously extended the realm of systems that can be treated in electrochemical terms. 

Here SC stands for a semiconductor material, (SC + for the product of its oxidation, Ox for the oxidizer, and Red for the reduced form of Ox. The above reactions, called the conjugated reactions, proceed at a solid-solution interface simultaneously and with equal rate. In electrochemistry of metals they are considered as quite independent from each other. Once the kinetic parameters of these reactions are known, one can determine the rate (current) and potential of corrosion, using the condition, which follows from the above considerations ... 

Most detailed studies of water photodissociation on SrTi03 and Ti02 have concentrated on photoelectrochemical cells (PEC cells) operating under conditions of optimum efficiency, that is with an external potential applied between the photoanode and counterelectrode. We have become interested in understanding and improving reaction kinetics under conditions of zero applied potential. Operation at zero applied potential permits simpler electrode configurations (11) and is essential to the development of photochemistry at the gas-semiconductor interface. Reactions at the gas-sold, rather than liquid-solid, interface might permit the use of materials which photocorrode in aqueous electrolyte. 

The basic requirement of the redox oxidant in contact with n-type semiconductors is that it has an equilibrium potential more negative than the decomposition potential of the semiconductor and more positive than the lower edge of the semiconductor conduction band. The basic requirement of the reductant electrolyte is that its redox equilibrium potential be negative of the oxidant electrolyte and more positive than the lower edge of the semiconductor conduction band. More work will be necessary at characterizing solid electro-lyte/semiconductor interfaces with those solid electrolytes available before satisfactory solid-state devices capable of photocharge can be realized. 

Several excellent reviews of liquid-phase epitaxy have appeared in the literature over the past 15 years (1-12). The discussion in this chapter will be limited in scope but will supplement the material discussed in previous reviews. In particular, issues that can be analyzed by traditional methods of chemical engineering are addressed for this chemical process. Because the growing solid-liquid interface is near equilibrium, the calculation of multicomponent compound-semiconductor phase diagrams will be emphasized. 

Heteroepitaxy. Heteroepitaxy (e.g., deposition of Al Ga As on GaAs) is somewhat different, because the solid and liquid cannot initially be in equilibrium, that is, a chemical potential difference exists across the solid-liquid interface. In compound semiconductors, the chemical potential of each element is constrained by compound stoichiometry. For example, for a ternary solid (A B C) in equilibrium with a ternary liquid, the conditions of equilibrium are given by equations 8 and 9 ... 

Analysis of the growth process by LPE usually stipulates an equilibrium boundary condition at the solid-liquid interface. The solid-liquid phase diagrams of interest to LPE are those for the pure semiconductor and the semiconductor-impurity systems. Most solid alloys exhibit complete mis-... 

Similar to the molecular photosensitizers described above, solid semiconductor materials can absorb photons and convert light into electrical energy capable of reducing C02. In solution, a semiconductor will absorb light, and the electric field created at the solid-liquid interface effects the separation of photo-excited electron-hole pairs. The electrons can then carry out an interfacial reduction reaction at one site, while the holes can perform an interfacial oxidation at a separate site. In the following sections, details will be provided of the reduction of C02 at both bulk semiconductor electrodes that resemble their metal electrode counterparts, and semiconductor powders and colloids that approach the molecular length scale. Further information on semiconductor systems for C02 reduction is available in several excellent reviews [8, 44, 104, 105],... 

In bulk semiconductor electrodes, the electric field at the solid-liquid interface causes the electrons and holes to move in opposite directions within the material. Then, depending on the type of material, n-type or p-type, the electrons or holes... 

Figure 1. Band Structure in a n-type Semiconductor A. Solid State. B. In contact with a liquid phase redox couple (0/R). IL=energy of the conduction band. Vertical line indicates solid-liquid interface. CB= conduction band VB = valence band.

Some of the very interesting applications of these layered intercalates are in material design [3], ion exchange [4], catalysis [5], in the study of quantum-sized semiconductor particles [6], assembly of molecular multilayers at solid-liquid interfaces [7], designer electrode surfaces [8], preparation of low-dimensional conducting polymers [9], and so forth. 

Minerals, electrochemistry of — Many minerals, esp. the ore minerals (e.g., metal sulfides, oxides, selenides, arsenides) are either metallic conductors or semiconductors. Because of this they are prone to undergo electrochemical reactions at solid solution interfaces, and many industrially important processes, e.g., mineral leaching and flotation involve electrochemical steps [i-ii]. Electrochemical techniques can be also used in quantitative mineral analysis and phase identification [iii]. Generally, the surface of minerals (and also of glasses) when in contact with solutions can be charged due to ion-transfer processes. Thus mineral surfaces also have a specific point of zero charge depending on their sur-... 

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