Semiconductor Band Bending Research Articles

Solar Photoelectrochemical Cells Based on Photoinduced Hydrogen Atom Abstraction Reactions in Solution

AbstractSolar irradiation of a two‐component photoelectrochemical cell containing aqueous alcohol (2‐propanol, ethanol or methanol) solutions of a “mediator,” such as methyl viologen or diquat, and benzophenone in the photoanode compartment and aqueous acid in the cathode leads to the generation of moderately stable photocurrents in the 400–1500 μA range. Similar results are obtained, but with generally lower currents, when benzophenone‐free solutions are irradiated. The reactions in the photoanode compartment all originate with an excited state hydrogen atom abstraction from the alcohol to generate two reducing free radicals; the ketyl radicals formed are rapidly oxidized by the mediator to give an electrochemically active but relatively stable semi‐reduced mediator species which is oxidized at the electrode surface. The basic photochemical and subsequent dark reactions are reasonably efficient and well‐understood; the overall peak efficiencies of the present cell system are relatively low (1–2%) reflecting the low fraction of the solar spectrum absorbed by the photochemically reactive reagents. The net chemical change occurring is endothermic and the products are in principle readily recyclable. The solutions are photochromic due to transient generation of the semi‐reduced mediator. A primary advantage of this system is that the use of a mediator to avert radical combination or other energy‐wasting processes provides the system with a directionality analogous to that obtained via band bending in semiconductors.

Raman spectroscopy as a surface sensitive technique on semiconductors

It is demonstrated that the measurement of a bulk phonon property, i.e., the Raman intensity of the resonantly excited symmetry forbidden LO-phonon via its dependence on the surface electric field, is a sensitive method to measure surface band bending in polar semiconductors. The observation of coupled phonon-plasmon modes shows the existence of flat band conditions on clean, UHV-cleaved (110)GaAs. This condition can also be reached on pinned surfaces by photoexcitation with laser power densities in excess of 500 W/cm2. A curve fitting of the observed L+/L−-spectrum with the Lindhard–Mermin dielectric function yields information on the wave-vector smearing due to strong light absorption and impurity scattering.

Theory and experiment for silicon Schottky barrier diodes at high current density

Metal-silicon Schottky barrier diodes exhibit n values which theoretically vary as a function of doping and applied voltage. The expected variation depends on which theoretical model is used to describe the current transport. Titanium n-type silicon barriers were prepared. At a doping level of 3 × 10 15 cm −3 the barrier height and n-value measured at 100 mV were 0.485±0.005 V and 1.02±0.01 whereas for a doping level of 2 × 10 14 cm −3 the corresponding values were 0.500±0.005 V and 1.18±0.05. The experimental variation of the diode n value as a function of semiconductor band bending showed good agreement with the thermionic-diffusion model of Crowell and Beguwala: n values increased rapidly as the band bending β → 2, and n values were highest at a given β for diodes with the lowest doping concentration. Similar results were obtained by measurements on magnesium and aluminium barriers on n-type silicon. An analysis of the results has shown that the variation of the diode saturation current I s follows the predictions of the thermionic-diffusion theory, although there were some anomalies at high current densities. The anomalies did not result from variation of the width of the undepleted region of the epitaxial silicon layer or from diode self-heating effects.