Alternative Semiconductor Substrates

Another impedance-based imaging technique for laterally resolved characterization of thin films or electrochemical systems is Scanning Photo-induced Impedance Microscopy (SPIM) [44]. It is based on photocurrent measurements at field-effect structures. In their simplest arrangement, field-effect structures consist of a semiconductor substrate with a thin insulator, and a gate electrode. This gate electrode can be a metal film resulting in the structure Metal Insulator Semiconductor (MIS) or, alternatively. Electrolyte Insulator Semiconductor structures are used, in which the electrolyte is in direct contact with the insulator, and a reference electrode is required to fulfill the function of the gate electrode. 

A modified SILAR system has been used to grow CdSe in CdS/CdSe core shell semiconductor nanocrystals.12 A cadmium precursor solution, with CdO dissolved with oleic acid in octadecane, was injected onto the substrate, and the Se solution (Se powder dissolved with tributylphosphine in octadecane) was similarly injected. The temperature of the reaction solution was 185 °C. A CdS outer layer in the CdS/CdSe/CdS colloidal quantum wells was deposited by alternating injections of cadmium and sulfur both in octadecane solutions at 230-240 °C. These structures showed high PL quantum yields (20-40%), relatively narrow emission bands, and tunable emission colors from about 520 to 650 nm depending on the number of CdSe monolayers. 

With regard to its unique properties, a carbon substrate can be considered an excellent alternative material to continuous metal conductors and semiconductors for the construction of DNA sensors and chips. 

The technique of alternating polyelectrolyte film construction has also been adapted to incorporate semiconductors into layered films. For example, multilayer films have been constructed by alternately dipping a quartz substrate into a solution of poly(diallylmethylammonium chloride) and then a solution of a stabilized CdS or PbS colloid (41). The layer-by-layer self-assembly of alternating polymer and metal sulfide is at least partially driven by the electrostatic attraction of the cationic polymer and the negative charge of the stabilized MC colloid particles. 

Ion-etching is used to form the parallel semiconductor strips 1 from a single semiconductor body and to form the separate electrodes and their connections for each strip 1 from a metal layer deposited on the semiconductor body and on the substrate 2 (see EP-A-0007668). Alternatively, the strips 1 are formed from an epitaxial layer of one conductivity type material which is deposited on an intrinsic substrate 2 or a substrate 2 of cadmium telluride. 

An alternative route to solution-processible organic semiconductors is to use precursors to small molecule semiconductors, such as pentacene [59] or tetrabenzo-porphyrin [60], which can be converted into their fully conjugated, insoluble form by thermal or irradiative [61] treatment on the substrate. Pentacene precursors have been shown to yield field-effect mobilities of 0.01-0.1 cm2 V-1 s 1 [62], and 0.1-0.8 cm2 V-1 s 1 [63] after thermal conversion at 150-200 °C. Small molecule organic semiconductors can also be rendered solution processible by attachment of flexible side chains [64-66], Due to the relatively low solubility of these molecules the growth of uniform thin films of these molecules remains challenging, however. 

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