Other Crystalline Semiconductors

Etch Profiles. The final profile of a wet etch can be strongly influenced by the crystalline orientation of the semiconductor sample. Many wet etches have different etch rates for various exposed crystal planes. In contrast, several etches are available for specific materials which show Httle dependence on the crystal plane, resulting in a nearly perfect isotropic profile. The different profiles that can be achieved in GaAs etching, as well as InP-based materials, have been discussed (130—132). Similar behavior can be expected for other crystalline semiconductors. It can be important to control the etch profile if a subsequent metallisation step has to pass over the etched step. For reflable metal step coverage it is desirable to have a sloped etched step or at worst a vertical profile. If the profile is re-entrant (concave) then it is possible to have a break in the metal film, causing an open defect. 

Preparation of Semiconductors 17.3.8.6. Other Crystalline Semiconductors 17.3.8.6.5. Transition Metal Chalcogenides. 

To make a breakthrough in household appliances and other consumer product markets UV sensors have to become significantly cheaper while spectral selectivity as a major key feature must be guaranteed. Most of today s UV photodiodes are made from crystalline semiconductor materials. The cheaper materials (Si) lack spectral selectivity, and the wide band gap materials are very expensive. What they all have in common their top performance regarding sensitivity and speed. Crystalline photodiodes have risetimes of often below 1 s. However, the described processes to be sensed here are not faster than some milliseconds or even much slower. In order to obtain a reasonably-priced SiC or GaN photodiode, the photoactive area is often reduced to below 1 mm2 and barely fills the sensor housing. So far, the top sensitivity offered by the semiconductor has been sacrificed for a competitive... 

In crystalline semiconductors, the most common technique for the measurement of carrier mobility involves the Hall effect. However, in noncrystalline materials, experimental data are both fragmentary and anomalous (see, for example. Ref. [5]). Measured HaU mobility is typically of the order of 10 - 10 cm A /s and is frequently found to exhibit an anomalous sign reversal with respect to other properties providing information concerning the dominant charge carrier. Thus, apart from some theoretical interest, the Hall effect measurements are of minimal value in the study of macroscopic transport in amorphous semiconductors. 

While considering trends in further investigations, one has to pay special attention to the effect of electroreflection. So far, this effect has been used to obtain information on the structure of the near-the-surface region of a semiconductor, but the electroreflection method makes it possible, in principle, to study electrode reactions, adsorption, and the properties of thin surface layers. Let us note in this respect an important role of objects with semiconducting properties for electrochemistry and photoelectrochemistry as a whole. Here we mean oxide and other films, polylayers of adsorbed organic substances, and other materials on the surface of metallic electrodes. Anomalies in the electrochemical behavior of such systems are frequently explained by their semiconductor nature. Yet, there is a barrier between electrochemistry and photoelectrochemistry of crystalline semiconductors with electronic conductivity, on the one hand, and electrochemistry of oxide films, which usually are amorphous and have appreciable ionic conductivity, on the other hand. To overcome this barrier is the task of further investigations. 

The n parameter equals 1 for direct bandgap semiconductors or 4 for indirect bandgap semiconductors in the case of allowed fundamental transitions [22], Other values of n, 2 or 3, are valid only for forbidden transitions. The proper transformation allows estimation of the bandgap energy, Eg, for both types of crystalline semiconductors. Figure 7.7 presents the procedure of Eg evaluation. 

Nevertheless, the first functional working TFT was demonstrated by Weimer in 1962 (Ref 2). He used thin films of polycrystalline cadmium sulfide, similar to those ones developed for photodetectors. The simplified structure is shown in Fig. 1(b). Other TFT semiconductor materials like CdSe, Te, InSb and Ge were investigated, but in the mid-1960 s the emergence of the metal oxide semiconductor field effect transistor (MOSFET) based on the crystalline silicon technology and the possibility to perform integrated circuits, led to a decline in TFT development activity by the end of the 1960s. 

Thermally and mechanically stable thin film formation is indispensable for fabrication of practical electronic devices such as organic light-emitting diodes, field-effect transistors, and particularly for flexible electronic papers. There are two approaches for formation of thermally and mechanically stable thin films. One is formation of glassy semiconductors retaining ordered structures by cooling from liquid-crystalline states [94]. The other approach is polymerization of liquid-crystalline semiconductors with a reactive moiety. 

Intrinsic semiconductors. The group fourteen elements carbon, silicon, germanium, and tin can be found to adopt the diamond-type crystal structure shown in Figure 3 a. Other crystalline structures are also found for example, graphite and diamond are different crystal structures of the same element, carbon. Because of its size and orbital energies, carbon forms very... 

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