Mobility in single crystals

J. Y. Lee, S. Roth and Y. W. Park, Anisotropic field effect mobility in single crystal pentacene, Appl. Phys. Lett,... 

At room temperature for instance, the mobility in single crystals of aromatic compounds like anthracene or perylene is very high, i.e. in the range of 10 cm A s [5], In amorphous polymers the carrier mobilities are orders of magnitude smaller and typical values are in the range of 10 and 10 cm A s [6]. 

Calculations of this type are carried out for fee, bcc, rock salt, and hep crystal structures and applied to precursor decay in single-crystal copper, tungsten, NaCl, and LiF [17]. The calculations show that the initial mobile dislocation densities necessary to obtain the measured rapid precursor decay in all cases are two or three orders of magnitude greater than initially present in the crystals. Herrmann et al. [18] show how dislocation multiplication combined with nonlinear elastic response can give some explanation for this effect. 

Overview of Unit Operations. To maximize the electron or hole (carrier) mobility and thus device speed, ICs are built in single-crystal substrates. Methods of bulk crystal growth are therefore needed. The most common of these methods are the Czochralski and float-zone techniques. The Czochralski technique is a crystal-pulling or melt-growth method, whereas the float-zone technique involves localized melting of a sintered bar of the material, followed by cooling and, thus, crystallization. 

Figure 96 Temperature dependence of the charge carrier mobility in organic single crystals, (a) The electron mobility in a crystal grown from moderately purified perylene (see Ref. 28), and (b) the electron (/ ) and hole (// ) mobilities in synthetic ultrapurified anthracene crystals at an electric field E 2.3x10 V/cm directed along the crystallographic axes b (adapted from Ref. 436a).

This result is useful in understanding the variation of the field dependence of the TOF measured mobility from sample to sample, following the carrier density gradients (Fc dn/cbc). For example, the role of the diffusion carrier stream would explain the field dependence of jx in single crystals whenever their near-surface layer is strongly populated... 

Ordered conjugated organic materials exhibit low trap densities and high charge carrier mobilities, for both electrons and holes. As an example, in single crystals of anthracene [87], the hole and electron mobilities are approximately 1 cm V s and the crystal photoluminescence quantum yield at room temperature is almost unity. Both electroluminescence [87] and stimulated emission... 

FIGURE 2.1.23 Pressure dependence of the field-effect mobility (a) and the threshold voltage (b) in single-crystal rubrene OFETs (solid and open symbols correspond to the increasing and decreasing pressure). (From Rang, Z. et ah, Appl. Phys. Lett., 86, 123501, 2005.)... 

J. Takeya, M. Yamagishi, Y. Tominari, R. Hirahara, Y. Nakazawa T. Nishikawa, T. Kawase, T. Shimoda and S. Ogawa, Very high-mobility organic single-crystal transistors with in-crystal conduction channels, Appl. Phys. Lett, 90, 102120 (2007). 

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