Two donor-acceptor (D-A) copolymer PDVTs based on diketopyrrolopyrole and (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) units are synthesized for solution-processed field-effect transistors (FETs). The highly π-extended TVT units strengthen the coplanarity of the polymer backbone. FETs based on PDVTs show high mobilities above 2.0 cm(2) V(-1) s(-1) with a current on/off ratio of 10(5)-10(7) , high shelf storage, and operation stability.

Employing density-functional theory in combination with scanning tunneling microscopy, we demonstrate that a thin metallic film on a semiconductor surface may open an efficient and hitherto not expected diffusion channel for lateral adatom transport: adatoms may prefer diffusion within this metallic layer rather than on top of the surface. Based on this concept, we interpret recent experiments: We explain why and when In acts as a surfactant on GaN surfaces, why Ga acts as an autosurfactant, and how this mechanism can be used to optimize group-III nitride growth. DOI: 10.1103/PhysRevLett.90.056101 PACS numbers: 68.35.Fx, 68.37.Ef, 81.15.Aa Most known transport mechanisms on semiconductor surfaces have rather high activation temperatures: typically a ratio between activation and melting temperature well above 0.5 is found (T act =T melt > 0:5). Growing at lower temperature typically results in a rough surface morphology. Recently, a number of interesting transport mechanisms having low activation energies have been discussed. One example is the diffusion of transition metal atoms into bulk Si, which may actually be more efficient than diffusion on the surface [1]. Surface morphology changes mediated by bulk vacancy transport [2] and vacancy mediated surface diffusion [3] have also been discussed recently for metals. In this Letter, we discuss a growth mechanism in which the surface transport involves adatom diffusion below a metallic adlayer on a semiconductor surface. Employing state-of-the art first principles calculations and scanning tunneling microscopy, we demonstrate that adatoms may prefer to incorporate and diffuse between the adlayer and the substrate. This mechanism, which we call adlayer enhanced lateral diffusion (AELD) becomes activated already at rather low temperatures, thereby enabling the growth of materials having a high melting temperature even at modest temperatures.The materials system for which this mechanism is demonstrated are the group-III nitrides (GaN, InN, AlN, and their alloys). The members of this technologically important class of materials are strongly bonded and exhibit high melting temperatures. For example [4], GaN has a melting temperature of 2791 K and so the optimum growth temperature is expected to be higher than 1400 K. However, since GaN decomposes in vacuum when the temperature exceeds 1200 K, low pressure growth methods such as molecular beam epitaxy (MBE) can be performed only at temperatures well below the decomposition temperature. One might therefore expect that MBE growth results in a poor surface morphology and indeed, this is found for a large range of possible growth conditions. Only under extreme Ga-rich conditions, close to the onset of Ga-droplet formation, can smooth surfaces be achieved [5,6]. Recent experimental studies indicate that the presence of an In adlayer also leads to a smooth surface morphology and a better crystal quality [7][8][9][10].In order to understand why the presence of In or excess Ga leads to smoother surfaces, it is essential to k...

Rational heteroatom engineering is applied to develop high-performance electron-transporting naphthalenediimide copolymers. Top-gate field-effect transistors fabricated from selenophene-containing polymers achieve an ultrahigh electron mobility of 8.5 cm V s and excellent air-stability. The results demonstrate that the incorporation of selenophene heterocycles into the polymers can improve the film-forming ability, intermolecular interaction, and carrier transport significantly.