The ferroelectric nature of polymer ferroelectrics such as poly(vinylidene fluoride) (PVDF) has been known for over 45 years. However, its role in interfacial transport in organic/polymeric field-effect transistors (FETs) is not that well understood. Dielectrics based on PVDF and its copolymers are a perfect test-bed for conducting transport studies where a systematic tuning of the dielectric constant with temperature may be achieved. The charge transport mechanism in an organic semiconductor often occurs at the intersection of band-like coherent motion and incoherent hopping through localized states. By choosing two small molecule organic semiconductors - pentacene and 6,13 bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) – along with a copolymer of PVDF (PVDF-TrFe) as the dielectric layer, the transistor characteristics are monitored as a function of temperature. A negative coefficient of carrier mobility is observed in TIPS-pentacene upwards of 200 K with the ferroelectric dielectric. In contrast, TIPS-pentacene FETs show an activated transport with non-ferroelectric dielectrics. Pentacene FETs, on the other hand, show a weak temperature dependence of the charge carrier mobility in the ferroelectric phase of PVDF-TrFE, which is attributed to polarization fluctuation driven transport resulting from a coupling of the charge carriers to the surface phonons of the dielectric layer. Further, we show that there is a strong correlation between the nature of traps in the organic semiconductor and interfacial transport in organic FETs, especially in the presence of a ferroelectric dielectric.
It is of great technological importance to develop high quality III-Nitride layers and optoelectronic devices on Si substrates due to its low cost and wide availability as well as use of the highly matured Si microtechnology. Here we report on a novel scheme of substrate engineering to obtain high quality GaN layers on Si substrates. An ion implanted defective layer is formed in the substrate that partially isolates the III-Nitride layer from Si substrate and helps to reduce the strain in the film. The experimental results show substantial decrease in crack density, indicative of high interfacial tensile strain, with an average increase in the crack separation of 190 μm with crack free regions of 0.18 mm<sup>2</sup> for a 2 μm thick GaN film. The optical quality and strain reduction in GaN film show strong dependence on the implantation conditions and the thickness of buffer layer. Moreover the GaN film grown on implanted AlN/Si substrate has better optical properties as compared to non implanted AlN/Si. In this paper we will show how the above mentioned scheme can resolve the issues related to cracks and dislocation density in the film that are detrimental to GaN based optoelectronic devices.
Para-phenylene type molecules are efficient photoluminescence emitters in the ultraviolet-blue-green spectral range. They are used in light emitting diodes (LEDs) and photopumped lasers. Photoexcited para-phenylene type molecules give rise to strong emission from singlet excitons, bleaching of the singlet exciton absorption, induced absorption from triplet excitons and induced absorption from polarons. Since the latter two processes represent absorption of the emitted light of singlet excitons, the presence of polarons and triplet excitons might be a fundamental problem for laser diodes made from para-phenylene type molecules. In our experiments we modify the molecular geometry by the application of hydrostatic pressures up to 80 kbar in a temperature range of 10 to 300 K. In particular we show how triplet and polaron states, which are present in LEDs under operation, react to the induced geometric changes. The spectra of ground state absorption, excited state emission, bleaching of the singlet exciton absorption, induced absorption from triplet excitons and induced absorption from polarons are significantly broadened and shifted in energy. In order to explain the observed behavior we have performed three-dimensional bandstructure calculations within density functional theory for the planar poly(para-phenylene). By varying the intermolecular distances and the length of the polymer repeat unit pressure effects can be simulated.
(100) composition modulation as well as (101) and (101) tweed strain contrast were observed in Zn<SUB>1-x</SUB>Mg<SUB>x</SUB>S<SUB>y</SUB>Se<SUB>1-y</SUB> epitaxial films grown on ZnSe buffer layers. Surface buckling of the TEM plan-view specimens along the (110) direction is induced by the modulated structure in the Zn<SUB>1-x</SUB>Mg<SUB>x</SUB>S<SUB>y</SUB>Se<SUB>1-y</SUB> films. High quality Zn<SUB>1-x</SUB>Mg<SUB>x</SUB>S<SUB>y</SUB>Se<SUB>1-y</SUB> films were obtained by growing a ZnSe buffer layer on As-stabilized GaAs substrates with Zn treatment of the substrate prior to the growth of the film. In these samples, no vacancy-contained Ga<SUB>2</SUB>Se<SUB>3</SUB> interfacial layer was found at the ZnSe/GaAs interface. Samples with rough ZnSe/GaAs interfaces contained a high density of Frank partial dislocations originating at the ZnSe/GaAs interface. The interface roughness is believed to result from an As-rich GaAs surface after the oxide desorption.