Reflectance difference spectroscopy during CdTe/ZnTe interface formation

We have investigated the first stages of epitaxial growth of CdTe on ZnTe and ZnTe on CdTe with reflectance difference (RD) spectroscopy. Spectroscopic RD data show strong optical anisotropy responses at the critical points of the bulk dielectric function at the E₀, E₁ and E₁+Δ₁ interband transitions of ZnTe and CdTe, respectively, which indicate that anisotropic in-plane strain occurs during epitaxial growth. Kinetic RD data taken at the E₁ transition of the respective material exhibit with an accuracy of 1 ML the onset of the formation of misfit dislocations for these material systems.     

The role of metal contacts in the stability of n-type organic field effect transistors

We are presenting a long-time bias stress stability of C₆₀-based n-type organic field effect transistors (OFETs), in bottom gate, top contacts configuration, with aluminium (Al), silver (Ag) and gold (Au) source–drain contacts. The results clearly shows that the bias stress effects in C₆₀-based n-type OFETs is similar to p-type OFETs and it can be reduced by using an appropriate metal for the source–drain contacts. During the bias stress time, the threshold voltage shift and an increase in the contacts resistance have also been measured. On the basis of the stability of the device parameters, it is proposed that the Al source–drain contact-based devices gives better stability as compared to the devices with Ag and Au source–drain contacts. Our results show that the bias stress-induced threshold voltage shift is due to the trapping of charges in the channel region and in the vicinity of the source–drain contacts.     

Quantitative luminous efficiency determination for large-area light-emitting devices

During recent decades organic light-emitting devices became of increasing interest in many fields of applications. A broad spectrum of available organic molecules can be used to fabricate organic light-emitting diodes (OLEDs) emitting at various photon energies. One of the commonly used quality criteria to specify the light output of OLEDs is the luminous efficiency. The standard measurements are based on the assumption of a point source, which is not applicable in the case of large-area devices, or can be achieved only approximately by long distances between emitter and detector. Therefore, we developed a data evaluation procedure, taking into account the extended area of the emitter as well as the geometry of the detector and its spectral sensitivity. This mathematical model delivers correction factors for the luminous efficiency as a function of the distance between a large-area emitter and a detector. The model was tested experimentally for rectangular-shaped OLED structures and delivered accurate results for three different standard types of detectors.     

Growth of highly doped p-type ZnTe layers on GaAs using a nitrogen DC plasma cell

In this paper we report about the use of a DC plasma cell for nitrogen doping of ZnTe layers grown on GaAs substrates by molecular beam epitaxy (MBE). The samples, characterized by high resolution X-ray diffraction, photoluminescence and Hall measurements, show good crystalline quality and carrier concentrations as high as 1×10²⁰ cm^-3 with mobilities up to 30 cm²/V·s at room temperature. To our knowledge, this is the highest hole doping level reported so far in the literature for any wide-gap II–VI compound. The achieved hole concentration is one order of magnitude larger than values reported using an RF plasma source.     

Spontaneous rearrangement of para-sexiphenyl crystallites into nano-fibers

In the growth of para-sexiphenyl films on mica(001), an interesting phenomenon was observed where small individual crystallites spontaneously arrange into parallel running high-aspect ratio chains with a length in the micrometer range. A statistical analysis of the chain dimensions reveals a very narrow length distribution and the existence of a threshold length. The observation of denuded zones around the chains and the interior chain structure suggest a rearrangement of the crystallites as entities. It is proposed that this spontaneous rearrangement is driven by the formation of a one-dimensional defect array which evolves within the monomolecular para-sexiphenyl wetting layer when a critical crystallite density is reached. PACS 81.07.Nb; 81.10.Bk; 81.16.Dn; 68.37.Ps