A. Krier

Optical properties of lead phthalocyanine (PbPc) thin films

Abstract The optical properties of thin films of lead phthalocyanine in both monochlinic and triclinic forms have been studied. Optical parameters have been determined from both absorption and reflectivity data for wavelengths in the UV and visible regions. Monoclinic films show two absorption bands, identified as exciton absorption at low energy and fundamental absorption above 2.4 eV. A direct allowed transition at 2.42 eV was oserved. A doublet of energies in the absorption spectrum at 1.86 and 1.74 eV is in agreement with observations for other metal phthalocyanines. The spectrum for triclinic materials shows a strong absorption band at 1.61 eV and two shoulders at 1.72 and 1.95 eV. The present results support the view that a band model analysis of this material is valid despite the very low mobility values found for both monoclinic and triclinic films.

Powerful interface light emitting diodes for methane gas detection

Powerful (light emitting diodes) LEDs which exhibit more than 3.5 mW of output power at room temperature have been fabricated by liquid phase epitaxy (LPE) and characterized. These LEDs are well matched to the CH4 absorption spectrum and confirm the potential of the devices as a key component for use in an infrared CH4 gas sensor. We report on the efficient interface electroluminescence in our LEDs across the InAs/InAsSbP heterojunction consistent with type II emission. This directly suppresses the Auger recombination and enables these sources to emit maximum powers in excess of 3 mW at room temperature. Furthermore, the use of Yb rare earth ion gettering in these devices was found to be effective in increasing the LED output power. We attribute this to a reduction in the residual carrier concentration arising from the removal of unintentional donors and point defects in the InAs active region material. The ring contact geometry was also found to be superior when compared to the dot contacts in our structures. The LEDs demonstrated in this work are sufficiently powerful to be used in a practical methane gas sensor. (Some figures in this article appear in colour in the electronic version; see www.iop.org)

Structure, electrical conductivity and electrochromism in thin films of substituted and unsubstituted lanthanide bisphthalocyanines

The structure, electrical conductivity and electrochromic properties of thin films of lutetium bisphthalocyanine, prepared mostly by the Langmuir–Blodgett technique, have been investigated with particular reference to the differences between unsubstituted (LuPc2) and substituted (LuPc2 (OBu)16) materials. Scanning tunnelling microscopy images on graphite show the difference in the two structures and give molecular dimensions of 1.5×1.0 nm and 2.8×1.1 nm respectively, in agreement with previous estimates. The in-plane d.c. conductivity was studied as a function of film thickness and temperature. LuPc2 is approximately 106 times more conductive (σ≈5×10−1 Ω−1 m−1, Ea=0.25 eV) than the substituted material. The conduction normal to the film plane and the electrochromism were very sensitive to the choice of bottom electrode. The electrochromism on indium–tin oxide coated glass was studied in LuPc2, ErPc2 and LuPc2 (OBu)16 using cyclic voltammetry and spectroscopy. The green/red oxidative step is seen in all cases but the green/blue reductive step is abolished in LuPc2 (OBu)16. The importance of molecular packing and π-orbital overlap in these effects, and the possible relevance to more complex layer structures and devices, is discussed.

Room-temperature InAs0.89Sb0.11 photodetectors for CO detection at 4.6 μm

An InAs0.89Sb0.11 photovoltaic detector that operates at room temperature in the 2.5-5 mu m mid-infrared wavelength region is reported. The photodiode has an extended spectral response compared with other currently available III-V room-temperature detectors. In order to accommodate the large lattice mismatch between the InAs0.89Sb0.11 active region and the InAs substrate, a buffer layer with an intermediate composition was introduced into the structure. In this way, we obtained room-temperature photodiodes with a cutoff wavelength near 5 mu m, a peak responsivity of 0.8 A/W, and a detectivity of 1.26 x 10(9) cm Hz(1/2)/W. These devices could be effectively used as the basis of an optical sensor for the environmental monitoring of carbon monoxide at 4.6 mu m, or as a replacement for PbSe photoconductors

Room temperature midinfrared electroluminescence from InSb/InAs quantum dot light emitting diodes

Self-assembled InSb submonolayer quantum dots (QDs) in an InAs matrix have been grown by molecular beam epitaxy using Sb2 and As2 fluxes. The structures exhibit bright midinfrared photoluminescence up to room temperature. Intense room temperature electroluminescence with a peak at wavelength near 3.8 μm was observed from p-i-n light emitting diode structures containing ten InSb submonolayer QD sheets inserted within the InAs active region.

Purification of epitaxial InAs grown by liquid phase epitaxy using gadolinium gettering

Very pure InAs epitaxial layers of high quantum efficiency have been grown by liquid phase epitaxy using Gd gettering of the growth solution. The residual carrier concentration was reduced to ∼6×1015 cm−3 and the peak photoluminescence intensity increased considerably (between 10 and 100 times) using this technique. The low-temperature (4 K) photoluminescence spectra exhibited sharp bound exciton and donor-acceptor lines. The linewidth of the exciton was measured to be only 3.8 meV which is narrower than for undoped epitaxial InAs grown by other techniques.

Room temperature photoluminescence at 4.5μm from InAsN

Nitrogen incorporation in InAsN epilayers grown by radio-frequency plasma-assisted molecular beam epitaxy was investigated as a function of growth conditions. Reduced growth rate, growth temperature, and arsenic flux significantly enhance the nitrogen incorporation. Optimal growth conditions allowed us to obtain high quality InAsN with nitrogen composition of up to 2.5%. The epilayers exhibit intense 4K photoluminescence (PL) with double-peak features, which were attributed to free carrier recombination and localized carrier recombination. Strong room temperature PL emission up to a wavelength of 4.5μm is obtained.

ZnO-based thin film transistors employing aluminum titanate gate dielectrics deposited by spray pyrolysis at ambient air.

The replacement of SiO2 gate dielectrics with metal oxides of higher dielectric constant has led to the investigation of a wide range of materials with superior properties compared with SiO2. Despite their attractive properties, these high-k dielectrics are usually manufactured using costly vacuum-based techniques. To overcome this bottleneck, research has focused on the development of alternative deposition methods based on solution-processable metal oxides. Here we report the application of spray pyrolysis for the deposition and investigation of Al2x-1·TixOy dielectrics as a function of the [Ti(4+)]/[Ti(4+)+2·Al(3+)] ratio and their implementation in thin film transistors (TFTs) employing spray-coated ZnO as the active semiconducting channels. The films are studied by UV-visible absorption spectroscopy, spectroscopic ellipsometry, impedance spectroscopy, atomic force microscopy, X-ray diffraction and field-effect measurements. Analyses reveal amorphous Al2x-1·TixOy dielectrics that exhibit a wide band gap (∼4.5 eV), low roughness (∼0.9 nm), high dielectric constant (k ∼ 13), Schottky pinning factor S of ∼0.44 and very low leakage currents (<5 nA/cm(2)). TFTs employing stoichiometric Al2O3·TiO2 gate dielectrics and ZnO semiconducting channels exhibit excellent electron transport characteristics with low operating voltages (∼10 V), negligible hysteresis, high on/off current modulation ratio of ∼10(6), subthreshold swing (SS) of ∼550 mV/dec and electron mobility of ∼10 cm(2) V(-1) s(-1).

Molecular beam epitaxial growth of InAsN:Sb for midinfrared Optoelectronics

We report molecular beam epitaxial growth and characterization of dilute nitride InAsN:Sb. X-ray diffraction, energy dispersive x-ray spectrometry, and electron probe microanalysis revealed that nitrogen incorporation is significantly enhanced by introduction of Sb flux during growth, together with a dramatic improvement of the photoluminescence. These observations were attributed to the surfactant effect of Sb which suppresses the surface diffusion length of nitrogen and improves the homogeneity of the alloy. Sb incorporation is enhanced with the presence of nitrogen which was associated with the surface kinetic of growth. InAsN:Sb∕InAs p-i-n light emitting diodes operating near 4.0μm were also realized.

Linear magnetoresistance due to multiple-electron scattering by low-mobility islands in an inhomogeneous conductor

Linear transverse magnetoresistance is commonly observed in many material systems including semimetals, narrow band-gap semiconductors, multi-layer graphene and topological insulators. It can originate in an inhomogeneous conductor from distortions in the current paths induced by macroscopic spatial fluctuations in the carrier mobility and it has been explained using a phenomenological semiclassical random resistor network model. However, the link between the linear magnetoresistance and the microscopic nature of the electron dynamics remains unknown. Here we demonstrate how the linear magnetoresistance arises from the stochastic behaviour of the electronic cycloidal trajectories around low-mobility islands in high-mobility inhomogeneous conductors and that this process is only weakly affected by the applied electric field strength. Also, we establish a quantitative link between the island morphology and the strength of linear magnetoresistance of relevance for future applications.