Peculiarities of radiative recombination in amorphous molecular semiconductors doped with polymethine dye

Processes of charge carrier photogeneration and recombination are investigated in films of poly-N-epoxypropylcarbazole doped with polymethine dye. Films with blocking contacts were illuminated with light from either the region of dye absorption or beyond this region. The kinetics of accumulation and relaxation of electron–hole pairs with lifetimes greater than tens or hundreds of seconds was studied. It is presumed that the reason for the growth of recombination luminescence intensity in an external electric field is connected with the increase in efficiency of radiative recombination stimulated by electrons captured from photogenerated excitons. © 1998 John Wiley & Sons, Ltd.     

Features of the exciplex mechanism of charge photogeneration in amorphous molecular semiconductors doped with the dye rhodamine 6G

Results of investigation of the processes of charge carrier photogeneration and relaxation within the exciplex absorption region in films of poly-N-epoxypropylcarbazole doped with the dye rhodamine 6G are given. When the films are illuminated with light from the dye absorption region, new centres of charge photogeneration appear that absorb within the exciplex absorption region. It is shown for the first time that these centres have a long lifetime at room temperature and their temporal concentration changes correlate with EPR photosignal changes. The concentration of centres is independent of the external electric field strength during their generation. These centres are identified as triplet exciplexes. © 1997 John Wiley & Sons, Ltd.     

Photoconductivity of organic polymer films doped with porous silicon nanoparticles and ionic polymethine dyes

Features of electrical conductivity and photoconductivity of polyvinylbutyral films containing porous silicon nanoparticles and similar films doped with cationic and anionic polymethine dyes are studied. Sensitization of the photoelectric effect by dyes with different ionicities in films is explained by the possible photogeneration of holes and electrons from dye molecules and the intrinsic bipolar conductivity of porous silicon nanoparticles. It is assumed that the electronic conductivity in porous silicon nanoparticles is higher in comparison with p-type conductivity.     

Dissociation of electron–hole pairs interacting with triplet excitons in amorphous molecular semiconductors

The photoconductivity of films of amorphous molecular semiconductors increases upon simultaneous photogeneration of singlet electron–hole pairs (EHPs) and triplet excitons and decreases upon photogeneration of triplet EHPs and triplet excitons. An increase in external electric field strength leads to a decrease in the effect of triplet excitons on the film photoconductivity caused by EHP dissociation. It is concluded that under increasing external electric field strength the mobility of charge carriers increases and the velocity of EHP dissociation becomes comparable with the velocity of spin conversion of EHPs interacting with triplet excitons. © 1997 John Wiley & Sons, Ltd.     

Achieving time-of-flight mobilities for amorphous organic semiconductors in a thin film transistor configuration

We observe bulk-like hole transport in amorphous organic semiconductors in a thin film transistor (TFT) configuration. Five different organic hole transporters (HTs) commonly used in organic light-emitting diodes are investigated. When these HTs are deposited on SiO₂ gate dielectric layer, the TFT mobilities are 1–2 orders of magnitude smaller than those obtained from bulk films (3–8 μm) using time-of-flight (TOF) technique. The reduction of hole mobilities can be attributed to the interactions between the organic HTs and the polar SiO bonds on the gate dielectric layer. Detailed temperature dependence studies, employing the Gaussian disorder model, indicate that the SiO₂ gate dielectric contributes between 60 and 90 meV of energetic disorder in the charge hopping manifold. Besides SiO₂ gate dielectric, similar effects can also be observed for other polar insulators including polymeric PMMA and BCB, or HMDS-modified SiO₂. However, when a common non-polar polymer, polystyrene (PS), is employed as the dielectric layer, the dipolar energetic disorder becomes negligible. Holes effectively experience bulk-like transport on the PS gate dielectric surface. TFT mobilities extracted from all five organic HTs are in excellent agreements with TOF mobilities. The present study should have broad applications in the transport characterization of amorphous organic semiconductors.     

Measurement of the lowest unoccupied molecular orbital energies of molecular organic semiconductors

The lowest unoccupied molecular orbital (LUMO) energies of a variety of molecular organic semiconductors have been evaluated using inverse photoelectron spectroscopy (IPES) data and are compared with data determined from the optical energy gaps, electrochemical reduction potentials, and density functional theory (DFT) calculations. A linear fit to the electrochemical reduction potential (relative to an internal ferrocene reference) vs. the LUMO energy determined by IPES gives a slope and intercept of ?1.19 ± 0.08 eV/V and ?4.78 ± 0.17 eV, respectively, and 0.92 ± 0.04 and ?0.44 ± 0.11 eV, respectively, based on the DFT calculated LUMO energies. From these fits, we estimate the LUMO and exciton binding energies of a wide range of organic semiconductors.     

Transient luminescence in organic light-emitting diodes explained by trap-assisted recombination of stored charges

We show that after voltage turn-off the recombination of the charges which are stored in the on-state may follow a trap-assisted mechanism instead of the Langevin formula in organic light-emitting diodes. A microscopic model based on this form of recombination is introduced, which not only fits the transient electroluminescence very well in the whole range of the off-state but also provides parameters quantitatively characterizing the stored charges in the on-state. In the last part, we briefly compare the work by Weichsel et al. [1] and our model, trying to constitute a comprehensive picture of the stored charges in OLEDs. As an OLED model system we have chosen a host/guest system co-doped with red and green phosphorescent emitters.     

Contacts between amorphous metals and semiconductors

The letter describes the relative enhancement of the thermal stability of Schottky barriers on both silicon and gallium arsenide that can be achieved using thin films of amorphous Ni-Nb and Ta-Ir alloy, the latter alloy being among the most stable fabricated to date, with less than 5% degradation of the barrier height after 2.5 h at 500°C.     

Photoinduced structural changes in amorphous semiconductors

A variety of photostructural changes observed in tetrahedral and chalcogenide amorphous semiconductors are reviewed from physical and chemical points of view. In particular, observations of the photodarkening and related phenomena in chalcogenide glasses are summarized, and structural models which have been proposed so far are criticized. Fiz. Tekh. Poluprovodn. 32, 964–969 (August 1998)     

High-quality dielectric suitable for use with amorphous semiconductors

Very-high-quality Al/SUB 2/O/SUB 3/ films have been grown by anodizing aluminum films in a bath of tartaric acid, buffered with NH/SUB 4/OH and diluted with propylene glycol. Several thousand films were evaluated to demonstrate high resistivities and breakdown voltages, as well as dielectric properties stable with respect to film thickness, frequency, and applied voltage. It is shown that an optimum solution, an optimum buffer, an optimum pH, and an optimum anodization time exist.     

Characteristics and device applications of erbium doped III-V semiconductors grown by molecular beam epitaxy

We have studied the properties of molecular beam epitaxially (MBE)-grown Erdoped III-V semiconductors for optoelectronic applications. Optically excited Er³⁺ in insulating materials exhibits optical emission chiefly around 1.54 μm, in the range of minimum loss in silica fiber. It was thought, therefore, that an electrically pumped Er-doped semiconductor laser would find great applicability in fiber-optic communication systems. Exhaustive photoluminescence (PL) characterization was conducted on several of As-based III-V semiconductors doped with Er, on bulk as well as quantum-well structures. We did not observe any Errelated PL emission at 1.54 μm for any of the materials/structures studied, a phenomenon which renders impractical the realization of an Er-doped III-V semiconductor laser. Deep level transient spectroscopy studies were performed on GaAs and AlGaAs co-doped with Er and Si to investigate the presence of any Er-related deep levels. The lack of band-edge luminescence in the GaAs:Er films led us to perform carrier-lifetime measurements by electro-optic sampling of photoconductive transients generated in these films. We discovered lifetimes in the picosecond regime, tunable by varying the Er concentration in the films. We also found the films to be highly resistive, the resistivity increasing with increasing Er-concentration. Intensive structural characterization (double-crys-tal x-ray and transmission electron microscopy) performed by us on GaAs:Er epilayers indicates the presence of high-density nanometer-sized ErAs precipitates in MBE-grown GaAs:Er. These metallic nanoprecipitates probably form internal Schottky barriers within the GaAs matrix, which give rise to Shockley-Read-Hall recombination centers, thus accounting for both the high resistivities and the ultrashort carrier lifetimes. Optoelectronic devices fabricated included novel tunable (in terms of speed and responsivity) high-speed metal-semiconductor-metal (MSM) photodiodes made with GaAs:Er. Pseudomorphic AlGaAs/ InGaAs modulation doped field effect transistors (MODFETs) (for high-speed MSM-FET monolithically integrated optical photoreceivers) were also fabricated using a GaAs:Er buffer layer which substantially reduced backgating effects in these devices.     

Electrical characteristics and threshold switching in amorphous semiconductors

The relative role of thermal and electronic effects on the threshold switching mechanism in the amorphous semiconductor Ge₁₇Te₇₉Sb₂S₂, has been investigated, using virgin sandwich type devices.Evidence is presented in support of a critical field bulk switching mechanism, in which secondary heating effects play a role only because of the variation of the critical switching field with temperature. Switching delay time measurements are interpreted in terms of adiabatic and steady state heating, for times shorter and longer, respectively, than the thermal time constant of the device. An isothermal switching field is calculated by correcting the measured switching field for the estimated change in temperature of the device prior to switching. Switching voltage measurements were made with pulse widths from 1 nsec. to d.c. In this range the isothermal switching field is independent of the pulse width. The d.c. switching field is shown to be a function of the thermal constraints of the device structure. In contrast, the isothermal switching field is a fundamental material parameter. It is established, from short pulse (1·5 nsec) measurements of the current voltage characteristic that the critical switching field marks the onset of a previously unreported high field conduction region. This characteristic, in the vicinity of the switching field, is similar to that of a reverse biased diode in the vicinity of its breakdown voltage.     

Effect of crystal density on sublimation properties of molecular organic semiconductors

Thermal evaporation is an essential process employed in the fabrication of optoelectronic devices based on small molecular organic materials. Knowing the evaporation properties (e.g. sublimation enthalpy, vapor pressure) of archetypal compounds and being able to predict these properties of new compounds is therefore important for the design of processes and deposition apparatus. To address this lack of reliable, easily reproducible information we used thermogravimetry to characterize the sublimation properties of pentacene; tris(8-hydroxyquinolino) aluminum (Alq₃); 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA); metal-free phthalocyanine (H₂Pc); boron subphthalocyanine chloride (SubPc); iron phthalocyanine (FePc); copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc). A linear relationship was found between enthalpy and vapor pressure, and crystal density, allowing for the estimation of thermophysical properties of untested compounds of a similar class using a simple model.     

Horizontal orientation of linear-shaped organic molecules having bulky substituents in neat and doped vacuum-deposited amorphous films

Organic amorphous films fabricated by vacuum deposition have been widely used in organic light-emitting devices, making use of their high-performance optical and electrical characteristics and taking advantage of the easy fabrication of pinhole-free thin smooth layers of a desired thickness. However, random orientation in amorphous films often makes it difficult to utilize their best optical and electrical potential. Here the authors demonstrate that the linear-shaped molecules of fluorescent styrylbenzene derivatives are horizontally oriented in organic amorphous films fabricated by conventional vacuum deposition even when the molecules are doped in an isotropic host matrix film. The longer the molecular length is, the larger the anisotropy of the molecular orientation becomes. The weak interaction between adjacent molecules and the linear-shaped molecular structure probably cause the horizontal orientation. The fact that the horizontal molecular orientation occurs on any underlying layers shows the high versatility of the horizontal orientation for various applications. Their findings will provide a new guideline for molecular designs that can be used to improve optical and electrical characteristics of organic optoelectronic devices, such as organic light-emitting diodes and organic laser devices.     

Photoluminescence of erbium in amorphous hydrogenated phosphorus-doped silicon

The photoluminescence of erbium ions in phosphorus-doped a-Si:H films has been investigated. The observed increase in the Er photoluminescence with increasing defect density in the samples and the correlation of the temperature variation of the Er photoluminescence and defect-associated photoluminescence intensities are explained on the basis of a model of excitation of Er ions as a result of Auger recombination with defect participation. Fiz. Tekh. Poluprovodn. 31, 869–871 (July 1997)     

Infrared luminescence from silicon nanostructures heavily doped with boron

Intense highly polarized radiation from silicon nanostructures heavily doped with boron to 5 × 10²¹ cm⁻³ is studied as a function of temperature, forward current, and an additional lateral electric field. The features of the radiation intensity and degree of polarization suggest that an important role in the formation of the luminescence spectra is played by the ordered system of B⁺-B⁻ dipoles, formed as a result of the reconstruction of shallow boron acceptors as centers with negative correlation energy. The results obtained are interpreted within a proposed model based on two-electron adiabatic potentials, according to which radiation results from donor-acceptor recombination via boron dipole center states, involving shallow phosphorus donors.     

Specific features of Hall measurements in doped semiconductors

Semiconductors - The limits of applicability of the Hall method for separate determination of the impurity concentration are considered for the case of doping with only one main impurity. A diagram...     

Screened impurity scattering in heavily doped covalent semiconductors

A method is presented for determining the relative strengths of acoustic lattice scattering and screened impurity scattering in heavily doped semiconductors from measurements of their Hall mobility and thermoelectric power. Screening of the impurity centres by the free carriers results in an appreciable reduction in the impurity-scattering contribution, and this effect is estimated for heavily doped n type germanium.     

Carrier recombination and lifetime in highly doped silicon

The predominance of phonon-assisted band-band Auger recombination in highly doped silicon is demonstrated by showing that no recombination mechanism involving common (unavoidable) defects in silicon can yield carrier lifetimes that are consistent with the measured lifetimes, which exhibit an inverse-quadratic doping-density dependence, and/or with their temperature dependence. Both trap-assisted-Auger and Shockley-Read-Hall recombination mechanisms are considered, and dependences of the defect density on the doping density, which are implied by theory and experiment, are accounted for.