GaInN/GaN heterostructures and quantum wells have been grown by low pressure metalorganic vapor phase epitaxy on sapphire using an AIN nucleation layer. We found a significant In incorporation only for growth temperatures of 700°C, although still very high In/Ga ratios in the gas phase had to be adjusted. The In content could be increased by reducing the H2/N2 flow ratio in the main carrier gas. GaInN layers typically show two lines in low temperature photoluminescence which are identified as excitonic-like (high energy peak) and impurity-related-like (low energy) by time-resolved spectroscopy. Quantum wells with a thickness between 8 and 0.5 nm showed only one emission line. The peak of the thinnest wells shows excitonic-like behaviour, whereas we found a smooth transition to an impurity-related-like type with increasing thickness. By scanning transmission electron microscopy studies we found indications for composition fluctuations in these thicker quantum wells which may cause localization effects for the excitons and thus be responsible for the observed optical spectra.

Surface compositions and work functions (Φ) of commercially available indium tin oxide (ITO) substrates were measured by photoelectron spectroscopy (UPS/XPS). Whereas substrates cleaned by organic solvents are significantly contaminated and have low Φ values (3.9–4.2±0.1 eV), substrates cleaned by Ar+ sputtering typically have values of Φ=4.3±0.1 eV. Even higher Φ values (up to 4.7±0.1 eV) are obtained by reactive ion etching with oxygen, likely related to oxygen-containing surface impurities. Evaporated TPD is physisorbed on ITO, but causes a drop of the vacuum potential by 0.2–0.4 eV (depending on the ITO pretreatment) directly at the TPD/ITO interface, in contradiction to the common-vacuum level rule. The TPD highest occupied molecular orbital (HOMO) is found 1.1–1.3 eV below the Fermi level of the ITO, which indicates the presence of a significant barrier for hole injection.

We have measured the current-voltage characteristics and device efficiency of organic Light emitting diodes (OLEDs) based on 8-hydroxyquinoline aluminum (Alq(3)) in combination with several cathode layer setups. The electron injection properties of cathode metals evaporated under high vacuum (HV) and ultra-high vacuum (UHV) conditions are compared. Further, cathodes incorporating a thin layer of lithium fluoride, which is covered with a metal capping layer, are investigated. It will be shown that aluminum is an outstanding capping metal and significantly improves both electron injection and device efficiency. Quasi-static and transient current-voltage measurements on single-layer devices will be presented. It will be demonstrated that cathodes, comprising 0.2 nm LiF and aluminum, are able to sustain space charge limited currents in Alq(3). Additionally, the efficiency and lifetime data of multi-layer devices using this cathode layer setup are discussed. (C) 2000 Elsevier Science S.A. All rights reserved.

We investigate electron injection and transport in single-layer devices of 8-hydroxyquinoline aluminum sandwiched between two electrodes. Electrodes comprising a thin lithium fluoride layer are compared with co-evaporated magnesium–silver cathodes and with pure aluminum cathodes. By employing both transient and quasistatic current measurements, the impact of the LiF-layer thickness on electron injection is investigated. It is demonstrated that contacts comprising 0.1–0.2 nm LiF and an aluminum capping layer are able to sustain space-charge-limited currents in 8-hydroxyquinoline aluminum. Further, steady-state current–voltage measurements as a function of temperature are discussed with respect to trap distributions in 8-hydroxyquinoline aluminum.

Bright white light emission in rubrene-doped organic LEDs has been demonstrated using spiro compounds—such as spiro-TAD, shown in the Figure—with high glass transition temperatures. These materials show high morphological stability for hole transport and emission of blue light, and thus still operate at temperatures well above 100 °C. The devices give a maximum luminance of 11800 cd/m2.

The lowest obtainable operating voltage for organic light emitting diodes (OLEDs) utilising a predefined organic layer setup can only be achieved with ohmic contacts both for electron and hole injection. We have investigated dark current transients of unipolar single-layer samples, and we have found ohmic contacts both for hole injection at indium tin oxide (ITO)/4,4′,4″-tris{N-(1-naphtyl)-N-phenylamino}-triphenylamine (1-Naphdata) interfaces and for electron injection at 8-hydroxyquinoline aluminum (Alq3)/LiF/Al interfaces. Therefore, the properties of OLEDs comprising these two interfaces are governed only by bulk material properties and internal organic/organic interfaces. In order to identify the dominating mechanisms concerning the temperature-dependent behaviour of prototypical double layer OLEDs, we have measured (with respect to the applied electric field) the activation energies of the charge carrier mobility and of the steady state current density in 1-Naphdata (holes) and Alq3 (electrons), the activation energies of the steady state current density and of the luminance in OLEDs comprising an 1-Naphdata/Alq3 heterojunction, plus the activation energy of the luminance onset. These experimentally activation energies are discussed with respect to device performance in the typical operating temperature range of flat panel displays including implications for further device optimisation.

We investigate the impact of the deposition of low work function metals such as calcium on thin layers of fluorene-type polymers by time-of-flight secondary ion mass spectroscopy. An implantation process rather than a slow metal diffusion is found to be the most probable source of metal contamination within the polymer layers. This contamination extends to a range of several tens of nanometers in the organic layers. Photoluminescence and electroluminescence measurements are performed with varying calcium layer thicknesses. The luminescence efficiency exhibits a strong correlation with the depth profile of the calcium present within the polymer. The results are discussed with respect to the exciton diffusion length in the fluorene polymer. A numerical model including exciton formation, migration, and quenching is proposed in order to describe the observed phenomena.

Balanced injection of positive and negative charge carriers is a key issue for the operation of highly efficient organic light emitting devices at low operating voltage. In this article, we will give an overview of our investigations on the optimization of charge carrier injection from the anode and the cathode into organic semiconductors. These investigations include proper pretreatment of the indium-tin oxide substrate, and stacking of several organic hole transporting layers to increase the hole injection current from the transparent anode into the emissive layer. On the cathode side, binary metal alloys, and the effect of an insulating layer between the respective metal cathode and the first electron transporting layer are investigated.

Using two complementary methods, we have investigated the individual contribution of the space charge-limited hole transport in vapor-deposited films of 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (m-MTDATA) to the temperature behavior of organic light-emitting diodes. In single-layer indium tin oxide/m-MTDATA/Ag structures, we have measured the activation energies of the current density and of the hole mobility as a function of the applied electric field. Both activation energies obtained under steady-state and pulsed conditions are comparable, which confirms that the temperature behavior of the current density is predominantly governed by the hole mobility.

The impact of oxygen plasma treatment of indium tin oxide anodes on performance and durability of vapor-deposited organic electroluminescent devices is shown. Investigations focused on the long-term stability using driving conditions suitable for passive matrix driven displays. Reliability studies of solvent only cleaned samples indicate the presence of a predominating degradation process at the interface between indium tin oxide and the hole injection layer which results in a drastic rise of the operating voltage. This voltage increase could be reduced to 0.31 mV/h by oxygen plasma treatment. As hole injection layer copper phthalocyanine is compared with a star-shaped amine derivative.

A one-dimensional numerical model for the quantitative simulation of multilayer organic light emitting diodes (OLEDs) is presented. It encompasses bipolar charge carrier drift with field-dependent mobilities and space charge effects, charge carrier diffusion, trapping, bulk and interface recombination, singlet exciton diffusion and quenching effects. Using field-dependent mobility data measured on unipolar single layer devices, reported energetic levels of highest occupied and lowest unoccupied molecular orbitals, and realistic assumptions for experimentally not direct accessible parameters, current density and luminance of state-of-the-art undoped vapor-deposited two- and three-layer OLEDs with maximum luminance exceeding 10000 cd/m2 were successfully simulated over 4 orders of magnitude. For an adequate description of these multilayer OLEDs with energetic barriers at interfaces between two adjacent organic layers, the model also includes a simple theory of charge carrier barrier crossing and recombination at organic–organic interfaces. The discrete nature of amorphous molecular organic solids is reflected in the model by a spatial discretization according to actual molecule monolayers, with hopping processes for charge carrier and energy transport between neighboring monolayers.

The efficiency of organic light-emitting devices is significantly influenced by the performance of the electron-injecting contact. Lowering the energetic barrier between the metal contact and the lowest unoccupied molecular orbital of the adjacent organic electron transport layer should facilitate the injection of negative charge carriers, and, thus, improve the electroluminescence yield by increasing the electron density in the emitting zone. Therefore, it is widely believed that lowering the work function of the cathode metal will improve the quantum efficiency of the devices and, concomitantly, reduce the operating voltage. Here, we report on measurements of devices with tris(8-hydroxyquinolinolato)aluminum-(III) as electron transport and emissive layer. The latter layer is contacted with a variety of chemically very different cathode metals (including some lanthanides), which cover a range from 2.63 eV up to 4.70 eV on the work function axis. We demonstrate the existence of an efficiency maximum at a work function of about 3.7 eV which, to the best of our knowledge, has not been reported yet. These results are of practical importance with respect to the choice of pure cathode metals for organic electroluminescent display applications.

GaN layers with different nucleation techniques have been grown by metalorganic vapor phase epitaxy. At atmospheric pressure, no epitaxial layer growth occurred, whereas at low pressure the GaN quality could be drastically improved by an AlN nucleation layer. Excellent material properties have been found by photoluminescence, X-ray diffraction and Hall experiments. The incorporation of In in GaInN could be enhanced by a reduction of both growth temperature and reactor pressure. GaInN quantum wells grown at 700°C show fairly strong photoluminescence peaks.

GaN single layers and GaInNGaN heterostructures have been grown by low pressure metalorganic vapor phase epitaxy on sapphire substrates. We found best growth conditions and the highest growth rate for GaN to be at about 1000°C, whereas the growth rate decreased for both, higher and lower temperatures. In contrast, GaInN with a significantly high In content could only be grown at lower temperatures around 700°C. Besides growth temperature and reactor pressure, the composition of the carrier gas was found to play an important role: the In incorporation rate is about doubled when reducing the hydrogen/nitrogen ratio. GaInNGaN quantum wells show even higher In contents compared to bulk layers.

By using picosecond time-resolved photoluminescence we have measured the lifetime of excess charge carriers in GaN epitaxial layers grown on sapphire at temperatures up to 300 K. The decay time turns out to be dominated by trapping processes at low excitation levels. The radiative lifetime derived from our data is dominated by free excitons at temperatures below 150 K, but also clearly shows the gradual thermal dissociation of excitons at higher temperatures. From our data, we are able to determine the free exciton binding energy and the free carrier radiative recombination coefficient. By combining these data with optical absorption data, we find the interband momentum matrix element and an estimate for the hole effective mass, which is much larger than previously thought.

We have studied the optical gain in nitride-based laser structures. We find evidence for excitonic gain at room temperature. A strong polarization dependence of the gain is observed, in accordance with the band structure. We have used optical gain spectroscopy employing the stripe excitation method in order to elucidate the mechanisms of optical gain in GaInN-GaN and GaN-AlGaN double heterostructures and quantum wells.

By optical gain spectroscopy we have studied the fundamental laser properties of GaInN/GaN heterostructures grown on sapphire. Utilizing the stripe excitation method we have measured optical gain spectra at room temperature. Due to the low symmetry of the wurtzite structure and the resulting splitting of the uppermost valence bands, we find optical gain only for the TE mode. Our analysis shows that the optical gain is due to direct band-to-band transitions in an electron-hole plasma. For gain amplitudes typically found in lasers, we find carrier densities up to 3×1019 cm−3, which are likely to lead to rather large threshold current densities.