Soo-Jin Chua

Degradation and failure of organic light-emitting devices

The degradation and failure of organic light-emitting device are observed via optical microscopy. The “degraded area” has been identified to be made up of three regions: (1) a dark spot at the center, (2) a nonemitting area forming the core, and (3) a weakly emitting area surrounding the core. It is found that due to metal migration, as evidenced from the secondary ion mass spectrometry profiles, the indium tin oxide/polymer interface roughens during operation. The intense local current at sharp points degrades the polymer causing the formation of the dark center. Further current stress caused the central core to carbonize which may lead to short and/or open circuits accompanied by fluctuations in the device current.

Lithium–fluoride-modified indium tin oxide anode for enhanced carrier injection in phenyl-substituted polymer electroluminescent devices

Phenyl-substituted polymer electroluminescent (EL) devices using an insulating lithium–fluoride (LiF) layer between indium tin oxide (ITO) and poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT) hole transporting layer have been fabricated. By comparing the devices made without this layer, the results demonstrate that the former has a higher EL brightness operated at the same current density. At a given constant current density of 20 mA/cm2, the luminance and efficiency for devices with 1.5 nm LiF-coated ITO were 1600 cd/m2 and 7 cd/A. These values were 1170 cd/m2 and 5.7 cd/A, respectively, for the same devices made with only an ITO anode. The ultrathin LiF layer between ITO and PEDOT modifies the hole injection properties. A more balanced charge carrier injection due to the anode modification by an ultrathin LiF layer is used to explain this enhancement.

Correlation between dark spot growth and pinhole size in organic light-emitting diodes

Our in situ experimental observations of dark spot growth in organic light-emitting diodes using optical microscopy show a linear rate of growth for the area of all the dark spots. We used uniformly sized silica micro particles to intentionally create size-controllable pinholes on the cathode protective layer. Subsequently, we observed initial formation of dark spots as a result of these pinholes and then monitored their growth. Due to usage of particles of various diameters, we were able to linearly correlate the growth rate with pinhole size. This allows us to estimate the original pinhole sizes that gave rise to the dark spots, which we believe were initiated by “dust” particles. Our studies verify that dark spot formation is due to pinholes on the protective layer that creates pathways for water or oxygen permeation, and that dark spot growth is dependent on the pinhole sizes.

Plasma-induced damage to n-type GaN

The effects of plasma etching on 1/f noise and photoluminescence (PL) characteristics of n-GaN have been investigated. A reduction of 1/f noise was observed after plasma exposure, a result of enhanced passivation of the reactive surface. This is attributed to the removal of carbon and the creation of a Ga-rich surface by the etching process. Nevertheless, the formation of nonradiative recombination centers impaired the PL intensity. Reconstruction of a stoichiometric surface was achieved by annealing. This induced the incorporation of carbon into GaN, deteriorating the PL performance further, but it could be restored by a chemical treatment of 10:1 HF:H2O.

Stabilization of electrode migration in polymer electroluminescent devices

A thin 3-nm-thick parylene layer is deposited by chemical vapor deposition at room temperature on the indium tin oxide (ITO) coated glass substrate to form a bilayer anode of an organic light emitting diode. The parylene layer forms a conformal film to cover the spikes present in the ITO film. This parylene film presents a smoother surface to the subsequent organic layers. The parylene film not only reduces the occurrence of dark spots, acting as a barrier for oxygen diffusion from either the ITO or from the atmosphere and stabilizing the migration of the electrodes during electrical stress, but also improves the injection of holes from the anode. By inserting another parylene layer in between the organic and cathode layers, the probability of formation of nonemissive areas is further reduced.

Photoluminescence studies on InGaN/GaN multiple quantum wells with different degree of localization

It has been found, by using photoluminescence (PL) studies, that both the localized states and nonradiative recombination centers in In0.06Ga0.94N/GaN multiple quantum wells (MQWs) can be greatly suppressed by inserting a monolayer of AlN before the growth of each well layer. While inserting a monolayer of AlN before each well layer does not have any effect on the growth rate or on the indium content of MQWs, it does improve room-temperature PL intensity of the In0.06Ga0.94N/GaN MQWs. The physics behind the suppression of localized states and nonradiative recombination centers is discussed.

Outgoing multiphonon resonant Raman scattering and luminescence in Be- and C-implanted GaN

We have performed outgoing resonant Raman scattering and photoluminescence measurements on as-grown, Be- and C-implanted GaN in the temperature range of 77–330 K. In implanted GaN after postimplantation annealing at 1100u200a°C, the A1(LO) multiphonons up to the seventh order were observed with the very strong four longitudinal optical (LO) and five LO modes at ∼2955 and ∼3690 cm−1, respectively, showing extraordinary resonance behavior. With the sample temperature, these two modes significantly decreased and increased in intensity, respectively. The phenomenon is attributed to the variation of resonant conditions due to the shift of the band gap energy. Meanwhile, the combination of E2(high) and quasi-LO phonons was strongly enhanced by quasi-LO phonon involvement and thus the corresponding overtones can be clearly observed even up to the sixth order (m=6). The mechanisms that such strong outgoing multiphonon resonance Raman scattering occurred to implanted GaN instead of high-quality as-grown GaN samples ca...

Influence of electrical stress voltage on cathode degradation of organic light-emitting devices

Our in situ experimental observations of the influence of electrical stress voltage on organic light-emitting device growth in dark spot areas are presented. We demonstrate the use of microsized silica particles to create uniformly sized defects on the protective layer. This is an efficient way to control the location and the number of dark spots. The growth in dark spot area was studied at different driving voltages from 0 up to 11 V. Dark field microscopy was used to monitor the dark spot size below the turn-on voltage. The bright field was used at or above the turn-on voltage. Our observations indicate that dark spot growth was strongly affected by the electrical stress voltage. A linear growth rate with respect to the voltage was observed with a fitting parameter better than 99.7% when the device is driven above the turn-on voltage. We interpret the dark spot growth in terms of the diffusion of moisture and oxygen accompanied by cathode layer chemical and physical changes.

Electronic and vibronic properties of Mg-doped GaN: The influence of etching and annealing

We report a systematic study of the effects of wet chemical treatment, inductively coupled plasma etching, and thermal annealing on the surface and optical properties of Mg-doped p-type GaN. The chemical bonding and surface stoichiometry of the GaN surface subjected to different processing steps are analyzed based on the results of x-ray photoelectron spectroscopy. Atomic force microscopy has been employed to characterize the surface morphology. Photoluminescence (PL) and micro-Raman techniques have been used to investigate the electronic and vibrational properties of plasma etched surface. We have correlated the surface changes induced by dry etching of p-type GaN to the corresponding changes in the defect and impurity related states, through their manifestation in the PL spectra. We have observed several local vibrational modes (LVMs) in p-type GaN subjected to various processing steps. A broad structure in the low-temperature Raman spectra around 865u2009cm−1 is attributed to the electronic Raman scatterin...

Micro-raman scattering in laterally epitaxial overgrown GaN

In this study, micro-Raman spectroscopy has been used to investigate the vibrational properties of laterally epitaxial overgrown (LEO) GaN. The LEO GaN films were grown by metal organic chemical vapor deposition on a 2 in. sapphire substrate with SiN mask. Photoluminescence and polarized Raman scattering measurements have been performed in the two regions of GaN growth (wing and window regions). Raman scattering results are consistent with the lateral growth of GaN in the overgrown region. We have observed second-order Raman scattering in the wing and window regions of GaN. The observations of longitudinal optical phonon plasmon modes in the overgrown region demonstrate that LEO GaN is doped. We have carried out micro-Raman mapping of the local strain and free carrier concentration in the LEO GaN. Anharmonicity due to temperature in LEO GaN has also been investigated. The anharmonicity was found to increase with increasing temperature, and such temperature-induced anharmonicity introduces changes in the l...