J. N. Miller

Organic electroluminescent devices

Electroluminescence from organic materials has the potential to enable low-cost, full-color flat-panel displays, as well as other emissive products. Some materials have now demonstrated adequate efficiencies (1 to 15 lumens/watt) and lifetimes (>5000 hours) for practical use; however, the factors that govern lifetime remain poorly understood. This article provides a brief review of device principles and applications requirements and focuses on the understanding of reliability issues.

FORMATION AND GROWTH OF BLACK SPOTS IN ORGANIC LIGHT-EMITTING DIODES

We report electroluminescence (EL) degradation studies of thin‐film organic light‐emitting diodes under ambient conditions. Bilayer organic ITO/TPD/Alq3/Mg/Ag devices were studied via EL and photoluminescence (PL) microscopy. In situ imaging of device luminescing areas and measurement of sample luminance were performed, allowing for a detailed study of black spot formation and luminance reduction under constant voltage stress conditions. Post‐stress devices were further characterized using PL microscopy, and it was found that black spots result from delamination of the metal at the Alq3/Mg interface initiated by pinholes on the cathode, caused by substrate defects.

Cathodoluminescence atomic scale images of monolayer islands at GaAs/GaAlAs interfaces

Direct images of growth islands differing by 2.8 A [1 monolayer (ML)] height at GaAs/AlGaAs heterointerfaces and of the columnar structure of quantum wells are reported for the first time. The structures are grown by molecular‐beam epitaxy (MBE) with interruptions of the growth of ≊2 min at the interfaces. The method used to obtain these images is scanning cathodoluminescence. The dependence of the lateral extension of these islands on growth conditions is investigated. For fixed growth rate rs≊0.5 ML/s the mean island size decreases from 6–7 μm to 2 μm upon an increase of growth temperature from Tg=600 to 660 °C. Apparently the growth process changes from a planar to a three‐dimensional one. For low‐growth temperature and rate the lateral extension of such islands can be larger than the carrier diffusion length. Under these conditions interisland thermalization of carriers is largely suppressed. Quantitative information on the reduction of roughness of the quantum well interfaces with increasing growth i...

Structural changes of the interface, enhanced interface incorporation of acceptors, and luminescence efficiency degradation in GaAs quantum wells grown by molecular beam epitaxy upon growth interruption

A comparison of the luminescence of GaAlAs/GaAs/GaAlAs quantum wells (QW) grown by molecular beam epitaxy with and without a 1–100 s interruption of the growth at the interfaces is presented. Well widths of 2 and 5 nm are studied as model systems. The luminescence from the noninterrupted samples consists of Gaussian shaped doublets. A line shape theory is outlined, allowing for the first time a determination of the distribution of the microscopic chemical and crystallographic disorder of the interfaces from the spectra. All samples were grown under nominally identical conditions and all show the same Gaussian distribution of the interface position. The full width at half maximum of the distribution function is 1.25 A. The interfaces in these samples are believed to have an island‐like character with a typical island size of much less than 17 nm. Interruption of the growth by a few seconds changes the luminescence line shape function qualitatively. The spectrum splits into doublets, with doublet fine struc...

Anisotropic and inhomogeneous strain relaxation in pseudomorphic In0.23Ga0.77As/GaAs quantum wells

The structural properties of pseudomorphic In0.23Ga0.77As/GaAs single quantum wells are investigated with x‐ray double‐crystal diffractometry. Anisotropic tilting of lattice planes along dislocations and anisotropic reflectivity of fully relaxed domains coexisting with strained domains are reported for the first time. Due to the anisotropic strain relaxation the crystal symmetry changes from tetragonal in the fully strained case to monoclinic in a partially relaxed quantum well.The structural properties of pseudomorphic In0.23Ga0.77As/GaAs single quantum wells are investigated with x‐ray double‐crystal diffractometry. Anisotropic tilting of lattice planes along dislocations and anisotropic reflectivity of fully relaxed domains coexisting with strained domains are reported for the first time. Due to the anisotropic strain relaxation the crystal symmetry changes from tetragonal in the fully strained case to monoclinic in a partially relaxed quantum well.

In situ grown‐in selective contacts to n‐i‐p‐i doping superlattice crystals using molecular beam epitaxial growth through a shadow mask

Highly selective n‐ and p‐type contacts to GaAs doping superlattices have been achieved by using molecular beam epitaxial growth through a silicon shadow mask to form interdigital grown‐in contacts. Low contact resistance, excellent diode characteristics, and efficient lateral injection electroluminescence are obtained.

Oxygen gettering by graphite baffles during organometallic vapor phase epitaxial AlGaAs growth

The effects of baffles upon the incorporation of oxygen into organometallic vapor phase epitaxial AlxGa1−x As have been investigated. Both graphite and SiC baffles have been studied. A simple model can be used to describe this effect as being dependent on the adsorption of trimethylaluminum onto the graphite, followed by reaction with oxygen to remove, or getter it from the gas stream.

Control of Be diffusion in molecular beam epitaxy GaAs

After molecular beam epitaxy GaAs is grown at 585 °C, substantial diffusion of Be occurs during growth of subsequent layers or during subsequent in situ annealing at 700 °C. By using an order of magnitude larger As4 flux than commonly used during growth and annealing, we were able to lower the Be diffusion coefficient by an order of magnitude. We also show that the Be diffusion coefficient is strongly dependent on the Be concentration. Different specimens were prepared with varying Be doping density, As4:Ga beam flux ratio, and annealing temperature. In these experiments, the Be concentration profiles were measured by secondary ion mass spectrometry and from them Be diffusion coefficients were calculated.

Dependence of structural and optical properties of In0.23Ga0.77As/GaAs quantum wells on misfit dislocations: Different critical thickness for dislocation generation and degradation of optical properties

The critical layer thickness Lc for dislocation generation and the characteristic thickness for consequent modification of the optical properties of strained quantum wells are shown to be remarkably different. We visualize the misfit dislocations directly using scanning cathodoluminescence and transmission electron microscopy. The dislocations are found to be asymmetrically distributed within the (001) interface plane. The critical thickness for In0.23Ga0.77 As on GaAs is determined to be Lc ≊15 nm in agreement with the theory of Matthews and Blakeslee [J. Cryst. Growth 27, 118 (1974)]. The deterioration of quantum efficiency, a change of recombination dynamics, the increase of spectral broadening and a shift of the band gap occur at much larger thickness than Lc since these quantities are barely affected by low dislocation densities. The relaxation is heterogeneous; strained and fully relaxed domains coexist. Using double crystal x‐ray diffraction we find that partially relaxed layers with thickness larg...

Dopant redistribution during titanium silicide formation

For advanced metal‐oxide‐semiconductor structures it is highly desirable to have a self‐aligned silicide structure on source and drain regions as well as on the gate in order to simplify lithography while reducing parasitic resistances. The reproducible formation of metal silicides on highly doped n+ and p+ regions with shallow junctions is required for those structures. Therefore, it is essential to understand the dopant behavior, silicide phase formation, and grain growth during high‐temperature annealing. Titanium silicides were formed on arsenic and boron difluoride implanted and annealed Si substrates. Arsenic atoms showed a significant redistribution and loss during titanium silicide formation. Arsenic atoms diffused out of the silicide surface with an activation energy of 0.95 eV. Boron atoms segregated to the silicide surface, and some boron atoms were lost from the surface; on the other hand, fluorine atoms were retained in the silicide layer. After prolonged 900 °C annealing, the thin titanium d...