Keith B. Kahen

Non-blinking semiconductor nanocrystals

The photoluminescence from a variety of individual molecules and nanometre-sized crystallites is defined by large intensity fluctuations, known as ‘blinking’, whereby their photoluminescence turns ‘on’ and ‘off’ intermittently, even under continuous photoexcitation. For semiconductor nanocrystals, it was originally proposed that these ‘off’ periods corresponded to a nanocrystal with an extra charge. A charged nanocrystal could have its photoluminescence temporarily quenched owing to the high efficiency of non-radiative (for example, Auger) recombination processes between the extra charge and a subsequently excited electron–hole pair; photoluminescence would resume only after the nanocrystal becomes neutralized again. Despite over a decade of research, completely non-blinking nanocrystals have not been synthesized and an understanding of the blinking phenomenon remains elusive. Here we report ternary core/shell CdZnSe/ZnSe semiconductor nanocrystals that individually exhibit continuous, non-blinking photoluminescence. Unexpectedly, these nanocrystals strongly photoluminesce despite being charged, as indicated by a multi-peaked photoluminescence spectral shape and short lifetime. To model the unusual photoluminescence properties of the CdZnSe/ZnSe nanocrystals, we softened the abrupt confinement potential of a typical core/shell nanocrystal, suggesting that the structure is a radially graded alloy of CdZnSe into ZnSe. As photoluminescence blinking severely limits the usefulness of nanocrystals in applications requiring a continuous output of single photons, these non-blinking nanocrystals may enable substantial advances in fields ranging from single-molecule biological labelling to low-threshold lasers.

Properties of Ga vacancies in AlGaAs materials

Intermixing of AlGaAs‐based interfaces is known to be enhanced by capping wafers with a layer of SiO2. Assuming that this enhancement results from the introduction of additional Ga vacancies into the sample, it is possible to obtain the temperature‐dependent equilibrium Ga vacancy diffusivity. Experiments are performed whereby SiO2‐capped quantum well samples are annealed at temperatures ranging from 800 to 1025 °C. Calculated photoluminescence shifts are compared with the measured spectra, and a relation for the Ga vacancy diffusivity of the form 0.962 exp(−2.72/kBT) cm2/s is obtained. Using this relation, the equilibrium Ga vacancy concentration can be computed via an ensemble Monte Carlo simulation. The resulting expression is 1.25×1031 exp(−3.28/kBT) cm−3.

Rigorous optical modeling of multilayer organic light-emitting diode devices

We present an exact classical solution to the problem of dipole emission in a planar multilayer light-emitting device. The inputs to the model are the photoluminescence and quantum yield of the emitter material, and the device layer thicknesses and indices of refraction. The results of the model are applied to predicting the radiant intensity of organic light-emitting diodes as a function of varying device layer thickness. It is shown that the predicted radiances are in excellent agreement with the data. We also present results for the Poynting power distribution from a randomly aligned dipole for positions both internal and external to the diodes.

Two-dimensional simulation of laser diodes in the steady state

A fully self-consistent steady-state two dimensional model of laser diodes is presented. The model consists of the simultaneous solution of the Poisson and the electron and hole drift-diffusion equations, the wave equation, and the photon rate equation. Excellent agreement with experiment is obtained for both gain-guided and index-guided laser diodes. Specific results are given for channeled-substrate planar (CSP) lasers. It is shown that comparable electron current confinement is provided by both internal strips (p-GaAs barriers) and zinc-diffused planar stripes. The confinement in the first case is due to energy barriers and in the latter case is due to lateral electric fields. For the holes, current spreading is shown to be reduced substantially for the planar stripes because of the use of high-resistivity n-cap layers. It is demonstrated that the thickness of the p-GaAs layers can be smaller than the minority carrier diffusion length since only a very small fraction of the laser light passes through the barriers. >

Effect of ion implantation dose on the interdiffusion of GaAs‐AlGaAs interfaces

Experimental results of enhanced interdiffusion of GaAs‐AlGaAs interfaces are reported. These are obtained by implanting Ar ions at doses ranging from 2×1013 to 5×1014 cm−2 into heterostructure samples followed by rapid thermal annealing at 950 °C for 30 s. The degree of intermixing decreases from the surface up to the projected ion range and is a function of the implantation dose. It is postulated that this variation results from the coalescence of some of the excess vacancies into extended defects, which are then unavailable to assist in the enhanced interdiffusion process. By assuming that the concentration of mobile vacancies at any depth is proportional to the ion’s electronic energy loss and inversely proportional to the ion’s nuclear energy loss, the calculated intermixing results are shown to be in good agreement with the experimental data.

Model for the diffusion of zinc in gallium arsenide

The anomalous shape of the Zn diffusion profile in GaAs has been quantitatively explained. The Frank–Turnbull mechanism is invoked to govern the interchange between interstitial and substitutional Zn, via the Ga vacancies. These vacancies are proposed to be either neutral or singly ionized, depending on the position of the Fermi level. In addition, two physical phenomena are proposed. Substitutional Zn thermally generates interstitial Zn‐Ga vacancy pairs and there is pairing between the donor, interstitial Zn, and the acceptor, substitutional Zn. The model is found to be in good agreement with the experimental data.

STUDIES OF ZINC DIFFUSION IN GALLIUM ARSENIDE BY RAPID THERMAL PROCESSING

Rapid thermal annealing has been used to diffuse Zn into GaAs from a thin film zinc silicate source prepared by atmospheric pressure chemical vapor deposition. For a diffusion temperature of 650 °C, comparisons were made with conventional, open‐tube furnace annealing and the diffusivities were found to be similar, in contrast to previous experimental work. In the temperature range 650–750 °C, sharp zinc diffusion profiles are observed. Above 750 °C, kinks are seen in the diffusion profiles. These kinks are also observed when semi‐insulating substrates are used instead of silicon doped n+ ‐substrates. Previously, we have introduced a zinc diffusion model based on the pairing of interstitial Zn with all acceptor species present during diffusion. The dominant species are found to be substitutional Zn and the gallium vacancy, where the concentration of the latter is a function of the background doping concentration. The results of this model are shown to agree with all of our experimental evidence and are als...

Mechanism for zinc diffusion in n-type gallium arsenide

A model is presented that is able to account successfully for the Zn diffusion profiles in n+‐GaAs. The model is based on a previously developed formalism whose basis is the retarding of the interstitial Zn diffusivity because of Coulomb pairing between interstitial and substitutional Zn. By extending the model to include Coulomb pairing of interstitial Zn with all acceptors present during diffusion, the resulting theoretical profiles are shown to be in very good agreement with the data while using only one adjustable parameter.

Analysis of distributed-feedback lasers using a recursive Green's functional approach

A method is presented for obtaining the exact propagation constants (resonance wavelengths and threshold gains) for distributed-feedback lasers. The method is based on computing the exact Greens function for resonant cavities using a recursive Greens function approach. The net threshold gains of distributed-feedback lasers are calculated and compared with the results obtained using conventional coupled-mode theory. The comparison reveals that coupled-mode theory is weakest for aperiodic structures. >

Analysis of distributed‐feedback lasers: A recursive Green’s function approach

A recursive Green’s function method is introduced for obtaining the exact modal characteristics of aperiodic distributed‐feedback lasers. Additionally, based on scaling arguments, a new expression for the coupled‐mode theory coupling coefficient, κ, is derived, resulting in increased accuracy for coupled‐mode calculations.