Jozsef Hegedus

Structure and photoinduced volume changes of obliquely deposited amorphous selenium

Atomic scale computer simulations on structures and photoinduced volume changes of flatly and obliquely deposited amorphous selenium (a-Se) films have been carried out in order to understand how the properties of chalcogenide glasses are influenced by their preparation method. Obliquely deposited a-Se thin films contain more coordination defects and larger voids than the flatly deposited ones. To model the photoinduced volume changes the electron excitation and hole creation were treated independently within the framework of tight-binding formalism. Covalent and interchain bond breakings and formations were found. The obliquely deposited samples containing voids showed a wide spectrum of photoinduced structural changes in the microscopic level and volume changes in the macroscopic level.

Impurity-induced step interactions: a kinetic Monte Carlo study

A one-dimensional (1D) continuum description of growth on vicinal surfaces in the presence of immobile impurities predicts that the impurities can induce attractive step interactions when they suppress the diffusion of adatoms on the surface, thus destabilizing the equidistant step train. In the present communication, we verify this prediction by kinetic Monte Carlo simulations of a 2D solid-on-solid model. We identify the conditions where quasi-1D step flow is stable against island formation or step meandering, and analyse in detail the statistics of the impurity concentration profile. The sign and strength of the impurity-induced step interactions is determined by monitoring the motion of pairs of steps. Assemblies containing up to 20 steps turn out to be unstable towards the emission of single steps. This behaviour is traced back to the small value of the effective, impurity-induced attachment asymmetry for adatoms. An analytic estimate for the critical number of steps needed to stabilize a large step bunch is derived and confirmed by simulations of a 1D model.

Embedded multi-domain LED model for adaptive dimming of streetlighting luminaires

Temperature dependence of solid state lighting products is often not considered. Lighting products are designed either to be too robust to fulfil the requirements under any possible environmental conditions, or fail to perform the minimal expectations due to high ambient temperature. However, temperature dependent nature of LEDs even could be a new benefit, taking it into account in the design stage. Keeping the light output values at the desired level at any ambient temperature would be a cost efficient solution. This paper shows power saving effects of a smart adaptive system using a controlled current source and describes a method to gain the controlling functions. A case study with real meteorological temperature dataset is carried out.

Aging tendencies of power MOSFETs — A reliability testing method combined with thermal performance monitoring

Die attach degradation in power electronic devices is a common failure mode besides bond wire damage. This paper describes the chip and packaging level effects of high cycle count power cycling on novel mid-power automotive MOSFETs. The related investigation is carried out in controlled ambient conditions. The thermal transient measurements during the active temperature cycling reliability test were evaluated along with K-factor calibration to identify different failure modes. The described measurement method gives the opportunity to distinguish between electrical and thermal related structural error modes. The newly developed thermal interface used in the DUTs was found to withstand the 150 °C thermal amplitude well beyond 100,000 cycles without fatal failures. The thermal performance degradation was less than 28.5% after 70,000 cycles for the complete assembly.

Aging tendencies of power leds under different humidity conditions during thermal reliability testing

The standard LED characterization methods do not consider changes in relative humidity of the test environment. This paper describes the effects of a high relative humidity drop on the measured light output of undomed test LED samples. The related investigation is carried out in controlled ambient conditions. Spectra of the test LED samples and of a reference light source were captured in discrete relative humidity steps. The relative humidity dependence is identified for both the light output of the undomed LED samples and for the reflectance of the optical measurement system itself. 2-2.5% of change is found in the optical parameters in the 35-80% and 35-100% ranges of relative humidity.