Bronius Saulys

A DETAIL ANALYSIS OF ELECTRICAL AND OPTICAL FLUCTUATIONS OF GREEN LIGHT-EMITTING DIODES BY CORRELATION METHOD

A detail analysis of electrical and optical fluctuations of large power (1 W) green light-emitting diodes (LEDs) is presented. Special attention was directed to measurement and interpretation of correlation coefficient between electrical and optical fluctuations. The correlation coefficient was measured not only over frequency range from 10 Hz to 20 kHz, but also in every one-octave frequency band by using digital filters. It is shown that correlated part of electrical and optical fluctuations for investigated green LEDs is related with random potential fluctuations of parameters of quantum well due to charge carrier capture by the defects in the active layer, while uncorrelated part of electrical noise is caused by parallel leakage channel which resistance is many times higher than that of p-n junction.

Reliability investigation of light-emitting diodes via low frequency noise characteristics

Abstract Investigation of changes of operation and noise characteristics during aging process of light-emitting diodes (LEDs) has been carried out. Several groups of different design (different optics) LEDs based on different materials (nitride-based blue and white LEDs, phosphide-based red LEDs) have been investigated. It is found that leakage current components appear due to LED’s defects and their affect is observed as increase of both the low frequency electrical noise intensity and non-ideality factor of current-leakage characteristic in small current region. No considerable changes of light intensity characteristics during LEDs aging have been observed. Noise modeling, spectral and correlation analysis of optical and electrical fluctuations show on partly correlated optical and electrical fluctuations caused by defects in the active region of the LED. Degradation processes of investigated LEDs foremost occur in the diode chip and lead to the leakage current that has important affect to the electrical fluctuation level, but practically has a weak influence to the light emission properties of LED. Phosphorous layer of white LEDs and additional optical elements have no significant influence to the reliability of investigated LEDs under given aging conditions.

Noise characteristics and radiation spectra of multimode MQW laser diodes during mode-hopping effect

Comprehensive investigation of noise (optical and electrical fluctuations and cross-correlation factor between them) and radiation spectrum characteristics have been carried out for InGaAsP/InP Fabry-Perot (FP) multiple quantum well laser diodes (LDs). Mode-hopping occurs at particular operation conditions (injection current and temperature) in FP LD operation. LD radiation spectrum is outspread and intensive highly correlated optical and electrical fluctuations are observed during mode-hopping. Physical processes during mode-hopping have 1 µs - 1 ms characteristic times and are caused by generation-recombination and carrier capture processes in centers formed by defects in barrier layer.

Noise characteristics and reliability of high power white light emitting diodes based on nitrides

High power white light emission diode reliability and aging processes have been investigated. Optical, electrical and noise characteristics have been carried out for initial devices and during their aging. Analysis of noise characteristics help revealing of light emission diode aging processes and reliability problems. It is found that optical and electrical noise spectra changes reflect light emission diode aging. Noise characteristics, especially correlation factor between optical and electrical fluctuations, and current-voltage characteristics at low bias reveal physical processes that take place during investigated device aging and rapid its degradation. It is shown that reason of high power light emission diode degradation is related with defects presence in the device structure. Additional defects appear during LED operation and lead to the leakage current and non-radiative recombination increase.

Characterization of degradation process in white light nitride-based LEDs by low-frequency noise

Noise characteristics (optical and electrical fluctuations and cross-correlation factor between two noise signals) of high power nitride-based white light emitting diodes (LEDs) have been investigated for initial samples and after accelerated aging. Different spectrum parts from white LED radiation have been investigated separately. Electrical fluctuations and low frequency optical noise of investigated LEDs distinguish by 1/fα-type spectrum (above 100 Hz shot noise prevails in optical noise). 1/fα-type optical and electrical fluctuations are partly correlated. After accelerated aging optical 1/fα-type noise intensity increased, and cross-correlation factor between blue and red light intensity fluctuation increased, too. Origin of increased 1/fα-type optical noise is defects in the active layer of investigated white LED, and their increase during aging.