Ilya E. Titkov

Temperature-Dependent Internal Quantum Efficiency of Blue High-Brightness Light-Emitting Diodes

Internal quantum efficiency (IQE) of a blue high-brightness InGaN/GaN light-emitting diode (LED) was evaluated from the external quantum efficiency measured as a function of current at various temperatures ranged between 13 and 440 K. Processing the data with a novel evaluation procedure based on the ABC-model, we have determined the temperature-dependent IQE of the LED structure and light extraction efficiency of the LED chip. Separate evaluation of these parameters is helpful for further optimization of the heterostructure and chip designs. The data obtained enable making a guess on the temperature dependence of the radiative and Auger recombination coefficients, which may be important for identification of dominant mechanisms responsible for the efficiency droop in III-nitride LEDs. Thermal degradation of the LED performance in terms of the emission efficiency is also considered.

AlGaInP red-emitting light emitting diode under extremely high pulsed pumping

Efficiency of commercial 620 nm AlGaInP Golden Dragon-cased high-power LEDs has been studied under extremely high pump current density up to 4.5 kA/cm2 and pulse duration from microsecond down to sub-nanosecond range. To understand the nature of LED efficiency decrease with current, pulse width variation is used. Analysis of the pulse-duration dependence of the LED efficiency and emission spectrum suggests the active region overheating to be the major factor controlling the LED efficiency reduction at CW and sub-microsecond pumping. The overheating can be effectively avoided by the use of sub-nanosecond current pulses. A direct correlation between the onset of the efficiency decrease and LED overheating is demonstrated.

Efficiency of True-Green Light Emitting Diodes: Non-Uniformity and Temperature Effects

External quantum efficiency of industrial-grade green InGaN light-emitting diodes (LEDs) has been measured in a wide range of operating currents at various temperatures from 13 K to 300 K. Unlike blue LEDs, the efficiency as a function of current is found to have a multi-peak character, which could not be fitted by a simple ABC-model. This observation correlated with splitting of LED emission spectra into two peaks at certain currents. The characterization data are interpreted in terms of non-uniformity of the LED active region, which is tentatively attributed to extended defects like V-pits. We suggest a new approach to evaluation of temperature-dependent light extraction and internal quantum efficiencies taking into account the active region non-uniformity. As a result, the temperature dependence of light extraction and internal quantum efficiencies have been evaluated in the temperature range mentioned above and compared with those of blue LEDs.

Correlation between p-GaN growth environment with electrical and optical properties of blue LEDs

Two blue (450 nm) light–emitting diodes (LED), which only differ in top p-GaN layer growth conditions, were comparatively investigated. I-V, C-V, TLM, Electroluminescence (EL) and Photoluminescence (PL) techniques were applied to clarify a correlation between MOCVD carrier gas and internal properties. The A-structure grown in the pure N2 environment demonstrated better parameters than the B-structure grown in the N2/H2 (1:1) gas mixture. The mixed growth atmosphere leaded to an increase of sheet resistances of p-GaN layer. EL and PL measurements confirmed the advantage of the pure N2 utilization, and C(VR) measurement pointed the increase of static charge concentration near the p-GaN interface in the B structure.

Novel evaluation procedure for internal and extraction efficiency of high-power blue LEDs

Internal quantum efficiency (IQE) of a high-brightness blue LED has been evaluated from the external quantum efficiency measured as a function of current at room temperature. Processing the data with a novel evaluation procedure based on the ABC-model, we have determined separately IQE of the LED structure and light extraction efficiency (LEE) of UX:3 chip.

Temperature effects on optical properties and efficiency of red AlGaInP-based light emitting diodes under high current pulse pumping

In this paper, current-dependent emission spectra and efficiency measured on the same AlGaInP red light-emitting diode (LED) pumped with the current pulses of very different durations are recorded. This enabled for the first time distinguishing between high-carrier concentration and self-heating effects on the efficiency decline at high current magnitudes. The electron leakage to the p-side of the LED structure, which is the major mechanism of the efficiency reduction, is found to rise substantially when the device self-heating starts to develop. As a result, in comparison to continuous-wave excitation, driving the LED with sub-microsecond current pulses allows suppressing the device self-heating and, eventually, increasing the operating current by an order of magnitude without noticeable efficiency losses. Based on the reduced ABC-model, neglecting Auger recombination, the light extraction efficiency, injection efficiency, and internal quantum efficiency of the LED are estimated, suggesting light extraction to be the most critical factor for the overall efficiency of the LED. The coupled spectral/power LED characterization using the variable-duration current pulse pumping is found to be an effective approach for analyzing mechanisms of the device operation.

Internal quantum efficiency and tunable colour temperature in monolithic white InGaN/GaN LED

Internal Quantum Efficiency (IQE) of two-colour monolithic white light emitting diode (LED) was measured by temperature dependant electro-luminescence (TDEL) and analysed with modified rate equation based on ABC model. External, internal and injection efficiencies of blue and green quantum wells were analysed separately. Monolithic white LED contained one green InGaN QW and two blue QWs being separated by GaN barrier. This paper reports also the tunable behaviour of correlated colour temperature (CCT) in pulsed operation mode and effect of self-heating on device performance.