A. E. Chernyakov

Failure mechanisms in blue InGaN/GaN LEDs for high power operation

Unpredictable fast failure of blue power InGaN/GaN LEDs is caused by redistribution of In under action of injection currents between nano-scale regions of InGaN alloy with non-equilibrium composition. Unreliable LEDs can be recognized by the increase in forward current values at U < 2 V which is not accompanied by simultaneous reversed current increase during short aging tests (less than 100 h).

High-efficiency InGaN/GaN/AlGaN light-emitting diodes with short-period InGaN/GaN superlattice for 530–560 nm range

A new method of forming the active region in high-efficiency InGaN/GaN/AlGaN light-emitting diode (LED) structure for long-wave green range is described. The introduction of a short-period InGaN/GaN superlattice situated immediately under the emitting quantum well and overgrown with GaN layer at reduced temperature leads to a more than tenfold increase in the efficiency of emission. For the proposed LEDs, the maximum quantum efficiency was 12% at 552 nm and 8% at 560 nm.

Localized states in the active region of blue LEDs related to a system of extended defects

Blue light-emitting diodes (LEDs) based on InGaN/GaN quantum wells (QWs) with different characters of the system of extended defects (SEDs) threading through the active region have been studied using the current-voltage (I–U), capacitance-voltage (C–V), and deep-level transient spectroscopy (DLTS) measurements in the dark and under illumination with white light in a temperature range from 100 to 450 K. The DLTS curves exhibit broad E1 and E2 peaks with amplitudes dependent on the illumination. This behavior can be explained assuming the presence of localized states related to SEDs in the active region of the LED. The LEDs with more developed SEDs are characterized by a greater concentration of donor-type traps, which leads to an increase in the density of free charge carriers in QWs, which screen the electron-hole interaction. This circumstance can be among the factors responsible for a severalfold decrease in the quantum efficiency of such LEDs.

Low-frequency noise in as-fabricated and degraded blue InGaAs/GaN LEDs

Study of the spectral noise density and its dependence on current density in as-fabricated and degraded blue light-emitting diodes (LEDs) based on InGaN/GaN quantum-well structures are reported. It is shown that defects are generated nonuniformly in the course of degradation, being concentrated along extended defects penetrating into the active region of LEDs. It is demonstrated that the decrease in the external quantum efficiency in the course of aging is due to the enhancement of charge transport uniformity, which leads to the formation of shunts and local overheating regions. Typically, in blue LEDs, these effects are responsible for the ambiguous development of the degradation process, which hinders prognostication of LED service life. The effect of noise suppression is observed in a narrow current density range (10−2 to 10−1 A cm−2) corresponding to the onset of radiative recombination.

Comparison of the properties of AlGaInN light-emitting diode chips of vertical and flip-chip design using silicon as the a submount

Vertical and flip-chip light-emitting diode (LED) chips are compared from the viewpoint of the behavior of current spreading in the active region and the distribution of local temperatures and thermal resistances of chips. AlGaInN LED chips of vertical design are fabricated using Si as a submount and LED flipchips were fabricated with the removal of a sapphire substrate. The latter are also mounted on a Si submount. The active regions of both chips are identical and are about 1 mm2 in size. It is shown that both the emittance of the crystal surface in the visible range and the distribution of local temperatures estimated from radiation in the infrared region are more uniform in crystals of vertical design. Heat removal from flip-chips is insufficient in regions of the n contact, which do not possess good thermal contact with the submount. As a result, the total thermal resistances between the p-n junction and the submount both for the vertical chips and for flip-chips are approximately 1 K/W. The total area of the flip-chips exceeds that of the vertical design chips by a factor of 1.4.

Features of the recombination processes in InGaN/GaN based LEDs at high densities of injection current

It is established that the low-frequency noise density S in InGaN/GaN based light-emitting diodes (LEDs) operating a current densities j > 20 A/cm2 depends on the current density as S ∼ j3. This dependence is indicative of the formation of new nonradiative recombination centers, which may account for a drop in the external quantum efficiency of LEDs operating at high current densities.

Experimental study of electroluminescence and temperature distribution in high-power AlGaInN LEDs & LED arrays

Comprehensive analysis of current spreading, temperature distribution, and near-field electroluminescence of high-power “face-up” AlInGaN LEDs and LED matrixes been performed by combination of different experimental and methods. A thermal resistance characterization consists in investigations of transient electrical processes in the diode sources under heating by direct current and analysis using a thermal equivalent circuit (the Cauer model). By the involved method, thermal resistances of internal elements of the LEDs are determined. At the same time high resolution mapping of EL and thermal radiation was obtained by optical microscope and infra-red images technique. It has been established correlation of the thermal resistance with a change in the current distribution at high excitation levels (current crowding).

Experimental study of electroluminescence and temperature distribution in high-power AlGaInN LEDs & LED matrixes

Comprehensive analysis of current spreading, temperature distribution, and near-field electroluminescence of high-power “face-up” AlInGaN LEDs and LED matrixes been performed by combination of different experimental and methods. A thermal resistance characterization consists in investigations of transient electrical processes in the diode sources under heating by direct current and analysis using a thermal equivalent circuit (the Cauer model). By the involved method, thermal resistances of internal elements of the LEDs are determined. At the same time high resolution mapping of EL and thermal radiation was obtained by optical microscope and infra-red images technique. It has been established correlation of the thermal resistance with a change in the current distribution at high excitation levels (current crowding).

The relationship between the reliability of transistors with 2D AlGaN/GaN channel and organization type of nanomaterial

The first experimental results demonstrating that the carrier mobility in the AlGaN/GaN 2D channel of transistor structures (AlGaN/GaN-HEMT) is correlated with the manner in which the nanomaterial is organized and also with the operation reliability of transistor parameters are presented. It is shown that improving the nature of organization of the nanomaterials in AlGaN/GaN-HEMT structures, evaluated by the multifractal parameter characterizing the extent to which a nanomaterial is disordered (local symmetry breaking) is accompanied by a significant, several-fold increase in the electron mobility in the 2D channel and in the reliability of parameters of transistors fabricated from these structures.

Specific features of proton interaction with transistor structures having a 2D AlGaN/GaN channel

It has been shown that the interaction of 1 MeV protons at doses of (0.5–2) × 1014 cm–2 with transistor structures having a 2D AlGaN/GaN channel (AlGaN/GaN HEMTs) is accompanied not only by the generation of point defects, but also by the formation of local regions with a disordered nanomaterial. The degree of disorder of the nanomaterial was evaluated by multifractal analysis methods. An increase in the degree of disorder of the nanomaterial, manifested the most clearly at a proton dose of 2 × 1014 cm–2, leads to several-fold changes in the mobility and electron density in the 2D channel of HEMT structures. In this case, the transistors show a decrease in the source–drain current and an order-of-magnitude increase in the gate leakage current. In HEMT structures having an enhanced disorder of the nanomaterial prior to exposure to protons, proton irradiation results in suppression of the 2D conductivity in the channel and failure of the transistors, even at a dose of 1 × 1014 cm–2.