Power blue InGaN/GaN light emitting diodes (LEDs) are the basis for the solid-state lighting. In spite of excellent success in the light-emitting devices technology, the physical mechanisms of non-radiative recombination and ones that responsible for the unpredictable failure after a comparatively shot operation time of some part of LEDs with the same values of external quantum efficiency as LEDs having long life time have not been clarified until now. The difficulties arisen under the study of the physical mechanisms are related to numerous forms of nano-structural arrangement (NA) in these LEDs and nano-scale phase separation (NPS) in InGaN both depending strongly on growth conditions. NA forms are determined by coalescence crystallites of near 3D growth mode to near 2D one and result in formation of the extended defect system (EDS) piercing the LED active region. EDS includes threading dislocations, their accumulations and numerous dilatation and dislocation boundaries. To study such complicated system not enough the traditional methods. It seems that the application of low frequency noise (LFN) techniques allows one to fully characterize these peculiarities. The methods are known to be an effective tool to obtain a comprehensive information not only on the generation-recombination (G-R) noise due to point defects, but also on charge fluctuations at surface states, presence of several barriers with close energy levels, nano-material disorder or local field fluctuation [1]. The behaviour of the defects can be investigated at the large current density range.

In this abstract we present the results demonstrating the efficiency of LFN methods for separation of the contribution of EDS and point defects to non-radiative recombination to clarify the causes of unpredictable failure of power blue InGaN/GaN LEDs. All results have been obtained on InGaN/GaN LEDs with the values of external quantum efficiency (η) 45-50% at wavelength 450-460 nm. Low frequency noise measurements in the frequency range 1 Hz – 10 kHz and spectral voltage S_V(j) and current S_I(j) noise density dependences at 10^-4 – 50 A/cm² and η(j) dependencies were performed on all LEDs.

To separate the contribution of EDS and point defects in non radiative recombination the classification of LEDs on their leakage current (LC) values were used. The LC values at U < 2 V integrally characterize electrical properties of EDS. The lower LC values are, the smaller concentration of extended defects is [2]. It has been revealed that low-frequency noise peculiarities such as the shape of noise spectral density (S_I) on frequency (f) as 1/f and increase in S_I with an LC values growth are caused by EDS presence. The point defects (PD) contribution to non-radiative recombination processes is observed in current density region 10^-2 – 10 A/cm² where radiative recombination prevails. The non-radiative recombination caused by EDS at j < 10^-2 A/cm² and at j > 10 A/cm² is predominated. The complicated behavior of EDS with an injection current change has been observed. The shape of the decreasing part at S_I(j) dependences in the LEDs differs noticeably from the S_I(j) ~ 1/j² being typical for local trap filling mechanism.

In order to clarify the causes of unpredictable failure in LEDs with the same η and LC but different values of forward current at U < 2.5 V and remarkable discrepancy in the shape of I-V forward branches, which might be attributed to the presence of local regions with smaller E_g, were studied by LFN methods before and after 10-1000 hours aging at 35 A/cm² and 100^oC p-n junction temperature. No considerable change in η, I-V characteristics and S_V(j), S_I(j) dependences were observed for majority of LEDs under the study even after 1000 hours, except fort he LEDs with a remarkable discrepancy in the shape of I-V forward branches. S_V(j) dependencies of these LEDs differ strongly from the law S_V (j)~1/j which is typical for p-n junction with uniform current distribution. Moreover S_I(j) dependences at j < 10^-2 A/cm²  increases as S_I ~ j⁴. According to [1], this type of S_I(j) is caused by appearance of local overheating regions. These peculiarities usually appear after 10-100 hours of aging process. Although any noticeable changes in but η values has not been observed, the life time of these LEDs are smaller than 1000 hours. The results of electroluminescence study enable us to suppose that redistribution of In between nano-scale regions of InGaN alloy with non-equilibrium composition takes place.

S_I(j) dependences for InGaN/GaN LEDs at 77 K has been obtained. Their analysis allows us to separate the charge carriers transport along EDS from the one along nano-scale fluctuations of InGaN composition. Thus LFN methods let us obtain important and useful information about recombination and degradation mechanism in power blue InGaN/GaN LEDs.

References

[1] N.V. Dyakonova at al., Physics and techniques of semiconductors, 25(12), 2065 (1991)

[2] A.V. Kamanin al. Phys. stat.sol.(c), 3, 2129 (2006).

[3] Schubert EF. Light-emitting diodes. 2nd ed. UK: Cambrige University Press (2006).

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1. Multifractal analysis in diagnostics of nanostructures