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Low-frequency noise in diagnostics of power blue InGaN/GaN LEDs

Evgeniia Shabunina ,  Anton Chernyakov ,  Michael Levinshtein ,  Natalia M. Shmidt ,  Nadezhda Talnishnikh 

A.F.Ioffe Phisico-Technical Institute (FTI), Politekhnicheskaja 26, S.Petersburg 194021, Russian Federation

Abstract

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 SV(j) and current SI(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 (SI) on frequency (f) as 1/f and increase in SI 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 SI(j) dependences in the LEDs differs noticeably from the SI(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 Eg, were studied by LFN methods before and after 10-1000 hours aging at 35 A/cm2 and 100oC p-n junction temperature. No considerable change in η, I-V characteristics and SV(j), SI(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. SV(j) dependencies of these LEDs differ strongly from the law SV (j)~1/j which is typical for p-n junction with uniform current distribution. Moreover SI(j) dependences at j < 10-2 A/cm2  increases as SI ~ j4. According to [1], this type of SI(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.

SI(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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Related papers

Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 5, by Evgeniia Shabunina
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-04-01 13:25
Revised:   2013-04-01 13:27