Influence of polarization fields on carrier lifetime and recombination rates in InGaN-based light-emitting diodes

Abstract

We study differential carrier lifetimes in InGaN light-emitting diodes ͑LEDs͒ of varying wavelengths. Increase in wavelength is correlated with an increase in lifetime, due to the impact of the polarization fields on carrier overlap. This effect explains the early onset of droop in longer-wavelength LEDs.Efficiency droop is a nonthermal process of disputed origin which quenches the internal quantum efficiency ͑ IQE ͒ of GaN-based light-emitting diodes ͑LEDs͒ at high current density ͑J͒. One point of particular interest is the relationship between emission wavelength ͑͒ and droop. Early on, it was noticed that in LEDs emitting at longer the onset of droop occurs at lower J. 1 This is an important limitation for longwavelength high-power LEDs, especially in the green range. It has been suggested that this could be due to increased carrier leakage 2 with In content or to a carrier delocalization 3 from In-rich areas.We have recently shown evidence that droop is a bulk phenomenon occurring even in photoluminescence in bulk GaN. 4 We have also shown that droop is related to a shortening of the nonradiative carrier lifetime, manifesting the onset of a nonradiative recombination phenomenon behaving roughly ϳn 3 where n is the carrier density. 5 In this letter, we explore the relationship between and carrier lifetime, and show how the aforementioned effects can largely be explained by the influence of the polarization fields on carrier wave functions.We study a set of five ͑0001͒ InGaN LEDs with multiquantum-wells ͑MQW͒ of varying , whose characteristics are indicated in Table I. The first four samples have the same nominal active region ͑AR͒ design and a varying In content, while the fifth sample has nominally the same In content as the fourth one but slightly different AR layer thicknesses, yielding a longer . We measure IQE for each sample as described in Ref. 5 ͓Fig. 1͑a͔͒. We note that its peak value peak is similar for all samples, while the onset of droop happens at lower J for longer-wavelength samples. We stress the importance of comparing samples of similar IQE , because variations in crystal quality influence the shape of the efficiency curve-namely, larger Shockley-Read-Hall ͑SRH͒ recombinations push the curve toward higher J which can mask a more fundamental variation in droop. Figure 1͑b͒ shows the same curves, normalized to unity and with a linear J axis, making the effect of droop onset more obvious.We perform differential carrier lifetime ͑͒ measurements on these samples, as described in Ref. 5. By integrating , we obtain the carrier density n. By coupling this measurement to that of IQE , we access the radiative and nonradiative lifetimes R and NR , and the corresponding recombination rates G R and G NR . As in Ref. 5, we can now derive three quantities: a = G NR / n, b = G R / n 2 , and c = G NR / n 3 . These are useful because they enable comparison to an ABC model, where the coefficients A, B, and C stand for SRH, radiative, and Auger coefficients, respectively. 6 Namely, in an ideal ABC model, a sho...

First-principles calculations of indirect Auger recombination in nitride semiconductors

“…Our present results show that the rate of decrease of the Auger coefficient with density in the 2 × 10 18 − 5 × 10 19 cm −3 range (which includes the peak of the internal quantum efficiency of LEDs [9]) is smaller than the rate of decrease of the radiative coefficient (B) [36]. The more rapid decrease of the B coefficient in comparison to the C coefficient with increasing free-carrier density may explain the asymmetry of the internal quantum efficiency curve reported in the literature [68], without invoking an additional fourth-order or higher-power carrier-loss mechanism.…”

“…for E gap = 3.5 eV) and the corresponding coefficients cannot account for the experimentally measured values in nitride devices (10 −31 -10 −30 cm 6 s −1 ) [3][4][5][6][7][8][9][10][11]. The coefficients increase drastically for decreasing band-gap values in the range of 2.4-3.5 eV, which is the typical band-gap range for nitride devices.…”

“…As a result, it becomes an important carrier recombination mechanism at high carrier densities and reduces the efficiency of solid-state light emitters at high currents. In particular, Auger recombination has been shown to be involved in the efficiency reduction of nitride LEDs and lasers at high power [2][3][4][5][6][7][8][9][10][11][12][13]. The direct Auger recombination process [ Fig.…”

Comparative studies of efficiency droop in polar and non-polar InGaN quantum wells

“…This removes the problem that arises in CW photoluminescence (PL) and electroluminescence (EL) experiments that under steady-state excitation conditions the equilibrium carrier densities in c-plane and m-plane structures can be very different due to the dependence on the different recombination rates. The importance of the role of the recombination rate in determining droop, hence excited carrier density, has already been shown by David et al 29 for polar InGaN LEDs with varying In fraction. In this work, it was shown that decreases in the recombination rate as the In fraction increased lead to the occurrence of efficiency droop at progressively lower current densities.…”

Intervalley energy of GaN conduction band measured by femtosecond pump-probe spectroscopy

“…The pump pulse power was scaled with the photon energy between 0.60 and 0.68 nJ, corresponding to an average photoexcited carrier density of 1 × 10 18 cm −3 . For such carrier densities, hot carrier generation related to Auger recombination can safely be ignored [17].…”