Ultrafast Carrier Dynamics and Recombination in Green Emitting InGaN MQW LED

Abstract

Ultrafast spectroscopy, in particular time-resolved photoluminescence, can help in the development of InGaN/GaN heterostructures for long wavelength visible emitters. In this paper, we present recent results that underscore the current understanding of two approches to achieve desired emission characteristics and recombination mechanisms in InGaN/GaN MQWs for green LEDs. Specifically, the photoluminescence decay of samples grown using two different design approaches is discussed. In one approach, samples, with high indium incorporation, were grown on a high quality AlN substrate to achieve green emission. The resulting photoluminescence decay of the green luminescence is long-lived and non-exponential. Quantitative analysis showed that the decay is non-exponential and consists of a stretched-exponential decay component, at short times, followed by a power law decay component at longer times. This non-exponential decay, resulting in significant elongation of the radiative lifetime, is indicative of inhomogeneity in the quantum wells. Thus carrier localization, in a structure with low defect density, proves to be an effective means to achieve green emission. In another approach, a piezoelectric Stark-like ladder effect is used. In this case, a methodical layer-by-layer growth homogeneity optimization process was adopted to achieve an optical transition below the electron to heavy-hole (e 1 hh 1 ) transition when the quantum well is subjected to the presence of the strong piezoelectric polarization dipole. An advantage of this approach is that it has proven successful in achieving green luminescence on conventional sapphire substrates. The resulting photoluminescence decay at 14 K of a sample grown by this approach is single exponential and shorter in duration than the decay observed in the first approach. This exponential decay further supports previous AFM studies that revealed a homogeneous active region.

Development of 2" AlN Substrates Using SiC Seeds

“…The comparison of photoluminescence of the GaN layers deposited on the Al face and on the N face of the single-crystal AlN substrate showed that in the former case photoluminescence is consistent with that of the homoepitaxial Ga-face GaN while in the latter an existance of the tail localized states was found (Tamulatis et al, 2003). The studies of deep-UV emission of AlGaN quantum wells (Gaska et al, 2002) as well as AlGaN UV (Nishida et al, 2004a;Ren et al, 2007;Xi et al , 2006a) and InGaN MQW green (Cartwright et al, 2006) LEDs grown on the bulk AlN substrates and blue and UV LED on the bulk GaN substrates (Cao et al, 2004;Cao et al , 2004a;Du, Lu, Chen, Xiu, Zhang & Zheng, 2010) as well as cyan and green LEDs grown on a-plane (Detchprohm et al, 2008) and m-plane (Detchprohm et al, 2010) GaN bulk substrates prove the superiority of the native substrates, e.g., the luminescence intensity of the quantum well grown on bulk AlN was higher that that of the quantum well grown on SiC by a factor of 28, the noticeable improvement over LEDs grown on sapphire in device impedance and thermal characteristics (Ren et al , 2007a), the reduction in current-voltage differential resistance and in turn-on voltage (Paskova et al, 2010). The emission spectrum of AlGaN-based UV-LEDs on a bulk AlN substrate under the high current injection is much more stable than that of LEDs fabricated on the conventional substrate (Nishida et al, 2004a;.…”