Degradation Mechanism of GaN-based LEDs With Different Growth Parameters
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1195-B08-06
Degradation Mechanism of GaN-based LEDs With Different Growth Parameters K.K. Leung1, W.K. Fong1, P. K. L. Chan2 and C. Surya1 1 Department of Electronic and Information Engineering and Photonics Research Centre, The Hong Kong Polytechnic University, Hong Kong, China 2 Department of Mechanical Engineering The Hong Kong Polytechnic University Hong Kong, China. ABSTRACT We investigated the degradation mechanism of GaN LEDs due to the application of a high d.c. stressing current. To identify the underlying process for device failure we examined the effects of the InGaN quantum well growth parameters on the hot-electron hardness of the devices. Systematic characterizations on the degradations in the microstructural, thermoreflectance, and low frequency noise properties of the devices were performed. INTRODUCTION III-nitrides are the materials of choice for the development of high-power LEDs [1-3]. Device reliability is an important issue for such high-power applications which is strongly affected by the crystal quality of the films. To date sapphire is the most commonly used substrates for the growth of III-nitride heterostructures. Due to the large lattice mismatch between GaN and the sapphire substrate high dislocation density is typically found in the GaN films. This may significantly affect the reliability and lifetimes of the devices. It is therefore important to optimize the growth conditions to enhance the reliability performance of the LEDs. In this paper we present experimental data on the microstructural, thermoreflectance and lowfrequency noise properties, as a function of the stress time, for GaN LEDs fabricated using different growth conditions for the active regions of the devices. The degradation mechanism will be investigated based on the experimental results. EXPERIMENT GaN LEDs were grown on c-face (0001) sapphire substrates using a Thomas Swan metalorganic chemical vapor deposition (MOCVD) system. A 30 nm thick GaN nucleation layer was deposited at 520°C. This was followed by a 100 nm thick of undoped GaN epilayer deposited at 1035°C and a pressure of 200 Torr during. A ditertiarybutyl silane source was used for the growth of n-doped GaN [4,5]. The carrier concentration of the n-layer is 3×1018cm-3. The active layers consist of five GaN/InGaN quantum wells (QWs). The InGaN MQWs were grown at 700°C and the GaN barriers were grown at 850°C. The doping concentration for the top p-layer is 1017cm-3. Finally, a 150nm thick p-GaN layer of doping concentration ~4×1017 cm-3 was grown at 1010°C on top of the MQWs. The main difference between the devices is in the growth paratmeters for the MQWs as summarized in Table I below. Close to three-fold increase
in the TEG flux was used for device A leading to three times increase in the growth rate for the MQWs in device A. Table I: Two different types of devices with different growth conditions for the MQWs Device A InGaN well
GaN barrier
TEG=10.56µmol/min TMI=19.39µmol/min TMI/(TMI+TEG) = 64.7% 27.54 µmol/min
Device B TEG=3.52 µmol/min TMI=13.96µm
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