Role of deep levels in DC current aging of GaN/InGaN Light-Emitting Diodes studied by Capacitance and Photocurrent Spect
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Role of deep levels in DC current aging of GaN/InGaN Light-Emitting Diodes studied by Capacitance and Photocurrent Spectroscopy. A. Castaldini1, A. Cavallini*1, L. Rigutti1, M. Meneghini2, S. Levada2, G. Meneghesso2, E. Zanoni 2, V. Härle3, T. Zahner3, and U. Zehnder3 1 Department of Physics, University of Bologna, V.le Berti Pichat 6/2, 40127 Bologna, Italy 2 Dept. of Information Engineering, University of Padova, Via Gradenigo 6A, I-35131 Padova, Italy 3 OSRAM OS, Wernerwerkstr. 2, D-93049 Regensburg, Germany * email: [email protected] ABSTRACT We present a combined Capacitance-Voltage (C-V), Deep Level Transient Spectroscopy (DLTS) and Photocurrent (PC) study of short-term instabilities of InGaN/GaN LEDs submitted to forward current aging tests at room temperature. C-V profiles detect changes consisting in apparent doping and/or charge concentration increase within the quantum wells. This increase is correlated to dramatic modifications in the DLTS spectrum when the reverse bias and filling pulse are properly adjusted in order to probe the quantum well region. The new distribution of the electronic levels detected by DLTS could explain the observed decrease in the light emission efficiency [1,2] of the device, as the deep levels generated during the stress may provide alternative recombination paths for free carriers. The photocurrent spectra do not change in shape during stress, although their amplitude slightly decreases. This is related to a decrease of the device yield, in this photodetector configuration, with increasing aging time. Thus, we can suggest that the introduction of new defect levels in the bulk material lowers the free carrier mobility. INTRODUCTION New applications like general lighting, TV backlighting, and automotive headlamps require an extended long term stability. The long term behaviour of commercial LEDs is quite good as verified by long term tests of OSRAM products for modifications of designs and processes. In this work we investigate special test structures with worse degradation behaviour in order to get information about microscopic and macroscopic degradation mechanisms associated with carrier transport. In particular, we study the kinetics of the short-term instabilities of InGaN/GaN devices submitted to relatively low forward current aging tests (15-20mA), focussing particularly on the role that deep levels play in the degradation of the optical efficiency. Other studies have identified thermally activated failure mechanisms of InGaN LEDs [3-5]: this paper describes short-term instabilities which can not be attributed to thermal effects only, since at the relatively low forward current values adopted selfheating induces only a moderate temperature increase. We performed capacitance, DLTS and Photocurrent measurements and used the results to interpret the modification of the apparent doping profile and of the related trap characteristics [6]. EXPERIMENTAL Test LED structures grown by metal-organic vapour phase epitaxy on silicon carbide substrates have been inve
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