Recombination Mechanism in Short-Wavelength GaN/AlGaN Quantum Wells
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Recombination Mechanism in Short-Wavelength GaN/AlGaN Quantum Wells D. Fuhrmann, T. Retzlaff, U. Rossow, and A. Hangleiter Institut f¨ur Technische Physik, Technische Universit¨at Braunschweig, Mendelssohnstr. 2, D-38106 Braunschweig, Germany E-mail: [email protected] ABSTRACT To date, light emission by AlGaN-based heterostructures and LED’s operating in the ultraviolet region is far less efficient than emission from longer wavelength structures based on GaInN. We have realized GaN/AlGaN quantum well structures emitting in the 360 - 320 nm range with peak room-temperature internal efficiencies reaching more than 20 %. From detailed studies of the temperature and excitation power dependence of the efficiency we find that excitons play a crucial role enhancing radiative recombination in such structures. Except for the peak internal efficiency, which reaches 73 % in GaInN/GaN, the overall behavior in GaN/AlGaN and GaInN/GaN is very similar, suggesting that the main difference is the nonradiative recombination mechanism. Introduction So far, light emission by AlGaN-based heterostructures operating in the ultraviolet spectral region is far less efficient than visible light emission by their GaInN-based counterparts. This is most evident from the rather poor efficiency of ultraviolet light-emitting diodes (LED’s). As to date, external quantum efficiencies even on low-defect-density substrates (e.g. bulk AlN) hardly exceed 1 % [1]. On the other hand, visible LED’s based on GaInN exhibit external efficiencies up to 40 % [2]. It is frequently argued that localization by random potential fluctuations in GaInN-based structures makes them more efficient. On the other hand, recent detailed studies of high-efficiency GaInN/GaN quantum wells [3] have led us to the conclusion that at room temperature carriers are mobile rather than localized in such structures. In order to study the effect in ultraviolet emitters, we have grown GaN/AlGaN quantum well structures on sapphire and SiC substrates. The emission wavelengths of the QW’s was varied in the 320-360 nm range by adjusting the QW width. Using temperature dependent photoluminescence measurements at variable excitation power we have determined the internal quantum efficiency (IQE) and analyzed its temperature and excitation power dependence. Experimental The substrate material used in this study was commercially available n-type doped, Si-face, on-axis 6H-SiC(0001) (Cree Inc.) or c-plane sapphire. The samples were grown by low pressure metal-organic vapor phase epitaxy (MOVPE) in a horizontal reactor (Aixtron AIX 200RF) with trimethylgallium (TMG), trimethyaluminum (TMAl), and ammonia (NH ) as precursors and hydrogen as a carrier gas. Diluted SiH was used for n-type doping. On SiC, growth started with a thin (10-50nm thick) AlGaN nucleation layer ramped down to or an AlN buffer layer of thickness grown at 1190 C with 100nm. AlGaN layers were grown afterwards at temperatures of typically 1190 C, a total pressure of 50 mbar, and total flux of 6 slpm. For growth on sapphi
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