Igh Indium Content InGaN Films and Quantum Wells.
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ABSTRACT InGaN films and InGaN/GaN quantum wells with high indium content have been grown by MOVPE and characterised to evaluate the growth process and the indium incorporation efficiency. The characterisation techniques include photoluminescence, DC X-ray and TEM. The closed spaced vertical rotating disk reactor configuration results in a very high Indium incorporation for InGaN material, compared to other configurations. InGaN layers with an indium composition up to 56 % have been deposited which still exhibit very good optical properties (intense PL emission). The influence of various growth conditions on the InGaN composition and quality have been investigated to optimize the layer quality. TEM diffraction patterns have shown that the ternary InGaN layer can be chemically ordered. The In and Ga atoms occupy respectively the two simple hexagonal sublattice sites related by the glide mirrors and helicoidal axes of the P6 3mc symmetry group of the wurtzite GaN. INTRODUCTION Since the demonstration of a CW room temperature operating nitride based laser diode by Nichia Chemicals the ternary alloy InGaN has received increasing attention. Light emitting diodes and laser diodes use InGaN as the active layer because it exhibits the possibility of controlling the emission wavelength from the visible to the near UV. However, the exact underlying physics of these devices (for example the role of InGaN quantum dots in the emission mechanism) are not yet fully understood. Similarly the OMVPE growth of InGaN needs to be explored more thoroughly to be able to improve device efficiencies. The growth of InGaN, especially with a high indium content, still shows some fundamental problems: the formation of indium droplets on the surface, the low indium incorporation and phase separation. To be able to incorporate the desired mole fraction of InGaN the layers have to be grown at relatively low temperatures (700 'C to 850 'C), which is limiting the crystalline and optical quality [1]. A better understanding of the growth conditions limiting the incorporation is required. Therefore we have focused our efforts on trying to grow InGaN fdms with a maximum Indium mole fraction and to identify the parameters that enhance the Indium incorporation. The conclusions made at these extreme conditions can then be applied in growing low indium content (20%) InGaN active layers at the higher temperatures required for good crystalline and optical quality. It has been suggested and confirmed in the past that a large difference in atomic spacing can introduce ordering in semiconductor compound material. Atoms re-arrange on an atomic scale to reduce the strain energy involved in stretching the compound lattice. The ordering was found to be dependent on several growth parameters including growth temperature, growth rate and substrate orientation [2]. This phenomenon has already been observed in other material 107
Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
systems [3], but up to now no ordering had been found in InGaN lay
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