Luminescence characterization of InGaN/GaN vertical heterostructures grown on GaN nanocolumns
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1144-LL16-04
Luminescence characterization of InGaN/GaN vertical heterostructures grown on GaN nanocolumns Robert Armitage Panasonic Electric Works Co., Ltd. 1048 Kadoma Osaka, 571-8686 Japan ABSTRACT The photoluminescence of MBE-grown InGaN/GaN vertical heterostructures on c-axis oriented GaN nanocolumns is investigated. Nanocolumnar InGaN heterostructures exhibit luminescence efficiencies greater than 20% for peak emission wavelengths as long as 540 nm. Compared to otherwise identical InGaN samples with larger median column diameters, the luminescence is blue-shifted and exhibits reduced efficiency for diameters less than about 50 nm. Growth of InGaN on GaN columns with a broad distribution of diameters results in broad-band photo-luminescence that appears white to the eye and has efficiency as high as 23%. INTRODUCTION GaN and its alloys with InN and AlN are widely used in visible light-emitting diodes (LEDs) and other devices. Compared to conventional planar epilayers, synthesis of these materials in the form of nanocolumnar crystals offers some advantages including low densities of extended defects [1] and partial relaxation of strain in lattice-mismatched heterojunctions via the free surfaces of the columns [2,3]. The latter effect could potentially be exploited to extend the operation of InGaN LEDs to yellow and red wavelengths, which are difficult to realize with planar epilayers due to the 11% lattice mismatch between GaN and InN. In this work we study the photoluminescence of InGaN/GaN heterostructures on top of c-axis oriented GaN nanocolumns grown spontaneously by molecular-beam epitaxy (MBE) on Si substrates. EXPERIMENTS Growth was performed in an Eiko EL-10A MBE machine with activated nitrogen supplied by an Oxford MPD-21 rf plasma source with a 276-hole PBN aperture plate and Ga and In evaporated from effusion cells. The base pressure is in the 10-10 torr range. Prior to growth the n-type Si(111) substrates were heated at 1050°C for 20 min. The rf source was operated at 450W with N2 pressure of 4.5×10-5 torr. In some cases a 100-200 nm GaN buffer layer was deposited at 500°C before the GaN nanocolumns growth. Nanocolumns were grown at a substrate temperature of 850-920°C with a Ga cell temperature of 950-1000°C. The temperature was then lowered to 680-720°C for the growth of a thin (
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