We investigate the properties of InGaN-based vertical-type solar cells having wavelengths ranging from the ultraviolet to green regions. It is well known that InGaN-based solar cells require a high indium composition to obtain high conversion efficiency. However, although InGaN-based solar cells with a high indium composition have been fabricated, their conversion efficiency has not sufficiently increased. Therefore, to further understand carrier transport, we measured the bias-dependent external quantum efficiency. For vertical-type green solar cells with a high indium composition, we confirmed that they have a higher short circuit current than other samples tested due to their broader overlapping region with the solar spectrum, though their fill factor remained low due to their high barrier height and strong piezoelectric field, which caused a reduction in the carrier tunneling rate.
We report the growth and characterization of a Si-doped, n-type AlGaN layer with 45% Al composition. For the application of n-type AlGaN layers with high Al composition in ultraviolet emitters, we fabricated an n-Al0.45Ga0.55N layer with high crystalline quality and high electrical conductivity by inserting Al0.85Ga0.15N/AlN superlattices (SLs) to prevent cracks prior to growing the n-type AlGaN layer. The dislocation density in the n-AlGaN layer with 45% Al composition and SLs was less than 2.4 x 1010 cm-2, which was lower than the dislocation density of 5.3 x 1010 cm-2 for the n-AlGaN layer without SLs. The resistivity, mobility, and free electron concentration in the n-type Al0.45Ga0.55N layer with SLs were 2.2 x 10-2 Ω×cm, 55.0 cm2 /V-s, and 5.0 x 1018 cm-3 at room temperature, respectively.
We investigated the surface plasmon coupling behavior in InGaN/GaN multiple quantum wells at 460 nm by
employing Ag nanostructures on the top of a roughened p-type GaN. After the growth of a blue light emitting diode
structure, the p-GaN layer was roughened by inductive coupled plasma etching and the Ag nanostructures were formed
on it. This structure showed a drastic enhancement in photoluminescence and electroluminescence intensity and the
degree of enhancement was found to depend on the morphology of Ag nanostructures. From the time-resolved
photoluminescence measurement a faster decay rate for the Ag-coated structure was observed. The calculated Purcell
enhancement factor indicated that the improved luminescence intensity was attributed to the energy transfer from
electron-hole pair recombination in the quantum well to electron vibrations of surface plasmon at the Ag-coated surface
of the roughened p-GaN.
We have reported the growth and characterization of quaternary AlGaInN bulk layer and AlGaInN/InGaN MQWs
grown by metal-organic chemical vapor deposition (MOCVD) for high efficiency ultraviolet light-emitting diodes. The
inclusion of the small fraction of the indium in AlGaInN layer was found to lead a fewer structural defects and reduction
of strain in the layer. From the temperature dependent photoluminescence (TDPL) and time-resolved PL (TRPL), the
internal quantum efficiency at 300K was obtained as 60 % for sample grown with quaternary AlGaInN barrier in MQWs
and 25 % for the sample with ternary AlGaN barrier. It was resulted that the dominant optical transition in
AlGaInN/InGaN MQWs was due to localized exciton recombination and reduction of strain in the QW stack with
indium incorporation in the barriers, resulting the longest decay lifetime from quaternary AlGaInN alloys. We measured
the optical output power from the UV LED device grown with quaternary AlGaInN barriers.
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