Photoluminescence of single InGaN quantum dots grown at low surface densities by MOVPE

Abstract

We report the observation of photoluminescence due to single InGaN dots using samples containing very low surface dot densities. InGaN dot samples were grown by metalorganic vapour phase epitaxy with varying surface dot densities, the lowest being below 1 per µm 2 . The dots are typically 30-40 nm in diameter and 4-6 nm in height. Photoluminescence spectra taken from large dot ensembles indicate a low energy tale to the InGaN layer emission, which increases in intensity as the dot density increases. The energy resolved photoluminescence lifetime indicates that low energy feature is long lived, a significant rise time for the low energy photoluminescence also indicates slow carrier relaxation. The use of 400 nm apertures in aluminium shadow masks allows the observation of sharp line features due to individual exciton localisation centres. Under low excitation powers the broadband InGaN epilayer emission is found to break up into very large number of sharp line features consistent with the presence of strong localisation excitons within the InGaN epilayer due to alloy fluctuations. In addition, on the low energy wing of the quantum well band, strong, isolated sharp emission lines are observed. Many such apertures containing single line emission spectra have been observed. 1 Introduction Semiconductor quantum dots are an area of great current interest. The zero dimensional potential created by the formation of nanoscale inclusions in a surrounding matrix offers the possibility of controlling the generation and transmission of individual carriers. Moreover, the presence of isolated (in energy) carrier eigenstates permits a wide range of fundamental physics experiments. The formation of quantum dots by the so-called Stranski-Krastonov growth mode has been observed in several III-V semicondcutor systems such as InAs/GaAs and InP/GaP. In these systems the formation of pyramidal quantum dots is essentially strain driven with the dot density and size only weakly dependent on the exact growth conditions. The formation of dots in the GaN/InGaN system is potentially very important for optoelectronic devices such as quantum dot lasers which are predicted to have a lower threshold current than conventional quantum well lasers. InN and GaN are immiscible in the bulk and the associated alloy In x Ga 1-x N can only be formed under highly non-equilibrium epitaxial growth conditions.

Effects of depletion on the emission from individual InGaN dots

“…In a previous report 6 we observed ͑using conventional shadow masks͒ that the narrowest single exciton lines in a similar sample to the one studied here have a full width half maximum ͑FWHM͒ of ϳ5 meV. The slightly larger SOQDs studied in this letter have larger linewidths, with a minimum FWHM of ϳ8 meV.…”

“…12 Previously we measured the carrier lifetimes in our system, which were in the nanosecond regime, thus ruling out lifetime broadening as a significant mechanism. 6 Further evidence for the clear inhomogenous nature of the broadening process in our structures can be seen in the details of the bias dependence of the micro-PL. Under forward bias at around +2 V the spectrum changes to show at least one extra line ͑marked I 1 ͒ close in energy to the excitonic emission ͑marked I͒.…”

“…Atomic force microscopy ͑AFM͒ studies carried out on an uncapped sample 6 show that the surface is covered with a very low density ͑ϳ5 dot/ m 2 ͒ of large nanostructures, with a nonuniform distribution of the background InGaN layer. The average diameter and height of the SOQDs are 30 and 5 nm, respectively.…”