Optical spectroscopy of single non-polar InGaN quantum dots

Thesis

Experimental investigations of single InGaN/GaN quantum dots grown on the non-polar (11-20) plane are presented.

The electrical driving of non-polar nitride quantum dots is demonstrated. Correlation measurements on a single dot prove single-photon emission with a second order correlation value of g²(0) = 0.18(18), taking into account the detector time response. The dot’s emission exhibits a high degree of linear polarisation, in agreement with a study on 76 randomly chosen quantum dots. Current-dependent measurements reveal three biexciton-exciton pairs with a small binding energy of 3meV, indicating dot heights below 5 nm. Electroluminescence of a single dot is demonstrated up to 130 K.

A lateral electric field is applied to quantum dots with the aim of estimating their lateral size and confining potential from linewidth broadening. A measurement on a single dot yields 6(1) nm and 28(3) meV. However, 60 other emission lines do not show a broadening, but their energy shifts allow to determine the magnitude of the in-built dipoles. As expected, these are small compared to those of polar InGaN dots, ranging between -0.4eÅ and +0.3eÅ. The existence of both parallel and anti-parallel dipoles is attributed to competition between the first and second order piezo-electric component. A mean in-built field of -3 kV/cm with a standard deviation of 11 kV/cm is deduced, demonstrating a reduction by more than two orders of magnitude compared to polar InGaN dots. Further observations are unexpected non-parabolic energy shifts, attributed to device fabrication issues, and similar shift behaviour of some emission lines in the same spectrum, attributed to multi-excitonic complexes and excited states.

A correlation method is used along with continuous laser excitation to measure the fast timescale of spectral diffusion of several dots. Only a part of a dot’s linewidth is selected. One randomly chosen dot exhibits a spectral diffusion time of 860(160) ns at low excitation power. The inverse of the spectral diffusion time increases with increasing excitation power, in line with previous reports. Other quantum dots exhibit shorter, but also longer spectral diffusion times, up to 1170(50) ns. This is at least 3.5 times longer than for any previous measurement on a nitride dot.

Authors: Kocher, C

• DOI: 10.5287/ora-zrmdnodkb
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: nitride; quantum optics; quantum dot; non-polar; single-photon generation
• Subjects: Quantum optics; Quantum dots