Optical and microstructural properties of InGaN/GaN self-assembled quantum dots
GaN and its related ternary alloys have been extensively investigated in recent years due to their excellent optical and electrical properties. Further progress in this field is expected to revolutionise the optoelectronics and electronics industry. Properties such as high thermal conductivity, high luminous efficiency and mechanical robustness have made GaN the superior material system for light emitting devices operating from the UV to the entire visible spectrum region. In addition, new applications for GaN in high temperature, high frequency electronics are being investigated.
The MOVPE (Metal-organic-vapour-phase-epitaxial) growth of GaN layers on sapphire substrates has lead to the successful design and production of the first quantum well (QW) LED's and LD's operating in the blue and violet parts of the spectrum. Commercial drive for applications in Blu-Ray and HD-DVD systems have helped accelerate their development. Despite this progress, the successful production of light emitters operating in the green and UV parts of the spectrum remains elusive. The lack of success in this area originates in a low understanding of the intrinsic material properties, optical properties and the required growth techniques.
In recent years, GaN layers grown on the c-face of a sapphire substrate have been of poor quality, containing many vacancies, traps and non-radiative recombination centres. Despite the high density of defects, high quantum efficiencies have been observed in InGaN/GaN QW's. The reasons for this are not fully understood, but many people believe they are the result of the self-organisation or phase separation of the InGaN layer during growth, essentially creating 3D potential fluctuations or quantum dots (QD's). Charge carriers become trapped in the QD's, preventing them from diffusing towards non-radiative recombination centres, thus increasing the observed quantum efficiency. It is believed that the use of self-organised QD's in LD's will improve device performance, increasing temperature stability and reducing threshold current densities, even in devices will high defect densities.
Our research is centred on the tuning of the growth parameters in order to aid the self-organisation of an InGaN layer into QD's. Samples are grown by MOVPE processes at the EPSRC National Centre for III-V Technologies. Spectroscopy experiments to determine the optical properties include photoluminescence (PL), PL excitation (PLE), time resolved PL and pump-probe measurements. Microstructural properties will be determined by TEM and AFM imaging techniques.
|Academic Staff:||David Mowbray|
|PhD Students:||Sam Davies|
This research is supported by our collaborations with the EPSRC National Centre for III-V Technologies, University of Sheffield and the School of Electrical Engineering and Electronics, University of Manchester.