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Prem Kumar Kandaswamy

Al(Ga)N/GaN nanostructures for intersubband optoelectronics in the near- and mid-infrared

Published on 29 June 2010
Thesis presented June 29, 2010

This work reports on the design, epitaxial growth and characterization of Al(Ga)N/GaN quantum wells (QWs) and quantum dots (QDs) which constitute the active region of intersubband (ISB) devices operating in the near-infrared (NIR) and mid-infrared (MIR) spectral regions. The optimization of the Nextnano3 software for design of GaN/AlN QWs for ISB absorption in the 1.55 μm telecom region is presented. Growth of these structures was performed using plasma-assisted molecular beam epitaxy. The deposition process required fine tuning due to the large lattice mismatch. Minimum strain relaxation was obtained by adopting Ga-excess growth of both GaN and AlN layers. Staking faults embedded in the AlN layers were identified to be the cause of periodic relaxation of the structure. Infrared optical characterization demonstrates that the polarization-induced internal electric fields introduces a blue shift of the transitions and can critically modify the absorption magnitude.
Three-dimensionally confined GaN/AlN QDs introduces many novel properties for application as active region in ISB devices. The growth of QDs was performed under both Ga-rich and N-rich conditions. Dilution of QDs with required size was achieved using the enhanced-mobility condition of the Ga-rich method. Spectroscopic studies reveal absence of non-radiative recombination even in long lived QDs. Photodetectors fabricated on GaN/AlN QD superlattices present photocurrent at NIR and MIR, assigned to s-pz and s-pxy transitions, respectively. The dark current depends on the QD density due to hopping transport.
Foreseeing the importance of ISB devices in MIR and far-infrared regions of the spectrum, we have achieved ISB wavelength extension up to ~ 10 μm. This was performed basically by decreasing the internal electric-field and reducing the quantum confinement in GaN/AlGaN QW superlattices. Doping can induce a blue shift of more than 50% of the ISB transition energy, due to many-body effects.

nitrides, intersubband, molecular beam epitaxy, infrared, quantum well, quantum dot, intersubband, Nanostructures

On-line thesis.