GaN/AlN Multiple Quantum Well Structures

Doctoral thesis, 2007

The III-nitride semiconductors: InN, GaN and AlN are promising for photonic, high power and high temperature electronic devices. Their large and direct band gaps cover the range 0.7 to 6.2 eV, i.e. infrared to ultraviolet wavelengths. Due to the large conduction band-offset, ~ 2 eV, in GaN/AlN heterostructures, there is also a potential to use intersubband transitions for wavelengths above 1 µm. Examples of intersubband devices are light emitters, modulators, detectors and amplifiers, which can operate at 1.55 µm for telecommunication applications. GaN layers were grown by plasma assisted molecular beam epitaxy on different substrates: sapphire (-Al2O3), silicon (111) and the GaN template. The surface morphology and crystalline quality were studied by reflection high energy electron diffraction, atomic force microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction. It was found that GaN layers grown on sapphire had very smooth surfaces (root-mean-square roughness less than 0.5 nm) and very high quality (full width at half maximum of X-ray diffraction (0002) scan less than 50 arcsec). Molecular beam epitaxy regrown GaN on templates exhibited step and terrace features with clear atomic steps. The similar surface roughness and crystal quality as the templates indicated that this is an excellent substrate to further develop GaN based heterostructure devices by MBE. The GaN growth on the Si(111) substrate, however, showed inferior quality, due to the difficulty in controlling the Si/AlN interface without disordered SiN formation. This know-how about GaN growth on different substrates was employed to prepare GaN/AlN multiple quantum wells structures. These in turn were used to fabricate modulators for intersubband transitions. Different periods (1, 5, 10 and 20), well widths (1.5 - 5.4 nm) and barrier widths (1.2 – 5.1 nm) were grown on sapphire and templates, respectively. X-ray diffraction 2 symmetrical scans along the GaN (0002) plane showed large number of superlattice peaks, which indicated very high quality of the grown multiple quantum well structures. Intersubband transitions, with wavelengths from 1.5 to 3.5 μm and peak width of 93 meV, were observed by Fourier transform infrared spectroscopy. The layer thickness, relaxation status, Si doping efficiency as well as the intersubband absorptions were systematically investigated. Moreover, the intersubband transition energies were calculated by the envelope function approach considering the conduction-band nonparabolicity, built-in fields, strain, and many-body effects. It was found from experiments and calculations that the AlN barrier width can affect the intersubband transition energy through its effect on the built-in fields, especially when the barrier width is below 3 nm. With Si δ-doping in the middle of the AlN barrier layers, the intersubband absorbance was greatly improved compared to homogeneous doping in barriers and/or wells. Due to monolayer fluctuations of the quantum well layer thickness, the intersubband absorption spectrum was perfectly fitted as a superposition of several Lorentzian peaks. The energies fitted to the intersubband transition of quantum well thickness differed corresponding to an integer number of monolayer and the typical peak widths were ~ 60 meV. Moreover, multiple quantum well structures grown on template had higher intersubband absorbance and smaller peak width as compared with those grown on sapphire. The very good control of the quantum well layer thicknesses and the intersubband transition wavelengths as well as the demonstration of intersubband transitions with peak widths below 100 meV are promising for devices such as high speed electroabsorption modulators.