Physics and applications of intersubband transitions in nitride based heterostructures

In the past decades, III-nitride semiconductor compounds have attracted an increasing amount of interest due to their applications to blue-violet laser diodes and white light emitting diodes, or for their use as ultraviolet emitting devices for biomedical applications. In addition, these materials can also be used to achieve high power, high frequency electronic devices, such as high electron mobility transistors. Furthermore, thanks to the large conduction band offset of 1.75 eV in the AlN/GaN system, it is also possible to develop active optoelectronic devices working in the telecom range using intersubband transitions (ISBT). This is quite interesting as in nitride compounds these transitions are characterized by fast recombination dynamics, which should open new prospects for optoelectronic devices operating in the THz regime. Another property of interest of nitride compounds is their mechanical hardness and good thermal behavior, which enables foreseeing applications in harsh environment (space). Up to this work, the only reported results of ISBT in the telecom range were achieved using molecular beam epitaxy (MBE) growth technique. On the other hand, ISBT energies reported on samples grown with metal-organic vapor phase epitaxy (MOVPE) were far below the telecom limit. Nevertheless, MOVPE being the technique of choice for the production of nitride based optoelectronic devices, it is worth from a technological point of view to investigate whether or not it is possible to reach the telecom range with this technique. In this work, the origins of the discrepancies between results obtained with the two techniques will be investigated. First the effect of different growth parameters over ISBT will be studied for MOVPE grown samples. This will lead to the conclusion that with this growth technique, strain plays a critical role on the AlN/GaN heterostructure properties. In fact, it will be shown that strain induced interface instability occurring at the well/barrier interface is detrimental to reach the telecom range. It will be proven that, for MOVPE grown samples, such wavelengths can be reached if the active region is deposited on AlN templates. Following investigations carried out on MOVPE structures, MBE grown samples will also be studied and it will be shown that for complex heterostructures ( e.g. with coupled QWs), this growth technique is better suited. This is notably due to its lower growth temperature leading to a decrease of interdiffusion/segregation effects, also detrimental to ISBT energy. In addition, AlInN lattice-matched to GaN will be investigated in order to achieve ISBT in the mid-infrared range with low defect density heterostructures. It will also be shown that thick LM AlInN waveguide layers operating in the telecom range with low propagation losses can be grown by MOVPE. Finally, the development of ISB based nitride devices will be studied and a first electro-optical modulator working up to a frequency of 2.5 GHz will be demonstrated.


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