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GaN Quantum Devices for T-Ray Sources

Final Report Summary - TERAGAN (GaN Quantum Devices for T-Ray Sources)

Infrared optoelectronic devices are critical elements not only for optical-fiber telecommunications, but also in the fields of industrial control, medical diagnostics, security screening or environmental monitoring. The aim of TeraGaN was the development of a new infrared technology using III-nitride semiconductors (GaN/AlGaN). These semiconductors are nowadays the materials of choice for optoelectronic devices in the green-to-ultraviolet spectral region, and find also application in high-power radio-frequency electronic devices. The outstanding physical and chemical stability of III-nitrides enables them to operate in harsh environments, and their bio-compatibility and piezoelectricity render them suitable for the fabrication of chemical sensors. Moreover, III-nitride semiconductors are the most “environment-friendly” technology available in the market.

We have demonstrated that III-nitrides can be tuned to emit and detect in the whole infrared spectrum by engineering the electron quantum confinement in nanostructures. Within this project, two approaches were initially considered, namely the fabrication of planar structures (quantum wells) on recently-available high-quality bulk GaN substrates, and the insertion of heterostructures in a GaN nanowire, forming a GaN/AlGaN nanodisk sequence.

As a first conclusion of our research, the most promising approach towards a GaN-based far-infrared (THz) technology consists of using quantum wells in a nonstandard crystallographic orientation, the so-called “m-plane”. This orientation allows the realization of structures without the internal electric field that is intrinsic to III-nitride semiconductors. Furthermore, the m-plane it is particularly favourable from the point of view of crystalline growth, so that micrometer-size defect-free GaN/AlGaN quantum well stacks can be synthesized on bulk GaN. These results have leaded to the fabrication of the first m-plane quantum-well THz photodetector.

TeraGaN has also explored extreme miniaturization of GaN/AlGaN heterostructures within nanowires. Introducing nanowires, instead of planar structures, as active media in optoelectronic devices would enable to control separately the electrical and the optical device cross section. Furthermore, the large surface-to-volume ratio of nanowires allows misfit strain to be elastically released, thus broadening the viable size and composition range for the active region. Within this project, we have demonstrated the feasibility to observe optically-excited electron transitions within quantum states in nanowires containing GaN/AlGaN nanodisks. These transitions occur under excitation with near-infrared or mid-infrared light, depending on the axial dimensions of the disks and the density of dopants like silicon or germanium. Interestingly, this phenomenon is rather insensitive to the nanowire external environment. We have also confirmed that the reduction of the amount of material in a nanowire array in comparison to a planar layer with the same thickness do not have any effect on the total absorption of the structure. Exploiting these features, we have fabricated the first infrared photodetector based in transitions between electron quantum states in nanodisks within a single nanowire. These developments set the basis of a new infrared technology with the potential to overcome current commercial devices in terms of speed, thanks to the ultra-fast relaxation time of electrons in excited quantum states.