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Zinc Oxide For TeraHertz Cascade Devices

Periodic Reporting for period 3 - Zoterac (Zinc Oxide For TeraHertz Cascade Devices)

Reporting period: 2018-03-01 to 2020-02-29

Why Zoterac?
The terahertz (THz) spectral region, located between the infrared and the microwave regions, is known as “the THz gap” because of the lack of compact semiconductor devices. This spectral domain is currently intensively explored in view of its potential for medical diagnostics, security screening, trace molecule sensing, astronomical detection, space-borne imaging, non-invasive quality control or wireless communications. A prerequisite for public-domain applications to emerge in the strategic THz frequency range is the availability of compact size semiconductor sources operating at room temperature, which is out of range of the current technology based on GaAs quantum cascade lasers.

What is Zoterac?
ZOTERAC is a project which proposes a disruptive approach based on ZnO-based nano-engineered semiconductors in order to realize THz emitters operating at room-temperature with milliWatt output power capability as well as THz quantum detectors with unprecedented large operating temperatures. These devices are based on the quantum cascade concept and take benefit of the large optical phonon energy of ZnO (twice that of GaAs) for achieving high temperature operation.

Benefits of Zoterac
Establishing a new state-of-the-art for the design, growth and processing of ZnO/ZnMgO heterostructures, and developing an advanced know-how on oxide-based devices are major challenges of the project. The consortium regroups world–class academic experts on ZnO technologies, quantum cascade lasers and detectors as well as THz optoelectronics. The strategies have been chosen based on a careful assessment of the risk attached to all tasks and achievement of targeted objectives at each stage of the project. This project which implies a strong expertize in basic physics, chemistry and engineering, is expected to generate high impacts in terms of scientific and technological achievements.
During this period, all partners have started their activities within ZOTERAC. The planned tasks have been launched following the description of action of the GA. The scientific deliverables for the reporting period have been met, while the two milestones achieved. The work of each package has followed the planning and yielded the significant progress.

• WP1 work has been focused on the designs of quantum cascade devices with targeted wavelengths in the THz range. The designs have been optimized in terms of perfomance and growth using different methods and have been provided to WP2 (D1.4). The designs set the state of the art for ZnO-based QC devices.

• The work on the growth of the ZnO/(Zn,Mg)O quantum structures has been carried out in WP2. 140 samples have been produced during this period. During this period the first ZnO-based THz devices have been fabricated. QCD and QCLs with a perfectly controlled thickness (with a sub monolayer deviation) have been delivered. All the devices have been grown in the new MBE system, which has been evaluated in terms of lifetime of all its components. A quick, precise and non-destructive method has been developed to quickly asses these complicated sample structures.

• WP3 work has been focused on the process for the samples. All the technological steps have been optimized. A total of 169 devices have been processed. Many critical steps have been successfully implemented (Substrate removal and wafer bonding, etch stop layers, surface passivation, metal metal waveguide…) and are reported in Deliverable D3.3. THz and QCLs has been successfully processed.

• • WP4 work has been focused on two different tasks: the assessment of basic physical material parameters on one hand, and detector and emitter prototype characteristics on the other hand. As a result of the optimization in ZOTERAC we have demonstrated that ZnO based could be an ideal candidate for optical metamaterials, whose properties can be controlled by the intersubband transitions. THz QCDs clearly show absorption at the targeted wavelength. Several THz ZnO QCLs have been thoroughly investigated in order to understand the particular behavior of ZnO-based QCL. They exhibit electroluminescence in the THz range, close to the wavelength expected from the experimental structure. The observation of EL is a huge step towards the realization of QCLs. But, to achieve the lasing action, the waveguide had to be optimized in order to reduce the optical losses in the THz. A new batch of samples has been processed with double-metal waveguide structure which yields a large mode overlap factor, and thus a higher modal gain. With the optimized gain medium and waveguide structure, we would expect to observe the lasing action from ZnO-based THz QCLs

WP 5 has delivered two deliverables (the plan for communication D5.2 and the plan for dissemination and exploitation D5.3 updated M36.
ZOTERAC will have a strategic impact for European research and will offer a world leadership in the very attractive field of the THz. The ZOTERAC objectives address directly the H2020 FET-open call by exploring novel fundamental physics, by developing a future and emerging technology, and by offering large interdisciplinary prospective.
In addition to the large scientific impact directly related to the project, the THz emitters and detectors developed in ZOTERAC will generate a great breakthrough in a variety of application domains. Among them, one could retain the following striking examples:

- Since many materials (clothes, paper, plastic, walls,…) are transparent to the THz, these radiations can be employed for the detection of hidden weapons and to detect traces of explosives having absorption lines in the THz range. THz technology represents the most suitable candidate for fabricating systems in the field of security, such as people and luggage scanners in airports and other public places.
- THz radiations are non-ionizing and eye safe, they could partially replace the X-ray in medical diagnostics. In particular, the detection of skin and breast cancers will be at a so early stage that it will considerably increase the success probability of the treatment. The very high sensitivity of enamel and dentine properties to THz radiations is calling for the use in dental screening.
- From an environmental point of view, gas phase spectroscopy at THz frequencies allows to study the chemical processes in the upper atmosphere (atmospheric molecules like water, oxygen, carbon monoxide,... have strong absorption bands in this frequency range), which are crucial in ozone formation and destruction.
- The THz radiations will become at the center of material science, industrial quality control and product inspection because it represents a non-destructive technique to inspect plastics, polymers, ceramics... For instance, the pharmaceutical industry is very demanding for non-destructive and chemical analysis of tablets, capsules and other dosage forms.
ZOE: our new MBE System for ZOTERAC
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