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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - FLASH (Far-infrared Lasers Assembled using Silicon Heterostructures)

Teaser

Summary

The Terahertz (THz) region is defined as the range of the electromagnetic spectrum comprised in between the microwaves and the infrared, with its characteristic wave frequency thus extending from 0.3 THz (corresponding to a wavelength of 1 mm) to 10 THz (a wavelength of 30 Î¼m). The blooming interest in this spectral region is driven by several reasons. First, THz waves can penetrate a wide variety of nonconducting materials, passing through e.g. clothing, paper, cardboard, wood, plastic, and ceramics almost undisturbed whereas is absorbed by metals. This enables the use of THz radiation into security screening systems, for example, to detect concealed weapons on a person. Moreover, many substances of interest for security and defense, such as plastic explosives, exhibit spectral fingerprints in the THz range.
Another very interesting feature is that THz radiation is strongly attenuated in water. Therefore, its propagation in living tissues is very sensitive to the water content of cells, suggesting potential healthcare applications such as 3D medical imaging, including the detection of epithelial cancer and wound inspection through bandages. Moreover, THz radiation energy, being four orders of magnitude smaller than x-rays, cannot damage DNA. As a consequence, THz light-based diagnostic is potentially much safer than conventional x-rays.
In addition, since the atmosphere is highly transparent to THz light in selected frequency windows, and THz radiation is eye safe, the latter finds interesting applications to short range, high-bandwidth telecoms as testified by the constant push to rise the frequency of LANs.
However, no technological platform based on THz radiation (light emitters, detectors and I/O electronics) is available to date, featuring the costs and performances required for the mass-market demand that all these applications could potentially trigger. Leveraging on the Si-based microelectronics infrastructure for the manufacturing of a THz photonic platform, could ultimately respond to these needs. In fact, the well-established Si-based microelectronics accounts for >98% of the semiconductor market, allowing for an enormous cut of cost per device and granting mass-production capabilities.
In particular, the achievement of an electrically pumped Si-based laser constitutes the Holy Grail of modern photonics and it would be a long sought-after scientific and technological breakthrough.
Quantum cascade lasers (QCL) are unipolar intersubband lasers that use the charge carrier transitions between subband states in or between QWs to generate population inversion and lasing. By means of a suitable band structure engineering of semiconductor heterostructures, light emission in the THz region can be achieved. While QCL based on compound semiconductors have been demonstrated and are commercially available, electrically pumped Si-based QCLs have not been achieved yet. The main challenges to be solved in the development of such devices are related to the lattice strain, the doping segregation, and the parasitic valleys.
By addressing significant material science challenges with innovative experimental and theoretical approaches, the FLASH project aims to develop a cost-effective and compact THz quantum cascade laser integrated on Si using manufacturing processes and materials compatible with the â€œstandardâ€ complementary-metal-oxide-semiconductor (CMOS)-compatible technology. The successful delivery of the proposed THz photonic platform on Si would be foundational in enabling the fabrication of THz sources, integrated optics, electronics and non-linear elements for THz circuits, performing better than their III-V counterpart thanks to the lack of polar optical phonon scattering combined with the lower free carrier absorption typical of group IV elemental semiconductors.

Work performed

In the first year of the project, we demonstrated that that the CVD growth technology available to the consortium allows the deposition of hundreds of strain-compensated Ge/SiGe heterolayers having tunneling barriers of 1-2 nm and interface roughness < 0.2 nm. Double plasmon waveguide with losses of 10 cmâ€“1 at 4.84 THz with a modal overlap of 0.98 have been designed using an average Si0.05Ge0.95 material for the quantum cascade stack. Different metal-contact solutions have been investigated. Electron tunneling through thin barriers has been studied by measuring intersubband absorption in asymmetric coupled quantum well samples. The modeling accuracy in reproducing experimental spectra guarantees high precision of design and high reliability for the calibration of material parameters for future SiGe-based THz electronic and photonic devices. Non-equilibrium Green function simulations of a QCL design adapted from the III-V material system to the Ge/SiGe one demonstrates the robustness of the latter QCL structure as a function of temperature. They indicate that the gain in the proposed Ge/SiGe QCL featuring the interface roughness typically measured on our samples is expected to overcome the estimated losses at room temperature, leading to lasing.

Final results

In FLASH, we want to demonstrate, for the first time, that an industrial-viable technological platform is achievable exploiting electronic transitions in the conduction band of Ge quantum wells (QWs) produced with Ge-rich GeSi quantum structures. We will demonstrate 3 key components of the platform: integrated source, low loss waveguides and integrated MEMS optics. In particular, the innovative quantum cascade laser will leverage on the non-polar nature of Si and Ge crystal lattices to potentially enable room-temperature operation and will emit > 1 mW power in the 1-10 THz range. Unfortunately, there is a limited precision available as to the knowledge of the material parameters necessary for the design of the devices. This is one of the main problems for the development of this kind of devices, since the design relies on a delicate balance of several resonant tunneling processes for carrier injection and extraction. Therefore, the results obtained in the first year of the project on the coupling of states in adjacent quantum wells as well as the development of a non-equilibrium Green function code for the design and the simulation of Ge/SiGe devices, are important steps towards the realization of Ge/SiGe THz quantum cascade lasers and may be interesting for further devices in this materials system relying on resonant tunneling.
The proposed technology would be a game changer for many of the proposed and already developed THz applications where the potential mass market requires sources at the â‚¬100-â‚¬1k price level, to be benchmarked against the typical â‚¬50k-100k now available on the market. The cheap and practical THz platform proposed could enable the wide-scale deployment of THz systems for healthcare (e.g. oncology imaging, for early stage detection), security imaging (explosive detection), THz bandwidth telecoms, non-destructive production monitoring, and astronomy.