Far-infrared Lasers Assembled using Silicon Heterostructures

Periodic Reporting for period 2 - FLASH (Far-infrared Lasers Assembled using Silicon Heterostructures)

Période du rapport: 2018-11-01 au 2021-07-31

The Terahertz (THz) region is the range of the electromagnetic spectrum comprised in between the microwaves and the infrared, with frequencies extending from 0.3 to 10 THz (i.e. wavelength of 1 mm to 30 µm). The interest for THz light is driven by a variety of possible technological applications. One example is that THz light can be used for security screening, e.g. to detect concealed weapons or plastic explosive, since it penetrates several nonconducting materials (clothing, paper, wood, plastic) whereas it is absorbed by metals. In addition, many substances of interest for security and defense exhibit THz spectral fingerprints.
In the healthcare domain, THz light could enable new medical imaging capabilities, including oncology for epithelial cancer detection and wound inspection through bandages, since its propagation in living tissues is highly sensitive to the water content of cells. THz radiation has photon energies many orders of magnitude below x-rays but care has to be taken that electric fields are maintained below threshold levels to prevent damage to DNA and other biological materials. Below such thresholds, THz radiation has been shown to be safe to use on epithelium tissue and ophthalmology applications.
Furthermore, THz radiation has potential in the short range/high-bandwidth telecommunications field, since the Earth's atmosphere has sufficient transparency in selected spectral windows especially below 0.5 THz.
An integrated THz technology (comprising emitters, detectors and I/O electronics), which is essential to achieve the required performances and reduce the device costs for the above-mentioned mass-market applications, is still missing. The development of a Si-based THz technology, nonetheless, could ultimately respond to these market requirements, since Si-based microelectronics accounts for >98% of the semiconductor global market, granting mass-production capabilities and low prices at volume.
In this perspective, the realization of an electrically pumped Si-based THz laser would be a long sought-after scientific and technological breakthrough.
By addressing significant material science challenges with innovative experimental and theoretical approaches, the FLASH project targeted the demonstration of a cost-effective and compact THz quantum cascade laser (QCL) integrated on Si. QCLs are unipolar devices exploiting carrier transitions between subband states in or between quantum wells (QWs) to generate population inversion and lasing. Lasing in the THz region can potentially be achieved by means of suitable band-structure engineering of selected semiconductor heterostructures. While QCLs based on III-V materials have already been demonstrated and are commercially available, a Si-based THz QCL is still elusive. FLASH’s team proposed to leverage on the electronic transitions occurring between L-valley energy subbands formed in the conduction band of Ge QWs featuring Ge-rich GeSi barriers to achieve this ambitious goal and demonstrate an industrial-viable technological THz platform comprising a Si-based THz source, low-loss waveguides, and integrated MEMS optics.

The main challenges in the development of the proposed Si-based QCL are related to the strain due to the lattice and thermal mismatch existing between Si and Ge lattices, the dopant (P atoms in our case) segregation, and the parasitic conduction band valleys (mostly valleys). These difficulties, common to other Ge/SiGe heterostructures, are greatly magnified in QCL structures due to their large thickness, made of hundreds of identical modules comprising of tens of different nanometric layers. In fact, since the QCL gain is proportional to the number of the cascade periods and owing to the laser modal size, the optimal THz QCL thickness lies at and above the 10 µm range. Consequently, the heteroepitaxial strain management and the growth reproducibility are of the greatest importance. The FLASH consortium managed to achieve an unmatched degree of control over the growth obtaining QCL active layer structures with a limited density of defects (threading dislocation density <2×106 cm-2), record-low interface roughness (<0.2 nm) and abrupt interfaces (1 nm). The control over the growth rate was reached for the entire extension of about 10 µm thick active layer regions as demonstrated in Fig. 1a, with module-to-module thickness fluctuations kept below 3%. These successes lead to the demonstration, for the first time, of electroluminescence at 3.4 and 4.9 THz from electrically pumped Ge/SiGe heterostructure LEDs (Fig. 1 b,c).
The modeling accuracy demonstrated in FLASH 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. In particular, we developed a simulation code based on the non-equilibrium Green’s function (NEGF) formalism that accounts for the material and structural parameters derived by the insight experimental characterization of the samples. The estimated gain values are on-par or even higher than what was predicted and measured in analogous III-V systems in a wide range of temperatures. More importantly, the simulated gain values are higher than the predicted losses in the double-metal (Cu) waveguides we planned to use as a laser cavity (see Fig. 2), even at room temperature. Consequently, we have developed the high-quality microfabrication processes for demonstrating the first fabricated double-metal waveguides in this material system. The subsequent step will see the final integration of our high-quality QCL active regions in the developed double metal waveguides to finalize the laser realization.
The relevance of these results is evidenced by their wide dissemination through the publication of 20 peer-reviewed articles and 39 oral presentations at important conferences of the field. Moreover, a spin-off company (nextnano Lab SAS, France) has been founded to further develop the nextnano.NEGF software that was used throughout the project.

During the FLASH project, we improved the quality of the deposited Ge/SiGe heterostructure on Si to a high degree and, thanks to their thorough characterization, we established the material parameters impacting the performance of optoelectronic devices based on this material system. In turns, this allowed us to refine and make more reliable the simulation tools developed within the project, a fundamental step for accurate QCL design and modelling. The progress made in FLASH is of paramount importance for the realization of Ge/SiGe THz quantum cascade lasers but also certificates the potentiality of this material system for the development of a THz photonic platform on Si. This cheap and practical THz platform would be a game changer for many of the proposed and already developed THz applications where the potential mass market requires sources at the €100-€1 price level, to be benchmarked against the typical €50k-100k now available on the market. The wide-scale deployment of THz systems has the potential to improve the quality of life of European and international citizens through improved healthcare and security. Additional applications of great social impact include high-bandwidth telecommunications, proteomics, drug development, and non-destructive production monitoring.

Figure 1: (a) STEM image at different magnifications of the QCL active region. (b)Schematics of the

Figure 2. (a) Conduction band-edge profile and carrier density of the designed Ge/SiGe QCL (b): pr

Periodic Reporting for period 2 - FLASH (Far-infrared Lasers Assembled using Silicon Heterostructures)

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