INtegrable Thz si-basEd quaNtum caScade opEration (INTENSE)
Description
High-frequency microwave and THz radiation are increasingly used in medical diagnostics, material inspection, security, and next-generation 6G telecommunications. However, the “THz gap” persists due to the lack of cost-effective, compact, and high-performance sources and detectors. Existing THz emitters remain too bulky and expensive for widespread adoption.
This project aims to develop a semiconductor-based THz laser using CMOS-compatible, non-toxic Ge-rich SiGe heterostructures. The device, based on a quantum cascade laser (QCL) architecture, will feature multiple Ge/SiGe quantum wells (QWs) coupled by ultra-thin tunneling barriers. The non-polar nature of SiGe could enable room-temperature, broadband operation (1–10 THz), covering spectral regions inaccessible to III-V materials due to the Reststrahlen band (5–14 THz).
The consortium includes experts in SiGe quantum structures, heteroepitaxial growth, and microfabrication. Recent breakthroughs in THz electroluminescence from Ge/SiGe QCL structures, combined with advanced modeling and fabrication techniques, position the team to demonstrate lasing.
By integrating THz emitters with mainstream electronics, this project could enable compact, cost-effective, and scalable THz systems, fostering new applications in existing and emerging fields.
Aims
The goal of the project is to develop a compact, cost-effective THz light source by demonstrating lasing from a silicon-based QCL at room temperature (RT) in the 1-10 THz range. Such a breakthrough would enable affordable, integrable THz sources for applications in healthcare, security, and high-bandwidth telecoms.
The active layer will use n-type Ge/SiGe heterostructures, leveraging their non-polar lattice to achieve RT operation with long subband lifetimes. Compatibility with silicon photonics will be ensured through deposition on Si(001) or SOI substrates. Despite progress in Ge/SiGe THz electroluminescence, key challenges remain, including mitigating heteroepitaxial strain and controlling atomic-scale composition for ultra-thin tunneling barriers. Thermal strain must also be managed to prevent structural degradation during cooling.
Addressing these challenges is crucial not only for demonstrating a Ge/SiGe QCL but also for advancing quantum devices in photonics, quantum computing, and microelectronics. Intermediate goals include growing high-quality Ge-rich SiGe heterostructures with low defect densities and developing low-loss THz waveguides, with potential applications in lab-on-chip biosensing.
Expected Results
- Fabrication of thick Ge-rich SiGe heterostructures on Si and SOI substrates with a low threading dislocation density (TDD) around 10^6 cm⁻². (completed)
- Development of a 10-micron-thick, strain-compensated QCL structure, designed for expected gain in the THz range at RT greater than 20 cm⁻¹, with a TDD of approximately 10^6 cm⁻². (completed)
- Demonstration of THz electroluminescence from the above-mentioned QCL structures.
- Creation of low-loss waveguides (<20 cm⁻¹) in the THz range at room temperature.
- Successful demonstration of THz lasing.
State of the art
Unlike “conventional” semiconductor lasers, QCLs are unipolar devices. This means that either electrons or holes are involved in the photon generation. In fact, the stimulated emission process occurs through electron (hole) transitions between quantized states in the conduction (valence) band, known as subbands. The optically active material of QCL comprises a stack of hundreds of identical periods. Each consists of different quantum wells coupled by tunnel barriers, obtained by a controlled variation, at the nanometer scale, of the multi-layer composition. By applying a suitable bias, an energy slope is induced in the band edge profile and the confined subbands of each individual period align, in a way that an electric current can flow through the structure by means of subsequent resonant tunneling with radiative transitions occurring in each module of the periodic structure. Therefore, the light emission efficiency of QCLs per injected carrier can outperform diode lasers by a factor of ×103. QCLs were first demonstrated in 1994 in the Mid-IR and in 2002 in the THz and, since then, have mainly been manufactured utilizing III-V semiconductors. However, the polar nature of these materials limits their use in the THz range. In fact, due to strong phonon interaction, it is not possible to have emission in the range between 5 and 10 THz (Reststrahlen band) and it is necessary to cool the device to achieve lasing below 5 THz. Moreover, at the record temperature of 250 K the required large current density and dissipated power still prevent integration into more complex photonic structures. To address the limitations of current THz QCLs, the project suggests using the non-polar, CMOS-compatible SiGe material system to develop cost-effective QCLs that can operate at room temperature across the THz range. N-type Ge/SiGe multi-quantum wells on Si substrates are considered the most promising architecture for Si-based QCLs, though previous models overlooked practical challenges like lattice mismatch and thermal expansion differences between Si and Ge. These issues require careful strain management and balancing in the active layer. Therefore the growth of thick Ge-rich SiGe quantum cascade structures on Si remains a significant challenge.
Riferimento: PRIN 2022 PNRR – Codice progetto: P2022EE94T – CUP: F53D23010400001
Investimento totale del progetto: 227915 euro
Partner/proponente: Università di Pisa (Dipartimento di Fisica) (PI)
Coordinatore dell’UdR Università degli Studi Roma Tre: Monica De Seta.
