Optical and electrical properties of highly strained silicon towards extended infrared photodetection
(2025)
Files
thesis_nicolas_ROISIN.pdf
Open Access - Adobe PDF
- 21.2 MB
Details
- Authors
- Supervisors
- Abstract
- Silicon photodetectors are commonly used for visible and near-infrared photodetection. They are relatively easy to manufacture and can be produced in large quantities at relatively low cost, making them well suited for use in a wide range of devices. However, the ability of silicon-based photodetectors to detect infrared radiation beyond 1.1 μm is limited by their indirect band gap of 1.12 eV. Strain engineering has been proven to reduce the band gap of semiconductors, but its application to improve optoelectronic performance has been lacking. In this work, a thorough study of the optical and electrical properties of highly strained silicon is proposed to evaluate its potential for infrared photodetectors. First-principles calculations using density functional theory is performed to obtain the straininduced changes in the silicon band structure for all major strain configurations. Uniaxial strain along the [110] direction is found to be promising for reducing the band gap, with a reduction of about 200 meV at 2%-strain, while being compatible with high-strain fabrication schemes. The band gap reduction is experimentally validated by photoluminescence measurements on silicon beams strained up to 1% with silicon nitride actuators. In the second chapter, the variations in electron mobility in n-type silicon resistors have first been studied experimentally for uniaxial strain along [001] using a four-point bending scheme. The experimental results highlight the important increase in the longitudinal resistivity, with a 50% increase at 1%-strain, while showing the limitations of existing theories to predict the mobility of strained silicon. First-principles calculations are performed to retrieve the mobilities of strained silicon by solving the Boltzmann transport equation, considering electron-phonon interactions. The method is used for [001] and [110] uniaxial strains. The theoretical calculations show the limitation of existing theories for the electron mobility while calculating, for the first time with this level of theory, the hole mobility of strained silicon. A significant increase in hole mobility up to 1340 cm2V−1s−1 and 1761 cm2V−1s−1 as been found for uniaxial strain of 2% along [001] and [110], respectively. The electron mobility shows an increase up to 2200 cm2V−1s−1 for uniaxial strain along [001] of 2%, but is negatively impacted by [110] uniaxial strain with a longitudinal and transverse mobilities of 1221 cm2V−1s−1 and 168 cm2V−1s−1, respectively. The Bethe-Salpeter equation and the EPW code is used to predict fully theoretically the complex refractive index and absorption coefficient of highly strained silicon, taking into account the electron-phonon interactions, without the need for a semi-empirical model containing experimental biases. The results for the absorption coefficient show a significant increase with [110] uniaxial strain, following the band gap reduction. As the band gap is reduced to 0.66 eV at 4%-strain, the wavelength of light that can be absorbed is extended to 1.88 μm. The absorption coefficient calculated theoretically at 1.2 μm goes from 1.5 × 10−2 cm−1 to 168 cm−1 at 4%-strain. The absorption results are compared with the absorption coefficient of conventional infrared materials such as Ge, SiGe and InGaAs, showing the potential of strained silicon to compete with them. Finally, the first-principles results are used in combination with technology computer aided design (TCAD) simulation to investigate the potential of strained silicon for infrared photodetection, showing equivalent performances to standard infrared materials but with the necessity of achieving high levels of strain. The increase of the absorption coefficient for [110] uniaxial strains is found to directly enhance the infrared sensitivity of the device, while the mobility variations impact the time response of the detector. A sensitivity of 0.8 AW−1 at 1400 nm is finally predicted theoretically for a 100μm-long photodiode strained up to 4%. This work demonstrates for the first time the potential of using strained silicon for near and shortwave infrared applications, although the fabrication of such devices remains a challenge.
- Affiliations
Citations
Roisin, N. (2025). Optical and electrical properties of highly strained silicon towards extended infrared photodetection. https://hdl.handle.net/2078.5/241706