The progress towards terahertz quantum cascade lasers on silicon substrates

Abstract

A review is presented of work over the last 10 years which has been aimed at trying to produce a Si-based THz quantum cascade laser. Potential THz applications and present THz sources will be briefly discussed before the materials issues with the Si/SiGe system is discussed. Waveguide designs and waveguide losses will be presented. Experimental measurements of the non-radiative lifetimes for intersubband transitions in Si Ge quantum wells will be presented along with theory explaining the important scattering mechanisms which determine the lifetimes. Examples of p-type Si/SiGe quantum cascade designs with the experimental electroluminescence will be reviewed and examples of n-type Si-based designs will be presented. In the conclusion designs and structures will be discussed with the greatest potential to achieve an electrically pumped Si-based THz laser.

Mid-infrared intersubband absorption from p-Ge quantum wells grown on Si substrates

“…This has been due to the difficulty in growing the high quality Ge rich superlattices on Si, due to the large lattice mismatch of 4.2% between Si and Ge. 8 Instead of using the !5 lm thick graded buffers, it is now possible to grow the high quality pure Ge QWs on a thin ($1 lm) SiGe strain relaxed virtual substrate 9,10 using techniques such as low energy plasma enhanced chemical vapor deposition (LEPECVD). The benefit of using p-Ge QWs over p-Si 1Àx Ge x QWs for x < 0.85 is that it will provide larger absorption coefficients, due to the smaller effective masses for both the heavy hole (HH) and light hole (LH) bands.…”

“…In addition, whilst conduction band designs allow only the TM (z)-polarized intersubband transitions, the valence band allows both TE (x-y) and TM polarizations to be observed. 8 This arises due to non-parabolicity effects and therefore strong coupling of the bands, which relaxes parabolic selection rules. 8,11,12 Hence, both surface normal and waveguide geometry devices can be realized.…”

“…8 This arises due to non-parabolicity effects and therefore strong coupling of the bands, which relaxes parabolic selection rules. 8,11,12 Hence, both surface normal and waveguide geometry devices can be realized. In this paper, surface-normal p-type Ge QW intersubband absorption is investigated for a range of QW widths designed by 8-band k.p theory 13 and analyzed using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Raman spectroscopy, and scanning transmission electron microscopy (STEM).…”

Silver antimony Ohmic contacts to moderately doped n-type germanium

“…6 Ge has also been used for spintronic devices 7 along with thermoelectrics 8 and there have been proposals for Ge/SiGe quantum cascade lasers. 9 Poor electrical contacts to n-Ge are a potential roadblock to the majority of these applications. Low resistive Ohmic contacts to n-Ge are difficult to achieve because of strong Fermi level pinning at the charge neutral level (0.08-0.09 eV), 10,11 just above the valence band (Fig.…”

A study of the impact of dislocations on the thermoelectric properties of quantum wells in the Si/SiGe materials system

“…5 Thus, strained balanced quantum well structures must be utilised, as all practical layer thicknesses for a complete thermoelectric generator are significantly greater than the critical thickness of the heterolayers. 6 Although, some/many aspects of the thermal and electrical transport maybe considered to be independent of the exact fabrication process implemented, two important scattering mechanisms which can depend on the exact nature of the quality of the epitaxial growth are interface roughness and dislocation scattering, these two mechanisms are related to some degree. 7,8 roughness scattering on thermal conductivity has been studied previously, 9,10 although little has been done in the context of thermoelectrics directly where it is important to consider both electrical and thermal transport.…”