Growth variation effects in SiGe-based quantum cascade lasers

Abstract

Abstract. Epitaxial growth of SiGe quantum cascade (QC) lasers has thus far proved difficult, and nonabrupt Ge profiles are known to exist. We model the resulting barrier degradation by simulating annealing in pairs of quantum wells (QWs). Using a semiclassical charge transport model, we calculate the changes in scattering rates and transition energy between the lowest pair of subbands.We compare results for each of the possible material configurations for SiGe QC lasers. The effects are most severe in n-type (001) Si-rich systems due to the large effective electron mass, and in p-type systems due to the coexistence of light-and heavy-holes.The lower effective mass and conduction band offset of (111) oriented systems minimises transition energy variation, and a large interdiffusion length (L d = 1.49 nm) is tolerated with respect to scattering rate. Ge-rich systems are shown to give the best tolerance with respect to subband separation (L d = 3.31 nm), due also to their low effective mass.

The progress towards terahertz quantum cascade lasers on silicon substrates

“…This system has a far lower effective mass of about which is crucial to not only improving the gain through higher wavefunction overlap and matrix elements, but also as lower effective masses improve the tolerance of subband alignment to growth inaccuracies [108]. Indeed Valavanis et al [108] have used the diffusion profiles measured by TEM [122] to provide more accurate modelling of the subband positions due to rounding of the heterostructure barrier profiles from Ge segregation and diffusion. A number of designs have been suggested with Ge quantum wells with gain well above the waveguide losses not only at 4 K but also at room temperature.…”

“…This was due to the growth not being accurate enough to provide alignment between the subband states in the injector at any electric field. The effect of growth variations has been studied by Valavanis who has shown that for such LH to HH structures to align, the heterolayer thickness requires to be within 1 monolayer and the Ge content must be accurate to within better than 1 part in 10 3 [108]. The present growth tools are not yet capable of meeting these tolerance requirements unless a significant number of calibration samples are used to accurately set the growth parameters.…”

“…For miniband formation, barriers of only a few monolayers would be required which is beyond the present growth technology. In addition, Valavanis et al [108] has shown that the higher the , the less tolerant is the material to growth inaccuracies. Low is therefore key not just for high tunnel currents and miniband formation but also to make the material more tolerant to growth inaccuracies.…”

Extended density-matrix model applied to silicon-based terahertz quantum cascade lasers

“…44,45 This leads to significant changes in the bandstructure and scattering lifetimes, 46,47 and can therefore degrade the performance of devices. In this section, we investigate the impact of interdiffusion on the QCL gain and we attempt to recover the lost performance through design optimization.…”

Si/SiGe quantum cascade superlattice designs for terahertz emission

“…Clearly a move to electron based QCL designs which could use the more parabolic conduction bands would not only make the characterization of spontaneous emission easier but should also show significantly higher gain mainly through the lighter m ‫ء‬ , according to a number of proposals. [36][37][38] Further work has also determined that lighter m ‫ء‬ systems such as n-type Ge quantum well QCLs have more tolerance to growth fluctuations and inaccuracies 38 and a number of multiple Ge quantum well heterostructures have now been demonstrated experimentally. [39][40][41][42][43] The present designs have also suffered by the upper radiative state not being inside the energy range of the miniband thereby reducing the coupling between the injector and the upper radiative state.…”