On the Possibility of an Intersubband Laser in Silicon-on-Insulator

Abstract

As thin Si quantum wells with oxide barriers have become an experimental reality in silicon-on-insulator technology, we discuss the feasibility of a terahertz laser based on an intersubband transition in such a Si quantum well. Electrons tunnel into an upper twodimensional subband from a thin polysilicon gate through an ultrathin tunneling oxide and are extracted laterally from the Si well by diffusion. Population inversion arises because lateral diffusion to the contacts can be a faster process than intersubband relaxation and also because the in-plane diffusivity in the upper subband is suppressed by the interaction with a nearby subband characterized by a heavy in-plane mass. Thick oxide layers provide optical confinement and the main optical loss mechanism is the weak free-carrier absorption in thin doped regions in the polysilicon gate and substrate. A voltage on the substrate may provide some tunability of the gain peak.

Nonclassical devices in SOI: Genuine or copyright from III–V

“…As long as the energy separation (E 2 À E 1 ) lies below the Si optical phonon energy, nonradiative relaxation to E 1 is suppressed. Assuming uniform injection from the gate, the diffusion equation in the QW can be solved, leading to carrier densities n 2 and n 1 for the upper and lower subband, respectively [37]. Defining L D (D 2 s) 1/2 as the diffusion length in the upper subband with respect to relaxation to E 1 and setting L G = 2L symmetrically about the x = 0 midpoint of the gate, one obtains in the limit of L D > L:…”

“…Estimating the energy broadening in the Si QW to be $5 meV, corresponding to monolayer fluctuations in t Si and assuming strong population inversion (n 2 À n 1 ) % n 2 as predicted by Eq. (1) for L D < L, we obtain a gain a $ 350(n 2 /10 11 ) cm À1 [37]. This gain must overcome the optical losses due to free-carrier absorption in the polySi emitter, in the narrow doped region under the buried oxide, and in the lateral contacts, as well as possible absorption in the SiO 2 cladding layers [40].…”