Direct Nanoscale Imaging of Evolving Electric Field Domains in Quantum Structures

Abstract

The external performance of quantum optoelectronic devices is governed by the spatial profiles of electrons and potentials within the active regions of these devices. For example, in quantum cascade lasers (QCLs), the electric field domain (EFD) hypothesis posits that the potential distribution might be simultaneously spatially nonuniform and temporally unstable. Unfortunately, there exists no prior means of probing the inner potential profile directly. Here we report the nanoscale measured electric potential distribution inside operating QCLs by using scanning voltage microscopy at a cryogenic temperature. We prove that, per the EFD hypothesis, the multi-quantum-well active region is indeed divided into multiple sections having distinctly different electric fields. The electric field across these serially-stacked quantum cascade modules does not continuously increase in proportion to gradual increases in the applied device bias, but rather hops between discrete values that are related to tunneling resonances. We also report the evolution of EFDs, finding that an incremental change in device bias leads to a hopping-style shift in the EFD boundary – the higher electric field domain expands at least one module each step at the expense of the lower field domain within the active region.

Ignition of quantum cascade lasers in a state of oscillating electric field domains

“…Thus, avoiding NDC is a common design paradigm of the related, technologically extremely successful Quantum Cascade Laser (QCL) [10,11], as already pointed out in the precursory work [12]. While NDC and the formation of stationary domains have been observed and analyzed in some QCL structures [13][14][15][16], they are commonly considered to impede the desired lasing action.…”

Implementation of non-uniform mesh in non-equilibrium Green’s function simulations of quantum cascade lasers

“…For the medium-value fields, a plateau rather than oscillatory behavior is observed. This discrepancy is most probably caused by the evolving electric field domains [22]. In structures like the THz QCL, such domains are very likely to form due to the incomplete relaxation of carrier energy within one module [4].…”

Monte Carlo modeling applied to studies of quantum cascade lasers

“…Even if one assumes that the sequence of material layers repeats identically, the periodicity may be broken when the device is under bias, with possible creation of space charge regions. This phenomenon is known to have a profound influence on device behavior (Dhar et al 2014). The propagation of such domains and the resulting unstable behavior is often mentioned to explain why a QCL device, in some regime, does not work as expected.…”