title: 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.
authors.affiliation: Department of Electrical and Computer Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Ave. West, Waterloo, N2L3G1, Ontario, Canada.
authors.forename: Rudra Sankar
authors.initials: RS
authors.lastname: Dhar
authors.affiliation: Department of Electrical and Computer Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Ave. West, Waterloo, N2L3G1, Ontario, Canada.
authors.forename: Seyed Ghasem
authors.initials: SG
authors.lastname: Razavipour
authors.affiliation: National Research Council, Bldg. M-50, 1200 Montreal Rd, Ottawa, Ontario K1A0R6, Canada.
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Dupont
authors.affiliation: Department of Electrical and Computer Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Ave. West, Waterloo, N2L3G1, Ontario, Canada.
authors.forename: Chao
authors.initials: C
authors.lastname: Xu
authors.affiliation: National Research Council, Bldg. M-50, 1200 Montreal Rd, Ottawa, Ontario K1A0R6, Canada.
authors.forename: Sylvain
authors.initials: S
authors.lastname: Laframboise
authors.affiliation: Department of Electrical and Computer Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Ave. West, Waterloo, N2L3G1, Ontario, Canada.
authors.forename: Zbig
authors.initials: Z
authors.lastname: Wasilewski
authors.affiliation: Department of Electrical Engineering and Computer Science, Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, USA.
authors.forename: Qing
authors.initials: Q
authors.lastname: Hu
authors.affiliation: Department of Electrical and Computer Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Ave. West, Waterloo, N2L3G1, Ontario, Canada.
authors.forename: Dayan
authors.initials: D
authors.lastname: Ban