Terahertz Ionization of Highly Charged Quantum Posts in a Perforated Electron Gas

• Published in: Nano letters 2012-03, Vol.12 (3), p.1115-1120
• Main Authors: Morris, Christopher M, Stehr, Dominik, Kim, Hyochul, Truong, Tuan-Anh, Pryor, Craig, Petroff, Pierre M, Sherwin, Mark S
• Format: Article
• Language: English
• Subjects: Absorption; Amplitudes; Arsenicals - chemistry; Charging; Computer Simulation; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Electron gas; Electrons; Exact sciences and technology; Fullerenes and related materials; Gases - chemistry; Indium - chemistry; Low-dimensional structures (superlattices, quantum well structures, multilayers): structure, and nonelectronic properties; Materials science; Mathematical analysis; Models, Chemical; Nanocrystalline materials; Nanoscale materials and structures: fabrication and characterization; Nanostructure; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Perforating; Physics; Quantum Dots; Quantum wells; Semiconductors; Static Electricity; Strain; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Terahertz Radiation; Visible and ultraviolet spectra
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl1044154

“Quantum posts” are roughly cylindrical semiconductor nanostructures that are embedded in an energetically shallower “matrix” quantum well of comparable thickness. We report measurements of voltage-controlled charging and terahertz absorption of 30 nm thick InGaAs quantum wells and posts. Under flat-band (zero-electric field) conditions, the quantum posts each contain approximately six electrons, and an additional ∼2.4 × 1011 cm−2 electrons populate the quantum well matrix. In this regime, absorption spectra show peaks at 3.5 and 4.8 THz (14 and 19 meV) whose relative amplitude depends strongly on temperature. These peaks are assigned to intersubband transitions of electrons in the quantum well matrix. A third, broader feature has a temperature-independent amplitude and is assigned to an absorption involving quantum posts. Eight-band k·p calculations incorporating the effects of strain and Coulomb repulsion predict that the electrons in the posts strongly repel the electrons in the quantum well matrix, “perforating” the electron gas. The strongest calculated transition, which has a frequency close to the center of the quantum post related absorption at 5 THz (20 meV), is an ionizing transition from a filled state to a quasi-bound state that can easily scatter to empty states in the quantum well matrix.