University of Illinois Urbana-Champaign

Light emission and current instabilities in transistor-injected quantum cascade structures

Kumar, Raman

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https://hdl.handle.net/2142/121345

Description

Title
Light emission and current instabilities in transistor-injected quantum cascade structures
Author(s)
Kumar, Raman
Issue Date
2023-07-14
Director of Research (if dissertation) or Advisor (if thesis)
Dallesasse, John Michael
Doctoral Committee Chair(s)
Dallesasse, John Michael
Committee Member(s)
  • Leburton, Jean-Pierre
  • Eden, James Gary
  • Bogdanov, Simeon
Department of Study
Electrical & Computer Eng
Discipline
Electrical & Computer Engr
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
  • Quantum Cascade Lasers
  • Heterojunction Bipolar Transistor
  • Semiconductor Device Physics
Abstract
Quantum cascade lasers (QCLs) have emerged as an important coherent light source in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) region of the electromagnetic spectrum. The applications range from molecular spectroscopy to defense countermeasures. The basic device structure of a typical QCL consists of a superlattice sandwiched between two heavily doped n-type contacts. The light emission from such a structure is a result of intersubband transitions of electrons between quantized electronic states in the conduction band. An appropriate application of electric field is necessary to align the quantum states inside the structure that establish the emission wavelength, which is therefore directly related to the applied voltage across the device terminals. The two-terminal nature of the device further causes the applied voltage and injected current to be inherently coupled. Therefore, the output of a conventional QCL cannot be scaled without changing the emission wavelength which leads to undesirable spectral instability. The transistor-injected quantum cascade laser (TI-QCL) is a three-terminal device that seeks to overcome the limitations of a conventional QCL by placing the active quantum cascade region in the reverse-biased base-collector junction of an n-p-n heterojunction bipolar transistor. The current injection is independently controlled by forward-biased base-emitter junction and the applied reverse bias on the base-collector junction sets up the quantum state alignment in the quantum cascade region and therefore the emission wavelength. This makes the TI-QCL a versatile coherent MWIR light source where the wavelength can be tuned by varying the reverse bias on the base-collector junction and output power can be independently scaled by injecting current from the forward-biased base-emitter junction. In this work, electronic and optical properties of an InP-based transistor-injected quantum cascade structure are described. The current-voltage characteristics at cryogenic temperatures are shown to exhibit bistability, negative differential resistance and collector current oscillations which are related to current transport through the superlattice. NIR spontaneous emission due to band-to-band recombination in the base region and associated bistable switching is experimentally demonstrated. The spontaneous emission spectrum is measured using Fourier Transform Infrared (FTIR) spectroscopy. Coherent near-infrared light emission from forward-biased BC junction is experimentally demonstrated with blue shift in spectrum at higher injection currents which is attributable to increase in separation of energy levels with the accumulation of charge in the quantum impedance matching (QIMA) region. Preliminary results on spontaneous MWIR light emission results are also presented. The current bistability is physically interpreted using a quantitative self-consistent quantum mechanical model. The QIMA region is modeled as a half-harmonic oscillator. The accumulated charge in the QIMA is shown to induce an electric field that dynamically aligns the ground of the harmonic oscillator with superlattice resonant levels. This dynamic alignment/misalignment is quantitively shown to give rise to bistability which can find potential applications in ultra-sensitive detectors.
Graduation Semester
2023-08
Type of Resource
Thesis
Copyright and License Information
Copyright 2023 Raman Kumar

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