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Nano physics

The term 'Nanophysics' defines the physics of mechanical, electric or optical systems of which the dimensions are on the nanometer scale. The group of Prof. Bimberg focuses its work on the electro-optical properties of epitactically grown semiconductor nanostructures. Some representatives of these nanostructures are quantum wells, quantum wires, and quantum dots. According to their names, their dimensions are below the de Broglie wavelength in one, two or three spacial directions, which leads to an energy quantization in the respective directions. In the following one representative will be presented, which currently is of the greatest significance as far as opto-electronical applications and fundamental research are concerned - the quantum dots (Fig. 1). Due to full energy quantization (Fig. 2) (i.e. no bandstructure with E = f(k)) they resemble atoms in a dielectric matrix.

Semiconductor quantum dots

Fig. 1: Three stacked InAs (semiconductor 1) quantum dots in a GaAs (semiconductor 2) matrix. The base length of the quantum dots is 20 nm (high-resolution cross-sectional scanning tunneling microscopy (XSTM)).

are formed by a nanometer-sized embedding of a semiconductor 1 in another semicondutor 2 with a greater band gap.

The main point here is that the electronic and optical properties are now a function of the quantum dot structure. By adjusting the size, the shape, and the material of the quantum dot it is thus possible to design the energy levels and the overlap of the electron and hole wave functions - the perfect construction kit for opto-electronical applications.

Fig. 2: Diagram of a quantum dot energy scheme, e.g. for InAs in GaAs.

Practical applications are found in the control of the emission wavelength of the emitted photons in laser structures, large localisation energies e.g. of holes for memories, and the optimization of the electron-hole-wavefunction overlap for large oscillator strength. Another interesting and current application are single photon emitters for the secure transmission of cryptographical keys. Here, the use of quantum dots allows the generation of single linear polarized photons, or even of entangled photon pairs at optimal quantum efficiency and high emission rate.

Besides the already presented Type-I quantum dot, which forms a quantum well for electron and holes, there are also Type-II quantum dots, in which only one of the two charge carriers is localized. One representative is GaSb in GaAs. Here only holes are confined in the quantum dot. This is a promising material system for memory applications, for due to the lacking electrons no e-h recombinations take place and the information (the holes) can be stored for a long period of time.

Nanophysics research in the group of Prof. Bimberg

  • Investigation of semiconductor quantum dots by means of optical and eletrical methods
  • Single dot spectroscopy and ensemble spectroscopy
  • Modeling of the optical/electronic properties of semiconductor quantum dots

Optical investigation of quantum dot ensembles:

  • Methods: PL, PLE
  • Material systems: In(Ga)As/Ga(Al)As, InP/GaP, GaSb/Ga(Al)As, In(Ga)N/Ga(Al)N
  • Energy levels and recombination channels
  • Dependency of electronic/optical properties on structural parameters
  • Lateral carrier transport and redistribution processes
  • Recombination dynamics
  • Interaction with phonons
  • Spectral hole burning and spin storage

Optical investigation of single quantum dots:

  • Methods: CL, µ-PL, µ-PLE (possibly with the help of mesas and shadow masks)
  • Material systems: In(Ga)As/Ga(Al)As, InP/GaP, GaSb/Ga(Al)As, In(Ga)N/Ga(Al)N
  • Excitonic complexes: bond energy, exchange interaction, polarization characteristics
  • Dependency of the excitonic complexes on structural parameters
  • Optimization for applications, such as single photon emitters
  • Correlation measurements
  • µ-cavity structures
  • Phonon interaction, dephasing processes

Electrical investigation of quantum dots:

  • Methods: IV, CV, DLTS (electrical, optical), photo current
  • Material systems: In(Ga)As/Ga(Al)As, GaSb/Ga(Al)As
  • Storage mechanisms, retention times, memory concepts
  • Transport properties
  • Charge carrier capture and emission processes
  • Temperature and electric field dependent effects
  • Material characterization (doping profiles, carrier mobilities)

Modeling of optical and electronic properties of single quantum dots and quantum dot ensembles:

  • Methods: 8-band k.p theory, tight-binding theory, configuration-interaction method
  • Material systems: In(Ga)As/Ga(Al)As, InP/GaP, In(Ga)N/Ga(Al)N, GaSb/GaAs
  • Single and coupled quantum dots
  • Single particle energy levels, excitonic complexes, oscillator strengths, polarization behavior, exchange interaction

Further literature:

  • Bimberg, D., M. Grundmann, et al. (1999). Quantum dot heterostructures. Chichester, [Eng.]; New York, John Wiley.
  • Grundmann, M. (2002). Nano-optoelectronics : concepts, physics, and devices. Berlin ; New York, Springer.

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