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Nano materials

Our research consists of the investigation of growth and optimization of semiconductor nanostructures, especially quantum dots, for optoelectronic application and fundamental research. Since the lateral size of such nanostructures is in the nanometer range, the fabrication is not trivial. The current methods can be divided into three groups:

  • Wet-chemical methods for the precipitation of colloidal quantum dots (Figure 1A)
  • Growth of higher dimensional structures (e.g. layers) and the following etching methods which apply high resolution lithography (Figure 1B)
  • Self-organized growth using epitaxial facilities (Figure 1C-E)

In our group, the self-organized epitaxial growth of nanostructures is studied. Self-organization describes the process of depositing a semiconductor 1 in a certain amount on a semiconductor 2 leading to a formation of coherent 3-dimensional nanostructures, (quantum dots). The formation of these structures is a consequence of the minimization of the total energy of the system. Structural properties of quantum dots such as shape, size, composition and areal density can be controlled by a variation of growth parameters.

Abb. 1: Quantum dot structures made with different fabrikation processes
Lupe

Also, it is to be noted that there is a difference between sole 3-dimensional growth and the decomposition processes of a (ternary) layer. In the self-organized, sole growth, there are 2 distinct growth modes. The Stranski-Krastanow growth mode utilizes a wetting layer for the formation of the quantum dots. This wetting layer is usually 1 monolayer thick and consists of the same material as the quantum dots. In the Volmer-Weber growth mode, the quantum dots grow directly on the substrate without any wetting layer. The 3-dimensional growth results in a lower strain energy compared to a complete layer.

For some ternary materials such as InGaN on GaN, quantum dots are not formed as free-standing 3-dimensional objects but they emerge from decomposition effects. InGaN deposited on GaN does not stay in a stöchiometric distribution instead In-rich regions are formed and surrounded by InGaN with lower In-concentration. Again, the strain energy is reduced.

Metal-organic chemical vapor-phase deposition (MOCVD) and molecular beam epitaxy (MBE) are the most common epitaxial methods for the fabrication of quantum dots. MOCVD uses carrier-gases saturated with the required materials in a low pressure atmosphere (20-200 mbar). In comparison, MBE utilizes effusion cells containing the solid material in a high vacuum environment (10-5-10-7 mbar). Especially for the industrial mass production of semiconductor heterostructures, MOCVD is more preferable than MBE because of the less critical working-pressure.

An important characteristic of self-organized growth methods is the possibility to produce huge amounts of quantum dots in a short time. (For example: In the InAs/GaAs system quantum dots are formed within ~10 seconds. With lateral quantum dot density of ~ 4x10^10 cm-2 on a 2 inches wafer, this would yield a rate of 2x10^10 quantum dots per second!)

Nanomaterials - main research of the AG Bimberg:

Fabrication and optimization of quantum dot structures for:

  • Laser and amplifier (edge emitting laser, VCSEL, SOA)
  • VECSEL (1220 nm, 1040 nm, 940 nm) for application in the visible spectral range (doubled frequency)
  • Single photon emitter
  • Memories


Further reading:


  • D. Bimberg, M. Grundmann und N.N. Ledentsov, Quantum Dot Heterostructures (John Wiley & Sons, UK, 1998).
  • V.A. Shchukin, N.N. Ledentsov und D. Bimberg, Epitaxy of Nanostructures (Springer Heidelberg, Germany, 2003).



Contact:

  • Dr. Andre Strittmatter, EW 539
  • Gernot Stracke, EW 540
  • Jan-Hindrik Schulze, EW 438
  • David Quandt, EW 438

 

 

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