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Novel 3-dimensional Ge dot crystals grown on templated Si substrates

Great attempts are being made to create new artificial materials for realizing prospective devices with novel functionalities. Templated self-assembly of quantum dots is regarded one possible avenue to realize such devices. Here we demonstrate an artificial 3-dimensional Ge quantum dot crystal hosted in a Si matrix, which is realized by means of templated self-assembly.

The Si substrates were templated by means of x-ray interference lithography (XIL), harnessing a wavelength of 13.2 nm to realize the prepatterning [1]. After exposure and photoresist development, shallow holes (4-10 nm in depth) were etched into the Si substrates by reactive ion etching. Subsequently, the resist was removed and the substrates were cleaned to use them for the deposition of Si and Ge. The deposition was accomplished by means of molecular-beam epitaxy (MBE) in order to realize the Si/Ge quantum dot structures. By choosing suitable growth conditions, perfect alignment of Ge dots in the etched grooves is found [2-5]. Moreover, stacking of quantum dot layers separated by narrow Si spacer layer leads to the formation of 3-dimensional arrays of ordered Ge quantum dots, i.e. a 3-dimensional Ge dot crystal (QDC) [6,7]. Figure 1 depicts the pristine quality of such Ge QDC, which consists of one atomic force image and two transmission electron micrographs. The lateral and vertical periodicities amount to 32 nm and 10 nm, respectively.

Ge-dot-array-Fig1Figure 1: Image of a 3-dimensional Ge QD crystal. The image is composed of two TEM and one AFM image.

Photoluminescence measurements of the Ge QDC were carried out at 10 K, which can be seen in figure 2. Besides the PL peaks on the high energy side that originate from the Si substrates, two peaks on the low energy side are found, which are ascribed to the no phonon and TO-phonon emission of the Ge QDs [6]. To confirm the assignment, the energy structure of the Ge QDC is determined by means of the NextNano simulation package [8], which is illustrated in figure 3. The calculations show that the heavy holes are located inside the Ge QDs, whereas the electrons situated in the Si matrix around the QDs. The energy difference between the heavy hole and Dz electron ground state amounts to 885 meV, which is in good agreement with the PL data.

Future work will concentrate on reducing the lateral and vertical periodicities of the Ge QDCs. The NextNano simulations show that by reducing the lateral dimensions to 20 nm, which lies well in the capability of the XIL technique, the Ge QDs will interact with each other, which has a substantial impact on the energy structure of the Ge QDC. Consequently, the delocalized Dxzelectrons will constitute the energy ground state, which causes significant modifications in optical as well as transport properties of the Ge QDCs.

Ge-dot-array-Fig2Figure 2: PL spectrum of a Ge QD crystal. Besides the PL signal from the Si substrate, a strong signal from the Ge QDs is seen on the low energy side.

Ge-dot-array-Fig3Figure 3: Simulation of the energy structure of a Ge QD in a Si matrix. The simulation was carried out using the NextNano simulation package.


[1] H. H. Solak,
Nanolithography with coherent extreme ultraviolet light,
J. Phys. D: Appl. Phys. 39(10), R171–R188 (2007)

[2] C. Dais, H. H. Solak, Y. Ekinci, E. Müller, H. Sigg, D. Grützmacher,
Ge quantum dot molecules and crystals: Preparation and properties,
Surface Science 601 2787 (2007)

[3] C. Dais, G. Mussler, H. Sigg, T. Fromherz, V. Auzelyte, H. H. Solak, D. Grützmacher,
Photoluminescence studies of SiGe quantum dot arrays prepared by templated self-assembly,
European Phys. Lett. 84, 67017 (2008)

[4] C. Dais, G. Mussler, H. Sigg, E. Müller, H. H. Solak, D. Grützmacher,
Evolution and stability of ordered SiGe islands grown on patterned Si(100) substrates,
J. Appl. Phys. 105, 122405 (2009)

[5] G. Rinke, G. Mussler, J. Gerharz, J. Moers, D. Grützmacher,
Growth of Ge dots on templated Si substrates with diffusion-altered holes,
European Phys. Lett. 85, 58002 (2009)

[6] D. Grützmacher, T. Fromherz, C. Dais, J. Stangl, E. Müller, Y. Ekinci, H. H. Solak, H. Sigg, R. T. Lechner, E. Wintersberger, S. Birner, V. Holy, G. Bauer,
Three-dimensional Si/Ge quantum dot crystals,
Nano Lett., 7, 3150 (2007)

[7] V. Holy, J. Stangl, T. Fromherz, R. T. Lechner, E. Wintersberger, G. Bauer, C. Dais, E. Müller, D. Grützmacher,
X-ray diffraction investigation of a three-dimensional Si/SiGe quantum dot crystal,
Phys. Rev. B 79, 035324 (2009)

[8] S. Birner, T. Zibold, T.Andlauer, T. Kubis, M. Sabathil, A. Trellakis, P. Vogl,
nextnano: General purpose 3-D simulations,
Transactions on Electron Devices 54, 2137 (2007)