24. Oct. 2013, 16:15 Uhr, Gebäude NW1, Raum H3
High Pressure in Solid State Physics and Material Sciences
Prof. Tadeusz Suski, Polish Academy of Science, Warszawa, Poland
In the first part of my talk I will give a short overview of the hydrostatic pressure application in various aspects of materials science. In particular, sintering of ceramics and fabrication of nanomaterials (metals and semiconductors), well developed in the Institute of High Pressure Physics, will be presented.
Then, I will concentrate on nitride semiconductors. Gallium nitride and its alloys with InN and AlN represent a new family of semiconductors that currently takes the second place after silicon in terms of the range of possible applications and contribution to the world market. Their quantum structures, band gaps and related light emission properties are crucial in optoelectronic applications from ultraviolet to infrared range of the spectrum. The main condition for obtaining high quality nitride epitaxial structures used in optoelectronics is an employment of high quality GaN substrates. Growth of GaN single crystal is very difficult and the fabrication of the best substrates requires an application of hydrostatic pressure. I will introduce details of such GaN-substrate growth techniques and explain a role played by high pressure.
In spite of wide applications of nitride semiconductors their physical properties are not well understood. Due to polar crystallographic structure (hexagonal-wurtzite) of nitrides a spontaneous polarization appears representing one cause of the strong internal electric fields present in GaN-based quantum structures. The second contribution comes from the lattice mismatch between consecutive nitride layers and related piezoelectric effect. Strength of resulting internal electric fields can be as high as few MV/cm. As a result the wavelength of emitted light demonstrates a large red shift with respect to the band gap value of the thick layers of these materials. Corresponding changes are seen in the pressure response of the emitted wavelength. Width of the quantum well becomes a very important parameter in determining luminescence energies and its pressure dependence in nitride quantum structures. This is an unique property of the polar structures of nitrides. I will show how the properties of emitted light depend on: i) the related built-in electric field and its screening and ii) how application of pressure can be used for tuning the laser diode wavelength and for determination of the internal electric field magnitude.