Thin Film Growth
The nucleation and coalescence stages of film formation are largely decisive for the constitution and properties of a material. This holds true both for film/substrate interfaces and for layer/layer interfaces in heteroepitaxy and multilayer film growth. Particularly sensitive are the phase formed, crystallographic texture, threading defects in the layers, and in-plane residual stress. Subsequent growth typically requires growth to an appreciable film thickness before steady-state conditions are obtained if different from the nucleating layer. During the initial stages, the nuclei selection will be sensitive to the incoming flux (species and energy/flux distribution and their interaction with the substrate) and any contamination (e.g. residual gas in the vacuum system).
Since many years, we have taken a special interest in the growth and characterization of superlattices and multilayers the high density of constrained layers and interfaces alters the physical properties of the materialsAt the same time it offers possibilities to study the effects of reduced dimensionality and interfaces. We study superlattice systems from ceramic materials of nitrides, oxides, and carbides. The studies are aimed at understanding the growth mechanisms and thermal stability of as-deposited material, and explaining the mechanical behavior including large hardness enhancements of the layers.
As for film materials and model systems, we have selected ceramic materials of nitrides, oxides, and carbides, but study also metals.
AUTO-ORGANIZED NANOSTRUCTURES; Ageing Phenomena with Spinodal Decomposition in Thin Films & Self-Organization during Deposition of Nanocomposites
Lars Hultman and Jens Birch
2007-2013, The Swedish Research Council (VR)
The project aims to achieve a fundamental understanding of nanostructure formation in metastable nitride alloys by thermodynamic segregation during PVD deposition and annealing. Functional films are exiting since both surface and in-depth decomposition can be controlled in alloys quenched into the miscibility gap. Self-organizing nanostructures have impact for age hardening of films as an original concept by us, but also for band-gap engineering in III-N semiconductors. Model systems are MAlN (M=Ti,Zr,Hf,Cr,In,Ga) and MSiN (M=Ti,Zr,Mo,W) as well as other combinations with positive heat of mixing. In order to follow spinodal decomposition (or nucleation) as a function of composition, temperature and pressure, we apply state-of-the-art Atom Probe Tomography, He Ion Microscopy, DSC/TGA, in situ XRD furnace, FIB, Cs-corrected TEM/EELS, nanoindentation, ERDA/RBS and VT-STM. Integrated with the experiments are leading first-principles calculations together with alloy theory and thermodynamics. The effect of coherency strain between the decomposition phases and (any) crystallographic anisotropy will be explored. Beyond polycrystalline and epitaxial alloys, we are also initiating studies on secondary phase transformation in amorphous alloys (ZrSiN). Finally, we address superhard TiN/Si3N4 nanocomposites shown by us to contain strain-stabilized cubic-SiNx tissue phase.
Physics of defects in bulk GaN
Galia Pozina and Carl Hemmingsson
Energimyndigheten (2011-2013)
GaN is recognized as one of the most important materials for modern optoelectronics and electronics. For example, GaN-based white light emitting diode (LED) bulbs will last at least 60 years, will emit light, which is similar to sun light, and will be much cheaper to produce than the present illumination systems. Low energy solid state lighting is very beneficial for the society, both in terms of energy savings, environment impact and safety aspects. Today about 20 % of the global electricity is used for lighting and by improving the energy efficiency of the light sources huge energy savings can be done (about 20 TWh annually). Thus, research is going on worldwide to develop light emitting diodes with better energy efficiency. To get a better performance with long operation lifetime and high external quantum efficiency the active nitride layer in the LED devices must have a low defect density. This is a severe problem at present, since due to the lack of GaN substrates, the devices are grown on foreign substrates, like sapphire or SiC. This leads to an enormous defect density in the active region of the device and this cannot be tolerated for many devices such as high brightness LEDs. Using GaN substrate, the defect density in the active layer can be reduced significantly, and it will be of the same order as in the substrate. However, there is still insufficient fundamental understanding of the problems related to the extended and point defects in bulk GaN. The main aim of the project is to get a comprehensive knowledge about defects, which influence the quantum efficiency, and a way to reduce their density in the bulk GaN.The work requires development of synthesis using analytical simulation of growth and such advanced characterization techniques as optical spectroscopy, electron microscopy including cathodoluminescence and atomic force microscopy.
Unsolved Problems for the Diffusion in Ceramics with potential for Realizing Exotic Phases using New Reaction Pathways
Lars Hultman, Jens Birch, Valeriu Chrita, and Björn Alling
2006-2012, The Swedish Research Council (VR)
The project is inspired by the condition that diffusion in nitrides and carbides is largely terra
incognita. We make fundamental research on unexplored diffusion phenomena in functional
ceramics with the aims to realize and determine properties of newly predicted, not yet synthesized
phases, e.g., rare inverse nitride perovskites, polytypism in nitrides (putting DFT to test) and MAX
phases). We seek to obtain transport parameters and apply theories for bulk and surface atom
migration in ceramics related to the above. We have thus discovered ip-Sc3AlN and vacancystabilized
cubic-like Si3N4. We employ in situ laboratory and synchrotron growth with neutron and
XRR/XRD as well as MD simulations and ab initio calculations. We also employ analytical FEGTEM/
STEM/EELS and delocalization-free sub-Å resolution TEM.
