Polarons in CdTe and TiO2
The aim of this proposal is to increase the understanding of polaron properties as a function of the host material. Despite intensive theoretical work on polaron properties there are many open questions both from a fundamental point of view and from an applied point of view. First principles calculations based on density functional theory within the local density or generalized gradient approximation are not sufficient to describe polaronic solutions. A new class of so called hybrid functionals allows to describe small polaron solutions within DFT calculations. This has opened for many recent studies of small polarons. Moreover, the development within device technology towards nanodevices has increased the importance of understanding polaron behaviour as a function of not only materials properties in general but of the detailed relationship between the symmetry and extension of the involved electronic orbitals. This will be achieved in this proposal by studying polarons in a variety of semiconductor materials and constrained geometries at T=0 with the help of density functional theory. We aim at studying polarons even at finite temperatures by studying the influence of phonon modes on the polaron stability. CdTe is used to make thin film solar cells. CdTe can be alloyed with Hg to make a versatile infrared detector material. CdTe alloyed with a small amount of Zn makes an excellent solid-state X-ray and gamma ray detector. Detection efficiency and high energy resolution at room temperature make CdTe detectors important for many X-ray and gamma ray detection applications, such as medical and industrial imaging, non-destructive testing, security and monitoring and nuclear safeguards. TiO2 is a functional material with widespread applications in technology. Its wide band gap (3 eV) is for example utilized in sunscreening and chemical solar cells. Also it is used in photocatalytic splitting of water and degradation of organic molecules in polluted air or water. It turns out that the understanding of the polaron behaviour has direct impact on the functionality of these devices. The goals of this proposal are summarized in the following: 1. to tailor wanted polaron properties: this we will achieved by an increased understanding of small polarons in TiO2 which in turn will provide us with a toolbox how to tailor wanted polaron properties. 2. to improve device performance: this will be achieved by studying hole polaron properties in II-VI semiconductors, which in turn will allow us to suggest recipes for improvement of device performance. 3. to manipulate charge carrier properties by controlling the polaron properties: this will be achieved by tailoring low dimensional systems of CdTe or TiO2 which in turn allow us to manipulate charge carrier properties by controlling the polaron properties. 4. to provide measurable quantities associated with different polarons which hopefully can assist in the experimental characterization of the device properties. 5. to understand the influence of phonons on the stability of polarons. 6. to investigate the influence of polarons on the stabilization of the long rang magnetic order among dissolved magnetic ions in a semicondutor host.