Dynamic interplay between defects and dopants in Zinc Oxide

Zinc Oxide (ZnO), perhaps best known from Zinc-ointments and buckets, is an easily accessible, reasonably cheap and safe material. In addition to the use in ointments etc., ZnO can be made semiconducting and grown in large single-crystal, super-pure structures. Based on its semiconducting properties, ZnO has a potential for being used in a wide range of applications, amongst others in transparent conductor junctions for solar cells, white light-emitting diodes, room-temperature UV laser diodes, and as a component in piezoelectric and thermoelectric circuits. It is only when one can introduce other elements in a controlled manner, in order to give the material a specific surplus or deficit of loosely bound electrons, which the semiconductor becomes useful. A pure semiconductor is nothing else than an electric isolator, whilst our goal is to control the number of available electrons that may take part in the conduction of current. If one can create a surplus of electrons in one part of the material, and at the same time create a deficit of electrons in another part, you have effectively formed a diode. The main challenge with ZnO, however, is that creating a deficit of electrons (p-type), in order to successfully make an effective diode, has proven extremely difficult. Nor is it trivial to precisely control the surplus (n-type) electrons. Roughly speaking, as of today one is able to create either small or large surpluses of electrons, but for use in, for instance, transparent conductor junctions for solar cells the aim is an even larger surplus to ensure the lowest possible resistance in the junction, without compromising with how fast each electron can move (mobility). In order to understand more of how other elements (impurities/dopants) act in ZnO, and thereby create ways to control their behavior, we willingly want to introduce a number of these impurities to high-quality super-pure ZnO. Once the impurities have been introduced we can study, how quickly they move (diffusion), how they move, and how they dynamically interact with other dopants and defects in this project we have called "Dynamic interplay between defects and dopants in Zinc Oxide". In the first stage of the project the main focus has been to create a new diffusion model to describe diffusion through vacancies (vacancy mediated diffusion) with a detailed physical description. This model will have a wide applicability to this type of diffusion in a number of semiconductor materials. In this project the model have been applied to understand diffusion of Aluminum (Al) in Zinc Oxide, and study how Al interact with available Zinc vacancies in the material using Lithium as a vacancy marker. Furthermore, also other n-type dopants, like Ga, In and B, have been studied and evaluated using the same model. As part of the project a research stay in Columbus, Ohio, USA at The Ohio State University has been completed. The purpose of the stay has been to learn an optical characterization technique - Cathodoluminescence (CL). This technique utilize an electron beam to induce additional energy into the material. This deposited energy is absorbed by exciting electrons up to a higher lying energy position. This condition is, however, not stable and after a short period this electron will again decay either by emitting heat or light. By monitoring the energy of the emitted light, one can reveal details of electronic defect states in the material. This makes i possible to monitor defects with a high spatial resolution, and by varying the energy of the electron beam, one may also probe different depths of the sample. From this it is in principle possible to reconstruct a 3D model of the defect distribution in the sample. It is, however, not possible to directly identify the origin of the defects. By understanding the interplay between defects and dopants it may after all be possible to justify the identity of the involved defects. One example of a complex interaction between defects and dopants has been suggested in a recent paper. The concentration of Fe in ZnO seems to have a profound impact on the apparent stability of a well know Li - H complex. In samples with a relatively high Fe-concentration the there are indications that there is a ongoing defect reaction between Fe and Li, resulting in a removal of the Li-H complex.

Fundamental studies of semiconductor physics have provided basic scientific knowledge and lead to life altering technologies. Semiconductors continue to play a leading role in the development of our modern society - with applications ranging from integra ted circuits to sensors and energy converting materials like solar cells, light emitting diodes, piezo- and thermoelectric materials. Zinc Oxide (ZnO) is a promising semiconducting material. It is abundant and non-toxic and can be utilized as a wide ban d gap semiconductor with potential applications in several fields; as a transparent conductive oxide, thermoelectric material and as ultraviolet light emitting diode, to mention a few. ZnO is regarded as an environmentally safe and biocompatible alternati ve. To obtain such material properties ZnO is doped by different elements, e.g., n-type (Al, Ga, Si), p-type (Ag, Cu), neutral (Mg) and magnetic active dopants (Ag, Ni, Fe and Cu). However, it has proven to be a challenging material to fully understand an d control. Little is known about the diffusion properties, solubility limits and thermal stability of these and other common dopants. There are few available reports on dopant diffusion and solubility and most of them are based on indirect methods (e.g., H, Li, Ga and Al). Furthermore, such knowledge is essential to be able to do controlled and reproducible processing of ZnO in general. E.g., it is not possible at this stage to engineer bulk ZnO with a predefined electrical resistivity. In this project we will use secondary ion mass spectrometry to study diffusion of common dopants in Zn-/O-rich ambient. Appropriate diffusion models will be developed to describe the mechanisms involved and H and Li will be used as probe elements. The resulting material will be characterized by electrical and optical techniques. From this we will learn more about the dynamic interplay between dopants and intrinsic defects in ZnO and be able to tailor electrical/optical properties.