Growth and characterization of phosphorus doped n-type CVD diamond films of various orientations and devices/applications based on these

The fact that diamond is a fabulous material with special properties, which are explored in a wide range of applications, is already known by now. The aim of this thesis is to exploit the material’s characteristics, namely the semiconducting property. The deposition and characterization of n-type Chemical Vapour Deposition (CVD) diamond thin films are presented in this study. The bottom-up approach used in this survey starts with chapter 1, in which an overview of the diamond structure and its growth history is given. The reasons behind choosing Microwave Plasma Enhanced Chemical Vapor Deposition (MW PE CVD) as the appropriated method for producing our layers are properly explained. Also, diamond doping and the selection of the dopants, used for either n or p-type doping, are discussed. Special attention is given to phosphorous as an element, since it’s the only component which can trigger a substitutional doping process in diamond leading to useful n-type conduction. The chapter finishes with a section dedicated to applications and a correlation between diamonds properties and specific types of application is made. The simplest application, which makes use of the semiconducting properties of diamonds, is the “pn-junction”, which is the main device addressed in my thesis. The second chapter pursues an accurate description of all the experimental techniques used in this research for growth and characterization of P-doped diamond layers. As expected, the growth procedure employed in obtaining ntype diamond opens chapter 2. Experimental setups for discovery and identification of impurities in CVD diamond, such as photocurrent and FourierTransform Photocurrent Spectroscopy (FTPS), are described next. The techniques which can characterize the quality of the obtained layers (from surface topography and morphology up to its electrical properties) are also explained in this chapter. The description of the necessary procedure, used for fabrication of diamond-based pn-junctions, is closing this chapter. With chapter 3 the experimental part of the thesis starts. This study segment is dedicated to the P incorporation into diamond thin films when {100} and {111}- oriented diamond substrates were used. The doping process on {111}-oriented samples obtained at IMO is discussed in the context of the existing literature and the results presented here show the progress made over the first year of my thesis. Motivated by the need to optimize the quality of the doping process, a specific O2/H2 pre-treatment plasma step was developed. This procedure allowed the reduction of crystallographic defects incorporated in the n-type films and originating from the substrate. This finding enabled a slight improvement in quality of our P-doped {111}-oriented diamond films. The development of n-type doping on {100}-oriented diamonds is presented in the second half of chapter 3. For this purpose a new doping strategy, different from the current trend present in literature, was introduced. The main differences of our growth procedure were: the elevated substrate temperature and the increased methane concentration. After a careful inspection of the results it was concluded that the doping of P in our {100}-oriented samples failed when this set of plasma parameters was used. The reproducible and efficient n-type P-doping of diamond was successfully achieved for {100} or {111}-oriented substrates in various groups located all around the world. In contrast, only a few attempts reported doping into {110}- oriented films and the plasma parameters employed there were similar as for {111}-oriented films. Chapter 4 is dedicated exclusively to the doping process into {110}-oriented diamond substrates, either in single crystalline or polycrystalline specimens. For polycrystalline substrates, the layers were first grown using the so-called {111}-oriented strategy. For comparative reasons, a second doping scheme was tried. This time the P incorporation was addressed by atypical plasma parameters, which were previously tested for doping of P in our {100}-oriented samples. For probing the resulting layers several investigations were carried out. This inquiry includes surface and microstructural analyses, defect spectroscopy and electrical measurements. The preliminary surface images showed the presence of microfacets at the diamond surface. Also cathodoluminescence data and Raman spectra are presented. Based on the aforementioned results the quality of the P-doped layers was established and a confirmation of P incorporation was done. The overall estimation of the polycrystalline n-type doping point out to a higher quality of our layers when compared with the films grown using the {111}-oriented strategy. Thus, a general conclusion for polycrystalline {110}-oriented diamond films can be drawn: the novel set of plasma conditions is suitable for P incorporation. The successful results presented for P doping onto polycrystalline diamond specimens inspired the work towards doping of single crystalline {110}-oriented layers. After the growth of the CVD diamond layers, the presence of P-related features was firstly revealed from PC and FTPS investigations. The results were confirmed by cathodoluminescence spectra. Thus, the successful P incorporation into the single crystalline {110}-oriented diamond layers was achieved. This points to the general conclusion of the chapter: this particular set of plasma parameters is appropriate for P doping of {110}-oriented diamond layers, even though some questions regarding the influence of the microfacets upon the incorporation process are still to be addressed. The last chapter of the thesis describes the necessary fabrication steps used in obtaining various applications based on the n-type diamond layers described in the previous chapters. Obtaining pn-junctions devices, based on single or polycrystalline diamond layers, is not an easy task and complicated techniques are requested, such as lithography. The development of such applications requires quality layers. The characterization of single crystalline diamond pnjunctions reveals a successful preparation process of diamond-based devices at IMO. Based on this training, polycrystalline pn-junctions were fabricated. Their characterization proves that the quality of our layers is high enough to obtain devices. Moreover, the procured devices can be compared with the results from literature and thus it was validated that the new set of plasma growth conditions are appropriate for applications. The chapter ends with extending the range of the diamond-based devices. From thermionic electron emission measurements of the polycrystalline P-doped films grown on metallic substrates it was established that this material may be suitable for thermionic electron emitters. Finally, the conclusions of this work are presented in the chapter called “general conclusions and outlook” together with some perspectives on the research topic.

Jaar van publicatie:2009

Trefwoorden:Diamonds, Semiconductors

Toegankelijkheid:Open