The Application of Resonance Ionisation Spectroscopy to III-V Semiconductor Surface and Depth Analysis

Resonance Ionisation Spectroscopy (RIS) is a relatively new analytical technique which is gaining increasing significance in the field of quantitative trace analysis. This work is principally concerned with the particular application of Resonance Ionisation Spectroscopy to surface and depth analysis of semiconductor material. For any new technique to succeed amidst a plethora of competing, proven technologies, it must offer some added advantage. The introductory chapter assesses some of the common electron, ion and laser based techniques already in existence, with which RIS is in direct competition, stressing fundamental problems in use which limit their degree of success. As a preamble to Chapter 2, the process of Resonance Ionisation Spectroscopy is also introduced at this stage. Chapter 2 deals with the physical principles behind RIS and its mass analysis derivative, RIMS, introducing the concept of atomically selective ionisation schemes used to discriminate between elements. A rate equation model, restricted to a three level system, allows determination of the necessary experimental conditions for efficient implementation of the process. In its most common form, with regards to its application to solid sample analysis, RIS is applied in a post-ionisation mode, in which case ion sputtering or laser ablation are employed in the sample vaporisation process. Of the two methods, ion sputtering is by far the more versatile and better understood. Chapter 3 introduces the physical concepts of ion sputtering and erosion and includes the necessary theory for modelling of the experimental conditions. With regard to depth profiling in particular, the factors contributing to the broadening of known concentration profiles are described in detail. A prerequisite for such analysis, is the ability to detect and recognise the species being analysed. To this end, a major portion of the project was devoted to the design and construction of a time-of-flight mass spectrometer, responsibility for which was placed solely upon the author. Chapter 4 deals specifically with the instrumentation aspect of the project, detailing the operational principles of a time-of-flight mass spectrometer, the various components of the analysis system constructed, the vacuum and laser systems, the ion optical assembly and the sputter ion gun to name but a few. Chapter 5 is the first relating to actual experiments, and provides the basis for an estimate of the operational usefulness of the instrument. Both pulsed secondary ion (SIMS) and resonant ion (RIMS) analyses were carried out on Aluminium and Gallium by the author. The wavelength spectra allow an investigation of the effects of experimental parameters on the resonant process. Chapter 6 describes attempts at depth profiling of aluminium concentration through multilayered structures of GaAs and AlGaAs. During the design stage, familiarity with the problems associated with the secondary ion background prevalent in these experiments, led the author to devise a novel approach to experimental procedure and ion optical design. Chapter 7 discusses the solution in detail, and produces experimental data and computer simulation as confirmation of the usefulness of the method. Recent investigations by the author have led to the development of what has been termed Resonant Laser Ablation (RLA). Chapter 8 introduces this new concept which is derived from the combined mechanisms of laser ablation and resonant ionisation to offer enhancements in sensitivity and selectivity. The technique has generated a great deal of interest due to the possibilities it offers in the field of surface analysis. Experiments, carried out initially by the author and at a later stage in collaboration with Dr. Wang Li of the Department of Physics and Astronomy, on semiconductor and metal samples indicate that RLA may prove to be an interesting alternative technique to existing forms of laser spectrometry. Consequently RLA has become a subject for further investigations within the group. Finally, Chapter 9 discusses briefly the project as a whole and describes some interesting future developments which should enhance the position of RIS/RIMS in the fields of spectrometry and surface analysis