TEM Studies of III-V MOSFETs for Ultimate CMOS
Over the past half-century electronic industry has enormously grown changing the way people live their lives. Such growth has been driven by the miniaturisation and development of the transistors which are the main components in an integrated circuit (IC) commonly referred as a chip. Until today electronic industry has been based on the use of Si and its native oxide SiO2 in transistors. However, the performance limit of conventional Si based transistors is rapidly being approached and alternatives will soon be required. One of the proposed alternatives is GaAs. n-type GaAs has a mobility 5 times higher than Si. This makes it a suitable candidate for MOSFETs devices. So far, GaAs has not been used for practical MOSFETs because of the difficulties of making a good dielectric oxide layer in terms of leakage current and unpinned Fermi Level. Using processes pioneered by Passlack et al, dielectric gate stacks consisting of a template layer of amorphous Ga2O3 followed by amorphous GdGaO have been grown on GaAs substrates. Careful deposition of Ga203 can leave the Fermi Level unpinned. The introduction of Gd is important in order to decrease the leakage of current. The electrical properties of the Ga2O3/GdGao.4XOo.(, dielectric stack are related to the Gd concentration and the quality of the GaAs/Ga2O3 interface. Over the past years in a unique partnership several research groups from the Physics and the Electronic and Electrical engineering Department have collaboratively worked for the realisation and development of such new generation of GaAs based transistors using the technology described above. The properties of such devices depend on structures at the nanoscale which is only few atoms across. Thus the characterization using the transmission electron microscope (TEM) becomes essential.
In this project TEM has been used to study several MBE grown Ill-V semiconductor nanostructures. In particular most of the thesis is focussed on the chemical characterisation of the GaAs/Ga2O/GGO dielectric gate stack, mainly using electron energy loss spectroscopy (EELS) and high-resolution scanning Transmission electron microscopy (STEM) imaging. As said above the quality of such interfaces affects the properties of the whole device. Hence the results presented herein represent an important feedback for the realisation of world performance GaAs devices.
Background information on the type of Ill-V materials that were studied in this project is given in chapter 1. This information involves a description of the GaAs/Ga2()3/GGO interface. In addition the molecular beam epitaxy technique employed for the growth of these structures is explained.
Chapter 2 deals with the experimental apparatus and techniques employed to characterise the various materials. For instance, the chapter begins with a short introduction to the foundations of electron microscopy such as electron lenses and guns. The conventional transmission electron microscopy (CTEM) and STEM imaging techniques are explained along with electron energy loss spectroscopy (EELS). An introduction to the theory of energy loss near edge structure (ELNES) is also given. ELNES is dependent on the local chemical and structural environment of the absorbing atom. The three electron microscopes utilised in this project are also described. These instruments comprise a Tecnai F20 and T20 and SuperSTEMl. In the case of the SuperSTEMI the method of the aberration correction is also outlined. However, most of the results presented in this thesis were obtained using the F20 and partly the T20. Finally a summary of the specimen preparation techniques are given at the end of this chapter.
Chapter 3 deals with modelling of the fine structure business. As said in chapter, the interactions which cause ELNES are complex and not well understood, making the interpretation of the fine structure from complex materials very difficult. By modelling the fine structure and investigating how changes in the model affect the spectral features, can improve the understanding of the data experimentally collected. A detailed description of the multiple scattering technique which was exclusively used is given.
However the Ga203/GGO dielectric gate stacks turned out to be electron beam sensitive. High electron dose changes the chemical structure. In chapter 4 some examples of e damage effects are shown. In particular phenomena such crystallisation and phase separation usually occur when the dose is high. These problems make almost impossible to acquire any ELNES data as a high dose is required in order to get a well-resolved spectrum. Thus the simulation work was halted as it did not seem to offer useful support for the main project.
Chapter 4 deals with the compositional analysis across the GGO layer of a wide range of samples using EELS spectrum imaging (SI). By recording EELS spectra over the energy range from the 0 K-edge to the As L23-edges allows characteristic edges of Ga, As, Gd and 0 to be extracted. Their intensity can be converted to atomic fraction by using proper standards. The results shown in this chapter show excellent agreement with the ones obtained using Rutherford backscattered spectroscopy as analytical technique.
Chapter 5 deals with investigation of the effects of three different etching processes on the GaAs/Ga203/GGO dielectric gate stack. This investigation was mainly carried out using energy filtering transmission electron microscopy (EFTEM) in the Tecnai T20. EFTEM provides a quick analysis of the structure. Thus it is really useful when large areas need to be analysed. One of the three etching processes has been characterised also using EELS SI. EELS SI offers much better spatial resolution and the possibility to apply the data processing developed in chapter 4 to quantitatively characterise the material and obtain information on the composition. Background information on the EFTEM and the effects of the objective aperture on the acquisition of filtering images are also given.
The aim of the analysis described in chapter 5 is to develop a way to study the effects of etching processes on the GaAs/Ga203/GGO structure. This obviously represents a very important issue for fabrication purposes.
Finally in chapter 6, a detailed analysis of the interface GaAs/Ga203/GGO was carried out using HAADF STEM imaging. High angle annular dark field (HAADF) imaging carried out in the STEM has revealed to be an excellent method for characterizing the interfaces as the contrast is sensitive to the atomic number. Images were acquired using both F20 and SuperSTEMI electron microscopes. The use of the second one has allowed to get a better insight on the interface as it is fitted with an aberration corrector. Hale’s model [8] predicts that the interface appears to be different whether it is observed along the [110] or the [1-10] direction. Thus TEM specimens of two different samples have been prepared cutting the original wafer off along two perpendicular directions. This ensured the possibility to look at the interface along [110] and [1-10] directions. While some differences are seen, the period dribbling predicted by the Hale mode is absent. A more detailed investigation is required.