Films of ε-Ga2O3: thermal stability, physical properties and defects

Abstract

Oxide semiconductors are generally characterized by good thermal and chemical stability, they can be grown by various growth techniques and be doped to obtain the necessary conductivity for optoelectronic devices, power electronics and gas sensors, which makes them very popular and intensively studied. Gallium oxide, which belongs to this group of semiconductors, was first observed when the gallium element and its principal compounds were discovered at the end of 19th century. Rapidly, the study and the interest about this material moved from a basic research to first simple applications, i.e. catalyst for chemical reaction, gas sensor in different environments, transparent conductive electrodes. Over time the crystal growth technology improved significatively allowing to achieve both bulk and epitaxial thin films of high quality Ga2O3. Further technological developments, together with a controlled n-type doping led to a wider range of applications. The wide bandgap (4.6-4.9 eV), makes it one of the most interesting materials for UV optoelectronics. It simultaneously attracts the global attention for power electronics due to its high critical breakdown field, estimated of about 8 MV/cm, which leads to a BFOM (Baliga Figure of Merit) four times larger than the value observed in SiC and GaN compounds. Among the six known Ga2O3 polymorphs, the research was particularly focused on the β-phase that is the thermodynamically stable phase. Nevertheless, in the last years an increasing attention was turned to other polymorphs as well, especially towards the ε-Ga2O3, which results the second most stable phase. It can be epitaxially grown at lower temperature with respect to β-polymorph and exhibits a good matching with the most diffused commercial substrates. Although the ε-polymorph showed multiple interesting properties, it still remains rather unexplored compared with the most investigated β-phase. These assumptions provided the principal motivations for the present Ph.D. work, that is focused on the epitaxial growth of high quality undoped and n-type doped films of ε-Ga2O3, carried out at IMEM-CNR institute, and on relevant characterization performed at the Department of Mathematical, Physical and Information Sciences (Physics Unit) of Parma University, and completed by national and international collaborations during the thesis period, in order to obtain new information on still unknown fundamental properties. The first introductive chapter describes the state-of-the-art of the ε-polymorph, starting from the crystal and electronic structures, summarizing then the main physical properties and the epitaxial growth techniques, and concluding with a brief comparison with the stable β-phase. Chapter 2 introduces the basic theory behind the epitaxial growth, focusing on the MOCVD method employed to obtain the samples analyzed in this thesis. Thus, chemical, physical and thermodynamic mechanisms are discussed. Finally, the experimental setup and the representative growth parameters are presented, along with the most significant results related to undoped and silicon doped films. In chapter 3, the study about the thermal stability of ε-Ga2O3 layers, which resulted confirmed up to a temperature of ~700°, is reported. The phase transition was determined by using X-Ray diffraction on different specimens of the same sample, thermally treated at temperatures between 700°C-900°C. Additionally, TEM analysis and DSC characterization, performed by external collaborators, further confirmed the crystal phase conversion mechanism. The purpose of the fourth chapter is to describe the electrical and optical properties of the ε-polymorph measured by different methods. Initially, after an introductive overview of the β-Ga2O3 electrical characteristics, the contact deposition techniques with the related parameters and the best electrode structures are reported. The contact development represents an important parallel field of study considering that electrical and photo-electrical measurements strongly depends on the contact performance. Then, the central part of the chapter is focused on the electrical investigation on Si and Sn doped samples, reporting the conditions and the configurations involved and presenting the obtained results. These data are discussed also taking into account the supplementary information provided by EPR, ToF-SIMS and RBS characterization, from external cooperating groups. The chapter ends with optical and photo-electrical analysis of undoped films which highlight a significant response to UV illumination, making the ε-Ga2O3 a promising candidate for UV solar-blind photodetector fabrication. Chapter 5 deals with defect-related deep energy states within the energy gap. Initially, the presence of these deep levels was argued looking at the absorption tail as well as to photocurrent for illumination with photons below the energy bandgap. However, a specific investigation, later performed at Valladolid University by cathodoluminescence, provided more detailed information, which led to a schematic model of the deep level-related optical transitions. Finally, a conclusive chapter summarizes the whole topics and the results presented in this work.