Title Ohmic Contact Formation of Gallium Nitride andElectrical Properties Improvement Author ( s )

of Thesis Silicon (Si) is still now used mainly as material for semiconductor devices. But, it is important to seek for better alternative materials for use in the next-generation semiconductor devices due to the physical limitation of silicon. Gallium nitride (GaN) is one of the most promising candidates. Also, GaN has an advantage as a high frequency communication element. GaN-based semiconductor devices promise higher energy efficiency devices with capability of handling higher power and longer service life compared with silicon devices. However, in spite of having a significant development in the crystal growth and device processing technology of light emitting diodes (LED) and laser diodes (LDs), the application of GaN semiconductor as semiconductor devices is still far from real. In the present study, the strategy to improve the electrical properties of n-type and p-type GaN contacts has been carried out. In the case of n-type GaN contact, to seek for the most effective methods to improve the electrical properties of the contact, an alternative method to form nitrogen-vacancies (N-vacancies) near the interface to lower the contact resistance have been developed. Additionally, to clarify the most suitable surface orientation of n-type GaN contact, unconventional contact orientations have been observed and compared. In the case of p-type GaN contact, a different approach to finding an appropriate contact material for p-type GaN has been used in the present study. GaN shows a high degree of similarity with SiC. The present study demonstrates the idea by using the same SiC contact material (Ti3SiC2) contacts on p-type GaN and examining the properties of the contacts. For comparison, the contact properties of conventional monolayer contact of p-type GaN such as Au, Ni and also have been observed. Additionally, in the present study, in order to improve the electrical properties of p-GaN contact, the enhancement of hydrogen (H) release from p-GaN is attempted by applying current flow through the p-GaN substrates during low temperatures annealing. By enhancing the H release from p-GaN, the electrical properties of p-GaN contact interface can be improved. The higher carrier (accepter) concentration achieved by enhancing the H release from p-GaN will reduce the width of the Schottky barrier (SB) formed at the contact interface. By thinning the SB width, the probability of carrier to tunneling through the SB can be increased, resulting in improvement of the electrical properties of the p-GaN contact. The present study is consisting of 7 chapters. In the chapter 1, the background of semiconductor devices and wide band-gap semiconductor are explained. Next, the present issues related to p-type and n-type GaN is pointed out. The objective and research flow are explained. In the chapter 2, the important theories related to ohmic contact formation of GaN and improvement of electrical conductivity are explained. In the chapter 3, the experimental procedure used in present study is described. There are specimen preparations, heat treatment and, structural and electrical analysis. In the chapter 4, results related to electrical conductivity improvement of n-type GaN are discussed. The first topic is effect of N-vacancies formation to electrical conductivity. Ti contact films were formed on n-type GaN after Ar ion irradiation of the substrate for 300, 600, 1200, 2400 and 3600 s. Ti2N is formed adjacent to substrate by the Ti deposition. It is likely that a huge amount of N-vacancies is formed within the GaN sub-surface near to the contact. I-V conduction profiles show that ohmic conduction is achieved. Furthermore, the conductance of the contacts is increased by prolonging the irradiation time. However, extensive irradiation more than 3600 s shows no further improvement in contact conductance. It is likely due to the phase transformation from GaN to Ga-rich 2 at the GaN sub-surface. The Ga-rich phase enhanced the formation of Ti-Ga compound at the interface during the Ti deposition and reduce the number of N-vacancies within the sub-surface of GaN. The second topic in this chapter is effect of n-type GaN crystal orientations to electrical conductivity. Ti contact films were formed on various orientation of GaN surface. The contact formed on (0001) Ga-face shows the highest electrical conduction in the as-deposited state. However, the conduction deteriorates by annealing even at a low temperature of 773 K. On the other hand, the conductance of the contacts on other three surfaces shows some improvement by annealing at 973 K. However the conductance deteriorates by further annealing at 973 K for 300s. The deterioration of the electrical conductance is attributed to the formation of Ti-Ga compound at the interface, as shown in the XRD patterns and TEM observation. In the chapter 5, results related to contact formation and interfacial structure observation of p-type GaN are discussed. To the investigation of the interfacial structure of GaN and Ti3SiC2, Ti-Si-C ternary film with a composition stoichiometrically close to Ti3SiC2 has been deposited on p-type GaN. Polycrystalline Ti3SiC2 phase has been formed on the substrate after annealing at 973 K and 1073 K. However, after the annealing at 973 K and 1073 K, the dominant carrier-type of p-type GaN has inverted to n-type. This is likely due to the increasing of N-vacancies in the p-type GaN sub-surface resulting from the out-diffusion of N atoms during the annealing at high temperature. The direct-current conduction test shows that ohmic-like contacts have been achieved after the annealing. These ohmic-like contacts are likely formed between n-type GaN (inverted from p-type by the annealing) and the byproducts of Ti3SiC2 formation (Ti5Si3 and TiSi2), the residual Ti phase and/or the TiN phase. Additionally, to improve the electrical conduction of p-type GaN contact by thinning the SB at the interface, contact formation processes of Au and Ni with a low annealing temperature have been investigated. The investigation of the interfacial structure and electrical conduction of p-type GaN/Au and p-type GaN/Ni contacts after annealing at 673 K for 3600 s show some identical results. The dominant carrier-type of both contacts are maintained after the annealing. XRD analysis and TEM observation of both contacts revealed that the no intermetallic compound is formed at the interface after the annealing. Furthermore, the electrical conduction profile of these contacts show some improvement after the annealing at 573 K and 673 K for 3600 s. From these investigations, it can be conclude that annealing at 673 K for 3600 s is an adequate annealing condition to form Au and Ni contact on p-type GaN, while improving the electrical conduction and while maintaining the p-type carrier. In the chapter 6, the results related to H release enhancement from p-type GaN are discussed. In order to improve the electrical conduction of Mg-doped p-type GaN contacts, enhancement of H release from GaN substrates is attempted by applying current flow during annealing. The electrical conduction profiles of the p-type GaN/Ni contact annealed at 573 K and 673 K for 3600 s while subjected to current flow show some improvement compared to the contact annealed without applying the current flow. From these results, it can be understood that by applying current flow through the GaN substrates during annealing process, H release form GaN substrates can be enhanced by even annealing at low temperature. To understand the mechanism of H release by applying current flow during annealing, the change in current values p-type GaN contacts during annealing has been observed. By using regression analysis and kinetic model, the electrical conduction improvements achieved by applying current flow through GaN substrate during annealing has been analyzed. The results suggest that that by applying current flow during annealing and by forming a contact with material that H can diffuse into such as Pd, the H release from GaN substrate can be enhanced and the electrical conduction of p-type GaN contact can be significantly improved. Finally in the chapter 7, the conclusions of the present study are explained. 様 式 7 論文審査の結果の要旨及び担当者 氏 名 ( Aiman bin Mohd Halil )