Harvey Hall, Ph.D. (2003) 

Dissertation Topic: Fabrication and Electrical Characterization of Contacts to GaN Films

Major Professor:  Dr. Kalyan Kumar Das, Associate Professor of Electrical Engineering

M.S.:  Electrical Engineering, Tuskegee University

Employment:  NAVAIR,  Power Systems-NAWCWD China Lake, CA 

Dissertation Abstract:

The primary objective of this study was to develop rectifying and ohmic contacts for use in GaN based devices, and to acquire an understanding of the basic processes involved in the formation of these contacts.  Electrical contacts to both n and p-type GaN films have been investigated using sputtered films of metals such as Al, Au, Cr, Cu, Ni, Pt, and Ti. The metals deposited on the n-type films with doping levels of 1 X 1017/cm3 showed rectifying characteristics with all the metals studied with the exception of Al and Ti.  The ideality factors for the metal contact diodes were much greater than 1.  Barrier heights of the contacts and the doping concentration of the film were extracted from computer simulated I-V characteristics, and capacitance-voltage (C-V) measurements.  Logarithmic plots of the I-V characteristics in the forward direction indicated that carrier transport was dominated by space charge limited current conduction influenced by the presence of deep-level states, as opposed to thermionic emission. Therefore, the use of an ideality factor to describe the electrical behavior of the contacts, based on the assumption that thermionic emission prevailed might not be justified. The GaN films investigated showed the presence of deep-levels in the energy range 0.27 to 0.43 eV below the conduction band.  Current-voltage characteristics also yielded the concentration of surface states and their position in the bandgap of GaN films.  Observed variation of the barrier heights with the metal work function was interpreted in terms of the electronegativity model, taking into account the effects of the metal induced gap/surface states.

In order to obtain ohmic contacts on GaN, the carrier concentration in the surface region of the film was increased by high dose implantation of Si (1 X 1015/cm2, 35 keV), for n-type and Mg (1 X 1015/cm2, 35 keV) for p-type films.  After implantation, the GaN samples were annealed at 1100 oC to activate the dopant and remove implantation damage.  Rutherford backscattering spectroscopy and channeling studies indicated that the anneal step did not completely remove the implantation damage.  In the n-type material the anneal process resulted in a high electron carrier concentration at the surface, due to donor activity of the implanted Si, as well as donors contributed by N vacancies generated.  Specific contact resistance ( ) of 1.4 X10-5 and 1 X 10-7  were obtained for Ni/Au and Ti/Au metallization, respectively.

The high annealing of Mg implanted samples resulted in an over compensation of the p-type dopants due to the creation of N vacancy/donors, and hence the film converted into n-type.  Implantation of N ions into Mg implanted-annealed samples was conducted in order to restore the N lost during the annealing process.  A dose of approximately 5 X 1015/cm2 of N implant appeared to be sufficient to remove the compensation effects of N vacancy/donors.  Annealing after N implantation was conducted at 900 oC, as it had been established that an anneal at this temperature introduced negligible N vacancies.  The contact resistivities obtained with Ni/Au and Pt/Au on the Mg and N implanted samples were in the range 8 - 21 ; however,  for as-grown p-type GaN (1 X 1018/cm3) was 3 X 10-2  for Ni/Au contacts and  4 X 10-3  for Pt/Au contacts, after alloying at 900 oC.