Michael Awaah, Ph.D. (2006)
Dissertation Topic: Electrical Contacts to ALGaN/GaN
Major Professor: Dr. Kalyan Kumar Das, Associate Professor of
Electrical Engineering (Retired)
M.S.: Electrical Engineering, Tuskegee University
Employment: Intel Corporation, Chandler, AZ
Group III-nitrides, in particular GaN and its heterostructures with AlGaN, have some unique electronic material properties that make these material systems almost ideally suited for the fabrication of a number of high-performance electronic and optoelectronic devices.
Heterojunction devices generally provide greater advantages than homojunction devices, since in addition to the doping type and concentration, one can tailor the energy bandgap as well as other material properties such as mobility and effective mass to achieve superior performance. The present research involves a study of AlGaN/GaN heterostructures. Fabricated test structures included Ni/AlGaN/GaN rectifying diodes, representative of the gate element of high electron mobility transistors (HEMTs) with annealed ohmic "back contacts" and HEMT devices of various geometries. Test masks also included a set of transfer length method (TLM) pads for the evaluation of ohmic contacts employed in the fabrication of HEMT devices. Electrical measurements on these test structures yielded a contact resistivity of 2.0 x 10-3 Ω.cm2, a barrier height of ~1.1 eV for the rectifying diodes, a sheet charge density of ~1.1 x 1013 cm-2 at the AlGaN/GaN interface, a conduction band offset of 0.27 eV. These data are in close agreement with those reported by other researchers. The HEMT devices yielded a drift mobility of ~511 cm2/V.s and a transconductance of 10 - 25 mS.
Devices comprised of AlGaN/GaN/AlGaN double heterostructures, namely, blue LEDs, were also characterized electrically. The blue LEDs were highly non-ideal with several approximately linear regimes in the semi-logarithmic plots of the forward characteristics. Calculated values of the ideality factor range from 2.1 to 6.7, indicating that the recombination process in these diodes cannot be conveniently described by the Schockley-Read-Hall statistics. Logarithmic plots of the forward characteristics indicate a space-charge-limited-current (SCLC) conduction through the active region of the diodes. Observed changes in slope of these logarithmic plots are representative of a high density of deep-level states. An analysis of these characteristics yielded approximate concentrations of deep states of 5.6 x 1016 cm-3, 8.2 x 1016 cm-3, and 1.5 x 1017cm-3 , located at 0.29 eV, 0.37 eV and 0.52 eV, respectively, above the valence bandedge. The major peak in the electroluminescent spectrum from these diodes was observed at 433 nm and a smaller peak at 378.4 nm. The 433 nm peak corresponds to a photon-energy of 2.86 eV that relates closely to a transition from the conduction band minimum to the deep-level states located at 0.52 eV above the valence band edge.
A recombination lifetime of ~25 ns has been determined from the observed reverse recovery storage times for these LEDs. It is likely that this experimentally determined lifetime arises from a combination of radiative and nonradiative processes occurring in the active region of the LEDs, where carriers are confined by the two heterointerfaces.