T. Yokogawa, S. Miyake, A. Yamashita
Keywords: carbon nanotube, GaN, electrical propertiy, band-structure, Shottky barrier height
Summary:Recently power devices using GaN have attracted much attention because of high power and high efficiency due to the wide band-gap. These power devices are generally used in high temperature operation by the heat dissipation of devices. Therefore, for high device reliability, it is important to design the mounting structure for heat sink to obtain efficient heat spreading. Carbon nanotube is expected to be excellent heat conductor for heat spreading of the power device because of extremely high thermal conductivity. Therefore, it is thought that carbon nanotube is very useful for electrode material with high thermal conductivity in the GaN power device. However, there are no reports on detail electrical properties and band-structure for the hetero-interface between the carbon nanotube and the GaN. In this paper, we present results of electrical characterization for band-structure in carbon nanotube/GaN hetero-interface. Multi-wall carbon nanotubes (MWCNTs) were prepared electrophoretically on the n-type GaN. Commercial MWCNT particles were acidified with a mixture of nitric and sulfuric acid. Acid-treated MWCNTs were washed thoroughly with water and then centrifuged until the supernatant became neutral. After drying in an oven, the MWCNTs were dispersed in isopropyl alcohol to be charged with a positive charge. A constant DC voltage of 80 V was applied typically. The positively charged MWCNTs colloids were deposited on the negative GaN surface. The GaN films used in this experiment were grown on sapphire substrates by MOCVD. The carrier concentration was 1×1019 cm-3 by Si doping and the thickness was 5.4 μm. Measurements of I-V, C-V and its temperature dependence were performed to estimate the barrier height of the interface between MWCNTs and GaN films. Usual Ti(30 nm)/Al(100 nm)/Ti(30 nm)/Au(50 nm) electrode was used for ohmic contact of n-GaN with annealing treatment (850 ℃, 20 s). We obtained I-V characteristics between the Ti/Al/Ti/Au ohmic contact electrode and the MWCNT electrode on the n-GaN film. And Schottky diode property was observed. From the Schottky diode property, the barrier height φB was estimated. φB=0.34 eV was determined from thermionic emission model. C-V measurements were also performed at a frequency of 1 MHz. From the plot of 1/C2 vs V, φB=0.4 eV was estimated. Previously, the work function of the MWCNTs was reported to be the value of 4.3 eV. Different group also reported the value of 4.8 eV measured by photoelectron emission. Therefore the average value is 4.55 eV. On the other hand, the work function of Ni metal is 5.04～5.35 eV. Then the average value is 5.2 eV. And the barrier height φB was reported to be a value of 0.95 eV for the Ni/GaN Schottky barrier. From this barrier height of Ni/GaN, the difference of the barrier height between Ni and MWCNT is 0.57 eV. This value almost agrees well with the difference of the reported work function between Ni and MWCNT (0.65 eV). In conclusion, Schottky diode property using the interface between MWCNT and GaN were obtained. Barrier height was determined to be about 0.34 eV.