[15p-503-1~16] 15.4 III-V-group nitride crystals

[15p-503-6] Control of Carbon in MOCVD-grown GaN for Power Devices by Supersaturation

〇Seiji Mita¹, Felix Kaess², Jingqiao Xie², Luis H. Hernandez-Balderrama², Shun Washiyama², Pramod Reddy¹, Andrew Klump², Alexander Franke², Ronny Kirste¹, Axel Hoffmann³, Erhard Kohn², Tomasz Sochack⁴, Michal Bocowski⁴, Ramon Collazo², Zlatko Sitar¹ (1.Adroit Materials, 2.NC State Univ., 3.Tech. Univ. Berlin, 4.IHPP Unipress)

Keywords:GaN

Achieving high electron mobility in the low doping regime (n < 5x10¹⁶ cm^-3) is still difficult for metalorganic vapor deposition (MOCVD) grown GaN, however, indispensable to realize a high quality drift region for GaN-based power Schottky diodes, where high electron mobility at controllable low doping is desired, typically to carrier concentrations of less than 2x10¹⁶ cm^-3. By utilizing the thermodynamic Ga supersaturation model which was developed by following the simplified GaN chemical reaction, Ga + NH₃ = GaN + 3/2H₂, we identified that carbon was the main defect attributing to the sudden reduction of the electron mobility, the electron mobility collapse, in n-type MOCVD-GaN. SIMS has been performed in conjunction with C concentration and the Ga supersaturateion model. Our base growth conditions consisted of a TEGa flow rate of 134 µmol/min and a growth temperature of 1040 °C in low growth pressure of 20 Torr at a constant total mass flow of 7.5 slm. Both H₂ and N₂ were used as diluent gases. By controlling the ammonia flow rate, the input partial pressure of Ga precursor, and the diluent gas within the Ga supersaturateion model, the C concentration in Si-doped GaN was controllable from 6x10¹⁹ cm^-3 to values as low as 2x10¹⁵ cm^-3. It was found that the electron mobility collapsed as a function of free carrier concentration, once the Si concentration closely approached the C concentration. Lowering the C concentration to the order of 1x10¹⁵ cm^-3 by optimizing Ga supersaturation achieved controllable free carrier concentration down to 5x10¹⁵ cm^-3 with a peak electron mobility of 820 cm²/Vs without observing the mobility collapse. Further carbon control led to the achievement of the necessary high carrier concentrations in the back contacts while allowing for controllable, low carrier concentrations in the drift layers; a carrier concentration of 2x10¹⁶ cm^-3 with a mobility of 1100 cm²/Vs was obtained for GaN on sapphire. GaN homoepitaxially grown on HVPE/ammonothermal GaN substrates were carried out by using the growth condition resulting in low C concentration and showed a narrow PL DBX peak of 100 µeV, suggesting the higher quality of this material. Results on the performance and further limitations of Schottky diodes based on these achievements will be discussed.