Effects of Downstream Plasma Exposure on β-Ga₂O₃ Rectifiers

Abstract

The effects of downstream plasma exposure with O2, N2 or CF4 discharges on Si-doped Ga2O3 Schottky diode forward and reverse current-voltage characteristics were investigated. The samples were exposed to discharges with rf power of 50 W plasma at a pressure of 400 mTorr and a fixed treatment time of 1 min to simulate dielectric layer removal, photoresist ashing or surface cleaning steps. Schottky contacts were deposited through a shadow mask after exposure to avoid any changes to the surface. A Schottky barrier height of 1.1 eV was obtained for the reference sample without plasma treatment, with an ideality factor of 1.0. The diodes exposed to CF4 showed a 0.25 V shift from the I–V of the reference sample due to a Schottky barrier height lowering around 14%. The diodes showed a decrease of Schottky barrier height of 2.5 and 6.5% with O2 or N2 treatments, respectively. The effect of plasma exposure on the ideality factor of diodes treated with these plasmas was minimal; 0.2% for O2 and N2, 0.3% for CF4, respectively. The reverse leakage currents were 1.2, 2.2 and 4.8 μA cm−2 for the diodes treated with O2, and CF4, and N2 respectively. The effect of downstream plasma treatment on diode on-resistance and on-off ratio were also minimal. The changes observed are much less than caused by exposure to hydrogen-containing plasmas and indicate that downstream plasma stripping of films from Ga2O3 during device processing is a relatively benign approach.

A Comprehensive Review on Recent Developments in Ohmic and Schottky Contacts on Ga2O3 for Device Applications

“…On the other hand, significant surface degradation was found after plasma exposure, and the damage did not recover fully even after annealing at 500 °C. Xia et al . investigated the effects of downstream plasma exposure with O 2 , N 2 , and CF 4 discharges on Si-doped β-Ga 2 O 3 Schottky barrier diodes fabricated on HVPE-grown 10 μm thick β-Ga 2 O 3 to separate chemical effects from physical damage due to ion bombardment and to simulate processes like photoresist ashing, dielectric removal, and surface cleaning that occur during device fabrication.…”

“…191 On the other hand, significant surface degradation was found after plasma exposure, and the damage did not recover fully even after annealing at 500 °C. Xia et al 192 194 They observed a near-ideal reverse leakage current attributed to the TFE tunnelling mechanism in surface electric field in the range from 0.8 MV/cm to 3.4 MV/cm. The SBHs in the range 1.37−1.40 eV extracted from reverse I−V characteristics were consistent with values extracted from forward I−V and C−V characteristics.…”

“…It was observed that only CF 4 exposure leads to a significant change with a decrease of 14% in SBH to 1.1 eV SBH without any plasma exposure, while O 2 and N 2 exposures decreased the SBH by 2.5% and 6.5%, respectively. Minimal changes were observed in ideality factors, turn-on resistance, and rectification ratio 192. Luo et al also reported reverse leakage current reduction in vertical β-Ga 2 O 3 SBDs fabricated on 8 μm thick HVPE-grown (001) β-Ga 2 O 3 epilayers through the Schottky barrier modification by low power CF 4 plasma surface treatment 193.…”