Effects of Oxygen Annealing of β-Ga₂O₃ Epilayers on the Properties of Vertical Schottky Barrier Diodes

Abstract

Electrical characteristics of vertical Schottky barrier diodes (SBDs) fabricated on as-grown and oxygen annealed β-Ga 2 O 3 (001) epilayers were investigated. SBDs on as-grown epilayer showed anomalous reverse leakage characteristics. Annealing of β-Ga 2 O 3 epilayers in an oxygen-containing environment up to 40 min immensely reduced the reverse leakage current. The specific onresistance (R on ) of the SBD remained very close to that of the as-grown sample for annealing up to 20 min and increased almost by 25 times for annealing up to 40 min. Simulations of reverse tunneling characteristics, assuming oxygen vacancy type surface states, explained both the magnitude and the shape of anomalous reverse leakage. The diffusion of oxygen during annealing passivated the oxygen vacancy type surface states at first (for 20 min annealing)-resulting in two orders of reduction in leakage with minimal change in R on . Further annealing (up to 40 min) subsequently reduced the epilayer net carrier concentration from 3.3 × 10 16 to 2.9 × 10 15 cm −3 -resulting in immense change in both reverse leakage and R on . Thus, oxygen annealing proved to be a vital technique for the passivation of surface states and to reduce the net carrier concentration, which allows the modulation of reverse leakage of β-Ga 2 O 3 SBDs.

Identification and Suppression of Majority Surface States in the Dry-Etched β-Ga2O3

“…) and the ionized donors, leading to the additional barrier and making surface semi-insulating, 34 which is consistent with the widened depletion region and the lowest N D − N A for sample S3. 6 In summary, the majority surface states induced by the dry etching have been identified to be E2* traps (E C −0.75 eV) related to divacancies (V Ga −V O ) in β-Ga 2 O 3 , which are subsequently suppressed via the optimized surface treatments. With the combined oxygen annealing and piranha treatment, the resultant SBDs exhibit the negligible hysteresis current− voltage characteristics and a unity ideality factor.…”

“…Figure a shows the semilogarithmic J–V of diodes S1–S4. All diodes exhibit high rectification ratios >10 9 at ±2 V. Notably, the oxygen-annealed sample S3 exhibits the lowest reverse leakage current due to the charge compensation effect in a formed ultrathin semi-insulating (SI) surface layer . All forward J–V features obey strictly the thermionic emission (TE) model, as given by where J 0 = (1.50–2.43) × 10 –11 A/cm 2 is the saturation current density obtained by fitting, A * = 33.65 A/(cm 2 K 2 ) is the Richardson constant of β-Ga 2 O 3 , and R s is the series resistance.…”

“…All diodes exhibit high rectification ratios >10 9 at ±2 V. Notably, the oxygen-annealed sample S3 exhibits the lowest reverse leakage current due to the charge compensation effect in a formed ultrathin semi-insulating (SI) surface layer. 6 All forward J−V features obey strictly the thermionic emission (TE) model, as given by 3…”

“…The 1/C 2 slope difference results from the charge density variation of surface states in the UID β-Ga 2 O 3 epilayer. 6 In particular, sample S3 exhibits the lowest capacitance corresponding to its minimum leakage current, 2 as the SI interfacial layer contributes an additional capacitance (C I ) in series with the capacitance (C L ) of depletion region the β-Ga 2 O 3 epilayer. In terms of 1/C = 1/C I + 1/C L , the SI thickness (d) is estimated to be 13 nm, which is exactly equal to the difference of the depletion widths for samples S3 and S4.…”

“…Dry etching is a crucial fabrication process to define device patterns or remove surface contaminants but in turn inevitably induces surface damages and deteriorates device performance . To overcome this barrier, various surface treatments have been reported, which had substantial impacts on the electrical properties of β-Ga 2 O 3 power diodes. − Notably, the majority traps ( E C –0.75 eV, named E2*) were attributed to an intrinsic defect that has been reported to be related to gallium vacancies (V Ga ) , or divacancy complexes (V Ga –V O ) , in β-Ga 2 O 3 . Recently, the E2* trap was also reported as a dominant contributor to limit the Schottky diode performance and as the primary source of the subthreshold instability of β-Ga 2 O 3 MESFETs. − So far, the underlying trap evolution and the associated correlation of carrier transport and surface electronic structures still lack exploration.…”