The passivation of modulation-doped Al 0.1 Ga 0.9 N/GaN Schottky surfaces by treatment with ͑NH 4 ͒ 2 S and P 2 S 5 /͑NH 4 ͒ 2 S has been investigated. The current-voltage ͑I-V͒ curves of these Schottky diodes demonstrate that the Ti/Pt/Au-Al 0.1 Ga 0.9 N/GaN Schottky diodes prepared by P 2 S 5 /͑NH 4 ͒ 2 S treatment have the lowest reverse-leakage current ͑7.5 nA/cm 2 ͒ and the highest Schottkybarrier height ͑0.98 eV͒. Detailed X-ray photoelectron spectroscopy analysis indicates that a thin sulfide layer on the samples with the ͑NH 4 ͒ 2 S treatment has an improvement for these favorable characteristics. Further, a thin phosphide layer on the samples with the P 2 S 5 /͑NH 4 ͒ 2 S treatment exhibits a larger improvement on the Schottky-barrier height and the reverse-leakage current.The wide-bandgap Al x Ga 1−x N/GaN high-electron mobility transistors ͑HEMTs͒ have great potential for use in high-power and high-frequency microwave power applications 1,2 due to their high temperature, high breakdown, and excellent electron-transport properties. However, reliable Schottky contacts are required before Al x Ga 1−x N/GaN HEMTs can be extensively used. In literatures, many researchers reported effective enhancement of barrier height and reduction of leakage current for Schottky contacts on AlGaN by the insertion of a thin Al layer, 3 the surface control process, 4 and the postannealing process. 5 Additionally, sulfide treatment with the solutions of ͑NH 4 ͒ 2 S 6 and P 2 S 5 /͑NH 4 ͒ 2 S 7,8 is well known to form thin sulfide layers on GaAs-or InP-related semiconductors. Recently, Liu et al. 9 reported that treating Pt/Au Schottky contacts ͑formed on modulation-doped AlGaN/GaN heterostructures͒ with ͑NH 4 ͒ 2 S solution could maximize the Schottky barrier height ͑SBH͒. This proves that ͑NH 4 ͒ 2 S treatment is an effective method in the passivation of Al 0.1 Ga 0.9 N surface. However, the use of P 2 S 5 /͑NH 4 ͒ 2 S to passivate Al 0.1 Ga 0.9 N/GaN Schottky diodes still lacks investigation. Thus, the Al 0.1 Ga 0.9 N/GaN Schottky surfaces prepared by the ͑NH 4 ͒ 2 S and P 2 S 5 /͑NH 4 ͒ 2 S treatment are studied in this work.The Al 0.1 Ga 0.9 N/GaN epi-samples were grown by metallorganic chemical vapor deposition ͑MOCVD͒ on 2 in. sapphire wafers. The structure of the samples comprises a sapphire substrate, a GaN buffer layer, a 2 m undoped GaN layer, and a 0.5 m Si-doped Al 0.1 Ga 0.9 N. The AlGaN/GaN interface is an abrupt from secondary-ion mass spectrometry ͑SIMS͒ analysis. The carrier concentration was 1.29 ϫ 10 17 cm −3 for n-Al 0.1 Ga 0.9 N. All the samples were cleaned by standard processes. The ohmic contacts were initially patterned on the n-Al 0.1 Ga 0.9 N surface using the standard photolithographic lift-off technique. A multilayered Ti/Al/Ti/Au ͑20/100/40/150 nm͒ ohmic contact was deposited on the grown sample by electron-gun evaporation at an applied pressure of 3.2 ϫ 10 −7 Torr. Then, those ohmic contacts were annealed at 900°C for 5 s in ambient N 2 . Three kinds of sulfidation samples were prepared as fo...
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