Effect of Various Pre-Treatments on Ti/Al/Ti/Au Ohmic Contacts for AlGaN/GaN HFET Devices

Abstract

The effect of a number of different treatments on the contact resistance for Ti/Al/Ti/Au (30/80/120/ 55 nm) contacts to Al 0.3 Ga 0.7 N (20 nm)/GaN HFET structures were investigated. The structures were grown by MBE. Neither preannealing of the surface at temperatures up to 1100 C in N 2 , nor etching of the surface using chemically assisted ion beam etching (CAIBE) was effective in reducing the contact resistance. Both light CAIBE etching (%5 nm) to partially remove the AlGaN layer, and deeper etching (%50 nm and 100 nm) to remove the AlGaN and contact the GaN channel layer directly, resulted in a substantial increase in the measured contact resistance. The only treatment that was found to decrease the contact resistance was a brief wet etch in K 2 S 2 O 8 /KOH solution under 254 nm UV irradiation. A resist stabilization treatment was developed so that the process was self-aligned. The contact resistance in W mm was decreased by a factor of 1.7 for the optimum etch time. Scanning electron microscopy showed that the etching resulted in the formation of discrete holes perforating the AlGaN layer.Introduction Both the rf and dc characteristics of AlGaN/GaN HFET devices are strongly dependent on the ohmic contact resistance of the source/drain contacts. Ti/Al based contact schemes (such as Ti/) have been successfully used by many workers in the fabrication of high power devices. Various schemes have been proposed to improve these ohmic contacts. Inductively coupled plasma N 2 discharges [1], or O 2 reactive ion etching [2] have been demonstrated to reduce the contact resistance in some work. Pre-annealing, prior to metal deposition, has been also shown to reduce the contact resistance for GaN [3], and also in some cases, for AlGaN/GaN HFET structures [4], although very long pre-anneals were used in the latter case.

Investigation of surface treatment schemes on n-type GaN and Al0.20Ga0.80N

“…Investigations of ohmic contact formation to GaN-based semiconductors using different metallization schemes such as Ti/ Al/ Ti/ Au, Ti/ Al/ Pt/ Au, Ti/ Al/ Ni/ Au, and V / Al/ Pt/ Au have been reported. [1][2][3][4][5][6][7][8][9][10][11] In conjunction with efforts to achieve low-resistance ohmic contacts to n-GaN using low work-function metal contacts, surface treatment techniques have also been proposed as an additional route to improve ohmic contacts, as the nature of the GaN surface is crucial to the formation of high quality ohmic contacts. Gallium oxide is formed on GaN and AlGaN surfaces when epilayers are exposed to ambient atmosphere.…”

Review—Progress in Electrochemical Etching of Third-Generation Semiconductors

“…Geng et al 75 studied effects of the types of metal catalyst used in electrodeless photoelectrochemical etching, and found that the promotion order of metals to the etching rate was Ag < Au < Ir < Pt, and the difference to the band bending caused by the metal-semiconductor contact. Additionally, Bardwell et al 76,77 also used KSO system as the etchant, and carried out a series of studies on the effect of parameters on the etching morphology in the electrodeless photoelectrochemical etching of GaN.…”