Ti/Al/Ti/Au and V/Al/V/Au Contacts to Plasma-Etched n-Al0.58Ga0.42N

Abstract

Cross-sectional transmission electron microscopy was used to study annealed Ti/Al/Ti/Au and V/Al/V/Au ohmic contacts to as-received and plasma-etched n-Al 0.58 Ga 0.42 N. The reaction depth of low-resistance V-based contacts to as-received n-Al 0.58 Ga 0.42 N is very limited, unlike previously reported Ti-based contacts to n-Al x Ga 1-x N. In the present study, the Ti/Al/Ti/Au contacts to as-received n-Al 0.58 Ga 0.42 N required much higher annealing temperatures than the V-based contacts and also exhibited deeper reactions on the order of 40 nm. To achieve a low contact resistance on plasma-etched n-Al 0.58 Ga 0.42 N, different metal layer thicknesses and processing conditions were required. The Ti-and V-based contacts to plasma-etched n-Al 0.58 Ga 0.42 N exhibited both similar contact resistances and limited reaction depths, along with the presence of an aluminum nitride layer at the metallization/semiconductor interface. Metal channels penetrate the aluminum nitride layer connecting the top of the metallization to the n-Al 0.58 Ga 0.42 N. The similarity in phase formation in the contacts to plasma-etched n-Al 0.58 Ga 0.42 N is likely the reason behind the similarity in specific contact resistances.

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“…Cr, Zr, Nb, …) or the substitution of Ti by V. In the latter case, the generation of a heavily ndoped thin layer at the AlGaN interface is driven by the Al in the metal scheme while V acts as an indiffusion barrier [68]. Circumstantial evidence reported by several authors of the N extraction from the n-AlGaN layer and the consequent low ncontact resistance was the formation of a continuous thin AlN layer (~2-5 nm thick) at the interface [62,68]. Nevertheless, the driving force for the reaction metal/AlGaN weakens as AlGaN becomes more energetically stable with the increase in Al mole fraction: N atoms create a more stable compound with Al than with Ga due to the lower formation enthalpy of AlN (−318.1 kJ mol −1 ) in comparison to GaN (−110.9 kJ mol −1 ) [58,61].…”

“…While investigating the n-contact formation for higher Al mole fractions, the most common approach has been to transfer the already established metallization scheme for n-GaN to the n-AlGaN material system. Unfortunately, Ti-based electrodes have shown to form ohmic contacts with n-AlGaN only up to an AlN molar fraction of 0.58 [62]. Typically, the best reported values of contact resistivities are lower than 10 −6 Ω cm 2 on n-GaN.…”

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“…3,6,8,90 Ohmic contact to n-type Al x Ga 1-x N.-For n-type Al x Ga 1-x N, ohmic contacts can be easily formed by using Ti/Al-based or V/Albased metal schemes. [91][92][93][94][95][96][97][98][99][100][101][102][103][104][105][106] The Ti/Al-based metal schemes are generally used as contacts to n-Al x Ga 1-x N, producing contact resistivities of 10 −4 −10 −6 cm 2 after annealing at high temperatures. [91][92][93][94][95][96] The ohmic contact formation was explained by the formation of AlN and TiN phases, generating donor-like nitrogen vacancies (V N ) and hence increasing donor concentrations near the surface region.…”

“…[91][92][93][94][95][96] The ohmic contact formation was explained by the formation of AlN and TiN phases, generating donor-like nitrogen vacancies (V N ) and hence increasing donor concentrations near the surface region. 92,98,104 However, Ti/Al-based contacts had a tendency to form Schottky behavior as the Al mole fraction increases, 104,105 while V/Al-based schemes provided lower contact resistance due to the formation of vanadium nitride (VN). The generation of V N and the low work function (3.56 eV) of VN were shown to be responsible for the lower contact resistance.…”

Ohmic Contact to High-Aluminum-Content AlGaN Epilayers

“…6,7 However, there are very few reports on similar studies for silicon-doped n-AlGaN with an aluminum percentage of greater than 50%. [8][9][10] Miller et al 11 had earlier reported on ohmic contact to 58% AlGaN, by increasing the annealing temperature and also by utilizing a vanadium-based metallization scheme. With increasing aluminum percentage it is widely expected that the difficulty of making ohmic contacts will increase owing to the increased ionization energy and decreased mobility of carriers.…”