by Laura Peters, contributing editor

November 23, 2010 – Deeply scaled MOSFETs require a self-aligned source/drain (S/D) manufacturing strategy. For the upcoming III-V semiconductors with high-mobility channels, low-resistance contact with self-aligned S/D is essential, but has not been successfully demonstrated. However, at the upcoming International Electron Devices Meeting (IEDM, 12/6-8 in San Francisco, CA), researchers from the University of Tokyo, the National Institute of Advanced Industrial Science and Technology (AIST, Ibaraki, Japan) and Sumitomo Chemical Co. (Ibaraki), will present promising results on the fabrication of a Ni-InGaAs alloy, formed by direct reaction of nickel and InGaAs, to yield low-resistance, self-aligned S/D InGaAs MOSFETs for the first time.

A major challenge the researchers faced is the low electron Schottky barrier height (SBH) with InGaAs. In their studies of the I-V characteristics of n-type and p-type InGaAs diodes, among the metals Ni, Ti, Sn and Al, nickel provided the highest current in the n-InGaAs contact and good rectifying behavior with an on/off ratio of 10⁵ in the p-InGaAs contact. Also, a 1-min 250°C anneal is sufficient to provide a uniform Ni-InGaAs alloy layer.

The Schottky barrier height was determined to be 83-120meV. To lower this further, the team fabricated Ni-InGaAs/In_xGa_1-xAs Schottky diodes with varying concentrations of indium (x=0.4, 0.53, 0.6, 0.7, 0.8). The indium layers, with a thickness of 500nm, were grown on InP substrate by metalorganic vapor phase epitaxy. The nickel was deposited by evaporation. The I-V characteristics (Figure 1) showed an enhancement of the reverse current with increasing indium content, but the rectifying behavior was weakened because of the bandgap lowering. The Schottky barrier height was zero for indium content of 0.7 and 0.8 (Figure 2). Sheet resistance was low and constant with indium content.

Figure 1: I-V characteristics of Ni-InGaAs alloy/n- InxGa1-xAs and Ni-InGaAs/p- InxGa1-xAs diodes with x=0.4, 0.53, 0.6, 0.7, 0.8.

With favorable electrical results, the team went on to fabricate In_0.7Ga_0.3As MOSFETs.

With higher indium content came simultaneous increases in drive current and lowering of S/D resistance. The lower Schottky barrier height and low sheet resistance of Ni-InGaAs alloy delivers reduced S/D resistance in InGaAs MOSFETs with high electron mobility.

Figure 2: Increasing indium content brings down the Schottky barrier height of the as-deposited diode and Ni-InGaAs/ InxGa1-xAs after RTA.