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Annealing accelerates electron mobility

In-situ annealing nearly doubles electron mobility in InGaAs MISFETs
�A team of researchers from Japan has almost doubled electron mobility in InGaAs MISFETs by performing an annealing step prior to the deposition of a HfO2 gate dielectric.

These efforts by engineers from the National Institute of Advanced Industrial Science and Technology and Sumitomo Chemical will help to enhance the credentials of III-V channels as alternatives to silicon for maintaining the march of Moore's Law. Higher mobilities allow a reduction in operating voltage of the transistor while maintaining its current, thus enabling ICs to maintain performance while consuming less power.

Traditionally, researchers have paired InGaAs MISFETs with an Al2O3 dielectric to create transistors with high mobility and a low interface state density. But the dielectric constant of this oxide is just 9, which is half that of HfO2, the gate dielectric used within the silicon industry today.

A higher dielectric constant is preferred, because it should make it easier to shrink transistor dimensions while maintaining performance. However, replacing Al2O3 with HfO2 has been hampered by a reduction in inversion electron mobility.

The team from Japan has overcome this by loading the epiwafers in an atomic layer deposition tool, and prior to deposition of the dielectric, annealing them in an argon atmosphere for 30 minutes at 300 degC. After this, a film of HfO2 is deposited using a 50-cycle process, before gates are patterned using a standard reactive-ion etching process and metal contacts are added.

X-ray photoelectron spectroscopy measurements on this device, and also on a control that had not been annealed, revealed that the former structure produced a stronger signal at an energy associated with gallium oxides. This occurred because annealing is estimated to increase the thickness of the GaOx from 0.15 nm to 0.20nm.

A series of different electrical measurements by the team showed that the annealed device had a much smaller frequency dispersion around the threshold voltage, which was the result of a decrease in the density of border traps in the HfO2 dielectric.

Mobility in the annealed sample peaked at about 1250 cm2 V-1 s-1 for a surface carrier density of around 2.5 x 1012 cm-2. In comparison, the mobility for the control was just shy of 700 cm2 V-1 s-1 for a surface carrier density of around 3.5 x 1012 cm-2.

The team attributes superior mobility to effective GaOx passivation at the interface between InGaAs and HfO2

�Reference: M. Oda et al. Appl. Phys. Express 7 061202 (2014)

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