Demonstration of Large Flatband Voltage Shift by Designing Al2O3/SiO2 Laminated Structures with Multiple Interface Dipole Layers
Authors: Kita Koji , Kamata Hironobu 
Published: February 2020

The presence of the dipole layer often results in a flatband voltage (VFB) shift in metal/high-k/SiO2/Si gate stacks typically in the order of a few hundreds of mV [1]. Enlargement of the attainable range of VFB shift caused by those interface dipole layers would be advantageous in some applications including power MOSFETs, which need high VTH for the stable operation of the devices. However, the amount of positive VFB shift attainable by a single interface dipole layer formation is limited within ~0.6 V using Al2O3-on-SiO2 stacks. One of the simple solutions to overcome this limitation would be adding up the VFB shifts induced by multiple interface dipole layers in a laminated dielectric stack. In this study we investigated the VFB shifts of MOS capacitors employing laminated stacks with Al2O3 and SiO2 layers ((Al2O3/SiO2)n) as the gate dielectric [2], to demonstrate a possible design of a stack with a large positive shift of VFB (>1 V).
Silicon wafers with a resistivity of 1-5 Ωcm were thermally oxidized to grow 5 nm-thick-SiO2 followed by the fabrication of three kinds of samples (i)-(iii). The sample (i) is Au/Al2O3(2-8 nm)/SiO2/Si stack obtained by Al2O3 deposition by rf-sputtering on thermally grown oxide. The sample (ii) is Au/SiO2(2-7 nm)/Al2O3(3.0 nm)/SiO2/Si stacks, prepared by a SiO2 growth by the oxidation of metallic-Si with different thicknesses on the identical structure with the sample (i), to investigate the effects of the dipole layer formation by the deposition of top-SiO2. The metallic-Si was deposited by electron beam evaporation method followed by the oxidation conducted at 800°C in O2 ambient. The sample (iii) is Au/Al2O3(2-6 nm)/SiO2(3.0 nm)/Al2O3(3.0 nm)/SiO2/Si stack fabricated by the deposition of Al2O3 on the identical structure with the sample (ii), to evaluate the effects of dipole layer formation at top-Al2O3-on-SiO2 interface.
The VFB shift caused by each dipole layer was determined from capacitance-voltage characteristics by excluding the effect of fixed charges. From the first-Al2O3 thickness-dependence of VFB for Al2O3/SiO2/Si structure of sample (i), it was found that a dipole layer inducing +0.47±0.05 V VFB shift was formed. For SiO2/Al2O3/SiO2/Si structure of sample (ii), one might expect a formation of the dipole layer at the top SiO2-on-Al2O3 interface should induce VFB shift with the same magnitude but the opposite direction to the one observed for the sample (i), because both are the interfaces between Al2O3 and SiO2 but with the opposite stacking sequences. However, the experimentally observed VFB shift due to dipole layer at the top SiO2-on-Al2O3 was determined to be only 0~+0.2V in our experimental condition. We speculate that the dipole effects for this interface would be suppressed because the thermal oxidation process of metallic-Si on Al2O3 results in a reduction of the stress at this interface, taking account of the model [1] that the dipole layer would be triggered by structural relaxation by an adjustment of oxygen density at the interface. For Al2O3/SiO2/Al2O3/SiO2/Si structure of sample (iii), the VFB shift caused by the dipole layer at the top Al2O3-on-SiO2 interface was estimated as approximately +0.8±0.2 V to the positive direction. As a result, a large VFB shift ~+1.2 V in total was demonstrated for this stack due to the additivity of multiple dipole layers in the laminated stack [2], thanks to the selective formation of dipole layers at Al2O3-on-SiO2 interfaces and the effective suppression of those at SiO2-on-Al2O3 interfaces. From these results we can conclude that by designing the number of laminated layers and tuning the stacking conditions, a very large VFB shift (>1V) is attainable with ((Al2O3/SiO2)n) laminated dielectric layers.
[1] K. Kita and A. Toriumi, Appl. Phys. Lett. 94, 132902 (2009).
[2] H. Kamata and K. Kita, Appl. Phys. Lett. 110, 102106 (2017).