[Paper Review] Formation of Strain-Induced Quantum Dots in Gated Semiconductor Nanostructures

Ted Thorbeck, Neil M. Zimmerman|arXiv (Cornell University)|Sep 11, 2014

Semiconductor materials and devices2 references4 citations

TL;DR

This paper demonstrates that elastic strain from metal gates in silicon nanostructures can induce quantum dots (QDs) due to strain-induced band structure modifications, explaining unexplained many-electron QDs in devices where electrostatic barriers alone cannot account for their formation. The strain-induced QDs are eliminated by replacing metal gates with highly doped poly-silicon, highlighting strain as a critical factor in QD device design.

ABSTRACT

Elastic strain changes the energies of the conduction band in a semiconductor, which will affect transport through a semiconductor nanostructure. We show that the typical strains in a semiconductor nanostructure from metal gates are large enough to create strain-induced quantum dots (QDs). We simulate a commonly used QD device architecture, metal gates on bulk silicon, and show the formation of strain-induced QDs. The strain-induced QD can be eliminated by replacing the metal gates with poly-silicon gates. Thus strain can be as important as electrostatics to QD device operation operation.

Motivation & Objective

• To investigate the role of elastic strain from metal gates in unintentionally forming quantum dots in silicon-based nanostructures.
• To resolve the long-standing puzzle of unexplained many-electron quantum dots appearing at consistent locations across different devices.
• To demonstrate that strain-induced band structure changes can produce tunnel barriers and localized states equivalent to electrostatically defined QDs.
• To propose a solution—replacing metal gates with highly doped poly-silicon—to eliminate strain-induced QDs in device architectures.
• To establish strain as a critical design parameter on par with electrostatics in low-temperature semiconductor nanostructures.

Proposed method

• Simulating strain distributions in a metal-gate-on-bulk-silicon device architecture using finite element methods, accounting for thermal expansion coefficient mismatch between metal and silicon.
• Calculating the change in the conduction and valence band structure due to strain using the deformation potential approximation, particularly focusing on the sum of strains (ε_y + ε_z) for light holes.
• Applying the strain-induced band shift to predict effective potential barriers and quantum dot formation in the band structure.
• Modeling a silicon nanowire with nickel contacts to analyze strain-induced tunnel barriers and QDs, assuming no Schottky barrier formation.
• Using the strain-induced potential to estimate tunneling resistance and assess the stability of charge localization in the QD.
• Comparing results between metal-gate and poly-silicon-gate architectures to demonstrate the elimination of strain-induced QDs.

Experimental results

Research questions

• RQ1 Can elastic strain from metal gates in silicon nanostructures produce quantum dots that are indistinguishable from electrostatically defined QDs?
• RQ2 Why are many-electron quantum dots consistently observed at the same locations in different devices, despite efforts to reduce defects?
• RQ3 To what extent does strain alter the band structure in a way that creates potential minima capable of localizing charge carriers?
• RQ4 Can replacing metal gates with highly doped poly-silicon eliminate strain-induced quantum dots?
• RQ5 How does the magnitude of strain-induced potential compare to electrostatic potential in typical QD device operation?

Key findings

• Strain from metal gates on bulk silicon produces a localized potential minimum in the conduction and valence bands, forming a strain-induced quantum dot with a peak in the band energy at the center of the device.
• The strain-induced quantum dot in the nanowire system forms a potential well between two tunnel barriers, each with a height of 5 meV and a width of 30 nm, resulting in an estimated tunneling resistance of 45 MΩ.
• The calculated strain-induced potential is sufficient to localize holes and create a stable, quantized QD even in the absence of Schottky barriers.
• Replacing metal gates with highly doped poly-silicon eliminates the strain-induced QD by reducing the thermal expansion coefficient mismatch, thus removing the source of elastic strain.
• The strain-induced QD explains the persistent observation of QDs at fixed locations across devices, inconsistent with random defect-based models.
• Strain effects are as significant as electrostatic effects in determining QD formation, necessitating strain engineering in future QD device design.